Direct Production of Na₂WO₄-Based Salt by Scheelite Smelting

Abstract

Tungsten is one of the critical materials with important applications in many areas. Electrolysis of Na₂WO₄-based salt is a short and green process for the production of tungsten metal and alloys. The conventional process for producing Na₂WO₄ is expensive and time-consuming. Scheelite (CaWO₄) is becoming the most important resource for the extraction of tungsten. Based on thermodynamic calculations and phase equilibrium studies, a novel process is proposed to prepare Na₂WO₄-based salt directly from scheelite through a high-temperature process. By reacting with silica and sodium oxide, immiscible layers of liquid salt and slag are formed from scheelite between 1200 and 1300 °C. High-density salt containing Na₂WO₄ is separated from the silicate slag, which is composed of impurities and fluxes. The effects of fluxing agents, smelting temperature, and reaction time on the direct yield of WO₃ and purity of sodium tungsten are investigated in combination with thermodynamic calculations and high-temperature experiments. The salt containing up to 99% Na₂WO₄ is obtained directly in a single process, which can be used for the production of other tungsten chemicals. This study provides a novel research method and detailed information to produce low-cost sodium tungstate directly from scheelite.

1. Introduction

Tungsten is considered a critical raw material in many countries due to its important applications in hard alloys, military equipment, industrial catalysis, and the aerospace, nuclear, energy, and information technology industries [1,2,3,4]. Scheelite (CaWO₄) and wolframite ((Fe,Mn)WO₄) are the most important W-containing minerals for the production of tungsten [5,6,7,8]. In the conventional process, tungsten is leached from concentrates of scheelite, wolframite, or their mixtures using an aqueous solution of sodium carbonate or sodium hydroxide to obtain a W-containing solution. The impurities in the solution are removed, and pure ammonium paratungstate (APT), (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O, is obtained as an intermediate compound, which can be used to produce final products such as metallic tungsten and tungsten carbide [5,6,7,8]. Leaching tungsten from its minerals requires an autoclaving process at pressures between 1.2 and 2.6 atm and temperatures around 200 °C. Great excesses of reagents are consumed, which are difficult to recover. Over 25 tons of high-salinity wastewater is generated per ton of APT product in the process, which incurs high investment and operation costs [5,9].

Different approaches have been suggested to overcome the problems associated with the conventional soda leaching process. Orefice et al. [10] used a solvometallurgical solution of 2 mol L⁻¹ HCl_aq in ethylene glycol to extract tungsten from scheelite, and over 95% recovery was achieved for low-grade minerals. Xu and Zhao [11] used Na₂O and SiO₂ to react with wolframite at 1050–1200 °C for 60 min. Over 99% of tungsten was recovered by leaching the resulting materials. Although different decomposition agents and conditions were applied, APT was still the intermediate product to be used for further processing. A green and short process was proposed to produce tungsten metal and alloys without APT [12,13,14,15,16,17]. The first step was to prepare tungstate-containing salt at temperatures above 1100 °C. The salt was then used to produce tungsten metal or alloys by reduction or electrolysis at high temperatures [14,15,16,17]. In these studies, Na₂CO₃, NaCl, and SiO₂ were used to react with tungsten minerals. Two immiscible liquids were formed: low-density salt containing Na₂O, NaCl, and WO₃ and slag containing Na₂O, SiO₂, and impurities from tungsten minerals. The WO₃ content in the salt is approximately 30%, which has limited applications due to high Na₂O and NaCl contents.

A phase equilibrium study on a Na₂O-WO₃ system shows that a series of sodium tungstate Na₂WO₄, Na₂W₂O₇, Na₂W₄O₁₃, and Na₂W₆O₁₉ is present containing WO₃ from 79 to 95.8 wt% [18]. Sodium tungstate has been used in many areas such as coated electrodes for electrocatalysis and as a fire retardant for fabrics [19], energy storage [20], and raw materials for other tungsten products [21,22]. However, the synthesis of sodium tungstate from APT is a long process with high cost. Recent studies [23,24] indicate that sodium tungstate Na₂WO₄ can be formed at high temperatures when both Na₂O and SiO₂ are present. Low-density silicate-based slag is immiscible with the high-density sodium tungstate. High-purity sodium tungstate containing up to 79.8 wt% WO₃ was obtained by smelting wolframite at 1100–1300 °C [24]. The wolframite ore, which is easily concentrated and decomposed, is gradually declining and scheelite is becoming the dominant tungsten resource [25,26]. The different composition of scheelite compared with wolframite requires different decomposition conditions and results in a different slag composition. The present study focuses on a fundamental investigation of the smelting of scheelite concentrate in air. FactSage 8.4 [27] was used to predict the reactions between the scheelite and decomposition reagents. The effects of Na₂CO₃, SiO₂, temperature, and reaction time on the distribution of the elements between salt and slag were investigated by high-temperature experiments.

