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oleophilicity of pyrite on the separation of pyrite from coal by flotation
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Int. J. Miner. Process. 41 (1994) 227-238
ELSEVIER
L
Role of collector and frother, and of hydrophobicity/ oleophilicity of pyrite on the separation of pyrite from coal by flotation D. Liu, P. S o m a s u n d a r a n * Henry Krumb School of Mines, Columbia University New York, NY 10027 USA (Received 7 August 1992; accepted after revision 21 October 1993 )
Abstract
The effect ofdodecane and three different alcoholic frothers [n-butylalcohol, n-hexylalcohol and methylisobutylcarbinol (MIBC)], on the flotation separation of pyrite from Pittsburgh No. 8 coal was investigated. Addition of dodecane resulted in a decrease of selectivity and this was attributed on the basis of the induction time measurements to the smaller difference between the oleophilicities of coal and pyrite than the difference between their hydrophobicities. The results of induction time measurements also showed pyrite to be hydrophilic at the air-water interface and hydrophobic at the oil-water interface. Frother consumption was found to be dependent on the molecular weight and chemical structure with MIBC, the larger and more complex one, being the most effective. However, the frother type was found to have no measurable effect on the separation itself of pyrite and non-pyritic minerals from coal.
1. Introduction Removal of pyrite from coal is currently the most economic and efficient way for coal desulfurization. Flotation separation, which exploits the differences between the interfaeial properties of coal and those of mineral impurities, is a promising method for fine coal cleaning. In coal flotation, alcoholic reagents such as MIBC and pine oil are usually used as frothers, and fuel oil as the collector. Use of the collector, usually in conjunction with the frother, is known to enhance coal flotation significantly (Horsely and Smith, 1951 ). Also, it has been reported that the pyrite content in the froth concentrate increased with the collector and the *To whom correspondence should be addressed. 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. Allrightsreserved
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D. Liu, P. S o m a s u n d a r a n ~Int. J. Min. Process. 41 (1994) :~7~,_ ~ 2~.,,,
frother addition (Aplan, 1977). However, the underlying mechanism of pyrite carry over and the individual roles of collector and frother in the process are not clear. A study of the natural hydrophobicity/oleophilicity of pyrite (Kocabag et al., 1990) showed that the mineral remained hydrophilic when contacted with a gas bubble, but became hydrophobic (or oleophilic) in the presence of an oil drop. Since oily reagents do adsorb on oleophilic surfaces due to hydrophobic interactions, the natural oleophilicity of pyrite, relative to that of coal, can influence the flotation separation of pyrite from coal. In the present work, the hydrophobicity and oleophilicity of coal-pyrite, relative to those of coal, have been monitored using induction time measurement and its role on the separation of pyrite from coal by flotation was investigated. The effects of dodecane and alcoholic frothers of different molecular weight and structure on both the reagent consumption and the selectivity has been examined.
2. Experimental 2.1. Materials Coal Coal sample used in this study is a Pittsburgh No. 8 seam supplied by R and F Coal Co., OH. The proximate, ultimate and sulfur forms analyses of this sample are listed in Table 1. The 2-4 inch sample supplied was crushed to nominal 1/4 inch size in a laboratory roll crusher under an argon atmosphere, and the product was stored under argon. Minerals Coal-pyrite was obtained from a sample rejected from Pocahontas No. 3 coal by Deister tables, supplied by Island Creek Coal Company, Oakwood, VA. The coal refuse supplied has a high content of pyrite and was concentrated further by Table 1 Analysis of the coal sample - - dry basis a Ultimate analysis (%)
C H N O S Ash Heating value, Btu/lb
Proximate analysis and sulfur-forms analysis (%) 69.87 5.10 1.45 6.40 4.58 12.60
12,420
Moisture Volatile matter Fixed carbon Ash Sulfatic S Pyritic S Organic S Total sulfur
2.30 35.55 51.86 12.59 0.28 2.72 1.64 4.64
aData provided by Department of Mineral Engineering, University of California at Berkeley.
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
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washing, sieving, shaking, and magnetic separation. The 60 × 100 mesh size fraction of the final concentrate contained about 90% by weight of pyrite. This sample was stored under argon atmosphere in a freezer. Before use, the stored sample was washed three times with triply distilled water (TDW) to remove the suspected oxidation products and was ground in TDW. A desirable size fraction obtained from the ground sample was used as coal-pyrite sample for induction time measurements. Ore pyrite of > 99.5% purity was purchased from VWR Scientific Co. The 50 × 100 mesh size of this sample was stored under argon atmosphere in a freezer. The procedures for sample preparation and usage of the ore-pyrite sample were the same as those for coal-pyrite. Quartz of > 99.5% purity was purchased from Johnson Matthey Inc. and was used as received.
Reagents n-Dodecane and n-hexylalcohol, specified to be of > 99% purity, were purchased from Aldrich Chemicals. Methylisobutylcarbinol (MIBC) and n-butylalcobol, also specified to be of > 99% purity, were purchased from American Tokyo Kesai and Fisher Scientific respectively.
