Molten storage and carbonization characteristics of premium grade coal-tar pitch: Influence of crude tar type and distillation conditions

Abstract

A growing trend towards the transport and use of molten rather than solidified pitch has caused tar distillers to examine the relative stability of pitches while held in storage tanks. The scope of laboratory storage-stability tests can be extended to higher temperatures as a method of studying the earliest stages of pitch polymerization/carbonization. Some indications of the ultimate performance of the pitch in the end-use process can be gained. Crude tar has a strong influence on the ultimate properties and performance of a pitch. Seven tars have been made into 120 °C Mettler softening-point pitches by two distillation methods to study their subsequent storage and carbonization behaviour. The behaviour exhibited by each pitch allows a judgement on overall ‘reactivity’ and on the sensitivity to thermal conditions in the distillation of each tar. Two distillation schemes were adopted: batch distillation followed by heat treatment; and batch distillation at a pressure which was progressively reduced. The results show that time at elevated temperature has a major effect on the properties and performance of the resulting pitch. Vacuum distillation allows lower processing temperatures. Larger amounts of secondary quinoline-insolubles were formed under the ‘distillation with heat treatment’ scheme. Complete wetting of coke by pitch took place at lower temperatures for the vacuum-distilled pitches. Heat-treated pitches increased more rapidly in softening point and lost more mass when thermally soaked. Distillation under vacuum conditions resulted in significantly modified pitch carbonization characteristics.