Mineral matter distribution

Analyses of mineral matter distributions as a function of parti-... 

Investigations of many other coal seams and coals within a particular seam indicate that indeed the mineral matter distribution... 

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. 

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. 

Albertsson (Paiiition of Cell Paiiicle.s and Macromolecules, 3d ed., Wiley, New York, 1986) has extensively used particle distribution to fractionate mixtures of biological products. In order to demonstrate the versatility of particle distribution, he has cited the example shown in Table 22-14. The feed mixture consisted of polystyrene particles, red blood cells, starch, and cellulose. Liquid-liquid particle distribution has also been studied by using mineral-matter particles (average diameter = 5.5 Im) extracted from a coal liquid as the solid in a xylene-water system [Prudich and Heniy, Am. Inst. Chem. Eng. J., 24(5), 788 (1978)]. By using surface-active agents in order to enhance the water wettability of the solid particles, recoveries of better than 95 percent of the particles to the water phase were obsei ved. All particles remained in the xylene when no surfactant was added. 

Cellulose is the most abundant of naturally occurring organic compounds for, as the chief constituent of the eell walls of higher plants, it comprises at least one-third of the vegetable matter of the world. The cellulose eontent of such vegetable matter varies from plant to plant. For example, oven-dried cotton contains about 90% cellulose, while an average wood has about 50%. The balance is composed of lignin, polysaccharides other than cellulose and minor amounts of resins, proteins and mineral matter. In spite of its wide distribution in nature, cellulose for chemical purposes is derived commerically from only two sources, cotton linters and wood pulp. 

The processes that govern the formation of ash particles are complex and only partially understood (Figure 7.12). The mineral matter in pulverized coal is distributed in various forms some is essentially carbon-free and is designated as extraneous some occurs as mineral inclusions, typically 2-5 pm in size, dispersed in the coal matrix and some is atomically dispersed in the coal either as cations on carboxylic acid side chains or in porphyrin-type stmctures. The behavior of the mineral matter during combustion depends strongly on the chemical and physical state of the mineral inclusions. 

The size distribution of the kaolinite is quite variable and again the Kingston sample differed in that most of the kaolinite has sodium associated with it and could therefore be considered a smectite rather than kaolinite. Marcasite is commonly present as a replacement of original coal particles. The gypsum is present mainly in the -20 ym size range. Overall the mineral matter content of the lignites is of the order of 5%-10% (i.e. a medium distribution density). 

The SEM-AIA results contain very detailed information for the composite coal/mineral particles and their component parts (i.e., information on size, phase identification, and associations) which can be presented in a number of ways. Tables can be prepared to show the distribution of the sample as a function of particle size and to show the coal-mineral association in terms of bulk properties or in terms of surface properties. For bulk properties, the distribution of coal and minerals is prepared as a function of the total mineral content of the individual particles which can be related to particle density. For surface properties, coal and mineral data are tabulated as a function of the fraction of particle surface covered by mineral matter which can be used to predict the surface properties of the particles and their behavior during surface-based cleaning. Examples of these distributions are given below. 

The first method chosen to express the coal-mineral association results is in terms of the weight fraction of mineral matter in the individual particles, as determined from their cross section. The resulting distribution is comparable to the so-called "grade distributions" used in the mineral industry [8,9]. Such a distribution is included in Table II for the Upper Freeport coal. The data in the table indicate that pyrite is preferentially liberated as compared to quartz or kaolinite. About 78% of the pyrite is in particles containing more than 80% mineral matter, which should be easily removed by density-based separations. 

Figure 4. Distribution of coal and mineral matter in Upper Freeport coal as a function of the mineral matter content of the particles.

However, Figure 5 illustrates that the general distribution of mineral matter is not the same as that of the pyrite which is liberated to a larger degree. 

In addition to expressing coal-mineral association as a function of particle mineral matter content, our most recent efforts have emphasized the determination of coal-mineral association based on particle surfaces. While the previous distributions were measured in terms of bulk properties to provide an indication of the probable cleaning behavior of a coal in a density-based process, they do not lend much insight into cleaning behavior during surface-based processes, such as froth flotation or oil agglomeration. There is no a priori reason to expect that association measured and expressed in terms of bulk composition is generally indicative of association measured in terms of particle surfaces. Therefore, it would also be useful to express results in terms of the proportion of coal (or minerals) present on the surface of the particles. 

Results for the Indiana coal show a distinctly different character. The association distributions are presented in terms of bulk composition in Figures 8 and 9. In general, the mineral matter does... 

The above conclusions based on SEM-AIA measurements of association in terms of particle surfaces are somewhat more tentative than the conclusions drawn from bulk association distributions. There are more analytical difficulties when characterizing the particle surfaces than when characterizing the bulk sample. In addition, the factors determining the overall surface nature of a particle are more complex than just the relative amount of the phases present on the particle surface. However, the SEM-AIA results can still provide a useful and heretofore unavailable insight into the nature of mineral matter in coal. 

Simultaneously with the efforts to determine the origin of mineral matter in coal, systematic efforts were underway to estimate the quantitative distribution of trace and minor elements in American coals. The early analyses were performed on high-temperature ashes, and as a consequence, the investigators had to be content with determining the nonvolatile metallic oxides. However, with the advent of the low temperature asher and improvisations and advances in wet chemical, radiochemical, and instrumental analytical techniques, we not only can analyze nondecomposed mineral matter but also can study the composition of whole coal. 

Watt, J. D., The Physical, and Chemical Behaviour of the Mineral Matter in Coal Under Conditions Met in Combustion Plant. Part I, The Occurrence, Origin, Identity, Distribution, and Estimation of the Mineral Species in British Coals, British Coal Utilization Research Association, Literature Survey, 121 p., Leatherhead, Surrey, England, 1968. 

An electronmicroprobe x-ray analyzer was used to determine how and what type of mineral matter is distributed on the surface of the various lithotypes. Results indicated that the mineral matter located at the surface is vastly different among the various lithotypes. The mineral matter appearing on the flat, homogeneous vitrain surface (Figure 4) was found to be distributed... 

Samples of raw and cleaned coals produced at the Ames Laboratory and obtained from TRW have been characterized for the mineral matter content and distribution among various particle sizes. The sulfur forms before and after treatment have been determined by conventional ASTM techniques and a direct SEM method. 

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