Mention must be made of the ore rejected as too small by spatters and cobbers; it is picked over, and the richest pieces are passed to the best-ore heap. The remainder is subjected to a riddle with 1-in. meshes, and again through one with 1/2-in. meshes. The large from both these processes is picked over, and, along with the small, subjected to dressing operations similar to those already described.
The processes herein involved may be classed into 2 principal groups, the English and the German. Taken on the whole, these groups have a common feature in that they both consist of a succession of roastings and fusions which gradually eliminate the sulphur and other foreign matters. But they possess distinct characteristics. The English method adopts a reverberatory furnace, proceeds rapidly, but entails complex manipulations requiring skilled labour; it terminates in an operation wherein the reduction of the products to a metallic state is effected by a double decomposition of oxygenated and sulphuretted combinations. The method is appropriate to the nature of the materials generally treated, rich ores of various composition. The German method, on the contrary, is slower, and consists of roastings and fasions carried on in different apparatus. The roasting is generally done in heaps, or better in stalls; the fusion in low blast furnaces. This latter method is considered preferable for poor and impure ores, and is more economic for districts where fuel is scarce and dear.
Summarized, the German method, taking for example that practised in the Hartz for impure sulphides, comprises the following operations:-
1. Roasting the ore in heaps of 50 to 100 tons.
3. Roasting the crude matte (generally 3 fires).
4. Concentrating smelting of roasted crude matte, raising the copper to 45 or 50 per cent.
5. Roasting the concentrated matte, which is performed more thoroughly than the previous one, up to 5 or 6 fires.
6. Smelting for black copper, giving a thin matte of 65 per cent. copper, a little impure black copper, and cinders holding 1 1/2 per cent. copper.
7. Roasting the thin matte at 7 or 8 fires.
8. Renewed smelting for black copper and thin matte.
9. 10. Fresh roasting and fresh smelting.
11, 12. First fining of black copper, and fining at low heat.
As to the two operations of roasting and smelting, they offer no particular difficulty of execution. In the roasting, it suffices to mix the ore and fuel (mineral or vegetable) in successive layers, whose arrangement carefully preserves access of air. The amount of fuel and number of successive operations are determined by the degree to which the roasting is to be carried. For the smelting, the materials, properly proportioned and well mixed, are charged in at the top of the furnace, in alternate layers with the fuel; draught is effected by a blast at the bottom; a tap-hole situated near the base allows of the outflow of molten metal.
The English method comprises:-
1. Roasting the ore in a reverberatory furnace.
2. Smelting for crude matte, in the reverberatory.
3. Roasting the crude matte, also in reverberatory.
4. Smelting for rich matte, also in a reverberatory.
Successful economical management of smelting operations is dependent principally on due assortment of the ores for the various operations involved in their preparation, and for the final production of metallic copper. These operations are 10 in number; and for them the crude ores are assorted into 5 classes, viz.:-
1. Such ores as contain 3 to 15 per cent. of copper combined with sulphur, and with iron also mineralized with sulphur, forming iron pyrites and arsenic associated with quartz and other siliceous and earthy minerals.
2. Ores of similar constitution, but containing 15 to 25 per cent. of copper.
3. Sulphides of copper, with less of the sulphides of iron, containing 15 to 20 per cent. of copper, a portion of which is in the state of oxide, principally associated with siliceous minerals.
4. Principally oxides and carbonates of copper with some of the sulphides, containing 20 to 30 per cent. of pure metal, the associated minerals being principally siliceous.
5. Rich oxides of copper, free from sulphur and arsenic. or other metals which can have an injurious effect on the metal, obtainable from these ores, which contain 60 to 80 per cent. of copper, the accompanying minerals being chiefly quartzose.
The 10 operations involved in the treatment of these ores are as follow:-
2. Fusion of the calcined product of the first operation with minerals of the second class not previously calcined. This operation is termed melting for coarse metal.
3. Roasting of coarse metal.
4. Melting for white metal. In this operation the coarse metal is fused together with ores of the fourth class.
5. Melting for blue metal. The calcined coarse metal is fused with roasted ores rather rich in copper.
6. Remelting of slags from operations 4, 7, and 8.
7. Roasting of white metal for the production of white metal of superior quality.
8. Roasting for regulus.