Reversible collapse and Mg release of de- and rehydroxylated homoionic cis-vacant montmorillonites

Homoionic Na⁺, Ca²⁺, Sr²⁺, Li⁺, Cu²⁺ and Zn²⁺ samples of the <2 μm fraction of a cis-vacant montmorillonite from Linden (Bavaria) were steam treated at 200°C (≈1.5 MPa), 240°C (≈3.3 MPa) and 300°C (≈8.0 MPa) after dehydroxylation at temperatures up to 630°C. Cation exchange capacity (CEC) measurements, determination of exchangeable cations and X-ray diffraction (XRD), supplemented by thermoanalytical investigations of the evolved water in a thermobalance linked to a mass spectrometer, infrared (IR) and electron spin resonance (ESR) spectroscopy were employed to obtain information about the state of expandability and structural changes of swellable montmorillonite and the sites of interlayer and octahedral cations after heating and rehydroxylation.The XRD pattern of the initial samples showed a well-defined (001) reflection according to the interlayer cation and its hydration state under laboratory atmosphere. After dehydroxylation the pattern exhibited (001) reflections between 9.6 and 9.8 Å, corresponding to a collapsed structure for all samples. The Na⁺-, Ca²⁺- and Sr²⁺-rich montmorillonites regained partial expandability after rehydroxylation at 200°C and full expandability after rehydroxylation at 300°C if the dehydroxylation temperature was less than 630°C. Rehydroxylation at 300°C of the Cu²⁺- and Zn²⁺-rich montmorillonites did not cause reexpansion, whereas the Li⁺-rich samples recovered a partial swellability after rehydroxylation at 240°C and nearly the full swellability after rehydroxylation at 300°C.The Li⁺-, Cu²⁺- and Zn²⁺-rich samples underwent a strong CEC reduction due to migration of the interlayer cations into the 2:1 layer before dehydroxylation started. After rehydroxylation under water steam Cu²⁺- and Zn²⁺-rich samples released 16–30 meq/100 g of Mg²⁺ from the structure, increasing with the steam temperature. Mg²⁺ release was not observed for the Li⁺-rich montmorillonite.     

Dissolution kinetics of dehydroxylated nickeliferous goethite from limonitic lateritic nickel ore

The dissolution kinetics in 2 M H₂SO₄ of variously dehydroxylated nickeliferous goethites was investigated for five oxide-type lateritic nickel deposits. Goethite was the main constituent with minor amounts of quartz, talc, kaolinite and Mn oxides. Dissolution of Fe from heated materials followed the Kabai equation. There was a 9–34-fold increase in the Kabai dissolution rate constant (k) for samples heated at 340–400 °C due to both the increased surface area (1.5–2.6 fold) and higher density of structural defects (5–10 fold) in the variously dehydroxylated products. The presence of structural Al and Cr in goethite appears to reduce dissolution rate possibly through the greater M³⁺–OH, O bond strength relative to Fe³⁺, Ni²⁺–OH, O. Nickel showed congruent dissolution with Fe indicating that Ni was uniformly incorporated in the goethite structure. Pre-heating goethite to 600–800 °C for 30 min resulted in incongruent dissolution of Fe and Ni. It is postulated that some Ni is ejected from the neo-formed hematite structure and resides on the crystal surface or in voids. These results may contribute to the development of more efficient procedures for Ni extraction including heap leaching of lateritic nickel ores.     

Effect of dehydroxylation on the specific heat of simple clay mixtures

Specific heats of four clays (standard reference kaolins, commercial kaolin and montmorillonite) before and after dehydroxylation have been measured. The results were compared with handbook data for the thermal chemical properties of solids. Good agreement has been obtained for the reference kaolin before any thermal treatment. Then, following thermal treatments at 500 °C, 600 °C and 700 °C, dehydroxylation leads to a progressive decrease of heat capacity per unit mass. After dehydroxylation, heat capacity values for all the studied materials are rather similar and agree closely with those estimated by the rule of mixtures. Finally, an empirical relation describing the specific heat capacity (C) in J kg⁻¹ K⁻¹ of dehydroxylated kaolin from 40 °C to 1100 °C is proposed: C = 1128 + 0.102T − 36 × 10⁶T⁻² where T is in K.     

