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Solubility of Sodium Tungstate in Molten Sodium Hydroxide

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Solubility of Sodium Tungstate in Molten Sodium Hydroxide Received: October 31, 2017 Electrochemistry Accepted: December 22, 2017 Published online: January 31, 2018 The Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.17-00079 Article Electrochemistry, 86(2),61–65 (2018) Solubility of Sodium Tungstate in Molten Sodium Hydroxide Tetsuo OISHI* and Miki YAGUCHI Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan * Corresponding author: [email protected] ABSTRACT The solubility of sodium tungstate (Na2WO4) in a molten sodium hydroxide (NaOH) bath and its dependence on the partial pressure of water vapor were measured. This was done as part of an ongoing study of a new tungsten (W)- recycling process using molten NaOH. First, the nature of the chemical compound in the equilibrium solid phase in a molten NaOH bath was confirmed by adding excess amount of tungsten oxide as a tungstate ion source into the bath. X-ray diffraction analysis indicated that anhydrous Na2WO4 is the solid phase in equilibrium with the liquid phase of molten NaOH saturated with Na2WO4. Then, the solubility of the anhydrous Na2WO4 was measured under various partial pressures of water vapor. The results revealed that the solubility of anhydrous Na2WO4 drastically increased with the partial pressure of water vapor and the bath temperature from a molar fraction of 0.06–0.17. This value was much higher than those previously reported for molten nitrate and nitrites. © The Electrochemical Society of Japan, All rights reserved. Keywords : Tungsten, Recycling, Molten Sodium Hydroxide, Solubility 1. Introduction of WC and binder metal of cobalt (Co) is anodically dissolved in a molten NaOH, and tungstate ions are accumulated in the melt. Tungsten (W) is widely used in industries due to the high hardness At the same time, dissolved Co is reduced on the cathode and is of the W metal and tungsten carbide (WC). In Japan, most part of separately recovered as Co powder,14 although Kamimoto et al. W is used in super-hard alloys, which are currently indispensable claimed that Co remained in the super-hard alloy under a similar for machining and other industries.1 However, mineral sources of condition.17 Then, a solution of sodium tungstate is obtained by W are relatively scarce. The recycling of W from super-hard alloys dissolving the solidified NaOH melt and W could be recovered is therefore receiving great interest worldwide, and a variety of via the conventional hydrometallurgical steps. The solubility of 2–5 recycling processes has been proposed and/or industrialized. Na2WO4 in an NaOH melt is also of relevance for other molten salt Among these recycling processes, those involving molten salts processes, such as those mentioned above, due to the ubiquity of stand out as a result of the combination of the high chemical stability molten NaOH in this field. In addition, even if NaOH is not used at of WC and the excellent feature of molten salts as reaction media, the initial condition, related compounds, such as sodium oxide, can particularly under harsh conditions.6 One typical example of molten be formed during the oxidation step of WC in pure molten sodium salt processes utilizes sodium nitrate as the oxidizing reagent with nitrate, as described in reaction (1). Prior to the measurement of the the assistance of oxygen in air in which WC in a super-hard alloy is solubility, the nature of the solid chemical compound in equilibrium 2–4,7 oxidized according to the following reaction: with the NaOH melt saturated with Na2WO4 was confirmed. Then, the solubility of Na WO was evaluated under various partial WC þ 3NaNO þ 1=4O 2 4 3 2 pressures of water vapor (P ) since it is known that the water ! þ þ þ = ð Þ H2O Na2WO4 3NO CO2 1 2Na2O 1 content in the melt play an important role in determining the nature However, the above reaction is strongly exothermic and cannot be of molten NaOH and that the activity of water in the melt is 2,8 P 15,18 easily controlled, which has led to the use of sodium carbonate, controllable by H2O. Finally, anodic dissolution of a commer- sodium hydroxide (NaOH), or mixtures of alkali chlorides as cially available throw-away tip (super-hard alloy tool) was carried 2,4,8–10 diluents. Sodium sulfate as well as oxygen in air has also been out in order to confirm the importance of the solubility of Na2WO4 examined as oxidizing reagents in molten sodium carbonate or and the effectiveness of introduction of water vapor. hydroxide baths.11–13 In recent research, anodic dissolution of WC in a NaOH melt in which WC is anodically oxidized and dissolved into 2. Experimental the melt with high current efficiency