Manufacturer and builder 7, 1870
The term "vitriols" comprisest the soluble sulphates of all the metals. The most common, and the same time the most prominent, are the sulphate of iron (copperas,) the sulphate of copper, (blue vitriol,) and the sulphate of zinc, (white vitriol.) These three are very extensively used in the arts and manufactures; and they are therefore themselves made in large quantities, and by many different methods. The mode followed varies in different localities, according to the facilitics presented for economical manufacture, and is, of course, the most appropriate, the cheaper, and at the same time purer, the article can be produced. The latter quality especially is at present very essential, while formerly vitriols were often made which varied in composition and the relative amounts of impurities with almost every campaign. The manufacture of one or more there vitriols as products, apparently secondary in importance, is in many establishments really the only means of keeping the whole business in a paying and prosperous condition; and, considered in this light, it is, especially in the United States, of the utmost importance. This applies particularly to many mining and smelting establishments; and for the benefit of those who may profit from the perusal of this article, as description of the methods employed for the manufacture of these substances will be given in the following. To treat the subject fully and exhaustively, and in such a manner that even those who are not versed in chemistry and technology may become sufficiently acquainted with the details of the various operations, we will first describe the manufacture of each one of the three above-mentioned vitriols in a separate article, and, in conclusion, point out in which manner the manufacture of all three may, under certain circumstances, be profitably combined in the same run of operations.
Green vitriol, or copperas.
This substance, the sulphate of protoxide of iron, is, when pure, composed of 25,9 parts of sulphuric acid, and 45.3 parts of water. One part of this vitriol is dissolved by 2 parts of water at O, by 1½ parts at 15, and by 0,27 parts at 90 degrees C. The common copperas of the market contains, however, besides the above sonstituents, small quantities of oxide of zinc, copper, aluminium, magnesium, potassium, etc., and always more or less peroxide of iron. These substances do not impair the value of the vitriol for some purposes, while for others they are highly injurious. The contents of copper, for instance, render copperas unfit to use for the manufacture of Prussian bue, while the presence of peroxide of iron is injurious in dyeing with indigo, but desirable in making ink. Prussian blue, and in coloring black by means of tannic acid.
The manufacture of vitriol on a large scale is conducted in various ways. If an entirely pure product is to be made, the best results are undoubtedly reached by dissolving iron in dilute sulphuric acid. This is either used right from the lead-chambers of sulphuric-acid works, before it is concentrated, or after it has previously been used for the manufacture of ether, in petroleum-refineries, etc. In the latter case, it is diluted so as to show not above 1,150 specific gravity, then warmed in copper or lead vats, into which old iron is subsequently introduced. The acid is saturated, when the evolution of hydrogen gas ceases. After this, the iron is left in contact with the solution for a short time, in order to precipitate foreign metals, like copper, etc., while the liquid is gently heated until a sample, dopped on a piece of glass, shows, on cooling, a thin, crystalline skin. The solution is then drawn off, by means of a siphon, into a large reservoir, where it is carefully covered up and left to settle for eighteen to twenty-four hours. When all the suspended particles have sunk to the bottom, the clear solution is drawn off by siphons, and run into the crystallization vessels by means of lead-lined troughs. These vessels were used formerly in the shape of truncated cones, about three and a half feet deep and seven feet in diameter; but lately the flat, rectangular vat, made of planks and lined with sheet-lead, is generally preferred. Across the tops of the vat slats are laid, which are perforated with many holes.At the expiration of a fortnight, and in cold weather before that time, the crystallization is completed, and the product is only washed with very little water (which is not permitted to run fof, but used again in diluting sulphuric acid for another operation and then dried on inclined shelves at ordinary temperature. On the proper execution of the washing and drying process depends mainly the appearance of the product. If the crustals have not been dried perfectly, they are inclined to become covered by a coating of yellowish-brown basic sulphate of peroxide of iron in a short time, while they keep their transparency and beautiful bluish-green color for months if properly dried. As copperas has, however, the property of taking upoxygen from the air eagerly, it will, sooner or later, change its color (and composition) unless the air is excluded. For large quantities this is however, rarely done, as the newly-formed salt is not injurious in the most prominent applications of iron vitriol, and in some absolutely advantageous. To keep copperas unaltared for laboratory purposes, the bottle intended to hold it is generally, before filling it, washed out with ether, or the air is driven out by carbonic acid. When filled, it is tightly stoppered, and the cork covered with wax. It is, however, not alone the property of copperas to take up oxygen from the air, which acts injuriously, and has to be guarded against as much as possible. If, before drying, the mother-liquor has not been permitted to drain off a sufficient lenght of time (eight or ten days) on suitable shelving, some of the liquid, or, if evaporated, the salts contained in it, will remain in the angles between the different crystals. This makes the copperas totally unfit for transportation, because it attracts moisture from the air very rapidly, to such a degree as to liquefy the vitriol and allow it to run out of the barrels. Great loss is of course hereby accasioned.
In making the pure green vitriol, only those crystals are generally taken out of the vats and packed for shipment which have adhered to the cane, strips of lead, etc., as the yare much the purest and lagerst, while the portions deposited back into the dissolving vessels and crystallized over.
A very pure copperas is also made in some localities by dissolving spherosiderite (carbonate of iron). In diluted sulphuric acid; the proceedings in this method are essentially the same as those above described, the only difference being in the additional expense of the removal, from time to time, of the gangue from the dissolving vessels. The resulting copperas is not as pure an article as that made from metallic iron.
By fat the largest quantities of copperas are made directly from the various sulphurets of iron, occurring mineralized in great masses in nature, and from mine waters. In a subsequent article these methods, which are so important for many of our mines in the Eastern and Southern States, will be fully discussed.