Binding Mechanism And Improvement Of High-temperature Thermal/mechanical Properties Of Self-binding Iron Ditch Castable

The iron ditch is a vital facility of the blast furnace.It is also an important channel for separating liquid slag from molten iron and making molten iron led to steelmaking process at the same time.The quality and performance of its working lining determine whether the blast furnace tapping can be carried out continuously.The binding systems of Al₂O₃-SiC-C castables used for the lining of iron ditch are varied due to the influence of the size,shape and environmental conditions of the iron runner.The binding system not only determines the construction performance and mechanical strength of Al₂O₃-SiC-C castable,but also affects the interface state,microstructure,and service performance of the bonded material during baking process and under high temperature service state.The main problems faced by iron ditch castable in the service process are as follows:higher and higher tapping temperature,increased flow rate and velocity of the liquid slag and molten iron.Meanwhile,the casting construction and maintenance time of iron ditch castable is short,the working temperature of iron ditch castable fluctuates greatly before and after iron tapping,and it continues to bear the scouring and erosion of molten iron and slag under high temperature.In view of the above factors,in addition to excellent high-temperature mechanical properties,iron ditch castable must also have good construction performance,high early strength and hot modulus of rupture,good thermal shock stability and excellent oxidation resistance and slag erosion resistance.This undoubtedly puts forward more stringent requirements for the binding system of iron ditch castable.At present,pure calcium aluminate cement or sol is mainly used as the binding agent in the Al₂O₃-SiC-C iron ditch castable binding system.However,in the traditional pure calcium aluminate cement（PCAC）or low cement（LCC）bonded iron ditch castables,due to the introduction of Ca O component in the calcium aluminate cement,it is bound to react with Al₂O₃ and SiO₂ to form anorthite and other low melting substances at high temperature,which will reduce the high temperature strength of the iron ditch castables at high temperature,thus affecting its resistance to molten iron and slag scouring under high temperature.The selection of silica sol or aluminum sol as the binding agent in iron ditch castable has the problems such as low early strength and uncontrollable hardening rate,poor construction performance and thermal/mechanical properties.Based on this,in this paper,the densed fused alumina and SiC were used as the main raw materials,the imported ultra-high purity submicron SiO₂ powder was used as the sole self-binding binder,and the ultra-high purity submicron SiO₂ powder combined with activeα-Al₂O₃ powder were served as the composite self-binding binder.The effects of submicron SiO₂ powder and SiO₂-Al₂O₃ composite self-binding system on the bonding properties of Al₂O₃-SiC-C castables were studied respectively.The effects of ultra-high purity submicron SiO₂ powder andα-Al₂O₃ powder on sintering properties and high temperature strength were systematically studied by measuring the rheological properties of the prepared castable with self-binding system.The bonding behavior of Al₂O₃-SiC-C castables with different binding systems were simulated,calculated,analyzed and compared by using X-ray diffraction,scanning electron microscope,energy spectrum analyzer and other analysis and testing techniques,combined with the test and analysis results of mechanical properties under different temperature conditions and theoretical calculation by Fact Sage thermodynamic software.Meanwhile,ordinary blast furnace slag and titanium containing blast furnace slag were selected to compare and analyze the erosion resistance of conventional bonded,self-bonded Al₂O₃-SiC-C castables,which revealing the erosion mechanism of iron ditch castables and the improvement mechanism of erosion resistance.The following meaningful results were obtained:（1）Ultra-high purity submicron SiO₂ powder is pure white powder with high purity（w（SiO₂）=99.9%）.Under the microscope,it presents a spherical structure,and the particle size distribution range is 0.1～1.0μm with a median diameter of D₅₀=0.242μm.This fine spherical structure is easy to disperse in the dispersion system and achieve close filling in the matrix system.Submicron SiO₂ powder could be hydrolyzed to form silicic acid with p H value of 3.51.The polycondensation reaction between silicic acids could form a stable"-Si-O-Si-"structure network to generate strength of the castable,so as to realize the self-binding of Al₂O₃-SiC-C castable.In the Al₂O₃-SiC-C castable system,the addition amount of sole submicron SiO₂ powder was5wt%.The hot modulus of rupture of the sample under the condition of 1400℃×1h could reach 11.07MPa.（2）In Al₂O₃-SiC-C castable,the study of self-binding composite powders（submicron SiO₂ powder combined with activeα-Al₂O₃ powders）showed that the viscosity of the castable binding system decreased with the increase of the addition amount of activeα-Al₂O₃ powder,which was beneficial to improve the fluidity of the castable.However,the rheological properties of Al₂O₃-SiC-C castable changed from pseudoplastic fluid to Newtonian fluid when the addition amount of activeα-Al₂O₃powder exceeded a certain value,which was not conducive to improve the binding properties of castables.On the basis of adding 5wt%submicron SiO₂ powder,the optimum addition amount of activeα-Al₂O₃ powder was 4wt%.（3）The comparative study on the sintering properties of Al₂O₃-SiC-C castable samples with three different binding systems showed that,after heat treatment under110℃for 24h,the modulus of rupture of the composite micropowder self-binding castable samples at room temperature was lower（4.70MPa）than that of the pure calcium aluminate cement-bonded（9.00MPa）and silica sol-bonded（5.20MPa）samples,however,the hot modulus of rupture of the self-binding castable sample under 1400℃for 1h was up to 15.63MPa,which was obviously higher than that of Al₂O₃-SiC-C castable samples bonded with pure calcium aluminate cement（3.20MPa）and silica sol（6.40MPa）.The excellent hot modulus of rupture of the self-binding Al₂O₃-SiC-C castable was mainly due to the decrease of the content of low melt phase at high temperature and the enhancement of the in-situ formation of mullite phase in the binding system.（4）The calculation results of Al₂O₃-SiO₂-MO（M=Ca、Mg、Fe、Ti）system by Fact Sage thermodynamic theory showed that the Ca O content was the main factor affecting the amount of liquid phase in Al₂O₃-SiO₂-MO system.At the temperature of1500℃,for every 1wt%increased of Ca O content,the amount of liquid phase in the system increased by about 5%,and the amount of liquid phase gradually increased with the rising of temperature.（5）Study on the slag erosion resistance of Al₂O₃-SiC-C castable bonded with three binding systems by two kinds of blast furnace slag showed that,the erosion effect of titanium-bearing slag on Al₂O₃-SiC-C castable was weaker than that of ordinary furnace slag,which was attributed to the chemical components of Ti O₂ and Ca O in the titanium-bearing blast furnace slag and chemical components of Al₂O₃ in the working face of castable could form Ca-Al-Ti-O compounds with high melting point at high temperature,which could hinder the corrosion of slag.The slag erosion resistance of the composite micropowder self-binding Al₂O₃-SiC-C castable was obviously better than that of pure calcium aluminate cement and silica sol binding Al₂O₃-SiC-C castable because of its high strength and less liquid phase at high temperature.