KINETICS OF IRON OXIDE DIRECT REDUCTION BY COAL E.R. ABRIL 1

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1 KINETICS OF IRON OXIDE DIRECT REDUCTION BY COAL E.R. ABRIL 1 CIMM- Av.Velez Sarsfield 1561 C.P.5000 Córdoba, Argentina. aabril@intiemor.gov.ar Abstrat - A new interpretation to the kinetis of iron oxide diret redution, based on the influene of probability of ativation of the reation on the flux of transformation, is presented in this paper. This interpretation is applied to justify a new proess of diret redution of iron oxides, by using arbon as the only reatant. Keywords - Diret redution; kinetis of hemial reations; ativated omplex; flux of reation; arbon ativity I. INTRODUCTION In order to analyze the diret redution of iron oxides, a new kineti interpretation, based on onepts similar to those of Eyring ativated omplex theory (Glasston et al., 1941 and Darken and Gurry 1953), is applied. This new interpretation does not relate to a thermodynami equilibrium ondition but to a probabilisti approah. The interpretation was developed by Abril (1978) and improved by Abril (1997). Additional innovations are inluded in this paper, mainly to explain the reason why the empirial "speifi rate of reation" is generally onstant, at a given temperature. II. CHARACTERISTICS OF THE NEW INTERPRETATION A "flux of transformation" will be defined -for a given reation- as the number of moles of reatants whih are transformed into produts, at a unit time. This flux is assumed to depend on an average "probability of ativation" of a reation, related to a onstant frequeny for the interhange of energy. A group of reatants may be defined as the minimum amount of reatants, grouped near enough among themselves, as to be able to be transformed into an ativated omplex, when all the omponents of the group are simultaneously ativated. 1 Investigador Superior Emérito del CONICET. The number of groups of reatants to be onsidered is assumed to be proportional to the number of defiient omponents of reatants, from an estequiometri point of view. The probability of ativation of a reation is onsidered as a ompound probability, whih means that its value is equal to the produt of probabilities orresponding to eah one of the reatants of a group. The flux of transformation, J, is defined by the following formula: J f n W K t, (1) where f is the assumed onstant number of energy hanges of the omponents per seond; n the number of defiient omponents of the reatants; W the probability of ativation of the reation and K t a fator of proportionality between the number of defiient reatants and the total number of reatant groups of the system. With homogeneous reations, to any ativated omplex formed, a group of produts will be produed in the system. When dealing with heterogeneous reation -as the one to be analyzed in this paper- only the ativated omplexes formed in a small volume, faing the interfae an be transformed into produts. The reason for this will be later analyzed. From the value of J given by formula (1), orresponding values of rate of reation, and of speifi rate of reation, may be obtained, as explained by Abril (1997) 2 III. PROBABILITY OF ACTIVATION OF A SINGLE COMPONENT In order to obtain the W value of formula (1) the meaning of the probability of a single omponent will be defined. Based on statistial onepts, the following expression will be aepted to be proportional to the probability of ativation of a omponent i. 2 J and the reation rate are onsidered equivalent but J is not neessarily related to a unit volume of a system, as happens with reation rates 145

2 e e e Ei RT Gi RT Ei RT, (2) where E i and G i relate respetively to internal energies, and Gibbs free energies, of a mole of omponent i, both in the system and when i forms part of an ativated omplex. Aording to the definition of internal energy E, the value E' i beomes: E TS PV, (3) i i i where T is the absolute temperature (assumed onstant in a given reation), P the external pressure (also assumed onstant), S i relates to molar entropies of i, and V i to molar volumes of i. In both formulae (2) and (3), the symbol represents differenes of molar energy -related to omponent i- between those orresponding to the ativated omplexes and those orresponding to omponent i alone in the system. At a given temperature and pressure, the molar values of E i and G i may be onsidered onstant when analyzed in an ativated omplex. Gi e The expression represents a partiular value of the ativity of omponent i, whih is proportional to any other value of the ativity of the same omponent, partiularly to the ativity related to a pure omponent i. This last ativity may be related to the onentration of omponent i in the system. It will be postulated that, in many ases, the value of E' i - given by (3)- is pratially onstant, if the proess is performed at onstant temperature and pressure. This onstany is assumed to be related to the fat that, generally, every hange of the thermoentropi energy of a omponent of a system is pratially ompensated by a orresponding hange of thermodynami work. If the values of TS i and PV i are onsidered onstant in the ativated omplexes -at a given temperature and pressurethe value of E' i may be onsidered onstant at this ondition. The reason why E i ' does not always appear as onstant, may be related to an important hange of the molar entropy of a omponent of the system, whih may not be ompensated by a orresponding hange of the molar volume of the omponent. As it was said before, expression (2) is proportional to orresponding probability of ativation. If the onstant of proportionality is onsidered inluded in the value of f, of formulae (1), this probability may be onsidered defined by: W e e Gi E'/ RT i. (4) From this expression, the orresponding values of W may be obtained by applying the ompound probability onept. IV. CONSTANCY OF THE SPECIFIC RATE OF REACTION From the value of E i, for any reatant, a orresponding value, to be alled E', may be obtained in relation to all omponents of a reation, by applying the onept of ompound probability mentioned above. If the value of E i ' is assumed onstant for any omponent of a group of reatants, E' will also be onstant. The onstany of E' explains why, in many ases, the experimental speifi rate of reation appears to be onstant, for a given temperature and pressure. It is so if we onsider that E' defines the exponential of the speifi rate of a reation. When temperature inreases, the value of TS i PV i also inreases both in the ativated omplexes and in the system. It appears logial to assume that similar inreases will be produed in both ases, at any temperature, and so values of E' i (and orresponding values of E') may be independent of temperature, as found in the experimental values of speifi rates of many reations. V. STOPPAGE OF HETEROGENEOUS REACTIONS IN SOLIDS It will be aepted that any reation between a gaseous reatant and a solid omponent is produed when the gas reatant is previously inorporated to the solid, through its external vaanies. Any obstale whih avoids this inorporation, ats as stoppage of the reation. When a metal is in ontat with a gas ontaining oxygen, an oxidation of the metal will be produed. This may be stopped by the produts of the reation, if they are fixed to the external vaanies of the metal. This is what happens in the oxidation of aluminum at normal temperature, but not in the oxidation of iron. What is assumed to happen is that the aluminum oxide is greatly fixed to the external vaanies of the metal, while this does not happen with the iron oxide. In the redution of iron oxide by hydrogen, the produt of the reation is water vapor. If this water reats with some foreign omponent of the iron ore, the redution may be stopped. This is assumed to happen with some types of iron ore pellets, whih appear diffiult to be redued with atual proesses of diret redution, whih employ hydrogen. In relation to the proess to be analyzed in this paper, the only omponent that may stop the reation is the arbon powder that may appear over the surfae of the iron oxide pellets. This phenomenon is produed when the ativity of arbon, 146

