0620 · 9.6Extraction of metals

Reactivity and extraction methods

0:00An iron beam and an aluminium drink can both began with materials taken from the ground. However, obtaining these metals requires very different processes. Iron can be extracted using carbon monoxide, while aluminium needs electrolysis. By the end of this video, you will understand why those methods differ and how each process produces the metal. An ore is a naturally occurring rock from which a metal can be extracted economically.

0:24That means obtaining the metal must be financially worthwhile. Most ores contain metal compounds mixed with unwanted material. Extraction therefore involves producing the metal from its compound, not simply separating pieces of metal from surrounding rock. For this topic, we need two ways of recognising reduction. Removing oxygen from a substance is reduction, and gaining electrons is also reduction. Both descriptions will help us understand extraction. Iron(III) oxide loses oxygen in the blast furnace, while aluminium ions gain electrons during electrolysis.

0:57The reactivity series places metals in order of their chemical reactivity. More reactive metals form compounds that are generally harder to reduce to the metal. Less reactive metals are easier to obtain from their ores. Carbon provides a useful dividing line when deciding which extraction method is suitable. Metals below carbon can generally be extracted from their oxides by reduction using carbon. Iron is below carbon, so a carbon based process works.

1:23Metals above carbon cannot be extracted from their oxides this way. Aluminium is above carbon, so electrolysis is needed instead. Carbon appears here as a useful reference substance, even though it is not a metal. Its position helps predict whether it can remove oxygen from a metal oxide. Think of the series as a guide to the strength of the chemical challenge. A more reactive metal requires a more demanding extraction method.

1:48Very unreactive metals, such as gold, can occur naturally as the element. There may then be no compound that needs reducing. This reinforces the general relationship between reactivity and extraction. The higher a metal lies in the series, the harder it usually is to obtain from its compounds.

Iron extraction in the blast furnace

2:04Iron is extracted from hematite in a blast furnace. The compound we need to reduce is the iron(III) oxide in hematite. Coke supplies carbon, and limestone helps remove an impurity. These three materials enter at the top, while hot air is blown into the furnace near the bottom. The furnace is not simply a container where everything melts together. Different reactions have different jobs, and the products of one reaction help the next.

2:29Coke provides heat and helps produce the reducing agent. Limestone deals with silica, an impurity that would otherwise remain mixed with the furnace contents. Near the bottom, carbon in the coke burns in oxygen from the hot air. The word equation is carbon plus oxygen gives carbon dioxide. This reaction releases heat, keeping the furnace hot enough for the extraction process. It also produces carbon dioxide, which becomes a reactant in the next stage.

2:55As carbon dioxide passes through hot coke, it reacts with more carbon. Carbon plus carbon dioxide gives carbon monoxide. The carbon dioxide is reduced because it loses oxygen as carbon monoxide forms. This stage produces the substance that removes oxygen from the iron(III) oxide. Carbon monoxide passes upwards through the furnace and reacts with the hematite. Iron(III) oxide plus carbon monoxide gives iron plus carbon dioxide. Oxygen is removed from the iron oxide, so the oxide is reduced.

3:25Carbon monoxide gains that oxygen and becomes carbon dioxide. Carbon monoxide is therefore the reducing agent in this reaction. A reducing agent causes another substance to undergo reduction. Here, it takes oxygen away from the iron oxide. The iron produced becomes molten in the furnace and collects at the bottom, where it can be tapped off. Keep the sequence connected rather than learning three unrelated statements. Burning coke provides heat and carbon dioxide.

3:53Reaction with additional coke then produces carbon monoxide. That gas reduces the iron oxide, so carbon supports extraction even though carbon monoxide performs the main reduction described here.

Slag formation and furnace equations

4:04The other part of the process removes silica from the ore. Silica is silicon dioxide, and it is an unwanted impurity. Limestone contains calcium carbonate, which breaks down when heated. This is thermal decomposition, meaning that heat causes a compound to break down into simpler substances. Calcium carbonate gives calcium oxide plus carbon dioxide. The calcium oxide then reacts with silica. Calcium oxide plus silicon dioxide gives calcium silicate. Calcium silicate forms the slag, separating this impurity from the iron rather than allowing it to remain mixed into the product.

