Progress in the Preparation of Calcium Carbonate by Indirect Mineralization of Industrial By-Product Gypsum

: To avoid the long-term pollution of land and water by industrial gypsum by-products, the exploitation of this resource has become a priority. The indirect synthesis of calcium carbonate from the industrial by-product gypsum has received substantial attention as a viable method for resource utilization. Currently, the primary problems in the indirect manufacture of calcium carbonate from the industrial by-product gypsum are additive recycling and process simpliﬁcation. This paper describes the present state of development and compares various indirect mineralization systems. The factors affecting leaching and mineralization in the indirect mineralization of CO 2 from by-product gypsum and the management of CaCO 3 crystallinity are discussed, and the current additive regeneration cycle is summarized. The applications of other technologies in the indirect mineralization of by-product gypsum are also summarized, as are the obstacles, and required future work. This review provides guidelines for the laboratory indirect mineralization of by-product gypsum as well as practical applications.


Introduction
Owing to its low cost, abundance, and excellent biocompatibility, calcium carbonate (CaCO 3 ) is a frequently used filler in the paper, plastic, rubber, and food sectors [1][2][3]. Carbonation [4], complicated decomposition [5], and pyrolysis [6] are the most common techniques used to produce calcium carbonate. Although well developed, the conventional production methods have drawbacks such as high energy consumption and the depletion of natural ores [2,7,8]. To address these drawbacks, a novel method for producing calcium carbonate using calcium-containing industrial solid waste as a raw material rather than natural minerals has been developed [9,10]. Reacting CO 2 from flue gas with calcium from the industrial by-product gypsum (CaSO 4 ) produces calcium carbonate more economically than the traditional approach does by eliminating the additional stages of mining, grinding, and separation of natural minerals [11]. Additionally, the industrial by-product gypsum can be obtained from a variety of sources located close to the source of CO 2 emissions, such as thermal power plants [12]. In addition to being an innovative application of gypsum, this method also sequesters CO 2 . Three significant industrial gypsum by-products are flue-gas desulfurization gypsum (FGDG) [13], which is produced from coal combustion after the wet desulfurization of flue gas; phosphogypsum (PG) [14], which originates from phosphate fertilizer production; and red gypsum (RG) [15], a by-product of the neutralization of spent sulfuric acid in titanium dioxide production. In 2020, China produced approximately 200 Mt of industrial gypsum by-products, 90% of which was FGDG, phosphogypsum, and titanium gypsum [16]. Similarly, in the EU and the US, phosphogypsum from fertilizer production is a gigantic deal [17]. The major components and chemical properties of by-product gypsum are identical to those of natural gypsum, and it has been extensively researched for usage The main components of phosphogypsum and red gypsum are similar to those of FGDG, but they contain more impurities. In addition to other minor impurities, phosphogypsum can contain silica, free H3PO4 [38], and phosphate, whereas red gypsum contains iron oxide (Fe2O3) and titanium oxide (TiO2) [39]. The sulfur contents of FGDG, phosphogypsum, and red gypsum range from 12.80 to 18.60 wt.%, 11.99 to 18.40 wt.%, and 12.64 to 15.42 wt.%, respectively, with similar ranges. However, the different calcium and impurity contents indicate differences among the three types of waste gypsum. The main components of phosphogypsum and red gypsum are similar to those of FGDG, but they contain more impurities. In addition to other minor impurities, phosphogypsum can contain silica, free H 3 PO 4 [38], and phosphate, whereas red gypsum contains iron oxide (Fe 2 O 3 ) and titanium oxide (TiO 2 ) [39]. The sulfur contents of FGDG, phosphogypsum, and red gypsum range from 12.80 to 18.60 wt.%, 11.99 to 18.40 wt.%, and 12.64 to 15.42 wt.%, respectively, with similar ranges. However, the different calcium and impurity contents indicate differences among the three types of waste gypsum.   Table 1 shows that by-product gypsum contains a large amount of calcium (22.38-34.13 wt.