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BIA-Brukerstyrt innovasjonsarena

Process Innovations for High Current Density

Tildelt: kr 7,7 mill.

Prosjektnummer:

174276

Prosjektperiode:

2006 - 2011

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Hydro has over the past ten years succeeded with very ambitious programmes for continuous productivity increase in existing potlines, reaching world-leading performance levels. The limits for possible improvements by a continuous approach have now nearly been reached, as the performance of many existing electrolysis cell technologies (designed in the 70's and 80's) has been pushed close to their maximum. Through introduction of novel, innovative technology elements, combined with holistic cell redesign an d a more advanced understanding of the Hall-Heroult process, one is now aiming for a step from the anodic current densities of the potlines of today (0.8-0.9 A/cm2) into the range 1-1.1 A/cm2. This is a new approach to productivity increase with an inhere ntly large potential, but at a larger technological risk. The PI-HCD project will unite researchers from academia, focusing on the fundamentals of aluminium production, with the industrial R&D community in Hydro Aluminium, and will be at the very forefr ont of the field internationally. The project will educate 5 Ph.D. candidates to the Norwegian aluminium industry and continue to strengthen the Norwegian research community in primary aluminium production. In particular, the cross-disciplinary capabiliti es of the research groups involved will be strengthened through the approach taken. Sub-project one covers studies of anode reactivity and carbon dusting, and implications for operation. At high current densities (higher than 0.9 A/cm2) experience shows that the carbon anode may represent a severe limitation to stable, high performance cell operation. Accumulation of carbon dust in the cells and an increased occurrence of anode deformations (spikes) may constitute a serious obstacle. Carbon dust under the anodes will reduce the interpolar distance since the effective area for current conduction is reduced. The work in this sub-project will focus on preparation of low-dusting carbon anodes, including raw material considerations and the possible use of additives, development of an effective analytical tool for quantitative determination of carbon dust in the electrolyte of Hall - Héroult cells, anode reactivity (chemical/electrochemical) and its effect on the formation and growth of deformations, and th e origin of carbon dust (anode and anode cover material) and its operational effect through interpolar distance reduction. In sub-project two, fundamental studies will be undertaken of the electrochemistry at the anode, and of flow patterns and mass tran sport in the electrolyte under high current density and low interpolar distance. Improved prediction of the turbulent electrolyte flow, under the influence of both electromagnetic forces and drag forces from gas bubbles, is crucial for better prediction o f mass transfer mechanisms, including both the side ledge dynamics and the distribution of alumina in the cell. Quantitative models based on computational fluid dynamics (CFD) will be further developed as guides to stabilize the performance of the cell, w ith respect to the current efficiency and the heat balance of the cell. Sub-project three covers modelling of fracture of carbon anodes and cathodes. In order to understand failure due to fracture, it is important to understand the generation of tension by thermal gradients and thermal mismatch of materials as well as the materials resistance to fracture (fracture toughness). The influence of raw materials and additives and the resulting microstructure on the mechanical performance will be elucidated. Th ermo-mechanical simulation by finite element methods combined with measurements and models for fracture mechanics of carbon materials will aim at understanding the relationship between microstructure and mechanical properties. Sub-project four seeks to establish of mechanisms for the electrochemical and mechanical cathode wear. The main reason for disengagement of aluminium reduction cells is the chemical/mechanical wear of the graphitic and graphitized carbon cathode bottom blocks. This type of materi als is necessary at high current densities, due to its improved electrical conductivity. The aim is to clarify the wear mechanism with respect to current density, metal/bath velocities, carbon and pore structure and possible presence of additions. Industr ial measurement campaigns and collection of industrial data for wear behaviour will be emphasized in this sub-project. The laboratory-study approach currently undertaken in the CarboMat project will also be continued, at high-current-density conditions. The topic of sub-project five is development of tools for simulation of degradation of cathode lining. The non-carbon based cathode lining materials are mainly silicon carbide, refractory bricks and insulation bricks. Pressing the performance of reductio n cells, as in the case of increasing current density, makes control of the cell's heat balance of utmost importance. A small error in the

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BIA-Brukerstyrt innovasjonsarena