4:40The molten slag lies above the molten iron because it is less dense. Separate outlets allow these layers to be removed independently. Limestone does not reduce the iron oxide, and slag is not another name for molten iron. Its role is to carry away the silica impurity. This next part is for the Extended paper. You need the balanced symbol equations for the five furnace reactions. The first three are shown on the board.

5:04Their coefficients tell us the relative amounts reacting, while each equation preserves the number of atoms of every element. For coke burning, the equation reads C plus O₂ gives CO₂. A single carbon atom reacts with one oxygen molecule to form carbon dioxide. Carbon monoxide then forms in the reaction C plus CO₂ gives 2CO. Each side has two carbon atoms and two oxygen atoms, so the product needs the coefficient two.

5:30Let us work through the iron reduction equation instead of simply memorising its coefficients. Begin with one formula unit of iron(III) oxide. It contains two iron atoms and three oxygen atoms. We therefore need two iron atoms on the product side, and all three oxygen atoms must be transferred elsewhere. Each carbon monoxide molecule gains one oxygen atom when it becomes carbon dioxide. Removing three oxygen atoms from the iron oxide therefore requires three carbon monoxide molecules.

5:58Those molecules become three carbon dioxide molecules. The balanced symbol equation is on the board, with two iron atoms produced. Now check the complete equation, including oxygen already present in carbon monoxide. The left side has three oxygen atoms in the oxide and another three in carbon monoxide. The right side has six oxygen atoms across three carbon dioxide molecules. There are also three carbon atoms and two iron atoms on each side.

6:26This example shows why balancing must not involve changing a compound’s formula. Altering the formula would describe a different substance, rather than a different amount. We change only the coefficients placed before formulae. Those numbers allow the reaction to conserve atoms while keeping the identities of the substances unchanged. The remaining two equations describe limestone decomposition and slag formation. Both are already balanced with one of each formula shown. Notice that calcium oxide is a product of the first equation and a reactant in the second.

6:56This connection explains why adding limestone helps remove silica from the furnace. Check the decomposition equation atom by atom. Calcium carbonate contains one calcium atom, one carbon atom and three oxygen atoms. Calcium oxide holds a calcium atom and an oxygen atom, while carbon dioxide holds the carbon and two oxygens. That gives three oxygen atoms on each side, so no extra coefficients are needed. A common mistake is to say that limestone reacts directly with silica in the required slag equation.

7:27First, heating produces calcium oxide from calcium carbonate. It is the calcium oxide that reacts with silicon dioxide to form calcium silicate. Giving these steps separately makes the role of limestone clear.

Aluminium extraction at the cathode

7:39The main ore of aluminium is bauxite, and aluminium is extracted by electrolysis. Bauxite is first processed to obtain purified aluminium oxide. You do not need the details of that purification here. The important distinction is that bauxite is the ore, while aluminium oxide is the compound used in the extraction cell. Heating aluminium oxide with carbon does not provide a suitable extraction method because aluminium is above carbon in the reactivity series.

8:06Instead, an electrical supply drives its reduction. Electrolysis uses electricity to decompose an ionic compound. The process produces aluminium by supplying electrons to aluminium ions. This next part is for the Extended paper. Aluminium oxide is dissolved in molten cryolite before electrolysis. Pure aluminium oxide would require a very high operating temperature to melt. Using cryolite allows the process to operate at a lower temperature and reduces the energy needed to maintain the liquid mixture.

8:37Cryolite is not the ore, and it is not the source of the aluminium being extracted. Its role is to provide a molten medium that dissolves aluminium oxide at a lower operating temperature. Within that mixture, ions can move and carry charge. Solid aluminium oxide would not work because its ions cannot move freely. The cell contains positive carbon anodes and a negative cathode formed by the carbon lining. Aluminium ions have a 3+ charge, so they are attracted to the cathode.

9:05Oxide ions carry a 2− charge and move towards the anodes. Opposite charges attract, which explains the directions of ion movement. At the cathode, aluminium ions gain electrons and become aluminium atoms. This is reduction because electrons are gained. Molten aluminium collects at the bottom of the cell and is removed. The electrical supply continually provides the electrons needed for this reaction to continue. The word description is an aluminium ion plus electrons gives aluminium.

