Phosphoric acid purification sludge: Potential in heavy metals and rare earth elements

The present study was carried out to show the potential in heavy metals (HM) and the rare earth elements (REE) which presents the residues of phosphoric-acid(PA) purification. Three different cadmiferous solid residues (according to the nature of the purification process of the PA: BG, BC and BS) were collected from an industrial site located in the south of Tunisia. The mineralogical study showed the predominance of anhydrite, accompanied by quartz, malladrite; calcium sulfate hemihydrate and fluorapophyllite. The microanalysis showed (i) the association of cadmium and zinc, (ii) as well as the presence of associated REEs. The chemical analysis showed that (i) the calcium sulfate concentrations are majority in samples BS, BG and BC (44, 34 and 44%, respectively), (ii) significant concentrations of phosphoric acid (28, 18 and 21% P₂O₅, respectively), (iii) the HM: Cd, Zn, Cr, Ni, V, Cu, Pb, Co, Mo, Mn and U have proportion in the order of 0.1%. The concentrations of Cd, Zn and Cr are respectively in the order of: 230, 149 and 189 mg/kg for BS, 346, 243 and 153 mg/kg for BG and 183, 129 and 440 mg/kg for BC and (iv) the REEs: La, Ce, Nd, Eu, Y et Yb present considerable mass percentages able to reach 0.2%. A series of extraction tests was led on the cadmiferous sludges to evaluate the rates of HM (Cd, Zn) and REE dissolution, using two solvents (deionized water (DW) and aqueous sodium based alkaline metal solution). The results showed that the dissolution rates of Cd and Zn are respectively in the order of (12-29% and 41-45% for DW; 67-86% and 83-93% for Na₂SO₄ solution). The extractability of HM and REE is strongly influenced by pH, solvent nature and mineral load in the cadmiferous sludges. The water-soluble metals represent a significant mobile fraction, making the toxic elements more sensitive to mobilization processes, such as leaching and erosion. Whereas, the metals extractable by the Na₂SO₄ solution represent a very important exchangeable and "co-crystallization" fraction, which reflects the bioavailability of these metals.