Abstract
Hard Grey Scale (HGS) is a hard and strongly adhering fouling material that forms on solid surfaces impinged by the off-gas generated in the pot cells of aluminium production plants. Different theories on HGS formation mechanisms have been suggested, involving particle re-crystallization or formation of a binder phase between the deposited particles. However, no conclusive proofs have yet been presented. This study focuses on statistical quantification of HGS growth rates and particle deposition characteristics from analysis of fouling experiments in a primary aluminium production plant. The experiments were performed with a cylindrical cold-finger inserted in a duct upstream of the Gas Treatment Center (GTC) with durations ranging from a few hours to several months. Detailed characterization of cold-finger deposits and off-gas particle size distributions was performed using image analysis (IA) and a Particle Diffraction Sizer (PDS) to investigate particle capture efficiencies on both sides of the cold-finger. In addition, an old HGS sample from a secondary alumina transport pipe, with different appearance than cold-finger HGS, was analyzed. Results show that HGS particles have a log-normal size distribution centered at 1.2 μm with 35% of the particles below 1 μm. A similar granular structure and size distribution was found for the HGS sample from the secondary alumina transport pipe, suggesting that a similar formation mechanism exists for the two types of HGS. Particle capture efficiency results for cold-finger HGS at low Stokes numbers are in good agreement with numerical studies considering inertial impaction efficiencies. At larger Stokes numbers, i.e. larger particle sizes, rebounding and re-entrainment effects result in lower capture efficiencies than those predicted by the impaction models. The present results suggest that HGS formation is due to inertial deposition of small particles, with the submicron-sized fraction generating a closely packed structure responsible for the HGS bulk properties.