Recovery of thorium from phosphogypsum by acid leaching

Phosphogypsum (PG), a waste material readily available in large quantities, is generated in the wet process of phosphoric acid production. Due to the contaminants in the material and the radioactivity of PG, the use of this material has limitations. South African phosphogypsum (from Phalaborwa) contains high levels of thorium that could be recovered for industrial use. The study was conducted to determine the best acid for leaching thorium from phosphogypsum; to investigate the effect of solid to liquid ratios and of concentration on the efficiency of the process; and to study the effect of contact time on the thorium recovered from the leaching process. Thorium was recovered from phosphogypsum by leaching the samples with strong acids under specified controlled and manipulated variables. 98% sulphuric, 32% hydrochloric and 55% nitric acid were compared to identify the best leaching reagent. The solid to liquid ratios studied were 2%, 5% and 10%. The acid concentrations investigated were 2 M, 5 M and 10 M. Further study was conducted to measure the radioactivity of phosphogypsum after treatment (using citric acid) and recommend possible applications for the material. Nitric acid proved to be the best leaching reagent among the investigated acids, recovering 0.07486 g of Th/kg of PG. The lowest concentration of 2 M recovered more thorium (0.06117 g of Th/kg of PG); while a higher solid loading yielded more thorium (10% loading recovered 0.04453 g of Th/kg of PG). With application of citric acid, the activity indexes of phosphogypsum for different applications were reduced to I_{1, building} = 1.21, I_{2, street and playground} = 0.487 and I_{3, landfilling}=0.132.