The contents of this article will provide you with everything you will need to know about graphite blocks and their use.
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Graphite blocks are made from crystalline carbon and are engineered to have specific properties such as density, electrical resistance, hardness, porosity, compressive strength, flexural strength, thermal expansion, and thermal conductivity. While graphite naturally occurs, most industrial-grade graphite blocks are synthesized from petroleum coke or coal tar pitch. High-purity graphite blocks, also known as molded graphite, contain up to 99.99% carbon and exhibit the specialized properties and characteristics for which graphite is renowned.
Natural graphite (NG) is a form of crystalline carbon found in metamorphic and igneous rocks. It is utilized as a thermal management material and mold lubricant due to its heat dissipation properties. Graphite, along with diamonds, is one of the most common naturally occurring forms of carbon.
Graphitization is the process that transforms carbon into graphite. This involves heating amorphous carbon to rearrange its atomic structure into a crystalline form. During graphitization, carbon atoms are reorganized to fill atomic gaps, improving the arrangement of atoms. This rearrangement occurs in the presence of oxidizing gases, which break the bonds in amorphous carbon, facilitating the formation of graphite.
In its natural form, graphite appears grayish-black and opaque with a metallic sheen, displaying both metal and non-metal properties. It is chemically inert and highly refractory, offering high thermal and electrical conductivity. These unique properties stem from its crystalline structure, where carbon atoms are arranged in hexagonal rings within layers that are stacked parallel to each other.
There are two main types of graphite: natural and synthetic. Natural graphite is composed of graphitic carbon and varies in crystallinity. Synthetic graphite, on the other hand, is produced from coke and coal pitch. It has a less crystalline structure compared to natural graphite and contains graphitic carbon that is formed through the process of graphitization.
The first step in manufacturing graphite blocks is selecting the appropriate raw materials. The choice between natural and synthetic graphite depends on the specific requirements of the application and the desired properties of the final product. For optimal results, the highest purity graphite is preferred, as it yields the highest quality products.
Natural graphite comes in three main forms: amorphous graphite, flake graphite, and crystalline vein graphite. Amorphous graphite is a microcrystalline form created from the metamorphosis of anthracite coal combined with a metamorphic agent. Flake graphite forms under high pressure and temperature conditions within metamorphic rocks. Crystalline vein graphite is a naturally occurring type that has been pyrolyzed, melted, and flowed into the cracks and crevices of rocks.
Synthetic graphite, also known as artificial graphite, exists in two main forms: primary and secondary graphite. Primary synthetic graphite is produced through the high-temperature treatment of coke, while secondary synthetic graphite is a byproduct of manufacturing graphite electrodes and other components. Both types of synthetic graphite are graphitized at temperatures up to 3000°C (5432°F) and exhibit high purity, excellent lubrication properties, and superior electrical conductivity.
To ensure the quality of a graphite block, whether it is derived from natural sources or synthesized, the raw graphite undergoes a purification process. This process includes chemical treatments, thermal processing, and mechanical methods. Because the types and amounts of impurities can vary from batch to batch, it is essential to analyze the raw graphite to determine the level of impurities present.
Common impurities in graphite include potassium, sodium, aluminum, calcium, magnesium, and various silicate minerals. Most natural graphite, except for vein graphite, has a relatively low carbon content. Flake graphite, in particular, is purified to enhance its carbon content. Before purification, the raw graphite is crushed to increase the efficiency of the process.
Thermal purification is conducted at temperatures up to 2500°C (4532°F) and produces high-purity graphite. This method is costly due to the specialized furnaces and high energy consumption required. Chemical purification involves the use of hydrofluoric acid, alkali acid, and chlorination roasting to remove impurities.
Chemical purification effectively removes hydrophobic impurities through an acid wash, which reacts with the impurities to separate them from the graphite. This process not only purifies the graphite but also serves as a pretreatment before flotation. The purity achieved with chemical methods can reach up to 99.5%, making it a cost-effective option.
