As large industrial process heat users, the CO2 emissions of alumina refineries are considered “hard to abate”. The digestion area, as the largest single energy consumer in the form of process steam, is critical to both minimising overall refinery energy consumption and delivering net zero carbon in the most economical way. In future, the traditional goal of maximising heat recovery from digestion will be supplemented with a similarly important goal of recovering waste heat from other refinery sources to digestion via mechanical vapour recompression (MVR) as a direct substitute for fossil-fuel based steam. Reviewing the digestion energy balance, single and dual stream heating technologies and incorporating refinery waste heat recovery, this paper explores digestion configurations that lower refinery energy consumption and carbon footprint. It is shown that incorporating waste heat recovery can reduce digestion energy input by more than half with single stream significantly outperforming dual stream. Increasing the number of live steam heating stages further improves this energy benefit.

Electric motors are indispensable in alumina Refineries, consuming a significant portion, typically 70% or more, of the total electricity. Despite various energy-saving initiatives like the use of Variable Frequency Drives (VFDs), energy efficient motors, power factor improvements, etc., there is still room for improvement. The operational hours of small and unmonitored motors remain largely unaddressed which ultimately increases the specific energy. Surprisingly the number and utilization of small motors (15 to 37 kW) is high in alumina refinery, especially those used for sump pit cleaning purposes. This type of area is typically controlled manually and not monitored on a regular basis which ultimately increases the overall specific energy consumption of the plant. Traditional pit level controllers, such as float and capacitance sensors, installed to optimize sump pump operation, often fail due to sedimented solids because of the nature of the process slurry. Consequently, the associated energy consumption increases. Monitoring motor load and underload currents can play crucial role in controlling operational hours. In our research, we have successfully utilized motor underload current monitoring to reduce more than 50% pump runtime, resulting in substantial energy savings by 70 kW to 80 kW per day per pump within the alumina refinery, thereby contributing to a carbon footprint reduction.

Improving the alumina calcination process with the objectives to reduce specific fuel energy consumption and enhance product quality has been the driver of recent technological advancements. Alumina quality in terms of parameters such as LOI, surface area and phase composition impact the efficiency of downstream smelting process. Gamma alumina is often considered the most desirable phase from a pure dissolution perspective; the operating parameters of the calciner has profound impact on the phase transformation reactions and final phase composition. In this work, calciner operation is optimized using computational fluid dynamics (CFD). The optimum thermal profile is obtained by simulating cases with various air-to-fuel ratios. Fairly good agreement has been obtained with the available temperature measurements that were recorded for model validation. The physical and phase analysis reveals that the final alumina is over-calcined with the current operational strategy. In addition, a non-uniform solid and gas flow is observed in furnace at a wide range of operating parameters. The CFD simulations indicate that for the particular calciner examined; more uniform air/solid distribution and mixing are achieved by maintaining an air to fuel ratio of approximately ~23. Onsite trials have been conducted to reduce the calciner temperature from 1140 deg.C to 1075 deg. C. The result indicated an increase of LOI and specific surface area from 0.4 to 0.7, and from 58 m2/g to 74 m2/g, respectively, thereby meeting the quality requirements of smelting grade alumina.

The chloride route for alumina production has recently gained renewed interest in R&D and industry. The EU-funded “SisAl Pilot” (no 869268) exemplifies this, utilizing HCl and AlCl3 for Al2O3 production. Using HCl allows for alternative raw materials such as anorthosite, kaolin, and by-products such as bauxite residues or fly ash to be utilized for alumina production. A 1977 study by the US Bureau of Mines identified the chloride route as a promising alternative to the Bayer process. As of late, this alumina production method has been used to produce specialized alumina products such as polishing suspensions.

However, handling HCl poses significant challenges, particularly in the roasting step where aluminium chloride hexahydrate (ACH) is converted to Al2O3 and HCl, with subsequent capture and recycle of the HCl. Scaling this process to an industrial level presents additional difficulties, such as feeding solid material into a reactor in an HCl gas atmosphere.

This paper aims to reintroduce this technology to the alumina community and to demonstrate the properties of Al2O3 samples produced by two calcination methods: direct calcination of ACH precipitate and spray roasting after re-dissolving ACH or using another AlCl3 source. One sample is produced by acid processing of calcium aluminate slag together with ACH precipitation, and the other by direct spray roasting of a Polyaluminiumchloride (PAC) solution. X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses are used to examine their distinctive properties.