2.2. Methods Flotation feed preparation A laboratory rod mill was used to obtain a minus 200 mesh wet ground feed for flotation. Rod mill dimensions and details of various grinding parameters are given in another paper (Liu et al., 1994). The ground product from the rod mill was divided into four fractions using a mechanical splitter. Each of these fractions contained about 125 grams of solids and about 400 grams of water (more water was added after grinding for cleaning purpose ). This sample (coal slurry) was stored/aged in air at ambient temperature for about two hours, and thereafter was used as flotation feed.
Flotation A two-liter Denver D-1 impeller/stator assembly flotation machine, with provisions for mechanical froth removal and air flow control, was used to conduct batch flotation tests. Details of flotation machine parameters and other pertinent variables can be found in another paper (Liu et al., 1994). Before a flotation test was conducted, the aged coal slurry was transferred into the cell, and about 1300 grams of distilled water was added so that the total volume of the slurry was 1800 ml. The slurry was then agitated for three minutes prior to the addition of the flotation reagents. Following this preconditioning, desirable amounts of the coal collector and a frother were added in sequence. After each reagent addition (within 30 seconds), the slurry was conditioned for one minute before the next was added. Conditioning with the reagents was continued until the total conditioning time, including the preconditioning time, reached twelve minutes. The natural pH of the coal slurry was measured, being
230
D. Liu, P. Somasundaran / lnt. J Min. Process. 41 (1994)22 7-238
about pH 4. Flotation was then started by introducing air into the cell while simultaneously switching on the froth removal paddles. Flotation was carried out for five minutes; whereupon, the float and sink products were filtered, dried, weighed, and analyzed for pyrite and ash contents. Most of the flotation tests were regular one stage operation. In some cases, two stage flotation tests were conducted. The first stage flotation for the two stage tests was the same as that for the one stage tests. In the second stage, the froth concentrate obtained from the first stage flotation was reconditioned in frother solution (at its natural pH of about 5.5 ) and floated again. The percentage of froth product for the second stage flotation was controlled so that it was about the same as that for the first stage. No oily reagents (frothing reagents only) were used in the second stage of flotation.
Hydrophobicity /oleophilicity evaluation Hydrophobicity and oleophilicity of coal and pyrite were determined by measuring their induction times using an APT-100 type induction timer. A captive bubble/drop, held at the tip of a glass tube, was pushed downward through the aqueous solution and kept it in contact with the bed of particles for a pre-selected period of time, which could have ranged from 1 to 4000 milliseconds. After each contact, the bubble/drop was withdrawn to the original position and was visually examined to determine if any particles had become attached to the bubble/drop (or if a fruitful contact between the bubble/drop and the particles had occurred). The important device parameters that could have affected the measured induction time were the voltage which controlled the speed of vertical movement, bubble/drop size (fixed at about 1.5 m m diameter), the distance travelled by the bubble/drop (0.2 to 0.3 mm ), and the distance between the bubble and the bed of particles (about 0.1 m m ) . The induction time was taken as the contact time for which fifty percent of the contacts were fruitful. A minimum of fifteen measurements (contacts) for each selected contact time was attempted. The 100 × 140 mesh fractions of the research samples of coal, coal-pyrite, ore pyrite, or quartz were used in the induction time measurements. The samples were first washed triply distilled water (TDW) and then conditioned for ten minutes in TDW at a solid concentration of 10 percent, pH was controlled throughout the conditioning time. The conditioned slurry was transferred to a special sample cell for induction time measurements. The induction times measured for the attachment of particles to an air bubble or to an oil (n-dodecane) drop were used to evaluate the particle hydrophobicity or oleophilicity.
3. Results and discussion In this study, the results of the flotation tests have been presented in the form of selectivity curves which are plots of percent pyrite or non-pyritic minerals rejection versus percent coal. Increase in coal substance recovery along the selectivity curve is obtained by increasing the frother dosage, while maintaining the col-
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
231
lector dosage and other parameters the same. A shift of the curve towards the upper left corner indicates increased selectivity.
3. I. Effect of dodecane on the separation of pyrite and non-pyritic minerals from coal The effect of dodecane addition on the separation of pyrite from coal is shown in Fig. 1. It can be seen that the selectivity decreases with an increase in dodecane dosage. The effect is more pronounced in two stage tests (Fig. 2). Decreased selectivity obtained at higher dodecane dosage can be due to: a) decrease in the difference between the hydrophobicities of coal and pyrite due to dodecane adsorption and/or b) increased pyrite entrainment due to possible increase of froth viscosity caused by the presence of the oily reagent. The latter possibility, however, seems unlikely since dodecane dosage has no effect on the separation of non-pyritic minerals from coal (Figs. 3 and 4). Since non-pyritic minerals in coal such as carbonates and clays are hydrophilic, dodecane is not likely to adsorb on these minerals and, therefore, has no effect on the separation of these minerals from the coal.
3.2. Hydrophobicity/oleophilicity of coal and pyrite Results of the flotation studies suggest that the difference between the hydrophobicities of coal and pyrite decreases with increase in dodecane addition. Adsorption of dodecane on the surfaces of both coal and pyrite can, therefore, be expected to occur under the coal flotation conditions. Adsorption behavior of 90
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Fig. 1. Effect of dodecane addition on the separation of pyrite from coal - - single stage flotation tests.
