Stronger 3D prints

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3D printing is a manufacturing process used for both consumer and industrial applications in the aeronautics, automotive, rail and medical industries. The Shoryuken project being developed at IMT Nord Europe aims to improve the mechanical performance of the objects printed using plastic and composite materials. To accomplish this, it combines 3D printing with laser welding technology.

In the industrial world, certain parts of cars, trains, airplanes, prostheses and orthoses are now manufactured using 3D printing. This manufacturing method enables the small-scale production of customized, geometrically complex parts using 3D digital models, without requiring expensive, specifically designed molds. This procedure saves time and materials when producing prototypes and products to be marketed. However, 3D printing has its limits, especially in relation to structural composite materials, which are plastic materials reinforced with fibers with a high level of resistance and rigidity.

3D printing processes that use composites with yarn containing cut reinforcing fibers generally produce materials with relatively weak mechanical properties. In order to improve the mechanical performance of the printed parts, manufacturing processes using yarns reinforced with continuous fibers are now in high demand in the industry. These yarns are made of thermoplastics, heat-sensitive materials, and continuous carbon or glass yarns. During the printing process, the yarns are melted in order to add the materials they contain layer by layer. The carbon fibers contained in the thermoplastic yarns do not melt and provide the object with solidity and resistance.  

However, the required level of resistance and rigidity can only be obtained in the direction of the fibers, since they are all positioned on a single printing plane. “The current composite 3D printing technology does not allow for the production of parts containing continuous fibrous reinforcements oriented in all the directions desired in space. This is a disadvantage when there are mechanical constraints in three dimensions,” says André Chateau Akue Asseko, researcher in Materials Science at IMT Nord Europe and winner of the Young Researchers call for projects by the French National Research Agency (ANR).

Hybridization of innovative technologies

This is precisely the technological barrier that the new Shoryuken* project seeks to overcome. To accomplish this, the initiative is studying the pairing of 3D printing with laser welding. This combination makes it possible to print two or more components for the same composite part in different printing directions and then use laser welding to assemble them.

“The difficulty stems from the presence of fibers or porosity, which disrupt the laser beam path due to the heterogeneity, which introduces thermal and optical diffusion phenomena,” the scientist explains. This assembly process therefore requires that the small areas filled with thermoplastics be treated differently during 3D printing. The laser radiation melts the thermoplastic polymer in a targeted manner with the composite material surrounding it. Once they are welded together, the two components become inseparable. This makes it possible to produce objects containing reinforcing fibers positioned in ways that allow them to resist mechanical loads in different directions.

Virtual engineering to optimize production

Modeling and simulation tools integrating multiphysics coupling are being developed to optimize these innovative design and production processes. These tools therefore contain information on the interaction between the laser and materials and their thermal and mechanical behavior. “Virtual engineering makes it possible to define the optimal assembly conditions that will ensure the quality of the welding interface,” says André Chateau Akue Asseko. The software, populated with information on the materials of interest, such as melting points, is used to simulate the behavior of two materials that are welded together in order to prevent spending too much time and materials on 3D printing tests.

The user can therefore adjust the laser parameters in order to conduct an optimal weld right away. “These simulations allows us to identify the optimal temperature and speed ranges for welding,” the researcher explains. The development of this type of tool would allow companies to reduce their development and industrialization costs before production by avoiding potential assembly problems. This would ensure the mechanical performance of the manufactured goods.

Read more on I’MTech: 3D printing, a revolution for the construction industry?

Multisectoral applications

 “For this project, we chose to focus on the health sector by producing a prosthetic arm as a demonstrator,” says the scientist, who is currently in contact with companies specialized in prosthesis design. André Chateau Akue Asseko explains that he initially chose to prioritize this sector for pragmatic reasons. “There is strong demand in this field for customized items, adapted to the users’ morphology. The parts are reasonably sized and compatible with the capabilities of our experimental equipment,” the researcher says.

The Shoryuken project will end in 2026. By that time, the future process and digital tool could convince other industries, such as the rail and automotive sectors, of the benefits of customizing parts and tailoring their functionalization for small and medium-scale production runs. For transportation companies, the significantly lighter weights of the parts designed and produced help to cut down on fuel consumption and thereby reduce carbon emissions, which are a key concern in the current global environmental context.

Rémy Fauvel

* The ANR JCJC SHORYUKEN project on the “Assembly of Hybrid Thermoplastic and Thermosetting Carbon Composite: Customization of Complex Structures” is funded by the French National Research Agency (ANR) as part of the 2021 Generic Call for Proposals (AAPG 2021 – CE10) on “Industry and Factories of the Future: People, Organization, Technology.”

New technologies to prevent post-operative hernias

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Baptiste PILLET, Mines Saint-Etienne – Institut Mines-Télécom

The abdomen experiences intra-abdominal pressure, which varies according to the volume of organs, respiration, muscle activation and any physiological activity. As a consequence, the abdomen must resist forces exerted by this pressure, which can at times be high when coughing or vomiting, or during pregnancy. Certain illnesses such as obesity, paired with high intra-abdominal pressure, can lead to a hernia forming.

A hernia is when an organ, such as the small intestine, pushes through a natural opening, leaving its original cavity. It is a pathological protrusion, most often caused by weakness in the tissue that fails to resist the pressure from the organ. Factors such as obesity or repeatedly carrying heavy loads can increase this internal pressure, thereby making it more likely for the balance between tissue and organs to be disrupted.

It is a common condition, accounting for over 100,000 operations in France in 2020. If a hernia worsens, it can lead to bowel obstruction, which is why surgery is often preferred as a prophylaxis (preventively). Surgery involves reducing the protrusion and returning the intestine to its cavity.

Inguinal hernias are when the hernia is located just above the groin crease, whereas femoral hernias are located below the groin crease. Umbilical hernias occur near the navel, and lastly, epigastric hernias are located between the abdominal muscles, above the navel. In general, femoral hernias are more common in women and more complicated than inguinal hernias, which are more common in men. Umbilical hernias often occur after the umbilical orifice does not close correctly, and are therefore more common in infants.

Reducing hernias after abdominal surgery

After abdominal surgery, the resistance and mechanical behavior of the abdominal wall may be disrupted, which can lead to an incisional hernia (also known as an ‘eventration’). During a laparotomy (vertical incision of the abdomen), the linea alba (connective tissue between the rectus abdominis) presents areas of weakness after scarring over, which may later reopen. Such incisions of the abdomen may be necessary in around a hundred operations (organ transplant, cesarean section, etc.) and yet they lead to up to 11% of incisional hernias.

Although at present there is no means to detect and prevent abdominal hernias (natural or incisional), efforts have been made to reduce the rate of complications. From now on, in the majority of abdominal reconstructions (during a laparotomy or hernia repair), mesh is inserted between the various layers of muscle to strengthen the abdominal wall and therefore reduce the risk of recurrent hernia or eventration.

