Sintering of Indian Iron Ores - Technical Considerations

Transcription

1 Sintering of Indian Iron Ores - Technical Considerations Chandan Barman Ray, Bhaskar Roy, Tushar Kanti Mandal & Subhas Das* M. N. Dastur & Company (P) Ltd, Consulting Engineers P-17 Mission Row Extension Kolkata , India Phone: (+91 33) & Fax: (+91 33) & Key words: Raw material quality, mix homogenization, bed permeability, size segregation, monitoring & control, sinter quality INTRODUCTION The sintering of iron ore fines is a century old technology. There has been gradual improvement in sintering process, sinter plant engineering, process control & automation and several others. Improvement in sintering technology and close control on sinter quality has enabled sinter to be the most favoured burden material in the blast furnace around the world. Blast furnace burden constituting high proportion of superior quality sinter enables low cost operation. In the recent past, several important developments have been made in plant design features, sintering technology, which have substantially increased sinter machine size and improved plant efficiency, viz. productivity, product quality and its consistency, reduced energy consumption, lower cost and reduced emissions. This paper discusses the major technical considerations of iron ore sintering for improved sinter plant performance. Technology status of the new plants vis-à-vis that of the old units and suitable measures envisaged for upgradation of some old units in India for improved performance as well as the operating practice and the major impediments experienced in Indian sinter plants with remedial measures have also been discussed. SINTERING PROCESS AND IMPORTANT TECHNICAL CONSIDERATIONS Sintering is a process in which iron ore fines, coke breeze, limestone fines, dolomite/dunite fines, iron and flux bearing fine waste products of the steel plant are converted into coarse agglomerates on a permeable grate by fusion. The upper surface is raised to a high temperature by burning fuel gas with burners and air for sintering is drawn from the top through the grate by creating draft with the help of undergrate suction fan. After a short ignition period, heating of the top surface is discontinued and a narrow combustion zone move downwards through the bed and each layer in turn being heated to C. Melt makes bond between the grains in combustion zone and agglomerate is formed. The waste gas drawn through bottom of the bed is treated and released to the atmosphere. The use of superior and uniform quality sinter is a pre-requisite to achieve stable operation, higher productivity, improved hot metal quality, reduced fuel rate, lower CO 2 generation and extended campaign life of blast furnace. The characteristics of high quality sinter are given in Table-1. AISTech 2012 Proceedings 521

2 Table-1. Characteristics of high quality sinter Fe content, % 58 and above Al 2 O 3 content, % <1.5 FeO content, % ~7 MgO content, % <2 Grain size, mm 5-50 Mean grain size, mm Tumbler index (+6.15 mm), % > 76 Reducibility index, % > 65 Reduction degradation index (-3.15 mm), % <20 Porosity High Softening temperature, 0 C 1250 Fusion to melting range, 0 C The major technical considerations that have a bearing on the performance of sinter plant e.g. productivity, sinter quality and specific energy consumption are enumerated below: Superior and consistent quality raw materials High bed permeability Optimising bed height Intensive and efficient ignition of bed top surface Optimising heat patterns along bed height Optimising gas to solid heat transfer along bed height Optimising melt formation along bed height Elimination of false air through pallet edge Reliability of equipment and facilities Monitoring and control system Energy recovery Environment control Superior and consistent quality raw materials The chemical and physical characteristics of the raw materials and their uniformity have significant effect on productivity and quality of the sinter. Particle size, gangue contents like SiO 2, Al 2 O 3, MgO, alkalis and combined water are of great importance. Controlling of Al 2 O 3 content, maintaining optimum level of SiO 2 and MgO contents and ensuring higher CaO/SiO 2 ratio enable higher production and good quality sinter. The FeO content of sinter and fuel consumption rise with increasing proportions of iron ore grain size <0.1 mm and >8 mm. Increase in <0.1 mm iron ore fractions affect nodulisation of the mix and decreases bed permeability. Heat transfer is adverse. The heating and melting of +8 mm fractions is slow and heat requirement is higher. Thus, the higher proportion of <0.1 mm and >8 mm ore fractions increase fuel consumption and FeO content in the sinter. Proper size fractions of coke facilitate uniform combustion, heat output as well as heating pattern conducive for decreasing FeO level in sinter. The grain size of the additives should be limited to approximately 2 mm for uniform distribution in the mix to achieve higher strength, reducibility and porosity of the sinter. Sizing and bed blending of the raw materials are important to ensure uniformity of physical and chemical characteristics of the raw materials. High bed permeability The uniform development of sintering reactions through out the bed of solids requires a homogenous raw mix of high permeability to enable higher production of superior and uniform quality sinter. Proper storage, accurate proportioning, intense mixing and effective nodulisation of the raw materials, suitable storage of material mix and proper feeding of the mix on the grate collectively influence permeability and homogeneity of the material bed. The raw material bins are designed to avoid build-up on the wall to reduce segregation of coarse and fine particles. Low level in the bins aggravates size segregation. The discharge of materials by dosing weigh feeders shall be exact to conform pre-determined ratios. The uniform distribution of additives and solid fuel is essential for preparation of the homogenous sinter mix which is achieved with the use of high intensity mixer. Preparation of raw mix in intensive type mixer ensures intimate mixing and proper wetting of the materials. Calcined lime is a good binder, its addition in the mixer and proper control of moisture content in the mix facilitates formation of stable micro pellets during nodulisaion of the sinter mix in 522 AISTech 2012 Proceedings

