Mixer Mills Fast homogenization of small sample volumes

Mixer mills grind and homogenize small sample volumes quickly and efficiently by impact and friction. These ball mills are suitable for dry, wet and cryogenic grinding as well as for cell disruption for DNA/RNA or protein recovery. For special applications such as mechanosynthesis, they offer unique solutions. Mixer mills are well known for their ease of use and small foot-print compared to other types of ball mills.

Cooling & Heating

Mixer Mill MM 500 control

Material feed size*: <= 10 mm
Final fineness*: ~ 0.1 µm
Vibrational frequency: 3 - 30 Hz (180 -1800 min-1)
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Background information

Details on the fields of application, working mechanisms and materials used in mixer mills.
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Nanoscale Grinding

Mixer Mill MM 500 Nano

Material feed size*: <= 10 mm
Final fineness*: ~ 0.1 µm
Vibrational frequency: 3 - 35 Hz (180 - 2100 min-1)
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Six samples in one batch

Mixer Mill MM 500 Vario

Material feed size*: <= 8 mm
Final fineness*: ~ 5 µm
Vibrational frequency: 3 - 35 Hz (180 - 2100 min-1)
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The classic allrounder

Mixer Mill MM 400

Material feed size*: <= 8 mm
Final fineness*: ~ 5 µm
Vibrational frequency: 3 - 30 Hz (180 - 1800 min-1)
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Grinding with liquid nitrogen

CryoMill

Material feed size*: <= 8 mm
Final fineness*: ~ 5 µm
Vibrational frequency: digital, 5 - 30 Hz (300 - 1800 min-1)
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Measuring System GrindControl

Measurement of pressure and temperature in the grinding jar
Pressure measurement 0-5 bar
Temperature measurement: -25 °C - +90 °C
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Mixer Mills - Function Principle

The grinding jars of mixer mills perform radial oscillations in a horizontal position. The inertia of the grinding balls causes them to impact with high energy on the sample material at the rounded ends of the jars and pulverize it. High energy milling is possible by operating at high frequencies up to 35 Hz. The movement of the jars and balls causes further size reduction effects through friction and additionally leads to effective mixing of the sample. The degree of mixing can be increased by using several smaller balls.

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Mixer Mills - Fields of application

Mixer mills are used for the pulverization of soft, hard, brittle, and fibrous materials in dry and wet mode. With their small footprint, ease of use, and very short processing times, they are true allrounders in the laboratory.

Mixer mills are ideally suited for tasks in research like mechanochemistry (mechanosynthesis, mechanical alloying and mechanocatalysis), or ultrafine colloidal grinding on a nanometer scale, as well as for routine tasks such as mixing and homogenizing.

They are also widely used for cell disruption for DNA/RNA extraction via bead beating. Up to 240 ml of cell dispersions can be processed for protein extraction or metabolome analysis.
A crucial advantage of mixer mills is their great versatility – in some models combined with the capacity to actively cool or heat material, allowing for more controlled configurations than in other ball mills. In the field of mechanochemistry, the possibility to control the reactions inside the jar is very beneficial.

Depending on the models, temperatures down to -196°C or up to 100°C can be applied. Mixer mills are available with 1, 2 or 6 stations. Jars and balls are available in various sizes, designs and materials.

Typical Sample Materials - titanium oxide

titanium oxide
wet grinding

metal alloy
dry grinding

hair
dry grinding

tyre rubber
cryogenic grinding

Mixer Mills - cooling and heating options

The CryoMill is designed for cryogenic grinding at -196°C, whereas the MM 500 control covers a temperature from -100°C to +100°C, with a temperature regulation from -100°C to 0 °C.

Cooling is beneficial, e. g., for:

preserving temperature-sensitive analytes, like volatile substances or pharmaceutical and food ingredients
embrittlement of sample materials which cannot be pulverized at room temperature
wet grinding, to stay below a certain temperature (in some cases below room temperature is possible)
mechanochemistry, to stop reactions and stabilize intermediate products, so the final product differs from the result obtained without cooling

Some applications are improved if the sample material is heated during the process, for example:

Paste making (food industry)
Intensifying mechanochemical reactions

The required temperatures and the operational setup depend on the specific application.

Mixer Mills - Grinding jar and ball materials

How to choose the most suitable material

The material of the grinding tools needs to be selected with the subsequent analysis in mind. If, for example, a sample is analyzed for its heavy-metal content, the abrasion of a steel jar and balls might introduce chromium into the sample which would lead to falsified analysis results. Consequently, only a metal-free material like zirconium oxide is suitable for the purpose.

The material of the tools also has an impact on their efficiency. The two most important aspects are:

Energy input (related to the density of the material)
Hardness

Energy input

The higher the density of a material, the higher is the energy input. This means that the acceleration of, for example, tungsten carbide grinding balls at a given speed is higher compared to jar and ball materials of lower density. The energy input is higher when the ball hits the sample, causing a better crushing effect which is beneficial for pulverizing hard-brittle samples.

For soft materials, on the other hand, too much energy input may prevent effective crushing. In such cases, the sample is not really pulverized into a fine powder but rather forms a layer that sticks to the jar walls and covers the balls. Homogenization is not possible that way and sample recovery is difficult. That is why for soft materials other mill types, for example rotor mills, are better suited.

Hardness

To find a jar and ball material with suitable hardness, the consideration is simple: The material must be harder than the sample. If the material is less hard, the grinding balls could be ground by the particles of the sample material.

