Cryogenic Grinding

A

Seminar report on

“CRYOGENIC GRINDING”

Submitted by

Manoj Kumar Naik

0901294165

“Mechanical Engineering”

2009-13

Supervised by

“Prof. K.Mahapatra”

(Dep’t. Of Mechanical Engineering)

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ACKNOWLEDGEMENTBehind every student who ascends the height of success and

achievement has a group effort and it is reflected in this

project. We cannot undermine the role and responsibility of

the people who were instrumental in extending all possible

support for preparation of this project report.

We express our deep sense of gratitude and appreciation to

Head of Mechanical department Prof. K.Mahapatra and our

guide Krutibash Khuntia for their constant valuable guidance

and help in implementing our project report.

We further take this opportunity to thank all the staff

members of our college for taking active participation and

providing us all the necessary data and statistics during the

preparation of our report so as to make it a great success.

Manoj Kumar Naik

0901294165

Mech. Engg.

CERTIFICATEPage | 2

This is to certify that the project report CRYOGENIC GRINDING

entitled is being submitted by Manoj Kumar Naik in partial

fulfillment of Degree of Bachelor in Technology in Mechanical

Engineering to the Biju Patnaik University of Technology, is a

record of bonafied work carried out by them under my

guidance and supervision. The results embodied in this project

report have not been submitted to any other University or

Institute for the award of any degree.

Guide Prof. K.Mahapatra

Krutibash Khuntia HOD (Mech. Engg.)

Dept. Of Mech. Engg.

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ABSTRACT

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 materials at sub-zero temperatures ranging from 0 to minus 70°F. The particles are frozen with liquid nitrogen as they are being ground. This process does not damage or alter the chemical composition of the materials. Normal grinding processes which do not use a cooling system can reach up to 200°F.

CONTAINS

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1. CRYOGENIC GRINDING PROCESS

2. CRYOGENIC GRINDING PROCESS

3.METHODS AND APPARATUS FOR CRYOGENIS

GRINDING

4. Cryogenic Screw Feed Conveyor

5. CRYOGENIC PREPARATION OF SAMPLE MATERIALS

6. TRADITIONAL/ CRYOGENIC GRINDING

7. BENEFITS

8. APPLICATION OF CRYOGENIC GRINDING

9. Conclusion

10. BIBLIOGRAPHY

INTRODUCTION

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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 materials

at sub-zero temperatures ranging from 0 to minus 70°F.

The materials are frozen with liquid N2 as they are being

ground. This process does not damage or alter the

chemical composition of the material in any way. Normal

grinding processes which do not use a cooling system

can reach up to 200°F. These high temperatures can

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reduce volatile components and heat-sensitive

constituents in materials. The cryogenic grinding

process starts with air-dried material, rather than freeze-

dried materials.

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 N2, these may be embrittled

and easily fractured into small particles.

CRYOGENIC GRINDING PROCESS

The material is feed into a feeder hopper and

dropped into a conveyor when the material to be

processed enters the pre-chilled conveyor; liquid N2

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is sprayed and blended directly onto the material. The

material is conveyed via a stainless steel special

design auger. The auger not only transports the

grinding media, but also mixes with liquid N2 for greater

cooling efficiencies.

The liquid N2, a cryogenic fluid with a boiling

temperature of –196 0C absorbs heat from the

material and vaporized to a gaseous state. The N2

gas exits the system conveying the process heat away

from the process

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Liquid N2 is added until the temperature of the

material is reduced to a predetermined set point.

This set point is the glass transition temperature of

the material finally the brittle material enters an

impact (pin) mill where it is ground to a desired

particle size. Computer controls the entire process of

cryogenic grinding system. Since almost all materials

embrittle when exposed to cold temperatures,

cryogenic size reduction utilizes the cold energy

available from liquid N2 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 N2 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 N2 is at a temperature of -

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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. N2 is a non-flammable, non toxic and

inert gas which makes up 78.09% of the air we breathe.

Raw material passing along a conveyor is cooled using

controlled amounts of liquid N2 which allows for finer

grinding and increased throughputs.

METHODS AND APPARATUS FOR CRYOGENIS

GRINDING :

The production rate of a conventional cryogenic

grinding system incorporating an impact mill may be

increased by (a) providing means to allow at least 70%

of the embrittled material entering the mill to leave the

mill before it passes the inlet; and (b) providing means

to restrict the flow of the cold gas through the impact

mill.

The product leaving the impact mill is screened and any

oversize is preferably recycled to the impact mill.