3 E. R. ABRIL related to pure arbon, is equal or greater than one. The reasons for this stoppage are later analyzed. VI. CARBON ACTIVITY The hemial reation between CO and CO 2 in a solid vessel is normally assumed to be given by:. (5) 2CO( in gas) CO2 C ( in gas) ( in solid ) Aording to this, the reation onstant is related to the ativity of arbon in solid. By aepting the ativity equivalene priniple, stated by Abril (1978), the ativity of arbon will be onsidered to be, at equilibrium, equal in solid and in gas, if the same system of referene is used. It will be assumed that atoms of arbon exist in the gas, what defines its ativity. The number of suh atoms may be very small, if the orresponding oeffiient of ativity is quite large. Aording to the above, the preipitation of arbon powder omes from the gas, when the ativity of arbon, related to pure arbon, beomes equal or larger than one. VII. CHARACTERISTICS OF IRON ORE DIRECT REDUCTION BY A CO AND CO 2 ATMOSPHERE If only the redution of FeO iron oxide is analyzed, 1 the following two types of reations may be onsidered to apply, when using a gas formed by CO and CO 2. FeO CO CO2 Fe (6) and FeO C CO Fe. (7) In any ase the reative omponent is onsidered to be previously fixed on the superfiial vaanies of the pellets. It will be assumed that, due to the size of arbon atoms, reation (7) is mainly produed. The two above reations are additive so, if we apply reation (7), reation (6) effet may be onsidered inluded by affeting the value of J, of formula (1), by an additional onstant fator. This is so, as the same parameters will affet the obtained formula, as it will be later seen. The arbon will be the defiient reatant of reation (7) and, due to that, the n value of formula (1), may be 3 As explained in Abril (1978), a similar proedure an be applied to analyze any other type of iron oxides, as they will be previously transformed to FeO -at the surfae of the pellets, in a short time- at the temperatures normally used in the proesses. aepted to depend on the amount of vaanies of the iron oxide, where the arbon atoms may be introdued. Not all suh arbon atoms may be able -after being ativated- to produe produts of the reation, as the orresponding ativated omplexes have to be formed near the interfae between gas and iron oxide, in order for the CO formed to pass from solid to gas. It will be aepted that only the atoms of arbon loated in a small volume, near the interfae, should be onsidered in the value of n. The above mentioned volume will be onsidered defined by the interfae surfae, times a normal distane, whih is assumed to be a onstant, at a given temperature. The relationship between this distane -to be alled Xand the temperature, is assumed to be suh, that the volume X remains onstant when temperature hanges but the interfae surfae is the same. The number of arbon atoms whih defines de value of n of formula (1) is onsidered proportional to the number of vaanies in the volume X. In order to obtain this number, a fator that relates -in a rystalline system- the number of lattie atoms to orresponding vaanies will be established. This fator is, aording to Reed Hill (1966), defined by: Q v e (8) where Q v is a onstant. In relation to our problem, the lattie atoms are the oxygen atoms of the iron oxide, as the iron behaves as an interstitial omponent. The number of oxygen atoms of the volume X is related to orresponding number of vaanies (on the iron oxide phase) by means of fator (8). As not all suh vaanies may be filled with arbon atoms, it will be aepted that a onstant perentage will always be oupied by arbon atoms. From all the above, the number n of formula (1) may be defined by: n e Qv (9) where is a onstant independent of T, whih depends on the onstant perentage of arbon filled vaanies and, also, on the onstant number of oxygen atoms in the volume X, whih, as stated above, is the same for any value of. The value of n given by (9), has to be introdued in formula (1) where only W has to be obtained, as f and K t are onsidered as unknown onstants. Aording to what was said before, the value of W may be found by multiplying the probabilities of ativation of arbon and that of FeO. 147

