Interlaboratory Comparison Test for Fixed Carbon Content Determination: Standards, Methods and Stati

Fixed carbon content is a decisive quality parameter for coal, coke, graphite, and activated carbon, directly affecting commercial valuation, combustion behavior, and metallurgical performance. An interlaboratory comparison test for fixed carbon content determination verifies that participating laboratories, applying standardized proximate analysis methods, produce consistent, accurate, and traceable results. This article explains the sample design, determination items, test methods, statistical evaluation criteria, and typical applications of fixed carbon comparison programs.

Technical Overview

Fixed carbon (FC) is the solid, non-volatile combustible residue that remains after moisture, ash, and volatile matter are removed from a carbonaceous material. In routine industrial analysis, fixed carbon is not measured directly; it is calculated by difference from the three measured components of proximate analysis. Because each of those three determinations carries its own analytical uncertainty, the reliability of the final fixed carbon value depends heavily on strict procedural control, well-maintained equipment, and competent operators.

A fixed carbon comparison test (interlaboratory comparison or proficiency testing round) is an organized exercise in which multiple qualified laboratories analyze identical, homogenized samples using the same standardized method. The reported results are then evaluated statistically against an assigned reference value to identify bias, poor precision, or methodological deviations. The main objectives of such a program include:

• Verifying the technical competence of laboratories and analysts in proximate analysis and fixed carbon calculation
• Validating the equivalence of different standard methods, such as GB/T, ISO, and ASTM procedures, on the same material
• Meeting the quality-assurance-of-results requirements of ISO/IEC 17025, which expects accredited laboratories to monitor performance through interlaboratory comparison or proficiency testing
• Calibrating and verifying rapid instrumental proximate analyzers against classical gravimetric reference methods
• Resolving commercial disputes between producer, trader, and end-user laboratories

The statistical design and evaluation of the round typically follow ISO 13528, ISO 5725, ASTM E691, or ASTM E1601, ensuring that conclusions about laboratory performance are objective and defensible.

Testing Samples

The credibility of any comparison test begins with the samples. Candidate materials are collected, dried, crushed, and divided into general analysis samples, then thoroughly homogenized and split into coded blind units. Homogeneity is verified statistically before dispatch, typically following ISO 13528 and ISO Guide 35, so that any variation between reported results can be attributed to laboratory performance rather than to sample non-uniformity. Common material categories included in fixed carbon comparison rounds are described below.

Bituminous Coal, Anthracite, and Thermal Coal

Coal samples are prepared to a nominal 0.2 mm general analysis sample in accordance with GB/T 474 or ISO 18283. Fixed carbon levels typically range from approximately 40 % to 90 % on a dry basis depending on rank, with anthracite occupying the upper end of the range. Coal samples are the most frequent matrix in fixed carbon proficiency programs because they represent the largest commercial volume of traded solid fuels.

Coke, Semi-Coke, and Petroleum Coke

Metallurgical coke, foundry coke, semi-coke (lan char), and calcined or green petroleum coke are prepared per GB/T 1997 and analyzed per GB/T 2001 or the corresponding ISO and ASTM methods. Fixed carbon values commonly fall between 80 % and 99 %. These materials require careful control of volatile matter determination because even small crucible or furnace deviations produce noticeable shifts in the calculated fixed carbon value.

Graphite and Carbon Products

Natural flake graphite, spherical and purified battery-grade graphite, electrode graphite, and carbon paste frequently exhibit fixed carbon contents above 99 %. At such high levels, the difference method becomes insensitive, so comparison programs for these materials often incorporate a direct gravimetric burn-out determination, for example per GB/T 3521, or a total carbon determination by combustion-infrared detection as a cross-check.

Activated Carbon and Other Porous Carbons

Coconut-shell, coal-based, and wood-based granular or powdered activated carbons are evaluated using the applicable national series (for example, GB/T 7702) and international methods such as ASTM D2866 for ash and ASTM D2867 for moisture. Porous carbons adsorb atmospheric moisture rapidly, so scheduled weighing and desiccator discipline are emphasized in the comparison protocol.

