Semi-coke (also known as lantan or lan coal) fixed carbon content testing is a critical quality control procedure that determines the combustible carbon residue in this low-temperature carbonization product, directly influencing its value as a reducing agent and fuel in ferroalloy, silicon metal and calcium carbide production. Fixed carbon is typically determined by proximate analysis in accordance with recognized standards such as GB/T 212, GB/T 2001, ISO 562 and ASTM D3172, and serves as a key contractual index in domestic and international semi-coke trade.
Technical Overview
Semi-coke, commonly referred to as lantan or blue charcoal, is a high-quality carbonaceous solid produced through the low-temperature pyrolysis (approximately 600–700 °C) of weakly-caking, low-rank Jurassic coal in vertical internal-heating retorts. Compared with conventional metallurgical coke, semi-coke is characterized by high fixed carbon, low ash, low sulfur, low phosphorus, low aluminum content, high chemical reactivity and high specific electrical resistance. These properties make it a preferred reducing agent and fuel in several metallurgical and chemical processes.
Fixed carbon (FC) is defined as the combustible, non-volatile solid residue that remains after moisture, ash and volatile matter have been accounted for. It is the primary index for evaluating the fuel value and reduction capacity of semi-coke, and it typically ranges from 80% to 90% on a dry basis for commercial products. Because fixed carbon is the dominant component of semi-coke, even small deviations in its reported value have a direct impact on pricing, furnace performance, electrode consumption and overall smelting economics.
Fixed carbon is normally obtained by calculation from proximate analysis results, using the fundamental mass-balance relationship:
- FC(ad) = 100 − M(ad) − A(ad) − V(ad), where M, A and V are the moisture, ash and volatile matter contents on an air-dried basis, expressed in percent by mass.
- Results can be converted to other reporting bases, for example FC(d) = FC(ad) × 100 / (100 − M(ad)), to eliminate the influence of transport moisture.
Commercial semi-coke specifications frequently classify products into grades such as fixed carbon (dry basis) ≥ 82%, ≥ 84%, ≥ 86% and ≥ 88%, combined with limits on ash, volatile matter, sulfur and moisture. Reliable determination of these values requires strict adherence to standardized sampling, sample preparation and instrumental analysis procedures.
Testing Samples
Representative sampling and sample preparation are prerequisites for accurate fixed carbon determination. The following sample types are commonly submitted to accredited third-party laboratories for testing.
Commercial Semi-Coke Lumps
Sized lump products, typically in fractions such as 6–13 mm, 13–25 mm and 25–50 mm, are sampled from stockpiles, conveyor streams, shiploads or truckloads. Sampling is generally performed in accordance with GB/T 1997 for coke materials, GB/T 475 and GB/T 475 for commercial coal lots, ISO 18283 or ASTM D2234, with subsequent reduction and division of the gross sample to a laboratory sample of 0.2 mm fineness following GB/T 474, ISO 18283 or ASTM D2013.
Semi-Coke Fines and Breeze
Fraction sizes below 6 mm, including semi-coke powder generated during production, screening and transport, are tested both for commercial sale and for process control in furnace injection applications. Fines require special attention during sample division because of segregation effects.
In-Process and Production Control Samples
Samples taken at the retort discharge, after wet or dry quenching, and after screening are analyzed to monitor carbonization uniformity, quenching efficiency and the stability of volatile matter release across production shifts.
Feed Coal and Coal Blend Samples
The raw Jurassic coal or blended feedstock is tested in parallel with the product semi-coke to establish carbonization yield, fixed carbon enrichment and mass balance across the pyrolysis unit.
Testing Items
Fixed carbon determination is inseparable from the full proximate analysis suite. A typical test program for semi-coke includes:
- Fixed carbon content, reported on air-dried basis (FCad), dry basis (FCd) and, where required, dry ash-free basis (FCdaf).
- Total moisture (Mt) and air-dried moisture (Mad).
- Ash content (Aad, Ad) and, where applicable, ash composition including SiO2, Al2O3, Fe2O3, CaO and MgO.
- Volatile matter content (Vad, Vdaf), a key indicator of carbonization completeness.
- Total sulfur (St,d) and, on request, forms of sulfur, phosphorus and chlorine.
- Gross and net calorific value (Qgr,d, Qnet,ar) for fuel valuation.
- Ultimate analysis: carbon, hydrogen, nitrogen and oxygen, to distinguish total carbon from fixed carbon.
- Ash fusibility temperatures (DT, ST, HT, FT) for slagging assessment.
- Optional indices such as chemical reactivity (CO2 reactivity), electrical resistivity, true relative density and bulk density, which are relevant to ferroalloy furnace operation.
Testing Methods
Moisture Determination
Air-dried moisture is determined by drying a 1 g test portion of 0.2 mm semi-coke at 105–110 °C under nitrogen or air until constant mass, following GB/T 212, ISO 11722 or ASTM D3173. Total moisture of the as-received lot is determined on a crushed representative sample, usually in a drying oven at 105–110 °C.
