Article

The effect of alkali halides on the thermal hydrolysis of magnesium chloride and magnesium bromide

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Abstract

DTA, TG and DTG curves were recorded for ground mixtures of hydrated MgCl2 and MgBr2 with NaCl, NaBr, KCl, KBr, CsCl and CsBr. Infrared spectra of the ground mixtures were recorded after they had been heated to several temperatures. Substitutional solid solutions of hydrated magnesium in potassium and cesium salts were formed during grinding of the salt mixtures. However, a solid solution of MgCl2 is always obtained with KCl and CsCl, as a result of grinding the alkali chloride either with MgCl2 or with MgBr2. Also, a solid solution of MgBr2 is always obtained with KBr and CsBr, either from MgCl2 or from MgBr2. Cesium halides form stable solid solutions with the magnesium halides, and thermal hydrolysis of the hydrated magnesium is prevented. The solid solutions formed with potassium halides are less stable than those formed with cesium salts and hydrated Mg is thermally hydrolyzed. The temperature at which this hydrolysis occurs in the presence of potassium halide is above 300 °C, whereas in its absence this reaction occurs at temperatures below 250 °C. No hydrated solid solution of Mg is formed with sodium halides during the grinding process and the thermal behavior of magnesium chloride or bromide in sodium chloride or bromide, respectively, is similar to the thermal behavior of the pure magnesium salt. The thermal behavior of MgBr2 in NaCl is similar to that of pure MgCl2 but the thermal behavior of MgCl2 ground with NaBr shows some similarities with each of the two pure magnesium salts, the chloride and the bromide.

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... In practice, the hydration of MgCl 2 at ambient temperature and its further hydrolysis reactions upon heating are unavoidable. The hydrolysis reactions of MgCl 2 have been thoroughly investigated using differential thermal analysis (DTA) as well as differential scanning calorimetry (DSC) in the literature and the following reactions may occur upon heating [26][27][28] with the progressive dehydration of hydrated phases: ...
... At about 182 • C, the partial hydrolysis of MgCl 2 following reaction (4) can also take place [26][27][28]. ...
... Upon further heating at temperatures above 350 • C and among a wide range of possible reactions the complete hydrolysis of MgCl 2 into MgOHCl and its further dehydroxylation above 554 • C occur [26][27][28]: ...
Article
Molten chlorides are promising heat transfer fluids (HTF) and thermal energy storage (TES) materials for third-generation concentrated solar power (CSP) plants. Despite their low cost and wide operating temperature ranges (400–800 °C), structural materials experience severe corrosion in commercially available chloride salts. The understanding of the corrosion mechanisms is therefore critical for successful plant design to ensure constant power generation over the 30 years of expected lifetime. This work investigates the corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten NaCl-KCl-MgCl2 (24.5-20.5-55.0 wt%) under isothermal conditions at 700 °C in Ar. For both alloys, the corrosion attack is associated with the selective dissolution of Cr and the removal of Cr-rich carbides from the alloy matrix leaving subsurface voids. The initial “impurity-driven” corrosion mechanism associated with the cathodic reduction of MgOH⁺ impurities leads to the formation of insoluble MgO on the substrates’ surface. Compositional and microstructural features of the alloys, especially the distribution of Cr-rich carbides as well as the solubility and mobility of carbon within the metallic matrix, are found to significantly affect their corrosion resistance. The experimental observations indicate, that the formation of galvanic pairs and solid-state diffusion mechanisms play a major role regarding the corrosion attack of the alloys. The systematic corrosion experiments conducted in this study indicate a lower corrosiveness of NaCl-KCl-MgCl2 ternary mixture than that of NaCl-KCl binary mixture in static conditions at 700 °C under Ar. This work gives significant insights into corrosion issues that may be expected in third-generation industrial CSP plants. Share Link – a personalized URL providing 50 days' free access: https://authors.elsevier.com/a/1bZbI3In-97BEg
... ACS grade) and MgCl 2 (>98% purity, ACS grade) -were procured from Alfa Aesar and Sigma- (MgCl 2 ) is extremely hygroscopic, and it should be handled with utmost care to avoid moisture 104 absorption [28,29]. The presence of water can lead to several hydrolyses and partial hydrolysis 105 reactions [30]. The final products of the hydrolysis reaction at high temperature are magnesium 106 oxide (MgO) and corrosive hydrochloric acid (HCl) gas [29][30][31][32]. ...
