A brief review on flue gas desulfurization gypsum recovery toward calcium carbonate preparation

Abstract

The past several years have witnessed great progress in utilization of industrial waste gypsum. Newly developed carbonation technology toward CaCO₃ preparation also reveals a significant utilization way to recover high-value products from waste gypsum, whereas there is a shortage of systematic reviews reporting the most recent progress in carbonation of flue gas desulfurization gypsum (FGDG). This review provides a timely and comprehensive summary of major achievements regarding FGDG carbonation and calcium carbonate production to address future investigation directions. We start with a brief introduction of FGDG production and utilization approaches in practical use with their advantages and disadvantages. Then we systematically summarize two types of carbonation, including a direct way and an indirect way. The direct way typically involves three steps: CO₂ capture and CO₃²⁻ formation; CaSO₄·2H₂O dissolution; CaCO₃ crystallization. High purity CaCO₃ is prepared and the polymorph of precipitated CaCO₃ is affected by many factors, such as the Ca²⁺/CO₃²⁻ ratio, reaction conditions, impurities, and additives. The indirect way involves gypsum thermal reduction, carbonation, and sulfur recovery. Finally, challenges of current work and perspectives are presented to expedite future industrialization progress and provide a promising research direction for FGDG carbonation.

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Environmental significance

The accumulation of a large amount of flue gas desulfurized gypsum (FGDG) not only occupies valuable land resources, but also the harmful substances such as fluorine and heavy metals in FGDG may increase environmental risks. In addition, plants need large quantities of limestone as a desulfurizer. Limestone mainly comes from mining, and the mining process causes significant damage to the regional ecological environment and easily causes personal injury and death accidents. Newly developed technology can transform FGDG into calcium carbonate while capturing CO₂ in the flue gas. The development of FGDG carbonation can recover Ca and S resources and contribute to the circular economy.

1. Introduction

Flue gas desulfurization gypsum is produced from the capture of SO₂ in the flue gas of fossil fuel combustion using Ca-based absorbents.^1–3 Limestone or lime slurry is a widely used Ca-based absorbent.⁴ This desulfurization process is as follows:^5,6

SO₂ + H₂O = 2H⁺ + SO₃²⁻ (1)

Ca²⁺ + SO₃²⁻ = CaSO₃ (2)

1/2O₂ + CaSO₃ = CaSO₄ (3)

CaSO₄ + 2H₂O = CaSO₄·2H₂O (4)

Finally, FGDG is produced from the suspension liquid after dewatering and washing. With strict limits on SO₂ emissions, every year a huge amount of FGDG is produced.^7,8 According to statistics, FGDG production is about 255 million tons.⁹ In China, the yield of FGDG has exceeded 100 million tons since 2017.⁵ In India, approximately 20 million tons of FGDG are expected to be generated by the year 2040.¹⁰ Due to the huge amount of production, the treatment of FGDG has become a hot issue in solid waste management in recent years.

The most common application of FGDG is to directly use it in civil infrastructure materials.^11–14 The main component of FGDG is CaSO₄·2H₂O with purity more than 90%.^15,16 Thus, it is an ideal material to replace natural gypsum.^17,18 Currently, about 70% of the total FGDG is used as raw material to produce wallboard, gypsum board, whitewashing, cement and concrete in the USA and China.^19–24 However, the supply of FGDG significantly exceeds the demand for building materials, leading to FGDG price dropping. Another application of FGDG in practice is as a resource material in agriculture and soil improvement.^25–28 For example, FGDG can provide S, P, and K fertilizers for plant growth and amend saline–alkali soil by exchange of Na.^29,30 Compared to natural gypsum, FGDG has a higher level of heavy metals.^31,32 The content ranges of Hg, Pb, Sb and Zn in FGDG are 0.198–1.33, 1.33–1.84, 4.57–10.9, and 4.26–29 mg kg⁻¹, respectively. Hence there is an environmental risk that heavy metals will migrate from FGDG to soil and plants.³³ Nowadays, the consumption of FGDG in agriculture is very limited.

The lack of a utilization way gives rise to serious economy and environmental problems. FGDG treatment is going to be an extra expense for plants. Besides, the accumulation of excessive FGDG will occupy a lot of land, as well as increase the risk of heavy metal leaching. To solve these problems, new approaches have been developed recently.³⁴ New approaches are aimed at preparing high-value chemicals such as hemihydrate, calcium carbonate, adsorptive materials and composites.^35–42 Among them, preparation of calcium carbonate from FGDG has received great attention. Calcium carbonate is widely used as a filler in polymer composites, paper, plastic, rubber, and pharmaceutical applications.^43–49 Besides, it can also function as a SO₂ absorbent in the FGD process. In this way, FGDG is regenerated by the carbonation reaction and then reused in desulfurization, as shown in Fig. 1. Preparation of calcium carbonate from FGDG not only solves the problem of FGDG accumulation and saves the natural resources of CaCO₃, but also provides by-products such as sulfur and ammonium sulfate. Furthermore, carbonation with FGDG is conducive to reducing carbon emission, by using CO₂ in flue gas as a carbon source. There are mainly two kinds of methods developed for FGDG carbonation, i.e. a direct way and an indirect way. The direct way refers to the direct carbonation reaction of FGDG with CO₂ and an alkaline reagent at low temperature. The indirect way is first to reduce CaSO₄ to CaS or CaO at high temperature and then react it with CO₂ to prepare CaCO₃.^50,51 However, a summary of recent progress in flue gas desulfurization gypsum recovery toward calcium carbonate preparation is still lacking.

