An Approach to CO₂ Emission Reduction in the Iron and Steel Industry: Research Status and Development Trends of Integrated Absorption-Mineralization Technologies

Abstract

With the acceleration of global industrialization, the issue of carbon dioxide (CO₂) emissions has become increasingly severe, highlighting the urgent need to develop effective CO₂ capture and utilization technologies. CO₂ absorption-mineralization technology, as an emerging method, can convert CO₂ into solid minerals, achieving both long-term storage and emission reduction goals. This paper systematically reviews the latest research progress in CO₂ absorption-mineralization technology, with a particular focus on its application potential and sustainability in the steel industry. Additionally, it summarizes the research status and optimization strategies of various monoamine and mixed amine absorbents and explores the main process technologies, reaction mechanisms, and key parameters of industrial CO₂ mineralization. Through multiscale modeling analysis, the study delves into the reaction mechanisms and influencing factors of the mineralization process, providing theoretical support for the industrial application of the technology. The research indicates that CO₂ absorption-mineralization technology not only effectively reduces greenhouse gas emissions but also offers raw materials for industries such as construction, thus promoting sustainable resource development. Although this technology shows good application prospects, it still faces key challenges in economic viability and technical feasibility during practical implementation. This paper aims to clarify the current research hotspots and challenges, providing theoretical and practical support for future large-scale application.

1. Introductory

As global industrialization continues and fossil energy is consumed in large quantities, emissions of greenhouse gases (GHGs), especially carbon dioxide (CO₂), continue to rise, leading to increasing global climate change. According to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), global CO₂ emissions will reach about 34 billion tons in 2019, an increase of more than 50% compared to the pre-industrial period [1]. This continuing trend of increasing emissions has triggered a series of serious environmental problems, such as the high incidence of extreme weather events, sea level rise, ecosystem degradation and the sustainable development of society [2]. To address this global challenge, the Paris Agreement clearly sets out the goal of limiting the increase in the global average temperature to 2 °C above pre-industrial levels. However, to achieve this goal, it is projected that the world will need to reduce emissions by 10–20 billion tons of CO₂ per year by 2050, which poses a huge challenge to existing emission reduction technologies [3].

In order to cope with the increasingly severe pressure to reduce emissions and realize the sustainable development of society, a variety of CO₂ capture technologies have been developed internationally, including chemical absorption, physical adsorption, membrane separation, etc. [4]. Among them, chemical absorption has become the dominant technology for industrial-scale CO₂ capture due to its high separation efficiency and mature process [5]. Among many chemical absorption solutions, amine absorbents (such as MEA (monoethanolamine), DEA (diethanolamine), etc.) are widely used because of their high absorption efficiency and selectivity, and have been demonstrated in the thermal power, iron and steel, cement, and other industries [6,7]. However, with the deepening of industrial practice, chemical absorption technology exposes three key issues: Firstly, the final attribution of CO₂: Traditional amine absorption is essentially a reversible process, and the absorbents’ regeneration process not only requires the consumption of large amounts of steam (accounting for about 70% of the total operating cost), but also the release of high concentrations of CO₂ needs to be further compressed, transported, and sequestered. This not only increases the complexity of the process but also poses a potential risk of secondary emissions [8,9]. The second is the sustainability of the technology: Amine absorbents in the recycling process will occur thermal degradation, oxidative degradation, and acid gas-induced chemical degradation, resulting in a continuous decline in the absorption performance, not only needing to regularly replenish fresh solvents (annual depletion rate of about 1.5–2.5 kg/tCO₂), but also leading to corrosion of the equipment and other engineering problems [10]. The third issue is economic feasibility. Currently, the cost of CO₂ capture using the amine method is in the range of 60–100 USD/ton. And if the subsequent compression, transportation, and storage costs are taken into account, the total cost will climb further, which seriously restricts the large-scale application of the technology [11]. These problems are enough to prove that the chemical method is not a sustainable and environmentally friendly technology, and there is an urgent need for new technologies to emerge.

In this context, CO₂ absorption-mineralization technology is an innovative integrated solution, the principle of which is shown in Figure 1. It not only inherits the high efficiency of chemical absorption but also realizes the permanent fixation of CO₂ through the mineralization process. It is characterized by sustainability, which fundamentally solves the key problems faced by traditional absorption technology [12,13]. Firstly, the technology utilizes the world’s abundant calcium and magnesium mineral resources and a wide range of industrial solid wastes as reaction raw materials, breaking through the limitations of the traditional amine method in terms of raw material supply [14]. Also, by directly converting CO₂ into thermodynamically stable carbonate minerals (such as CaCO₃, MgCO₃, etc.), the risk of secondary release of CO₂ in traditional absorption technology is avoided, and true permanent fixation is achieved [15]. The irreversibility of the mineralization reaction ensures the durability of the fixation effect, without the need to consider the degradation problems faced by traditional absorbents. Most importantly, this technology realizes “waste for waste” by using industrial solid waste (such as iron and steel metallurgical slag, power plant fly ash, red mud, etc.), and the mineralized products can also be used as building materials, fillers, and other high-value products. This creates a significant economic benefit and fundamentally solves the economic problems of traditional absorption technology, such as large investment and small output [16]. This approach effectively realizes the recycling of resources and the sustainable development of the greenhouse gas reduction industry.

The unique advantages and sustainability of CO₂ absorption-mineralization technology have driven continuous exploration by the global research community. As one of the major carbon emission sources in the world, the flow and transformation of carbon elements in the production process of the iron and steel industry constitute a complex network system. Especially in the blast furnace process, its carbon emissions account for about 50% or more of the total emissions, and the blast furnace gas is characterized by high flow rate, moderate temperature (about 150–200 °C), stable CO₂ concentration (9–12%), etc. This provides a good foundation for the large-scale application of CO₂ capture technology. Meanwhile, the iron and steel industry has a large amount of waste heat resources and solid waste by-products, which are highly compatible with the process characteristics of CO₂ absorption-mineralization technology. In the current research, for the field of CO₂ absorption, the research mainly focuses on monoamine and mixed amine absorbents. Monoamine absorbents, represented by MEA, are widely used for their high reactivity and good absorption capacity, and the research focuses on the optimization of absorption kinetics, analysis of degradation mechanism, and control of regeneration energy consumption [17]. However, the problems of equipment corrosion, solvent loss, and regeneration energy consumption of monoamine absorbents have prompted researchers to turn their attention to mixed amine systems [18]. Mixed amine technology achieves an optimal balance between absorption rate and capacity through the synergistic effect of components, such as MEA-MDEA (N-methyldiethanolamine), DEA-MDEA, and other mixed systems, which significantly reduce the regeneration energy consumption while maintaining a high absorption rate. Especially with the addition of active agents and blocking agents, the mixed amine system showed significant improvement in terms of oxidation resistance, thermal stability, cycling performance, and sustainability. In terms of mineralization technology, the research mainly focuses on the analysis of the reaction mechanism, process optimization, and process enhancement. The mineralization activity and transformation characteristics of metallurgical solid waste such as steel slag were systematically studied. The activation mechanism, reaction pathway and kinetic characteristics of solid waste were gradually elucidated through multi-scale modelling and analysis [19]. Faced with low mineralization efficiency and poor mineralization effect, researchers thoroughly investigated the influence of key process parameters such as temperature, pressure and liquid–solid ratio on the reaction efficiency through macroscale, which provided a theoretical basis for process optimization. New processing technologies such as ultrasonic and microwave enhancement were also analyzed to improve the reaction rate and conversion efficiency. At the same time, the mineralization reaction faces the problem of raw material selection, and different minerals have great influence on the mineralization effect. At present, micro- and mesoscale studies are used to screen minerals that are more suitable for industrial environments by studying interfacial reactions and molecular dynamics. At present, researchers have used multi-scale analysis of the mineralization process to gradually improve the mineralization mechanism, influencing factors and molecular dynamics, and to provide optimization ideas and ways to improve the mineralization process. Several countries have conducted engineering demonstrations, and through amplification tests and process optimization, the engineering feasibility of the integrated absorption-mineralization process in the iron and steel industry has been preliminarily validated, accumulating experience has been accumulated for the promotion of the technology. These measures also establish a foundation for assessing the sustainability potential of the technology.

