Fundamental Study On Carbonization And Nitridation Of Vanadium-Titanium Magnetite In The Process Of Coal-based Direct Reduction Followed By Magnetic Separation

Vanadium-titanium magnetite is an important strategic resource in China and the main carrier of vanadium and titanium.Because iron is closely coexisting with titanium,and vanadium occurs in titanium magnetite in form of isomorphism state,so it is unable to separate iron from vanadium and titanium by beneficiation.At present,the blast furnace has become the main technology for processing vanadium-titanium magnetite which has the enormous processing capacity at relatively low cost.However,it still exists the problem that the difficult utilization of TiO₂ in blast furnace slag.In view of the carbonation/nitride of titanium can be chlorinated at a low temperature,this paper proposes to reduce and carbon/nitride the concentrate in the way of carbon-burdened,using vanadium-titanium magnetite concentrate as raw material,then iron powder and tailings which is rich in titanium carbide or titanium nitride can be obtained by magnetic separation.Iron powder can be used as raw material for steelmaking and carbon/nitriding slag can be chlorided to produce titanium tetrachloride.The thesis focuses on the basic science and technical problems of carbothermal reduction,carbonization and nitriding of vanadium-titanium magnetite,the relative theory and technological researchs are carried out,in order to provide fresh ideas for the utilization of vanadium-titanium magnetite.First of all,the thermodynamic behavior of iron and titanium oxide reduction,carbonization,and nitridation reactions were calculated and the conditions and difficulty of each reactions were compared through the thermodynamics analysis,iron oxides are easy to be reduced,while titanium iron oxides are relatively difficult to be reduced,requiring higher partial pressure of CO.The initial temperature of carbonization reaction of iron oxide and titanium iron oxide was below 850?,and the initial temperature of nitridation reaction was below 950?when carbonaceous reducing agent coexisted with N₂.The temperature of carbon reduction of titanium oxide was higher than that of iron oxide,and the CO partial pressure of titanium oxide was higher than that of iron oxide.The sequence of titanium oxide carbonizing to form TiC was:TiO>TiO₂>Ti₂O₃>Ti₃O_5,contrary to the ease or complexity of reduction,the initial temperature of TiO carbonization was the lowest under standard condition,which was 1191.58?,and the initial temperature of Ti₃O₅ carbonization was the highest,which was 1348.03?.It was difficult for titanium oxide to react directly with N₂,when reducing agent C existed,the initial temperature of Titanium oxide nitridation was below 1250?under standard condition,and the initial temperature of TiO nitridation was774.25?.When?_N2>49.88%,the sequence of titanium oxide nitridation was:TiO>Ti₂O₃>TiO₂>Ti₃O_5.The nitridation reaction of TiC could occur when?_N2?5%,the Gibbs free energy of the chlorination reaction of TiC and TiN was low,and the reaction could proceed spontaneously at low temperature.On the basis of thermodynamics research,the carbonation/nitridation behavior of vanadium-Titanium magnetite concentrate under Ar and N₂ atmosphere was studied respectively by reductive roast,and the influence law of process schedule on carbonation/nitridation was investigated.Regardless of Ar atmosphere or N₂ atmosphere,there was no obvious iron bead forming on the surface of the product at low temperature(<1150?),after the temperature rises to 1250?under Ar atmosphere,iron beads precipitated on the surface of the pellets.When the temperature raised to 1150?under N₂atmosphere,obvious iron beads begined to form on the surface of the pellets,and the higher the temperature,the larger the particle size of the iron beads.Increasing reduction temperature,prolonging roasting time or increasing C/O molar ratio were all beneficial to improving the metallization rate of iron and the carbonization rate of titanium under Ar atmosphere,Roasting 2.5h at a C/O ratio of 1.3,the rate of metallization and carbonization exceeded 95%within the range of 1250??1300?.Increasing temperature and prolonging time were beneficial to carbonization/nitridation reaction by carbothermal under N₂atmosphere.Increasing the C/O ratio would raise the percentage of TiC in the product.TiC and TiN existed in the form of TiCx Ny.It took about 3h for the nitriding rate to reach more than 90%at 1150?,the roasting time and the C/O ratio should be controlled in 1.5?2.0h and 1.0?1.1,respectively.The paper focused on the study of the phase and structure evolution law of the carbon reduction/nitridation process in combination with MLA mineral parameters automatic analysis system.It can be conclued by analyzing phase change of reduced carbide that the evolution law of the titanium-containing phase is:FeTiO₃?FeTi₂O₅?(Fe_0.33Ti_0.46Mg_0.21)(Ti_1.9Mg_0.1)O₅?TiC,when the C/O ratio was 1.3 under Ar atmosphere.The evolution law of the titanium-containing phase under N₂ atmosphere was:FeTiO3?FeTi2O5?(Fe_0.33Ti_0.46Mg_0.21)(Ti_1.9Mg_0.1)O₅?TiN?TiC_xN_y.The microstructure of reductive carbonization was similar to that of reductive nitridation products,the main phase and gangue phase was metallic iron phase and spinel phase,respectively.TiC and TiN are mostly granular and presented a semidiomorphic-it-shaped structure,while TiN presented a complete cubic crystal.The granularity of both was small,generally 1-7 m,and they were often embedded in the interior or edge of Fe(Si)phase.Compared with TiN,major TiC was embedded in the iron phase,and it was difficult to separate from iron due to its small particle size and being wrapped by iron,in addition to its certain magnetic properties.The Sorting effect of reductioncarbon/nitridation products was studied by magnetic separation test.The reasonable breaking time and magnetic field strength for magnetic separation of reductive carbide was 15min and 60m T,the reasonable magnetic field intensity for magnetic separation of reducing nitride was 150m T.The saturation magnetic strength of TiN was about 1/4 of TiC,which was beneficial to magnetic separation.the proportion of titanium that entered into the slag phase after magnetic separation increases with the extension of the reductive carbonization time,under the condition of a reduction temperature of 1300?and a C/O ratio of 1.3.The content of TiC in the tailings reached26.24%after magnetic separation of the reductive carbide at 1300?for 3 hours.However,only 21.53%TiC entered into tailings,with a low recovery rate.When the reduction temperature decreased from 1300?to 1250?,the proportion of titanium entering into tailings increased,while TiC reduced slightly in tailings,under the circumstance of C/O ratio is 1.3 and holding time is 1h.The iron recovery rate in the magnetic material was over 98%,while the proportion of TFewas only about 80%,when reductive nitridation was performed for 1?2h on the condition that C/O ratio is 1.3 at 1300?.The recovery rate of TTiand TiN in the tailings was 60%?64%and 62%?65%after magnetic separation,respectively.While the content of TiN in the tailings reached about 34%,indicating that separation was not complete,the recovery rate of TTiand TiN entering into the magnetic separation tailings showed a trend of first rising and then falling with the extension of holding time at 1300?.