Mineral matter

Mineral matter content or ash yield varies widely, from <6% in thick deposits as found in AustraUa and Germany, to >40% in deposits in Turkey. 

Drying. In many cases, the high moisture content of young coals dictates significant drying (qv) before use. In some cases, partial removal of mineral matter, especially water-soluble species, is desirable. 

Sheet form, composed of organic roofing felt, saturated with asphalt and coating on both sides with asphalt compound that may or may not contain mineral stabiHzer, surfaced with powdered talc, mica, or other tine mineral matter to prevent sticking. Classified, in mineral net mass per unit area of roofing, as Type I, 1943 g/m (39.8 lb/100 fC) Type II, 2666 g/m (54.6 lb/100 fC) Type III, 2495 g/m (51.1 lb/100 fC) and Type IV, 1943 g/m (39.8 Ib/lOOfC). 

Trinidad asphalt has a relatively uniform composition of 29% water and gas, 39% bitumen soluble in carbon disulfide, 27% mineral matter on ignition, and 5% bitumen that remains adsorbed on the mineral matter. Refining is essentially a process of dehydration by heating the cmde asphalt to ca 165°C. The refined product averages 36% mineral ash with a penetration at 25°C of about 2 (0.2 mm), a softening point (ring and ball method) of 99°C, a flash point (Cleveland open cup) of 254°C, a sulfur content of 3.3%, and a saponification value of 45 mg KOH/g. The mineral matter typically contains... 

Bitumen Soluble in Carbon Disuffide (ASTMD4). Asphalt is defined as a mixture of hydrocarbons that are completely soluble in carbon disulfide. Trichloroethylene or 1,1,1-trichloroethane have been used in recent years as safer solvents for this purpose. The procedure for these and other solvents for asphalt with Htde or no mineral matter are described in ASTM D2042. 

If the mineral matter in the coal exceeds about 40%, then the material is referred to as a coaly or carbonaceous shale. If the mineral matter is a finely divided clay, well dispersed in the coal, then the material may be described as a stony coal or bone coal. 

The ASTM Glassification. The ASTM classification system was adopted in 1938 as a standard means of specification. This system is used in the United States and in many other parts of the world, and is designated D388 in the ASTM Standards (18). The higher rank coals are specified by fixed carbon >69%, or for volatile matter <31%, on a dry, mineral-free basis. Lower rank coals are classified by calorific value on the moist, mineral-matter-free... 

Mineral Matter in Goal. The mineral matter (7,38) in coal results from several separate processes. Some comes from the material inherent in all living matter some from the detrital minerals deposited during the time of peat formation and a third type from secondary minerals that crystallized from water which has percolated through the coal seams. 

Pieces of coal are mixtures of materials somewhat randomly distributed in differing amounts. The mineral matter can be readily distinguished from the organic, which is itself a mixture. Coal properties reflect the individual constituents and the relative proportions. By analogy to geologic formations, the macerals are the constituents that correspond to minerals that make up individual rocks. For coals, macerals, which tend to be consistent in their properties, represent particular classes of plant parts that have been transformed into coal (40). Most detailed chemical and physical studies of coal have been made on macerals or samples rich in a particular maceral, because maceral separation is time consuming. 

Dry, mineral-matter-free basis except for moisture value. 

Coke Production. Coking coals are mainly selected on the basis of the quaUty and amount of coke that they produce, although gas yield is also considered. About 65—70% of the coal charged is produced as coke. The gas quaUty depends on the coal rank and is a maximum, measured in energy in gas per mass of coal, for coals of about 89 wt % carbon on a dry, mineral matter-free basis, or 30% volatile matter. 

Reactions of Goal Ash. Mineral matter impurities have an important effect on the utili2ation of a coal. One of the constituents of greatest concern is pyrite because of the potential for sulfur oxide generation on combustion. The highest concentrations of pyrite are associated with coal deposition under marine environments, as typified by the Illinois Basin, including parts of Illinois, Indiana, and Kentucky. Additionally, the mineral matter... 

The Pott-Broche process (101) was best known as an early industrial use of solvent extraction of coal but was ended owing to war damage. The coal was extracted at about 400°C for 1—1.5 h under a hydrogen pressure of 10—15 MPa (100—150 atm) using a coal-derived solvent. Plant capacity was only 5 t/h with an 80% yield of extract. The product contained less than 0.05% mineral matter and had limited use, mainly in electrodes. 

Minor elements contribute >1 wt % to the ash trace elements contribute <0.1 wt %. The degree of de-ashing achievable by physical cleaning depends on the distribution of mineral matter in the coal. In some cases, a considerable amount of the mineral matter can be removed in other cases, especially where the mineral matter is distributed throughout the coal as microscopic particles, deashing by physical cleaning is not practical. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

Coal Hquefaction iavolves raising the atomic hydrogen-to-carbon ratio from approximately 0.8/1.0 for a typical bituminous coal, to 2/1 for Hquid transportation fuels or 4/1 for methane (4). In this process, molecular weight reduction and removal of mineral matter and heteroatoms such as sulfur, oxygen, and nitrogen may need to be effected. 

Other variations of catalytic and noncatalytic coal Hquefaction schemes were also developed (27,28). Additionally, bench-scale and semiworks systems have been operated in Germany by researchers at Bergbau-Forschung in Essen (29). A 2.5 ton per day pilot plant is being operated by the National Coal Board in the United Kingdom at Point of Ayr in Wales (30). This facdity is notable for the use of semibatch or candle filters for removal of mineral matter and unreacted coal from the primary Hquefaction products. 

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