Comprehensive Guide to the Extraction, Properties, and Uses of Metals

Properties and Basic Principles of Metal Extraction

• Definition and Description of Metals: A specific definition for a metal does not exist, but they are characterized by the following set of physical properties:

  • They are generally solids at room temperature, with the notable exception of mercury.

  • They possess high strength and hardness.

  • They are characterized by high melting points and boiling points.

  • They serve as excellent conductors of both heat and electricity.

• Economic Factors in Metal Production: The market price of products manufactured from different metals varies based on two primary factors:

  • The availability and accessibility of the metal ore.

  • The cost associated with the specific reducing agent required for extraction.

• Ores and Pre-treatment: Metals are extracted from ores, which are defined as impure forms of a metal. Most ores exist as oxides. If an ore is not an oxide, it must be converted into one by burning the material in oxygen (a process known as roasting).

• Chemical Reduction: To obtain the metal, oxygen must be removed from the metal oxide using a suitable reducing agent. The choice of extraction method is dictated by the metal's position in the reactivity series.

• The Reactivity Series: The sequence from most reactive to least reactive is as follows:

  • Potassium

  • Sodium

  • Calcium

  • Magnesium

  • Aluminium

  • (Carbon)

  • Zinc

  • Iron

  • Lead

  • Copper

• Reducing Agent Selection: For metals positioned below carbon in the reactivity series, the most cost-effective reduction method is heating the ore with carbon.

Extraction of Iron: The Blast Furnace Process

• The Blast Furnace Environment: Iron extraction occurs in a blast furnace, which features a temperature gradient: it is cooler at the top and significantly hotter at the bottom.

• Main Ore: The primary ore used for iron extraction is haematite, known chemically as Iron (III) oxide ($Fe_2O_3$).

• Raw Materials: The four essential raw materials introduced into the blast furnace are:

  • Haematite (Iron ore).

  • Coke (an impure form of carbon).

  • Limestone (Calcium carbonate, $CaCO_3$).

  • Air.

• Chemical Reactions within the Furnace:

  • Production of Carbon Dioxide: Coke burns in the presence of air to produce carbon dioxide. This reaction is highly exothermic and provides the heat necessary for the furnace.       $C(s) + O_2(g) \rightarrow CO_2(g)$

  • Production of Carbon Monoxide: At high temperatures, the carbon dioxide reacts with additional coke to produce carbon monoxide, which serves as the primary reducing agent in the cooler upper regions of the furnace.       $CO_2(g) + C(s) \rightarrow 2CO(g)$

  • Reduction of Iron Ore (Cooler Parts): The iron ore is reduced by carbon monoxide to yield molten iron and carbon dioxide gas.       $Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$

  • Reduction of Iron Ore (Hotter Parts): In the hotter lower regions, some iron ore is reduced directly by carbon, yielding molten iron and carbon monoxide.       $Fe_2O_3(s) + 3C(s) \rightarrow 2Fe(s) + 3CO(g)$

  • Collection: The molten iron flows to the bottom of the furnace where it is periodically tapped off.

Removal of Impurities and Slag Formation

• The Main Impurity: The primary impurity found in iron ore is silicon dioxide ($SiO_2$), which is the main component of sand.

• Role of Limestone: Limestone is added to remove silicon dioxide through a multi-step chemical process:

  • Thermal Decomposition: The heat of the furnace decomposes limestone into calcium oxide and carbon dioxide.       $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$

  • Neutralization Reaction: Calcium oxide ($CaO$), a basic oxide, reacts with the acidic silicon dioxide ($SiO_2$) to form molten calcium silicate (slag).       $CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l)$

• Slag Management: Molten slag is less dense than molten iron, allowing it to float on top of the iron. This facilitates its removal and separation.

• Waste Gas Recycling: Waste gases produced during the process are piped back into the furnace to heat the system, increasing efficiency.

Varieties of Iron and Steel

• Cast Iron and Pig Iron:

  • Pig Iron: The raw, molten iron collected directly from the blast furnace, then cooled in molds.

  • Cast Iron: Produced by melting and cooling pig iron under specific controlled conditions. It contains approximately $4\%$ carbon.

  • Physical Properties: Cast iron is hard but extremely brittle, meaning it breaks easily upon impact.

  • Uses: Used for manhole covers, guttering, drainpipes, and engine cylinder blocks.

• Relationship between Carbon Content and Properties:

  • Higher carbon content leads to increased hardness but also increased brittleness.

  • Lower carbon content results in a stronger metal that is less prone to breaking.

• Comparison Table of Iron and Steel Types:

  • Wrought Iron: Pure iron; used for decorative work like gates and railings.

  • Mild Steel: Contains up to $0.25\%$ carbon; used for nails, car bodies, shipbuilding, and guiders.

  • High-carbon Steel: Contains $0.25-1.5\%$ carbon; used for cutting tools and masonry nails.