When such mesh is not used, the rate of recurrence is around 50%. Approximately 400,000 abdomen repairs using mesh take place each year in Europe, representing a cost of around €3.2 billion. This makes it one of the most common general surgeries, and yet, the rate of recurrence is still far too high (between 14 and 44%). Even 1% fewer recurrences would save €32 million a year.

The reinforcement mesh used has the purpose of strengthening areas of weakness during scarring and filling orifices to rebuild the abdominal wall. The surrounding biological tissue will then colonize the implant to return to a state close to the original. At present, the mesh is manufactured with resorbable or non-resorbable synthetic fibers, sometimes with derivatives from organic tissue (dermis or submucosa from human, porcine or bovine small intestine). It is characterized by the size of the pores, fiber diameter and thickness, etc. as well as mechanical characteristics, such as its resistance to stretching, bending, rupture, etc.

Better understanding recurrence

Mechanical tests and postoperative monitoring with imaging are taking place to understand the rate of recurrence, which remains too high. Often the mesh does not present the same mechanical behavior and therefore does not reproduce and adapt to that of the abdominal wall in the best way (mesh too rigid, for example). While the mechanical behavior of the mesh and abdominal wall has been relatively well studied in the literature, there remains a lack of understanding around the mesh’s integration in the abdomen environment. The initially implanted mesh will evolve in its behavior and effect on the abdominal wall over time as it integrates into the surrounding tissue. Moreover, it has been observed that the mesh has a tendency to contract or even deteriorate over time.

Digital models representing the abdomen and its repair are starting to be developed. Similarly, while more and more innovative research is appearing, there remains a lack of understanding around the high rate of recurrence, due to a shortage of data on these digital models. Specifically, there is no simulation that makes it possible to study and faithfully predict how the abdominal wall reopens, even when the mesh has been implanted.

With the aim of filling this knowledge gap, an animal study is underway to observe the role of mesh in reconstructing the abdominal wall following an incisional hernia.

The mechanical characteristics will be studied at multiple postoperative intervals through mechanical tests, and the integration of the mesh will be closely monitored thanks to medical imaging. At the same time, a digital model will be developed to represent the abdomen and its components (various layers of muscle, connective tissues, etc.) as accurately as possible.

The mechanical data will be then implemented into the model to analyze the mesh’s integration into its environment, as well as its effects over time. According to the placement of the textile, how it is attached and the physiological activity, it will be able to predict whether or not a reopening will occur, where it will arise and whether it will spread. This digital model could allow for better understanding of the abdominal wall mesh repair process and thereby improve implants, surgical techniques and consequently, treatment outcomes.

Baptiste Pillet, Lecturer-Researcher and Biomechanics PhD student, Mines Saint-Etienne – Institut Mines-Télécom

This article was republished from The Conversation under the Creative Commons license. Read the original article here (in French).

Déchets verre, waste

An oasis of waste reconverted into ceramic materials

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Transforming industrial waste and unused by-products could make it possible to respond to issues of scarcity for civil engineering resources, recyclability and even reducing use of fossil fuels. Doan Pham Minh, process engineering researcher at IMT Mines Albi, explains several avenues for recycling and reusing materials explored by his work.

One man’s trash can be another man’s treasure. Turning rubbish into resources is the aim of the circular economy. And it is also the issue at the heart of the Innovative Ceramic Materials for Energy Storage and Construction (MACISEB)[1] project, launched in 2019, with the participation of researchers from IMT Mines Albi[2]. “Our objective is to transform inorganic, industrial waste and by-products, which can be found around us, into something that is useful for society,” describes Doan Pham Minh, process engineering researcher. The solutions identified as part of the project will then be transferred to companies in the Occitania region. From finding other uses for unrecyclable waste to replacing raw materials that are running out, the principle of ‘second life’ can be applied to a large range of unexpected situations.

Sand reserve seeks replacement for time to rest and recuperate

The French Agency for Ecological Transition (Ademe) reports that between 27 and 40 billion tons of sand are extracted each year around the world. It can be found in our buildings and windows, as well as our computers. “The demand for this resource is even more critical than that for noble metals. And the reserves are running out so quickly that they are arriving at breaking point,” emphasizes Pham Minh. Extracted from quarries or taken from riverbeds, natural sand is formed by the lengthy process of erosion. Too long, therefore, to meet society’s needs. However, this material is indispensable for the civil engineering sector (its main consumer) and therefore the economic stability of many countries.

Read more on I’MTech: Sand, an increasingly scarce resource that needs to be replaced

This is why the MACISEB project is seeking sand replacement products from inorganic by-products, i.e. industrial waste that is not currently being used. “The idea is not to completely change our means of manufacturing, but to replace a critical raw material using a circular economy approach,” specifies Pham Minh. With his team, the researcher has created resource maps for the entire Occitan territory. He identified and located deposits with high potential and similar properties to sand. He also ensured that these products are sustainable, by noting the quantity and availability of this waste. In this way, multiple candidates were selected, including glass residue.

During the recycling process, glass is ground up into grains fine enough to be reused by glass factories. However, a portion of this glass, too fine, coarse or contaminated, is not reused. “We are recovering this leftover glass to replace part or all of the sand needed to make ceramic bricks or tiles,” specifies Pham Minh. Sand from foundries, slag from blast furnaces, and ashes from biomass thermal power plants are also promising.

Using these materials, researchers have suggested formulas to create bricks and tiles with the same mechanical and thermal properties as those made with clay and natural sand. Moreover, the formulas comply with industrial specifications. The products are therefore guaranteed to be able to be manufactured using equipment that companies already possess, without extra investment. The first bricks will be made in 2022, and then tested by the Scientific and Technical Center for Building (CSTB).

From wind to heat: reusing wind turbine blades

The operating lifespan of a wind turbine is estimated at around twenty years. This means that the first French facilities are now arriving at end-of-life, and there will have to be more dismantling in the coming years. In short, recycling is becoming a major challenge for the wind energy industry. While the parts made from metal (pole and rotor) and concrete (base) recycle well, the blades – made from glass fiber mixed with organic resin – are not so lucky. Another part of the MACISEB project involves researchers recycling this waste into thermal storage materials. “Our objective is to reuse glass fiber from the blades to develop ceramics used by concentrated solar power (CSP) plants,” explains the researcher. This means of energy production transforms solar energy first into heat, then electricity. To do so, it uses systems made up of mirrors that concentrate the sun’s rays at one point, generating extremely high temperatures (from 200 to 1,500°C). The heat is transported by fluid, to propel the turbines and produce. It can be stored in ‘thermal batteries’, to later be released during the night to ensure continuity of service.

