aluminum hydroxides

Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2002 Electronic ReleaseALUMINUM OXIDE - Bayer Process (L. Keith Hudson, Chanakya Misra, Anthony J. Perrotta, Karl Wefers, F. S. Williams)

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1. General Aspects

Almost 4000 years ago Egyptians and Babylonians used aluminum compounds in various chemicals and medicines. HERODOTUS mentioned alum in the fifth century B.C. and PLINY referred to "alumen," now known as alum, as a mordant to fix dyes to textiles around 80 A.D. In 1754 MARGGRAF showed that a distinct compound existed in both alum and clays. In 1761 the French chemist GUYTON DE MORVEAU proposed the name "alumine" for the base in alum, identified in 1787 by ANTOINE LAVOSIER as the oxide of a then-undiscovered element. By the 1700s the earthy base alumina was recognized as the potential source of a metallic element.GREVILLE (1798) described a mineral from India that had the composition Al2O3 and named it corundum [1302-74-5], derived from the native name of this stone. HAÜY (1801) called a mineral diaspore [14457-84-2] (from the Greek "diaspora" meaning dispersion) because it decrepitated on heating. Its composition, Al2O3 · H2O, was determined by VAQUELIN in 1802. Gibbsite [14762-49-3], named after the American mineralogist G. GIBBS, was found by DEWEY in 1820; TORREY (1822) showed this mineral to have the composition Al2O3 · 3 H2O. The name hydrargillite [14762-49-3] was given to a similar mineral found later in the Ural Mountains. Using the newly developed technique of X-ray diffraction, BÖHM and NICLASSEN [13] identified a crystalline aluminum oxide hydroxide, Al2O3 · H2O, later named böhmite (boehmite). BÖHM discovered a second type of trihydroxide, Al2O3 · 3 H2O, a year later. FRICKE [14] suggested the name bayerite [20257-20-9] for this compound, believing it to be the product of the Bayer process, which he later identified as gibbsite. Only few occurrences of natural bayerite have been reported [15]. VAN NORDSTRAND et al. [16] reported a third form of trihydroxide, which was later named nordstrandite [13840-05-6] in his honor.

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1.1. Aluminum Hydroxides

A general classification of the various modifications of aluminum hydroxides is shown in Figure (1). The best defined crystalline forms are the three trihydroxides, Al(OH)3: gibbsite, bayerite, and nordstrandite, and two modifications of aluminum oxide hydroxide, AlO(OH): boehmite and diaspore. Besides these well-defined crystalline phases, several other forms have been described in the literature [2], [3]. However, there is controversy as to whether they are truly new phases or simply forms with distorted lattices containing adsorbed or interlamellar water and impurities.Identification of the different hydroxides and oxides is best carried out by X-ray diffraction methods [3]. Mineralogic and structural data are listed in Tables (1) and (2).

Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2002 Electronic Release © Wiley-VCH. Weinheim. Germany.Page: 1


Gelatinous hydroxides may consist of predominantly X-ray indifferent aluminum hydroxide or pseudoboehmite. The X-ray diffraction pattern of the latter shows broad bands that coincide with strong reflections of the well-crystallized oxide hydroxide boehmite.The aluminum hydroxides found abundantly in nature are gibbsite, boehmite, and diaspore. Gibbsite and bayerite have similar structures. Their lattices are built of layers of anion octahedra in which aluminum occupies two thirds of the octahedral interstices. In the gibbsite structure, the layers are somewhat displaced relative to one another in the direction of the a axis. The hexagonal symmetry of this lattice type (brucite type) is lowered to monoclinic. Triclinic symmetry was found in larger gibbsite single crystals from the Ural Mountains [17].In bayerite the layers are arranged in approximately hexagonal close packing. Because of shorter distances between the layers, the density is higher than in the case of gibbsite. The crystal class and space group of bayerite have not yet been established clearly.The individual layers of hydroxyl ion octahedra in both the gibbsite and the bayerite structures are linked to one another only through weak hydrogen bonds. Bayerite does not form large single crystals. The most commonly observed growth forms are spindle- or hourglass-shaped somatoids. The long axis of these somatoids stands normal to the basal plane; i.e., the somatoids consist of stacks of Al(OH)3 layers (Fig. (2)). The effect of alkali ions on the structures of Al(OH)3 types was investigated by several workers [3], [18], [19]. Intercalation of Li+ ions transforms gibbsite to the hydrotalcite structure [20].Gibbsite crystals of appreciable size are not uncommon. Clear pseudohexagonal platelets about 1 mm in diameter are known from Arö in Norway. Prismatic crystals 0.5–1 mm in length are occasionally produced in the Bayer process.Nordstrandite, the third form of Al(OH)3, was described by VAN NORDSTRAND [16] and others [21]. The structures of nordstrandite and bayerite were investigated [22] and compared with those of monoclinic and triclinic gibbsite, which had been determined previously [17].The lattice of nordstrandite is built of the same, electrically neutral Al(OH)3 octahedral layers that form the structural elements of gibbsite and bayerite [14]. The lattice period amounts to 1.911 nm in the direction normal to the layer. This corresponds to the sum of identical layer distances of bayerite plus gibbsite. The identical nordstrandite structure consists of alternating double layers, in which the OH octahedra are arranged once in the packing sequence of bayerite, and then in that of gibbsite. Material containing continuous transitions from bayerite through nordstrandite to gibbsite has been prepared through the proper selection of precipitation conditions [18].

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1.2. Aluminum Oxide Hydroxides

Pseudoboehmite is formed during aging of X-ray indifferent hydroxide gels as a precursor of trihydroxide. The reflections of pseudoboehmite are broadened not only because of the very small particle size, but also because of variable distances of the AlO(OH) double chains, which form the



structural element of pseudoboehmite as well as of well-crystallized boehmite. The lattice spacing in the direction of the c axis increases by 0.117 nm for each mole of excess water [22].Boehmite consists of O, OH double layers in which the anions are in cubic packing. The aluminum ions are octahedrically coordinated. These layers are composed of chains of [AlO(OH)]2 extending in the direction of the a axis [23]. The double layers are linked by hydrogen bonds between hydroxyl ions in adjacent planes. Boehmite crystals exhibit perfect cleavage parallel to the (010) plane.In the diaspore structure the oxygen ions are nearly equivalent, each being joined to another oxygen center through a hydrogen ion. The anions are hexagonally close packed [24]. The position of the hydrogen ion has been established by neutron diffraction [25]. The O–H–O distance is 0.265 nm. By infrared studies, the bond energy for the hydrogen bridges in diaspore was determined to be 28.7 kJ/mol, compared with 20.1 kJ/mol for boehmite [26].

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1.3. Aluminum Oxide, Corundum

The hexagonally closest packed -Al2O3 modification is the only stable oxide in the Al2O3–H2O system. Corundum is a common mineral in igneous and metamorphic rocks. Red and blue varieties of gem quality are called ruby and sapphire, respectively. The lattice of corundum is composed of hexagonally closest packed oxygen ions forming layers parallel to the (0001) plane. Only two-thirds of the octahedral interstices are occupied by aluminum ions. The structure may be described roughly as consisting of alternating layers of Al and O ions. The corundum structure was determined in the early 1920s [27]; numerous workers later confirmed and refined these data [3]. Properties of corundum are listed in Tables (1) and (2).

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1.4. The Al2O3–H2O System

Under the equilibrium vapor pressure of water, crystalline Al(OH)3 converts to AlO(OH) at about 375 K. The conversion temperature appears to be the same for all three forms of Al(OH)3. At temperatures



lower than 575 K, boehmite is the prevailing AlO(OH) modification, unless diaspore seed is present. Spontaneous nucleation of diaspore requires temperatures in excess of 575 K and pressures higher than 20 MPa. In the older literature, therefore, diaspore was considered the high-temperature form of AlO(OH). The first reaction diagram of the phase transitions in the Al2O3–H2O system was published in 1943 [28]. These workers determined the gibbsite boehmite conversion temperature to be 428 K. Boehmite transformed to diaspore above 550 K; diaspore converted to corundum, -Al2O3, at 725 K. Similar results were reported in 1951 [29].The system was reinvestigated in 1959 [30] and in 1965 [31]. A phase diagram based on these data is shown in Figure (3). Diaspore is the stable modification of AlO(OH); boehmite is considered metastable, although it is kinetically favored at lower temperatures and pressures. This is because the nucleation energy is lower for boehmite than for the considerably more dense diaspore. Nucleation is additionally facilitated by the possibility that boehmite can grow epitaxially on Al(OH)3. In the Al2O3–Fe2O3–H2O system, the presence of the isostructural goethite, -FeO(OH), lowers the nucleation energy for diaspore so that this AlO(OH) modification crystallizes at temperatures near 373 K [32]. This observation explains the occurrence of diaspore in clays and bauxite deposits that have never been subjected to high temperatures or pressures.Nomenclature. Although there is fairly good agreement in the more recent literature on phase fields and structures of the crystalline phases in the Al2O3–H2O system, the nomenclature is still rather unsystematic.Bayerite, gibbsite (hydrargillite), and nordstrandite are trihydroxides of aluminum, and not oxide hydrates. The designation "aluminum oxide monohydrate" for boehmite and diaspore is also incorrect. Both are true oxide hydroxides. Molecular water has been determined only in poorly crystallized, nonstoichiometric pseudoboehmite.The designation of the modifications of aluminum hydroxides and oxides lacks uniformity just as much as does the nomenclature of the compounds. According to the general usage in crystallography, the most densely packed structures are designated as -modifications [3]. Bayerite, diaspore, and corundum fall within this class. The compounds with cubic packing sequence, gibbsite and boehmite, have been designated by the symbol . Nordstrandite can be classified as -Al(OH)3 when regarding this compound not as an intergrowth of bayerite and gibbsite, but as an independent modification.

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1.5. Thermal Decomposition of Aluminum Hydroxides

When aluminum hydroxides or oxide hydroxides are heated in air at atmospheric pressure, they undergo a series of compositional and structural changes before ultimately being converted to -Al2O3. These thermal transformations are topotactic. Despite a loss of 34 or 15% of mass for the trihydroxides or oxide hydroxides, respectively, the habit of the primary crystals and crystal aggregates changes very little. This leads to considerable internal porosity, which may increase the specific surface area of the material to several hundred m2/g. Structural forms develop that, although not thermodynamically



stable, are well reproducible and characteristic for a given temperature range and starting material. These transition aluminas have been the subject of numerous investigations because of their surface activity, sorptive capacity, and usefulness in heterogeneous catalysis. The literature in this field of physical chemistry has been reviewed up to 1987 [3].The simplest transformation is that of diaspore to corundum. As the structures of these two compounds are very similar, the nucleation of -Al2O3 requires only minor rearrangement of the oxygen lattice after the hydrogen bonds are broken. A temperature below 860–870 K is sufficient for complete conversion. The newly formed corundum grows epitaxially on the decomposing diaspore, with the (0001) plane of Al2O3 parallel to the (010) plane of AlO(OH) [33]. Recent work [34] has shown that transformation to corundum (-Al2O3) proceeds through an intermediate '-Al2O3 phase.

The thermal transformation, at ambient pressure, of boehmite and the trihydroxides to -Al2O3 requires considerably more structural rearrangements and is generally not completed until the temperature reaches at least 1375–1400 K. The first step in the reaction sequence is the diffusion of protons to adjacent OH groups and the subsequent formation of water [35], [36]. This process begins at a temperature near 475 K. If this water cannot diffuse rapidly out of larger trihydroxide particles, hydrothermal conditions may develop locally, resulting in the formation of -AlO(OH). With increasing loss of water, a large internal porosity develops. The lattice voids left by the escaping water are not readily healed because of the slow diffusion in this low temperature range. The voids are oriented parallel and perpendicular to the basal plane of the trihydroxide crystals (Fig. (4)). The highest surface area and lowest crystalline order of the solid (not counting newly formed boehmite) is obtained at a temperature around 675 K. With increasing temperature the surface area decreases, while the density of the solid shows progressively higher values (Fig. (5)). This trend is the result of progressive reordering and consolidation of the solid.During the thermally driven consolidation and reordering, the solid goes through structural stages that are influenced by the nature of the starting material as well as by heating rates, furnace atmosphere, and impurities. The general reaction paths are illustrated in Figure (6), which shows the various intermediate transition forms that have been identified during the reordering process.Transition oxides formed at lower temperatures are mostly two-dimensional, short-range ordered domains within the texture of the decomposed hydroxides. Extensive three-dimensional ordering begins at about 1050 K. Until completely converted to corundum, the solid retains considerable amounts of OH– ions. Most likely protons are retained to maintain electroneutrality in areas deficient of cations. Therefore, the presence of protons may retard the reordering of the cation sublattice. The high surface area (>75 m2/g) of -Al2O3 has been shown to provide thermodynamic stability [37]. Addition of fluorine to the furnace atmosphere removes protons. As a result, rapid transition to -Al2O3 occurs at temperatures as low as 1150 K. Markedly tabular corundum crystals form, possibly because the preceding transition alumina is mostly two-dimensionally ordered [38].Transition forms other than those shown in Figure (6) can be obtained by hydrothermal treatment [3]. The structures of various transition forms have been investigated [17], [22], [39].

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1.6. Aluminates and Related Compounds

Sodium oxide forms several compounds with aluminum oxide. These so-called -aluminas represent a group of aluminates having the same, or very similar, structures but variable chemical composition. Their molar ratios (Na2O:Al2O3) can vary between 1:1 and 1:11. The 1:1 sodium aluminate, NaAlO2 [1302-42-7], exists in at least two allotropic modifications. Orthorhombic -NaAlO2 is stable below 750 K; the higher temperature modification is tetragonal. For preparation and properties of technical sodium aluminate, Aluminum Compounds, Inorganic - 2.1. Sodium Aluminate.

The 1:11 -Al2O3 crystallizes from melts containing aluminum oxide and sodium oxide or other sodium compounds. A 1:5 -alumina has been prepared by heating -Al2O3 with NaAlO2 or Na2CO3 at about 1325 K.

Several other -aluminas containing CaO, BaO, or SrO in 1:6 ratio also have been reported [3].

Recent interest in -alumina is related to its use as a solid electrolyte in sodium–sulfur secondary batteries ( Batteries).

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2. Bauxite, the Principal Raw Material

2.1. Definition and Geology

The term bauxite [1318-16-7] is used for sedimentary rocks that contain economically recoverable quantities of the aluminum minerals gibbsite, boehmite, and diaspore. The name derives from the description by BERTHIER, in 1821, of a sediment that occurred near the village of Les Baux in the Provence, France. Originally assumed to be a dihydrate of aluminum oxide, bauxite was later recognized as being composed of aluminum hydroxide, iron oxide and hydroxide, titanium dioxide, and aluminosilicate minerals [2].Early in this century, major bauxite deposits were found in various parts of the Tertiary and Cretaceous limestone formations of the European Alps; also in several locations on the North American continent, e.g., in Arkansas, Alabama, and Georgia. Since the 1920s, extensive deposits have been discovered in the tropical and subtropical climate belts.The oldest known bauxites developed in the Precambrian; the youngest are of recent origin. Deposits may occur as extensive, flat bodies blanketing areas of many square miles; they may form irregularly shaped fillings of dolinas in old karst surfaces, or lenses several hundred meters in diameter and tens of meters thick. Allochtonous, i.e., displaced bauxites, are also common; erosion of primary deposits and redeposition of the detritus in valleys or along mountain slopes has frequently led to mineable accumulations of ore.



