ISO/PRF 6791

ISO/PRF 6791 is an ISO standard for carbonaceous materials used in aluminium production. It specifies a conventional gravimetric method for determining the quinoline-insoluble material in pitch for electrodes. Laboratories, quality teams and buyers use it to measure and report this property on a prepared pitch sample in a consistent way.

What does ISO/PRF 6791 specify?

ISO/PRF 6791 specifies how to determine the content of quinoline-insoluble material in pitch used in aluminium production. The document is organized into clauses on scope, normative references, principle, reagents and materials, apparatus, sample preparation, procedure, expression of results and test report.

Clause 4 explains that the method works by extracting the pitch with quinoline at a defined temperature and time, then weighing the insoluble fraction. In practice, this gives a mass-based result that is suitable for routine laboratory control.

What are the key requirements of ISO/PRF 6791?

Sample preparation and handling

ISO/PRF 6791 requires the sample to be taken in accordance with ISO 6257, and the test sample is prepared immediately before the determination. If the pitch is hard, it is ground to a particle size not exceeding 200 µm; if it is too soft, it is melted and mixed under controlled conditions, with the temperature kept below 150 °C and the melting period limited to 10 min.

This matters because pitch condition affects the result. A representative, freshly prepared sample reduces bias from settling, softening or uneven particle size.

Principle of the test

ISO/PRF 6791 measures the fraction of pitch that remains insoluble in quinoline after extraction at a specified temperature and time. The result is obtained by weighing the residue after filtration, washing and drying.

In practice, the operator is not measuring a chemical concentration directly. The method translates solubility behavior into a mass percentage that can be compared between batches and suppliers.

Reagents and materials

ISO/PRF 6791 calls for analytical-grade reagents, freshly distilled quinoline, toluene, and a diatomaceous-silica filter aid with controlled particle size. The filter aid is dried to constant mass before use.

This matters because solvent purity and the state of the filter aid affect extraction and filtration. The procedure also includes safety warnings for quinoline and toluene, so the work must be done with proper ventilation and handling controls.

Apparatus and test conditions

ISO/PRF 6791 uses a water bath controlled at 70 °C to 80 °C, a Hartley funnel or equivalent, glass fibre filter pads, and an oven controlled at 105 °C to 110 °C. The filter pad must retain 98 % of particles of size 1.2 µm, and a larger funnel and pad may be used if available.

These conditions matter because the extraction temperature, filtration efficiency and drying temperature all influence the measured residue. The apparatus section gives the laboratory the hardware needed to keep the method repeatable.

Procedure and calculation

ISO/PRF 6791 weighs about 1 g of sample, adds about 1 g of filter aid and 25 mL of quinoline, heats the mixture for 15 min to 20 min, filters it through the prepared pad, washes the residue with hot quinoline and then with toluene, and dries the residue to constant mass. The calculation in Clause 9 uses the masses of the test portion, the dried pad, the filter aid and the final dried residue.

In practice, the analyst must record masses carefully to 0.005 g and keep the suspension hot during filtration. The result is reported as quinoline-insoluble material, in percent by mass.

Precision and report

ISO/PRF 6791 gives repeatability of 1.0 % and reproducibility of 1.5 %. Clause 10 requires the test report to identify the sample, cite the document, give the test date, state the result with reference to Clause 9.1, and note any deviations or unusual features.

These items help a buyer, plant or laboratory judge whether two results are close enough to compare and whether the test was run under the stated method.

Who uses ISO/PRF 6791?

ISO/PRF 6791 is used in carbon paste plants, aluminium primary production, pitch laboratories and quality control functions that work with pitch for electrodes. It is relevant when teams need to control pitch content, compare incoming material, or assess how quinoline-insoluble material may affect anode quality.

The introduction notes that quinoline-insoluble material can be primary or secondary, and that its level is important when optimizing pitch content in a carbon paste plant. That makes the standard useful not only for testing, but also for process control and supplier evaluation.

What changed in ISO/PRF 6791 from the previous edition?

ISO/PRF 6791 is the second edition and replaces ISO 6791:1981. The stated changes are added analysis procedure details, text corrected to formal SI formatting, and an updated test report description.

Which standards are used with ISO/PRF 6791?

ISO 6257, Carbonaceous materials used in the production of aluminium - Pitch for electrodes - Sampling, provides the sampling method used before the test. ISO/PRF 6791 relies on it so the laboratory begins with a representative pitch sample.

What does the ISO/PRF 6791 document contain?

The document contains the test method from sample preparation through reporting. It gives the apparatus list, including the Hartley funnel, filter pads, water bath and oven, and it includes a figure showing the Hartley funnel details.

It also provides the reagent and filter-aid requirements, the heating and filtration sequence, and the constant-mass drying steps. Clause 9 contains the calculation formula and precision statement, and Clause 10 gives the required report content.