3 pelletiser. The size of surge hopper should avoid crushing effect on the micro pellets. The material level in the hopper should be optimum and the filling to be uniform to prevent segregation. High and uniform permeability of the material bed decreases specific electrical energy consumption. Optimising bed height A high and uniform permeability allows the bed height on the sinter machine to be increased, resulting in lower specific fuel consumption. The incorporation of suitable mix feeding system can avoid excessive high sintering temperatures, which favourably contributes to the sinter strength, controls FeO content and improves reducibility of the sinter. Sinter with FeO content less than 7% is possible with a material bed height of higher than 600 mm. An increased sintering time positively influences mean size of the sinter product. Intensive and efficient ignition of bed top surface An intensive and efficient ignition of complete surface area of the sinter mix is critical for proper sintering. The modern vertical burner system using high calorific fuel gas improves ignition efficiency and reduces heat consumption. Optimising heat patterns along bed height The feeding of raw mix on sinter machine by the conventional drum feeder system forms piles on the pallets, and results in an avalance action when the slope of the piles become the angle of repose. Fine particles segregate at the top and coarse particles at the bottom. At the same time a sandwich like structure of fine and coarse particles is formed and cause uneven air flow and improper sintering of the mix. Heat generation at the top bed level is inadequate, as fuel content is deficient and some regions at the top surface are not properly sintered. In addition, top surface is rapidly cooled by ambient air drawn through the bed and become fragile. These deficiencies increase proportion of the return fines and decrease sinter yield. Solid fuel content increases in layers below and reaches maximum at the bottom level. The generation of excessive high temperatures in layers below has adverse effect on sintering and increases FeO content of the sinter. To counter the heat shortfall in the upper part and optimising heat pattern in the layers below, certain techniques have been developed. These are Intensified Sifting Feeder (ISF), Segregation Plate, Segregating Slit Wire (SSW), twin layer charging system and others. These facilities eliminate material avalance and facilitate size segregation of the materials along the bed height. The coarser and heavier materials with greater heat requirement accumulate in the lower part of bed. The fine particles are deposited on the top of the material bed. The percentage of fine ore particles is lower in the middle and lower layers. As the finer particles have a higher fuel content, concentration of fuel decreases from the top to the bottom of the material bed. In the twin layer charging system, the coarse fraction with a lower solid fuel content is first charged at the bottom layer via a special charging chute. The second layer comprising smaller grain sizes and higher fuel content is fed on the top of the first layer by drum feeder system. Optimising gas to solid heat transfer along bed height The finer particles have a higher heat transfer coefficient from the gas to solid than the coarser particles. The change in specific surface areas in the material bed increases heat transfer in the upper layer and decreases heat transfer in the lower layer. This change not only improves ignition on the sinter mix surface but also uniformise the heat of sintering reaction, which used to be insufficient in the upper layer and excessive in the lower layer of material bed in the conventional sinter plant. Optimising melt formation along bed height The finer particles have high lime and low iron content and melt easily. The presence of more fuel, higher heat transfer rate, increased temperature facilitates intense sintering of the surface. In the middle and lower layers, fine ore particles are not present except the ones adhered on the micropellets. The result is reduction of excessive heat, melting and nonuniform sintering and thus optimizes sintering reactions. The overall permeability, heat and melt balances improve in the vertical direction of the sinter bed and enable uniform sintering. The quality and yield of sinter improves. Further, burn through point (BTP) control is an important requirement for improved plant performance. The location should be at correct position and uniform across the machine width. The distance of BTP from the sinter machine discharge has a direct influence on sinter machine productivity, average FeO content and mean size of the product sinter. The homogenous sinter mix with higher level of permeability and its even distribution in the bed facilitate the location of BTP at correct position as well as uniform across the machine width. AISTech 2012 Proceedings 523

