Mills are chosen by material behaviour, not by target fineness. How brittle, tough, elastic and heat sensitive samples each need a different grinding action, and where a second machine is unavoidable.
Laboratories buy mills by target fineness. Catalogues are written that way, and it is the wrong starting point. Two samples that both have to reach 50 micrometres, one a fired clinker and one a polypropylene offcut, need entirely different machines, and no amount of running time on the wrong one will get either there.
The right starting point is how the material fails. Brittle material fractures under impact. Tough and elastic material deforms instead and has to be cut. Fibrous material wraps around anything that spins. Heat sensitive material changes while it is being ground. Those four behaviours name the machine before fineness is discussed at all.
Write down two numbers before looking at any mill: the largest piece that will be fed in, and the fineness required at the end. The ratio between them decides whether one machine will do.
A single mill covers roughly one to two orders of magnitude of size reduction well. Beyond that the sample spends a long time in a machine designed for a different stage, which heats it, contaminates it and wears the grinding tools. A 60 mm lump of rock going to 40 micrometres is a two step job. The jaw material matters for the same reason: if the laboratory will analyse the sample for the element the jaws are made of, the crusher is a contamination source and the analysis quietly reports the machine.
Two supporting items belong in the same order and are usually forgotten. The DR 100: Vibratory Feeder meters pourable material into a mill at a steady rate, which is what makes throughput and grinding results reproducible when a mill runs unattended. The TG 200: Fluid Bed Dryer dries bulk material in warm air before milling, so moisture does not distort the result. Material that has sat through a monsoon week in an unconditioned store will not grind the way it did in February.
Jaw crushers take medium hard, hard, brittle and tough material and reduce it by compression between a fixed and a moving jaw. They are the first stage for rock, ore, clinker, slag, ceramics and construction material, and they are chosen by feed size and throughput rather than by final fineness.
Plastics, rubber, textiles, cable, plant material, packaging and paper cannot be crushed. They absorb the impact and spring back, so they are cut against a stationary knife until the fragment passes a bottom sieve, and that sieve sets the final fineness.
The SM 50: Cutting Mill is the benchtop unit for small quantities, with a grinding chamber designed so a high proportion of the sample can be recovered, which matters when there is only a little of it. The SM 100: Cutting Mill is the basic routine machine at a fixed 1500 rpm, with the bottom sieve setting fineness between 0.25 mm and 20 mm. The SM 200: Cutting Mill has a stronger drive and a chamber arranged for straightforward cleaning, which is the real bottleneck when a laboratory runs mixed material types back to back.
The SM 300: Cutting Mill is a different proposition. Its RES technology stores energy in a flywheel and releases it into the cut, so the rotor holds speed through tough material that would stall a conventional mill, and speed is adjustable from 100 to 3000 rpm, which lets heat sensitive samples be run slowly. For a plastics or textiles laboratory handling recycled and mixed feedstock, that combination is worth the difference.
Where the sample is wet, fatty or oily, cutting gives way to knife milling. The GRINDOMIX GM 200: Knife Mill homogenises food and feed containing water, fat or oil as well as dry and fibrous samples, with two blade speeds for coarse cutting then fine homogenisation in one vessel and a boost setting to 14,000 rpm. The GRINDOMIX GM 300: Knife Mill takes batches up to 4500 ml and has autoclavable sample contact parts, which is the deciding feature where microbiological testing follows. For near infrared work the TWISTER: Cyclone Mill draws ground material out through a cyclone separator, limiting the heating and moisture loss that shift an NIR result.