1. A method of cryogenically grinding pieces of material,

with substantially less consumption of cryogenic

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refrigerant than heretofore required, comprising the

steps of:

(a) Pre- cooling the pieces of material to be ground in

counter-current heat exchange with a cold gaseous

cryogenic refrigerant,

(b) further cooling and embrittling the pre-cooled pieces

of material in direct contact with a liquid cryogenic

refrigerant, and thereby vaporizing said liquid cryogenic

refrigerant and generating said cold gaseous cryogenic

refrigerant,

(c) Continuing said cooling and embrittling step for a

sufficient contact time to thoroughly embrittle said

pieces throughout,

(d) Introducing said thoroughly embrittled pieces of

material through an inlet of a rotary impact mill,

(e) Passing said thoroughly embrittled pieces of material

through said mill in an arcuate path comprising more

than 270° of travel through said mill,

(f) discharging on the first pass through said mill at

least 70% of the entering embrittled material through a Page | 11

no screened discharge opening subtending an angle of

between 5° and 60°, and

(g) Restricting the flow of said cold refrigerant gas

which is discharged from said mill to a sufficient degree

to cause the majority of said cold gaseous refrigerant

which is generated to flow in counter-current heat

exchange with said pieces of material according to

step(a).

2. The method according to claim 1, including the step

of removing at least 85% of the embrittled material

entering said mill on the first pass there through.

3. The method according to claim 1, including the step

of removing at least 95% of the embrittled material

entering said mill on the first pass there through.

4. The method according to claim 1, including the step

of separating oversize pieces discharged from said mill,

and recycling said oversize pieces to step (a).

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4.2- Apparatus for cryogenically grinding

material comprising:

(a) a pre-cooling hopper,

(b) mixer-conveyor means connected to said hopper

and including means for injecting liquid cryogenic

refrigerant into direct contact with material therein for

embrittling said material and generating cold gaseous

refrigerant,

(c) a rotary mill having an inlet connected to receive

embrittled material from said mixer-conveyor means,

and including non-screened outlet means located more

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than 270° degree apart from said mill inlet in the

direction of rotation of the mill,

(d) said non-screened outlet means subtending an

angle of between 5° and 60° for discharging at least

70% of the material on the first pass through said mill,

and

(e) means for receiving ground material from said mill

outlet means and including gas flow restricting means

for reducing the flow of gas discharged from said mill to

a minor portion of the cold gas generated in the mixer-

conveyor and thereby forcing the major portion of the

cold gas in counter-current heat exchange with the

material in said mixer-conveyor means and in said feed

hopper.

6. The apparatus as claimed in claim 5 in which said

non-screened outlet means subtend an angle of

between 20° and 50°.

7. The apparatus as claimed in claim 5 in which said

non-screened outlet means is positioned adjacent said

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mill inlet.

8. The apparatus as claimed in claim 5 in which a

multiple deflector lining extends around the entire

arcuate path between said mill inlet and said mill

outlet means.

9. The apparatus as claimed 5 in which said gas flow

restricting means comprise gas flow control valve

means having variable positions for varying the

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amount of cold gas flowing through and discharged

from said mill v/s that flowing in counter-current heat

exchange with said pieces of material being cooled

thereby.

According to one aspect of the present invention,

there is provided a method of cryogenically grinding

material, which method comprises the steps of

advancing the material to be ground towards an

impact mill, embrittling said material by bringing said

material into direct contact with liquid N2 and a

stream of cold vaporized N2 the major portion of which

is travelling away from said impact mill in

countercurrent flow with said material; introducing

said embrittled material through the inlet of said

impact mill; removing at least 70% of the embrittled

material entering said mill before it passes said inlet;

and providing means to restrict the flow of gaseous N2

from said mill so that only a minor portion of said N2

passes through said impact mill.

The present invention also provides apparatus for

cryogenically grinding material which apparatus

comprises means for cryogenically embrittling. means

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for restricting the flow of cold gas leaving said impact

mill; and means for screening the material leaving said

mill.

Preferably the apparatus also includes a conveyor

for returning the material which will not pass through

said screen to the means for cryogenically embrittling

said material.

Advantageously, said impact mill consists of a

hammer mill although the invention is applicable to

other mills in which grinding is achieved by impacting

the material to be ground against a moving member,

for example, a rotary beater mill, a fan mill, a turbo

mill and a pin disc mill.

Cryogenic Screw Feed Conveyor

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The cryogenic feeder is a specially constructed

stainless steel heat exchanger designed to optimize

heat transfer, using liquid N2 as a refrigerant. The

feeder is suitable for pre-cooling a wide variety of

materials, including plastic, rubber, food and drugs to

their embrittlement temperature for cryogenic

grinding operations. Heavy-Duty components and

simplicity of construction assure a durable and reliable

cryogenic cooling system.

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The cooling conveyor operates with high thermal

efficiency resulting in lower N2 consumption. The

material is cooled to it's embrittlement temperature

traveling through the cooling conveyor. When the

material is passed through the pulverizer it is reduced

to the required size. Some products can be reduced to

as low as -325 mesh.

