| Steel industry is energy-intensive,and the blast furnace(BF)ironmaking is crucial for energy conservation and emission reduction.Currently,the research of low carbon BF technologies mainly focus on utilization of carbon iron composite,hydrogen-bearing materials injection,top gas recycling,and oxygen BF.An innovative burden technology of iron coke hot briquette(ICHB,a kind of carbon iron composite)was proposed in this study based on the domestic raw materials.In terms of the new burden,the emphatically studied issues include the preparation process of ICHB,the optimization of metallurgical properties of ICHB,the gasification reaction behavior and kinetics of ICHB,the evaluation of high temperature metallurgical properties of ICHB,the influence mechanism of ICHB on the reaction process of BF ironmaking burdens,and the effects of ICHB on the softening-melting and dripping properties of BF comprehensive burdens.Furthermore,the synergic optimization mechanism for the metallurgical properties of ICHB,the catalytic mechanism of metal iron on the gasification of ICHB,and the coupling effect mechanism between the gasification of ICHB and the reduction of iron-bearing burden were revealed.The preparation process of ICHB was investigated.It was showed that the ICHB with high compressive strength are achieved through reasonably adjusting the process parameters such as iron ore and coal ratios,hot briquetting temperature,carbonization temperature and time.The appropriate preparation conditions are 15%iron ore,65%soft coal A,20%anthracite,300? hot briquetting temperature,1000? carbonization temperature,and 4 h time(constant temperature time).Under these conditions,the compressive strengths of ICHB before and after carbonization are 736 N and 5049 N,respectively,and the metallization degree is approximately 75.72%.The optimization for the metallurgical properties of ICHB by iron ore and coal blending was studied.It was indicated that the compressive strength,drum strength,reactivity,and post-reaction strength of ICHB can be synergistically optimized through adjusting the composition of blended coal and the type of iron ore.The optimized preparation conditions are 15%iron ore B,55%bituminous coal A,10%bituminous coal C,20%anthracite,300? hot briquetting temperature,1000? carbonization temperature,and 4 h carbonization time(constant temperature time).Under the optimization conditions,the compressive strengths of ICHB before and after carbonization are 775 N and 4970 N,respectively,and the drum strength is about 95.97%.Moreover,the reactivity of ICHB is approximately 61.19%and the post-reaction strength is 51.23%.The evaluation methods for the high temperature metallurgical properties of ICHB were researched.The results showed that the reactivity of ICHB should be evaluated under the modified conditions based on GB/T 4000-2017 or under the simulating BF conditions,and the post-reaction strength should be estimated by the condition of constant weight loss ratio or by the condition of simulating BF atmosphere.Simultaneously,the calculation equation of ICHB reactivity based on the weight loss ratio should be modified due to the effect of metal iron.For the high temperature metallurgical properties of ICHB with adding 15%iron ore B,the modified reactivity under GB/T 4000-2017 is approximately 61.19%,which is 68.35%under the simulating BF conditions.Furthermore,the post-reaction strength of ICHB under the simulating BF atmosphere is about 53.42%,which is approximately 80.70%(the value of coke is about 73.41%)with 20%weight loss ratio.Compared with the conventional coke,it has good high temperature metallurgical properties for ICHB under the new evaluation method.The gasification reaction behavior and kinetics of ICHB were studied.It was demonstrated that the starting temperature of ICHB reacted with CO2 is gradually decreased from 919? to 839? with the addition ratio of iron ore increasing from 0%to 20%.The most probable mechanism function for the non-isothermal gasification of ICHB with adding different ratios of iron ore is the phase boundary reaction model,whose integral form and differential form are G(x)=(1-x)-1-1 and f(x)=(1-x)2,respectively.Under this model,the activation energy is reduced from 312.78 to 249.83 kJ/mol,and the natural logarithm of the pre-exponential factor is decreased from 26.29 min-1 to 21.81 min-1 with increasing the ratio of iron ore from 0%to 20%.Meanwhile,they display the kinetics compensation effect.In addition,the shrinking core model is the best mechanism to describe the isothermal gasification process of ICHB.The integral form and differential form are g(x)=1-(1-x)1/2 and f(x)=2(1-x)1/2,respectively.The activation energy for this model is 144.77 kJ/mol,and the pre-exponential factor is 999.37 min-1.The catalytic mechanism of metallic iron on the gasification reaction of ICHB was investigated.It also showed that the metallic iron acts as a catalyst to promote the gasification of ICHB,which is used to transfer oxygen through redox reaction.In this process,metallic iron is oxidized to FeO by CO2 gas,and then FeO is immediately reduced by surrounding carbon to generate metal iron again.The generated metal iron is recombined with the surrounding CO2 again to cycle this process.Under this circumstance,the gasification of ICHB is continuously promoted by this process.The influence mechanisms of ICHB on the reaction process of ironmaking burdens were investigated.It was indicated that ICHB can protect the conventional coke from gasifying with CO2,but the protection has a certain limitation.Also,the reduction process of iron-bearing burdens can be accelerated by ICHB,and the enhancement is more obvious with mixed charging ICHB in comparison to layered charging.Simultaneously,the promotion of ICHB on the reduction of iron-bearing burdens is more remarkable compared with conventional coke.Moreover,the increase in the reduction temperature can strengthen the reduction of iron oxide and the gasification of ICHB,so the coupling effect between the reduction of iron oxide and the gasification of ICHB was enhanced.In addition,with increasing the charging ratio of ICHB from 0%to 30%,the reduction degree of pellet and sinter is increased from 23.69%to 33.72%and from 25.72%to 29.71%,respectively.In this process,the increase in the reduction degree of iron-bearing burdens with adding ICHB is gradually leveled off,which declare that the acceleration of ICHB on the reduction of iron-bearing burdens has a limitation due to the low reducibility of iron-bearing burdens.The coupling effect between the reduction of iron-bearing burdens with a higher reducibility and the gasification of ICHB is more significant.Therefore,it is better in practical application that ICHB is used with iron-bearing burdens with good reducibility.The effects of ICHB on the softening-melting and dripping behaviors of BF comprehensive burdens were studied.It was showed that the softening interval of comprehensive burdens is increased from 206.3? to 218.9?,while the melting interval is decreased from 171.1? to 124.8? with increasing the charging ratio of ICHB from 0%to 40%.Additionally,the dripping ratio is first increased and then decreased,which reach to 40.58%with charging 30%ICHB.The S-value of comprehensive burdens is gradually reduced,which indicated the permeability becomes better.In terms of softening and dripping properties of comprehensive burdens,the charging ratio of ICHB should not exceed 30%of coke.The study will benefit to enrich the theory and technology system of the new burden of ICHB,and lays the foundation for the practical application of ICHB.Also,it can effectively promote the progress and development of low carbon BF ironmaking technology in China. |
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