The Coal Handbook: Towards Cleaner Production || Surface chemistry fundamentals in fine coal processing

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Woodhead Publishing Limited, 2013347 12 Surface chemistry fundamentals in fine coal processing J. S. LASKOWSKI, University of British Columbia, Canada DOI : 10.1533/9780857097309.2.347 Abstract : It is argued that the wettability which is fundamental for fl otation also determines the properties of fi ne coal aqueous suspensions and thus controls not only fl otation but also fl otation products dewatering and handling either as dry products or as suspensions (e.g. coal-water slurries). Typical fi ne particle technology problems appear also in gravity separation methods in which fi ne magnetite aqueous suspensions are used as a medium. In this chapter an attempt is made to look at these various unit operations in some unifi ed way based on the fact the main aspects of these unit operations result from the fundamental fact that all these are aqueous suspensions of fi ne particles characterized by the same rheological phenomena. Key words : fi ne coal, coal fl otation, settling, fi ltration, fl occulation, oil agglomeration, pelletization, fi ne coal handleability, coal-water slurries, magnetite dense media, rheology. 12.1 Surface properties of coal Coal is an organic sedimentary rock whose composition changes with coal-ifi cation. Since metamorphic development of coal, also referred to as coal-ifi cation, is synonymous in chemical terms with progressive enrichment of the coal substance in organically bound carbon, all coals, regardless of their origin or type, can be arranged in an ascending order of carbon content (Fig. 12.1). As this fi gure shows, coal is a highly cross-linked polymer con-sisting of a number of stable fragments connected by relatively weak cross-links. Coal also contains heteroatoms, such as oxygen (which appears in coal in the form of phenolic, etheric, and carboxylic groups), nitrogen, and sulfur, and their presence in coal structure strongly affects coal surface properties. Coal surface properties, like the properties of any other solid, can be stud-ied via wettability measurements. This involves measurement of contact angle ( ) with the use of liquid with known surface tension ( L ). The work of adhesion of liquid to solid ( W SL ) is given by W W WSLWW SWW LLW SLWW AB( ) +WSWW LLW L (1 [12.1] 348 The coal handbook Woodhead Publishing Limited, 2013 where WSLWW LW and WSLWW AB stand for the Lifshitsvan der Waals contribution to the work of adhesion and the acid-base interactions energy contribution, respectively (please note that in older publications the term WSLWW d , the disper-sion forces contribution, was used instead of WSLWW LW as is common today). In order to evaluate the dispersion forces contribution to the wettabil-ity of coals, Gutierrez-Rodriquez et al . (1984) used methylene iodide and showed that the values of the contact angle measured with this compound do not depend on coal rank, or on its oxidation. These contact angle values for various coals were in the range of 28 9 irrespective of the experimen-tal technique (captive-bubble or sessile-drop). As shown by Fowkes (1964) W WSLWW d SLWW LW d Ld=WSLWW LW 2 sdd Ldd [12.2] For water L d 22 mJ/m 2 . Methylene iodide, as saturated hydrocarbons, is a useful reference liquid because its intermolecular attraction is entirely due to London dispersion forces. For methylene iodide L d = L = 50.8 mJ/m 2 , and for methylene iodide wetting coal surface, one can obtain: W WSLWW SLWW d d Ld L=WSLWW d 2 d Ld = L ssdd Ldd L ( c+1 os ) [12.3] Coal rank%CdafPeatLignite60708791Sub-bituminousHigh-volatile bituminousMedium-volatile bituminousLow-volatile bituminousSemi-anthraciteAnthraciteGraphite 12.1 Variation in coal structure and carbon content with coal rank. Surface chemistry fundamentals in fi ne coal processing 349 Woodhead Publishing Limited, 2013 This gives for coal s d 44 mJ/m 2 . For coal interacting with water, if it is assumed that coal is a homogenous hydrocarbon matrix that is unoxidized, is mineral matter free, and interacts with water only via dispersion forces: W WSLWW SLWW d =WSLWW d ( ) L ( ))1+ [12.4] and thus cos= +1 1+ = 2W d L dSLWW dL L [12.5] Putting for water L = 72 mJ/m 2 one can derive the contact angle on such a coal surface would have been about 98 . Any smooth coal surface having a water contact angle of less than 98 contains, therefore, various hydrophilic areas (polar functional groups, inorganic impurities, etc.) on the hydropho-bic hydrocarbon matrix (Laskowski, 1994, 2001). 12.1.1 Effect of coal rank on wettability In the 1940s, Brady and Gauger (1940) observed that the contact angle values measured on Pennsylvania bituminous coals were larger than on anthracite, while North Dakota lignites were very hydrophilic. The results of comprehensive wettability studies on coal from the Donbass Basin (Ukraine) were published by Elyashevich (1941), while further details were provided by Horsley and Smith (1951) in the 1950s. The analysis of the wet-tability of coals as a function of coal rank was offered by Klassen in his coal fl otation monograph (Klassen, 1963) in which he used Elyaschevichs data. This relationship is shown in Fig. 12.2 using more recent data of coal analysis for oxygen content of Gutierrez-Rodriquez et al . (1984) and Bloom et al . (1957). As Fig. 12.2 shows, low-rank coals that possess a lot of oxygen are quite hydrophilic, while low-volatile matter bituminous coals are the most hydrophobic of all. Comparison of the contact angle values shown in Fig. 12.2 with the calculated value for pure coal organic matrix (about 98 ) indicates that while the contact angles measured on bituminous coals are not that different from this calculated value, the difference increases with decreasing coal rank. This is an obvious effect of increasing oxygen content in coal with decreasing rank (also shown in Fig. 12.2). The contact angle measured on bituminous coal is smaller than the calculated values for pure coal organic matrix because coal always contains some hydrophilic inor-ganic matter (ash). 350 The coal handbook Woodhead Publishing Limited, 2013 Coal is a very heterogeneous solid. Figure 12.3 is a schematic represen-tation of coal surface. Coal can be depicted as a hydrocarbon matrix that contains various functional groups (Fuersteau et al ., 1982). The composi-tion of the matrix varies with the coalifi cation (Fig. 12.1). Coal also contains mineral matter and is porous. As Fig. 12.1 shows, with increasing coalifi ca-tion degree hydrocarbons building coal become more aromatic. Rosenbaum and Fuerstenau (1984) assumed that coal may be modeled as composite material, the non-wettable portions of which are made up of paraffi ns and aromatic hydrocarbons, and whose wettable portions are represented by functional groups and mineral matter. To calculate the contact angle on such a composite surface they used the Cassie-Baxter equation and assumed that 65 70 75 80 85Carbon (%, daf)90OxygenWater-captivebubbleWater-sessiledrops95 100001020304050607080510Oxygen (%)Contact angle ()15202530 12.2 Relationship between coal rank and wettability by water measured by the captive-bubble and sessile-drop methods (Source: After Gutierrez-Rodriquez et al ., 1984 with permission of Elsevier), and the relationship between coal rank and the total oxygen content (Source: After Bloom et al ., 1957 with permission of Elsevier). CoalMineralmatterCoohOHPores 12.3 Schematic representation of coal surface. Surface chemistry fundamentals in fi ne coal processing 351 Woodhead Publishing Limited, 2013the maximum values for contact angles on paraffi nic hydrocarbons can be as high as 110 , while those for aromatic hydrocarbons are only 85 . This explains why the wettability of very aromatic anthracites is lower than that of bituminous coals. This concept was further developed in the patchwork assembly model by Keller (1987). Such an analysis must also include coal porosity. For example, Horsley and Smith (1951) observed that some petrographic constituents (e.g. fusain), lose good natural fl oatability after prolonged immersion in water. More recent results (He and Laskowski, 1992) entirely prove the effect of porosity. However, while on less hydrophobic surfaces water is sucked into capillaries by capillary forces and this makes such a coal even more hydrophilic, the capillaries on the surface of a hydrophobic coal will stay fi lled with air and this will make such a surface more hydrophobic. 12.2 Coal flotation Coal fl otation is the only fi ne coal cleaning process that is effective in treat-ing 0.15 mm size coal. Because of coal high natural hydrophobicity it may appear to be easy to fl oat, but the wide range of surface properties of coals from various ranks, and various degrees of liberation of the treated particles make the process very often diffi cult. Flotation of low rank/oxidized coals and desulfurizing fl otation are still challenging problems awaiting for solution. 12.2.1 Effect of rank on fl otability In accordance with what has been said, coal fl oatability should strongly depend on rank as has been extensively discussed (Laskowski, 2001). In 1951, Horsely and Smith concluded that in order to obtain equal recoveries a larger quantity of reagents were required for anthracites and lignites than for bituminous coals. In practice, this requires the use of different combina-tions of reagents in fl oating different coals. Xu and Aplan (1993) demon-strated it in a very simple way. Figure 12.4 shows that while MIBC alone is suffi cient to fl oat the very hydrophobic bituminous coals, a combination of MIBC (frother) and an oil (collector) is needed to fl oat lower-rank coals. Aplan noted a semi-logarithmic relationship between the fuel oil consump-tion and the carbon content in coal. As has already been pointed out, coal is heterogeneous and it contains organic matter and mineral matter. The former appears in the form of mac-erals, and the latter as minerals. Macerals are classifi ed into three groups: vitrinite, exinite (liptinite) and inertinite. The vitrinite group comprises the most abundant macerals in coal. Macerals do not appear in isolation, but occur in associations in various proportions and with variable amounts of mineral matter to give rise to the characteristic banded or layered character 352 The coal handbook Woodhead Publishing Limited, 2013of most coals. These associations are referred to as lithotypes and can be distinguished macroscopically. The lithotypes include vitrain (bright bands in coal), clarain (bright, lustrous constituent, which in contrast to vitrain has dull intercalations), durain (dull) and fusain (black or gray in color with fi brous structure similar to that of charcoal). Since macerals have different chemical compositions, their surface and fl otation properties also vary. As Fig. 12.5 taken from Klassens monograph (Klassen, 1963) shows, coal particles varying in size and petrographic com-position behave differently in the process. Fine bright particles dominate in the fi rst products and only with time coarse particles and dull constituents start fl oating. Large particles, including particles that are not liberated, fl oat only when the fi ne particles are removed from the cell. These data correlate very well with Horsley and Smiths observations (Horsley and Smith, 1951), which indicate that bright petrographic components (vitrain) are more hydrophobic and fl oat better than dull components (durain). Arnold and Aplan (1989) claim that hydrophobicity of coal macerals fol-lows the pattern: exinite>vitrinite>inertinite. 70 80 90 100Carbon (%)0.010.1110100Minimum amount of frother plus collector for optimumrecovery (kg/t)MIBC onlyMIBC and oilMIBC and naphthenic acid 12.4 Minimum amount of frother and collector for optimum recovery of coals of various carbon contents. (Source: After Aplan, 1993.) Surface chemistry fundamentals in fi ne coal processing 353 Woodhead Publishing Limited, 2013 The conclusions regarding the behavior of coal macerals in fl otation are further complicated by mineral matter content. The effect of petrographic composition of coal particles on their fl otation properties can be studied only for fresh (unoxidized) and low ash samples (Holuszko and Laskowski, 1995). For samples containing more than 15% ash, the surface properties are predominantly determined by mineral matter. 12.2.2 Flotation reagents The behavior of coal in the fl otation process is determined not only by a coals natural fl oatability (hydrophobicity), but also by the acquired fl oat-ability resulting from the use of fl otation reagents. The general classifi cation of the reagents for coal fl otation is shown in Table 12.1 (Laskowski, 2001). The use of liquid hydrocarbons (oils) as collectors in fl otation of coal is characteristic for the group of inherently hydrophobic minerals (graphite, sulfur, molybdenite, talc, coals are classifi ed in this group). Since oily collec-tors are water-insoluble, they must be dispersed in water to form an emul-sion. The feature making emulsion fl otation different from conventional fl otation is the presence of a collector in the form of oil droplets, which must 2 4 6 8 10 12 14 16 18 20Flotation time (min)020406080100Yield (%)4213 12.5 Effect of petrographic composition and particle size on coal fl otation kinetics. (1) bright coal; (2) dull coal; (3) shale interlocked with dull constituents; (4) gangue. (Source: After Klassen, 1963.) Woodhead Publishing Limited, 2013 Table 12.1 Coal fl otation reagents Type Flotation use as Functional group Examples Action Nonpolar (Water-insoluble) Collectors _ Kerosene Fuel oil Selective wetting and adhesion of oil drops to coal particles Surface active (Water soluble) Frothers Hydroxyl Nitrogenous Aliphatic alcohols Polyglycols Frothers with some collecting abilities. Also improve emulsifi cation of oily collectors Emulsifi ers (Soluble in oily collector) Promoters Hydroxyl Carboxyl Nitrogenous Polyethoxylated alcohols, fatty acids, etc. Facilitate collector emulsifi cation and spreading over coal Inorganic (Water soluble salts) Modifi ers _ NaCl, Na 2 SO 4 H 2 SO 4 , Ca(OH) 2 Ca(OH) 2 Promoters pH regulators Sulfi de depressants Protective Colloids Depressants Hydroxyl Carboxyl Polymers: starch, dextrin, carboxymethyl cellulose, etc. Modifi ers, Coal depressantsSurface chemistry fundamentals in fi ne coal processing 355 Woodhead Publishing Limited, 2013collide with mineral particles in order to enhance the probability of particle-to-bubble attachment. The process is based on selective wetting: the drop-lets of oil can adhere only to particles that are to some extent hydrophobic. The effect of emulsifi cation on fl otation has been studied, and its benefi cial effect on fl otation is known (Sun et al ., 1955). Coal fl otation is commonly carried out with a combination of an oily col-lector (e.g. fuel oil) and a frother (e.g. MIBC). All coal fl otation systems require the addition of a frother to generate small bubbles and to create a stable froth (Table 12.2). Typical addition rates for frothers are in the order of 0.050.3 kg of reagent per tonne of coal feed. Depending on the hydrophobic character of the coal particles, an oily collector such as diesel oil or kerosene may or may not be utilized. When required, dosage rates commonly fall in the range of 0.21.0 kg of reagent per tonne of coal feed, although dosage levels up to 2 kg/t or more have been known to be used for some oxidized coals that are diffi cult to fl