Extraction of cadmium from phosphoric acid by trioctylphosphine oxide/kerosene solvent using factorial design

Abstract

Cadmium is present in all types of phosphates in concentrations ranging from 1 - 90 ppm. During digestion of phosphates by concentrated sulfuric acid to produce phosphoric acid by the wet method 70-80% wt of cadmium is transferred to the phosphoric acid while 20-30% wt remains in solid phosphogypsum. Cadmium is then transferred from the phosphoric acid to the fertilizers. Most environmental regulations try to reduce the percentage of cadmium in the fertilizers. Several studies were carried out to reduce the concentration of cadmium in phosphoric acid. Some used ion-exchange techniques. Others used liquid-liquid extraction by alkaline solvents such as trioctylamine (TOA). Several investigators used commercial phosphoorganic solvents such as Cyanex 302 and Cyanex 93. This paper studies the extraction of cadmium from phosphoric acid by trioctylphosphine oxide (TOPO) solvent in kerosene. The extraction was enhanced by the presence of Cl^- in the solution. The temperature had a negative effect on the extraction. I ncreasing the concentration of cadmium while keeping chloride ion concentration constant decreased the extraction. The extraction was increased by increasing the concentration of the phosphoric acid.