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Contents
 
Manufacturing and Production of Graphite
 
1. Mining of Graphite
2. Modern Manufacture
 
 

1. Mining of Graphite

 

Natural graphite is not very plentiful. Carbon forms strong bonds with oxygen in carbon monoxide and especially carbon dioxide i.e. it oxidises easily. Nevertheless natural deposits do exist, and artificial graphite can be made in large quantities. Some time before 1565 an enormous deposit of graphite was discovered at the site of Seathwaite Fell near Borrowdale, Cumbria, England, which the locals found very useful for marking sheep. This particular deposit of graphite was extremely pure and solid, and could easily be sawn into sticks.

 

World production of natural graphite in 2006 is estimated at 1.03 million tonnes and in 2005 1.04 million tonnes, of which the following major exporters produced: China produced 720,000 tonnes in both 2006 and 2005, Brazil 75,600 tonnes in 2006 and 75,515 tonnes in 2005, Canada 28,000 tonnes in both years, and Mexico 12,500 tonnes in 2006 and 12,357 tonnes in 2005.

 

According to the USGS, U.S. (synthetic) graphite production in 2006 was 132,000 tonnes valued at $495 million and in 2005 was 146,000 tonnes valued at $391 million, and high-modulus graphite (carbon) fiber production in 2006 was 8,160 tonnes valued at $172 million and in 2005 was 7,020 tonnes valued at $134 million.

 

 

Fig 41. Graphite output in 2005

 

2. Modern Manufacture

 

Modern graphite manufacture commences with a high molecular weight hydrocarbon, often natural pitch or a residue of crude oil distillation, which is first converted to coke by heating in the absence of air. This is long and complex process, usually taking several weeks to perform. The result of the process is that the carbon atoms order themselves in extensive hexagonal clumps and create a Good Coke.

 

The coke is the calcinated, crushed and sieved to get a specific distribution of particle sizes. Next these particles are bound together using hot pitch, and the mixture extruded or moulded to form rough blocks of the shape eventually desired.

 

The binder is then itself coked by baking, and an extensive pore network is created. The baked article is therefore reimpregnated and rebaked until an adequate density is reached. Finally the material is converted to graphite at around 3000°C by passing a current through a conducting coke bed surrounding the blocks. At 1600°C the localised regions of order begin to link up and graphitic properties begin to appear. Beyond 2400°C individual crystallines grow to some extent, but large scale re-orientation does not occur. Most impurities are volatile and so disappear at graphitisation temperatures.

 

As the newly made graphite cools the individual crystallines contract anisotropically creating large stresses in the overall matrix. As a result a network of fine cracks (known as Mrozoski cracks) is created. Together with the remaining volatilisation pores, the graphite is networked with pores, occupying about 20% of the overall volume, and as the pores are interconnected they provide access to the external atmosphere. The porous structure plays an important role in the behaviour of graphite under irradiation.

 

The brick manufacturing processis illustrated below:

The auxiliary Polygonal Brick line
Machining the bore of a Polygonal Brick
1. A view along the auxiliary Polygonal Brick Line.

2. Machining the bore of a Polygonal Brick. This is one of the most critical operations as the bore of the brick is then used as a Datum for all subsequent operations.

 

Machining the flats
Final inspection of a Polygonal Brick

3. Machining the flats on the sides of the Polygonal Bricks on a Duplex machine.

4. Final inspection pf a Polygonal Brick. Great emphasis is placed on the quality of the product in this shop.

A minimum of 100% inspection of all dimensions on all bricks is carried out.

 

This machine is putting the Keyways into the faces of a Polygonal Brick
Lowering a brick onto the Inspection Table

5. This machine is putting the Keyways into the faces of a Polygonal Brick. This operation is extremely critical as the accuracy with which it is carried out determines the correct build up of the core.

6. Lowering a brick onto the Inspection Table. Note the special handling equipment.

 

 

 

 

 

Although several varieties of graphite exist, only two types will be discussed here. Gilsocarbon graphite offers the most reliable choice in graphite: it is derived from coke of the USA origin and has a well proven high degree of dimensional stability under irradiation together with a high degree of isotropy. It is available in a moulded form in a variety of grades dependent on the number of impregnations carried out in the production process. Although also available as an extruded product, extensive testing has shown that no significant difference has been found between extruded and moulded versions, apart from the obvious differences in grain direction. All gilsocarbon grades are available in either a purified or un-purified condition, and significant raw material cost differences apply depending on the degree of purification and impregnation.

 

Pitch coke graphite offers a more economical alternative range of materials for graphite bricks. It tends to be slightly cheaper than the equivalent purified grades of gilsocarbon graphite material but it is inferior in strength, less isotropic and has worse dimensional stability under irradiation. Production experience of pitch coke graphite materials in available in the UK, but only in relative small sizes.

 

Gilsocarbonite graphite is used in the modern fleet of AGR cores.