ISO 501:2025

Overview

ISO 501:2025 specifies a standardized laboratory method for determining the crucible swelling number (CSN) - also known as the free swelling index (FSI) - of hard coal. The test quantifies how a ground coal sample behaves (swells and cokes) when heated in a covered silica crucible under controlled heating and atmosphere. The CSN/FSI is a widely used indicator of coal coking properties and is important for coal characterization, quality control and process selection.

Key topics and technical requirements

• Sample preparation

  • Use the general analysis test sample prepared per ISO 13909-4 and ISO 18283.
  • Grind to pass a 212 µm sieve; avoid over-pulverization and ring mills.
  • Perform test within 24 h of grinding to avoid oxidation.
  • Test sample mass: 1,00 g to 1,01 g.

• Apparatus

  • Silica crucible and lid with specified dimensions (e.g., external height 26 mm ± 1 mm, top diameter 41 mm ± 1 mm, capacity 16–17.5 ml, mass 12 g ± 1 g).
  • Pierced lid and thermocouple for temperature monitoring.
  • Burner (Teclu or Meker) or alternative electrical heating if equivalent to gas method (within ±0,5 CSN unit).
  • Draught shield, silica triangle support, 500 g mass and sight tube for profile comparison.

• Heating profile and conditioning

  • Empty crucible base temperature: 800 °C ± 10 °C after 1.5 min, 820 °C ± 5 °C after 2.5 min.
  • Precondition apparatus and check temperature profile regularly.

• Procedure and result classification

  • Heat coal in covered crucible until volatile flame dies out (minimum 2.5 min), cool, and examine residue.
  • Classify results as 0, 0.5, 1, 1.5–9, or >9 based on coke button form and standard profile templates.
  • Perform tests in triplicate (or up to 5/10 depending on spread rules); report mean to nearest 0.5 unit.

• Precision

  • Repeatability: maximum acceptable difference for 3 determinations = 0.5 unit.
  • Reproducibility: for 5 determinations, maximum acceptable difference between labs = 1 unit.

• Safety

  • The standard includes a safety warning; users must establish appropriate health and safety practices.

Applications and users

ISO 501:2025 is used by:

• Coal laboratories and quality control teams for coal characterization, coking potential assessment and blending decisions.
• Coke producers and steelmakers assessing feedstock suitability for coke ovens.
• Researchers and standards bodies comparing coal swelling behavior.
• Regulatory or procurement teams specifying coal quality.

Practical benefits include consistent CSN/FSI measurement for process control, product specification, and comparative testing across laboratories.

Related standards

• ISO 13909-4 - Coal and coke - Mechanical sampling - Part 4: Preparation of test samples of coal
• ISO 18283 - Coal and coke - Manual sampling

Keywords: ISO 501:2025, crucible swelling number, CSN, free swelling index, FSI, hard coal testing, coal coking properties, crucible test, coke button, coal laboratory standard.