David S. Wong

Anode Effect Phenomena during Conventional AEs, Low Voltage Propagating AEs & Non‐Propagating AEs

Anode effect (AE) phenomena in aluminium cells can be separated into several categories. Firstly, ‘conventional’ AEs (>8V) are typically initiated on one or two localized anodes and then, due to an abrupt increase in current density, rapidly propagate to the other anodes in the cell thereby providing the typical emission spectrum of PFCs. Secondly, ‘low voltage propagating’ AEs (<8V) result from localized AEs rapidly propagating to a limited section of anodes with the cells remaining below conventional AE voltage; these AEs often undergo electrical shorting, especially at narrow ACDs, resulting in rapid self-termination. In contrast, the continuous background emission of PFCs should be categorized as a third type of AE or ‘nonpropagating’ AEs. The fundamental mechanisms that initiate continuous PFCs very likely still apply, but the localised AEs do not propagate sufficiently to other anodes for a cell to exhibit a voltage signature characteristic of a low voltage AE.

New Generation Control for Daily Aluminium Smelter Improvement Generation 3 Process Control for Potlines

Aluminium smelting is facing serious challenges in reducing energy consumption, increasing current efficiency and meeting constantly changing environmental expectations. Traditional control systems aim to achieve and maintain pre-determined smelter targets through adjusting process parameters in order to compensate for changes in inputs, operations and special causes of variation. These control systems are not designed to remove the causes of variation and cannot address the pace and complexity of the challenges in the industry.

Fluoride Emissions Management Guide (FEMG) for Aluminium Smelters

All smelters worldwide operate under strict fluoride emission limits and the reduction of fluoride emissions is further driven by health and environmental considerations. A Fluoride Emissions Management Guide (FEMG) has been written by the Light Metals Research Centre (LMRC) on the invitation of Australian Aluminium Council (AAC), under the Asia-Pacific Partnership (APP) on Clean Development and Climate. The aim of the FEMG is to provide a better understanding of the factors affecting and ways of reducing fluoride evolution and emissions in smelters, and further, to provide smelters with an operational guide for reducing and managing fluoride emissions.

Evaluation of Time Consistency When Quantifying Emissions of Perfluorocarbons Resulting from Low Voltage Anode Effects

Low voltage anode effects (LVAE) have been recognized in aluminium smelters since mid 2000s. Such events are now known to generate perfluorocarbons (PFC) emissions over time and no methodology currently exists to adequately quantify such emissions for inclusion in greenhouse gas (GHG) inventories. A novel methodology has been proposed to estimate the annual LVAE emissions from the aluminium industry (Marks & Nunez) based on a set of existing measurement data from multiple in situ gas measurement campaigns. There is currently no estimate of the contribution of LVAE emissions using this methodology for past years. This paper investigates the potential effects of adopting this approach and applying it to historic emissions inventories of PFCs. Based on historical data supplied by a select set of aluminum smelters, the total respective PFC emissions resulting from high and low voltage anode effects were calculated, allowing evaluation of the potential impact of LVAE on the total amount of PFCs reported.

The Nature of Particles and Fines in Potroom Dust

Potroom dust or particulates are major contributors to a smelter’s total environmental burden. A wider study on the environmental contribution of these particulates was conducted across multiple prebake smelters, part of which was to determine the composition and particle size distribution of this dust and its material sources. This has provided an understanding of the fate of particles within the potroom, after the point of emission. In general, anode cover material and feed alumina were found to be contributors of coarser dust that tends to settle on various surfaces in the potroom (floor, pot superstructures, rafters), thereby becoming a source of recirculating dust. In contrast, bath fume was found to be the dominant contributor to fines/ultrafines, from operations involving open cells and hot, fuming materials in the potroom. Such fines are fluoride-based, highly mobile and readily emitted from the potroom. Particles also tend to decrease in size at higher potroom elevations.

Visualising the Sources of Potroom Dust in Aluminium Smelters

‘Potroom dust’ comprises one of the major sources of particulate emissions from a smelter to the environment. With regulatory emission limits for particulates continually tightening, there is a need for smelters to understand the sources and pathways by which dust is generated in a potroom. Only armed with this understanding can smelters develop targeted strategies to counter these emissions. Methodologies to sample and analyse the composition of potroom dust (both settled on surfaces and airborne) have been applied in four smelters. By taking samples across a range of potroom locations and elevations, an overall compositional picture of dust can be built and visualised for any potroom. In general, settled dust is dominated by cover material and alumina — the role of each, however, is influenced by the granulometry of cover and how alumina is delivered to the pot. In contrast, airborne dust in a potroom is typically dominated by bath-related compounds.

PFCs from the Chinese Aluminium Sector—Challenges in Emissions Accounting and Further Characteristics

PFCs from high voltage anode effects (HV-AE ≥8 V) in aluminium smelters are currently accounted for by 2006 IPCC methodologies. Industry accounting of global PFCs is done by the IAI via anode effect (AE) surveys, however reporting from Chinese facilities remains limited, contributing to a 55% non-participation rate in 2015. A Chinese AE survey was conducted to help provide further data. However responses (supported by data from a case study smelter) highlighted that AE frequency (AEF) and duration (AED) statistics in China often do not include all detected AEs (only longer duration AEs e.g. ≥60 s) which is inconsistent with IPCC Tier 2 PFC accounting methodologies. The paper also presents PFC measurement data (2011–12) from 10 potlines in China, showing that unaccounted emissions from low voltage anode effects (LV-AE <8 V) can also be significant for low-amperage (235–240 kA) cells and that total PFC emissions were negatively correlated with current efficiency.

Transforming the Way Electricity is Consumed During the Aluminium Smelting Process

This paper examines the development of the new EnPot heat exchanger technology for aluminium smelters and the potential impact it could have on the sustainability and economics of primary aluminium production. The EnPot technology can be used to help the aluminium smelting industry be part of the solution to accommodate increased intermittency in our future renewable energy generation, post COP 21. The EnPot system provides for the first time, dynamic control of the heat balance of aluminium smelting pots across the potline, so that energy consumption and aluminium production can be increased or decreased by as much as plus or minus 30% almost instantaneously. This enables a new way of thinking to emerge when considering the relationship the aluminium smelter plays in connection to the power grid. The EnPot technology provides smelters with the means to free up power back to the grid, transforming the smelter from only an end user of power into a ‘virtual battery’ for the electricity network. This fundamentally changes the way electricity is consumed by the smelter and is a concept already being tested by several smelters.

Detecting, Identifying and Managing Systematic Potline Issues with Generation 3 Process Control

Due to the nature of the process, aluminium smelting can have many issues/abnormalities/problems. One type is systematic issues, which are those that occur on multiple pots simultaneously and can result in prolonged issues across a potline if not identified early. However, detecting and identifying these issues can be a difficult task for smelter engineers and management, particularly when information sources are not recorded, integrated and available in a centralised location. In addition, they are often not displayed in a way that is convenient for tracking and identifying systematic problems. For example, an increasing trend in cryolite ratio (CR) on all pots can be difficult to spot if graphical CR trends are only displayed for individual pots, or worse, in a tabulated format. This paper presents two case studies in detecting systematic issues in smelting process using the Generation 3 process control system that highlights the importance of a centralised data location and appropriate data presentation. The cause of the systematic issues and the actions taken to manage the impact will also be discussed.