Green Process for the Preparation of MnCO₃ and Recovery of By-Product Mg-Containing (NH₄)₂SO₄ Solution

Abstract

The conventional manganese carbonate preparation process faces challenges such as low resource utilization efficiency and difficulties in treating by-product Mg-containing ammonium sulfate solution. In this study, a two-stage leaching process was developed to efficiently extract Mn and Mg from the ore. NH₄HCO₃ was used as a precipitant to convert Mn²⁺ in the leachate to MnCO₃, achieving a Mn precipitation efficiency of 99.89%, and the resulting product contained 44.45% Mn, meeting the first-class product indicators of HG/T 4203-2011 (Chinese standard on manganese carbonate for industrial use). To further enhance resource utilization, a combined stripping–adsorption process was designed to treat the Mg-containing ammonium sulfate solution generated during the carbonization process. Subsequently, the economically valuable gypsum and magnesium oxide products were prepared. Additionally, 88.20% of the NH₃ in the solution was stripped and recycled to prepare NH₄HCO₃ and then used during carbonization. Finally, a purified solution free of ammonia nitrogen was obtained using 001×7 resin to dynamically adsorb the filtrates obtained during the stripping process, and the maximum adsorption capacity of resin for ammonia nitrogen was 51.14 mg/g. This process provides a novel approach to achieving clean production in the manganese carbonate production industry.

1. Introduction

Manganese carbonate products are integral to various industries, primarily in the electrolytic metal manganese (EMM) industry, and also play significant roles in the steel, battery, electronics, and chemical industries [1,2,3,4]. In recent years, new energy industries have led to the development of manganese-based battery materials, such as LiMn₂O₄ and LiMn_xFe_1–xPO₄ [5]. Consequently, the demand for manganese in the battery industry is expected to rise [6].

Manganese carbonate is typically prepared by the carbonization precipitation of a manganese sulfate solution [7,8]. China’s manganese resources are predominantly composed of low-grade manganese carbonate ores [9,10]. The manganese sulfate solution obtained by leaching contains a high concentration of impurities, especially Mg²⁺. Due to the chemical similarity between Mg²⁺ and Mn²⁺, efficient separation is difficult to achieve [11]. During the process of purifying a manganese sulfate solution, magnesium is typically removed as an impurity, resulting in the waste of magnesium resources [12]. Various purification processes such as chemical precipitation, solvent extraction, and crystallization are employed to remove magnesium [13]. The crystallization of the manganese sulfate solution has the advantage of being simple and easy to operate, but it can only achieve partial separation of manganese and magnesium [14,15]. The high cost of extractants and the potential generation of organic wastewater limit the application of solvent extraction in manganese purification [16]. For chemical precipitation, excess precipitating agents, typically fluoride, are added to the solution to remove magnesium [17]. However, the use of excess fluoride can lead to the presence of excessive F⁻ in the solution, which exhibits strong corrosive properties in acidic solution, further increasing the burden of impurity disposal [18].

The carbonization precipitation method is an effective way to prepare manganese carbonate, with the precipitating agents classified as sodium salts and ammonium salts [19,20]. When sodium salts are used as the precipitating agent, a large amount of low-value sodium sulfate solution is produced, resulting in high treatment costs [21]. In contrast, the ammonium sulfate solution produced when ammonium salts are used has higher economic value, making ammonium salts more suitable for the carbonization process. NH₄HCO₃ can effectively separate manganese and magnesium from the solution, allowing for the preparation of manganese carbonate products [22,23]. This method does not remove magnesium from the solution, thereby avoiding the introduction of difficult-to-handle anions such as F⁻ and creating conditions for the recovery of magnesium resources. The byproduct after precipitation is a high-concentration Mg-containing ammonium sulfate solution.

