High-Pressure Carbonation of Phosphogypsum for Calcium Carbonate Preparation and Crystal Modification Regulation

Abstract

Phosphogypsum (PG) was used as a calcium source for preparing calcium carbonate (CaCO₃) through NH₄Cl leaching followed by high-pressure carbonation. The effects of NH₄Cl concentration, liquid-to-solid mass ratio, temperature, and leaching time on Ca²⁺ extraction were investigated, and the effects of CO₂ pressure, carbonation time, and NH₃·H₂O dosage on Ca²⁺ conversion were evaluated. The optimal conditions were an NH₄Cl concentration of 1.5 mol/L, a liquid-to-solid mass ratio of 60:1, a leaching temperature of 25 °C, a leaching time of 60 min, a CO₂ pressure of 1 MPa, a carbonation time of 10 min, and 12 vol% NH₃·H₂O addition. Under these conditions, the Ca²⁺ leaching rate and conversion rate reached 81.25% and 97.36%, respectively. The product obtained without organic additives was mainly spherical vaterite with partial particle agglomeration. Based on the optimized process, aspartic acid, glutamic acid, ethanol, and glycerol were introduced to regulate CaCO₃ crystallization. Appropriate additive dosages further improved Ca²⁺ conversion, promoted calcite as the dominant polymorph, and produced well-dispersed spherical CaCO₃ particles. Among the tested additives, glutamic acid and glycerol showed the strongest effects on crystal morphology regulation.

1. Introduction

Phosphogypsum (PG), a solid waste by-product generated by the wet-process phosphoric acid industry, is produced in large quantities annually in Yichang, Province, China, a major phosphate-resource region in the Yangtze River Basin [1,2]. In the context of China’s “Dual Carbon” goals, namely carbon peaking and carbon neutrality, the conversion of PG into calcium carbonate (CaCO₃) through carbonation offers a promising strategy for both CO₂ utilization and solid-waste valorization [3,4]. However, the practical utilization of PG for CaCO₃ preparation depends largely on developing a simple, low-cost, and efficient route for extracting calcium from the gypsum phase.

CaCO₃ exists in three major polymorphs, among which calcite is the thermodynamically stable phase under ambient conditions [5,6]. Carbonation processes are generally classified into direct and indirect routes. Direct carbonation is suitable for calcium-rich systems containing a high content of free calcium oxide (f-CaO). Because calcium in PG mainly exists as calcium sulfate, an indirect carbonation route was adopted in this study to facilitate calcium extraction and subsequent control of CaCO₃ precipitation. It should be noted that direct carbonation of calcium sulfate has also been reported to be feasible, even at room temperature [7]. Previous studies [8,9,10,11] have commonly employed strong acid or alkali leaching for calcium recovery; however, these approaches may cause severe equipment corrosion and co-dissolution of impurities. In comparison, NH₄Cl is a low-cost and widely available salt leaching agent. During NH₄Cl leaching, CaSO₄·2H₂O, the dominant gypsum phase in PG, partially dissolves and releases Ca²⁺ into the leachate [12]. Qiao et al. and Chen et al. [13,14] independently used NH₄Cl solutions to leach PG and subsequently prepared CaCO₃ by bubbling CO₂ into the Ca²⁺-rich leachate, achieving Ca²⁺ conversion rates of approximately 80%. The resulting products were predominantly vaterite, characterized by spherical particles and fine, loose CaCO₃ aggregates on their surfaces, with noticeable agglomeration. After Ca²⁺ leaching in the indirect process, the subsequent carbonation stage using conventional CO₂ bubbling is limited by the low solubility of CO₂ and slow gas–liquid mass transfer, resulting in slow reaction kinetics, prolonged carbonation times, and relatively low Ca²⁺ conversion efficiencies. High-pressure carbonation can effectively alleviate these limitations. López-Periago et al. [15] carbonated a Ca(OH)₂ suspension under ultrasonic stirring at 40 °C and 13 MPa for 10 min, achieving a Ca²⁺ conversion rate of 88%. The obtained CaCO₃ was calcite with a rhombic morphology formed through polycrystalline aggregation.

