Application of a jet loop reactor to enhance removal of sulphates from mine water using coal fly ash, lime and aluminium hydroxide

摘要

Recent studies have shown that a combination of coal fly ash (FA) and aluminium hydroxide (A1(0H)3) can be used to treat neutral mine drainage (NMD) and reduce sulphate concentration to within South African drinking water quality standards. The shortcoming of this method is the large amounts of coal FA required to raise the pH to greater than 11(3:1 liquid to solid ratio) so that A1(0H)3 can be added to facilitate further removal of sulphate to less than 400 mg/L through ettringite precipitation. This means that very large silos will be required to store FA. Too much FA in the water makes the mixture difficult to mix and pump using normal methods, making scaling up of this treatment technology impracticable. In the current study, the amount of fly ash needed to increase the pH to greater than 11was reduced, by adding 0.25 % of lime (w/v ratio) with a combination of 6:1 mine water to coal FA ratio using a jet loop reactor. Addition of 83.2 g of amorphous Al(OH)_3 to 80 L of NMD resulted in a decrease in the sulphate concentration to less than 500 mg/L, which is within the allowable limit of South African drinking water Class II standard (400-600 mg/L). Bench scale studies using 0.25 % (w/v) of lime and 6:1 coal mine water to coal FA ratio could not reduce the sulphates to below 500 mg/L. Therefore the impingement and cavitation mixing techniques in a jet loop reactor played an important role in enhancing sulphate removal.