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.     

Thermogravimetric Analysis of Coal Char Combustion Kinetics

Four chars prepared from pulverized coals were subjected to non-isothermal and isothermal combustion tests in a thermogravimetric analysis(TGA)device.Three different test methods,i.e.,non-isothermal single heating rate(A),non-isothermal multiple heating rate(B),and isothermal test(C),were conducted to calculate the kinetic parameters of combustion of coal char.The results show that the combustion characteristics of bituminous coal char is better than that of anthracite char,and both increase of heating rate and increase of combustion temperature can obviously improve combustion characteristics of coal char.Activation energies of coal char combustion calculated by different methods are different,with activation energies calculated by methods A,B and C in the range of 103.12-153.77,93.87-119.26,and 46.48-76.68kJ/mol,respectively.By using different methods,activation energy of anthracite char is always higher than that of bituminous coal char.In non-isothermal tests,with increase of combustion temperature,the combustion process changed from kinetic control to diffusion control.For isothermal combustion,the combustion process was kinetically controlled at temperature lower than 580℃ for bituminous coal char and at temperature lower than 630℃ for anthracite char.     

CHARACTERIZATION OF PROPERTIES AND REDUCTION BEHAVIOR OF IRON ORES FOR APPLICATION IN SPONGE IRONMAKING

Studies on the chemical and physical properties, and the reduction behavior (in coal) of hematite iron ores procured from 10 different mines of Orissa, were undertaken to provide information for the iron and steel industries (sponge iron plants in particular). The majority of the iron ores were found to have high iron and low alumina and silica contents. All these iron ores were free from the deleterious elements (S, P, As, Pb, alkalies, etc.). The results indicated lower values of shatter and abrasion indices, and higher values of tumbler index in all the iron ore lumps except Serazuddin (previous) and Khanda Bandha OMC Ltd. For all the fired iron ore pellets, the degree of reduction in coal was more intense in the first 30 min, after which it became small. Slow heating led to higher degree of reduction in fired pellets than rapid heating. All the iron ores exhibited more than a 90% reduction in their fired pellets in 2-h time interval at a temperature of 900°C. Iron ore lumps showed a lower degree of reduction than the corresponding fired pellets.     

Influence of carbon structure and mineral association of coals on their combustion characteristics for pulverized coal injection (PCI) application

The influence of carbon structure and mineral matter of three pulverized coals on their char characteristics including reactivity was studied under a range of combustion conditions in a drop tube furnce (DTF) and thermogravimetric (TGA) furnace for PCI application. Physical and chemical properties of coals and their combustion derivatives were characterized by automated reflectogram, X-ray diffraction, scanning electron microscope, and BET N₂ adsorption. The QEMSCAN technique was used to characterize the heterogeneous nature of minerals of discrete coal particles. The TGA char reactivity was related to the proportion of coal particles displaying strong association of calcium/sulfur phases with carbon matrix to highlight the catalytic influence of minerals on char reactivity at low temperatures. The study suggested that during DTF combustion tests at 1200 °C, char reaction rates might have been catalyzed by coal minerals, particularly due to illite and its association with carbon. Under the same combustion conditions, most of the coal minerals did not transform significantly to slag phases. Coal burnout was found to improve significantly in a combustion temperature range of 1200 °C to 1500 °C. The improvement of coal burnout with temperature appeared to be influenced by coal properties, particularly as a function of the chemical nature of minerals, as well as the degree of associations with other minerals. The study implies that coals with similar mineral compositions might not necessarily reflect similar combustion behavior due to the differences in their associations with other phases. The study highlighted the significance of the characterization of the heterogeneity of coal particles including mineral associations for a comprehensive and reliable assessment of the combustion performance of PCI in an operating blast furnace. QEMSCAN is a trademark of Intellection Pty Ltd., Milton, Queensland, Australia.     

