Structural Evolution of Olivine during Mechanochemically Assisted Mineral Carbonation under CO₂ Flow

Abstract

The mechanism of the mechanically assisted mineral carbonation of commercial olivine under the flow of a carbon dioxide (CO₂)/nitrogen (N₂) mixture has been elucidated by ex situ powder X-ray diffraction and Fourier-transform infrared spectroscopy. The overall CO₂ conversion depends on the rotational frequency of the mill’s engine, and it reaches 85% within 90 min of mechanical treatment at a flow rate of 2.5 L min⁻¹. By tuning the frequency of rotation, the kinetics of CO₂ conversion unveil a complex reaction pathway involving subsequent steps. Structural analyses suggest that clinochlore, a magnesium (Mg-)- and iron (Fe-)-containing aluminosilicate gathered among the components of olivine, is formed and consumed in different stages, thus promoting the CO₂ sequestration that eventually results in the formation of hydrated and anhydrous Mg-based carbonates.

1. Introduction

The unceasing production of anthropogenic carbon dioxide (CO₂) has been raising serious concerns among the scientific community. Since the signing of the Kyoto Protocol in 1997, tight policies have been implemented in order to decrease the amount of CO₂ generated from industrial processes and domestic activities, as well as to reduce the overall CO₂ concentration in the atmosphere, whose value above 420 ppm is targeted to play a role in global warming and climate change [1]. In this scenario, “carbon capture, utilization, and storage” (CCUS) encompasses the set of activities in the fields of basic science, technology development, regulatory affairs, etc., aimed at subtracting CO₂ from the atmosphere by permanent confinement through the implementation of physical, chemical, and biological CO₂ fixation/conversion, as well as developing processes with a zero net impact of CO₂ by directly capturing the CO₂ when it is produced [2,3]. Indeed, the release of CO₂ into the atmosphere primarily occurs from specific sources, including thermoelectric plants, foundries, cement factories, and so on. This category of CO₂, known as “capturable CO₂”, is emitted near its production source, making it more manageable to trap using targeted technologies [4]. Among these, the postcombustion approach involves CO₂ captured from the exhaust gases of the combustion process, and it is the most promising sequestration route, as it comes just after the process that produces CO₂ [5,6]. The concentration of CO₂ in exhaust gases is typically between 4–14 v/v%, with unfavorable conditions for gas separation at atmospheric pressure [7]. In these operating modes, the denitrified and desulphurized CO₂ reacts with liquid, such as MEA (monoethanolamine), the only widely used and established commercial solution, or filtered by specially designed membranes, raising concerns about the whole environmental sustainability of the process [8]. A more viable alternative to removing CO₂ being considered now is the so-called mineralization of CO₂, i.e., its transformation into carbonates of alkaline and alkaline earth metals starting from mafic and ultramafic minerals, as well as metal oxides or hydroxides [9,10]. Among the minerals, olivine, serpentine, talc, and basalt are some of those investigated that have gained wide attention due to their low cost and large availability. In particular, olivine is an abundant mineral in the earth’s crust (>50% by upper mantle volume), commonly found in mafic igneous rocks [11]. The mineralization process using Ca-Mg-Fe natural silicates was proposed in the pioneering reports of Seifritz [12] and Lackner [13], mimicking the phenomenon that naturally happens as described above. The whole set of reaction schemes accompanying CO₂ mineralization is quite complex, and it is well known in the literature [8,12]. For the sake of brevity, two main processes can be considered: (i) serpentinization of olivine, a redox process involving Fe²⁺ oxidation and H₂O reduction, which leads to the serpentine formation, flanked, in the presence of CO₂ by (ii) carbonate precipitation via exchange reaction and possibly a Sabatier-type CO₂ hydrogenation, according to the following scheme:

$$6(\mathrm{M}\mathrm{g},\mathrm{F}\mathrm{e}{)}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4+12{\mathrm{H}}_2\mathrm{O}+6\mathrm{C}{\mathrm{O}}_2\to 2(\mathrm{M}\mathrm{g},\mathrm{F}\mathrm{e}{)}_3\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5(\mathrm{O}\mathrm{H}{)}_4+2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+8{\mathrm{H}}_2+6\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{O}}_3+2\mathrm{S}\mathrm{i}{\mathrm{O}}_2$$

Moreover, serpentine itself could undergo mineral carbonation, according to the reaction below:

$$(\mathrm{M}\mathrm{g},\mathrm{F}\mathrm{e}{)}_3\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5({\mathrm{O}\mathrm{H})}_4+3\mathrm{C}{\mathrm{O}}_2\to 3(\mathrm{M}\mathrm{g},\mathrm{F}\mathrm{e})\mathrm{C}{\mathrm{O}}_3+2\mathrm{S}\mathrm{i}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$$

