Sulphate ions removal from an aqueous solution: I. Co-precipitation with hydrolysed aluminum-bearing salts

The removal of sulphate ions constitutes one of the main challenges in the mining, metallurgical and chemical industries, among others. Sulphate removal from aqueous streams is by far one of the most difficult and onerous tasks in these industries, and most existing processes are inefficient and costly. This work presents a method to remove sulphate ions in an acidic medium based on co-precipitation with aluminum salts, namely AlCl3 and a polyaluminum salt (PAC), followed by filtration. Herein, the different Al species react differently with sulphate, forming soluble and insoluble complexes, the latter of which causes insolubility and allows for the removal. The reaction kinetics depend on the activity of the so-called Alb species (which are classified as polymeric) and was proved here by different techniques. Better results were obtained with the inorganic salt AlCl3, when compared to PAC, probably because of the higher activity of the Al species. Co-precipitation of sulphate depends highly on the pH (which is optimal at 4.5), the mass ratio between reagents and sulphate ions (which is optimal at 7:1) and time (10 min). Finally, it was concluded that the technique proposed here has good potential for the treatment of sulphate-bearing effluents, including acid mining drainage (AMD).

Research highlights
► Sulphate ions interact strongly with aluminum species at pH 4.5.
► Co-precipitation of sulphate ions depends highly on the pH.
► Co-precipitation of sulphate ions depends the mass ratio between aluminum reagents and sulphate ions.