Advances on resource utilization of semi-dry desulfurization ash by thermal decomposition: a high-efficiency and low-temperature method for large-scale processing

Abstract

The semi-dry flue gas desulfurization ash (SFGDA) is an industrial waste generated by the semi-dry desulfurization process, and its resources have been continuously attracted attention. Through the method of heat decomposition, the SFGDA decomposed into CaO and SO₂ has emerged as a prominent research topic. This paper summarizes various of research workers, who revealed that the decomposition temperature of CaSO₄ in SFGDA is greater than 1678 K and 1603 K in the air atmosphere and N₂ atmosphere, respectively, presenting challenges such as high energy consumption and limited economic feasibility. On the one hand, the effects of CO and C regulating the pyrolysis atmosphere on reducing the pyrolysis temperature were reviewed. On the other hand, the impact of additives such as Fe₂O₃ and FeS₂ was considered. Ultimately, the joint effects of regulating atmosphere and additives were discussed, and an efficient and low-temperature decomposition route was obtained; adding solid C source and Fe₂O₃ for pyrolysis reaction, the decomposition temperature of CaSO₄ can be reduced by at least 230 K and desulfurization efficiency exceeds 95% under the condition of micro-oxidizing atmosphere. Moreover, the CaO resulting from SFGDA decomposition can be further synthesized into calcium ferrite, while the enriched SO₂ can be utilized for the production of industrial sulfuric acid, which holds promising prospects for large-scale industrial applications.

Introduction

SO₂, the primary culprit behind acid rain, poses significant threats to the Earth's ecosystem, human society, and economy, with the steel and electricity sectors being major contributors (Liu et al. 2020; Shi et al. 2017). Given to its severe environmental contamination, SO₂ emission is currently under rigorous regulation by the State Environmental Protection Agency, creating flue gas desulphurization an enormously relevant issue. It is worth noting that the implementation of desulfurization technology in coal-fired power plants and the steel industry has contributed to SO₂ emissions in 2021 accounting for 12.97% of those in 2012, but desulfurized ash will increase proportionately (Wang et al. 2023; Yi et al. 2023), as illustrated in Fig. 1.

Fig. 1

Sulfur dioxide emissions in China (2012–2021)

The prevailing desulfurization technologies in the current scenario can be categorized into wet desulfurization technology, dry desulfurization technology, and semi-dry desulfurization technology based on the type of desulfurization agent employed (Zhai et al. 2017; Mjalli et al. 2014; Li et al. 2022). However, the dry desulfurization method exhibits significantly lower desulfurization efficiency compared to wet and semi-dry techniques, leading most enterprises to abstain from its adoption. Although the wet desulfurization process achieves a desulfurization effectiveness exceeding 90% (Li et al. 2022), it suffers from drawbacks such as a protracted process flow and the generation of substantial waste liquid and desulfurized gypsum, severely impeding its marketability and utilization. As a result, the conventional desulfurization process is being increasingly replaced by the more advantageous semi-dry desulfurization method, which entails lower costs and eliminates the generation of excess waste liquid.

Nonetheless, the implementation of the semi-dry desulfurization method by steel mills and power plants gives rise to a considerable quantity of SFGDA, comprising primarily of CaSO₃, CaSO₄, f-CaO, Ca(OH)₂, CaCl₂, and CaCO₃. (Liang et al. 2011; Zhang et al. 2012; Li et al. 2019). Due to its intricate chemical composition, SFGDA finds limited application in conventional sectors such as cement and building materials, and is predominantly regarded as solid waste, leading to its extensive storage and landfilling. One the one hand, the activated CaO and CaCl₂ present in SFGDA negatively impact on the stability and strength of construction materials. The long-term hydration process involving CaO and H₂O generates Ca(OH)₂, the Ca(OH)₂, which leads to volumetric expansion of buildings and structures. In addition, the presence of deliquescent properties and the release of chloride ions from CaCl₂ in SFGDA can cause yellow stains on building walls and corrosion of steel bars, posing a significant threat to building safety.

On the other hand, in accordance with national standards, the mass percentage content of SO₃ in ordinary cement or non-fired bricks should not exceed 3.5 wt.%, restricting the usages of desulfurization ash (Koralegedara et al. 2019; Wang et al. 2005). The prospect of secondary pollution arising from the release of SO₂ from SFGDA under heated or acidic conditions is deeply concerning. Excessive incorporation of SFGDA into building materials is inappropriate, and a substantial amount of unused SFGDA remains stockpiled, thereby necessitating urgent attention to address the utilization of SFGDA.

Recently, the high-temperature pyrolysis process of decomposing CaSO₄ and CaSO₃ in SFGDA into CaO and SO₂ has garnered significant attention in research. The CaO generated from the pyrolysis of the desulfurization ash is recycled as a calcium source, while the SO₂-rich flue gas undergoes dust removal, washing, and catalytic conversion to produce sulfuric acid, enabling comprehensive utilization of Ca and S in SFGDA. However, the high decomposition temperature of pure CaSO₄ reaching 1673 K, poses challenges in practical applications due to elevated production costs and energy consumption. To reduce the decomposition temperature of CaSO₄, experts have conducted extensive research using various methods to reduce the decomposition temperature of CaSO₄. Currently, there is a lack of comprehensive comparison and evaluation of these methods. Therefore, the primary aim of this review is to summarize recent advancements in the low-energy and low-environmental-risk thermal decomposition of SFGDA. This includes examining the fundamental characteristics of SFGDA, the decomposition behavior of SFGDA in inert, oxidizing, and reducing atmospheres, as well as the effects of solid additives. The present review will serve as a valuable reference for future investigations focused on highly efficient pyrolysis and the subsequent utilization of SFGDA, thereby contributing to the progress of sustainable materials and energy production research.

Physicochemical properties of SFGDA

Wei et al. (Wei et al. 2021) reported that the SFGDA generated by the iron and steel industry exhibits a varied color palette, encompassing shades of pink, light gray, and dark red, attributed to the presence of iron oxide compounds originating from the raw materials employed in the smelting process. Figure 2 depicts SFGDA from a steel industry with an inconsistent shape, a smooth surface, and an unconsolidated construction (Liu et al. 2010). What is more, the obscuration of SFGDA in steel plants is 15.05%, which is not conducive to apply untreated (Yu et al. 2018).

Fig. 2

SEM image of SFGDA from steel factory. (Liu et al. 2010)

Furthermore, Liu et al. (2010) confirmed that the particle-size distribution of the SFGDA is from 3.42 to 13.77 μm and the median size is 4.18 μm, shown in Table 1. Surface analysis revealed that the SFGDA possesses a substantial specific surface area of approximately 7940 m²∙kg⁻¹, indicating its pronounced chemical reactivity.

