aluminium compound

c© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a01 527.pub2

Aluminum Compounds, Inorganic 1

Aluminum Compounds, Inorganic

Otto Helmboldt, Giulini Chemie GmbH, Ludwigshafen, Germany (Chap. 1 and 2)

L. Keith Hudson, Aluminum Company of America, Alcoa Center, Pennsylvania, USA (Chap. 2.1)

Chanakya Misra, Aluminum Company of America, Alcoa Center, Pennsylvania, USA (Chap. 2.1)

Karl Wefers, Aluminum Company of America, Alcoa Center, Pennsylvania, USA (Chap. 2.1)

Wolfgang Heck, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 3)

Hans Stark, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 4.1 and 5)

Max Danner, Hoechst Aktiengesellschaft, Werk Gersthofen, Augsburg, Germany (Chap. 4.2, 4.3 and 5)

Norbert Rosch, Clariant Produkte (Deutschland) GmbH, Gersthofen, Germany

Related Articles → Aluminum Oxide is a separate keyword. Aluminum fluoride and cryolite → Fluorine Com-pounds, Organic

1. Aluminum Sulfate and Alums . . . . 11.1. Aluminum Sulfate . . . . . . . . . . . 11.1.1. Properties . . . . . . . . . . . . . . . . . . 11.1.2. Production . . . . . . . . . . . . . . . . . 21.1.3. Uses . . . . . . . . . . . . . . . . . . . . . 41.2. Alums . . . . . . . . . . . . . . . . . . . . 51.2.1. Potassium Aluminum Sulfate . . . . . 61.2.2. Ammonium Aluminum Sulfate . . . . 71.2.3. Sodium Aluminum Sulfate . . . . . . . 82. Aluminates . . . . . . . . . . . . . . . . 82.1. Sodium Aluminate . . . . . . . . . . . 82.2. Barium Aluminates . . . . . . . . . . . 9

3. Aluminum Alkoxides . . . . . . . . . . 104. Aluminum Chloride . . . . . . . . . . 104.1. Anhydrous Aluminum Chloride . . 104.1.1. Properties . . . . . . . . . . . . . . . . . . 104.1.2. Production . . . . . . . . . . . . . . . . . 114.1.3. Quality Specifications and Analysis . 114.1.4. Handling, Storage, and Transportation 124.1.5. Uses . . . . . . . . . . . . . . . . . . . . . 124.2. Aluminum Chloride Hexahydrate . 134.3. Basic Aluminum Chlorides . . . . . . 135. Toxicology . . . . . . . . . . . . . . . . . 156. References . . . . . . . . . . . . . . . . . 15

1. Aluminum Sulfate and Alums

1.1. Aluminum Sulfate

Aluminum sulfate is the second most impor-tant industrial compound of aluminum, after alu-minum oxide. Aluminum sulfate was first usedin Paris in 1844 to replace potassium alum. To-day, it has taken over almost all areas of appli-cation that potassium alum originally had.

1.1.1. Properties

Aluminum sulfate is almost insoluble in anhy-drous alcohol but readily soluble in water; aque-ous solutions are acidic. Literature data on solu-bility and the structure of the precipitate in waterdiffer markedly and should be used with caution(see Table 1). Previously, Al2(SO4)3 · 18 H2O

was thought to crystallize from aqueous solu-tion under normal conditions (20 ◦C, 1bar). Thisis now doubted, because such factors as hydro-lysis, oversaturation, shifts in equilibrium, and,particularly, poor crystal formation make defini-tive characterization difficult [5]. The commonform of aluminum sulfate, generally consideredto be Al2(SO4)3 · 18 H2O [7784-31-8], occursin nature as alunogen (hair salt) and can becrystallized from hydrochloric acid solution asmicroscopically small, white needles. However,some researchers ascribe a hydrate water con-tent of 17 mol to this form of aluminum sulfate[6]. A hydrate containing 27 mol of water canbe prepared readily and in high purity [1]. Otherwell-defined aluminum sulfates contain 16, 10,and 6 mol of water. A total of 39 basic and 3acidic aluminum sulfates, as well as 13 differ-ent hydrates of the neutral salt, are describedin the literature [7]. The existence of aluminum

2 Aluminum Compounds, Inorganic

sulfates with 14, 13, 12, 9, 7, 4, 2, and 1 molof water can be concluded from the vapor-pres-sure curves and the dehydration curves of theAl2(SO4)3 – H2O and Al2(SO4)3 – Al(OH)3 –H2O systems [8, 9].

In industrial practice, the hydrate water con-tent is unimportant because “crystalline” alu-minum sulfate is a ground, microcrystallinesolid with variable water content, which is ob-tained from a melt. The desired Al2O3 con-tent is adjusted within certain limits by heating.At a temperature above 340 ◦C, anhydrous alu-minum sulfate [10043-01-3] is formed, a whitepowder, � 2.71 g/cm3, that decomposes above770 ◦C to aluminum oxide [9].

1.1.2. Production

In Germany and most European countries, alu-minum sulfate is produced on a large scale onlyfrom aluminum hydroxide and sulfuric acid bythe Giulini process. In this process, aluminumsulfate is obtained relatively easily with highpurity. Production by the action of sulfuric acidon aluminum-containing ores (clays and baux-ite with high silicon and low iron content, e.g.,bauxite with SiO2 > 5%, Fe2O3 ca. 1%) is stillimportant in some countries.

Giulini Process [10]. The Giulini methodfor producing various grades of aluminum sul-fate, containing between 8 and 23% Al2O3, isshown schematically in Figure 1.

A pressure-resistant, stirred vessel (a) is filledwith aluminum hydroxide (moist or dry). Thecalculated quantity of warm sulfuric acid isadded from a preheater, and the mixture isstirred. In calculating the sulfuric acid concen-tration required to obtain an SO3 content of ca.1% below stoichiometric, all process steps in-volving the introduction or the removal of watermust be accounted for. Generally, acid of density1.6 g/cm3 is used. The reaction starts after 60– 300 s and is complete after 10 – 12 min. Theheat of reaction causes the temperature to rise toca. 170 ◦C, while the pressure rises to 5 – 6 bar.The mixture must not be stirred for more than1 h because otherwise the aluminum sulfate canhydrolyze to give insoluble basic aluminum sul-fate and strongly acidic sulfate melt. An auto-

clave unit allowing batches of 2.5 t can produceca. 50 t of aluminum sulfate in a 10-h shift.

The melt is led into a copper container whereit is concentrated by flash evaporation (b). Fromthe evaporator, the melt is sucked into a well-isolated vacuum tank (c), which is evacuated tothe vapor pressure of the aluminum sulfate melt.This vacuum cooling avoids incrustation of theheat exchanger surfaces.

