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28-12-2009, 05:00 PM
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cryogenic grinding

.doc  Cryogenic Grinding.DOC (Size: 141 KB / Downloads: 852)
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.

Chapter I
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 todayâ„¢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 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.
Chapter II
APPLICATION OF CRYOGENICS
The major areas in which cryogenics find its applications are : -
1. Gas Industry “ in air separation. The volume of production of nitrogen and Oxygen by cryogenic separation of air is the important of the separation of air, refrigeration and separation. In the separation column, the difference in the boiling points of the constituents of air is used to separate them out.
2. As the source of gas. For example, the breathing oxygen needed for the pilots of the fighter aircraft is supplied by vaporizing liquid oxygen on board. In this way is a weight reduction of 65% and space reduction of 85%.
3. In space research “ as rocket propellant and for space simulation. The most important advantage of cryogenic fuels is that these have very high specific impulse when compared to other fuels (specific impulse is kgs of thrust produced per kg of propellant per sec). The value is approximately 500 for cryogenic fuels whereas it is about 250 for alcohol oxygen mixture.
4. In biology “ for preservation and in treatment of diseases.
5. In food industry “ for food handling and processing
6. In electronics “ both semiconductor and superconductor electronics for better signal to noise ratio speed etc
7. In miscellaneous applications such as cryogenic grinding , freezing pipelines for repairs, shrink fitting, fire fighting, etc
8. In medicine “ Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy (MRS), Magneto Cardio Graphy (MCG), etc.
9. In nuclear and high “ energy physics
10. Metal fabrication
Chapter III
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.
Physical properties of liquid nitrogen is produced by the separation of air into its components in an air separation plant and is distributed in vacuum insulated transport vessels to the end user where it is stored in a vacuum insulated storage vessel till it is used. At atmospheric pressure liquid nitrogen is at a temperature of “320 deg F and possesses a latent energy content of 94 BTU/LB resulting in a total cooling energy content of 179.6 BTU/LB. Nitrogen is anon-flammable, non toxic and inert gas which makes up 78.09% of the air we breathe. It has the characteristics of an inert gas, except at highly elevated temperatures, and does not form any compound under normal temperatures and pressure. Drawn from the liquid phase, nitrogen generally has a purity of 99.998 % with a dew point less than “ 100 deg F and is very dry.


Rapid embritlement of tough materials
Liquid Nitrogen at 77.6 K is used to embrittle a material prior to size reduction. Once brittle the material is much easier to grind. When CRYO-GRIND system is used to recycle composite or multi component materials, two separate phenomena occur. First, since each component generally would have a different coefficient of thermal contraction, high thermal stresses are created at the interface between the components due to rapid cryogenic cooling. Second, because each component material embrittles at different temperatures, it allows selective embritlement, which further enhances separation effectiveness. The most brittle components will undergo greater size reduction. Through careful control of thermal stress and embritlement with operating temperature, cleaner separation and recovery of individual components are achieved.
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.

Cryogenic Grinding System
Chapter IV
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.
Cryogenic grinding also reduces the material to particle sizes difficult or impossible to attain with ambient temperature grinding. The dry, cold, inert atmosphere in which the grinding occurs minimizes reaction with the material and reduces the loss of volatile components. When processing composite materials, cryogenic grinding usually makes it easy to separate the various materials.
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.

Chapter V
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
Chapter VI
APPLICATIONS OF CRYOGENIC GRINDING
5.1 CryoGrinding of steel
The large amount of thermal energy generated during machining and grinding at high speed and feed rate raises the temperature at the cutting zones excessively. This elevated temperature level under large cutting stresses accelerates plastic deformation and wear of cutting edges leading to increased cutting forces and premature tool failure. Cooling with conventional cutting fluids in the form of jet or mist is unable to solve the problem. In such cases cryogenic cooling by agents like liquid nitrogen will improve the situation. In the case of cryogenic grinding, the liquid nitrogen from a reservoir under air pressure comes out a jet from a nozzle to the spot where cooling is desired. The jet impinges on the work surface at the grinding point from a suitable distance and angle. The amount of temperature reduction due to cryocooling will increase the grinding depth. Because of the extreme cooling action, the life of the grinding wheel\s will be increased.
5.2 Thermoplastics
To which Nylon, PVC, Polyethylene, and polypropylene belong are commonly used in powdered form, for but not limited to, a variety of applications such as adhesives, powdered coatings, fillers, resins and plastics sintering and molding. These powders generally can only be produced in high production rates and fine particle sizes utilizing cryogenic size reduction.
5.3 Thermo sets
To which natural and synthetic rubbers belong are important recyclable materials. Under cryogenic size reduction these materials can economically and at high production rates be ground into fine powders, used as filler, be recycled.
5.4 Adhesives & Waxes
These materials at ambient temperature are generally pliable and sticky and when ground would form excessive deposits in the mill building up heat, increasing energy requirement and eventually shutting down the size reduction process. Under cryogenic temperatures these products become brittle and can be pulverized with much less energy and without forming deposits.
5.5 Explosives
Explosives explode when their ignition temperature, in the presence of oxygen, is achieved. Cryogenic size reduction performs two tasks when grinding explosives; it reduces the temperature of the material well below its ignition temperature and removes the oxygen from the system thereby eliminating the possibility of combustion. The product to be ground is filled into the volumetric screw feeder where it is metered at a specific rate into the cryogenic pre-cooler. In the cryogenic pre-cooler liquid nitrogen is injected and combines with the product thereby cooling and embrittling the product. The product is then transported, along with the cold gas generated by the evaporation of the liquid nitrogen, to the grinding mill where it is pulverized. The pulverized product then goes through a classifier where it is separated into various particle sizes and packaged. Should oversize material exist this can be fed back into the volumetric feeder and recycled into the system. The cold gas from the mill is recycled through the filter or bag-house and makeup air back into the mill. Excessive cold gas is vented out. In addition the cold dry nitrogen gas keeps both the classifier and bag-house free of moisture and inert, preventing the possibility of dust explosions and buildup of product.
5.6 Spices
Spices like Pepper, cinnamon, chilly, Ginger, Cumin seed, Nutmeg, Glove etc., have a characteristic taste and aroma. These characteristic qualities are essential in them to have their value as Ëœspiceâ„¢. These qualities exist in them due to the presence of etheric oils within. The etheric oils have their boiling points ranging down to 50oC. During conventional grinding, due to the heat produced by friction, the temperature of ground spices shoots up to about 90oC, where by most of the etheric oils oil off resulting in inferior quality of the ground product. This inferior quality is evident by the reduced taste and aroma.
Chapter VII
PROBLEMS WITH CONVENTION GRINDING
6.1 Loss of etheric oil
The applied energy gets dissipated in the form of heat (>99%) and hence the temperature in the grinding zone rises to more than 90oC resulting in loss of etheric oils whose boiling point vary from 50oC to 320oC. This results in the inferior quality of the ground product.
6.2 Clogging and gumming of the mill
Spices like nutmeg, clove, cinnamon, etc.., contain high level of fat while capsicum, chilli, etc, contain high moisture content. These cause clogging and gumming of mill thus affecting the throughput and quality of the ground product. High moisture content materials often stick to the parts of the mill.
6.3 Oxidation and related degradation:
Due to intimate cyclone effect of the air in the vicinity of grinding zone, aromatic substances in materials oxidize and become rancid. In addition the formation of fresh and exposed surfaces due to grinding, accelerates the process of oxidation.
Chapter VIII
ADVANTAGES OF CRYOGRINDING WITH LIQUID NITROGEN
7.1 Higher retention of etheric oils
Due to lower operating temperatures, the etheric oils will be retained in the product almost to the original level. Hence the ground product will obviously be better in taste and aroma, with the improved value as Ëœspiceâ„¢.
7.2 Prevention of oxidation and rancidity
The heat developed during grinding will be absorbed by liquid nitrogen which in turn will be converted to vapour. These vapours expel any air in the mill and produce an inert atmosphere during grinding. This eliminates the possibility of oxidation. To add to this, an inexpensive, dry and inert atmosphere for storage and package of ground product is created.
7.3 Increased throughput and power saving
Due to usage of liquid nitrogen, the raw material becomes brittle. This in turn keeps the oil and moisture content in the crystallized condition during grinding and avoids clogging. At the same time when the material is brittle, less power is required to crush. Thus cooling with liquid nitrogen increases the throughput due to reduction in specific energy requirement.
7.4 Finer particle size
CryoGrinding results in finer particle sizes of the ground spices. This eliminates speckling problems and reduces the settling rate of spice powders in liquid preparation.
7.5 Reduction in microbial load
By cooling with liquid nitrogen, some bacteriaâ„¢s which some others may become dormant. Thus, when the ground product is warmed to the room temperature (in the inert atmosphere of nitrogen) one expects considerable reduction in microbial load.
7.6 Possibility of fine grinding of difficult spices
By use of low temperatures, the raw materials become brittle which causes fibers to shatter. Thus fibrous spices like ginger can be ground easily to finer particle size. High oil content spices like nutmeg can be ground easily. CryoGrinding can be employed for grinding green spices like chilies with no pre-drying and also with the retention of its original colour.
Chapter IX
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.
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.

