The Manufacture of Vitriols. IV.

Manufacturer and builder 10, 1870

Of the sulphurets of copper occurring in nature, chaleocite, covellite, and chalcopyrite are the best suited for the manufacture of blue vitriol. Out of the two former the copper can be extracted in the form of vitriol, in the cheapest and completest manner, by roasting in reverberatory furnaces and subsequent leaching with hot water, strongly acidulated with sulphuric acid. A portion of the copper, already converted into a sulphate of copper by the carefully-conducted roasting process, will, in this way, be extracted immediately by the water, while the portion which has become oxide of copper by roasting forms sulphate of copper with the sulphuric acid, and passes into aqueous solution. The small amount of iron which is nearly always present in the above two minerals, though it is sometimes not in their crystallized varieties, will not pass into solution, as oxide of iron becomes, after exposure to red-heat, insoluble in sulphuric acid; and a very pure vitriol can thus be secured.

To produce sulphate a copper, however, by the roasting process alone, without any addition of sulphuric acid in subsequent operation, chalcopyrite is the most desirable mineral. In this process the copper pyrites is first pulverized, and, without previous separation from the gangue, (such as is almost always messy for smelting operations,) it can be directly introduced into the roasting furnace. This is one reason why copper ores too poor to be treated for metallic copper (in which case the expense of the necessary mechanical concentration before smelting is often fatal to profitable operations,) may be in many cases used to advantage for the manufacture of vitriol.

The furnaces employed are always reverberatories, either open-hearth or muffle-furnaces. They are generally very long seed comparatively narrow; or two or more are placed one above the other, and so constructed that the flame and heat pass successively from the lowest to the highest before the waste gases enter the flues leading to the acid-chambers or to the chimney. The charge can at intervals be lowered from the upper furnace into the one below, a new charge being introduced into the highest hearth, and thus a continuous renting secured. In the flues are dampers for the regulation of the temperaturem which must be as much as possible in the power of the workmen. The object of the above management of the roasting furnaces is, on the one hand, economy of heat, and, on the other hand, a gradual preparation of the ore and its prolonged contact with the developed sulphurous acid gas. The heat necessary for the process is very low, and little fuel, except for the test heating of the furnace, is required. In fact, the burning sulphur supplies even more than is desired during the roasting, so that the dampers must be partly closed, in order to retard the otherwise too rapid draught, and to keep the sulphurous acid in contact with the ore as long as possible. By stirring the ore, either by hand or by machinery, new surfaces are exposed from time to time, and in order to make these as large as possible, the ore is thrown up in ridges across the furnace. The whole process is based on its well-known relations of the temperatures at which sulphates of iron and copper are formed and decomposed. It is not necessary to enter here into a detailed statement of the theory of this beautiful metallurgical process, which has been to ably expounded by Plattner, Keel, and others: suffice it to say thet all explanations agree as to the final results established in practice. The sulphate of iron it formed long before even a cherry-redd heat is reached in the furnace; at this temperature the sulphate of iron is decomposed, the sulphurons acid developed from the iron salt aiding materially to produce, in conjunction with that from the oxidation of the disulphide of copper, the sulphate of copper, while the iron remains as an oxide. If the heat be carried higher, the sulphate of copper will also be decomposed; and it requires therefore much attention, and the frequent taking of samples, to maintain the correct degree of heat. The formation of sulphate of iron takes place when the double reverberatory is used in the upper furnace. In the muffle-furnace (which is charged at the flue-end, and the charge gradually moved toward the fire-bridge) sulphate of iron is found at about the middle. In the lower furnace, (when two are used,) or in the part of the muffle nearest to the fire-bridge, the copperas is decomposed and the sulphate of copper formed.