2. Materials and Methods

The scheelite concentrate used in the present study was supplied by Chongyi Zhangyuan Tungsten Co., Ltd (Ganzhou, China). and the composition is given in

Table 1. Composition of scheelite used in this study, measured by XRF (wt%).

WO3	CaO	Fe2O3	MnO	SO3	As2O3	SiO2
71.2	24.1	0.5	0.2	0.7	0.2	3.1

. The size of the scheelite concentrate was smaller than 0.075 mm. Analytical-grade sodium carbonate and silicon dioxide were employed as decomposition and flux reagents for the smelting of scheelite. In addition to CaO and WO₃, small amounts of Fe₂O₃, MnO, SiO₂, sulphur, and arsenic are also present in the scheelite. Sulphur and arsenic are presented as SO₃ and As₂O₃ for presentation purposes.

The flow chart of the scheelite smelting experiment is shown in Figure 1. As per the experimental design, the required masses of scheelite, sodium carbonate, and silica were weighed and thoroughly mixed in an agate mortar. Following uniform mixing, the mixture was pelletized using a tableting press and placed in an alumina crucible (inner diameter 30 mm, height 60 mm). The crucible was then placed in a muffle furnace under an air atmosphere. The furnace was heated at a rate of 10 °C/min to the predetermined temperature and held isothermally for a specified duration. Upon reaction completion, the crucible was cooled in the furnace and cracked to obtain the samples. The silicate slag and the salt were carefully separated and ground for compositional analysis. The compositions of the silicate slag and the salt were analyzed by a PANalytical Axios X-ray fluorescence spectrometer (PANalytical B.V., Almelo, The Netherlands) where the samples were prepared by the fusion bead method.

FactSage 8.4 was used to predict possible reactions as a base for the experimental plan. The databases of “FactPS”, “FToxid”, and “FTsalt” were selected in the “Equilib” module. The solution phases selected in the calculations included “FToxide-SLAGB”, “FToxide-SPINC”, “FToxide-MeO_A”, “FToxide-Mel_A”, “FToxide-OlivA”, and “FToxid-NAShA”.

3. Results and Discussion

Thermodynamic calculations by FactSage show that, at high temperatures, WO₃ from scheelite reacts with Na₂O from Na₂CO₃ to form liquid Na₂WO₄ (melting point 698 °C [18]), while CaO in scheelite combines with SiO₂ and Na₂O to form a molten slag. The immiscible salt and slag both remain in a liquid state and separate into two layers due to their density difference. The density of Na₂WO₄ is 4.18 g/cm³ [27] and the densities of the slags are 2.42–2.58 g/cm³, estimated from the partial molar density of individual oxides. The lower-density slag floats on top of the higher-density salt. The Na₂O from the thermal decomposition of Na₂CO₃ serves two functions in the smelting process. On one hand, Na₂O reacts with WO₃ to form Na₂WO₄. On the other hand, Na₂O dissolves into the slag to decrease its liquidus temperature.

3.1. Thermodynamic Calculations for High Temperature Smelting

Based on the chemical composition of the scheelite shown in Table 1, thermodynamic calculations were performed using FactSage 8.4 to predict the phase changes with varying experimental parameters. The starting material is 100 g of scheelite (Ore) with different amounts of Na₂O and SiO₂. Additions of Na₂O or SiO₂ are represented as Na₂O/Ore or SiO₂/Ore, respectively. In the results of the FactSage calculations, most of the sulphur is present in the “Slag-liq” phase as “Na₂SO₄” and most of the arsenic is present as a compound “Na₃AsO₄_solid”. Tungsten is not included in the “Slag-liq” phase of the current database. All tungsten is present as compounds including liquid “Na₂WO₄” or solid “MWO₄” where M = Ca, Fe, or Mn. Na₃AsO₄ is assumed to behave as a salt like Na₂WO₄. In the following sections associated with the FactSage calculations, the weight of “Salt” includes both Na₂WO₄ and Na₃AsO₄.

