What is Cryogenic Grinding?

Freezing for Fineness – The Dos and Don'ts of Cryogenic Grinding

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Mills for cryogenic operation are specifically designed for the operation at very low temperatures and have a number of special features. For instance, the grinding tools have to be constructed from tough materials to be able to work at sub-zero temperatures. The mill has to be gas-tight to the highest possible degree. Sealing materials used have to be suitable for use in low temperatures. Furthermore, a special bearing and lubrication system specifically designed for cryogenic operation is necessary. Also, the mill housing has to be insulated to avoid icing-up and to save energy.

The Following Types of Mills are Used in Cryogenic Grinding

Fine impact mills: These are widely used with cryogenic grinding of plastics as size reduction technology. Both mills are typically operated with LN2 at negative temperatures of up to -175°C.

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UPZ mechanical impact mills: These are designed for dry grinding soft to mediumhard materials. The feed material is charged to the center of the rotor equipped with grinding elements and is comminuted by impact against the rotor and stator elements. After passing through the grinding zone, the product enters the mill housing and is then discharged from the mill by gravitational force. The rotation generates an air flow from which the end-product is extracted and charged to a filter.

Customise your Cryogenic Process for Maximum Efficiency

Different grinding elements can be employed, e.g., a pin disc or a plate beater unit with profiled grinding track or a sieve grate. The fineness is set by adjusting the rotor speed and the feed rate. The Contraplex Wide Chamber Mill is a fine impact mill with two driven pin discs. In counter rotating mode, much higher relative speeds are possible than with the UPZ fine impact mill, which has only one driven pin disc.

Fine Impact Mill 800 UPZ stainless steel design
Fine Impact Mill 800 UPZ stainless steel design
(Picture: HOSOKAWA ALPINE)

The highest relative speed develops at the outermost pin rows and can be up to 250 m/s. The fineness can be adjusted by altering the pin disc speeds. The centrifugal forces acting on both discs and the design with the wide-chamber housing is ideal for processing sticky materials.

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