Copper-Smelting

Copper is often found in nature in the metallic state, and occasionally in considerable quantity, such as the masses recently found at Lake Superior; but the principal source is from ores; the metal in them being in combination with other matters, such as oxygen, sulphur, carbonic acid, &c., forming an extensive variety of minerals of distinct forms and character, and of great scientific interest.

The principal sources of copper ore in this country are Cornwall and Devonshire: great quantities, however, are imported from Ireland, Cuba, Chile, and Australia. The average quantity of ore raised in this country during these last five years is about 155,000 tons, yielding about 12,000 tons of metal; and the quantity imported about 60,000 tons, yielding about 10,000 tons of metal. The ores are mostly all taken to Swansea and neighbourhood, where the principal smelting-works are situated.

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 to certain qualities which they are known to possess, such as having much silica, iron pyrites, tin, arsenic, &c.: some consideration is also given to the quantity of copper they contain. The object of these arrangements and classifications of ores in the yard is to enable the operative smelter to select from and make up a constant working mixture, having the following characters:—

1. That the copper in the mixture be not under 9 nor above 13 per cent.: if under the former, it is unprofitably poor; if over the latter, the slags have a tendency to contain copper, creating a loss.

2. That 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. That the mat or coarse metal obtained from fusion contain as nearly as possible 30 per cent. of copper. And

4. That the mixture do not contain ores having impurities calculated to make the copper of a lower quality than is desired. There is no definite or fixed rule to guide the smelter in these classifications, except a practised eye in distinguishing the character of ores, and the report of the assayer.

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 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 six to eight inches. The quantity of ore put in varies, according to the size of the furnace, from three tons to six tons. The fire of the furnace is kept low at first: after two or three hours the ore on the surface becomes visibly red, the heat is gradually increased to a yellow red; but this heat only penetrates to the depth of about two 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. This calcination lasts generally nine hours; but when ores are known to be stiff, containing much silica and sulphuret of iron, twelve hours are allowed. The following changes and chemical actions take place: the sulphur is partly burned off by combining with oxygen and forming sulphurous and sulphuric acids, and partly volatilized as sulphur uncombined; arsenic is volatilized either as metal or oxide; the copper and iron lose a part of their 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 follow-

ing analysis of ore, before and after calcining, will give an idea of the changes that have taken place.

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 slag from more advanced operations.

From 2 to 3 cwt. cobbling.¹

When the charge is let down into the furnace it is spread equally over the bottom, the doors are all closed, every air-hole is stopped with clay, and the 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 fused contents to make sure that the whole is perfectly fused. This being the case, the workman begins the operation of skimming, that is, drawing off the scoriae or slag, which from its less specific gravity floats on the surface of the mat: this is effected by means of a long rake, the scoriae being drawn out at the front door. When the surface is 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 a large pit of water which granulates it. 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.

The average composition of good coarse metal is

and the slag or scoriae, as

The next operation is the calcination of the granulated coarse metal. This is done in the same manner as the calcination of the ore. The charge of metal covers the bottom of the hearth to the depth of about 4 inches, making about 4 tons to the charge, which is put in through the hoppers fitted upon the top of the furnace, 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 both to the calcination and the furnace. When the charge is let into the furnace, it is slowly brought to a visible red, and the fire is gradually and cautiously increased until the metal acquires a bright red heat, which should take about fourteen hours. 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 or vaults beneath, and a quantity of water is thrown upon it to prevent dust, &c., from accumulating above it.

¹ Cobbling is a term given to old bricks and bottoms of furnaces that have absorbed copper, and are broken down into pieces.

Copper-smelting. The following analysis gives an average result of the changes effected in this operation:—

The next operation is fusing the calcined coarse metal. The charge for an ordinary-sized furnace of 8 feet by 13 feet is—

25 cwt. of calcined metal,
6 to 7 cwt. slags from the roasters,
2 to 3 cwt. of cobbing.

In 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-blue 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.

The following is the composition of good blue metal and sharp slag.

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 calcination 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 subsulphuret—a condition necessary for reduction by the present method of smelting. The produce of this fusion is a mat termed pimpled metal, from its having small rough granules on the surface of the ingots. The average composition of this metal is—

The composition of this slag is very irregular: it always contains copper, and has to be remelted.

The next operation is roasting—a process generally identified in books on chemistry with calcining, but which is distinct. The roasting furnace differs from the fusing furnace by having a large opening in the side for putting in the charge, and is furnished with more air-holes in the bridge. The charge for an ordinary-sized furnace is three 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 semifluid state. This is continued for about twenty-four hours, during which a great portion of the sulphur is driven off with the whole of the iron, which, with silica and other matters, forms scoria, and is from time to time skimmed off. When all these impurities are removed, and when the composition of the mat or

regulus is a subsulphuret of copper $\backslash text\{Cu\}\_2\backslash text\{S\}$, then begins what may be termed roasting proper; and which, unless when the regulus has been very rich, constitutes another operation termed the second roasting, requiring other 24 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 take place during this effervescence may be explained thus: if we take 1 equivalent of subsulphuret of copper, which is composed of 16 sulphur and 64 copper, and 2 equivalents of oxide of copper, which is composed of 16 oxygen and 64 copper—incidentally equal weights,—mix them together in a crucible, and expose them to a heat sufficient to melt copper, the whole of the copper will be reduced, and the sulphur evolved as sulphurous acid, $\backslash text\{Cu\}\_2\backslash text\{S\}\; +\; 2\backslash text\{CuO\}\; =\; \backslash text\{SO\}\_2\; +\; 4\backslash text\{Cu\}$.

The reactions which occur in the process of roasting are the same. 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, and instantly reacts upon another portion of the subsulphuret, reducing the metal, as shown above. 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, pimpled 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 with a smooth hollow 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 or blisters of oxide of copper, it is termed blistered; which only takes place when the copper is good and ready for refining. When making 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 select.

The reader will not have failed to remark that the reduction and purifying of the copper are effected without the aid of carbonaceous matters of any sort, contrary to what is stated in all chemical books in reference to the reduction of copper; indeed, the addition of carbonaceous matters would be detrimental.

The next operation is that of refining—bringing the metal into a state fit for the market. The refining-furnace is similar in form to a roasting-furnace, but a little smaller, and the bottom is made to incline a little to the front door, where a small well is made. The copper being ladled from the furnace, this well allows of the ladles being dipped into the metal when the quantity in the furnace is small, so that the last portions of the metal can be taken out.

About six 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 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 we have 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 con-