Fig. 1 Schematic diagram of the FGD process with calcium recycling.

Here we mainly focus on the most advanced development of FGDG application toward CaCO₃ preparation. Specifically, we exclusively focus on the carbonation reaction under different conditions and the prepared CaCO₃. To deepen comprehension of influence factors, the corresponding mechanism is discussed. We first introduce the direct way to prepare calcium carbonate from FGDG. Then the indirect way with the recovery of sulfur is presented. Finally, a succinct summary of this review and perspectives for future development are provided to lead to an investigation of cost-effective and high-efficiency technology for industrial application.

2. Direct way

2.1 Carbonation system and pathway

The direct way is to prepare a calcium carbonate precipitate with waste gypsum at near room temperature. Soluble carbonates such as Na₂CO₃ and NH₄HCO₃ were used as CO₃²⁻ sources for CaSO₄ to CaCO₃ conversion in the early research studies.^52,53 Nowadays, with the strong demand for reducing carbon emission, more researchers are devoted to directly convert FGDG to CaCO₃ using CO₂ gas. The main component of FGDG is CaSO₄·2H₂O. The carbonation reaction of CaSO₄·2H₂O and CO₂ proceeds only if the solubility product of CaSO₄·2H₂O is larger than that of CaCO₃ in the solid–liquid system. At atmospheric CO₂ pressure, the solubility products of CaSO₄·2H₂O and CaCO₃ are affected by pH.⁵⁴ According to theoretical calculations, the solubility product of CaSO₄·2H₂O is larger than that of CaCO₃ at pH > 7.5.⁵⁵ This will lead the reaction (5) to proceed rightwards. In contrast, if pH < 7.5, CaCO₃ is going to dissolve and the reaction (5) proceeds leftwards.

CaSO₄·2H₂O + HCO₃⁻ ↔ CaCO₃ + H⁺ + SO₄²⁻ + 2H₂O (5)

Hence, the control of pH conditions is very important for FGDG direct carbonation. In order to achieve high pH conditions, FGDG is mixed with alkaline solution. The alkaline solution is commonly made using sodium hydroxide or ammonia. Accordingly, carbonation systems are named NaOH-FGDG-CO₂ and NH₃-FGDG-CO₂ respectively. As shown in Fig. 2, when using sodium hydroxide as an alkaline reagent, the carbonation processes involve three main steps:⁵⁶

Fig. 2 Schematic diagram of FGDG carbonation processes with sodium hydroxide and CO₂ (reprinted with permission from Luo et al.⁵⁶ Copyright Elsevier 2023).

(1) The dissolution of FGDG in alkaline solution to produce Ca²⁺ and SO₄²⁻.

(2) The capture of CO₂ gas using alkaline solution and generation of HCO₃⁻ and CO₃²⁻.

(3) The nucleation of CaCO₃ and formation of supersaturation in the solution phase, followed by precipitation.

Similarly, Gong et al. put forward multistep carbonation of the NH₃–gypsum–CO₂ system.⁵⁷ It involves CO₃²⁻ formation, CaSO₄·2H₂O dissolution, and CaCO₃ crystallization. CO₂ is absorbed by ammonia to form NH₂COO⁻, which then hydrolyzes to NH₃ and CO₃²⁻. Ca²⁺ in the bulk solution is consumed rapidly with the formation of a CaCO₃⁰ ion pair. The concentration of the CaCO₃⁰ ion pair increases and reaches supersaturation. After that, the ion pair concentration decreases gradually, whereas CaCO₃ precipitates obviously in the bulk solution. The rates of CaSO₄·2H₂O dissolution and CO₂ absorption determine the carbonation rate. According to the recent work about FGDG carbonation as listed in Table 1, both sodium hydroxide and ammonia are efficient alkaline reagents for FGDG carbonation with more than a 98% conversion ratio achieved. The alkali concentration will affect the conversion ratio. As the NaOH concentration increased from 0.5 mol L⁻¹ to 3 mol L⁻¹, the conversion ratio increased from 96.4% to 99.94%.⁵⁶ Compared to ammonia, sodium hydroxide is more environment-friendly, because it can avoid ammonium volatilization from ammonia.⁶⁷ In addition, sodium hydroxide can shorten conversion time of FGDG. However, the by-product of the NaOH-FGDG-CO₂ carbonation system is sodium sulfate, and its economic value is relatively low. In comparison, the ammonium sulfate by-product from the NH₃-FGDG-CO₂ system is a high-value chemical. Besides sodium hydroxide and ammonia, researchers developed FGDG carbonation methods by using amine as the alkaline reagent.^66,68,69 Wang et al. tested several kinds of amine for FGDG carbonation, including monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), 1,3-diaminopropane (DAP), piperazine (PZ), 3-amino-1-propanol (MPA), and N,N-dimethylethanolamine (DEEA).⁶⁶ Amines can enhance the absorption of CO₂ as well as promote gypsum dissolution by binding amino groups with Ca²⁺. The purity of CaCO₃ reached 93.8% when using DAP as the alkaline reagent, which was comparable to 94.4% purity when using the NaOH alkaline reagent. Interestingly, after the carbonation process, the reagent solution was successfully treated by bipolar membrane electrodialysis for amine regeneration and recovery of H₂SO₄. This could largely reduce the cost of alkaline consumption.

