Iron Smelting

» Introduction

Three types of iron may be distinguished according to carbon content. Wrought iron contains virtually no carbon, while cast iron contains about 2-4% carbon.1,2 Steel lies in the middle, with anywhere from a fraction of a percent to about 2% carbon.3,4

An alloy is formed by mixing a metal with one or more other elements. Thus, steel and cast iron are actually iron-carbon alloys. Adding carbon to iron makes it harder, yet more brittle. Steel lies in the ideal hardness range for most purposes, including tools, weapons, and architectural supports.

Wrought iron and steel can be shaped by heating and hammering, whereas cast iron is so hard that it must be cast into shape (melted and poured into a mould). While steel is by far the predominant metal of the modern age, cast and wrought iron continue to be produced for specialized purposes.1,6

» The Chemistry

Elements are the basic building blocks of matter; they cannot be separated into simpler substances. Most elements are metals, which are shiny and good conductors of heat and electricity; other elements are simply called nonmetals. Elements can fuse to form compounds. Elements and compounds are known as pure substances.

A compound cannot be broken apart via a physical change (e.g. smashing with a hammer); only via a chemical change. A physical change does not break/form electromagnetic bonds between substances, whereas a chemical change does.33 Chemical changes can be achieved in various ways, such as applying heat or electricity, or by simply mixing the right substances together.

The solid, inorganic matter that makes up the Earth is referred to as rock, which is a mixture of pure substances. These individual substances are termed minerals. A few minerals are pure elements (e.g. copper, silver, gold), but most are compounds.

The Earth’s crust is comprised almost entirely of oxide minerals, which feature oxygen combined with a metal (e.g. iron oxide, silver oxide). By far the most abundant is silicon dioxide (aka silica), which makes up over half the Earth’s crust.

Rock that is rich in a valuable mineral is referred to as ore. Most iron ore contains some form of iron oxide (e.g. Fe₂O₃, Fe₃O₄).3 In order to extract iron from iron ore, one must first remove the unwanted minerals, then remove the oxygen from the iron. The latter step can only be accomplished via a chemical reaction.

The extraction of metal from an oxide ore is accomplished via smelting. Smelting is the removal of a metal from ore via the application of heat. More specifically, smelting is accomplished by mixing ore with some form of carbon fuel (e.g. charcoal) in a low-oxygen chamber, and burning the fuel.

Normally, the combustion of carbon is described by:

In a low-oxygen environment (such as a car engine or smelting furnace), however, the carbon atoms are willing to settle for a single oxygen partner:

The result is carbon monoxide, which still wants to bond with another oxygen atom. Given the opportunity, it will tear that atom away from a molecule of iron oxide.

» Wrought Iron

The earliest smelting furnaces were stone-lined pits in the ground. Draft was achieved by running pipes to the bottom and pumping air through with a bellows.27 (In order to reach the temperatures needed to melt ore, a smelting furnace must be supplied with air.7)

Three ingredients are mixed in a smelting furnace: ore, carbon, and flux. Prior to the Industrial Revolution, the main form of carbon was charcoal.31 Since the interior of a smelting furnace is a densely packed, low-oxygen environment, the burning charcoal produces mainly carbon monoxide. As the carbon monoxide rises through the molten ore, it strips away the oxygen atoms from the iron oxide.23 Since pit furnaces were not hot enough to melt the iron itself, it would sink to the bottom in a solid mass called a bloom.27

Flux is any material that tends to bind with the unwanted minerals in an ore, collecting them into globs (termed slag) for easier removal. The most commonly-used material for flux is limestone.20

Thus, when the pit furnace was allowed to cool, the iron bloom was dotted with globs of slag. The bloom was subsequently worked (“wrought”) by hammering to remove the slag.1 Yet a small amount of slag inevitably remained, distributed throughout the iron; this gives wrought iron its characteristic fibrous composition.1,6 In fact, it is critical that this residual slag remain in wrought iron, as it provides strength; pure iron is too soft for most practical purposes.5

When iron and carbon are heated together, the iron tends to absorb some of the carbon (a phenomenon termed carburization). Although a pit furnace is not hot enough for this to happen significantly, it was discovered by the earliest civilizations that steel can be (very slowly) produced by heating wrought iron and charcoal together in a sealed vessel for a prolonged period.4,27

» Cast Iron

The pit furnace was succeeded by the blast furnace, essentially a vertical metal chamber into which the ore-carbon-flux mixture is continuously poured, while air is forced (“blasted”) up through the molten mixture from the bottom of the chamber. The iron quickly melts and sinks to the bottom while the molten slag floats on top, from where it is drained off via a pipe connected partway up the chamber. The iron is then drained from the bottom of the furnace into a mould and allowed to harden.16 The blast furnace remains the principal method of smelting iron to this day.14

There is a tradeoff for this speed, however. Melting the iron causes it to take on a very high (2-4%) carbon content, thus resulting in brittle cast iron. (Cast iron, prior to being cast in its final form, is often referred to as pig iron.) Like wrought iron, cast iron can be painstakingly converted to steel, either by prolonged heating in the open air (which causes decarburization, as carbon in the iron bonds with oxygen in the air) or by melting cast iron together with wrought iron.27

In Europe, the blast furnace was developed in the High Middle Ages. It should be noted, however, that this invention first appeared in ancient China. It is unknown whether the technology was transmitted westward or independently invented in the West.27

The next great leap in iron processing came with the steam power of the Industrial Revolution (ca. 1750-1850), which allowed for a much greater flow of air, thus enabling much larger, faster blast furnaces.27 Charcoal was replaced by coke, which remains the standard smelting fuel today.16,27 (Charcoal is obtained by baking wood at a high temperature in an airless environment, which reduces it to near-pure carbon; coke is obtained by treating coal in the same manner.)

With cast iron production booming throughout the Industrial Revolution, a mechanized process was developed to convert cast iron into steel. This was the puddling process, in which molten cast iron is stirred with rods, thus rapidly burning away the carbon.17 Yet even the puddling process was too inefficient to make steel an economic alternative to cast iron.19

» Steel

Steel could only be feasibly mass-produced with the development of the Bessemer process at the end of the Industrial Revolution. The Bessemer process refines cast iron into steel by bringing the molten iron to a very high temperature (in a pear-shaped furnace called a Bessemer converter) and directing a continuous stream of air through it. This rapidly reduces carbon to the desired level (and burns away residual slag).19

The contemporary steel-making process is basically a refined version of the Bessemer process; notably, pure oxygen has replaced the stream of ordinary air.19 Steel finally supplanted cast iron as the world’s predominant metal ca. 1880. Indeed, the period ca. 1750-1880 may be dubbed the age of iron and steam, whereas 1880 onward may be called the age of steel and electricity.32

» Sources Cited