Currently, the main method for recovering a high-concentration ammonium sulfate solution is evaporation crystallization, which produces ammonium sulfate products [24,25,26,27]. However, due to the presence of Mg in the ammonium sulfate solution after the carbonization process, direct evaporation crystallization will result in the precipitation of both MgSO₄ and (NH₄)₂SO₄, which negatively affects the quality of the ammonium sulfate product [28]. Additionally, the high energy consumption during the evaporation crystallization process limits its application in manganese carbonate production. Therefore, how to use an economical and energy-efficient consumption process to treat a Mg-containing sulfate solution while achieving the recovery and utilization of magnesium and ammonia is an issue worthy of study.

This study explored a green process for recovering Mn and Mg from manganese carbonate ore, focusing on the recovery of a by-product Mg-containing ammonium sulfate solution obtained from the process of manganese carbonate preparation. Mn was effectively leached and recovered from the ore, resulting in the formation of manganese carbonate. Subsequently, the Mg-containing ammonium sulfate solution produced during the carbonization process was efficiently treated using a combined stripping–adsorption process. Magnesium and ammoniacal nitrogen in the solution were recovered. Finally, a preliminary economic evaluation of the proposed process was conducted.

2. Materials and Methods

2.1. Materials

The manganese carbonate ore was obtained from Guangxi, China. It was ground to a mesh of 100 and subsequently used for leaching experiments. The ore contained 13.81% Mn, with the main impurity elements being Ca (19.33%), Si (4.91%), Mg (2.93%), and Fe (0.77%). The X-ray diffraction (XRD) pattern (Figure 1) shows that the main phases of the ore were kutnohorite, calcite, quartz, and gypsum.

2.2. Green Process for Manganese Carbonate Ore Resource Utilization

A green process was developed for the resourceful utilization of manganese and magnesium from manganese carbonate ore and the recovery of the Mg-containing ammonium sulfate solution. This process encompasses the leaching, carbonization, stripping, adsorption, and preparation of gypsum and magnesium oxide, as shown in Figure 2.

2.2.1. Leaching Experiments

For each experiment, 100 g of manganese ore was used. Sulfuric acid and water were added to a beaker containing the ore and the mixture was stirred for a predetermined period. The leaching residues were then filtered from the leachate. The leaching efficiencies of Mn and Mg were studied during the direct and two-stage leaching processes. The leachate was purified and subsequently used in the carbonization process. A detailed description of the purification process is provided in Text S1. The leaching efficiency E was determined using Equation (1).

$$E(\%)=\left(1-{\displaystyle \frac{{\omega }_1{m}_1}{{\omega }_0{m}_0}}\right)\times 100\%$$

(1)

where

E: leaching efficiency of Mn or Mg (%).

${\omega }_0$: mass fraction of Mn or Mg in ore (%).

${\omega }_1$: mass fraction of Mn or Mg in leaching residues (%).

$m_0$: mass of the ore (g).

$m_1$: mass of the leaching residues (g).

2.2.2. Carbonization Experiments

The purified leachate (300 mL) was transferred to a beaker. Next, a 20% NH₄HCO₃ solution was then added to the beaker and stirred for 30 min. Subsequently, the mixture was filtered to separate the filter residue from the filtrate, and the filter residue was the manganese carbonate product.

2.3. Stripping and Adsorption Tests

2.3.1. Stripping Test

Based on the concentrations of Mg and NH₃ in the filtrate from the carbonization process, a simulated solution was prepared using analytical-grade (NH₄)₂SO₄ and MgSO₄·7H₂O. For each experiment, 50 mL of the simulated solution was added to a 100 mL beaker with magnetic stirring, followed by the addition of calcium hydroxide powder for the ammonia-stripping process. The stripping process was carried out in a fume hood. The effects of calcium hydroxide dosage, temperature, and reaction time were investigated. The precipitation efficiency of Mg was determined using Equation (2). Three experiments were carried out under the same conditions.

$$\alpha (\%)={\displaystyle \frac{C_1{V}_1}{C_0{V}_0}}\times 100\%$$

(2)

where

$\alpha$: precipitation efficiency of Mg (%).

$V_0$: volume of solution before stripping (L).

$V_1$: volume of solution after stripping (L).

$C_0$: concentration of Mg in solution before stripping (g/L).