%). As the solubility product of calcium carbonate (K sp = 3.36 × 10 −9 ) [47] is much lower than that of calcium sulfate (K sp = 3.14 × 10 −5 ) [48], by-product gypsum has the potential to be mineralized to calcium carbonate. As the acidity of CO 2 dissolved in water is not sufficient to dissolve minerals [49,50], various physical and chemical schemes have been proposed to facilitate the dissolution of calcium from by-product gypsum [51,52]. Calcium carbonate was prepared from by-product gypsum via indirect mineralization using the following steps: First, Ca 2+ was extracted from the gypsum by-product using leaching agents (e.g., ammonium salts (NH 4 Cl) and acid and bases (such as H 2 SO 4 and NaOH). Next, CO 2 was passed through the calcium-rich solution to form calcium carbonate and complete the CO 2 sequestration. Finally, the leaching agent was regenerated and recycled [53][54][55]. The technical process is shown in Figure 2, corresponding to reaction Equations (1)-(3).  Table 1 shows that by-product gypsum contains a large amount of calcium (22.38-34.13 wt.%). As the solubility product of calcium carbonate (Ksp = 3.36 × 10 −9 ) [47] is much lower than that of calcium sulfate (Ksp = 3.14 × 10 −5 ) [48], by-product gypsum has the potential to be mineralized to calcium carbonate. As the acidity of CO2 dissolved in water is not sufficient to dissolve minerals [49,50], various physical and chemical schemes have been proposed to facilitate the dissolution of calcium from by-product gypsum [51,52]. Calcium carbonate was prepared from by-product gypsum via indirect mineralization using the following steps: First, Ca 2+ was extracted from the gypsum by-product using leaching agents (e.g., ammonium salts (NH4Cl) and acid and bases (such as H2SO4 and NaOH). Next, CO2 was passed through the calcium-rich solution to form calcium carbonate and complete the CO2 sequestration. Finally, the leaching agent was regenerated and recycled [53][54][55]. The technical process is shown in Figure 2, corresponding to reaction Equations (1)-(3). CaSO4·2H2O(s) + Leaching agents ↔ Ca 2+ + SO4 2− + 2H2O (1)

Main Technical Routes for Indirect Mineralization of By-Product Gypsum
The preparation of calcium carbonate via the indirect mineralization of industrial byproduct gypsum can be divided into four processes: alkali leaching, pH swing, salt leaching, and complexation.

Alkali Leaching
Blencoe et al. [56] first proposed the use of NaOH to leach calcium and magnesium from silicate minerals to prepare carbonates via mineralization and CO 2 sequestration. Because calcium hydroxide is less alkali than sodium hydroxide or ammonia, the calcium in the industrial by-product gypsum can be converted to Ca(OH) 2 by the action of the alkali. According to the principle of making a weak alkali from a strong alkali, the transfer of calcium from calcium hydroxide to calcium carbonate is then achieved via the passage of CO 2 . Corresponding to reaction Equations (4)-(6), the process of preparing calcium carbonate via the indirect mineralization of phosphogypsum with NaOH as a leaching agent is shown in Figure 3.

Main Technical Routes for Indirect Mineralization of By-Product Gypsum
The preparation of calcium carbonate via the indirect mineralization of industrial byproduct gypsum can be divided into four processes: alkali leaching, pH swing, salt leaching, and complexation.

Alkali Leaching
Blencoe et al. [56] first proposed the use of NaOH to leach calcium and magnesium from silicate minerals to prepare carbonates via mineralization and CO2 sequestration. Because calcium hydroxide is less alkali than sodium hydroxide or ammonia, the calcium in the industrial by-product gypsum can be converted to Ca(OH)2 by the action of the alkali. According to the principle of making a weak alkali from a strong alkali, the transfer of calcium from calcium hydroxide to calcium carbonate is then achieved via the passage of CO2. Corresponding to reaction Equations (4)-(6), the process of preparing calcium carbonate via the indirect mineralization of phosphogypsum with NaOH as a leaching agent is shown in Figure 3.
Based on the principle of alkali leaching, Altiner et al. [60] investigated the effect of reactor design on the mineralization time and change in the calcium carbonate particle size. The reaction time required to produce calcium carbonate particles in a Venturi reactor was half of that without the Venturi tube, and all the calcium carbonate particles produced were calcite. However, the crystal structure was affected by the experimental conditions.
