Extraction of metals Revision Notes

The reactivity series is an ordered list of metals ranked from most reactive to least reactive based on their tendency to lose electrons and form positive ions. Metals like potassium, sodium, and calcium are very reactive, while metals such as copper, silver, and gold are much less reactive.

This ranking reflects how strongly metals hold onto their electrons and how readily they react with other substances, including oxygen and water, to form compounds called ores. The more reactive a metal, the more stable its ore tends to be, making the metal harder to extract.

• Reactivity series orders metals by reactivity.
• Higher reactivity means metals form more stable compounds.
• Lower reactivity metals form less stable compounds.

Metals high in the reactivity series form very stable compounds with oxygen or other elements in their ores. Extracting these metals requires breaking strong chemical bonds, which demands more energy and complex methods.

For example, potassium and sodium are so reactive that their ores cannot be reduced by carbon. Instead, they require electrolysis, an energy-intensive process that uses electricity to separate the metal from its compound.

In contrast, metals lower in the series, such as iron or copper, form less stable compounds. These metals can often be extracted by heating their ores with carbon or by simple heating, which is less energy-demanding.

• High reactivity metals form stable ores, needing complex extraction.
• Low reactivity metals form less stable ores, easier to extract.
• Extraction difficulty increases with metal reactivity.

The method used to extract a metal depends on its reactivity. Metals less reactive than carbon, such as iron, zinc, and lead, can be extracted by reduction with carbon. This involves heating the ore with carbon, which removes oxygen from the metal oxide to yield the pure metal.

Metals more reactive than carbon, like aluminium and sodium, cannot be extracted by carbon reduction because carbon is not reactive enough to displace them from their compounds. Instead, these metals are extracted by electrolysis, which uses electrical energy to break down the ore.

Very unreactive metals like gold and silver are often found in their native state and can be extracted by simple physical methods such as melting or chemical methods like displacement reactions.

• Metals less reactive than carbon: extracted by reduction with carbon.
• Metals more reactive than carbon: extracted by electrolysis.
• Very unreactive metals: often found native, extracted by simple methods.

Iron is extracted from hematite (iron(III) oxide) by reduction with carbon in a blast furnace because iron is less reactive than carbon.

Aluminium is extracted from bauxite by electrolysis because aluminium is more reactive than carbon and cannot be reduced by it.

Copper, which is less reactive, can be extracted by heating its ore or by displacement reactions, making its extraction easier compared to highly reactive metals.

• Iron (less reactive than carbon): extracted by carbon reduction.
• Aluminium (more reactive than carbon): extracted by electrolysis.
• Copper (low reactivity): extracted by heating or displacement.

The position of a metal in the reactivity series directly influences the extraction method and difficulty. Higher reactivity metals require more energy and advanced techniques, while lower reactivity metals can be extracted by simpler, less costly methods.

Understanding this relationship helps predict the most suitable extraction process for a given metal and explains why some metals are more expensive to produce.

• Higher reactivity = more energy-intensive extraction.
• Lower reactivity = simpler extraction methods.
• Reactivity guides choice of extraction technique.

Hematite is a naturally occurring iron ore with the chemical formula Fe2O3. It contains iron combined with oxygen and is the primary source of iron metal extracted for industrial use.

The iron in hematite is in the form of iron(III) oxide, which must be chemically reduced to obtain pure iron metal.

Mining hematite provides the raw material needed for iron production.

• Hematite formula: Fe2O3
• Contains iron in oxidised form
• Main ore used for iron extraction

A blast furnace is a large industrial furnace used to extract iron from hematite by chemical reduction.

It operates at high temperatures, allowing chemical reactions to occur that separate iron from oxygen.

The furnace is charged with hematite, coke (a form of carbon), and limestone, and hot air is blasted in to sustain combustion and generate heat.

• High-temperature furnace for iron extraction
• Charged with hematite, coke, limestone
• Hot air blast supports combustion and reactions

Coke burns in the presence of hot air to produce carbon dioxide and heat: C + O2 → CO2.

The carbon dioxide then reacts with more coke to form carbon monoxide: CO2 + C → 2CO.

Carbon monoxide is crucial because it acts as the reducing agent that removes oxygen from iron(III) oxide.

• Coke combustion produces CO2 and heat
• CO2 reacts with coke to form CO
• CO reduces iron(III) oxide to iron

The main chemical reaction in the blast furnace is the reduction of iron(III) oxide by carbon monoxide: Fe2O3 + 3CO → 2Fe + 3CO2.

In this reaction, carbon monoxide removes oxygen atoms from iron(III) oxide, producing iron metal and carbon dioxide gas.

This process converts the ore into molten iron, which collects at the bottom of the furnace.

• Fe2O3 + 3CO → 2Fe + 3CO2
• Carbon monoxide removes oxygen from iron oxide
• Molten iron is produced and collected

Iron is extracted from hematite by heating it in a blast furnace with coke and limestone.

Coke burns to produce carbon monoxide, which reduces iron(III) oxide to iron.

The limestone helps remove impurities as slag, while molten iron is collected for further processing into steel or other products.

• Hematite reduced by CO in blast furnace
• Coke provides carbon monoxide
• Limestone removes impurities as slag

Aluminium is not found in its pure metallic form in the Earth's crust because it is highly reactive. Instead, it occurs mainly as bauxite, a rock composed primarily of aluminium oxide minerals mixed with impurities such as iron oxides and silica.

Bauxite is the main source of aluminium worldwide because it contains a high percentage of aluminium oxide (Al₂O₃), which can be processed to extract aluminium metal.

