Application of magnesia chrome bricks in various industries

During the production of magnesia chrome bricks, there will be many additives, such as chromium oxide, aluminum oxide, zirconia, etc. These additives have a very important impact on the performance of magnesia chrome bricks, as shown below:

1. Effect of chromium oxide on magnesia chrome refractories

Chromium oxide is one of the main components of magnesia chrome refractories. The influence of properly adding chromium oxide with high purity and small particle size to magnesia chrome refractories on magnesia chrome refractories mainly includes the following three aspects:

(1) Increase direct binding rate

When the content of Cr203 in periclase increases, the dihedral angle between silicate melt and periclase increases, which also leads to the worse wettability between silicate and magnesia chrome spinel compared with silicate and magnesia iron spinel; This phenomenon also causes that silicate can only be distributed in island form between magnesia chrome spinels, and the direct bonding in magnesia chrome bricks also increases.

Chromium oxide

(2) Increase strength

In the process of sintering, the solid solution of periclase in magnesia chrome refractories is dissolved in spinel during the re dissolution of silicate Re dissolution. Because of their similar crystal structures, spinel can form a large number of secondary spinels around periclase crystals, which improves the strength and slag resistance of products; Adding high-purity small grain chromium oxide into magnesia chrome bricks can promote the formation of spinel and secondary spinel.

(3) Increase slag viscosity

When magnesia chrome refractories contain more chromium oxide, due to the low chemical activity of chromium oxide, when chromium oxide is present in the slag, the viscosity of the slag component increases.

Magnesia chrome brick

2. Effect of alumina on magnesia chrome refractories

The addition of alumina in magnesia chrome refractories has different effects depending on the raw materials used. The raw materials contain more impurities such as CaO and SiO2. Adding appropriate aluminum oxide can promote the sintering of magnesia chrome refractories, and the brick structure will become denser and deeper. This is because alumina can form low melting materials with calcium silicon and other components in the brick, which accelerates the process of sintering and densification.

However, due to the different content of CaO and SiO2 in the raw materials and the influence of the crystal structure of the raw materials such as periclase, the content and diffusion of CaO and SiO2 in the bricks are different; When the amount of CaO and SiO2 diffused to the boundary is not enough to meet the reaction speed of alumina, the remaining alumina will react with MgO in periclase crystal as follows:

MgO+Al2O3=MgO·Al2O3

That is, spinel is generated at the grain boundary and other places in the brick. Due to the large difference in volume and density between the reactants MgO, Al2O3 and the product magnesia alumina spinel, the generated spinel is accompanied by a large volume expansion, which largely hinders the progress of magnesia chrome brick sintering reaction. Therefore, the porosity in the brick increases and the strength decreases.

In other words, when adding alumina to magnesia chrome refractories, it is necessary to consider the CaO and SiO2 components in the raw materials and add alumina appropriately; If most Al2O3 is added, it can react with the silica calcium component in the brick to form a low melting phase, and it can show a continuous distribution in the brick. At this time, as the amount of liquid phase increases in the sintering process, it can promote the material transmission in the sintering process, promote the sintering of the brick, and improve the density of the products; On the contrary, if the content of CaO and SiO2 is too low, it is not enough to meet the conditions for consuming Al2O3 to generate liquid phase. At this time, Al2O3 will react with the MgO component in the brick to generate spinel. At this time, the volume expansion caused by the formation of spinel cannot be well alleviated, the density of magnesium chromium products will be reduced, and the normal temperature compressive strength will be affected.

3. Effect of zirconia on magnesia chrome refractories

The addition of zirconia can improve the performance of magnesia chrome refractories to a certain extent: (1) ZrO2 has strong chemical stability, is chemically inert to general glass melts and acids and alkalis, and is not easy to be wetted by metal solutions; (2) ZrO2 can change the aggregation state and crystal shape of crystal boundary phase in magnesia chrome refractories, reduce the dihedral angle between crystals, and promote the combination between crystals.

However, excessive ZrO2 addition is unfavorable to magnesia chrome refractories, because the solid solubility of ZrO2 in magnesia is small. If excessive ZrO2 is added, the residual ZrO2 will remain between grains, hindering mass transfer during sintering and not conducive to densification of bricks.

