1. Introduction
The increasing greenhouse gas emissions and particularly anthropogenic carbon dioxide (CO
2) in the atmosphere are known to play a major role in climate change [
1]. Mitigation solutions are needed more than ever. Among the methodologies proposed for mitigation, mineral carbonation appears to be one of the most sustainable [
2,
3]. This natural and spontaneous phenomenon involves the reaction between CO
2 (aqueous or gas) and divalent cations bearing minerals in order to form the associate carbonates [
3]:
Equation (1) Carbonation reaction [
4]
The reaction products are stable and inert solids where CO
2 is sequestered. The composition of the resulting carbonates depends on the major cations present in the reactant mineral [
5]. Carbonation reaction can be divided in three main steps: (i) the CO
2 dissolution in water (ii) the material dissolution and (iii) the precipitation of carbonates as final products. The process is essentially controlled by the first two steps [
6]. Serpentine minerals, due to their high amount of Mg
2+ [
7] are considered for carbonation [
8]. Thermal treatment acts on serpentine dissolution by enhancing Mg
2+ availability, making it a key step for the process [
9]. Serpentine dissolution first results in a rapid exchange of surfacing Mg
2+ with protons (H
+) before being extracted from the structure into the solution, during a much slower phase [
10,
11]. The dissociation of CO
2 added to the solution will generate protons and HCO
3- ions, therefore enhancing Mg
2+ availability (Pasquier et al., 2014b).
Lizardite, antigorite, and chrysotile are the main minerals of the serpentine group (Mg
3Si
2O
5(OH)
4), belonging to the phyllosilicate class [
7,
12,
13,
14]. Serpentine structure is made of stacked layers composed of two sheets: the tetrahedral layer composed of silicon tetrahedral (SiO
4), linked to the lateral Mg of the octahedral layer by its apical oxygen atoms, forming a covalent bond [
14,
15]. Outer hydroxyl groups contribute to Van der Waals interactions between the two layers, whereas inner hydroxyl groups contribute to intrafoliar Van der Waals interactions [
15,
16,
17].
Under high temperatures, hydroxyl groups, linked to Mg atoms, escape the structure. During this dehydroxylation process, serpentine transformed into amorphous phases (between 550 and 750 °C—Equation (2)), and then recrystallized into forsterite (Mg
2SiO
4 > 750 °C), associated with enstatite (MgSiO
3 > 800 °C) as the temperature increased (Equation (3)) [
18,
19,
20]. Two types of amorphous phases have been described [
21]: pseudo-amorphous phases, named α-meta-serpentine, appearing at 50% of the total dehydroxylation reaction, and amorphous meta-serpentine, appearing at 90% of the total dehydroxylation. The formation of αmeta-serpentine component can be observed at a temperature close to 580 °C visualized on a diffractogram by a feature in the lower angle domain (2Ɵ = ± 6°) [
21].
Equations (2) and (3): Serpentine dihydroxylation
It has been observed that amorphous meta serpentine tends to promote Mg
2+ leaching and thus carbonation [
21,
22,
23]. Therefore, optimized conditions for carbonations have been prescribed to be between 630 °C and 650 °C for 30 to 120 min [
22,
24,
25]. However, in the previous studies, carbonation reactions have essentially been performed using pure CO
2 gas at high temperature and high pressure [
21,
25,
26,
27], strong acids or salts to promote dissolution [
22,
28]. To date no studies have been conducted on optimizing thermal activation from the mineralogical point of view, especially for direct aqueous mineral carbonation using diluted gas. In these conditions, serpentine dissolution is only promoted by carbonic acid at room temperature and low/mild CO
2 partial pressure and a good activation is more than ever critical for reaction.
This study is part of the follow-up work on direct flue gas carbonation process initiated by Mercier et al. at INRS, Québec [
29]. Using mining residues available in the Province of Québec, the process uses a simulated cement plant flue gas to perform direct flue gas aqueous carbonation [
30]. Carbonation reaction parameters have been optimized by Pasquier [
31], optimized conditions for the precipitation of carbonates have been determined by Moreno [
32] whereas a technical and economical evaluation of the process have shown its feasibility and sustainability in the Province of Québec [
33]. However, a pilot scale test revealed that thermal treatment conditions needed to be optimized for the INRS process as well [
34,
35].
