Overview
ISO 502:2025 - Hard coal - Determination of caking power - Gray‑King coke test - specifies the standardized laboratory method for assessing the caking power of hard coal using the Gray‑King coke test. The method carbonizes a representative coal sample under controlled heating and compares the resulting coke residue with standard reference residues. For highly swelling coals, the procedure uses an additive (standard electrode carbon) to determine the minimum admixture required to produce a standard coke type.
Keywords: ISO 502:2025, Gray‑King coke test, hard coal, caking power, coal testing, coke type.
Key topics and technical requirements
- Scope and purpose: Method applies to hard coal (not brown coal/lignite) and is used to assess caking behaviour under standard conditions; it is a parameter adopted in international coal classification (UNECE).
- Principle: Heat a 20 g sample (ground to pass 212 µm) in a horizontal retort to 600 °C at a controlled heating rate; evaluate the coke residue visually against standard reference figures.
- Sample preparation: Use the general analysis sample prepared per ISO 13909‑4; air‑dry and mix immediately before testing.
- Apparatus: Horizontal electric furnace (uniform temperature zone), heat‑resistant retort tube with side arm, distance rod, tar receiver and outlet (fume management required).
- Reagents: Standard electrode carbon specifications (examples included):
- Moisture < 1 %, volatile matter < 1.5 %, ash < 5 %
- Bulk density at 25 °C: 1.00–1.05 g/cm³ (determined per Annex A)
- Relative density: 2.05–2.09
- Particle size distribution specified (fractions through 212, 125, 63 µm)
- Procedure: Controlled ramp to 325 °C, then 5 °C/min to 600 °C, hold 15 min, cool and examine residue. For coals that over‑swell, repeat with graded additions of electrode carbon to identify the minimum grams needed to reach type G.
- Precision: Repeatability and reproducibility criteria defined per ISO 5725‑2; typical allowable differences summarized (A–G: one letter; >G: one unit in subscript).
Applications
- Quality control and classification of coking coals for steelmaking and metallurgical coke production.
- Coal blending assessment to predict coking behaviour and optimize blast‑furnace feedstocks.
- Laboratory testing for coal producers, research institutes, coke plants, and trading/specification verification.
Who should use this standard
- Coal testing laboratories and petrographic/analytical labs
- Coke plant/process engineers and metallurgical quality managers
- Coal merchants, commodity inspectors, and regulators involved in coal classification
Related standards
- ISO 1213‑2 - Coal and coke - Vocabulary (sampling, testing, analysis)
- ISO 5725‑2 - Accuracy (trueness and precision) - Basic method for repeatability/reproducibility
- ISO 13909‑4 - Mechanical sampling - Preparation of test samples of coal
Note: The method requires appropriate fume control (toxic gases/tars) and adherence to laboratory safety practices.
