Aluminium is one of the most commonly produced metals in the world. It’s produced from alumina, which is extracted from bauxite through a chemical process that – owing to its high iron oxide content – leaves a residue that resembles red mud and gives the residue its name. More than 160 million tons of bauxite residue are produced every year. In Europe, alumina refineries can be found in Ireland, Greece, Romania, Spain, France and Germany, among other countries.
For decades, bauxite residue was disposed of in the sea by a few plants around the world. Professor Yiannis Pontikes watched this with sorrow whilst a bachelor's student in Greece. Today, most of bauxite residue is preserved in large sedimentation basins. In 2010, the dam burst of one of these basins in Ajka, Hungary, leading to an environmental disaster in which hundreds of thousands of cubic meters of caustic red mud flooded several villages and ended up in local waterways.
For some time now, researchers have been looking for ways to valorise bauxite residue. That research gained momentum after the accident in Ajka, Pontikes and postdoctoral researcher Tobias Hertel explain. Both are part of the SREMat (Sustainable REsources for Engineered Materials) research group that belongs to the SeMPeR (Sustainable Metals Processing and Recycling) group at the department of Materials Engineering. Most of the work in the group revolves around developing ways to make new materials from waste. Another incentive for the valorisation of bauxite residue is the storage problem encountered by many alumina refineries.
A third factor relates to the restrictions China regularly imposes on rare-earth exports. Various KU Leuven research groups, collaborating today at the KU Leuven Institute on Sustainable Metals and Minerals (SIM2), have been investigating how to extract metals and rare earths from bauxite residue.
“If you do that, however, there will still be a residual product,” says Tobias Hertel. “We’re investigating possibilities for converting bauxite residue into building materials, using bauxite residue in its entirety, so that no waste is left behind. We chose to look at building materials because the volumes of bauxite residue produced are too large to be processed in any other way.”
Other groups at other universities worldwide mix bauxite residue with a binding agent such as cement. “But then your bauxite residue is no more than a filling material and it doesn’t contribute to the characteristics of the end product. We’re trying to transform bauxite residue with the intention of creating a reactive material with particular characteristics. Then there’ll be a greater payoff to using that material. It’s then no longer about getting rid of unwanted waste, but about something that offers added value.”
More than 160 million tons of bauxite residue are produced every year.
In his doctoral research, Tobias Hertel mixed bauxite residue with additives such as carbon, sand and limestone. “Then it’s heated to 1,100 to 1,200 °C, which causes it to melt. Afterwards, we pour the viscous material into water to ‘quench’ it. That results in a glass-like material, which we then mill down to a fine powder. To that powder we add an alkaline solution that’s able to dissolve the material, similar to what happens when you add water to cement powder.”
Just as with cement, a paste is created that, after some time, hardens into a solid block. Depending on the mould you use, you can create bricks, shingles, tiles and building panels. “Or porous blocks that can be used as insulation material. Porous or dense aggregates are also possible: rocky chunks that, for example, you can add to make concrete."
It’s not the only technique, adds Professor Pontikes. “For example, we’ve patented a process called ‘The solid state Bayer process’ that uses autoclaves to deliver final products incorporating more than 80 percent bauxite residue. And we were the first to my knowledge to make a new type of cement with more than 40 percent of bauxite residue. Bauxite residue has been added to cement for some time now, but this is limited to 3 percent. This is already significant, considering the annual cement production of more than 4 billion tons.”
We’re trying to transform bauxite residue into a reactive material with particular characteristics. Then it’s no longer about getting rid of unwanted waste, but about something that offers added value.
Commercial production of building materials based on bauxite residue has not yet begun, says Professor Pontikes. “But what’s unique about our group is that we scale up. A few months ago, together with Michiel Giels, also performing his PhD at the group, we managed to produce 2.5 tons of the glassy material, as described in Tobias's doctoral research, at RWTH Aachen where they have a very large furnace. This is a promising step, and soon we will be taking our own upscaling facilities to Transfarm, the place where KU Leuven’s pilot facilities will be located.”
The researchers combine fundamental and applied research. “At the back of our mind is always the question where fundamental research can lead,” says Tobias Hertel. Professor Pontikes talks about research that is ‘problem-driven’ but ‘science-deep’. “Some researchers are driven by curiosity, they want to understand why. We start from a problem we want to solve, and we go towards understanding why. With such an approach, there is no dilemma between fundamental and applied research. It’s merely about the starting point, because you need to go deep anyway if you really want to make a contribution.”
As part of the H2020 project RemovAl, work is scheduled to erect a house that will be made entirely out of materials from bauxite residue. SREMat group wants to take this even further; they have also designed interlocking building blocks that fit into each other: “We’re striving for a house that can be dismantled, deconstructed, instead of demolished. That’s why we don’t use cement mortar, and we use building elements that you can take apart and reuse. A bit like Lego but smarter (laughs).”
Most likely this house will be in the ‘Aspra Spitia’ area of Greece, which means ‘white houses’ in Greek. “So we might end-up building a house with green materials made from red mud in the Casablanca of Greece (laughs). The final building elements will actually have a wide range of colours. The materials we develop are black because the iron oxide that provides the red colour in bauxite residue turns black due to the processes involved. But other materials, developed by partners participating in this project, might be more red or grey. We definetely look forward to it.”
SREMat is strongly committed to communicating what they’re working on and one way to do this is by demonstrating what is possible. “In the EIT-KIC RawMaterials project RECOVER we developed mobile production units in bespoke containers, where we produce up to 1000 kg a day of final products showing to the public what you can do with bauxite residue and other residues.” This approach is also appreciated by industry. For some time now, most of the alumina refineries in Europe have collaborated and shown sincere interest in developing solutions. And these collaborations occur outside Europe as well. The research group has, for example, bilateral projects with EGA as well as with Rio Tinto, the world's second largest metals and mining corporation. “A lot is possible, we just need to go outside of our comfort zone and show commitment.”