Metallurgy                                                                               

1.0General Principles and Processes of Isolation of Elements- Metallurgy

Metallurgy is the process of extracting metals from rocks. It's a fancy word for the process. We use chemicals and technology to separate the metal from the rock. Although different ways to do this depend on the metal, some basic rules always apply. These rules help us ensure we're doing it right, no matter what kind of metal we're trying to get.

Extracting and isolating an element from its combined form relies on different chemistry principles, yet common general principles are shared across all metal extraction processes.

2.0Occurrence of Metals

Ores are minerals in the Earth's crust from which metals can be economically extracted. While metals like gold and silver are found in their pure state, others like potassium and sodium are typically bound within compounds. Extracting pure metals from these compounds is crucial. Not all minerals qualify as ores, as only those from which metals can be readily extracted are considered such.

• Native Ores: These ores are found in their elemental form, with the metal occurring naturally in its pure state. Examples include silver, gold, and platinum.
• Oxide Ores: These ores are composed of oxides or oxysalts of metals, such as carbonates, phosphates, sulfates, and silicates.
• Sulfide Ores: These ores are primarily made up of sulfides of metals like iron, lead, zinc, and mercury.
• Halide Ores: These ores comprise halides of metals, forming compounds with elements like fluorine, chlorine, bromine, or iodine.

3.0Principal Ores of Some Important Metals 

Ores typically contain more than the metal we want; they often come with unwanted materials called gangue. Extracting metals from ores involves three main steps:

• Concentrating the ore,
• Separating the metal from the concentrated ore, and
• Purifying the metal.

4.0Concentration of Ores

By Physical Separation Method:

Ores often contain gangue impurities, which are rocky substances and siliceous matter. To extract metals, the ore is first concentrated to increase the amount of desired metal or its compounds. This step is called ore concentration. Different methods, chosen based on the ore's characteristics, type of impurity, and environmental concerns, are used for this process.

 Hydraulic washing or Gravity separation 

This process relies on the variation in specific gravities between the ore and gangue particles, making it a form of gravity separation. In this approach, a stream of running water flows upwards to rinse the powdered ore. Consequently, the lighter gangue particles are carried away while the heavier ore particles remain.

Magnetic Separation

In this operation, a powdered ore travels along a conveyor belt that passes over two rollers, one of which is magnetic. During this process, the magnetic particles are drawn towards the magnetic roller and are the first to fall, followed by the non-magnetic particles.

Froth Floatation Method

The froth flotation process relies on the distinction in the wetting properties between the ore and gangue particles. It's exclusively employed for sulfide ores.

By Chemical Separation Method

Leaching

Leaching is a process where the ore is treated with a specific reagent, like acids, bases, or other chemicals, to make the desired component soluble while leaving impurities insoluble. The soluble part is then recovered from the solution using appropriate chemical methods.

Bayer's process (Leaching of alumina from bauxite)

• In the leaching process for extracting alumina from bauxite, powdered ore is heated with a concentrated solution of NaOH at high temperature and pressure. This process, called digestion, extracts Al₂O₃ as sodium aluminate. Sodium aluminate is neutralized with CO₂ to precipitate hydrated Al₂O₃, which is then filtered, dried, and heated to obtain pure Al₂O₃. Sodium silicate, formed as an impurity, remains in solution.

Cyanide Process

• In silver and gold metallurgy, the metal or ore is treated with NaCN or KCN in the presence of air or oxygen. This results in the formation of a complex compound with the metal
• 4M(s) + 8CN^⊝(aq) + 2H₂O + O₂(g)➜ 4[M(CN)₂]^⊝ (aq)^Å + 4OH^⊝(aq) (M= Ag or Au)
• 2[M(CN)₂]^⊝(aq) + Zn(s)  ➜ [Zn(CN)₄]^2⊝(aq) + 2M(s)

5.0Extraction of Crude Metal from Concentrated Ore

To extract metal from its concentrated ore, it's typically converted into a form suitable for metal reduction. Sulphide ores are often first converted to oxides because oxides are more easily reduced. Therefore, isolating metals from concentrated ore involves two main steps:

(a) Conversion to oxide.

(b) Reduction of the oxide to metal.

• Conversion to oxide.

Calcination

Calcination involves heating ore without air to remove water from hydrated oxides or carbon dioxide from carbonates below their melting points.

Advantages of Calcination:

• Removes moisture.
• Destroys organic matter.
• Converts hydroxide and carbonate ores into oxides.
• It makes the metal porous and easier to work with.

E.g.

