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... magnesium chloride is very hygroscopic. Furthermore, as shown in Figure 2, water can become chemically bound to form a series of hydrate compounds with magnesium chloride (23,24). Actually, Figure 2, which is the water-rich half of the MgCl 2 -H 2 0 phase diagram, corrects several errors that have become entrenched in the magnesium literature over the years and displays the data in a metallurgical format. ...
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... as shown in Figure 2, water can become chemically bound to form a series of hydrate compounds with magnesium chloride (23,24). Actually, Figure 2, which is the water-rich half of the MgCl 2 -H 2 0 phase diagram, corrects several errors that have become entrenched in the magnesium literature over the years and displays the data in a metallurgical format. It is evident that, except for the dodecahydrate, the hydrates all melt incongruently. ...
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... to the phase diagram of Figure 2, at room temperature the most fully hydrated form of magnesium chloride is the hexahydrate, MgCl 2~ 6H 2 0. Its dehydration proceeds by the following reaction: ...
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... 3 shows this graphically as a plot of log P H20 versus T. The broken line, FG, represents the estimated water vapor pressure in equilibrium with the solid tetrahydrate and "molten hexahydrate," or more precisely, the melt at the respective liquidus composition. The points F, G, and H represent the same conditions in Figure 3 as they do in the phase diagram of Figure 2. ...
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... enlarged simplified version of the electrode assembly of the anhydrous cell is shown in Figure 12(a). The overall reaction is that given by equation (51), while the individual electrode reactions may be written as follows: ...
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... study indicated that separation of anode and cathode products in the existing magnesium cells is unsatisfactory due to entry of magnesium into the anode compartment followed by its eventual chlorination. A cell design that included sloping semi-walls which are claimed to improve separation was proposed (80) and is shown in Figure 12b. This work also pointed to the value of comparative aqueous-molten salt studies in determining similarity criteria. ...
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... 6 and in equation [11] are the partial molar enthalpies of mixing of A 2 MCl 6 and ACl with respect to the pure solids as reference states, while 1 0 is the standard enthalpy of decomposition of solid A 2 MCl 6 into MCl 4 vapour and solid ACl, according to reaction [4], where the reactant and products are in their standard states. According to equation [11] and under the assumption that = 0a plot of 4 versus 1 should be linear with slope − 1 0 + 2 6 − 2 ) / . 1 0 for the decomposition reaction [9] is expected to be large relative to the partial molar enthalpies 2 6 and . ...
Most of the phase diagrams reported in the literature have been determined in open atmospheric conditions indicating that the substances involved are not influenced by the presence of air and moisture. In these cases, the Gibbs phase rule is applied in its open condition of 1 atm pressure, and no special conditions need to impose. However, for many elements, such as all reactive metals, the phase diagrams are determined by conditions imposed to remove all the reactive actions of the presence of an atmosphere. In these cases, a special cell is needed to be constructed in a way that the material of construction of the cell and the absence of air is secured. The Gibbs phase rule is applied in its full mathematical formulation in those cases. The present publication reports on the determination of correct conditions to obtain meaningful results on the phase diagrams.
... Currently, thermal reduction and electrolytic processes are being used for primary Mg metal production. Depending on the type of process used, different treatments are conducted on Mg resources, i.e. dolomite (CaCO 3 ·MgCO 3 ), magnesite (MgCO 3 ), seawater, brine, and carnallite (MgCl 2 ·KCl·6 H 2 O) [3,4]. For example, in the Pidgeon process, the most representative thermal reduction process, calcination at 1423-1473 K is conducted to dolomite before using it as a feedstock [3,5]. ...
... For example, in the Pidgeon process, the most representative thermal reduction process, calcination at 1423-1473 K is conducted to dolomite before using it as a feedstock [3,5]. In the electrolytic processes, Mg resources are converted into anhydrous magnesium chloride (MgCl 2 ) before using it as a feedstock [3,4]. ...
A molten salt electrolysis process using a silver (Ag) cathode, followed by vacuum distillation was systematically investigated with the purpose of producing high-purity magnesium (Mg) metal from magnesium oxide (MgO). The electrolysis of MgO was conducted using Ag cathode and carbon (C) anode in magnesium fluoride (MgF2)–lithium fluoride (LiF) molten salt at 1083–1163 K with the cathodic current density of 0.213–0.223 A cm−2. After the electrolysis, 8.53–21.5 mass% Mg–Ag alloys were produced through the electroreduction of MgO with the average cell voltage of 1.95–2.11 V, and the current efficiency was 74.6–85.1%. When the Mg alloys obtained after the electrolysis of MgO were distilled at 1300 K under vacuum, Mg metal with a purity of 99.9998% was produced under certain conditions. Therefore, the developed Mg production process shows the feasibility of the environmentally friendly and efficient production of high-purity Mg metal from MgO through the molten salt electrolysis using an Ag cathode, followed by vacuum distillation.Graphical Abstract
... To construct this figure, data points from figures in literature have been processed using the Webplotdigitizer tool [95]. with the estimates based on experiments proposed by Kipouros et al. [91]. Transition temperatures proposed by others [92][93][94] also coincide with the simulated data. ...
