SOCIETY FOR MINING, METALLURGY, AND EXPLORATION, INC.

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1 SOCIETY FOR MINING, METAURGY, AND EXPORATION, INC. PREPRINT NUMBER P.O. BOX 6252 ITTETON, COORADO RECOVERY OF PYRITE IN COA FOTATION: ENTRAINMENT OR HYDROPHOBICITY? S. K. Kawatra T. C. Eisele Michigan Technological University Houghton, Michigan For presentation at the 1991 SME Annual Meeting Denver, Colorado - February 25-28, 1991 Permission is hereby given to publish with appropriate acknowledgments, excerpts or summaries not to exceed one-fourth of the entire text of the paper. Permission to print in more extended form subsequent to publication by the Society must be obtained from the Executive Director of the Society for Mining, Metallurgy, and Exploration, Inc. If and when this paper is published by the Society for Mining, Metallurgy, and Exploration, Inc., it may embody certain changes made by agreement between the Technical Publications Committee and the author, so that the form in which it appears is not necessarily that in which it may be published later. These preprints are available for sale. Mail orders to PREPRINTS, Society for Mining, Metallurgy, and Exploration, Inc., P.O. Box 6252, ittleton, Colorado PREPRINT AVAIABIITY IST IS PUBISHED PERIODICAY IN MINING ENGINEERING

2 1 Abstract. Under normal conditions, a significant amount of pyrite is recovered in the froth during flotation of high-sulfur coal. In order to reduce this pyrite recovery, it is first necessary to determine the primary recovery mechanism, as different measures are required for reducing entrainment, lockedparticle flotation, or true hydrophobic flotation. In this paper, evidence is presented \Ihich suggests that hydrophobic flotation is not the major mechanism for recovery of liberated pyrite, and that the bulk of the recovered pyrite is there either as a result of simple entrainment or by being physically locked \lith floatable coal particles. Introduction In many coal flotation operations, a significant amount of apparently liberated pyrite is seen to report to the froth, thus raising the inorganic sulfur content of the clean product and reducing its economic value. Obviously, a method of preventing this unwanted recovery of pyrite is desirable. However, the best means for preventing liberated coal pyrite recovery Hill be primarily controlled by the recovery mechanism for these particles, thus this mechanism should be determined before selecting a particular method for removing or suppressing pyrite. In general, there are two \Jays for a liberated particle to enter the froth phase: particle entrainment andor bubble attachment. The separation of these t\jo effects in a real experiment is very difficult, due to the presence of locked particles. Since coal pyrite does not have the same properties as pure mineral pyrite [Chernosky and yon, 1972], and it is not practical to produce coal pyrite Vlhich is completely free of locked coal particles, experiments to demonstrate conclusively Hhether pyrite from a given coal either does or does not float by hydrophobic bubble attachment are impractical, and the best which can be done is to estimate the relative probable quantity of liberated pyrite Vlhich can be accounted for by the tho mechanisms. For this paper, a conventional flotation machine was used to compare the floatability of tvlo different mineral pyrites and a coal pyrite, using only nonpolar oil as collector. Experiments \lere also carried out to attempt to measure the floatability of liberated pyrite in coal using a conventional flotation cell. Theoretical Discussion It has long been known that mineral pyrite has some hydrophobic tendency; hol'lever, this tendency is normally very slight, and significant flotation of pyrite require3 addition of a collector of some type. The natural flotability of pyrite is also strongly pll dependent, with the highest floatability in acidic solutions. Depression of mineral pyrite is quite easy, I-lith a variety of highly effective depressants being available. In most selective sulfide mineral flotation circuits the bulk of the floated pyrite does so as a result of entrainment and locking to hydrophobic particles [SHE liineral Processing Handbook, 1985]. l'ihile the responses of mineral pyrite in flotation are fairly Hell knmffi, the behavior of coal pyrite is poorly understood [Chernosky and yon, 1972]. This is due to its fonnation and long residence in the presence of a variety of organic compounds. It is often reported that coal pyrite exhibits surface chemistry radically different from that of mineral pyrite. In addition, the extreme heterogeneity of coal deposits apparently result in the properties of the associated pyrite also varying over a Hide range. As a result, experim~nts \lith coal pyrite from a specific source may not be particularly relevant to pyrites from other sources. Many investigators have attempted to use various chemicals as depressants for coal pyrite in order to reduce the sulfur content of the product, Hith some success [Purcell, 1982; Hirt, 1973]. HO\Jever, in most reported experiments it has not been clear \'lhether the decrease in pyrite flotation Has due to reduced pyrite hydrophobicity or improved froth drainage, as the quantity of entrained \yater in the froth is often not measured. Also, since pyrite depressants reduce the floatability of coal slightly, it is reasonable to expect that feher locked particles ~ Hill float in the presence of these reagents, thus reducing the quantity of pyrite floating by this means as \Yell. Detennination of the quantity of entrained material is difficult, particularly \hen a small number of experiments are conducted, as the Hater recovery in the froth must be determined and correlated Hith the recovery of the entrained species. Since entrainment is, in theory, linearly related to Hater recovery, the entrainment constant can be determined by linear regression when the data set available is sufficiently large. The floated material which is not accounted for by the regression line is then assumed to be floating either by hydrophobic bubble attachment or as part of locked particles. Since the entrainment constant varies \-li th particle size, any changes in the particle size of the entrained species Hill tend to significantly change the results [ynch, et al., 1981]. EA~erimental Procedures and Results In order to determine whether the pyrite in a particular Pittsburgh Seam coal was floating by lockingentrainment or by true flotation, three sets of experiments Here carried out. These experiments provided data for conditions in which pure mineral pyrite flotation could be expected Hith only #2 fuel oil as a collector, the ext~nt to which coal pyrite flotation occurs during normal flotation of coal, and the quantity of Hater entrained in the froth. Pure Pyrite Flotation Relatively pure pyrite from three sources (Custer, S.D., and Rico, CO, mineral pyrites obtained from Ward's Natural Science Establishment, and coal pyl'ite collected as large nodules from Panther Valley, PAl Here ground to the size distribution given in Table 1, using a steel rod mill, at 6% solids and a

