Ironmaking & Steelmaking: Processes,Products and Applications

The present paper develops a one-dimensional model of a novel coal based iron ore direct reduction process. In this process, a mixture of iron ore, coal fines and small amount of binder is made into pellets and these are placed in a bed. Air is forced upward through the pellet bed and provides oxygen for the volatiles and part of the coal in the pellets to be burnt. Initially the pellet bed is heated from the top. As the temperature of the top level of pellets increases, they start to evolve pyrolytic matter which is ignited and, as a consequence, the pellets at lower levels in the bed are heated. In this way, a flame propagates downward through the bed. The iron ore reacts with the gases evolved from the coal (including volatiles) and carbon in the coal and undergoes reduction. The model presented in the article simulates the processes occurring in the solid and gaseous phases. In the solid phase, it uses a novel porous medium model consisting of porous pellets in a porous bed with two associated porosities. The model includes equations for energy balance, reactions of iron oxide with carbon monoxide and hydrogen, coal pyrolysis and reactions between the gas components in the voids. The model shows that a rapidly increasing temperature front can travel downward through the bed if the air is supplied for long enough. The predictions of the modelling are discussed and compared with observations obtained from an experimental rig.

A semiempirical kinetic model has been developed to determine the course of reduction of iron ore–graphite composite pellets over time in a laboratory scale side heated packed bed reactor attached with a tailor made bottom hanging thermogravimetric set-up. The rate parameters in the model, especially the three sets of apparent activation energy values and frequency factors associated with the reduction of iron oxides in three elementary steps, namely hematite to magnetite, magnetite to wustite and wustite to iron, have been estimated based on experimental data by employing an optimisation tool, the genetic algorithm (GA). The difference between the predicted and experimental degree of reduction is minimised to obtain the rate parameters. The experimental degree of reduction is calculated based on mass loss data during reduction and the exit gas analysis. Estimated values of apparent rate parameters were found to be of the same order of magnitude to their intrinsic counterparts reported in literature. Finally, by using the predicted rate parameters the temporal evolution of various oxide phases as well as pure iron has been evaluated.

Regressive orthogonal design experiments have been carried out based on the analysis of the oxidation and induration behaviours of individual magnetite and hematite pellets. The research shows that the optimal pellet preheating time and temperature, and roasting time and temperature change with hematite/magnetite (H/M) ratio. Under the conditions of preheating temperature 900°C, preheating time 10 min, roasting temperature 1275°C and roasting time 15 min, the compression strength of preheated and roasted pellets is found to be respectively more than 444 N/pellet (N/P) and 3993 N/P when the magnetite content in the pellets exceeds 20%. The pellet strengths meet the requirements of rotary kilns and blast furnaces respectively.

Chromite ore pellets are used for ferrochrome production by the smelting reduction process. The strength and porosity of the sintered pellets play a significant role in smelting reduction. Granulometry of the chromite fines used for pellet production is an imperative enabler which controls the pellet strength and porosity by packing of particles and formation of bonds due to melting of gangues at high temperature sintering. The effect of chromite ore granulometry and sintering process parameters was evaluated to improve the metallurgical performance of pellets in a submerged arc furnace. The drop test, compressive strength and porosity of sintered pellet samples from plant and laboratory tests were evaluated. Mathematical correlations of the pellet properties (strength and porosity), ore granulometry and sintering parameters were determined using Statistica (ver7), statistical software package. It was found that ore granulometry, i.e. particle size, plays a vital role in the pelletisation process by controlling the number of contact points for bond formation and due to different liberation and smelting reduction characteristics of gangue minerals. It was concluded that for Sukinda chromite ores, required pellet properties can be achieved by maximising the 74–37 μm fraction of chromite fines in the charge and sintering at 1000°C.

The blast furnace has been, and is likely to remain, the dominant technology for ironmaking. Coke is fed to blast furnace as a fuel and its quality plays a significant role in controlling the performance of the furnace. The quality of coke depends on the quality of coal or its blend, coking parameters and precarbonisation techniques, if any. With decreasing availability and increasing cost of good quality hard coking coal, coke makers face a tough challenge for production of metallurgical coke at competitive rates. Coke quality has been enhanced in recent times by introduction of precarbonisation techniques, such as compaction of the blend into cakes, so as to improve its bulk density. JSW Steel has adopted the newly developed vibrocompaction precarbonisation technique, along with non-recovery ovens, having a capacity to produce 1˙2 Mtpa of coke. Optimisation of coal blend and bulk density of cake produced from vibrocompaction has helped JSW use inferior coals up to 35% in the coal blend, without adversely affecting the coke quality. The present paper discusses the optimisation of bulk density and coal blend, and use of non-coking coals in the coal blend to obtain the desired coke with properties: coke strength after reaction (CSR) exceeding 64%, coke reactivity index (CRI) <25% and Micum index (–10 mm) (M10) <6%.

A study has been undertaken to identify the source of coke fines sampled from the deadman area of the blast furnace. Using measurements of the coke crystallite dimension L _C, it was established that some of the fines found in the deadman area of a blast furnace were not simply the degradation products of the lump coke in this area. The L _C was measured using standard X-ray analysis techniques. The coke fines had a higher L _C than the coke lump and therefore have experienced a higher maximum temperature than the associated coke lump. This finding has been interpreted as at least some of the coke fines found in the deadman area (lower temperature region) have been blown there from the raceway by the high velocity hot blast.

