Decreased gas consumption of a fluidized bed furnace

The feasibility of utilizing a closed circulatory system to generate gases for a fluidized bed furnace was investigated with the primary concentrations of both economizing on the raw materials used for producing furnace atmospheres and decreasing the air pollution caused by exhaust gases. Air humidified with water vapor was first introduced into a charcoal furnace for causing a reaction with hot charcoal to form a carburizing atmosphere. This atmosphere was then introduced into a fluidized bed furnace to carburize steels. The exhaust gases from the fluidized bed furnace were recycled by repassing them through the hot charcoal layer in the charcoal furnace with a gas pump. The charcoal furnace and the fluidized bed furnace formed a closed circulatory system during the carburization of steels. Experiments were performed with various parameters of this system, including content of water vapor in the humid air, temperature of the charcoal, rate of recirculation of the atmosphere,etc. The effect of each parameter on the carburizing behavior in the fluidized bed furnace was investigated on the basis of the rate of carburization and the carbon potential of the atmosphere. The feasibility of applying this system to a fluidized bed furnace was assessed from the aspects of the fluidization of A1₂O₃ powder, the result of carburizing steel, and the rate of consumption of charcoal. The closed system employed in generating atmosphere was demonstrated by the experimental results to have enabled the fluidized bed furnace to operate normally and to have significantly decreased both the consumption rate of charcoal and the environmental pollution.     

bdGas carburizing of steel with furnace atmospheres formedin situ from methane and air and from butane and air

Carburizing experiments were conducted at 927°C (1700°F) and 843°C (1550°F) using furnace atmospheres formed from methane and air and from butane and air introduced directly into the carburizing furnace. Gas flow rates were low to promote equilibration of the reaction products within the furnace. The air flow rate was held constant while the methane or butane flow was automatically regulated to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. In comparing the results of these experiments with earlier results obtained using propane and air, several differences were noted: (a) The methane content of the furnace atmosphere, measured by infrared analysis, was about twice as great when methane was the feed gas rather than propane or butane. This was true despite the fact that the mean residence time of the gas within the furnace was greater in the methane experiments. Methane appears to be less effective than propane or butane in reducing the CO₂ and H₂O contents to the levels required for carburizing. (b) There was a greater tendency for the CO content of the furnace atmosphere to decrease at high carbon potentials when methane is used instead of propane or butane. The decrease in CO content is due to hydrogen dilution caused by sooting in the furnace vestibule. These differences in behavior make propane or butane better suited than methane forin situ generation of carburizing atmospheres. However, there is no difference in the amount of carburizing occurring at a specified carbon potential when methane, propane, or butane are used as the feed gas in this process. J.A.Pieprzak, formerly a member of the Engineering and Research Staff     

JPMA development awards

Abstract

The challenges in controlling carbon potential during sintering of steel powder have been discussed in many experimental and theoretical studies. The main issues lie within the complex thermodynamics and kinetics of processing atmosphere chemistry in continuous sintering furnaces. Although many models have been proposed to address the problem, these have rarely come to reality and entered industry practice. The purpose of this article is to summarise these discussions and investigate the interaction of the atmosphere constituents with the sintered compact within a sintering furnace. An important aim is to provide the PM industry with a fresh understanding of furnace operations and to provide recommendations to improve the control of furnace conditions. A case study is given of an existing furnace installation using Sinterflex technology which allows continuous monitoring and/or control of the furnace atmosphere. The reduction of oxides and carbon potentials to optimise the production parameters is described.     

Power increase of plasma heating systems by CO2 addition into the furnace atmosphere: Investigations in view of the plasma ladle furnace

For certain steel grades the treatment in conventional ladle furnaces equipped with graphite electrodes may not be advisable because of the risk of carbon pick-up. In these cases metallic plasma torch systems operated with argon as plasma gas may be preferable. However, the maximum active power available from a 3 torch AC argon plasma system is at present limited to about 4 MW. Tests were carried out with 200 kg heats in a pilot furnace equipped with 2 AC argon plasma torches to investigate the arc voltage increase by adding CO₂ to the furnace atmosphere. With 20% CO₂, the voltage of a 30 cm arc was raised from 100 to 173 V. The transfer rate of oxygen from the CO₂ in the atmosphere via the plasma arc to the steel melt was measured by means of steel and off-gas analyses. After formation of a slag layer, about 10% of the oxygen supply injected into the furnace as CO₂ was transferred to the melt. The measured values concerning power increase and oxygen transfer were extrapolated to ladle capacities of 50 to 150 t and 3-torch 12 kA AC plasma heating systems. In a furnace atmosphere containing 60% CO₂, the active power available from a plasma system would be 10 MW as compared to 3.5 MW in pure argon. The oxygen transfer rates proved to be relatively small due to the low gas flow rates of such systems and to the low mass transfer efficiency. The addition of 40% CO₂ would raise the power to heat 110 t of steel at a heating rate of 3 K/min, while the oxygen level would increase within 30 min by less than 10 ppm. The specific CO₂ demand would in that case be 0.12 m³(STP)/t.     