3 Table 1. 2 Size distributions for the samples used in the pyrite flotation experiments I detennined by Hicrotrac particle size analyzer. Cumulative % passing, Cumulative % passing, Size, lm mineral pyrite samples coal pyrite samples ( Table 2. Conditions and results for pure pyrite experiments. Test Pyrite Charge wt. 1A Custer, S.D. 15 2A 15 3A 15 4A 15 5A 75 6A 75 7A 75 BA 75 1B Panther Valley 15 2B 15 1C Custer, S.D. 15 2C " 15 3C Panther Valley 15 4C " 15 5C Rico, CO 15 6C 15 1 Silica 15 2 Silica 15 ph adjusted to 12 \iith NaOH to prevent corrosion and subsequent dissolution of iron, which is reported to be a pyrite depressant [Hiller and Baker, 1972]. The pyrite was then filtered, divided into charges of the desired weight, repulped in a Denver laboratory flotation cell (volume of 1.9 liters), and the ph was adjusted to the desired level with sulfuric acid. Finally,.25 gm HlBCliter and the noted quantities of #2 fuel oil were added as frother and collector, respectively. The pulp was then conditioned for 2 minutes, and floated for 5 minutes with froth removed manually. The results of these experiments are given in Table 2, along \-lith two control runs using pure silica Hhich had been prepared in the same manner as the pyrite. The ph values Here selected on the basis of earlier tests, which shmved that mineral pyrite floats \.ell below ph = 4, and poorly at higher ph values. The pyrite appeared strongly to buffer the ph at ph = 4, consuming approximately 3 grams sulfuric acid per kilogram of pyrite to reduce #2 fuel oil, kgmt ph % Floats B.B ph below this level. Upon lowering the ph below 4, the pyrite became strongly floatable, simultaneously releasing a strong odor of H2S. The charges for Tests 7A and 8A were allowed to air-dry before repulping for flotation, and became \.arm to the touch while drying. The reduced recovery for these tests was most likely due to oxidation of the pyrite. Unlike the two mineral pyrites, the Panther Valley coal pyrite never became strongly floatable, and also never released the characteristic H2S odor. The froth product from the coal pyrite appeared, from qualitative examination at 4x magnification of the loose powder, to be predominantly composed of locked coal-pyrite particles, floating as a result of the hydrophobicity of the coal portion. Additional experiments were attempted using coal pyrite from a second source, the Empire Coal mine, Gnadenhutten, Ohio., However, it was found that even \vhen apparently pure pyrite nodules were collected and cleaned by hand, the pyrite still contained a residue of 27% by wt.

