4 Frocreding.s of the Roijol Society of Vtctorld. 



ultimately penetrates the mass ; sulphur at this stage is mainly 

 eliminated, and the material may be thrown into wave-like ridges 

 to expose as large a surface as possible to the air. When moved 

 to the fourth door, no sparks may be seen on stirring, but the ore 

 appears to be porous and offers resistance to the rabbling tools. 



When opposite the fifth door the ore is only stirred occasion- 

 ally, since the sulphides have practically disappeared, and all 

 that I'emains to Ijc done is to decompose any sulphate of iron 

 formed, or to oxidise the magnetic oxide to the higher state. 



After this the ore is moved down to the hottest part of the 

 furnace, or next to the fire-bridge, care being taken to exclude 

 reducing gases at this stage ; a few minutes' exposure on this 

 hearth is generally sufficient to decompose any injurious sulphates 

 and to raise the whole of the iron to its highest state of oxidation. 

 Should it be kept too long on this hearth, especially in the 

 pi'esence of silica, it is again reduced to the magnetic form, and 

 afterwards will partly unite with silica to form a black sintery 

 mass from which it is difficult to extract the gold. The tempera- 

 ture should on no account reach the melting point of the bullion 

 to be treated, otherwise the gold will be transformed from a 

 porous, spongy, well annealed state, in a condition readily 

 attacked by chlorine, to a spherical form offi-ring but little 

 surface to be acted on. Another danger is that, in the case of 

 liraonite ores, which, though they open up and lose their coml)ined 

 water at a calcining temperature, yet, at too high a temperature, 

 the mass closes again and seals up the contained gold. 



Should the ore contain much zinc, arsenic, or antimony, or low 

 grade bullion, the introduction of a small quantity of salt is 

 beneficial on the last hearth ; it has been found that from 5 to 

 10 lbs. is sufficient for a ton of ore, and if introduced and stirred 

 vigorously into the oie, when heated up to about 800° C, all 

 ferrous oxide is converted almost instantly into ferric oxide, 

 while hydrochloric acid, chlorine, and volatile chlorides are given 

 oti in abundance ; below this temperature no perceptiljle action 

 goes on. The ore is discharged fi-oin the furnace ahnost imme- 

 diately. 



Samples taken all over the heai'th, Ijefore introducing the salt 

 and after withdrawing the ore from the furnace, show no losses 

 of gold due to volatilisation. As a matter of fact, gold is veiy 