However, several challenges remain to be addressed in current research. In absorption technology, the selection of components for mixed amine systems is still largely empirical and lacks systematic theoretical guidance. Additionally, the mechanisms of active agents and blocking agents have not yet been fully elucidated, which hinders further optimization of absorption performance. In terms of mineralization technology, the activation mechanism and reaction kinetic model of industrial solid waste are not perfect enough, and it is difficult to accurately predict and regulate the reaction process; the mechanism and energy utilization efficiency of the reinforcement means also need to be studied in depth. In the mineralization process, the current multi-scale study of minerals, reaction conditions, and so on, for industrial applications shows there is still a certain gap. In the future, multi-scale modeling studies should be established from the aspect of more practical applications. The origin of the raw materials and the destination of the products have yet to be investigated, which is the key to sustainability. In the application of the iron and steel industry, although the CO₂ absorption-mineralization technology shows the potential for application, it still faces two key scientific issues: (1) The technical feasibility needs to be systematically verified, especially in terms of the matching degree between the technology and the existing iron and steel process, the efficiency of the system integration, and optimization of the process parameters; (2) The economic feasibility needs to be evaluated in depth, including the investment cost, operational efficiency, and the value of carbon emission reduction. The systematic solution of these problems is of great significance to promote the large-scale application of this technology in the iron and steel industry.

Based on the above problems, this paper systematically organizes the relevant research advancements in the field of CO₂ absorption-mineralization technology, conducting a comprehensive review from several key aspects. First, it examines the latest developments in the field of CO₂ absorption and mineralization technology by thoroughly analyzing and summarizing pertinent literature from recent years. Second, it details the current status and optimization strategies for monoamine and mixed amine absorbents. Third, it delves into the mineralization efficiency by exploring, the reaction mechanisms and key process parameters associated with the mineralization of industrial solid waste. Fourth, it assesses the feasibility of applying these processes in industrial contexts, particularly within the iron and steel industry. Finally, it evaluates sustainability by considering economic efficiency, energy consumption, and environmental impact. Through a multi-dimensional and multi-level literature analysis, this paper aims to clarify current research hotspots and challenges, identify key scientific issues, and provide theoretical guidance for the future development of this technology. It also aims to provide a decision-making framework for addressing technical bottlenecks in engineering applications, ultimately promoting the large-scale implementation of CO₂ absorption-mineralization technology for industrial emission reduction.

2. Status of CO₂ Absorbents Research

Currently, the most widely used CO₂ absorption technology is the chemical absorption method, which primarily depends on the selection and development of absorbents. Based on their composition, absorbents can be categorized into two types: monoamine and mixed amine. Monoamine absorbents are known for their high reactivity and rapid absorption rates [20]. In contrast, mixed amine systems enhance performance through the synergistic effects of various types of amines. The research advancements related to these two types of absorbents will be discussed in detail below.

2.1. Monoamine CO₂ Absorbents

Among various monoamine absorbents, MEA, DEA and MDEA are the three most representative amine absorbents. Their historical and industrial application experiences have established a significant foundation for the advancement of CO₂ absorption technology [21]. As the first amine absorbent to be industrialized, MEA serves as an important reference for subsequent studies due to its strong alkalinity and rapid reaction kinetic characteristics [22]; DEA has demonstrated the advantages of secondary amines in reducing energy consumption and inhibiting corrosion through the development of the S.N.P.A-DEA process. MDEA has created a new direction in activated amine absorption, owing to its higher absorption capacity and lower desorption energy requirements. The characteristics of these three typical amine absorbents and their process applications are described in detail below.

(1) Monoethanolamine (MEA)

The MEA absorption process was developed by the Union Carbide Corporation and is one of the earliest industrialized methods. The process flow is shown in Figure 2. MEA reacts with CO₂, the absorption of CO₂ is very efficient, and the product can be decomposed through appropriate heating to desorb the MEA solution. The main advantages of this method include the strong basicity of MEA, high gas purity, rapid reaction with CO₂, and the relatively low molecular weight of MEA, which allows for a greater absorption capacity compared to other amines at the same mass concentration [23].

The feed gas is cooled to approximately 40~50 °C at the bottom of the absorption tower, where it reacts with MEA solution in a countercurrent flow. After purification, the gas exits from the top of the tower. The rich liquid at the bottom of the absorption tower and the lean liquid at the bottom of the desorption tower are transferred to the top of the desorption tower via a heat exchanger, and contact with steam generated by the reboiler in a countercurrent manner, allowing for the absorption of CO₂ from the rich liquid. The resulting lean liquid is then returned to the top of the absorption tower for recycling [24].

However, the heat generated during the absorption of MEA is relatively high, necessitating a substantial desorption capacity. The corrosion of equipment by MEA is significant, and when the gas mixture contains oxygen, MEA can degrade, producing oxalic acid, formic acid, and other by-products. These acidic substances not only increase the corrosiveness of the solution towards the equipment but also contribute to the formation of insoluble iron salts. Equipment by MEA corrosion process by-products will interact with MEA to generate some amino compounds, resulting in additional loss of MEA, which will lead to a vicious cycle of MEA degradation process, thus accelerating the degradation of MEA [25].

(2) Diethanolamine (DEA)

Diethanolamine (DEA) is a secondary amine, and its reaction mechanism of DEA with CO₂ is similar to that of MEA. It has a higher boiling point and can absorb CO₂ at elevated temperatures. It is less prone to degradation than MEA, exhibiting a lower vapor pressure, which makes it suitable for low-pressure operations. exhibiting a lower vapor pressure, which makes it suitable for low-pressure operations. The energy consumption required for the desorption of DEA is significantly lower than that of MEA, and it is easy to be desorbed [26]. The disadvantages of DEA are also more obvious, as it is a secondary amine and is not as good at absorbing CO₂ as MEA [25]. It is also more expensive and not economically sustainable. The use of DEA to absorb CO₂ was firstly proposed by Berthier, and later Wendt, Daily, Bailleul, and so on, and more detailed research has since been done. The French Petroleum Institute (S.N.P.A) proposed a new method-S.N.P.A-DEA method by combining the characteristics of DEA. According to the relevant information of the S.N.P.A, a more concentrated aqueous solution of DEA (mass concentration of 25% to 30%) is utilized to absorb acidic gases, as long as the partial pressure of acidic gases in the feed gas is sufficiently high to achieve the stoichiometric ratio, usually 1.0 to 1.3 g of DEA per gram molecule of acid gas [27]. Also, if the rich solution is desorbed sufficiently, the working pressure of the absorption tower is high enough to produce a standard purified gas. The advantage of the S.N.P.A-DEA method is that the presence of impurities such as CS₂ and CO_S does not damage the solution, and in addition, the decomposition products formed can be removed by filtration using activated carbon. Typically, DEA is less corrosive than MEA, because of its lower desorption temperature, the ease with which acidic gases can be desorbed, and the lack of the need for vigorous reboiling [28]. In addition, the decomposition products formed by the side reactions are essentially non-corrosive. The DEA specific flow chart is shown in Figure 3.

(3) N-Methyldiethanolamine (MDEA)

The process of absorbing carbon dioxide by MDEA solution has both chemical absorption and physical absorption. Theoretically, the CO₂ load absorbed by MDEA solution is relatively large, twice as much as that of MEA, and the energy consumption for desorption is very small. Therefore, it can be operated at low temperature, and the solution is not easy to be degraded and volatilized [29]. However, its main disadvantage is its slow reaction rate with CO₂, which is usually increased by adding an activator (e.g., piperazine) or increasing the operating pressure, i.e., the activated MDEA method [30]. The method was developed by the German BASF (BASF). In the early seventies, Germany, the United States, and other countries industrialized the process flow as shown in Figure 4.

The main equipment of the activated MDEA method includes flash desorption columns, vapor desorption columns, and absorption columns [31]. The feed gas is pressurized into the bottom of the absorbents tower, and the semi-poor liquid obtained by secondary flash vaporization absorbs most of the CO₂. The semi-purified gas enters the upper section of the absorbents tower and is absorbed in depth with the poor liquid from the steam desorption tower, absorbing the residual CO₂. Water is added from the top of the absorbents tower to recycle the MDEA in the center of the gas, and the energy released from flash vaporization of the high-pressure section is recovered by the turbine and used to drive the semi-poor liquid pump. The flash liquid section is reduced in pressure by its own pressure into the low-pressure section for another pressure flash, desorbing most of the CO₂. Most of the semi-poor liquid returns to the lower section of the absorption tower, and a small portion of it exchanges heat with the poor liquid sent to the steam desorption tower is steam desorbed for further desorption. The solution at the bottom of the desorption tower through the reboiler to produce a large amount of water vapor, extracting residual CO₂, and the gas at the top of the tower is returned to the flash desorption tower of the low-pressure section for the use of the degassing medium. The lean solution is returned to the upper part of the absorption tower after heat exchange and cooling [32].