In the present paper, only the proportion of magnesium prior to precipitation will be studied and considered as an intermediate product of the carbonation, as thermal activation can only acts on enhancing serpentine dissolution. Therefore, post-carbonation solids were not considered in the present study for the given reasons. Furthermore, it serves to give a novel approach of evaluating the influence of amorphous phases on serpentine dissolution and thus Mg2+ leaching during direct flue gas aqueous mineral carbonation by introducing a new quantifying method of those phases. Those new mineralogical data will provide a further understanding of the relation between thermal activation and serpentine dissolution and therefore, improve this step in the INRS carbonation process.
3. Results and Discussion
3.1. Mass Loss
The proportion of mass lost by each sample during thermal treatments has been registered and is presented in
Table 4. As expected, the proportion of mass lost during treatment increased with the temperature. It reached a peak at 14.5% for sample F and was treated at 750 °C for 15 min. This value is in agreement with the expected one, between 12.0% and 14.0% [
48,
49].
3.2. Mineralogical Transformations Along with Activation Temperatures
The evolution in the mineral composition at different temperatures and residence times has been studied using XRPD. Serpentine shows high crystallinity in samples U, A, and B, respectively untreated, treated at 550 °C for 15 min, and treated for 60 min. It then decreases in samples C, D, E, and F, respectively treated at 650 °C for 15 min, 30 min, and 60 min, and at 750 °C for 15 min. Crystalline features disappear in samples F and G, respectively, at 750 °C for 15 and 30 min. Amorphous contents can be identified in all of the treated samples as the crystallinity decreases. Forsterite is observed in samples E, G, and H, shown by highly crystalline peaks.
The remaining magnetite (the small proportion not removed during gravimetric separation) shows peaks in all samples, whereas the hematite (Fe
2O
3) which appears during the duration and temperature of the test increases. Due to the tests being performed in atmospheric conditions, the iron in the ferrous form (Fe
2+) contained in the serpentine structure is oxidized into ferric iron (Fe
3+) [
50]. As iron rich olivine (fayalite—Fe
2SiO
4) can essentially incorporate Fe
2+ in its structure [
51], hematite (Fe
2O
3) is preferentially formed.
Table 5 presents phases quantification as measured using the Rietveld refinement. Three issues are faced: (i) these values do not consider the mass loss occurring during thermal treatment (ii) amorphous components are identified in the untreated sample, due to the stacking disorder of serpentine, making the identification of thermally induced amorphous components difficult, and finally (iii) a small peak is observed in the low angle that can be attributed to illite thus undermining the observation of the formation of meta-serpentine as described by [
21]. Wilson et al. [
38] determine that absolute quantification errors (wt %) for serpentine (chrysotile) and non-serpentine phases, regardless of their abundance in a sample, to be under 5.0 wt %. Consequently, illite is not considered in the Rietveld refinement as their peaks are too low and would fall under the estimation limit.
In an attempt to overcome these issues, a mass factor (MF in Equation (5)) is computed based on the mass loss of each sample (
Table 4). Using this factor, the abundance of each phase can be expressed as grams per 100 g of starting material as given in Equation (5).
Equation (5): Proportion of phases expressed in mass
As dehydroxylation is considered to be the loss of H2O from the structure, the mass of H2O lost per gram of serpentine is computed in order to obtain the proportion of dehydroxylated serpentine (Equation (6)). The value used as maximum mass loss “%mass loss max” was obtained experimentally and found to be 14.2% for this material.
Equation (6): Proportion of dehydroxylated serpentine
The initial remaining material is decomposed into a non-reacted serpentine (serpentine(i)) associated with a non-reacted amorphous phase (amorphous(i)) induced by the layered structure of the serpentine. Their masses are calculated according to Equation (7), assuming that amorphous phase and crystalline initial serpentine both dehydroxylated in the same proportion.
