Coal Preparation, 1993 Vol. 13, pp. 63-72Photocopying pennitted by license only@ 1993 Gordon and Breach Science Publishers S.A.Printed in the United States of America

Effect of Wet Versus Dry Grinding onRejection of Pyrite and Non-PyriticMinerals from Pittsburgh No.8 Coal byFlotationD. UU, T. V. V ASUDEV AN, P. SOMASUNDARAN* and C. C. HARRIS

Henry Krumb School of Mines, Columbia University, New York, NY 10027

(R~c~iv~d January 18, /991)

The effect of grinding mode on the efficiency of separation of pyrite and non-pyritic minerals fromPittsburgh No.8 seam coal by froth flotation was investigated. In the ease of minus 6(KJ micron feed.grinding mode had no effect on the efficiency of separation of either pyrite or non-pyritic minerals.With minus 75 micron feed. however. the selectivity achieved with the wet ground feed was higherthan that obtained using the dry grind and this was attributed to the dry state aggregation of coal withmineral matter. Comparison of flotation selectivity and washability curves showed that entrainment offines due to water flow was the reason for the loss of selectivity during notation of coal from non-pyritic minerals. In the case of pyrite. in addition to entrainment. notation of pyrite due to its naturalhydrophobicity contributed to the loss of selectivity.

Key words: Coal cleaning. flotation. dry grinding. wet grinding. Pittsburgh No.8 coal. coal pyrite.washability

INTRODUCTION

Deep cleaning of coal by physical separation processes requires fine grinding toliberate coal from pyrite and other ash-forming impurities. Froth flotation is themost commonly used technique for beneficiating fine coal. Since the mode ofgrinding, dry or wet, can affect differently the surface properties of coal and thegangue minerals it is important to understand its role in the flotation process.Majority of the recent investigations on flotation of fine coals have been conductedwith coal feeds obtained by either dry grinding or wet grinding. 1-1 Therefore, theeffect of different grinding methods on the separation of coal from pyrite fromother ash forming minerals is not clear from these studies. Miller and Guzzo~ havecompared the effect of dry and wet grinding methods for deep cleaning ultrafinecoal by flotation. Their study was preliminary in nature, however, and also theeffect of grinding method on pyrite rejection-the primary objective of recent

"To whom correspondence should be addressed

63

D. UU dill.64

investigations on advanced coal cleaning-was not reported. Further. the effect ofanother important parameter. namely the grind size. was not considered in theirinvestigation. In the current study. the effect of wet and dry grinding methods onthe separation of pyrite and non-pyritic ash from coal has been tested on a com-parative basis with both coarse (minus 600 microns) and fine grinds (minus 75microns).

'Y'

EXPERIMENTAL

MaterialsCoal sample used in this study is from Pittsburgh No.8 scam supplied by RandF Coal Co., OH. The proximate, ultimate and sulfur forms analyses of this sampleare listed in Table I. The 2-4 inch sample supplied was crushed to nominal 1/4 inchsize in a laboratory roll crusher under argon atmosphere and the product was storedin polyethylene bags under argon. Typically, these samples were stored for aboutsix months prior to testing.

MethodsGrinding and particle size analysis: A laboratory rod mill was used to obtain minus(iX) micron (coarse grind) and minus 75 micron (fine grind) flotation feeds. t2 Rodmill dimensions and details of various experimental parameters used are given inTable II. Size distribution of plus 75 micron particles was determined using U.S.Standard Testing sieves whereas that of minus 75 micron fines was obtained usingLeeds and Northrup Microtrac particle size analyzer. Washability analysis wasconducted using sink-float tests carried out in accordance with ASTM-D4371 pro-ccdure.:I:'

TABLE IAnalysis of the coal S8mpie-dry basis

Proximate analysis andsulfur-forms (%)Ultimate analysis (%)

69.875.101.456.4()

4.5812.W

2.~35.5551."12.590.282.721.644.64

MositurcVolatile matterFixed CarbonAshSulfatic SPyritic SOrganic STotal Sulfur

CHN0SAshHeating value.

Btu/lb 12.420

't95% passing ~ microns and 75 microns respectively.'*Samples for washability studies were prepared in an outside lahoratory (Geochemical Testing Lab-oratory. Somerset. PA) arranged hy Praxis Engineers. Inc. The size distribution of washability samples.however. were similar to those of flotation feeds (Table IV\.

WET VERSUS DRY GRINDING 6S

TABLE II

Rod mill dimensions and grinding parameters

25.4 cm (10") length x 22.8 cm (9") i.d.

