Removal and Resource Utilization of High Concentration Flue Gas Sulfur Dioxide Using Manganese Carbonate Ore

Abstract

The removal of high concentration flue gas sulfur dioxide (SO₂) using manganese carbonate ore desulfurization (MCO-FGD) is a promising route that combines economic benefits and pollution control. However, the problems of intermediate oxidation and by-product control have plagued the industrial application of the MCO-FGD technique for a long time. Based on the fact that there is symbiosis of manganese and iron in natural manganese ore, in this study, small amounts of Fe(III) and MnO₂ were introduced into the MCO-FGD reaction system to enhance the oxidation of SO₂ to SO₄⁻ and suppress the manganous dithionate (MnS₂O₆) by-product generation. The results suggested that the addition of Fe(III) led to the generation of potent oxidant Mn(III) in the reaction system, which accelerated the generation of SO₃^−• radicals and, thus, enhanced the oxidation of SO₂. Under the optimum reaction conditions, the 2.0% of inlet SO₂ could be removed to 62 ppm, obtaining 90.1% manganese leaching efficiency, and the concentration of MnS₂O₆ in the desulfurized liquid was kept below 2.5 g/L after a six-stage desulfurization. The results are of great importance for the sustainable development of the manganese metallurgical industry, which provides theoretical and technical support for the recycling of sulfur and manganese. The influences of different operational conditions on SO₂ removal, the catalytic mechanism, and manganese leaching were studied to provide theoretical and technical support for resourceful MCO-FGD technology.

1. Introduction

In recent decades, the popularity of flue gas desulfurization (FGD) has substantially reduced SO₂ emissions in China [1,2]. As the technology has matured, FGD has enabled resourceful recycling of part of the flue gas SO₂, such as using desulfurization gypsum as construction material [3,4] and adsorption desulfurization to sulfuric acid, etc. [5]. Nevertheless, the resource utilization of flue gas SO₂ is still limited, especially in the case of high concentration flue gas SO₂, which is currently only capable of being used for the production of sulfuric acid. With the proposal of “carbon neutrality”, pollution control with the goal of resource utilization will be receiving more attention [6,7]. Therefore, developing high-value-added resource utilization FGD technology is of great significance to the sustainable development of society.

Manganese is an important resource in industry and human life, widely used in steelmaking, non-ferrous metallurgy, chemical engineering, agriculture, and medicine, as well as new energy [8,9,10]. Manganese carbonate ore (MCO) and manganese oxide ore (MOO) are the most two important types of manganese resources found in nature, of which MCO accounts for more than two thirds, about 73%, of the total manganese reserves [11,12]. Direct acid leaching using sulfur acid is the most widely used method for MCO from the perspective of both energy consumption and economics [13,14]. However, the increasing demand for manganese coupled with the depletion of high-grade manganese ore has posed great challenges for the industry [15,16]. The use of medium- and low-grade (<30%) manganese ore, due to the high impurities and lower manganese content, causes increases in acid consumption, resulting in a stronger flue gas emission and more solid waste [15,16,17]. Consequently, traditional acid leaching suffers the problems of high energy consumption and significant environmental contamination [18,19]. Therefore, the development of an eco-friendly and cost-effective process for using medium- or low-grade MCO is imperative.

Manganese ore-based flue gas desulfurization attracted considerable attention in the past two decades, which adopts MOO in wet flue gas desulfurization technology (MOO-FGD) to remove flue gas SO₂ and obtain manganese sulfate simultaneously [20,21]. Our research group has also made significant strides in adapting this pollution control process to industrial electrolytic manganese production (EMP) [8,22]. If MCO can replace MOO in MOO-FGD, using the flue gas desulfurization process to facilitate manganese leaching would be very beneficial. This method could help achieve a low-cost and eco-friendly manganese leaching of MCO, particularly for the utilization of medium- and low-grade ores [23]. However, some bottlenecks remain, limiting the applications, such as an unreasonable balance between desulfurization performance and manganese leaching, insufficient oxidation of SO₂ in the aqueous phase, the generation of by-product MnS₂O₆, and the inconsistency between experimental research and industrial production. These issues directly impact the utilization of the desulfurization liquid and determine the cost-effectiveness of the technique.