Chapter X
RESULTS OF EXPERIMENTAL STUDIES WITH PEPPER
Experimental studies were carried out on the samples under different conditions:
9.1 Volatile oil content and flavor components
The pepper contains the oil-bearing cells mainly in the skin and towards the tip of the cortex. When the pepper is ground these cells are broken and volatile oils and flavour components evaporate causing aroma. In CryoGrinding higher percentage of oils and flavor components are retained when compared with conventionally ground products. The comparison is shown below.
Sl. No. Components Cryoground (%) Conventional Ground (%)
1. Moisture 13.00 11.00
2. Volatile Oil 2.61 1.15
3. Flavour compounds (relative concentration) -Pining
Limonene 1.40
8.30 0.29
1.18
9.2 Throughput of the Mill
For a constant current of 6A, the throughput of the mill by CryoGrinding was 50 Kg/hour (at-50oC), which was 2.25 times than that of conventional grinding producing a throughput of 22 Kg/hour. This analysis clearly illustrates a throughput of 225 kg/hour (against the rated throughput of 100 kg/hour by conventional method at room temperature) could be easily be attained by CryoGrinding by suitable modification of the screw cooler and by vibratory feeder.
Chapter XI
FUTURE PROSPECTS
As the cost of raw materials and energy is increasing day by day, it is very necessary to use optimum quantity and at the same time getting the required quality. By using CryoGrinding technology these aspects can be met efficiently. By using this we can also recycle tough and composite materials. It has many significant advantages over conventional grinding. This also leads to value addition to the product. CryoGrinding is economically viable, if liquid nitrogen costs are not formidable. By adopting CryoGrinding technology the leading spice industries of our country will earn considerable foreign exchange by exporting more value added processed spices, in place of exporting whole spices. The technique can be easily extended to processing of PVC and industrial waste plastics in view of recycling of non-biodegradable materials.

BIBLIOGRAPHY
¢ A short term QIP course on cryogenic technology (Center for continuing education IIS Bangalore)
¢ AICTE-ISTE Short-term programme on contribution to technology development from space research
¢ Mc Graw Hill Encyclopedia of science & technology: 7th Edition VOL-4
¢ Proceeding of the 18th International Cryogenic Engineering Conference (Edited by K.G. Narayankhedhar, Narosa Publishing House)
¢ http://www.csa.fnal.gov Cold Facts (The quarterly magazine of the Cryogenic Society of America).


CONTENTS
1. Introduction
2. Application of Cryogenics
3. Cryogenic Grinding Process
4. Cryogenic Grinding Technology
5. Advantages of Cryogenics
6. Application of Cryogenic Grinding
7. Problems with Conventional Grinding
8. Advantages of Cryogrinding with Liquid Nitrogen
9. Working of Cryogrinding Plant
10. Results of experimental studies with Pepper
11. Future Prospects
12. Bibliography

ACKNOWLEDGEMENT
First of all I thank the almighty for providing me with the strength and courage to present the seminar.
I avail this opportunity to express my sincere gratitude towards
Dr. T.N. Sathyanesan, head of mechanical engineering department, for permitting me to conduct the seminar. I also at the outset thank and express my profound gratitude to my seminar guide Mr. A. Ajithkumar and staff incharge Asst. Prof. Mrs. Jumailath Beevi. D., for their inspiring assistance, encouragement and useful guidance.
I am also indebted to all the teaching and non- teaching staff of the department of mechanical engineering for their cooperation and suggestions, which is the spirit behind this report. Last but not the least, I wish to express my sincere thanks to all my friends for their goodwill and constructive ideas.

SUNILKUMAR NADUVILAKANDI
21-03-2010, 02:07 PM
Post: #2
RE: cryogenic grinding

.doc  Cryogenic Grinding.DOC (Size: 131.5 KB / Downloads: 226)


Chapter I
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 todayâ„¢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 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.
Chapter II
APPLICATION OF CRYOGENICS
The major areas in which cryogenics find its applications are : -
1. Gas Industry “ in air separation. The volume of production of nitrogen and Oxygen by cryogenic separation of air is the important of the separation of air, refrigeration and separation. In the separation column, the difference in the boiling points of the constituents of air is used to separate them out.
2. As the source of gas. For example, the breathing oxygen needed for the pilots of the fighter aircraft is supplied by vaporizing liquid oxygen on board. In this way is a weight reduction of 65% and space reduction of 85%.
3. In space research “ as rocket propellant and for space simulation. The most important advantage of cryogenic fuels is that these have very high specific impulse when compared to other fuels (specific impulse is kgs of thrust produced per kg of propellant per sec). The value is approximately 500 for cryogenic fuels whereas it is about 250 for alcohol oxygen mixture.
4. In biology “ for preservation and in treatment of diseases.
5. In food industry “ for food handling and processing
6. In electronics “ both semiconductor and superconductor electronics for better signal to noise ratio speed etc
7. In miscellaneous applications such as cryogenic grinding , freezing pipelines for repairs, shrink fitting, fire fighting, etc
8. In medicine “ Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy (MRS), Magneto Cardio Graphy (MCG), etc.
9. In nuclear and high “ energy physics
10. Metal fabrication
Chapter III
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.
Physical properties of liquid nitrogen is produced by the separation of air into its components in an air separation plant and is distributed in vacuum insulated transport vessels to the end user where it is stored in a vacuum insulated storage vessel till it is used. At atmospheric pressure liquid nitrogen is at a temperature of “320 deg F and possesses a latent energy content of 94 BTU/LB resulting in a total cooling energy content of 179.6 BTU/LB. Nitrogen is anon-flammable, non toxic and inert gas which makes up 78.09% of the air we breathe. It has the characteristics of an inert gas, except at highly elevated temperatures, and does not form any compound under normal temperatures and pressure. Drawn from the liquid phase, nitrogen generally has a purity of 99.998 % with a dew point less than “ 100 deg F and is very dry.

