GOLD EXTRACTION METHODS. 97 



It is now becoming' usual to arrange lO stamps to be driven 

 from one cam-shaft, but the mortar-box for each 5-stamp unit 

 is still retained. Breakage of these cam-shafts is still a con- 

 stant source of expense and of stoppage, though this has been 

 considerably reduced as a result of careful investigation of the 

 quality of the steel used by means of a microscopic examina- 

 tion and the utilisation of the latest knowledge in production 

 and heat treatment of this metal. It is proposed at the City 

 Deep mill to put an inch hole right through the cam-shaft 

 centre and to supply extra bearings between the cams, in ac- 

 cordance with Mr. Laschinger's designs, in order to obviate 

 breakage as far as possible. 



The greatest changes that have taken place in the batterv 

 are in the weight of the stamp and the size of the screen used. 

 The average weight of the stamps running in 1903 was about 

 1,100 lb., and no samp heavier than 1,500 lb. was in opera- 

 tion. The duty, 4'9i tons crushed per stamp per 24 hours, was 

 thus small compared with the average for 1909, which 

 amounted to 6'79 tons. This increase of duty is not only due 

 to the heavier weight of the stamp employed or to the fact 

 that this weight is rendered more effective by concentration 

 near the crushing point by increasing the weight of the head 

 and using shorter stems; it is also due to the fact that, since 

 the introduction of tube mills, it is possible to use a much 

 coarser screen in the mortar-box. The average weight of the 

 stamps now pounding rock within 20 miles of Johannesburg 

 is probably not more than 1,350 lb., but stamps up to 1.800 lb. 

 are now at work, and in the near future a weight of 2,000 lb. 

 will be employed as at the City Deep mill, with a duty as high 

 as 20 tons a day or possibly more, the limit being set by the 

 coarseness of screen employed. 



In 1903 a screen mesh of 600 to 1,000 holes to the square 

 inch was usual, and anvthing coarser than 500 mesh was rare. 

 To-day screens with nine holes to the square inch are actually 

 in use, and the highest efficiency has been determined by the 

 Mines Trials Committee to be correlated with this aperture. 

 The same stamps which give a duty of 5"88 tons with a screen 

 having an aperture of 0"0i5" show the high figure of 20*03 

 tons with the 9-mesh screen, having an aperture of 0*27". 

 Even this result may be improved by separating from the ore 

 fed to the battery that portion already fine enough to pass 

 the screen employed (about 30 per cent.), so that in future 

 it is possible that a duty of 30 tons per stamp per diem may 

 be fairly common. 



It may be pointed out that the present economical limit of 

 nine holes to the square inch is to some extent governed by 

 the diameter of the tube mills employed. The standard size 

 at present is 5 feet 6 inches in diameter, and 22 feet long, 

 but it is possible that in the future tubes of greater diameter 

 may be successfully employed. This will tend to increase the 

 " size of unit," which is a favourable means of decreasing- 

 both capital required and working cost, and may render 

 economic the use of even coarser screens. Proceeding on 



