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

Author / Editor: Sylvia Bräunlein / Dominik Stephan

According to the All India Plastics Manufacturers’ Association (AIPMA), the Indian plastics industry grows at an annual rate of 12-15 per cent. Manufacturers are increasingly focusing on customizing products in accordance with end-user needs. Here’s an overview of how cryogenic grinding is the means to a useful and cost-effective solution for fine grinding of plastics.

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

For the production of functional polymers, the demand for fine and ultrafine particle sizes is an ongoing market trend. Besides technical feasibility, the grinding process needs to be cost-effective and energy-efficient. There are many industrial applications where fine grinding of plastics is required, including textile coating of fitted carpets, mixing of compounds, hotmelt powders, additives for paints and recycling of recoverable materials such as rubber.

Cryogenic grinding is a method of powdering products at sub-zero temperatures ranging from 0°C to -175°C. The products are cooled down with liquid nitrogen before they are ground. This process does not damage or alter the chemical composition of the products in any way. Normal grinding processes that do not use a cooling system can result in heat generation of up to the softening or even melting point of the product.

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Why Cryogenic Grinding?

Many plastic materials need to be ground before they are suitable for further processing or applications, and several of these are of a viscoelastic nature. Size reduction can often only be achieved by cooling the material to below the glass transition point; thus embrittling the material before grinding. Cryogenic grinding is suitable for materials such as thermoplastics, elastomers, waxes, paint additives and even some metals.

Besides the enhanced grindability, there are a number of further advantages brought about by the cryogenic grinding of plastics. Owing to the fact that the particles are embrittled at low temperatures, impact mills can achieve throughput rates that are two to three times higher and thus operate at reduced power consumption. Higher particle fineness values are also achieved. A valuable side effect of cryogenic grinding is that the particle shape is smoother and therefore the flow properties of the fine powder are enhanced. This again leads to continuous, problem-free operation even with sticky products.

Cold vs. Warm – What is the Best Grinding Process?

A cryogenic process — listing all aspects of the system
A cryogenic process — listing all aspects of the system
(Picture: HOSOKAWA ALPINE)

Figure 1 shows a direct comparison between the grinding results achieved by means of cryogenic grinding and those achieved with a warm grinding process. It was possible in this example to improve the D50 value from approximately 480 μm down to approximately 180 μm. Depending on the grinding process, the nitrogen consumption is between 0.7 and 1.2 kg of LN2 per kg of the product.

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.

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.

The Cryogenic Process – a System Example

After the plastic granules have been metered out of the silo, the coarse material from the screening machine is added and this mixture is then fed to the whirling motion screw cooler, the granules are conveyed to the Contraplex pin mill CW (or the fine impact mill UPZ) and are ground. The product is discharged via a silo-mounted filter to the downstream screening machine, where the fine product is screened and discharged as end-product via an automatic filter. The coarse fraction of the screening machine is conveyed back to the cryogenic feeder for recirculation.

A Cryogenic Process — Listing all Aspects of the System

An automatic cryogenic control unit controls the supply of the LN2 to the entire system. The system is insulated to minimize the consumption of nitrogen. All-important process parameters, e.g., mill speed, power consumption, throughput rate and nitrogen consumption are measured and evaluated.

Figure 1: Comparison between results achieved by cryogenic grinding and warm grinding processes
Figure 1: Comparison between results achieved by cryogenic grinding and warm grinding processes
(Picture: HOSOKAWA ALPINE)

A Practical and Economical solution for Viscoelastic Materials

In view of the fact that the cost of raw materials and energy is increasing day by day, it is important to optimize the capacity and at the same time achieve the required quality. The cryogenic grinding process provides extremely high particle fineness values along with high product quality. Cooling the grinding process with liquid nitrogen prevents the temperatures of plastics from rising above the softening point. This rules out product melting and the subsequent formation of build-up, and additionally enhances the particle shape and thus the flow property of the fine powder.

These aspects constitute the significant advantages of cryogenic grinding compared with conventional grinding and furthermore add value to the product.

* The author is Operations Director- Chemicals Division, HOSOKAWA ALPINE Aktiengesellschaft

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