Overview
ISO 157:1996 - "Coal - Determination of forms of sulfur" specifies standardized laboratory methods to quantify the sulfate sulfur and pyritic sulfur in coals (including brown coals and lignites) and to calculate organic sulfur by difference. The second edition (1996) replaces the original 1975 edition and describes sample preparation, chemical separations (acid reflux extractions), analytical finishes and result expression for sulfur speciation in coal.
Key topics and technical requirements
- Scope and principle
- Uses differential solubility under reflux: dilute hydrochloric acid extracts sulfates and non‑pyritic iron while pyritic iron/sulfur remains insoluble; the residue is then extracted with dilute nitric acid to dissolve pyritic iron.
- Organic sulfur is calculated as: total sulfur − (sulfate sulfur + pyritic sulfur).
- Sample preparation
- Analysis sample prepared to standardized particle size (ground to pass 75 µm) and equilibrated for moisture; test portion mass selected relative to expected total sulfur.
- Analytical methods
- Sulfate sulfur: determined gravimetrically by precipitation as barium sulfate (BaSO4) with prescribed reagents, filtration, ignition and weighing.
- Pyritic sulfur: pyritic iron in the nitric‑acid extract is determined by titrimetry, calorimetry or atomic absorption spectroscopy; pyritic sulfur is calculated assuming FeS2 stoichiometry.
- Apparatus & reagents
- Reflux equipment (cold‑finger condenser), muffle furnace, Gooch/G4 crucibles, class A glassware, certified reagents (dilute HCl ~15%, dilute HNO3 ~9%, barium chloride, etc.).
- Quality and precision
- Blank tests and duplicate determinations required.
- Repeatability limit: results from duplicates in same lab should not differ by more than 0.02% (absolute).
- Reproducibility critical difference between labs: 0.03% (absolute) at 95% confidence.
- Safety
- Warnings on handling corrosive/toxic reagents and use of appropriate lab practices.
Applications
- Routine coal quality analysis for fuel buyers and sellers to characterize sulfur forms.
- Environmental compliance and emissions control planning (sulfur speciation informs SO2 control strategies).
- Coal cleaning and beneficiation - identifying pyritic vs. organic sulfur helps choose cleaning processes.
- Power plants, coke ovens and metallurgical operations use these data for process control and compliance.
- Research laboratories and standards testing labs performing contract or regulatory testing.
Who uses this standard
- Coal testing laboratories and quality control teams
- Power generation and utility fuel analysts
- Coal traders and classification agencies
- Environmental regulators and emissions modelers
- Research institutions studying coal chemistry and beneficiation
Related standards
ISO 157 references and complements other ISO standards on coal sampling and sulfur analysis, for example ISO 331, ISO 334, ISO 351, ISO 1988 and ISO 5069‑2 (sample preparation and total sulfur methods). Use ISO 157 together with these sampling and total‑sulfur methods for reliable sulfur speciation in coal.