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Zeta Potential Measurements on Three Clays from Turkey and Effects of Clays on Coal Flotation
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
184, 535–541 (1996)
0649
Zeta Potential Measurements on Three Clays from Turkey and Effects of Clays on Coal Flotation SYED ABID HUSSAIN,* S¸AHIg NDE DEMIg RCIg ,† ,1
AND
¨ ZBAYOG˘LU‡ GU¨LHAN O
*Mining Engineering Department, University of Engineering and Technology, Lahore-31, Pakistan; †Department of Chemistry, Art and Science Faculty, Middle East Technical University, Ankara, Turkey; and ‡Department of Mining Engineering, Engineering Faculty, Middle East Technical University, Ankara, Turkey Received February 16, 1996; accepted August 6, 1996
There is a growing trend of characterizing coal and coal wastes in order to study the effect of clays present in them during coal washing. Coarse wastes from the Zonguldak Coal Washery, Turkey, were characterized and found to contain kaolinite, illite, and chlorite. These three clays, obtained in almost pure form from various locations in Turkey, have been subjected to X-ray diffraction (XRD) analysis to assess their purity and zeta potential measurements in order to evaluate their properties in terms of their surface charge and point of zero charge (pzc) values. It was found from XRD data that these clays were almost pure and their electrokinetic potential should therefore be representative of their colloidal behavior. All three clay minerals were negatively charged over the range from pH 2.5 to 11. Chlorite and illite have pzc at pH 3 and pH 2.5, respectively, whereas kaolinite has no pzc. The effect of these clays in Zonguldak coal, wastes, and black waters on coal flotation was studied by floating artificial mixtures of Zonguldak clean coal (4.5% ash) and individual clay. The flotation tests on coal/individual clay revealed that each clay influences coal flotation differently according to its type and amount. Illite had the worst effect on coal floated, followed by chlorite and kaolinite. The loss of yield in coal was found to be 18% for kaolinite, 20% for chlorite, and 28% for illite, indicating the worst effect of illite and least for kaolinite during coal flotation. q 1996 Academic Press, Inc. Key Words: coal wastes; coal washing; zeta potential; X-ray diffraction; kaolinite; illite; chlorite; flotation.
INTRODUCTION
Clay minerals in coal not only are a source of ash but also harmfully affect a number of processes during coal washing, such as flotation, flocculation, and dewatering. There are very few studies describing the effect of clays on flotation and its relationship with their zeta potential (1a, b). Froth flotation of coal is a physicochemical process based on surface properties of coal which can be measured through electrokinetic potential (zeta potential). In general, zeta potential measurement of the solid–liquid interface can be used 1
To whom correspondence should be addressed.
to study the nature of a solid surface. The electrokinetic measurements are therefore the relative values for the semiquantitative estimation of surface charge and adsorption. Stability and coagulation of colloidal dispersion are considered in terms of electrokinetic data for systems that cannot be tested by electrochemical techniques (2). As flotation depends on water chemistry the presence of clays may change it through an electrical double layer. Therefore, zeta potential measurements of the clays can be of value to process engineers for controlling the washery processes. It was aimed to determine the zeta potentials of illite, kaolinite, and chlorite obtained from various sources in Turkey. Hussain et al. (3, 4) and Hussain (5) characterized clay minerals in coal, wastes, and black waters from Zonguldak Coal Washery in Turkey and found that only three clays, namely kaolinite, illite, and chlorite, are present. Semiquantitative XRD analysis also indicated dominance of kaolinite in coal and of illite in black waters (5–8). These clays, in the forms of slimes, caused loss of recovery in coal flotation through slime coatings on coal and/or bubble and increased reagent consumption (9–11). Arnold and Aplan (1a) investigated the effect of clays on artificial coal–clay mixtures and concluded that different clays influence coal flotation differently, and therefore slimes should be well characterized with respect to clays to determine their effect on actual floating systems involving high ash coals. MATERIALS AND METHODS
The clays used in this study and their particulars are given in Table 1. In X-ray diffraction analysis (XRD) a Jeol JSDX-100S X-ray spectrometer diffractometer was used. All the analysis were done using CuKa radiation with Cu/Ni electrodes and goniometer speed of 17 (2u ) per minute. The slit widths used were 17 for the divergence slide, 0.27 for the receiver slide, and 17 for the scatter slide. For XRD analysis samples were prepared as unoriented mounts, no cation saturation
535
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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536 TABLE 1 Turkish Clays Studied Clay type
Location
Variety and preparation
Kaolinite
Eskis¸ehir
Illite
Ordu
Chlorite
Ordu
Esk 4045, Kaolin 64.6% passing 5 mm 81% kaolinite XRD trace obtained showed mainly peaks of kaolinite and quartz (Fig. 1) Crushed in jaw and rolls to minus 10 mesh, then ground to minus 325 mesh Tyler in porcelain ball mill XRD trace showed peaks of micaceous clay minerals and quartz (Fig. 2) Concentration after magnetic separation, crushed and ground in porcelain mortar grinder to minus 325 mesh. XRD trace showed mainly chlorite peaks (Fig. 3)
was used, and samples were not subjected to heating. The cavity filling method of sample preparation was used. During this preparation the finely ground sample is put in a cavity and compressed with a spatula or a microscope slide is moved back and forth. This may cause orientation of the flaky clays such as chlorite and an unoriented mount may seem to be an oriented mount. The XRD traces of kaolinite, illite, and chlorite are shown in Figures 1–3. A Rank Brothers particle electrophoresis apparatus MKII (Cambridge, England) was used to measure the electrophoretic mobilities. During the measurement a quartz rectangular cell and platinum electrodes were used. After the cell was filled with the prepared suspension a voltage of 100 V
FIG. 2. XRD trace of illite (unoriented mount): I Å illite, Q Å quartz, M Å micaceous clay minerals.
was applied across it. The velocity of the particles in suspension was measured at both the front and back stationary levels in the chamber using the two inner surfaces as reference settings for focusing the microscope. By altering the direction of the current after each mobility measurement, polarization effects were minimized. The mobility of particles was computed after taking the averages of readings at the front and back levels. Each reported mobility was the mean mobility of 20 particles (10 on each level and 5 in each direction), yielding a standard deviation of about 5% (5). The equation used for converting observed mobilities U (U Å q /E, where q is the particle velocity and E is the applied field strength) into the effective electrokinetic potential (zeta potential) q depends upon the value of the
FIG. 1. XRD trace of kaolinite (unoriented mount); A Å anhydrite, K Å kaolinite, Q Å quartz, G Å geothite, R Å green rust.
