Fundamental Study On Coal-based Reduction Of Vanadium-titanium Magnetite Concentrates With Multi-layer Pellets In Rotary Hearth Furnace

There are huge amount of vanadium-titanium magnetite in Panxi area, China,containing iron, vanadium and titanium as main valuable elements and with thecoexistence of other strategic metal elements, such as chromium, cobalt, nickel andscandium, which has high value for comprehensive utilization. When these ore arebeneficiated, vanadium-titanium magnetite concentrates and ilmenite concentrates areproduced. Conventionally, titanomagnetite concentrates are treated by blast furnace.Most of iron and partly of vanadium can be extracted, however, almost all of titaniumgo into the slag containing22²5%TiO₂. There is no an appropriate and economicmethod so far to deal with the slag to recovery the titanium components from it,resulting in some issues such as resource waste and enviroment pullution. In recentyears, most of the studies focus on developing an alternative route to use thetitanomagnetite concentrates. One potential choice is the rotary hearth furnace process,which involves the reduction step of composite briquette of titanomagnetiteconcentrates with noncoking coal and the smelting of the reduced sample in anelectric-arc furnace. This process has many features such as high temperature, coke freeand no other iron ores need to be added. However, the reduction rate ofvanadium-titanium magnetite concentrates is slow due to the complexity of mineralstructure and high reduction temperature. Moreover, the low bed height of agglomerates,about20²5mm, is prevailing in present RHF due to insufficient heat supply for a highbed height. These factors leading to the low productivity and low yeild for RHF process.It is a important way that increase the bed height to solve it. It is very significant toachieve the high bed height operation for the development and scale enlargement ofRHF process dealing with vanadium-titanium magnetite concentrates in China.The aim of present study is to achieve the reduction of multi-layer pellets madefrom vanadium-titanium magnetite concentrates and coal. The reduction behavior andkenitics of carbothermic reduction of vanadium-titanium magnetite concentrates andthat of multi-layer reduction were investigated by isothermal reduction andnon-isothermal reduction experimental methods.The carbothermic reduction behavior of vanadium-titanium magnetite concentrateswere investigated using isothermal reduction and non-isothermal reductionexperimental methods. The following conclusions are obtained: The carbothermal reduction behavior of vanadium-titanium magnetite concentratesand the effect of temperature and reduction time on the phase transformation duringreduction of vanadium-titanium magnetite concentrates were investigated bynon-isothermal reduction and isothermal reduction experimental methods. Followingconclusions are obtained:（1） Five stages were found in the carbothermic reduction ofvanadium-titanium magnetite concentrates briquette with coal. The devolatilization ofcoal occurred in the first stage, and reductions of iron oxides mainly in the second andthird stages. The reduction rate of iron oxide in the third stage was much higher thanthat in the second stage because of the high carbon gasification rate. The iron in theilmenite （FeTiO₃） was reduced in the fourth stage. In the final stage, TiO₂was partiallyreduced to lower valence oxides.（2） The phase transformation of the briquettes wasobtained. The main phases of sample reduced at900℃are metallic iron、ilmenite（FeTiO3） and titanomagnetite （Fe₃-xTi_xO₄）. The traces of rutile （TiO₂） were observed at1000℃. The iron carbide （Fe3C） and ferrous-pseudobrookite （FeTi2O5） appeared at1200℃. The titanium carbide was found in the sample reduced at1350℃. Wustite （FeO）and ulvospinel （Fe₂TiO₄） were not found in present study. The vanadium-titaniummagnetite concentrates was reduced to iron at1623K in argon along a stepwisesequence with Fe_2.75Ti_0.25O₄,Fe_2.5Ti_0.5O₄, Fe_2.25Ti_0.75O₄, ilmenite（FeTiO₃）, wustite（FeO）and ferrous-pseudobrookite （FeTi₂O₅） as intermediates.（3） Iron and titanium achievedseparation in the composite briquette reduced at1300℃. Nuggets were observedoutside of briquette reduced at1300℃and a higher temperature.The kenitics of carbothermic reduction of vanadium-titanium magnetiteconcentrates under isothermal and non-isothermal heating conditions were study bymodel-free isoconversional method. The following conclusions are obtained:（1）Forisothermal conditions, the reduction rate is controlled by phase boundary reaction for Rless than0.2with the apparent activation energy of about68KJ mol-1and bythree-dimensional diffusion for R greater than0.75with the apparent activation energyof about133.3KJ mol-1. For R in the range from0.2to0.75, the reaction rate is undermixed controlling and the apparent activation energy increases with increase inreduction degree.（2）For non-isothermal conditions, it is found that the activationenergy changes with the reaction fraction increasing. T＜885℃，the overall reducitonrate is controlled by carbon gasification reaction, the activation energy increasegradually.885℃≤T＜985℃, the rate controlling step turns to the reduction of wustite, the activation energy decrease gradually.985℃≤T＜1190℃, reduction of ilmenite anddiffusion become the rate controlling step, the activation energy increase again.The effect of Fe-Si on the carbothermic reduction of vanadium-titanium magnetiteconcentrates was investigated by isothermal and non-isothermal experiment,respectively. The conclusion can be summarized as follows:（1） The addition of Fe-Siaccelerates the carbothermic reduction rate of vanadium-titanium magnetiteconcentrates. A part of silicon in the Fe-Si substitutes for carbon to participate thereduction of vanadium-titanium magnetite concentrates.（2） The addition of Fe-Sifacilitates the nucleation and coalescence of metallic iron formed by reduction. Thegreater the ferrosilicon dosage, the larger the metallic iron particle.（3） A reactionmechanism for the carbothermic reduction of vanadium-titanium magnetite concentrateswith Fe-Si addition was proposed. In the first stage （lower than1000℃）, the solid phasereactions with carbon and silicon as reductants are dominant. The exdothermicreduction by silicon, to certain extent, promotes the reduction of vanadium-titaniummagnetite concentrates. In the second stage （1000¹150℃）, the rate of reduction byCO is much faster than that of reduction by silicon, resulting in little influence of Fe-Sion the reduction of vanadium-titanium magnetite concentrates. Fe-Si has little influenceon the reduction of vanadium-titanium magnetite concentrates. In the final stage（1150℃≤T）, the reduction by silicon markedly occurs again, which further facilitates thereduction of vanadium-titanium magnetite concentrates and the coalescence of metalliciron.The reduction of composite pellet of vanadium-titanium magnetite concentratesand coal with one layer and multi-layer were investigated in a heating furnace usingnature gas as fuel. It is found that the reduction of composite pellets depend ontemperature greatly. For the multi-layer reduction, the reduction rate of top layer isfaster than other layers. In the later stage of reduction, reoxidation of iron occurs in toplayer. The metallizaiton degree for each layer can not reach to a high valuesimultaneouslyThe triple layer reduction of composite pellets and the enhencement of that wereinvestigated in a three-zone heating furnace. It is found that there are little effect on thesynchronous triple layer reduction by increase the pellet size of top layer alone.Ferrosilicon addition can significantly promote the reduction of composite pellet inlower layer. Under present experimental conditions, the synchronous triple layerreduction can be achieved with a metallizaiton degree of about88%. By the synergistic effect of non-equidimension distribution and ferrosilicon enhancement, themetallization degree of each layer can reach to about92%simultaneously. For specifymetallization degree and reduction time made according to the actual industrialproduction condition, the operation with high bed hegiht for RHF can be achieved bynon-equidimension distribution and ferrosilicon enhancement.