Natural manganese minerals are typically associated with iron, a troublesome impurity in conventional manganese metallurgy due to their comparable geochemical properties [11]. In hydrometallurgy, however, the dissolved Fe(III) is also an ionic catalyst that can speed chemical oxidation through the Fe(III) ↔ Fe(II) cycle [24]. In addition, Fe(III) is also capable of catalyzing additive reactions with other metal ions [25,26]. Current research on using Fe(III) and Mn(II) for oxidizing SO₂ in the liquid phase for low-concentration flue gas desulfurization is well-developed [24,27]. Ibusuki et al. discovered that the combination of Fe(III) and Mn(II) significantly enhances the autoxidation of SO₂ when compared to employing only individual metal ions [24]. Chen et al. subsequently determined that the coexistence of Fe(III) and Mn(II) triggers the production of strongly oxidizing Mn(III), which expedites the oxidation of SO₃²⁻ to SO₄²⁻ [27]. However, these studies are solely based on low-concentration SO₂ removal and do not account for high concentration flue gas SO₂, and are conducted without the presence of any solid components. Most importantly, none of these studies have been evaluated for practical applications.

Considering the association of Mn and Fe in natural manganese ores, and to effectively utilize resources of desulfurization liquids, a novel strategy for high concentration flue gas SO₂ desulfurization combined with efficient manganese leaching using the manganese carbonate ore process, based on MCO−FGD technology, has been developed and systematically studied in this experiment. In this technique, Fe(III) is used as an additive to speed up the liquid-phase catalytic oxidation of SO₂, while water in the traditional wet flue gas desulfurization is substituted with an anode liquid of electrolytic manganese production (AEMP) to facilitate the re-utilization of flue gas SO₂ and achieve low-cost manganese leaching at the same time. The influences of different operation conditions on SO₂ removal, the catalytic mechanism, and manganese leaching were studied to provide theoretical and technical support for the resourceful MCO−FGD technology, and the results are of great importance for the sustainable development of the manganese metallurgical industry.

2. Materials and Methods

2.1. Materials

The chemical reagents, e.g., manganese sulfate (MnSO₄•H₂O, AR, 99%), ferric sulfate (Fe₂(SO₄)₃, AR, Fe³⁺ > 21%), magnesium sulfate (MgSO₄, AR, 99%), sulfuric acid (H₂SO₄, AR, 98%), ammonium sulfate ((NH₄)₂SO₄, AR, 99%), nitric acid (HNO₃, AR, 65–68%), phosphoric acid (H₃PO₄, AR, 85%), sodium hydroxide (NaOH, AR, 98%), hydrogen peroxide (H₂O₂, AR, 30%), ammonium ferrous sulfate (AFS) ((NH₄)₂Fe(SO₄)₂·6H₂O, AR, 99.5%), perchloric acid (HClO₄, AR, 70–72%), and manganese carbonate (MnCO₃, AR) were purchased from Chengdu Kelong Chemical Co., Ltd. The manganese carbonate ore (MCO) originated from Ghana, and its main ingredients, based on X-ray fluorescence (XRF) analysis, are listed in

Table 1. Main ingredients of the manganese carbonate ore (MCO) based on XRF analysis (%).

Components	Mn	O	C	Si	Al	Fe
Content (%)	24.97	46.27	9.88	7.41	2.07	1.19
Components	K	Na	Ca	Mg	S	Ti
Content (%)	0.41	0.32	4.72	2.39	0.11	0.07

. The AEMP solution used in this study was laboratory-prepared in accordance with the engineering actuality, which contains 40.0 ± 1.0 g·L⁻¹ of H₂SO₄, 38.0 ± 0.5 g·L⁻¹ of MnSO₄, 18 ± 0.5 g·L⁻¹ of MgSO₄, and 60.0 ± 0.5 g·L⁻¹ of (NH₄)₂SO₄. The other low-concentration components were not considered.

2.2. Experimental Setup

The homemade flue gas desulfurization setup comprised four components, the flue gas generation system, the reaction system (including the bubbling reactor and water bath), the measurement system, and exhaust gas treatment, as depicted in Scheme 1. During standard operation, SO₂, O₂, and N₂ from gas cylinders were controlled by a mass flowmeter and uniformly mixed in a gas mixer to produce a simulated flue gas with a specific SO₂ concentration. The flue gas was subsequently introduced into a bubbling reactor containing a desulfurizing liquid. The bubbling reactor was placed in a 60 °C water bath and vigorously stirred with a magnetic stirrer. The SO₂ removal rate, manganese leaching rate, and other relevant technical indexes were measured at specified time intervals.

