Leaching of Scheelite Concentrate for Tungsten Extraction

Abstract

As a critical raw material, tungsten plays a broad role in machining, electronics, aerospace, and other high-tech industries. The extraction of tungsten from tungsten concentrates is a prerequisite for the production of high-purity products. Approximately 70% of China’s tungsten resources are in the form of scheelite. The extraction method of low-quality scheelite is crucial for the production application of the tungsten process as resources of high-quality wolframite are gradually being depleted. This article systematically reviews the processes and challenges faced in the hydrometallurgical process of scheelite concentrates and provides useful insights. Typical leaching processes for scheelite concentrate have shown excellent leaching efficiencies, with tungsten trioxide (WO₃) recoveries exceeding 90%. Alkaline leaching processes are promising, but temperature and pressure are crucial for this method. The sintering–leaching process is energy-consuming and costly. Meanwhile, leaching with hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) often results in the formation of tungstic acid (H₂WO₄) on the mineral surface, which inhibits further leaching and leads to a low extraction rate. In contrast, the mixed-acid leaching method is more promising, with recovery close to 100%, a short process, and low-cost, and the acid leaching solution is recyclable.

1. Introduction

In recent years, the expanding global economy and continued exploitation of mineral resources have driven a dramatic surge in the demand for metal commodities. This demand spans raw materials critical to everyday metal goods, cutting-edge technologies, and innovative applications that underpin modern life and industrial progress [1,2]. Renowned for its exceptional durability and heat resistance, tungsten serves as a strategic metal employed across critical sectors [3,4,5], including industrial manufacturing, civilian technologies, military systems, and nuclear energy applications [6,7,8]. According to U.S. Geological Survey (USGS) data (Figure 1), global tungsten reserves exhibited a steady upward trajectory from 2014 to 2024, peaking at 4.6 million tons in 2024—the highest level in the past decade—and witnessed a 4.55% increase over this period. These reserves are concentrated in a few key regions: China dominates with 2.4 million tons (52.53% of global reserves), Australia holds 0.57 million tons (12.48%), and Russia accounts for 0.4 million tons (8.76%). While tungsten concentrate production spans multiple countries, including China, Australia, Russia, Vietnam, Spain, Korea, Austria, Portugal, and other countries. China remains the unrivaled leader, contributing over 80% of the global output (Figure 2b). In 2024, global tungsten mine production reached 8.14 million metric tons, with China contributing 6.7 million tons, commanding a dominant 82.31% share of the world total (see Figure 2a). This underscores China’s pivotal role in the tungsten supply chain. Additionally, in 2023, China exported 17.5 thousand tons of tungsten products (Figure 3a), with export revenues totaling 683 million dollars (Figure 3b), reflecting its leading position in both production and international trade. China holds the world’s largest tungsten reserves and dominates global production and consumption, solidifying its position as the primary metal supplier and a market powerhouse [9,10,11].

Currently, economically viable tungsten ores primarily consist of wolframite, scheelite, and mixed wolframite [12]. Due to its superior ore grades and simpler mineral composition compared to scheelite, wolframite is the preferred source for extraction [13]. As high-quality wolframite continues to be mined and utilized, the availability of high-grade quartz vein wolframite is gradually declining [14,15]. Consequently, low-grade tungsten resources are increasingly contributing to tungsten concentrate production, particularly low-grade scheelite [16] and wolframite mixed with high molybdenum content [17,18,19]. The extraction of scheelite plays a crucial role in supplementing tungsten ore production, thereby contributing to a more sustainable balance between resource utilization and environmental preservation [20,21].

The increasing complexity of tungsten resources has led to a decline in WO₃ grade, which has fallen below 50% in tungsten concentrates [22,23]. The complexity of tungsten raw materials imposes higher demands on the tungsten extraction process, requiring significant energy input to break the mineral structure. These factors complicate tungsten recovery and pose hurdles to sustainable production [24]. Therefore, technologies capable of efficiently processing low-grade, complex tungsten ores offer significant cost advantages [25]. Molten salt electrolysis has been shown to extract and recover key metals efficiently [26]. Almost all existing methods for processing tungsten ores were developed before the 1950s and fall into two main categories: pyrometallurgy and hydrometallurgy. Hydrometallurgy dominates the market due to its advantages, including effective separation and relatively simple, mild operating conditions [27].

This paper provides an overview of the current state of tungsten resources and a systematic review of common hydrometallurgical process flows for low-grade scheelite concentrates. The objective is to synthesize the current understanding of scheelite concentrate wet leaching and offer relevant insights, ultimately contributing to the sustainable advancement of this technology.

2. Scheelite Resources

Natural tungsten occurs chiefly as tungstate. Although over twenty tungstate and tungsten-bearing minerals were identified, the main ones with mining value are wolframite, scheelite, and wolframite–scheelite mixed ore. Scheelite accounts for about 70% of China’s tungsten reserves, wolframite for about 20% [28], and the rest are mixed tungsten ores, which mainly exist in four types of deposits: skarn, quartz vein, porphyry, and pegmatite [9,29].

Table 1. Major large global tungsten deposits [31].

