This article mainly describes the industrial chain, classification, production indicators, production characteristics, production process and current industry status of graphite electrodes.
Key words: graphite electrode; classification and characteristics; process flow; development status
Graphite electrode industry chain
Graphite electrode can also be called artificial graphite electrode, which refers to the use of petroleum coke and needle coke as raw materials, coal tar pitch as binder, after raw material calcination, crushing and grinding, batching, kneading, molding, roasting, impregnation, graphitization and mechanical A high temperature resistant graphite conductive material made by processing. Taking advantage of the excellent physical and chemical properties of graphite electrodes, the carbon product industry, which mainly produces graphite electrodes, has become an important part of the contemporary raw material industry.
Graphite electrode industry chain The upstream of the industrial chain is mainly raw material suppliers such as petroleum coke and needle coke, as well as production equipment suppliers. Raw materials account for a large proportion of graphite electrode production costs, accounting for more than 65%. The middle reaches are graphite electrode manufacturers. Downstream is the field of electric furnace steelmaking, electric furnace silicon smelting, electric furnace yellow phosphorus smelting and other fields.
Classification of graphite electrodes
According to the different raw materials used and the differences in the physical and chemical indicators of the finished product, graphite electrodes are divided into:Common power graphite electrode (RP), high power graphite electrode (HP) and ultra high power graphite electrode (UHP).
Ordinary power graphite electrode
It is allowed to use graphite electrodes with a current density lower than 17A/cm2; the upstream raw materials are mainly petroleum coke and coal tar pitch, and the downstream applications are mainly used in ordinary power electric furnaces for steelmaking, silicon smelting, and yellow phosphorus smelting.
High Power Graphite Electrode
It is allowed to use graphite electrodes with a current density of 18~25A/cm2; the upstream raw materials are petroleum coke and coal tar pitch, and needle coke is added, and the downstream applications are mainly used in high-power electric arc furnaces for steelmaking.
Ultra High Power Graphite Electrode
Graphite electrodes with a current density greater than 25A/cm2 are allowed; the upstream raw materials are petroleum coke, needle coke, and coal tar pitch, and the content of needle coke is greater than that of high-power graphite electrodes. The downstream applications are mainly used in ultra-high-power steelmaking electric arc furnaces.
Graphite electrode index
The main indicators to measure the quality of graphite electrodes are resistivity, bulk density, mechanical strength, thermal expansion coefficient, elastic modulus, etc. The oxidation resistance and thermal shock resistance of graphite electrodes in use are related to the above indicators. The accuracy and reliability of the connection are also important testing items.
Resistivity
The resistivity of graphite electrode is an important physical performance index and a parameter to measure the conductivity of the electrode. Refers to the resistance of the conductor to the current when the current passes through the conductor. It is numerically equal to the resistance of a conductor with a length of 1m and a cross-sectional area of 1m2 at a certain temperature, which reduces consumption during use. Usually measured by the voltage drop method, the resistivity can measure the degree of graphitization of the graphite electrode. The lower the resistivity of the graphite electrode, the higher the thermal conductivity and the better the oxidation resistance.
Bulk density
Increasing the bulk density is beneficial to reduce the porosity and improve the mechanical strength, and improve the oxidation resistance, but if it is too large, the thermal shock resistance will decrease. Therefore, other measures need to be taken to make up for this deficiency, such as increasing the graphitization temperature to increase the heat of the electrode. Conductivity and the use of needle coke as raw material to reduce the coefficient of thermal expansion of the finished product.
Mechanical Strength
The mechanical strength of graphite electrodes is divided into three types: compression resistance, flexural resistance and tensile strength. Graphite electrodes are often in danger of being broken when they are in contact, or due to the impact of slumping materials, damage caused by strong vibrations, etc. Graphite electrodes with high flexural strength are not easy to be broken.
Elastic modulus
Elastic modulus is an important aspect of mechanical properties, it is an index to measure the elastic deformation capacity of materials, and it refers to the stress-strain ratio within the elastic deformation range. The greater the modulus of elasticity, and the greater the stress required to produce elastic deformation, it is simply, the greater the modulus of elasticity for brittle materials, the smaller the modulus of elasticity for flexible materials.
Coefficient of thermal expansion
The coefficient of thermal expansion of graphite as an electrode is a very important thermal performance parameter. The lower the value, the stronger the thermal stability of Chinese products and the higher the oxidation resistance, which can be reflected in the use. lower.
The quality of graphite electrodes depends on four aspects: raw material performance, process technology, management and production equipment, among which raw material performance is the primary condition.
Ordinary power graphite electrodes are produced by ordinary grade petroleum coke, which have low physical and mechanical properties, such as high resistivity, large linear expansion coefficient, and poor thermal shock resistance, so the allowable current density is low.
