Lorentz Petter Lossius

A review of Coke and Anode Desulfurization

During calcination, petroleum coke desulfurizes depending on the sulfur content and final temperature. This has a negative effect on coke properties such as real density, porosity, and reactivity. Additional sulfur loss occurs during anode baking and this can further deteriorate anode properties such as density and reactivity. The sulfur level of high sulfur cokes used by the calcining industry has increased since 2003. Although the average sulfur content of cokes used by smelters has not increased significantly due to environmental limits, the difference in sulfur level of cokes used in blends has increased. Calciners must avoid “over-calcining” high sulfur cokes and smelters must be wary of additional desulfurization during baking. This paper presents a review of past and recent data on coke and anode desulfurization, and gives recommendations on how to best counter the problems that can arise from use of higher sulfur cokes in blends.

The Equivalent Temperature Method for Measuring the Baking Level of Anodes

The equivalent temperature is a measure of baking level using a temperature scale (°E) based on heat treatment temperatures.

Coke and Anode Desulfurization Studies

The sulfur level of high sulfur cokes used by the calcining industry for blending is increasing. During calcination, petroleum coke desulfurizes and the rate of sulfur loss is dependent on both the sulfur level and final temperature. Desulfurization negatively affects coke properties such as real density and porosity and additional desulfurization during anode baking can negatively affect anode properties. This paper is a follow-up to a 2007 TMS paper on desulfurization and presents the results of additional studies on anode and coke desulfurization versus equivalent baking level and discusses the effect of the level of calcination of high sulfur cokes on anode reactivity. The results indicate that the coke calcination levels must be set with coke sulfur levels in mind. It also shows that blending with high sulfur cokes need not be detrimental to anode properties.

Characterization of Prebake Anodes by Micro X-ray Computed Tomography

As part of the continuous work in improving anode quality at Hydro Aluminium, series of pilot scale anodes have been manufactured with systematic changes in coke type and green paste production including recipe. The anodes also support M.Sc., Ph.D. and PostDoc work in programs supported by Hydro and the Norwegian Research Council. In addition to regular analysis, the pore, void and grain distribution has been investigated using Micro X-ray Computed Tomography (µCT). This non-destructive 3D imaging is now implemented at a cost to allow larger numbers of samples, and a methodology has been developed by SINTEF that yields surprising sharp detail, suited to interpret important structural factors for relatively large anode volumes of 10–130 mm diameter. Given a better cost to information yield than image analysis and mercury porosimetry, Hydro will continue to support academic work with CT analysis. Examples will be shown of baked anodes before and after electrolysis testing plus crack patterns after mechanical testing.

Relationships between Coke Properties and Anode Properties — Round Robin 19

This paper discusses the preparation and production of bench scale and pilot scale anodes with five different calcined coke samples prepared for a world-wide calcined coke round robin (RR). A key objective of the RR and anode testing was to look for relationships between calcined coke properties and anode properties, particularly coke bulk density/apparent density results and anode densities. The calcined coke RR was the 19th organized by Rain CII Carbon, but this time it was a collaborative effort with Hydro Aluminium and R&D Carbon. Calcined coke results for RR19 are discussed in greater detail in another paper in these proceedings. Bench scale anodes with the five cokes were prepared at R&D Carbon and used to select optimum pitch levels for production of pilot scale anodes. This is the first time a RR with such a broad scope has been coordinated and published and it has provided some useful data for the industry.

Bulk Density ‐ Overview of ASTM and ISO Methods with Examples of Between Laboratory Comparisons

The bulk density of petroleum coke is an important property when evaluating a coke for use in anodes in primary aluminum metal electrolysis. It is also an important property in petroleum coke trade. There are international standards for testing coke bulk density; ASTM has two vibrated bulk density (VBD) methods, D4292 and D7454, and ISO has a tapped bulk density (TBD) method, ISO 10236. There is a concern in anode production that it is difficult to obtain sufficiently good between-laboratory comparisons with any of these methods, both for use in comparisons, and when used for distinguishing coke qualities from different producers. The paper will present the methods, give results from several interlaboratory studies and discuss the between-laboratory comparison results.

Determination of Coke Calcination Level and Anode Baking Level ‐ Application and Reproducibility of L‐sub‐c Based Methods

The average crystallite size (L-sub-c or LC) is an important property of carbon materials for aluminium electrolysis; LC is used for characterizing the petroleum coke calcination level and sometimes also to estimate the baking level of anodes. This paper discusses problems when comparing LC results from different laboratories using precision statements from ASTM and ISO standards. The main cause is peak broadening errors introduced by the XRD instrument and sample preparation. The LC standards ASTM D5187 and ISO 20203 neglect these errors. Two ways are demonstrated to minimize the peak broadening effect to improve the standards, 1) by using thin sample thickness and 2) by embedding the coke in a high absorptive medium. Using LC to determine the anode baking level is discussed and three practices are discussed; measurement on the anode directly or two methods for using a reference coke that is baked with the anode. It is shown that precision is better for the latter methods. Especially for underbaked anodes a baking level estimated from measurement of the anode LC can be misleading.

Understanding the Anode Porosity as a Means for Improved Aluminium Smelting

The anode pore structure affects gas diffusion as well as air and carboxy reactivity burn-off during the aluminium electrolysis. The coarse porosity was investigated using X-ray computed tomography, the mid-range porosity using optical microscopy, and the finest porosity using mercury intrusion porosimetry. These methods were combined to gain a deeper understanding of porosity and how this affects anode quality and the potential for energy savings. Test materials were received from Hydro Aluminium with different coke sources, particle sizes, binder levels, and mixing temperatures. The paper will present measurement results and link these to production and process factors. The findings indicate that direct comparison for each technique is challenging because of the physical basis of the measurement and the different measurable pore size ranges. However, the combination of different techniques gives valuable insight to understanding anode porosity. Coke type, granulometry, pitch content and baking process affecting anode porosity, properties and reactivity.

Interaction Between Anode Aggregate and Binder in the Sessile Drop Wetting Test

Coal Tar Pitch (CTP), recycled butts and Calcined Petroleum Coke (CPC) are the main raw materials used for anode fabrication. Over the last decades, considerable changes in CTP and CPC quality have been observed. Changes in metallurgical coke production technology have affected coal tar quality, and crude oil qualities have changed causing severe changes of CPC properties. This can have consequences for the mixing step during anode production, and the final anode performance. Hydro Aluminium has systematically followed these raw material changes and studied the effect on the interaction between coke and binders by applying a sessile drop measurement wetting test. The results are reviewed in terms of relevance for the anode production process. This article focuses on equipment, routines and sample preparation as they are crucial factors for obtaining reliable and repeatable results during these experiments. Wetting dependence on particle size and butt impurity level was found. Additionally, a method where hot pitch is added to a hot coke substrate is demonstrated.

Effect of Coke Properties on the Bubble Formation at the Anodes During Aluminium Electrolysis in Laboratory Scale

The anodic reaction of aluminium electrolysis cells leads to the formation of CO2 bubbles, which partly screen the anode surface and leads to an increase in the cell voltage. An advantage of these bubbles is that the formation and release contribute to the stirring of the electrolyte, however, the screening of the surface increases the irreversible energy losses. The voltage and current oscillation due to the bubble evolution during electrolysis for different anode materials have been determined in a laboratory cell. The effect of coke sulphur content and grain sizes were investigated. Anodes with finer coke fraction showed lower oscillations than coarser fraction equivalents. Additionally, the influence of current density on the amplitude of the anode potentials was measured. A 64% increase of current density caused an increase of anode potential oscillations from 79 to 170%.