2.3. Analytical Methods

The flue gas SO₂ before and after the bubbling reactor was adsorbed by a 3% H₂O₂ solution, and the formed H₂SO₄ was then determined by titrating with NaOH solution (0.01 mol/L), using bromcresol green and methyl red as indicators to determine the ending point [28]. The SO₂ removal efficiency X (%) was calculated by using Equation (1), where $C_{S{O}_2,in}$ and $C_{S{O}_2,out}$ (ppm) are the inlet and outlet SO₂ concentrations.

$$X=\frac{C_{S{O}_2,in}-C_{S{O}_2,out}}{C_{S{O}_2,in}}\times 100\%$$

(1)

The dissolved manganese [Mn(II)] ($C_{[M{n}^{2+}]}$, g/L) in the filtrate was determined using the ammonium iron(II) sulfate titrimetric method with a certain improvement based on the GB/T 1056-2016 (Chinese National Standard) [29], and the [Mn(II)] content was calculated using Equation (2), where C_AFS (mol/L) is the concentration of standard Fe(NH₄)₂·(SO₄)₂ solution and V₀ and V₁ (mL) are the volume of Fe(NH₄)₂·(SO₄)₂ used for the blank test and sample titration.

$$C_{[M{n}^{2+}]}=\frac{C_{AFS}\times 54.94\times 25.0\times {10}^{-3}}{V_1-V_0}$$

(2)

The concentration of MnS₂O₆ in the filtrate was determined using the distillation−iodometric method with some improvements [30]. The MnS₂O₆ concentration $C_{(Mn{S}_2{O}_6)}$ (g/L) was calculated using Equation (3), where v is the volume of filtrate (mL), C is the concentration of standard Na₂S₂O₃ solution (mol/L), V₂ and V₃ (mL) are the volume of Na₂S₂O₃ used for the blank test and sample titration, and 215.07 is the molar mass of MnS₂O₆.

$$C_{(Mn{S}_{2}O6)}=\frac{C·(V_3-V_2)}{2v}\times 215.07$$

(3)

The concentration of SO₃²⁻ in the filtrate was determined using the iodometric method [31]. We measured the quantitative 0.01 mol/L concentration of iodine solution into the iodine bottle, quickly added 1 mL of the sample, then added 2 mol/L HCl and shook well, placed it away from light for 5 min, and titrated with Na₂S₂O₃ standard solution until the solution was light yellow. Then, we added 1 mL of starch solution and continued titrating until the blue had just faded away. The concentration of SO₃²⁻ ($C_{[S{O}_3{}^{2-}]}$, g/L) was calculated using Equation (4), where $C_{Na{S}_2{O}_3}$ (mol/L) is the concentration of Na₂S₂O₃ standard solution, V₄ and V₅ (mL) are the volume of Na₂S₂O₃ solution consumed by the blank test and sample measurement, v₁ (mL) is the volume of the sample measured, and 80.06 is the molar mass of SO₃²⁻.

$$C_{[S{O}_3{}^{2-}]}=\frac{C_{Na{S}_2{O}_3}\times (V_5-V_4)}{2v_1}\times 80.06$$

(4)

A rapid titration method was used for the determination of SO₄²⁻ ($C_{[S{O}_4^{2-}]}$, g/L) in the filtrate, and the results were calculated using Equation (5) [32]. Typically, we took a certain volume of the sample in a triangular flask and added 40 mL of 1 + 1 ethanol solution. When the mixture was well stirred, 3–4 drops of 1.0 g/L bromophenol blue solution were added, and it was titrated to yellow with 0.10 mol/L perchloric acid. Then, we overdosed 8–10 drops, added 5 drops of alizarin indicator, and titrated with 0.10 mol/L BaCl₂ standard solution until pink.

$$C_{[S{O}_4^{2-}]}=\frac{(V_7-V_6)\times C_{BaC{l}_2}\times 96.06}{v_2}-1.2C_{[S{O}_3^{2-}]}$$

(5)

where V₆ and V₇ (mL) are the volume of BaCl₂ consumed by the blank test and sample measured, $C_{BaC{l}_2}$ (mol/L) is the concentration of BaCl₂ standard solution, v₂ (mL) is the volume of the sample measured, and 96.06 is the molar mass of SO₄²⁻.