4 Elimination of false air through pallet edge Proper filling near the pallet s side walls with sinter mix is difficult. The more loosely packed mix near the side plates and the gap formed by the shrinkage of sinter bed; results in flow of the good amount of air, known as secondary/false air through the pallet edge. The secondary air does not take part in the sintering process and reduces temperature and sintering become inferior in the zone. This increases return fines load, waste gas volume and enhances power consumption. To prevent the ingress of secondary air in a modern sinter machine, rim zone covers of approximately 300 mm width are installed along the sidewalls of pallet cars. Several sinter plants in Japan have adopted pallet cars with modified bottom retaining grate bars at the pallet edge to arrest false air ingress, and have achieved large reduction of waste gas volume and improved sintering of material mix at the sidewall regions. As a result, return fines generation has reduced, electrical power consumption is lower and productivity of the sinter plant is higher. Reliability of equipment and facilities The equipment and facilities are designed and manufactured with due consideration of its severity of application to ensure longer availability for uninterrupted plant operations. Presently, modern sinter plant operates for days. The incorporation of accurate gadgets to monitor equipment condition, e.g. temperature, vibration, sound etc. is important to carry out timely preventive maintenance to avoid unplanned outage. Uninterrupted plant operations ensure process stability to achieve higher production, good quality sinter, reduced energy consumption and lower cost. Monitoring and control system High level of automation, effective monitoring and control of the process and equipment are adopted to ensure stable process conditions, improved equipment availability, higher productivity, uniform sinter quality and low production cost. The introduction of process models and expert system has enabled effective control of the process parameters, which contributes to enhanced levels of plant performance. The important monitoring and control functions carried out are given below: Control functions: - Feed proportioning control - Return fines balance control - Sinter mix moisture control - Surge bin level control - Hearth layer hopper level control - Combustion control of ignition furnace - Feed drum speed control - Burn-through point (BTP) control - Sinter cooler speed control - FeO control in sinter Monitoring functions: - Proportioning bins level - Permeability of material bed in sinter machine - Temperature of wind boxes - Temperature of cooled sinter - Winding and bearing temperature of critical motors - Bearing temperature of critical fans - On line dust and emission monitoring in stacks Energy recovery In the sinter plant, the waste gas and sinter cooling air carry approximately 20% and 30% of the total heat respectively. The use of a part of hot air recovered from the front portion of sinter cooler as combustion air in ignition furnace reduces gaseous fuel combustion. The use of remaining portion of the heated air as process air in the strand with the installation of a hood reduces solid fuel consumption and improves sinter quality. Alternatively, the heated air can be used to generate steam and electricity. The collection of hot waste gas from the selective wind boxes can be used to generate steam and electric power. The recirculation of the process gas mixed with hot air from the cooler in the extended hood decreases solid fuel consumption because of utilisation of sensible heats of waste gas and cooler off gas and combustion of CO to CO 2. Environment control Particulate and gaseous emissions are generated at all phases of the sintering process. The pollutant load from sinter plant constitutes a major part of the total emissions from an integrated iron and steel plant. The process gas contains dust, NOx, SOx, CO 2, CO, dioxine, furan etc. and requires treatment in ESPs and other facilities like DeSOx, DeNOx etc. to limit their emission loads. 524 AISTech 2012 Proceedings

