Written and maintained by CASRAI Editorial Board
Last updated
A ball mill is a grinding machine that reduces a solid sample to fine powder by tumbling, shaking or spinning it in a closed jar together with hard grinding balls. In laboratories it is a standard sample-preparation instrument for homogenizing and pulverizing solids before analysis. Lab staff, core-facility users and equipment buyers mostly meet three designs: planetary, mixer and drum mills.
What is a ball mill?
A ball mill (sometimes called a ball grinder) grinds material by impact and friction between grinding balls, the jar wall and the sample. Manufacturer Retsch describes its laboratory ball mills as covering dry and wet grinding, with listed final fineness from below 15 µm for large drum mills down to 0.1 µm for planetary and mixer models (Retsch ball-mill overview, read 2026-11-07). Typical users are analytical, materials, geology and biology laboratories that need a homogeneous powder or, with bead beating, disrupted cells. For small-volume manual grinding see mortar and pestle; for liquid or tissue samples see homogenizer and sonicator.
How does a ball mill work?
The jar is filled partly with sample and grinding balls, sealed, and moved so the balls strike and rub the sample. What differs between designs is the motion. In a planetary ball mill the jar sits eccentrically on a rotating sun wheel and turns in the opposite direction at a 1:-2 ratio; the speed difference between balls and jar produces both friction and impact (Retsch PM 100 page, read 2026-11-07). In a mixer mill the jar oscillates radially in a horizontal position. In a drum mill the jar spins on its own axis.
Planetary ball mill
A planetary ball mill is the high-energy option for fine and colloidal grinding. The Retsch PM 100 runs at 100-650 rpm, reaches a maximum acceleration of 33.3 g and lists final fineness below 1 µm, and below 0.1 µm for colloidal grinding. Its jars are pressure-tight and dust-proof thanks to an O-ring seal. Process parameters such as ball properties, filling ratio and speed, and guidelines on wear contamination, are reviewed in Burmeister and Kwade, Chemical Society Reviews 2013, 42, 7660-7667 (DOI 10.1039/C3CS35455E).
Mixer mill
The Retsch MM 400 shakes jars at 3-30 Hz, takes jars of 1.5, 5, 10, 25, 35 and 50 ml, accepts feed up to 8 mm and lists a final fineness of about 5 µm, with a maximum of 2 x 20 ml of sample. It also supports cryogenic grinding: with the CryoKit, jars are pre-cooled in a liquid nitrogen bath for about 2 minutes (Retsch MM 400 page, read 2026-11-07).
Types of laboratory ball mill
| Type | Motion | Capacity (Retsch range) | Final fineness listed |
|---|---|---|---|
| Planetary | Jar on a circular path with counter-rotation | Up to 4 x 220 ml | 0.1 µm |
| Mixer mill | Jar shakes horizontally | Up to 6 x 20 ml | 0.1 µm |
| High-energy ball mill | High-energy impact, up to 76 g | Up to 2 x 45 ml | 0.08 µm |
| Drum mill | Jar spins on its axis | Up to 35 l | Below 15 µm (feed below 20 mm) |
Source: Retsch, Ball Mills overview, read 2026-11-07. Figures are one manufacturer’s product range, not an industry-wide classification. Retsch also lists a dedicated XRD-Mill McCrone for X-ray diffraction sample preparation.
Jar and ball sizes that matter
For the planetary PM 100, nominal jar sizes are 12, 25, 50, 80, 125, 250 and 500 ml, with the 12-80 ml sizes stackable. Jar and ball materials offered include hardened steel, stainless steel, tungsten carbide, agate, sintered aluminium oxide and zirconium oxide. Mixer-mill jars add PTFE and PMMA (Retsch product pages, read 2026-11-07).
How to choose jar and ball material, and ball size
What size balls for a ball mill? Retsch gives a rule of thumb for the PM 100: the balls should be about three times the size of the largest sample piece. For other applications, consult the manual and the process literature cited below.
- Feed size: match ball diameter to the largest piece in the sample (about 3x for the PM 100).
- Wear and contamination: jar and ball wear can add material to the powder; the Burmeister and Kwade review includes guidelines on wear contamination. The Retsch pages read give no contamination figures for its jar materials.
- Temperature: heat-sensitive or tough samples can be ground cryogenically in a mixer mill with pre-cooled jars.
- Dry or wet: Retsch lists both modes for its planetary and mixer models.
Safe use and handling basics
Use only the jars, lids and balls the manufacturer specifies for the model, close the jar with its seal, and follow the instrument manual for filling level, speed and run time. On the PM 100, jars of agate, aluminium oxide, zirconium oxide or tungsten carbide are fitted in a stainless protective jacket. For liquid nitrogen cooling, follow your institution’s cryogen handling rules and the manual.
Common mistakes
- Choosing balls without regard to the largest feed particle.
- Ignoring wear contamination when the powder goes to trace-element or phase analysis.
- Expecting planetary-level fineness from a drum mill; the listed finenesses differ by design.
- Using jars or closures not specified for the model.
Frequently asked questions
What is a ball mill?
A grinding machine that pulverizes solids inside a closed jar using hard balls, by impact and friction.
How does a ball mill work?
The jar is moved (planetary counter-rotation, horizontal oscillation or spinning on its axis) so balls strike and rub the sample until it reaches the desired fineness.
What is a planetary ball mill?
A mill whose jar rides on a sun wheel and turns in the opposite direction (1:-2 on the Retsch PM 100), combining friction and impact for fine grinding.
What is a ball grinder?
The same concept as a ball mill: a grinder that uses balls as the grinding medium.
What size balls for a ball mill?
For the Retsch PM 100, about three times the size of the largest sample piece.
Sources
- Retsch, Ball Mills, read 2026-11-07.
- Retsch, Planetary Ball Mill PM 100, read 2026-11-07.
- Retsch, Mixer Mill MM 400, read 2026-11-07.
- Burmeister CF, Kwade A. Process engineering with planetary ball mills. Chem Soc Rev 2013;42:7660-7667. DOI 10.1039/C3CS35455E.