Table 1 List of published work about FGDG carbonation

Works	Alkaline reagent	Condition	Solid-to-solution ratio	CO2 gas	Conversion rate	CaCO3 crystal
Luo et al.^56	3 mol per L NaOH	Stirring	1 : 8	0.0 MPa CO2	99.94%	Calcite
Luo et al.^56	3 mol per L NaOH	Stirring	1 : 8	0.2 MPa CO2	99.95%	Aragonite
Wang et al.^58	25 wt% ammonia solution	Stirring + ultrasound	—	99.99% CO2	98%	Vaterite
Wang et al.^59	25 wt% ammonia solution	Stirring for 1 h	10%	99.99% CO2	—	60% vaterite and 40% calcite
Tan et al.^60	Aqueous ammonia	—	molar ratio of NH3 to CaSO4 = 2.0	99.99% CO2	90% at 40 °C, close to 100% at 80 °C	—
Lee et al.^61	25 wt% ammonia solution	Stirring	15%	15 wt% CO2, 85 wt% N2	96%	Calcite (40–90%)
Altiner et al.^62	NaOH or NH4OH	Stirring	1 : 13, 1 : 9, and 1 : 7	99.99% CO2	—	Calcite in NaOH and calcite and vaterite in NH4OH
Ding et al.^63	3 mol per L ammonia solution	Stirring	20%	75% N2, 15% O2, 10% CO2	90%	Mainly calcite with some vaterite
Lee et al.^64	NH4OH, 25 wt%	Stirring	15–50%	15% CO2 and 85% N2	95%	vaterite and calcite
Song et al.^51	Ammonia + ethanol	Stirring	20%	99.9% CO2	—	Calcite and vaterite
Liu et al.^65	Glycine + ammonia	Stirring for 1 h	—	Pure CO2	97% purity	Vaterite
Wang et al.^66	1.0 mol per L amine	Stirring	6.8%	40 vol% CO2 in N2	59.8–96.4%	Vaterite (DAP), calcite (PZ)

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2.2 Mineralization principle and modeling

In an alkaline medium, the driving force of CO₂ mineralization of desulfurized gypsum is due to the difference in solubility products between CaCO₃ and CaSO₄·2H₂O in water. As the solubility product of CaCO₃ (K_sp = 4.8 × 10⁻⁹, 298 K) is much smaller than that of CaSO₄·2H₂O (K_sp = 2.6 × 10⁻⁵, 298 K), the driving force of the reaction is large, and the theoretical conversion rate can reach more than 99%. Several studies have been conducted on modeling the CO₂ adoption, ammonium bicarbonate hydrolysis and CaCO₃ formation process. Tan et al. studied the process parameters of direct wet mineralization of CO₂ with desulfurized gypsum and established a reaction model of direct wet mineralization of CO₂ with desulfurized gypsum in an ammonia medium system.⁶⁰ There is a competition between the CO₂ absorption reaction and NH₂COO⁻ hydrolysis reaction, in which the CO₂ absorption reaction dominates over the latter. The HCO₃⁻ (or CO₃²⁻) from NH₂COO⁻ hydrolysis is consumed fast to form calcium carbonate, which would enable the HCO₃⁻ (or CO₃²⁻) concentration to vary little. Liu et al. investigated CaCO₃ crystal nucleation and growth processes in the gas (CO₂)–liquid (NH₃·H₂O)–solid (CaSO₄) three-phase system.⁷⁰ The research revealed that temperature affected CaCO₃ crystal growth more than the nucleation process. They established a model to predict the CaCO₃ particle size. Gong et al. studied modeling of multistep Ca²⁺ transfer in CaSO₄ mineralization using a gypsum disk.⁵⁷ The CaCO₃⁰ ion pair was the intermediate product in CaCO₃ induction and nucleation periods. The ion pair determined nucleation and formation of supersaturation. The modeling revealed that NH₂COO⁻ hydrolysis was the rate-limiting step in induction and nucleation periods, and the dissolution of CaSO₄·2H₂O became dominant in the growth period. An increase in the CO₂ flow rate can improve NH₂COO⁻ hydrolysis, leading to more nuclei and high supersaturation. The impeller speed can affect the dissolution rate in the growth period, but it had no remarkable effect on the concentrations of components in induction and nucleation periods.

2.3 CaCO₃ crystal and utilization

Calcium carbonate is an important raw material in many industrial processes, such as plastics, rubber, tires, paper, building materials, coatings, food, medicine and feed. Certainly, calcium carbonate from FGDG mineralization can be directly functioned as a desulphurization sorbent without impurity separation, because a desulphurization sorbent does not require highly purified CaCO₃.⁷¹ Considering the large amount of FGDG, it is a promising way to consume the CaCO₃ products. Besides, it can avoid long distance transportation. It's worth noting that calcium carbonate has three polymorphs, namely, calcite, aragonite, and vaterite,^72,73 as shown in Fig. 3. The crystal structure of CaCO₃ for FGDG carbonation has received a lot of attention because it determines specific purposes. Because vaterite CaCO₃ has good smoothness, fluidity, dispersion and wear resistance, it is widely used in the fields of rubber, paint, ink, medicine, toothpaste and cosmetics.⁷⁴

Fig. 3 Crystal structures of (A) calcite, (B) aragonite, and (C) vaterite. Ca atoms are displayed as large yellow balls, and carbonate groups are illustrated with gray (carbon) and red (oxygen) balls. Vaterite is depicted with a hexagonal P6₃/mmc structure that accounts for a partial occupancy of one-third of the carbonate groups. (Reprinted with permission from Chang et al.⁷² Copyright American Chemical Society 2017.).