22.8 cm (9") length x 1.9 cm (3/4.) i.d.. # I22.8 cm (9") length x 1.6 cm (5/8") i.d.. # 222.8 cm (9") length x 1.3 cm (1/2") i.d.. # 3

Rod mill dimensions

Rod dimensions(three different sizes)

Material:Mill and rods

Number of rods:

316 stainless steel

Dry grinding-minus 600 micronsWet grinding-minus 6(XI micronsDry grinding-minus 75 micronsWet grinding-minus 75 micronsMill speed~:

24 (8 each of #1. #2 and #3)24 (8 each of #1. #2 and #3)42 (14 each of #1, #2 and #3)48 (16 each of #1. #2 and #3)56 rpm (~% critical speed)

500 grams, 80% passing 0.635 cm700 cc

SolidsLiquid (wet grinding)

Grindinl! time:Dry grinding-minus 6(X) micronsWet grinding-minus 600 micronsDry grinding-minus 75 micronsWet grinding-minus 75 microns

Grinding medium

4 minutes and 50 seconds3 minutes31 minutes18 minutes

Air

Flotation: Flotation tests were conducted in a two-liter Denver D-1 flotationmachine with provisions for mechanical froth removal and air flow control. Theground product from the rod mill was divided in to four fractions using a mechanicalsplitter. Each of these fractions contained about 125 grams of solids and was usedas the flotation feed. After charging the cell with the solids (42 weight percentslurry in the case of the wet ground product), a specified amount of distilled waterwas added so that the total volume of liquid was 1000 cc. The slurry was agitatedfor three minutes prior to the addition of the reagents (preconditioning step). Inthe case of the dry ground product the slurry had to be preconditioned for anadditional three minutes (to completely wet the sample) in order to obtain repro-ducible results. Following preconditioning, a known amount of collector was addedand the slurry was agitated for one minute after which a specified amount of frotherwas added. Conditioning with the reagents was carried out for an additional fiveminutes after which flotation was started by introducing air in to the cell andsimultaneously switching on the froth removal paddles. Flotation was carried outfor five minutes following which the float and sink products were filtered, dried,weighed and analyzed for ash and pyrite contents. Details of flotation machineparameters and other pertinent variables are given in Table III.

Analysis: The ash and pyritic sulfur contents of the coal samples were determinedusing the ASTM procedure (D 3174-82 for ash and D 2492-84 for pyrite). The non-

68 D. UUetol.

TABLE IV

Particle size distribution of various grinds and washability samplesI

Minus 600Minus 600Minus 600

Minus 75Minus 75Minus 75

340340

180195

6262W

324250

212327

..1

5.6

5.5

*-subscript represents percentile value

not this difference can produce significant effect on the separation efficiency, selec-tivity curves obtained for dry ground feeds of minus 600 microns and minus 75microns are compared in Figure 3. It can be seen from this figure that there is nosignificant difference in the separation achieved although the minus 600 micronfeed is much coarser (average particle size = 180 microns) than the minus 75 micronfeed (average particle size = 21 microns). This suggests that in the size rangestudied, entrainment is independent of particle size. The other possibility for the

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~ REcovmYFIGURE 2 Effect of the grinding mode on the separation of non-pyritic minerals from coal andcomparison of washability and flotation selectivity curves for minus 75 micron feed.

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WET VERSUS DRY GRINDING 69

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FIGURE 3 Comparison of flotation selectivity and washability curves for the dry ground minus 600micron and minus 75 micron feeds.

overlap of the selectivity curves obtained with the coarse and the fine grinds is thatfiner grinding could result in an increase of both the liberation and the entrainmentwith the two effects cancelling each other out. This, however, is unlikely due tothe following reason. A decrease of particle size from 180 microns (average sizeof the dry coarse grind) to 21 microns (average size of the dry fine grind) resultsin only about a 2.5 percent increase in the entrainment of non-pyritic minerals(Figure 3). This suggests that the increase in entrainment due to a decrease ofparticle size from 23 microns (average size of the wet fine grind) to 21 microns(average size of the dry fine grind) would be much less than 2.5 percent. Also,since the particle size distributions of wet and dry ground feeds are similar (TableIV), heterocoagulation effects can be considered as not significant. To examinewhether or not the degree of liberation can contribute to the observed differencebetween the separation efficiencies achieved with dry and wet ground minus 75micron feeds, the washability curves of minus 600 micron feed and minus 75 micronfeed are compared in Figure 3. It can be seen from this figure that the degree ofliberation of the non-pyritic mineral matter in the minus 75 micron feed is onlymarginally higher than in the minus 600 micron feed. This suggests that differencesin the liberation characteristics also do not contribute in any significant manner tothe observed difference in the separation efficiency.

D. UUet.t70

Other possible reasons for the relatively poor separation achieved with the dryground feed are the oxidation of coal surface and dry state aggregation of themineral matter with coal during grinding. To determine whether or not oxidationof the coal surface can result in loss of selectivity, tests were conducted in which,after wet grinding, the rods from the mill were removed, a known amount of 30weight percent hydrogen peroxide (oxidizing agent) was added to the mill and theslurry conditioned for 30 minutes maintaining the mill speed at the same level asthat used during grinding (56 rpm). Flotation results obtained with the chemicallyoxidized feed are compared with the unoxidized feed in Table V. The results showthat, although four times higher dosage of frother is required for oxidized coal toachieve a yield equivalent to that of unoxidized coal the ash content of the frothproducts are the same indicating that oxidization of coal surface diminishes onlythe flotability of coal and not the selectivity.