Deposit Type	Associated Gangue Minerals	Associated Metallic Minerals	Deposit	Region	Resource Quantity (104 t)	Average Grade of WO3 (%)
Skarn	Garnet, diopside, fushanite, epidote, etc.	Shalcopyrite, pyrrhotite, molybdenite, galena, sphalerite, and other sulfides, as well as tin, wolframite, and so on.	Zhuxi tungsten mine	Jiangxi, China	>100	0.64
			Malipo tungsten mine	Yunnan, China	53	0.43
			Sandaozhuang molybdenum–tungsten	Henan, China	42	0.12
			Mactung	Canada	59	1.08
			Mittersill tungsten mine	Austria	610	0.5
			Tmyauz	Russia	504	/
Quartz vein	Quartz, followed by feldspar, muscovite, biotite, fluorite, etc.	Molybdenum, pyrite, arsenopyrite, cassiterite, and other metal minerals symbiosis; some deposits contain natural bismuth and niobium tantalum minerals.	Shizhuyuan polymetallic mine	Hunan, China	71	0.34
			Hemerdon	United Kingdom	401	0.19
			Verkhne-Kayrakt	Russia	62	/
			Mt. Carbine	Australia	223	0.12
Porphyry	Quartz, potassium feldspar, sericite, chlorite, fluorite, etc.	Molybdenite, chalcopyrite, pyrite, galena, sphalerite, etc.; some deposits contain cassiterite or rare metal minerals.	Xintianling tungsten–molybdenum–mismuth mine	Hunan, China	32	0.37
			Xinluokeng tungsten mine	Fujian, China	30	0.23
Pegmatite	Quartz, potassium feldspar, albite, muscovite, spodumene, beryl, niobium tantalite, tourmaline and so on.	Cassiterite and molybdenite metal minerals; some deposits contain natural bismuth, bismuth sulfide minerals, or rare earth minerals.	Dahuting tungsten mine	Jiangxi, China	106	0.2
Other type	/	/	Chuangkou tungsten mine	Hunan, China	33	/
			Jerseyemerald	Canada	504	/
			Logtung	Canada	401	/
			Sisson	Canada	334	/
			Northern Dancer	Canada	223	0.1
			Laparilla	Spain	334	/
			Dolphin	Australia	87	0.55
			Nuiphao	Vietnam	87	/

Note: “/” represents the absence of precise data.

represents large tungsten deposits of different deposit types around the world. China, the world’s leading tungsten producer, operates over ten major mines, each yielding more than 1300 tons of WO₃ annually. These mines are widely distributed and concentrated, mostly located in Jiangxi and Hunan in southern China, like Xiangfu Mountain and Shizhuyuan [30].

The main component of scheelite is CaWO₄, an orthorhombic crystal system. Owing to the geochemical similarity between tungsten and molybdenum, molybdenum ions readily undergo isomorphic substitution for tungsten within the scheelite lattice, forming calcium molybdate (CaMoO₄) [32]. Minerals such as quartz, pyroxene, calcite, molybdenite, fluorite, bismuthite, and apatite are generally associated with scheelite. These minerals predominantly consist of calcium–magnesium silicates and carbonates, forming the primary matrix of the ore deposits [33]. The concentration, spatial site resistance, and electrochemical properties of calcium ions are the main reasons for the differences between calcium-bearing minerals [34]. Following decades of extensive mining, tungsten resource grades have progressively declined, while mineral compositions have become increasingly complex. Challenges such as intricate symbiotic mineral relationships, finer embedded grain sizes, and difficulties in the comprehensive utilization of resources have further compounded these issues [14,35,36]; relying on flotation alone makes it very difficult to enrich tungsten in large quantities [37,38].

3. Scheelite Hydrometallurgical Leaching Process

The hydrometallurgical processing of tungsten ore can be classified in multiple ways [39]. However, research has extensively explored various wet recovery methods for tungsten, which are broadly grouped into three primary categories. The first methodology employs strong acids, such as HCl, H₂SO₄, and nitric acid (HNO₃), as leaching agents to facilitate tungsten extraction at low temperatures. The second approach involves leaching calcium tungstate from scheelite into a sodium tungstate (Na₂WO₄) solution and insoluble calcium salts using sodium carbonate (Na₂CO₃) and sodium hydroxide (NaOH) as reagents. A key challenge in the current hydrometallurgical processing of scheelite lies in the inherent insolubility and mineralogical complexity of tungsten ores, which demand significant energy input to disrupt their structural integrity. To address this, a third category of innovative leaching technologies has emerged, aiming to optimize leaching conditions and enhance efficiency in tungsten recovery.

Table 2. Main technologies for scheelite leaching.

Method		Main Process Parameters	Leaching Recovery	References
Alkaline leaching process	Na2CO3 high-pressure leaching	Temperature: 200–230 °C, L/S: 5 mL/g, 2–3 times NaCO3 dosage, reaction: 5h.	95%–98%	[40]
	NaOH high-pressure leaching	Temperature: 180 °C, L/S: 0.8 mL/g, 2.5 times NaOH, stirring speed: 400 rpm, reaction: 2 h.	98%	[41]
Acid leaching process	HCl leaching	Temperature: 28–100 °C, pH: 1.5–3.	99%	[42]
	H2SO4 leaching	Temperature: 100 °C, mineral particle size range: 32–44 μm, stirring speed: 400 rpm, L/S: 0.8 mL/g, sulfuric acid concentration: 3 mol/L.	90%	[27]
Sintering–leaching process	Na2CO3 sintering–H2O leaching	Sintering temperature: 800 °C, 3 times Na2CO3, 2 h; Leaching temperature: 90 °C, 4 mol NaOH.	98%	[5]
Na3PO4-CaF2 mixed leaching process		Temperature: 180 °C, 1.6 times of the theoretical dosage of Na3PO4, 2 times of the theoretical dosage of CaF2, initial NH3·H2O concentration 30 g/L, L/S: 2 mL/g, reaction: 3h.	98%	[43]
		Temperature: 180 °C, 2 times the theoretical amount of Na3PO4, 1.5 times the theoretical amount of CaF2, 14% NaOH, L/S: 4 mL/g.	98%	[44]
		Temperature: 16 °C, L/S: 2 mL/g, stirring speed: 350 rpm, ore: CaF2 = 21.25, 1 mol/L Na3PO4, 1.67 h.	98%	[45]
New leaching process	H2SO4–H2O2 leaching	Temperature: 45–50 °C, H2SO4 concentration: 2–3 mol/L, H2O2 concentration: 1.5–2.5 mol/L, L/S: 6–10 mL/g, reaction: 60–90 min; Temperature: 90 °C, reaction: 4 h.	80%–99%	[46,47,48]
	H2SO4–H3PO4 leaching	Temperature: 90 °C, 1.0–2.5mol/L H2SO4, 0.5–1mol/L H3PO4, L/S: 5–10 mL/g.	99%	[27]
	HCl–H2O2 leaching	Temperature: 30 °C, 1.6 mol/L H2O2 and 2 mol/L HCl, stirring speed: 400 rpm, reaction: 3 h.	79%	[48]

summarizes the main technologies for wet leaching of scheelite for tungsten recovery.

