METALLURGICAL PRACTICE. §t 



Treatment Of the tailings.— The tailings from the mill are run 

 into settling pits immediately outside the mill. Here the sands and 

 the greater portion of the slimes are caught. The overflow goes to 

 the catch-pits, where during three weeks the remainder of the slimes 

 is allowed to settle, after which the water is pumped back to the mill 

 reservoir. 



As soon as the pits are full they are emptied by coolies who scrape 

 the wet sand up into baskets which they carry on their heads to the 

 decomposition heaps (see Pis. 15 and r6). The stacked tailings 

 are there left exposed to the action of the atmosphere for three months, 

 during which time the percentage of moisture is reduced from 16 or 17 

 per cent, to 7 or 8 per cent., and the gold contained in the sands is 

 said to be rendered more easily amenable to the cyanide treatment. 

 They are then filled into the percolating vats and submitted to the 

 MacArthur process of treatment. The strength of the cyanide 

 solution used is o - 26 for mill-tailings and 0*15 for slimes. After a 

 three-days' treatment the solutions are drawn off and conducted 

 through the extractor boxes where the gold is precipitated by zinc 

 shavings. When sufficient gold has been accumulated, the boxes 

 are opened and cleaned up, and the gold recovered from the zinc 

 slimes by the usual method of reduction with borax and alkaline 

 carbonates, the bullion thus obtained having a fineness of 650 to 820 

 per 1,000. 



The above is a brief outline of the process as carried on, with few 

 variations, at the mines. The drawback to the method is the 

 amount of handling the tailings undergo from the time they leave 

 the mill to the moment of their final deposition on the residue heaps ; 

 and one cannot help comparing it with the almost automatic process 

 in use in the Transvaal. There the pulp from the mill is elevated 

 by a tailings. wheel, or some form of sand-pump, to the height 

 required for the automatic conduct of the subsequent operations. 

 By means of launders and Spitzkasten (pointed boxes) the sands 

 are classified into two or more grades of fineness, which are run 

 direct into the treatment tanks, the final overflow carrying the finely 

 suspended slimes being mixed with lime and run into vats where the 



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