The Miller process can be used to produced gold of 99.99% purity or even higher. Nevertheless, the low activity of the remaining traces of impurities, mainly copper and silver, when the chlorination process completion, results in an ever increasing proportion of the chlorine reacting with the gold. The gold chlorine being volatile passes from the reaction zone and consequently needs special collection arrangements for its recovery. This renders the process costly and therefore when high purity gold has to be produced, electrolytic refining is preferred. However, in this case a lock-up of gold in the electrodes and electrolyte has to be faced and the interest on capital represented by this immobilized gold can amount to a considerable sum of money if large scale electro-refining is practiced. As a result, gold for monetary purposes is refined by chlorine and the demands for high purity gold are by electro-refining.
It is common to find traces of platinum group metals, which are not removed by chlorination. Iridium in particular is not very soluble in gold and may appear as black spots or specks on the surface of the refined bars. For example, in some gold operations in South Africa is high the platinum group metals content and chlorine refined gold from that material is preferred as raw material for the electrolytic plant. A common number is 30 ppm of platinum group metals in the chlorine refined gold. The reasons are two, first, to reduce as far as possible the presence of platinum group metals in the good delivery bars and second, to recover these metals from the anode sludge for sale in the platinum market.
Gold electrolysis is performed in a gold chloride solution. Electrolyte can be prepared by dissolving metallic gold with chlorine gas in the presence of hydrochloric acid,
2Au + 2HCl + 3Cl2 = 2HAuCl4
This reaction is performed in a 100 liter glass flask. Cathode deposited and acid are placed in the flask, chlorine bubbling in, and the flask heated to 70oC, with agitation. On complete dissolution, the solution is diluted and further acidified to obtain a gold concentration of 75-85 g/L gold and 100 g/L HCl, which is suitable for use as electrolyte. The acid gold ionizes thus,
HAuCl4 = H+ + AuCl4-
AuCl3 = Au3+ + 3Cl-
At the anode, the following reaction stake place,
Au -3e = Au3+
Au3+ + 4Cl- = AuCl4-
At the cathode,
AuCl4- = Au3+ + 4Cl-
Au3+ + 3e = Au
Some gold is dissolved in the monovalent state,
Au = Au+ + e
Au+ + 2Cl- = AuCl2-
Subsequently some of this gold is precipitated closet o the anode as a fine sludge,
3HAuCl2 = 2Au + 2HCl + HAuCl4
The disproportionation reaction is known to take place since the fine particles of gold so precipitated have been found to be of a higher purity than the anode. Similarly the existence of HAuCl2 in solution is verified by the fact that cathode efficiencies based on Au3+ are higher than 100%. On the assumption that only trivalent gold exists in solution, 2.45 g must be dissolved and plated per ampere-hour. In practice plating rates some 3% higher than this are found, with the dissolution rate at the anode being slightly higher than the plating rate at the cathode.
Form these reactions it can be noted that hydrochloric acid is produced at the cathode and consumed by the gold chloride produced at the anode. Uniform concentration of electrolyte to eliminate depletion at the cathode is achieved in practice by gently agitating the solution with paddles. Heating the solution to between 50-65 oC improves ion mobilities and enables a greater current to be passed without polarization. Unfortunately this increases the volatilization of acid with an increase in the threat of corrosion in the plant.