Extraction of Sodium Tungstate from Tungsten Ore by Pyrometallurgical Smelting

Abstract

Tungsten is one of the strategic metals produced from tungsten ores through sodium tungstate. The hydrometallurgical process is a common technology for extracting sodium tungstate from high-grade tungsten concentrates. The grade of tungsten ore is decreasing, and the mineral processing to produce a high-grade concentrate suitable for the hydrometallurgical process is becoming more difficult. It is desirable to develop a new technology to effectively recover tungsten from the complex low-grade tungsten ores. A fundamental study on the pyrometallurgical processing of wolframite was carried out through thermodynamic calculations and high-temperature experiments. The wolframite was reacted with Na₂CO₃ and SiO₂ at 1050–1200 °C and then leached with water to obtain a sodium tungstate solution as a feed for the traditional process of APT (Ammonium paratungstate). The factors affecting the extraction rate of tungsten from wolframite were investigated in air and neutral atmosphere. The extraction rate of tungsten was found to increase with increasing Na₂O content and decrease with increasing SiO₂ addition and temperature. The extraction rate in argon was higher than that in air for wolframite.

1. Introduction

Tungsten is regarded as a strategic metal by many countries due to its properties and applications [1,2]. The most common tungsten minerals are wolframite (Fe,Mn)WO₄ and scheelite CaWO₄. The production of tungsten includes several steps: a mineral process to obtain over 50% WO₃ concentrates from the ores containing 0.1–1% WO₃, leaching from the concentrates to obtain sodium tungstate, conversion and precipitation of intermediate tungsten compound ammonium paratungstate (APT), calcination of APT to obtain WO₃ and reduction of WO₃ at high temperature to obtain tungsten metal powder, which is used for the production of tungsten carbide or other W-containing alloys [3,4,5,6]. Tungsten ore is considered as one of the critical minerals due to itseconomic importance and short supply [7]. It is important to use the critical minerals efficiently by maximizing the recovery rate during extraction. The overall recovery rate of tungsten includes the recovery rates during mineral processing and leaching. The recovery rate of leaching tungsten concentrates relies on their WO₃ content. High-grade concentrate can result in a higher recovery rate of leaching. In recent years, the quality of the tungsten ores has been changed significantly with excessive exploitation [8]. In order to obtain high-grade tungsten concentrate, the recovery rate during mineral processing has to be reduced, which will decrease the overall recovery rate of tungsten from the ores [9,10]. On the other hand, different conditions are used for leaching of wolframite or scheelite. Mixed ores are becoming the major source of tungsten, which makes beneficiation and leaching more difficult [11,12]. A number of leaching techniques have been developed to improve the recovery rate for the complex tungsten concentrates [13,14,15]. However, these hydrometallurgical processes still have to treat high-grade concentrate to achieve a high recovery rate.

Recent studies on phase equilibria of the Na₂O-WO₃ and Na₂O-SiO₂-WO₃ systems show that several compounds, including Na₂WO₄, Na₂W₄O₁₃ and Na₂W₆O₁₉, can be formed at high temperature, and they are insoluble with the sodium silicate slag [16,17]. These studies provide a base for pyrometallurgical processing of tungsten ores, as more intensive reaction conditions can be obtained compared to the hydrometallurgical process. The aim of this study is to explore the possibility of recovering tungsten from the ore through a high-temperature process.

2. Experimental Methods and Materials

The materials used in high-temperature extraction of tungsten included wolframite concentrate, analytical sodium carbonate (Na₂CO₃) and silicon dioxide (SiO₂). The chemical composition of the wolframite concentrate used in the present study is shown in

Table 1. The composition of wolframite concentrate analyzed by XRF (wt%).

Wolframite	WO3	CaO	FeO	MnO	SiO2	S
wt%	78.04	1.06	10.69	7.88	1.73	0.61

. The concentrate mainly contained WO₃, FeO and MnO, with small amounts of CaO, SiO₂ and sulfur. The concentrate was dried at 120 °C for 24 h and ground for the experiments. An amount of 10 g concentrate was mixed with the required sodium carbonate and silicon dioxide in an agate mortar. Well-mixed samples were pelletized and placed in an alumina crucible. The conditions of the high-temperature experiments are shown in

Table 2. Experimental conditions for high-temperature processing of wolframite concentrate.