$C_1$: concentration of Mg in solution after stripping (g/L).

The removal efficiency of NH₃ was determined using Equation (3).

$$\mathrm{\epsilon }(\%)=\left(1-{\displaystyle \frac{C_3{V}_1}{C_2{V}_0}}\right)\times 100\%$$

(3)

where

$\mathrm{\epsilon }$: removal efficiency of NH₃ (%).

$V_0$: volume of solution before stripping (L).

$V_1$: volume of solution before stripping (L).

$C_2$: concentration of NH₃ in solution before stripping (g/L).

$C_3$: concentration of NH₃ in solution after stripping (g/L).

2.3.2. Adsorption Test

Batch adsorption experiments were conducted by adding 50 mL of the (NH₄)₂SO₄ solution to a 100 mL conical flask. The adsorption process was performed in a thermostatic oscillator at 25°C with a shaking speed of 120 r/min. The effects of the adsorbent dosage, initial ammonia concentration, and adsorption time were examined. Batch adsorption experiments were performed in triplicate to ensure reproducibility. The recyclability of the resin was investigated under the conditions established in the batch adsorption experiments. The 001×7 resin was added to a 300 mg/L ammonia solution and shaken at 25°C with a speed of 120 r/min for 4 h. Subsequently, the resin was filtered and prepared for the adsorption-desorption cycle. Seven cycles were performed to evaluate the regeneration performance of the resins. The equilibrium adsorption capacity (q_e) and removal efficiency (R) toward NH₃ (ammonia nitrogen) were calculated as follows:

$$q_e\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{\left(C_{\mathrm{i}}-C_{\mathrm{e}}\right)V}{m}}$$

(4)

$$R(\%)=\left(1-{\displaystyle \frac{C_{\mathrm{i}}-C_{\mathrm{e}}}{C_{\mathrm{i}}}}\right)\times 100\%$$

(5)

where

$q_{\mathrm{e}}$: equilibrium adsorption capacity toward ammonia nitrogen (mg/g).

R: removal efficiency toward ammonia nitrogen (%).

$V_1$: volume of solution before adsorption (L).

$m$: mass of the 001×7 resin (g).

$C_{\mathrm{i}}$: initial concentration of ammonia nitrogen (mg/L).

$C_{\mathrm{e}}$: equilibrium concentration of ammonia nitrogen (mg/L).

2.4. Analytical Techniques

The elemental composition of manganese carbonate ore was analyzed using the following methods: Mn content was analyzed according to China standard GB/T 1506-2016 [29]; Ca and Mg contents were analyzed according to China standard GB/T 26416.3-2022 [30]; Si content was measured according to China standard GB/T 6730.10-2014 [31]; Fe content was analyzed according to China standard GB/T 6730.73-2024 [32]. The concentrations of Mn, Mg, and other metal ions in the solution were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES; Optima5300DV, Perkin Elmer, Waltham, MA, USA). The NH3 concentration was measured using Nessler’s reagent spectrophotometric method according to the Chinese environmental protection standards (HJ 535-2009 [33]). XRD (UltraX TTR III, Rigaku Company, Akishima, Japan) was used to study the material composition of the manganese carbonate ores. A microscope was used to acquire scanning electron microscope (SEM; JEOL/JSM-7610FPlus, Tokyo, Japan) images. The microdomain composition was measured using energy-dispersive X-ray spectroscopy (EDS; JEOL/JSM-7610FPlus, JEOL, Tokyo, Japan). The Fourier-transform infrared (FT-IR) spectra of the 001×7 resin were recorded before and after adsorption using an FT-IR spectrometer (Nicolet 6700, Thermo Electron Scientific Instruments, Waltham, MA, USA) in the range of 4000 to 500 cm⁻¹.