In summary, metal carbonate may be formed at the expense of corresponding silicates or of other metal precursors, and these reactions can occur in parallel with metal oxidation, hydrogen (H₂) evolution, and CO₂ reduction processes. However, the natural process occurs with slow kinetics that limit its feasibility for large-scale purposes. Despite this main hurdle, several methods have been studied to increase the reaction rate of silicates in carbonation, such as chemical and electrochemical activation [14], thermal activation [15], the use of additives in the reaction fluid with high temperatures and pressures [16], or mechanochemical activation [16,17]. Thermal activation leads to dehydrating hydrosilicates, such as serpentine minerals, but is ineffective on olivine [15]. Chemical activation allows for the removal of metal cations from silicates but involves a high additional cost for the disposal of hazardous acid sludge [18]. On the other hand, mechanical activation reduces the average size of the crystallites with a corresponding increase in the surface area and, eventually, in the active–reactive sites of the silicate. Moreover, it allows fast leaching of divalent cations gathered in silicate phases, avoiding the need for further chemicals, thus providing more efficient routes [19]. Even though this technique is energy-demanding, it could become effective if integrated with pre-existing processes [20,21]. In this regard, mechanical milling, with repeated fracturing on the reagent powders, would induce structural defects in materials, reduce the average size of mineral grains, and expose new active surfaces in the reaction environment [22]. Mechanochemical treatment was found to be an effective way to enhance the reactivity of natural silicates [23,24], and several reports [11,17,25,26] have highlighted how the microstructural parameter and texture properties were influenced by the type of grinding, grinding duration, and the presence of liquid, such as water or ethanol. A further step forward was taken by highlighting that grinding natural silicates (such as forsterite, serpentine, and wollastonite) in the presence of CO₂ (reactive milling, RM) [27] promoted mechanochemical absorption of CO₂ in the form of metal carbonate. More recently, studies based on the mechanical treatment of olivine minerals conducted by our research group [28,29] have highlighted effective CO₂ mechanosorption on silicates, although in batch conditions (under static CO₂ atmosphere). Nonetheless, there are no applications for mechanochemically activated processes carried out in a continuous-gas-feeding mode. This work aims to study the behavior of olivine as a solid-state CO₂ store in flow conditions, driven by mechanochemical activation, thus simulating a mechanochemically induced process of postcombustion CO₂ capture and transformation, ideally applicable to an industrial process that continuously emits CO₂. Moreover, the conversion process is investigated by a structural characterization to gain more insight into the carbonation process in dynamic conditions.

2. Results and Discussion

Figure 1 showcases the P-XRD performed on the pristine olivine sample. From the profile matching and further Rietveld refinement, it arises that the commercial olivine is mainly constituted by a Mg-based nesosilicate with some Fe inclusion, namely forsterite (~92.5 wt.%), flanked by an inosilicate, ferrous enstatite (~5 wt.%), and clinochlore (~2.5 wt.%). The forsterite, crystallizing in an orthorhombic space group, Pbnm, features isolated SiO₄ tetrahedra pointing in opposite directions along the c-axis. The tetrahedra are only connected to MO₆-distorted octahedra by oxygen corner sharing. The central atom of these octahedra, which are usually labeled in literature as the M2 coordination sites, is the metallic bivalent cation, which could be both Mg²⁺ and Fe²⁺. The other octahedral site for cation, namely M1, is less distorted and connects two M2 octahedra along the b-axis, creating a tetrahedral interstitial site whose fourth face is shared with the SiO₄ tetrahedron.

Enstatite crystallizes in the orthorhombic Pbca space group, and its structure showcases a series of edge-sharing SiO₄ tetrahedra, which indefinitely creates chains along the c-axis. These chains are perpendicularly connected to the layers in which Mg²⁺ and Fe²⁺ randomly sit in both regular octahedral and distorted octahedral sites, whose coordination distance for the latter with the farthest O atoms exceeds 2.5 Å.