At present, thermal power plants store heat using molten salt – a mixture of potassium nitrate and sodium nitrate. “These compounds can also be found in agricultural fertilizer. There is therefore a conflict of use between the two sectors. However, there is currently no commercial alternative that is economically and environmentally viable,” explains Pham Minh. Transforming turbine blades into ceramics would therefore provide a new solution for this sector. With this in mind, researchers are developing materials capable of handling intense, repeated cycles of heating and cooling for multiple years. This solution would eventually make it possible to reuse a waste product that promises to grow. But it will also give a technological boost to the thermodynamic solar energy sector, which could allow it to establish itself in the renewable energy market. As part of the MACISEB project, this research is being undertaken by the PROMES laboratory, a partner of the project and academic reference body in the area of thermal storage. ART-DEV, partner and social sciences laboratory, is also looking into the social conditions for recycling wind turbine blades and the possibility of implementing a recycling ecosystem for the blades at a regional scale.

Industrial fumes: an idea to get the turbines going

Another application could make use of ceramic materials made from inorganic waste to capture heat. At present, the industry squanders over 30% of the energy it consumes in the form of so-called waste heat, released into the atmosphere in industrial fumes. Researchers at IMT Mines Albi are collaborating with company Eco-Tech Ceram, specialist in thermal storage, in order to recover this energy, store it and use it to supply industrial processes. For example, ceramicists and metal-working factories use high-temperature ovens, often running on natural gas. Reusing the heat captured from their fumes would make it possible to partially heat their equipment and therefore reduce their fossil fuel consumption.

Like for thermodynamic solar, the challenge is therefore to develop ceramic materials adapted for companies’ conditions of use. “Nevertheless, here another issue arises: industrial fumes contain pollutants. Such acidic, corrosive gases accelerate the aging of ceramics and therefore alter their performance,” explains the researcher. Moreover, the composition of fumes varies according to the industrial operations. The first thing to do will therefore be to characterize the kind of fumes, their temperature, etc. sector by sector, in order to develop sustainable materials while keeping costs under control[3].

Anaïs Culot

[1] Project funded by the European Regional Development Fund (ERDF), part of European policy aiming to strengthen economic, social and territorial cohesion in the European Union by supporting development in regions such as here, in the Occitania region.
[2] The project brings together researchers from the RAPSODEE center, the PROcesses, Materials and Solar Energy (PROMES) laboratory, the Actors, Resources and Territories in Development (ART-DEV) laboratory and the company Eco-Tech Ceram.
[3] This is part of the objectives of certain projects, Eco-Stock® solutions to recycle complex industrial waste heat (SOLUTEC, launched in 2021) and developing monolithic materials from local clay blends to reuse industrial waste heat in Occitania (CHATO, launched in 2021), led by IMT Mines Albi.
Photographie montrant plusieurs blocs de béton

Improve the quality of concrete to optimize construction

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Since the late 20th century, concrete has become the most widely used manufactured material in the world. Its high level of popularity comes alongside recurring problems, affecting its quality and durability. Among these problems is when one of the components in concrete, cement paste, sweats. Mimoune Abadassi, civil engineering PhD student at IMT Mines Alès, aims to resolve this problem.

“When concrete is still fresh, the water inside rises to the surface and forms condensation,” explains Mimoune Abadassi, doctoral student in Civil Engineering at IMT Mines Alès. This phenomenon is called concrete sweating. “When this process takes place, some of the water will not reach the surface and remains trapped inside the concrete, which can weaken the structure,” adds the researcher, before specifying that “sweating does not only have negative effects on the concrete’s quality, as water allows the material to be damp cured, which prevents it from drying out and cracks appearing that would reduce durability”.

In his thesis, Abadassi studies the sweating of cement paste, one of the components of concrete alongside sand and gravel. In analyzing cement paste prepared with varying amounts of water, the young researcher has remarked that the more water incorporated in the cement paste, the more it sweats. He has also looked into the effect of superplasticizers, chemical products that when included in the cement paste, make it more liquid, more malleable when fresh and more resilient when hardened. “When we increase the amount of superplasticizer, we have observed that the cement paste sweats more as well,” indicates Abadassi. “This is explained by the fact that superplasticizers disperse suspended cement particles and encourage the water contained in clusters formed by these particles to be released,” he points out, before adding that “this phenomenon causes the volume of water in the mixture to increase, which increases the sweating of the cement paste”.

Research at the nanometric, microscopic and macroscopic level

By interfering with the sweating, superplasticizers also affect the permeability of cement paste. To study its permeability when fresh, Abadassi used an oedometer, a device mainly used in the field of soil mechanics. Oedometers compress a sample, extract the water contained inside and measure the volume, to determine how permeable it is. The larger the volume of water recovered, the more permeable the sample. In the case of cement paste, if it is too permeable, more water will enter, which reduces cohesion between aggregate particles and weakens the material’s structure.

By varying certain parameters when preparing the cement paste, such as the amount of superplasticizer, Abadassi aims to observe the changes taking place within the paste, invisible to the naked eye. To do so, he uses a Turbiscan. This machine, generally used in the cosmetics industry, makes it possible to analyze particle dispersion and cluster structure in the near-infrared. By observing the sample at scales ranging from the nanometer to the millimeter, it is possible to identify the formation of flocks: groups of particles in suspension which adhere to one another, and that, in the presence of superplasticizers, separate and release water into the cement paste mixture.    

To understand the consequences of phenomena in cement paste at the microscopic and mesoscopic scale, Abadassi uses a scanning electron microscope. This method makes it possible to observe the paste’s microstructure and interfaces between aggregate particles at a nanometric and microscopic scale. “With this technique, I can visualize internal sweating, shown by the presence of water stuck between aggregate particles and not rising to the surface,” he explains. When concrete has hardened, a scanning microscope can be used to identify fissuring phenomena and cavity formation caused by the sweating paste.

Abadassi has also studied the effects of an essential stage in cement paste production: vibration. This process allows cement particles to be rearranged, leaving the smallest possible gaps between them and therefore making the paste more durable and compact. After vibrating the cement paste at various frequencies, Abadassi concluded that sweating is more likely at higher frequencies. “Vibrating cement particles in suspension will cause them to be rearranged, which will lead to the water contained in flocks being released,” he describes, adding that “the greater the vibration, the more the particles will rearrange and the more water will be released”.

Once these trials are finished, the concrete’s mechanical performances will be analyzed. One way this will be done is by exerting mechanical pressure on an object, in this case, a sample of concrete, to measure its resistance to said pressure. The results obtained from this experiment will be connected with microscope observations, Turbiscan tests and trials varying the parameters of the cement paste formula. All of Abadassi’s results will be used to create a range of formulas that can be utilized by concrete production companies. This will provide them with the optimal quantities of components, such as water and superplasticizers, to include when preparing cement for use in concrete. In this way, the quality and durability of the most widely used manufactured material in the world could be improved.

