II. Roasting to Sulphate, and extracting with Water. 1. Silver .(X). 2. Copper (II). (Ziervogel process.) The metal is precipitated by iron. III. Roasting with Salt, Soluble Chloride being formed. 1. Silver .(IX). 2. Copper (Augustin and Von Patera processes.) (Longmaid and other pro cesses.) IV. Formation of Soluble Chloride by Gaseous Chlorine. At the present time, electrolysis is mainly employed in the refining of copper. Numerous electrolytic methods of extracting metals from their ores have frequently been brought forward; few, however, have advanced beyond the experimental stage. It is impossible in a work of this description to deal fully with the methods that are adopted in extracting even the principal metals from their ores, nor is it desirable to attempt to do this, for the student has an abundant technical literature at his disposal. It is far more important that at the outset of his work he should make himself familiar with the nature of metallurgical operations generally, so that he may see in what points they differ from those conducted either in the laboratory or in purely chemical works. With this object in view, a few typical metallurgical processes have been chosen for somewhat full treatment, care having been taken to select those which present considerable complexity of detail, since a knowledge of such groups of processes as those adopted in industrial centres like Swansea, Denver, and Freiberg gives the student a clearer insight into the practice of metallurgy than desultory reading affords. Welsh Method of Copper-smelting. This method, in modified forms, conducted in reverberatory furnaces, is employed for a large proportion of the world's production of copper. In Germany a blast-furnace method is employed. The one broad distinction between this method and the Welsh method consists in the fact that in the latter copper sulphide is the reducing agent, whilst in the German process it is carbonic oxide. Both processes depend essentially on the affinity which copper has for sulphur, which enables it to form a regulus while the associated metals are eliminated in an oxidised form. In the Welsh method (Fig. 78) comparatively poor ores, consisting of copper pyrites with a considerable proportion of iron pyrites, containing 5 to 15 per cent. of copper, richer ores of the same composition containing 15 to 25 per cent. of copper, and copper pyrites with oxidised ores, are mixed so as to contain 9 to 15 per cent. of copper. The smelting method varies with * Dingler's Polytechnisches__Journal, vol. cclxvi. (1887), p. 283. On electrolytic methods consult Balling, Die Electro-Metallurgie. the nature of the ore, but it usually comprises six distinct processes: 1. Calcination of Mixed Ores. -This consists in roasting the pyritic ores, whether rich or poor, containing iron pyrites, arsenic, and antimony. The roasting is always incomplete, and the extent to which it is carried depends on the proportion of sulphides available for the subsequent operations. FIG. 78. 2. Fusion of the Roasted Ores for Regulus.-The ores are fused with slag rich in cuprous oxide, known as metal-slag, from the fourth operation. Fluorspar is useful as a flux, and is added with oxides and carbonates. The operation gives two products-a regulus, coarse-metal, in which it has been attempted to concentrate all the copper, and a slag, ore-furnace slag which seldom contains more than 0.5 per cent. of copper and is mostly thrown away. The regulus contains 30 to 34 per cent. of copper and 23 per cent. of sulphur. It also contains a notable quantity of arsenic, antimony, and tin. 3. Calcination of Coarse-metal. This calcination is always incomplete, but its degree of perfection depends on the quantity of oxidised ores which it is possible to reserve for the subsequent operation. 4. Fusion of Calcined Coarsemetal.-In this operation roasted regulus and sulphuretted ore rich in copper, and containing but little sulphide of iron, are employed. Oxidised ores of copper, free from arsenic, antimony, and tin, are also added. Two products are obtained, regulus, containing 65 to 80 per cent. of copper, and 18 to 22 per cent. of sulphur. The slag, metal-slag, is a ferrous silicate, containing a little oxide of copper and shots of metallic copper, the total amount of copper it contains being 2 to 3 per cent. This slag is melted in operation 2. 