4 3 which \>las insoluble in nitric acid, the residue being predominantly composed of coal and clay. As a result, it Ivas not possible to conduct the desired experiments using coal pyrite which had been isolated from the associated organic inclusions, and the results obtained \lere similar to those for the Panther Valley coal for this reason. Since no conclusive technique was available for distinguishing liberated particles from locked particles in quantity, the role of locked coalpyrite particles in pyrite flotation, tion, relative to liberated pyrite, could not be confirmed. Pittsburgh Coal Conventional Flotation For experiments, this coal was ground in a steel rod mill to the size distribution given in Table 3. The freshly ground coal ~las then divided into 25 gm charges for flotation, which were stored at -2~C to retard oxidation. Timed flotation experiments Here carried out in a 4-liter flotation cell, \lith gravity-feed pulp level control and mechanical froth scrapers (Figure 1), under the conditions given in Table 4. The frother Has Dowfroth 2, added at a rate of.25 gliter. Froth was collected over the time intervals -3 sec, 3 sec-1 min, 1 min-2 min, 2 min-3 min, 3 min-5 min, and 5 min-9 min, filtered, dried, and weighed. All products Ivere analyzed for ash content, and total sulfur content was determined using a ECD SC-132 sulfur determinator. The results for all six tests are given in Table S. Table 3. Size distribution of Pittsburgh seam coal used in the coal flotation experiments, produced by grinding 9 g charges of coal at 4% solids in a steel rod mill for 45 minutes. Size distribution Ivas measured using a Hicrotrac particle size analyzer. Size, pm Cumulative % Passing Table 4. Flotation conditions for timed tests using conventional flotation and Pittsburgh seam coal. Test # ph collector (fuel kgmt.4 kgmt.8 kgmt Water Reservoir Froth Removal Paddles ~ 1'- \ ~ OverflOW Weir fiyj) Figure 1. Schematic diagram of flotation cell with automatic pulp level control.

5 Table 5. Proauct 4 Results from timed flotation experiments using a constant-level conventional flotation cell % l'lt Test 1 3 sec Froth min min min min min 6.73 Final Tails 16.7 Test 2 3 sec Froth min min min min min 4.66 Final Tails Test 3 3 sec Froth min min min min min 6.1 Final Tails Heasured Head Test 4 3 sec Froth min min min min min 6.6 Final Tails 18.4 Test 5 3 sec Froth min min min min min 5.2 Final Tails 12.3 Test 6 3 sec Froth min min min min min 5.5 Final Tails lfeasured Head % AsFi % S.{. sohas ~ Discussion The results from the pyrite flotation experiments show very clearly that pure mineral pyrite loses its natural floatability nearly completely \hen the ph is greater than 5., which is comparable to the floatability of silica, which is known to be hydrophilic. This indicates that under normal conditions for coal flotation (ph c 5-9) unmodified mineral pyrite is not noticeably recovered by true hydrophobic flotation. Even large dosages of neutral oil collectors do not render mineral pyrite floatable over this ph range, up to 3 kg collectorl metric ton pyrite. This has been found to be true for pyrites from two distinct sources, and is likely to be generally true for all pyrites which are not associated Hith organic matter. Therefore, true hydrophobic flotation of coal pyrite should only be expected when the pyrite has been heavily modified by organic material, or when flotation is being carried out under acid conditions. The coal pyrite, although prepared in the same fashion as the mineral pyrites, did not exhibit similar behavior, as it maintained essentially constant levels of flotation recovery (31-43%) over the ph range from 2.2 to 8.8, while the mineral pyrite dropped from nearly 99% recovery to in some cases less than 4% over the same ph range. Qualitative microscopic examination of the coal pyrite flotation product indicated that much of the floating material was coal and locked coall pyrite particles. It is therefore hypothesized that, in this case, the coal pyrite is floating due to attachment to coal. This hypothesis is supported by recent work by Chander and Aplan [1989], where great care was taken to isolate

