Semi-coke, widely known as lan tan, is a high-fixed-carbon solid fuel and metallurgical reductant produced by the low-temperature pyrolysis of weakly-caking Jurassic coals in northern China. Quality determination of semi-coke covers moisture, ash, volatile matter, fixed carbon, sulfur, calorific value, mechanical strength, particle size, and electrical resistivity, and is performed using recognized GB, ISO, and ASTM test methods. This article explains the key sample types, testing items, analytical methods, and industrial applications of semi-coke quality determination for producers, traders, and end users.
Technical Overview
Semi-coke is manufactured by carbonizing lump or crushed low-rank coal at relatively low temperatures, generally between approximately 550 °C and 750 °C, in vertical retorts or rotary furnaces. The principal production regions are the Yulin area of Shaanxi Province, together with Inner Mongolia, Ningxia, and Xinjiang, where thick seams of low-sulfur, low-ash Jurassic coal provide an ideal feedstock. The resulting product retains a controlled amount of volatile matter while developing the high fixed carbon, open porosity, and high chemical reactivity that distinguish it from conventional high-temperature metallurgical coke.
Typical commercial properties include fixed carbon above roughly 80 percent on a dry basis, volatile matter below about 10 percent, ash commonly between 6 and 12 percent, and total sulfur frequently below 0.5 percent for premium material. One of the most valued characteristics of semi-coke is its high specific electrical resistivity, which improves electrode burial and energy efficiency in submerged-arc reduction processes such as silicon metal and ferrosilicon smelting. Because semi-coke is traded in large volumes and priced against measured properties, systematic quality determination underpins process control, contract settlement, customs classification, and export inspection. In China, product quality is additionally guided by dedicated national standards such as GB/T 25211 for semi-coke product technical requirements and GB/T 25212 for the coal used in semi-coke production, while the individual test methods are drawn from the established GB, ISO, and ASTM standards families for coal and coke analysis.
Testing Samples
Lump Semi-Coke Grades
Coarse and small lump grades, commonly traded in fractions such as 25–80 mm and 8–25 mm, represent the mainstream commercial products. Lump samples are used for particle-size screening, shatter and drum strength testing, and, after crushing and division, for all chemical analyses. Representative sampling is critical because size segregation in stockpiles and during vessel loading is pronounced.
Semi-Coke Fines and Breeze
Material below about 8 mm, often referred to as semi-coke fines or breeze, is sampled for moisture, ash, volatile matter, sulfur, and calorific value. Fines are increasingly briquetted or used as sintering and injection fuel, so size distribution and ash chemistry are routinely reported alongside the basic quality parameters.
Prepared Laboratory Samples
From the gross sample, a general analysis sample is prepared by air-drying, stage-crushing, and division to a powder passing 0.2 mm for proximate, ultimate, calorific, and ash-composition determinations. Separate coarser fractions, typically in the 1–6 mm range, are reserved for reactivity and electrical resistivity measurements, while intact lump fractions are kept for mechanical testing. Sampling and sample preparation follow GB/T 475 and GB/T 474 or their international equivalents ISO 18283 and ISO 13909, with documented precision and bias checks for each lot.
Testing Items
A complete semi-coke quality determination typically combines the following parameter groups:
- Proximate analysis: total moisture (Mt), air-dried moisture (Mad), ash (Ad), volatile matter (Vdaf), and fixed carbon (FCd) calculated by difference.
- Sulfur and phosphorus: total sulfur (St,d) and phosphorus (Pd), both critical quality and cost drivers in ferroalloy and silicon smelting.
- Calorific value: gross calorific value on a dry or as-received basis (Qgr,d or Qgr,ar) and, where required, net calorific value for fuel applications.
- Ash chemistry and fusion behavior: SiO2, Al2O3, Fe2O3, CaO, MgO, TiO2, K2O, and Na2O contents, together with ash fusion temperatures (deformation, softening, hemisphere, and flow).
- Mechanical strength: shatter strength indices and drum strength indices, such as M25 and M10, describing resistance to breakage and abrasion during handling and furnace charging.
- Particle size distribution: oversize and undersize percentages determined by sieve analysis against the contracted size fraction.
- Physical properties: specific electrical resistivity, true relative density, porosity, and CO2 reactivity indices.
- Trace and minor elements: chlorine, arsenic, mercury, and alkali metals where environmental regulations or downstream process constraints apply.
Testing Methods
Moisture Determination
Total moisture is determined by staged air-drying and oven drying of a crushed sample to constant mass at 105–110 °C, following GB/T 211, GB/T 2001, ISO 589, or ASTM D3302. Air-dried moisture on the 0.2 mm analysis sample is measured as the first step of the proximate analysis per GB/T 212, ISO 11722, or ASTM D3173, and is required to convert all other results to a consistent reporting basis.
Ash, Volatile Matter, and Fixed Carbon
Ash is determined by slow incineration of the analysis sample at 815 °C to constant mass in accordance with GB/T 212, GB/T 2001, ISO 1171, or ASTM D3174. Volatile matter is measured by heating a covered crucible at 900 °C for exactly seven minutes per GB/T 212, GB/T 2001, ISO 562, or ASTM D3175, with special procedures applied to decrepitating samples. Fixed carbon is then calculated by difference as 100 minus moisture, ash, and volatile matter, and is the primary commercial index of semi-coke quality.
