THE ROLE OF BOUDOUARD S REACTION IN THE PROCESS OF FEO. Jan Mróz

Transcription

1 THE ROLE OF BOUDOUARD S REACTION IN THE PROCESS OF FEO REDUCTION FROM LIQUID SLAGS OF CAO-FEO-SIO 2 AND CAO-FEO TYPES Jan Mróz Czestochowa University of Technology, Al. Armii Krajowej Czestochowa, Poland, janmroz@mim.pcz.czest.pl Abstract The reduction of iron oxides from liquid slag is the fundamental stage of many newly developed, future-oriented technologies of obtaining liquid iron (pig iron), which are referred to as reduction iron smelting processes, whose main distinguishing feature is the use of noncoking coals. The reaction mechanism of FeO reduction from liquid slags is very complicated and the meaning of Boudouard reaction as limiting factor of the reduction rate, is not sufficiently clear. It was established that the limiting role of Boudouard reaction depends on the overall rate of FeO reduction. In the case of basicities of slag in the range of Bouduard reaction does not play any limiting factor, except conditions when intense agitation of melt is used by means of rotating graphite disc [1]. There exists a definite, terminal reduction rate, exceeding of that causes that Boudouard reaction becomes a limiting factor for overall reduction process. This terminal reduction rate at temperature 1350 o C is approx mol FeO cm -2 s -1, and at temperature 1420 o C it is approx mol FeO cm -2 s -1. These values refers to the graphite reducer. Moreover, it was established, that at higher slag basicities (1.8 and higher), reduction rate increases to the extent when Boudouard reaction is not sufficiently effective and becomes a limiting factor of reduction reaction [2]. In the paper investigations on the reduction of FeO from liquid slags of type CaO-FeO- SiO 2 and CaO-FeO are described. The basicities (CaO/SiO 2 ) of investigated slags is equal to 2.6 and 3.0. The development of Boudouard s reaction in the processes of FeO reduction from these slags depends on the wettability of the carbon reducer by the slag. With increasing CaO concentration and with related increase in the surface tension of slags, the CO/CO 2 ratio in post-reaction gases increases. I. INTRODUCTION An intensive development of studies on new technologies for the production of liquid iron (pig iron) outside the blast furnace has been observed in the recent decade or so. The main concept underlying those processes, which are referred to as iron reduction smelting processes, is the use of non-coking hard coals as the reducing agent, which, along with lower investment outlays (due to the elimination of the coking plant and, in some instance, also the sintering plant) makes these processes competitive to the blast-furnace process. The major role in these processes is played by Boudouard s reaction of coal gasification (CO 2 +C=2CO) by means of carbon dioxide, which is a source of a gaseous reducing agent, carbon monoxide. At present, the following sequence of partial processes is regarded as the most probable mechanism of reduction of iron oxides in the liquid phase [3]: 1 1

2 a) diffusion of FeO from the bulk of slag to the slag-gas interface, b) chemical reaction of reduction at the slag-gas interface, c) diffusion of CO 2 from the slag-gas interface to the carbon-gas interface, d) chemical reaction at the carbon-gas interface. The issue of to what extent Boudouard s reaction can be a reaction controlling the overall rate of the process of iron oxide reduction in the liquid phase is the subject of numerous studies [3-9]. Study [4] that draws on the results by Gulbrannsen [5] argues that this reaction has a minor effect on the reduction rate. However, in many cases a significant difference in reduction rate is found depending on the type of reducing agent used (e.g. coke, graphite, pitch coke, et.) [6], which indicates that this reaction has in these instances a major effect on the overall reduction rate. This is also confirmed by studies by other authors [3,7]. On the other hand, also quite a number of works [8,9] have different conclusions, stating that the reduction reaction, and more specifically the diffusion of FeO to the slag-graphite (or slaggas) interface, constitutes a stage that limits the overall reduction rate. So differing investigation results allow one to presume that the role of Boudouard s reaction in the overall process of iron oxide reduction from the liquid phase depends on the process conditions which determine the rates of particular partial processes, and in particular two reactions: the reduction reaction and Boudouard s reaction. This study presents some results of investigation using a graphite rotating disk that allows (within the diffusion control or reduction) to accelerate the reduction rate [1], the investigations results of reduction from higher-basicity slags, which is distinguished by high process rates [2] and the results of FeO reduction from slags of high basicities of 2.6 and 3.0 and from slags of type CaO-FeO free from silica. II. EXPERIMENTAL II.1. Investigations of reduction with a rotating disk The reduction experiments were carried out at temperatures of 1350 o C and 1420 o C. The basicity of slags used, as defined by the ratio CaO/SiO 2, was The slags contained 20% and 60% of FeO. The main element of the testing stand is a graphite reducer in the form of a so called rotating disk. The reducer had the form of graphite cylinder 19.5 mm in diameter and 45 mm long. To isolate the side walls of the reducer from the slag, it was covered with an alundum shield in the form of a sleeve. The working surface was the flat face of the cylinder (referred to as a disk). The rotations of the graphite cylinder suspended on a molybdenum rod were counted by a photoelectric sensor mounted above the rotating disk of the driving motor. An FeO slag sample of a total mass of 50 g was placed in an iron crucible and then put into the alundum retort of a Tammann furnace. Argon was passed through the reaction system at a rate of 118 dm 3 /h. Then, the graphite rod was lowered to a depth of 7 mm above the iron crucible bottom. For the initial 120 s, the reduction was conducted with the disk in a stationary state, and then the disc was set in rotary motion at speeds of 100, 400, 800, 1200, 1600 and 2000 rpm in intervals of 120 s. The rate of reduction was determined based on the readings of an automatic oxygen and carbon dioxide analyzer which analyzed the contents of these gases in the carrier gas, i.e. argon. A computer was connected to the measuring system, which recorded the analyzer s readings at intervals of 10 s. The testing results are presented graphically in the reduction rate reduction time (disk rotation) coordinate system in Fig

