244 Report S.A.A. Advancement of Science. 



Cement vats succeeded wooden ones in some instances, but after ai 

 few years steel vats, already common in the United States, took their 

 place. The collection by means of hoses or rotating distributors of 

 the sands and concentrates in vats supported on masonry and provided 

 with bottom discharge doors, instead of in dams or pits, previous to 

 transfer to the leaching or treatment vats, was universally adopted. 

 During all this time plants were steadily increased in size and vats 

 of larger d-ameter were adopted until at the present time a 400-ton 

 vat forty feet in diameter has become a standard size unit. Separa- 

 tion of the coarser and richer pyritic portion of the pulp by means, 

 of crude spitzlutte for longer treatment than the sands became com- 

 mon, and within the last few years the decantation process of treat- 

 ing the low-grade Rand slimes was developed and accepted as profit- 

 able practice. The use of lime for settling current slimes led to two< 

 unexpected advantages in amalgamation; first in allowing a prompt 

 return to the mill of nearly all the water in the battery pulp, and 

 secondly in improving the percentage recovery of gold on the plates. 

 The precipitation of gold from the extremely dilute cyanide solu- 

 tions used in slime treatment was first practically accomplished by 

 the Siemens-Halske process, to be followed in turn by the lead-zinc 

 couple, which had been originally patented by J. S. MacArthur ni 

 connection with the cyaniding of cupriferous gold ores. 



Besides the direct help cyaniding has been to milling, it has 

 materially modified the view that the percentage recovery of gold 

 on the plates is the main point. Now the value of the residues 

 ultimately discarded forms the criterion, and no manager hesitates 

 to sacrifice a slight extra recovery in the mill, if reduction costs can 

 be thereby reduced and no higher residues are ultimately discharged 

 from the cyanide plant. In fact, the function of the mill has become 

 that of an ore reduction machine for the subsequent operations of 

 amalgamation and cyaniding, and questions of screens, height of 

 discharge, ratio of water to ore are all considered in reference to 

 their influence on the total extraction. Combined with all the main 

 principles which have thus crystallized out have come endless im- 

 provements in detail. Cyanide bullion was at one time not worth 

 more than ;^3 per ounce, but the introduction of acid treatment and 

 manganese dioxide refining rendered any fineness desired obtain- 

 able, and of late the lead smelting of zinc gold slimes promises to 

 supersede this on the score of economy. 



The metal lead is playing an increasingly important part in the 

 metallurgy of gold. In addition to the use of the lead-zinc couple 

 for precipitation and lead smelting of zinc gold slimes above referred 

 to, it is becoming the practice in some plants to add lead salts to 

 all leaching and dissolving solutions with considerable advantage in 

 assisting the solution of the gold and ensuring lengthy efficiency of 

 gold precipitation by the zinc shavings from all weak solutions. 



The chemistry of the cyanide process has been the subject of 

 much research and speculation and is even now in many respects 

 obscure. It is probable, however, that the solution of gold is really 

 an oxidation process and its precipitation one of reduction. In the 