3. Results and Discussion

3.1. Preparation of Manganese Carbonate

In the manganese industry, the direct leaching process is employed to extract Mn from manganese carbonate ores [34]. However, there are issues with low manganese leaching efficiency and incomplete acid utilization, leading to acid waste [35]. This paper proposes a two-stage leaching process to address these issues. The reaction conditions and cycling process for both leaching processes are presented in Text S2 and Figure S1a. Compared with the direct leaching process, the Mn leaching efficiency increased from 94.11% to 97.49%, the acid consumption (in terms of acid/ore ratio) used in the leaching process decreased from 0.9 to 0.828 g/g, and the pH of the leachate increased from 0.18 to 5.60. This process achieved higher Mn and Mg leaching efficiencies and lower acid consumption. Additionally, the weak acidity of the leachate effectively reduced the consumption of neutralizing agents during the subsequent pH adjustment process. The XRD patterns of the ore before and after leaching are shown in Figure S1b. The leaching residue was mainly composed of gypsum and quartz with no observable Mn- and Mg-related components.

The leachate was neutralized and purified, resulting in a solution primarily consisting of MnSO₄ and MgSO₄. NH₄HCO₃ was used as the precipitant to separate Mn and Mg. The mechanism of separation was as follows: The solubility product constant (K_sp) of MgCO₃ is 10^−5.17, which is significantly higher than that of MnCO₃ (10^−10.63). When NH₄HCO₃ dissolves, the CO₃²⁻ in the solution is mainly generated through the ionization of HCO₃⁻. During this process, the concentration of CO₃²⁻ remains low, which allows it to preferentially react with Mn²⁺ in the solution to form MnCO₃. The reaction conditions were as follows: the pH of the solution was 6.4, the molar ratio of ammonium bicarbonate to Mn²⁺ was 2.2:1, the reaction time was 30 min, and the reaction temperature was 30°C. The Mn precipitation efficiency was 99.89% under these conditions. The Mn content of the resulting manganese carbonate was 44.45%, whereas the Mg content was 0.53%, meeting the first-class product indicators of HG/T 4203-2011 (Chinese standard on manganese carbonate for industrial use). The XRD patterns and SEM images of the products were shown in Figure 3. The results indicated that the products were manganese carbonate and the particles were spherical. Owing to its low Mg content, the product can serve as a Mn source for preparing EMM, reducing the rate of Mg accumulation in the electrolyte, and thereby improving the electrolytic efficiency [36].

The use of NH₄HCO₃ as the precipitant enabled the efficient separation of Mn and Mg. However, the filtrate obtained from the carbonization process primarily consisted of ammonium sulfate and magnesium sulfate, referred to as the Mg-containing ammonium sulfate solution. The concentrations of Mg and NH₃ were 4.51 and 15.64 g/L, respectively. Therefore, further treatment is required to recover valuable components and prevent environmental contamination.

3.2. Treatment of Mg-Containing Ammonium Sulfate Solution

Ammonia mainly exists in two forms in the solution: free NH₃ and NH₄⁺. Due to its low Henry’s law constant, free NH₃ readily transfers from the liquid phase to the gas phase [37]. Therefore, the stripping process is commonly employed to recover ammonia nitrogen from ammonium sulfate solutions [38,39,40]. The efficiency of ammonia nitrogen removal during stripping is largely influenced by factors such as pH, temperature, and gas flow rate [39,41]. The stripping process typically operates under strongly alkaline conditions, as a high pH enhances the thermodynamic driving force for ammonia stripping [42]. Additionally, Mg²⁺ readily forms Mg(OH)₂ precipitates under alkaline conditions. Given these properties, at a high pH, ammonia is easily stripped, while magnesium tends to precipitate. Thus, the stripping process presents a promising approach for treating Mg-containing ammonium sulfate solutions, enabling the separate recovery of both magnesium and ammonia. In alkaline conditions, NH₄⁺ in the solution is converted to NH₃·H₂O and released as NH₃, achieving ammonia recovered by air stripping. Magnesium can be fully precipitated from the solution and further processed into magnesium products.

