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

Acylindrical 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 forma tight network keeping together the larger particles. The deposition of those particles is suggested to be a key in scale formation.