Inspection of Prebaked Carbon Anodes Using Multi-Spectral Acousto-Ultrasonic Excitation

Abstract

Reduction cell operation in primary aluminum production is strongly influenced by the properties of baked anodes. Producing consistent anode quality is more challenging nowadays due to the increasing variability of raw materials. Taking timely corrective actions to attenuate the impact of raw material fluctuations on anode quality is also difficult based on the core sampling and characterization scheme currently used by most anode manufacturers, because it is applied on a very small proportion of the anode production (about 1%), and long-time delays are required for lab characterization. The objective of this work is to develop rapid and non-destructive methods for the inspection of baked anodes. Previous work has established that sequential excitation of smaller parts collected from an industrial sized anode using acousto-ultrasonic signals at different frequencies allowed detecting and discriminating anode defects (pores and cracks). This was validated qualitatively using X-ray computed tomography. This work improves the method by using frequency-modulated excitation and building quantitative relationships between the acousto-ultrasonic signals and defects extracted from tomography images using Wavelet Transforms and Partial Least Squares (PLS) regression. The new excitation approach was found to provide similar or better inspection performance compared with sequential excitation, while requiring a shorter cycle time.

Interpolation of Pathway Based Non-Destructive Testing (NDT) Data for Defect Detection and Localization in Pre-Baked Carbon Anodes

“…First, it provides a simpler way to illustrate and visually appreciate the method's performance in a simpler environment before moving to more complex 3D situations. Second, the proposed interpolation technique is generic, and could be applied to other materials for which a 2D structure can be assumed, such as in [21,22].…”

“…For example, Alcoa's 4-point-probe (4PP) [8], SERMA [9][10][11][12][13][14], and MIREA [15][16][17][18] technologies were developed and implemented to measure electrical resistivity in multiple points using a similar concept to Seger's, but using different probe arrangements for application on green and/or baked anodes. Finally, mechanical properties-based measurements were also investigated, namely Modal Analysis (MA) [19], which measures the vibration response of the whole anode body at once after some mechanical excitation, and acousto-ultrasonic (AU) based methods [20][21][22][23][24] applied in multiple anode positions.…”