ASC castables for iron troughs are generally low-cement or ultra low cement castables, which are mainly composed of Al2O3 aggregate, SiC, carbon, cement and various additives.
Al2O3 aggregate is the main component of the castable and is the particle skeleton of the castable. Al2O3 aggregate mainly includes fused white corundum, brown corundum, sub-white corundum, sintered alumina and high-alumina bauxite clinker. When used, it should be selected comprehensively according to the use conditions of the material. High-grade materials use fused dense corundum, mid-range materials use brown corundum, and low-grade materials use sintered corundum or bauxite clinker.
The main reasons for adding SiC to castables are: (1) it can effectively prevent carbon oxidation and improve the oxidation resistance of the refractory material of the iron trough; (2) SiC has a low expansion coefficient, which is only half of Al₂O₃, which can prevent cracking during the heating and cooling process of ASC castables; (3) SiC has high thermal conductivity, which can improve the thermal shock resistance of ASC castables; (4) SiO2, CO and CO2 produced after SiC oxidation can effectively inhibit the oxidation of materials; (5) SiC can effectively improve the anti-scouring performance of materials. However, when the amount of SiC added is too large, the high-temperature strength of the material decreases, so the amount of SiC added needs to be controlled at 10%~25%. Research and field use have shown that a high SiC content can improve the castable's resistance to slag erosion. Therefore, the SiC content of castables is often as high as 20% or more.
In Al2O3-SiC-C castables, carbon can prevent slag from penetrating into the material; it can confine slag to the surface of the refractory material, thereby improving the material's corrosion resistance; at the same time, carbon can also improve the material's thermal conductivity, improve the material's thermal shock resistance, and reduce the material's structural peeling and cracking. Carbon can be added in the form of graphite, carbon black, asphalt coke, and other raw materials. Of course, the effect of carbon in the castable is related to the amount of carbon added and the type of carbon raw material. Carbon is usually added in the form of asphalt balls or coke, with an addition amount of about 5%.
Al2O3-SiC-C castables for iron ditch usually use high-alumina cement and pure calcium aluminate cement as binders. Cement is added to maintain the low-temperature and medium-temperature strength of the material; when cement is added, a small amount of CaO will be introduced, which is not conducive to the material's corrosion resistance; in addition, as the amount of cement increases, the water demand of the castable increases, resulting in an increase in the castable's porosity, a decrease in bulk density, and a decrease in corrosion resistance. Therefore, the Al2O3-SiC-C castable used for iron ditch is generally low cement castable and ultra-low cement castable, and the total content of CaO in the castable is controlled below 1.0%~2.5%.
Adding silicon powder can react with carbon in the material to generate SiC at a certain temperature. The generated SiC is in two states in the matrix. One is very fine SiC whiskers with a diameter of about 0.1~0.5μm. It is distributed between the particles of the matrix and plays a bridging role. It has a strong reinforcement effect and can improve the high-temperature strength of the castable. The other is worm-like or flocculent SiC, which can improve the microstructure of the castable and form Al2O3-SiC-C material combined with SiC, which can improve the oxidation resistance and slag resistance of the castable.
(6) Metal aluminum powder
Because metal aluminum powder can react with water in the castable to generate H2, after the H2 is removed, fine exhaust holes are left, which is conducive to the discharge of internal moisture, can remove some free water, and at the same time prevent bursting during baking. The heat released during the reaction can also speed up the dehydration speed, accelerate the coagulation and hardening process of the castable, and improve the strength of the castable; in addition, the Al(OH)3 gel generated after the reaction can form a new binding phase, which can also improve the strength of the castable.
However, the amount of metal aluminum powder added should not be too much, otherwise it will release a large amount of hydrogen, leaving too many pores, making the material structure loose, reducing strength, and worsening corrosion resistance.
(7) Organic fiber
Organic fiber can prevent the castable from bursting during the baking process, because the organic fiber is burned away during the drying process of the castable, leaving an exhaust channel, which is conducive to the discharge of water in the castable.
(8) Sodium polyphosphate
After sodium polyphosphate is added to the castable, it can increase the volume density of the castable, reduce the porosity of the material, increase the strength, and improve the construction performance of the castable due to its dispersing and water-reducing effect. The selected sodium polyphosphates are mainly sodium tripolyphosphate and sodium hexametaphosphate.
(9) Retarder or accelerator
Retarder or accelerator is added to the castable to adjust the use time of the castable and make the castable have better construction performance. Commonly used accelerators for calcium aluminate cement are: NaOH, KOH, Ca(OH)2, Na2CO3, K2CO3, Na2SiO3, etc.; commonly used retarders for calcium aluminate cement are: NaCl, BaCl2, MgCl2, CaCl2, citric acid, tartaric acid, gluconic acid, ethylene glycol, phosphates and wood iodates, etc.
IPv6 network supported
español
العربية