The analysis of three typical amine absorbents, MEA, DEA, and MDEA, shows that they have their own characteristics and mutual advantages and disadvantages [33]. In order to comprehensively compare the performance differences of various amine absorbents, this paper further summarizes the performance parameters of other major monoamine absorbents, including AMP (2-amino-2-methyl-1-propanol), MDEA, PZ (piperazine), etc., and systematically compares them in terms of CO₂ uptake capacity, reaction rate, desorption temperature, regeneration energy consumption, thermal stability, etc. [34], as shown in

Table 1. Monoamine absorbents properties.

Absorbents	CO2 Absorption Capacity Mol CO2/mol	Reaction Rate k1/(106 mol·m−2sPa)	Absorption of Reaction Heat kJ/mol	Regeneration Energy Consumption GJ/t CO2	Ref.
AMP	1–1.5	0.7	73	2.0–2.5	[22]
MDEA	1–1.68	0.26	54.6	2.5–3.5	[27]
PZ	0.8–0.9	2.5	70	3.15	[22]
MEA	0.5–0.7	1.6	147	3.3–4.4	[24]
DEA	0.25–0.5	1.1	79	2.1–2.6	[5]

.

2.2. Mixed Amine CO₂ Absorbents

According to the functional characteristics and design purpose of mixed amine systems, mixed amine absorbents can be categorized into three groups: activated mixed amines, synergistic mixed amines, and functional mixed amines [24]. Activated mixed amines are primarily used to enhance the absorption rate of the system by adding activators, such as piperazines, while maintaining low energy consumption of the main absorbents [35]; synergistic mixed amines utilize the complementary advantages of different types of amines to achieve a balance between absorption rate and energy consumption through a reasonable ratio. On the other hand, functional mixed amines introduce amines with special functional groups to provide additional properties, such as antioxidant and corrosion-resistant capabilities, in addition to the basic absorption function of the system [36]. The characteristics of these three typical amine absorbents and their applications in the process are described in detail below.

(1) Activated mixed amines absorbents

Activated mixed amine absorbents is an important CO₂ capture technology; the core idea is to improve the system performance by incorporating an activator (e.g., PZ) into the primary absorbents (e.g., MDEA) [37]. Regarding the reaction mechanism, the active groups in the activator (typically primary or secondary amines) can rapidly react with CO₂ to form carbamate intermediates. Meanwhile, the primary absorbents provide a greater absorption capacity and lower desorption energy consumption, as illustrated in Figure 5. This synergistic effect not only enhances the overall absorption performance of the system but also mitigates the limitations associated with using a single amine absorbent to a certain extent [38].

From the current study status, the research of activated mixed amines primarily focuses on the following aspects: firstly, the screening and optimization of activators. In addition to the traditional piperazine (PZ), researchers have investigated other highly effective amine compounds as activators, such as EDA, TETA (triethylenetetramine) and so on. It has been demonstrated that the CO₂ absorption capacity of the DEEA-TETA system can reach 3.1 mol/L, which was much higher than that of the traditional MEA absorbents. The DETA-PMDETA (5 M, 4:1) system showed an initial absorption rate of 0.7 mol/(L·min) at 50 °C, which was significantly higher than that of the MEA system (0.3 mol/(L·min)) [40].

Secondly, the optimization study of the ratio of activator to primary absorbents significantly influences the performance of the mixed amine system. Taking the DEEA-MAPA system as an example, it was observed that CO₂ and MAPA (methylamine propionate) were primarily enriched in the lower layer, while the upper layer mainly consisted of DEEA (N,N-diethylethanolamine) and a small amount of CO₂. This layering phenomenon was consistent with the phase transition mechanism of the 2B4D system. It was found by NMR spectroscopy revealed that MAPA reacted with CO₂ before DEEA, which led to the phase separation due to the limited solubility of the reaction product in DEEA [41].

In terms of process condition optimization, the researchers concentrated on the optimization of operating parameters such as absorption temperature (40–60 °C), desorption temperature (100–120 °C), and the gas–liquid ratio. Studies have demonstrated that temperature has a significant effect on phase separation behavior. For example, the temperature range of phase separation for temperature-controlled biphasic absorbents is usually between 10–20 °C, which puts high demands on the operating temperature of the absorption tower [42].

In terms of performance evaluation, activated mixed amines demonstrate significant a advantages: the regeneration energy consumption can be reduced to 2.5–3.0 GJ/tCO₂, making them 20–30% more energy efficient than traditional MEA systems [43]. Additionally, their chemical stability is better than that of single amine system; the equipment corrosion is relatively low. However, this technology still faces some challenges: (1) the problem of volatile loss of activator, some activators exhibit high volatility, which is easy to be lost during the recycling process; and (2) the problem of controlling the viscosity of the system, as the viscosity of mixed amine systems tends to increase significantly after absorbing CO₂, which adversely affects mass transfer efficiency.

(2) Synergistic mixed amine absorbents

The synergistic mixed amine system utilizes different types of amine compounds to complement each other to achieve an overall enhancement in CO₂ capture performance [44]. From the point of view of process characteristics, the technology adopts the traditional absorption–desorption cycle flow, CO₂ absorption occurring at 40–60 °C, desorption at 100–120 °C, and can be integrated with industrial waste heat recovery systems to realize the energy gradient utilization [45]. At the core of this lies a significant difference in the reaction mechanisms of different types of amines with CO₂: primary and secondary amines react rapidly with CO₂ through the formation of carbamate intermediates; tertiary amines react with CO₂ via base-catalyzed hydration [46]. Site-blocking amines (e.g., AMP) exhibit unique reaction pathways. The synergistic effect of this multiple reaction mechanism not only enhances the overall absorption performance of the system but also allows for the balancing of different performance indices.

From the performance analysis, the synergistic mixed amines demonstrate significant advantages. For instance, in the DEEA-TETA system, the CO₂ uptake capacity can reach 2.14 mol/L, while the regeneration energy consumption is only 13.5 kJ/mol. The DETA-DEEA system (comprising 2 mol/L DETA + 3 mol/L DEEA) exhibits excellent cycling performance. Additionally, the CO₂ uptake capacity of the DEEA-AEEA system reaches up to 2.58 mol/L [47]. Furthermore, system performance can be enhanced by introducing a third or additional components to the diamine system, thereby achieving multiple synergistic effects. For example, the incorporation of a polyamine component can significantly increase the absorption rate of a tertiary amine-based absorbent and reduce costs. It has also been observed that the viscosity of some synergistic mixed amines increases significantly after CO₂ absorption, which affects mass transfer efficiency and needs to be addressed by optimizing the partition ratio [48].

Current research on synergistic mixed amines primarily focuses on the following aspects: First, the screening and optimization of amine components. Researchers have explored the potential applications of various combinations of amines, including cyclic amines and site-resistant amines, to diversify the range of absorbents. Second, the study of multi-component synergistic effects aims to enhance performance further by introducing a third or additional components. In terms of optimizing process conditions, researchers concentrate on refining operating parameters such as temperature and gas-liquid ratio. However, the technology still faces several challenges: (1) optimizing the distribution ratio of components is complex and requires balancing multiple performance indicators; (2) some systems experience excessively high viscosity; and (3) in actual industrial flue gas environments, system stability and cycling efficiency can be compromised by the presence of impurities such as SO₂ and NO_x.

(3) Functional mixed amines absorbents

The functional mixed amine system represents a novel approach to CO₂ capture technology. This system enhances the performance of traditional amine absorbents by introducing specific functional groups or incorporating functional components. In terms of process characteristics, the technology utilizes a conventional absorption–desorption cycle, with CO₂ absorption occurring at temperatures between 40–60 °C and desorption at 100–120 °C [49]. The researchers have developed a variety of highly efficient functional mixed amine systems: MEA and sulfolane with a mass ratio of 5:4, the CO₂ absorption capacity of up to 2.67 mol/L; DEEA-TETA system through the introduction of sulfolane as a regulator, showing excellent energy-saving characteristics, CO₂ absorption capacity of up to 2.14 mol/L, and the regeneration of only 13.5 kJ/mol energy consumption. The energy consumption of regeneration was only 13.5 kJ/mol; the DETA (diethylenetriamine)-propanol-water system using a ratio of 30 wt% DETA to 50 wt% propanol exhibited a CO₂ uptake capacity of 2.12 mol/L at 8 °C [13]. When the MEA-propanol-water system was formulated in the ratio of 3:4:3 by volume, the viscosity of the system was relatively high, reaching 16 mPa·s, although the absorption capacity could reach 2.4 mol/L [50].