Flotation is another method used for graphite purification, based on the difference in wettability between graphite and other minerals. During flotation, graphite floats to the surface with bubbles, while hydrophilic, undesirable minerals remain in the water. This process, though widely used, is adapted to the specific type of graphite being processed and typically involves multiple grinding and flotation steps.
The powder from the purification stage is mixed with synthetic resins, coal tar pitch, or petroleum pitch, which act as binders to create a graphite paste. These binders hold the graphite particles together during the shaping and forming stages, where the graphite powder is compressed into blocks. The amount and type of binder used can vary depending on the specific type of graphite and its intended application, as different types have different binding requirements.
Several factors are crucial in the blending and mixing process, including the application requirements for the graphite block. These factors include particle size, the shaping method, and the specific needs of the end user. Adjustments in these parameters ensure that the final graphite blocks meet the necessary performance and quality standards for their intended use.
The shaping and forming of graphite blocks are accomplished through methods such as isostatic pressing, extrusion, or die molding. The choice of method depends on the manufacturer and the specific requirements of the customer. Each method offers different benefits and is selected based on the desired properties and dimensions of the final graphite blocks.
Extrusion – Extrusion is a process where the graphite paste is forced through a die that has the desired shape of the graphite block. The paste is squeezed along the cylinder of the extruder toward a die that has the shape of the graphite block. During the process, the paste is compressed under pressure as the baffle moves along the cylinder. As the shaped paste exits the cylinder through the die, it is cut to the desired length, checked, and cooled. Extrusion is a continuous process that can produce any number of graphite blocks continuously.
Compression Molding – With compression molding, as with vibration molding, the mold has the shape and size of the graphite block. The mold is filled with the graphite paste and pressure is applied above and below the mold to compress and form the paste into the shape of the mold. It is a slow method for forming graphite blocks with one graphite block being formed during each cycle. Compression molding is used as an alternative to extrusion since it can produce any size of graphite block. The graphite blocks formed by compression molding have exceptional mechanical strength, friction resistance, density, hardness, and conductivity.
The compacted graphite blocks are heat-treated in a furnace at temperatures ranging from 900°C to 1200°C (1650°F to 2200°F). During this baking process, carbonization and thermal decomposition of the binder occur, transforming it into elemental carbon and releasing volatile components. This baking cycle aims to convert the graphite blocks into solid carbon. The process is meticulously controlled and requires an extended period to ensure the desired quality and properties of the final product.
Graphite is inherently porous and can absorb water slowly, which may be unsuitable for some applications. To address this issue, graphite blocks are often impregnated with materials that enhance their characteristics and properties. Impregnation involves various processes used to inject materials into the graphite blocks. The choice of impregnation method depends on the intended use of the graphite blocks. The impregnating material typically has a lower viscosity than the binder, allowing it to penetrate and fill the gaps within the graphite. Petroleum pitch is commonly used for this purpose. High-density graphite blocks may undergo multiple impregnation and rebaking cycles to achieve the desired properties.
Graphitization is the process that crystallizes carbon into crystalline graphite by applying high temperatures ranging from 2700°C to 3000°C (4900°F to 5450°F). This process causes the carbon atoms to rearrange into a stacked, parallel plane structure, significantly altering the properties of the graphite blocks. Additionally, the high temperature of graphitization purifies the graphite by causing impurities, such as binder residues, gases, oxides, and sulfur, to vaporize.
Graphitization occurs in an Acheson furnace, which features a central rectangular chamber surrounded by walls made of refractory materials. The furnace is designed to maintain the heat generated through electrical resistance. To ensure the process is effective, oxygen is removed by covering the graphite blocks with oxygen scavenging materials.
The extensive use of graphite across various industries necessitates strict adherence to manufacturing and production protocols, as each graphite block is tailored to meet specific industrial requirements. After completing the production stages, each graphite block undergoes meticulous inspection to ensure it meets the industry standards for which it was produced. The blocks are labeled, documented, and assigned identifying data to facilitate traceability for integration into assemblies.
