A Closer Look at Iron Smelting
or Extractive Metallurgy
The Smelting Process
• Early smelting operations in Britain
involved a two step process: smelting in a
charcoal-burning furnace and then refining
in a forge (to reduce non-iron content).
• These required considerable capital, raw
materials (iron ore, wood to make charcoal
– later coal to make coke, limestone) and
a transportation system to distribute the
products.
Historical Timeline
Date Technology
12th c. Tools and weapons made from Fe in
BCE meteorites by smelting in bloomeries
1200 BCE Fe + charcoal wrought iron
1st c. BCE Oldest existing blast furnace in China
1500 Steel production appeared in Europe
1709 Fe + coke cast iron (A. Darby)
2008 Top steel producers: China, US, Japan
Bloomery
• A bloomery was the earliest form of a
smelter.
• Bloomery consists of a heat resistant
chimney + pipes at the bottom for air +
way to remove product called bloom
(reduced iron).
• Air is added via natural draft or bellows
which required power (e.g. Water power)
Bloomery - 2
• Process: Heat bloomery by burning
charcoal or coke; when hot, add iron ore +
more fuel + limestone.
• During the process, iron in the ore is
reduced as pure iron pieces fall to bottom
of chimney and weld together in a spongy
mass to form the bloom.
Bloomery - 3
• The rest of the ore (impurities) form the
slag. It also ends up at the bottom of the
chimney including becoming embedded in
the bloom.
• To remove the slag and thus further purify
the iron product, the bloom is reheated
and then hammered.
• The product of this process is called
wrought iron.
Charcoal and Coke
• Charcoal: Residue left when wood
(carbohydrate) is heated in absence of
oxygen (anerobic) to drive off water and
other volatile components. The porous
residue is about 85% carbon and burns
hotter and cleaner than wood.
• Coke: Residue left when coal is heated in
absence of oxygen (anerobic) to drive off
water and other gases (H2, CH4, CO).
Charcoal to Coke
• Clow and Clow p 331, diagram on p 336
• A process for converting coal to coke was
patented in 1627.
Blast Furnace
• Again the required components are iron
ore, fuel (originally charcoal), oxygen or air
and a way to recover the product.
• Process: Add iron ore and fuel from top
and air (the blast) from the bottom
resulting in iron reduction occurring
continuously throughout the furnace.
• Reduced iron and slag fall to the bottom
and are recovered.
Blast Furnace - 2
• The process of using coke instead of
charcoal as the fuel was first developed
and sustained by Abraham Darby I in
Coalbrookedale in 1709. This was a major
contributor to the Industrial Revolution.
• This furnace originally made cast iron
pots, kettles, and later steam engine
cylinders.
• Later pig iron was produced.
Coke Iron
• A second blast furnace was built in 1715
where work culminated in 1754 (A. Darby
II) successfully producing iron bars from
pig iron. This led to a huge expansion of
the iron industry.
• 1768 – first iron rails
• 1778 – A. Darby III started building the first
Ironbridge and completed it in 1780.
The Pace of Technology
• The transition to coke-based cast iron
production from charcoal-based malleable
iron production took over 50 years even as
forests were depleted, new cast-iron
technologies were developed and
limitations overcome:
– Lack of high quality coal (i.e. low in S)
– Lack of high quality coke
– Developing more and new sources of power
– Resistance to change to new technology
• The first Darby furnace was excavated
and is on display at Ironbridge.
• The following slide shows a modern balst
furnace.
http://www.pigiron.org.uk/index.p
hp?nav=furnaceprocess
Iron and its Alloys
Fe Oxides Fe2O3(hematite), Fe3O4(magnetite), FeO
Pig Fe 3.5%-4.5% C + Si, Mn, P, S impurities.
Very brittle (why called “pig” iron?)
Cast Fe 2%-4% C + 1%-6% Si + Mn impurities.
Produced by heating pig iron to reduce
P and S.
C Steel 2% C with Mn, S, P and Si
Wrought 0.2%, iron becomes
brittle and unstable at low T.
Al Mainly impacts slag viscosity (fluidity).