Rémy Fauvel

composites

Technology for improving the recycling of plastics and composites

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Plastics and composites aren’t recycled as often as we might wish, as a result of a lack of facilities, the right technologies not yet existing or not being profitable, or hazardous waste deposits. IMT Nord Europe have been working in partnership with manufacturers to develop and improve the available technologies.

Plastics and composites get a lot of bad press, but it is hard to do without them for many objects we use every day, including our cars. In order to minimise their polluting effect, they must be recycled, but this is complicated from both a technological and an economic perspective. Two researchers from IMT Nord Europe are seeking to improve processes with a view towards industrialisation.

In order to recycle plastic, outlets have to be found for these recycled materials. One of the main stumbling blocks is the presence of pollutants, including volatile organic compounds (VOCs), which can produce unpleasant and even toxic odours. There are also very strict standards governing the emission of VOCs and odours in vehicle passenger compartments. Marie-France Lacrampe, a researcher at IMT Nord Europe, is working on a solution which is striking in its simplicity: water-assisted extrusion.

Eliminating odours

Extrusion is a process traditionally used to manufacture objects made from plastic, involving pushing a doughy material through a die of the desired cross-section. Water is injected into the extruder and the steam washes the plastic, extracting the majority of any VOCs. “A few changes need to be made to the extruder”, explains Marie-France Lacrampe. Professor Lacrampe is working alongside three industrial partners and another laboratory, with the industrial pilot expected to be operational within two years.

In order to further improve this process, the researcher intends to combine water with supercritical CO2 – pressurised CO2 which becomes a highly effective solvent. The advantage is that it removes different molecules from those removed using water.

Process organisation and eco-design

Efficient recycling normally starts with designing materials which are easy to recycle. This is particularly true when it comes to food packaging, which is often made using several different materials (cartons, thermoformed tubs or re-heatable pouches, for example). “The ideal solution is to mix compatible polymers which can then be integrated into existing recycling processes”, explains Marie-France Lacrampe.

When it comes to recycling, it’s not just a question of the technology used, but how the whole process is organised. Waste must be used as locally as possible in order to cut transport and logistics costs, requiring intelligent analysis and handling of flows.

“If we want to boost recycling rates then we have to tackle what we don’t know how to do. This is particularly true for small quantities (hazardous waste deposits) and materials which we are unable to recycle or aren’t very good at recycling such as opaque PET (the plastic used to make milk bottles, for example). We are working on recycling small quantities through additive manufacturing, the industrial version of 3D printing, extruding them again with additives so that they be reused.” 

Composites – rarely recycled

If recycling plastics isn’t always easy, just imagine what it must be like for composites, materials which are generally comprised of glass or carbon fibre and a polymer matrix. A modern aircraft such as the Airbus A350 is half-made of composites, which are used in whole sectors of industry, from transport (not just aircraft, but also cars, boats and bikes) to electronics, leisure and wind power.

Once they have reached the end of their life, composites are primarily burned in order to produce energy, which isn’t ideal from either an environmental or an economic point of view. “Solutions are being developed in the aeronautics sector to recover carbon fibres”, points out Mylène Lagardère, who is also a researcher at IMT Nord Europe. “It is mostly carbon-based composites which are used in aeronautics, which are more “noble”, making them easier to recycle.” Technology for recycling fibreglass composite does exist, but it is not yet profitable.

Developing more affordable methods

There are two possible processes for recovering fibres: a chemical process in which the matrix is dissolved in a solvent (allowing the matrix to be reused) and a thermal process in which the matrix is damaged. Matrices themselves are either thermoplastic, meaning they can be melted, or thermosetting, meaning they are damaged when heated. As a result, as Mylène Lagardère explains, “each fibre-matrix combination is processed differently,  with a different process for each product.” This is what makes recycling composites so complicated. The purer the material, the easier it is to recycle.

As we can see, improving recycling is essential, and research into this subject is rightly being prioritised. “Our aim is to develop methods which are both simple and affordable”, explains Mylène Lagardère. “Our basis is the industrial problem: if we have a deposit of materials with certain properties, then we can recover a recycled material with such properties.” The issue is that, during recycling, the properties of the material always deteriorate, as the fibres are shortened.

The recycling of composites is still very much in its infancy, but a few processes are starting to emerge,  whether in water sports, where the association APER – funded by an eco-tax on new crafts – dismantles abandoned boats, or in the wind power industry. The automobile industry is also having to adapt, with legislation requiring recycled materials to be used in the production of new vehicles.

Cécile Michaut

Large quantities of composites for recycling on the horizon?

10 million tonnes of composites are produced each year worldwide, and the market is continuing to grow at a rate of 5% year on year. But recycling is set to really accelerate: composites whihe arrived on the market 20 to 30 years ago are now reaching the end of their lives. 50,000 tonnes of wind turbine rotors will need to be recycled between 2021 and 2022. In 2023, 25,000 boats, three-quarters made from composites, are to be dismantled. 4,000 railway carriages are also awaiting dismantling. Although resources remain limited (15,000 tonnes of production waste and 7,000 tonnes of materials at end of life in 2017), significant growth is anticipated. Processes mut develop and organise in order to become sustainable.

Also read on I’MTech

“We must work now to recycle composites”
Recycling carbon fibre composites: a difficult task

3D printing, a revolution for the construction industry?

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Estelle Hynek, IMT Nord Europe – Institut Mines-Télécom

A two-story office building was “printed” in Dubai in 2019, becoming the largest 3D-printed building in the world by surface area: 640 square meters. In France, XtreeE plans to build five homes for rent by the end of 2021 as part of the Viliaprint project. Constructions 3D, with whom I am collaborating for my thesis, printed the walls of the pavilion for its future headquarters in only 28 hours.

Today, it is possible to print buildings. Thanks to its speed and the variety of architectural forms that it is capable of producing, 3D printing enables us to envisage a more economical and environmentally friendly construction sector.

3D printing consists in reproducing an object modeled on a computer by superimposing layers of material. Also known as “additive manufacturing”, this technique is developing worldwide in all fields, from plastics to medicine, and from food to construction.

For the 3D printing of buildings, the mortar – composed of cement, water and sand – flows through a nozzle connected to a pump via a hose. The sizes and types of printers vary from one manufacturer to another. The “Cartesian” printer (up/down, left/right, front/back) is one type, which is usually installed in a cage system on which the size of the printed elements is totally dependent. Another type of printer, such as the “maxi printer”, is equipped with a robotic arm and can be moved to any construction site for the direct in situ printing of different structural components in a wider range of object sizes.

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Pavilion printed by Constructions 3D in Bruay-sur-l’Escaut. Constructions 3D, provided by the author

Today, concrete 3D printing specialists are operating all over the world, including COBOD in Denmark, Apis Cor in Russia, XtreeE in France and Sika in Switzerland. All these companies share a common goal: promoting the widespread adoption of additive manufacturing for the construction of buildings.

