A filter is simply a large drum slowly rotating on a horizontal shaft. The drum is porous and partially submerged in a semicircular steel tank, into which pulp from the bottom of the thickener is pumped.
As the drum rotates, a vacuum is applied, causing the pulp to adhere to the drum. Further vacuuming then sucks out the solution.
Water is sprayed on to the outside top of the rotating drum to wash out any entrapped solution. The vacuum also catches this solution. Some mill operators filter the pulp twice to be sure all that’s left of the valuable gold-cyanide solution is recovered.
The remaining solid material, or filtercake, is mixed with water and pumped outdoors to a tailings pond. In the past, mill tailings were pumped into swamps and small lakes. Today, they must be adequately contained so that they can not drain into surrounding waterways where they can damage the surrounding ecosystem. Dams and other barriers are often constructed to contain tailings.
All the gold in the ore is now contained in solutions, either from the thickener overflow or the filtering circuit, These solutions are collected in a tank and then pumped through canvas sheets to remove any fine clay particles in a process known as clarification. Clarified solutions are sparkling clear, with a light green tint. Fine zinc dust is added to the solution, and it combines with the gold to form a precipitate which is caught between leaves of canvas in a filter press.
This gold precipitate, which resembles black mud, is quite impure. It must be refined to remove the zinc and any iron, copper or other contaminants it may contain.
The modern approach is to avoid much of the above process of thickening and filtering in favor of direct gold recovery using activated carbon granules. This is called the carbon-in-pulp (CIP) process, and it is used in most newer mills because it avoids many of the solid/liquid separation stages, there by keeping recovery costs low.
In the CIP process, the cyanide pulp is treated in four to six smaller tanks into which are added coarse, activated carbon granules (usually ground and burnt coconut shells). The gold in the solution is absorbed on to these granules and the granules containing the gold are screened from the pulp, there by recovering the gold.
The gold is recovered from the carbon by washing with a small amount of hot, strong sodium hydroxide and sodium cyanide solution. Gold is recovered from this concentrated solution by electrolysis, which causes it to be deposited onto steel wool cathodes. Just as in the Merrill-Crowe process, a final refining step is necessary before pure gold is produced.