Isostatic pressing graphite is developed in the 1960 s a new type of graphite materials, has a series of excellent performance. Isostatic pressing graphite heat resistance good, for example, in an inert atmosphere, with its mechanical strength not only the rise of temperature, increase, in the 2500 ℃ maximum when left to the right; Compared with ordinary graphite, fine structure and uniformity; Low thermal expansion coefficient, excellent thermal shock resistance; Isotropic; Thermal conductivity and chemical corrosion resistance is strong, conductive good performance; Excellent machining performance. It is precisely because has a series of excellent performance, such as static pressure graphite in metallurgical, chemical, electrical, aerospace and atomic energy industry and other fields is widely used, and, with the development of science and technology, application field continues to expand.
1 the isostatic pressing graphite production process
Very apparently, production technology of isostatic pressing graphite and graphite electrodes. Isostatic pressing graphite need the structure of isotropic materials, you need to raw material grinding into a fine powder, need to apply cold isostatic pressing molding technology, roasting cycle is very long, in order to achieve the target density, need many times
Dip - roasting cycle, cycle of graphitization is much longer than ordinary graphite.
1.1 raw material to produce isostatic pressing graphite materials including aggregate and
Adhesives. Aggregate is usually with petroleum coke and pitch coke and pitch coke also have land, such as the POCO AXF series isostatic pressing graphite, pitch coke is land Gilsonitecoke production. In order to according to the different USES to adjust the product performance, also useful carbon black, artificial graphite additives. In general petroleum coke and pitch coke calcined under 1200 ~ 1400 ℃, to use to remove moisture and volatile matter. But in order to improve the mechanical properties and structure of the product density, also has a direct made of raw material to produce the isostatic pressing graphite raw coke. Raw coke is characterized by containing volatile, has since the sintering, and the binder coke synchronous expansion and contraction. Adhesives usually use coal tar, according to the different equipment condition and technical requirements of various enterprises, using the coal asphalt softening point from 50 ℃ to 250 ℃. Isostatic pressing performance of graphite under the influence of raw material is great, the selection of the raw material is needed to produce a key link in the process of the final product. Prior to feeding uniformity of material characteristics and strict inspection.
1.2 milling isostatic pressing graphite aggregate size usually require up to 20 microns below. At present, the most elaborate isostatic pressing graphite, maximum particle diameter of 1 mu m, it is very fine. The aggregate coke grinding into fine powder, so need to use superfine mill. Grinding of average particle size of 10 ~ 20 microns powder need to use the vertical roller mill, the grinding powder of average particle size less than 10 microns will need to use the airflow mill.
1.3 kneading will manufacture fine powder and coal asphalt binder in proportion into the mix and pinch machine heating type kneading, make coke powder particle surface evenly attached to a layer of asphalt. After kneading, paste, to make it cool. Compared with graphite electrode production, the production of the isostatic pressing graphite kneading asphalt quantity is more, when the temperature is higher, some time longer.
1.4 secondary flour paste after crushing, grinding, and the screen is divided into several dozens to hundreds of micron particle size of particles after mixing, used as a molding material, called pressed powder. Secondary grinding equipment is usually use vertical roller mill and ball mill.
1.5 molding is different from the ordinary and extrusion molding, such as static pressure is graphite by cold isostatic pressing technology molding (figure 2). Powdered materials will be filled to the rubber mold, high frequency electromagnetic vibration, pressure powder is close-grained, after sealing process of pumping air into vacuum state, discharge the air between powder particles, in the containing water or oil liquid medium, such as high pressure vessel, pressure to 100 ~ 200 mpa, compressed into cylindrical or rectangle products. According to PASCAL's principle, through the liquid medium such as water pressure to the rubber mold, pressure is equal in all directions. In this way, the pressure in the mold powder particles is not according to fill in the direction of orientation, but is compressed by the irregular arrangement, therefore, in spite of graphite on the crystallographic features is anisotropic, but overall, isostatic pressing graphite Is isotropic. After molding products in addition to the cylindrical, rectangular, including the shape of cylinder, crucible, etc. Isostatic pressing molding machine is mainly used in powder metallurgy industry. Due to the aerospace, nuclear industry, hard alloy, high voltage electromagnetic and other high-end industry demand, isostatic pressing technology is developing very fast, already have manufacturing work cylinder inner diameter 3000 mm, 5000 mm, maximum working pressure 600 MPa cold isostatic pressing machine. At present, the isostatic pressing graphite carbon industry used in the production of cold isostatic pressing machine specification is the largest Φ 2150 mm * 4700 mm, and maximum working pressure of 180 mpa.
1.6 roasting in the roasting process, complex chemical reaction taking place between aggregate and caking agent, adhesives, release a large number of volatile, at the same time polycondensation reaction. At low temperature preheating stage, raw products, because of the heat in the subsequent heating process, the volume contraction due to condensation reaction. The greater the volume of raw products, the release of volatile, the more difficult, and raw products surface and internal easy to produce temperature difference, thermal expansion, the phenomenon such as uneven contraction, which may lead to crack of raw products. Isostatic pressing graphite because of its fine structure, especially slow roasting process requires you to be, and have to be very uniform furnace temperature, especially in the asphalt volatile discharge temperature stage, sharply to caution, heating process, heating speed cannot be more than 1 ℃ / h, furnace temperature difference requires less than 20 ℃, the process need about 1 ~ 2 months.
1.7 dipping in the process of roasting, volatile bituminous coal is expelled. Gas discharge and volume shrinkage when leaving tiny pores in the products, and are almost open porosity. In order to improve the products of the volume density, mechanical strength, electrical conductivity, thermal conductivity, resistance to chemical reaction, can be treated with pressurized impregnation method, namely through opening stomatal coal tar dipping into the products. Products should be preheated, then in the macerate tank vacuum degassing, again good molten coal pitch to join the impregnating tank, pressure make impregnant pitch into the products inside. Usually, the isostatic pressing graphite to after many times of dipping - roasting cycle ring.
1.8 graphitization The products are heated to about 3000 ℃ after roasting, carbon atoms in crystal lattice ordered arrangement, completed by carbon to graphite, called graphitization. Graphitization method has acheson method, internal combination method, high frequency induction method, etc. Usually acheson method, products from furnace charging to emerge, needs to yellow four letters about isostatic pressing graphite production technology, main USES and the analysis of the domestic market 5, 34, 1-1.5 months. Each furnace can handle several tons to dozens of tons of calcined product. After graphitization, products of the volume density, conductivity, thermal conductivity and corrosion resistance have been improved greatly, machining performance is also improved. However, graphitization reduces flexural strength of products.
1.9 check after graphitization, also need to products of density, hardness, strength, resistivity, ash content, such as index to check, to determine whether meet the requirements of indicators. 1.10 purification isostatic pressing graphite when used in semiconductor, monocrystalline silicon, atomic energy, and other fields, has a high requirement on the purity and must be used a chemical method to remove impurities, can be used in these areas. To remove impurities in the graphite, usually put graphitization products in halogen gas heated to about 2000 ℃, the impurity was halide halide of low boiling point and volatile. Almost all of the products graphitization of the impurity elements can use chlorine gas halide. But exceptions boron element, it can only get rid of fluoride. Used for purification of halogen gas have chlorine, fluorine, or is that under the condition of high temperature decomposition of these halogenated hydrocarbon gas, for example, carbon tetrachloride (CCl4), dichlorodifluoromethane (CCl2F2). The purification method can make full use of the graphite in the heat does not react with halogen, and the unique properties of porous graphite.