5 In view of the pollution norms becoming more stringent, suitable provisions shall be incorporated in all the new sinter plants and pollution abating measures provided in the existing plants should be adequately upgraded to conform stipulated emission levels. The installation of these facilities involves considerably high investment and the power consumption is very high. The adoption of the recirculation of process gas from the waste gas main or selective recirculation of waste gas from the wind boxes mixed with heated air from the cooler in some sinter plants has not only reduced emission levels of the pollutants considerably but also improved sinter quality, yield and reduced solid fuel and electrical energy consumption. As a result, capital investment will be lower and power consumption will be less. Thus, it is important that proper techno-economic evaluation of the two options needs to be undertaken to arrive at optimum techno-economic solution. INDIAN SCENARIO Indian has a large reserve of iron ore. The Fe content of ore is high and the strength is lower. The generation of fines is more during normal processing having proportion of lump and fines 40:60. Therefore, the role of sintering of iron ore fines is critical in India to conserve rich hematite reserve. Sintering of iron ore commenced in India with the installation of two sinter machines, each with 75 m 2 grate area, at Tata Steel, Jamshedpur in Since then, a number of sinter plants have been installed. Presently, the total installed capacity of sinter plants is about 60 million ton per year (mtpy). New iron ore sintering capacity of about 30 mtpy is being installed in the country. The new sinter plants are of large size and provided with all the features of a modern plant. The size of the largest sinter machine is 492 sq m effective area. Presently, sinter constitutes 70-80% of the blast furnace burden in India. Sinter will continue to constitute major portion of the blast furnace burden in future. In order to conserve high quality hematite ore reserve and also to mitigate certain deficiencies in sinter quality, it is envisaged to use 60-65% sinter and 20-30% pellet in the blast furnace burden in future. Huge accumulation of inferior quality iron ore fines in the mines will be beneficiated to produce pellets. Raw materials Although, the ore is rich in iron content, the alumina content is high and alumina/silica ratio is generally adverse. The quality of BF grade limestone is moderate. The CaO content is lower and insoluble level is higher and increases inputs of alumina and alkalis in the system. The ash content of coke has been high. Moreover, the variation in quality of the raw materials has been wide. As a result, alumina and alkali contents remained higher, iron content became lower, strength, RDI and reducibility inferior and the quality characteristics of sinter varied widely. In order to improve the sinter quality with such raw materials, the following measures have been generally adopted in Indian plants: - Washing of iron ore fines to reduce alumina content and percentage of fines content - Use of pyroxenite/dunite to dilute alumina content with the increase of silica content - Base mix preparation facility are installed to enable consistency in sinter feed quality. The base mix comprises 100% all iron bearing materials, 80% of solid fuel and flux. The balance 20% solid fuel and flux is added in the sinter plants. The first base mix preparation unit was installed in By the mid nineties the base mix preparation units were provided in other integrated steel plants. The range of chemical compositions of the major raw materials being used in the different plants is given in Table-2. Table-2. Chemical composition of major raw materials Iron ore fines Limestone Dolomite Pyroxenite Fe, % SiO 2, % Al 2 O 3, % CaO, % MgO, % List of sinter plants A list of sinter plants in India is given in Table-3. Table-3. Sinter plants in India Name of plant Area, m 2 Bhilai Steel Plant 4 x 50; 3 x 75; 1 x 80; 1 x 320; 1 x 360 (1) Durgapur Steel Plant 2 x 142.8; 1 x 198 Rourkela Steel Plant 2 x 125; 1 x 192; 1 x 360 (1) AISTech 2012 Proceedings 525

