THE KAND COLD MINING INDUSTRY. I23 



about two-thirds of the crushing is done in tube-mills, of which 

 about three hundred have been erected on the Rand, having a 

 crushing capacity of over a million tons of ore monthly. In 

 ordinary battery practice, reduction of the ratio of water to ore, 

 combined with high stamp duties, has greatly decreased the cost 

 of pulp elevation and return water pumping costs, whilst the 

 lesser volume of pulp has likewise decreased the classifier 

 capacity necesary per ton of ore crushed. Owing to the tube- 

 mill hydraulic classifiers determining the size of particles leav- 

 ing the crushing plant instead of the aperatures of the battery 

 screen, exactitude of the latter has become of minor importance. 

 Grading analyses of various crushed ore products have become 

 as much routine tests as are assays, in view of the fact thalt 

 percentage of gold extraction is a function of crushing, whilst 

 the relative crushing capacity of stamps and tube-mills is deter- 

 mined from grading analyses on the " nominal crushing luiit " 

 system.* In cyanide practice the influence of temperature upon 

 the rate of slime settlement and of gold precipitation upon the 

 lead-coated zinc shavings has been fully realised. The tempera- 

 ture of mill service water and cyanide solutions is therefore 

 regularly recorded, and artificial heating employed in winter 

 where economically practicable. 



Another development which has become generally accepted 

 in Rand reduction works is the system of circuits, whereby over- 

 size ore particles are returned for further comminution, or water 

 and cyanide solution for re-use. A reference to a flow-sheet 

 diagram t will show that in addition to the tube-mill circuit and 

 the mill service water circuit, both sand and slime solutions and 

 residue discharge trucks have their own circuits, with the result 

 that only the ore passes through the plant, carrying with it to 

 waste, when discharged as residue, a certain amount of water 

 in the form of dilute poor cyanide solution. This circuit sys- 

 tem is largely the result of a limited water supply, and of the 

 flat or gently sloping mill sites on the Rand, and is a distinct 

 variation from the steeply inclined mill site in favour under 

 other conditions, w'here the ore descends by gravity through the 

 various stages of treatment, and the final tailings or residues are 

 disposed of in a creek at the foot of the plant. 



The author's sincere thanks are due to Air. D. W. Rossiter 

 for furnishing much of the statistical data contained in this 

 paper. 



Brevium, a New Element. — Uranium-.Y consists 

 of two elements, Uranium-A', and Uranium-.Y^, wnth half-periods 

 of 24.6 days and 1.15 minutes respectively. To the latter the 

 name Brevium has been given : it is a near analogue of tantalum, 

 and occupies the last line in the fifth group of the periodic 

 system. | 



* " Rand Metallurgical Practice," 1. 136-7. 

 f'Rand Metallurs,ncal Practice." 2. 6. 7. 

 tJoiini. Chciii. Soc. (1915). T08. Abs. 12]. 665. 



