Dissolution Behavior of Rare-Earth Metal in H2SO4-H3PO4-H3PW12O40 Solution

The dissolution behavior of a rare-earth element in H2SO4-H3PO4-H3PW12O40 solution has been investigated. The results showed that La2(SO4)3 dissolution increased with increasing H3PO4 or H3PW12O40 concentration in single H3PO4 or H3PW12O40 solution. The presence of H3PW12O40 can improve La2(SO4)3 dissolution owing to its coordination toward La3+ in H3PO4-H3PW12O40 mixed solution. However, La2(SO4)3 dissolution decreased gradually with increasing H2SO4 concentration in H2SO4-H3PW12O40 mixed solution. In these solutions, increasing the temperature showed an adverse effect on La2(SO4)3 dissolution. In H2SO4-H3PO4-H3PW12O40 solution, when the concentration of H2SO4 or/and H3PO4 was lower, increasing the H3PO4 concentration or decreasing the temperature was beneficial to La2(SO4)3 dissolution. However, when H2SO4 or H3PO4 reached a certain concentration, some H3PW12O40 precipitated from the mixed solution. Furthermore, increasing the H3PO4 concentration was found to be harmful, while increasing the temperature could increase La2(SO4)3 dissolution. Nevertheless, increasing the H2SO4 concentration showed an adverse effect on La2(SO4)3 dissolution regardless of the H3PO4 concentration.