This article was originally published in the Cosmos Print Magazine, September 2024.
THE DEVICE CAN recycle batteries and solar panels, make magnets and pharmaceuticals, and pull individual gas molecules out of a mixture.
It can do all of these things with less energy, and fewer additives, than industry currently uses. The tool could dramatically improve the environmental impact of chemical manufacturing, while making it more productive. It might even end up providing water and air in space.
This technological marvel, capable of so much, is a ball mill.
It’s a 19th century invention: a rotating cylinder filled with steel ball bearings, designed to grind materials into fine powder. The ball mill is more systematic than your kitchen mortar and pestle, but it’s doing essentially the same thing.
Yet ball milling – and the field it belongs to, mechanochemistry – is enjoying a renaissance.
“There is inherent greenness to it,” says Professor Tomislav Friščić, a chemist at the UK’s University of Birmingham. Friščić says this sustainability is one reason mechanochemistry is getting so much attention.
“And another reason is that it actually works.”
A simple ball mill. Credit: Bundesanstalt Für Materialforschung (BAM).
Trapping gas and circular batteries
The first time Dr Srikanth Mateti used a ball mill, the experiment worked so well that he thought he’d made a mistake. The results were just too good to make sense.
“I was a second year PhD student,” Mateti, now a research fellow at Deakin University, recalls. He was using a ball mill to combine a hydrocarbon gas with a compound called boron nitride (also called white graphene), made from boron and nitrogen.
“It’s like using a front loader washing machine. You pull it closed, you put liquid inside, and it starts,” says Mateti. Instead of water, he was pumping in gas, monitoring it by checking the pressure inside the mill.
“But after some time, the gas disappeared,” he says. According to the meter, the pressure inside the chamber was zero. Mateti’s supervisor, Professor Ying Chen, suggested things he could be doing wrong.
Was there a leak? Was the gas cylinder empty? Did he not fill the mill properly? Should he redesign the experiment? He retried the process “20 or 30 times”.
“Whatever I did, it’s a zero. Zero, zero, zero.”
After a couple of “frustrating” months, verifying every piece of equipment was working properly, Mateti reached another conclusion. The gas was being completely absorbed by the boron nitride. He theorised: “If all the gas goes into the material, I should expect this kind of gas will be there when I heat it. It should release.”
Lo and behold, he had not discovered an error, but a shockingly efficient way to store and transport gas. Pump it into a ball mill with some boron nitride, run the mill for up to a day, and your gas is now a solid, safe, easy-to-transport powder. When you want the gas back, just heat the powder and collect it.
Dr Srikanth Mateti and Professor Ying (Ian) Chen. Credit: Supplied.
The method started with hydrocarbons, but Mateti’s team has shown it can work with a variety of other gases, including carbon dioxide, ammonia, and hydrogen. The storing and transport of these three gases is an increasingly important problem: CO2 captured from the atmosphere and industrial processes needs to go somewhere, while hydrogen and ammonia both have big roles to play in energy and agriculture. In gaseous form, they’re leaky and dangerous. But combined with boron nitride in a ball mill, they become far more malleable.
The process is also selective: you can tune it to just suck one type of gas, like CO2, out of a mixture.
“We are happy with the results, because the method and the material is versatile,” says Mateti. He and his colleagues are now pursuing several different applications for the ball mill and boron nitride method, filing patents as they go.
Ball milling is a more familiar concept to battery researchers, particularly those interested in recycling. Lithium-ion batteries are complicated mixtures of precious metals, with no two manufacturers using exactly the same recipe. Plants normally need to disassemble each battery, then mill them into a sand-like substance called black mass. The black mass is submitted to either pyrometallurgy or hydrometallurgy – a series of high temperatures, or corrosive solvents, respectively – to extract the lithium and other precious metals for re-use.