otate. The benefi cial effect of a frother on fl otation with an oily collector was demonstrated and explained by Melik-Gaykazian et al . (1967). Frother adsorbs at the oil/water interface, lowers the oil/water interfacial tension and hence improves emulsifi cation. However, frother also adsorbs at the coal/water interface (Frangiskos et al ., 1960; Fuerstenau and Pradip, 1982; Miller et al ., 1983) and provides anchorage for the oil droplets to the coal surface. Chander et al . (1994), after studying various non-ionic surfactants, concluded that the fl otation of coal can be improved in their presence because of the increased number of droplets, which leads to an increase in the number of droplet-to-coal particle collisions. While the use of oily col-lectors and frothers is the most common, also a group of fl otation agents known as promoters have found application in coal fl otation. In gen-eral, these are strongly surface-active compounds and are mostly used to enhance further emulsifi cation of water-insoluble oily collectors in water. Because of environmental concerns associated with tailing ponds, the method for disposing of fi ne refuse from coal preparation plants by under-ground injection has been gaining wide acceptance. Unfortunately, many common fl otation reagents, including diesel oil, are not permitted when fi ne refuse is injected underground into old mine works. This is the main driv-ing force for fi nding replacement for the crude-oil based fl otation collectors (Skiles, 2003). An alternative to fuel oil may be biodiesel, a product created by the esterifi cation of free fatty acids generally from soy oil, with an alcohol such as methanol, and subsequent transesterifi cation of remaining trigly-cerides. Water, glycerol and other undesirable by-products are removed, to produce a product that has physical characteristics similar to diesel oil. The use of some vegetable oils was demonstrated to provide equivalent (and even superior) fl otation results when compared with diesel fuel (Skiles, 2003). These are the results of commercial scale tests on a circuit that has 356 The coal handbook Woodhead Publishing Limited, 20134.25 m in diameter columns. The product concentrate ash was 13.5%. The consumption of the tested vegetable oil was about two times lower from the consumption of diesel oil in these tests. 12.2.3 Flotation of low rank coals The subject of fl otation of low-rank coals was tackled by Wheeler (1994) in his interesting paper on the effect of frothers on coal fl otation. Frothers have by far the largest effect on coal recovery and they do not only act in their tradi-tionally accepted function of bubble makers. In his tests he used anthracite, medium-volatile bituminous, high-volatile B bituminous, and subbituminous A coals, and different frothers in combination with fuel oil. For easy-to-fl oat medium-volatile bituminous coal the aliphatic alcohols such as MIBC were found to be excellent. Going down in the order of natural fl oatability, medium- Table 12.2 Frothers utilized in coal fl otation Name Formula Solubility in H 2 O (1) Aliphatic alcohols Methyl isobutyl carbinol (MIBC) 2-ethyl hexanol 2,2,4-trimethyl-pentanediol 1,3-monoisobutyrate (TEXANOL) R OH C H3 CH CH2 CH CH3CH3 OHCH3 CH2 CH2 CH2 CH CH2 OHCH2 CH3CH3 CH3 O CH3CH3 CH CH C CH2 O C CHOH CH3 CH3 Low Low Insoluble (2) Polyglycol-type frothers DF 250 DF1012 Aerofroth 65 DF 400 DF-1263 CH 3 (PO) 4 OH CH 3 (PO) 6.3 OH H(PO) 6.5 OH CH 3 (PO) 4 (BO)OH Total 32% Total Very good PO stands for propylene oxide (-CH 2 -CH 2 -CH 2 -O-), and BO for butylene oxide (-CH 2 -CH 2 -CH 2 -CH 2 -O-) Cresylic acids (mixture of cresols and xylenols) that in the past were commonly used in coal fl otation are not in use any more because of their toxicity. Surface chemistry fundamentals in fi ne coal processing 357 Woodhead Publishing Limited, 2013volatile bituminous > anthracite > high-volatile B bituminous > subbitumi-nous A. MIBC quickly loses its effectiveness, fi rst to 2-ethyl hexanol, then to texanol and glycol frothers (e.g. DF-1012). On the bvBb coal 2-EH fl oated 15% more coal than MIBC. Wheelers results confi rm that, while short chain aliphatic alcohols possess only frothing properties, other frothers also exhibit collecting properties, and the properties of oil emulsifi ers. So, the fi rst conclu-sion is that in the fl otation of lower-rank/oxidized coals, one single reagent is not suffi cient. A combination of properly selected frother and an oil, and good emulsifi cation, in most cases leads to a satisfactory fl otation. The use of the specifi cally selected promoters may also be helpful. The use of properly selected reagents under the best possible conditions is especially important when coals are diffi cult to process. An emulsifi cation of the reagents and their stage addition are particularly useful in such cases. An obvious practical solution is shown in Fig. 12.6. 12.2.4 Desulfurizing fl otation The need to reduce the sulfur content of coal to low levels is one of the more pressing needs facing coal preparation engineers if they are to actively assist power-generating stations in reducing their overall sulfur emissions. Yet, as of now, effective pyrite depression for a wide variety of coals remains out of reach. FrotherOilWater1/31/3 1/31/3Emulsification2/3Feed 12.6 A general concept showing a coal fl otation circuit with emulsifi cation of oily collector and stage addition of reagents. (Source: After Laskowski, 2001 with permission of Elsevier.) 358 The coal handbook Woodhead Publishing Limited, 2013 Coal, a sedimentary organic rock, contains organic matter (macerals) and inorganic matter (minerals). Coal preparation upgrades raw coal by reduc-ing its content of mineral matter; the particles with lower ash content are separated from those with higher ash content. The most common minerals that occur in coal are clay minerals, carbonates (e.g. dolomite, calcite, sider-ite), oxides (e.g. quartz) and sulfi des (e.g. pyrite). The last one is especially important. Convincing pieces of evidence indicate that most of the mineral matter in coal down to micron grain sizes is indeed a distinct separable phase that can be liberated by fi ne crushing and grinding. Keller (1984) measured the particle-size distribution of mineral matter obtained by low-temperature ashing of a few samples of coal from the Pittsburgh seam. Most mineral grains were in the size range from 1 to 10 m, while the rest were coarser than 100 m. Cleaning these coals by the Otisca-T oil agglomeration pro-cess demonstrated that the ash content can be reduced to 1% ash by grind-ing coal down to a few microns. This indicates that the inherent ash content may be as low as about 1% if coal is fi nely ground to obtain proper libera-tion, and then concentrated using a highly selective method. Sulfur is the constituent of coal that most affects coal marketing. Three types of sulfur in coals can be distinguished by chemical analysis: sul-fi des (pyrite and marcasite), sulfates (mostly gypsum) and organic sul-fur. Organic sulfur in coal appears in different organic compounds, such as thiophenes, sulfi des (aliphatic R-S-R and aromatic -S- and thiols (R-SH and -SH). The majority of the organic sulfur in high rank coals is thiophenic (Attar. 1979; Attar and Dupuis, 1981). Typical sulfur analyses of coals from different regions throughout the world set out by Mayers (1977) varied from 0.38% to a high of 5.32%. The pyritic sulfur content of these selected coals varied from a low of 0.09% to a high 3.97%, while the organic sulfur content varied from a low of 0.29% to a high of 2.04%. In general, organic sulfur levels greater than 2% or much less than 0.3% are almost never encountered, and pyritic sulfur contents greater than 4% are also uncommon. The distinction between inorganic and organic sulfur is of great impor-tance. Inorganic sulfur content in coals can be reduced by physical sepa-ration methods. In general, coal and pyrite can be separated either by depressing pyrite and fl oating coal, or by depressing coal and fl oating pyrite. But since pyrite density is over 5 g/cm 3 and coal density is in the range from 1.3 to 1.5 g/cm 3 , the rejection of gangue (and thus also pyrite) can be improved by better circuitry and machinery. Gravity separation methods are quite effi cient in separating coal particles from pyrite particles. Due to the very high density of pyrite, even very small amounts of pyrite are suf-fi cient to increase the density to a point where the coal particles can be Surface chemistry fundamentals in fi ne coal processing 359 Woodhead Publishing Limited, 2013rejected. Therefore, particles containing small amounts of pyrite are more easily rejected by density processes than by surface-based processes such as fl otation. However, the effi ciency of separation by gravity methods falls rap-idly for particles fi ner than 100 m. Therefore, coal desulfurization depends critically on pyrite grain size, and hence on the dissemination of pyrite in coal, since only pyrite that is liberated can be separated from coal. Statistical analyses performed by Zitterbart et al . (1985) revealed that the percentage of liberated pyrite is inversely correlated to the mean particle size for sev-eral seams. For example, for the Pittsburgh seam coal about 55% of pyrite was liberated after grinding coal down to 50 m. Since coal is not ground down to liberation sizes before fl otation (as it is done in the case of ores), the fl otation feed is a mixture of poorly and well liberated particles. A combination of gravity separators and fl otation cells in the same fi ne coal processing circuit is therefore essential; such combined fl owsheets are very characteristic for coal fl otation. In practice, the maximum particle size for coal fl otation is generally 28 mesh (0.6 mm) for highly fl oatable coals. The conditions required to recover coarse particles (i.e., high aeration rates and high reagent dosages) must often be avoided since they also favor the fl otation of impurities. Consequently, most coal fl otation is applied to minus 100 mesh (0.15 mm) particles as more cost-effective spiral concentrators or multiple stages of water-only cyclones can be used to upgrade the plus 100 mesh (0.15 mm) fractions. In such cir-cuits, the fractions high in sulfur that contain pyrite are rejected with high density reject. Pyrite almost always appears in polymetallic sulfi de ores. Since it is treated as a gangue, which has to be depressed in the differential fl otation of sulfi des, its fl oatability has been extensively studied. In the fl otation of sulfi de ores with thio-collectors (e.g. xanthate), it is common to depress iron sulfi des (pyrite/marcasite/pyrrhotite) by carrying out fl otation in an alkaline environment using lime. However, similar conditions do not depress pyrite in the fl otation of coal (Fig. 12.7). The collectorfrother system in coal fl o-tation is dictated by coal rank and fl oatability, and the selected reagents for best coal fl otation apparently also promote pyrite fl otation. It is known that some sulfi des display so-called self-induced fl otation in moderately oxidiz-ing conditions. It is of interest to point out that Baker et al . (1990) observed the synergistic action of coal and oxygen in coal pulps. This mechanism, as well as incomplete liberation from organic matter, may be responsible for the different behaviors of coal-pyrite and ore-pyrite. It is known that coal-pyrite fl oats well in the presence of hydrocarbon collectors (Olson and Aplan, 1984). Since the fi rst option, namely fl otation of coal and depression of pyrite has not been very successful, the second option, that is the reverse fl otation 360 The coal handbook Woodhead Publishing Limited, 2013in which coal is depressed and pyrite is fl oated, was also investigated. In the two-stage reverse fl otation process, developed by the US Department of Energy (Miller and Deurboruck, 1982), after the fi rst conventional fl ota-tion stage, the froth product that contains coal but also pyrite particles is repulped with dextrin and the pyrite is fl oated off with xanthate in acidic pH from depressed coal. Although the pilot plant tests were very encouraging, the process has not been commercialized. 12.3 Solidliquid separation Unit operations in the coal preparation plant can broadly be divided into four distinct groups: classifi cation (that is mostly screening), benefi ciation, dewatering of the benefi ciation products, and water clarifi cation (Fig. 12.8). As Figs 12.8 and 12.9 demonstrate, with the exception of screening (and some crushing), all other unit operations are carried out in water. Therefore, what the coal prep plants are dealing with are aqueous suspensions. As the term fi ne indicates, in the fi ne coal cleaning circuit the solid particles are fi ne. These fi ne suspensions, whether fl otation froth products or fl otation tail-ings, are subjected to the solidliquid separation since clarifi ed water must be recycled back to the process. The properties of these suspensions, and our ability to control them, determine the outcome of such unit operations. Coal-pyriteCoalOre-pyrite202040Flotation recovery (%)60801004 6 8pH10 12Fuel oil: 50 mg/tMIBC: 50 mg/t 12.7 Comparison of fl otation recovery of fi ne coal, coal-pyrite, and ore-pyrite as a function of pH using fuel oil and MIBC. Coal 74 m, coal- and ore-pyrite 45 m. (Source: After Jiang et al ., 1993 with permission of Elsevier.) Surface chemistry fundamentals in fi ne coal processing 361 Woodhead Publishing Limited, 2013 12.3.1 Stability of mineral suspensions The solid-in-liquid dispersed systems are commonly classifi ed as colloids or suspensions. In the former, the particle size is below 1 m; in the lat-ter case, these particles are larger than 1 m. Brownian motion of particles suspended in a liquid, characteristic for colloidal systems, ceases when par-ticles are coarser than 1 m. However, irrespective of their size, particles ROM coalRecycled waterWaterclarificationProductsDewateringBeneficiationClassification(Screening) 12.8 Water circuit of coal preparation plant. MiningFine storageGrindingSlurry transportationClassificationConcentrationThickeningDrying0 10 20 30 40 50Water, volume %60 70 80 90 100FiltrationScreeningCrushingCoarse storage 12.9 Variation in water content during various stages of processing (Holland and Apostolides, 1969). 362 The coal handbook Woodhead Publishing Limited, 2013suspended in a liquid frequently collide, not only because of their Brownian movement, but also because these particles, depending on their size, settle at different rates. As a result of such particleparticle collisions, the stability of the system is determined by the interaction between the particles during these encounters. The system is considered to be stable when the particles are dispersed and settle as individual units; the system is unstable, and coag-ulates, when the particles form aggregates and settle quickly. In the case of colloidal systems, detection of the coagulation is fairly sim-ple. In the case of mineral suspensions, the size of the particles is such that they are not subjected to the Brownian motion and thus the solid particles settle due to gravity (at the rate given by Stokes equation). The state of aggregation of such a slurry can be judged by visual inspection of samples left to stand in tall glass cylinders. The typical behavior of A dispersed (sta-ble) or B coagulating (or fl occulating) polydisperse and multicomponent slurries is shown schematically in Fig. 12.10. 