The Gold Refining Process by Aqua Regia was introduced at the Pretoria Mint after the Miller process had been tried and abandoned owing to the alleged difficulty of treating the gold bullion extracted by the cyanide process. In the aqua regia process the gold is dissolved and precipitated. It is made very difficult if the silver exceeds 100 parts per 1,000, and at Pretoria bullion was not treated if the silver exceeded 50 parts per 1,000. Mill gold with 80 to 110 parts of silver, and cyanide gold if it contained more than 50 parts, were melted with gold obtained by the chlorination process to reduce the silver to less than 50 parts, and granulated. Charges of 500 grammes of the granulations were then placed in each of 40 boiling flasks of 3,250 c.c. capacity, and treated with a mixture of 6 parts HCl to 1 part HNO3. Some gold was always left undissolved to avoid loss of acid. The flasks were heated on sand baths, and at the end of the day poured into porcelain vessels holding 100 litres each.

Next morning the clear liquid was siphoned off, leaving the silver chloride at the bottom of the vessel, and transferred to a tank containing a solution of ferrous chloride. The gold thus precipitated was separated and washed, and the ferrous chloride regenerated by the addition of iron. The gold could not be toughened on melting, even by repeated additions (up to 40 times) of a few grammes of CuCl2, and it was necessary to pass chlorine through it. The gold thus obtained was from 996 to 999 fine. About 300 kilogrammes of gold per month was thus refined at a cost, including subsequent coinage, of 1s. 10½d. per standard oz., 916.9 fine. The loss on the transaction was 5½d. per oz., as the Mint bought gold at £3 16s. 9½d. per standard oz., with a deduction of 4d. for refining. The floor of the refinery was slated and drained to carefully constructed gutters, and no liquids were thrown away until they were declared free from gold. The method of testing was to take 20 litres, add some acetate of lead, pass sulphuretted hydrogen, allow to settle, decant, roast the precipitate, and cupel the lead. In spite of all precautions, however, the loss of gold was heavy. This cumbrous and inconvenient process was brought to an end in 1899 by the South African War.

Parting of Gold from Platinum & Iridium

The method of separating osmiridium, platiniridium from gold by remelting. A platinum manufacturers separateS platinum from gold by dissolving in aqua regia, driving off acids, and precipitating the platinum with ammonium chloride. At Freiberg, platiniferous gold is purified by melting with twice its weight of sodium bisulphate, and subsequently with a little nitre. The slags contain most of the platinum, the resulting gold being 997 to 998 fine.
The best and most modern method, however, is the electrolytic one described above.