From the perspective of mechanism and performance optimization, researchers have discovered that system viscosity can be effectively reduced by weakening intermolecular hydrogen bonding. For instance, Liu et al.’s study of the DEEA-AEEA system demonstrated that the volatility of the system is primarily determined by DEEA, which is due to its high number of nonpolar groups. Consequently, when selecting functional mixed amines, the impact of volatility can be mitigated by adjusting the proportion of nonpolar groups. In addition, the introduction of specific functional groups can enhance the chemical stability and oxidation resistance of the system, thereby reducing the degradation loss of the absorbents. Li et al. attempted to design PZ/DMF phase change absorbents using N,N-dimethylformamide (DMF) as the solvent, which resulted in a 22.9% increase in CO₂ uptake capacity and a 29.0% improvement in the maximum uptake rate compared to its aqueous solution. Currently popular configurations of mixed amine carbon dioxide absorbers are shown in Table 2.

However, functional mixed amines still encounter several challenges in practical applications. Firstly, the introduction of functional groups may increase the synthesis costs of absorbents. Secondly, the stability and selectivity of these functional groups require further enhancement to perform effectively in the complex environment of industrial flue gas. Thirdly, the viscosity of certain systems significantly increases after CO₂ absorption, which adversely affects mass transfer efficiency. Future research will focus on the following: (1) Developing new, cost-effective materials with functional groups, emphasizing the design of multifunctional groups; (2) Conducting in-depth studies on the mechanisms of action of functional groups to provide theoretical guidance for molecular design; (3) Optimizing process parameters to enhance the overall efficiency of the system; (4) Implementing industrial demonstration studies to validate the practical application of the technology. Additionally, researchers need to establish a standardized performance evaluation system and develop guidelines for solution formulation for improved clarity, readability, and technical accuracy while correcting grammatical and punctuation errors [51].

Table 2. Mixed amine CO₂ absorbents.

Absorbents			Regeneration Energy (GJ/t CO2)	Viscosity/cp	CO2 Absorption/ (mol/kg)
Name		Distribution Ratio
Activated mixed amines absorbents	DEEA-MAPA [12]	2 mol/L MAPA + 5 mol/L DEEA	0.75	2.4	/
	BDA-DEEA [13]	4 mol/L DEEA + 2 mol/L BDA	0.505	/	/
Synergistic mixed amines absorbents	DEEA-HMDA [20]	V(DMSO):V(PMDETA) = 1:4	/	0.841
	DEEA-AEEA [27]	25 wt.% + AEEA25 wt.%DEEA+	0.64	2.58	/
	DETA-DEEA [30]	3 mol/L DEEA + 2 mol/L DETA	1.94	2.14	13.5
Functional mixed amines absorbents	DMSO-PMDETA [34]	V(DMSO):V(PMDETA) = 4:6	2.5	0.97	13.92
	MEA-1-propanol-H2O [37]	V(MEA):V(1-propanol):V(H2O) = 3:4:3	2.4	2.4	16
	MEA-Sulfolane [45]	M(MEA):M(Sulfolane) = 5:4	2.67	2.67	8
	DETA-1-propanol-H2O [52]	30 wt%DETA + 50 wt%1-propanol	2.12	2.12	8

Table 2. Mixed amine CO₂ absorbents.

Absorbents			Regeneration Energy (GJ/t CO2)	Viscosity/cp	CO2 Absorption/ (mol/kg)
Name		Distribution Ratio
Activated mixed amines absorbents	DEEA-MAPA [12]	2 mol/L MAPA + 5 mol/L DEEA	0.75	2.4	/
	BDA-DEEA [13]	4 mol/L DEEA + 2 mol/L BDA	0.505	/	/
Synergistic mixed amines absorbents	DEEA-HMDA [20]	V(DMSO):V(PMDETA) = 1:4	/	0.841
	DEEA-AEEA [27]	25 wt.% + AEEA25 wt.%DEEA+	0.64	2.58	/
	DETA-DEEA [30]	3 mol/L DEEA + 2 mol/L DETA	1.94	2.14	13.5
Functional mixed amines absorbents	DMSO-PMDETA [34]	V(DMSO):V(PMDETA) = 4:6	2.5	0.97	13.92
	MEA-1-propanol-H2O [37]	V(MEA):V(1-propanol):V(H2O) = 3:4:3	2.4	2.4	16
	MEA-Sulfolane [45]	M(MEA):M(Sulfolane) = 5:4	2.67	2.67	8
	DETA-1-propanol-H2O [52]	30 wt%DETA + 50 wt%1-propanol	2.12	2.12	8

Monoamine absorbents are the most widely commercialized chemical absorbents due to their established technology and low cost, but at the same time reflect the significant problems of substantial solvent loss and high energy consumption during regeneration. In contrast, mixed amine absorbents can better fulfill the requirements for high absorption rates, high absorption capacities, and low energy consumption. This is achieved through the synergistic effects of different amines, which combine the advantages of various single amines. In summary, mixed amine absorbents demonstrate benefits in reducing regeneration energy consumption and enhancing absorption performance. Nevertheless, further research and development are necessary to expand their industrial applications. Additionally, phase change absorbents and anhydrous absorbents, as a new generation of absorbents, hold great potential but also require further investigation and improvement.

3. Current Status of CO₂ Mineralization Research

The concept of CO₂ mineralization, which began with Seifritz in 1990, is centered around the idea of simulating and accelerating the natural process of rock weathering and CO₂ uptake [52]. In this process, CO₂ dissolves in water to form carbonic acid, which then neutralizes with alkaline minerals to form stable solid carbonates. These carbonates will not decompose in nature over long geological ages, thus realizing the permanent sequestration of CO₂. It is worth mentioning that the standard Gibbs free energy of carbonates is 0 to 180 kJ/mol lower than that of CO₂, demonstrating that carbonates are the most stable form of elemental carbon. Furthermore, the mineralization reaction, being an exothermic process, can proceed spontaneously, which significantly reduces the energy consumption of CO₂ mineralization technology. CO₂ mineralization and storage not only simplify the CCUS technology chain by achieving the capture and storage of CO₂ in one step, but also reduce the technology’s costs through the sale of the mineralized products and potentially create an economic income [53,54].

In terms of raw material selection, CO₂ mineralization technology makes extensive use of natural ores, such as serpentine, magnesium olivine, and wollastonite, as well as alkaline solid wastes such as steel slag, fly ash and construction waste [55,56]. These resources are abundant, widely distributed, and less geographically restricted, providing a wide range of applications for coal-fired power plants, steel mills, and other scenarios [57]. At the same time, from the waste utilization perspective, tailings can also be used as a raw material for mineralization. Tailings, which are an important pollutant but also a mineral resource, can realize the permanent fixation of CO₂ through the carbonation reaction of silicate minerals in the tailings [58,59]. In addition, tailings are characterized by small particle size and large specific surface area, which can reduce the cost of pre-treatment of mineralized raw materials [60,61]. The treatment of tailings also combats pollution, which can improve the environment and promote ecological development. The equation for the reaction for mineralization with alkaline magnesium-bearing minerals such as serpentine is as follows:

6HCl + Mg₃SiO₅(OH)₄ → 3MgCl₂ + 2SiO₂ + 5H₂O

(1)

CO₂ + NaOH → NaHCO₃

(2)

MgCl₂ + 2NaHCO₃ → MgCO₃ + 2NaCl + H₂O + CO₂

(3)

Currently, the CO₂ mineralization and sequestration process is mainly divided into two types: direct mineralization and indirect mineralization, and the flow chart is shown in Figure 6. Direct mineralization means that CO₂ reacts directly with minerals in a single reactor to produce carbonates [62,63]. It is further subdivided into dry and wet direct mineralization, indirect mineralization with CO₂ consists of two main steps: firstly, extraction of Ca²⁺/Mg²⁺ from the mineralized raw material by means of a leaching agent, followed by the reaction with CO₂ under alkaline conditions to produce the corresponding carbonate. The versatility and flexibility of this technology have laid a solid foundation for its widespread application in practice [64,65]. Below is a detailed description of the two types of mineralization.

