ABSTRACT

The kinetics of the interaction of MnO₂, Mn₂O₃, and Mn₃O₄, with sulphuric acid solutions and the effect of oxalic acid on this process are studied. As the sulphuric acid concentration is increased from 0.1 to 5 N, the dissolution rate of Mn₂O₃, Mn₃O₄ to MnO₂ and Mn²⁺ ions decreases, whereas it increases with the concentration of Mn²⁺ ions. Upon the addition of H₂O_x, the complete dissolution of Mn₃O₄ occurs more quickly. The reaction order with respect to the H⁺ and ions is +0.5 ± 0.1. A mechanism of MnO₂ dissolution promotion by is proposed. The dissolution rate was found to depend on the concentrations of and ions and was highest at pH 1.6 ± 0.2. A rate law and mechanism are suggested for manganese oxides dissolution.

1. Introduction

To accelerate the etching of manganese alloys and optimise the dressing and leaching of manganese oxide ores, it is necessary to understand the mechanisms of these processes [1-12]. The most promising method of improving the effectiveness of the etching of manganese oxides is reductive dissolution by various complexing agents [5-10]. However, the rate-limiting step has not been identified in most works on the reductive dissolution of oxides.

2. Material and Method

In the study of dissolution kinetics, we used extra-pure γ-MnO₂, Mn₂O₃ and Mn₃O₄ powder with a particle size of 60 - 80 μm. A sample of oxide weighing 0.362 ± 0.005 g was placed in a temperature-controlled reaction vessel containing 0.750 ± 0.005 l of an aqueous solution of sulphuric and oxalic acids. During dissolution, the mixture was stirred magnetically at ν = 600 - 700 rpm. In preliminary runs, the dissolution rate was almost independent of ν for ν > 500 rpm. The pH of the solution was adjusted by adding sulphuric acid. The concentration of sulphuric acid was varied between 0.005 and 2.5 mol/l. Most measurements were carried out at 353 K. The solution was sampled at regular intervals using an N16 Schott filter. The manganese in solution was determined photocolourimetrically as the complex with formaldoxime [13]. The non-stoichiometric composition change during the dissolution of oxides phases in sulphuric acid was measured by a technique described in a previous study [14].

3. Results and Discussion

3.1. Analysis of the Kinetic Data of Manganese Oxide Dissolution (Mn₂O₃, Mn₃O₄ and MnO₂) in Sulphuric Acid

As seen in Figures 1-3, a remarkable feature of the interaction of manganese oxide with sulphuric acid is that the oxide is dissolved incompletely. The process is terminated at α = 0.66 (α is the ratio of the concentration of Mn²⁺ ions in the solution to the total manganese in the oxide sample).

Based on the data shown in Figures 1 and 2, the dissolution rates of Mn₂O₃ and Mn₃O₄ decrease with increasing sulphuric acid concentration.

3.2. Calculation of the Specific Rate of Dissolution

Using these curves (Figures 1-3), we derived the specific dissolution rate W, the apparent orders of reaction with respect to hydrogen ions and manganese (II) ions, and

the activation energy of dissolution by testing various heterogeneous kinetic models (Table 1) using affine transformations. First, we plotted α against t/t_0.5, where t_0.5 is the time required for 50% of the sample to dissolve.

To select the heterogeneous kinetics equations that best fit the experimental curves, the obtained α versus t/t_0.5 dependences were described by various theoretical dependences.

The choice of the equation routinely began with the most simple equation, (e). The compressed sphere equation is not applicable because the curves are s-shaped and correspond to the Fisher criterion.

The best results were obtained with the Hill function (1), which corresponds to the formation of active sites at the dislocations and grain boundaries of the oxide phase [15].

The theoretical lines well describe the experimental points (Figures 1 and 2), proving the applicability of the chosen Equation (1).

(1)

(2)

The obtained A values (A = 0.2) were used to calculate the specific rate of dissolution (disproportionation).

(3)

where τ_0.5 = Wt_0.5(τ_0.5 = 0.65).

Using Equation (1) it is possible to calculate the specific rate of dissolution for the given concentration (Tables 2 and 3).

The activation energy of the process calculated from the log W versus 1/T dependence gives E_a = 80 ± 0.5 kJ/mol.

We found that the dissolution rates of Mn₂O₃ and Mn₃O₄ decrease as [H⁺] is increased, which is attributed to the disproportionation reactions, resulting in incomeplete dissolution.

The process can be represented as disproportionate according to the following scheme:

(4)

where x₁ and x are the initial and final x = [O]/[Mn] ratios for the oxide phase.