Previous studies have mainly focused on optimizing leaching and carbonation conditions and have shown that CaCO₃ polymorphs and morphologies can be regulated either by adjusting carbonation parameters or by introducing additives. Nevertheless, the regulation of CaCO₃ polymorphs and morphologies in PG-derived calcium leachate systems, particularly under high-pressure carbonation conditions, remains insufficiently understood. Seo et al. [16] investigated the effect of ethanol concentration on CaCO₃ formation by adding different amounts of ethanol to a Ca(OH)₂ solution prior to CO₂ bubbling. They found that increasing ethanol concentration gradually decreased the average particle size of the resulting CaCO₃ and shifted the dominant polymorph from calcite to vaterite. Jin et al. [17] conducted carbonation in a Ca(OH)₂–ethanol system at 40 °C, 10 MPa, and 600 rpm for 2 h. The products were mainly vaterite, appearing as porous microspheres with uniform morphologies and no obvious agglomeration. Konopacka-Łyskawa et al. [18] performed CO₂ bubbling carbonation in a Ca(OH)₂ system containing 0–20% ethanol or glycerol at 20 °C and 900 rpm. Pure calcite was obtained under all tested conditions. Without additives, the CaCO₃ particles had an average size of 9 µm. As the organic solvent concentration increased, the solution viscosity increased, leading to a gradual reduction in particle size; in particular, the 20% glycerol system produced particles with a size distribution of 0.1–0.59 µm. Luo et al. [19] introduced aspartic acid solutions at different concentrations into Ca(OH)₂ suspensions and prepared CaCO₃ by CO₂ carbonation, obtaining products dominated by vaterite. Yang et al. [20] prepared CaCO₃ by mixing Ca(OH)₂ powder with 0–10% aspartic acid or glutamic acid solutions, followed by CO₂ aeration at 20 °C for 2 h. In the absence of amino acids, pure calcite with a particle size of 1–2 µm was obtained. Aspartic acid and glutamic acid can selectively adsorb onto specific crystal faces of thermodynamically stable calcite [20,21], thereby inhibiting its preferential growth. As a result, the dominant polymorph shifted to vaterite after amino acid addition, and the particle size decreased. These studies indicate that additives can significantly affect CaCO₃ crystallization. Alcohols can regulate solution viscosity, stabilize the calcite phase, and control particle size [15,16,17]. In contrast, amino acids can coordinate with Ca²⁺ through their carboxyl groups during the prenucleation stage, thereby affecting the formation and stability of calcium-containing precursor species. In addition, their selective adsorption on specific calcite facets can inhibit the preferential growth of calcite, which may favor the retention of metastable vaterite. However, most of these conclusions were derived from atmospheric-pressure bubbling carbonation experiments using Ca(OH)₂ as the calcium source [18,19]. This system differs substantially from the PG-derived calcium leachate system under high-pressure carbonation used in the present study. Therefore, the corresponding polymorph and morphology regulation mechanisms require further clarification.

NH₄Cl was selected as the leaching agent because of its low cost, wide availability, and mild leaching conditions. During NH₄Cl leaching, CaSO₄·2H₂O in PG partially dissolves, releasing Ca²⁺ and SO₄²⁻ into the NH₄Cl-containing leachate. The filter cake mainly consists of undissolved PG residues and less soluble impurity phases, while minor amounts of soluble impurity species may also enter the leachate. Therefore, the main objective of this study was to develop a simple and cost-effective route for extracting Ca²⁺ from PG and converting it into CaCO₃ through high-pressure carbonation. First, the effects of the liquid-to-solid mass ratio, NH₄Cl concentration, reaction temperature, and leaching time on the Ca²⁺ leaching rate were investigated. Subsequently, the effects of CO₂ pressure, carbonation time, and NH₃·H₂O addition on the Ca²⁺ conversion rate were examined to optimize the carbonation process. Finally, aspartic acid, glutamic acid, ethanol, and glycerol were introduced to evaluate their effects on the Ca²⁺ conversion rate, CaCO₃ polymorphs, and microscopic morphologies.

2. Materials and Methods

2.1. Materials

PG samples were collected from a phosphogypsum stockpile in Yichang, Hubei Province, China, and appeared as a grayish-brown solid. Before use, the samples were dried at 100 °C to constant mass, pulverized using a ball mill, and passed through a 0.15 mm sieve to obtain the pretreated feedstock.

The chemical composition, microstructure, and X-ray diffraction (XRD) pattern of the raw PG are shown in

Table 1. Chemical composition of raw PG (wt%).