3 This figure shows the results of reduction from a 1.27-basicity slag at temperatures of 1350?C and 1420?C. As can be seen, no carbon dioxide exist while the disk in a stationary state; it only begins to appear during reduction with the rotating reducer at higher reduction rates, i.e. in this case at approx. 10.0?10-6 mole FeO?cm -2 s -1 (Fig. 1a). In the case of reduction at 1420?C (Fig. 1b), CO 2 appears at a reduction rate of approx. 15.0?10-6 mole FeO?cm -2 s -1. In the variant of reduction from slags containing a lower FeO content, i.e. 20%, the process rate did not exceed 10.0?10-6 mole FeO?cm -2 s -1, both at 1350?C and 1420?C, because of which no carbon dioxide appeared in post-reaction gases. II.2. FeO reduction from high-basicity slags in the stationary state of the reducer Further investigations of FeO reduction from slags with increased CaO/SiO 2 basicity equal to 1.8 and 2.0, respectively, showed that also in the stationary state of the reducer (without rotations), at a sufficiently high reduction rate, carbon dioxide appeared in the reaction system [2]. This encouraged the author to continue the tests on slags of even higher basicity. Within this study, tests were carried out on CaO-FeO-SiO 2 -type slags of basicities equal to 2.6 and 3.0, respectively, and in a slag system free from silica, i.e. in the CaO-FeO system. The CaO-FeO-SiO 2 -type slags contained 60% FeO, whereas the CaO-FeO slag contained 70% FeO. Reduction was conducted at 1420?C in a Tammann furnace. The reduction rate was determined based on the concentration of CO and CO 2 in argon that flowed through the alundum reaction retort at a rate of 141 dm 3 /h. The dimensions of Armco iron crucibles and the dimensions of the alundum retort were identical as in the tests with the rotating disk. Figure 2 shows the results of reduction rate and the amounts of carbon dioxide released during the reaction for all the three slag types. In all the three variants, a very high reduction rate is visible in the initial stage. For the CaO-FeO-SiO 2 -type slags, the maximum reduction rate is approx ?10-6 mole FeO?cm -2 s -1 for the 2.6-basicity slag and 284.0?10-6 mole FeO?cm -2 s -1 for the 3.0-basicity slag. The rate of reduction of the CaO-FeO slag is even higher, amounting to approx ?10-6 mole FeO?cm -2 s

4 In all the three cases, carbon dioxide is seen to occur in the reaction system. This is in agreement with the results of the rotating disk tests, where the critical rate of reduction at 1420?C was found to be approx. 15.0?10-6 mole FeO?cm -2 s -1, above which the rate of Boudouard s reaction is slower that the reduction rate and part of CO 2 is not used in the process of coal gasification. Similarly as in the case of reduction with the rotating disk, the variation of carbon dioxide concentration in reaction gases is a strict reflection of the reduction rate. III. DISCUSSION The occurring close correlation between the behaviour of reduction rate and CO 2 concentration in gases reflects the interrelation of two basic reactions: FeO (l) + CO (g) = Fe (s,l) + CO 2(g) (1) C (s) + CO 2(g) = 2CO (g) (2) At a too high rate of the reduction reaction according to reaction (1), part of the gaseous product of reduction, i.e. carbon dioxide, fails to react with the graphite carbon, according to reaction (2). From this moment on, the overall reduction rate starts to be controlled not only 4 4

