122 THE RAND GOLD MINING INDUSTRY. 



solution from residual slime, the power and maintenance costs 

 of maintaining large tonnages of pulp in motion for long were 

 excessive, and the ordinary methods commonly employed as a 

 result of a quarter of a century's experience and development 

 have gradually attained a degree of economy and efficiency which 

 are difficult to rival.* As everywhere with every ore, the best 

 method is that which conforms most closely with the characteris- 

 tics of the ore, and utilises most fully local conditions. Another 

 method which has been practised on a working scale is the use 

 of a dilute cyanide solution in place of water for crushing, which 

 is frequently associated elsewhere with the treatment of silver 

 sulphide or gold telluride ores. In the case of banket ore, how- 

 ever, this procedure involves the abandonment of a cheap and 

 simple means of recovering half to three-quarters of the gold 

 in the ore, and necessitates a much larger cyanide plant and 

 more prolonged and expensive cyanide treatment. In addition 

 to the foregoing objections, there is liability to loss of gold- 

 bearing cyanide solution, and difficulty in obtaining accurate 

 screen values. The retention of amalgamated plates, when 

 crushing wdth cyanide solution, results in their gradual corrosion, 

 and the deposition of the dissolved copper on the zinc shavings. f 



The importance of good classification upon crushing so as 

 to prevent oversize particles escaping from the crushing plant 

 has already been referred to ; but the necessity is no less for 

 ensuring proper separation of sand and slime, so that each may 

 receive the cyanide treatment by leaching or settlement to which 

 it is adapted, and in order that the sand residue be ultimately 

 well suited for mine filling. The presence of either product in 

 appreciable quantities in the other interferes with the extraction, 

 slime in sand causing non-permeable sand charges, and sand in 

 slime causing pump wear and slow dissolution of gold in slime 

 charges. At the present time a tailing pulp classifier should 

 yield a slime overflow, of which 99 per cent, passes a screen 

 of 200 holes to the linear inch. 



Among the features of ore treatment practice, in which 

 progress has been much marked, is the increased weight of 

 stamps, and their duty in tons of ore crushed per 24 working 

 hours. In 1903 a 1,250 lb. stamp with a 5-ton duty was con- 

 sidered to be doing good work with fine screening, whereas 

 most recently erected stamp mills have been equipped with 

 2,000 lb. stamps, giving a 20-ton duty with coarse screening up 

 to ^-inch aperture. The number of stamps installed on a mine 

 has therefore long ceased to be any criterion of the tonnage 

 of ore crushed monthly. This advance has been rendered possible 

 by the use of tube-mills for re-crushing the coarser particles in 

 the screen pulp, and has great advantages both in saving capital 

 expenditure and operative crushing costs. In a modern plant 



* Letter by F. L. Bosqui, in Mining and Scientific Press of 2nd May, 

 1914. and in S'.A. Mining Journal of 6th June. TQ14 



t E. L. Bateman in Metallurgical and Chemical Engineering, p. 672, 

 Dec, iQi,3; and S.A. Mining Journal, p. 46g, loth January, TQT4. 