SO3	CaO	SiO2	A12O3	Fe2O3	K2O	CuO	P2O5	F	Loss
50.22	35.31	10.94	0.57	0.68	0.56	0.02	0.59	0.87	0.24

, Figure 1, and Figure 2, respectively. All chemical reagents used in this study were of analytical grade unless otherwise stated. Ammonium chloride (NH₄Cl, analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), ammonia solution (NH₃·H₂O, concentration or purity, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), ethanol (C₂H₅OH, analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and glycerol (C₃H₈O₃, analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used. Aspartic acid (C₄H₇NO₄, analytical grade, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) and glutamic acid (C₅H₉NO₄, analytical grade, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) were also used.

2.2. Preparation of CaCO₃

Leaching of Ca²⁺ from PG

Using a one-factor approach, 5 g of PG was added to NH₄Cl solutions with different concentrations of 0.1–2.0 mol/L to prepare suspensions with liquid-to-solid mass ratios of 10:1–60:1. The suspensions were reacted at different temperatures of 25–85 °C for 10–120 min. After leaching, vacuum filtration was performed for solid–liquid separation. The filtrate was collected as the Ca²⁺-rich leachate, while the filter cake mainly consisted of undissolved PG residues and less soluble impurity phases.

Before carbonation, NH₃·H₂O solution was added to the Ca²⁺-rich leachate at different dosages of 4–16 vol%. The mixture was then transferred into a high-pressure reactor for carbonation. The carbonation reaction was conducted at 25 °C using CO₂ with a purity of 99 vol% under pressures of 0.1–2.0 MPa for 4–15 min. After the reaction, the slurry was allowed to stand at room temperature for 2 h. The CaCO₃ solids were then separated using a 0.1 µm membrane filter, thoroughly washed with deionized water, and dried at 80 °C to constant mass.

Aspartic acid, glutamic acid, ethanol, and glycerol were separately introduced into the Ca²⁺-rich leachate. The mixtures were carbonated for 10 min under the optimized conditions of 12 vol% NH₃·H₂O addition, 25 °C, and a CO₂ pressure of 1.0 MPa. After carbonation, the slurry was filtered using a 0.1 µm membrane filter to collect the CaCO₃ solids. The collected solids were thoroughly washed with deionized water and dried at 80 °C to constant mass.

2.3. Experimental Methods and Characterization

2.3.1. Determination of Ca²⁺ Leaching Rate

The Ca²⁺ leaching rate reported in this study represents the experimentally measured apparent recovery of calcium from PG, rather than a theoretical equilibrium solubility value. First, 25 mL of the Ca²⁺-rich leachate was diluted tenfold with deionized water. Then, 25 mL of the diluted solution was collected, and the Ca²⁺ concentration was determined by EDTA titration. The amount of Ca²⁺ in the leachate, denoted as n₂, was calculated from the volume of EDTA consumed. Finally, the Ca²⁺ leaching rate, η, was calculated using Equation (1):

$$\eta ={\displaystyle \frac{n_2}{n_1}}\times 100\%$$

(1)

where n₁ is the amount of Ca²⁺ in CaSO₄·2H₂O in PG (mol), and n₂ is the amount of Ca²⁺ in the leaching solution (mol).

2.3.2. Determination of Ca²⁺ Conversion Rate

After carbonation, the slurry was allowed to stand for 2 h and then filtered through a 0.05 µm membrane filter. The residual Ca²⁺ concentration in the filtrate was determined by EDTA titration, and the amount of residual Ca²⁺, denoted as n₃, was calculated from the volume of EDTA consumed. The Ca²⁺ conversion rate, α, was then calculated using Equation (2):

$$\alpha ={\displaystyle \frac{n_2-n_3}{n_2}}\times 100\%$$

(2)

where n₂ is the amount of Ca²⁺ in the leaching solution (mol), and n₃ is the amount of Ca²⁺ in the filtrate after carbonation (mol).

2.3.3. Characterization Methods

The composition of PG and CaCO₃ was qualitatively identified by X-Ray diffraction (XRD) using a SmartLab (9 kW) diffractometer (Rigaku Corporation, Tokyo, Japan). The morphology of PG and CaCO₃ was observed using a 3200A scanning electron microscope (SEM) (Gwqnt Quantum, Hefei, China) operated at an accelerating voltage of 0.2–30 kV.

