Study On The Formation Mechanism Of Titanium Carbide,Titanium Carbonitride And Composite Materials In The Carbothermic Reduction Of Panzhihua Ilmenite

Vanadium-titanium-bearing magnetite of Panzhihua is a complex iron ore,which is rich in the elements of vanadium and titanium.It accounts for more than 90%of the titanium reserves in China.By the beneficiation process of the ore,three hundred thousand tons of ilmenite concentrate are produced every year.With the exhaustion of the high grade titanium ores,more attention have been paid on the titanium rich material produced by ilmenite as an alternative raw material for rutile.Titanium carbide and titanium carbonitride have face centered cubic NaCl type structures.Due to the high melting point,high hardness,good electrical conductivity,high chemical and thermal stability,both of them have been found tremendous applications in various fields,such as wear-resistant material,cutting tools,anodes in lithium-ion batteries and so on.Considering the factor of resource,environment protection and economic benefits,the preparations of titanium carbide and titanium carbonitride by the ilmenite concentrate have been receiving more and more attentions.Although many works have been done,the phase evolution regularity,dynamic analysis and reaction mechanism during the carbothermic reduction process are still unclear and controversial.In addition,compared to other foreign or domestic origin of ilmenite resources,there are lots of impurity elements conexisting in the ilmenite,such as Mg,Si,Al,Ca and Mn.The composition,morphology and distribution of the impurities in the reduction products are not clear.It is necessary to further study how to remove the impurities and improve the purity of the reduction products.Therefore,the formation of titanium carbide,titanium carbonitride and composite materials by Panzhihua ilmenite has been studied in this paper by using the thermodynamic software Factsage,X-ray diffraction,scanning eletron microscopy,infrared gas analyzer and vacuum pressure control system.The main research contents include the effects of the carbon ratio,reaction temperature,reaction time and pre-oxidation treatment on the phase evolution,kinetics,reaction mechanism and impurity phase composition;the reaction mechanism of the carbothermic reduction of ilmenite in vacuum;the preparations of Fe-TiC composite powder and Fe-TiC composite ceramic by the carbothermic reduction of ilmenite in vacuum.The obtained conclusions are shown as follows:1)The reaction sequence of the ilmenite concentrate from 1200 to 1500? in argon was found as:FeTiO₃?Fe+Ti₃O₅?Ti₂O₃?TiC_1-xO_x?TiC.TiC_1-xO_x did not appear until the temperature was raised to 1300?.With the increase of the reaction temperature,most of the iron existed in the form of continuous phase with TiC particles and impurity phases?mainly composed of MgO,MgAl₂O₄ and Mg₂SiO₄?tightly inlaid on it.The reaction sequence of the ilmenite concentrate from 1200 to 1500? in nitrogen was found as:FeTiO₃?Fe + Ti₃O₅?TiC_1-xO_x?TiC_1-xN_x.The lowest temperature for the generation of TiC_1-xO_x and TiC_1-xN_x was 1300?Ti₂O₃ did not appear as an intermediate phase due to its instability in nitrogen atmosphere.The products of the pre-oxidation at 800? in the air atmosphere were Fe₂O₃,TiO₂ and Fe₂Ti₃O₉,while Fe₂O₃,TiO₂ and Fe₂TiO₅ at 1000?.The pre-oxidation could not change the phase evolution during the carbothermic reduction process,but increased the specific surface area and reduced the activation energy,which was beneficial to the carbothermic.For both pre-oxidations,the temperature for the generation of TiC_1-xO_x decreased from 1300? to 1200?.2)The solid-liquid reaction was proposed during the carbothermic reduction process.When the reaction temperature was higher than 1300?,the ilmenite was quickly reduced to Fe and TiO₂ at the beginning of the carbothermic reduction process.When the reduction product Fe appeared on the surface of the mineral particles,part of carbon dissolved in Fe to form a liquid phase.Due to the good fluidity,the Fe-C droplets gathered and formed a liquid layer out of the unreacted core,which cut off the direct contact between the solid carbon and the titanium suboxides.It made liquid Fe form a diffusion channel for carbon to contact with the unreacted core.Although the content of dissolved carbon in the liquid layer was small,there were a large amount of solid carbon on the outside of product layer.Once the dissolved carbon in the liquid iron was consumed by the titanium suboxides in the reaction interface,there would be a diffusion of carbon from solid carbon to liquid phase continuously.Finally,the titanium suboxides were reduced to titanium carbide.3)During the carbothermic reduction process in vacumm,the impurity elements of Mg,Ca and Mn were removed from the mixture in gaseous state.The contents of Mg,Ca and Mn decteased significantly with the increase of the reaction temperature and vacuum degree.When the reaction temperature was higher than 1400?,the impurity phases of MgO,M-Al₂O₄ and Mg₂SiO₄ disappeared in the reduction products.After the carbothennic reduction process,the impurity elements of Si and Al were dissolved in the iron.4)The cavuum degree of the sintering process was higher than that of the carbothennic reduction process.As a result,part of Fe evaporated in gaseous state,which decreased the content of Fe in the Fe phase.At the same time,the element of Al in the Fe phase formed Al₄C₃,and part of Ti in the TiC phase was dissolved in the Fe phase,which led to the decrease of Al content and the increase of Ti content in the Fe phase.When the carbon ratio was 0.391 and the reaction temperature was 1500? the properties of the Fe-TiC composite ceramics was the best.After the leaching process,the high purity of TiC powder was obtained.The particle size of the TiC powder was about 4?m and most of the TiC powder has a cube shape.The contents of Si,Fe,Al,Ca and Mg in the TiC powder was 4.53wt%,0.15wt%,0.12wt%,0.089wt%and 0.0088wt%,respectively.After sintering with a vacuum degree of 0.002Pa at 1400? for 6h,its density was 5.3g·cm-3,the bending strength was 508.82Mpa and the microhardness was 949.9HV.Lots of transgranular TiC cracking was found after bending testing.