6 5 liberated pyrite from seven coal sources for a variety of detailed analyses. Using a microflotation cell aerated with nitrogen gas, all of the purified pyrites were found to shoh very little inherent floatability (average = 1.9% \-'t. floating, range = -7% Vt.. ). The procedure Hhich Chander and Aplan describe for purifying the pyrites does provide opportunity for the pyrite to oxidize somewhat, Hhich may conceivably destroy its inherent floatability. Nevertheless, it appears likely that coal pyrites have a very low inherent floatability, and are therefore floated primarily as either locked or entrained particles. Unfortunately, there does not appear to be any available method for conclusively measuring the relative quantities of locked and liberated pyrite particles in coal flotation, nor is there an effective method for isolating coal-free pyrite from coal which does not also either contaminate or oxidize the pyrite surfaces. At the present time, it is therefore not possible to prove that coal pyrite either does or does not enter the froth as a result of true hydrophobic attachment. Based on the behavior of the pure mineral pyrite samples, it is hypothesized that coal pyrite is recovered in the froth by entrainment and by mechanical attachment to floatable coal particles. HOHever, as a result of the fine scale of the organic contaminants in coal pyrite, it is clear that mineral pyrite is not a good model for the behavior of coal pyrite, and should not be treated as such. An approximate estimation of Hhether coal, mineral, and pyrite particles are being recovered by true flotation, locking, or entrainment can be achieved by correlating the recovery of a given component v,ith the recovery of water in the froth. This correlation is shown in Figures 2, 3, and 4 for ash, liberated pyritic sulfur, and ash-free coal for the Pittsburgh Seam coal sample. iberated pyritic sulfur (SP) was obtained from the formula %SP = %S - 1.8(%vt.; ash-free coal), Hhere 1.8% sulfur is the lowest sulfu~ content obtained for this coal by flotation. Figure 2 ShO\"IS that ash-free coal recovery does not correlate \Vith Hater recovery, while Figures 3 and 4 show that ash and liberated sulfur correlate linearly with the water recovery, with correlation coefficients of.952 and.767, respectively. It is immediately seen that the correlation of liberated pyrite and water recoveries are unaffected by the collector dosage over the range studied, which confirms that the liberated pyrite is being recovered by entrainment and not true flotation. This is further borne out by the correlations of liberated pyrite recovery with ash (Figure 5) and ash-free coal (Figure 6) recoveries. The behavior of the liberated pyrite is nearly indistinguishable from that of the strongly hydrophilic ash (\~hich is predominantly clay slimes that are known to float by entrainment), but is wildly variant from the behavior of the ash-free coal. Since the relationship of pyrite recovery to ash " QJ QJ > 8 - u QJ : QJ u I 6, g... QJ QJ... c _ 4 2QJ.!!E._. I If) ::l.1 E.~ 2 o u::l... fi. III O~ ~--~--~~ o Water Recovery During a Time Interval, % of Total Cell Vol ume II III III III f~gure 2. Compar~son ot ash-tree combustible matter recovery with.later recovery during timed conventional flotation experiments.

7 6 6 Ash Rec. =.38(Water Rec.) Correlation Coefficient =.952..c (J) <{ +-' 3: rJ "", rji III ". o~--~--~--~--~--~--~~ o Water Recovery During Time Interval J of Total Cell Volum~ Figure 3. Correlation of ash recovery with water recovery in timed conventional flotation experiments..2 6 :::J t) U Ash Rec. =.27(Water Rec.) Correlation Coefficient =.767 ~4 1J +-' o 2 2 :.J Water Recovery During Time Interval, % of Total Cell Volume Figure 4. Correlation of pyrit1.c sulfur recovery \lith \later recovery in tilll.:d convt::nt.ional flotation experiments.