12.3.2 DLVO theory In the DLVO theory (developed independently by Deryaguin, Landau, Verwey and Overbeek) the energy of interaction between solid particles MicrographsB1 2 3A(a)(b) 12.10 Visual appearance of mineral suspensions (schematic). (a) stable; (b) coagulating. (1) initial; (2) short time; (3) long time. Circles: appearance of samples in the optical microscope (Kitchener, 1978). Surface chemistry fundamentals in fi ne coal processing 363 Woodhead Publishing Limited, 2013is estimated as a sum of the London-van der Waals attraction, and electro-static repulsion (when the interacting particles have the same either neg-ative or positive electrical charge) resulting from overlap of the electrical double layers surrounding the interacting particles (Fig. 12.11). In the classical DLVO theory, the total energy of the interaction between two particles is given by V = V R V A [12.6] where V R is the energy of electrostatic double layer repulsion (positive value means repulsion) and V A is the van der Waals attraction (negative value means attraction). In practical situations, the electrokinetic potential (known as zeta poten-tial) is measured to characterize the electrical charge of solid particles. Figure 12.12 shows the calculated value of the total interaction energy ( V ) for the system at two different values of the zeta potential. The repulsion between two particles depends on the electrical charge of these particles. Figure 12.12 shows repulsion curves at two different val-ues of zeta potential of the interacting particles. Since zeta potential can be changed, for instance by changing pH, and the van der Waals attraction forces, which depend on the chemical composition of the particles, are not changed when pH is altered, and the two total interaction curves for this system are different. In Case 1, the zeta potential is large and the repulsion curve V R (1) is very positive, giving large positive values of the total inter-action curve (the maximum on the total interaction curves is referred to as energy barrier). If the kinetic energy of the interacting particles is not large enough, the particles will not be able to overcome the energy barrier and the particles will not attach to each other. The system will then be stable. In Case 2, the energy barrier does not exist; each collision between the par-ticles leads to attachment, and the system will coagulate. Hence, coagulation +++++ ++++++++++++++++++++ ++ +++++++++++++++++++++ + + + ++ ++ +++++++++++++++++++ 12.11 Schematic picture of the two identically charged solid particles which double layers overlap when they approach each other (Gregory, 2006). 364 The coal handbook Woodhead Publishing Limited, 2013is the process in which the particles aggregate when the electrical repulsion between the particles is lower than the energy of attraction. For particles which are hydrophobic, the attraction energy also includes the hydrophobic force, which is larger than the van der Waals attraction. As shown by Xu and Yoon (1989, 1990), the coagulation curve in such a case looks like that shown in Fig. 12.13 (top part). The experimental curves were obtained for a bituminous coal. Since with increasing hydrophobicity the additional hydrophobic attractive force becomes larger, which together with the van der Waals force is balanced by the electrostatic repulsion dependent on pH, the pH range of coagulation increases with increasing hydrophobic-ity of the interacting particles. As Fig. 12.14 shows, the iso-electric points of coals (the pH at which the zeta potential values are zero) are more negative for lower-rank and oxi-dized coals. Such particles acquire more negative zeta potentials in water, are less hydrophobic (Fig. 12.2), and thus form more stable suspensions. Fine coal suspensions of hydrophobic bituminous coals coagulate eas-ily. Honaker et al . (2004) have even shown that the naturally hydrophobic material (such as coal and graphite) can be selectively coagulated and sepa-rated from hydrophilic impurities without the use of oily agglomerants and fl occulants. VR (1)VR (2)V (1)V (2)VADistance between particles (H )0Potential energy of interaction (V)+ 12.12 Total interaction energy obtained by summation of an attraction curve, V A , with two different repulsion curves, V R (Shaw, 1970). Surface chemistry fundamentals in fi ne coal processing 365 Woodhead Publishing Limited, 2013 12.3.3 Flocculation The merit of modern polymeric fl occulants is their ability to produce larger, stronger fl ocs than those obtained by coagulation. Flocculants are polymers with high molecular weight that are soluble in water. It is generally accepted that polymers used as fl occulants aggregate suspensions of fi ne particles by a bridging mechanism. The bridging is considered to be a consequence of the adsorption of the segments of the fl occulant macromolecules onto the surfaces of more than one particle. Such bridging links the particles into loose fl ocs (Fig. 12.15). 2040400401 3 5pHpHc7 9 11Ec (%)Zeta potential (mV)60800Fresh Oxidized 12.13 Effect of pH, and hydrophobicity, on coagulation of fi ne coal particles. (Source: Xu and Yoon, 1989 with permission of Elsevier.) 366 The coal handbook Woodhead Publishing Limited, 2013 The polymers used in fl occulation can be classifi ed into coagulants, which are highly charged cationic polyelectrolytes with molecular weights in the 50 000 to 10 6 daltons range, and fl occulants, with molecular weights up to 20 10 6 daltons. It is known that fl occulants are not very effective for treat-ing stable suspensions and, as pointed out by Kitchener (1972), the fl oc-culation is much more effi cient if the suspension is fi rst destabilized by coagulation. This can be achieved by changing pH or by addition of some inorganic coagulants (e.g. lime or alum). Also, low molecular weight cationic polymers can be used to destabilize suspensions, as most mineral particles in 0Heavily oxidizedSubbituminouspHBituminousAnthracitesZeta potential (mV)+ 12.14 Generalized zeta potential vs pH diagram for coals of various rank. (Source: Laskowski and Parfi tt, 1989 with permission of Taylor & Francis) (a) (b) 12.15 Schematic illustration of (a) bridging fl occulation, (b) restabilization at high concentrations by adsorbed polymer. Surface chemistry fundamentals in fi ne coal processing 367 Woodhead Publishing Limited, 2013water carry negative electrical charge. In general, the destabilization process is strongly dependent on process water chemistry. Adsorption of the polymer is generally necessary for fl occulation to occur. It is important, however, to realize that adsorption and fl occulation are not separate sequential processes, but occur simultaneously (Hogg, 1999). There is general agreement as to the basic mechanism involved in the process; the optimum fl occulation occurs at fl occulant dosages corresponding to a par-ticle coverage that is signifi cantly less than complete. Incomplete surface coverage ensures that there is suffi cient unoccupied surface available on each particle for the adsorption of segments of the fl occulant chains dur-ing collision of the particles. The bridging takes place at fl occulant dosages corresponding to a particle surface coverage that is signifi cantly less than complete, and thus at higher concentrations, the polymers stabilize suspen-sions by the mechanism referred to as steric stabilization. Hogg et al . (1993) showed that the appropriate choice of fl occulants is determined primarily by chemical factors (mineral composition, solu-tion chemistry, etc.), but the performance of the fl occulant depends more on physical variables, such as agitation intensity and the rate of fl occulant addition. Flocculants The vast majority of commercial fl occulants are based on copoly-mers of acrylic acid and polyacrylamide (also referred to as hydrolyzed polyacrylamide): (-CH 2 -CH-) m (-CH 2 -CH-) n | | CONH 2 COO - Na + As a result of hydrolysis even non-ionic polyacrylamides contain some anionic groups. This is expressed as the degree of anionicity (the degree of anionicity of completely hydrolyzed polyacrylamide is 100%, so it is a polyacrylic acid). Another important group of fl occulants is polyethylene oxide, (-CH 2 CH 2 O-) n C (Rubio, 1981; Scheiner and Wilemon, 1987). Cationic poly-electrolytes such as copolymers of acrylamide and quaternary ammonium compounds are also available (e.g. Poly-DADMAC). Naturally occurring materials such as polysaccharides (e.g. carboxymethyl cellulose, starch, guar gum, etc.) have also been used as fl occulants. According to Kitchener (1978), the fi rst use of fl occulants involved the application of starch in combination with lime for the clarifi cation of a coal mines effl uent (the patent was fi led in 1928). 368 The coal handbook Woodhead Publishing Limited, 2013 The effectiveness of polymers as fl occulants depends on their molecular weight, the sign of their charge (e.g. anionic or cationic), and the relative charge density (for polyacrylamides this is expressed by the degree of anio-nicity). Depending on molecular weight, the same compounds can operate as dispersants (e.g. dextrin, low molecular weight) or fl occulants (e.g. starch, high molecular weight). Low molecular weight copolymers of polyacrylate type are manufactured as dispersants (e.g., Dispex manufactured by Allied Colloids (now CIBA), etc.). Xu and Cymermans (1999) data confi rmed that the best fl occulants for the clay-containing wastes (Syncrude tailings) are moderately anionic high molecular weight polyacrylamides (optimum around 2030% anionicity). Hamza et al . (1988) reported that anionic polyacrylamides were the best for enhancing the settling rate of fi ne coal. Polymer molecular weight The molecular weight of fl occulants is commonly characterized through vis-cosity measurements. This is based on the Flory-Huggins (Flory, 1953): [ ]]] = KMa [12.7] where [ ] is the intrinsic viscosity of the polymer solution (has units of reciprocal concentration), M is the polymer molecular weight, and K and a are constants. After Henderson and Wheatley (1986) Fig. 12.16 shows the effect of poly-acrylamide intrinsic viscosity (that is indirectly molecular weight) on the sedimentation rate of fl occulated tailings for polyarylamides with varying anionicities. Because of the relationship between polymer intrinsic viscosity and its molecular weight (Equation [12.7]), what Fig. 12.16 shows is the effect of fl occulant molecular weight on fl occulation. It must be born in mind, however, that the intrinsic viscosity of a polymer increases with rising solvent quality. This is shown in Fig. 12.17 for various polymers in various solvents. In a good solvent polymer macromolecules are in extended form, but coil when the solvent quality decreases. This may hap-pen when the ionic strength of the system is increased, or the pH is changed. The exponent a in Equation [12.7] is a measure of the solvent quality and, as Fig. 12.17 shows, it is large (larger than 0.5) for a good solvent, and smaller than 0.5 for poor solvents. Since conformation of polymer macromolecules in solvent depends on the solvent quality, also polymer adsorption onto solid particles depends on it. Adsorption is generally higher from a poor solvent than a good solvent (Koral et al ., 1958). Surface chemistry fundamentals in fi ne coal processing 369 Woodhead Publishing Limited, 2013000.5Log (Tailings sedimentation rate) (cm/min)11.522.510Polymer intrinsic viscosity (L/g)20 30 12.16 Effect of polyacrylamide intrinsic viscosity (molecular weight) on sedimentation rate of the fl occulated tailings. (Source: After Henderson and Wheatley, 1986). Copyright Taylor & Francis Group, LLC. (http://www.taylorandfrancis.com), reproduced with permission.) 101102[]/(ml g1)103104 105 106 107M /(g mol1)a = 0.0a = 0.5a = 1a = 2 12.17 Intrinsic viscosity [ ] as a function of the molar mass Mw for different polymer-solvent systems as given by Eq. [7]. In addition to the straight line relationships indicating the range of good solvents (a > 0.5) and bed solcents (a370 The coal handbook Woodhead Publishing Limited, 2013 Klein and Conrad (1980) derived the following empirical equation that can be used to determine polyacrylamide molecular weight. This relation-ship holds for polyacrylamide samples ranging in molecular weight from 5 10 5 daltons to 6 10 6 daltons when measurements are conducted in 0.5 M NaCl solution at 25 C. [ ] .]] = 7 1. 9 1 0 3 0 77Mw [12.8] Testing fl occulation Solubilities and rates of dissolution of high molecular weight fl occulants in water are generally low, and preparation of the polymer solution is a very important fi rst stage (Brown, 1986). The following step, mixing the polymer solution with the suspension, is critical (Owen et al ., 2009). Since fl occulants are either used to enhance solids settling rates to maxi-mize thickener capacity, to enhance dewatering by fi ltration, or to improve water clarifi cation, various tests are utilized. They include measurements of solids settling rate, sediment density, fi ltration characteristics, and superna-tant turbidity. Several techniques have been proposed to determine the settling velocity in laboratory experiments, the jar tests being the most common (Coe and Clevenger, 1916; Richardson and Zaki, 1954; Michael and Bolgers, 1962). Jar testing involves homogenization of the suspensions in settling cylinders, introduction of the fl occulant, and mixing by moving a plunger up and down in the cylinders (or by inverting the cylinders several times). This procedure is claimed not to be satisfactory because of the local over-dosing that can occur when the relatively concentrated fl occulant solution meets the slurry (Kitchener 1978); but more important is that the agitation in this method does not produce the optimum fl occulation. Farrow and Swift (1996) dem-onstrated that the jar test has several problems. It is important to realize that adsorption and fl occulation are not separate sequential processes, but occur simultaneously (Hogg, 1999). The commonly used improved experimental procedure includes addition of the fl occulant to a vigorously agitated sus-pension, which is immediately stopped after addition of the reagent (Keys and Hogg, 1979). Different mixing/polymer addition conditions may result in very different fl oc sizes and settling rates. Owen et al . (2009) showed that mixing of the slurry with a dilute fl occulant solution within the feedwell determines the performance of commercial thickeners. It was also shown that under certain conditions intense agitation for short times may even change the nature of fl occulation, from total fl occulation to a selective fl oc-culation of only some mineral constituents (Ding and Laskowski, 2007). Surface chemistry fundamentals in fi ne coal processing 371 Woodhead Publishing Limited, 2013 The use of the shear vessel, as described by Farrow and Swift (1996) and Rulyov 1999, 2004), in the fl occulation tests was recently tested by Rulyov et al . (2011) and Concha et al . (2012). Their set-up is shown in Fig. 12.18. The use of a shear vessel (similar to rotational Couette viscometers) in assessing fl occulation effi ciency has the advantage of quantifying the mixing intensity through the shear rate. Using the shear vessel, Rulyov (1999) and Rulyov et al . (2004) have demonstrated that the mixing time in fl occulation can be reduced, from minutes to 56 s, by the appropriate hydrodynamic treatment of the suspension at a given shear rate. In the set-up shown in Fig. 12.18 the Couette-type vessel is rotating in the external vessel, the gap being 1.5 mm. The reactor is fed continuously with the suspension by a measuring peri-staltic pump. Before entering the Couette reactor, the pulp receives contin-uously a diluted fl occulant solution, at a fl ow rate to give a pre-determined dosage. After 6 s treatment at a pre-determined shear rate in the Couette reactor, the fl occulated suspension is discharged from the ultra-fl occulator through a 3 [mm] inner diameter transparent tube equipped with an opto-electronic sensor that registers the fl uctuation of the intensity of the light beam passing normally through the tube (in accordance with techniques proposed by Gregory and Nelson (1984)). The electronic signal is processed ComputerMotorFeed slurryLevelsensorTo wasteSettlingcolumnFlocculantVariablespeed motorPeristalticpumpDiversionwhensamplingPPP BAC200 mm 12.18 Schematic illustration of the set-up used to test fl occulation. (Source: After Concha et al ., 2012 with permission of Elsevier.) 