The Rand practice is to use hot water for this purpose for the more thorough removal of acid and sulphates.
The thickened sludge is then pumped into the press and when the latter is filled the cakes are further washed by forcing water through them from cloth to cloth by means of the special washing ports. The clear liquor is decanted and more water added, agitated, settled and decanted as before. When all action has ceased more water may be run in to fill the tank, and the charge may be allowed to settle. The acid sulphate of soda, NaHSO4, is sometimes used as a cheap substitute for sulphuric acid. Water should be present to the amount of about 10 parts by weight to one of acid necessary, and the latter is added to the charge little by little, care being taken to see that the action is not sufficiently violent to cause it to boil over the agitator meanwhile keeping the contents of the tank well stirred. When the precipitate is to be acid-treated it is advisable to displace the cyanide solution in the zinc boxes with water before beginning the clean-up so as to avoid the formation of hydrocyanic acid on the addition of the sulphuric acid. If the clean-up tank is to be used for acid treatment, it should be fitted with an adjustable hood and chimney to carry off the gases which are often poisonous and also with a decanter for removing the supernatant liquor after settlement.
In some plants it is the custom to treat the whole of the precipitate with sulphuric acid before sending it to the filter press and in that case some or all of the “zinc shorts” may be allowed to enter the tank and share the acid treatment. The broken zinc which is too big to pass the screen is usually returned to the zinc room to be utilized in one or other of the ways already described, but some metallurgists prefer to get rid of it either by dissolving it in acid, leaving a rich residue to be washed and filtered or by melting down in crucibles with a special flux. The suction pipe of a small plunger pump is connected with the bottom of the tank and the product delivered into a filter press of the ordinary metallurgical type.
#Gold precipitate free
For a large plant the screening of the zinc shorts is more conveniently done by running the contents of the zinc boxes to a revolving trommel provided with the desired mesh of wire cloth and a spray of clean water to wash the short zinc free from precipitate. The latter after leaving the precipitation box runs to this screen where the coarse zinc is removed while the precious metal mud together with some fine zinc falls through into the tank, the operation being assisted by a stream of water or solution under pressure from a hose. At the top, on one side, is placed a large screen of from 20 to 60 mesh, which maybe supported on springs to assist in working the sludge through it. The clean-up tank may be six feet in diameter and four feet deep with arrangements for keeping the charge in agitation by means of stirring arms. In some plants it is customary to replace the solution in the boxes with water before beginning the clean-up so as to avoid the troublesome effect of the cyanide upon the hands and arms of the workmen when this is not done it is usual to supply rubber gloves for their use. As soon as each compartment is cleaned and repacked the solution flow is opened and allowed to run long enough to cover the zinc and protect it from the air. The long zinc is removed from the tub of water and carefully packed in, and the next compartment proceeded with in a similar manner, the shortage of zinc in the first being supplied by that removed from the second, and so on down the line. The interior of the compartment is then carefully hosed out and cleansed with brushes or sponges, the outlet closed, and the screen-bottomed tray replaced. A plug or cock is then opened at the bottom of the compartment and the solution, sludge, and short zinc allowed to run out into a launder which conveys it to the clean-up tank. The zinc is then removed and after draining may be placed in a tub of water to protect it from oxidation by the atmosphere. With zinc shaving precipitation the usual method of cleanup is to shut off the flow of solution and starting at the head compartment of a box to wash the shavings gently in the solution avoiding any action that would tend to break up the attenuated threads into short pieces.
0 kommentar(er)
