How Do We Get Iron and Steel Out of Stone?

Iron processing is a benchmark of civilization for good reason -- this metal makes a lot of technology possible. Learn about the history and science of refining iron and steel in today's episode of BrainStuff, based on this article: https://science.howstuffworks.com/iron.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren Vogelbaum. Here.
If you were to follow humanity's genetic trail back through
the millennia, you'd find primitive creatures fumbling for a foothold
on a primeval earth, lacking the natural physical advantages of

(00:22):
other animals. It's a marvel that humans were able to
claw their way out of the Cenozoic era at all.
Of course, Homo sapiens had an advantage over most of
the other animals the ability to make and use tools.
While they lacked a lion's teeth and claws or a
deer's defensive antlers, they learned to craft their own tools

(00:44):
from the world around them. The oldest known tools date
back two point six million years to a time when
humans used shaped stone to carry out a variety of tasks.
After all, a sharpened rock can potentially stab and bludget,
but also slice, scrape, and pound. In time, humans began

(01:05):
to specialize their tools, creating everything from arrowheads to pestles
for grinding grain. But stone is a brittle and inflexible medium. Eventually,
our ancestors were able to pinpoint more durable and malleable
materials a first copper, than bronze, than iron. Their capability

(01:26):
with these materials is one of the modern measures of civilization,
and between the fifteenth and twentieth centuries, some countries had
an industrial leg up on the competition due to the
availability of iron ore deposits. For example, China, India, England,
the US, France, Germany, Spain, and Russia all have substantial

(01:47):
iron ore deposits. When you think of the historical importance
of all of these societies, you can see the correlation.
Even today, iron makes possible a huge array of products,
especially the carbon rich commercial iron, which we call steel. Cars, tractors, bridges,
trains and their rails, tools, skyscrapers, guns and ships all

(02:11):
depend on iron and steel to make them strong. But
as common as iron is an Earth's crust, and it's
our fourth most common element there, making up about five
percent of the crust by weight, it comes bound up
in ores, which are basically rocks that contain iron and
other stuff. So how do we turn a slab of

(02:33):
rock into a set of stainless steel, surgical instruments or
a locomotive. Today, let's talk about how iron and steel
built the modern world. Iron is incredibly useful. It's less
brittle than stone butt compared to wood or copper, extremely
strong when properly heated. It's relatively easy to shape into

(02:55):
various forms using simple tools. It can handle high temperatures,
wowing us to build fire tongs and furnaces out of it.
You can also magnetize iron, making it useful in the
creation of electric motors and generators, which we talked a
bit about in our recent episode on induction cooking, and
again it is common in some areas. Iron concentrates and

(03:17):
ores make up seventy percent of the stuff. Its ease
of manipulation is why iron and steel were so important historically.
To refine aluminum, for example, you need access to huge
quantities of electricity, and to shape it you have to
cast or extrude it. That's why iron has been used

(03:37):
for thousands of years, while aluminum didn't really exist in
any meaningful way until the twentieth century. The reason that
the tip of the Washington Monument is a pyramid made
of aluminum is that aluminum was more valuable than gold
in eighteen eighty four. There may come a day when
humans become so technologically advanced that iron is completely repla

(04:00):
placed by materials like aluminium or various plastics or fibers
made of carbon or glass, but as of now, iron
and steel are less expensive for many purposes. The only
real issue with iron and steel is rust, although we've
come up with lots of solutions for controlling that by painting, galvanizing,
chrome plating, or even just coating the iron with what's

(04:22):
called a sacrificial anode that will corrode faster, sort of
like hiring a bodyguard who will take a bullet for you. However,
before iron can be put to any of these uses,
it has to be mined and refined right after the ground.
Raw iron ore is a mix of what's called ore
proper and gang, which is soil. The ore proper can

(04:44):
usually be separated by crushing the raw ore and simply
washing away the lighter gang. Breaking down the ore proper
is more difficult. It's a compound of carbonates, hydrates, oxides, silicates, sulfides,
and various impurity all bound up together. To get to
the bits of iron in the ore, you have to

(05:05):
smelt it out. Smelting involves heating up or until the
metal becomes spongy, and the different compounds begin to break
down or break out. Most importantly, it releases oxygen from
the iron ore, which usually makes up a high percentage
of it. The most primitive facility to smelt iron is
a bloomery. There, a blacksmith burns charcoal with iron ore