372 The coal handbook Woodhead Publishing Limited, 2013and displayed in a three digital format, thus showing, in relative units, the values of fl occulation effi ciency (or mean fl oc size) and the mean shear rate . In the tests designed to measure the settling rate of the treated suspen-sion, the suspension from the outlet of the tester is continuously fed to a 14 mm diameter settling cylinder and, as soon as the suspension fi lls the cyl-inder, the cylinder is inverted once and the initial settling rate is measured. The fl occulation of fl otation tailings from one of the major Chilean copper mines and Orifl oc-2010 polycarylamide fl occulant in a Couette-type reactor was recently reinvestigated by Concha et al . (2012). By varying the shear rate from 100 to 2000 [s 1 ], the solid concentration from 1 to 15 % by volume, and the fl occulant dosage from 0 to 20 g/ton, it was shown that an important interaction exists between these variables. At the optimal fl occulant dosage, the optimal suspension concentration and the optimal fl occulation time, an increase by 50% in the solid fl ux density function is possible when the shear rate of = 100 1s is changed to the optimum value of around 400 1s . It is worth pointing out that ultrafl occulator-type devices have already been installed at some coal preparation plants (Rulyov, 2004; Rulyov et al ., 2009). Testing the use of fl occulants in fi ltration In present day practice, disposal of the fi ne waste fraction (for instance, coal fl otation tailings) is usually accomplished by: 1. addition of fl occulant to the slurry and thickening of the resulting fl ocs in a thickener, with the underfl ow from the thickener pumped to an impoundment area; 2. treatment of the thickener underfl ow with fl occulant and dewatering on a mechanical device, such as vacuum fi lter or fi lter press. The former method requires the availability of enough land environmen-tally suitable for construction of impoundments and, with the latter, the dewatered material can be mixed with coarse waste fraction and discarded by stacking. This latter method requires dewatering by fi ltration. The dewatering process during fi ltration comprises several stages, as illus-trated in Fig. 12.19 (Lockhard and Veal, 1996). Flocculants reduce the seg-regation of fi ne and ultrafi ne particles, which causes blinding of the fi lter medium, and thus increases dewatering rates. Experimental determination of the effect of fl occulants on fi ltration usu-ally involves fl occulation of the tested slurry, transfer of the fl occulated material to a funnel-type fi lter that is operated under controlled vacuum, and determination of both the fi ltration rate as well as the cake moisture content. The fi ltration rate is described by Darcys equation: Surface chemistry fundamentals in fi ne coal processing 373 Woodhead Publishing Limited, 2013 ddVtKA PH= [12.9] where d dtdV / is the volumetric fl ow rate of fi ltrate through a fi lter cake in time t , A is the fi lter area, P is the pressure drop across the fi lter cake, is the liquid viscosity, H is the cake thickness, and K is the rate constant referred to as permeability. KkS= 32 2( ) ( )1 [12.10] where is the cake porosity, S is the specifi c surface area of the particles, and k is the Kozeny constant. The effect of fl occulants on the fi ltration rate is related to changes in fi lter-cake properties; the permeability, which is determined by porosity of the cake, is the most important. As it is determined by particle size and shape, the appearance of the fl ocs improves the cake porosity. In the simple fi ltra-tion tests in a funnel-type fi lter the particles segregate. Figure 12.20 shows Initial bridging startscake formation andfiltration commencesCake forming Cake formedCake compacted Desaturation by air Air breakthrough(d) (e) (f)(a) (b) (c)12.19 Schematic representation of stages in dewatering by fi ltration. (Source: After Lokhart and Veal, 1996). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) 374 The coal handbook Woodhead Publishing Limited, 2013a better set-up (Tao et al ., 1999), which has been introduced to avoid this problem. In the device shown in Fig. 12.20, the slurry is continuously circulated to avoid sedimentation and the fi lter leaf is submerged in well-mixed slurry. The tests allow determination of the fi ltration rate vs fi lter-cake thickness, and the cake moisture content vs the fi lter thickness. As Fig. 12.19 shows, after the formation of the cake, the cake compres-sion and water expression follow. The situation depicted in Fig. 12.19d is the saturated capillary state, with all the pores fi lled with water. Under these conditions, a capillary pressure opposes air entry, and only when the applied pressure exceeds the capillary pressure does the removal of water from the cake commence (Hogg, 1995). = 2 cosr [12.11] where is the surface tension of the liquid, is the contact angle at the solidliquid interface (receding angle should be used). Equation [12.11] describes the applied pressure necessary to expel liquid from the pores in a packed bed. The equation shows that more hydrophobic coals are easier to dewater, and that the use of surfactants to lower water CirculationsumpCirculationpumpStirrer MoisturetrapFiltercellMoisturetrapVacuumpump 12.20 Illustration of experimental set-up for fi ne coal dewatering by fi ltration. (Source: After Tao et al ., 1999). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) Surface chemistry fundamentals in fi ne coal processing 375 Woodhead Publishing Limited, 2013surface tension may also have some merit. From this point of view, the fi ltra-tion aids can be classifi ed into: 1. Flocculants (to increase cake porosity and thus fi ltration rate). 2. Oily hydrocarbons (to make the coal particle more hydrophobic, agglom-erate fi ne particles, and thus increase not only cake porosity but also par-ticle hydrophobicity). 3. Surfactants (to lower water surface tension and thus reduce capillary retention forces). While polyacrylamide fl occulants are commonly used to improve the set-tling rate, their application in fi ltration circuits has some specifi c functions. The use of high molecular weight non-ionic and anionic fl occulants shows that there exists a fl occulant addition range where there is a signifi cant reduc-tion in the moisture content of the fi lter cake. Increased polymer addition results in an increased moisture content. This can be explained by different fl ow modes of liquid through the fl occulate fi lter cake. While the increased fl occulation of fi ne particles may improve the interfl oc fl ow, the drainage of liquid from the fl ocs becomes increasingly diffi cult (Mishra, 1987). This sug-gests that, while at certain doses the cake moisture content can be reduced, it may increase again at high fl occulant doses. When moisture reduction is the objective, for high molecular weight polymer, over-dosing can lead to an increase in the moisture content. With low molecular weight polymers, a greater degree of moisture reduction can be achieved (Mishra, 1987). Since fl occulants are hydrophilic macromolecules, their adsorption makes solid particles hydrophilic and this also increases the ability of the solid to retain moisture. Emulsifi ed oil can also be used as a fi ltration aid. Such emulsion not only agglomerates fi ne coal particles but also renders them hydrophobic. Nicol and Rayners (1980) results demonstrate that an oil addition of 1% can lead to a lowering of the fi lter-cake moisture from 26% to 16% (wet basis). This considerable lowering of fi lter-cake moisture is accomplished by an increase in both fi ltration rate and solids pick-up rate as oil droplets agglomerate fi ne coal particles. A dramatic improvement in fi ne coal dewatering with the use of asphalt emulsions was reported by Wen et al . (1994). The effect of fl occulants, emulsifi ed oil, and the addition of surfactants on fi ne coal fi ltration have been studied by Laskowski and Yu (1998, 2000). They have shown that the emulsifi cation of kerosene in the presence of surfactants can dramatically reduce the size of oil droplets and, more importantly, can entirely change the electrokinetic properties of such droplets. The particles of low-rank/oxidized coals are negatively charged in water and are diffi -cult to agglomerate with oil. However, when the oil is emulsifi ed in solu-tion with a cationic surfactant, the obtained cationic-emulsion agglomerates 376 The coal handbook Woodhead Publishing Limited, 2013such a suspension effi ciently, improves fi ltration, and also reduces the cake moisture content. Novel dewatering aids were patented by Yoon and Basilio (U.S. Patent 5,670,056). As pointed out by Yoon et al . (2003, 2006), while the addition of 0.5 kg/t of kerosene could reduce the cake moisture content by 5%, the use of their novel additives was able to reduce the moisture con-tent from 2030% down to the 814% range using vacuum fi ltration. The improved performance of new hydrophobizing agents has been attributed to a simultaneous decrease in fi ltrate surface tension and increase in particle hydrophobicity. As disclosed in a patent (U.S. Patent 5,670,056) these new dewatering additives include fatty acid esters; the examples provided indi-cate that they are mostly used as a solution in butanol. These agents seem to be similar to the EKT agent patented as a coal fl otation promoter (Polish Patent 104,569). Use of fl occulants in coal processing In the most common applications, anionic polyacrylamide fl occulants are applied with molecular weight of 10 6 10 7 daltons. While fl occulation of coal-clay fi ne waste generated by coal preparation plants that is then pumped to a pond or impoundment provides a method for rapidly recovering most of the water contained in the waste slurry, this technique also requires the availability of enough land. A method of disposal that minimizes the land requirements is that whereby the underfl ow from the thickener is treated with a fl occulant and dewatered on a mechanical device such as fi lter or belt press. Such treatment yields a material that can be mixed with the coarse waste fraction and disposed by stacking. However, this technique heavily depends on whether the treated material reslurry during a wet season. As was shown by the US Bureau of Mines (Brown and Scheiner, 1983; Scheiner, 1996), an interesting alternative is the use of polyethylene fl occulant (PEO). In the developed process of coal-clay waste fl occulation with PEO, addition of calcium or magnesium salts (for instance lime) is required. Their results indicate that prior coagulation before appli-cation of PEO is very important for effi cient bridging. In this process lime is added up to pH 9 or higher and the PEO dosage required to get optimum results varied from 50 to150 g/t. A dewatering system developed by the US Bureau of Mines uses PEO to fl occulate the waste, followed by dewatering on a static screen. The waste is dewatered (using about 0.05 kg/t of PEO) to produce a material having a solid content ranging from 55% to 60%. It is important that the dewatered material is quite stable and does not re-slurry easily when brought into contact with water. Comparison of PEO with PAM (with about 40 Da) in the dewatering of the fl occulated coal-clay wastes on a belt fi lter press showed that the required dosages of PEO were much lower than those with PAM to obtain about 70% solids content. Surface chemistry fundamentals in fi ne coal processing 377 Woodhead Publishing Limited, 2013 12.3.4 Oil agglomeration Insoluble oily collectors are utilized in coal fl otation. Such oils appear in the pulp in the form of droplets when oil is emulsifi ed in aqueous environ-ment. The droplets attach to the particles that are hydrophobic and then bridge these particles when the system is vigorously stirred. This is so-called oil agglomeration. Three major factors control oil agglomeration (Capes, 1991): (a) solid wettability; (b) the amount of the oil; and (c) the type and intensity of conditioning. The amount of agglomerating oil is critical. At low oil levels, discrete lenses-shaped rings are formed at the points of contact of the particles (Fig. 12.21a and 12.21b). At higher doses, the oil rings begin to coalesce and form a continuous network (Fig. 12.21b and 12.2c). Since droplets of pure aliphatic hydrocarbons attach only to very hydro-phobic particles, they are very selective. Droplets of the oils containing polar hydrocarbons can also attach to the high-ash coal particles. This trans-lates into higher yields of the agglomerated product and higher ash con-tent. Heavier oils were shown to yield excellent recoveries under the more intense mixing conditions needed to disperse more viscous oil in water (Capes and Germain, 1982; Capes 1991). Fuel oils with addition of aromatic hydrocarbons were found to be very good agglomerants (Agus et al ., 1996; (a) (b) (c)(d) 12.21 Oil distribution on moist agglomerates: (a) pendular state; (b) funicular state; (c) capillary state; (d) oil droplets with particles inside or at its surface. 