From the careful selection of raw materials to the shaping, baking, and molding processes, every step is executed with attention to detail and adherence to stringent quality standards. Different industries have their own sets of standards for graphite blocks, requiring manufacturers to comply with these regulations to ensure that the final products are of the highest quality.
There are certain factors that differentiate synthetic graphite blocks from natural graphite blocks. Both forms have carbon as their base material but exhibit distinctly different characteristics, processing methods, and applications. Natural graphite blocks are generally less expensive, have high capacity, and require less energy for production. In contrast, synthetic graphite blocks have a higher density and thermal conductivity ranging from 700 W to 1500 W.
The production processes for both natural and synthetic graphite blocks share many similarities, with the primary difference being the raw materials used. Synthetic graphite production begins with green petroleum coke, which is derived from the refining or catalytic cracking of heavy oils.
Manufacturing synthetic graphite involves methods similar to those used for producing ceramic materials. Coke and graphite are ground and mixed with carbon-based pitches to form a homogeneous mass. This mass is then subjected to the standard steps in graphite block production.
The process for manufacturing synthetic graphite was first introduced in 1893 by Charles Street, who graphitized amorphous carbon. Petroleum coke, a byproduct of crude oil refining, is the preferred material for producing synthetic graphite. Another less expensive raw material is pitch coke, derived from coal tar.
All graphite is based on carbon, found in both diamonds and graphite. The key difference lies in their atomic structures: diamonds have tetrahedral atomic arrangements, while graphite features a hexagonal crystalline structure forming planar layers.
To produce synthetic graphite, amorphous carbon is heated over an extended period in a process known as graphitization. This process rearranges the atomic structure, filling atomic gaps and adjusting the layout of the atoms. Gradual temperature increases alter the crystal structure of carbon, transforming it closer to graphite’s characteristic layered structure. The result is enhanced lubrication properties, oxidation resistance, and improved thermal performance in the final product.
| Graphitization Process | |
|---|---|
| Temperature | Changes Caused by Temperature Increases |
| Room Temperature - 1300°C | There are no changes in the carbon atoms, but minimal structural changes begin. |
| 1300°C - 2000°C | With the increase in temperature, crystal structures grow, which indicates movement and the rearrangement of atoms. As the atoms rearrange and change, the spacing between them changes and they shrink. |
| 2000°C - 3000°C | As the crystal growth increases, spacing continues to decrease and open, spaces diminish and are filled. |
Electrographite synthetic graphite is produced from pure carbon sourced from coal tar pitch and calcined petroleum coke, which is heated in an electric furnace. Another type of synthetic graphite is made from heated calcined petroleum pitch. Despite the different raw materials used in production, synthetic graphite does not exhibit the same crystalline structure as natural graphite but maintains exceptionally high purity.
The essential feature of synthetic graphite is the graphitic carbon formed through the process of graphitization. Synthetic graphite is characterized by high electrical resistance, notable porosity, and very low density. Its porosity makes it unsuitable for refractory applications.
| Characteristics of Synthetic Graphite | ||
|---|---|---|
| Property | Effect of Graphitization on the Properties of Graphite | Reason for the Change |
| Lubricity | Increase | Van der Waals forces in graphite’s atomic structure are broken to allow the layers of graphene to slide off and deposit onto a counter surface to provide lubrication for applications. |
| Oxidation Resistance | Increase | As graphitization progresses, a crystal structure forms and there are less non-bonded atoms leaving fewer places for oxidation, giving graphitized graphite oxidation resistance. |
| Thermal Conductivity | Increase | The structure of graphite allows for heat to flow through the material to avoid heat buildup. |
| Coefficient of Friction | Decrease | The layered structure of the graphite allows graphene to rub off when placed against a counter surface because the Van der Waals forces connecting graphene layers are easily broken. |
| Hardness | Decrease | The Van der Waals forces in graphite can be easily broken compared to its intertwined amorphous layout, which makes graphite a much softer material. |
| Strength | Decrease | Graphite’s layered structure results in it being softer and having less strength compared to the harder and stronger carbon graphite that has amorphous carbon in it. |
Graphite blocks are available in various types, each tailored for specific manufacturing and industrial applications. The primary classification is based on the grain structure of the blocks, which can be fine, medium, or coarse. Additionally, graphite blocks can be categorized by their purity, crystalline structure, and specific characteristics and properties.
