Coal cleaning physical fine

In any development of advanced physical coal-cleaning (PCC) techniques, an important consideration is the heterogeneous nature of coal and, in particular, the variable manner in which pyrite occurs in coal. This variability influences the behavior of coal with regard to cleaning (26). In some coals, pyrite is distributed throughout the coal matrix as particles only microns in size. Thus, to separate pyrite from these coals, the coal must be crushed to very fine size in order to "liberate the pyrite from the coal particles. However, conventional commercial PCC techniques cannot... 

The final session responded to the question If the mineral matter is in the coal, what can be done to remove the mineral matter This session dealt with a number of techniques to physically remove the mineral matter in processes called coal cleaning. These processes involve crushing and in some cases pulverizing to fine and even ultrafine sizes to permit the liberation of the mineral matter from the coal. An efficient separation requires removal of mineral matter with a minimum removal of the desirable combustible material. 

The scheme used in physical coal cleaning processes varies among coal cleaning plants but can generally be divided into four basic phases initial preparation, fine coal processing, coarse coal processing, and final preparation (Figure 6.1). 

Current commercial coal cleaning methods are invariably based on physical separation chemical and biological methods tend to be too expensive. Typically, density separation is used to clean coarse coal while surface property-based methods are preferred for fine coal cleaning. In the density-based processes, coal particles are added to a liquid medium and then subjected to gravity or centrifugal forces to separate the organic-rich (float) phase from the mineral-rich (sink) phase. 

Physical methods can effectively remove some trace constituents from coal, especially if deep cleaning methods are employed (7 ). However, such methods do not adequately remove finely disseminated minerals or organically bound elements, thereby necessitating chemical treatments for removing such components. 

Physical cleaning of various coals by oil 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). Oil agglomeration appeared to be more effective at removing trace elements than the wet concentrating table or float/sink density separations. This may be related to an increase in the liberation of mineral matter associated with grinding to produce the relatively fine particle sizes required in the oil agglomeration technique. 

C.E. Capes and R.L. Germain, Selective oil agglomeration in fine coal beneficiation, In Y.A. Liu and T.D. Wheelock (Eds.), Physical methods of cleaning coal, Marcel Dekker, New York, in press. 

The removal of both mineral matter and sulfur species to very low values would provide premium solid fuels and possibly new chemical feedstocks. Several techniques are being explored to achieve these goals. The mineral matter in a physically cleaned coal can be further reduced by the solubilization of the aluminosilicate minerals. This can technically be accomplished with the use of alkaline and then acid treatments. A variety of studies are under way to define the conditions required for effective removal of the mineral matter and establish the amount of sulfur reduction that can be accomplished. Others involve the use of fine grinding to liberate the coal from the mineral matter. Then an agglomerant is used to separate the coal matter from the aqueous phase containing suspended mineral matter. A new approach uses microwave energy to selectively decompose the clays into species that can be solubilized and removed. Still another technique involves treatment with carbon dioxide to reduce the particle size and permit the liberation of the mineral matter. Over the next few years these will be studied further and it is hoped that coal will become available in a form with less of these interesting, but not entirely desirable mineral species. 

This classification is useful in that the removal of minerals from coal in preparation plants is strongly influenced by the mineral s physical mode of occurrence. Fine-grained quartz, clay, and pyrite disseminated within macerals are least susceptible to removal by physical cleaning methods whereas rock fragments and minerals in layered, nodular, and fissure modes break free and are more easily removed. 

Physical cleaning methods can remove inorganic sulfates (gypsum) and most of the coarse pyrite the finely disseminated microcrystalline pyrite and organic sulfm are usually not separable by such processes. This means that in the case of coal containing 70% of sulfm in pyritic form and 30% as organic sulfm, the physical cleaning can rednce the sulfur content by about 50%. 

The chemical methods are not effective in removing major amounts of organic sulfur. Thus, removal of total sulfur to produce a clean-burning fuel is very difficult. It is envisioned that the optimum desulfurization of coal will include physical, chemical, and microbial treatment. The raw coal will be ground fine to liberate most of the pyritic sulfur and then processed by using the column... 

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