6 Bokaro Steel Ltd. 3 x 312 Nilachal Ispat 1 x 180 Visakhapatnam Steel Plant 2 x 312; 1 x 400 (1) Bhusan Steel 1 x 177 Tata Steel Jsr 2 x 75; 1 x 192; 2 x 204 Tata Steel, KPO 1 x 500 (1) Jindal Steel & Power Ltd. 2 x 204 Jindal South West 1 x 198, 2 x 204; 1 x 492 Jindal South West - Ispat 1 x 198 Essar Steel 1 x 120 NOTE: (1) Under construction Technology status of the sinter plants The technology status of old sinter plants and that of the new plants in India are given below in Table Raw materials - Coke crushing - Blending Table-4. Technology status of old and new sinter plants in India Items Old sinter plants New sinter plants Proportioning section - Raw materials feeding - Calcined lime addition Mixing and nodulising - Primary mixing - Nodulising Sinter mix charging - Drum feeder - Provision for size segregation of charge mix Bed depth Ignition furnace - Ignition facility - Combustion air Pallet - Sealing system Open circuit Not available Disc feeder Not provided Mixing drum Nodulising drum Provided with single gate Initially not provided, segregation plate having fixed inclination & permeability bars provided later Bed depth mm which increased to 500 mm in some plants where exhaust fan suction adequate Less efficient burners, long furnace Ambient air Closed circuit Base mix preparation Belt weigh feeder Provided Mixing drum; high intensity mixing facility being installed Nodulising drum; intensive nodulising unit being installed Provided with motor driven multiple sector gates Segregation plate with adjustable inclination and permeability bars provided mm Efficient burners, short furnace Hot air from cooler Wear bars mounted on springs and sealing bars fixed rigidly Spring loaded pallet sealing system - Pallet edge effect No measure to prevent air Measures incorporated to infiltration prevent air infiltration Hot screen Provided Not provided, provision of AISTech 2012 Proceedings

7 Plant electrics - Machine drive - Waste gas - Sequential control Items Old sinter plants New sinter plants special chute to maintain good permeability of cooler bed MG set with DC motor Synchronous motor with conventional excitation control Through relay logic Variable speed drive with squirrel cage induction motor Full capacity variable speed drive with synchronous motor/ squirrel cage induction motor PLC based starting, stopping & sequential control Dust extraction system Less efficient systems Efficient systems Waste gas cleaning Electrostatic precipitator (ESP) to control dust load at 100 mg/nm 3 Improved ESPs to control dust load of waste gas at 50 mg/nm 3 ; space provision for installation of DeSOx & DeNOx facilities Energy recovery - Hot air recovery from cooler Monitoring and control - Architecture - Return fines balance control - BTP control - Permeability of material bed in sinter machine Not provided Recovered and used as combustion air in ignition furnace and in extended hood Unit architecture instrument Manual Manual Manual measurement PLC/DCS based Auto Auto Auto measurement Performance indices of sinter plants The chemical constituents of the major raw materials being used vary considerably among the plants. Performance indices of the different plants as well as among the different units in a plant have been affected due to wide variation in raw material characteristics and considerable difference in technology status of the operating units. The critical performance indices of different sintering units are given in Table-5. Table-5. Critical performance indices of Indian sinter plants Productivity, t/m 2 /d Working days in a year Solid fuel consumption, kg/ton Gaseous fuel energy consumption, MJ/ton Electrical energy consumption The important quality characteristics of sinter in different plants are given in Table-6 below. Table-6. Quality characteristics of Indian sinter Fe SiO 2 Al 2 O 3 CaO MgO FeO % % % % % % Chemical composition Size, mm 5-50 Tumbler index, +6.3 mm, % Physical & metallurgical properties RDI, mm, % >30 Reducibility, % AISTech 2012 Proceedings 527