5 by the transfer of reagents, but also by Boudouard s reaction of carbon gasification, and the reduction begins to be a process of a mixed mechanism. According to the authors of references [10], Boudouard s reaction is composed of two stages, namely: C + CO 2 = C(O) + CO (3) C(O) = CO (4) where reaction (4) represents the stage of coal gasification, and this stage is the slowest. Stage (3), called the reaction of oxygen exchange, is faster by many times, and it is assumed to reach a state close to equilibrium during the course of the process. This has been determined very precisely by using isotope methods. It should also be assumed that a significant factor in Boudouard s reaction, particularly in the presence of liquid phases, becomes the reducer s surface availability to CO 2 as a reduction product. As the wettability of graphite by slag is different depending on chemical composition of the latter, it can be presumed that a change in slag basicity should have an influence on the progress of Boudouard s reaction. For this purpose, the carbon monoxide-to-carbon dioxide concentration ratio was analyzed during the reaction for all three variants. The obtained results are shown in Fig. 3. It can be seen from this figure that the carbon monoxide-to-carbon dioxide ratio is the smallest for reduction from the 2.6-basicity slag, and the largest in the case, where the slag contains no silica at all. This means that increasing CaO concentration in the slag favours the development of Boudouard s reaction. This relationship may be ascribed to the increased availability of the reducer s surface in the case of slags with higher CaO contents, as it is known that increase in CaO concentration results in an increase in surface tension in the CaO-FeO-SiO 2 slag system [11] and a decrease in graphite wettability. The obtained results confirm that the process of iron oxide reduction from liquid phases is a very complex process. In the development of new reduction smelting technologies using carbon reducers, the understanding of factors influencing the development of Boudouard s reaction is a problem of paramount importance. 5 5

6 IV. SUMMARY 1. A limiting rate of FeO reduction from liquid slags of CaO-FeO-SiO 2 type by means of graphite at a temperature of 1420 o C is a rate of approx mol FeO cm -2 s In stationary reaction conditions (a motionless reducer), the limiting value of mol FeO cm -2 s -1 is exceeded in the case of reduction from slags with a CaO/SiO 2 basicity = 1.8 and higher. In these conditions, Boudouard s reaction becomes a factor limiting the reduction rate. In the case of FeO reduction from slags with basicities below 1.8, Boudouard s reaction is not a reduction rate-limiting factor. 3. The development of Boudouard s reaction in the processes of FeO reduction from liquid slags depends on the wettability of the carbon reducer by the slag. With increasing CaO concentration and with related increase in the surface tension of slags, the CO/CO 2 ratio in post-reaction gases increases. This phenomenon should be ascribed to an increase in the graphite surface available to post-reaction gases as a result of the decreasing wetting abilities of slags with increasing basicity. V. LITERATURE 1. Mróz, J. Reduction of iron oxides from liquid slags using a graphite rotating disc. Steel Research, 2000, nr. 1+2, pp Mróz J. Some phenomena of FeO reduction from slags of higher basicity. In the Proceedings from the Mills Symposium: Metals, Slags, and Glasses: High Temperature Phenomena, London, 2002, pp Story S. R., Sarma B., Fruehan R. J. Cramb A. W., Belton G. R. Communications: Reduction of FeO in smelting slags by solid carbon: Re-examination of the influence of the gas-carbon reaction. Metall. a. Mat. Trans.B, 1998, vol. 29B, August, p Sarma A., Cramb A. W., Fruehan R. J. Reduction of FeO in smelting slags by solid carbon: Experimental results. Metallurgical and Materials Transactions B, 1996, vol 27B, pp E. A. Gulbransen, K. F. Andrew, F. A. Brassart: Reaction of graphite with carbon dioxide at o C under flow conditions. Carbon, 1965, vol.2, pp Sabela W., Stec R., Mróz J., Budzik R. Okreslenie modelu i optymalnych warunków przeplywu gazów w strefie miekniecia i topnienia wsadu w wielkim piecu, In the nonpublished report BZ-10-1/83/P/MR, Politechnika Czestochowska, 1983, Czestochowa. 7. Belton G. R., Fruehan R. In the J. Proc. Ethem. T. Turkdogan Symp., Iron a. Steel Society, Pittsburgh, PA, 1994, pp Davies M. W., Hazeldean G. S. F., Smith P. N. Physical Chemistry of Process Metallurgy, The Richardson Conference, (1973), Publ. Inst. Min. Metall., 1974, p Sato A., Aragane G., Kamihira K., Yoshimatsu S. Reducing rates of molten iron oxide by solid carbon or carbon in molten iron. Trans. ISIJ, 1987, vol. 27, pp Ptak W., Sukiennik M. Phenomenological description of the Boudouard s reaction rate. Archiwum Hutnictwa, 1981, t.26, z.4, pp Mills K.C., Keene B.J. Int.Mat. Rev., 32, (1-2), (1987), 1 according to Slag Atlas 2 nd Edition, Verlag Stahleisen GmbH