Slag Bath Research Articles

Smelting reduction reactions by solid carbon using induction furnace: foaming behaviour and kinetics of FeO reduction in CaO–SiO2–FeO slag

Smelting reduction process technology is progressing rapidly, and research to understand the reduction of FeO in molten slag and the associated foaming behaviour has gained importance. The present paper reports experimental data on the reduction of FeO in molten slag generated in a 30 kW capacity induction furnace. The influence of FeO content in the slag and temperature on the foaming and kinetics is discussed. The foaming index, a parameter describing the travel time of gas in the reactor, is shown to decrease with an increase in the superficial gas velocity. The quantitative dependence of the foaming index on slag properties viscosity, surface tension and density has been studied. The data have also been analysed to give an estimation of the activation energy for the reduction reaction. The reduction reaction, initiated by direct slag–graphite contact, produces CO gas, which spreads into the molten slag bath causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction is found to depend directly on the initial FeO content. An increase in temperature increases the rate of reduction, which has an activation energy of 118 kJ mol−1 of FeO. The results indicate that transport of FeO in the liquid phase is the rate controlling step. The major findings are in agreement with those reported by earlier investigators.

Influence of Slag Composition on Slag–Iron Interfacial Tension

Interfacial tensions between liquid iron and slag were examined for CaO–SiO2 binary and CaO–SiO2–Al2O3 ternary slag systems. The influences of FeO, MgO, B2O3, TiO2, BaO and ZrO2 in slag on the interfacial tension were also investigated by varying these oxides from 1–30 wt% in the CaO–SiO2 binary or CaO–SiO2–Al2O3 ternary slag systems.The interfacial tensions were determined by the sessile drop technique under argon atmosphere at 1 853 K. An X-Ray radiographic technique was employed to image the profile of molten iron drop in a slag bath and the interfacial tensions between slag and iron were obtained from the drop shape analysis.In examining the effect of slag composition on the interfacial tension, the oxygen in liquid iron and its impact on the interfacial tension was taken into consideration. The interfacial tension was found slightly increased with increasing CaO/SiO2 ratio in CaO–SiO2 slag, and with increasing MgO content in the slag. A decrease of interfacial tension was found with increasing FeO in slag. Only minor changes in the interfacial tension were observed with varying the slag composition in the CaO–SiO2–Al2O3 ternary slag system and the influences of B2O3, TiO2, BaO, and ZrO2 in slag on the interfacial tension were found insignificant.

Twin-electrode DC smelting furnaces—Theory and photographic testwork

Use of twin-electrode DC arc furnaces for the purposes of smelting is a growing trend in the research being performed at Mintek. Even though twin-electrode furnaces add complexity to aspects of the mechanical design of the furnace roof and electrode mechanisms (as well as power supply configurations and control systems), the design and operational advantages gained in moving to dual electrodes are significant. Some small-scale testwork on a twin-electrode furnace was performed at Mintek in late 2003, with the aim of enabling direct observation of the plasma arcs and their interactions both with each other and the molten slag bath beneath them. Still and video photography of the twin arcs was performed. The results are presented here, along with some theoretical exploration. The arc trajectory is shown to follow a circular path, with the radius of curvature being directly proportional to the electrode separation, and approximately independent of electrical variables such as current. The influence of the arc deflection on the true arc length, and the dependence of arc voltage on electrode height have been calculated.

Modelling of an ilmenite-smelting DC arc furnace process

Ilmenite smelters must be operated with the slag bath contained in a freeze lining of solidified slag to prevent refractory damage and ensure feasible furnace life. This makes the process an interesting and challenging one to operate and control. The study focused on interactions between the freeze lining and slag bath. A mathematical model was developed to describe heat transfer, solidification and melting in the freeze lining and furnace wall. This model was incorporated into another model to yield a dynamic simulation-type model of the entire smelting furnace process. Simulation experiments with the models aimed to study the dynamic response of the freeze lining to changes in power input, slag composition, and operating parameters. Due to confidentiality considerations, it was not possible to incorporate industrial data for comparison into the study. Regardless of this, the results still remain useful to practitioners when considering the process aspects that were investigated.