From the laboratory to full scale

3D printing requires mortars with very specific characteristics that enable them to undergo rapid changes.

In fact, these materials are complex and their characterization is still under development: the mortars must be sufficiently fluid to be “pumpable” without clogging the pipe, and sufficiently “extrudable” to emerge from the printing nozzle without blocking it. Once deposited in the form of a bead, the behavior of the mortar must change very quickly to ensure that it can support its own weight as well as the weight of the layers that will be superimposed on it. No spreading or “structural buckling” of the material is permitted, as it could destroy the object. For example, a simple square shape is susceptible to buckling, which could cause the object to collapse, because there is no material to provide lateral support for the structure’s walls. Shapes composed of spirals and curves increase the stability of the object and thus reduce the risk of buckling.

These four criteria (pumpability, extrudability, constructability and aesthetics) define the specifications for cement-based 3D-printing “inks”. The method used to apply the mortar must not be detrimental to the service-related characteristics of the object such as mechanical strength or properties related to the durability of the mortar in question. Consequently, the printing system, compared to traditional mortar application methods, must not alter the performance of the material in terms of both its strength (under bending and compression) and its longevity.

In addition, the particle size and overall composition of the mortar must be adapted to the printing system. Some systems, such as that used for the “Maxi printer”, require all components of the mortar except for water to be in solid form. This means that the right additives (chemicals used to modify the behavior of the material) must then be found. Full-scale printing tests require the use of very large amounts of material.

Initially, small-scale tests of the mortars – also called inks – are carried out in the laboratory in order to reduce the quantities of materials used. A silicone sealant gun can be used to simulate the printing and enable the validation of several criteria. Less subjective tests can then be carried out to measure the “constructable” nature of the inks. These include the “fall cone” test, which is used to observe changes in the behavior of the mortar over time, using a cone that is sunk into the material at regular intervals.

Once the mortars have been validated in the laboratory, they must then undergo full-scale testing to verify the pumpability of the material and other printability-related criteria.

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Mini printer. Estelle Hynek, provided by the author

It should be noted that there are as yet no French or European standards defining the specific performance criteria for printable mortars. In addition, 3D-printed objects are not authorized for use as load-bearing elements of a building. This would require certification, as was the case for the Viliaprint project.

Finding replacements for the usual ingredients of mortar for more environmentally friendly and economical inks

Printable mortars are currently mainly composed of cement, a material that is well known for its significant contribution to CO₂ emissions. The key to obtaining more environmentally friendly and economical inks is to produce cement-based inks with a lower proportion of “clinker” (the main component of cement, obtained by the calcination of limestone and clay), in order to limit the carbon impact of mortars and their cost.

With this in mind, IMT Nord-Europe is working on incorporating industrial by-products and mineral additives into these mortars. Examples include “limestone filler”, a very fine limestone powder; “blast furnace slag”, a co-product of the steel industry; metakaolin, a calcinated clay (kaolinite); fly ash, derived from biomass (or from the combustion of powdered coal in the boilers of thermal power plants); non-hazardous waste incineration (NHWI) bottom ash, the residue left after the incineration of non-hazardous waste, or crushed and ground bricks. All of these materials have been used in order to partially or completely replace the binder, i.e. cement, in cement-based inks for 3D printing.

Substitute materials are also being considered for the granular “skeleton” structure of the mortar, usually composed of natural sand. For example, the European CIRMAP project is aiming to replace 100% of natural sand with recycled sand, usually made from crushed recycled concrete obtained from the deconstruction of buildings.

Numerous difficulties are associated with the substitution of the binder and granular skeleton: mineral additions can make the mortar more or less fluid than usual, which will impact the extrudable and constructable characteristics of the ink, and the mechanical strength under bending and/or compression may also be significantly affected depending on the nature of the material used and the cement component substitution rate.

Although 3D printing raises many issues, this new technology enables the creation of bold architectural statements and should reduce the risks present on today’s construction sites.

Estelle Hynek, PhD student in civil engineering at IMT Nord Europe – Institut Mines-Télécom

This article has been republished from The Conversation under a Creative Commons license. Read the original article (in French).

Graphene, or the expected revolution in electronics: coming soon

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Thibaut Lalire, IMT Mines Alès – Institut Mines-Télécom

“Material of the 21st century,” a “revolutionary material”: these are some of the ways graphene has been described since it was discovered in 2004 by Konstantin Novoselov and Andre Geim. The two scientists’ research on graphene won them the Nobel Prize in Physics in 2010. But how do things stand today – seventeen years after its discovery?

Graphene is known worldwide for its remarkable properties, whether mechanical, thermal or electrical. Its perfect honeycomb structure composed of carbon atoms is the reason why graphene is a high-performance material that can be used in numerous fields. Its morphology, in the form of a sheet just one atom thick, makes it part of the family of 2D materials. Manufacturers have stepped up research on this material since its discovery, and a wide range of applications have been developed, in particular by taking advantage of graphene’s electrical performance. Many sectors are targeted, such as aeronautics, the automotive industry and telecommunications.

Is there graphene in airplanes?

Graphene is used for its status as a champion of electrical conductivity, as well as for its low density and its flexibility. These properties allow it to join the highly exclusive club of materials used in  aeronautics.

Lightning and ice buildup are problems frequently encountered by airplanes at high altitudes. The impact of a lightning strike on a non-conductive surface causes severe damage that can even include the aircraft catching fire. The addition of graphene, with its high electrical conductivity, makes it possible to dissipate this high-energy current. Airplanes are designed in such a way so as to route the current as far as possible from risk areas – fuel tanks and control cables – and therefore prevent loss of control of the aircraft, or even explosion.

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The history of graphene starts here. Umberto/Unsplash, CC BY

A coating composed of a resin reinforced with graphene, which is referred to as a “nanocomposite,” is used as an alternative to metal coating, since its low density makes it possible to obtain lighter materials than the original ones – limiting the aircraft’s mass, and therefore, its fuel consumption. But the electrically conductive materials required to dissipate the energy of the lightening strike have the drawback of reflecting electromagnetic waves, meaning that this kind of material cannot be used for stealth military applications.

To overcome this shortcoming, different forms of graphene have been developed to conserve its electrical conductivity while improving stealth. “Graphene foam” is one of these new structures. The wave penetrates the material, which creates a phenomenon in which the wave is reflected in all directions, trapping it and gradually suppressing its traces. It is not possible for the wave to return to the radar, so the aircraft becomes stealth. This is referred to as electromagnetic shielding.

Graphene for energy storage

Graphene has also become widely used in the field of electrical energy storage.

