Sensitivity of carbon anode baking model outputs to kinetic parameters describing pitch pyrolysis

Carbon anode blocks, used in aluminum electrolysis cells, are usually baked in furnaces for several days, during which they release volatiles due to pitch pyrolysis. Therefore, numerical modeling of anode baking furnaces has to include some representation of pitch pyrolysis via a set of kinetic parameters. These kinetic parameters can vary with raw materials and baking parameters and are tedious to determine experimentally. In this work, we studied how the main outputs of an anode baking model are affected by the variance of the kinetic parameters. Results show that certain model outputs are not considerably influenced by changes in the kinetic parameters (e.g., spatial variation of anode porosity, maximum heating value from volatiles), while others are significantly affected (e.g., time evolution of anode porosity, time of maximum heating value of volatiles, internal pressure of anode), in particular by activation energy variability

Application of boron oxide as a protective surface treatment to decrease the air reactivity of carbon anodes

The oxidation of a carbon anode with air and CO2 occurs during the electrolysis of alumina in Hall-Héroult cells, resulting in a significant overconsumption of carbon and dusting. Boron is well known to decrease the rate of this reaction for graphite. In this work, the application of boron oxide has been investigated to evaluate its inhibition effect on the air oxidation reaction, and to provide an effective protection for anodes. Different methods of impregnation coating have been explored. Impregnated anode samples were gasified under air at 525 C according to the standard measurement methods. X-ray tomography was used to obtain the microstructural information of the samples before and after air-burning tests. The impregnated samples showed a very low oxidation reaction rate and dust generation

Acoustic emission techniques to measure the properties of coke particles: a first foray

The performance of the Hall-Héroult aluminium reduction process is strongly influenced by fluctuations of the baked carbon anode properties. The currently decreasing quality and increasing variability of the anode raw materials, coke and pitch, combined with the frequent supplier changes by anode manufacturers to meet their specifications and reduce purchasing costs make it very challenging to produce anodes with consistent properties. Furthermore, the coke quality control scheme traditionally used by aluminium smelters involving infrequent coke sampling and characterization in the laboratory is inadequate for tracking coke variability when formulating the anode paste, and applying timely corrective actions when necessary. Developing new rapid and non-destructive sensors for measuring key coke properties such as density and porosity directly from the production line is highly desirable. This work investigates the possibility of using acoustic emission techniques for measuring physical and/or mechanical properties of coke particles. A set-up was developed for recording the sound made by coke particles dropped on a metal sheet. The potential of the approach was tested on coke samples having different physical properties (several sizes and suppliers). The acoustic signature of each type of coke particle was correlated with their physical properties using regression analysis

Aluminium Production Process: Challenges and Opportunities

Aluminium, with more than 50 Mt annual production in 2016, is an essential material in modern engineering designs of lightweight structures.[...

Effects of Charcoal Addition on the Properties of Carbon Anodes

Wood charcoal is an attractive alternative to petroleum coke in production of carbon anodes for the aluminum smelting process. Calcined petroleum coke is the major component in the anode recipe and its consumption results in a direct greenhouse gas (GHG) footprint for the industry. Charcoal, on the other hand, is considered as a green and abundant source of sulfur-free carbon. However, its amorphous carbon structure and high contents of alkali and alkaline earth metals (e.g., Na and Ca) make charcoal highly reactive to air and CO2. Acid washing and heat treatment were employed in order to reduce the reactivity of charcoal. The pre-treated charcoal was used to substitute up to 10% of coke in the anode recipe in an attempt to investigate the effect of this substitution on final anode properties. The results showed deterioration in the anode properties by increasing the charcoal content. However, by adjusting the anode recipe, this negative effect can be considerably mitigated. Increasing the pitch content was found to be helpful to improve the physical properties of the anodes containing charcoal

Reuse of Acid Bioleachate in Bacterial Oxidation of a Refractory Gold Sulfide Concentrate

