GRAPHITE

Graphite is one of the three allotropic forms in which the element carbon exist in nature, the other two being diamond and coal. It is a good conductor of electricity and heat. It has a high refractory and can with stand temperature up to 3000 C in an insert atmosphere. In the presence of oxygen it burns between 6200 C and 7200 C. it remains unaffected by most of the acids and reagents but yields graphitic acid on treatment with a mixture of potassium nitrate and nitric acid. Another valuable property of graphite is that it leaves a black smear on the hands when touched because of its extreme softness and greasiness. This property is found best in cryptocrystalline variety. It is, therefore, used in the manufacture of the lead of the pencils.

Crude graphite seldom finds direct application in industry. It is invariably beneficiated to produce the required quality. Its principal industrial uses are in graphite crucible, foundry, brake lining, brushes, lubricant batteries, paints, refractory and pencils. It is being used to increase the carbon content of steel and as mag-carbon bricks in steel making.

The specifications for various industries are discussed below: -

1. Graphite Crucibles: -

BIS (IS: 11321 – 1985) has prescribed specifications for graphite for graphite crucibles, the chemical requirements are prescribed as fellows:

S. No.

Characteristics

Requirement % by mass

i

Moisture

1.0

ii

Analysis on dry basis

a)Volatile matter max.

b) Ash

Fe2O3 Max.

TiO2 Max.

CaO Max.

SiO2 + Al2O3 Max.

Alkalies, Max.

c)Fixed carbon min.

 

1.5

 

1.2

0.5

0.02

Balance

0.2

85.0

iii

Sulphur – max.            

0.25

Physical Requirements: - All crystalline graphite shall be free from oil, uniform in quality, free from visible impurities, such as mica, pyrites and free from other contamination’s. The fineness of graphite shall confirm to the requirement given below:

Sieve No. - 1.20mm
+ 180 m
- 300m - 1.20mm
+ 125m
- 1.20mm
300m

Present

100

15

75

95

Flakiness: - The flakiness index shall not be less than 125 and oxidation temperature shall be approximately 7500 C.

Silicone Carbide Crucibles: - The traditional clay crucibles are progressively being substituted by carbon bonded silicon carbide crucibles. This change over is reported to have occurred together with introduction of isostatic pressing technology where small flakes could be used. This has helped the use of medium sized graphite flakes.

2. Foundry:

The BIS (IS: 1305 – 1984) has prescribed specifications for graphite for use in foundry coating. The physical requirements of natural graphite shall be as under:

S.No. Characteristics
Requirement % by mass
Grade-I   Grade-II  Grade-III

i

Moisture

1.0

1.0

1.0

ii

Analysis on dry basis

a) Volatile matter max.

b) Ash, - max.

c) Fixed carbon

 

3.0

12

by different

 

3.0

20

by different

 

3.0

30

by different

iii

Non-graphitic carbon - mad.

0.5

2.5

2.5

The requirements of synthetic for use as foundry-coating materials shall be as under:

S. No.

Characteristics

Requirement % by mass

i

Moisture

1.0

ii

Analysis on dry basis

a)Volatile matter max.

b) Ash

c)Fixed carbon min.

 

2.0

3.0

by different

iii

Non-graphitic carbon - mad.    

3.5

The fineness shall be as given below:

Sieve No.

150m

75m

Present

100

75

3. Brake lining:

Most of the manufactures use a blend of nature and synthetic graphite in the ration of 3:2. The leading manufacturers require a minimum Ca bon content of 98%. At least 95% of the particles should pass through 200 BSS mesh (76 micron). Graphite having carbon content of 90% could be used provided the level of the abrasive impurities is low.

 4.Brushes:

Natural and artificial graphite in different combination are used along with other raw materials in the manufacture of brushes the carbon content in graphite should be 95% is preferred. Bulk of the graphite should pass 300 mesh (53 microns).

 5.Lubricants:

BIS specifications (IS: 495 – 1967) prescribed the requirement and method of sampling and test for flaky graphite intended chiefly for use in the manufactures of lubricants. The materials should be good quality graphite, dry, steel grey in color, having metallic lusture and smooth and soapy feel. It should be free from grit, grease and other impurities. It should wholly pass through 700 microns. IS sieve and not more than 50% by weight of the materials shall be retained on 125 microns IS: Sieve. The detailed requirement, which the material should comply with are mentioned below:

Requirement of graphite flake for lubricant:

S. No.

Characteristic

Requirement

1.

Pass on heating percent by wt. max.

1.25

2.

Petroleum either soluble matter percent by wt. max.

0.50

3.

Water soluble matter present by wt. max.

1.25

4.

Acidity of water extract            

To specify the requirement of the test.

5.

Ash, percent by wt. max.

5.00

6.

Non-graphite carbon   

3.00

7.

Freedom for abrasive matter present by wt. max.

To pass the test.

The BIS standard does not specify the carbon content of the graphite flake. However, carbon content varying from 96 to 99% is usually considered suitable for the purpose.

6.Batteries:

It is reported that cell performance is influenced by carbon content, structure, crystallographic orientation and size of the particles. Generally speaking the Indian consumers are using graphite having 90% carbon content 8 – 13% ash, less than 0.5% iron and H2O less than 3% and sedimentation volume 4cc.

7.Paints:

The specifications for graphite for use in the paint industry as given in the BIS specification IS: 62 – 1950 as given below:

Requirement of graphite for paints:

S. No. Characteristic
Requirement
1. Volatile matter
Note more than 2%
2. Oil absorption  
With in 5% of the approved sample.
3. Color
Close math to approved samples.
4. a)Staining power
b)Tone
Not inferior to the approved samples. Equal to the approved samples.
5. Matter soluble in H2O
Not mare than 1%
6. Ash
Not mare than 40%
7. Marking & Delivery
As agreed with the purchaser subject to the special requirement for the lead free pigment when specified.

