REMOVAL OF ARSENIC FROM SULPHURIC ACID.—In many French works the sulphuric acid is freed from arsenic by sulphide of barium. The sulphate of barium formed falls to the bottom of the vessel with the sulphide of arsenic. In Germany, sulphuretted hydrogen gas is more generally used. At the ochre mines in the Harz Mountains, where sulphuric acid is made as a bye-products and contains besides -11-'14 per cent. of arsenious acid, per cent. of sulphate of lead, and smaller proportions of antimony, copper, iron, lime, and potash, the following method of purification is adopted : The precipitation of the arsenic, lead, antimony, and copper is performed in a leaden pan about 8 ft. long by 3 ft. 6 in. wide and 1 ft. 9 in. deep, in which the acid is diluted to 93° Tw. and heated to 75° (167° F.). The dilution of the acid to that degree is considered advisable because stronger acid decomposes more readily with sulphuretted hydrogen. At the bottom of the pan lies a leaden false bottom, or tray, perforated with small holes, whose edges are turned down about 2 in., so that the tray is suspended at about that height from the floor of the pan. The upper surface of the false bottom measures about 3 ft. 6 in. x 1 ft. Underneath it and within the down-turned edges, a leaden pipe is introduced by which the sulphuretted hydrogen gas is ad mitted, and which streams up through the holes and into the supernatant acid. The pan is closed by a leaden luted cover in which is a pipo for conveying away the excess of sulphuretted hydrogcu. Thu pan holds about 2 tons of acid, whose purification will be complete in about six hours. The completion of the precipitation of the arsenic may be recognized in that the acid then conuneuces to assume a milky appearance. Acid thus purified is said to contain but •0003 per cent. of arsenic.
The sulphuretted hydrogen required is prepared from sulphide of iron (produced by smelting 280 parts of old iron with 115 parts of sulphur) and sulphuric acid of about 110^ Tw. from the chambers. For this purpose four cylin drical leadon vessels are used, about 14 in. in diameter and 18 in. in height. In the arched cover of each vessel, besides the gas pipe for carrying the sulphuretted hydro gen to the pan, is an aperture for the in sertion of the sulphide of iron which may be tightly closed by means of a screw and a funnel-topped bent tube for admitting the necessary sulphuric acid. For the purification of the 2 tons of acid which the pan holds, about 100 lb. of sulphide of iron and 110 lb. of sulphuric acid at 110° Tw. are needed.
At Freiberg, the precipitation of the arsenic by sulphuretted hydrogen gas is conducted iu au apparatus by which the loss of this gas is much lees than iu the preceding method. It is also unnecessary to dilute the sulphuric acid to be treated, as it is not subjected to heat. Fig. 76 shows a vertical section, and Fig. 77 a plan of this apparatus. A is the so-called precipitation tower formed of lead, and about 7 ft. 6 iu. high and 2 ft. 6 iu. iu
diameter. The sulphuretted hydrgeu is conveyed to the tower from the generating vessel by the leaden pipe o. The sul phuric acid to be purified runs from the leaden chambers direct into the lead-lined cisterns D. These are in con nection with the leaden tube b running down the centre of the tower and terminating below in a shallow box, provided with eight little holes o, through which the acid is forced in fine jets by the hydrostatic pressure in the pipe b. The great distri bution thus achieved consider ably assists the precipitation of the arsenic. The jet holes can he closed by raising tho load coated iron rod i, which is fur nished with little cones fitting exactly into the holes. The purified acid flows through the pipes d, which can be shut by pinch-cocks on the indiarubber parts e, into the cisterns C beneath. From here it is either run into the concentrating pans after having deposited the sulphide of arsenic, or it is let into the forcing apparatus D, from which it is again raised into the cisterns B, should it he found necessary to treat it once more. For this purpose the valves', made of iudiarubber and furnished with screw wheels, are opened. As soon as the forcing apparatus is full the valves are closed again, and air is forced into the apparatus at h through the pipe E, which is provided with a valve g, and this forces the acid up the pipes k into the upper cisterns. The forcing apparatus are formed of strong iron cylinders lined with sheet lead, the valves and other parts are of spelter.
More recently an improvement has been made in this apparatus, consisting in substituting horizontal prisms, such as in Gerstenhoefer's kiln, for the jets, these prisms being flat-side upper most and formed of hollow sheet lead.
The treatment of the acid with the sulphuretted hydrogen is repeated as many times as necessary, until after remaining for some time in contact no more signs of arsenic are shown. Generally three repetitions of the process are sufficient to render the acid quite free from arsenic. The precipitated sulphide of arsenic is allowed to settle in large lead-lined tanks, and the clear liquid acid is forced by a leaden pump into the reservoir which feeds the leaden concentrating pans. The yellow sediment of sulphide of arsenic is well washed, and then sent to the arsenic smelters.
Formerly the gas generating apparatus consisted of several leaden vessels communicating with each other, and provided with an outer jacket, in which steam was made to circulate in order to warm the apparatus; now, however, a single large square leaden tank is used for the purpose. The gas passes from this tank into a leaden washer half-filled with water, provided with two little glass windows through which the process may be observed. From the washer the gas is taken direct to the precipitating tower.