The melt falls from the vacuum containerinto the mixer, where it is seeded at 85 ◦C with1 – 2% aluminum sulfate powder. The pulp-like product reaches the “crystallization belt,” asmooth, heat-resistant, trough-form rubber con-veyor belt (e), and crystallizes there in ca. 30min. Because of the high heat of crystallization,the material has a temperature of ca. 90 ◦C andcannot be broken to fine size in one step (f). Itpasses over air-cooled conveyor belts until it hascooled to 40 ◦C, after which it is ground (h) andsieved (i). The goods are filled (j) into paper orjute sacks or transported loose in silo cars. Alu-minum sulfate with 17.2% Al2O3 produced inthis way contains only 0.01% insoluble mate-rial; therefore, digestion of the aluminum hy-droxide is almost complete. For transportationas a solution the Al2O3 content is adjusted to ca.8% to avoid crystallization during transport.

Production from Bauxite. Finely groundbauxite (for example, 60%Al2O3, 1.5%Fe2O3,1.6% TiO2, 3.0% SiO2, 32% H2O) also canbe used as starting material. In this case, 3 molH2SO4 are charged onto 1 mol Al2O3. TheFe2O3 component in bauxite is disregarded be-cause the Al2O3 is only 97 – 98% digested andtherefore sufficient sulfuric acid is availablefor production of aluminum sulfate containing17.5% Al2O3. The aluminum sulfate obtainedby digestion of bauxite contains ca. 0.5%Fe2O3and ca. 2.2% insoluble residue.

Production fromLess Pure StartingMate-rials. Acid digestion of predominately silicon-rich raw materials gives a solution of alu-minum sulfate. The purity depends on the pro-cess and starting materials. Iron, which stronglyinterferes, is precipitated with calcium hexa-cyanoferrate (II) as Berlin blue (iron hexa-cyanoferrate (II)), with calcium sulfide as ironsulfide, or by hydrolysis as basic iron sulfate.For details, see [11]. The clear solution is de-


Table 1. Solubility of aluminum sulfate as a function of temperature (grams of anhydrous salt per 100 g water)

t, ◦C 0 10 20 30 40 50 60 70 80 90 100 Reference31.3 33.5 36.15 40.36 45.73 52.13 59.10 66.23 73.14 80.83 89.11 [1]31.2 33.5 36.4 40.4 46.1 52.2 59.2 66.1 73.0 80.0 89.0 [2]27.5 27.6 28.0 28.8 29.9 31.0 32.8 36.6 38.74 46.85 [3]23.9 25.0 26.9 31.5 36.4 41.7 47.0 [4]

Figure 1. Production of aluminum sulfate by the Giulini process [10]

canted and sold as a liquid or concentrated to61.5 ◦Be, allowed to solidify, andmilled. The fi-nal product, which contains ca. 0.5%Fe2O3 and0.1% insoluble material, is the technical grade.In addition, there is an iron-free grade having anFe2O3 content of ca. 0.005%.

The Kretzschmar process is used to producevery pure, iron-free aluminum sulfate by diges-

tion of clay (for example, 40 – 43% Al2O3, 53– 56% SiO2, 2 – 4% Fe2O3) with sulfuric acid.The greater part of the impurities is removedand crystals are separated from the solution bystirring. The formation of colloids is avoided byusing vacuum apparatus. Pure, large crystals ofAl2(SO4)3 · 18H2O (15.3%Al2O3) can be sep-arated easily from the impure mother liquor by


centrifuging [12]. The residue from the digestionprocess (SiO2) can be convertedwith lime to cal-ciumhydrosilicate,which increases the hardnessof and also plasticizes lime mortar.

In a process developed by the U.S. Bureauof Mines, alcohol is used to reduce the viscos-ity of the oversaturated aluminum sulfatemotherliquor [13].

Olin Mathieson Chemical Corp. developedan economical process for producing high-quality aluminum sulfate from the clay or wasteshale of coal mines. Large crystals (1.5 – 3 mm)with iron contents of less than 0.03% are pro-duced in a patented crystallizer [14]. A processfor producing aluminum sulfate from alum-con-taining ores is given in [15].

Aluminum sulfate is produced from waste“red mud” (from the aluminum oxide industry)by suspending the mud in water and passing sul-fur dioxide through the suspension until pH 2 isreached. After filtration and removal of the sul-fur dioxide in vacuo to give a pH of 4.5 – 5.0,Al(OH)SO3 and SiO2 · n H2O precipitate. Theprecipitate is filtered and treated with sulfuricacid, whereby aluminum sulfate dissolves. [16].

Commercial Grades. Whereas Al2(SO4)3 ·18 H2O theoretically contains 15.3% Al2O3,commercial grades of aluminum sulfate con-tain 14 – 15%, 15 – 16%, 17 – 18%, 18%,or 22 – 23% Al2O3 (Table 2). Generally, thealuminum sulfate containing 17 – 18% Al2O3(water-soluble aluminum content, calculated asAl2O3) is used most frequently. The calculatedhydratewater content of this aluminum sulfate is13 mol. The commercial grade containing 17 –18% Al2O3 is delivered also with various spe-cial qualities, such as low arsenic or low ironcontent.

The basicity of the product is defined as theexcess in Al2O3 over the stoichiometric SO3 :Al2O3 ratio. The 17 – 18% Al2O3 grade has abasicity of 0.1 – 1%, i.e., the Al2O3 content ex-ceeds the stoichiometric amount by 0.1 – 1%.Occasionally, the term “basicity” is defined asthe ratio % SO3 to % Al2O3. For commercialaluminum sulfate, this ratio is ca. 2.30, whereasthe theoretical value is 2.35. This weakly ba-sic aluminum sulfate has the advantage that itis less hygroscopic and therefore hardly affectseither the metal parts of the apparatus during use

or paper sacks during transport. Also, free aciddamages cellulose fibers in the paper industry.

1.1.3. Uses

About two thirds of the total aluminum sulfateproduction is used for treating water. About onehalf of the total production goes into the paperindustry (paper sizing, pH adjustment, wastewa-ter purification) [17, p. 246].

Paper Industry. Aluminum sulfate is usedfor precipitating and fixing sizing agents, wet-strength agents, and basic dyes; for improvingretention; for dispersing resin particles that sticktogether and block sieves; as the starting mate-rial of a high-quality slip for coating glossy pa-per (satin white); and for the production of lakedyes and wallpaper. For an overview of uses innewspaper factories, see [18]; for a detailed dis-cussion on the importance of aluminum sulfatein paper production, see [5]. Aluminum sulfatewith more than 0.2% Fe2O3 gives paper a yel-low tinge and cannot be used for good-qualitywhite paper.

