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.
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.