3. Results and Discussion

3.1. Process for CaCO₃ Preparation via High-Pressure Carbonation of Phosphogypsum

3.1.1. Leaching Rate of Ca²⁺ in PG

Figure 3 shows the effects of the liquid-to-solid mass ratio, NH₄Cl concentration, reaction temperature, and reaction time on the Ca²⁺ leaching rate.

After reaction with 0.5 mol/L NH₄Cl at 25 °C for 60 min, the Ca²⁺ leaching rate increased markedly with increasing liquid-to-solid mass ratio. The maximum leaching rate of 69.5% was obtained at a liquid-to-solid mass ratio of 60:1. NH₄Cl can hydrolyze in water to release H⁺, thereby lowering the solution pH [22]. As the liquid-to-solid mass ratio increased, the amount of NH₄Cl solution available per unit mass of PG increased, which provided more favorable conditions for the dissolution of CaSO₄·2H₂O. In addition, the increased solution volume reduced the accumulation of dissolved Ca²⁺ and SO₄²⁻ near the solid surface, thereby promoting mass transfer and facilitating further dissolution of the gypsum phase. These effects contributed to the increase in the Ca²⁺ leaching rate.

At 25 °C, with a liquid-to-solid mass ratio of 60:1 and a reaction time of 60 min, the Ca²⁺ leaching rate first increased significantly and then decreased slightly with increasing NH₄Cl concentration. The maximum leaching rate of 81.25% was achieved at an NH₄Cl concentration of 1.5 mol/L. With increasing NH₄Cl concentration, the ionic strength of the leaching solution increased, and ion interactions such as Ca²⁺–Cl⁻ association became more pronounced. These effects can reduce the effective concentration of free Ca²⁺ in solution and shift the dissolution equilibrium of CaSO₄·2H₂O toward further dissolution, thereby increasing the Ca²⁺ leaching rate. However, when the NH₄Cl concentration was further increased to 2.0 mol/L, the excessive ion concentration increased the solution viscosity [22], which hindered the diffusion of Ca²⁺ and SO₄²⁻ and led to a slight decrease in the Ca²⁺ leaching rate.

At a liquid-to-solid mass ratio of 60:1 and an NH₄Cl concentration of 1.5 mol/L, the Ca²⁺ leaching rate decreased with increasing reaction temperature after 60 min of leaching. The maximum leaching rate of 81.25% was obtained at 25 °C. This decreasing trend may be attributed to the reduced solubility of PG at elevated temperatures [23], which lowers the concentration of dissolved Ca²⁺ in the leaching solution and consequently decreases the Ca²⁺ leaching rate.

At 25 °C, with an NH₄Cl concentration of 1.5 mol/L and a liquid-to-solid mass ratio of 60:1, the Ca²⁺ leaching rate initially increased and then tended to stabilize with increasing reaction time. The maximum leaching rate of 81.25% was achieved at 60 min. Sufficient reaction time is required for NH₄Cl diffusion and its interaction with PG particles. Therefore, extending the leaching time promotes more complete calcium extraction and increases the Ca²⁺ leaching rate. However, excessive leaching time may lead to local supersaturation of Ca²⁺ and SO₄²⁻, thereby promoting the reprecipitation of CaSO₄. In addition, trace Fe³⁺ in PG may gradually dissolve during prolonged leaching and interact with SO₄²⁻ to form hydrated ferric sulfate species, such as Fe₂(SO₄)₃·xH₂O [24]. These processes may reduce the amount of dissolved calcium remaining in solution, resulting in a slight decrease in the Ca²⁺ leaching rate.

In summary, the optimal conditions for Ca²⁺ extraction from PG using NH₄Cl leaching were determined to be a reaction temperature of 25 °C, an NH₄Cl concentration of 1.5 mol/L, a liquid-to-solid mass ratio of 60:1, and a leaching time of 60 min.

3.1.2. Conversion Rate of Ca²⁺ in the Leaching Solution

Figure 4 shows the Ca²⁺ conversion rate as a function of CO₂ pressure, carbonation time, and NH₃·H₂O addition at 25 °C.

