Methods of Gold Extraction. 245 



former case some oxidizer, usually oxygen from the atmosphere, lib- 

 erates nascent cyanogen which has the property of combining with 

 metallic gold, and in the latter case nascent hydrogen, liberated by 

 the combination of zinc with the oxygen of water, owing to oxygen 

 having a greater affinity for zinc than for hydrogen, replaces the 

 gold existing in the solution as potassium auro-cyanide and causes 

 its precipitation in the metallic state. 



The chief feature of Rand practice is as indicated the carrying 

 out of comparatively cheap and simple methods on a very large scale. 

 The bulk of the improvements resulting in reduction of working costs 

 may be attributed to mechanical advances in plant and appliances. 

 In fact the metallurgy of gold, like any other applied scientific 

 process, is carried out very largely by mechanical means, and the 

 qualifications for successful work, beyond the individuality of the 

 operator, depend at least as much upon his acquaintance with 

 engineering possibilities and processes as upon purely chemical or 

 abstract knowledge. Since ver}' large amounts of low-grade ore have 

 to be handled at a low working cost to be profitable, complex and 

 expensive methods whatever the ultimate extraction are impractic- 

 able, though in other countries where richer and more refractory 

 material is handled they may be economically sound. For instance, 

 there is no doubt that the method of treating slimes by filter-presses, 

 as developed in Westralia, yields a considerably higher percentage 

 extraction than is common on the Rand by decantation methods. 

 But in Westralia the ore treated is so high grade that, in spite of the 

 high percentage extractions obtained by filter-pressing, the slimes 

 residues discharged after treatment frequently carry more gold per 

 ton than does our product before any treatment at all. 



Concurrent with progress in actual recovery methods sampling 

 and assaying have been developed to a high pitch of accuracy, and 

 every stage of the recovery work is daily checked ; to such an extent 

 is this carried that one grain of gold per ton in the cyanide solutions 

 (or one part in fourteen millions) is determined, and a close watch 

 even kept on the assay value of the mill water, lest it become con- 

 taminated with cyanide and consequently gold. At the same time 

 elaborate and detailed systems of costs on each of the Companies 

 are kept in the head offices of the mining corporations, so that by 

 comparison any favourable results may be investigated and generally 

 adopted when practicable, whilst any unduly high expenditure on 

 working costs is similarly checked. The immense scale of operations 

 renders this close attention to and watch upon details of the greatest 

 importance, since in a modern 200-stamp mill one grain of gold or 

 twopence per ton of ore amounts to ^£3,000 per annum. The " group 

 system " renders the systematizing and comparing of results possible, 

 whilst each department of a subsidiary Company has the assistance 

 in any difficulty of the knowledge and experience of a consulting" 

 specialist. Another factor in the rapid development of Rand metal- 

 lurgy is the fact that owing to the price of gold being fixed there is 

 no inducement for the keeping of " trade secrets " such as exist in 

 the metallurgy of a metal like copper, whose value depends upon 