Equation (7): Mass of initial phases
The amount of dehydroxylated serpentine and amorphous phase corresponding to the first amorphous observed, (respectively named serpentine(d) and amorphous(d)) are given by Equation (8).
Equation (8): Mass of intermediate amorphous phases
Further dehydroxylation leads to the formation of meta-serpentine, whose mass is obtained by Equation (9) This formation is marked by the total loss of the hydroxyls groups at close to 10 wt % of the starting material mass.
Equation (9): Mass of meta-serpentine
As a result, three phases emerge from this calculation: first an initial serpentine, resulting from the sum of amorphous(i) and serpentine(i), then an intermediate amorphous components which is the sum of amorphous(d) and serpentine(d) corresponding to the first stage of amorphization, and finally meta-serpentine. Forsterite and iron oxides (magnetite and hematite) remain unaltered by the calculation.
As shown in
Table 6, Serpentine is gradually replaced by intermediate amorphous phases in samples treated at temperatures lower than 650 °C and peaks for 60 min treatment at 70.3 g/100 g of starting material. Meta-serpentine is first found in samples treated at 650 °C for 15 min. Its proportion increases with the temperature and peaks at 27.2 g/100 g of starting material in the sample treated at 750 °C for 15 min. The increase of meta-serpentine is combined with a decrease of intermediate amorphous components contents. As seen previously (
Table 5), forsterite is observed in samples E, G and H, respectively treated at 650 °C for 60 min and at 750 °C for 30 and 60 min. A treatment at 750 °C for 15 min produced a sample with no initial serpentine and no forsterite but only amorphous phases, associated with iron oxides. These observations are in agreement with previous studies which observed the formation of an intermediate amorphous component, α meta-serpentine, progressively replacing serpentine below 580 °C. It is then followed by the appearance of an amorphous meta-serpentine material by 650 °C prevailing by 750 °C [
21].
3.3. Impact of Mineralogy on Dissolution
3.3.1. Two Batches Dissolution
It is known that the amount of Mg
2+ available for leaching will directly control, along with the amount of CO
2 treated, the quantity of carbonates being precipitated from the liquid phase after carbonation [
30,
31]. This study focuses on the proportion of Mg
2+ leached from thermally treated serpentine samples.
Figure 3 shows the mass of intermediate amorphous components and of meta-serpentine added up and plotted against the proportion of Mg
2+ leached into the liquid phase during the carbonation reaction. As the amount of amorphous components increases from none (U- untreated sample) to 77 g/ 100 g of starting material (F—750 °C 15 min), the proportion of Mg
2+ leached during two batches of gas increases too, respectively from 3.3 wt % to 13.5 wt % of initial Mg
2+ concentration in solid. Samples D, G and H show similar proportions of Mg
2+ leached and a close amount of amorphous components. However, initial serpentine constitutes a third of the former composition, whereas forsterite is formed in the two latter. As observed in previous studies at 650 °C, the solubility of Mg
2+ ions is first increased by thermal treatment until it is reduced with the decreasing content of amorphous phases and the formation of forsterite. The amount of Mg
2+ leached from the heat activated serpentine appears to be linearly dependent on the proportion of amorphous phases.
3.3.2. Successive Batches Dissolution
Thermal treatment conditions of samples D and F (650 °C for 30 min and 750 °C for 15 min) are chosen to be tested on successive batches as they respectively are the recommended conditions in literature [
22] and the conditions giving the highest proportion of Mg
2+ leached after two batches of gas in our conditions.
