Tellurium recovery from the thermoelectric materials bismuth telluride and antimony telluride by chemical vapour transport

Abstract

This work investigates the recovery of elemental tellurium from the thermoelectric materials Bi₂Te₃ and Sb₂Te₃ via chemical vapour transport (CVT) using the abundant element sulfur. A one-step process is established, involving a redox reaction between the respective tellurides and sulfur, followed by transporting elemental tellurium with the transport agent sulfur at mild temperatures. Differential scanning calorimetry identifies the redox reaction process for both systems (Bi₂Te₃/S and Sb₂Te₃/S) to occur in the temperature range of 175–200 °C. Additionally, the reaction conditions were optimised, and recovery rates, as well as transport rates for closed and open experimental setups, were determined. A temperature gradient of 425 °C → 325 °C with a Te : S molar ratio of 1 : 1.25 is proposed as optimum for a closed experimental setup. For an open experimental setup, a temperature gradient of 500 °C → 300 °C allows establishing a way with short reaction time of 1 h, identifying it as the best experimental recycling setup. For the system Bi₂Te₃/S, these reaction conditions resulted in a recovery rate up to 76% h⁻¹ (transport rate 151.2 mg h⁻¹) for the open experimental setup. Additionally, the possibility of instrumental upscaling was shown on a laboratory scale. In all cases, the purity of the recovered tellurium was analysed using PXRD, Raman spectroscopy, EDX and DTA/TG + MS. The purity of Te was increased to >99.9 wt% through a purification step of heating to 500 °C under an Ar atmosphere, surpassing the detection limit used.