Physical and mechanical characterizations of carbon anodes produced from different vibro-compactors

The aluminum industries in Quebec consume about 1.27 million tonnes per year of carbon for anode production. The anode quality is widely influenced by the quality of raw materials and the parameters of the manufacturing process which involves mixing, vibro-compacting, and baking. The anodes are regularly replaced at an interval of 20 to 30 days. This interval reduces when the anode quality is low. A better understanding of the vibro-compaction process would reduce the variation in the properties of formed anodes and thereby improve the anode performance during aluminum production. This, in turn, would reduce the cost and the greenhouse gas emissions. The focus of this work is to study the influence of vibro-compaction parameters on anode properties. The physical and mechanical characterization of anodes produced by different vibro-compactors was carried out. The article will present the results of this study

Prediction of Low-Voltage Tetrafluoromethane Emissions Based on the Operating Conditions of an Aluminium Electrolysis Cell

Greenhouse gas (GHG) generation is inherent in the production of aluminium by a technology that uses carbon anodes. Most of those GHG are composed of CO2 produced by redox reaction that occurs in the cell. However, a significant fraction of the annual GHG production is composed of perfluorocarbons (PFC) resulting from anode effects (AE). Multiple investigations have shown that tetrafluoromethane (CF4) can be generated under low-voltage conditions in the electrolysis cells, without global anode effect. The aim of this paper is to find a quantitative relationship between monitored cell parameters and the emissions of CF4. To achieve this goal, a predictive algorithm has been developed using seven cell indicators. These indicators are based on the cell voltage, the noise level and other parameters calculated from individual anode current monitoring. The predictive algorithm is structured into three different steps. The first two steps give qualitative information while the third one quantitatively describes the expected CF4 concentration at the duct end of the electrolysis cells. Validations after each step are presented and discussed. Finally, a sensitivity analysis was performed to understand the effect of each indicator on the onset of low-voltage PFC emissions. The standard deviation of individual anode currents was found to be the dominant variable. Cell voltage, noise level, and maximum individual anode current also showed a significant correlation with the presence of CF4 in the output gas of an electrolysis cell

Réactivité de l'anode et désulfuration : effet du niveau de calcination du coke

Les propriétés du coke et la performance des anodes sont affectées par le niveau de calcination du coke. Une densité de coke (VBD) élevée implique une température de calcination élevée (jusqu'à la désulfuration) correspondant à une gamme de longueurs cristallines « Lc » entre 25 et 30 A. Par contre, des études antérieures ont démontré que la calcination à plus basse température (sous-calcination, Lc 20 - 25 A) permettrait de réduire le poussiérage des anodes [1,2]. D'autre part, l'évolution du marché du coke force l'industrie de l'aluminium à utiliser des cokes à teneurs plus élevées en soufre. La perte en soufre survient durant la calcination à des températures plus élevées que 1250 °C [3-7]. Des pores se forment lors de cette désulfuration produisant ainsi une diminution de la VBD. Par conséquent, le niveau de calcination du coke et le comportement lors de la désulfuration doivent être évalués. Cette étude a pour but d'évaluer l'effet du niveau de calcination et du niveau de cuisson sur la réactivité du coke de pétrole, du brai de charbon et de la pâte anodique. Ainsi des analyses thermogravimétriques ont été faites sur des échantillons de coke, de pâte et de brai. La réactivité au CO2 de ces matériaux à 963°C a été déterminée et les résultats démontrent que, dépendamment du type d'échantillon (coke, pâte ou brai), la réactivité au CO2 est reliée soit à la surface spécifique, soit à longueur cristalline ou soit à une combinaison des deux. Dans le but de comprendre la façon dont le soufre est émis lors de la désulfuration, une seconde étude a été effectuée sur du coke de pétrole calciné en laboratoire ainsi que sur des cokes verts (coke de pétrole non-calciné). Les analyses de type XPS montrent que certains liens chimiques semblent majoritairement présents lors de la calcination à de hautes températures. De plus, la désulfuration se produit lentement jusqu'à une température supérieure à 1400°C puis la réaction s'accélère. Finalement, le soufre n'est pas réparti de façon similaire entre les différents cokes verts lorsque différentes espèces chimiques du souffre sont considérées

Prediction of low-voltage tetrafluoromethane emissions based on the operating conditions of an aluminium electrolysis cell

Greenhouse gas (GHG) generation is inherent in the production of aluminium by a technology that uses carbon anodes. Most of those GHG are composed of CO2 produced by redox reaction that occurs in the cell. However, a significant fraction of the annual GHG production is composed of perfluorocarbons (PFC) resulting from anode effects (AE). Multiple investigations have shown that tetrafluoromethane (CF4) can be generated under low-voltage conditions in the electrolysis cells, without global anode effect. The aim of this paper is to find a quantitative relationship between monitored cell parameters and the emissions of CF4. To achieve this goal, a predictive algorithm has been developed using seven cell indicators. These indicators are based on the cell voltage, the noise level and other parameters calculated from individual anode current monitoring. The predictive algorithm is structured into three different steps. The first two steps give qualitative information while the third one quantitatively describes the expected CF4 concentration at the duct end of the electrolysis cells. Validations after each step are presented and discussed. Finally, a sensitivity analysis was performed to understand the effect of each indicator on the onset of low-voltage PFC emissions. The standard deviation of individual anode currents was found to be the dominant variable. Cell voltage, noise level, and maximum individual anode current also showed a significant correlation with the presence of CF4 in the output gas of an electrolysis cell

