Transformation of red mud from aluminium industry into a coagulant for wastewater treatment

Red mud is a waste produced in very large quantities by the aluminium industry. This study relates to a process of transformation of red mud, by treatment in acid and saline medium, into a concentrated liquid or solid but soluble product, containing high amounts of Al and Fe that is usable as a coagulant for water/wastewater treatment. The optimal conditions for the production of liquid or solid coagulant include the treatment of red mud pulp (20% w/v) using 1765 kg H2SO4/ton of treated red mud (trm) and 469 kg NaCl/trm at 110 °C for 2 h. Under these conditions, final extraction and recovery yields of 75% Fe and 74% Al from red mud were measured by analyzing the solid coagulant (after water evaporation). Production rates of 222 kg Fe/trm and 78.9 kg Al/trm were measured during production of the solid coagulant. X-ray diffraction analysis revealed that this solid coagulant is mainly made up of NaFe(SO4)2, NaHSO4 and Al5Cl3(OH)12·2H2O. Finally, precipitation tests using synthetic solutions have shown that the solid coagulant produced from red mud has a phosphorus removal capacity similar to commercial coagulants (alum, ferric sulfate, ferric chloride).