Water Purification. Aluminum sulfate is animportant flocculating agent for purifying water[19]. The mechanism is as follows: Positivelycharged aluminum ions, which are hydrated orhydroxylated with water via numerous interme-diate stages, neutralize the negative charge onthe colloidal material in the water. As a resultof mutual adsorption, the material flocculates,sediments at various rates, and finally settles assludge after 15 – 150 min. To neutralize the hy-drogen atoms formedonhydrolysis of aluminumsulfate, some carbonate hardness is consumed:

Al2(SO4)3+6 HCO−3 →2 Al (OH)3+6 CO2+3 SO2−

4

For 50 mg/L of the 17 – 18% Al2O3 grade0.25 mmol/L of carbonate hardness (based onalkaline-earth ions) is consumed. Generally, 5 –50 g aluminum sulfate is sufficient to purify 1m3 of water. An advantage of using aluminumsulfate is the lack of postflocculation, providedthe correct amount is added. Disadvantages arethe formation of free carbon dioxide, the in-crease of non-carbonate hardness, such as cal-cium sulfate, and the pH remaining between5 and 7. If no natural bicarbonate hardness is


Table 2. Commercial grades of aluminum sulfate

Grade Al2O3, wt% Fe2O3, wt% Insoluble material, wt% Basicity * , %8% (solution) 8 0.004 – 114 – 15% 14 – 15 0.006 – 0.008 0.04 0.1 – 1.015 – 16% 15.1 – 16.0 0.006 – 0.008 0.04 0.1 – 1.017 – 18% ca. 17.2 < 0.01 0.03 0.1 – 1.018% ca. 18.0 < 0.01 < 0.03 0.8 – 1.522 – 23% ca. 22.8 < 0.02 ca. 0.03 1.0 – 1.8

* Defined in text

present, an alkaline substance that also has a dis-persing effect, such as lime or sodiumaluminate,must be added. Aluminum sulfate, used in com-bination with calcined leached clay, is suitablefor binding mercury ions, for example, in seawater [20].

Other Uses. Aluminum sulfate with a verylow iron content (below0.01%) is used as amor-dant in dyeing; a higher iron content is unaccept-able because this leads to color changes. Furtheruses are for pickling of seeds, deodorizing ofmineral oils, tawing, and producing aluminumhydroxide gel employed, for example, as a fillerfor synthetic rubber.Aluminum sulfate has someimportance also as a catalyst support. Finally, itis the starting material for almost all other alu-minum compounds.

Production and Capacity Data. Alu-minum sulfate production in the USAwas 1 168000 t in 1980 and 1 075 000 t in 1982 [21]; thatin Japan was 757 000 t in 1980 [22]. The mainU.S. producer of aluminum sulfate is AlliedCorp. The main Japanese producers are NikkeiKako Co. and Scintoma Aluminum SmeltingCo.

1.2. Alums

Historical Aspects. Alumen, fromwhich theword alum is derived, was known to the an-cient Greeks and Romans as an astringent andalso as a mordant in dyeing wool. Alum wasemployed in processing skins, for embalminganimal and human corpses, and for fireproof-ing wood. However, what was known at first asalum was not a defined substance but referredto both alum-containing minerals and mixturesof alum with iron vitriol. Paracelsus was thefirst to distinguish true alum from iron vitriol. In

the years 1776 – 1798, Chaptal and Vauquelinestablished that alum was a double salt of potas-sium and aluminum sulfates and that the potas-sium ions could be replaced by ammonium ions.The raw material of the ancient alum industrywas alum stone (alunite) or alum shale. Alumstone contains all components of the alum in thecorrect ratios and is processed by roasting andleaching. Roasting, aging for several years, andleaching of alum shale gave an aluminum solu-tion from which the alum was precipitated withalkali. The alum industry played an importantrole during the whole of the Middle Ages.

The industrial production of the alums ceasedto have particular importance when it becamepossible to produce aluminum sulfate econom-ically in high purity, because only the alu-minum content is of practical importance. Mostof the alum production methods now have onlylimited, partly only historical, interest. Clayand other alkali-containing silicates, particularlybauxite, were used as rawmaterials and were di-gested using alkali or acid. Today, alums are pro-duced only from aluminum hydroxide, which isobtained by the Bayer process from bauxite (→Aluminum Oxide).

General Properties. Alums are crystallinedouble salts of the general formula (cation 1)+

(cation 2)3+ (anion2−)2 · 12 H2O. The mostuseful alums are those with trivalent aluminumcations and sulfate anions, M+Al3+(SO2−

4 )2 ·12 H2O. The individual components of the alumcan be replaced, retaining both the regularcrystalline form and the hydrate water. Alumswith the following trivalent metals are known:iron, chromium, cobalt, manganese, titanium,vanadium, gallium, indium, scandium, rhodi-um, and iridium. The monovalent componentcan be an alkali metal, or it can be ammoni-um, alkylammonium, arylammonium, or thal-lium. The existence of a lithium alum is uncer-


tain. Known combinations include potassium–aluminum, cesium–aluminum, potassium–chro-mium, and hydroxyammonium–aluminum. Aselenate alum also is known: KAl(SeO4)2 · 12H2O.

All alums crystallize in strongly refractingoctahedra or cubes [1]. They have an astrin-gent taste and rot-proofing, protein-precipitatingproperties.

In aqueous solution, alums show all thechemical properties that their components showseparately. Physical properties, such as color,electrical conductivity, and freezing-point low-ering, are the sum of the properties of the com-ponents, provided the solutions are very dilute.At higher concentrations, complexes, such as[Al(SO4)2 · (H2O)2]−, are formed. The solubil-ities of the alums in water decrease from sodiumto cesium (see Table 3). On heating, the alumslose their water of crystallization either partiallyor completely. However, part of the water istaken up again at normal temperature and hu-midity.

1.2.1. Potassium Aluminum Sulfate

Potassium aluminum sulfate [7784-24-9],known as potassium alum, KAl(SO4)2 · 12H2O, � 1.75 g/cm3, Mr 474.4, and ammoniumaluminum sulfate are the aluminum compoundsknown longest. Potassium alum occurs in natureas efflorescence on alum shale and in volcanicareas on trachyte and lava as feather alum.

Potassium alum crystallizes in large, col-orless, transparent octahedra, which melt at92.5 ◦C in their own water of crystallization.The hardness of the crystals on the Mohs scaleis 2. The octahedra absorb long-wavelength IRradiation almost completely, but are transparentto visible light. Certain substances (hydroxides,carbonates, borates, carbamides, alsometals andorganic dyes) promote the formation of basicaluminum sulfate by binding free sulfuric acidin the mother liquor. Under these conditions, thecubic form is favored (cubic or Roman alum).

Potassium alum is stable in air of normalhumidity. Dehydration does not begin below30 ◦C, but at 65 ◦C nine moles of water are lost.Literature data on the dehydration are contradic-tory. Investigations on thermal decompositionare given in [23]. According to these results,

K2SO4, γ-Al2O3, and 3 K2SO4 · Al2(SO4)3form at 780 ◦C, whereas K2SO4, α-Al2O3, andK2O · 12Al2O3 are produced at 1400 ◦C.Whenheated above its melting point, potassium alumdehydrates, forming calcined alum (alum us-tum), KAl (SO4)2 [10043-67-1]. At red heat,SO3 is released.

Potassium alum is soluble in dilute acid butalmost insoluble in anhydrous alcohol, acetone,and methyl acetate. Because of the marked in-crease in solubility in water with temperature,potassium alum can be purified more easily thanother aluminum salts by recrystallization and, inparticular, can be freed from iron sulfate. Mixedcrystals form readily with ammonium sulfate.

