High-Purity Tungsten Oxide Production from Low-Grade Scheelite Concentrates at Pilot Plant Scale

Abstract

Tungsten is a critical raw material with increasingly important industrial applications. It is primarily found in minerals such as scheelite and wolframite (0.5% W), which are extracted and processed at the mine site to produce a high-grade scheelite concentrate (60% W). This process results in significant tungsten losses in the form of tailings, currently not utilized at the EU level. Deep eutectic solvents and imidazolium-based ionic liquids have been shown to possess excellent utility for recovering tungsten from low-grade concentrates, achieving tungsten oxide (96% purity) at high global yields (80%). In this study, an optimized ionic liquid-based process (involving leaching, solvent extraction, crystallization, and calcination) was developed at the laboratory scale. Important issues such as solvent flammability or the commercial availability of ionic liquids were addressed to ensure the safety and industrial feasibility of the process. Furthermore, a pilot plant was designed, constructed, and operated for a significant period (3 days). Tungsten oxide was produced with improved purity (>99%) and global yield (91.6%) in continuous operation.

1. Introduction

Tungsten (W) is a highly sought-after metal, with a wide range of applications stemming from its exceptional physical properties. Its high melting point (3422 °C) and high electrical conductivity (18 × 10⁶ S·m⁻¹) [1] make it an ideal material for medical devices, such as X-ray tubes and welding electrodes, as well as for aerospace and electronic applications, including smartphones, computers, and televisions. Furthermore, its high tensile strength (980 MPa) and hardness (294 Brinell) [2] render it a valuable material for the manufacture of machine tools, such as cutting and drilling equipment, and for use in the military and automotive sectors. Notably, China dominates global tungsten production, accounting for approximately 84% of the world’s total output, which was estimated at 79,900 tonnes in 2024 [3]. In contrast, the European Union’s tungsten production is significantly lower, at around 2000 tonnes per year, comprising only 2.5% of global production in 2024. This disparity, combined with increasing demand, has led to tungsten’s classification as a Critical Raw Material in the EU.

In nature, tungsten is primarily found in the form of two minerals: Scheelite (CaWO₄) and Wolframite ((Fe,Mn)WO₄). However, the presence of these minerals in tungsten ores is typically very low, with concentrations usually below 0.5% W. As a result, the initial step in tungsten production involves the liberation of Scheelite (or Wolframite) from the ore, which is typically achieved through crushing at the mine site, followed by gravity separation using shaking tables and flotation technologies [4]. These processes yield a high-grade Scheelite concentrate, containing approximately 60% W in the form of CaWO₄. Nevertheless, this process also generates substantial quantities of unvalorized tailings, which contain around 0.02% W and represent approximately 10–40% of the tungsten present in the original mineral ore. Notably, the subsequent transformation of CaWO₄ into high-value tungsten-based products necessitates the production of high-purity tungsten trioxide (WO₃).

The most widely used process for recovering tungsten from mineral ores is alkaline digestion with sodium carbonate or caustic soda, conducted under high pressure (ranging from 1.2 to 2.6 atm) and temperature (around 200 °C) [5,6]. The primary advantage of this well-established method is that sodium tungstate remains in solution, while impurities such as Ca, Fe, and Mn are precipitated as hydroxides. Soluble impurities, including SiO₂, P, As, and Mo, can be removed through a subsequent solvent extraction step [5]. Tungstic acid (WO₃H₂O) is then precipitated by adjusting the pH with HCl. Purification of WO₃ involves redissolution in ammonia, followed by crystallization of ammonium paratungstate (APT) ((NH₄)₁₀(H₂W₁₂O₄₂)·H₂O) and calcination at 600 °C. However, this process has several major drawbacks, including the generation of large quantities of waste [7] (ranging from 20 to 100 tons of sodium salt water per ton of APT), as well as the requirement for elevated pressure and temperature. In an effort to reduce the temperature and pressure, Yang et al. [8] proposed the use of ultrasound (US)-assisted heating. Although the activation energy of the process was not reduced by the US heating method, the rate of reaction was enhanced, as the ultrasounds facilitated the elimination of the product layer, thereby transforming the kinetic controlling step.