5. Roasting the Regulus. This operation is very complex, and comprises a series of fusions and roastings in an oxidising atmosphere. Its object is to purify the regulus and to scorify the iron, and to bring as much as possible of the copper to the metallic state. To operations of this character the Welsh smelter applies the term roasting, as distinguished from calcining. Two products are obtained, blister-copper, containing about 98 per cent. of copper, and a slag, roaster-slag, which always contains oxide of copper, shots of metallic copper, and often a small proportion of arsenic, antimony, and tin. This slag is usually added to the charge in operation 4. 6. Refining and Toughening.-The object of this operation is to purify the copper by oxidising the foreign bodies that it contains. The refining should bring the metal to the malleable state of marketable copper. The slag obtained, refinery-slag, is very rich in copper, about 55 per cent., and is added to the charge in operation 4. In the Welsh method of copper smelting, reverberatory furnaces are exclusively employed, the area of the_grate being generally about one-fifth of that of the hearth. The furnaces used for calcination have very deep fireplaces, so as to give reducing gases; but they have an abundant supply of oxygen from the air, which is admitted through holes in the fire-bridge. The ore is charged-in through a flat hopper, in which it is allowed to dry for some time before being introduced into the furnace. The internal dimensions of the calcining furnaces are usually 30 feet by 12 feet, or 21 feet by 12 feet. The charge is 3 to 6 tons of ore, the calcination occupying 12 to 24 hours. The doors are then closed, and the temperature raised in order to decompose the sulphates. The gas evolved may consist of 10.6 per cent. of sulphurous and carbonic anhydrides, 9 per cent. of oxygen, and 80 per cent. of nitrogen. The furnaces, in which the fusion for regulus is effected, are so arranged as to give a high temperature, and the fireplace is consequently large in comparison with the laboratory portion. The bed consists of a series of layers of compressed sand cemented with metal-slag. It is about 15 to 20 inches in thickness, and slopes in all directions towards a tap-hole. There is a door in proximity to the flue, and through this door the slag is skimmed without allowing the air to pass over the regulus and oxidise it. In front of the tap-hole is a perforated receptacle, which can be lowered by a crane into a tank full of water. The bed is about 14 feet long, the distance between the top of the bridge and the roof being 23 inches. The charge is usually 2 tons of ore. Slags from subsequent operations are added, and fluorspar, if much earthy gangue is present. The materials used in this first fusion for regulus are chiefly oxides and sulphides of copper and iron, and the operation consists essentially in obtaining copper sulphide, thus: 2Cu2O + 2FeS + 2SiO2 = 2Cu,S + 2FeSiO, The sulphides must be in excess, otherwise copper will be absorbed in the slags, and copper will be obtained. The calcination* of the regulus is effected in a furnace similar to that used for the first calcination. The charge is 3 to 6 tons of regulus. The reactions taking place are similar to those in the first calcination. The process occupies twenty to thirty-six hours, and 12 per cent. of the sulphur is eliminated. The second fusion is conducted in a similar manner to the first fusion for coarse-metal. The charge is 2 tons of calcined coarsemetal and 12 cwt. of slag and oxidised ores. The regulus obtained consists of white-metal (75 per cent. of copper), or bluemetal (60 per cent. of copper), or pimple-metal (81 per cent. of copper). The slag is a silicate of iron, with about 4 per cent. of copper. The reactions are similar to those that obtain in the first fusion for regulus. Blue-metal contains some sulphide of iron, and pimple-metal contains an excess of the oxidised copper compounds. When very impure ores are dealt with, it is often found advisable to allow a little coarse copper to form as bottoms. The roasting of these different varieties of regulus is conducted in a reverberatory furnace with a deep hearth. The process is prolonged as much as possible, and thus the arsenic and antimony may be caused to escape before the actual reducing action commences. When the pasty mass has collected at the bottom of the furnace the temperature is raised, and the reaction between * Copper smelters call these operations calcination, but roasting would be more accurate. |