It can be concluded from Table 1 that vaterite will be produced from FGDG mineralization when using ammonia. The possible principle might be as follows:⁵⁸ In the first stage, the initial Ca²⁺ concentration in the system is low (the solubility of CaSO₄·2H₂O is 2 g L⁻¹@20 °C, so the initial Ca²⁺ concentration in the solution is about 0.01 M). In the ammonia medium, when CO₂ is introduced into the reaction system, CO₂ and ammonia will react quickly to produce a large amount of CO₃²⁻; at this time, a very low Ca²⁺/CO₃²⁻ ratio is conducive to the formation of vaterite.⁷⁵ In the second stage, the mixture of (NH₄)₂CO₃ and NH₄HCO₃ is the main substance.⁷⁶ In addition, the concentration of SO₄²⁻ gradually increases with the extension of reaction time, and a higher concentration of SO₄²⁻ can stabilize vaterite in aqueous solution.⁷⁷ Therefore, the synergistic effect of low Ca²⁺/CO₃²⁻ with a higher concentration of SO₄²⁻ may be a key factor in the formation of vaterite in the second stage. In the third stage, the reaction of HCO₃⁻ with Ca²⁺ to produce vaterite may be due to the high concentration of SO₄²⁻, because a high concentration of SO₄²⁻ can stabilize vaterite in aqueous solution and inhibit its conversion to calcite. Therefore, the combined effect of the above factors promotes the formation of vaterite in the process of FGDG mineralization in the ammonia medium system.

2.4 Effect of factors on the CaCO₃ polymorph

Much effort has been made to control the polymorph of calcium carbonate from carbonation. The polymorph of CaCO₃ is affected by many factors, such as the Ca²⁺/CO₃²⁻ratio, reaction conditions, impurities, and additives. The Ca²⁺/CO₃²⁻ ratio is a key factor related to CaCO₃ crystallization growth.^78,79 A high ratio of Ca²⁺/CO₃²⁻ favors the formation of the calcite.⁷⁵ Experiments demonstrated that NaOH solution has a better Ca²⁺ extraction effect than NH₃·H₂O solution, resulting in a higher Ca²⁺/CO₃²⁻ ratio in NaOH solution.⁸⁰ Thus, it has been found that CaCO₃ tends to be pure calcite in the NaOH system, whereas vaterite was achieved in the NH₃·H₂O system.⁸¹ Another way to reduce the Ca²⁺/CO₃²⁻ratio is using high CO₂ pressure. Experiments demonstrated that calcite is generated by carbonation with NaOH and atmospheric CO₂, while aragonite is generated at CO₂ pressure 0.2 MPa.⁵⁶ As CO₂ pressure increases, CaCO₃ particle size becomes larger and purity is 0.7% lower.

Using additives is an effective way to control the CaCO₃ polymorph. Liu et al. added glycine in the NH₃-FGDG-CO₂ system.⁶⁵ As a result, calcite content in CaCO₃ reduced and vaterite content increased. When no glycine is added, around 40% is calcite and 60% is vaterite. When glycine concentration is 20 wt%, vaterite purity reached 97%. They attributed it to the formation of a Ca(NH₂CH₂COO)₂ intermediate via reactions (6) and (7), which reduced the concentration of Ca²⁺ in bulk solution and provided a lower local Ca²⁺/CO₃²⁻ ratio. Song et al. introduced ethanol into the NH₃-FGDG-CO₂ system during the induction period.⁵¹ Ethanol might block the surface and inhibit the perfect growth of calcite, by binding more strongly at the calcite surface than with water. Under stoichiometric ammonia conditions, the addition of ethanol gave rise to polymorph change from calcite to vaterite. Under excess-ammonia conditions, peanut-like aragonite crystals with dandelion-like heads were formed when 30 and 50 vol% ethanol were used. However, with ethanol more than 70 vol%, the reaction products were not CaCO₃, but were rather compounds composed of (NH₄)₂SO₄ and (NH₄)₂Ca(SO₄)₂·H₂O. Polyacrylic acid is proved to enhance gypsum dissolution.⁸² With the addition of 2.7 g per L polyacrylic acid in the NH₃-FGDG-CO₂ system, the amount of dissolved Ca²⁺ increased to 60% of the gypsum. The prepared CaCO₃ was amorphous, which could completely crystallize to calcite after exposure to air for 2.5 hours.

NH₂CH₂COOH + NH₄OH → NH₂CH₂COONH₄ + H₂O (6)

2NH₂CH₂COONH₄ + CaSO₄·2H₂O → Ca(NH₂CH₂COO)₂ + (NH₄)₂SO₄ + 2H₂O (7)

