Introduction to Manufacture
Contents
1. Introduction
2. Overview
3. Detailed reports
1. Introduction
Most electrographites are produced from a petroleum-coke filler and a coal-tar-pitch binder. Nuclear grade of electrographites are produced by modifications of conventional manufacturing methods, with special care being taken to exclude impurities that have significant neutron-absorption cross sections. The processing steps in the manufacture of a conventional nuclear graphite are summarized in the figure below. Various purification techniques, derived chiefly from similar processes employed in the production of spectrographic-arc carbons, have been introduced into the manufacture of nuclear-grade graphite to achieve purities that may exceed 99.999 per cent carbon.
2. Overview
a) Raw Materials
i. Petroleum Coke
All commercially produced carbons are composed of a carbonaceous filler material bonded by a carbonized binder, usually coal-tar pitch.The most commonly used filler materials are petroleum coke, metallurgical coke, anthracite, and lampblack. Of these, the most easily graphitized material is petroleum coke; i.e., when heated to a temperature of 2800 to 3000 °C the carbon from petroleum coke achieves a higher degree of crystallinity than that of other materials. For this reason almost all commercial graphites are made from a petroleum-coke base filler. Other easily graphitized cokes can be prepared from certain aromatic hydrocarbons, but economic considerations prevent their use in any commercial practice involving tonnage quantities.
ii. Coal-tar pitch
The search for a material having suitable properties as binder for the petroleum-coke filler leads quickly to the choice of coal-tar pitch, the heavy residue derived from the destillation of coal tar from by-product coke ovens. The availability of this relatively inexpensive material in tonnage quantities has made possible the rapid growth of the carbon industry. One of the important requirements of a binder for electrographite is that it be a thermoplastic material, being solid at room temperature and filler with the binder, facilitates the forming of the filler-binder mixture and permits storage and handling of the formed articles at room temperature without adverse effects on the shape of the product. The coal-tar pitch commonly used as a binder in graphite production should have a softening point of apporximately 100 °C so that mixing and forming can be accomplished in convenient steam-heated equipment. Handling of the formed articles is facilitated by the high viscocity of the pitch ( of the order of 109 poises) at room temperature.
3. Detailed Reports