Ash Determination
The test portion is heated in a muffle furnace in a programmed sequence to a final temperature of 815 °C, where all combustible matter is burned off and the residual inorganic mass is weighed, in accordance with GB/T 212, ISO 1171 or ASTM D3174. Slow, staged heating is essential for semi-coke to prevent ignition losses and splashing of fines.
Volatile Matter Determination
The test portion, with a drop of moisture-free condition as required, is heated out of contact with air at 900 ± 10 °C for exactly 7 minutes in a covered vitreous silica crucible, following GB/T 212, GB/T 2001 (coke-based procedure), ISO 562 or ASTM D3175. The mass loss, corrected for moisture, is reported as volatile matter. Strict control of temperature, timing, crucible placement and crucible lid fit is critical, because volatile matter carries the largest uncertainty contribution to the calculated fixed carbon value.
Calculation of Fixed Carbon and Result Reporting
Fixed carbon is calculated by difference from the three proximate determinations. Typical repeatability limits under GB/T 212 are 0.20% for moisture, 0.30% for ash (below 15%) and 0.50% for volatile matter on an air-dried basis; the corresponding reproducibility limits between laboratories are wider. The laboratory report should clearly state the analytical basis, the standards applied, the individual proximate values and the calculated fixed carbon value with its effective precision.
Instrumental and Automated Methods
Automatic thermogravimetric proximate analyzers, which reproduce the standard temperature programs for moisture, ash and volatile matter in a controlled atmosphere, are widely used for high-throughput routine testing. All automated instruments must be validated against the reference furnace methods using certified reference materials and regular inter-laboratory checks.
Distinction from Total Carbon
Fixed carbon must not be confused with total carbon determined by ultimate analysis (combustion with infrared detection of CO2 per GB/T 476 or ISO 17247). Total carbon includes the carbon present in volatile matter, so it is always numerically higher than fixed carbon. Contract specifications should state unambiguously which parameter is required.
Quality Assurance
Accredited third-party laboratories ensure result reliability through duplicate determinations, blank and check samples, certified reference materials, calibrated balances and furnaces, defined repeatability acceptance criteria and participation in proficiency testing schemes.
Applications
Fixed carbon test results for semi-coke are applied across the entire value chain, from production process optimization to final commercial settlement:
- Silicon metal and ferrosilicon smelting: semi-coke with high fixed carbon (typically FCd ≥ 82–86%), high resistivity and high reactivity serves as the principal reducing agent, improving furnace permeability and reducing electricity consumption.
- Silicomanganese and other ferroalloy production: fixed carbon data are used to balance the carbon-to-ore ratio and control slag chemistry.
- Calcium carbide manufacturing: high fixed carbon and low ash semi-coke directly increase carbide yield and reduce lime consumption.
- Blast furnace injection and foundry applications: semi-coke fines partially replace anthracite or pulverized coal injection, with fixed carbon defining replacement ratios.
- Gasification and civilian fuel: fixed carbon and fuel ratio (FC/V) govern heat output and burnout behavior.
- Trade and logistics: fixed carbon on a dry basis is a core settlement index in purchase contracts, export inspection and customs declaration, with moisture determination used for weight adjustment.
- Research and development: benchmarking of feed coals, carbonization temperature optimization and evaluation of new retort technologies.
FAQ
What is the typical fixed carbon content of semi-coke?
Commercial semi-coke generally contains 80–90% fixed carbon on a dry basis, with premium grades reaching 86–88% or higher. The exact value depends on the rank and ash content of the feed coal and on the final carbonization temperature.
Is fixed carbon measured directly in the laboratory?
No. Fixed carbon is a calculated value obtained by subtracting moisture, ash and volatile matter from 100%. Its accuracy therefore depends on the precision of the three underlying proximate analysis determinations, particularly volatile matter.
Why is the fixed carbon result lower than the total carbon result?
Total carbon from ultimate analysis includes carbon bound in volatile tars and gases driven off during heating, whereas fixed carbon represents only the solid combustible residue. Total carbon is always higher than fixed carbon, and the two parameters must not be interchanged in contracts.
Which reporting basis should be used for contracts?
Dry basis (FCd) is recommended for quality clauses because it removes the effect of quenching and transport moisture. As-received or air-dried basis values may be used for weight settlement provided total moisture is determined simultaneously on the same lot.
How are results for semi-coke different from those for metallurgical coke?
Semi-coke retains more volatile matter (commonly 4–10% dry ash-free basis) because it is produced at lower temperatures, while metallurgical coke is carbonized above 1000 °C. Testing procedures follow coal or coke proximate analysis standards as appropriate, with careful control of heating conditions.
How can sampling error in fixed carbon testing be minimized?
Semi-coke is heterogeneous in size and moisture. Correct sampling plans per GB/T 1997, GB/T 475, ISO 18283 or ASTM D2234, adequate increment numbers, prompt moisture determination after sampling and standardized air-drying and reduction to 0.2 mm are essential to keep the total uncertainty of the reported fixed carbon within contractual tolerances.