Many of the geologically younger bauxites are covered only by thin layers of soil; others are buried by coastal or alluvial sediments. Older deposits, especially in the Balkans or the Ural Mountains, often are overlain by carbonate rocks hundreds of meters thick. Regardless of their geologic age, all bauxites were formed during continental periods [40].

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2.2. Composition and Properties

The chemical composition of bauxites of various origin is given in Table (3) [2]. Aluminum oxide, iron oxide, and titanium and silicon dioxides are the major chemical components of all bauxites. Alkali and alkaline earth compounds are rarely found.

Gibbsite, -Al(OH)3, is the predominant aluminum mineral in the geologically young bauxites of the tropical climate belt. Mesozoic and older bauxite contain mostly boehmite (-AlO(OH)) or diaspore (-AlO(OH)). Since their formation, many of the older bauxites have been buried under considerable layers of younger sediments and often were subjected to tectonic stress. Boehmite and diaspore formation appears to be related to an increasing degree of metamorphism. However, both minerals also occur in young deposits, although in minor quantities. The chemical environment obviously plays as important a role in the formation of boehmite and diaspore as do pressure and stress [40].Dissolution of gibbsite requires the mildest conditions in the Bayer process (see Section 3.1.2. Digestion). Higher temperatures and alkali concentrations are necessary for the digestion of boehmite and diaspore. Technically, both oxide hydroxides can be processed without difficulties. The current abundance of high-grade gibbsitic bauxites, however, has made boehmite- and diaspore-rich ores economically less attractive.

Goethite, -FeO(OH) [1310-14-1], and hematite, -Fe2O3 [1309-37-1], are the most prevalent iron minerals in bauxites. They are practically inert under the conditions of the Bayer process. In both materials, some of the iron may be replaced isomorphically by aluminum ions. This amount of aluminum is included in the chemical analysis of the bauxite but is normally not extracted in the digest. Magnetite (Fe3O4) is found in some European bauxites; pyrite (FeS2) and siderite (FeCO3) also may occur. Decomposition of pyrite may lead to high sulfur levels in the process solutions.Anatase, TiO2 [1317-70-0], is the titanium mineral found most frequently in bauxites. Rutile, TiO2 [1317-80-2], occurs in some European deposits; FeTiO3 is also present in minor quantities, especially in titanium-rich bauxites. Titanium dioxide minerals are attacked under only the most severe conditions of the Bayer process.Silicon dioxide may occur as quartz [14808-60-7], as in the bauxites of the Darling Range in Western Australia. Most commonly, however, SiO2 is associated with the clay minerals kaolinite, halloysite, or montmorillonite. These aluminosilicates react with sodium aluminate solutions to form insoluble sodium aluminum silicates during digest, causing loss of sodium hydroxide and extractable alumina. The amount of so-called reactive silica is one of the major factors determining quality and price of the ore.



Minor constituents, such as chromium, vanadium, zinc, and gallium, have little effect on the Bayer process or on the quality of the final product. Some tend to accumulate in the recirculated process solutions (e.g., gallium and vanadium) and must be removed periodically by appropriate treatments.The physical properties of bauxites, i.e., texture, hardness, and density, can vary widely. Geologically old diaspore bauxites, especially those high in iron oxide, are very hard and can reach densities of 3.6 g/cm3. Young tropical deposits, in contrast, may have an earthy, soft texture and a density around 2.0–2.5 g/cm3. Allochtonous bauxite often consists of hard nodules embedded in a soft, usually clayey matrix. Porous textures also occur. The color of bauxites is largely determined by the type and particle size of the prevalent iron mineral. Highly dispersed goethite tends to be yellow to orange, whereas dark brown tones usually are associated with coarser hematite. Colors can vary greatly within a single ore body.Hardness, texture, and the amount of overburden determine the methods applied for bauxite mining. Deposits in Greece, former Yugoslavia, Hungary, and the former Soviet Union require deep mining to depths of several hundred meters, often complicated by the difficulties of controlling water levels in the porous limestone formations. Tropical bauxites frequently are located so close to the surface that they can be recovered with normal earth-moving equipment.

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2.3. Genesis of Bauxites

Many of the geologically young bauxite deposits are located in the savannah region, which extends north and south of the tropical rain forest belt. The climate of this region is characterized by a high mean annual temperature and abundant precipitation during the rainy season. Deposits occur on gently sloping hills or on peneplains. The stratigraphic evidence shows that these bauxites have formed in situ. Parent rocks may be coarse-grained, igneous rocks such as syenite, phonolite, basalt, or gabbro. However, large deposits also developed on kaolinitic sandstones, on phyllites, and on schists. A layer of kaolinitic clay is frequently found between the ore body and the parent rock.Bauxite probably forms during long periods of low geologic activity when the combination of high temperature, abundant precipitation, and good vertical drainage favors intensive chemical weathering. The sequence of leaching begins with removal of alkali followed by the removal of alkaline earths. Oxides of iron, aluminum, titanium, and silicon are mobilized and reprecipitated as hydroxides and oxides. Aluminum hydroxide and silica form kaolinite, Al4(OH)8Si4O10. This sequence first leads to the formation of tropical soils (laterites). Bauxitization follows when the climatic, chemical, and topographic conditions prevail long enough to allow the removal of silicon dioxide as well.The bauxite deposits of the Mediterranean region, the Caribbean Islands, and many other locations that are associated with tertiary and older limestone formations (karst bauxites) were formed by a similar weathering process. Parent materials were lateritic soils and clays transported into the karst region and deposited in depressions. During extended terrestrial periods, high mean temperatures, copious precipitation, and good vertical draining through the porous limestone bedrock facilitated a thorough



desilification. In the older literature, the parent material for karst bauxitization was reported to be clayey residue left after weathering of substantial layers of carbonate rocks [41]. Researchers have shown conclusively that igneous rocks were the source of this material [42][43][44].Comprehensive reviews on the geology, mineralogy, and genesis of bauxites have been published [40], [45].

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2.4. Major Bauxite Deposits

Until the 1950s, the European aluminum industry was supplied from the karst bauxite deposits of France, Hungary, former Yugoslavia, and Greece. United States sources (Arkansas) and ore from Surinam provided the raw material for North American production. Since then, the picture has changed dramatically. The four largest bauxite producers of 1990, namely, Australia, Guinea, Jamaica, and Brazil (Table (4)), hardly would have been mentioned in 1950. Today, they provide more than half of the world's total bauxite output.Africa. The Savannah region covers an area of the African continent that has experienced low geologic activity for a long time. In this belt, which stretches from the Ivory Coast to Madagascar, very large bauxite deposits were found. The major production is currently concentrated in Guinea and Ghana. Cameroon, Sierra Leone, Mali, and the Congo region, among other areas, have substantial reserves.Australia. Major deposits are located in the Darling Range in Western Australia, on the Gove Peninsula in the Northern Territory, and on the Cape York Peninsula in Queensland. Bauxite also occurs in New South Wales, in Victoria, and on the island of Tasmania. The Australian deposits developed between the Eocene and Pliocene epochs on substrates ranging from Precambrian sandstones to Tertiary basalts. Gibbsite is the predominant aluminum mineral, although some boehmite occurs in all but the Western Australian deposits.South America. On the outer slopes of the old Guyana Shield, many economically important deposits have been found. They are located in the Amazon Basin of Brazil, in Colombia, Venezuela, Surinam, Guyana, and French Guyana. Surinam and Guyana have been producing for more than 60 years, whereas the Brazilian Amazon deposits have been mined only recently. Bauxite also is produced in the Pocos de Caldas area in Southern Brazil, state of Minas Gerais. The South American bauxites generally are geologically young, gibbsitic ores.Caribbean. Jamaica and the Dominican Republic have major reserves of karst bauxites that occur on Tertiary limestones under generally very thin overburden. Although gibbsite is the main aluminum mineral, some boehmite also is present. Jamaica has been one of the world's leading bauxite producers for the past 20 years.North America. The only economically important deposit is located in Arkansas, where gibbsitic bauxite developed on nepheline syenite during the Eocene. Less than 5 × 107 t of bauxite remain, and the grade is declining.Europe. Except for a few commercially insignificant occurrences, all European bauxites are of the karst



type. The oldest deposits (Devonian/Mississippian) are those of the Tikhvin area in the former Soviet Union; the youngest are the Eocene bauxites of former Yugoslavia. Most European deposits developed during the Lower and Upper Cretaceous, e.g., the diaspore and boehmite bauxites of France, Greece, and Romania, and the gibbsite and boehmite bauxites found in Hungary, former Yugoslavia, and Italy. Today, all European mines combined contribute only about 15% of world production.Asia. Major deposits of gibbsite bauxites occur in India; the island of Kalimantan (Indonesia) has large potential reserves. Numerous bauxite deposits, most of them containing geologically old, diaspore-rich ore, were found in China. Substantial deposits also are located in Western Siberia.

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2.5. Economic Aspects

The proved reserves of bauxite shown in Table (4) are sufficient to supply the world aluminum industry for a few centuries. Total resources are estimated by the U.S. Geological Survey at 55 to 75 × 109 t. Because of the worldwide distribution of significant ore deposits, a disruption of bauxite supply for political reasons appears highly unlikely. In 1974 several major bauxite-producing nations formed the International Bauxite Association (IBA) with the intent of increasing control over the exploitation of their bauxite deposits. Although levies were increased substantially, competition from countries not associated with the IBA helped maintain a reasonable price structure.Dissolution of gibbsite requires the mildest conditions in the Bayer process (see Section 3.1.2. Digestion). Higher temperatures and alkali concentrations are necessary for the digestion of boehmite and diaspore. Technically, both oxide hydroxides can be processed without difficulty. The current abundance of high-grade gibbsitic bauxites, however, has made boehmite- and diaspore-rich ores economically less attractive.Economic and political considerations currently favor refining of bauxite near the deposit and shipment of either alumina or aluminum ingot. Brazil, Surinam, and Australia have smelters, although their refining capacity by far exceeds the demands of domestic metal production. Jamaica and Guinea refine a substantial portion of their own bauxite production.

About 25% of all bauxite mined is used for producing refractories, abrasives, catalysts, adsorbents, and other industrial chemicals.

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References

3. Bayer Process

3.1. History and Procedure

In 1855 the French mining engineer LOUIS LE CHATELIER obtained alumina from bauxite by sintering with sodium carbonate at 1200 °C and leaching the sodium aluminate with water. Aluminum hydroxide was then precipitated from the sodium aluminate solution by carbon dioxide. Early use of aluminum hydroxide was chiefly as a mordant in the textile dyeing industry. The demand for pure alumina increased rapidly when it became the raw material for aluminum production upon development of the Hall–Héroult cell. The Austrian chemist KARL JOSEPH BAYER received German patent 43977 in August, 1888 for a new, improved process for production of aluminum hydroxide from bauxite, and the process became known as the Bayer process in his honor. Bayer initially worked on his process at the Tentelev chemical plant near St. Petersburg in Russia. Between 1888 and 1900 he supervised the construction of Bayer process plants in Germany, England, France, Italy, and the United States. The Merrimac Chemical Company in Massachusetts first used the Bayer process in the United States, and the Alcoa plant in East St. Louis, Illinois, was constructed in 1901. Since then, plants have been built in more than 25 countries, and the present world alumina capacity is over 40 × 106 t/a (Table (5)).The important features exploited in the Bayer process are that boehmite, gibbsite, and diaspore can be dissolved in NaOH solutions under moderate hydrothermal conditions; the solubility of Al2O3 in NaOH is temperature dependent; most other components of the bauxite are quite inert in the process; and the silica that does dissolve subsequently forms a nearly insoluble compound. These features permit formation of a sodium aluminate solution, physical separation of the impurities, and precipitation of pure Al(OH)3 from the cooled solution.The main features of the Bayer process have remained unchanged for the last 100 years, although the scale of operations has been enlarged considerably due to chemical engineering developments. Figure (7) shows the flow sheet of the process as it is now practiced. Each operation in the process is carried out in a variety of ways. The process begins with preparation of the bauxite by blending for uniform composition followed by grinding. In most plants the bauxite is ground while suspended in a portion of the process solution. This slurry is mixed with the balance of the heated NaOH solution, then treated in a digester vessel at well above atmospheric pressure. The digest reaction is:

Al(OH)3 + Na+ + OH– Na+ + Al(OH)4–

Additional reactions convert impurities such as SiO2, P2O5, and CO2 to relatively insoluble compounds. The slurry leaves the digester at a temperature above its atmospheric boiling point and is cooled by flashing off steam as the pressure is reduced in several stages. The flashed steam is used to heat the slurry and the solution going to the digester. The bauxite residue solids are separated from the sodium aluminate solution in two steps so that the coarse fraction is processed separately from the fine. Both residue fractions are washed and discarded. The solution, being free of solids, is cooled and seeded with fine crystals of Al(OH)3; this causes the Al(OH)4

– ions to decompose to Al(OH)3, thereby reversing the reaction that previously had taken place in the digester. Again, the heat removed in cooling the solution is used to heat a colder stream in the process. After the precipitation reaction has proceeded to the point that about half of the Al2O3 in the solution has been removed, the mixture of solids and solution is sent to classifiers. The fine Al(OH)3 particles are returned to the process to serve as seed. The coarse particles are washed and calcined to Al2O3. Excess solution introduced in washing the product and the residue must be removed by evaporation. In some cases the solution is treated to remove both organic and inorganic impurities before the solution is recycled through the plant.



3.1.1. Bauxite Preparation

The bauxite entering the refinery must be uniform and sufficiently fine that extraction of the Al2O3 and the other operations are successful. The chemical composition of bauxite varies. Uniformity is improved by blending material mined from several pits and, if necessary, by adding bauxite from storage piles. A more recent development is to store ground bauxite slurry in surge tanks before it is pumped to digestion. These agitated tanks are operated so that plant feed is uniformly blended for several hours. Sometimes bauxites are dried to improve handling or washed to remove clay. Hard bauxite is reduced to particles finer than 2 cm in roll or cone crushers and hammermills. Before it enters the process it is ground further to less than 0.15 cm.Previously, fine grinding was done in dry mills operating in closed circuit with vibrating screens. Such operation required very dry bauxite to avoid blinding the screens. This resulted in a dusty working environment. In most modern plants, the bauxite is mixed with a portion of the process solution and is ground as a slurry. Rod mills and ball mills are used most frequently. The ground slurry may be passed over screens or through cyclones, with the fine particles progressing and the coarse ones being returned to the mills.In all-wet grinding the bauxite feed and the flow of solution are controlled to keep the solids content of the slurry between 45 and 55%. The power consumed by the mill drive is an indicator of the amount of grinding being done and may be used to control the feed to the mill. On a longer time scale the particle size of the product can be used to make changes in the grinding operation.

