Rhenium (Re) is a scarce, strategically vital metal with unique properties that make it indispensable in high-temperature superalloys for jet engines and power generation, as well as in platinum-rhenium catalysts for the petrochemical industry. Its limited natural abundance, with an estimated average crustal concentration of just 0.4 parts per billion, and projections of resource depletion within a century, underscore the critical need for efficient recycling from secondary sources.
The demand for rhenium is steadily increasing with technological advancements, yet its substitution in key applications remains challenging without incurring significant performance losses or higher costs. Recycling not only alleviates the pressure on primary resource extraction but also avoids substantial mining costs, waste generation, and associated environmental impacts.
Secondary resources for rhenium generally contain significantly higher concentrations of the metal compared to primary ores. These sources primarily encompass Ni-Re superalloys, which are a major contributor, typically containing 1–20 wt% Re, with common commercial alloys having 3–6 wt% Re; used turbine blades from jet engines serve as a prime example of this category.
Another key secondary resource is W-Re and Mo-Re alloys, which usually contain 3–5 wt% Re and find use in applications such as heating elements and X-ray tubes. Finally, spent Re-containing catalysts, predominantly from the petroleum refining industry, represent a significant stream, typically containing around 0.3 wt% Re, often alongside platinum, on an alumina support. Globally, it's estimated that at least 10 tons of rhenium are recycled annually from such spent scrap, with a potential ultimate recyclability of over 80%.
Rhenium recycling is a multi-faceted process that typically combines pyrometallurgical (high-temperature processing) and hydrometallurgical (aqueous solution-based processing) techniques. The choice of specific methods is heavily influenced by the composition of the secondary resource and the need to also recover other valuable metals present in the scrap. The overarching goal is to efficiently separate rhenium and other metals from the scrap matrix, after which the purification steps often mirror those used in extracting rhenium from primary resources.
Over the years, various methods have been developed for recovering rhenium from superalloy scrap. One notable approach is the electrolytic decomposition method proposed by Stoller et al. in 2008. This process involves low-frequency electrolysis in a cell containing a 15–25 wt% hydrochloric acid solution. Once the electrolytic process is complete, the remaining scrap undergoes an oxidizing leach using a sodium hydroxide-hydrogen peroxide solution. Rhenium is then recovered from the resulting filtrate through ion exchange.
Another technique, described by Olbrich et al. in 2009, employs molten salt digestion. In this method, superalloys are digested in a molten salt mixture consisting of 60–95 wt% sodium hydroxide and 5–40 wt% sodium sulfate at temperatures between 800 and 1200 °C. The digestion process is often enhanced with additives like sodium carbonate and oxidizing agents such as sodium nitrate. The end product is subsequently dissolved in water for the hydrometallurgical separation of metals.
Further advancements were made by Palant et al. between 2011 and 2014, who developed a complex electrochemical process utilizing electrolytes like sulfuric acid, a combination of sulfuric and hydrochloric acids, or nitric acid. In this setup, about 70% of the rhenium concentrates in anode slimes, which are then leached using ammonia, while 25–30% transfers to the electrolyte and is recovered through solvent extraction, leading to the production of KReO4 crystals.
Additionally, Dasan et al. introduced a method in 2011 that begins with grinding the scrap into fine particles (~5 µm) to increase the surface area. The ground material is then oxidized, converting rhenium into a volatile oxide form. Following this, Luederitz et al. in 2013 patented a process where all metals are solubilized using hydrochloric acid or mixtures of hydrochloric and nitric acids. This technique allows for the selective precipitation of rhenium as Re2S7, which later gets oxidized and sublimated into Re2O7.
In 2016, Srivastava et al. devised a two-step leaching process where base metals like nickel, aluminum, chromium, cobalt, and tungsten are first leached using 4 mol/L hydrochloric acid. Rhenium remains in the residue, and it is subsequently leached using in-situ generated chlorine in the same hydrochloric acid solution before undergoing solvent extraction.
Britton and Markarian, in 2017, patented another method involving digestion with a 50–1000 g/L sulfuric acid solution along with a halide-free oxidant, such as air, ozone, oxygen, or peroxide. This is followed by further digestion of the filter cake and the separation of rhenium and platinum through ion exchange.
In a different approach, Kim et al. in 2018 explored a pyrometallurgical pretreatment followed by leaching. In this process, superalloy scrap with a particle size of less than 150 μm is pretreated with aluminum granules at temperatures around 1500 °C, resulting in the formation of Al3Ni. This is followed by a two-step hydrochloric acid leaching, first extracting nickel, aluminum, cobalt, and chromium, and subsequently leaching rhenium from the residue using electrogenerated chlorine, leaving tantalum behind.
