Leaching of rare earth elements from phosphogypsum via citric acid medium: optimization through central composite design and kinetics studies

Due to limited resources of rare earth elements (REEs) and their high demand, these are subjected to supply constraints. So it is important to recover REEs from potential secondary resources. Phosphogypsum is the waste generated on an enormous scale (300 million metric tons per year) from the fertilizer industry. It contains valuable REEs and its disposal has become a major environmental issue. The aim of this research work is to optimize the leaching parameters and determine the kinetics of leaching of REEs from phosphogypsum waste using citric acid. Response surface methodology (RSM) through central composite design (CCD) has been used to assess the influence of three key input variables, i.e., leachant concentration, temperature, and leaching time on the dissolution efficiency of REEs. The results of regression analysis and response surface plots revealed that with the increase in citric acid concentration as well as leaching time, the dissolution ratio of REEs would increase significantly. Under the optimum leaching conditions of 2 mol/L citric acid, 343 K temperature, and leaching duration of 180 min, 90.11% dissolution efficiency was achieved. The kinetics of the leaching was elucidated with the use of shrinking sphere model and was found to be diffusion-controlled. This shows that the high leaching of REEs under moderate conditions is attainable, offering eco-friendly citric acid as a substitute to recover these critical elements from PG waste. The present work tends to develop efficient leaching protocols for the recovery of REEs, which are indispensable raw materials in numerous technological applications.