Chemical Structure and Properties of Heat Treated Coal in the Early State of Carbonization (VI)

Knoop Hardness of Originals and Heat Treated Coals

Abstract

The measurments of hardness are a very available mean in the study of rheological properties of coal. It is well known that the hardness of coal, changes with rank and that the hardness of heat-treated coals has a maximum at heat treatment temperature (HTT) of about 1, 200°C. It apPears, however, that very few detailed inves tigations of hardness of heat-treated coals in the early state of carbonization have yet been made. The present authors measured the Knoop hardness of 17 coals of every rank and that of coals heat-treated over the range from HTT 200°C to HTT 500°C.
The relation between Knoop hardness number of original coals and rank shows in Fig. 5.The cohesive energy density of coals has a minimum at about 85-90% C. It is supposed, therefore, that the hardness of coals shows a minimum at about 90%C. Although the intermolecular forces tend to increase below about 85%C, the hardness shows a maximum at about 80%C and decreases with the decrease of rank. This phenomenon may be explained by the increase of micro-pore structure or texture in the lower rank coals. On the other hand, the unreactive oxygen has a maximum at about 80%C (Fig. 8). Assu ming that the major part of the unreactive oxygen is an ether-type, it is supposed that the other-type oxygen in coals increases with rank up to about 80%C. It may be said, therefore, that the oxygen bridge in coals increases and the hardness of coals increases with rank in the range of about 70%C to about 80%C.
The Knoop hardness number of heat-treated non-caking coals below about 80%C is almost the same as that of original coals over the range of HTT about 200-250°C and then increases suddenly with the increase of HTT. It seems that this sudden increase of hardness depends on the chemical change of constitution in original coals caused mainly by dehydration.
The Knoop hardness number of heat-treated caking coals in the range of about 80% C to about 90%C is almost the same as that of original coals up to HTT about 300°C, gradually decreases with the increase of HTT, reaches a minimum at HTT about 400-450°C, and then suddenly increases with HTT. It is seen that the Knoop hardness of heat-treated caking coals is closely related to the lower molecular substances in original coals and to the thermal softening and thermal fusing properties of original coals.
The Knoop hardness number of heat-treated anthracite above about 90%C is almost the same as that of original coal up to HTT about 450°C and then increases with HTT. It is suggested that the sudden increase of Knoop hardness in heat-treated coals above a certain HTT depending on the rank of original coals has an intimate relation with the development of cross-linking, aromaticity and ring condensation in heat-treated coals.