4.1. Effect of temperature and CO2 capture
The XRD analysis of the 45 °C_n, 60 °C_n, and 80 °C_n samples are shown in Fig. 3A-C, respectively.
Upon reaction, only Ca(OH)2 (ICSD collection code: #191851), Na2CO3 (ICSD collection code: #5009), Na2CO3·H2O (ICSD collection code: #1852), and unreacted CaCO3 (ICSD collection code: #80869) could be detected, with respective main reflection angles at 2θ of 29.5°,27 34.1°,28 16.9° (ref. 29) and 30.1°.30 No additional phases were detected, suggesting the absence of secondary and competing reactions. The TG data for the 45 °C_n, 60 °C_n and 80 °C_n series is shown in Fig. 4A-C, respectively.
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The phase quantification was performed through the weight losses observed in the TG data for CaCO3 (at 560-800 °C), Ca(OH)2 (at 310-470 °C) and Na2CO3·H2O (at 50-130 °C). The estimated quantities for all the samples discussed are summarised in Table 2, where the processing conditions used are also reported.
It must be mentioned that the content of Na2CO3 was calculated by subtracting the sum of the other quantified phases Ca(OH)2, CaCO3 and Na2CO3·H2O from the total mass (100%). In fact, since Na2CO3 would start decomposing above 851 °C,31 it could not be directly quantified by TG analysis (up to 800 °C) through the detection of the relevant peak. A higher content of Na2CO3 could also be suggested by the lower LOI registered for samples with similar reaction efficiencies (Table 2). This aspect will be extensively discussed in Section 3.2. However, the quantification was considered reliable since the XRD analysis confirmed the absence of any additional phases in the solid products. Moreover, the ratio between the precipitated Na2CO3·H2O/Na2CO3 and Ca(OH)2 (mol%/mol%) revealed a good accordance to the stoichiometry expressed in eqn (1) (Table 2). In fact, 1 mol of both Ca(OH)2 and Na2CO3·H2O/Na2CO3 should precipitate for each mol of CaCO3 reacted, and the resulting molar ratio between Na2CO3·H2O/Na2CO3 and Ca(OH)2 was expected to be close to unity. Specifically, the ratios were suggesting a slight over-precipitation of Na2CO3·H2O and Na2CO3 with respect to Ca(OH)2 for all the systems studied and that could possibly be reflecting the distribution of the positive (Ca2+) and negative (CO32−) charged sites on the surface of the CaCO3. Statistically, a 27% excess of negatively charged sites may be found on the CaCO3 surface,32 justifying the greater affinity of the CaCO3 to interact with the cationic species Na+ in the liquid bulk to form Na2CO3·H2O/Na2CO3.
Based on the TG data, the extent of reaction (α) was calculated for each system, and the outcomes are reported in Fig. 5.
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The conversion of CaCO3 was high (0.7 < α < 0.8) in the tested reaction conditions, in line with the qualitative XRD data (Fig. 3A-C), showing progressive decrease in the intensity of the CaCO3 main peak at 29.5° 2θ. During the first minute, the system temperature had a significant impact on the extent of the reaction; enhanced conversion efficiencies were gained at higher temperatures, while limited effects were observed at longer residence times. It seems likely that the higher conversion registered at short residence times and higher temperatures could be linked to the lower viscosity of the NaOH solutions, favouring the ionic mobility and the enhanced interaction between the dissolved species and the solid surface and bulk.
The efficiency of the system may also be expressed in terms of CO2 capture, expressed as moles of CO2 precipitated as Na2CO3·xH2O per second of reaction progression. As reported in Fig. 6, the CO2 capture rate was decreasing from ∼4.5 × 10−4 mol sec−1 of CO2 in the first minute of reaction down to two orders of magnitude below (∼10−6 mol sec−1 of CO2) after 5 min of contact time. In other terms, around the 80% of the total process CO2 initially introduced was effectively captured after 1 min of reaction.
The samples reacted at ambient conditions (45 °C_5 min_RT, 60 °C_5 min_RT, and 80 °C_5 min_RT) indicated extent of reactions like those reacted at a constant temperature after 5 min (Fig. 5). Decreasing temperature trends could be detected for 45 °C_5 min_RT, 60 °C_5 min_RT, and 80 °C_5 min_RT, with final temperatures of 21.5, 42.1, and 53.6 °C, respectively, after 5 min of residence time. Evidently, the temperature of the system was not significantly influencing the progression of the reaction for a residence time of 5 min.
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