Predicting Tungsten Oxide Reduction with the Extended Discrete Element Method

During technical reduction of tungsten trioxide powder in hydrogen atmospheres, the local temperature and the ratio of water vapor to hydrogen partial pressures govern the conversion rate. Water vapor removal rate not only affects the conversion progress, but also drives the final metallic tungsten powder size distribution. The amount of water vapor inside the bed depends on the hydrogen flow, the height of powder beds and the size characteristics of the initial oxide. The chemically aggressive environment and high temperatures make it difficult to do the measurements inside the reactors for studying or control the process. On the other hand, multi-physics computational techniques help to understand the evolution of the complex phenomena involved in the process. This contribution presents the eXtended Discrete Element Method as a novel approach to investigate the complex thermochemical conversion of tungsten oxides into tungsten metal. The recently emerged technique is based on a coupled discrete and continuous numerical simulation framework. In the study, an advanced and consolidated two-phase Computational Fluid Dynamics (CFD) tool for porous media represents gaseous phase penetration and transport. The discrete feedstock description includes one-dimensional and transient distributions of temperature and species for each powder particle. This allows gaining a new and valuable insight into the process, which may lead into finer tungsten powder production, and consequently more resistant tungsten carbide products. Transient and spatial results for powder composition, gas species as well as a mass loss comparison with experimental data for non-isothermal hydrogen reduction of tungsten trioxide are demonstrated and discussed.