Liquid nitrogen (LIN) has been used for more than 30 years in cryogenic grinding to “super-refrigerate” size reduction processes in industries such as food processing, plastics recycling, powder coating, viscosity control additives and tire recycling. Liquid nitrogen offers a number of unique benefits to size reduction processes.
- Liquid nitrogen is one of the coldest
liquids on earth with a boiling point of -320°F (-195°C). This means that
liquid nitrogen can be used to cool materials that require very low processing
temperatures such as polyolefins or similar tough-to-grind materials.
- Because nitrogen does not support combustion, it
can help make size reduction operations safer.
- Nitrogen is inert and does not react with other
substances under normal conditions.
- Unlike liquid carbon dioxide, which forms carbonic acid when it dissolves in the moisture inherent to food products, nitrogen does not affect the pH of food products. In fact, liquid nitrogen has a lower boiling point than liquid carbon dioxide.
Both spice grinding and thermoplastic grinding require equipment designed with the following features:
- Ease of cleaning.
- Ability to operate at extremely cold temperatures.
- Good control of the liquid nitrogen and material
- Capable of being demonstrated in the manufacturer’s test laboratory.
As the cryogenic grinding system cools down from ambient temperatures during startup, the system components will shrink. For example, a 10' length of stainless steel piping will shrink by more than 0.125" (3 mm) as it cools from ambient to -100°F (-73°C). A longer pipe will shrink even more. Therefore, the grinding system must be designed to accommodate this shrinkage - and the stresses generated. Because many metals such as carbon steel as well as some plastics become brittle at low temperatures and can fracture, the grinding system must be designed with components that can tolerate the low temperatures required without failure.
Maintaining good control of the relative flows of the material being processed and the liquid nitrogen being used to cool the material is crucial to the successful commercial operation of a cryogenic grinding system. For example, spices are temperature sensitive. The heat generated from the actual grinding can cause spices to lose their essential oils, distort their aroma and flavor, and alter their color, all of which reduce the quality of the spices. Additionally, if the spices are not processed at a low enough temperature, the oils and fats in the spices can cause agglomeration and clog up the mill. Good control of the operating temperature via good liquid nitrogen flow control can help prevent these issues.
In recycling processes, for example, off-spec and post-industrial waste of thermoplastics are reprocessed and added to other compounds as fillers to increase toughness and strength characteristics of the resin matrix. This process requires consistent fine particles to ease blending and achieve an eco-friendly process. Thus, fluctuations in particle size may interfere with the efficiencies of such processes. Production rates will also fall as the processing temperature increases. If the temperature gets too high, the resin may even partially melt and clog up the mill.
Testing is an important aspect of any size reduction system. Size reduction, especially of thermoplastics or elastomers, is still a very empirical technology. The equipment supplier should be able to demonstrate the performance of their equipment with the substrate being ground.
While grinding applications may differ, the desired end result is often the same - achieving the finest particle size and most uniform particle distribution while maximizing production rates and minimizing overall operational costs. Cryogenic size reduction technology using liquid nitrogen to remove heat produced during the grinding process can result in a finer, more consistent particle size distribution and higher throughputs for many products compared to conventional grinding methods. These products include adhesives and waxes, carpets, color concentrates and pigments, composites, grains, pharmaceuticals, plastics, powder coatings, metal, multi-component materials, rubber, and spices and herbs. PC