This paper focuses on the application of the mineralization process in the steel industry. Minerals such as Ca(OH)₂, CaO, Ca₂SiO₄, and Ca₃AlO₆ in steel slag can undergo carbonation to form CaCO₃. Here are some possible reaction equations:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(4)

CaO + CO₂ → CaCO₃

(5)

2CaO⋅SiO₂ + 2CO₂ + 2H₂O → 2CaCO₃ + SiO₂⋅2H₂O

(6)

3CaO⋅Al₂O₃ + 3CO₂ → 3CaCO₃ + Al₂O₃

(7)

3.1. Direct Mineralization

(1) Gas-solid dry direct mineralization technology

Gas-solid dry direct mineralization, first proposed by Lackner et al. in 1995, is the simplest MC (Mineral Carbonization) technology. The core of the process lies in a one-step gas-solid mineralization reaction of CO₂ gas directly with natural minerals such as serpentine and magnesium peridotite, etc., in a specific reactor without solvent to produce stable carbonates. According to thermodynamic calculations, the Gibbs free energy of the reaction is less than zero, indicating that it is theoretically spontaneous under natural conditions. Despite the directness and relative simplicity of this pathway, the reaction rate is significantly limited in natural environments due to the low levels of carbon dioxide in the atmosphere; to increase efficiency, the raw ore needs to undergo pre-processing steps such as crushing and milling [66,67]. According to relevant studies, the reaction process can be effectively accelerated by increasing the reaction temperature and initial CO₂ partial pressure. For example, the experimental data of Lackner et al. indicated that the maximum conversion of fine-grained (100 μm) serpentine was only 25% after 2 h of reaction at 500 °C and a partial pressure of 3.4 × 10⁷ Pa CO₂ [68]. DaCosta et al. further improved the gas-solid direct mineralization process by using flue gas moistened with water vapor to react with fine-grained (2.5–60 μm) natural ores, and showed that magnesium peridotite was converted up to 18% in 30 min at 150 °C, 15% CO₂, and 8.3% water vapor [64]. However, even though these improvements enhance the reaction rate, the carbonation reaction rate and conversion rate are still difficult to meet the demands of industrial applications even under high temperature and high pressure conditions, so the technology does not have the prospect of industrialized application [65].

(2) Wet direct mineralization technology

Unlike the gas-solid dry direct mineralization technique, the wet direct mineralization technique involves proceeding in a gas-liquid-solid three-phase system, where CO₂ is first dissolved in water to form carbonic acid, and then subsequently reacts with the ore in a weakly acidic environment by dissolution and precipitation. The generation of carbonates captures CO₂ more efficiently under ambient conditions due to the presence of water [69]. This process was first proposed by O’Connor et al. and successfully applied to the mineralization of serpentine and magnesium olivine. This method significantly increases the reaction rate compared to the direct dry method [70]. However, this method is only suitable for industrial solid wastes with high reactivity and is too costly for applications in mineralization of natural ores, because in the process of direct carbonation, as silicate minerals carbonate, the passivation layer of amorphous silica will gradually accumulate and attach to the surface of the reactants, impeding the leaching of metal ions and thus affecting the carbonation reaction process. In order to further enhance the efficiency of the reaction, the ore material needs to undergo pre-treatment steps, such as crushing, milling, and even thermal excitation, and maintain high temperature and CO₂ partial pressure during the reaction process, while adding catalysts such as sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃) and sodium chloride (NaCl) [71,72]. O’Connor et al. showed that the conversion of magnesium olivine, thermally excited serpentine, and wollastonite could reach 49.5%, 81.8%, and 73.5%, respectively, under the conditions of crushing of raw ore to less than 100 μm, temperatures of 100 to 200 °C, CO₂ partial pressures of 100 to 150 bar, and 0.64 mol NaHCO₃/mol NaCl. Although the wet direct mineralization technology demonstrates high ore conversion and CO₂ sequestration capacity with a significant reduction in reaction time, its high temperature and pressure requirements, as well as the high energy consumption of ore pretreatment, significantly increase the process cost. Although there are no studies proving that wet direct mineralization is economically feasible for use in industrial production, it is still considered the most promising technology for CO₂ sequestration.

3.2. Indirect Mineralization

The indirect mineralization process is an efficient method for utilizing calcium and magnesium ions extracted from ores or solid wastes. Initially, these ions are leached from the ore or waste using an acid leaching agent. Following a solid-liquid separation step, a solution enriched with calcium and magnesium ions is obtained. This solution is then mixed with CO₂ gas or carbonates (e.g., NaHCO₃/Na₂CO₃, NH₄HCO₃/(NH₄)₂CO₃) sourced from the capture process, which initiates a mineralization reaction that ultimately produces carbonates (CaCO₃, MgCO₃), thereby creating a mineralization seal for CO₂ [73]. The advantage of the indirect carbonation process lies in its ability to produce high-purity, high-value carbonate products, thereby reducing overall process costs. This efficiency is attributed to the effective removal of impurities, such as silica and iron, prior to carbonate precipitation. However, the technology also presents certain challenges. The ore must undergo energy-intensive pre-treatment steps, including crushing and milling, before processing. Additionally, the recovery and recycling of acidic leaching agents pose significant challenges. Therefore, in the research and development of indirect mineralization technology, it is essential to identify suitable acidic leaching agents. The ideal leaching agent should not only leach calcium and magnesium ions quickly and effectively but also be easy to recover and recycle, thereby minimizing environmental impact and enhancing the economic viability of the process.

(1) Mineralization by hydrochloric acid leaching

Lackner et al. in 1995 proposed an indirect mineralization process utilizing hydrochloric acid (HCl) as a leaching agent. The core steps of this process include the extraction of calcium and magnesium ions from fine-grained ore into solution using hydrochloric acid, followed by solid-liquid separation techniques. The calcium and magnesium-rich filtrate is then heated to completely evaporate the water, resulting in the formation of hydrated magnesium chloride solids (MgCl₂·6H₂O) [74]. Further heating at 150 °C converts the hydrated magnesium chloride into magnesium hydroxide solids (Mg(OH)₂), which are subsequently employed in the mineralization reaction with CO₂ [75,76]. Notably, the hydrochloric acid used in this process can be recycled. Another study conducted by Maroto-Valer et al. employed a different approach. They activated serpentine in sulfuric acid (H₂SO₄) at temperatures ranging from 20 to 65 °C for a duration of 3 to 12 h. Subsequently, the magnesium sulfate (MgSO₄) obtained through leaching was reacted with sodium hydroxide (NaOH) to produce solid magnesium hydroxide (Mg(OH)₂) [77]. This magnesium hydroxide was then mineralized with CO₂ at 20 °C and 40 bar, and the experimental results indicated that serpentine could be converted up to 55%. A summary of the results from various studies reveals that the effectiveness of leaching agents on the leaching efficiency of the ore follows a specific order: sulfuric acid (H₂SO₄) is the most effective [78], followed by hydrochloric acid (HCl), and finally nitric acid (HNO₃). Although indirect mineralization processes utilizing strong acids as leaching agents have demonstrated technical feasibility, the regeneration and recycling processes of these leaching agents are energy-intensive and can lead to chlorine depletion [79,80]. Neglecting the regeneration and recycling of the leachate will result in significantly increase the overall cost, thereby limiting the industrial application of this process.

(2) Mineralization by leaching of strong acid and weak base salts

To address the challenges associated with leachate regeneration, researchers have begun to investigate the potential application of various strong acids and weak base salts that exhibit low energy consumption during regeneration in the indirect mineralization process. These include acetic acid, ammonium salts, and other substances utilized as leaching agents [81,82]. The reaction between natural calcium silicate ore and acetic acid demonstrated negative Gibbs free energy changes at each stage of the entire reaction system during thermodynamic analysis. This result indicates that the entire reaction process is thermodynamically spontaneous and does not require significant external energy input, thus facilitating the use of acetic acid as a leaching agent in the indirect mineralization process [83,84]. Considering the high similarity in the composition of iron and steel metallurgical solid wastes to that of natural calcium silicate ore, Eloneva et al. synthesized non-commercial grade CaCO₃ using blast furnace slag. Firstly, an acetic acid solution was used to leach the waste residue and extract calcium ions from it [85,86]. Subsequently, the leachate containing calcium ions was separated from the waste stream and subjected to carbonation. During the carbonation step, in order to control the reaction conditions, the researchers also added an appropriate amount of acid neutralizer (sodium hydroxide) [87,88]. This process induced the precipitation of calcium carbonate crystals. For each ton of CO₂ sequestered, approximately 4.4 tons of biofilter material is consumed (which translates to approximately 227 kg of CO₂ sequestered per ton of biofilter), accompanied by the consumption of 3.6 tons of acetic acid and 3.5 tons of sodium hydroxide.

