INDUSTRIAL APPLICATION OF GRAPHITE
Graphite it is an allotropic form of carbon with characteristic hexagonal form of crystals the consequence of which is one of the highest strengths known in nature and unique physical properties. Graphite materials applied in industry never have fully shaped structure being rather a transient form between amorphous carbon and graphite crystal. Their properties, and therefore quality, are precisely correlated with the degree of graphitization, internal structure and impurities content which does not exceed 10ppm in the best graphites.
Graphite – a commonly known substance, for instance as pencil inserts, has gained considerable significance in technique. It has multiple forms and industry applications of key importance for machine construction very often. For these purposes we can classify three the most known graphite types.
Natural graphite which is a result of metamorphism of rocks reach in organic compounds like hard carbon, however, this process runs in considerably longer geologic time. Graphite deposits occur in large dispersion, both in amorphous form (black graphite) and in crystal form as flake graphite (silver), having characteristic scaly structure.
Acheson graphite manufactured in industrial process as various types of products shaped from materials of high carbon content like petroleum coke and anthracite. Raw material is ground, and then shaped and sintered into required shapes which after impregnation are graphitized in high temperatures, over 2500°C.
Expanded graphite – soft and flexible material made by exfoliation of graphite crystals intercalated by sulphur or fluorine atoms. Pressing of expanded mass gives materials in the form of profiles, yarn or foil which inherit high resistance of graphite and at the same time flexibility and elasticity similar to elastomers and fibre based materials.
Unique properties of graphite
Graphite combines both ceramic materials features and some properties of metals, having lubrication properties and extraordinary wear resistance and resistance to external environment influences.
High thermal conductivity of graphite is on the average level of most metals. It exceeds conductivity of ferrous metals and steel but it is no match for copper and aluminium. Good conductivity of graphite has an influence on its high thermal resistance and protects against damage or overheat in dynamic applications as sealing, bearing, sliding elements.
Low reactivity and resistance to most chemical agents. Graphite is chemically stable in normal conditions and resistant to most acids and alkalies except strongly oxidizing ones as aqua regia (nitrohydrochloric acid) and liquid oxygen. However, at the temperatures of over 450°C graphite can react with oxygen or water steam from environment and can also dissolve in some liquid metals and create carbides. Graphite doesn’t melt at high temperatures like most solid bodies but it undergoes sublimation. In reduction atmosphere it can work successfully at the temperature up to 2500°C.
Fire-resistance and resistance to thermal shocks. Fire-resistance of materials is normally determined with the use of the following empirical formula:
In the case of graphite, combination of good thermal conductivity with mechanical strength places it at the very high level among the best fire-resistant materials. Additionally, its low thermal expansion coefficient in comparison with other high temperature materials causes that it is exceptionally resistant to thermal shocks.
Mechanical strength at high temperatures. Opposite to the other high temperature structural materials, the mechanical strength of graphite is rising together with temperature rise and it doubles its value at the temperature of 2400°C. This peculiar feature causes that graphite is often z better structural material in high temperature conditions than alloys on the basis of infusible metals. Furthermore, graphite has relatively low density at the level of 1.6 to 1.9 g/cm3 and because of this its relative strength and effectiveness in constructions might be even higher.
Electrical conductivity. Graphite’s ability to conduct electrical power causes that it is the only high temperature conductor which is in general use, besides tungsten, molybdenum and vanadium group elements. This feature in connection with high wear resistance causes wide application of graphite in electrical engineering, metallurgy and machinery industry. What is important, graphite’s conductivity rises together with temperature rise at the temperature range from 400 to 600°C which is its peculiar and key feature for its application as electrodes in ultra high power arc furnaces. The graphite resistivity can be also controlled in the production process by the use of modified batch materials or by saturating finished products with metals.
Self-lubrication and high wear resistance. Because of the scaly structure of graphite crystals and easiness of planes breaking off and sliding, graphite covers surfaces of machine working elements with thin graphene film. This causes the effect of laminar lubrication in all conditions even after other types of lubricants go dry, also at high temperatures. Graphite keeps it scaly structure even in case of considerable apparent wear and the biggest fragmentation.
Amorphous and flake graphite
Amorphous graphite is the most common and cost efficient form of natural graphite. Unfortunately it is strongly contaminated and its structure is a transient form between amorphous carbon and graphite. The flake graphite hasn’t these defects. It is a form of natural graphite with highly ordered crystalline structure. It has characteristic metallic gloss and it is often called silver graphite or crystal graphite. The nature of spatial structure of flake graphite results in ease of breaking bonds between adjacent planes and creating a scaly structure. The consequence of this structure is high anisotropy of graphite physical properties. Hardness, strength and conductivity in a direction parallel to the axis of crystallite exceed several times the value in a direction perpendicular to that axis.
The basis of natural graphite classification is its carbon content and granulation. Most types exist in a form of powders but the flake graphite is also available in the form of liquid dispersion, pastes and ready to use greases. Among the most important application areas of graphite there are:
Foundry engineering – flake graphite in a form of dispersion and powders is used for foundry covers where it also serves as a refractory covering protecting castings against formation of external defects and sand buckles. Graphite is also an excellent additive to moulding sands and core sands; it increases their thermal resistance, ductility and tendency to release lustrous carbon.
