Coal-tar-pitch
(CTP) is a fossil carbon material, currently used
as a the binder in carbon anode manufacturing process. Regardless
of the technical benefits of coal-tar-pitch, it contains polycyclic
aromatic hydrocarbons (PAHs), known to be carcinogenic for humans
and detrimental to the environment. To overcome this challenge, research
was carried out to search for a suitable substitution with health-
and environmental-friendly properties. Bio-pitch, produced from bio-oil,
could be a potential alternative binder for the carbon anode manufacturing
process, due to its similarity with CTP. However, the properties of
bio-pitch could be significantly different from those of CTP depending
on its origins and process conditions. This study focuses on the synthesis
of bio-pitches from bio-oil under different vacuum extraction conditions
and characterization of its physical and chemical properties, aiming
at the determination of the conditions that may result in suitable
properties for anode formulation. Both typical characterization and
profound chemical analysis of bio-pitches were carried out, including
the determination of density, softening point, coking value, quinoline
insolubles, PAH content, molecular weight, viscosity, and elemental
composition and the identification of chemical structures. In addition,
the condensed fraction produced was also analyzed to identify the
reaction mechanisms occurring during the pyrolysis process.
Aluminum industry depends on coal-tar-pitch as a binder to produce carbon anodes used for aluminum electrolysis. This binder is nonrenewable and mainly composed of polycyclic aromatic hydrocarbons which are harmful for the human health and the environment. The biomass-driven pitch is considered to be a green and abundant binder. In this study, we synthesized biopitch from bio-oil by heat treatment of the bio-oil at 160 and 180 °C. The biopitch was then baked at 1100 °C to produce its carbonized form. In comparison to the carbonized coal-tar-pitch, the baked biopitch showed amorphous microstructure, higher air reactivity and higher specific electrical resistivity. Biopitch is known to have high wettability and adhesion with the coke particles which could reduce the negative effect of inferior characteristics of its baked form on the resulting anodes. Biopitch was used to replace 100 % of the conventional coal-tar-pitch binder in the anode recipe. Green anodes were produced by mixing the coke aggregates with the biopitch at 178 °C. The baked anode properties were measured and compared to those of classical reference anodes (made of coal-tar-pitch and calcined coke). Although, the air and CO 2 reactivity of the biopitch anodes is slightly higher than that of the reference ones, the biopitch anodes showed similar density, coefficient of thermal expansion, specific electrical resistivity, mechanical strength, and lower air permeability in comparison to those of the reference anodes.
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).
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