|The element is used only to a small extent in commerce. None at all has been produced in recent years in the United States, where the annual demand rarely exceeds 100 tons. In Europe it is generally produced by the sublimation of native arsenic or by heating arsenopyrite or lollingite. The mineral is heated in the absence of air at 650° to 700° C. in earthenware retorts or in tubes laid horizontally in a long furnace. The arsenide minerals decompose thus:|
FeS2.FeAs2 = 2FeS + 2As
4FeAs2 = Fe4S + 7As
A furnace usually holds 24 to 30 retorts, about feet long and 6 inches in diameter, made of clay and powdered bricks, with a glazed exterior which makes them impervious to arsenical vapours, the charge for each retort being about 20 to 25 lbs. of ore. A spiral piece of thin sheet iron is inserted about 4 inches into the mouth of the retort and projects for the same distance into a cylindrical earthen condenser, the joint between the retort and the condenser being luted with clay. A door at the end of the condenser allows access to the retort and enables observation of the operation. On distilling, most of the volatilised arsenic condenses on the iron spirals as nearly white crystalline crusts or as glistening grey scales. Some finely divided arsenic and volatilised sulphide collect on the walls of the condensers, but the volatilisation of sulphide may be minimised by the addition of potash or lime to the charge. The end of the operation, which may take up to 12 hours, is judged by observing the fumes through the condenser door. When cool, the spiral iron sheet is removed and unrolled and the crystalline mass detached. The yield is about 50 per cent, of the arsenic present in the arsenopyrite, while it is somewhat larger in the case of leucopyrite. The residues may be roasted in order to recover the remaining arsenic as arsenious oxide. The arsenic is ready for sale without further treatment, but may be purified by resublimation, after mixing with powdered charcoal.
Metallic arsenic is sometimes obtained from arsenious oxide. Thus in Altenberg (Silesia) the oxide is heated at 650° to 700° C. with charcoal in an earthenware crucible covered with a conical iron cap which acts as receiver, while in Chicago retorts composed of steel pipes large enough to take a charge of 450 lbs. were at one time used for the same purpose, the arsenic being collected in water-cooled pipe condensers. This reduction method is not satisfactory, however, as the product is largely amorphous and is not so desirable as the crystalline form, being suitable only for making arsenic compounds. Moreover it always contains arsenious oxide. A Japanese method consists in fixing the vapour of arsenious oxide by means of ferric oxide or alumina at a temperature above 218° C. and then reducing the product with water-gas, Mond gas or producer gas above 100° C.; the arsenic thus freed is then sublimed.
The element may also be prepared by heating the sulphides with carbon and alkali carbonate or cyanide, but in this case also the product is largely amorphous.
Various attempts have been made to employ electrolytic methods for the extraction of arsenic from arsenical minerals. Thus Siemens and Halske suggested the treatment of sulphide ores with sulphides or hydrosulphides of the alkali metals so that the arsenic passed into solution as a double sulphide or thioarsenite, thus:
As2S3 + 6NaHS = As2S3.3Na2S + 3H2S
Electrolysis of the resulting solution in a diaphragm cell with carbon or platinum cathodes resulted in liberation of arsenic in the cathode compartment. Extraction of the ores with sulphydroxides of the alkaline earth metals and subsequent electrolysis has also been suggested. These methods have little application, however, since the arsenic in the common ores, such as arsenopyrite and leucopyrite, cannot be extracted as sulphide.
Arsenic may be deposited by electrolysis of a solution of arsenic trichloride in glacial acetic acid; it is always obtained in the amorphous form. It has been successfully plated on copper and brass from a solution of arsenious acid containing alkali chlorides.
Pure arsenic has been prepared by reducing carefully purified ammonium dihydrogen arsenate at 1000° C. in a current of ammonia, the arsenic being finally resublimed in a vacuum. The element may also be obtained as an amorphous precipitate by reduction of aqueous arsenious acid, for example, by means of sodium hypophosphite, or by the addition of a few drops of phosphorus trichloride. The reaction in the latter case probably takes the following course:
As2O3 + 3PCl3 + 9H2O = 2As + 3H3PO4 + 9HCl
A similar reaction occurs also with arsenates and arsenites, and is sufficiently delicate to reveal the presence of as little as 0.000075 gram of arsenic per cubic centimetre.
Compact pieces of metallic arsenic may be obtained by pressing powdered arsenic at temperatures approaching 500° C., with exclusion of air.