Cleaning coal

ROM coal generally has 5%-40% of ash content and 0.34% of sulfur content depending on the geological conditions and mining technique used. Coal cleaning, therefore, is often required to remove excessive impurities for efficient and environmentally safe utilization of coal. In the United States, the coal cleaning is most common at Eastern and Midwestern mines. 

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. 

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 

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Concern over the release of hazardous trace elements from the burning of coal has been highlighted by the 1990 Clean Air Act Amendments. Most toxic elements are associated with ash-forming minerals in coal (5). As shown in Table 1, levels of many of these toxic metals can be significantly reduced by physical coal cleaning (6). 

Table 1. Effect of Coal Cleaning on Trace Elements ...

Conventional coal cleaning processes can remove about 50% of pyritic sulfur and 30% of total sulfur. For northern Appalachian region coals it has been shown that a greater sulfur reduction can be achieved by applying physical coal cleaning to finer size coals (Table 3) (8). 

Flotation. The appfication of flotation (qv) to coal cleaning is a relatively new development iu the United States. In 1960, only 0.6% of the clean coal produced came from flotation. However, by 1983 flotation accounted for about 5% of the clean coal production (Table 2). Utilization of the flotation process is expected to grow rapidly because more fine size coal is produced as a result of beneficiation schemes that require significant size reduction of the taw coal prior to cleaning to enhance the fiberation of pyrite and ash minerals. 

Although froth flotation is recognized as the best available fine coal cleaning technique, it becomes iueffective when the particle size is much smaller than 0.1 mm or when the feed contains a large amount of clay, resulting iu low coal recovery or poor selectivity. A solution to these problems is the use of modified flotation devices. 

Agglomeration-Based Fine Coal Cleaning. Most recently a search for nonaqueous collectors or reagents for fine coal cleaning has been undertaken. A number of Hquids have been tested and found to be suitable as agglomeration agents. These include heavy oil. Freon, pentane, hexane, heptane, 2-methylbutane, methyl chloride, and Hquid carbon dioxide. 

Dry Coal Cleaning. Developments in the areas of magnetic and electrostatic separation as a means of cleaning coals in the dry state include high gradient magnetic separation (HGMS), triboelectrostatic separation (TESS), and dry coal purifier (D-CoP). 

C. T. Fold and A. A. Price, "Evaluation of the Effect of Coal Cleaning on Eugitive Elements," Phase II, Part I, Bituminous Coa/Research, BCR Report L-1082,1980. 

R. E. Hucko and co-workers, "Status of DOE-sponsored Advanced Coal Cleaning Processes," ia R. R. K1 impel and P. T. Luckie, eds.. Industrial Practice of Fine Coal Processing, SME, Inc., Littieton, Colo., 1989. 

FIG. 19-33 Typical dense-media flow sheet for a coal-cleaning plant. Couiiesy of Frocess Machineiy Division, AtihurG. McKee Co.)... 

Coal-cleaning plants (with thermal dryers) Kraft-pulp mills Portland-cement plants Primary zinc smelters Iron and steel mill plants Primary aluminum-ore-reduction plants Primary copper smelters ... 

Other methods which should be mentioned because they show potential benefits for pollution reduction from utility stacks include (1) coal cleaning... 

Mineral Products (e.g.. Cement Manufacturing, Coal Cleaning, and Stone Quarrying and Processing)... 

Rein-kohle,/. pure coal, clean(ed) coal solid carbon, -koks, m. pure coke, clean coke. >kultur, kultivierung, /. Bact.) pure culture, pure cultivation. 

The Lambda Group Inc. was an industrial organization involved in coal business, which supported the development of a coal cleaning process led by Jo Davison, during the second half of the 1980s. 

In an open report, this company informed how the process evolved into three different applications, depending on where the coal was cleaned and what part of the coal was being cleaned. Only the latter is fully based in the awarded patent. The process was presented internationally as the Lambda Coal Cleaning Process [142] in 1987, but after that announcement there is no additional information on whether it reached commercial demonstration. 

Gravimelt A coal cleaning process in which coal is heated with molten sodium hydroxide and then washed with acid. The process removes 90 percent of the sulfur and 95 percent of the ash. It was planned to be piloted in 1987. 

Mineral products (e.g., cement manufacturing, coal cleaning, and stone quarrying and processing)... 

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