Graphite electrode can also be called artificial graphite electrode, refers to petroleum coke, needle coke as raw materials, coal tar pitch as binder, after calcination of raw materials, crushing grinding powder, batching, mixing, molding, roasting, impregnation, graphitization and mechanical processing and made of a high-temperature resistant graphite conductive material.
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Using the excellent physical and chemical properties of graphite electrode to produce the main varieties of graphite electrode carbon products industry has become an important part of the contemporary raw material industry.
The upstream of the industrial chain are mainly suppliers of petroleum coke, needle coke and other raw materials and production equipment suppliers. Raw materials account for a large proportion of graphite electrode production cost, accounting for more than 65%. The middle reaches are graphite electrode production enterprises, and downstream is the electric furnace steelmaking, electric furnace smelting silicon, electric furnace smelting yellow phosphorus and other fields.
According to the difference of raw materials and physical and chemical indicators of finished products, graphite electrodes are divided into: Ordinary power graphite electrode (RP), high power graphite electrode (HP) and ultra-high power graphite electrode (UHP).
Graphite electrode classificationApplicationsOrdinary power graphite electrodeGraphite electrodes with current densities below 17A/ cm 2 are allowed; The upstream raw materials are mainly petroleum coke and coal tar pitch, and the downstream applications are mainly used in ordinary power electric furnaces such as steel making, silicon smelting and yellow phosphorus smelting.High power graphite electrodeGraphite electrodes with current densities of 18 to 25A/ cm 2 are allowed; The upstream raw materials are petroleum coke and coal tar pitch, adding needle coke, and the downstream application is mainly used in high power electric arc furnace for steelmaking.Ultra-high power graphite electrodeGraphite electrodes with current densities greater than 25A/ cm 2 are allowed; The upstream raw materials are petroleum coke, needle coke and coal tar pitch, whose needle coke content is higher than that of high-power graphite electrode, and the downstream application is mainly used in ultra-high power steel making arc furnace.The main indicators to measure the quality of graphite electrode are resistivity, volume density, mechanical strength, thermal expansion coefficient, elastic modulus, etc. Graphite electrode in the use of oxidation resistance and thermal shock resistance are related to the above several indicators, the accuracy of machining products and the reliability of the connection is also an important test item.
● The resistivity
It is the resistance of a conductor to current when it passes through it. It is numerically equal to the resistance of a conductor with a length of 1m and a cross-sectional area of 1m2 at a given temperature, reducing consumption in use. Usually measured by voltage drop method, the size of resistivity can measure the graphitization degree of graphite electrode, the lower the resistivity of graphite electrode, the higher the thermal conductivity, the better the oxidation resistance.
● Density of volume
Increasing bulk density is beneficial to reduce porosity, improve mechanical strength and improve oxidation resistance. However, if the bulk density is too large, the thermal shock resistance will decrease. Therefore, other measures should be taken to make up for this deficiency. Such as increasing the graphitization temperature to increase the thermal conductivity of the electrode and using needle coke as raw material to reduce the thermal expansion coefficient of the finished product.
● Mechanical strength
Mechanical strength of graphite electrode is divided into compressive, flexural and tensile strength of 3 kinds, the main determination of flexural strength, flexural strength is the graphite electrode in use and break related performance indicators. In the electric furnace, when the electrode and the non-conductive object contact, or due to the collapse of the collision, strong vibration and other reasons, the graphite electrode often has the risk of being broken, the graphite electrode with high flexural strength is not easy to be broken.
● Modulus of elasticity
Elastic modulus is an important aspect of mechanical properties. It is an index to measure the elastic deformation ability of materials, and refers to the stress-strain ratio within the elastic deformation range. The larger the elastic modulus, and the greater the elastic deformation to produce the desired stress, the larger the elastic modulus of the brittle material, and the smaller the elastic modulus of the flexible material.