Graphene is an ideal candidate as an electrode for Li-ion batteries and supercapacitators. Its high electrical conductivity and high specific surface area (corresponding to the available surface on the graphene that can accommodate ions and facilitate the exchange of electrons between the graphene electrode and the lithium) makes it possible to obtain a large “storage capacity.” A large number of ions can easily insert themselves between the graphene sheets, which allows electrons to be exchanged with the current, increasing the battery’s electricity storage capacity, and therefore battery life. The ease with which ions can insert themselves into the graphene electrode and the high electrical conductivity of this material (for rapid electron transfer) result in a battery with a much shorter discharge/charge cycle. Graphene’s high conductivity makes it possible to deliver a great quantity of energy in a very short time, resulting in more powerful supercapacitators. Graphene is also a good thermal conductor, which limits temperature rise in batteries by dissipating the heat.

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Electric batteries are increasingly pervasive in our lives. Graphene could help improve their performance. Markus Spiske/Unsplash, CC BY

At the industry level, Real Graphene has already developed an external battery that can completely recharge a mobile phone in 17 minutes. In an entirely different industry, Mercedes is working on a  prototype for a car with a battery composed of graphene electrodes, proclaimed to have a range of 700 kilometers for a 15-minute recharge  – at present, these values are quite surprising at first glance, especially for electric vehicles which require batteries with high storage capacity.

Making its way into the field of electronics

One area where graphene has struggled to set itself apart compared to semi-conductors is the field of electronics. Its electronic properties – due to its “band structure” – make it impossible to control electrons and graphene therefore behaves like a semi-metal. This means that the use of graphene for binary  – digital – electronics remains challenging, especially for transistors, which are instead composed of semi-conductors.

In order for graphene to be used in transistors, its band structure must be modified, which usually means degrading its honeycomb structure and other electrical properties. If we want to conserve this 2D structure, the chemical nature of the atoms that make up the material must be modified, for example by using boron nitride or transition metal dichalcogenides, which are also part of the family of 2D materials.

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Microscopy of the interface between graphene and boron nitride(h-BN). Oak Ridge National Laboratory, Flickr, CC BY

If, however, we wish to use graphene, we must target applications in which mechanical properties (flexibility) are also sought, such as for sensors, electrodes and certain transistors reserved for analog electronics, like graphene field-effect transistors. The leading mobile phone companies are also working on developing flexible mobile phone screens for better ergonomics.

The manufacturing of the coming quantum computers may well rely on materials known as “topological insulators.” These are materials that are electrical conductors on their surface, but insulators at their core. Research is now focusing on the topological phase of graphene with electric conduction only at the edges.  

The wide variety of applications for graphene demonstrates the material’s vast potential and makes it possible to explore new horizons in a wide range of fields such as optoelectronics and spintronics.

This material has already proved itself in industry, but has not revolutionized it so far. However, ongoing research allows new fields of application to be discovered every year. At the same time, synthesis methods are continually being developed to reduce the price of graphene per kilogram and obtain a higher-quality material.

Thibaut Lalire, PhD student in material science, IMT Mines Alès – Institut -Télécom

This article has been republished from The Conversation under a Creative Commons license. Read the original article (in French).

air intérieur

Our indoor air is polluted, but new materials could provide solutions

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Frédéric Thévenet, IMT Lille Douai – Institut Mines-Télécom

We spend 80% of our lives in enclosed spaces, whether at home, at work or in transit. We are therefore very exposed to this air, which is often more polluted than outdoor air. The issue of health in indoor environments is thus associated with chronic exposure to pollutants and to volatile organic compounds (VOCs) in particular. These species can cause respiratory tract irritation or headaches, a set of symptoms that is referred to as “sick building syndrome.” One VOC has received special attention: formaldehyde. This compound is a gas at room temperature and pressure and is very frequently present in our indoor environments although it is classified as a category 1B CMR compound (carcinogenic, mutagenic, reprotoxic). It is therefore subject to indoor air quality guidelines which were updated and made more restrictive in 2018.

The sources of volatile organic compounds

VOCs may be emitted in indoor areas by direct, or primary sources. Materials are often identified as major sources, whether associated with the building (building materials, pressed wood, wood flooring, ceiling tiles), furniture (furniture made from particle board, foams), or decoration (paint,  floor and wall coverings). The adhesives, resins and binders contained in these materials are clearly identified and well-documented sources.

To address this issue, mandatory labeling has existed for these products since 2012: they are classified in terms of emissions. While these primary sources related to the building and furniture are now well-documented, those related to household activities and consumer product choices are more difficult to characterize (cleaning activities, cooking, smoking etc.) For example, what products are used for cleaning, are air fresheners or interior fragrances used, are dwellings ventilated regularly? Research is being conducted in our laboratory to better characterize how these products contribute to indoor pollution. We have recently worked on cleaning product emissions and their elimination. And studies have also recently been carried out on the impact of essential oils at our laboratory (at IMT Lille Douai) in partnership with the CSTB (French National Scientific and Technical Center for Building) in coordination with ADEME (French Environmental and Energy Management Agency).

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Emission, deposition and reactivity of essential oils in indoor air (Shadia Angulo-Milhem, IMT Lille Douai). Author provided

In addition to the primary sources of VOCs, there are also secondary sources resulting from the transformation of primary VOCs. These transformations are usually related to oxidative processes. Through these reactions, other kinds of VOCs are also formed, including formaldehyde, among others.

What solutions are there for VOCs in indoor air?

Twenty years ago, an approach referred to as a “destructive process” was being considered. The idea was to pass the air to be treated through a purification system to destroy the VOCs. These can either be stand-alone devices and therefore placed directly inside a room to purify the air, or integrated within a central air handling unit to treat incoming fresh air or re-circulated air.

Photocatalysis was also widely studied to treat VOCs in indoor air, as well as cold plasma. Both of these processes target the oxidation of VOCs, ideally their transformation into CO2 and H2O. Photocatalysis is a process that draws on a material’s – usually titanium dioxide (TiO2) – ability to  adsorb and oxidize VOCs under ultraviolet irradiation. Cold plasma is a process where, under the effect of a high electric field, electrons ionize a fraction of the air circulating in the system, and form oxidizing species.

The technical limitations of these systems lie in the fact that the air to be treated must be directed and moved through the system, and most importantly, the treatment systems must be supplied with power. Moreover, depending on the device’s design and the nature of the effluent to be treated (nature of the VOC, concentration, moisture content etc.) it has been found that some devices may lead to the formation of by-products including formaldehyde, among others. Standards are currently available to oversee the assessment of this type of system’s performance and they are upgraded with technological advances.

Over the past ten years, indoor air remediation solutions have been developed focusing on the adsorption – meaning the trapping – of VOCs. The idea is to integrate materials with adsorbent properties in indoor environments to trap the VOCs. We have seen the emergence of materials, paint, tiles and textiles that incorporate adsorbents in their compositions and claim these properties.

