Overview
ISO 157:2025 - Coal - Determination of forms of sulfur specifies standardized analytical methods for quantifying sulfate sulfur and pyritic sulfur in coal and for calculating organic sulfur by difference. The third edition updates procedural clauses and adds a formula for organic sulfur. The procedures rely on differential solubility under reflux and subsequent analytical finishes to deliver reproducible sulfur speciation for coal quality and research purposes.
Key topics and technical requirements
- Scope: Methods to determine sulfate and pyritic sulfur mass fractions and to calculate organic sulfur (organic sulfur = total sulfur − (sulfate + pyritic)).
- Principle: Differential solubility of sulfates and pyrites in dilute hydrochloric and nitric acids under reflux; successive dissolution and direct determination of sulfur species.
- Sample preparation:
- Use the general analysis test sample prepared per ISO 13909-4 or ISO 18283.
- Grind about 25 g to pass a 75 µm sieve and equilibrate moisture with laboratory atmosphere.
- Separation procedure: Extraction with dilute hydrochloric acid (≈15 %) dissolves sulfate sulfur and non-pyritic iron; pyritic iron/sulfur remain in the residue for filtration and further treatment.
- Reagents (examples given in the standard):
- Concentrated HCl (~36 %, ρ ≈ 1.18 g·ml−1) and dilute HCl (≈15 %).
- Concentrated nitric acid (~70 %) and dilute nitric acid (≈9 %).
- Analytical finishes for pyritic sulfur: titrimetric, colorimetric, and atomic absorption spectrometric options are specified to suit laboratory capability.
- Quality assurance: Blank tests, expression of results, and stated precision (repeatability and reproducibility limits) are included.
- Reporting and calculations: Test-report requirements, calculation to different bases (air-dry, dry, ash-free) per Clause 10, and Annex A describing derivation of calculation factors.
Practical applications and users
ISO 157:2025 is intended for:
- Coal testing laboratories and quality-control teams in mining and utilities.
- Researchers studying coal chemistry and sulfur speciation.
- Coal cleaning and coal preparation operations that need to assess sulfur removal potential.
- Environmental compliance and emissions modelling where sulfur form affects combustion behavior and SOx emissions.
- Blending and classification workflows that require sulfur form data to optimize fuel mixes.
Benefits include consistent, comparable sulfur-speciation data for process optimization, regulatory reporting, emissions estimation, and product specification.
Related standards
Keywords: ISO 157:2025, coal sulfur determination, sulfate sulfur, pyritic sulfur, organic sulfur, coal analysis, sulfur speciation, coal quality control.
