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Decarbonising Aluminium

Decarbonisation of the aluminium industry is in the news (1,2) with Rio Tinto recently announcing a $1.2 million feasibility study aimed at decarbonising part of the alumina refining process.  The Australian Renewable Energy Agency, an independent agency funded by the Federal government, will contribute $0.6 million to the cost of this project.  In the overall scheme of global decarbonisation this is not a major development but it prompts us to look at the carbon footprint and decarbonisation pathways of the aluminium industry.

Aluminium is an interesting decarbonisation case study from a global perspective for a number of reasons: 

(1) it is an important material that plays a key role for all advanced economies (ie aluminium will continue to be needed in a post carbon world), 

(2) like many major industrial products, it’s contribution to global CO2 emissions in not insignificant –  according to the World Aluminium Association about 2% of global GHG emissions result from aluminium production processes (3) 

(3) there are real technical challenges that need to be overcome to make the aluminium zero carbon.

It is also relevant from an Australian perspective because not only do we have a large domestic aluminium industry that employs lots of Australians and contributes meaningfully to export earnings but also because it is an industry that, according to many, often conservative, commentators, is at risk from net zero carbon policies .  

To help us understand the challenges facing the Aluminium industry – on both a global and national basis – let’s first summarise the carbon generating activities associated with the production of aluminium.  Once this is done we can look at what work is needed to reduce and eliminate these emissions.

Aluminium production overview – Australia is big in Bauxite and Alumina but China is the dominates the overall industry

There are three basic steps in the production of primary aluminium

Bauxite mining – bauxite is the ore used to produce aluminium.  Australia is the leading global bauxite producer by volume.  Mines in Queensland and Western Australia are responsible for 33% of the world’s total production, two thirds of which is processed domestically in Australia with the balance exported, mostly to China

Alumina Refining – bauxite is processed (refined) into alumina as an intermediate step prior to the production of aluminium metal.  Australia is the world’s second largest alumina producer with about 15% of global output.  Most of this is exported – again mostly to China.  China is the largest alumina producer with about 50% of total production.  

Aluminium smelting – alumina from the refining step is the feedstock for the aluminium smelting process.   Unlike bauxite mining and alumina production where Australia is a major player, our share of metal production is only roughly 3% of the global total.  China is the largest producer with close to 60% of total world output.

Emissions profile – Smelting is the heavy hitter but there are meaningful emissions from other parts of the process chain

Bauxite mines – bauxite mines are open cut, surface mines meaning heavy, mobile equipment (trucks, shovels, draglines etc) is used to remove overburden, extract the bauxite and transport it for crushing and storage.  The mines in Australia have relatively low levels of overburden so they are smaller in terms of material moved/tonne of product when compared with other mining operations – for example copper, gold, coal and iron ore.  Bauxite mines have other pretty standard mine site mineral processing equipment – large scale crushers, sizing and storage infrastructure allowing finished product to be loaded onto ocean going vessels.  

Alumina Refineries – the refining of bauxite into alumina uses a preferential dissolution process that is more or less unique to the aluminium industry.  This process relies on the fact that aluminium oxide dissolves in Sodium Hydroxide (NaOH) at temperatures of around 150 – 250C allowing it to be separated from undesirable components of the ore which do not dissolve.  After the dissolved aluminium oxide has been cooled and converted back into solid form, it is then calcined to remove chemically bound water and stored prior to transport. One feature of Alumina refineries is the generation of an environmentally nasty by-product called ‘red mud’ – this is highly alkaline and contains the bauxite impurities that are removed in producing alumina.  It is typically stored in settling ponds adjacent to the refinery and gradually (and hopefully)  remediated into inert landfill. 

Aluminium Smelters – aluminium is often called “solid electricity” because the electrochemical process to convert aluminium oxide (alumina) into metal uses massive quantities of electricity.  In addition to the consumption of electricity, aluminium smelting produces other GHG emissions.  These include CO2 as well as small volumes of fluoride containing gases that have a very powerful climate warming impact.

Emissions estimates and decarbonisation pathways – mining and refining

The International Aluminium Institute (3) estimates that the industry – from mining to smelting – produced 1,068 billion tonnes of GHG emissions per year.  The split between processing steps was estimated to be

Mining – 3 billion tonnes (0.3%)

Refining – 204 billion tonnes (18.4%)

Smelting (including anode production and casting) –  862 billion tonnes (81.3%)

Note that these emission estimates exclude transport between the different phases of the production cycle and also exclude downstream processing into finished products – rolling, extrusion, casting etc.  While emissions from these steps are non zero they are relatively minor compared to the key processing steps outlined above.

