Superalloys are a class of high-performance materials designed to operate in extreme conditions, typically characterized by their ability to retain mechanical strength, resist oxidation, and prevent creep under high-temperature environments. These alloys are primarily composed of base metals like nickel (Ni), cobalt (Co), and iron, which are alloyed with other elements such as chromium (Cr), molybdenum (Mo), and tungsten (W) to enhance their mechanical and thermal properties. Due to their exceptional properties, superalloys are used in critical applications like aerospace engines, gas turbines, and chemical reactors. However, despite their robustness and importance, superalloys present considerable challenges when it comes to recycling.
The recycling of superalloys is complex due to their multi-element composition and the high performance of their constituent metals. The difficulty lies in separating these elements in an efficient manner. Superalloys are composed of a mixture of valuable, high-demand metals such as nickel, cobalt, rhenium (Re), and tungsten, alongside less desirable metals that may be present in trace amounts. Recycling processes must selectively recover these valuable metals while leaving behind refractory elements like tantalum (Ta) and tungsten. Traditional recycling methods, such as smelting or electrorefining, often fail to achieve this separation due to the similar chemical properties of many elements in the superalloy, leading to a loss of valuable metals or contamination. This inefficiency not only makes the recycling process costly but also increases the environmental impact by requiring additional raw material extraction. The need for a more effective and selective recycling method is thus essential in improving both the economics and sustainability of superalloy recovery.
One promising approach for cleaning and recycling superalloys is the use of molten magnesium (Mg) in a process known as dealloying. The use of molten magnesium in dealloying superalloys presents a promising and efficient method for cleaning and recycling these valuable materials, as demonstrated in the research by (Li et al. 2024). Molten magnesium has the unique ability to selectively dissolve specific metals from superalloys based on differences in their reactivity. The process involves immersing a superalloy into molten magnesium at high temperatures, typically around 850°C, where magnesium preferentially reacts with and dissolves more reactive elements, such as nickel and cobalt, leaving behind less reactive and more refractory metals like rhenium, tantalum, and tungsten. This selective dissolution process is driven by the significant difference in the chemical reactivity of the elements within the superalloy.
By controlling various parameters such as temperature, reaction time, and the mass ratio of superalloy to molten magnesium, the extraction of valuable metals can be optimized. For example, at a temperature of 850°C and with a reaction time of approximately 300 minutes, the extraction rate of nickel from the DD5 superalloy has been shown to exceed 88%. This selective extraction helps concentrate high-value metals into a Ni-rich alloy, while the residual superalloy, now depleted of nickel and cobalt, retains a higher concentration of refractory elements.
During the dealloying process, the residual superalloy undergoes significant microstructural changes. The extraction of nickel and cobalt leads to the formation of pores and voids within the alloy, which enhances its surface area. The porosity of the residual superalloy is typically in the range of 2–30 nm, as revealed by nitrogen adsorption and desorption tests. These changes in microstructure are crucial for subsequent recycling steps as they make the residual material more brittle and easier to crush. This transformation is beneficial for the subsequent recovery of metals such as rhenium and tungsten, which are present in the residual superalloy in higher concentrations after the nickel extraction.
The microstructural alterations induced by molten magnesium dealloying are further reflected in the mechanical properties of the residual superalloy. The compressive strength of the residual material is drastically reduced, often to approximately one-tenth of the original superalloy’s strength. This reduction in mechanical integrity is advantageous for the recycling process as it facilitates the breaking down of the residual superalloy into smaller particles, which can be further processed for the extraction of remaining high-value metals.
The dealloying process using molten magnesium not only facilitates the extraction of nickel but also makes the residual superalloy easier to process. After the initial dealloying step, the residual material, now rich in refractory metals, can be subjected to additional processes, such as hydrometallurgical treatment, to recover valuable metals like rhenium and tungsten. The porosity and reduced mechanical strength of the residual superalloy allow for efficient grinding and milling into fine powders, typically in the range of 5–100 μm, after a short ball-milling process. These fine powders can then be subjected to further refining techniques to recover the remaining valuable elements.
One of the benefits of using molten magnesium is the creation of a separation between the Ni-rich alloy and the residual superalloy, effectively concentrating the high-value metals. This separation is crucial, as it allows for the recycling of multiple elements from a single superalloy without the need for complex multi-step processes. The remaining molten magnesium can also be recycled, making the entire process more sustainable.
To maximize the efficiency of the dealloying process, certain parameters must be carefully controlled. The temperature, reaction time, and mass ratio of the superalloy to molten magnesium are key factors in determining the extraction efficiency of valuable metals. It has been observed that increasing the temperature to around 850°C significantly enhances the extraction rate of nickel, but excessive heating can lead to undesirable effects, such as the volatilization of magnesium or back-diffusion of metals like nickel into the residual superalloy. Thus, the optimal reaction temperature is typically in the range of 800–850°C.
The reaction time also plays a significant role in determining the extent of metal extraction. Dealloying durations of around 300 minutes are considered optimal for maximizing nickel extraction, while longer durations may lead to the evaporation of magnesium and a reduction in extraction efficiency. Similarly, the mass ratio of superalloy to molten magnesium affects the overall extraction process, with the highest nickel extraction rate typically occurring at a mass ratio of 1:10.
The use of molten magnesium in dealloying superalloys presents a promising and efficient method for cleaning and recycling these valuable materials. By selectively dissolving more reactive elements such as nickel and cobalt, this process helps recover high-value metals while leaving behind a residual superalloy rich in refractory elements like rhenium and tungsten. The microstructural changes induced by dealloying, including the formation of pores and the reduction of mechanical strength, enhance the ease with which the residual superalloy can be processed in subsequent stages. As such, this approach offers a more sustainable and economically viable solution to the challenges of superalloy recycling, enabling the recovery of critical metals in a more efficient and environmentally friendly manner.
Li, H., Wang, J., Liu, F., Guo, X., Wang, Z., Yu, D., & Tian, Q. (2024). Clean recycling of spent nickel-based single-crystal superalloy by molten magnesium. Journal of Materials Research and Technology, 30, 3960-3966.