Table 1 Particle size characteristic parameters of SFGDA in the steel factory

Table 2 presents the composition of semi-dry flue gas desulfurization ash (SFGDA) obtained from steel plants, which includes components such as CaO, CaSO₃, CaSO₄, f-CaO, SiO₂, Al₂O₃, Fe₂O₃ and MgO. and magnesium oxide (MgO). Furthermore, the SFGDA produced by iron and steel enterprises shows an average content of 32.70 wt.% CaO, 16.81 wt.% CaSO₃, and 16.91 wt.% CaSO₄, highlighting the abundance of calcium and sulfur resources. Predicated on the thermophysical properties of SFGDA, several studies have been initiated with the thermal decomposition for SFGDA.

Table 2 Chemical characteristics of SFGDA from steel plants. (Liu et al. 2010)

Thermal decomposition of SFGDA in inert and oxidizing atmosphere

The elucidation of the thermal decomposition mechanism of carbonate, sulfite, and sulfate compounds is not only intriguing in its own right but also provides valuable insights into the physical and chemical transformations occurring within SFGDA (Yu et al. 2018). Yang et al. (2019a, b) and Zhou et al. (Zhou et al. 2017) discovered that temperature plays a significant role in the decomposition of desulfurization ash under inert or oxidizing conditions. In a nutshell, the thermogravimetric experiment confirmed the existence of five distinct stages during the heating process of SFGDA in an N₂ atmosphere. In reaction stage I, thermodynamic analysis revealed a 2 wt.% reduction in SFGDA at 473 K, attributed to the desorption of adsorbed water and the loss of crystalline water from compounds such as CaSO₃∙0.5H₂O, CaSO₄∙2H₂O.

Consequently, the transformation of Ca(OH)₂ into CaO occurred below 673 K. In stage IV, as depicted in Fig. 3 and summarized in Table 3, the thermal decomposition of CaCO₃ occurred within the temperature range of 953–1073 K. Characterized as Fig. 3 and Table 3, the thermal decomposition of CaSO₃ in SFGDA commenced around 1083 K and concluded approximately at 1313 K during stage IV. In stage V, the release of SO₂ can be attributed to the minor decomposition of CaSO4 at 1473 K, shown in Table 3. What’s more, the decomposition pattern of SFGDA in an N₂ atmosphere exhibits similarities to its calcination in an Ar atmosphere.

Fig. 3

TGA curves of semi-dry sintering flue gas desulfurized ash in N₂ and air. (Zhou et al. 2017)

Table 3 Chemical reaction of SFGDA under different temperature changes

The pyrolysis process of CaSO₃ in SFGDA also can be divided into five stages in air. Initially, the SFGDA undergoes the release of both free water and structured water below 473 K. Subsequently, Ca(OH)₂ decomposes into CaO and H₂O below 673 K. Between 673 and 953 K, the primary reaction observed is the oxidation of CaSO₃, resulting in the formation of CaSO₄ as the major product. In stage IV, as the temperature range up to 953–1073 K, the CaO and CO₂ will be generated by CaCO₃ component in the SFGDA. During stage V, CaSO₄ decomposes, yielding CaO and SO₂, which mirrors the reactions observed in an inert atmosphere. In comparison to the former stages, the generation of CaSO₄ from CaSO₃ in the SFGDA during the oxidation stage is not conducive to low-temperature and high-efficiency decomposition.

Figure 4 (a) illustrates the decomposition efficiency and f-CaO content of CaSO₃ in SFGDA following calcination. At 1073 K, in SFGDA undergoes decomposition, resulting in the production of CO₂ and CaO. And then, CaO could rapidly adsorb surrounding SO₂ to synthesize CaSO₃, which reduces the decomposition efficiency of CaSO₃. The calcined product of SFGDA in an argon atmosphere reveals the presence of CaS between 1073 and 1373 K, indicating the decomposition of calcium sulfite and the formation of CaS and CaSO₄. Evidently, achieving comprehensive desulfurization of CaSO₃ in an inert atmosphere presents a challenge due to the occurrence of Eq. 8.

Fig. 4

Effect of temperature on decomposition of SFGDA and phase transition in Ar. (Yang et al. 2019a, b)

$$4{\mathrm{CaSO}}_{3}(\mathrm{s})\to \mathrm{CaS}(\mathrm{s})+3{\mathrm{CaSO}}_{4}(\mathrm{s})$$

(8)

Finally, through thermogravimetric non-isothermal experimental research, it was observed that the initial decomposition temperature of analytically pure anhydrous CaSO₄ powder in a high-purity N₂ atmosphere is 1433 K, while the final decomposition temperature is 1603 K (Ping et al. 2004). Concomitantly, the results indicate that the initial decomposition temperature is 1506 K and the final reaction temperature is 1678 K under an air stream. It is intriguing to note that the prolonged duration of the initial and final reactions is facilitated by an elevated oxygen concentration. Maintaining a specific oxygen concentration in the reaction gas leads to the decomposition of approximately 92 wt.% of CaSO₄. As Eq. 5, achieving a higher desulfurization rate poses significant challenges due to the partial coating of CaO on the unreacted CaSO₄, thereby impeding the complete decomposition of CaSO₄.

Considering the current predicament of incomplete and high-temperature decomposition of CaSO₄ in SFGDA, there is an urgent need to reduce the initial reaction temperature for synthesizing CaO with minimal energy consumption. Such an endeavor would signify a noteworthy advancement in the quest for sustainable and energy-efficient methodologies in the production of materials possessing desirable attributes.

SFGDA decomposed by reducing gas

Given the distinctive attributes of SFGDA, a plethora of applications can be envisaged for enhancing gas decomposition processes in the realm of low-temperature pyrolysis. Especially, reducing gas (Robbins 1966; Tian et al. 2010; Yang et al. 2019a, b; Shen et al. 2008; Diaz-bossio et al. 1987; Zheng et al. 2017; Song et al. 2008; Song et al. 2009; Sunphorka et al. 2019; Zhang et al. 2013; Simei et al. 2017), CO is considered to be of particular efficiency to reduce and decompose CaSO₃ and CaSO₄ to prepare CaO, CaS and SO₂ excellently.

