The recovery of metals from scrap products, long a tradition driven by their inherent value, is a cornerstone of sustainable economic practices. Tungsten, a critical raw material with unique properties, has been recycled since the early 1930s. Modern discussions around a circular economy, aimed at mitigating resource depletion, further highlight the importance of tungsten recycling. While metal atoms are not lost from the planet through use and can theoretically be reused indefinitely, the primary challenge lies in managing them to prevent dissipation and dilution to levels where recovery becomes economically unviable. This report will examine the global flow of tungsten, with a specific focus on tungsten metal products, and critically assess the economic limitations that affect its recycling potential.
In 2016, the total input for the production of tungsten intermediates was 108,500 metric tons (t) of tungsten content. Of this, 37,500 t originated from scrap (both new and old), resulting in a recycling input rate of 35%. Approximately 98,000 t of tungsten were consumed in end-use products, and with an estimated 29,000 t of old scrap generated, the end-of-life recycling rate stood at 30%. This positions tungsten among the metals, achieving a recycling input rate above 25%, a feat accomplished by only a third of sixty metals studied in a recent UNEP report.
Tungsten metal products, also known as tungsten mill products, constitute 10% of global tungsten use. This category includes powder metallurgical (PM) tungsten used in lighting, electronics, high-temperature applications, electrical contacts, and alloys for high-performance switches (e.g., W-Ag, W-Cu, WC-Ag). These products are almost exclusively manufactured using powder metallurgical techniques.
The estimated global recycling rate for tungsten metal products is relatively low, at 22%. Within this segment, heavy metal products achieve the highest individual recycling rate at 30%. A significant portion of this recycled heavy metal is production scrap, such as turnings from shaping, which is largely reused within the same product area.
The majority of PM tungsten scrap is new scrap, including items like coils, mesh, wires, blanks, trim, turnings, rods, switches, contacts, and sinter-bar ends. Unlike in the steel and superalloy sector, this new PM scrap cannot be directly recycled back into PM products. However, high-purity scrap from this category is well-suited for direct addition in superalloy manufacturing, often fetching the highest scrap prices. Less pure PM scrap can be used to produce cast eutectic carbide (WC-W2C), a hardfacing material, or for manufacturing tool steel, which has a higher tolerance for impurities. The amount of old scrap that can be collected from end-use applications in this segment is comparatively small, contributing to the low overall recycling rate. Contaminated parts can, however, be readily recycled through oxidation and subsequent alkaline digestion.
Several economic factors influence whether end-of-life tungsten products are recycled. Under open market conditions, recycling primarily occurs if it is economically viable. The key drivers are processing costs versus the revenue generated from recyclates, making metal prices a direct influence on collection and processing efficiencies.
The concentration of a metal in a product plays a crucial role in the economic viability of its recycling. The Sherwood plot, a concept introduced by Thomas Sherwood in 1959 and later applied to recycling by Allen and Behmanesh in 1994, illustrates the relationship between the dilution of materials and their market prices. Generally, materials are more likely to be recycled if they fall above the Sherwood line, indicating either a high concentration in a product or a high market price.
Studies have shown that for many metals, including tungsten in certain applications like automobiles, their position on the Sherwood plot can be in the non-recycling area due to high dilution (Fig. 9 in the source document). While some metals like Cr, Cu, and Ni are recycled despite this, it's because components containing higher concentrations of these metals (e.g., batteries, catalytic converters, wiring) are dismantled, effectively reducing their dilution and moving them into the recycling area of the plot. Tungsten parts in vehicles are often not separated, remaining diluted and thus unrecycled.
A significant increase in metal price could theoretically move a metal across the Sherwood line into the recyclable zone. However, for tungsten at very high dilutions, the price would need to reach hypothetical levels (e.g., US$1000/kg plus) to cross this threshold.
Small tungsten components, such as coils in energy-saving lamps, balls in pens, or vibration alarm weights in mobile phones, are often distributed within complex multi-material systems. These parts represent a very small fraction of the overall device weight, leading to extremely high dilution and placing them firmly in the non-recycling area of the Sherwood plot.
The total mass of tungsten per unit in such items is exceptionally small, resulting in an extremely low material value, sometimes less than 0.01 cent per part. At such low values, establishing economically viable collection and recycling systems is not feasible, as the costs of collection and separation far exceed the material's worth. This leads to the loss of several hundred tons of tungsten globally through discarding, as these small parts are widely distributed.
Even if a product like a mobile phone is in the recycling area (though close to the Sherwood line), the tungsten value (e.g., 0.76 cents for an unbalanced motor weight) is often too low to justify expensive dismantling processes. Future developments, such as robotized disassembling lines, might alter this situation if sufficient device volumes can be collected to operate such lines economically.
The increasing complexity of tools presents another economic challenge. Large wear parts used in mining, construction, oil & gas drilling, and metal forming are often composites of cemented carbide and steel. If carbide parts are pressed into a steel matrix, they can be separated relatively easily via thermal treatment. However, brazed parts require solder removal.
Often, steel tool parts or cutters are protected by tungsten-containing hardfacing materials that are sprayed, welded, or infiltrated onto the surface. In such cases, the high cost of manual pre-processing (transport, dismantling, material separation, solder removal) can render recycling economically unfeasible, despite a comparatively high tungsten value.
Modern high-performance drill bits, such as Polycrystalline Diamond Compact (PDC) cutters, are highly complex multi-material systems. They consist of components like copper alloy-infiltrated coarse WC or cast carbide drill bodies, cemented steel parts, PDC cutters bonded to tungsten carbide substrates, cemented carbide nozzles, solders, and intricate hardfacing layers. The mechanical and chemical treatments required to separate these components before recycling are extremely elaborate and expensive. Consequently, such drill bits are often only partially recycled; easily dismantled parts like PCD cutters might be separated, but the large remaining body containing significant tungsten mass may remain untouched. This necessitates new recycling strategies and technologies for these complex material combinations. Limits to recycling also arise when tungsten-bearing materials, such as DeNOx catalysts, become heavily contaminated at their end-of-life. The hydro-metallurgical multi-stage processing needed to separate valuable materials from contaminants is often more expensive than disposing of the waste in a hazardous waste disposal site.
The global flow of tungsten demonstrates a significant reliance on recycling, particularly for new scrap generated during production. Tungsten metal products, while accounting for 10% of global use, exhibit a comparatively low overall recycling rate of 22%, with heavy metal production scrap seeing higher internal reuse.
The economic limitations to tungsten recycling are multifaceted. The Sherwood plot effectively illustrates how high dilution and low market prices can render recycling unviable for many tungsten-containing products, especially small components in complex devices where the tungsten value per unit is negligible. Furthermore, the increasing complexity of modern tools and products, such as advanced drill bits and hardfaced components, introduces significant pre-processing costs related to dismantling and separation, often outweighing the value of the recoverable tungsten. Finally, heavily contaminated materials may face disposal as a more economical option than costly purification processes.
Addressing these economic hurdles will require innovations in collection logistics, automated disassembly technologies, and cost-effective separation techniques. While a wide array of recycling technologies for tungsten exists, their application is often constrained by these economic realities. Future improvements in recycling rates will depend on overcoming these limitations to make the recovery of tungsten from a broader range of end-of-life products economically feasible.