TREATMENT OF SULPHATES-RICH SOLUTIONS THROUGH ETTRINGITE PRECIPITATION WITH INDUSTRIAL REAGENTS

Mine wastewaters are often characterised with high concentrations of sulphates, heavy metals and increased electrical conductivity. It is mandatory for the industry to implement convenient treatment methods in order to reduce these parameters. The aluminium precipitation with the simultaneous formation of ettringite was chosen for laboratory testing as it is appropriate when the concentrations of sulphates have to be reduced–up to 300 mg/l according to the Bulgarian regulatory framework. In the present study synthetic waters were treated, as their composition was similar to that of typical mining effluents. Initially, the pH of the solutions was adjusted to 9.0 with Ca(OH)2 and thus the heavy metals precipitated in the form of hydroxides. Then, an aluminium source was added as Al-cement, industrial Na-aluminate or aluminium oxychloride. The best results in terms of sulphates and electrical conductivity decrease were obtained with Al-cement (69 – 100 % sulphates removal rate), but the quantities of the generated sludges were large, which raises the question of their proper disposal. With the industrial Na-aluminate the rate of removal of sulphates was very low and with AlOCl it was 96%. However, the use of both reagents showed an increase in the electrical conductivity of the treated solutions