Cryogenic grinding, also known as cold grinding, is a specialized technique that involves cooling materials to very low temperatures using cryogenic fluids like liquid nitrogen before grinding them into fine particles.
This method is particularly effective for materials that become brittle at low temperatures, allowing for more efficient and effective grinding.
The primary benefits of cryogenic grinding include the preservation of volatile oils and flavors, better product appearance, and reduced discoloration.
It addresses common issues in conventional grinding, such as heat generation, oxidation, and material clumping, by embrittling the material at cryogenic temperatures, making it easier to grind without the drawbacks associated with ambient temperature grinding.
7 Key Points Explained
1. Definition and Basics of Cryogenic Grinding
Cryogenic Grinding Definition: Cryogenic grinding is the process of cooling materials to extremely low temperatures using cryogenic fluids such as liquid nitrogen (LN2) before grinding them into fine particles.
Cryogenics Origin: The term "cryogenics" is derived from the Greek words "kryos" (cold or freezing) and "genes" (born or produce).
Materials Suitable for Cryogenic Grinding: This technique is used in various fields including cryogenic steel, thermoplastics, thermosets, explosives, and spices.
2. Benefits of Cryogenic Grinding
Preservation of Volatile Oils and Flavors: In industries like spice processing, cryogenic grinding significantly reduces the loss of volatile oils and flavors, which are crucial for product quality.
Better Product Appearance: The process helps in maintaining the original appearance of the product, preventing discoloration that often occurs during conventional grinding.
Reduction of Thermal Damage: By controlling the temperature during grinding, cryogenic grinding minimizes thermal damage, which is a common issue in conventional grinding processes.
3. Mechanism of Cryogenic Grinding
Embrittlement of Materials: Almost all materials become brittle when exposed to cold temperatures. Cryogenic grinding leverages this property by using cold energy from cryogenic fluids to embrittle the material prior to and during the grinding process.
Overcoming Ambient Grinding Issues: Traditional grinding at ambient temperatures can cause elastic materials to soften, adhere, and clump together, leading to clogging and reduced efficiency. Cryogenic grinding addresses these issues by embrittling the material, making it easier to grind into fine particles without clumping.
4. Cryogenic Fluids Used
Common Cryogenic Fluids: Dry ice and liquid nitrogen are the most popular cryogenic agents used in the spice grinding industry. Liquid nitrogen is particularly effective due to its extremely low temperature, which can reach as low as -196°C.
Effect on Material Properties: When materials are cooled with liquid nitrogen, their properties change, making them more brittle and easier to grind without the drawbacks of heat and oxidation.
5. Applications and Development
Spice Processing Industry: Cryogenic grinding is extensively used in the spice processing industry to preserve the aroma and flavor compounds present in spices. It helps in maintaining a continuous low temperature within the mill, thereby retaining most of the flavor strength per unit mass of spice.
Development of Cooling Systems: The development of low-temperature grinding mills involves designing cooling coils (evaporators) and selecting refrigerants and compressors based on the refrigeration load calculated from the grinding chamber's dimensions.
6. Comparison with Conventional Grinding
Energy Efficiency: In conventional grinding, only 1% of the total applied energy is used for the grinding process, with the remaining 99% dissipated as heat. Cryogenic grinding, by contrast, leverages the cold energy of cryogenic fluids to make the grinding process more efficient.
Problem Solving: Cryogenic grinding overcomes common issues in conventional grinding, such as heat generation, introduction of tensile stresses, reduced tool life, clogging, and gumming of mills, and oxidation.
7. Practical Applications
Fine Grinding of Viscoelastic and Plastic Materials: Cryogenic grinding is particularly useful for fine grinding of viscoelastic and plastic materials, which are often energy-consuming and cost-intensive to grind at ambient temperatures. By cooling these materials with liquid nitrogen, they become brittle and can be ground more effectively.
Innovative Grinding Plants: Companies like the Messer Group use innovative grinding plants with paddle screw coolers and supply cryogenic liquid nitrogen or carbon dioxide to maintain product quality during the grinding process.
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