... It belongs to the series of hydrated magnesium chlorides, MgCl 2 ·nH 2 O, n = 1, 2, 4, 6, 8, 12, with n = 6 (bischofite) being the most fully hydrated of these mineral phases at room temperature. 39 As temperature increases, these hydrated phases decompose according to the following reactions: This decomposition produces an increase of volume because the total volume of the solid, dehydrated phase and that of the released water is larger than the initial volume of the equimolar amount of the hydrated phase. This volume increase has been computed by using a function obtained by fitting the recent results from slow thermal gravimetry experiments (TGA) during these transformations 28 (see the Supporting Information for a detailed description of these procedures and the data used to compute phase density from crystallographic data). ...
A genetic model is proposed for the formation and evolution of volcano-like structures from materials other than molten silicate rocks. The model is based on Mount Dallol (Afar Triangle, Ethiopia), currently hosting a conspicuous hydrothermal system with hot, hyper-acidic springs, forming a colorful landscape of unique mineral patterns. We reason that Mount Dallol is the last stage of the formation of a salt volcano driven by the destabilization of a thick sequence of hydrated minerals (the Houston Formation) after the emplacement of an igneous intrusion beneath the thick Danakil evaporitic sequence. Our claim is supported by field studies, calculations of the mineral/water volume balance upon mineral dehydration, and by a geothermal model of the Danakil basin predicting a temperature up to 220 °C at the Houston Formation after the intrusion of a basaltic magma without direct contact with the evaporitic sequence. Although insufficient for salt melting, this heating triggers mineral dehydration and hydrolysis, leading to a total volume increase of at least 25%. The released brine is segregated upward into a pressurized chamber, where the excess volume produced the doming of Mount Dallol. Later, the collapse of the dome formed a caldera and the emission of clastic flows. The resulting structures and materials resemble volcanic lava flows in distribution, structure, and texture but are entirely made of salty materials. This novel mechanism of the generation of pressurized brines and their later eruption extends the relevance of volcanologic studies to lower temperature ranges and unanticipated geologic contexts on Earth and possibly also on other planets.
... Magnesium metal is produced via two main processing routes i.e., electrolysis of molten magnesium chloride and thermal reduction of magnesia [1]. The proportion of magnesium produced via electrolysis is smaller than that of the thermal reduction as the former is an energy intensive process. ...
... 2MgO(s) + 2CaO(s) + Si(Fe)(s) = 2CaO⋅SiO 2 (s) + 2 Mg(g) + (Fe)(s) (1) The vapor pressure of magnesium produced by the reduction was measured using entrainment method under hydrogen gas flow. The magnesium pressures at temperatures between 1100 and 1190 • C were found to be between 10.1 and 30.2 mmHg, respectively. ...
The extraction of magnesium from calcined dolomite via vacuum metallothermic reduction process has been evaluated thermodynamically and investigated experimentally. The equilibrium phases formed during the metallothermic reduction as a function of pressure, reductant, temperature, and additive combination were thermodynamically analyzed. The experimental work consisted of calcination of dolomite – (Ca,Mg)CO3 and metallothermic reduction of the calcined dolomite/dolime – (Ca,Mg)O. The metallothermic reductions of the dolime with different reductants and additives were carried out in a laboratory scale vertical retort at a final holding temperature of 1200 °C and pressure lower than 10⁻⁴ atm using different reductant types (ferrosilicon or aluminum) and different additive combinations (CaF2 and B2O3). Magnesium metal was extracted from the dolime through metallothermic reduction by either ferrosilicon or aluminum. The residue from reduction with ferrosilicon disintegrated into fine powder, while the residue from reduction with aluminum remained as briquette. The physical degradation of the residue from reduction with ferrosilicon was found to be minimized by the B2O3 addition. The CaF2 addition increased the magnesium extraction, while the B2O3 addition decreased the magnesium extraction.
