The Manufacture of Vitriols II

Manufacturer and builder 8, 1870

The principal minerals used for the manufacture of copperas are magnetic pyrites, marcasite, and common iron pyrites. The conversion of the former two into sulphate of iron is the simplest, because these two are easily decomposed and sulphatized by the air and moisture alone, without requiring any additional assistance from man: the latter requires artificial help by a preparatory roasting.

Copperas, espacially the impure article, commands only a low price in the market; and for this reason the operations connected with its manufacture must be carried out on a large scale in order to make profitable returns possible. In making it from magnetic pyrites or marcasite, which abound in all parts of the globe, where metamorphic strata crop out on the surface, very large quantities of these minerals are therefore subjected to the necessary manipulations at the same time. The ores are broken into lumps of small size, not larger than hens' eggs, and dumped upon extensive water-tight floors, in amounts of 300 to 500 tons in a heap. The floors are made impenetrable by evenly spreading several layers of clay, which has been freed from admixed small stones and organic matter as much as possible, over the surface of the ground intended to serve as the floor, and tamping it carefully all over so firmly that, after the operation is completed, the finger can not make an impression on the clay. The material possesses generally the right amount of moisture for this purpose, when dug out of the ground; but, if not, it must be prepared so as to stick together when pressed in the shape of a ball in the hand, without leaving the palm wet or even moist. The floor is made either round, square, or in the shape of a parallelogram, and inclined from all sides toward a point outside; at the same time it is elevated at least a foot aboce the surrounding surface of the ground. Its lowest point is connected by wooden troughs or pipes, with wooden reservoirs, or settling-tanks. Wooden floors would not answer the purpose, because, during the subsequent decomposition and oxidation of the ores, much heat is developed, which sometimes reaches such a height as to set the heaps actually on fire. As soon as the tamping of the clay foor is completed, the broken pyrites may at once be thrown on. The shape of the heaps follows that of the floor, leaving about two feet free all along the outside. Their sides are thrown up as steep as the nature of the material will permit, and their height ought not to exceed seven feet. In this manner, they are left exposed to the weather for some time. Frequent changes of rain and warm weather favor the decomposition and oxidation greatly; and soon cracks will be notived in the pieces of pyrites; hair-like bunches of crystals will be formed on the outside, and the whole pile will gradually be covered by a white effflorescence of iron vitriol. As soon as this decomposition is found to have progressed for some distance, say two or three feet into the interior of the heap, the lixiviating process is commenced. For this purpose, a suitable scaffolding is erected above the heap, carrying troughs perforated with very small holes in the bottoms and sides. Into these, water is brought and distributed over the heap as equally as possible. It percolates through the mass, slowly dissolving on its way to the impermeable floor the sulphate of iron, and carrying it, mixed with small particles of ore, into the settling tanks. After these have been filled, the solution is pumped back over the heap, and permitted again to penetrate it, dissolving new portions of vitriol. This operation is repeated several times, and finally the liquid is left to settle in the reservoirs. After this, it is drawn off from the sediment, and conducted into concentrationvats, (best constructed of masonry,) which are arched, and heated by means of fire-places at one end, the flames passing over the surface of the liquid. Evaporation is thus materially accelerated by the draught. As soon as the liquid shows 20 per cent of vitriol, firing is discontinued, and the solution is drawn into large vats to permit the deposition of the basic sulphate of iron, a yellowish-brown powder, which is generally formed in considerable quantities. When the liquid has become clear, it is run into a second concentrationvat, usually constructed of sheet-led, arched and heated like those mentioned before. Here the solution is evaporated until it contains 48 per cent of vitriol, and after the settling of the basic salt, which is this time fomred in less quantity than before, it is drawn over into the crystallization vessels, where large quantities of the vitriol are deposited in from 10 to 14 days, in the same manner as in the case formerly spoken of, with the only exception that the best vitriol from the strips of lead, canes, etc. and the inferior kinds from the bottoms and sides, are mixed before packing int othe barrels and sold together, because the low price of the vitriol prepared in this manner does not permot recrystallization of the inferior article. Such copperas contains, according to the composition of the ores from which it has been manufactured, from 16 to 24 per cent of protoxide of iron, combined with sulphuric acid, while the balance consists of the deteriorating sulphates of copper, zinc, manganese, alumina, magnesia, etc. Pure copperas would contain 25.9 per cent of protoxide of iron.

The basic sulphate of iron, deposited during the settling processes, and the scum formed on the top of the solutions while evaporating, are heated strongly in reverberatory furnaces, and converted into red oxide of iron, which is much used as paint, and known as Venetian red.

The mother-liquor is either again concentrated and crystallized for itself, yielding an inferior quality of vitriol, or it is mixed with water and used again for lixiviation of the heaps, after they have been left for several months to form a new quantities of sulphate of iron.

Works of this kind, to be kept in continual operation, require a large working area; since many heaps are required to supply withtou interruption the necessary solutions. It takes, of course, a long time before all the iron in the ore is transformed into sulphate. In some coutnries such heaps have continued to yield a paying lye for twenty years. The cost of the manufacture is of course extremely small, as no handling of the raw material is necessary beyond its deposition on the lixiviating floors, and the amounts of fuel used, as well as the labor required, are so small that they form but a trifling item in the cost. The main expense in this method lies in the mining of the ore and its breaking to the proper size. It has also been attempted to do away with the expense of the first concentration, by conducting the weak vitriol solution upon thorny brush, in trickling through which it is rapidly concentrated by evaporation, even at the average temperature, since it thus presents the largest possible surface; but the loss in this method by the formation of large quantities of basic sulphate has been found to balance the gain in fuel.

*) The ease with which marcasite, a bisulphuret, decomposes in the air, without artificial roasting, while the ordinary bisulphuret does not, is a matter not fully explained. The chemical constituion of the two minerals being the same, this difference in behavior is probably due to the different arrangement of molecules, indicated by the different crystal forms.When common pyrites is used as raw material for the manufacture of copperas, it must be submitted to a roasting operation, in order to drive off one equivaltent of sulphur; because the bisulphuret of iron is not at all, or but very slowly, decomposed, and subsequently oxidized by atmorpheric influences. But after it has been changed by roasting to a sulphuret, decomposition takes place rapidly, It is then treated the same as magnetic pyrites or marcasite,* while the sulphur eliminated during the roasing process is used either for the manufacture of sulphuric acid, or condensed, remelted repeatedly, and cast into moulds to be sold in the form of short bars.

Considerable sulphate of iron is also made of different localities from mine-waters, as, for instance Fahlun, (Sweden,) Goslar, (Prussia,) etc. These waters contain always variable proportions of sulphate of copper, which must be removed before the copperas is crystallized. The solutions as they come from the mine are always very weak, and various designs have been tried to effect concentration at a low rate of expense. At Fahlun, gradation-walls of thorns are used; at the Rammelsberg mines, the mine-waters are used for the leaching of what is there called "the old man" - the refuse of former mining stowed away in the mine and used for filling-in. Now copperas and zinc-vitriol are made out of this material. After the mine-waters are sufficiently concentrated, say to twenty per cent, they are conducted into vats filled with scrap-iron, in order to precipitate the copper from the solutions. This effected, they are condensed further, and the copperas crystallized in the manner already described.