The oxidation rate (η, %) of SO₃²⁻ was calculated by measuring the changes in SO₃²⁻ and SO₄²⁻ in the desulfurization filtrate (6).

$$\eta =(\frac{C_{[S{O}_4^{2-}]}}{C_{[S{O}_4^{2-}]}+1.2C_{[S{O}_3^{2-}]}})\times 100\%$$

(6)

3. Results

3.1. Mechanism Study of MCO-FGD

3.1.1. The Role of Mn(II) and Fe(III) in SO₂ Removal

Based on the known chemistry property that MnCO₃ is not involved directly in SO₂ conversion in the desulfurization system, the wet FGD process using MnCO₃ can be divided into three specific steps: (i) absorption of SO₂ from gas to desulfurization slurry, (ii) oxidation of SO₂ and acid generation in the liquid, and (iii) reaction between MnCO₃ and acid. Therefore, the effectiveness of SO₂ removal mainly depends on the absorption and oxidation of SO₂ in the desulfurization system. Numerous operational parameters affect the desulfurization performance to varying degrees. Among these, the pH of the desulfurizing liquid and the reactant concentration are the most significant. In addition, the pH also affects the species of SO₂ in the aqueous phase; specifically, the SO₂ exists mainly as H₂O·SO₂ at pH < 2 and as HSO₃⁻ at pHs higher than this [33,34], which also impacts the performance of SO₂ removal.

The influence of pH on the role of Mn(II) and Fe(III) in SO₂ removal was firstly evaluated, where the Fe₂(SO₄)₃ and/or MnSO₄ dissolved in D.I. water were used as the desulfurizing liquid, and the experimental conditions were controlled at 1.0 vol.% of SO₂ (10,000 ppm) and 10.0 vol.% of O₂ with a total 2.0 L/min flow rate, 5.0 g/L of [Fe(III)] (when used), and/or 20.0 g/L of [Mn(II)] (when used) in 400 mL of desulfurizing liquid, and the reaction temperature was maintained at 60 °C in a water bath. According to the results shown in Figure 1, the SO₂ removal performance of the Mn(II) and Fe(III) reaction systems showed a tendency of first strengthening and then weakening with the pH increasing from 1 to 5 (Figure 1a,b). The optimum pH of both reaction systems was 4, and the relatively higher detected oxidation rate at a lower pH could be attributed to the unstable existence of H₂O−SO₂ or HSO₃⁻. Nevertheless, the SO₂ removal efficiency of both the Mn(II) and Fe(III) reaction systems decreased fast to almost zero in 30 min.

The coexistence of Mn(II) and Fe(III) exhibited a synergistic effect, as is shown in Figure 1c. The Mn(II)−Fe(III) reaction system demonstrated superior SO₂ removal performance to the Mn(II) and Fe(III) reaction systems at different pHs, and the oxidation of SO₂ was more efficient (>86.5%). The higher SO₄²⁻ content in the desulfurization liquid, as seen in Figure 1f, confirms the substantial improvement after adding Fe(III) due to this synergistic effect. These indicate the substantial improvement after the addition of Fe(III) due to the formation of the synergistic effect between Fe(III) and Mn(II). In addition, the optimum pH for SO₂ removal in the Mn(II)-Fe(III) reaction system decreased to 3. This is important for the FGD process with MnCO₃, since a lower pH is preferable to obtain high manganese leaching (MnCO₃ + 2H⁺ → Mn²⁺ +H₂O + CO₂). Meanwhile, the lower pH of the desulfurization liquid enables a lower SO₃²⁻, which is beneficial for the subsequent resource utilization of the desulfurization liquid. Thus, the pH of the desulfurizing liquid was maintained at 3, except where specified.

The effects of ionic concentration on SO₂ removal are shown in Figure 2, where the pH of desulfurizing liquid was 3 and the other conditions were kept unchanged. The results visually demonstrated the change in SO₂ removal efficiency with different ionic contents, and a higher Mn(II) or Fe(III) content was favorable for SO₂ removal. For the Mn(II) reaction system (Figure 2a), the increase in Mn(II) concentration not only improved the SO₂ removal activity but also slowed down the depletion of the FGD capability. The oxidation rate of SO₂ was also enhanced somewhat, and the calculated oxidation rate ([SO₄²⁻]/([SO₄²⁻] + [SO₃²⁻])) increased from 53.71% to 62.65%, which means the removed SO₂ was more oxidized to SO₄²⁻. The SO₂ removal by Fe(III) showed a similar change to that of Mn(II) with the increase in content, but demonstrated stronger oxidability (Figure 2b), with an oxidation rate higher than 94.0% when the concentration was ≥10.0 g/L. The optimum content was 5.0 g/L in the Fe(III) reaction system, and the desulfurization performance remained stable as it continued to increase to 7.0 g/L.