Another current practice for obtaining Na₂WO₄ is the roasting of scheelite and wolframite with Na₂CO₃ at 800–900 °C [9]. This process facilitates the removal of impurities such as Ca, Fe, and Mn, which are not soluble in water. However, additional purification steps are still necessary to eliminate soluble impurities like SiO₂, P, As, and Mo. Notably, a significant excess of Na₂CO₃ (150–200% of the stoichiometric amount) is required for efficient roasting [9]. Furthermore, the subsequent production of tungstic acid from Na₂WO₄ involves the use of HCl, resulting in the generation of substantial quantities of sodium salt wastewater. To address the issue of soluble impurities, Yang et al. proposed the addition of SiO₂ to remove Mo and enhance the W yield [10]. Similarly, Sreenivas et al. [11] demonstrated that the addition of Na₂CO₃ can prevent the back precipitation of CaWO₄, thereby increasing the W yield. More recently, Spooren et al. [12] developed a novel approach involving microwave (MW)-assisted fusion of a low-melting eutectic alkali salt system consisting of a 1:1 (m/m) NaOH:KOH mixture. This modification enabled a significant reduction in the reaction temperature to 150–200 °C, leading to substantial energy savings. Despite these improvements, the alkali roasting method remains plagued by the production of large volumes of wastewater and high energy consumption.

Tungstic acid can be directly produced from Scheelite and Wolframite through acid leaching [5,9,13]. However, this process involves the precipitation of tungstic acid on the surface of the particles, which can hinder the reaction between HCl and the particles [13]. To mitigate this issue, the material must be ground to a very small size (<44 μm), thereby increasing the operational and investment costs. Nevertheless, the acid leaching method yields a highly pure tungstic acid (WO₃·H₂O), which can be easily dissolved in an ammonia solution. Subsequent steps, including crystallization (APT) and calcination, can then be employed to obtain WO₃, similar to the alkaline method described earlier. It is worth noting, however, that the acidic method has some significant drawbacks, including the use of strong and toxic acids like HCl and HNO₃, which can lead to the production of substantial quantities of salt (chlorides) waters [5,9,13].

In the authors’ previous research papers [14,15], an ionic liquid (IL)-based process involving multiple technologies (leaching, solvent extraction, stripping, crystallization, and calcination), was developed for the production of pure tungsten oxide under mild conditions (60 °C and atmospheric pressure). In this work, the feasibility of the IL-based technology developed for tungsten oxide production from low-grade scheelite concentrates (LGSC) was demonstrated at pilot plant scale (TRL7). To achieve this, the basic engineering was developed and the pilot plant built to validate the process. Prior to the experimental campaign, some laboratory-scale experiments were conducted to ensure compliance with the requirements of the pilot plant. The present paper proposes an innovative tungsten oxide production process that can be applied to LGSC (1–4% W), with lower environmental impact compared to traditional leaching processes. Unlike the alkali/acid digestion or alkali roasting methods, which involves tungstic acid precipitation and subsequent purification as ammonium paratungstate (APT) resulting in the generation of substantial amounts of salt waters (chlorides), the proposed IL-based process eliminates the need for acid precipitation, thereby avoiding the generation of this waste. Furthermore, whereas traditional alkali/acid digestion or alkali roasting methods often require high energy consumption due to high temperature and pressures, which can be partially mitigated by the use of ultrasound (US) or microwave (MW) technologies, the proposed IL-based process takes place at mild temperatures and atmospheric pressures, achieving high yields in a very short time (approximately 1 h). The environmentally friendly characteristics of this IL-based process make it an attractive solution for the valorization of tungsten from tailings, which are currently stockpiled at the mine sites and can be valorized at mild operation conditions.

2. Materials and Methods

2.1. Material Characterization

A low-grade scheelite concentrate (LGSC) was supplied by Saloro, a tungsten mining company based in the Northwest of Spain (Barruecopardo mine site). For characterization purposes, reagents such as HCl (37%), HNO₃ (65%), and HF (48%) were obtained from Scharlab (Expert Q^®, Barcelona, Spain).