Rapid embritlement of tough materials
Liquid Nitrogen at 77.6 K is used to embrittle a material prior to size reduction. Once brittle the material is much easier to grind. When CRYO-GRIND system is used to recycle composite or multi component materials, two separate phenomena occur. First, since each component generally would have a different coefficient of thermal contraction, high thermal stresses are created at the interface between the components due to rapid cryogenic cooling. Second, because each component material embrittles at different temperatures, it allows selective embritlement, which further enhances separation effectiveness. The most brittle components will undergo greater size reduction. Through careful control of thermal stress and embritlement with operating temperature, cleaner separation and recovery of individual components are achieved.
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.

Cryogenic Grinding System
Chapter IV
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.
Cryogenic grinding also reduces the material to particle sizes difficult or impossible to attain with ambient temperature grinding. The dry, cold, inert atmosphere in which the grinding occurs minimizes reaction with the material and reduces the loss of volatile components. When processing composite materials, cryogenic grinding usually makes it easy to separate the various materials.
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.
Chapter V
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
Chapter VI
APPLICATIONS OF CRYOGENIC GRINDING
5.1 CryoGrinding of steel
The large amount of thermal energy generated during machining and grinding at high speed and feed rate raises the temperature at the cutting zones excessively. This elevated temperature level under large cutting stresses accelerates plastic deformation and wear of cutting edges leading to increased cutting forces and premature tool failure. Cooling with conventional cutting fluids in the form of jet or mist is unable to solve the problem. In such cases cryogenic cooling by agents like liquid nitrogen will improve the situation. In the case of cryogenic grinding, the liquid nitrogen from a reservoir under air pressure comes out a jet from a nozzle to the spot where cooling is desired. The jet impinges on the work surface at the grinding point from a suitable distance and angle. The amount of temperature reduction due to cryocooling will increase the grinding depth. Because of the extreme cooling action, the life of the grinding wheel\s will be increased.
5.2 Thermoplastics
To which Nylon, PVC, Polyethylene, and polypropylene belong are commonly used in powdered form, for but not limited to, a variety of applications such as adhesives, powdered coatings, fillers, resins and plastics sintering and molding. These powders generally can only be produced in high production rates and fine particle sizes utilizing cryogenic size reduction.
5.3 Thermo sets
To which natural and synthetic rubbers belong are important recyclable materials. Under cryogenic size reduction these materials can economically and at high production rates be ground into fine powders, used as filler, be recycled.
5.4 Adhesives & Waxes
These materials at ambient temperature are generally pliable and sticky and when ground would form excessive deposits in the mill building up heat, increasing energy requirement and eventually shutting down the size reduction process. Under cryogenic temperatures these products become brittle and can be pulverized with much less energy and without forming deposits.
5.5 Explosives
Explosives explode when their ignition temperature, in the presence of oxygen, is achieved. Cryogenic size reduction performs two tasks when grinding explosives; it reduces the temperature of the material well below its ignition temperature and removes the oxygen from the system thereby eliminating the possibility of combustion. The product to be ground is filled into the volumetric screw feeder where it is metered at a specific rate into the cryogenic pre-cooler. In the cryogenic pre-cooler liquid nitrogen is injected and combines with the product thereby cooling and embrittling the product. The product is then transported, along with the cold gas generated by the evaporation of the liquid nitrogen, to the grinding mill where it is pulverized. The pulverized product then goes through a classifier where it is separated into various particle sizes and packaged. Should oversize material exist this can be fed back into the volumetric feeder and recycled into the system. The cold gas from the mill is recycled through the filter or bag-house and makeup air back into the mill. Excessive cold gas is vented out. In addition the cold dry nitrogen gas keeps both the classifier and bag-house free of moisture and inert, preventing the possibility of dust explosions and buildup of product.
5.6 Spices
Spices like Pepper, cinnamon, chilly, Ginger, Cumin seed, Nutmeg, Glove etc., have a characteristic taste and aroma. These characteristic qualities are essential in them to have their value as Ëœspiceâ„¢. These qualities exist in them due to the presence of etheric oils within. The etheric oils have their boiling points ranging down to 50oC. During conventional grinding, due to the heat produced by friction, the temperature of ground spices shoots up to about 90oC, where by most of the etheric oils oil off resulting in inferior quality of the ground product. This inferior quality is evident by the reduced taste and aroma.
Chapter VII
PROBLEMS WITH CONVENTION GRINDING
6.1 Loss of etheric oil
The applied energy gets dissipated in the form of heat (>99%) and hence the temperature in the grinding zone rises to more than 90oC resulting in loss of etheric oils whose boiling point vary from 50oC to 320oC. This results in the inferior quality of the ground product.
6.2 Clogging and gumming of the mill
Spices like nutmeg, clove, cinnamon, etc.., contain high level of fat while capsicum, chilli, etc, contain high moisture content. These cause clogging and gumming of mill thus affecting the throughput and quality of the ground product. High moisture content materials often stick to the parts of the mill.
6.3 Oxidation and related degradation:
Due to intimate cyclone effect of the air in the vicinity of grinding zone, aromatic substances in materials oxidize and become rancid. In addition the formation of fresh and exposed surfaces due to grinding, accelerates the process of oxidation.
Chapter VIII
ADVANTAGES OF CRYOGRINDING WITH LIQUID NITROGEN
7.1 Higher retention of etheric oils
Due to lower operating temperatures, the etheric oils will be retained in the product almost to the original level. Hence the ground product will obviously be better in taste and aroma, with the improved value as Ëœspiceâ„¢.
7.2 Prevention of oxidation and rancidity
The heat developed during grinding will be absorbed by liquid nitrogen which in turn will be converted to vapour. These vapours expel any air in the mill and produce an inert atmosphere during grinding. This eliminates the possibility of oxidation. To add to this, an inexpensive, dry and inert atmosphere for storage and package of ground product is created.
7.3 Increased throughput and power saving
Due to usage of liquid nitrogen, the raw material becomes brittle. This in turn keeps the oil and moisture content in the crystallized condition during grinding and avoids clogging. At the same time when the material is brittle, less power is required to crush. Thus cooling with liquid nitrogen increases the throughput due to reduction in specific energy requirement.
7.4 Finer particle size
CryoGrinding results in finer particle sizes of the ground spices. This eliminates speckling problems and reduces the settling rate of spice powders in liquid preparation.
7.5 Reduction in microbial load
By cooling with liquid nitrogen, some bacteriaâ„¢s which some others may become dormant. Thus, when the ground product is warmed to the room temperature (in the inert atmosphere of nitrogen) one expects considerable reduction in microbial load.
7.6 Possibility of fine grinding of difficult spices
By use of low temperatures, the raw materials become brittle which causes fibers to shatter. Thus fibrous spices like ginger can be ground easily to finer particle size. High oil content spices like nutmeg can be ground easily. CryoGrinding can be employed for grinding green spices like chilies with no pre-drying and also with the retention of its original colour.
Chapter IX
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.
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.
Chapter X
RESULTS OF EXPERIMENTAL STUDIES WITH PEPPER
Experimental studies were carried out on the samples under different conditions:
9.1 Volatile oil content and flavor components
The pepper contains the oil-bearing cells mainly in the skin and towards the tip of the cortex. When the pepper is ground these cells are broken and volatile oils and flavour components evaporate causing aroma. In CryoGrinding higher percentage of oils and flavor components are retained when compared with conventionally ground products. The comparison is shown below.
Sl. No. Components Cryoground (%) Conventional Ground (%)
1. Moisture 13.00 11.00
2. Volatile Oil 2.61 1.15
3. Flavour compounds (relative concentration) -Pining
Limonene 1.40
8.30 0.29
1.18
9.2 Throughput of the Mill
For a constant current of 6A, the throughput of the mill by CryoGrinding was 50 Kg/hour (at-50oC), which was 2.25 times than that of conventional grinding producing a throughput of 22 Kg/hour. This analysis clearly illustrates a throughput of 225 kg/hour (against the rated throughput of 100 kg/hour by conventional method at room temperature) could be easily be attained by CryoGrinding by suitable modification of the screw cooler and by vibratory feeder.
Chapter XI
FUTURE PROSPECTS
As the cost of raw materials and energy is increasing day by day, it is very necessary to use optimum quantity and at the same time getting the required quality. By using CryoGrinding technology these aspects can be met efficiently. By using this we can also recycle tough and composite materials. It has many significant advantages over conventional grinding. This also leads to value addition to the product. CryoGrinding is economically viable, if liquid nitrogen costs are not formidable. By adopting CryoGrinding technology the leading spice industries of our country will earn considerable foreign exchange by exporting more value added processed spices, in place of exporting whole spices. The technique can be easily extended to processing of PVC and industrial waste plastics in view of recycling of non-biodegradable materials.
BIBLIOGRAPHY
¢ A short term QIP course on cryogenic technology (Center for continuing education IIS Bangalore)
¢ AICTE-ISTE Short-term programme on contribution to technology development from space research
¢ Mc Graw Hill Encyclopedia of science & technology: 7th Edition VOL-4
¢ Proceeding of the 18th International Cryogenic Engineering Conference (Edited by K.G. Narayankhedhar, Narosa Publishing House)
¢ http://www.csa.fnal.gov Cold Facts (The quarterly magazine of the Cryogenic Society of America).
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.
CONTENTS
1. Introduction
2. Application of Cryogenics
3. Cryogenic Grinding Process
4. Cryogenic Grinding Technology
5. Advantages of Cryogenics
6. Application of Cryogenic Grinding
7. Problems with Conventional Grinding
8. Advantages of Cryogrinding with Liquid Nitrogen
9. Working of Cryogrinding Plant
10. Results of experimental studies with Pepper
11. Future Prospects
12. Bibliography
ACKNOWLEDGEMENT