Grinding tools of different materials

It is not recommendable to use tools of different materials, e. g. a jar made of steel used with balls made of zirconium oxide. First, abrasion from both materials will influence the analytical result, and second, wear of the tools is increased.

Mixer Mills - Jar types and sizes

Classic mixer mills work with screw-top jars which are designed for quick handling and pulverization of small sample amounts. The jars are available in hardened steel, stainless steel, tungsten carbide, agate, zirconium oxide, and PTFE.

The MM 500 nano and MM 500 control are operated with screw-lock jars. These jars are pressure-tight up to 5 bar, the integrated safety closure allows for convenient handling. The new jar design is very beneficial for wet grinding and pulverizing fibrous samples like hair.

Thanks to the flat lid, the nominal volume can be fully used, for instance when milling fibrous samples, or to ensure the optimum mixture of material, small balls and liquid for wet grinding.

Available materials include hardened steel, stainless steel, tungsten carbide and zirconium oxide ensuring contamination-free processing. Aeration lids for all mixer mill jar sizes and materials are available, e.g. for processing under inert atmosphere.

^{1. Screw-top jar}
^{2. Screw-lock jar}
^{3. Filling volume}

< << <<

	Screw-top jars MM 400, MM 500 vario, CryoMill	Screw-lock jars MM 500 nano, MM 500 control
Different jar materials	7 (4)	4
Jar sizes	1.5 \| 5 \| 10 \| 25 \| 35 \| 50 ml	50 \| 80 \| 125 ml
Aeration lids	no	yes
GrindControl	no	yes
Integrated safety closure	no	yes
Suitable for dry grinding	yes	yes
Suitable for wet grinding	Limited - jar design is not optimal for applying the 60% filling rule	Yes, designed to apply the 60% rule
Grinding of fibrous samples	yes	Yes, very easy handling, as the lids are flat and the full volume of the jar can be used to fill in voluminous sample

Mixer Mills - Recommended jar fillings

For dry grinding

For dry grinding, the best results are usually obtained with the so-called one-third-rule. This means, that approximately one third of the jar volume should be filled with balls. Following this rule, the smaller the balls are, the more must be taken to fill a third of the jar. Another third of the jar volume should be filled with sample material. The remaining third is free space to allow the ball movement inside to achieve the required comminution energy for fast pulverization of the sample.

Following this rule, the required crushing energy is provided while at the same time sufficient sample material is in the jars to prevent wear.

^{1. One third free space
2. One third sample
3. One third grinding balls}

For fibrous samples

For fibrous materials, or materials which lose their volume drastically when pulverized, a higher sample filling level is advisable. Sufficient material needs to be in the jar to minimize wear. If necessary, it is possible to add more material after some minutes to maintain the minimum required volume.

^{1. Two third sample
2. One third grinding balls}

For wet grinding

To produce particle sizes down to 100 nm or less, wet grinding and friction is required rather than impact. This is achieved by using many small balls with a large surface and many friction points. Consequently, the one third filling level, which is recommended for dry milling processes, is exchanged by the 60 % rule, meaning that 60 % of the jar are filled with small balls. The sample amount should be approx. 30 %. First, the small balls are added to the jars (by weight!) and then the material is added and mixed. Finally, the dispersant liquid is mixed carefully.

^{1. One sixth to one third sample + liquid
2. Two thirds grinding balls}

How to choose the correct grinding ball size

To ensure that the grinding balls rapidly pulverize the sample, they should have at least three times the size of the biggest sample piece. Typically, a factor of approximately 1000 can be applied to ascertain the suitable ball size for the intended final fineness. If a grind size of 30 µm (D90) is the objective, ball diameters between 20 mm and 30 mm are best suited. If smaller particles are required, a second process step with smaller balls is required.

As larger balls could crush smaller ones, it is not advisable to combine different ball sizes in a single milling process.

Wet and Nanoscale grinding in mixer mills

Nanotechnology deals with particles in a range from 1 to 100 nm. These particles possess special properties due to their size, as their surface is greatly enlarged in relation to their volume (so-called “size-induced functionalities”). Ultrafine particles are, for example, harder and more break-resistant than larger particles.

Small particles tend to get charged on their surfaces and agglomerate during dry grinding, which is a limiting factor for size reduction. For nanoscale grinding, liquid or dispersant is used to keep the particles separated and salt solutions help to neutralize the surface charges. Long chain molecules in the liquid can keep the particles separated thanks to steric hindrance.

Mixer Mills - FAQ

What is a mixer mill?

Mixer mills belong to the family of ball mills and are characterized by their small footprint, fast processing times and great versatility.

They are used for mixing, pulverizing and homogenizing hard, medium-hard, brittle, soft, elastic and fibrous sample materials.

Size reduction is effected through impact and friction. Mixer mills from Retsch are available with one, two or six grinding stations.

Which applications require a mixer mill?

Mixer mills are used for dry, wet and cryogenic pulverization of small sample volumes within seconds. They generate the required energy input for nanoscale grinding.

A typical field of application is cell disruption by bead beating for DNA/RNA and protein extraction.

Mixer mills are also frequently used in the field of mechanochemistry, particularly those models which provide cooling and heating options.

How does a mixer mill work?

Sample material and grinding balls are filled into the jar which is clamped into the mill. The radial oscillations performed by the mill lead to the pulverization by impact and friction of the balls. The sample is also thoroughly mixed by the movements of jar and balls.