High alumina brick is one of the commonly used refractory bricks. It belongs to the silicon-aluminum refractory material and the neutral refractory brick. It is commonly used in various large and medium-sized thermal kiln equipment or smelting kilns, according to GB/T2988-2012 Divided into 5 grades: LZ-80, LZ-75, LZ-65, LZ-55, LZ-48. In the production process of high alumina bricks, the type of waste caused by cracks on the surface and inside of the product. Therefore, it is necessary to analyze the various reasons for the cracks of high alumina bricks and make adjustments and improvements to reduce the aluminum caused by the cracks of high alumina bricks and improve the qualification rate of high alumina bricks.
(1) Reasons for raw materials
The impurity (especially K2O and NazO) and the degree of sintering of high alumina bauxite clinker, the main raw material for producing high alumina bricks, will cause cracks on the surface of the product. The poorly sintered clinker continues to shrink and become uneven during the firing process, causing the product to crack and produce cracks; at the same time, during the firing process of the product, the poorly sintered clinker itself is secondary mullite (3a-AlzO: +2Si02~3Alz0-2SiO2, 1200~1500℃) is still going on. The secondary mullite petrochemical mainly refers to the siO combined with clay in the production of high alumina bricks: it reacts with Alz03 of the clinker to form mullite, and is accompanied by volume expansion (About 10%), resulting in inconsistent shrinkage of the product, resulting in an increase in the number of surface cracks and an increase in the degree of cracking on the surface of the product.
Improvement measures: The main mineral phases of high alumina bricks are mullite, corundum and glass phases. As the content of A0 in the product increases, the number of mullite and corundum phases also increases, and the glass phase decreases accordingly, and the refractoriness and high-temperature performance of the product increase accordingly. In the actual production of high alumina bricks, attention should be paid to the impurity content of the high bauxite clinker used for control. According to the requirements of YB/T5179-2005 (high bauxite clinker), the content of K20 and NaO<0.35%~0.6% , Try to use raw materials with low impurity content and good sintering degree for production.
(2) Causes of mud
The critical particle size in the material, the amount of fine powder added, and the mixing quality of the mud will all affect the cracks of the semi-finished high alumina brick. For example, the matrix material is generally composed of fine powder, which shrinks during the firing process, while the particles generally swell. There is a large difference in deformation between the two, which causes the inside to usher, which causes cracks in the product.
(3) Reasons for molding
Aluminum bricks are usually formed by friction presses or automatic brick presses above 300T. The biggest feature of cracks caused by product bricks, shaped abrasives, uneven cloth, operation and pressure, etc., is the regularity, and the depth and location of the cracks are basically the same.
The reason for the brick type is that different brick types are pressed with different molds, and the mold structure design with holes, tongues, grooves or corners is unreasonable. There are many tongues and grooves and it is not easy to form on the multi-mold upper surface, and the mold sides on both sides are loose plates, which can easily lead to cracks after being ejected from the mold.
Unreasonable molding die design: due to the lack of vent grooves and vent holes in the molding die design, the bricks are prone to cracks when they are molded out of the mold.
Reasons for uneven cloth: uneven cloth easily makes the sides and middle parts of high alumina bricks different in density. During the drying process of the green body, because the density of the surrounding areas is lower than the middle, the temperature of the sides is higher than the middle temperature. As a result, the edge water evaporates quickly and the middle water evaporates slowly, resulting in a situation where the edge dehydration rate is greater than that of the middle. When the edge syneresis is too fast, the edges of the green body will crack.
The reason for the forming operation and the forming pressure: During the forming operation, "short stroke is too fast" and "full stroke is hit hard". Although most of the gas is eliminated, a small part of the gas is still compressed and not completely discharged, and the grade pressure exceeds the fracture of the brick. Strength, it will cause internal transverse cracks in the brick.
Tracy Liu:
In the middle part of the situation, when the edges are dehydrated
The reason for the forming operation and the forming pressure: During the forming operation, "short stroke is too fast" and "full stroke is hit hard". Although most of the gas is eliminated, a small part of the gas is still compressed and not completely discharged, and the grade pressure exceeds the fracture of the brick. Strength, it will cause internal transverse cracks in the brick.
(4) Reasons for firing
The sintering of aluminum bricks is liquid phase sintering, the formation temperature and amount of the liquid phase, the heating rate during firing, the atmosphere conditions, the shrinkage of the green body during the firing process, and the recrystallization of the secondary mullite compound corundum It will cause inconsistent shrinkage and cracks on the surface of the product.
Pre-heating cracks: After the dried brick enters the roasting kiln, there are still two kinds of moisture that need to be removed slowly. One is free water and the residual moisture after drying will cause netting on the surface of the product. The fringe pattern produced by the pre-heating zone of the roasting kiln mostly occurs on the large surface and middle part of the brick. The cracks are rough and deep. The parts where the billet is not dry (leeward side, middle and lower parts) are the most severely fired with cracks: in firing The shrinkage that occurs during the process, the secondary mulliteization reaction and the recrystallization of corundum will cause inconsistent shrinkage and cracks on the surface of the product. Cracks caused by chemical changes in the firing process are difficult to avoid, and the reactants can be uniformly dispersed, and the local expansion will not be too large or too small to aggravate the cracks. The expansion caused by the actual sintering chemical change is beneficial. For example, the formation of mullite can offset the shrinkage of high-temperature products and keep the volume stable.
The secondary mullite petrochemical process generally starts at 1200°C, and the reaction is completed at 1400°C to 1500°C. This reaction is the mullite formed by the reaction of silicon and corundum, which is precipitated when kaolinite forms mullite (primary mullite). It is called the secondary mullite phase and has a volume expansion of about 10%. . Generally speaking, if the amount of secondary mullite is small, the reaction temperature is low.
Cooling zone cracks: The cracks produced by excessive cooling are thinner and straighter, and the cracks are relatively smooth, and some are not obvious in appearance. They are dark lines, and there is a dull sound when knocking. The windward side of the billet edge is more serious than other parts.
There are many reasons for the cracks of high alumina bricks, but there are one or several factors that work together, but in essence, it is mainly because the effect of stress exceeds the stress that the high alumina brick itself bears that causes the cracks to occur. In the actual production process, the micro-cracks caused by the physical and chemical changes of the product are sometimes beneficial to improve the thermal shock resistance of the product. It is necessary to analyze the various factors that cause the high-alumina brick cracks, and make improvements to increase the qualified rate of the finished product. .
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