By Rebecca Todesco, Editor, Mining Magazine
Efforts to attain global net zero targets are seeing the mining industry’s focus shift from traditional commodities – like coal and oil – to other minerals that Australia has in abundance, including alumina. A two-prong approach that mining contractors can take, in addition to focusing mining efforts on critical minerals, is ensuring that the processes involved in mineral refining are not emissions intensive. A report commissioned by the Australian Renewable Energy Agency (ARENA) provides a roadmap the industry can take to slash carbon emissions in alumina refining.
Australia is the world’s second-largest producer of alumina and the largest exporter of alumina globally, with the industry contributing $7-$8 billion to the national economy each year. Alumina’s main use is as raw material for aluminium smelters, although it has additional industrial uses. It can be used in porcelain, glass and other metallic paint manufacturing. It is also used as the fuel component for solid rocket boosters, in the manufacturing of spark plug insulators, as an abrasive, a filler for plastics, and in metal refineries to convert hydrogen sulphide waste gases into elemental sulphur.
There are a number of stages in the aluminium production chain, with the three main steps being bauxite mining, alumina refining and aluminium smelting. Bauxite mining is the least energy intensive of the three stages, with aluminium smelting, which relies heavily on electricity, using the most energy.
In 2021, as the largest producer of bauxite in the world, Australia mined over 100 million tonnes of the mineral and exported close to 40 million tonnes. More than 90 per cent of the world’s mined bauxite is used in alumina production.
The second stage in the production chain is alumina refining, and this can be broken down into two steps; the Bayer process and a high-temperature calcination process. The Bayer process is responsible for 70 per cent of the energy used at a refinery, with the calcination process claiming the remaining 30. Currently, alumina refining relies on fossil fuels to provide both heat and electricity.
The third stage is aluminium smelting and Australia is the sixth largest producer of aluminium globally. Aluminium is one the world’s most widely used metals, after iron and steel.
Australian alumina refining
The alumina refining process is extremely emissions and energy heavy. Compared to bauxite production and aluminium smelting, the emissions abatement path for alumina refining in Australia is less evident.
Australia has six alumina refineries – four in Western Australia and two in Queensland. Alumina refining and its associated processes are responsible for up to three per cent of the country’s annual emissions. To put it into perspective; Australia’s six alumina refineries consume more than twice the energy used by the entirety of Tasmania.
These kinds of figures have prompted alumina producers to seek out ways to cut emissions in the alumina refining process, including turning to Australia’s renewable energy for solutions.
Australia has an abundance of low-cost renewable energy resources and significant local expertise, both of which are necessary for this sector to achieve its net zero ambitions. Despite this, the process of transitioning alumina refining to renewable energy is not easy.
The roadmap
Published in 2022, A Roadmap for Decarbonising Australian Alumina Refining underscores the importance of alumina refining to Australia and its economy while also acknowledging the process’ difficulties in decarbonising.
The report, commissioned by ARENA and prepared by Deloitte, features insight and contributions from Australia’s three alumina producers – Rio Tinto, South32, and Alcoa.
Key decarbonisation technologies
The Roadmap identifies and explores four key decarbonisation technologies that could change the way refineries consume and use energy by enabling the uptake of renewable energy and eliminating the need for fossil fuels.
The four technologies are:
- Mechanical vapour recompression (MVR)
- Electric boilers
- Electric calcination
- Hydrogen calcination
Each of these technologies comes with its own benefits and constraints, with each at different levels of readiness for application in alumina refining.
According to the Roadmap, employing these technologies could provide credible pathways to cutting alumina refining emissions by up to 98 per cent.
The key decarbonisation technologies mentioned in the Roadmap are currently under development and investigation. As such, the report offers two abatement pathways: the Innovator Abatement Pathway and the Gradual Abatement Pathway.
The former would see the technologies applied by refineries over the course of five years, once they have reached technology maturity. As part of this pathway, the assumption is that there are no commercial or technical barriers to investment.
The latter of the two involves technologies deployed over ten years once they have reached technology maturity. This pathway takes into consideration staged deployment, evolving regulatory frameworks and potential capital constraints that may emerge. Despite this, the pathway is still in line with achieving net zero by 2050.
Mechanical Vapour Recompression (MVR)
The Bayer process of an alumina refinery is when bauxite is mixed with caustic soda – or sodium hydroxide – and then heated under pressure. In order to meet this heat demand, fossil fuel boilers are used to produce steam.
This steam pressure is between 5,000kPa and 10,000kPa – 325°C to 400°C – for refineries with high temperature digestion, and 600 to 800kPa –175°C to 230°C – for low temperature digestion systems.
The difference between traditional operating systems and MVR is that the MVR process captures waste water vapour at relatively low pressure and temperatures, and recompresses it through a series of turbo fans and compressors, returning it to the temperature and pressure needed for the Bayer process.
The compressors in the MVR process capture the energy in the waste vapour system, acting as a highly effective means of providing heat. Compared to an electric boiler, only a relatively small amount of energy is required to recompress the captured vapour to the necessary pressure and temperature for the Bayer process – one third to be exact.
Without the MVR process, this waste heat would otherwise be lost to the atmosphere. In addition to this, the MVR process reduces water losses incurred throughout the Bayer process, and reduces demand for fossil fuel-fired boiler steam. Estimated water savings of approximately 5.2GL per year are expected if MVR processes are adopted by all Australian refineries.
