Overview
ISO 21687:2007 specifies a standardized volumetric method for the determination of the true density of solid carbonaceous materials used in aluminium production. The method uses helium gas pyknometry (volumetric) to measure the mass per unit volume of green and calcined petroleum coke, pitch-derived materials and similar solids (for example electrodes). The procedure is not suitable for graphitized materials.
This standard supports accurate material characterisation, helps estimate degree of calcination and provides reproducible results for quality control and process monitoring.
Key Topics
- Scope: Density determination for green and calcined petroleum coke, hydrocarbons with high boiling range and other solid carbonaceous materials. Not applicable to graphitized material.
- Principle: A ground and sieved test portion (particle size ≤ 0.063 mm) is dried, weighed and placed in a gas pyknometer. Helium is used to pressurize the analysis chamber and then expanded into a calibrated expansion chamber; pressures are recorded and the sample volume is calculated volumetrically.
- Apparatus & materials: crusher, grinder, porcelain bowl, copper plate, 0.063 mm sieve (ISO 3310-1), drying oven (preferably vacuum-capable), desiccator, gas pyknometer, calibration balls, tared sample cup, analytical balance (0.1 mg), helium (≥ 99.996 % purity).
- Sample preparation: Sampling follows ISO 6257, ISO 6375 or ISO 8007-1 as applicable. Samples are crushed, ground to ≤ 0.063 mm, sieved and dried (typical drying at 110 °C under reduced pressure for coke/anode materials) before analysis.
- Calculation: Density ρ = m / V, where m is the dry mass and V the volume determined by pyknometry. Instruments usually report volume automatically; the standard gives the formula for manual calculation from measured pressures and calibrated chamber volumes.
- Precision: Repeatability and reproducibility were established via round-robin tests (ASTM E 691). Example repeatability: pitch r = 0.003 g/cm³; coke r = 0.004 g/cm³; anodes r = 0.002 g/cm³. Reproducibility values are also provided in the standard.
Applications
- Routine quality control of petroleum coke and electrode materials
- Estimation of degree of calcination for process control and specification
- Material acceptance testing and specification compliance
- Research, development and failure analysis where true solid density is required
This method is particularly valuable when pore exclusion (accessible pore volume) and accurate solid-phase density are needed for process or product evaluation.
Related Standards
For implementation, follow the apparatus calibration, sample handling and reporting requirements given in the full ISO 21687:2007 text to ensure repeatable, comparable density measurements across laboratories.
