Synergistic CO₂ mineralization using coal fly ash and red mud as a composite system

Abstract

CO₂ mineralization plays a critical role in the storage and utilization of CO₂. Coal fly ash (CFA) and red mud (RM) are widely utilized as CO₂ mineralizers. However, the inert calcium species in CFA limit its carbonation capacity, meanwhile the substantial Ca²⁺ releasing of RM is hindered by a covering layer of calcium carbonate. In this study, CO₂ mineralization in a composite system of CFA and RM was investigated to enhance the carbonation capacity. Multiple analyzers were employed to characterize the raw materials and resulting mineralization products. The results demonstrated that a synergistic effect existed in the composite system of CFA and RM, resulting in improving CO₂ mineralization rate and efficiency. The produced calcium carbonate was ectopically attached the surface of CFA in the composite system, thus slowing down its coverage on the surface of RM. This phenomenon facilitated further releasing Ca²⁺ from the internal RM, thereby enhancing CO₂ mineralization efficiency. Meanwhile, the inclusion of RM significantly improved the alkalinity of the composite system, which not only promoted the dissolution of Ca²⁺ of the inert CaSO₄(H₂O)₂ in CFA, but also accelerated CO₂ mineralization rate. The investigation would be beneficial to CO₂ mineralization using industrial solid wastes.

1 Introduction

Escalating greenhouse gas emissions, especially CO₂, have inflicted significant harm on global ecosystems, resulting in devastating climate changes. Various strategies have been proposed for the utilization and storage of CO₂, encompassing land filling (Afra et al. 2023), chemical conversion (Fernández-González et al. 2022), biotransformation (Bajracharya et al. 2017) and mineralization (Koytsoumpa et al. 2018). Among these approaches, CO₂ mineralization stands out as an effective and sustainable method for the permanent and safe storage of CO₂ in a stable carbonate mineral form, emulating natural rock weathering processes (Zevenhoven et al. 2008; Seifritz 1990).

CO₂ mineralization involves the carbonation of industrial alkali metal wastes and natural minerals (Romanov et al. 2015). The direct reaction between CO₂ and natural minerals is hampered because of needing energy-intensive pretreatment to release their alkaline components (Wang and Maroto-Valer 2011). In contrast, industrial alkali metal wastes like coal fly ash (CFA) and red mud (RM) have gained prominence due to their low cost, high reactivity, and proximity to CO₂ emission sources (Sanna et al. 2012). While CFA exhibits a low Ca²⁺ content and inert nature, limiting its carbonation capacity during CO₂ mineralization. RM has a high Ca²⁺ content, but the Ca²⁺ cannot be quickly and fully released because the resulting calcium carbonate would cover the remained RM, thereby reducing its overall carbonation potential and efficiency. Consequently, there is a pressing need for further advancements to enhance the carbonation capacity of both CFA and RM for CO₂ mineralization.

As a byproduct of coal combustion, CFA accounts for 60 wt%–88 wt% of the total combustion residue of coal-fired power plants (Yao et al. 2015; Ram and Mohanty 2022; Brent et al. 2012). CFA contains various inorganic components such as SiO₂, Al₂O₃, CaO, Fe₂O₃, Na₂O, MgO, and CaSO₄ (Yuan et al. 2022). Despite being used as a filler for bricks, roads, or dams, a significant amount of CFA is still discarded in soil or ash ponds, raising serious environmental concerns (Liu et al., 2018). Exploring the potential of CFA for CO₂ mineralization presents a promising opportunity to permanently and securely store CO₂ in stable carbonate minerals.

Previous studies have primarily focused on the feasibility of CFA carbonation, employing gas-solid and aqueous routes (Pei et al. 2017; He et al. 2013), with the latter being more effective (Noack et al. 2014; Ding et al. 2022; Dananjayan et al. 2016). Ho et al. (2021) employed low-Ca²⁺ containing CFA for direct wet mineralization of CO₂, achieving a maximum carbonation capacity of 31.0% (6.3 g-CO₂/kg-CFA). Yuan et al. (2022) found that CFA exhibited higher carbonation capacity under supercritical CO₂ conditions than that low-pressure conditions, reaching a maximum of 54.9 g-CO₂/kg-CFA. Ji et al. (2017) discovered that CFA containing active Ca/Mg crystal phases exhibited enhanced CO₂ solidification, and the reactivity of calcium phases surpassed that of magnesium phases.