378 The coal handbook Woodhead Publishing Limited, 2013Blaschke, 1990). Labuschange (1987) and Good et al . (1994) described how addition of alcohols to oil can improve oil agglomeration. New light was shed on the mechanism of the oil agglomeration process in a series of papers by Wheelock and his co-workers (Drzymala et al ., 1986; Drzymala and Wheelock 1997; Milana et al ., 1997). Their results revealed that the agglomeration process of a moderately hydrophobic coal with hep-tane is triggered by a small amount of air present as a separate phase. The rate of agglomeration increases as more air is admitted to the system, or as the amount of agglomerant or agitation increases. Oil droplets can attach only to the particles that are hydrophobic, and the oil agglomeration of low-rank, subbituminous coals is not very effi cient. Pawlak et al . (1985) reported that subbituminous coals can be agglomerated with heavy refi nery residues. Since particles of low-rank coals are charged negatively in water, it was also demonstrated that kerosene with 1% addi-tion of dodecylamine when emulsifi ed in water produces oil droplets charged positively in a very broad pH range that agglomerate subbituminous coal very well (Laskowski and Yu, 2000). Various oil agglomeration processes have been developed (Mehrotra et al ., 1983). These include Trent, Convertol, NRCC (National Research Council of Canada), Shell, Olifl oc, CRFI (Central Fuel Research Institute in Dhanbad, India), and BHP processes. In the process developed at the National Research Council in Ottawa (Capes, 1991), the light-oil addition and high-intensity conditioning produces micro-agglomerates, and this is followed by the addition of heavy oil and low-intensity conditioning to pro-duce a handleable product that can be recovered by screening. In the more recent developments, the addition of oil was reduced to a few percent, and this provides better selectivity. In these more recent fl owsheets, the high-intensity mixing is followed by a low-intensity mixing, which enables the coal-oil agglomerates to enlarge and strengthen. The agglomerated slurry is then passed to a Vor-Siv screen. The recovered agglomerates are further dewatered in a screen bowl centrifuge. In a fi nal stage, the agglomerates can be further enlarged by pelletization. Solidliquid separation in fi ne coal cleaning circuits is always diffi cult (and expensive) and the fl owsheet shown in Fig. 12.22 constitutes a very inter-esting alternative to conventional dewatering and drying. In this process classifying hydrocyclones are used to split the fl otation feed into 0.1 mm and +0.1 mm fractions. The former is processed by oil agglomeration while the latter goes to fl otation. The Oilfolc process offers a means for avoiding thermal drying. The fl otation concentrate obtained when fl oating coarser feed dewatered by vacuum fi ltration contains 2% less moisture and the oil agglomeration of the fi ne fraction produces 68% ash clean coal products. It is worth noting that such a process can dramatically improve handleability of the fi nal product. Surface chemistry fundamentals in fi ne coal processing 379 Woodhead Publishing Limited, 2013 12.3.5 Pelletization Coal preparation plants rely mostly on gravity separation techniques, and in the past the coarse products constituted main saleable products. Introduction of new separation methods such as fl otation resulted in a wide utilization of fi ne size fractions as well. This in turn has resulted in a considerable increase in the amount of fi ne clean coal products which poses severe storage, trans-portation and pollution problems. One possible method by which these problems can be alleviated is pelletization of fi ne coal. As reported in literature (Leonard and Newman, 1989; Holuszko and Laskowski, 2010), the addition of a few percent of water to a dry coal can substantially increases its bulk density. This indicates that in the presence of water droplets some sort of consolidation takes place in the bed of fi ne dry particles. Such a process can be further intensifi ed by tumbling the particles in a disk or drum (pelletization). The pelletization process is controlled by capillary phenomena, and the driving force for the pelletization is the lowering of the total surface free energy of the system through a reduction of the effective airwater interfacial area. As shown by Kapur and Fuerstenau (1966) and Sastry and Fuerstenau (1977), the kinetics of pellet formation proceed through three stages: the formation of nuclei agglomerates, the transitional stage, and the ball growth stage. The rolling action of the drum brings the individual particle into prox-imity with each other, so that the physical forces become operative and cause 0.5 mm0.5 + 0.1 mm0.1 mmOilCCW 12.22 Flowsheet of the Oilfl oc test set-up in the Monopol Dressing Plant of Ruhrkohle AG. (Source: After Bogenschneider and Jasulaitis, 1977.) 380 The coal handbook Woodhead Publishing Limited, 2013the particles to rearrange, and surface tension reduction brings about nuclei formation via the bridges of wetting liquid. In the agglomeration of granu-lar materials by capillary forces, the wettability of the aggregated particles by the applied liquid plays an important role (Schubert, 1984). Figure 12.23 shows the coal pelletizing circuit (Sastry and Mehrota, 1981). Only a limited number of research results on pelletization of fi ne coal are available, and Sastry and Fuerstenaus EPRI report (1982) is probably the most valuable source of information in this area. Some results of this report will be used to illustrate the principles. Sastry and Fuerstanu (1982) found that it is possible to pelletize coal of different ranks and widely different size distributions. Rive et al . (1964) reported that even coal as coarse as 1.2 mm could be pelletized, provided that it contained at least 50% of 74- m mate-rial. The rate of the process is governed by the operating conditions and the type of coal. The rate of pellet growth is strongly affected by the amount of feed moisture. The amount of moisture required to pelletize is different for different coal samples, and was found to be mainly determined by the ash content. Figure 12.24 shows the pelletization results obtained with different sam-ples of raw coal. The coal fi nes for these experiments were prepared by stage-crushing in a jaw crusher, and dry grinding in a laboratory ball mill. Two hundred FeedconveyorBinderbinMixer andflufferWater supplyLiquid binderstorageSpraysPelletizingdiscRev. conveyorShredderRoller-screenDry pellets to silo or stockpileBelt dryer 12.23 Flowsheet of a coal pelletizing circuit with pelletization disk. Surface chemistry fundamentals in fi ne coal processing 381 Woodhead Publishing Limited, 2013gram batches of the dry coal utilized in the pelletization experiments were premixed with the desired amount of distilled water (10 min) to pre-pare a moist feed. The moist feed was then placed in a 150-mm length by 225-mm diameter pelletizing drum, enclosed in a humidifi ed chamber and tumbled at a rotational speed of 40 rpm. At the end of each experiment, a portion of the pellets was used for determining their moisture content and compressive strength. As Fig. 12.24 demonstrates, pelletization of fi ne coal is possible only within a narrow range of moisture additions. Outside that range, the coal fi nes are either too dry to agglomerate or too wet and form lumpy and weak agglomerates. Ash content was found to be the most signifi cant property that determines moisture requirements for pelletization. Sastry and Fuerstenaus (1982) report also provided information on the effect of binders on fi ne coal pelletization. The dry compressive strength of the pellets produced without a binder is from 1 to 10 kg strength. Pellet strength was found to decrease with decreasing ash and sulfur content. Corn starch was found to be a very good binder, in that it improves the compres-sive strength of the pellets by several hundred percent; however, such pellets exhibit poor resistance to moisture penetration. On the other hand, asphalt emulsion, which was found not to improve pellet qualities much, renders DekkerBelle ayre southShamokin anthraciteIllinois No. 6UpperFreeportSewickleyFreeportWestKentuckyLowerFreeportOhio no. 9Pittsburgh no. 8LowerKittanning01520Moisture range for pelletization wt (%)30405020Ash content wt (%) (dry basis)40 60 12.24 Infl uence of ash content (dry basis) of coal samples on their moisture requirements for pelletization. (Source: After Sastry and Fuerstenau, 1982.) 382 The coal handbook Woodhead Publishing Limited, 2013pellets waterproof. The pellets produced with 1% by weight of corn starch that were subsequently sprayed with asphalt emulsion produced strong pel-lets which were also water resistant. 12.4 Fine coal handleability The coal must fl ow by gravity in each operation in the distribution system in order to keep these operations and the whole transportation process effi cient. Handleability is commonly defi ned as the ability of the coal to pass through a handling system without causing blockages and hold-ups (Brown, 1997). Preparation plants produce coal as blends of various size fractions. Metallurgical coal fi nes are treated by fl otation, and fi ne coal products are recovered from fl otation circuits by fi ltration. The fi lter cake commonly has a moisture content in excess of 20%. This, in combination with the rest of the blend, can result in handling problems. In the preparation of thermal coals, untreated fi nes are usually recombined with the cleaned coarse fractions from wet gravity separation, which also contain a considerable amount of moisture. Many handleability studies carried out over the years have related some of the physical parameters of coal samples, such as amount of fi nes, mineral matter type, and moisture content, to the handling characteristics. In the 1950s British coal scientists (Cutress et al ., 1960) developed a method to provide a means of assessing the ease of discharge of washed coals and blends from the hopper at the bottom of rail wagons. The Durham Cone test offered a quick and relatively simple way to determine coal fl owability. The vibrating cone was designed to imitate train movement (Fig. 12.25); as a result, the behavior of coal during transport by trains could be reproduced using the Durham Cone. Over the years the method became used as the Coal blendCoal handleabilityCollectingbinLift springloaded leverto releasedischarge plateVibration inhorizontaldirectionDischarge plate500 mm dia.Stainlesssteelhopper 12.25 Schematic illustration of Durham Cone. Surface chemistry fundamentals in fi ne coal processing 383 Woodhead Publishing Limited, 2013standard test to examine handleability, and was used in many studies with varied success. A fi rst comprehensive study on handleability of coal using Durham Cone was published by Hall and Cutress (1960), and was followed by many others (Vickers, 1982; Mikka and Smitham, 1985; Arnold, 1992; Brown, 1997). The major drawback of the method was that the reproduc-ibility of the results was questionable. It was also pointed out that mixing of the sample before the test was extremely important, and that any mixing involving rolling produced a balling effect and altered the fl ow properties of the mixture as measured by Durham Cone. The test results are expressed as Durham Cone Index, DCI = mass sam-ple/average time, or as a fl ow rate (or Durham Cone discharge rate), with the units being kg/s. The analysis of the data reported by several researchers failed to detect any simple correlations (Vickers, 1982; Brown et al ., 1996). Pretty good cor-relation was, however, found between the DCI and the product of the sim-ple moisture content and the fi nes ash fraction, where the fi nes ash fraction is defi ned as: 0 038 0 038100. .038 0AshAshwhole 0.038 is the weight percent yield of the 0.038 mm size fraction, Ash 0.038 is the ash content in the 0.038 mm material, and Ash whole is the ash content in the whole sample. This correlation is shown in Fig. 12.26 (Brown et al ., 1996, 1997, 2000). R = 0.94800600400Moisture * Ash fraction of the fines 384 The coal handbook Woodhead Publishing Limited, 2013 Handleability problems can easily be solved by removing wet fi nes from the power station blend, but this solution is not conducive to maximizing the coal industry saleable output. Better understanding of the factors that con-trol fi ne coal handleability is vital for fi nding a long term solution that would allow further dewatering/reconstitution of the fi lter-cake products into any saleable product. In the tests described by Jones (1990) it was considered that the recon-stitution of the 0.5 mm size fraction into coarser granules capable of with-standing shear in storage and handling had to be an essential part of any fi nal solution. The results are given in Fig. 12.27. As this fi gure indicates, the fi lter cake addition in excess of 8% of the total blend placed the blend han-dleability below the target level and into the problem zone. By pelletizing the 0.5 mm fi nes and adding it in equal parts together with non-pelletized fi nes the handleability was drastically improved. Handleability has recently been re-examined and it has been shown that it critically depends on coal surface properties (Holuszko and Laskowski, 2004). In this research program, pelletizing was used to characterize fi ne coal tendency to aggregate when it is transported/tumbled. As Fig. 12.28 shows, depending on coal surface wettability the adhesion of water droplets onto the coal surface will be different. While in the case of weakly hydrophobic (subbituminous or oxidized) coal the water will tend to 0024Durham Cone discharge rate (kg/s)610 20Filter cake addition (% wt)30 40Filter cake onlyTarget handleabilityFilter cake/Pellet mix ratio 1:150 12.27 Durham Cone discharge rate for various percentages of fi lter cake addition to the power station blend. (Source: After Jones, 1990 with the permission of the Mining and Materials Processing Institute of Japan.) Surface chemistry fundamentals in fi ne coal processing 385 Woodhead Publishing Limited, 2013spread on the surface and strongly bridge coal particles, in the case of very hydrophobic bituminous coal (upper part of Fig. 12.28) the adhesion of the droplets to coal surface will be weak and this will lead to a formation of very frail pellets. It is rather well established that the content of fi nes and the moisture content are the most important factors (Wawrzynkiewicz, 2003; Arnold, 2004) determining coal handleability. However, tests on the effect of mois-ture content clearly indicate that this may be very different depending on coal surface properties. Figure 12.29 shows the fl ow curves determined with the use of the Durham Cone for two coal samples. Medium-volatile bituminous coal samples from British Columbia were used in these tests. The oxidation degree of the tested samples was deter-mined using the transmittance measurement of the alkali extracts (ASTM D 526393), and their wettability was characterized by the equilibrium mois-ture (ASTM D 141293). The high equilibrium moisture indicates hydro-philic coal, while low (around 1%) moisture indicates hydrophobic coal. The high transmittance values indicate non-oxidized coal (hydrophobic), the values well below 90% pointing to oxidized coal (hydrophilic). The equi-librium moisture for LC 3 sample was 1.30%, while for the LC 8U sample it was 7.34%; the transmittance values for these samples were, respectively, 95.25% and 26.06%. Water dropletCoal surfaceWater layerCoalsurface 12.28 Schematic illustration of the effect of coal wettability on the behavior of water droplets on coal surface; upper part very hydrophobic bituminous coal, bottom part lower-rank/oxidized coal. 386 The coal handbook Woodhead Publishing Limited, 2013 As Fig. 12.29 shows, the fl owrate patterns for hydrophobic LC 3 and hydrophilic LC 8U are different. The fl ow rates were plotted vs surface moisture for these samples. The surface moisture was obtained as the dif-ference between actual moisture content of the sample and its equilibrium moisture. It can be noticed that at the same amount of surface moisture, e.g. at 10% surface moisture (Fig. 12.29), the LC 8U coal sample ceases to fl ow, while the LC 3 coal is still handleable. For lower-rank and oxidized coals, the equilibrium moisture is usually much higher than for high rank and non-oxidized coals. Therefore, these coals can tolerate higher moisture levels before they become diffi cult to handle. However, at moisture levels exceeding their equilibrium moistures, a rapid deterioration of their handling behavior is observed, leading to practically non-fl owing conditions. In the case of hydrophobic coals, only the mineral matter affects handleability signifi cantly. For such coals, an increase in mois-ture content tends to affect handleability to a certain level, but these coals do not cease to fl ow completely. Apparently a high amount of clay material, even in very hydrophobic coals, can have a very damaging effect on handle-ability. These effects can also be tested using pelletization experiments, as it was shown that coals that can easily be pelletized may become diffi cult to handle at a given level of moisture (Holuszko and Laskowski, 2004). The picture that emerges from the discussed available results indicates that whenever fi ne coal particles are ready to aggregate (as a result of their surface properties and the moisture content) they will make the whole blend of various saleable size fractions aggregate, and that will cause handleability 0.000.501.001.502.002.503.000.00 5.00 10.00 15.00 20.00Surface moisture content (%)Flow rate (kg/s)LC8U LC 3 12.29 Durham Cone discharge rate for LC 3 (36% fi nes) and LC 8U (37% fi nes) coal samples as a function of surface moisture content. (Source: After Holuszko and Laskowski, 2006.) Surface chemistry fundamentals in fi ne coal processing 387 Woodhead Publishing Limited, 2013problems. The readiness of such a material to aggregate is what creates the problem and this readiness degree can be discharged only by either ren-dering such particles more hydrophobic, or by allowing them to aggregate by pelletizing this material. 