Fast Pulverization and Long-Term Nanoscale Grinding

Mixer mills are true all-rounders which are utilized in laboratories globally. These versatile, compact bench-top units for wet, dry, and cryogenic grinding of small sample amounts are easy to handle. They mix and homogenize powders in just a few seconds.

Planetary ball mills have been the instruments of choice so far when it comes to long-term grindings of several hours with high energy input to obtain particles sizes <1 µm, e. g. for chemical reactions or mechanical alloying. Despite their advantages for this type of application, they have the drawback of needing cooling breaks and they are not as easy to handle as mixer mills.

RETSCH has now introduced the new mixer mill – MM 500 (see Figure 1) which operates with a maximum frequency of 35 Hz. Therefore, it is the first mixer mill in the market with the crushing power to produce particles down to the nanometer range.

The MM 500 holds two grinding jars sized 50 ml, 80 ml or 125 ml and is suitable for long-term grinding processes up to 99 hours. It is a real alternative to planetary ball mills with all of the advantages a mixer mills provides - such as comfortable handling and less warming of sample and grinding jar.

RETSCH

Figure 1: RETSCH's new Mixer Mill MM 500.

High Energy Input Results in High Final Fineness

Thanks to the increased frequency of 35 Hz, the new MM 500 achieves extraordinary grind sizes. Classic mixer mills typically have a maximum frequency of 30 Hz or even less. Hence, those mixer mills are unsuitable for applications that require high energy input.

The effect of the higher energy input on the final fineness after grinding basalt for a few minutes is shown in Figure 2. During the first two minutes the influence of the additional 5 Hz is extremely impressive: the particles obtained at 35 Hz are factor 2 smaller than at 30 Hz.

Grinding of basalt (2-5 mm initial grain size) in the MM 500 results in better fineness compared to classic Mixer Mills thanks to the increased frequency of 35 Hz instead of max 30 Hz (50 ml jar + 12 x 12 mm grinding balls, similar results in 80 ml or 125 ml jars).

Figure 2: Grinding of basalt (2-5 mm initial grain size) in the MM 500 results in better fineness compared to classic Mixer Mills thanks to the increased frequency of 35 Hz instead of max 30 Hz (50 ml jar + 12 x 12 mm grinding balls, similar results in 80 ml or 125 ml jars).

Moreover, the MM 500 accommodates two jars with a maximum usable volume of 45 ml each which is twice the volume a smaller mixer mill can process but comes near to what is possible with planetary ball mills. The below examples show that the new MM 500 is ideally suited for applications which are performed traditionally using planetary ball mills, like ultra-fine grinding down to <100 nm

Pulverization in the Nanometer Range Without Heat Build-up

Typical process times for ultra-fine grinding are several hours; in conventional ball mills extra time needs to be added for grinding breaks to avoid overheating of the sample. The new MM 500 creates particles in the nanometer range without requiring cooling breaks, unlike planetary ball mills. This is because the grinding mechanism is based on impact instead of friction, so generates less heat and is more effective than that of a planetary ball mill.

Figure 3 shows that while grinding aluminum oxide, the temperature inside the grinding jars is very similar in the planetary ball mill and the MM 500 – the difference being that the planetary ball mill requires a four-minute break after every minute of grinding. Without it, the temperature inside the jar could rise to 100 °C in just the first few minutes of the grinding process.

Particle fineness and temperatures during wet grinding of aluminum oxide with 0.1 mm grinding balls of zirconium oxide. The MM 500 was operated without cooling breaks, the total process time therefore equals the net grinding time. 2 h net grinding time was required in the MM 500 to obtain particles of 0.14 µm, whereas 5 h total process time including cooling breaks (1 h net grinding) were required in the Planetary Ball Mill to obtain a particle size of 0.18 µm. The temperature was comparable in both mills.

Figure 3: Particle fineness and temperatures during wet grinding of aluminum oxide with 0.1 mm grinding balls of zirconium oxide. The MM 500 was operated without cooling breaks, the total process time therefore equals the net grinding time. 2 h net grinding time was required in the MM 500 to obtain particles of 0.14 µm, whereas 5 h total process time including cooling breaks (1 h net grinding) were required in the Planetary Ball Mill to obtain a particle size of 0.18 µm. The temperature was comparable in both mills.

The maximum attainable grind size of 0.14 µm of the aluminum oxide sample was obtained much faster in the MM 500 than in the benchmark planetary ball mill (two hours instead of five hours process time). A further example for ultra-fine grinding can be observed in Figure 4: Grinding of titanium dioxide resulted in much better fineness in the MM 500 (90 nm) compared to the benchmark planetary ball mill (130 nm) in only a quarter of the time.

Best grinding result of wet grinding of titanium dioxide with 0.1 mm grinding balls zirconium oxide. The MM 500 was operated without cooling breaks, the total process time is therefore the net grinding time. After 2 h net grinding time the MM 500 produced particles sized 90 nm. In the benchmark planetary ball mill the highest fineness of 130 nm particles was reached after 2 h net grinding time (10 h total process time including cooling breaks).

