Henry Krumb School of Mines, Columbia University, New York, NY 10027. (R~c~iv~d January 18, /991) ... Miller and Guzzo~ have compared the effect of dry ...
Coal Preparation, 1993 Vol. 13, pp. 63-72 Photocopying pennitted by license only @ 1993Gordon and Breach Science Publishers S.A. Printed in the United States of America
Effect of Wet Versus Dry Grinding on Rejection of Pyrite and Non-Pyritic Minerals from Pittsburgh No.8 Coal by Flotation D. UU, T. V. V ASUDEV AN, P. SOMASUNDARAN* and C. C. HARRIS
HenryKrumbSchoolof Mines, ColumbiaUniversity,New York,NY 10027 (R~c~iv~dJanuary 18, /991)
The effect of grinding mode on the efficiency of separation of pyrite and non-pyritic minerals from Pittsburgh No.8 seam coal by froth flotation was investigated. In the easeof minus 6(KJmicron 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 higher than that obtained using the dry grind and this was attributed to the dry state aggregation of coal with mineral matter. Comparison of flotation selectivity and washability curves showed that entrainment of fines due to water flow was the reason for the loss of selectivity during notation of coal from nonpyritic minerals. In the case of pyrite. in addition to entrainment. notation of pyrite due to its natural hydrophobicity 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 processesrequires fine grinding to liberate coal from pyrite and other ash-forming impurities. Froth flotation is the most commonly used technique for beneficiating fine coal. Since the mode of grinding, dry or wet, can affect differently the surface properties of coal and the gangue minerals it is important to understand its role in the flotation process. Majority of the recent investigations on flotation of fine coals have been conducted
with coal feedsobtainedby either dry grinding or wet grinding.1-1 Therefore, the effect of different grinding methods on the separation of coal from pyrite from other ash forming minerals is not clear from these studies. Miller and Guzzo~have compared the effect of dry and wet grinding methods for deep cleaning ultrafine coal by flotation. Their study was preliminary in nature, however, and also the effect of grinding method on pyrite rejection-the primary objective of recent
"To whom correspondence should be addressed
63
64
D. UU dill.
investigations on advanced coal cleaning-was not reported. Further. the effect of another important parameter. namely the grind size. was not considered in their investigation. In the current study. the effect of wet and dry grinding methods on the separation of pyrite and non-pyritic ash from coal has been tested on a comparative basis with both coarse (minus 600 microns) and fine grinds (minus 75 microns). EXPERIMENTAL Materials Coal sample used in this study is from Pittsburgh No.8 scam supplied by Rand F Coal Co., OH. The proximate, ultimate and sulfur forms analysesof this sample are listed in Table I. The 2-4 inch sample supplied was crushed to nominal 1/4 inch size in a laboratory roll crusher under argon atmosphere and the product was stored in polyethylene bags under argon. Typically, these samples were stored for about six months prior to testing. Methods Grinding 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 Rod mill dimensions and details of various experimental parameters used are given in Table 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 using Leeds and Northrup Microtrac particle size analyzer. Washability analysis was conducted using sink-float tests carried out in accordance with ASTM-D4371 proccdure.:I:'
TABLE I Analysisof the coal S8mpie-dry basis Proximate analysis and sulfur-forms (%)
Ultimate analysis (%)
C H N 0 S Ash Heatingvalue. Btu/lb
69.87 5.10 1.45 6.4()
4.58 12.W 12.420
Mositurc Volatile matter Fixed Carbon Ash Sulfatic S Pyritic S Organic S Total Sulfur
2.~ 35.55 51." 12.59 0.28 2.72 1.64 4.64
't95% passing ~ microns and 75 microns respectively. '*Samples for washability studies were prepared in an outside lahoratory (Geochemical Testing Laboratory. Somerset. PA) arranged hy Praxis Engineers. Inc. The size distribution of washability samples. however. were similar to those of flotation feeds (Table IV\.