Basic potassium aluminum sulfate,K[Al(OH)2]3(SO4)2 · 3/2 H2O, occurs in na-ture as lowigite. The compound is made syn-thetically as a rather insoluble, amorphous pow-der by heating aluminum sulfate, water, andan excess of potassium sulfate or by heatingpotassium alum with water in the ratio 1 : 4 to200 ◦C[24].Another basic potassiumaluminumsulfate, K[Al3(OH)6(SO4)2] [1302-91-6], con-taining less water, is the alum stone (alunite)occurring in nature.

Production. Aluminum hydroxide (wet hy-drate; Al2O3 ca. 57%, Fe2O3 0.016 – 0.023%,H2O 42.5%) and sulfuric acid (� ≈1.6 g/cm3;Fe2O3 0.020 – 0.040 g/L) react in a stirred,corrosion-resistant pressure boiler at 5 – 6 barto form aluminum sulfate as shown in Figure 1.The aluminum sulfate melt is then led, with re-lease of pressure, into a copper container. Here, astoichiometric quantity of potassium sulfate (aschloride-free as possible) is added. The solutionis heated to ca. 100 ◦C for 2 – 3 h and adjusted to40 – 44 ◦Be with mother liquor. After filtrationfrom insoluble material, the alum melt is left incrystallization boxes for 10 days, after which thealum is removed in blocks. The product obtainedin this way contains < 0.001% Fe2O3, 0.001 –0.004% chloride, and < 0.01% insoluble mate-rial.

For the production of alum crystals, the alummelt is brought into a stirred crystallization bath,where it is cooled to ca. 40 ◦C by air bubbles.The pulp is then separated from the motherliquor by centrifuging and washed. The motherliquor is collected and returned to the coppercontainer. The potassium alum is dried at 50 –


Table 3. Solubility of some aluminum alums as a function of temperature (grams of anhydrous salt per 100 g water)

t,◦C 0 10 20 30 40 50 60 70 80 90 100Sodium 56.2 60.5 61.5 66.7Potassium 3.0 4.0 5.9 7.9 11.7 17 25 40 71 109 154Ammonia 2.6 4.5 6.6 9.0 12.4 15.9 21.1 26.9 35.2 50.3 70.8Rubidium 0.71 1.09 1.40 3.1 4.98 21.6Cesium 0.2 0.3 0.4 0.8 1.24 5.3 22.8Thallium 3.05 4.4 6.2 12.6 26.0

60 ◦C. The salt, which forms lustrous crystals,is sieved and packed in paper bags that are linedwith polyethylene.

A process for obtaining potassium alum fromalum-containing ores is given in [15], and theproduction of basic potassiumand sodiumalumsfrom synthetic alum is described in [25].

Uses. As already mentioned, the industrialimportance of potassium alum has declined con-siderably. However, it is still employed in tawingskins, as a mordant in dyeing, and as a coagu-lating agent for latex. Because of its astringentand protein-precipitating properties, potassiumalum is used in the pharmaceutical and cosmet-ics industries. The use of potassium alum as astyptic pencil, because of its blood-stanchingproperty, is very widespread and popular. Themost important application today is in the gyp-sum industry, which employs potassium alumas hardening agent and setting accelerator forthe production of marble cement and alabasterplaster. However, for purifying water and sizingpaper, potassium alum has been replaced com-pletely by aluminum sulfate. In the paper indus-try, aluminum sulfate is designated traditionally,although incorrectly, as an alum.

1.2.2. Ammonium Aluminum Sulfate

Ammonium aluminum sulfate [7784-26-1], alsocalled ammonium alum, NH4Al(SO4)2 · 12H2O,Mr 453.33, � 1.64 g/cm3,mp 93.5 ◦C, oc-curs in nature as shermigite. Crystals dopedwithother alums show birefringence. The solubilityof ammonium alum in water is similar to that ofpotassium alum, with which it forms a continu-ous series of mixed crystals. Ammonium alumis slightly soluble in dilute acids and glycerolbut insoluble in absolute alcohol. In aqueous so-lution, ammonium alum is neutral.

Data on loss of water on heating are in dis-agreement. Gel’Perin and Chebotkevich [26]

have reported that water of crystallization is re-leased in three stages: first to give the hydratewith 21 mol water, then to that with 3 mol wa-ter, and finally to the anhydrous product, so thatit is in fact more correct to formulate ammoniumalumwith 24 mol water. Above 193 ◦C, decom-position with the release of ammonia begins. Onglowing above 1000 ◦C, sulfur trioxide is lost,leaving an aluminum oxide residue.

Production. Ammonium alum is mademainly by dissolving aluminum hydroxide insulfuric acid and adding ammonium sulfate butsometimes by the reaction of ammonia gas withaluminum sulfate and sulfuric acid. The proce-dure is analogous to the potassium alumprocess.To obtain a quality particularly low in iron oxideor other metal oxides (< 0.0001% Fe2O3; nor-mal quality ca. 0.001% Fe2O3), as is requiredfor production of synthetic gems, very pure start-ing materials must be used. Ammonium alumoccurs as an intermediate in the “aloton” pro-cess, which operates in a sulfuric acid medium.This process had some importance prior to 1945in Germany and the United States for aluminumhydroxide and aluminum oxide production (→Aluminum Oxide)

Uses. In Europe, ammoniumalum is not usedin large quantities. Applications include thosein dressing furs in tanning, in the production ofvery fine aluminum oxide particles for polish-ing metallographic surfaces, and, in some coun-tries outside Europe, as a disinfectant. In theUnited States, ammonium alum is important asan additive in baking powder; ca. 500 t/a are pro-duced for this purpose. More recently, ammoni-um alum has gained considerable importance asstarting material for the production of the finelypowdered, loose aluminum oxide of high pu-rity that is required for synthesizing corundumgems, such as rubies and sapphires. This quality


of aluminum oxide is made by heating ammo-nium alum (or ammonium aluminum selenatealum) to 1000 ◦C.

1.2.3. Sodium Aluminum Sulfate

Sodium aluminum sulfate [7784-28-3], sodiumalum, NaAl(SO4)2 · 12 H2O, melts at 61 ◦C inits water of crystallization. In nature, it occursas the mineral mendozite. Again, the data onthermal dehydration disagree. Sodium alum isinsoluble in absolute alcohol but is much moresoluble inwater than all other alums. Alum pow-der (very fine crystalline form) is not availablecommercially. These two facts make it difficultto obtain sodium alum free from iron. Becauseof this and also because of its strong tendencyto age, sodium alum has never gained the sameimportance as the other alums. In Europe, its usehas been abandoned. In the United States, how-ever, sodium alum is still utilized in relativelylarge quantities (ca. 3000 t/a) in baking powder.

Production. In the United States, sodiumalum is produced by adding a clear solution ofsodium sulfate to aluminum sulfate. After dilu-tion to 30 ◦Be and subsequent heating, a sludgeof potassium sulfate, sodium silicate, and causticsoda is added to improve the purity of the prod-uct. The mixture is pumped into a stirred vessel,where it is mixed for several hours. During thisstage, the ratio of aluminum sulfate to sodiumsulfate is adjusted to the stoichiometric amount.Afterwards, the melt is pumped into an evapo-rator and concentrated to such an extent that itsolidifies to a hard cake on pouring into a coolingtank. This sodium alum cake is then heated andfinally ground to the desired size (99% througha sieve of 100 mesh) [17, p. 250].

