WET PROCESSES. - Severa] methods of extracting copper by the wet way have been more or less in practice at various periods ; but it is only of recent years that one of these has been established on a scale of great commercial extent and importance. From a very early time it has been known that the water which drained from mines containing pyritous copper ores, and which from the oxidation of the sulphide of copper contained some proportion of cupric sulphate, yielded metallic copper by precipitation in the presence of malleable or cast iron. The copper obtained in this way is known as cementation copper, and from the Spanish and Portuguese pyrites mines a considerable amount of metallic copper has long been so precipitated. The process now very extensively adopted for treating Spanish and Portuguese pyrites, and some ores of similar composition from other countries is that patented by Mr William Henderson in 1859. Mr Henderson's process is in several essential particulars the same as one patented in 18.12 by Mr William Longmaid, which, however, was chiefly designed for the production of sulphate of soda, copper being only a by-product. There can be no doubt that Mr Henderson is the practical originator of the wet process, which in Great Britain now occupies a most important position among metallurgical industries.

The ores treated by the Henderson process are remarkably constant in character, and the following may be taken as representing their average composition : - The pyrites is first employed by alkali manufacturers and other consumers of sulphuric acid as a source of that sub. stance, in burning for which the ore loses about 30 per cent, of its weight. It is this burnt pyrites which forms the raw material of the process. The various stages it undergoes are briefly as under.

Grinding. - The burnt ore, as received from the acid burners, is first mixed with about 15 per cent. of common salt, and ground to a fine powder by passing it between a pair of heavy cast-iron rolls. As the amount of sulphur left in the burnt ore is apt to vary, it is necessary to ascertain its proportion in each parcel of burnt pyrites. When the sulphur falls short of the proportion necessary for effecting the decomposition which follows, a sufficient quantity of " green " or unburned pyrites, is added to produce a proper balance. If, on the other hand, the sulphur has been insufficiently extracted, " dead " roasted ore is added.

Calcination. - This operation is accomplished in several kinds of furnaces, that used by the Tharsis Sulphur and Copper Company being a large muffle or close furnace. By others a patent furnace with a revolving hearth and mechanical stirring arrangement has been adopted with good results ; and some use open reverberatory furnaces heated by gas from Siemens's generators. During the roasting the mixture is frequently stirred, and, in the case of hand-worked furnaces, turned with long rabbles, and the completion of the operation is ascertained by test assays. When the copper has been brought into a soluble condition, the charge is raked out of the furnace and permitted to cool under a screen at its mouth. By the calcination the sulphur in the compound is first oxidized, sulphate of sodium is formed, and at the same time the chlorine from the sodium chloride unites with the copper to form cupric chloride. A small proportion of cuprous chloride is also formed, and special precautions have to be taken to prevent the extensive formation of this compound, which is dissolved only with difficulty. The hydrochloric acid and other gaseous products evolved during the calcination are condensed as " tower liquor" in ordinary condensing towers, and the product is used in the subsequent process of lixiviation.

Lixiviation. - The calcined ore is conveyed to tightly-caulked wooden tanks, in which it receives repeated washings with hot water, tower liquor, and dilute hydrochloric acid, till all the soluble copper is thereby extracted. The product of the later washings is pumped or drawn up by a modification of Giffard's injector, to serve as a first liquor for subsequent charges of the lixiviating tanks, and no solution under a definite strength is permitted to pass on to the next stage in the process. The insoluble residue in the tanks consists of "purple ore," an almost pure ferric oxide, largely used in " fettling " blast furnaces, and for smelting purposes ; besides which it is available as jeweller's rouge.

Precipitation. - The precipitation of metallic copper from the solution of its chloride is accomplished in large

It has been found possible to extract in this process with profit the small proportions of lead, silver, and gold which Spanish pyrites is known to contain. Two processes are in operation for this purposes--one devised by Mr F. Claudet and the other by Mr W. Henderson, the original patentee of the wet process. The liquors from the first three washings contain practically all these metals, and they alone are treated. Mr Claudet precipitates them from the solution by means of iodide of potassium. Mr Henderson dilutes his solutions to from 20° to 25° Twaddell, and adds a very weak solution of a lead salt, such as the acetate, by which he obtains a cream-coloured precipitate containing about 53 per cent. of lead, 5 or 6 per cent. of silver, and 3 oz. of gold to each ton of the precipitate.

The importance of the wet process may be estimated from the fact that although it originated only in 1860, already 14,000 tons of copper are annually produced by it in Great Britain alone, out of an annual production for the whole world estimated at from 126,000 to 130,000 tons.