Abstract
Cadmium and iron are common impurities in wet process phosphoric acid (WPA). These impurities should be minimized to the acceptable levels before the commercialization of the WPA. Organic extractant such as trioctylammine (TOA) will be protonated in acidic media and can act as a liquid anion exchanger for separation of anionic chloro-species of Cd2+ and Fe3+ from WPA. Synthetic solutions containing phosphoric acid, 40 ppm Cd2+ and 3% Fe3+ (calculated as Fe2O3) were prepared and the different parameters affecting the extraction of these metal ions with TOA were investigated. The extraction of the two metal ions was found to be neglected in absence of chloride ions and it sharply increased by increasing HCl concentration. More than 98% of Cd2+ was extracted with 20% TOA in kerosene from 30% H3PO4 in presence of 1-3% HCl. Almost complete extraction of Fe3+ was achieved in presence of 10% HCl at similar experimental conditions. The TOA concentration of about 10% and 30% could completely extract Cd2+ and Fe3+ at 10% HCl for 10 min, respectively. A third phase formation was observed when TOA in kerosene was contacted with acidic aqueous solutions and this was eliminated by modification of TOA with 10% n-octanol but the extraction efficiency was slightly declined. The extraction process was quite fast, where 3 minutes was found to be sufficient for equilibrium extraction of both metal ions. Increasing the H3PO4 concentration enhanced the extraction of Fe3+ but little affected that of Cd2+. Most of the two metal ions can be easily stripped by contacting the loaded TOA with water but emulsion formation was observed and the phase separation was difficult. Acidic solutions such as 0.5 M HClO4 can solve the problem and strip about 90% of both metal ions. Small amount of H3PO4 (about 1% of the started acid) was co-extracted and stripped together with Cd2+ and Fe3+ under same conditions.