Oxidative Dissolution of Cemented Tungsten Carbides in Molten Sodium Carbonate by Addition of Copper(I) Oxide as Oxidizing Agent for Tungsten Recycling

Due to the monopolized supply of tungsten resource, it is important to efficiently recycle tungsten scrap for use as a secondary resource. The recycling of tungsten from cemented carbide tools by the molten carbonate method was investigated using simulated hard and soft scrap (carbide tool tips and WC powder, respectively). The oxidative dissolution of tungsten was examined in molten Na2CO3 under Ar–O2–CO2 atmospheres at 1173 K. Based on the immersion potentials of Cu, W, Co, C, and WC–Co, Cu2O was suggested to work as an oxidizing agent for tungsten dissolution. The oxidative dissolution rate for carbide tool tips with 12.8 mol% Cu2O addition reached 57 mg h−1 for the reaction time of 2.5 h, equivalent to 0.32 mm h−1. The decrease in the dissolution rate after 2.5 h was attributed to the decrease in the Cu(I) ion concentration in the melt and the inhibition of ion diffusion by the deposited metallic Cu. No violent reaction leading to explosion was observed, even for the oxidative dissolution of fine WC powder with a large surface area. Thus, this method provides significant safety improvements compared to the molten nitrate method.


Introduction
Tungsten is one of the critical metals with high hardness, wear resistance, and thermal resistance. It is widely used in high-speed steel, heat-resistant steel, cemented carbide tools, and processed materials (e.g., plates, wires, and bars) for lighting and electronic components. Cemented carbide tools composed of tungsten carbide (WC) particles and cobalt metal binder are utilized as cutting and abrasive tools in The contributing editor for this article was Hongmin Zhu. diverse industrial fields, including automobile, aircraft, and civil engineering. The application for cemented carbide tools accounted for 65% in 2016 of the tungsten consumptions in the world, and 77% in 2019 in Japan [1,2].
In terms of global supply, China is the major supplier of tungsten resources, with 51% of the world's tungsten reserves and 84% of global tungsten mining production in 2021 [3]. This monopolized situation would lead to unstable supply of tungsten resources in the event of mining accidents as well as financial and political circumstances. Tungsten is designated as a critical metal, at least in some regions including the EU, the U.S., and Japan. To prepare for supply difficulties, it is important to secure resources and achieve secondary use of tungsten scrap. However, the recycling rate of tungsten scrap in Japan was only 9.0% in 2019 [2]. Therefore, it is important to efficiently recycle cemented carbide scrap and use it as a secondary resource. Waste from cemented carbide tools is classified as hard or soft scrap. Hard scrap consists of relatively large pieces of cemented carbide, such as spent drill bits and tips from cutting and abrasive tools. Soft scrap is a fine powder generated during the powder molding process of these tools.
The commercially operated recycling methods for cemented carbide tool scrap are classified into direct and indirect [1,4,5]. The direct method gives a separation by either physical or chemical treatment, or a combination of both. One of the typical processes is zinc alloying method, in which WC particles are recovered by crushing cemented carbide scrap after the alloying/dealloying reaction of metallic Zn and Co, which has a difficulty of composition adjustment. Indirect method such as thermal oxidation method and molten nitrate method [6,7] is appropriate for strict purity control. Various pyrometallurgical and hydrometallurgical processes have been reported as laboratory-level experiments. Typical processes are oxidative roasting and carbothermic reduction at high temperature [8,9] and acid leaching and electrolysis methods near room temperature [10,11]. In the thermal oxidation method, cemented carbide scrap is calcined in air at high temperature, then the surface oxide layer is leached in an alkali solution by a treatment known as "peeling." The surface layer formed in a thermal oxidation contains CoWO 4 in addition to WO 3 [12], and the oxidation stops for large scraps owing to the whole coverage with CoWO 4 layer [6,7]. Then, the repeated calcination and leaching steps are necessary for large scraps. In the molten nitrate method, cemented carbide scrap is oxidatively dissolved in molten NaNO 3 at 973-1173 K using the oxidizing power of nitrate [6,7,13,14]. The resulting Na 2 WO 4 is then purified by hydrometallurgical processing. However, the reaction in molten nitrate is strong and fast with vigorous gas evolution, which poses a risk of explosions, especially for the treatment of soft scrap owing to its large specific surface area. Other molten salt methods using electrolysis in hydroxide [15,16] and dissolution in mixture of sulfates and hydroxides [17] have also been reported for recycling tungsten materials, which were summarized in [15].