Zirconia

4. Effect of iron oxide on magnesia chrome refractories

Due to the existence of magnesium iron spinel, iron oxides can promote the sintering of magnesia chrome refractories to a certain extent. However, due to the existence of variable valence of iron oxides, and the slightly different solid solubility of the two oxides FeO and Fe2O3 in periclase, the magnesia chrome products with more iron oxides are not suitable for copper smelting production with unstable atmosphere and temperature.

If magnesia chrome refractories with high iron content are used in copper converters, it is possible to generate swelling and loose layers due to the following phenomena: under high temperature reduction, Fe2O3 in periclase solid solution will be reduced to FeO, and low iron spinel will be generated in bricks; When the temperature drops or the oxidation atmosphere is used, the low iron spinel will be oxidized again to form MgOFe2O3; In this process, the volume changes, which will cause the explosion of magnesia chrome refractories and the formation of evacuation layer.

The effects of the above substances on magnesia chrome refractories in the use of copper smelting process are not only their own effects on magnesia chrome refractories, but also their interactions with iron silicon slag and SO2 atmosphere should be noted.

Nanotechnology can design materials at nanometer scale. By using nanomaterials, it can also change the performance of traditional materials and optimize their microstructure. By applying nanotechnology to refractories, the mechanical properties and heat resistance of refractories can be improved. However, there are still problems in the cost and process of using nanotechnology in refractories. Therefore, it is necessary to strengthen the research on nanotechnology to meet the performance requirements of refractories.

Nanotechnology and refractory materials

1.1 Overview of nanotechnology

Nanotechnology, also known as nanotechnology, is a technology to study the properties and applications of materials in the scale of 1~100nm. Its purpose and idea is to directly use atoms or molecules to manufacture products with specific functions. It is a technology that uses a single atom or molecule to complete manufacturing work. Nanotechnology is a highly interdisciplinary discipline, integrating different contents such as physics, nano chemistry, materials science, biology, electronics, etc. The comprehensive use of nanotechnology can improve the utilization efficiency of raw materials, and also enable nanomaterials to have more outstanding performance.

1.2 Nanotechnology and refractory materials

Applying nanotechnology to the manufacture of refractories can obtain amorphous refractories and oxides with excellent performance. According to the requirements of fire prevention and fire resistance, nanotechnology can directly design the microstructure of materials and improve the quality and performance of refractories. At present, the dispersion methods of nano powder include mechanical stirring dispersion, ultrasonic dispersion, surface grafting modification technology and so on. However, using nanotechnology to obtain refractories can control the production cost of refractories. By designing the microstructure of the materials, the durability of the materials is improved. It has a very good application effect for improving the performance of refractories and the level of fire protection technology.

Application of Nanotechnology in Refractories

2.1 Application of nano carbon materials in refractories

After graphite is added to the refractory, the thermal shock resistance and corrosion resistance of the refractory can be improved. However, if too much graphite is added, the oxidation resistance of the refractory will become worse, because the graphite will react with oxygen at high temperature, resulting in an increase in the porosity of the refractory. According to the amount of graphite, it can be divided into multi carbon refractories and low carbon refractories. Generally, the boundary is 8%. Multi carbon refractories are those with carbon content equal to or higher than 8%, and low carbon refractories are those with carbon content below 8%. Multi carbon refractory:

Carbon nanotubes and graphene can be added into multi carbon refractories, which can improve the refractory mechanical properties and thermal shock resistance of refractories. For example, at present, carbon nanotubes are used instead of flake graphite. Under the same preparation process, refractories can have a more compact microstructure and mechanical properties. Through experimental analysis, after using carbon nanotubes as carbon sources, the mechanical properties of refractories after sintering are much better than those of graphite carbon sources. Second, by adding carbon nanotubes as carbon sources into magnesia carbon refractories, refractories have more tenacity. Combined with the current research, the mechanical properties of refractories can be significantly improved by adding nano materials or in-situ synthesizing nano materials under the condition of multi carbon. After heat treatment, nano carbon materials have a more obvious improvement effect on the materials. When a certain amount of antioxidant is added to the refractory, the nano carbon material can form ceramic whiskers with the antioxidant material in the microstructure, so that the refractory can have higher toughness. However, in the current application, under the condition of multi carbon, nano carbon materials still have certain defects and are prone to oxidation and alteration, so they cannot completely replace the graphite carbon source. It is also necessary to judge the use of nano carbon materials through experiments.