Figure 2 shows the weights of the phases as a function of Na₂O/Ore ratio at 1200 °C and SiO₂/Ore = 0.3. With increasing Na₂O/Ore, CaWO₄ decomposes continuously. CaO goes to slag and WO₃ goes to salt. The weight of CaWO₄ decreases rapidly and the weight of salt increases rapidly with increasing Na₂O/Ore. When Na₂O/Ore reaches approximately 0.34, the CaWO₄ is completely decomposed, and the salt reaches a maximum. All excess Na₂O dissolves into liquid slag, resulting in a continuous increase in slag weight. According to the thermodynamic calculations, 34 g of Na₂O is sufficient to decompose the scheelite at 1200 °C and a SiO₂/Ore ratio of 0.3. When temperature and SiO₂/Ore ratio vary, the minimum Na₂O required to decompose the scheelite will also change.

SiO₂ is a key slag-forming component which absorbs the CaO from the decomposition of scheelite. Na₂O is a necessary component in the slag to lower the liquidus temperature. As shown in Figure 3, the weight of salt remains constant when the SiO₂/Ore ratio is between 0.1 and 0.33. The weight of slag increases with increasing SiO₂/Ore ratio as all SiO₂ is present in the slag. However, when the SiO₂/Ore ratio exceeds 0.33, the CaWO₄ starts to be stable and the weight of salt decreases with increasing SiO₂/Ore ratio. The reason is that SiO₂ and WO₃ always have competitive reactions with Na₂O. The amount of Na₂O is fixed in the reaction system, and excess SiO₂ reacting with Na₂O impairs scheelite decomposition. It can be seen from Figure 3 that when the SiO₂/Ore ratio is above 0.33, the weight of salt decreases and the weight of CaWO₄ increases with increasing SiO₂/Ore. 0.33 is the maximum SiO₂/Ore ratio in the present conditions to ensure the complete decomposition of scheelite.

The salt is always a liquid due to its low melting point. Complete separation of the slag from the salt requires a liquid slag as the solid phase in the slag can increase the viscosity significantly. Figure 4 shows that the proportion of liquid phase in the slag increases with SiO₂/Ore and reaches a maximum of 99 wt% at a SiO₂/Ore ratio of 0.21. Solid MeO (solid solution of CaO with small amounts of Fe₂O₃, Mn₂O₃, and Na₂O) is always present in the slag as a primary phase. The results shown in Figure 3 and Figure 4 identify an optimum SiO₂/Ore ratio between 0.21 and 0.33 to ensure a liquid slag and avoid the formation of CaWO₄.

3.2. Experimental Results

Based on the thermodynamic predictions and preliminary experiments, the detailed experimental plan is shown in

Table 2. Experimental conditions for scheelite smelting in air.

Number	Ore (g)	Temp (°C)	Time (min)	Na2CO3 (g)	SiO2 (g)	Na2O/Ore
C1	10	1200	60	10	6	0.58
C2	10	1200	60	10	9	0.58
C3	10	1200	60	8	6	0.47
C4	10	1200	60	12	6	0.70
C5	10	1200	30	10	6	0.58
C6	10	1200	120	10	6	0.58
C7	10	1300	60	10	6	0.58
C8	10	1250	60	10	6	0.58
C9	10	1200	60	10	7.5	0.58
C10	10	1200	60	6	6	0.35
C11	10	1200	60	4	6	0.23
C12	10	1200	60	10	10.5	0.58
C13	10	1200	60	10	12	0.58

, including variable parameters such as the weights of scheelite (ore), Na₂CO₃, and SiO₂, in addition to temperature, and reaction time. Na₂O/Ore ratio is also presented in the table for easy calculations as CO₂ will leave the condensed system at high temperatures. According to the conditions set in Table 2, the effect of each variable parameter is discussed when other variables are fixed. For example, the effect of Na₂CO₃ addition is discussed at 1200 °C for 60 min with 6 g SiO₂ addition (C1, C3, C4, C10, and C11). It can be seen from the table that the Na₂O/Ore and SiO₂/Ore ratios used in the experiments are all higher than those predicted by Factsage 8.4 shown in Figure 2, Figure 3 and Figure 4. The preliminary experiments show that more Na₂O and SiO₂ is required to form two clear layers of liquid slag and salt. The current databases in FactSage 8.4 cannot accurately predict the actual reactions in WO₃-containing systems.

