Semi-Coking Coal, also referred to as Weakly Coking Coal (WCC) or Semi-Soft Coking Coal (SSCC), is a grade of bituminous coal that exhibits limited to moderate caking properties. When heated in the absence of air, it partially softens and fuses but does not form strong metallurgical coke like hard coking coal (HCC).
Due to its cost efficiency, acceptable carbonization behavior, and relatively low impurity levels, semi-coking coal is widely used in ferroalloy production, sponge iron (DRI), calcium carbide manufacturing, foundries, industrial coke blending, and chemical industries.
Coal Classification by Coking Strength
Hard Coking Coal (HCC): Strong coking behavior – produces high-strength
metallurgical coke
Medium Coking Coal (MCC): Moderate coking behavior – acceptable coke quality
Semi-Coking / Weakly Coking Coal: Weak to moderate coking –
produces semi-coke or weak coke
Non-Coking Coal: No coking behavior – no coke formation
Key Characteristics of Semi-Coking Coal
Moderate Caking Ability
Coal particles soften and partially fuse during carbonization, forming a weak,
porous coke structure. The coke strength is insufficient for standalone blast
furnace use.
Low to Medium Volatile Matter
Typically ranges between 18% and 28%, resulting in controlled devolatilization,
improved thermal stability, and reduced smoke or flame instability during
carbonization.
Higher Fixed Carbon Content
Fixed carbon typically ranges from 55% to 70%, providing high heat value and
good reduction capability in metallurgical and chemical processes.
Low Sulfur & Phosphorus
Sulfur content generally ranges from 0.3% to 0.8%, while phosphorus is usually
below 0.05%. These low impurity levels are critical for ferroalloy, foundry,
and metal reduction applications.
Moderate Ash Content
Typically between 8% and 15%, resulting in lower slag volume and easier ash
disposal compared to high-ash coal.
Typical Mechanical & Chemical Properties
Fixed Carbon: 55% – 70%
Volatile Matter: 18% – 28%
Ash Content: 8% – 15%
Moisture (ARB): 5% – 12%
Calorific Value: 6,000 – 7,200 kcal/kg
Sulfur: 0.3% – 0.8%
Phosphorus: < 0.05%
Free Swelling Index (FSI): 1.0 – 3.0
Gieseler Fluidity: Low to moderate
Refining & Carbonization Properties
Carbonization Behavior
Plastic transformation begins at approximately 350–400°C, with semi-coke
formation occurring between 500–650°C. The resulting coke has a porous carbon
matrix and lower mechanical strength compared to coke produced from hard
coking coal.
Semi-Coke Production
Semi-coke produced from weakly coking coal is characterized by high fixed
carbon, low ash, and high reactivity. It is suitable for ferroalloy smelting,
calcium carbide production, and gasification feedstock.
Reactivity
Semi-coking coal and its derived semi-coke are more reactive than
metallurgical coke, improving reduction efficiency in submerged arc furnaces,
rotary kilns, and shaft furnaces.
By-Product Yield
Produces lower tar and coke oven gas compared to prime coking coal, resulting
in reduced chemical recovery potential but a cleaner carbonization process.
Processing & Handling
Beneficiation
Crushing and washing reduce ash content and enhance coke yield and consistency.
Blending Behavior
Commonly blended with hard coking coal to reduce cost or with medium coking
coal to control plasticity. Blending improves coke CSR/CRI balance and charge
stability.
Storage & Handling
Semi-coking coal has a lower spontaneous combustion risk compared to
sub-bituminous coal, but moisture control is required to prevent excessive
fines generation.
Applications of Semi-Coking Coal
Ferroalloy Industry
Used as a reductant and heat source in silicon, manganese, and chrome alloy
production.
Sponge Iron (DRI)
Used in rotary kiln processes, providing a stable temperature profile with
lower impurity transfer.
Calcium Carbide Production
Preferred due to high reactivity and low ash content.
Foundry Coke Blending
Blended with stronger coke to improve porosity and permeability.
Industrial Coke Ovens
Cost-effective blends for applications where high CSR coke is not mandatory.
Chemical & Gasification Industry
Feedstock for coal gas, syngas, and carbon-based materials.
Advantages of Semi-Coking Coal
Lower cost than hard coking coal, suitability for multiple metallurgical processes, production of reactive semi-coke, low sulfur and phosphorus content, and excellent blending characteristics.
Limitations
Cannot independently produce blast furnace-grade coke, lower coke strength (CSR), limited chemical by-product recovery, and requires blending for steelmaking applications.
Semi-Coking vs Hard Coking Coal
Semi-coking coal exhibits weak to moderate coking strength and produces semi-coke, while hard coking coal produces strong metallurgical coke. Semi-coking coal is more reactive and lower in cost, making it suitable for ferroalloy and DRI applications rather than blast furnaces.
Semi-Coking / Weakly Coking Coal is a versatile, cost-effective metallurgical coal that bridges the gap between non-coking and prime coking coals. Its ability to produce reactive semi-coke with low impurity levels makes it highly valuable for ferroalloy, sponge iron, calcium carbide, and industrial coke blending applications.