Phase transformation mechanism of spodumene during its calcination

This contribution provides a detailed in-situ account of transformation reactions during calcination of a typical high-grade α-spodumene (α-LiAlSi₂O₆) concentrate, a pre-treatment step required to refine spodumene into commercial lithium chemicals. We observe four reaction pathways during the transition of spodumene, employing in-situ high-temperature powder XRD measurements using both cathode-tube and synchrotron radiation. At a relatively slow heating rate of 8 °C min⁻¹, we observe a close relationship between the development of γ-spodumene, with an onset temperature of 842 °C, and reduction of the amorphous background, in the collected XRD spectra. This demonstrates that, initially γ-spodumene recrystallises from amorphous spodumene. At the fast initial heating rate of 100 °C min⁻¹, γ-spodumene first appears at a higher temperature of 1025 °C. This mineral subsequently transforms into β-spodumene at high temperatures along the reaction pathways denoted as pathway (1) amorphous spodumene → γ-spodumene → β-spodumene and (3) crystalline α-spodumene → γ-spodumene → β-spodumene. The stability of γ-spodumene strongly depends on the mechanical treatment of the sample, and the heating rate of the calcination process, suggesting high and low activation energies for pathways (3) and (1), respectively. In another experiment, we observe rising peaks of β-quartz, a minor gangue mineral in the spodumene concentrate, that reflect the substitution of Li⁺ and Al³⁺ for Si⁴⁺ above 875 °C. This phase ultimately transforms to β-spodumene at 975 °C. The same experiment demonstrates the spectrum of β-spodumene continuously increasing in magnitude, above 975 °C, with decreasing abundance of α-spodumene, indicating a direct conversion of α- to β-spodumene. Thus, the two other reaction corridors comprise: (2) crystalline α-spodumene → β-quartz_ss → β-spodumene; (4) crystalline α-spodumene → β-spodumene. Heating of a finely ground sample results in faster and more-complete conversion of α-spodumene compared to a coarser specimen. Our experiments establish the characteristic temperatures of phase transformations during spodumene calcination and reveal the influence of amorphous material and thermal history on reaction sequences. Approaches that integrate the optimisation of grinding and heating thus bear the potential to reduce the energy requirements of the calcination process, including the extraction of lithium from γ-spodumene formed at a lower temperature.