Cyanidation
More gold is recovered by cyanidation than by most other processes. In cyanidation, the metallic gold is oxidised and then dissolved in an alkaline cyanide solution. When the gold dissolution process is complete, the gold-bearing solution is separated from the solids.
Ores with very high gold content of 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 cyanide, and this percolates down through the piled ore, dissolving the gold. With this method there are very strict environmental guidelines to prevent the cyanide from seeping into the ground water.
These strict defined rules, are laid out by the International Cyanide Code, for the safe and responsible use of cyanide.
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Gold refining
This refers to processes where the pure precious metals are extracted from mined material, doré, and from recycled products such as jewellery and electronics.
The main techniques to extract the impurities from the 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 | |||||
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The two gold refining methods most typically used to derive pure gold are: the Wohlwill and the Miller process.
With the Miller process gaseous chlorine is used 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 straightforward, but it produces gold of only about 99.5 percent purity.
During the Wohlwill process, electrolysis increases purity to about 99.99 percent. In this process, a casting of impure gold is lowered into an electrolyte solution of acid and gold chloride. Under the influence of an electrical current, the gold migrates to a charged electrode (cathode), where it’s restored to a highly pure metallic state, leaving the impurities as a separate solution or residue.
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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 valuable 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 |
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