As shown in Table 2, the alkali leaching of gypsum byproducts does not use additives, and the final calcium carbonate crystal formed is mainly calcite. However, the mineralization efficiency and calcium carbonate purity differ because of the difference in gypsum by-products. In addition, the indirect mineralization of the gypsum byproduct and other wastes (such as caustic waste liquid [58], bauxite slag [61], waste brine [62,63], and seawater [64]) will play a significant role in the comprehensive utilization of waste resources in the future. However, alkali leaching still requires a large amount of alkali liquor, which is corrosive to equipment, and the remaining alkali and Na 2 SO 4 in the filtrate after mineralization are difficult to separate. Therefore, achieving efficient circulation is difficult.

pH Swing
Extracting calcium and magnesium ions from raw minerals or alkali solid waste is easier in acidic environments than in alkali environments; however, high-purity carbonate precipitates can be prepared in alkali environments [65,66]. Changing the pH from low to high or from high to low during leaching-mineralization is called the pH swing method. Alissa et al. [67] first mineralized and sequestered CO 2 from serpentine using the pH swing method to obtain high-purity magnesium carbonate. Preparing calcium carbonate via the pH swing indirect mineralization of by-product gypsum includes dissolving calcium or iron at low pH to separate insoluble impurities and then adding NH3·H2O to increase the pH to remove the main impurities (iron or silicon). Finally, carbonation of the high-purity Ca-rich solution with CO 2 is performed at pH 10. Figure 4 shows a schematic of the indirect mineralization of red gypsum using the pH swing method.
Based on the principle of alkali leaching, Altiner et al. [60] investigated the effect of reactor design on the mineralization time and change in the calcium carbonate particle size. The reaction time required to produce calcium carbonate particles in a Venturi reactor was half of that without the Venturi tube, and all the calcium carbonate particles produced were calcite. However, the crystal structure was affected by the experimental conditions.
As shown in Table 2, the alkali leaching of gypsum byproducts does not use additives, and the final calcium carbonate crystal formed is mainly calcite. However, the mineralization efficiency and calcium carbonate purity differ because of the difference in gypsum by-products. In addition, the indirect mineralization of the gypsum byproduct and other wastes (such as caustic waste liquid [58], bauxite slag [61], waste brine [62,63], and seawater [64]) will play a significant role in the comprehensive utilization of waste resources in the future. However, alkali leaching still requires a large amount of alkali liquor, which is corrosive to equipment, and the remaining alkali and Na2SO4 in the filtrate after mineralization are difficult to separate. Therefore, achieving efficient circulation is difficult.

pH Swing
Extracting calcium and magnesium ions from raw minerals or alkali solid waste is easier in acidic environments than in alkali environments; however, high-purity carbonate precipitates can be prepared in alkali environments [65,66]. Changing the pH from low to high or from high to low during leaching-mineralization is called the pH swing method. Alissa et al. [67] first mineralized and sequestered CO2 from serpentine using the pH swing method to obtain high-purity magnesium carbonate. Preparing calcium carbonate via the pH swing indirect mineralization of by-product gypsum includes dissolving calcium or iron at low pH to separate insoluble impurities and then adding NH3·H2O to increase the pH to remove the main impurities (iron or silicon). Finally, carbonation of the high-purity Ca-rich solution with CO2 is performed at pH 10. Figure 4 shows a schematic of the indirect mineralization of red gypsum using the pH swing method. Different leaching agents have different leaching efficiencies for gypsum by-products during the pH swing process. Amin et al. [69] compared the effects of acid and alkali leaching agents on red gypsum. Acidic leaching agents (H2SO4, HCl, and HNO3) extracted calcium and iron more efficiently than alkali leaching agents (NaOH, KOH, and NH3·H2O) did. Additionally, HCl extracted calcium more efficiently than HNO3 and H2SO4 did. Thus, Different leaching agents have different leaching efficiencies for gypsum by-products during the pH swing process. Amin et al. [69] compared the effects of acid and alkali leaching agents on red gypsum. Acidic leaching agents (H 2 SO 4 , HCl, and HNO 3 ) extracted calcium and iron more efficiently than alkali leaching agents (NaOH, KOH, and NH 3 ·H 2 O) did. Additionally, HCl extracted calcium more efficiently than HNO 3 and H 2 SO 4 did. Thus, calcium carbonate from red gypsum mineralization is generally obtained via the swing process of adding H 2 SO 4 for leaching and then adding an alkali to adjust the pH [10,12,42].