Formation of CaO by reducing gas

In a previous investigation undertaken by Robbins (1966), the adsorption of gas and polymorphic transitions were scrutinized throughout the CaSO₄ decomposition process employing reducing gas for CaO synthesis. The production of CaO can be conceptualized as involving dissociation and nucleation phenomena. According to the findings of the adsorption experiments, it was noted that CO does not display notable adsorption onto the CaSO₄ surface; instead, it undergoes direct surface reactions. The gaseous CO molecules engage in surface reactions with oxygen molecules present on the CaSO₄ surface, resulting in the adsorption and formation of SO₂ and CO₂ compounds on CaO, as depicted in Eq. 9. And then, CO₂ would rapidly desorb into the gas phase, as Eq. 10. The final control step is the direct desorption of SO₂ and the formation of CaO nuclei, as Eq. 11. Once an adequate number of CaO nuclei are formed, the decomposition rate increases. As hypothesized by Robbins (1966), the surface of the CaO core offers a rapid pathway for the desorption of CO₂ and SO₂, facilitating the rapid growth of the nucleus and enhancing the release of SO₂. Thus, the desorption rate is heavily influenced by factors such as gas velocity, particle size, and CO concentration.

$$\mathrm{CO}(\mathrm{g})+{\mathrm{CaSO}}_{4}\to \mathrm{CaO}\cdot {\mathrm{SO}}_{2}\cdot {\mathrm{CO}}_{2}$$

(9)

$$\mathrm{CaO}\cdot {\mathrm{SO}}_{2}\cdot {\mathrm{CO}}_{2}\to \mathrm{CaO}\cdot {\mathrm{SO}}_{2}+{\mathrm{CO}}_{2}(\mathrm{g})$$

(10)

$$\mathrm{CaO}\cdot {\mathrm{SO}}_{2}\to \mathrm{CaO}+{\mathrm{SO}}_{2}(\mathrm{g})$$

(11)

Conversely, it was observed that the reductive decomposition rate of CaSO₄ diminishes as the gas velocity surrounding the particles increases. According to Robbins’s proposed mechanism for CaSO₄ reduction, it is suggested that a portion of the SO₂ species undergoes reduction to elemental S on the CaO surface, leading to a reduction in the SO₂ concentration at the interface. It is plausible that higher flow rates could contribute to an elevation in the SO₂ concentration on the CaO surface, considering the abundance of SO₂ in the gas stream. The higher concentration of SO₂ on the surface hinders the direct desorption of SO₂ from CaSO₄ and the nucleation of CaO. Additionally, decreasing the particle size increases the maximum desulfurization rate of CaSO₄, while having no significant impact on the initial desulfurization rate. On the other hand, Robbins (1966) and Wheelock (Wheelock 1960) stated that an increase in CO₂ concentration slightly decreases the maximum desulfurization rate, but does not noticeably affect the initial rate. Furthermore, it is important to maintain a low concentration of CO in the CO-CaSO₄ reaction system, as excessive CO concentration is unfavorable for the formation of CaO. Zhao et al. (Zhao et al. 2016) investigated the relationship between SO₂ concentrations, the fraction of released S, and different CO concentrations, specifically 0.10%, 0.25%, 0.50%, and 1.00%, Fig. 5.

Fig. 5

Effect of CO concentrations on SO₂ concentrations and fraction of S released. (Zhao et al. 2016)

The results demonstrated that as the concentration of CO increases, the release of SO₂ occurs earlier, and the amount of sulfur released is directly proportional to the CO concentration. Specifically, under 1.00% CO, more than 21.07% of S was released, approximately double when under 0.25% CO. In a separate study, Okumura et al. (Okumura et al. 2003) investigated the impact of CO concentration (2%, 5%, and 10%) on CaSO₄ desulfurization at 1273 K. As Fig. 6(1) shown, it was observed that the initial decomposition reaction rate of CaSO₄ increases and the desulfurization rate equilibrium point time of CaSO₄ is shortened with an augmentation in the CO concentration. Nevertheless, the desulfurization rate value is diminished by the increase of CO, implicating low CaO productivity (Zheng et al. 2011a, b; Zhang et al. 2012; Kuusik et al. 1985). the conversion of CaSO₄ to CaO is contingent upon the concentrations of CO and CO₂, with a favorable outcome observed when the CO concentration is low and the CO₂ concentration is high. Furthermore, Gruncharov et al. (Gruncharov 1985) and Okumura et al. (2003) have indicated that the initial decomposition rate of CaSO₄ decreased when the CO₂ concentration was varied from 10 to 30 vol.% while maintaining a constant CO concentration of 2.00 vol.%. A comparison between Fig. 6(1) and Fig. 6(2) elucidates that the conversion of CaSO₄ to CaO is contingent upon the concentrations of CO and CO₂, with a favorable outcome observed when the CO concentration is low and the CO₂ concentration is high.

Fig. 6

The effects of CO, CO₂, PCO / PCO₂ and reaction time on desulfurization efficiency: (1) effect of the concentration of CO; (2) CO₂ on the decomposition of CaSO₄ at 1273 K; (3) Effect of P_CO/P_CO2 on the final conversion of CaSO₄ to CaO; (4) Effect of reaction time on CaSO₄ decomposition. (Okumura et al. 2003)

By integrating the methodologies employed by Oh et al. (1990) and Okumura et al. (2003), the pseudo-equilibrium desulfurization rate (wt.%) and reduction potential P_CO/P_CO2 are graphed. Figure 6(3) illustrates that as the P_CO/P_CO2 value decreases, the conversion (X) increases. At low reduction potentials (P_CO/P_CO2 < 0.10), CaSO₄ exhibits near-complete regeneration to CaO. However, discrepancies in the X values are observed, particularly for P_CO/P_CO2 > 0.2. These discrepancies may arise due to variations in the reaction conditions employed. Moreover, as depicted in Fig. 6(4), the CaO obtained from the decomposition of CaSO₄ demonstrates superior performance in terms of SO₂ and CO₂ re-adsorption compared to the CaO derived from limestone decomposition. At 1273 K, an apparent conversion value of 91% for the decomposition of CaSO₄ to CaO was obtained in a 2 vol.% CO and 30 vol.% CO₂ atmosphere.

Irrespective of the reduction reaction of CaSO₄ in the CO system, CO-N₂ system, or CO-CO₂-N₂ system, the interaction between CO and CaSO₄ resulting in the formation of CaO is accompanied by a highly endothermic reaction. (Kuusik et al. 1985; Gruncharov et al. 1985; Ghardashkhani et al. 1991). The desorption of SO₂ and the formation of CaO necessitate 66 kcal/g·mol, as indicated by Wheelock's (1960) equilibrium measurements. Whereas, the initial reaction between CO and CaSO₄, resulting in the production of CO₂ and adsorbed SO₂, releases 27 kcal/g·mol. Therefore, the total heat required for the overall reduction and decomposition of CaSO₄ and CO is 39 kcal/g·mol.

