100 president's address — SECTION B. 



exposure to air, the latter afford an opportunity for arsenic and an- 

 timonj' to form arsenates and antimonates with the cuprous oxide, 

 salts which are not volatile at the roaster temperature. In the con- 

 verter the elements mentioned are volatilised from the very beginning 

 of the operation, roughly one-half being removed by the time white 

 metal is formed. The pressure of blast in itself has no purifying effect ; 

 b\it, secondarily, by creating a higher temperature and energising the 

 blows, it will tend in that direction. 



While thus, as a whole, the rapid pneumatic treatment of mattes 

 may present no paramount chemical advantages over the older work 

 in point of purification — especially when really dirty mattes are 

 Bessemerised (which, so far, appears not to have been done) — its 

 tremendous advantages in point of simplification cannot be gainsaid. 

 And not the least important corollary for the practical metallurgist 

 is the corresponding mental relief which it affords from the otherwise 

 ever-present vexatious question of the impurities, aside altogether 

 from ex;"<edition, econom}', &c. 



Chemical Features of the Process. — As remarked above, the chemical 

 features of the process have as yet scarcely received the attention which 

 they deserve. The complete secret of the slag formation is not yet 

 lifted. It is not understood how the oxidation of the iron is effected ; 

 and, though the end result is, of course, the formation of protoxide, 

 which forms a silicate with the quartz of the lining, it is a moot point 

 what becomes of the combination of oxygen and iron, or what is its 

 behaviour between the time of its first formation and its scorifi cation. 

 It has been thought that it remained in solution in the ferrous sulphide 

 of the matte as protoxide. On the other hand, there is a likelihood of 

 the formation of a higher oxide, which is subsequently at once reduced 

 to protoxide in the presence of silica, with which iron can unite in no 

 other permanent form. More positive knowledge is to hand regarding 

 the behaviour of the white m.etal in the after-blow. The assumption 

 that the cuprous sulphide is oxidised directly to metallic copper and 

 sulphurous anhydride is not tenable. There is, on the contrary, a 

 formation of cuprous and cupric oxide, which play a reducing role with 

 each other, wholly analogous to the roaster process, though, of course, 

 the reactions are run through with greater speed. 



The evidence of the gases points to the fact that both during the 

 first blow, or the slag-forming period, and the after-blow, practically the 

 whole of the oxygen in the air blown in is (or can be) utilised. In this 

 respect the converting process and pyrite smelting, in its purest form, 

 act exactly alike. There are moments during the blowing for slag when 

 there are leakages of air through the molten column, easily permitted 

 by the very irregular turmoil going on within the vessel, and free 

 oxygen may also appear towards the end oi the after-blow, just before 

 the copper finish. But, on the whole, the theoretically desirable con- 

 dition of things is actually enacted during such portion of the two 

 periods when the most work is being done. 



There appears to be a kind of preferential oxidation of the iron of 

 the matte during the first period. Of course, only that portion of the 



