Tungsten is a vital raw material used across various sectors, including energy, materials science, information technology, and heavy industries, with a notable presence in the form of tungsten carbide. Its unique properties and the absence of effective substitutes have led to its classification as a critical and strategic mineral in the United States, the European Union, and other regions.
For over seventy years, China has maintained a dominant position in the global tungsten market, accounting for approximately 82% of the world’s mine production and 56% of its reserves as of 2020. This concentration, combined with increasing environmental regulations in China affecting supply, highlights the pressing need to investigate alternative and secondary sources of tungsten. One promising avenue is the exploration of mine tailings, which are substantial byproducts generated during ore beneficiation. While leveraging these materials presents challenges, it also offers significant opportunities for securing tungsten resources.
Tungsten ore processing is characterized by its inherent inefficiency, primarily due to the low concentrations of tungsten typically found in these ores, averaging around 0.4–0.6%. As a result, significant amounts of tungsten are often discarded during processing. Estimates suggest that for every tonne of tungsten concentrate produced (ranging from 50–65% WO3), between 7 and 10 tonnes of tailings are generated. Currently, global tailing accumulations are believed to exceed 100 million tonnes, which may contain approximately 96,000 tonnes of WO3. In 2010, it was estimated that around 25.6 kilotonnes of tungsten were lost to tailings, underscoring a major untapped resource.
A notable aspect of tungsten in these tailings is its particle size. The primary economically valuable minerals, wolframite [(Fe,Mn)WO4] and scheelite (CaWO4), are mainly found in fine and ultrafine fractions; wolframite particles often measure below 25 µm, while scheelite particles typically fall below 74 µm. This fine particle size poses challenges for recovery using conventional mineral processing techniques. Additionally, the weathering of primary ores can lead to the formation of secondary tungsten minerals, which complicates reprocessing efforts and may contribute to lower tungsten recovery rates.
Beyond the economic incentive of resource recovery, the reprocessing of tungsten tailings is crucial for environmental remediation. While tungsten itself generally exhibits low toxicity and limited mobility in the environment (with around 93% in soil bound in the residual fraction ), tungsten mine wastes can be hazardous. The primary environmental threat from tailings stems not from tungsten but from other associated contaminants like arsenic (As), copper (Cu), zinc (Zn), and lead (Pb)-bearing sulfide minerals, carbonates, and sulfates.
A major environmental concern is Acid Mine Drainage (AMD). When sulfide minerals within tailings are exposed to air and water, they oxidize, generating acidic conditions. This acidic water then leaches heavy metals and arsenic, contaminating soil and water bodies. The Lianhuashan mine in China serves as a stark example, where AMD from tailings released significant quantities of Cu, Cd, Zn, Pb, Hg, and particularly As, into the local environment, while tungsten itself was not a major contaminant element. Reprocessing tailings offers a pathway to mitigate these risks, decontaminate affected areas, and rehabilitate abandoned mine sites.
The unique characteristics of tungsten tailings, particularly the fine particle sizes, require specialized or enhanced extraction methods for effective recovery. One of the traditional methods employed is gravity separation, which is favored for wolframite beneficiation due to its high density. This method is advantageous because it is low-cost and environmentally friendly, as it does not require chemical reagents. However, its effectiveness dramatically declines when it comes to fine wolframite particles, with recovery rates for particles below 20 μm reported to be less than 45%. Enhanced gravity concentrators, such as the Hang and vibrate cone concentrators and the Falcon concentrator, have shown improved recovery rates of 76 to 83% for fine tungsten minerals within the +10 to 75 μm range, but they still struggle with ultrafine particles measuring less than 10 μm.
