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

The silver-binding properties of two strains of the alga Chlorella vulgaris (211/11b and 211/12) have been investigated. Rapid Ag+ uptake from an aqueous solution has been proven. The relative affinity of the strains 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 56.7 mg/g of algae when contacted with 200 mg/l AgNO3 solutions. The binding of the Ag+ ion is largely pH-independent when present in a low concentration but pH-dependent when present in a high concentration. When contacted with a simulated photoprocessing effluent, over 80% of the silver can be removed by strain 11b. Froth flotation can be an efficient 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 most efficient flotation; however when floating the algae from the growth medium the presence of Al3+ ions actually inhibited flotation. All the flotation experiments were carried out in a Hallimond tube using the amine collector ARMAC-T. The optimum concentration for this collector was 0.4 g/l. Owing to the alga's high sorbtive capacity for the silver thiosulphate complex at low concentrations, retrieval systems based on Chlorella vulgaris would be best applied for final cleansing of effluents from existing silver recovery systems.