CHAPTER-6: SUMMARY AND CONCLUSIONS

CHAPTER-6: SUMMARY AND CONCLUSIONS

190 6. SUMMARY AND CONCLUSIONS 6.1 Summary of laboratory test work Based on the entire laboratory test work, findings are summarized as following; 6.1.1 Characterization of iron ore fines and fluxes Iron ore fines from the Noamundi captive mines of Tata Steel contain high iron values (64 to 66% Fe). Total gangue content ranges from 3.5 to 4%. Around 25% of the fines are minus 150 microns size that can be readily used for the pelletizing. From the characterization studies of iron ore fines using QEMSCAN; it was found that primary iron minerals are hematite and goethite. Kaolinite, limonite, gibbsite and quartz are present as gangue minerals. Hematite and goethite are present in the proportion of 68% and 30% respectively. 90% of the alumina in the sample is associated with goethite, gibbsite and kaolinite. Ore is massive in nature showing no clear layering described by different mineral phases. Goethite is mostly associated with hematite and the latter is present as inclusion within goethite and vice versa. Size wise mineralogical study indicated that finer fraction is rich in goethite whereas hematite is concentrated more in coarse fractions. Liberation analysis indicated that goethite is more liberated in the finer fractions and hematite in the coarser fractions. Deportation analysis showed that Al is predominantly contributed by goethite and the average Al content in the goethite is around 3%. XRD analysis of fluxes showed that limestone is composed of calcite, dolomite and magnesite are composed of magnesite and small quantities of quartz whereas pyroxenite is primarily composed of enstatite.

191 From thermo gravimetric analysis (TGA) it was evident that magnesite dissociation occurs at lower temperatures (around 500 o C) followed by dolomite (around 700 o C) and limestone (around 750 o C). Pyroxenite, which is a magnesium silicate, does not dissociate unlike the above carbonate fluxes. Devolatilization of coal was found to start from around 315 o C. 6.1.2 Grinding, green pelletizing and induration studies Grinding of the iron ore fines, up to mean particle size (MPS) of 55 microns, was found to be optimum. Over grinding beyond this MPS generated more amount of ultrafines, <25 micron particles. Pelletizing feed with MPS of 55 microns exhibited optimum green pellet properties viz., drop number, green compression strength and moisture content. Green pellets prepared from pellet feed of different fineness exhibited self-preserving behaviour that was used for predicting the size distribution of green pellets. D 50 of green pellets was found to be high at 46 micron MPS of pelletizing feed. Firing studies of pellets prepared from pellet feed of varying fineness indicated that firing temperature of 1300 o C and MPS of 55 microns for green pellet feed are required to attain desired cold crushing strength of pellets.

192 6.1.3 Effect of pellet basicity (CaO/SiO 2 ) and MgO content on quality and microstructure of the fired pellets using limestone and dolomite flux Effect of pellet basicity (CaO/SiO 2 ) and MgO content on the melt formation and microstructure during the induration of iron ore pellets was examined. Fired pellets with varying basicity (0 to 0.8) and MgO (0 and 1.5%) content were tested for cold strength, reduction degradation index, reducibility, swelling and softening-melting characteristics. Optical microscope studies with image analysis software were carried out to estimate the amount of different phases. SEM-EDS analysis was done to record the chemical analysis of oxide and slag phases. X-ray mapping was also carried out to understand the distribution of CaO, MgO, SiO 2 and Al 2 O 3 in different phases. With increasing basicity, the amount of silicate melt, which acts bonding phase, was found to increase in the fired pellets. FeO content of the silicate melt was decreased with increased basicity of the pellets. Addition of MgO to both acid and limestone-fluxed pellets resulted in the formation of high melting point slag during their induration. Acid pellets exhibited highest swelling, whereas maximum swelling in the MgO-free pellets was observed at 0.6 basicity. Addition of MgO to both acid and limestone-fluxed pellets at all the basicity levels considerably reduced the swelling tendency of pellets due to the formation of high melting point slag that gives sufficient bond strength to withstand the reduction stresses. With increasing basicity, MgO-free and MgO pellets exhibited considerably lower reduction degradation compared to acid pellets due to the formation of more amount of silicate melt, which is more stable under the reducing conditions in blast furnace. Reducibility of MgO-free pellets is slightly lower compared to acid pellet due to the formation of silicate melt in the former, which softens and impedes the flow of reducing gas within the pellet thereby retarding the reduction. Addition of MgO to both acid and limestone-fluxed pellets at

