Purification of precursors of calcium orthophosphates synthesis by co-precipitation method

Abstract

The purification of the synthesis precursors of calcium phosphates from the toxic microimpurities Cu(II), Cd(II), As(III) and Pb(II) by co-precipitation with a part of the target product was studied. It was found that a maximum extraction of Cu(II), Cd(II) and As(III) from the CaCl2 solutions was achieved in the acidic and alkaline media. When precipitating calcium phosphates from the H3PO4 solutions, the following patterns regarding the degree of co-precipitation of the microimpurities with increase of pH were observed: the degree of co-precipitation of Cd(II) decreases rapidly, whereas the degree of the co-precipitation of As(III) goes through the maximum and Cu(II) is removed completely. Pb(II) ions are also completely removed in both CaCl2 and H3PO4 solutions. It was shown that in order to purify the CaCl2 solution, it is necessary to add at least 15 g of H3PO4 per 1 dm3 of the solution and then adjust pH to the value of 2.5 to 3.0 by neutralizing the solution with ammonia. In order to remove the impurities from the phosphoric acid solution, it is recommended to carry out the co-precipitation in two following stages: firstly, the ions of Cd(II), Cu(II) and Pb(II) are removed at pH of 6.0–6.5, which requires at least 4 g of CaCl2 per 1 dm3 of the acid, and then the arsenic residues are removed at pH of 8.0–8.25, which requires at least 13 g of CaCl2 per 1 dm3 of the solution.