3.1. Acid Leaching Processes

The principle of acid leaching is to utilize the reaction between acid and tungsten ore and control the temperature, concentration, time, and stirring speed, to make the tungsten in the tungsten ore enter the solution as soluble H₂WO₄, to achieve tungsten extraction [49]. The specific process flow is shown in Figure 4. As HCl, H₂SO₄, and HNO₃ are inexpensive and applicable, they are common acid leaching agents [50].

3.1.1. HCl Leaching Process

HCl is among the most extensively studied leaching agents for scheelite [51]. HCl leaching involves reacting raw materials such as scheelite with HCl and converting tungsten into a solution as H₂WO₄, as illustrated in Equation (1). The resulting H₂WO₄ is directly calcined to obtain WO₃ products. Then, metallic tungsten powders are produced by the calcination and reduction of WO₃. When CaCl₂ reacts with H₂SO₄, gypsum is formed. If the impurity content of H₂WO₄ in the leaching solution is high, it is redissolved in ammonia to produce an ammonium tungstate solution. This solution is then used for the production of ammonium tungstate (APT) [52,53]. The chemical equilibrium of HCl leaching of scheelite and the treatment of H₂WO₄ products have been studied in detail [42]. There are also several other follow-up methods, such as scheelite acid leaching and sustainable extraction of tungsten [54]. During the leaching process, Fe, As, P, Mo, Mn, etc., may be dissolved by acid into a leaching solution, increasing the difficulty of the subsequent removal of impurities.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)={\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{WO}}_4\left(\mathrm{s}\right)$$

(1)

The leaching of scheelite is mainly influenced by the chemistry of tungsten, calcium, and the anions used in the leaching process, mainly in terms of temperature, pH, and leaching agent concentration. Typical operations for HCl leaching comprise processing scheelite in 1.5–3 mol HCl in excess relative to stoichiometry at 85–100 °C. The limited solubility of H₂WO₄ produced from scheelite in the HCl solution causes solid H₂WO₄ to coat the surface of the particles. This coating blocks additional HCl from reacting with the CaWO₄, which results in decreased tungsten recovery [55]. Consequently, the conversion efficiency of scheelite to H₂WO₄ remains suboptimal. To address the issue of the H₂WO₄ passivation layer impeding WO₃ recovery in HCl leaching, extensive research efforts have focused on enhancing the process. Several approaches have been explored to improve this situation, such as decreasing particle size, employing substantial amounts of concentrated HCl, adopting a heated ball mill reactor, and dissolving H₂WO₄ using organic acids [56].

Common organic acid additives include phosphoric acid (H₃PO₄) and hydrogen peroxide (H₂O₂) [45]. The addition of H₃PO₄ facilitates the formation of soluble heteropoly-tungstic acid, which enhances tungsten dissolution by stabilizing it in the aqueous phase [51]. The reaction formula is shown in Equation (2). It can significantly improve the tungsten leaching rate and inhibit the leaching of impurities [57]. However, the addition of H₃PO₄ increases the amount of P in the leach solution. The leaching efficiency of scheelite concentrate was markedly enhanced, achieving a tungsten recovery rate of 99.6% through a HCl-H₃PO₄ complexation system [58]. Scheelite with HCl and H₃PO₄ produces a leaching solution containing phosphotungstic acid (H₃[PW₁₂O₄₀]) and calcium chloride (CaCl₂). Subsequent addition of ammonia (NH₃·H₂O) or ammonium chloride (NH₄Cl) precipitates ammonium phosphotungstate ((NH₄)₃[PW₁₂O₄₀]·9H₂O), a high-purity intermediate for tungsten extraction. Notably, the acidic leaching solution can be reproduced and reused in subsequent cycles, enabling both efficient tungsten recovery and sustainable utilization of by-products such as CaCl₂.

$$12{\mathrm{CaWO}}_4\left(\mathrm{s}\right)+24\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)=12{\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_3\left[{\mathrm{PW}}_{12}{\mathrm{O}}_{40}\right]\left(\mathrm{aq}\right)+12{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)$$

(2)

Leaching of scheelite in a HCl-H₂O₂ system converts tungsten to soluble peroxotungstic acid (H₂[WO₃(O₂)]) [17,48], as described in Equation (3). The leaching of 99.4% tungsten from scheelite HCl leaching slag at 1.95 mol/L H₂O₂ for 60 min and 30 °C has also been studied [17]. Impurity migration with this method is similar to that in HCl leaching, and the addition of H₂O₂ promotes the oxidation of Mo, making it more soluble in the leach solution. However, there are no subsequent reports on subsequent processes for treating soluble metatungstate, soluble aqueous hydrogen oxalate tungstate, or peroxotungstate solutions.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)={\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left[{\mathrm{WO}}_3{(\mathrm{O}}_2)\right]\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)$$

(3)

Overall, HCl is effective because of its ability to easily break down the calcium tungstate structure. HCl leaching is relatively simple but may be limited by the high concentration and high temperature required for effective tungsten recovery. The addition of organic acid not only promotes the leaching of scheelite ore and the formation of heterogeneous tungstic acid but also prevents the problem where the mixed precipitation of H₂WO₄ and calcium chloride leads to difficulty in subsequent separation.

3.1.2. H₂SO₄ Leaching Process

H₂SO₄ is another widely utilized leaching agent for tungsten ores [59]. The H₂SO₄ leaching process of scheelite is similar to that in the HCl leaching process; the reaction is represented by Equation (4) and illustrated schematically in Figure 5a. The migration pattern of impurities is similar to that in the HCl leaching process, and the main impurity elements in the leach solution are Fe, As, P, Mo, and Mn.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)={\mathrm{Ca}\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{WO}}_4\left(\mathrm{s}\right)$$

(4)

The scheelite with low WO₃ content can be fully converted into H₂WO₄ after stirring and reacting for 2 h at a temperature of 85 °C, acid concentration of 1.35, and liquid–solid (L/S) ratio of 2. This process also faces a situation where the produced H₂WO₄ cover prevents further tungsten leaching, and the H₂SO₄ leaching of scheelite produces a large amount of CaSO₄ by-products [60]. To address these limitations, researchers have adopted the addition of chelating agents to enhance leaching efficiency and mitigate by-product interference.