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19-10-2010, 04:20 PM
Post: #3
RE: cryogenic grinding
CRYOGENIC GRINDING :


CRYOGENIC GRINDING PRESENTED BY BINAY SHAW 4TH YEAR MECHANICAL ENGINEERING ROLL NO. 10207051016 KALYANI GOVT. ENGINEERING COLLEGE GUIDED BY PROF. S. DAS Head, DEPT. OF MECHANICAL ENGINEERING KALYANI GOVT. ENGINEERING COLLEGE KALYANI NADIA

GRINDING :
GRINDING Grinding is one of the most popular finishing processes. This process is carried out with a grinding wheel made up of grits for removing materials from workpiece surface.

The problems in conventional grinding process: :
The problems in conventional grinding process: High heat generation Introduction of tensile residual stress and micro-cracks occur High level of surface quality can not be reached Problems in hard materials. Less tool life Sometimes M.R.R. is not satisfactory.

Now… elimination of all those problems are required…but… :
Now… elimination of all those problems are required…but… HOW…???

Here comes the introduction of _ :
Here comes the introduction of _ C R Y O G E N I C G R I N D I N G …also known as Freeze Grinding

How it is done… :
How it is done… The cryogen like liquid nitrogen is applied onto the cutting point mostly in the form of jet. At low temperature (-196°C) of the cryogen, the temperature at the cutting zone is controlled more effectively than the other methods of applying cutting fluid.

Experimental set-up :
Experimental set-up

Slide 8:
. Ground chips of different steels under different environments: (a) MS 30 µm; (b) HCS 40 µm; © CDS 40 µm; (d) HDS 30 µm; (e) HSS 40 µm

Slide 9:
Surface conditions of different steels ground under different environmental: (a) MS 30 µm; (b) HCS 40 µm; © CDS 40 µm; (d) HDS 30 µm; (e) HSS 40 µm.

discussing in course of viewing the advantages… :
discussing in course of viewing the advantages… The cryogenic grinding lowers the thermal related problems Reduced grinding forces High M.R.R. can be achieved Minimizing residual stress and micro-cracks Retention of wheel grit sharpness (i.e. low rate of grit wear) Surface finish, dimensional accuracy and tool life improved Difficult-to-machine materials can also be machined Moreover, use of liquid nitrogen as a grinding fluid facilitates clean and pollution free working environment

Cryo-grinding do have distinctive advantages over the conventional cooling processes, but… :
Cryo-grinding do have distinctive advantages over the conventional cooling processes, but… The application of the cryogen in the moist atmosphere may cause formation of ice and around the delivery nozzle and the piping system carrying the cryogen. This may result in the possible blockage of the liquid nitrogen delivery system. Economic considerations should be solved

Hence we can conclude… :
Hence we can conclude… Cryo-grinding improves product quality by controlling thermal effects Oxidation and surface burning are eliminated Surface damage is absent The surface roughness in cryo grinding is found to be greater than in conventional grinding Reduction in forces and specific energy The benefits of cryo-grinding are greater for harder materials and higher infeeds





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09-01-2011, 05:48 PM
Post: #4
RE: 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.



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TABLE OF CONTENTS

Chapter No. Title Page No.
1 Introduction 5
2 Application of Cryogenics 7
3 Cryogenic Grinding Process 8
4 Cryogenic Grinding Technology 11
5 Advantages of Cryogrinding 13
6 Applications of Cryogenic Grinding 13
7 Problems with Convention Grinding 16
8 Advantages of Cryogrinding with Liquid Nitrogen 17
9 Working of Cryogrinding Plant 19
10 Results of Experimental Studies with Pepper 21
11 Cryogenic preparation of sample materials 22
12 For which material is cold grinding advisable 23
13 Future Prospects 32
14 Bibliography 33

TABLE OF FIGURES


Figure No. Figure Name Page No.
1
2 Cryogenic Grinding System 11
3 Working of Cryogenic plant 20
4 Mixer Mill MM 301 with grinding jars of Stee. 25
5 Kryo-Kit with containers tongs and Goggles. 25
6 Cooling the jars in liquid nitrogen 26
7 Clamping the cooled jars into the MM 301 27
8 Granulated rubber before grinding 27
9 Granulated rubber and after grinding with LN2 28
10 Plant leaves before grinding 28
11 Plant leaves and after grinding with LN2 29
12 Adapters for disposable reaction vials 30
13 Ultra-Centrifugal Mill ZM 200 31


1. 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 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.