The content and form of Ca²⁺ in CFA play a pivotal role in determining its CO₂ sequestration potential. Highly active calcium crystal phases in CFA include lime (CaO), slaked lime (Ca (OH)₂), calcium silicate (CaSiO₃), and gypsum (CaSO₄). Lime (CaO), slaked lime (Ca (OH)₂), and calcium silicate (CaSiO₃) can directly participate in the mineralization reaction with CO₂, while the inert gypsum (CaSO₄) would generate CaCO₃ with CO₂ only in an alkaline environment (Wang et al. 2020). Therefore, during CFA carbonation, the introduction of another alkaline solid waste, such as RM or calcium carbide slag, may be beneficial to the mineralization reaction between CaSO₄ in CFA and CO₂ under alkaline conditions. The approach has the potential to significantly enhance the utilization rate of Ca²⁺ in CFA and improve its effectiveness for CO₂ sequestration.

RM is an alkaline solid waste generated during the alumina refining process of bauxite. RM poses significant environmental hazards and incurs high disposal costs (Genç-Fuhrman et al. 2004; Yang and Xiao 2008; Tsakiridis et al. 2004). Due to its high content of alkaline components, such as CaO, RM exhibits potential for CO₂ sequestration through carbonation processes. Mucsi et al. (2021) found that extending the reaction time and performing mechanical grinding increased the carbonation capacity of RM by 1.7%, while significantly reducing the pH of the resulting suspension. Revathy et al. (2021) demonstrated that temperature, CO₂ pressure, liquid-to-solid ratio, and reaction time had a significant impact on the carbonation capacity of RM.

Currently, research efforts primarily focus on utilizing CO₂ absorption to neutralize the alkalinity of RM, with limited attention paid to its carbonation capacity. One of the major challenges associated with RM is its low carbonation capacity and the generation of water-soluble Na₂CO₃. Adding gypsum or concentrated brine could increase the content of free and soluble Ca²⁺ and Mg²⁺ in RM system, thus enhancing its ability to mineralize CO₂ (Han et al. 2017).

Due to the huge output and containing abundant calcium-bearing compounds, CFA and RM are remarkably promising for CO₂ mineralization in a large scale. The combination of CFA and RM holds great promise for enhancing the carbonation capacity for CO₂, given that the strong alkalinity of RM would facilitate releasing inert Ca²⁺ from CFA, and thus reducing the overall alkalinity of RM (Liang et al. 2018). This approach provides a feasible solution for CO₂ sequestration and solid waste management.

In the study, the potential of utilizing CFA and RM as a composite system was investigated to improve the carbonation capacity without other additional chemicals. The impact of the CFA and RM compound system on CO₂ mineralization rate was monitored using a pH meter, while the effect of the compound system on CO₂ mineralization efficiency was determined through thermogravimetry. Additionally, mineral phases, microstructures, and particle distributions of the original samples and the mineralization products were analysed using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2 Experimental section

2.1 Materials

CFA and RM were collected from a power plant and an alumina plant in Shanxi Province, China, respectively. They were dried at 105 ℃ for 24 h prior to use. Then, RM was crushed with a pulverizer and passed through 200-mesh sieve. CO₂ used in the experiments had a purity of 99.99%. The chemical composition of the CFA and RM samples was presented in Table 1 according to X-ray fluorescence spectrometry (XRF) analysis.

Table 1 Chemical composition of the raw materials (wt%)

2.2 Experimental section

The CO₂ mineralization process was conducted as follows. Initially, 100 mL of deionized water was added to a three-necked flask and placed on a thermostatic magnetic stirrer. The reactions were conducted at a specific temperature for a certain duration of time, with varying CO₂ flow rate and amount of reactants. The raw materials underwent a 5-minute hydrolysis in a three-necked flask. Subsequently, CO₂ was injected into the flask while continuously monitoring the pH value. CO₂ injection continued until the pH value remained stable for 10 min, upon which the reaction was terminated. Finally, the carbonated solid product was filtered, and the residue was dried at 105 ℃ for 4 h. The experimental process and equipment used are depicted in Figs. 1 and 2, respectively.

Fig. 1

Diagram representing the experimental procedure

Fig. 2

Schematic diagram of the experimental apparatus

2.3 Sample characterization

Multiple analytical techniques were employed to investigate the mineralization behavior and product characteristics. The mineralization rate was determined in real-time using an online pH meter (PHS-201 F, China). The microstructure and elemental composition of the raw materials and mineralization products were observed using scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS, HITACHI, S-4800, Japan). XRD (D2-Phaser, Bruker, Germany) was used to identify the mineralogy of the raw materials and mineralization products. The chemical composition of the raw materials was analyzed using X-ray fluorescence spectrometry (XRF, S8 Tiger, Bruker, Germany). The thermal stability of the raw materials and mineralization products was determined using thermogravimetric analysis (TG, Pyris 1, Perkinelmer, USA) with an Al₂O₃ crucible. The structural changes of the materials before and after mineralization were analyzed using Fourier transform infrared spectroscopy (FT-IR, Shimadzu IR Affinity-I). The infrared spectra of all samples were collected in the wavenumber range of 400–4000 cm^− 1 using the KBr pellet method.