3.1.2. Digestion

In digestion, all of the Al2O3 in the bauxite must be extracted. A solution is produced that contains the maximum Al2O3 concentration that can remain stable through the rest of the process. This must be accomplished while using a minimum amount of energy.The conditions for digestion can and do vary widely. The first consideration is whether the available Al2O3 is present as gibbsite, boehmite, diaspore, or a mixture of these minerals. The dissolution rates of the three are quite different. Generally, if the bauxite contains mixed phases, the digestion conditions will be chosen on the basis of the least soluble compound. Any Al(OH)3 or AlOOH left undissolved can act as seed in the clarification step, causing precipitation of Al(OH)3 while the residue is still in contact with the solution. In the sweetening process, boehmitic bauxite is digested under relatively mild conditions, producing an intermediate Al2O3 concentration. In a separate vessel, gibbsitic bauxite is added to the flow from the first digest to raise the Al2O3 concentration to the desired level. Another approach is to digest only the gibbsite in a mixed bauxite. The residue is separated from the solution and redigested under more severe conditions to recover the boehmite. This method reduces the flow to the high-temperature digest and so requires lower capital and operating costs.The solubility data (Fig. (8)) show that the Al2O3 concentration in the process solution can be increased by increasing the temperature, the NaOH concentration, or both [1][2][3][4]. As a result, operating conditions in plants vary widely. Higher digest temperatures result in higher pressures, making the equipment more expensive. There also is the need for more heat-exchange equipment, which further increases capital costs. High concentrations, on the other hand, permit increased production from a given flow rate and, hence, from a given plant installation. Precipitation is thought to occur better at lower concentrations, but use of low precipitation concentrations while digesting at high concentrations requires dilution and additional operating costs for subsequent evaporation. Choosing digester



conditions involves balancing these physical factors with local economics, together with the designer's experience. This has resulted in the spectrum of operating conditions given in Table (6).The conditions listed in the first line of Table (6) are those for digesting bauxite at the atmospheric boiling point. Quite high alkali concentrations are required and the evaporation requirement is an extraordinary 5.3 t of water for a tonne of Al2O3. Most plants digesting gibbsitic bauxite use the conditions on the second line of Table (6). Boehmitic bauxite is digested using one of two general sets of conditions. The first is European practice, in which higher concentrations and dilution prior to precipitation are preferred to higher digest temperatures. When American companies began processing boehmitic bauxite from the Caribbean area, most chose the same concentrations used for gibbsitic bauxite; therefore, a higher temperature was required.The second important reaction in digestion is desilication. In the equation below, kaolinite (1) is used to illustrate the reaction of siliceous minerals with the process solution:

Al2O3 · 2SiO2 · 2H2O + 6NaOH 2NaAlO2 + 2Na2SiO3 + 5H2OThe soluble products react to form a series of precipitates with zeolite structure, having a composition of approximately Na8Al6Si6O24(OH)2.

Depending on temperature and concentration, the ratio of Al2O3:×SiO2 in the zeolite (tectosilicate) structure can vary. For each Al3+ replacing Si4+ in the lattice, one Na+ is taken up to maintain charge neutrality. Anions, such as SO4

2– or CO32– may substitute for OH– in the structure. The formation of the

zeolitic desilication product (DSP) therefore leads to costly loss of sodium hydroxide. This reaction, however, is necessary for lowering the level of dissolved SiO2 to less than 0.6 g/L, the maximum acceptable concentration.The rate-determining step in the desilication reaction is the nucleation and crystallization of the desilication product. Therefore, the presence of seed particles is important; without seeds, solutions containing 0.75 g/L SiO2 will not react in 40 min at 415 K. This has led to the paradoxical situation that some bauxites contain too little reactive SiO2 for good desilication. The slurry blending and storage operation discussed earlier can be an important part of the desilication process. If the storage temperature is above 355 K, about 80% of the reaction takes place in 8 h. More important, seed is formed so the desilication reaction in the digest is not delayed. Very low SiO2 concentrations can be achieved if an excess of CaO is charged. At high temperatures, and with the lime additions, a less soluble desilication compound (cancrinite) is formed. European plants use longer holding times during or after digestion to facilitate desilication.Some CaO is added to the digest even when extreme desilication is not required. The calcium reacts with the carbonate and phosphate compounds as follows:

CaO + H2O Ca(OH)2

Ca(OH)2 + Na2CO3 CaCO3 + 2 NaOH (causticization)

5 Ca(OH)2 + 3 Na3PO4 Ca5(PO4)3OH + 9 NaOHThe last reaction controls the level of phosphates in the process solution; this impurity can affect clarification adversely. Causticization in the digest was more important when prices were such that the sodium required by the process was supplied more cheaply as Na2CO3. Its current importance is in control of the CO3

2– concentration, which, at high levels, can affect precipitation. In modern practice, the causticization reaction is carried out on dilute process solutions outside the digester. Maintaining low concentrations and having much of the NaOH combined as NaAlO2 are favorable to more complete reaction of CO3

2– with Ca2+.

3.1.3. Equipment



The equipment for digestion includes the reactor vessel, heat-exchange equipment, and pumps. The first reactors were horizontal vessels with crude agitators mounted on axial shafts. These were filled with a slurry of bauxite, lime, and process solution. They were closed and heated individually by injecting high-pressure steam. At the end of the designated holding time, a discharge valve was opened and a slurry was forced into another vessel at atmospheric pressure. Only a small portion of the steam flashing from the slurry as the pressure was reduced could be recovered. Continuous operation was introduced in the 1930s in which heat could be exchanged between the hot stream leaving the digester and the cooler, incoming stream. This greatly reduces the energy requirements.Reactors. Digester vessels, whether batch or continuous, provide intimate contact between the bauxite and the solution for a period long enough to complete the extraction and desilication processes. The equipment designed for this purpose is quite diverse. Agitated horizontal vessels have been replaced by vertical units to avoid the mechanical problems with sealing the agitator shaft. Both designs have used a series of vessels to minimize short circuiting of the flow. European practice includes use of a series of vertical agitated vessels, each operating at a higher temperature than the preceding one. These units are heated by flashed steam in internal steam coils. A German digester design uses concentric pipes; the liquid being heated flows through the inner pipe while the hot slurry from the digester is returned through the outer pipe. This tube digester has been modified by placing several tubes for the cold slurry in parallel inside a larger outer tube. This reduces the length of the unit. The variety of digester design is illustrated in Figures (9), (10). The flow through single-digester units may be as high as 12 m3/min.The usual construction material for digesters is mild (low-carbon) steel, despite data showing that, at the temperatures and NaOH concentrations involved, there may be stress corrosion cracking resulting from alkali embrittlement. The presence of the [Al(OH)4]

– ions reduces the activity of OH– in the solution. Finely divided Fe2O3 in the bauxite quickly saturates the solution with Fe3+, suppressing corrosion of the metal. Some digestion equipment has been nickel plated for safety.Flashing. In most plants the hot slurry from the digester is flashed in a series of pressure vessels until it reaches the atmospheric boiling point. The steam generated in flashing is used in heat exchangers to heat the flow of liquor and bauxite coming to the digester. Flashing also removes water from the process stream, thereby accomplishing a significant portion of the evaporation needed. Normally, heaters are tubular, but coils of tubing also are used. As few as three stages of flashing may be needed when the digestion temperature is 418 K, and this may be increased to ten stages for a unit operated at 515 K. Figure (9) is a schematic diagram of a high-temperature digestion unit.Heat Exchangers. Operating practice has been divided as to whether the bauxite slurry should be heated separately or whether it should be mixed with the rest of the solution and then heated. These two modes of operation have been designated the single- and the double-stream processes. Those favoring separate heating of the bauxite slurry, the double-stream option, feared rapid deposition of desilication product on the heat-transfer surfaces. Experience has shown that, particularly if the bauxite slurry is stored for blending, those fears are unfounded. The rate of fouling is no higher than for heating clean liquor. In both options there is slow deposition of desilication product on the heat-exchanger surface because the desilication reaction does not reach equilibrium in the digester and continues at a slow rate wherever the solution temperature is raised. The encrustation must be removed periodically by washing with dilute HCl or H2SO4, or by mechanical means to maintain a high heat-transfer rate.In some plants using the double-stream system, the bauxite slurry is heated in contact heaters. These heaters are designed much like barometric condensers. The liquid is forced to fall through the steam in the heater in droplets or in thin films. The large surface area exposed results in high heat-transfer rates, and because there is no metallic surface, there is no fouling. The penalty is higher dilution.The heat needed to raise the temperature above that achieved in the flashing system can be introduced by direct injection of steam. More often the final heating to the digest temperature is done in a separate heat exchanger, with the energy coming from steam, hot oil, or molten salts.Pumps. Centrifugal pumps are used in most digester systems. Those used for pumping slurries are



built of wear-resistant alloys. A great deal of maintenance is required to keep the shaft seals operating well. In some plants, concern about pump wear has led to the use of diaphragm pumps. In such pumps a check valve admits the slurry to a chamber on one side of a flexible diaphragm. When that chamber is full, oil at high pressure is forced into a chamber on the other side of the diaphragm. This moves the slurry through another check valve into the digester. Therefore, no rapidly moving parts are in contact with the slurry. In a Japanese modification, called the Hydrohoist, the diaphragm is replaced by a sphere that is free to move the length of a cylinder. Slurry is kept in one end of the cylinder and the drive fluid, which is clean process solution, is in the other end. Because of this choice of drive fluid, leakage past the sphere is of no consequence and tight clearances are not required.Measurement and Control. Operating variables that must be controlled are temperature, solution concentration, and degree of desilication. The holding time is fixed by the design of the digester vessels. As noncondensable gases are formed by the reaction of the solution with organic materials in the digest, it is necessary to vent these gases so that the digester remains full. Venting can be controlled by sensing the liquid level with a float or a radioactive sensor. Temperature measurement and control are relatively simple because only the temperature from the final heater is critical. Concentration control comes from blending the bauxite and careful proportioning of the slurry and clean solution flows. Originally, feedforward control was used in which the composition of the incoming materials was used to set the rates at which they were added. In modern practice feedback control is based on the composition of the exiting solution. Chemical analyses of the solution can be used, but the time lag involved makes this control imprecise. Nearly real-time control has been achieved by using the fact that the conductivity of the solution at a given temperature is a linear function of the mass ratio of Al2O3 and NaOH. This is the variable of interest. In modern plants the output of the sensor goes to a computer, which in turn controls the amount of bauxite slurry pumped. Such control has reduced the variance in concentrations to about a third of the values experienced earlier. This permits operation closer to the maximum safe concentration with less risk of premature precipitation of Al2O3 in clarification.

3.1.4. Residue Separation

The next step in the process is the separation of bauxite residue solids from the solution. A wide variety of equipment and procedures is used in this operation. The methods chosen depend on the quantity and properties of the residue.

The particle size distribution of the residue solids is usually bimodal. The coarse fraction over 100 m in diameter is termed sand, whereas the rest of the solids are finer than 10 m. The sand fraction may range from 5 to nearly 50% of the total. The lowest amounts are in the Caribbean bauxites and the highest are in the residue from Western Australian bauxite. In the plant the sand fraction is separated from the process stream in liquid–solids cyclones or in more primitive settling devices. A variety of equipment, including Dorr, Hardinge, and Aikens classifiers, have been used to wash the sand free of process solution before it is discarded. In all of these units the sand moves countercurrently to the wash water so that a maximum of washing is done with a minimum of water. The wash solution is added to the balance of the process flow and proceeds to further clarification steps for removal of the fine solids.In most plants the fine residue fraction is settled in raking thickeners of the type illustrated in Figure (11). These vessels may exceed 49 m in diameter. Some older plants used multideck thickeners. Difficulties in keeping the multideck units in balanced operation have led to almost exclusive use of single-deck units in new construction.The desanded slurry is fed at the center of the thickener and clarified solution overflows at the perimeter. As the solution flows radially across the thickener, both the horizontal and vertical velocities



become very low. The solids, having a higher specific gravity than the solution, sink to the bottom of the thickener. A settling rate of 1.5 m/h is sufficient for commercial operation. The solution overflowing the thickener usually contains less than 0.3 g/L solids, whereas the underflow ranges from 15 to 35 wt% solids. The rotating mechanism has plows mounted at an angle to the arm. These slowly move the settled pulp across the bottom of the thickener to a discharge port. A few units have been designed with the discharge at the perimeter rather than at the center. The objective was to avoid the need to elevate the units to provide access to a center discharge.The fine solids behave as a relatively stable colloidal suspension, so they settle slowly if not treated further. The addition of flocculants improves the clarity of the thickener overflow, the settling rate of the solids, and the solids content of the underflow pulp. Flocculants act by binding the very fine particles into flocs that may be several millimeters in diameter. The ratio of mass to drag forces is increased so the flocs settle more rapidly.Starch from grains or roots was the first flocculant; the dosage varied from 0.5 to 3 kg per tonne of bauxite. The amount required increases as the surface of the residue increases and is affected by the mineral composition of the residue. Starch has the advantages of being inexpensive and ubiquitous. It does react with the NaOH in the solution to add organic compounds to the solution. In the 1950s water-dispersible polymers were proposed as flocculants. The first compounds were effective when added at 10% of the starch charge, but they were expensive so they offered no economic advantage. More recently, flocculants based on acrylate–acrylamide copolymers have begun to replace starch. The functional groups of these anionic flocculants can be modified to suit specific operating conditions. Different materials may be used in the thickeners and in the washing operation. In some cases a small amount of starch is added to the synthetic flocculant to improve overflow clarity. These synthetic materials now offer an economic advantage because they have been made less expensive and more effective.With the residue concentrated in the thickener underflow, the next task is to wash it free of process solution so that it can be discarded. This is usually done in countercurrent decantation systems using washing thickeners that are similar in design to those used for settling. The washing operation is done in as many as seven stages with the solids moving countercurrent to the wash stream. This is illustrated in Figure (12). An equation can be written for each stage expressing a material balance between soluble materials and dilution. Computer models of the washing system allow accurate prediction of performance.In residue washing, the objective is to minimize both the value of the soluble salts lost and the cost of evaporating the added wash water. The solubles in the discarded pulp can be reduced by increasing the solids content of the underflow. This reduces the amount of solubles carried from one stage to the next so that dilution is more effective and can be accomplished by using more flocculant or by increasing the holding time in the thickener. There is some risk in the latter change. If some undissolved gibbsite or boehmite remains in the residue, it may induce precipitation with a resulting loss of Al2O3.In some plants vacuum drum-type washing filters are used to replace some or all of the washing thickener stages. The solids content of the residue may be increased to 50–60 wt% by filtration. Therefore, only about 33% as much solution is needed compared to plants using thickeners and this results in a great increase in the effectiveness of washing. Calculations for countercurrent washing show that two stages discharging at 55% solids are more effective than six stages operating at 20% solids.Another approach only recently introduced into plant practice is called the deep thickener. This equipment was developed in the British coal industry to concentrate the slimes from coal washing. In the deep thickener, increased quantities of flocculant are used, and a column of residue pulp up to 10 m deep is maintained. The hydraulic pressure of the deep column appears to densify the pulp so that underflow solids approaching the concentration of filter cake are obtained. The overflow from these units contains more solids than the flow from standard thickeners, but the units are finding use in



