A cleaning approach for the preparation of nano tungsten oxide from waste tungsten powder by hydrogen peroxide leaching and spray pyrolysis

In the production process of tungsten powder, waste tungsten powder with excessive impurities is generated due to the contact between tungsten oxide and iron boats. Currently, there is no cost-effective direct utilization method. This study proposes to use the green oxidant H2O2 to selectively leach waste tungsten powder to obtain a pure peroxotungstic acid solution and then to directly prepare nano-WO3 by spray pyrolysis. First, the leaching of tungsten was investigated in detail. Under the conditions of a reaction time of 60 min, temperature of 40 °C, raw material particle size of 5–13 μm, liquid-to-solid ratio of 15:1 ml/g, and H2O2 concentration of 16 mol/L, the leaching efficiency of tungsten reached 97.89%, where the impurity elements (Fe, Ni and Cr) were all below 1%. The analysis of the leachate confirmed that W was leached in the form of peroxotungstic acid (PTA). Second, the thermal decomposition mechanism of PTA was investigated. The decomposition process was divided into three stages. The first stage (50–135 °C) was [WO2(O2)H2O]·nH2O to remove crystal water; the second stage (137–230 °C) was when the WO2(O2)H2O crystals decomposed into amorphous tungsten oxide and released H2O and O2. The third stage (350–420 °C) was when amorphous tungsten oxide transformed into WO3 crystals. Finally, under the conditions of an initial PTA concentration of 20 g/L, temperature of 240 °C and air velocity of 90 L/min, spray pyrolysis of the PTA leachate was performed, and hollow nanosphere WO3 with an average particle size of less than 500 nm was obtained. This novel method realizes the clean and efficient preparation of nano-WO3 and the short-process reuse of waste tungsten powder.