Among these adsorbent materials, there are two types of approaches. Some trap the VOCs, and do not re-emit them – it’s a permanent, irreversible process. The “VOC” trap can therefore completely  fill up after some time and become inoperative, since it is saturated. Today, it seems wiser to develop materials with “reversible” trapping properties: when there is a peak in pollution, the material adsorbs the pollutant, and when the pollution decreases, for example, when a room is ventilated, it releases it, and the pollutant is evacuated through ventilation.

These materials are currently being developed by various academic and industry players working in this field. It is interesting to note that these materials were considered sources of pollution 20 years ago, but can now be viewed as sinks for pollution.

How to test these materials’ ability to remove pollutants

Many technical and scientific obstacles remain, regardless of the remediation strategy chosen. The biggest one is determining whether these new materials can be tested on a 1:1 scale, as they will be used by the end consumer, meaning in “real life.”  

That means these materials must be able to be tested in a life-size room, and with conditions that are representative of real indoor atmospheres, while controlling environmental parameters perfectly. This technical aspect is one of the major research challenges in IAQ since it determines the representativeness and therefore the validity of the results we obtain.  

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Experimental IRINA room (Innovative Room for Indoor Air studies, IMT Lille Douai). Author provided

We developed a large enclosed area in our laboratory for precisely this purpose a few years ago. With its 40 square meters, it is a real room that we can go into, called IRINA (Innovative Room For Indoor Air Studies). Seven years ago, it was France’s first fully controlled and instrumented experimental room on a 1:1 scale. Since its development and validation, it has housed many research projects and we upgrade it and make technical updates every year. It allows us to recreate the indoor air composition of a wood frame house, a Parisian apartment located above a ring road, an operating room and even a medium-haul aircraft cabin. The room makes it possible to effectively study indoor air quality and treatment devices in real-life conditions.

Connected to this room, we have a multitude of measuring instruments, for example to measure VOCs in general, or to monitor the concentration of one in particular, such as formaldehyde.

Frédéric Thévenet, Professor (heterogeneous/atmospheric/indoor air quality physical chemistry), IMT Lille Douai – Institut Mines-Télécom

This article has been republished from The Conversation under a Creative Commons license. Read the original article (in French).

What do we know about the environmental impact of biocomposites?

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Bio-based materials are an alternative to those derived from fossil resources. They have been increasingly promoted in recent years. However, given the recent development of this sector, their real environmental impact is still relatively unknown. Joana Beigbeder, researcher in environmental analysis at IMT Mines Alès, provides an update on the life cycle analysis (LCA) of these emerging materials.

Although bio-based materials are presented as an effective alternative to their petroleum-based predecessors, the details of their environmental impact are still not well known. Plant-based materials are certainly an improvement for some impacts, such as carbon footprint, but it can be trickier when it comes to soil use or soil pollution, for example. To get to the bottom of this, life cycle analysis is an indispensable tool.

Joana Beigbeder, researcher in environmental impacts at IMT Mines Alès, is working on the life cycle analysis of these new bio-based materials, particularly biocomposites. The objective is to compare the environmental impact of different scenarios, either between bio-based and petroleum-based materials, or according to the different stages of the life cycle of the same material. The researcher says, “the focus of life cycle analysis is to study several environmental impacts in parallel and to identify possible transfers of pollution throughout the life cycle of a product”.

Different sources of impact

With regard to environmental impact, some points seem obvious. If we only look at carbon footprint, it seems obvious that bio-based materials are more beneficial than their petroleum-based cousins. But the reality is more complex because there are many variables to consider. “Carbon impact is a well-known indicator, but we must also take into account the different impacts of pollution, human toxicity, soil or water pollution or global warming”, says Joana Beigbeder. Take the automotive sector, for example. If for the same function, a part made of bio-based material has a higher weight, it will require more energy to use and will not necessarily be beneficial. “It may be a better solution from an environmental point of view, but that’s precisely what the LCA will tell us,” she says.

Some points seem more obvious, such as the consumption of fossil resources and the impact on global warming in general. Plant materials absorb CO2 during their growth, which gives them an undeniable advantage. That said, their growth implies an agricultural impact, especially on land use. “Depending on the type of crop, this can lead to soil pollution. Generally, the more fertilizers and additives that are required for growth, the less beneficial some of the impacts will be,” says Joana Beigbeder. This brings up a new issue that does not exist with petroleum-based materials.

“It’s always a case of compromising, and we look for ways to improve the process or the product,” says the researcher. “Plants, such as flax or hemp, require little or no input and are therefore a preferred option. But the material is only one area for improvement when it comes to eco-design”, she notes. To reduce the environmental impact of a product, lightening its weight and extending its lifespan are key points. This can include the reparability of the product, or even new concepts such as prioritizing the use of the product over the purchase of the product.

“There is also a question of ecological common sense, if we study a disposable product, reusable or compostable materials will be favored,” says Joana Beigbeder. One research topic includes the development of compostable diapers and band-aids, a promising step in reducing plastic pollution.

Predicting potential impacts

“Life cycle analysis study is really done on a case-by-case basis depending on the service provided by a product,” says Joana Beigbeder. To estimate the consumption of solvents or the chemical synthesis of materials, the researcher uses previous scientific publications and mathematical models. But in some areas, data is missing. “We sometimes have to use fairly strong approximations on certain impacts, but this still allows us to map trends and areas for improvement,” she maintains. This can be the case for the production of plant fibers, as well as for their disposal at the end of their use.

As these materials do not yet have a major presence on the market, there are currently no channels for processing the resulting waste. In fact, there is no data on the environmental impact of the incineration, landfill, or pollution from these newly emerging materials. “To estimate these impacts, we assimilate them to other, better-known closely-related materials on which we have data, to develop our hypotheses,” she explains. This work also generates new data for LCA such as recycling simulations for end-of-life treatment.

“Some bio-based materials are at the laboratory stage today, and it’s still hard to imagine what will happen at the industrial stage,” says Joana Beigbeder. The researcher then works on predictions: she imagines scenarios for materials that are not yet on the market and analyzes whether this has any impact on an environmental level. Also, given the recent nature of the bio-materials sector, their production will be less optimized than that of traditional petroleum-based materials which have evolved and improved. “This works against the newer materials, as they will need to go through several developmental stages,” she points out.

This research is essential for the development of sustainable alternatives to conventional materials and is part of an overall vision to meet the challenges of plastic pollution. “A lot of research is focused on the development of new materials with a smaller environmental impact. The VOICE project, for example, launched in 2018 in partnership with Innovation Plasturgie Composites, Silvadec, IMT Mines Alès and funded by ADEME, focuses on the recycling of biocomposite decking. Or the Agrobranche project, which brings together 8 French partners[1], and focuses on, among other things, the study of bio-based reinforcements from agroforestry,” she concludes. These two projects reflect the blossoming of new emerging materials and the current development of research to find sustainable alternatives to the plastic problem.

