α → γ → β-phase transformation of spodumene with hybrid microwave and conventional furnaces

Spodumene is the most important lithium containing hard rock mineral. In order to extract lithium from spodumene concentrate by leaching, the crystal structure of spodumene must be converted from the natural monoclinic α-form to the tetragonal β-form. The technical possibilities to generate the heat to the conversion process of spodumene concentrate via microwaves were studied. The heat treatment experiments were carried out with a domestic microwave furnace (700 W) with silicon carbide susceptor and with the conventional resistance heated furnace as a reference. In the microwave furnace the phase transformation of spodumene began after 110 s of heating and samples were converted almost completely to β-spodumene after 170 s. Partial melting of gangue minerals was observed in samples after 170 s of heating. Heating in microwave furnace for 170 s corresponded with the heating of approximately 480-600 s at temperature of 1100 °C in the conventional furnace. In addition to α- and β-forms an intermediate phase, hexagonal γ-spodumene, was identified from samples heated with both furnaces. The conversion of spodumene samples was verified with X-ray diffraction (XRD) and with field emission scanning electron microscope (FESEM).