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 TimelineDate Technology

12th c. BCE

Tools and weapons made from Fe in 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.php?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 Fe

< 0.25% C. Very malleable

Alloy Steel Low C, but with added Cr, Mb, Ni, W

HSLA Microalloyed, high strength, low alloy

Properties and Uses of Iron = f(composition)

• At least 3 types of pig iron were produced in the early stages of smelting. They differ in % Fe, C, etc– White crude iron: most brittle, not malleable,

very hard, bright white crystals when fractured.

– Grey crude iron: dark granular fractures, not as hard or brittle, more easily shaped

– Black cast-iron: rough fractures

Effect of Iron Impurities on Properties

C As %C increases, hardness increases, but malleability decreases.

S Causes hot iron to be brittle causing cracks limited integrity, weld failure.

Si Leads to gray iron which is less brittle, preferred for casting.

P Increases hardness and strength if 0.05% < %P < 0.2%. But > 0.2%, iron becomes brittle and unstable at low T.

Al Mainly impacts slag viscosity (fluidity).