[1] Scop Agroof (coordinator), IMT Mines Alès, CIRAD BiowooEB, LERMAB, FRD, CRA Bretagne, CIRAD AMAP, INRA SPO

Tiphaine Claveau

Decontaminating and treating waste from the steel industry

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The manufacture of steel produces mineral residues called steel slags, which are stored in large quantities in slag dumps. These present a dual challenge. On the one hand, they are potentially harmful for the environment and health, and on the other hand they are a useful resource for certain industries. The HYPASS project at Mines Saint-Étienne aims to address both of these issues. Launched in 2018, it offers new solutions for extracting heavy metals and managing pollution from steel slag dumps.

During the steel manufacturing process, iron ore is heated to high temperatures. A lighter residue phase forms on the surface, like whey. When it has cooled, this artificial rock, the slag, is poured into slag dumps which can spread over several hectares.

In France, there are some 30 million tons of steel slag accumulated in dumps. This residue contains heavy metals that are a danger for health and the environment on a large scale because polluting particles can be transmitted through erosion. It is therefore important to limit the diffusion of these particles. 

However, this slag can be used! There are a wide range of fields of application for steel slag today, including the production of concrete and cement, the glass industry, ceramics and even agriculture. Unfortunately, the presence of heavy metals in the slag can be a stumbling block because they can have a negative impact on the spaces where they are used. These metals, such as chrome, molybdenum or tungsten, each have different industrial uses. By efficiently and optimally extracting the heavy metals from the steel slag, these slag dumps could be decontaminated and new ways of reusing the materials could be developed. 

To address these challenges, the HYPASS (HYdrometallurgy and Phyto Management Approaches for Steel Slag management) project, financed by the French National Research Agency and certified by AXELERA, a competitiveness cluster for the chemical and environmental sectors, was launched in 2018. It includes Mines Saint-Étienne.1 The HYPASS methodology has been implemented at the slag dump in Châteauneuf, in the Loire, which is listed as a member of the SAFIR (French Innovation and Research Sites) network. The project aims to develop an innovative technological approach to allow recovery of strategic metals from slag and, at the same time, a more environmentally-friendly management of steel slag dumps.

Extracting heavy metals                                 

The first part of the project consists in extracting the heavy metals from the slag using hydrometallurgy. This technique extracts minerals using a solution during a process called leaching. “Hydrometallurgy dates from the early 20th century and was originally used on ores with a high metal content, such as gold extraction by cyanidation”, explains Fernando Pereira, a researcher on the HYPASS project at Mines Saint-Étienne. “However, over the last 30 years or so, hydrometallurgy has also been increasingly used for the treatment of waste which could be considered as low metal content ores.”

Although the first stages of the technique can sometimes differ, it generally involves physical pretreatment of the mineral matrix, dissolution of the metals in acid or alkaline reagents, high-temperature roasting and then purification and refining.

As part of the HYPASS project, the researchers from Mines Saint-Étienne, in association with the French Geological Survey (BRGM), have made several important adjustments in the laboratory to the stages in the hydrometallurgical extraction process. These adjustments had to take account of the efficiency of the techniques as well as their financial aspects, because some processes can prove to be particularly expensive when used at an industrial scale.

“The most expensive aspect isn’t the fact of using the different reagents during the leaching stage, but the preliminary grinding and roasting stages [heating to make the metal oxides more soluble] because they require large amounts of energy”, says Fernando Pereira. An important modification to the temperature adjustment during the roasting stage was therefore necessary to optimize the efficiency in relation to cost. Another original adjustment was made in the choice of reagents used to extract the metals. Usual methodologies are based on the use of acids to dissolve the ores, but this implies a non-selective extraction, meaning the different compounds are mixed in the extraction solution. The HYPASS methodology has developed the use of alkaline reagents that allow the specific extraction of strategic metals, thus facilitating their subsequent use as well as that of the mineral matrix. 

Finalization of the phytostabilization method 

The second part of the project consists in stabilizing the slag dump pollution by covering it with plants. However, it is very difficult to grow plants on slag dumps for a number of reasons: the soil of these sites is extremely alkaline, it has no organic matter and few essential elements for growth such as nitrogen and phosphorus. In addition, this soil has poor rainwater retention. It is also highly toxic, due in particular to the presence of a specific form of chromium, known as chromium VI, which is a known carcinogen.

Read more on I’MTech: When plants help us fight pollution

It is therefore a real challenge to grow plants in an environment as hostile as a slag dump. Mathieu Scattolin, in charge of the phytostabilisation part of the HYPASS project, has made important adjustments to the growing conditions of plants in these environments. “Experiments have shown that pH is a key factor for the success of implementing phytostabilization on slag heaps,” says Pereira.

When the soil is too alkaline, certain chemical elements that are important for plant growth (such as manganese, zinc and phosphorus) develop properties that make them less phytodisposable, meaning that less is transferred from the soil to the plant. On the other hand, toxic chemical elements, such as Chrome VI, tend to be assimilated more. 

To resolve these difficulties, a species of fungus called Rhizophagus irregularis was inoculated with the plants. “Wherever we were in the slag dump, the inoculation of Rhizophagus Irregularis led to relatively fast colonization of the root systems”, says the researcher. This symbiosis notably allows the soil to be made less alkaline, thus increasing the phytodisposability of important elements and reducing that of chromium VI. The presence of this fungus in the soil also allowed the supply or organic matter and the increase of the water retention of the soils.

The perfection of the optimal growth conditions was tested in the laboratory and then in experimental sections of Châteauneuf slag dump, which led to a rapid colonization of the root systems. It also led to the launch of a new component in the HYPASS project.

A decision support tool is being developed to compare different scenarios for managing steel slag dumps. Fernando Pereira explains that “this tool will allow us to compare and choose management scenarios for steel slag based on criteria concerning environmental impact, financial costs and support for the ecosystem.” The design work is based on the principles of life cycle analysis (LCA) to allow the tool to provide an estimation of the global environmental impacts for each possible scenario.

In terms of hydrometallurgy, there are still a few phases of development in the laboratory. Kinetic monitoring is envisaged in order to minimize the leaching time of the metals. Metal oxide capture tests and solutions using microwave technology – to see if it is possible to get rid of the roasting stage, which is particularly energy-intensive – are also being developed. The phytostabilization part, on the other hand, seems to be finalized.

A European project is envisaged as a follow-up, including a scale-up and the development of more important laboratory trials. “This would be part of a Horizon Europe-type project” says the researcher, to give a broader perspective of the project.

By Antonin Counillon.

1 Mines Saint-Étienne is part of the HYPASS project through the Environment, City, Society mixed research unit.