2. Aluminates

Only the aluminates of barium and sodium haveimportance in industry. Aluminates occur alsoin cement and in spinels.

2.1. Sodium Aluminate

Sodium aluminate [1302-42-7] is an impor-tant commercial inorganic chemical. It func-

tions as an effective source of aluminum hy-droxide for many industrial and technical appli-cations. Commercial grades of sodium alumi-nate are available in solid and liquid forms. Puresodium aluminate (anhydrous) is a white crys-talline solid having a formula variously givenas NaAlO2, Na2O · Al2O3, or Na2Al2O4. Com-mercial grades, however, always contain morethan the stoichiometric amount of Na2O, the ex-cess being on the order of 1.05 to 1.50 times theformula requirement. Hydrated forms of sodiumaluminate are crystallized from concentrated so-lutions.

Sodium aluminate has no defined meltingpoint; it softens above 1700 ◦C when sodiumbegins to evaporate slowly, leaving aluminumoxide.

Production. The chief commercial processfor the manufacture of sodium aluminate isthe dissolution of aluminum hydroxides insodium hydroxide solution. Aluminum trihy-droxide (gibbsite) from the Bayer process (→AluminumOxide) can be dissolved in 10 – 30%aqueous NaOH solution at a temperature nearthe boiling point.

The use of more concentrated NaOH solu-tions leads to a semisolid product. The processis carried out in steam-heated vessels of nickel orsteel, and the aluminumhydroxide is boiledwithca. 50% aqueous sodium hydroxide until a pulpforms.After this is poured into a tank and cooled,a solid mass containing about 70% NaAlO2 isformed. After being crushed, this product is de-hydrated in a rotary oven heated either directlyor indirectly by burning hydrogen. The resultingproduct contains ca. 90% NaAlO2 and 1% wa-ter, together with 1% free NaOH. The solubilityof the salt produced in this way depends stronglyon how much excess sodium hydroxide is used.

Alternatively, bauxite can be used directlyas the alumina source. Bauxites containinggibbsite are extracted at 150 ◦C and 5 bar,whereas boehmite containing bauxite requireshigher temperatures (230 ◦C) andpressures. Thesodium aluminate solution obtained from the di-gestion process is separated from any impuritiesand then concentrated to the commercial liquidgrade by evaporation. The solid product is ob-tained by drying the liquid.


The sinter method also has been used to pro-duce sodium aluminate. By sintering sodiumcarbonate directly with Bayer aluminum trihy-droxide in rotary sintering kilns at 1000 ◦C, anessentially anhydrous product can be obtained.When sintering bauxite, it is essential to leachthe sinter mass with water and separate the im-purities.

Uses. The major use of sodium aluminate isfor water treatment, including both potable andindustrial waters. Sodium aluminate is used asan adjunct to water softening systems, as a co-agulant aid to improve flocculation, and for re-moving dissolved silica. Sodium aluminate dis-solves in water to give a solution that has a pHvalue of 8. The aluminum hydroxide that precip-itates from this solution has excellent floccula-tion properties and coagulates other impuritiespresent in the water. Depending on the impuri-ties, the conditions of precipitation can be im-proved by adding aluminum sulfate [27].

In construction technology, sodium alumi-nate is employed to accelerate the solidifica-tion of concrete, mainly when working duringfrost, under water, and in humid soil (see Table4). Compared to calcium chloride, which actsin a similar manner, sodium aluminate has theadvantage of greater setting acceleration at lowwater-to-cement ratios [28]. Because it does notattack the reinforcing metals, it can be used forwork with steel-reinforced concrete. Too rapidsetting can cause cracks in the concrete, af-fecting the final strength of the building. Byadding substances such as oxoacids [29], sugar,naphthene [30], K2CO3, or Na2SO4 [31] thefinal strength increases without affecting thesetting rate significantly. Sodium aluminate in-creases the resistance of mortar to water, al-kali, and acid. For the production of expandedconcrete, sodium aluminate serves to activatenitrogen-releasing substances [32]. To stabilizefoam formation in the production of light refrac-tory bricks, up to 5% (relative to the total drymass) sodium aluminate is added.

Increasingly, sodium aluminate is being usedfor the production of synthetic zeolites, used ascatalyst supports or as catalysts [33 – 36] andas adsorbents. Sodium aluminate is one of theprincipal sources of alumina of the preparationof alumina adsorbents and catalysts.

Table 4. Acceleration of setting by sodium aluminate in a mixtureof 6 parts sand, 2 parts concrete, and 1 part water

%NaAlO2 in concreteSetting started Setting completed0 4 h 30 min 6 h1 30 min 1 h 40 min1.5 15 min 30 min2 10 min 15 min10 instantly

In the paper industry, sodium aluminate in-creases the opacity [37], retention of fibers andfilling materials [38], and the paper strength[39]. Also, it stabilizes the pH value of the wa-ter circulation [40] and increases the dispersionstability of titanium dioxide [41].

Another application of sodium aluminate isas a pickle to protect metallic surfaces (copper[42], aluminum [43], among others). This effectis based on the formation of a thin, very firmlyadhering layer of aluminum hydroxide or, afterheat treatment, of aluminum oxide on the sur-face, which does not affect the metallic luster. Inthe enamel industry, sodium aluminate is addedto enamel mixtures to achieve low-melting cov-ers [44].

Further applications of sodium aluminate arein lithography for the production of printing inks[45] and print forms [46], in the detergent andvarnish industries, in the production of water-insoluble floor waxes [47], as a dispersion agentfor production of high-purity asbestos [48], as anadditive to drilling fluids [49], and in the textileindustry as a mordant in dyeing and in printingcloth.Manyother uses of sodiumaluminate havebeen reported. These include inhibition of glassetching by alkaline solutions; protection of steelsurface during galvanizing; improving dyeing,antipiling, and antistatic properties of polyestersynthetic fibers; as an additive to foundry sandmolds and cores; and as a binder in the ceramicsindustry.

2.2. Barium Aluminates

The industrially important barium aluminatesare BaO · 6 Al2O3 [12254-17-0], mp 1915 ◦C;BaO · Al2O3 [12004-04-5], mp 1815 ◦C; and 3BaO · Al2O3 [12004-05-6], mp 1425 ◦C [50].The first two crystallize hexagonally.

Barium aluminates are produced by melt-ing bauxite with coal and barite. Leaching themelt gives a solution of barium aluminate, from


which the salt is obtained by evaporation. Verypure barium aluminates can be produced by sin-tering mixtures of aluminum oxide with bar-ium carbonate. All barium aluminates, includ-ing such hydrated compounds as BaO · Al2O3 ·4 H2O, 2 BaO · Al2O3 · 5 H2O, BaO · Al2O3 ·7 H2O, and 7 BaO · 6 Al2O3 · 36 H2O, hydro-lyze in water, forming hydrargillite (gibbsite),Al(OH)3, and relatively soluble barium hydrox-ide.