In practical engineering applications, incorporating an appropriate quantity of O₂ into the reaction environment can elevate the reaction temperature through the exothermic interaction between O₂ and CO. The introduction of O₂ not only adjusts the partial pressure ratio of CO₂ to CO in the reaction atmosphere but also modifies the reaction mechanism associated with the reduction of CO and the decomposition of CaSO₄ into CaO. Xia et al. (Xia et al. 2022) investigated the impact of O₂ on the regeneration process of CaO from CaSO₄ through CO reduction. In this investigation, the reduction and decomposition characteristics of CaSO₄ were examined and compared under various process conditions in a laboratory-scale fixed-bed reactor utilizing CO-N₂/CO-CO₂-N₂ atmospheres. The results revealed that CaSO₄ can be completely decomposed into CaO when the reaction temperature exceeds 1273 K, CO% ≥ 2, and P(CO₂)/P(CO) ≥ 8. Furthermore, the addition of an appropriate amount of O₂ enhances the yield of CaO in the product. In the O₂-CO-N₂ atmosphere with O₂% of 7 and CO% of 16, CaSO₄ can be entirely decomposed into CaO without the need for CO₂ supplementation. The physical properties of CaO produced through the reduction and decomposition of CaSO₄ exhibit superior characteristics compared to those obtained through the calcination of CaCO₃, which aligns with the conclusions of Okumura (2003).

In addition to the aforementioned factors, Wheelock (1960) investigated the influences of temperature, particle size, gas flow rate, and SO₂ concentration in the gas stream. He concluded that as the temperature increases, the sulfur removal efficiency improves, but accompanied by higher energy consumption, as shown in Fig. 7(a). Moreover, as shown in Fig. 7(b), the maximum desulfurization rates, ranging from 0.1 to 0.3 lb·s⁻¹·m⁻², exhibited an almost linear increase with the increase of mass velocity in the 2%CO-98%N₂ system. Additionally, as depicted in Fig. 7(c), the initial desulfurization rate showed little dependence on particle size, whereas the maximum desulfurization rate experienced a sharp decline as particle size increased. Hence, it is most probable that the rate is controlled by internal diffusion of the reaction. Eventually, the highest desulfurization rate was achieved at three distinct levels of CO concentration. In Fig. 7(d), it can be witnessed that at a CO concentration of 2%, the maximum desulfurization rate initially decreased with increasing SO₂ concentration from 0 to 0.5%, but subsequently exhibited a slight increase with further increases in SO₂ concentration.

Fig. 7

Effects of temperature, particle size, flow rate and SO₂ concentration in gas stream on CaSO₄ desulfurization rate (wt.%). (Wheelock 1960)

At a CO concentration of 3%, the effect was reversed. And a level of 4%, a different effect was still observed. Although the results are scattered, the maximum desulfurization rate appears to increase slightly with increasing SO₂ concentration across the entire investigated range. A minor concentration of SO₂ in the feed gas exerts a substantial impact on the initial desulfurization rate.

Formation of CaS by reducing gas

Undoubtedly, both reaction temperature and CO concentration have a significant impact on the formation of undesired CaS during the desulfurization process. Wheelock (1960) determined that the decomposition reaction into CaS is negligible at temperatures below 1422 K, but the reaction can be entirely avoided if the temperature reaches 1478 K. In addition, the production of CaS is also suppressed when the P_CO/P_CO2 value in the system is low. To obtain high-purity CaO, it is essential to comprehend the mechanism of CaS generation and manipulate the reaction conditions to minimize its formation. In contrast to the formation reaction of CaO, CaS can be produced when CaO⋅SO₂ continues to react with CO prior to SO₂ desorption.

According to the mechanism of CaSO₄ decomposition proposed by Robbins (1966), the most stable intermediate in reductive decomposition is SO₂ adsorbed on CaO, and the adsorbed SO₂ can be reduced to elemental sulfur adsorbed on CaO, Eq. 12.

$$\mathrm{CaO}\cdot{\mathrm{SO}}_2+2\mathrm{CO}\left(\mathrm g\right)\rightarrow\mathrm{CaO}\cdot S+{2\mathrm{CO}}_2\left(\mathrm g\right)$$

(12)

Both Wheelock (1960) and Xia et al. (2022) have demonstrated that at temperatures exceeding 1273 K and with a high concentration of CO₂, CO₂ can displace S on the surface of CaO and be adsorbed. This process effectively hinders the formation of CaS (as described in Eq. 13) by reducing the available amount of adsorbed sulfur for reaction with CO.

$$\mathrm{CaO}\cdot S+\mathrm{CO}\left(\mathrm g\right)\rightarrow\mathrm{CaS}+{\mathrm{CO}}_2\left(\mathrm g\right)$$

(13)

SFGDA decomposed by solid reductant

Several researchers have investigated the reductive decomposition of CaSO₄ using a solid reductant, including coke (Yan et al. 2014; Kale et al. 1992; Van der Merwe et al. 1999; Ma et al. 2010), lignite (Zheng et al. 2013a, b), charcoal (Chaalal et al. 2020), biomass (Rebbling et al. 2016; Zheng et al. 2011a, b), and graphite (Zheng et al. 2013a, b; Böke et al. 2002; Feng et al. 2019; Ma et al. 2012; Zhou et al. 2011). Presently, there is no universally accepted comprehensive theory that explains the mechanism of thermal decomposition of CaSO₄. However, it is recognized that both solid–solid and gas–solid reaction mechanisms are involved in the reduction of CaSO₄ to CaO by C.

Formation of CaO by solid reductant

Initial period, it was observed that the decomposition temperature of CaSO₄ could be decreased in the presence of coke in the reaction mixture. Figure 8 presented by Factsage 8.1 demonstrates that when delta G < 0 kJ∙mol⁻¹, the reduction of CaSO₄ by C can occur spontaneously, resulting in the production of CaO, CO₂ and SO₂, as shown in Eq. 14. The CO generated from the reaction between C and CO₂ is participated in the decomposition of CaSO₄ to CaO at 1200 K approximately, as shown in Eq. 15.

Fig. 8

Delta G − T relationship lines of the major chemical reactions

Hence, based on the findings presented in Fig. 8, it can be inferred that the conversion of CaSO₄ to CaO and SO₂ through C takes places at a higher temperature compared to the decomposition of CaSO₄ to CaS, indicating a solid–solid reaction mechanism, as described in Eq. 16. Consequently, at a temperature of 1453 K, the reaction between CaS and CaSO₄ leads to the formation of CaO and SO₂, as illustrated in Eq. 17. Moreover, CaSO₃ undergoes decomposition to CaS in the presence of C, as demonstrated in Eq. 18.