Exp No	Ore (g)	Temp (°C)	Time (min)	Na2CO3 (g)	SiO2 (g)	Atmosphere
1	10	1200	60	10.5	3	Air
2	10	1200	60	10	3	Air
3	10	1200	60	8.05	3	Air
4	10	1200	60	6.81	3	Air
5	10	1050	60	6.81	4	Air
6	10	1100	60	6.81	4	Air
7	10	1200	60	6.81	4	Air
8	10	1200	60	9.29	3	Air
9	10	1200	60	9.29	4	Air
10	10	1200	60	9.29	5	Air
11	10	1200	60	10.52	3	Ar
12	10	1200	60	9.29	3	Ar
13	10	1200	60	8.05	3	Ar
14	10	1200	60	6.81	3	Ar

. Experiments 1–4 investigated the effect of Na₂CO₃ on the extraction rate of wolframite. Note that Na₂O is an effective component at high temperature as a result of Na₂CO₃ decomposition. Experiments 5–7 compared the effect of the reaction time, and experiments 8–10 looked at the effect of SiO₂ on the extraction rate of wolframite. The iron from wolframite can be Fe²⁺ or Fe³⁺ during a high-temperature process. Experiments 11–14 were carried out in argon gas at different Na₂CO₃ additions.

The experiments in air were carried out in a muffle furnace. The samples were cooled down in the furnace after the reaction time, as shown in Table 2. The experiments in argon were carried out in a sealed vertical tube furnace similar to the one described in previous studies [16,17]. The crucible with the sample was initially suspended on a Mo wire at the bottom end of the furnace. After the furnace was flashed by 400 mL/min Ar gas flow for 30 min, the sample was raised to the hot zone of the furnace. After the reaction, the sample was lowered to the bottom of the furnace and removed after cooling. The heated sample was ground and leached in water at 50 °C. The water to solid ratio was 5:1, and the mixture was stirred for 120 min before separating the sodium tungstate solution from the residue by filtration. The residue was analyzed by XRF (X-Ray Fluorescence), and the content of WO₃ in the residue was used to evaluate the extraction rate of tungsten from the concentrate.

3. Results and Discussion

3.1. Thermodynamic Considerations of High-Temperature Reactions

FactSage 8.2 [18] is a powerful thermodynamic software and was used to predict high-temperature reactions in the present study. The composition of the wolframite given in Table 1 was used for thermodynamic calculations. The databases of “FactPS” and “FToxid” were used in the “Equilib” module. The solution phases selected in the calculations were “FToxide-SLAGA”, “FToxide-SPINC”, “FToxide-MeO_A” and “FToxide-OlivA” and “FToxide-Mull”.

Figure 1 shows the changes of the phases as a function of sodium oxide addition calculated by FactSage 8.2. SiO₂ addition to 100 g wolframite is 30%, and the temperature is 1200 °C. As can be seen, wolframite decreases and sodium tungstate increases with increasing Na₂O. With approximately 32% Na₂O addition, all wolframite is decomposed and converted to Na₂WO₄. On the other hand, SiO₂ only starts to react with the wolframite when Na₂O addition is greater than 12%, as Na₂O reacts with wolframite first. Na₂O is an essential slag-forming component together with SiO₂. With 16% Na₂O addition, all SiO₂ is dissolved into the liquid slag with the oxides of sodium, iron and manganese. More than 33% Na₂O will only enter the silicate slag. Both Na₂WO₄ and the silicate slag are liquid at 1200 °C, which enables the products to be removed from the furnace easily. During the high-temperature reaction, other components from the concentrate, such as CaO and sulfur, are dissolved in the slag phase.

Figure 2 shows the effect of temperature on the decomposition of wolframite at 30 wt% SiO₂ and various Na₂O additions. It can be seen from Figure 2a that decomposition of wolframite increases with increasing temperature at a given Na₂O addition. With 20% and 25% Na₂O addition, the wolframite cannot be completely decomposed even at 1600 °C. With 30% and 35% Na₂O addition, the wolframite can be completely decomposed at 1350 and 1080 °C, respectively. It can be seen from Figure 2b that, at a given Na₂O and SiO₂ addition, a minimum temperature is required to completely decompose the wolframite. For example, 32% Na₂O is required to fully decompose the wolframite at 1200 °C, which is the case shown in Figure 1. In other words, the lowest temperature is required to completely decompose the wolframite at a given Na₂O addition. For example, the temperature must be over 1350 °C to fully decompose the wolframite if 30% Na₂O is added. Na₂O has a much lower melting temperature (1132 °C) than other components. Sufficient Na₂O can form liquid Na₂WO₄ and liquid slag Na₂O-FeO-MnO-SiO₂ to fully decompose the wolframite. If Na₂O is not sufficient, a higher temperature is required to ensure the liquid slag Na₂O-FeO-MnO-SiO₂ is formed.

