Amorphous Graphite Fixed Carbon Testing: Standards, Methods, and Quality Assessment

Amorphous graphite fixed carbon testing is the core quality control procedure used to determine the commercial grade and purity of earthy (microcrystalline) graphite for refractory, foundry, metallurgical, and carbon product applications. Accredited third-party laboratories perform proximate analysis in accordance with recognized standards such as GB/T 3521 and relevant ASTM methods, accurately measuring moisture, volatile matter, ash, and fixed carbon. This article explains the technical principles, sample requirements, test methods, and practical applications of fixed carbon determination for amorphous graphite.

Technical Overview

Amorphous graphite, also known as earthy graphite, microcrystalline graphite, or cryptocrystalline graphite, is a naturally occurring form of graphite produced by the regional or contact metamorphism of coal seams and carbon-rich sediments. Unlike flake or vein graphite, its crystals are too small to be distinguished under an optical microscope, giving the material a dull, earthy appearance. Run-of-mine amorphous graphite typically contains 60–85% graphitic carbon together with mineral impurities such as silica, alumina, iron oxide, and calcite.

Fixed carbon (FC) is the primary quality and pricing parameter for amorphous graphite. It represents the non-volatile, combustible carbonaceous material remaining after moisture, volatile matter, and ash are removed. Because each percentage point of fixed carbon directly affects commercial value, accurate determination is essential for producers, traders, and downstream users.

In routine industrial analysis, fixed carbon is determined indirectly by proximate analysis using the difference method:

Fixed Carbon (%) = 100 − [Moisture (%) + Volatile Matter (%) + Ash (%)]

Commercial amorphous graphite is commonly classified into fixed carbon grades such as 70%, 75%, 80%, 83%, and 85%, while upgraded concentrates and chemically purified products can reach 90–99% carbon. Testing is generally performed by accredited third-party laboratories following GB/T 3521 for graphite chemical analysis, together with ASTM ash and instrumental proximate analysis methods where applicable.

Testing Samples

Raw Ore and Run-of-Mine Material

Lump or run-of-mine amorphous graphite is tested to evaluate deposit quality, guide beneficiation flowsheets, and establish purchase specifications between mines and processors. Samples are crushed, divided, and pulverized before analysis to ensure representativeness.

Graphite Concentrates and Milled Powders

Flotation concentrates and air-classified or ball-milled powders, commonly supplied in particle sizes from 100 mesh down to sub-325 mesh, are the most frequently tested products. Fixed carbon verification confirms that each production batch meets the declared grade and particle size specification.

Processed and Blended Graphite Products

Graphite-clay blends, spherical or purified grades, recarburizers, and graphite-containing premixes are analyzed to verify carbon content, screen for contamination, and support customs clearance, trade settlement, and supplier qualification.

Representative sampling is critical because amorphous graphite is naturally heterogeneous. Typical submission requirements include:

• A minimum of 100–300 g of homogenized test material per sample
• Pulverization to a nominal particle size below 0.15 mm (approximately 100 mesh) for chemical analysis
• Division by coning and quartering or rotary splitting to obtain representative test portions
• Sealed, moisture-tight packaging to prevent moisture pickup or loss during transport
• Clear labeling of grade, batch number, and declared specification for comparison with measured results

Testing Items

A complete characterization package for amorphous graphite normally covers the following parameters, with fixed carbon as the headline result:

• Fixed carbon content – the principal grade-defining parameter, reported on an air-dried, dry, or as-received basis
• Moisture – free and inherent water affecting weight-based commercial settlement
• Volatile matter – thermally released hydrocarbons and gases, critical for refractory and foundry performance
• Ash – the inorganic mineral residue, reported at 850–900 °C combustion
• Sulfur content – determined by high-temperature combustion infrared detection, relevant to metallurgical quality and emissions
• Ash chemistry – SiO2, Al2O3, Fe2O3, CaO, MgO, and alkalis by X-ray fluorescence or ICP-OES
• Particle size distribution – by laser diffraction or sieve analysis according to ISO 13320 or ASTM B214
• Trace elements – Pb, As, Cd, Hg, and others for regulatory compliance in consumer and battery applications

Testing Methods

Proximate Analysis by Difference (GB/T 3521)

The standard and most widely accepted procedure follows GB/T 3521, the national standard for the determination of moisture, volatile matter, ash, and fixed carbon in graphite. The method consists of three gravimetric determinations:

• Moisture: a weighed test portion is dried at 105–110 °C to constant mass; the mass loss is reported as moisture.
• Volatile matter: the sample is heated in a covered, gas-tight crucible at 900 ± 10 °C for seven minutes to exclude air, preventing oxidation of the carbon; the mass loss, corrected for moisture, is the volatile matter.
• Ash: the sample is burned in an open vessel in a muffle furnace at 850–900 °C until constant mass; the inorganic residue is the ash content.

