ASSESSMENT OF REDUCTION BEHAVIOR OF HEMATITE IRON ORE PELLETS IN COAL FINES FOR APPLICATION IN SPONGE IRONMAKING

Studies on isothermal reduction kinetics (with F grade coal) in fired pellets of hematite iron ores, procured from four different mines of Orissa, were carried out in the temperature range of 850–1000°C to provide information for the Indian sponge iron plants. The rate of reduction in all the fired iron ore pellets increased markedly with a rise of temperature up to 950°C, and thereafter it decreased at 1000°C. The rate was more intense in the first 30 minutes. All iron ores exhibited almost complete reduction in their pellets at temperatures of 900 and 950°C in < 2 hours' heating time duration, and the final product morphologies consisted of prominent cracks. The kinetic model equation 1 ? (1 ? α)^1/3 = kt was found to fit best to the experimental data, and the values of apparent activation energy were evaluated. Reductions of D. R. Pattnaik and M. G. Mohanty iron ore pellets were characterized by higher activation energies (183 and 150 kJ mol^?1), indicating carbon gasification reaction to be the rate-controlling step. The results established lower values of activation energy (83 and 84 kJ mol^?1) for the reduction of G. M. OMC Ltd. and Sakaruddin iron ore pellets, proposing their overall rates to be controlled by indirect reduction reactions.     

STUDIES ON THE REDUCTION KINETICS OF HEMATITE IRON ORE PELLETS WITH NONCOKING COALS FOR SPONGE IRON PLANTS

In the present investigation, fired pellets were made by mixing hematite iron ore fines of ?100, ?16 + 18, and ?8 + 10 mesh size in different ratios and studies on their reduction kinetics in Lakhanpur, Orient OC-2 and Belpahar coals were carried out at temperatures ranging from 850°C to 1000°C with a view toward promoting the massive utilization of fines in ironmaking. The rate of reduction in all the fired iron ore pellets increased markedly with an increase in temperature up to 1000°C, and it was more intense in the first 30 min. The values of activation energy, calculated from integral and differential approaches, for the reduction of fired pellets (prepared from iron ore fines of ?100 mesh size) in coals were found to be in the range 131–148 and 130–181 kJ mol^?1 (for α = 0.2 to 0.8), indicating the process is controlled by a carbon gasification reaction. The addition of selected larger size particles in the matrix of ?100 mesh size fines up to the extent studied decreased the activation energy and slightly increased the reduction rates of resultant fired pellets. In comparison to coal, the reduction of fired pellets in char was characterized by significantly lower reduction rates and higher activation energy.     

Reduction and Swelling of Fired Hematite Iron Ore Pellets by Non-coking Coal Fines for Application in Sponge Ironmaking

In the present investigation, the reduction and swelling behaviors (in low grade coal) of fired iron ore pellets, prepared by blending hematite iron ore fines of ?100, ?18 + 25, and ?10 + 16 mesh sizes in different proportions, have been studied in the temperature range of 850–1000°C with an objective to promote massive utilization of fines in sponge ironmaking. An increase in temperature up to the range studied (850–1000°C) substantially enhanced the reduction rate and the rate was found to be highest in the first 15–30 min at all these temperatures. All the fired pellets, made by mixing iron ore particles of ± 100 mesh size, have shown approximately the same reduction rates and slightly higher swelling indices than those made from fines of ?100 mesh size only. In all the fired pellets reduced at temperatures of 850°C and 900°C, the results indicated an increase in the extent of swelling with reduction time. Reduction of fired pellets at temperatures of 950°C and 1000°C exhibited shrinkage in their reduced products, and the extent of this shrinkage increased with increase in exposure time.     