and a relatively low applied voltage of 0.4 V has been proposed.14,15 One of the common points 2.1 Identification of the equilibrium solid chemical compound of the abovementioned processes is that the reaction product is The nature of the solid-state chemical compound in equilibrium sodium tungstate (Na2WO4) dissolved in the melt. Therefore, the with the liquid phase, i.e., the NaOH melt saturated with Na2WO4, solubility of Na2WO4 is regarded as one of the most important was confirmed. A schematic of the experimental apparatus is shown parameters to determine the performance of these processes and in Fig. 1. In a glassy carbon (GC) crucible (i.d. = 40 mm and optimize the operating conditions. However, the solubility of height = 40 mm), 15 g of NaOH and 10 g of WO3 (Wako Pure Na2WO4 in molten salts has been hardly reported. To the best of Chemical Co., Ltd.) were placed and the entire sample was fused at our knowledge, the values of nitrate or nitrite melts reported by 723 K. Since our preliminary experiment indicated that the solubility 16 P Yurkinskii et al. constitute the only example to date. increases with H2O, the added WO3 was thoroughly dissolved P P This background prompted us to investigate the solubility of under high H2O. This H2O was controlled by passing argon (Ar) Na2WO4 in an NaOH melt as part of our ongoing study of a new gas through a water bath maintained at 303–363 K and introducing W-recycling process. In this process, a super-hard alloy tool consists the Ar stream with the water vapor into the reaction chamber. The 61 Electrochemistry, 86(2),61–65 (2018) 2 The experimental setup for anodic dissolution of a throw-away 1 tip was essentially the same as that reported,14,15 that is, a commercially available throw-away tip with no coating was placed on a nickel (Ni) plate and used as the anode. The cathode was an Ni plate of dimensions of 10 mm © 20 mm. In a glassy carbon crucible, a 60 g of NaOH and 12 g of WO3 was placed and the entire sample P was fused at 723 K under H2O of 0 or 0.7 atm. Then, a constant cell P voltage of 0.4 or 1.0 V, for the conditions of H2O of 0 and 0.7 atm, respectively, was applied, and the passed current was measured. 6 3. Results and Discussion 3.1 Identification of the solid-state chemical compound in equilibrium with the liquid phase 4 Our preliminary experiment suggested Na2WO4 as the most possible solid chemical compound under equilibrium conditions. 3 Therefore, WO was chosen as one of the other tungstate ion sources 7 3 5 to avoid misidentification of the added and the precipitated compounds. The added WO3 dissolves in molten NaOH according to the following reaction: Figure 1. Schematic of the experimental apparatus. 1: Ar gas and ð Þþ ð Þ! þ ð ÞðÞ water vapor outlet. 2: Flexible stainless tube with heaters and gas WO3 s 2NaOH l Na2WO4 in the melt H2O l 2 inlet. 3: Equilibrium solid chemical compound. 4: NaOH melt. 5: Since H2O formation increases the solubility of Na2WO4 as will be Glassy carbon (GC) crucible. 6: Ar gas inlet. 7: Water bath in a described later, a significant amount of WO3 could be dissolved for a mantle heater. while, even under Ar atmosphere. According to the vaporization of excess H2O and the resultant decrease in the solubility of Na2WO4 P H2O value was assumed to be same as that of the water vapor in the melt, a solid phase precipitated. As shown in Fig. 2, the pressure at the temperature of the water bath. Then, dry Ar gas was obtained solid phase exhibited some diffraction peaks that were introduced into the reaction chamber in order to precipitate the solid coincident with those of anhydrous Na2WO4. In addition, no phase. The solid phase thus obtained was washed with ethanol to diffraction peaks other than those attributed to NaOH were observed dissolve the solidified NaOH melt and was subjected to X-ray in the sample before being washed with ethanol. These data diffraction (XRD) analysis. indicated that the solid-phase chemical compound in equilibrium Since the solid phase was confirmed to be Na2WO4 anhydrate, with the Na2WO4-saturated NaOH melt was anhydrous Na2WO4. we tackled its synthesis for the subsequent experiments from commercial sodium tungstate dihydrate (Na2WO4·2H2O; ChemPur) 3.2 Solubility of Na2WO4 in the NaOH–Na2WO4–H2O system by vacuum drying at 473 K for more than 24 h. The general The main constituents of the current system were NaOH and experimental procedure was as follows. As mentioned in the Na2WO4. In addition, a small amount of H2O coexisted in the melt. previous experiment, a certain amount of NaOH and an excess However, the H2O content was supposed to be 3 wt% or less amount of Na2WO4 were set in a GC crucible and fused at a given according to our preliminary experiment in a pure NaOH melt.
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