'Y'
WET VERSUS DRY GRINDING
6S
TABLE II Rod mill dimensionsand grindingparameters 25.4cm (10") length x 22.8cm (9") i.d. 22.8cm (9") length x 1.9cm (3/4.) i.d.. # I 22.8cm (9") length x 1.6cm (5/8")i.d.. # 2 22.8cm (9") length x 1.3cm (1/2")i.d.. # 3
Rod mill dimensions Rod dimensions (three different sizes) Material: Mill and rods Number of rods: Dry grinding-minus Wet grinding-minus Dry grinding-minus Wet grinding-minus Mill speed ~: Solids Liquid (wet grinding) Grindinl! time: Dry grinding-minus Wet grinding-minus Dry grinding-minus Wet grinding-minus Grinding medium
316 stainless steel 600 microns 6(XImicrons 75 microns 75 microns
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% passing0.635 cm 700 cc
6(X)microns 600 microns 75 microns 75 microns
4 minutes and 50 seconds 3 minutes 31 minutes 18 minutes Air
Flotation: Flotation tests were conducted in a two-liter Denver D-1 flotation machine with provisions for mechanical froth removal and air flow control. The ground product from the rod mill was divided in to four fractions using a mechanical splitter. Each of these fractions contained about 125 grams of solids and was used as the flotation feed. After charging the cell with the solids (42 weight percent slurry in the case of the wet ground product), a specified amount of distilled water was added so that the total volume of liquid was 1000cc. The slurry was agitated for three minutes prior to the addition of the reagents (preconditioning step). In the case of the dry ground product the slurry had to be preconditioned for an additional three minutes (to completely wet the sample) in order to obtain reproducible results. Following preconditioning, a known amount of collector was added and the slurry was agitated for one minute after which a specified amount of frother was added. Conditioning with the reagents was carried out for an additional five minutes after which flotation was started by introducing air in to the cell and simultaneously switching on the froth removal paddles. Flotation was carried out for five minutes following which the float and sink products were filtered, dried, weighed and analyzed for ash and pyrite contents. Details of flotation machine parameters and other pertinent variables are given in Table III. Analysis: The ash and pyritic sulfur contents of the coal sampleswere determined using the ASTM procedure (D 3174-82for ash and D 2492-84for pyrite). The non-
68
D. UUetol. TABLE IV Particle size distribution of various grinds and washability samples
Minus 600 Minus 600 Minus 600
340 340
180 195
32 42 50
21 23 27
62 62 W
Minus 75 Minus 75 Minus 75
I
..1 5.6 5.5
*-subscript represents percentile value
not this difference can produce significant effect on the separation efficiency, selectivity curves obtained for dry ground feeds of minus 600 microns and minus 75 microns are compared in Figure 3. It can be seen from this figure that there is no significant difference in the separation achieved although the minus 600 micron feed is much coarser (average particle size = 180microns) than the minus 75 micron feed (average particle size = 21 microns). This suggests that in the size range studied, entrainment is independent of particle size. The other possibility for the
N
. ~ tU W w a:
~ u
-a: >.-
a.. I 6 z
00
8J
70
~
S]
m
100
REcovmY
FIGURE 2 Effect of the grinding mode on the separation of non-pyritic minerals from coal and comparison of washability and flotation selectivity curves for minus 75 micron feed.
WET VERSUSDRY GRINDING
69
~
. ~ ..-c u w to; '&
ffi a: u too 0: >0..
I
§
m
m
M
CCR; ~YeRY.
m
8J
100
~
FIGURE 3 Comparison of flotation selectivity and washability curves for the dry ground minus 600 micron and minus 75 micron feeds.
overlap of the selectivity curves obtained with the coarse and the fine grinds is that finer grinding could result in an increase of both the liberation and the entrainment with the two effects cancelling each other out. This, however, is unlikely due to the following reason. A decrease of particle size from 180 microns (average size of the dry coarse grind) to 21 microns (average size of the dry fine grind) results in 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 of particle 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 (Table IV), heterocoagulation effects can be considered as not significant. To examine whether or not the degree of liberation can contribute to the observed difference between the separation efficiencies achieved with dry and wet ground minus 75 micron feeds, the washability curves of minus 600 micron feed and minus 75 micron feed are compared in Figure 3. It can be seen from this figure that the degree of liberation of the non-pyritic mineral matter in the minus 75 micron feed is only marginally higher than in the minus 600 micron feed. This suggeststhat differences in the liberation characteristics also do not contribute in any significant manner to the observed difference in the separation efficiency.
