7 October 2009

… Sometimes oxygen makes pollution worse

Oxidation of sulfides in mining wastes produces high concentrations of sulfate, iron, and other metals, and frequently also very low pH values.

Compared to fine-grain mine tailings, produced in ore treatment by flotation and other techniques, waste rock is just material comprising large size particles and deposited in waste rock piles. In waste rock piles with high permeability and sulfide content, the oxidation of sulfides produces heat, and temperatures may reach very high values. When that occurs, air with oxygen sucks into the pile in convection, and this accelerates the pyrite oxidation rate. GenFeration of contaminants is then much higher than in the oxygen diffusion typical for mine tailings, and environmental impact may be severe.

There is a program out there called THERMOX that enables the evaluation of environmental impact of waste rock piles under different scenarios (e.g., varying climatic conditions, different geometries of a pile, etc.). On a subsequent stage, different mitigation and remediation options can also undergo evaluation using the program. The program is highly versatile and may see use in other applications as well, including sequestration of carbon dioxide and multiphase transport of organic contaminants.

Researchers used the waste rock pile at the Doyon Mine site in northern Québec, Canada, for the analysis. Field studies and numerical modeling revealed a zone of fast convective oxygen supply close to the pile slope and a zone of much slower oxygen supply in the pile core. Convective oxygen supply close to the pile slope resulted in high temperatures and concentrations of dissolved contaminants. Internal evaporation further enriches the already very concentrated pore solutions. Researchers confirmed the presence of internal evaporation by applying stable isotopes deuterium and oxygen-18 at the Doyon Mine site.