Mathematical analysis of reactions in metal oxide/carbon mixtures

A mathematical study has been made of the rates of reactions in metal oxide/carbon mixtures. The rate equations derived take into account the rate control by carbon-CO₂ oxidation and by diffusive and viscous flow of the reaction products, CO₂ and CO, through the packed bed where a pressure gradient exists. As the thickness of the powder mixture increases, the pressure buildup during reduction increases. If the furnace atmosphere is a neutral gas, the rate of reduction of metal oxides by carbon decreases, because of the dilution of CO₂ in the interparticle pores of the bed by back diffusion of the furnace atmosphere.     

Softening and Melting Behavior of Mixed Burden for Oxygen Blast Furnace

 The behaviors of mixed burden in the cohesive zone of oxygen blast furnace were studied by softening and melting tests, and the influence of reducing gas and burden basicity on the softening and melting behaviors of mixed burden was also investigated. The results indicated that the softening range became wide, however, the melting range narrowed sharply in the atmosphere of oxygen blast furnace. The permeability of burden in the oxygen blast furnace was obviously improved comparing with the conventional blast furnace. In addition, the content of sulphur in the dripping iron of oxygen blast furnace was much lower than that of conventional blast furnace, however, the content of carbon increased. An optimum basicity of burden, which could lead to the appearance of the narrower melting range and better permeability of burden, was obtained in the atmosphere of oxygen blast furnace.     

Effect of Slag on Inclusions During Electroslag Remelting Process of Die Steel

Many factors influence the non-metallic inclusions in electroslag steel including furnace atmosphere and inclusions’ content in the consumable electrode, slag amount and its composition, power input, melting rate, filling ratio, and so on. Fluoride containing slag, which influences the non-metallic inclusions to a great extent, has been widely used for the electroslag remelting process. The current paper focuses on the effect of fluoride containing slag on the inclusions in electroslag ingots based on the interaction of the slag-metal interface and electroslag remelting process. In this work, die steel of CR-5A and several slags have been employed for investigating the effect of slag on inclusions in an electrical resistance furnace under argon atmosphere in order to eliminate the effect of ambient oxygen. Specimens were taken at different times for analyzing the content, dimensions, and type of non-metallic inclusions. Results of quantitative metallographic analysis indicate that a multi-component slag has better capacity for controlling the amount of inclusions; especially protective gas atmosphere has also been adopted. The findings of inclusions in electroslag steel by SEM–EDS analysis reveal that most non-metallic inclusions in electroslag steel are MgO-Al₂O₃ inclusions for multi-component slags, but it is Al₂O₃ inclusions when remelting using conventional 70 wt pct CaF₂-30 wt pct Al₂O₃ slag. The maximal inclusions’ size using multi-component slags is less than that using conventional binary slag. Small filling ratio as well as protective gas atmosphere is favorable for controlling the non-metallic inclusions in electroslag steel. All the results obtained will be compared to the original state inclusions in steel, which contribute to choice of slag for electroslag remelting.     

Kinetics of the reaction of SiO(g) with carbon saturated iron

The reaction of SiO(g) with carbon saturated iron plays an important role in silicon transfer in the iron blast furnace. In the tuyere zone SiO(g) is generated from the reaction of coke with its ash, and the SiO(g) reacts with carbon saturated iron droplets as they pass through the furnace. This reaction may also play a role as a gaseous intermediate reaction for the reduction of SiO₂ from slags by carbon dissolved in iron. The rate and controlling mechanism for the SiO reaction with carbon dissolved in liquid iron was determined in the temperature range 1823 to 1923 K. A constant pressure of SiO(g) was generated by the reaction of CO with SiO₂; the reaction of carbon with SiO₂ gave higher but decreasing pressures of SiO with time. The SiO(g) generated was then reacted with carbon saturated iron under conditions for which the gas phase mass transfer conditions are clearly defined. By a systematic variation of the appropriate variables it was demonstrated that the rate was controlled by gas phase transfer of SiO to the gas-metal interface. The rate changed with gas diffusional distance but was not a strong function of temperature or of melt composition. The rate calculated for gas phase mass transfer was in good agreement with the experimental results. An erratum to this article is available at .     