Feasibility of manufacturing phosphoric acid by smelting reduction technology

In this paper, a new process for directly making phosphoric acid from phosphorus ore by smelting reduction with postcombustion is presented based on the experience in smelting reduction for iron-making. Theoretically, the new process can save energy when compared with the traditional pyro-process for the manufacture of phosphoric acid. Laboratory simulation shows that it is possible to make phosphoric acid by smelting reduction with postcombustion process. Fundamental research on phosphate reduction in molten slag is carried out. Experimental results show that the reduction rate increases with the increase of temperature, basicity of slag, and stirring intensity of the molten slag. Carbon dissolved in the liquid Fe–P–C alloy has a greater reduction potential than solid carbon. Under the condition of slag basicity (CaO/SiO2) equal to 1˙1, and the concentration of Al2O3 at 8˙3%, the reduction rate constant by solid carbon can be expressed as k = 1˙1 × 103 exp(–217 360/RT). The rate constant for reduction by carbon in liquid Fe–P–C alloy is about 10 times larger than that for solid carbon under similar conditions. The reduction by bubbling CO gas is close to the thermodynamic equilibrium assuming that bubbles escape from the slag bath.

Heat transfer and banks formation in a slag bath with embedded heat sources

A study was conducted for the heat transfer and the formation of solid banks inside a bath of molten slag. This study is motivated by the need to predict the formation of a protective thermal barrier for the refractory brick walls inside smelting furnaces. A mathematical model for natural convection dominated solid liquid phase change with embedded heat sources is presented. A scale analysis is conducted yielding algebraic expressions for the steady-state minimal side bank thickness and location, molten volume fraction and average Nusselt number at the surface of the bath in terms of the Rayleigh number. The predictions of the scale analysis are validated with numerical results and their range of application are delineated.

TECHNOLOGY FOR ELECTROSLAG REMELTING WITH ROTATION OF THE CONSUMABLE ELECTRODE

It is known that metal which is produced by the method of electroslag remelting (ESR) has service properties superior to those of metal made by open refining and casting. This can be attributed to the better conditions which exist for refi ning in the specially chosen slag and for the crystallization of the metal in the case of ESR. However, the need for a two-stage metallurgical conversion (essentially requiring repeat refining) makes ESR metal more expensive than metal made by electric-arc refining. By resorting to out-of-furnace treatments, the content of harmful impurities in the latter product can be reduced to nearly the same level as in ESR metal. In addition, the consumption of electric power in the production of ESR ingots is more than three times greater than the level characteristic of steelmaking in electric furnaces. It is this circumst ance that is presently impeding the wider use of ESR [1]. The costs of producing ESR ingots could be reduced significantly by using an electroslag remelting technology that involves rotation of the consumable electrode around its axis. The General Metallurgy Department of South-Ural State University has already had many years of experience in developing and studying an ESR process that takes place in a centrifugal force field [2‐8]. Such a technology has several important advantages. Most significantly, it makes it possible to appreciably increase the productivity of the process (more than 25% in some cases) without increasing the amount of power that must be supplied to the slag bath. This is accomplished through a change in the hydrodynamic situation in the bath, a reduction in the height of the bath, and the forced removal of liquid metal from the electrode. The hydrodynamic pattern which exists during the melting of a rotating electrode is different from the pattern characteristic of an electrode that moves only vertically. An ascending slag flow is created in the zone near the consumable electrode as it rotates. While rising, the slag is heated and reaches its highest temperature in the immediate vicinity of the elec trode end that is being melted (Fig. 1 b). In the standard technology, slag washes over the melting conical portion of the electrode after it gives up heat to the water-cooled wall of the mold. Moving through the near-electrode region, the slag reaches its highest temperature at the boundary with the liquid metal bath (Fig. 1 a). The removal of liquid metal from the electrode as the latter rotates occurs mainly under the influence of centrifugal forces rather than gravitational forces, which is typical of the given (standard) remelting technology. When the electrode is melted in a centrifugal force field, the liquid phase (drops) are more shielded from the melting surface than when the electrode is not rotated. The melting end of the consumable electrode is flat when it is rotated. The slag level in the mold is thus lower than in the standard technology. In the latter, the slag bath is higher because slag is displaced by the conical end immersed in it. The decrease in the area of contact between the slag and the water-cooled wall of the mold also reduces heat losses by 9‐13%, the exact reduction depending on the parameters of the remelting process. The presence of a flat end makes it possible to ensure that the required distance is maintained between the electrode and the liquid bath with a 10‐15% decrease in the amount of flux which is used. In addition, rotation of the consumable electrode significantly improves the refining ability of the remelting process. This is due to the fact that the thickness of the layer of liquid metal (film) on the end of the electrode is uniform and mini