2.4 Calculation of CO₂ mineralization efficiency and rate

The mineralization efficiency of the alkaline solid waste was calculated using Eqs. (1)-(3) (Vassilev and Vassileva 2020; Miao et al. 2021). The theoretical carbonation capacity of the solid waste was determined using Eq. (1):

$${Th}_{\_{m}_{{\text{CO}}_{2}}}= \,\left[ {{{44} \over {56}}\left( {{m_{\text{CaO}}} + {{44} \over {40}}{m_{\text{MgO}}} - {{56} \over {80}} \times {m_{\text{SO}_3}}} \right) + {{44} \over {40}}{m_{\text{MgO}}} \times 1000} \right]$$

(1)

where ${Th}_{\_{m}_{{\text{CO}}_{2}}}$ (g-CO₂/kg- solid waste) represents theoretical CO₂ storage mass fraction, while ${m}_{\text{CaO}}$ (g-CaO/g- solid waste), ${m}_{{\text{SO}}_{3}}$ (g-SO₃/g- solid waste), and ${m}_{\text{MgO}}$ (g-MgO/g- solid waste) are the mass fractions of CaO, SO₃, and MgO in the sample, respectively.

The weight loss of the mineralization product obtained from the thermal gravimetric analysis (Fig. 3) indicated that the weight loss before 105 °C was due to the evaporation of free water in the product (Ji et al., 2019), while the weight loss between 105 and 500 °C was due to the loss of bound water in the product. The weight loss between 500 and 900 °C was attributed to the decomposition of calcium carbonate in the CFA, resulting in the release of CO₂ (Ni et al. 2017).

Fig. 3

TG-DTG curve of sample during carbonation reaction

Equation (2) was used to determine the carbonation capacity of the sample (Wang 2019):

$${m}_{{\text{CO}}_{2}}{=}\frac{{\varDelta m}_{500-900{^{\circ}C}}}{{m}_{105{^{\circ}\text{C}}}-{\varDelta m}_{500-900{^{\circ}\text{C}}}}\times 1000$$

(2)

where ${m}_{{\text{CO}}_{2}}$ represents the carbonation capacity, ${\varDelta m}_{500-900{^{\circ}\text{C}}}$ is the weight of fixed CO₂ after reaction, and ${m}_{105{^{\circ}\text{C}}}-{\varDelta m}_{500-900{^{\circ}\text{C}}}$ is the weight of the raw material. ${\varDelta m}_{105{^{\circ}\text{C}}}$ is the mass before the decomposition of the mineralization product.

The mineralization efficiency was defined using Eq. (3), The results obtained from the mineralization efficiency calculations were used to assess the potential of the alkaline solid waste as a CO₂ storage material:

$$\delta \left( \% \right)\, = \,{{{m_{\text{CO}_2}}} \over {T{h_{ - m{\text{CO}_2}}}}} \times \,100\%$$

(3)

Equation (4) defines CO₂ mineralization rate, and the outcomes from these calculations could evaluate the speed at which the alkaline solid waste could be utilized as a CO₂ sequestration material (Ma et al. 2021).

$$\vartheta =\frac{{\text{pH}}_{\text{f}\text{i}\text{n}\text{a}\text{l}-}{\text{pH}}_{\text{i}\text{n}\text{i}\text{t}\text{i}\text{a}\text{l}}}{t}$$

(4)

pH_final signifies the pH level at the end of CO₂ mineralization, pH_initial denotes the pH level at the begin of CO₂ mineralization, while t denotes the duration of the mineralization reaction.

3 Results and discussion

3.1 Comparison of CO₂ mineralization in the single and composite systems

As displayed in Fig. 4, the initial pH of aqueous solution with dispersing CFA was approximately 11.0, which rapidly dropped to 5.6 at the end of the reaction. The dispersed RM-containing aqueous solution had a initial pH of 12.2, which underwent mineralization to achieve a pH of 6.7. For the composite system of RM and CFA, the pH initially reached 11.7, which was due to OH⁻ concentration increase resulting from alkaline species hydrolysis in RM. The composite system exhibited a pH close to 6.3 at the end of the reaction, surpassing the pH of the single CFA system.