concentrating residue for washing and for disposal. The simplicity of the equipment adds to its appeal.The final stage of clarification is polish filtration of the solution, sometimes called clear pressing. Most of the residue solids have been removed from the process solution in the previous steps, but product purity must be protected by removing the few particles of solids remaining. Filter presses are used most widely.Another approach to polish filtration is use of a stationary filter in which the medium is uniform, fine sand a meter deep. Gravity provides the force to move the solution through the sand bed. During operation, the fine solids are mixed with the sand, from which they are elutriated at the end of the cycle. Such filters can be automated and are especially effective with residues that are difficult to process.Close attention is required to keep the solids content of the filtrate down to 0.5 mg/L to protect product purity. Light transmission of the filtrate is used as an index of clarity. Purity originally was determined visually, but nephelometers are used now. If evidence of solids is found, the filtrate from each filter is scanned to locate the source. Some solids may pass through the filter cloth before a layer of solids forms to serve as a barrier, so it is common practice to recycle the first portion of filtrate.Control in clarification has two major objectives; the solids must be removed and the washed residue must be prepared for discard, using a minimum of wash water. The first objective is achieved by measuring the cloudiness of the filtrate. High solids in the underflow are dependent in part on the retention time in the thickener, so measurements must be made of the pulp level in the unit. Automatic determination of the interface between clarified liquid and pulp has been done by ultrasonic and by optical scanning. The density of the underflow pulp is monitored by a device that measures absorption of radiation or by gravimetric measurement. If the pulp is too dilute the rate of withdrawal is decreased. This control can be overridden if the pulp level in the unit rises too high so that there is concern that the solids in the filter feed may become excessive. The flows of solids and wash water also must be measured and proportioned for efficient operation.

3.1.5. Precipitation

The filtered solution has a temperature of 375 K and it must be cooled to 335–345 K before precipitation. This cooling is usually done in a flashing system analogous to that used in recovering heat in digestion. Because the temperatures are below the atmospheric boiling point, the flash vessels and the heat exchangers must operate under vacuum. Most of the heat removed by flashing is transferred to the colder solution returning from precipitation. Plate-type liquid–liquid heat exchangers have also been used. A recent development is to cool the solution further midway through the precipitation operation. This allows higher recovery of Al2O3 without using conditions that lead to excessive nucleation of product.Recovery of the Al(OH)3 from the process solution is known as precipitation, decomposition, or Ausrührung. The reaction is the reverse of the digester reaction given earlier. The cooling done after digestion and filtration has moved the solution into an area of the solubility diagram known as the metastable region. The concentration and temperature are such that the solution is supersaturated with Al(OH)3 but is not saturated enough for spontaneous crystallization. This is illustrated by Figure (13). At the digest temperature, T1, the Al2O3 concentration is increased to point P on the diagram. Cooling to temperature T2 results in crossing the solubility curve into the metastable region. Addition of Al(OH)3 seed at this point causes precipitation and the concentration of Al2O3 approaches the solubility curve at T2. The additional cooling recently adopted moves the solution conditions further to the left in the diagram above a lower point on the solubility curve.The kinetics of the precipitation reaction are represented by the equation:



where:

c = Al2O3 concentration, g/L

k = constant

E = activation energy, about 59000 J/mol

R = 8.31441 J mol–1 K–1

T = temperature, K

A = seed area, m2/L

c = concentration at equilibrium

The reaction is second order, i.e., the rate is affected by the square of the difference between the actual Al2O3 concentration and the equilibrium concentration. The operating temperature affects the equilibrium concentration and the reaction rate. Because seed area is important, the seed charge must be as high as can be maintained while meeting other operating objectives.The first objective in precipitation is to produce Al(OH)3 that, when calcined, meets the specifications for metallurgical alumina. These specifications are discussed in Section 5.2. Typical Specifications for Metallurgical Alumina. The second objective is to obtain a high yield from each volume of solution. This increases the plant capacity and reduces the amount of energy spent in heating and pumping the solution. At the same time, the processes serving to create new Al(OH)3 particles must be controlled so that the number of seed particles created equals the number of particles leaving the system as product. This requires balancing nucleation, agglomeration, growth, and particle breakage, so a combination of science and art has developed.About 2 × 1010 nuclei must be formed in each liter of solution to balance the process. The rate of nucleation is a strong function of both the degree of supersaturation and of temperature. Very few nuclei are formed if the temperature is above 350 K. A change in the operating temperature of only 5 K will change a system from a net producer to a net consumer of nuclei. Temperature changes of 2–3 K are the most frequently used control measures.The nuclei grow by accumulation of Al(OH)3 on their surface if they become large enough to be viable seeds. The rate of growth can be as high as 9 m/h, but it is generally much lower. The growth rate is independent of the particle diameter. Particles also increase in size by agglomeration. By a process that is not well understood, the nuclei form clusters in the first few hours of the cycle in batch precipitation. When the supersaturation is low, this agglomeration does not occur. Some speculate that when the rate of precipitation is high, the Al(OH)3 is not well crystallized when it is first deposited on the seed. This later may serve as the bond for agglomeration. At low supersaturation the deposited material is better ordered. The action of mechanical equipment in precipitation can break fragments off crystals or break up agglomerates, creating secondary nuclei, but this is not a major factor in most plants.Figure (14) is an electron micrograph of some Al(OH)3 produced in a plant operation. The particles are obviously the result of agglomeration. Those agglomerates having fewer particles can be shown by thin-section micrographs to have grown radially from an agglomerate of relatively few nuclei.In Figure (15), the Al2O3 concentrations in experimental precipitations is shown as a function of time and temperature. The slopes of these curves indicate the rates at which precipitation is progressing. All curves show a high rate at the start and appear to be approaching a final value asymptotically. The final concentrations were well above the saturation level, but in these as in plant operation the rate became unusually low, so the process was terminated.

Precipitators are vertical cylinders; the height, which may be 30 m, is usually 2.5–3.0 times the



diameter. The seeded slurry is circulated to maintain intimate contact between solids and solution. Early precipitators circulated the material by means of a central air lift pipe. Air introduced at the bottom of the pipe reduced the apparent density of the slurry within the pipe. This caused slurry to flow into the bottom of the pipe to establish circulation. Modern precipitators are circulated mechanically by impellers up to 3 m in diameter operating in a draft tube. In these units the flow is downward through the tube, so the tank bottoms are nearly flat to reverse the flow and cause upward motion of the slurry. Tanks with air lifts have conical bottoms to direct the flow to the central air lift.Originally, precipitators were filled individually in batch operation. This method had several disadvantages, chief of which was the need for many operators because the operating cycle of a single precipitator required at least 15 separate operations. Control was difficult and batch operation left equipment out of service part of the time.Nearly all plants built in the last 30 years have used the continuous system. Up to 13 tanks are placed in series so that the flow of seed and solution moves by gravity through channels connecting the tank tops. Continuous flows are much easier to measure and to automate. The number of flows is reduced, so control is better and the labor requirement is reduced. There are some operating problems. One is to avoid passage of the slurry across the tank tops, thereby reducing the retention time. Movement of solids from tank to tank is also a problem. An ideal system would retain fine particles in the tanks and move only the coarse material to the discharge. This is not possible, so a compromise method of operation must be found. Precipitation takes place at constant conditions in each tank instead of following a continuous curve, as in Figure (16). This tends to decrease production per tank, but the loss is more than recovered because the tanks are always in use.Several patterns have been used to supply the seed for precipitation. The earliest was to neutralize a portion of the solution. An Al(OH)3 gel was precipitated that changed into fine crystallites on aging. This very fine material was grown through at least one cycle to provide an active seed for product precipitation. The more common approach is to use fine particles, produced in the operation, as seed. The slurry leaving precipitation is classified into a coarse product fraction and one or more fine fractions. The fines are returned to the process to grow to product size. Classification is done by controlling the flow rate through tanks so that the fines are elutriated. Liquid–solid cyclones are coming into use because they provide more accurate separations. The seeding and classification systems have become increasingly complex. In modern plants at least three fractions are separated. In some plants the agglomerates formed from fine seed must pass through the system once more before becoming product. This reflects the belief that the agglomerates must be made tougher by deposition of additional Al(OH)3 so that they do not disintegrate in calcination and subsequent handling.In a modification [47], a relatively small charge of fine seed is added to the first tank in a series. This promotes agglomeration. Then, additional coarser seed is added to the second or third tank to provide the large seed area needed for high production. As indicated previously, cooling may be done between units in a series to increase the supersaturation in the final tanks, thereby increasing the production rate. A further change in modern plants is to filter the seed slurry being recycled. This decreases the amount of spent solution returning to the system and increases both the Al2O3 concentration in the first tank of the series and the overall production.The control variables available to the precipitation operator are the temperatures in the system; the flow per unit, which translates into holding time; the amount of seed; and the particle size of the seed. Some of these variables are difficult to control, so the system usually cycles slowly from coarser than desired to finer. There is enough inventory in the process so that minor cycles can be tolerated without affecting plant output.

3.1.6. Impurities



The impurities in the product Al2O3 are affected by all of the foregoing operations, including selection of the bauxite. There are at least three classes of impurities. The first is residue solids that are not removed in clarification. The large surface area of Al(OH)3 in precipitation effectively sweeps up these solids into the product. The preventive measure is increased vigilance in filter maintenance and operating. The second class is materials that are in true solution, such as sodium and gallium. The third group includes materials that are in solute form at the temperature of digestion but precipitate later in the process. Their solubilities may be only milligrams per liter and return of the solution to equilibrium at lower temperature may be slow. If precipitation is not completed before filtration, the remaining impurities appear in the product. There are also other impurities that do not fall neatly into these categories.Sodium is the main component of the process solution and is also the largest contaminant of the product. The Na2O content of alumina can be decreased by increasing the seed charge and by increasing the precipitation temperatures. The real variable that these operating changes help control is the rate of crystal growth. Higher temperatures, greater seed area, and lower Al2O3 concentrations all slow growth and give impurities more time to diffuse from the surface. By proper control of these variables an order of magnitude decrease in the Na2O content of a commercial product has been achieved. Some sodium is lost in the Hall–Héroult cells during smelting by absorption into the cell lining and by conversion to sodium metal, so complete absence of sodium in metallurgical alumina is not necessary. The target is to balance the input to the cells with the losses. This requires Al2O2 containing about 0.4–0.5 wt% Na2O.Gallium is a ubiquitous component of aluminous ores. Its chemistry is similar to that of aluminum, so it accumulates in Bayer process solutions until an equilibrium is reached at about 0.2 g/L. The gallium content of Al2O3 is a linear function of the gallium concentration in the solution. Therefore, the amount leaving in the product equals the input. Because gallium is in true solution, the same changes that lower the Na2O content of the product lower the gallium content. Only in the production of high-purity metal has there been concern about the gallium content of alumina. About 10 t/a of gallium is recovered from Bayer process solutions. This is the best source for the production of gallium ( Gallium and Gallium Compounds).

Silicon is a component of many aluminum alloys, yet the specification for Al2O3 is less than 0.02% SiO2. This value is only 25% of that specified 50 years ago. The SiO2 is in solution, but it appears to be added to the product through a slow continuation of the desilication reaction (Section 3.1.2. Digestion). There is also evidence that if the rate of Al2O3 deposition on seed is not high, the seed surface becomes at least partially covered with desilication product, thereby becoming less active as a seed for Al(OH)3 precipitation and more active toward SiO2. This impurity is controlled by driving the desilication reaction nearer to completion. Lowering the SiO2 concentration of the solution below 0.1 g/L gives a product containing less than 0.003% SiO2.

Potassium is undesirable in the Al2O3 because it may destroy the graphite in Hall–Héroult cells by intercalation, i.e., it diffuses between the layers of the graphite structure, thus expanding its volume. Although it is soluble in Bayer solutions, there has not been a recorded instance of K2O concentrations becoming high enough to affect the Al2O3 quality.Iron(III)oxide (Fe2O3) as an impurity in Al2O3 has been the subject of much investigation. It is soluble up to 0.1 g/L in the solutions at digest temperatures (400–500 K), but only to 0.001 g/L at 333 K [48]. This reconciles the observations that Fe2O3 behaved at times as if it were in solution and as a colloid at others. There are several iron minerals that can serve as sources for the impurity. Hematite (-Fe2O3) and goethite (-FeO(OH)) are present in most bauxites. Goethite is slowly converted to hematite in high-temperature digests, particularly if CaO is present. This conversion improves clarification because hematite settles better than goethite. The mineral siderite (FeCO3) present in some



bauxites not only is a source of iron but also reacts with NaOH to increase the amount of causticization needed. Pyrite (FeS2) is almost insoluble in Bayer solutions; its adverse effects occur when the sulfur is oxidized to sulfate in the presence of air and water. The sulfate will react with NaOH to form Na2SO4 which has to be removed periodically.Paradoxically, there is less difficulty with Fe2O3 as an impurity in processing bauxites containing a large amount of iron oxides. This is probably because the iron oxides serve as seed and hasten hydrolysis of the NaFeO2 formed in the digest. Running the solution over iron oxide particles before filtration has been effective in controlling this impurity. Another process for lowering the iron content of Al2O3 is termed step precipitation. The filtered Bayer solution is lightly seeded with Al(OH)3 for a short time to precipitate up to 3 g/L Al2O3. The seed and the small amount of precipitation effectively sweep the colloidal iron hydroxide particles from the solution, leaving an Fe2O3 concentration of less than 0.005 g/L.Calcium also is a common impurity. The presence of increasing amounts of Na2CO3 in the Bayer solution seems to increase the CaO in the product. The calcium content of alumina, as with sodium, needs to be controlled such that the Hall–Héroult cell bath can come to an equilibrium level acceptable for cell performance. Calcium can be controlled by step precipitation or by lowering the carbonate content of the solution.