In accordance with the gas–solid process theory, the initial step involves the oxidation of C to CO, whereupon fully contacts CaSO₄ to generate CaO. The reaction between C and CaSO₄ do not instantaneously but rather proceeds until completion. Previous studies by Ma et al. (2010) and Su et al. (Su et al. 2019) have highlighted the influence of temperature and C content on the decomposition of CaSO₄. Supported by thermodynamic analysis, Ma et al. (2010) determined that the main reaction represented by Eq. 14 could occur within a temperature range of 1273–1423 K. It appears that maintaining a prolonged high temperature and utilizing raw materials with low carbon content, or operating under weak reducing conditions, promotes the decomposition of CaSO₄ into CaO. Additionally, Jia et al. (Jia et al. 2016) and Zheng et al. (2011a, b) also investigated the reduction of CaSO₄ to CaO using lignite, coal, biomass, and other materials as reducing agents.

To achieve a substantial concentration of SO₂ gas, CaSO₄, CaS, S, and FeS₂ can be counted as an abundant sources of sulfur to enhance sulfuric acid production. The interaction between CaSO₄ and CaS or S is recognized as a solid-state reaction mechanism, leading to the generation of SO₂ in the gaseous phase. Song et al. (Song et al. 2019) conducted an analysis, affirming the technical feasibility of utilizing FeS₂ as a solid reductive agent in the decomposition of CaSO₄ for the production of SO₂. It is noteworthy that the SO₂ capture is not limited to FeS₂ and CaSO₄, as Ca₂Fe₂O₅ and CaO can also be incorporated into the decomposition slag, thereby enhancing the economic feasibility of the process. The main solid–solid reactions between CaSO₄ and FeS₂ are as follows: Eq. 20 and Eq. 21. Significantly, the conducive conditions for obtaining low-sulfur solid products are hindered by the occurrence of Eq. 20. To effectively eliminate residual sulfur from the slag, it is imperative to comprehensively consider the maintenance of an appropriate oxygen pressure within the reaction system.

$$4CaS{O}_{4}(s)+2Fe{S}_{2}\to C{a}_{2}F{e}_{2}{O}_{5}(s)+CaO+5S{O}_{2}(g)+CaS(s)$$

(20)

$$7CaS{O}_{4}(s)+2Fe{S}_{2}\to C{a}_{2}F{e}_{2}{O}_{5}(s)+5CaO+9S{O}_{2}(g)$$

(21)

Formation of CaS by solid reductant

Based on the thermodynamic calculations illustrated in Fig. 8, it can be inferred that the reduction of CaSO₄ and CaSO₃ to CaS can be accomplished using C and CO at temperatures below 1173 K. Li et al. (Li et al. 2015) revealed that the thermal decomposition of CaSO₃ to produce CaS employing coal as a reducer, was carried out with a molar ratio of C to CaSO₃ at 2.5 (n(C)/n(S) = 2.5), a reaction temperature of 1173 K, a reaction time of 1.5 h and a coal particle size 100 mesh, shown in Fig. 9. Both thermodynamic calculations and experimental results indicate that the formation of CaS during CaSO₃ decomposition can be attributed to Eq. 17 and Eq. 18, involving solid–solid and gas–solid reactions. Additionally, according to Eq. 16, CaS is consumed and reacts with the remaining CaSO₄ to produce CaO. XRD and SEM analyses conducted by Li (2015) have validated that CaS is the only solid product formed under appropriate reaction conditions. Consequently, in the reaction system where CaSO₃ is reduced by coal with an n(C)/n(S) ratio of 2.5, the decomposition of CaSO₃ to CaO can be considered almost negligible.

Fig. 9

Effects of holding time and coal particle size on CaSO₄ decomposition (Jia et al. 2016)

Analogously, Jia et al. (2016) discovered that the presence of coal significantly enhances the high-temperature decomposition of CaSO₄, resulting in the decomposition of 87% CaSO₄ into CaS, which is on account of the solid–solid reaction between C and CaSO₄. Additionally, the duration of calcination and the particle size of coal influence the decomposition of CaSO₄. As indicated in Fig. 9, the decomposition of CaSO₄ is more favorable when a reducing agent with an extended reaction time and finer particle size is employed. This observation aligns with the formation reaction of CaS and CaO.

Realistically, an elevation in carbon content tends to promote the formation of CaS, which is unfavorable for sulfur removal. Su et al. (2019) conducted an experimental investigation utilizing a tubular resistance furnace to to explore the influence of carbon content on the desulfurization rate at various temperatures. They observed that an increase in the n(C)/n(CaSO₄) ratio did not result in an improvement of the desulfurization rate at the same temperature, as depicted in Fig. 10(a). Elevated temperature plays a pivotal role in augmenting the desulfurization rate, as indicated by the reaction mechanism wherein decomposed CaS reacts with CaSO₄ to form CaO, and the presence of CO in the gaseous phase accelerates the conversion of CaSO₄ to CaO. These two factors are recognized the primary catalysts for improving the desulfurization rate.

Fig. 10

Variation curve of desulfurization rate with carbon content at different temperatures and gas phase pristine test results. (Su et al. 2019)

Tian, Zheng, and Zhong et al. (Tian et al. 2010; Diaz-bossio et al. 1985; Simei et al. 2017) have investigated that CaSO₄ serves as a promising oxygen carrier, readily reducible to form CaS under high concentrations of CO, while remaining stable in a reducing atmosphere. Within the C-CaSO₄ reaction system, the concentration of CO gradually increases as the reaction proceeds, and the potency of the reducing environment is contingent upon the carbon content in the raw material. Furthermore, the conversion rate of CaSO₄ and CO into CaS is higher at temperatures below 1273 K compared to their reaction in the solid-state, which results in the production of CaO.

