102 GOLD EXTRACTION METHODS. 



involved. When it is considered that a shilhng will cover all 

 cost by the standard methods, on the large scale now usual, 

 with an extraction of 85 to 89 per cent, of the total gold, it is 

 clear that the field for economy, either by increasing extrac- 

 tion, where residues are now worth is. per ton or less, or in 

 decreasing working cost, is not a wide one. The fact must 

 also be borne in mind that in all feasible methods so far 

 suggested the large vats employed for collection of the slime 

 are still necessary, and even those for solution of the gold are 

 either still required or a more expensive treatment in smaller 

 vessels must be substituted. 



With reference to the precipitation of the gold from the 

 working solutions, it may at once be remarked that little 

 change has been made during the last seven years. As, how- 

 ever, this work is performed at a cost of less than one penny 

 per ton and as the value is brought so low as o'oi dwt., here 

 again the field of future economy is restricted. Experiments 

 have been made with zinc dust, in place of zinc shavings, 

 and the result obtained so far promise economy of space, 

 decrease in capital cost and greater security of the bullion pro- 

 duced. Certain difficulties in the clean-up may possibly be 

 evaded, and it is this weak point in our present methods that 

 leads to the pious hope that in the future the plan of using 

 zinc dust and filter presses may have a brighter prospect. 



A glance at the whole series of improvements effected in 

 the past few years will render evident the fact that progress 

 has been almost entirely in the domain of mechanics, and that 

 nearly all proposals for the future emphasise the reduction in 

 capital cost, and in some cases this is pressed so far as to 

 I)artly ignore the fact that the increased working cost involved 

 is more than counterbalancing. The origin of this state of 

 things can no doubt be referred to the fact of the demon- 

 strated economy in providing a much greater ratio of reduc- 

 tion capacity to mine area, and to the fact that much larger 

 areas are now worked under one company. These facts com- 

 bine to render necessary very huge capital sums, and all pos- 

 sible means of reducing the amounts necessary are so much 

 the more welcome. 



The most interesting question at the present time refers to 

 the disposal of the residue after extraction of the gold has 

 been pushed to its economic limits. Up to the present time 

 nothing has been proposed or done with the slime but store 

 it in dams, where it is useless, unbeautiful. and a source of 

 expense. The coarser portion of the pulp, that is the sand, is 

 now being sent back direct to the mine at the Simmer and 

 lack, which pioneered this improvement, and at several mines 

 in the Central Rand sand is being removed from the old dumps 

 and sluiced into the worked-out stopes below. This method 

 of sand-filling was inaugurated this year at the Village Deep, 

 and the success obtained is causing the adoption of similar 

 methods on many of our leading mines. The disadvantages 

 of using old residue for this purpose are the double handling 

 of material involved and the expense of supplying lime to 

 neutralise the large amount of acid and acid salts generated 



