Aluminum Reduction Technology: Electrolyte and Fundamentals, Anode Effects and PFC Emissions
Sponsored by: TMS Light Metals Division, TMS: Aluminum Committee
Program Organizers: Mark Dorreen, Light Metals Research Centre, The University of Auckland
Monday 2:00 PM
February 27, 2017
Location: San Diego Convention Ctr
Session Chair: Jayson Tessier, Alcoa; Thor Aarhaug, SINTEF
2:00 PM Introductory Comments
Bauxite Processing via Chloride Route to Produce Chloride Products and Subsequent Electrolysis of Aluminium Chloride to Produce Aluminium Metal: Sankar Namboothiri1; Subash Mallick1; 1Gharda Scientific Research Foundation
Based in Mumbai, India, Gharda Scientific Research Foundation (GSRF) is working to produce aluminium chloride and primary aluminium via carbochlorination of bauxite followed by electrolysis step. In this paper, a literature review of the chloride route based on published literature and concept proof experiments in various small setups are discussed. Bauxite was chlorinated with calcined petroleum coke in an externally heated small bubbling fluidised bed reactor. The AlCl3 thus produced was purified by reductive distillation in a small setup and used for aluminium chloride electrolysis in a 50 lit cell. The study aimed to generate scale up data for optimizing the chloride conversion, recovery and purity as well as electrolysis efficiency. The findings are used for process scale up to a kg scale setup and then to a larger demonstration plant. Presently, GSRF is operating a demonstration plant for carbochlorination of bauxite.
Stability of Chlorides in Cryolitic Electrolyte: Luis Espinoza-Nava1; Xiangwen Wang1; 1Alcoa Technical Center
Chloride is considered one of the most important contaminants in cryolitic electrolyte system for aluminum smelting. Chloride salts such as NaCl, MgCl2 and others have also been chosen for potential additives to change the electrolyte physical chemical properties for smelting efficiency improvement. This paper studied the chloride stability in the cryolitic electrolyte system. The behavior of NaCl and MgCl2 was investigated when there is a moisture present with or without electrolysis or when there is anode effect. HCl will be generated along with HF during a normal electrolysis even when chloride in the electrolyte is at a low level. CF3Cl gas is generated along with CF4 and C2F6 when anode effect occurs. The results confirm that even though chlorides (NaCl/MgCl2) are considered the most important contaminants, the level of the contaminants remains relatively low in the smelting electrolyte without obvious accumulation over time.
Sodium in Aluminium as a Cell Performance Indicator: A Quantitative Framework: Asbjorn Solheim1; 1SINTEF
The relationship between the content of sodium in the metal in industrial aluminium production cells and the current efficiency (CE) was analysed, using available theories and experimental data. The sodium concentration as well as the CE varies with the bath composition (the bath ratio) and the cathodic overvoltage. The mentioned parameters are all functions of the same type of ratio between the activities of sodium fluoride and aluminium fluoride, thus enabling an internally consistent and quantitative description. The "ideal" relationship between the sodium concentration in the metal, the CE, and the cathodic overvoltage is illustrated. It is suggested that the sodium content in the metal can indeed be used as an indicator of the cell performance, but it is necessary to use adequate sampling and analysis techniques. The exact cell conditions during sampling should also be taken into account, such as the bath composition and the overfeeding/underfeeding cycle.
Role of Heat Transfer in the Formation of Carbon Oxides in Smelting Cells: Mark Dorreen1; N.E. Richards2; Barry Welch3; 1Light Metals Research Centre, The University of Auckland; 2Retired; 3University of Auckland; University of New South Wales
Following publication of the proposed Pearson-Waddington equation for current efficiency, smelting operators have considered trends in the CO/CO2 ratio to be a direct indicator of poor cell performance. The common assumption has been that only a small amount of CO is formed electrochemically relative to CO2 even though the participating species (carbon and oxyfluoride anions) are the same for both products. In combustion reactions involving carbon and oxygen, thermodynamic equilibrium is reached rapidly at cell temperatures, suggesting CO should be the dominant product evolved. Background PFC trends where there is an increase in CO content place greater doubt on existing mechanisms. Results from several studies demonstrate process conditions that satisfy the entropic energy play an important, if not dominant role in determining the product gas mix. This provides a clear path for formulating process, operating and control conditions that minimise the evolution of carbon oxides from smelting cells.