Low carbon refractories: Since too much carbon will lead to insufficient oxidation resistance of refractories, low carbon refractories will be used to solve the problem of high carbon content and corrosion. In the low carbon environment, the use of nano materials can meet the requirements for the performance of refractories to a certain extent. At present, low carbon magnesium refractories can withstand the impact caused by stress and meet the mechanical property requirements of refractories. As carbon nano materials have relatively high strength and toughness, they can absorb or release the stress at the crack tip through bridging and crack deflection, which can improve the mechanical properties, fracture resistance and other properties of refractory products. According to the current situation, the amount of nano carbon materials used in low carbon refractories is far less than that of multi carbon refractories, either by in-situ growth or by direct addition. In actual processing, the catalyst can be used to form high-temperature ceramic phase in the refractory. By forming whiskers, the refractory and mechanical properties of the refractory can be further improved.

2.2 Application of carbon nano materials in oxide products

In oxide products, the use of nano powder mainly focuses on the influence of nano powder on the sintering behavior of oxides, and the influence of nano powder on the microstructure of materials. The strength and toughness of the material are improved, and the nano powder can be used as a mineralizer to promote the crystal phase transformation of the material. For example, adding nano curing rate and nano silica into corundum refractories can reduce the sintering temperature of refractories by more than 100 ℃. For magnesia chrome refractories, the sintering temperature of magnesia chrome refractories can also be reduced by adding appropriate nano Fe2O3, and the obtained refractories have a more compact microstructure, which improves the strength of magnesia chrome refractories. After adding less than 3% nano powder into the corundum refractory matrix, the corundum material can be sintered more effectively, and the bending strength has also been significantly improved, as well as the thermal shock resistance of the corundum brick. At present, the main obstacle to the use of nano powder oxidation materials is the high price, which limits the large-scale industrial investment. Therefore, it is necessary to strengthen the research of nano powder to meet the development needs.

2.3 Application of nanotechnology in pouring materials

Nanotechnology is mainly introduced into the pouring materials in the form of nano powder, nano sol, gel, etc. The principle is to use the small size effect of nano materials, use the surface and interface effects, control the water addition, reduce the content of low melting point substances in the pouring refractory, enhance and improve the microstructure of the materials, and improve the overall pouring performance of the materials. For example, the use of nano aluminum oxide powder in corundum spinel can improve the mechanical properties of materials at different temperatures. Through observation, the microstructure of materials has been significantly improved, so the stability of materials has been significantly improved. The role of sol and gel is similar to that of nano powder, but they have the advantage of low cost. In the pouring materials, both sol and gel can be easily diffused, so that nano materials can be evenly distributed in the pouring materials, meeting the requirements of improving the microstructure of the pouring materials.

Specific technologies for application of nanomaterials

3.1 Nano composite corundum brick and magnesia chrome brick

The material uses a mixing process, adding a small amount of nano powder in the ingredients, thus changing the properties of the material, so that the material can have higher strength and stability. By adding a small amount of nano alumina and nano silica into the ingredients of corundum bricks, and adding a small amount of ferric oxide into the magnesia chrome bricks, the compressive resistance at room temperature is obviously strengthened, and the mechanical indexes can better meet the application requirements. Combined with the actual experimental situation, the use of nano powder silicon dioxide can significantly change the mechanical properties of the sample, especially when the addition amount is about 1%~2%. After high temperature sintering at 1550 ℃, the strength is 1.5~2 times higher than that of the traditional sample, which has extremely high mechanical properties. While adding aluminum oxide nano powder into the magnesia chrome brick, according to the observation of the microstructure, it can be found that the microstructure has changed significantly before and after adding the nano powder. Depending on the change of the microstructure, the change effect in mechanics can be determined.

