Park states that this method is in use in many plants in Victoria for precipitating gold from cyanide solutions the procedure being to pass the solution through barrels packed with finely broken carbon/charcoal, which is afterward burned, and the ash melted down with fluxes. He does not consider it suitable for large tonnages of solution.
H. R. Edmands describes a process of charcoal precipitation which shows promise of great usefulness in certain circumstances. For this process the charcoal is ground to a fine powder. The author says,
“Fine grinding is very beneficial; it must be carried beyond anything measurable by screen tests, but there would appear to be a point beyond which it is inadvisable to proceed. This point can be best determined by precipitation tests, or perhaps by noticing the color of an emulsion of a known weight of charcoal in distilled water.
“In practice it is arrived at by the time allowed for crushing with a given weight of balls. Charcoal quenched in water, when red hot and then wet ground, is superior to the air-cooled dry crushed product. In most cases 2 to 3 hours suffice for precipitation, when the charcoal is finely ground and agitation given, but the bulk of the precipitation takes place much sooner. At the Yuanmi plant precipitation is effected by filtering through thin layers of charcoal and the time of contact must be measured in seconds.
“ Carbon/Charcoal is a much less effective precipitant of silver and copper than of gold.”
The apparatus used, known as the “ Moore-Edmands Extractor ” is a rectangular box with V-shaped bottom, in which are suspended filter leaves after the manner of the Butters filter except that there are horizontal propellers in the bottom for producing agitation. The method of procedure is as follows:
The charcoal after removal from the kiln is washed to free it from ash, and is charged moist to the tube-mill with about 1/3 its weight of water, where it is ground with small iron balls for about 18 hours. Measurement of charcoal in pulp is effected by sp. gr. determination. Solution is run into the box until the leaves are well covered and then charcoal pulp is pumped in and the propellers started. After a few minutes solution is run in and a cake of charcoal built up on the leaves. It takes about 20 minutes for the solution in the box to become clear and then pregnant solution is added and drawn through the cake for 6 hours. At the end of that time the charcoal cake is dropped and re-emulsified and the cakes re-formed.
Three precipitation tanks are employed and are run on the counter-current principle. A given charge of charcoal remains in its unit for 9 days. A new charge forms the 3d unit in the series for the first 3 days, it then becomes the 2d unit, and for the final 3 days is the 1st unit, the object being to enrich the unit as much as possible with pregnant solution before cleaning up, and to have charcoal that is more or less barren at the end of the series to strip the solution before going to the barren sump. The charcoal cake, after being three days in the 1st unit is discharged and burnt and the unit charged with fresh charcoal. The clean up is made by emulsifying the cake and pumping to a plate and frame press, where it is washed, the cake discharged from this being ready for burning. This is done in braziers in the bottom of which a few hot coals are laid and the damp cake shovelled in on top. It needs about 48 hours to consume all the carbon. The ash is then melted down in the usual way, the flux recommended by Edmands being 40-50 parts of borax, 50 of sand and 10 of salt for every 100 pounds of ash.