Under the conditions of 12 vol% NH₃·H₂O addition and a carbonation time of 10 min, the Ca²⁺ conversion rate increased markedly as the CO₂ pressure increased from 0.1 to 1.0 MPa. However, when the pressure was further increased to 2.0 MPa, the additional improvement was limited, with the conversion rate reaching 98.34%. Compared with previous studies [12,13,14] using CO₂ bubbling at atmospheric pressure, the present high-pressure carbonation process achieved a higher Ca²⁺ conversion rate within a shorter reaction time and under milder temperature conditions. In a sealed reactor, according to the ideal gas law, increasing the CO₂ pressure at a constant temperature increases the amount of CO₂ per unit volume. The higher CO₂ concentration provides a stronger driving force for gas–liquid mass transfer [25], thereby promoting CO₂ diffusion into the bulk liquid. In addition, according to Henry’s law, increasing the pressure enhances the solubility of CO₂ in the solution. Dissolved CO₂ reacts with water and undergoes stepwise dissociation to form carbonate species. Under alkaline conditions provided by NH₃·H₂O, the concentration of CO₃²⁻ increases, which promotes its reaction with Ca²⁺ to form CaCO₃ precipitates and thus increases the Ca²⁺ conversion rate. However, excessive CO₂ dissolution at higher pressure may generate more carbonic acid and lower the system pH, shifting the carbonate equilibrium toward HCO₃⁻. This may favor the formation of soluble calcium bicarbonate species and limit the further increase in the Ca²⁺ conversion rate.

Under the conditions of 12 vol% NH₃·H₂O addition and a CO₂ pressure of 1.0 MPa, the Ca²⁺ conversion rate increased rapidly as the carbonation time was extended from 4 to 10 min. When the carbonation time was further extended to 15 min, the additional increase was limited, with the conversion rate ultimately reaching 98.51%. During the initial stage of carbonation, the Ca²⁺ concentration in the leachate was high, and the carbonate species generated from dissolved CO₂ rapidly reacted with Ca²⁺ to form CaCO₃ precipitates. At this stage, the reactant concentrations were relatively high, the diffusion distance was short, and the mass-transfer resistance was low, resulting in a rapid increase in the Ca²⁺ conversion rate. As carbonation proceeded, Ca²⁺ in the solution was progressively consumed, leading to a decrease in ion concentration and a reduced driving force for precipitation. Therefore, the increase in the Ca²⁺ conversion rate gradually slowed at longer carbonation times.

After carbonation for 10 min at a CO₂ pressure of 1.0 MPa, the Ca²⁺ conversion rate initially increased substantially and then plateaued with increasing NH₃·H₂O addition. A Ca²⁺ conversion rate of 97.36% was achieved at 12 vol% NH₃·H₂O addition. As the NH₃·H₂O concentration increased, dissolved CO₂ reacted with NH₃·H₂O to form ammonium carbonate species, which subsequently dissociated to provide CO₃²⁻. Meanwhile, the increased NH₃·H₂O addition maintained an alkaline environment, promoting the conversion of dissolved CO₂ into carbonate species. The resulting increase in CO₃²⁻ concentration allowed more Ca²⁺ to precipitate as CaCO₃, thereby increasing the Ca²⁺ conversion rate. However, at 16 vol% NH₃·H₂O addition, most of the Ca²⁺ had already precipitated as CaCO₃, and the formed CaCO₃ particles increased the mass-transfer resistance for the remaining unreacted Ca²⁺ and CO₃²⁻. Consequently, the Ca²⁺ conversion rate showed little further increase. Therefore, 12 vol% NH₃·H₂O addition was selected as the optimal dosage because it enabled efficient Ca²⁺ conversion while avoiding reagent waste and the undesirable particle-size increase associated with excessive NH₃·H₂O addition [26].

In summary, a Ca²⁺ conversion rate of 97.36% was achieved under the optimized carbonation conditions: 12 vol% NH₃·H₂O addition, a carbonation temperature of 25 °C, a CO₂ pressure of 1.0 MPa, and a carbonation time of 10 min.

3.1.3. Crystal Morphology Under Optimal High-Pressure Carbonation Conditions

Figure 5 and Figure 6 show the SEM image and XRD pattern of calcium carbonate prepared from PG under the optimized high-pressure carbonation conditions, respectively. The obtained CaCO₃ crystals were predominantly spherical, and band-like aggregates formed by the assembly of spherical particles were also observed. The XRD results indicated that the product was mainly composed of vaterite, with only trace amounts of calcite. In addition, the product contained a relatively high proportion of large particles, whereas fine particles were less developed.