More formidably, the residual of CaS does not contribute to effective sulfur removal in SFGDA, resulting in low CaO production. Therefore, the conversion of CaS to CaO is extremely important to achieve a high desulfurization rate correspondingly. Propitiously, the introduction of O₂ into residual CaS deemed feasible for industrial application, leading to additional CaO generation (Anthony et al. 2003; Turkdogan et al. 1974; Song et al. 2007). The oxidation of pure CaS (with a particle size of 45 µm) was investigated by Davies, Yrjas, and Marban et al. (Davies et al. 1994; Yrjas et al. 1997; Marban et al. 1999) via an atmospheric thermobalance. They observed that the predominant amount of SO₂ was liberated within the initial 25-min period at temperatures ranging from 1123 to 1323 K. Upon oxidizing CaS with 4.2 vol% O₂ for 30 min at 1123 K and 1223 K, the reaction described in Eq. 22 occurred, resulting in CaSO₄ conversion rates of approximately 8% and 32%, separately. Meanwhile, the oxidation reaction of CaS and O₂ can be represented by Eq. 23, with corresponding CaO conversion rates of approximately 2% and 30%, respectively. Davies et al. (1994) attributed the release of SO₂ to the apparent solid–solid reaction between CaS and CaSO₄, which was confirmed in their study. Furthermore, they discovered that no solid–solid reaction occurred at 1123 K, whereas at 1323 K, complete conversion to CaO was achieved, indicating the transformation from CaS to CaO. Outstandingly, Yrjas et al. (1997) detected that excessively high concentrations of O₂ immediately reacted with CaS, forming impermeable product layers of CaSO₄ or CaO, thereby impeding further sulfur release reactions.

$$CaS\left(s\right)+2{O}_{2}\left(g\right)\to CaS{O}_{4}$$

(22)

$$CaS\left(s\right)+3/2{O}_{2}\left(g\right)\to CaO+S{O}_{2}\left(g\right)$$

(23)

Maintaining a moderately low level of O₂ concentration and a temperature greater than 1323 K will benefit the conversion of CaS to CaO (Turkdogan et al. 1974; Song et al. 2007; Wang et al. 2020). Thence, optimizing factors such as the reduced particle size of the SFGDA, thorough mixing of C, maintaining a reaction temperature above 1323 K, and utilizing low C content prove more advantageous in preventing the formation of CaS, thus enabling the production of high-purity CaO.

SFGDA decomposed by compound additive

Apart from solid reductant, solid iron oxides, which are non-reducing agents, also play a significant role in facilitating the decomposition of SFGDA. Higuchi et al. (Higuchi et al. 2016) proposed a method for synthesizing calcium ferrite from waste gypsum board (mainly composed of CaSO₄·2H₂O) by incorporating iron oxides. Through thermodynamic estimations and small-scale heating tests, three distinct synthetic routes for calcium ferrite were identified: (1) calcium ferrite production in an air environment, (2) single-step heat treatment, and (3) two-step heat treatment, as shown in Fig. 11. The results indicated that calcium ferrite is partially synthesized through the decomposition of the gypsum board in an air atmosphere at 1453 K, but achieving complete decomposition of CaSO₄ is exceedingly challenging. Equation 24 is considered the primary reaction, indicating that reducing the P_O2 or P_SO2 contributes to the near-complete formation of calcium ferrite.

Fig. 11

The phase diagram of chemical equilibrium in the Ca-Fe-S–O system (Higuchi et al. 2016)

$$xCaS{O}_{4}(s)+yF{e}_{2}{O}_{3}(s)\to xCaO\cdot yF{e}_{2}{O}_{3}(s)+xS{O}_{2}(g)+0.5x{O}_{2}(g)$$

(24)

As a consequence, through the addition of C in a specific proportion, the P_O2 decreases, resulting in desulfation rates of over 95% for CaSO₄. Subsequently, there would be more calcium ferrite generated than processes if SO₂ was removed after reaction and slightly oxidative atmosphere in the reaction system was maintained throughout the reaction period (Zhao et al. 2020; Liu et al. 2021). Although some CaSO₄ in waste gypsum board decomposes into CaS, it fortunately undergoes oxidation to yield CaO in the presence of trace amounts of oxygen. Whereafter, a portion of the calcium ferrite is produced through the reaction between CaO and Fe₂O₃, as described by Eq. 25 and Eq. 26.

$${\mathrm{CaSO}}_{4}(\mathrm{s})+{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(\mathrm{s})+1/2\mathrm{C}(\mathrm{s})\to \mathrm{CaO}\cdot {\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(\mathrm{s})+{\mathrm{SO}}_{2}(\mathrm{g})+1/2{\mathrm{CO}}_{2}(\mathrm{g})$$

(25)

$$2\mathrm{CaS}(\mathrm{s})+{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(\mathrm{s})+3{\mathrm{O}}_{2}(\mathrm{g})\to 2\mathrm{CaO}\cdot {\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(\mathrm{s})+2{\mathrm{SO}}_{2}(\mathrm{g})$$

(26)

Actually, Eq. 27 to Eq. 30 demonstrate that carbon would reduce Fe₂O₃ to Fe and simultaneously convert CaSO₄ into CaS and CaO. Subsequently, Fe is oxidized to Fe₂O₃ by O₂, which then interacts with CaSO₄ to form calcium ferrite.

Meanwhile, C undergoes additional oxidation and is consumed in the processes of iron oxide reduction. Further research on low-carbon consumption is warranted. Gratifyingly, the burning of the C reaction and the oxidation of iron provide a significant amount of energy to the reaction system.

$$3C(s)+F{e}_{2}{O}_{3}(s)\to 2Fe(s)+3CO(g)$$

(27)

$$3CO(g)+F{e}_{2}{O}_{3}(s)\to 2Fe(s)+3C{O}_{2}(g)$$

(28)

$$3/2C(s)+F{e}_{2}{O}_{3}(s)\to 2Fe(s)+3/2C{O}_{2}(g)$$

(29)

$$CaS{O}_{4}(s)+2Fe(s)+{O}_{2}(g)\to CaO\cdot F{e}_{2}{O}_{3}(s)+S{O}_{2}(g)$$

(30)

To reduce carbon consumption, the two-step heat treatment is considered a reasonable and comprehensive approach. Higuchi et al. (2016) proposes the decomposition of gypsum board with carbon as the first step, leading to the formation of CaO or a small amount of CaS. The second step involves the formation of calcium ferrite from the decomposed material and Fe₂O₃, resulting in the production of calcium ferrite with varying breakdown-product contents. Equation 31 illustrates this process.

$$zCaO(s)+wF{e}_{2}{O}_{3}(s)\to zCaO\cdot wF{e}_{2}{O}_{3}(s)$$

(31)

Two distinct processes are present: single-step and two-step processes, with the latter having the potential to reduce carbon consumption theoretically, as depicted in Fig. 12. Furthermore, the two-step process is more advantageous for synthesizing calcium ferrite with low sulfur content compared to the one-step process. Notwithstanding, the two-step process is complicated with high expenses and high facility requirements, and so whether it is environmental protection, energy-saving or economy should be considerately thought in future industrial applications. On the contrary, the one-step production of calcium ferrite involves challenging conditions for precise control of process parameters. Nevertheless, Route remains the preferred choice for commercial production due to its straightforward process flow.