The extraction of aluminium begins with the purification of bauxite to obtain pure aluminium oxide, commonly called alumina. This is done by the Bayer process, which removes impurities and produces a white powder of aluminium oxide.

• Bauxite contains aluminium oxide and impurities.
• It is the main ore used to obtain aluminium.
• Purification of bauxite yields aluminium oxide (Al₂O₃).

Aluminium is a very reactive metal, positioned high in the reactivity series. Because of this, it cannot be extracted by heating its ore with carbon (reduction) as is done with less reactive metals like iron.

Instead, aluminium oxide must be broken down by electrolysis, a process that uses electrical energy to separate aluminium metal from oxygen.

This method requires a large amount of electrical energy, making aluminium extraction energy-intensive and costly, but it is the only practical way to obtain pure aluminium metal from its ore.

• Aluminium is too reactive for reduction by carbon.
• Electrolysis uses electricity to separate aluminium from oxygen.
• The process is energy-intensive but effective.

The purified aluminium oxide is dissolved in molten cryolite (Na₃AlF₆) to lower its melting point from about 2050°C to around 900°C, reducing energy consumption during electrolysis.

The molten mixture is placed in an electrolytic cell with carbon electrodes. When a direct current passes through, aluminium ions (Al³⁺) gain electrons at the cathode to form aluminium metal, while oxide ions (O²⁻) lose electrons at the anode to form oxygen gas.

The overall reactions are: At the cathode: Al³⁺ + 3e⁻ → Al (liquid aluminium collects at the bottom) At the anode: 2O²⁻ → O₂ + 4e⁻ (oxygen gas is released) The carbon anodes react with oxygen to form carbon dioxide, so the anodes gradually wear away and must be replaced regularly.

• Aluminium oxide is dissolved in molten cryolite to reduce melting point.
• Electrolysis separates aluminium ions and oxide ions.
• Aluminium forms at the cathode; oxygen forms at the anode.
• Carbon anodes react with oxygen, producing CO₂ and requiring replacement.

Cryolite acts as a solvent for aluminium oxide, lowering the melting temperature of the mixture from over 2000°C to about 900°C, which significantly reduces the energy needed for electrolysis.

It also increases the electrical conductivity of the molten mixture, allowing the electric current to pass efficiently through the electrolyte.

Without cryolite, the process would be impractical due to the extremely high temperatures required to melt pure aluminium oxide.

• Cryolite lowers the melting point of aluminium oxide.
• It improves electrical conductivity of the electrolyte.
• It reduces energy consumption during electrolysis.

Extracting aluminium from bauxite involves first purifying the ore to obtain aluminium oxide, then using electrolysis of molten aluminium oxide dissolved in cryolite to produce aluminium metal.

The process requires large amounts of electrical energy and uses carbon electrodes that are consumed during electrolysis.

The aluminium produced is collected as a molten metal at the bottom of the electrolytic cell and can be cast into various shapes for industrial use.

• Extraction involves purification and electrolysis steps.
• High electrical energy input is necessary.
• Carbon electrodes are consumed and replaced regularly.
• Molten aluminium is collected for further processing.

Iron is primarily extracted from its ore hematite (Fe2O3) using a blast furnace. Hematite contains iron in the form of iron(III) oxide, which must be chemically reduced to obtain pure iron metal. The blast furnace provides the high temperatures and chemical environment necessary for this reduction process. The main reducing agent in the furnace is carbon monoxide, which is produced inside the furnace by reactions involving coke (a form of carbon).

• Hematite is the main ore of iron, containing iron(III) oxide (Fe2O3).
• Extraction requires reduction of Fe2O3 to Fe.
• A blast furnace is used to achieve the necessary conditions.

Coke, a form of almost pure carbon, is added to the blast furnace and combusted in the presence of hot air blown into the furnace. This combustion reaction is exothermic and produces the high temperatures (around 1500°C) needed for the extraction process. The reaction also produces carbon dioxide gas.

• C + O2 → CO2 (combustion of coke)
• This reaction releases heat, raising the furnace temperature.
• Carbon dioxide is formed as a product.

The carbon dioxide produced in the combustion zone reacts further with more coke at high temperatures to form carbon monoxide. This reaction is important because carbon monoxide acts as the reducing agent that removes oxygen from iron(III) oxide. The reaction is reversible and endothermic, requiring the high temperature maintained by coke combustion.

• CO2 + C → 2CO (formation of carbon monoxide)
• Carbon monoxide is a key reducing agent in the blast furnace.
• This reaction helps maintain a supply of CO gas in the furnace.

The carbon monoxide gas produced then reacts with the iron(III) oxide in the ore. This reaction removes oxygen from the iron oxide, reducing it to molten iron metal. The oxygen combines with carbon monoxide to form carbon dioxide, which can cycle back to form more carbon monoxide. The molten iron collects at the bottom of the furnace and can be tapped off for further processing.

• Fe2O3 + 3CO → 2Fe + 3CO2 (reduction of iron oxide)
• Carbon monoxide removes oxygen from iron oxide, producing iron.
• Molten iron collects at the furnace base for extraction.

The extraction process involves a sequence of linked reactions: combustion of coke produces heat and CO2; CO2 reacts with coke to form CO; CO reduces Fe2O3 to Fe. Each step is essential to maintain the temperature and chemical environment for efficient iron extraction. Understanding these reactions helps explain why coke is used both as a fuel and a reducing agent source in the blast furnace.

• C + O2 → CO2 (heat generation)
• CO2 + C → 2CO (carbon monoxide formation)
• Fe2O3 + 3CO → 2Fe + 3CO2 (iron oxide reduction)