Behaviour of kaolinite coated olivine pellets in blast furnace

Abstract

Production olivine fluxed blast furnace pellets were coated with a water based slurry of kaolinite to give a coating of 5 kg t^-1 pellets. The LKAB experimental blast furnace in Lulea? was used to compare the behaviour of kaolinite coated olivine pellets (MPBO-KA) with that of regular olivine pellets coated with a water spray (MPBO-WA). Blast furnace operation with the kaolinite coated pellets was more stable, with higher productivity and slightly lower reductant consumption, than operation with the normal water coated pellets. In the lower shaft of the furnace, significant quantities of potassium oxide were found to have reacted with the olivine pellets and also with the kaolinite coating.     

Fluxless laser beam joining of aluminium with zinc coated steel

Abstract

A laser welding–brazing (LWB) process to join zinc coated steel and aluminium sheets in two different flange geometries is reported. The deep drawing steel sheets are covered by a zinc layer of maximum thickness 10 μm, and a zinc based filler wire was used in the welding experiments with a Nd–YAG laser. Because of the differences in melting temperatures between iron (1808 K), aluminium (933 K), and zinc (693 K), it is possible to weld the aluminium alloy only. Owing to the zinc coating on the steel side, a Zn–Al alloy can be brazed onto the steel without any flux agent. The inevitable formation of a Fe–Al intermetallic phase at the bondline of the weld seam and the steel can be limited to a thickness of less than 5 μm and to a proportion of the contact area only. Mechanical as well as dynamic tests show results comparable to those obtained via other joining techniques. Salt chamber corrosion tests of varnished specimens display minor damage and no decline in tensile strength.     

Experimental study of coal based direct reduction in iron ore/coal composite pellets in a one layer bed under nonisothermal,asymmetric heating

Abstract

The reduction of hematite iron ore fines by different carbonaceous materials in composite pellets in a one-layer bed under nonisothermal asymmetric heating from above is studied. Temperature measurements at various points in the bed show that there are large gradients both inside and outside the pellets, with the reduction fastest at the top where the temperature is highest. The relative difference between the degree of metallisation between the top and the bottom of the pellet can be even greater during the reduction. The pellets undergo nonuniform swelling/shrinking towards the end of the reduction, and their height is less than their width. Reoxidation can start at the top of the pellet while the bottom has not yet been fully reduced. Those with a higher initial volatile content but with the same amount of fixed carbon as other pellets, gave a significantly higher degree of reduction without the involvement of fixed carbon gasification reactions.     

Raw materials for Corex and their influence on furnace performance

Abstract

The scarcity of good quality coking coal for the blast furnace (BF) has made steel makers look for an alternative iron-making process that requires little or no coke. The Corex process has been developed as an alternative to BF iron-making, which uses non-coking coal and a small amount of coke as fuel, and pellet/lump ore as iron-bearing feed. JSW Steel operates two Corex units each of 0?8 Mtpa, commissioned in 1998 and 2001 respectively. Iron oxides and non-coking coals have to meet certain physical, chemical and high temperature properties for stable operation and to attain high performance levels. Experience of the Corex operation with various coals and iron oxides over the years has helped in understanding the influence and sensitivity of raw materials on its performance and develop new raw material specifications. Statistical analysis of plant data showed that the significant parameters affecting fuel rate and production are moisture, volatile matter, char strength after reaction of coal, reduction disintegration index (RDI; %, –6?3 mm) of pellets and slag rate. This helped achieve the most efficient operating parameters, surpass rated capacity and utilise steel plant waste. The present paper brings out the impact of various raw material properties, and the modified specifications of coal and iron oxides for Corex.     

Recycling of steel plant mill scale via iron ore pelletisation process

Abstract

Mill scale is an iron oxide waste generated during steelmaking, casting and rolling. Total generation of mill scale at JSWSL is around 150 t/day and contains 60–70%FeO and 30–35%Fe₂O₃. To recover the iron, the mill scale must be smelted in a blast furnace or other reduction furnace; however, it is usually too fine to use without previous agglomeration such as via pellet or sinter mix. JSWSL operates a 4·2 Mtpa pellet plant to produce pellets for Corex and BF ironmaking units. The aim of this study is to determine the effect of mill scale on pellet properties. Detailed laboratory basket trials were conducted using up to 40% of mill scale in the pellet mix. The addition of mill scale up to 10% is considered to provide the optimum balance of chemical, physical and metallurgical properties of the pellet.     