Fig. 12

Diagram of one-step or two-step synthesis of calcium ferrite (Zhao et al. 2020; Liu et al. 2021)

Based on the study of Higuchi (2016), calcium ferrite also was prepared from desulfurization gypsum, iron ore, blast furnace dust, and graphite by Zhao (2020). As shown in Fig. 13(a), the raw materials for the reaction were initially introduced into a tube furnace in both powder and block forms, followed by calcination at a temperature of 1373 K under an air atmosphere. It was observed that the powder exhibited a superior desulfurization effect compared to the block form. The decomposition and desulfurization process of CaSO₄ involve both solid–solid and gas–solid reactions, with the latter exhibiting significantly faster kinetics than the former. The presence of gaps between the powder particles facilitates the entry of reducing gases such as CO, allowing for gas–solid reactions with CaSO₄. Consequently, the use of powdered reactants promotes sulfur removal and leads to a higher desulfurization rate.

Fig. 13

Curve of desulfurization rate and sulfur content with the ratios of raw materials under different conditions: (a) different morphologies, (b) different atmospheres, (c) different carbon content, (d) different iron oxide content. (Zhao et al. 2020)

It can be seen from the Fig. 13(b) that when the molar ratio of CaO (CaSO₄) to Fe₂O₃ is constant, the desulfurization rate of gypsum in argon atmosphere is significantly higher than that in air atmosphere. The sulfur content in the product is lower than that in air atmosphere under argon atmosphere, illuminated that the desulfurization reaction of gypsum hardly occurs in an oxidizing atmosphere. The higher n(CaO): n(Fe₂O₃) contributes to the lower the desulfurization rate of gypsum in argon, which shows that the increase of iron oxide promotes the decomposition and desulfurization of gypsum.

As shown in Fig. 13(c), it is evident that maintaining a constant ratio of n(CaO) to n(Fe₂O₃), a higher carbon content corresponds to a higher desulfurization rate. However, the desulfurization rate shows minimal changes beyond a carbon content of 30%, with the highest achieved rate being approximately 95%. As Fig. 13(d), when the carbon content is fixed and the ratio of n(CaO): n(Fe₂O₃) is 1:1, the product exhibits the highest amount of calcium ferrite, exceeding 95%. Ultimately, the optimal reaction conditions for the one-step synthesis of pure calcium ferrite involve using powdered raw materials in an inert atmosphere, with a CaO/Fe₂O₃ ratio of 1:1 and a carbon content of 30%. To mitigate the presence of CaS and minimize its impact on the desulfurization rate, Zhao et al. (2020) controlled the temperature at 1423 K and utilized the following composition: 30.4 wt.% desulfurization gypsum (with CaSO₄ as the primary component), 31.7 wt.% iron ore, 11.1 wt.% blast furnace dust, and 26.8 wt.% graphite. Experimental results demonstrated a significant formation of calcium ferrite under these optimal reaction conditions, resulting in a desulfurization rate of 98.9% and suggesting its feasibility as a recyclable sintering additive.

Summary and perspective

The production of extensively purified CaO and the concentration of SO₂ through pyrolysis are widely acknowledged as promising strategies that have captivated an increasingly large cohort of researchers in their quest to solve the issue of harmless, bulk, and value-added disposal of SFGDA. Achieving efficient and low-temperature sulfur removal in SFGDA necessitates the establishment of favorable reaction conditions. Currently, investigations are underway to optimize the desulfurization efficacy of semi-dry desulfurization ash by controlling variables such as the reaction atmosphere, temperature, reaction duration, and the type and dosage of additives.

SFGDA demonstrates suboptimal pyrolysis efficiency when subjected to temperatures of 1678 K in an air and 1603 K in an N₂, respectively. Furthermore, the presence of CaSO₄ significantly prolongs the duration required for complete decomposition of SFGDA, thereby diminishing the desulfurization efficacy of CaSO₄ and raising the temperature threshold for sulfur removal. This impedes the large-scale industrial production of CaO and SO₂. Therefore, there is a pressing need to identify a low-temperature, high-efficiency, and environmentally friendly desulfurization additive that enables the recycling of chemical substances such as S and Ca.

As discussed previously (Fig. 14 and Table 4), the decomposition temperature of CaSO₄ in the SFGDA can be reduced, leading to an improved desulfurization rate under a reducing atmosphere. In the CO-N₂ atmosphere, the CaS is more effortlessly formed blew 1273 K than CaO generated from SFGDA. This unfavorable occurrence hinders the reduction of sulfur content in solid products. Relatively speaking, there is almost 100% desulfurization rate or decomposition rate in the CO-CO₂-N₂ and CO-CO₂-SO₂-N₂ at 1323–1423 K, but the unalterable truth is that the reduction reaction of CaSO₄ with CO is a strongly endothermic reaction, requiring more external heating. Therefore, in practical industrial applications, it is high-priority to facilitate endothermic reactions through the exothermic reaction between O₂ and CO. What's more, CaS generated from the reduction of CaSO₄ can be oxidized by O₂ at 1323–1373 K, reducing residual sulfur and improving desulfurization rate. It is worth noting that the complexity of the system can be simplified by eliminating the presence of SO₂ or CO₂. However, maintaining CO pressure stablely and combustion safely is a tricky issue for industrialization process henceforth.

Fig. 14

The synthetic route of CaO and Ca₂Fe₂O₅ with various additives

Table 4 Summary of current research status

It is noteworthy that the endeavor to reduce CaSO₄ with CO for the production of CaO is currently at the laboratory stage, primarily due to its high raw material cost and expensive equipment investment. In contrast, solid-phase reducing agents such as coke, graphitic carbon, high-sulfur carbon, pyrite, and CaS are more economically viable options for the reductive decomposition of SFGDA. When the molar ratio of CaSO₄ to C is 1:0.8, CaSO₄ can be effectively decomposed to yield predominantly CaO at temperatures exceeding 1273 K, culminating in a desulfurization rate of over 95%. Additionally, CaSO₄ is reduced to form CaO and CaS calcined at 1273 K during the reaction with FeS₂, and Fe₂O₃ is combined with CaO to synthesis calcium ferrite product reused as a sintering additive. The S in the refractory pyrite and SFGDA is migrated to the gas phase, enabling the separation of the sulfur and calcium iron salts. This extraction and utilization of low-grade pyrite not only demonstrates the effective utilization of resources but also holds promising prospects for future applications.