70
D. UUet.t
Other possible reasons for the relatively poor separation achieved with the dry ground feed are the oxidation of coal surface and dry state aggregation of the mineral matter with coal during grinding. To determine whether or not oxidation of 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 30 weight percent hydrogen peroxide (oxidizing agent) was added to the mill and the slurry conditioned for 30 minutes maintaining the mill speed at the same level as that used during grinding (56 rpm). Flotation results obtained with the chemically oxidized feed are compared with the unoxidized feed in Table V. The results show that, although four times higher dosage of frother is required for oxidized coal to achieve a yield equivalent to that of unoxidized coal the ash content of the froth products are the same indicating that oxidization of coal surface diminishes only the flotability of coal and not the selectivity. Above discussions suggest that dry state aggregation of coal with thc mineral matter. which can result in the formation of hard agglomerates, is the most probable reason for the relatively poor separation achieved with the dry ground feed. It is possible 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 rejection Pyrite flotation selectivity curves obtained for minus ~ micron and minus 75 micron feeds are shown in Figures 4 and 5 respectively. Corresponding washability curves for pyritic sulfur are also shown in these figures for comparison. It can be seen from Figure 4 that, as observed for non-pyritic ash rejection, the mode of grinding does not playa significant role in determining the selectivity achieved during flotation in the caseof minus ~ micron feed whereas with minus 75 micron feed higher selectivity can be obtained with the wet ground feed in comparison to that with the dry ground feed. Another important observation is that the shifts of the selectivity curves obtained for pyrite from the washability curves are even farther away than the shifts in the curves for non-pyritic ash (compare Figures 1 and 2 with Figures 4 and 5 respectively). For a given coal recovery, the difference between the washability and the selectivity curves in terms of percentage non-pyritic ash rejections is 10-15 percent compared to 15-25 percent for pyrite. The additional loss of selectivity in the case of pyrite may be due to the natural hydrophobicity of pyrite as well as due to incomplete liberation.
TABLE V Effect of hydrogenperoxideadditionon thc notation pcrformanceof wct minus75 micronfced from PittsburghNo.8 coal ('.oil. g/kg
Frot. g/kg
H,O~ wt%
Rot. Yield wt%
Ash in float wt%
0.0 3.0
n.s n.o
5.1 5.1
~
WET VERSUS DRY GRINDING
71
ro
~ . z a tU W
8)
~ ~ ~ u
.:
a>
ta: t
8)
50
m
70
8)
9J
100
COAL RECOVERY~ Z FIGURE 4 Effect of grinding mode on the separation of pyrite from coal and comparison ofwashability and flotation selectivity curvcs for minus 600 micron feed.
CONCLUSIONS flotation tests were performed to evaluate the effect of wet versusdry grinding on the separation of both pyrite and non-pyritic minerals from a coal sample obtained from Pittsburgh No.8 seam. At coarser grind size, minus 600 microns, flotation selectivity was not significantly affected by grinding mode whereas at the finer grind size, minus 75 microns, selectivity achieved with the wet ground feed was higher than that obtained with dry grinding. Relatively poor separation achieved with dry grinding was attributed to dry state aggregation of mineral matter with coal. Our study also showed that rejection of pyrite obtained by flotation was 15-25 percent less than that predicted by washability data whereas in the ease of non-pyritic minerals the difference from the predicted value was only 10--15percent. Relatively poor performance of flotation process with respect to the rejection of pyrite may have been caused by the natural hydrophobicity of pyrite.
72
D. UUetal.
1
z a
.
tJ w ..., w a:
~ LL ~
U t~ 0.. 50
~
70
8)
g)
1m
CCR. rECOVERY. Z FIGURE 5 Effeci of grinding mode on the separation of pyrite from coal and comparison of washability and flotation selectivity curves for minus 75 micron feed.
Acknowledgements The authors wish to acknowlcdge the United States Department of Energy (USDOE) for their financial support (DOE Projcct # DE-AC22-88PC88878), Prof. O. W. Fuerstenau and his group at University of 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 Processingand Utilization of High Sulfur Coals (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 Ullrafine Coal by Flotation. Department of Energy Technical Progress Report. DOEJPETcrrR-84/IO. September 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. DEAC22-88PC88878. February I~). K. J. Miller. Proc 1st Inter. Con! on Processingand Utilization of High Sulfur Coals (Y. A. Attia. ed.). Elsevier. 1985. D. 239.
