Porous CaO Research Articles

Fabrication of Iron Particles from Porous CaO and Molten Iron Sulfide using Spontaneous Wettability Conversion

Fabrication of Iron Particles from Porous CaO and Molten Iron Sulfide using Spontaneous Wettability Conversion

Heterogeneous Thermochemical Decomposition of a Semi-Transparent Particle Under High-Flux Irradiation—Changing Grain Size Versus Shrinking Core Models

An unsteady numerical model coupling radiative-conductive-convective heat and mass transfer to chemical kinetics of heterogeneous decomposition is developed for a semi-transparent and optically large CaCO3 reacting particle exposed to direct high-flux irradiation. Two decomposition models are studied: a shrinking core model, with a well-defined CaO–CaCO3 interface between the spherical unreacted CaCO3 core and the porous CaO spherical shell around the core, and a volumetric model, with changing grain size throughout the particle. The Rosseland diffusion approximation is employed to solve for internal radiative transfer in the particle. The mass and energy equations are solved numerically by employing the finite volume method and the explicit Euler time integration scheme. For fixed CO2 partial pressure and total gas phase pressure, the computed temperature profiles show the features typical for a heat transfer limited reaction. The volumetric reaction model leads to a faster chemical conversion as compared to that for the shrinking core model, by 41.3%. For CO2 pressure increasing due to the chemical reaction and diffusion of CO2 being the only mass transfer mode inside the porous particle, the reaction becomes mass transfer limited. The increasing partial pressure of CO2 inhibits the chemical reaction, leading to an increase of the total reaction time by a factor of 433 and 734 for SCM and VM, respectively, compared to the case with fixed partial pressure of CO2.

Electrophoretic deposition of porous CaO–MgO–SiO2 glass–ceramic coatings with B2O3 as additive on Ti–6Al–4V alloy

The sub-micron glass-ceramic powders in CaO-MgO-SiO(2) system with 10wt% B(2)O(3) additive were synthesized by sol-gel process. Then bioactive porous CaO-MgO-SiO(2) glass-ceramic coatings on Ti-6Al-4V alloy substrates were fabricated using electrophoretic deposition (EPD) technique. After being calcined at 850°C, the above coatings with thickness of 10-150μm were uniform and crack-free, possessing porous structure with sub-micron and micron size connected pores. Ethanol was employed as the most suitable solvent to prepare the suspension for EPD. The coating porous appearance and porosity distribution could be controlled by adjusting the suspension concentration, applied voltage and deposition time. The heat-treated coatings possessed high crystalline and was mainly composed of diopside, akermanite, merwinite, calcium silicate and calcium borate silicate. Bonelike apatite was formed on the coatings after 7days of soaking in simulated body fluid (SBF). The bonding strength of the coatings was needed to be further improved.

Heterogeneous thermochemical decomposition of a semi-transparent particle under direct irradiation

Transient heat and mass transfer in a non-uniform emitting, absorbing and anisotropically scattering medium inside a semi-transparent, optically large and chemically reacting particle directly exposed to an external source of high-flux radiation is analyzed numerically. Thermal decomposition of calcium carbonate is selected as the model chemical reaction. The unsteady mass and energy equations are solved numerically using the finite volume technique and the explicit Euler time-integration scheme. Radiative transport is modeled using the Rosseland diffusion approximation and the Monte Carlo ray tracing method. Direct irradiation and internal radiative transfer in the particle are highly favorable for particle heating and the decomposition reaction, decreasing the total reaction time by a factor of 15, as compared to the case with external and internal radiation neglected in the analysis. In the latter case, the temperatures at the particle center and the particle surface increase monotonically to 1406 and 1417 K for the reacting particle, and 1428 and 1432 K for the non-reacting particle, respectively, after 179 s—the total reaction time of the reacting particle. With radiation included in the analysis, the surface temperatures of both reacting and non-reacting particle increase from the initial 300 to 1300 K in less than 2 s, and at the same rate until the onset of the endothermic chemical reaction at t=1.1 s. The surface temperature of the reacting particle increases further up to 2000 K after 12 s, when the whole particle is calcined. Weak dependence of the temperature, the overall reaction extent, and the total reaction time on the CaO grain size is observed in spite of strong dependence of the radiative properties of porous CaO on the CaO grain size.