3.2.1. Stripping of Mg-Containing Ammonium Sulfate Solution

To increase the pH, the stripping process typically requires the addition of alkaline chemicals, with sodium hydroxide being the most commonly used. However, the addition of sodium hydroxide in this sulfate system introduces a large amount of Na⁺ into the solution, leading to the stripped solution mainly consisting of Na₂SO₄, which requires high costs for treatment. In the hydrometallurgical industry, calcium-based oxides, primarily calcium oxide and calcium hydroxide, are commonly used to neutralize acidic leachate. For sulfate systems, adding calcium oxide to the solution mainly results in the formation of a CaSO₄ precipitate. The slightly soluble nature of CaSO₄ and calcium-based oxides in the solution ensures that the stripping process will not introduce excessive Ca²⁺ into the solution, minimizing the burden of impurity removal. Among calcium-based oxides, calcium hydroxide exhibits stronger alkalinity than calcium oxide, making it more effective for increasing the solution’s pH. Additionally, calcium hydroxide is relatively inexpensive, further enhancing its suitability for the stripping process. Therefore, calcium hydroxide was selected as the preferred reagent for pH adjustment in this study.

The stripping experiment was carried out in a fume hood. During the stirring of the solution, NH₃ transferred from the liquid surface to the surrounding air and was subsequently removed through the fume hood. Notably, no gas was injected into the solution during this experiment. The effects of calcium hydroxide dosage, reaction temperature, and reaction time on the NH₃ removal efficiency and Mg precipitation efficiency were systematically investigated. The results indicated that both NH₃ removal efficiency and Mg precipitation efficiency increased with a higher dosage of calcium hydroxide (Figure 4a). The effect of dosage on the pH of the solution was examined, and the results are provided in Figure S3. An increase in calcium hydroxide dosage raised the solution pH. At a high pH, the fraction of free NH₃ in the solution became higher, making it easier for ammonia to be removed from the solution [37]. Although reaction temperature had no significant impact on Mg precipitation efficiency, it enhanced NH₃ removal efficiency (Figure 4b). An increase in temperature can raise the molecular kinetic energy, further promoting molecular diffusion and enhancing the removal of NH₃. However, high temperatures could promote water evaporation, which was unfavorable for the subsequent recovery of ammonia. The Mg precipitation was faster, reaching 99.98% within 20 min, whereas the NH₃ removal was comparatively slower, reaching 97.96% after 240 min (Figure 4c). Initially, the NH₃ concentration in the solution was higher than the gas phase, leading to a large concentration gradient and faster ammonia transfer. However, as the stripping process progressed, the NH₃ concentration in the solution decreased and the gas phase concentration increased, reducing the concentration gradient and slowing NH₃ removal.

Based on these findings, the optimal reaction conditions were determined as follows: calcium hydroxide dosage of 1.2, reaction temperature of 30 °C, and reaction time of 240 min. Under these conditions, 400 mL of actual Mg-containing ammonium sulfate solution was treated. The NH₃ removal efficiency and Mg precipitation efficiency were 88.20% and 99.99%, respectively, and the concentrations of NH₃ and Mg in the filtrate were 2.05 g/L and 0.0017 g/L, respectively. The decrease in NH₃ removal efficiency is primarily due to the larger volume and higher total ammonia nitrogen content in the actual Mg-containing ammonium sulfate solution. In the fume hood, due to no air being injected into the solution during the stripping process, ammonia removal relies on natural transfer at the liquid–air interface, which limits contact between ammonia and air, resulting in reduced removal efficiency compared to the smaller-volume simulated solution. According to the experimental results, the stripping process can effectively recover ammonia nitrogen from Mg-containing ammonium sulfate solutions, achieving the separation of magnesium and ammonia. The stripped NH₃ can be absorbed by a carbon dioxide solution to produce ammonium bicarbonate, which can be returned to the carbonization process to recycle the ammonia. The CO₂ originates from the leaching process, contributing to a reduction in carbon emissions. Although the stripping process recovers most of the ammonia, a portion of ammonia nitrogen remains in the solution and must be removed to prevent potential environmental issues.

