Reflectance petrographic analyses

Seam correlations, measurements of rank and geologic history, interpretation of petroleum (qv) formation with coal deposits, prediction of coke properties, and detection of coal oxidation can be deterrnined from petrographic analysis. Constituents of seams can be observed over considerable distances, permitting the correlation of seam profiles in coal basins. Measurements of vitrinite reflectance within a seam permit mapping of variations in thermal and tectonic histories. Figure 2 indicates the relationship of vitrinite reflectance to maximum temperatures and effective heating time in the seam (11,15). 

This investigation relied on petrographic analysis of polished sections using reflected light and the scanning electron microscopy (SEM) and electron microprobe (EPMA) analyses to identify minerals and to document the distribution of gold. The mineralized zone is coincident with a distinct bleached alteration zone that contains fine- to coarse-grained, subhedral arsenopyrite and pyrite in quartz-carbonate veins. 

Briquets prepared at BCR for petrographic analysis are maintained in a desiccator for an extended period prior to microscopic analysis. Specific high moisture coals have been analyzed for reflectance at regular intervals following their insertion in the desiccating atmosphere under these conditions of continued desiccation, reflectance remained relatively constant. 

The maceral content defines the coal type sapropelic, with >50% liptinite, or humic, more abundant, usually presenting a banded structure. On the other hand, based on the different optical properties of macerals, the reflectance of vitrinite is an essential characteristic used in coal identification and related to rank. A good analysis of maceral content provides knowledge about the chemical composition of a coal, their behavior in different conversion processes, and can also be used as a parameter of coal rank (see section on Petrographic analysis). 

The clear knowledge about the purity of a coal is decisive for the correct design of gasification processes. No other method than the petrographic analysis, especially the reflectance analysis, gives a more precise statement regarding composition of a coal sample. Other bulk parameters, such as fraction of volatile matter, carbon content, or heating values, are inappropriate to determine whether a pure coal or a mixture (coal blend) are on hand. 

The heat-treated samples undergo a standard petrographic analysis determining the mean and maximum reflectance. 

The samples collected from the technical system experience the same petrographic analysis as did the heat-treated samples collecting mean and maximum reflectance readings. 

Coal petrography (Chapter 4) has become widely used for predicting coke quality based on coal analysis and has led to a system for predicting coke stability based on petrographic entities and reflectance of coal (Schapiro and Ciray, 1960). Thus, an optimum blend of coals could be selected to produce desired coke quality. 

The most widely used petrographic analyses of coal are maceral analysis and vitrinite reflectance analysis. Both are performed on representative samples ground to < 1 mm in size and embedded in resin. The polished surfaces are then examined under a white reflected light microscope. 

Saeidi et al. (2013) have proposed a modified empirical criterion to determine the strength of transversely anisotropic rocks. They used the tests from intact anisotropic slate obtained from three different districts of Iran and the data of Saroglou Tsiambaos (2008). Based on their analysis, they conclude that the parameter kp reflects well the strength anisotropy of the slates tested, as shown by its variation presented in Figure 8. They also suggest that the abnormal variation of kp for slate S can be attributed to its petrographical properties (inter-beddings of clay, detritus, volcanic ash). 

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