Dephosphorization of Nitric Acid Solutions from the Leaching of Monazite Ferrous Ores under Hydrothermal Conditions

The precipitation of phosphoric acid from nitric acid solutions in the presence of iron(3+) oxides and hydroxyoxides under hydrothermal conditions was studied. It was shown that the dephosphorization process proceeds with the formation of iron(3+) hydroxyphosphate (giniite) Fe5(PO4)4(OH)3∙2H2O. The most efficient and fast process is implemented at temperatures of 180-200 °C and above. It was found that the content of phosphoric acid in the solution as a result can be reduced to 10-3 mol/l and below. Lanthanide, uranium, and thorium nitrates are stable under conditions of hydrothermal dephosphorization of solutions, in contrast to iron(3+) nitrates decomposing to hematite and nitric acid. The features of the studied hydrothermal process were used to increase the efficiency of direct nitric acid leaching of monazite iron-bearing ores of the Chuktukon rare metal deposit.