Drug Discovery & Pharmaceuticals CryoMill - Cryogenic Grinding was Never More Convenient or Efficient Dr Gerhard Beckers, Retsch GmbH, Email:
g.beckers@
retsch.com
Some sample materials have properties which make size reduction at ambient temperature impossible. If, for example, very elastic materials need to be ground or volatile components have to be preserved for further analysis, it is essential to carry out cryogenic grinding. The use of liquid nitrogen helps to embrittle the sample, thus improving its breaking properties, and preventing volatile substances from escaping due to the frictional heat produced by the grinding process.
Retsch has developed the CryoMill (Figure 1) especially for this application. An integrated cooling system ensures that the grinding jar is continually cooled with liquid nitrogen before and during the grinding process. Thus the sample is embrittled and volatile components are preserved. Thanks to the Autofill System, liquid nitrogen is continually refilled in the exact amount which is required to keep the temperature at –196°C.
Grinding in the CryoMill is effected by impact and friction. The grinding ball moves freely inside the grinding jar which allows for thorough pulverisation of the sample, in contrast to other cryogenic mills with differently shaped grinding tools.
Based on an extensive dialogue with users of the CryoMill, Retsch was able to integrate a variety of suggestions into the further development and optimisation of the new product generation. As a result, the new CryoMill makes grinding with liquid nitrogen as convenient and effective as never before.
Example Plastic (PE-LD)
LD polyethylene is a highly elastic material which is very difficult to grind with impact and friction, even at very low temperatures. Compared to a grinding process at 25 Hz, the results at 30 Hz have improved by 50%.
Parameters:
6 g PE-LD granulate in grinding jar 50 ml, stainless steel, 1 grinding ball Ø 25 mm, stainless steel,
5 cycles of 2 min each, with 1 min intermediate cooling at 25 Hz (blue curve) and 30 Hz (red curve)
Figure 1. Retsch’s CryoMill with 50 l liquid nitrogen tank Example Leather Benefits
• Increased oscillating frequency of up to 30Hz allows for 50% higher energy input • Reinforced housing and optimised liquid nitrogen duct • Clearly structured user interface, memory for 9 SOPs • Additional accessories such as adapter for 6 reaction vials
Improved grinding results thanks to increased vibrational frequency
The maximum vibrational frequency of the new CryoMill generation is 30 Hz. This allows for a considerably higher energy input which results in smaller grind sizes. This is particularly true for extremely tough and elastic materials.
Leather is a very tough and fibrous material. These properties make it very robust for usage but also fairly difficult to grind. Processed at 25 Hz for a short interval, the material is only slightly torn up whereas at 30 Hz, after the same grinding time, it is completely pulverised.
Parameters:
8 g leather (approx. 8x8 mm) in grinding jar 50 ml, stainless steel, 1 grinding ball Ø 25 mm, stainless steel,
3 cycles of 2 min each, with 1 min intermediate cooling at 25 Hz (left) and 30 Hz (right)
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