Barium aluminate is used for purifying waterbecause the Ba2+ ion precipitates both the sul-fates and the carbonates, whereas the aluminateions form insoluble calciumaluminate.A furtheruse of barium aluminates is as a special cement,for example, as binding agent for the produc-tion of high-temperature refractories, and in theproduction of radiation shields [51].

3. Aluminum Alkoxides

Aluminum alkoxides (alcoholates) are solid orliquid compounds of covalent character. Theyare readily soluble in hydrocarbons, but spar-ingly soluble in alcohols. Hydrolysis with wa-ter occurs readily, giving aluminum hydroxideand the corresponding alcohol. For a generalreview of aluminum alkoxides, see [52]. In-dustrially, only the isopropoxide (isopropylate)and sec-butoxide (sec-butylate) are important.These compounds are used to adjust the viscos-ity of varnishes, to impregnate textiles, as in-termediates in the production of pharmaceuti-cals, and as antitranspirants in cosmetics. In theindustrial production of ketones and aldehydes,aluminum alkoxides are employed as reducingagents (Meerwein-Ponndorf reaction).

Aluminum isopropoxide [555-31-7],Al(OCH(CH3)2)3, is a white solid, Mr 204.25,mp 118 ◦C, bp 125 – 130 ◦C at 533 Pa,d204 1.0346, flash point 26 ◦C. It is usually pro-

duced by direct reaction of aluminum and iso-propyl alcohol in the presence of mercury(II)chloride catalyst. In a later version of the pro-cess [53], the alcohol was heated under reflux ina column filledwith aluminum chips; no catalystwas required and nearly quantitative conversionwas obtained. The alkoxide can be purified bydistillation. In an alternative productionmethod,excess isopropyl alcohol is added to a solution

of aluminum chloride in benzene; the hydrogenchloride formed is removed by introducing dryammonia into the reactor and filtering off theammonium chloride that precipitates.

Aluminum sec-butoxide [2269-22-9],Al(OC4H9)3, is a colorless liquid,Mr 246.3, bp180 ◦C at 533 Pa, d20

4 0.9671, flash point 26 ◦C,and is produced in amanner similar to aluminumisopropoxide.

4. Aluminum Chloride

4.1. Anhydrous Aluminum Chloride

Oerstedt first prepared anhydrous aluminumchloride [7446-70-0] in 1825 by the reaction ofchlorine gas with a mixture of alumina and car-bon. This compound has acquired great signif-icance in organic chemistry as a catalyst, par-ticularly for Friedel-Crafts syntheses and alliedreactions for the production of alkylated aromat-ics, dyestuffs, pharmaceuticals, and perfumerychemicals. For general literature, see [1] and[54].

4.1.1. Properties

Physical Properties. In the solid and gasphases at temperatures up to 400 ◦C, aluminumchloride is present as the dimer, Al2Cl6. Disso-ciation of the dimer progresses with rising tem-perature and is quantitative above 800 ◦C. Solidaluminum chloride crystallizes to form a mono-clinic layer lattice. Pure aluminum chloride iswhite, but the commercial product usually hasa yellowish or grayish tinge because of smallamounts of iron chloride or aluminum impuri-ties. Physical properties of anhydrous aluminumchloride [55]:

Mr 133.34� at 25 ◦C 2.44 g/cm3

Sublimation temperature 181.2 ◦Cat 101.3 kPa

Triple point, at 233 kPa 192.5 ◦CHeat of formation at 25 ◦C –705.63 ± 0.84 kJ/molHeat of sublimation of dimer 115.73 ± 2.30 kJ/molat 25 ◦C

Heat of fusion 35.35 ± 0.84 kJ/molHeat of solution at 20 ◦C –325.1 kJ/mol


Chemical Properties. Anhydrous alu-minum chloride reacts extremely violentlywith water, evolving hydrogen chloride. Thehexahydrate [7784-13-6], AlCl3 · 6 H2O, Mr241.44, is formed in this reaction. In aqueoussolution, aluminum chloride is partially hy-drolyzed to hydrochloric acid and aluminumoxychloride, AlClO [13596-11-7]. For this rea-son, anhydrous aluminum chloride cannot beobtained by concentrating the solution and cal-cining the hydrate. When aluminum chlorideis heated with γ-alumina, aluminum oxychlo-ride, AlClO, forms, but this reaction goes in thereverse direction at temperatures above 700 ◦C:

AlCl3+Al2O3�3 AlClO

If aluminum chloride vapor is passed at1000 ◦C under reduced pressure over moltenaluminum, volatile aluminum monochloride[13595-81-8], AlCl, is formed, but this decom-poses immediately into the elements in coolerzones of the reactor. This method has beenadopted for purifying aluminum.

Reaction between aluminum chloride andother metal halides, such as CaCl2, CrCl3, andFeCl3, gives mixed halides. Eutectic melts withother metal chlorides are of industrial signifi-cance, for example, that with sodium chloride isused as a solvent in chlorination reactions.

Anhydrous aluminum chloride dissolvesreadily in polar organic solvents. As a Lewisacid it forms addition compounds with numer-ous electron donors, such as hydrogen chloride,hydrogen sulfide, sulfur dioxide, sulfur tetra-chloride, phosphorus trichloride, ethers, esters,amines, and alcohols.

4.1.2. Production

The starting materials are either aluminum orpure aluminum oxide. Bauxite now has no eco-nomic significance as a raw material because ofthe iron chloride always present in the product.

Chlorination of Aluminum [56]. Today,most anhydrous aluminum chloride is made bychlorinating aluminum.

Chlorine is passed through molten aluminumin ceramic-lined, tube-shaped reactors. The re-action is highly exothermic:

2 Al (s) +3 Cl2 (g) →Al2Cl6 (s) ∆H◦

= −1411 kJ/mol

The temperature in the reactor is maintainedat 670 – 850 ◦C by controlling the admissionrates of chlorine and aluminum and by coolingthe reactor walls with water. The aluminum usu-ally is replenished in the form of lumps. The dif-ficulty of controlling the large heat of reactioncan be overcome also by dividing the processinto a number of small units.

The aluminum chloride vapor leaving the re-actors is passed through ceramic-lined tubes intolarge, air-cooled iron chambers. Solid aluminumchloride is withdrawn from the condenser wallsat regular intervals, ground (insuring exclusionof moisture), and classified by sieving. Chlorinein the off-gas is removed by conventional meth-ods, such as absorption in caustic soda solution.

Chlorination of Pure Aluminum Oxide.Chlorination of alumina is advantageous overthe formerly widely used bauxite process (lessreactor corrosion and higher-purity product),and at the same time it avoids the high raw ma-terial costs involved in metal chlorination [57].

Carbon monoxide and chlorine are partiallyconverted to phosgene over an activated charcoalcatalyst. The gaseous reaction mixture enters abrick-lined, fluidized-bed reactor,where it reactswith finely divided γ-alumina to yield aluminumchloride. The reaction is exothermic enough (ca.300 kJ/mol, based on AlCl3) to maintain thetemperature at 500 – 600 ◦C without externalheat. Consequently, the process permits largeunits, and the low reaction temperature insuresthat the brick lining has a long life.

