How can gold be extracted

At the present day, the wet methods produce little more than a tenth of the total output of gold, while mechanical improvements in the old processes, made during the last half-century, are probably answerable for four or five times as much.

You can also search for this author in PubMed Google Scholar. Reprints and Permissions. ROSE, T. The Extraction of Gold by Chemical Methods. Nature 55, — Download citation. Next, mill operators thicken the slurry with water to form pulp and run the pulp through a series of leaching tanks. Leaching dissolves the gold out of the ore using a chemical solvent. The most common solvent is cyanide, which must be combined with oxygen in a process known as carbon-in-pulp. As the cyanide and oxygen react chemically, gold in the pulp dissolves.

When workers introduce small carbon grains to the tank, the gold adheres to the carbon. Filtering the pulp through screens separates the gold-bearing carbon. The carbon moves to a stripping vessel where a hot caustic solution separates the gold from the carbon. Another set of screens filters out the carbon grains, which can be recycled for future processing.

Finally, the gold-bearing solution is ready for electrowinning , which recovers the gold from the leaching chemicals. In electrowinning, operators pour the gold-bearing solution into a special container known as a cell. The zinc causes a chemical reaction which separates the gold from the ore. The gold is then removed from the solution with a filter press. For the carbon-in-pulp method, the ground ore is mixed with water before cyanide is added. Then carbon is added to bond with the gold.

The carbon-gold particles are put into a caustic carbon solution, separating out the gold. In heap-leaching, the ore is placed on open-air pads and the cyanide sprayed over it, taking several weeks to leach down to an impervious base. The solution then pours off the pad into a pond and is pumped from there to a recovery plant where the gold is recovered. Heap-leaching helps recover gold from ore that would otherwise be too expensive to process. Another process involves ground ore being passed over plates which are coated with mercury.

The gold and the mercury form an amalgam, leading to the name of the process, amalgamation. The amalgam collects at the bottom of the mixing container and is easily separated from the other minerals. Subsequently, the amalgam is heated until the mercury evaporates leaving the pure raw gold.

This method also involves dangers to health and the environment experienced when vaporizing highly poisonous mercury. Elecrowinning is in the meantime a frequently used process whereby gold is extracted without the use of toxic substances. This method uses an electrochemical approach. Here advantage is taken during the extraction of other precious metals such as f.

In the process of the electrolytic cleaning of the rock sludge the anode activates the precious metal bearing rock sludge. The cathode is made of a piece of pure metal. During electrolysis, the gold in the raw rock material is not dissolved and collects as a sludge under the anode.