The recovery of germanium (Ge) from fly ash (CFA) is an important task in the mining and waste recycling sectors due to germanium’s valuable applications in the semiconductor, optical, and solar industries. Fly ash, a byproduct of coal combustion in power plants, contains trace amounts of various metals, including germanium. Despite its low concentration, germanium recovery from fly ash can contribute significantly to the global supply of this rare metal.
Fly ash primarily consists of aluminosilicate minerals, including aluminum (Al), silicon (Si), calcium (Ca), and iron (Fe) oxides, along with other trace metals. The germanium content in fly ash is generally low but still valuable enough to be recovered. Roasting of fly ash and subsequent chemical leaching are the most common methods for recovering germanium from CFA.
Chemical leaching is used to dissolve metal ions from the solid fly ash matrix. However, germanium is often present in its oxidized form within the silicate or aluminosilicate minerals, making its recovery challenging. Therefore, pretreatment steps such as roasting are necessary to enhance the solubility of germanium by altering its chemical state, facilitating its dissolution during the leaching process.
Chemical leaching of metals from roasted CFA typically involves the use of strong acids or organic acids to solubilize the target metals. The roasting process usually employs alkaline reagents such as sodium carbonate (Na2CO3) to break down the silicate structure of the ash. This process, known as alkaline roasting, converts the aluminosilicate phases into more soluble forms, enhancing the extraction efficiency during the subsequent acid leaching.
The first step in the chemical recovery of germanium is the roasting of fly ash with alkaline reagents. This process helps break down the complex mineral matrix in fly ash, converting metal-containing phases into more soluble compounds. Roasting typically occurs at high temperatures (above 700°C) in the presence of sodium carbonate, which reacts with the silicate and alumino-silicate minerals in the fly ash, forming sodium aluminate and sodium silicate. This conversion increases the solubility of germanium, making it easier to extract during the acid leaching phase.
The roasted fly ash is then subjected to leaching with acids to extract germanium and other valuable metals like vanadium (V) and lithium (Li).
In the chemical leaching process, organic acids like citric acid, oxalic acid, and gluconic acid are typically used to dissolve metals from roasted fly ash. These acids act as chelating agents, forming stable metal-organic complexes that help solubilize germanium and other metals from the solid matrix.
For instance, gluconic acid has been found to be particularly effective in dissolving germanium from fly ash. Other organic acids like oxalic acid and citric acid can also assist in the leaching process by lowering the pH of the leaching solution and improving the solubility of metal ions.
The leaching efficiency is influenced by several factors, including:
The efficiency of the chemical leaching process can vary depending on the type of acid used and the roasting conditions. For instance, gluconic acid has shown high efficacy in the leaching of germanium, while other acids like citric acid and oxalic acid also contribute to the dissolution of metals from roasted fly ash.
Several parameters must be optimized to maximize the recovery of germanium from roasted CFA:
When comparing chemical leaching with other recovery methods, such as bioleaching or physical extraction, chemical leaching generally results in higher recovery rates of germanium, particularly when organic acids are used. However, chemical leaching also has environmental drawbacks, including the use of toxic chemicals and the production of waste streams that must be managed properly.
In terms of metal recovery, chemical leaching with commercial acids yields recoveries of germanium, vanadium, and lithium that can be further optimized through careful adjustment of leaching conditions. However, studies have shown that the recovery of germanium in chemical leaching processes is slightly lower compared to bioleaching methods due to the additional involvement of bacterial metabolites, which can enhance the dissolution of metals beyond what is achievable with organic acids alone.
The recovery of germanium from real fly ash using chemical leaching methods has proven to be effective, particularly when organic acids like gluconic acid, citric acid, and oxalic acid are used. The process typically requires roasting of the fly ash to enhance metal solubility, followed by acid leaching to extract germanium and other valuable metals. The optimized conditions for chemical leaching include the proper acid concentration, leaching time, temperature, and agitation speed.
Despite its effectiveness, chemical leaching does have certain environmental concerns, primarily due to the use of harsh chemicals and the generation of toxic waste. However, it remains one of the most reliable methods for the extraction of germanium from fly ash, contributing to the recycling of valuable metals and the reduction of waste materials.
Torralvo, F. A., & Fernández-Pereira, C. (2011). Recovery of germanium from real fly ash leachates by ion-exchange extraction. Minerals Engineering, 24(1), 35-41.
Rezaei, H., Shafaei, S. Z., Abdollahi, H., Ghassa, S., Boroumand, Z., & Nosratabad, A. F. (2023). Spent-medium leaching of germanium, vanadium and lithium from coal fly ash with biogenic carboxylic acids and comparison with chemical leaching. Hydrometallurgy, 217, 106038.