Coolant strategies for BOF steelmaking

Abstract

Thermal balance of basic oxygen furnace (BOF) steelmaking is controlled primarily by the scrap added as coolant. In the present steel scenario, insufficient availability and high cost of prime scrap has led us to explore the possibilities to use alternative coolants. Until now, choice of alternative coolants used in converter is determined by availability, price and operational strategies of the shop. These coolants have varying cooling effects and significantly influence the blowing parameters and turn-down performance. With the aim to reduce cost and dependence on prime scrap, various alternative coolants, such as iron ore lumps, pig iron, iron ore pellets, direct reduced iron and lime have been experimented with in 130 t converters under different conditions. The heat absorption behaviour and melting rate of these coolants were significantly different and found to be dependent on physical properties and addition patterns. Each coolant was individually compared for its effect on turn-down chemistry, tap temperature, slag condition and refractory wear pattern. Use of some of these coolants had beneficial effect of forming foamy slag and improved the dephosphorisation. Combined use of these coolants has helped in minimising the prime scrap requirement. This paper describes the utility of blow patterns, optimisation of coolant mix and addition strategies contrived for the synergic use of alternative coolants.     

Mixed burden softening and melting phenomena in blast furnace operation Part 3 – Mechanism of burden interaction and melt exudation phenomenon

Abstract

This research was focused on expanding the current understanding of the mechanism of softening and melting of ferrous materials including liquid exudation and high temperature material interaction. The bulk softening and melting experiments were interrupted at temperatures of interest and samples were examined for morphological changes. The results of these experiments were complimented with viscosity, surface energy and deformation rate calculations which were helpful in understanding the melt exudation phenomenon. The microstructure of the materials showed a transition from heterogeneous to homogenous state with increasing temperatures. The melt dripping was predominantly observed in olivine fluxed pellets. The exuded slag was primarily composed of an alkali rich phase which was found least viscous of the slags present in this system. The viscosity of the liquid and the structure of metallic shell jointly control the flow of liquid from the ferrous materials. Addition of magnesia in lieu of lime was found to provide beneficial impact, in particular on the rate of liquid evolution at high temperature. Based on the metallographic examination of the samples a mechanism of burden interaction is proposed.     

Formation of potassium slag in olivine fluxed blast furnace pellets

Abstract

Mineralogical evaluation of olivine pellets coated with kaolinite, taken from the LKAB experimental blast furnace, shows significant reactions with potassium. Sampling has revealed strong potassium deposition in pellets in the lower shaft close to the wall, but much less deposition towards the furnace centre. Iron reduction and the deformation of the pellets were enhanced in the zone of high alkali deposition. Thin sections of pellet samples were prepared to distinguish amorphous and crystalline slag phases for a better understanding of the formation of the potassium rich slag. Olivine breaks down to various extents to form a SiO₂–FeO–MgO–K₂O glass. The kaolinite coating shows strong reaction throughout the cross-section of the lower shaft to form kalsilite (KAlSiO₄) and K₂O rich glassy slag. Studies of thin sections of the slag products were shown to be very useful in separating amorphous phases such as the K₂O rich glass from the crystalline olivine rim.     

Mixed burden softening and melting phenomena in blast furnace operation Part 2 – Mechanism of softening and melting and impact on cohesive zone

Abstract

The present work was designed to improve the current understanding of the softening and melting (SM) mechanism of ferrous materials and to identify their potential impact on the cohesive zone in the blast furnace (BF). The lump ore, direct reduced iron (DRI) and hot briquetted iron (HBI) were individually subjected to industrial trials and their results were compared with those obtained in the laboratory (SM) tests described in first part of the present paper. The difference between melting and 10% bed shrinkage temperature obtained from the results of laboratory and industrial tests was compared and it was found within 10% limits. The SM temperatures were also correlated to the basicity, gangue and flux contents of the mixed burden. The liquid slag mass fraction was computed using FactSage software and compared with the deformation of the burden bed. These results indicated presence of a small amount of liquid at temperatures close to the softening temperature of the burden suggesting that the deformation of solid phases is the primary cause for softening of the burden. The melting of the mixed burden is dependent on the melting point of DRI and melt exudation occurs close to the meltdown of the burden. The impact of ferrous burdens on the cohesive zone was predicted on the basis of experimental results which indicated DRI/HBI is an excellent blast furnace feed material to improve the productivity and decrease the coke rate in the BF.     

Reaction rate and product morphology in carbon composite iron ore pellets with and without Portland cement

Abstract

The aim of this work is to study the reaction rate and the morphology of intermediate reaction products during iron ore reduction when iron ore and carbonaceous materials are agglomerated together with or without Portland cement. The reaction was performed at high temperatures, and used small size samples in order to minimise heat transfer constraints. Coke breeze and pure graphite were the carbonaceous materials employed. Portland cement was applied as a binder, and pellet diameters were in the range 5·6–6·5 mm. The experimental technique involved the measurement of the pellet weight loss, as well as the interruption of the reaction at different stages, in order to submit the partially reduced pellet to scanning electron microscopy. The experimental temperature was in the range 1423–1623 K, and the total reaction time varied from 240 to 1200 s. It was observed that above 1523 K the formation of liquid slag occurred inside the pellets, which partially dissolved iron oxides. The apparent activation energies obtained were 255 kJ mol^–1 for coke breeze containing pellets, and 230 kJ mol^–1 for those pellets containing graphite. It was possible to avoid heat transfer control of the reaction rate up to 1523 K by employing small composite pellets.