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FIG. 3. XRD trace of chlorite (unoriented mount).
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EFFECT OF CLAYS ON COAL FLOTATION
dimensionless quantity xa, in which a is the radius of the particle (assumed spherical) and x is the quantity
F
4pe 2Snz 2 ekT
G
1/2
,
familiar in electric double layer theory. From the expression given, x É 1 1 10 6 cm at 257C in water containing 1 mM of a 1:1 electrolyte. If xa ú 200 it will usually be sufficiently accurate to use the Smoluchowski formula, which in the (original) unrationalized form is UÅ
ej , 4ph
where e is the permittivity of the suspending liquid. Typical units in the older system would be U in micrometers per second under 1 V cm (usually then in the range from 0 to 6), in which case for water at 257C q in mV would be given by z Å 12.83U.
If xa õ 0.1 then the Hu¨ckel equation would be relevant: UÅ
ej (unrat.). 6ph
But this will hardly ever be useful in the case of water as suspending medium since even water with no added electrolyte (seen as air equilibrated) has x á 10 6 , so that particles would have to be as small as 10 nm (0.01 mm) radius to reduce xa to 0.1. In the intermediate range of xa (0.1 to 200) the Henry equation UÅ
ej [1 / f ( xa)] (unrat. form) 6ph
U r j conversion. If a large (approximately spherical) particle is an agglomerate of small spheres then the question of which radius to use is seen to arise. If ( x 1 smaller radius) is small ( õ0.1) then again it is fairly evident that the larger radius governs the effective xa. In all the above it has been assumed that the particle itself is rigid (i.e., of infinite viscosity). In the absence of firm evidence it seems best to treat all particles of radius 1 mm or less as rigid. Under all the above considerations it is assumed that the Smoluchowski equation given above was used in the calculation of the nominal zeta potentials. Each clay sample before measurement was prepared as a master suspension in deionized distilled water containing particles less than 10 mm after sedimentation of an already ground material in an agate mortar to minus 300 mesh British standards. The mobility of each particle was measured in suspension after soaking it for 10 min at each pH between pH 2.5 and 11. The pH adjustment was done by adding a small amount of HCl and NaOH and by using a Fisher (Accumet Model 230) pH meter. In order to study the effects of clays, namely kaolinite, illite, and chlorite, a series of artificial mixtures of clean coal (4.5% ash) and clays (ground to 0325 mesh B.S.) were prepared and flotation was carried out using kerosene oil as collector (340 g/t) and methyl isobutyl carbinol as frother (85 g/t), which were previously optimized (5). The flotation tests were conducted in a Wedag laboratory flotation machine having a fiberglass cell (capacity Å 500 ml) manufactured locally. The speed of the impeller was kept constant at 1420 rpm. All the tests were conducted in distilled water (pH 6.5). Agitation and conditioning time were also constants as three minutes each in all the tests. For each test 50 grams of sample consisting of coal/clay mixture according to the amount of clays (0–25%) in each case was used, keeping the pulp density constant (10% solids by weight). The coal samples were ground to minus 28 mesh first in a jaw crusher and then in rolls. Three clays found in Zonguldak coal, namely kaolinite, illite, and chlorite, were used in this study (3–5). These clays were obtained from various sources in Turkey and on XRD analysis were found to be almost pure (4, 5).
can be seen if the z potential is small (say õ20 mV), where f ( xa) is a correction factor taking the values given below: xa
0
0.1
1.0
5
10
50
100
`
f ( xa)
0
0.001
0.027
0.160
0.239
0.424
0.458
0.5
If the z potential is not small, then in the range xa Å 0.1 to 200 the best available computations of the U: z relationship are those of Wiersema, Loeb, and Overbeek (12). These authors also considered the effect of different electrolyte charge types. If particles are not spherical then, unless xa is everywhere large, there must be some doubt about the
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RESULTS AND DISCUSSION
The XRD traces showed that each clay selected for this study contains only one clay mineral, along with nonclay impurities associated with clay, such as quartz. In the traces the broad reflections starting from 3–47 (2u ) and terminating at about 13–147 (2u ) might be due to the amorphous materials and organic matter present in the samples or to the somewhat poor alignment of the goniometer (13) (Figs. 1, 2, and 3). As expected from the preparation method, in chlorite the XRD trace shows a regular pattern (Fig. 3). The chlorite
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FIG. 4. Zeta potentials of Turkish clays as a function of pH.