高铝铁矿和高锰铁矿共还原行为的研究

高铝铁矿和高锰铁矿是两种储量丰富但又极难分选的铁矿资源,实现铁、锰、铝的高效综合利用具有重要意义.本文研究了这两种铁矿石工艺矿物学,考查了单矿种及两者的混合矿种的直接还原行为及还原过程中的矿物组成演变,揭示了相应的还原机理.结果表明:高铝铁矿难还原,其机理为经还原后仅部分铁氧化物转化为金属铁,其余的铁与铝、硅矿物形成难还原的铁橄榄石和铁尖晶石;高锰铁矿易还原,其中的铁氧化物大部分被还原成金属铁,锰氧化物与铝、硅矿物结合形成锰尖晶石和锰橄榄石,促进了铁氧化物的还原.而且在相同还原条件下,高锰铁矿球团金属化率比高铝铁矿高30个百分点,前者还原性明显优于后者.两种矿进行共还原,当高锰铁矿配比达到60%时,球团金属化率就可大于90%.锰氧化物的存在对高铝铁矿石中铁氧化物的还原具有显著的促进作用.     

Studies on the Reduction–Swelling Behaviors of Hematite Iron Ore Pellets with Noncoking Coal

Studies on the reduction and swelling behaviors of fired pellets, made by mixing hematite iron ore fines of ?100, ?18 + 25, and ?10 + 16 mesh sizes in different proportions, were carried out with low-grade coal in the temperature range of 850–1000°C with an aim to promote the massive utilization of fines in ironmaking. The rate of reduction in all the fired iron ore pellets increased markedly with an increase in temperature up to 1000°C and it was more intense in the first 15-min soak time. Relatively higher reduction rates and swellings/shrinkage were observed in the pellets made by the addition of larger size (+100 mesh) particles in the matrix of ?100 mesh size fines. In general, highest swelling was observed in the fired pellets at a reduction temperature of 850°C, followed by a decrease at 900°C. At both these temperatures, the percentage of swelling increased with reduction time up to the range studied (120 min). The fired pellets reduced at temperatures of 950°C and 1000°C, showed shrinkage, and the extent of this shrinkage increased with increase in exposure time at 950°C. The percentage swelling/shrinkage in the fired pellets was found to be related to their crushing strengths and porosities.     

Influence of microwave heating on the microstructures of iron ore pellets with coal during reduction

Iron ore oxidised pellets as the burden of blast furnace present many advantages, such as uniform size, high iron grade and high physical strength. A comparison of the iron ore oxidised pellets with coal (out-proportioning) by conventional heating and microwave heating was carried out in this paper. Microstructure transformations during reduction process were investigated by optical microscopy and scanning electron microscope with energy dispersive spectrometry analysis. Micro-hardness of metallic iron phase formed in the reduction was tested with digital micro-hardness tester. The influences of microwave heating on reduction degree, morphology, iron phase and gangues were investigated, respectively. The results show that reduction time can be greatly shortened by microwave heating even at lower temperatures. The fine cracks generated, as the pellets were heated by microwave, were irradiated due to the selectivity of microwave heating. Densification of the metallic iron phase and the separation of the iron and gangues were both found to be enhanced by microwave heating.     

CHARACTERISTICS OF INDIAN NON-COKING COALS AND IRON ORE REDUCTION BY THEIR CHARS FOR DIRECTLY REDUCED IRON PRODUCTION

Studies on the chemical and physical properties (proximate analysis, sulphur content, reactivity, iron ore reduction potential, caking index, and ash fusion temperatures) of coals, procured from 16 different mines in Orissa, India, were undertaken for their judicial selection in Indian sponge iron plants. These coals were found to have low sulphur (range of 0.40–0.66%) and a moderate-to-high ash (range: 22–53%) contents. The results indicated that there were no caking characteristics in any of the coals except Basundhara. The majority of the studied coal ashes were found to have higher fusion temperatures (ST: 1349–1547°C; HT: 1500–1663°C; and FT: 1510–1701°C). An increase in the fixed carbon content in the coal char, in general, led to a decrease in its reactivity toward CO₂. The majority of the chars exhibited significantly higher reactivities (>4.0 cc of CO/g·sec). Further reduction studies in coal chars at 900°C indicated an increase in the degree of reduction of fired hematite iron ore pellets with an increase of char reactivity and reduction time. The authors recommend using the majority of the studied coals as such and some of them (Lakhanpur, Samleshwari, Orient OC–4, and Dhera coals) after blending or beneficiation.     