Wolframite can be decomposed with or without SiO₂:

(Fe,Mn)WO₄ + Na₂O → Na₂WO₄ + FeO + MnO

(1)

(Fe,Mn)WO₄ + Na₂O + SiO₂ → Na₂WO₄ + slag (Na₂O-FeO-MnO-SiO₂)

(2)

If SiO₂ is not present, the FeO and MnO formed from decomposition of the wolframite may cover the surface of the remaining wolframite to stop further reaction between wolframite and Na₂O. In contrast, SiO₂ can react with Na₂O, FeO and MnO to form a liquid slag, which can enhance the decomposition of wolframite. However, it can be seen from Reaction (2) that SiO₂ can also consume Na₂O to form a slag. Figure 3 shows the changes of the phase fractions as a function of SiO₂ addition calculated by FactSage 8.2. At 1200 °C with 30% Na₂O, Na₂WO₄ decreases and slag increases with increasing SiO₂ addition. However, when SiO₂ exceeds 21%, wolframite starts to appear. SiO₂ is a stronger acidic oxide than WO₃. Strong basic oxide Na₂O first reacts with SiO₂. When the Na₂O addition is fixed, more SiO₂ addition consumes Na₂O to form a slag first. The remaining Na₂O is not sufficient to fully decompose the wolframite. A certain amount of SiO₂ is necessary to form a liquid slag with FeO, MnO and Na₂O. Excess SiO₂ will consume more Na₂O and influence the decomposition of wolframite.

3.2. Experimental Results

High-temperature experiments were carried out to evaluate the effects of Na₂O addition, SiO₂ addition, temperature and oxygen partial pressure on the extraction rate of tungsten from wolframite. The melted samples were leached with water, and the leaching residue was analyzed by XRF. The remaining WO₃ concentration in the leaching residue is used to represent the extraction rate. High WO₃ in the residue means low extraction rate. Figure 4 shows the effect of Na₂O on the WO₃ in the leaching residue in air. An amount of 30% SiO₂ was added, and the temperature was 1200 °C. It can be seen that WO₃ in the leaching residue decreases with the increase in Na₂O addition. When the Na₂O addition is higher than 58.5%, the WO₃ in the leaching residue is 0.36%, which is much lower than that (>1%) in the conventional hydrometallurgical process [19]. The experimental results confirmed the trend predicted by FactSage 8.2, as shown in Figure 1. However, the actual Na₂O required to fully decompose the wolframite is much higher than the predictions. This indicates that the thermodynamic database for a WO₃-containing system needs to be improved.

Figure 5 shows the experimental results on the effect of SiO₂. As can be seen in the figure, the WO₃ in the leaching residue increases with the increase in SiO₂, which confirms the trend predicted by FactSage, as shown in Figure 3. Too much SiO₂ consumed more Na₂O; the remaining Na₂O was not sufficient to fully decompose the wolframite.

Figure 6 shows the effect of temperatures on the remaining WO₃ in the leaching residue with additions of Na₂O and SiO₂ at 39.8% and 40%, respectively. It can be seen in the figure that within the range of 1050–1200 °C, WO₃ in the leaching residue increases with increasing temperature. This can be explained by the solubility of WO₃ in the slag. It seems that the solubility of WO₃ in the silicate slag increases with increasing temperature. The WO₃ dissolved in the silicate slag cannot be leached with water. In the current FactSage database, WO₃ is not included in the slag phase, and the solubility of WO₃ in the slag cannot be predicted.

Reaction (2) shows the decomposition of wolframite without a variation in the valence state. Iron is initially present in the wolframite as Fe²⁺, which can be oxidized to Fe³⁺ in air. The above discussions focus on the reactions in air. Figure 7 shows the decomposition of wolframite by Na₂O in argon at 1200 °C, as predicted by FactSage 8.2. The general trend of the reactions in argon is similar to those in air, as shown in Figure 1. However, it can be seen that the Na₂O required to fully decompose wolframite is 30%, which is lower than that in air (30%).

A series of experiments were conducted in argon gas for comparison with those in air. Under the neutral gas flow, iron is present as Fe²⁺, which is the same as in the wolframite. It can be seen in Figure 8 that WO₃ in the leaching residue is 0.22% with 54% Na₂O addition. If the Na₂O addition is more than 58%, the WO₃ in the leaching residue approaches zero, and the recovery of WO₃ is almost 100%. It is easy to decompose wolframite under reducing conditions.

4. Conclusions

Thermodynamic calculations and high-temperature experiments confirmed that WO₃ can be effectively recovered from wolframite by the pyrometallurgical process. The recovery of tungsten from wolframite is much higher than that in the conventional process. SiO₂ can form liquid slag with the oxides of iron, manganese and sodium to enhance the decomposition of wolframite. However, excess SiO₂ consumes Na₂O and reduces the decomposition rate. High Na₂O addition and low temperature are beneficial for maximizing the recovery of tungsten. It is easy to decompose wolframite under reducing conditions where iron is present as Fe²⁺.