Lastly, clinochlore crystallizes in a triclinic space group, $P\overline{1}$. Using the c-axis as a guide, its structure is constituted by two layers of isolated SiO₄ tetrahedra, oriented in opposite directions, which share three edge O atoms with an inner layer of octahedra, whose center mainly hosts Al³⁺ atoms. Due to the strong Si-O bonds, these three layers are tightly bonded. The remaining O in each of the SiO₄ groups in both layers is shared with octahedra belonging to the so-called brucite layer [30], which is constituted by more distorted octahedra that host in their centers, Mg²⁺, Fe²⁺, or Al³⁺ connected to O-H groups. The brucite layer is less tightly bonded to the three T-O-T group of layers previously described. Therefore, the overall hardness of the mineral is considerably lower than that of both forsterite and enstatite [31]. A more comprehensive structural description of the crystallographic phases found in the olivine is gathered in the Table 1, below.

Some morphology hints have been gained by SEM analyses: the images presented in Figure 2 reveal that olivine is constituted of a homogeneous distribution of millimetric and submillimetric particles with no regular shape. More interestingly, EDX analyses further confirm the presence of the typical elements that constitute the crystallographic phases listed above. In particular, Fe distribution fairly overlaps those of both Si and O, validating the presence of Fe in the main component of the olivine, i.e., the forsterite.

Figure 3 showcases the comparison of the CO₂ conversion kinetics over olivine treated at different rotational frequencies of the mill. Regardless of the speed, the increase in the CO₂ conversion over time is noticeable. The overall processes appear to not be linear, albeit for the first 30 min of reactive milling, a quasi-constant, steep increase in the conversion is visible. Indeed, after this period, a sudden change in the conversion behavior is witnessed by the appearance of a plateau, which interestingly lasts longer as the rpm of the mill decreases. Subsequently, a new steep increase in conversion happens, followed by a second and eventually a third steady region that sets the maximum value of CO₂ conversion. In particular, the reaction carried out at 1000 rpm (red curve in Figure 3) reaches 85% of CO₂ conversion, which remains stable within the further monitored time interval.

By varying the rpm of the mixer/mill engine, is it possible to tune the frequency of the impacts between the milling spheres and the reagents and, eventually, the mechanical energy transferred during each impact per unit of time [32,33]. The variation in these two parameters occurs simultaneously with the increase in the rpm value and defines the intensity of mechanical treatment. The trends shown in Figure 3 comply with composite reaction mechanisms involving solid–gas interactions, regardless of the activation method (thermal, mechanical, etc.). Indeed, several elementary steps characterize these reactions, i.e., physical and chemical adsorption of gases, diffusion, dissociation, and conversion phenomena [34,35]. In particular, these mechanisms have been highlighted in various mechanochemically activated heterogeneous processes [36], such as the hydrogenation of CO₂ [37] or CO on metal oxide catalysts [38,39]. In the latter, beyond the dependency on the chemical system, the reactivity is driven by the conditions of the mechanical treatment [28,40,41]. This evidence has also been confirmed by the present study, in which the reduction in the rotation frequency unveils a multistep process for CO₂ conversion upon olivine. Since the final conversion value within the observation time interval (180 min) increases with the rpm value, and the shape and extent of the intermediate plateau appear to be more pronounced as the rpm value decreases (particularly at 745 rpm; see the green curve in Figure 3), it is worth gaining more insight into the reaction mechanism by exploring the possible relations between crystallographic phase evolutions and the kinetics of the solid–gas reaction.

Therefore, the phase evolution during the mechanical reaction carried out at 745 rpm was followed by ex situ P-XRD at subsequent milling times up to the end of the process, i.e., 180 min. The results, gathered in Figure 4 and detailed in Table 1, reveal the formation, yet at the earliest stages of milling of a hydrated Mg-based carbonate phase, i.e., nesquehonite, MgCO₃⋅3H₂O, and, subsequently, the crystallization of the anhydrous carbonate, magnesite, MgCO₃, accompanied by variations of a few relative percentage points in clinochlore relative abundance. In particular, for milling times lying on the plateaus, the clinochlore slightly decreases but is partly reformed at the end of each steep region (i.e., 140 and 180 min), whereas nesquehonite reaches its maximum during the first, longest plateau, reaching the value of almost 1 wt.% up to 60 min. The formation of these carbonate phases complies with the serpentinisation process [42] involving the oxidation of Fe²⁺ to Fe³⁺. Nesquehonite could be formed yet at room temperature with a slight CO₂ overpressure, and its conversion to magnesite occurs at a slightly higher temperature (around 90 °C) [43], which is a condition that can be locally achieved during a high-energy ball-milling. Indeed, mechanical milling has proven to be effective in various systems containing iron in order to trigger reactivity, leading to solid solutions in binary systems, as well as to promote cationic substitution in ionic compounds [39,44]. Fe³⁺ can be included in a substitutional form in the sites of the Al³⁺ species in clinochlore, as reported in studies on the products of metamorphism of mafic rocks, which lead to the formation of minerals such as serpentines and chlorites [45]. Likewise, as already observed in the literature, the fixation of carbonate-based species is promoted by the basic environment arising from the minerals gathered in the olivine and is therefore responsible for the CO₂ chemical sequestration reported in Figure 3.

