Comprehensive Study Notes on Metallurgy and Metal Alloys

Fundamentals of Metallurgy: Definitions and Key Terminology

Metallurgy is defined as the science and practice of working with metals, specifically the processes involved in obtaining metals from rocks and enhancing their strength or utility for everyday applications like tools, machines, and buildings. Within this field, several key terms are used to describe materials and processes. A mineral is any substance obtained by excavation from the Earth's crust. It is explicitly noted that a mineral does not necessarily need to contain a profitable amount of an element to be classified as such.

An ore is a specific type of mineral from which a pure metal can be obtained easily and economically through metallurgical processes. For a mineral to be considered an ore, its extraction must be both economical and profitable. Examples of ores include Bauxite with the formula $Al_2O_3 \cdot 2H_2O$ and Hematite with the formula $Fe_2O_3$.

In the context of ore extraction, gangue refers to the unwanted materials present in an ore, which typically include sand, clay, and various rocky impurities. This gangue must be removed before the metal extraction process begins. To facilitate this removal, a substance called a flux is added during extraction. The flux reacts with gangue to make impurities easier to separate. The resulting byproduct from the reaction between flux and gangue is known as slag. Slag is a glass-like material that can be easily removed from molten metal. It is generally a mixture of metal oxide and silicon dioxide. An example equation for slag formation is $FeO + SiO_2 \rightarrow FeSiO_3$.

Basic Principles and Methods of Ore Concentration

Metallurgy follows a sequence of basic principles to extract and purify metals: Concentration, Roasting and Calcination, Reduction (converting metal oxide to metal), Refining, and Alloying. Concentration is the first major step and includes several physical and chemical methods.

Gravity separation, also known as the hydraulic separator method, is based on the difference in specific gravities between the metallic ore and the gangue particles. Generally, metal ores are heavier than the associated gangue. By applying a flow or current of water over powdered ore, lighter rocky impurities are washed away, leaving the heavier ore particles behind. There are two specific methods of gravity concentration: Jigging and Tabling. Jigging involves the removal of lighter portions by means of a stream of water or air rising through a bed of coarse ore particles. Tabling, or using a Wilfley table, involves passing a mixture of ore and water over an inclined vibrating table to separate fine ore particles from gangue.

Magnetic separation is utilized when either the ore itself or the impurities within it are magnetic. In this process, powdered impure ore is dropped as a thin layer onto a moving rubber belt. One of the rollers driving the belt contains an electromagnet. Magnetic particles are attracted toward the magnet and collected in a separate heap, while non-magnetic particles fall off into a different container.

Froth Flotation is the preferred method for concentrating sulphide ores. The ore is crushed into a fine powder and mixed with water in a tank. A frothing agent, such as pine oil or eucalyptus oil, is added to make the ore particles water-repellent. Compressed air is then passed through the mixture to create bubbles. The sulphide ore particles attach to these air bubbles and rise to the surface as foam or froth, because the ores are lighter than the impurities. This froth is then collected to obtain the concentrated ore.

Chemical processes involve treating powdered ore with specific reagents that dissolve the ore while leaving the impurities behind. These undissolved impurities are then removed via filtration. An example of this is the process for extracting silver using sodium cyanide:
$Ag_2S + NaCN \rightarrow Na[Ag(CN)_2]$
$2Na[Ag(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Ag \downarrow$

Thermal Processes: Roasting and Calcination

Roasting is the process of heating ores in the presence of excess air to convert them into oxides. This process is generally applied to sulphide ores. During roasting, volatile impurities are evaporated, and the ore is oxidized. Examples of chemical reactions during roasting include:
$FeCuS_2 + O_2 \rightarrow FeO + Cu_2O + SO_2 \uparrow$
$4As + 3O_2 \rightarrow 2As_2O_3 \uparrow$
$2P_4 + 8O_2 \rightarrow 2P_4O_8 \uparrow$

Calcination is the process of heating ores in a limited supply of air to convert them into oxides. This is typically used for oxide or carbonate ores. The process removes moisture and volatile impurities. Typical reactions include:
$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$
$ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$

Smelting and Reduction Processes

Smelting, or reduction, involves heating metal oxides to transform them into metal. Specifically, smelting is the process of reducing an oxide with carbon in the presence of flux at high temperatures, usually carried out in a blast furnace. Key reactions in this stage include:
$2C + O_2 \rightarrow 2CO$
$Cu_2O + C \rightarrow 2Cu \downarrow + CO$
$Cu_2O + CO \rightarrow 2Cu \downarrow + CO_2$

Metallurgy of Copper: From Pyrite to Blister Copper

Copper ($Cu$) has an atomic number of $29$, an atomic weight of $63.5$, and a valency of $1$. Its electronic configuration is $2, 8, 18, 1$. The term is derived from the Latin word 'Cuprum.' Major copper ores include Copper Pyrite ($CuFeS_2$), Ruby Copper ($Cu_2S$), and Malachite ($Cu(OH)_2 \cdot CuCO_3$). The principal ore used for extraction is copper pyrite.

