Cryomilling

Cryomilling, also known as cryogenic grinding or freeze grinding, is a size reduction process that involves cooling materials to very low temperatures before milling. Typically, liquid nitrogen is used as the cooling agent, bringing the material's temperature down to below its glass transition point or embrittlement temperature. This cooling makes many materials, which are otherwise tough or elastic at room temperature, brittle and easily fractured.

The process begins by introducing the material and a cryogenic agent, usually liquid nitrogen, into a milling chamber. The liquid nitrogen rapidly cools the material, reducing its molecular mobility and making it more susceptible to fracture. Once sufficiently cooled and embrittled, the material is subjected to high-energy milling. Various milling equipment can be used, including ball mills, hammer mills, or jet mills, depending on the material properties and desired particle size. The brittle material fractures into smaller particles under the impact and shear forces within the mill. As the material is milled, more cryogenic agent may be added to maintain the low temperature and prevent thermal degradation or oxidation.

Cryomilling is particularly beneficial for processing materials that are difficult to mill at ambient temperatures. This includes polymers, elastomers, food products, pharmaceuticals, and some natural products. The low temperatures minimize degradation, preserve volatile compounds, and prevent the melting or smearing of heat-sensitive materials. The resulting fine particles often exhibit improved characteristics, such as increased surface area and enhanced reactivity, which can be advantageous for subsequent processing steps or applications. The process requires specialized equipment to handle cryogenic temperatures safely and efficiently.