02-08-2014, 02:49 PM
“CRYOGENIC GRINDING”
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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 eq uipment. 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 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.
Solid materials are ground or pulverized by way of hammer mills, attrition mills, granulators
or other equipment. A smaller particle size is usually needed to enhance the further
processing of the solid, as in mixing with other materials. A finer particle also helps in
melting of rubber and plastics for molding. However, many materials are either very soft or
very tough at room temperatures. By cooling to cryogenic temperatures with liquid nitrogen,
these may be embrittled and easily fractured into small particles.
A scientifically controlled study using four herbs was conducted at Frontier Herbs in the Fall
of 1996, comparing cryogenic grinding methods with normal grinding methods. The herbs
tested included feverfew, goldenseal, valerian and echinacea. In all cases the cryogenically
ground herb contained greater amounts of the constituents tested. Feverfew herb showed the Cryogenic Grinding
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greatest difference, with the cryogenically ground herb containing 21.8% higher levels of
parthenolide, the primary active constituent. Valerian root showed an 18.7% increase in
valerenic acid when cryogenically ground. Goldenseal root showed a 16.4% increase in
berberine and 10.7% increase in hydrastine. Lastly, Echinacea purpurea root showed a 12.1%
increase in total phenolic content in the cryogenically ground root. Test results were obtained
by HPLC (high performance liquid chromatography) methods.
Cryogenic grinding was shown to significantly affect active constituent levels in herbs. Test
results showed an average increase of 15.6% in constituents tested in four medicinal herbs
when they were ground cryogenically. The range was 10.7% to 21.8%, indicating that some
herbs are affected more than others by the temperatures at which they're ground.
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.
TYPES OF CRYOGENIC GRINDING
1. Freezer milling
Freezer milling is a type of cryogenic milling that uses a solenoid to mill samples. The
solenoid moves the grinding media back and forth inside the vial, grinding the sample
down to analytical fineness. This type of milling is especially useful in milling
temperature sensitive samples, as samples are milled at liquid nitrogen temperatures. The
idea behind using a solenoid is that the only "moving part" in the system is the grinding
media inside the vial. The reason for this is that at liquid nitrogen temperatures (-196°C)
any moving part will come under huge stress leading to potentially poor reliability.
Cryogenic milling using a solenoid has been used for over 50 years and has been proved
to be a very reliable method of processing temperature sensitive samples in the
laboratory.
2. Cryomilling
Cryomilling is a variation of mechanical milling, in which metallic powders or other
samples (e.g. temperature sensitive samples and samples with volatile components) are
milled in a cryogen (usually liquid nitrogen or liquid argon) slurry or at
a cryogenics temperature under processing parameters, so a nanostructured microstructure
is attained. Cryomilling takes advantage of both the cryogenic temperatures and
conventional mechanical milling. The extremely low milling temperature suppresses
recovery and recrystallization and leads to finer grain structures and more rapid grain
refinement. The embrittlement of the sample makes even elastic and soft samples
grindable. Tolerances less than 5 µm can be achieved. The ground material can be
analyzed by a laboratory analyzer.
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.