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