Liquid-liquid extraction of cadmium ions from 55-65 w% “black” phosphoric acid from a Nissan H process with diphenyldithio-phosphinic acid (DPP) or dicyclohexyldithiophosphinic acid (DCP) dissolved in an alkane (C12 or C16) at 90°C was investigated. The cadmium concentration could be reduced from 25 ppm to below 2 ppm at a ligand/(Cd+Cu) ratio of 6 (mol/mol). Cd and Cu were removed with a high selectivity. At lower ligand/(Cd+Cu) ratios Cu is removed more selectively than Cd. DPP forms precipitates with Cd, Ni, Zn and Cu, whereas the Cd and Zn complexes of DCP are soluble in the organic phase. For DCP the equilibrium constant Ke =[CdL2] / ( [Cd] [L¯]2) in 55 w% c.p. H3Po4 (90°C) has a value of 5∗105l2/mol2. Ke decreases slightly with decreasing % H3PO4 A fast decomposition of DPP was observed in “black” phosphoric acid at 90°C.
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Tjioe et al. (1989) studied this question.
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