... The presence of water can lead to several hydrolyses and partial hydrolysis 105 reactions [30]. The final products of the hydrolysis reaction at high temperature are magnesium 106 oxide (MgO) and corrosive hydrochloric acid (HCl) gas [29][30][31][32]. Therefore, the presence of 107 small quantities of metal oxide in the mixture is unavoidable. ...
... Removal of oxide impurities from 108 magnesium salt requires a process that involves adding Cl 2 or HCl gas over the melt [28,29]. A 109 wide range of reactions are possible during a thermal treatment of hydrated MgCl 2 [30]: were conducted with a 50 mL/min nitrogen purge at a heating rate of 10 °C/min and 20 °C/min for melting points and specific heat respectively. Specific heat capacities of the samples were 131 determined using the ASTM E1269 standard, with sapphire as the reference material. ...
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A novel ternary eutectic salt mixture for high-temperature sensible heat storage, composed of sodium chloride, potassium chloride and magnesium chloride (NaKMg–Cl) was developed based on a phase diagram generated with FactSage®. The differential scanning calorimetry (DSC) technique was used to experimentally validate the predicted melting point of the ternary eutectic composition, which was measured as 387 °C, in good agreement with the prediction. The ternary eutectic was compared to two binary salts formulated based on prediction of the eutectic composition by FactSage, but unfortunately DSC measurements showed that neither binary salt composition was eutectic. Nonetheless, the measured thermo-physical properties of the ternary and the two binary mixtures are compared. Liquid heat capacities of both the ternary and binary salts were determined by using DSC with sapphire as the standard reference. The average heat capacity of the ternary mixture was recorded as 1.18 J g−1 K−1. The mass loss of the molten eutectic salts was studied up to 1000 °C using a thermogravimetric analyser in nitrogen, argon and air. The results showed a significant mass loss due to vaporisation in an open system, particularly above 700 °C. However, simulation of mass loss in a closed system with an inert cover gas indicates storage temperatures above 700 °C may be feasible, and highlights the importance of the design of the storage tank system. In terms of storage material cost, the NaKMg-Cl mixture is approximately 4.5 USD/kWh, which is 60% cheaper than current state-of-the-art nitrate salt mixtures.
... The tentative reaction at the first step might be written as 3KMg(SO 4 )Cl⋅2.75H 2 O → 8.25H 2 O↑ + K 2 Mg 2 (SO 4 ) 3 + KMgCl 3 (or KCl + MgCl 2 ). While magnesium chloride is rather sensitive to the thermal hydrolysis, its thermal behaviour in the presence of alkali chlorides is more complex and hydrolysis is somewhat retarded or even suppressed (Shoval and Yariv, 1985;Shoval et al. 1986). Unfortunately, this behaviour has been studied only by DTG and IR spectroscopy, and it was not reported whether the intermediate products are amorphous or crystalline. ...
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Kainite, KMg(SO4)Cl⋅2.75H2O, is one of the most common hydrated sulfate minerals, and it plays an important role as a source of potassium. However, its properties and structure have, to date, been insufficiently studied. In our present work, kainite was investigated using multiple techniques, including single-crystal and powder X-ray diffraction, thermogravimetry, differential scanning calorimetry (DSC), and infrared spectroscopy (IR). The mineral is monoclinic, C2/m , a = 19.6742(2), b = 16.18240(10), c = 9.49140(10) Å, β = 94.8840(10)°, V = 3010.86(5) Å ³ and Z = 16. The structure was refined to R 1 = 0.0230 for 3080 unique observed reflections with | F o | ≥ 4σ F . The complex hydrogen bonding system for kainite is described for the first time. The structure of kainite contains seven symmetrically independent sites occupied by water molecules, six of which are strongly bonded to Mg ²⁺ cations while the seventh resides in the framework cavities. The acceptors of the hydrogen bonds are either chloride anions, neighbouring water molecules or oxygens atoms of sulfate groups. A bifurcated hydrogen bond was described for one of the water molecules. Based on the analysis of the crystal structure, we have confirmed and propose the correct formula for kainite as KMg(SO4)Cl⋅2.75H2O. The thermal studies of kainite in the temperature range of –150°C to +600°C indicate its stability to 190°C. The decomposition products are K2Mg2(SO4)3 , KCl and K2SO4 . The thermal expansion was calculated, which for kainite has a character typical for monoclinic crystals and similar to the compressibility tensor described earlier.