In general, impurity elements such as Si, Mg, Al, Fe, F and K exist in FGDG,⁸³ and it's worth noting the influence of impurities. CaCO₃ generated from pure CaSO₄·2H₂O with NH₃–CO₂ is 100% vaterite, while from FGDG-NH₃-CO₂ it is a mixture of vaterite and calcite.⁵⁹ According to the XRD results in Fig. 4,⁵⁹ dolomite (CaMg(CO₃)₂) in CaSO₄·2H₂O will lead to the formation of calcite using NH₃–CO₂. The dolomite particles have a negative charge in the process of carbonation of CaSO₄·2H₂O, which preferentially adsorb Ca²⁺ through the electrostatic attraction force leading to a higher local ratio of Ca²⁺/CO₃²⁻. It is reported that CaSO₄·2H₂O carbonation at a high ratio of Ca²⁺/CO₃²⁻ tends to form calcite.^84,85 This may be the reason why different polymorphs are observed. High content of Mg²⁺ or MgO can stabilizing the amorphous CaCO₃ produced from precipitate crystallization.^86–88 The presence of F, Fe, and Mg accelerates the conversion rate of calcium carbonate from thermodynamically unstable vaterite to thermodynamically stable calcite.^89,90 The effect of F is more obvious, in all samples with F, and only calcite and no vaterite are present. In the reaction of FGDG carbonation with ammonium carbonate, CO₃²⁻ in ammonium carbonate replaces SO₄²⁻ in gypsum to form CaCO₃. Therefore, the greater concentration of CO₃²⁻ ions in the solution will be conducive to the reaction. F, Fe and Mg impurities will affect the progress of ammonium carbonate hydrolysis. The hydrolysis of F⁻ produces OH⁻, leading to the increase of CO₃²⁻ ions, and the conversion rate is increased. The hydrolysis of Mg²⁺ and Fe³⁺ produces H, which reduces the concentration of CO₃²⁻. Fe(OH)₃ and Mg(OH)₂ generated from hydrolysis will cover the surface of FGDG with precipitation and reduce the reaction rate and conversion rate. The higher the concentration of Mg²⁺ and Fe³⁺, the more serious the hydrolysis and deposition, and the more the conversion rate is reduced.⁹¹

Fig. 4 XRD pattern of CaCO₃ obtained in the presence of varying amounts of dolomite particles: (a) CaCO₃ sample prepared in the absence of dolomite; (b–f) CaCO₃ prepared with 0.5 wt%, 1.0 wt%, 3.0 wt%, 5.0 wt% and 7.0 wt% dolomite particles, respectively. Abbreviations: V, vaterite; C, calcite; D, dolomite. (Reprinted with permission from Wang et al.⁵⁹ Copyright Elsevier 2019.).

Reaction conditions such as temperature, CO₂ flow rate, solvent ratio, alkaline concentration and stirring rate are important factors. Temperature has a great influence on CaCO₃ polymorphs according to the work by Lee et al.⁶¹ At 20 °C and 40 °C, the carbonation product is a mixture of spherical vaterite and calcite with poorly developed edges. At 60 °C, most vaterite will transform into calcite. At 80 °C, needle-flower-like aragonite is newly formed. The use of ultrasound can help control the crystallization process and the formation of CaCO₃ polymorphs due to sonocrystallization.^92,93 Application of ultrasound technology in FGDG carbanion can achieve high conversion efficiency and pure vaterite in the ammonia system.^58,94 The CaSO₄·2H₂O conversion efficiency is about 60% at 25% ultrasonic amplitude (20 kHz, 650 W), while both conversion efficiencies at 50% and 75% ultrasonic amplitudes were around 98%. The stirring speed has a relatively minor effect on the formation of CaCO₃ polymorphs. It mainly affects the particle size and morphologies. Increasing stirring speed can reduce CaCO₃ particle size. The particle size at 450 rpm and 675 rpm is 23.45 μm and 18.38 μm, respectively.⁵⁶

2.5 FGDG leaching and mineralization technology

As the low solubility of FGDG inhibited the carbonation reaction, researchers developed new mineralization technology with leaching. First, Ca²⁺ is leached from FGDG using a leaching agent. Secondly, the leachate and solid residue are separated. Finally, the Ca²⁺ in the leachate is carbonated to produce CaCO₃. By leaching technology, the impurities in FGDG are removed, and thus high purity CaCO₃ can be obtained. Several leaching agents have been developed, including inorganic salts and organic leaching agents.

NaCl, NH₄Cl and CH₃COONH₄ were inorganic salt based leaching agents. Chen et al. explored the mineral mineralization process of phosphogypsum under the action of NaCl and NH₄OH.⁹⁵ The dissolution efficiency of phosphogypsum under the optimal conditions was 49.42%. One ton of phosphogypsum can chelate 115 kg of CO₂ during mineralization and produce 262 kg of CaCO₃. In the whole preparation process, NaCl was recycled 4 times, and the corresponding reaction efficiency was above 60%. Ding et al. experimentally and theoretically studied the mineral mineralization of phosphogypsum with ammonium acetate.⁹⁶ Under the optimal conditions, the dissolution rate of calcium is 98.1% and the mineralization efficiency is 98.32%. The structure of carbonated products was controlled by adjusting the reaction temperature and the amount of ammonia. The phosphogypsum leached with NH₄Cl showed that the optimum dissolved amount of CaSO₄·2H₂O was 18.7 g L⁻¹ and the carbonation rate was 98.22%.⁹⁷

Organic leaching agents contain carboxyl (–COOH) and amino (–NH₂) groups in their molecular structures that can form a soluble chelate with Ca²⁺. Using sodium gluconate as a phase transfer agent, Yang et al. synthesized calcium carbonate using phosphogypsum as a raw material at room temperature and atmospheric pressure through a simple and effective “phase transfer – precipitation” route.⁹⁸ The results showed that the presence of sodium gluconate inhibited the nucleation and growth of calcite and promoted the formation of vaterite. Gong et al. used aspartic acid as the leaching agent, and the results showed the amount of dissolved CaSO₄·2H₂O and the total carbonation efficiency during cycling were determined to be 16.3 ± 0.4 g L⁻¹ and 46.5 ± 1.9%, respectively.⁹⁹

3. Indirect way

3.1 Thermal reduction of CaSO₄

The indirect way for calcium carbonate preparation from FGDG is quite different from the direct way. Firstly, CaSO₄ in gypsum is converted to CaS via thermal reduction. Secondly, calcium carbonate is produced via CaS carbonation. In the thermal reduction reaction, carbon, carbon monoxide gas or hydrogen gas is used as a reductant and mixed with gypsum at high temperature.^100–102 Typically, carbon is more accessible and thus it has been widely used. The thermal reduction process using carbon as a reductant is present in reaction (8).

