The effect and implication of impurities on the calcination of α spodumene for lithium extraction

A suite of spodumene samples from albite-spodumene type pegmatites, mined for their lithium content, in the Archaean North Pilbara and Yilgarn Cratons regions of Western Australia, were examined to assess the influence of trace element impurities in spodumene and associated gangue phases on the thermal transformation of spodumene. Calcination of spodumene is required to convert the natural, monoclinic α-spodumene form into the tetragonal β-spodumene form, which is more amenable to recovering lithium during hydrometallurgy processing.

Spodumene contains minor concentrations of Fe (500–10,000 mg/kg), Mn (200–1400 mg/kg) and other trace element impurities incorporated within the crystal structure. Primary gangue mineralogy comprises quartz, Na/K-feldspar and mica, with secondary alteration predominantly as ‘sericitic’ phyllosilicates (muscovite-lepidolite, chlorite/cookeite mixtures) variably enriched in Fe, Mn, Mg and K relative to spodumene.

Primary and secondary mica undergo thermal dehydroxylation at temperatures (<950 °C) below the spodumene transformation temperature (970–1100 °C). Decomposed micas form melts that coat the surface and partially encapsulate the calcined spodumene grain surfaces. Feldspar decomposition at 1060 to 1200 °C, coincides with spodumene transformation, and can also result in melt formation, depending upon the composition of the feldspars (K-feldspar, albite). The thermal degradation of other mineral contaminants, such as biotite, pyroxene and amphibole from the presence of country rock (mafic, ultramafic) in the concentrate also coincides with the α- to β-spodumene phase transformation. The generated melts that coat grains can reduce the rate of α-β spodumene conversion and the subsequent ability to extract lithium from calcined spodumene.

Primary Fe and Mn impurities in spodumene, and those hosted by mica impurities within spodumene have a marked effect in decreasing the temperature of the α-γ-β spodumene conversion. Spodumene is not a strong conductor of heat, and the highly exothermic reaction of Fe and Mn oxidation within both mica and spodumene during thermal alteration affects thermal conductivity, leading to increased heat transfer within spodumene particles, which promotes the thermal transformation of spodumene at a lower temperature.

However, calcined spodumene particles with high Fe and Mn contents (> ∼ 0.5 wt%) showed black, open sintered regions, accompanied by the generation of fine (<5 μm) particles, and exsolution of Fe/Mn-oxides particles. The sintering and the generation of fines in the calciner will lead to reduced lithium recovery from calcined products.

This study illustrates the importance of minimising micaceous and feldspar components in the concentrate during the beneficiation stage, which can potentially lead to a decrease in lithium recovery during the extraction process. However, the fine-integrated nature of micas associated with spodumene alteration and those associated with fine fractures within spodumene are unlikely to be liberated without costly fine grinding and cleaning processes before the calcination process.

Alternative calcination processes that target reducing the effect of gangue materials on clinker formation and encapsulation of spodumene particles are available and discussed.