SOLUTION TREATMENT

Refractory Gold Concentrates

As described above, a portion of the recirculating solution stream is bled continuously from the circuit and either neutralized or advanced to downstream recovery processes. The purpose is to maintain acidity and metals concentrations at target levels. Operation at very low pH values or high iron levels may result in inhibition of bacterial activity.

Concentrates in which pyrite constitutes the major sulfide mineral generate acid during biooxidation. Unless this acid is neutralized in-situ by carbonates or other acid-consuming constituents in the concentrate or support rock, the acid concentration in the recirculating solution will increase as biooxidation proceeds. Replacing the solution bled to neutralization with fresh water allows the pH to be controlled to the desired value.

Concentrates containing arsenic, usually as arsenopyrite, generate soluble As(III) during biooxidation. Neutralization of the solution under appropriate conditions results in precipitation of arsenic as ferric arsenate or scorodite. Sludges from the neutralization of biooxidation solutions containing arsenic have been shown to meet the requirements of the USEPA's TCLP test for stability.

Neutralization is carried out in a series of four or more agitated tanks, using ground limestone and lime. The cost of ground limestone is typically lower than that of lime with equivalent neutralizing capacity. Overall neutralization costs can be minimized by using limestone to raise the pH to approximately 5, and then adding a small quantity of lime to further raise the pH to 7-8, if necessary. Retention time in the neutralization tanks is typically 4-6 hours.

Neutralized effluent may be pumped directly to a tailings impoundment for disposal. Alternatively, the effluent may be thickened to separate the precipitated solids, allowing the thickener overflow to be reused as make-up water. The thickened sludge is pumped to the tailings impoundment.

Base Metal Concentrates

In the GEOCOAT® processing of base metal concentrates, the solution bleed stream is the feed to downstream metals recovery processes. The flow rate of the bleed stream is selected to maintain the concentration of the metal to be recovered at the optimum for downstream processing. The high permeability of the GEOCOAT® heap allows recirculation of solution to build up metals concentrations without fear of losses by retention in the heap.