Emissions-Free Aluminium
Aluminium is a lightweight metal with a ton of different uses: from aircraft, to buildings, to drink cans. Most important is that it’s used in a number of technologies related to the energy transition. These include electricity networks, battery packs, solar power and electric vehicle chassis.
Decarbonising aluminium industry is challenging but essential, as it accounts for 3% of global carbon emissions.
Demand for these technologies is projected to increase enormously over the coming decades, and aluminium production will have to increase along with it. Unfortunately, aluminium production is also responsible for more than a gigaton of CO2 globally in 2021, nearly 3% of total global emissions.
Since we’re going to need aluminium as a part of the transition, we can’t exactly stop producing it. That means we need to find ways of producing it without the CO2. Fortunately, engineers are figuring out ways of doing just that, and in this article, we’ll explore what technologies we’ll use to eliminate emissions from aluminium production.
Aluminium production
Mining
The first stage of aluminium process is mining. Aluminium is the most abundant metal in the Earth’s crust, but it’s almost never found in its elemental state. It is much more commonly found as a part of bauxite, which is an ore made up of about half alumina – that’s aluminium oxide – alongside various other minerals like silicon, iron oxide, and titanium. Bauxite was named after Les Baux in France, where it was first discovered and the largest deposits, we know about today are located in Australia, Guinea, and China.
Bauxite is mined to extract alumina, which makes up about half of its composition.
Bayer Process
Once the bauxite has been mined, it’s refined into alumina (aluminium oxide). The most common way this takes place is through the Bayer process, which is used for 90% of aluminium production. The Bayer process involves heating the bauxite in a pressure vessel along with caustic soda to form sodium aluminate, NaAlO2 and then aluminium hydroxide is precipitated out from that. At around 150 to 200 degrees C, the aluminium hydroxide crystallizes, while other materials either don’t, or take much longer. Then the aluminium hydrate particles are calcined at around 1200 C to remove the water, resulting in anhydrous alumina.
This stage is responsible for about 15% of the emissions from aluminium production, mostly because of the high temperatures needed for those reactions, and traditionally that’s come from burning fossil fuels to generate heat.
During Bayer process, bauxite is refined into alumina.
Smelting
90% of the alumina we produce goes on to be made into aluminium through the smelting process. This part uses electrolysis, and it is by far the most energy and emissions intensive part of producing aluminium. The smelting process takes place in small cells, a large smelter would have hundreds of these cells lined up in rows. Inside the cells, alumina is dissolved in molten cryolite that creates a conductive environment at around 960 degrees C.
The bottom of the cell works as the cathode for the process, while the anode is made up of carbon. An electric current is applied to the anode and cathode and electrolysis occurs in the conductive mixture. The charge causes oxygen atoms to join with the carbon in the anode, leaving molten aluminium on the cathode and carbon dioxide on the anode. This process requires an enormous amount of electricity, around 15 MWh to produce one tonne of aluminium.
Smelting uses the most energy among all processes in aluminum production due to the extremely high temperatures required.
The aluminium then goes on to the cast house. There it’s remelted to remove any remaining impurities, typically metals like iron or copper, and at the same time other metals like manganese may be added to make alloys. The pure aluminium or alloy is then cast into moulds where it solidifies. This ends the production process, but not the aluminium’s lifecycle. After aluminium ingots leave the smelter, they will be manufactured into drink cans or solar panel frames, or bikes or planes.
Recycling Aluminium
After an aluminum product is no longer usable, it will probably be sent to a recycling plant. Aluminium can be recycled an infinite number of times without degradation, and it is one of those good news stories where not only can we recycle it, we actually do.
That’s because the energy needed to recycle aluminium, including transport and sorting, is dramatically less than that needed to make virgin aluminium. Dramatically less energy use equals dramatically lower prices and therefore about 75% of all aluminium produced in human history is still being used in one form or another. It’s an ideal material for a circular economy.
But despite this, recycled production currently only makes up about a third of the global total. The biggest producer, China, has one of the lowest recycling rates; recycled production there is only about 20%. This isn’t because Chinese people don’t participate in cash-for-cans programs, but because most Chinese aluminum is exported, usually as consumer goods. When these goods reach the end of their lifecycles, other countries recycle the aluminum domestically instead of sending it back to China.
China is the biggest aluminium producer in the world, but most of the aluminium is exported.
Increasing the amount of aluminium from recycled production is going to be quite hard in the short term because recycling rates are already pretty high. Today, about 70% of aluminium from end-of-life scrap is recovered. This can be improved by better sorting methods at waste management facilities; taking efforts to channel end-of-life scrap back to aluminium producers; and by providing incentives for product manufacturers to make their products easier to recycle (IEA).
Other actions can be taken to improve material efficiency include things like improving manufacturing yields in cast-houses, reducing how much we need to produce, and using stronger aluminium alloys for certain applications, allowing for less aluminium to be used overall.
Growth in demand for aluminium is increasing though, and it’s expected to continue to. Even if recycling rates went to 100%, and other material efficiency strategies are introduced, primary aluminium production will need to continue for decades at least. Therefore, we need to figure out how to get rid of the emissions in primary production.
Sources of emissions in aluminium production
The main sources of emissions mentioned earlier are in alumina refining, process emissions from carbon anodes, and fossil fuel combustion used to generate electricity for the electrolysis process, accounting for about 60% of all emissions. The latter contributes by far the most emissions, but happily those are the easiest to eliminate.
Demand for aluminium is growing, so it is crucial to address emissions at their source, which is the production process.
The amount of electricity required for aluminium smelting is enormous. Globally smelters use about 3% of global electricity, and locally it’s even more significant. In New South Wales, Australia, a single smelter in Tomago uses about 12% of the state’s electricity, where 60% of its electricity from coal. Globally, the situation is similar. More than half of the electricity used in aluminium production comes from coal. Most of this is because of China, which produces a little over half of the world’s aluminium, and still has a grid that primarily relies on coal.