2. APPLICATION OF CRYOGENICS

The major areas in which cryogenics find its applications are: -
1. Gas Industry – in air separation. The volume of production of nitrogen and Oxygen by cryogenic separation of air is the important of the separation of air, refrigeration and separation. In the separation column, the difference in the boiling points of the constituents of air is used to separate them out.
2. As the source of gas. For example, the breathing oxygen needed for the pilots of the fighter aircraft is supplied by vaporizing liquid oxygen on board. In this way is a weight reduction of 65% and space reduction of 85%.
3. In space research – as rocket propellant and for space simulation. The most important advantage of cryogenic fuels is that these have very high specific impulse when compared to other fuels (specific impulse is kgs of thrust produced per kg of propellant per sec). The value is approximately 500 for cryogenic fuels whereas it is about 250 for alcohol oxygen mixture.
4. In biology – for preservation and in treatment of diseases.
5. In food industry – for food handling and processing
6. In electronics – both semiconductor and superconductor electronics for better signal to noise ratio speed etc
7. In miscellaneous applications such as cryogenic grinding , freezing pipelines for repairs, shrink fitting, fire fighting, etc
8. In medicine – Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy (MRS), Magneto Cardio Graphy (MCG), etc.
9. In nuclear and high – energy physics
10. Metal fabrication


3. 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.
Physical properties of liquid nitrogen is produced by the separation of air into its components in an air separation plant and is distributed in vacuum insulated transport vessels to the end user where it is stored in a vacuum insulated storage vessel till it is used. At atmospheric pressure liquid nitrogen is at a temperature of –320 deg F and possesses a latent energy content of 94 BTU/LB resulting in a total cooling energy content of 179.6 BTU/LB. Nitrogen is anon-flammable, non toxic and inert gas which makes up 78.09% of the air we breathe. It has the characteristics of an inert gas, except at highly elevated temperatures, and does not form any compound under normal temperatures and pressure. Drawn from the liquid phase, nitrogen generally has a purity of 99.998 % with a dew point less than – 100 deg F and is very dry.








Rapid embritlement of tough materials



Liquid Nitrogen at 77.6 K is used to embrittle a material prior to size reduction. Once brittle the material is much easier to grind. When CRYO-GRIND system is used to recycle composite or multi component materials, two separate phenomena occur. First, since each component generally would have a different coefficient of thermal contraction, high thermal stresses are created at the interface between the components due to rapid cryogenic cooling. Second, because each component material embrittles at different temperatures, it allows selective embritlement, which further enhances separation effectiveness. The most brittle components will undergo greater size reduction. Through careful control of thermal stress and embritlement with operating temperature, cleaner separation and recovery of individual components are achieved.

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.


Figure 2: Cryogenic Grinding System



4. 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.
Cryogenic grinding also reduces the material to particle sizes difficult or impossible to attain with ambient temperature grinding. The dry, cold, inert atmosphere in which the grinding occurs minimizes reaction with the material and reduces the loss of volatile components. When processing composite materials, cryogenic grinding usually makes it easy to separate the various materials.
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.


5. 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


6. APPLICATIONS OF CRYOGENIC GRINDING

6.1 CryoGrinding of steel
The large amount of thermal energy generated during machining and grinding at high speed and feed rate raises the temperature at the cutting zones excessively. This elevated temperature level under large cutting stresses accelerates plastic deformation and wear of cutting edges leading to increased cutting forces and premature tool failure. Cooling with conventional cutting fluids in the form of jet or mist is unable to solve the problem. In such cases cryogenic cooling by agents like liquid nitrogen will improve the situation. In the case of cryogenic grinding, the liquid nitrogen from a reservoir under air pressure comes out a jet from a nozzle to the spot where cooling is desired. The jet impinges on the work surface at the grinding point from a suitable distance and angle. The amount of temperature reduction due to cryocooling will increase the grinding depth. Because of the extreme cooling action, the life of the grinding wheel\s will be increased.

6.2 Thermoplastics
To which Nylon, PVC, Polyethylene, and polypropylene belong are commonly used in powdered form, for but not limited to, a variety of applications such as adhesives, powdered coatings, fillers, resins and plastics sintering and molding. These powders generally can only be produced in high production rates and fine particle sizes utilizing cryogenic size reduction.

6.3 Thermo sets
To which natural and synthetic rubbers belong are important recyclable materials. Under cryogenic size reduction these materials can economically and at high production rates be ground into fine powders, used as filler, be recycled.

6.4 Adhesives & Waxes
These materials at ambient temperature are generally pliable and sticky and when ground would form excessive deposits in the mill building up heat, increasing energy requirement and eventually shutting down the size reduction process. Under cryogenic temperatures these products become brittle and can be pulverized with much less energy and without forming deposits.
6.5 Explosives
Explosives explode when their ignition temperature, in the presence of oxygen, is achieved. Cryogenic size reduction performs two tasks when grinding explosives; it reduces the temperature of the material well below its ignition temperature and removes the oxygen from the system thereby eliminating the possibility of combustion. The product to be ground is filled into the volumetric screw feeder where it is metered at a specific rate into the cryogenic pre-cooler. In the cryogenic pre-cooler liquid nitrogen is injected and combines with the product thereby cooling and embrittling the product. The product is then transported, along with the cold gas generated by the evaporation of the liquid nitrogen, to the grinding mill where it is pulverized. The pulverized product then goes through a classifier where it is separated into various particle sizes and packaged. Should oversize material exist this can be fed back into the volumetric feeder and recycled into the system. The cold gas from the mill is recycled through the filter or bag-house and makeup air back into the mill. Excessive cold gas is vented out. In addition the cold dry nitrogen gas keeps both the classifier and bag-house free of moisture and inert, preventing the possibility of dust explosions and buildup of product.

6.6 Spices
Spices like Pepper, cinnamon, chilly, Ginger, Cumin seed, Nutmeg, Glove etc., have a characteristic taste and aroma. These characteristic qualities are essential in them to have their value as ‘spice’. These qualities exist in them due to the presence of etheric oils within. The etheric oils have their boiling points ranging down to 50oC. During conventional grinding, due to the heat produced by friction, the temperature of ground spices shoots up to about 90oC, where by most of the etheric oils oil off resulting in inferior quality of the ground product. This inferior quality is evident by the reduced taste and aroma.

7. PROBLEMS WITH CONVENTION GRINDING
7.1 Loss of etheric oil
The applied energy gets dissipated in the form of heat (>99%) and hence the temperature in the grinding zone rises to more than 90oC resulting in loss of etheric oils whose boiling point vary from 50oC to 320oC. This results in the inferior quality of the ground product.

7.2 Clogging and gumming of the mill
Spices like nutmeg, clove, cinnamon, etc.., contain high level of fat while capsicum, chilli, etc, contain high moisture content. These cause clogging and gumming of mill thus affecting the throughput and quality of the ground product. High moisture content materials often stick to the parts of the mill.

7.3 Oxidation and related degradation:
Due to intimate cyclone effect of the air in the vicinity of grinding zone, aromatic substances in materials oxidize and become rancid. In addition the formation of fresh and exposed surfaces due to grinding, accelerates the process of oxidation.


8. ADVANTAGES OF CRYOGRINDING WITH LIQUID NITROGEN

8.1 Higher retention of etheric oils
Due to lower operating temperatures, the etheric oils will be retained in the product almost to the original level. Hence the ground product will obviously be better in taste and aroma, with the improved value as ‘spice’.

8.2 Prevention of oxidation and rancidity
The heat developed during grinding will be absorbed by liquid nitrogen which in turn will be converted to vapour. These vapours expel any air in the mill and produce an inert atmosphere during grinding. This eliminates the possibility of oxidation. To add to this, an inexpensive, dry and inert atmosphere for storage and package of ground product is created.

8.3 Increased throughput and power saving
Due to usage of liquid nitrogen, the raw material becomes brittle. This in turn keeps the oil and moisture content in the crystallized condition during grinding and avoids clogging. At the same time when the material is brittle, less power is required to crush. Thus cooling with liquid nitrogen increases the throughput due to reduction in specific energy requirement.



8.4 Finer particle size
CryoGrinding results in finer particle sizes of the ground spices. This eliminates speckling problems and reduces the settling rate of spice powders in liquid preparation.

8.5 Reduction in microbial load
By cooling with liquid nitrogen, some bacteria’s which some others may become dormant. Thus, when the ground product is warmed to the room temperature (in the inert atmosphere of nitrogen) one expects considerable reduction in microbial load.

