Reduction of High-Chromium-Containing Wastewater in the Leaching of Pyritic Waste Rocks from Coal Mines

Coal is an abundant resource which can be used to produce low-cost energy; however, its usage causes great environmental damage. Before mineral coal can be used, it must be processed to remove coal tailings. These tailings contain pyrite and accumulate in large dumps, presenting significant environmental liabilities, such as acid mine drainage. Another industry that generates environmental liabilities is the chrome-plating industry, mainly because it produces hexavalent chromium (Cr 6+ ) waste. The main aim of this work was to evaluate Cr 6+ as a reduction agent in trivalent chromium (Cr 3+ ) conversion in the leaching of coal-mine waste containing pyrite. Cr 3+ is about 100 times less toxic than Cr 6+ and can be easily removed from industrial effluents by alkaline precipitation. There are several sources of effluents containing Cr 6+ —a compound which is known worldwide to be toxic, carcinogenic, and mutagenic. A leaching and treatment device was developed and tested for waste treatment. The results indicated that the developed treatment system reduced 100% of Cr 6+ to Cr 3+ through pyrite leaching in a Cr 6+ wastewater sample from the electroplating industry. In addition, the chromium sludge resulting from the treatment process, after calcination, was tested in a ceramic glaze as a pigment and, when compared with an industrial pigment, showed similar mineralogical characteristics.