After high-temperature smelting, it is observed that in all experiments, slag and salt were clearly separated into two layers. Figure 5 shows typical macroscopic photographs of the samples after high-temperature smelting. Low-density slag on the top has a high viscosity where the liquid has been converted to glass. The slag and salt show a clear boundary, which makes it easy to distinguish between the two phases. No significant reactions are observed between the sample and the alumina crucible. The amount of salt appears to be the same in C10 and C12 samples. More slag is present in C12 than in C10, and the different colors between the slags indicate their difference in composition.

The compositions of the slag and salt analyzed by XRF are listed in

Table 3. Weight and composition (wt%) of slag.

Exp. No.	Weight (g)	WO3	CaO	Fe2O3	MnO	SO3	As2O3	SiO2	Na2O
C1	14.4	13.6	14.0	0.4	0.1	0.0	0.2	43.3	28.4
C2	16.4	6.7	12.4	0.5	0.1	0.0	0.2	56.1	24.0
C3	13.4	12.6	15.2	0.5	0.1	0.0	0.2	47.0	24.4
C4	15.6	14.9	13.2	0.5	0.1	0.0	0.2	39.3	31.8
C5	14.4	14.1	14.1	0.5	0.1	0.0	0.2	42.7	28.3
C6	14.1	11.3	14.8	0.5	0.1	0.0	0.2	44.5	28.6
C7	13.9	10.8	14.9	0.5	0.1	0.0	0.2	45.3	28.2
C8	14.0	10.9	14.8	0.5	0.1	0.0	0.2	44.9	28.6
C9	15.2	9.0	13.7	0.5	0.1	0.0	0.2	50.7	25.8
C10	11.1	8.1	16.7	0.6	0.1	0.0	0.3	56.6	17.6
C11	10.1	9.0	15.7	0.7	0.1	0.0	0.3	62.4	11.8
C12	18.5	6.4	11.1	0.4	0.1	0.0	0.2	58.3	23.5
C13	20.2	6.5	10.2	0.4	0.1	0.0	0.2	60.7	21.9

and

Table 4. Weight and composition (wt%) of salt and direct yield of WO₃.

Exp. No.	Weight (g)	WO3	CaO	Fe2O3	MnO	SO3	As2O3	SiO2	Na2O	Direct Yield of WO3 (%)
C1	7.4	74.5	0.6	0.0	0.0	0.5	0.0	1.0	23.4	77.9
C2	8.5	76.2	0.6	0.0	0.0	0.5	0.0	1.4	21.3	90.6
C3	7.3	76.8	0.6	0.0	0.0	0.6	0.0	0.3	21.7	78.7
C4	7.5	72.8	0.7	0.0	0.0	0.5	0.0	2.6	23.4	76.2
C5	7.4	74.0	0.5	0.0	0.0	0.5	0.0	1.9	23.1	76.9
C6	7.8	75.3	0.2	0.0	0.0	0.5	0.0	0.6	23.4	82.2
C7	8.0	75.5	0.2	0.0	0.0	0.6	0.0	0.2	23.5	84.3
C8	7.9	75.3	0.2	0.0	0.0	0.6	0.0	0.4	23.5	83.2
C9	8.2	75.9	0.4	0.0	0.0	0.5	0.0	1.4	21.8	87.1
C10	8.4	77.4	2.8	0.0	0.0	0.5	0.0	0.4	18.9	91.5
C11	8.3	78.6	6.2	0.0	0.0	0.5	0.0	0.3	14.4	91.3
C12	7.8	76.4	0.7	0.4	0.0	0.5	0.0	0.3	21.7	84.2
C13	7.6	76.9	0.9	0.0	0.0	0.5	0.0	0.4	21.3	82.3