Utilization of High Sulfur Raw Materials in Iron Ore Pellets

Pyrite cinder and high sulfur magnetite were used as raw materials to produce iron ore pellets. Good qualities of green balls and fired pellets were obtained from the feed comprising 50% pyrite cinder and 50% high sulfur magnetite concentrate at a small scale. Small-scale tests were proven by pilot-scale tests. The high grade fired pellets, assaying 63.22% Fe, were analyzed, and the compressive strength of fired pellets was over 2500 N/pellet. The fired pellets possessed excellent metallurgical performances, such as reducibility index higher than 67%, reduction swelling index lower than 15% and low temperature reduction degradation index (+ 3.15 mm) higher than 1%, which can be used as the burden for blast furnace.     

不同类型铁矿石的低温还原粉化特征

以炼铁厂常用烧结矿、球团矿以及块矿为研究对象,考察了不同含铁原料在高炉炼铁工艺和COREX非高 炉炼铁工艺2种煤气条件下的低温还原粉化行为。研究结果表明:①铁矿石的结构组成决定了其粉化形式,块矿 和烧结矿的粉化形式均表现为“裂开式”,粉化产生的细小粒级颗粒较均匀地分布于-6.3 mm范围;不同于烧结矿 和块矿,球团矿则属于“剥落式”粉化,粉化产物粒径主要集中在-0.5 mm范围;②在还原气氛和温度相同的情况 下,烧结矿粉化程度最高,球团矿次之,块矿最低;③还原温度对3种含铁炉料的还原粉化程度均具有重要影响,当 还原煤气条件相同时,3种含铁炉料的低温还原粉化指数均在550℃时达到最大值;④COREX炼铁工艺的煤气条 件加剧试验用铁矿石的低温还原粉化程度,但对不同类型铁矿石的影响有所差异,综合粉化产生的细小颗粒的增 幅可知,块矿受其影响程度较大,其次是球团矿,而烧结矿对煤气性质改变的敏感性最小。     

Reduction Behaviour of Iron Ore–Coal Composite Pellets in Rotary Hearth Furnace (RHF): Effect of Pellet Shape,Size, and Bed Packing Material

Reduction of iron ore–coal composite pellets in multi-layers at rotary hearth furnace (RHF) is limited by heat and mass transfer. Effect of various parameters like pellet shape, size, and bed packing material that are supposed to influence the heat and mass transfer in the pellet bed, have been investigated, on the reduction behaviour of iron ore–coal composite pellets at 1250 °C for 20 min in a laboratory scale RHF. Reduced pellets have been characterised through weight loss measurement, estimation of shrinkage, porosity, and qualitative, quantitative phase analysis by XRD. A significant difference in the degree of reduction is observed layer-wise in the pellet bed with the variation in pellet shape and size. Pellet bed without any packing material or packed with coal have demonstrated higher degrees of reduction compared to the pellet bed packed with graphite and sand.     

Effect of alumina on slag–metal separation during iron nugget formation from high alumina Indian iron ore fines

Abstract

Iron nuggets can be obtained from ore–coal composite pellets by high temperature reduction. Alumina in the ore plays a vital role in slag–metal separation during nugget formation, as it increases the liquidus temperature of the slag. In this study, the effect of carbon content, reduction temperature and lime addition on slag–metal separation and nugget formation of varying alumina iron ore fines were studied by means of thermodynamic modelling. The results were validated by conducting experiments using iron ore fines with alumina levels ranging from 1·85 to 6·15%. Results showed that increase in reduction temperature enhances slag metal separation, whereas increasing alumina and carbon content beyond the optimum level adversely affects separation. Carbon below the required amount decreases the metal recovery, and carbon above the required amount reduces the silica and alters the slag chemistry. Optimum conditions were established to produce iron nuggets with complete slag–metal separation using iron ore–coal composite pellets made from high alumina iron ore fines. These were reduction temperature of 1400°C, reduction time minimum of 15 min, carbon input of 80% of theoretical requirement and CaO input of 2·3, 3·0 and 4·2 wt-% for 1·85, 4·0 and 6·15 wt-% alumina ores respectively.     