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oily agents, such as dodecane, from aqueous solution on mineral surfaces is known to be determined not only by the natural hydrophobic characteristics of minerals but also by their oleophilic properties. Therefore, in order to understand the mechanisms involved in determining the effect of oil addition on pyrite separa-
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tion, both the natural hydrophobicity and the natural oleophilicity of coal and pyrite warranted a study. It should be mentioned that the olcophilicity of minerals at oil-water interfaces is usually considered also as a kind of hydrophobicity, which is different from the hydrophobicity of the minerals observed at airwater interfaces. To distinguish them from each other, the hydrophobicity observed at oil-water interfaces is defined as oleophilicity of minerals and the hydrophobicity observed at air-water interfaces as intrinsic-hydrophobicity. Induction time measurements were carried out to determine the relative intrinsic-hydrophobicities and oleophilicities of coal and pyrite. The intrinsic-hydrophobicities of the minerals were evaluated by the induction times measured for bubble-particle attachment, the oleophilicities were determined by the induction times for oil-particle attachment. The induction times of coal, coal pyrite, ore pyrite, and quartz for bubbleparticle attachment are given in Table 2. As expected, coal is found to be the most hydrophobic mineral with the shortest induction time, while quartz is the most hydrophilic mineral with the longest induction time. The hydrophobicity of ore pyrite is critical because it is more hydrophilic than coal, but more hydrophobic than quartz. Coal pyrite is found to be more hydrophobic than ore pyrite, possibly due to the presence of microsize coal particles as impurities on the surface of coal pyrite. Table 2 also gives the induction times of coal, coal pyrite, ore pyrite, and quartz measured using an oil drop. It can been seen that the induction time for each of the minerals obtained with oil drop is much shorter than that with air bubble. This suggests that minerals such as coal and pyrite are more hydrophobic at the oil-water interface than at the air-water interface. Since the induction times
D. Liu, P. Somasundaran ~Int. J. Min. Process, 41 (1994) 227-238
234
Table 2 Hydrophobicity and oleophilicity of coal, coal-pyrite, ore pyrite, and quartz: induction time measurements pH
Coal (ms) Air
4 5 6
32.1 31.5 31.4
Coal-pyrite (ms)
Ore pyrite ( ms )
Quartz ( ms )
Oil
Air
Oil
Air
Oil
Air
Oil
4.0
89.6 90.6 91.2
10.3
303
21.2
612
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COAL RECOVFRY, %
Fig. 5. Effect of frother type on reagent consumption.
of coal-pyrite and ore pyrite measured with oil drops ( 10.3 and 21.2 milliseconds respectively) are even shorter than that of coal measured with air bubbles (31.4 milliseconds), pyrite (especially the coal-pyrite) can be oleophilic in the presence of oil. A close examination of the work done by Kocabag et al. (1990) suggests that the above observation on hydrophobicity/oleophilicity of the pyrite samples (slightly oxidized) agrees with Kocabag's results, although the authors did not clearly identify the distinction between the two affinities. Also, it is found that the pH did not affect the induction times of coal and coal-pyrite to a large extent in the pH range of 4 to 6 (Table 2).
3.3. Effect offrothers on coal flotation recovery The effect of different frothers (MIBC, hexanol and butanol) on coal flotation recovery is shown in Fig. 5. It can be seen that the level of consumption of n-
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
235
butylalcohol is higher than those of the alcohols with higher molecular weights, n-hexylalcohol and MIBC. Also, the consumption of n-hexylalcohol is higher than that of MIBC, which due to branching can cover a larger area than the former. These results suggest that under the present test conditions, branching and/or larger molecules are more efficient as frothers possibly due to their higher frothing ability. Also, Frangiskos et al. (1960) have reported that frothing agents can be effective only if adsorbed at the gas-liquid interface and will lose their effectiveness if removed from solution by adsorption on the coal surface. Furthermore, they have shown that the frother adsorption process consists of rapid chemisorption on the external surface of the coal particle followed by slower physical adsorption by diffusion into the pores. The molecular dimensions of the frother, relative to the pore size, will determine the rate of penetration, as well as the extent of frother loss by sorption into the pores. Both of these factors will be lower for the larger frother. Thus, it is possible that larger amounts of n-butylalcohol may have penetrated into the pores as compared to the higher molecular weight alcohols, n-hexylalcohol and methylisobutylcarbinol (MIBC). Similarly, n-hexylalcohol can be expected to adsorb on coal surface more than MIBC.
3.4. Effect offrothers on the separation of pyrite and non-pyritic minerals The effect of different frothers (MIBC, hexanol, and butanol ) on the efficiency of separation of pyrite from coal is illustrated in Fig. 6. It can be seen that selectivities obtained with these three reagents are almost the same. This suggests that the wettabilities of coal and pyrite are either not influenced or affected to the same extent by the frothers. Frangiskos et al. (1960) have reported that frothing 95
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- \
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Fig. 6. Effect o f f r o t h e r t y p e o n s e p a r a t i o n o f p y r i t e f r o m c o a l - - s i n g l e s t a g e f l o t a t i o n tests.