Last year, Dr Oleksandr Dolotko, an engineer in battery recycling, and his colleagues at the Karlsruhe Institute of Technology (KIT) in Germany, published the details of a new technique. The team found that adding battery cathodes to a ball mill, along with a reducing agent like aluminium, yields a mixture rich in an oxide of lithium. Conveniently, the lithium is the only part of the mixture that dissolves in water.
“By simple water leaching and filtration, we can separate lithium from all other byproducts,” says Dolotko.
The “universal” battery recycling method requires no high temperatures, and no additional chemicals. Aluminium is already a component of lithium-ion batteries, which simplifies the process further. The other precious metals in the battery are also changed into forms that are much easier to recover.
“Other critical components such as cobalt, nickel and manganese, are also chemically reduced,” says Dolotko. They can be extracted at room temperature, using acids a tenth as strong as those currently used in industry.
Dolotko is continuing to collaborate with the KIT researchers to optimise and scale the technique.
“Currently, the scalability of our process is constrained by the design of the milling machines,” he says. The milling and leaching works perfectly at the biggest size they have, in the lab. Now, the game is to iron out safety concerns – fine particles of metal do, unfortunately, carry some ignition risk – and find an industry partner to help make the process even bigger.
Inside the mill
So why does it work?
Reactions – or at least, reactions that chemists are interested in – don’t usually happen spontaneously. Extra energy is needed to break and make bonds between atoms.
If you’ve ever baked a cake, you understand this intuitively. Cake mix will stay just that – a mixture – until it’s put in a hot oven. Heat is necessary to make the ingredients react into cake.If you’re putting clothes in a washing machine, you’re also using a chemical reaction: one between soap in the washing powder, and dirt on the fabric. But this reaction doesn’t necessarily need heat; the “cold wash” setting still gets your stuff clean. Instead, washing machines use mechanical energy to work. The machine spins, and clothing, water and washing powder crash into each other. This movement spurs the soap–dirt reaction.
Ball mills employ the washing machine method.
“In the ball milling process, balls collide with each other and the vial walls, generating mechanical energy from these impacts,” says Dolotko. The energy and high pressure forces chemical bonds to break and reform.
“The materials undergo stress, friction, deformation… a fresh surface area is created, you’re getting shear force, and different impacts on the materials,” says Mateti. “This mechanical energy triggers the chemical reactions.”
That’s the heart of mechanochemistry: mechanical energy triggering chemical reactions. Mateti and Dolotko are far from the only researchers who have discovered its value.
An illustration of a process for making perovskite, a chemical used in solar cells. Two salts and grinding balls are put into a container and rotated continuously. The slow reactions in rotating mills divide the process into several steps, which allows researchers to see the intermediate phases of the reactions. The photo in the bottom–right shows the experimental setup. Credit: BAEK, KY., LEE, W., LEE, J. ET AL. MECHANOCHEMISTRY-DRIVEN ENGINEERING OF 0D/3D HETEROSTRUCTURE FOR DESIGNING HIGHLY LUMINESCENT CS–PB–BR PEROVSKITES. NAT COMMUN 13, 4263 (2022).
Mechanochemistry moves onwards
“Fifteen years ago, there were maybe 30–40 papers per year in mechanochemistry,” says Professor James Batteas, a chemist at Texas A&M University, USA. “Now what we’re seeing is more on the order of 800 papers a year, and it’s continuing to grow.”
Batteas, who takes his martinis shaken and not stirred, is the co-editor-in-chief of the journal Mechanochemistry, which was launched by the UK’s Royal Society of Chemistry in March 2024. It’s the first journal dedicated solely to the field.
“We’d heard quite a lot from people working in mechanochemistry that they’d struggled to get peer review on their papers from mechanochemistry experts,” says Dr Laura Fisher, executive editor of the journal. Mechanochemistry crosses many different disciplines, from engineering to biology. Alongside conferences and associations, the journal is aiming to coalesce the community.
Friščić, the other editor-in-chief of the journal, says that people frequently approach him after conference presentations with revelations about the value of grinding.
