Recovery of silver@ from suspension after enzymatic sepn. from film waste - by acidifying, flocculating with sodium sulphite and precoat filtration

摘要

Method involves acidifying a silver suspension to pH 4.5-5.5, treatment with 0.05-0.2 g/l Na2SO3 and filtration through a precoat layer after flocculation for 30-60 min.. The precoat layer pref. consists of acicular CaSO4.2H2O crystals with a length of 20-150 microns and width of 2-10 microns. USE/ADVANTAGE - Relatively small amts. of Ag can be recovered from a large vol. of aq. soln. with min. loss. Up to 99.5% recovery of Ag (as Ag and AgBr) is attained. The coat is favourable and most of the gelatin and its decomposition prods. is sepd. in the filtrate, instead of accumulating in the Ag sludge. In an example, 140 g comminuted film waste were treated with 1000 ml water with an enzyme activity of ca. 100 Kunitz units. The plastic fraction was sifted out and washed well with a total of 800 ml water in several portions, giving very pure-waste suitable for recycling. The suspension was adjusted to pH with 1 ml semi-conc. H2SO4, then 0.15 g Na2SO3 were stirred in. Flocculation started immediately and was complete after ca. 20 min.. The suspension was filtered through a 20-25 mm thick precoat layer. The light yellow filtrate contained partly decomposed gelatin and was passed to biological clarifying plant. The filter cake consisted of CaSO42H2O, water, Ag, AgBr and a small amt. of organic impurities (gelatin, dyestuffs etc.). The Ag could be worked up in the usual way.