As an additional step, renewable electricity-powered MVR for the Bayer system could pave the way for zero emissions heat in the Bayer process.
The recovery of waste energy means the MVR process is substantially more efficient than a traditional boiler and the renewable energy required is far less than the fossil fuel energy being displaced. Displacing the combustion of fossil fuels with renewable electricity for process heating and steam generation in the Bayer process could reduce refining emissions by up to 70 per cent.
In order to realise the full abatement potential of MVR technologies, significant capital is required. However before this investment can occur, the technology must be proven.
In May 2021, ARENA announced $11.3 million in funding to Alcoa to demonstrate MVR technology at its Wagerup alumina refinery in Western Australia, with this project a first-of-its-kind demonstration of MVR in Australia.
The technical and commercial viability of integrating renewable energy-powered MVR into Australian alumina refining processes was studied, with the results finding that the technology is feasible. Alcoa will progress to the next stage of the project by installing a 4MW MVR module at the Wagerup alumina refinery.
Electric boilers
Steam generation is responsible for approximately 70 percent of the carbon emissions of an alumina refinery. Current practice employs the use of fossil fuel fired boilers to generate the steam required. However, in lieu of fossil fuel boilers, electric boilers can be used to provide the primary steam needed in the alumina-refining process, with electric boilers suitable for use instead of traditional gas or coal fired boilers in both high and low temperature refineries.
As it stands, electric steam generation technology operating at the necessary pressure for high temperature refining has not yet been commercially proven. Despite this, it is still considered to be the most prospective decarbonisation technology to produce steam for high temperature refineries.
Electric boilers tend to have greater operating costs than current fossil fuel burners, and because they are quite a mature technology, there is less potential for further cost reductions through innovation. To make electric boiler technology feasible, other financial support processes, such as access to low-cost renewable energy may be necessary.
Whether used alongside MVR technologies or as an independent option in situations where MVR is not possible, utilising electric boilers – especially when powered by renewable energy – instead of fossil fuel fired boilers could significantly reduce or eliminate emissions in the Bayer process of alumina refining.
Electric calcination
The chief purpose of electric calcination is to replace the burning of fossil fuels with electric heating. Similar to hydrogen calcination, electric calcination produces pure steam that can be captured and reused in the Bayer process. This mitigates the need for other MVR-generated or other boiler steam.
Upon implementation, electric calcination is expected to need thermal storage to guarantee a continuous supply of thermal energy to calciners, with this thermal energy also offering refineries added flexibility to decrease grid electricity imports during peak pricing events. This, in turn, improves the overall economics of electric calcination. With this additional load flexibility, refineries are also able to participate in demand management.
Electric calcination is currently in the early stages of development, with low technology readiness. It is capital intensive and needs large-scale, low-cost, renewable electricity to be considered viable.
Investigations into the techno-economic feasibility of electric calcination technology are in progress through Alcoa’s pilotscale demonstration at its Pinjarra Alumina Refinery.
Alcoa’s project is the first of its kind and secured $8.6 million of funding from ARENA and $1.7 million from the Western Australia Clean Energy Fund. As well as seeking to increase understanding of the techno-economic feasibility of renewable energy-powered electric calcination, the study is aiming to improve understanding of the economic benefits that renewable powered electric calcination can achieve.
When used in conjunction with MVR technologies, electric calcination has the potential to reduce a refinery’s carbon emissions by up to 98 per cent and energy intensity by approximately half, as well as reducing water consumption.
Hydrogen calcination
The calcination process accounts for up to 30 per cent of alumina refining missions, and replacing natural gas in this process with renewable hydrogen is a significant step towards eliminating emissions.
Oxy-firing – the process by which hydrogen is combusted directly with oxygen – releases pure steam as a combustion product, with supplementary steam also generated as a consequence of the calcination process by removing chemically-bound water from the alumina trihydrate.
This steam is suitable for potential use in the alumina refinery, whereby MVR can capture and recycle this steam in the Bayer process, ultimately reducing steam production and water consumption, and improving energy efficiency.
Hurdles for decarbonisation
Contributions and input into the development of ARENA’s report were made through a series of workshops. During these workshops, participants raised and discussed some key barriers.
One of the consistently raised barriers to achieving decarbonisation was the uncertainty concerning the treatment of emissions under emerging regulatory or market frameworks, and the uncertainty of future energy price trajectories. These gave rise to concerns about the risks associated with being an early mover.
Another key concern was the need for significant investment in generation, and transmission and technology infrastructure. The necessity of such high investment might be off putting for refineries.
The ongoing labour and skills shortages around the country were highlighted as another potential barrier, as the skills required for the energy transition are in high demand across many industries in Australia, especially with the increase in hydrogen and solar projects and critical minerals exploration.
The key technologies’ requirement of a firmed renewable energy supply was also identified as a barrier, as each of the technologies require renewable energy to power the low emissions technologies and/or to produce renewable hydrogen.
The alumina industry, while playing a crucial role in Australia’s economy is one of the nation’s hard-to-abate industries. The need to cut emissions in alumina refining is recognised and significant technologies are in development or undergoing feasibility studies to assess their viability in alumina refining.
Although A Roadmap for Decarbonising Australian Alumina Refining offers some great insight into decarbonisation, a united effort is needed by industry in order to reduce carbon emissions in alumina refining, further helping Australia to meet its zero emissions targets.