Recently, we proposed a molten carbonate method for recycling tungsten from hard cemented tool scrap [18]. This method involves the oxidative dissolution of cemented carbide scrap in molten Na 2 CO 3 under an Ar-O 2 -CO 2 atmosphere. The resulting Na 2 WO 4 is then treated using the same wet processing and hydrogen reduction of WO 3 as in the molten nitrate method. The reaction is easy to control by changing the partial pressures of O 2 and CO 2 , which are the parameters for oxidation and basicity, respectively. Importantly, this provides significant safety improvements compared to the molten nitrate method. In our previous study [18], tungsten metal was selected as a model sample for the fundamental study of this process, and the thermodynamics and kinetics of the oxidative dissolution of tungsten metal into molten Na 2 CO 3 at 1173 K were investigated. Oxidative dissolution and Na 2 WO 4 formation were confirmed in an Ar-O 2 -CO 2 atmosphere at various partial pressures of CO 2 (pCO 2 ), and the results verified that oxidative dissolution proceeds via two types of oxidation mechanisms involving O 2 2− /O 2 − and CO 3 2− ions as oxidizing agents. In the present study, we propose a revised recycling process of tungsten from cemented tool scrap using metal ions as a mediator for oxidative dissolution. We explore the use of oxidizing agents for tungsten dissolution with reference to the principles of oxidizing agents used in aqueous solutions. Table 1 compares the characteristics of various oxidizing agents in molten Na 2 CO 3 with those in aqueous solutions. In aqueous solutions, physically-dissolved oxygen gas is one of the typical oxidizing agents such as for metal corrosion. While chemically-dissolved and physically-dissolved species have different dissolution forms, the O 2 2− /O 2 − ions in molten Na 2 CO 3 belong to this category because they are oxygen species chemically-dissolved by reaction with O 2− ions. In addition, H + ions, which are the constituent ions of the electrolyte in aqueous solution, also serve as oxidizing agents such as in the dissolution of the Al and Zn metals in acids. The CO 3 2− ions in molten Na 2 CO 3 are oxidizing agents in this classification. However, these oxidizing agents lack either oxidizing power or solubility as indicated in the table. Therefore, metal oxides (MO x ) were considered as candidates of oxidizing agents with improved oxidizing power and solubility, with reference to the use of oxidizing agents such as KMnO 4 in aqueous solution systems. In this method, the oxidation power and solubility can be controlled by changing the metal oxide and gas atmosphere.
The flowchart of the proposed process is shown in Fig. 1a, and the principles are schematically shown in Fig. 1 (b). Hard tool scraps are added into molten Na 2 CO 3 with an addition of MO x as oxidizing agents. The added MO x is dissolved in molten carbonate to form M n ions (Reactions (1) and (2)).
Here, M n denotes ions of metal M with oxidation state n, that is, cation M n+ or complex oxoanion M a O b (2b−an)− .
The dissolved M n ions function as an oxidizing agent by their reduction to M metal or M m ions (where n > m) with a lower oxidation state. Tungsten in the carbide tool is therefore oxidized and dissolved in the melt as WO 4 2− ions. Carbon in the carbide tool can be also oxidized and dissolved depending on the oxidizing power of the agent.
The formed M metal and M m ions are oxidized by oxygen gas to be recycled into M n ions.
Here, the concentration of O 2− ions can be adjusted based on the partial pressure of CO 2 in the system in equilibrium with CO 3 2− ions.
Thus, a constant O 2− concentration can be maintained, regardless of the formation or consumption of O 2− ions during the reaction. In total, M n ions are not consumed and work as a mediator of the oxidative dissolution since oxygen gas is the oxidizing agent. The obtained salt is dissolved into water to recover the tungsten component as ammonium paratungstate (APT, (NH 4 ) 10 In this study, we selected Cu 2 O as an example of oxidizing agent and experimentally verified the concept of the function of an oxidizing agent for the oxidative dissolution of simulated scrap (WC-Co cemented carbide tips and WC powder) using the molten carbonate method at 1173 K. Previously, Na 2 CO 3 slag has been widely investigated for copper smelting to eliminate impurities in crude copper [20][21][22][23][24][25][26]. However, to the best of our knowledge, there are no reports on the utilization of Cu 2 O as an oxidizing agent in molten Na 2 CO 3 . Therefore, the first contribution of this study is to present the potential of Cu 2 O as an oxidizing  agent in molten Na 2 CO 3 . High oxidizing power and high solubility in molten carbonate are both necessary for Cu 2 O to function as an oxidizing agent. We investigated the oxidizing power based on the immersion potential of Cu metal and discussed its solubility based on literature. The second contribution of this study is to demonstrate the feasibility of the oxidative dissolution of tungsten using Cu 2 O as an oxidizing agent based on experiments with simulated hard and soft scrap (cemented carbide tips and WC powder, respectively). Finally, the rate and characteristics of the reaction system are evaluated.