Mining bauxite is clearly a negligible part of the aluminium story.  Decarbonisation will require replacement of diesel powered mobile equipment with a new fleet powered by batteries, bio-diesel or hydrogen and electricity supplied by the grid will need to come from zero carbon generation.  While there are plausible options to reduce the carbon footprint of large scale surface mines, this is still in the development / prototype stage and the cost to retrofit the global fleet of haul trucks and excavators, not just for bauxite mining but all large scale mining operations, will not be easy or cheap.  

Energy usage in alumina refining is mostly to create the heat inputs needed for the process. The Bauxite dissolution process requires millions of litres of caustic solution to be continuously  heated to temperatures in the range 150 – 250C and the final product calcination step requires temperatures in excess of 1000C.  Heat for the dissolution process is typically provided by steam generated by on site coal or natural gas boilers while the high temperatures required for calcination invariably mean direct fossil fuel combustion (i.e. large, furnace scale natural gas or coal burners). 

Neither industrial steam production or achieving high temperatures by direct combustion are unique to alumina refining.  Many other industries require either process steam (e.g. food, pharmaceutical and chemical production) or high temperatures approaching 1000C  (e.g. cement, glass, chemicals and other industrial minerals).  Conceptual decarbonisation pathways include electrification using zero carbon generation (most suited for smaller scale applications) and where higher temperatures or higher intensity inputs are required, green hydrogen, concentrated solar thermal energy and carbon capture utilisation and storage (CCUS) are being considered.  The Rio / Arena study will look at adapting one of these ideas to the central Queensland refineries.

The good news for those seeking to decarbonise alumina refining is that conceptual decarbonisation pathways exist and the generic nature of the energy requirements means that technology developments and innovations made in unrelated industries will be transferable and customised to specific refineries in Australia and elsewhere.  The bad news is that even with these conceptual pathways, decarbonisation remains a complex and challenging task for large scale, energy intensive industrial installations like alumina refineries.

One challenge is that large process plants often need reliable and consistent energy inputs.  Processes that run at elevated temperatures – using furnaces, kilns, digesters – typically want to avoid being turned on and off.  As a general rule operational efficiencies, productivity, equipment life and product quality will be negatively impacted if the equipment is forced to run intermittently.  This is a key feature of aluminium smelting, as will be discussed further below, but it also applies to alumina refining and indeed many other processes.  In broad terms this makes supply from intermittent sources like solar inherently problematic and will force plant managers to seek energy supply options and locations with the lowest likelihood of interruption.

A second, perhaps related point, is that retrofitting new, cutting edge technology into a complex, large scale, mature operation whose cost structure and safe operation relies on maintaining a stable process with a consistent, high quality output will not be easy.  The fact that a spreadsheet suggests an aluminium refinery can be converted to run on green hydrogen (a fuel source that in practical terms does not exist) or concentrated solar (another “in the pipeline” product) will not make replacement or retrofits easy, cheap or low risk.  Climate advocates will make the point that a national plan for getting to net zero is a good place to start when addressing these challenges.  That is probably correct but it will be interesting to see if there is condemnation of government assistance to “big polluters” who, in many cases, will need both carrot and stick regulations if they are to make massive investments in new, potentially untried equipment.  

Bottom line – decarbonisation can and will happen for complex industrial applications but it won’t be easy, cheap or without some facility owners either choosing to close up and move elsewhere or losing cost or quality advantages.

Emissions estimates and decarbonisation pathways – smelting (a major challenge)

The smelting process is responsible for the bulk of the GHG emissions associated with the production of aluminium metal.  As noted above, smelting is estimated to be responsible for 862 million tonnes of GHG emissions out of the industry’s total of 1068 billion tonnes with 671 million tonnes coming from emissions associated with electricity supplied to the smelting process.  The data below shows that 60% of smelter capacity is linked to coal fired generation and 93% of the 671 million tonnes of GHG emissions associated with electricity supply.  There is no mystery where the decarbonisation priority lies.

Coal fired generation (eg China) = 60% smelter output / 93% GHG emissions

Gas fired generation (eg Middle East) = 11.8% smelter output / 5% GHG emissions

Others (nuclear, renewables, mixed generation) = 5.9% smelter output / 2% GHG emissions

Hydro generation (eg Canada) = 22.5% smelter output / 0% GHG emissions

Electricity Supplyif the first step in decarbonising aluminium production is to move away from coal then instinctively one would think that converting to renewables is an obvious move strongly  aligned with broader national and global grid decarbonisation activities.  

