Magnesium Carbon Refractory Production Process And Commonly Used Raw Materials in Detail

Magnesium carbon refractories were developed for electric furnace application in Japan in the 1970s, and the first applicability test was carried out on the electric furnace in 1970. After six years of applicability test, magnesium carbon refractories were formally popularized and applied on the electric furnace. Compared with other carbon materials, magnesium carbon refractory materials added natural scale graphite and carbon binding agent, so that it has excellent thermal conductivity, smaller thermal expansion rate, greatly enhanced the performance of magnesium carbon bricks, especially to improve its resistance to slag erosion and thermal shock stability. It has been widely used in ultra-high-power electric arc furnace furnace wall, roof, badly eroded high-temperature hot spots, slag line and steel outlet parts, and also used in converter mouth, steel outlet side, trunnion wall and melting pool, etc., as well as the slag line of ladle refining furnace. Magnesium and carbon refractories production materials and processes are as follows:

The main raw material for the production of magnesium carbon refractories is magnesium sand. As the quality of magnesium sand plays a great role in influencing the performance of magnesium carbon refractories, so in the production, the selection of reasonable magnesium sand has become the first step in the production of high-quality magnesium carbon refractories. Commonly used magnesium sand for fused magnesium sand and sintered magnesium sand, they have different characteristics, its mineral composition is mainly magnesite. In the production of magnesium carbon refractories, the performance parameters of magnesium sand are mainly considered as follows:

①Purity of magnesium sand (MgO content).

② impurity phase and its content;

③ magnesia bulk density, porosity and magnesite grain size, etc.

The purity of magnesium sand plays an important role in the slag erosion resistance of magnesium carbonaceous refractories, which is because when the MgO content is very high, the impurity phase is relatively reduced, the MgO crystals are divided by the silicate phase as the impurity phase to reduce the degree of MgO crystals as the impurity phase is a direct bond, so it improves the slag erosion resistance of magnesium carbonaceous refractories.

(High-purity magnesium sand balls)

The impurity phases in magnesium sand are mainly SiO₂, CaO, B₂O₃, Fe₂O₃ and so on, if magnesium sand contains high impurities, especially B₂O₃, it will have a negative impact on the refractoriness and high-temperature performance of magnesium carbonaceous refractory materials, and the impurity phases will have an effect from the following aspects:

① The high content of impurity phase will reduce the degree of direct bonding of MgO crystals;

② SiO₂, CaO, etc. will form eutectic with MgO at high temperature;

③ SiO₂, Fe₂O₃ and other impurities will preferentially react with C at high temperatures, making pores in the magnesium-carbon bricks and reducing the resistance of magnesium-carbon refractories to slag erosion.

(Fused large crystalline magnesia)

Magnesia carbonaceous refractories in the process of use, slag will penetrate into the magnesia particles through the pores and magnesite grain boundaries with magnesite crystals to produce a reaction, resulting in its destruction, especially when the magnesia also has a high level of impurities such as CaO, SiO₂, etc., which will accelerate the rate of destruction, resulting in the magnesia in the magnesia of magnesia crystals are continuously eroded, flaking off into the slag.

Therefore, magnesia with high bulk density has less relative impurity content, which can reduce the pathway to be eroded by dissolved slag and improve the slag erosion resistance of magnesium carbonaceous refractories. At the same time, larger magnesite grains can improve the degree of direct bonding between grains, reduce the area of grain boundaries, and reduce the path of slag penetration to the grain boundaries. Fused magnesia grain size is larger and the degree of direct bonding between the grains is higher, in the production of total general choice of fused magnesia as raw material to improve the slag erosion resistance of products. Therefore, in the production of high-quality magnesium carbonaceous refractories, must be selected with high bulk density and high purity of magnesium sand, for example, the MgO content greater than or equal to 97%, CaO/SiO₂ is not less than 2, the bulk density is not less than 3.34g/cm³, the porosity is not greater than 3% and the grain boundaries of the magnesium sand is well developed. However, in actual production, due to the magnesium carbonaceous refractories used in different parts of the different requirements for its performance. Therefore, according to the actual situation to choose the magnesium sand of comparable quality, in line with the principle of reducing costs, reducing the consumption of high-quality resources, in favor of sustainable development.

