Overview of Mine Processes at Vatukoula
Vatukoula is a multi-shaft underground mine. The gold recovery process begins with the crushing and grinding of ore and the removal of the resulting primary slime through screening and rake classification. Next, the bulk gold-telluride/pyrite concentrate and primary slime are sent through separate flotation circuits and then re-combined after achieving a specified level of concentration (Sinclair Knight Merz Pty Ltd, 1994). The combined flotation concentrates then undergo roasting and a process known as cyanidation. Because gold occurs in extremely small concentrations, cyandiation is the only economical way to extract gold particles from the ore. This process involves the application of a cyanide solution to dissolve the gold into an aqueous medium through several leaching circuits, followed by the adsorption of the resulting gold-cyanide complex onto activated carbon. Silver is also recovered at a later stage using sodium chloride (Sinclair Knight Merz Pty Ltd, 1994). Residual cyanide is destroyed through the gassing of pulp from the carbon adsorption circuit with SO2. Tailings are subsequently pumped off for disposal into a tailings dam.
Worldwide, 90 % of gold is recovered using cyanidation (Akcil, 2006). As is the case with any industrial chemical, cyanide can pose unacceptable risks if it is not managed properly. Cyanide is known to react with a wide variety of elements in solution, resulting in the formation of many cyanide-related complexes. Because they process such large quantities of ore, gold mining facilities also produce large quantities of effluent from the cyanidation process, which can contain free cyanide and a variety of metal-cyanide complexes including zinc, nickel, cadmium, copper, arsenic, and iron, depending on the composition of the water that is used in the solution. Effluent is also likely to contain the oxidized products of these compounds, including ammonia, cyanate and thiocyanate (Muezzinoglu, 2003).
Chemical Transportation and Storage
The chemicals used in the mine processes at Vatukoula are imported, transported to the mine, and subsequently stored on-site. The chemicals present at Vatukoula at the time of this study included sodium cyanide (NaCn; solid tablets and liquid solution), sulfuric acid, lead nitrate, zinc powder, nitric acid (HNO3), various flotation reagents, and approximately 100 tons of elemental sulfur (Metallurgy mill worker, name withheld, personal communication, July 27, 2007).
Disposal of Mine Wastes
Effluent is eventually stored in large tailing dams, where some of the cyanide solution is recycled. The dams are left open to the atmosphere to allow the evaporation of hydrogen cyanide (HCN) gas, which is accelerated by direct solar radiation (photolytic dissociation of cyanide) (Muezzinoglu, 2003). Many ores contain pyrites and sulfur-containing inorganic compounds that can make controlling pH extremely difficult. Variations in pH in tailings dams or effluent streams may increase the solubility of metal cyanide complexes and thus create the potential for heavy metal contamination of the ground water, surface water, and soil (Muezzinoglu, 2003).