This instinct would be broadly correct but there is plenty of nuance in navigating this shift.

Green electrons will work just as effectively as those from coal or gas power stations so at a fundamental level there are no problems running an aluminium smelter on renewable and/or low carbon power.  In fact, the smelting industry historically (i.e. prior to China’s domination of the industry) relied heavily on hydro power. 

Smelter operators will, however, be somewhat nervous about an over reliance on intermittent wind and solar sources.  Aluminium smelting is extremely intolerant to even short interruptions to the supply of electricity. Losing power for more than an hour risks catastrophic damage to the smelting furnaces (pots) and even a partial decrease in current will cause major process disruptions and loss of efficiency.  Variations in power supply also result in an increase in fluoride byproducts that are very active from a GHG perspective. 

For situations where power is supplied by a large, third party generator it seems inevitable that commercial power supply negotiations will drive an industry realignment.  Power producers being forced to reduce their carbon footprint will need to assess whether they can continue to agree to the supply guarantees that smelters require.  This commercial tension will logically drive smelters away from locations where generators have concerns over supply continuity in favour of jurisdictions with a more reliable power supply.  One imagines this will favor regions with strong hydro and nuclear generation though it is possible that a high capacity storage capability will support smelting capacity running on renewables.

Adding an additional layer of complexity is the fact that only 45% of smelter power purchases (4) come from third party power providers.  The remainder – 55% of smelter output – use self generated power, typically from an adjacent coal mine/power station facility.  In many instances, this configuration effectively monetises a (potentially stranded) coal asset by turning the coal into aluminium. In addition, the smelter owned power station will often also supply power to nearby communities.  Unlike a large scale grid operator with multiple generation options and with a diverse customer base, smelter specific coal fired generators are likely to have very little scope or capability to shift to low carbon sources.  A significant portion of global aluminium smelting is therefore literally tied to high emissions generation supply and if the world is to meet its decarbonisation targets this smelting capacity will need to close.  Adjacent communities who lose access to power will be hoping for government support to fund replacement power supply.

Process Emissions from Aluminium Smelting – while decarbonising the electricity supply is the first decarbonisation priority for an aluminum smelter here are also some other emissions streams that need to be considered.

The basic chemistry of the smelting process is the reduction of aluminium oxide in the presence of carbon anodes to produce aluminium metal and carbon dioxide (CO2)

2Al2O3 + 3C → 4Al + 3CO2

This basic chemistry produces CO2 emissions and in combination with other carbon losses due to air burn as well as emissions from the production of anode raw materials produces roughly 150 million tonnes of CO2/year.  This is well short of the overall generation emissions but means that even with zero carbon electricity the current aluminium smelting process is not net zero.  Smelter operators will also be conscious that sourcing the raw materials for consumable anodes, petroleum coke and coal tar pitch, is likely to become increasingly problematic as fossil fuel usage is constrained.  To address these risks and to reduce smelter GHG emissions, research is underway to replace consumable carbon anodes with non consumable, inert anodes.  This approach will eliminate GHG emissions from a chemistry perspective by producing oxygen gas (O2) rather than CO2 and also ease future raw material supply issues.  

The commercialisation of non consumable inert anode technology is, however, a huge technical challenge – the Hall Herout smelting process, based on carbon anodes, has fundamentally not changed in over 100 years.  The primary challenge is finding materials that have the right combination of electrical conductivity, high temperature resistance, low reactivity with molten aluminium and fluoride salts while being economically viable.  For smelting technologists this is a bit like the quest for the holy grail.  At the moment it seems more likely than not that aluminium smelting will need to rely on carbon anodes and either use carbon capture and storage to eliminate the CO2 emissions or have governments accept that these are a category of unavoidable emissions that can be netted out against land usage sequestration activities.

The final source of smelter GHG emissions are the fluoride containing gases that are by-products of the smelting process, particularly when the process is not well controlled. These gases, typically a mixture of carbon tetrafluoromethane (CF4), and carbon hexafluoroethane (C2F6), have GHG warming factors of 6,500 and 9,200 respectively.  This means that 1 kg of these gases is equivalent to 6.5 or 9.2 tonnes of CO2.  Small quantities of these gases therefore have a disproportionate warming impact.  

Production of these carbon based gases will obviously be eliminated if inert anode technology can be commercialised but even without this innovation the volumes of these gases emitted due to poor cell operation has decreased dramatically over the past few decades with the advent of advanced computer controls and the replacements of aging smelters with newer facilities.  