To determine the elemental composition of the solid materials (LGSC, leaching residues, and tungsten oxide), a microwave digestion process was performed using a Milestone Ethos-up, in accordance with the UNE-EN 13656:2020 standard [16]. The resulting eluates were then analyzed by means of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) using an Agilent spectrophotometer mod. 7900 (Santa Clara, CA, USA). This analytical technique was also employed to examine the eluates from leaching and solvent extraction.

Prior to leaching, the LGSC material was ground to achieve an optimal particle size distribution, ensuring effective leaching behavior. Grinding was carried out using a Retsch MM500 ball mill (Haan, Germany). The granulometry of the material was determined using a CISA RP-200-N digital sieve shaker (Barcelona, Spain), with sieves of various sizes (25 μm, 50 μm, 100 μm, 200 μm, and 425 μm). The sieving process was performed for 15 min, with an amplitude of 2.5 mm and a cycle time of 10 s. The material was then divided into distinct granulometry fractions.

2.2. Laboratory Scale Methods

2.2.1. Laboratory Scale Leaching

A Deep Eutectic Solvent (DES) was prepared using a previously established composition (Choline Chloride/Oxalic Acid/water, ChCl/OA/water, 1:1:33 in mol) [15]. The DES was synthesized by combining Choline Chloride (≥98%, Indagoo, Barcelona, Spain) and Oxalic Acid (99.6%, Vadequimica, Barcelona, Spain) in the molar ratio of 1:1 with water (69% wt), ensuring a homogeneous mixture was obtained under stirring. To optimize leaching performance, the material was ground to a particle size of −200 μm in a Retsch MM500 ball mill.

Leaching experiments were conducted in a 1 L Premium HME-R Minireactor (Scharlab, Barcelona, Spain), equipped with a condenser, under controlled temperature conditions. The DES was heated to 60 °C, and the material was added under vigorous stirring. The reaction mixture was maintained at these conditions for a predetermined time, after which the bulk of the reaction was cooled to ambient temperature. Subsequently, the mixture was filtered under vacuum, and the cake was carefully washed and dried at 100 °C.

The resulting fractions were analyzed by means of Inductively Coupled Plasma Mass Spectrometry (ICP-MS), following the procedure described in Section 2.1, to determine the tungsten (W) yields and concentrations of impurities. The leachate was stored for subsequent solvent extraction experiments.

2.2.2. Laboratory Scale Solvent Extraction

The solvent extraction step is composed of IL extraction, where tungsten (W) is loaded from the leachate to the organic phase, and stripping, where W is recovered in an aqueous solution.

For IL extraction, the organic phase was a 20:80% wt mixture of the commercially available ionic liquid Cyphos IL 101 (trihexyl(tetradecyl)phosphonium chloride) (Syensqo, Brussels, Belgium) [17,18] and Brenntsolv 150 ND (Brenntag, Essen, Germany), a non-flammable aromatic solvent (flash point > 61 °C), combined with 1-octanol (Sigma-Aldrich, St. Louis, MO, USA) as a phase modifier to prevent third-phase formation. The use of Cyphos IL 101 with aromatic solvents, such as Brenntsolv 150 ND, is consistent with previous studies that have employed similar solvents, such as Shellsol A150 (Shell, London, UK) (flash point 63 °C) [6,19]. Unlike the ionic liquid Cyphos IL 104 (Cytec Industries, Woodland Park, NJ, USA) [15,20] reported in the previous publication [15], Cyphos IL 101 is commercially available in the quantities required for pilot plant operation. The IL extraction yield was enhanced by increasing the number of equilibrium steps. The equilibrium steps were performed in a mixer-settler unit, where the organic and aqueous phases were mixed and separated in a controlled manner, allowing the tungsten to reach equilibrium distribution between the two phases. The required number of steps in counter-current mode was calculated using the McCabe–Thiele method [21], which involves building an equilibrium curve from solvent extraction experiments at different organic-to-aqueous phase ratios, plotting the operation line, and calculating the number of theoretical mixer-settlers by building steps between the operation line and the equilibrium curve.