Lithium is also acceptable in Hall–Héroult cells. It can be removed by step precipitation.Other metallic impurities that appear in many bauxites in small amounts are chromium, copper, manganese, titanium, vanadium, and zinc. Generally, these impurities are not so concentrated that removal processes are required. Sulfide salts have been added to precipitate copper and zinc. The chromium is a problem only if it is oxidized; the organic salts in the solution are usually a sufficient reductant. The addition of manganese compounds as oxidants for organic material has been reported.Anions, such as carbonate, chloride, fluoride, and sulfate, are known to interfere with Al(OH)3 precipitation. The carbonate ion is controlled by addition of lime, although processes have been reported in which Na2CO3 is removed by evaporative crystallization or by cooling the solution to 260 K. The sulfate and chloride ions usually are controlled naturally, as their salts are incorporated in the desilication product in sufficient quantities to maintain an equilibrium. There are commercial operations in which the sulfate salt is removed by evaporation and crystallization. The least soluble salt is schairerite (Na2SO4 · NaF), and when the fluoride ion is depleted, burkeite (2Na2SO4 · Na2CO3) appears. The fluoride ion is almost never present in quantities large enough to be harmful.Many bauxites contain phosphate minerals. Apatite (3Ca2P2O8 · CaF2) does not react in the process; indeed, this compound or its hydroxy counterpart is formed when soluble phosphates react with calcium in the digest. Aluminum phosphates, such as wavellite (4AlPO4 · Al(OH)3 · 9H2O) do react, forming Na3PO4 as the soluble phosphate. The phosphate ion interferes with flocculation of bauxite residues containing goethite, probably by competing for attachment sites [49]. The concentration of phosphates never is high enough to interfere with Al2O3 purity. Addition of CaO to the digest is the control measure.Organic matter in bauxite, whether it be roots and twigs from plants or humic acids from decayed matter, reacts in the digest to form a wide variety of organic compounds. Some operation process streams have exceeded 15 g/L organic carbon in solution. There is evidence that larger molecules are oxidized in the process all the way to Na2CO3 and sodium oxalate (Na2C2O4). In addition some, but not all, of the organic compounds may interfere with precipitation [4].The effect of sodium oxalate on processing has not been quantitatively determined, yet many plants incorporate equipment for crystallization of the oxalate salt from the solution. Others wash this salt from the Al(OH)3 seed and recover it from the wash water. The evidence is overwhelming that operations are improved by the oxalate removal techniques. Early removal processes included evaporation to high concentrations followed by centrifugal separation of the gelatinous salt mass. In other plants, a portion of the solution was mixed with bauxite or Al(OH)3 and heated to 1280 K. This



destroyed the organic compounds and formed NaAlO2, which could be recovered in the process. A German process adds magnesium salts to the digest to remove the deleterious organic compounds. Still others are investigating methods of oxidizing the organic material in the solution [50].

3.1.7. Calcination

The final operation in production of alumina is calcination. The temperature of the Al(OH)3 is raised above 1380 K resulting in the reaction:

2 Al(OH)3 Al2O3 + 3 H2OAs discussed in Section 2.2. Composition and Properties, this reaction can take several pathways and several transition forms may appear. The end of all pathways is -alumina. In older European practice, a major portion of the alumina was converted to the -phase, sometimes by the addition of a fluoride salt to lower the temperature of transformation. In American practice, calcination always has been less severe; so, normally, 20% of the alumina is in the -phase.Before calcination, it is necessary to wash the process solution from the coarse Al(OH)3. This is done countercurrently, using storage tanks and filters. As in residue washing, increasing the number of washing stages and the concentration of solids leaving each stage improves the effectiveness of washing. Vacuum filters of several designs have been used for the final separation. The early Oliver and Dorr drum filters have been replaced by horizontal rotary filters because better washing can be achieved. The quality of the wash water is of some concern because such impurities as calcium and magnesium can be adsorbed on the surface of the Al(OH)3.Earlier, calcination was done in reverberatory furnaces. These had neither the capacity nor the thermal efficiency required, so they were soon replaced by rotary kilns. These kilns are cylinders that may be 3.5 m in diameter and over 80 m long. They are mounted on bearings and rotate about an axis inclined at a small angle to the horizontal. Damp Al(OH)3 from the filters enters the upper end and slowly tumbles toward the lower end, traveling against a stream of hot gas formed by combustion of natural gas or oil at the discharge end. Early kilns were less than 40 m long, but longer units have better thermal efficiency and higher capacity.Product coolers were added to kilns so that part of the heat in the incandescent product could be transferred to the combustion air. Early coolers followed designs developed in the cement industry and were themselves rotary units much like, although smaller than, the kilns. Improved coolers fluidize the product Al2O3 around heat-exchange surfaces, often in several stages. In such coolers, the heat can be recovered in combustion air or in wash water and the product can be cooled readily to 425 K.The progressive adoption of large-capacity stationary calciners has reduced fuel consumption for alumina calcination to ca. 3.1 GJ per tonne of alumina, which compares to ca. 4.5 GJ per tonne for earlier rotary kilns. Several stationary calciner designs with capacities of > 2500 t/d alumina are now commercially available. In stationary calciners, a combination of fluid beds and stages in which the solids are transported in suspension is used. Figure (17) is a schematic diagram of a commercial unit capable of calcining 1500 t/d.The Al(OH)3 is washed and dewatered on the filter (a) and is conveyed into the flash dryer (b), where it encounters hot gas from cyclone (g). The gas evaporates the free water from the particle surfaces. Because the temperature exceeds 480 K, a portion of the water of hydration also is driven from the crystals. Cyclone (c) separates the solids from the gas. The gas, cooled almost to its dew point, leaves the unit through the electrostatic precipitator (j), where dust is recovered. Vessel (d) serves as a surge buffer to smooth the flow of solids through the unit. The solids are separated in cyclone (g) to enter the vessel (f) where combustion of fuel is taking place. Vessel (e) allows a holding time at the calcination temperature so that the water content of the product is low. A small amount of fuel can be burned in



this vessel if a higher temperature is desired. In this design the flow of solids is counter to the flow of the gas.The solids are cooled, also in counterflow, by the combustion air in unit (h). Final cooling is in a fluidized bed unit (i). In the fluidized unit a small amount of air is introduced through a membrane at the bottom. This lifts but does not entrain the solids, so they behave much like a fluid. Heat is transferred at a high rate to air or water flowing through tubes submerged in the bed. The product Al2O3 can be cooled below 400 K with all of the recovered energy being used in the process.The great advantage of the stationary units is that the heat required for calcination is 3100 MJ/t. The capital costs are lower than for rotary kilns of equal capacity, and the maintenance costs are lower primarily because the refractories are not subjected to rotational stresses. Because the capacity per unit is high and automation is quite easy, the labor requirements are low. The stationary units are being installed in retrofitting older plants as well as in new construction.

Alumina calcined in stationary kilns has a lower -alumina content and a higher surface area than alumina calcined at the same temperature in a rotary kiln. This reflects the shorter residence time and the absence of a high-temperature flame in the static units. Photomicrographs show that the rapid temperature rise creates vapor pressure within the particles that tends to fracture large crystals.The characteristics of the metallurgical alumina are controlled mostly by the time and temperature of calcination. The important parameter is the reaction between the Al(OH)3 feed and the flow of fuel. The fuel must be free of impurities that can contaminate the alumina. Sulfur and vanadium in fuel oil are limited by concern for product purity. The amount of combustion air must be sufficient to burn the fuel completely without the use of a great excess that increases the gas flow and thereby reduces the thermal efficiency. Factors affecting fuel economy and alumina quality in modern stationary calciners are reviewed in [51].

3.1.8. Evaporation

As is shown on the flow sheet for the Bayer process (Fig. (7)), the solution continuously cycles through the plant. Consequently, any wash water used must be evaporated so that the solution volume can be controlled. About 10% of the flow is evaporated in the two cooling areas by being flashed into steam. In high-temperature digesters, the amount of flash evaporation is even larger. At least one plant processing high-grade bauxite was designed without any additional evaporation capacity. In most cases, the economics require installation and operation of evaporators. The objective is to minimize the summed costs of evaporation and the value of the soluble salts lost by incomplete washing. The minimum is usually attained with a net dilution of the residue of 1.5–2.0 kg/kg.Table (7) lists the input and exit streams that compromise the dilution balance in an operating plant. Efficiency demands that all of these flows be monitored.Several evaporator designs are used in Bayer plants, but nearly all designs are used in multiple-effect configurations. In such units each stage operates at a lower pressure than the preceding one. Therefore, the vapor evaporated from the solution in the first stage is at a temperature high enough to heat the solution in the second stage, causing it to boil. This continues to the final stage, where the vapor is condensed. The condenser operates under vacuum so a jet pump is necessary to remove noncondensable gases from it. The more times the latent heat is used, i.e., the more stages that are present, the more water can be evaporated by the fuel steam used in the first effect. With Bayer solutions the maximum number of effects is six. This is because the boiling point of the solution is elevated 5–8 K by the dissolved solids. This reduces the steam temperature in each effect; with many effects there is little temperature difference available to cause evaporation.An unconventional design called continuous regenerative evaporators has been used in some plants.



These units are similar to the heating and flashing equipment used in digestion in that the feed is heated through up to ten stages without evaporation; the hot solution is then flashed through an equal number of stages and the steam is used for heating the feed. These units are not as efficient as most multiple-effect evaporators. The design choice is based on economics. Special evaporator designs may be used when it is necessary to crystallize organic or inorganic salts from the solution. In such designs pumps are used to increase the thermally induced flow through the tubes in the heaters. The heat-transfer surfaces in evaporators must be cleaned periodically to remove encrustations of desilication product and soluble salts.Evaporators are controlled primarily by changing the amount of steam used. The flow of feed solution is regulated so that crystallization of soluble salts is either induced or avoided, depending on the operating mode. Temperature readings at each effect are compared to design values to indicate operating difficulties.

3.1.9. Residue Disposal

The most important environmental problem in the Bayer process is disposal of the bauxite residue. The solution left with the residue after an economical amount of washing is still very alkaline and cannot be allowed to contaminate ground water. Furthermore, the desilication product in the residue which is in contact with water has the capacity to exchange sodium for hydrogen. An aqueous slurry of the residue that had been washed with 1000 times its mass of distilled water still reached a pH value of 10.5 on standing. The undrained fine residue, even after years of consolidation, does not have enough strength to support buildings or equipment. These properties make disposal a difficult problem.Previously, disposal was to a marine environment where the alkalinity was diluted by large quantities of water. This method has been used in the seas off Europe and Japan and in a river in the United States. Studies by environmentalists have indicated little damage to flora or fauna by the residue in a disposal area [52]. Today, however, environmental concern is so great that any new refinery is unlikely to be permitted to use marine disposal.Early inland refineries simply dammed a convenient valley or built retaining dikes on flat land to form residue disposal areas. In some cases, the sandy portion of the residue was used to build the dikes. This method can be effective and cheap if care is taken to protect the surroundings by proper sealing techniques. The compacted residue has a lower permeability than many clays; yet, there have been isolated leaks into aquifers from such impoundments. Improved designs have been developed.In Germany, retaining dikes are built with a portion of the structure designed to be porous. The dilute solution draining from the residue is channeled by the porous sections into water-treatment facilities before being discarded. In some cases, the dike material exchanged ions with the solution so that little treatment was needed. The draining allowed consolidation of the residue so that it could support equipment. An American firm used a drained lake in which the bottom of the disposal area was covered with sand in which draining pipes were laid. The residue was pumped to these areas in the traditional manner as a dilute slurry. Most of the conveying water was decanted, while some percolated through the deposit to the drains. This design has two advantages: the residue consolidates better, so the storage capacity of an area is increased, and the hydraulic head on the bottom is reduced to zero, so that leakage is unlikely. Recent lakes have been built using the sand drains on top of seals of clay and plastic film for additional environmental safety.Another modern disposal method, called dry stacking, takes advantage of the thixotropic nature of the residue. By some method, usually vacuum filtration, the residue is concentrated to 35–50 wt% solids. The slurry is agitated to reduce its viscosity by as much as two orders of magnitude and pumped to a disposal area. There it flows in lava-like fashion over the surface, establishing a slope away from the



discharge point. In the absence of shear, the viscosity of the slurry increases and flow stops. Water does not separate from the slurry and the slope causes rain to run off rapidly, so the surface usually is losing water to the atmosphere. The surface becomes deeply fissured, further assisting drying. In about 90 days the residue may dry to 75 wt% solids, far drier than in any of the other disposal methods. In this state, it can support heavy earth-moving equipment and can be recovered or used for increasing the height of retaining dikes. This method maximizes the storage capacity of a given area and seems to pose the minimum threat to the environment. It does require construction of a permanent lake area for water storage and for cooling water. If any use is to be made of the residue, recovery is relatively easy.Many investigations have been directed toward finding a commercial use for bauxite residue. The high iron content of some residues suggested production of pig iron. The quality of the iron was poor and the amount of slag formed exceeded the original amount of residue. Similarly, chemical processes have been developed for recovery of Al2O3, Na2O, and TiO2 from residue. Although all are technically possible, none has been feasible economically. Small quantities of residue have been used in making Portland cement, and smaller quantities have been used as a mold wash, as an insulating material, and, after reaction with H2SO4, as a water treatment. No chemical use is likely to consume a significant part of the residue [1], [5].The residue is claylike and can be used in ceramic materials. The sodium causes formation of glasses at 1450 K, giving a vitreous bond. Bricks have been made commercially, but economic factors and other shortcomings of the brick have eliminated this use. Sintering the residue into aggregate for concrete may be economical where natural aggregate is not available.In Texas, agronomists have shown that the residue can be used to neutralize acidic soil, replacing limestone. The cost of preparing the material for application and the logistics argue against extensive use. The conclusion is that large-scale use of bauxite residue is unlikely, so efforts should be directed toward reclamation of disposal areas.

3.1.10. Energy in the Process

Approximately 16 MJ are required to produce a kilogram of Al2O3. The worldwide range is 7.4–32.6 MJ/kg [48]. Variations in the quality of bauxite, plant design, and the size of the plants are the reasons for the large differences. Even with existing plants, large improvements in energy usage can be made. The Aluminum Association (USA) reports that the energy used per unit of alumina production in 1980 was 68% of the energy used in 1972 [54]. The energy used in mining is less than 5% of the total and is difficult to change. The energy for transporting bauxite is double that for mining it, because the average tonne of bauxite used in the United States is transported 4700 km. The situation worldwide is different because much of the bauxite is refined close to the mines.Within the refinery, more than half of the energy is used for pumping and heating the solution and for evaporation. The previous discussion (Section 3.1.3. Equipment) has shown that plant design can affect heat recovery and minimize energy use. Emphasis is being placed on increasing the amount of Al2O3 recovered from less than 50 g/L to over 65 g/L; some claim yields as high as 100 g/L. Because energy is more closely related to the flow of solution than to yield, the energy savings are large. The static calciners are nearly as efficient as they can be made because the temperatures of the gas and of the product leaving the units are very low. The change from rotary kilns has made up a large portion of the savings. There are still opportunities to improve operating practice and design to reduce energy consumption [55].

