Kinetic study on the leaching of Pt, Pd and Rh from automotive catalyst residue by using chloride solutions

Recycling platinum group metals (PGMs) from waste and/or secondary resources is becoming quite important due to the limitation of the natural resource deposits of PGMs. Among the processes of PGMs' recycling from secondary resources, the hydrometallurgical method draws extensive concerns, particularly for the selection of leaching solution. The present work evaluated the ability of chloride leaching solutions, i.e. HCl-H2O2 and NaClO-HCl-H2O2 to extract Pt, Pd and Rh from a kinetic point of view. The effect of temperature in the range of 277-307 K and time were also examined under the conditions of -500 mu m of particle size and 100 g L-1 of pulp density. The kinetic model (I): ((1 - alpha)(-1/3) - 1) + 1/3ln(1 - alpha) = k(1)t was found to be the most suitable to describe the leaching process of Pt, Pd and Rh in the NaClO contained solution. In this model the interface transfer and diffusion across the leached support layer both affect the rate of leaching reaction. Model (II): (1 - (1 - alpha)(1/3))(2) = k(2) ln(t) fits the kinetic data very well for the dissolution of Pd in the HCl-H2O2 solution, showing that the diffusion across the leached support layer mainly affect the kinetic behaviors and the diffusion coefficient is inversely proportional to leaching time t. The NaClO contained solution and HCl-H2O2 Solution leaching of Pt, Pd and Rh are strongly dependent on the leaching temperature. In the case of the NaClO contained solution, the activation energy based on model (I) for Pd, Pt and Rh was 63.5, 59.1 and 77.9 kJ mol(-1), respectively. It is probably because of that the diffusion of NaClO contained solution across the leached support layer was very slow due to the micro-porous structure of the catalyst; and a high activation energy was needed in effect. Regarding to the leaching in the HCl-H2O2 Solution, the activation energy for Pd was 13.4 kJ mol(-1) on the basis of model (II).