... The corrosive impurities could be oxygen-based (O 2 ) and moisture-based (H 2 O). Depending on the salt's stability at ambient temperatures, the moisture in the salt is usually be in the form of hydrates (M⋅nH 2 O where M: MgCl 2 and n: 1, 2, 4, 6, 8 and 12 [7]). When the salt is heated most of this moisture easily vaporizes, while the residual moisture reacts with the salt to form oxide (-O), hydroxide (− OH) and hydroxychloride (− OHCl) compounds, and the hydrogen chloride (HCl) gas. ...
... showed that the dehydration of MgCl 2 ⋅6H 2 O is a six-step complex process which involves reducing MgCl 2 ⋅6H 2 O to MgCl 2 ⋅4H 2 O, MgCl 2 ⋅2H 2 O, MgCl 2 ⋅H 2 O and MgCl 2 with a possibility of also producing MgOHCl and HCl gas from the hydrolysis of MgCl 2 ⋅2H 2 O and MgCl 2 ⋅H 2 O. However, Kipouros et al. [7] noted that complete dehydration of MgCl 2 is still possible and achievable by providing a blanket of Cl 2 or Cl 2 based (HCl) gas with proper partial pressure ratios (i.e. P (H 2 O)/P (HCl). ...
The impurity-driven corrosion behavior of HAYNES® 230® alloy in the molten KCl-MgCl2-NaCl salt was studied at 800 °C for 100 hours with different salt purity conditions. It was found that HAYNES® 230® alloy exhibited better corrosion resistance in the salt with lower impurities. Furthermore, the corrosion (non-immersion) due to molten salt vapors was more severe at higher temperature. The attack was localized and driven by outward diffusion of chromium through the grain boundaries. The presence of Mg in its metal form in the salt resulted in an even higher mass-loss in HAYNES® 230® alloy due to Mg-Ni interaction.
... Based on the above equation, the equilibrium line of the reaction between molten MgCl 2 ⋅6H 2 O and solid MgCl 2 ⋅4H 2 O could be plotted in the Clapeyron diagram as shown in Fig. 9 (red line), where other equilibrium lines are referred from literature [49,50]. To experimentally verify the reliability of this predicted equilibrium characteristic, the MgCl 2 ⋅6H 2 O was put in a closed vacuum chamber and heated from room temperature to 140 • C with a slow heating rate of 0.5 K/min, where the vapor pressure and temperature of the sample are recorded. ...
Reversible thermal dehydration reaction of MgCl 2 ⋅6H 2 O has been studied as a potential working way for ther-mochemical heat storage with high energy density. Understanding its complex multistep dehydration behavior is significant for guiding practical applications; however, there is a lack of deep understanding about the phase transition of MgCl 2 ⋅6H 2 O during its dehydration reaction process. In this work, the reaction mechanism, dehydration kinetics, and equilibrium behaviors of MgCl 2 ⋅6H 2 O are studied in detail with the help of thermal analytical measurements and morphological observations. The thermogravimetric analysis coupled with differential scanning calorimetry (TG-DSC) shows the dehydration behavior and reaction pathways varying with gas atmosphere, heating rate, water vapor pressure, and sample mass. The formation of intermediate molten MgCl 2 ⋅6H 2 O is identified that it makes strong effects on the dehydration kinetics and reaction equilibrium. Futhermore, a revised Clapeyron equilibrium equation is developed and experimentally verified for describing the reaction equilibrium of molten MgCl 2 ⋅6H 2 O. It is expected that the findings in this study pave paths for efficient thermal energy storage application based on MgCl 2 ⋅6H 2 O.
... The easiest way to prevent the contamination of O 2 and H 2 O is to thoroughly dry the hygroscopic MgCl 2 salt. The temperatures at each drying stage were determined from the vapor-pressure curves in Fig. 2. Some authors [23][24][25] proposed a multi-step heating to purify MgCl 2 salts and to reduce the formation of MgOHCl. In this research we have analyzed different thermal treatments, at different dwelling times at each temperature, to compare those available in the literature as well as new ones proposed in this study, with a more specific stepwise heating at T1-T5 shown in Fig. 2. Specific conditions for thermal treatments performed are shown in Table 3. ...
... Vidal and Klammer [26] stablished different key temperatures (treatment #1) using long dwelling times for each temperature, but heating ramps were not reported. On the other hand, Ding et al. [23] stablished a quick thermal treatment only using a heating step at 200°C, using a heating ramp of 5°C/min. After this step, no heating ramps or additional temperature steps were reported before reaching the testing temperature (700°C). ...
