The role of P(V) salts in inhibiting CaCO3 precipitation in the uranium in-situ leaching process using CO2 and O2

Abstract

Addition of chemical agents to inhibit the CaCO₃ precipitation process is a potential way to solve the blocking problem in the industrial in situ leaching process for uranium recovery. This research investigated the role of P(V) added as sodium hexametaphosphate ((NaPO₃)₆), anhydrous sodium pyrophosphate (Na₄P₂O₇), and sodium tripolyphosphate (Na₅P₃O₁₀) as inhibitors for the precipitation of CaCO₃ in supersaturated solutions of Ca²⁺ and HCO₃⁻. The results indicated that the three inhibitors prevented the formation and growth of CaCO₃ and Na₅P₃O₁₀ showed the best performance. In the aqueous phase Ca²⁺ coordinated with P(V) ions to form complex anions of Ca-phosphates including CaHP₂O₇^—, CaP₂O₇²⁻, CaHP₃O₁₀²⁻, and CaP₃O₁₀³⁻. The role of P(V) as an inhibitor was attributed to the adsorption of P(V) species including CaP complex anions and phosphate anions on the calcite surface to form an amorphous protective layer which prevented further precipitation of CaCO₃ phase. Moreover, dosage threshold of Na₅P₃O₁₀ was experimentally determined to be 2 mg/L. This dosage gave a CaCO₃ precipitation inhibition efficiency higher than 90 %. Injection of inhibitors had no negative effect on the in situ leaching of uranium by CO₂ and O₂ and subsequent ion exchange adsorption process for uranium recovery.