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3.2. Economic Aspects

The largest cost elements in alumina production are raw materials, energy, and capital-related costs for the production equipment. Labor, operating supplies, and miscellaneous costs are much smaller than these three.Bauxite is the most important raw material. Its cost includes those for mining, transport, levies, and taxes. In addition, the relative quality of the bauxite ore influences the expenditures for necessary reagents. Mining is relatively inexpensive because most deposits are covered with only a shallow overburden and can be mined with efficient equipment. Since the formation of the International Bauxite Association (IBA), levies and taxes have become a large part of the cost of bauxite. These costs have become relatively stable because most are related to the selling price of aluminum. The availability of bauxite from outside the IBA countries has allowed the effect of supply and demand to influence this cost. Where levies are not imposed, some system of taxation provides income to the producing nation. Transportation costs vary from very small to as much as half the delivered cost of the bauxite. Some bauxite travels less than 5 km by truck or conveyor belt to the refinery. Other bauxite may travel more than halfway around the world. The characteristics of bauxite most important in influencing its value are the available alumina and the reactive silica contents. The latter quantity has a great effect on the amount of sodium hydroxide and lime required in processing. Bonuses are given for high available alumina values, and penalties are charged for excessive silica.In Section 3.1.10. Energy in the Process, values were given showing the fourfold range in energy used to produce alumina. The most important variables are the plant design and the quality of the bauxite being processed. The most energy-efficient bauxites are gibbsitic, with very low residue content.

The capital costs also show wide variation. In 1990 U.S. dollars new installations may cost from $700 to $900 per annual tonne of capacity. The variables are design factors, location, capacity, and the properties of the bauxite to be processed. Much of the world capacity was constructed before construction costs were inflated and so has lower capital costs. The effect of capacity is also large, and plants producing 2 × 106 t/a have costs well below those of plants producing less than 105 t/a.Alumina is traded on the world market as a commodity. Not all smelters have alumina capacity dedicated to them; they buy alumina on contract. Long-term contracts are common. In other instances, alumina is bought when needed at spot market prices. Since 1980, alumina has sold on the spot market for less than $150 to over $280 per tonne, reflecting a change in the relationship between supply and demand. Economics do not always control production because many corporations own refineries to supply their smelters. In other instances, national governments own a significant portion of a refinery and for political reasons may choose to operate in a noncompetitive situation. Costs per tonne of alumina can, under unfavorable conditions, exceed $200, although efficient plants may produce at roughly half this figure.

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4. Other Processes for Alumina Production

4.1. Raw Materials

Many investigations have sought processes to replace the Bayer process by using raw materials other than bauxite [2]. Clay, primarily kaolinite, has been most considered because it can contain up to 39% Al2O3 ( Clays). Other materials, including anorthosite, nepheline, coal wastes, and fly ash, have been candidate raw materials. Yet, less than 2% of the world supply of alumina is not made by the Bayer process. This happens only where use of a domestic raw material and production of a desirable byproduct change the economic picture. Alumina from ores other than bauxite normally costs 1.5–2.5 times that from the Bayer process.Because aluminum is amphoteric, both acid and alkaline processes have been developed. Most of the processes in each class follow the same general flow sheets.

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4.2. Alkaline Processes

Sodium compounds, such as Na2CO3, react at 1280 K with the Al2O3 in aluminous ores to produce water-soluble NaAlO2. This compound can be leached from the sinter with water and the solution treated to remove impurities; the purified solution is neutralized with CO2 to recover Al(OH)3. The last step regenerates the Na2CO3 for recycle. Difficulty arises because the aluminous ores contain SiO2, which also reacts in the sinter to form soluble Na2SiO3. In processing, the desilication reaction discussed in Section 3.1.2. Digestion takes place, so the net recovery of Al2O3 is small or zero. If 2 mol of CaO, usually as limestone, is charged for every mole of SiO2 in the raw material, insoluble calcium silicate compounds are formed. Under proper leaching conditions the NaAlO2 can be recovered from such sinters with only slight loss.The process has been used commercially in two American plants to recover most of the Na2O and Al2O3 lost in the desilication product formed while treating high-silica native bauxite. A schematic flow sheet is given in Figure (18). The same process was investigated by the U.S. Bureau of Mines in a large pilot plant using anorthosite as the raw feed [51]. [Anorthosite is a mixture of the minerals anorthite (CaAl2SiO8) and albite (NaAlSi3O8).] Unless the composition, sintering, and leaching were



closely controlled, the residue formed a gel in leaching and became very difficult to filter.In a variation of the sintering approach, calcium replaces the sodium so that calcium aluminate (3CaO · Al2O3) is formed as well as calcium silicate (CaSiO3). The calcium aluminate reacts with Na2CO3 in an aqueous leach to form NaAlO2.

3 CaO · Al2O3 + 3 Na2CO3 + 2 H2O 2 NaAlO2 + 4 NaOH + 3 CaCO3

In carbonation, Al(OH)3 is formed and the Na2CO3 solution is regenerated. Nepheline, (Na,K)AlSiO4, is treated by this process in the former Soviet Union. The calcium silicate residue is processed to make about 10 t of cement per tonne of alumina. The two products make the process viable here.In the Pedersen process, sintering was replaced with a reducing fusion so that ferric ions in the ore were reduced to metal. Iron and the calcium slag were separated by decantation. During cooling, the CaSiO3 passes through a crystalline phase change and the resulting stresses reduce the slag to powder [1], [2].The sinter processes suffer economically for several reasons. The energy for sintering and for evaporation of the leach solution must be added to the energy required from operations analogous to those in Bayer processing. The capital investment is increased by the need for sintering equipment, and although limestone is inexpensive, the quantity required is so large that the expense is considerable.

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4.3. Acid Processes

All of the acid processes follow the general flow sheet given in Figure (19). Usually the clay is prepared by grinding and roasting to about 1000 K. The roasting changes the kaolinite to meta-kaolin, from which the aluminum can be dissolved as the acid salt. The roasted clay is leached in an acid solution, usually at the atmospheric boiling point. Some investigators have chosen to leach at elevated temperature for processing advantages even though corrosion problems become more severe [1]. The siliceous residue is separated using sedimentation and filtration as in the Bayer process. Small amounts of iron salts remain in the clarified solution. These salts are removed by extraction with an organic compound that forms a complex with the iron but not with the aluminum salts. The organic solution is decanted from the aqueous phase and is treated to separate the iron salt and to regenerate the organic extractant.A hydrated aluminum salt is recovered from the aqueous solution, usually by evaporation and cooling to cause crystallization. In the hydrogen chloride process, advantage is taken of the low solubility of AlCl3 in HCl. The HCl gas is absorbed in the solution both to regenerate the acid for leaching and to precipitate AlCl3 · 6H2O. In all acid processes the hydrated salt is decomposed to Al2O3 by heating to 1300 K. Both water and the acid radical are driven off, and the acid is absorbed for recycling.The U.S. Bureau of Mines sponsored a cooperative effort with several aluminum companies to investigate acid processes for recovery of alumina from clay [56]. For several reasons the HCl process was preferred: the reagent is inexpensive, processing conditions are not severe, and the salt can be recovered without evaporation. The HCl is not decomposed in calcination, and the water content of the acid salt is half that of other acid salts. Despite these features the process is not competitive



economically with the Bayer process. A major fault is the amount of energy used to roast the clay, to evaporate the solution, and to decompose the acid salt. The investment is increased because of the corrosive solutions. A process disadvantage is that the Al2O3 is physically different from Bayer alumina, so smelting practice has to be changed for this aluminum source.Pechiney and Alcan jointly operated a pilot plant using the H+ process, a modification of the HCl process in which H2SO4 is added to the digest [52]. The combination of acids eliminated the need to roast the clay. Other processes use H2SO4, H2SO3, HNO3, or NH4HSO4 as reagents to attack the clay, but no commercial use has been made of any of them.Clay is a major impurity of coal, so the ash from coal may be considered an overroasted clay. The acid processes, with minor modifications, will extract alumina from ash [57]. Anorthosite that is high in anorthite reacts with boiling HCl; this approach has been investigated in Norway and Canada [58]. So far, all of these methods have served only to enrich the technical literature.Two aluminous minerals that do not contain silica have attracted attention and, because of their composition, require different technology. Alunite, K2SO4 · Al2(SO4)3 · 2Al2O3 · 6H2O [12588-67-9], has been used commercially in Russia. Both H2SO4 and K2SO4 are useful byproducts of alumina production by this method. The alunite is heated to drive off the hydrated water; additional heating under reducing conditions decomposes the aluminum sulfate without adversely affecting alumina recovery. The solid residue from the roast is a mixture of K2SO4, Al2O3, and gangue. The K2SO4 is dissolved in water and recovered. Alumina is extracted from the gangue in a modification of the Bayer process. In other approaches using alunite, the solutions are kept acidic so the alumina is recovered as a salt [59].Dawsonite, Na2O · Al2O3 · 2 CO2 · 2 H2O [12011-76-6], is found in some oil shales. The temperatures used to recover the oil from the shale decompose the dawsonite to NaAlO2, which is soluble in dilute NaOH. The Al(OH)3 and Na2CO3 can be recovered from the solution by carbonation. The economics are such that recovery of the oil must be competitive before mining and byproduct recovery can be considered.

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5. Metallurgical Alumina

Aluminum production is the principal application for alumina; more than 92% of world alumina production is used for this purpose. The property requirements for commercial metallurgical alumina are therefore of considerable importance to the alumina industry. Specifications have responded to changes in energy costs, aluminum cell design, cell gas scrubbing techniques, environmental regulations, working conditions in smelters, and the technology for alumina calcination. In general, there has been a shift away from relatively small-particle-size, highly calcined, "floury" alumina to a coarse, free flowing, dust-free, less calcined, "sandy" alumina of narrower particle sizing and higher chemical purity ( Aluminum).



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5.1. Alumina Properties Required for Smelting

Five developments have had a significant impact on alumina properties.Cell Design. New, high current efficiency, prebaked anode cells with automatic center-feed systems require that the alumina be consistent, with trouble-free handling properties to insure proper conveying and volumetric metering from the feeder. These criteria are best met by a free-flowing, moderate- to low-calcined alumina with relatively coarse and narrow particle size distribution.Cell Gas Dry Scrubbing. The use of dry scrubber systems employing cell-feed alumina as adsorbent for fluorides in effluent gas from the cells dictates other requirements for metallurgical alumina:1) High adsorption capacity for hydrogen fluoride. This property is closely related to the specific

surface area of the alumina, which is higher for the lower calcined aluminas.2) Attrition resistance.3) Free flowability.4) Higher chemical purity to compensate for capture in the dry scrubber of impurities which are

recycled to the cell.

Pot Room Working Conditions. Use of low-calcined, high surface area alumina as a cover for the cell bath reduces fluoride evolution within the pot room. Working conditions in the smelter are degraded by dust caused by fine particles of alumina. Reduction of the fines fraction (<44 m) in the alumina and high attrition resistance are important for reducing dust.Use of Stationary Calciners and Pneumatic Handling Systems for Alumina. The replacement of rotary alumina kilns by the energy-efficient stationary calciners has resulted in producing a different type of calcined alumina for smelter feed. This difference is reflected in the interrelationship between degree of calcination (measured by the mass loss on ignition), specific surface area, and the -Al2O3 content [60].The strength of the alumina particles has become of concern not only because of relatively higher breakdown in the stationary calciners, but also because of attrition occurring in pneumatic unloading and conveying equipment and in fluid-bed dry-scrubbing systems. The generation of fine particles in such equipment, apart from causing unacceptable dusting conditions in the smelters, often results in troublesome segregation problems in alumina storage bins and bunkers.Electrolyte Composition and Temperature. The trend to operate cells at lower temperatures with electrolyte having a lower bath ratio (NaF:AlF3) has decreased the solubility and rate of dissolution of alumina in the electrolyte. The rate of dissolution is greater for aluminas having higher surface areas and low content of -Al2O3. Both properties can be achieved by a low degree of calcination.

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5.2. Typical Specifications for Metallurgical Alumina

The considerations discussed in Section 5.1. Alumina Properties Required for Smelting have contributed to the evolution of the general specifications used in production and international trading (Table (8)). These values are only representative and considerable variation exists in actual practice depending on price, availability, smelting practices, and many other factors. Consistency of quality is a major consideration.

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6. Industrial Alumina Chemicals

Alumina, in various forms, is one of the inorganic chemicals produced in greatest volume today. Although production of aluminum metal currently consumes ca. 90% of all alumina, an increasing amount is being applied in the chemical industry for fillers, adsorbents, catalysts, ceramics, abrasives, and refractories. With the development and growth of applications and markets for alumina chemicals, all the major alumina producers have, over the years, converted a part of their capacity to produce various alumina chemicals. In fact, some of the older, smaller alumina refining plants have been totally converted to alumina chemicals production in order to be economically viable. Chemical uses account for nearly 8% of the world production.

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6.1. Aluminum Hydroxides



Aluminum hydroxides constitute a versatile group of industrial chemicals. Important uses requiring large quantities are as fillers in plastic and polymer systems and for the production of aluminum chemicals. A moderate amount is used for the production of alumina-based adsorbents and catalysts.Aluminum hydroxide meets most of the requirements for an effective filler: white or near-white color; large volume production base, resulting in price and supply stability; consistency of physical and chemical properties; a wide range of particle size distributions; chemical inertness; and nontoxicity. However, its increasing popularity as a plastics filler is strongly related to its fire-retardant and smoke-suppressant properties, which justify the somewhat higher price compared with calcium carbonate and other mineral fillers. Aluminum hydroxide acts as a fire retardant by adsorbing heat through endothermic dehydration and dilution of pyrolytically produced combustion gas by the released steam. Dehydration and water release processes become significant at temperatures above 500 K. The smoke-inhibiting activity of aluminum hydroxide filler has been attributed to promotion of solid-phase charring in place of soot formation. Although offering these desirable features, aluminum hydroxide has certain disadvantages that impose some limitations on its uses as a filler. Like other nonreinforcing mineral fillers, it generally lowers strength. Because it undergoes thermal decomposition, it is not suitable for processing above 500 K. These factors are responsible for the larger use of aluminum hydroxide in latex carpet backings and in glass-reinforced polyesters, where processing temperatures below about 480 K generally are used. These are also chemically cross-linked or fiber-reinforced systems, in which loss of strength caused by a nonreinforcing filler may not be very important. Aluminum hydroxide filler has been used less extensively in thermoplastics, e.g., poly(vinyl chloride) and polyethylene, and elastomeric materials. For use as a filler, the crystalline aluminum hydroxide from the Bayer process is dried and ground to particles 10 m size. Special grades with increased whiteness and a variety of both particle size ranges and chemical purity also are available commercially.

Fine, precipitated aluminum hydroxide having a uniform particle size (1 m) is used in paper making as a filler pigment and as a coating. As a filler, it disperses rapidly with low sedimentation. Improved printing properties are reported for the hydroxide-filled paper. The application of fine, platy aluminum hydroxide as a paper coating is well established in the paper industry. It gives a coating of high brightness, opacity, and gloss ( Paper and Pulp).