Fig. 4

t-pH curve of carbonation reaction for CFA, RM + CFA and RM.

The mineralization equilibrium time for CFA, RM + CFA, and RM was found to be 357 s, 1464 s, and 4742 s, respectively. The expected mineralization equilibrium time (∆t) and mineralization rate of CFA + RM were calculated to be 2549.5 s ($\frac{357+4742}{2}$) and 0.0021 units/s ($\frac{5.4}{2549.5}$) (Ma et al. 2021), respectively. However, the actual ∆t and mineralization rate of CFA+RM during the pH drop process were 1464 s and 0.0037 units/s ($\frac{5.4}{1464}$), respectively. The mineralization rate increased by 76.2% ($\frac{0.0037-0.0021}{0.0021}\times 100\%$). When the RM:CFA ratio was 1:1, the expected carbonation capacity should have been 120.35 (g-CO₂/kg-solid waste) $(\frac{237.2+3.5}{2}\text{)}$ (Fig. 5) (Ni et al. 2017), while the actual carbonation capacity was 135.51 (g-CO₂/kg-solid waste). The carbonation capacity increased by 12.60% $\text{(}\frac{135.51-120.35}{120.35}\times 100\%\text{)}$. These findings demonstrated the presence of a synergistic effect between RM and CFA, resulting in heightening mineralization rate, carbonation capacity, and mineralization efficiency.

Fig. 5

Performance of CFA, RM + CFA and RM in carbonation reaction

The temperature and CO₂ flow rate were generally assumed to be the factors that impacted the mineralization reaction (He et al. 2013). As shown in Fig. 6a, the mineralization efficiency initially increased and then decreased with increasing temperature. The phenomenon could be attributed to the exothermic nature of the mineralization reaction, which was not conducive to the reaction at high temperatures (Lee et al., 2018). Additionally, excessively high temperatures could lead to decreasing the solubility of CO₂ in the aqueous solution. Furthermore, increasing CO₂ flow rate resulted in the stabilization of mineralization efficiency, as exhibited in Fig. 6b. This observation could be explained by the low solubility of CO₂ in water, i.e., a small amount of CO₂ could saturate the aqueous solution, making the effect of CO₂ flow rate not obvious.

Fig. 6

a Effects of temperature and b CO₂ flow rate on CO₂ mineralization

3.2 Influence of different CFA:RM ratios in the composite system on CO₂ mineralization

The carbonation capacity of the sample at an RM:CFA ratio of 5:5 was expected to be 120.35 g-CO₂/kg-solid waste based on calculations. However, the actual carbonation capacity was found to be 135.51 g-CO₂/kg-solid waste, indicating a 12.60% increase in carbonation capacity. This fact highlighted the synergistic effect of RM and CFA on CO₂ mineralization.

Figure 7 illustrates the changes in mineralization capacity and efficiency of RM and CFA at various ratios. The mineralization capacity of the composite obviously decreased as the ratio of RM and CFA reduced. The mineralization efficiency gradually increased when the ratio of RM and CFA ranged from 10:0 to 5:5, and it decreased when the ratio of RM was less than 5:5. These results provided compelling evidence of the enhanced mineralization reaction resulting from the synergistic effect of RM and CFA. Interestingly, a gradual decrease in RM content within the composite system was correspond to a gradual decrease in the theoretical carbonation capacity. However, contrary to expectations, the mineralization efficiency demonstrated a gradual increase until the RM to CFA ratio reached 5:5. Remarkably, the maximum mineralization efficiency was achieved at this specific ratio.

Fig. 7

Performance for CO₂ mineralization in the composite of CFA and RM with different ratio

Figure 8 illustrates the growth rate of carbonation capacity under different RM and CFA ratios. A negative growth rate of -22.91% was observed at the 1:9 of RM and CFA ratio, indicating a negative synergistic effect on mineralization. In contrast, positive growth rates were all observed for other ratios, signifying a synergistic effect under those conditions. Notably, the highest growth rate of carbonation capacity, reaching 12.60%, was achieved at the 5:5 RM:CFA ratio. These findings further confirmed the synergistic effect of RM and CFA on CO₂ mineralization, and underscored the critical role of an optimized RM and CFA ratio in maximizing mineralization efficiency.

