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The common causes and solutions for the damage of refractory castable

Words:[Big][Medium][Small] Mobile Page Two-Dimensional Code 2018-10-15     
1. Castable parts used in cement kiln

Refractory castable is widely used in cement industry because of its low production cost and convenient construction.At present, it is mainly used in preheater, blanking pipe, before and after kiln mouth, three-time air pipe and cooling machine and other parts with complex shapes and difficult to lay bricks (as shown in FIG. 1). The normal service life is generally more than one year.

Abnormal damage is common in use

& emsp;& emsp;Castable on the top of the grate cooling machine falls off abnormally, such as falling off after a few hours of production, and then the steel plate burns through;Rotary kiln mouth castable service life is short, such as shandong taian a cement plant less than three months time began to appear obvious damage, first is the end surface appeared to fall off, the main body appeared slight crack and a little fall off, then in the short term appeared larger crack and obvious block fall off.These all cause stop kiln maintenance, thus affecting production.

3. Abnormal damage cause analysis

& emsp;& emsp;3.1 pieces of castable will fall off when anchoring parts fall off

& emsp;& emsp;In the process of using the castable, the anchoring parts can closely adhere to the vessel wall, maintain the integrity of the castable and increase the bending resistance of the castable.In the place where the piece of castable falls off, it was found through inspection that the anchor pieces fell off with roots in many places, and there were only some welding spot marks left. It was obvious that the strip of castable fell off because the gripper was welded at the welding spot.

& emsp;& emsp;3.2 repeated and drastic changes in temperature cause rapid damage to castable

& emsp;& emsp;The thermal shock stability of castable is poor, and it is easy to be damaged by drastic temperature change.In rotary kiln cooling zone (especially the kiln mouth) as an example, it is not only to the clinker abrasion unceasingly, but also constantly by the effect of secondary air and high temperature furnace clinker, each turn, kiln lining material sometimes buried under the high temperature materials, sometimes exposed to flame, the surface temperature of the lining synchronism changes along with the rotation of the kiln, the temperature difference of above 200 ℃, influence depth of 15 ~ 20 mm, according to the kiln speed 3.5 r/min, the cyclical temperature rise and fall repeatedly in the surface layer of the castable because thermal fatigue;If you have a cold kiln, the temperature changes more dramatically.They are all causes of castable damage.During maintenance and elimination, it can be found that spalling and dropping blocks often appear on the surface of residual castable in the depth range of 20~50mm, which is mainly caused by thermal fatigue damage.

& emsp;& emsp;3.3 construction reasons

& emsp;& emsp;Refractory castable has a set of strict construction requirements, must be operated according to this requirement, in order to ensure the good physical properties of castable.For example, a company decided to replace the top castable in the high temperature area of the grate cooling machine. In order to ensure the ignition and production according to the predetermined plan, the construction team adopted the overall casting, and did not reserve expansion joints in accordance with the provisions. At the same time, in order to facilitate the construction, the water consumption was significantly larger than the required amount.More serious is the end of the construction will immediately fire heating, and as scheduled.Due to the rapid temperature rise, the temperature of castable increases sharply, and a large amount of internal moisture evaporates rapidly, generating large expansion stress, which causes the castable to burst and flake off, causing large area of falling off accidents.

& emsp;& emsp;The above operation obviously violates the construction requirements of refractory castable.Firstly, the amount of water added is not well controlled, which has a great influence on the strength of castable.Second, there is no reserved expansion joint (generally, about 1.5m expansion joints should be left at intervals), which will inevitably generate a large expansion force when the temperature rises.

& emsp;& emsp;3.4 abnormal operation of kiln

& emsp;& emsp;In the ignition or stop kiln and kiln skin peeling and other abnormal production conditions, the temperature often appears sharp changes, so that castable produced a huge internal stress, because of thermal fatigue damage.Where the air gun is installed, the service life of the castable is reduced due to the frequent opening and stopping of the air gun.In addition, during the operation of rotary kiln, the huge mechanical (shear) stress caused by the excessive deformation of the cylinder body causes the kiln lining to be subjected to periodic compression and relaxation, resulting in the overall depth damage of refractory castable.

4 solutions

& emsp;& emsp;4.1 the material of anchoring parts and welding rods shall be properly selected to avoid the anchoring parts falling off

& emsp;& emsp;Due to the different materials of steel plate, anchor and welding rod of kiln equipment, the amount of thermal expansion of each part is not consistent when the furnace is heated, resulting in huge thermal stress and easy to lead to welding spot cracking, thus causing the anchor and casting material to fall off together.Therefore, the choice of anchorage and welding rod should be paid attention to be the same as the steel plate material of kiln equipment, to avoid welding joints due to different expansion and cracking.In addition, try to use a single anchor with many welding spots or increase the length of welding spots to improve the welding firmness.As can be & other;Y”V anchor is replaced with “L”Even to put “L”Form a “X”V welding, can also increase the anchoring pieces in the casting material is very thin when the ability to fix, prevent falling off.

