German China

Sample Preparation Size Matters: From Fast Pulverization to Long-Term Grindings

Author / Editor: Dr. Tanja Butt* / Dipl.-Chem. Marc Platthaus

Homogeneity is an important requirement for many samples in the labo­ratory — regardless of whether they are required for analyses or syntheses. Read how you can use a new mixer mill to produce powders with particle sizes down to the nanometer range.

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Fig.1: The new MM500 mill enables grinding down to the nanometer range.
Fig.1: The new MM500 mill enables grinding down to the nanometer range.
(Source: Retsch)

Mixer mills are true all-­rounders which are used in laboratories all over the world. These compact versatile bench-top units for dry, wet and cryo­genic grinding of small sample amounts mix and homogenize powders in only a few seconds. When it comes to long-term grindings of several hours with high energy input to obtain particles sizes < 1 µm, e.g. for mechanical alloying or chemical reactions, planetary ball mills have been the instruments of choice so far. Despite their benefits for this type of application, they have the drawback of requiring cooling breaks and are not as easy to handle as mixer mills.

Retsch has now introduced the new MM 500 which features a maximum frequency of 35 Hz and is therefore the first mixer mill in the market that generates enough crushing power to produce particles in the nanometer range. As it accommodates two grinding jars sized 50 ml, 80 ml or 125 ml and with its suitability for long-term grinding processes up to 99 hours, for the first time a mixer mill has become a real alternative to a planetary ball mill — with more comfortable handling and less warming effects.

The MM 500 provides maximum operational convenience and safety. The innovative design of the Screw-Lock grinding jars ensures optimum exploitation of the usable volume; the lids are easily screwed, closing the jars tightly for applications like wet grinding and mechanochemical reactions up to 5 bar. The jars are easily inserted into the user-friendly clamping system and may even remain in the system for extraction of a sub-sample or a quick visual check of the sample. A 4.3-inch touch display enhances operating convenience. By using the new Retsch-App, the user can control the mill via smart phone or tablet, create application routines, access the Retsch application database or get in touch with the Retsch service team.

High energy input results in high final fineness

The application examples in this article show impressively that the new MM 500 achieves extraordinary grind sizes thanks to the increased frequency of 35 Hz, whereas classic mixer mills usually have a maximum frequency of 30 Hz or even less. Thus, those mixer mills are not suitable for applications which require high energy input. Figure 2 shows the influence of the higher energy input on the final fineness after grinding basalt for a few minutes.

The influence of the additional 5 Hz is very impressive in the first two minutes where the particles obtained at 35 Hz are factor 2 smaller than at 30 Hz. Additionally, the MM 500 accommodates two jars with a maximum usable volume of 45 ml each which is more than twice the amount a smaller mixer mill can process but comes close to what is possible with planetary ball mills. The following examples demonstrate that the new MM 500 is good suited for applications which are traditionally performed in planetary ball mills, like ultra-fine grinding down to <100 nm.

Pulverization to nanometer range without heat build-up

Typical process times for ultra-fine grinding are several hours; in conventional ball mills additional time for grinding breaks to avoid over-heating of the sample must be added. In contrast to planetary ball mills, the new MM 500 produces particles in the nanometer range without requiring any cooling breaks; the grinding mechanism based on impact rather than friction generates less heat and is more effective than that of a planetary ball mill. The example in figure 3 shows that the temperature inside the grinding jars, while grinding aluminum oxide, is very similar in the MM 500 and the planetary ball mill — with the difference that the planetary ball mill requires a four-minute break after each minute of grinding. Otherwise, the temperature inside the jar could rise to 100 °C already in the first few minutes of the grinding process. The maximum achievable grind size of 0.13 µm of the aluminum oxide sample was obtained much quicker in the MM 500 than in the benchmark planetary ball mill (2 h against of 5 h process time).

Another example for ultra-fine grinding is shown 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.

Conclusion: combination of mixer and planetary ball mill

The new MM 500 is a combination between a classic mixer mill and a planetary ball mill. On the one hand, it achieves grinding results within a few minutes, on the other hand, it is powerful and robust enough to carry out long-term grindings of many hours, for example for mechanical alloying. On top of that, uncomplicated handling ensures safe operation for the user.

* T. Butt Retsch GmbH, 42781 Haan, Germany

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