Figure 4: Best grinding result of wet grinding of titanium dioxide with 0.1 mm grinding balls zirconium oxide. The MM 500 was operated without cooling breaks, the total process time is therefore the net grinding time. After 2 h net grinding time the MM 500 produced particles sized 90 nm. In the benchmark planetary ball mill the highest fineness of 130 nm particles was reached after 2 h net grinding time (10 h total process time including cooling breaks).

Grinding of Fibrous Sample Material

Beside processing brittle and hard samples, planetary ball mills are typically utilized to pulverize fibrous sample materials like hair or straw. For effective grinding in ball mills, a rule of thumb is to fill 30 % of the jar volume with sample. A further 30 % is filled with balls (dry grinding). It is advised to fill more sample in the jar to decrease wear, as fibrous samples tend to lose volume during grinding.

The jar design of planetary ball mills as well as the new screw-lock jar design of the MM 500 (Figure 5) is extremely convenient to get enough fibrous, fluffy, voluminous samples into the jars – because the lids of those jars are flat. In classic mixer mills, a part of the volume is located in the lids which makes it difficult to fill in enough fibrous sample material – the part volume in the lid cannot be utilized effectively.

New screw-lock jar design.

Figure 5: New screw-lock jar design.

In this example (see Figure 6), animal hair was ground. 1.2 g of the very light material was filled in 80 ml grinding jars and ground for 7 minutes with 30 balls with size 10 mm. The result in the new MM 500 was a lot better than in the planetary ball mill.

Final fineness after 7 min continuous grinding of animal hair in the new MM 500 and the benchmark Planetary Ball Mill with 30 x 10 mm grinding ball stainless steel.

Figure 6: Final fineness after 7 min continuous grinding of animal hair in the new MM 500 and the benchmark Planetary Ball Mill with 30 x 10 mm grinding ball stainless steel.

Cryogenic Grinding of Tough, Elastic Samples Materials

Classic mixer mills are the instruments of choice when it comes to pulverization of tough, elastic sample materials like rubber, plastics, or elastomers . However, those sample materials cannot be pulverized at room temperature, they only get deformed by impact effects . To improve their breaking properties they are embrittled with liquid nitrogen, then pulverization is possible.

The grinding ball(s) and the sample material are filled in the grinding jar  which is then tightly closed before it is placed in a bath with liquid nitrogen for indirect sample embrittlement. The advantage of the new MM 500 over classic mixer mills is again the increased crushing power of 35 Hz, plus the larger volume of the grinding jars of up to 125 ml.

Figure 7 shows the result of cryogenic grinding of the two sample materials polypropylene and polyamide. Again, the increased energy input of 35 Hz instead of 30 Hz results in finer particles obtained in the new MM 500.

At maximum frequency the mixer mill MM 500 produces finer particles during cryogenic grinding than classic mixer mills. Left: grinding results after 4 x 2 minutes with intermediate cooling of 3 mm polypropylene particles. Right: grinding results after 7 x 2 minutes with intermediate cooling of 2 mm polyamide particles.

Figure 7: At maximum frequency the mixer mill MM 500 produces finer particles during cryogenic grinding than classic mixer mills. Left: grinding results after 4 x 2 minutes with intermediate cooling of 3 mm polypropylene particles. Right: grinding results after 7 x 2 minutes with intermediate cooling of 2 mm polyamide particles.

Safe and Easy Operation

The MM 500 supplies maximum safety and operational convenience. The grinding jar lids are screwed tight easily for applications such as wet grinding and mechanochemical reactions up to 5 bar. The innovative design of the screw-lock jars ensures optimum exploitation of the usable volume.

This is a huge benefit over the jars of classic mixer mills where the lid makes up part of the jar volume, making it difficult to apply ball fillings of 60 %, which is required for wet grinding. Additionally, when the jar volume is not part of the lid it is more convenient for the user to fill in fibrous sample materials.

The clamping system of the new MM 500 is very user-friendly, especially when compared to the clamping of planetary ball mills. The jars are inserted into the clamping system easily and can even stay in the system for extraction of a sub-sample or a quick visual check of the material (see Figure 8). The large 4.3 inch touch display is another benefit of the new MM 500 which enhances operating convenience.

The innovative screw-lock grinding jars are easy to handle and clamping is a matter of seconds.

Figure 8: The innovative screw-lock grinding jars are easy to handle and clamping is a matter of seconds.

The user can control the mill via smart phone or tablet by using the new RETSCH App. The app allows a user to access the RETSCH application database, create application routines, or get in touch with the RETSCH service team.

In addition to the described advantages, the mill is ideal for classic mixer mill applications: cryogenic grinding plus quick pulverization of hard and brittle samples, but also for new types of applications such as ultra-fine grinding down to <100 nm or mechanical alloying – typically with no need for cooling breaks.

The application examples in this article show that the new MM 500 produces better grind sizes than classic mixer mills and even beats planetary ball mills in terms of total process time and fineness.

Conclusion

The new MM 500 is the perfect combination between a planetary ball mill and a classic mixer mill. On the one hand, it is powerful and robust enough to carry out long-term grindings in the nanometer range or mechanical alloying processes, on the other hand it achieves excellent pulverization results within a few minutes. On top of that, uncomplicated handling ensures safe operation.

This information has been sourced, reviewed and adapted from materials provided by RETSCH GmbH.

For more information on this source, please visit RETSCH GmbH.

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