Nitrogen in SL/RN direct reduced iron : origin and effect on nitrogen control in EAF steelmaking

In the steel plant considered here, direct reduced iron (DRI), produced by the coal based Stelco-Lurgi/Republic –National (SL/RN) process, makes up 50% or more of the total iron charge. The SL/RN DRI samples from a kiln cooler had high nitrogen contents (50-250 ppm, depending on particle size), contributing to elevated nitrogen levels in liquid steel produced in the electric arc furnaces. The proposed mechanism of nitriding of SL/RN DRI involves gaseous nitrogen (present within the rotary cooler) diffusing into the solid bed and is supported by a simple diffusion model. A strong correlation was found between the melt-in carbon content of the liquid steel and the final tap nitrogen content, with melt-in carbon of 0.3% C or higher resulting in nitrogen levels below 50 ppm at tap, even when charging DRI material that is high in nitrogen.Department of Materials Science and Metallurgical Engineering of the University of Pretoriahttp://www.maney.co.uk/index.php/journals/irs/ai201

Wear mechanisms of carbon-based refractory materials in silicomanganese tap holes : Part I : Equilibrium calculations and slag and refractory characterization

Silicomanganese (SiMn) as an alloy supplies silicon and manganese to the steelmaking industry. It is produced through carbothermic reduction in a submerged arc furnace. The slag and metal are typically tapped through a single-level tap hole at 50 K (50 C) below the process temperature of 1873 K to 1923 K (1600 C to 1650 C). In one tapblock refractory design configuration, the tap hole is installed as a carbon tapblock and rebuilt during the life of the lining using carbon-based cold ramming paste. The carbon tapblock lasts for a number of years and ramming paste only for months. The purpose of the study presented here was to determine to what extent chemical reactions between carbon-based refractory and slag or metal in the tap hole of a SiMn furnace can contribute to wear of tap-hole refractory. The results of the study are reported in two parts. In Part I, the results of thermodynamic calculations of the potential for chemical reaction between carbon-based refractory material and slag or metal are reported. The results were tested experimentally using pure graphite and synthetic SiMn slag (produced from pure oxides). The paper also reports the composition, microstructure, and phases of industrial SiMn slag, and commercially available carbon block and cold ramming paste refractory materials. These compositions were used in predicted equilibria of refractory–slag reactions. Thermodynamic calculations suggest that reaction between SiMn slag and carbonbased tap-hole refractory is possible, and experiments with nominally pure materials support this. However, practical refractory materials are by no means pure materials, and contain secondary phases and porosity which can be expected to affect reaction with slag. Such reactions are examined in Part II.National Research Foundation of South Africa (Grant TP2011070800005).http://link.springer.com/journal/116632016-04-30hb201

Thermal conductivity of solidified manganese-bearing slags : a preliminary investigation

The thermal conductivity of slag is an important parameter in the design of furnace-containment systems based on freeze-lining technology. Literature indicates that the crystal content of a slag has a significant influence on its thermal conductivity. Industrial, rich, manganese-bearing slag was cooled at different rates to create samples with different microstructures. The coefficients of thermal conductivity of these samples were measured in a nitrogen atmosphere from room temperature to 990°C at 100°C intervals. The laser-flash measurement technique was used for this purpose. Follow-up investigations included XRF and XRD and modelling in FACTSage.http://www.saimm.co.za/ai201

Wear mechanisms of carbon-base refractory materials in SiMn tap-holes : Part II : In situ observation of chemical reactions

The purpose of the study presented here is to determine to what extent chemical reactions between carbon-based refractory and slag or metal in the tap-hole of a SiMn furnace can contribute to wear of tap-hole refractory. The results of the study are reported in two parts. In Part I, thermodynamic calculations suggested that reaction between silicomanganese slag and carbon-based tap-hole refractory is possible, and experiments with nominally pure materials support this. However, practical refractory materials are by no means pure materials and contain secondary phases and porosity which can be expected to affect reaction with slag. In Part II, such reactions are examined experimentally, in cup and wettability tests, using commercially available carbon block and cold-ramming paste refractory materials and mainly industrial SiMn slag. Clear evidence was found of chemical reaction at approximately 1870 K (approximately 1600 C), forming SiC and, it appears, metal droplets. Both carbon block and ramming paste refractory reacted with slag, with preferential attack on and penetration into the binder phase rather than aggregate particles. The two types of carbon-based refractory materials showed similar extents of chemical reaction observed as wetting and penetration in the laboratory tests. The differences in refractory life observed practically in industrial furnaces should therefore be attributed to wear mechanisms other than pure chemical wear as studied in this work.National Research Foundation of South Africa (Grant TP2011070800005).http://link.springer.com/journal/116632016-04-30hb201

Modeling of manganese ferroalloy slag properties and flow during tapping

Stable operation of submerged-arc furnaces producing high-carbon ferromanganese (HCFeMn) and silicomanganese (SiMn) requires tapping of consistent amounts of liquid slag and metal. Minimal effort to initiate and sustain tapping at reasonable rates is desired, accommodating fluctuations in especially slag chemical composition and temperature. An analytical model is presented that estimates the tapping rate of the liquid slag-metal mixture as a function of taphole dimensions, coke bed particulate properties, and slag and metal physicochemical properties with dependencies on chemical composition and temperature. This model may be used to evaluate the sensitivity to fluctuations in these parameters, and to determine the influence of converting between HCFeMn and SiMn production. The model was applied to typical HCFeMn and SiMn process conditions, using modelled slag viscosities and densities. Tapping flow rates estimated were comparable to operational data and found to be dependent mostly on slag viscosity. Slag viscosities were generally lower for typical SiMn slags due to the higher temperature used for calculating viscosity. It was predicted that flow through the taphole would mostly develop into laminar flow, with the pressure drop predominantly over the coke bed. Flow rates were found to be more dependent on the taphole diameter than on the taphole length.http://link.springer.com/journal/116632016-12-31hb201

