Investigation on Scale Formation in Aluminium Industry by Means of a Cold-Finger

A cylindrical fouling probe or “cold-finger” has been used to investigate fouling from aluminium production off-gas. The probe was located upstream from the off-gas cleaning system. Surface deposits have been collected for further analysis by EPMA and XRD, and compared with off-gas dust and old scale samples collected in the same experimental site. Cross-section micrographs of the deposit surfaces have been obtained to highlight the differences in surface structures formed on the upstream and downstream faces of the cold-finger. Strongly adhered hard scale formed after only two days in the upstream face of the probe. Loosely attached deposits accumulated downstream, which consisted of distinguishable particles of Al₂O₃, spherical Cryolitic bath condensates and Ni-S phases. The hard scale was rich in small bath condensates (NaAlF₄) that form a tight network keeping together the larger particles. The deposition of those particles is suggested to be a key in scale formation.