In petrochemical, pharmaceutical, metallurgical, food, electronics, waste treatment and many other fields of equipment, due to the corrosiveness of the medium, the equipment is required to have both corrosion resistance and good heat transfer performance, coupled with good economy. Graphite equipment has become an ideal equipment. The main material used in making graphite equipment is graphite. The chemical equipment made of graphite material has the following advantages: excellent chemical stability, high thermal conductivity, small linear expansion coefficient, good mechanical processing performance, relatively lightweight equipment and so on.
Graphite materials can be divided into natural graphite and artificial graphite according to their formation. However, natural graphite has low purity, high impurity content and loose structure. Therefore, artificial graphite is mainly used in chemical equipment. Synthetic graphite can be divided into many categories according to the processing technology. Therefore, the choice of graphite material for the manufacturing process of graphite blank material for chemical equipment is a problem that needs to be considered by all production units at present. In this paper, the current situation of materials used by graphite equipment manufacturers in China is analyzed and compared comprehensively, as a reference for the selection of materials in the production of graphite equipment.
Classification of graphite materials
The materials used in graphite equipment must be impermeable graphite. According to impermeable materials, it can be divided into impregnated graphite, pressed graphite and cast graphite.
Graphite substrates used for impregnating graphite are further divided into the following categories.
1. According to the material forming method, it is divided into extrusion forming graphite, vibration forming graphite, mould pressing forming graphite, isostatic pressing forming graphite, etc.
These materials can be used in the manufacture of graphite equipment parts only after they are processed by roasting, impregnating asphalt and graphitizing. For the same paste, different molding methods have obvious differences in internal structure, physical and chemical properties, thermal expansion coefficient, etc. Among them, isostatic pressing method is superior to other molding methods. The advantage and disadvantage of the molding material; due to the influence of internal and external friction, the density of each part of the green blank is not uniform, in addition to the uneven feeding, the volume density of the place with more feeding increases. Anisotropy is significant. Vibration molding material products have density inhomogeneity along the height direction, and the density of the middle part is lower than the two ends.
2. According to the particle size of material, the maximum particle size of ultrafine graphite is 0.015-0.08 mm, the maximum particle size of fine graphite is 0.1-0.3 mm, and the particle size range of coarse graphite is more than 0.5-2 mm.
Graphite green body will have different physical and chemical properties under the same extrusion process because of different particle size. Such as particle size become finer, density increases, specific resistance decreases, mechanical properties such as compression and bending resistance increased, thermal expansion coefficient reduced and so on. Therefore, for graphite equipment, the finer material is, the better the product is. Of course, the premise is to ensure that each process meets the requirements.
3, According to the purpose: Metallurgy graphite, EDM graphite, special graphite for chemical industry.
Metallurgical graphite is usually hot extruded graphite with large particle size, loose structure, large porosity, mechanical and thermal properties are inferior to EDM graphite and chemical special graphite. EDM graphite and special graphite for chemical industry belong to ultrafine graphite or fine graphite because of their fine particles. They have the advantages of compact structure and high mechanical strength, but the yield is low and the cost is high.
For domestic graphite equipment manufacturers, metallurgical electrode graphite is still used more at present, especially molded graphite is used most. Due to the fierce market competition, manufacturers have to do so in order to reduce costs as much as possible.
4. The above-mentioned graphite materials are all through high temperature graphitization process, and are the preferred materials for graphite equipment manufacturing. There is also a kind of material called regenerated graphite on the market at present. The raw material used is the crushed graphite produced by machining graphite products, which is crushed, shaped and baked. Although the cost is low, but brittleness, processing performance is not good, and contains a considerable number of impurities, and some even mixed with iron, such as for graphite equipment heat exchanger or pressure components, in use will soon corrode perforation, the quality of graphite equipment there are hidden dangers. It can only be used for processing general graphite parts.
Several main graphite materials used in graphite equipment.
1. Common graphite materials
Ordinary graphite products have the advantages of low cost, wide range of application, comprehensive specifications and so on, and are now widely used. Especially in metallurgical industry, in China, because of its low price, it is also the main raw material for graphite equipment manufacturing.
2. High purity graphite materials
High-purity graphite has the advantages of good thermal stability, high volume density, high precision, strong oxidation resistance, and its purity, flexural strength and compressive strength are higher than other graphite. However, due to the special production process of high-purity graphite, the product cost is higher, and the product specifications are easy to be limited by materials. High-purity graphite products are widely used in mold products, mercury rectifiers, semiconductor melting crucibles and so on in the smelting industry. The small-size pure graphite heater components matched with each index of power transformer in the production process are widely used in special processes. It is restricted by the influence of price and material specification in chemical graphite equipment.
3. Fine grain high quality graphite materials
Fine-grained high-quality graphite materials, high-power graphite and high-purity graphite part of the production process in one, with bulk density, uniform product layer structure, product specifications are comprehensive and other advantages, but the bending strength of large-size products is relatively low, high cost. Fine-grained graphite products are widely used in sintering process, the product requirements are higher, the specifications are larger boat selection, and the production process of cemented carbide rod, sheet, strip products boat, is the most choice of graphite materials. In the production of graphite equipment, the equipment using this kind of material has increased in recent years.
Discussion on material selection
1. The linear expansion coefficient, thermal conductivity, strength and modulus of elasticity of various kinds of graphite have different values with different orientations. Furthermore, different materials can be selected for the design and manufacture of graphite equipments, and the products produced by different materials have different structural strength and so on because of the action situation and working conditions of the products performance. At present, the State Administration of Quality Supervision, Inspection and Quarantine has published the Regulations for Safety and Technical Supervision of Non-metallic Pressure Vessels, which require the selection of materials. Therefore, the selection of graphite materials should also be controlled in product design.
2. For manufacturers, because of the needs of the market, they can also produce different cost-effective products for users to choose. For example, the equipment produced with high strength fine particles is used for advanced or special purposes, while the ordinary chemical medium such as non-strong acid, strong alkali, pressure and temperature requirements are not high equipment, ordinary electrode graphite is enough.
3. The manufacturer should check the quality of the material when purchasing. The bad became mixed with the good seriously in the market. Once the material is used improperly, the quality of the graphite equipment will be greatly affected.
Author: Chen Honglin
