ISO 502:2025

Hard coal - Determination of caking power - Gray-King coke test

This document specifies the Gray-King coke method of assessing the caking power of hard coal under standard conditions.

Titre manque

General Information

• Status: Published
• Publication Date: 08-May-2025
• ICS: 73.040 - Coals
• Technical Committee: ISO/TC 27/SC 5 - Methods of analysis
• Drafting Committee: ISO/TC 27/SC 5 - Methods of analysis
• Current Stage: 6060 - International Standard published
• Start Date: 09-May-2025
• Due Date: 20-May-2025
• Completion Date: 09-May-2025

Relations

• Revises: ISO 502:2015 - Coal - Determination of caking power - Gray-King coke test
• Effective Date: 27-May-2023

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.

Standards Content (Sample)

International
Standard
ISO 502
Fourth edition
Hard coal — Determination of
2025-05
caking power — Gray-King coke test
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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or ISO’s member body in the country of the requester.
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagent . 1
5.1 General .1
5.2 Standard electrode carbon .1
6 Apparatus . 2
7 Preparation of sample . 3
8 Procedure . 3
8.1 Hard coals with a Gray-King coke type within the range A to G (see 11.3) .3
8.2 Hard coals with a Gray-King coke type greater than G (see 11.3) .3
9 Expression of results . 3
10 Precision . 4
10.1 General .4
10.2 Repeatability .4
10.3 Reproducibility . .4
11 Notes on procedure . 4
12 Reference figures . 6
13 Test report . 10
Annex A (normative) Determination of the bulk density of electrode carbon .12
Bibliography .15

iii
Foreword
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of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5, Methods
of analysis.
This fourth edition cancels and replaces the third edition (ISO 502:2015), which has been technically revised.
The main changes are as follows:
— changed application of document from coal to hard coal, because the method is not applicable to brown
coals and lignites;
— replaced reference from Roga Index to Caking Index;
— added Clause 3;
— added normative reference ISO 5725-2;
— moved location of Table 1 and added title;
— added title to Table 2;
— updated CSN units from fractions to decimals.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The purpose of the Gray-King coke test, which is one of the parameters adopted for the International
Classification of Hard Coal by Type by the United Nations Economic Commission for Europe, is to assess the
caking properties of a type of coal or a blend of coals by carbonizing under standard conditions.
Although the Gray-King test and the Caking Index test both assess the caking properties of a hard coal, they
do not measure precisely the same parameters and are not alternative methods.

v
International Standard ISO 502:2025(en)
Hard coal — Determination of caking power — Gray-King
coke test
1 Scope
This document specifies the Gray-King coke method of assessing the caking power of hard coal under
standard conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 1213-2, Coal and coke — Vocabulary — Part 2: Terms relating to sampling, testing and analysis
ISO 5725-2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method for
the determination of repeatability and reproducibility of a standard measurement method
ISO 13909-4, Coal and coke — Mechanical sampling — Part 4: Preparation of test samples of coal
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1213-2 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principle
The sample is heated under standard conditions to a final temperature of 600 °C. The coke residue obtained
is classified by reference to a series of standard residues. If the coke residue produced is so swollen that it
fills the cross-section of the retort tube, the determination is repeated with the coal admixed with a suitable
quantity of electrode carbon or equivalent material. For these highly swelling coals, the Gray-King coke type
is defined by the minimum amount of electrode carbon required to produce a strong hard coke residue of
the same volume as the original coal and electrode carbon mixture.
5 Reagent
5.1 General
For notes on procedure, see 11.1.
5.2 Standard electrode carbon
High temperature electrode carbon:

Moisture less than 1 %
Volatile matter less than 1,5 %
Ash less than 5 %
3 3
Bulk density at 25 °C (determined 1,00 g/cm to 1,05 g/cm
in accordance to the procedure in Annex A)
Relative density at 25 °C (see 11.2) 2,05 to 2,09
Size analysis:
Retained on 212 μm test sieve less than 1 %
Through 212 μm test sieve, retained on 125 μm test sieve less than 26 %
Through 125 μm test sieve, retained on 63 μm test sieve 10 % to 40 %
Through 63 μm test sieve 50 % to 85 %
It is recommended that a dust mask is used while using the standard electrode carbon which can contain
undesirable trace elements from the original processing of this material.
6 Apparatus
6.1 Furnace, horizontal electric, 50 mm internal diameter and 300 mm long, with one end closed and
the other carrying a plug of insulating material which is bored centrally with a hole 25 mm in diameter.
The winding of the furnace shall be such that the middle 200 mm is at a uniform temperature within ±5 °C
at both 300 °C and 600 °C. Alternatively, the furnace may be constructed from an electrically-heated
aluminium-bronze block, with one or several, bores of 25 mm diameter. The furnace shall be insulated and
located in a cover of metal or other suitable material, and shall be equipped with a suitable thermocouple,
lying above the retort tube when the latter is in position and with the junction at the centre of the furnace.
An indicator shall be provided for showing the furnace temperature with an accuracy of ±5 °C. A suitable
means of controlling the energy input shall also be provided to permit an increase in temperature at a rate of
5 °C/min. A multiple tube furnace to allow simultaneous determinations is convenient. The furnace may be
of the fixed type or mounted on rails. Suitable furnaces are shown in Figure 3 and Figure 4.
6.2 Retort tube (see Figure 5), a heat-resistant glass or transparent silica tube, 20 mm internal diameter
and 300 mm long, closed at one end, with a side arm, 8 mm internal dia
...