Theoretical interpretation of the decarburization mechanism in convective oxygen steelmaking

The theory of gas absorption is introduced in order to understand the decarburization mechanism theoritically. When the existing decarburization kinetic data are applied to instantaneous reaction regimes of the theory, it can be clearly explained. The decarburization mechanism in convective oxygen steelmaking can be described sufficiently by the overall reaction l/2O₂ +C → CO whether decarburization follows a direct or indirect path. The reason for this phenomenon is that the feature of instantaneous reaction remains unchanged by the intensive convection in the liquid phase. The decarburization mechanism depends only on the two stages of carbon concentration. The criterion for transition from a high-carbon to a low-carbon regime is estimated to be C < (k_a/k_L) ·P _{O 2}. In the high-carbon regime, which shows zeroth-order kinetics with respect to carbon concentration, oxygen absorption into a liquid phase is always considered to be the controlling step, and the controlling parameters area, k_G, andP _{O 2}, among which the most important parameter is the gas/liquid interfacial area,a, in the case of high-speed injection of pure oxygen. In the low-carbon regime, the controlling step is the transport of carbon in molten steel, and first-order kinetics in terms of (k_L·a) · C can be approximated. It indicates that if the outer controlling parameter (k_L · a) is conserved, the intrinsic change in carbon concentration is more important than the kinetics itself. Formerly Graduate Student at Seoul National University. This paper is a part of the dissertation of Dr. Zong in partial fulfillment of his Ph.D. degree.     

Metallurgical results from a 30-t AC plasma ladle furnace

In a 30-t ladle furnace equipped with three A.C. transferred arc-plasma torches for a power input of 6 MW, the influence of argon plasma heating on the chemical composition of a number of steel grades from carbon to chromium-nickel steels was investigated. The concentrations of carbon, manganese, chromium and nickel remained virtually constant even for extra-long heating periods of 2 hours. Silicon melting loss was appr. 0.03%/h, comparable to conventional ladle furnaces. Nitrogen pickup proved to be negligibly small, although the N₂ partial pressure in the furnace was about 0.5 bar. Since the slag is well heated up by the plasma flames, a metal desulphurization is noticed, the extent of which depends on the basicity of the slag. At the slag/gas interface, equilibrium is established with the slightly oxidizing furnace atmosphere, leading to sulphur transfer from slag to gas via SO₂.     

Application of an Off‐Gas Analysing System to Control Oxidation during Stainless Steelmaking in an EAF

During stainless steelmaking in the electric arc furnace at Deutsche Edelstahlwerke GmbH, oxygen is injected to oxidize unwanted tramp elements mainly carbon and silicon. Unfortunately the injected oxygen also oxidizes valuable elements such as chromium and iron, which causes an economical loss and has a negative environmental effect. Since the temperature and dilution techniques to minimise chromium oxidation are seldom applied in the electric arc furnace, a new strategy to minimise chromium oxidation has to be developed. This paper proposes a new strategy which involves the use of a continuous off‐gas analysing system to minimise chromium oxidation by monitoring the oxidation products in the off‐gas, i.e. CO and CO₂. During stainless steelmaking in the electric arc furnace, for which initial carbon and silicon input cannot be precisely known due to imprecise scrap analysis, the installed off‐gas analysing system should provide precise information concerning an efficient oxygen injection. This would then directly prevents excess chromium and iron oxidation. A continuous off‐gas analysing system installed at Deutsche Edelstahlwerke GmbH delivers a promising result for future applications. During the plant trial, the efficiency of oxygen injection as well as the chromium and iron yields were increased.     

Power increase and metallurgical effects during arc heating of liquid steel due to the addition of molecular gases

Molecular gases were laterally injected into the gas atmosphere of a pilot furnace containing 200 kg of steel melt and heated by two AC argon plasma arcs. The power increase in the arcs obtained by the injection was 12 % for 20 % N₂, 70 % for 20 % CO₂, 32 % for 3 % CH₄, and 62 % for 3 % C₃H₈. Mixtures of CO₂ and hydrocarbons were also tested. CO₂ acted as an oxidizing agent for steel components with oxygen transfer efficiencies of 28 % in the case of slag-free steel melts and of 10 % in the presence of slag, while the addition of CH₄ did not cause any noticeable carbon transfer to the melt.     