8 7 >. 8 > u n:: :J :J (f) 6 U >. a.. " l Ash Recovery Figure 5. Correlation of pyritic sulfur recovery with ash recovery in timed flotatation tests. 8 >. > u n:: :::J 6 -:::J (f) u 4 +oj >. a.. " 2 +oj. -I Combustibles Recovery Figure 6. Correlation of pyritic sulfur recovery \-lith couibustible llict.ttl.;.i," recovt:ry in tim<!d flotation tests.

9 8 recovery is virtually unaffected by fuel oil dosage, the most reasonable assumption is that the liberated pyrite is floating by entrainment and has no particular hydrophobic character [Kal-latra and Eisele, 1988]. Further, if any liberated pyrite is floating by true hydrophobic flotation, increasing the fuel oil dosage does not increase the quantity of pyrite which floats in this fashion. Conclusions From the results presented, the follolving conclusions may be dral-ln: 1. Mineral pyrite is readily floatable by fuel oil alone at acid ph «4), but this native floatability is entirely lost at the higher ph levels Hhere coal flotation is most often carried out. 2. Determination of the floatability of coal pyrite in the absence of coal particles is not practical, due to the intimate intermixing of carbonaceous and pyritic components even in pyrite nodules which appear visually to be relatively pure. As a result, pyrite I-lhich floats by true hydrophobic bubble attachment cannot readily be distinguished from that ~hich floats due to mechanical attachment to coal particles. While it can be hypothesized that completely liberated coal-pyrite particles behave similarly to mineral pyrite, and are therefore not recovered by flotation at near neutral ph, this is very difficult to prove. 3. The results of timed flotation of a Pittsburgh seam coal using conventional flotation are consistent Hith the pyrite being recovered in the froth predominantly by entrainment and locked particles, \lith any contribution from hydrophobic flotation of liberated pyrite being minimal. References SHE-AllIE, 1985, SHE Hineral Processing Handbook, Section 5, "Flotation," Society of I-lining, Hetallurgical, and Petroleum Engineers, ittleton, CO. Chander, S. and Aplan, F. F., 1989, "Surface and Electrochemical Studies in Coal Cleaning," Final Report to the U.S. Department of Energy, DOEIPCI 8523-T11 (DE 9-763). Chernosky, F. J. and yon, F. ll., 1972, "Comparison of the Flotation and Adsorption Characteristics of Ore and Coal-Pyrite \-lith Ethyl Xanthate," AllIE Transactions, vol. 252, pp Hirt, W. C., 1973, Separation of Pyrite from Coal by Froth Flotation, unpublished Haster's thesis, Pennsylvania State University. Kal-latra, S. K. and Eisele, T. C., 1988, "Studies Relating to Removal of pyritie Sulfur from Coal by Column Flotation," Chapter 22, Column Flotation '88 (K.V.S. Sastry, ed.), S~IE, ittleton, CO, pp ynch, A. J., Johnson, N. \t., Hanlapig, E. V., and Thorne, C. G., 1981, Hineral and Coal Flotation Circuits: Their Simulation and Control, Elsevier, Amsterdam. Hiller, K. J. and Baker, A. F., 1972, "Flotation of Pyrite from Coal," U.S. Bureau of Hines Technical Progress Report TPR51. Purcell, R. J., 1982, Circuitry Variations for the Rejection of Pyritic Sulfur during Coal Flotation, unpublished Ilaster' s thesis, Pennsylvania State University. AcknoHledgments The authors would like to thank Hs. Jacqueline F. Bird, Director of the Ohio Coal Development Office, for her useful suggestions and critical discussion; Mr. \-1. Kukura for providing the plant facilities, personnel, the valuable signatures and the engineering services; and Dr. R. R. Klimpel, DO\-l Ch~mical Company, for his repeated patience and for the chemical reagents to operate the flotation circuit at Empire Coal.