Kinetics of spodumene calcination (α-LiAlSi₂O₆)

Kilns consume about half of the energy necessary to operate lithium refineries and their decarbonisation requires accurate modelling of the calcination of spodumene concentrates fed to the process. This contribution applies the isoconversional methodology to investigate the kinetic parameters of the transition of α-spodumene to its high-temperature polymorph of β-spodumene, using the heat flux measurements from the differential scanning calorimetry (DSC). The normalised energy demand (α_H), presented as a function of temperature, characterises these measurements. The activation energy E_α and the product of the reaction model f_α and the frequency factor A_α, (fA)_α, depend on α_H. As the process involves multi-step reactions, we deploy the Friedman differential method and the accurate flexible integral method of Vyazovkin to obtain the kinetic parameters. We also modify the method of Ortega to acquire additional estimates of E_α and (fA)_α and apply the rigid integral method of Starink for comparison. The Friedman, Vyazovkin and modified methods deliver the same estimates of the kinetic parameters within their error bands. The Starink method works surprisingly well for predicting the conversion time despite the inaccuracies in the derived values of E_α and (fA)_α. This comes to pass because of the compensation effect between these parameters. The activation energy declines rapidly from around 1000 kJ mol⁻¹ at the commencement of the heat treatment to 668 kJ mol⁻¹ at α_H = 0.22, then, decreases gently to 577 kJ mol⁻¹ at α_H = 0.98, during successive recrystallisation events. Average uncertainties in these results amount to 13 kJ mol⁻¹. The frequency factors fall between 58.5 (±1.0) min⁻¹ and 51.0 (±3.2) min⁻¹, as computed at α_H = 0.23 and 0.98, respectively. The so-called false compensation analysis reveals that the first-order reaction model (in α_H) governs the energy demand for calcination for α_H ≥ 0.23, but, initially, the transformation proceeds through the dissociation-diffusion regime that is not part of the established reaction models. This regime must not be ignored in modelling the calcination of α-spodumene, as it consumes around 20 % of energy required for the transformation reactions. The results reveal significant differences in the predictions of the treatment time, by more than two orders of magnitude, from the existing kinetic models, and explain the differences by the experimental conditions to collect the data for the models. The dissociation of Si-O bonds and diffusion of Si⁴⁺ ions out of their tetrahedral cages govern the onset of the thermal treatment of α-spodumene and account for the elevated values of the activation energy in the dissociation-diffusion regime. The two recrystallisation events are limited by the multicomponent diffusion, especially Si⁴⁺, in partly crystallised structure. The recrystallisation of γ- to β-spodumene defines the required retention time of concentrate particles in the kiln, to maximise the effectiveness of the subsequent recovery of lithium from the treated material. Fitting E_α and ln(fA)_α to polynomials allows a convenient integration of the model equation for making predictions under any heating program. The model forecasts well the transformation of particles of α-spodumene characterised by d₈₀ = 315 µm, studied in additional experiments, using the X-ray powder diffraction to quantify the conversion of α-spodumene. The predictions from the isoconversional model also concur well with other conversion measurements available in the literature, within the expected variability of different spodumene concentrates.