Cryogenic grinding permits heat-sensitive, thermoplastic, and elastic materials to be economically ground to very small particle sizes. The cryogenic process actually embrittle a material prior to size reduction and controls heat buildup in the grinding equipment. The result is high product quality and system productivity.
Cryogenic grinding involves cooling a material below its embitterment temperature with a cryogenic fluid, typically liquid nitrogen or, in certain applications, carbon dioxide. After cooling, the material is fed into an impact mill where it is reduced in size primarily by brittle fracture
Cryogenic grinding is used for grinding spices, thermoplastics, Elastomers, color concentrates, and similar materials. It is also used to recover a variety of scrap materials, such as factory scrap rubber and scrap tires, and to separate the components in composite materials.
Introduction to Cryogenic Grinding :
Cryogenic grinding technology can efficiently grind most tough materials and can also facilitate Cryogenic recycling of tough composite materials and multi component scrap. The heart of this technology is the CRYO-GRIND SYSTEM. It employs a cryogenic process to embrittle and grind materials to achieve consistent particle size for a wide range of products. The cryogenic process also has a unique capability for recycling difficult to separate composite materials.
Cryogenic grinding is a method of powdering herbs at sub-zero temperatures ranging from 0 to minus 70°F. The herbs are frozen with liquid nitrogen as they are being ground. This process does not damage or alter the chemical composition of the plant in any way. Normal grinding processes which do not use a cooling system can reach up to 200°F. These high temperatures can reduce volatile components and heat-sensitive constituents in herbs. The cryogenic grinding process starts with air-dried herbs, rather than freeze-dried herbs.
CRYOGENIC GRINDING TECHNOLOGY
For pulverizing many materials, cryogenic grinding technology increases productivity and lowers power costs. Many elastic or “soft” materials are very difficult to pulverize, requiring long cycle times and high energy consumption. This combination decreased productivity and increased costs unnecessarily. Cryogenic grinding involves cooling a material below its embrittlement temperature with a cryogenic fluid, typically liquid nitrogen or, in certain applications, carbon dioxide. After cooling, the material is fed into an impact mill where it is reduced in size primarily by brittle fracture. This process has several benefits:
• Ability to process relatively “soft” or elastic materials that cannot otherwise be ground
• Increased throughput
• Reduced power consumption
• Smaller size particles
• Minimal loss of volatile components
• Lower capital investment
Probably the greatest benefit provided by cryogenic grinding is the ability to grind “soft” or elastic materials that otherwise could not be ground, or could be ground only with long cycle times and high energy use. By embrittling the material, fine powder or crumb can be obtained easily and with a minimum expenditure of energy. Because embrittled material grinds easily, the throughput for a given mill is substantially increased and less power is used per pound of material ground.
ADVANTAGES OF CRYOGRINDING
1. Higher production rate
2. Lower energy consumption
3. Finer particle size
4. More uniform particle distribution
5. Lower grinding cost
6. No heat generation which is good while grinding spices, pharmaceuticals and scrap plastics
7. Provides an inert atmosphere thus eliminating the possibility of oxidation
WORKING OF CRYOGRINDING PLANT
The spice to be ground is cleaned manually and fed in to the hopper. From the outlet of the hopper the spice enters in to the vibratory feeder, which is positioned with a small inclination towards the entry of the helical screw conveyor. The vibratory feeder has a provision to control the feed rate.
The grinding mill is driven by a 5.5 KW, 3 – phase,50 Hz motor. The mill is connected with the motor by a flat belt between the motor pulley and the mill pulley. Inside the mill, stud disc is mounted on the shaft. The circularly projecting studs fit between the similar projecting studs from the rear of the front door. The spice to be ground gets locked between the studs of the rotating disc and studs of the stationary disc. When the mill is running, the spice gets crushed between the studs and comes out through an optional sieve as a ground product. The helical screw conveyor has a total length of a metre where the fed material travels horizontally to the grinding mill. The screw conveyor is driven by a 0.75 KW drive with reduction gear and inverter control. Liquid nitrogen from a storage container is sprayed into the screw conveyor. The time of stay of spice in the conveyor can be adjusted by varying the speed of the drives. A censor monitors the temperature of the ground spice and the liquid nitrogen spray is optimized using automatic feed back control.
To the bottom of the mill a collecting bin is housed where the ground product gets collected. The bottom of this tapering collecting bin, a rotary valve is mounted which is driven by a 0.37 KW motor. The rotary valve has 8 compartments mounted radially in the same plane. The product, which comes out of the rotary valve, is lifted to the storage container by the Hapman Helix conveyor. This is driven by a 0.37 KW motor. The vaporized nitrogen from the mill is sucked by a centrifugal blower and through the filter assembly if fed back to the mill.
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.