The variation of the Knoop hardness number (
Hk) in a temperature range of 20°C to 350°C was investigated for eight kinds of coal of different ranks.
The H
k of brown coals (Tempoku and Taiheiyo) increased remarkably with the increase of tempearature. The
Hk determined at a room temperature after the heating at 350°C was higher than that before the heating. The increase of
Hk with temperature is considered to be mainly due to the intrinsic changes (the evolution of moisture and the increment of crosslinkages) in brown coals.
The
Hk of anthracite (Omine) began to increase slightly above about 200°C. This would be due to the increase in the elastic recovery, because the
Hk at a room temperature after heating at 350°C was identical to that before the heating.
The
Hk of bituminous coals (Miike, Yubari, Hashima and Itmann), on the other hand, decreased with the temperature. The temperature change of Yubari coal and Hashima coal was reversible. For Miike coal, the
Hk obtained at a room temperature after the heating at 350°C was slightly lower than that before the heating, but for Itmann coal it was slightly higher. It is interpreted that the decrease of
Hk is due to the increase of free volume, the effect of bitumen as plasticizer and the decomposition of coal.
The
Hk of durite was lower than the value of vitrite except Itmann coal. In case of bituminous coals, both the
Hk of durite and vitrite decreased with the increase of temperature. The former depended more apparetly on temperature than the latter.
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