For stripping, tungsten can be selectively recovered with ammonia solutions in the form of ammonium paratungstate [6,15]. However, due to the low distribution factors, it was not feasible to operate in counter-current mode. The tungsten stripping yield was optimized by increasing the number of equilibrium steps in cross-current mode. The organic phase was loaded with hydroxyl ions after the first stripping step. A regeneration step with a hydrochloric 1M solution was necessary. Ammonia 25% wt and hydrochloric acid 35% (Vadequimica, Barcelona, Spain) were used for the preparation of stripping and regeneration solution respectively.

2.3. Pilot Plant Equipment and Operation

The pilot plant comprised five distinct sections: pre-treatment, leaching, solvent extraction, crystallization and calcination. The following paragraphs describe the equipment and operating conditions used in each section of the pilot plant.

2.3.1. Pilot Plant Pre-Treatment

As mentioned in the laboratory scale experiments, milling was performed to achieve an optimal particle size prior to the leaching reaction. A Retsch model TM-300 mill (Haan, Germany) was used, capable of grinding batches of up to 5 kg. The milling process employed 40 kg of stainless steel balls, each with a diameter of 30 mm (Ø).

2.3.2. Pilot Plant Leaching

Leaching was conducted in a 100 L NORMAG spherical glass reactor (Bad Homburg, Germany) equipped with electrical heating up to 150 °C (Figure 1). For the leaching reaction, a mixture of 70 kg of Deep Eutectic Solvent (DES) was prepared in a separate tank, comprising 11.4 kg ChCl, 10.3 kg OA, and 48.3 kg water. This DES mixture was then pumped into the reactor, where it was heated to 60 °C under agitation. Subsequently, 14 kg of milled material was added, and the reaction was initiated. The reaction was allowed to proceed for 2.5 h to ensure maximum yield was attained. Following this, the resulting solid–liquid mixture was transferred to a HPLE filter press (TEFSA, Barcelona, Spain) with 300 × 300 mm filter plates, where the liquid (leachate) and solid (wet cake) phases were separated. To recover any residual tungsten, a portion of clean DES was pumped through the wet cake. A total of five leaching reaction batches were performed during the experimental campaign to generate sufficient eluate for the subsequent solvent extraction step. All leachate batches were stored in a 1000 L container for this purpose.

2.3.3. Pilot Plant Solvent Extraction

The leachate was then directed to the mixer-settler module for solvent extraction. For this purpose, mixer-settlers (model MSU-5, Figure 2) were supplied by MEAB (Gothenburg, Sweden). Each module consisted of a mixing chamber (1 L) where the aqueous and organic phases were mixed under vigorous agitation, and a settling chamber (5 L) where the phases were separated. The mixer-settlers and all associated equipment (pumps, containers) were fabricated from PVDF to ensure chemical compatibility with the components of the organic phase. The pH and temperature were continuously monitored in all mixer-settlers. The solvent extraction process was carried out in a three-stage configuration: (1) Ionic Liquid (IL) extraction, where tungsten was transferred to the IL-based organic phase; (2) Stripping, including a regeneration step, where tungsten was recovered in an ammonia solution as ammonium paratungstate.

2.3.4. Crystallization and Calcination

The stripping solution from the solvent extraction process was then fed into the crystallization module. A spray dryer (model ESDT1) supplied by European Spray Dry Technologies (Harlow, UK) was utilized for this purpose. The air inlet temperature was set at 230 °C, resulting in an exhaust air temperature above 100 °C, which prevented the condensation of droplets on the equipment walls. The crystallized ammonium paratungstate (APT) was subsequently calcined in a Nabertherm NR 80/11 furnace (Lilienthal, Germany) under the optimized conditions established in laboratory-scale studies. In previous research by the authors [15], a calcination temperature of 650 °C for 3 h was employed. However, in this study, it was found that calcination at the same temperature for 1 h was sufficient to achieve the complete transformation of APT to WO₃.

3. Results

This chapter is divided into two sections. First, the tungsten oxide production process described in our previous papers [14,15] was adapted to meet the requirements of the pilot plant. Second, the main results of the experimental campaign in the pilot plant are presented and discussed.

