31 Behavior of Mineral Matter during Coal Beneficiation Harold L. Lovell
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Department of Mineral Engineering, The Pennsylvania State University, University Park, PA 16802
Coal beneficiation involves a series of steps to separate the mineral matter from the combustible portion of the coal. Current coal characterization for beneficiation is usually limited to measurements of the particle specific gravity distribution (washability). It is further assumed that the properties of the coal feed stream and related mineral matter remain constant during the separation or cleaning process, but the compositions of the streams do change. These changes are important in understanding the lack of expected separations. The effects of specific mineral constituents on different unit operations are described. Better measurement and analytical systems will permit improved control of the processes and better separations.
Each coal beneficiation unit o p e r a t i o n responds to the p h y s i c a l and c h e m i c a l p r o p e r t i e s o f i t s f e e d . The r e s p o n s e o f t h e b e n e f i c i a t i o n steps to the " c o l l a g e " o f the i n d i v i d u a l p a r t i c l e s depends s p e c i f i c a l l y on t h e f r e q u e n c y d i s t r i b u t i o n o f t h e component particle properties. The p a r t i c l e r e s p o n s e s a r e n o t t o t a l l y i n d e pendent o f p a r t i c l e p r o p e r t y - p r o c e s s parameter i n t e r a c t i o n s , but a r e a l s o s i g n i f i c a n t l y c o n t r o l l e d by t h e o v e r a l l d i s t r i b u t i o n o f particle properties. Diligence i s appropriately applied to e s t a b l i s h t h e " c o l l a g e " d i s t r i b u t i o n by c e r t a i n p a r t i c l e p r o p e r t i e s o f t h e p l a n t f e e d . T h i s d i s t r i b u t i o n p r o v i d e s t h e d e s i g n bases f o r each u n i t o p e r a t i o n i n t h e f l o w sheet - i n c l u d i n g i t s c a p a c i t y , f l o w r a t e , and u n i t l o a d i n g - thus u l t i m a t e l y d e f i n i n g the s e p a r a tional performance, efficiency, and u n i t costs. Each unit o p e r a t i o n s h o u l d be s i z e d and o p e r a t e d such t h a t i t s optimum p e r formance i s w e l l w i t h i n t h e s e n s i t i v i t y range o f i t s f e e d p r o p e r t y distribution. The c o n c e r n t h a t t h e f e e d d i s t r i b u t i o n s may, a t t i m e s , exceed t h e s e n s i t i v i t y range f o r some u n i t o p e r a t i o n w i t h i n 0097-6156/ 86/0301 -0438S06.00/0 © 1986 American Chemical Society
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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the system, i s n o r m a l l y r e l a t e d to i n s i t u c o a l v a r i a b i l i t y , which can be extreme* The concern may extend to c o a l f e e d s from d i f f e r e n t seams and/or p a l e o g e o l o g i c a l o r i g i n s , m i n i n g systems, or h a n d l i n g - s t o r a g e systems. U n f o r t u n a t e l y , seldom a r e the c o n c e r n s e x t e n s i v e and r a r e l y a r e the f e e d c h a r a c t e r i z a t i o n s d e t a i l e d t o each downstream o p e r a t i o n . The p l a n t f e e d c h a r a c t e r i z a t i o n u s u a l l y i s l i m i t e d to p a r t i c l e s i z e d i s t r i b u t i o n and, i f d e t a i l e d , w i l l i n c l u d e a p a r t i c l e g r a v i t y d i s t r i b u t i o n ( w a s h a b i l i t y ) f o r s e v e r a l s i z e g r o u p i n g s g r e a t e r than 28 or 16 mesh. There i s no g e n e r a l l y a c c e p t e d range f o r e i t h e r the s i z e or d e n s i t y g r o u p i n g s . The c h a r a c t e r i z a t i o n o f the i n d i v i d u a l particle fractions are usually limited to moisture, with temperature ash, and total sulfur content. In some unusual i n s t a n c e s , the low temperature ash and s u l f i d e s u l f u r c o n t e