Determination of coke calcination level and anode baking level - Application and reproducibility of L-sub-c based methods

Abstract

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.