Processing industrial wastes with the liquid-phase reduction romelt process

The Romelt technology for liquid-phase reduction has been developed for processing metallurgical wastes containing nonferrousmetal components. Thermodynamic calculations were made to investigate the behavior of silver, copper, zinc, manganese, vanadium, chrome, and silicon when reduced from the slag melt into the metallic solution containing iron. The process can be applied to all types of iron-bearing wastes, including electric arc furnace dust. The distribution of elements between the phases can be controlled by adjusting the slag bath temperature. Experiments at a pilot Romelt plant proved the possibility of recovering the metallurgical wastes and obtaining iron.

The effect of injection parameters on slag fuming

Many slag treatment processes use injection of solid reductants into slag baths to recover metal values. In the present laboratory study, the effect of selected injection parameters such as lance diameter, gas flowrate. coal particle size and slag bath temperature have been investigated, and the behaviour of different types of reductant in slag fuming has also been considered. Coal laden gas discharge through a lance into a slag bath has been monitored by a pressure transducer. The pressure traces have revealed that the slag bath viscosity changes as fuming progresses. High momentum coal injection, a thin wall lance, intense bath agitation and increasing temperature all improve the fuming rates. Natural gas was found to be a better reductant than coal and brown coal char.

Thermodynamic simulation model of the isasmelt process for copper matte

A computer model has been constructed to simulate thermodynamically the behavior of the minor elements Zn, Pb, As, Sb, and Bi as well as the major elements Cu, Fe, Si, O, and S in the Isasmelt process, producing copper matte. The model is based on the new concept that there are two independent reaction sites in a slag bath: one for fast oxidation and the other for slow reduction. The oxidizing reaction at the first site produces matte, magnetite-rich slag and gas from chalcopyritic concentrate and siliceous flux. The slag is then partially reduced with lump coal at a site removed from the first site. The oxidizing and reducing reactions are assumed to proceed under a separate set of equilibrium conditions. The process heat balance and thermodynamic distribution of the minor elements are united and expressed as functions of varying weights and compositions of concentrate, flux (silica, limestone), coal, oil, and oxygen-enriched air. The process chemistry was analyzed in terms of Fe3O4, FeO, and FeS activities, as well as SO2 partial pressure. The thermodynamic model explains well the minor element distributions observed in the 15 tons per hour pilot furnace, and it is used to project the optimal smelting conditions for the full-scale 100 tons per hour Isasmelt furnace.

新製鉄法への多様な試み

In the first part of the paper, various attempts on the development of new ironmaking processes in Japan are briefly reviewed.The decade of 80's is assumed to be the third period of the intensive R & D on new ironmaking and is featured by an anticipation of an appreciable structure change in crude iron supplying system. The blast furnace system itself is changing by introduction of extensive use of coal injection as a possible alternative of cokemaking on which blast furnace process is inevitably based.The differentiation of the functions of blast furnace and their proper recombination would be the second route of new ironmaking processes, which are called smelting reduction processes (SRP)In the second part, smelting reduction processes under development in the world are briefly reviewed.SC, Corex and XR are classified into the shaft type processes and Corex is the only SRP which has been developed to the commercial level. So called in-bath type SRP should be classified in two groups, iron bath and slag bath type.HIsmlt process has characteristics of iron bath type and DIOS and LPR have features of typical slag bath type process.