The process conditions also significantly affect the reaction kinetics and crystal form of the calcium carbonate. Amin et al. [70] observed that an equilibrium always exists between Ca 2+ and CO 2 during mineralization. At a specific calcium concentration, increasing the CO 2 pressure improves the mineralization efficiency. They also found that the mineralization efficiency was much higher while using ammonium bicarbonate as a CO 3 2− source rather than CO 2 [71]. At the same time, it is confirmed that the mixture of vaterite, calcite and aragonite can be formed below 80 • C, while vaterite alone can be formed only above 170 • C [71]. To promote the dissolution of CO 2 in the mineralization stage, Omeid et al. [29] added monoethanolamine (MEA) as a CO 2 absorbent, which yielded a mineralization efficiency of 98.9% for calcite calcium carbonate. A positive correlation also existed among the mineralization efficiency of calcite, MEA consumption, and CO 2 dissolution. Ding et al. [72] found that, in the pH swing process of phosphogypsum, vaterite can be prepared at a lower temperature (20-40 • C) when ammonia water is added during mineralization.
As the temperature increases, some of the vaterite is transformed to aragonite and calcite, decreasing the purity of vaterite ( Figure 5).
calcium carbonate from red gypsum mineralization is generally obtained via the swing process of adding H2SO4 for leaching and then adding an alkali to adjust the pH [10,12,42]. The process conditions also significantly affect the reaction kinetics and crystal form of the calcium carbonate. Amin et al. [70] observed that an equilibrium always exists between Ca 2+ and CO2 during mineralization. At a specific calcium concentration, increasing the CO2 pressure improves the mineralization efficiency. They also found that the mineralization efficiency was much higher while using ammonium bicarbonate as a CO3 2− source rather than CO2 [71]. At the same time, it is confirmed that the mixture of vaterite, calcite and aragonite can be formed below 80 °C, while vaterite alone can be formed only above 170 °C [71]. To promote the dissolution of CO2 in the mineralization stage, Omeid et al. [29] added monoethanolamine (MEA) as a CO2 absorbent, which yielded a mineralization efficiency of 98.9% for calcite calcium carbonate. A positive correlation also existed among the mineralization efficiency of calcite, MEA consumption, and CO2 dissolution. Ding et al. [72] found that, in the pH swing process of phosphogypsum, vaterite can be prepared at a lower temperature (20-40 °C) when ammonia water is added during mineralization. As the temperature increases, some of the vaterite is transformed to aragonite and calcite, decreasing the purity of vaterite ( Figure 5). Amin et al. [65,[69][70][71]73,74] used the reaction model PHREEQC-2.18 program to simulate the mineralization and precipitation of various solid phases during the indirect mineralization of red gypsum via the pH swing method, so as to predict CO2 absorption. The results showed that calcium ions were rapidly released from CaO at pH 12.5. In addition, Amin et al. [65,[69][70][71]73,74] used the reaction model PHREEQC-2.18 program to simulate the mineralization and precipitation of various solid phases during the indirect mineralization of red gypsum via the pH swing method, so as to predict CO 2 absorption. The results showed that calcium ions were rapidly released from CaO at pH 12.5. In addition, the simulation results directly showed the saturation indices of CaO and CaCO 3 , indicating that their dissolution and precipitation depend on the saturation state and pH value.
Ding et al. [72] removed the main impurity in phosphogypsum, SiO 2 , using the pH swing process of alkali leaching, acid washing, and mineralization in an alkali environment. The calcium leaching rate reached 99.6%, and the mineralization efficiency reached 98.57%. The relevant reactions are shown in Equations (9)- (11).
