graphite scrap block factory

Pubdate: 07-30 2021

Graphite scrap block function

Magnesia-carbon brick is a graphite scrap block factory using high-melting alkaline oxide magnesium oxide (melting point 2800℃) and high-melting carbon materials that are difficult to be infiltrated by slag as raw materials, adding various non-oxide additives. A non-burning carbon composite refractory material combined with a carbon binder. Magnesia-carbon bricks are mainly used for the lining of converters, AC electric arc furnaces, and DC electric arc furnaces, and the slag line of ladle.

As a composite refractory material, magnesia carbon bricks effectively utilize the strong slag corrosion resistance of magnesia and the high thermal conductivity and low expansion of carbon, which compensates for the biggest shortcomings of poor spalling resistance of magnesia.

Its main characteristics are: good high temperature resistance, strong slag resistance, good thermal shock resistance, and low high temperature creep.

graphite scrap block factory

graphite scrap block preparation process

The traditional magnesia carbon brick manufactured by the graphite scrap block factory using synthetic tar binders according to the cold mixing process hardens during the tar damage process and obtains the necessary strength, thus forming isotropic glassy carbon. This kind of carbon does not show thermoplasticity, which can relieve a large amount of stress in a timely manner during the baking or operation of the lining. The magnesia-carbon brick produced with the asphalt binder has high high temperature plasticity due to the anisotropic graphitized coke structure formed during the asphalt carbonization process.

graphite scrap block factory

graphite scrap block production process

The main raw materials of MgO-C bricks include fused magnesia or sintered magnesia, flake graphite, organic binders and antioxidants.

Magnesia

Magnesia is the main raw material for the production of MgO-C bricks, which can be divided into fused magnesia and sintered magnesia. Compared with sintered magnesia, fused magnesia has the advantages of coarse periclase crystal grains and large particle volume density. It is the main raw material used in the production of magnesia carbon bricks. The production of ordinary magnesia refractories requires high-temperature strength and corrosion resistance for magnesia raw materials. Therefore, attention is paid to the purity of magnesia and the C/S ratio and B2O3 content in the chemical composition. With the development of the metallurgical industry, the smelting conditions are becoming more and more demanding. In addition to the chemical composition, the magnesia used in the MgO-C bricks used in the metallurgical equipment (converter, electric furnace, ladle, etc.) requires high density and Great crystallization.

Carbon source

Whether in traditional MgO-C bricks or low-carbon MgO-C bricks that are used in large quantities, flake graphite is mainly used as its carbon source. Graphite, as the main raw material for the production of MgO-C bricks, mainly benefits from its excellent physical properties: ① non-wetting to slag. ②High thermal conductivity. ③Low thermal expansion. In addition, graphite and refractory materials do not eutectic at high temperatures, and have high refractoriness. The purity of graphite has a greater impact on the performance of MgO-C bricks. Generally, graphite with a carbon content of more than 95%, preferably more than 98% is used.

In addition to graphite, carbon black is also commonly used in the production of magnesia carbon bricks. Carbon black is a highly dispersed black powdery carbonaceous material produced by the thermal decomposition or incomplete combustion of hydrocarbon hydrocarbons. The carbon black has fine particles (less than 1μm), large specific surface area, and the mass fraction of carbon is 90~ 99%, high purity, high powder resistivity, high thermal stability, low thermal conductivity, it is difficult to graphitize carbon. The addition of carbon black can effectively improve the spalling resistance of MgO-C bricks, increase the amount of residual carbon, and increase the density of the bricks.

Binding agent

Commonly used binders for the production of MgO-C bricks include coal tar, coal tar and petroleum pitch, as well as special carbon resins, polyols, pitch-modified phenolic resins, synthetic resins, etc. The binding agent used has the following types:

1) Asphalt substances. Tar pitch is a kind of thermoplastic material. It has the characteristics of high affinity with graphite and magnesium oxide, high residual carbon rate after carbonization, and low cost. It has been used in large quantities in the past; but tar pitch contains carcinogenic aromatic hydrocarbons, especially the content of benzopyridine. High; due to the strengthening of environmental awareness, the use of tar pitch is now decreasing.

2) Resin substances. Synthetic resin is made by the reaction of phenol and formaldehyde. It can mix well with refractory particles at room temperature. After carbonization, the residual carbon rate is high. It is currently the main binder for the production of MgO-C bricks; but it is formed after carbonization. The glassy network structure is not ideal for the thermal shock resistance and oxidation resistance of refractory materials.

3) On the basis of asphalt and resin, the substance obtained after modification. If the bonding agent can be carbonized to form an inlaid structure and form carbon fiber material in situ, then this bonding agent will improve the high temperature performance of the refractory material.