Figure 1c illustrates the impact of adding Fe(III) to the Mn(II)-Fe(III) reaction system, where Mn(II) was kept at 20.0 g/L and Fe(III) varied from 1.0 to 7.0 g/L. The SO₂ removal of the Mn(II)-Fe(III) reaction system (20.0 g/L + n g/L) was found to be different from that of the Mn(II) and Fe(III) reaction systems over time. Increasing the Fe(III) addition enhanced the capability of SO₂ removal; the SO₂ removal efficiency was still 41.6% after 30 min when 7.0 g/L of Fe(III) was added. In addition, the oxidation property of the Mn(II)-Fe(III) reaction system was also greatly improved, and the detected SO₃²⁻ concentrations after 30 min reaction decreased from 1.09 to 0.08 g/L when the Fe(III) addition increased from 0 to 7.0 g/L.

All these results confirm the catalytic activity of Mn(II) and Fe(III) under acidic conditions [24], and the higher SO₂ removal efficiency and oxidation rate suggest a remarkable catalytic synergy between them. These provide both the theoretical and experimental foundation for the implementation of manganese carbonate desulfurization coupled with effective manganese leaching.

3.1.2. The Influences of Fe(III) on SO₂ Removal by MnCO₃

Based on the results of Section 3.1, the flue gas desulfurization by Fe(III)-added MnCO₃ was conducted, where Fe₂(SO₄)₃ was used as the Fe(III) source, and it was added according to the stoichiometry based on the conditions of the desulfurization liquid configuration. The results are shown in Figure 3a,b. The SO₂ removal efficiency decreased with the reaction runs for both the MnCO₃-only and the Fe(III)-added MnCO₃ (Fe(III)-MnCO₃) reaction systems, while it should be noted that the introduction of Fe(III) improved the desulfurization activity of MnCO₃. At reaction times of 20, 40, 60, and 90 min, the SO₂ removal efficiencies of the Fe(III)-MnCO₃ reaction system were 91.78, 81.78, 76.78, and 57.14%, respectively, which were greatly superior to the MnCO₃-only reaction system (81.42, 62.14, 46.78, and 20.71%). This difference is also consistent with Section 3.1.1. Corresponding to the better SO₂ removal performance, the Fe(III)-MnCO₃ system leached more manganese than the MnCO₃-only system. In just 20 min, the Fe(III)-MnCO₃ system achieved 54.3% manganese leaching efficiency, which was twice as high as that of the MnCO₃-only system. After 90 min of desulfurization, the manganese of MnCO₃ was completely leached with the assistance of Fe(III), whereas it was only 78% without Fe(III).

Figure 3c is the SO₄²⁻ and SO₃²⁻ concentrations in the desulfurizing liquid at different reaction times. In the MnCO₃-only reaction system, both the SO₄²⁻ and SO₃²⁻ concentrations were increased with the reaction time. However, the desulfurization liquid had a low concentration of SO₃²⁻ in the Fe(III)-MnCO₃ reaction system throughout the desulfurization process. The highest SO₃²⁻ concentration of 0.24 g/L was obtained after 20 min desulfurization, which then gradually decreased to 0.06 g/L after 90 min. In contrast, the SO₄²⁻ showed a rapid increase followed by a relatively stable change, with concentrations of 3.73, 7.70, 11.12, and 11.77 g/L at reaction times of 20, 40, 60, and 90 min, respectively. The change in the acidity of the desulfurizing liquid may account for the variation in SO₃²⁻ concentration. Furthermore, the Fe(III)-MnCO₃ system had a higher total content of SO₃²⁻ and SO₄²⁻, providing evidence of its superior SO₂ removal performance.