The injection rate of fossil fuels in the blast furnace is limited because of a drop in the flame temperature in the raceway as well as problems in deadman region and the cohesive zone owing to the unburnt char. An alternative option for coke saving, a clean deadman as well as increase in blast furnace productivity is injection of hot reducing gases (HRG) which are produced by low grade coal gasification or top gas recycling after CO₂ and N₂ removal. Calculations using a mathematical model show that HRG injection at higher temperature is desirable. Hot reducing gas injection is possible up to 300 Nm³ thm^–1, above which the top gas temperature shoots up beyond practical limits. Furthermore, it also shows that if the flame temperature is maintained constant by varying steam and oxygen injection, the productivity is increased by 16% and coke rate is reduced by 84 kg thm^–1 with the replacement ratio of 1˙4 kg coke/kg gasified coal at 300 Nm³/thm of HRG injection. It was also observed that the complete replacement of pulverised coal (PC) injection with HRG injection is more effective over the coinjection of PC and HRG in terms of coke rate saving. However, oxygen enrichment is possible up to 75% with the coinjection of HRG and PC, with a resultant of rise in productivity. Injection of HRG in the form of top gas (blast furnace gas) is more effective over the injection of HRG generated from coal gasification. The productivity is increased by 25% and coke rate is reduced by 83 kg thm^–1 with the replacement ratio of 1˙7 kg coke/kg HRG at 250 Nm³ thm^–1 of HRG injected from top gas.

The blast furnace is a countercurrent reactor in which a reducing gas is produced by coke gasification with the oxygen blown in via tuyeres. The reducing gas flows upwards, reducing the iron ores charged at the top of the furnace. It is a very complex process with many influencing and correlating factors. Its productivity is the quotient between possible gas throughput per unit of time and required specific gas generation for 1 tonne of hot metal obtained, and its permeability is a measure of the gas ability to pass through the bed of solid materials. The objective of 'high levels of injection of pulverised coal' is not only compatible with productivity, but also even necessary to increase blast furnace productivity. In this sense the helium tracing technique consists of injecting He at the tuyeres with its arrival at the blast furnace top being detected by a mass spectrometer. With this measurement it is possible to define the transfer time as the delay between the injection moment and the time when the helium concentration reaches 10% of the maximum detected level. Calculated variables from the measurements allow a concise characterisation of the blast furnace state. These gas transfer measurements can be considered as a new tool to evaluate the state of a furnace at a specific moment. The main advantage will be that by employing only one measurement it will be possible to evaluate the furnace state.

6Cr21Mn10MoVNbN is a new type of alloy used for heavy duty engines. Thermal simulation, metallography, X-ray diffraction and theoretical analysis have been used to study the deformation behaviour of the alloy at elevated temperatures. By introducing an internal variable parameter, a constitutive equation has been suggested for the dynamic calculation of the stress/strain curve for this alloy based on the Zerrilli–Armstrong equation used for fcc materials. The data calculated using the equation developed agree fairly well with the experimental data. The microstructure of the alloy is found to depend greatly on temperature and strain rate. When the alloy is deformed at 850°C, Cr₂₃C₆ precipitates from the matrix. The lower the strain rate, the more Cr₂₃C₆ precipitated from the matrix. At the same time, the amount of Cr₂₃C₆ decreases with increasing temperature. Between 850 and 1050°C, the amount of NbN in the alloy increases with increasing temperature and reaches a peak at 1050°C. NbN begins to decrease when the temperature gets to 1150°C. At this temperature, carbides and nitrides in the alloy, especially those distributed on the grain boundaries, begin to be dissolved into the matrix in great deal. The microcracks propagate easily along the grain boundaries of the alloy.

A study has been made of the cold deformation aging susceptibility of dual phase steel with 14 and 22% martensite by volume. It was found that yield strength (YS) and ultimate tensile strength (UTS) increased with increasing testing temperature up to 200°C and then decreased at 250°C which indicated that static strain aging is dominant up to 200°C. This aging is associated with free dislocations in the ferrite as a result of the austenite–martensite transformation, which become preferential sites for solute atom diffusion. At higher temperature (250°C) a softening effect is dominant due to tempering of martensite. It was also observed that at a given aging temperature YS, UTS and ΔY (increase in stress due to strain aging) increased with by volume of martensite.

Flow field characteristics within an elliptical liquid bath, agitated by a top submerged gas injection lance system, are investigated experimentally using laser diagnostics techniques and high speed photography. The model is designed to simulate the flow behaviour of a conceptual AusIron furnace design which consists of an elliptical perspex vessel and two vertically supported lances. The results indicate that higher flow rate created a strong recirculation zones at the bottom of the bath and hence better mixing. The lower level of submergence caused a rapid spread of the gas jet at the top section of the bath and the higher submergence levels improved the agitation and flowfields in the bath. In general, the findings from the present study enabled us to gain more knowledge of the overall physical phenomena of such a complex system, which in turn assist furnace designers in determining the optimum operating conditions for complex industrial systems.

Twin roll strip casting is regarded as a prospective technology offering many economic benefits. The control of fluid flow in the pool is, however, particularly difficult due to the high casting speed and small pool volume. In the present study, a three-dimensional mathematical model has been developed for the coupled analysis of fluid flow, heat transfer and solidification in the pool using the finite difference method. The characteristics of transport phenomena in the pool of a twin roll strip caster using a wedge type melt delivery system were analysed by numerical simulation. The results show that it is desirable for the wedge melt delivery system to provide the uniformity of flow and temperature in the pool to maintain the casting process and improve the strip quality.