Antioxidants

In order to improve the oxidation resistance of MgO-C bricks, a small amount of additives are often added. Common additives are Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B4C , BN and the recently reported Al-BC and Al-SiC-C additives [5-7]. The principle of action of additives can be roughly divided into two aspects: On the one hand, from the perspective of thermodynamics, that is, at working temperature, additives or additives react with carbon to form other substances, and their affinity for oxygen is greater than that of carbon and oxygen. , It takes precedence over carbon to be oxidized to protect carbon; on the other hand, from the perspective of kinetics, the chemical Density, block pores, hinder the diffusion of oxygen and reaction products, etc.

graphite scrap block factory

graphite scrap block application

The refractory materials used in the early ladle slag line were directly combined with magnesia-chrome bricks, and fused and combined with high-quality alkaline bricks such as magnesia-chrome bricks. After the successful use of MgO-C bricks on the converter, the refining ladle slag line also began to use MgO-C bricks, and achieved good results. my country and Japan generally use resin-bonded MgO-C bricks with a carbon content of 12% to 20%, while Europe mostly uses asphalt-bonded MgO-C bricks with a carbon content of about 10%.

Japan’s Sumitomo Metals Kokura Iron and Steel Plant uses MgO-C bricks with a MgO content of 83% and a C content of 14-17% in the VAD slag line instead of directly bonded magnesia chrome bricks. The life of the slag line is increased from 20 to 30 -32 times [9]. The graphite scrap block factory LF refining ladle uses MgO-C bricks instead of magnesia-chrome bricks, and the life of the slag line is increased from 20-25 times to 40 times, which has achieved good results. Osaka Kiln Refractories Company studied the effects of carbon content and antioxidant types on the oxidation resistance, slag resistance and high temperature flexural strength of MgO-C bricks. Research believes that: MgO-C bricks made of a mixture of fused magnesia and sintered magnesia, plus 15% phosphorous flake graphite and a small amount of magnesium aluminum alloy as antioxidants, have a good use effect, and the capacity is 100 tons. The LF ladle slag line is used, compared with the MgO-C brick with 18% C content without antioxidant, the damage rate is reduced by 20-30%, and the average erosion rate is 1.2-1.3mm/furnace.

Since the use of MgO-C bricks instead of magnesia-chrome bricks in my country’s refined ladle slag line bricks, the comprehensive use effect is obvious. The 300t ladle slag line of Baosteel Co., Ltd. started to use MT-14A magnesia-carbon bricks in July 1989, and the life of the slag line was maintained at more than 100 times; the 150T electric furnace ladle slag line used low-carbon magnesia-carbon bricks to smelt cord steel, and the tapping temperature 1600℃~1670℃, the obvious effect has been achieved.

graphite scrap block factory

graphite scrap block

With the advancement of smelting technology and the new requirements for refractory materials, traditional magnesia-carbon bricks have found the following problems in the long-term application and practice process:

①The high thermal conductivity increases the heat loss and increases the tapping temperature, which brings about an increase in energy consumption, and at the same time increases a series of problems such as corrosion of refractory materials;

②As the lining material of special refining furnaces, such as smelting high-quality clean steel and ultra-low carbon steel in VOD refining ladle, it will cause carbon increase problems;

③ Consume a lot of precious graphite resources. In view of the above situation, in recent years, the development of low-carbon magnesia-carbon bricks with low carbon content and excellent performance for refining ladle has attracted the attention of domestic and foreign industries.

The main problems caused by the decrease of carbon content in magnesia carbon bricks are the decrease of thermal shock stability and slag resistance. As we all know, after the carbon content in the magnesia-carbon brick is reduced, the thermal conductivity of the brick is reduced, and the elastic modulus is increased, which makes the thermal shock resistance of the brick worse. After the carbon content is reduced, the wettability of the slag and molten steel with the material is enhanced, and the resistance of the slag and the permeability of the molten steel to the material deteriorates.

The understanding of solving these problems mainly includes the following three aspects:

①Improve the thermal shock stability of magnesia-carbon bricks by improving the carbon structure of the bonded carbon: the traditional magnesia-carbon bricks are mostly phenolic resin. Carbon bricks are brittle and have a high elastic modulus, which is unfavorable to the thermal stability of the product, and the high-temperature strength of the product is also low. After introducing graphitized carbon precursor into phenolic resin, this composite binder can be carbonized into secondary carbon with fluid or mosaic structure under the environment of magnesia carbon brick use, or nano carbon fiber can be formed in situ. The improvement of the carbon structure and the reinforcement of the formation of nano-carbon fibers can improve the thermal shock stability and high-temperature strength of low-carbon magnesia-carbon bricks;

②Optimizing the matrix structure of magnesia-carbon bricks: the thermal shock stability and slag permeability resistance of magnesia-carbon bricks mainly depend on the composition and structure of the matrix. How to increase the ratio of aggregate particles and carbon particles when the carbon content is greatly reduced Contact frequency, that is, reducing the scale of carbon particles and ensuring their high dispersion, is one of the important measures to improve the thermal shock stability and slag penetration resistance of low-carbon magnesia-carbon bricks. Controlling the size, shape and distribution of pores by adjusting the particle size composition of the matrix ingredients will also have a significant impact on the thermal conductivity of the material;

③Using high-efficiency antioxidants: As the carbon content in magnesia-carbon bricks decreases, the oxidation protection of carbon is particularly important, so it is also necessary to use suitable high-efficiency antioxidants.


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