The generation of CF4 and C2F6, despite their high impact on atmospheric warming, is currently a relatively low priority issue.  It is worth noting, however, that maintaining a stable electricity supply is a key parameter in minimising the generation of these gases. 

Conclusion 

Technical – Decarbonising the aluminium industry will be an interesting mix of the evolution of a zero carbon electricity, the development of low carbon technologies for generic processes such as industrial steam and high temperature calcination and potentially the roll out of some transformative smelting changes that will be unique to the industry.  None of this will be easy but work in all these areas is underway and one assumes aluminium will become progressively greener in the next few decades.  Whether this gets the industry to net zero by 2050 or not remains to be seen and progress will depend on both regulatory changes and successful technical innovation and implementation.  Carbon capture and storage remains a possible remedy for some emission categories that can’t be eliminated by other means. 

Political Implications – Australia

Australia should continue to play a leading role in bauxite mining with future risks more a function of mine life and cost than carbon pressures.  Bauxite mining looks to be well down the list of decarbonisation concerns for Australia.  

The future of Australia’s large alumina refining franchise should also be reasonably secure although investment in low carbon technology and continued community and government support will be needed.  The motivation of refinery owners, dominated by Alcoa/Alumina, Rio Tinto and South 32, will be critical – especially if there is a suggestion that building a new greenfield refinery (possibly outside Australia) could be cost effective compared with complex and expensive brownfield retrofits.  The owners will also be aware that alumina refineries are not pretty places with waste streams that create genuine, localised environmental challenges.  Investment decisions will be sensitive to opposition from community groups or changes to site permits issued by state or federal regulators.  More optimistically, refineries could become customers for a local green hydrogen industry or even advanced solar technology, a scenario that governments may be keen to encourage.

Australia’s aluminium smelting capacity, on the other hand, looks to have a less certain medium term future.  As has been discussed elsewhere in this blog (5), without nuclear and with a relatively small hydro generation capacity, a decarbonised Australian grid will have limited baseload generation and be very reliant on renewables with storage.  Given the inevitable risk of supply droughts during the winter, this is not a configuration that suits aluminium smelting. It will be very surprising if the three Australian smelters that use coal fired electricity last into the 2030’s, while the future of the aging Bell Bay smelter in Tasmania may depend on competing needs for its roughly 300 MW of hydro power (1% of Australia’s total power and about 15% of its hydro capacity).

International perspective

Internationally, decarbonisation of the aluminium industry starts (and probably stops) with China.  With over half of global refining and, more importantly, smelting capacity,  China’s actions will determine how quickly the industry overall can decarbonise.  There are obviously clear parallels and overlaps with decarbonisation more generally and prompt questions like 

Can China decarbonise its electricity grid? 

Will China seek to maintain its dominant world position as an aluminium producer or close some of its capacity as it turns away from fossil based power?  

As other countries develop low carbon electricity will they see aluminium smelting as an attractive industry?

Will China (or perhaps middle eastern nations) take the lead in trialing carbon capture and storage for aluminium smelting?

One possible game changer would be the introduction of tariffs related to the carbon footprint of goods crossing international borders.  Aluminium produced from hydro power has perhaps one third of the GHG emissions of that produced from a coal fired smelter – if this results in lower import tariffs it could encourage a mitigation away from coal linked smelters.  The fact that a significant amount of Chinese aluminium is consumed internally will make this more complex and would require the tariffs to include manufactured goods containing high carbon aluminium.

At a higher level, a discussion of the decarbonisation challenges facing the aluminium industry showcases the types of issues that many industries and governments are facing in implementing net zero policies.  Technically almost anything is possible with enough time and money but for some refineries and smelters the journey to zero carbon means they will need to close causing collateral damage (and some celebration) in local communities.  For other refineries and smelters, decarbonisation should mean massive investment, with new jobs and opportunities.  This dynamic will be replicated for dozens of other industries all of which face a mixture of generic and industry specific challenges.

  1. https://reneweconomy.com.au/rio-tinto-to-investigate-use-of-renewable-hydrogen-in-alumina-processing/
  1. https://www.smh.com.au/politics/federal/rio-tinto-push-for-green-aluminium-gets-600-000-government-boost-20210615-p5818c.html
  1. https://www.world-aluminium.org/media/filer_public/2021/04/01/iai_ghg_pathways_position_paper.pdf
  1. https://www.world-aluminium.org/statistics/primary-aluminium-smelting-power-consumption/#data
  1. https://journeytozerocarbon.com/?p=467

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