In the aqueous carbonation system, NH₄⁺ may interact with the hydrated surfaces of CaCO₃ crystallites, thereby affecting particle aggregation and crystal growth. The adsorption of NH₄⁺ may alter the surface interactions among CaCO₃ particles, leading to crystal agglomeration, and may also modify the growth rates of specific crystal faces. These effects can change the crystal growth direction and contribute to the formation of the branched structures observed in the product [22]. In addition, NH₄⁺ may adsorb onto active sites on calcite surfaces, such as the {104} facet, thereby blocking growth sites and inhibiting calcite development. According to the Ostwald ripening process, metastable vaterite can gradually transform into thermodynamically stable calcite with prolonged reaction time [5]. This transformation may partially offset the inhibitory effect of NH₄⁺ on calcite growth, which explains the presence of weak calcite diffraction peaks in the XRD pattern.

3.2. Modulation of CaCO₃ Polymorphs via Organic Additives

The CaCO₃ crystals prepared under the optimized high-pressure carbonation conditions were predominantly vaterite; however, particle agglomeration and a non-uniform size distribution were observed. As a metastable CaCO₃ polymorph with relatively low thermodynamic stability and crystallinity [5,6], vaterite is prone to phase transformation during storage or application, which may lead to performance degradation. Therefore, organic additives were introduced before carbonation of the Ca²⁺-rich leachate to regulate CaCO₃ crystallization and improve the morphological uniformity of the products. In this study, the effects of these additives on the Ca²⁺ conversion rate, polymorph composition, and crystal morphology of the resulting CaCO₃ were investigated.

3.2.1. Ca²⁺ Conversion Rate in Leachate with Different Organic Additives

Figure 7 shows the effect of additive concentration on the Ca²⁺ conversion rate.

With increasing aspartic acid and glutamic acid concentrations, the Ca²⁺ conversion rate initially increased slightly and then decreased. The maximum conversion rates of 97.51% and 97.83% were achieved at 1 wt% aspartic acid and 1 wt% glutamic acid, respectively. Aspartic acid and glutamic acid can provide nucleation sites for CaCO₃ formation [27]. Therefore, at low concentrations, these additives promote CaCO₃ nucleation and slightly enhance the Ca²⁺ conversion rate. However, when their concentrations exceed a certain threshold, steric hindrance may occur [27], which hinders the diffusion of Ca²⁺ and CO₃²⁻ toward the growing crystal surfaces. In addition, aspartic acid and glutamic acid can form stable complexes with Ca²⁺ [20,27,28], thereby reducing the concentration of free Ca²⁺ in solution. According to the solubility product relationship of CaCO₃, Ksp = [Ca²⁺][CO₃²⁻], a decrease in free Ca²⁺ requires a higher CO₃²⁻ concentration to initiate precipitation. This increases the precipitation barrier, inhibits CaCO₃ formation, and ultimately leads to a decrease in the Ca²⁺ conversion rate.

The Ca²⁺ conversion rate also showed an initial increase followed by a decrease with increasing ethanol and glycerol concentrations. The maximum conversion rates of 98.96% and 98.43% were obtained at 5% ethanol and 3% glycerol, respectively. At low additive concentrations, both alcohols can increase solution supersaturation and reduce the nucleation barrier, thereby accelerating the early-stage nucleation of CaCO₃ [29,30,31]. This promotes the reaction between Ca²⁺ and CO₃²⁻, resulting in a slight increase in the Ca²⁺ conversion rate. However, as the alcohol concentration further increases, excessive alcohols significantly increase the solution viscosity [16,18]. The increased viscosity reduces the diffusion coefficients of Ca²⁺ and CO₃²⁻ and slows their transport in the solution. Consequently, these ions cannot diffuse to the crystal growth sites in time, which retards the growth of CaCO₃ nuclei. The abundant nuclei formed at the initial stage cannot obtain sufficient reactants for continuous growth, eventually leading to a decrease in the Ca²⁺ conversion rate.