Finally, Mamo et al. in 2019 investigated the use of aqua-regia for rhenium recovery, which included a two-stage precipitation process. This process involved precipitating oxides of aluminum, chromium, molybdenum, and titanium at a pH of 5.05, followed by mixed hydroxides of cobalt and nickel at a pH of 7.0, ultimately resulting in a rhenium-enriched solution.
Spent catalysts typically contain around 0.3 wt% rhenium (Re) and 0.3 wt% platinum (Pt) on an alumina carrier, and recovery methods often focus on selectively extracting these valuable metals. Initially, before the 1980s, historical caustic dissolution was a common method for recovery, but it posed significant challenges in effectively retrieving rhenium.
In more recent practices, sulfuric acid dissolution has emerged as a more favorable approach. This method generally involves leaching rhenium and aluminum using sulfuric acid or sodium hydroxide, often with reducing agents included to keep platinum within the residue. Following this step, rhenium can be separated from the aluminum sulfate solution through ion exchange or solvent extraction, as detailed by El Guindy in 1997.
Another method discussed by Elutin et al. in 1997 is selective roasting and leaching. This technique entails roasting the materials, followed by acid or alkali leaching and ion exchange to produce high-purity ammonium perrhenate (NH4ReO4). Additionally, Han and Meng patented a process in 1996 known as selective pressure leaching. This involves using ammonium halogen salts—specifically iodide or bromide—combined with oxygen and sulfuric acid at elevated temperatures and pressures, allowing for the recovery of both rhenium and platinum through various methods.
Research by Angelidis et al. in 1999 introduced a simplified dissolution process using dilute sodium bicarbonate. This method allows for up to 97% recovery of rhenium initially, followed by sulfuric acid leaching to extract any remaining platinum and aluminum.Furthermore, Thomas in 2008 patented a process for halogen acid dissolution followed by resin extraction.
This method targets the extraction of rhenium, gold, and platinum group metals from an acid solution—preferably a combination of a halogen acid and a halogen element—and utilizes a non-cross-linked polyamine composite resin along with solvent extraction techniques.
In 2003, Allison et al. patented a unique oxidation and sublimation method. This technique converts rhenium into a sublimable oxide by heating it in an oxidizing atmosphere, allowing for the isolation of rhenium from the resulting volatilized oxide. There are also general acid leaching techniques that have been applied in the processing of superalloys, as reported by researchers such as Luederitz et al. (2013), Ferron and Seeley (2015), and Britton and Markarian (2017).
Lastly, Kasikov and Petrova in 2008 offered a categorization of catalyst decomposition methods, distinguishing between those that selectively extract rhenium without decomposing the alumina carrier and those that involve carrier decomposition as part of the process.
The recycling of rhenium from secondary resources is paramount for establishing a sustainable, low-carbon, and resource-efficient economy. The initial crucial step involves the effective collection of end-of-life products and meticulous classification of different types of rhenium-containing scrap. Based on this classification, a suitable and efficient process flowsheet can be determined. This selection process must thoroughly consider several factors.
Economic feasibility is a primary concern, ensuring the cost-effectiveness of the recovery process. Recycle efficiency is also key, aiming to maximize the yield of rhenium and other valuable metals. Environmental factors must be addressed by minimizing waste and harmful emissions. The potential recycling of other metals present in the scrap should be integrated into the process. Finally, the utilization of existing industrial equipment can help to reduce capital investment.
The metallurgy of rhenium, encompassing both primary extraction and secondary recycling, has made significant strides. While primary extraction focuses on enriching rhenium fractions from molybdenum, copper, lead, and uranium processing, recycling efforts are centered on efficiently processing diverse and often complex spent materials.
The continued development and optimization of recycling technologies are essential to ensure a stable and sustainable supply chain for this critical metal, supporting high-tech industries and emerging innovations.
For organizations looking to responsibly manage their rhenium-containing scrap and contribute to a circular economy, Quest offers state-of-the-art recycling solutions. By partnering with Quest, you can ensure that your rhenium scrap is processed with high efficiency, environmental consideration, and economic viability, transforming waste streams into valuable resources.
Critical evaluations of all recycling processes remain necessary to identify the most sustainable options and build a resilient rhenium industry for the future, and Quest is committed to being at forefront of these advancements.