Another technique is magnetic separation, which leverages the paramagnetic nature of wolframite. This method typically operates in high-intensity systems, but, similar to gravity separation, its efficiency is highly dependent on particle size. The magnetic forces acting on wolframite decrease quickly with size, leading to the loss of finer particles. For example, even with high magnetic intensities of 1.5 T, recovery rates for particles below 10 μm may only reach about 60%, compared to approximately 90% for larger particles at 1.3 T. Wet High-Intensity Magnetic Separation (WHIMS) has come to prominence as a more effective approach for recovering fine and weakly magnetic minerals from tailings, showing promising modeling predictions of 80 to 90% recovery rates for new tailings.
In terms of flotation, which separates minerals based on their surface physicochemical properties, this method is mainly utilized for processing scheelite ores. Wolframite, however, has low floatability and is not generally processed via flotation on an industrial scale. Nonetheless, for ultrafine wolframite that is poorly recovered by gravity or magnetic methods, flotation has demonstrated success. The use of highly selective reagents—such as alkyl hydroxamates, phosphonic acid derivatives, benzohydroxamic acid (BHA), sodium oleate (NaOl), and the new surfactant N-(6-(hydroxyamino)-6-oxohexyl) octanamide (NHOO)—can significantly enrich fine and ultrafine wolframite from ore slimes. One particular study reported achieving a concentrate of 36.87% WO3 with a recovery rate of 62.90% from fine wolframite slime. However, a notable drawback of flotation is the possibility of new environmental contamination stemming from residual flotation reagents, especially if they carry arsenic.
Chemical leaching, specifically heap leaching, represents another extraction method that has shown efficacy for low-grade ores, like copper and gold. However, it is generally considered impractical for reprocessing tungsten tailings at ambient conditions. The decomposition of scheelite is hindered under standard temperatures due to the formation of a passivating layer of tungstic acid (H2WO4). Effective breakdown of both scheelite and wolframite necessitates harsh conditions—such as high temperatures exceeding 180°C for scheelite with NaOH or over 125°C for scheelite treated with HCl—along with the application of significant pressure or concentrated chemicals, making it economically unviable for heap leaching application on tailings.
Lastly, bioleaching offers a promising approach that utilizes microorganisms for metal extraction, gaining recognition for being cost-effective, safe, and environmentally friendly compared to conventional methods. It has been successfully applied in extracting various metals from ores and is increasingly utilized to remove heavy metals from solid wastes like mine tailings. For tungsten tailings, bioleaching has shown notable success in decontamination processes, including the removal of arsenic with recovery rates up to 96.7%, and nearly 100% recovery of manganese using mixed cultures of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Although direct bioleaching of tungsten from minerals, such as scheelite, through Metallosphaera sedula, is an emerging field, its large-scale application for tungsten recovery from tailings is still in the early development stages.
Despite the recognized potential, the reprocessing of tungsten tailings faces substantial hurdles, primarily economic viability. The low concentration of tungsten in tailings means that the yield of concentrate per tonne of reprocessed material can be very small. For instance, a reprocessing trial at the Panasqueira mine in Portugal, while technically successful in producing 50–55% WO3 wolframite concentrates, was deemed uneconomical due to the low overall recovery volume.
A successful strategy for tungsten tailings reprocessing must therefore adopt a comprehensive approach. The economic assessment should not only consider the market value of the recovered tungsten (and any other co-recovered valuable minerals) but also factor in the significant societal and environmental benefits derived from tailings decontamination and the rehabilitation of abandoned mine sites.
Tungsten mine tailings represent a considerable, yet largely unexploited, secondary resource. The impetus for their reprocessing is twofold: to augment the supply of a critical metal and to mitigate the serious environmental hazards associated with their historical disposal. While conventional methods are often ill-suited for the fine-grained nature of tungsten in these wastes, enhanced techniques like WHIMS and flotation with specialized reagents show technical promise. Chemical leaching is largely unviable, but bioleaching is emerging as a strong candidate for decontamination and potentially, in the future, for direct metal recovery. Ultimately, unlocking the value in tungsten tailings will require overcoming economic challenges through innovative technologies and a holistic valuation that includes environmental stewardship benefits. Such efforts are crucial for a more sustainable and secure tungsten supply chain.