Showing papers by "Houshang Alamdari published in 2014"

Clear microstructure–performance relationships in Mn-containing perovskite and hexaaluminate compounds prepared by activated reactive synthesis

Abstract: Microstructural properties of mixed oxides play essential roles in their oxygen mobility and consequently in their catalytic performances. Two families of mixed oxides (perovskite and hexaaluminate) with different microstructural features, such as crystal size and specific surface area, were prepared using the activated reactive synthesis (ARS) method. It was shown that ARS is a flexible route to synthesize both mixed oxides with nano-scale crystal size and high specific surface area. Redox properties and oxygen mobility were found to be strongly affected by the material microstructure. Catalytic activities of hexaaluminate and perovskite materials for methane oxidation were discussed in the light of structural, redox and oxygen mobility properties.

Automated Crack Detection Method Applied to CT Images of Baked Carbon Anode

Abstract: Computed tomography (CT) is a powerful non-destructive technique providing a large amount of useful data to characterize carbon anodes Previous works employed this technique to characterize baked anode samples and some relationships have been proposed for apparent density and total porosity as a function of X-ray attenuation coefficients In this paper, an existing method of crack detection in 2D was applied on CT images of full-scale baked anode to estimate the amount of cracks The crack detection method has however been modified to improve the termination procedure of the algorithm which was essentially based on the calculation of circularity of the percolated region The improvement consists of calculating other percolated region properties in order to end adequately the percolation process The proposed method has been applied to anode slices with and without stub holes

Characterization of Homogeneity of Green Anodes through X‐Ray Tomography and Image Analysis

Abstract: Homogeneity of green anode has a direct effect on final properties of baked carbon anodes. This work aimed to study the influence of mixing parameters on the homogeneity of laboratory-scale anodes. Anode pastes were made using different mixing times and temperatures and then underwent a compaction procedure. Homogeneity was characterized by the distribution of coke, pitch and porosity throughout the anodes as well as the variations in binder matrix thickness (fine coke + pitch). X-ray tomography was used as a non-destructive tool to evaluate the material distribution within the samples. The microscopic images of the green samples were analyzed to measure the mean thickness of binder matrix. The lowest variations in material distribution and the minimum thickness of binder matrix were obtained for the sample mixed at 178 °C for 10 minutes. The most homogeneous sample with the lowest binder matrix thickness had the maximum green and baked apparent density.

High Temperature Compression Test to Determine the Anode Paste Mechanical Properties

Abstract: In order to develop a constitutive law for Hall process anode finite element modelling, a compression test method was developed to identify the hot paste properties which evolve with the density. A thin stainless steel cylindrical mould was placed in a furnace mounted on a press. The mould was instrumented with strain gages in the axial and tangential directions to capture the strain as a function of the axial load applied on the paste. The compression test meets the need for characterizing the paste mechanical properties. Loadings were designed to excite the specific mechanical properties, as a function of density, useful for the numerical model. The deformation of the mould shell allowed evaluation of the paste Young’s modulus. A compaction test was performed and a tomography scan was realized on the resulting sample to confirm that the green compact density is almost uniform.

Characterization of Packing Ability of Coke Particles

Abstract: This work aimed to investigate the reliability of the vibrated bulk density (VBD) test for estimating the void fraction and packing ability of coke particles. VBD is conventionally used in the anode industry to characterize the calcined coke and to determine the required amount of pitch and fine coke. High VBD may be achieved by dense particles while they do not necessarily result in a highly packed bed of particles since packing properties depend on particle shape factors. The apparent density of coke fractions was measured using image analysis. Then the inter-particle void fraction was calculated from the VBD. This void fraction indicates the packing ability of particles. VBD results did not follow the void fraction trend. It is shown that different cokes with equal VBD values might present different inter-particle void fractions. Nor is the VBD necessarily in agreement with intraparticle porosity. It is thus suggested to consider complementary parameters such as shape factors and particle porosity along with the VBD.

Viscoplastic Modeling of the Green Anode Forming Process

Abstract: To model the forming process of green anodes, a nonlinear viscoplastic constitutive law using the concept of natural configuration has been developed. For this purpose, a Helmholtz free energy was proposed in order to take into account the nonlinear compressible behaviour of the anode paste and a dissipation potential was introduced to characterize the irreversible deformation process. An experimental study based on compaction tests of the anode paste at 150 °C was carried out. To characterize the Poisson’s effect, which leads to a non-negligible radial pressure, a thin steel instrumented mould was used. An inverse identification procedure using finite elements analysis showed that the model is able to reproduce experimental results in a good agreement.

Tribological behaviour of the green anode paste with a steel plate at 150 °C

Abstract: In order to accurately predict the mechanical behaviour of paste during forming process, the friction law between the carbon paste and the mould wall is an important parameter to be determined. This paper presents the tribological behaviour of the lubricated paste/steel interface subjected to high stress conditions at the anode forming temperature of 150 °C. A method to characterize the tribological behaviour has been developed and an apparatus was built. The method is based on the comparison of two successive experiments. In the first experiment, the paste is in contact with the friction plate. In the second one, a layer of Teflon is placed under the paste in order to excite another parameter thereby allowing the identification of the friction coefficient between the paste and steel wall. These experiments were performed with a paste under different normal loads. The static and kinetic friction coefficients of the Teflon/steel, steel/steel and paste/steel interfaces have been estimated. The static and kinetic friction coefficients of the Teflon/steel are respectively 0.17 and 0.13. The steel/steel friction coefficients were evaluated twice which gave a static coefficient that varies between 0.22 and 0.30. The kinetic coefficient varies between 0.18 and 0.25. The static and kinetic paste/steel friction coefficients obtained from both experiments are clearly similar. Their values are 0.15 and 0.13 respectively.

Room Temperature Creep Behaviour of Ramming Paste Baked at Different Temperatures

Abstract: To simulate the thermo-mechanical behaviour of ramming paste in aluminium electrolysis cells, experimental data are required to feed 3D constitutive models. Both axial and radial strain measurements are necessary to design constitutive laws and to identify the parameters. However, radial strains are difficult to obtain at high temperature due to limitations of measuring devices. Moreover, samples need to be sufficiently large to acquire significant radial strain amplitude. Hence, the creep behaviour of ramming paste was studied at room temperature so the radial behaviour could then be extrapolated for the high temperature case using the axial strain evolution as a function of the temperature. Mechanical properties of ramming paste, baked at temperatures ranging from 250 °C to 1000 °C, were measured at room temperature. Uniaxial creep tests were performed at three different stress levels for each baking temperature.

Characterisation of the Material Behaviour of Cathode Steel Collector Bar at High Temperatures and Low Stress Levels

Abstract: The study of the deformation behaviour of the collector bar at conditions experienced within the aluminium reduction cell is of great importance to optimizing the efficiency and increasing the life span of the cell. This paper communicates the results of an experimental program carried out on the steel collector bar material (AISI 1006) to investigate its behaviour in relation to its thermal, mechanical and the creep properties. Tests were carried out in compression at low stresses, 0.5 to 2 MPa and high temperature, 900 °C. Different behaviour was observed at low stresses below 2 MPa, which can be characterised by time and applied stress level. For the test at 2 MPa, a conventional creep curve with dominating secondary creep region was obtained. Oxidation and corrosion were factors considered due to the aggressive environment of the test condition. Metallographic inspection showed effect of oxides on tested sample.