The pulp from the tube mills is elevated to two classifiers (Fig. 9), the overflow from which passes to the amalgamated plates, water being added en route. There are ten plates, each 4 foot 9 inches by 12 foot long with 1 1/8th inch drop in the centre. The fall of the plates is 1 3/8th inches per foot, and at the end of the plates there are two riffles 3½ inches wide by ½ inch deep.
About 28lb. of mercury is used every morning in cleaning and dressing the plates and the loss sustained is 0.25 ounces per ton of ore crushed. It is still too early to say what the effect of fine grinding will be upon the percentage recovery by amalgamation. The tube mills started work on the 12th September, 1910, and a fire on the 15th September put two compressors out of service and consequently stopped the pumps below No. 13 level. Rich ore from development work was therefore cut off and the tonnage had to be maintained from above No. 13 level. The amalgamation and cyanide returns immediately before and after starting the tube mills are therefore not comparable. In addition to this the ore varies so much in nature and in its gold-silver ratio that it would be necessary to take the average over periods of at least six months in order to get reliable figures.
Percentage Recovery of Values by Amalgamation.
1910 31.0 for 3 months after starting tube mills.
The reduction of the recovery by 10% since September is probably due to the alteration in the character of ore treated. No amalgam traps are used, but each Vanner has a muntz-metal box inside its feed-distributing box, which catches mercury, becomes coated with amalgam and is cleaned up every six months.