Ca(OH) 2 (s) + CO 2 (g) → CaCO 3 (s) + H 2 O(l) (11) As shown in Table 2, in the pH swing process of by-product gypsum, the conditions without additives in the mineralization stage are more stringent than those with additives. Therefore, considering the energy consumption and cost, a more efficient additive selection or adjustment to produce higher value and purer calcium carbonate will be a future development trend. Although the pH swing process for leaching by-product gypsum improves leaching efficiency and removes impurities [15], it has obvious disadvantages. First, the process is complex and requires large amounts of acids and alkalis. Second, recycling the leaching agent is difficult, which is expensive and harmful to the environment.

Salt Leaching
The addition of a strongly soluble electrolyte to increase the solubility of insoluble substances is called the salt effect. Salt leaching applies this principle to the indirect mineralization of gypsum by-products to calcium carbonate. The gypsum byproduct extracted via the salt effect is mainly divided into ammonium salt and other salts according to the salt type. Chen et al. [54] proposed leaching calcium from phosphogypsum using sodium chloride followed by mineralization in an ammonia water environment to prepare calcium carbonate. For reaction equations such as 13 and 14, the results show that under the optimized conditions (3 mol/L, 50: 1 mL/g, 30 • C, 60 min), the leaching rate of Ca 2+ is 49.42% and the carbonation rate is 96.31%. Ding et al. [75] used ammonium acetate to separate Ca 2+ from PG and concentrate impurity ions. For reaction Equations (14)-(18), the leaching and mineralization efficiencies of calcium were more than 98%. The phosphogypsum was leached with NH 4 Cl and mineralized in ammonia water. The results showed that the optimum dissolved amount of CaSO 4 ·2H 2 O was 18.7 g/L and the carbonation rate was 98.22% [76]. PG(s) → Ca 2+ (aq) + SO 4 2− (aq) + SiO 2 (s) Ca 2+ (aq) + CO 3 2− (aq) → CaCO 3 (s) The regenerative stability also differed after leaching and mineralization with different salts. When NaCl was used as the leaching agent, it could be recycled four times, and the corresponding reaction efficiency could exceed 60% [54]. When NH 4 Cl was used as the leaching agent, the mineralized filtrate was recycled up to nine times, as shown in Figure 6a, and the reaction efficiency was maintained above 50%. The crystal phase of the obtained product was calcite, and no other phases of CaCO 3 were found (Figure 6b) [76,77].
In salt leaching, the experimental conditions also have an important influence on the mineralization efficiency and final calcium carbonate morphology. Chen et al. [54] found that during the mineralization of phosphogypsum in a NaCl-NH 3 ·H 2 O system, increasing the recovery time of the mineralized filtrate caused the morphology of the calcium carbonate to change gradually from calcite to vaterite. Ding et al. [75] first produced pure vaterite calcium carbonate at 20 • C in an ammonia water environment. With increasing temperature, vaterite was converted into aragonite without any crystal-form regulator [75]. This confirms that increasing the amount of ammonia is beneficial for vaterite formation [54,75,76]. Additionally, the mineralization efficiency decreased with increasing temperature, and the  In salt leaching, the experimental conditions also have an important influence on the mineralization efficiency and final calcium carbonate morphology. Chen et al. [54] found that during the mineralization of phosphogypsum in a NaCl-NH3·H2O system, increasing the recovery time of the mineralized filtrate caused the morphology of the calcium carbonate to change gradually from calcite to vaterite. Ding et al. [75] first produced pure vaterite calcium carbonate at 20 °C in an ammonia water environment. With increasing temperature, vaterite was converted into aragonite without any crystal-form regulator [75]. This confirms that increasing the amount of ammonia is beneficial for vaterite formation [54,75,76]. Additionally, the mineralization efficiency decreased with increasing temperature, and the effects of ammonia addition, CO2 gas velocity, and reaction time on the mineralization efficiency increased to a plateau.
