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Silver Recovery — Hickman and Hyndman 703
a negative pressure slightly less than the head of a stationary column of Hquid of height LM, is developed at L. Since L communicates with a trough of liquid R (sodium sulphide solution), sulphide will be sucked in during the flushing period. Now this period is one of constant duration ; the time taken to empty a crock from one predetermined level to another through a tube of constant dimensions. Therefore each time the crock flushes, the same amount of spent hypo will be delivered and the same quantity of sulphide administered through the side tube. All that is needed is that the level in the sulphide trough shall be approximately constant. This has been accomplished by mounting two 20-liter glass bottles Si and S2 a little above R and allowing stout rubber tubes Ti and T2 to hang from tubulures to below the surface in'R. Pinch cocks close the tubes at will. To fill the bottles, the rubber tubes are pinched, the corks withdrawn, the sulphide solution poured in, corks inserted tightly, and the tubes undipped. Whenever the level in R falls below the ends of the tubes, air leaks in and allows a little sulphide to flow down. Two bottles are used so that when one is empty the other can function during filling. It is convenient to have one tube Ti a Httle longer than T2.
The action of the measuring unit is to give a series of intermittent rushes of treated hypo solution. Fluctuations in discharge from the film processing machinery are taken care of by variations in the time the top crock takes to fill. The function of the second crock is to store the mixed hypo and let it gently into the settling tank so that the sludge shall not be unduly disturbed. This crock is therefore fitted with a constricted orifice so that each unit of solution can leak away in the interval between two flushes at peak load.
Fig. 1 depicts the whole assembly. The crocks are mounted on the top of the settling tank and when once adjusted need no attention whatsoever. The film base funnel is made of material sold for side curtains of automobiles. It is 4 feet long, 18 inches wide at the base, and 2 inches wide at the top and is joined at the seam by ordinary film splicing cement. It is suspended by a saddle from the second crock orifice. Its action in settling the precipitate is best seen from the shading in the diagram.
Collecting the sludge of alumina and silver sulphide is quite straightforward. The desilvered hypo, brown from developer but quite transparent and free from precipitate, flows to waste from the top of the tank. The sludge is best drawn off daily from the