has a micaceous nature and it is difficult to grind the sample to obtain a flakeless powder. The results obtained for zeta potential values measured against pH are plotted in Fig. 4 (5). All clays in Fig. 4 were found to have negative zeta potential in the pH range observed (2.5 to 11). The kaolinite was the most negative clay, ranging in zeta potential value from 024.0 to 049.5 mV. The illite was the least negative clay, giving a range of 0 to 042.0 mV. The chlorite gave a range of 0 to 036.5 mV. A lower pzc value at pH 2.5 for illite and at pH 3.0 for chlorite was obtained. The zeta potential measurements of Turkish clays showed that kaolinite (unlike others) was more negatively charged (lowest current) than chlorite and illite and therefore no pzc value could be found, as this permanent negative charge of kaolinite due to the isomorphic substitution remains independent of pH in the acidic range (14–16). In the basic range, the edge surface at which the octahedral sheet is broken may be compared with the surface of an alumina particle. Hydroxyl ions act as potential-determining ions and cause the edge surfaces to be negatively charged (16). Therefore, in kaolinite along the pH range studied no point of zero charge (pzc) was obtained. Kaolinite was always negatively charged, but it appeared less negative at about pH 5. A noticeable increase in negative zeta potential at pH ú5 is thought to be related to the pzc of the clay edge at about
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neutral pH (17, 18). Changes in zeta potential values in the pH range 2.5–11 showed the presence of complex ionexchange reactions on heterogeneous 0SiOH and 0AlOH sites. Arnold and Aplan (1b) determined zeta potentials of coal and clay as a function of pH in both tap and distilled water. On comparison with zeta potentials of kaolinite determined by Arnold and Aplan (1b) it was found that three types of kaolinite, namely, A, B, and C, were used by them. Kaolinite A and C showed completely different behavior than kaolinite from Turkey, whereas kaolinite B showed no pzc value, like kaolinite used in this study. The pzc value of illite was 2.5, which is similar to the value determined for Turkish illite. Chlorite could not be compared, as no chlorite zeta potential value was determined by them (1b). It is evident that the zeta potential of pure clays in distilled water is an intrinsic property of clays. Clay is always negatively charged; however, an increase in zeta potential is noted at around pH 6 due to the pzc of the clay edge. ¨ zbayog˘lu (19) that a decrease in It was reported by O zeta potential means better floatability, and it was found by Arnold and Aplan (1b) that the presence of ions in water (tap water) can reduce the overall negative charge of particles in the pH range from 2.5 to 10. This means that less negative clays are probably less harmful to coal flotation. Therefore, in this case illite should give less depression of coal in flota-
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EFFECT OF CLAYS ON COAL FLOTATION
tion than chlorite and kaolinite. The use of cations such as Al 3/ and Fe3/ may easily reverse the charge of those clays which are less negative (furnish pzc values easily), thus enabling these clays to be depressed by the use of dispersants and depressants with advantageous use in flotation (1a). As coal is usually negatively charged and gives a negative zeta potential in distilled water (5, 17, 19) due to its anisotropic nature, and clays are also negatively charged, only clays having charge opposite to that of coal should cause greater problems by adsorbing their positively charged portions on the coal surface, thereby preventing coal flotation. Hussain (5) measured the zeta potentials of clean coal (4.5% ash) with three dispersants, sodium silicate, calgon (sodium hexametaphosphate), and marsperse CB (sodium lignin sulfonate), and three depressants, CMC (carboxymethyl cellulose, Tylose C30), dextrin, and Auro Depressant 633, and found that only dextrin decreased the negative zeta potentials of coal, with increased dosage showing better depressing effects and better adsorption on coal surfaces. All others were not found advantageous in floating the coal. Only dextrin, in the light of zeta potential measurement, depressed fine high ash coal in single stage reverse flotation. Effects of various molar concentrations of MgCl2 , CaCl2 , Ca(OH)2 , AlCl3 , and Al2 (SO4 )3 on zeta potentials of clean coal were also studied by Hussain (5). The calcium cations were found to be less negative than the magnesium cations due to higher adsorption with increasing cation radius. Aluminum chloride and aluminum sulfate reverse the charge of coal by adsorbing on negatively charged surfaces by electrostatic forces even at lower concentrations. These findings were in line with others (19–21). The results obtained by floating clean coal (4.5% ash) and three different clays (kaolinite, illite, and chlorite) are given in Table 2 and are plotted in Fig. 5. This figure shows the effect of addition of kaolinite, illite, and chlorite (0–25%) on flotation of clean coal. It can be seen that illite has the worst effect both on yield and ash when floated with clean coal. Chlorite was found to have an intermediate effect on yield and ash, and kaolinite the least. It can be seen that up to 10% kaolinite and chlorite have no appreciable effect on flotation of coal. On the other hand, up to 25% (maximum amount) of kaolinite and chlorite clays reduced the yield to 18 and 20%, respectively. Illite addition up to 25% gave a loss of yield to about 28% and increase of ash to 6.5%, showing that coal flotation is affected more by illite than by kaolinite. Comparatively higher depression of coal by illite than by kaolinite on U.S. coals was indicated by Arnold and Aplan (1a), in agreement with the findings of this study. In order to compare the effects of clays in amounts determined by XRD analysis of coal samples from Zonguldak region (K Å 52%, I Å 31%, Ch Å 17%), these were converted to their proportions of maximum amount 25% (5). This gave the amount of each clay (K Å 13%, I Å 8%, Ch
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TABLE 2 Effects of Clays on Coal Flotationd Name of clay added Kaolinite
Chlorite
Illite
a
Clay added %
Yield %
Ash %
0
86.7
2.5
5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
83.3 79.5 76.9 72.1 68.9 79.0 76.0 73.4 70.2 66.5 77.5 73.2 68.2 64.3 59.1
3.0 4.0 5.0 6.0 7.0 3.5 4.5 6.0 7.0 8.0 4.0 5.5 7.0 8.0 9.0
Reagents; kerosene oil Å 340 g/t, methyl isobutyl carbinol 85 g/t.
