Plastic and Agglutinating Properties

Plasticity refers to the melting and bonding behavior of the coal and (1) is an indication of the initial softening, chemical reaction, gas liberation, and resolidiflcation process within the coke oven. 

When coal is heated, an unstable intermediate phase (metaplast) is formed after the moisture is driven from the coal. This intermediate phase is responsible for the plastic behavior of coal. On further heating, a cracking process takes place in which tar is vaporized and nonaromatic groups are split off followed by recondensation and formation of semicoke. 

When coal or a blend of coals is coked in slot-type ovens, two principal layers of plastic coal are formed parallel to the oven walls. As carbonization proceeds, the plastic layers move progressively inward eventually meeting at the oven center. It is within these plastic layers that the processes which result in particulate coal being converted into porous, fused semicoke take place. The semicoke undergoes further devolatization and contracts which result in fissures in the final coke product. 

All coals undergo chemical changes when heated but there are certain types of coal which also exhibit physical changes when subjected to the influence of heat. These particular types of coals are generally known as caking coals whereas the remaining coals are referred to as noncaking 

The plastic behavior of coal is of practical importance for semiquantitative evaluation of metallurgical coal and coal blends nsed in the production of coke for the steel industry. When bituminous coals are heated in the absence of air throngh over the range of 300°C-550°C (570°F-1020°F), volatile materials are released and the solid coal particles soften (to become a plastic-like mass) which swells and eventnaUy resolidifies. 

The thermal conductivity of coal generally increases with an increase in the apparent density of the coal as well as with volatile matter content, ash content, and temperature. In addition, the thermal conductivity of the coal parallel to the bedding plane appears to be higher than the thermal conductivity perpendicular to the bedding plane. 

There is little information about the influence of water on the thermal conductivity of coal, but since the thermal conductivity of water is markedly higher than that of coal (about three times), the thermal conductivity of coal could be expected to increase if water is present in the coal. 

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