Brigitte Morais

A Dynamic Process Model for Predicting the Performance of Horizontal Anode Baking Furnaces

Anode manufacturing is an important step during the production of primary aluminum, and baking is the costliest stage of the anode manufacturing process. The industrial challenge resides in obtaining a good anode quality while keeping the energy consumption, environmental emissions, and cost to minimum.

Use of Mathematical Modelling to Study the Behavior of a Horizontal Anode Baking Furnace

Large numbers of carbon anodes are used in aluminum industry. The manufacture of carbon anodes involves the preparation of a paste (a mixture of coke, pitch, and recycled material), the production of green anodes via mixing and compaction of this paste, followed by cooling and baking of the green anodes. Anode baking is carried out in large furnaces. Any modification to design or operation would require a careful study of its impact on anode quality. In recent years, mathematical models have been used effectively to complement the experimental work in order to improve furnace operation and design. A design model and a process model are being developed to study the behavior of a horizontal anode baking furnace and to determine the necessary improvements. In this article, these models and their use for the study of a furnace will be described, and the results of the numerical simulations will be presented.

Review of Different Techniques to Study the Interactions Between Coke and Pitch in Anode Manufacturing

The quality of carbon anodes, consumed in electrolysis during the primary aluminum production, has an important impact on the electrolytic cell performance. Coke and pitch are the raw materials used in anode manufacturing. The raw material properties and the process parameters during production determine the anode quality. A plant receives these materials from different sources, and the variability in their properties is usually a major concern during anode production. The interaction between coke and pitch influences strongly the anode properties. Study of coke and pitch individually as well as the interactions between them using different techniques (spectroscopic, optical, etc.) such as XRD, FTIR, XPS, and SEM help identify their compatibility. Each technique gives information on different aspects of the raw materials. In this article, the use of a number of these techniques for studying coke, pitch, and their interactions will be discussed. Results will be presented for a number of cases.

Measurement of The Electric Current Distribution in an Anode

Carbon anode, which carries the electricity and provides the carbon necessary for the electrochemical reaction in the electrolytic cell, is an essential component of the primary aluminum production. Energy efficiency and environmental impact are still the key issues in modern aluminum smelters; thus, it is important to control the quality of anodes which strongly influences the energy consumption and the greenhouse gas emissions.

Characterization of Cracking Mechanisms of Carbon Anodes Used in Aluminum Industry by Optical Microscopy and Tomography

The objective of this work is to understand the different mechanisms of crack formation in dense anodes used in the aluminum industry. The first approach used is based on the qualitative characterization of the surface cracks and the depth of these cracks. The second approach, which constitutes a quantitative characterization, is carried out by determining the distribution of the crack width along its length as well as the percentage of the surface containing cracks. A qualitative analysis of crack formation was also carried out using 3D tomography. It was observed that mixing and forming conditions have a significant effect on crack formation in green anodes. The devolatilization of pitch during baking causes the formation and propagation of cracks in baked anodes in which large particles control the direction of crack propagation.

Effect of Heating Rate on the Crack Formation During Baking in Carbon Anodes Used in Aluminum Industry

The quality of carbon anodes used in aluminum industry depends on the raw material properties and the manufacturing process parameters. It is one of the key factors directly related to the aluminum production cost. The degradation in anode quality such as crack formation increases the energy consumption, the environmental emissions, and the smelter’s overall operating cost.

Quantification of Sodium Present in Dry Aggregates and Anodes

In aluminum industry, it is important to determine the concentration of contaminants present in anode raw materials as rapidly as possible in order to adjust the anode recipe. Sodium, which is an impurity coming largely from anode butts, significantly influences the anode reactivity, and an increase in its concentration increases the anode consumption. A simple and inexpensive method was developed to quantify the sodium content in dry aggregates and anodes without grinding the samples. The method is based on potentiometric principles using a sodium-ion specific electrode. A sample can be analyzed easily within thirty minutes. In order to prevent the rapid degradation of the specific electrode due to experimental conditions, sodium is extracted from samples by electrophoresis prior to the test. The comparison shows that the measured sodium concentrations obtained are similar to the results determined by other test methods.

Study of the Wetting of Coke by Different Pitches

Prebaked anodes are made of dry aggregates (coke, recycled butts and rejected green/baked anodes) and the binder pitch. During the mixing process, the wettability of coke by pitch influences the anode paste behavior. Coke particles (-125 +100 μm) were prepared from the same source using two different procedures, and the wettability of cokes by five pitches with different properties was studied using a sessile-drop system at 170oC. The contact angle results show that the wettability of coke by all pitches follows a similar trend for both cases, but the actual contact angles are different. The cokes and the pitch-coke interfaces were also investigated by the optical and scanning-electron microscopy (SEM) techniques. The image analysis results indicated that the coke particle size distribution depends on the coke preparation, which seems to influence its wettability.

Description and Applications of a 3D Mathematical Model for Horizontal Anode Baking Furnaces

In aluminum industry, carbon anodes are consumed continuously during alumina reduction in the electrolysis cells. Anodes are made of calcined coke, butt, and recycled anode particles and pitch as the binder. Green anodes are baked in large furnaces where they attain specific properties in terms of density, mechanical strength, and electrical conductivity. Baking is an important and costly step in carbon anode production. The proper operation of the furnace provides the required anode quality.

Characterization of Dry Aggregates in Carbon Anodes by Image Analysis

Carbon anode is one of the key components in the electrolytic production of primary aluminum. Anodes are mainly composed of dry aggregates such as calcined petroleum coke and recycled materials with pitch as the binder. Granulometry of the dry aggregates is important to obtain good physical, chemical, electrical, and mechanical anode properties. Sieving can indicate the size range of particles, but it does not reveal much information about the shape or nature of the individual particles. This article presents an image analysis technique to study not only the granulometry, but also the physical characteristics (e.g. aspect ratio, roundness, form factor, etc.) of each and every particle. The custom-made software for the image analysis can also separately identify the butt and coke particles based on different shape parameters. This technique could help track changes in granulometry at different stages of the anode production and consequently improve the quality.