Optimisation of the Gieseler Fluidity, Dilatation and Crucible Swelling Number of Australian Coking Coals

Crucible Swelling Number, Audibert-Arnu dilatometer and Gieseler plastometer residua were collected for a range of Australian coals. The samples were examined at the optical microscope and hand specimen level, to understand the processes that influenced the results. This was with an aim to understand the relationships between these indices and the results obtained from coal carbonisation and coke testing.

The high heating rate in the Crucible Swelling Number [CSN] test produced residua that was least like coke, both in morphology and molecular ordering. During the CSN test, the rate of volatile production within the individual particles exceeded the rate of diffusion through the particles resulting in the formation of numerous pores. As a result of pore formation and development the semi-plastic particles expanded until they compressed against the surrounding particles, forming pore wall material. Larger pores evolved by coalescence with surrounding pores and by further migration of volatile matter from the pore wall material. With increased molecular ordering of the pore wall material, a more dense pore wall microstructure developed. As the pore wall microporosity decreased, the remaining volatile matter did not easily diffuse through the forming coke matrix and a set of smaller pores formed in the pore wall material. The interconnection of the macropores and venting of the volatiles in the latter stages resulted in partial collapse of the foamy structure.

In the Audibert-Arnu Dilatometer test, between the initial softening temperature and the temperature of maximum contraction, softening of grain boundaries with the associated devolatilisation and loss of inter-particle pores resulted in volume loss. In the latter part of this stage intra-particle pore formation was initiated. At the temperature of maximum contraction, inter-particle space had been mostly eliminated and the mass loss due to the devolatilisation process was offset by the volume gain due to intra-particle pore formation. This resulted in a halt in the contraction rate. With a further increase in temperature, intra-particle porosity significantly exceeded mass loss due to devolatilisation and the pencil expanded. The cessation in dilatation was initially not due to resolidification of the pencil, as the temperature at which this occurs is generally lower than the resolidification temperature determined in the Gieseler dilatometer test. It is most probably a result of the force being applied to the piston by the expanding pencil being equal to the force due to the mass of the piston.

It was found that the Gieseler Plastometer results were influenced primarily by the expansion of the transforming coal/coke due to the formation of gasification pores. A strong relationship was found between the density of the residua and the maximum Gieseler plasticity, indicating that this test was not measuring the fluidity of the transforming material but rather the amount of material in the path of the stirrer arms. The residua also showed that the stirrer arms ploughed voids in the transforming material. These voids persisted for up to ? of a revolution behind each arm. For high plasticity samples, the adhesion of the transforming material to both the spindle and the wall of the retort produced a different shaped [spiral] residuum compared to the more dome-like structure of less plastic samples. It may be that the viscosity of the transforming material may be being measured in the Gieseler plastometer test. However, rather than at the point of maximum "Fluidity", it is being measured prior to the formation of gasification pores.

No significant relationships were found between the three tests examined and any of the coke quality indices including cold coke drum strength indices, CO2 reactivity indices, maximum coking wall force measurements or maximum internal gas pressures.