15— THE DEVELOPMENT OF GOLD EXTRACTION 

 METHODS ON THE WITWATERSRAND. 



By W. a. Caldecott, B.A., F.C.S. 



The general principles upon which gold extraction work upon 

 the Rand is carried out are so simple, and some acquaintance with 

 them so common, that only a brief outline of the operations as at 

 present conducted will be given. Following upon this, some 

 features in the development of Rand practice, and the considerations 

 which have led to the adoption of present methods will be discussed. 

 Messrs. Williams, Reuneit, Yates, and Goldmann have all dealt in 

 some detail with recovery methods as practised on the Rand, and 

 to the works of these members of our Association I would refer any- 

 one desirous of further information. 



The ore as it comes from the mine is mechanically tipped over 

 a " grizzly " or inclined screen composed of parallel bars at short 

 distances apart. By this means it is separated into " fines " and 

 " coarse." The former pass at once to the mill, whilst the latter, 

 after washing by a spray of water, gravitates to a belt or rotating 

 circular table, where the pieces of rock other than banket are picked 

 out by hand, and eventually find their way to a waste dump. The 

 auriferous banket is delivered automatically to huge breakers, which 

 reduce it to pieces not exceeding 2|" in diameter, and it then passes 

 into trucks, which convey it to the mill bins. From the bins it 

 gravitates through automatic feeders into the batteries, where it is 

 crushed. The mill consists of a series of 5-stamp batteries ; a 

 battery comprises five stamps, each weighing one-half to two-thirds 

 of a ton. Each stamp is lifted and allowed to fall from a height 

 of about eight inches nearly a hundred times a minute upon the ore 

 resting on the dies, contained in what is called a mortar-box. As 

 the ore is automatically delivered into the mortar-box about seven 

 times its weight of water is also allowed to flow in, and the pulp 

 produced by crushing is splashed against a screen of 600 to 1,000 

 holes per square inch in the front of the mortar-box. As may be 

 imagined the noise in a stamp mill is tremendous and all conversa- 

 tion in the building impossible. Heard from a distance it much 

 resembles the roar of breakers on the sea-shore. As the ore is gradu- 

 ally reduced to a sufficient degree of fineness it is carried by the 

 water through the screen and travels as pulp over inclined amalgamat- 

 ed copper plates, to which the liberated coarser metallic gold adheres 

 and is periodically scraped off as amalgam. The amalgam contains 

 about two-thirds of its weight of mercury which is recovered for re- 

 use by retorting, and the spongy metallic gold left after this operation 

 cast into bars. The bullion thus obtained usually contains about 