236
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238 85
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agents, even after adsorption, have tittle influence on the wettability of coal since the increase in hydrophobicity due to their adsorption is masked by the large contact angle of the natural coal surface. Selectivity for the separation of non-pyritic minerals was also not affected by the type of frother used (Fig. 7). This suggests that the type of the alcohols has no measurable effect on water flow out of the cell since entrainment, and thus separation efficiency, can be expected to change if water flow changes.
3.5. Role of relative hydrophobicity and oleophilicity of pyrite to those of coal on pyrite separation It is recognized that the natural surface properties of a mineral are primarily determined by the molecular structure of the solid and the types of bonding involved. Pyrite structureisknown to resemble that of the simple cubic NaCI structure, where CI is replaced by the S-S group (Leja, 1982; Wyckoff, 1963). In the structure,the bonding is wholly covalent. It is believed that the covalent bonds present on the surface structure of pyrite are responsible for the hydrophobic characteristicof the surface in comparison to a non-covalent bonded mineral, such as quartz. O n the other hand, since the covalent bonding in pyrite is not as symmetric as that in an oil-typemolecule, pyriteshould be more hydrophilic than coal. Because an oil phase such as that of n-dodecane is known to be more hydrophobic than a gas phase, the attraction,due to hydrophobic interaction,between the oil phase and a slightly h y ~ b i c surface of a solid such as pyrite could be
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
237
stronger than that between a gas phase and the solid surface. Thus, it can be understood that pyrite is oleophilic (appearing to be hydrophobic) at the oilwater interface and hydrophilic at the air-water interface. In the case of flotation using only a frother, such as MIBC or hexanol, as the flotation reagent, it is not expected to adsorb significantly on the surface of pyrite. Also, the adsorption of the frother reagent on the coal surface does not cause significant change in the hydrophobicity of coal, as has been discussed before. In other words, the hydrophobicities of coal and pyrite have not been modified to a measurable extent by frother alone. Therefore, the pyrite separation in this case is basically determined by the intrinsic-hydrophobic differences between coal and pyrite. When an oily reagent such as dodecane is used in conjunction with a frother, the oily reagent is expected to adsorb on the surfaces of coal and pyrite because both coal and pyrite were found to be oleophilic. Under these conditions, the separation could be affected by the differences in oleophilicity between coal and pyrite. Since such difference in oleophilicity (6.3 milliseconds between their induction times) between coal and pyrite was found to be much less than the difference in their intrinsic-hydrophobicity (59.8 milliseconds), the separation by the difference in oleophilicity should be more difficult than by that in intrinsic-hydrophobicity.
4. Conclusions I. Addition of an oily agent, n-dodecane, resulted in a decrease in flotationselectivity for removal of pyrite from coal and appeared to have no effect on the separation of non-pyritic minerals. 2. Pyrite (especiallycoal-pyrite) was found to be hydrophilic at the air-water interface and oleophilic (appearing to be hydrophobic) at the oil-water interface. Also, the difference between oleophilicitiesof coal and coal-pyrite was found to be much smaller than that between their intrinsichydrophobicities. This is proposed to be the reason for the decrease in the selectivityfor pyrite separation with the addition of dodecane, since an oily reagent such as dodecane can adsorb on the pyrite surface under the coal flotationconditions. 3. It was found that only the recovery of coal depended on the type of the frother used, while the frother type had no effecton the selectivityitself.Lower effectiveness of frothers with smaller molecular size could be attributed to their increased loss by adsorption into the pores.
Acknowledgement The authors acknowledge the United States Department of Energy and University of California at Berkeley (primary contractor) for financial support (DEAC22-88PC88878) and Prof. C.C. Harris of Columbia University for helpful suggestions.
238
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-23~
References Aplan, F.F., 1977. Use of the flotation process for desulfurization of coal. In: T.D. Wheelock (Editor), ACS Symposium Series No. 64: Coal Desulfurization. Am. Chem. Society, Washington, DC. Frangiskos, N.Z., Harris, C.C. and Jowett, A., 1960. The adsorption of frothing agents on coal. In: Proceedings of the International Surface Activity Congress, Cologne. Vol. 4, pp. 404-416. Horsely, R.M. and Smith, H.G., 1951. Principles of coal flotation. Fuel, 30 (3): 54-63. Kocabag, D., Shergold, H.L. and Kelsall, G.H., 1990. Natural oleophilicity/hydrophobicityof sulphide minerals, II. Pyrite. Int. J. Miner. Process., 29:211-219. Leja, J., 1982. Surface Chemistry of Froth Flotation. Plenum Press, New York, pp. 133-150. Liu, D., Somasundaran, P., Vasudevan, T.V. and Harris, C.C., 1994. Role ofpH and dissolved mineral species in Pittsburgh No. 8 coal flotation system - - I. The floatability of coal. Int. J. Miner. Process., 41: 201-214. Moudgil, B.M., 1983. Effect of polyacrylamide and polyethylene oxide polymers on coal flotation. Colloids Surf., 8 (2): 225-229. Wyckoff, R.W.G., 1963. Crystal Structures, Vol. 1 ( 2nd Ed. ). Interscience, New York, pp. 346-347.