Pyrolytic graphite blocks are created through the decomposition of hydrocarbon gas, typically methane, in a vacuum furnace to produce extremely pure graphite. This process is slow, time-consuming, and costly. During production, methane or hydrocarbon gas is heated under low pressure at 2000°C (3632°F), resulting in the formation of layers of graphite with a non-porous, easy-to-machine surface.
A notable property of pyrolytic graphite is its diamagnetism, which allows it to repel or be repelled by a magnetic field. Pyrolytic graphite is used in various applications, including heating and cooling conductors in the rocket industry, neutron modulators for nuclear reactors, and high-power vacuum lamps. It is also utilized in products such as sputtering targets, ion beam grids, ion implant hardware, liquid phase epitaxy hardware, ultra-high vacuum crucibles, thermal insulators, rocket nozzles, and heater elements.
Amorphous graphite blocks, also known as aphanitic or cryptocrystalline graphite, are composed of microcrystalline graphite. They form a dense aggregate of tiny natural graphite crystals, giving them a gray-black or steel-gray color with a shiny metallic appearance.
Despite their metallic look, amorphous graphite blocks are soft to the touch, with a smooth texture that can easily color your hands. In addition to their appearance, amorphous graphite blocks offer several beneficial properties, including chemical stability, thermal and electrical conductivity, high-temperature resistance, and resistance to acid, alkali, corrosion, and oxidation. These characteristics make amorphous graphite blocks well-suited for applications such as casting, coatings, batteries, and carbon products, due to their small crystal size, plasticity, and excellent adhesion.
Flake graphite blocks are derived from natural graphite found in metamorphic rock. These blocks feature a layered structure where carbon atoms are arranged in a hexagonal lattice, with each layer consisting of carbon atoms in an sp2 configuration. The layers are loosely bonded by Van der Waals forces, which contributes to the flaky nature of the graphite blocks.
Flake graphite blocks have a shiny appearance and surface that makes them excellent at reflecting light. This property, combined with their ability to provide lubrication at high temperatures for extended periods, makes them highly effective in various applications. When chemicals are introduced, the Van der Waals bonds in the layers weaken, causing the volume of the graphite blocks to expand up to 300 times. This expansion capability is why flake graphite blocks are often referred to as expandable graphite.
Crystalline vein graphite is a natural form of pyrolytic carbon that can vary in particle size, appearing either flake-like with fine particles or in medium-sized particles. It is one of the most crystalline forms of graphite, with carbon purities ranging from 80% to 90%. This graphite is available in powder form, with particle sizes as small as 3 µm, and in lumps ranging from 8 cm to 10 cm.
Crystalline vein graphite, also known as plumbago, Sri Lankan graphite, or Ceylon graphite, is challenging to describe due to its unique characteristics, leading to various theories about its origins. Unlike amorphous graphite or other minerals, crystalline vein graphite is found in veins and fissures within rocks. It is formed from the deposition of graphitic carbon that has been melted by naturally occurring high temperatures. The deposits of crystalline vein graphite are exceptionally pure, often exceeding 90%, with most reaching up to 99.5% purity.
The primary application of crystalline vein graphite blocks is in electrical applications, where it is used in brushes for current-carrying electrical motors. It is also utilized in brake and clutch systems, where it lines brake shoes as a substitute for asbestos.
Synthetic graphite blocks have gained widespread popularity due to their high purity and availability. The specific form of synthetic graphite blocks often dictates their application in various industries. For instance, synthetic graphite blocks, also known as isotropic graphite, are commonly used in energy storage solutions within the solar industry. These blocks are made from petroleum coke, resulting in a graphite structure that differs slightly from other types.
