Taking Gold Out of Solution

Traditionally, recovering the gold from the cyanide solution was achieved by separating the pregnant, or gold-laden, solution from the barren solids present and afterwards precipitating the gold.

The Merrill-Crowe method is what the traditional approach is called. The first step is to move the pulp from the agitators to one or more thickeners – large, shallow tanks. The solution flows over the top of the tank and is collected in a launder around the tank's perimeter, while the worthless rock particles sink to the bottom and are slowly raked to the center by mechanical arms which operate in a continuous manner. This material is discharged through a pipe at the bottom of the tank but it contains too much valuable material to be discarded, so it is filtered to recover the additional gold.

Filtering
A filter is just a large drum which is slowly rotating on a horizontal shaft. The drum is porous and is partially submerged in a steel tank that is semi-circular, into which pulp from the bottom of the thickener is pumped. A vacuum is applied as the drum rotates, causing the pulp to adhere to the drum. Additional vacuuming afterwards sucks out the solution.

Water is sprayed on to the outside top of the rotating drum to wash out any solution that is entrapped. This solution is also caught by the vaccuum. Some operators of mills filter the pulp a couple of times to be sure all that is left of the valuable gold-cyanide solution is recovered.

The solid material which remains, or the filtercake, is mixed with water and pumped outdoors to a tailings pond. Mill tailings in the past were pumped into small lakes and swamps. Nowadays, they must be adequately contained so that they will not be able to drain into surrounding waterways where they can harm the ecosystem that surrounds them. Dams and other barriers are often built to contain tailings.

All the gold in the ore is now contained in solutions, either from the thickener overflow or the fil-tering 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 precipítate which is caught between leaves of canvas in a filter press.

This gold precipitate, which resembles black mud, is very impure. It must be refined to remove the copper and any iron, zinc or other contaminants it is likely to contain.

The modern approach is to avoid a lot of the previous process of filtering and thickening in favor of direct gold recovery using activated carbon granules. This process is called the carbon-in-pulp (CIP), and it is used in most of the newer mills because it avoids many of the solid/liquid separation stages, this way keeping recovery costs very low.

In the process of carbon-in-pulp, the cyanide pulp is treated in four to six smaller tanks into which are added coarse, activated carbon granules which are 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, in such a way as to recover the gold.

The gold is recovered from the carbon by washing with a small quantity of strong, hot sodium hydroxide and sodium cyanide solution. Gold is recovered from this concentrated solution by elec-trolysis, which causes it to be deposited onto steel wool cathodes. A final refining step is necessary before pure gold is produced just as in the Merrill-Crowe process.