Pascal Lavoie

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.

Increasing the Power Modulation Window of Aluminium Smelter Pots with Shell Heat Exchanger Technology

With power prices constantly rising, and varying aluminium prices requiring operating flexibility, the financial incentive for smelters to adopt a power modulation strategy is becoming larger. However, the power modulation window, in which a smelter can safely operate its reduction cells, is limited. The Light Metals Research Centre has developed the Shell Heat Exchanger (SHE) technology for controlling the heat dissipation from aluminium smelting pot shells. By varying the air flow through the SHE, the heat removal from the shell can be increased or decreased as desired, doubling the previous power modulation window or allowing power modulation with minimal disturbance to the pot thermal balance.

A Review of Alumina Feeding and Dissolution Factors in Aluminum Reduction Cells

Modern aluminum reduction cells use point feeding technology to replenish alumina as it is consumed by the electrolytic process. The dissolution of alumina has become increasingly difficult to control as the cell sizes and electrolysis intensity have increased. The mass of alumina added per unit time is now much higher than a decade ago, and must take place within a smaller electrolyte mixing volume. In order to replenish the alumina concentration evenly, the alumina needs to be delivered, dispersed, dissolved, and distributed throughout the reduction cell. The dissolution itself follows a 4-step process that can be limited by a multitude of factors. The status of the research on each of these factors is reviewed in the present paper. Although research in laboratory cells has been conducted many times, and the impact of many factors on dissolution has been measured, published observations of alumina feeding on industrial cells are very sparse, especially regarding the dissolution dynamics in the space–time domain and the impact of the feeder hole condition. The present paper therefore presents a qualitative model of the factors governing alumina dissolution in industrial cells and offers the hypothesis that maintenance of the feeder hole condition is central to ensuring alumina dissolution and prevention of sludging.

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.

The Impact of Alumina Quality on Current Efficiency and Energy Efficiency in Aluminum Reduction

Current efficiency and energy efficiency are the two most important metrics for assessing the overall performance of a given potline. The influence of alumina quality on these parameters is however poorly understood although likely influential. This interplay is considered here with the construction of multiple regression models from daily data of test groups of 60 cells each for a number of important parameters, pertinent alumina properties from detailed materials characterization of judiciously chosen aluminas and relevant weather parameters for approximately 20 months of effective sampling. The concentration of α-Al2O3 in the fines is found to be the single most influential parameter affecting current efficiency and energy efficiency. Models of superheat and noise indicate that this is predominantly due to issues associated with alumina dissolution. Energy efficiency is also correlated with gibbsite content of the fines, likely due to the added energy required for the phase transformation from hydroxide to sesquioxide.

Application of Multivariate Statistical Process Control with STARprobeTM Measurements in Aluminium Electrolysis Cells

Due to the multivariate nature of the aluminium electrolysis process, the usual univariate control algorithms coupled with the low visibility of the process used in the aluminium industry intrinsically causes control errors resulting in suboptimal process control. A trend in the industry consists in applying multivariate statistical process control with specific responses to the cause of variations. Such a system in the form of a Bath Temperature and Chemistry Control Module (BTCM) was developed to be used in conjunction with state-of-the-art STARprobeTM measurements of electrolyte properties, providing the ability to respond quickly to causes of abnormalities detected from the immediate synchronous bath measurements. This paper presents the system and results from a smelter’s pilot section.

Alumina Concentration Gradients in Aluminium Reduction Cells

The length of aluminium electrolysis cells have constantly increased over the last decades. The drive to increase productivity resulted in the need to feed and dissolve more alumina in less electrolyte. There is mounting evidence that these two trends are pushing the electrolysis cells above their capability to maintain alumina concentration, through time and space, at levels preventing both conventional and non-propagating anode effects. Alumina concentration gradient measurements were performed within large industrial cells and showed that large gradients occurred between locations in cells.

Determination of the Effect of Pitch-Impregnation on Cathode Erosion Rate

A number of smelters have adopted or have trialed high density, pitch impregnated cathode blocks as one measure to counter the trend in decreasing cell life due to line current increases. To date the true benefits of pitch impregnated cathode blocks are not fully understood and therefore a joint collaboration between SEC CARBON Limited and the Light Metals Research Center has endeavored to understand the effect of pitch impregnation on cathode block performance. The initial results of this project showed that pitch impregnated cathode blocks had no benefit in regards to electrochemical wear resistance, and it was proposed that this was due to the pitch impregnation increasing the reactivity of the cathode material [1]. This paper reports on recent work conducted to firstly characterize the difference between the pitch impregnation phase and other phases present in the bulk cathode matrix and secondly to understand the relative reactivity of these phases under electrolysis conditions.

Managing Potlife through Statistical Analysis

A significant portion of ongoing costs in aluminium smelters comprises of electrolysis cell reconstruction. The cells, also referred as pots, undergo different wear mechanisms and failure modes determining their finite lifespan. Smelters have to carefully manage all aspects determining potlife to maximize it while minimizing reconstruction costs. Pot design, materials and supplier selection, construction quality and electrolysis operations all affect potlife.