Chemical Cleaning of Coal - American Chemical Society

6 Chemical Cleaning of Coal Effect on the Removal of Trace Elements

Downloaded by UNIV LAVAL on July 8, 2014 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch006

G. A. Norton, Richard Markuszewski, and H. G. Araghi Ames Laboratory, Iowa State University, Ames, IA 50011

Levels of many trace elements in Illinois No. 6 and Pittsburgh No. 8 coals were significantly reduced by treatments with molten caustic mixtures or aqueous alkali solutions, followed by washing with acid. Treating runof-mine Illinois No. 6 coal with aqueous Na C0 at elevated temperatures and pressures and washing the product coal subsequently with acid reduced Mn, Pb, and Zn concentrations by 75% or more, while levels of Cd and Ni were reduced by lesser amounts. In one of these samples, substantial reductionsinlevels of Ba, Cr, Rb, and Sr were also observed. Coals which were treated with molten NaOH/KOH mixtures at atmospheric pressure showed substantial reductionsinconcentrations of As, Cd, Hg, Pb, Rb, Se, Sr, and Zn. The amounts of several elements, such as Cr, Cu, and N i , were elevatedinsome of the treated coals, but t h i s can be attributed to corrosion of reactor components. 2

3

Coal contains most of the elements of the periodic table, the majori t y of which are present in concentrations of 100 ppm or l e s s . Many of these trace elements are toxic to plant and animal l i f e , even at low l e v e l s . Because U.S. power plants consume on the order of 600 m i l l i o n tons of coal annually f o r the production of e l e c t r i c i t y U ) , coal combustion can mobilize thousands of tons of p o t e n t i a l l y hazardous trace elements into the environment each year. Due to the large quantities of coal combusted, even trace amounts of toxic elements present in the coals can accumulate to hazardous l e v e l s . Also, p o t e n t i a l l y deleterious effects of particulate emissions from coal combustion may be enhanced since many trace elements are surface-enriched (2) and concentrate p r e f e r e n t i a l l y in the smaller, more respirable p a r t i c l e sizes ( 3 ) . Substantial amounts of some elements, such as As, Hg, and Se, areinthe vapor phase in f l u e gases from coal combustion and are e s s e n t i a l l y unaffected by most p a r t i c l e cont r o l devices. Aside from the potential detrimental environmental aspects, trace elements in coal can pose adverse technological 0097-6156/86/0319-0063$06.00/0 © 1986 American Chemical Society

In Fossil Fuels Utilization; Markuszewski, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

64

problems, such as catalyst poisoning in c a t a l y t i c hydrogénation and g a s i f i c a t i o n reactions (4»5,6). Physical methods can e f f e c t i v e l y remove some trace constituents from coal, e s p e c i a l l y i f deep cleaning methods are employed (7). However, such methods do not adequately remove f i n e l y disseminated minerals or o r g a n i c a l l y bound elements, thereby necessitating chemical treatments for removing such components.


Background A l i t e r a t u r e survey was conducted to acquire information pertinent to the removal of trace elements from U.S. coals during physical and chemical cleaning. This survey included trace element associations with minerals and their a f f i n i t i e s for organic or inorganic components in c o a l . L i t e r a t u r e was also searched to ascertain the trace elements of concern with respect to coal u t i l i z a t i o n and for data reported previously on the removal of trace elements during physical and chemical coal cleaning. Results of these l i t e r a t u r e searches are discussed below. Trace Elements of Environmental Concern. In a l i s t prepared by a panel of s c i e n t i s t s and engineers involved with environmental aspects of coal u t i l i z a t i o n , As, B, Cd, Hg, Mo, Pb, and Se were considered to be of primary environmental concern ( 8 J . The elements Cr, Cu, Ni, V, and Zn were of moderate concern, while Ba, Co, Ge, and Mn were believed to be of minor concern. Despite the t o x i c i t y of Ag, Be, Sn, and T l , the panel believed concentrations of these elements in coal and coal residues are n e g l i g i b l e and that the likelihood of health hazards associated with these constituents is remote. This l i s t served as a guide in choosing the elements to be determined in raw and treated coals in this study. However, some of the elements, such as B, were not studied in this work due to the inherent d i f f i c u l t i e s with the analytical procedures and instrumental methods employed. Occurrence of Trace Elements in Coal. Based on the reviewed l i t e r a ture, it is evident that modes of occurrence in U.S. coals vary considerably, although several generalizations can be made. Many e l e ments of environmental concern, including As, Cd, Hg, Pb, and Zn, have an a f f i n i t y for the mineral portion of the coal and tend to be associated with p y r i t e or accessory s u l f i d e minerals (6,9-12). Other elements which have been reported as being at least p a r t i a l l y associated with pyrite include Co,ton,and Se. The element Zn appears to occur as s p h a l e r i t e in most c o a l s . Although Mo has a general inorganic a s s o c i a t i o n ,itsmode of occurrence is uncertain. The element As has been found to occur in s o l i d solution in pyrite (13), while Cd has been found in s o l i d solution in s p h a l e r i t e (14,15). The predominant mineral species containing a p a r t i c u l a r element can vary. In one example, Pb seems to be present mostly as PbSe in Appalachian Basin coals, but it tends to exist in the form of PbS in coals from other regions (13,16). Although most trace elements appear to be largely i n o r g a n i c a l l y associated, several of them have a strong a f f i n i t y for the organic portion of the coal and are believed to be present as metal chelates. Among the elements in this category are B, Be, and Ge (6,9-12).


6.

NORTON ET

AL.