12.5 Rheology effects in fine coal processing The importance of this topic has further increased with introduction of coal-water slurries as another way of fi ne coal utilization. 12.5.1 Viscosity of suspensions The viscous nature of the fl uid, i.e. its internal friction, is manifested only when one region of the fl uid moves relative to another. The measurements may involve the fl ow of liquid through capillary under a given pressure, or movement of the liquid placed in between two cylinders (Couette) and caused by the rotation of one of them. In all such cases, the liquid fl ow rate (shear rate) is related to the applied shear stress (pressure) via Newtons low of viscosity: D [12.11] where is the shear stress (Pa), is the dynamic viscosity (Pa s, Pascal second = Nm2s), and D is the shear rate (s 1 ). In the old system a con-venient measure was the centipoise (cP = 0.01 poise), because water has a viscosity of about 1 cP at room temperature (on the SI scale this cor-responds to 1 mPa s). Figure 12.30 is a sketch of the shear stress vs shear rate relationship for several different fl uids. For a stable (no aggregation) diluted suspension of solid spherical par-ticles in Newtonian liquid, Einstein derived the following expression = o( )+ k [12.12] where is the viscosity of the suspension, o s is the viscosity of the sus-pending medium and is the volume fraction of the suspended solid particles. Rheological measurements, when relative viscosity (relative viscosity = / o ) is plotted vs solid concentration (usually v/v %), provide the curve displayed in Fig. 12.31. As this plot demonstrates, the viscosity of a suspen-sion of spherical particles signifi cantly deviates from Einsteins law above = 0 1 , and beyond a volume fraction, ma x , called the maximum packing 388 The coal handbook Woodhead Publishing Limited, 2013fraction, the dispersed particles lock into a rigid structure and fl ow ceases. The packing fraction for hexagonally packed monodisperse spheres is 0.74. As pointed out by Barnes et al. (1989), these values are much smaller for other particles (only 0.3 for grains, and 0.2 for rods). 01Relative viscosity (r)1maxVolumetric solid fraction () 12.31 Schematic graph showing the concentration dependence of the relative viscosity of a typical suspension. The slope at the origin equal intrinsic viscosity. PlasticPseudoplasticNewtonianShear stress, Shear rate, DDilatant 12.30 Rheological curves for Newtonian and several non-Newtonian fl uids. Surface chemistry fundamentals in fi ne coal processing 389 Woodhead Publishing Limited, 2013 The relative viscosity function depicted in Fig. 12.31 in the dilute region is determined by the requirement that the slope at the origin equals the intrin-sic viscosity (2.5 for spheres). The function is best described by the KriegerDougherty equation (Krieger and Dougherty, 1959; Kierger, 1972): ro= = 1ma x[ ] ma x. [12.13] where r is the relative viscosity, is the viscosity of suspension/colloid, o is the viscosity of suspending medium, is the volume fraction of the solid, ma x is the maximum packing fraction, and [ ]]] is the intrinsic viscosity. Taking [ ]]] = 2 5. and ma x = 0 7. 4 for close-packed uniform spheres the exponent in Equation [12.13] is 2.5 0.74 = 1.85. The value of [ ]]] for non-spherical particles is much larger than 2.5. The rheology of fi ne particle systems depends on many factors: particle size, shape and solids concentration. Another factor that strongly infl uences rheological behavior is particleparticle interaction. In general, rheologi-cal behavior becomes more non-Newtonian as the particle size decreases. While well-dispersed systems of spherical particles at low-to-moderate solid concentration (below 40%) exhibit Newtonian behavior, the aggregating systems are non-Newtonian. As Equation [12.13] indicates, the measured relative viscosity becomes very large when is near to the close-pack condi-tion and the effect of the factors discussed above becomes particularly evi-dent in this high solid content range. The utility of the KriegerDougherty equation is in the fact that it takes into account the effect of particle-size dis-tribution and particle shape, as both affect the maximum packing fraction. 12.5.2 Coal-water slurries (CWS) Typically coal that has been mined and cleaned in a coal preparation plant is shipped either to a power generation plant (thermal coal) or to a coke-mak-ing plant (metallurgical coal). This needs large areas for dry sample storage, handling, and transportation, and poses all sorts of environmental problems (e.g. dusting, spontaneous combustion, etc.). Coalwater slurries (CWS) (also referred to as coalwater fuels (CWF), or as coal-water mixtures (CWM)) are highly loaded suspensions of fi ne coal in water. Since these are mixtures of coal and water, CWS is free from some of the major problems of solid coal, such as dusting and spontaneous combustion during storage and transportation. Unlike solid coal, CWS is a liquid, so it does not require large handling facilities. Utilization of fi ne coal in the form of CWS also simplifi es the fi ne coal preparation circuit, in 390 The coal handbook Woodhead Publishing Limited, 2013that it does not need deep dewatering and drying. The fi ne coal in a fi lter cake is directly converted into CWS and is pipelined to a power-generating plant, where it is burned like a heavy oil; the coarse coal does not have to be blended with the troublesome fi nes and so its handling is improved as well. Since CWS is utilized as a fuel to generate power, its calorifi c value is an important factor. Because of the loss of energy on water evaporation (latent heat of evaporation for water is about 2300 kJ/kg) the presence of water in CWS reduces its heating value. For instance, for one kilogram of CWS con-taining 60% of bituminous high-volatile coal with a heating value of 27 MJ/kg, the simplifi ed calculation gives: Evaporation lossCombustion heat = =1000 4 2 30 6 27100 5 64 2. %68 Of course, the evaporative heat loss can be reduced by increasing coal content in the CWS as shown in Fig. 12.32. For a highly loaded CWS with 70% coal the minimum evaporative heat loss will be about 3.65% for high-volatile bituminous coal (27 MJ/kg), and 4.9% for subbituminous coal (20 MJ/kg). However, the most important requirement for the CWS is that it must be pumpable. Consequently the problem is how to increase solids concentration in CWS without raising viscosity above an acceptable level. The answer to this question has two components: (i) the effect of coal par-4824Evaporative heat loss (%)68101252 56 60% coal64 68 72: 20 MJ/kg: 27 MJ/kg 12.32 Effect of coal content in CWS and coal heating value on evaporative heat loss during combustion of CWS. (Source: After Laskowski, 2001 with permission of Elsevier.) Surface chemistry fundamentals in fi ne coal processing 391 Woodhead Publishing Limited, 2013ticle size and particle-size distribution, and (ii) the effect of coal surface properties and chemical additives (referred to as viscosity reducers). Effect of coal particle-size distribution on rheology of CWS The maximum packing fraction, ma x in Equation [12.13], increases with increasing polydispersity of the suspension. Broader particle size distribu-tions have higher values of ma x because the small particles fi t into the gaps between the larger ones. On the other hand, non-spherical particles lead to poorer space-fi lling and hence lower ma x . Particle aggregation, for exam-ple fl occulation, also leads to a low maximum packing fraction because, in general, the fl ocs are not close-packed. This was experimentally confi rmed by Farris (1968). Since coal content in CWS is the most important factor determining its utility, the effect of the coal particle-size distribution on the rheology of CWS was thoroughly examined. The top particle size in CWS is determined by boiler requirements; on the other hand, the size and the yield of the fi nes fractions are determined by CWS viscosity and grinding cost. Because of the burning process limitations, it is usually assumed that the top size cannot exceed 250 m with a particle-size distribution of 7080% below 75 m. With these limits in mind, Ferrini et al . (1984) showed how the coal particle-size distribution could be opti-mized with regards to viscosity of the highly loaded CWS. Figure 12.33 shows particle-size distribution of the samples tested by Ferrini et al . (1984). 1020406080Cumulative % retained100107831211109Particle size (m)10012.33 Particle-size distribution of a family of bimodal particle samples derived from the continuous reference curve No. 3. (Source: After Ferrini et al ., 1984.) 392 The coal handbook Woodhead Publishing Limited, 2013 Curve 3 in Fig. 12.33 shows the particle-size distribution of the sample prepared by grinding. The bimodal suspensions were prepared by screen-ing 20 m and +45 m size fractions from this sample and mixing them at different ratios. Apparent viscosity for the samples varying in particle-size distribution is shown in Fig. 12.34. As this fi gure demonstrates, the changes in viscosity with particle-size distribution are quite dramatic. Samples 9, 10, and 11, that is the samples that contain about 40% of the fi ne fraction, have much lower viscosities than Sample 3. Ferrini et al .s (1984) results clearly demonstrate that a highly loaded CWS can be obtained only by properly adjusting the coal size distribution. The benefi cial effects that can be obtained by manipulating the particle-size distribution are considerable, and therefore grinding and selection of optimum particle-size distribution is an import aspect in all CWS-patented technologies. The Chinese authors (Zhang Rong-Zen et al ., 1984; Zuna Wang et al ., 1993) claim that the Chinese experience indicates that if a fi ne coal is unsuit-able in its as-received form for CWS preparation, in most cases this is due to low content of ultrafi ne particles. Effect of coal surface properties on rheology of CWS Rheology of fi ne particle systems depends on many factors, particleparticle interactions being one of them. As shown by Xu and Yoon (1989, 1990), hydrophobic particles tend to coagulate in aqueous environment (Fig. 12.13). We studied such effects with the use of rheological methods. 20 30 40Fine fraction (%)114008001200Viscosity (mPa .s)1600200024001098371250 60Continuoussize dist.70 12.34 Effect of 20 m fraction content on apparent viscosity of bimodal coal-water slurries at constant overall coal content (72% by wt.). (Source: After Ferrini et al ., 1984.) Surface chemistry fundamentals in fi ne coal processing 393 Woodhead Publishing Limited, 2013 The tests were carried out with F4 sample of a bituminous coal (ash 11.7%, moisture 0.6%, fi xed carbon d.m.m.f 73.3%) from a mine in British Columbia. The contact angle measurements used to characterize its wettability by water gave 90 (advancing contact angle). The suspensions were prepared after dry pulverization of the sample below 212 m and all the rheological tests were carried out at 59% solids by wt. As Fig. 12.35 reveals, for the suspensions prepared from the fresh F4 coal sample, very high yield stress values were obtained over a very broad pH range (from about 4 to 9). For the sample oxidized at 8590 C over 24 h, high yield stress values were recorded only around pH of 4.5, while for the sample oxidized over 221 h, the yield stress maximum is obviously situated at pH much lower than 3.5. The point is that the iso-electric points for these three samples were found to be at pH 8 for the fresh sample, around pH of 4.5 for the sample oxidized for 24 h, and at pH of 2.5 for the heavily oxidized sample. Thus, while the hydrophobic fresh sample coagulates over a broad pH range from about 4 to 9 (Fig. 12.35) and it is not much affected by the position of the iso-electric point (i.e.p.), the oxidized sample (24 h) coagu-lates only around its i.e.p. The heavily oxidized sample has i.e.p. around pH 2.5 and it can be expected that the maximum coagulation pH is also situated around this pH value. This agrees remarkably with the data published by Xu and Yoon (1989, 1990). 3048Casson yield stress (Pa)1216204 5 6 7 8pH9 10 11 12 13F4 Fresh F4 Oxidized (24 h)F4 Oxidized (221 h) 12.35 Yield stress values for aqueous suspensions of the bituminous coal as a function of pH. (Source: After Melo et al ., 2004, with permission of the Metallurgical Society of CIM.) 394 The coal handbook Woodhead Publishing Limited, 2013 In the rheological measurements, whenever suspended particles coag-ulate and the network of the interacting particles is formed, high yield stress values are recorded. The maximum values correspond well with the maximum coagulation (Johnson et al ., 2000). While strongly hydrophobic particles coagulate over a broad pH range (see Fig. 12.13), the hydrophilic particles coagulate only over the pH range at which these particles do not carry electrical charge. In total agreement with these conclusions are our rheological measure-ments carried out with CWS prepared from the same F4 bituminous coal in the presence of humic acids. Some of these results are shown in Fig. 12.36. Direct measurements of the wettability of the tested coal in the presence of humic acid indicate that this hydrophobic bituminous coal becomes hydrophilic as a result of humic acid adsorption. This is not surprising, as humic acids are hydrophilic polymers. This compound resulting from oxidation of organic matter, or obtained directly by extraction of low-rank coals, is a complex anionic polyelectro-lyte that contains both carboxylic and phenolic functional groups. There is evidence that irrespective of the original coal surface properties, humic acids impart properties similar to low-rank coals to all coals (Laskowski, 100 102030Contact angle ()405060708090200400Yield stress (Pa)60080010001200100 1000Humic acid conc. (ppm)10 000F4 advancing F4 receding F4 yield stress 12.36 Effect of humic acids on wettability of bituminous coal and on the yield stress of 65% (wt.) CWS prepared from this coal. (Source: After Pawlik et al ., 1997 with permission of the Metallurgical Society of CIM.) Surface chemistry fundamentals in fi ne coal processing 395 Woodhead Publishing Limited, 20132001). The electrokinetic measurements revealed that the coal particles in the humic acids solutions acquire a very negative charge, as evidenced by very negative zeta potential values (almost 70 mV) (Pawlik et al ., 1997). The increased hydrophilicity of coal particles and their increased negative charge must stabilize such particles against coagulation. As a result, the rhe-ological properties of such suspension become more Newtonian, as indi-cated by decreasing values of the yield stress. Coals varying in rank differ in surface properties. While bituminous coals are quite hydrophobic, the subbituminous coals are not (Fig. 12.2). The rhe-ological measurements depicted in Figs 12.35 and 12.36 indicate that these differences in surface properties must affect the rheological properties of the suspensions prepared from such coals. Since hydrophobic solid particles suspended in aqueous environment tend to aggregate, the rheological prop-erties of such suspensions should be different from the rheological proper-ties of the suspensions prepared from low-rank coals. Figure 12.37 shows, after Schwartz (1985), the apparent viscosity of the coalwater slurry prepared using three different coal samples. This fi gure indicates that while it is possible to obtain highly loaded CWS from Coal 3 (76% wt.) at the limiting viscosity of 1000 mPa.s, for Coal 1 the maximum solids content at this viscosity is only about 65%. The wettability tests gave for the three coal samples the following contact angles: 90 for Sample 3, 40 for Sample 2, and 20 for Sample 1. These samples were also characterized by chemical analyses that gave the following C/O contents: 4.6 (Sample 1), 550500Apparent viscosity (mPa.s)1000150060Coal 1Coal 2Coal 365 70Concentration of coal (wt. %)75 80 12.37 Apparent viscosity of coalwater slurry prepared from three different coal samples with the same stabilizer (0.05% of non-ionic ethylene oxide/propylene oxide copolymer). (Source: After Schwartz, 1985.) 