Measurement of Immersion Potential in Na 2 CO 3 Molten Salt
Na 2 CO 3 (Fujifilm Wako Pure Chemical Corp., 99.8%, 300 g) was placed in an alumina crucible (As One Corp., > 99%, outer diameter: 90 mm, inner diameter: 80 mm, height: 140 mm) and dried under vacuum at 453 K for more than 12 h to remove moisture. The crucible was placed at the bottom of a quartz vessel in an airtight quartz container and heated in a vertical furnace. The flow rate of the mixed Ar-O 2 -CO 2 gas was controlled using a mass flow controller (Horiba STEC Co. Ltd., SEC-E40 or PE-D20). The flow rate was fixed to 50 mL min −1 as sccm (standard cubic centimeter per minute) and the partial pressure was controlled by each flow rate; a CO 2 partial pressure of 6 × 10 −4 atm (60 Pa) was attained at a flow rate of 0.03 mL min −1 . The measurements were conducted in an Ar-O 2 -CO 2 atmosphere (pO 2 : 0.2 atm (2 × 10 4 Pa), pCO 2 : 6 × 10 −4 atm) at 1173 K by a three-electrode method using an electrochemical measurement system (Hokuto Denko Corp., HZ-7000). The working electrodes were W plate (Nilaco Corp., 4 × 5 × 0.1 mm, 99.95%), Co plate (Nilaco Corp., 4 × 5 × 0.1 mm, 99.95%), C plate (Nilaco Corp., 4 × 5 × 2 mm, 99.5%), WC-Co tip (Big Daishowa Seiki Co., Ltd., TPGD070202FN), and Cu plate (Nilaco Corp., 4 × 5 × 0.1 mm, 99.6%). Au wire (Japan Metal Service, Ltd., 99.99%, diameter: 0.5 mm) was immersed in the molten Na 2 CO 3 as a quasi-reference electrode (QRE). Because the atmosphere and molten salt contain O 2 gas and O 2− ions, respectively, the potential of the QRE corresponds to the O 2 /O 2− equilibrium at a given O 2 partial pressure, and the activity of O 2− ions determined by the CO 2 partial pressure [18]. Figure 2 illustrates the experimental setup used for oxidative dissolution experiments. An outline of the experimental setup is described elsewhere [18]. Powdered Na 2 CO 3 (3.07 g) and Cu 2 O (0.265 g or 0.529 g, Fujifilm Wako Pure Chemical Corp., 99.5%) were placed in an alumina crucible (Nikkato Corp., SSA-S, C1, volume: 30 cm 3 , height: 24 mm) and dried overnight under vacuum at 453 K. The Na 2 CO 3 powder was weighed such that the depth of the molten salt was 6 mm. The as-purchased WC-Co tip (Big Daishowa Seiki Co., Ltd., TPGD070202FN, 480 mg, thickness: 2.5 mm) or WC powder (0.100 g, Kojundo Chemical Laboratory Co., Ltd., 99%, average particle diameter: 150 μm) was embedded into the crucible containing the powdered Na 2 CO 3 and Cu 2 O. The composition of the tip, as determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Hitachi Ltd., SPECTRO BLUE), was 88.5 wt% W-5.0 wt% Co. Others are carbon and minor additives. The temperature of the alumina reaction tube was raised from 293 to 1173 K at a rate of 5 K min −1 and maintained for a reaction time of 0-25 h. This is sufficiently above the melting temperature of Na 2 CO 3 (1131 K). The 0 h experiment was conducted by raising the temperature to 1173 K and then immediately lowering the temperature at a rate of 5 K min −1 . The flow rate was fixed at 50 mL min −1 , and the partial pressure of CO 2 was 6 × 10 −4 or 0.8 atm (60 or 8 × 10 4 Pa). The recovered tip was analyzed using X-ray diffraction (XRD, Rigaku Corp., MiniFlex, Cu Kα line, 30 kV, 10 mA). The recovered salts were crushed in a mortar and dissolved in deionized water using a sodium tartrate chelating agent to avoid the precipitation of WO 3 in acidic solution. After filtration, the solution was adjusted to pH = 1 using HNO 3 solution. The amount of tungsten dissolved in the molten carbonate was determined using ICP-AES.

