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MDMW-Iron59 | Sintering | Glasses

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The objective of this investigation was to study the influence of lime, silica, alumina and magnesia on the strength of sinters for which detailed micro structural studies were undertaken in the laboratory and regression analyses were done to establish relationship between the existing phases and strength properties.
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  134    Mineral Processing Technology (MPT 2007) Strength and Microstructural Studies on Iron Ore Sinters Rajib Chakravorty  Department of Metallurgical & Materials Engineering National Institute of Technology, Durgapur  ABSTRACT: For iron ores, sintering is one of the most important methods of agglomeration in which a widevariety of chemical constituents are present in the final sinter depending upon the presence of these constituentsin the blend mix of the sinter. The content of these chemical and mineral constituents greatly influence thestrength and reducibility characteristics of the sinter. Higher the amount of hematite and lower the amount of magnetite, greater will be the reducibility. Presence of calcium silicate and calcium ferrite increases the strengthof the sinter while glassy matrix decreases it. Presence of pores in the sinter may increase its reducibility but willhamper its strength property. It has been found that higher alumina content enlarges the solidification range of thedolomite sinters and improves the low temperature breakdown characteristics while improving the reducibilityonly upto a certain value. However, it deteriorates the reduction degradation index. The objective of the presentinvestigation was to study the influence of lime, silica, alumina and magnesia on the strength of sinters for whichdetailed micro structural studies were undertaken in the laboratory and regression analyses were done to establishrelationship between the existing phases and strength properties. 1. SINTER MINERALOGY In acid sinters where no lime is present the major constituent is fayalite. The strength of sinter after an initial higher side, starts decreasing after optimum fayalite content is reached.As basicity of the sinter mix increases theslag volume increases and the crystalline silicatesare increasingly replaced by glassy matrix. Limemakes the glass lessviscous requiring lower temperature and hence, less fuel, thus increasingthe degree of oxidation and hematite content bothsrcinal and secondary hematite. Hence the sinter strength decreases and reducibility increases.With increasing basicity the amount of glassfirst increases and the fuel rate decreases andhence strength continues to decrease andreducibility to increase. At a ratio of 1.2, calciumferrites begin to appear and at 1.4 to 1.6, thecrystallization of CaO.SiO2 occus. At this stagethe volume of the glass is maximum and thestrength of the sinter minimum.As the basicity increases above 1.4, theglassy matrix is replaced by Ca-silicates andferrites, the latter increasingly taking over as bonding medium with simultaneous steadyincrease in sinter strength.The susceptibility of sinter breakdown at basicity 1.4-1.6 can be somewhat decreased bythe use of Al2O3 and MgO. Alumina enlarges thesolidification range of dolomite sinters, relievesthe cooling stress and increases the mechanicalstrength. However, alumina content betters the properties only upto a certain limit and Al2O3upto 6% was used till the early 80s due to better cold strength and low temperature breakdown properties. Recent investigations have confirmedthe reduction-degradation values of sinter areadversely affected by increased alumina contentand at Tata Steel, the alumina content wasattempted to be kept at 2-2.5% only for thesereasons. 2. METHODOLOGY, RESULTS ANDDISCUSSION Laboratory samples and sinters from Durgapur Steel Plant were subjected to optical microscopy.Samples were of 1.7-1.8 basicity. Themicrophotographs confirmed the presence of  both primary and secondary hematite, Ca andMg-ferrites, magnetite and slag in the sinter samples. Wide variations in proportions of different phases even in the same type of sinter were observed, but by and large, in both type of sinters, ferrites were found to be the predominant phase followed by hematite, silicates andmagnetite. Data from 30 heats of sinters from thesintering plant of DSP were collected andsubjected to a regression analysis in which the  135    Mineral Processing Technology (MPT 2007) variation of the cold strength or Tumbler Indexwas correlated with the percentage of CaO, MgO,SiO2 and Al2O3 and the following equation wasobtained:TI (Tumbler Index) = 2.3934 (%CaO) +6.2593 (%MgO) + 9.9912 (%Al2O3) – 0.8654(%SiO2).The above equation shows that %SiO2 insinter has a negative coefficient which impliesthat the TI value of sinter decreases with increasein silica content. The DSP data shows that %Sio2and %FeO in sinter are comparable and there is probability of silicates formation of calcium or fayalite type also.Positive coefficent of CaO in the equationindicates a higherCaO content causes replcementof glassy matrix by Ca-ferrites increasinglytaking over as the bonding medium resulting inincrese in the TI values; whereas the coefficientfor %Al2O3 indicates a positive effect on coldstrength in this dolomitic sinter but only upto acertain limit as stated earlier. A higher %MgOalso leads to a high TI value in a manner similar to that for the CaO. 3. CONCLUSION The basicity of the sinter under investigation isabout 1.8; the type required for the blast furnacesunder DSP conditions and hence the glassymatrix is predominantly replaced by Ca-ferritesand silicates. Ca-ferrite is acting as the bondingmaterial in addition to the secondary hematitewhich causes the sinter strength required for the process. Needle like ferrite precipitates hold thegrains together. The regression equation showsindividual effects of the constituents. BIBLIOGRAPHY [1]Principles of Blast Furnace Ironmaking : A K Biswas.[2]Sintering at Tata Steel: Tata Steel publication.[3]Modern Ironmaking: V R Tupkary and R HTupkary.[4]R. Chakraborty: Private communications onsinter project. DSP grains showing hematite as fine irregular grainsSamples showing lathe and dendritic slags  Strength and Microstructural Studies on, Iron ore Sinters 136  Lab samples showing dendritic eutectic mode of occurrence of ferriteLab samples showing hematite as coarse crystals
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