Hydration kinetics of recarbonized lime

The hydration rate of partially recarbonized lime pellets has been studied by employing a single pellet into a stream of air/vapor water mixtures using a computer aided thermo-gravimetric technique. The hydration reaction takes place on a sharp, well defined interface between the product layer and the unreacted core. It has been found that the hydration mechanism consists in two steps. The first observes very slow hydration rates and the second higher ones. During the first step the water molecules diffuse through a close porous carbonate structure and the second one begins when these molecules reach the calcium oxide-calcium carbonate interface to continue through the lime core. A theoretical model was derived to explain the results of this study. The experimental results were acceptably well predicted with calculations performed using this model. On the other hand, it was found also that the higher is the lime calcination temperature the thinner is the recarbonized layer formed even under high CO₂ partial pressures. However, these thin layers are more effective to increase the hydration resistance of lime pellets than thicker ones. This is an important experimental finding since it indicates that low recarbonization extents are good enough to increase the hydration resistance of steelmaking lime without impairing its quality.