193 all basicity levels increased their reducibility by forming high melting point slag which does not soften at reduction temperatures and keeps the pores open for reducing gas thereby enhancing reduction. Inferior softening and melting characteristics of the acid pellets could be attributed to the formation of FeO rich low melting fayalitic liquidus slag. MgO-free pellets with increasing basicity exhibited increased softening temperatures and low softening-melting range due to the formation of burden slag with high liquidus temperature. To relatively compare pellet quality based on vital quality parameters, a new dimensionless index called composite quality index was formulated. Higher composite index indicates the improved pellet quality and vice versa. Limestone fluxed MgO-free pellets at 0.8 basicity, and dolomite fluxed pellets at 0.4 basicity & 1.5% MgO exhibited optimum metallurgical quality parameters among all the pellets studied.

194 6.1.4 Effect of pellet MgO content on quality and microstructure of fired pellets using magnesite flux Effect of magnesite addition, to increase the MgO content, on the melt formation and microstructure during the induration of the iron ore pellets was examined. Fired pellets with varying MgO content (from 0 and 3.0%) were tested for cold compression strength, swelling, reduction degradation index and reducibility. Optical microscope studies with image analysis, and SEM-EDS analysis was done to record the amount of phases and their chemical analysis. Addition on magnesite resulted in the formation of magnesioferrite in the fired pellets. FeO content of the silicate melt /slag phase in the pellets decreased from 30% in the acid pellets to around 5% in the magnesite fluxed MgO pellets. Lower FeO in the melt increases its melting point. CCS of both acid pellets and magnesite fluxed pellets was within the acceptable limit for the blast furnace. In magnesite pellets, CCS decreased with increasing MgO due to the formation of low strength silicate melt phase. Acid pellets exhibited highest swelling. Addition of MgO considerably reduced the swelling tendency of the pellets due to the formation of high melting point slag that results sufficient bond strength to withstand the reduction stresses. Reduction degradation of the pellets was found to be reduced with increasing MgO, due to the formation of more amount of magnesioferrite and silicate melt, which are more stable under the reducing conditions in the blast furnace. MgO addition considerably improved the reducibility of the pellets, especially in the range of 0.5 to 1.5% MgO. Formation of less amount of liquid slag due to the presence of MgO could be attributed to this improved reducibility of magnesite pellets. At 1.0 to 1.5% MgO content, fired pellets exhibited optimum metallurgical properties.

195 6.1.5 Effect of pellet MgO content on quality and microstructure of fired pellets using pyroxenite flux Effect of pyroxenite addition, to increase the MgO content, on the melt formation and microstructure during the induration of iron ore pellets was examined. Fired pellets with varying MgO content (from 0 and 10.0%) were tested for cold compression strength, swelling, reduction degradation index and reducibility. Optical microscope studies with image analysis, and SEM-EDS analysis was done to record the amount of phases and their chemical analysis. Addition of pyroxenite up to 5% to get 1.5% MgO in the pellets was found to be an optimum flux dosage for blast furnace grade hematite pellets. FeO in the slag phase reduced from 30% in acid pellets to around 3% in the pyroxenite fluxed pellets containing 1.5% MgO. Pyroxenite addition beyond 5% showed its poor assimilation in the pellet matrix and increased the amount of relict magnesium silicate phase resulting in no further drop in slag FeO. Strength of the pyroxenite pellets is comparable to acid pellets in spite of high amount slag phase that filled up the pores between the oxide grains thereby reducing the porosity. The negative effect of slag phase on pellet strength is counteracted by the reduced porosity of pyroxenite pellets. RDI of pyroxenite pellets was found to be superior, compared to acid pellets, due to the formation of magnesioferrite and more amount of silicate melt, which are more stable under the reducing conditions in the blast furnace. Swelling index of acid pellets was poor due to the formation of low melting point fayalitic slag whereas the pyroxenite pellets exhibited less swelling by forming high melting slag with low FeO that gives sufficient bond strength to withstand the reduction stresses. Pyroxenite pellets resulted in better softening-melting characteristics compared to the acid pellets due to the formation of high melting point slag and magnesio-wustite phase during reduction.