“They might say: ‘Hey, 15 years ago my grandmother was grinding something in a mortar and pestle, and it changed colour – I think that was mechanochemistry’. I say ‘yeah, absolutely!’”.
Results in Grandma’s mortar and pestle are very difficult to reproduce – there are just too many variables involved. But ball mills are much more replicable. This has made them a staple of the field.
“Ball mills have come out on top as a simple, and relatively affordable, solution to do mechanochemistry that’s reproducible,” says Friščić.
“It’s very easy to scale up. If somebody has parameters which work in the lab scale, it’s a cakewalk to fine tune the parameters in the large scale,” says Mateti of ball mills. He adds that it typically only needs a one-step reaction to get the product you want – the fewer steps, the higher your yield and the lower the risk of contamination.
It’s such a successful method that it prompts another question: given grinding is a prehistoric technique, why are chemists only discovering its value now?
“I for one am quite certain we will see mechanochemical reactors on the Moon and Mars,” says Batteas.
One reason is sustainability: ball milling is a textbook example of green chemistry. “10–20 years ago, that wasn’t such a hot topic,” says Fisher.
Chemists, like many makers, are increasingly motivated by environmental concerns. Mechanochemistry can be done at room temperature, and it doesn’t need reagents to be in liquid form to mix. This removes the need for chemicals to dissolve the reagents – solvents are a huge drain on resources, and a huge source of waste, in chemistry.
The pharmaceutical industry is another influence. “There’s a big drive to make new medicines and new drugs, particularly after COVID,” says Fisher. Mechanochemistry offers new reaction pathways to new molecules.
But central to the interest is the ability to properly understand mechanochemistry.
“What does it mean to shake, to vibrate, to compress, to move materials around, to push particles together, to create compression waves? What does it really mean for chemistry?” asks Friščić.
“We now have different toolsets that allow us to start to really understand what’s happening in these reactor systems on the molecular level,” says Batteas. “I like to talk about mechanical effects as martini shaker chemistry – I throw some things in there and get some things out. But scientifically, there’s a real drive to understand: how do you make mechanochemistry controllable and predictable?”
Chemists need newer analytical technology to do this. Spectroscopy – shining different types of light through molecules to understand their shapes – became much more useful for these purposes over the 2010s.
For example: “There’s a lot of really nice work that’s been going on for almost a decade by sticking ball mills into synchrotron X-ray paths,” says Friščić.
At Deakin, Mateti has one student who is dedicating a PhD to studying the boron nitride and gas reactions inside a ball mill in real-time, using X-Ray and Raman spectroscopy. At the moment, the team has to “reverse-engineer” reactions to figure out why they work. If they could watch them proceed, they’d be able to optimise all sorts of parameters – including speed and type of rotation, ball-to-powder ratio, temperature and pressure – to get the fastest results.
“Instead of doing 10 hours blindly, maybe you’d do one hour of ball milling. You’d save a lot of time, energy, everything,” says Mateti.
Ball mills are a compellingly simple piece of technology. But they conceal a world of far more complicated molecular interactions. Chemists are just beginning to commercialise their grinding work, but they understand the field has potential.
“I believe this could be the best way to do manufacturing, and even synthesis,” says Friščić.
“I for one am quite certain we will see mechanochemical reactors on the Moon and Mars, because that’s how you’re going to do manufacturing there,” says Batteas. No one wants to ship tonnes of solvents into space. So if you’re going to turn the rocks up there into anything, it will probably need to happen with grinding.
“Can we do mechanochemistry to produce water or oxygen from the materials that are there? You’ve got lots of silicates, there’s lots of oxygen bound up in there. You’re going to find materials with trapped water crystals,” suggests Batteas. “Hopefully, someone will fund this!”
Whether or not the sky is the limit, it’s clear that mechanochemistry, and the humble ball mill, has many more successes rolling inside it.
“It can be simple, it can be fast. And sometimes you get things out of it that you don’t quite expect,” says Friščić. “The element of surprise keeps us busy.”