Ammonium salts such as NH₄Cl are frequently employed as leaching agents in indirect mineralization techniques due to their exceptional selectivity for the leaching of Ca²⁺ ions [89]. Kodama et al. successfully developed a pH swing mineralization technique based on NH₄Cl and elaborated the main reaction equations involved when NH₄Cl is used as a leaching agent, along with the associated energy changes. The reaction equations indicate that the continuous production of NH₃ during the leaching process resulted in the leach solution gradually becoming alkaline. This eliminates the need for additional lye additions to adjust the pH and effectively addresses the challenge of CO₂ being difficult to exist in the liquid phase in ionic form under acidic conditions. The whole system establishes a buffer system of NH₄⁺/NH₃-H₂O, which significantly enhances the stability of the system and allows the reaction to proceed for a long period of time. This results in a greater conversion of CO₂ to carbonate, which reacts with Ca²⁺ ions, leading to excellent mineralization efficiency. Experimental results show that the overall leaching rate approaches 60%, while the mineralization rate reaches approximately 80% when using a 1 mol concentration of NH₄Cl. It is worth mentioning that NH₄Cl can be regenerated at the end of the mineralization reaction, thus realizing the recycling of the leaching agent and reducing the consumption of chemicals for indirect mineralization techniques. This feature offers significant economic and environmental advantages for large-scale industrial applications and is one of the key considerations in practical applications [90,91]. These studies not only demonstrate the potential for reusing of industrial waste, but also provide new directions for environmentally friendly CO₂ fixation technologies.

$$4N{H}_{4}Cl+2CaSi{O}_3\to 2CaC{l}_2+Si{O}_2+4N{H}_3+2H_{2}O$$

$$4N{H}_3+2C{O}_2+2CaC{l}_2+4H_{2}O\to 2CaC{O}_3+4N{H}_{4}C$$

Figure 7 [68] demonstrates the processes and mechanisms of direct and indirect mineralization.

CO₂ mineralization technology is an emerging carbon sequestration technology that achieves the permanent fixation of CO₂ by simulating the natural weathering process of rocks. This technology has the dual advantage of being an environmentally friendly and spontaneous process, while facilitating the use of waste resources and reducing emissions by using industrial solid wastes as raw materials. A comparison of the main mineralization processes is shown in

Table 3. Comparison of mineralization processes.

Mineralization Type	Minerals and Chemicals	Advantages	Disadvantages
Direct mineralization	Gas-solid dry direct mineralization technology	The process is simple and does not require the use of solvents. The reaction is spontaneous and consumes less energy.	Reaction rates are slow and require high temperature and pressure conditions to improve efficiency. Carbonation reaction rates and conversions are difficult to meet the demands of industrial applications.
	Wet direct mineralization technology	Faster reaction rate than dry process. Suitable for industrial solid waste with high reactivity.	Costly, requires ore pre-treatment (crushing, milling). High temperature and pressure requirements increase energy consumption.
Indirect mineralization	Mineralization by hydrochloric acid leaching	Capable of producing high purity, high value carbonate products. Can effectively remove impurities such as silica and iron.	Requires energy-intensive ore pretreatment steps. Recovery and recycling of acid leachate is difficult.
	Mineralization by leaching of strong acid and weak base salts	The regeneration of leaching agent consumes less energy. Recycling of leaching agent can be realized and chemical consumption can be reduced.	Fine control of the reaction conditions is required to maintain the stability and selectivity of the leaching agent.

. Future research should focus on the following directions for technological breakthroughs: first, the development of new functional mixed amine systems, optimizing the absorption performance by introducing modifiers, designing multiple functional group materials to improve the CO₂ absorption capacity; second, in the mineralization process, ultrasound, microwave and other means of enhancement should be used to improve the reaction rate, develop a new type of leaching agent system to improve the leaching efficiency of metal ions, and optimize the three-phase gas–liquid–solid contact; third, in terms of industrial integration, the focus should be promoting deep integration with the iron and steel industry, utilizing low-grade waste heat for desorption, and developing a new type of separation device to improve system efficiency. As the 2030 carbon peaking timeline approaches, it is expected that the government will introduce more supportive policies, including fiscal and tax incentives and carbon market construction, which will further promote the industrialization of the technology. Especially in the iron and steel industry, CO₂ absorption-mineralization technology can not only achieve emission reduction targets but also gain considerable economic benefits through industrial solid waste resourcing and value-added by-products.

3.3. Multi-Scale Theoretical Calculation Methods for Studying Mineralization Mechanisms

At the forefront of research on carbon capture and mineralization technologies, multi-scale theoretical calculation methods have become revolutionary scientific tools for analyzing complex reaction mechanisms. These advanced computational methods can precisely span multiple research dimensions, from the electronic scale to the macroscopic scale, providing scientists with unprecedented observational and simulation capabilities in the microscopic realm. Compared to traditional experimental methods, which are limited by observational technologies and scale constraints, multi-scale theoretical calculation methods can overcome these limitations by accurately reconstructing the complete kinetic processes of chemical reactions through numerical simulations. The primary advantage of this computational approach lies in its ability to systematically simulate and reconstruct the microscopic interaction mechanisms within complex chemical systems. By integrating various theoretical methods, such as quantum mechanics, statistical mechanics, and molecular dynamics, scientists can comprehensively analyze the essence of the CO₂ mineralization reaction, examining aspects from electronic energy levels and atomic structures to molecular dynamics. In current research on CO₂ mineralization, multi-scale theoretical calculation methods mainly encompass three levels:

• Electronic Scale Calculation: Represented by first-principles calculations based on fundamental principles of quantum mechanics, methods like Density Functional Theory (DFT) are employed to accurately describe the electronic structure and microscopic interactions of the system. This level primarily focuses on the microscopic features of electronic energy levels, electron density distributions, and chemical bonding.
• Atomic/Molecular Scale Simulation: This includes Ab Initio Molecular Dynamics and Classical Molecular Dynamics. Through tracking atomic/molecular trajectories, this level reveals the dynamic processes at the atomic/molecular level, including atomic/molecular migration as well as local structural evolution.
• Mesoscopic and Macroscopic Scale Simulation: Utilizing methods such as coarse-grained molecular dynamics or dissipative particle dynamics, this level studies the overall behavior of larger-scale systems. It explores thermodynamic properties and structural evolution at the macroscopic scale.

In electronic scale simulations, precise simulations of electronic structures and quantum-scale interactions can provide a more microscopic perspective for analyzing complex mineralization reaction mechanisms. Currently, the primary focus of research is on the electronic dynamics of surface adsorption processes, the electronic reconstruction mechanisms of active sites in catalysts, and the analysis of reaction pathways at the quantum scale. For instance, Nie et al. [92] built a kinetic model for the mineralization of the MEA+MDEA system, which was validated using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) methods, identifying the frontier molecular orbital energy levels of the reaction products. Wang et al. [93] conceptualized the CO₂ mineralization process as an organic molecular battery, analyzing the role of organic reductive catalysts and achieving accurate predictions of open-circuit voltage. Liu et al. [94] focused on the carbonation behavior of C-S-H, constructing a kinetic model for the carbonation process. This model was validated through microscopic characterization, which revealed the influence of Ca/Si on the reaction rate. Wang et al. [95] further refined the mechanism of mineralization by investigating the effects of temperature, partial pressure of carbon dioxide, and sodium ion concentration on the mineralization kinetics. These studies, through precise simulations at the electronic scale, not only provide deep insights into the microscopic mechanisms of the CO₂ mineralization reaction but also offer critical theoretical guidance and innovation pathways for developing efficient low-carbon capture and mineralization technologies. However, there are limitations. Nie et al. [92] only studied the MEA+MDEA system, and the samples and analytical methods are relatively single, resulting in greater constraints on the experimental findings. Liu et al. [92] conducted experiments under ideal conditions, revealing significant discrepancies between the microscopic characteristics of the samples and those observed under natural conditions. This disparity complicates the accurate representation of the mineralization process. Additionally, there are some key data in the paper that were not measured, which significantly impacts the calculations. Wang et al. [91] suggest that incorporating CO₂ into the battery system is different from conventional mineralization scenarios. Consequently, the calculated data may not be particularly meaningful for the conventional direction. Furthermore, Wang et al. [95] only considered the mineralization of olivine as a raw material, and the single nature of the data is not conducive to adequate analysis, as the reactions conducted in the laboratory differ considerably from those in an industrial environment. Future research should choose more realistic materials and the experimental data should be more accurate and complete to avoid too much calculation errors. DFT is very significant for the kinetic study of the mineralization reaction and can be further improved through the combination of micro-, meso-, and macroscopic scales.