Int. J. Miner. Process. 41 (1994) 227-238
ELSEVIER
L
Role of collector and frother, and of hydrophobicity/ oleophilicity of pyrite on the separation of pyrite from coal by flotation D. Liu, P. S o m a s u n d a r a n * Henry Krumb School of Mines, Columbia University New York, NY 10027 USA (Received 7 August 1992; accepted after revision 21 October 1993 )
Abstract
The effect ofdodecane and three different alcoholic frothers [n-butylalcohol, n-hexylalcohol and methylisobutylcarbinol (MIBC)], on the flotation separation of pyrite from Pittsburgh No. 8 coal was investigated. Addition of dodecane resulted in a decrease of selectivity and this was attributed on the basis of the induction time measurements to the smaller difference between the oleophilicities of coal and pyrite than the difference between their hydrophobicities. The results of induction time measurements also showed pyrite to be hydrophilic at the air-water interface and hydrophobic at the oil-water interface. Frother consumption was found to be dependent on the molecular weight and chemical structure with MIBC, the larger and more complex one, being the most effective. However, the frother type was found to have no measurable effect on the separation itself of pyrite and non-pyritic minerals from coal.
1. Introduction Removal of pyrite from coal is currently the most economic and efficient way for coal desulfurization. Flotation separation, which exploits the differences between the interfaeial properties of coal and those of mineral impurities, is a promising method for fine coal cleaning. In coal flotation, alcoholic reagents such as MIBC and pine oil are usually used as frothers, and fuel oil as the collector. Use of the collector, usually in conjunction with the frother, is known to enhance coal flotation significantly (Horsely and Smith, 1951 ). Also, it has been reported that the pyrite content in the froth concentrate increased with the collector and the *To whom correspondence should be addressed. 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. Allrightsreserved
SSDIO301-7516(93)EO065-S
228
D. Liu, P. S o m a s u n d a r a n ~Int. J. Min. Process. 41 (1994) :~7~,_ ~ 2~.,,,
frother addition (Aplan, 1977). However, the underlying mechanism of pyrite carry over and the individual roles of collector and frother in the process are not clear. A study of the natural hydrophobicity/oleophilicity of pyrite (Kocabag et al., 1990) showed that the mineral remained hydrophilic when contacted with a gas bubble, but became hydrophobic (or oleophilic) in the presence of an oil drop. Since oily reagents do adsorb on oleophilic surfaces due to hydrophobic interactions, the natural oleophilicity of pyrite, relative to that of coal, can influence the flotation separation of pyrite from coal. In the present work, the hydrophobicity and oleophilicity of coal-pyrite, relative to those of coal, have been monitored using induction time measurement and its role on the separation of pyrite from coal by flotation was investigated. The effects of dodecane and alcoholic frothers of different molecular weight and structure on both the reagent consumption and the selectivity has been examined.
2. Experimental 2.1. Materials Coal Coal sample used in this study is a Pittsburgh No. 8 seam supplied by R and F Coal Co., OH. The proximate, ultimate and sulfur forms analyses of this sample are listed in Table 1. The 2-4 inch sample supplied was crushed to nominal 1/4 inch size in a laboratory roll crusher under an argon atmosphere, and the product was stored under argon. Minerals Coal-pyrite was obtained from a sample rejected from Pocahontas No. 3 coal by Deister tables, supplied by Island Creek Coal Company, Oakwood, VA. The coal refuse supplied has a high content of pyrite and was concentrated further by Table 1 Analysis of the coal sample - - dry basis a Ultimate analysis (%)
C H N O S Ash Heating value, Btu/lb
Proximate analysis and sulfur-forms analysis (%) 69.87 5.10 1.45 6.40 4.58 12.60
12,420
Moisture Volatile matter Fixed carbon Ash Sulfatic S Pyritic S Organic S Total sulfur
2.30 35.55 51.86 12.59 0.28 2.72 1.64 4.64
aData provided by Department of Mineral Engineering, University of California at Berkeley.
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
229
washing, sieving, shaking, and magnetic separation. The 60 × 100 mesh size fraction of the final concentrate contained about 90% by weight of pyrite. This sample was stored under argon atmosphere in a freezer. Before use, the stored sample was washed three times with triply distilled water (TDW) to remove the suspected oxidation products and was ground in TDW. A desirable size fraction obtained from the ground sample was used as coal-pyrite sample for induction time measurements. Ore pyrite of > 99.5% purity was purchased from VWR Scientific Co. The 50 × 100 mesh size of this sample was stored under argon atmosphere in a freezer. The procedures for sample preparation and usage of the ore-pyrite sample were the same as those for coal-pyrite. Quartz of > 99.5% purity was purchased from Johnson Matthey Inc. and was used as received.
Reagents n-Dodecane and n-hexylalcohol, specified to be of > 99% purity, were purchased from Aldrich Chemicals. Methylisobutylcarbinol (MIBC) and n-butylalcobol, also specified to be of > 99% purity, were purchased from American Tokyo Kesai and Fisher Scientific respectively.
2.2. Methods Flotation feed preparation A laboratory rod mill was used to obtain a minus 200 mesh wet ground feed for flotation. Rod mill dimensions and details of various grinding parameters are given in another paper (Liu et al., 1994). The ground product from the rod mill was divided into four fractions using a mechanical splitter. Each of these fractions contained about 125 grams of solids and about 400 grams of water (more water was added after grinding for cleaning purpose ). This sample (coal slurry) was stored/aged in air at ambient temperature for about two hours, and thereafter was used as flotation feed.
