Fundamentals And Intensifying Techniques Of Solid-state Reduction Of Chromite Ores

To utilize chromite fines effectively, improve cost effectiveness and supply good burden materials for ferrochromium-making, solid-state reduction behaviors of chromite ores and intensifying techniques of the reduction had been investigated, the mechanisms were studied further by using TG, SEM, XRD,optical microscope and chemical phase analysis.Following conclusions are achieved:1)Solid-state Reduction Behaviors of Chromite ores(1)Thermodynamic Calculation and Analysis As the reduction temperature is below 1200℃,the reduction products of chromite are metallic iron and Cr7C3;when reduction temperature is higher than 1200℃,the products are Fe and (Cr,Fe)7C3,metallic iron formed during the initial reduction is transformed to (Cr,Fe)7C3 eventually. Some (Cr,Fe) appear above 1300℃,and more forsterite and other olive separate out evidently in the reduction products at the same time.(2)Kinetics of Reduction Experiments were carried out to investigate reaction process of anthracite-chromite pellet in nitrogen atmosphere with different isothermal and non-isothermal heating rates.The results indicate that the whole reaction can be expressed in a first order reaction formula. The weight loss of anthracite-chromite pellet can be considered as a function of the temperature in the pellet. The reduction kinetic equation is:1n [-1n(1-f)/T2p]=1n[AE(1-2RTp/E)/HR]-E/RTp.(3)The experiment indicates that indirect reduction with the help of the boudouard reaction of carbon, comparing with the solid-solid DR; take a favorable position in the reduction process.2) Intensifying Techniques and mechanism of Solid-state Reduction of Chromite Ores(1)Solid-state Reduction Techniques of Chromite ores The metallization ratios increase with the increasing of reduction temperature from 1150℃to 1450℃,and the increasing tendencies are more obvious between 1200℃and 1350℃.As a result of strong endothermic reaction, chromic oxide, comparing with ferric oxide, needs higher reduction temperature. The study on reduction time shows that the ferric oxide has greater reduction rate, and so extending reduction time is more benefit for chromic oxide reduction. The amount of carbon-containing of chromite ores plays a significant role for the reduction of chromic oxide, especially from 1200℃to 1300℃. The metallization of iron, chromium and pellet increase greatly with the increasing of C/O mole ratios, when the C/O mole ratio is 1.2,the metallization indexes of prereduction pellet obtain great amplification. The types and qualities of reductants can affect the result of the reduction of chromite ores.There are better C/O mole ratios at different temperatures for the reduction, and those are 1.2 and 1.4 at 1150℃and 1200℃,respectively, when reductant is anthracite; however, the total reduction reactions need 4h and 3h, respectively, duing to the restraining of reduction rate of chromic oxide.(2)The study of the effects of additives on solid-state reduction properties of no-containing carbon chromite ores discovered the catalytic performances of different additives at different temperatures.The laws are as follows:when the reduction temperature is 1300℃,The catalytic abilities of additives for the reductions of ferric oxide and chromic oxide are T3>T2>T5>T4>T1 and T2>T3>T1>T5>T4,respectively;and those are T3>T2>T5>T4 and T3 >T4>T2>T5 at 1200℃.T3 and T2 can take perfect synergistic catalytic effect on the reduction of chromite ores.The experiment of effects of additives on solid-state reduction properties of carbon-containing chromite ores pellet revealed that the laws are as follows: when the reduction temperature is 1150℃,the catalytic abilities of additives for the reduction are T5>T3>T4>no additive>other additives;T3>T4> T7>T5>T6>T2>T8>T9>no additive are showed between 1200℃and 1250℃;moreover, the catalytic abilities of additives for the reduction are T3 >T4>T6>T5>T7>T2>T8>T9>no additive at 1300℃.T3,T4, G, T1 and F-agglomerant form admixture, and which can take perfect synergistic catalytic effect on the reduction of chromite ores.(3) Mechanism of Intensification Solid-state Reduction of Chromite oresMicrostructure The prereduction pellet of chromite ores was observed with an optical microscopy, and the influences of reduction temperature and time, the amount of carbon, as well as additive on the microstructure of prereduction pellet were studied. It can be shown clearly that increasing temperature, time and C/O ratios can enhance metallization ratio;a small amount of ferrochromium gets together and grows up to distribute more evenly in the pellet;and forsterite and other gangue minerals get to separate out from chromites and gather up.The additive can make carbothermal reduction homogeneous, accelerate diffusion in the reduction process, and promote ferrochromium grain growth; and so the metal grains concentrate densely and equably.XRD Patterns The Fe3O4 phase has disappeared at 1150℃;and the (Mg,Fe)Cr2O4 phase is not found from 1250℃to 1350℃.It is shown that (Cr,Fe) solid solution appear above 1300℃,increasing with reduction temperature rising;and more much forsterite separate out evidently in the reduction products at the same time.However, as a result of no carbon-containing, although there are (Cr,Fe) solid solution and (Cr,Fe)7C3 at 1400℃,the total amount is small.This can make further evidence for that carbon-containing play a vital role in the reduction. At same time, the weaker original main phases and wilder reduzate phases confirm the advantage of additive for the reduction from minerals composition in the pellet.(4) The process that additives intensify the reduction of coal-bearing chromite ores is very complex, including promoting the volatile separate out and carbothermal reduction homogeneous and accelerating diffusion in the reduction process.