Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/378

348 C P P E ores containing less than 4 per cent, of copper, for which and for still poorer ores the wet process is preferred. SMELTIXG. In Great Britain ordinary copper smelting is almost entirely centred at Swansea in Wales, although it is also practised in Lancashire. The processes there employed for extracting copper are technically known as the " English method, " in contradistinction to numerous other modified processes adopted at Continental and other foreign smelting centres. The following is an outline of the English method as conducted at Swansea. The ores are divided by the smelter into two general classes those containing sulphur, and those having little or no sulphur. The former are subdivided according as they contain much silica, iron pyrites, tin, arsenic, &c., or a larger or smaller quantity of copper. The object of this classification of ores in the yard is to enable the operative smelter to make up a constant working mixture, having the following characters : 1. The copper present is not under 9 nor above 14 per cent.; if under the former it would be unprofitably poor ; if over the latter, the slags would have a tendency to retain copper, creating a loss. 2. After being calcined for an ordinary length of time, it will fuse easily without the necessity of adding flux, giving a clean and easily fused slag. 3. The mat or coarse metal obtained from fusion contains as nearly as possible 30 per cent of copper. 4. The mixture does not contain ores having impurities cal culated to make the copper of too low a quality. There is no definite or fixed rule to guide the smelter in these classifications, except a practised eye in distinguish ing the character of ores, and the report of the assayer. I. Calcination of the Ores. The mixture of ores being selected according to these rules, it is carried to hoppers on the top of a large reverberatory furnace, termed the calcining furnace, and is then let down into the hearth, where, after drying a little, it is spread equally over the bottom, and covered to a depth of from 6 to 8 inches. The quantity of ore put in varies, according to the size of the fur n. ice, from 3 tons to 4 tons. The fire of the furnace is kept low at first; after two or three hours the ore on the sur face becomes visibly red, and the heat is gradually increased to a yellow red ; but this heat penetrates to the depth of only about 2 inches, consequently the ore has to be stirred and turned over by means of long iron paddles every hour, so as to expose a new surface to the action of the air and fire. The calcination lasts generally from twelve to twenty- four hours, the length of time being dependent on the proportions of silica and sulphide of iron in the charge. Calcining furnaces are now very commonly provided .with Siemens regenerators and heated with gas. The following changes take place : the sulphur is partly burned off, form ing sulphurous and sulphuric acids, and partly volatilized in the free state ; arsenic is volatilized and oxidized ; and part of the copper and iron lose sulphur and combine with oxygen, forming oxides. When the ore is sufficiently calcined, it is let down into the cubs or vaults beneath, by openings in the floor. Water is added to the hot ore in the cubs to prevent dust and assist further oxidation ; the ore is then removed to a yard, and there stored up, ready for the fusing furnace. The following analysis of ore, before and after calcining, will give an idea of the changes that have taken place : Before Calcination. Copper 12 3 Iron 327 Sulphur 31-0 Silica 24 100-0 After Calcination. Topper 12 2 Iron 22 7 Oxide of Iron 18 5 Sulphur 16-2 Silica .. 30-4 100-0 IT. Fusion of Calcined Ore. The next operation is the fusing of the calcined ore, which is done in a reverberatory furnace, termed an ore fusing furnace, fitted also with a hopper on the top for charging it. The charge consists of From 25 to 30 cwt. of calcined ore ; From 7 to 9 cwt. of sharp or metal slag from operation IV. ; From 2 to 8 cwt. cobbing. 1 When the charge islet down into the furnace it is spread equally over the bottom, the doors are all closed, every air hole is stopped with clay, and rhe heat of the furnace increased as rapidly as possible. After about five hours firing, when the furnace has reached a white heat, the door-plate is removed, and a long iron rake passed through the contents to make sure that the whole is perfectly fused. This being the case, the workman begins the operation of slimming, that is, drawing off the slag, which floats on the surface of the mat, and removing it at the front door. When the surface i.s skimmed, the common practice is to let down a second charge of ore, and to fuse and skim in the same manner, before tapping the furnace to let out the metal or mat, which is generally tapped into large pits of water, and so granulated. These pits are from 6 to 8 feet deep, and from 4 to 5 feet square, and into them a perforated box is lowered, which receives the charge of metal, and is raised by a crane or pulley. The metal is then removed to a yard for the next operation. This mat is termed granulated coarse metal. In many cases the coarse metal is first run into moulds and subsequently crushed for the next operation. The average composition of good coarse metal is given by Le Play as Copper 33-7 Sulphur 29-2 Iron 33 6 Foreign metals 2 Slag, mechanically mixed 1 ~1 99 -6 That of the slag or scoriae is Silica, mixed and combined 60 - 5 Protoxide of iron 28 - 5 Alumina, Lime, &c 11 -Q 100-0 III. Calcination of Coarse Metal. This operation is performed in the same manner as the calcination of the ore. The charge of metal, which is about 4 tons, covers the bottom of the hearth to the depth of 4 inches or so. It is put in through the hoppers fitted upon the top of the fur nace, as described for the ore. The coarse metal being easily fused, great care is required not to raise the heat of the furnace too high, otherwise the metal will cake, and by adhering to the bricks will prove prejudicial bcth to the calcination and to the furnace. When the charge is let into the furnace, it is slowly brought to a visible red, which during the nest fourteen hours is gradually increased to a bright i - ed heat. This temperature is continued until the charge has been altogether twenty-four hours in the furnace, when it is let down through the bottom into the cubs, and water is thrown upon it. The following analyses give an average result of changes effected in this operation : the Metal put into Calciner. Copper 32 Iron 39 Sulphur 25 Other matters and loss, 4 Metal after Calcination. Copper 33 Iron 38 Sulphur 13 Oxygen, &c 16 100 100 IV. Fusion of Calcined Coarse Metal. In this operation the charge for an ordinary-sized furnace of 8 feet by 13 feet is 1 Cobbing is the name given to broken pieces of old bricks and bot

toms of furnaces that have absorbed copper.