CaSO₄·2H₂O + 2C → CaS + 2CO₂ + 2H₂O (8)

The reduction reaction is an endothermic reaction. Enthalpy (ΔH°), entropy (ΔS°), and Gibbs-free energy (ΔG°) decrease over the temperature and the proportionality constant (k) increases, as shown in Table 2 (data from Tewo et al.¹⁰³).

Table 2 The effect of temperature on ΔH°, ΔS°, ΔG°, and k (data are reproduced with permission from Tewo et al.¹⁰³ Copyright Elsevier 2019)

T (°C)	ΔH° (kJ mol^−1)	ΔS° (kJ mol^−1)	ΔG° (kJ mol^−1)	k
25	171.0	367.0	61.5	1.7 × 10^−11
156	172.0	367.0	13.5	2.3 × 10^−2
286	170.1	365.6	−34.3	1.6 × 10^3
417	167.3	361.2	−81.9	1.6 × 10^6
547	162.9	355.4	−128.7	1.6 × 10^8
678	157.0	348.8	−174.7	3.9 × 10^9
939	141.1	334.1	−263.8	2.4 × 10^11
1069	131.0	326.2	−307.0	8.8 × 10^11
1200	119.7	318.1	−349.0	2.4 × 10^12

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Tewo et al. used the Pyrosim Mintek model to predict that CaSO₄ to CaS conversion could be increased from 26.8 to 85.0% when the temperature was raised from 500 to 1100 °C.¹⁰³ The appropriate temperature range is 700−900 °C for a high conversion ratio.¹⁰⁴ If the temperature exceeds 1000 °C, self-decomposition reactions and re-reactions probably occur according to reactions (9)–(13). Tan et al. conducted thermal reduction experiments with a 2 : 1 weight ratio of FGDG to carbon powder, and gypsum was completely decomposed into calcium sulfide by calcining at 900 °C for 30 min.¹⁰⁵ However, only the oldhamite phase has been observed after in the temperature range of 900 to 1100 °C. Liu et al. achieved a FGDG conversion ratio of 97.89% and CaS purity of 90.42% with 30% carbon content after thermal reduction at 900 °C for 2 hours.¹⁰⁴ They indicated that thermal reduction was completed via step-by-step reactions, and SO₄²⁻ was transformed into SO₃²⁻–S₂²⁻–S²⁻. Confirmed by a scanning electron micrograph, a hollow structure was formed gradually from the outside to the inside. Phosphogypsum is also a major source of industrial waste gypsum. Laasri et al. tested thermal reduction of phosphogypsum by CO gas.¹⁰⁶ At 600 °C, calcium sulfide (CaS) was mainly formed with high CO partial pressure (>50%), and calcium oxide (CaO) was mainly formed with lower CO partial pressure (<20%). At 1000 °C and above, CaSO₄ was completely converted to CaS, CaO, and minor co-products due to the presence of impurities in phosphogypsum. One should note that other products were also found after thermal reduction apart from CaS and CaO, such as Ca₂Fe₂O₅ and Ca₂Al(AlSiO₇).^107–109 The reason is that CaSO₄ will react with the impurities (Al, Fe, Si etc.) in gypsum at high temperature.

CaSO₄ → CaO + SO₂ + 0.5O₂ (9)

CaS + 3CaSO₄ → 4CaO + 4SO₂ (10)

3CaS + CaSO₄ → 4CaO + 2S₂ (11)

CaS + 2O₂ → CaSO₄ (12)

CaS + 1.5O₂ → CaO + SO₂ (13)

The carbon to gypsum ratio is a vital factor. Insufficient carbon content will lead to a low conversion efficiency. However, too much carbon will result in carbon waste and low CaS purity. It is reported that there is an increasing trend in conversion efficiency from 57.4 to 83.8% when the carbon ratio is increased from 1 : 1 to 4 : 1.¹⁰³ Liu et al. investigated the relationship between carbon content and conversion.¹⁰⁴ At 10% carbon content, conversion efficiency was about 41% with 25% CaS purity. The conversion efficiency increased to above 97% and the CaS purity was about 90% at 30% carbon content. Further increasing the carbon content had a slight influence on conversion efficiency, but the CaS purity reduced less than 80% due to carbon residues. Motaung et al. made pelletized gypsum from acid mine drainage (AMD) neutralization by adding starch or cellulose as a binder into a mixture of gypsum and coal.¹¹⁰ The CaS yield of pelletized gypsum thermo-reduction was improved from 60% without starch or cellulose to 71% or 67% with 8% starch or cellulose, respectively. The CaS yield could further reach above 90% with gypsum and starch at a ratio of 1 : 2.9 at 1050 °C for 20 min.

3.2 CaS carbonation and sulfur recovery

The produced CaS can react with H₂O and CO₂ for carbonation via reaction (14). However, the produced CaCO₃ is low-grade (i.e. <90 mass%) which comprised a mixture of calcite and vaterite.¹⁰² Carbonation of CaS is difficult and sluggish in aqueous solution, and more than 3 hours are needed for CaCO₃ to occur.¹⁰⁵ This is due to CaS being a sparingly soluble salt in water.¹¹¹ Accordingly, de Beer developed a new carbonation method by sparging H₂S gas to aqueous CaS suspensions.⁵⁰ H₂S reacted with CaS to form water-soluble Ca(HS)₂via reaction (15). Thus 91.7% CaS was extracted into the aqueous phase and separated from the impurities in the solid phase. Then CO₂ gas is introduced for carbonation by reaction (16). High purity (99.5%) CaCO₃ is produced. Rhombohedral structured calcites with different particle sizes were demonstrated after Ca(HS)₂ carbonation with different initial Ca²⁺ concentrations, as shown in Fig. 5.

