Kinetics of Solid–Solid Reactions in a Screw Reactor

This paper reports a detailed study on the analysis of reaction kinetics for solid–solid reactions relevant to continuous mechanochemistry. The reaction between crystals of NaCl and CuSO4.5H2O is carried out in a screw reactor. Interface mass transport and relation to shear are modeled. A mechanochemical reaction is dictated by two distinct rates: one being a constant inherent property of the reaction system, the time-dependent chemical diffusivity, and a variable rate governed by the time-invariant shear diffusivity provided by a constant shear rate in the screw reactor. The magnitude of the mechanochemical rate constant is predicted, and its dependence on the screw speed, screw diameter, and particle size are established. The mechanochemical reaction during and after removal of shear is studied, and the rate equation for the mechanochemical process and reaction handling in a screw reactor is tested. The sensitivity of the rate constant to shear and particle size depends upon the ionic interdiffusion rate at rest. The reaction in materials with a low activation energy of ionic jumps into an adjacent vacancy and thereupon high jump rates are less sensitive to shear, and conversely, in lattices with a high free energy of activation and low ionic transport rates, the rate constant is highly sensitive to shear, similar in behavior to the temperature sensitivity of the reaction, which increases with activation energy.