Bacterial pre-oxidation of refractory gold concentrates generates large volumes of leachate and requires a significant supply of nutrients to support bacterial growth. Therefore, bioleachate reuse reduces both water consumption and the nutrients required for the process. However, the efficiency of this method and its benefit need to be further explored. In the present study, two tests on the reuse of bioleachate in new cycles of bacterial oxidation were carried out to evaluate the efficiency and the benefit of bioleachate reuse. Our results showed that the reuse of bioleachates could reduce nitrogen and phosphorus requirements by 40% and 36%, respectively, after a 14-day biooxidation stage in a stirred tank bioreactor. We also showed that the reuse of bioleachate had a positive effect on the recovery of gold in a subsequent 48 h treatment by cyanidation. The gold recovery rate (initial concentration of 44 mg/kg) remained unchanged at 90% after the two bioleachate recirculation loops. The reuse of bioleachate also made it possible to increase the solubilization rates of other metals from the sulfide concentrate. Thus, the solubilization yields of copper (initial concentration of 3587 mg/kg) and zinc (initial concentration of 27,315 mg/kg) increased, respectively, from 14.8% and 40.2% to 37.5% and 99.6% after the two bioleachate recirculation loops

Electrical Resistivity Measurement of Carbon Anodes Using the Van der Pauw Method

The electrical resistivity of carbon anodes is an important parameter in the overall efficiency of the aluminum smelting process. The aim of this work is to explore the Van der Pauw (VdP) method as an alternative technique to the standard method, which is commonly used in the aluminum industry, in order to characterize the electrical resistivity of carbon anodes and to assess the accuracy of the method. For this purpose, a cylindrical core is extracted from the top of the anodes. The electrical resistivity of the core samples is measured according to the ISO 11713 standard method. This method consists of applying a 1 A current along the revolution axis of the sample, and then measuring the voltage drop on its side, along the same direction. Theoretically, this technique appears to be satisfying, but cracks in the sample that are generated either during the anode production or while coring the sample may induce high variations in the measured signal. The VdP method, as presented in 1958 by L.J. Van der Pauw, enables the electrical resistivity of any plain sample with an arbitrary shape and low thickness to be measured, even in the presence of cracks. In this work, measurements were performed using both the standard method and the Van der Pauw method, on both flawless and cracked samples. Results provided by the VdP method appeared to be more reliable and repeatable. Furthermore, numerical simulations using the finite element method (FEM) were performed in order to assess the effect of the presence of cracks and their thicknesses on the accuracy of the VdP method

Discrete Element Method Investigation of Bulk Density and Electrical Resistivity of Calcined Coke Mixes

Packing density and electrical resistivity of particles assemblies are important factors for a variety of applications of granular materials. In the present work, a three-dimensional imaging technique is coupled with the discrete element method (DEM) to model anode grade calcined coke particles. Three-dimensional DEM models of samples with different size distribution of particles were studied to obtain the inter-particle contact information. As the content of fine particles increased, a higher inter-particle contact density and smaller average contact radius was observed in the samples. Confronting the DEM data and experimental measurements of electrical resistivity showed the simultaneous effects of packing density and contact density. Samples with higher contact density and smaller contact radius in general held high electrical resistivities. However, if increasing the contact density does not modify contacts between large particles, this will have a positive effect on packing density, so a lower electrical resistivity was obtained

Electrochemical Corrosion Behavior of Fe₃Al/TiC and Fe₃Al-Cr/TiC Coatings Prepared by HVOF in NaCl Solution

Adding TiC particles into iron aluminide coatings has been found to improve its wear resistance, but its corrosion behavior is less known. In this study, the corrosion behavior of Fe3Al/TiC and Fe3Al-Cr/TiC composite coatings, prepared by high velocity oxy fuel (HVOF) spraying, was studied in 3.5 wt. % NaCl solution by means of electrochemical techniques and surface analysis. Results revealed that adding TiC particles into Fe3Al matrix to improve the wear resistance does not deteriorate the corrosion behavior of Fe3Al coating. It was also showed that addition of chromium to Fe3Al/TiC composite provides a more protective layer