(05:30):
and a good supply of oxygen provided by bellows or
a blower. Charcoal is essentially pure carbon. The carbon combines
with oxygen to create carbon dioxide and carbon monoxide, and
releasing lots of heat in the process. Carbon and carbon
monoxide combined with the oxygen in the orb and carry
it away, leaving iron metal. However, in a bloomery, the

(05:54):
fire doesn't get hot enough to melt the iron completely. Instead,
the iron heats up into a spongy mass containing iron
and silicates. Heating and hammering this mass, called the bloom
forces impurities out and mixes the glassy silicates into the
iron metal to create wrought iron. A wrought iron is

(06:16):
hardy and easy to work, making it perfect for creating tools.
Different ancient civilizations around the world were up to different stuff,
but in general, toolmakers were learning to smelt copper around
five thousand BCE, bronze around three thousand BCE. And iron
around two thousand BCE. However, before many of these civilizations

(06:39):
began to transition from their bronze age to an iron age,
some toolmakers were already creating iron implements from meteorites called
black copper by the ancient Egyptians. Meteoric iron isn't the
sort of thing one finds in huge, consolidated locations, but
by collecting bits and pieces of it. This this cosmic

(07:00):
metal was put to use in jewelry and other ornamentation.
While blacksmiths occasionally used meteoric iron two craft swords. These
prized weapons were relegated to people of great power, such
as the caliphs of these six hundred CE. It would
take almost one thousand years of iron smelting before iron
became the dominant metal, because furnace technology had to improve,

(07:25):
and later by the mid thirteen hundred CE, blast furnaces
that burn hot enough to not just soften iron but
actually melt it hit the scene. A blast furnace is
charged with iron ore, limestone, and charcoal or coke, a
coke being charcoal made from coal which burns very hot.

(07:45):
Huge quantities of air blast in at the bottom of
the furnace, and the calcium in the limestone combines with
the silicates to form slag. Liquid iron collects at the
bottom of the blast furnace underneath a layer of slag.
The bla Broxsmith periodically lets the liquid iron flow out
and cool, typically in a bed of sand. Once it cools,

(08:07):
this metal is known as pig iron. Pig iron contains
four to five percent carbon and is so hard and
brittle that it's almost useless. It was considered a waste
product of the advanced bloomeres before people figured out what
to do with it. Basically, you have three options. First,
you can melt it, mix it with slag, and hammer

(08:29):
it out to eliminate most of the carbon and create strong,
malleable wrought iron. A second, you can melt the pig
iron and combine it with scrap iron, smelt out impurities,
and add alloys to form cast iron. This metal contains
two to four percent carbon, along with quantities of silicon, manganese,
and trace impurities. Cast iron, as the name implies, is

(08:51):
typically cast into molds to form a wide variety of
parts and products. The third option for pig iron is
to push the refining process even further and create steel.
Steel is iron that has most of the impurities removed
and has a consistent small concentration of carbon throughout, generally

(09:12):
less than two percent, but often less than zero point
three five percent. Impurities like silica, phosphorus, and sulfur weaken
steel tremendously, so they must be removed. The advantage of
steel over iron is greatly improved strength. Throughout the eighteen hundreds,
engineers developed technologies like the Bessemer process and the modern

(09:33):
open hearth furnace, which burn hot enough to melt steel
and use oxidation in limestone, respectively to help separate out
molten iron and steel from everything else. However, most modern
steel plants use what's called a basic oxygen furnace to
create steel. The advantage is speed, as the process is

(09:54):
about ten times faster than the open hearth furnace. In
these furnaces high peer the oxygen blows through the molten
pig iron, lowering carbon, silicon, manganese, and phosphorus levels. The
addition of chemical cleaning agents called fluxes helped to reduce
the sulfur and phosphorus levels. A variety of other metals

(10:16):
can be alloyed with the steel at this point to
create different properties. For example, the addition of ten to
thirty percent chromium creates stainless steel, which is very resistant
to rust. The addition of chromium and molybdenum creates chromemulley steel,
which is strong enough that less can be used for
parts where weight matters, like in bicycle and automotive manufacturing

(10:40):
and aerospace engineering. When you think about it, two accidents
of nature made it much easier for human technology to
advance and flourish. One is the prominent availability of iron ore.
The other is the accessibility of coal and oil to
power the production of iron. Without both of these working

(11:01):
for us, our societies would look very different today. Today's
episode is based on the article how iron and Steel
Work on HowStuffWorks dot com, written by Marshall Brain and
Robert Lamb. Brain Stuff is a production of iHeartRadio in
partnership with how Stuffworks dot Com and is produced by

(11:21):
Tyler Klang. Four more podcasts my heart Radio, visit the
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