... Doping of halides in salt hydrates is used to improve their dehydration kinetics and retard hydrolysis. 19 Doping of various salts like chlorides, acetates, sulfates, and nitrates has been investigated in order to assess their effect on dehydration temperature of Mg(OH) 2 and Ca(OH) 2 . Among these dopants, nitrates are found to be effective in decreasing the dehydration temperature and accelerating the dehydration kinetics of hydroxides. ...
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Chloride-based salt hydrates form a promising class of thermochemical materials (TCMs), having high storage capacity and fast kinetics. In the charging cycles of these hydrates however hydrolysis might appear along with dehydration. The HCl produced during the hydrolysis degrades and corrodes the storage system. Our GGA-DFT results show that the enthalpy charge during proton formation (an important step in hydrolysis) is much higher for CaCl2·2H2O (33.75 kcal/mol) than for MgCl2·2H2O (19.55 kcal/mol). This is a strong indicator that hydrolysis can be minimized by appropriate chemical mixing of CaCl2 and MgCl2 hydrates, which is also confirmed by recent experimental studies. GGA-DFT calculations were performed to obtain and analyze the optimized structures, charge distributions, bonding indicators and harmonic frequencies of various chemical mixtures hydrates and compared them to their elementary salts hydrates. We have further assessed the equilibrium products concentration of dehydration/hydrolysis of the chemical mixtures under a wide range of operating conditions. We observed that chemical mixing leads to an increase of the onset hydrolysis temperature with a maximum value of 79 K, thus increasing the resistance against hydrolysis with respect to the elementary salt hydrates. We also found that the chemical mixing of CaCl2 and MgCl2 hydrates widens the operating dehydration temperature range by a maximum value of 182 K (CaMg2Cl6·2H2O) and lowers the binding enthalpy with respect to the physical mixture by ≈ 65 kcal/mol for TCM based heat storage systems
... Mixture of salt hydrates is used to improve the kinetics and hinder undesired side reactions. Hydrolysis can be hindered by mixing with other halides [4]. To decrease the dehydration temperature of Mg(OH) 2 and Ca(OH) 2 , various salts like chlorides, acetates, sulphates, and nitrates have been doped and nitrates was found to be effective in decreasing the dehydration temperature [6]. ...
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Chloride based salt hydrates are promising materials for seasonal heat storage. However, hydrolysis, a side reaction, deteriorates, their cycle stability. To improve the kinetics and durability, we have investigated the optimum operating conditions of a chemical mixture of CaCl2 and MgCl2 hydrates. In this study, we apply a GGA-DFT to gain insight into the various hydrates of CaMg2Cl6. We have obtained the structural properties, atomic charges and vibrational frequencies of CaMg2Cl6 hydrates. The entropic contribution and the enthalpy change are quantified from ground state energy and harmonic frequencies. Subsequently, the change in the Gibbs free energy of thermolysis was obtained under a wide range of temperature and pressure. The equilibrium product concentration of thermolysis can be used to design the seasonal heat storage system under different operating conditions.
... It was experimentally observed that the hydrolysis reaction could be avoided by use of external HCl pressure 14 or by adding other halides. 19 However, there is no fundamental understanding of the conditions for the hydrolysis. ...
