Process for lanthanides-Y leaching from phosphogypsum fertilizers using weak acids

Phosphogypsum (PG) precipitates as a by-product of the phosphoric acid industry when treating phosphate rocks with H₂SO₄. Lanthanides and trivalent yttrium content, Ln-Y, in the rock are mostly found in the PG. After a slight modification of the precipitated PG, some portion of the PG containing ˜2.0% P₂O₅ is used as a low-grade phosphogypsum fertilizer (PGF). In this work, studies on Ln-Y, leaching from PGF were carried out using different concentration of weak acids such as boric acid, malic acid and citric acid at different retention time (Rt). It was found that the leaching efficiency of the total Ln-Y by citric acid solution is more favorable than that using boric or malic acid solutions. In this concern, 1.0 mol/L citric acid solution leach about 40.9% of Ln-Y, when the acid volume (L) to solid mass of PGF (S) ratio equals 2.0, L/S = 2.0, after Rt of 15 min at temperature of 298.0 K. Therefore, leaching of total Ln-Y from PGF by citric acid was investigated in terms of the acid concentration, Rt, and L/S as well as temperature. It is found that one leaching cycle by 1.0 mol/L citric acid solution using L/S ratio of 5.0 and Rt 15.0 min at temperature of 358 K, produced 53.3% from the total Ln-Y present in the used PGF sample. From these result a flow sheet was developed based on three leaching cycles to give a maximum leaching efficiency of 83.4%. Analysis of different Ln-Y in the final product indicated that 1.0 mol/L citric acid solution has high leaching selectivity for Er (89.4%), Ce (88.2%) and La (81.8%) and lower leaching selectivity for Pr (71.9%) and Y(40.7%).