Indirect aqueous carbonation of CaSO4·2H2O with aspartic acid as a recyclable additive

Abstract

Calcium leaching using additives is the most critical step in the indirect aqueous carbonation process of CaSO₄·2H₂O. However, recovery of the soluble additives from the sulfate-rich carbonation filtrate limits the large-scale industrial implementation of current carbonation technologies. To address this issue, we employed aspartic acid (Asp) as a leaching additive. The dissolution capability of CaSO₄·2H₂O in aqueous ammonia was found to improve significantly owing to the complexation effect between Asp and the Ca²⁺ ions. The maximum amount of dissolved CaSO₄·2H₂O was determined according to the competitive relationship between the complexing effect and the inhibitory effect of free ammonia molecules on the dissociation of CaSO₄·2H₂O, and the solution pH influences such competition. The precipitation of CaCO₃ was examined by monitoring the variations in the pH and conductivity of the carbonation reaction system. As a result, the shift in the Asp dissociation equilibrium extended the induction period, and the growth period was divided into three stages according to the relative difference between the consumption and formation rates of CO₃²⁻. Moreover, it was determined that the carbonation products consisted of stable spherical vaterite particles. The recovery of Asp was also demonstrated at its isoelectric point, with a recovery efficiency of >80% being achieved, and recycling experiments confirmed the stability of the recycled Asp. Finally, the amount of dissolved CaSO₄·2H₂O and the total carbonation efficiency during cycling were determined as 16.3 ± 0.4 g L⁻¹ and 46.5 ± 1.9%, respectively.