炼焦工艺条件对铁焦性能影响的试验

 在加拿大铁矿粉配比为10%的条件下,系统研究了炼焦工艺条件（堆积密度、升温速度和焖炉时间）对铁焦性能（气孔率、机械强度、热性质和铁矿石还原程度）的影响。结果表明,炼焦工艺条件对铁焦性能的影响显著：堆积密度为1.1 t/m3时,铁焦的性能最佳;升温速度为2.5 ℃/min时,焦炭的机械强度和热性质最佳,而铁矿石还原程度随升温速度的提高而增大;焖炉时间为120～150 min时,铁焦的各种性能均最佳;改变炼焦工艺条件可以得到具有不同性能的铁焦。     

煤中氢对含碳球团还原的影响

针对含碳球团还原过程中的煤种选择问题,研究了烟煤和无烟煤挥发分中氢对含碳球团还原的作用以及温度、加热速度对氢还原过程的影响。结果表明,含碳球团中煤热解产生的氢对铁氧化物有还原作用。由于煤中挥发分的热解析出温度与氢还原铁氧化物的还原温度不一致,氢在还原初期迅速放出,导致氢的还原作用率低;提高温度和加热速度可提高煤中氢的还原作用率和挥发分的利用率。综合考虑,含碳球团实际生产选择煤种时,应选择反应性好的无烟煤。     

Effects of Temperature and Atmosphere on Pellets Reduction Swelling Index

 After taking into account the conditions of the domestic iron resources and the non-coking coal resources, the process of coal gasification-shaft furnace is an effective way to develop direct reduction iron in China. The following tasks are very critical to choose suitable process of shaft furnace and gasification, including the production of oxidized pellets with excellent comprehensive properties as well as the study of the reaction behavior and mechanism of swelling. The results showed that the oxidized pellets of using domestic magnetic iron concentrate as raw materials have favorable comprehensive properties, including higher mechanical strength both before and after reduction, faster reduction rate and lower reduction swelling index (RSI). All of these properties can meet the shaft furnace yielding requirement. When the temperature was below 1223 K, the pellets′ RSI was lower than 20%. With increasing of the content of H2 in atmosphere, the pellets reaction rate accelerated, crushing strength enhanced and RSI decreased. The RSI dropped to 10.26% at 1323 K in 100% H2 atmosphere, and it is up to 39.88% in 100% CO atmosphere. The iron grains mainly presented in platelike when pellets were reduced by H2, however, in CO atmosphere the iron grains were precipitated in flocculent. The whisker shape of iron grains and heating effects of reduction reaction are the major factors leading to the poor pellets strength and increase of RSI. Appropriately controlling the temperature and increasing the ratio of H2 to CO in atmosphere are good for dropping the RSI.     

煤的性质对铁矿—煤压块反应过程的影响

在升温条件下进行了不同性质的煤制成的铁矿－煤压块反应实验。结果表明：反应初期主要是挥发分的析出,在还原过程中,铁矿－无烟煤压块中挥发分先将铁氧化物还原至FeO,然后硕将FeO还原到Fe。铁矿－烟煤压块中挥发分和碳能够边疆将铁氧化物还原至Fe,无烟煤和烟煤混合制成的铁矿－混合煤压块中先是烟煤中的碳将铁氧化物部分还原至Fe,然后无烟煤中的碳将剩余的FeO还原到Fe。     

Application of Modified Humic Acid (MHA) Binder in the Pelletizing Of Fluxed Hematite Concentrate

Modified humic acid (MHA) binder consists of high molecular weight organic molecules and inorganic part. It is extracted from lignite coal with sodium hydroxide and used in pelletization of iron ore concentrates. Our previous study shows that MHA binder is also a potential binder instead of bentonite for pelletizing of fluxed hematite. For evaluating the use of MHA binder in pelletization of fluxed hematite, pelletizing tests were conducted to optimize parameters, such as dose, firing temperature and time. The results show that the qualities of green/dried balls with 0.6 wt% MHA binder are equal to or even better than that of 0.66 wt% bentonite pellets, and that both are much higher than the minimum requirements of the pellets’ strengths. The compression strength of fired pellets also suggested that MHA binder is promising to completely replace bentonite in pelletizing of fluxed hematite concentrate. However, the abrasion rate of the fired pellets with MHA binder is slightly higher than that of bentonite pellets.     

