364 Report S.A.A. Advancement of Science. 



processes. The reverse should be the case. The mechanical pre- 

 paration of the ore should be directed to providing a product especi- 

 ally suited to the Cyanide treatment. 



It was soon found that the crushed material generally produced 

 by stamps was not well suiter] to cyanide treatment. 



It contains too large a percentage of extremely fine material 

 which fills up the interstices between the larger grains, not only im- 

 peding the filtration and efficient draining off of the cyanide solu- 

 tions from the mass of sands, but also checking the free entrance of 

 air which supplies the greater part of the oxygen required for the 

 efficient dissolving of the gold. 



The following table will gi\e an idea of the effect pro<luced l)y 

 crushing b\ different .systems. 



The percentages given indicate the amount of crushed ore or 

 sand which passed through sieves of certain number of meshes per 

 square inch, but remained on the next smaller size. 



Meshes per s(}uare inch 400 900 i6oo 3600 8100 



0/ 



/o 



Wet Stami:)s. 700 Mesh — 11.15 -8.53 9.21 51.11 



Dry Stamp.s, 400 Mesh — 20.30 9.80 21.80 49.10 



Rolls. 400 Mesh 29.00 35.00 14.00 22.00 



Ball Mill. 500 Mesh 20.07 -4-38 13-88 41.67 



Rand experience shews that about 25';,, of the Mill product 

 should be classed as " Slimes," i.e., a material ^\•hich does not afford 

 free passage to solutions. It is clear, therefore, that wet crushing 

 stamps make a far too' large proportion of a finer product than is 

 necessary for efficient extraction of the gold, the excess of which must 

 be removed and treated separately. 



Dr\' crushing by stamps gives even worse results : see the example 

 given where the crushing was stopped at 400 me.sh instead of 700. 



Ball Mills follow clo.sely on stamps. Rolls, however, give a pro- 

 duct, the whole of which theoretically should offer no difficult}- to 

 percolation, and this has been found to be the case in practice. 



When the ore has been crushed sufficiently fine tO' allow the 

 cyanide solution to attack the gold and extract a sufficient percentage, 

 any finer crushing is not only a needless waste of power and wear of 

 materials, but is also actually detrimental to efficient treatment. 



A distinction must be made between an economically satisfac- 

 tory extraction and a very high extraction, as the latter ma}' be ol)- 

 tained at a greater cost than the additional gold is worth. 



It has been shewn that a very large amount of gold is won from 

 the Mines on the Rand, but it must be remembered that the ore 

 mined, milled, and treated to produce this quantity reached the 

 enormous total of over six million tons, the gold won from the ton 

 (2,000 lbs.) being on the average but 9.81 dwts. (say, 235?, grains) 

 worth, say, thirty-nine shillings and threepence (39s. 3d.). 



For this comparatively small value to provide for cost of working 

 and give profit as well, it will be readily conceded that great care 