In the H₂SO₄–H₂O₂ leaching system, the efficient and clean treatment of scheelite can be achieved by utilizing the transition of CaWO₄ to CaSO₄-nH₂O and H₂WO₄ as well as H₂O₂ with H₂WO₄ complexation, and the increase in the density of H₂O₂ promotes it; a comparison with a system not using H₂O₂ is shown in Figure 5b. Peroxytungstic acid has poor thermal stability, and pure solid H₂WO₄ can be obtained by leaching with heating. The disadvantage is that iron and manganese ions will catalyze the leaching of peroxytungstic acid and affect the leaching effect of tungsten, so it is not suitable for the leaching of wolframite [61]. A tungsten leaching rate of 98.0% was achieved at a temperature of 90 °C with a H₂SO₄ concentration of 2.0 mol/L, a H₂O₂ concentration of 2.5 mol/L, an L/S ratio of 6 mL/g, and a leaching time of 4 h [60]. It can be seen that scheelite can be leached with high efficiency in H₂SO₄–H₂O₂ at atmospheric pressure [62].

Under atmospheric pressure, scheelite was selectively leached in a mixed H₂SO₄–H₃PO₄ acid system, producing a soluble heterotungstic acid (H₃[PW₁₂O₄₀]) solution alongside the precipitation of solid calcium sulfate (CaSO₄) [49]; the reaction formula is shown in Equation (5), and the reaction flow is shown in Figure 6. Furthermore, tungstate solutions with low degrees of polymerization have been shown to promote the efficient dissolution of H₂WO₄, as demonstrated in prior studies [63]. While producing heteropoly-tungstic acid, the calcium ions reacted with phytate to produce calcium phytate, which had a good leaching effect and solved the problem of producing a large amount of CaSO₄ by-products. A maximum tungsten leaching rate of 98.8% was achieved using H₂SO₄ solution with H₃PO₄ as a chelating agent under the following concentrations: 2.5 mol/L H₂SO₄, 2.0 mol/L H₃PO₄, an L/S ratio of 10 mL/g, a reaction time of 4 h, and a reaction temperature of 140 °C [18]. This H₂SO₄–H₃PO₄ mixed-acid leaching method effectively extracts tungsten from scheelite, achieving nearly 100% leaching efficiency [27,64].

$${12\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{12\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)= 12{\mathrm{Ca}\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_3\left[\mathrm{P}{\mathrm{W}}_{12}{\mathrm{O}}_{40}\right]\left(\mathrm{aq}\right)+{12\mathrm{H}}_2\mathrm{O}$$

(5)

The same effect can be achieved by using relatively high-cost oxalic acid (H₂C₂O₄) to react with scheelite, because the complexing coordination of oxalic acid is similar to that of H₂O₂ [65,66], forming soluble (H₂[WO₃(C₂O₄)H₂O]) and solid CaC₂O₄, and about 99.2% of tungsten was leached at low pressure [67]; the reaction is given in Equation (6). Another scholar studied the acidic leaching of scheelite by sodium organic phytate at atmospheric pressure.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{2\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4\left(\mathrm{aq}\right)={\mathrm{H}}_2\left[{\mathrm{WO}}_3{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)\mathrm{H}}_2\mathrm{O}\right]\left(\mathrm{aq}\right)+{\mathrm{CaC}}_2{\mathrm{O}}_4\left(\mathrm{s}\right)$$

(6)

When H₂SO₄ alone was employed for leaching, the precipitated H₂WO₄ formed a passivating layer on the particle surfaces, significantly reducing scheelite conversion efficiency. However, the introduction of H₂O₂, H₃PO₄, and H₂C₂O₄ overcame this limitation by facilitating complete reaction kinetics, resulting in the formation of acicular CaSO₄ crystal whiskers. NH₄Cl was incorporated into the leaching solution to obtain a heteropoly-tungstate precipitate. Then, ammonium hydroxide ((NH₄)OH) was used to dissolve heteropoly-tungstate precipitate; this was followed by solution purification. D301 resin was used to remove the hindering effect of CaSO₄ in the H₂SO₄-H₃PO₄ process with an extraction rate of 99.9% [68]. Likewise, the solvent extraction demonstrated effective recovery of materials such as tungsten and rare earth elements in mixed H₂SO₄–H₃PO₄ leachate [54,68,69]. While these measures effectively mitigate the passivation caused by the solid H₂WO₄ layer, they inevitably introduce trade-offs, such as elevated production costs or the need for specialized equipment fabricated from corrosion-resistant materials. The development and application of H₂O₂, H₃PO₄, and H₂C₂O₄ have broadened the applicability of the H₂SO₄ system for decomposing tungsten concentrates under hydrometallurgical conditions [70].

3.1.3. HNO₃ Leaching Process

The use of HNO₃ for scheelite leaching has been studied less extensively but has shown potential when used in combination with other reagents; the reaction is represented by Equation (7).

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2\mathrm{H}{\mathrm{NO}}_3\left(\mathrm{aq}\right)={\mathrm{H}}_2{\mathrm{W}\mathrm{O}}_4\left(\mathrm{s}\right)+{\mathrm{Ca}\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)$$

(7)

HNO₃ leaching can not only be used for scheelite concentrate but also tungsten secondary resources. A separate investigation has shown that the leaching process of scheelite with HNO₃ and H₃PO₄ is controlled by surface chemistry [71]. Compared with HCl, HNO₃ is less corrosive to equipment, and the wastewater can also be used to produce nitrogen fertilizer. Due to its strong oxidizing property, it is very effective for the removal of flotation chemicals remaining on the surface of scheelite concentrate. HNO₃ alone often requires auxiliary chemicals to enhance the leaching process. In HNO₃–H₃PO₄ leaching of scheelite, the leaching solution of H₃PO₄ is recycled, which can not only reduce emissions but also facilitates recovery. Tungsten extraction exceeding 97% can be achieved through leaching under optimized conditions: 80–90 °C, 3.0–4.0 mol/L HNO₃, an L/S ratio of 10:1, a H₃PO₄ dosage chemical ratio of 3, and a leaching time of 3 h [72]. Na₂WO₄ was obtained as the final product, with 98.5% recovery of APT and 95.6% total recovery of tungsten [36].