To further study the mineralization reaction mechanism of phosphogypsum in the CH3COONH4-NH3·H2O system, Ding et al. [75] calculated the thermodynamic parameters under standard reaction conditions. The results show that the three-step reaction mechanism of phosphogypsum mineralization in this system is as follows: (a) leaching Ca 2+ from the original phosphogypsum, (b) conversion of calcium acetate into calcium hydroxide via ammonia water, and (c) conversion of calcium hydroxide into calcium carbonate via the dissolution of gaseous CO2 (g) in water (Figure 7).  To further study the mineralization reaction mechanism of phosphogypsum in the CH 3 COONH 4 -NH 3 ·H 2 O system, Ding et al. [75] calculated the thermodynamic parameters under standard reaction conditions. The results show that the three-step reaction mechanism of phosphogypsum mineralization in this system is as follows: (a) leaching Ca 2+ from the original phosphogypsum, (b) conversion of calcium acetate into calcium hydroxide via ammonia water, and (c) conversion of calcium hydroxide into calcium carbonate via the dissolution of gaseous CO 2 (g) in water (Figure 7). In salt leaching, the experimental conditions also have an important influence on the mineralization efficiency and final calcium carbonate morphology. Chen et al. [54] found that during the mineralization of phosphogypsum in a NaCl-NH3·H2O system, increasing the recovery time of the mineralized filtrate caused the morphology of the calcium carbonate to change gradually from calcite to vaterite. Ding et al. [75] first produced pure vaterite calcium carbonate at 20 °C in an ammonia water environment. With increasing temperature, vaterite was converted into aragonite without any crystal-form regulator [75]. This confirms that increasing the amount of ammonia is beneficial for vaterite formation [54,75,76]. Additionally, the mineralization efficiency decreased with increasing temperature, and the effects of ammonia addition, CO2 gas velocity, and reaction time on the mineralization efficiency increased to a plateau.
To further study the mineralization reaction mechanism of phosphogypsum in the CH3COONH4-NH3·H2O system, Ding et al. [75] calculated the thermodynamic parameters under standard reaction conditions. The results show that the three-step reaction mechanism of phosphogypsum mineralization in this system is as follows: (a) leaching Ca 2+ from the original phosphogypsum, (b) conversion of calcium acetate into calcium hydroxide via ammonia water, and (c) conversion of calcium hydroxide into calcium carbonate via the dissolution of gaseous CO2 (g) in water (Figure 7).  Although as mentioned above, leaching agents such as NaCl and NH 4 Cl can be regenerated and recycled. Figure 6 shows that the leaching efficiency of the mineralized filtrate gradually decreases as the number of cycles increases. Therefore, the development of additives with better regenerative stabilities is an important objective for future research on the indirect mineralization of byproduct gypsum to calcium carbonate.

Complexation
The application and kinetic analysis of organic acids containing carboxyl or hydroxyl groups that can promote mineral dissolution via complexation has been extensively researched. Fredd et al. [78] found that chelators such as cyclohexanediamine tetraacetic acid (CDTA), diethyltriamine pentaacetic acid (DTPA), and ethylenediamine tetraacetic acid (EDTA) significantly increased the dissolution of calcite. The chelating agent promotes the dissolution of the mineral phase via a ligand-exchange reaction, yielding a highly soluble metal complex [79].
Owing to their different complexation constants for calcium ions, different organic acids have different efficiencies for leaching and final mineralization of the same raw material [80], and they produce different purities and calcium carbonate morphologies [81]. Yang et al. [53] used sodium gluconate as a phase-transfer agent to indirectly mineralize phosphogypsum and prepare calcium carbonate microspheres. The presence of sodium gluconate inhibited the nucleation and growth of calcite but promoted the formation of vaterite. The interaction between sodium gluconate and Ca 2+ plays a key role in the formation of monodisperse vaterite CaCO 3 . These are summarized in reactions 19 and 20. Yuan et al. [82] used the same "phase transfer-precipitation" route to mineralize gypsum scales attached to evaporator walls to prepare calcium carbonate micron rods.
