(2) Kopp found certain disadvantages in working by the above method, and adopted instead the reaction of antimony chloride upon a dilute solution of hyposulphite of lime.
Experiencing much difficulty in the decomposition of antimony sulphide by hydrochloric acid on an industrial scale, he experimented on roasting the sulphide at a moderate temperature in contact with air and steam, whereby most of the antimony sulphide is converted into oxide, while the sulphurous acid driven off is utilised for making the hyposulphite of lime. This proved a most successful plan, and the resulting antimony oxide is readily dissolved by commercial hydrochloric acid.
During the oxidation of the antimony sulphide, a certain proportion of antimonious acid may be produced. This is but slightly soluble in hydrochloric acid. It may be collected, however, by saving the residues from the treatment by hydrochloric acid, and washing them with chloride or hyposulphite of lime, which will dissolve the adherent antimony chloride; they are then dried, and melted with a little antimony sulphide and quicklime, so as to transform the whole into antimony green, the quicklime having the effect of decomposing any small residue of antimony chloride.
The preparation of the hyposulphite of lime is cheaply effected by the action of sulphurous acid on sulphides of lime, the sulphurous acid being derived either from the roasting of the antimony sulphide, or from pyrites or brimstone in the usual way.
Calcium polysulphide is prepared by boiling finely powdered sulphur and newly slaked lime in water. Certain advantages arise from the addition to this solution of a little powdered calcium oxysulphide, or some quicklime.
In the reaction of sulphurous acid on calcium sulphide and oxysulphide, sulphur is set free and forms a sulphite of lime, which, in the presence of sulphur and undecomposed sulphide, is soon transformed into hyposulphite, the reaction being facilitated by the rise of temperature which takes place in the apparatus.
As soon as the liquor has become slightly acid, it is drawn off into a large settling tank. If, after agitating for some time, the liquor has not become neutralised by the undecomposed calcium oxysulphide contained in it, this is brought about by addition of a little calcium sulphide, and is recognisable by the appearance of a black precipitate of sulphide of iron. After due settlement, the clear liquor is decanted, and forms a solution of nearly pure hyposulphite of lime.
The production of antimony vermilion is effected from the foregoing solutions of antimony chloride and hyposulphite of lime, in apparatus consisting simply of a series of wooden tanks raised conveniently above the floor, holding about 500 gals. each, and provided with steam coils for heating their contents.
Sufficient hyposulphite of lime solution is run into the tanks to fill about seven-eighths of their depth; and then into the first tank is poured the chloride of antimony solution, in quantities of a few pints at a time. A white precipitate is formed, and rapidly dissolves at first; when it is slow in going into solution, even though stirred, the addition of antimony chloride should be stopped, as an excess of hyposulphite of lime is essential. The liquor in the tank must be perfectly clear and limpid, and should any white precipitate remain it must be dissolved by making small additions of hyposulphite.
At this stage steam is admitted into the coils, and thereby the temperature of the solutions is gradually raised to 120° or 140° F., or even to 160° F., while stirring is unceasingly carried on. The reaction is soon manifested by the successive colours of the liquor, passing from straw-yellow to lemon-yellow, orange-yellow, orange, orange-red, and lastly a very deep and brilliant red. The steam is shut off from the coil before the desired tint is arrived at, as the acquired heat and the agitation complete the development of the colour. If the heating is carried too far, the red gradually passes to a brown and later to nearly black. With experience, almost any desired shade of red can be produced.