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Magnesium-lime process for decolourization of kraft mill effluents Rush, Richard John


A magnesium-plus-lime coagulation process, incorporating magnesium recovery by sludge carbonation, recently developed for water treatment has been tested for its application to the decolourization of kraft mill effluents. Total bleached kraft mill effluent (BKME), after biological treatment and effluent from the first caustic extraction stage of a conventional bleachery (E₁ or T-20 effluent) were examined in the laboratory. It was shown that a combination of low magnesium and low lime dosages can achieve better decolourization of kraft mill effluents than 3-5 times as much lime alone, (i.e.: 30-60 mg/1 Mg⁺⁺ plus 375-475 mg/1 lime (as Ca0) at pH ≃11.1 for BKME; and 150-300 mg/1 Mg⁺⁺ plus 1875-3750 mg/1 lime (as Ca0) at pH ≃12.1 for T-20 effluent.) Colour removals of 90-95% were achieved using either fresh or recycled magnesium. Results of the study showed that greater than 90% magnesium recovery can be attained by operating the sludge carbonator to a final pH ≃7.5, with complete mixing and a good C02 diffuser system provided. (Key words: magnesium, lime, kraft mill effluent, decolourization, colour removal, Magnesiurn Carbonate Process.)

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