In order to extend the service life of the refractory bricks of cement rotary kiln lining, a layer of clinker is attached to its surface as a protective layer, which is the kiln skin. Under high temperature conditions, the interaction between cement clinker and refractory materials to form kiln skin is a complex phenomenon. From the perspective of refractory materials, this problem involves many issues such as refractory "sweating", damage to refractory materials when hanging kiln skin, and the hot and cold physical and chemical properties of cement clinker-refractory materials. The rotary kiln often maintains a high temperature of more than 1450℃, and the steel plate of the cylinder cannot withstand such high temperature. In order to protect the kiln cylinder, a layer of refractory firebricks is inlaid on its inner wall, but the refractoriness and thickness of fire bricks are limited, and they cannot withstand the invasion of long-term high temperature and corrosion of chemical reactions of materials. The kiln skin is not only conducive to extending the service life of refractory bricks, but also reduces the heat dissipation of the kiln cylinder and improves thermal efficiency.
How can the kiln skin be made to stick to the refractory firebricks?
The material moves from the cold end to the hot end in the rotary kiln. When it enters the firing zone, a liquid phase appears, and the amount of liquid phase increases with the increase of temperature. The liquid phase of the material has adhesive properties, but the adhesiveness will decrease with the increase of temperature, so the kiln skin cannot be hung when burning a high fire. When the surface temperature of the refractory bricks does not make the liquid phase in an overheated state, the material has the greatest viscosity. When the refractory brick is pressed under the material, the two stick together and undergo chemical changes. Later, as the temperature rises, they solidify to form the first layer of kiln skin. The same principle is used to form the second, third, and... layers of kiln skin. As the kiln skin is hung for a longer time, the kiln skin becomes thicker and thicker. As the kiln skin continues to thicken, the surface temperature of the kiln skin continues to rise, the viscosity of the liquid phase gradually decreases, and the material that sticks to it also decreases. At the same time, due to the gravity of the kiln skin itself and the friction and mechanical vibration of the material, the kiln skin sticks and falls off, and the amount is almost equal, so a kiln skin of a certain thickness is formed.
Relationship between refractory materials and kiln skin performance
First, the chemical mineral composition, porosity and firmness of the organizational structure of the refractory materials determine whether the clinker components are easy to enter the brick body, and what substances are produced after the reaction with the refractory materials. Therefore, it determines whether the refractory materials are easy to "sweat" and whether they are easy to bond cement clinker particles to form kiln skin.
Second, the chemical composition, phase composition and physical properties of the refractory surface have changed due to the high temperature and the erosion of clinker. For example, magnesia-iron spinel refractory bricks and ferroalumina spinel bricks contain a large amount of Fe2O3 to help burn. If the kiln skin cannot be formed quickly, the Fe2O3 in the bricks will react with the clinker to form C4AF and C2F, resulting in an increase in the bulk density of the hot end of the refractory, a decrease in the apparent porosity, and an increase in the thermal expansion coefficient, making it easy to be damaged by thermal shock. Third, the physical properties of the cement clinker-refractory reaction Fourth, the reaction product of cement clinker with dolomite bricks is C3S, and the reaction product with magnesia-chromium and magnesia-alumina refractories is C2S. C2S has the possibility of phase change and pulverization. Among the three elements of Cr, Al and Fe, only Cr can be stable. Therefore, dolomite bricks have the best kiln cladding performance, followed by magnesia chrome bricks, and magnesia alumina bricks have the worst kiln cladding performance.