High-power graphite electrodes are produced with high-quality petroleum coke (or low-grade needle coke), and their physical and mechanical properties are higher than those of ordinary power graphite electrodes, allowing greater 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 resistivity and linear expansion coefficient of the joint blank should be smaller than the electrode body, but also the joint blank should have higher tensile strength and thermal conductivity. In order to strengthen the electrode connection To ensure reliability, the joint should be equipped with a joint bolt.
Production characteristics of graphite electrodes
The main raw material for the production of graphite electrodes is petroleum coke (including needle coke). A small amount of pitch coke can be added when producing ordinary power graphite electrodes. The binder is coal tar pitch. The production characteristics of graphite electrodes are as follows:
(1) There are many production processes and a long production cycle. The production cycle of ordinary power graphite electrodes is about 45 days, and the production cycle of ultra-high power graphite electrodes requires more than 70 days. Joints that require multiple dips have a longer production cycle.
(2) Energy consumption is relatively high. 1 ton of ordinary power graphite electrode needs to consume about 6000 kW·h of electricity, thousands of cubic meters of gas or natural gas, and about 1 ton of metallurgical coke particles and metallurgical coke powder (secondary energy).
(3) There are many processes in the production of graphite electrodes, which require a lot of special mechanical equipment and kilns with special structures. The construction investment is large and the investment recovery period is long.
(4) The production process of graphite electrodes produces a certain amount of dust and harmful gases, so it is necessary to take comprehensive environmental protection measures for ventilation, dust reduction and elimination of harmful gases.
Graphite electrode production process
The process flow of producing graphite electrodes is shown in the figure, and several main production processes are as follows:
Both petroleum coke and pitch coke need to be calcined, and the calcining temperature should reach 1300°C to fully remove the volatile matter in the raw materials and improve the true density and conductivity of the coke.
(2) Crushing, screening and batching
The calcined raw materials are crushed and sieved into aggregate particles of specified size, a part of the raw materials are ground into fine powder, weighed according to the formula and aggregated to form a mixture of various particles.
(3) kneading
In a heated state, mix and knead a quantitative amount of various particle mixtures with a quantitative binder to form a plastic paste.
(4) Molding
Under the action of external pressure (compression molding or extrusion molding) or vibration (vibration molding), the paste is pressed into a green body with a certain shape and higher density.
(5) Roasting
Put the green body in a specially designed high-temperature furnace, cover the green body with filler (coke powder or river sand), and gradually raise the temperature to about 900-1100°C to carbonize the binder to obtain a roasted product.
(6) Dipping
In order to improve the bulk density and mechanical strength of the product, the roasted product is put into an autoclave, 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 a high-density and high-strength joint blank, impregnation requires Repeat 2 to 3 times.
(7) Graphitization
Put the roasted product into the graphitization furnace (covered with insulation material), and use the heating method of direct energization to convert the roasted product into a graphite crystal structure, so as to obtain the physical and chemical properties required for artificial graphite electrodes.
(8) Machining
According to the requirements of use, the graphitized blank is subjected to surface turning, end surface processing and screw hole processing for connection, and the joint for connection is processed again.
(9)Packing after passing the inspection is the finished product.
Status quo of graphite electrode industry
My country’s current graphite electrode enterprises are mainly distributed in North China, Central China, East China, Northeast China, Northwest China and other regions. Among them, North China is the most concentrated area of graphite electrode production capacity in my country, accounting for 29.5% of domestic production capacity, mainly small and medium-sized enterprises; followed by Northeast China, which accounts for 22% of domestic production capacity; and then Northwest China, which accounts for 14% of domestic production capacity. The other is Central China and East China. The participants are mainly private enterprises, and China’s graphite electrode production accounts for about 50% of the global graphite electrode production.
Graphite electrodes are high-energy and high-emission industries, and future expansion may be limited under the background of restrictions on double highs. The production of one ton of graphite electrode consumes about 1.69 tons of standard coal. According to the conversion method of 2.66 tons of carbon dioxide per ton of standard coal, the carbon emission of a single ton of graphite electrode is 4.48 tons, so future expansion may be limited.
Domestic mainstream graphite electrode manufacturers include Fangda Carbon, Jilin Carbon, Kaifeng Carbon, Liaoning Dantan, Nantong Carbon, etc. As a leading enterprise in China’s graphite electrode industry, Fangda Carbon’s graphite electrode market share in China exceeds 20%, and its graphite electrode production capacity ranks third in the world. In the ultra-high-power graphite electrode market, due to the high technical requirements for ultra-high-power graphite electrodes, leading companies in the industry with corresponding technical strength have released production capacity, and leading companies account for more than 80% of the market share of ultra-high-power products. Large-scale graphite electrode companies in the middle reaches have strong bargaining power over the downstream steelmaking industry, and require downstream customers to pay for delivery without providing an account period.