STUDY OF THE SOLUBILITY OF ZINC FERRITE IN A MIXTURE OF CITRIC AND SULFURIC ACIDS

ABSTRACT

In the paper, since the main component containing zinc in zinc cake is zinc ferrite, the research work studied the thermodynamic aspects of the process to reduce ferric iron in zinc ferrite to ferrous state under the influence of citric acid. Based on the results of thermodynamic analysis, it was established that the limiting stage among the chemical reactions occurring in the system of zinc ferrite and a mixture of citric and sulfuric acids is the stage of mutual chemical interaction of zinc ferrite and citric acid. Accordingly, the optimal temperature required for the complete reaction of zinc ferrite with citric acid was 338 K (65 °C). By studying the kinetics of the process, it has been proven that the equilibrium constants of the reactions of dissolution of zinc and iron oxides in sulfuric acid, transformed into individual forms due to the reduction of zinc ferrite at the same temperature, have high values.

АННОТАЦИЯ

В статье, поскольку основным компонентом, содержащим цинк в цинковом кеке, является феррит цинка, в ходе исследовательской работы были изучены термодинамические аспекты процесса с целью восстановления трехвалентного железа в феррите цинка до двухвалентного состояния под воздействием лимонной кислоты. По результатам термодинамического анализа установлено, что лимитирующей стадией среди химических реакций, протекающих в системе феррита цинка и смеси лимонной и серной кислот, является стадия взаимного химического взаимодействия феррита цинка и лимонной кислоты. Соответственно, оптимальная температура, необходимая для полной реакции феррита цинка с лимонной кислотой, составила 338 К (65 ^оС). Путем изучения кинетики процесса доказано, что константы равновесия реакций растворения оксидов цинка и железа в серной кислоте, перешедших в отдельные формы за счет восстановления феррита цинка при той же температуре, имеют высокие значения.

Keywords: thermodynamics, zinc ferrite, pyrite, citric acid, reduction process, Gibbs free energy, equilibrium constants, temperature, kinetics.

Ключевые слова: термодинамика, феррит цинка, пирит, лимонная кислота, процесс восстановления, свободная энергия Гиббса, константы равновесия, температура, кинетика.

Finding a means for hydrometallurgical processing of zinc ferrite in the presence of a reducing agent that can satisfy all the requirements for a reducing agent is one of the important tasks of zinc metallurgy today [1]. Taking this into account, in this research work, the use of citric acid (C₆H₈O₇) was recommended and its effect on the process was studied [2]. Citric acid (or citric acid) is a water-soluble, harmless, and relatively inexpensive restorative [3]. Citric acid is preferred over other regenerating agents as it maintains an acidic environment in the solution [4]. The complete molecular equation for the chemical reaction of zinc ferrite by reduction with citric acid followed by selective dissolution of the remaining solid in sulfuric acid is as follows:  

9ZnFe₂O₄ + H₃C₆H₅O₇ + 27H₂SO₄ → 18FeSO₄ + 9ZnSO₄ + 6CO₂ + 31H₂O

Thermodynamic values of all substances involved in the reactions were calculated for standard conditions, and their initial values are listed in Table 1 [5].

Table 1.

Corresponding thermodynamic quantities of substances (at 298 K)

Using the values given in Table 1, the results of chemical reactions of sulfidation of magnetite with technical sulfur under standard conditions were calculated according to Hess’s law and are presented in Table 2.

Table 2.

Values of zinc ferrite reduction with citric acid at standard conditions (298 K)

From the values of thermodynamic calculations presented in Table 2 under standard conditions, it can be seen that the reaction of recovery of zinc ferrite with citric acid in a sulfuric acid environment is an exothermic reaction, and its standard Gibbs energy values are negative, that is, the reaction occurs spontaneously at 298 K. The increase in entropy in the particles formed in the system of reductive selective melting reactions increases the diffusion of the formed gases and prepares a favorable ground for the next stage of oxidation-reduction processes [6-7].

Based on the conclusion drawn from the values in Table 2, the corresponding mathematical equation of the relationship between the effects of the increase in temperature on the recovery reaction was constructed, and it looks like this:

∆G^T = – 2589,5 – 0,5552 · T

Based on the calculated mathematical expression, the probability of occurrence of the chemical process of regeneration was determined when the temperature in the reaction system increases by every 10 units. The obtained results are presented in Figure 1.

Figure 1 shows the corresponding Gibbs energies of the selective dissolution chemical reaction of reducing zinc ferrite with citric acid in a sulfuric acid environment in the temperature range of 298 - 438 K (i.e., 25 - 165 ^oC). This can also be seen clearly in Figure 1 in the form of an Ellingham diagram.

Figure 1. Ellingham diagram of the reaction flow for the reduction of zinc ferrite with citric acid

In the graph shown in Fig. 1, the equilibrium constants of the chemical reactions at the given temperatures for the reduction reaction were determined based on the equation of the relationship between temperature and Gibbs energy and the values of free energies of the chemical reaction of the addition of zinc ferrite with citric acid, and these values are presented in Fig. 2.

From the graph depicted in the form of histograms in Fig. 2, it can be understood that when the temperature used in typical hydrometallurgical processes reaches 338 K (65 ^oC), the equilibrium constant for the formation of metal sulfates in the selective melting of zinc ferrite in the presence of citric acid in a sulfuric acid environment reaches its optimal value (K_E=2.688).     

The result of the thermodynamic and kinetic analysis of zinc ferrite reduction with citric acid showed that with increasing temperature, the probability of reaction increases, but the equilibrium constant of the reaction decreases. This can be attributed to the fact that the selective melting process is exothermic. Increasing the temperature above 65 ^oC has a negative effect on the equilibrium constant of the reaction and causes a decrease in the rate of the chemical reaction [8-9].

Figure 2. Variation of the equilibrium constant during the reduction of zinc ferrite with citric acid as a function of temperature

The importance of studying this process is that the zinc sulfate formed as a result of the reaction is separated electrolytically. Before that, it is necessary to clean the solution from additional elements. In addition, due to the dissolution of iron and zinc in the solids remaining from the selective melting of zinc smelting cakes, the amount of rare metals in it increases and reaches a level that is effective in production. These metals can also be extracted using selective smelting. This makes it possible to introduce waste-free technology and increase the level of environmental protection.

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