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CSEC Chemistry: Extraction of Metals (Iron)

Updated: Jun 1, 2021

If you read our last post, you already know that metals are found naturally as ores, ionic compounds in the earth's crust contaminated by other elements. We humans have gotten really good at creating efficient systems for the extraction of these ores from the earth, and it follows that we needed a way to get the pure metal (the only part we really care about) into a form we can use.

As we said before, more reactive metals (that is, metals towards the top of the reactivity series) require electrolytic reduction to be extracted from their ores. Electrolytic reduction uses a lot of energy though, because the compounds formed by these very reactive metals are very stable, and need a lot more energy to be separated.

The less reactive metals (below aluminium on the reactivity series) do not need to be reduced using electrolysis, and can instead be reduced by chemical methods.

CSEC requires that you have an understanding of the chemical reduction method used in the extraction of iron.

The Extraction of Iron (Chemical Reduction)

Iron is the most abundantly produced metal on earth, and most of it converted to steel, an iron alloy. In the production of the iron, we require the following raw materials:

-Iron Ore

Iron is found most commonly in ore as iron oxide. Different types of iron ores contain different iron compounds, but the most common iron ores are hematite (iron III oxide, Fe2O3) and magnetite (iron II,III oxide, Fe3O4)

-An energy source

-A reducing agent

The reducing agent used is usually coke (carbon) or hydrogen gas

-A flux

The 'flux' is a substance (usually limestone) which forms a slag with silicates and other impurities which can be removed from the final product easily.

Traditionally, humans produce iron in blast furnaces. In our current times, blast furnaces are completely automated.

A blast furnace is a closed system into which iron-bearing materials (iron ore lump, sinter and pellets), flux (slag formers such as limestone) and reducing agents (i.e. coke) are continuously fed from the top of the furnace shaft through a charging system that prevents escape of blast furnace gas (BF gas).

The extraction of iron can be divided into about 5 stages.

1) Dried hot iron ore, limestone and coke are fed into the top of the blast furnace.

2) Near the base of the furnace, hot air is blown in. The coke burns in the hot air, producing carbon dioxide and heat:

C(s) + O2(g) → CO2(g)

3) The carbon dioxide is reduced to carbon monoxide by the hot coke being fed in:

CO2(g) + C(s) → 2CO(g)

4) The newly formed carbon monoxide reduces the hot iron ore to molten iron:

For hematite:

Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

For magnetite:

Fe3O4(s) + 4CO(g) → 3Fe(l) + 4CO2(g)

The molten iron, Fe(l), will run to the bottom of the furnace.

5) Limestone (CaCO3, calcium carbonate) breaks down at the high temperatures in the furnace to calcium oxide and carbon dioxide:

CaCO3(s) → CaO(s) + CO2(g)

6) The calcium oxide combines with sand (silicon dioxide), the main impurity in iron ore, to form a molten slag (calcium silicate) that can be tapped off separately:

CaO(s) + SiO2(s) → CaSiO3(l)

The molten slag is immiscible with the molten iron and floats on top of it, so they can be run off separately.

The iron formed in the blast furnace is usually allowed to cool in shallow trays called 'casts.' For this reason, this type of iron is called cast iron or pig iron, and contains impurities such as carbon, phosphorous and sulphur. Because of its impurity, it is very brittle and is usually converted to steel.

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