ISO 11256:2015

Overview

ISO 11256:2015 defines a laboratory test method to determine the clustering index (CI) of iron ore pellets intended as feedstock for shaft direct-reduction processes. The standard provides a validated, repeatable procedure that simulates the thermal and reducing conditions in shaft reactors and quantifies the tendency of hot‑bonded pellets to form clusters after reduction and mechanical disaggregation.

Key topics and technical requirements

• Scope: Applicable to hot‑bonded iron ore pellets used in shaft direct‑reduction feedstocks. Test results are intended for quality control, contractual specification and research.
• Principle: Isothermal reduction of a fixed bed of pellets to 95% degree of reduction, followed by controlled disaggregation in a tumble drum; the clustering index is the mass fraction of material remaining in clusters after disaggregation.
• Sample and test portions:
  • Representative sample per ISO 3082; sized pellets (typical size fractions described in the standard).
  • Test portions: at least five portions ≈ 2 000 g each; one reserved for chemical analysis.
• Test atmosphere and gas control:
  • Reducing gas composed of CO, CO2, H2 and N2 with specified volume fractions (document specifies approx. 30% CO, 15% CO2, 45% H2, 10% N2) and strict impurity limits.
  • Reducing gas flow typically maintained at 40 L/min (reference conditions) with specified preheating and nitrogen for heating/cooling.
• Temperature and loading:
  • Entire test portion held at 850 °C ± 5 °C.
  • Static load applied after initial reduction: 147 kPa ± 2 kPa (via ram and porcelain ball bed).
• Apparatus essentials:
  • Reduction tube assembly with perforated support plate, furnace with balance, gas supply and flow control, tumble drum (1 000 mm diameter × 500 mm length) rotated at ~25 r/min, weighing equipment.
• Result expression:
  • Calculation of the Clustering Index (CI) as mass of clustered material after defined tumbler cycles; procedures for repeatability, acceptance and reporting are included.

Practical applications and users

• Steelmakers and direct‑reduction plant operators: assess pellet behaviour in shaft reactors and anticipate handling/operational issues.
• Pellet producers and metallurgical QA/QC teams: specify product quality, ensure consistency and support commercial contracts.
• Test laboratories and R&D groups: compare pellet formulations, bonding methods, and process adjustments with a standardized, reproducible metric.
• Engineering consultancies evaluating feedstock suitability for DRI (direct reduced iron) processes.

Related standards

• ISO 3082 - Sampling and sample preparation for iron ores
• ISO 2597-1 / 2597-2 - Determination of total iron content
• ISO 9035 - Acid‑soluble iron(II) determination
• ISO 11323 - Vocabulary for iron ore and direct reduced iron

Keywords: ISO 11256:2015, clustering index, iron ore pellets, shaft direct‑reduction, hot‑bonded pellets, tumble drum, reducing gas, pellet quality, DRI feedstock.