Isothermal Solid-state Reactions Research Articles

Comparative study of the isothermal solid-state reaction systems of kaolinite–Na2CO3 and kaolinite–quartz–Na2CO3 for coal gangue activation

A clear understanding of the solid-state reaction of kaolinite (Kln), quartz (Qtz), and sodium carbonate (Na2CO3) is of great significance for the process optimization of coal gangue calcined with Na2CO3. In this work, a comparative study of the isothermal solid-state reaction systems of Kln–Na2CO3 and Kln–Qtz–Na2CO3 was performed by means of X-ray diffraction (XRD), scanning electron microscope, and energy dispersion spectroscopy (SEM-EDS). The results showed that the calcined products both for these reaction systems mainly contain different kinds of sodium aluminum silicates (e.g., NaAlSiO4, Na1.55Al1.55Si0.45O4, and Na1.95Al1.95Si0.05O4) and various kinds of sodium silicates (e.g., Na2Si3O7, Na2SiO3, and Na6Si2O7). The mass percentage of Na2CO3 played a key role in the phase transformation, determining the Na/Al/Si molar ratio of the formed sodium aluminum silicates. Compared with the reaction system of Kln–Na2CO3, the existence of Qtz inhibited the formation of sodium aluminum silicates in the reaction system of Kln–Qtz–Na2CO3. It should be noted that the formed phases both for these reaction systems were slightly different from that of the thermodynamical calculated results of Na2O–SiO2–Al2O3 using FactSage™ software. According to both the experimental and calculated results, a reasonable batching area for coal gangue activation was proposed that the addition of Na2CO3 should be in the range of 20–50% of the total mass of Kln, Qtz, and Na2CO3.

COMF: Comprehensive Model-Fitting Method for Simulating Isothermal and Single-Step Solid-State Reactions

It is well known that the implementation of the conventional model-fitting (CMF) method leads to several indistinguishable ‘best’ candidate models (BCMs) for a single-step isothermal solid-state reaction (ISSR), meaning that subjective selection becomes unavoidable. Here, we developed a more robust comprehensive model-fitting method (COMF) which, while maintaining the mathematical simplicity of CMF, utilizes a ranking criterion that enables automatic and unambiguous determination of the BCM. For each model evaluated, COMF, like CMF, fits the integral reaction rate, but, unlike CMF, it also fits the experimental conversion fraction and reaction speed. From this, three different determination coefficients are calculated and combined to rank the considered models. To validate COMF, we used two sets of experimental kinetic data from the literature regarding the isothermal desolvation of pharmaceutical solvates: (i) tetrahydrofuran solvates of sulfameter, and (ii) methanol solvates of ciclesonide. Our results suggest that from an algorithmic perspective, COMF could become the model-fitting method of choice for ISSRs making the selection of BCM easier and more reliable.

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe2O3 is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH4, CO and H2 respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe2O3 (20wt.%)/ZrO2 was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH4, CO and H2, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH4, but the phenomenon was not remarkable for the reduction by CO and H2. The reduction rate achieved using CO and H2 was relatively faster than achieved using CH4. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe2O3 to FeO by CH4, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH4. The reduction of Fe2O3 to Fe by CO and H2 can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe3O4 by water and FeO to Fe2O3 by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.

Synthesis of NbC and NbB2 by MA‐SHS in Air Process.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of NbC and NbB 2 by MA-SHS in air process

When the powder mixture of Nb/B/C = 2/2/1 in molar ratio was mechanically activated by grinding in a planetary ball mill and then exposed to air, it self-ignited spontaneously and the self-propagating high-temperature synthesis of NbC and NbB 2 was induced. In this mechanical activation assisted self-propagating high-temperature synthesis (MA-SHS) in air, it was essential that graphite was transformed into fine, disordered carbon by mechanical activation, and intimately covered the particles of Nb metal. On the other hand, the isothermal solid-state reaction of the powder mixture of Nb/B/C = 2/2/1 in argon formed NbB 2 at 1000 °C and then NbC at 1200 °C. The reaction mechanism and the morphology of the products obtained in both processes are discussed. The product with fine, homogeneous microstructure obtained in the MA-SHS in air process is expected to be a promising candidate as precursor of NbC–NbB 2 composites.

Effect of gas composition on the kinetics of iron oxide reduction in a hydrogen production process

Fe 2 O 3 is a suitable oxygen transfer agent for converting CO present in syngas to CO 2 for its eventual separation from H 2 . However, H 2 also reacts with iron oxide to form H 2 O . In order to evaluate the reactions for hydrogen enrichment, investigations into Fe 2 O 3 to FeO reduction kinetics in the presence of syngas constituents were conducted. The reaction kinetic parameters were estimated based on the thermogravimetric data. Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions, based on Johnson–Mehl–Avrami–Erofe’ev equation describing nucleation and growth processes, was applied. The experimental results indicate that the reduction is initially a surface-controlled process, but once a thin layer of lower iron oxides (magnetite, wüsite) is formed on the surface, the mechanism shifts to diffusion control. It was concluded that this initial stage of the reaction process could be interpreted as a both phase-boundary-controlled reaction and the two-dimensional nucleation and growth (transformation of the crystallographic lattices of higher oxide to lattices of lower oxide) at the gas/iron oxide interface. Comparison of the reaction courses for both the reducing agents ( H 2 , CO) independently and for their mixture was performed. It was found, that the reaction rate increases with, both, temperature and the hydrogen content in inlet gas. The activation energy values were estimated and compared.