, respectively. The weights of the slag and salt were determined by mass balance. The direct yield of WO₃ collected in salt is calculated by the following formula:

$$\mathrm{D}\mathrm{i}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{t} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{o}\mathrm{f} {\mathrm{W}\mathrm{O}}_3={\displaystyle \frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{W}\mathrm{O}}_3 \mathrm{i}\mathrm{n} \mathrm{s}\mathrm{a}\mathrm{l}\mathrm{t}}{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{W}\mathrm{O}}_3 \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{u}\mathrm{n}\mathrm{g}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{n} \mathrm{o}\mathrm{r}\mathrm{e}}}\times 100\%$$

(1)

It can be seen from the tables that CaO, Na₂O, SiO₂, and WO₃ are the major components of the slag, and Na₂O and WO₃ are the major components of the salt. All iron, manganese, and arsenic are present in the slag, and all sulphur is present in the salt. WO₃ content in the salt is in the range of 72.8–78.6 wt%, which is much higher than that (≈30% WO₃) in the tungsten salts extracted by Na₂CO₃, NaCl, and SiO₂ [12,13,14,15,16,17]. Small amounts of CaO and SiO₂ are also present in the salt. Direct yield of WO₃ and salt composition as a function of experimental variables are discussed in the following sections. It can be seen from Table 3 that 6.7–14.9 wt% WO₃ is present in the slag. The direct yield of WO₃ calculated according to Equation (1) is up to 91.5%. Direct yield of WO₃ is not the only measure for optimum conditions. It can be seen from Table 4 that the direct yields of WO₃ are 91.5 and 91.3 in C10 and C11, respectively. However, CaO contents are 2.8 and 6.2 wt% in C10 and C11, respectively. The purity of Na₂WO₄ is significantly reduced in the presence of CaO. If the purity of Na₂WO₄ is the target, C7 seems to be an optimum condition where CaO and SiO₂ contents are low (0.2 wt%) and the direct yield of WO₃ is reasonable (84.3%).

Water leaching the smelting slag at 90 °C can reduce the WO₃ content in the residue as low as 0.5 wt%. The solution containing WO₃ can be treated in a traditional process to recover tungsten. The overall recovery of tungsten in both salt and solution is above 99%.

In the current database of FactSage 8.4, the FToxide-SLAGB phase does not include WO₃. It can be seen from Table 3 that 6.4–14.9 wt% WO₃ is present in the slag. On the other hand, the sodium tungstate is a Na₂WO₄ compound with a constant composition. Table 4 shows that the Na₂O/WO₃ ratio varies in salt and other oxides such as CaO, SiO₂, and SO₃ are also present. The different compositions of slag and salt between FactSage predictions and measurements can explain why the experimental results to be discussed below may not be the same as the calculations. The FactSage database needs to be further optimized using accurate experimental data.

3.2.1. Effect of Na₂O/Ore Mass Ratio on Direct Yield of WO₃ and Salt Composition

Figure 6 shows the effects of the Na₂O/Ore ratio on the direct yield of WO₃ and major components in salt (SiO₂/Ore mass ratio = 0.6, 1200 °C, 60 min). As shown in Figure 6a, the direct yield of WO₃ slightly increases with Na₂O/Ore below 0.35 and then decreases with increasing Na₂O/Ore. At a Na₂O/Ore ratio of 0.7, the direct yield of WO₃ decreased to 76.2%. The WO₃ content remained in the slag shows an opposite trend to the direct yield of WO₃ with increasing Na₂O/Ore.

Figure 6b shows that in the salt, Na₂O increases and WO₃ decreases with increasing Na₂O/Ore. When Na₂O/Ore increases from 0.23 to 0.7, the WO₃ content in salt decreases from 78.6 to 72.8 wt%. It was shown in Figure 2 that 100% WO₃ can be recovered theoretically when the Na₂O/Ore ratio is above 0.34. The experimental results shown in Figure 6a confirm that a maximum recovery of WO₃ can be reached at a Na₂O/Ore ratio of 0.35. However, the maximum recovery of WO₃ is only 91.5% due to the loss of soluble WO₃ in slag. Different from FactSage predictions, an increase in Na₂O/Ore ratio above 0.35 decreases the direct yield of WO₃ as a result of decreased WO₃ content in the salt.