One of the major applications of synthetic graphite blocks is in steel furnaces and aluminum smelters. Their high energy density, low cost, and scalability offer significant advantages in these industries. Synthetic graphite blocks are used to drive turbines, where the infrared radiation they emit is converted into electricity. Although synthetic graphite tends to be more expensive than natural graphite, the cost is mitigated by the high volume production of these blocks for energy applications, which significantly reduces their price.
One of the key methods for distinguishing between different types of graphite blocks is by their grain size. This characteristic is crucial in determining the suitability of a graphite block for a specific application. During the selection process, the grain size of the graphite block plays a significant role in assessing its appropriateness for various uses.
Fine grain graphite is characterized by its high density and is known for producing precision-machined details with exceptional finishes, which helps reduce wear. To be classified as fine grain, the graphite material must have particles ranging in size from 0.0001 inches to 0.005 inches (0.00254 mm to 0.127 mm). These particles are milled to achieve the desired size and then pressed into the shape of the graphite block. Fine grain graphite contains approximately 5% to 15% openings between the particles, although these are often too small to be easily visible. Due to its high density, fine grain graphite is commonly produced in small cross-sectional blocks.
Fine grain graphite's formability and high density make it suitable for a wide range of applications. Some common components produced from fine grain graphite blocks include crucibles, continuous casting dies, rocket nozzles, electrical brushes, heating elements, seals, and jigs.
Medium grain graphite blocks are suited for both roughing and finishing applications. They have a grain size ranging from 0.020 in to 0.062 in (0.508 mm to 1.5748 mm), with 12% to 20% of their volume being porous and visible to the naked eye. The production of medium grain graphite blocks is more cost-effective compared to fine grain graphite, as it typically involves extrusion or compression molding rather than isostatic molding. Common applications for medium grain graphite include furnaces, trays, extrusion guides, heating elements, crucibles, and self-lubricating bearings, which is one of its major uses.
Coarse grain graphite is a cost-effective option for processes that require large quantities of raw materials. The grain size of coarse grain graphite ranges from 0.040 in to 0.25 in (1.016 mm to 6.35 mm), with porosity levels between 12% and 20%. Its ability to withstand thermal shock and rapid temperature changes from molten metals makes it ideal for manufacturing crucibles, large ingot molds, and pouring troughs. The large particles of coarse grain graphite are easily visible to the naked eye, and its strength and stability are well-suited for producing large parts.
The primary application of graphite blocks is in furnaces due to their ability to withstand thermal shock and their low thermal expansion. These specific properties make graphite blocks essential for various industrial uses. Their chemical stability, ease of machining, and lightweight nature have made them a critical component in manufacturing processes.
Graphite block producers offer graphite blocks in a wide range of sizes to fit the needs of any size company. In many cases, graphite blocks are custom ordered to meet specific needs and requirements. It is this flexibility that has made graphite blocks so important and an easy way to put graphite to use.
In powder metallurgy, graphite blocks are used in sintering, where raw materials are placed on a graphite block and melted. The high temperature and oxidation resistance of graphite blocks meet the demanding requirements of the powder metallurgy industry. The blocks can be used repeatedly, which saves users on production costs.
Metals can be heated in a graphite crucible up to 2732°F (1500°C) to convert them into liquid form for graphite mold casting, a method used for casting various industrial products. Graphite molds, similar to metal molds, offer good thermal conductivity and thermochemical stability. When casting with tin bronze and aluminum iron bronze, graphite molds help eliminate defects such as shrinkage, porosity, and pinholes, while also providing improved mechanical properties.
Graphite blocks used in the manufacture of electrodes possess high electrical conductivity and refractory properties, including thermal shock resistance and low thermal expansion. These blocks are uniquely capable of withstanding the required electrical conductivity for electric arc furnaces, as well as enduring the extreme heat levels involved in the process.
Synthetic graphite blocks are primarily used as moderators or reflectors in nuclear reactors. For uranium fission to take place effectively, the neutrons generated must be slowed down by a neutron moderator, which is typically a material with low atomic weight. Initially, heavy water was used for this purpose, but it was later replaced by graphite due to its high purity. Graphite blocks used in nuclear fusion must be of exceptional purity and free of boron, as boron absorbs neutrons.
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