Simulator of non-homogenous alumina and current distribution in an aluminum electrolysis cell to predict low-voltage anode effects

Perfluorocarbons are important contributors to aluminum production greenhouse gas inventories. Tetrafluoromethane and hexafluoroethane are produced in the electrolysis process when a harmful event called anode effect occurs in the cell. This incident is strongly related to the lack of alumina and the current distribution in the cell and can be classified into two categories: high-voltage and low-voltage anode effects. The latter is hard to detect during the normal electrolysis process and, therefore, new tools are necessary to predict this event and minimize its occurrence. This paper discusses a new approach to model the alumina distribution behavior in an electrolysis cell by dividing the electrolytic bath into non-homogenous concentration zones using discrete elements. The different mechanisms related to the alumina distribution are discussed in detail. Moreover, with a detailed electrical model, it is possible to calculate the current distribution among the different anodic assemblies. With this information, the model can evaluate if low-voltage emissions are likely to be present under the simulated conditions. Using the simulator will help the understanding of the role of the alumina distribution which, in turn, will improve the cell energy consumption and stability while reducing the occurrence of high- and low-voltage anode effects

Influence of hooding conditions on gas composition at duct end of an electrolysis cell

Aluminum smelters are known to be important producers of perfluorocarbons (PFC). These gases are generated when the localized overvoltage in the cell exceeds the threshold necessary to electrolyze the cryolite, hence generating an anode effect. When it remains localized, this event is difficult to identify and it can generate only a small amount of PFC for several hours. Under these conditions, the cell behavior is almost undisturbed and no action is initiated from the cell control system to correct the situation. To understand this phenomenon, it is common to extract the gas from the duct end of specific cells - where dilution is minimal - and measure the gas composition continuously using a Fourier-transformed infrared spectrometer (FTIR). However, air infiltration can affect the measured PFC concentration, and the gas flow rate in the duct of the cell. In this study, a tracer gas was injected into the cell under multiple scenarios to assess the impact of the hooding conditions on the flow rate and concentration of the measured gases. This investigation quantified the uncertainty associated with the measurements of the gas composition for six specific scenarios compared to optimal hooding conditions

Recent on-line measurements of individual anode currents at Alouette

Since early 2014, Alouette has used a system provided by Wireless Industrial Technologies (WIT) to measure individual anode currents on two pots. The system works by measuring the adjacent magnetic field generated by the current for each anode hanger. This paper summarizes initial difficulties and how they have been overcome. Recent current measurements show good agreement with alternative methods for measuring currents (e.g. mV drop along anode hangers). An algorithm has been developed for discerning an imminent anode effect from changes in the measured magnetic fields due to changes in anode currents. Practical reductions of anode effect frequency, compared to cells of reference, have been achieved by using the results of this algorithm to trigger corrective action through the pot control computer. Some additional potential benefits of anode current measurement are described in the paper

Quantification of perfluorocarbons emissions during high voltage anode effects using non-linear approach

Significant amounts of greenhouses gases are produced annually by aluminium smelters around the globe. Most of these emissions are carbon dioxide but perfluorocarbons are nonetheless an essential part of GHG inventories for aluminium smelters. The total mass of perfluorocarbons declared is estimated using a linear relationship between some process parameters and a specific emission coefficient. However, this linear method does not accurately represent the observed behaviour of PFC emissions. Continuous gas measurements were performed using a Fourier-transformed infrared spectrometer connected to the gas treatment centre for several days. With the data collected from individual high voltage anode effects, four new models are proposed to estimate the emissions of tetrafluoromethane, along with three new models to estimate hexafluoroethane emissions. These non-linear models are compared to the existing methods and the overall accuracy of each model is calculated in comparison to in-situ measurements. The accuracy of each model to predict emissions associated to individual anode effects have been investigated as well and the results indicate that tetrafluoromethane emissions can be more accurately predicted by the proposed non-linear models. Models proposed to predict hexafluoroethane emissions are optimistic, but further refinements are necessary to optimise the accuracy of individual predictions

New approach for quantification of perfluorocarbons resulting from high voltage anode effects

The methodology used for accounting PFC emissions resulting from aluminum smelting have been developed nearly 20 years ago. There has not been any update on this methodology since 2008 and recently published research demonstrated possible avenues for improvements resulting in increased accuracy. Current models used in the aluminum industry quantify PFC emissions linearly, based on the monthly average polarized anode effect duration for entire pot lines. To potentially increase the precision of the quantification process, other approaches, composed of non-linear models, were used to estimate the emissions of PFC during individual high voltage anode effects. Gas monitoring was performed in different smelters and the efficiency of these methods was evaluated and discussed. The potential benefits and drawbacks related to those quantification methodologies were evaluated by extrapolating results for significant operation periods. Additionally, with this analysis, it was possible to assure time-consistency between the original quantification methodology and the new approaches explored in this paper

On-line monitoring of anode currents to understand and improve the process control at Alouette

In 2014, Alouette acquired a system to monitor the on-line anode current on two pots. This system, developed and supplied by WIT, reports all anodes current and the pot voltage for every second of operation. The following paper describes some of the resulting improvements that apply to the process control of the aluminum electrolysis cell. Current monitoring of the anodes easily indicates the generation of localized anode effects (AE) prior to their propagation into a “voltage triggered” AE. Basic concepts, algorithms, results and optimization to improve the detection rate are discussed in the first part of the paper. Moreover, AE are directly influenced by the alumina distribution in the cell. A better understanding of the dissolution patterns based on the feeder’s position was achieved by using the monitoring system