Å 4%) to be included in the artificial mixture of clean coal. Flotation results showed that yield and ash values as percentages were 70.0 and 9.0, respectively. Without clay, clean coal flotation gave a yield value of about 90% with an ash value of 2.5%, showing the effect of clays in reducing the yield and increasing the ash of the product (coal) (5, 22). In the literature there are two opposite views about the effect of clay slimes on coal flotation. Most investigators (10, 12) believe that slime coatings are the real cause of loss in mineral flotation recoveries. The slimes are heaviest when the slimes are uncharged or oppositely charged to the material being floated and thus badly affect the flotation results. Similarly other researches (13, 23, 24) note depression of coal due to the clay slimes. However, Firth and Nicol (25) found no effect of kaolin (clay) on coal flotation. Another explanation about the bad effects of clays on flotation through the entrainment or water carryover mechanism is reported by some investigators (26, 27). The controversy on two opposite views regarding the effect of clays on flotation by slime coatings on mineral or coal and water carryover is still unresolved. It may be said that both these mechanisms play a part in spoiling the results of coal flotation. The type of clay and the amount have major roles in coal depression, whereas the water carryover mechanism has a minor effect. It was noted that water carryover in coal mainly contributed toward ash increase of the floated product due to clay slimes and the coal depression was negligible (i.e., yield was not affected). The depression of coal can therefore be related mainly to the clay type and its slime coatings on coal particles, which mask them from the reagent adsorption. However, both the mechanisms, one (clay type and amount) reducing the yield and the other (water carryover) lowering
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FIG. 5. Effect of kind and amount of clay on yield and ash of clean coal (4.5% ash) in flotation.
the grade by increasing the ash of the floated product, are operative in coal flotation. It is fortunate that Zonguldak washery coal did not contain any montmorillonite type clays, which are very harmful for U.S. coal flotation (12, 28–30), and therefore a less gloomy picture has emerged. However, the effects of illite, kaolinite, and chlorite in Zonguldak coal cannot be ignored while this coal is floated. CONCLUSION
The conclusions drawn from the present investigation are summarized as follows: Zeta potentials of all Turkish clays (kaolinite, illite, and chlorite) determined in distilled water over a pH range from 2.5 to 11.0 were all negative, ranging from 013.1 to 049.5 mV. Kaolinite was the most negative clay, compared to chlorite and illite, found electronegative in decreasing order. No point of zero charge of kaolinite could be found because of its permanent negative charge due to isomorphic substitution and/or structural defects. Illite surface exhibited a point of zero charge (pzc) at pH Å 2.5 and chlorite surface at pH Å 3.0. Although zeta potential measurements of these three clays do not give an absolute clue to their behavior toward flotation or any other process, they may reflect their nature and hence these clays may be dealt with accordingly by washery engineers.
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Flotation tests on artificial mixtures of clean coal and individual clays revealed that each clay influences coal flotation differently according to its type and the amount used for flotation. Illite was found to have the worst effect on coal flotation by reducing the yield and increasing the ash of the floated coal, followed by chlorite and kaolinite in decreasing order. Clay coatings on coal and water carryover of ultrafine clays were both responsible for deteriorating yield and ash of floated coal. REFERENCES 1. (a) Arnold, B. J., and Aplan, F. F., Int. J. Miner. Process. 17, 225 (1986); (b) Int. J. Miner. Process. 17, 243 (1986). 2. Leja, J., ‘‘Surface Chemistry of Froth Flotation.’’ Plenum, New York, 1982. ¨ zbayog˘lu, G., in ‘‘The First Interna3. Hussain, S. A., Demirci, S¸., and O tional Conference on Modern Process Mineralogy and Mineral Processing,’’ p. 228, 1992. ¨ zbayog˘lu, G., Appl. Clay Sci. 7, 471 4. Hussain, S. A., Demirci, S¸., and O (1993). 5. Hussain, S. A., ‘‘Characterization of Coal and Wastes from Zonguldak Coal Washery and Effect of Clays on Coal Flotation,’’ Ph.D. Thesis. Middle East Technical University, Ankara, Turkey, 1992. ¨ zbayog˘lu, G., and Demirci, S¸., in ‘‘International Con6. Hussain, S. A., O ference on Recent Advances in Materials and Mineral Resources, 3– 5 May, Penang, Malaysia,’’ p. 554. Pevak Foundation Publication, 1994. ¨ zbayog˘lu, G., and Atabey, E., Int. J. 7. Hussain, S. A., Demirci, S¸., O Environ. Issues Miner. Energy Ind., 101 (1993). ¨ zbayog˘lu, G., and Demirci, S¸., in ‘‘Proceedings of 8. Hussain, S. A., O
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9. 10. 11. 12. 13.
14. 15.
16. 17. 18. 19.
Second National Symposium on Modern Trends in Contemporary Chemistry, April 4–6, 1995, Islamabad, Pakistan,’’ p. 317. Sun, S. C., Trans. A.I.M.E. 153, 479 (1943). Fuerstenau, D. W., Gaudin, A. M., and Miaw, H. L., Trans. A.I.M.E. 211, 792 (1958). Iwaski, I., Cooke, S. R. B., Harraway, D. H., and Choi, H. S., Trans. A.I.M.E. 223, 97 (1962). Wiersema, P. H., Loeb, A. L., and Overbeek, J., J. Colloid Interface Sci. 22, 78 (1966). Brindley, G. W., and Brown, G., ‘‘Crystal Structures of Clay Minerals and Their X-Ray Identification.’’ Mineralogical Soc. Monograph No. 5, London, 1980. Ferris, A. P., and Jepson, W. B., J. Colloid Interface Sci. 51, 245 (1975). Wierer, K., Eckl, J., Oberndorfer, J., and Dobias, B., in ‘‘XVI International Mineral Processing Congress’’ (E. Forssberg, Ed.) p. 819. Elsevier, Amsterdam, 1988. Aplan, F. F., and Feurstenau, D. W., in A.I.M.E. 50th Anniversary Volume,’’ p. 170, 1962. Van Olphen, H., ‘‘An Introduction to Clay Colloid Chemistry.’’ Wiley, New York, 1963. White, G. N., and Zelanzy, L. W., Clays and Clay Miner. 36, 141 (1988). ¨ zbayog˘lu, G., ‘‘Determination of Flotation Characteristics of Several O
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20. 21.
22.
23. 24.
25. 26. 27. 28. 29.