CaS + H₂O + CO₂(g) → H₂S(g) + CaCO₃ (14)

CaS + H₂O + H₂S → Ca(HS)₂ + H₂O (15)

Ca(HS)₂ + H₂O + CO₂ → CaCO₃ + H₂S (16)

Fig. 5 SEM images (top row 1000× and bottom row 5000× magnification) of high-grade CaCO₃ produced at initial Ca²⁺ concentrations of (a) 450 mmol L⁻¹, (b) 900 mmol L⁻¹ and (c) 1800 mmol L⁻¹. (Reprinted with permission from de Beer et al.⁵⁰ Copyright Elsevier 2015.).

The H₂S produced in reactions (14) and (16) can be further oxidized to high-value elemental sulfur.^112,113 High pH (9.0) is required for the dissolution of hydrogen sulfide, and Fe-based chelates are widely used to promote the reaction. The main chemical reaction can be simplified in reaction (17):¹¹⁴

H₂S + 2Fe³⁺Lⁿ⁻ → S⁰ + 2Fe²⁺Lⁿ⁻ + 2H⁺ (17)

where Lⁿ⁻ denotes an organic ligand with n− charges and S⁰ represents a zero-valence sulfur product. Tan et al. successfully recovered elemental sulfur with 0.5–3 μm in diameter from FGDG.¹⁰⁵

4. Perspectives

4.1 Technical and economic analyses

Although many studies have reported that a high FGDG conversion rate can be achieved, the results are obtained from bench and pilot scale units. There are still many challenges to further development of practical engineering projects. Cost and profit mainly depended on the raw materials (ammonia or sodium hydroxide) and byproducts (ammonium sulfate, sodium sulfate, and elemental sulfur). However, the prices of these chemicals fluctuate wildly, leading to an economic uncertainty. Raw material sources and byproduct utilization deserve further investigation for practical engineering projects. Advanced technologies to produce high quality CaCO₃via impurity separation and polymorph control are also beneficial for practical economic viability. Table 3 summarizes the comparison of FGDG carbonation approaches. The challenges that each face in approaching practical application are discussed in detail in this section.

Table 3 Comparison of the direct way and indirect way

Approaches	Reaction processes	Technical and economic analyses
Direct way	NaOH-FGDG-CO2 or NH3-FGDG-CO2	Advantages of simple processes, high conversion rates and mild reaction condition. Issues of ammonia escape, device corrosion and by-product application
Indirect way	1. Thermal reduction; 2. CaS carbonation; 3. elemental sulfur preparation	Insufficient conversion rate. High purity CaCO3. Suitable for factories with cheap energy sources

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The direct way has many advantages such as convenient operation, low energy cost and high conversion rates, and it is promising for scaling up. FGDG carbonation can be accomplished in a single step, which is very beneficial to reduce equipment investment and post-production operation. The direct way is carried out under mild reaction conditions, with temperature less than 100 °C and pressure less than 0.2 MPa. According to economic analysis, energy cost for 1 ton of FGDG is about 1.4 ton of steam and 150 kW h of electricity, including the crystallization process of the ammonium sulfate by-product.³⁶ According to the data in Table 1, conversion rates of CaSO₄ can reach more than 90%, and some even approach 100%. Developing large reactors on an industrial scale is a technical key point to achieve satisfactory conversion rates on a large scale. It is worth noting that Cl and F are enriched in FGDG from flue gas.^115,116 The issues of equipment corrosion by Cl and F should be taken into consideration in practical use. Another challenge is the value of the sulfate by-product. For the NaOH-FGDG-CO₂ approach, Na₂SO₄ is produced. For the NH₃-FGDG-CO₂ approach, (NH₄)₂SO₄ is produced. We have conducted a market survey on the price of raw materials in China, and the price of (NH₄)₂SO₄ is three to five times that of Na₂SO₄. Thus, the NH₃-FGDG-CO₂ approach is more scalable in economy. However, if a large amount of (NH₄)₂SO₄ is produced from FGDG, there would be a risk of price drop. Besides, the ammonia escape is still a common environmental problem.^117–119

The indirect way is a multistep approach, which is more complicated than the direct way. The FGDG thermal reduction step requires high temperature around 1000 °C, resulting in large energy consumption. The energy input is about 4060 kJ per kg-FGDG for thermal reduction.¹²⁰ As for the indirect way, it is meaningful to combine it with other industrial processes which produce plenty of cheap methane, hydrogen, carbon dioxide and petroleum coke by-products.^121–123 Beneficially, the indirect way can avoid the evaporative crystallization process and save energy. In comparison, the conversion rate of the indirect way is lower than that of the direct way. The FGDG to CaS efficiency is usually less than 90%. Efforts should be made to develop optimum reaction conditions as well as efficient reaction devices. One advantage of the indirect way is that it is possible to prepare high purity CaCO₃ from CaS. By forming water-soluble Ca(HS)₂, water-insoluble impurities are removed, and the CaCO₃ purity reaches 99.5%.⁵⁰

