Metallurgy: Detailed Explanation

Metallurgy is a multifaceted scientific process that encompasses a range of techniques with the objective of extracting metals in their elemental state from mineral deposits known as ores. Metallurgy encompasses an extensive number of principles and technological procedures utilized to effectively extract and refine metals, ultimately yielding them in a state of exceptional purity.

The technique used to extract metal is determined by factors such as its activity, the characteristics of the ores, and the presence of impurities within the ore. Therefore, metallurgical operations are carried out through three distinct routes or branches, namely:

(i) Hydrometallurgy

(ii) Electrometallurgy

(iii) Pyrometallurgy

The classification mentioned above does not suggest that a metal can be completely extracted from the minerals using only one metallurgical process. This statement highlights the notion that when a metal ore undergoes a particular metallurgical procedure, it can produce a resulting substance that may require the utilization of an alternative metallurgical procedure for its extraction. The extraction of aluminum from bauxite ore is a complex procedure that encompasses a combination of hydrometallurgical operations and partial calcination using pyrometallurgy techniques.

The process of obtaining metallic aluminum is ultimately accomplished through the electrometallurgical reduction method. While the extraction of a metal involves multiple metallurgical processes that may have overlapping aspects, it is crucial to examine and evaluate each of these processes separately.

Hydrometallurgy

Hydrometallurgy is a process that encompasses the extraction of metals from aqueous solutions derived from ores or recycled materials. Hydrometallurgy necessitates the implementation of three distinct operations. The following operations are examined and analyzed in this discussion:

(i) Leaching

Leaching is a chemical process in which finely ground ore is dissolved in water by reacting with the right aqueous reagent. This leaves the impurities behind as a solid residue that doesn’t dissolve. The metal that is intended for extraction remains in a solution that is composed of water, known as an aqueous solution. This solution is referred to as leach liquor. It is worth noting that any impurities that are present in the leach liquor can be readily eliminated.

One example involves the leaching process of Bauxite ore using concentrated Sodium Hydroxide (NaOH) at elevated temperatures. This particular method selectively dissolves aluminum while leaving behind undissolved residues of Fe₂O₃, SiO₂, TiO₂, and a few other noble metals.

Al₂O₃.2H₂O + Fe₂O₃ + SiO₂ + TiO₂+NaOH (Δ)→ NaAlO₂ (aq.) + H₂O + solid impurities

^{(Bauxite) {Impurities} (leaching)}

Ag₂S + Impurity + 4 NaCN → 2 Na[Ag(CN)₂] + Na₂S + Solid impurities

^(leaching)

(ii) Concentration And Purification of Leach Liquor

The leach liquor, which contains the target metal, is subjected to a concentration process. In cases where the solution also contains unwanted metal ions, various techniques such as precipitation, ion exchange, and solvent extraction are employed to eliminate these unwanted ions.

(iii) Recovery of Metal or Metal Compounds

The concluding phase of the hydrometallurgical process is known as recovery. The leach solution, which contains metal ions such as Cu²⁺ or Ag⁺, may exist in a complex state and can be effectively recovered through a displacement reaction involving a precisely determined quantity of more electropositive metals, such as Zinc (Z).

Cu²⁺ (aq.) + Zn → Cu↓ + Zn²⁺

2 Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] +2 Ag↓

Electrometallurgy

• Electrometallurgy is a subfield of metallurgy that focuses on the deposition of electropositive metals, such as Sodium (Na), Potassium (K), Calcium (Ca), Magnesium (Mg, Aluminium (Al), and others, onto the cathode surface.
• This process is achieved by introducing an external electrical energy source into the electrolytic cell, where the metals are present in their molten or fused salt form.
• The method of extracting metals through electrolytic reduction is commonly referred to as electrowinning. The extraction of these metals through electrolysis of their aqueous salt solution is not feasible. Conversely, these metallic elements exhibit a significant propensity to combine with carbon in order to produce carbides, rendering the carbon reduction method of extraction impractical.
• The extraction of these substances is limited to the process of electrolysis involving their fused salts. In addition to this, the application of the principle of electrometallurgy extends to the electrorefining process of various impure metals, including but not limited to calcium, zinc, and silver.

Pyrometallurgy

• Pyrometallurgy, a subfield of metallurgy, focuses on the extraction and purification of metals with lower electropositivity, including copper (Cu), iron (Fe), zinc (Zn), tin (Si), chromium, nickel, cobalt, mercury, and others. This is achieved through the application of heat during the mining and processing processes of their respective ores.
• In this particular scenario, the powdered ore undergoes an initial concentration process through the utilization of appropriate physical and chemical techniques.
• In the majority of cases, the concentrated ore undergoes a process of calcination and roasting in order to obtain it in the form of an oxide. 
• Following the completion of smelting and reduction processes, the resulting metal typically exists in an impure state. The process of achieving the purification of metal is ultimately accomplished through the implementation of appropriate thermal or electro-refining techniques.

References

• https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_(Averill)/22%3A_The_d-Block_Elements/22.03%3A_Metallurgy
• https://www.vedantu.com/chemistry/processes-of-metallurgy
• https://www.britannica.com/science/metallurgy
• https://byjus.com/chemistry/processes-of-metallurgy/
• https://en.wikipedia.org/wiki/Metallurgy#:~:text=Metallurgy%20is%20a%20domain%20of,which%20are%20known%20as%20alloys.