THE EXTRACTION OF GOLD, ETC. 311 



The limit of difficulty of dissolution is reached when the 

 ore contains some rapid absorbent of oxygen such as ferrous 

 sulphide, when, instead of encountering delay the operator 

 finds that the gold is not dissolved at all. In such a case 

 it is necessary to saturate the substance completely with an 

 excess of oxygen before applying the cyanide solution. It 

 is in this way that Caldecott solved the problem of getting 

 into solution the gold contained in accumulated slimes on 

 the Rand. 1 The gold in slimes fresh from the battery is 

 readily soluble in cyanide solution, being in a very fine 

 state of subdivision, but it is quite otherwise with slimes 

 which have accumulated in dams and settling pits and have 

 been exposed to the weather for some time. Under these 

 conditions the iron pyrites is rapidly decomposed into fer- 

 rous sulphide and free sulphur, the impervious nature of 

 the materials preventing free access of air which would 

 result in the formation of sulphates and free acid. To 

 prepare these slimes for treatment Caldecott supplies oxy- 

 gen artificially in the form of air delivered from a perforated 

 pipe fixed near the bottom of the agitation vat containing 

 the pulp. Aeration of the slimes has now been regularly 

 used since the end of 1896 at the Rand Central Ore 

 Reduction Company's Works, accelerated when this opera- 

 tion takes too long or when much organic matter is present 

 with from two to eight ounces of permanganate of potash per 

 ton of dry slimes. After the presence of ferrous sulphide 

 can no longer be detected in the slimes, the aeration is still 

 carried on for an hour or more, in order to oxidise any 

 ferrous hydrate remaining and cyanide is then added, the 

 gold being now found to be readily soluble. 



Such treatment represents what is necessary in an 

 extreme case, and the addition of potassium permanganate 

 or sodium dioxide is often made in the United States and 

 elsewhere merely to increase the rate of action of cyanide. 

 In the Sulman Teed process, now in operation on arsenical 

 pyritic ore at Deloro in Canada, 2 oxygen is dispensed with 



1 Proc. of the Chem. and Met. Soc. of S. Africa, 17th July, 1897. 



2 See paper by Hugh K. Picard read at the London meeting of the 

 Federated Institution of Mining Engineers, May, 1898. 