Carbonaceous materials for the preparation of MgO-C refractories are mainly flake graphite.

Flake graphite is divided into four categories according to fixed carbon content: high purity graphite, high carbon graphite, medium carbon graphite and low carbon graphite.

The main parameters affecting the performance of magnesium carbonaceous refractories are fixed carbon content, particle size, ash composition and its content, particle shape, volatile matter and moisture. Among them, the fixed carbon refers to the graphite in addition to the volatile fraction, ash components; volatile fraction refers to the graphite volatile organic and inorganic substances. In general, the higher the fixed carbon content of graphite is required, the magnesium carbon refractories prepared have excellent structure during use at high temperatures, and the mechanical properties of the products such as high temperature flexural strength are better.

With different purity of graphite as carbon raw materials produced magnesium carbonaceous refractories, there are obvious differences in the structure. The purer the graphite, the magnesium carbon refractories produced the more excellent resistance to slag erosion, the lower the purity of graphite, the production of magnesium carbon refractories at high temperatures, due to the impurity phase melting into the glass phase and magnesium sand or carbon reaction, resulting in defects within the deterioration of the products of the structure of the localization of the product, reducing the high temperature strength of the product and so on.

The volatile components in graphite will produce more volatiles in the heat treatment process, resulting in increased porosity of the products, reducing the performance of the products.

The particle size of graphite has a great influence on the thermal shock stability and oxidation resistance of the products. It is generally believed that the larger the scale of flake graphite, the better the thermal shock stability and oxidation resistance of the product, which is due to the large scale graphite has a higher thermal conductivity and smaller specific surface area. General requirements, the production of magnesium carbonaceous refractories with flake graphite particle size greater than 115 mesh. The thickness of the flake graphite also has an impact on the performance of the products, the smaller the thickness of the flake graphite, the effective area of its end surface oxidized is reduced, can improve the oxidation resistance of the products. In recent years, due to the development of low carbon magnesium carbon bricks, the carbon content is reduced, in order to ensure the uniform distribution of graphite in the products, the particle size has a tendency to reduce.

(Scalar graphite)

Ash is the residue of graphite after oxidation treatment. The more ash in graphite, the lower the slag resistance of magnesium-carbon products. In addition, impurities also have a certain effect on the oxidation resistance of graphite. Its role can be divided into two aspects. On the one hand, some of the interspersed oxides have a catalytic effect on the oxidation of graphite; on the other hand, the ash of graphite has an effect on the thickness of the decarburized layer formed after the oxidation of magnesium carbonaceous refractories, thus affecting their oxidation resistance. However, it is not the case that the higher the purity of the graphite produced by the MgO-C refractories have better oxidation resistance.

The development of binding agent plays a crucial role for magnesium carbon bricks. The binding agent will not only affect the preparation process of magnesium-carbon bricks, but also affect the microstructure of the products and thus affect their performance. Therefore, the selection of suitable binding agent for the preparation of magnesium carbon bricks plays an important role.

As the binding agent on the performance of magnesium-carbon bricks and process has a great impact, according to the preparation process of magnesium-carbon bricks, its requirements for the binding agent are: small wetting angle of graphite and magnesium sand, can be well combined with graphite and magnesium sand particles, viscosity is small and strong mobility; high temperature heat treatment after the products to maintain high strength, so that it does not produce expansion or contraction; the rate of residual carbon should be high, and no pollution to the environment. Magnesium carbonaceous refractories commonly used binding agent has the following three main types:

① Asphalt binding agent: the main use of tar pitch, it is a thermoplastic material, can be very good with magnesium sand, graphite and other combinations and high temperature heat treatment after a high rate of carbon residue, the price is cheap and low, had been widely used by refractory enterprises. However, due to the increase of people's awareness of environmental protection and tar pitch on the environmental pollution of the intensification of tar pitch, so that now the use of tar pitch decline.

