ACCSESS innovation could help industry manage their CO₂ emissions and mineral residues at the same time

ACCSESS is currently working on an innovation that uses mineral residues and CO₂ emitted from industrial processes to produce high purity calcium carbonate. This technology has the potential to reduce industrial CO₂ emissions while creating value for industrial actors through the production of calcium carbonate, and contributing to managing their waste. We spoke to Antonio Gasós, PhD candidate at ETH Zürich’s Separation Processes Laboratory under Prof. Dr. Marco Mazzotti, to find out more.

An economic incentive for handling CO₂ emissions and waste

Industries such as cement, pulp and paper, and waste-to-energy produce goods that are necessary for society. However, they also produce a lot of CO₂, which is currently released into the atmosphere, contributing to climate change, and mineral residues, which are landfilled or badly recycled. The ACCSESS project is currently developing an innovation that can deal with both at the same time by using the emitted CO₂ and the mineral residues to produce precipitated calcium carbonate, which can then be used or stored. This is known as “indirect mineral carbonation”, and is a collaboration between Neustark and ETH Zürich, led by Prof. Dr. Marco Mazzotti.

Precipitated calcium carbonate has a market value of 10-400 EUR per tonne depending on its quality, and its production emits 200-350 kg of CO₂ equivalent per tonne. Therefore, this innovation provides an excellent economic incentive for industrial actors to handle their CO₂ emissions and residues by producing high purity calcium carbonate while avoiding those additional emissions.

“I would say there are three axes to the technology: one is capturing and storing CO₂ from industry. Two, is neutralising and upgrading the wastes from industry. Three is producing high purity precipitated calcium carbonate that can replace current production,” said Antonio Gasós, PhD candidate at ETH Zürich’s Separation Processes Laboratory, who is currently working on this innovation, supervised by Prof. Dr. Marco Mazzotti.

But how do you produce calcium carbonate from CO₂ and mineral residues?

Mimicking a natural process on an industrial scale

If you’ve ever looked inside a water boiler, you might have noticed a white powder lining the walls. This white powder is calcium carbonate. It forms because hard water contains high levels of calcium (and magnesium) that it gathered before it ended up in the boiler – for example, when water flows over and through rocks in mountains, it can dissolve small amounts of these minerals. When the boiler heats the water, these minerals react with dissolved CO₂ from the air, which produces calcium carbonate.

“We want to mimic this, but at an industrial scale,” said Antonio. “So instead of mountains, we use other sources of minerals, which are actually waste residues from industries.”

Industries produce residues as a byproduct of their main activities, such as cement kiln dust from the cement industry, slag from the steel industry, and paper sludge incineration ash from the pulp and paper industry. While these byproducts – also known as “alkaline industrial residues” – are currently considered mineral waste, they are a key component of this innovation. 

“These residues contain similar chemical elements to those leached by water from the mountains, but they can react more easily with CO₂,” he explained. In ACCSESS, the calcium is removed from these industrial residues, just as water would dissolve calcium from mountain rocks. Ammonium nitrate is used to increase the water’s calcium uptake capacity. The calcium-rich water and CO₂ are then added into a reactor at ambient conditions to produce calcium carbonate.  The process also produces the leached solid residues, now without calcium.

Creating value from industrial residues while avoiding additional CO₂

These leached solid residues also have the potential to create value for industry while avoiding additional CO₂ emissions.

For example, for some materials, removing some of the mineral phases can give them properties that cause them to resemble cement. Therefore, they could be used by the construction industry to replace part of the cement clinker in new buildings.

“Clinker produces around 700 kilograms of CO₂ per tonne produced. Among industrial emissions, cement is the biggest emitter – and is responsible for 7% of global emissions. So, replacing cement could also have quite a considerable and interesting impact,” said Antonio.

This potential application is also being investigated in ACCSESS by VDZ: “VDZ are taking the products from this process, both the calcium carbonate and the leached materials, to their facilities to see how it can be reused in construction. They are making sure that the materials meet regulations, and they are building blocks of concrete and then testing the properties of these blocks over multiple months to ensure good reusability,” Antonio explained.

Neustark is currently commercialising another form of recarbonation

Outside of ACCSESS, Neustark is currently commercialising another form of carbonation, wherein minerals and CO₂ are mixed directly in a reactor, without the liquid component. This is known as “direct mineral carbonation”, and it can not only potentially enable certain materials to be reused, but also even upgraded, such as demolition concrete.  

