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(From Calcining, Crushing, Sieving, Grphitization, Maching, Inspection-shipping)
Graphite production & further processing
• Introduction
• Graphitising
• Graphitisation furnaces
• Coating
• Costner
• Inductively heated furnaces
• Purification
• Machining
• High grades surface finishing
• Coating
• Chemical vapor deposition
Some applications, such as graphite electrodes for the electric arc furnace require a higher thermal and electrical conductivity than that of baked carbon materials. These synthetic graphites normally follow a production process similar to that of baked carbon (forming, impregnation, rebuking) but require an additional process step, that of graphitisation where temperatures of around 3000°C are achieved.
Heat treatment Graphitising
The final step in graphite manufacture is a conversion of baked carbon to graphite, called graphitising, i.e. heat-treating the material at temperatures in the region of 2600°C - 3300°C. During the graphitising process, the more or less pre-ordered carbon (turbostratic carbon) is converted into a three-dimensionally ordered graphite structure. Depending on the raw materials and the processing parameters, various degrees of convergence to the ideal structure of a graphite single crystal are achieved. Since graphitisation increases the lattice order and produces smaller layer distances, it simultaneously leads to a considerable growth of ordered domains. However, the degree of order that can be reached depends largely on the crystalline pre-order of the solid used. These reduced lattice layer distances are macroscopically noted as a contraction in volume. This graphitisation-shrinkage is approximately 3 to 5%. Due to this shrinkage, density of the graphite increases.
The Graphitisation Process
The final step in the production o f graphite is the graphitization process. • Amorphous or baked carbon is converted to electro graphite by a thermal treatment at approximately 3 000°C. • Essentially any amorphous carbon material can be graphitised. The potential crystallite growth and ordering are latent within the baked carbon structure. • Under the influence o f temperature the crystallites grow and rearrange in an ordered pattern of stacked parallel planes. This transformation is accompanied by a change in the physical properties o f the material (See change during graphitisation ). • T he greater the degree of crystallite growth during heating up, the better the graphitability (gaphitization degree), which effect s the final resistivity achieved. • There is a variation among different need le cokes concerning the graphitability. • The graphitisation degree depends on the structure o f the basic material (graphitability) and the applied graphitisation temperature. It is determined by x-ray measurements.
Changes during graphitisation
1. Crystal development
2. Material gets softer and machine able
3. Impurities vaporize
4. Becomes a better electrical and thermal conductor
5. Physical property changes
Heat treatment Graphitization
Acheson furnace the stock is arranged in blocks within a horizontal bed, usually perpendicular to the axis of the furnace (click here for the picture of the Acheson furnace). The space between these segments is filled with a resistor material consisting of a coke/graphite granular mixture. The current is supplied to the load by two water-cooled head electrodes at the narrow sides of the furnace, which is thermally insulated by a mixture of coke, sand, carbon black and/or sawdust, thus protecting the material against oxidation. As the electrical resistance of the furnace decreases with an increasing degree of graphitisation, the power to the head electrodes is controlled and adjusted by transformers. Nowadays, nearly all graphitising furnaces are DC-operated since powerful rectifiers are available. For fine-grained material, an uncontrolled release of decomposition products from the charge does not cause problems, so that the normal maximum temperature of 2800°C can be reached within a few days. Depending on the size of the furnace and its operation mode, one working cycle including cooling lasts two to three weeks.
Heat treatment Graphitisation
Graphitisation Castner furnace/longitudinal graphitisation Lengthwise graphitisation (longitudinal array) was first described by Castner, and is characterised by the direct connection of the stock one to another in a row without a resistor material in between (click here for the picture of the Castner furnace). The artifacts are clamped between the head electrodes of the furnace and heated by passing the current directly through the load. The contact area of the material to be graphitised has to fit well and therefore requires plan e-parallel machining and an adjustable clamping device in order t o secure a constant, tight electrical contact for a low-contact resistance. The insulation against oxidation and high heat losses is the same as for Acheson furnaces. This graphitising method has the advantage of shorter heating periods, less power consumption and smaller furnace dimensions. Comparison with Archeson furnace.
Production inductively heated
Inductively heated furnaces inductively heated furnaces are normally used for parts, which need to be graphitised with a very well controlled temperature profile. Indirectly heated furnaces consist of a horizontal or vertical graphite tube of rectangular or cylindrical cross-section. The outside of these tubes is insulate d with carbon black surrounded by a carbon felt jacket. The material is passed through the furnace, either continuously or at intervals. It is protected from oxidation by using water-cooled seals at the ends of the furnace and by applying a stream of inert gas. The main advantage of such furnaces is very close temperature control.
Production Purification of Graphite
The graphitising process is also accompanied by a purification of the material treated, normally reducing the content of impurities to considerably less than 1000 ppm . For many applications, this purity is insufficient, so that a thermal purification at higher temperatures up to 3100°C with longer residence times is carried out to reduce the impurities to a concentration of less than 200 ppm . If still low each value is required, at herm o -chemical purification is necessary.
Production Purification
Thermo-chemical purification even after graphitisation at around 3000°C most graphite contain small amounts of metallic impurities. If the ash values in the material have to be below 200ppm, thermal purification is applied. By adding gaseous halogens or halogen compounds, all hetero-atoms forming stable carbides are transferred into volatile halogen compounds and thus removed. By means of this procedure, impurities may be lowered to less than 1ppm.
Fabrication Machining
Machining graphite can be machined wet or dry with ordinary machine tools but is normally machined dry. The different steps are dust extraction, cutting, turning, grinding, bonding and polishing .
Fabrication - dust
Extraction Individual extraction arrangements are essential on all machines when dry machining is carried out. Centralised extraction equipment is not worth installing except in large graphite machine shops. Where workshops are predominantly engaged in metal machining it is enough to install industrial vacuum cleaners on individual machines (vacuum 300mm head of water or air velocity of about 18 m/sec). When machining carbon and graphite it is important to prevent ingress of dust into electrical motors and control panels.
Fabrication Cutting
Cutting of graphite does not necessarily require specialist tools. However optimum results may be achieved using diamond cutting wheels.
Fabrication Drilling
Drilling For drilling graphite, ha rd metal drills are used. To avoid chipping at t h e drilling exit the point angle should be 70-100° and the clearance angle 10-15°.
Fabrication Turning
Turning Specialist machine tools for the turning of graphite are not absolutely necessary and a wide range of machines are in use, from wood turning lathes to CNC lathes. The machines normally have to be adapted t o accommodate graphite machining, in particular protection of electrical equipment from dust.
Fabrication Roughening
Roughening The surface of the graphite is mad e rough by silicon carbide wheels with a grain size of 20-46 µm and hardness F-K depending, on the hardness of the carbon material.
Fabrication Milling
Milling Specialised milling machines is not required but optimum results may b e obtained using hard metal or diamond tools.
Fabrication Finishing
Super finishing Depending on the hardness of the graphite material electro-corundum wheels with grain size 120-160 µm and hardness P-Z are used.