3.2. Alkaline Leaching Processes

The commonly used alkali leaching processes for scheelite extraction include Na₂CO₃ high-pressure leaching, NaOH high-pressure leaching, phosphate pressure leaching, and sodium fluoride high-pressure leaching. The alkali leaching process is a widely employed technique for tungsten ore processing, particularly effective for minerals such as wolframite and scheelite. These alkali leaching processes involve introducing alkaline reagents to initiate leaching reactions under elevated pressure and temperature, thereby solubilizing tungsten for recovery [73]. The detailed process flow is illustrated in Figure 7.

3.2.1. NaOH High-Pressure Leaching Process

NaOH high-pressure leaching is a method in which sodium hydroxide reacts with tungstate minerals in tungsten ore to produce soluble Na₂WO₄ [41,74,75], enabling efficient tungsten extraction. The reaction is represented by Equation (8). This process offers several advantages, including a short process cycle, simple equipment requirements, high leaching efficiency, and strong adaptability to diverse raw materials [76]. Compared to acid leaching, alkaline leaching is more adaptable to high-impurity raw materials. Most of the impurities will stay in the leaching slag in the form of precipitation because of the solubility, and the impurity elements in the leaching solution are mainly P, Mo, and As.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)={\mathrm{Na}}_2{\mathrm{W}\mathrm{O}}_4\left(\mathrm{aq}\right)+{\mathrm{Ca}\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)$$

(8)

Alkali pressure leaching is a hydrometallurgical process producing tungsten from scheelite by reacting NaOH with the ore under elevated temperatures and high alkali concentrations. This process is particularly effective for treating tungsten concentrates with a WO₃ content exceeding 20%. Key operational parameters—including temperature, NaOH concentration, and mineral particle size—critically influence the leaching efficiency of scheelite. Generally, the NaOH pressure-cooking method is carried out in a solution under the conditions of 160–200 °C, a concentration of NaOH of ≥50%, a ratio of L/S of 4–5, a reaction time of 2–4h, and a total pressure of 8–12 bar. High tungsten extraction rates from scheelite concentrate have been achieved by using NaOH leaching under various autoclaving conditions. For example, a 2 h leaching process under the conditions of 160 °C, a NaOH stoichiometric ratio of 2.2, an L/S ratio of 0.8 mL/g, and a stirring speed of 400 rpm resulted in 98.8% WO₃ extraction [41]. Tungsten has also been extracted from scheelite concentrate with NaOH under the conditions of 120 °C, a spiral speed 160 rpm, a reaction time of 3.5 h, and 2.2 doses of NaOH; the resulting WO₃ content in the leaching solution was 99.18% [77]. Additional studies have shown that under the conditions of 200 °C, an amount of Na₂CO₃ two times the theoretical calculation, an L/S ratio of 3 mL/g, and an alkali mineral ratio 35%, the recovery rate of tungsten can reach 99.5%.

While the alkaline leaching process avoids surface passivation by H₂WO₄ precipitation, its efficiency is constrained by secondary reactions. Specifically, Ca(OH)₂, a by-product of NaOH–scheelite interactions, can trigger a reversible reaction during post-leaching dilution and filtration. This reversibility reproduces scheelite from Na₂WO₄ solutions, limiting tungsten leaching rates to below 90% in many cases. Consequently, optimizing the recovery of tungsten from Na₂WO₄ solutions remains a persistent challenge in hydrometallurgical processing [78]. To mitigate undesirable side reactions during alkaline leaching, additives such as Na₂CO₃, Na₃PO₄, or fluoride salts are typically introduced. These agents inhibit calcium redeposition by forming calcium compounds with lower-solubility products. For instance, phosphate reacts with Ca₂₊ to produce Ca₃(PO₄)₂, which stabilizes the system and enhances scheelite dissolution. High-pressure NaOH leaching of low-grade scheelite, enhanced by phosphate additives, yielded a WO₃ leaching rate of 94.45% in a case study. Optimized conditions included a stirring speed of 400 r/min, a reaction temperature of 100 °C, 6.76 mol/L NaOH, 1.69 mol/L Na₃PO₄, and a reaction time of 540 min. The phosphate effectively suppressed calcium interference, improving tungsten recovery [79].

To promote the leaching of scheelite from NaOH, it was found that high WO₃ recovery could be achieved by a hot grinding process or mechanical activation of a vibrating ball mill and autoclave at the same time [80]. The microwave-assisted heating alkaline melting of scheelite improves the reaction time and the leaching efficiency after melting [81]. This technology is currently employed mainly in China. Ultrasonic waves are able to strip and eliminate the product layer, thus improving the recovery of tungsten [82]. Alkaline leaching has been proposed as an alternative to acid leaching because of its environmental advantages. Sodium hydroxide can dissolve scheelite, but the process is usually slow compared to acid leaching. Crude Na₂WO₄ needs to be decontaminated, and conventional techniques are difficult to use because conventional chemical purification introduces new impurities [83,84]. In addition, the presence of impurities such as calcium carbonate (CaCO₃) complicates the process by forming insoluble by-products.

3.2.2. Na₂CO₃ High-Pressure Leaching Process

The Na₂CO₃ high-pressure leaching process is suitable for treating scheelite and wolframite mixed ores, and the process has been perfected and has become a common method for treating scheelite in many countries [40,85]. The reaction formula of scheelite and Na₂CO₃ solution is shown in Equation (9). The migration pattern of impurities is similar to that in the NaOH leaching process.

