Ore Treatment

Gold.

The metallurgy of gold owes its characteristic features to two principal factors:—the chemical inactivity of gold and the high value of the product. It is probable that the most com plete recovery of gold could be obtained by smelting methods, i.e., by subjecting the ore to furnace treatment in which, however, the relatively great mass of siliceous and other matter accom panying the gold would have to be fluxed or molten. Less costly methods, although they do not yield so full a recovery, are pre ferred. This is an interesting example of the factors which de termine the choice of metallurgical methods. At every stage of extraction it becomes a matter for careful calculation whether the cost of more complete extraction will be compensated by the value of the metal recovered. The conclusion in each partic ular case will depend upon the methods of treatment and re covery available. Improvement not only in the nature of the methods available, such as the introduction of flotation, cyaniding, etc., but also improved methods of mechanical handling, design of machines, etc., all of which tend to cheapen the cost of re covery, affect the result so that, as methods are improved, it not only becomes profitable to carry extraction further but to treat poorer ores or even to re-treat the "tailings"—i.e., the rejected matter—from earlier reduction processes. In the case of gold, the amalgamation and cyaniding processes are the most important, although chlorination is also applied.

Amalgamation is the older process but is still widely used for the extraction of the gold from certain types of "free milling" ores in which it occurs in a form readily absorbed by mercury. In other cases amalgamation is used as a preliminary to cyanid ing for the purpose of extracting in a cheap manner those por tions of the gold which are most readily accessible, at the same time leaving the ore, of ter this treatment in a more uniform con dition so far as gold content is concerned, and thus better adapted to cyaniding. Amalgamation itself is carried out either during actual grinding (pan amalgamation) or during the pas sage of the pulp produced by grinding over amalgamated copper plates. The amalgam thus formed is heated in retorts, when the mercury is driven off and spongy gold is left behind.

Mercury has the power of forming amalgams with silver, lead and other base metals and if these are present in a gold ore they rapidly contaminate the mercury which is said to "sicken" and becomes incapable of taking up gold from the ore pulp. Mercury is also lost by becoming so finely divided that it is carried off with the ore pulp.

Cyaniding is a wet-way process of the "leaching" type. It depends upon the fact that a well-aerated dilute solution of potassium or sodium cyanide will dissolve gold and, by sufficiently prolonged exposure can be made to extract a very large percent age of the gold present from a finely-divided pulp which is agitated with it. From this solution the gold is afterwards pre cipitated by means of finely-divided zinc.

Silver.

Both amalgamation and cyaniding are applicable to silver, so far as chemical principles are concerned. Their practi

cal application, however, offers serious difficulties. Silver is much more slowly amalgamated by mercury than is gold, while the presence with it of lead and zinc tends to foul the mercury too rapidly. Further, silver frequently occurs in the form of minerals which do not directly lend themselves to amalgamation. As re gards cyaniding, on the other hand, silver requires much stronger solutions of the cyanide and even in these dissolves far more slowly. As a result, the extraction of silver is more frequently carried out by entirely different methods which depend upon the solubility of certain silver salts. The chloride, for instance, is soluble under certain conditions in strong brine, while a solu tion of sodium thio-sulphate (commonly called "hyposulphite") is capable of extracting certain forms of silver from the ore pulps. From these solutions the silver can be readily precipitated —for example, as the chloride—which is then readily reduced to the metallic state by the action of Copper.—While the treatment of copper ores by wet-way ex traction processes occupies an important part in the metallurgy of that metal, the most typical processes applied to it come under the heading of "smelting"—i.e., treatment in furnaces. Apart from the roasting processes to which reference has already been made, three types of furnace treatment are applied to copper ores. These are the blast-furnace, the reverberatory smelting and refin ing furnace and the "converter." In this respect, the metallurgy of copper resembles that of steel but in principle rather than detail. Ferrous metallurgy is on by far the larger scale.

The blast-furnace (q.v.) as used for the treatment of copper ores is a much smaller thing than the huge furnaces used for iron. It has also to carry out somewhat different functions. These differ according to the nature of the ore or concentrate to be treated. If this is of the oxidised type then it is fed into the blast furnace together with fuel (coke) and a suitable flux, with the object of bringing about the reduction of the oxides to the metallic state. The carbon of the fuel combines with the oxygen of the ore, while the other constituents of the ore combine with the flux to form a slag. The resulting products are an impure form of metallic copper and a slag containing very little copper. If, on the other hand, the ore contains much sulphur, together with—as a rule—sulphide of iron or iron pyrites, the function of the blast furnace is to burn away the sulphur and to produce a copper "matte" containing only a small proportion of sulphur. This matte is afterwards treated in the "converter" to form crude copper. In the copper blast furnace as used for "pyritic smelting" the minimum amount of fuel is employed to generate the neces sary heat, while a large excess of air is blown through in order to oxidise the sulphur. The fluxing materials are designed to combine with the oxides or iron formed by the oxidation of the iron pyrites, so that the iron passes into the slag. This treatment, however, is only practicable when the sulphur content of the material is not too high.