8 The alumina and FeO contents of Indian sinter are high. In view of the adverse effects of higher alumina content on the tumbler index and RDI of sinter, as illustrated in Figure-1 and Figure-2 respectively, the tumbler index of Indian sinter is low; RDI is high. Sinter reducibility is also inferior Tumbler Index Al 2 O 3 in sinter, % Figure1. Effect of Al 2 O 3 on tumbler index of sinter 32 RDI, mm % Al 2 O 3 in sinter, % Figure 2. Effect of Al 2 O 3 on RDI of sinter AISTech 2012 Proceedings

9 Hot metal cost with varying sinter charge The effect of varying proportion of sinter in the blast furnace burden on the hot metal cost has been examined. The basic considerations are as follows: - Hot metal production mtpy - Iron ore quality % Fe; 1.9% SiO 2 ; 2.0% Al 2 O 3 - Sinter quality % Fe; % SiO 2 ; 2.1% Al 2 O 3 - Pellet quality % Fe; 2.15% SiO 2 ; 1.87% Al 2 O 3 The analyses of raw materials are based on beneficiated iron ore. The operating cost per ton of hot metal expressed as indices, for the different alternatives are presented below: Parameters Alt-1 Alt-2 Alt-3 Alt-4 Alt-5 Sinter, % Pellet, % Ore, % Operating cost index From the above results the following observation emerge: - Sinter proportion in the blast furnace should be in the range of 70-80% in order to minimize hot metal cost - Cost of pellet being higher than that of sinter, its proportion in the burden has to be limited. Beneficiation of the inferior quality iron ore fines will reduce cost of pellet as well as the alumina content, and will facilitate use of increased proportion of pellet in BF burden in future Upgradation of old sinter plant The possibility exists to upgrade several old plants by incorporating modern features to improve performance indices. Major modifications and their effects are enumerated below: Aspect Modification Effect Machine grate area Increase of grate area Higher production Solid fuel crushing Close circuit system Process improvement Weighing system in Belt weigh feeder Improved process stability proportioning section and sinter quality Binder Calcined lime addition Stable micro pellets and Sinter raw mix charging Drum feeder having motorised multiple sector gates; segregation plate of adjustable inclination improved bed permeability Optimisation of bed permeability, heat patterns and melt formation along bed height Bed depth Increase of bed depth to 650 mm Reduction of solid fuel consumption and control of FeO content in sinter Pallet car Modified pallet cars to accommodate increased bed depth and to facilitate grate area extension and suction area remaining unchanged Elimination of secondary air ingress; higher production Ignition furnace Incorporation of efficient vertical burners and short furnace Reduction of gaseous fuel consumption Screening of hot sinter Segregation chute in place of hot screen Improved plant availability and extension of machine length Energy recovery Hot air recovery from front portion of cooler for use as combustion air and recirculation in extended hood; or W.h.r.b based power generation Reduction of heat energy consumption in furnace and solid fuel consumption; or electric power generation AISTech 2012 Proceedings 529

10 CONCLUSION In order to improve performance of the sinter plant further, the following aspects need careful consideration: Selection of proper beneficiation technology to limit alumina content of ore fines below 1.8% Use of superior quality limestone Use of serpentine or dunite instead of dolomite, and addition of sand Use of process models and support of expert system for effective monitoring and control of the process Use of cooler hot air in the extended hood to reduce specific fuel consumption and improve yield or power generation Recirculation of process gas in the extended hood to reduce solid fuel consumption and reduce emission levels REFERENCES 1. D.C. Goldring and T.A. Fray, Ironmaking & Steelmaking 1989, Vol 16, No. 2, pp M. Fujimoto, T. Inasumi, et al, 1990 Ironmaking Conference Proceedings, pp H. Beer, K. Kersting, et al, 1995 Ironmaking Conference Proceedings, pp G. Cip, L Gould, et al, Iron & Steel Review March 2002, pp B.K. Das, A. Ranjan, et al, Tata Search, 2007, pp D. Bettinger, M. Widi, et al, Ironmaking & Steelmaking 2007, Vol 34, No. 5, pp Al Fulgencio, C. Archinger, et al, AISTech 2010 proceedings, Vol. I, pp AISTech 2012 Proceedings