Elsevier

Calphad

Volume 62, September 2018, Pages 232-237
Calphad

Thermodynamics of fluoride-based molten fluxes for extraction of magnesium through the low temperature solid oxide membrane (LT-SOM) process

https://doi.org/10.1016/j.calphad.2018.07.006Get rights and content

Abstract

The melt quenching experiments and thermodynamic calculations of phase diagrams were carried out to investigate potential additives for the low temperature solid oxide membrane (LT-SOM) magnesium extraction process. The solubility of MgO, which is a major source of magnesium extraction, was also measured in the molten fluoride fluxes. The solubility of MgO in the 46.5MgF2-46.5CaF2-7LiF and 45MgF2-45CaF2-10NaF (wt%) systems reached 3.4 and 1.9 wt% at 1473 K, respectively, and 1.5 wt% MgO in both fluxes at 1223 K. In addition, the 45MgF2-55CaF2 binary eutectic flux, which has been widely used in SOM process, could dissolve up to 2.3 wt% MgO at 1473 K. This value is significantly lower than the literature value, i.e. 10 wt% MgO. From the evaluation of the activity coefficient of MgO in the 46.5MgF2-46.5CaF2-7LiF and 45MgF2-45CaF2-10NaF fluxes under MgO saturation, it was confirmed that the stability of MgO in the 7LiF flux is greater than that in the 10NaF flux. Hence, the driving force of MgO dissolution into the 7LiF flux is higher than that into the 10NaF flux. The newly developed molten flux for magnesium extraction using the LT-SOM process with an operating temperature lower than 1273 K is the 46.5MgF2-46.5CaF2-7LiF system.

Introduction

There are various types of metallurgical processes for magnesium extraction, among which the Pidgeon process and the electrolysis process are commonly employed in the magnesium industry [1], [2], [3]. In the Pidgeon process, dolomite is calcined to MgO and CaO, and these oxides are reduced by silicon to yield magnesium gas. In the magnesium chloride electrolysis process, molten magnesium is produced by chlorination of magnesium oxide. These methods have several disadvantages, including being energy intensive and having lower efficiency for the Pidgeon process and toxic gas emissions and thus environmental risks for the electrolysis process [1], [2], [3].

As a more environmentally friendly and efficient extraction process, the solid oxide membrane (SOM) process has arisen due to its various advantages, such as lower cost, lower energy consumption, and zero emissions. The SOM process is an advanced electrolysis method for magnesium extraction. The target metal is produced at the cathode through direct electrolysis of metal oxides using a membrane with selective oxygen ion permeability [4]. The pioneering research for the SOM process originated from Pal et al., Boston University. Since the late 1990′s, the authors have investigated the electrolysis process to extract metal from its oxides using an oxygen ion conducting membrane [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. They designed a whole cell used to extract metal with the SOM process [6], [7]. They have also studied the production of aluminum [9], titanium [10], and silicon [11], [12]. The authors proposed six requirements of a flux system for the SOM process [8]. Among these requirements (e.g., higher electronegativity of the target metal cations, lower electronic conductivity, higher chemical stability of the membrane, etc.), the most significant factor is higher solubility of the metal oxide in the molten flux media.

Most of the previous attempts focused not only on the corrosion mechanism of the membrane in the molten flux but also on designing a whole cell, including the anode assembly [8], [13], [14], [15], [16], [17], [18], [19], [20], [21]. However, even though it is essential to determine the proper flux compositions for the whole process that are feasible at the desired conditions, there are few thermodynamic studies regarding the molten flux system for the SOM process.

In addition, lowering the operating temperature below 1273 K would bring various advantages, including energy- and cost-saving effects. Correspondingly, ceria-based membranes have become of interest for anode assembly because of their superior ionic conductivity at lower temperatures (around 1273 K) compared to yttria-stabilized zirconia (YSZ) membrane [22]. Therefore, in the present study, we investigated potential additives to introduce into the MgF2-CaF2 flux to lower the melting temperature, and we proposed a new flux system for the modified low temperature SOM (LT-SOM) process using a ceria-based membrane. Moreover, the solubility of MgO in fluoride-based molten fluxes was quantitatively evaluated based on thermodynamic calculations and experimental confirmation.

Section snippets

Experimental procedure

Direct quenching experiments were performed to evaluate the effective additives for achieving a higher solubility of MgO in fluoride-based molten fluxes. The materials and specific procedures for the quenching experiments are shown in Table 1. Graphite crucibles were employed with a sealed lid (Fig. 1) to avoid volatilization loss and to control the compositions of the fluoride-based fluxes. Approximately 2 g of flux powder and 0.5 g of high purity (> 99.9%) MgO poly-crystalline chunk were

Selection of potential molten flux systems for LT-SOM process

In general, the SOM process employs a flux system with a eutectic composition of 45MgF2-55CaF2 (wt%) (eutectic temperature: approx. 1273 K), which was proposed by a Boston University research group [23]. However, some elements need to be added into the flux system so that the operating temperature of the SOM process can be decreased. In order to determine which potential additives to introduce into the flux, the reduction potentials must be considered. In the SOM process, magnesium gas is

Conclusions

In the present study, melt quenching experiments were employed in conjunction with thermodynamic calculations of phase diagrams to investigate potential additives for the low temperature SOM (LT-SOM) process. Moreover, the solubility of MgO, which is a major source of magnesium extraction, was measured in the molten fluoride fluxes. The major findings of this study are as follows:

  • 1.

    The solubility of MgO in the 46.5MgF2-46.5CaF2-7LiF and 45MgF2-45CaF2-10NaF systems reached 3.4 wt% and 1.9 wt% at

Acknowledgement

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20152020105950).

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