Leaching Characteristics of Lanthanum from a Secondary Resource Using Inorganic and Organic Acids: Emphasizing the Citric Acid Kinetics

Spent hydro-processing catalysts are classified as toxic and dangerous materials because they still contain heavy metals, such as lanthanum (La). Thus, these spent catalysts need to be processed so they do not pollute the environment. This study focused on studying the effect of several parameters in the lanthanum leaching process and continued studying the leaching process’ kinetics. A selective leaching process towards lanthanum was carried out using inorganic acids (1 M H2SO4, HCl, HNO3) and organic acids (1 M C6H8O7, CH3COOH). The effect of leaching temperature (30 °C, 50 °C, and 70 °C), pulp density (5% w/v, 10% w/v, 20% w/v, 30% w/v, 40% w/v, and 50% w/v), and different citric acid concentrations (0.01, 0.05, 0.5, and 1 M) were also studied to obtain the optimum conditions. From this study, citric acid was an effective reagent for leaching lanthanum. Up to 100% of lanthanum can be recovered at 1 M acid, a 20% pulp concentration, and a 70 °C leaching temperature. As a green leaching agent compared with H2SO4, HCl, and HNO3, citric acid could give a prospectus method to recover the lanthanum from the spent hydro-processing catalyst. Moreover, the kinetics of lanthanum leaching was studied to determine the controlling mechanism and the kinetics factors using the shrinking core model. The obtained values of Ea and Ao were 29.81 kJ/mol and 30.51 min−1, respectively. The application of citric acid in the lanthanum leaching process provides significant input in the scale-up production of lanthanum.