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)={\mathrm{Na}}_2{\mathrm{W}\mathrm{O}}_4\left(\mathrm{aq}\right)+{\mathrm{CaCO}}_3\left(\mathrm{aq}\right)$$

(9)

The production of CaCO₃ consumes CO₃²⁻, releases OH⁻, raises pH, inhibits CO₃²⁻ hydrolysis, and improves leaching efficiency [86,87]. Na₂CO₃ high-pressure leaching is characterized by wide applicability, high tungsten leaching rate, difficult-to-dissolve veinstone, and low impurity leaching rate, and the tungsten leaching rate increases with the increase in Na₂CO₃ concentration. However, too high a concentration of Na₂CO₃ will lead to a decrease in tungsten leaching rate, and when the concentration of Na₂CO₃ exceeds 230g/L, the tungsten recovery rate will decrease, because Na₂CO₃ forms the insoluble complex carbonate Na₂CO₃-CaCO₃ on the surface of scheelite particles, hindering the leaching process. Kinetic and thermodynamic analyses of Na₂CO₃ leaching [62,88] define optimal operational parameters as follows: temperature of 190–225 °C, Na₂CO₃ concentration of 10%–18%, reaction time of 1.5–4 h, 2.5–4.5 times the stoichiometric ratio of Na₂O to WO₃; under these conditions, the process achieves tungsten recovery rates exceeding 98%. Post-leaching, the crude Na₂WO₄ solution undergoes purification to be free of excess tungstate impurities [83], followed by crystallization into high-purity APT for industrial applications.

While Na₂CO₃ pressure leaching is generally regarded as more environmentally sustainable than acid leaching methods, its operational demands—such as elevated temperatures and pressures—are significantly higher for achieving efficient tungsten extraction. A critical drawback of this method lies in its leaching residues, which are classified as hazardous waste due to their residual alkalinity and heavy metal content [89]. Notably, these residues also contain recoverable metal species, and inadequate metal recovery poses risks of resource wastage and significant economic losses [90,91]. The novel hydrometallurgical process employing quaternary ammonium-based direct solvent extraction in alkaline media demonstrates exceptional versatility. It is compatible not only with autoclave-assisted Na₂CO₃ leaching but also with processing NaOH leach solutions derived from diverse tungsten ores, enabling efficient metal recovery across feedstocks [92]. Furthermore, tungsten extraction from mine tailings presents untapped potential, offering a pathway to valorize low-grade or discarded resources while mitigating environmental liabilities [93,94]. Concurrently, recovering high-value metals from tungsten processing residues can enhance existing recycling frameworks. This integration not only supplements primary extraction but also advances toward closed-loop material flows within the tungsten industry, aligning with circular economy principles [95,96,97].

3.2.3. Phosphate High-Pressure Leaching Process

In phosphate high-pressure leaching of scheelite, Na₃PO₄ is the predominant leaching agent, with the reaction mechanism represented by the following equations, Equations (10) and (11):

$${3\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{2\mathrm{Na}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)={\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2\left(\mathrm{s}\right)+3{\mathrm{Na}}_2{\mathrm{W}\mathrm{O}}_4\left(\mathrm{aq}\right)$$

(10)

$${5\mathrm{CaWO}}_4\left(\mathrm{s}\right)+{3\mathrm{Na}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)={\mathrm{Ca}}_5{\left({\mathrm{PO}}_4\right)}_3\left(\mathrm{O}\mathrm{H}\right)\left(\mathrm{s}\right)+5{\mathrm{Na}}_2{\mathrm{W}\mathrm{O}}_4\left(\mathrm{aq}\right)$$

(11)

The significantly lower-solubility products of Ca₃(PO₄)₂ and Ca₅(PO₄)₃(OH) compared to Ca(OH)₂ and CaCO₃ thermodynamically favor the use of Na₃PO₄ for decomposing scheelite. This is because PO₄³⁻ preferentially sequesters Ca²⁺ into insoluble calcium phosphate phases, driving the dissolution of scheelite more efficiently than hydroxide or carbonate ions. However, the addition of phosphate increases the concentration of P in the solution, and the high temperature and pressure promote the dissolution of impurity ions. The leaching effect of sodium phosphate was studied by comparing whether sodium hydroxide was added or not. When the amount of sodium phosphate was 1.8 times the theoretical amount, the leaching rate of WO₃ could reach 99.98% at 135 °C and under a reaction time of 2 h. It was found that, by adding sodium hydroxide and sodium phosphate together to decompose scheelite, the leaching effect was better.

When the high-calcium tungsten mineral is treated with alkali pressure boiling, adding a certain amount of phosphate reduces the leaching rate of tungsten ore, which is also based on the principle that phosphate and calcium easily form stable insoluble matter. Although the leaching effect of this process is good, there are also some shortcomings: when using sodium phosphate as the leaching agent, the content of the harmful impurity P in Na₂WO₄ solution obtained from the leaching of scheelite is too high, which increases the difficulty of purifying the solution and removing P in the subsequent process. The amount of sodium phosphate will increase greatly for tungsten medium ore with more calcium salt. Sodium phosphate is more expensive than sodium hydroxide and soda. Therefore, at present, sodium phosphate is rarely used alone as a leaching agent to treat scheelite in industrial production, and it is more often used as an additive in the alkali leaching process.

3.2.4. NaF High-Pressure Leaching Process

The NaF high-pressure leaching process uses sodium fluoride to react with scheelite at a high temperature and high pressure to produce soluble Na₂WO₄, thus realizing tungsten extraction; the reaction formula is shown in Equation (12). The addition of NaF at high temperature and pressure may promote the dissolution of impurity elements such as P, Mo, Fe, Mn, A, and so on.

$${3\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2\mathrm{NaF}\left(\mathrm{aq}\right)={\mathrm{CaF}}_2\left(\mathrm{s}\right)+{\mathrm{Na}}_2{\mathrm{WO}}_4\left(\mathrm{aq}\right)$$

(12)

