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 DateTechnology 12th c. BCE Tools and weapons made from Fe in meteorites by smelting in bloomeries 1200 BCEFe + charcoal  wrought iron 1 st c. BCEOldest existing blast furnace in China 1500Steel production appeared in Europe 1709Fe + coke  cast iron (A. Darby) 2008Top 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 (H 2, CH 4, 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 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 – 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.

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Iron and its Alloys Fe Oxides Fe 2 O 3 (hematite), Fe 3 O 4 (magnetite), FeO Pig Fe3.5%-4.5% C + Si, Mn, P, S impurities. Very brittle (why called “pig” iron?) Cast Fe2%-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 SteelLow C, but with added Cr, Mb, Ni, W HSLAMicroalloyed, 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 CAs %C increases, hardness increases, but malleability decreases. SCauses hot iron to be brittle causing cracks  limited integrity, weld failure. SiLeads to gray iron which is less brittle, preferred for casting. PIncreases hardness and strength if 0.05% 0.2%, iron becomes brittle and unstable at low T. AlMainly impacts slag viscosity (fluidity).