Potential of Cu 2 O as an Oxidizing Agent
The oxidizing power of Cu(I) ions was estimated from the immersion potential. In the same way that the immersion potential in an aqueous solution correlates with the ionization tendency, the immersion potential of a metal in molten Na 2 CO 3 corresponds to the M n /M potential. Therefore, to proceed with the dissolution of W by using MO x as an oxidizing agent for WC-Co tips according to Reactions (3) and (4), the immersion potential of metal M should be more positive than that of W. The immersion potentials of Cu (as the metallic component of the additive); W, Co, and C (as components of the cemented carbide tip); and the WC-Co tip itself were measured in molten Na 2 CO 3 in Ar-O 2 -CO 2 (pO 2 : 0.2 atm, pCO 2 : 6 × 10 −4 atm) at 1173 K. The measured immersion potentials are listed in Table 2. While the immersion potentials for the components of the cemented carbide tip and the tip itself were in the range of − 0.6 to − 1.0 V vs. Au QRE, the value for Cu was − 0.39 V. The fact that Cu has a more positive immersion potential than W and WC in molten Na 2 CO 3 at 1173 K suggests Cu 2 O will function as an oxidizing agent. The details of the electrochemical behavior of Cu, such as cyclic voltammetry, will be reported in a separate paper.
Several researchers have studied the refining of crude copper in molten Na 2 CO 3 -based slag in different atmospheres. Takeda et al. [23] confirmed the phase separation of Cu 2 O-Na 2 O slag (> 50 wt% Cu) and molten Na 2 CO 3 with low solubility of Cu 2 O (< 1 wt% Cu) in the presence of molten Cu metal at 1523 K in an Ar atmosphere. Kojo et al. [21] and Fukuyama [27] reported that the solubility of Cu 2 O in molten Na 2 CO 3 coexisting with molten Cu in a CO 2 atmosphere was dependent on the oxygen partial pressure. Thus, Cu 2 O has low solubility in molten Na 2 CO 3 in both CO 2 and Ar atmospheres, with representative values of 1 wt% solubility of Cu at pCO 2 = 0.01 MPa and pO 2 = 0.1 Pa at 1523 K. Claes et al. [28] investigated the solubility of Cu 2 O in molten Na 2 CO 3 -K 2 CO 3 at a lower temperature (1073 K) in a pure CO 2 atmosphere, and found a value of > 10.5 g L −1 . According to these reports except for the study by Claes et al., the solubility of Cu 2 O in molten Na 2 CO 3 is expected to be smaller than 1 wt% at 1173 K. When a larger amount of Cu 2 O is added, Cu 2 O-Na 2 O liquid phase is expected to form via release of CO 2 gas in addition to the Na 2 CO 3 -Cu 2 O sat. (< 1 wt% Cu) phase (reaction (8)).
At 1173 K, the Cu 2 O-Na 2 O system exists as a liquid phase, with a eutectic point at 1076 K and 72 wt% Cu 2 O [29]. The Cu 2 O-Na 2 O liquid phase is expected to have greater density than Na 2 CO 3 -Cu 2 O, because solid Cu 2 O and solid Na 2 CO 3 (the main components of the respective systems) have densities of 6.0 and 2.53 g cm −3 , respectively, at 298 K. Therefore, the bottom side of the melt would possess high Cu 2 O solubility as Cu 2 O-Na 2 O phase.

Oxidative Dissolution of WC-Co Tips in Na 2 CO 3 -Cu 2 O Molten Salt
The dissolution experiments of WC-Co tip were conducted by a reaction with 6.4 mol% of Cu 2 O (12.8 mol% CuO 0.5 ) with respect to Na 2 CO 3 at 1173 K for 25 h under a CO 2 partial pressure of 6 × 10 −4 or 0.8 atm (denoted as low and high pCO 2 conditions, respectively). Figure 3a-c show photographs of the tips before and after the reaction. Red-brown deposits adhered to the surface of the remaining tips under each condition. Under low pCO 2 conditions (Fig. 3b), the deposits were firmly adhered to the tip. In contrast, a brittle black layer existed between the tip and deposit under high pCO 2 conditions (Fig. 3c). Moreover, the tip and deposit peeled apart easily off, as shown in Fig. 3d. The peeled black layer was analyzed by XRD (Fig. 3e). The XRD peaks were attributed to metallic Cu and Co, which confirmed the precipitation of Cu on the tip surface and the presence of residual Co from the carbide tip. In addition, sharp and broad peaks attributed to graphite and amorphous carbon, respectively, are not observed. The cross-section of the tip after the reaction under high or low pCO 2 conditions is shown schematically in Fig. 3f. Figure 4 compares the dissolved weight of W in molten Na 2 CO 3 with and without 6.4 mol% Cu 2 O, as determined by ICP-AES. At each CO 2 partial pressure, the dissolution amount drastically increased with the addition of Cu 2 O. In conjunction with the XRD results, it was confirmed that dissolved Cu(I) ions acted as oxidizing agents for W and C with Cu precipitation, which is reasonable from the immersion potentials listed in Table 2. The reason for the lack of dissolution of the Co residue, despite its more negative immersion potential than Cu, is expected to the stable oxide layer in his melt.
