Tungsten is a critical metal known for its high melting point and excellent hardness, making it essential in various industrial applications, including the production of hard metals, electrical contacts, and light bulb filaments. As the demand for tungsten continues to rise, the tungsten extraction industry is undergoing significant advancements, yet it faces several challenges. This overview explores the current extraction methods, recent innovations, and the recycling of tungsten scrap.
Tungsten exists in low concentrations in the Earth's continental crust, estimated at 1.0 ppm (Liu & Ma, 1987). Two primary tungsten minerals, scheelite (CaWO₄) and wolframite ((Fe, Mn)WO₄), are valuable for industrial purposes, accounting for 70% and 30% of the resources, respectively. Countries like China, Russia, the US, and Canada have significant tungsten reserves, with China controlling around 60% of global resources and 80% of the world’s mine production (Gunn, 2014).
scheelite (CaWO₄)
wolframite ((Fe, Mn)WO₄)
Tungsten ore concentration, which is decreasing, is now around 50% or lower, making extraction more complex. The tungsten concentrate, with 65% WO₃, undergoes a pretreatment process involving roasting or leaching to remove impurities.
Tungsten recycling is another significant source, offering environmental and economic benefits. Recycling tungsten from scrap material, which contains up to 99% tungsten, has been practiced since the 1960s (Kieffer & Baroch, 1981). Tungsten scrap comes from new scrap (waste from processing) and old scrap (end-of-life products), with an estimated 34% of world tungsten supply in 2013 coming from recycled sources (European Commission, 2014).
The hydrochloric acid (HCl) decomposition method is one of the prevalent techniques for tungsten extraction. This process is characterized by a relatively short flowsheet and lower production costs, making it attractive for treating scheelite concentrates with minimal impurities. However, when dealing with concentrates containing higher impurity levels, such as wolframite, the process becomes more complex.
Key Challenges:
Thermodynamically, the conversion of scheelite to tungstic acid in HCl solutions is favorable; however, the formation of solid tungstic acid can hinder further reactions. Various studies have aimed to improve conversion efficiency by optimizing particle size, utilizing excess HCl, employing heated ball mill reactors, and exploring alternative organic acids for dissolution.
Modern tungsten extraction processes are designed to accommodate different raw materials and achieve higher yields of uniform quality products. These processes, often involving soda or caustic soda digestion, entail elevated temperatures and excessive reagent use to maximize recovery rates of WO3. While these methods offer some advantages, they also present significant drawbacks:
Efforts to recover NaOH and Na2CO3 from leaching solutions have been explored, but these methods often result in a loss of WO3 or increase processing costs.
Innovative laboratory processes are emerging to address the environmental challenges and inefficiencies associated with traditional extraction methods. These new techniques aim to reduce wastewater discharge and improve reagent recycling:
This method employs stoichiometric amounts of sulfuric and phosphoric acids while recycling the spent acid solution to produce ammonium tungstate. However, separating phosphorus from the ammonium tungstate solution remains a challenge, often requiring additional reagents and resulting in waste generation.
This technique focuses on achieving zero-waste discharge through a closed-circuit system, allowing for the treatment of various tungsten raw materials. While promising, it involves high energy consumption during the roasting phase and can release exhaust gases. Furthermore, managing impurities that dissolve into the leaching solution complicates purification.
This approach can effectively treat scheelite, wolframite, and mixed concentrates. It allows for recycling of spent H2SO4 with additional acid supplementation. However, concerns about accumulating impurities and optimizing recovery efficiency warrant further investigation.
These innovative processes mark significant progress in tungsten metallurgy, particularly in terms of chemical reagent recycling and reducing environmental impact. Large-scale industrial piloting will be crucial for assessing their feasibility and refining process flowsheets.
The recycling of tungsten scrap is increasingly critical due to resource scarcity. Efficient recycling practices can help mitigate the environmental impact of tungsten extraction while ensuring a sustainable supply. Key considerations in tungsten scrap recycling include:
Recent reviews highlight the advantages and limitations of various tungsten recycling methods, emphasizing the need for effective classification and the deployment of comprehensive recycling strategies.
The tungsten extraction industry is at a crossroads, facing challenges related to environmental sustainability, resource management, and production efficiency. Ongoing advancements in extraction methods, coupled with innovative recycling practices, hold promise for a more sustainable future in tungsten metallurgy. Continued research and development efforts will be crucial in addressing these challenges and ensuring the long-term viability of tungsten as a critical resource.
Shen, Leiting; Li, Xiaobin; Lindberg, Daniel; Taskinen, Pekka . (2019). Tungsten extractive metallurgy: A review of processes and their challenges for sustainability. Minerals Engineering, 142(), 105934–. doi:10.1016/j.mineng.2019.105934
Liu, Y. J., & Ma, D. S. (1987). Geochemistry of tungsten.
Gunn, G., 2014. Critical Metals Handbook, Chapter 16, Tungsten. British Geological Survey/American Geophysical Union/John Wiley & Sons, Ltd., pp. 385–413
Kieffer, B.F., Baroch, E.F., 1981. Extractive Metallurgy of Refractory Metals, Proc. TMSAIME Annu. Meeting, pp. 273.
European Commission, 2014. Report on critical raw materials for the EU, Report of the Ad-hoc working group on defining critical raw materials. European Commission.