Jet loop reactor application for mine water treatment using fly ash, lime and aluminium hydroxide

Recent studies have shown that a combination of coal fly ash (FA) and Al(OH)₃ can be used to treat neutral mine drainage (NMD) and reduce sulphate concentration to within South African drinking water quality levels, Class II (400–600 mg/L). The shortcomings of this method were the large amounts of FA required to raise the pH to greater than 11 (3:1 liquidto- solid ratio) so that Al(OH)₃ can be added to facilitate removal of sulphate ions through ettringite precipitation. This requires large silos to store FA, making upscaling of this treatment technology using normal mixing methods to be unrealistic. In the current study, a jet loop reactor was used to reduce the amount of FA needed to increase the pH to greater than 11. The pH was raised to greater than 11 by mixing 0.25 % of lime (w/v ratio) and 13 kg of coal FA with 80 L of NMD in a jet loop reactor. After the pH of the mixture was above 11, amorphous Al(OH)₃ (83.2 g) was added to the mixture. This resulted in the sulphate concentration decreasing to less than 500 mg/L. Bench-scale studies using 0.25 % (w/v) of lime and 6:1 coal mine water to FA ratio could not reduce the sulphate concentration to below 500 mg/L. Therefore, the impingement and cavitation mixing techniques that happen in a jet loop reactor played an important role in enhancing sulphate removal.