Table 1. Cell parameters and relative abundance of the crystallographic phases obtained from Rietveld refinement on P-XRD patterns collected at subsequent milling stages, as well as the parameter Rwp%, which represents the weight ratio of the difference between the observed pattern and the XRD calculation [46].

← Sample	Phase name →	Forsterite Ferroan	Enstatite Ferrous	Clinochlore		Nesquehonite		Magnesite	Iron
	Formula →	Mg1.8Fe0.2SiO4	Mg0.8Fe0.2SiO3	(Mg,Fe(II))5Al(Si3Al) O10(OH)8		MgCO3·3H2O		MgCO3	α-Fe
	Sp. Group→	Pbnm	Pbca	P$\overline{1}$		P21c		$R\overline{3}c$	$Im\overline{3}m$
Pristine	a (Å)	4.76	18.26	5.15	α = 93.95	–		–	–
	b (Å)	10.22	8.83	9.58	β = 95.60	–		–	–
	c (Å)	5.99	5.20	14.42	γ = 89.58	–		–	–
	Weight %	92.5	5.0	2.5		–		–	–
	Rwp %	11.43
2 min BM	a (Å)	4.76	18.25	5.38	α = 91.55	7.70		–	–
	b (Å)	10.22	8.83	9.52	β = 102.24	5.37	β = 89.99	–	–
	c (Å)	5.99	5.19	14.63	γ = 88.67	12.11		–	–
	Weight %	91.0	4.4	3.4		1.1		–	–
	Rwp %	11.59
20 min BM	a (Å)	4.76	18.26	5.40	α = 92.21	7.70		5.03
	b (Å)	10.22	8.83	9.52	β = 101.71	5.38	β = 90.01
	c (Å)	5.99	5.19	14.61	γ = 88.91	12.09		17.95
	Weight %	91.1	5.2	2.9		0.4		0.2	0.2
	Rwp %	11.57
60 min BM	a (Å)	4.76	18.26	5.32	α = 93.24	7.71		5.05	2.86
	b (Å)	10.22	8.84	9.54	β = 99.99	5.36	β = 90.31
	c (Å)	5.99	5.19	14.55	γ = 91.47	12.10		17.85
	Weight %	90.4	6.0	1.8		0.9		0.2	0.6
	Rwp %	11.34
90 min BM	a (Å)	4.76	18.26	5.34	α = 90.68	7.74		5.05	2.86
	b (Å)	10.22	8.84	9.39	β = 98.11	5.36	β = 90.71
	c (Å)	5.99	5.19	14.47	γ = 92.01	12.10		17.85
	Weight %	90.6	6.5	1.8		0.3		0.2	0.6
	Rwp %	9.66
140 min BM	a (Å)	4.76	18.26	5.42	α = 91.67	7.73		5.43	2.87
	b (Å)	10.22	8.82	9.43	β = 101.79	5.36	β = 89.64
	c (Å)	5.99	5.21	14.60	γ = 88.37	12.02		16.62
	Weight %	90.7	5.6	2.3		0.3		0.1	0.9
	Rwp %	11.90
180 min BM	a (Å)	4.76	18.26	5.42	α = 91.69	7.36		5.41	2.86
	b (Å)	10.22	8.82	9.42	β = 101.75	5.52	β = 90.78
	c (Å)	5.99	5.25	14.60	γ = 88.35	11.7		16.79
	Weight %	88.8	6.3	2.4		0.4		0.2	1.8
	Rwp %	10.62

Table 1. Cell parameters and relative abundance of the crystallographic phases obtained from Rietveld refinement on P-XRD patterns collected at subsequent milling stages, as well as the parameter Rwp%, which represents the weight ratio of the difference between the observed pattern and the XRD calculation [46].

