Some thermodynamic aspects of the reduction of magnetite in the presence of carbon

ABSTRACT

The article presents a study of the thermodynamic parameters of the process of reduction of the mineral magnetite in the presence of carbon and their relationship. Accordingly, as the temperature rises, the likelihood of the magnetite reduction reaction increases. When the temperature reaches 928 K, the reaction system reaches absolute equilibrium, and from 929 K the reduction reaction shifts to the right according to the Le-Chatelier principle. When the temperature reaches 1273 K, the equilibrium constant of the chemical reaction reaches its maximum value.

АННОТАЦИЯ

В статье представлено исследование термодинамических параметров процесса восстановления минерала магнетита в присутствии углерода и их взаимосвязи. Соответственно, с повышением температуры увеличивается вероятность реакции восстановления магнетита. Когда температура достигает 928 К, реакционная система достигает абсолютного равновесия, а от 929 К реакция восстановления смещается вправо по принципу Ле-Шателье. Когда температура достигает 1273 К, константа равновесия химической реакции достигает максимального значения.

Keywords: magnetite, reduction, carbon, carbothermy, wustite, thermodynamics, enthalpy, entropy, Gibbs energy, equilibrium constant, temperature.

Ключевые слова: магнетит, восстановление, углерод, карботермия, вюстит, термодинамика, энтальпия, энтропия, энергия Гиббса, константа равновесия, температура.

When carbon is used as a reducing agent to recover metals from metal oxides, such reactions are called carbothermic reactions in metallurgy. Carbothermic reactions can occur gradually depending on the amount of oxygen in the metal oxide [1]. For example, in the first stage of the recovery of high oxides of iron - hematite with the help of carbon, magnetite is formed, in the second stage - wustite, and finally in the third stage - metallic iron. Carbothermic reactions usually take place at temperatures of several hundred degrees Celsius [2]. These processes are used to separate the elemental (pure) forms of many metals. The carbothermic method cannot be used for oxides of some active metals, such as alkali and alkaline-earth metals [3]. This is because in order for carbothermic reactions to take place, the oxygen content of the oxide-containing metal must be less than that of the reducing carbon. For example, oxides of sodium, potassium, and calcium cannot be reduced with carbon. Because these metals are more susceptible to oxygen than carbon. Therefore, such chemical reactions do not occur in practice. The ability of metals to participate in carbothermic reactions can be understood in more detail through Ellingham diagrams [4].

The second stage in the recovery of iron oxides is the recovery of the mineral Fe₃O₄ (magnetite) [5]. This reduction reaction is of practical importance, as it reduces the amount of magnetite in the converter slag. The magnetite mineral is reduced to the mineral FeO (wustite):

Fe₃O₄ + C = 3FeO + CO↑                                                                   (1)

The temperature-dependent formula for the change in Gibbs energy for chemical reaction 1 is written as follows [6]:

∆G_reac = 211,61 – 0,22801T                                                                 (2)

Table 1 presents the thermodynamic values calculated from formula (2).

Table 1

Results of thermodynamic analysis of carbothermic reduction reaction of magnetite

Of all the carbothermic reactions, the most energy-intensive is the reduction reaction of magnetite [7]. The reason why magnetite recovers more energy than hematite is that magnetite is a combination of two oxides (FeO ∙ Fe₂O₃). Given the formation of wustite during the reduction of magnetite with carbon, it follows that wustite in magnetite does not participate in the carbothermic reaction, only the hematite part is involved in the reaction [8]. Then the question naturally arises: why the same energy is not used in the recovery of magnetite and hematite? In response, we can say that the only reason for the absorption of a lot of energy in the recovery of magnetite is the formation of a chemical bond between the two oxides of iron [9]. It takes extra energy to break that bond. In general, large amounts of external energy are required to recover magnetite due to the energy required to break the chemical bonds in the hematite and to break the chemical bond between the hematite and the wurtzite [10]. The high liquefaction temperature of magnetite can also be explained by this situation [11,12].    

Figure 1. The change in Gibbs energy during the carbothermic reduction of magnetite

As can be seen from Figure 1, no chemical reaction takes place at a temperature of 928 K, and the Gibbs energy of the system has a positive value. When the temperature reaches 928 K, the system reaches absolute equilibrium (∆G_reac = 0), i.e. the rate of forward and reverse reactions is equal (K_e = 1). At a temperature of 929 K, a chemical reaction begins at the contact boundaries of the magnetite and carbon systems [13,14,15].  

At 930 K, the chemical equilibrium constant in the reduction reaction of magnetite is greater than 1, but the reaction is very slow [16]. This is because one of the most important conditions for a chemical reaction to take place is the separation of the products of the reaction from the reaction surface. In hydrometallurgical processes, various mixing methods are used to overcome this problem [17]. For example, mechanical (blade) mixing, pneumatic (air) mixing, and so on. However, in pyrometallurgical processing, it is more difficult to mix the reaction system, especially when it is almost impossible to mechanically mix high-temperature liquid converter slag. Because metal flakes melt in liquid slag at high temperatures [18,19]. Therefore, gas bubbling method is used in the processing of high temperature liquid slag. This means that in the method we are considering, carbon reacts with all the iron and copper oxides to form CO₂ gas and begins to rise [20].

Figure 2. Temperature-dependent changes in the equilibrium constant of magnetite during carbothermic reduction

The CO₂ gases released from several reaction surfaces combine to form larger bubbles, which pneumatically stir the liquid bath as they rise. In this case, the FeO formed after the recovery of the magnetite leaves the reaction surface due to the bubbling of the liquid bath, opening the surface of the next layer of magnetite and the continuation of the recovery reaction.

According to the production practice, the reduction of magnetite with solid carbon occurs in the temperature range of 820 - 1000 ^oC (1093 - 1273 K) (Fig. 2). In this temperature range, the equilibrium constant of the recovery process ranged from 100 to 1693.      

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