Analytical models for the gas carburizing process

Mathematical models for carburizing in batch and continuous furnaces are described. Both the steady-state model for continuous furnaces and the time-dependent model for batch furnaces are based on material balances, with thermodynamic equilibrium of the constituents of the furnace atmosphere assumed. The instantaneous rate of carburizing is taken to be proportional to the difference between the carbon potential of the furnace atmosphere and the surface carbon content of the work load. Computer programs incorporating these models were written which predict furnace operating characteristics for any assumed process. The continuous furnace model predicts the pattern of internal gas flow within the furnace and computes the natural gas (or air) additions to each zone needed to achieve the desired carbon potentials and satisfy the carbon demand. The batch furnace model describes how the furnace atmosphere changes in composition during carburizing as a result of the interaction of the instantaneous carbon demand and the rate of supply of carburizing gases to the furnace. Examples of the use of these programs are given, and the limitations of the predictions are discussed.     

Study of Pellets and Lumps as Raw Materials in Silicon Production from Quartz and Silicon Carbide

The use of high-purity carbon and quartz raw materials reduces the need for comprehensive refining steps after the silicon has been produced carbothermically in the electric reduction furnace. The current work aims at comparing the reaction mechanisms and kinetics occurring in the inner part of the reduction furnace when pellets or lumpy charge is used, as well as the effect of the raw material mix. Laboratory-scale carbothermic reduction experiments have been carried out in an induction furnace. High-purity silicon carbide and two different high-purity hydrothermal quartzes were charged as raw materials at different molar ratios. The charge was in the form of lumps (size, 2–5 mm) or as powder (size, 10–20 μm), mixed and agglomerated as pellets (size, 1–3 mm) and reacted at 2273 K (2000 °C). The thermal properties of the quartzes were measured also by heating a small piece of quartz in CO atmosphere. The investigated quartzes have different reactivity in reducing atmosphere. The carbothermal reduction experiments show differences in the reacted charge between pellets and lumps as charge material. Solid–gas reactions take place from the inside of the pellets porosity, whereas reactions in lumps occur topochemically. Silicon in pellets is produced mainly in the rim zone. Larger volumes of silicon have been found when using lumpy charge. More SiO is produced when using pellets than for lumpy SiO₂ for the same molar ratio and heating conditions. The two SiC polytypes used in the carbothermal reduction experiments as carbon reductants presented different reactivity.     

Pore nucleation in solidifying high-purity copper

High-purity copper (6 or 7 N) was melted and solidified unidirectionally in the atmosphere of a H₂-Ar gas mixture for the purpose of studying the mechanism of pore nucleation in solidifying metal. Hydrogen content in the melt was controlled by changing the partial pressure in the atmosphere. Pores were formed when the hydrogen partial pressure in the atmosphere was 0.3 atm or more. Oxides of aluminum and silicon were observed at the bottom of the pores and the pores were nucleated heterogeneously. Water vapor with a very low partial pressure existed in the furnace atmosphere, and the melt must have contained a small amount of oxygen in equilibrium with this water vapor. The solid/liquid (S/L) interface was planar and convection was eliminated. The redistribution of the solute during solidification can, therefore, be estimated. The concentration of oxygen in the liquid at the S/L interface is estimated to be much larger than its initial concentration, due to the very small equilibrium distribution coefficient of oxygen in copper, and aluminum and silicon were oxidized even though their concentrations were very low. The probability of homogeneous nucleation by α particles was very small in these experiments.     

Theoretical Approach to Change Blast Furnace Regime With Natural Gas Injection

The operation of blast furnace using natural gas and oxygen enriched blast (composite blast technology) is considered in many countries to be standard operation for a modern blast furnace particularly in certain countries with cheap and stable supply of natural gas. The theoretical flame temperature (TFT) of combustion and the degree of direct reduction of iron oxides (r_d) arc considered as the main controlling parameters of composite blast technology. The calculated values of these parameters are mainly dependent on the amount of air blast consumption. This amount of air blast is measured before entering into blast stoves. Actually, some of air blast is lost through valves of air stoves. Consequently, the real volume of air blast in the furnace is less than the recorded value by amounts of 5% ? 15% which is not considered in the estimation of r_d and TFT. The purpose is to analyze the different methods for estimation of air blast inside the blast furnaces and develop a theoretical model to calculate air blast consumption with high accuracy. Based on the calculation of air blast consumption, a complete roadmap is demonstrated to change the operation regime parameters of blast furnaces working on composite blast technology.     