← Sample	Phase name →	Forsterite Ferroan	Enstatite Ferrous	Clinochlore		Nesquehonite		Magnesite	Iron
	Formula →	Mg1.8Fe0.2SiO4	Mg0.8Fe0.2SiO3	(Mg,Fe(II))5Al(Si3Al) O10(OH)8		MgCO3·3H2O		MgCO3	α-Fe
	Sp. Group→	Pbnm	Pbca	P$\overline{1}$		P21c		$R\overline{3}c$	$Im\overline{3}m$
Pristine	a (Å)	4.76	18.26	5.15	α = 93.95	–		–	–
	b (Å)	10.22	8.83	9.58	β = 95.60	–		–	–
	c (Å)	5.99	5.20	14.42	γ = 89.58	–		–	–
	Weight %	92.5	5.0	2.5		–		–	–
	Rwp %	11.43
2 min BM	a (Å)	4.76	18.25	5.38	α = 91.55	7.70		–	–
	b (Å)	10.22	8.83	9.52	β = 102.24	5.37	β = 89.99	–	–
	c (Å)	5.99	5.19	14.63	γ = 88.67	12.11		–	–
	Weight %	91.0	4.4	3.4		1.1		–	–
	Rwp %	11.59
20 min BM	a (Å)	4.76	18.26	5.40	α = 92.21	7.70		5.03
	b (Å)	10.22	8.83	9.52	β = 101.71	5.38	β = 90.01
	c (Å)	5.99	5.19	14.61	γ = 88.91	12.09		17.95
	Weight %	91.1	5.2	2.9		0.4		0.2	0.2
	Rwp %	11.57
60 min BM	a (Å)	4.76	18.26	5.32	α = 93.24	7.71		5.05	2.86
	b (Å)	10.22	8.84	9.54	β = 99.99	5.36	β = 90.31
	c (Å)	5.99	5.19	14.55	γ = 91.47	12.10		17.85
	Weight %	90.4	6.0	1.8		0.9		0.2	0.6
	Rwp %	11.34
90 min BM	a (Å)	4.76	18.26	5.34	α = 90.68	7.74		5.05	2.86
	b (Å)	10.22	8.84	9.39	β = 98.11	5.36	β = 90.71
	c (Å)	5.99	5.19	14.47	γ = 92.01	12.10		17.85
	Weight %	90.6	6.5	1.8		0.3		0.2	0.6
	Rwp %	9.66
140 min BM	a (Å)	4.76	18.26	5.42	α = 91.67	7.73		5.43	2.87
	b (Å)	10.22	8.82	9.43	β = 101.79	5.36	β = 89.64
	c (Å)	5.99	5.21	14.60	γ = 88.37	12.02		16.62
	Weight %	90.7	5.6	2.3		0.3		0.1	0.9
	Rwp %	11.90
180 min BM	a (Å)	4.76	18.26	5.42	α = 91.69	7.36		5.41	2.86
	b (Å)	10.22	8.82	9.42	β = 101.75	5.52	β = 90.78
	c (Å)	5.99	5.25	14.60	γ = 88.35	11.7		16.79
	Weight %	88.8	6.3	2.4		0.4		0.2	1.8
	Rwp %	10.62

In some of the patterns, the phase identification procedure also allowed for the identification of α-Fe, whose source could be likely the contamination from the stainless-steel jar.

The interpretation of the diffraction data of such a multiphase system, characterized by a high number of Bragg reflections over the entire explored 2θ angular range, is not straightforward, given that the precipitation of crystalline carbonates likely occurs at the surface of the particles, and the further milling action hampers the continuous enlargement and growth of crystalline phases, while favoring the occurrence of metastable conditions. Moreover, even though the data reported in Figure 3 may suggest a discrepancy between the conversion values and the amount of carbon sequestered in the form of mineral carbonates (Table 1), the comparison of the absolute amounts of involved chemical systems seems to clarify the point under the approximation of the ideal behavior of the gas. The absolute CO₂ amount fed into the mechanochemical reactor within the time interval that was investigated, 180 min, approaches 1.84 mmoles, and the amount of converted CO₂, which can be roughly estimated according to the reported kinetic trend, approaches 1.30 mmoles. The mass of reactant olivine, considering the lightest phase in the solid mixture, Forsterite, as a whole, is about 13.6 mmoles, which means that the ratio between CO₂ converted molecules and olivine units is less than 1/10, and the maximum amount of carbonate phase which might be obtained is less than 10% v/v. It is then clear that the overall amounts of crystalline clinochlore and carbonates represent only a small fraction of the whole solid mixture at the end of each run (see Table 1), also taking into account that a fraction of carbon-based compounds may still occur in metastable phases, whose amount corresponds to the complement to the crystalline phases estimated by Rietveld method. Lastly, it should be pointed out that, according to the reaction scheme gathered in the Introduction section, CO₂ reduction processes can also take place during milling, thus affecting the stoichiometry of the reaction by reducing the available carbon for carbonate precipitation. The quantification of these phases is beyond the scope of this paper and will be thoroughly assessed in further communications. Nonetheless, given that the relatively low percentages of carbonate-based phases (less than 2 wt.%) are fairly close to the quantification limit of Rietveld refinement, the validation of the proposed conversion mechanism arises from FT-IR measurements, which were carried out on the powders treated for different grinding times (Figure 5)”.