n t may be d e t e r m i n e d . Pragmatism, p r o c e d u r e s , and economics p r e v e n t the direct determination of minerals whose c o n c e n t r a t i o n s will be m o d i f i e d i n the p r o c e s s . These l e v e l s a r e e x p r e s s e d i n terms o f ash c o n t e n t . I f t h e r e i s any c h a r a c t e r i z a t i o n of the i n d i v i d u a l macérais o r m i n e r a l s , they a r e e v a l u a t e d on the b a s i s of the "total-composite" feed sample. Should flotation processes be anticipated, some " l a b o r a t o r y f l o a t a b i l i t y " s t u d i e s (_1) may be c a r r i e d out on some minus 28 mesh or f i n e r s i z e d f r a c t i o n . The p a r t i c u l a r f r a c t i o n thus t e s t e d i s seldom the same i n a l l d e t a i l s as t h a t which w i l l exist w i t h i n the p l a n t f l o w s h e e t . Feed h a r d n e s s or f r i a b i l i t y (Hardgrove G r i n d a b i l i t y Index, D r o p - S h a t t e r , e t c . ) may a l s o be determined on the composite f e e d sample. I f any comminution e v a l u a t i o n s a r e made f o r the p a r t i c u l a r f e e d c o a l i n the c o u r s e of s e l e c t i n g a p a r t i c u l a r comminution d e v i c e , they a r e u s u a l l y made on a sample p u r p o r t e d to be r e p r e s e n t a t i v e o f the plant feed. Attempts to e v a l u a t e v a r i a t i o n s of p a r t i c l e s t r e n g t h o r s t a b i l i t y , m a c e r a l , m i n e r a l , or e l e m e n t a l c o m p o s i t i o n , have been l i m i t e d to r e s e a r c h c h a r a c t e r i z a t i o n s , as those r e p o r t e d by the a u t h o r (2_). We a r e almost t o t a l l y d e v o i d o f p r a g m a t i c t e c h n i q u e s to e s t a b l i s h the p a r t i c l e s i z e s or volumes of i n d i v i d u a l components within a given i n d i v i d u a l coal p a r t i c l e (3). These o b s e r v a t i o n s can but l e a d t o the c o n c l u s i o n t h a t i n c o n t r a s t to t y p i c a l u n i t o p e r a t i o n s i n c h e m i c a l e n g i n e e r i n g , the c o a l processing engineer assumes a f e e d to each u n i t o p e r a t i o n within the plant system based upon the defined plant feed. F u r t h e r , i t i s assumed t h a t the c o l l a g e of p a r t i c l e s e n t e r i n g the p l a n t as f e e d DOES NOT CHANGE i n p a s s i n g through the p r o c e s s i n g system, and t h a t the same number o f p a r t i c l e s 2 by 1-inch h a v i n g d e n s i t i e s between 1.40 and 1.45 gm/cc ( o r any o t h e r f r a c t i o n ) l e a v e the p l a n t as e n t e r . We know t h a t t h i s i s not c o r r e c t even i f we i g n o r e the comminution o p e r a t i o n s w i t h i n the p l a n t d e s i g n e d t o make such changes. The composition o f any individual coal beneficiation feed p a r t i c l e ranges from a n e a r l y u n i f o r m metamorphized p l a n t component t h r o u g h an almost i n f i n i t e m i x t u r e s e r i e s w i t h m a c e r a l s - m i n e r a l s to an o p p o s i t e end member as a n e a r l y u n i f o r m m i n e r a l component. The b e h a v i o r of a b e n e f i c i a t i o n f e e d d u r i n g p r o c e s s i n g i s d e t e r m i n e d by
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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c o m p o s i t i o n s whose r e s p o n s e s to p r o c e s s parameters a r e e s t a b l i s h e d by the p a r t i c l e p r o p e r t i e s such as s i z e , shape, d e n s i t y , h a r d n e s s , p o r o s i t y , and g a s - l i q u i d - s o l i d i n t e r f a c e s . The p a r t i c l e r e s p o n s e e s t a b l i s h e s i t s d i r e c t i o n and r a t e of movement toward one of the process product p o r t s . W i t h c o a l s as s e d i m e n t a r y rocks of p a l e o b o t a n i c a l origin, t h e i r p l a n t i n o r g a n i c c o n t e n t s i n c o r p o r a t e those components which