Effect of CaO on the reduction behaviour of iron ore–coal composite pellets in multi-layer bed rotary hearth furnace

The effect of CaO on the reduction behaviour of iron ore–coal composite pellets has been studied in a laboratory scale multi-layer bed rotary hearth furnace at 1250°C for 20?min. Reduced pellets have been characterised through weight loss, porosity measurement, phase analysis by XRD, and morphology study by SEM. The addition of CaO to the composite pellets showed different effects at different carbon levels. For higher carbon-containing pellets (C/Fe₂O₃ molar ratio at the upper stoichiometric level of 3), the addition of CaO increased the extent of reduction for all three layers significantly up to a certain limit (4?wt-%); and thereafter the degree of reduction is decreased with a further increase in CaO percentage in the pellets. For low carbon-containing pellets (C/Fe₂O₃ molar ratio of 1.66), the addition of CaO to the pellets did not show any beneficial effect.     

球团配煤试验研究

以铁矿球团为研究对象,根据成球理论,在实验室进行了球团配煤的试验研究.结果表明：添加适量固体燃料可以提高球团矿强度,降低焙烧燃耗,同时改善球团矿的冶金性能；混合料中配加一定比例的煤,生球的落下强度有所提高,但是抗压强度随着配煤量的增加略有降低,而爆裂温度则变化不明显；当配煤量为2.0％（质量分数）时,生球指标达到了质量要求；配煤量对焙烧球强度的影响比较显著,配加2.0％（质量分数）的煤使焙烧温度降低了40℃,焙烧时间缩短了3 min,效果显著.     

Cold bonded ore–coal composite pellets for sponge ironmaking Part 1 Laboratory scale development

Abstract

There is a need for the development of efficient industrial processes to use iron ore fines of high grade. Attention is particularly drawn to rotary kiln sponge ironmaking technology using lump iron ore, where productivity is low and energy consumption high compared with gas based processes. Fundamental studies carried out elsewhere indicate that the reduction of lump iron is accelerated if a limited amount of carbonaceous material is incorporated in the agglomerate of iron ore fines. Based on these considerations, cold bonded ore–coal composite pellets have been developed for sponge ironmaking in a rotary kiln. These composite pellets were tested in the laboratory and found to reduce very quickly, compared with lump iron ore. Composite pellets were also tested in an 8 t/day rotary kiln sponge iron plant giving enhanced productivity and lower coal consumption, and these results will be presented in Part 2 of this paper (next issue).     

Study of nonisothermal reduction of iron ore-coal/char composite pellet

Cold-bonded composite pellets, consisting of iron ore fines and fines of noncoking coal or char, were prepared by steam curing at high pressure in an autoclave employing inorganic binders. Dry compressive strength ranged from 200 to 1000 N for different pellets. The pellets were heated from room temperature to 1273 K under flowing argon at two heating rates. Rates of evolution of product gases were determined from gas Chromatographie analysis, and the temperature of the sample was monitored by thermocouple as a function of time during heating. Degree of reduction, volume change, and compressive strength of the pellets upon reduction were measured subsequently. Degree of reduction ranged from 46 to 99 pct. Nonisothermal devolatilization of coal by this procedure also was carried out for comparison. It has been shown that a significant quantity (10 to 20 pct of the pellet weight) of extraneous H₂O and CO₂ was retained by dried pellets. This accounted for the generation of additional quantities of H₂ and CO during heating. Carbon was the major reductant, but reduction by H₂ also was significant. Ore-coal and ore-char composites exhibited a comparable degree of reduction. However, the former showed superior postreduction strength due to a smaller amount of swelling upon reduction. Formerly Graduate Student, Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India