Aqua Regia Refining

Aqua regia (royal water) is the name given to a mixture concentrated hydrochloric and nitric acids (in a ratio of 3:1 respectively). It has been known about for more than 1,000 years, firstly by Islamic alchemists and later by European alchemists from around 1300.

It is so named because it can dissolve gold (which neither acid can do on its own) as well as some of the platinum group metals. As soon as these acids are mixed together, a slow chemical reaction starts which produces the volatile products nitrosyl chloride and chlorine gas:

HNO₃ (aq) + 3 HCl (aq) → NOCl (g) + Cl₂ (g) + 2 H₂O (l)

The mixture is sufficiently stable that it has a reasonable shelf-life. However, as the acid gradually loses its potency to dissolve gold because of this reaction, it is normally produced by mixing the two acids just before use.

The mechanism that allows gold to dissolve involves the nitric acid acting as an oxidizing agent to produce trivalent gold ions (Au³⁺) which then combine with the chlorine ions to form tetrachloroaurate ions (AuCl₄^-). The resulting solution is known as chloroauric acid.

Industrial plants employ one of two types of digesters: glass (which allows the acid to be heated in the final stages of dissolution) and polypropylene (in the form of a drum, as shown here, which rotates to encourage the reaction kinetics). The precious metal feedstock is usually granulated to form grain or 'popcorn' to provide a large surface area to help speed the dissolution.

Because silver chloride is insoluble, silver does not dissolve in aqua regia. If the feedstock contains too high a proportion of silver, the dissolution reaction may be incomplete (because some of the gold ends up being encapsulated in silver).

Once the dissolution is complete, the liquid is separated from the solid residues (from which silver is recovered) and then gold is selectively precipitated using reducing agents such as sulphur dioxide or ferrous sulphate.