9:33The ionic half equation is on the board. One aluminium ion needs three electrons because its charge is 3+. Adding three negative charges produces a neutral atom, so both atoms and overall charge are balanced. A half equation describes what happens at one electrode, including the transfer of electrons. Electrons belong on the left side for this reduction because they are gained. Writing only one electron would leave an overall positive charge.

9:58That would not describe the formation of a neutral aluminium atom.

Anode reactions and process differences

10:02At the anode, oxide ions lose electrons and form oxygen. Loss of electrons is oxidation. Two oxide ions are needed to supply the two oxygen atoms in one oxygen molecule. Each ion loses two electrons, so four electrons are released altogether. The word description is oxide ions give oxygen plus electrons. The ionic half equation is shown on the board, with electrons on the right because they are lost. Two oxide ions initially carry a total charge of 4−.

10:31The four released electrons account for that charge, while the oxygen molecule is neutral. Be careful to distinguish an oxide ion from an oxygen molecule. An oxide ion is one oxygen atom carrying a 2− charge. An oxygen molecule contains two oxygen atoms joined together and has no overall charge. That distinction explains both the coefficient of two and the four electrons in the anode equation. The oxygen formed reacts with the hot carbon anodes to produce carbon dioxide.

10:59Carbon is therefore used up, and the anodes gradually burn away. They must be replaced regularly to keep the process operating. Their loss is caused by this chemical reaction, not simply by the passage of electric current. The electrode half equation describes oxygen formation from oxide ions. The subsequent reaction with carbon explains why carbon dioxide leaves the cell. These statements are not contradictory because they describe different stages. First oxygen forms at the electrode, and then it reacts with the anode material.

11:29Keep the two extraction processes separate when answering questions. Hot air, coke, limestone and slag belong to the blast furnace description. Cryolite, carbon anodes and electrode half equations belong to aluminium electrolysis. Both produce metals by reduction, but the methods of achieving that reduction are different. Another common mistake is to describe extraction as melting the ore to release the metal. Melting alone changes physical state; it does not turn a metal compound into its element.

11:57A chemical change is required. In our examples, either carbon monoxide removes oxygen or aluminium ions gain electrons.

Exam answers and recap

12:05An examination might ask you to describe how limestone removes silica in a blast furnace. Start with what happens when limestone is heated, then explain which product reacts with the impurity. Finish by naming the substance formed. This follows the chemical sequence and avoids confusing limestone with the calcium oxide it produces. Calcium carbonate undergoes thermal decomposition to form calcium oxide and carbon dioxide. Calcium oxide reacts with silicon dioxide.

12:32Calcium silicate forms as slag. The slag separates from the molten iron and is removed. For an Extended question requesting symbol equations, write the two balanced equations shown earlier. You could also be asked to explain why aluminium requires electrolysis rather than reduction using carbon. State that aluminium is above carbon in the reactivity series. Its oxide cannot be reduced using carbon, so electrolysis is used. To state its main ore, give bauxite, not cryolite or purified aluminium oxide.

13:03An Extended question may ask you to explain why the carbon anodes need regular replacement. Oxygen is produced at the anodes. That oxygen reacts with carbon to form carbon dioxide. The carbon anodes are consumed and burn away. Each sentence adds a separate step in the explanation. Another Extended question may ask for the role of cryolite. Aluminium oxide is dissolved in molten cryolite. This lowers the operating temperature of the electrolysis.

13:30Less energy is therefore needed, which reduces the cost of extracting aluminium. Do not describe cryolite as the ore or as a catalyst. The central idea is that reactivity determines how difficult extraction will be. Carbon based reduction works for suitable metals below carbon, including iron. More reactive aluminium needs electrolysis instead. The furnace supplies heat, makes carbon monoxide and removes silica, while the electrolysis cell supplies electrons to aluminium ions.

13:58For iron, connect coke to carbon monoxide and limestone to slag. For aluminium, connect cryolite to a lower operating temperature and the cathode to reduction. Oxygen formation explains why the carbon anodes are consumed. Understanding those connections turns the equations into descriptions of a process, rather than facts to memorise.