EXTRACTION AND RECOVERY OF SILVER FROM LOW-GRADE LIQUORS USING MICROALGAE

The silver-binding properties of two strains of the alga Chlorella vul garis (211/11b and 211/12) have been investigated. Rapid Ag+ uptake fr om an aqueous solution has been proven. The relative affinity of the s trains based on varying silver concentrations has been measured. Over a silver concentration range of 10 mg/l to 200 mg/l strain 211/11b had the greater affinity for the Ag+ ion, reaching a maximum loading of 5 6.7 mg/g of algae when contacted with 200 mg/l AgNO3 solutions. The bi nding of the Ag+ ion is largely pH-independent when present in a low c oncentration but pH-dependent when present in a high concentration. Wh en contacted with a simulated photoprocessing effluent, over 80% of th e silver can be removed by strain 11b. Froth flotation can be an effic ient technique in the removal of silver-bound Chlorella vulgaris from a reaction solution with efficiencies of recovery attaining 97.5% for Ag+ bound strain 211/11b and 89% for silver thiosulphate-bound strain 211/11b. The flocculant aluminium sulphate was usually required for mo st efficient flotation; however when floating the algae from the growt h medium the presence of Al3+ ions actually inhibited flotation. All t he flotation experiments were carried out in a Hallimond tube using th e amine collector ARMAC-T. The optimum concentration for this collecto r was 0.4 g/l. Owing to the alga's high sorbtive capacity for the silv er thiosulphate complex at low concentrations, retrieval systems based on Chlorella vulgaris would be best applied for final cleansing of ef fluents from existing silver recovery systems.