In 2008, China produced more than 1.3 billion tons of cement annually, and most of the clinkers used were calcined in rotary kilns. In the past 30 years, PC and SP kilns have been put into production. The rotary speed of the kiln has increased by 3-4 times, the working temperature is high, the heat load is large, the composition of raw materials is easy to change, and the amount of evaporation and condensation of harmful alkali metals, sulfur and chlorine is large, resulting in serious alkali corrosion. Therefore, this kind of kiln has a high demand for refractory materials and quality, so we should face it seriously.
In traditional cement rotary kilns, unshaped refractories are mainly used in front and rear kiln portals and chain belts of wet-process kilns, such as high alumina refractory castables and steel brazing reinforced refractory castables, etc. In PC and SP kilns, besides front and rear kiln portals, unshaped refractories are used in splitter, preheater, tertiary air duct, kiln door cover, etc. Amorphous refractories, such as corundum and high alumina refractory castables, steel fiber reinforced refractory castables, alkali-resistant castables, self-flowing refractory castables and heat-insulating castables, are also widely used in parts such as chillers and so on. The dosage of amorphous refractories accounts for about one third of the total refractories used in kilns. Therefore, the amorphous refractory plays an important role in the new type of kiln.
1. use conditions of refractory castables for rotary kiln mouth
The refractory lining of rotary kiln mouth is the weak link of kiln lining. Especially the refractory lining of front kiln mouth is used at about 1100 C. It needs to withstand the impact of quench and hot air flow, high temperature cement clinker wear and high temperature harmful gas alkali corrosion. The damage of liner is mainly cracking, spalling and abrasion, so refractory must have good thermal shock resistance, wear resistance and alkali resistance.
When the cylinder of rotary kiln mouth is not deformed, it can be generally masonry with anti-stripping high-alumina brick, spinel brick and directly bonded magnesia-chrome brick and other stereotyped products, and use high-quality mud as joint material, so as to form a whole, prevent brick dropping, and improve life. However, the deformation of the cylinder is widespread, so the refractory castable is used to pour, the steel shell is welded with rivets, and the castable is mixed with steel fiber reinforced refractory castable. The castable is constructed and baked according to the regulations to meet the production needs of the kiln mouth cylinder lining.
In foreign countries, corundum and high alumina steel fiber reinforced refractory castables are widely used in front of cement kilns. Their service life can reach 2-8. The castables have high aluminum content and high strength. Therefore, the thermal shock resistance is poor, the cracks are easy to peel off, and the service life is reduced. The production practice shows that the effect of steel fiber reinforced high alumina refractory castable is better than corundum castable.
Development of refractory castables for 2. rotary kiln mouth
Steel fiber reinforced corundum and high alumina refractory castables for rotary kiln entrance are prone to crack and spalling during years of use, i.e. poor thermal shock resistance and low service life. Accordingly, Beijing Tongda, Zhejiang Jincheng and Kaifeng Special Resistance Units have developed high alumina refractory castables, high alumina mullite castables and mullite refractory castables. The service life of these castables is more than one year and good results have been obtained.
It is well known that clay brick is superior to high alumina brick in thermal shock resistance and alkali resistance because of its low alumina content, high strength, good wear resistance and poor seismic resistance. In castables for kiln mouth, Al2O3
The content is 50%-70%. The strength is moderate (both wear resistance and thermal shock resistance are suitable). The drying and compressive strength (1100 C) are about 40 MPa and 70 MPa, respectively. It should be pointed out that refractory castables are composed of aggregates, powders, binders and admixtures. The phase composition of the matrix is in a non-equilibrium state before use. Under the action of high temperature when used, the components of the matrix will follow the phase equilibrium relationship to reconstitute a new phase equilibrium (i.e. obtain excellent performance and in-situ reaction). It should be). Therefore, it is necessary to select suitable compositions and optimize the batching design to achieve good sintering, volume stability, corrosion resistance and high temperature mechanical properties.