The extraction process begins with concentration via the froth flotation method. Large pieces of $CuFeS_2$ are crushed to powder and agitated in a mixture of water and pine oil/eucalyptus oil. Air is passed through the solution, causing the copper pyrite to rise as foam while impurities stay in the water.

During Roasting, the concentrated ore is heated in a reverberatory furnace at temperatures below its melting point. Moisture and volatile impurities like sulphur, phosphorus, and arsenic are driven out as volatile oxides:
$S + O_2 \rightarrow SO_2 \uparrow$
$P_4 + 5O_2 \rightarrow P_4O_{10} \uparrow$
$4As + 3O_2 \rightarrow 2As_2O_3 \uparrow$
Additionally, metal sulphides decompose and convert:
$CuFeS_2 + O_2 \rightarrow Cu_2S + FeS + SO_2 \uparrow$
$FeS + 2O_2 \rightarrow 2FeO + SO_2$
$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$

In the Smelting stage, roasted ore, coke, and silica ($SiO_2$ acting as flux) are charged into a blast furnace. Iron sulphide ($FeS$) oxidizes to $FeO$, which then reacts with the silica flux to form iron silicate slag ($FeSiO_3$). Because $FeS$ has a lower melting point than $Cu_2O$, it forms the slag first. The remaining molten mixture of $FeS$ and $Cu_2S$ is known as "Matte."

Bessemerisation involves mixing the molten matte with sand in a Bessemer furnace. The inner lining of the furnace is acidic (made of $SiO_2$) because the impurities are basic. The process is a "self-reduction" process where no external reducing agent is added. Reactions include:
$FeS + O_2 \rightarrow FeO + SO_2$
$FeO + SiO_2 \rightarrow FeSiO_3 \downarrow$
$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$
$Cu_2O + Cu_2S \xrightarrow{\text{self reduction}} 6Cu + SO_2 \uparrow$
During the cooling of this molten copper, $SO_2$ gas escapes, forming blisters on the surface of the metal. This results in "Blister Copper," which is $98\%$ pure.

Refining Copper: Electro-Metallurgy and Purity

To achieve high purity ($99.99\%$), Blister copper (containing $2\%$ impurities) is refined using electrolysis. In this process, a slab of Blister copper is made the anode (positive terminal), and a small piece of $100\%$ pure copper serves as the cathode (negative terminal). The electrolyte is a solution of sulphuric acid ($H_2SO_4$) and copper sulphate ($CuSO_4$).

During electrolysis, the blister copper at the anode is oxidized:
Anode: $Cu_{(s)} \rightarrow Cu^{2+} + 2e^-$
Simultaneously, copper ions from the solution are deposited at the cathode:
Cathode: $Cu^{2+} + 2e^- \rightarrow Cu_{(s)}$
This process effectively transfers pure copper to the cathode while impurities are left behind.

Extraction of Iron from Hematite

Iron is extracted primarily from Hematite ($Fe_2O_3$) and Magnetite ($Fe_3O_4$). The process follows three main steps: Concentration, Calcination, and Smelting. Concentration is performed using the gravity separation method (hydraulic separator) because Hematite is heavier than its impurities, which are washed away by high-pressure water.

Calcination involves heating the concentrated ore in a limited supply or absence of air to remove moisture and volatile impurities like sulphur and phosphorus:
$S + O_2 \rightarrow SO_2 \uparrow$
$P_4 + 5O_2 \rightarrow P_4O_{10} \uparrow$
$FeCO_3 \xrightarrow{\Delta} FeO + CO_2$

Smelting is done by mixing calcined ore with limestone and coke and charging it into a blast furnace heated by pre-heated compressed air. This gives rise to four distinct zones based on temperature and chemical reactions: the Zone of Reduction ($300^{\circ}C - 800^{\circ}C$), the Zone of Heat Absorption ($800^{\circ}C - 1200^{\circ}C$), the Zone of Combustion ($1300^{\circ}C - 1500^{\circ}C$), and the Zone of Fusion ($1500^{\circ}C - 1900^{\circ}C$).

