Unit 9: Extraction of Metals

Most metals are found in the Earth's crust combined with other elements in rocks. An ore is a rock that contains a sufficient concentration of a metal compound to make its extraction economically viable. The process of extraction involves separating the metal from the other elements in its ore, which is primarily a reduction process (gain of electrons) for the metal ion.

The choice of extraction method depends on several factors, mainly:

• Position in the Reactivity Series: The reactivity of a metal determines the stability of its compounds. More reactive metals form more stable compounds and require more powerful, energy-intensive extraction methods.
• Cost of the Process: Economic viability is key. This includes the cost of energy, raw materials (reducing agents), and the equipment required.
• Required Purity of the Metal: Some applications require very pure metals, which may necessitate additional refining steps after the initial extraction.

The main methods are:

1. Electrolysis: This is the most powerful method and can extract the most reactive metals. It uses a large amount of electrical energy to decompose the molten metal compound. It is expensive but produces a very pure metal. Used for metals high in the reactivity series (e.g., Aluminum).
2. Reduction with Carbon: This is a cheaper method where the metal oxide is heated with carbon (in the form of coke). Carbon acts as a reducing agent, removing oxygen from the metal oxide. This method can only be used for metals that are less reactive than carbon (e.g., Iron, Zinc, Tin, Lead).

Aluminum is the most abundant metal in the Earth's crust, but it is very reactive. It is extracted from its main ore, bauxite, which is purified to yield aluminum oxide ($Al_2O_3$), also known as alumina.

Alumina has a very high melting point (~2072 °C), making it impractical to electrolyze directly. Instead, it is dissolved in molten cryolite ($Na_3AlF_6$), which acts as a solvent and lowers the operating temperature to a more manageable ~950 °C.

The process, known as the Hall-Héroult process, takes place in a large electrolysis cell with carbon electrodes.

• At the Cathode (Negative Electrode): Aluminum ions gain electrons (are reduced) and form molten aluminum, which sinks to the bottom of the cell and is tapped off.
  $$ Al^{3+}(l) + 3e^- \rightarrow Al(l) $$
• At the Anode (Positive Electrode): Oxide ions lose electrons (are oxidized) to form oxygen gas.
  $$ 2O^{2-}(l) \rightarrow O_2(g) + 4e^- $$

A significant issue is that the oxygen produced at the high temperature reacts with the carbon anodes, burning them away to form carbon dioxide.
$$ C(s) + O_2(g) \rightarrow CO_2(g) $$ Because of this, the anodes must be replaced regularly, adding to the cost. The process consumes vast amounts of electricity, making aluminum production expensive and recycling very important.

Iron is extracted from its ore, usually haematite ($Fe_2O_3$), in a giant tower called a blast furnace. The raw materials are iron ore, coke (a source of carbon), and limestone ($CaCO_3$), which are added in layers from the top. A blast of hot air is forced in from the bottom.

Several key reactions occur at different temperatures inside the furnace:

1. Production of the Reducing Agent: The coke burns in the hot air to produce carbon dioxide, which then reacts with more hot coke to form carbon monoxide, the main reducing agent.
   $C(s) + O_2(g) \rightarrow CO_2(g)$
   $CO_2(g) + C(s) \rightarrow 2CO(g)$
2. Reduction of Iron Ore: The carbon monoxide reduces the iron(III) oxide to molten iron, which trickles to the bottom of the furnace.
   $Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_2(g)$
3. Removal of Impurities: The main impurity in the ore is silicon dioxide ($SiO_2$). The limestone decomposes in the heat to form calcium oxide. This basic oxide reacts with the acidic silicon dioxide to form molten calcium silicate, known as slag.
   $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$
   $CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l)$ (Slag)

The molten slag is less dense than the molten iron, so it floats on top and can be tapped off separately. The molten iron produced, called pig iron, is impure and brittle due to a high carbon content. Most of this is converted to steel in the Basic Oxygen Process, where pure oxygen is blown through the molten iron to oxidize the excess carbon and other impurities.

Extraction of Tin

Tin is less reactive than iron and can also be extracted by carbon reduction. Its main ore is cassiterite ($SnO_2$). The ore is crushed and heated with carbon in a furnace.
$$ SnO_2(s) + 2C(s) \rightarrow Sn(l) + 2CO(g) $$

Extraction of Gold

Gold is extremely unreactive and is found as the native element (not in a compound). However, it is usually found in very low concentrations mixed with large amounts of rock. The extraction process involves separating these tiny flakes from the rock. A common modern method is cyanide leaching.

The crushed ore is treated with a dilute solution of sodium cyanide (NaCN) while air is bubbled through it. The gold is oxidized and dissolves to form a soluble complex ion.
$$ 4Au(s) + 8NaCN(aq) + O_2(g) + 2H_2O(l) \rightarrow 4Na[Au(CN)_2](aq) + 4NaOH(aq) $$ The gold is then recovered from this solution by reducing it with a more reactive metal, usually zinc powder.
$$ 2Na[Au(CN)_2](aq) + Zn(s) \rightarrow Na_2[Zn(CN)_4](aq) + 2Au(s) $$