were p a r t of the o r i g i n a l p l a n t system and a s s o c i a t e d s u b s t a n c e s , as well as those that have been i n t r o d u c e d t h r o u g h a l l the subsequent g e o l o g i c e v e n t s . A c c o r d i n g l y , the m i n e r a l components found i n c o a l s r e f l e c t the n a t u r e of the o r i g i n a t i n g p l a n t systems, their environment, reduction conditions, temperatures, and p r e s s u r e s , as w e l l as those c o n d i t i o n s to which the coal-forming s t r a t a have been s u b j e c t d u r i n g a l l the e n s u i n g g e o l o g i c epochs, i n c l u d i n g p a s t and c u r r e n t c i r c u l a t i n g ground w a t e r s . Thus, the o b s e r v e d c o m p l e x i t y of the r e s u l t i n g p h y s i c a l c h a r a c t e r i s t i c s i s to be e x p e c t e d . The minerals found i n U n i t e d States coals continue to be s t u d i e d w i t h the a v a i l a b i l i t y of improved i n s t r u m e n t a l p r o c e d u r e s such as x-ray diffraction, i n f r a r e d absorption, and scanning e l e c t r o n m i c r o s c o p y beyond the t r a d i t i o n a l o p t i c a l and chemical mineralogical techniques as a p p l i e d t o t h i n s e c t i o n s , p o l i s h e d p e l l e t s , and i s o l a t e d p a r t i c l e s . The m i n e r a l s may be grouped i n t o the s i l i c a t e s ( k a o l i n i t e , i l l i t e m o n t m o r i l l o n i t e , and chlorite), the o x i d e s (quartz, chalcedony, hematite); the s u l f i d e s ( p y r i t e , m a r c a s i t e , and s p h a l e r i t e ) ; the s u l f a t e s ( j a r o s i t e , gypsum, b a r i t e , and numerous i r o n sulfate minerals); the c a r b o n a t e s (ankerite, c a l c i t e , d o l o m i t e , and s i d e r i t e ) ; and numerous a c c e s s o r y minerals ( a p a t i t e , phosphorite, zircon, rutile, chlorides, nitrates, and trace minerals). The g r e a t e s t i n t e r e s t i n m i n e r a l o c c u r r e n c e s i n c o a l p a r t i c l e s f o r p r o c e s s i n g e n g i n e e r s r e l a t e s to t h e i r p o t e n t i a l l i b e r a t i o n as an e s s e n t i a l f i r s t s t e p toward t h e i r p h y s i c a l removal. Further, the c o n c e r n r e l a t e s to the m i n e r a l b e h a v i o r i n each of the u n i t operations within the preparation plant and environmental i m p l i c a t i o n s w i t h i n the p r e p a r a t i o n o p e r a t i o n s , f o r u t i l i z a t i o n o f the c l e a n c o a l p r o d u c t and the d i s p o s a l of the r e f u s e m a t e r i a l s . The g r e a t e s t a t t e n t i o n has been g i v e n to the former i n t e r e s t s , e s p e c i a l l y as a p p l i e d t o the l i b e r a t i o n of p y r i t e i n e f f o r t s to a c h i e v e the g r e a t e s t p o s s i b l e s u l f u r r e d u c t i o n d u r i n g p r o c e s s i n g . S p e c i f i c Responses of C o a l M i n e r a l Components d u r i n g
Processing:
The " c o l l a g e " of p a r t i c l e s e n t e r i n g a c o a l p r o c e s s i n g p l a n t i s s u b j e c t t o a s e r i e s of u n i t o p e r a t i o n s d e s i g n e d to a c h i e v e the desired l e v e l of q u a l i t y improvement. The development of the initial set of particles is determined by the mining and preprocessing storage and handling systems. Undoubtedly these o p e r a t i o n s i n t r o d u c e s t r e s s e s w i t h i n the c o a l p a r t i c l e s t h a t l e a d to subsequent f r a c t u r e f a i l u r e s . Any p o t e n t i a l c o n t r o l of the n a t u r e of t h i s p a r t i c l e s e t i s u s u a l l y e x t r e m e l y l i m i t e d - b e i n g
