100 GOLD EXTRACTION METHODS. 



avoids the frequent choking" up by small pebbles of the smaller 

 nozzles necessarily employed with less efficient arrangements. 



The increased efficiency of the cone used in place of an 

 inverted pyramid is largely connected with the fact that it 

 is always arranged to overflow all round the circumference, 

 thus ensuring greater tranquility for settlement and a smaller 

 height of overflow. The spitzkasten has been invariably used 

 with an overflow over one edge only, though of course it 

 would be quite simple to arrange it to be used, similarly to the 

 cone, with central inflow and baffle, and overflow over all 

 four sides, where only wood is available for construction 

 material, and in some other cases such an arrangement would 

 present considerable advantages. 



It has always been understood that successful treatment 

 of crushed ore by cyanide solutions depended not only upon 

 the ultimate fineness attained, but also upon the completeness 

 of the separation of the leachable sand from the finer slime 

 which tends to clog up the material to be percolated and to 

 render the access of the air required more difficult. 



By the older methods, still largely in use, it is found im- 

 practicable to separate sand containing less than 3 to 5 per 

 cent, of slime, and even this efficiency is rarely attained. This 

 difficulty is incidentally largely obviated in the method of col- 

 lection by means of the Caldecott table. This appliance con- 

 sists of a slowly-moving table with a peripheral band from one 

 to three feet wide, upon which the thick pulp, containing less 

 than 30 per cent, of moisture as separated at the underflow 

 of a deep cone provided with a diaphragm, is allowed to fall. 

 This band is provided with a filter-bottom, under which a 

 vacuum is maintained by suitable air and water pumps. By 

 these means the excess moisture is sucked out, and the dried 

 material, with about 12 per cent, or less water, is continuously 

 removed by a plough-scraper just before the point where the 

 new material is being deposited. A few inches of sand are 

 allowed to remain to assist the filtering medium, and this has 

 to be removed at intervals when cleaning is necessary. The 

 overflow of the separating cone employed carries away the 

 greater portion of the slime, but in some cases a second cone 

 is emploved in which the underflow of the first is washed in 

 clean water, and by this means less than i per cent, of slime 

 is allowed to remain with the sand. The clean sand removed 

 from the table is washed by means of a stream of weak cyanide 

 solution to a centrifugal pump, and thus delivered to the col- 

 lecting vats through Butters distributors, a large portion of 

 the gold getting dissolved in transit. The treatment of the 

 charce mav be completed in this collecting vat. as is done at 

 the East Rand Mines, but some little advantage is found in 

 transfering it to one of a series of second treatment vats as at 

 the Simmer and Jack and elsewhere. 



The advantages gained in the cleanness of the sand and the 

 fact that it is in contact with cyanide solution a. few minutes 

 after being crushed enable a considerable saving in capital 

 expenditure to be made in the vat capacity required. Inci- 