Total Sulfur
Two approaches are accepted. Routine quality control relies on high-temperature combustion with infrared detection in accordance with GB/T 214, ISO 19579, or ASTM D4239, while the classical Eschka gravimetric method under ISO 334 or GB/T 214 remains suitable for arbitration and verification. Accurate sulfur data are essential because sulfur transfers into ferroalloys, silicon products, and emissions.
Calorific Value
Gross calorific value is measured in an adiabatic or isoperibolic oxygen bomb calorimeter calibrated with certified benzoic acid, following GB/T 213, ISO 1928, or ASTM D5865. Results are reported on a dry or as-received basis, and net calorific value is calculated from the gross value, hydrogen content, and moisture when semi-coke is sold or taxed as a fuel.
Phosphorus, Ash Composition, and Ash Fusion
Phosphorus is determined by spectrophotometry per GB/T 216 or by inductively coupled plasma techniques. Ash composition is measured using the classical chemical methods of GB/T 1574 or by X-ray fluorescence and ICP-OES, with particular attention to potassium and sodium, which influence furnace operation and ash behavior. Ash fusion temperatures are determined in controlled reducing and oxidizing atmospheres per GB/T 219, ISO 540, or ASTM D1857 to assess slagging tendency in combustion applications.
Mechanical Strength and Particle Size
Shatter strength is evaluated by dropping a sized lump sample from a standard height onto a steel plate and screening the product, while drum strength indices such as M25 and M10 are obtained with tumbler drum procedures adapted from GB/T 2006 for coke. Particle size distribution is determined by mechanical sieve analysis in accordance with GB/T 477, ISO 1953, or ASTM D4749, and oversize or undersize limits are compared against the contractual specification.
Electrical Resistivity and Reactivity
Specific electrical resistivity is measured on a packed or compressed bed of a defined particle fraction using standardized direct-current procedures; semi-coke characteristically exhibits resistivity several times higher than that of metallurgical coke, which is a key advantage in submerged-arc furnaces. CO2 reactivity can be assessed by procedures adapted from GB/T 4000, using reduced temperature or shortened duration to accommodate the high chemical activity of semi-coke. All determinations can be performed by accredited third-party laboratories operating under ISO/IEC 17025 quality systems.
Applications
Silicon Metal and Ferroalloy Production
High fixed carbon, low ash, low phosphorus, and high electrical resistivity make semi-coke a preferred reductant for silicon metal, ferrosilicon, and silicomanganese furnaces, where it improves electrode penetration and reduces specific energy consumption.
Calcium Carbide Manufacture
Consistent size, low ash, and controlled volatile matter allow semi-coke to replace a portion of metallurgical coke in calcium carbide smelting, improving furnace permeability and reducing raw material cost.
Clean Solid Fuel
Compared with raw coal, semi-coke burns with lower smoke and volatile emissions, so it is widely used as a domestic and industrial fuel for heating, boilers, and civil applications, supported by calorific value, sulfur, and moisture determinations.
Chemical and Metallurgical Reduction Uses
Semi-coke serves as a reducing agent in chemical processes, as a feedstock for activated carbon and recarburizer production, as sintering fuel in iron ore plants, and as a carbon source in specialized metallurgical operations, each with its own tailored testing profile.
FAQ
What is the difference between semi-coke and metallurgical coke?
Semi-coke is produced at 550–750 °C, roughly half the coking temperature of metallurgical coke. It retains 4–10 percent volatile matter, has far higher electrical resistivity and chemical reactivity, and is somewhat weaker mechanically, which suits reductant and fuel duties rather than blast furnace burden support.
Which parameters most influence semi-coke pricing?
Fixed carbon, ash, sulfur, moisture, and particle size are the primary pricing parameters, with phosphorus, mechanical strength, and electrical resistivity becoming decisive for silicon metal and premium ferroalloy contracts.
What do the reporting bases ad, d, daf, and ar mean?
These abbreviations denote air-dried, dry, dry-ash-free, and as-received bases, respectively. Laboratories always state the basis alongside each result, and moisture data are required to convert between bases for contract comparison.
How is sampling uncertainty controlled?
Sampling plans define the number and mass of increments per lot according to GB/T 475, ISO 18283, or ISO 13909, with mechanical sampling preferred at loading points. Duplicate sampling and periodic bias tests verify that overall precision meets the specified targets.
Can semi-coke be tested by accredited third-party laboratories?
Yes. Accredited third-party laboratories routinely perform full semi-coke quality determination under ISO/IEC 17025 systems, applying GB, ISO, or ASTM methods and issuing reports used for trade settlement, factory acceptance, and export inspection.
How long does a routine test suite take?
A standard suite covering proximate analysis, sulfur, calorific value, and size distribution is typically completed within three to seven working days after sample receipt, while strength, ash fusion, and trace element testing may require additional time for sample preparation and instrument scheduling.