Indeed, the first carbonate phase found, after just 2 min of reactive grinding, is nesquehonite, identified by the three characteristic bands whose wavenumbers can be found at approximately 1518, 1478, and 1430 cm⁻¹, respectively. These signals also persist in the ground sample for 20 and 60 min. Once the milling time surpasses 60 min, it is possible to notice the conversion from hydrated Mg carbonate to the related anhydrous magnesite (MgCO₃), whose presence can be witnessed until the end of the experimental observation. More in detail, the signals falling in that region correspond to the bond vibrations of CO₃²⁻ groups, as reported in the literature [47]. Specifically, for the analyzed samples, two characteristic bands are observed at 1423.9 and 1484.3 cm⁻¹, attributable to the stretching of the carbonate group of the magnesite, confirming the formation of the metallic carbonates as a consequence of the mechanical treatment. In addition, the signals at approximately 3400 cm⁻¹ and 1680 cm⁻¹ are attributable to the stretching of O-H groups and to the bending of H₂O, respectively [48]. The continuous CO₂ reaction at the surface of the olivine further consumes H₂O, which is progressively removed from the environment of the reaction and thus also drained from the freshly formed nesquehonite [42].

Lastly, in order to assess the possible direct effect of the high-energy grinding on the reactivity of the crystallographic phases that compose olivine and eventually untangle it from the transformations due to the reactivity towards CO₂, a series of mechanical treatments were performed under argon (Ar) atmosphere on olivine fresh samples for increasing milling time, ranging from 1 to 10 h, operating at 750 rpm (Figure 6 and

Table 2. Structural and microstructural data from Rietveld refinement on olivine powders mechanically treated under Ar atmosphere. It is worth noting that by increasing the milling time (up to 10 h), a partial amorphization of the sample occurs; therefore, the overall weight % of the phases could not be 100. For the 5 h and 10 h samples, clinochlore as a crystalline phase was no longer observable.

← Sample	Phase Name →	Forsterite Ferroan	Enstatite Ferrous	Clinochlore
	Chemical formula →	Mg1.8Fe0.2SiO4	Mg0.8Fe0.2SiO3	(Mg,Fe(II))5Al(Si3Al)O10(OH)8
	Space group →	Pbnm	Pbca	P$\overline{1}$
1 h BM	a (Å)	4.76	18.25	5.15
	b (Å)	10.22	8.83	9.58
	c (Å)	5.99	5.20	14.42
	Grain size (nm)	105	54	123
	Weight %	93.7	4.0	2.3
	Rwp %	7.74
2 h BM	a (Å)	4.76	18.25	5.13
	b (Å)	1.23	8.83	9.55
	c (Å)	5.99	5.20	14.45
	Grain size (nm)	134	69	134
	Weight %	88.9	7.7	3.4
	Rwp %	8.17
3 h BM	a (Å)	4.76	18.26	5.11
	b (Å)	10.22	8.83	9.59
	c (Å)	5.99	5.20	14.47
	Grain size (nm)	124	78	121.15
	Weight %	88.8	8.3	1.7
	Rwp %	9.60
4 h BM	a (Å)	4.76	18.26	5.10
	b (Å)	10.22	8.83	9.59
	c (Å)	5.99	5.20	14.46
	Grain size (nm)	109	79	124
	Weight %	89.9	8.0	0.9
	Rwp %	7.79
5 h BM	a (Å)	4.76	18.26	-
	b (Å)	10.23	8.82	-
	c (Å)	6.00	5.20	-
	Grain size (nm)	88	29	-
	Weight %	88.7	10.3	Not found
	Rwp %	9.38
10 h BM	a (Å)	4.76	18.29	-
	b (Å)	10.23	8.78	-
	c (Å)	6.00	5.28	-
	Grain size (nm)	78	21	-
	Weight %	85.4	14.6	Not found
	Rwp %	8.71

).

