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Germanium, a critical and rare metal, is increasingly in demand due to its applications in high-tech industries such as telecommunications, electronics, solar energy, and fiber optics. Given its strategic importance, extensive research has focused on developing efficient, environmentally benign, and economically viable methods for its recovery from a range of raw materials. These efforts are particularly focused on secondary resources, such as zinc oxide dust, coal, and end-of-life (EoL) products, for which traditional extraction processes have often been inefficient. This review provides an in-depth look at recent advances in germanium recovery, highlighting key methods and their applications in both laboratory and industrial settings.
A novel and highly promising method for germanium recovery involves the functionalization of TiO2 nanoparticles with organic acids containing hydroxyl groups, which significantly enhances the adsorption capacity for germanium. Among the various organic acids tested, tartaric acid (dihydroxysuccinic acid)-modified TiO2 nanoparticles (TA-TiO2-OH) have demonstrated excellent adsorption properties. These nanoparticles, with hydroxyl functional groups, exhibit a high germanium adsorption capacity of approximately 122 mg/g at a pH of 3. The presence of nearly 4.48 mmol/g of hydroxyl groups in TA-TiO2-OH nanoparticles facilitates strong interactions with germanium ions, thereby enhancing their uptake.
However, while the initial adsorption performance is impressive, challenges remain in the desorption phase. Germanium can be desorbed using 1 M HCl solutions; however, after multiple cycles, desorption efficiency decreases due to incomplete removal, reducing adsorption capacity. This issue suggests that although organic acid-functionalized TiO2 nanoparticles show potential for germanium recovery, further research is needed to optimize the desorption process and ensure their repeated use without significant loss of efficiency.
Zinc oxide dust, often produced in metallurgical processes, is a significant source of germanium. Still, its recovery is hindered by the adsorption of germanium onto colloidal iron hydroxides (Fe(OH)3) during the neutralization leaching process. Recent studies have explored the use of ultrasonication to improve the recovery of germanium from zinc oxide dust. Ultrasonication reduces the system’s redox potential and inhibits the formation of colloidal Fe(OH)3, which otherwise traps germanium and causes its loss.
Experimental data have demonstrated that under conditions such as 30 g/L sulfuric acid, 80°C, 500 W ultrasonic power, and a short pH adjustment time of 3 minutes, ultrasonication significantly reduces germanium loss rates compared with conventional processing. In traditional methods, high losses of elements occur due to the formation of colloidal Fe(OH)3, which leads to the entrapment of germanium. However, ultrasonication not only minimizes germanium loss but also improves overall recovery efficiency, offering a more sustainable and efficient alternative for germanium recovery from zinc oxide dust.
The search for the most effective adsorbent for germanium recovery has led to the development and testing of a variety of materials, including modified TiO2 nanoparticles, activated carbon, and other organic-functionalized adsorbents. These adsorbents exhibit a wide range of germanium uptake capacities, which vary based on the specific material and the conditions under which they are tested. The maximum germanium uptake capacity is an important factor for determining the feasibility of these materials for large-scale applications.
However, a significant issue arises when applying these adsorbents in real-world leachates, which are often highly acidic (sulfuric acid concentrations exceeding 100 g/L). Many studies evaluating germanium recovery focus on synthetic solutions with controlled pH values, which do not always reflect the challenging conditions encountered during industrial processing. Furthermore, the adsorption capacity of these materials tends to decrease after multiple adsorption-desorption cycles, making it difficult to assess the long-term effectiveness of various adsorbents. The disparity in performance between synthetic solutions and real-world leaching conditions underscores the need for further optimization of adsorbent materials and processes to operate efficiently under the acidic, complex conditions typically encountered in germanium-bearing industrial waste.
Coal and lignite are significant sources of germanium. However, the extraction of this metal from these materials has traditionally been hindered by the formation of germanium silicates, specifically GeO2-SiO2 solid solutions, which are difficult to separate. Recent advancements have focused on a combined process of gravity separation and low-temperature sintering to enhance germanium recovery from coal and lignite.
In this process, gravity separation is first used to concentrate the germanium in lignite, yielding a metal concentration factor of approximately 2. Subsequently, the lignite is subjected to sintering at temperatures ranging from 300–500°C. This treatment yields a residual ash containing 1.8 g of germanium per kilogram of lignite, with a concentration factor of 10.6. This combined approach enables the recovery of more than 90% of the germanium in lignite, which can then be extracted by chlorinated distillation. This process not only enhances germanium recovery but also helps reduce the environmental impacts of coal mining and combustion by recycling valuable metals from coal ash.
The combustion of coal generates ash that contains valuable elements such as germanium, arsenic, and tungsten. Recent research has explored the use of sequential vacuum distillation to recover these metals, particularly germanium, from coal ash. In this process, arsenic is volatilized first at temperatures below 550°C, while germanium and tungsten are volatilized as sulfides by the addition of sodium sulfite.
The optimal conditions for this process are a temperature of approximately 1050°C, a pressure of 1 Pa, and a reaction time of 2 hours. Under these conditions, germanium is recovered as germanium sulfide (GeS), while arsenic and tungsten are volatilized as arsenic sulfide (As2S3) and tungsten sulfide (WS2), respectively. This method has shown promise in recovering germanium from both coal fly ash and bottom ash, offering a sustainable way to recycle germanium while also removing toxic elements such as arsenic. The sequential vacuum distillation process thus provides an efficient and environmentally friendly method for recovering germanium from coal ash.
Despite promising advances in germanium recovery methods, significant challenges remain in scaling these processes to industrial scale. Many of the methods discussed above, including adsorption using modified TiO2 nanoparticles and ultrasonication, have been tested primarily in laboratory or small-scale settings. Translating these methods into large-scale, continuous operations capable of processing complex, highly acidic leachates remains a key hurdle.
Furthermore, while bio-hydrometallurgical methods using microorganisms or bio-based processes to recover metals have demonstrated potential in other metal recovery processes, their application to germanium recovery remains limited. The development of bio-hydrometallurgical approaches for germanium recovery presents an exciting avenue for future research, particularly given the increasing focus on sustainability and minimizing the environmental impact of mining and industrial waste processing.
Recent advancements in germanium recovery have significantly improved efficiency, sustainability, and environmental performance. Key innovations include the use of organic acid-functionalized TiO2 nanoparticles, ultrasonication for neutralizing zinc oxide dust, and advanced recovery techniques for coal and lignite. These developments offer new pathways for recovering germanium from secondary resources, such as industrial waste and coal, which have traditionally been difficult to process.
However, several challenges remain, particularly in optimizing these processes for industrial-scale applications. The gap between laboratory-scale successes and real-world conditions, such as highly acidic and complex leachates, must be addressed to ensure the commercial viability of these technologies. As demand for germanium continues to grow, these advancements will play a critical role in ensuring a reliable, sustainable supply of this valuable metal. Further research into bio-hydrometallurgical methods, continuous processing systems, and process optimization is essential for bridging this gap and ensuring that these recovery technologies can meet the needs of the industry.