Waihi Borough Council Diamond Jubilee Booklet 1902-1962
By GEO. CHAPPELL
An engine and rake of trucks conveyed the quartz from the mine to Waikino, where it was crushed to a very fine powder. The gold, silver and other metals were dissolved by cyanide solutions and precipitated into metallic zinc shavings in the form of a black slime. This was conveyed in a steel truck under escort back to Waihi and the Refinery at the east end. Here the slime was treated with sulphuric acid to dissolve the surplus zinc contained therein. The slime was now washed in hot water, dried in a very large drying furnace and then melted with borax, soda and sand, in an oil burning furnace. The resulting bullion (which was alloy of gold, silver, copper, lead, iron and other metals), was bailed out into moulds, each bar weighing 1000 ozs. This was the first stage of refining.
The bullion was now conveyed to a large capel furnace where as much as 25,000 ozs were melted at one time. In this furnace a large percentage of the metals, other than gold and silver, were oxidised off with the aid of added lead, the process reducing the base metal down to about 2 per cent. From here the bullion went to the silver cells, so called because it was here that the silver was separated from the gold, by the aid of nitric acid, and an electric current. During this process the silver was dissolved and precipitated in the form of pure silver crystals which were washed in boiling water, dried and melted into 1000 oz bars ready to be shipped away.
The gold from the silver cells was now collected, washed and melted into slabs and then put into the gold cells. Here by electrolysis the gold was dissolved and re-deposited as chemically pure gold on strips of pure gold foil hung in the cells. When this process was finished the deposited gold was collected, washed in boiling water, dried and melted and cast into 400 oz bars, worth about £5,000 each. These bars were packed into specially made boxes and sent to the Mint.
Before being sent away, the gold and silver had to be assayed. In the case of gold a sample was taken either by chipping a piece off the ends of the bars, or by boring a hole right through. A small quantity (about 10 grains) of the sample was now weighed out on very sensitive scales, capable of weighing down to one thousandth of a grain. It was wrapped in a small piece of sheet lead, together with about 2½ times its weight of silver. This package was now placed in a small cupel made of Morganite and thence placed in a cupel furnace, the cupel absorbing the lead and any impurity contained in the gold. When the process was finished all that remained on the cupel was a small button of gold and silver. This button was flattened out on a small anvil and then rolled into a strip about two to two and a half inches long in a small rolling mill. The strip was then rolled round a small piece of glass rod, and put into a flask of boiling nitric acid, which dissolved the silver and left the gold in a pure state. It was then washed in distilled water, dried and weighed and the value of the gold calculated at so much per oz. In the case of the silver the value was obtained by a chemical analysis.