● Thermal expansion coefficient
Graphite as the thermal expansion of the electrode influence coefficient is very important thermal performance parameters, the lower the value, indicates that the thermal stability of Chinese products is stronger and stronger, the higher the oxidation resistance, performance in use can reflect the less broken, the lower the consumption.
The quality of graphite electrode depends on raw material performance, process technology, management and production equipment, among which raw material performance is the primary condition. Ordinary power graphite electrode, using ordinary grade petroleum coke production, its physical and mechanical properties are low, such as high resistivity, large linear expansion coefficient, poor thermal shock resistance, so the allowed current density is low.
High power graphite electrode using high quality petroleum coke (or low grade needle coke) production, its physical and mechanical properties are higher than ordinary power graphite electrode, allowing a larger current density. Ultra-high power graphite electrodes must be produced using high grade needle coke. The joint quality of high-power and ultra-high-power graphite electrodes is particularly important. Not only the electrical resistivity and linear expansion coefficient of the joint billet are smaller than the electrode body, but also the joint billet should have higher tensile strength and thermal conductivity. In order to enhance the reliability of the electrode connection, the connector should be equipped with a connector bolt.
The main raw material for the production of graphite electrode is petroleum coke (including needle coke). A small amount of asphalt coke can be added to the production of ordinary power graphite electrode. The bonding agent is coal asphalt.
(1) The production process is multiple and the production cycle is long. The production cycle of ordinary power graphite electrode is about 45 days, and the production cycle of ultra-high power graphite electrode is more than 70 days. Joints that require multiple impregnations have longer production cycles.
(2) high energy consumption, 1t ordinary power graphite electrode needs to consume about kW•h power, gas or natural gas thousands of cubic meters, metallurgical coke particles and metallurgical coke powder (secondary energy) about 1t.
(3) The production of graphite electrode process, need a lot of special machinery and equipment and special structure of the kiln, construction investment is large, investment payback period is long.
(4) Graphite electrode production process produces a certain amount of dust and harmful gases, so it is necessary to take sound ventilation dust removal and elimination of harmful gas environmental protection measures.
The production process of graphite electrode is shown in the figure. The main production processes are as follows:
(1) Calcination
Petroleum coke or asphalt coke need to be calcined, calcination temperature should reach ℃, in order to fully remove volatile in the raw material, improve the true density and electrical conductivity of coke.
(2) Crushing, screening and ingredients
The calcined raw material is broken and sieved into aggregate particles of specified size, and a part of the raw material is ground into fine powder. After weighing according to the formula, the mixture of various particles is assembled.
(3)Kneading
In the state of heating, the quantitative mixture of various particles and quantitative binder are mixed and knead into plastic paste.
(4) Shape
Under the action of external pressure (molding molding or extrusion molding) or vibration (vibration molding), the paste is pressed into a certain shape and a higher density of the blank.
(5) Roasting
The billet was placed in a specially designed high temperature furnace, and the billet was covered with filling material (coke powder or river sand), and the temperature was gradually raised to about 900 ~ ℃ to make the binder carbonized, so as to obtain the roasted product.
(6) Soaking
In order to improve the bulk density and mechanical strength of the product, the roasted product is loaded into the autoclast, and the liquid impregnating agent is pressed into the pores of the roasted product. After impregnation, it should be roasted again. In order to obtain the joint blank with high density and high strength, the impregnation needs to be repeated 2 ~ 3 times.
(7) Graphitization
Load the calcined product into the graphitization furnace (covered with insulation material), and use direct electrifying heating method. The physical and chemical properties of artificial graphite electrode were obtained by transforming the calcined product into an ink-crystal structure.
(8) Mechanical processing
According to the requirements of use, the graphitized blank is processed for surface turning, end face processing and screw hole processing for connecting, and then processed for connecting joints.
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(9) The finished product shall be packaged after passing the inspection.