Coal Cleaning: Effect on Trace Element Removal

65


Although the majority of Β appears to be associated with the organic matrix in most coals, Β has also been reported occurring in i l l i t e (13-17). Many elements, such as Cr, Cu, Ni, and Se, appear to have a mixed or highly variable organic/inorganic a f f i n i t y in some coals. Again, these are only generalizations, since organic/inorganic associations of trace elements can vary widely from coal to coal and can show both extremes in t h e i r a f f i n i t i e s . Trace Element Removal During Physical Cleaning. Commercial coal cleaning processes employ physical means for b e n e f i c i a t i o n and are aimed at removing ash forming minerals and s u l f u r , although removal of the mineral matter also r e s u l t s in reduced levels of some trace elements. Trace element extraction e f f i c i e n c i e s for various physi cal cleaning processes have been reported. In one study, eight geographically diverse coals were ground and cleaned subsequently with a wet concentrating table (18). Clean ing e f f i c i e n c i e s with respect to trace elements varied widely between c o a l s . For the elements of environmental concern, average percent removals were less than 50%. In studies where coal was p h y s i c a l l y cleaned by f l o a t / s i n k separation in s t a t i c baths of known s p e c i f i c gravity, reductions in trace element concentrations were also gener a l l y less than 50% (1,7,19). Physical cleaning of various coals by o i l agglomeration reduced levels of As, Cr, Pb, Mn, Mo, Ni, and V by 50-80%, while levels of some other trace elements were reduced by lesser amounts (20). O i l agglomeration appeared to be more e f f e c t i v e at removing trace e l e ments than the wet concentrating table or f l o a t / s i n k density separa t i o n s . This may be related to an increase in the l i b e r a t i o n of min eral matter associated with grinding to produce the r e l a t i v e l y f i n e p a r t i c l e sizes required in the o i l agglomeration technique. Levels of various elements such as As, Cd, Cr, Cu, Pb, Mn, Se, and Zn were reduced by 50% or more in some coals cleaned by combina tions of heavy media cyclones, froth f l o t a t i o n c e l l s , and hydraulic c l a s s i f i e r s (21-24). Cleaning was less e f f e c t i v e for other coals. In general, average reductions in overall trace element content cor responded to reductions in ash l e v e l s . Trace Element Removal During Chemical Cleaning. The l i t e r a t u r e on trace element removal from coal by chemical cleaning methods alone is sparse. There are some i n d i c a t i o n s , however, that leaching physical l y cleaned coals with various acids can e f f e c t i v e l y remove many trace elements of i n t e r e s t . In one study, for example, various coals from eastern, c e n t r a l , and western coal regions were cleaned by a combi nation of physical and chemical means, and extraction e f f i c i e n c i e s f o r numerous elements were ascertained by analyzing the raw and treated coals (25). Cleaning was performed by f l o a t i n g the coals at a s p e c i f i c g r a v i t y of 1.40, grinding the f l o a t f r a c t i o n to -325 mesh, and then successively leaching with 10% HNO,, 49% HF, and 25% HC1. Based on average concentrations in raw and treated coals from a given region, levels of Be were reduced by 90% or more, while levels of As, B, Cr, Cu, Mo, Ni, Se, and V were reduced by at least 70%. In that study, physical separation by f l o t a t i o n followed by acid leachings was found to be much more e f f i c i e n t at removing trace elements than f l o t a t i o n alone, although t h i s may be p a r t i a l l y attributed to the


66

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

l i b e r a t i o n of mineral matter during grinding prior to the chemical leaching step. Only a few of the chemical cleaning processes studiedinthe past or c u r r e n t l y being developed present trace element data in addi t i o n to the conventional s u l f u r , ash, and heating values. One of the exceptions is the Jet Propulsion Laboratory C h l o r i n o l y s i s Process, in which cleaning was generally accomplished by bubbling chlorine through a mixture containing 100 grams of f i n e l y ground coal, 200 grams of solvent (either methyl chloroform, carbon t e t r a c h l o r i d e , or tetrachloroethylene), and 20-70 grams of water (26). The c h l o r i n a tion step was conducted at 50-100 C at atmospheric pressure for 10-20 minutes. The coal was then hydrolyzed by washing with water f o r 60-120 minutes at 60-100°C and subsequently dechlorinated at 350-550 C in a steam atmosphere. For some coals treated under these conditions, levels of As, Be, Pb, and V were reduced by 50-90% (26,27). Analytical data on the raw and treated coals are shown in Table I. In another one, the Battel l e Hydrothermal Process (28-30), an aqueous s l u r r y of f i n e l y ground coal, NaOH, and Ca(0H) was heated for 10-30 minutes at 250-350 C at pressures of 600-2500 psig. Trace element data for several Ohio coals are shown in Table I I . Based on averages from these coals, this process reduced levels of a l l re ported trace elements by 70-90%. In yet another chemical cleaning process, the Meyers Process, crushed coal was leached with an acidic solution of f e r r i c sulfate at 100-130 C for several hours (31). I f the leaching time was long enough, almost a l l of the p y r i t i c sulfur was extracted. At the same time, levels of As, Cd, Cr, Mn, Ni, Pb, V, and Zn were s i g n i f i c a n t l y reduced (32). S t i l l , the data discussed above represent only a small f r a c t i o n o f the chemical processes that have been studied. e


e

2

e

e

Table I. Elemental Data for Coals Before and After Treatment by the Low-Temperature C h l o r i n o l y s i s (26,27) 1

Kentucky #4 (26) Raw As Ba Be Cd Hg Li Ρ Pb Se Ti V 1 2

73 5 8 1

Chemical Cleaning of Coal - American Chemical Society

Recommend Documents