MnS₂O₆ is an unavoidable by-product of manganese-based flue gas desulfurization that can compromise not only the purity of manganese sulfate but also the subsequent electrolysis progress [35,36]. Therefore, control of MnS₂O₆ is of great importance for practical applications. As is shown in Figure 3d, the MnS₂O₆ gradually increased with the reaction time whether or not the Fe(III) was added. Nonetheless, the MnS₂O₆ concentration in the Fe(III)-MnCO₃ reaction system of 0.24 g/L was much lower than that in the MnCO₃-only reaction system (0.92 g/L). In the MnCO₃-based desulfurization system, the MnS₂O₆ generated was largely attributed to the hydrolysis of Mn(II), which resulted in a small amount of MnO₂ and further generated MnS₂O₆ because of the SO₂ incomplete oxidation. The details, however, need to be explored further. As depicted in Figure 3a,b, the addition of Fe(III) enhanced the oxidation of SO₃²⁻ to SO₄²⁻, thereby reducing the level of SO₃²⁻, the precursor of S₂O₆²⁻ generation, and finally resulted in lower MnS₂O₆ generation. In summary, Figure 3 shows that the MnCO₃-based desulfurization coupled with effective manganese leaching is technically feasible, as evidenced by improved SO₂ removal efficiency, manganese leaching, oxidation rate, and reduced MnS₂O₆ concentration.

3.1.3. Mechanism Analysis

The catalytic oxidation of SO₂ by iron and/or manganese ions in an acid solution has been commonly considered to follow a free radical reaction mechanism [24,25,26,27]. The free radical theory suggests that Mn(II) undergoes a chelation reaction with HSO₃⁻ to form Mn₂HSO₃⁺ and MnHSO₃⁺, which further reacts with dissolved oxygen to form SO₃^•− [33]. According to the solution chemistry theory, SO₂ in the liquid will have existed as HSO₃⁻ in the pH range from 1.89 to 7.20. The precipitation of Fe(III) begins at pH 1.62 and is complete at pH 3.20, while the Mn(II) does not hydrolyze at pH < 6.58. Therefore, a proper pH of the desulfurizing liquid is important to have a relatively higher concentration of HSO₃⁻, and this is consistent with the experimental studies shown in Figure 1 and Figure 2.

The enhancement of SO₂ removal is largely due to the strong oxidizing property of the introduced Fe(III), which could trigger chain catalytic reactions to generate the SO₃^•− free radical under acidic conditions (7) [36,37]. After that, the SO₃^•− reacted with the dissolved O₂ to become SO₅^•−, which oxidized the Mn²⁺ to Mn³⁺ and further accelerated the generation of SO₃^•− (8) and (9). The consumed Fe(III) could be regenerated through the Fe(III) ↔ Fe(II) cycle in the presence of dissolved oxygen (11). Combined with the previous research, therefore, the desulfurization mechanism of Fe(III)-enhanced SO₂ removal by MnCO₃ was proposed, and the main reactions are shown in Equations (7)–(12) [37,38]. The acceleration in desulfurization and the improvement in the oxidation rate of SO₂ observed through the concentration of SO₄²⁻ and SO₂ oxidation rate confirm the deduction of the proposed mechanism for the addition of Fe(III) in the Mn(II)-based reaction system.

$${\mathrm{Fe}}^{3+}+{\mathrm{HSO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}^{2+}+{\mathrm{SO}}_3^{•-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(7)

$${\mathrm{SO}}_3^{•-}+{\mathrm{O}}_2\to {\mathrm{SO}}_5^{•-}$$

(8)

$${\mathrm{Mn}}^{2+}+{\mathrm{SO}}_5^{•-}+{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{3+}+{\mathrm{HSO}}_5^{-}$$

(9)

$${\mathrm{Mn}}^{3+}+{\mathrm{HSO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Mn}}^{2+}+{\mathrm{SO}}_3^{•-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(10)

$${\mathrm{Fe}}^{2+}+{\mathrm{SO}}_5^{•-}+{\mathrm{H}}^{+}\to {\mathrm{Fe}}^{3+}+{\mathrm{HSO}}_5^{-}$$

(11)

$${\mathrm{HSO}}_3^{-}+{\mathrm{HSO}}_5^{-}\to 2{\mathrm{HSO}}_4^{-}$$

(12)

