Effects of mechanical activation on the kinetics of terbium leaching from waste phosphors using hydrochloric acid

The effect of mechanical activation (MA) on the kinetics of terbium (Tb) leaching from waste phosphors using hydrochloric acid was investigated. Leaching kinetics, such as apparent reaction rate, activation energy and reaction order, were determined using the shrinking-core model and the Arrhenius equation. Results obtained from experiments with different concentrations of HCl and under different leaching temperatures were used for the determinations. The impacts of factors such as rotational speed, HC1 concentration and leaching temperature on the leaching rate of Tb were also discussed. The results showed that MA could dramatically increase the leaching rate of Tb from waste phosphors, and the apparent reaction rate (k) of leaching was accelerated as well. For inactivated waste phosphors, the apparent activation energy (Eap) was 52.82+/-3.95 kJ/mol, indicating that the rate-controlling step of the leaching process was the chemical reaction. The Eap dropped to 25.96+/-3.90 kJ/mol and 10.96+/-2.79 kJ/mol when the waste phosphors were mechanically activated at rotational speeds of 400 and 600 r/min, respectively; the leaching process transformed to a hybrid (chemical-reaction and diffusion) control process, and even a reagent-diffusion (through the product layer) control process. The apparent reaction order for Tb leaching from 400 r/min-activated waste phosphors was 2.49+/-0.11, and it decreased to 1.16+/-0.17 when the rotational speed of 600 r/min was used. Kinetics results indicated that MA could make Tb leaching occur spontaneously, and the activation intensity of waste phosphors was strengthened with higher rotational speed.