Analysis of Blast Furnace Zones in Iron Production

In the Zone of Reduction ($300^{\circ}C - 800^{\circ}C$), Carbon Monoxide ($CO$) reduces iron oxides into "spongy iron":
$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
$2Fe_3O_4 + CO \rightarrow 6FeO + CO_2$
$FeO + CO \rightarrow Fe + CO_2$

In the Zone of Heat Absorption ($800^{\circ}C - 1200^{\circ}C$), which appears bright red, $CO_2$ is reduced by coke into $CO$, absorbing heat in the process:
$CO_2 + C \rightarrow 2CO - 39\,kcal$

In the Zone of Combustion ($1300^{\circ}C - 1500^{\circ}C$), which appears white-red hot, coke combines with $O_2$ to produce heat and $CO_2$:
$C + O_2 \rightarrow CO_2 + \text{Heat}$

In the Zone of Fusion ($1500^{\circ}C - 1900^{\circ}C$), spongy iron melts along with impurities ($S, C, P, Si, Mn$). Limestone reacts with silica to form slag:
$CaO + SiO_2 \rightarrow CaSiO_3 \downarrow \text{(slag)}$
Molten iron, known as cast iron or pig iron, is collected at the bottom. It contains approximately $4\%$ carbon and has $90\%-92\%$ purity. The waste gases from the furnace, called flue gas, contain $CO$ ($60\%$), $CO_2$ ($4\%$), $N_2$, and $H_2$.

Properties and Classifications of Metal Alloys

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. The purpose of alloying includes increasing strength, improving hardness, creating desirable properties like those of stainless steel, and enabling the use of metals as catalysts in chemical reactions.

Non-ferrous alloys do not contain iron as their primary component. Examples and compositions include:

• Brass: An alloy of Copper ($70\%Customs$) and Zinc ($30\%$). It is softer and more ductile than its parents, making it easier to work or weld. It is used for cartridge cases and household articles. If the composition is $90\%$ $Cu$ and $10\%$ $Zn$, it is called French Gold, which is harder and used for bullet jackets and jewellery.
• German Silver: Composition is Copper ($25\%-50\%$), Zinc ($10\%-35\%$), and Tin ($5\%-35\%$). It possesses silver-like appearance, excellent ductility, and corrosion resistance. It is used for coinage, utensils, and cutlery.
• Bronze: Alloy of Copper ($90\%$) and Tin ($10\%$). It is strong, corrosion-resistant, and used for coins, bearings, and photo frames.
• Gun Metal: A type of bronze containing Copper ($85\%$), Zinc ($4\%$), Tin ($8\%$), and Lead ($3\%$). It withstands high pressure and is used in marine engines and power plants.
• Duralumin: Composed of Aluminum ($95\%$), Copper ($4\%$), Magnesium ($0.5\%$), and Manganese ($0.5\%$). It is a lightweight, strong, and hard alloy used in aircraft structures and truck wheels.
• Solder: Standard solder compositions include $Sn:Pb$ ratios of $50:50$, $40:60$, or $67:33$. It has a very low melting point and is used for soldering circuits and cables.
• Wood Metal (Lipowitz's Alloy): Composed of Bismuth ($50\%$), Lead ($26.7\%$), Tin ($13.3\%$), and Cadmium ($10\%$). It is solid at room temperature but has a very low melting point, making it suitable for safety devices (safety balls) and low-temperature casting.

Ferrous Alloys: The Classification and Utility of Steel

Steel is a ferrous alloy composed of iron and carbon, with carbon content ranging from $0.15\%$ to $1.5\%$. It is classified into four types based on carbon percentage:

1. Dead carbon steel: Contains up to $0.15\%$ carbon.
2. Low carbon steel (Mild steel): Contains $0.15\%-0.45\%$ carbon. It is tough, malleable, and ductile. It has a strength of $555\,N/mm^2$ and a hardness of $140\,BHN$. It is used for automobile bodies, gears, and railway axles.
3. Medium carbon steel: Contains $0.45\%-0.80\%$ carbon. It has good compressive strength and is better for hardening than wrought iron. It is used for turbine rotors, spring clips, and hammers.
4. High carbon steel: Contains $0.80\%-1.5\%$ carbon. It possesses high hardness and wear resistance but loses hardness between $200^{\circ}C - 250^{\circ}C$. Its peak hardness is reaching $450-500\,BHN$. It is used for making chisels, wheels, and automatic clutch disks.