Metallurgy involves extracting valuable metals from their ores and refining the extracted metals to a purer, more commercially viable form. This article will examine the fundamentals of metallurgy.
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Metals occur in either their free or combined state. For example, non-reactive metals, such as gold, platinum, and silver, are usually found in their free state, while more chemically reactive metals, such as iron, copper, or aluminum, are found in their combined state. Minerals from which these metals are extracted are called ores.
Metallurgy typically refers to commercial methods, which are different from laboratory methods. The science of metallurgy also includes the principles of making alloys by combining metals and the atomic, physical, and chemical properties of metals.
Classification of Ores
Ores are classified based on the type of compound that the metal forms, as below:
- Native Ores: Those that contain metals in their free state
- Sulphurised and Arsenical Ores: These contain metals such as sulfides or arsenides
- Oxide Ores – These ores contain the oxide form of metals
- Halide Ores – These ores consist of metallic halides
Metallurgical Processes
There are three general steps in metallurgical processes:
- Mining and removing the ore from the ground
- Separating and concentrating the metal or compound
- Reduction of the ore to produce the final metallic product
The chief metallurgical processes are described below.
Mining and Extracting Ore
Mining is one of the oldest economic activities in human history. While the principles of mining and extracting ore for concentration and metal extraction have remained mainly unchanged for millennia, technological progress means that today's methods vastly differ from those used in ancient times.
Metal-containing ores are extracted from surface or deep underground mines. They are then transported to locations where the concentration and purification processes occur. This complex process involves drills, ore trucks, and conveyor belts.
Concentration of Ore
The concentration of ore involves the removal of impurities and other physical processes, such as magnetic separation, hydraulic washing, and froth floatation.
Conversion of Ore Concentrate to Oxide
The concentrated ore is then converted into its oxide form through roasting, calcination, and reduction.
Metal Refining
The metal is refined via liquidation, electrolysis, and distillation.
Extractive Metallurgy
Metallic compounds are complex mixtures and cannot usually be extracted via simple processes. Before extractive metallurgy can commence, the compound must typically be converted into a more easily treatable form. Once this conversion has been performed, the metallic constituents of the compound can be separated from the other constituent materials.
In this process, metal sulfides are converted into oxides, chlorides, or sulfides; oxides are converted to chlorides or sulfides; carbonates are converted to oxides.
This stage of the metal extraction process employs several processes, including hydrometallurgical and pyrometallurgical practices. Pyrometallurgy includes smelting and roasting, whereas methods such as electrowinning (which applies an electric current to the compound solution) and leaching (selective diffusion using an aqueous solution) are used in hydrometallurgy.
Pyrometallurgy
Oxidation and reduction are two of the most common pyrometallurgical processes. In oxidation, metallic oxides are formed due to metals' affinity for oxygen, which are then further treated to separate pure metals from waste products. Reduction involves feeding a metallic oxide into a furnace along with a reducing agent, releasing combined oxygen and leaving a carbonaceous oxide and uncombined metal.
Roasting involves specialized roasters such as rotary kilns and blast, flash, and fluidized-bed roasters. Different types of specialized roasts include chloridizing roasts, volatilizing roasts, oxidizing roasts, and calcination. Roasting can be exothermic or endothermic.
Reduction and matte smelting are the two main types employed in pyrometallurgy. Blast furnaces reduce smelting to produce oxides from the metallic charge and slag. Matte smelting produces matte, a homogenous metallic sulfide, and the slag produced is in its oxide form.
Blast furnaces are universally employed in reduction smelting, differing slightly in construction depending on which type of metal is produced. Matte smelting uses equipment such as reverberatory furnaces and electric furnaces. As well as reduction and matte smelting, flash smelting has emerged in recent years and is becoming widely accepted in commercial metallurgy. Electrolytic smelting is also used.
Hydrometallurgy
Hydrometallurgical processes are employed for low-grade ores, which would make more expensive extraction processes less economically viable. A solution is formed from which the pure metal can be extracted and refined. Leaching usually involves preliminary processes such as controlled roasting to reduce the ore and produce a calcine that can easily dissolve in the leaching solution.
Pressure oxidation is a common treatment for sulfides. This involves preheating a concentrated slurry, which is then pumped into agitator-containing compartments in an autoclave with gaseous oxygen added to it. Oxidation typically takes about two hours.
Different solvents are used depending on the metallic compound being leached. For instance, sodium carbonate or sulfuric acid is used to leach oxides, whereas sulfuric acid or water is used for sulfinates. Precious metals are leached using a cyanide solvent.
Physical Metallurgy
Physical metallurgy involves the production of commercial products from metals, producing metal parts with various desired electrical, physical, chemical, structural, and aesthetic properties. Depending on the desired product, various forming methods are used, such as sheet metal for automotive fenders or powder metallurgy for small parts.
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In Summary
Metallurgy is a complex, interdisciplinary field encompassing various processes, from the extraction of ores to the production of refined metals.
Understanding the principles and techniques of metallurgy, such as pyrometallurgy, hydrometallurgy, and physical metallurgy, provides insight into how raw materials are converted into essential components for modern life. From the initial mining of ores to the complex techniques used to refine and shape metals, metallurgy remains a crucial foundation for numerous industries, powering technological advancements and economic growth worldwide.
Continue Reading: How Has Mining Changed Over Time?
References and Further Reading
Encyclopedia Britannica (2024) Metallurgy [online] Available at: https://www.britannica.com/science/metallurgy
Knight, J (2024) Metallurgy through the ages [online] Encyclopedia.com. Available at: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/metallurgy-through-ages
Jiang, L.Y (2017) Chapter 1 – Metallurgy: Importance, Processes, and Development Status Membrane-Based Separations in Metallurgy pp. 3-18. [online] Available at: https://www.sciencedirect.com/science/article/abs/pii/B9780128034101000013
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