Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/379

Rh COPPER. 349 25 cwt. of calcined metal , 5 to 7 cwt. slags from the roaster and refinery furnaces ; 2 to 3 cwt. of cobbing la this mixture the oxide of iron is in excess in relation to the silica, and it is therefore much more easily fused than the ore ; but the reactions which take place are similar : the silica and oxide of iron combine to form slag, which floats upon the surface of the mat and has to be skimmed off, after which the mat is tapped out into sand-moulds. Two charges are generally fused before the metal is tapped out. This mat is termed blue metal from its being of a slate-bine colour ; the scoria is termed sharp slag, from its containing an excess of oxide of iron, and being consequently used as a flux for fusing the ore in operation II. The following is the composition of good blue metal and sharp slag : Blue Metal. Copper 53 8 Sulphur 20-5 Iron 12 6 Insoluble 4 2 Oxygen and loss 3 9 100 Sharp Slag. Oxide of iron..., 53 Oxide of copper 2 Silica, &c 45 100 Should there be no ores such as carbonates or oxides on hand to smelt, the blue metal, instead of being tapped into sand-beds as described, is run into pits of water in the same manner as coarse metal, and subjected to another cal cination and fusion. When oxides and carbonates, such as the Australian ores, are on hand, they are generally fused with the calcined coarse metal, by which means a double advantage is obtained ; the excess of oxide of iron in the calcined metal fluxes the silica of the ore which has little iron, and the copper in the ore is converted into cupric sulphide, a con dition necessary for reduction by the present method of smelting. The produce of this fusion is a mat termed pimpled or white metal, from its having small rough granules on the surface of the ingots. The average com position of this metal is Copper 78 Sulphur 18 Iron, 2 Silica 2 100 The composition of the slag from this operation is very variable; it always contains copper, and has to be remelted. V. Roasting. This operation has been often identified with calcining, from which, however, it is distinct. The roasting differs from the fusing furnace by having a large opening in the side for putting in the charge, and it is fur nished with more air-holes in the bridge. The charge for an ordinary-sized furnace is 3 tons. When the metal is brought to fusion, the air-holes of the furnace are all opened, and a free current is allowed to pass over the surface of the fused mass : the heat of the fire is then regulated so as to keep the charge in a sort of semi-fluid state. This is con tinued for about twenty-four hours, during which a great portion of the sulphur is driven off&quot;, and the iron, by unit ing with silica and other matters, forms scoria, which is from time to time skimmed off. When all the impurities are removed, and the mat or regulus acquires the composi tion of sulphide of copper, Cu 2 S, then (except when the regulus has been very rich) begins another operation termed the second roasting, or roasting proper, requiring other twenty-four hours. In this last roasting, when the air-holes are opened, a brisk effervescence ensues over the surface of the fluid mass. The chemical reactions which give rise to this effer vescence may be explained thus. The oxygen of the air combines in the first place with a portion of the sulphur. forming sulphurous acid. A portion of the copper is also oxidized, to form the sub-oxide, and instantly reacts upon another portion of the sulphide, reducing the metal. The reactions are chemically represented thus : 2Cu 2 O + Cu 2 S = 6Cu + SO 2. The process is a very beautiful one, and exhibits a nice adaptation of principles to practice. The sponge regulus has a specific gravity of 5, the reduced copper of about 8 ; so that the copper sinks to the bottom, where it is protected, and a new surface of regulus becomes exposed to the action of the air. If the ore be pure, or if no select copper be required, the operation of roasting is continued until the whole of the copper is reduced ; when it is tapped out into sand-moulds, forming coarse copper, bed copper, pii^led copper, or blistered copper, according to quality. The term- coarse copper is applied occasionally to all these kinds except the blistered. If the ingot sets with contraction and exhibits a smooth hoUow surface, it is termed bed, and generally indicates the presence of other metals, as tin. When the surface of the ingot is covered with pimples, it is termed pimpled copper, and indicates the presence of sulphur. When covered with large scales of oxide of copper, it is termed blistered ; but this is only when the copper is good and ready for refining. The following analysis of blister copper is given by Le Play : Copper. 98 4 Iron -7 Nickel, Cobalt, and Manganese 3 Tin and Arsenic 4 Sulphur -2 lOO O To make select copper, the roasting is carried on until about one-fourth of the copper in the regulus is reduced ; the furnace is then tapped, and the reduced metal is obtained at the bottom of the first and second ingots, or pigs, as copper bottoms, which contain most of the metallic impurities. The regulus is collected and again roasted, which produces the purest metal the ordinary process of smelting can give ; it is termed best selected. VI. Refining. In this operation, the remainder of the sulphur and foreign metals present in the copper is removed, and tha metal is brought into a condition fit for the market. The refining furnace is similar in general form to a roasting furnace, except that the bottom inclines gradually down from all sides towards a deep part, or well, which is near the end door. It has also a large door on one side, but neither opening in the roof nor side tap-hole. Siemens s regenerative furnace has been very generally intro duced for refinery purposes. When the copper is to be finally ladled out of the furnace the deep part, or well, allows of the ladle being dipped into the metal till the last portions are quite baled out. From 6 to 8 tons of copper from the roasting furnace are put into the refining furnace, the doors and air-holes of which are closed, and the heat is raised until the metal is in fusion, when the air-holes are opened. A short roasting is generally required, which is done in the manner above described, and the scoria which collects is carefully skimmed off. The separation of impurities is facilitated by occasionally stirring the metal with a rake. Some refiners throw pieces of green wood upon the surface, under the impression that it assists the escape of sulphur. The roasting is continued until a ladleful of the metal taken out sets with contraction. If the metal be very coarse, ife will set with a surface having a frothy appearance ; if finer, it sets with expansion, first round the edge, then swelling towards the centre, forming a little mound or cone, and occasionally boiling over and throwing up jets of metal, forming a miniature volcano. When the setting of the