Below roughly a millimetre the choice divides by how the energy is delivered, and by whether the sample can be exposed to air.
| Action | Machine | Reaches |
|---|---|---|
| Impact and shearing against a ring sieve | ZM 300 ultra centrifugal mill, speed selectable to 23,000 rpm with temperature monitoring | Below 40 micrometres on soft, medium hard, brittle and fibrous material |
| Impact against fixed beaters | SR 300 rotor beater mill, speed adjustable in 500 rpm steps | Below 50 micrometres on soft to medium hard material |
| Cross beating with an exchangeable bottom sieve | SK 300 cross beater mill | Below 100 micrometres on coal, coke, minerals and slag |
| Swinging hammers past a bottom sieve | HM 200 hammer mill at up to 1500 kg per hour | Below 0.8 mm on large sample volumes |
| Shearing between two discs, gap 0.05 to 5 mm | DM 200 disc mill | Preliminary and fine grinding of medium hard, hard and brittle material |
| Rings and a puck oscillating in a sealed jar | RS 200 vibratory disc mill, or RS 300 XL for 35 ml to 2000 ml batches | Below 20 micrometres in seconds, which is what spectral analysis needs |
Ball mills are the other family, and they are chosen by batch size and by how much energy has to go in. The PM 100: Planetary Ball Mill is the single station machine, reaching below 1 micrometre and 0.1 micrometre for colloidal grinding. The PM 200: Planetary Ball Mill takes two stations so two samples run under identical conditions, and can measure input energy. The PM 300: Planetary Ball Mill takes jars up to 500 ml at a centrifugal acceleration of 64 g, and the PM 400: Planetary Ball Mill is the floor standing four station machine at a maximum of 4 x 220 ml, with a speed ratio selectable at 1 to minus 2, 1 to minus 2.5 or 1 to minus 3, which changes the balance between impact and friction in the grinding action.
For nanoscale work the limit is heat, not energy. The Emax: High Energy Ball Mill reaches below 80 nm by combining high frequency impact with intensive friction, and its integrated water cooling and temperature control are what allow long runs without the sample overheating. Where the sample is large rather than demanding, the TM 300: Drum Mill and Bond Index Tester takes drums from 5 to 43.3 litres and doubles as a Bond Index Tester with the ball or rod module fitted; the TM 500: Drum Mill handles up to 35 litres down to about 15 micrometres.
Two special cases deserve naming. The XRD-Mill McCrone uses 48 cylindrical grinding elements to reach below 1 micrometre with an action gentle enough to preserve the crystal lattice, which is what a diffraction measurement depends on. And the RM 200: Mortar Grinder reproduces the manual mortar and pestle at a set pestle pressure and a fixed 100 rpm, wet or dry, to below 10 micrometres, so the result stops depending on who is doing the grinding.
Elastic and temperature sensitive materials become grindable when they are cold enough to be brittle. Rubber, waxes, fatty foods, polymers, tissue and pharmaceutical actives all fall into this group, and so does anything whose volatile content is part of the analysis.
The CryoMill cools the grinding jar continuously with liquid nitrogen to minus 196 degrees Celsius before and throughout the cycle. The important detail is that the jar is cooled, not the sample: the sample stays dry, so there is no condensation to interfere with a moisture or volatiles determination afterwards.
Mixer mills cover cryogenic work alongside dry and wet grinding. The MM 400: Mixer Mill oscillates two jars radially at 3 to 30 Hz with typical grinding times of 30 seconds to 2 minutes, and also disrupts cells in reaction vials. The MM 500 control: Mixer Mill holds jar temperature anywhere from minus 100 to 100 degrees Celsius, so the thermal conditions of a mechanochemical run can be set rather than tolerated. The MM 500 nano: Mixer Mill reaches about 0.1 micrometre from batches of up to 2 x 45 ml, and the MM 500 vario: Mixer Mill has six stations and processes up to 50 samples in one run with adapter racks.
Before committing to cryogenic milling in Bangladesh, settle the nitrogen supply. A CryoMill consumes liquid nitrogen every run, and a laboratory without reliable local dewar exchange will find it idle for weeks. Settle the practicalities in cryogen supply for a research laboratory in Bangladesh, and they should be resolved before the mill is ordered, not after.
One last item that belongs with the ball mills rather than the crushers. GrindControl: Pressure and Temperature Measuring System records pressure and temperature inside a sealed grinding jar and transmits it wirelessly. For mechanochemistry it turns a mill into an instrument. For everyone else it is the fastest way to discover that a grinding programme is cooking the sample.