A kinetic model of the ‘Fe2+ oxidisation’ process for colour enhancement in natural marble

In this work, a study of the solid-phase, colouring reaction was undertaken in pure calcite and natural marble. Inorganic iron redox couples (Fe2+/Fe3+) and their redox product hematite (Fe2O3) produce a natural red colour on the treated surface of the marble. Pure calcite was used to simulate the marble matrix under colouring treatment. This simulation quantified the kinetics of the colour enhancement process as well as minimised the complexity of analysis caused by trace impurities in natural marble. The practical importance of this work was verified by following the colour change in a natural marble sample annealed at 380 °C. The observed kinetics is modelled using the well-known Avrami equation. A change in reaction mechanism was observed at around 400–420 °C and it is proposed that an annealing temperature of 380 °C be adopted. This provides a good practical compromise between maximising rate and the constraint imposed by excessive acid-gas evolution at higher temperatures. The activation energy of the reaction was estimated to be 398 kJ mol−1. This is within the normal range expected for solid-phase, redox reactions.