At present, China’s graphite electrode enterprises are mainly distributed in North China, Central China, East China, Northeast China, Northwest China and other regions. North China is the most concentrated area of graphite electrode production capacity in China, accounting for 29.5% of China’s production capacity, mainly in small and medium-sized enterprises;
The northeast region accounts for 22% of China’s production capacity. Then there is the northwest, which accounts for 14% of China’s capacity. The other is central China, East China. The participants are mainly private enterprises, and China accounts for about 50% of the global graphite electrode production.
Graphite electrode is an industry with high energy consumption and high emission, and its future production expansion may be limited under the background of double high limit. It takes 1.69 tons of standard coal to produce one ton of graphite electrodes. According to the conversion method of 2.66 tons of carbon dioxide for 1 ton of standard coal, the carbon emission of a single ton of graphite electrode is 4.48 tons, so the future expansion of production may be limited.
China’s mainstream graphite electrode manufacturers include Fangda carbon, Jilin carbon, Kaifeng carbon, Liaoning Dan carbon, Nantong carbon and so on. As a leading enterprise in China’s graphite electrode industry, Fangda Carbon’s graphite electrode market share exceeds 20% in China, and its graphite electrode capacity ranks the third in the world.
In the ultra-high power graphite electrode market, due to the high technical requirements of ultra-high power graphite electrode, leading enterprises with corresponding technical strength in the industry account for more than 80% of the market share of ultra-high power products by releasing capacity.
Large graphite electrode companies in the midstream have strong bargaining power over the downstream steelmaking industry, requiring downstream customers to receive payment and delivery without providing account period.
The supply side of the graphite electrode market is still expected to continue to shrink. On the one hand, the cost is high, the demand is not good, and the production enthusiasm of graphite electrode enterprises is insufficient. In order to avoid inventory accumulation, there are plans to reduce production or suspend the production of pressure type and other processes.
Graphite can prove tricky to machine, particularly for EDM electrodes that require outstanding accuracy and structural consistency. Here are five key points to keep in mind when working with graphite:
Graphite grades are visually difficult to distinguish, but each features unique physical characteristics and properties. Grades of graphite are grouped into six classifications by average particle size, but only the smaller three (with particles measuring 10 microns or smaller) see regular use in modern EDM. How grades rank within classifications is an indicator of potential applications and performance.
According to an article by Doug Garda (of Toyo Tanso when he contributed to our sister publication, MoldMaking Technology, but now of SGL Carbon), roughing uses grades with particle sizes ranging from 8 to 10 microns. Less-precise finishing and detailing applications employ grades with 5- to 8-micron particle sizes. Electrodes made from these grades are often used to create forging dies and die-cast molding dies, or used in less-complex powdered and sintered metal applications.
Fine detailing work and smaller, more intricate features are better suited for grades with 3- to 5-micron particle sizes. Applications for electrodes in this range include wire cutting and aerospace.
Ultra-fine, precision electrodes, which employ graphite grades with particle sizes of 1 to 3 microns, are often required for exotic aerospace metal and carbide applications.
Writing for MMT, Jerry Mercer of Poco Materials identifies particle size, flexural strength and Shore hardness as three key determinants of performance during electrode machining. However, the graphite’s microstructure is often the limiting factor in how the electrode performs during final EDM operations.
In a different article for MMT, Mercer says flexural strength should be above 13,000 psi to ensure graphite machinability into deep, thin ribs without breakage. The graphite electrode manufacturing process is lengthy and can require detailed, difficult-to-machine features, so ensuring durability like this helps to keep costs low.
Shore hardness measures the machinability of graphite grades. Mercer warns that graphite grades which are too soft can clog cutter flutes, slowing down the machining process or packing the hole with dust that can stress the hole walls. Reducing feeds and speeds in these situations can prevent errors, but this will increase machining time. Hard, small-grain graphites can also cause material at the rim of the hole to chip during machining. These materials can also be very abrasive to cutters, causing wear that impacts the integrity of the hole diameter and increases the cost of the job. Generally, avoiding deflection at high hardness values requires reducing machining feeds and speeds by 1% for each point of Shore hardness higher than 80.