Equation (4), Figure 4, and Table 4 both indicate a dependence between the extent of dissolution (α) and non-stoichiometric composition (х) with time, which can be expressed using the following equation [14]:

(5)

The orders of the disproportionation reaction determined from logW versus pH and logw versus log[Mn²⁺] are as follows:

(6)

A preliminary consideration of the effect of different factors on the kinetics of the process shows that the obtained curves are rectified in the 1/W versus [H⁺] and 1/W versus [Mn²⁺] coordinates. This observation allows us to describe the interaction rate of manganese oxides with H₂SO₄ by the equation:

(7)

where K_al, K_a2 and W⁰ are kinetic constants.

We propose a Mn₂O₃ dissolution mechanism that explains the negative orders for protons and positive orders for manganese ions [6]:

1) Hydration of an oxide surface:

(8)

2) Adsorption of protons:

(9)

3) Dissolution from active sites:

(10)

4) Volumetric reaction:

(11)

Integrally:

(12)

The disproportionation of Mn₃O₄ characterised by two-stage kinetics (Figure 1) can be represented by two overall reactions:

, (13)

(14)

In both cases, low-solubility MnO₂ is formed on the oxide surfaces.

Therefore, it is important to determine the conditions that promote the efficient dissolution MnO₂.

The kinetics of manganese dioxide dissolution were then studied in sulphuric acid solutions containing oxalic acid.

3.3. Kinetics Analysis of the Effect of Oxalic Acid on the Rate of the Interaction and Dissolution of MnO₂ in Sulphuric Acid Solutions

Influence of concentration of oxalic acid. The results of the dissolution of manganese dioxide in various concentrations of oxalic acid are shown in Figure 5. Based on Figure 4, increasing the oxalate ion concentration increases the extent of dissolution (α). Thus, full dissolution is observed.

The specific dissolution rate increases with oxalic acid concentration at a constant sulphuric acid concentration. The plot of this dependence (Figure 5) resembles that of an adsorption isotherm. The dissolution rate follows the empirical equation:

(15)

The Langmuir and Freundlich isotherms describe the experimental data with identical accuracies; therefore, the following is used as a first approximation:

(16)

where n is the reaction order with respect to oxalic acidand is the total concentration of all forms of oxalic acid (n = 0.66, K_a = 0.026 mol/l, W_max = 0.66 min^–1).

When tends to infinity, the process rate reaches its maximum, W_max.

Influence of рН at constant oxalic acid concentration. The dissolution rate of manganese (IV), (III) oxides was studied as a function of pH in solutions containing 0.005 M oxalic acid. We found that W is highest at pH 1.6 ± 0.2 (Figure 6).

From the kinetic data obtained, the following relation was derived for the dissolution of MnO₂ in oxalic acid [16-19]:

, (17)

where θ_i is the coverage of the MnO₂ surface by species i.

3.4. Modelling the Manganese (IV) Oxide Dissolution Mechanism

According to the site-binding model [16-19], dissolution proceeds via the formation of intermediates when the surface is positively charged.

Scheme 1:

here, θ₁ is the fraction of the unbound oxide surface and θ₂ and θ₃ are the coverages of the surface by and complexes, respectively. In this case, the dissolution rate may be limited by the passage of the intermediate complex into solution, which occurs at a rate proportional to θ₃. W and θ₃ can be determined as functions of various parameters by applying the method based on a steady-state approximation and the principle of detailed balancing [16-19] to the steps preceding the rate-limiting step.

The dissolution rate at constant pH is given by:

(14)

with W_max = 0.37 ± 0.05 min^–1, K_al = {k_–1k_–2 + k₃(k_–1 + k₂)}/(k₁k₂) = 2 × 10^–3 mol/l and K_a2 = (k_–2 + k₃)/k₂ = 4 × 10^–3 mol/l. Here, K_a1 and K_a2 are the equilibrium constants of adsorption at the dioxideelectrolyte interface., where is the fraction of ions in solution:

(20)

where β₄ and β₅ are the dissociation constants of oxalic acid: log β₄ = 1.271, log β₅ = 4.266 [12-16].

4. Conclusions

The dissolution of manganese oxides in sulphuric acid proceeds to partial completion, whereas the addition of oxalic acid allows it to proceed to completion.

Thus, the performed experimental and theoretical investigation of the dissolution and disproportionation processes of manganese oxides in H₂SO₄ solutions in the presence of H₂O_x indicated that the dissolution mechanism involves the adsorption of H₂O_x followed by the reduction of the surface layers of the oxide.

The dissolution of manganese dioxide in sulphuric acid solutions containing oxalic acid can be accelerated considerably. The dissolution rate is affected by the, , and H⁺ ions. The maximum dissolution rate is observed at pH 1.6 and a 1:5 concentration ratio for the manganese ions and oxalic ions.

The first step in manganese dioxide dissolution is the coadsorption of and ions, followed by charge transfer. The resulting ions pass into solution. The rate-limiting step is three-dimensional nucleation on the MnO₂ surface.

5. Acknowledgements

This work was supported by the Ministry of Education of the Russian Federation (state contracts № 14.740.11. 1095, № 3.5258.2011).

REFERENCES