Hydrometallurgical Recovery of Germanium from Coal Gasification Fly Ash: Pilot Plant Scale Evaluation

In this article, a hydrometallurgical method for the selective recovery of germanium from fly ash (FA) has been tested at pilot plant scale. The pilot plant flowsheet comprised a first stage of water leaching of FA, and a subsequent selective recovery of the germanium from the leachate by solvent extraction method. The solvent extraction method was based on Ge complexation with catechol in an aqueous solution followed by the extraction of the Ge−catechol complex (Ge(C6H4O2)32−) with an extracting organic reagent (trioctylamine) diluted in an organic solvent (kerosene), the relevant reaction probably being 2(C8H17)3N(org) + Ge(OH)4o(aq) + 3(C6H6O2)(aq) ↔ ((C8H17)3NH)2Ge(C6H4O2)3(org) + 4H2O(aq) followed by the subsequent stripping of the organic extract. The process has been tested on a FA generated in an integrated gasification with combined cycle (IGCC) process. The paper describes the designed 5 kg/h pilot plant and the tests performed on it. Under the operational conditions tested, approximately 50% of germanium could be recovered from FA after a water extraction at room temperature. Regarding the solvent extraction method, the best operational conditions for obtaining a concentrated germanium-bearing solution practically free of impurities were as follows: extraction time equal to 20 min; aqueous phase/organic phase volumetric ratio equal to 5; stripping with 1 M NaOH, stripping time equal to 30 min, and stripping phase/organic phase volumetric ratio equal to 5. 95% of germanium were recovered from water leachates using those conditions.