The aluminum chloride vapor is filteredthrough a bed of coarse pumice chips, con-densed, and further processed as describedabove. The off-gas contains chlorine, phosgene,and large amounts of carbon dioxide. The chlo-rine is removed by scrubbing, and the phosgeneis hydrolyzed with water. Extremely pure alu-minum chloride is obtained by resublimationfrom molten sodium aluminum chloride.

4.1.3. Quality Specifications and Analysis

Anhydrous aluminum chloride is ground andmarketed as powder or granules. Aluminum is


usually determined by atomic spectrometry, thechloride by argentometry. The assay is 98 –99% AlCl3. The main impurity is iron (0.05wt%max. or 0.01 wt% for resublimed product).Sampling and analysis of anhydrous aluminumchloride must be carried out in an atmosphere ofdry air or nitrogen.

4.1.4. Handling, Storage, andTransportation

Because of its corrosive and irritant action,anhydrous aluminum chloride is classified asa dangerous substance. National and interna-tional regulations must be observed, such as67/548/EEC, the key European Economic Com-munity directive dealing with the classifica-tion, packaging, and labeling of dangerous sub-stances.

Handling. Goggles, gloves, and protectiveclothingmust bewornwhen handling aluminumchloride. A fume cupboard or a respirator withfilter typeB/St against acid gases should be used.Because hydrogen chloride is evolved when alu-minum chloride is exposed to water, any spillsmust be taken up dry, and only small, residualamounts can be washed away with plenty of wa-ter. Sodium bicarbonate or slaked lime shouldbe used for neutralization.

Storage and Transportation. Aluminumchloride is dispatched in vented steel drumsor in tank trucks or rail tankcars that can beemptied pneumatically with dry air or nitrogen(dew point below − 40 ◦C). Because the prod-uct tends to cake, it should not be stored formorethan six months. International marine trans-portation is governed by the IMDG code, class8, UN no. 1726. RID, ADR, ADNR: Class 8,no. 11b, Rn 801, 2801, 6801, respectively. EEC:Yellow Book 78/79, EG no. 013−003−00−7.UK: Blue Book, Corrosives & IMDG code E8031. USA: DOT regulations, corrosive solid,CFR 49, 172.101.

4.1.5. Uses

Anhydrous aluminum chloride is an impor-tant Friedel-Crafts catalyst in the chemical andpetrochemical industries [58, 59]. A principalapplication is the alkylation of benzene by alkyl

halides to form alkylbenzenes that are consumedin the production of synthetic detergents, suchas alkylbenzenesulfonates. Aluminum chloridealso catalyzes the liquid-phase ethylation of ben-zene with ethylene to yield ethylbenzene, mostof which is used in the production of styrene.

Ethyl chloride is produced mainly by the re-action of hydrochloric acid and ethylene in thepresence of aluminum chloride; the consump-tion has significantly decreased because of thedeclining demand for tetraethyl lead as an anti-knock additive.

In the dyestuffs industry, aluminum chlorideiswidely used as catalyst in the production of an-thraquinone and its derivatives and of pigments,such as phthalocyanine green. A further appli-cation of aluminum chloride is as a finish for ti-tanium dioxide pigments, which are in this wayprotected from oxidation by an oxide layer.

Until 1960, aluminum chloride was used ex-tensively in petroleum refining for cracking andisomerization, but it has now been replaced byzeolite catalysts. It is still employed in reform-ing hydrocarbons, as a polymerization catalystin the production of hydrocarbon resins, and forthe production of butyl rubber.

It is a catalyst also in the production of com-pounds, such as aromatic aldehydes, ketones,and 2-phenyl-ethanol, used in fragrances. Otherapplications for anhydrous aluminum chlorideinclude the production of aluminum borohy-dride, lithium aluminum hydride, as well ascompounds of phosphorus and sulfur.

Most of these reactions with aluminum chlo-ride produce a solution that is often used as aflocculating agent in the treatment of waste wa-ter.

Aluminum chloride also is an intermediate inthe production of aluminum by the Alcoa smelt-ing process [60] (→ Aluminum).

Data on the consumption of anhydrous alu-minum chloride in the USA are given in Table5.

Table 5. U.S. anhydrous aluminum chloride consumption in 1993(not including the amounts consumed in the production ofaluminum) [61]

Alkylate detergents 2200 tEthylbenzene 4000 tHydrocarbon resins 3900 tPharmaceuticals 4000 tTitanium dioxide 2200 tMiscellaneous 4900 tTotal 21200 t


4.2. Aluminum Chloride Hexahydrate

Properties. The white hydrate AlCl3 · 6H2O [7784-13-6], Mr 241.43, crystallizeshexagonally and dissolves exothermally in wa-ter (133 g hexahydrate per 100 g water at 20 ◦C)[62]. The solubility increases only slightly withtemperature. The aqueous solutions are stronglyacidic because the salt hydrolyzes. Hydrogenchloride is evolved on heating concentrated so-lutions. The structure of aqueous solutions ofAlCl3 · 6 H2O is discussed in [63].

Production and Uses. Aqueous solutions ofAlCl3 · 6 H2O can be obtained by dissolvingaluminum hydroxide in hydrochloric acid: thehexahydrate crystallizes when hydrogen chlo-ride passes into cold (about 20 ◦C) saturated so-lution. In this way, impurities, particularly iron(III) chloride, can be removed [62]. Aluminumchloride hexahydrate has onlyminor importanceas such; for example, it is used for hardeningphotographic layers. However, it is of industrialimportance as an intermediate in the productionof aluminum oxide [64, 65].

4.3. Basic Aluminum Chlorides

Properties. Basic aluminum chlorides[1327-41-9], aluminum hydroxychlorides, alu-minum chloride hydroxides, poly(aluminumhydroxychlorides), have the general formulaAl2(OH)6−nCln · x H2O. The individual com-pounds are best defined either by the molarratio of aluminum to chloride (2/n) or by theirbasicity, defined as (1 – n/6) · 100%. Below aminimum water content, which depends on n,the compounds are unstable; they decomposeto aluminum hydroxide and aluminum oxide onheating to above 80 ◦C, releasing water and hy-drogen chloride. All basic aluminum chlorideswith n = 1 – 5 can be isolated, and all are white,in some cases crystalline substances, partiallysoluble in the lower alcohols [66, 67], but readilysoluble in water. For example, 170 g of the com-pound Al2(OH)5Cl · 2.5 H2O dissolves in 100g water at 20 ◦C. However, the viscosity of thesolution prevents further amounts from dissolv-ing. The electrical conductivities of the aqueoussolutions, shown in Figure 2, are characteristicof the basic aluminum chlorides.

Figure 2. Electrical conductivity of freshly prepared ba-sic aluminum chloride solutions: ◦) from Locron L (ca.50% aqueous solution of Al2(OH)5Cl · 2.5 H2O) and hy-drochloric acid at constant aluminum oxide concentration(11.5%) and 20 ◦C; �) prepared directly from AlCl3 · 6H2O

Aqueous solutions of basic aluminum chlo-rides are acidic because the compounds hydro-lyze (see Table 6). The stability of the solutionsdepends on the basicity: compounds with alu-minum to chloride molar ratios of ca. 0.5 : 1to about 1.6 : 1, corresponding to basicities ofca. 30 – 75%, decompose to give insoluble ba-sic aluminum chloride at a rate that dependson the temperature and concentration of the so-lution [68]. On the other hand, compounds ofbasicity greater than 75% are very stable inaqueous solution. For example, compoundswiththe approximate composition Al2(OH)5Cl · 2 –3 H2O [12042-91-0] are so stable at concentra-tions> 50% that no decomposition occurs, evenon boiling for days.

