Coking propensity tests: The Gray–King Assay and the Free Swelling Index (FSI) of a coal primarily measure its

Introduction / Context:
For metallurgical coke production, a coal’s ability to soften, swell, and re-solidify into a coherent porous mass (coke) is critical. Laboratory indices help screen coals for coking blends. Two common tests are the Gray–King Assay and the Free Swelling Index (FSI), both of which relate to caking or agglutinating behavior under heating.

Given Data / Assumptions:

• Gray–King Assay grades the type of coke formed by heating a standard coal charge in a silica tube under specified conditions.
• FSI (also called Swelling Index) measures the volume increase of a coal button upon heating under prescribed conditions.
• Both tests correlate with coking potential in blends.

Concept / Approach:

While “swelling” is observed, these tests are not merely about expansion; they indicate whether plasticity develops and particles fuse (agglutinate) into a strong coke. High indices usually mean better coking behavior. Non-caking coals show little swelling and poor Gray–King ratings; prime coking coals show significant plasticity and coherent coke formation.

Step-by-Step Solution:

Verification / Alternative check:

Industrial practice uses Gray–King, FSI, Gieseler plastometry, and Ruhr dilatometry together to characterize coking coals; these collectively evaluate plastic range, fluidity, and resulting coke strength after reaction (CSR/CRI tests downstream).

Why Other Options Are Wrong:

Pure “swelling characteristics only” misses the caking interpretation; abradability and impact resistance are mechanical handling properties; ash fusion temperature is determined by separate tests.

Common Pitfalls:

Assuming high swelling alone guarantees good coke—excessive expansion without coherence can be detrimental; blending strategies balance plasticity and stability.

Final Answer:

Agglutinating (caking/coking) properties