Flotation A two-liter Denver D-1 impeller/stator assembly flotation machine, with provisions for mechanical froth removal and air flow control, was used to conduct batch flotation tests. Details of flotation machine parameters and other pertinent variables can be found in another paper (Liu et al., 1994). Before a flotation test was conducted, the aged coal slurry was transferred into the cell, and about 1300 grams of distilled water was added so that the total volume of the slurry was 1800 ml. The slurry was then agitated for three minutes prior to the addition of the flotation reagents. Following this preconditioning, desirable amounts of the coal collector and a frother were added in sequence. After each reagent addition (within 30 seconds), the slurry was conditioned for one minute before the next was added. Conditioning with the reagents was continued until the total conditioning time, including the preconditioning time, reached twelve minutes. The natural pH of the coal slurry was measured, being
230
D. Liu, P. Somasundaran / lnt. J Min. Process. 41 (1994)22 7-238
about pH 4. Flotation was then started by introducing air into the cell while simultaneously switching on the froth removal paddles. Flotation was carried out for five minutes; whereupon, the float and sink products were filtered, dried, weighed, and analyzed for pyrite and ash contents. Most of the flotation tests were regular one stage operation. In some cases, two stage flotation tests were conducted. The first stage flotation for the two stage tests was the same as that for the one stage tests. In the second stage, the froth concentrate obtained from the first stage flotation was reconditioned in frother solution (at its natural pH of about 5.5 ) and floated again. The percentage of froth product for the second stage flotation was controlled so that it was about the same as that for the first stage. No oily reagents (frothing reagents only) were used in the second stage of flotation.
Hydrophobicity /oleophilicity evaluation Hydrophobicity and oleophilicity of coal and pyrite were determined by measuring their induction times using an APT-100 type induction timer. A captive bubble/drop, held at the tip of a glass tube, was pushed downward through the aqueous solution and kept it in contact with the bed of particles for a pre-selected period of time, which could have ranged from 1 to 4000 milliseconds. After each contact, the bubble/drop was withdrawn to the original position and was visually examined to determine if any particles had become attached to the bubble/drop (or if a fruitful contact between the bubble/drop and the particles had occurred). The important device parameters that could have affected the measured induction time were the voltage which controlled the speed of vertical movement, bubble/drop size (fixed at about 1.5 m m diameter), the distance travelled by the bubble/drop (0.2 to 0.3 mm ), and the distance between the bubble and the bed of particles (about 0.1 m m ) . The induction time was taken as the contact time for which fifty percent of the contacts were fruitful. A minimum of fifteen measurements (contacts) for each selected contact time was attempted. The 100 × 140 mesh fractions of the research samples of coal, coal-pyrite, ore pyrite, or quartz were used in the induction time measurements. The samples were first washed triply distilled water (TDW) and then conditioned for ten minutes in TDW at a solid concentration of 10 percent, pH was controlled throughout the conditioning time. The conditioned slurry was transferred to a special sample cell for induction time measurements. The induction times measured for the attachment of particles to an air bubble or to an oil (n-dodecane) drop were used to evaluate the particle hydrophobicity or oleophilicity.
3. Results and discussion In this study, the results of the flotation tests have been presented in the form of selectivity curves which are plots of percent pyrite or non-pyritic minerals rejection versus percent coal. Increase in coal substance recovery along the selectivity curve is obtained by increasing the frother dosage, while maintaining the col-
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
231
lector dosage and other parameters the same. A shift of the curve towards the upper left corner indicates increased selectivity.
3. I. Effect of dodecane on the separation of pyrite and non-pyritic minerals from coal The effect of dodecane addition on the separation of pyrite from coal is shown in Fig. 1. It can be seen that the selectivity decreases with an increase in dodecane dosage. The effect is more pronounced in two stage tests (Fig. 2). Decreased selectivity obtained at higher dodecane dosage can be due to: a) decrease in the difference between the hydrophobicities of coal and pyrite due to dodecane adsorption and/or b) increased pyrite entrainment due to possible increase of froth viscosity caused by the presence of the oily reagent. The latter possibility, however, seems unlikely since dodecane dosage has no effect on the separation of non-pyritic minerals from coal (Figs. 3 and 4). Since non-pyritic minerals in coal such as carbonates and clays are hydrophilic, dodecane is not likely to adsorb on these minerals and, therefore, has no effect on the separation of these minerals from the coal.
3.2. Hydrophobicity/oleophilicity of coal and pyrite Results of the flotation studies suggest that the difference between the hydrophobicities of coal and pyrite decreases with increase in dodecane addition. Adsorption of dodecane on the surfaces of both coal and pyrite can, therefore, be expected to occur under the coal flotation conditions. Adsorption behavior of 90
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COAL RECOVERY, %
Fig. 1. Effect of dodecane addition on the separation of pyrite from coal - - single stage flotation tests.