高炉喷吹混煤时碳与二氧化碳反应的探索

 通过采用热重及反应物岩相观察,研究不同煤种在不同配比下混煤残碳与CO2反应行为,由热重曲线可知：2种煤混合时,在900~1100℃,混煤实际与加权失重率相差不大,甚至实际失重率小于加权失重率;在1100~1200℃,混煤的实际失重率大于加权失重率,最大时相差10%。3种煤混合时,煤粉的实际失重率既与温度也与挥发分有关。得出结论：在1100~1200℃时,高炉喷吹混煤反应性更强,焦炭的保护作用更为突出,高炉内煤粉利用率更高。从煤粉碳素熔损反应后形态来看,高煤质煤粉残碳先于低煤质煤粉残碳反应完全。煤粉的挥发分含量不同时,其反应形态也不同：低挥发分的混煤中,烟煤的内孔反应和气泡产生不剧烈;而在高挥发分的混煤中,烟煤残碳反应现象较为剧烈。     

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.     

Development in smelting reduction processes

Corex process is a smelting reduction process to produce hot metal of blast-furnace quality. Coal is used instead of coke, and this replacement makes production costs of hot metal decrease. Iron ore reduction and melting is separated into two steps: in a melter gasifier reducing gas is generated and melting energy is produced by coal gasification; iron ore is reduced in a shaft furnace. Due to this separation, a great variety of untreated coals can be used. The Corex process is designed to operate under elevated pressure, up to 5 bar. Reducing gas is generated in a fluidized bed by partial oxidation of coal. After leaving the melter gasifier, the gas is mixed with cooling gas to obtain a temperature suitable for direct reduction, i.e. approximately 850–900°C. The fines captured in a hot cyclone are re-injected into the gasifier. Reducing gas is fed into the reduction furnace and ascends through the iron burden according to the counterflow principle. The hot DRI having a temperature of 800–900°C is continuously charged into the melter gasifier, where further reduction is effected and melting occurs. Hot metal and slag drop to the bottom of the melter-gasifier. Analogous to blast-furnace practice hot metal and slag are discharged by conventional tapping.     

Reduction behaviour of agglomerated iron ore nuggets by devolatisation of high-ash,high-volatile lignite coal for pig iron production

High-quality coking coals all over the world are gradually approaching extinction. These days, steel industries are trying to focus more on the utilisation of non-coking grades of coal. The present work involving high-ash, high-volatile lignite coal can be used indirectly in iron-making processes. Direct use is not possible due to low amount of carbon and high value of ash. High ash content leads to huge sulphur content, and this leads to high cost involvement in secondary processes. On the other hand, huge amount of iron ore fines are generated during mechanised mining, sizing, screening, transportation, beneficiation and sintering processes. Iron ore nuggets are formed from inferior quality iron ore fines using suitable binders with the applied pressure. Mechanical properties of iron ore nuggets are also assessed through shatter and abrasion test. A furnace was designed, to indirectly utilise high-ash, high-volatile lignite coal, for pre-reduction iron ore nuggets. Iron ore nuggets were partly reduced by CO, H₂ and fine carbon produced from volatilisation of coal. Optimized pre-reduced nuggets, having high mechanical stability was directly charge in the raising hearth furnace for pig iron production.     

炼焦煤灰分对其结焦性的影响规律

 炼焦煤灰分是决定其价格的重要因素,为比较同一矿点煤种灰分对结焦性质的影响规律,对各炼焦煤进行黏结性、煤岩特征、灰分等分析并进行40 kg试验焦炉炼焦试验分析。研究表明,不同煤种炼焦煤灰分降低对黏结性改善幅度不同,黏结性较好的炼焦煤当其灰分降低时,黏结性改善不明显;黏结性较差的炼焦煤当灰分降低时,其黏结性改善较为明显;且不同煤种炼焦煤灰分降低时灰组成变化规律不同。不同煤种炼焦煤灰分降低时所炼焦炭其强度变化程度不同,黏结性较好的炼焦煤,当其灰分降低,所炼焦炭强度变化幅度不大;黏结性较差的炼焦煤,当其灰分降低时,所炼焦炭强度改善较大。     

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.     