Renewables as solutions?
How about we transition global electricity grids to renewable energy instead of fossil fuels? But we are planning on doing that anyway. Regarding the emissions that come from the electrolysis part of the process, it is nearly that simple.
Outside of China, hydroelectric power is by far the largest source of electricity for smelters. This is because, in favorable locations, it has historically been very cheap and reliable. While you can't move a hydro dam to be near major electricity users, you can strategically locate a smelter in areas with excellent hydro potential.
Because of this, there is a strong correlation between regions with high aluminum production and abundant, cheap hydroelectricity, such as Quebec, Norway and Iceland. Despite having relatively small populations and no bauxite mining, Canada is the fourth-largest producer of aluminum in the world, with nearly all of it coming from Quebec. Norway ranks eighth, and Iceland ranks tenth.
Challenges in Australia
Manapouri hydro power station in New Zealand, the second-largest power plant in the country, was built specifically to provide cheap power for New Zealand’s only aluminum smelter. However, recent fluctuations in electricity prices have at times rendered the smelter unprofitable, threatening its closure and leaving people to wonder how to utilize all that hydropower if the smelter shuts down.
This also explains why Australia, which mines more bauxite than any other country, and produces the second most alumina, is only the sixth largest producer of aluminium, as the Australian grid relies mostly on more expensive fossil fuels. As a result, we export much more alumina and bauxite than aluminium, and the aluminium we do produce is pretty emissions intensive. But we’re transitioning to renewables pretty rapidly, so these emissions will decrease accordingly.
Australia, despite being the world's largest bauxite miner, is only the sixth-largest aluminium producer because fossil fuel-based aluminium production is expensive and highly polluting.
Technologies to reduce emissions from smelting
Technologies are being developed to help smelters see a smooth transition towards a net-zero economy. Aluminium smelters traditionally cannot vary their energy use much. That means that the cost of their operation varies with the cost of electricity and in Australia that price varies a lot, from negative $1000/MWh when there is a whole lot of solar power in the grid and not much demand, to over $15000 when there’s a supply shortfall.
Therefore, for a large electricity user connected to the electricity grid, there’s a strong incentive to be able to reduce power usage during shortfalls when prices are very high. To a certain extent smelters with standard technology can do this, and have.
Tomago Aluminium mentioned earlier uses a constant 950 MW which is about 10% of New South Wales’ total. But when needed, it can shut down around 600 MW within minutes, by cycling individual potlines through a carefully controlled curtailment.
To take it further than this and make it an everyday tool to follow renewable generation and the associated low electricity prices, new technologies are needed. One that’s in development is the EnPot system which would allow smelters to turn their energy usage up or down by 30%.
The EnPot system covers the sides of each pot with heat exchangers, connected to an external ducting and suction. This system allows power usage to be changed while maintaining pot temperatures and preventing process disturbances.
EnPot enables smelters to achieve stable, efficient pot operation and adjust energy consumption indefinitely to produce the exact amount of aluminum needed. Image source
Once we get to the point where aluminium smelters can be turned up or down by 30% or more, they will be a great match for variable renewables, and help achieve several goals at the same time: lowering emissions, increasing the efficiency of the grid, and lowering the cost of smelting. So things are looking promising on the electricity side.
Technologies to reduce emissions from carbon anodes
Emissions from carbon anodes and alumina refining are going to be much harder to eliminate, but solutions have begun emerging. One way to deal with these process emissions is through carbon capture and storage (CCS). This tends to be much more difficult for aluminium than other facilities, as the CO2 concentration in the offgas for aluminium facilities is only about 1%, while for fossil fuel power plants it tends to be at least 4%.
Despite this, Norsk Hydro, the largest aluminium producer outside of China has announced that they plan to use CCS to capture emissions, with the goal of an industrial scale pilot by 2030. The plan involves using closed cells for the smelting process, resulting in more concentrated emissions which can be captured more easily, with any remaining emissions being captured through direct air capture.
Another method to reduce emissions would be to eliminate anode emissions entirely. An anode is needed for the electrolysis process to pull the oxygen atoms away from the aluminum, but it doesn’t necessarily have to be a carbon anode. Inert anodes have been developed that can perform the same role as carbon anodes, but release oxygen instead of CO2 as they degrade, and do not result in CO2 emissions from the baking process of carbon anodes.
The anodes were developed in two separate pilots: Rusal’s Krasnoyark plant in Russia, and Elysis, a joint venture of Alcoa and Rio Tinto, located in Quebec, have both successfully used inert anodes to make aluminium for the first time in 2021. Assuming clean electricity is used for the smelting process, the result is near-zero emission aluminium smelting.
Technologies to reduce emissions from refining
That still leaves alumina refining, which you might remember needs temperatures around 150 to 200 degrees Celsius. This is actually not such a hard temperature to reach without fossil fuels, and a range of zero emissions technologies can reach that high, like electric resistance heating.
The Yarwun refinery is replacing natural gas in the calcination process of alumina refining and is studying the construction of a hydrogen pilot plant at the site. Image source
Several Australian initiatives have been undertaken to this end, including a Worsley refinery that’s successfully used 30% biomass in its fuel mix, and a Yarwun refinery that is attempting to pilot the use of hydrogen, which can be emissions-neutral depending on how it’s made. Additionally, a Pinjarra refinery is set to in 2023 begin piloting a process for electric calcination, which remember needs temperatures of around 1000 degrees Celsius.
Once a trial like this succeeds, the zero-emissions alumina could then go on to a smelter powered by zero-emissions electricity using inert anodes, and then we’d get zero-emission aluminium for a first time.
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