Both the carboxyl and amino groups of amino acids form complexes with calcium ions. However, researchers have long focused on the regulation of amino acids in the crystal form of calcium carbonate during mineralization [83,84]. In recent years, amino acids have attracted considerable attention as new additives for leaching-mineralization cycles. Zheng et al. [85][86][87] found that amino acids act as not only leaching agents via complexation effects in the leaching stage of indirect mineralization but also CO 2 absorbers and calcium carbonate crystal inducers in the mineralization stage. The related reaction Equations are (21)- (23).
Gong et al. [55] leached and mineralized CaSO 4 ·2H 2 O by exploiting the solubility difference of aspartic acid at different pH values. The results showed that the solubility of CaSO 4 ·2H 2 O was 16.3 g L −1 in 7 wt.% ammonia and a liquid-to-solid ratio of 50. Owing to the absorption of CO 2 by aspartic acid during mineralization, the induction time of calcium carbonate crystallization was significantly increased (as shown in Figure 8b), and the obtained calcium carbonate was a homogeneous vaterite type (as shown in Figure 8c), which confirmed that a relatively high content of amino acids could aid in the formation of the vaterite phase. The process route is illustrated in Figure 8a.
The regeneration of the leaching agent is also different when the industrial by-product gypsum is indirectly mineralized via complexation. Gong et al. [55] combined the dissolution characteristics of amino acids with the leaching-mineralization of desulfurized gypsum. Because amino acids are sparingly soluble at their isoelectric points, aspartic acid can be precipitated and then recycled by adjusting the pH to the isoelectric point after mineralization. After 10 cycles, the recovery efficiency was maintained at 80%, while the dissolved CaSO 4 ·2H 2 O and total carbonation efficiency were 16.3 ± 0.4 g L −1 and 46.5% ± 1.9%, respectively (as shown in Figure 9a), and the spherical vaterite particles with diameters of 2-5 mm were recovered after each stage (as shown in Figure 9b). which confirmed that a relatively high content of amino acids could aid in the formation of the vaterite phase. The process route is illustrated in Figure 8a. The regeneration of the leaching agent is also different when the industrial by-product gypsum is indirectly mineralized via complexation. Gong et al. [55] combined the dissolution characteristics of amino acids with the leaching-mineralization of desulfurized gypsum. Because amino acids are sparingly soluble at their isoelectric points, aspartic acid can be precipitated and then recycled by adjusting the pH to the isoelectric point after mineralization. After 10 cycles, the recovery efficiency was maintained at 80%, while the dissolved CaSO4·2H2O and total carbonation efficiency were 16.3 ± 0.4 g L −1 and 46.5% ± 1.9%, respectively (as shown in Figure 9a), and the spherical vaterite particles with diameters of 2-5 mm were recovered after each stage (as shown in Figure 9b). The complexation of the gypsum by-product will be the preferred choice for indirect mineralization in the future because it uses significantly less acid and alkali, is a multifunctional additive (leaching agent, CO2 absorbent, crystal form regulator), and has a high efficiency of regeneration and circulation. However, the leaching kinetics of complexation remain unclear, and the complexation via aspartic acid requires further study. The complexation of the gypsum by-product will be the preferred choice for indirect mineralization in the future because it uses significantly less acid and alkali, is a multifunctional additive (leaching agent, CO 2 absorbent, crystal form regulator), and has a high efficiency of regeneration and circulation. However, the leaching kinetics of complexation remain unclear, and the complexation via aspartic acid requires further study.

Application of Other Technologies in the Indirect Mineralization of Industrial By-Product Gypsum
Other technologies have also been applied to improve the indirect mineralization of by-product gypsum. If the sulfate solution produced after mineralization is not used, it is equivalent to producing secondary waste. Recently, the application of bipolar membrane electrodialysis (BMED) to waste salts has attracted considerable attention [89,90]. Ho et al. [91] regenerated a mineralized NaNO 3 solution using BMED and obtained NaOH and HNO 3 solutions for recycling, which further improved the process economy ( Figure 10). Many studies have shown that combining BMED with indirect mineralization is promising [92,93].

