Thermodynamics, Kinetics Model, and Reaction Mechanism of Low Vacuum Phosphate Reduction Process for Germanium Recovery from Optical Fiber Scraps

Germanium, because of its unique performance, has been widely used in the manufacture of optical fiber. Recycling germanium is significant because of geographical scarcity and technical barriers. In this study, a novel low vacuum phosphate reduction process of GeO2 was proposed to recover germanium from optical fiber scraps. The thermodynamics, kinetics model, product analysis, and reaction mechanism were studied. Thermodynamic results indicated that the decomposition products were mainly the P(V) compounds Na4P2O7 and Na5P3O10,(,) PH3, O-2, and H-2. The reaction order models of d alpha/dt = 1.44 X 10(8) exp(-212000/8.314T)(1 - alpha)(3.63) min(-1) can describe the kinetic results of the phosphate reduction process well. The average activation energy (E) over bar (alpha) located around 212 kJ mol(-1). The chemical species of Ge in the condensed products were metal Ge, Ge(II), and a small amount of Ge(IV) by XPS analysis. A hydrolysis reaction followed by a nucleophilic addition-elimination mechanism can explain the decomposition of NaH2PO2. Then, a mechanism of collapse of the GeO2 octahedron by the action of PH3(g) and H-2 was presented to clarify the formation of Ge and its oxides. Finally, a nucleation mechanism under low-vacuum conditions explained the distribution of condensed products. We quantitatively investigated the vacuum phosphate reduction process and can successfully guide germanium recovery from optical fiber scraps.