... XRD analysis (Table 5) confirmed the compounds detected in this layer and MgOHCl, MgO and KCl were obtained. It is important to highlight that MgOHCl was pointed in the literature as one of the most corrosive impurities present in chloride salts [4,23,24,25]. In the following tests with the different thermal treatments performed (Fig. 4b-e) before the corrosion test, this compound was reduced, obtaining lower content in chloride from the EDX carried out in the steel surface. ...
The operating temperature of a steam turbine is limited to 565 ºC by the molten nitrate heat-transfer fluid; therefore, a new molten salt chemistry is needed to increase the maximum operating temperature in the new generation of CSP plants and improve the thermal-to-electrical energy conversion efficiency in the turbine block, such as chloride molten salts. Nevertheless, the prevention of high-temperature corrosion on containment materials using chlorides plays a critical role and a corrosion mitigation plan is needed to achieve the target plant lifetime of 30 years. This paper presents a corrosion mitigation strategy focused on different thermal treatments performed in the eutectic ternary chloride molten salt composed by MgCl2/NaCl/KCl (55.1 wt.%/24.5 wt.%/20.4 wt.%). Corrosion rates were obtained through linear polarization resistance technique in a conventional commercial stainless steel (AISI 304) at 720 ºC during 5 h of immersion after the different thermal treatments carried out. Scanning electron microscopy and XRD analysis were used to confirm the corrosion rates and corrosion layer proposed by electrochemical techniques, obtaining a minimum corrosion rate of 6.033 mm/year for the best thermal treatment performed.
... In order to evaluate the accuracy of this peak method, the results obtained from equation (3) are compared with the general integration calculation results for pure MgCl 2 $6H 2 O. Fig. 12 shows the dehydration process of pure MgCl 2 $6H 2 O. Although many studies have reported TG-DSC testing of MgCl 2 $6H 2 O [16,34,43], it is difficult to totally distinguish dehydration step from each other. The clear thermal dehydration plateaus of TG and distinct endothermic peaks of DSC are obtained as shown in Fig. 12 by carrying out experiment under 0.2 C/min heating rate and 2000 Pa vapor pressure. ...
A novel multi-form thermochemical energy storage method is proposed for high energy-density thermal energy storage based on multi-step sorption processes. The proposed multi-form thermochemical energy storage combines the physisorption energy storage of a porous matrix, the chemisorption energy storage of a salt hydrate, and the absorption energy storage of the salt solution. High-performance composite sorbent of MgCl2@zeolite was prepared to demonstrate the feasibility of the proposed multi-form thermochemical energy storage. The water uptake contributions of physisorption, chemisorption and absorption of the composite sorbent were measured by a “three-step” hydration method. The multi-step desorption processes measured by TG at an extremely slow heating rate shows the apparent decrease of decomposition temperature of MgCl2 hydrates in zeolite matrix. The maximum sorption capacity of the MgCl2@zeolite composite sorbent without solution leakage is as high as 0.55 g/g and its gravimetric and volumetric thermal energy densities reach 1368 kJ/kg and 308 kWh/m³ respectively with charging temperature of 200 °C. This gravimetric energy density is about 2.26 times higher than that of pure zeolite 13X. The experimental results verified that the proposed multi-form thermochemical energy storage is an effective method to improve sorption capacity and to achieve high energy-density thermal storage.
... MgO particles are weighed on a Mettler Toledo weighing device, and distributed into the reactor homogeneously. High purity nitrogen is selected as the inert gas and Fig. 2 e An illustration of temperature effect on the MgCl 2 ·6H 2 O dehydration process [16,17]. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2 fed into reactor by bubbling in the HCl solution with a controlled flow. ...
... At this temperature, almost all of the MgCl 2 hydrates are liberated compared to that of 250 C sample. This is an expected outcome, considering controlled dehydration thermodynamics of the MgCl 2 hydrates [16,17]. Here, the predominant substance is found to be MgOHCl at reasonable rates where a considerable amount of Mg(OH) 2 is also observed. ...
In this paper, an experimental study is undertaken to capture HCl in dry form in order to decrease the power consumption of the four-step Mg-Cl cycle using MgO as the capturing agent. A new experimental method is developed to capture HCl from its mixture with steam, and liberate HCl in a dry form. Several cases are studied to observe HCl capture performance, including testing of the resulting substances in detail using Thermogravi-metric Analysis (TGA), and X-ray diffraction (XRD) tests. The results of these experiments show 30.8% HCl capture by solid MgO particles in a semi-batch packed bed reactor design with an uncertainty value of ±1.17%. The XRD results indicate that an optimum reactor temperature of ~275 C is critical to prevent the process from side reactions and undesired products. The experimental results are adapted to the four-step Mg-Cl cycle to form a final design with HCl capture.
