GOLD EXTRACTION METHODS. lOI 



dentally also an increase in the tonnage of slime separated 

 this way, which may amount to 8 to 12 per cent., permits a 

 saving' in working cost due to the fact that slime can be more 

 cheaply treated than sand. 



It is probable, as the greater part of the advantages of this 

 method of collection are really due to the increased efficiency 

 of separation, that its principles will be more largely availed 

 of in the future, either by increased use of the whole apparatus 

 or by other means of dealing with the same problem. 



A radically different method of solving this problem of most 

 economic treatment of the crushed ore is that being tried at 

 the East Rand IMines under the name of the Arbuckle Process. 

 This involves the treatment of the pulp as a whole without 

 separation of slime from sand, and thus soves that difficulty by 

 t'le method of evasion. The pulp, mixed with about three 

 times its weight of weak cyanide solution, is agitated by means 

 of compressed air in deep vats with conical bottoms, circula- 

 tion being maintained by means of the " Pohle " air-lift prin- 

 ciple. About 20 hours is required for a sufficient solution of 

 the gold, when the whole pulp and solution are transferred to 

 an ingenious separating apparatus, the solid portion with as 

 little as 25 per cent, moisture is sent to a further apparatus in 

 order to wash out all the dissolved gold, and the clean solu- 

 tions are sent to the precipitation plant. 



The possible drawbacks are extra cost for power for air com- 

 pression, and for the mechanical arrangements involved, and 

 the inherent defect that the light slime, which reallv needs only 

 about one hour's treatment, must remain, undergoing needless 

 circulation for the full time required by the most refractory 

 portions of the ore. The general decrease in working costs, 

 owing to larger units, greater skill in handling material and 

 other important reasons, render it probable that in future this 

 method, perhaps modified in the direction of greater simpli- 

 city in the mechanical details, which promises lower capital 

 cost for plant, which is very suitable for the finer grinding, 

 gradually coming into the region of practical economy, 

 and wliich evades the difficultv of successful percola- 

 tion with extremely fine sand, will in this countrv 

 become of as great importance as very similar methods 

 in other parts of the world. It is oossible that, as in Aus- 

 tralia and elsewhere, some method of mechanical filtration 

 may be found suitable for extracting the dissolved gold from 

 the material treated, and alreadv on these fields the vacuum 

 filter is undergoing successful trial at the Crown Mines, where, 

 however, it is in use for handling slime onlv. It is yet too 

 earlv to form an opinion as to whether the more compact 

 apparatus w-ill aifect the abolition, from future plants, of the 

 enormous vats now used for the slime decnntation process. 

 It is certainly generally felt among metallurgists that the large 

 and expensive plants used for treatment of slime by the stan- 

 dard process offer tempting opportunities in the matter of re- 

 duction of capital cost, but so far it is the general experience 

 that a more than countervailing' increase of working cost is 



