This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1919" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1919.
The zinc method of silver recovery which has been worked out in our Research Laboratory is in no way objectionable. The zinc used is the metal in granulated form, similar to metal filings. Your stock house will be able to procure some for you.
Place your exhausted hypo solutions in wooden kegs or barrels. If the bath is alkaline, it should be neutralized with acetic acid, using litmus paper as an indicator. Then add an excess of acid in the proportion of 1 oz. glacial or 3 ozs. 28% acetic acid to the gallon of solution. Then add the granulated zinc in the proportion of 2 lbs. to the gallon of solution and stir occasionally.
After twenty-four hours, the silver should all be precipitated as a black sludge of metallic silver. The progress of the extraction can be followed by taking a small portion of the liquid, making acid with a few drops of acetic and then adding a little sodium sulphide solution. A white precipitate indicates that all the silver has been removed - a black precipitate that some silver is still in solution and that the solution should stand longer.
When all the silver has precipitated and the solution is clear, decant it off. A convenient method is to have a spigot near the bottom of the barrel. The barrel or other container should then be filled up again and the operation repeated seven or eight times until the recovery becomes slow, owing to the exhaustion of the zinc.
The estimate of twenty-four hours for the recovery of silver is based on a fresh charge of zinc. As the extraction proceeds and the zinc is used up, the rate of precipitation falls off so that when more than two or three days are required to precipitate the silver, more zinc should be added.
As the silver is removed from the hypo solution, the zinc continues to dissolve in the acetic acid, but, allowing for this loss, a pound of zinc, costing but a small sum, will recover a pound of silver, worth approximately $17.00, so the recovery is certainly worth while.
When sending silver residues to a refinery, it is more economical to save until a quantity has been accumulated, as a fixed assay charge is made regardless of the quantity submitted. In gathering the sludge from the bottom of the barrel, save every particle of the solid matter, draining it carefully before packing for shipment.
It might be added that zinc dust was tried out thoroughly but was not found practical as it remains in suspension in the liquid and does not allow the liquid to be decanted readily. The coarser zinc settles rapidly and takes the silver with it, making decanting a simple matter.
Sales of Eastman Portrait Film in the United States, for the first eight months of 1919, were 122% in excess of those for the corresponding period of 1918. In Canada, the same comparison is not possible, because Film was put on the Canadian market early in 1919. However, the Canadian figures are surprisingly good, despite handicaps. The explanation is that film results are, in every instance, better than plate results.