CaO and SiO₂ contents in the salt are shown in Figure 7a as a function of Na₂O/Ore ratio. At low Na₂O/Ore ratios of 0.23 and 0.35, up to 6.2 wt% CaO is present in the salt. The salt is composed of both Na₂WO₄ and CaWO₄. WO₃ in CaWO₄ (80.5 wt%) is higher than that in Na₂WO₄ (78.9 wt%). A decrease in WO₃ in salt with an increase in Na₂O/Ore ratio from 0.23 to 0.47 (Figure 6b) is a result of conversion from CaWO₄ to Na₂WO₄. Above a Na₂O/Ore ratio of 0.47, SiO₂ content in salt increases with increasing Na₂O/Ore ratio accompanying a decrease in WO₃ and an increase in Na₂O in the salt (Figure 6b). Figure 7b shows that the weight of salt decreases and the weight of slag increases with increasing Na₂O/Ore ratio. This indicates that more Na₂O is dissolved in the slag than in the salt. It is concluded from Figure 6 and Figure 7 that, if CaO in salt is not an issue and a high direct yield of WO₃ is important, a Na₂O/Ore ratio of 0.23 is sufficient, which also reduces the cost of the reagent.

An ideal reaction to describe the decomposition of scheelite by sodium oxide in the presence of silica can be shown as the following equation, which is the case predicted by FactSage.

CaWO₄ + Na₂O + SiO₂ → Na₂WO₄ (salt) + CaO-Na₂O-SiO₂ (slag)

(2)

However, it is seen from Figure 6 and Figure 7 that not only WO₃ is present in the slag, but also CaO and SiO₂ are present in the salt. Both products, salt and slag, are different between the experiments and predictions. The actual chemical reaction can be represented by the following equation.

CaWO₄ + Na₂O + SiO₂ → Na₂O-WO₃-CaO-SiO₂ (salt) + CaO-Na₂O-SiO₂-WO₃ (slag)

(3)

When the amounts of reactants CaWO₄ and SiO₂ are constant, excess Na₂O moves CaO and WO₃ from salt to slag. More Na₂O enters the slag than the salt, resulting in an increased slag/salt weight ratio.

3.2.2. Effect of SiO₂/Ore Mass Ratio on Direct Yield of WO₃ and Salt Composition

Figure 8 shows the effect of SiO₂/Ore mass ratio on the direct yield of WO₃ and major components in salt at a fixed Na₂O/Ore mass ratio of 0.58, at 1200 °C for 60 min. As shown in Figure 6a, as the SiO₂/Ore ratio increases, the direct yield of WO₃ first increases rapidly and then decreases slowly. It is shown in Figure 6a that the WO₃ content in slag shows an opposite trend to the direct yield of WO₃ with increasing Na₂O/Ore, which is not the case, as shown in Figure 8a. The reasons can be explained by Figure 8b and Figure 9b. It can be seen from Figure 8b that the WO₃ content in salt continuously increases with an increasing SiO₂/Ore ratio. Increased SiO₂ absorbs more Na₂O into the slag resulting in a lower Na₂O in the salt. Figure 9b shows that the weight of salt increases with increasing SiO₂/Ore ratio when the SiO₂/Ore ratio is in the range between 0.6 and 0.9. An increase in the direct yield of WO₃ is the result of a high WO₃ content in salt and salt weight at a SiO₂/Ore ratio from 0.6 to 0.9. Different from the expectation, it can be seen from Figure 9a that increasing SiO₂ addition does not always increase the SiO₂ content in salt. After reaching a maximum of 1.4 wt%, the SiO₂ content in salt decreases to 0.3 wt% when the SiO₂/Ore ratio increases from 0.75 to 1.05. A SiO₂/Ore ratio of 0.9 seems to be an optimum value where the direct yield of WO₃ and WO₃ content in salt are around 90% and 76 wt%, respectively. The CaO content in salt, shown in Figure 9a, varies in a small range between 0.4 and 0.9 wt%. As predicted in Figure 3, increased SiO₂ results in the formation of CaWO₄, which dissolves in the salt, raising the CaO content.