Analogous to sintering flue gas desulfurization gypsum (the main component is CaSO₄), the SFGDA decomposition product CaO is combined with Fe₂O₃ to synthesize calcium ferrite using a rotary kiln device (as Fig. 15). Nonetheless, the decomposition temperature of CaSO₄ is immobile high, and the desulfurization rate is not satisfactory. By introducing a solid carbon source into the system containing CaSO₄ and Fe₂O₃, the optimal decomposition temperature of CaSO₄ can be lowered to 1373 K, and the carbon in the reaction system plays the role in providing heat source and reducing agent for the reaction. Within the C-CaSO₄-Fe₂O₃ system, CaSO₄ is deviously reduced by carbon to form CaS. Moreover, under normal pressure, CaSO₄ can be reduced to CaO in a slightly oxidizing atmosphere at a temperature of 1323 K, and then combined with Fe₂O₃ to form calcium ferrite. Therefore, in the C-CaSO₄-Fe₂O₃ reaction system, it is crucial to maintain a controlled atmosphere of a mildly reducing atmosphere within the rotary kiln device in order to obtain the calcium ferrite product at a lower temperature.

Fig. 15

Industrial flow chart of SFGDA to produce calcium ferrite and sulfuric acid

Notwithstanding, despite the increasing interest in the synergistic preparation of calcium ferrite using calcium sulfate, iron, and carbon-containing solid waste materials, there is a lack of reports on small-scale industrial experiments. What’s more, the research on the reaction mechanism of C-CaSO₄-Fe₂O₃ involves heterogeneous reactions, including gas–solid and solid–solid reactions, circumlocutionarily complicated. Thus far, there has been no clear mechanistic explanation for the C-CaSO₄-Fe₂O₃ reaction system. Furthermore, there is a discrepancy between the results obtained from laboratory experiments and industrial tests, and the scarcity of qualified technical personnel has hindered the industrial application of the semi-dry method. Consequently, further verification and elucidation of the microscopic mechanism underlying the C-CaSO₄-Fe₂O₃ reaction are essential to guide actual industrial production. Indispensablely, it is crucial to prioritize the technical training of relevant operators, as it significantly impacts the industrialization process of this method.

Approximately estimated, in the CO-CaSO₄-N₂, CO-CO₂-CaSO₄-N₂, and C-CaSO₄-N₂ reaction systems without oxygen involvement, the reaction temperature is set at 1100℃, the calcination time is 4 h, and the power of the calciner is 2500 W, resulting in an external energy consumption of approximately 3.6 × 10⁴ kJ, equivalent to 1.10 $/ton in terms of power cost. Introducing O₂ to facilitate the combustion reaction of CO and coke provides heat for the decomposition of SFGDA, reducing this cost component.

Therefore, coke powder offers a cost advantage as a reducing agent. From a product perspective, if only carbon monoxide or carbon reducing agent is added, and the calcined products of SFGDA are all calcium oxide and sulfuric acid products, the benefits per ton of calcium oxide and sulfuric acid are 72.99 and 58.39 $/ton. In contrast, the calcium ferrite product obtained by adding carbon and iron oxide to the SFGDA brings a profit of 437.96 $/ton simultaneously, enabling the comprehensive utilization of calcium, iron, and sulfur resources. Henceforth, the preparation calcium ferrite from SFGDA produced during sintering holds significant potential for multitudinous, innocuous, and high-value consumption of SFGDA, paving the way for its promising commercial application.

Conclusions

Pyrolysis holds great promise as a technology to decompose SFGDA into CaO and SO₂, facilitating the recycling of chemical elements. The purity and properties of the CaO product significantly vary depending on factors such as raw material particle size, composition of the reducing gas, types of additives, calcining temperature, and reaction termination time. In current research endeavors, it is a forefront challenge to achieve high efficiency and low temperature desulfurization for SFGDA to produce CaO at 1273–1373 K within 120 min. Appropriate particle size enhances the desulfurization efficiency of SFGDA, shorten the reaction time and improved the purity of CaO.

(1). SFGDA undergoes decomposition in the presence of reducing gas or a weak oxidizing atmosphere. CaSO₄, the main refractory pyrolysis component in SFGDA, can be converted to CaO with a conversion rate of 0.91 in a 2%CO-30%CO₂-68%N₂ atmosphere. Under low reducing atmosphere (P_CO/P_CO2≤0.10), CaSO₄ is completely decomposed into to CaO, but external heat is required for the decomposition process. Interestingly, the adding of 1.0% SO₂ in a CO-CO₂-N₂ (P_CO/P_CO2≤0.10) the atmosphere achieves a 100% conversion rate of CaSO₄ to CaO, and then external heating is needed to provide the required heat for the reaction. Propitiously, it is feasible to attain the necessary reaction temperature by utilizing the exothermic reaction between O₂ and CO, enabling the complete conversion of CaSO₄ to CaO in a 7% O₂-16%CO-77% N₂ atmosphere. Unfortunately, there are currently no reports on the industrial application of SFGDA for CaO production CaO due to the high cost of reducing gas, equipment investment, and security concerns.

(2). The addition of solid reducing agents such as C and FeS₂ to SFGDA allows for a decrease in the temperature required for CaO preparation. Besides, these solid additives are more cost-effective compared to reducing gases. While more evidence is required to fully comprehend the precise action mechanism of C on SFGDA, it is recognized that the reaction between C and SFGDA involves solid-solid and gas-solid reactions. When C is added in the ratio of n(CaSO₄): n(C) = 1: 0.8, the conversion rate of CaSO₄ to CaO reaches 95.16% in an N₂ atmosphere above 1273K, resulting in a significant amount of CaO product formation. FeS₂ introduced to a reducing agent, the decomposition temperature of CaSO₄ in the SFGDA can be reduced to 1093-1273 K under low-pressure conditions. However, the content of CaS in the solid product is relatively high, and when the temperature is higher than 1273 K, more encouraging to the decomposition of CaSO₄ to form CaO.

(3) The addition of iron oxide as an auxiliary agent reduces the decomposition temperature of SFGDA, albeit to a small extent. Therefore, carbon-based reducing agents are often added simultaneously to promote the decomposition of CaSO₄, reducing the reaction temperature below 1373K. Under the catalysis of composite additives, CaO can be completely generated, which combines iron oxide to produce calcium ferrite, enhancing the application value of the process. Ultimately, the one-step method of decomposing SFGDA and producing calcium ferrite represents an industrial pyrolysis technology for efficient and economical utilization of SFGDA.