Mineral Processing And Extractive Metallurgy

April 02, 2025

Mineral Processing & Extractive Metallurgy

Complete Guide for GATE Metallurgy (MT) - Section 4 (TestUrSelf)

Table of Contents

1. Mineral Processing
2. Extractive Metallurgy
3. Iron & Steel Making
4. Slag Properties
5. Steel Making Processes
6. Continuous Casting

4.1 Mineral Processing

Comminution Techniques

Comminution is the process of reducing the size of ore particles to liberate valuable minerals from waste rock.

Crushing and Grinding Equipment

Equipment	Size Reduction Range	Application
Jaw Crusher	150-250 mm → 25-50 mm	Primary crushing
Cone Crusher	50-100 mm → 10-25 mm	Secondary crushing
Ball Mill	1-5 mm → 75-150 μm	Fine grinding
Rod Mill	5-20 mm → 300-1000 μm	Coarse grinding

Bond's Law

W = 10W_i(1/√P - 1/√F)

Where:

• W = work input (kWh/ton)
• W_i = Bond work index (kWh/ton)
• P = 80% passing size of product (μm)
• F = 80% passing size of feed (μm)

Size Classification

Screening

Screening efficiency = (mass of undersize in product)/(mass of undersize in feed) × 100%

Hydrocyclones

d₅₀ = k(D_c^0.67D_i^0.45)/(D_o^0.71h^0.38Q^0.45(ρ_s-ρ)^0.5)

Where d₅₀ = cut size, D_c = cyclone diameter, D_i = inlet diameter, D_o = overflow diameter, h = pressure head, Q = flow rate

1

2

3

Typical comminution circuit: (1) Primary crushing → (2) Secondary crushing → (3) Grinding

Flotation

Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic by using differences in their surface properties.

Flotation Reagents

Type	Function	Examples
Collectors	Render mineral surface hydrophobic	Xanthates, Dithiophosphates
Frothers	Stabilize bubbles	Pine oil, MIBC
Modifiers	Control pH and selectivity	Lime, Na2CO3, Na2S

Flotation Kinetics

R = R_∞[1 - exp(-kt)]

Where R = recovery at time t, R_∞ = maximum recovery, k = rate constant

Gravity and Other Beneficiation Methods

Gravity Separation

V_t = [2g(ρ_s-ρ)d²]/(9μ)

Where V_t = terminal settling velocity, ρ_s = particle density, ρ = fluid density, d = particle diameter, μ = fluid viscosity

Equipment

• Jigs: Pulsating water flow separates heavy and light minerals
• Spirals: Centrifugal force separates particles by density
• Shaking tables: Stratification and differential movement

Other Methods

• Magnetic separation: For ferromagnetic minerals (magnetite)
• Electrostatic separation: For conductive vs non-conductive minerals
• Leaching: Chemical dissolution of valuable minerals

Agglomeration

Sintering

Process of compacting and forming a solid mass by heat or pressure without melting

% Sinter = (mass of sinter)/(mass of raw mix) × 100%

Pelletizing

Process of rolling moist fines into balls (green pellets) which are then hardened by firing

Drop number = Number of drops from 45 cm height before breakage

Briquetting

Compaction of fine materials into larger lumps using binders under high pressure

Process	Temperature Range	Typical Binders
Sintering	1200-1400°C	None (self-fluxing)
Pelletizing	1250-1350°C	Bentonite (0.5-1%)
Briquetting	Ambient	Pitch, tar, cement

4.2 Extractive Metallurgy

Material and Energy Balances

Input = Output + Accumulation + Losses

Heat Balance Components

1. Sensible heat of reactants
2. Heat of reactions
3. Sensible heat of products
4. Heat losses

Example: Copper Smelting

For a copper concentrate (30% Cu) smelted to matte (50% Cu):

Mass balance: 100 kg concentrate → 60 kg matte + 40 kg slag

Cu recovery = (60×0.5)/(100×0.3) × 100% = 100%

Non-Ferrous Metal Extraction

Aluminium Production

Bauxite

Al₂O₃

Al

Bayer Process → Hall-Héroult Process

Al₂O₃ + 3C → 2Al + 3CO (Hall-Héroult)

Conditions: 950°C, cryolite (Na₃AlF₆) electrolyte, 4-5 V, 150-300 kA

Copper Extraction

2CuFeS₂ + 5.5O₂ → Cu₂S·FeS (matte) + 3SO₂ + FeO (slag)

Cu₂S + O₂ → 2Cu + SO₂ (converting)

Titanium Production

TiO₂ + 2Cl₂ + 2C → TiCl₄ + 2CO (chlorination)

TiCl₄ + 2Mg → Ti + 2MgCl₂ (Kroll process)

Key Metals

Na Al Ti Cu Zn Pb

Extraction methods vary based on metal reactivity and ore type

4.3 Iron and Steel Making

Blast Furnace Process

The blast furnace is a counter-current reactor where iron ore is reduced to molten iron using coke as both fuel and reductant.

