Raw materials/precursors of molded graphite.

Raw materials selection. The selection of the appropriate raw materials is the first and critical step in the manufacturing process. It determines to a great degree, the properties and the cost of the final product. The characteristics of these raw materials such as the particle size and ash content of cokes, the degree of carbonization of pitch, the particle structure of lampblack, and the impurities and particle size of natural graphite must be taken into account.
To use high-grade, expensive raw materials to produce an undemanding product, such as a grounding anode for electrolytic protection, would be wasteful and economically unsound since these electrodes do not require optimum properties and cost is the overriding factor. On the other hand, nuclear applications demand a graphite with the lowest-possible impurities and the highest-possible mechanical properties. This requires the selection of premium-grade precursor materials with cost somewhat secondary.
Raw materials can be divided into four generic categories: fillers, binders, impregnants, and additives.
Fillers: The filler is usually selected from carbon materials that graphitize readily. As mentioned in Cha.4, such materials are generally cokes, also known in industry as “soft fillers”. They graphitize rapidly above 2700C. Other major fillers are synthetic graphite from recycled electrodes, natural graphite, and carbon black.
Petroleum coke is the filler of choice in most applications. It is a porous by-product of the petroleum industry and an almost-pure solid carbon at room temperature. It is produced by destructive distillation without the addition of hydrogen, either by a continuous process or, more commonly, by a batch process.
The batch process consists of heating high-boiling petroleum feed-stocks under pressure to approximately 430C, usually for several days. This promotes the growth of mosophase-liquid polycylic crystals. The material is then calcined up to 1200C, to remove almost all the residual hydrogen, and finally ground and sized.
By varying the source of oil and the process parameters, it is possible to obtain various grades of petroleum-coke filler with different properties. The industry commonly uses three grades:
– Needle coke, a premium grade with distinctive needle-shape particles, produced by delayed coking from selected feedstocks with low concentration of insolubles. It is used in applications requiring high thermal-shock resistance and low electrical resistivity.
– Anode coke for less demanding applications.
– Isotropic coke in applications where isotropic properties and a fine-grained structure are required.
Binders: The most common binder is coal-tar pitch which is a hard, brittle and glassy material. It is a by-product of metallurgical-coke production and is obtained by the distillation or heat treatment of coal-tar. From 35 to 60 kg of pitch are produced from every metric ton of coal.
The composition of coal-tar pitch is complex and may vary considerably since it depends on the degree of refinement of the available coke-oven tars. Two factors can noticeably influence the quality and graphitization characteristics of the pitch: (a) its softening point and (b) the content of insoluble complexes of quinoline. This content may vary widely from one pitch to another.
Other binders such a petroleum pitch and thermosetting resins are used for specialty applications.