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MgCl2 hydrates are considered as high-potential candidates for seasonal heat storage materials. These materials have high storage capacity and fast dehydration kinetics. However, as a side reaction to dehydration, hydrolysis may occur. Hydrolysis is an irreversible reaction, which produces HCl gas thus affecting the durability of heat storage systems. In this study, we present the parameterization of a reactive force field (ReaxFF) for MgCl2 hydrates to study the dehydration and hydrolysis kinetics of MgCl2·H2O and MgCl2·2H2O. The ReaxFF parameters have been derived by training against quantum mechanics data obtained from Density Functional Theory (DFT) calculations consisting of bond dissociation curves, angle bending curves, reaction enthalpies, and equation of state. A single-parameter search algorithm in combination with a Metropolis Monte Carlo algorithm is successfully used for this ReaxFF parameterization. The newly developed force field is validated by examining the elastic properties of MgCl2 hydrates and the proton transfer reaction barrier, which is important for the hydrolysis reaction. The bulk moduli of MgCl2·H2O and MgCl2·2H2O obtained from ReaxFF are in close agreement with the bulk moduli obtained from DFT. A barrier of 20.24 kcal mol(-1) for the proton transfer in MgCl2·2H2O is obtained, which is in good agreement with the barrier (19.55 kcal mol(-1)) obtained from DFT. Molecular dynamics simulations using the newly developed ReaxFF on 2D-periodic slabs of MgCl2·H2O and MgCl2·2H2O show that the dehydration rate increases more rapidly with temperature in MgCl2·H2O than in MgCl2·2H2O, in the temperature range 300-500 K. The onset temperature of HCl formation, a crucial design parameter in seasonal heat storage systems, is observed at 340 K for MgCl2·H2O, which is in agreement with experiments. The HCl formation is not observed for MgCl2·2H2O. The diffusion coefficient of H2O through MgCl2·H2O is lower than through MgCl2·2H2O, and can become a rate-limiting step. The diffusion coefficient increases with temperature and follows the Arrhenius law both for MgCl2·H2O and MgCl2·2H2O. These results indicate the validity of the ReaxFF approach for studying MgCl2 hydrates and provide important atomistic-scale insight of reaction kinetics and H2O transport in these materials.
... The thermal decomposition of double salts of the type Mg[H 2 O] 6 XY 3 (X = Rb + , Cs + , NH 4 + ,…; Y = Cl -, Br -, I -) has been subject of thermal analysis for many years, since the process is important for industrial purposes. In magnesium double salts containing water, a wide variety of possible reactions can occur during heating (Shoval et al., 1986). The most probable ones are: ...
Thesis
X-ray powder diffraction is a well-established technique to analyse structural and microstructural properties of materials. The possibility to record in-situ powder diffraction data allows studying changes within the structure and microstructure of a sample that occur in dependence on the applied external conditions (e.g. temperature, pressure). In the present thesis, in-situ X-ray powder diffraction was used to study structural and microstructural changes of different samples occurring at elevated temperature or upon UV illumination. Several structural phase transitions were studied using the approach of parametric Rietveld refinement. In parametric Rietveld refinement a set of powder diffraction pattern is refined simultaneously, constraining the evolution of some parameters using mathematical models, so that only the variables of the model need to be refined. In order to model and analyse the behaviour of structural parameters, Landau theory and its corresponding equations were used, owing to the fact that structural parameters (e.g. lattice strain, changes in atomic positions or occupancy) comprise an order parameter as defined in Landau theory. For description of the crystal structure of materials, several different approaches were tested, e.g. atomic coordinates, symmetry modes, rigid body rotations or rigid body symmetry modes. The dependence of preparation conditions on the properties of nanomaterials and their growth kinetics was studied using Whole Powder Pattern Modelling. This method allows modelling X-ray powder diffraction pattern using the microstructure of the sample without the use of arbitrary profile functions. The Fourier transforms of frequently observed effects as crystallite shape and size distribution or density of various defects, like dislocations and stacking faults, are utilised in order to get the resulting diffraction profile. Two different systems with industrial application, CeO2 and Cu2ZnSnS4, which were produced using a sol-gel approach, were investigated.
... Since magnesium chloride is severely hygroscopic it must be handled with extreme care to ensure that it does not absorb moisture [12]. The presence of water can result in the following hydrolysis reaction that would produce corrosive HCl gas and a metal oxide upon heating [13,14]: ...
Conference Paper
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