The temperature in the furnaces has been correct, and the ore nearest to the fire-bridge must be drawn out, when a sample is taken, and, after cooling a little, brought in contact with water, gives an abundant deep blue solution, without greenish tinge. The sample is taken by withdrawing from the furnace, opposite the working-door nearest to the fire-bridge, and about half way across, a portion of the ore with a small test-shovel, or large spatula. It is then put, in the form of a ridge or dam, across a porcelain saurcer, care being taken that on at least one side of this ridge no ore is scattered. At the opposite side, after the temple has partially cooled, water is added in very small quantity. This first water is absorbed by the anhydrous sulphate of copper; but if a little more is added on the same side, a clear blue concentrated [] of copper-vitriol will at once appear on the other side of the ridge. This solution ia then diluted by an additional supply of water, in order to permit of better judgment in regard to the color, and of an easier discovery of a greenish tinge, which would annouce the presence of iron-vitriol in small quantity. A dark green color of the solution shows the presence of much copperas; while a dirty, pale green assures us that no appreciable quantity of blue vitriol has yet been formed. In both cases, the roasting must be continued until the sample presents the desired characteristics. If the heat has been carried too high near the fire-bridge, the solution obtained from the temple presents a light blue color already with the firth addition of water, and the presence of much oxide of copper is thus revealed to us. This should never occur, and can not, if samples are taken at the proper intervals, of five to ten minute, toward the latter part of the process. But if this undesirable result has been reached, the oxide of copper can only be transformed into vitriol by direct leaching with hot, diluted sulphuric acid, or by smelting the charge, together with unrouted copper pyrites, into matte, which must then be pulverized and parts through a more careful roasting process. If a satisfactory result has been reached in the roasting process, the hot ore is drawn out into iron cars, and dumped into lead-lined leaching vessels. These have double bottoms, (over the upper perforated one of which coarse cloth is spread,) and are half filled with water. Here the ore remains for twelve houra being occasionally stirred, and at the expiration of that time the blue solution is drawn of by means of wooden faucets at the bottom. It undergoes then the usual operations of settling, concentration, (if necessary,) and crystallization, as described in a former article. The crystals obtained after the first concentration are very pure vitriol; a product mixed to a small degree with sulphide of iron is obtained by concentrating the mother-liquor, and crystallizing again; and out of the secondmother-liquor the copper must be precipitated by iron, as another crystallization would produce vitriol too impure for the market. The cement copper is either dissolved in sulphuric acid and manufactured into vitriol or sold in metallic form. Where smelting-works are connected with the establishment, the cement copper is added in the black copper smelting.

For the processs above described, only such pyrites as contain only sulphurets of iron and copper should be employed, since these give, without troublesome and expenelve purifications, the best product, Copper-vitriol is, however, often made from artificially-prepared sulphurets of copper; and in tide case the original impurities of the ore, being incidentally removed in the preparatory treatment, exert no injurious influence. Of the processes introduced for this manufacture, two are especially prominent. In one of these, the artificial sulphuret is produced by heating old copper (sheet-copper, scraps, etc.) in a reverberatory, and afterward adding sulphur while the draught is cut off. In the other, the sulphuret is produced in the regular course of copper-smelting in the shape a matte. In the latter case, the highest grade mattes are the most suitable, as they contain the least iron.

To convert old copper into sulphuret, it is, as we have said, heated in reverberatories, and, when red-hot, about one sixth of its weight of coarsely pulverized sulphur is added, while the flues are shut. The disulphide of copper formed on the surface of the copper sheets is taken ont, and the remaining copper is again treated as before. The resulting disulphide is pulverized and roasted in the same furnace. Much sulphate is formed; but owing to the absence of sulphate of iron, or some other sulphate which is decomposed at a lower temperature than sulphate of copper, much oxide of copper is also formed. It is, therefore, necessary to leach the product of the roasting with dilute sulphuric acid, in order to extract all the copper. The solution undergoes subsequently the operations of settling, etc., and a very pure blue vitriol is the result. In order to make sulphate of copper from furnace product, the highest grade matte from the last smeltings are generally selected, because they contain the least foreign subetances. They must be very finely pulverized before roasting, and the product must also be leached with dilute sulphuric acid in order to dissolve the oxide of copper which, besides the sulphate, is formed in large quantity. The final product is pure vitriol, specially from the two first crystallizing operations. The last mother-liquor is treated by the manipulations we have described in connection with the manufacture of vitriol from copper pyrites.

In the regular course of operation, copper vitriol is produced in many establiehments as a by-product, in various other ways than those above mentioned; but none of these are imitable where vitriol manufacture is the main object of the works, and we than therefore omit them here.

At present, sulphate of copper commands a very high price in the market compared with the value of the raw material; and it is now, in most localitice in this country, more profitable to make blue vitriol from the ore, or even from matte, than to sell the ingot-copper. Only excessive charges for transportation may alter this fact, since the bulk of the vitriol is four times as great as that of the ingots; but where freights are at all reasonable, (indeed, if the tramportation tee market does not exceed 1½ cents per pound,) this manufacture can scarcely fail to be extremely profitable.

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