3.2 Nano composite Al O SiC-C castable

AlO-SiC-C has extremely excellent performance, so it is widely used in iron hooks and has a very stable performance in fire protection. In order to further improve the performance of AlO-SiC-C, especially its high temperature performance, researchers adopted the method of using silica alumina gel powder to replace pure calcium aluminate cement as the binder. By this method, the synthesis temperature was significantly reduced, and the β Phase formation. In practical use, the use of gel powder makes the refractories have higher bending strength. Combined with XRD analysis, it can be seen that after the gel powder is added, the nano composite AlO-SiC-C material has a lower formation temperature and has become a transition phase at 1100 ℃. After industrial application, this kind of nano materials has achieved good results. In the blast furnace, the one-time iron flux has reached 157900 tons, which is about 40000 tons higher than that without nano powder AlO-SiC-C material, so very good economic benefits can be obtained.

3.3 ZrO2 composite and Cr O composite

In the application process of ZrO2 material, the rapid rate of sizing nozzle determines the life of billets. Combined with research, the main reasons for its diffusion are low strength and large porosity. Therefore, using nanotechnology can better reduce the porosity of sizing nozzle and improve its performance. After the use of nano materials for ZrO, its microstructure has changed significantly, especially with higher density after use. Through SEM and XRD tests, it was found that the small pores of the nozzle were filled with nano particles. After being sintered at 1500 ℃ for 6 hours, the bulk density and porosity of Jingkou green body after nano ZrO composite are the same as those of Jingkou green body sintered at 1800 ℃ for 6 hours under MSG nano treatment. If it is sintered at 1800 ℃ for 6 hours, the porosity will drop to 11%, which is more excellent than the previous 19%. Compared with the pore diameter change of ZrO2 water inlet before and after nanocomposition, before nanocomposition, the pore diameter of the sample was mostly concentrated at 100 nm, and the pore diameter and pore volume were significantly smaller, and the pore diameter was only one tenth of the previous one. Therefore, the use of nano particle filling has a very good effect, and can promote the sintering of materials and reduce the sintering temperature. For Cr O composites, the preparation will go through the evaporation and condensation process. Because Cr O composites are volatile in the process of high-temperature sintering and have a relatively high evaporation rate, the obtained refractories generally have high porosity, large pore diameter, large volume, density and other problems, and poor slag corrosion resistance will occur in practical applications. At present, nano materials are generally used for adjustment to improve porosity. For example, Cr O nano materials can make magnesia chrome refractories more powerful.

According to the pore volume distribution map of magnesia chrome refractories after nanotechnology treatment, it can be determined that after nanotechnology treatment, the pore volume distributed at 20 microns in the sample decreases significantly, which proves that the use of nano materials can greatly help reduce the porosity of refractories and improve the bulk density, so it is possible to improve the slag resistance of magnesia chrome refractories. Combined with the microstructure analysis, it can be found that the surface of the material also forms a dense deposition layer, which improves the surface density.

3.4 Coated nano oxide film graphite

Carbon can not be wetted by molten steel and slag, and has extremely high thermal conductivity. Adding carbon into oxides can greatly change its performance. Therefore, carbon pouring refractories have become one of the hot directions in the development of refractories. At present, the commonly used material is graphite. Because of its non wetting characteristics, graphite castables are difficult to disperse, which will also affect the fluidity of castables and affect the actual application. Therefore, the surface of graphite needs to be modified. First, the hydrolysis of various inorganic salts is used to coat the surface of graphite with oxide film using natural scales. Ensure that the oxide coated graphite can exist in an amorphous state after being treated at 500 ℃, and then the oxide coated on the graphite surface can form C-O-M bond with the graphite, which can have the characteristics of chemical adsorption. After graphite coating, the surface of particles changed obviously. First, the average strength increased, and the fractal dimension of particles increased, thus increasing the specific area of particles. Combined with SEM photo analysis, the graphite coated with nano oxide and the wetting angle are greatly reduced compared with the treated graphite ζ The potential also exhibits a behavior similar to that of nano oxides. After being coated with graphite, Al2O3 has a stronger hydrophilicity. The viscosity and deposition volume of the suspension solution are lower than those of the materials without graphite treatment, and the dispersion stability is significantly improved after the addition of dispersants.

Conclusion

Nanotechnology can obviously improve the microstructure and performance of refractories when it is put into refractories. For technicians and researchers, we should strengthen the research on nanotechnology, manufacture more low-cost nanomaterials, change the performance of refractories, and fully apply them to industrial production.