Thermal, mechanical and chemical factors contribute to the stress and damage of refractory bricks in kiln lining. The destructive effect of the above factors varies with the kiln type, operation and the position of the kiln lining in the kiln. The decisive factors are the deformation of flame, kiln material and kiln shell, which make the kiln lining bear various stresses.
For magnesia bricks, there are 8 damage factors, mainly including:
① Clinker melt infiltration
Clinker melt mainly comes from kiln material and fuel, and the infiltrating phases are mainly C2S and c4af. C2S and c4af infiltrated into the metamorphic layer will strongly dissolve periclase and other parts in the alkaline brick, and precipitate secondary CMS, c3ms2 and other silicate minerals, and sometimes even potassium nepheline. The melt will fill the pores in the refractory brick lining, making this part of the brick layer densified and brittle. In addition, the dual effects of thermal stress and mechanical stress make the brick very easy to crack and peel off. Since C2S and c4af begin to form when the temperature is above 550 ℃, and the temperature of the materials entering the precalcining kiln has reached 800~860 ℃, the clinker melt penetrates into the whole precalcining kiln, that is, the clinker melt has a certain infiltration and erosion effect on each kiln lining of the precalcining kiln.
② Condensation of volatile components
In the precalcining kiln, components such as alkaline sulfate and chloride are volatilized, condensed and recycled repeatedly, resulting in the enrichment of these components in the raw meal. The R2O and SO3 contents of raw meal in the hottest preheater at the kiln tail are often increased to 5 times and 3~5 times of the original raw meal respectively. When the hot material enters the rear 1/3 part of the kiln shell, the volatile components in the material will condense and deposit in all brick surfaces and layers, making the place highly dense and eroding the adjacent components except periclase, resulting in a significant weakening of the thermal shock stability of the brick infiltration layer, forming expansive nepheline and leucite, The brick will be damaged by alkali cracking, and will crack and peel off under the combined action of thermal mechanical stress. Since the precalcining kiln has no kiln belt from the kiln tail to the firing zone, the closer it is to the high temperature zone, the deeper the kiln lining is eroded by alkali salt, and the more serious the kiln lining damage is. Therefore, special attention should be paid to the type selection of the kiln lining at this position.
③ Reduction or reduction oxidation reaction
When the thermal system in the kiln is unstable, it is easy to produce reduction flame or incomplete combustion, so that fe3+ in the magnesia chrome brick is reduced to fe2+, resulting in volume shrinkage. Moreover, the migration and diffusion ability of fe2+ in periclase crystal is much stronger than that of fe3+, which further intensifies the volume shrinkage effect, resulting in holes, weakening of structure and reduction of strength in the brick. At the same time, the alternating change of reduction and oxidation atmosphere in the kiln causes the volume effect of shrinkage and expansion to occur repeatedly, and the bricks will produce chemical fatigue. This process mainly occurs in areas without kiln skin protection.
④ Overheating
When the kiln heat load is too high, and the brick surface loses the protection of the kiln skin for a long time, the matrix of the hot surface layer melts at high temperature and migrates to the cold surface layer, which densifies the cold surface layer of the brick lining, while the hot surface layer is loose and porous (generally prone to the normalizing point area of the firing zone), so it is not resistant to abrasion, impact, vibration and thermal fatigue, and is easy to be damaged. In recent years, in the cooling zone and transition zone, many enterprises have used silica fume bricks. Most accidents of silica fume bricks are caused by over burning, and there are few other reasons. Silicon mullite brick is mainly composed of silicon carbide and mullite, and silicon carbide plays a very important role. Theoretically, when the temperature rises to about 2500 ℃, silicon carbide begins to decompose into silicon steam and graphite. In fact, under the reducing atmosphere in the kiln, silicon carbide has begun to decompose at about 1700 ℃, which constitutes fatal damage to silicon mullite brick.
⑤ Thermal shock
When the kiln operates abnormally or the kiln skin is unstable, the alkaline bricks are easy to be damaged by heat shock. The kiln skin suddenly collapses, causing the temperature of the brick surface to rise suddenly (even up to thousands of degrees), resulting in great thermal force in the brick. In addition, the frequent start-up and shutdown of the kiln causes frequent alternating thermal stress in the brick. When the thermal stress exceeds the structural strength of the brick lining, the brick begins to crack, increases and deepens along its structural weakening, and finally breaks the brick. When the kiln skin falls, the broken bricks on the hot surface layer are taken away, causing continuous damage to the bricks. Thermal shock is easy to occur in the transition zone near the kiln tail.
⑥ Thermal fatigue
During the operation of the kiln, when the brick lining is under the feeding layer, the surface temperature will decrease. When the brick lining is exposed to the flame, the surface temperature will increase. The rise and fall range of the brick lining surface temperature can reach 150~230 ℃ with an impact depth of 15~20mm for each revolution of the kiln. If the speed of the precalcining kiln is 3r/min, this periodic temperature rise and fall can reach 130000 times per month. This kind of temperature rise and fall repeatedly leads to thermal fatigue of the surface layer of the alkaline brick and accelerates the peeling damage of the brick.
⑦ Extrusion
During the operation of the rotary kiln, the kiln lining receives the comprehensive effect of mechanical stresses such as pressure, tension, torsion and shear. Among them, the rotation of the kiln, the ovality of the kiln shell and the collapse of the kiln skin make the bricks subject to dynamic loads: the quality of the bricks and the kiln skin and the thermal expansion of the bricks themselves make the bricks bear static loads. In addition, the relative movement between the brick lining and the kiln shell, between the brick lining and the brick lining, as well as the welds on the brick retaining ring and the kiln body, will make the brick lining bear various mechanical stresses. When the sum of all stresses exceeds the structural strength of the brick, the brick will crack and be damaged. This phenomenon occurs in the whole kiln lining of the precalcining kiln.
For bricks close to the brick retaining ring, most of the damage is caused by extrusion.
⑧ Wear and tear
When the discharge area at the kiln mouth of the precalcining kiln is not protected by the kiln skin, the clinker and large kiln skin are hard, which will cause serious impact and wear damage to the refractory brick lining at this position.