A metal is commonly understood as a material that exhibits high electrical and thermal conductivity, a characteristic luster, and notable strength and malleability. The term refers both to a pure chemical element, such as iron or gold, and to manufactured materials like steel. The majority of metals found in use today are not recovered in their pure elemental state. The industrial process of “making metal” involves separating a metal element from its natural compounds and then purifying it. This complex process, known as metallurgy, converts raw earth materials into usable forms through mechanical, thermal, and chemical transformations.
The Raw Materials
The journey of metal begins with ore, which is a naturally occurring rock or sediment containing a metal compound in a concentration high enough to be economically recovered. After mining the ore, the initial step is mechanical processing to physically prepare the material for chemical separation. This preparation involves crushing the large chunks of rock and then grinding them into a fine powder, a process known as pulverization, which increases the surface area for later chemical reactions.
Next, the valuable mineral is separated from the unwanted earthy material, or “gangue,” through concentration. Methods like froth flotation are used extensively for sulfide ores, such as those containing copper or zinc. This technique relies on surface properties, causing metal-bearing particles to attach to air bubbles and float to the surface as a froth while the gangue sinks. This process yields a concentrated mineral product, significantly reducing the amount of material treated in later energy-intensive extraction steps.
Extracting Pure Metal
The concentrated ore still contains the metal chemically bound to other elements, often as oxides or sulfides, requiring a chemical reaction to isolate the pure metal element. One of the oldest and most common extraction methods is pyrometallurgy, which uses extreme heat and a chemical reducing agent. In this process, the metal compound is heated in a furnace, often with carbon (in the form of coke), which acts as the reducing agent by chemically bonding with the oxygen or sulfur in the compound. For example, in the production of iron, carbon monoxide strips oxygen from iron oxide, leaving behind molten iron in a reaction commonly referred to as reduction.
For less reactive metals, such as gold and silver, or for low-grade ores, hydrometallurgy offers an alternative approach using aqueous solutions. This method involves leaching, where a chemical solvent, such as a cyanide solution, is used to selectively dissolve the desired metal from the ore into a liquid phase. Once the metal ions are in solution, they can be precipitated out or recovered by other means, often requiring much lower temperatures than smelting.
Highly reactive metals, like aluminum, require electrometallurgy, which uses a powerful electrical current to force the chemical separation. Aluminum oxide, extracted from bauxite ore, is dissolved in a molten salt bath and subjected to a current in the Hall-Héroult process. The electricity drives a non-spontaneous reaction, supplying the energy needed to chemically reduce the metal ions into pure aluminum metal. The choice of extraction method depends entirely on the metal’s chemical reactivity and the nature of its ore compound.
Refining for Purity
The metal produced immediately after extraction is often called “crude” or “unrefined” because it still contains small amounts of residual impurities. These trace elements can negatively affect the metal’s mechanical and electrical properties, making a final purification step necessary for most commercial applications. This process of removing minute contaminants is called refining.
One of the most effective and widely used refining techniques is electrorefining, particularly for copper. In this method, the crude metal is cast into an anode and placed in an electrolytic cell opposite a thin sheet of pure metal acting as the cathode. When an electric current is applied, the impure metal at the anode dissolves, and only the pure metal is deposited onto the cathode, leaving most of the unwanted elements behind.
For metals with relatively low boiling points, such as zinc or mercury, refining can be achieved through vacuum distillation. The crude metal is heated until it vaporizes, leaving behind the non-volatile impurities. The pure metal vapor is then collected and condensed back into a highly purified liquid or solid form.
Mixing Metals to Create Alloys
For most modern applications, the purified metal element is rarely used in its pure state; instead, it is intentionally combined with other elements to form an alloy. An alloy is a substance with metallic properties that results from mixing two or more elements, with at least one being a metal. This blending is performed to engineer materials with properties superior to those of the individual components, often increasing strength, hardness, or corrosion resistance.
The properties of the final alloy are often a function of the different atomic sizes of the constituent elements. For instance, the addition of carbon to purified iron creates steel, where the small carbon atoms fit into the spaces between the larger iron atoms in the crystal lattice. This atomic disruption makes it harder for the metal’s layers to slide past each other, resulting in a much stronger and harder material than pure iron.
Common alloys illustrate this principle of enhanced performance. Bronze, a combination of copper and tin, is known for its hardness and resistance to wear, making it suitable for bearings and sculptures. Brass, a blend of copper and zinc, is valued for its pleasing color and superior workability compared to copper alone. By carefully controlling the composition, metallurgists create materials tailored for specific industrial needs.