Due to the way EDM produces a mirror image of the electrode in the part being machined, Mercer also says a tightly packed, uniform microstructure is vital for graphite electrodes. Uneven particle boundaries increase porosity, thus increasing the erosion of particles and hastening the failure of the electrode. Non-uniform microstructures also result in uneven surface finishes during initial electrode machining — a problem exacerbated on high-speed machining centers. Hard spots in graphite can also cause tool deflection, throwing the final electrode out-of-spec. This deflection can be slight enough that angled holes appear deceptively straight at the point of entry.
There are dedicated machines for graphite machining, but while these will greatly speed along production, they are not the only machines manufacturers can use. Aside from dust control (covered later in the article), past MMS articles have reported on the benefits of a machine with a fast spindle and a control with a high processing speed for graphite manufacturing. Ideally, the fast control should also have look-ahead functionality, and the user should utilize toolpath optimization software.
When impregnating graphite electrodes — that is, filling the holes of the graphite microstructure with micron-sized particles — Garda recommends using copper as it can stabilize machining of exotic, copper and nickel alloys, such as those used in aerospace applications. Copper-impregnated graphite grades create finer finishes than non-impregnated grades of the same classification. They also enable stable machining when working with unfavorable conditions such as poor flushing or inexperienced operators.
Although synthetic graphite — the kind used to fabricate EDM electrodes — is biologically inert and thus not as initially hazardous to humans as some other materials, improper ventilation can still cause problems, according to a third article from Mercer. Synthetic graphite is electrically conductive, posing issues around equipment that can short out upon contact with foreign conductive materials. Also, graphite impregnated with materials like copper and tungsten requires additional care.
Graphite dust is invisible to the human eye in small concentrations, Mercer explains, but it can still cause stinging, watering and redness. Contact with the dust may be abrasive and mildly annoying, but absorption is unlikely. The exposure guideline for time-weighted average (TWA) for graphite dust over an 8-hour period is 10 mg/m3, which is a visible concentration that should never occur with a dust collection system in use.
Excessive exposure to graphite dust over extended periods of time can lead to inhaled particles of graphite being retained in the lungs and bronchi. This causes a serious, chronic form of pneumoconiosis called graphitosis. Graphitosis is typically associated with natural graphite, but has happened in rare circumstances with synthetic graphite.
Accumulated dust in the workplace is highly flammable, and (in a fourth article) Mercer says it can explode under certain circumstances. Dust fires and deflagration occur when ignition meets a sufficient concentration of fine particulates suspended in air, with explosions more likely to occur if the dust is dispersed in significant volume or is in a contained area. Controlling any one hazardous element (fuel, oxygen, ignition, dispersion or confinement) drastically lowers the possibility of a dust explosion. Most times, the industry focuses on fuel by removing dust at the source through ventilation, but shops should consider all factors for maximum safety. Dust control equipment should also have explosion relief vents or an explosion suppression system, or be installed in an oxygen-deficient environment.
Mercer identifies two primary methods for controlling graphite dust: high-velocity air systems with dust collectors — which can be fixed or portable, depending on the application — and wet systems that saturate the area around the cutter with fluid.
Shops doing a limited amount of graphite machining can use a portable unit with a high-efficiency particulate air (HEPA) filter that can be moved from machine to machine. However, shops that machine significant amounts of graphite should generally use a fiixed system. The minimum air speed for capturing dust is 500 feet per minute, with speed increasing inside the duct to a minimum of feet per second.
Wet systems flush away dust at the risk of the fluid “wicking” (being absorbed) into the electrode material. Not removing the fluid prior to placing the electrode in the EDM can result in contamination of the dielectric oil. Operators should use water-based solutions, as these wick less easily than oil-based solutions. Drying the electrode before EDM use typically involves placing the material in a convection oven for about one hour at a temperature slightly exceeding the vaporization point of the solution. The temperature should never exceed 400 degrees, as this makes material oxidize and erode. Operators should also never use compressed air to dry the electrode, as the air pressure merely forces the fluid deeper into the electrode structure.
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