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d e t e r m i n e d by p r o d u c t i o n and economic f a c t o r s . S i t u a t i o n s which lead t o o x i d a t i o n , d e c r e p i t a t i o n , and absorption of excessive l e v e l s o f m o i s t u r e may be m o d i f i e d . The i n t r o d u c t i o n o f m o i s t u r e prior to p r o c e s s i n g p r o b a b l y enhances c l a y s w e l l i n g , tends t o i n c r e a s e the amount of m i n e r a l f i n e s ( u s u a l l y c l a y s ) i n t o the p l a n t stream, and may enhance localized heating, swelling, and oxidation. I n i t i a t i o n o f d i s p e r s i o n o f c l a y s may begin here. U n c o n t r o l l e d comminution d u r i n g the h a n d l i n g and s t o r a g e due to d r o p p i n g from s t a c k e r s , compaction by g r a d e r s , e t c . tend to c r e a t e fines and probably selectively f a v o r s r e d u c t i o n of the softer particles, especially certain clays. O t h e r h a n d l i n g s t e p s such as particle movement through jigs, etc., tend to increase the production of fines. The o x i d a t i o n of c o a l and temperature i n c r e a s e s may f a v o r the p r o d u c t i o n of water s o l u b l e s a l t s l e a d i n g to a c i d p l a n t waters and p o t e n t i a l c o r r o s i o n . Primary c r u s h i n g which u s u a l l y i n v o l v e s b r e a k e r s or single r o l l c r u s h e r s may be p r e c e d e d by a s c a l p i n g o p e r a t i o n to remove l a r g e p a r t i c l e s of hard s h a l e s and s a n d s t o n e s . Such o p e r a t i o n s , though not r e j e c t i n g l a r g e amounts o f r e f u s e , do p r e v e n t wear, save energy, and p r e v e n t the i n t r o d u c t i o n of a d d i t i o n a l r e f u s e i n t o the plant feed. The primary comminution d e v i c e s are designed to control top size rather than achieve particle liberation. T h e r e f o r e , they o f f e r an o p p o r t u n i t y to m i n i m i z e f i n e s p r o d u c t i o n and show some s e l e c t i v i t y toward the l a r g e h a r d e r p a r t i c l e s such as s h a l e s and sandstones. P r e - o p e r a t i o n a l t e s t i n g of comminution devices should minimize the production of fines. In coal p r o c e s s i n g systems where c l a s s i f y i n g r o t a t i n g m i l l s may be used, the s e l e c t i v e b u i l d u p o f h a r d e r components w i t h i n the m i l l can a f f e c t system performance. In c o a r s e c o a l s o r t i n g equipment, such as j i g s and heavy media vessels, the softer minerals will tend to comminute due to a t t r i t i o n a l a c t i o n s o f p a r t i c l e movement and r e s u l t i n f u r t h e r d i s p e r s i o n i n t o the p l a n t c i r c u l a t i n g water system. In j i g s , the p r o d u c t i o n o f m i n e r a l f i n e s may be d e s i r a b l e to enhance h i n d e r e d settling effects. In the Haldex heavy media system (4) and watero n l y c y c l o n e s , the p r e s e n c e o f m i n e r a l f i n e s a r e e s s e n t i a l to s e r v e as an autogenous heavy media system. O p e r a t i o n a l c a r e must be taken to p r e v e n t the b u i l d u p of u n a c c e p t a b l e l e v e l s of fines l e a d i n g to u n a c c e p t a b l e f l u i d v i s c o s i t y l e v e l s . Although quartz, clays, and other very fine mineral particles enhance these conditions, several types of clays, notably kaolinite and montmorillonite ( t h o s e c o n t a i n i n g l a r g e r amounts o f sodium), a r e especially responsible. Suspended c l a y l e v e l s above f i v e p e r c e n t i n such systems a r e most u n d e s i r a b l e and may l i m i t c o n t r o l o f the d e n s i t y i n m a g n e t i t e heavy media systems. The v i s c o s i t y e f f e c t i n c r e a s e s w i t h d e c r e a s i n g p a r t i c l e s i z e and w i t h s p h e r i c a l shape which enhances s e t t l i n g r a t e s . These c o n c e r n s can a l s o become c r i t i c a l i n c o a l - w a t e r t r a n s p o r t systems. In f i n e c o a l s o r t i n g systems, such as heavy media w a t e r - o n l y c y c l o n e s , t a b l e s , and s p i r a l s , the d e n s i t y and responses of suspensions are even more c r i t i c a l .