Studies on the Catalyzing Transesterification of Soybean Oil with Methanol for Biodiesel Using a Novel Porous CaO Microsphere Catalyst

A novel porous CaO microsphere catalyst was synthesized by calcining spherical CaCO3 precursor which was prepared easily by mixing CaCl2 with Na2CO3. After characterized by scanning electron microscope and X-ray diffraction analysis, the synthesized CaO was studied on Studies on the catalyzing transesterification of soybean oil with methanol for biodiesel. The effects of calcining temperature, reaction time and temperature, amounts of catalyst and methanol on the catalytic activity of CaO microsphere had been evaluated by catalyzing transesterification of soybean oil with methanol for biodiesel. It expressed excellent catalytic activity with a transesterification yield of more than 98% at 65 °C in one hour with 3 wt% of CaO microsphere calcined at 800 °C.

Fabrication and osteoregenerative application of composition-tunable CaCO3/HA composites

A series of porous CaCO3/HA composites have been produced with an increased molar percentage of HA from 0, 10, 25, 50, 75 to 100%. The compression strength of the CaCO3/HA composites increases proportionally before reaching 50% and gradually levels off with a further increase up to 100%. The 50% CaCO3/HA composite, with a similar compression strength to natural bone, has been evaluated in vitro and in vivo, showing great potential for osteoregenerative applications. A three-step chemical process for preparing the CaCO3/HA composites has been revealed by time-dependent XRD analysis: (i) CaCO3 → CaO + CO2↑, (ii) 10CaO + 6(NH4)2HPO4 → Ca10(PO4)6(OH)2 + 12NH3↑ + 8H2O↑, and (iii) CaO + CO2 → CaCO3/Ca(OH)2 + CO2 → CaCO3 + H2O. The initial formation of porous CaO by a fast release of CO2 at high temperature leads to the production of the porous CaCO3/HA composites. Such understanding may allow us to extend this solid-phase fabrication approach to various other porous composites with controlled compositions for promising applications.

Structure of Na2O–CaO–P2O5–SiO2 Glass–Ceramics with Multimodal Porosity

We have investigated the evolution of the structure of nano–macro porous CaO–Na2O–P2O5–SiO2 bioactive glass–ceramics by Fourier transform infrared (FTIR) and Raman spectroscopies, and X‐ray diffraction (XRD). A controlled devitrification, followed by a chemical leaching treatment is used to produce a multimodal distribution of nano/macro pores that are expected to improve cell attachment. Data show that the leaching process removes the sodium‐ and calcium‐containing crystalline phases that are formed during the ceramming heat treatment. The primary Si–O peaks in the infrared spectra blue shift with leaching, indicating that the sample becomes SiO2 rich. In parallel, the fraction of nonbridging oxygen decreases. These results suggest a restructuring of the glass network far below the glass transition temperature. The stresses from leaching, capillary forces, and subsequent restructuring develop and grow, eventually producing cracks in the sample.

Decomposition and Calcination Characteristics of Calcium-Enriched Bio-oil

The decomposition and calcination characteristics at high temperatures of calcium-enriched bio-oil (CEB) were investigated from 900 to 1100 °C in the study. The CEB were produced by reacting bio-oil with calcium hydroxide, which combined the functions of desulfurization and providing heat during the decomposition and calcination processes. The decomposition of CEB consisted of four steps, partly similar to the case of pure calcium acetate (CA). The rate of the final step, i.e., the calcination rate to derive CaO, was higher in the case of CEB than in the case of CA. The alkali metal migrated from bio-oil might accelerate the calcination rate of CEB-derived CaCO 3 and sintering rate of CaO. The porosities of CEB-derived CaO particles were higher than that from CA, but the Brunauer−Emmett−Teller (BET) surface areas were lower. Dried CEB of pH 10.0 (CEB10) were typical amorphous solids, which should be the optimum bifunctional material with a medium heating value. The decomposition of CEB10 yielded porous CaO particles of moderate surface area and sintering rate for potential SO 2 capture.

CO2 absorption of CaO coated on aluminosilicate foam

CaO-coated aluminosilicate foam, Cs0.9Al0.9Si2.1O6, was fabricated by utilizing a polyurethane foam, and CO2 absorption behavior of the foam was investigated. Aluminosilicate foam was fabricated by two-step heat treatment at 873 K for 20 h and 1523 K for 5 h following the coating of aluminosilicate slurry on the polyurethane foam. CaO-coated foam was fabricated by the decomposition of Ca(OH)2 pillar crystals prepared by coating CaO slurry on the aluminosilicate foam at 1073 K for 5 h, which resulted in the formation of porous CaO on the foam. The CaO-coated foam showed good CO2 absorption ability and a long lifetime for the cyclic process of CO2 absorption and desorption. The CaO-coated foam continued to maintain a CO2 absorption ratio of approximately 90% for 15 CO2 absorption and desorption cycles.

