Methods of Gold Extraction. 243 



In principle the process has remained practically unchanged to 

 the present date, the chief advances made being mechanical devices 

 and arrangements of plant for reducing cost of handling on a 

 large scale, the gradual reduction of strength of working solu- 

 tions combined with the adoption of leaching in place of agitation 

 for all products through which solutions could percolate, the introduc- 

 tion of the decantation method of slimes treatment, the recognition 

 of the part oxygen plays in the solution of the gold and the use of 

 zinc couples for facilitating the precipitation of gold from weak solu- 

 tions. The net result has been that the process now costs to operate 

 hardly more pence per ton than it originally did shillings. Though 

 gradually increasing knowledge has developed greater efficiency in 

 all departments, yet even at the present day the saying is fully 

 justified that a Company relies upon its battery returns for its work- 

 inT costs and upon its cyanide plant for its profits. Simple and gener- 

 ally uniform as is the composition of banket ore from a metallurgical 

 standpoint, in that besides a minute proportion of metallic gold it is 

 practically composed of silica and a little iron pyrites, yet the average 

 value is so low that even with the 90% recovery' from all sources now 

 attainable, the margin between profit and loss is by no means great. 

 Owing to the comparatively fine nature of the gold and its distribution 

 in banket it is hardly likely that any improvements possible in amalga- 

 mation, concentration and chlorination methods would have rendered 

 it possible to work the present grade of ore at an appreciable profit. 

 From the imperfect fluidity of mercury the recovery by amalgamation 

 cannot be compared with that of a liquid solvent which dissolves gold 

 even when only partially exposed. Close concentration is costly as 

 well as the subsequent roasting of the concentrates ; it may be noted 

 that after roasting cyaniding yields as good an ultimate extraction 

 as chlorination though about double the time of treatment is required. 

 Many unsuccessful attempts have been made to concentrate the gold 

 values of the battery pulp so as to discard a worthless gangue, as is 

 done in treating the ores of many base metals. Owing to the 

 distribution of the gold through the quartzose as well as pyritic por- 

 tion of the crushed ore these attempts have failed, with the result 

 that as described the whole of the ore now undergoes double treat- 

 ment, first by amalgamation and then by leaching. 



After the period of demonstration was complete, the Robinson 

 cyanide plant started work on a large scale at the end of December, 

 1890, and the results obtained therefrom and those from the Sheba 

 plant, which began work a couple of months later, caused any linger- 

 ing doubts as to the importance of the new factor in the metallurgy 

 of gold to be dispelled. At once the erection of plants composed 

 of square wooden vats was begun by various Companies for the 

 treatment of their tailings heaps and the current tailings collecting 

 in dams. The heavy cyanide consumption caused by the acid nature 

 of this accumulated material was reduced by the use of lime. Gradu- 

 ally further improvements were made; large circular wooden vats 

 were successfully constructed and used in spite of the fears expressed 

 that on account of their large diameter the staves would collapse. 