Testing Items

The comparison test reports all components required for the fixed carbon calculation, so that any disagreement in FC can be traced back to the responsible parameter. Standard reporting items include:

• Air-dried basis moisture, determined at 105–110 °C (or 104–110 °C under ASTM procedures)
• Ash yield, determined by controlled incineration at 815 °C under GB/ISO conditions or at approximately 750 °C under ASTM conditions
• Volatile matter, determined by heating a covered crucible at 900 °C for 7 minutes per GB/T 212 and ISO 562, or at 950 °C for 7 minutes per ASTM D3175, with coke-specific conditions applied per GB/T 2001
• Fixed carbon calculated by difference on the air-dried basis, dry basis, or dry-ash-free basis
• Total moisture, when conversion between as-received and air-dried bases is required
• Direct fixed carbon or burn-out residue and total carbon by combustion-infrared analysis, included as optional cross-checks for high-purity materials

The core calculation is performed as follows: on an air-dried basis, FC equals 100 minus moisture minus ash minus volatile matter; on a dry basis, FC equals 100 minus ash minus volatile matter. All participants must report results on the same basis, with the calculation chain documented, to ensure a valid comparison.

Testing Methods

Reference Standards and Methods

The comparison protocol designates one primary reference method and, where relevant, permits alternative methods for equivalence assessment. The most widely used standards include:

• GB/T 212 and ISO 17246 for proximate analysis of coal
• ISO 562 for volatile matter of hard coal and coke, and ISO 1171 for ash of solid mineral fuels
• ISO 5068 for moisture in brown coals and lignites, and ISO 687 for moisture in coke analysis samples
• GB/T 2001 for industrial analysis of coke
• ASTM D3172 together with ASTM D3173, D3174, and D3175 for proximate analysis of coal and coke, and ASTM D5142 for instrumental procedures
• GB/T 3521 for chemical analysis of graphite, and the GB/T 7702 series plus ASTM D2866 and ASTM D2867 for activated carbon
• ISO 13528, ISO 5725 (adopted nationally as GB/T 6379), GB/T 28043, ASTM E691, and ASTM E1601 for the statistical design and evaluation of the comparison itself

Determination Procedure

Each participant performs duplicate determinations under the designated method. The gravimetric sequence comprises: drying the 1 g analysis portion to constant mass for moisture; step-heating in an open dish to the specified final ashing temperature, holding to constant mass, and cooling in a desiccator before weighing; and heating the covered volatile matter crucible at the specified temperature for the exact prescribed time before cooling and reweighing. Fixed carbon is then computed by difference, and both duplicate values together with their mean are reported. Parallel blanks, calibrated balances, and verified furnace temperature profiles are mandatory elements of a valid determination.

Comparison Test Protocol

A representative round is designed with the following elements:

• Participation by at least six to eight qualified laboratories, including one or more anchor laboratories with demonstrated reference-level capability
• Three to five sample levels covering the commercial concentration range of the material class
• Blind-coded, homogeneity-verified split samples issued simultaneously to all participants
• A certified reference material included as an undisclosed control sample where available
• A fixed reporting template specifying basis, individual determinations, mean values, equipment type, and any deviations, with a defined submission deadline

Statistical Evaluation of Results

The assigned value is derived either from the certified value of a reference material or from the robust consensus of participant data, for example using the Algorithm A approach in ISO 13528. Precision is characterized through repeatability (r) and reproducibility (R) limits in accordance with ISO 5725 and ASTM E691. Individual laboratory performance is judged using the standard tools of proficiency testing:

• z-score evaluation, where an absolute z-score of 2 or less is satisfactory, between 2 and 3 is a warning signal, and 3 or more is an action signal
• En number evaluation, where an absolute En value of 1 or less confirms agreement with the assigned value within the stated uncertainties
• Cochran and Grubbs tests to detect outlying variances and means before final statistics are computed
• Pairwise difference checks against the reproducibility limit R when comparing results between two specific laboratories, as in commercial arbitration

Where multiple methods are included, method-level bias is assessed by comparing group means, confirming whether GB, ISO, and ASTM procedures deliver statistically equivalent fixed carbon values on the same material.

