592 EEPORT — 1891. 



brium. As soon as the sulphurous anhydride reaches a certain tension the 

 oxidation of the sulphide is arrested, even though an exce«s of oxygen be 

 present, and the oxidation is not resumed until the action of the draught 

 changes the conditions of the atmosphere of the furnace, when the lower 

 sulphides remaining are slowly oxydised, the copper sulphide being converted into 

 copper sulphate mainly by the intervention of the sulphuric anhydride formed as 

 indicated. Probably by far the greater part of the iron sulphide only becomes 

 sulphate for a very brief period, being decomposed into the oxides of iron, mainly 

 ferric oxide, the sulphur passing off. Anj^ silver sulphide that is present would 

 have been converted into metallic silver at the outset were it not for the simuj- 

 taneous presence of other sulphides, notably those of copper and of iron, which 

 enables the silver sulphide to become converted into sulphate. The lead sulphide 

 is also converted into sulphate at this low temperature. The heat is now raised 

 still further witli a view to split up the sulphate of copper, the decomposition of 

 which leaves oxide of copper. If, as in this case, the bases are weak, the sulphuric 

 anhydride escapes mainly as such ; but when the sulphates of stronger bases are 

 decomposed the sulphuric anhydride is to a great extent decomposed into a mix- 

 ture of sulphurous anhydride and oxygen. The sulphuric anhydride, resulting from 

 the decomposition of this copper sulphate, converts the silver into sulphate, and 

 maintains it as such, just as, in turn, at a lower temperature, the copper itself had 

 been maintained in the form of sulphate by the sulphuric anhydride eliminated 

 from the iron sulphide. When only a little of the copper sulphate remains unde- 

 composed, the silver sulphate begins to split up, and the furnace charge must 

 therefore be immediately withdrawn, or the whole of the silver sulphate would be 

 converted into metallic silver, partly by the direct action of heat alone, and partly 

 by reactions such as those shown in the following equations : — 



Ag2 SO, + i¥efi, = 2Ag + GFe.Oa + SO^ 

 Ag.,SOi+ Cu.O =2Ag + CuSO< + CuO. 



If the charge were not withdrawn, the silver would thus be effectually removed 

 from the solvent action of water, and the smelter's efforts would have failed 

 entirely. The charge still contains lead sulphate, which cannot be completely 

 decomposed at any temperature attainable in the roasting furnace, except in the 

 presence of silica, and it is well to leave it where it is if the residue has subse- 

 quently to be smelted with a view to the extraction of the gold. The elimination 

 of arsenic and antimony gives rise to problems of much interest, and again con- 

 fronts the smelter with a case of chemical equilibrium. For the sake of brevity it 

 will be well for the present to limit the consideration to the removal of antimony, 

 which may be supposed to be present as sulphide. Some sulphide of antimony is 

 distilled off, but this is not its only mode of escape. An attempt to remove 

 antimony by rapid oxidation would be attended with the danger of converting it 

 into insoluble antimoniates of the metals present in the charge. In the early 

 stages of the roasting it is therefore necessary to employ a very low temperature, 

 and the presence of steam is found to be useful as a source of hydrogen, which 

 removes sulphur as hydrogen sulphide, the gas being freely evolved. The reaction 



Sb2S3 + 3H,= 3H2S + 2Sb 



between hydrogen and sulphide of antimony is, however, endothermic, and could 

 not, therefore, take place without the aid which is afforded by external heat. The 

 facts appear to be as follows : sulphide of antimony, when heated, dissociates, and 

 the tension of the sulphur vapour would jiroduce a state of equilibrium if the 

 sulphur thus liberated were not seized by the hydrogen and removed from the 

 system. The equilibrium is thus destroyed and fresh sulphide is dissociated. The 

 general result being that the equilibrium of the system is continually restored and 

 destroyed until the sulphide is decomposed. The antimony combines with oxygen 

 and escapes as volatile oxide, as does also the arsenic, a portion of which is vola- 

 tilised as sulphide. 



The main object of the process which has been considered is the formation of 