cyclones, viscosity In froth
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f l o t a t i o n , the p r e s e n c e o f c l a y f i n e s i s u n d e s i r a b l e and u s u a l l y r e q u i r e s a d e s l i m i n g s t e p ahead of f l o t a t i o n i f t h e i r c o n c e n t r a t i o n becomes e x c e s s i v e * It i s i n the water c i r c u i t t h a t the b u i l d u p of fines, e s p e c i a l l y c l a y s , must be c o n t r o l l e d . The responses become e v i d e n t i n dewatering d e v i c e s and as c e n t r i f u g e s and f i l t e r s . In the l a t t e r c a s e , c l a y s may cause b l i n d i n g r e s u l t i n g i n u n a c c e p t a b l e water c o n t e n t s i n the f i l t e r cake, t h i n watery c a k e s , and unacc e p t a b l e performance. D i f f i c u l t i e s i n the f i l t r a t i o n of r e f u s e f i n e s has l e d r e c e n t l y t o the i n t r o d u c t i o n of e x p e n s i v e p r o c e s s e s such as p r e s s u r e and b e l t f i l t e r s to meet e n v i r o n m e n t a l s t a n d a r d s . In t h i c k e n e r s , e x c e s s i v e c l a y f i n e s may reduce s e t t l i n g rates, m i n i m i z e the f o r m a t i o n o f d e s i r a b l e u n d e r f l o w s l u r r y d e n s i t i e s , and l e a d to p l a n t f a i l u r e . I t i s i n the d e w a t e r i n g s t a g e s and water c i r c u i t , those p r o c e s s e s a t the end o f the f l o w s h e e t , t h a t these r e s p o n s e s become a c u t e . A l t h o u g h the use o f one o r more p o l y m e r i c flocculants can usually control these situations, unexpected changes i n p l a n t f e e d s may r e q u i r e f e e d r a t e r e d u c t i o n s o r p l a n t shutdown. Recent environmental r e g u l a t i o n s which e s s e n t i a l l y r e q u i r e c l o s e d water c i r c u i t s make the problems of m i n e r a l f i n e s b u i l d u p e s p e c i a l l y severe. S i m i l a r d i f f i c u l t i e s a r e a s s o c i a t e d w i t h the d i s p o s a l of r e f u s e f i n e s . These examples d e s c r i b e some of the more prominent r e s p o n s e s of m i n e r a l components i n c o a l p r o c e s s i n g o p e r a t i o n s . Control of t h e s e problems can be a c h i e v e d w i t h b e t t e r d e t e c t i o n and a n a l y t i c a l systems t o i d e n t i f y the problems.
Literature Cited 1a. Cavallaro, J.A. and A.W. Deurbrouck, Froth Flotation Washability Data of Various Appalacian Coals using the Time Release Analysis Technique. Report of Investigations 6652. U.S. Bureau of Mines, 1965. 1b. Alderman, J.K., Evaluating Flotation Washability Data. Coal Mining and Processing, pp. 70-73, 1981. 2. Lovell, H.L., The Characteristics of American Coals, Final Report, U.S. Dept. of Energy, FE-2030-11, 1976. 3a. Richardson, D. and H.L. Lovell, Pyrite Liberation - Key to Sulfur Reduction during Beneficiation. Proceedings of Coal Conference and Expo V, Symposium on coal Preparation and Utilization, October 1979. 3b. Richardson, D.L. A study of the Occurrence of Pyrite in Coal and Its Relationship to Liberation in Coal Preparation and Mine Drainage Formation. M.S. Thesis in Mining Engineering, 1979, The Pennsylvania State University. 4. Anon. Investigation of the Haldex (Simdex) Process for Beneficiating Coal Refuse: Hungarian Practice - 1969. Special Report 80, The Coal Research Section, The Pennsylvania State University, University Park, Pennsylvania. RECEIVED November 4, 1985
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
