Kinetics of vanadium dissolution from black shale in pressure acid leaching

The leaching kinetics of vanadium from black shale in the sulphuric acid-oxygen system is presented. The effects of agitation speed, leaching temperature in the range of 110-150 °C, sulphuric acid concentration, oxygen partial pressure and particle size on the rate of vanadium leaching were determined. The results indicate that the rate is nearly independent of agitation above 200 rpm and increases with increasing temperature, sulphuric acid concentration and oxygen partial pressure. As leaching occurs, there is a progressive dissolution of a vanadium-bearing alumino-silicate phase, while the inert quartz phase assembles onto the mineral surface and remains as an “ash” layer. The leaching kinetics was analyzed by using a new variant of the shrinking core model (SCM) in which both the interfacial transfer and diffusion across the product layer affect the leaching rate. The determined activation energy was found to be 40.14 kJ/mol and the reaction orders with respect to sulphuric acid concentration and oxygen partial pressure were 0.61 and 1.67, respectively. A semi-empirical rate equation was derived to describe the process.