Desulphurization of Placer Dome’s Musselwhite mine gold cyanidation tailings
Mill tailings with a potential to generate acid mine drainage should be properly disposed of to avoid environmental problems. Acid may be produced in a tailings site through reactions such as the following:
2FeS2 + 71/2 O2 + H2O Æ Fe2(SO4)3 + H2SO4
Fe2(SO4)3 + 6H2O Æ 2Fe(OH)3 + 3H2SO4
Although bacterial activity is known to promote the process, the principal requirement for these reactions to occur is the exposure of the sulphide mineral to oxygen and moisture. Thus, the placement of an inert cover over previously deposited tailings could provide sufficient protection from these agents and inhibit acid generation. The inert material needed for this purpose might be derived from production tailings by sufficiently reducing their sulphur content, i.e. by desulphurizing them, most probably through flotation. The suitability of desulphurized tailings as a cover material may be assessed by determining their net neutralization potential (NNP), which is the neutralization potential (NP) minus the acid generation potential (AP) of the tailings, all expressed in terms of tonnes of CaCO3/1000 tonnes of tailings. Any positive NNP value would theoretically imply that the tailings would be non-acid generating, but in practice higher values should be aimed at to be more certain.
Placer Dome Canada operates a 4000 tpd gold cyanidation facility at Musselwhite mine, in northwestern Ontario, where the mill tailings have been classified as acid generating due to their higher than acceptable pyrite (FeS2) and pyrrhotite (Fe8S9) content. Currently, the tailings are not desulphurized, but are treated for cyanide destruction using the INCO SO2/Air method prior to their disposal.
As part of the company’s tailings management program, a laboratory investigation was undertaken to examine the effectiveness of flotation in desulphurizing the tailings. Experiments were conducted, using the Box-Wilson experimental design method, to study the effect of pH, collector (KAX – potassium amyl xanthate) concentration, and air flow rate, and a computer simulation program was developed for the flotation process using Microsoft Excel and its Visual Basic Editor. Both the test data and simulation results indicated that the Musselwhite mine gold cyanidation tailings can be successfully desulphurized by froth flotation of the sulphide minerals contained in them. It was found that flotation could readily reduce the sulphur content of the tailings to well below 0.5% and raise their NNP values to 10 or more. The concentrate removal rate necessary to achieve these NNP levels would depend on the flotation conditions employed, however, according to the simulation assessments, it may amount to as low as 5% of the flotation feed. Among the process variables, the air flow rate proved to be the most critical, with pH and collector concentration showing lesser importance, as illustrated in the figure. It was therefore concluded that the amount of air admitted into the flotation cell needs to be carefully optimized and closely monitored in an actual plant operation. The use of copper sulphate for the purpose of reactivating the sulphide minerals was found unnecessary, and the reason for this has been attributed to the fact that copper sulphate is employed during the cyanide destruction process and thus reactivation might already have taken place at that stage. Pilot plant tests are presently underway at the mine site to further investigate the tailings desulphurization process in preparation for full-scale implementation.