Stepwise Separation for Calcium Recovery from an Ammonium Sulfate Leaching System

Calcium recovery from high calcium solid waste is driven by the latter’s growing environmental impact, emphasizing the need to develop a high value-added utilization approach. Inspired by our previous study, we develop a stepwise separation strategy for calcium recovery from an ammonium sulfate leaching system to efficiently remove iron and unburned carbon residue. In this study, tributyl phosphate (TBP) used as the carbon residue adsorbent for calcium recovery exhibits high selectivity and good recycling stability. Adsorption measurements show that TBP yields a superior carbon residue adsorption ability with a maximum adsorption ratio greater than 95%. Moreover, the adsorption percentage is also kept more than 90% after 10 cycles, which substantiates the good potential of TBP for reuse. Besides, the low concentration (1 mol/L) of H₂SO₄ provides an acidic environment, which makes TBP more feasible to be applied in carbon residue adsorption and effective iron(III) stripping. Furthermore, it is inferred from the Fourier transform infrared spectroscopy/X-ray photoelectron spectrometer and density functional theory analyses that the enhanced carbon separation performance may be ascribed to the carboxyl in carbon residue coordinated to the donor oxygen atom of TBP at the position of P-O. This work provides the foothold for further research on the recovery of calcium from industrial by-products.