Table 6. pH values of aqueous solutions of Al2(OH)5Cl · 2.5 H2Oat 20 ◦C (mass fractions, w, in %)

w pH60 3.430 4.015 4.210 4.35 4.43 4.4

In contrast to the aluminum chlorides oflow basicity (below 65%), the compounds ofhigh basicity (above 65%) do not crystal-lize from their aqueous solutions, but form


glassy masses. When solutions in the basicityrange 30 – 65% are evaporated, the compoundAl2(OH)3.7Cl2.3 · 6.05 H2O (basicity 62%) al-ways crystallizes [69]. On addition of alkali tobasic aluminum chlorides, aluminum hydroxideprecipitates. Hydrochloric acid converts all ba-sic aluminum chlorides to the hexahydrate:

Al2(OH)5Cl+5 HCl+7 H2O→2 AlCl3·6 H2O

Chemical Structure. Basic aluminum chlo-rides have been known for a long time [1,p. 205], but only recently have investigationsgiven significant insight into their chemicalstructures [70 – 73]. Basic aluminum chloridesare mixtures of complex compounds of vari-ous degrees of polymerization. Spectroscopicand kinetic investigations of the basic alu-minum chlorides and of their solutions [70 – 73]lead to the conclusion that the complex ion[Al13O4(OH)24(H2O)12]7+ is present and is inequilibriumwith its polymeric forms. Variationsobserved in the properties of these aqueous so-lutions such as viscosity and pH are caused bypolymerization and depolymerization [74].

Analysis. Basic aluminum chlorides are sostable that the aluminum content can be deter-mined only after decomposition of the com-plex ion [75, 76]. The usual procedure is tomix the solution containing aluminum with60% sulfuric acid and 0.1 M disodium ethyl-enediaminetetraacetate, followed by ca. 20%sodium hydroxide. The pH is adjusted to 4.7 –4.9 at 60 – 70 ◦C. After the solution is cooled toabout 25 ◦Cand buffer solution (acetic acid, am-monium acetate), acetone, and indicator (dithi-zone in ethanol) are added, the solution is back-titrated with 0.1 M zinc sulfate. One milliliter of0.1 M disodium ethylenediaminetetraacetate isequivalent to 2.698 mg of aluminum.

Production. Basic aluminum chlorides aremade by dissolving either aluminum hydroxideormetallic aluminum inhydrochloric acid.Agedaluminum hydroxide leads only to compoundsof basicity up to 65% [68]. To obtain aluminumchlorides of high basicity, freshly precipitatedaluminum hydroxide [77] must be used. How-ever, the preferred method is to dissolve alu-minum in hydrochloric acid either thermally orelectrochemically:

2 Al+HCl+5 H2O→Al2(OH)5Cl+3 H2

This method is exemplified by the followingprocess [78]:

Aluminum electrodes are set up at distancesof 40 mm in a corrosion-resistant electrolysiscell. Hydrochloric acid (� 1.04 g/cm3) is pouredinto the cell at a rate such that the reaction tem-perature remains at ca. 80 ◦C. After all the hy-drochloric acid has been added, the voltage bet-ween the first and last electrodes is set so thatan average voltage of 0.6 V per electrode pair isobtained at a current density of ca. 170 A/m2.The formation of explosive gas mixtures mustbe avoided throughout the reaction by dilutingthe hydrogen with air or nitrogen to below theexplosion limit. After about 70 h, the densityof the electrolyte increases to about 1.2 – 1.3g/cm3, and the pH value to about 3.5. At thesame time the cell voltage falls. The electrolytethen consists of a solution containing aluminumand chloride in themolar ratio 2 : 1, correspond-ing to a basicity of 83%. The corresponding bro-mides aswell as iodides are obtained by the samemethod. The solid halides are obtained from thesolutions by careful evaporation or by spray dry-ing [79]. Recently another method may be ofinterest: Controlled thermic decomposition ofAlCl3 × 6 H2O [86].

Uses and Quality. The most important ba-sic aluminum chloride with regard to con-sumed quantities are the polyaluminum chlo-rides (PAC) with basicities in the range of 35 –60 %. PAC is widely used as a flocculating agentfor purifying water. In water treatment PAC canbe used for purifying surface water, sewage, andwastewater from industry andmunicipalities andfor water treatment in swimming pools. A sec-ond important application for PAC is as fixingsizing agent in the paper industry. Because of thechange from acidic to neutral or alkaline sizing,large quantities of aluminum sulfate had to besubstituted by PAC. Various qualities are avail-able with aluminum contents between 7.5 and18.0 %, calculated as aluminum oxide.

Another important basic aluminium chloridehas the composition Al2(OH)5Cl · 2 – 3 H2Oand is known in the literature as aluminumchlorohydrate. Very stringent purity specifica-tions apply to this substance. Themaximum lev-els permitted are Fe 100 ppm, SO4 500 ppm,


Pb 20 ppm, As 3 ppm. These are required forone of its major applications, namely, that inthe cosmetics industry as the effective compo-nent of antiperspirants [80, 81]; trade names areChlorhydrol (Reheis, USA) and Locron (Clari-ant). Other important areas of application forthis aluminum chloride are in the production ofcatalysts and highly temperature-resistant fibersbased on Al2O3, as a hydrophobic agent for im-pregnation of cotton textiles, for tanning leather,as a retention agent in paper production, as abinding agent for fire-resistant ceramic productsas a hardening agent for rapid fixing baths in thephotographic industry and as a flucculant for pu-rifying swimming-pool water.

5. Toxicology

Aluminum sulfate is classified as nontoxic;an LD50 value of 6207 mg/kg ( mouse, oral) isreported [82].Alum solutions are known for theirastringent (tissue-contracting) effects. A TLV of2 mg/m3 was established for water-soluble alu-minum salts [83].

Anhydrous aluminum chloride has a corro-sive and irritant effect on the skin, respiratorytract, and eyes. The toxic effect is caused by theevolution of hydrochloric acid when the productis exposed to moist air. The resulting respiratorydifficulties vary from mild irritation to cough-ing. TheMAK value (1995) and TLV (1983) forhydrochloric acid are both 7 mg/m3 [83, 84].

The extensive use of the basic aluminumchloride Al2(OH)5Cl · 2 – 3 H2O in the cos-metic industry worldwide over more than 30years has shown that this compound is not irri-tating to the skin in the concentrations used com-mercially. Although intermittent application of150 mg Al2(OH)5Cl · 2 – 3 H2O over a periodof 3 days was found to cause mild skin irritationin humans, a dose of only 7.5 mg of AlCl3 · 6H2O caused the same degree of irritation [85].

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