Thermogravimetric study of GaAs chlorination between -30 and 900 oC

Highlights: → The GaAs chlorination with Cl2 is feasible under several conditions. → The reaction products are different according to the working conditions. → The Cl2 partial pressure has a marked influence on the chlorination rate. → The reaction starts by three ways according to the interval of temperature used. - Abstract: Gallium (as GaAs) is at present an essential part of electronic devices, and the recovery of this element from electronic wastes is fundamental for the metallurgic industry. In this work, with the aim of recovering Ga by chlorination, the following reaction was investigated: GaAs(s) + 3Cl2(g) → GaCl3(s,g,l) + AsCl3(s,g,l) A thermogravimetric study of the previous reaction was carried out, and the following variables were investigated: chlorine partial pressure, in the range between 0.2 and 1 atm, by dilution with N2 and reaction temperature in the -29 oC to 900 oC interval. The thermodynamic calculations predicted that the reaction was feasible during the entire analyzed temperature range, and the experimental results were in agreement with the thermodynamic estimations. The results showed that the chlorination rate did not change significantly with temperature, but it increased with the chlorine partial pressure. It was found that AsCl3 was partially evaporated at temperatures above 20 oC, while the GaCl3 started evaporating at 100 oC. The experimental observations showed that the mechanism originating the reaction was different in each temperature range ranging between -30 and 160 oC, 200-400 oC, and above 500 oC. In the first case the reaction was caused by adsorption of Cl2 on the solid surface. In the second case, the solid started reacting when little quantities of GaCl3 and AsCl3 vapor were injected in the entrance of the system, indicating that the chlorination kinetics responded to an autocatalytic mechanism. Above 500 oC, the reaction was caused without adding chlorination products. The results of the analysis by scanning electron microscopy and X-ray diffraction showed a preferential attack on one of the crystal planes of GaAs. Besides, the formation of Ga2O3 and GaAsO4 was observed as a consequence of the presence of small quantities of oxygen accompanying Cl2. The results showed that chlorination is a selective and economic methodology for the recovery of gallium from electronic wastes.