3.2.2. Crystal Morphology of CaCO₃ with Different Organic Additives

Figure 8 and Figure 9 show the SEM images and XRD patterns of CaCO₃ prepared in the presence of different organic additives, respectively. With the addition of aspartic acid, glutamic acid, ethanol, or glycerol, calcite became the dominant polymorph, accompanied by trace amounts of vaterite. Previous studies [16,17] also reported that the addition of small amounts of ethanol or glycerol as organic additives can facilitate calcite formation during CaCO₃ synthesis. The morphologies of the CaCO₃ crystals varied noticeably under different additive conditions. Compared with the additive-free sample, the crystals prepared with organic additives were more fully developed, and their particle sizes were slightly reduced. In the presence of aspartic acid, the crystals were mainly spherical, with a small fraction of dumbbell-shaped particles. With glutamic acid, the crystals were predominantly quasi-spherical. In the ethanol-containing system, the crystals consisted of spherical particles, fine loose aggregates, and a small amount of cubic particles. In the glycerol-containing system, the crystals were mainly spherical.

After dissociation in solution, the negatively charged carboxylate groups of aspartic acid and glutamic acid can bind with Ca²⁺, thereby providing active sites for CaCO₃ crystallization [32]. This interaction may lower the nucleation barrier of metastable vaterite and promote its formation. Owing to its crystal structure, vaterite tends to grow relatively uniformly in different directions, which favors the formation of spherical particles. In addition, amino acid molecules adsorbed on the surfaces of CaCO₃ crystallites may impart a negative surface charge, enhancing electrostatic repulsion and weakening van der Waals attraction between particles [33]. As a result, well-dispersed spherical or quasi-spherical CaCO₃ particles were obtained. Aspartic acid and glutamic acid can also selectively adsorb onto specific facets of thermodynamically stable calcite [20,21], thereby inhibiting the preferential growth of calcite. This may explain the presence of weak vaterite peaks in the XRD patterns. However, the carbonation process was followed by continuous stirring of the slurry for 2 h before vacuum filtration, during which dissolution and recrystallization may occur [34]. As aspartic acid and glutamic acid complex with dissolved Ca²⁺, the local ionic activity product may decrease, which can facilitate the transformation of metastable vaterite into thermodynamically stable calcite. This explains why calcite became the dominant phase in the final products.

Ethanol and glycerol can reduce the surface tension of the solution, and their hydroxyl groups may interact with or adsorb onto CaCO₃ crystal surfaces. These effects can inhibit anisotropic growth and promote the simultaneous development of multiple crystal faces, resulting in smaller particles and the formation of fine, loose CaCO₃ aggregates on the surfaces of spherical particles. Moreover, the addition of ethanol and glycerol increases the solution viscosity, which reduces interparticle collisions and slows Ostwald ripening [16,18]. This further suppresses excessive crystal growth and agglomeration, thereby improving the dispersion of CaCO₃ particles. In addition, the hydroxyl groups of ethanol and glycerol may preferentially adsorb onto calcite facets [35,36]. This can stabilize calcite nuclei and inhibit vaterite growth, explaining the dominant calcite diffraction peaks observed in the XRD patterns.

In summary, the addition of all four organic additives led to a slight decrease in particle size, more complete crystal development, and a polymorphic transformation from vaterite to calcite. Among the additives tested, glutamic acid and glycerol showed stronger effects on crystal morphology regulation.

4. Conclusions

(1)

The optimized process for preparing CaCO₃ from PG by high-pressure carbonation was as follows. PG was first leached for 60 min at 25 °C using 1.5 mol/L NH₄Cl with a liquid-to-solid mass ratio of 60:1, achieving a Ca²⁺ leaching rate of 81.25%. The resulting Ca²⁺-rich leachate was then carbonated for 10 min at 25 °C, a CO₂ pressure of 1.0 MPa, and 12 vol% NH₃·H₂O addition, resulting in a Ca²⁺ conversion rate of 97.36%. The obtained CaCO₃ crystals were predominantly vaterite with a quasi-spherical morphology, although particle agglomeration was observed.

(2)

High-pressure carbonation was further performed using Ca²⁺-rich leachates containing organic additives, including aspartic acid, glutamic acid, ethanol, and glycerol. In all additive systems, the Ca²⁺ conversion rate initially increased and then decreased with increasing additive concentration. The optimal dosages were 1 wt% for both aspartic acid and glutamic acid, 5 vol% for ethanol, and 3 vol% for glycerol. The resulting CaCO₃ products were predominantly calcite with quasi-spherical morphologies. The particles exhibited relatively uniform sizes and good dispersion. Among the tested additives, glutamic acid and glycerol showed stronger effects on crystal morphology regulation.