Electrode Technology: Anode Characterization
Sponsored by: TMS Light Metals Division, TMS: Aluminum Committee
Program Organizers: Houshang Alamdari, Laval University
Monday 2:00 PM
February 27, 2017
Location: San Diego Convention Ctr
Session Chair: Duygu Kocaefe , University of Quebec at Chicoutimi
2:00 PM Introductory Comments
Characterization of Prebake Anodes by Micro X-ray Computed Tomography: Stein Rørvik1; Lorentz Lossius2; 1SINTEF Materials & Chemistry; 2Hydro Aluminium
As part of the continuous work in improving anode quality at Hydro Aluminium, several series of pilot scale anodes have been manufactured with systematic changes in coke type, recipe and green paste production. The anodes support MSc, PhD and PostDoc work in research programs supported by Hydro and the Norwegian Research Council. In addition to regular analysis; pore-, void- and grain-distribution is investigated using Micro X-Ray Computed Tomography (CT). This non-destructive 3D imaging is implemented at a cost allowing larger numbers of samples, using a methodology developed by SINTEF that yields very sharp detail, suited to interpreting micro-structural factors for relatively large volumes of 10mm to 130mm diameter. Given a better cost-to-information yield than image analysis and mercury porosimetry, Hydro will continue to support academic work with CT analysis. Examples will be shown of baked anodes before and after electrolysis testing plus crack patterns after mechanical testing.
Development of Techniques and Tools for the Determination of Carbon Anode Quality: Duygu Kocaefe1; Yasar Kocaefe1; Dipankar Bhattacharyay1; Bazoumana Sanogo1; Yao Ahoutou1; Hang Sun1; Patrick Coulombe2; 1University of Quebec at Chicoutimi; 2Aluminerie Alouette Inc.
The quality of prebaked carbon anodes, consumed in electrolysis during the primary aluminum production, has an important impact on the cell performance. The anode quality depends on the raw material quality and operating conditions in the anode plant. Development of simple, quick, and inexpensive techniques and tools for anode quality control will help industry identify the source of problems and take the necessary corrective actions rapidly. In this article, different quality control tools developed to find optimum vibration time, pitch content in green anode, metallic impurity content, wettability of coke by pitch, effect of mixing on coke particle size distribution, and measurement of green and baked anode electrical resistivities will be presented. In parallel, data analysis using the artificial neural network (ANN), a powerful statistical tool for such applications, provides complementary information on quality and process. This article will present also the potential utilization of ANN in quality control.
Non-destructive Testing of Baked Anodes Based on Modal Analysis and Principal Component Analysis: Moez Ben Boubaker1; Donald Picard1; Carl Duchesne1; Jayson Tessier2; Houshang Alamdari1; Mario Fafard1; 1Laval University; 2Alcoa Primary Metals Smelting Center of Excellence
Baked anodes quality control is still mostly based on core sampling and characterization. Only a small proportion of the production can be tested by this method due to the costly, time consuming, and destructive nature of the analytical techniques. Furthermore, the core properties are not necessarily representative of those of the whole block. A rapid and non-destructive method for anode quality control based on vibration modal analysis is proposed. A number of baked anodes produced under different conditions at the Alcoa Deschambault smelter were selected. These were excited mechanically at different locations and the vibration signals of the blocks were measured by accelerometers. Principal Component Analysis (PCA) was used to cluster the vibration modes of the anodes. It is shown that the proposed approach allows detecting and distinguishing different types of anode internal defects. Some of the tested anodes were cut and imaged in order to confirm the results visually.
3D Automated Anode Stub Inspection System: Jean-Pierre Gagne1; Remi St-Pierre1; Pascal Coté1; Harold Frenette2; 1STAS; 2Alcoa
During the production of aluminium from conventional prebaked Hall Héroult electrolysis cells, anodes have to be replaced on a regular basis. The anode rod assemblies, submitted to multiple heating/cooling cycles, tend to deform over time. Moreover, attacks from HF gas and liquid bath erode the stubs to a different extent at each cycle. The stubs thus require regular inspection to determine if they need to be repaired. Repair costs are often very expensive and difficult to follow and forecast. In 2016, STAS commissionned its new ASIS3D for the 3D analysis of stubs in any configuration (like hexapods). All anode rods in the rodding shop are automatically inspected in less than 30 seconds. A work order is automatically generated for each anode rod that needs to be repaired, after which it is registered in the database and printed on the anode rod. This paper describes the industrial performances of the ASIS3D.
