Aluminum Reduction Technology: Continue Environment (Material and Equipment) & Fundamental Studies (Alumina Dissolution and Bath)
Sponsored by: TMS Light Metals Division, TMS: Aluminum Committee
Program Organizers: Nadia Ahli, Emirates Global Aluminium; Nancy Holt, Hydro Aluminium As

Thursday 2:00 PM
March 18, 2021
Room: RM 28
Location: TMS2021 Virtual

Session Chair: Bertrand Allano, RIO TINTO


2:00 PM  
Instant Monitoring of Aluminum Chemistry in Cells Using a Portable Liquid Metal Analyzer: Sveinn Hinrik Gudmundsson1; Birna Björnsdóttir2; Kristjan Leosson1; 1DT Equipment; 2Nordural ehf
    A portable analyzer for monitoring the aluminum chemistry in production cells is presented. With the portable battery-powered device, trace element concentration analysis can be obtained within 45 seconds of manually extracting a liquid metal sample from the cell. The device uses laser-induced breakdown spectroscopy (LIBS) to directly analyze the composition of the liquid metal, down to concentrations of approx. 10 ppm for a range of elements. We compare the performance of the portable analyzer with conventional off-line laboratory analysis of cast process samples. In addition to the significant time savings and reduced chance of human error, we show that direct analysis in the liquid metal is particularly advantageous in the case of trace elements that display significant segregation upon solidification, such as silicon.

2:20 PM  
Dissolution Characteristics and Concentration Measurements of Alumina in Cryolite Melts: Luis Carlos Bracamonte1; Vegard Aulie1; Christian Rosenkilde2; Kristian Einarsrud1; Espen Sandnes1; 1Ntnu University; 2Hydro Aluminium
    Acquirement of reliable data is a key factor to better understand alumina dissolution, one of the most important processes in the Hall-Héroult process. Simultaneous video recording of the alumina addition and dissolution and in situ measurements of alumina concentration in cryolite melts in a see-through oven, can be a powerful method to reveal crucial information such as the behavior and the variation of the alumina concentration during the entire dissolution process. The sensor data reflecting the actual dissolved alumina in the melt, is interesting individually, but especially when analyzed together with the alumina-bath interaction obtained from the see-through cell. In this work, video recording using a high-speed camera and electromotive force (emf) based alumina sensor measurements were performed during the addition of alumina and throughout the entire dissolution process. The measurements were performed in a see-through cell changing different variables such as alumina quality, alumina particle size and bath composition.

2:40 PM  
On Gaseous Emissions during Alumina Feeding: Sindre Engzelius Gylver1; Ĺste Follo2; Vegard Aulie1; Espen Sandnes1; Helene Marie Granlund3; Anders Sřrhuus4; Kristian Etienne Einarsrud1; 1Norwegian University of Science and Technology; 2Elkem; 3Alcoa Mosjřen; 4GE Power
     Aluminium electrolysis involves feeding of alumina into a cryolite based bath. Water originating from alumina as well from air reacts with fluorides and results in HF evolution. Untreated HF gas is a significant environmental and economical issue. HF is however effectively adsorbed in (primary) alumina before being fed back to the cell as secondary alumina, thereby recycling the fluoride. As alumina is fed to the cell, it forms a raft, delaying the dissolution process and linked to several operational challenges. The goal of the current work is twofold; first, water content is investigated in a lab scale setting, aiming to explain raft porosity, ultimately causing it to float rather than disperse in the bath. Secondly, the evolution of HF is investigated for different aluminas in both a lab scale setting and in industrial measurements performed at Alcoa Mosjřen, aiming to identify correlations between gas evolution and alumina properties.

3:00 PM  
On the Feasibility of Using Low-melting Bath to Accommodate Inert Anodes in Aluminium Electrolysis Cells: Asbjorn Solheim1; 1SINTEF Industry
    Inert anodes for aluminium production can be made of ceramics, metals, or a mixture of those (cermets). Regardless of the type of anode, the surface will be an oxide. At high enough anode potential, the surface oxide will be decomposed upon formation of the corresponding fluoride, eventually leading to a catastrophic defect. The decomposition voltage was calculated for materials based on Ni, Fe, Cu, Co, and Cr in terms of the activities of alumina and aluminium fluoride at 960 °C and at 800 °C. The risk of failure was higher at low temperature, partly because low-melting baths have high activity of aluminium fluoride, and partly because it will be challenging to maintain high enough activity of alumina, even in a "slurry cell" where the bath is a suspension of tiny solid alumina particles. It was estimated that the alumina particles in the slurry cannot be larger than about 10 microns.

3:20 PM  
Electrochemical Reduction and Dissolution of Aluminium in a Thin-layer Refinery Process: Andrei Iasinskii1; Peter Polyakov1; Ilya Moiseenko1; Sai Krishna Padamata1; 1Siberian Federal University
    The liquid metals refining industry has a demand for new technologies operated at low energy consumption and environmental impact nowadays. This work addresses an approach for Al purification and extraction from scrap in a thin layer of the multiple-capillary molten salt electrochemical system. The two types of single-capillary cells with quasi-reference electrodes were used to study the kinetics of aluminium reduction and dissolution in a narrow (Ř x length) 1x1 and 1x5 mm channel filled with liquid LiF-AlF3 or equimolar NaCl-KCl with the AlF3 addition at 850 °C. A multiple-capillary Al refinery process can be designed to significantly reduce the specific energy consumption. The new refinery process can be performed at high current densities. The single capillary electrolysis can be used for kinetics studies. Thin-layer cells should be operated at a thickness no more than 5 mm to compensate the high resistance which may vary in the range from 0.7 to 2.5 Ω at this capillary length. The 64LiF-36AlF3 melt can be used as an electrolyte for the thin-layer electrolysis due to the high electrical conductivity, the wide potential window between Al and Li reduction and the low liquidus temperature.

3:40 PM  
Influence of Additives on Alumina Dissolution in Superheated Cryolite Melts: Jonathan Alarie1; László Kiss1; Sándor Poncsák1; Renaud Santerre2; Sébastien Guérard3; Jean-François Bilodeau3; 1University of Quebec at Chicoutimi; 2Technical Advisor, Retired from Rio Tinto; 3Arvida Research and Development Centre, Rio Tinto
    Alumina dissolution is of first concern in modern electrolysis cells. The rate of dissolution strongly depends on the physical and chemical conditions in the cryolitic melts. In this work, the alumina dissolution rate has been monitored with the gravimetric methods in NaF-AlF3-CaF2-Al2O3 systems. To evaluate the main influences of the additives and the superheat, a parametric study has been conducted. A wide range of additives content have been studied to fit industrial conditions. Thus, bath compositions with different liquidus temperatures, in the range from 924 °C to 970 °C, were used to dissolve alumina at fixed superheat. Results show consistency with physicochemical expectations so that the additives content decreases the alumina dissolution rate and the superheat increase this rate. Then the experimentally obtained correlations can help to adjust the parameters to obtain the desired rate of dissolution.

4:00 PM Question and Answer Period