Application of Other Technologies in the Indirect Mineralization of Industrial By-Product Gypsum
Other technologies have also been applied to improve the indirect mineralization of by-product gypsum. If the sulfate solution produced after mineralization is not used, it is equivalent to producing secondary waste. Recently, the application of bipolar membrane electrodialysis (BMED) to waste salts has attracted considerable attention [89,90]. Ho et al. [91] regenerated a mineralized NaNO3 solution using BMED and obtained NaOH and HNO3 solutions for recycling, which further improved the process economy ( Figure 10). Many studies have shown that combining BMED with indirect mineralization is promising [92,93]. To make the reaction more efficient or to control the crystal form of the calcium carbonate produced, researchers have used bubble, Venturi, and membrane electrolysis reactors to indirectly mineralize gypsum by-products. Altiner et al. [60] applied a microbubble generator to the mineralization process with gypsum as the raw material [64] and used Venturi tubes with different diameters as mineralization reactors. Both reactors shortened the reaction time and produced nano-sized calcium carbonate. Altiner et al. [10] used a Venturi reactor with an ultrasonic probe to improve the agglomeration of calcium carbonate. Konopacka-Łyskawa et al. [94] used a bubble reactor to generate homogeneous vaterite calcium carbonate. Heping [95] used membrane electrolysis to achieve low-energy phosphogypsum mineralization.

Conclusions and Prospects
In summary, indirect mineralization, which uses by-product gypsum as a calcium source, can not only facilitate industrial "double waste" treatment but also produce highvalue calcium carbonate with a higher raw material utilization rate and product purity than those of other processes. However, there are many difficulties and challenges too. On the one hand, reducing the capital cost and reagent consumption to make the entire CO2 mineralization process more economically viable. First of all, further development of leaching agents that with better cycle stability to simplify the process, such as aspartic acid, can not only facilitate the preparation of calcium carbonate products in a single-crystal form but also aid in the recovery and recycling of the leaching agents, which has both environmental and economic benefits. Secondly, coupling the indirect mineralization of by-product gypsum with the treatment of other industrial wastes will be a future development trend. On the other hand, the mineralization conditions for desulfurized gypsum and phosphogypsum are mild, whereas those for red gypsum are harsh. Therefore, understanding the internal differences in the mineralization of different gypsum by-products and providing clear information for selecting the best mineralization route will be an To make the reaction more efficient or to control the crystal form of the calcium carbonate produced, researchers have used bubble, Venturi, and membrane electrolysis reactors to indirectly mineralize gypsum by-products. Altiner et al. [60] applied a microbubble generator to the mineralization process with gypsum as the raw material [64] and used Venturi tubes with different diameters as mineralization reactors. Both reactors shortened the reaction time and produced nano-sized calcium carbonate. Altiner et al. [10] used a Venturi reactor with an ultrasonic probe to improve the agglomeration of calcium carbonate. Konopacka-Łyskawa et al. [94] used a bubble reactor to generate homogeneous vaterite calcium carbonate. Heping [95] used membrane electrolysis to achieve low-energy phosphogypsum mineralization.

Conclusions and Prospects
In summary, indirect mineralization, which uses by-product gypsum as a calcium source, can not only facilitate industrial "double waste" treatment but also produce highvalue calcium carbonate with a higher raw material utilization rate and product purity than those of other processes. However, there are many difficulties and challenges too. On the one hand, reducing the capital cost and reagent consumption to make the entire CO 2 mineralization process more economically viable. First of all, further development of leaching agents that with better cycle stability to simplify the process, such as aspartic acid, can not only facilitate the preparation of calcium carbonate products in a single-crystal form but also aid in the recovery and recycling of the leaching agents, which has both environmental and economic benefits. Secondly, coupling the indirect mineralization of by-product gypsum with the treatment of other industrial wastes will be a future development trend. On the other hand, the mineralization conditions for desulfurized gypsum and phosphogypsum are mild, whereas those for red gypsum are harsh. Therefore, understanding the internal differences in the mineralization of different gypsum by-products and providing clear information for selecting the best mineralization route will be an important future research direction. Meanwhile, an in-depth study of leaching and mineralization kinetics is required to understand indirect carbonation and provide basic data for its large-scale application.