Typical Sources of Discrepancy

Experience from comparison rounds shows that most disagreements in fixed carbon trace back to a limited set of causes, including:

• Uncalibrated muffle furnaces, drift of thermocouples, or incorrect temperature-sensing positions inside the furnace chamber
• Ill-fitting or damaged volatile matter crucible lids allowing air ingress and partial oxidation of the char
• Deviations from the prescribed sample mass, heating rate, residence time, or cooling period
• Moisture re-adsorption during delayed weighing, particularly for porous and high-surface-area carbons
• Loss of fine particles during ashing due to drafts or overly rapid insertion of dishes into the hot furnace
• Drift in rapid instrumental analyzers that have not been verified against gravimetric reference determinations

Applications

Fixed carbon comparison tests serve a broad range of technical and commercial purposes across the solid fuel and carbon materials industries:

• Proficiency testing and ongoing performance monitoring for ISO/IEC 17025 accredited third-party laboratories and in-house quality laboratories
• Commercial arbitration and result reconciliation between producer, purchaser, and inspection laboratory data in coal and coke trade
• Verification and periodic recalibration of rapid instrumental proximate analyzers used for daily production control in coal preparation plants and coke plants
• Multi-plant laboratory harmonization within large steel, coke, graphite, and activated carbon production groups
• Product certification support for exported anthracite, foundry coke, semi-coke, and battery-grade graphite shipments
• Method validation and standardization research, including statistical demonstration of equivalence between GB, ISO, and ASTM procedures
• Operator training assessment, where individual analysts are evaluated against reproducibility limits on blind samples

FAQ

Why is fixed carbon calculated by difference instead of measured directly?

During proximate analysis, moisture, ash, and volatile matter can each be isolated and weighed as a distinct gravimetric fraction. The material remaining after these fractions are removed is, by definition, the fixed carbon plus the ash, so fixed carbon is obtained arithmetically by difference. This approach is stipulated in GB/T 212, ISO 17246, and ASTM D3172. Direct burn-out determination is reserved for specific high-purity materials such as graphite, where the difference method loses sensitivity.

What criteria determine whether a laboratory passes the comparison?

Under ISO 13528 evaluation, a laboratory result is satisfactory when its absolute z-score is 2 or less, or when its absolute En number is 1 or less relative to the assigned value. In two-party commercial comparisons, the difference between results is judged against the reproducibility limit R of the designated standard; a difference within R confirms acceptable agreement.

How many laboratories and samples are required for a valid round?

A statistically robust round generally involves at least six to eight qualified laboratories testing three to five sample levels that span the relevant concentration range. Larger participation improves the stability of the consensus assigned value and strengthens the conclusions drawn from the precision statistics.

Can rapid instrumental analyzers take part in the comparison?

Yes. Instrumental analyzers operated per ASTM D5142 or equivalent validated procedures can participate, provided their results are reported with a clear method identifier. Comparison against gravimetric reference data then quantifies any instrument bias and supports recalibration where necessary.

What corrective actions follow an unsatisfactory result?

Laboratories receiving an action signal should investigate furnace temperature calibration, crucible condition and lid fit, timing accuracy, balance calibration, desiccator and weighing discipline, and calculation basis. A root-cause analysis, corrective action, and re-testing on retained comparison samples or a certified reference material should be completed and documented before the next round.

Which reporting basis should be used for fixed carbon?

All participants must report on the basis specified in the protocol, most commonly the air-dried basis. However, because moisture varies between laboratories and over time, the dry-basis fixed carbon is often used for the statistical comparison itself, as it removes the influence of ambient moisture differences on the evaluated parameter.