412. Recovery of Silver from Exhausted Fixing Solutions. On the average, about threr quarters of the silver contained in sensitive materials pass into the fixing baths. There is from 15 to 30 gr. of fine silver per dozen plates (7 X 5 in.) and about fib gr. per hundred sheets of paper of the same size. Assuming that the necessarily imperfect process of recovery in a small works only permits of a yield of 75 per cent of these quantities, one can estimate the value of this recovery, allowing for the cost of precipitation (never very high) and for the cost of extracting the silver by the smelters.' The silver in fixing baths is generally precipi tated in the form of sulphide. Under these conditions, and provided that the silver is not completely precipitated, fixing baths may be used again, at least once, though not indefinitely, because the accumulation of soluble bromides and particularly of iodides 2 in the solution con siderably retards fixation in solutions so regenerated.
The silver may also be recovered in the metal lic state by precipitation on plates or scraps of zinc, iron, or copper. In this case the silver is contaminated with various impurities and needs to be refined.
To the residues thus collected are added the ashes of clippings from prints and films stripped from waste negatives.
413. When the silver is precipitated as sulphide, the exhausted baths must be stored and treated in a yard or shed which is far enough from dark-rooms, store-rooms, etc., to avoid any risk of sulphuretted hydrogen coming in contact with sensitive The used fixing baths are thrown into a barrel having wooden and with the top re moved. This barrel should be mounted on bricks ; it must be provided at about a quarter of the way from the bottom with a wooden outlet or tap, so that the greater part of the liquid may be run off, and at the bottom with a bung-hole, from which the sludge of silver sulphide may be periodically run off. Old developing solutions may also be poured into the same vessel, since they contain a little silver. These solutions will reduce a small quantity of the silver in the fixing solutions to the metallic state, and they will also tend to neutralize the residual acidity of the fixing baths. When the barrel is about three-quarters full, about loo gr.
oz. approximately) of sodium mono sulphide' is added for every gallon of liquid to be treated (io grrn. of the sulphide for every litre of liquid). This sulphide should be previously dissolved in a little boiling water. The mixture is stirred with a stick and left to settle. The next day a little of the clear supernatant liquid is taken in a test tube (if the liquid is turbid it must be filtered) and a few drops of a solution of sodium monosulphide are added 2 ; if no black precipitate forms, the silver has been completely precipitated ; if, on the contrary, a black preci pitate forms, add to the liquid in the barrel about half as much of the sodium monosulphide as before and repeat this process if necessary until all the silver has been precipitated. 3 After a final settling, the liquid is run off by the tap through a filter-bag of close felt placed so as to trap any silver sulphide which might be lost.
After the cask has been emptied in this way several times, as successive lots of fixing bath are treated, the black sludge is run off through the lower bung-hole and is collected in a tray. After drying, this mud is stored in a box until a sufficient quantity has been collected for sending to the smelter.' Pure dry silver sulphide contains 87 per cent of its weight of fine silver. On account of numerous impurities which accompany it, silver sulphide obtained in the manner described works out at about 6o per cent if it is precipitated from baths free from alum, and at about 4o per cent when it is contaminated with alumina or chromium hydroxide resulting from the inter action of sulphide with alums.
414. To recover the silver from old fixing solutions by another method, a barrel of 5 to io gallons capacity, and equipped as in the previous case, may be used. Before starting the recovery process, the liquid must be neutralized, by means of caustic soda or milk of lime (lime first wetted with a little water and then left to slake) if it reddens blue litmus paper, or with sulphuric acid if it turns red litmus blue ; at the neutral point practically no change of colour takes place with either kind of paper. Then the liquid is acidified by adding 350 gr. (or a little more than 31 oz. (fluid) ) of ordinary sulphuric (66° Baurne). After stirring, about 2 lb. of granulated or scrap zinc for every gallon of liquid (200 gm. per litre) are thrown into the barrel ; this quantity causes rapid precipitation (in about 24 hours, if the mixture is stirred from time to time) of silver (partially in the form of sulphide), which forms a black deposit on the zinc and in the bottom of the barrel.
An appreciable economy of zinc may be effected by enclosing the latter in a bag of coarse cloth suspended in the liquid.
On the next day, in order to find whether or not the liquid still contains any dissolved silver, proceed as follows : In a test-tube take a little of the clear, reddish, supernatant liquid, acidify with a few drops of sulphuric acid, shake (make sure that the liquid reddens blue litmus paper), and add about one-fifth of its volume of a io per cent solution of sodium monosulphide ; if no black precipitate forms, the silver is completely precipitated (a black precipitate obtained with out the liquid being acidified has no significance) ; if silver still is present, keep the zinc in contact with the liquid for another day. As soon as all the silver is deposited, decant the liquid by the side opening.
One charge of zinc suffices for seven or eight such recovery processes without any appreciable slowing of the action. When the precipitation. needs from two to three days, it may be acceler ated by adding about 3 oz. of zinc per gallon of liquid (zo grrn. per litre). Under these conditions, i lb. of zinc allows of the recovery of about lb. of silver.
From time to time the black deposit is col lected and dried.
These various operations should be carried out in a well-ventilated place, as far as is convenient from dark-rooms and any sensitive materials.'