In gold recovery, the most used hydrometallurgical process is where gold is dissolved by alkaline cyanide solutions in the presence of oxygen.
The ore must be reduced in size, until reaching a granulometry that allows the leaching solution to trap the target metal and bring it to the solution.
The granulometry for this operation is achieved, in ball mills. Cyanide of sodium and lime is commonly added to the mill, so that when the ore is broken, the gold particle is exposed and the solution starts to act and dissolve the gold particle.
In most of the cases this pulp that came from the mill undergoes a process called, CCD or counter current decantation.
The CCD system for washing cyanide pulps is the first step of washing the pulp in thickeners in series.
The pulp is introduced in the first thickener, and the water in the last thickener.
The flow of pulp and water is in opposite directions. As a result, the pulp becomes progressively lower in soluble gold content as it passes the discharge. On the contrary, the water added in the last thickener passes forward, increasing its concentration of lime, cyanide, and gold. The CCD system is used as a supplement of the filtration.
The cake that comes out of the filters, presents approximately 10% humidity. This humidity, is nothing more than a solution with some dissolved gold, which will be sent together with the solids, previous destruction of the cyanide, to the tailings dam.
Finally the rich solution is clarified, deoxygenated, and gold precipitated with very fine zinc powder, and of high purity.
The precipitate Zn-Au-Ag is separated from the solution by means of filters, then it is dried and sent to smelting, where a doré bar with gold and silver contents is obtained, to send to refining. The solution with dissolved gold, which did not recover, returns to the leaching circuit.
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