MA-SHS of ZrC and ZrB<sub>2</sub> in Air from the Zr/B/C Powder Mixtures

MA-SHS of ZrC and ZrB<sub>2</sub> in Air from the Zr/B/C Powder Mixtures

Mechanical activation assisted self-propagating high-temperature synthesis of ZrC and ZrB2 in air from Zr/B/C powder mixtures

Abstract When the powder mixture of Zr/B/C=1/1/1 in molar ratio was mechanically activated by grinding in a planetary ball mill and then exposed to air, it self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) of ZrC and ZrB 2 was simultaneously occurred. In this mechanical activation assisted self-propagating high-temperature synthesis in air (MA-SHS in Air), it was found that high reactivity of disordered carbon formed by mechanical activation of graphite played a decisively important role. On the other hand, the isothermal solid-state reaction of the unground powder mixture of Zr/B/C=1/1/1 in argon formed ZrB 2 at 800 °C and then ZrC at 1000 °C. The reaction mechanism and the morphology of the products in both processes are discussed. The product with fine, homogeneous microstructure obtained in MA-SHS in Air is expected to be a promising candidate as precursor of ZrC–ZrB 2 composites.

Formation of amorphous alloys by solid state reactions

Amorphous metallic alloys have traditionally been prepared by the rapid solidification of molten alloys. Recently, new synthesis methods have been developed which are based on isothermal solid state reactions between pure metals. These reactions are carried out at temperatures that kinetically favor the interdiffusion of the species but preclude the nucleation of intermetallics. Other novel synthesis methods rely on the mechanical attrition of crystalline alloy powders. This paper reviews the basic concepts behind these novel synthesis methods and compares the amorphous alloys prepared by these techniques with those obtained by the rapid solidification of melts.

Formation of Amorphous Metallic Alloys by Solid-State Reactions

AbstractFor the last 25 years, amorphous metallic alloys have been prepared by the rapid quenching of melts. Recently, new methods of synthesis based on isothermal solid-state reactions have been developed. It has further been shown that the reaction products can be predicted from free energy diagrams that treat the amorphous alloy as an undercooled liquid. These discoveries have opened new windows to the synthesis of novel metastable materials, both amorphous and crystalline. This paper reviews the basic concepts behind amorphization by solid-state reactions and discusses our current understanding of the nucleation and growth of the amorphous alloy.

The distinguishability of selected kinetic models for isothermal solid-state reactions

Data generated from selected model expressions for solid-state kinetics have been analysed in terms of expressions of closely-related form to test distinguishability of the models on which the expressions are based. Random errors have then been introduced into these data and the data have been re-analyzed in terms of the original and closely-related expressions. As is to be expected, introduction of errors decreases distinguishability further and estimates have been made of the accuracy of measurement of the extent of reaction, α, required, the most suitable ranges of α to use, from plots of residuals, and the acceptable levels of various statistical parameters, for reliable distinction between alternative models.

Kinetic studies of the solid state reaction involving intermediate phases System BaCO 3SiO 2

The effect of particle size on the isothermal solid state reaction between BaCO 3 and SiO 2 (Ba/Si = 2/1) has been studied in air from 850° to 950°C by thermogravimetric and X-ray diffraction techniques. X-ray diffraction analysis showed that the overall reaction basically consists of the two consecutive reactions: 1. 1. 2BaCO 3 + SiO 2 = BaSiO 3 + BaCO 3 + CO 2 2. 2. BaSiO 3 + BaCO 3 = Ba 2SiO 4 + CO 2. The rate constants and activation energies for both reactions were obtained from early and later parts of the isotherms, respectively. Reaction isotherms and relative amounts of BaSiO 3 and Ba 2SiO 4 during the reaction for different combinations of reactant powders have been interpreted in terms of the particle size ratio of BaCO 3 to SiO 2.

Method of Comparing Solid‐State Kinetic Data and Its Application to the Decomposition of Kaolinite, Brucite, and BaCO3

A method of comparing the kinetics of isothermal solid‐state reactions based on the classical equation for analysis of nucleation‐and‐growth processes is described. In this method, plots of In In (1‐α) vs In (time), where α is the fraction reacted, are used to distinguish reaction mechanisms. Even nonintegral slopes obtained for values of the fraction reacted from 0.15 to 0.50 may indicate whether the reaction rate is diffusion‐ or phase‐boundary‐controlled. The problems of ascertaining zero time and self‐cooling (or heating) of the reacting sample can be observed in the analysis but do not cause severe difficulties in interpretation, as they can for analyses based on reduced‐time plots. The analysis is applied to the dehydroxylation of kaolinite and of brucite and the decomposition of BaCO3.