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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).
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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.
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.
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
