232 CHARLES E. MUNROE 



The leaching or percolation vats vary much in form, 

 dimensions, and construction. Thus they may be of wood, 

 barrel shaped, 22.5 feet in diameter and 4 feet deep, and 

 holding a charge of 30 tons; or of concrete, rectangular, 50 

 by 40 feet in area and 4 feet deep, and holding 150 tons. 



The following facts indicate the wide ranges of variation 

 in methods : 



In the Mercur district the ore is covered with solution, 

 which is allowed to stand from thirty minutes to six hours 

 and then drawn off. This operation is repeated from eight 

 to thirty five times. Here the material leached is so coarse 

 that there is no danger of packing. Each operation of cover- 

 ing takes from two to six hours. A few mills cover the pulp 

 with solution, allow it to stand forty eight to ninety six hours, 

 draw it off, and wash. Many of the mills follow the strong 

 solution with a wash of weak solution (one tenth per cent 

 or less). This is in turn followed by a water wash, which 

 flows through the zinc boxes into the weak solution tank 

 and becomes the first wash for the next charge. 



In the extraction processes the treatment with cyanide 

 solutions is accomplished by percolation. This method is 

 widely and successfully used, but it has its disadvantages. 

 If the material treated is a clean sand, the solution penetrates 

 throughout the mass, exerting its full solvent effect, and 

 the subsequent draining and washing are easity accompHshed. 

 But if the ore contains some kind of rock which is converted 

 by the crushing into a powder that when moistened produces 

 slimes — a formation which is most marked when the rock 

 is of a clayey nature — the presence of these ore slimes in 

 the leaching vats may retard or even prevent percolation, 

 according to their amount and character. In working ores of 

 this kind, the difficulty has been obviated by coarse crushing, 

 but in that case gold is lost, because the solvent can not pene- 

 trate through the coarser granules of ore to the inclosed grains 

 of metal. To overcome these defects in the process, resort 

 is had — as in the Pelatan-Clerici process, or in the use of the 

 Aurex sluice — to agitation during exposure to the cyanide 

 solution. Furthermore, since in these processes amalgama- 

 tion and precipitation are carried on nearly simultaneously 