In atomic/molecular scale simulations, precise tracking of atomic/molecular motion trajectories, along with the reconstruction of dynamical processes at this scale, can provide a more dynamic perspective for understanding the microscopic structural evolution of complex mineralization reactions. Currently, the emphasis is on migration mechanisms and real-time evolution processes of interfacial reactions. For example, M. J. Abdolhosseini et al. [96] studied the impact of a water film coating mineral surfaces of nanometer thickness on the mineralization process, explaining the mechanisms by which ionic solvation and hydration behaviors are altered under nanoconfinement. Gu et al. [97] constructed a polymer network hydrogel mineralization model with interpenetrating polyethyleneimine (PEI) chains and inorganic calcium chloride (CaCl₂), obtaining the reactive pathways for synergistic functions through molecular-level theoretical computations. Dai et al. [98] investigated the molecular dynamics of clay mineralization of carbon dioxide at the molecular level. Through the movement and reaction of individual molecules in the reaction, the feasibility of clay treatment of carbon dioxide was derived, providing a new idea for mineralization. These detailed simulations at the atomic/molecular scale not only enhance the understanding of the microscopic mechanisms of mineralization reactions but also yield significant scientific breakthroughs for developing cross-scale computational techniques.

In mesoscopic and macroscopic scale simulations, reconstructing the overall behavior and thermodynamic evolution of complex mineralization systems can provide a more comprehensive system perspective for understanding the macroscopic regulatory mechanisms of mineralization reactions. For example, Dai et al. [99] derived the kinetic parameters of the CO₂ mineralization reaction using a tubular reactor equipped with a static mixer, analyzing the effects of turbulence on conversion rates. Mehdipour et al. [100] constructed models of carbonate cement concrete components, studying how gas space distribution (velocity and flow quantified through computational fluid dynamics analysis) and processing conditions (temperature, relative humidity, and flow quantified through factorial design) affect drying and carbonation. Bhardwaj et al. [101] built a fluidized bed reactor model, verifying the significance of moisture on the mineralization of serpentinite particles. Peng et al. [102] investigated the effects of temperature and pressure on mineralization efficiency by means of a self-developed dynamic static reaction test device. It was found that the mineralization efficiency increased parabolically with temperature, with 72 °C identified as the turning point for the growth rate of mineralization efficiency, and 82 °C as the optimal temperature for enhancing the mineralization efficiency of supercritical CO₂. Changes in supercritical CO₂ pressure had a small effect on the mineralization efficiency. These studies at the macroscopic scale systematically reveal the overall regulatory mechanisms of mineralization reactions and provide critical theoretical guidance for developing efficient CO₂ mineralization technologies. The theoretical guidance processes of different methods are illustrated in Figure 8 below.

The interplay of microscopic, mesoscopic, and macroscopic approaches allows us to refine the CO₂ mineralization mechanism across multiple scales. The mineralization mechanism is generally divided into three stages: (1) CO₂ dissolution and ionization to produce CO₃²⁻; (2) dissolution of calcium ions; and (3) formation of precipitation [103]. Further refinement is carried out through these three scales based on this framework. On the microscopic scale, gaseous CO₂ enters the mineral slurry, rapidly dissolving in water to generate H₂CO₃. This compound ionizes to produce CO₃²⁻, which is continuously transported into the interior of the mineral under the effect of the concentration difference, while calcium ions dissolve at this time and are simultaneously transported outward. The two species meet during transport and react. It has now been found that the products resulting from the reaction affect the propagation of subsequent substances, reducing the rate and efficiency of the reaction [102]. The microscopic scale provides valuable insights into how each substance is affected and hindered in terms of kinetics. Utilizing the mesoscopic scale provides a complete understanding of the migration and transformation of the medium at the interface, where the substances and individual ions produced by the reaction can be clearly observed [103]. Finally, the macroscopic scale is utilized to refine and understand the various factors affecting the mineralization reaction such as temperature, turbulence conditions, air pressure, and reactant state during the reaction. By adjusting these factors and observing the effects on the kinetics, these three scales complement each other and work together to improve the theory [95]. Through these three scales, we can also study the optimal reaction state. Taking the pore size of minerals as an example, in the process of carbon dioxide mineralization using minerals, the pore structure on the surface of minerals is observed in the microscopic scale. Theoretically, interconnected matrix porosity favors dissolution and precipitation [94]. Building on this, the migration and transformation of carbon dioxide on the surface of medium pores are observed at the mesoscopic scale. Finally, the mineralization efficiency is verified in macroscopic scale, which proves the theory of the role of pore size in mineralization after full observation and verification across all three scales [91].

Wang et al. [95] investigated the effect of CO₃²⁻ ion kinetics in solution by adjusting the sodium ion content, media particle size, and gas partial pressure, and the formation and diffusion of the carbonate molecular layer was observed from the mesoscopic scale. Wang et al. [95] combined these three scales to successfully examine both the kinetic and chemical aspects of the mineralization reaction. Peng et al. [102] shaped the mineralization theory by establishing a microdynamic model, on the basis of which the parameters of temperature, pressure, water vapor diffusion coefficient, and supercritical CO₂ were varied to explore the mesoscopic scale pore changes, and the effect on the mineralization efficiency. From the microscopic model to the macroscopic regulation to the mesoscopic pore exploration, the study of the factors affecting the mineralization rate and efficiency is completed from three aspects. Finally, it is concluded that the efficiency and rate of mineralization is grossly overestimated if pore multiscale benefits are not taken into account. The multiscale model further reveals the transport law of CO₂ under multi-field coupling, providing a powerful tool for injection scheme design, real-time monitoring and prediction. This model is a powerful tool for injection program design, real-time monitoring and prediction. The above examples give us good reference and inspiration. Studying from multiple scales can more accurately simulate the role played by each factor in the overall mineralization reaction. Through the above methods, future researchers can gain a more comprehensive and in-depth understanding of mineralization studies by combining microscopic and macroscopic models. In the future, such cross-scale analysis and modeling will be important for optimizing the mineralization process, improving the efficiency of mineralization, and developing new mineralization techniques [93,96,100].

4. Feasibility Analysis of CO₂ Absorption-Mineralization for Steel Industry Applications

The steel industry is one of the world’s major sources of carbon emissions, and the flow and transformation of carbon in its production processes form a complex network system [104]. As shown in Figure 9, starting from the bottom process, washed coal is converted into coke by high-temperature dry distillation in a coking oven, which also produces coke oven gas containing CO, CO₂, and CH₄, as well as chemical products such as tar containing C-H compounds [105,106]. After the coke is crushed, it enters the sintering process together with anthracite coal and carbon-containing solvents. During the ore sintering process, sintering flue gas containing CO and CO₂ and carbon-containing sintering dust is generated. In the core ironmaking process, coke and iron ore undergo a reduction reaction in the blast furnace. On one hand, the process serves as a reductant to reduce iron ore to carbonaceous pig iron, and on the other hand, it acts as a heat source to provide energy for the reaction [107,108]. This process produces large quantities of blast furnace gas containing CO and CO₂, while the hot blast furnace also produces carbonaceous dust and flue gases containing CO and CO₂. Subsequently, in the converter steelmaking process, carbon is oxidized and removed from the molten pig iron by blowing oxygen into the molten pig iron, generating converter flue gas and exhaust gases. Finally, in the continuous casting and hot rolling process, the steel is solidified and rolled into carbon-containing steel products, and its heating furnace also generates CO and CO₂ fumes in this process [109,110,111]. In the whole process, the blast furnace process is responsible for the largest carbon emissions, accounting for more than 50% of the total emissions, and its CO₂ concentration is significantly higher than that of the flue gas from conventional coal-fired power plants, which provides a strong foundation for the large-scale application of carbon capture technology [112,113].

