The kinetics of the thermal dehydration of compacted composite grains of Na2CO3·H2O (SC-MH) comprising columnar SC-MH crystalline particles and an SC-MH matrix were investigated as a model system for composites of the same compound with a porphyritic texture. The presence of an induction period was confirmed as a novel finding for the thermal dehydration of SC-MH. The subsequent mass loss process was characterized as a partially overlapping two-step process attributed to the consecutive reactions of SC-MH matrix and columnar SC-MH crystalline particles. The overlapping nature of two reaction steps was revealed by determining the contributions and kinetic parameters of the individual reaction steps via a kinetic deconvolution analysis. Furthermore, the initial mass loss process caused by the thermal dehydration of the SC-MH matrix was characterized as a physico-geometrical consecutive process comprising a surface reaction and a subsequent three-dimensional (3D)-phase boundary-controlled reaction. The subsequent thermal dehydration of the columnar SC-MH crystalline particles compacted in the grains was characterized as being geometrically constrained by 3D-interface shrinkage, forming two reaction interfaces during the overlapping stage of the two reaction steps. It was expected from the kinetic results that the linear advancement rate of the second reaction interface was influenced by the water vapor produced at the reaction interface of the first reaction step. This caused the linear advancement rate of the second reaction interface to accelerate as the reaction proceeded due to contraction of the first reaction interface and completion of the first reaction step.

Physico-geometrical kinetics of the thermal dehydration of sodium carbonate monohydrate as a compacted composite of inorganic hydrate comprising crystalline particles and matrix.

This study examined the effects of atmospheric water vapor and CO2 on the thermal decomposition of granular malachite as a model process for the thermal decomposition of large and compact agglomerate solids. In previous studies in a dry N2 gas stream, the thermal decomposition of the granular malachite exhibited physico-geometrically constrained two-step mass loss behaviors accompanied by the swelling of granular particles and crack formation in the surface product layer of each granule. In the presence of atmospheric water vapor, the reaction was shifted systematically to lower temperatures with increasing atmospheric water vapor pressure (p(H2O)) by maintaining the two-step mass loss behavior. Kinetic analyses of the two-step heterogeneous process and subsequent universal kinetic description for each reaction step over different p(H2O) values demonstrated that the catalytic effect of atmospheric water vapor is more significant in the first reaction step because of the surface reaction. Conversely, in the presence of atmospheric CO2, the reaction shifted systematically to higher temperatures with increasing partial pressure of CO2 where the surface and internal reaction steps were more clearly separated, and additional mass-loss steps appeared to complete the reaction. The enhanced retardation effects of atmospheric CO2 as the mass-loss process advanced were confirmed by kinetic analyses of the empirically deconvoluted five-step reaction process. This phenomenon was explained by the effects of atmospheric CO2 on the construction of the surface product layer that helps block the diffusional removal of the gaseous product and increases the internal gaseous pressure.

Individual effects of atmospheric water vapor and carbon dioxide on the kinetics of the thermal decomposition of granular malachite.

The origin of the multistep thermal dehydration of calcium hydrogen phosphate dihydrate (dibasic calcium phosphate dihydrate (DCPD)) to form γ-calcium diphosphate (γ-calcium pyrophosphate (γ-CPP)) via calcium hydrogen phosphate anhydride (dibasic calcium phosphate anhydride (DCPA)) was investigated from a specific viewpoint of physico-geometrical constraints generated during the reaction. The overall thermal dehydration was separated into five partially overlapping steps through systematic kinetic analysis. The first three steps and the residual two steps were attributed to the thermal dehydration of DCPD to form DCPA and of DCPA to form γ-CPP, respectively. The first to third steps were kinetically characterized by the surface reaction of plate-like particles controlled by nucleation and growth, the movement of the reaction interface inward to the plate by releasing water vapor through voids formed in the surface product layer, and the rapid escape of water vapor accompanied by the cleavage of plate-like particles into slices, respectively. The contributions of each component step varied with the heating conditions and atmospheric water vapor pressure. The subsequent dehydration of DCPA proceeded in two steps by the release of trapped water molecules in amorphous DCPA induced by its gradual crystallization and the dehydration of DCPA to form poorly crystalline γ-CPP, which continued to grow during the fifth mass loss step and exhibited a detectable exothermic phenomenon after the mass loss was completed. The possible causes of the variation in the multistep reaction features with reaction conditions were discussed by correlating the kinetic analysis results with the crystallographic and morphological findings.

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Physico-geometrical kinetic insight into multistep thermal dehydration of calcium hydrogen phosphate dihydrate.

The thermal decomposition of spherically granulated malachite particles was investigated to unveil the specific kinetic features of the reaction in samples in granular form toward the improvement of the thermal processing of malachite as a precursor of functional CuO. Granular malachite underwent thermal decomposition via a partially overlapping two-step mass loss process upon heating the sample in a stream of dry N2 gas. Morphologically, the process was characterized by swelling of the granular particles and cleavage divisions of the surface layer. The kinetics of the thermal decomposition was investigated through step-by-step kinetic analyses of the systematically recorded thermoanalytical curves. Finally, the kinetics of the component reaction steps was separately characterized by performing a kinetic deconvolution analysis. The first reaction step, which contributed approximately 25% to the overall reaction and followed pseudo-first-order kinetics, was attributed to the thermal decomposition of the granular particle surface. The as-produced surface product layer impeded the diffusional removal of the gaseous products, i.e., CO2 and water vapor, from the interior of the granular particles, which caused swelling of the granular particles owing to an increase in the internal gaseous pressure and the cleavage division of the surface product layer by crack formation. The second mass loss step occurred inside the granular particles under significant variations in the self-generated reaction conditions and geometrical constraints and reached its maximum rate midway through the reaction. Possible causes of the observed specific rate behavior are discussed from the viewpoint of physico-geometrical kinetics in the solid–gas system.

Yuta Aoki

Papers

Thermal decomposition of spherically granulated malachite: physico-geometrical constraints and overall kinetics