3.2.2. Resource Utilization of Magnesium

To prevent the waste of magnesium resources, the filter residue obtained from the stripping process was transformed into gypsum (CaSO₄·2H₂O) and magnesium oxide (MgO) products through the steps shown in Figure 2. The specific reaction processes and conditions are listed in Text S3. The CaSO₄·2H₂O, Mg, and Mn content of the gypsum product was 99.36%, 0.014%, and 0.057%, respectively, which meets the premium-grade product indicators of GB/T 5483-2008 (Chinese standard on natural gypsum). The MgO, Ca, and Mn content in the magnesium oxide products was 98.87%, 0.2%, and <0.01%, respectively, which meets the first-class product indicators of HG/T 2573-2012 (Chinese standard on light magnesium oxide for industrial use). The XRD patterns of the products are presented in Figure 5. This process not only converts the precipitated magnesium into magnesium oxide products but also utilizes the calcium hydroxide added during the ammonia-stripping process to produce economically valuable gypsum products. This approach achieves the resource utilization of both magnesium and calcium, thereby further enhancing the economic value of the process.

3.3. Adsorption Experiments

For the removal of residual ammonia nitrogen from the stripped solution, the adsorption method offers advantages such as effectiveness and simplicity [43,44]. Several studies have investigated the application of adsorption to treat ammonia nitrogen in wastewater [45,46,47,48]. While these methods effectively remove ammonia nitrogen from wastewater, they have disadvantages, such as the high cost of adsorbent preparation, poor recycling performance, and low adsorption capacity, which limit their industrial application.

3.3.1. Batch Adsorption Experiment

Among the commercially available acidic cation exchange resins, 001×7 resin is the least expensive and, as a widely used material, has stable physical and chemical properties. Therefore, it was selected as the adsorbent for treating the residual ammonia nitrogen in the solution after stripping.

As shown in Figure 6a, the effect of adsorbent dosage on the adsorption of ammonia nitrogen was investigated. The removal efficiency gradually increased with increasing resin dosage owing to the availability of numerous adsorption sites. However, when the resin dosage was too high, the adsorption sites on the resin surface became underutilized, leading to a reduction in the resin adsorption capacity and resin waste. Therefore, the subsequent resin dosage was set to 10 g/L.

The effects of varying the initial ammonia nitrogen concentration from 100 mg/L to 700 mg/L on removal efficiency and adsorption capacity were investigated (Figure 6b). As the initial ammonia concentration increased, the adsorption capacity and removal efficiency exhibited opposite trend. A higher ammonia nitrogen concentration intensified the concentration gradient between the liquid phase and the resin, thereby enhancing the driving force for ammonia nitrogen transfer to the resin. This facilitated the transfer of ammonia nitrogen into the resin, resulting in an increase in the adsorption capacity.

As depicted in Figure 6c, the effects of adsorption time on ammonia nitrogen removal and adsorption capacity were investigated. At 30 min, the removal efficiency reached 74%, while the adsorption capacity reached 88% of its maximum value. As a strongly acidic cation exchange resin, 001×7 resin possesses a more negative surface charge, which enhances its electrostatic attraction to NH₄⁺ in the solution. After 30 min, the adsorption sites on the resin were progressively occupied by ammonia nitrogen, leading to a gradual decrease in the adsorption efficiency. Eventually, the adsorption sites became saturated and equilibrium was reached.

3.3.2. Adsorption Kinetic and Isotherm Study

To investigate the kinetics of ammonia nitrogen adsorption by the 001×7 resin, we utilized the pseudo-first-order (PFO) and pseudo-second-order (PSO) models for fitting purposes [49]. The results are shown in Figure 7a,b and

Table 1. Kinetic and isotherm parameters of ammonia nitrogen adsorption by the 001×7 resin.

