Electrode Technology: Baking Furnace/Electrode Design
Sponsored by: TMS Light Metals Division, TMS: Aluminum Committee
Program Organizers: Houshang Alamdari, Laval University
Wednesday 8:30 AM
March 1, 2017
Location: San Diego Convention Ctr
Session Chair: Donald Ziegler, Alcoa
8:30 AM Introductory Comments
Flow Detection Module – A New Model to Predict the Flow in Open Pit Anode Baking Furnaces: Detlef Maiwald1; Domenico Di Lisa1; Frank Heinke1; Florian Krummrich1; 1Innovatherm
The continuous measurement of the flow inside the flues of baking furnaces is still a challenge and not solved. As a substitute, the draught is measured in one position of each flue to determine the existence of a volume or flow. Knowing the real flow would be of the essence for a more accurate and optimized control of the combustion inside the furnace. A mathematical model for flow detection, based on fuzzy logics was developed to determine the actual flow in each flue. With the introduction of this flow detection module, each flue in a furnace is continuously evaluated. The on-line mathematical model calculates the actual flow in each flue by correlation of the relevant process data available in the firing system. As a result, the baking process is further optimized for lowest emissions and best fuel efficiency. The paper illustrates the mathematical approach and shows the results.
Formation of Carbon Build-Up on the Flue Wall of Anode Baking Furnace: Zhaohui Wang1; Arne Petter Ratvik1; Tor Grande2; Stein R°rvik1; 1SINTEF Materials and Chemistry; 2Norwegian University of Science and Technology
A hard carbon build-up layer often formed on the flue wall surface in anode baking furnaces. The layer accumulated over thermal circles and regularly needed to be removed mechanically to ensure sufficient space for the anodes between flue walls. The underlying mechanisms are still unknown, but the extent of the carbon build-up varies from plant to plant. Build-up on the flue wall from an autopsy of an open top furnace has been studied. Microstructure and phase compositions of carbon build-up, especially at the refractory interface, have been studied by optical microscopy, X-ray computed tomography, SEM/EDS, and XRD. Pyrolytic carbon was found to be the main carbon source in carbon build-up layer. The transport of silicon source from the refractory material to the flue wall surface is proposed to contribute to the formation of carbon build-up. Proposed formation mechanisms of carbon build-up and the reaction schemes are supported by thermodynamic calculations.
Investigation of Spent Refractory Lining in an Anode Baking Furnace: Trond Brandvik1; Zhaohui Wang2; Arne Petter Ratvik2; Tor Grande1; 1Norwegian University of Science and Technology, NTNU; 2SINTEF Materials and Chemistry
The refractory lining in anode baking furnaces is exposed to harsh chemical environment and thermal cycling. Here, we report on an autopsy of spent refractory lining in an anode baking furnace after 4000 days operation. Variation in the density and porosity across the bricks revealed carbon build up in pores had taken place towards the anode side. The spent refractory bricks were investigated with respect to chemical and mineralogical composition by X-ray diffraction, optical and electron microscopy. No significant change in the mineralogical composition across the brick could be detected by X-ray diffraction. Fluoride was not found in the bricks, while traces of sodium was shown to accumulate together with calcium impurities in the bricks. The changes in the chemical and mineralogical composition across the bricks are discussed in relation to possible chemical reactions taking place during anode baking.
25 Years of Naural Gas Purged Infrared Pyrometer Temperature Measurement For The Operation Of Open-Top Anodes Baking Furnaces: Yvon Menard1; 1Retired Process Specialist
To improve a low flue walls life, a natural gas purged infrared pyrometer system was implemented to replace original thermocouples based temperature measurement system on firing system, in 1991. Flue wall life increased to be over 100 cycles, on Pechiney furnaces, by preventing flue walls overheating. Anodes baking quality is adequate for two burners ramps fire technology. Some problems had to be corrected over time as they appeared: calibration drift, pinched injector tubes, frosted pyrometers lenses, etc. Pyrometers and thermocouples measurement technologies can both successfully bake anodes to specifications, but original cost, ease of operation, ease of maintenance and effect on refractory may favor one system over the other, depending on plant situation.
10:15 AM Break
Impact of Cast Iron Degradation and Cathode Block Erosion on the Current Path in the Cathodic Assembly of Aluminum Production Cells: Martin Brassard1; Marc LeBreux1; Martin Desilets1; Gervais Soucy1; Martin FortÚ2; Jean-Franšois Bilodeau2; 1UniversitÚ de Sherbrooke; 2Rio Tinto
Carbon-cast iron electrical contact degradation is still considered one of the main cause for the CVD increase over the lifetime of the electrolysis cell. An Ansys numerical model was developed to evaluate the effect of the carbon-cast iron electrical contact degradation and the cathode erosion on the CVD and the current path. Chemical degradation data from laboratory and industrial samples were used to calibrate the cast iron and the contact resistivities. Results demonstrate that the contact degradation at the cast iron interface has a direct impact on the current distribution at the carbon block surface. Current density that is concentrated at the ledge toe is pushed back toward the center of the block as the electrical contact quality decreases. When coupled with the carbon block erosion, the resulting current distribution may give a clue about the double W shape erosion.
Reducing Cathode Voltage Drop and Reducing Peak Current Density by Use of Cathode Nails across the Carbon to Cast Iron Interface: Will Berends1; Stephen Haley1; 1Hatch
The solidification shrinkage of cast iron between the collector bar and carbon slot surfaces of a cathode assembly introduces low contact pressure and high electrical contact resistance, particularly at the top of the cathode slot. The use of steel nails to bridge the cast iron shrinkage gap lowers the contact resistance significantly. Recent laboratory testing of varying densities and diameters of nails in the cathode assembly demonstrates the reduction of resistance that is possible through the top of the cathode slot, and the corresponding reduction of resistance through the cathode. The selective use of cathode nails towards the centre of the collector bar is modeled to demonstrate the potential of improved current distribution with reduced peak current density.
Production of NiFe2O4 Nanocermet for Aluminium Inert Anode: Wu Xianxi1; Zhu Weidong1; Luo Kunlin1; Jia Hefeng1; 1Guizhou University
The NiFe2O4 nanocermet developed in this paper was prepared by homogeneous precipitation, mould pressing and sintering process. And the nanocermet was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The experiments and tests on corrosion and electrical conductivity show that the nanocermet can be used for inert anode of aluminum electrolysis. The mechanical properties and processability of cermet for inert anode of aluminum electrolysis can be improved.
Gas Anodes Made of Porous Graphite for Aluminium Electrowinning: Babak Khalaghi1; Henrik Gudbrandsen2; Ole Kjos2; Karen Osen2; Ove Paulsen2; Tommy Mokkelbost2; Geir Martin Haarberg1; 1Norwegian University of Science and Technology (NTNU); 2SINTEF
Greenhouse gas (GHG) emission is one of the main challenges of the current aluminium production process. Consumable carbon anodes have a major share in GHG emissions from the electrolysis cells. On the other hand, a commercial inert oxygen-evolving anode has not been found yet. Another possibility is to supply the anodes with a reducing gas, such as methane. Indeed, the anodes must be porous. The reducing gas can alter the anodic reaction which in turn, decreases the CO2 emission. Besides, participation of methane in the anode reaction should give a lower anode potential, which may assist to reduce partial perfluorocarbons emissions during industrial operation. Different porous graphite grades were tested as gas anode material for aluminium electrolysis. The effect of graphite properties on the gas flow in the porous structure was studied. Electrolysis experiments were also carried out to detect the changes in the anodic reaction and subsequent gas emission.