 According to the IS specifications only flake graphite is prescribed for the paint industry. However, the amorphous graphite generally containing between 50 – 55% carbon, which also contains 20 – 25% silica, is considered of the suitable for paints. Silica enhances the distribution of the graphite particles throughout the paint, whereas impurities like, pyrite and mica tend to affect the longevity of the paint.

In the manufacture of conductive paint for electrodes graphite of the type used in the dry cell battery manufacture is required.

8. Refractory:

The refractory manufactures in India require graphite having fixed carbon 80 to 95% ash 4:3 to 20%. The graphite should be flaky of size 1m. The following 4 grades have been specify by the sub-committee on refractory raw materials appointed by the DGTD in their report dated August 1985.

 

Grade-I

Grade-II

Grade-III

Grade-IV

Fixed carbon

95% min.

90% min.

85% min.

80% min.

Fusion point of ash

15500 C

15500 C

14000 C

--

Physical specifications

the mineral should be flaky, the size range being –40 to +60 mesh.

9. Pencils:  

The BIS specification (IS: 2079 – 1982) is for pencil slips. It prescribed the requirements for graphite of two grades intended for use in the manufacture of pencil slips. Depending on the fineness and ash content, graphite has been classified in to two grades. For grade-I graphite should have ash content of not more than 5% by mass. The particle size should be such that minimum 90% of the material pass through sieve of apparatus size 53 micron. Grade-II should have as content of about 5% bot not more than 10% by mass.

The particles size should be such that minimum 90% of the material pass through sieve aperture size 53 microns. The pencils are manufactures from crystalline flaky graphite. The details regarding requirements have been mentioned in BIS specifications (IS: 2079 – 1983).

The general requirements of the materials are that it shall be of good quality, dry, soft, dark, powder with smooth feel. It should be free from grit, grease and visible impurities. The specific requirement is as follows:

1.Loss in mass on heating shall not exceed 3%
2.Petroleum either soluble shall not exceed 0.50% by mass.

3.Mater soluble matter shall not exceed 1.25% by mass.
4.Ash content of the material for Grade-I shall not be more than 5% by mass, and for Grade-II .above 5% but not more than 10% by mass.
5.Ash should not fuse below 11000 C.
6.The non-graphitic carbon content of the material shall not exceed 3% by mass.
7.The blackness of smear shall not be more than 30%

It has been indicated by some consumers of graphite the in pencil industry graphite having fixed carbon content of 96 – 97%, Ash 3 – 4%, SiO2 0.95%, Al2O3 0.75%, CaO 0.22%, and MgO 0.05% is required. About 90% of the material should pass through – 325 mesh.

Future Trends:
The use of graphite in the manufactures of refractories like magnesite – carbon bricks and graphitized alumina products is comparatively a new application of graphite.

As graphite gives the brick, a high thermal conductivity the magnesite carbon bricks are used in conjugation with water-cooled panels in electric-arc furnaces. The advancement in the technology has brought a change from using predominantly large flakes with 85% carbon content to using a variety of blended graphitic of smaller flakes with a carbon content of 87 to 90%. The desirable ash content is 2% through graphite with ash content up to 10% is also used. Impurities such as silica, alumina and iron are detrimental in mag-carbon refractory manufactures as these promote unfavorable reactions with magnesia by the formation of compounds of lower melting point, thus decreasing the life of bricks.

Graphitized alumina refractories are used in the process of continuous casting of crude steel, and secondary refining. They are essentially used to control and protect the metal flowing from the ladle to the water-cooled mould. Graphite, which imparts two most important properties to alumina refractories, is a combination of corrosion resistance and thermal shock resistance. Graphite with a minium carbon content of 80% is used. As the flake size is more sign significant than the purity in this case, the graphite should be of the BSS mesh size range of –30 to +100 (500 microns to 152 microns).

10. Synthetic Graphite: -

Synthetic graphite is being produced in India. The total installed capacity is 31,300 tonnes. Production was reported as 20.026 tonnes and 24,210 tonnes in 1983 and 1984 respectively. During 1985 production rose to 28,000 tonnes (estimated) of required at the level during 1986 as well. Synthetic graphite industry in India is heavily dependent upon petroleum coke. It is manufactured by three companies, namely Graphite India Ltd., carbon Corporation Ltd., and Hindustan Electro Graphite Ltd., Synthetic graphite in blend with natural graphite is used in some industries, but electrode industry is its dominant consumer and uses synthetic graphite exclusively.

Practically the entire ROM is being beneficiated before use in different industries. In the inventory as on 01.01.1985 reserves classification has been made as given below:

1.

+ 40% F. C. This is the lowest grade that has been reported to be used directly for foundry facing.

2.

10 – 40% F. C. Techno economic feasibility for beneficiation has been established and this grade is actually being accepted as feed for beneficiation plants in India.

3.

-10% F. C. (Conditional resources)

Only technology for beneficiation is available, but economic feasibility is yet at be established in India.

The group discussed the trends in graphite production a an use and came to the conclusion that the classification of reserves as adopted in the mineral inventory as on 01.01.1985 should be retained. The same is reproduced below:

1.

+ 40% F. C. This is the lowest grade that has been reported to have been used directly for foundry facing.

2.

10 – 40% F. C. Techno economic feasibility for beneficiation has been established and this grade is actually being accepted as feed for beneficiation plants in India.

3.

-10% F. C.
(Conditional resources)
Only technology for beneficiation is available, but economic feasibility is yet at be established in India, although some plants are using ROM graphite as low as 6% fired carbon.
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