Model	Parameter	Value	Model	Parameter	Value
Pseudo-first-order	qe	24.38	Langmuir	qm (mg/g)	51.14
	k1	0.6147		KL (L/mg)	0.0192
	R2	0.9285		R2	0.9598
Pseudo-second-order	qe	25.63	Freundlich	KF (L/g)	5.1930
	k2	0.0081		1/n	0.3899
	R2	0.9998		R2	0.9956

. The PFO model is typically associated with physical adsorption, whereas the PSO model is related to ion exchange and chemical adsorption [50]. Karadag et al. [51] fit the adsorption process of ammonia nitrogen on zeolite using the PSO model; the R² of the model was >0.999, indicating that the adsorption process was mainly ion-exchange-dominated chemical adsorption. In this study, the R² values of the PSO and PFO models were 0.9998 and 0.9285, respectively, indicating that the PSO model fit the adsorption process better. Therefore, the adsorption of ammonia nitrogen by the 001×7 resin is considered to be chemical, mainly via ion exchange.

The Langmuir and Freundlich isotherms were applied to analyze the adsorption behavior of ammonia nitrogen onto the 001×7 resin, with the results being shown in Figure 7c,d and Table 1. The Freundlich model had a higher R² than the Langmuir model, and thus could better describe the adsorption process. The results demonstrated that the adsorption of ammonia nitrogen by the 001×7 resin was not simple monolayer adsorption but might have involved multiple adsorption layers, and the active adsorption sites on the resin surface were not uniform. Additionally, the 1/n value in the Freundlich isotherm was 0.3899 (0 < 1/n < 1), indicating that the adsorption of ammonia nitrogen by the 001×7 resin was favorable [52].

3.3.3. Adsorption Mechanism

FT-IR analysis was performed on the 001×7 resin before and after adsorption to investigate the ammonia nitrogen adsorption mechanism. (Figure 8). Two new peaks appeared at 3197 cm⁻¹ and 3062 cm⁻¹ in the FT-IR spectrum of the resin after adsorption (Figure 9), which correspond to the antisymmetric and symmetric telescoping vibrations of NH₄⁺ [53], respectively. Peaks at 1160 cm⁻¹ and 1035 cm⁻¹ were categorized as antisymmetric and symmetric stretching of the sulfonic acid group (-SO₂-OH) [54], respectively; the intensity and position of these two peaks changed after adsorption, suggesting that the structure of the sulfonic acid group may have changed. Combined with the newly generated NH₄⁺ peaks at 3197 cm⁻¹ and 3062 cm⁻¹, this suggests that the H⁺ of the sulfonic acid group underwent an ion-exchange reaction with free NH₄⁺ in solution.

In addition, we recorded the pH of the solution before and after adsorption and observed that it decreased from 6.5 to 1.5 after adsorption. This phenomenon indicates that NH₄⁺ displaced the H⁺ on the sulfonic acid group into the solution during adsorption, resulting in a decrease in pH. To visualize this process, we used methyl red as an indicator and observed that the solution changed from orange to red after adsorption. This phenomenon visually demonstrated the adsorption of ammonia nitrogen by the resin.

3.3.4. Regeneration

Seven consecutive adsorption–desorption cycle experiments were performed with 5% sulfuric acid as the eluent (Figure 9a). The results demonstrated that the ammonia nitrogen removal efficiency of the resin decreased from 83.88% to 79.08% during the second cycle. This decline in efficiency could be attributed to the presence of irreversible ammonia nitrogen adsorption sites within the resin. After several cycles, the ammonia nitrogen removal efficiency remained above 78%, retaining 78.62% by the seventh cycle, indicating good cycling performance. Its ability to maintain a high ammonia nitrogen removal efficiency over multiple cycles makes it a promising option for industrial applications.

3.4. Adsorption Experiment of an Actual Ammonia Solution

Before adsorption, the Ca dissolved in the filtrate obtained from the stripping process must be removed. The adsorption of Ca on the resin can lead to the formation of CaSO₄ precipitates during the resin elution process, which can plug holes and affect the recycling performance of the resin. The CO₂ generated during the leaching process can be used to remove Ca [55], which helps to reduce carbon emissions. The filtrate was injected with CO₂ until the pH approached 7.0, followed by sedimentation and filtration. Subsequently, the concentration of Ca in the solution was reduced to less than 0.05 g/L, which had little impact on the subsequent adsorption process. The XRD pattern of the generated calcium carbonate precipitate is shown in Figure S4.

