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

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 Ca-P 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.