232
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-
-
single stage
oily agents, such as dodecane, from aqueous solution on mineral surfaces is known to be determined not only by the natural hydrophobic characteristics of minerals but also by their oleophilic properties. Therefore, in order to understand the mechanisms involved in determining the effect of oil addition on pyrite separa-
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D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238 90
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tion, both the natural hydrophobicity and the natural oleophilicity of coal and pyrite warranted a study. It should be mentioned that the olcophilicity of minerals at oil-water interfaces is usually considered also as a kind of hydrophobicity, which is different from the hydrophobicity of the minerals observed at airwater interfaces. To distinguish them from each other, the hydrophobicity observed at oil-water interfaces is defined as oleophilicity of minerals and the hydrophobicity observed at air-water interfaces as intrinsic-hydrophobicity. Induction time measurements were carried out to determine the relative intrinsic-hydrophobicities and oleophilicities of coal and pyrite. The intrinsic-hydrophobicities of the minerals were evaluated by the induction times measured for bubble-particle attachment, the oleophilicities were determined by the induction times for oil-particle attachment. The induction times of coal, coal pyrite, ore pyrite, and quartz for bubbleparticle attachment are given in Table 2. As expected, coal is found to be the most hydrophobic mineral with the shortest induction time, while quartz is the most hydrophilic mineral with the longest induction time. The hydrophobicity of ore pyrite is critical because it is more hydrophilic than coal, but more hydrophobic than quartz. Coal pyrite is found to be more hydrophobic than ore pyrite, possibly due to the presence of microsize coal particles as impurities on the surface of coal pyrite. Table 2 also gives the induction times of coal, coal pyrite, ore pyrite, and quartz measured using an oil drop. It can been seen that the induction time for each of the minerals obtained with oil drop is much shorter than that with air bubble. This suggests that minerals such as coal and pyrite are more hydrophobic at the oil-water interface than at the air-water interface. Since the induction times
D. Liu, P. Somasundaran ~Int. J. Min. Process, 41 (1994) 227-238
234
Table 2 Hydrophobicity and oleophilicity of coal, coal-pyrite, ore pyrite, and quartz: induction time measurements pH
Coal (ms) Air
4 5 6
32.1 31.5 31.4
Coal-pyrite (ms)
Ore pyrite ( ms )
Quartz ( ms )
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Air
Oil
Air
Oil
Air
Oil
4.0
89.6 90.6 91.2
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COAL RECOVFRY, %
Fig. 5. Effect of frother type on reagent consumption.
of coal-pyrite and ore pyrite measured with oil drops ( 10.3 and 21.2 milliseconds respectively) are even shorter than that of coal measured with air bubbles (31.4 milliseconds), pyrite (especially the coal-pyrite) can be oleophilic in the presence of oil. A close examination of the work done by Kocabag et al. (1990) suggests that the above observation on hydrophobicity/oleophilicity of the pyrite samples (slightly oxidized) agrees with Kocabag's results, although the authors did not clearly identify the distinction between the two affinities. Also, it is found that the pH did not affect the induction times of coal and coal-pyrite to a large extent in the pH range of 4 to 6 (Table 2).
3.3. Effect offrothers on coal flotation recovery The effect of different frothers (MIBC, hexanol and butanol) on coal flotation recovery is shown in Fig. 5. It can be seen that the level of consumption of n-
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
235
butylalcohol is higher than those of the alcohols with higher molecular weights, n-hexylalcohol and MIBC. Also, the consumption of n-hexylalcohol is higher than that of MIBC, which due to branching can cover a larger area than the former. These results suggest that under the present test conditions, branching and/or larger molecules are more efficient as frothers possibly due to their higher frothing ability. Also, Frangiskos et al. (1960) have reported that frothing agents can be effective only if adsorbed at the gas-liquid interface and will lose their effectiveness if removed from solution by adsorption on the coal surface. Furthermore, they have shown that the frother adsorption process consists of rapid chemisorption on the external surface of the coal particle followed by slower physical adsorption by diffusion into the pores. The molecular dimensions of the frother, relative to the pore size, will determine the rate of penetration, as well as the extent of frother loss by sorption into the pores. Both of these factors will be lower for the larger frother. Thus, it is possible that larger amounts of n-butylalcohol may have penetrated into the pores as compared to the higher molecular weight alcohols, n-hexylalcohol and methylisobutylcarbinol (MIBC). Similarly, n-hexylalcohol can be expected to adsorb on coal surface more than MIBC.
3.4. Effect offrothers on the separation of pyrite and non-pyritic minerals The effect of different frothers (MIBC, hexanol, and butanol ) on the efficiency of separation of pyrite from coal is illustrated in Fig. 6. It can be seen that selectivities obtained with these three reagents are almost the same. This suggests that the wettabilities of coal and pyrite are either not influenced or affected to the same extent by the frothers. Frangiskos et al. (1960) have reported that frothing 95
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1 O0
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Fig. 6. Effect o f f r o t h e r t y p e o n s e p a r a t i o n o f p y r i t e f r o m c o a l - - s i n g l e s t a g e f l o t a t i o n tests.
