The effect of optical properties and heating rates on the solar-driven Bayer process: An experimental and numerical analysis

To transition to a sustainable energy society, renewable energy must become predominant across all sectors of our economy, including metals and minerals refining. Refining processes are very energy intensive, motivating further research and development (e.g., solar calcination for converting aluminium hydroxide derived from bauxite ore into anhydrous alumina at >1,000°C). Using direct concentrated solar energy for this process is theoretically possible, but the effect of transient optical properties and the impact of heating rates on chemical phase transitions are under-explored. This experimental study investigates the close interconnection between these aspects and the potential for solarisation of aluminium refining by examining: (i) The spectral optical properties of aluminium hydrates and anhydrous aluminas; (ii) The dehydration pathways of high-purity aluminium tri-hydrate under concentrated solar conditions; (iii) The impacts of solar additives on heating rates and system efficiency. It was found that low solar absorption of aluminium hydroxide (only ∼15% solar weighted absorbance) restricts desirable alumina phase transitions. However, non-reactive, re-usable additive absorbing particles (i.e., 5wt.% CarboHSP, 92% solar weighted absorbance) can significantly enhance heating rates, temperatures, and thermal efficiency. Importantly, controlled optical properties boost the proportion of valuable γ and θ alumina phases, thereby overcoming a techno-economic bottleneck to solar-driven refining.