A novel approach for removing cadmium from synthetic wet phosphoric acid using sulfide precipitation process operating in batch and continuous modes

The removal of heavy metals, especially cadmium, from wet phosphoric acid is still a challenging stage during the manufacturing of phosphate fertilizers. In this work, a novel approach was developed to improve the performance of the sulfide precipitation process as a purification technique. This is based on NaHS rather than Na₂S as a precipitating agent in both batch and continuous modes for cadmium removal from synthetic wet phosphoric acid. First, the respective performance of Na₂S and NaHS was compared in the batch mode. NaHS was shown to improve decadmiation in comparison to Na₂S even though the same trends were observed for both precipitating agents as a function of operating conditions. An increase in Sulfur/Cd²⁺ molar ratio improved cadmium sulfide precipitation. Conversely, increasing temperature and phosphoric acid concentration impaired the effectiveness of cadmium removal efficiency. An apparent kinetic model was developed taking into consideration the reaction mechanisms and supersaturation to predict the residual cadmium concentration in phosphoric acid solutions. The feasibility of the continuous process based on NaHS was also assessed. The results showed that a residence time of 40 min was needed to achieve 85.6 % of Cd²⁺ elimination at 25 °C. As aggregation seemed to be the main mechanism governing particle size, this increased with the Sulfur/Cd²⁺ ratio, which could improve downstream solid separation. Eventually, these findings of this study provide useful insights into the feasibility of chemical precipitation using NaHS as a technology of phosphoric acid decadmiation in the industry.