In spite of all precautions, however, the loss of gold was heavy. 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.
#X32 COMPACT VS X AIR FREE#
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. per standard oz., with a deduction of 4d. per oz., as the Mint bought gold at £3 16s.
About 300 kilogrammes of gold per month was thus refined at a cost, including subsequent coinage, of 1s. The gold thus obtained was from 996 to 999 fine. 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 precipitated was separated and washed, and the ferrous chloride regenerated by the addition of iron. 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 flasks were heated on sand baths, and at the end of the day poured into porcelain vessels holding 100 litres each. Some gold was always left undissolved to avoid loss of acid. capacity, and treated with a mixture of 6 parts HCl to 1 part HNO3. Charges of 500 grammes of the granulations were then placed in each of 40 boiling flasks of 3,250 c.c. 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. 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.
In the aqua regia process the gold is dissolved and precipitated. 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.
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