From 1 to 3 h of milling time, no qualitative difference can be observed with respect to the pristine sample; indeed, the three phases composing olivine can be found, i.e., forsterite, enstatite, and clinochlore. Nonetheless, slight changes in their relative abundance are noticeable from Rietveld refinement due to the partial loss of crystallinity induced by the mechanical grinding (Table 2). For further milling time, from 3 to 10 h, the contribution of the clinochlore phase to the pattern is lost. This could be due to a further amorphization that can be noticed in the amorphous shoulder at low scattering angles that eventually hides the presence of clinochlore. It is worth noting that, conversely, the Bragg reflections belonging to clinochlore are clearly visible after 3 h of milling under CO₂ reactive flow (see Figure 4). Moreover, the Bragg reflections ascribed to the enstatite widen, whereas the signals belonging to the forsterite remain narrow, even up to 10 h of mechanical grinding (Figure 6). This evidence demonstrates that the observed process of mineral carbonation is only driven by the presence of CO₂ in the reactor, since the mechanochemical treatment of olivine under inert atmosphere did not promote any further chemical reaction or phase transformation but only a partial amorphization and a slight loss of crystallinity, as above described. More likely, high-energy ball-milling promotes CO₂ conversion by fracturing particles, thus exposing fresh surfaces in which solid–gas reactions can further take place. Lastly, the formation of clinochlore and H₂, according to the redox reaction reported above, in the presence of CO₂, allows for the production of light hydrocarbons, as revealed by the gas chromatograph output data, as could be expected by considering the results obtained in batch conditions [28]. Nevertheless, the different conditions of reactions (batch vs. flow) could impact the kinetics and the production of H₂ and light hydrocarbons, which will be the object of further investigations by our research group.

3. Materials and Methods

3.1. Synthesis

Commercial olivine was provided by SATEF-HA S.p.A (Vicenza, Italy), and it was employed for CO₂ mechanochemical conversion without any further pretreatment. The experimental setup, shown in Figure 7, and reported in more detail elsewhere [49], consisted of a prototype steel reactor for ball-milling, equipped with gas transfer lines, connected upstream to the CO₂/N₂ reaction mixture tank and downstream to a gas-chromatograph (GC). The feed gas flow into the mechanochemical reactor was tuned by a mass flow controller. The GC was equipped with a GS-Q column and both hot wire and flame ionization detectors, HWD and FID, respectively, using He as carrier gas. A gas sampling valve, possessing a volume of 0.5 mL for typically 2 g of olivine and 0.3 mL of deionized water (silicate:H₂O = 6.67:1 weight ratio), was placed inside a stainless-steel mechanochemical reactor together with three spheres of 3.7 g each, made by the same material. The reactor (vessel), packaged in air, was inserted into the mixer clamp of a commercial 8000 Mixer/Mill, Spex SamplePrep (Metuchem, NJ, USA). The mill was customized in order to tune the engine rotational frequency; in particular, 745, 875, and 1000 rpm were selected for this work.

3.2. Reactant Gas Mixture: Setup

The reactant gas mixture consisted of 10% CO₂ (purity = 99.995%), balanced in N₂. First, the reaction chamber was saturated by flowing 20 mL min⁻¹ of the reagent gas mixture for 1 h to remove air. Subsequently, the feed gas flow, kept at atmospheric pressure, was set to the nominal value of interest (typically 2.5 mL min⁻¹) before the reaction started. The reactor was directly connected to the gas sampling valve of the GC, whose loop volume was 0.5 mL. Gas sampling was then performed at selected time intervals, usually 5 min. In order to monitor the disappearance of CO₂ during the mechanically activated process, its initial concentration was determined by averaging 10 samplings collected during 1 h, which is considered a blank. As a general procedure, the GC gas analyses of the reactant and product gaseous mixture were performed at selected times once the mechanochemical process started: the first gas sampling was carried out after 1 min, and the subsequent ones were collected at regular steps of 5 min for at least 3 h of monitoring.

3.3. Percentage Conversion of CO₂

This parameter indicates the rate of CO₂ transformation compared with that initially present:

$$\mathrm{\%}\mathrm{C}{\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{[{\mathrm{C}{\mathrm{O}}_2]}_{\mathrm{i}}-[{\mathrm{C}{\mathrm{O}}_2]}_{\mathrm{G}\mathrm{C}}}{[{\mathrm{C}{\mathrm{O}}_2]}_{\mathrm{i}}}}\times 100$$

where [CO₂]_i and [CO₂]_GC are the initial and measured CO₂ concentration expressed in v/v%, respectively.

