Impact breakage processes are used to reduce the size of hard, brittle materials but are not useful for reducing soft or elastic materials unless the materials can be cooled to the point at which they become brittle. This is where cryogenic breakage is becoming very important. Its main use is in recycling processes for plastic and rubber, mainly thermoplastics and scrap tires, and it works by cooling the materials below their embrittlement points so they can be shattered in an impact mill. Liquid nitrogen and occasionally carbon dioxide are used as the coolants.
One advantage of cryogenic grinding is that the process operates at a low temperature (normal dry-grinding processes often operate at temperatures above 80°C, and it’s used to grind herbs and spices to reduce the loss of volatiles and to grind drugs and chemicals to reduce chemical reactions. Its main use, however, is to make rubber crumb from scrap tires.
The first patent for cryogenic breakage was granted in 1909 to Gaston Galy to increase crushing efficiency by using liquid air to cool and embrittle the material being crushed. At that time, rubber was a valued commodity, particularly for its use in the growing automobile market. But it was still comparatively scarce, and its price was so high in 1910 that 0.45 kg of rubber cost nearly as much as the same amount of silver. It is not surprising that every effort was made to recycle rubber products and that the average recycled content of all rubber products was more than 50%. By 1960, the recycled content had decreased to 20% and by 1995 to 2%. Much of the decrease resulted from the difficulty of grinding steel-belted radial tires, but recycling is increasing. In 1999, 228,800 tons, or 10% of rubber available for disposal, was recycled in the European Union and 180,000 tons, or 7%, was recycled in the United States. Tires are recycled by shredding into chips about 5 cm in size and then grinding the chips into rubber crumbs. Ambient grinding or cryogenic grinding can be used to produce the crumbs (Reschner 2002).
Ambient temperature grinding is carried out in a series of granulators, which work by a cutting and shearing motion, with the product size controlled by a screen in the mill and by cracker mills, which are counter-rotating rolls with serrations cut in them. The lower practical limit for the ambient process is 40 mesh. Cryogenic breakage requires the rubber to be cooled to less than -600°C, at which point it becomes as brittle as glass and is shattered readily in a hammer mill. Figure 5.24 shows a typical cryogenic grinding circuit. Cryogenic grinding also liberates the fiber and steel more completely from the rubber than ambient grinding, so the final product is cleaner. The temperature of liquid nitrogen is -2,000°C at atmospheric pressure, and the choice of temperature depends on the crumb size required. The consumption of liquid nitrogen is 0.5 lb/lb of rubber at -1,000°C and 0.34 kg at -2,000°C. Table 5.2 gives the sizing distributions of typical 60-mesh ground rubber from ambient and cryogenic processes.