Comparing the results of the reactions with added Cu 2 O, the dissolution rate was higher under high pCO 2 conditions.  This was ascribed to the higher solubility of Cu 2 O in molten Na 2 CO 3 at higher pCO 2 [21,27]. Since all the Cu(I) ions were likely to be consumed before the end of the 25 h reaction time, oxidative dissolution experiments were conducted with shorter reaction time to investigate the rate of oxidative dissolution.
In these experiments, the amount of Cu 2 O was doubled to 12.8 mol% (25.6 mol% CuO 0.5 ), and the pCO 2 partial pressure was fixed at 6.0 × 10 −4 atm. The amount of W that dissolved in the absence of Cu 2 O was also measured for comparison. Figure 5 shows the amount of dissolved W with respect to the reaction time. The arrow indicates the equivalent amount of W dissolution for 12.8 mol% Cu 2 O according to Reaction (9). The plot for the reaction with 12.8 mol% of Cu 2 O shows that the reaction is mostly complete after 2.5 h, after which the extent of reaction increases gradually toward the equivalent value. The reaction rate after 2.5 h was 57 mg h −1 . Since the rate of weight loss is affected by the shape of the carbide tool, the reaction rate was evaluated as a one-dimensional value under a simplified assumption. From the initial thickness of 2.5 mm, initial weight of 480 mg, and W ratio of 88.5 wt% contained in the tip, this rate is equivalent to 0.32 mm h −1 under the assumption that the dissolution reaction only proceeded from the upper surface of the tip with a reaction area of 0.55 cm 2 , as shown in the photographs in Fig. 3.  (1) and (2)) and by stripping and removing deposited Cu (against factor (3)). It would also be effective to construct a M n /M m reaction system for the oxidizing agent, whereby the reduction products are not metal but ions with a lower oxidation state; this would prevent metal from depositing on the tip surface (factor (3)). The recycle of Cu(I) ion by O 2 gas injection to the melt and product is a future effective challenge.

Oxidative Dissolution of WC Powder in Na 2 CO 3 -Cu 2 O Molten Salt
Oxidative dissolution experiments were also conducted using WC powder to elucidate the applicability of the molten carbonate method with Cu 2 O oxidant for recycling soft scrap. The dissolution of 76.5 mg of W from a 100 mg WC sample was confirmed through the reaction in molten Na 2 CO 3 at 1173 K for 2.5 h with the addition of 12.8 mol% Cu 2 O in an Ar-CO 2 (pCO 2 : 6.0 × 10 −4 atm) atmosphere. Notably, no salt splash was observed in the crucible after the reaction (see Fig. 6). Therefore, safe and efficient processing is expected not only for hard scrap but also for soft scrap. This is particularly valuable given the risk of explosions in (10) 4Cu + O 2 → 4Cu(I) + 2O 2− the molten nitrate method owing to vigorous NO x gas evolution for samples with a high specific surface area, such as soft scrap.

Conclusions
This study investigated and experimentally verified the oxidative dissolution of cemented carbide tips and WC powder as simulated scraps by utilizing metal ions as a reaction mediator in molten Na 2 CO 3 in an Ar-CO 2 atmosphere at 1173 K. A significantly higher reaction rate was achieved compared to that in a previous study by adding Cu 2 O as an oxidizing agent to oxidize W component. Cu metal has a more positive immersion potential than W metal and WC-Co tips, providing it with high oxidizing power. During the reaction, Cu metal was deposited and W was oxidatively dissolved from the carbide tips, whereas Co metal remained as a residue. A high oxidative dissolution rate of 57 mg h −1 or 0.32 mm h −1 was achieved. The removal of deposits and establishment of a reaction system in which the reduction products become ions are desirable to improve the reaction rate. W was also oxidatively dissolved from WC powder with the Cu 2 O oxidizing agent. Recycle of Cu into Cu(I) ions by O 2 gas injection and dissolution reaction at different CO 2 partial pressure need to be investigated. In addition, although Cu 2 O was selected as the oxidizing agent to demonstrate the concept, its low solubility is undesirable. Exploration of