When the dosage of sodium fluoride was 170%–180% of the theoretically calculated amount, the leaching rate of scheelite concentrate could reach 99.5%–99.7% at 225 °C and under a reaction time of 2 h. The alkalinity of the Na₂WO₄ solution obtained after the leaching was low. Compared with the soda pressure-cooking method, the fluoride salt leaching method not only shortens the leaching time but also reduces the acid used for neutralization, and in the sodium salt waste solution produced by neutralization, the waste residue contains 75%–80% CaF₂, which can be used to manufacture fluorite pellets required by the iron and steel metallurgy industry. Recent research has explored leveraging the common ion effect involving Na₊; to separate and recover NaF from fluorine-containing solutions. This approach holds significant promise for advancing fluorine-containing wastewater treatment and enabling sustainable recovery of fluorine resources [98]. In parallel, Fe₂₊-based salts have demonstrated substantial potential for industrial-scale separation of W and molybdenum Mo, owing to their ability to exploit the distinct solubility behaviors of these metals in hydrometallurgical systems [99].

$${\mathrm{CaWO}}_4\left(\mathrm{s}\right)+2{\mathrm{NH}}_4\mathrm{F}\left(\mathrm{aq}\right)={\mathrm{CaF}}_2\left(\mathrm{s}\right)+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{WO}}_4\left(\mathrm{aq}\right)$$

(13)

Scholars from the former Soviet Union milled scheelite first using planetary ball mill mechanical activation and then in a rotary pressure cooker for ammonium fluoride (NH₄F) leaching; the reaction formula is shown in Equation (13). When dosing NH₄F at 2.5 times the theoretical amount and using 10% free NH₃·H₂O, alongside an L/S ratio of 4:1, a reaction temperature of 150 °C, and a holding time of 4 h, the tungsten leaching rate reached 99.3%. This process reduces the number of steps in the conversion of Na₂WO₄ to ammonium tungstate, making the process much simpler.

3.3. Other Leaching Processes

3.3.1. NaOH/Na₂CO₃ Sintering–Water Leaching Process

Conventional CaO sintering of tungsten-containing materials to extract tungsten produces exhaust gases and requires a high CaO:WO₃ stoichiometric ratio (approximately 3), leading to cumbersome recirculation of auxiliary materials [100]. Moreover, high-temperature roasting can introduce impurities into the leach solution, complicating the purification. To address these issues, the NaOH/Na₂CO₃ sintering–water immersion method was proposed.

The NaOH/Na₂CO₃ sintering–water leaching process uses tungsten ore and NaOH/Na₂CO₃ mixed sintering to convert tungsten into soluble Na₂WO₄ and, through water extraction of Na₂WO₄, to achieve tungsten extraction; the reaction process is shown in Figure 8. For example, employing the NaOH fusion–water leaching method on tungsten concentrate (61.5% WO₃) involves sintering with excess NaOH at 650 °C for 1 h, followed by aqueous leaching. This process achieves a tungsten recovery rate of 98.6% by effectively decomposing scheelite into soluble Na₂WO₄ [101].

Based on the Na₂CO₃ sintering–water immersion method, the proposed Na₂CO₃ sintering–(NH₄)₂CO₃ leaching process is a new technology for tungsten extraction that recycles wastewater and prevents its discharge (Figure 9).

The Na₂CO₃ sintering–(NH₄)₂CO₃ leaching process usually involves calcining fully mixed tungsten concentrate and CaCO₃ at 800–850 °C, so that tungsten minerals are converted into Na₂WO₄. The calcined product is leached at 30–80 °C in ammonium carbonate solution to obtain a (NH₄)₂WO₄ solution, and then, after the removal of impurities, the solution undergoes evaporation and crystallization to yield APT. The main component of the leaching residue is CaCO₃, which can be recycled. This can be performed by sintering tungsten-containing material without Na₂CO₃ at 700 °C, followed by (NH₄)₂CO₃ leaching for 60 min, resulting in a 61% tungsten leaching rate. In contrast, the tungsten recovery rate can be increased to 79% after sintering with 2% Na₂CO₃ and then leaching with (NH₄)₂CO₃ for 15 min. This is indicated by the fact that the added Na₂CO₃ effectively suppresses scheelite precipitation by sequestering Ca²⁺ into soluble complexes.

3.3.2. H₂SO₄ Conversion–NH₄HCO₃ Leaching Process

Aimed at the characteristics of low volatility of H₂SO₄, low cost, and easy recovery of ammonium carbonate solution, the process of H₂SO₄ conversion–(NH₄)₂CO₃ leaching to produce APT has received much attention. However, the presence of impurities requires additional chemical precipitation, ion exchange, or solvent extraction steps, increasing process length and cost. To improve the process route for the production, a new leaching system of H₂SO₄ conversion–NH₃·H₂O–NH₄HCO₃ leaching has also been proposed [102], and the reaction process is shown in Figure 10. Under this system, most of the impurities will stay in the leaching slag in the form of precipitation, and the impurity elements in the leaching solution are mainly P, Mo, and As.

Under the conditions of 70–90 °C and normal pressure, tungsten concentrate can be completely converted into H₂SO₄ solution by controlling the concentration of H₂SO₄, and the converted solution can be recycled by adding a small amount of H₂SO₄. By adding NH₄HCO₃ solution into the conversion solution, Ca²⁺ is converted into CaCO3 precipitation and removed, and the purified (NH₄)₂WO₄ solution is evaporated and crystallized to obtain APT products. The H₂SO₄ conversion–NH₄HCO₃ leaching process can treat scheelite, wolframite, and mixed concentrate. The production of CO₃²⁻ in the reaction process can prevent the secondary combination of WO₄²⁻ and Ca²⁺ and ensure the leaching effect of tungsten. The ion exchange method is easy to apply [103], has a fast adsorption rate, has a high recovery rate, is environmentally friendly, and has good application prospects [104,105]. The H₂SO₄ conversion–NH₄HCO₃ leaching process is characterized by the leaching agent circulation, which simplifies the conversion step from Na₂WO₄ to (NH₄)₂WO₄. However, the recovery rate of the leaching system needs further study.

