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This paper reports on the application of an acousto-ultrasonic (AU) scheme for the inspection of industrial-size carbon anode blocks used in the production of primary aluminium by the Hall-Héroult process. A frequency-modulated wave is used to excite the anode blocks at multiple points. The collected attenuated AU signals are decomposed using the Discrete Wavelet Transform (DTW) after which vectors of features are calculated. Principal Component Analysis (PCA) is utilized to cluster the AU responses of the anodes. The approach allows locating cracks in the blocks and the AU features were found sensitive to crack severity. The results are validated using images collected after cutting some anodes.

Carbon anodes, used in Hall–Heroult electrolysis process, are subject to carboxy gasification. It is generally thought that the baked pitch of anode is more chemically reactive than coke and thus it preferentially reacts with CO 2 , resulting in premature disintegration of anode. In this study, the apparent reaction rates of anode and its constituents (coke, pitch and butt) were measured separately. The reaction conditions were set to minimize the mass transport limitations (sample size: 2 mg, CO 2 flow rate: 100 ml/min and particle size: about 30 μm). The activation energies of anode, coke, pitch and butt were found to be 192, 175, 207 and 182 kJ/mol, respectively. The evolution of specific surface areas in micro-, meso- and macropores as a function of gasification percentage was revealed using CO 2 and N 2 adsorptions and the ratio of catalyst/inhibitor was obtained using XPS analysis. It was found that the apparent reaction rate of anode and its constituents increased with the progress of the gasification under CO 2 atmosphere at 960 °C. The reaction rate of 0.033 min −1 at 50% of gasification, was obtained for pitch, which is similar to those of anode, coke and butt, suggesting that a pitch, baked separately, is not the most reactive constituent of anode.

As a result of tightening of environmental legislation, there is a renewed interest in the technology of alkali activated concrete also known as geopolymer concrete (GPC). Yet, for GPC to assume its own niche in the construction market, it must display equal or better properties at elevated temperatures than Ordinary Portland Cement (OPC) concrete. Currently there is sufficient evidence that GPC has a better resistance to fire; however not all of its behavioural aspects at high temperatures have been investigated. The aim of this research therefore is to investigate its deformational behaviour, in particular the existence or lack of transient creep, which OPC is known to undergo under first time heating when subject to compressive stresses. It was found that not only GPC undergoes transient creep, but also exhibits both expansive and contractive volume changes during heating. It was also found that GPC maintains its structural integrity at 1000 °C.

The quality of anode electrodes is an important issue in aluminum production plants. Therefore, not only raw material conditions but also the conditions of different production steps could affect the anode quality. Analyzed in this paper is the die compaction of some anode paste samples with specified compositions. Portions of fine coke and coarse coke particles were mixed and some coal tar pitch was added to this mixture in order to make the anode paste sample. The Blaine number of the fine coke portion and the pitch content were selected as two variables to make different anode paste samples. Thereafter, each sample was compacted in a rigid die while applying a uniaxial stress up to 60 MPa. The compaction behavior of different samples was studied and an analytical relationship between the green relative density and the axial pressure applied in the die was suggested. After having identified the parameters of the suggested expression for each sample, the contribution of the rearrangement phenomenon to the material densification was studied. Regarding the good agreement between the suggested relationship and the experimental results, it would be possible to predict the compactability of different anode pastes based on the analytical equation.

The anodes used in aluminum production are formed by compaction of a paste composed of binder matrix and coarse particles of petroleum coke (aggregates). Binder matrix composed of a coal tar pitch and fine calcined petroleum coke is usually characterized by coke and/or pitch content and also by the fineness of the coke particles. Since the coke particles are rigid and assumed to be non-deformable during compaction, the deformation behavior of the binder matrix plays a crucial role in the anode paste compaction process. Compaction of binder matrix with different compositions in a rigid closed die was studied in this work. Binder matrix compositions were compacted to a maximum uniaxial pressure of 70 MPa at 150°C. Different strain rates of 2.9 × 10 − 4 s − 1 and 2.9 × 10 − 3 s − 1 enabled us to evaluate the contribution of viscous behavior of the material to the compaction of binder matrix as a function of its composition and deformation rate. A similar experimental compaction procedure with strain rates of 1.8 × 10 − 4 s − 1 and 1.8 × 10 − 3 s − 1 was applied on paste samples with different pitch contents. This study revealed that the compaction of binder matrix and anode paste with conventional compositions is not significantly a time dependent process. Viscous behavior may therefore not have a significant contribution to the compaction of the material.

article i nfo The quality of anode electrode used in aluminum electrolysis industry and its dependence on the production parameters are studied in this work. Using a calcined petroleum coke and a coal tar pitch, anode pastes were made at laboratory scale at four mixing temperatures and four mixing times. The pastes were compacted in a cylindrical mold at a maximum pressure of 60 MPa. The green samples were then baked at 1130 °C for 12 h. Measurement of density variations as well as porosity and pore size distribution was carried out on the green and baked samples as a function of mixing temperature and time. For the setup used, an optimum mixing time and temperature were suggested, resulting in a better mixing effectiveness, maximum density, and min- imum air permeability and specific surface area.

The long term, visco-elastic behaviour of carbon cathode materials used into the Hall–Heroult cell linings has been investigated. The uniaxial compressive creep tests with free lateral strains have been conducted on three different carbon cathode materials (semi-graphitic, graphitic and graphitized) currently used in aluminium reduction cells. The evolution of mechanical properties caused by damaging material has been estimated through cyclic loading tests. Measurements were made of the axial and the radial strains at ambient temperature. The “key” operating mechanisms behind the observed creep phenomenon are evaluated.