Mills and grinders might sound like tools that peaked in laboratory uptake years ago, but scientists keep finding new uses for them. “There are lots of new applications for mills and grinders,” says Brian Vargo, product manager at Retsch, which operates its headquarters in Haan, Germany. “For one thing, folks are trying to do advanced materials research, like nanotechnology.”
In general, says Refika Bilgic, managing director at IKA Works in Wilmington, North Carolina, “Mills are commonly used in sample preparation for quality control—for example, tablets, seeds, waste samples, bio-fuels, foods, etc.” Those examples alone cover a wide range of applications for mills and grinders, but even more exist.
Getting finely ground materials also matters in areas that might not be expected, even by many scientists. For instance, Scott L. Anderson, distinguished professor of physical and analytical chemistry at the University of Utah, uses ball mills to make additives for rocket fuel. In some cases, Anderson takes micron-size boron, which can be purchased commercially, and grinds it to particles that are 50- 100 nanometers across. Then he coats the particles with an organic layer. “That prevents the metal from oxidizing prematurely and makes the metal particles readily dispersible in the fuel,” Anderson says. “The smaller particles heat up and burn faster, which is important for things like maneuvering thrusters that need pretty fast combustion rates.”
The list of new mill-and-grinder users goes on from there. Vargo describes examples in energy, especially working with biomass and even recycling nuclear fuel rods, municipal solid waste, and more.
Behind the grind
In today’s mills and grinders, many users expect new features. “People are looking more at automation in mills and grinders, giving them better integration in work-flow solutions,” says Tim Osborn-Jones, director of marketing and international sales at SPEX SamplePrep in Metuchen, New Jersey. “They also look for user interfaces that let them save programs and parameters.”
The ways to grind materials mirror the wide range of applications. For example, some devices grind materials at very low temperatures, even in the cryo range. A variety of companies offer cryogenic grinders, and they have for some time. “The cold helps grind some samples that are too malleable to grind to a powder at room temperature but can be ground at a cryogenic temperature,” says Osborn-Jones. “The cryogenic temperature also helps to preserve some samples, like proteins or volatile compounds.”
Sometimes, getting the most from a mill or grinder comes from following counterintuitive advice. For instance, Vargo points out that one of his company’s products offers a speed range up to 18,000 revolutions per minute. “We generally run it flat out,” he says. “That seems counterintuitive, so I’ll verify that a customer has that knowledge.”
If counterintuitive isn’t enough, sometimes you need to just listen to some savvy advice from the Rolling Stones: “you can’t always get what you want.” Many users want to rock and roll through every grind in one step. “Sometimes that’s possible,” Vargo says, “and sometimes you can do it in one step, but it puts extra strain on the machine.” As an example, he says, “If you want to go from 3-centimeter particles to 250-micron ones, doing it in two steps might be better.”
Osborn-Jones also sees users overloading mills. To make a mill run right longer, he says, “Follow the device’s guidelines for sample size and weight.”
On top of the need for more than one step in some applications, labs might also need more than one mill or grinder. As Vargo says, “Everyone wants one machine that does it all, but that’s not necessarily the best approach if one mill is really designed for one thing and being used for another.” Using the wrong mill for the job can even destroy the device.
Matching the mill
How does a lab manager get the right mill for the job? Vargo suggests asking three questions:
• What is the initial particle size?
• What size do you want to get down to?
• What is the batch size?
“The answers to those questions narrow the focus on the appropriate mill for a specific application,” Vargo says.
Bilgic adds that a lab manager can get the most from a mill by using one “that will allow for sample processing to the desired particle size in a very short period of time.”
In some applications, scientists must experiment to get the desired results. As Anderson says about making rocket-fuel additives, “We played with the milling conditions to get these very small particles.” The programmability of his mills helped with that. He says, “For example, we mill for a while, then rest, and then reverse direction. So we set up automated milling runs that may go 15 or 16 hours.” He adds, “So program control comes in very handy.”
To allow such experimentation, lab managers should not be required to buy a different mill or grinder for every test. “Scientists expect some flexibility in the way that a machine can be used,” says Osborn-Jones. “You can get a device that works with more than one type of sample or application.” For example, some grinders work with several vial formats and sizes—where the sample goes. “That allows the user to perform different applications and throughput in one machine,” Osborn-Jones explains.
So getting the best performance for the longest time in today’s mills and grinders depends on a balance between getting the right machine for the job at hand and knowing when a machine can provide more flexibility in its applications. It’s possible that a researcher will even come up with new ways to use a mill—even turning it into actual rocket science. To achieve that, a mill needs precise controls to keep experiments accurate.
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