Alluvial gold refers to fine particles of elemental gold found in riverbeds, streambeds, and floodplains. Alluvial deposits are either dredged from pond and river bottoms or sluiced from banks and floodplains with high-pressure hydraulic hoses. They are usually concentrated by gravity techniques and require little or no comminution.

An alluvial gold dust buyer will then assay the elemental gold before releasing it for refining. Gold refining begins with amalgamation or cyanidation.

Amalgamation is the process of combining gold ore with mercury by either slurry mixing techniques or grinding. The resulting amalgam is heated to distill off the mercury.

Cyanidation is a process of oxidizing gold and dissolving it in an alkaline cyanide solution, based on the Elsner reaction, to allow the gold-bearing solution to be separated from the solids. This is accomplished for higher grade ores in large tanks (vat leaching) or for low-grade ores, by spraying a dilute cyanide solution over the ore (heap leaching).

Granular activated carbon can be added to the ore slurry during or upon the completion of gold solubilization to remove it from the solution. Gold is then leached by chemical solution or deoxygenation and filtering. Gold ores not amenable to cyanidation (refractory ores) can be treated with various oxidizing processes, using high temperature and high pressure to remove interfering substances prior to cyanidation.

Gold extracted by amalgamation or cyanidation is then melted into gold dore’ bars of about 90 percent purity. The gold dust buyer or gold dore’ purchaser then transports the bars to a refinery for further refining.

Using the Miller process, the gold is melted and gaseous chlorine is blown onto the liquid, causing impurities to form chloride compounds that separate into a layer on the surface of the gold. This produces gold bars of 99.5 percent purity.

The gold is further refined by the Wohlwill process, which uses electrolysis with a hydrochloric acid/gold chloride solution, or by a wet chemical process using a nitric acid/hydrochloric acid mixture, to produce gold bars of 99.99 percent purity.

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Sources: Encyclopedia Britannica, National Mining Association