196 6.1.6 Advanced metallurgical testing of the pyroxenite fluxed pellets Based on resultant pellet quality observed during the test work and availability of fluxes at the captive mines, pyroxenite fluxed pellets were suggested as suitable burden material for the blast furnaces at Tata Steel. It was decided to fine-tune the pyroxenite pellet chemistry, before commercial production in the pellet plant, to find out the minimum amount of MgO in pellets to get the desired high temperature properties. Addition of MgO in the form of pyroxenite improved the quality of pellets as compared to acid pellets. Acid pellets exhibited inferior metallurgical properties; free swelling index~ 49%, softening temperature~1074 o C, disintegration -3.15mm ~28% against the desired target of <17%, >1150 o C and <5% respectively. It was observed that minimum of 0.6% MgO is required in the pyroxenite pellets to control the swelling index with in the target range. Pellets with < 0.6% MgO resulted in swelling >17% As per the advanced swelling and softening test, at least 0.3 to 0.6% MgO% is required in the pellets to obtain desired softening temperature and lower pressure drop. Beyond this level, there was no appreciable improvement in quality. Advanced reduction degradation tests indicated that MgO>0.6% is required to reduce the disintegration of pellets during reduction in the stack zone of blast furnace. Considering the target pellet quality parameters with respect to the above test results, it was concluded that 0.6% to 0.9% MgO is desired in the pyroxenite pellets to obtain required high temperature properties. A patent was also filed on the use of magnesium silicate (pyroxenite) as flux in the pelletizing for improved metallurgical properties.

197 6.2 Conclusions on green pelletizing and effect of fluxes on pellet quality The investigations carried out during this test work and the results obtained thereof conclude the following salient points; In the Noamundi iron ore fines, hematite and goethite minerals were found to be distributed in coarser and finer fractions respectively. To produce desired quality pellets, these fines need to be blended thoroughly to achieve homogenies mineral distribution in the pellet feed. Grinding of the ore fines in the ball mill needs to ensure that the mean particle size of the pelletizing feed is maintained at 55 microns. At this fineness, both green pellet quality and the fired pellet strength found to be optimum. Pellets need to be fired up to 1300 o C to achieve desired cold crushing strength to withstand handling & storing and loads inside the shaft of the blast furnace. Acid pellets prepared from the Noamundi iron ore fines, without any flux addition, exhibited inferior metallurgical properties. Insufficient formation of silicate melt phase during pellet firing and high amount of FeO in the melt phase found to deteriorate the acid pellet quality. Substantial improvement in the quality of pellets, as compared to acid pellets, was observed due to the addition of fluxing agents. Selection of fluxing agents depends on the overall burden chemistry of the blast furnace that includes sinter, pellets and lump ore. In the limestone fluxed pellets, (CaO/SiO 2 ) needs to be maintained at 0.8 to obtain desired metallurgical properties for the blast furnace. In case of dolomite fluxed pellets, basicity of 0.4 and MgO content of 1.5% found to be optimum for the desired pellet quality. MgO addition to the pellets found to be more effective in improving their metallurgical properties. It forms magnesioferrite and high melting point bonding phase within the pellets. These phases helped in considerably reducing the undesired swelling and reduction degradation of pellets.

198 Pellets fluxed with magnesite exhibited improved metallurgical properties at 1.0 to 1.5% MgO content. Magnesite addition improved the reducibility of pellets due to the formation of fewer amounts of liquid slag and uniform porosity. For the first time, pyroxenite (magnesium silicate mineral) was established as suitable flux for the pelletizing. Unlike carbonate fluxes like limestone or dolomite, pyroxenite does not undergo any endothermic reaction for dissociation. 0.6 to 0.9% MgO was found to be optimum in the pellets when pyroxenite is used as fluxing agent. A new dimensionless index called composite quality index (also called p-index ) was formulated for the pellets. This index can be used as a tool to relatively compare quality of different pellets based on vital metallurgical quality parameters. Based on resultant pellet quality observed during this entire test work and availability of fluxes at the captive mines, pyroxenite fluxed pellets were suggested as suitable burden material for the blast furnaces at Tata Steel. Use of pyroxenite flux for pelletizing of the Noamundi iron ore fines, as suggested, resulted in improved metallurgical properties of pellets, especially swelling index and reducibility, at the 6 MTPA capacity iron ore pelletizing plant of Tata Steel at Jamshedpur.