236
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238 85
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I
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60
70
80
90
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Fig. 7. Effect o f frother type on separation o f non-pyritic minerals from coal - - single stage flotation tests.
agents, even after adsorption, have tittle influence on the wettability of coal since the increase in hydrophobicity due to their adsorption is masked by the large contact angle of the natural coal surface. Selectivity for the separation of non-pyritic minerals was also not affected by the type of frother used (Fig. 7). This suggests that the type of the alcohols has no measurable effect on water flow out of the cell since entrainment, and thus separation efficiency, can be expected to change if water flow changes.
3.5. Role of relative hydrophobicity and oleophilicity of pyrite to those of coal on pyrite separation It is recognized that the natural surface properties of a mineral are primarily determined by the molecular structure of the solid and the types of bonding involved. Pyrite structureisknown to resemble that of the simple cubic NaCI structure, where CI is replaced by the S-S group (Leja, 1982; Wyckoff, 1963). In the structure,the bonding is wholly covalent. It is believed that the covalent bonds present on the surface structure of pyrite are responsible for the hydrophobic characteristicof the surface in comparison to a non-covalent bonded mineral, such as quartz. O n the other hand, since the covalent bonding in pyrite is not as symmetric as that in an oil-typemolecule, pyriteshould be more hydrophilic than coal. Because an oil phase such as that of n-dodecane is known to be more hydrophobic than a gas phase, the attraction,due to hydrophobic interaction,between the oil phase and a slightly h y ~ b i c surface of a solid such as pyrite could be
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-238
237
stronger than that between a gas phase and the solid surface. Thus, it can be understood that pyrite is oleophilic (appearing to be hydrophobic) at the oilwater interface and hydrophilic at the air-water interface. In the case of flotation using only a frother, such as MIBC or hexanol, as the flotation reagent, it is not expected to adsorb significantly on the surface of pyrite. Also, the adsorption of the frother reagent on the coal surface does not cause significant change in the hydrophobicity of coal, as has been discussed before. In other words, the hydrophobicities of coal and pyrite have not been modified to a measurable extent by frother alone. Therefore, the pyrite separation in this case is basically determined by the intrinsic-hydrophobic differences between coal and pyrite. When an oily reagent such as dodecane is used in conjunction with a frother, the oily reagent is expected to adsorb on the surfaces of coal and pyrite because both coal and pyrite were found to be oleophilic. Under these conditions, the separation could be affected by the differences in oleophilicity between coal and pyrite. Since such difference in oleophilicity (6.3 milliseconds between their induction times) between coal and pyrite was found to be much less than the difference in their intrinsic-hydrophobicity (59.8 milliseconds), the separation by the difference in oleophilicity should be more difficult than by that in intrinsic-hydrophobicity.
4. Conclusions I. Addition of an oily agent, n-dodecane, resulted in a decrease in flotationselectivity for removal of pyrite from coal and appeared to have no effect on the separation of non-pyritic minerals. 2. Pyrite (especiallycoal-pyrite) was found to be hydrophilic at the air-water interface and oleophilic (appearing to be hydrophobic) at the oil-water interface. Also, the difference between oleophilicitiesof coal and coal-pyrite was found to be much smaller than that between their intrinsichydrophobicities. This is proposed to be the reason for the decrease in the selectivityfor pyrite separation with the addition of dodecane, since an oily reagent such as dodecane can adsorb on the pyrite surface under the coal flotationconditions. 3. It was found that only the recovery of coal depended on the type of the frother used, while the frother type had no effecton the selectivityitself.Lower effectiveness of frothers with smaller molecular size could be attributed to their increased loss by adsorption into the pores.
Acknowledgement The authors acknowledge the United States Department of Energy and University of California at Berkeley (primary contractor) for financial support (DEAC22-88PC88878) and Prof. C.C. Harris of Columbia University for helpful suggestions.
238
D. Liu, P. Somasundaran ~Int. J. Min. Process. 41 (1994) 227-23~
References Aplan, F.F., 1977. Use of the flotation process for desulfurization of coal. In: T.D. Wheelock (Editor), ACS Symposium Series No. 64: Coal Desulfurization. Am. Chem. Society, Washington, DC. Frangiskos, N.Z., Harris, C.C. and Jowett, A., 1960. The adsorption of frothing agents on coal. In: Proceedings of the International Surface Activity Congress, Cologne. Vol. 4, pp. 404-416. Horsely, R.M. and Smith, H.G., 1951. Principles of coal flotation. Fuel, 30 (3): 54-63. Kocabag, D., Shergold, H.L. and Kelsall, G.H., 1990. Natural oleophilicity/hydrophobicityof sulphide minerals, II. Pyrite. Int. J. Miner. Process., 29:211-219. Leja, J., 1982. Surface Chemistry of Froth Flotation. Plenum Press, New York, pp. 133-150. Liu, D., Somasundaran, P., Vasudevan, T.V. and Harris, C.C., 1994. Role ofpH and dissolved mineral species in Pittsburgh No. 8 coal flotation system - - I. The floatability of coal. Int. J. Miner. Process., 41: 201-214. Moudgil, B.M., 1983. Effect of polyacrylamide and polyethylene oxide polymers on coal flotation. Colloids Surf., 8 (2): 225-229. Wyckoff, R.W.G., 1963. Crystal Structures, Vol. 1 ( 2nd Ed. ). Interscience, New York, pp. 346-347.