Strengthening Mechanism Of Magnetic Field In Low Temperature Reduction Of Iron-bearing Minerals

In view of the low utilization rate of low-grade and composite co-associated mineral resources,the?low temperature reduction-magnetic separation or melting?idea is applied to separate iron from other valuable elements.In order to improve reaction efficiency of low-temperature reduction,The thesis proposes to apply static magnetic field to the iron ore reduction process for the first time,aiming to realize strengthening effect of magnetic field on iron ore reduction,and providing theoretical basis and technical support for developing low-grade,composite co-associated iron ore low-temperature magnetic field enhanced reduction technology.Based on classical thermodynamic model,solid phase transformation theory and magnetic energy theory,the thesis introduces magnetic energy to solid-state reduction process of iron-bearing minerals,and proposes magnetic energy reaction thermodynamic model and magnetic energy nucleation thermodynamic model.It is found that the magnetic field is able to affect equilibrium state of chemical reaction,and it is favorable for inducing new phase to overcome the critical nucleation energy.The effect degree depends on magnetic field intensity,electronic structure of different valence iron oxides and interaction with the applied magnetic field.For the reactions of Fe₂O₃?Fe₃O₄ and Fe_xO?Fe,the magnetic field reduces Gibbs free energy of the reaction and increases the equilibrium constant;for Fe₃O₄?Fe_xO,the magnetic field reduces equilibrium constant of the reaction.The magnetic field can reduce the critical nucleation radius of metallic iron,and the nucleation-promoting effect of magnetic field is more significant under low reduction temperature conditions.On the other hand,thermodynamic analysis on stress magnetic effect of magnetic medium indicates that the magnetic field increases lattice constant of the iron oxide and expands its crystal.The isothermal weight loss experiment was applied to study the reaction characteristics of iron oxides under the action of magnetic field,and evaluate the effect of magnetic field on iron oxide gradual reduction process from reaction efficiency,phase composition and structure morphology.It is found that application of magnetic field can significantly improve reaction efficiency of iron oxide reduction and shorten the reduction time.For the Fe₃O₄?Fe_xO reaction,when reduction degree reaches 96%,the reaction time is only 1/2 of that under the non-magnetic condition;for the Fe_xO?Fe reaction,when t=10 min,the metallization rate is 2.26 times of that under the non-magnetic condition,and when the metallization rate reaches 70%,reduction time is shortened by 60 min.The effect of the magnetic field weakens as the reaction temperature and time increase,namely,the magnetic field promotes iron oxide solid-state reduction more significantly under low temperature and in the initial stage of the reaction.Under action of the magnetic field,reduction of iron oxide is carried out in the order of Fe₂O₃?Fe₃O₄?Fe_xO?Fe to reduce the sample structure to a loose state,and a small amount of new phase inside the dense reactant shows a dotted growing.On the basis of the gas-solid reaction kinetic model,the reaction kinetics of iron oxide reduction under magnetic field conditions was analyzed.It is found that the strengthening effect of magnetic field on iron oxide reduction is mainly achieved by increasing the interfacial chemical reaction rate and improving the gas diffusion kinetic condition.During the formation of Fe,when the temperature is 750-850°C,with magnetic field applied the reaction rate constant increases by 9%-27%and the diffusion coefficient of gas in porous media increases by 17%-85%.Consequently,magnetic field has a more significant effect on reducing the diffusion resistance of reducing gas.Based on the classical nucleation theory and diffusion theory,the reactant activated molecule concentration and Fe atom diffusion were studied to explore the effect of magnetic field on Fe atom nucleation growth kinetics,and the effect of magnetic field on Fe nucleation growth orientation was analyzed from the perspective of crystal structure.Studies showed that the magnetic field significantly increases concentration of the activated molecules in reactant.During the conversion of Fe_xO?Fe,the concentration of Fe_xO activated molecules involved in the reaction increases by nearly two orders of magnitude.Magnetic field promotes diffusion of the Fe atoms by reducing the vacancy formation energy and increasing the vacancy concentration.Compared with circumstances of that under normal conditions,under the magnetic field the diffusion coefficient of Fe atom is increased by 29%and the diffusion activation energy is 72.55 KJ/mol.It proves that the static magnetic field accelerates the nucleation rate of Fe atoms and promotes growth of the metallic iron layer.The relation between the ratio of Fe nucleation rates under magnetic and non-magnetic conditions?35?I and the reduction temperature T?°C?is?35?I?28?65.402e^{?7?-.0002T?8?}.When it comes to Fe crystal structure,Fe_xO preferentially precipitates?Fe crystal nucleus on{100}crystal plane under the action of magnetic field,which is consistent with the conventional metallurgical conditions;growth direction of?Fe deflects gradually from the preferential<100>direction to<100>,<101>and<111>directions.Studies on reaction behaviors and occurrence states of mineral oxides and associated elements under magnetic field showed that the effect of magnetic field on Ca₃?PO₄?₂,Ca O and Si O₂ accelerates the reduction of iron oxide.It is mainly reflected in that the stress magnetic effect on reaction system generated by the applied magnetic field changes microscopic morphology of the iron-bearing minerals,promotes the diffusion of Ca in Fe O_x,and makes the morphology and the extent of aggregation of Ca₃?PO₄?₂ and the iron-bearing phase,favorable for the separation of Fe and P.The enrichment degree of P in the slag phase under magnetic field is higher than that under normal conditions.Studies on characteristics of solid-state reduction of low-grade Bayan Obo mine showed that the magnetic field is able to significantly improve reduction efficiency of the Bayan Obo mine carbon-containing pellets achieving the reduction degree of93.18%at 60 min,which is 2.22 times of that under non-magnetic condition.This clearly indicates that the low-temperature rapid reduction of iron ore can be realized by applying magnetic field.Meanwhile,it is also found that the effect of magnetic field on reduction of low-grade iron ore is obviously different from that of iron oxide,which indicates that multiple simultaneous solid reactions in iron ore,such as iron oxide,other element oxides and gangue minerals,have their own characteristics and interact with each other,jointly promoting the formation of metallic iron.Strengthening effect of the magnetic field on iron ore reduction will decrease as the TFe increases,revealing that magnetic field is an effective strengthening method to process low-grade refractory iron ore through direct reduction technology.