Methodology of statistical study of the chemical composition of by-products of coal mining to assess their suitability as materials for reclamation

The results of the research presented in earlier works with the participation of the authors (Klojzy-Karczmarczyk et al. 2016a, b, c) indicate the possibility of the use of mining waste from Janina Coal Mine (Upper Silesian Coal Basin, Poland, TAURON Wydobycie S.A.) or products (aggregates) produced on the basis of gangue for technical reclamation. It is essential for this process to obtain a material with low total sulfur, and it has been shown that the best quality parameters are obtained for the material subjected to modification by the rejection of fine fractions. In this work, the lower limit of the material’s fraction was determined using statistical analysis, wherein with the predetermined but low probability of error, the content of contaminants (sulfur, as well as other elements) does not exceed the allowable limit (reference value). To solve this task, 3 different statistical methods were used: evaluation of guaranteed contents (the maximum for a given degree of probability) of the polluting components, the results of statistical hypotheses testing and analysis of correlation and regression. The applied statistical methods produce results very proximate to each other. In the presented case study for the Janina coal mine based on the results of a statistically small sample ( 20 mm may be considered as safe for the purposes of reclamation, and the material fraction >25 mm as very safe. It cannot be ruled out, however, that this limit may be reduced to a diameter of 16 mm. The content analysis of 11 elements (As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, V, Zn determined as total content in the secondary samples for the fraction 10-200 mm (the fraction >200 mm was not present) showed that they are lower than the allowable limits specified by relevant legal regulations. For this fraction, exceedance of the limit values of sulfur content is, however, highly probable. For this reason, the primary criterion for determining the suitability of the waste material for the purposes of reclamation should be the total sulfur content, and the determination of the lower limits of the material fraction that meets the environmental requirements should be conducted using statistical methods based on the results of experimental sampling of rock remaining after coal preparation. The use of advanced methods of statistical analysis may facilitate the design of technological processes, leading to obtain industrial quantities of high quality, appropriately graded reclamation material.