The material to be ground is loaded into a feed

hopper. From the hopper, the material enters the

cooling conveyor where liquid N2 at -320 F is sprayed

directly onto the material. The liquid N2 vaporizes to a

gas by absorbing the required heat of vaporization

from the feed material. Seeking thermal equilibrium,

the cold N2 gas continues to cool the feed material to

its embrittlement point.

A thermocouple and temperature controller is used

to control and monitor the temperature within the

cooling conveyor and the grinding mill discharge

hopper. The material enters the Pulva - Sizer Grinding

Machine where the material is ground into a powder.

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CRYOGENIC PREPARATION 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 N2 (LN2 ;T =-196 °C) or dry

ice (CO2 ;T =-78 °C) which promote the breaking

behavior of such materials.

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For which materials is cryogenic

grinding advisable?

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 N2 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

precooled 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.

The cryogenic process produces fairly smooth

fracture surfaces. Little or no heat is generated in the

process. This results in less degradation of the rubber.

In addition, the most significant feature of the process

is that almost all fiber or steel is liberated from the Page | 21

rubber resulting in a high yield of usable product and

little loss of rubber. The price of liquid N2 has come

down significantly recently and cryogenically ground

rubber can compete on a large scale with ambient

ground products.

Cryogenic grinding: an

independent voice

CRYOGENIC grinding is a proven technology that is

extremely effective, especially for plastics and

rubbers, according to Frank Burmester, an engineer

with industrial gas supplier Air Products in Germany.

But as he pointed out: “Unfortunately, all too often

it’s put in a box marked ‘expensive’”.

Probing the fundamental:

Conventional cryogenic grinding uses liquid N2 to

cool the material to be ground, making it brittle and so

reducing the amount of energy required by the mill. It

also increases throughput often by 100% or more for

the same particle size distribution or enables finer

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grinding at the same throughput, and it tends to

narrow the product particle size distribution.

Liquid N2 consumption varies from around 0.7 kg

for every kg of material ground, for low-value products

such as recycled rubber, right up to 2 0 kg/kg for

difficult, high-value products that cannot be ground by

any other method.

One reason liquid N2 consumption figures are so

hard to pin down, is that different users get different

performance from similar equipment: “One plant may

use 1 kg of liquid N2 per kg of product, while another

may use three or five times as much for the same

product in an identical mill. “The hard part has been

not in the software but in deciding what to measure

and what weighting factors to use. A conventional

cryogenic grinding process measures the temperature

at a single point, at the bottom of the mill.

Grinding quickly, grinding cool:Page | 23

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

N2 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 N2 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.

How low can we go?Page | 24

The liquid N2 is injected directly into the mill,

where it acts as a heat transfer medium rather than a

refrigerant. Liquid N2 injection rate is controlled by a

sensor that measures the temperature of the air and

N2 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.

To allow time for the material’s temperature to fall

sufficiently, the liquid N2 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 N2 needed to reduce the temperature

further.

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TRADITIONAL/ CRYOGENIC

GRINDING

Disadvantages of

Existing

Grinding System

Advantages of

cryogenic

Grinding System

The heat is developed

inside the grinding mill

Temperature below 0

0C inside the grinding

mill

High energy consumptionLow energy

consumption

Existing grinding

equipments more

than two times recycle

into

the mill for required

particle size.

Approx. 2 - 3 times

higher grinding capacity

Fire Risk No Fire Risk

High capacity motors

are required to grind the

Low capacity

motors are required to Page | 26

material grind the material

Air pollution due to

evaporating essential

oil into the atmosphere

evaporation of

essential oil into the

atmosphere

No control on particle sizeParticle size under

control

BENEFITS:

Smaller particles

More uniform particle size distribution 

Efficient process

Process cooling/temperature control

Trial Facilities

Can grind smaller rubber particles below 200

micron

Improved surface morphology

Low capacity motors required

Reduced power consumption

Approx. 2 - 3 times higher grinding capacity

APPLICATION OF CRYOGENIC GRINDING:

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Grinding of heat sensitive or friable materials

Rotational molding materials

Textile coatings

Adhesive coatings

Many twin screw extruder applications

Powder coatings

Electrostatic coatings

Additives for compounding

Specialty molding applications requiring smooth

finishes

Conclusion:

From the above report we concluded that

“Cryogenic grinding” technology can efficiently

grind most tough materials and can also

facilitate cryogenic recycling. The specific

precrushing energy is reduced meaning that the

mill achieves high levels of grinding

performance. Also, prevents grinding losses and

thermal damage to the feed material t hat would

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otherwise be caused by the volatisation or

overheating of constituent ingredients.

BIBLIOGRAPHY

I have prepared seminar report on

cryogenic grinding by the help of

followings books:

Production Engineering

Manufacturing Technology

Also taking data from from following

internet sites:

www.google.com

www.wikipedia.com

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