*3.3. Impact of Mineralogy on Dissolution*

## 3.3.1. Two Batches Dissolution

It is known that the amount of Mg2<sup>+</sup> available for leaching will directly control, along with the amount of CO2 treated, the quantity of carbonates being precipitated from the liquid phase after carbonation [30,31]. This study focuses on the proportion of Mg2<sup>+</sup> 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 Mg2<sup>+</sup> leached into the liquid phase during the carbonation reaction. As the amount of amorphous components increases from none (Uuntreated sample) to 77 g/ 100 g of starting material (F—750 ◦C 15 min), the proportion of Mg2<sup>+</sup>

leached during two batches of gas increases too, respectively from 3.3 wt % to 13.5 wt % of initial Mg2<sup>+</sup> concentration in solid. Samples D, G and H show similar proportions of Mg2<sup>+</sup> 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 Mg2<sup>+</sup> 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 Mg2<sup>+</sup> leached from the heat activated serpentine appears to be linearly dependent on the proportion of amorphous phases.

**Figure 3.** Extracted plotted against the quantity of amorphous phases, being the sum of the intermediate amorphous components and meta-serpentine. (Squares, triangle, and circles stands for test temperature of 550 ◦C, 650 ◦C, and 750 ◦C, respectively).

#### 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 Mg2<sup>+</sup> leached after two batches of gas in our conditions. Figure 4 shows the cumulative proportion of Mg2<sup>+</sup> leached after twelve batches of gas. After two batches of gas, the proportion of Mg2<sup>+</sup> 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 Mg2<sup>+</sup> 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 Mg2<sup>+</sup> leached higher by 5 wt % compared to the other sample. At the end of the 12 batches, 44.6 wt % of Mg2<sup>+</sup> 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 Mg2<sup>+</sup> leached reached a plateau close to 0.5 wt % during the tenth batch, suggesting that almost all of the Mg2<sup>+</sup> 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 CO2 could allow a higher proportion of Mg2<sup>+</sup> leached. As the solution is refreshed for every two batches of gas, the limiting factor is the availability of the Mg2<sup>+</sup> and not the saturation of the solution.

**Figure 4.** The Mg2<sup>+</sup> leached (expressed in percent of initial Mg2<sup>+</sup> concentration in the solid) for samples treated at 650 ◦C for 30 min (blue) and at 750 ◦C for 15 min (green).

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 Mg2<sup>+</sup>. 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.
