Overview
ISO 502:2015 - "Coal - Determination of caking power - Gray‑King coke test" specifies a standardized laboratory method to assess the caking power of coal by producing and classifying coke residues under controlled conditions. The Gray‑King coke test is widely used in coal testing and coal classification to evaluate how a coal or coal blend behaves when carbonized, and to inform blending and process decisions in coke production and metallurgical applications.
Key topics and technical requirements
- Test principle: Heat a 20.0 g sample (ground to pass 212 μm) in a horizontal retort to produce a coke residue that is compared with standard Gray‑King reference residues. Final temperature is 600 °C, with a heating rate of 5 °C/min after an initial ramp; hold at 600 °C for 15 min.
- Sample and preparation: Analysis sample ground to 212 μm, mixed and tested on the same day; typical sample mass 20.0 g ± 0.1 g.
- Apparatus: Horizontal electric furnace (50 mm ID × 300 mm long; uniform temperature ±5 °C across middle 200 mm), heat‑resistant glass or silica retort (20 mm ID × 300 mm, with side arm), distance rod, receiver and outlet tube.
- Electrode carbon / additive: For highly swelling coals that block the retort, the coal is blended with standard electrode carbon (or anthracite equivalent) in integer gram additions until a type G coke residue is produced. Electrode carbon specifications (moisture, volatile matter, ash, bulk and relative density, and particle size distribution) are defined.
- Result expression: Gray‑King coke types A–G. For coals requiring electrode carbon, the result is shown as a letter with a subscript indicating the minimum grams of electrode carbon needed (e.g., G2).
- Precision: Repeatability and reproducibility limits are defined (maximum acceptable differences typically one letter for A–G; one subscript unit for >G).
- Safety: Fumes are toxic - test should be performed in a fume cupboard; tar disposal and dust precautions (dust mask) are recommended.
Practical applications and users
ISO 502:2015 is used for:
- Quality control in coal testing laboratories
- Coke producers and metallurgical plants assessing feedstock caking behavior
- Coal blending decisions to meet coking requirements
- Regulatory and classification bodies (e.g., international coal classification)
- Research and development on coal properties and coke formation
Keywords: ISO 502:2015, Gray‑King coke test, caking power, coal testing, coke residue, electrode carbon, coal blending, furnace retort
Related standards
- ISO 1014 - referenced for determination of true relative density (used when evaluating electrode carbon).
This standard provides a reproducible, internationally recognized procedure for determining coal caking behavior, supporting consistent quality assessment and informed operational decisions in industries using coking coals.