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Turkish Bituminous Coal Seams in Zonguldak Basin.’’ Ph.D. Thesis, M.E.T.U. Ankara, 1977. Wen, W. W., and Sun, S. C., Trans. A.I.M.E. 262, 174 (1977). Sarikaya, M., ‘‘Determination of the Flotation Characteristics of Oxidized Coal from Zonguldak Coal Basin.’’ Ph.D. Thesis, M.E.T.U. Ankara, 1988. ¨ zbayog˘lu, G., in ‘‘International Conference on Hussain, S. A., and O Recent Advances in Materials and Mineral Resources, 3–5 May, 1994, Penang, Malaysia,’’ p. 562. Pevak Foundation, 1994. Mishra, S. K., in ‘‘Australasian I.M.M., Mill Operators Conf., Mt. Isa, 1978, p. 159. Lynch, A. J., Johnson, N. W., Manlapig, E. V., and Thorne, C. G., ‘‘Mineral and Coal Flotation Circuits: Their Simulation and Control.’’ Elsevier, Amsterdam, 1981. Firth, B. A., and Nicol, S. K., Int. J. Miner. Process. 8, 239 (1981). Warren, L. J., in ‘‘Principles of Mineral Flotation’’ (M. M. Jones and J. T. Woodcock, Eds.), The Wark Symp. Series, Vol. 40, p. 185, 1984. Subrahmanyan, T. V., and Fossberg, K. S. E., Int. J. Miner. Process. 30, 265 (1990). O’Gorman, J. V., and Walker, P. L., Jr., Fuel 50, 135 (1971). O’Gorman, J. V., and Walker, P. L., Jr., ‘‘Mineral Matter and Trace Elements in U.S. Coals.’’ U.S. Dept. of Interior, Office of Coal Research, Report No. 2, 1972. Gluskoter, H. J., J. Sed. Petrol. 37, 205 (1967).
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0649
Zeta Potential Measurements on Three Clays from Turkey and Effects of Clays on Coal Flotation SYED ABID HUSSAIN,* S¸AHIg NDE DEMIg RCIg ,† ,1
AND
¨ ZBAYOG˘LU‡ GU¨LHAN O
*Mining Engineering Department, University of Engineering and Technology, Lahore-31, Pakistan; †Department of Chemistry, Art and Science Faculty, Middle East Technical University, Ankara, Turkey; and ‡Department of Mining Engineering, Engineering Faculty, Middle East Technical University, Ankara, Turkey Received February 16, 1996; accepted August 6, 1996
There is a growing trend of characterizing coal and coal wastes in order to study the effect of clays present in them during coal washing. Coarse wastes from the Zonguldak Coal Washery, Turkey, were characterized and found to contain kaolinite, illite, and chlorite. These three clays, obtained in almost pure form from various locations in Turkey, have been subjected to X-ray diffraction (XRD) analysis to assess their purity and zeta potential measurements in order to evaluate their properties in terms of their surface charge and point of zero charge (pzc) values. It was found from XRD data that these clays were almost pure and their electrokinetic potential should therefore be representative of their colloidal behavior. All three clay minerals were negatively charged over the range from pH 2.5 to 11. Chlorite and illite have pzc at pH 3 and pH 2.5, respectively, whereas kaolinite has no pzc. The effect of these clays in Zonguldak coal, wastes, and black waters on coal flotation was studied by floating artificial mixtures of Zonguldak clean coal (4.5% ash) and individual clay. The flotation tests on coal/individual clay revealed that each clay influences coal flotation differently according to its type and amount. Illite had the worst effect on coal floated, followed by chlorite and kaolinite. The loss of yield in coal was found to be 18% for kaolinite, 20% for chlorite, and 28% for illite, indicating the worst effect of illite and least for kaolinite during coal flotation. q 1996 Academic Press, Inc. Key Words: coal wastes; coal washing; zeta potential; X-ray diffraction; kaolinite; illite; chlorite; flotation.
INTRODUCTION
Clay minerals in coal not only are a source of ash but also harmfully affect a number of processes during coal washing, such as flotation, flocculation, and dewatering. There are very few studies describing the effect of clays on flotation and its relationship with their zeta potential (1a, b). Froth flotation of coal is a physicochemical process based on surface properties of coal which can be measured through electrokinetic potential (zeta potential). In general, zeta potential measurement of the solid–liquid interface can be used 1
To whom correspondence should be addressed.