Reaction between calcium oxide and hydrogen sulphide and also its modelling for a fluidised bed coal gasifier

The reaction of solid particles of CaO with gaseous H2S in CaO+H2S→CaS+H2O has been studied in a hot, laboratory scale, fluidised bed. The reaction displays Langmuir–Hinshelwood kinetics, being zeroth order in H2S with large concentrations of H2S, but first order for tiny concentrations (≪ 10−4 mol m−3 at 950°C) of H2S; the first order rate constant was measured in a fluidised bed. An important and significant difference is found between the first order rate coefficients measured in a fluidised bed and in a thermogravimetric experiment. Two mathematical models for removing H2S by, for example, adding CaO to a coal gasifier are formulated, one with Langmuir–Hinshelwood kinetics, the other, an asymptotic model, with simplified first order kinetics. Both describe the experimental facts fairly well; interestingly, the simplified model works unexpectedly well for the sulphidation of CaO particles bigger than 0·5 mm, even at pressures up to 18 bar. A match between experiments and modelling succeeds in yielding values for the rate constant of reaction (1), the effective diffusion coefficient of H2S through the product CaS, the initial porosity of the particles of CaO and the initial effective diffusivity of H2S through porous CaO. The simpler model is slightly inaccurate for the initial stages of sulphidation of realistically sized particles (diameter≈0·5 mm) of CaO in a gasifier. This is because sulphidation is initially confined to the exterior of a particle and is actually zeroth order for the conditions of a gasifier. However, towards the end of reaction (1), the asymptotic model describes the observations well because in the centre of realistically sized particles of CaO, first order kinetics are eventually obeyed.

Nanocrystalline CaO as Adsorbent to Remove COD from Paper Mill Effluent

Porous nanocrystalline CaO has been prepared by the solution combustion process using calcium nitrate as oxidizer and glycine as a fuel. As prepared calcium oxide has been characterized using powder X-Ray Diffraction (XRD), BET surface area, and Scanning Electron Microscopy (SEM). The powder XRD pattern confirms the crystallinity and phase purity of the powder. The particle size of the powder obtained from Scherrer's formula lies in the range of 37-53 nm. The particles are loosely agglomerated and spherical in shape as observed by SEM. The specific surface area of the powder is 18.5 m2/g and the average pore diameter obtained from N2-desorption isotherm is approximately 5.2 nm. Batch shaking process was performed using CaO as adsorbent with effluent collected from pulp and paper mill to remove chemical oxygen demand (COD). The COD removal capacity of the CaO is approximately 93%.

Performance of Porous CaO Obtained from the Decomposition of Calcium-Enriched Bio-Oil as Sorbent for SO2 and H2S Removal

The performance of calcium oxide particles produced from the decomposition of calcium-enriched bio-oil (CEB), the product of the reaction of bio-oil and calcium hydroxide, as a sorbent for the in situ removal of SO2 and H2S from the flue gases of coal combustors and the coal gas produced in coal gasifiers, respectively, is investigated. Reactivity evolution experiments are carried out in a thermogravimetric analysis system using CaO samples prepared through decomposition of calcium-enriched bio-oil applied as a thin coating on a quartz pan or through calcination of two naturally occurring calcitic solids of high CaCO3 content. CEB is converted to CaO in one or two steps, that is, first conversion to CaCO3 in the presence of CO2 and then calcination. Calcination and sulfidation or calcination and sulfation are carried out both sequentially and simultaneously. Because of its high porosity, the CaO material that results from the decomposition of CEB is found to be capable of reaching much higher conversions ...

Measurement of equivalent diffusivity during the calcination of limestone

By using the expression for equivalent diffusivity during the calcination of large particles of limestone, measurements of the weight loss and calcination time yielded the equivalent diffusivity of CO 2 in porous CaO. Eight kinds of limestone were used. The experimental results showed that the equivalent diffusivity for different limestones and different particle sizes was almost the same, provided sintering did not occur in calcination.

