SOLVENT-EXTRACTION OF SILVER FOR POLLUTION-CONTROL OF PHOTOGRAPHIC FIXING SOLUTIONS

Silver recovery from photographic fixing solutions is investigated by applying solvent extraction techniques. Amines were shown to be the most promising extractants and quaternary ammonium chlorides selected for further studies because of high distribution coefficients and low pH-sensitivity. The distribution coefficient for silver from a 0.5 M ammonium thiosulphate solution increases from 2.3 to 46 when the extractant concentration is increased from 0.05 M to 0.5 M Aliquat 336. Further, the distribution of silver decreases with the square of the thiosulphate concentration. By dilution of the fixing solution 1:5 outlet silver concentrations below 1 mg/l can be achieved with 4 to 5 mixer settler units. Stripping can be performed by changing the thiosulphate concentration or by using an ammonium chloride solution. In order to recycle the organic phase between the extraction and stripping sections, co-extraction of other components in the solution, such as thiosulphate degradation products and halides, should be further studied.