- Time:Nov 06, 2023
The kiln lining materials in the cement kiln firing zone and transition zone are required to be resistant to high temperature, thermal change, chemical erosion and mechanical damage. The requirements are relatively demanding, and several factors compound and cross-destruct the lining bricks.
Si02 and silicates in the kiln material and kiln skin enter with the melt and fill the pores on the surface of the lining bricks, reducing the apparent porosity of the surface of the lining bricks to 7-9% and increasing the volume density to more than 3200kg/m³, causing the lining to The surface layer of bricks is densified. Generally, the thickness of the dense layer is about 20mm, which increases the expansion rate of the surface layer of the lining bricks, reduces the thermal stability of the surface layer of the lining bricks, and causes cracking and damage of the lining bricks.
The combined destructive factors caused the directly bonded magnesia chromium bricks and spinel bricks to develop cracks at 20-30mm of the hot surface layer and fall together with the kiln skin. This is the delamination and spalling of bricks we commonly see in kilns.
As cement kilns become larger, the requirements for refractory materials are higher and more stringent, which also promotes the development of refractory materials used in the cement industry. As the kiln diameter increases, the kiln lining life decreases. The relationship between kiln diameter and life is shown in the following formula, h=K 1/D2.
h: Lifespan D: Kiln diameter K coefficient (coefficient K is related to operating conditions, masonry, lining brick quality, usage, etc.).
Therefore, the larger the kiln diameter, the shorter the life of the lining bricks.
In view of the damage factors described above for the lining bricks used in the firing zone and transition zone of large cement kilns, after years of development and application, Zhengzhou Zhenjin Company has successfully developed special bricks suitable for the firing zone and transition zone of large cement kilns. Spinel bricks have been used in large cement kilns with good results and have been widely used.
High-end spinel bricks
The raw materials of high-grade spinel are high-purity sintered magnesia and high-temperature sintered spinel sand containing more than 65% alumina. The theoretical composition of spinel sand is A1203 71.68%, Mg0 28.32%. It is emphasized here that high aluminum must be used High-grade spinel bricks can be produced by molding spinel sand and commercially pure magnesia in a certain proportion and calcining them at high temperatures above 1750°C.
In the process of making bricks, the amount of spinel sand must be appropriate. Adding too little will not have the effect of spinel brick sand, and adding too much will be detrimental to the performance of the bricks, because theAl203 in spinel and cement Ga0 in the hot material will react to generate C12A7 low melting point mineral.
Characteristics of high-grade spinel bricks:
1. Good thermal shock resistance and stability
The important mineral components of high-grade spinel bricks are periclase (MgO) and magnesia-aluminum spinel (
MgAL204). Their expansion coefficients are different. When the bricks are fired at high temperatures, a solid-phase reaction will occur, which will also cause some spinels to form. There is an effective separation between the stone particles and the periclase matrix. The spinel particles are wrapped by pores. When the brick is subjected to stress and temperature changes during use, these micro-pores can absorb energy and prevent damage to the brick.
2. Strong volume stability
When the cement kiln uses coal as fuel, incomplete combustion often occurs in local areas in the kiln, and excessive carbon monoxide exists. Due to changes in oxygen content, an oxidation-reduction reaction will occur when directly combined with the iron component in the magnesia-chromium brick, such as Magnesia-iron spinel brick changes (Mg·Fe203→3Fe0.67). When the ferric iron is reduced to divalent iron, a 20% volume shrinkage occurs, and on the contrary, the volume increases. Repeated cycles will damage the structure of directly bonded magnesia-chromium bricks, but there is no iron in high-grade spinel bricks, so there is no such volume change effect.
3. High temperature resistance
High-grade spinel bricks have low SiO2 content, so there are very few silicate minerals with low melting points. They are all composed of minerals with high melting points, such as magnesia aluminum (melting point 2850°C) and magnesia-aluminum spinel (melting point 2135°C). For this reason it is resistant to high temperatures.
4. Strong corrosion resistance
High-grade spinel bricks are mainly composed of periclase and magnesia-aluminum spinel, with very little silicate phase, so they have strong resistance to alkali erosion.
5. Chrome-free pollution
High-grade spinel bricks contain no chromium and are an environmentally friendly product.
The physical and chemical properties of high-grade spinel bricks are superior to those of directly bonded magnesia-chromium bricks in many aspects, such as high compressive strength, greater than 5OMPa, high load softening temperature T0.6≥1650°C, good thermal shock stability, greater than 12 times, according to Its physical and chemical properties are suitable for the transition zone of large cement rotary kilns. However, the thermal conductivity of high-grade spinel bricks is larger than that of directly combined magnesia-chromium bricks. The thermal conductivity at 800°C is 3.0w/mk. Therefore, high-grade spinel bricks are used in the cement kiln transition zone with floating kiln skin or no kiln skin. After using for a long time, when the thickness of the remaining bricks reaches 100-120mm, it is best not to use a longer flame for calcination. If the temperature of the transition zone is too high, the cylinder may overheat due to the large thermal conductivity of the bricks.
6. Application
High-grade spinel bricks are used in the 3000T/D kiln of Shandong Shanaluminum Cement Factory with a cycle of more than 1 year. They are used in the 2500T/D and 5000T/D kilns of Inner Mongolia Wulan Cement Co., Ltd. with a cycle of more than 1 year. The 2500 T/D kiln in Jiangsu Helin lasted for 18 months and was used in the 6000 T/D kiln of Huaxin Cement Yangxin Co., Ltd. with a cycle of 456 days. Huaxin Group has 14 large kilns with a capacity of more than 5,000T/D, all of which have achieved significant results. It has also been used on large kilns such as Jiangsu Helin 6000T/D and Jiangxi Dieffenbachia 5000T/D and has achieved results of more than one year. High-grade spinel bricks solve the previous problem of short service life of transition zone bricks and inability to match fired zone bricks.