3.1. Laboratory Scale Results

The material LGSC was first subjected to milling (−200 μ) to ensure good homogeneity for ICP-MS characterization and to enhance the leaching reaction by increasing the particle surface area. The composition of LGSC, as determined by ICP-MS, is presented in

Table 1. Elemental composition of LGSC supplied by Saloro.

	W	Si	Al	Fe	K	Ca	Na
Conc. (%)	2.6	31.5	4.6	3.5	4.0	1.1	1.8

. Subsequently, a leaching reaction was performed according to the procedure described in Section 2.2.1. The leaching conditions were found to be feasible for this material in the pilot plant, achieving W yields of 95% in 1 h. The resulting leachate contained 5.26 g·L⁻¹ W (density 1066 g·L⁻¹) with a low presence of impurities, with iron being the major impurity (895 ppm), and all other elements present at concentrations below 100 ppm.

The leachate, with a tungsten concentration of 5.26 g·L⁻¹, served as the aqueous phase for the solvent extraction step. The solvent extraction conditions were established based on previous experience [15]. A mixture of Cyphos 101 (19.1%), Brenntsolv 150 ND (76.4%), and 1-octanol (4.5%) as a phase modifier was used as the organic phase. All experiments were conducted at ambient temperature. To determine the number of mixer-settlers (MS) required for the pilot plant in counter-current mode, the McCabe–Thiele method was employed (Figure 3). The equilibrium curve showed that the solvent extraction process was highly favorable, achieving a tungsten yield of 99.99% with just two MS at an organic-to-aqueous (O/A) ratio of 1:2. With two MS, an organic phase containing 10.52 g·L⁻¹ of tungsten would be produced.

The tungsten (10.52 g·L⁻¹) present in the organic phase from the IL extraction step needed to be recovered in the stripping step using NH₃ solutions. However, the equilibrium curve was not favorable for a counter-current stripping step. Specifically, the distribution factor (K_D) at an organic-to-aqueous (O/A) ratio of 1:1 was approximately 1, indicating that tungsten would be equally distributed between the aqueous and organic phases. Consequently, a mixer-settler configuration was employed, where clean stripping solution was fed into each mixer-settler. Figure 4 illustrates the tungsten stripping yields in each mixer-settler. A regeneration step was performed between the first and second steps using 1 M HCl. The stripping solutions used were 1 M NH₃ for the first step and 0.5 M NH₃ for the second and third steps.

It is noteworthy that, due to the high selectivity of the Cyphos IL 101 extractant in the IL extraction step, as well as the highly selective interaction between NH₃ and WO₄^2- that yields APT, the concentration of Fe in the stripping solution was negligible.

3.2. Pilot Plant Results

3.2.1. Flow-Sheet

A flow-sheet (Figure 5) of the pilot plant was designed based on the results of the experiments performed at laboratory scale. The following operational units were considered: leaching (comprising one reactor and a filter device), solvent extraction (consisting of six mixer-settlers (MS)), crystallization (utilizing one spray-dryer), and calcination (employing one furnace). Furthermore, the solvent extraction process was divided into three stages: IL extraction (two MS), stripping (three MS), and regeneration (one MS).

Based on the designed flow-sheet, the pilot plant was constructed. Figure 6 presents the layout of the plant, illustrating the organization and positioning of the various sections, including leaching, solvent extraction, crystallization, and calcination. Each of these sections can be clearly identified in the layout.

3.2.2. Pilot Plant Conditions

Table 2. Optimized operation conditions in the pilot plant for WO₃ production from LGSC.

Stage	Conditions
Leaching	T 60 °C time 1 h L/S ratio 5
IL Extraction Stripping Regeneration	IL Extraction Organic phase: - Extractant: IL Cyphos 101 (19.1% wt) - Dispersant: Brenntsolv 150 ND (76.4% wt) - Phase modifier: 1-octanol (4.5% wt) O/A ratio 1:2 Stripping NH3 1M (first mixer settler) NH3 0.5M (2nd and 3rd mixer settlers) O/A 1:1 Regeneration HCl 1M O/A 1:1
Crystallization	T 105 °C
Calcination	T 650 °C time 1 h

summarizes the pilot plant operation conditions based on the laboratory scale experiments.

