The most approved form of apparatus for the manufacture of magenta is that described by H. Cant, of the firm of Evans, Cant, and Co., of Stratford. It is shown in Figs. 488 and 489.
Aniline and arsenic acid are introduced together into a U-shaped boiler with flat ends, heated by a furnace, and set so as to avoid direct action of the fire. The furnace door F, and ashpit door A, allow of very nice regulation of the draught, which is a matter of great importance. The ends of the boiler are left exposed, and in one of them is an aperture o, for emptying it ; through them passes the spindle of the agitator G, and at the top is an opening P, and a " goose-neck" of copper S, connected with a copper condensing worm, contained in a suitable receptacle, so as to collect the aniline and water given off during the heating. The great advantage of this arrange ment is that the contents are removed by a small door, aecured by wrought-iron bolts and wedges, and the framework of which prevents waste trickling down when emptying. The charge for these boilers should not be more than half the capacity of the boiler ; the time required to work off a charge of 1200 lh. aniline and 2200 lb. arsenic acid is about eight hours. These proportions will yield approximately—water, 730 lb. ; aniline, 370 lb. ; crude " magenta melt," 2300 lb.
It is necessary that the agitat,ors be well worked even whilst the ingredients are being added, otherviise the mixture becomes solid, and has to be removed from the stills ; hence BM advantage in using a large proportion of aniline proper. The fans of the agitators should be so arranged as to stir every portion of the mixturo. Other oxidizing agents have been proposed, but none is so largely used as, nor moro successful than, arsenic acid. The heating should not rise above 200° (392° F.). The heavier unaltered aniline is removed from the melt, before emptying, by distillation with steam. The melt is sometimes slightly moistened with steam, so as to make it leave the stills more rapidly. By knocking out the wedges which secure the small door, the melt is received in an iron pan (see Fig. 489); a piece of sheet iron, acting aa a spout, prevents its trickling down over the furnace. The fire and asbpit doors being shut, the agitator is worked t,o assist its flowing out, the workmen then leave the spot so as to avoid the furnea. The aniline and water which come over are separated ;
the aniline, recovered and rectified by distillation with a little lime, is best suited for all purposes requiring a low boiling aniline. The rosaniline is separated from the crude melt either by con verting it first into a salt, or by first obtaining the base, purifying it, and then converting it into a salt.
During the heating in the stills, a kind of humus is formed ; this is removed from the colouring rnatter by boiling for four or five hours, with continuous agitation, in water heated by steam, with the addition of very dilute hydrochloric acid (7 lb. commercial acid to 350 gall. water). The hunaus separates out, and the liquor is drawn off, and passed through flannel in a filter press: the filtrate contains hydrochlorate of rosaniline, arsenious and arsenic acids, and some chlorides of arsenic ; the addition of carbonate soda neutralizes the free acid, and the colouring matter is precipitated, with a little arseniate. An excess of hydrochloric acid facilitates the separation of the rosaniline salt.
The rosaniline converted into hydrochlorate is purified as follows :—a quantity of solution con taining 1 ton of the crude salt is placed in a large iron tank, to this is added about ton of common salt, in small quantities at a time, and dissolved by a jet of steam let into the bottom of the tank by a tube; the hydrochlorate is almost insoluble in the solution of common salt, and separates on the surface of the liquid, the arsenic and arsenious acids being retained in solution as arseniates and arsenites of soda. At the end of a few days, the colouring matter remaining in suspension collects on the top, aS the liquid becomes cold, and is then removed. It is washed with a little boiling water, to remove adhering salts, and is then sufficiently pure for many purposes. Its further purification ia effected as follows :—It is dissolved in boiling water, filtered through flannel, and allowed to cool in large tanks, in which are suspended threads of wool ; at the end of several days, the salt haa Crystallized on the threads, and on the bottom of the tank ; the former is sold as "pure" or " refined," the latter is generally used for the manufacture of other dyes, as green or blue.