4.2 improve thermal shock performance of castable or adopt castable with better thermal shock stability

& emsp;& emsp;Corundum castable has good wear resistance at high temperature and liquid phase erosion resistance, but corundum castable has no micro cracks and large internal pores, and its thermal shock resistance is relatively poor compared with other aggregates.An enterprise USES corundum —After mullite composite aggregate refractory castable, the thermal shock resistance of castable is obviously improved.

& emsp;& emsp;4.3 improve the quality of castable, increase the heat capacity and reduce the temperature fluctuation

& emsp;& emsp;The internal thermal stress of castable is caused by the great change of casting body heat, and the change of casting body heat Q is mainly affected by three aspects: mass m, heat capacity Cp and temperature difference &Delta.T, their relationship Q=m·Cp·& Delta;T.The improvement of mass and heat capacity or the reduction of temperature difference in use can slow down the heat gradient of castable under the same heat transfer condition and greatly reduce the damage of thermal stress.The service temperature of castable parts is determined by the firing system, which will not change much in the whole service process.Similarly, heat capacity belongs to the property of castable itself. For the same type of castable, its heat capacity value is the same, and the heat capacity value of similar castable has little difference.Therefore, the most effective way is to increase the total enthalpy of casting body by improving the quality of casting body, so as to reduce the temperature fluctuation.Increasing the quality and volume of castable can also improve the overall mechanical strength of castable, so as to reduce the mechanical stress damage of some castable caused by equipment deformation during operation.For example, in December 2007, zhonglian taishan cement group co., LTD., for a 5000t/d production line rotary kiln mouth castable technical transformation, significantly increased the kiln mouth axial casting length, from the original 600mm to 1000mm, so that the actual quality of castable increased by 50% than the original.After modification, the service life of kiln mouth castable increased from less than 5 months on average and even less than 3 months on average to more than 1 year.

& emsp;& emsp;4.4 improve construction quality

& emsp;& emsp;Prior to construction of castable, the working area and surrounding environment must be kept clean to prevent contamination of refractory materials by unclean substances, and construction tools and equipment should also be kept clean.If hydraulic refractory castable is adopted, its fluidity is poor when the water content meets its strength, which will have certain influence on the construction.Any increase of water content on this basis will reduce the strength of castable.Therefore, for the convenience of construction, some operators will arbitrarily increase the water amount, resulting in a drop in pouring body performance.Besides adding water, stirring time also influences the fluidity of castable.Someone [1] has made statistics on the relationship between the fluidity of refractory castable with a certain water content and stirring time, as shown in figure 2.

& emsp;& emsp;As can be seen from figure 2, the flow of the castable must be stirred for a certain time (about 3 minutes) before it becomes effective, so it must follow the instructions for the construction of the refractory castable.This requires the factory before and after the construction, must be the masonry process of each specific link of the work procedures for acceptance.

& emsp;& emsp;In addition, curing and baking are also important factors affecting the performance of castable.Curing is to give the binder in the castable a setting and hardening time to obtain sufficient initial strength.The key to control during curing is the humidity of pouring material (which should be kept wet), curing temperature (which should be between 15 and 35℃) and time.Baking is to evaporate the remaining water out of the castable after hardening, and promote the sintering process at high temperature to further improve the physical properties of the castable.The main control in the baking process is the heating rate, generally 15 ~ 35℃ per hour.Once the temperature rise begins, the temperature rise process cannot be interrupted or stopped until the end of the temperature rise.Otherwise, great care should be taken to cool down the lining and, when reheating, follow the prescribed steps.For example, a company [2] temporarily decided to replace the top castable in the high temperature area of grate cooler.In order to ensure that the materials are delivered according to the predetermined plan, the construction team adopts the whole pouring, and no expansion joints are reserved as required.At the same time, in order to facilitate construction, the water consumption is obviously larger than the required usage.More serious is the end of the construction will immediately fire heating, and as scheduled.Due to the rapid temperature rise, the temperature of castable increases sharply, and a large amount of internal moisture evaporates rapidly, generating a large expansion force, which causes the castable to burst and flake off, causing a large area of falling off accidents.This is typically done without curing and baking, with serious consequences.

5 conclusion

& emsp;& emsp;In actual production process, the cause of the damage of refractory castable and solving measures may be more than the above mentioned, and they tend to be linked, therefore should according to particular case is particular analysis and flexible treatment, so as to give full play to the unique properties of refractory castable, achieve the best use effect.


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