126 president's address— section b. 



But an addition of silica is to be understood as having the effect of 

 raising this degree without affecting the composition of the slag. There 

 is more iron oxidised and slagged off, but the result is, not to make the 

 slag more ferruginous, but simply to increase its quantity. Too great 

 an excess of silica remains undigested, and chokes the fiirnace. Too 

 small an amount simply causes a lower grade of matte to fall, i.e., the 

 a,mount of matte increases, which depresses its copper tenor, and the 

 amount of slag decreases, but without altering its composition. 



This, in outline, is the action of the pyrite furnace. Needless to 

 say, the practice varies in important respects in different localities. 

 The chief point of difference, perhaps, is the use or non-use of a heated 

 "blast. A hot blast is only derivable from specially fired stoves, not 

 from the furnace gases themselves, though a tepidly heated blast may 

 be derived from hot-top furnaces. The extra paraphernalia and fuel- 

 consumption introduced by the stoves is, however, an economic encum- 

 brance, so that, after six years of experience, it was gladly discarded 

 at Mount Lyell in favor of a more energetic furnace manipulation, 

 which was found to be capable of more than replacing the extra incre- 

 ment of heat supplied by the hot blast. This radical improvement 

 was accomplished by simply supplying to four furnaces the entire 

 quantity of air which had formerly been sent into eight. The effect 

 was most marked, in every direction. The rate of concentration and 

 the furnace capacity increased, and costs decreased ; also, essentially, 

 the already small percentage of coke used. The present disposition 

 is to regard the application of hot blast in the development of true 

 pyrite smelting as a transitional phase fully outgrown, and in the light 

 of as unnecessary an adjunct as it is in converting. 



The grade of matte striven for naturally will depend on its destiny, 

 and, if designed for converting, will vary between 35 per cent, and 

 65 per cent, in copper, depending on local considerations of cost and 

 expediency. The original copper assay of the ore, and the individual 

 furnace practice decide whether the desired grade can, or is to be, 

 achieved in one, two, or three smeltings. The concentration of low- 

 grade mattes presents no difficulties, and it early became an estab- 

 lished rule at all works. Unfortunately, it appears to be so still, while, 

 as a matter of fact, a single smelting should suffice, unless the ore is 

 extremely poor. The single exception appears to be Mount Lyell. 

 That establishment probably treats the lowest average grade of ore 

 now being similarly furnaced anywhere, viz., 2 per cent, to 2J per cent, 

 of copper, but it concentrates this in the ratio of 18 to 20 into 1, with a 

 single smelting, without difficulty. The whole question is virtually 

 one of a great blast supply, assisted by a low coke percentage, a high 

 column, and constant watchfulness. Yet the blast delivery must not 

 be excessive, only adequate for the oxidation of sufficient of the sul- 

 phide. The Mount Lyell ore-supply permits of using about two parts 

 of heavy pyrite to one of silicious material (containing 70 per cent, 

 silica), and it will be of interest to learn that, of the large amount of 

 iron present, never less than 95 per cent, is burnt, or oxidised, the 

 small remainder going into the matte. This oxidation factor is of 

 great importance, since it controls the grade of the matte in copper. 