to study the nature of a solid surface. The electrokinetic measurements are therefore the relative values for the semiquantitative estimation of surface charge and adsorption. Stability and coagulation of colloidal dispersion are considered in terms of electrokinetic data for systems that cannot be tested by electrochemical techniques (2). As flotation depends on water chemistry the presence of clays may change it through an electrical double layer. Therefore, zeta potential measurements of the clays can be of value to process engineers for controlling the washery processes. It was aimed to determine the zeta potentials of illite, kaolinite, and chlorite obtained from various sources in Turkey. Hussain et al. (3, 4) and Hussain (5) characterized clay minerals in coal, wastes, and black waters from Zonguldak Coal Washery in Turkey and found that only three clays, namely kaolinite, illite, and chlorite, are present. Semiquantitative XRD analysis also indicated dominance of kaolinite in coal and of illite in black waters (5–8). These clays, in the forms of slimes, caused loss of recovery in coal flotation through slime coatings on coal and/or bubble and increased reagent consumption (9–11). Arnold and Aplan (1a) investigated the effect of clays on artificial coal–clay mixtures and concluded that different clays influence coal flotation differently, and therefore slimes should be well characterized with respect to clays to determine their effect on actual floating systems involving high ash coals. MATERIALS AND METHODS
The clays used in this study and their particulars are given in Table 1. In X-ray diffraction analysis (XRD) a Jeol JSDX-100S X-ray spectrometer diffractometer was used. All the analysis were done using CuKa radiation with Cu/Ni electrodes and goniometer speed of 17 (2u ) per minute. The slit widths used were 17 for the divergence slide, 0.27 for the receiver slide, and 17 for the scatter slide. For XRD analysis samples were prepared as unoriented mounts, no cation saturation
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536 TABLE 1 Turkish Clays Studied Clay type
Location
Variety and preparation
Kaolinite
Eskis¸ehir
Illite
Ordu
Chlorite
Ordu
Esk 4045, Kaolin 64.6% passing 5 mm 81% kaolinite XRD trace obtained showed mainly peaks of kaolinite and quartz (Fig. 1) Crushed in jaw and rolls to minus 10 mesh, then ground to minus 325 mesh Tyler in porcelain ball mill XRD trace showed peaks of micaceous clay minerals and quartz (Fig. 2) Concentration after magnetic separation, crushed and ground in porcelain mortar grinder to minus 325 mesh. XRD trace showed mainly chlorite peaks (Fig. 3)
was used, and samples were not subjected to heating. The cavity filling method of sample preparation was used. During this preparation the finely ground sample is put in a cavity and compressed with a spatula or a microscope slide is moved back and forth. This may cause orientation of the flaky clays such as chlorite and an unoriented mount may seem to be an oriented mount. The XRD traces of kaolinite, illite, and chlorite are shown in Figures 1–3. A Rank Brothers particle electrophoresis apparatus MKII (Cambridge, England) was used to measure the electrophoretic mobilities. During the measurement a quartz rectangular cell and platinum electrodes were used. After the cell was filled with the prepared suspension a voltage of 100 V
FIG. 2. XRD trace of illite (unoriented mount): I Å illite, Q Å quartz, M Å micaceous clay minerals.
was applied across it. The velocity of the particles in suspension was measured at both the front and back stationary levels in the chamber using the two inner surfaces as reference settings for focusing the microscope. By altering the direction of the current after each mobility measurement, polarization effects were minimized. The mobility of particles was computed after taking the averages of readings at the front and back levels. Each reported mobility was the mean mobility of 20 particles (10 on each level and 5 in each direction), yielding a standard deviation of about 5% (5). The equation used for converting observed mobilities U (U Å q /E, where q is the particle velocity and E is the applied field strength) into the effective electrokinetic potential (zeta potential) q depends upon the value of the
FIG. 1. XRD trace of kaolinite (unoriented mount); A Å anhydrite, K Å kaolinite, Q Å quartz, G Å geothite, R Å green rust.
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FIG. 3. XRD trace of chlorite (unoriented mount).
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EFFECT OF CLAYS ON COAL FLOTATION
dimensionless quantity xa, in which a is the radius of the particle (assumed spherical) and x is the quantity
F
4pe 2Snz 2 ekT
G
1/2
,
familiar in electric double layer theory. From the expression given, x É 1 1 10 6 cm at 257C in water containing 1 mM of a 1:1 electrolyte. If xa ú 200 it will usually be sufficiently accurate to use the Smoluchowski formula, which in the (original) unrationalized form is UÅ
ej , 4ph
where e is the permittivity of the suspending liquid. Typical units in the older system would be U in micrometers per second under 1 V cm (usually then in the range from 0 to 6), in which case for water at 257C q in mV would be given by z Å 12.83U.
If xa õ 0.1 then the Hu¨ckel equation would be relevant: UÅ
ej (unrat.). 6ph
But this will hardly ever be useful in the case of water as suspending medium since even water with no added electrolyte (seen as air equilibrated) has x á 10 6 , so that particles would have to be as small as 10 nm (0.01 mm) radius to reduce xa to 0.1. In the intermediate range of xa (0.1 to 200) the Henry equation UÅ
ej [1 / f ( xa)] (unrat. form) 6ph
U r j conversion. If a large (approximately spherical) particle is an agglomerate of small spheres then the question of which radius to use is seen to arise. If ( x 1 smaller radius) is small ( õ0.1) then again it is fairly evident that the larger radius governs the effective xa. In all the above it has been assumed that the particle itself is rigid (i.e., of infinite viscosity). In the absence of firm evidence it seems best to treat all particles of radius 1 mm or less as rigid. Under all the above considerations it is assumed that the Smoluchowski equation given above was used in the calculation of the nominal zeta potentials. Each clay sample before measurement was prepared as a master suspension in deionized distilled water containing particles less than 10 mm after sedimentation of an already ground material in an agate mortar to minus 300 mesh British standards. The mobility of each particle was measured in suspension after soaking it for 10 min at each pH between pH 2.5 and 11. The pH adjustment was done by adding a small amount of HCl and NaOH and by using a Fisher (Accumet Model 230) pH meter. In order to study the effects of clays, namely kaolinite, illite, and chlorite, a series of artificial mixtures of clean coal (4.5% ash) and clays (ground to 0325 mesh B.S.) were prepared and flotation was carried out using kerosene oil as collector (340 g/t) and methyl isobutyl carbinol as frother (85 g/t), which were previously optimized (5). The flotation tests were conducted in a Wedag laboratory flotation machine having a fiberglass cell (capacity Å 500 ml) manufactured locally. The speed of the impeller was kept constant at 1420 rpm. All the tests were conducted in distilled water (pH 6.5). Agitation and conditioning time were also constants as three minutes each in all the tests. For each test 50 grams of sample consisting of coal/clay mixture according to the amount of clays (0–25%) in each case was used, keeping the pulp density constant (10% solids by weight). The coal samples were ground to minus 28 mesh first in a jaw crusher and then in rolls. Three clays found in Zonguldak coal, namely kaolinite, illite, and chlorite, were used in this study (3–5). These clays were obtained from various sources in Turkey and on XRD analysis were found to be almost pure (4, 5).
