Fundamental Research On Gas-Solid Direct Reduction Of Titanomagnetite In A Fluidized Bed

The utilization of low-grade minerals such as titanomagnetite?TTM?has gained more attention in recent years due to the rapid depletion of high-grade natural resources.However,due to the limitation of blast furnace operating condition in the modern blast furnace-converter process,only iron and vanadium resource can be utilized,titanium in TTM cannot be effectively enriched and extracted.The direct reduction-electric arc furnace?EAF?melting separation process,?the two-step short process?,has been proposed as a more effective and promising approach for comprehensive utilization of the iron,vanadium,and titanium resources in TTM.The two-step process can be classified into the rotary kiln process,the rotary hearth furnace process,the shaft furnace process,and the fluidized bed direct reduction process according to the reduction reactor employed.By taking advantage of the high heat and mass transfer efficiency,and direct use of ore fines,the fluidized bed?FB?process exhibits a high potential to achieve high-efficient reduction of TTM.Compared with the direct reduction of common iron ores,the reduction of titania-ferrous oxides in TTM ores requires a much higher reduction potential than ordinary iron oxides,resulting in a lower reduction efficiency and high product cost.Therefore,how to break through the thermodynamic limitations of titanium-containing iron oxides and to improve the reaction efficiency in the direct reduction process are the basic key problems that must be solved for efficient utilization of titanium resources in the TTM two-step short process.In the present thesis,TTM from South Africa was used as the experimental material to study the FB direct reduction characteristic of the TTM.The influence of the pre-oxidation on the reduction efficiency as well as the fluidization characteristic was systematically investigated.The main findings and conclusions of this thesis are as follows:?1?Characteristic research on the phase transformation of TTM by gas-phase direct reduction.In the temperature range of 750-950?,the direct reduction of TTM can be divided into three sequential steps:the reduction of ordinary iron oxides?the first step?,ulvospinel reduction?the second step?and ilmenite reduction?the third step?.During the reduction process,FeO combines with FeO.TiO₂ to form 2FeO·TiO₂,which greatly reduces the equilibrium metallization degree of the first reduction step by about 15.4%.Also,solid solution impurity oxides?such as MgO,MnO?increase the required reducing gas potential for FeO to Fe reduction.Based on this,the phase transformation path of the impurity-containing titania-ferrous oxides in TTM and the corresponding quantitative model of the reduction potential-balance metallization degree have been built.?2?The mechanism of high temperature?825-950 ??pre-oxidation on the improvement of TTM direct reduction.By oxidation pretreatment,the titania-ferrous oxides difficult to be reduced in TTM are dissociated to easily reducible free Fe2O3,resulting in about 10%increase in the reduction efficiency.Additionally,the equilibrium metallization degree of the first and second reduction step can be relatively increased by 14.5 and 4.5%respectively.However,when the peroxidation temperature is greater than 900 ?,the improvement effect on the reduction rate became weak,due to the high-temperature sintering and the larger crystallite size of the oxidation product Also,Fe2O3 combines with TiO₂ to form Fe2TiO5 which decreases the amount of free hematite available for reduction.Thus,the equilibrium metallization degree at high oxidation temperature has a single optimum value.Based on this,the transformation path of the pre-oxidation enhanced the titania-ferrous oxides dissociation-direct reduction and the corresponding quantitative model of the reduction potential-balance metallization degree have been developed.?3?The mechanism of low temperature?700-825 ??pre-oxidation on the improvement of TTM direct reduction.At oxidation temperature<800 ?,where the formation of pseudobrookite phase can be prevented,the equilibrium metallization degree increased linearly with the increase in the oxidation time and can exceed the optimum metallization degree obtained at high pre-oxidation temperature.Whereas,at the oxidation temperature ?800 ?,there exist two peak values for the maximum effect of pre-oxidation on reduction improvement due to the competitive relationship between the amount of generated pseudobrookite and the free hematite.Finally,the enhanced titania-ferrous oxides dissociation-direct reduction transformation path pre-oxidized both at the low and high temperatures,and the corresponding quantitative model of the reduction potential-balance metallization degree have been built,which provides a theoretical guide for the optimal design of the direct reduction process.?4?Research on preventing defluidization by peroxidation method.In the previous study,it was found that in addition to improving the reduction efficiency of TTM,pre-oxidation can also improve the fluidization quality.It was found that peroxidation modifies the morphology of the as-reduced metallic iron on the particle surfaces,thereby changing the sticking behavior of the as-reduced particles.Based on the relationship between oxidation product morphology and the resultant iron morphology after reduction,the fluidization behavior of pre-oxidized SA TTM can be divided into three operating regions:defluidization?low oxidation condition?,stable fluidization?intermediate oxidation condition?,and unstable fluidization?high oxidation condition?.Even at the defluidization region,the fluidization quality is significantly better than that of the raw ore.At the stable fluidization region,the steady state fluidization gas velocity was reduced by 56%as compared with the raw concentrate under 100%CO gas at 900?.The peroxidation method provides a novel approach to suppressing defluidization via pre-oxidation treatments.