Dry Crushing of Ore. 367 



4. Expenses incurred in crushing fine enough to separate the 

 gold from the rock. 



5. Impossibility of dissolving coarse gold by cyanide. 



For some reason or other Engineers in South Africa have looked 

 upon any departure from the venerable process of wet milling with 

 little favour. 



In other parts of the World very large quantities of gold are 

 being produced from dry crushed ore. Alfred James in his work on 

 Cyanide Practice refers to the " Mammoth installations of the Metallic 

 Extraction Company " ; " The Economic Gold Extraction Company '" ; 

 the " De la Mar Mines " at Mercur, all in America, the latter plant 

 being able to crush 750 tons of hard ore per day to i/8th inch mesh. 

 At the Waihi Mines in New Zealand hard ore is crushed dr}' in the 

 ordinary stamp battery,, an application of an unsuitable machine to 

 a good system, but the results are nevertheless satisfactory. 



In the famous Mount Morgan Mine in Queensland (p. Argall on 

 Sampling and Dry crushing in Colorado) in one year 175,000 tons 

 were crushed by rolls and Krupp Ball Mills to a i/43rd inch, and in 

 Eomeo extremely coarse dn,^ cru.shing has given ver}' good results. 



Some of the American dn crushing mills deal successfully with 

 the troublesome telluride ores. 



It should be possible to deal with some of the South African 

 ores by dry crushing, although of course some are not suitable, but 

 the most important class, namely the Rand Conglomerates, do not 

 offer any special difficulty, as on the one hand they contain practically 

 an inapp)reciable amount of coarse gold and on the other they do 

 not require to be crushed to the extreme fineness of, say the Kalgoorlie 

 ores, whxh are commonly reduced to pass a sieve of 40,000 holes per 

 square inch. 



The " Marriner " and " Diehl "' processes which give such success- 

 ful results in Kalgoorlie ore require extremelv fine grinding as aji 

 essential. 



The " Objections " should be dealt with first, for if these can be 

 overcome the " Advantages " are too obvious to require much to be 

 said in their favour. 



1. Wear of crushing surface. This is given by Argall as 0.108 

 lbs. per ton of the rather soft and friable Mount Morgan ore. It is 

 crushed to pass sieves of 1,600 holes per square inch. 



At the Euipards Vlei Mine, Witwatersrand, the wear on a mixture 

 of free milling and pyritic ore was found to be 0.805 l^s. per ton 

 crushed to 200 meshes per square inch. The wear of shoes and dies 

 on a typical Rand mill (Wet stamps) crushing to say 600 meshes per 

 square inch is returned as 0.75 lbs. per ton of ore crushed. There 

 is therefore not much difference between the two processes as regards 

 waste of metal. 



2. Diisl. In a well-constructed Roller Mill the escaping dust 

 is practically inappreciable, especially if the ore is somewhat damp. 



3. Damp Ore. The cost of drj-ing is not alarming. Argall gives 

 it at 5 cents (ski-) per ton to bring moisture flown from 6% to 1%. 

 At the Wanderer Mine, Southern Rhudesia. damp ore containing 