② Resin binding agent: This is a kind of binding agent mainly used by current magnesium-carbon brick manufacturers, which is divided into thermoplastic phenolic resin and thermosetting phenolic resin. At room temperature, phenolic resin can combine well with magnesium sand, graphite and other particles, and has the advantage of high carbon residual rate after high temperature heat treatment. However, due to the phenolic resin after carbonization of the matrix formed by the glassy structure, so that the magnesium carbon brick oxidation resistance and thermal shock stability is reduced.

③ Modified asphalt and modified resin: the shortcomings of tar pitch and phenolic resin in the process of use, making people modify the existing tar pitch and phenolic resin. After modification of the asphalt and resin, after high temperature carbonization can form some in situ generation of carbon fibers or mosaic structure rather than poor glassy structure, which will help to improve the high-temperature performance of magnesium-carbon bricks, such as improving oxidation resistance and thermal shock stability.

Due to the addition of graphite in magnesium carbon bricks, it has many excellent properties, but due to the graphite in the use of magnesium carbon bricks is easy to be oxidized, so that its organizational structure is damaged, easy to be slag liquid and metal solution erosion, which makes the magnesium carbon bricks are destroyed, reducing the service life of the magnesium carbon bricks and the use of performance.

In order to ensure the performance of magnesium-carbon bricks, to protect its graphite from oxidation, often in the preparation of magnesium-carbon bricks to add a small amount of additives (also called antioxidants). The more commonly used additives are metal Al powder, metal Mg powder, Si powder, SiC powder and Al-Mg alloy, Al-Mg-Ca composite powder. In addition to preventing the graphite from being oxidized, the additives also affect the performance of magnesium carbon bricks in other ways, the main role is as follows:

①Antioxidant effect to stop the oxidation of carbon.

② Reduce the loss of carbon in carbon composite refractories by reducing CO(g) to produce solid carbon.

③Reduces porosity and improves the density of the product, and also improves oxidation resistance.

④Promote the crystallization of amorphous carbon generated by the binding agent.

⑤ Improve the oxidation resistance and slag resistance of the products by forming a surface protective layer.

The antioxidant effect of an antioxidant is usually considered in two ways: first, it is oxidized in preference to carbon and thus protects the carbon, and second, it forms some kind of compound to block the pores.

The preparation process of magnesium carbon bricks mainly includes the batching and mixing of mud, molding and heat treatment.

Magnesium carbonaceous refractories production process flow chart

In the preparation process of magnesium-carbon refractories, only by using the appropriate critical particle size of magnesium sand, graphite addition, mixing time and molding pressure, etc., can we get the optimal performance of magnesium-carbon refractory products.

In the production of magnesium carbon refractory materials, usually according to the need to use magnesium carbon refractory materials parts of the magnesium sand critical particle size. In the temperature gradient is larger, the product undergoes strong thermal shock parts of the use of critical particle size smaller magnesium sand, in order to enhance its thermal shock resistance; in the erosion of serious parts of the magnesium sand needs to be selected with a larger critical particle size, in order to improve its anti-erosion performance.

Adding a certain amount of magnesium sand fine powder in the matrix part of magnesium carbonaceous refractories will adjust the thermal expansion coefficients of its large particles and matrix part, make them match each other and reduce the thermal stress caused by the difference of thermal expansion coefficients; however, if the magnesium sand fine powder is too fine, it will accelerate the reduction of MgO, which will lead to the damage of magnesium carbonaceous refractory materials.

The amount of graphite added should be matched with the use of magnesium carbonaceous refractories. In general, if the graphite addition is less than 10%, it is difficult to form a continuous carbon network in the products, which can not effectively play the advantages of carbon; graphite addition is greater than 20%, the production of molding difficulties, easy to produce cracks, products are easy to oxidize, so the graphite addition is generally between 10-20%, according to different parts of the use of the different graphite additions, choose different graphite additions.

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