8.6 Possibility of fine grinding of difficult spices
By use of low temperatures, the raw materials become brittle which causes fibers to shatter. Thus fibrous spices like ginger can be ground easily to finer particle size. High oil content spices like nutmeg can be ground easily. CryoGrinding can be employed for grinding green spices like chilies with no pre-drying and also with the retention of its original colour.








9. 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.
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.

Figure 3: Working of Cryogenic Plant



10. RESULTS OF EXPERIMENTAL STUDIES WITH PEPPER

Experimental studies were carried out on the samples under different conditions:
10.1 Volatile oil content and flavor components
The pepper contains the oil-bearing cells mainly in the skin and towards the tip of the cortex. When the pepper is ground these cells are broken and volatile oils and flavour components evaporate causing aroma. In CryoGrinding higher percentage of oils and flavor components are retained when compared with conventionally ground products. The comparison is shown below.

Sl. No. Components Cryoground (%) Conventional Ground (%)
1. Moisture 13.00 11.00
2. Volatile Oil 2.61 1.15
3. Flavour compounds (relative concentration) -Pining
Limonene 1.40
8.30 0.29
1.18


10.2 Throughput of the Mill
For a constant current of 6A, the throughput of the mill by CryoGrinding was 50 Kg/hour (at-50oC), which was 2.25 times than that of conventional grinding producing a throughput of 22 Kg/hour. This analysis clearly illustrates a throughput of 225 kg/hour (against the rated throughput of 100 kg/hour by conventional method at room temperature) could be easily be attained by CryoGrinding by suitable modification of the screw cooler and by vibratory feeder.

11. CRYOGENIC PREPERATION OF SAMPLE MATERIALS
Within the context of sample preparation, size reduction plays an important role as it has a substantial influence on the results of the subsequent analysis. If the particles are too coarse or inhomogeneous the results of the analysis may turn out to be incorrect, especially if there is only a very small amount of sample material which represents the total amount.
Brittle materials like minerals, salt or slag can be easily crushed by applying high mechanical stress through impact, pressure or friction from outside. However, what can be done when the mechanical forces alone are not able to reduce the sample material to particles that are as small as possible? One solution to this problem is provided by the use of grinding aids such as liquid nitrogen ( LN2 ;T =-196 °C) or dry ice (CO2 ;T =-78 °C) which promote the breaking behavior of such materials.






12. FOR WHICH MATERIALS IS COLD GRINDING
ADVISABLE?

12.1 Samples with elastic behavior
With many polymers (plastics such as PP, PET, PA, etc.), as well as other materials, their viscoelastic behavior during grinding only results in a plastic deformation, i.e. crack initiation and therefore break-up does not occur. If objects such as elastic plastics are immersed in liquid nitrogen then their temperature falls below the so-called glass-transition temperature; this reduces the ability of the material to resist a high mechanical stress by elastic-plastic behavior or viscous flow. If this pre-cooled material is now placed in a suitable mill there is a build-up of stress peaks in the material matrix which results in brittle breaking behavior of the sample, i.e. the sample breaks like glass.

12.2 Samples with highly volatile constituents

Because of their thermal sensitivity materials that contain highly volatile constituents such as solvents (benzene, toluene, PCB, PCP, etc.) are difficult to prepare properly and reliably for analysis. These samples can be prepared in a similar way as elastic material. The influence of the low temperature considerably reduces the generally high vapor pressure of the constituents at room temperature and the sample matrix becomes embrittled. The increase in temperature that occurs during the size reduction process has no lasting influence on the analytical results through evaporation and loss of concentration. Cooling also counteracts the increased emission of the highly volatile constituents which would otherwise be favored by the larger surface area of the sample resulting from the size reduction process.


Cell Extraction

In the medical or biotechnical sectors for the extraction of DNA sequences from single cells or groups of cells (plant, human and animal tissues) the problem must be faced that their fragments react with extreme sensitivity toheat during and, in particular, after preparation and could be destroyed.
These applications require low-temperature grinding both in order to embrittle groups of cells and cell walls for easier disruption as well as to greatly slow down the rapid decomposition of the cell fragments. The higher the ambient temperature, the quicker the reaction kinetics of the oxidation and decomposition processes; this renders the samples unusable. In all the methods mentioned above we are talking about “cold grinding” or “cryogenic grinding”.

Suitable laboratory mills for cryogenic grinding
Mixer Mill MM 301
With the Mixer Mill MM 301 2 to 20 samples can be prepared within one single operation cycle. Pulverization is effected through the impact of the grinding ball on the sample material, homogenization is achieved through the horizontal oscillations of the jars in combination with the movement of the balls inside the jars. The oscillation frequency can be set continuously between 3 and 30 Hz.

Figure 4: Mixer Mill MM 301 with grinding jars of Stee.

The Mixer Mill MM 301 is outstandingly suitable for cryogenic applications. RETSCH offers a “Kryo-Kit” as an accessory which consists of two differently-sized insulated containers, two pairs of crucible tongs and protective goggles. For grinding with the aid of liquid nitrogen, grinding jars made from stainless steel and Teflon are suitable.



Figure 5: Kryo-Kit with containers, tongs and Goggles.


12.3 Using stainless steel grinding jars.

If steel grinding jars are used they are first filled with the ball charge and sample material, the cover is screwed on and the grinding jar is then immersed in the liquid nitrogen contained in one of the insulated containers for approx. 2 – 3 minutes using the crucible tongs. When nitrogen boiling has noticeably diminished (temperature equalization), the grinding jar is removed with the help of the tongs and safely clamped in the selflocking device in figure 4.
As the preparation time only takes approx. 1 – 3 minutes, the temperature in the jar hardly increases at all as a result of frictional heat, which means that the sample cannot be damaged by “thawing”. Additional intermediate cooling or post-cooling, such as is necessary with other mills, can normally be dispensed with. The short length of time required and the easy handling represent considerable advantages, which allow an extremely cost-saving use of the expensive liquid nitrogen.

Figure 6: Cooling the jars in liquid nitrogen



Figure 7: Clamping the cooled jars into the MM 301


Figure 5 shows granulated rubber before it is ground in the MM 301. It was pulverized to a final fineness of 0.1 – 0.5 mm within a few minutes for the subsequent analysis of harmful substances. This would not have been possible without the use of cryogenic grinding aids.


Figure 8: Granulated rubber before grinding


Figure 9: Granulated rubber and after grinding with LN2


If direct contact with the cooling medium is not harmful to the sample, grinding jar and sample material can be immersed into liquid nitrogen for a few minutes. After that, the material together with the balls is cooled down to the final temperature inside the jar by immersing it into liquid nitrogen and is then ground to the required fineness. Plant materials such as leaves (Figure 7) can be prepared like this for a subsequent cell extraction within the MM 301 (Figure8).

Figure 10: Plant leaves before grinding



Figure 11: Plant leaves and after grinding with LN2


12.4 Using Teflon grinding jars

If for contamination reasons it is not possible to use steel grinding jars, then grinding jars and balls made from Teflon (PTFE) can be used. However, because of their poorer thermal conductivity when compared with steel, it is advisable to carry out a separate pre-cooling process. If the sample is allowed to come into contact with the liquid nitrogen, then the grinding jar and sample material can also be directly cooled before the grinding process. After a few moments the sample and the grinding balls can be placed in the grinding jar which is then tightly sealed and cooled down to the final temperature before starting the grinding process.

