Review on resources and recycling of germanium, with special focus on characteristics, mechanism and challenges of solvent extraction

Abstract

Germanium is indispensable for various applications, especially in infrared optics, and has been designated as a strategic resource. However, independent germanium deposits are rare, and the total recovery efficiency for germanium is low. Hence, complete and rational utilization of existing germanium resources, including primary and secondary resources, is an extremely urgent requirement globally. To recover germanium from various aqueous solutions, diverse separation methods, such as tannin precipitation, solvent extraction etc., have been developed. Solvent extraction, being a highly selective and efficient process, is effective for recovering germanium. However, certain theoretical and practical challenges remain unaddressed with regard to germanium recovery via solvent extraction. Herein, the recovery of germanium from various resources via solvent extraction is discussed in detail, primarily from the following perspectives: (1) characteristics of different feeding solutions and the difficulties faced in recovering germanium from these solutions; (2) properties of different extractants (physical and chemical parameters, etc.); (3) germanium species in aqueous solution; and (4) extraction systems (advantages and disadvantages, mechanisms, challenges). The pH and composition of the aqueous phase and the price of the extractant determine the selection of extractant; the extractant characteristics determine the separation performance. The determination of the mechanisms of germanium extraction at the molecular (or atomic) level depends on the understanding of the germanium species. Based on these summaries, three potential research directions are proposed: control of co-extraction of non-target elements, determination of germanium species in various aqueous solutions, and environmental design and evaluation of solvent-extraction processes for germanium recovery. The significance of the review is to provide a reference for the development of a sustainable and more effective solvent-extraction process for recovering germanium from aqueous solutions.