• Al₂O_3..2H₂O   ➜Al₂O₃  + 2H₂O
• 2Fe₂O₃.3H₂O ➜2Fe₂O₃+ .3H₂O 

Roasting 

Roasting is a method for removing excess sulfur from metal sulfide ores by heating them in excess air. It's done below the ore's melting point in a furnace. Through oxidation, sulfides turn into metal oxides, releasing sulfur dioxide. Roasting differs from calcination in that it involves significant chemical alterations.

• 2ZnS + 3O₂  ➜2ZnO + 2SO₂
• 2Cu₂S + 3O₂➜2Cu₂O + SO₂

The sulphide ores of copper are heated in a reverberatory furnace.

Reduction of oxide to the metal

Reducing metal oxide typically involves heating it with a reducing agent, such as carbon (C), carbon monoxide (CO), or another metal. Like carbon, the reducing agent reacts with the metal oxide's oxygen.

M_xO_y + yC → xM + y CO

Some metal oxides are quickly reduced, while others are more challenging to reduce (meaning the metal ion gains electrons). Regardless, heating is necessary for the reduction process.

6.0Thermodynamic Principles of Metallurgy

Understanding metallurgical transformations relies on thermodynamic concepts, with Gibbs energy playing a crucial role. We analyze Gibbs energy changes to predict temperature requirements and suitable reducing agents for metal oxides (MxOy).

The reaction's Gibbs energy change (∆G) must be negative for a thermal reduction to occur feasibly at a given temperature The equation determines this change: ∆G = ∆H—T∆S, where ∆H is the enthalpy change, and ∆S is the process's entropy change.

For a reaction to proceed, ∆G must be negative. This happens in two cases:

1. When ∆S is positive, increasing temperature raises T∆S so that ∆H < T∆S, making ∆G negative.
2. If coupling reduction and oxidation yields a negative ∆G for the overall reaction, it becomes feasible. This coupling simplifies understanding.

• During metallurgy, 'flux' combines with 'gangue' to form 'slag', aiding gangue removal.
• Based on Gibbs energy vs temperature plots, Ellingham Diagrams help choose reducing agents for oxide reduction and aid in predicting thermal reduction feasibility.
• H.J.T. Ellingham pioneered the graphical representation of Gibbs energy, known as the Ellingham Diagram. It aids in selecting reducing agents for oxide reduction and predicting the feasibility of thermal ore reduction.

7.0Reduction of Ore to Metal

The calcined or roasted ore is typically reduced to its metallic state through one of the following methods.

Reduction by carbon (Smelting): 

It involves reducing the oxide with carbon at high temperatures, known as smelting.

• Less electropositive metal oxides, such as Pb, Zn, Fe, Sn, Cu, etc., are commonly reduced by intensely heating them with coal or coke in a blast furnace.

Some reactions

•    CuO + CO   ➜ CO₂ + Cu

 Gangue + acidic impurity  ➜(slag)

•   SiO₂ + CaCO₃ ➜ CaSiO₃ + CO₂

 Basic impurity + Flux  ➜ slag

•  MgCO₃ + SiO₂  ➜ MgSiO₃ + CO₂ ­
• Concentrated ore mixed with flux yields metal, slag, and gases. Coke is avoided in reducing s-block oxide Al2O3 due to the formation of metal carbides. E.g.   CaO + 2C ➜ CaC₂ + CO

Self-reduction

Self-reduction occurs in certain metal ores, such as Cu, Pb, Hg, etc. Sulphide ores are partially roasted to produce oxides, which are then reduced to metal by the remaining sulphide ore at high temperatures without additional reducing agents. This process is termed self-reduction.

•   2Pbs + 3O₂ ➜ 2PbO+2SO₂  ↑

         (Galena)    (air)

•       PbS   +       2PbO    ➜       3Pb +SO₂

          (unroasted ore)     (roasted ore)     (self reduction)

Metal displacement Method 

This method involves obtaining a water-soluble compound from the ore. The aqueous solution of this compound is then reacted with a more electropositive metal, displacing the metal from the solution.

(i) Zairvogel process for silver:

(ii) Separation of silver by complex formation (Cyanide process):

Electrolytic Reduction

Electrolytic reduction is primarily employed for extracting highly electropositive metals such as Na, K, Mg, Ca, Al, etc. It involves electrolysis conducted in large cells, with the addition of a small amount of another suitable electrolyte. This additional electrolyte serves several purposes:

(i) It decreases the melting point of the primary electrolyte.

(ii) It improves the conductivity of the solution.

(iii) It mitigates corrosion issues

e.g. Manufacture of metallic sodium (Down's process)

Reduction by Al (Thermite Reduction) :

Aluminium reduction, known as Thermite Reduction, is utilized for metals with exceptionally high melting points that require extraction from their oxides.