Blast Furnace Gas (BFG) is one of the most important by-product gases in blast furnace production [113]. The composition of blast furnace gas varies from one steel plant to another, and the composition and calorific value of blast furnace gas are affected by many factors, such as fuel, pig iron varieties, smelting process, and so on [114]. Modern ironmaking production process generally employ advanced production techniques, resulting in characteristics such as a large volume of gas and high wind temperatures. However, these factors also contribute to a lower calorific value of the produced blast furnace gas, which is characterized by high dust content, inappropriate for fire, unstable combustion utilization of blast furnace gas, which increases the difficulty of utilization. The main components of blast furnace gas in iron and steel enterprises include carbon monoxide (CO), carbon dioxide (CO₂), nitrogen (N₂), hydrogen (H₂), and methane (CH₄), etc., which can be seen in

Table 4. Blast furnace gas characteristics.

CO V/V%	H2 V/V%	CH4 V/V%	N2 V/V%	CO2 V/V%	O2 V/V%	Sulfide	Densities kg/m3	Calorific Value kJ/m3
25.0~30.0	1.5~3.0	0.2~0.5	55.0~60.0	9.0~12.0	0.2~0.4	<0.1%	1.29~1.30	3300~4200

.

From the technical level, CO₂ absorption-mineralization integrated technology has significant advantages when applied to blast furnace gas treatment. Firstly, blast furnace gas is characterized by high flow rate, moderate temperature (about 150–200 °C) and stable pressure, all of which contribute to reducing capture costs. Secondly, the high CO content (25.0–30.0%) of blast furnace gas can be utilized as an important chemical raw material. Although the CO₂ concentration (9–12%) is lower than that of cement kiln tail gas (15–20%), it remains significantly higher than that of other process flue gases [115,116]. In addition, this technology does not require high-temperature desorption, instead, it regenerates the absorbent solution through the pH change. The resulting product exists in the form of stabilized carbonate, thereby eliminating the need for gaseous CO₂ compression, which can significantly reduce energy consumption. Compared with the traditional MEA process, the energy consumption of the desorption and compression unit can be reduced by about 60%, which has obvious economic advantages [117,118].

From the process optimization point of view, this technology has multiple advantages in blast furnace gas treatment [22]. Firstly, the process route of “high reaction rate and low desorption energy consumption” can be adopted, with primary and secondary alcohol amines as the main body, and other amines can be added to form a mixed amine system. This not only ensures a high CO₂ reaction rate, but also reduces desorption energy consumption and corrosiveness. Secondly, the absorption performance can be optimized by introducing regulators (e.g., cyclobutanesulfone) [46], thereby increasing CO₂ absorption capacity. In addition, this technology can be effectively integrated with the waste heat resources of iron and steel enterprises, utilizing low-grade waste heat of 100–150 °C for desorption, which complements the traditional waste heat recovery system and improves the overall efficiency of the system [12]. In process design, gravity separators, membrane separators and other new phase separation devices can be used to strengthen the phase separation effect, accelerate the separation of the rich phase and the poor phase, and reduce the separation energy consumption.

From the perspective of economic operation, the integrated CO₂ absorption-mineralization technology shows significant cost advantages and market potential in blast furnace gas treatment. The technology can significantly reduce energy costs by avoiding the high temperature desorption process in the traditional process, in which the energy consumption of desorption and compression units can be reduced by more than 80% [107,119]. Furthermore, this technology has lower operation and maintenance costs. By using pH control instead of temperature control, it minimizes equipment corrosion, thereby extending the life of key components and reducing maintenance and replacement needs. The carbonate products generated by this technology have high industrial added value and can be used as raw materials for building materials, soil conditioners, etc., creating additional economic benefits. Under the background of the increasingly mature carbon trading market, each ton of CO₂ emission reduction can obtain about 50–60 yuan of carbon trading revenue. Considering the future trend of carbon price increases, the economic benefits of this technology will be further enhanced. Combining with the current steel industry “double carbon” target pressure, as well as the national policy support for low-carbon technologies [120], the technology has a good payback period and is expected to break even in 3–5 years, which is a strong investment attraction for steel companies. The economic comparison with the traditional scheme is shown in

Table 5. Comparison of energy consumption between conventional MEA capture technology and integrated mineralization technology.

Comparison of Energy Consumption Between Conventional MEA Capture Technology and Integrated Absorption Mineralization (in kJ/kgCO2)
Pre-treatment and absorption units
Equipment type	Energy consumption	/
Bellow	106.0
Pump	18.0
Subtotal	124.0
Desorption and compression units
Equipment type	Traditional MEA workers	Integration technology	Energy conservation
Reboiling of vapor stripping Tower	760.0	-	760.0
Compactor	397.0	-	397.0
Pump	9.9	9.9	0.0
Filter press	-	51.7	−51.7
Subtotal	1166.9	61.6	1105.3
Total system energy consumption
Traditional MEA workers	1290.9
Integration technology	185.6	1105.3
Percentage of energy savings	-	85.6%

below.

5. Summary and Outlook

This paper systematically reviews the recent progress of CO₂ absorption mineralization technology and provides an in-depth analysis of five aspects: absorber development, mineralization efficiency, mineralization process research, process application, and sustainable development. In terms of absorbents, monoamine absorbents (e.g., MEA) are widely used due to their high reactivity and low cost, but they also face challenges such as high solvent loss and high energy consumption for regeneration. The mixed amine system has a wide range of applications because it improves the absorption performance through the synergistic effect of the components and greatly reduces the regeneration energy consumption. In terms of mineralization efficiency, the main factors influencing the mineralization process were investigated through macroscopic analysis, and based on this, supercritical or suitable temperature and pressure working environments were explored to significantly improve mineralization efficiency. A comprehensive analysis of the mineralization process using multi-scale modelling has elucidated the mechanism of the reaction process and various influencing factors, providing many optimization ideas and ways for industrial applications. In the study of the mineralization process, it is mainly divided into direct mineralization and indirect mineralization, each of which has its own advantages and disadvantages. In view of specific industrial scenarios, it is necessary to select the mineralization technology after comprehensive consideration. In terms of industrial application, especially in the iron and steel industry, the technology is not only conducive to the reduction of carbon dioxide emissions, but also generates economic benefits through the use of industrial solid waste and the production of high value-added by-products, thus promoting resource recycling and sustainable development. In terms of sustainability, CO₂ absorption-mineralisation technology offers significant advantages in the treatment of blast furnace gas, including optimal process conditions (150–200 °C, 9–12% CO₂ concentration), a low-energy desorption process and good compatibility with existing systems. Overall, it is a sustainable and environmentally friendly technology in terms of treatment efficiency and economic viability.

The future development of CO₂ absorption-mineralization technology should focus on the following directions: in the development of absorbents, it is necessary to develop a new type of functional mixed amine system, optimize the absorption performance through the introduction of regulators, and design multi-functional group materials to improve the CO₂ absorption capacity and reduce the regeneration energy consumption; in the mineralization process, the main influencing factors are derived through the macroscopic scale and the reaction rate is increased through enhancement means such as ultrasound and microwave. Multi-scale modeling is used to study the reaction mechanism and molecular dynamics in various minerals and reaction environments in depth, providing a theoretical basis for various industrial scenarios. Improving the leaching efficiency of metal ions, optimizing the gas–liquid–solid three-phase contact method, and conducting in-depth research on the activation mechanism and reaction kinetic model of industrial solid wastes. In terms of industrial integration, the focus should be on promoting in-depth integration with the iron and steel industry, utilizing the low-grade waste heat (100–150 °C) for desorption, and developing a new type of separation device to improve the efficiency of the system, and at the same time, strengthening the assessment of the technical economics, including the investment cost, operational benefits and quantitative analysis of carbon reduction value. The last direction is to achieve better resource utilization and economic benefits, thus reducing the burden on the relevant industries and providing a guarantee for the vigorous implementation of the technology. As the 2030 peak carbon deadline approaches, governments are expected to introduce more supportive policies, including financial and tax incentives and carbon market construction, to promote the commercialization of the technology.