Composition and Structure Analysis of Hard Grey Scale (HGS) Formed on Cold Surfaces Exposed to Aluminium Production Off-gas

Abstract

Hard grey scale (HGS) is a strongly adhering fouling material forming on solid surfaces impinged by off-gas generated in the pot cells of primary aluminium production plants. Even though associated maintenance costs have a significant economic impact, the mechanisms behind HGS formation are not well understood. In the present work, a cooled fouling probe or “cold finger” placed in the off-gas duct, upstream of the gas treatment centre (GTC), at a Norwegian aluminium production site was used to study the formation mechanisms of HGS. Fouling experiments were performed with durations ranging from a few hours to several months. HGS formed on the windward side of the probe, whereas dusty and loosely attached deposits accumulated on the leeward side. The chemical composition and crystal phase evolution of the different deposits and off-gas particle samples were analysed by electron probe micro-analyser equipped with an energy-dispersive spectrometer (EPMA-EDS), quantitative x-ray diffraction (Q-XRD), LECO-C and transmission electron microscopy (TEM). Moreover, image analysis (IA) was used to investigate the particle size distribution and deposition properties of particles with different compositions. Inertial deposition of atmolite (NaAlF4) nanoparticles, produced by pot cell electrolyte vapour condensation, has been identified as the key mechanism in the formation of HGS.