The good news for rainbow trout and other aquatic species living downstream from gold mines is that the industry has largely succeeded in controlling deadly pollution from cyanide used as a leach reagent.
The bad news is that there are still plenty of potentially harmful components in mine wastewater that can be acutely toxic to fish and invertebrates. Among them is ammonia, produced through the use of ammonium-based explosives in blasting operations. Ammonia is also produced through the hydrolysis of the cyanates produced by oxidation of cyanide.
In response to the lethal threat that ammonia and other effluent components such as copper pose to aquatic environments, many countries have enacted comprehensive environmental regulations to control mine discharges. Until now, however, finding an approach effective in treating both ammonia and copper in cold climates has remained a challenge.
In a recently concluded eight month pilot test using mine water from an operating Canadian gold mine, Veolia Water Technologies has demonstrated that a two-part solution could eliminate effluent toxicity. The selected process combines precipitation to remove copper and biological oxidation to remove ammonia, cyanide and its derivatives. Although the two technologies have been used in hundreds of installations worldwide, their combination to remove toxicity in gold mine effluent is novel.
The precipitation was conducted using Veolia’s ACTIFLO® sand ballasted, high rate clarification process, which provides highly effective removal of suspended solids. For the biological oxidation, Veolia’s AnoxKaldnesTM Moving Bed Biofilm Reactor (MBBR) technology was used to remove the ammonia. Based on the biofilm principle, the biological process uses microorganisms grown on the surfaces of plastic carriers in the treatment reactor to remove contaminants from wastewater.
After experimenting with the sequencing, times and temperature of the process steps, Veolia’s team found that the toxicity of copper and ammonia in gold mine effluents could be eliminated through a three-step treatment consisting of:
- rough removal of copper using sand-ballasted clarification in the presence of ferric sulfate and sodium sulfide;
- biological oxidation in two moving bed biofilm reactors;
- final copper removal using sand-ballasted clarification in the presence of ferric sulfate.
At present, Veolia is working with the Canadian gold mine to design an optimal wastewater treatment plant.