17-03-2014, 03:37 PM
A Seminar Report On CRYOGENIC GRINDING
ABSTRACT
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
The term “Cryogenics” originates from Greek word which means creation or production by
means of cold. As prices for energy and raw materials rise and concern for the environment
makes safe waste disposal difficult and Costly, resource recovery becomes a vital matter for
today’s business. 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 sep arate
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 PROCESS
Since almost all materials embrittle when exposed to cold temperatures, cryogenic size
reduction utilizes the cold energy available from liquid nitrogen to cool, embrittle and inert
materials prior to and or during the grinding process. All materials which due to their specific
properties at ambient temperatures are elastic, have low melting points, contain volatile or
oily substances, have low combustion temperatures and are sensitive to oxygen, are ideal
candidates for cryogenic size reduction.
CRYOGENIC GRINDING SYSTEM
When using the system, measurable and repeatable results are obtained for lab or productions
calculations. Mills range in size from 7-1/2 HP to 200 HP. With our cryogenic grinding unit
an understanding develops with interaction of equipment components and operating
parameters. Factors such as consistent feed rate, precise temperature measurement, mill
operating parameters and pressure control are critical to the evaluation of cryogenic grinding
and cryogenic grinding systems.
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 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.
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.
CONCLUSION
Cryogenic grinding improves product quality by controlling thermal effects. In
this process, Oxidation and surface burning are eliminated. It also reduce wear
and Surface damages are eliminated. This helps in achieving finer particle size.
Material removal rate is high in cryogenic grinding. This process is economical
in the long run.