396 The coal handbook Woodhead Publishing Limited, 201312.7 (Sample 2), and 20.8 (Sample 3), with the following contents of C/H: 13.6 (Sample 1), 15.8 (Sample 2), and 18.5 (Sample 3). These results reveal that while Sample 3 is a very hydrophobic bituminous coal, the other two samples were clearly less hydrophobic, with Sample 1 being a low-rank sub-bituminous coal. Figure 12.37 illustrates well the existing controversy. As is seen from this fi gure, much higher coal content (loading) can be achieved with higher-rank coals than with low-rank coals, and this is sometimes interpreted that it is easier to prepare CWS from high rank coal. The problem is that coal is highly porous and heterogeneous. Coal moisture content (equilibrium mois-ture) is a function of coal rank. While it is very low for bituminous coals (around 1%) it can be as high as 20% for subbituminous coals. Since the maximum packing fraction, ma x , is around 0.75, the minimum content of water in suspension must be at least 25%. This is needed as water is a lubri-cating medium, without which this fi ne particle system cannot behave as a fl uid. For a low-rank coal, containing 20% equilibrium moisture (coal inter-nal moisture), the minimum content of water is then around 45%, while for the bituminous coal this can be around 26%. Since water content in CWS is the most important factor that determines its calorifi c value, a few processes have been developed to reduce the internal water content in the low-rank coals used in the preparation of CWS. Figure 12.38 shows apparent viscosity of CWS, measured at a shear rate of 100 s 1 , prepared from six different coals. In all cases, 1% of the same non-ionic additive was utilized. Several conclusions are evident from these plots. First, the experimentally measured apparent viscosity dramatically increases at a given solids content, and second, this limiting solids content is different for different coals. Since the particle-size distributions of these samples were similar, the conclusion is that the rheology of CWS depends on coal surface properties. Seki et al . (1985) derived the following relationship: M CMf cCc+1 1100 [12.14] where Mf is the free water in CWS, Cc is the dry coal content in CWS, and Mc is the coal equilibrium moisture content. So, obviously, for a highly loaded CWS (large Cc in Equation [12.14]), the free water content will sharply decrease if the coal moisture content is high, and this will raise the viscosity of CWS. The relationship between coal content in CWS at a viscosity of 1000 mPa.s and coal equilibrium moisture content is shown in Fig. 12.39. As these results demonstrate, the rheological properties of CWSs pre-pared from different coals characterized by similar particle-size distributions Surface chemistry fundamentals in fi ne coal processing 397 Woodhead Publishing Limited, 2013can be correlated with the coal moisture content, and then coal rank. The relationship between the CWS viscosity and coal moisture content can be used to estimate CWSs slurryability (maximum coal content in the CWS) at an acceptable viscosity. Additives in preparation of CWS CWS must be pumpable and this requirement translates into suffi ciently low viscosity, but CWS may be stored in tanks over a long period of time, and thus must also exhibit suffi cient sedimentation stability. Different addi-tives are required to reduce viscosity and to increase stability. Dispersants. Many low molecular weight polymers (MW < 100 000 daltons) adsorb onto coal surfaces and can be used as dispersants for coalwater suspensions. All good coal dispersants are also very effi cient depressants for coal fl otation (Pawlik, 2005). Both anionic (e.g., carboxym-ethyl cellulose, humic acid, polystyrene sulfonate, etc.) and non-ionic (dex-trin) depress coal fl otation and, by dispersing coal particles, improve CWSs 602001000Apparent viscosity (mPa.s)500065Coal content (%)70 75CoalUnder 74 mmparticlesize (%)788180798176771.01.01.01.01.01.01.01.42.43.33.34.95.68.6Additive% concentrationEquilibriummoisturecontent (%)Coal HCoal GCoal ECoal CCoal ACoal DCoal F 12.38 Relation between apparent viscosity and coal content in CWS for different coals. Non-ionic additive 1% on dry coal basis. (Source: After Seki et al ., 1985.) 398 The coal handbook Woodhead Publishing Limited, 2013stability. According to Hara et al . (1992), while good stability requires steric stabilization, a good slurryability requires strong electrostatic stabilization. Figure 12.40 depicts the effect of dispersant concentration on the viscosity of CWS prepared from three different coals (Higashitani et al ., 1990). As Fig. 12.40 shows, the CWS viscosity can clearly be reduced only at rather high dosages of dispersants (around 1000 g/t). Yoshihara (1999) syn-thesized graft copolymers and tested their effect on CWS rheology. The graft copolymers of sodium polyacrylic acid with polystyrene side chain showed higher adsorption onto coal and turned out to be better viscosity reducers for CWS. Polystyrene sulfonate (PPS) with molecular weight of 14 000 is commercially used in Japan for the preparation of CWS. Additives stabilizing CWS against settling (anti-settling additives). CWSs are made of relatively coarse particles. Such particles tend to settle under gravity and, when stabilized against aggregation, settle as individ-ual particles and form a compact sediment that may be diffi cult to re-disperse. Since CWSs may be stored in tanks over a considerable period of time, such a system must therefore also be stabilized against settling. Figure 12.41 shows schematically how the effect of the structure that develops as a result of particleparticle interactions affects the rheology and stability of CWSs. First, it must be borne in mind that the properties of CWSs are very different from the properties of the disperse systems dealt with by colloid 06065Coal content at 1000 mPa.s (%)70752 4 6Equilibrium moisture (%)8AnionicCoal-type:Coal BCoal DCoal GCoal ACoal ECoal FCoal CCoal HAdditive-Concentration:Anionic; 0.6%Nonionic; 1.0%AnionicNonionic ANonionic BNonionic10 12 12.39 Relation between coal content in CWS at 100 mPa.s and coal equilibrium moisture. (Source: After Seki et al ., 1985.) Surface chemistry fundamentals in fi ne coal processing 399 Woodhead Publishing Limited, 2013chemistry. The most obvious difference is the solid content. While in a very dilute disperse system any aggregation between particles results in a loss of stability and fast sedimentation, in a heavily loaded suspension the aggregation may prevent particles from settling and so it may stabi-lize the system. But aggregation creates a network of interacting parti-cles, and this will increase viscosity. Therefore, it is obvious that particles in CWS must be properly dispersed to reduce viscosity, and to increase maximum packing density. Dispersants are used for this reason. But since CWS must also be stable, some weak aggregation will also be required. This is achieved with the use of high molecular weight polyelectrolytes (for instance various natural gums with MW > 10 6 ) (Saeki et al ., 1999). CWS from low-rank coals There are many deposits of subbituminous coals and lignites that can easily be recovered by strip mining. Very often these coals constitute a very attrac-tive clean energy source, since they may contain 0.2% sulfur and 8% ash (e.g. Alaskan low-rank coals). However, the high inherent moisture content of these coals and their low heating value on an as-mined basis compromise the economics for rail haulage. Consequently, most low-rank coals are con-sumed by electric utilities located near mines sites. 104024 (Pa.s)68103 102Cs, kg-surfactant/kg- coal101D = 34.0 s1D D 59T H 61W W 63Coal Cc (wt %) 12.40 Effect of concentration of sodium salt of naphthalene formaldehyde sulfonate on CWS apparent viscosity at 34 s 1 . Symbols WW, DD and TH stand for different coal samples. (Source: After Higashitani et al ., 1990 with permission of the Mining and Materials Processing Institute of Japan.) 400 The coal handbook Woodhead Publishing Limited, 2013 In an effort to upgrade coal and produce a transportable fuel, several dewatering processes have been developed. Almost all of the coal-inher-ent moisture content can be removed by thermal drying; the fi nal mois-ture content achieved in the product is dependent on particle size and residence time. However, since low-rank coals depreciate due to shrink-age and loss of structural elasticity when they are dried in hot gases, the dried product is dusty and is subject to spontaneous combustion. When the dried coal is slurried in water, a signifi cant portion of the coal mois-ture removed during drying is reabsorbed onto coal particles. Such slur-ries have only slightly higher energy densities than similar slurries made from raw coals (usually less than 11.6 MJ/kg (Potas et al ., 1986)). To min-imize moisture reabsorption, the dried coal can be coated with a fuel oil or can be briquetted. Many so-called low-rank coal (LRC) drying processes have been devel-oped (Willson et al ., 1997). In one such process, the hot-water drying process HighWelldispersedPoorLowNoneHighGoodSedimentation stabilityViscosityYield stressMaximum solids contentHighHighLowWeaklyaggregatedExtensivelyaggregatedZeroSurface charge 12.41 Illustration of concentrated suspensions showing that at a given solids content the volume of sediment depends on particle aggregation. (Source: After Laskowski and Parfi tt, 1989 with permission of Taylor & Francis.) Surface chemistry fundamentals in fi ne coal processing 401 Woodhead Publishing Limited, 2013(HWD), coal is heated under pressure in water. Under such conditions, evolving tars remain on the coal surface and plug micropore entrances. The results shown for a subbituminous coal C from No. 4 seam of Usibelli Coal Mine, Alaska, confi rm the very strong effect of temperature on the process; equilibrium moisture content decreases from 25% down to 9.5%, and calo-rifi c value increases from 27.3 mJ/kg to almost 31 MJ/kg when the process temperature is increased from 275 C to 325 C (Walsh et al ., 1993). Ohki et al . (1999) used FTIR spectroscopy and showed that in the processing of an Indonesian LRC, Adaro (16.42% moisture, 45.6% volatile matter, 1.24% ash, 36.74% FC on as-received basis), the oxygen content of coal was signi-fi cantly reduced. This results from decomposition of carboxylic groups (and any reduction in the content of oxygen in coal translates into lower equilibrium moisture content). Potas et al . (1986) demonstrated that the maximum solids content in the CWS prepared from two Alaskan coal samples (lignite and subbitu-minous) can be increased from the 45% range to about 5560% range. In the vacuum drying/tar coating method (Usui et al ., 1999) the coal is dried under vacuum at 200 C, then at 270350 C and is coated with tar (5% wt./coal), and it is used to prepare CWS. Coal reverse fl otation Bituminous coals are hydrophobic (see Fig. 12.2), fl oat easily, and therefore forward fl otation is a common practice. But since subbituminous coals, and also the coals stored in old tailings ponds, which are not that hydrophobic, are diffi cult to fl oat the reverse fl otation may be quite an attractive option in such cases. The reverse fl otation of coal has recently been shown to be possible (Ding and Laskowski, 2006). There are fundamental differences between the forward and reverse coal fl otation. In the case of the forward fl otation, the clean coal is recovered as a froth product that is made hydrophobic with the use of various fl otation reagents. In the case of reverse fl otation, the clean coal product is what is ending up in the fl otation tailings; this product is kept as hydrophilic as pos-sible, to make its fl otation impossible (Fig. 12.42). While these differences may not seem to be very signifi cant, they are extremely important when these products are utilized to make a CWS. These differences are clearly seen when rheological curves for the clean coal product from the forward fl otation (froth product) (Fig. 12.43) are compared with the rheological curves obtained for the CWS prepared also for the clean coal but obtained from the reverse fl otation (these are fl otation tailings) (Fig. 12.44). The fi rst and the most obvious difference is that in the fi rst case, that is when the froth product from forward fl otation is used to prepare CWS, the dispersant is needed to increase the CWS loading (Fig. 12.43). PSS10 402 The coal handbook Woodhead Publishing Limited, 2013dispersant (polystyrene sulfonate with MW of 14 000) was from LION Corp., Japan. In the latter case, when the fl otation tailings from the reverse fl otation are used, high solids loading can be obtained without any dis-persant (Fig. 12.44). This product is suffi ciently hydrophilic and does not require the dispersant to prepare CWS. The reverse fl otation is not as simple as forward fl otation and in the case discussed required the use of cationic Raw coal Raw coalForward flotationReverse flotationConcentrate (coal)Concentrate (gangue)Tailings (gangue) (reject)Tailings (coal) 12.42 Schematic illustration of coal forward and reverse fl otation processes. 460200400Apparent viscosity (mPa.s)600800100012001400160048 50 52Solids content (%)54 56 58 60Forward in waterForward with 0.5% PSS10Forward with 1% PSS10 12.43 Effect of PSS10 dispersant on apparent viscosity (calculated at a shear rate of 100 s 1 ) of slurries prepared from the forward fl otation concentrates. (Source: After Ding and Laskowski, 2009). Copyright Taylor & Francis Group, LLC. (http://www.taylorandfrancis.com), reproduced with permission.) Surface chemistry fundamentals in fi ne coal processing 403 Woodhead Publishing Limited, 2013collector (Arquad 1250 from AKZO Nobel) along with some depressants/dispersants (dextrin, tannic acid and A-100 polyacrylamide fl occulant from Cytec). This, however, does not look so bad if compared with the dosages of dispersing additives that must be used in the preparation of CWS (they are usually in the range of 1%, that is 10 kg/t) when fl otation froth product is utilized. 