12.5 Using disposable reaction vials

Teflon (PTFE) adapters (Fig. 9) are available for preparing samples (e.g. tissue samples) in disposable reaction vials; these are also resistant to the low temperature of liquid nitrogen. The reaction vials containing the sample (and frequently also a digestion buffer solution) can be briefly pre-cooled together with the adapters in the insulated container and then clamped in the MM 301. The advantage of using disposable reaction vials is that no cross-contamination between the individual samples can occur. The heavier cell fragments are concentrated on the base of the reaction vial by centrifugation and in this way are separated from the lighter fragments in the lysate so that they can be easily extracted.


Figure 12: Adapters for disposable reaction vials


Ultra-Centrifugal Mill ZM 200

Another mill that is ideally suited for the cryogenic pulverization even of larger sample amounts is the Ultra- Centrifugal Mill ZM 200. Although with this high-speed rotor mill it is possible to pulverize some elastomers without cooling (e.g. polystyrene), in most cases pre-embrittlement with liquid nitrogen is necessary. This is done by immersing the sample in the form of granules or pre-crushed molded parts directly in the box containing LN2 und then grinding it. As a result of the temperature falling below the glass-transition temperature, the sample then “explodes” into particle sizes of 100-200 μm or less. For samples with a high fat content or thermally sensitive samples, embrittlement often results in improved grinding behavior as the frictional heat produced is considerably reduced. For powdery samples dry ice is often a more suitable grinding aid than liquid nitrogen. The dry ice is mixed with the sample at a ratio of 2:1 by volume and the whole mixture is ground in the ZM 200. As pure CO2 is involved the dry ice gradually evaporates from the sample without leaving any residue.


Figure 13: Ultra-Centrifugal Mill ZM 200









13. FUTURE PROSPECTS

As the cost of raw materials and energy is increasing day by day, it is very necessary to use optimum quantity and at the same time getting the required quality. By using CryoGrinding technology these aspects can be met efficiently. By using this we can also recycle tough and composite materials. It has many significant advantages over conventional grinding. This also leads to value addition to the product. CryoGrinding is economically viable, if liquid nitrogen costs are not formidable. By adopting CryoGrinding technology the leading spice industries of our country will earn considerable foreign exchange by exporting more value added processed spices, in place of exporting whole spices. The technique can be easily extended to processing of PVC and industrial waste plastics in view of recycling of non-biodegradable materials.

14. BIBLIOGRAPHY

• A short term QIP course on cryogenic technology (Center for continuing education IIS Bangalore)

• AICTE-ISTE Short-term programme on contribution to technology development from space research

• Mc Graw Hill Encyclopedia of science & technology: 7th Edition VOL-4

• Proceeding of the 18th International Cryogenic Engineering Conference (Edited by K.G. Narayankhedhar, Narosa Publishing House)

http://www.csa.fnal.gov Cold Facts (The quarterly magazine of the Cryogenic Society of America).
11-01-2011, 10:39 PM
Post: #5
RE: cryogenic grinding
Cryogenic Grinding

Presented by,
CYRIL GEORGE
Roll No:14
S7 ME


.ppt  Cryogenic grinding.PPT (Size: 708.5 KB / Downloads: 271)

Introduction
GRINDING PROCESS USING THE PRICIPLE OF CRYOGENICS.
USED FOR GRINDING RUBBER,PLASTICS,SPICES.
USES LIQUID NITROGEN AS CROGENIC FLUID.
MILL OPERATION WITH LIQUID NITROGEN CAUSES FEED MATERIAL TO BECOME BRITTLE.
PROVIDES DIMENSIONAL STABILITY AND INCREASED WEAR RESISTANCE.
PARTICLE CAN BE GROUND BELOW 400 MICRON PARTICLE SIZE.

Properties of Liquid Nitrogen


Molecular Weight : 28.01

Boiling Point @ 1 atm : -195.8°C, 77oK

Freezing Point @ 1 atm : -210.0°C, 63oK

Critical Temperature : -146.9°C

Critical Pressure : 33.5 atm
Properties of Liquid Nitrogen
Minimum effective temperature for some common refrigerants.
Grinding processes using liquid nitrogen


Working
Containers
Factors affect liquid nitrogen consumption in cryogenic grinding
Cryogenic grinding of Herbs
Cryogenic grinding plant for Spices
Cryogenic grinding plant for thermoplastics
Other applications
Pin Mill for Cryogenic Ultrafine Grinding
Advantages
Disadvantages
Grinding inorganic materials especially with large particle sizes is not economical.

Conclusion
Mill operation with liquid nitrogen causes the feed material to become brittle.
The specific precrushing energy is reduced meaning that the mill achieves high levels of grinding performance.
Additionally the cryogenic mode prevents grinding losses and thermal damage to the feed material that would otherwise be caused by the volatisation or overheating
of constituent ingredients.
The inert gas atmosphere provides a high degree of
safety when grinding flammable and potentially explosive products.
THANK YOU
11-01-2011, 11:15 PM
Post: #6
RE: cryogenic grinding
Introduction

In some size reducing equipments including high-speed hammer mills, air classifier mills, jet mills etc, a steady flow of ambient air is enough to dissipate the heat generated during grinding. The air flow is typically provided by the mill’s pumping action or supplemented by an auxiliary blower. The air volume required to dissipate the heat is based on the mill’s size, how much heat it generates, and the mill’s capacity for pumping the air. But for some applications, additional cooling is required. Providing the additional cooling whether to the material or the mill can be an expensive and complicated task.


.doc  cryogenic grinding report.doc (Size: 629 KB / Downloads: 99)
password:seminarprojects

Using cryogenic fluids to cool the grinding process can provide many benefits, including
1 Protective material quality,
2 Improving material characteristics,
3 Providing controllable cooling,
4 Enhancing processing,
5 Eliminating safety and environmental hazards, and
6 Minimizing equipment maintenance.


Cryogenics is the study of very low temperatures or the production of the same.
The word "cryogenics" comes from two Greek words; "kryos", which means cold
or freezing, and "genes" meaning born or produced.

Liquid nitrogen is the most widely used cryogenic grinding fluid.





Properties of Liquid Nitrogen

A. General
Liquid nitrogen is inert, colourless, odorless, non-corrosive, nonflammable, and extremely cold. Nitrogen makes up the major portion of the atmosphere (78.03% by volume, 75.5% by weight).
B. Physical Properties
1 Molecular Weight : 28.01
2 Boiling Point @ 1 atm : -195.8°C, 77oK
3 Freezing Point @ 1 atm : -210.0°C, 63oK
4 Critical Temperature : -146.9°C
5 Critical Pressure : 33.5 atm
6 Specific Gravity, Gas (air=1) @ (20°C), 1 atm : 0.967
7 Specific Gravity, Liquid (water=1) @ (20°C), 1 atm : 0.808
8 Expansion Ratio, Liquid to Gas, BP to 68°F (20°C) : 1 to 694
C. Health Effects
Although nitrogen is nontoxic and inert, it can act as a simple asphyxiant by displacing the oxygen in air to levels below that required to support life. Inhalation of nitrogen in excessive amounts can cause dizziness, nausea, vomiting, loss of consciousness, and death. Death may result from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. At low oxygen concentrations, unconsciousness and death may occur in seconds and without warning. Personnel, including rescue workers, should not enter areas where the oxygen concentration is below 19.5%, unless provided with a self-contained breathing apparatus or air-line respirator.


Minimum effective temperature for some common refrigerants.


