THERMODYNAMICS OF REDUCTION OF ZINC FERRITE BY PYRITE

ABSTRACT

The thermodynamics of reducing trivalent iron in zinc ferrite to divalent iron using pyrite as a reducing agent were investigated. This study aimed to elucidate the feasibility of the reduction process and assess the underlying thermodynamic principles. The Gibbs free energy change for the reduction reaction was calculated using thermodynamic data and equilibrium constants at various temperatures. The results indicate that the reduction of trivalent iron in zinc ferrite to divalent iron using pyrite is thermodynamically favourable under certain conditions. Factors such as temperature, pressure, and initial reactant concentrations play crucial roles in influencing the thermodynamic feasibility. The findings of this study provide valuable insights into the potential applications of pyrite as a reducing agent for transforming metal oxides in various industrial processes.

АННОТАЦИЯ

Исследована термодинамика восстановления трехвалентного железа в феррите цинка до двухвалентного железа с использованием пирита в качестве восстановителя. Это исследование было направлено на выяснение возможности процесса восстановления и оценку основных термодинамических принципов. Изменение свободной энергии Гиббса реакции восстановления рассчитывали с использованием термодинамических данных и констант равновесия при различных температурах. Результаты показывают, что восстановление трехвалентного железа в феррите цинка до двухвалентного железа с помощью пирита термодинамически выгодно при определенных условиях. Такие факторы, как температура, давление и начальная концентрация реагентов, играют решающую роль в влиянии на термодинамическую осуществимость. Результаты этого исследования дают ценную информацию о потенциальном применении пирита в качестве восстановителя для преобразования оксидов металлов в различных промышленных процессах.

Keywords: thermodynamics, redox reaction, trivalent iron, divalent iron, zinc ferrite, pyrite, reduction process, Gibbs free energy, equilibrium constants, temperature, kinetics.

Ключевые слова: термодинамика, окислительно-восстановительная реакция, трехвалентное железо, двухвалентное железо, феррит цинка, пирит, процесс восстановления, свободная энергия Гиббса, константы равновесия, температура, кинетика.

Zinc cake processing is an important process for extracting zinc from ores and producing high-quality zinc metal for use in a variety of industrial and commercial applications. Pyrometallurgical methods refer to processes that involve the use of high temperatures to extract metals from ores or other materials [1]. In the case of zinc cake, which is a byproduct of zinc refining, pyrometallurgical methods are commonly used to recover zinc [2].

The exact mineralogical content of zinc cake can be determined through mineralogical analysis techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) [3-5].

When the composition of zinc production cakes was analysed using the above methods of analysis, it was found that the main part of zinc oxide in its content is in the form of zinc ferrite [6]. Therefore, zinc production cakes were chosen as the research object.

Zinc ferrite, the main component of zinc cake, is resistant to acid and alkali, so it needs to be treated with various reducing agents before hydrometallurgical processing [7].

In this study, pyrite concentrates produced in copper beneficiation plants were selected as a reducing agent.

Thermodynamic analyzes were conducted taking into account the dependence of isobaric-isothermal potentials (Gibbs free energy) on temperature [8]. The Gibbs free energy change (∆G) is generally calculated using the following formula:

∆G_reac = ∆H_reac – ∆S_reacT

Where: ∆H_reac is the enthalpy of the corresponding chemical reaction, kJ/mol;

∆S_reac is the entropy of the corresponding chemical reaction, J/(mol·K);

T is the absolute temperature of the system, K.

Information about the standard thermodynamic values of the substances involved in the reactions under consideration was determined from the appendix presented in the work [9], and these values are presented in Table 1.

Table 1.

Thermodynamic values of substances

The temperature dependence of the reaction equilibrium constant (K_e) was determined by the following formula:

∆G = − R T lnK_e

Where: R is the universal gas constant, R = 8.31696·10^-3 kJ/(grad·mol);

K_e is the equilibrium constant of the corresponding chemical reaction.

