Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/265

 COP PER cent, of sulphur. The ore is under better control than is possible with the continuous feed and discharge, and when sufficiently roasted can be passed red-hot to the reverberatory furnace. These advantages compensate for the wear and tear and the cost of moving the heavy dead-weight. The M‘Dougall furnace is turret-shaped, and consists of a series of circular hearths, on which the ore is agitated by rakes attached to revolving arms and made to fall from hearth to hearth. It has been modified by Herreshoff, who uses a large hollow revolving central shaft cooled by a current of air. The shaft is provided with sockets, into which movable arms with their rakes are readily dropped. The Peter Spence type of calcining furnace has been followed in a large number of inventions. In some the rakes are attached to rigid frames, with a reciprocating motion, in others to cross-bars moved by revolving chains. Some of these furnaces are straight, others circular. Some have only one hearth, others three. This and the previous type of furnace, owing to their large capacity, are at present in greatest favour. The M‘Dougall-Herreshoff, working on ores of over 30 per cent, of sulphur, requires no fuel; but in furnaces of the Spence-O’Hara type fuel must be used, as an excess of air enters through the slotted sides and the hinged doors which open and shut frequently to permit of the passage of the rakes. The consumption of fuel, however, does not exceed 1 of coal to 10 of ore. The quantity of ore which these large furnaces, with a hearth area as great as 2000 feet and over, will roast varies from 40 to 60 tons a day. Shaft calcining furnaces like the Gerstenhoffer, Hasenclaver, and others designed for burning pyrites fines have not found favour in modern copper works. Kilns and stalls roast so small a quantity that they would cover too much ground and involve too much handling. The fusion of ores in reverberatory and cupola furnaces. —After the ore has been partially calcined, it is smelted to extract its earthy matter and to concentrate the copper with part of its iron and sulphur into a matte. In reverberatory furnaces it is smelted by fuel in a fireplace, separate from the ore, and in cupolas the fuel, generally coke, is in direct contact with the ore. When Swansea was the centre of the copper-smelting industry in Europe, many varieties of ores from different mines were smelted in the same furnaces, and the Welsh reverberatory furnaces were used. To-day more than eight-tenths of the copper ores of the world are reduced to impure copper bars or to fine copper at the mines; and where the character of the ore permits, the cupola furnace is found more economical in both fuel and labour than the reverberatory. The Welsh method, with its seven or eight operations, finds adherents only in Wales and Chile. In America the usual method is to roast ores or concentrates so that the matte yielded by either the reverberatory or cupola furnace will run from 45 to 50 per cent, in copper, and then to transfer to the Bessemer converter, which blows it up to 99 per cent. In Butte, Montana, reverberatories have in the past been preferred to cupola furnaces, as the charge has consisted mainly of fine roasted concentrates ; but even there the cupola is gaining ground. To smelt Butte ores there are about 75 reverberatories, but the number will be reduced by the replacement of small by large furnaces, heated by gas instead of solid fuel. At the Boston and Montana works tilting reverberatories, modelled after open hearth steel furnaces, were first erected; but they were found to possess objectionable features. Now both these and the egg-shaped reverberatories are being abandoned for furnaces as long as 43 feet 6 inches from bridge to bridge and of a width of 15 feet 9 inches, heated by gas, with regenerative checker work at each end, and fed with ore or concentrates, red-hot from the calciners, through a line of hoppers suspended above the roof. Furnaces of this

235

size smelt 200 tons of charge a day. But even when the old type of reverberatory is preferred, as at the Argo works, Colorado, where rich gold- and silver-bearing copper matte is made, the growth of the furnace in size has been steady. Richard Pearce’s reverberatories in 1878 had an area of hearth of 15 feet by 9 feet 8 inches, and smelted 12 tons of cold charge daily, with a consumption of 1 ton of coal to 2'4 tons of ore. His present furnaces are 35 feet by 16 feet, and smelt 50 tons daily of hot ore, with the consumption of 1 ton of coal to 3'7 tons of ore. The home of cupola smelting was Germany, where it has never ceased to make steady progress. In Mansfeld brick cupola furnaces are without a rival in size, equipment, and performance. They are round stacks, designed on the model of iron blast furnaces, 29 feet high, fed mechanically, and provided with stoves to heat the blast by the furnace gases. The low percentage of sulphur in the roasted ore is little more than enough to produce a matte of 40 to 45 per cent., and therefore the escaping gases are better fitted than those of most copper cupola furnaces for burning in a stove. But as the slag carries on an average 46 per cent, of silica, it is only through the utmost skill that it can be made to run as low on an average as 0‘3 per cent, in copper oxide. As the matte contains on an average 0-2 per cent, of silver, it is still treated by the Ziervogel wet method of extraction, the management dreading the loss which might occur in the Bessemer process of concentration, applied as preliminary to electrolytic separation. Blast furnaces of large size, built of brick, have of late years treated the richest and more silicious ores of Rio Tinto, and at present the Rio Tinto Company is introducing converters at the mine. This method of extraction contrasts favourably in time with the leaching process, which is so slow that over 10,000,000 tons of ore are always under treatment on the immense leaching floors of the company’s works in Spain. In the United States the cupola has undergone a radical modification in being built of water-jacketed sections. The first water-jacketed cupola which came into general use was a circular inverted cone, with a slight taper, of 36 inches’ diameter at the tuyeres, and composed of an outer and an inner metal shell, between which water circulated. As greater size has been demanded, oval and rectangular furnaces — as large as 180 inches by 56 inches at the tuyeres—have been built in sections of cast or sheet iron or steel. A single section can be removed and replaced without entirely emptying the stack, as a shell of congealed slag always coats the inner surface of the jacket. The largest furnaces are those of the Boston and Montana Company at Great Falls, Montana, which have put through 500 tons of charge daily, pouring their melted slag and matte into large wells of 10 feet in diameter. A combined brick- and water-cooled furnace was first used by the Oxford Copper Company, and has been adopted by the Iron Mountain Company at Kerwick, Cal., for matte concentration. In it the cooling is effected by water pipes, interposed horizontally between the layers of bricks. The Mt. Lyell smelting works in Tasmania, which are of special interest, will be referred to later. Concentrating matte to copper in the Bessemer converter. —As soon as the pneumatic method of decarburizing pig iron was accepted as practicable, experiments were made with a view to Bessemerizing copper ores and mattes. One of the earliest and most exhaustive series of experiments was made on Rio Tinto ores at the John Brown works by Mr John Holloway, with the ambitious aim of both smelting the ore and concentrating the matte in the same furnace, by the heat evolved through the oxidation of their sulphur and iron. Experiments along the same lines were made by Francis Bawden at Rio Tinto and Claude Yautin in Australia. The difficulty of effecting