Refractory materials – Large-size flake graphites (FG) are used for manufacturing of black refractory materials, melting pots, retorts, furnace lining elements. Addition of graphite plasticizes moulding sand and facilitates formation of the mass and moreover it increases the quality of finished products, improves their fire resistance, chemical resistance, considerably reduces friction and thermal expansion coefficient.
Special greases – for the production of greases in the form of pastes and suspensoids, the graphite micro-powders (MG) are used. Graphite may be an economical equivalent of molybdenum disulfide and improver of lithium greases, calcium based greases or bentonite masses, increasing their stability at high temperatures. In most applications the addition of graphite reduces friction coefficient, especially at high loads, and reduces the risk in devices exposed to shocks and overheating. The presence of graphite in lubricants guarantees that they keep their lubrication properties for a long time of operation, even after other components of grease get worn out and dry.
Antistatic coverings – due to good electrical conductivity, the graphite is used as charges passivator (oxidizing inhibitor) in anti-electrostatic coverings. It is a proven component during manufacturing of explosion-proof curtains, linings and mining foils. It is an additive to floors, to conductive and antistatic coverings.
Frictional and brake linings – during manufacturing of frictional and brake linings the fine-flake graphite (MG) with medium cleanliness is used. The graphite additive to frictional material increases its thermal conductivity, resistance to abrasive wear and quick temperature changes. It has positive influence on their reliability and life in long time.
Anti-flammable protectors – the modified graphite (EG) which has expanding properties is added as an agent increasing fire resistance (anti-pyrene) to various kinds of insulations, plastics and anti-fire barriers. Graphite content in the whole mass of plastic constitutes from 2% to 10%, depending on its type and intended use. Addition of graphite anti-pyrene lowers flammability of plastics on the basis of combining the effects of a few phenomena. In the initial phase the graphite screens thermal radiation protecting against excessive temperature rise and then the graphite expands and forms an insulation covering and finally the expanded graphite forms a refractory cover protecting against oxygen and flames access to deeper layers of material.
Other applications – flake graphite is also used to manufacture paints and anti-corrosion covers, to produce and make impregnation of sealing and insulating materials, to produce covers and blacksmith separators, to produce battery cores, electrodes and electrical engineering accessories, during manufacturing of composites and plastics, sintered carbides, cermets, explosive materials and in laboratory technique.
Synthetic moulded (Acheson) graphite
The moulded graphite is a synthetic carbon material which has crypto-crystalline structure. It is manufactured in the process of burning and graphitizing of materials on the basis of carbon precursors. Typical semi-finished products made of moulded graphite are the elements in the shape of plates, bars, rollers and big blocks weighing up to a few hundred kilograms. The semi-finished graphite products may be classified in three basic categories. These are:
Graphites moulded straight through and vibratory compacted which have large grain and high anisotropy, intended for manufacture of electrodes and structural members. This type of graphite is mass-produced and most commonly applied in industry.
Die stamped graphites which have fine-grained structure, grain at the level of 10 – 100 μm. The material is to a certain degree uniform but showing some anisotropy in direction of stamping. Medium cost of manufacture and a wide range of applications.
The isostatic graphites having uniform and isotropic structure and micro-grain at the level from 2 to 20 μm. Although they are manufactured with the use of very expensive CIP method, they are necessary and required in many up-to-date applications.
Pressed graphite applications
Growing expectations in relation to properties of structural materials of machines and devices stimulate continuous growth of demand for high quality products from carbon and graphite. These materials meet high strength criteria and moreover they are neutral for health and environment which makes them to be a modern and easy to use material. Above half of production of pressed graphite is used in the form of electrodes in arc furnaces (EAF) to produce melted metals. Graphite consumption is also quickly growing in many other industry sectors. This regards both traditional and the latest applications. These are:
vacuum furnace elements, rotors and blades of vacuum pumps,
rotary machine elements, rings, bearings and plain bearings,
industrial furnace electrodes, in metallurgy, chemistry and electroplating,
foundry accessories, meting pots, crystallizers, foundry moulds,
chemical apparatus elements, heat exchangers and measuring instrument elements,
high temperature heating elements, die blocks for electronics,
sunken electrodes to EDM and electric machine commutators,
secondary rafinators, linings and anodes for electrolysis of metals.
A choice of proper graphite kind for specified application is connected with specialist knowledge as regards its parameters and internal structure. It happens very often that graphite suitability, except physical parameters, depends on factors connected with specificity of production process, kind of precursor and impregnates used.
The manufacturing technology of this material was worked out relatively recently. It consists in fragmentation of graphite crystals into single graphene layers and blending it in homogeneous structure, most often in the form of foil. In the production process the precise milling lines are used which guarantee the dimensions and density stability in the whole mass what in consequence has decisive influence on the quality of various types of final products.
The products made of flexible graphite, as plates, seals and stuffing-box packings are intended for technical seals of the highest parameters, working mainly in power engineering and industry systems under a pressure up to 1000 bar. The flexible graphite is also used for manufacturing other seals, such as sealing tapes, spiral and multi-edge seals, packing rings of GDR type for industry fittings, flat seals in flange connections, various kinds of formed seals as well as seals for automotive industry.
Temperature resistance of pure graphite foil is above 2000°C which is the reason why this material is used also for high temperature elements in industrial furnaces, electrical contacts and heating electrodes in vacuum furnaces as a flexible and at the same conductive refractory material. Production volume of materials made of flexible graphite is continuously growing and now it exceeds 200 thousand tonnes per year. This constitutes about 10% of total graphite products consumption in industry..