The Potential of Acousto-Ultrasonic Techniques for Inspection of Baked Carbon Anodes

Abstract

High quality baked carbon anodes contribute to the optimal performance of aluminum reduction cells. However, the currently decreasing quality and increasing variability of anode raw materials (coke and pitch) make it challenging to manufacture the anodes with consistent overall quality. Intercepting faulty anodes (e.g., presence of cracks and pores) before they are set in reduction cells and deteriorate their performance is therefore important. This is a difficult task, even in modern and well-instrumented anode plants, because lab testing using core samples can only characterize a small proportion of the anode production due to the costly, time-consuming, and destructive nature of the analytical methods. In addition, these results are not necessarily representative of the whole anode block. The objective of this work is to develop a rapid and non-destructive method for quality control of baked anodes using acousto-ultrasonic (AU) techniques. The acoustic responses of anode samples (sliced sections) were analyzed using a combination of temporal features computed from AU signals and principal component analysis (PCA). The AU signals were found sensitive to pores and cracks and were able to discriminate the two types of defects. The results were validated qualitatively by submitting the samples to X-ray Computed Tomography (CT scan).

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

“…The results showed that the attenuated acoustic response of the materials obtained after frequency-modulated excitation was sensitive to the presence of cracks within anode samples, and to the density of pores distributed throughout the anode samples (slices). The performance of this approach was found to be similar to that obtained using sequential excitation [6], but with a significant reduction in cycle time.…”

“…Note that the following eight slices were selected for further AU analysis: 2, 3, 5, 7, 11, 15, 24 and 25 (indicated using red numbers in Figure 1a). These slices spanned a range of internal structures, and were selected from areas which were expected to contain different types of defects, such as pores and cracks (see [6] for more details). The CT-scan measured the attenuation coefficients of the material for a set of 512 × 512 3D voxel (volumetric pixel) per scan, which corresponded to the tomograph resolution.…”

“…These sensors allow measurement of a very important anode property related with energy efficiency, but their capacity to detect, locate and diagnose physical defects within the anode blocks has yet to be established. Alternatively, the performance of acousto-ultrasonic (AU) techniques for detection of various defects in carbon anodes has been demonstrated in previous work [6] using sequential acoustic excitation at different frequencies. Sliced anode samples were tested using this approach and their acoustic responses was analyzed using Principal Component Analysis (PCA).…”

“…For instance, the sequential excitation scheme used in previous work to obtain good inspection results consisted of testing the materials at seven excitation frequencies in the 100-250 kHz range for each measurement point [6]. A single frequency modulated wave in the same range would reduce cycle time by 7 to 1 for the same number of data points.…”

Aluminium Production Process: Challenges and Opportunities

“…Ben Boubaker et al [5] reported a work, exploring the potential of the acousto-ultrasonic (AU) technique as a non-destructive inspection method for prebaked anodes. Fast, reliable and non-destructive testing methods are of high importance in Al smelting process, making it possible to detect and discard the low-quality anodes before being set in the electrolysis cell.…”