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.
Highlights
Cadmium is present in all kinds of phosphates in concentrations from 1 to 90 ppm
During the manufacture of phosphoric acid by the wet method most of the cadmium in the phosphate is transferred to the phosphoric acid and to the fertilizers while 20-30% wt remains in the phosphogypsum
The distribution of cadmium between the acid and the solid phosphogypsum phase varies according to the operating conditions of the reactor, the process used for digestion and the properties of the crude phosphates used
Summary
Cadmium is present in all kinds of phosphates in concentrations from 1 to 90 ppm. Cadmium is responsible for the disease called itai-itai which was caused by the presence of 1 mgL−1 cadmium in water in Japan. Skorovarov stated that there is no relation between cadmium and uranium in the phosphates [2] He pointed out that the best method to remove cadmium from phosphoric acid was by liquid-liquid extraction with trioctylamine (TOA) in kerosene. He extracted cadmium from a laboratory prepared phosphoric acid solution containing chloride ion. Almela [3] extracted cadmium II from a chloride medium with a concentration of 1 molL−1 using the commercial Cyanex 302 in kerosene He deduced that substituting 1 atom of oxygen by 1 atom of sulfur in the solvent enhances the extraction.
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