THE RAND GOLD MINING INDUSTRY. II9 



ment of slime ; the application of the zinc-lead couple for the 

 precipitation of gold from very weak cyanide solutions ; the 

 recovery of water for re-use direct from the overflow of the 

 slime-collecting vats ; the building of very large and relatively 

 cheap plants composed of correspondingly large units grouped 

 together to deal profitably with low-grade ore ; the introduction 

 of heavy coarse-crushing stamps with secondary re-crushing 

 in tube-mills* ; and the gradual improvement in simplicity and 

 efficiency of pulp classification methods. The employment of 

 vacuum slime filters has enabled a better extraction of the gold 

 to be obtained from slime, particularly when relatively high in 

 value, than the ordinary decantation process ; and the use of 

 scoop discharges in tube-mills materially increases their crushing 

 capacity. f Although sand-tilling is essentially an underground 

 operation, yet the transportation on the surface as pulp of the 

 sand residue employed has been greatly developed, $ and pro- 

 blems such as the neutralisation of acid mine water under- 

 ground with limestone crushed to the fineness of cement, and 

 the allaying of dust from the sand dumps by covering with a 

 thin layer of mud from natural clay or slime residue, are still 

 engaging attention. § 



A very small proportion of the numberless proposals for 

 improvements in current ore treatment methods has proved of 

 sufficient utility to fulfil the hopes of those who brought them 

 forward. As a rule, most real advance has been of gradual 

 growth and development, || unsuited for protection by letters 

 patent, and hence benefitting the mining industry in general 

 rather than any individual. Besides proposals emanating from 

 an imperfect knowledge of actual local working conditions, many 

 schemes are merely inferior variants of common practice. Very 

 often more discernment is required to realise the need for an 

 improved process or device than ingenuity to supply the want. 

 New proposals can only be safely adopted as an essential part 

 of ore treatment after tests have been successfully and continu- 

 ously carried out on a working scale for a considerable time. 

 In the spread of technical knowledge and the advancement of 

 metallurgical practice the Chemical, Metallurgical, and Mining 

 Society of South Africa has played a very important part, and 

 the eight thousand pages of its Journal, published during the 

 twenty-one years of its existence, constitute a mine of informa- 

 tion for everyone engaged in the extraction of gold from its 

 ores. The price of gold being non-competitive, has facilitated 

 the publication and discussion of current practice or proposed 

 developments, and the realization of the obligation that each 



"" Journal Chcm. Met. and Min. Soc. of S.-L. 10, loS, 358. 



t W. R. Dowling: "The Use of Scoop Discharges in Tube-Mills," in 

 Journal Chem. Met. and Min. Soc. of S.A.. 15 (1915), 214. 



% Journal Chcm. Met. and Min. Soc. of S.A.. 14 (1913). 119. 



^Ibid. 15 (1915), OT. 174- 



II H. A. White: "Evolution in ]\[etallurgy." in 21st Anniversary 

 Number of South African Mining Journal (1912), 59. 



