Extraction of Cadmium From Phosphoric Acid With Diorganyldithiophosphinic Acids. I. Equilibrium Data

Liquid--liquid extraction of Cd ions from 55-65 wt.% "black" phosphoric acid from a Nissan H process with diphenyldithiophosphinic acid (DPP) or dicyclohexyldithiophosphinic acid (DCP) dissolved in an alkane (C sub 12 or C sub 16 ) at 90 deg C was investigated. The Cd concentration could be reduced from 25 to < 2 ppm at a ligand/Cd + Cu) ratio of 6 (mol/mol). Cadmium 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 K sub e = (CdL sub 2 )/((Cd)(L) exp 2 ) in 55 wt.% c.p. H sub 3 PO sub 4 (90 deg C) has a value of 5 x 10 exp 5 1 exp 2 /mol exp 2 . K sub e decreases slightly with decreasing percentage of H sub 3 PO sub 4 . A fast decomposition of DPP was observed in "black" phosphoric acid at 90 deg C. 27 ref.--AA