Mathematical Modelling of Heat Transfer During the Melting of Solid Particles in a Liquid Slag or Metal Bath

Mathematical Modelling of Heat Transfer During the Melting of Solid Particles in a Liquid Slag or Metal Bath

Improving solidification pattern of ESR ingots combined with energy savings

Most of the metallurgical effects resulting from electroslag remelting of metal may be divided into two groups, namely, the effects due to the slag/metal reactions taking place and the effects due to the special solidification conditions characteristic of this process. Solidification of ESR ingots takes place progressively as heat is removed from the liquid metal pool via the mold walls. By careful matching of the melting rate with the freezing rate, the desired shallow metal pool is attained, leading to the well known directional solidification pattern with consequent improvement in properties of the steel. The choice of power parameters is limited by a compromise between the need for a high melting rate for economic reasons (costs) which may tend to give a rather deep metal pool and the need for a shallow metal pool to obtain optimum metallurgical properties. In this process only a relatively small amount of the total energy input is actually utilized to melt down the metal. The major part of the energy is lost from the slag and metal pool to the water cooled mold.In this paper the results of numerical and experimental investigations are presented, setting out a simple method of saving energy and controlling the solidification pattern of the ingot. This method involves the addition of solid particles to the melt to utilize the surplus energy evolved in the central area of the slag bath.

A finite element iterative simulation of coupled electrothermal effects in an electric smelting furnace

The problem of predicting the electrothermal conditions within the slag bath of an electric smelting furnace is considered. A method of decoupling the electrothermal and fluid flow aspects of the problem is described. The steady state temperature and power distributions in the conducting slag bath are efficiently calculated using a Finite Element-simulation. A unique feature of the simulation is the incorporation of an impedance regulator that adjusts the immersion of the electrode as the electrothermal iteration proceeds. It is shown that the simulation predicts the terminal characteristics of the furnace with very good accuracy.

Kinetics of the zinc slag-Fuming Process: part II. mathematical model

A mathematical model of zinc slag fuming has been formulated based on the kinetic conception of the process developed in Part I of this paper. Each of the major reaction zones in the furnace — the slag bath where reduction of zinc oxide and ferric oxide takes place and the tuyere gas column where oxidation of coal and ferrous oxide occurs — have been characterized mathematically. The two zones and the water-jacketed furnace wall have been linked by overall heat and mass balances. Insufficient information is available, however, to characterize quantitatively two of the important kinetic processes occurring in the furnace: the division of coal between entrainment in the slag, combustion in the tuyere gas column and bypass; and oxygen utilization. To overcome this problem the model has been fitted to the data from eleven industrial fuming cycles. Consistent values have been obtained for these kinetic parameters over five different fuming operations indicating that the kinetic conception of the process is sound. The results indicate that about 33 pct of the injected coal is entrained in the slag, 55 pet combusts in the tuyere gas column, and 12 pct bypasses the bath completely. Oxygen utilization has been found to be high and can be correlated to bath depth.

A Mathematical Model Describing Forces and Circulation in an Electric Smelting Furnace

Conditions in the slag bath of an electric smelting furnace are examined as are several existing models for such furnaces. A digital model that provides an improved simulation of the slag bath is described. The model is used to determine the electrical resistance between electrodes and can be used to examine the nature of the electromagnetic forces that exist near the electrode tips. The mechanism by which these forces produce bath circulation is described.

The boronizing of parts from iron-base powders in borate slags

1. It has been established that, under both laboratory and industrial conditions, parts from iron-base powders can be successfully sintered in a protective medium consisting of molten AN-ShT1 and AN-ShT2 synthetic slags, whose high fluidity enables treated parts to be quenched direct from a slag bath. 2. The feasibility has been demonstrated in principle of combining the operations of sintering and electrolytic boronizing of parts from iron-base powders in molten synthetic borate slag electrolytes.

Properties of molten synthetic slag used for heating steel

1. The carryout of slag is larger than the carryout of salts, but can be reduced substantially by shaking the parts above the bath. 2. The distortion resulting from quenching is almost the same after heating in a slag bath or salt bath. 3. No attack on the surface of steel parts at operating temperatures was noted for slag baths, while a notable attack occurred in salt baths.

Crack formation during quenching with heating in salt and slag baths

Crack formation during quenching with heating in salt and slag baths

Heat treatment in slag baths

1. New slag baths have been developed with a low viscosity, good wetting capacity, easily removed from the surfaces of parts, and suitable for use as heat transfer agents at temperatures of 780–1150°C. 2. Heating in a slag bath makes it possible to eliminate decarburizing, obtain a bright clean surface after quenching, and reduce the difference in the hardness of the quenched part. Also eliminated are operations such as sandblasting or hydrobrasive cleaning; final machining is also reduced considerably.