Effect of Annealing on Properties of Carbonaceous Materials. Part II: Porosity and Pore Geometry

The pore structure of carbonaceous materials was studied using image analysis. The effect of annealing on the porosity and pore geometry of cokes, chars, and pyrolyzed coals (laboratory chars) was examined in the temperature range of 973 K to 1773 K (700 °C to 1500 °C). The porosity of chars and pyrolyzed coals significantly increased during annealing at temperatures below 1373 K (1100 °C) due to volatile matter release. Further increasing of the annealing temperature from 1373 K to 1773 K (1100 °C to 1500 °C) caused marginal porosity evolution. The porosity of cokes was not affected by annealing at temperatures below 1573 K (1300 °C) and slightly increased in the temperature range 1573 to 1773 K (1300 °C to 1500 °C). The increase in the porosity of chars and pyrolyzed coals during annealing at temperatures 1373 K to 1773 K (1100 °C to 1500 °C), and cokes at 1573 K to 1773 K (1300 °C to 1500 °C), was a result of reactions with oxides of their mineral phases. Annealing had a marginal effect on the pore shape (Feret ratio) of carbonaceous materials, but enlarged the pore size of chars and pyrolyzed coals and decreased their pore density.     

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.     

不同热解半焦与高炉喷吹煤粉燃烧性能差异性

 研究不同半焦与喷吹煤粉的燃烧性能有利于半焦进行高炉喷吹、降低炼铁成本。通过改变热解温度、热解升温速率、热解保温时间和热解气氛在管式炉中制备了不同的半焦样品,通过工业分析和热重试验、结合比表面积、发热量和活化能变化对不同热解半焦和两种喷吹煤粉燃烧性能差异进行了分析。结果表明,不同热解条件下制备的半焦,其燃烧性能大多介于两种喷吹煤之间,热解温度是影响半焦反应性能最重要的因素,热解温度对活化能的影响呈现先减小后增大的趋势。不同热解温度制备的半焦孔隙结构均比喷吹煤发达,且累积孔容积和比表面积均呈现先增加后减少的趋势,孔隙特点与燃烧性能基本对应。550 ℃是制备高反应性半焦较适宜的热解温度。可以通过控制热解条件制备高反应性半焦,这有利于半焦在高炉喷吹中的应用。     

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.     

加拿大焦煤和国内几种常用焦煤的结焦性差异

 研究了加拿大焦煤和国内常用主焦煤的结焦性差异,有利于国内焦化企业很好地利用加拿大炼焦煤,优化配煤生产。对加拿大煤和国内常用焦煤进行常规煤质分析和煤岩特征分析,探寻了二者黏结性存在差异的原因,并利用40 kg小焦炉进行加拿大焦煤替代国内优质焦煤实验。研究表明：与国内主焦煤相比,加拿大焦煤的单一性较好,灰分较低,变质程度较低;加拿大煤的基氏流动度lgMF和奥亚膨胀度[b]值较低,均质镜质组较少,且镜质组中含有较多裂纹和气孔,造成加拿大焦煤在成焦过程中熔融阶段流动性较小,膨胀压力较小;加拿大焦煤替代国内优质炼焦煤进行配煤炼焦,可以稳定焦炭质量。     