3:45 PM Break
Partial Anode Effect in a Two-Compartment Laboratory Alumina Reduction Cell: Henrik Ĺsheim1; Thor Aarhaug2; Wojciech Gebarowski1; Asbjřrn Solheim2; Geir Haarberg1; 1NTNU; 2SINTEF
Most laboratory systems investigating the aluminium production process utilize a single anode setup. When approaching alumina depletion under constant current conditions in such a system, the potential will increase to high levels (> 10 V) and initiate the anode effect and perfluorocarbon generation. However, it has been discovered by industrial measurements that perfluorocarbon generation may also occur at normal cell voltages. To be able to experience this phenomenon in the laboratory, a system with more than one anode was designed. The setup consists of a cylindrical graphite crucible with two separate chambers. The chambers have their own electrolyte and anode, while the crucible itself acts as the cathode. In addition, an electronic load is added in parallel with the anodes to further increase the degree of parallelism, simulating additional anodes as resistive elements. Gas analysis is employed on each chamber to add valuable information about the performance of each anode.
Co-evolution of Carbon Oxides and Fluorides during the Electrowinning of Aluminium with Molten NaF-AlF3-CaF2-Al2O3 Electrolytes: Mark Dorreen1; Margaret Hyland1; R. G. Haverkamp2; James Metson1; Ali Jassim3; B.J. Welch4; Alton Tabereaux5; 1University of Auckland; Light Metals Research Centre; 2University of Auckland; 3University of New South Wales; 4University of Auckland; University of New South Wales; 5Consultant
Recent finding of continuous, low level co-evolution of PFC during apparently normal operations of large operating cells has raised the question as to whether or not these arise from a different cause to a normal anode effect. Modern cell designs and operating conditions are dramatically different from the early laboratory and practical experiences, with increased numbers of anode blocks and slotting of anodes impacting electrolyte flow. Higher current densities and energy minimisation also introduce greater spatial variation in cells than ever before. The basic laws of electrochemistry and thermochemistry demonstrate that under these conditions each anode-cathode pair can operate under different anode potentials . Detailed analysis of a combination of studies shows the findings are consistent with all PFC emissions being triggered by the individual anode potential approaching that necessary for COF2(g) formation on the electrode surface, which then decomposes to release an increased amount of CO(g) and CF4(g).
Preventive Treatment of Anode Effects Using On-Line Individual Anode Current Monitoring: Lukas Dion1; François Laflamme2; Antoine Godefroy2; Charles-Luc Lagacé2; James Evans3; László Kiss1; Sándor Poncsák1; 1Université du Québec ŕ Chicoutimi; 2Aluminerie Alouette inc.; 3Wireless Industrial Technologies
Anode effects (AE) are considered a nuisance for aluminium production due to the numerous negative impacts that they generate in an electrolysis cell. Hence, great efforts have been deployed to minimize the occurrence of this event. Using online anode current monitoring, Alouette introduced an algorithm to detect abnormalities prior to an AE, allowing sufficient time to apply a corrective action before its occurrence. Several sets of strategies were tested to evaluate the best approach to correct the situation without generating additional problems in the cells. Finally, a communication was established between the individual anode current monitoring system and the cell control system of two pots to automatically launch preventive treatments of AE. A decrease in the total number of anode effects along with the cell instability is noticeable. Other indicators were also compared to make sure that no deterioration in the cells condition occurred over time.
Reduction in EGA Jebel Ali Potroom GHG Emissions: Daniel Whitfield1; Sergey Akhmetov1; Najeeba Al-Jabri1; 1Emirates Global Aluminium (EGA)
EGA's Jebel Ali smelter hot metal production has increased above 1 million tons per annum since 2010 and produced a record 1,045,255 tons in 2015 and with forecast total production of 1,065,280 tons in 2016. Jebel Ali Potrooms has also strived to minimize its environmental impact with significant effort to reduce the three main sources of GHG emissions; power consumption, anode carbon and CO2.eq emissions from anode effect PFC’s. Specific energy has decreased from 14.82 to 14.29 DC.kWh/kg, net carbon has reduced from 0.434 to 0.423 t / t Al and AE PFC emissions have reduced from 0.263 t to 0.071 t CO2.eq / t Al. This has resulted in reducing annual GHG emissions by 798,065 CO2.eq tons and was achieved despite increasing hot metal production by 63,266 tons since 2010. This achievement demonstrates EGA’s vision for operational excellence and continuing efforts to minimize environmental footprint.