Figure 4 shows the cumulative proportion of Mg
2+ leached after twelve batches of gas. After two batches of gas, the proportion of Mg
2+ leached demonstrates a significant discrepancy from the previous results and the present one. Indeed, the sample treated at 650 °C for 30 min shows a similar proportion of Mg
2+ leached to the one treated at 750 °C for 15 min. After 4 batches, the sample treated at 750 °C for 15 min is catching up with a proportion of Mg
2+ leached higher by 5 wt % compared to the other sample. At the end of the 12 batches, 44.6 wt % of Mg
2+ has been leached from the sample treated at 750 °C for 15 min against 32.4 wt % for the one treated at 650 °C for 30 min. For the sample treated at 650 °C for 30 min, the proportions of Mg
2+ leached reached a plateau close to 0.5 wt % during the tenth batch, suggesting that almost all of the Mg
2+ available in the present dissolution conditions might have been leached. This occurred with at 750 °C after 15 min, which indicates that the plateau has not been reached yet, suggesting that more batches of CO
2 could allow a higher proportion of Mg
2+ leached. As the solution is refreshed for every two batches of gas, the limiting factor is the availability of the Mg
2+ and not the saturation of the solution.
The increase in the slope of the curves between the batches 6 and 8 demonstrate the slight effect of the grinding on the material after the batch 6. Pasquier et al. [
30] demonstrated that the effect of the passivation silica layer, formed during dissolution, can be reduced by grinding and so revive the leaching of Mg
2+. Nevertheless, studies from the Carmex project [
52,
53]) show that a continuous mechanical exfoliation of the passivation layer as it forms on the grains would be a promising way to avoid the need for regrinding after six batches of gas.
3.4. Mineralogical Assemblage and Carbonation
As the hydroxyl groups escape the serpentine octahedral sheets, the remaining atoms of Mg and Si are reorganized through the amorphous components. As the temperature increases, serpentine is transformed into amorphous phases, then recrystallizes into forsterite.
The best mineralogical assemblage is shown to be a mixture of amorphous phases, as observed in sample F with the highest proportion of Mg
2+ leached after both two batches and successive batches carbonation tests. McKelvy et al. [
21] observed significantly higher carbonation reaction rates in the presence of amorphous meta-serpentine, formed above 610 °C, than in the presence of α-meta-serpentine formed below 580 °C and identified as intermediate amorphous components here. When structurally stable Mg-bearing phases are present in the assemblage, such as serpentine or forsterite, material reactivity decreases. In serpentine, Mg atoms are bonded to the hydroxyls groups whereas in forsterite, they are ionically bonded to silica tetrahedron [
54]. The differentiation of the two amorphous phases based on the proposed calculation in this paper were revealed to be accurate because the sample showing the highest value of metaserpentine appeared to be the one demonstrating the highest proportion of Mg
2+ leached, in accordance with observations proposed in the literature.
Moreover, reactivity appears to be affected more by mineralogy than by surface area. Larachi et al. [
15] provide surface area measurements on calcined samples, from 300 °C to 1200 °C, showing that it tends to decrease with increasing temperature as dehydroxylation occurs.
Furthermore, observations made here are in accordance to those made by other authors regarding the mineralogical transformations occurring during thermal activation and dehydroxylation [
21,
22]. Nevertheless, a shift of ideal temperature is observed, as observations made by others suggest that reactions at 650 °C were more likely to occur, rather than at 750 °C as occurred in the present study. Such a change can be attributed to numerous factors such as the initial material mineralogy, the experimental set up, and the methodology used to evaluate activation efficiency. For instance, Li et al., [
22] used hydrochloric acid to perform lixiviation tests, which is far from neutral pH and weak acid conditions used in the present study. On the other hand, McKelvy et al.’s [
21] work set the basis of serpentine dehydroxilation understanding using TGA/DTA and XRD. Conversely, their carbonation conditions used high temperature and supercritical CO
2, which is again far from the conditions tested here. Based on their results, past ideal activation conditions were shown to be effective, but not necessarily optimal. Therefore, the present results highlight the importance of considering the mineralogical assemblage alongside the thermal treatment parameters (temperature and residence time). Such an approach will allow us to take into account the effect of the initial material composition and potential specificity of the activation conditions/technique. Indeed, conditions in a rotary kiln will be very different from a furnace or a fluidized bed. As results, a study of the mineralogical assemblage can lead to an accurate optimization of heat activation operating conditions in accordance with the material activated and the equipment used.