5.2 Iron smelting and bloomery process

Iron smelting revolutionized ancient metalworking. The bloomery process, a key technique, used furnaces to extract iron from ore. This method produced a spongy mass called a bloom, which required further refining to create usable iron.

The process involved carefully designed furnaces, charcoal fuel, and iron ore. Air was pumped in to reach high temperatures, while chemical reactions reduced the ore to metal. The resulting bloom was then forged to remove impurities and create workable iron.

Bloomery Furnace Components

Essential Furnace Structure and Air Supply

Top images from around the web for Essential Furnace Structure and Air Supply

• 

  Metallurgy - Wikipedia View original

  Is this image relevant?

• 

  Bloomery - Wikipedia View original

  Is this image relevant?

• 

  Metallurgy | Introduction to Chemistry View original

  Is this image relevant?

• 

  Metallurgy - Wikipedia View original

  Is this image relevant?

• 

  Bloomery - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Essential Furnace Structure and Air Supply

• 

  Metallurgy - Wikipedia View original

  Is this image relevant?

• 

  Bloomery - Wikipedia View original

  Is this image relevant?

• 

  Metallurgy | Introduction to Chemistry View original

  Is this image relevant?

• 

  Metallurgy - Wikipedia View original

  Is this image relevant?

• 

  Bloomery - Wikipedia View original

  Is this image relevant?

1 of 3

• Bloomery furnace consists of a cylindrical or conical shaft made of clay or stone, typically 1-3 meters tall
• Tuyere serves as a nozzle or pipe inserted into the furnace wall to introduce air
• Bellows pump air into the furnace through the tuyere, increasing oxygen supply and temperature
• Tap hole located at the bottom of the furnace allows for the removal of molten slag during the smelting process

Furnace Design Considerations

• Furnace shape optimizes heat distribution and gas flow for efficient iron reduction
• Refractory lining inside the furnace withstands high temperatures (up to 1300°C) and protects the outer structure
• Multiple tuyeres may be used in larger furnaces to ensure even air distribution
• Furnace size varies based on desired iron output and available resources

Bloomery Process Inputs

Fuel and Raw Materials

• Charcoal serves as both fuel and reducing agent, providing heat and carbon monoxide for iron reduction
• Iron ore (typically hematite or magnetite) supplies the iron to be reduced
• Flux (limestone or sand) added to lower the melting point of gangue minerals and facilitate slag formation

Chemical Reactions and Gas Production

• Carbon monoxide generated from the incomplete combustion of charcoal
• CO acts as the primary reducing agent in the bloomery process
• Oxygen from the air supply combines with carbon to form CO2, which then reacts with more carbon to produce CO

Bloomery Process Outputs

Primary Iron Product

• Bloom forms as a spongy mass of iron and slag at the bottom of the furnace
• Sponge iron refers to the porous nature of the bloom, containing voids filled with slag and unreduced ore

Waste Products and Separation

• Slag consists of impurities from the ore, flux, and furnace lining
• Slag separation occurs through tapping molten slag from the furnace or by mechanical removal after cooling
• Bloom requires further processing (shingling and forging) to remove residual slag and consolidate the iron

Bloomery Smelting Mechanisms

Reduction Process and Temperature Zones

• Reduction process involves the removal of oxygen from iron oxides to produce metallic iron
• Temperature gradient forms within the furnace, creating distinct reaction zones
• Upper zone (400-800°C) preheats the charge and initiates reduction reactions
• Middle zone (800-1200°C) completes most of the reduction process
• Lower zone (1200-1300°C) allows for partial melting and agglomeration of reduced iron particles

Chemical Reactions and Iron Formation

• Iron oxide reduction occurs in stages: Fe2O3 → Fe3O4 → FeO → Fe
• Carbon monoxide reduces iron oxides according to the reaction: FeO + CO → Fe + CO2
• Direct reduction by solid carbon also occurs at higher temperatures: FeO + C → Fe + CO
• Reduced iron particles sinter and coalesce to form the bloom at the bottom of the furnace