Selection of main materials
The refractory lining at the front kiln mouth can withstand the abrasion and alkali corrosion of cement clinker in the range of 800 ~1250 ~C because of its frequent quench and heat. The selection of the main material of refractory castable must be decided according to its use conditions in order to achieve the purpose of low cost and easy use.
The refractory aggregate is made of bauxite clinker, pyroxene and mullite, the critical particle size is enlarged, and the grain gradation should be reasonably carried out to achieve densification, which is conducive to improving the thermal shock resistance and strength of the material.
Refractories, binders and additives are the matrix materials of refractory castables, and reasonable particle size distribution must be carried out in order to improve their properties. At room temperature, the strength of the construction can be obtained. Under high temperature, the phase transformation and sintering are easy to occur. At the same time, in-situ reaction occurs between the materials, and the micro-expansion can counteract the sintering shrinkage and improve the thermal shock resistance and strength of the castables.
The refractory powder is composed of corundum, bauxite clinker, mullite and silicon carbide, which are broken into various grain sizes. At the same time, the superfine powder should be filled with voids, and it has high activity and easy to produce in situ reaction to improve matrix properties.
The bonding agent is calcium aluminate cement, superfine powder and phosphate, etc. It is commonly used in combination, and its dosage should ensure the demoulding strength and make the drying compressive strength reach about 40 MPa.
Selection of additives (substances)
As amorphous refractories are typical in-situ reactive refractories, the selection of additives is very important. It is necessary to ensure the construction performance, baking performance and in-situ reaction at high temperature to obtain excellent material properties. The additives used in kiln mouth refractory castable are mainly dispersant, accelerator, plasticizer, sintering agent, expander and explosion-proof agent, etc. This paper mainly introduces the sintering agent, its varieties of materials are raw clay, zircon, titanium dioxide and pudding. This kind of material reacts in situ at service temperature to produce a certain number of mullite crystals, zircon phase, titanium aluminate crystals and inconsistent melting compounds (2Al2O3. B2O3), etc., accompanied by volume expansion and micro-cracks, which can compensate for its sintering shrinkage, thus resulting in sintering strength, corrosion resistance and good performance of castables. Good thermal shock resistance.
The amount of sintering agent varies with the variety of materials (generally 1%-7%), excessive use of sintering agent, in-situ reaction produces larger expansion and liquid phase, resulting in the castable thermal shock resistance and corrosion resistance reduced, but also affects the use effect. Therefore, the variety and quantity of sintering agent should be selected reasonably according to the requirements of using temperature, thickness of sintering layer, volume stability and corrosion resistance, so as to meet the actual use requirements.
The additives are mainly heat-resistant steel fibers. In the refractory castable for kiln mouth, 446 and 304 steel fibers are selected, which can meet the use requirements (dosage is 1%-4%). Its function is to suppress cracks in matrix or prevent crack propagation and improve thermal shock resistance of materials. Experiments show that the flexural strength of steel fiber reinforced refractory castables after 10 cycles is 3-4 times higher than that of castables without steel fiber (i.e. improving the thermal shock resistance). The service life of the refractory lining at the front kiln mouth is longer than one year. Therefore, heat-resistant steel fibers must be added to refractory castables at kiln outlet. It should be pointed out that when preparing steel fiber refractory castable, steel fiber must be mixed evenly in the castable to prevent steel fiber agglomeration, otherwise the service life of refractory lining at kiln mouth will be reduced.
In a word, the material composition of refractory castable for rotary kiln mouth should be determined according to its use conditions, and the in-situ reaction, sintering and volume stability between matrix materials should be guaranteed, so as to obtain good flexural strength, thermal shock resistance and corrosion resistance. At the same time, proper addition of additives (substances), and reasonable selection of its varieties and dosage, can improve the construction performance and usability of materials. Steel fiber reinforced alumina-mullite refractory castables have achieved initial results in kiln mouth and should be widely used. [author]: Han Hanglu |