3.2.3. Leaching Results

To validate the reaction kinetics during process scale-up, the leaching step was extended to 2.5 h. In an initial batch test, 14 kg of LGSC was mixed with 70 kg of DES, and the mixture was stirred at 60 °C. The resulting W and Fe yields (%) over time (h) are shown in Figure 7. As anticipated, the W yield reached 89% within 0.5 h and 94% within 1 h. The Fe yield remained at a maximum of 12% throughout the leaching reaction.

Five leaching reactions were performed under conditions similar to the previous experiment, and good repeatability was observed in terms of yields. Figure 8 illustrates the distribution of W and Fe in the three fractions (leachate, washing waters, and dry solid). Since the leachate concentrations were of the same order of magnitude, they were combined in a single container for the subsequent IL extraction experiments.

3.2.4. Solvent Extraction Results

To demonstrate the feasibility and robustness of the process, the mixer-settler module was operated for 3 days. During this period, the leachate flow rate was maintained at 3.6 L·h⁻¹, which is lower than the nominal capacity of 23.3 L·h⁻¹. This reduced flow rate was chosen to minimize reagent consumption during the experimental campaign. However, to verify the performance of the mixer-settlers at full capacity, additional tests were run at the nominal flow rate of 23.3 L·h⁻¹. The results, presented in

Table 3. Extraction yields (%) in the solvent extraction module.

	W		Fe
	Yield (%)	Conc. (g·L−1)	Yield (%)	Conc. (g·L−1)
Extraction	97.6	9.5	5.0	0.03
Stripping	98.3	3.2	14.2	0.01
Regeneration	−0.2	0.02	-	-
Overall	95.7	-	0.3	-

, show that the W extraction yields remained consistent across both operating regimes, with global W yields exceeding 95%. Moreover, Fe contamination was minimal, with a yield of only 0.3%. In the regeneration step, W losses were negligible, with only 0.2% of W being unrecovered, as indicated by the negative value in Table 3.

3.2.5. Crystallization and Calcination Results

The stripping solution (approximately 70 L) from the previous step was evaporated using the spray-dryer to produce ammonium paratungstate (APT). As part of this pilot plant operation, ammonia was not recovered; however, in a future industrial process, NH₃ could potentially be recovered for re-use through direct water absorption at high pressures and low temperatures [22]. Alternatively, the NH₃ could be valorized as a fertilizer in the form of ammonium sulphate or nitrate by scrubbing in acidic solution [23]. The APT was subsequently calcined at 650 °C in an air atmosphere for 1 h to produce 220 g of WO₃. The characterization of the final product is presented in

Table 4. Characterization of produced WO₃ in the pilot plant. All concentrations in %.

WO3	SiO2	P2O5	TiO2	Al2O3	CaO	Fe2O3	Rest
99.30	0.34	0.16	0.07	0.04	0.02	0.01	0.06

, which shows a WO₃ purity of 99.3%. The XRD spectra (Figure 9) are in good agreement with the expected WO₃ structure.

4. Conclusions

In this paper, a novel, environmentally friendly technology has been demonstrated at pilot plant scale (TRL7) for the production of high-purity (>99%) tungsten oxide using deep eutectic solvents (DES) and ionic liquids (ILs) at mild temperatures and atmospheric pressure. This technology has achieved excellent global W yields (91.6%) from low-grade scheelite concentrates (1–4% W). Furthermore, this approach enables the valorization of W tailings (0.02% W), currently stockpiled at mine sites, after a pre-concentration treatment. The successful operation of the pilot plant over a prolonged period (3 days) demonstrates the feasibility of scaling up this W production process to an industrial level.

The developed process offers a green alternative to existing technologies described in the state of the art, which typically involve the use of strong and hazardous acids (such as hydrochloric and nitric acid) and alkalis (such as sodium carbonate and sodium hydroxide), as well as high temperatures (150–900 °C) and pressures (up to 2.6 atm).