Fast back-reactions of shock-released CO2 from carbonates: an experimental approach

This work aims at investigating the processes leading to the liberation of CO2 and SO2/SO3 in the atmosphere after large meteorite impacts into sediments. Firstly, we review reactions and thermodynamic conditions to produce CO2 from carbonates and SO2/SO3 from sulfates. We show that decomposition of the carbonates and sulfates only occurs during shock pressure release.Secondly, we examine mineralogical and chemical data of natural impact breccias where pure CaO (lime) is always lacking and where secondary carbonates and probably sulfates occur abundant. This observation evidences the importance of back-reactions of CO2 and SO2/SO3 with the initially produced CaO.Third, we explore the kinetics and thermodynamics of the reactions involving CaO and CO2. We have performed 32 degassing and back-reaction experiments with fine-grained, chemically precipitated calcite, and with coarse-grained natural calcite, dolomite, and magnesite.Experiments with calcite confirm that residual CaO is highly reactive in the presence of CO2 in the 573–973 K interval: within less than 200 s, some 40 to 80% of CaO has back-reacted into CaCO3. These high reaction rates suggest that much of the impact produced CO2 may be highly transient. Scanning electron microscope observations show that these high reaction rates are enhanced by the exceptionally porous structure of the residual CaO. The kinetics of the CaO + CO2 reaction are explained by a gas-solid reaction model, in which the reaction rates are controlled by gas mass transfer through the porous CaO, the CO2-CaO surface interactions, and the diffusion of CO2 through CaCO3. Similar experiments conducted with dolomite and magnesite show that residual Mg-oxides do not react significantly at the 1000 s time scale and may, therefore, survive as witness of degassing in impact breccias.Published kinetic modeling of SO2/SO3 back-reactions with hot CaO to CaSO4 indicates typical conversion rates of around 50% after 1200 s. Hence back-reactions play also a crucial role in limiting the total amount of sulfur oxides released by an impact event into the Earth’s atmosphere and stratosphere. At low temperatures, residual CaO should react with water to yield Ca(OH)2 (another very efficient CO2 pump), or dissolve in natural waters strongly increasing the pH. This pH effect is globally compensated by the acid species (H2CO3, H2SO4) produced from liberated CO2 and SO2/SO3. Our experimental data, and the assessment of existing literature indicate that the amount of chemically active gases that have been released into the atmosphere by the Chicxulub impact event are most likely overestimated.

Modelling for Flash Calcination and Surface Area Development of Dispersed Limestone Particles

AbstractA mathematical model for calcination, sintering and surface area development of dispersed limestone particles is presented in the paper. The model describes the calcination by considering heat transfer, mass transfer of CO2 through a porous CaO layer, and the chemical kinetics. Diffusion of CO2 in a porous product layer is discussed in detail. The extent of calcination, BET surface area, temperature and pressure distribution in a particle were calculated. The extent of calcination and BET surface area, obtained from the simulation, are consistent with experimental data.

Kinetics of the reaction between gaseous sulfur trioxide and solid calcium oxide

Tiny particles (diameter < 100 µm) of porous CaO have been reacted with SO3 in inert N2 at atmospheric pressure and temperatures between 600 and 900 °C. These particles of CaO were well characterized by BET analysis and mercury porosimetry. Conditions were such that the kinetics of the reaction: CaO + SO3→ CaSO4, whereby one solid gives another, controlled the rate of production of CaSO4. Thus it was established that, for the initial stages of reaction, the diffusion of SO3 to a particle of CaO in the thermogravimetric balance used was relatively rapid, as was the diffusion of SO3 both inside the pores of a particle of CaO and through the layer of product, CaSO4. The reaction is first order in SO3, the rate constant (defined per unit area of solid CaO) being 3.2 ± 0.3 × 10–6 exp(–746 ± 108/T) m s–1. Thus the apparent activation energy is only 6.2 ± 0.9 kJ mol–1. The rate constant is almost identical to that for SO2 reacting with CaO to give CaSO3; in each case the steric factor is extremely low, being ca. 2 × 10–8. The diffusivity for SO3 through the product, CaSO4, is ca. 2 × 10–13 m2 s–1.

Reaction between gaseous sulfur dioxide and solid calcium oxide mechanism and kinetics

Sulfur dioxide has been reacted with tiny (diameter 35–85 µm) particles of porous CaO in a thermogravimetric balance. Kinetic measurements, together with infrared studies of the products, indicate that two reactions, viz: CaO + SO2→ CaSO3 and CaO + SO2→ 3/4CaSO4+ 1/4CaS occur. In the presence of both SO2 and O2, when CaSO3 and CaS both oxidise to CaSO4, these two reactions of SO2 provide the rate-determining steps for the initial stages of reaction for such tiny particles. Thus, the conditions used in this study succeed in preventing the rates of either of these reactions being controlled by the diffusion of SO2. The rates of both reactions per unit surface area of CaO have the general form: k([SO2]–[SO2]e), where [SO2]e is the concentration of SO2 for the reaction concerned being at equilibrium and k is the rate constant. Measurements of initial rates indicate that k= 7.2 × 10–6 exp(–1443/T) m s–1 and 1.2 × 10–6 exp(–481/T) m s–1 for the two reactions, respectively, correct to 25%. However, CaSO3 is unstable above ca. 1123 K, so that only the slower (second) reaction occurs above ca. 1123 K. That the rates of these reactions are independent of the concentration of oxygen confirms that SO3 plays no part in them. Measurements for large extents of reaction showed that the diffusion coefficient of SO2 through the solid products of reaction (mainly CaSO3) was Ds= 1.9 ± 0.5 × 10–14 m2 s–1. However, at high temperatures (> 1160 K) when only CaSO4 and CaS are produced, Ds falls to 2.3 × 10–15 m2 s–1.