4.2 Direct way coupled with NaOH regeneration

The direct way has many technical advantages and is promising for practical use. But stakeholders are usually more concerned about economic feasibility. Raw materials and energy consumption are the main operation costs. Raw materials depend on the price of NaOH or NH₃OH. Energy is mainly consumed in the crystallization of the Na₂SO₄ or (NH₄)₂SO₄ by-product. Wang et al. proposed a way to apply amine for carbonation and recycle the protonated amine by bipolar membrane electrodialysis (BMED).⁶⁶ Inspired by their work, it may be feasible to develop a NaOH–Na₂SO₄ circular system by bipolar membrane electrodialysis to reduce the cost of NaOH consumption and avoid energy consumption for crystallization.¹²⁴ As shown in Fig. 6, by introducing bipolar membrane electrodialysis into the NaOH-FGDG-CO₂ system, the by-product of Na₂SO₄ can be split into NaOH and H₂SO₄.¹²⁵ In this way, the in situ regenerated NaOH is reused for FGDG carbonation, instead of raw material procurement. Operation cost comparisons are shown in Table 4. The data of energy consumption for direct FGDG mineralization without BMED were obtained from a pilot plant at a scale of 3 ton per day located in Zhejiang Province, China. For 1000 kg of FGDG, reaction heating consumes about 400 kg of steam and evaporative crystallization consumes about 1000 kg of steam. The quantities of raw materials and products are calculated according to the stoichiometric ratio. When 1000 kg of FGDG was used, 476 kg of CaCO₃ and 676 kg of Na₂SO₄ were produced with the use of 381 kg of NaOH. Likewise, 629 kg of (NH₄)₂SO₄ was obtained when using 334 kg of ammonia. The price of chemicals is the market price in China. The operation costs for NaOH-FGDG-CO₂ and NH₃-FGDG-CO₂ approaches to process 1 ton of FGDG are $139 and $64, respectively. The difference is mainly due to the market price variance of Na₂SO₄ and (NH₄)₂SO₄. Although the NH₃-FGDG-CO₂ approach demonstrates better cost-effectiveness, the use of ammonia increases environmental risks. Interestingly, by coupling BMED with the NaOH-FGDG-CO₂ approach, operation cost is reduced to $67, close to that of NH₃-FGDG-CO₂. The energy consumption value of BMED is set to 3 kW h per kg-NaOH according to reported data.^118,126 This is mainly due to the saving of raw materials and steam consumed by evaporation crystallization. Considering that raw material prices are wildly affected by the market, BMED is a promising way to obtain sustainable alkaline supply and withstand market risk.

Fig. 6 Schematic diagram of FGDG carbonation coupled with bipolar membrane electrodialysis.

Table 4 Operation cost comparisons of the direct way with and without BMED

		NH3-FGDG-CO2	NaOH-FGDG-CO2	NaOH-FGDG-CO2 with BMED
Materials input
Ammonia	Input	334 kg	0	0
	Cost	$175 USD	0	0
NaOH	Input	0	381 kg	0
	Cost	0	$164 USD	0
Energy input
Steam	Input	1400 kg	1400 kg	400 kg
	Cost	$59 USD	$59 USD	$17 USD
Electricity	Input	0	0	1143 kW h
	Cost	0	0	$114 USD
Product output
CaCO3	Output	476 kg	476 kg	476 kg
	Income	$55 USD	$55 USD	$55 USD
(NH4)2SO4	Output	629 kg	0	0
	Income	$115 USD	0	0
Na2SO4	Output	0	676 kg	0
	Income	0	$29 USD	0
H2SO4	Output	0	0	460 kg
	Income	0	0	$10
Total cost		$64 USD	$139 USD	$66 USD

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5. Conclusion

In this review, we mainly summarize the most recent developments of calcium carbonate recovery from FGDG. There are mainly two kinds of methods developed for FGDG carbonation. i.e. the direct way and indirect way. The direct way is gypsum carbonation with CO₂ in alkaline solution at near room temperature. The indirect way is thermal reduction of gypsum first and then carbonation of the thermal reduction product.

The direct way typically involves three stages: CO₂ capture and CO₃²⁻ formation; CaSO₄·2H₂O dissolution; CaCO₃ crystallization. NaOH-FGDG-CO₂ and NH₃-FGDG-CO₂ are widely studied carbonation systems. The polymorph of CaCO₃ is affected by many factors, such as the Ca²⁺/CO₃²⁻ ratio, reaction conditions, impurities, and additives.

The indirect way involves gypsum thermal reduction, carbonation, and sulfur recovery. Thermal reduction is preferred to achieve a high conversion ratio at around 900 °C with the addition of carbon as the reductant. The carbonation of CaS is sluggish and produces low purity CaCO₃ and H₂S gas by-products. Transformation of CaS into Ca(HS)₂ by H₂S before carbonation can purify the CaCO₃ product. Finally, high-value elemental sulfur can be recovered from H₂S oxidation.

In comparison, the direct way has advantages of simple processes, a high conversion rate and mild reaction conditions. The indirect way can obtain high purity CaCO₃ and is suitable for factories with cheap energy sources. The combination of the direct way and BMED technique has significant advantages. Although there are several approaches developed for FGDG carbonation, pilot and industrial applications are needed to study technical and economic feasibility in the future.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

Wei Xu contributed to conceptualization, writing of the original draft, review & editing. Chunhong Liu contributed to conceptualization, writing – review & editing. Kaimin Du contributed to conceptualization and writing of the original draft. Qiangsheng Gao contributed to writing – review & editing. Zheming Liu contributed to writing – review & editing. Weijian Wang contributed to writing of the original draft.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Zhejiang Energy Group Science and Technology Project.

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