Energy aspects of a lead blast furnace

The energy effects accompanying the processing of the feed material to a lead blast furnace are considered in terms of a reversible model. Relative to this model the efficiencies of operating furnaces are found to be in the range 18 to 35 pct. The effects of the effluent gas CO₂/CO ratio and temperature and oxygen enrichment of the blast air in the thermodynamic efficiency are quantified. Improvements in efficiency achieved in industrial furnaces as a result of oxygen enrichment of the blast air are substantially greater than those predicted. Mass and enthalpy balances on an industrial lead blast furnace are presented from which it is estimated that approximately 9 pct of the carbon charged to the furnace is lost due to the solution loss reaction in the upper regions of the furnace. David R.Morris, currently on sabbatical leave, University of Cambridge, Cambridge, United Kingdom.     

Mathematical Modeling of the Energy Consumption and Carbon Emission for the Oxygen Blast Furnace with Top Gas Recycling

The ironmaking process is the most significant source of CO₂ emission in the iron and steel industry, which generates large quantities of greenhouse gases. Recently, oxygen blast and top gas recycling have been applied to the blast furnace to improve the energy efficiency and reduce the pollution from the ironmaking process. However, as a new ironmaking technology, the oxygen blast furnace with top gas recycling (TGR‐OBF) is still under development. This paper focuses on the investigation of the energy consumption and carbon emission for the TGR‐OBF process by modeling the stack, the bosh, the combustion zone, and the gas recycling system. Effects of the key parameters in the TGR‐OBF process on the carbon consumption of reactions and the energy consumption of the system are investigated by orthogonal experiments. Our results indicate that the TGR‐OBF process has the advantages of reducing energy consumption and CO₂ emission. The low temperature and high reducing environment in the new furnace is favorable to lower the coke gasification and increase the reaction rate of iron oxide. The recycling of the top gas can significantly reduce CO₂ emission, and the main advantage comes when the stripped CO₂ is stored.     

Thermodynamics of nitrogen in CaO-SiO2-AI2O3 slags and its reaction with Fe-Csat. melts

The solubility of nitrogen as the nitride ion in CaO-SiO₂-Al₂O₃ slags in equilibrium with N₂-CO gas mixtures and carbon was measured at 1823 K. The nitride capacity (C _N3-) was calculated to compare the nitrogen contents measured under different nitrogen and oxygen potentials.C _N3- decreased with increasing basicity and by replacing SiO₂ with A1₂O₃. The nitrogen partition ratio between carbon saturated iron and the slag was measured in CO gas at one atmosphere at 1823 K. By comparing the partition ratios with the corresponding nitride capacities measured by the gas-slag experiments, it was concluded that the oxygen partial pressure at the slag-metal interface was controlled by the Fe-FeO reaction. A new definition of nitride capacity was proposed based on the reaction between nitrogen and the network former,i.e., SiO₂ or A1₂O₃. This capacity could consistently explain the experimental results. Empirical equations were derived to estimate the activity coefficients of silicon and aluminum nitrides in the slags. On leave of absence from the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan.     

Desulfurization behavior of molten copper alloy by a soda ash

As a basic study on desulfurizing copper alloy scraps in the remelting process, desulfurization experiments were carried out with a molten Cu-8 pct Sn-0.1 pct P by using a Na₂CO₃ flux in a 2-kg high-frequency and a 5000-kg low-frequency melting furnace to investigate the influences of the melting atmosphere, the melting temperature, and the flux composition on the desulfurization behavior of the flux. Desulfurization and dephosphorization began simultaneously. Desulfurization ceased earlier than dephosphorization, and then the reversion of sulfur was found to proceed. Covering the melt with charcoal or a carbon crucible enhanced the desulfurizing ability of Na₂CO₃. The maximum desulfurizing ability was observed at 1473 K. Sulfur distribution between the melt and the flux increased with increasing the basicity of the flux and with decreasing Cu₂O content in the flux. The desulfurization rate was also evaluated based on a two-film model.