Study On The Defluidization Inhibition Of Iron Ore Fines During Multi-stage Fluidized Bed Reduction

Gaseous fluidized bed reduction directly uses iron ore powders to produce sponge iron,avoiding pelletizing,sintering,and coking,thereby becoming a promising direct reduction technology.Nowadays,due to the decrease of high-quality iron ore,the content of fine powders with a particle size around 100 ?m in the iron ore concentrates obtained after beneficiation increases.The reduction rate of fine powders is very high,which could maximize the advantages of fluidized bed reduction.However,during high temperature fluidized bed reduction,the fine powders readily stick to form big agglomerates due to the formation of fibrous iron or the increase of surface viscosity,and then deadly defluidization might happen,which would cause a great loss to industrial production.Therefore,in this work,taking the intermediates that appear during the multi-stage fluidized bed reduction,such as particles of low metallization degree,Fe3O4,FeO and particles of rich carbon,as the research subjects,a series of efficient methods for inhibiting the defluidization of iron ore fines are proposed by simulating the industrial multi-stage fluidized bed reduction processes,and the inhibition mechanisms are studied.The novelties and important conclusions of this work are summarized as follows:The influence of reduction conditions on the morphology of newly formed metallic iron and the corresponding agglomeration behavior are opened out,and the solutions for inhibiting defluidization through morphological control of iron are proposed.Addition of H2 in CO could not only accelerate the growth rate of iron grains,but also increase the amount of iron nuclei,resulting in the transformation of fibrous iron into dense one;addition of CO2 in CO could transform the "sharp-pointed" iron whiskers into "cactus-like" ones;decrease of reduction temperature could lower the strength of fibrous iron.Those could notably reduce the amount of agglomerates formed at the initial reduction stage.Furthermore,the transformation of iron morphology from fibrous to dense at the initial reduction stage could effectively reduce the required MgO content for inhibiting the defluidization of particles of high metallization degree at the final reduction stage.The role of MgO in preventing the defluidization of fine iron ores with different iron valences is addressed,and the optimal addition stage of MgO for inhibiting defluidization is suggested.Based on the studies of solid-state reactions between MgO and iron oxides at 700-900 ?,the inhibition effect of MgO is mainly due to the physical spacer effect at low and moderate temperatures(700 and 800 ?).The inhibition effect is valence-dependent at a high temperature(900 ?).For Fe2O3,the physical spacer effect is still the dominant reason.For Fe3O4 and FeO,the chemical spacer effect plays a decisive role in defluidization inhibition,and the chemical barrier layer formed by MgO and FeO is stronger than that formed by MgO and Fe3O4.Chemical spacer effect is more efficient than physical spacer effect for inhibiting defluidization.Therefore,for the multi-stage fluidized bed reduction process where Fe3O4 and FeO stably exist,the inhibition effect of MgO could be sequenced as FeO>Fe3O4>Fe2O3 at the high reduction temperature.The addition of MgO for inhibiting defluidization has little effect on the reduction rate of iron ore fines.A new defluidization-inhibiting additive CaO/Fe2O3 is prepared for enhancing the inhibition effect of Ca species,and the inhibition mechanism is clarified.Experimental results indicate that the Ca species coated on the surface of iron ore fines through pure CaO and Ca(NO3)2·4H2O show poor inhibition performance,while it is strengthened by the introduction of Fe(NO3)3·9H2O.Reduction results of CaO/Fe2O3 suggest that Ca species mainly inhibit defluidization through physical spacer effect.Microstructure observations indicate that CaO/Fe2O3 could not only suppress the growth of "sharp-pointed" iron,but also make Ca species coat tightly on the surface of sticky iron,thereby decreasing the cohesiveness.Moreover,Fe2O3 is demonstrated to be a general binder capable of enhancing the performance of poor additives for inhibiting defluidization.The deposition and evolution behaviors of carbon during multi-stage fluidized bed reduction of iron ore fines are opened out.Deposited carbon is not only an effective agglomeration-inhibitor,but also an excellent reductant.Experimental results show that high reduction potential,low reduction temperature,and H2 addition could accelerate the rate of carbon deposition,especially the formation of graphite.Graphite could suppress the formation of fibrous iron and decrease the surface viscosity,thereby inhibiting agglomeration at the high temperature reduction stage.Both graphite and Fe3C may be consumed through gasification reaction and solid-state reduction reaction during the final high temperature reduction,and graphite is more reactive than Fe3C.To reinforce the application of fluidization technology in producing sponge iron,a novel solid-state high temperature reduction method via deposited carbon is proposed and demonstrated to be feasible.In this thesis,we have focused on solving the problem of defluidization during multi-stage fluidized bed reduction of iron ore fines.Based on the evolution of iron ore fines under different reduction conditions,efficient solutions are proposed,such as morphological control of iron for preventing the agglomeration of particles of low metallization degree at the initial reduction stage,meanwhile decreasing the required MgO content for inhibiting defluidization at the final reduction stage;addition of MgO at FeO stage for efficiently inhibiting defluidization through the strong chemical spacer effect;preparation of the new defluidization-inhibitor CaO/Fe2O3 to enhance the inhibition effect of Ca species.Furthermore,through deep study on the deposition and evolution behaviors of carbon during multi-stage fluidized bed reduction,a novel solid-state high temperature reduction method via deposited carbon is proposed.Comparing with the previous studies,we have provided more practical,more efficient and more operable solutions for inhibiting defluidization.