EXTRACTION OF CADMIUM FBOM PHOSPHORIC ACID WITH DIORGANYLDITHIOPHOSPHINIC ACIDS. Part II: Rate of Extraction and Decomposition.

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(c12or c16) at 90°C was investigated.The rate of extraction of Cd from chemically pure phosphoric acid (55 w% H3Po4,90°C) is mainly determined by film diffusion of cadmium ions in the phosphoric acid phase to the interface. In the extraction from “black” acid, mass transfer in the organic phase or chemical reaction (ligand exchange) is probably the rate limiting step. An apparent mass-transfer coefficient (based on -5 [Cd] in the phosphoric acid phase) of about 1.6∗10−5 m/s was obtained for the extraction from “black” acid. A fast decompositon of DPP was observed and in a batch experiment the Cd concentration was already more than 50% of the initial value after 6 hours. This phenomenon is probably caused by the formation of solid complexes with Cu, Cd and Zn, which are dispersed in the phosphoric acid phase and then oxidized. The decomposition of DCP proceeded much slower and after 30 hours the Cd concentration was still below 50% of the initial value. Cu formed a solid complex with DCP, whereas t.he Cd and Zn complexes were soluble in the organic phase. A surface decomposition rate constant of the free ligand in the organic phase of 4∗10−8m/s was obtained.