3.2. SO₂ Removal of the Fe(III)-Enhanced MCO-FGD

To provide better guidance on the design of the MCO-FGD, the SO₂ removal performance of the Fe(III)-enhanced MCO-FGD technique was also evaluated. In this section, the simulated electrolytic anode solution was used to prepare the desulfurization solution to reduce the acid used in the practical application. The SO₂ removal efficiency of MCO-FGD at different inlet SO₂ concentrations is shown in Figure 4. With the increase in inlet SO₂ concentration, the SO₂ removal efficiency of the Fe(III)-enhanced MCO-FGD gradually decreased. After 30 min of reaction, as is shown in Figure 4a, the desulfurization efficiencies were 89.09, 76, 50.35, 44.88, and 31.34% for inlet SO₂ concentrations of 0.2%, 0.5%, 1.0%, 1.4%, and 2.0%, respectively, and even after 90 min of reaction, the desulfurization efficiency remained above approximately 20%. However, the desulfurization at 5.0% and 7.0% of inlet SO₂ indicate that MCO-FGD is less effective for the removal of ultra-high-concentration flue gas SO₂. Figure 4b is the manganese leaching corresponding to the desulfurization process. Higher inlet SO₂ concentrations were favorable for manganese leaching in the MCO-FGD process, with an increase from 26.93% to 39.57% after 30 min of reaction when the SO₂ concentration increased from 0.2% to 2.0%. After 90 min of desulfurization, the figures were 30.04% and 51.99%. These observations would help in selecting the inlet SO₂ concentration and retention time of each reaction stage in subsequent multi-stage MCO-FGD designs.

3.3. Technical Study of MCO-FGD

Previous research has demonstrated the feasibility of coupling the MCO-FGD process with effective manganese leaching. However, achieving a balance of SO₂ removal, manganese leaching, and economic feasibility in a single-stage FGD design, particularly under high concentration (>2%) SO₂-containing flue gas, has proved difficult. Based on the assumption that the desulfurized liquid is finally used for the electrolytic manganese production, therefore, a multi-stage tandem counter-current desulfurization approach was adopted, as shown in Scheme 2. To optimize the SO₂ removal, the electrolytic anodic solution (EAS) was used to prepare the desulfurizing liquid, given the known influences of pH on the SO₂ removal process. This design could further reduce pollution emissions from manganese manufacturing while improving the utilization efficiency of manganese.

3.3.1. Fe(III) Promoted Multi-Stage MCO-FGD

To test the feasibility and effectiveness of the proposed technique design, a five-stage tandem counter-current desulfurization experiment was conducted using flue gas SO₂ concentrations ranging from 0.2% to 2.0% (2000, 5000, 10,000, 14,000, and 20,000 ppm), with each stage performing desulfurization for 20, 20, 20, 60, and 60 min, respectively. MnCO₃ slurry was added in the final stage. The MCO-FGD process’s SO₂ removal efficiency and manganese leaching are shown in Figure 5a. The results indicate that the desulfurization process design was reasonable, despite the first- and second-stage outlet concentrations being slightly higher than the designated inlet concentrations. As shown in Figure 5b, after the five-stage desulfurization process was completed, the manganese leaching rate gradually increased to 73.9%.

In order to improve the desulfurization efficiency and achieve higher manganese leaching, the inlet SO₂ of each stage was readjusted to 0.3%, 0.6%, 1.0%, 1.5%, and 2.0% (3000, 6000, 10,000, 15,000, and 20,000 ppm), and the liquid−solid ratio was lowered from 10:1 to 8:1. As is shown in Figure 5c, the outlet concentrations of SO₂ after each desulfurization stage were lower than the inlet concentration of the next stage at the flue gas flow direction, thereby satisfying the design of the continuous desulfurization process. The pH of the desulfurization slurry throughout the desulfurization process was in the range of 2.5–3.0, which falls within the optimal pH range of the Mn(II)-Fe(III) desulfurization system. The outlet SO₂ was less than 200 ppm after the five stages of desulfurization, which can be easily thoroughly cleaned using the traditional FGD method. However, it can be seen that the manganese leaching decreased to 56.2% (Figure 5d), and the Mn²⁺ concentration of 32.1 g/L in the desulfurized liquid failed to meet the requirement for recovering manganese metal. (e.g., the production of electrolytic manganese metal requires a concentration of Mn²⁺ above 40 g/L). This indicates that a relatively low liquid−solid ratio is beneficial to desulfurization but not to manganese leaching.