Concrete consists of a mixture of gravel, sand and cement. Since cement is very porous, it breaks down after it has been used. This presents challenges to reusing the concrete after it has been demolished. However, if CO₂ is added back into the demolition concrete, the properties of the material change.

“The resulting solids have a less porous, crackly cement layer anymore, which means they can be reused as sand and gravel in new concrete. This means you can better reuse building materials while handling some CO₂,” explained Antonio. “This process enables the industrial actor to deal with their CO₂now, with the co-benefit of creating a new product that they can potentially sell.”

A couple tonnes of CO₂ successfully stored in tests with mobile demonstration plant

Through ACCSESS, Neustark, in collaboration with ETH Zürich, have built a mobile demonstration plant, with the aim of advancing this technology from laboratory scale to being ready for industrial integration.  The plant was successfully commissioned in January 2024, and can handle approximately 200 kg of CO₂ per hour. This is the second scaled-up plant of its kind, with the first being funded by Innosuisse in 2023.

As part of this work, the project partners are testing the produced calcium carbonate for use in the construction and paper industry, and the first testing campaigns were conducted using cement kiln dust and paper sludge incineration ash.

“The first part of the campaign was actually testing dry runs without solids to make sure nothing leaks, and then going one step back to improve it. We then moved towards making single-step tests, so testing the filters, testing the reactors, and comparing it to the lab-scale results. There wasn’t a 100% match, but we also didn’t expect that. They’re matching well enough, and that’s what’s important,” said Antonio. “Then Neustark operated the plant continuously for 24 hours, also at night, and ETH supported with experimental measurements. It was quite a big effort, and the first 24 hours of operation were successful. Around two tonnes of CO₂ were stored, so we also achieved a milestone there. And from there, we stopped and spent time analysing the results and understanding how to improve the technology.”

Next testing campaign to use flue gas instead of pure CO₂

While there is still work to be done, Antonio is very optimistic about the progress made so far.

“I think we’ve learned a lot. Starting up the plant took more time than expected because you need to handle solids that are fed continuously. They clog the pipes sometimes, and you need to find ways of preventing that. And then you have filters that need to be automated well – they need to be tightened all the time. When you work at this intermediate scale, you need to start automating equipment otherwise it’s too hard to operate, but it’s still not such a big scale; the equipment is still small, and clogging happens more easily. But I think Neustark have found a proper way of making it work,” he said.

The next testing campaign started in August 2024, this time using simulated flue gas, a mixture of CO₂ and air mimicking industrial exhaust gas, rather than pure CO₂. The results will help inform the project partners’ decisions on whether they should directly use their exhaust gases or add a pre-concentrating step to the plant: “I think that’s going to be an outcome of the project. We’re going to test different gas conditions, and then add a cost analysis too, understanding where the sweet spot is.”

We need to start storing CO₂ now

If successful, the benefits of this innovation in enabling industrial actors to handle both their CO₂ emissions and waste residues in a streamlined way cannot be understated. While this technology cannot handle the same amount of CO₂ as underground storage, for Antonio, it presents an additional means of storing some CO₂ emissions, particularly for places and industries without immediate access to underground storage – and doing so quickly. “We need to start storing CO₂ now. Switzerland, for example, will need to export its CO₂ for underground storage. Work on this has started with the DemoUpCARMA project, but the infrastructure and regulations aren’t there yet – they take a bit of time to establish. Then people say that we don’t start storing CO₂ because the infrastructure is not there. Now, we have a technique that’s not going to handle the full emissions from a cement plant, but it’s going to handle some of it, and you can start now. Start building your small-scale plants, and we can take care of your CO₂ now.”

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List of publications:

• Meijssen, M., Marinello, L., di Bella, C., Gasós, A., & Mazzotti, M. (2023). Industrial demonstration of indirect mineral carbonation in the cement and concrete sector. Journal of Environmental Chemical Engineering, 11(5), 110900.
• Gasós, A., Meijssen, M., & Mazzotti, M. (2024). Indirect mineral carbonation of recycled concrete aggregate: Enhancing calcium extraction using a packed bed reactor. Journal of Cleaner Production, 449, 141745.
• Gasós, A., Mazzotti, M. (2024). Comparative assessment of the indirect mineral carbonation of alkaline industrial residues. In preparation.