Methods of Gold Extraction. 247 



ing the small percentage additional recover)- economically possible- 

 While the extra recovery^ may be small measured in percentage, its 

 absolute amount may be very large on the aggregate existing and 

 future stamping power of these fields. But though the main lines 

 of practice may remain unchanged there will doubtless, as in the 

 past, be many changes in detail by process of steady growth and 

 evolution. Among foreshadowed improvements is finer crushing 

 before cyaniding of the coarse pyritic portion, separated by spitzlutte, 

 of the pulp leavin'^ the plates. This would appear more necessary 

 the deeper the level from which the ore is mined, as such ore, possibly 

 being more compacted from the greater pressure, seems to require 

 finer reduction in general than ore from nearer the surface, with which 

 in appearance and composition it otherwise corresponds. The 

 encased gold contents of this material after cyaniding are at present 

 considerable, the clean pyrites of course containing the most gold per 

 ton, whilst the quartzose portion, mechanically separated, contains 

 even a greater percentage of the gold contents of the charge. 



With a more assured political future increase of leaching plant 

 capacity is justifiable so that treatment can be continued until, on 

 the lines of old chlorination practice, no more gold can be profitably 

 washed out of the charge. The recent revival of schemes for the 

 treatment of old residue dumps, and the improved results obtained 

 since the war by the longer cyanide treatment allowed by the limited 

 number of stamps running, have both emphasized the fact that past 

 practice did not usually provide plants of sufficient leaching capacity. 



The same reasoning applies to slimes treatment by the only 

 process, that of decantation, hitherto demonstrated conomically 

 profitable with our low-grade product. Here extension of plant and 

 design of future plants on a more generous scale is likewise becoming 

 generally recognized as desirable, and the fact that with a 200-stamp 

 mill a vat fifty feet in diameter is becoming a standard size for slime 

 treatment indicates the scale on which this portion of the treatment 

 is carried out. Spitzkasten were at one time in general use for 

 thickening the slime-pulp before collection in a settling vat, but now 

 the entire slime-pulp is run direct into collecting vats with peripheral 

 overflow for the clear water. Possibly continuous slime treatment, 

 using exaggerated spitzkasten in the shape of vats with steeply in- 

 clined conical bottoms, may become adopted in certain cases; it 

 has often been discussed and experimented on and attempts are now- 

 being made to develop it on a large scale. 



The treatment of drainage from residue dumps has already been 

 referred to, and waste and surplus solutions from this and other 

 sources as well as by-products of every kind will as time goes on 

 be still more carefully watched and treated when of sufficient value. 



There still remains one class of material on the Rand whose 

 economic treatment is not yet possible, and this consists of the mil- 

 lions of tons of low-grade slimes accumulated in dams. Very ap- 

 preciable profits are derived by treating current slimes of low assay 

 value, but when slimes have once been stored the extra cost of 

 handling, the lime required to neutralize acid compounds formed by 



