The overflow from the sand vat is pumped up to two sets of spitzkasten, each having nine pyramidal hoppers 6 foot x 6 foot square and 6 foot deep, with 1 foot 6 inch depth above the hoppers. To facilitate settling, an addition of limewater is made. The lime is fed by a Challenge feeder to a Berdan pan, the water from which flows into the top end of the feed-distributing launder. About 10 tons of lime are used per month for 650 tons of slime (dry weight). The underflow from the spitz. passes to thickening vats, the first of which is a steel tank of the same shape and dimensions as the filter-frame tanks, viz. 20foot x 10 foot x 7foot deep, with two pyramidal hoppers at the bottom, 10 foot x 10 foot x 7foot deep.
The overflow from this passes to two settling vats 22 foot diameter and 7 foot deep, from which the overflow passes to two similar vats in the sand-treatment room. The overflow from the latter passes out to the river.
When slime is required for the tall agitator tanks, it is drawn from the steel tank and first two circular vats. The slime caught in the last two vats is drawn off twice a week and pumped to the agitators by a Gwyne 4 inch centrifugal pump of 200 gallon per minute capacity.
There are eight tall air-agitator tanks 12 feet in diameter and 30 feet high, the conical bottom occupying the bottom 5 foot 6 inch. Air is supplied near the base by two ½ inch pipes connected with a main carrying air at 40lb. pressure. These tanks have no central tube as they are unnecessary for agitation. The procedure is as follows- the tank is filled to 18 inches from the top with slime, which is allowed to settle for an hour or more and any clear water then decanted off, the amount varying from nil to 5 foot depth according to the thickness of slime available. When full to 18 inches from the top, the charge is about 120 long tons of 1 to 1 slime, the dry slime assaying about 2/3rd of the value of the sand from which it was derived. Five tons of week cyanide solution are run in ( 0.18 to 2.0% free KCN) through a wire basket containing from 200 to 250lb. sodium cyanide, thus making up 65 tons of 1.19 to 0.35% KCN. After 24 hours agitation the pulp is ready for filtration.
There are two filters of the fixed frame type, each set consisting of 31 leaves, each 9 foot x 4 foot 9 inches, equal to 2,650 square feet of filtering surface. The frames were spaced 7½ inches between centres, but 8 inches is found to be more convenient. They are connected at each end to a 4 inch manifold leading to the vacuum pump. A gauge glass is fitted in the connections to one of these manifolds so that a defective cloth is readily noticed. Any leaf may be disconnected and removed without interfering with the rest.
The details of the frame are shown in Figs. 10 and 11. Black corrugated iron of 1 inch corrugations forms the support for the cloth and is held by a frame formed of ¾ inch piping, having a ½ inch slot cut longitudinally to receive it. The leaf is 9 foot long and the supporting tube at the bottom sags slightly in the middle, but an extra pipe placed vertically in the centre to support the bottom tube would prevent this sag. This addition will be made in new frames. The tanks containing the filter frames are 20 foot x 10 foot and 7 foot deep and are of 3/16th inch steel plate stiffened by vertical and diagonal lengths of angle iron 4 inches x 4 inches x 3/8th inch. The bottom consists of two square pyramidal hoppers 7 feet deep, formed of ¼ inch steel plate and stiffened with a horizontal rib of 3 inch x 3 inch x ½ inch angle iron as shown in Fig. 10. The weight of the tank is supported by a 5 inch x 5 inch x ½ inch angle iron riveted on the sides and resting on an 18 inch x 7 inch H girder supported on cast-iron columns. At the ends of the tank similar angle iron rests upon 12 inch x 5 inch H girder, the ends of which are supported by the ends of the 18 inch x 7 inch girders (see section, Fig. 6).
The capacity of the filter tank is a little more than half that of an agitator. Slime from an agitator is run in by a 12 inch pipe, and is prevented from impinging on the filters by a 3/16th inch baffle plate. A full charge comes up to 6 inches from the top of the tank. Agitation is effected by compressed air, admitted from ½ inch pipes - two near the bottom of each hopper.
There are two vacuum pumps, designed by Mr. J. A. Turnbull, to work to a net lift of 40 feet. They are wet, double-acting, belt-driven, and either side of the cylinder can be disconnected. The cylinder is 8 inches in diameter x 18 inches stroke, and at 50 r. p. m. a vacuum of 23 to 25 inch of mercury is maintained.
An hour at this vacuum suffices to build up a ¾ inch to 1¼ inch cake when the cloths are new, and a 1 inch cake is equivalent to about 10 tons of dry slime. Old cloths work slowly owing to the formation of scale. During the formation of the cake the solution is pumped to a 15 foot x 15 foot x 4 foot settling tank, from which it goes to the weak-solution settler, prior to entering the zinc boxes. The solution runs from 0.18 to 0.2% KCN, and goes into stock as the weak solution for all work.
The slime remaining in the tank after the cake is formed is pumped out into a storage puddler-vat, 22 feet in diameter x 5 foot deep, (shown by dotted lines in Fig. 6). Weak-wash solution is run in from one of the two weak-wash vats, 22 feet in diameter x 7 feet deep, (shown by dotted lines in Fig. 6), and washing is continued for 1½ to 2 hours according to the grade of the material under treatment. The remaining wash solution is then pumped back to its tank and the vacuum continued for another 15 minutes to drain the cake as completely as possible, when it carries 18% to 25% of solution. During this 15 minutes the sluicing gates at the bottom of the hoppers are opened and the cake is sampled by taking a grab from one side of every other leaf. Vacuum is then released and the cake assisted to fall by the use of a wooden spatula and by hosing. This residue, which assays about 5 shillings per ton, is sluiced into the river. Discharging takes little more than 10 minutes with new cloths, but old cloths require an hour or more. For the pumping of slime and weak solution, two 8 inch Kershaw centrifugal pumps are used, capacity 1,400 gallons per minute, made by Thompson, Castlemaine, Victoria, Australia.