Considering that the CO₂ concentration evaluated at the GC will be equal to

$$\left[\mathrm{C}{\mathrm{O}}_2\right]=\mathrm{f}\mathrm{r}·\mathrm{C}{\mathrm{O}}_2 \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}$$

where “fr” is the response factor of the GC-column for CO₂ and “CO₂ Area” is the area of the integrated CO₂ signal in the chromatogram, the equation to calculate the CO₂ conversion rate can be rewritten as follows:

$$\mathrm{\%}\mathrm{C}{\mathrm{O}}_2\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{{\mathrm{C}{\mathrm{O}}_2 \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}_{\mathrm{i}}-{\mathrm{C}{\mathrm{O}}_2 \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}_{\mathrm{G}\mathrm{C}}}{{\mathrm{C}{\mathrm{O}}_2 \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}_{\mathrm{i}}}}\times 100$$

3.4. Assessment of Phase Evolution during Reactive Milling

Structural characterization was performed on both pristine and reacted samples by powder X-ray diffraction (P-XRD). The analyses were performed using a Smart Lab (Rigaku) diffractometer equipped with a Cu rotating anode in a Bragg–Brentano geometry. The P-XRD patterns were collected, setting a step size of 0.05° and a dwell time of 4 s per point. The identification of the crystallographic phases of the pristine olivine and of the formed products was performed using the software Xpert Highscore v. 1.0b (Philips Analytical B.V., Almelo, The Netherlands). Crystallographic index files (CIFs) were taken from the Crystallographic Open Database (COD) [50]. The relative abundance of the crystallographic phases in the samples, as well as their microstructural parameters, was obtained by Rietveld refinement performed using the software MAUD v. 2.999 [51]. In order to gather greater insight into the reacting mechanism of the CO₂ mineralization, ex situ P-XRD at selected milling time was also performed; in particular, in order to avoid any gas contamination or changes in reactant mixture or in the mass-to-powder weight ratio, six different runs were carried out, each one corresponding to a milling time indicated in Figure 4. P-XRD patterns were then analyzed according to the protocols described above. To further corroborate the results of the P-XRD analyses, the evolution of the crystallographic phases as a function of the milling time was flanked by infrared spectroscopy analyses. Spectra of the olivine samples were collected through a Fourier Transform FT-IR 480 Plus Spectrometer (Jasco, Easton, MD, USA) in the wavenumber region ranging from 4000 to 500 cm⁻¹. Microstructural characterization and elementary analysis of the olivine samples were performed by scanning electron microscopy (SEM) using a Quanta 200 microscope (FEI, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray spectroscopy (EDAX) detector (Ametek, Inc. Berwyn, PA, USA).

4. Conclusions

The kinetic profiles showcased by the reactions carried out at different rotational frequencies of the mill reveal that the CO₂ conversion process studied here cannot be described by a straightforward path. Indeed, changing the rotational frequency during the mechanochemically driven CO₂ conversion allowed us to unveil a more complex reaction mechanism that occurs through subsequent stages, whose features depend on the conditions of the mechanical treatment. The final conversion value within the observation time interval (180 min) increases with the rpm, while the shape and extent of the intermediate plateau appear to be more pronounced as the rpm decreases. This evolution suggests that the reaction rate of the CO₂ conversion lies on repeated formations and conversions of clinochlore, which act as a trigger for the reaction that eventually leads to the precipitation of carbonate phases, in agreement with the schemes reported in the Introduction section. The peculiar reactivity of clinochlore might be ascribed to its mechanical properties. Indeed, unless metallic cations (Mg²⁺ and Fe²⁺) needed for the formation of the carbonate phases are potentially available in all the phases gathered in the olivine, the relative softness of clinochlore (2–3 in the Mohs scale of hardness) compared with those of both forsterite and enstatite (6–7) might explain its enhanced reactivity under mechanical treatment. The repeated energy transfer due to the milling likely induces fractures in olivine where the softest component is present, thus exposing preferentially clinochlore to the incoming CO₂. Moreover, the magnesium carbonates, observed after the CO₂ conversion, arise from the Mg-containing brucite layers of clinochlore, the latter being loosely bonded to the aluminosilicate ones, therefore confirming the proposed topological description of the solid–gas reaction. Nonetheless, crystalline carbonate phases are not the only product of CO₂ conversion: the formation of light hydrocarbons, resulting from CO₂ reduction pathways, was also witnessed by GC analyses, showcased by the same mechanochemically driven reaction under batch conditions [28]. Investigations are ongoing to correlate the kinetics of CO₂ sequestration with the distribution of gaseous products, with the aim to further shed light on the complex mechanism behind mineral carbonation under flow conditions.

5. Patents

• Method for Converting Carbon Dioxide into High Added Value Chemical Compounds through a Mechanochemical Process under Continuous Gas Flow Conditions Int. Patent WO/2022200941 A1 2022
• Process for the Conversion of Carbon Dioxide into Value-Added Products by Means of a Process of Mechanochemical Activation of Industrial Processing Scraps Int. Patent WO/2023199254 A9 2023