FactSage predictions show in Figure 3 and Figure 4 that, at 1200 °C and a Na₂O/Ore mass ratio of 0.35, all tungsten can be recovered in salt when the SiO₂/Ore ratio is in the range of 0.21 to 33. However, experimental results shown in Figure 8a demonstrate that a maximum tungsten recovery of 90.6% is obtained at a SiO₂/Ore ratio of 0.9, which is much higher than that predicted by FactSage. This indicates that FactSage can predict certain reaction trends to support the experimental plan. Reliable predictions by the thermodynamic model require an advanced database optimized by accurate experimental data.

3.2.3. Effect of Temperature on Direct Yield of WO₃ and Salt Composition

Figure 10 shows the effects of temperature on the direct yield of WO₃ and major components in salt (Na₂O/Ore mass ratio = 0.58, SiO₂/Ore mass ratio = 0.6, reaction time = 60 min). Increasing the temperature from 1200 °C to 1250 °C significantly increases the direct yield of WO₃ from 77.9 to 83.2%. Further increase in temperature to 1300 °C only slightly increases the direct yield of WO₃ from 83.2% to 84.3%. On the other hand, both WO₃ and Na₂O contents in salt increase with increasing temperature, but the increment in WO₃ is more significant than Na₂O. Figure 11 shows that SiO₂ and CaO contents in salt decrease with increasing temperature. The weight of salt increases slightly with increasing temperature, as shown in Figure 11b. Considering the direct yield of WO₃, the purity of sodium tungstate, and energy consumption, 1250 °C seems to be an optimum temperature for economic purposes.

3.2.4. Effect of Reaction Time on Direct Yield of WO₃ and Salt Composition

The effect of reaction time on the direct yield of WO₃ and major components in salt is shown in Figure 12, where Na₂O/Ore mass ratio = 0.58, SiO₂/Ore mass ratio = 0.6, reaction temperature = 1200 °C. The direct yield of WO₃ and WO₃ content in salt increase continuously with increasing reaction time. Extending the reaction time from 30 min to 120 min increases the direct yield of WO₃ from 76.9% to 82.2% and raises the WO₃ content in salt from 74% to 75.2%. Appropriately extending the reaction time facilitates the separation of slag from salt. The impurities, CaO and SiO₂ contents in salt, show a decreasing trend with increasing reaction time, as shown in Figure 13a. A slight increase in salt weight is also observed with increasing reaction time, as shown in Figure 13b. Overall, a 120 min reaction time at 1200 °C seems to be necessary to achieve a high direct yield of WO₃ and a high WO₃ content in salt. At high temperatures such as 1250 °C, a short reaction time could be sufficient, which needs to be confirmed in the future.

4. Conclusions

High-temperature experiments for extracting tungsten from scheelite were conducted in air to systematically investigate the effects of Na₂O/Ore mass ratio, SiO₂/Ore mass ratio, reaction temperature, and reaction time on the direct yield of WO₃ and the WO₃ content in salt. FactSage 8.4 was used to predict high-temperature reactions, which assisted the experimental plan.

In the scheelite smelting process, Na₂O and SiO₂ were used to decompose scheelite and produce Na₂WO₄-based salt directly. WO₃ content in the salt is in the range of 72.8–78.6 wt%, which corresponds to 92.3%–99.6% Na₂WO₄. Iron, manganese, and arsenic from scheelite are all reported to be present in the slag, together with CaO, Na₂O, and SiO₂. All sulphur and small amounts of CaO and SiO₂ are present in salt. A minimum Na₂O/Ore was required to fully decompose CaWO₄. However, excess Na₂O decreased the direct yield of WO₃ and WO₃ content in salt. An optimum SiO₂/Ore range of 0.9 was determined to obtain 90% direct yield of WO₃ and 76.2 wt% WO₃ in salt. Both the direct yield of WO₃ in salt and the WO₃ content in salt increase with increasing temperature. Extending the reaction time can increase the direct yield of WO₃ and WO₃ content in salt.

Comprehensively considering the direct yield and purity of sodium tungstate, and the energy consumption and productivity, the optimal conditions for scheelite smelting are recommended to be a Na₂O/Ore mass ratio of 0.3–0.4, a SiO₂/Ore mass ratio of 0.9, 1250 °C, and 120 min. This study provides a novel research method and detailed information for the production of low-cost sodium tungstate directly from scheelite.