4 The probability of ativation of arbon may be written, aording to formula (4) as: E W a e (10) if subsript represents arbon, a defines the ativity of arbon related to pure arbon, and is a onstant fator G whih relates a to e. A similar expression may be obtained for the probability of ativation of the FeO. The ativity of FeO, related to pure FeO, is equal to one, if no solute exists in the iron oxide phase. By multiplying the probability of ativation of arbon by that of iron oxide, and inorporating the value of n, given by (9), on formula (1), the following definition of the flux J may be obtained: ( E Qv ) J d S a e, (11) n where d relates to the produt of all fators so far onsidered onstant (or pratially onstant), S n is the nominal interfae surfae (onsidered proportional to ), a is the ativity of arbon, and E' has the meaning assigned before to any reation. It may be proved that if reation (6) is onsidered a similar expression to (11) will be obtained, as a is diretly related to the ativity of reatant CO. Aording to that, only the values of d and E have to be hanged, to apply expression (11), when the two reations are onsidered. A formula similar to (11) was obtained in Abril (1987), for the redution of iron oxide by using hydrogen as reatant. In this ase partial pressure of hydrogen was used instead of the ativity of arbon. In the mentioned referene, experimental results, taken from the literature, were analyzed. It was observed that the value of E'+Q v was onstant, independent of T, and that d hanged slightly with the perentage of redution. It may be assumed that similar situations will appear when formula (11) is applied to the reation being analyzed here. The reason why d hanges with redution perentage was analyzed in the mentioned referene. Similar onsiderations may apply to our ase. VIII. INFLUENCE OF THE ACTIVITY OF CARBON Aording to equation (11) the ativity of arbon affets diretly the redution of iron oxide in the proess being analyzed. The larger this ativity, the faster the redution will be but, if this ativity beomes equal to or larger than one, the presene of arbon powder may stop the reation. The reason why the reation is stopped when a is one, or larger than one, is related to the struture of arbon powder, whih redues drastially the entropy of arbon introdued into the vaanies. Aording to what was said before, this hange of entropy affets the value of E' and so the probability of ativation of the reation. After many trials performed at our laboratories, it was found that the arbon powder may be avoided from the proess if iron pellets were heated, to the reation temperature, in ontat to the required arbon, and if the gases are reirulated by passing them trough a ooling area. These findings are reported mainly in Valdez (1999). IX. INFLUENCE OF CIRCULATION OF GASES If the gases are reirulated, and they pass through ooling areas, the ativity of arbon at the proess temperaturewill be inreased, due to the inrease of the amount of arbon atoms in the atmosphere, originated by the redution of temperature, and affeted by the gas veloity This inrease of ativity may allow to redue the time required for the reation to be ompleted, but, if the gases are in ontat with oal, the proess has to be regulated, in order to allow the reation to be finished, before the appearane of arbon powder. X. CONCLUSIONS Aording to test performed at our laboratories, a method may be established to eonomially redue iron oxide pellets, by using an atmosphere formed by heating oal, in presene of oxygen. It was found that the proess ould be applied to any type of pellets, what does not happen with atual diret redution methods. The reason is assumed to be related to the presene of hydrogen in atual methods, whih aording to what was said before may stop the reation with some types of pellets. As hydrogen is not used in the proposed method, this phenomenon is not produed. REFERENCES Abril,E.R., Termodinámia y Cinétia de las Transformaiones Metalúrgias. Aademia Naional de Cienias de Córdoba, Argentina (1978). Abril, E.R., Una Nueva Interpretaión de la Realidad. Editorial Blakpool. Córdoba, Argentina. (1997). Abril, E.R., Oxigen Transfer During Diret Redution of Iron Oxides. Latin Amerian Journal of Heat Mass Transformation. Buenos Aires, Argentina (1987). 148

5 E. R. ABRIL Darken, L.S., R.W. Gurry, Physial Chemistry of Metals. MGraw.Hill Book Co., New York. (1953). Glasston, S., K.J. Laider, H. Eyring, The Theory of Rate Proesses. MGraw-Hill Book Co., New York (1941). Reed-Hill, R. I., Physial Metallurgy Priniples. D. Van Nostrand Co., In..Prineton, New Jersey.(1964). Valdez, Gabriel, Nuevos avanes en la investigaión del proeso de reduión direta de óxido de hierro mediante arbono. Researh publiation, F.C.E.F. y N. of the Universidad Naional de Córdoba, Argentina (1999). Reeived: April 23, Aepted for publiation: February 4, Reommended by Subjet Editor L. T. Pinto. 149