Above discussions suggest that dry state aggregation of coal with thc mineralmatter. which can result in the formation of hard agglomerates, is the most probablereason for the relatively poor separation achieved with the dry ground feed. It ispossible that since the degree of aggrcgation will bc more severe for finer material,dry state aggregation does not playa significant role in the flotation of minus ~micron fced.

Effect of grinding mode on pyrite rejectionPyrite flotation selectivity curves obtained for minus ~ micron and minus 75micron feeds are shown in Figures 4 and 5 respectively. Corresponding washabilitycurves for pyritic sulfur are also shown in these figures for comparison. It can beseen from Figure 4 that, as observed for non-pyritic ash rejection, the mode ofgrinding does not playa significant role in determining the selectivity achievedduring flotation in the case of minus ~ micron feed whereas with minus 75 micronfeed higher selectivity can be obtained with the wet ground feed in comparison tothat with the dry ground feed. Another important observation is that the shifts ofthe selectivity curves obtained for pyrite from the washability curves are even fartheraway than the shifts in the curves for non-pyritic ash (compare Figures 1 and 2with Figures 4 and 5 respectively). For a given coal recovery, the difference betweenthe washability and the selectivity curves in terms of percentage non-pyritic ashrejections is 10-15 percent compared to 15-25 percent for pyrite. The additionalloss of selectivity in the case of pyrite may be due to the natural hydrophobicityof pyrite as well as due to incomplete liberation.

TABLE V

Effect of hydrogen peroxide addition on thc notation pcrformance of wct minus 75 micron fced fromPittsburgh No.8 coal

Frot.g/kg

('.oil.g/kg

H,O~wt%

Rot. Yieldwt%

Ash in floatwt%

n.sn.o

0.03.0

5.1

5.1

WET VERSUS DRY GRINDING 71

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50 m 70 8) 9J 100

COAL RECOVERY ~ Z

FIGURE 4 Effect of grinding mode on the separation of pyrite from coal and comparison ofwashabilityand flotation selectivity curvcs for minus 600 micron feed.

CONCLUSIONS

flotation tests were performed to evaluate the effect of wet versus dry grinding onthe separation of both pyrite and non-pyritic minerals from a coal sample obtainedfrom Pittsburgh No.8 seam. At coarser grind size, minus 600 microns, flotationselectivity was not significantly affected by grinding mode whereas at the finer grindsize, minus 75 microns, selectivity achieved with the wet ground feed was higherthan that obtained with dry grinding. Relatively poor separation achieved with drygrinding was attributed to dry state aggregation of mineral matter with coal. Ourstudy also showed that rejection of pyrite obtained by flotation was 15-25 percentless than that predicted by washability data whereas in the ease of non-pyriticminerals the difference from the predicted value was only 10--15 percent. Relativelypoor performance of flotation process with respect to the rejection of pyrite mayhave been caused by the natural hydrophobicity of pyrite.

72 D. UUetal.

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CCR. rECOVERY. Z

FIGURE 5 Effeci of grinding mode on the separation of pyrite from coal and comparison of washabilityand flotation selectivity curves for minus 75 micron feed.

Acknowledgements

The authors wish to acknowlcdge the United States Department of Energy (USDOE) for their financialsupport (DOE Projcct # DE-AC22-88PC88878), Prof. O. W. Fuerstenau and his group at Universityof California. Berkeley for providing ultimate and proximate analysis data, Praxis Engineers Inc.(Milpitas, California) for furnishing the washability data.

References

2.3.4.5.

7:

R. W. Perry and F. F. Aplan. Proc 1st Inter. Con! on Processing and Utilization of High SulfurCoals (Y. A. Attia. ed.). Elsevier. 1985. p. 215.J. D. Miller. Y. Ye and R. Jin. Coal Preparation. '.151 (1989).R. B. Read. C. Y. Meyers. L. R. Camp and R. Chan-Yu-King. Coal Preparation. 7. 85 (1989).D. C. Yang. Coal Preparation. 8. 19 (1990).K. J. Miller and R. F. Guzzo. II. Wet Grinding versus Dry Grinding for Deep Cleaning UllrafineCoal by Flotation. Department of Energy Technical Progress Report. DOEJPETcrrR-84/IO. Sep-tember 1984.D. W. Fuerstenau and K. V. S. Sastry. Coal Surface Control for Advanced Fine Coal Flotalio/t-.Univer.fily of California First Annual Progress Report. Department of Energy Project No. DE-AC22-88PC88878. February I~).K. J. Miller. Proc 1st Inter. Con! on Processing and Utilization of High Sulfur Coals (Y. A. Attia.ed.). Elsevier. 1985. D. 239.