Cryomiling

Cryomilling

In Europe and the USA, liquefied carbon dioxide is used in some cases, but the boiling point (sublimation point) of liquefied carbon dioxide is -78.5°C at atmospheric pressure, which is more than 100°C (!) higher than that of liquid nitrogen. In contrast to liquid nitrogen, which only slowly evaporates as a liquid when taken out of the storage container at atmospheric pressure, liquefied carbon dioxide gas solidifies due to the heat of evaporation, i.e. it forms dry ice, which makes it difficult to cool the material (since it is no longer a liquid, the contact area with the material is reduced). Therefore, liquid nitrogen is mainly used.

Now, it would seem that using such a pre-treatment would solve the problem, as the material could be put into a normal grinding mill as long as it is cooled down before feeding, but this is not the case.
The main reason for this is that the grinding operation generates a lot of heat.
Most of the energy (electricity) used by the crusher is converted into heat, for example by vibrations between atoms. It is said that at most a few percent of the energy is actually used to crush the material.

Therefore, if the material is only kept at a low temperature beforehand, the temperature of the crushed material will rise quickly during the crushing process, and the cooling process will be meaningless.
This is why we have developed a cryogenic milling system, which uses liquid nitrogen to cool the entire system, from the stage of keeping the material at a low temperature to the milling process. The system is operated in such a way that the liquid nitrogen and the vaporised nitrogen are circulated in order to make effective use of the liquid nitrogen, thus reducing the oxygen concentration in the system. This has the added advantage of reducing the oxidation of foodstuffs and ensuring that flavours, oils and pigments are preserved in the powder.

This is not a problem if the final product is stored in such a cold environment, but it is usually collected, filled or transported to other processes at room temperature. This means that the water and oil in the frozen material will melt. Condensation can also occur. In some cases, a paste is formed. It is therefore necessary to dry the product while bringing it slowly back to temperature, or to transport it on the assumption that it will form a paste.

There is an economic caveat to cryogenic grinding. The more liquid nitrogen is used, the cooler the temperature becomes and the more pulverisable and fine particles can usually be obtained. The problem is that when you want smaller particles, the amount of liquid nitrogen used becomes very high and the throughput is drastically reduced. Along with the reduction in throughput, the amount of liquid nitrogen used increases by a factor of 2 to 10. Liquid nitrogen is relatively expensive, costing around 1,000 yen per litre. For example, for polypropylene, a type of plastic, the cost of liquid nitrogen for grinding 1kg of material is about 37 yen, or 148 yen if you want a finer product. Therefore, commercially, it is necessary to select the appropriate particle size and to reduce the amount of liquid nitrogen used as much as possible.