With cryogenic grinding it is possible to produce very small grind sizes even with tough and elastic sample materials. The sample is embrittled with liquid nitrogen and can thus be ground more easily with impact, pressure and friction. Moreover, volatile components – which are often the object of an analysis – are preserved.
The author: Dr. Andreas Theisen Head of National Sales & Application Support, Retsch
Retsch’s Cryomill offers high grinding energy, convenient operation and a wide range of accessories. The mill is equipped with an integrated cooling system which continually cools the grinding jar with liquid nitrogen before and during the grinding process. The sample is thus embrittled and volatile substances preserved. An autofill system ensures that a sufficient quantity of LN2 is provided to keep the temperature at -196 °C. The user never comes into direct contact with the liquid nitrogen, so that a high degree of operational safety is guaranteed.
Size reduction in the Cryomill is effected by impact and friction. The grinding jar performs 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 grinding jar and pulverise it. In contrast to the grinding tools used in comparable cryogenic mills, the balls can move freely inside the jar, ensuring thorough pulverisation of the sample to a maximum final fineness of 5 µm.
High energy input
The maximum vibrational frequency was increased from 25 to 30 Hz for this new generation. This allows a much higher energy input resulting in improved grind sizes, especially when grinding particularly tough or elastic materials such as LD polyethylene. This plastic is very difficult to grind with impact and friction, even at very low temperatures. If the grinding results for this material at 25 Hz and at 30 Hz are compared, an approximately 50 % improvement can be seen at 30 Hz. The test involved grinding 6 g of PE-LD granulate in a 50 ml stainless steel jar with one 25 mm stainless steel grinding ball in five two-minute cycles, with one minute of intermediate cooling at 25 Hz (red curve) and 30 Hz (blue curve).
Field experience
The “Processing and Recycling of Raw and Residual Materials” department, which is part of the faculty of Environmental Sciences and Process Engineering of the Technical University of Cottbus (BTU), uses the Cryomill for grinding plastics. In the context of various national research projects, the Cottbus scientists study the treatment of plastics from electronic waste and car recycling. Single-variety separation of these plastics is carried out with the help of near-infrared methods. However, polymers which contain brominated flame retardants, and are therefore not allowed to recirculate in their existing form, cannot be detected in this way.
Extraction and subsequent analysis with a gas chromatograph/mass spectrometer (GC / MS) are the methods of choice for analysing reference samples, for example ABS, PS or PC-ABS. Until recently, the lab team at Cottbus University used an older model of Retsch’s ultra centrifugal mill to grind these samples. However, the plastic tended to heat up quickly during the grinding process, resulting in gas emission and chemical conversion of some of the volatile bromine compounds and hence incorrect analysis results. Permanent cooling of the plastic samples and the grinding chamber with liquid nitrogen likewise failed to achieve the desired effects.
Today, the researchers at Cottbus University use the Cryomill to grind brominated plastics. They start by embrittling the samples in a leak-proof grinding jar and reduce them to a size of less than 10 µm in a series of grinding cycles. Gas emission, thermal stress and sticking of the plastic particles during grinding are no longer a problem. Additionally, the homogeneity and extraction degree of the sample have been improved. Thanks to the increased vibrational frequency of the new Cryomill generation, the grind sizes have been successfully optimised.
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