98 GOLD EXTRACTION METHODS. 



these lines, a time may come when the stamp mill will be im- 

 proved entirely out of existence and the heavy rock-breakers 

 employed will be set to crush finely enough to pass directly 

 to the larger tube mills, or rolls might be used for the slight 

 reduction still required. 



No signs of such a progress are yet visible, though it 

 appears merely a logical deduction from the success which 

 has attended the efforts of our metallurgists in the direction 

 of improvement in the operation of the tube mill plant, which 

 is growing to be a more significant part of every new mill 

 erected. 



Coincident with the increase of coarseness of the pulp leav- 

 ing the mortar-box is the difficulty of directly amalgamating 

 the product without further crushing, and it is becoming clear 

 that the advantage of removing the apron plates entirely away 

 from the boxes will emphasize its necessity in the near future. 

 This removing of the amalgamating tables from proximity to 

 the mortar-boxes and the relegation of their duty to shaking 

 plates, placed after the tube mills, is contemplated in many 

 mines, though I believe it was first carried into effect at the 

 West Rand Central mill, where it has proved entirely success- 

 ful. The only possible drawback to this arrangement — that 

 the lighter portion of the crushed ore, called slime, largely 

 evades the tube mill shaking plates — can be obviated by special 

 arrangement as proposed for the 600-stamp mill being erected 

 at Randfontein. At the City Deep new plant the shaking- 

 plates are placed under the same roof as the extractor-boxes 

 of the cyanide plant, an arrangement which enables all gold 

 recovered to be superintended and handled, till it is in smelted 

 bars for the banks, in one building. 



The tube-mill is of course the principal novelty in the his- 

 tory of the reduction plant since Dr. Caldecott's paper was 

 read in 1903, though he foreshadow'ed it when he wrote 

 advocating the finer cnishing of the coarse and pyritic por- 

 tions of the mill pulp. 



Mr. Denny first advocated the use of the tube-mill for this 

 purpose in this country, and referred to its successful use in 

 Australia by Dr. Diehi. 



At first introduced cautiously, after careful observation of 

 its performance and of the improvement in extraction shown 

 after the finer crushing obtained by its means, it is now used 

 almost universally, not only on account of this increased 

 extraction of the gold, but also because it has been proved 

 here to be, in conjimction with the heavier stamps and coarser 

 screens, a much cheaper method of reducing ore to the fine- 

 ness demanded bv economic considerations. 



An endeavour has been made, on purely theoretic grounds, 

 to show that the crushing efficiency of the tube-mill is only 

 one-fourth of that of the" stamp battery, but it is certainly 

 true that its exended use is always accompanied by lower cost 

 for reducing the ore to the utmost fineness now considered 

 economicaliv sound. Thus one or two tube-mills were at 

 first emploved for every lOO stamps running, but at the end 



