Based on a systematic study of the calcination condition in the early stage [28,29], it was found that with an increase in calcination temperature, the decomposition speed of magnesite increased, but the maximum activity of magnesium oxide decreased. And at constant calcination temperature, the activity of magnesium oxide increased firstly with an increasing residence time up to a certain point, and then decreased. The activity of calcined products in this paper is shown in . The data shows that the activity of MgO decreased with the increase of calcination temperature. The magnesium oxide obtained at 550 °C was the most active among all products according to the chloride ion adsorption method, and the MgO obtained at 800 °C had the lowest activity.
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The simulation results of different active magnesium oxide hydration rates are shown in . No matter what the activity of magnesium oxide was, the hydration process underwent a rapid growth followed by a slowing down, as depicted in a,b and the curves&#; inflection points appeared at the hydration time of 60 min.
c illustrates that the R2 of the first-order kinetic model of magnesium oxide hydration was below 0.99 when the activity of magnesium oxide was higher than 10, and when the activity was less than 10, the R2 could reach 0.99 or higher. When the activity of magnesium oxide was tested, the R2 of the magnesium oxide hydration kinetic model was above 0.99. Therefore, the process of magnesium oxide hydration was consistent with the multi-rate kinetics model when the activity of magnesium oxide was high. The hydration process of magnesium oxide was not only consistent with the multi-rate kinetics model, but also consistent with the first-order kinetics model, when the activity of magnesium oxide was low.
Additionally, the associated parameter values of the first-order model and multi-rate model are shown as . It could be found that the α&#; of magnesium oxide plummeted and the k decreased to varying degrees with the decrease of the magnesium oxide activity, as shown in a. When the MgO activity ΔnCl&#; decreased from 21.50 mol/kg to 16.50 mol/kg, the α&#; dropped from 69.50% to 62.60% and the k fell from 0. to 0.. It could be seen that the activity ΔnCl&#; and α&#; respectively decreased by 23.26% and 9.21%; however, the k only decreased by 0.41%, which was a very small decline. While the magnesium oxide activity ΔnCl&#; decreased from 16.5 mol/kg to 7.5 mol/kg, the k fell from 0. to 0., and the ΔnCl&#; and k were reduced by 54.55% and 45.50%, respectively. In the b, the drop of the fast reaction constant kf was quicker than the slow reaction constant ks. Besides, the change of constant was positively correlated with the activity of magnesia, whether it was a fast reaction or slow reaction.
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When the activity of MgO was as high as 21.5 mol/kg, the first-order kinetics model was found to be inadequate to describe the hydration of MgO with high standard error, and the k value was close to that of MgO activity at 16 mol/kg. With the reduction of magnesium oxide activity, the first-order kinetics model gradually described magnesium oxide hydration better.
Because of the different activity magnesium oxide used in the present study, the hydroxide would be formed in different times ( ). Some magnesium hydroxide had been on the surface of magnesium oxide, and finally covered the whole magnesium oxide, which greatly reduced the purity of the final product, magnesium hydroxide. The growth direction of Mg(OH)2 was hampered by the attachment surface, and the morphology of the crystal became irregular. For the magnesium oxide with an activity of 21.5 mol/kg, the bulk of active magnesium oxide and plate-like Mg(OH)2 were observed in a at 10 min, and the conversion rate of MgO was 33.54%. With the extension of reaction time, the hydration rate of magnesium oxide and the content of sheet magnesium hydroxide increased continuously. After 40 min of hydration ( b), the surface of magnesium oxide particles was tightly wrapped by Mg(OH)2, resulting in a smaller area of magnesium oxide in contact with the solution. At this time, the conversion rate of magnesium oxide was 61.06%. The hydrated time was prolonged, and the conversion rate of magnesium oxide was slowed down since the package of the magnesium hydroxide particles prevented the internal magnesium oxide from hydrating continuously.
Compared to the product of magnesium oxide with an activity of 7.5 mol/kg ( c), the content of magnesium hydroxide flake particles in the product of magnesium oxide hydration with an activity of 21.5 mol/kg was higher at 10 min. When the hydration time was extended to 40 min, the content of flake magnesium hydroxide particles in the two active magnesium oxide hydration products increased significantly. Compared to the product of magnesium oxide with an activity of 7.5 mol/kg, not only was the hydration rate of magnesium oxide with an activity of 21.5 mol/kg higher, but also part of the magnesium hydroxide was relatively densely packed on the surface of magnesium oxide particles. The hydration rate of high active magnesium oxide was more affected by the wraps of magnesium hydroxide particles. Therefore, the higher the magnesium oxide activity, the greater the effect of magnesium hydroxide in-situ on the hydration rate of magnesium oxide, and the hydration was incompatible with the first-order kinetics model etc.
The XRD results ( ) revealed that none of the different active MgO was converted to Mg(OH)2 completely, which matched well with the SEM analysis results. The presence of the (200) face and (220) face of magnesium oxide could be clearly observed in a,b which illustrated that the two active magnesium oxide was not completely converted into magnesium hydroxide during 120 min. Furthermore, the intensity of magnesium oxide peaks in b was higher than that of a, indicating that the content of magnesium oxide ( b) was relatively greater. At the same time, the (200) face intensity of magnesium oxide ( d) with an activity of 7.5 mol/kg was significantly higher than that of magnesium oxide ( c) with an activity of 21.5 mol/kg. The full width at half maximum of the (200) face of the former ( d) was narrower than that of the latter ( c). It further proved that the active magnesium oxide had many lattice defects and was prone to hydrate.
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