THE EXTRACTION OF GOLD, ETC. 317 



then ready for cyaniding, and the superfluous water is 

 pumped back for use again in the batteries. 



The difficulties in further treatment have little to do 

 "with getting the gold into solution, but are due to the 

 impossibility of passing liquids through the clayey mass. 

 It is necessary to wash by decantation, and the amount 

 •of liquid thus obtained is from six to ten tons per ton of 

 slimes, the solutions containing only from o*oi to crooi per 

 cent, of cyanide and, after the ore has been treated by them, 

 from six to twenty-four grains of gold per ton, and the 

 recovery of gold from these solutions was an entirely new 

 problem for the metallurgists. 



Several methods are in operation in the Transvaal for 

 the solution of the gold, but the best is probably that in 

 which the slimes together with cyanide solution are passed 

 by a centrifugal pump from one tank to another. In this 

 passage of the pulp through the pump and pipes, air is 

 drawn in through the joints and glands, and is introduced 

 into the suction of the centrifugal pump. In this way a 

 large amount of oxygen is supplied to the pulp whilst it is 

 undergoing the most perfect agitation in its passage through 

 the pipes, and further aeration is effected when the stream 

 .falls into the vat, carrying air down with it. H. T. Durant 

 was responsible for the final modifications in this system, 

 which had been introduced by J. R. Williams, and the time 

 .required for the solution of the gold is now very short, 

 being mainly effected during the passage of the pulp from 

 •one vat to the other. 



It is found to be very undesirable to attempt to reduce 

 the amount of solution added at one time to the slimes. In 

 .thick slimes, even lumps of cyanide become coated over and 

 protected from the solvent action of the water, although 

 rapidly dissolved if there are four or five tons of water to one 

 ton of slimes, and the same effect is observed in dissolving 

 gold. To enrich the solution, however, two charges of slimes 

 are treated in succession by it before it is passed to the 

 precipitation plant. After settling for twenty-four hours the 

 solutions are decanted, and at the Rand Central Ore Re- 

 duction Company's works the slimes are finally transferred 

 to vats 50 feet in diameter and 16 feet deep, holding nearly 



