The de-silverizing of base bullion is carried out in accordance with the principles of the well-known Parkes process.
The base bullion produced by the blast furnace department carries (apart from silver values) impurities, the chief of which are copper, antimony, and arsenic.
In order to more clearly indicate the grade of the base bullion, the following analysis, representative of a half-year’s production of base bullion, is submitted:
Ag………………………………………………..68.6 oz. Au………………………………………………0.096
The purest gold obtainable is required for use as standards or check pieces in the assay of gold bullion. The following method of preparing it is now in use at the Mint. Gold assay cornets from the purest gold which can be obtained are dissolved in nitrohydrochloric acid, and the excess of nitric acid expelled by evaporation with additional hydrochloric acid on a water bath. The blackish-red fused product, smelling of chlorine and consisting chiefly of AuCl3. HCl, or HAuCl4 (chlor-auric acid), is then poured in a thin stream into a large glass vessel full of distilled water, and a solution of
The bead of silver and gold obtained by cupellation is squeezed between pliers, or flattened by a hammer on a clean anvil, to loosen the bone ash adhering to its lower surface, and is then cleaned by a brush of wires or stiff bristles. It is then weighed, the silver removed by solution in nitric acid, and the weight of the residual gold taken, when the difference between the two weighings represents the silver. If the bead contains more than one-fourth its weight of gold, more silver is added
Cupellation is conducted in a muffle furnace, the construction of which is shown in Figs. 62-63. The fire is lighted, a little bone-ash is sprinkled on the floor of the muffle to prevent its corrosion by litharge in case of the upsetting of a cupel, and the cupels introduced as soon as a bright red heat is attained. The cupels are cleaned by gentle rubbing or blowing before being charged-in, and are again cleaned with bellows before the lead is charged-in. They are placed in the furnace one by one, or, better, charged-in together on a tray, see below.
Cupels are little cups made of bone-ash
“Highgrading” a polite word for stealing gold and silver for your refinery, has been a way of life since the metals in native form have been mined or produced. Unfortunately, in the past, highgraders were not always severely prosecuted or even looked upon unfavorably in their communities. Recent high metal prices with attendant world wide publicity, however, has made highgrading or thievery again a serious problem in mines and plants.
When a small button of gold the size of the end of one’s little finger has a value equal to about one week’s wages, the safeguarding of
Fire assaying, in essence, is a miniature or small scale smelting process which recovers and reports the total gold content of the assay sample, including gold combined with other elements or locked in the ore particles. Because of this, a assay may report values that cannot be recovered by placer methods and it cannot be too strongly stressed that when dealing with gold placers, the sample values should not be determined by fire assay. Furthermore, no credence should be placed in placer valuations or reports that are based on the results of fire assays. Although
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
The Moebius Process of Purification of Gold by Electrolysis is now in successful operation and is said to be specially suitable for refining copper bullion containing large proportions of silver and gold with small quantities of lead, platinum, and other metals, but is chiefly used in parting dore silver containing not more than 20 per 1,000 of base metals.
The apparatus required consists of a number of wooden vats coated inside with graphite paint, and filled with a solution containing 1 per cent, of nitric acid, which constitutes the electrolyte. The anodes consist of plates of bullion of
The guard pot, with the clay pot in it containing 2 or 3 ozs. of fused borax, is placed in the furnace, and is heated gradually until the bottom of the clay pot is dull red. The ingots (of which the larger are slipper-shaped) to be refined, amounting in all to 650 to 720 ozs. in weight, are then placed loosely in the pot, the furnace filled with fuel, and the dampers opened. As soon as the gold is melted, which generally happens in about one and a-half hours, the boraxing of the pots being also affected at the same time, the perforated lid is put on, and the pipe-stem, previously brought carefully