Methods of Gold Extraction. 241 



87 % to 88 % fine gold, 10 % to 12 % silver and a little copper or 

 other traces of base metal. By means of the amalgamation process 

 above described 50 % to 60 % of the gold contents of the ore is 

 recovered, and the remainder contained in the tailings flows as pulp 

 to the cyanide plant. What is now considered a modem Rand 

 stamp-mill contains 200 stamps, and each stamp crushes about 5^ 

 tons of ore per 24 hours, so that, allowing for stoppages, upwards of 

 30,000 tons of ore are crushed per month. Before the war about 

 6,000 stamps were erected on the Witwatersrand, and this number is 

 likely to be doubled in a few years, if a sufficient supply of cheap 

 unskilled labour is forthcoming. 



The pulp leaving the mill is elevated by a tailings wheel or 

 pump and then flows into a spitzlutte or crude hydraulic classifier, 

 which separates out the heavier and richer pyritic material. The 

 concentrates thus produced are settled in vats, the poorer finer 

 sands remaining are collected in other vats, and the residual 

 slimes or unpalpably fine portion of the crushed ore is collected, 

 after mixing with lime to assist settlement, in still a third set of 

 vats with conical bottoms. The water only remains and is returned 

 to the mill for re-use. The tailings from typical half-ounce Rand 

 ore thus treated would yield about 10 % of 12 dwt. concentrates, 

 65 % of 4 dwt. sands, and 25 % of 2^ dwt. slimes. The concentrates 

 and sands, being porous permeable products, are treated by leaching 

 for several days will dilute cyanide solution in vats provided with 

 false filter-cloth bottoms. The cyanide solution gradually dissolves 

 and washes out the gold, and the drainings are passed through long 

 rectangular zinc boxes provided with several compartments contain- 

 ing lead-coated zinc shavings upon which the gold is precipitated. 

 The solution flows on into sumps, where it is made up to the requisite 

 strength by addition of solid cyanide and used again for fresh charges. 

 The slimes being non-leachable are treated by mixing with very 

 dilute cyanide solution, allowing to settle, decanting off the clear 

 supernatant gold-bearing solution and repeating this operation as 

 often as their richness warrants. The gold is precipitated from the 

 auriferous slimes solution in the same way as that from the sands or 

 concentrates. The finely divided metallic gold precipitated on the 

 zinc shavings is periodically washed off through a screen and, after 

 treating with dilute sulphuric acid to- remove the fine zinc mixed 

 with it, is calcined, smelted with flux and refining agents, and cast 

 into bars. 



The percentage recovery in the cyanide plant depends tO' some 

 extent upon how much gold has already been removed by the mill, 

 but assuming that 57 % of the total gold has been recovered by 

 amalgamation about 33 % further is recovered by cyaniding. Hence 

 the combined extraction by the mill and cyanide plant is about 90 %, 

 or 9 dwts. per ton from half-ounce ore and this is effected in a 

 modern plant at a total cost for all reduction and recovery charges 

 of about 6/- to 7/- per ton, or less than the value of 2 dwts. of bullion. 



The tailings after their treatment as described are called residues, 

 and their accumulation in immense piles of sand after removal from 



