During the processing of steel in steel mills, iron oxides will form on the surface of the metal. These oxides, known as mill scale, occur during continuous casting, reheating and hot rolling operations. Mill scale is a valuable metallurgical raw material since it contains 65-70% iron. JSW Steel Ltd is a 7 Mtpa integrated steel plant and generates 270 t of mill scale per day. Most of the materials of steel plant wastes are recycled through sinter making in most of the countries. Because of its physical, chemical and mineralogical properties, it can be used as a raw material in a process like sintering. The mill scale contains high amounts of Fe and low amounts of silica and alumina. Thus, recycling it through the sintering process helps in the saving of raw materials like iron ore and limestone. This paper presents preliminary findings of a study that investigates the potential for recycling steel mill scale in the sintering process. Experiments were conducted using the mill scale in sinter making from 0 to 70 kg/t of sinter. The total Fe and FeO contents of the sinter increased with the increase in mill scale addition. The sinter productivity decreased with the increase in mill scale addition due to a decrease in sinter bed permeability. The sinter strength and sinter mean size initially increased and reaches a maximum at mill scale addition of 40-50 kg/t of sinter and afterwards declines with the increase in mill scale addition. Sinter reduction degradation index and reducibility decreased with the increase in mill scale addition due to the increase in FeO content. Except sinter productivity, other desired sinter properties can be obtained with the use of 40-50 kg mill scale per tonne of sinter.
The sinter structure and its characteristics mainly depend on the raw material chemistry, size, size distribution and the sintering process parameters. In sintering process heat is supplied by coke breeze in the sinter mix to raise the bed temperature to achieve partial fusion and diffusion bonding. Airflow rate and flame front speed in sintering process has been found to guide the performance of the sinter plant and these parameters mainly depends on the sinter bed permeability. The flame front speed (bed permeability) has been considered as one of the important operating parameter and it depends on several factors; the feed size of the sinter being one of the most important parameter among them. Since iron ore proportion is at higher side in the sinter mix, its size fraction is very important. JSW Steel sinter plant receives iron ore fines of -10 mm size from Bellary -Hospet region which consist of 3 to 9% bigger than 10 mm and 30 to 35% smaller than 0.15 mm size fraction. It is well known that larger particles favour diffusion bonding and smaller particles favour slag bonding in sintering process. Accordingly, the study of the assimilation characteristics of different size range iron ore has an important role to control the reactions in the sinter bed and to obtain the target mineral structure. Too much variation in coarser and finer particle size range in sinter mix, the behavior of these +10 mm and -0.15 mm particles have been a subject of investigation and it is necessary to understand the role of iron ore particle size on sinter microstructure, sinter strength, sinter RDI, and productivity. In present work pot grate sintering experiments have been carried out in laboratory with different level iron ore size (mean particle size from 1.22 to 3.95 mm) to understand the influence of iron ore mean particle size on mineralogy, productivity, physical and metallurgical properties of the sinter. Sinter productivity increased with increase in iron ore mean particle size due to increased flame front speed (FFS) and improved bed permeability with lower sintering time. Sinter with iron ore mean particle size of 2.59 mm (Classifier fines) yielded better sinter strength with lower fines (-5 mm) and lower RDI. Higher sinter strength is due to effective distribution of acicular silico ferrites of calcium and alumina (SFCA) phases. The improvement in sinter RDI is due to the change in proportion of magnetite and hematite phase with flame front speed.KEY WORDS: iron ore sinter; iron ore feed size; microstructure; productivity; sinter strength; sinter RDI.
The quality of iron ore sinter mainly depends on sinter mineralogy, which in turn depends on the chemical composition of the sinter mix. The reduction properties of the mineral phases formed in the sinter influences the sinter reducibility. MgO has a varying effect on sinter reducibility at different silica contents. A recent trend in blast furnace operation shows that there is a considerable increase in usage of dolomite as a basic flux either directly or through sinter. Recently the silica levels in the sinter product of Sinter plant 1(SP1) of JSW Steel Limited have been fluctuating in the range of 5?5-9?6% due to variation in silica content of iron ore fines. At the same time, as per blast furnace requirement, the addition of dolomite has been changed from 2?4 to greater than 3?0% at SP1, and the reducibility of the sinter decreased (,60?0%). Laboratory pot grate sintering experiments have been carried out to determine the influence of MgO addition on microstructure and reducibility of low and high silica sinter. MgO additions have been varied from 1?4 to 3?2% for low silica (4?5%), and high silica (6?3%) iron ore fines. From the studies it was found that the reducibility of both sinters decreased with increase in MgO addition due to an increase in magnetite/magnesio spinel phase and silicate/slag phase. Reducibility of low silica sinter was greater with high silica sinter. High silica with high MgO sinter had lower reducibility compared to low silica with low MgO/high MgO and high silica with low MgO sinter.
From a sinter production point of view, it is important to optimise the sintering process with regard to both sinter quality and production rate. In sintering, airflow rate within the sinter bed decides the production rate and its physical and metallurgical properties. To study the influence of airflow rate (flame front speed) on sinter production and sinter quality, pot grate sintering experiments were conducted at sinter grate suction pressures ranging from 900 to 1700 mm water column over the sinter bed. During sintering, time-temperature data were recorded, and mineralogical studies were carried out. This study reveals that increase in sinter grate suction pressure through the sinter bed from 900 to 1700 mm water column significantly improved the sinter productivity from 34?37 to 48?90 t/m 2 /day; however, the physical and metallurgical properties of the sinter at higher suction pressure were not optimum with respect to blast furnace requirements. The maximum sinter productivity with desired physical and metallurgical properties was obtained at suction pressure 1300 mm water column. At this pressure, improvement in sinter quality was due to optimum firing temperature and enough retention time available for formation of mineral phases. At an airflow rate 1300 mm water column, sinter productivity was 41?0 t/m 2 /day, sinter strength (TI) was 73?10%, reduction degradation index was 25?0 and reducibility was 71?50%.
Bed permeability, rate of reductant and productivity of blast furnace (BF) performance mainly depends on both iron bearing material but also carbonaceous material. Most of the BFs have the sinter being a major burden; hence, in JSW Steel Ltd, four sinter plants are operating to fulfill the four BF's requirement. For efficient BF operations, sinter plants are key units whose proper performance is vital to produce desired sinter strength. The tumbler index of the sinter is an important property of the sinter, and sinter strength depends on the raw material composition and machine parameters. For smooth sinter plants operation, changes to the operating conditions should be few and precise. To achieve this, a much better understanding of the mechanisms relating control inputs to a sinter production rate and quality needs to be established. In the present work, a neural network based model has been developed and trained relating sinter strength with a set of nine process variables, namely, basicity, Al 2 O 3 /SiO 2 , MgO, MnO, FeO, moisture, coke breeze, burnthrough temperature and machine speed, to predict the tumbler index (26.3 mm) of the sinter. The variables to which strength of the sinter was most sensitive were Al 2 O 3 /SiO 2 , basicty, machine speed, and MgO, MnO and FeO. Tumbler index of the sinter was influenced by sinter porosity, which was itself determined by the firing temperature and green sinter mix carbon content. The predicted results were in good agreement with the actual data with ,3.5% error.
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