Fig. 8

Rate variation of carbonation capacity for the composite of CFA and RM with different ratio

3.3 Insight into of synergistic effect in the composite system for CO₂ mineralization

The cooperative impact of CFA and RM composite systems on CO₂ mineralization was examined by analyzing the mineralogical features of these composite systems before and after mineralization. As depicted in Fig. 9, the peaks corresponding to CaO and Ca(SO₄)(H₂O)₂ were observed by referencing standard PDF cards during XRD analysis, which was in accordance with the report (Ji et al., 2019). The existence of inactive Ca(SO₄)(H₂O)₂ could account for the low mineralization efficiency observed in CFA. The high reactivity of 2CaO·SiO₂ (C2S) in raw RM resulted in a substantial production of CaCO₃. In the composite system, the inclusion of RM resulted in the disappearance of the peak associated with Ca(SO₄)(H₂O)₂ after mineralization when the RM:CFA ratio exceeded 5:5. However, if the RM:CFA ratio was less than 5:5, the Ca(SO₄)(H₂O)₂ peak persisted, indicating that the alkalinity supplied by RM may not fully dissolve Ca(SO₄)(H₂O)₂, potentially accounting for the reduced mineralization efficiency at RM:CFA ratios below 5:5. Upon surpassing a 5:5 ratio of RM to CFA, the inactive Ca(SO₄)(H₂O)₂ of CFA could be fully dissolved in the composite system, which would lead to a sharp Ca²⁺ increase and the corresponding calcium carbonate. The produced calcium carbonate would cover the surface of RM besides CFA, which would hinder further releasing of other Ca²⁺ in the interior part of RM, thus leading to mineralization efficiency decrease.

Fig. 9

XRD patterns of a fresh and carbonated CFA and RM and b carbonated the composite of CFA and RM

As shown in Fig. 10, all five samples displayed peaks at 712 cm^− 1, indicating the presence of CaCO₃. Furthermore, peaks at 876 cm^− 1 and 1447 cm^− 1 were observed in carbonated RM: CFA of 5:5, carbonated CFA, and carbonated RM, which were assigned to the out-of-plane bending vibrations and anti-symmetric stretching vibration of the C-O group in CaCO₃, respectively. According to the previous literature (Wang 2019), these peaks corresponded to the in-plane and out-of-plane bending vibrations of the O-CO- group in CaCO₃, respectively.

Fig. 10

FT-IR spectra of fresh materials and carbonated products

SEM observations were carried out on the provided raw materials and mineralized products, as depicted in Fig. 11. After CO₂ mineralization in the single RM system, numberous fine needle-shaped calcium carbonate crystals were covered on RM surface, limiting further releasing of Ca²⁺ in the internal RM. Conversely, a small quantity of flake-like calcium carbonate was attached on CFA surface due to its low Ca²⁺ releasing in the single CFA system. Within the composite system, substantial Ca²⁺ was released from calcium species, leading to large amount of calcium carbonate formation. The generated calcium carbonate not just covert the RM surface but also attached to CFA surface. Reduced calcium carbonate coverage on RM surface was expected to enhance further releasing of Ca²⁺ in the internal RM, thus facilitating CO₂ mineralization in the composite system.

Fig. 11

SEM images of fresh materials and carbonated products

The synergistic effect of CFA and RM accelerated the efficiency and rate of CO₂ mineralization. In the composite system, CFA offered large amounts of surface attachment sites for the generated calcium carbonate, which would reduce the coverage of calcium carbonate on the RM surface, facilitating the releasing of Ca²⁺ in RM. Furthermore, alkaline oxides in RM underwent hydrolysis in the composite system, releasing high concentrations of OH⁻. The increased alkalinity encouraged the dissolution of Ca(SO₄)(H₂O)₂ within the CFA, increasing the free Ca²⁺ in the composite system, thereby elevating the CO₂ mineralization efficiency. Additionally, the increased alkalinity with the inflution of RM promoted CO₂ mineralization rate (Song et al. 2020).

4 Conclusions

In this study, the effect was investigated for combining CFA and RM for CO₂ mineralization, which revealed that the combined system of CFA and RM had a positive impact on the mineralization rate and efficiency. Notably, the mineralization rate and efficiency increased by 76.2% and 12.60% in a composite system of CFA and RM with mass ration of 5:5, respectively, compared to the calculated weighted averaged ones in the individual CFA and RM. The synergistic effect of RM and CFA was attributed to the ectopic adherence of the generated calcium carbonate to CFA surface and the increased alkalinity of the system caused by RM addition. The ectopic adherence of calcium carbonate to CFA surface promoted Ca²⁺ releasing from the internal RM. Moreover, the increased alkalinity leaded to the dissolution of the inactive CaSO₄(H₂O)₂ in CFA. This study would provide valuable insights into the synergistic effects of different components on CO₂ mineralization and pave the way for the development of effective and efficient CO₂ mineralization.