For example:

• Cr₂O_3​+2Al→2Cr+Al₂O₃
• 3Mn₃O₄ + 8Al→ 9Mn + 4A l₂O₃

8.0Refining 

Purification or refining of metals is necessary even after obtaining them through reduction methods, as they often contain undesirable impurities. Refining methods vary significantly depending on the type of metal and its intended use. These procedures are tailored to effectively remove impurities and enhance the metal's quality to meet specific application requirements.

• The Liquation Process

The liquation process is employed to refine metals that are easily fusible on their own but not with their existing impurities, which are typically infusible. In other words, the pure metal should have a lower melting point than each impurity. This method is commonly used to purify metals like tin (Sn) and zinc (Zn) and to extract lead (Pb) from zinc-mercury (Zn-Hg) alloys produced by Parke's process.

• The Distillation Process

The distillation process is applied to purify metals with volatile properties while containing non-volatile impurities or metals with non-volatile characteristics but having volatile contaminants. This technique is particularly effective in purifying cadmium (Cd), zinc (Zn), and mercury (Hg).

In this process, metals are transformed into volatile compounds, which are then collected and decomposed to yield pure metal. This method hinges on two crucial factors:

• When reacting with a suitable reagent, the metal must readily form a volatile compound.
• The volatile compound should be easily decomposed, facilitating straightforward recovery of the pure metal.

Let's delve into some examples to elucidate this technique below:

• Mond's Process: Used to purify nickel, it involves heating impure nickel with carbon monoxide to form nickel carbonyl, a volatile compound. At 180°C, the nickel carbonyl decomposes, yielding pure nickel and reusable carbon monoxide.
• Van Arkel Process: For ultrapure metal production, impurities are made inert as the impure metal is converted into a volatile compound. Electrical decomposition then isolates pure metal. This method is pivotal in purifying titanium, zirconium, hafnium, silicon, and other materials.

Amalgamation Process: 

They are utilized for noble metals such as gold (Au) and silver (Ag) extraction from native ores.

Other Methods:

• Kroll-Collen Process:    TiCl₄ + 2Mg -> 2MgCl₂ + Ti
• IMI Method: TiCl₄ + 4Na -> 4NaCl + Ti

Zone Refining or Fractional Crystallization

• This technique refines semiconductor elements like Si, Ge, Ga, and similar metals to achieve high purity. It exploits the difference in impurity solubility between molten and solid states. A mobile heater is moved along a metal rod, causing localized melting. As the heater progresses, pure metal crystallizes behind it while impurities migrate to the adjacent molten zone.

The Polling Process

• The poling process is employed in refining metals such as copper and tin. It involves passing an electric current through a molten metal, typically in the presence of suitable fluxes or additives. This current causes certain impurities, such as oxygen or sulfur, to react and form compounds that rise to the surface of the molten metal. These compounds are then removed, resulting in a purified metal.

Oxidation Process (Pyrometallurgical Oxidation) 

• It is utilized to refine metals with impurities with a greater affinity for oxygen than the metal itself. Cupellation, puddling, and bessemerization stand as significant methods within this category, employed for refining various metals.

Bessemerization is a pyrometallurgical process used to refine iron or steel by blowing air or oxygen through molten metal to remove impurities such as carbon, silicon, and manganese, producing high-quality steel.

Cupellation separates gold and silver from other metals or impurities. The metal sample is heated with lead in a porous container. The lead combines with oxygen, forming a layer of lead oxide that absorbs impurities, leaving behind pure gold or silver.

Electro–refining of Metals

Metals like Cu, Ag, Zn, Sn, Pb, Al, Ni, and Cr are refined through this method. 

• The impure metal serves as the anode in an electrolytic cell, while a thin plate of pure metal acts as the cathode.
•  The electrolyte consists of a solution of a double salt of the metal.

When an electric current is passed, the pure metal dissolves from the anode and deposits at the cathode. Soluble impurities enter the solution, while insoluble or less electropositive impurities settle below the anode as anode mud or sludge. For instance, impurities from blister copper deposits such as anode mud contain Au, Ag, Sb, Te, Se, and Pt.

9.0Uses of Aluminium, Copper, Zinc and Iron

• Aluminium: sed in food wrappers, paints, and alloys. It Conducts electricity and aids in metal extraction.
• Copper: Essential for electrical systems, pipes, and tough alloys like brass and bronze.
• Zinc: Galvanizes iron, used in batteries, alloys like brass, and manufacturing paints.
• Cast Iron: Cast stoves and railway sleepers contribute to wrought iron and steel production.
• Steel: Versatile metal alloy used in various applications, including automotive parts and utensils.