Filtering

The following is an excerpt from Mining Explained, published by The Northern Miner.

A filter is simply a large drum slowly rotating on a horizontal shaft. The drum is porous and partially submerged in a semi-circular 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 cannot 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 pumped through canvas sheets to remove any fine clay particles. this process is known as clarification. Clarified solutions are sparkling clear, with a light green tint. Fine zinc dust is added to the solution, and this 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 favour 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, thereby 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, thereby 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 on to steel wool cathodes. Just as in the Merrill-Crowe process, a final refining step is necessary before pure gold is produced.

Refining gold

Refining is the most spectacular part of the process. Silica, borax and soda ash are added to the dried precipitate (or steel wool, in the case of CIP), which is heated in a furnace. This results in a miniature smelting operation. On top of the melt is the slag containing the impurities, while the molten gold’s greater density causes it to sink to the bottom.

When the furnace’s contents are completely melted, the furnace is tilted and the molten material is poured into a conical mold. Worthless black slag forms on the top and is broken from the underlying gold button once it cools.

The button or buttons (there may be enough precipitate to necessitate more than one melt) are again placed in the furnace, melted and poured into bar molds.

Finally, the bars are weighed, small samples are removed to determine purity (expressed as fineness in parts per thousand), and the bars, called “dore bars,” are packed for shipping. In due course, the mine receives a cheque for the gold.