Processing, smelting and refining gold

From ore to doré

Cyanidation

More gold is recovered by cyanidation than by any other process. In cyanidation, metallic gold is oxidised and dissolved in an alkaline cyanide solution. When gold dissolution is complete, the gold-bearing solution is separated from the solids.

With ores of higher gold content (greater than 20 grams of gold per tonne of ore), cyanidation is accomplished by vat leaching, which involves holding a slurry of ore and solvent in large tanks equipped with agitators.

For extracting gold from low-grade ores, heap leaching is practiced; huge heaps are sprayed with a dilute solution of sodium cyanide, and this percolates down through the piled ore, dissolving the gold.

There are very well defined rules for the safe and responsible use of cyanide – as laid out in the International Cyanide Code.

Gold refining

This refers to processes used to processes used to extract and separate the precious metals in mined material, doré, and from recycled products (jewellery and electronics).

The main techniques used to remove the final impurities to create high caratage gold are summarised below:

Refining Technique Removes Base Metals Removes Silver Removes PGMs Large Scale Refiners Small Scale Refiners
Cupellation x x
Inquartation and Parting x x
Miller Process x x
Wohlwill Electrolytic Process* x
Fizzer Cell x
Aqua Regis Process**
Pyrometallurgical Process ✔*** x x x

*Can only be used when initial gold content is about 96%
**Only suitable when initial silver content is less than 10%
***Copper remains in gold

The two gold refining methods most commonly employed to derive pure gold are: the Miller process and the Wohlwill process. 

The Miller process uses gaseous chlorine to extract impurities when gold is at melting point; impurities separate into a layer on the surface of the molten purified gold. The Miller process is rapid and simple, but it produces gold of only about 99.5 percent purity. 

The Wohlwill process increases purity to about 99.99 percent by electrolysis. In this process, a casting of impure gold is lowered into an electrolyte solution of hydrochloric acid and gold chloride. Under the influence of an electric current, the gold migrates to a negatively charged electrode (cathode), where it is restored to a highly pure metallic state, leaving the impurities as a separate solution or residue.

Assaying
This refers to the processes used to determine the composition of a material or item (e.g. jewellery, bar or coin) and measure the proportion of precious metal content.

The main techniques used are:

Technique Versatility Accuracy Limitations
Fire Assay Only determines gold 0.2 parts per thousand Modifications needed when nickel and pgm's present
ICP Spectrometry Can determine other elements 1 part per thousand None
X-Ray Flourescence Can determine other elements 2-5 parts per thousand Restricted largely to flat specimens
Touchstone Only determines gold Largely a sorting test, 15 parts per thousand Unsuitable for high carat and hard white golds
Density Measurement Only determines gold Poor