Refractories containing Cr2O3 have good high temperature corrosion resistance. Among all oxide additives found in refractory materials, Cr2O3 can be said to be one of the most superior. The main reasons for the superior corrosion resistance of Cr2O3 are:
1) Cr2O3 can dissolve with some of the main oxides in refractories to form a solid solution with high melting point and eutectic point. The melting point of the solid solution formed is almost around 2000°C, so chromium trioxide has become an important raw material and additive for refractory materials;
2) The main causes of damage caused by the interaction of
refractory castables and high-temperature melt can be summarized as follows:
Mechanical action, chemical erosion of slag and thermal stress, and the damage of refractory materials under normal circumstances is the result of the combined action of the three. Mechanical action mainly refers to the erosion and wear of refractory materials by furnace charge, high-temperature melt and high-temperature air flow; chemical erosion refers to the damage caused by chemical reactions. Refractory materials will come into contact with slag, melt and smoke during service. When gas and dust and other substances undergo physical and chemical reactions, the structure is destroyed. The main process is the dissolution and penetration of the liquid phase, and the main manifestations are infiltration, slag corrosion and dissolution.
Cr2O3 can reduce the wettability of slag to refractory materials, reduce the erosion of slag to refractory materials, and at the same time reduce its penetration into refractory materials. Li Yong et al. studied the contact angles of 61.5%SiO2-17.8%Al2O3-20.7%MgO slag in magnesia refractories and magnesia chromium-containing refractories, and found that after holding at 1370℃ for 2 hours, the molten slag is in the chromium-containing refractories. The larger contact angle on the slag indicates poor wettability of the slag.
The corrosion of slag on refractory materials is mainly through penetration into the refractory material to form an erosion layer. When the temperature changes drastically, the minerals in the erosion layer expand and produce structural stress, which leads to the formation of cracks in the material, resulting in refractory Structural peeling of the material. When a certain amount of Cr2O3 is added to the refractory material, it can effectively reduce the penetration of slag in the refractory material. Even the copper smelting slag containing S and O with strong penetrating ability can also play the same role, thereby reducing The resulting structure peeled off.
When the
refractory contains Cr2O3, it will interpenetrate with the slag. After Cr2O3 enters the slag, it will affect the viscosity of the slag. Wang Chengxun and others studied the relationship between the viscosity of silicate slag and the content of Cr2O3, and showed that Cr2O3 not only has a low solubility in silicon hydrochloric acid slag, but also significantly increases the viscosity of the slag and reduces the fluidity of the slag, so it can reduce the slag Corrosive effect on refractory materials. The increase of slag viscosity can slow down its penetration into refractory materials, reduce the formation of a permeable layer in refractory materials, and thereby reduce the structural spalling of refractory materials. Therefore, chromium-containing refractories are widely used in non-ferrous metallurgy, steel metallurgy, coal water slurry gasifier and glass industry. Among all basic refractories, chromium-containing refractories are regarded as the best refractories.
Although chromium-containing refractories have excellent performance, since the late 1980s, their consumption has been greatly reduced, mainly because the trivalent chromium in Cr2O3 may be oxidized into poisonous materials during the service process of chromium-containing refractories The hexavalent chromium is very harmful to the human body and the environment. People began to use some chromium-free refractories to replace chromium-containing refractories, but in some environments with harsh service conditions, chromium-containing refractories are irreplaceable because of their excellent performance, such as the high-temperature firing zone of cement rotary kilns. The lining of the refining furnace outside the furnace, the coal water slurry gasifier, etc. still use a large amount of chromium-containing refractories.
In the cement industry, in order to reduce the pollution of hexavalent chromium, people have used dolomite, iron-aluminum spinel, magnesia dolomite, magnesia-aluminum spinel, and magnesia-calcium-zirconium refractories to replace the original magnesia-chromium refractories. However, the effect is not satisfactory, and there are problems of one kind or another that have not been solved. First of all, dolomite, magnesia dolomite and magnesia-aluminum spinel and other materials have poor kiln-hanging performance and high thermal conductivity. The use of refractory materials of these materials will increase the overall temperature of the kiln shell by 20-50°C, affecting the kiln itself The integral structure of the tyre also increases the requirements for tire expansion compensation; secondly, due to the high content of active calcium oxide in dolomite, it is easy to absorb water and hydrate, and there are special requirements during transportation and storage. The hydration of dolomite is particularly serious during the shutdown of the cement rotary kiln; although the iron-aluminum spinel refractory has excellent kiln-hanging properties, the change in the atmosphere of the cement rotary kiln will cause the iron to become The transition between valence and trivalence causes the structure of refractories to become loose, which affects the normal use of refractories. Based on the above reasons, magnesia-chromium refractories have the advantages of high high-temperature strength and good kiln-hanging performance, and are currently still widely used in the high-temperature firing zone of cement rotary kilns.
In the steel industry, chromium-free environmentally friendly refractory materials can be used in many parts of the steel industry. However, refractory materials in some places are severely corroded by molten steel and slag or need to withstand drastic temperature changes and environmental changes, such as furnaces. Outside the refining furnace, ordinary refractory materials are difficult to resist steel erosion and atmospheric changes.
In the coal water slurry gasifier, although silicon nitride, silicon carbide and silicon oxynitride have good compatibility and chemical inertness to most silicate solutions, their working environment is a strong reducing atmosphere. Make it unstable and easy to decompose. In addition, these materials easily react with the iron oxide in the coal slag to destroy the original composition and structure of the material. Therefore, carbides or nitrides are generally not used as lining materials for coal-water slurry gasifiers.
In the non-ferrous metallurgy smelting furnace, compared with the traditional steel smelting furnace, the non-ferrous metal smelting furnace has a higher oxygen partial pressure. Although the magnesia-carbon refractories have shown good performance in the converter, because the non-ferrous metallurgy smelting furnace has a higher oxygen partial pressure, the magnesia-carbon refractories are easily oxidized and damaged. In contrast, chromium-containing refractories are ideal refractories for non-ferrous metal smelting furnaces because of their good slag resistance and high high-temperature strength.
(1-Quartz;2-Zirconium Quartz;3-AZS;4-Corundum;5-Clay Brick;6-Dense Chrome Oxide Brick)
In the kiln for glass production, the B2O3 contained in the molten glass has a very strong corrosive ability to refractory materials, while the densely structured chromium oxide brick has a very good corrosion resistance to the borate glass melt, and its corrosion rate It is one-tenth to one-hundredth of other refractory materials. At the same time, the dense chromium oxide brick has low opening porosity, small specific surface area and high strength, which can effectively inhibit the volatilization of chromium oxide, the penetration of glass slag and the erosion and erosion. Figure 2 shows the corrosion resistance test results of refractory materials of different materials and alkali-free glass slag. It can be seen that only dense chromium oxide bricks basically have no volume loss under high temperature conditions of 1400-1600℃, and refractory materials of other materials follow The increase in temperature has different degrees of loss.
