Overview
ISO 4698:2022 - Iron ore pellets for blast furnace feedstocks - Determination of the free‑swelling index specifies a laboratory method to measure the free‑swelling index (FSI) of hot‑bonded iron ore pellets. The FSI is a relative measure of pellet volume increase when reduced in an unconstrained bed under conditions simulating the reduction zone of a blast furnace. The procedure delivers comparable results across laboratories to support quality control, contract specification and research.
Key topics and technical requirements
- Scope and applicability: Applicable to hot‑bonded pellets used as blast furnace feedstocks.
- Sample preparation: Use ISO 3082 sampling; pellet size range −12.5 mm / +10.0 mm; oven‑dry test sample to constant mass (105 °C ± 5 °C); minimum 1 kg test sample.
- Test portions: At least four test portions, each of 18 whole pellets selected at random.
- Reduction conditions:
- Isothermal reduction at 900 °C ± 10 °C in a fixed, unconstrained bed.
- Reducing gas composition: CO 30.0% ± 0.5%, N2 70.0% ± 0.5%.
- Gas purity limits for impurities (H2, CO2, O2, H2O) are specified.
- Reducing gas flow 15 L/min ± 1 L/min; heating/cooling N2 flow profiles given (e.g., 10 L/min to heat, 15 L/min during equilibration, 5 L/min for cooling).
- Default reduction time 60 min (shorter times, e.g., 40 min, permitted when appropriate).
- Apparatus: Reduction tube (internal diameter 75 mm ± 1 mm), test‑portion holder (three levels of 6 pellets), electrically heated furnace, gas‑supply system, and volumetric apparatus capable of 0.2 ml accuracy. Annex B describes volume measurement methods.
- Result calculation: FSI (%) = (V1 − V0) / V0 × 100, where V0 is volume before reduction and V1 after reduction. Annex A covers acceptance/repeatability rules.
- Safety: Carbon monoxide is toxic-tests require ventilation and safety controls.
Practical applications and users
- Steelmakers and blast furnace operators: Evaluate pellet behaviour in the reduction zone to predict furnace performance, permeability and burden behavior.
- Pellet producers: Quality control during production, batch certification and R&D for product optimization.
- Independent testing laboratories: Standardized method for contractual testing and inter‑laboratory comparison.
- Researchers: Studies of reducibility, swelling behaviour and effects of pellet chemistry or binder systems.
Keywords: ISO 4698:2022, free‑swelling index, iron ore pellets, blast furnace feedstocks, hot‑bonded pellets, reduction testing, CO/N2 reducing gas.
Related standards
- ISO 3082 - Iron ores: sampling and sample preparation procedures (referenced for sampling).
- ISO 11323 - Iron ore and direct reduced iron vocabulary (terms and definitions).