Ores of antimony which do not contain sulphur, such as the oxides and native antimony, are not commercially of much importance. When freed from earthy matters, they are readily reduced to the metallic state by being gently heated with powdered charcoal. The assay may be conducted in an earthenware crucible lined with charcoal, without the addition of any flux. If, however, the ore contain much siliceous impurity, it is necessary to use a flux. For this purpose, the ore may either be intimately mixed with 2 parts of black flux, or with 1 part of carbonate of soda and 0.25 of finely powdered charcoal. In this case, lining the crucible is unnecessary, and after it has remained in the firo until its conteuts are in a state of tranquil fusion, it should, on being withdrawn, be tapped gently against some hard body, to collect the fused metal into a compact button. When the crucible has become cold, it is broken, and the button extracted and weighed. Care is required in detach ing it from adhering slag, since from its brittleness it would otherwise be liable to become broken, and a portion consequently lost. This method is to ores containing sulphur in any quantity, as the sulphide when heated with black flux yields only oue-half of its metal, the rest being retained in the slag.
Assay of Bismuth.—When the ores of bismuth to be assayed consist of bismuth in the metallic state, or of oxides or sulphides free from admixture with other ores, the operation is a simple one. The powdered ore is mixed with the most fusible flux that can be obtained, and fused at a low heat in an ordinary fireclay crucible. A good flux consists of a mixture of 2 parts of carbonate of potash or soda, and 1 part of common salt, with a little red argol or cyanide of potash.
But when, as is generally the case, the ore contains a large proportion of copper, the above method cannot be adopted, because the copper is reduced also; and the necessity for a good method of separating the two metala has long been felt. A very simple means of effecting this has been devised by Hugo Tamm. The ores containing both bismuth and copper are the bismuth copper pyritee, or the sulphides of the two metals, and their double oxides or carbonates. Tho method employed in each case is varied slightly. It depends upon the fact that in presence of alkaline fluxes, carbonaceous reagents or carbon itself reduce sulphide of bismuth to the metallic state, leaving the sulphide of copper unaltered. In the case of tbe sulphides, all that is required is to flux the ore with a mixture of carbonate of soda or potash and common salt, to w hich a little flowers of sulphur and powdered charcoal have been added. In this operation, metallic bismuth is extracted easily and is tolerably free from copper. When the mixed metals consist of oxidee or carbonatea, the only difference to be observed is in the proportion of sulphur employed.
Three parts of ore should be mixed with from 2 to 3 parts of a flux composed of:— Carbonate of soda .. 5 parte Flowers of sulphur .. 2 parts Common salt .. .. 2 „ Powdered charcoal / „ Iron stirrers must not on any account be used in this operation. There is an unavoidable loss of about 8 per cent. of bismuth which should invariably be allowed for in the result. The com position of the flux and tbe amount employed may be varied to suit each particular case ; the operator will, after a few trials, be able to judge for himself of the most suitable proportions.
A good method in the wet way is to dissolve 100 grains of the pow& red ore in strong nitric acid. Evaporate the solution to dryness and add a little sulphuric acid. Evaporate again to drynesa, and then add a little water acidulated with sulphuric aeid. Filter ; add carbonate of ammonia in excess ; filter, wash and dry the precipitate, and then ignite it in a porcelain crucible, and weigh it. Every 100 parts correspond to 90 parts of bismuth.
Assay of Cadrnium.—Owing to its extreme volatility, cadmium cannot be assayed in the dry way. Except in the case of the mineral grcenockite, it is always found in ores of zine. It may be separated from the latter by dissolving the powdered ore in hydrochloric acid, and passing a stream of sulpburetted hydrogen through the solution. The cadmium is thrown down as sulphide, which must be dissolved by boiling with concentrated hydrochloric acid ; the chloride is thus formed, and this is converted into the carbonate by treating with an excess of ammonia. Finally, the carbonate ia converted by ignition into the oxide, which, when mixed with one-tenth of its weight of powd( red eoal and distilled in a glass or porcelain retort at a low red heat, yields a button of metallic cadmium.
Assay of Cobalt and Nicleel.—Theso two metals are almost invariably found associated t •g? tiler, and the operation of separating them from other metals with which they are found, and from each other, is one of the most useful, and also one of the most difficult that the assayer may have t ? perform. The following methods, for which we are indebted to 01iv(r North's Pmctical Assayer,' are quite accurate enough for ordinary purposes. Fifty grains of the finely powdered ore are calcined in a elay crucible until quite free from sulphur. The calcined residue is mixed with an equal m eight of metallic arsenic, and heated in a small French crucible, at a low heat, for ten or twelve minutes. A pellet of clay should be inserted in the tapering crucible, in order to prevent the rapid disengage ment of arsenical vapours. The residue is detached, broken up, and mixed with a flux of the following composition :— Carbonate of aoda .. 150 gmins.