Rheological Properties

Coking coals possess the ability, when heated in the absence of air, to soften, swell and then re-solidify to form a coherent, porous, hard coke structure. The Gieseler Plastometer and Arnu Dilatometer tests are used to evaluate the rheological or plastic properties of a coal or coal blend.

Gieseler Plastometer Test

For this analysis 5 grams of minus 40 mesh prepared coal are packed into a retort barrel along with a stirrer. A constant torque is applied to the stirrer and the coal is heated at 3°C/minute. As the coal softens, the stirrer begins to turn. The maximum fluidity value is expressed in dial divisions per minute (DDPM) of the stirrer rotation.

• High volatiles: 5,000 to >30,000 DDPM.
• Medium volatiles: <200 to 20,000 DDPM.
• Low volatiles: 20 to 1,000 DDPM.

The Gieseler Plastometer test is useful in determining the plasticity range of coals including the temperature at which: initial softening, maximum fluidity, and resolidification occurs. Plasticity range in OC and maximum fluidity in DDPM are key factors in determining which blends of coals will be optimal for coking.  

Arnu Dilatometer Test

The Arnu Dilatometer test is used to determine the swelling properties of coal when heated under standard conditions in a dilatometer. A thin 60 mm cylinder of coal formed under pressure from minus 60 mesh coal is inserted into a precisely calibrated retort tube with a graduated piston on top. The sample is then placed in a furnace. The apparatus is heated at 3°C/minute and the movement of the piston as the coal cylinder shrinks and expands is recorded.

The maximum dilatation value is the key parameter and for individual coals the highest value possible is considered optimal:

• High volatiles: +50 to >300%.
• Medium volatiles: +100 to 250%.
• Low volatiles: <0 to 200%.

Characteristic Arnu Dilatometer curves are generated when the piston movements (taken as a percentage of the total original coal cylinder length) are plotted against the corresponding temperatures. These charts provide valuable information regarding the suitability of your samples for use as coking coals.