CONCENTRATE SEPERATION

Rinsing the biooxidized heap with fresh water is an integral part of the GEOCOAT® process. In the treatment of refractory gold sulfide concentrates, rinsing reduces downstream cyanide consumption by removing soluble species from the oxidized coating before it is separated from the support rock. Additional removal of cyanide-consuming species takes place in the subsequent screening and thickening steps. Residual soluble species in the oxidized concentrate are further diluted by the addition of fresh water on the screen or trommel and removed with the thickener overflow.

Base metal GEOCOAT® heaps are rinsed to recover residual soluble metal values from the oxidized concentrate. Since the heap is typically not reclaimed after biooxidation, rinsing may need to be more thorough than for a refractory gold heap to ensure maximum recovery of valuable metals.

When a heap section, or panel, has been biooxidized to the target level, solution application is stopped and the panel allowed to drain. Because of the highly porous nature of the GEOCOAT® heap, draindown of most of the free moisture is completed in eight hours or less. The panel is then rinsed with fresh water. Rinse solution is directed to the on-solution pond or to a rinse solution pond from where it can be returned to the system as make-up water. The rinse water added to the system constitutes make-up, replacing water lost by evaporation and with the bleed solution discharged to the neutralization system or to the downstream metals recovery processes. After rinsing, the solution application piping is removed in preparation for heap reclaim.

HEAP RECLAIM

In the treatment of base metals concentrates, the heap is not reclaimed unless the support rock is to be recycled or the oxidized residue contains recoverable precious metals values. In processing refractory gold ores, since the values remain in the biooxidized concentrate residue, the heap is reclaimed and the material screened to separate the residue from the support rock.

A front-end loader transfers the oxidized material from the heap to the feed hopper of a grasshopper conveyor which feeds a horizontal conveyor running alongside the pad on the opposite side to the stacker feed conveying system. The loader operates on the drainage layer which protects the liner and solution collection and air distribution piping from damage. The conveyor delivers the material to the feed hopper of the washing screen or trommel. Large operations may justify the use of bucket wheel reclaimers and mobile conveyors.

SCREENING AND THICKENING

Where precious metals values are to be recovered from the oxidized concentrate, it is separated from the support rock on a horizontal vibrating screen or in a trommel. The tumbling action of the screen (or trommel) loosens the oxidized coating for removal from the support rock by water sprays. The washed support rock is conveyed to the stockpile and from there it is returned to the heap for recoating with fresh concentrate.

The screen (trommel) underflow is pumped to the oxidized concentrate thickener. The pH of the thickener underflow is adjusted by the addition of lime and the slurry pumped to the trash screen of the CIL plant. Trash screen oversize, mainly degraded support rock fragments, is discarded as waste.

The thickener overflow contains high concentrations of bacteria which have been stripped from the surfaces of the support rock and oxidized concentrate particles. To take advantage of these highly adapted bacteria, the thickener overflow is recycled to the on-solution pond for reapplication to the heap. This ensures that newly stacked panels are inoculated with adapted bacteria, minimizing the lag time for initiation of biooxidation.