Thermal evolution in air and argon of nanocrystalline MoS2 synthesized under hydrothermal conditions

Nanocrystalline molybdenum sulfide was synthesized between 150°C and 225°C under hydrothermal conditions starting from ammonium heptamolybdate and thiourea. Samples were characterized with X-ray powder diffraction, electron microscopy, nitrogen adsorption, thermoanalysis and infrared spectroscopy. The hydroxyls involved in the synthesis and adsorbed on crystals surface hindered crystallization and samples still recrystallized after the final dehydroxylation step above 300°C, just when hydroxyls were isolated from each other. This also promoted sulfur bond breaking that gave rise to partial transformation of the MoS₂ into MoO₂ when the annealing atmosphere was argon, and to the total transformation of the sulfide into MoO₃ when it was air. The initial MoS₂ crystals were bend; many of them were isolated, and others associated in bundles that formed worm-like grains interacting with each other to produce spherical grains aggregated in clusters. This morphology gave rise to samples with a low specific surface area.     

The influence of structure order on the kinetics of dehydroxylation of kaolinite

The structural order of kaolinite is an important factor that shows a substantial effect on the processes which take place during the thermal treatment of kaolin. The influence of structural order on the dehydroxylation process was investigated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA). The thermal analysis was performed on the samples with gradually decreasing structural order prepared by milling procedure. The apparent activation energy of dehydroxylation process decreases with decreasing structural order according to the exponential function. The extrapolation of experimental data leads to the estimation of apparent activation energy of 76.6 kJ mol^?1 and of frequency factor of 0.12 × 10⁴ s^?1 related to completely disordered form of kaolinite, while the ordered form shows the apparent activation energy of 216.17 kJ mol^?1 and the frequency factor of 9.26 × 10⁴ s^?1. The relationships between features such as the infrared pattern of treated material, the degree of structural order and the apparent activation energy were established.     

The kinetic analysis of the thermal decomposition of kaolinite by DTG technique

The thermal decomposition of kaolinite was studied by differential thermogravimetry (DTG) technique under non-isothermal conditions. Samples of industrially treated (washed) kaolin with high content of the medium ordered kaolinite were calcined using a heating rate from 1 to 40 K min^− 1. The apparent activation energy and frequency factor for the dehydroxylation of kaolinite was evaluated by Kissinger method as 195 ± 2 kJ × mol^− 1 and (8.58 ± 0.33) × 10¹⁴ s^− 1, respectively. Avrami exponent of the process was estimated using Kissinger empirical kinetic models and Carne equation.     

羟基磷灰石分解和恢复的研究

以羟基磷灰石（HA）为研究对象，研究了其在加热过程中相的分解以及冷却过程和不同气氛的后期热处理对HA相恢复的作用。结果显示：当温度从800℃升高至1200℃，伴随着结晶程度的升高，HA逐渐脱去羟基并转变成缺氧羟基磷灰石（OHA），大约在1200℃左右，缺氧羟基磷灰石发生分解，分解相为磷酸四钙（TTCP）和磷酸三钙（TCP）。慢冷和后期湿气热处理有利于分解相的恢复。在二氧化碳气氛中进行后期热处理时，碳酸根能进入到HA的晶格中，形成以A型替代为主的碳酸羟基磷灰石。     

The effect of dehydroxylation/amorphization degree on pozzolanic activity of kaolinite

The effect of heat treatment parameters on the dehydroxylation/amorphization process of the kaolinite-based materials such as natural and artificial kaolin clays with different amounts of amorphous phase (metakaolin) was investigated. The procedure for quantitative estimation of amorphous phase in the heat-treated kaolinite materials was developed. The process of dehydroxylation/amorphization of kaolinite was characterized by DTA/TGA with mass-spectrometry and X-ray powder diffraction. The influence of the heat treatment temperature and content of the amorphous phase on pozzolanic activity was studied. Finally, the relationships between the chemical activity, activity strength index and the amorphous phase content were found and discussed. The results obtained are important for an optimization of the process of the metakaolin large scale production and it's use as an active pozzolanic admixture.     

Thermal dehydroxylation of clay as alternative to organic modification: effects on properties of nanocomposites

Abstract

This study presents a novel approach towards clay modification via thermal dehydroxylation, for polymer nanocomposite application. The feasibility of modification of clay via thermal dehydroxylation and its influence on the polymer properties was investigated, with reference to the polyproplylene (PP) nanocomposites. In spite of ample available literatures on organophillic clay/polymer composites, use of organic compound for the clay modification has been found detrimental in various aspects. Therefore, in order to corroborate the advantage of the thermal dehydroxylation method over organophillic modification, a comparative aspect of the effect of the organically modified and thermally dehydroxylated clay on the polymer properties has been established. Wherein, remarkably greater mechanical, dynamic mechanical, thermal and microstructural properties were noticed in case of nanocomposites reinforced with thermally dehydroxylated clay. Modification of clay via thermal dehydroxylation method was found to be an effective approach for polymer nanocomposite application.