Influence Mechanism of Fluorine on the Preparation of Vaterite by Phosphogypsum Direct Mineralization: Insights from Experimental and Density Functional Theory Analyses

Phosphogypsum (PG), a byproduct of the phosphate fertilizer industry, is a promising candidate for CO2 mineralization due to its high calcium content. However, the direct mineralization of PG typically yields calcite rather than the metastable vaterite phase of CaCO3, which holds superior industrial value. This study investigates the influence of impurities in PG on vaterite formation through experimental and density functional theory (DFT) approaches. Through the analysis of diverse impurity types in PG, fluorine was identified as the dominant element influencing vaterite formation. Mineral carbonation experiments were conducted using pure CaSO4·2H2O and PG with varying fluorine species (NaF, Na2SiF6, and CaF2) and concentrations. It revealed that soluble fluorine significantly promotes calcite formation, even at a low concentration of 0.1 wt %, while sparingly soluble fluorides exhibit minimal effects. DFT calculations demonstrated that F– adsorption on vaterite (110) surfaces involves strong chemical interactions, destabilizing the structure and facilitating its transformation to calcite. In that case, a selective flotation process was used to remove soluble fluorine in PG, indicating that flotation pretreatment effectively reduced fluorine content in PG from 0.62 to 0.13%. Flotation PG enables the synthesis of high-purity vaterite with 70% content via direct mineralization. These findings elucidate the critical role of fluorine in governing CaCO3 polymorphism and provide a practical pathway for enhancing the value of PG-derived products through impurity control.