Key Reactions

C + O₂ → CO₂ (combustion)

CO₂ + C → 2CO (Boudouard reaction)

Fe₂O₃ + 3CO → 2Fe + 3CO₂ (reduction)

Material Balance

Input (per ton of hot metal)	Amount (kg)
Iron ore	1600-1800
Coke	300-400
Limestone	200-300
Hot blast	1400-1600 Nm3

Heat Balance

• Heat input: Coke combustion (75-80%), Hot blast (20-25%)
• Heat output: Hot metal (35-40%), Slag (10-15%), Top gas (25-30%), Losses (15-20%)

Alternative Iron Making (COREX, MIDREX)

COREX Process

Two-stage smelting reduction using coal instead of coke:

1. Reduction shaft: Pre-reduction to 85-90% metallization
2. Melter-gasifier: Final reduction and melting

MIDREX Process

Direct reduction using natural gas (H₂ + CO):

Fe₂O₃ + 3H₂ → 2Fe + 3H₂O

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Process	Reductant	Product	Energy (GJ/ton)
Blast Furnace	Coke	Hot Metal	14-16
COREX	Coal	Hot Metal	16-18
MIDREX	Natural Gas	DRI	10-12

4.4 Slag Properties

Structure and Properties of Slags

Slag is a molten mixture of oxides (CaO, SiO₂, Al₂O₃, MgO) that floats on molten metal, protecting it from oxidation and removing impurities.

Basicity Index

V-ratio = %CaO/%SiO₂

B₂ = (%CaO + %MgO)/(%SiO₂ + %Al₂O₃)

Sulfide Capacity

C_S = (%S)(P_O2/P_S2)^1/2

Phosphate Capacity

C_PO4 = (%PO₄^3-)(P_O2^5/4/P_P2^1/2)

Slag Type	Composition	Basicity	Viscosity (Poise)
Blast Furnace	35-45% CaO, 30-40% SiO2	1.0-1.2	5-15
Steelmaking	40-50% CaO, 10-20% FeO	2.0-3.5	0.2-2.0

Metallurgical Coke Production

Coking Process

Destructive distillation of coal at 1000-1100°C in absence of air

Coal

Oven

Coke

Coke Properties

Property	Value
Fixed Carbon	85-90%
Ash	8-12%
Volatile Matter	1-2%
CSR (Coke Strength after Reaction)	60-65%

4.5 Steel Making Processes

Primary Steel Making

Basic Oxygen Furnace (BOF)

Process dynamics:

• Oxygen (99.5% pure) blown at supersonic speed (Mach 2)
• Temperature: 1600-1650°C
• Process time: 40-50 minutes

Key Oxidation Reactions

C + ½O₂ → CO

Si + O₂ → SiO₂ (ΔH = -832 kJ/mol)

Mn + ½O₂ → MnO (ΔH = -385 kJ/mol)

Electric Arc Furnace (EAF)

Uses electrical energy (400-700 kWh/ton) to melt scrap:

Power input = √3 × V × I × cosφ

Secondary Steel Making

Ladle Processes

Process	Purpose	Reagents
Deoxidation	Remove dissolved oxygen	Al, FeSi, FeMn
Desulfurization	Reduce sulfur content	CaO, CaC2, Mg
Argon Stirring	Homogenize temperature and composition	Argon gas (2-10 Nm3/min)

Degassing Methods

• RH (Ruhrstahl-Heraeus): Recirculation degassing
• VD (Vacuum Degassing): Ladle degassing
• DH (Dortmund-Hörder): Lift degassing

Inclusion Shape Control

Calcium treatment modifies alumina inclusions:

3Ca + Al₂O₃ → 3CaO + 2Al

Forms liquid calcium aluminate inclusions (12CaO·7Al₂O₃)

Stainless Steel Manufacturing

Key Processes

1. Melting: EAF or AOD (Argon Oxygen Decarburization)
2. Decarburization: Reduce carbon while retaining chromium
3. Alloying: Add Ni, Mo, etc. for required properties

AOD Process

Uses mixed gas (O₂ + Ar/N₂) to reduce carbon without excessive chromium oxidation:

[C] + [O] → CO_(g)

3[C] + Cr₂O₃ → 2[Cr] + 3CO_(g)

Grade	Composition	Properties
304	18Cr-8Ni	General purpose
316	16Cr-10Ni-2Mo	Marine applications
430	17Cr	Ferritic, low cost

4.6 Continuous Casting

Fluid Flow in Tundish and Mold

Tundish Flow Control

Flow modifiers used to:

• Increase residence time (3-5 minutes)
• Promote inclusion floatation
• Minimize short-circuiting

Mold Flow Patterns

Froude number = v²/gD (should be 0.2-0.4)

Where v = nozzle exit velocity, D = nozzle diameter

Meniscus Control

• Level fluctuations < ±3 mm
• Mold powder consumption: 0.3-0.6 kg/ton
• Oscillation marks: 3-5 mm pitch

Heat Transfer in the Mold

Heat Flux

q = k(ΔT/δ)

Where δ = slag film thickness (1-2 mm), k = thermal conductivity (1-2 W/m·K)

Solidification

Shell thickness = K√t

Where K = solidification constant (20-30 mm/min^1/2), t = time in mold (1-2 min)

Parameter	Slab	Bloom	Billet
Mold length (mm)	900	800	700
Withdrawal speed (m/min)	1.0-1.5	0.8-1.2	2.0-4.0

Segregation and Inclusion Control

Segregation

Segregation ratio = C_max/C₀

Where C_max = maximum concentration, C₀ = bulk concentration

Inclusion Control

• Tundish: Ceramic filters, flow modifiers
• Mold: Electromagnetic braking (EMBr)
• Secondary cooling: Avoid reheating

Clean Steel Practices

• [O] < 20 ppm for most applications
• [N] < 50 ppm for drawing quality steels
• Total inclusions < 0.01% by volume