can be seen if the z potential is small (say õ20 mV), where f ( xa) is a correction factor taking the values given below: xa
0
0.1
1.0
5
10
50
100
`
f ( xa)
0
0.001
0.027
0.160
0.239
0.424
0.458
0.5
If the z potential is not small, then in the range xa Å 0.1 to 200 the best available computations of the U: z relationship are those of Wiersema, Loeb, and Overbeek (12). These authors also considered the effect of different electrolyte charge types. If particles are not spherical then, unless xa is everywhere large, there must be some doubt about the
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537
RESULTS AND DISCUSSION
The XRD traces showed that each clay selected for this study contains only one clay mineral, along with nonclay impurities associated with clay, such as quartz. In the traces the broad reflections starting from 3–47 (2u ) and terminating at about 13–147 (2u ) might be due to the amorphous materials and organic matter present in the samples or to the somewhat poor alignment of the goniometer (13) (Figs. 1, 2, and 3). As expected from the preparation method, in chlorite the XRD trace shows a regular pattern (Fig. 3). The chlorite
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538
FIG. 4. Zeta potentials of Turkish clays as a function of pH.
has a micaceous nature and it is difficult to grind the sample to obtain a flakeless powder. The results obtained for zeta potential values measured against pH are plotted in Fig. 4 (5). All clays in Fig. 4 were found to have negative zeta potential in the pH range observed (2.5 to 11). The kaolinite was the most negative clay, ranging in zeta potential value from 024.0 to 049.5 mV. The illite was the least negative clay, giving a range of 0 to 042.0 mV. The chlorite gave a range of 0 to 036.5 mV. A lower pzc value at pH 2.5 for illite and at pH 3.0 for chlorite was obtained. The zeta potential measurements of Turkish clays showed that kaolinite (unlike others) was more negatively charged (lowest current) than chlorite and illite and therefore no pzc value could be found, as this permanent negative charge of kaolinite due to the isomorphic substitution remains independent of pH in the acidic range (14–16). In the basic range, the edge surface at which the octahedral sheet is broken may be compared with the surface of an alumina particle. Hydroxyl ions act as potential-determining ions and cause the edge surfaces to be negatively charged (16). Therefore, in kaolinite along the pH range studied no point of zero charge (pzc) was obtained. Kaolinite was always negatively charged, but it appeared less negative at about pH 5. A noticeable increase in negative zeta potential at pH ú5 is thought to be related to the pzc of the clay edge at about
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neutral pH (17, 18). Changes in zeta potential values in the pH range 2.5–11 showed the presence of complex ionexchange reactions on heterogeneous 0SiOH and 0AlOH sites. Arnold and Aplan (1b) determined zeta potentials of coal and clay as a function of pH in both tap and distilled water. On comparison with zeta potentials of kaolinite determined by Arnold and Aplan (1b) it was found that three types of kaolinite, namely, A, B, and C, were used by them. Kaolinite A and C showed completely different behavior than kaolinite from Turkey, whereas kaolinite B showed no pzc value, like kaolinite used in this study. The pzc value of illite was 2.5, which is similar to the value determined for Turkish illite. Chlorite could not be compared, as no chlorite zeta potential value was determined by them (1b). It is evident that the zeta potential of pure clays in distilled water is an intrinsic property of clays. Clay is always negatively charged; however, an increase in zeta potential is noted at around pH 6 due to the pzc of the clay edge. ¨ zbayog˘lu (19) that a decrease in It was reported by O zeta potential means better floatability, and it was found by Arnold and Aplan (1b) that the presence of ions in water (tap water) can reduce the overall negative charge of particles in the pH range from 2.5 to 10. This means that less negative clays are probably less harmful to coal flotation. Therefore, in this case illite should give less depression of coal in flota-
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EFFECT OF CLAYS ON COAL FLOTATION
tion than chlorite and kaolinite. The use of cations such as Al 3/ and Fe3/ may easily reverse the charge of those clays which are less negative (furnish pzc values easily), thus enabling these clays to be depressed by the use of dispersants and depressants with advantageous use in flotation (1a). As coal is usually negatively charged and gives a negative zeta potential in distilled water (5, 17, 19) due to its anisotropic nature, and clays are also negatively charged, only clays having charge opposite to that of coal should cause greater problems by adsorbing their positively charged portions on the coal surface, thereby preventing coal flotation. Hussain (5) measured the zeta potentials of clean coal (4.5% ash) with three dispersants, sodium silicate, calgon (sodium hexametaphosphate), and marsperse CB (sodium lignin sulfonate), and three depressants, CMC (carboxymethyl cellulose, Tylose C30), dextrin, and Auro Depressant 633, and found that only dextrin decreased the negative zeta potentials of coal, with increased dosage showing better depressing effects and better adsorption on coal surfaces. All others were not found advantageous in floating the coal. Only dextrin, in the light of zeta potential measurement, depressed fine high ash coal in single stage reverse flotation. Effects of various molar concentrations of MgCl2 , CaCl2 , Ca(OH)2 , AlCl3 , and Al2 (SO4 )3 on zeta potentials of clean coal were also studied by Hussain (5). The calcium cations were found to be less negative than the magnesium cations due to higher adsorption with increasing cation radius. Aluminum chloride and aluminum sulfate reverse the charge of coal by adsorbing on negatively charged surfaces by electrostatic forces even at lower concentrations. These findings were in line with others (19–21). The results obtained by floating clean coal (4.5% ash) and three different clays (kaolinite, illite, and chlorite) are given in Table 2 and are plotted in Fig. 5. This figure shows the effect of addition of kaolinite, illite, and chlorite (0–25%) on flotation of clean coal. It can be seen that illite has the worst effect both on yield and ash when floated with clean coal. Chlorite was found to have an intermediate effect on yield and ash, and kaolinite the least. It can be seen that up to 10% kaolinite and chlorite have no appreciable effect on flotation of coal. On the other hand, up to 25% (maximum amount) of kaolinite and chlorite clays reduced the yield to 18 and 20%, respectively. Illite addition up to 25% gave a loss of yield to about 28% and increase of ash to 6.5%, showing that coal flotation is affected more by illite than by kaolinite. Comparatively higher depression of coal by illite than by kaolinite on U.S. coals was indicated by Arnold and Aplan (1a), in agreement with the findings of this study. In order to compare the effects of clays in amounts determined by XRD analysis of coal samples from Zonguldak region (K Å 52%, I Å 31%, Ch Å 17%), these were converted to their proportions of maximum amount 25% (5). This gave the amount of each clay (K Å 13%, I Å 8%, Ch
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TABLE 2 Effects of Clays on Coal Flotationd Name of clay added Kaolinite
Chlorite
Illite
a
Clay added %
Yield %
Ash %
0
86.7
2.5
5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
83.3 79.5 76.9 72.1 68.9 79.0 76.0 73.4 70.2 66.5 77.5 73.2 68.2 64.3 59.1
3.0 4.0 5.0 6.0 7.0 3.5 4.5 6.0 7.0 8.0 4.0 5.5 7.0 8.0 9.0
Reagents; kerosene oil Å 340 g/t, methyl isobutyl carbinol 85 g/t.