Fixed carbon is then calculated by difference to 100%. This framework is harmonized in principle with classical coal and coke proximate analysis practices such as ASTM D3172, D3173, and D3175, and ash may alternatively be verified according to ASTM C561 for graphite samples.

Direct Combustion–Infrared Absorption Method

For higher accuracy, total carbon can be measured directly by combusting the sample with accelerators in a high-frequency induction or resistance furnace in a stream of oxygen and quantifying the evolved CO2 by infrared detection. This technique, consistent with the principles of ASTM E1915 for carbon-bearing materials, provides an independent cross-check on the difference method. If the sample contains carbonate minerals, acid pre-treatment may be applied so that the result reflects graphitic carbon.

Instrumental Thermogravimetric Analysis

Automated thermogravimetric proximate analyzers apply programmed heating under nitrogen to release moisture and volatile matter, then switch to oxygen or air to burn off fixed carbon, leaving ash as residue. This approach follows the instrumental philosophy of methods such as ASTM D7582 and delivers rapid, reproducible multi-parameter results suitable for high-throughput batch control.

Complementary Analyses and Quality Control

Sulfur is commonly determined by combustion-infrared detection, ash composition by XRF or ICP-OES, and graphitic structure and degree of crystallinity by X-ray diffraction, which distinguishes well-ordered graphite from amorphous carbon. Reliable laboratories run duplicate determinations, matrix-matched reference materials, and blank corrections, and report results with the analytical basis clearly stated, since fixed carbon values differ between as-received, air-dried, and dry bases.

Applications

Fixed carbon test results directly govern how amorphous graphite is selected, priced, and applied across major industries:

• Refractories: graphite crucibles, ladle and foundry linings, and carbon-bearing refractory mixes, where low volatile matter prevents porosity and spalling
• Foundry engineering: mold facing and parting powders, core washes, and casting release agents
• Steelmaking and iron casting: recarburizers for adjusting carbon content in molten iron and steel
• Batteries: conductive carbon additive in zinc-carbon and alkaline dry cells, requiring consistent carbon purity and controlled impurities
• Pencil manufacturing: graphite-clay blends where fixed carbon determines darkness and writing quality
• Friction materials: brake linings and pads for automobiles and industrial machinery
• Lubricants and coatings: dry-film lubricants, forging die compounds, corrosion-resistant paints, and conductive coatings
• Rubber and plastics: functional filler providing conductivity, lubricity, and UV resistance
• Agriculture: seed coatings and fertilizer anti-caking agents

FAQ

What is fixed carbon in amorphous graphite?

Fixed carbon is the solid, combustible carbonaceous material remaining after moisture, volatile matter, and ash have been removed during proximate analysis. It is the key indicator of graphite richness and the primary basis for commercial grading and pricing.

How is fixed carbon calculated?

It is calculated by the difference method: 100% minus the sum of moisture, volatile matter, and ash percentages. Each component is determined gravimetrically under controlled temperature and atmosphere conditions specified in GB/T 3521.

Which standards apply to amorphous graphite fixed carbon testing?

GB/T 3521 is the principal standard for graphite proximate analysis. Supporting methods include ASTM C561 for ash in graphite, the ASTM D3172/D3173/D3175 coal proximate analysis series, ASTM E1915 for combustion-based carbon determination, and ISO 13320 for particle size measurement. Standards organizations themselves, rather than individual companies, define these procedures.

What fixed carbon grades are commercially available?

Common commercial grades include 70%, 75%, 80%, 83%, and 85% fixed carbon powders, with lump material and upgraded or purified concentrates reaching 90–99% carbon for specialty applications.

Why do results differ between laboratories or between report bases?

Fixed carbon values change depending on whether they are reported on an as-received, air-dried, or dry basis. Moisture variations, sample heterogeneity, and small differences in ash and volatile matter determinations shift the calculated result, so the analytical basis must always be compared like-for-like.

How is amorphous graphite testing different from flake graphite testing?

The proximate analysis framework is identical, but amorphous graphite contains more inherent mineral ash and finer particle sizes, while flake graphite is usually beneficiated to higher purity. X-ray diffraction may additionally be used to assess crystallinity and verify graphitic versus amorphous carbon content.

How much sample and time are required?

Typically 100–300 g of homogenized material is sufficient for a full proximate package with sulfur and particle size. Routine fixed carbon analysis by accredited third-party laboratories is generally completed within three to seven working days, depending on the scope of testing.