Isotope study on diffusion in CaSO4 formed during sorbent‐flue‐gas reaction

In sorbent-flue-gas reactions, porous CaO sorbent particles are used to capture SO{sub 2} by formation of CaSO{sub 4}. Because of the large molar volume of CaSO{sub 4}, the internal surface area which is originally available for reaction diminishes as CaSO{sub 4} forms. Once the CaSO{sub 4} layer forms, further sorbent sulfation is believed to be controlled by the product layer diffusion process. It has been suggested that the product layer diffusion occurs by gaseous diffusion (Simons and Garman, 1976) and by ionic diffusion (Bhatia and Perlmutter, 1981). In this work, a two-stage sulfation experiment using {sup 32}SO{sub 2} and {sup 34}SO{sub 2} was performed. For the first stage of sulfation, at 1,300 C, 5,000 ppm {sup 32}SO{sub 2}/air mixture was passed into the mullite tube and circulated out through the bubbler continuously. This stage lasted for 14 days. When the first stage was terminated, the tablets were removed from the furnace and examined. At the beginning of the second stage sulfation, 5,000 ppm {sup 32}SO{sub 2}/air mixture was first used during the heating period. As soon as the tube temperature reached 1,300 C, the mechanical pump was turned on and the pressure in the tube was reduced immediately. Upon themore » completion of the evacuation, isotope gas 75%{sup 34}SO{sub 2}-25%{sup 32}SO{sub 2} was introduced into the mullite tube. Appropriate amount of air was also introduced into the tube such that the total SO{sub 2} concentration was roughly 5,000 ppm. The second stage sulfation lasted for three days. The SIMS analysis was performed by Microelectronics Center in North Carolina.« less

CO&lt;sub&gt;2&lt;/sub&gt; Fixation by Ca(OH)&lt;sub&gt;2&lt;/sub&gt; at High Temperature

Carbon dioxide gas in a 10% CO2-90% N2 mixture was fixed with Ca(OH)2 granules under a dry condition between 100 and 800°C. The fraction of fixed CO2 was highest between 500 and 600°C. More than 90% CO2 was fixed under the controlled gas flow rate, weight of Ca(OH)2, temperature and time. The porous CaO particles, which were prepared by dehydration of Ca(OH)2, reacted with CO2 gas and CaCO3 was formed on the surface of CaO particles. The CO2 recovery system using Ca(OH)2 makes possible to CO2 fixation by the use of simple equipment and method, and this system is effective for the minor factory.

金属カルシウムによる酸化サマリウムの還元過程に関する研究

As a basic study for the application of the reduction-diffusion (R-D) process to the production of Sm2Co17 type magnet materials, reduction reactions of Sm2O3 and ZrO2 by matallic Ca, and the effects of requisite elements for the materials on the reactions were investigated by the differential thermal analysis. When Sm2O3 or ZrO2 alone was reduced, the reduction reaction began immediately after the fusion of Ca as has been reported for the case of the reduction of Nd2O3 by metallic Ca. When they were reduced in the presence of the requisite elements, however, Cu has a distinct effect to lower the reduction rate of Sm2O3, the reason of which has been attributed to the gradual formation of a Cu-Ca melt necessary to cause the reduction and also to the decrease in Ca activity due to the melt formation.A Sm2O3 block prepared by arc-melting Sm2O3 powder was reduced by Ca vapor to study the effect of CaO formed on the block during the reduction reaction. It has been found that the CaO layers formed on the Sm2O3 block were porous and did not obstruct the passage of Ca vapor. The formation mechanism of the porous CaO layers has been discussed and compared with the formation mechanism of dense CaO which plugs pores in a porous rare earth oxide pellet. Further, it has been concluded that, in the actual R-D process, the reduction reactions of rare earth oxides may not be hindered by the formed CaO.