After calcium removal, an adsorption column (the diameter is 5.0 cm, and the volume capacity is 25 mL) was packed with 7.5 g of 001×7 resin. Synchronously, the wastewater was delivered by down-flow to an adsorbing column using a peristaltic pump with a flow rate of 2.0 mL/min. The dynamic adsorption curves are shown in Figure 9b. The ammonia nitrogen in the solution was completely removed, and the concentrations of ammonia nitrogen in the solution before the breakthrough were below the detection limit. The resultant acidic wastewater, devoid of ammonia nitrogen, could be recycled in a leaching process or neutralized if it cannot be recycled. The results indicated that 001×7 resin had an excellent adsorption performance on ammonia nitrogen, further highlighting its potential for application in industrial ammonia-containing wastewater treatment and other related fields.

3.5. Preliminary Material Balance and Economic Evaluation

Preliminary material accounting and economic evaluation were performed under optimal conditions (Figure 10 and Table S1). The results revealed that processing 1 ton of manganese ore yielded 0.9333 t of ammonia-free manganese residue, 0.3005 t of manganese carbonate, 0.7475 t of gypsum, and 0.0355 t of magnesium oxide. An economic evaluation indicates a profit of USD 152.08 per ton of processed manganese ore. It is important to note that this economic analysis considers only the prices of the raw materials and final products. Actual production also involves other costs, such as investment in equipment, equipment maintenance, labor, storage, and sales.

The proposed treatment method for Mg-containing ammonium sulfate solution is ingeniously integrated with the manganese carbonate preparation process. Stripped NH₃ reacts with CO₂ generated during the leaching process to produce NH₄HCO₃, which is subsequently recycled in the carbonization process, significantly minimizing the cost of reagents for the process and reducing carbon emissions. The Mg²⁺ in the Mg-containing ammonium sulfate solution and the calcium hydroxide used in stripping is converted into magnesium oxide and gypsum, respectively, achieving the resource utilization of calcium and magnesium. Finally, the remaining ammonia-containing solution is treated by adsorption, resulting in an ammonia-free solution that can be recycled for the leaching process. The proposed process offers advantages such as being economical, mild reaction conditions, and high resource utilization efficiency. These inherent advantages make it highly applicable to industrial manganese carbonate production and contribute to the sustainable development of the manganese industry.

4. Conclusions

A green process was developed for recovering Mn and Mg from manganese carbonate ore, focusing on the resource utilization of both elements and the treatment of a Mg-containing ammonium sulfate solution. Based on these investigations, the following conclusions were drawn.

(1)

In the manganese carbonate preparation process, Mn and Mg in the ore were efficiently extracted via a two-stage leaching process. Compared to the direct leaching process, the leaching efficiencies of Mn and Mg increased from 94.11% to 97.49% and from 85.89% to 91.05%, respectively. The sulfuric acid from the leaching process was fully utilized, and the pH of the leachate was 5.60. The resulting manganese carbonate (Mn: 44.45%) obtained from the carbonization process met the first-class product indicators of HG/T 4203-2011 (Chinese manganese carbonate for industrial use).

(2)

A stripping–adsorption process for a Mg-containing ammonium sulfate solution was proposed. Under the optimized stripping conditions, a total of 88.20% of the ammonia nitrogen in the solution was stripped, and the stripped NH₃ was recovered to prepare ammonium bicarbonate. A total of 99.99% of the Mg was precipitated and converted into magnesium oxide, while the calcium hydroxide used in the stripping process was transformed into gypsum products.

(3)

The 001×7 resin demonstrated excellent ammonia adsorption performance, with a maximum capacity of 51.14 mg/g. The dynamic adsorption experiment with the actual ammonia-containing solution after stripping indicates that the remaining ammonia nitrogen in the solution was completely removed by adsorption.

(4)

An economic analysis shows that this process is economically feasible. This process offers environmental and economic advantages and serves as a reference for process innovation in the manganese industry.