Grinding quickly, grinding cool

GRINDING is a pretty inefficient process: up to 99% of the mechanical energy entering the mill ends up as heat. With industrial mills typically in the size range 20-100 kW, cooling is a significant issue - and for materials that deform or melt when they are warmed, temperature rise can also be a problem. Water or other liquids provide effective heat transfer, but not all materials are suitable for wet grinding. Indirect water cooling
of the mill is of limited effectiveness because of the lack of heat transfer area. Most dry mills therefore rely on a large flow of air or nitrogen to both cool and transport the product. But even with a large airflow, some particles can reach temperatures of up to
300°C, which is often high enough to cause a significant loss of quality.A clean and effective way to boost cooling is to inject liquid nitrogen at a temperature of -196°C into the product upstream of the grinding process. An example is the Cryo-grind system
offered by industrial gas supplier Air Products.




Grinding processes using liquid nitrogen

Grinding processes using liquid nitrogen fall into two types.


i. Temperature-controlled grinding

ii. True cryogenic grinding

In Temperature-controlled grinding, the liquid nitrogen is injected directly into the mill, where it acts as a heat transfer medium rather than a refrigerant. Liquid nitrogen injection rate is controlled by a sensor that measures the temperature of the air and nitrogen leaving the mill. The ground product leaves the mill at typically 10-30°C, and at no stage does its temperature fall low enough to cause embrittlement.

True cryogenic grinding is a different process designed to exploit the tendency of many materials to become brittle at low temperatures. In grinding plastics and rubbers, for example, the stresses needed to break up the material are dissipated by relaxation. As a result, plastics need 10 - 100 times as much energy to grind as inorganic materials, or typically 100 - 1000 kWh/t at room temperature for particles in the size range 100-1000 µm. Rapid impacts and low temperatures hinder the ability of polymers to relax. The effect is pronounced: for some grades of polypropylene, cooling from 20°C to -20°C increases stresses by an amount corresponding to a million-fold decrease in contact time during the impact. This is where cryogenic grinding comes in. To allow time for the material’s temperature to fall sufficiently, the liquid nitrogen is sprayed onto the feed in a
special contactor, such as a screw conveyor, well upstream of the mill. Large particles are harder to chill, so the feed is typically a granulate of around 3-6 mm size. Cooling shrinks the crystal lattice of the substance to be ground, and introduces microscopic cracks that greatly reduce the amount of energy needed to cause fracture. Usefully, the
heat capacity of the material decreases as the temperature falls, thus reducing the amount of liquid nitrogen needed to reduce the temperature further. In its glassy state the material is easier to grind, so throughput can be increased, particle size can be reduced, or both.

Wear on the mill is less, and the presence of inert nitrogen usually keeps the oxygen level below 6%, so providing useful protection against dust explosions. As well as being free from damage caused by temperature excursions, the cryogenically-ground product typically has a narrower particle size distribution than can be obtained from grinding at ambient temperature. The product generally flows more freely, and the mill is easier to clean.


Working
Raw material passing along a conveyor is cooled using controlled amounts of liquid nitrogen which allows for finer grinding and increased throughputs.


Cryo-Grind® Size Reduction Systems
For fine grinding plastics and pigments

Air Products Cryo-Grind® systems perform three essential tasks in the efficient particle size reduction of all types of plastics and pigments:

Cryogenic grinding
Grinding tough, thermoplastic materials- polypropylene, nylon etc. - at cryogenic temperatures ensures that plastics are brittle enough to be ground below 300 micron particle size: and even down to 50 micron and below with the improved new Cryo-Grind® systems.
Temperature control
Materials normally brittle enough to grind at ambient temperatures can start to degrade when the ambient temperature rises i.e. in summer, or if throughput increases are required .Cryo-Grind® systems inject controlled amounts of liquid nitrogen to regulate the heat of the grinding mill and allow higher material throughputs at any time
of the year.

Inert grinding

Organic materials (especially pigments) have an increased risk of explosion when very fine particles combine with air and the heat of the grinding mill. The use of nitrogen to make the atmosphere inert reduces the possibility of this occurrence.



Factors affect liquid nitrogen consumption in cryogenic grinding











Advantages

• Smaller particles
Plastics and pigments can be introduced into high performance materials with improved mixing and better final material quality.


• Regular particle size
Cryogenically ground materials have an arrower size differential and smoother surface area

• Efficient process
Nitrogen usage is minimized and throughput maximized by using Air Products Cryo-Grind® systems


Containers
Fig. 2 shows a typical cryogenic liquid cylinder. Cryogenic liquid cylinders are insulated, vacuum-jacketed pressure vessels. They come equipped with safety relief valves and rupture discs to protect the cylinders from pressure build-up. These containers operate at pressures up to 350 psig and have capacities between 80 and 450 liters of liquid. Product may be withdrawn as a gas by passing liquid through an internal vaporizer or as a liquid under its own vapor pressure.


So what’s best?

Not surprisingly, the best grinding technique depends on the product and the process. For easy-to-grind products such as many inorganic substances, especially with large particle sizes, cryogenic grinding may not be economic



Air Products Cryo-Grind™ systems enable rubber scrap materials to be ground into fine particles for re-use in compounding for new rubber and plastic products.
Benefits
1 Can grind smaller rubber particles down to below 200 micron
2 Regular particle size
3 Efficient process by introducing minimum nitrogen and maximum throughput
4 Improved surface morphology
Cryogenic grinding of Herbs
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 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 plant for Spices

A dosing wheel delivers the required quantity of spice, e.g. pepper, to the mill. The temperature-controlled supply of liquid nitrogen or carbondioxide ensures the material does not heat up from the heat arising during grinding. Finely pulverized, the material falls through the mill’s sieve and leaves the mill through a cellular wheel sluice .The mill gas is purified in a filter and the excess gas released by a butterfly valve. The remaining gas is returned to the mill. In this way gentle, dry and continuous grinding is possible .The controlled low temperature prevents the product from caking in the mill. The grinding process works continuously and the product retains a maximum of aroma.

Cryogenic grinding data,
e.g. for pepper

Particle size: 700 µm
Production rate: 750 kg/h
Nitrogen
consumption: 0.25 kg/kg pepper
Driving power: 35 kW

Cryogenic grinding plant for thermoplastics

A dosing wheel meters the plastic pellets , e.g. PE or PA, into the mill .The grinding heat would normally cause these thermoplastics to melt, rendering them ungrindable. The cold of the liquid nitrogen prevents this by embrittling the material in the cooling conveying screw .The cryogenic ground plastic and the gas drop into a collecting bin. The pulverized product leaves the mill for further processing through a cellular wheel sluice. The mill gas is purified in a filter and a quantity corresponding to the amount of nitrogen fed into the system released. The remaining gas is passed back to the mill for utilization of the residual cold.

Cryogenic grinding data,

e.g. for polyamide

Particle size: 80 µm (d50)
Production rate: 350 kg/h
Nitrogen
Consumption: 1.25 kg/kg polyamide
Driving power: 21 kW

Polyamide pellets Cryogenic ground polyamide


Schematic of a cryogenic grinding plant for thermoplastics


Pin Mill for Cryogenic Ultrafine Grinding



Conclusion

Mill operation with liquid nitrogen causes the feed material to become brittle. The specific precrushing energy is reduced meaning that the mill achieves high levels of grinding performance. Additionally the cryogenic mode prevents grinding losses and thermal damage to the feed material that would otherwise be caused by the volatisation or overheating of constituent ingredients. The inert gas atmosphere provides a high degree of safety when grinding flammable and potentially explosive products.

References
1 http://www.linde_gas.com
2 http://www.airproducts.com
3 http://www.safety.deas.harvard.edu

11-02-2012, 01:19 PM
Post: #7
RE: cryogenic grinding
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