3.3.2. Mn(II)-Fe(III) Promoted Multi-Stage MCO-FGD

Based on the abovementioned results and previous work [8], coupled with the fact that most of the natural manganese carbonate ore is accompanied by a certain amount of MnO₂, the MnO₂ and Fe(III)-co-promoted MCO-FGD process was carried out. Prior to the multi-stage desulfurization, the function of MnO₂ in the Mn(II)-Fe(III) reaction system was evaluated, and the desulfurization conditions were consistent with Figure 2c. As demonstrated in Figure 6a, the addition of only 1500 mg/L MnO₂ improved the desulfurization efficiency significantly, with the SO₂ removal efficiency after 10 and 30 min of desulfurization reaching 94.0% and 46.8%, whereas it was only 81.1% and 35.0% when without MnO₂, respectively. The pH of the desulfurized liquid decreased gradually from 1.54 to 1.25 (Figure 6b), indicating that sulfuric acid was generated during the desulfurization process. This finding aligns with previous research showing that MnO₂ addition enhances the catalytic activity of the reaction system, thereby accelerating SO₂ oxidation in liquid [39,40]. The reaction process of SO₂ catalytic oxidation of MnO₂ is complicated, and can be briefly described with Equations (13)–(16):

$${\mathrm{MnO}}_2+{\mathrm{O}}_2={\mathrm{MnO}}_2·{\mathrm{O}}_2$$

(13)

$${\mathrm{MnO}}_2·{\mathrm{O}}_2+{\mathrm{HSO}}_3^{-}={\mathrm{MnO}}_2·{\mathrm{O}}_2·{\mathrm{SO}}_3{\mathrm{H}}^{-}$$

(14)

$${\mathrm{MnO}}_2·{\mathrm{O}}_2·{\mathrm{SO}}_3{\mathrm{H}}^{-}={\mathrm{MnO}}_2+{\mathrm{HSO}}_5^{-}$$

(15)

$${\mathrm{HSO}}_5^{-}+{\mathrm{HSO}}_3{}^{-}=2{\mathrm{HSO}}_4^{-}$$

(16)

The MnO₂ and Fe(III)-co-promoted multi-stage desulfurization was a little different, modified to a six-stage process where the flue gas SO₂ of each stage from low to high was 3000, 6000, 10,000, 15,000, 18,000, and 20,000 ppm. In the desulfurization slurry, MnO₂ was added to achieve a 1000 mg/L concentration. The liquid−solid ratio was 10, and the reaction times of each stage were 20, 20, 20, 60, 90, and 90 min. The desulfurization efficiency and Mn leaching rate at all levels in the process are shown in Figure 6c,d. It can be seen that the outlet SO₂ concentration at each stage met the requirement for the next inlet, and the outlet SO₂ of the desulfurization system was only 62 ppm. meanwhile, the pH of the desulfurization liquid was kept between 2.5 and 3.0. Compared with Figure 5, the 1000 mg/L of MnO₂ improved the SO₂ removal performance and manganese leaching significantly. More than 90.0% of manganese was leached after the whole desulfurization process (90.1%), which could ensure that the manganese in the solid slag after desulfurization was less than 3%. To analyze the desulfurized liquid, the Mn(II) concentration reached 42.7 g/L, and the SO₃²⁻ and MnS₂O₆ concentrations were 3.9 g/L and 1.5 g/L, respectively, indicating that it can be used for manganese electrolysis production directly after a standard purification process.

4. Conclusions

In this study, the removal of high concentration flue gas SO₂ through manganese carbonate desulfurization and its resource utilization were studied. Based on the fact that there is symbiosis of manganese and iron in natural manganese ore, small amounts of Fe(III) and MnO₂ were introduced into the MnCO₃ desulfurization system to enhance oxidative desulfurization and suppress MnS₂O₆ by-product generation. The results indicate that the introduction of Fe(III) into the desulfurization system led to the generation of potent oxidant Mn(III) in the reaction system, which accelerated the generation of SO₃^−• radicals and, thus, enhanced the SO₂ oxidation to H₂SO₄ via the chain reaction mechanism. The enhanced oxidation also restrained the MnS₂O₆ generation. A feasibility analysis of the multi-stage desulfurization process revealed that a small amount of MnO₂ can significantly improve the performance of the Fe(III)-added MCO−FGD process. Under the condition of O₂ 10.0 vol.%, liquid−solid ratio 10, [Fe(III)] = 7.0 g/L, MnO₂ = 1000 mg/L, pH = 2.5, and reaction temperature 70 °C, 2.0% of the inlet SO₂ could be removed to 62 ppm, obtaining 90.1% manganese leaching efficiency, and the concentration of MnS₂O₆ in the desulfurized liquid was kept below 2.5 g/L after six-stage desulfurization. The Mn(II) and MnS₂O₆ concentrations in the desulfurized liquid were 42.7 and 1.5 g/L, meeting the requirements of electrolytic manganese metal production. These results are of great importance for the sustainable development of the manganese metallurgical industry, which provides theoretical and technical support for the recycling of sulfur and manganese.