Chemical wear analysis of a tap-hole on a SiMn production furnace

In April 2013 a 48 MVA submerged arc furnace producing silicomanganese was excavated in South Africa. Since the high shell temperatures recorded in the tap-hole area resulted in the furnace being switched out for relining, the tap-hole area was excavated systematically. A refractory wear profile of the tap-hole area with affected hearth and sidewall refractory was obtained in elevation. The carbon ramming paste in front of, above, and below the tap-hole was worn, as was the SiC with which the tap-hole was built. A clay mushroom formed but was detached from the refractories. Thermodynamic and mass-transfer calculations were conducted to quantify the potential for wear by chemical reaction between refractory and slag and refractory and metal in the tap-hole area. It was found that chemical reaction between refractory and slag or metal could offer only a partial explanation for the wear observed; erosion is expected to contribute significantly to wear.http://www.saimm.co.za/journal-papersam2016Materials Science and Metallurgical Engineerin

Partial slag solidification within an ilmenite smelter

Within ilmenite smelters, the slag is at a temperature just above its liquidus (because the furnace operates with a slag freeze lining), and the metal bath is at a temperature which is approximately 150°C lower than the slag temperature. As a result, solidification of the slag in contact with the metal is expected, and was indeed detected with sounding bars in operating furnaces. Samples of the solidified layer were obtained from a DC ilmenite smelter which had cooled with its contents intact. Analysis showed parts of the layer to be close to anosovite (Ti3O5) in composition; in all cases, the solid contained much less FeO than the primary pseudobrookite which forms during bulk slag solidification after tapping. Equilibrium calculations show that these compositions can result from reaction of initially FeO-rich pseudobrookite with dissolved carbon in the metal bath. The presence of this layer of near-anosovite material in the furnace has possible implications for settling of metal and for the furnace energy balance; the latter was explored with thermodynamic calculations.http://maney.co.uk/index.php/journals/mpm/ai201

Stability and metastability of corrosion pits on stainless steel

"Electrochemical noise" precedes the onset of stable pitting corrosion of stainless steel in chloride solutions. In this work, micro-electrodes have been used to capture single current noise events, which result from metastable pit growth on Type 304 stainless steel in chloride solutions. The form of the current transients has been studied in some detail, and the rate of occurrence of metastable pits has also been characterised as a function of potential, solution composition, and surface fmish. Stable pit growth requires the presence inside the pit of an acidic solution with a high chloride concentration; it is this aggressive solution which sustains rapid dissolution. This aggressive solution is formed by the hydrolysis of the metal cations which are produced by dissolution of the metal, and migration of chloride ions into the pit. Diffusion tends to dilute the pit anolyte, and a balance between the rate of dissolution (current density), and the rate of diffusion, is required to maintain the anolyte at an aggressive concentration. The rate of diffusion from concave hemispherical pits has been quantified. This analysis indicates that open hemispherical pits can only grow stably if the product of current density and pit depth is greater than a minimum value. This minimum product is not achieved in metastable pits - this implies that a (perforated) cover must be present over the pit mouth, to act as a barrier to diffusion. It is thought that the repassivation of metastably growing pits is caused by the rupture - due to the effect of osmotic pressure - of this cover. The experimentally-observed transition from metastable growth to stable propagation agrees with the predictions based on diffusion calculations. Experimental evidence indicates that metastable pit growth is under diffusion control, i.e independent of the electrode potential. The potential dependent nature of pitting is thought to originate with the pit nucleation process, which is considered to be fundamentally different from the pit growth process

Reduction in packed bed of iron ore and coal under one-dimensional heating : experimental results and modelling

Packed beds of coal and iron ore (mostly in the size range 425–850 mm) were heated by thermal radiation to the bed surface, to test possible rate determining steps, the effect of coal volatiles, and whether the previously predicted near constant and approximately linear relationship between heat transfer and reduction holds. Bed depths were mostly 40 mm and 16 mm in some cases. The results confirm that coal volatiles contribute significantly to reduction, reducing the bed temperature slightly, decreasing the energy requirement for reduction, and increasing the extent of reduction. A one-dimensional heat transfer and reaction rate model suggests that the thermal conductivity through reacted layers is higher than predicted, likely by the formation of continuous metal paths and of openings which support radiative heat transfer

Measurement of reduction in packed bed of iron ore and carbon under one-dimensional heating

An experimental configuration was developed to quantify the rate of heat transfer into an iron ore– carbon mixture during one-dimensional radiative heating. Repeatability and accuracy were evaluated by comparing the results of a radiation network calculation with a mass and energy balance, and by comparing predicted carbon contents after reaction with analysed carbon contents. For a sample consisting of prereduced ore and graphite, heat transfer is limited by both radiation to the sample surface and conduction into the sample; the off-gas analysis indicates that both the gaseous reduction reaction and carbon gasification are significantly away from equilibrium. The configuration is shown to yield reliable results for this complex combination of reaction steps