Introduction:

As carbon does not melt under normal pressure, carbon bodies can be manufactured the same way as ceramics, from mouldable mixes of carbonaceous solids and binding agents by the subsequent carbonization of the shaped articles. During the carbonizing process, the binder is converted into a solid coke residue, while the volatile matter (including degradation products) is evaporated. The conductivity of such material is considerably lower than that of graphitized grades, but is sufficient for many applications. The largest market for this type of carbon product is for anodes for aluminium production. There is a large variety of other applications.

Milling

As most solid raw materials do not have a grain size suited to direct processing, they have to be broken down to the desired particle size, sometimes in several steps using different equipment. Crushers and impact mills are used for producing grain sizes between 25 mm and 0.1 mm (0.04 and 0.004 in.). for finer milling to particle sizes less than 0.1 mm, pin, pendulum, tube, ball, impact and jet mills are used. In addition to the hardness of the material to be milled, its required grain size and particle size distribution play an important role in the selection of the appropriate mill.

Mixing

Mixers are used to homogenize and uniformly mix the solid compounds with the binding agent. In this process, the solids are placed into the mixer, and, during homogenizing heated by gas burners, steam or electric energy. In some special cases the solids are blended and homogenized in a separated dry mixer. The binder is added to the filler in liquid or solid form in an amount appropriate for the molding procedure selected. Mixes with identical grain size distributions have a higher binder requirement in the case of extrusion thn those, which will be die-moulded or iso-statically pressed. Predominantly low-speed sigma-blade mixers or paddle mixers are used. The mixer capacity may vary between several hundred to 2000 litres. The final mixing temperature is 120 to 200 C for a period of 20 to 120 minutes. The hot mixes are cooled to room temperature and then milled to the grain size requested for the shaping procedure. For mixes to be die-moulded, a technique called “evaporative mixing” is used to obtain a high level of density and strength, by vaporizing for up to several hours a proportion of the lower boiling fractions of the binder at the final mixing temperature.

Forming

For forming the green mixes, normally four methods are used, which are extrusion (continuous process), die-moulding (discontinuous process), isostatic pressing (discontinuous process) and vibrating (discontinuous process). The material properties and potential applications are greatly influenced by proper selection of the forming process. The mix consists of predominantly anisotropic particles, which obtain a distinct orientation according to the respective moulding process. The orientation is parallel to the forming direction in the case of extrusion, and perpendicular to it with die-moulding. Therefore, the formed artifacts show a varying anisotropy of their properties according to their grain configuration, thus making them unsuitable for certain applications.

Die-moulding

Die-moulding is a discontinuous forming process in which the mould is filled with the material to be moulded and is subsequently compacted by a ram to the desired height of the artefact. Very often one works simultaneously with upper and lower rams, since an ejection device is needed anyway in order to release the shape from the mould. Die-moulding generates a much less distinctive anisotropy than extrusion. Die-moulding is mostly used for small-dimensioned fine-grained carbon and graphite parts, and may be automated in the case of large quantities of the same shape. When the shrinkage in volume, due to subsequent thermal treatment, is properly taken into account, many parts may be manufactured in one step – either pressed to final size or at least to near net shape – so that a subsequent machining process may be partially or even completely.

omitted.

Die-moulding calls for very sophisticated tooling when more complicated parts have to be produced. Isostatic pressing is reserved for the manufacture of large-dimensioned isotropic artifacts, e.g. with cross-sections of 1000x1500 mm, and also requires high expenditure for producing the flexible moulds.

Extrusion

Extrusion is a continuous forming process in which the mix is filled into a large storage chamber of the press (mud chamber), compacted by means of a piston in the direction of the die exit and pressed out as a practically endless shape. When the pressing chamber is empty, the piston is retracted, and the storage section is filled again to repeat the pressing cycle. In order to overcome high friction forces, the inner diameter of theremovable die is reduced step by step. The die exist shape will differ according to the desired cross-section or profile of the extruded material. Extrusion generates a much more distinctive anisotropy than die-moulding. By means of the continuous extrusion process, cylindrical, rectangular/square, or even more complicated cross-sections of several square millimetres up to several thousand square centimetres are produced which are cut to the desired lenghth for further processing.

Isostatic pressing

Isostatic pressing is also a discontinuous forming procedure, having the advantage that the compacting force is applied uniformly from all directions. This is indispensable for the manufacture of isotropic large-dimensioned bodies. An elastic mould is filled with the material to be moulded, inserted into the liquid within a pressure vessel and compacted by applying high pressure to produce the desired shape. Isostatic pressing, even with highly anisotropic particles leads to nearly isotropic artifacts due to the uniform exertion of the pressing forces onto the flexible moulds.

Vibration

Vibrating is a discontinuous forming process for large dimension products. A mould is filled with the milled powder and a heavy metal plate is put on top of the powder. Then the material is compacted by vibrating the mould for a certain period of time. The formed bodies show a higher degree of isotropy compared to extruded or die-moulded material.

Baking

Carbon is oxidised at elevated temperatures. Therefore, the presence of air or other oxidising agents has to be excluded. During the baking process, the binder bridges between the solid filler particles are carbonised, i.e. converted into solid carbon, which produces the required strength of the artefact.

This process may be broken down into four steps:

1. Heat transfer from the outside into the interior

2. Porylosis/carbonization of the binding agent

3. Transport of volatile pyrolysis products to the outside

4. Cooling to room temperature

The purpose of the baking process is to

• Change the binder pitch chemically by heating

• Converting it into a solid carbon mass

• Which forms permanent carbon bonds between the coke particles

• Thus a permanent solid rigid body is formed

Purpose of baking in detail

• The purpose of baking is to cement or bond the coke particles together by converting the binder pitch to a solid carbon residue.

• The coke particles of the green stock are held by stickiness of the binder pitch in its solid state at room temperature. However, if the binder pitch is heated to its softening point, the pitch will melt and the green product will deform or fall apart.

• Thus, the baking process is used to change the binder pitch chemically by heating, converting it into a solid carbon mass which forms a permanent carbon bond between the coke particles. • Thus a permanent solid rigid body is formed. Liquid-phase

• Thus a permanent solid rigid body is formed.

Liquid-phase pyrolysis, is a controlled thermal treatment of molten condensed aromatic compounds under the exclusion of oxygen in the temperature range between 200 and 600 C. The carbonising process is aimed to maximise coke yield, which is accompanied by a low formation of volatile decomposition products. The rate of the temperature increase in the furnace and within the artefact plays a decisive role as a rapid pyrolysis leads to an increased generation of volatile pyrolysis products, and too strong a shrinkage of the shaped bodies results in the formation of cracks and ultimate destruction of the artifacts. During the pyrolysis phase, between 250 and 550 C, the temperature increase should be approximately