12.5.3 Rheology of magnetite dense media Dense medium separation In a dense-medium separation raw coal is introduced into the dense medium, whose density is higher than the density of the lighter constituent (coal) but lower than the density of the heavy constituent (inorganic rock). In a per-fect separator, the particles have an infi nite time to report to either sink or fl oat products. In practice, a limited time is available for the separation, and the rate with which the particles move (relative to the medium) determines the outcome of the process. Particles characterized by a small density dif-ferential or small size do not move in the medium fast enough and will be misplaced. Thus, the separation effi ciency is based on the phenomena that determine sedimentation of solid particles in liquids. 460200400Apparent viscosity (mPa.s)60080010001200140048 50 52Solids content (%)54 56 58 60Reverse in waterReverse with 0.5% PSS10Reverse with 1% PSS10 12.44 Effect of PSS10 dispersant on apparent viscosity (calculated at a shear rate of 100 s 1 ) of slurries prepared from the reverse fl otation tailings. (Source: After Ding and Laskowski, 2009). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) 404 The coal handbook Woodhead Publishing Limited, 2013 Settling phenomena A particle settling in a liquid under gravity is subject to two forces, a driv-ing force due to gravity and a drag force that opposes motion. The equation resulting from such considerations that describes the terminal velocity of solid particle in a liquid can be written in the following form: vd gCt p=43( )) [12.15] where d is the spherical particle diameter, is the particle density, is the liquid density, and C p is the dimensionless drag coeffi cient. C fd pf (Re )p [12.16] and since Repvd=vv [12.17] the drag acting upon the moving particle depends also on the medium vis-cosity ( ) but to the extent that depends on the prevailing hydrodynamic conditions. For small spherical particles ( d < 100 m) Cdp=24Re [12.18] and this, when substituted into Equation [12.15], gives the Stokes equation vd gt =218( )) [12.19] As this equation indicates, the settling rate of small particles strongly depends on medium viscosity. For large particles ( d > 1 mm) C d 0.44 and Equation [12.15] gives Newtons equation Surface chemistry fundamentals in fi ne coal processing 405 Woodhead Publishing Limited, 2013 vgdt =103( ))) [12.20] showing that in a turbulent fl ow regime the settling rate of spherical par-ticles does not depend on viscosity at all. There are a few empirical equations (e.g. Allens equation) that were derived for the intermediate Reynolds number range 1 406 The coal handbook Woodhead Publishing Limited, 2013 Dense media The most common media for separation is a fi ne magnetite suspension in water, ferrosilicon suspension, or a mixture of the two, depending on the required separation density. Magnetite alone is used when a density is required in the range from 1.25 to 2.2 g/cm 3 , a mixture between 2.2 and 2.9 g/cm 3 , and ferrosilicon alone for a density range 2.93.4 g/cm 3 . With magnetite density of about 5.0 g/cm 3 , 500 g of magnetite per liter of suspension, that is magnetite content of 10% by volume (35 % wt.) the suspension density is 1.4 g/cm 3 , and with doubled magnetite content to 20% by volume (55% wt) magnetite suspension density will be 1.8 g/cm 3 . The same calculations for ferrosilicon alloy, assuming that its density is 6.8 g/cm 3 (Collins et al . (1974) claims that density of ferrosilicon containing 1416% Si falls in the range from 6.6 to 7.0) give for the range of media den-sities from 2.9 to 3.4 g/cm 3 the ferrosilicon volume percent of 3341%. According to Mikhail and Osborne (1990), the following is the standard particle-size distribution of magnetite used in coal preparation: maximum 5% by mass coarser than 45 m, and 30% by mass fi ner than 10 m. Magnetite medium viscosity Bergh fer (1959) was probably the fi rst to obtain full rheological curves for magnetite dense-medium suspensions. His results clearly demonstrate plas-tic properties of such suspensions (see Fig. 12.30) and show that the devia-tion of the rheological properties of magnetite suspensions from Newtonian behavior quickly increases with fi neness of the magnetite. Meerman (1959), Whitmore (1958), and Yancey et al . (1958) sampled magnetite dense media from operating coal preparation plants and confi rmed that such suspensions behave like Bingham plastic fl uids. A few samples of magnetite used in our rheological studies (He and Laskowski, 1995) are listed in Table 12.3. Particle-size distributions of the tested samples (Table 12.3) are well described by the Rosin-Rammler-Bennett (RRB) particle-size distribution: d 63.2 is the size modulus (d 63.2 is that aperture through which 63.2% of the sample would pass), and m is the distribution modulus (the slope of the curves on the RRB graph paper), with diminishing value of the distribu-tion modulus the line representing distribution becomes less steep and that translates into larger differences between the sizes of fi ne and coarse par-ticles in the sample. Mag #1 was the Craigmont Mine commercial magnetite used in western Canadian coal preparation plants. Mag #2 was prepared by grinding Mag #1. Mag #3 was obtained by rejecting fi nes from Mag #1 in a classifying cyclone. Mag #4 and Mag #5 were micronized magnetites (70% and 90% below 5 Surface chemistry fundamentals in fi ne coal processing 407 Woodhead Publishing Limited, 2013 m, respectively), provided by the US Department of Energy in Pittsburgh (Klima et al ., 1990). Because of high magnetite density, in spite of fi ne size of the magnetite particles in the tested samples, these samples settle rather quickly and rhe-ological measurements require special equipment. A sensor system (ESSP) that was specially designed and built for magnetite suspensions (Klein et al ., 1995) was employed in the tests, along with a HAAKE Rotovisco RV 20 rheometer. The suspensions were demagnetized with the use of demagnetiz-ing coil before measurements. The results are shown in Fig. 12.45 (He and Laskowski, 1999). As these results indicate, the yield stress for coarse magnetite suspen-sions (Mag #1) at solids content below 10% by volume is very low and the existence of yield stress is more clearly manifested in the fi ne magnetite suspensions (Mag #4 and Mag #5). High yield stress values in these suspen-sions provide a proof for quite strong particle-particles interactions in such fi ne disperse systems. A few equations exist to describe rheological curves. Perhaps the most common is the Bingham equation, which describes the fl ow curves of plastic fl uids (see Fig. 12.30). =B p = lD [12.23] where is the shear stress, B is the Bingham extrapolated yield value (see Fig. 12.46), pl is the plastic viscosity, and D is the shear rate. The equation shown to describe rheological curves of magnetite suspen-sions particularly well is the Cason equation (Klein et al ., 1990; He and Laskowski, 1999; He et al ., 2001) = + ( ) C C+ (1 21 2 2/D [12.24] where C is the Casson yield stress and C is the Casson viscosity. Table 12.3 Particle-size distributions of the tested magnetites Sample d 63.2 ( m) M Mag #1 Mag #2 Mag #3 Mag #4 Mag #5 30.5 18.0 33.0 4.3 2.7 3.2 1.6 4.1 1.9 2.5 408 The coal handbook Woodhead Publishing Limited, 20130001020304050600102022%18%18%20%15%12%2%15%10%5%304012Shear stress (Pa)Shear stress (Pa)345601234586750 100 150 200 250 3005%15%22%25%28%Solid content 30%Solid content 28%Solid content 25% Solid content 22%25%22%20%18%12%5%Mag #1 Mag #2Mag #4 Mag #50 50 100 150 200 250 3000 50 100 150 200 250 300Shear rate (s1)Shear rate (s1)0 50 100 150 200 250 300 12.45 The fl ow curves of the tested samples of magnetite suspensions. (Source: After He and Laskowski, 1999). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) Surface chemistry fundamentals in fi ne coal processing 409 Woodhead Publishing Limited, 2013 Figure 12.47 shows the Casson yield stress determined from the experi-mental data displayed in Fig. 12.45. As Fig. 12.47 reveals, the Casson yield stress is very sensitive to the changes in the size of magnetite particles and solids content, while the Casson vis-cosity turned out not to be very sensitive to these changes over the studied medium density range (it is practically constant over the medium density range from 1.2 to 1.7 g/cm 3 ). The Casson yield stress and viscosity values for four tested magnetite samples at a medium relative density of 1.45 are given in Table 12.4. As these results demonstrate, the yield stress values are much larger than the viscosity, and while the yield stress increases with the fi neness of the tested magnetite samples, the Casson viscosity does not. Since the appar-ent viscosity of a non-Newtonian system (such as magnetite suspension) is determined by both yield stress and viscosity (that is either by Bingham yield stress and plastic viscosity, or Casson yield stress and Casson viscosity), it can be shown that the contribution coming from the yield stress exceeds by far the contribution of the viscosity (He et al ., 2001). Mineral particles that are treated in a dense medium must move in the medium in order to report to the proper products (low density concentrate or high density tailings). In order to start moving in the medium, in order to 000.5BC1Shear stress (Pa)1.5Bingham model Casson model Data22.5350 100Shear rate (1/s)150 200 250 12.46 Example of rheogram of slurry fi tted with either Bingham model or Casson model; as this example demonstrates, obtained B and C values are not identical. 410 The coal handbook Woodhead Publishing Limited, 2013shear the medium (in a static bath this happens under gravity), the particles must overcome the threshold drag. It is determined by the yield stress. In the dense-medium cyclone, the acceleration is dominated by centrifugal accel-eration; the results shown in Figs 12.48 and 12.49 were calculated assuming centrifugal acceleration of 40 m/s 2 . As Fig. 12.48 shows, while only particles with size smaller than 1 mm will not be able to shear the Mag #1 and Mag #2 media (in a dense-medium cyclone at 40 m/s 2 centrifugal acceleration), the near-density particles must be larger than 10 mm in order to be able to shear the Mag #4 medium. For 2 mm particles, the movement in the Mag #1 and Mag #2 media requires only density differential larger than 0.1 g/cm 3 . In Mag #4 the par-ticles with a size of 2 mm will be able to move in the medium only if their 1.10.0010.010.1Casson yield stress (Pa)110Temp. = 19C1001.2 1.3 1.4 1.5 1.6 1.7Medium density (g/cm2)1.8 1.9 2 2.1Mag#1 Mag#2 Mag#4 Mag#52.2 2.3 12.47 The effect of magnetite particle size and medium solid content on Casson yield stress. (Source: After He and Laskowski, 1995). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) Table 12.4 Casson yield stress and viscosity for four studied magnetite samples at a medium relative density of 1.45 Sample c (mPa) c (mPa.s) Mag #1 Mag #2 Mag #4 Mag #5 62 118 2110 2660 1.85 1.50 0.70 1.50 Surface chemistry fundamentals in fi ne coal processing 411 Woodhead Publishing Limited, 2013density is about 0.6 g/cm 3 larger than the medium density (Fig. 12.49). These results demonstrate how important are dense-medium rheological proper-ties for separation effi ciency. The medium capacity to maintain the homogeneity, the constant medium separation density 50 in the separating device, is a function of medium sta-bility. The medium used in dense-medium separation of coal is an aqueous Mag #2Mag #1Mag #4Mag #500400800Shear rate D (s1 )1200160020005 10 15Particle size (mm)20 25 30 12.48 Effect of particle size on effective shear rate of a particles with a relative density of 1.55 in various magnetite dense media with a relative density of 1.45. (Source: After He et al ., 2001 with permission of Elsevier.) 00200400Shear rate D (s1 )60080010000.3 0.6Mag #2Mag #1Mag #4Mag #5Relative density differential0.9 1.2 12.49 Effect of medium density differential on shear rate for a 2 mm particle in various magnetite dense media with relative density of 1.45. (Source: After He et al ., 2001 with permission of Elsevier.) 412 The coal handbook Woodhead Publishing Limited, 2013suspension of fi ne magnetite particles. Static stability of magnetite suspen-sion is defi ned as the reciprocal of the settling velocity that is easily deter-mined in the lab (Bozzato et al ., 1999). In a cyclone, magnetite particles are also subjected to centrifugal forces and tend to segregate creating den-sity gradients. This tendency to segregate is a measure of the magnetite medium stability under dynamic conditions prevailing in a cyclone. Practically, it is determined by the density differential between the cyclone underfl ow and overfl ow. As Fig. 12.50 indicates, the dynamic medium stabil-ity critically depends on magnetite particle size and medium density (that is magnetite concentration in the medium). These measurements confi rm that the measured density differential clearly depends on magnetite particle size, and it was shown to be a func-tion of the yield stress (He and Laskowski, 1994). As could be expected, Mag #3 turned out to be very unstable, as characterized by a huge density differ-ential of 1 g/cm 3 . For Mag #1 the maximum density differential is about 0.4 g/cm 3 . Collins et al . (1983) claim that separation effi ciency is satisfactory as long as the medium density differential is in the range of 0.20.5 g/cm 3 . All the studies confi rm that the behavior of the medium is process-deter-mining. In the empirical relationship proposed by Napier-Munn (1990) to describe the effect of medium rheology on DMC separation, the separation 100.050.10.15Density differential (g/cm3 )0.20.250.31.1 1.2 1.3 1.4Feed medium density (g/cm3)1.5Mag#4Mag#1Mag#2Mag#31.6 1.7 1.8 1.90.10.20.30.40.50.60.70.80.911.1 12.50 Effect of medium composition on medium differential measured in a 6 cyclone (D6B-12-S287 Krebs Engineering Int.) that was gravity fed at 10xD inlet pressure. (Source: After He and Laskowski 1995). Copyright Taylor & Francis Group, LLC.(http://www.taylorandfrancis.com), reproduced with permission.) Surface chemistry fundamentals in fi ne coal processing 413 Woodhead Publishing Limited, 2013effi ciency expressed by a value of E p was related to the medium apparent viscosity by: E dnp [12.25] where E p is the probable error, and is a function of apparent viscosity, = ( )ppf . Since the yield stress is a dominant factor that determines the apparent viscosity of fi ne magnetite suspensions, the experimentally measured yield stress can practically substitute for apparent viscosity in Equation [12.25] (He et al ., 2001). Optimizing medium composition is essential to achieving balanced sta-bility and rheological properties of the dense medium (He and Laskowski, 1995). The effect of medium composition on DMC performance depends on feed particle size. The separation of coarse particles (larger than 2 mm) is strongly affected by the medium stability and only slightly by medium rheology. Thus, the separation effi ciency improves when medium stability is improved by using fi ner magnetite dense medium. 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