Mineralogy and geochemistry of coal wastes from the Starzykowiec coal-waste dump (Upper Silesia, Poland)

• Isochemical and mass-transfer processes at the thermally altered coal waste dump.
• Relationship between chemical and mineral compositions in self-heated coal waste dump.
• The behavior of trace elements during self-combustion.

Coal wastes, the products of coal processing, deposited on dumps can undergo processes of self-heating and self-ignition, causing endogenic fires. Burning coal waste dumps are dangerous for the environment. The coal dump located at the Chwałowice Coal Mine (Upper Silesian Coal Basin, Poland) represents a long-term fire which affected the organic, mineral, and chemical composition of a huge amount of deposited wastes. Collected samples were divided into groups on the basis of kind, on the amount or absence of organic matter content, and on the basis of mineral-phase composition. Chemical composition of examined samples reveals heating/combustion processes in altered coal wastes. The organic matter present in the coal wastes changes through the following processes, all causing changes in C, N, O, and H content: (1) organic matter oxidation, (2) organic matter devolatilization, and (3) coke formation. Two processes alter the mineral phases and change the chemical composition of the combusting wastes: (4) mineral phase oxidation (e.g. pyrite and marcasite oxidation to Fe2O3) and (5) mineral dehydration (H2O or OH groups are released from clay minerals and micas). The latter are followed by isochemical processes: (6) mineral structure rebuilding while heating (enantiotropic transformations of e.g. SiO2) and (7) metastable phase formation. Processes 6 and 7 can change the chemistry of deposited wastes through (8) the formation of high-temperature minerals in combustion process (e.g. mullite) and (9) mineral crystallization from melt (e.g. indialite). After dump quenching, some minerals can become unstable because of (10) weathering processes. The behavior of trace elements is also strictly connected with the behavior of mineral phases which have undergone the burning processes. Most of them appear to stay unchanged. Moreover, organic matter content does not generally influence their concentration. Although the combustion processes are so spontaneous that their products cannot be predicted in all details, some dependence between chemical composition of the sample and its location within the dump or after burning were found.