3.3.3. Combined (NH₄)₃PO₄, NH₄·H₂O, and CaF₂ Leaching Process

(NH₄)₃PO₄, NH₄·H₂O, and CaF₂ have recently been applied in scheelite leaching. From previous analyses, it is known that fluoride, phosphate, and ammonium bicarbonate all promote the leaching of scheelite, and some scholars have proposed a new (NH₄)₃PO₄, NH₄·H₂O, and CaF₂ co-leaching process to extract tungsten from scheelite in a green and efficient way; the reaction formula is given in Equation (14).

$$9{\mathrm{CaWO}}_4\left(\mathrm{s}\right)+6{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)+{\mathrm{CaF}}_2\left(\mathrm{s}\right)=9{\left({\mathrm{NH}}_4\right)}_2{\mathrm{WO}}_4\left(\mathrm{aq}\right)+2\mathrm{Ca}{\left({\mathrm{PO}}_4\right)}_3\mathrm{F}\left(\mathrm{s}\right)$$

(14)

The decisive factors in the reaction process are dosage, initial ammonia concentration, temperature, L/S ratio, and holding time. The leaching efficiency of scheelite reaches 98.6% under the following conditions: 1.6–2 times the theoretical dosage of (NH₄)₃PO₄ and CaF₂, 30 g/L NH₄·H₂O concentration, 2 mL/g L/S ratio, 180 °C, and 3 h holding time [43]. In the efficient extraction of tungsten from scheelite by (NH₄)₃PO₄, NH₄·H₂O, and CaF₂, tungsten extraction from scheelite consistently exceeds 98% [45]. However, the addition of (NH₄)₃PO₄ significantly increased the P content of the leachate.

3.4. By-Product Composition and Treatment

Almost all scheelite leaching processes produce by-products with low toxicity, which can be reused after simple processing, or used in construction or soil improvement materials, etc. (listed in

Table 3. Toxicity and treatment of by-products from different leaching processes of scheelite.

Method	By-Products	Toxicity	Treatment
Na2CO3 high-pressure leaching	CaCO3	Low toxicity	Used as building material, in soil improvement, and in the paper industry.
NaOH high-pressure leaching	Ca(OH)2	Corrosive	Used in neutralization treatment, as building material, and in soil improvement.
Phosphate high-pressure leaching	Ca3(PO4)2 Ca5(PO4)3(OH)	Low toxicity	Used as fertilizer and in the production of H3PO4.
NaF high-pressure leaching	CaF2	Low toxicity	Used in the production of hydrogen fluoride (HF), fluoride products, cement retarder, etc.
Na2CO3 sintering–H2O leaching	CaCO3	Low toxicity	Used as building material, in soil improvement, and in the paper industry.
HCl leaching	CaCl2	Low toxicity	At low concentrations, dilution may be considered for discharge. Alternatively, calcium chloride may be recovered by evaporation, concentration, and crystallization.
H2SO4 leaching	CaSO4	Low toxicity	Used as gypsum building materials, in soil improvement, and in landfill disposal.
H2SO4-H2O2 leaching	CaSO4	Low toxicity
H2SO4-H3PO4 leaching	CaSO4	Low toxicity

).

4. Discussion

In this paper, we present the mainstream processes for wet leaching of scheelite concentrate and analyze these processes in detail. We found that the acid method has the advantages of being a simple process and low energy consumption, but the H₂WO₄ produced by the reaction will cover the surface of the mineral and hinder the reaction. The alkali method has strong adaptability to raw materials, but the reaction conditions are harsh and generally require high temperature and high pressure. It also faces some challenges in practical application, such as the reaction speed in the leaching process, the selectivity of the leaching solution, and the improvement of the metal recovery rate.

Regarding the problem wherein the H₂WO₄ produced by acid leaching of scheelite would hinder the reaction process, some scholars have sought to solve this problem by adding H₂O₂ or H₃PO₄ and found that the mixed-acid leaching process not only improves the tungsten leaching rate but also solves the problem of there being a large amount of wastewater and residue discharge in the process of production, through the recycling of acid leaching mother liquor. However, these results are limited to the wet leaching of scheelite, and the applicability of this method to other tungsten raw materials needs further research. Tungsten extraction from mine waste has some potential [93,94]. The extraction of high-value metals from tungsten residues can complement existing recycling processes and realize a closed-loop process for the tungsten industry [95,96,97]. To overcome this limitation, future research should be directed to the comprehensive utilization of wolfram, wolfram mixed concentrate, and tungsten secondary resources, and further research is needed to determine the feasibility of these processes for different tungsten raw materials while taking into account the cost and economy of industrial production.

Meanwhile, more attention should be paid to optimizing the leaching process, improving metal recovery efficiency, developing new environmentally friendly solvents, reducing the waste of resources, and lowering costs. With the progress of science and technology, especially the introduction of new materials, microbial leaching technology, ionic liquid technology, and other innovative technologies, the efficiency and environmental friendliness of scheelite hydrometallurgy are expected to improve significantly.

5. Conclusions

Possessing excellent physical and chemical properties, tungsten is a strategic choice of metal resource for daily life and scientific and technological development. As an effective way to recover tungsten resources, scheelite hydrometallurgical technology plays an increasingly important role in the utilization of tungsten resources. The acid leaching process is short, and the product cost is low. However, this process is mainly utilized to treat low-impurity scheelite. The presence of impurities such as P and As will affect the purity of tungsten, and HCl is volatile, corrosive, associated with poor operating environments, and incurs high investment costs. Compared with HCl, in current industrial production, the alkaline leaching process allows the use of a variety of raw materials to obtain higher WO₃ yields and uniform, high-quality products, but in industrial operations, high temperature and pressure are required, and conditions are harsh. At present, the leaching effect of H₂SO₄-H₂O₂ and H₂SO₄-H₃PO₄ leaching systems is good, which solves the problem of difficult treatment of three wastes. Compared with the traditional acid–base method, the mixed-acid method has significant advantages including high selectivity, low energy consumption, and environmental friendliness, and it can effectively reduce carbon dioxide emission and wastewater pollution, which is in line with the trend of green development of modern mines. The mixed-acid leaching technology can not only efficiently recover tungsten in a scheelite but also recover valuable by-products such as Mo, Mn, and so on, to realize the comprehensive utilization of resources.