Å 4%) to be included in the artificial mixture of clean coal. Flotation results showed that yield and ash values as percentages were 70.0 and 9.0, respectively. Without clay, clean coal flotation gave a yield value of about 90% with an ash value of 2.5%, showing the effect of clays in reducing the yield and increasing the ash of the product (coal) (5, 22). In the literature there are two opposite views about the effect of clay slimes on coal flotation. Most investigators (10, 12) believe that slime coatings are the real cause of loss in mineral flotation recoveries. The slimes are heaviest when the slimes are uncharged or oppositely charged to the material being floated and thus badly affect the flotation results. Similarly other researches (13, 23, 24) note depression of coal due to the clay slimes. However, Firth and Nicol (25) found no effect of kaolin (clay) on coal flotation. Another explanation about the bad effects of clays on flotation through the entrainment or water carryover mechanism is reported by some investigators (26, 27). The controversy on two opposite views regarding the effect of clays on flotation by slime coatings on mineral or coal and water carryover is still unresolved. It may be said that both these mechanisms play a part in spoiling the results of coal flotation. The type of clay and the amount have major roles in coal depression, whereas the water carryover mechanism has a minor effect. It was noted that water carryover in coal mainly contributed toward ash increase of the floated product due to clay slimes and the coal depression was negligible (i.e., yield was not affected). The depression of coal can therefore be related mainly to the clay type and its slime coatings on coal particles, which mask them from the reagent adsorption. However, both the mechanisms, one (clay type and amount) reducing the yield and the other (water carryover) lowering
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FIG. 5. Effect of kind and amount of clay on yield and ash of clean coal (4.5% ash) in flotation.
the grade by increasing the ash of the floated product, are operative in coal flotation. It is fortunate that Zonguldak washery coal did not contain any montmorillonite type clays, which are very harmful for U.S. coal flotation (12, 28–30), and therefore a less gloomy picture has emerged. However, the effects of illite, kaolinite, and chlorite in Zonguldak coal cannot be ignored while this coal is floated. CONCLUSION
The conclusions drawn from the present investigation are summarized as follows: Zeta potentials of all Turkish clays (kaolinite, illite, and chlorite) determined in distilled water over a pH range from 2.5 to 11.0 were all negative, ranging from 013.1 to 049.5 mV. Kaolinite was the most negative clay, compared to chlorite and illite, found electronegative in decreasing order. No point of zero charge of kaolinite could be found because of its permanent negative charge due to isomorphic substitution and/or structural defects. Illite surface exhibited a point of zero charge (pzc) at pH Å 2.5 and chlorite surface at pH Å 3.0. Although zeta potential measurements of these three clays do not give an absolute clue to their behavior toward flotation or any other process, they may reflect their nature and hence these clays may be dealt with accordingly by washery engineers.
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Flotation tests on artificial mixtures of clean coal and individual clays revealed that each clay influences coal flotation differently according to its type and the amount used for flotation. Illite was found to have the worst effect on coal flotation by reducing the yield and increasing the ash of the floated coal, followed by chlorite and kaolinite in decreasing order. Clay coatings on coal and water carryover of ultrafine clays were both responsible for deteriorating yield and ash of floated coal. REFERENCES 1. (a) Arnold, B. J., and Aplan, F. F., Int. J. Miner. Process. 17, 225 (1986); (b) Int. J. Miner. Process. 17, 243 (1986). 2. Leja, J., ‘‘Surface Chemistry of Froth Flotation.’’ Plenum, New York, 1982. ¨ zbayog˘lu, G., in ‘‘The First Interna3. Hussain, S. A., Demirci, S¸., and O tional Conference on Modern Process Mineralogy and Mineral Processing,’’ p. 228, 1992. ¨ zbayog˘lu, G., Appl. Clay Sci. 7, 471 4. Hussain, S. A., Demirci, S¸., and O (1993). 5. Hussain, S. A., ‘‘Characterization of Coal and Wastes from Zonguldak Coal Washery and Effect of Clays on Coal Flotation,’’ Ph.D. Thesis. Middle East Technical University, Ankara, Turkey, 1992. ¨ zbayog˘lu, G., and Demirci, S¸., in ‘‘International Con6. Hussain, S. A., O ference on Recent Advances in Materials and Mineral Resources, 3– 5 May, Penang, Malaysia,’’ p. 554. Pevak Foundation Publication, 1994. ¨ zbayog˘lu, G., and Atabey, E., Int. J. 7. Hussain, S. A., Demirci, S¸., O Environ. Issues Miner. Energy Ind., 101 (1993). ¨ zbayog˘lu, G., and Demirci, S¸., in ‘‘Proceedings of 8. Hussain, S. A., O
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