C.—3.

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(c.) IMPROVEMENTS IN MINING AND METALLURGICAL PLANT AND PROCESSES. A Paper on the " B. and M. Circulating-tank,"* by Mr. F. C. Brown, General Manager, Wa'ht Grand Junction Gold-mining Company (Limited). This apparatus fills a long-felt want in the cyanide and other metallurgical processes, where the treatment of the ore depends upon its metalliferous contents being extracted by means of chemical solvents. The tendency nowadays in the economic treatment of quartz-ores containing gold and silver by the cyanide process is to go in for exceedingly fine grinding, as it is becoming more, and more apparent that the extra extraction due to the fine grinding of the quartz-particles is far and away above the small extra cost of treatment due to the fine grindine. Fine grinding means a simplification of process, as the finely ground pulp, instead of having to be classified into sands and slimes as heretofore, and treated by two distinct methods—viz., percolation of the sands and agitation of the slimes—can be agitated direct, and the gold- and silver bearing solutions separated by filtration or decantation. The agitation of the whole pulp is carried on on some fields in the usual shallow agitators fitted with mechanically operated arms or stirrers, but there are serious drawbacks to this method, some of which are, — (1.) When the pulp is thick, instead of being properly aerated by the agitation, it is simply moved round and round in the agitator, and only its surface comes in contact with the air. This means that, in order to thoroughly aerate the charge and get the gold and silver into solution, a long period of agitation is necessary, unless special means are provided for introducing air into the gharge. (2.) With a thin pulp, especially if much sand is present, there is a great danger of settlement and packing on the bottom of the agitator. (3.) With a sandy charge, a sudden and unavoidable stop of the machinery means great trouble in restarting the agitation. (4.) The shafting and numerous belts for driving these agitators are a constant source of expense by wear-and-tear, as are also the agitation gears and arms. (5.) Shallow agitators require a large ground-space, and need expensive foundations. The apparatus herein described overcomes all these drawbacks, and by its use the following necessary conditions for efficient treatment are obtained :— (1.) The agitation or circulation of large charges of concentrates, sand, pulp, or slimes with cyanide or other solutions is most perfect. (2.) Every particle of the material under treatment is continually coming into contact with solution and air, and being subjected at the same time to a certain amount of scouring or washing. (3.) The agitation of the charge can always be readily started up, no matter how long it has been allowed to settle and become packed. (4.) The charge is always under control, and can be subjected to preliminary lime or other washes at the will of the operator. (5.) There arc no wearing-parts, no belts, and no moving gear. (6.) The consumption of power is about one-quarter of that required for mechanical agitation. (7.) The tanks are cheap to install, and require, a very small ground-space for a large tonnage— a most important feature in cold countries, where buildings have to be artificially heated. (8.) By means of a simple sampling device, a correct sample of the material under treatment can be obtained during agitation. (9.) The discharge of the material, after treatment, is accomplished quickly, and without any manual labour being necessary. General Description of the Apparatus. —The material to be treated can be run direct into the tanks from the tube mills or other grinding-machine in large quantities, say 50 to 100 tons of pulp (dry weight) for a charge, if necessary, allowed to settle so as to decant off the superfluous water, and then be perfectly agitated with cyanide-solution, and kept in circulation for such time as is necessary to dissolve the gold and silver. The apparatus consists of a tall tank with a conical bottom, and in the centre of the tank there is an upcast pipe open at both ends, and means is provided for introducing compressed air at the bottom of this pipe—practically, a tall tank with an air-lift pump in the centre of it. At the bottom of the tank, situated in the cone, there is a system of jets arranged to deliver air solution or water against the sides of the cone in such a manner as to soften or liquefy the sand or pulp packed tightly in the cone, and then prepare it to be lifted through the upcast pipe and discharged at the top of the tank. As the mixture] of sand or pulp and solution does not settle rapidly, it is found that in a comparatively short time the whole charge in the tank becomes a homogeneous mixture of sand, slime, and solution, the circulation of which can be kept going indefinitely by means of a comparatively small quantity of compressed air admitted at the bottom of the air-lift pipe. Method of starting the Agitation. —No matter how tightly the charge has become packed in the bottom of the tank, it can be readily started up. In experimenting with tanks 10 ft, in diameter by 39 ft. deep, charges of 60 tons of fine sand that had been allowed to settle for over a week were started up without any difficulty.

* In addition to the installation of these tanks at the Waihi Grand Junction Mine, there have been thirty-two tanks erected during the past year at the reduction-works of the Waihi Gold-mining Company (Limited), and a large number are in use at the Hacienda de San Franoisoo Company Mine and elsewhere in Mexico.