Technical aluminum hydroxide obtained from the Bayer process is 99.5% pure. It dissolves readily in strong acids and bases. For these reasons, aluminum hydroxide is the preferred raw material for the production of a large number of aluminum compounds. These include pure, iron-free aluminum sulfate (used in the paper industry and for water purification), aluminum fluoride, synthetic zeolites, and sodium aluminate. Aluminum hydroxide also is used in the glass industry and in cosmetic and pharmaceutical preparations. An important cosmetic use of aluminum hydroxide is in toothpaste. The mildly abrasive hydroxide cleans and polishes teeth.The price of aluminum hydroxides ranged from $0.20 to $0.60 per kg in 1990, the price range reflecting the cost of additional processing of the usual Bayer process product to suit application requirements. These include grinding, higher purity, classification, surface treatment, etc. Pharmaceutical-grade gel hydroxides were at the top of the price range.

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Title page



Previous References

6.2. Adsorbent and Catalytic Aluminas

Activated aluminas represent another group of technically important alumina chemicals. Principal uses are as drying agents, adsorbents, catalysts, and catalyst carriers. These products are obtained by thermal dehydration of different aluminum hydroxides in the 520–1070 K temperature range (see Section 1.5. Thermal Decomposition of Aluminum Hydroxides).

6.2.1. Preparation of Activated Aluminas

Bayer aluminum hydroxide is the chief source of commercial activated alumina products. Powder forms of activated alumina are produced by heating the hydroxide directly at 575–1825 K in ovens or in rotary or fluidized-bed calciners. The products have surface areas of 200–350 m2/g and losses on ignition of 3–12 wt% (at 575–1473 K). Such products are used as decolorizing agents for organic chemicals and as starting material for the production of aluminum fluoride ( Fluorine Compounds, Inorganic). Other uses include chromatographic and catalytic applications in organic chemistry.Granular activated alumina produced from Bayer plant crust is one of the oldest commercial forms of this product and still is used widely. A flow sheet of the production process is shown in Figure (20). The activated material is a hard, nondusting product. Table (9) lists some properties of a commercial product of this type (Alcoa F-1). A similar granular product has been produced by compacting Bayer hydroxide by mechanical pressure (Martinswerk GmbH, Federal Republic of Germany). The process utilizes a roll-type compactor. The product from the compactor is broken up and sieved to the required size fractions and activated at 675–875 K in a rotary calciner.Fast dehydration of Bayer aluminum hydroxide, either by vacuum or by exposure to high-temperature gas (1050–1300 K) for a few seconds, has been used for the production of ball-shaped, activated alumina having properties superior to the granular product. This process results in the formation of nearly amorphous -Al2O3. The product is finely ground and using water as binder, formed into spherical agglomerates in a rotating pan agglomerator. Rehydration of -Al2O3 with water leads to crystallization of bayerite, causing the agglomerates to harden. Reactivation of the hard balls at 675–775 K produces the activated product (5–20 mm in diameter) having a surface area of 320–380 m2/g.Alumina gels also have been used for the manufacture of activated aluminas. These gels are produced by neutralization of aluminum sulfate or ammonium alum by NH4OH, or from sodium aluminate by neutralization with acids, CO2, NaHCO3, and Al2(SO4)3. The gelatinous aluminum hydroxide precipitate is filtered and thoroughly washed and dried. The dried product is activated, milled, and agglomerated to a spherical product. Other forming processes, such as extrusion, pelletizing, and tableting, also can be used. The product is finally activated at 675–875 K to a loss on ignition value of about 6%. Although gels of various textures can be prepared, the usual industrial adsorbent products have very small pores (less than 4 nm in diameter) and surface areas in the range of 300–400 m2/g. A flow sheet of a manufacturing process is shown in Figure (21). Table (9) reports data on a commercial, gel-based product (Alcoa H-151).Aluminum oxide hydroxide (boehmite), a byproduct from the Ziegler process for linear alcohol production, is another source of activated alumina. The high purity of this material favors its use in catalytic applications. The fine-particle boehmite is normally extruded to various shapes. The material



is claimed to serve as its own binder when peptized with glacial acetic acid. The extrudate is cut to the required size, dried, and activated at 775–875 K. The surface area of the activated product is 185–250 m2/g. Commercial bayerite also has been used to produce activated alumina; the product is preferred in some catalytic applications.

6.2.2. Adsorbent Applications

A major application of activated alumina is in the field of adsorption, where its high surface area, pore structure, strength, and chemical inertness favor its use. The alumina performs important technical functions, such as gas and liquid drying, water purification, and selective adsorption in the petroleum industry.Gases that have been dried successfully by alumina desiccants are:

acetylene cracked gas hydrogen oxygen

air ethane hydrogen chloride propane

ammonia ethylene hydrogen sulfide propene

argon freon methane sulfur dioxide

carbon dioxide furnace gas natural gas

chlorine helium nitrogen

In many applications an alumina desiccant can dry gas to a lower dew point than any other commercially available desiccant. The static water adsorption capacities of two typical activated aluminas (Alcoa F-1 and H-151) in contact with air at different relative humidity conditions are shown in Figure (22).Moist gas usually is dried by passing it through a column (or tower) packed with the adsorbent. Adsorption of water on activated alumina is strongly exothermic, releasing between 45 and 55 kJ of heat per mole of water adsorbed. This factor must be considered in the design of the drying tower. Both granular and spherical forms of activated alumina can be used in desiccant beds. Sizes from 2 to 20 mm are available commercially. The granular products have lower surface areas and their application is based on low cost per kilogram, low water load, and low stream pressure. Spherical products, produced by either the gel or the fast dehydration processes, have larger surface areas, a narrower pore structure, and a high adsorption capacity, and they are relatively more expensive than the granular variety. The spherical kind usually is specified for high-pressure, high-moisture removal duties. The alumina desiccant is regenerated by passing a current of hot, dry gas (475 K) through the bed, usually countercurrent to the main gas flow.Liquids that can be dried with activated alumina include aromatic hydrocarbons, higher molecular mass alkanes, gasoline, kerosene, cyclohexane, power system coolants, lubricants, and many halogenated hydrocarbons. Liquids that are highly adsorbed on alumina (e.g., ethanol and methanol), react or polymerize in contact with activated alumina. Those containing components that tend to deposit on the alumina surface cannot be dried by activated alumina. Regeneration schemes for liquid dehydration units are varied and depend on the liquid being dried. In some applications the liquid being dried is vaporized, heated, and passed through the desiccant bed to adsorb water. Hot, dry gases also are used for regeneration.An evolving application of activated alumina is in water purification. Several important contaminants have been removed from water successfully and economically in pilot plants as well as in large-scale treatment plants. These include reduction of fluoride concentration in drinking water and in some industrial effluents, and removal of color and odor from effluent water from dye works and paper mills.



Removal of phosphate and arsenic also has been investigated.Activated alumina can be used to separate one or more components from a gas or liquid stream by taking advantage of differences in adsorption or desorption kinetics. For example, a short cycle process has been used to recover heavy hydrocarbons from a stream of lighter hydrocarbons. Often a regeneration scheme can be devised that permits cyclic use of the alumina. In other instances, such as removal of catalyst in polyethylene and hydrogen peroxide production, it is more economical not to recycle the alumina.

6.2.3. Catalytic Applications

Alumina is used in many industrial catalytic processes, both as the catalyst and as a support for catalytically active components. In many instances, the alumina support contributes to catalytic activity and so assumes an essential role in the catalyst system. Other catalytic uses of alumina take advantage of its strength, heat resistance, and inertness.Although the common adsorbent aluminas derived from Bayer hydroxide have many catalytic applications, high-purity materials (e.g., boehmite from the Ziegler process for linear alcohol production) are sometimes preferred. Because of the lower cost of Bayer process-based products, such techniques as water and acid washing of the activated product have been employed to reduce the alkali (Na2O) content of the Bayer material. The sodium oxide is known to have a negative influence in many catalytic processes. Catalyst-forming methods include tableting, pelletizing, compacting, ball forming (agglomeration), and extrusion. Such factors as purity, surface area, pore volume, and size distribution, and rate of deactivation influence catalyst performance and selectivity. In addition, crushing strength and resistance to attrition of the catalyst pellets are important considerations in practical operation of catalytic reactors. Although these physical aspects of alumina catalysts are well characterized, the surface structure and chemistry responsible for catalytic activity still remains unclear. Many investigators have attributed catalytic activity to the intrinsic acidity of the surface of activated alumina. First, the combination of two neighboring OH– groups to form water during the dehydration process leaves behind an exposed Al3+ ion, which, because of its electron deficiency, behaves as a Lewis acid site. In addition, hydroxyl groups are retained during thermal decomposition of aluminum hydroxide (see Section 1.5. Thermal Decomposition of Aluminum Hydroxides). These OH– ions on the surface may act as proton donors (Brønsted acids). These Lewis and Brønsted acid sites have been looked upon as the active catalytic centers. Further "defect" structures are formed with increasing degree of dehydration [56]. Of the types of defects created during dehydration, those assumed to have the greatest catalytic importance are the triplet vacancies. They provide unusual exposure of the aluminum ions in the underlying layer and constitute strong acid sites.Catalytic applications of alumina are extensive. Important industrial processes employing alumina as a catalyst by itself include alcohol dehydration and the Claus process for sulfur recovery ( Sulfur). Dehydration of alcohols over activated alumina is one of the oldest catalytic processes. Typical reaction conditions for olefin production are 575–675 K and atmospheric pressure. Lower temperatures favor the formation of ethers. The most suitable aluminas for alcohol dehydration catalysis are those that have large surface areas (150–200 m2/g) and possess good thermal and hydrothermal stability. Coke formation occurs over a period of several hundred operating hours and the catalyst must be regenerated by burning off the carbon with hot air at 773–873 K.The largest present-day catalytic application of activated alumina itself is in the Claus process, which is used to recover sulfur from hydrogen sulfide (H2S).

2 H2S + SO2 2 H2O + 3 S H = – 147 kJThe reaction is carried out catalytically in two or more conversion stages using alumina catalysts.



Reaction temperature in the first stage is around 625 K. At this temperature, the conversion of H2S is only about 65%. Subsequent lower temperature catalytic stages are used to further reduce the H2S concentration. Spherical, high-strength activated alumina catalysts are used in the Claus converters. Service life as high as 5 years has been reported. Deactivation of the alumina catalyst occurs by sulfation, thermal aging, and carbon and/or sulfur deposition. Regeneration of Claus catalyst involves removal of sulfur and burning off of carbon deposits.Alumina-supported catalysts are used extensively in the petroleum and chemical industries. In general, the petroleum industry catalysts have high surface area and high porosity; the support is mostly activated alumina. On the other hand, many typical chemical process catalysts (e.g., ammonia synthesis, steam reforming) are characterized by lower surface area (<20 m2/g) and are nonporous or have very large diameter pores. The carrier in this case is inert and consists mostly of calcined or sintered alumina products.Two different methods have been used commonly for preparation of alumina-supported catalysts: impregnation and coprecipitation. The alumina support used in the impregnation process generally has been formed into its final shape (extrudates, tablets) prior to the impregnation step. Impregnation with a salt solution of the active species is then carried out, followed by drying and thermal decomposition of the salt. In the coprecipitation procedure, hydroxides of aluminum together with the active component are precipitated from a salt solution by neutralization with ammonia or alkali. The washed precipitate is dried, powdered, and processed (e.g., by extrusion) to the desired shape and finally activated by thermal dehydration. The coprecipitation method is used when the active species must be present in high concentrations or when more uniform distribution of the active component is desired.The technical and patent literature contains innumerable examples of the use of alumina as a catalyst support. Some important examples are the catalytic dehydration of n-butane to butadiene (used in synthetic rubber), using a chromia-impregnated alumina catalyst; molybdenum–alumina catalysts, used in hydrorefining operations (e.g., desulfurization) in petroleum refining; and pelleted catalysts containing platinum, palladium, and rhodium on an alumina base, used in automobile exhaust catalytic converters.United States production of adsorbent-grade activated aluminas, both granular and spherical, amounted to nearly 250000 t in 1990. Price of the cheaper, granular product was quoted around $ 0.40–0.50 per kg. Price of the spherical product ranged from $ 0.55 to $0.65 per kg. Total catalytic applications of alumina in the United States were estimated to be around 400000 t in 1990. Price of preformed alumina for catalytic applications ranges from $ 0.60 to $ 4.00 per kg, depending on source and purity. Alumina-based Claus catalyst was priced between $ 3.00 and $ 3.50 per kg.

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7. Ceramic Uses of Alumina

Alumina is used extensively as a ceramic material ( Ceramics, Advanced Structural Products). Products range from relatively low-calcined grades of polishing aluminas to the extremely hard, fused



alumina and synthetically produced sapphire. The characteristics that make alumina valuable in ceramic applications are high melting point (2325 K), hardness (9 on the Mohs scale), strength, dimensional stability, chemical inertness, and electrical insulating ability. These, together with availability in large quantities at moderate prices, have led to extensive and varied uses of alumina as a ceramic material.

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7.1. Calcined Alumina

Ceramic aluminas are generally produced by calcining Bayer aluminum hydroxide at temperatures high enough for the formation of -Al2O3. By control of calcination time and temperature and by the addition of mineralizers, such as fluorine and boron, the crystallite size in the calcined product can be varied from 0.2 to 100 m. These calcined aluminas can be categorized broadly according to their sodium content. There are two general types: those having about 0.5% Na2O and low-soda grades with a content < 0.1%.Reactive alumina is a material manufactured by dry grinding calcined alumina to particle sizes smaller than 1 m. The large surface area associated with very fine particles and the high packing densities obtainable considerably lower the temperatures required for sintering.Tabular aluminas are manufactured by grinding, shaping, and sintering calcined alumina. The thermal treatment at 1900–2150 K causes the oxide to recrystallize into large, tabular crystals of 0.2–0.3 mm.

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7.2. Fused Alumina

For ceramic applications and for the production of abrasives, fused aluminas are manufactured by melting a suitable raw material in an electric arc furnace ( Abrasives). Calcined alumina from the Bayer process is used as a starting material for the highest quality fused alumina. Bauxites with varying levels of iron oxide, silicates, and titanium minerals are melted to produce the brown or less pure black qualities.



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8. Toxicology and Industrial Hygiene

For a discussion concerning the toxicology and industrial hygiene of aluminum oxides, Aluminum - 9. Toxicology and Occupational Health.

Continued ...

Title page Previous

9. References

[1] H. Ginsberg, F. W. Wrigge: Tonerde und Aluminium, Teil 1, Die Tonerde, W. De Gruyter, Berlin 1964.

[2] H. Ginsberg, K. Wefers: Aluminium und Magnesium, vol. 15, Die Metallischen Rohstoffe, Enke Verlag, Stuttgart 1971.

[3] K. Wefers, C. Misra: Oxides and Hydroxides of Aluminum, Alcoa Technical Paper no. 19, revised, Pittsburgh 1987.

[4] T. G. Parson: The Chemical Background of the Aluminum Industry; Lectures, Monographs, and Reports, no. 3, The Royal Institute of Chemistry, London 1955.