3:45 PM Break
Identification of the Stress Intensity Factor of Carbon Cathode by Digital Image Correlation: Donald Picard1; Luca Sorelli1; Julien Réthoré2; Houshang Alamdari1; Marc-Antoine Baril1; Mario Fafard1; 1Université Laval; 2Université de Lyon
Crack propagation in carbon cathode used in the aluminium industry has been investigated through flexural tests on notched specimens. The main parameters of interest were the geometrical evolution of the crack and the stress intensity factor at the end of the tips. The later, in the case of interfacial fracture in two-dimensional geometries, can be related to normal (mode I) and shear (mode II) stresses. In the present study, only the mode I (opening) has been solicited and has proved challenging for heterogeneous materials such as carbon/carbon composites. The technique used to measure those features is based on full-field measurement using digital image correlation. A new test methodology has been applied which optically measures the crack tip displacement fields by Digital Image Correlation. The stress intensity factors derived from the experimental data are consistent with results available in the literature.
The Impact of Anode Nails on the Stub to Carbon Electrical Contact Resistance of Anode Assemblies with Simulated Corroded Stubs: William Berends1; 1AluCellTech
The solidification shrinkage of cast iron causes low contact pressure and high contact resistance in the stub to carbon connection. The use of steel anode nails to bridge the shrinkage gap has previously been demonstrated to reduce contact resistance with full diameter stubs. New laboratory testing with reduced diameter stubs to simulate stub corrosion, and with anode nails, demonstrates the higher electrical resistances that result from the wider iron shrinkage gap, and how the anode nails can consistently provide a lower contact resistance over the full anode cycle. This lower and more consistent resistance between hot vs. cold stubs, and new vs. worn stubs can be used to predict a reduced variation in current distribution between anodes in a reduction cell.
Finite Element Analysis of Slot Size Effect on the Thermal-Electrical Behaviour of the Anode: Hicham Chaouki1; Mounir Baiteche1; Alain Jacques2; Edward Gosselin2; Mario Fafard1; Houshang Alamdari1; 1Laval University; 2SAWNODE
In the aluminium industry the anode quality is an important factor that influences the Hall-Heroult process efficiency. For this purpose, many efforts have been done in order to optimize the anode design such as the use of slots which allow gases, generated through the reduction operation, to escape. The aim of this work is to investigate the effect of the slots size on the thermo-electrical behaviour of the anode. To this end, a 3D thermo-electrical model taking into account the anode, the bath and the anode connector has been developed using Ansys software. Material parameters such as electrical resistivity, thermal conductivity and electrical contact resistance are temperature dependent. Numerical simulations were performed for several slots thicknesses. Furthermore, a new design based on a variable slots thickness is simulated.
Hydrodynamic and Thermoelectric 3D Mathematical Model of Aluminium Electrolysis Cell to Investigate Slotted Carbon Anode Efficiency: Mounir Baiteche1; Hicham Chaouki1; Edward Gosselin2; Alain Jacques2; Houshang Alamdari1; Mario Fafard1; 1REGAL, Aluminium Research Centre, University Laval; 2SAWNODE
Carbon anodes in Hall-Heroult electrolytic cell are the main source of carbon to reduce alumina into aluminum. Carbon dioxide resulting from the reduction reaction is evacuated through the lateral sides of the anode and through the slots. The shape and size of the slots is an important parameter for the anodes design. During manufacture, slots are formed in the anodes before or after baking. However, to avoid the stresses due to the introduction of slots in green anodes during molding, the solution is to saw the anodes post-baking. The challenge here is to remove minimum carbon from the anodes while having high energy efficiency during electrolysis process. This study is conducted to test numerically different sawn slots configurations in the anode using a 3D mathematical model of the electrolytic cell. The model takes into account the main thermoelectric phenomena and flow dynamics that govern the operation of the electrolysis cell.