In order to reduce trivalent iron in zinc ferrite, a concentrate containing pyrite mineral was used in the research work. It was recommended to use a rotary kiln to ensure sufficient interaction between the two solid phases. Because this is the first condition for the chemical reaction when the kiln rotates - zinc ferrite and pyrite particles diffuse well between each other. In this case, the rotation speed of the furnace was 0.4 revolutions/minute. The oxidation-reduction reaction between zinc ferrite and pyrite can be written as follows:

5ZnFe₂O₄ + FeS₂ = 5ZnO + 11FeO + 2SO₂↑

Using the thermodynamic values under standard conditions given in Table 1, the general formula for the change in Gibbs energy for a chemical reaction is written as follows:

∆G_reac = 734.3 – 0.56316T

Based on the above formula, the changes of the free energy and equilibrium constants of zinc ferrite reduction reaction at several temperatures were calculated and these values are presented in Fig.1. The values of the Gibbs energy and equilibrium constant of the reaction were calculated when the temperature in the system changes every 50 units. The results of the thermodynamic analysis presented in Fig.1 show that the reduction reaction of zinc ferrite in the presence of pyrite is an endothermic reaction, so the increase in temperature in the system accelerates the rate of the chemical reaction going in the right direction, and as a result, the value of the equilibrium constant increases according to the mathematical law.

Figure 1. Changes in Gibbs energy during the reaction of zinc ferrite with pyrite

The graph in Fig.1 shows the Gibbs energy versus temperature for the reduction of trivalent iron in zinc ferrite by pyrite, and it can be seen from this graph that the free energy (∆G) of the system for this case increases linearly with each 50 unit change in temperature decreases, i.e. becomes negative. This means that when the temperature rises, the probability of the reaction of zinc ferrite with pyrite to flow increases. The oxidation-reduction reaction between zinc ferrite and pyrite begins at a temperature of 1304 K (1031 ^oC), at higher temperatures the Gibbs energy in the reaction system has negative values.

Figure 2. Variation of the equilibrium constant during the reduction of trivalent iron in zinc ferrite using pyrite depending on the temperature

Figure 2 shows the graph of the temperature dependence of the equilibrium constant of the reduction reaction of trivalent iron in zinc ferrite using pyrite. In this graph, it can be seen that the chemical equilibrium constant is greater than 1 between the temperatures of 1323 and 1623 K. In the temperature range of 1423-1473 K, the equilibrium constant of the reaction was sufficient for this reaction to proceed. Even at higher temperatures, the equilibrium constant has high values, but when the temperature exceeds 1523 K, some silicate-like materials in the raw material begin to liquefy. Liquefied substances stick to other materials and form large grains. This prevents the raw materials from diffusing into each other. In addition, when the temperature exceeds 1523 K, the probability of liquefied raw materials sticking to the inner wall of the furnace increases. This reduces the productivity of the oven and has a negative effect on the principle of operation.

The above thermodynamic analyzes show that the rate-limiting step of the reaction of zinc ferrite with pyrite is both a kinetic and a diffusion regime. This type of reaction is one of the main problems of today’s non-ferrous metallurgy. In this case, increasing the productivity of the reaction depends on the number of collisions of molecules of reactants with each other and at the same time on the temperature. In order to increase productivity in such reactions, it is necessary to increase the rate of diffusion of substances to each other, the temperature and the size of the reaction surface.

In conclusion, the thermodynamic analysis of reducing trivalent iron in zinc ferrite to divalent iron using pyrite reveals that the process holds promise for practical implementation. The negative values of Gibbs free energy change (ΔG) obtained under certain temperature and pressure conditions indicate the spontaneous nature of the reduction reaction. This suggests that the utilization of pyrite as a reducing agent has the potential to efficiently convert trivalent iron species into divalent iron, contributing to the modification of metal oxide compounds. However, it is essential to carefully consider the specific operating conditions to ensure favourable thermodynamics and optimal conversion rates. Further experimental studies are warranted to validate the theoretical findings and to address kinetic and practical challenges that may arise during the reduction process. Overall, the thermodynamic insights provided by this study offer a foundation for the design and optimization of processes involving the reduction of trivalent iron species using pyrite as a reducing agent.

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