By whatever process gold may have been extracted from its ores, it is necessary to melt the crude bullion and cast it into bars so that its value may be ascertained, and that it may be put into a form convenient for transportation and sale. The name “ bullion ” may be conveniently restricted to … Read more
The operation of refining by one of the methods described above is necessary before gold can be exactly valued. When base metals other than copper are present, segregation occurs and the solidified metal is not uniform in composition. The result is that the exact value of the bullion cannot be determined, and the buyer must … Read more
The processes used for Gold Refining are as follows: Volatilisation. Oxidation (a) by air blowing or roasting. (b) by “ bessemerising” (c) by nitre. (d) by metallic oxides. (e) by cupellation. Chlorination. Sulphurisation. The use of iron or carbon. The method to be used depends partly on the composition of the bullion, and partly on … Read more
The furnace used for melting gold bullion is of simple construction. It may be round or square, with walls consisting of an outer layer of ordinary brick and an inner layer, at least 4 inches thick, of the best firebrick. There is often a complete outer casing of iron, which is useful in keeping the … Read more
The method given above is difficult to apply when solutions containing soluble cyanides of zinc and other metals require to be titrated. “ A white flocculent precipitate occurs at a certain stage, probably consisting of simple (insoluble) cyanide of zinc, formed by decomposition of the soluble double cyanide: K2ZnCy4 + AgNO3 = KAgCy2 + ZnCy2 … Read more
Since acidity of the ore causes decomposition of the cyanide, an obvious method of reducing the loss is to add alkali in some form. Before doing this, the free sulphuric acid and soluble salts may be removed by leaching with water, and then a solution of caustic soda or lime is run on to the … Read more
The ordinary gangue of most ores (silica and silicates of the alkalies and alkaline earths) exercises no direct influence on the cyanide solution. The carbonates of the alkaline earths are also probably without influence. The decomposing effects of sulphides of the heavy metals vary with the physical state of the sulphides. Metallurgists found that dilute cyanide … Read more
These methods are described here for convenience, as being more intelligible after the chemistry of the process has been discussed. The necessity of the presence of oxygen has already been dwelt on above. It has, however, been frequently pointed out that in the interior of a mass of ore undergoing treatment the conditions are not … Read more
When the pyrites occur in tailings which have been subjected to the action of the weather for some time before treatment, compounds are formed which are more prejudicial to the solution than the sulphides. Sulphide of iron, FeS2, is oxidised by air and water, ferrous sulphate and free sulphuric acid being formed, thus: FeS2 + … Read more
It was generally believed at one time that the precipitation of gold and silver by zinc was effected by simple displacement, according to the equation: 2KAuCy2 + Zn = K2ZnCy4 + 2Au Some considerations have been adduced, however, which throw doubt on this view. It is well known that a considerable excess of free cyanide must … Read more