The cupreous slate is sorted, according to its composition, into slate of lime, clay, iron, &c., by a mixture of which the smelting is facilitated. For example, 1 post or charge may consist of 20 cwt. of the ferruginous slate, 14 of the calcareous, 6 of the argillaceous, with 3 of fluor spar, 3 of rich copper slags, and other refuse matters. The nozzle at the tuyère is lengthened 6 or 8 inches, to place the melting heat near the centre of the furnace. In 15 hours 1 fodder of 48 cwts. of the above mixture may be smelted, whereby 4 to 5 cwts. of matte (crude copper, called Kupferstein in Germany) and a large body of slags are obtained. The matte contains from 30 to 40 per cent. of copper, and from 2 to 4 loths (1 to 2 oz.) of silver. The slags contain at times one-tenth their weight of copper.
The matte is composed of the sulphurets of copper, iron, silver, zinc, along with some arsenical cobalt and nickel. The slaty slag is raked off the surface of the melted matte from time to time. The former is either after being roasted six successive times, smelted into black copper; or it is subjected to the following concentration process. It is broken to pieces, roasted by brushwood and coals three several times in brick-walled kilns, containing 60 cwts., and turned over after every calcination; a process of four weeks’ duration. The thrice roasted mass, called spurrost, being melted in the cupola [fig. 313.] with ore-cinder, yields the spurstein, or concentrated matte. From 30 to 40 cwts. of spurrost are smelted in 24 hours; and from 48 to 60 per cent. of spurstein are obtained, the slag from the slate smelting being employed as a flux. The spurstein contains from 50 to 60 per cent. of copper, combined with the sulphurets of copper, of iron, and silver.
The spurstein is now mixed with dünnstein (a sulphuret of copper and iron produced in the original smeltings) roasted six successive times, in a quantity of 60 cwts., with brushwood and charcoal; a process which requires from 7 to 8 weeks. The product of this six-fold calcination is the Gahrrost of the Germans (done and purified); it has a colour like red copper ore, varying from blue gray into cochineal red; a granular fracture; it contains a little of the metal, and may be immediately reduced into metallic copper, called kupfermachen. But before smelting the mass, it is lixiviated with water, to extract from it the soluble sulphate, which is concentrated in lead pans, and crystallized.
The lixiviated gahröste mixed with from 1⁄4 to 1⁄5 of the lixiviated dünnsteinrost, and 1⁄6 to 1⁄10 of the copper slate slag, are smelted with charcoal or coke fuel in the course of 24 hours, in a mass of 60 or 80 cwts. The product is black copper, to the amount of about 1⁄4 the weight, and 1⁄6 of dünnstein, or thin matte. This black copper contains in the cwt. from 12 to 20 loths (6 to 10 oz.) of silver. The dünnstein consists of from 60 to 70 per cent. of copper combined with sulphur, sulphuret of iron and arsenic; and when thrice roasted, yields a portion of metal. The black copper lies undermost in the crucible of the furnace, above it is the dünnstein, covered with the stone slag, or copper cinder, resulting from the slate-smelting. The slags being raked off, and the crucible sufficiently full, the eye or nozzle hole is shut, the dünnstein removed by cooling the surface, and breaking the crust, which is about 1⁄4 to 1⁄2 inch thick. The same method is adopted for taking out the black copper in successive layers. For the de-silvering of this, and similar black coppers, see [Silver].
[Fig. 312.] is a vertical section through the form or tuyère in the dotted line A B of [fig. 314.] [Fig. 313.] is a vertical section in the dotted line C D of [fig. 315.] a is the shaft of the furnace, b the rest, c c the forms; d the sole or hearth-stone, which has a slope of 3 inches towards the front wall; e e, &c. casing walls of fire bricks; f f, &c. filling up walls built of rubbish stones; g g a mass through which the heat is slowly conducted; h h the two holes through one or other of which alternately the product of the smelting process is run off into the fore-hearth. Beneath the hearth-sole there is a solid body of loam; and the fore-hearth is formed with a mixture of coal-dust and clay; k is the discharge outlet. [Fig. 314.] is a horizontal section of the furnace through the hole or eye in the dotted line E F of [fig. 312.]; [fig. 315.] a horizontal section of the shaft of the furnace through the form in the dotted line G H of figs. 312 and 313. The height of the shaft, from the line E F to the top, is 14 feet; from E to G, 25 inches; from c to the line below b, 2 feet; from that line to the line opposite g g, 2 feet. The width at the line g g is 3 feet 3 inches, and at c 26 inches. The basins i i, [fig. 314.], are 3 feet diameter, and 20 inches deep.
The refining of copper is said to be well executed at Seville, in Spain; and, therefore, some account of the mode of operating there may be acceptable to the reader.
The first object is to evaporate in a reverberatory furnace all the volatile substances, such as sulphur, arsenic, antimony, &c., which may be associated with the sulphur; and the second, to oxidize and to convert into scoriæ the fixed substances, such as iron, lead, &c., with the least possible expense and waste. The minute quantities of gold and silver which resist oxidation cannot be in any way injurious to the copper. The hearth is usually made of a refractory sand and clay with ground charcoal, each mixed in equal volumes, and worked up into a doughy consistence with water. This composition is beat firmly into the furnace bottom. But a quartzose hearth is found to answer better, and to be far more durable; such as a bed of fire-sandstone.
Before kindling the furnace, its inner surface is smeared over with a cream-consistenced mixture of fire-clay and water.