The historical approach to treating refractory arsenic ores has been to use smelting and/or roasting. This entails subjecting the sulphide ore to intense heat, whereby the sulphides are burned off, leaving the desired metals for recovery. There are strict limits on the amount of arsenic that can be burned through a mineral treatment process imposed by the WHO. The industry rule is that smelters cannot treat base or precious metal concentrates that contain more than 0.5% arsenic. Furthermore, they are also subject to a total tonnage limit that can be processed in a given year.
Since bioleaching works in the absence of direct heat, there are no arsenic trioxide (As203) gases produced through the process. In fact, bioleaching can treat concentrates with much higher levels of arsenic than what would be allowed using a pyrometallurgical process.
Bioleaching can handle much higher levels of arsenic for neutralization. Bioleaching converts the arsenic present in the tailings into ferric arsenate, which is a stable and environmentally benign end product (US-EPA).
Previous testing of bioleaching precipitate residues using the Toxicity Characteristic Leaching Procedure (“TCLP”) has been undertaken in conformance with US Environmental Protection Agency (“EPA”) standards, which are recognized internationally as the test method for environmental stability of such materials. The results serve to confirm the environmental suitability of the precipitates for disposal. The EPA TCLP is the procedure used by Ontario Regulation 558 to determine the suitability of a byproduct for land disposal.
In addition to the avoidance of S02 airborne emissions, bioleaching can be an effective method for treating Acid Rock Drainage (“ARD”) through the oxidation of sulphides, which are responsible for leaching metals into surrounding areas. As opposed to “band-aid” solutions that treat contaminated water and/or soils indefinitely, bioleaching acts as a “cure” by targeting the source, the sulphides in tailings, and thus preventing future acid generation from occurring.