[5] P. Barrand, R. Gadeau: l'Aluminium, part 1, Editions Eyrolles, Paris 1964.

[6] A. N. Adamson, E. J. Bloore, A. R. Carr: Basic Principles of Bayer Process Design, Extractive Metallurgy of Aluminum, vol. 1, Interscience Publishers, New York 1963.

[7] A. N. Adamson: "Aluminum Production Principles and Practice," The Chemical Eng. (London), June 1970, 156–171.

[8] S. I. Kuznetsov, V. A. Derevyankin: The Physical Chemistry of Alumina Production by the Bayer Process, Moscow 1964.

[9] W. H. Gitzen: Alumina as a Ceramic Material, The Amer. Cer. Society, Columbus, Ohio 1970.

[10] C. Misra: Industrial Alumina Chemicals, ACS Monograph 184, The Amer. Chem. Society, Washington, DC 1986.

[11] L. Jacob, Jr. (ed): Bauxite, The Amer. Chem. Society, Westerville, Ohio 1990.



[12] L. D. Hart (ed): Alumina Chemicals Science and Technology Handbook, Society Of Mining Engineers (AIME), New York 1984.

[13] J. Böhm, H. Niclassen, Z. Anorg. Allg. Chem. 132 (1923) 1.

[14] R. Fricke, Z. Anorg. Allg. Chem. 175 (1928) 249; 179 (1929) 287.

[15] T. G. Gedeon, Acta Geol. Acad. Sci. Hung. 4 (1956) 95.

[16] R. A. Van Nordstrand, W. P. Hettinger, C. Keith, Nature (London) 177 (1956) 713.

[17] H. Saalfeld, Neues Jahrb. Mineral. Abh. 95 (1960) 1.

[18] H. Ginsberg, W. Hüttig, H. Stiehl, Z. Anorg. Allg. Chem. 309 (1961) 233; 318 (1962) 238.

[19] K. Wefers, Naturwissenschaften 49 (1962) 204.

[20] K. R. Poppelmeier, S. J. Hwu, Inorg. Chem. 26 (1987) 3297–3306.

[21] U. Hauschild, Z. Anorg. Allg. Chemie 324 (1963) 15.

[22] B. C. Lippens, Ph. D. Dissertation, University Delft (1961).

[23] H. D. Megaw, Z. Kristallogr. 87 (1934) 185.

[24] F. J. Ewing, J. Chem. Phys. 3 (1935) 203.

[25] W. R. Busing, H. A. Levy, Acta Crystallogr. 11 (1958) 798.

[26] O. Glemser, E. Hartert, Z. Anorg. Allg. Chem. 283 (1956) 111.

[27] W. H. Bragg, J. Chem. Soc. 121 (1922) 2766.

[28] A. W. Laubengayer, R. S. Weiss, J. Am. Chem. Soc. 65 (1943) 247.

[29] G. Ervin, E. F. Osborn, J. Geol. 59 (1951) 381.

[30] G. C. Kennedy, Am. J. Sci. 257 (1959) 563.

[31] A. Neuhaus, H. Heide, Ber. Dtsch. Keram. Ges. 42 (1965) 167.

[32] K. Wefers, Erzmetall 20 (1967) 13, 71.

[33] H. Schwiersch, Chem. Erde 8 (1933) 252.

[34] A. H. Carim et al., J. Am. Ceram. Soc., 80 (1997) 2677–2680.

[35] W. Feitknecht, A. Wittenbach, W. Buser: 4th Symposium on Reactivity of Solids, Elsevier, Amsterdam 1961, p. 234.

[36] F. Freund, Ber. Dtsch. Keram. Ges. 42 (1965) 23; 44 (1967) 141.

[37] J. M. McHale et al., Science 277 (1977) 788–791.

[38] K. Wefers, Erzmetall 17 (1964) 583.

[39] H. C. Stumpf, A. S. Russell, J. W. Newsome, C. M. Tucker, Ind. Eng. Chem. 42 (1950) 1398.

[40] I. Valeton: "Bauxites," Dev. Soil Sci. 1, (1972).

[41] J. G. deWeisse: Les Bauxites de l'Europe Centrale, Univ. Lausanne 1948.

[42] V. Z. Zans, Geonotes 5 (1959) 123.

[43] M. E. Roch: "Les bauxites du Midi de la France," Rev. Gen. Sci. Pures Appl. Bull. Assoc. Fr. Av. Sci.66 (1958) no. 5/6, 151–156.

[44] H. Erhart: "La Genèse des Sols," Evolution des Sciences, Masson, Paris 1956.

[45] G. Bardossy: Bibliographie des Travaux Concernant les Bauxites, ICSOBA, Paris 1966.

[46] U.S. Geological Survey, Mineral Commodity Summaries, January 1998.

[47] Nippon Light Metal Col., DE 2 807 245, 1977 (M. Kanehara, A. Kainuma, M. Fujiike).

[48] P. Basu, "Reactions of Iron Minerals in Sodium Aluminate Solutions," Light Met. (New York) 1983, 83–98.

[49] D. K. Grubbs, S. C. Libby, J. K. Rodenburg, K. Wefers, J. Geol. Soc. Jam. 4 (1980) 176–186.

[50] K. Yamada, T. Harato, H. Kato, Light Met. (N.Y.) 1981, 117–128.



[51] C. Misra, Travaux ICSOBA 24 (197) 194–204.

[52] UNEP/UNIDO Workshop Envir. Asp. Alumina Prod., Paris 1981.

[53] K. Bielfeldt, K. Kämpf, G. Winkhaus, Erzmetall 29 (1976) 120–125.

[54] The Aluminum Association, Inc., N.Y., Report no. 4, Sept. 15, 1981.

[55] G. Lang, K. Solymar, J. Steiner, Light Met. (N.Y.) 1981, 201–214.

[56] H. W. St. Clair, D. A. Elkins, W. M. Mahan, R. C. Merritt, M. R. Howcroft, M. Hayashi, Bulletin 577, U.S. Bureau of Mines, 1959.

[57] K. B. Bergston, Light Met. (N.Y.) 1979, 217–282.

[58] N. Gjelsvik, Light Met. (N.Y.) 1980, 133–148.

[59] V. S. Sazlin, A. K. Zapolskii, Tsvetn. Metall. UDK 669.712.2 (1968) 64.

[60] E. Barrilon, Erzmetall 31 (1978) 519–522.

[61] H. Knözinger, P. Ratnasamy, Catal. Rev. Sci. Eng. 17 (1978) 31–70.

first occurrence in article

Aluminum Oxide - Table 1

Mineralogic properties of oxides and hydroxides [3]

Phase Refractive index nD20 Cleavage Brittleness Mohs Luster

hardness

Average

Gibbsite 1.568 1.568 1.587 — (001) perfect tough 2.5 to 3.5 pearly vitreous

Bayerite — — — 1.583 — — — —

Boehmite 1.649 1.659 1.665 — (010) — 3.5 to 4 —

Diaspore 1.702 1.722 1.750 — (010) perfect brittle 6.5 to 7 brilliant pearly

Average

Corundum 1.7604 1.7686 — none tough when compact 9 pearly adamantine



Structural properties of oxides and hydroxides [3]

Phase Formula Crystal system Space Molecules Unit axis length, Angle Density



group per unit ×10–1 nm

cell a b c g/cm3

Gibbsite Al(OH)3 monoclinic C25

h 4 8.68 5.07 9.72 94°34 2.42

Gibbsite Al(OH)3 triclinic — 16 17.33 10.08 9.73 94°10 92°08 90°0 —

Bayerite Al(OH)3 monoclinic C25

h 2 5.06 8.67 4.71 90°16 2.53

Nordstrandite Al(OH)3 triclinic C i1 4 8.75 5.07 10.24 109°20 97°40 88°20 —

Boehmite AlO(OH) orthorhombic D21

h7 2 2.868 12.227 3.700 — 3.01

Diaspore AlO(OH) orthorhombic D21

h6 2 4.396 9.426 2.844 — 3.44

Corundum Al2O3 hexagonal (rhomb.) D36

d 2 4.758 — 12.991 — 3.98



Principal chemical constituents of various bauxites, wt%

Country and location Al2O3 SiO2 Fe2O3 TiO2 Loss on ignition

Australia

Darling Range 37 26.5 16.4 1.1 19.3

Weipa 58 4.5 6.9 2.5 26.8

Brazil

Trombetas 52 5.1 13.9 1.2 28.1

France

Southern districts 57 4.6 22.6 2.9 15.1

Guyana

Mackenzie 59 4.9 2.9 2.4 30.4

Guinea

Friguia 49 6.1 14.2 1.6 28.1

Boke 56 1.5 7.9 3.7 30.1

Hungary

Halimba 52 6.6 23.5 2.9 18.1

India

Orissa 46 2.7 22.4 1.1 24.2



Indonesia

Bintan 53.5 3.9 12.1 1.6 29.2

Jamaica

Clarendon 47.8 2.6 17.6 2.3 27.3

Surinam

Onverdacht 59 4.3 3.1 2.5 30.9

Moengo 54 4.2 10.4 2.8 28.9

United States

Arkansas 51 11.2 6.6 2.2 28.4

Former Soviet Union

Severouralsk 54 6.2 14.8 2.4 15.7

Former Yugoslavia

Mostar 52 3.9 21.2 2.7 16.2



World bauxite reserves and production, 103 t [46]

Country Mine production Reserves

1996 1997

United States n.a. n.a. 40000

Australia 43100 43500 7900000

Brazil 9700 9700 2900000

China 6200 7000 2000000

Guinea 14000 14000 900000

Guyana 2000 2000 900000

India 5100 5500 200000

Jamaica 11829 12000 2000000

Russia 3300 3300 200000

Surinam 4000 4000 600000

Venezuela 5600 5600 350000

Other countries 8928 8900 4400000

World total (rounded) 114000 115000 28000000





World alumina production, 103 t Source: Roskill's Metals Databook 1997)

Country Production

1996 1997

Australia 13008 13349

Brazil 2147 2800

Canada 1064 1100

China 2080 n.a.

France 425 430

Germany 994 1000

Greece 630 630

Guinea 630 622

Hungary 353 n.a.

India 1650 1660

Ireland 1186 1300

Italy 857 880

Jamaica 3030 3200

Japan 743 750

Spain 1095 1100

Surinam 1579 1600

Turkey 172 170

United Kingdom 110 110

United States 4530 4780

Venezuela 1641 1700

Yugoslavia 150 300

World total 43600 45000



Commercial digestion conditions

Bauxite type Temperature, K cNaOH, g/L Final cAl2O3, g/L

gibbsitic 380 260 165

415 105–145 90–130

boehmitic 470 150–250 120–160

510 105–145 90–130

diaspore* 535 150–250 100–150



* CaO is added to digests to accelerate dissolution of diaspore.



Dilution balance

Inputs Losses

residue wash evaporation

sand wash –

Al(OH)3 wash heat interchange flash

free moisture in bauxite Al(OH)3 to calcination

water in gibbsite and boehmite free water with Al(OH)3

injected steam water with residue

purge water vapor from solution surfaces

sodium hydroxide

cleanup water (maintenance)

uncontrolled dilutions



Typical properties of metallurgical alumina

Physical property

Particle size distribution, wt%

+ 100 mesh (Tyler) <5

+ 325 (44 m) >92

– 325 <8

Bulk density, kg/L

loose 0.95–1.00

packed 1.05–1.10

Specific surface area, m2/g 50–80

Moisture (to 573 K), wt% <1.0

Loss on ignition (573–1473 K), wt% <1.0

Attrition index (modified Forsythe–Hertwig method) increase in <44 m particles 4–15 wt%



-Al2O3 content (by optical or X-ray method), % <20

Chemical analysis wt%

Fe2O3 <0.015

SiO2 <0.015

TiO2 <0.004

CaO <0.040

Na2O <0.400



Typical properties of Alcoa activated aluminas

F-1 H-151 S-100

Al2O3, % 92 90 95

Na2O, % 0.58 1.6 0.35

SiO2, % 0.12 2.0 0.03

Fe2O3, % 0.06 0.03 0.05

Loss on ignition, % 7 6 5

Loose bulk density:

g/cm3 0.83 0.82 0.80

lb/ft3 52 51 50

Packed bulk density:

g/cm3 0.85 0.85 0.75

lb/ft3 55 53 47

Pore characteristics and surface area of typical Alcoa activated aluminas

Helium (true) density, g/cm3 3.25 3.40 3.15

Mercury (particle) density, g/cm3 1.42 1.38 1.24

Micro pore volume (pores <3.5 nm), cm3/g 0.017 0.023 0.012

Macro pore volume (pores >3.5 nm), cm3/g 0.023 0.020 0.037

Total pore volume, cm3/g 0.40 0.43 0.49

Total porosity, % 56.3 59.4 60.6

Pore diameter at 50% total pore volume, nm 17.7 3.5 4.7

Primary pore size range, nm 0–10000 0–40 0–500

BET surface area, m2/g 250 360 260

Pore diameter, nm

106–107 0.0031 0.0003 0.0000

Pore volume, cm3/g

105–106 0.0045 0.0001 0.0001

104–105 0.0059 0.0001 0.0004

103–104 0.0029 0.0004 0.0033

102–103 0.0045 0.0093 0.0047

35–102 0.0021 0.0099 0.0289



2–35 0.0165 0.0232 0.0115

0.0395 0.0433 0.0489


Aluminum Oxide - Figure 1

Classification of aluminum hydroxides (All figures courtesy of Aluminum Company of America)





Somatoids of bayerite



The Al2O3–H2O systemDashed lines [21], solid lines [22], [28]





Gibbsite heated to 573 K





Specific surface area, loss on ignition (LOI), and density of heated Al(OH)3



Decomposition sequence of aluminum hydroxide





Bayer process flow sheet



Phase diagram Na2O–Al2O3–H2O





High-temperature digestion and heat recovery



Tube digester



a) Piston pump; b) Heat; c) Condensate; d) Flow control valve; e) Steam



Residue thickenera) Feedpipe; b) Feedwell; c) Motorized lifting device; d) Liquid level; e) Overflow launder; f) Weir; g) Off-tank support; h) Tank; i) Cone scraper; j) Discharge cone



Flows at stage n in countercurrent decantation

(O=overflow; U=underflow)





Generalized solubility curve





Technical aluminum hydroxide (gibbsite) 200 ×



Effect of temperature on precipitation





Al2O3 concentration profiles in batch and continuous precipitation





Alcoa flash calcinera) Filter; b) Flash dryer; c) Cyclone; d) Fluidized-bed dryer; e) Holding vessel; f) Furnace; g) Cyclone; h) Multistage cyclone cooler; i) Fluidized-bed cooler; j) Electrostatic precipitator





Lime/soda sinter process flow sheet





Generalized acid process flow sheet





Flow sheet for production of activated alumina from Bayer plant crust



Production of gel-based activated alumina





Static water adsorption capacity of typical commercial activated aluminas at 25 °C (298 K)