Dissolution of carbon from coal-chars into liquid iron at 1550 °C

Carbon-dissolution studies were carried out on four coal-chars (ash content ranging from 9.04 to 12.61 wt pct), using the carburizer-cover method, and the rates of carbon transfer into liquid iron at 1550 °C were determined. A theoretical model was developed for estimating the interfacial area of contact between the chars and the liquid iron. Using a force-balance approach, the partial penetration of the particles was calculated numerically and the total solid/liquid contact area was evaluated for a range of system parameters. The wettability was found to have a very significant effect on the area of contact. An improvement in wetting reduced the upward force due to surface tension, thereby increasing the downward penetration of particles in the liquid and the contact area. While two chars showed a monotonic increase in carbon pickup by the liquid iron, a two-stage behavior was observed for the remaining two chars. Stage I, which corresponds to short times of contact, showed a much higher rate of carbon dissolution, as compared to stage II during later times. The slow rate of carbon dissolution in stage II was attributed to high levels of interfacial blockage by reaction products, which resulted in many fewer areas of contact between the carbonaceous material and the liquid iron. First-order dissolution rate constants (×10³ ms⁻¹) were computed for stage I in all chars, and the observed trend was as follows: 0.01795 (Char 1)>0.00954 (Char 4)>0.0061 (Char 3)>0.00274 (Char 2). These results compare well with the dissolution rate constants quoted in the literature. Char 1, which had the highest rate constant, also had the lowest concentration of reducible oxides (e.g., silica) among all chars. The consumption of solute carbon through silica reduction could affect the carbon levels in the liquid iron. Due to reduction reactions, the experimentally measured rates of carbon dissolution are expected to be slower than the inherent rates of carbon dissolution into the liquid metal. This study shows strong evidence that chemical reactions at the interface play an important role in determining the rate of char dissolution into liquid metal.     

添加铁矿粉对焦炭质量影响的研究

高反应性焦炭能够降低高炉焦比,减少CO2的排放量,改善高炉内铁矿石还原反应的反应效率。研究了在配合煤中添加碱性铁矿粉和酸性铁矿粉炼焦,铁矿粉的配入量对焦炭热性能的影响规律。试验结果表明,随着铁矿粉配入量的提高,焦炭的反应性增大,随后增幅放缓,且添加碱性矿粉的焦炭反应性高于添加酸性矿粉的焦炭,反应后强度与机械强度均有明显劣化;焦炭的灰分增大、硫含量逐步下降;焦炭的各向异性指数不断下降。     

Desulphurisation of high sulphur coal to improve industrial utilisation

Abstract

Coal from northeastern India is considered to be a very good quality coking coal owing to its low ash content (<8%). It has an excellent caking property, and after carbonisation it can provide a very good CSR (coke strength, after reaction with CO₂). On the other hand, its metallurgical application is not so encouraging as a result of its high sulphur content (3-7%). Normally the acceptable sulphur content in metallurgical coke is 0·7% maximum. About 90% of the sulphur is present in organic form, and the remaining 10% as pyrites and sulphates. Removal of sulphur from the organic types is somewhat complicated as it cannot be done by any physical process such as benificiation or flotation, whereas sulphur in pyrites and sulphates can be removed by physical means. With a view to lowering the sulphur content of the coal, desulphurisation studies were carried out at elevated temperatures ranging from 400 to 950°C under the flow of various reducing gases such as coke oven gas, water gas, steam and pure hydrogen. The retention time at each temperature of experimentation was usually varied at 15, 30 and 45 min. It was observed that the sulphur content in the coal was decreased in every experiment with the various reducing gases. The extent of sulphur removal in treatment of the coal with coke oven gas was up to 78·1%, that with steam 83·1% and that with water gas 85·7%. The minimum sulphur content found in desulphurised coal was 0·86%, which could not be used for metallurgical purposes unblended. In the case of experiments with steam, a severe loss of carbon (15-20%) was observed because of reaction with the steam, whereas no such losses were observed in experiments with the other two reducing gases. Hydrogen lowered the sulphur content at a faster rate in comparison with the other gases, but could not increase the degree of reduction. The present study may help to establish the technoeconomic feasibility of the processes; however, large scale experimental studies are required for confirmation. I&S/1721     

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.     

影响焦炭热性质因素的研究

利用13种粘结结焦性质指标相近的配合煤进行70 kg小焦炉试验,比较了所炼焦炭的热性质与矿物质催化指数、煤岩显微组分和焦炭光学组织的关系。结果表明:当配合煤粘结结焦性指标相近时,焦炭热性质主要由灰成分中矿物质催化指数MCI决定;煤岩显微组分含量也是影响焦炭热性质的因素之一;焦炭热性质中CSR不仅受CRI影响,而且受焦炭中光学各向异性指数OTI及光学组织含量的影响。该结论对指导配煤生产,稳定和提高焦炭热性质具有实际意义。