The cyclic CO2 capture, transient phases change, and microstructure appearance of a new kind of Ca-based regenerable CO2 sorbent, CaO/Ca12Al14O33, obtained by the integration of CaO as solid reactant with a composite metal oxide of Ca12Al14O33 as a binder, were investigated by thermogravimetric analysis, XRD, and SEM at different preparation calcination temperatures. When the calcination temperature in the preparation stage is higher than 1000 °C, the cyclic CO2 capture of this new sorbent declines. The lowered CO2 capture may mainly be attributed to the formation of Ca3Al2O6, which decreases the ratio of CaO to binder in sorbent, and the severe sintering of sorbent occurs when calcined at such high temperatures in the preparation processes. These results suggest that the calcination temperature for this new sorbent should not be higher than 1000 °C in order to obtain its high reactivity. The performance of the new sorbent over 50 cycles was evaluated under mild and severe regeneration conditions, respect...

Effect of Preparation Temperature on Cyclic CO2 Capture and Multiple Carbonation−Calcination Cycles for a New Ca-Based CO2 Sorbent

The Pechini process was used to produce high-purity, monocalcium aluminate (CaAl 2 O 4 ) powders at temperatures as low as 900 o C. Absorption spectrometry and BET measurements revealed particles with sizes ranging from submicrometer to 100 μm, with specific surface areas as high as 10 m 2 /g. Auger electron spectrometry (AES) was used to study the progressive elimination of surface carbon from the organic burnout as a function of temperature. The growth kinetics of calcium aluminate crystallites from a polymeric precursor were studied during calcination as a function of temperature, using transmission electron microscopy (TEM)

Chemical Synthesis and Characterization of Calcium Aluminate Powders

Interfacial reactions in hydroxyapatite/alumina nanocomposites

Starting from high energetic attrition milled mixtures of either -Al2O3 or amorphous (Al(OH)3) with CaCO3 the binary compound calcium monoaluminate (CaAl2O4) was synthesized with high purity. The processes were carried out on samples with the nominal CaAl2O4 stoichiometry. High energetic attrition milling of the reactants does not eliminate the starting carbonate, both batches showed a high degree of homogeneity and they behaved similarly on heating, forming CaAl2O4 at temperatures lower than 1300 ◦ C. These temperatures are lower than that required for the traditional solid state reaction process. The CaAl 2O4 synthesis path implies the formation of cryptocrystalline and crystalline Al2O3, CaO, Ca12Al14O33 and CaAl4O7 as transitory phases. The nucleation and growth of CaAl2O4 took place at 1300 ◦ C and shows a porous structure and small grain size (∼1.5m).

http://www.ibmc.up.pt/projectos/FJMonteiro/Researchers/AHDeAza/26%20JECS_Rivas_JM_25_14_3269_2005.pdf

Synthesis of CaAl2O4 from powders: Particle size effect

Kinetics and mechanism of formation of tricalcium aluminate, Ca3Al2O6

The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X-ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA2) in a 1:2 reaction mix of CaO and Al2O3. CA reacted further with Al2O3 to form CA2. The formation of CA2 obeyed the rate law equation 1 - (1 - x)1/3=Kt/r2. The activation energy for the system (140 kJ·mol−1 (33.4 kcal · mol−1)) was determined by the Arrhenius equation.

Formation Kinetics of Calcium Aluminates

Monocalcium aluminate and calcium dialuminate were prepared by heating different mixes of CaO and Al2O3, at 1400° and 1600°C respectively, evidence of which was found by X-ray diffraction study. Castables were made from the mixes of CA and CA2 in specific proportions, hydrated by keeping immersed in water and subsequently tested for strength.Hydrated cements on firing were found to be converted to higher aluminates in presence of Al2O3 including CA6. Fusion behaviours of CA, CA2and their mixes have been studied. Role of CA2 in high alumina refractory cements has also been discussed.

Calcium Aluminate Cement and Refractory Castable

CA, CA2 and their mixes with and without alumina aggregate were allowed to hydrate below 25°C and in not less than 90% relative humidity. The various phases formed were determined by X-ray analysis. The data obtained showed that the hydration products of CA were C3AH6 and AH3 whereas CA2, CA—CA2 mixes gave higher percentage of CAH10 and AH3. Hydrated CA mixes with Al2O3 after firing gave CA2 and CA6, whereas CA—CA2 mixes gave CA6. The strength value of CA—CA2 mixes increased with increase in CA2 content whereas PCE value increased with increase in CA2 content.

Hydration and some other Properties of CA, CA2 and their Mixes

Calcium aluminates are potentially useful ceramic materials. They are important constituents in the high alumina cements and castable refractory products. A literature survey reveals that very little work has been done towards understanding their sintering behaviour during firing. The present investigation has been conducted to determine the densiflcation characteristics of mixes of calcium dialuminate (CaAt4Ov) and calcium monoaluminate (CaA12 04). The material transport in the sintering of a powder compact is governed by several mechanisms under different conditions. Vapour transport and surface diffusion mechanisms were reported first to occur in various crystalline materials [1 ]. The surface curvature of the particles varies from point to point within a powder compact and thus the chemical potential of atoms or ions vary with position. At higher temperature, chemical potential gradients produce diffusion fluxes necessary for material transport. Although this possibility has been mentioned by earlier workers, Berrin and Johnson [2] used the chemical potential as the driving force in their equations. The first attempt in defining the mechanism of material movement was done by Frankel assuming that the material was able to move in a viscous manner because of the driving force provided by surface forces. Kuczynski [3] gave the quantitative approach on mechanism of material movement using an exponent method. The geometries of particles and pores in a

Densification of mixes of calcium dialuminate and monoaluminate

Mixes of CaCO3 and Al2O3 in the molar proportion of 3:1,1:1 and 1:2 respectively, were heated at different temperatures for various reaction times and the unreacted lime contents were determined. Phases formed in the reaction mixes have been studied by X-ray diffractograms of the partially reacted samples. Data show that lime is consumed in the three mixes before the equilibrium phase C3A, CA or CA2 is formed. The lime reactivity is in the sequence of CA2 < CA < C3A reaction mixes.*

Upendra Kumar Mandal

Papers

Formation Kinetics of Calcium Aluminates

Calcium Aluminate Cement and Refractory Castable

Hydration and some other Properties of CA, CA2 and their Mixes

Densification of mixes of calcium dialuminate and monoaluminate

Assimilation of Lime in the Formation of Calcium Aluminate from CaO and Al2O3