  • Cast Iron: Contains about $4\%$ carbon; used for manhole covers, guttering, and engine blocks.

  • Stainless Steel: An alloy of iron, chromium, and nickel; used for cutlery, cooking utensils, and kitchen sinks.

Rusting of Iron

• Conditions for Rusting: Iron requires the simultaneous presence of both oxygen and water to rust. The presence of electrolytes, such as salt, accelerates the process.

• Terminology: The term "rusting" applies specifically to the corrosion of iron, whereas other metals are said to "corrode."

• Chemical Formula of Rust: The chemical formula is given as $2Fe_2O_3 \times H_2O$, where $x$ represents a variable number of water molecules.

• Mechanism of Rusting:

  • Iron atoms lose electrons to form Iron (II) ions ($Fe^{2+}$).

  • These are further oxidized by air to form Iron (III) ions ($Fe^{3+}$).

  • Reactions involving water then produce the final rust product.

Methods of Preventing Rusting

• Barrier Methods: These work by physically blocking oxygen and water from reaching the iron surface. The effectiveness lasts only as long as the barrier is intact.

  • Painting.

  • Coating with oil or grease.

  • Covering with plastic.

• Alloying: A permanent method where iron is mixed with other elements. Even if the surface is scratched, the metal will not rust (e.g., stainless steel using chromium and nickel).

• Sacrificial Protection (Galvanization):

  • Iron is coated with a layer of zinc.

  • While the layer is intact, it acts as a barrier.

  • If the surface is broken, the iron still does not rust because zinc is more reactive than iron. Zinc corrodes instead, losing electrons to form zinc ions:       $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

  • These electrons flow into the iron, and any iron atom that has started to ionize regains electrons, preventing the formation of rust.

• Sacrificial Anodes:

  • Boats: Zinc blocks attached to hulls or keels act as sacrificial anodes to protect the iron hull.

  • Underground Pipelines: Sacks containing limps of magnesium are attached to the iron pipes. The magnesium ionizes, producing electrons that prevent the ionization (and thus the rusting) of the iron.

Extraction of Lead and Zinc

• Lead Extraction:

  • Ore: Galena (Lead sulphide, $PbS$).

  • Roasting: Lead sulphide is roasted in air to form lead oxide and sulphur dioxide.

  • Reduction in Blast Furnace: Lead oxide is reduced by carbon monoxide in cooler parts and by carbon in hotter parts.       $PbO(s) + CO(g) \rightarrow Pb(s) + CO_2(g)$       $PbO(s) + C(s) \rightarrow Pb(s) + CO(g)$

  • Collection: Molten Lead flows out through the bottom; impurity removal is identical to the iron process.

• Zinc Extraction:

  • Ore: Zinc blende (Zinc sulphide, $ZnS$).

  • Roasting: Zinc sulphide is roasted in air to form zinc oxide and sulphur oxide.       $2ZnS(s) + 3O_2(g) \rightarrow 2ZnO(s) + 2SO_2(g)$

  • Reduction: Zinc oxide is reduced by carbon monoxide (cooler parts) or carbon (hotter parts).       $ZnO(s) + CO(g) \rightarrow Zn(s) + CO_2(g)$

  • Collection: Unlike iron and lead, zinc is collected as a vapor from the top of the furnacec.

Extraction of Aluminium

• Method: Since aluminium is above carbon in the reactivity series, it cannot be reduced by carbon and requires electrolysis.

• Ore: Bauxite (Aluminium oxide, $Al_2O_3$). The bauxite is treated to produce pure aluminium oxide.

• Electrolysis Setup:

  • Solvent: Pure aluminium oxide has a melting point of approximately $2000^\circ\text{C}$, making direct electrolysis impractical. It is instead dissolved in molten cryolite (an alimunium compound).

  • Operating Temperature: The cryolite solution allows the process to run at about $1000^\circ\text{C}$.

  • Electrical Specifications: The cell operates at $5-6\,\text{volts}$ with a current reaching up to $1000,000\,\text{amps}$. The high current generates heat that keeps the ions ($Al^{3+}$ and $O^{2-}$) mobile.

  • Electrodes: Both the cathode and anode are made of carbon.

• Electrode Reactions:

  • At the Cathode: Aluminium ions gain three electrons to form molten aluminium metal, which sinks to the bottom and is siphoned off.       $Al^{3+} + 3e^- \rightarrow Al(l)$

  • At the Anode: Oxide ions lose four electrons to form oxygen gas.       $2O^{2-}(l) \rightarrow O_2(g) + 4e^-$

  • Anode Degradation: The oxygen produced reacts with the carbon anodes, causing them to burn and form carbon dioxide. Consequently, the anodes must be replaced regularly, increasing operational expenses.

Industrial Uses of Aluminium

• Overhead Electric Cables: Chosen for its excellent electrical conductivity.

• Aeroplane Bodies: Chosen for its low density (it is lightweight).

• Sauce Pans: Chosen for being a highly effective thermal conductor.