Mineral Processing Notes

April 03, 2025

Mineral Beneficiation & Agglomeration

Master the processes that transform raw ores into valuable concentrates

Table of Contents

• 1. Comminution Techniques
• 2. Size Classification
• 3. Concentration Methods
• 4. Agglomeration
• 5. Key Formulas
• 6. Process Diagram

1. Comminution Techniques

Comminution is the process of reducing ore size to liberate valuable minerals from gangue through crushing and grinding, accounting for 30-50% of total mineral processing costs.

1.1 Crushing (Coarse Size Reduction)

Primary Crushing

Jaw Crushers: Reduce run-of-mine ore (150-250mm) to 50-100mm using compressive force between fixed and moving jaws

Gyratory Crushers: For high-capacity primary crushing (5000+ tph), consisting of a conical head gyrating inside a crushing chamber

Secondary Crushing

Cone Crushers: Further reduce to 10-30mm using a gyrating cone inside a bowl liner

Impact Crushers: Best for softer ores, using high-speed impact forces (hammers or blow bars)

High Pressure Grinding Rolls (HPGR): Emerging technology that compresses feed material between two counter-rotating rollers

1.2 Grinding (Fine Size Reduction)

Equipment	Particle Size	Energy Consumption	Key Features
Ball Mills	75-250 µm	10-20 kWh/ton	Steel balls as grinding media, wet/dry operation
Rod Mills	1-5 mm	8-15 kWh/ton	Steel rods as grinding media, prevents overgrinding
SAG Mills	80% <2mm	15-25 kWh/ton	Semi-autogenous, uses ore itself as grinding media
HPGR	1-20 mm	30% less than ball mills	Energy efficient, generates micro-cracks in particles

Bond's Law of Comminution

The energy required to reduce particle size is proportional to the square root of the size reduction ratio:

W = W_i (10/√P₈₀ - 10/√F₈₀)

Where:

• W = Work input (kWh/ton)
• W_i = Work index (material-specific constant)
• P₈₀ = Product size (µm) at 80% passing
• F₈₀ = Feed size (µm) at 80% passing

Typical Work Index Values: Bauxite (8.8), Copper Ore (12.7), Iron Ore (13.5), Gold Ore (15.1)

What is the primary purpose of comminution in mineral processing?

To chemically separate minerals

To liberate valuable minerals from gangue

To increase the ore's chemical reactivity

To remove impurities through heating

Correct Answer: To liberate valuable minerals from gangue

Comminution breaks ore into smaller particles to free valuable minerals from the surrounding waste rock (gangue) for subsequent separation processes. The degree of liberation required depends on the mineral grain size and association with gangue minerals.

Which crushing equipment is most suitable for primary crushing of hard, abrasive ores?

Cone crusher

Gyratory crusher

Impact crusher

Hammer mill

Correct Answer: Gyratory crusher

Gyratory crushers are preferred for primary crushing of hard, abrasive ores due to their high capacity, robust construction, and ability to handle tough materials. They can process up to 12,000 tph and reduce feed sizes up to 1.5m down to 100-200mm.

2. Size Classification

Separating particles by size using screening or hydroclassification methods to ensure proper sizing for downstream processes.

2.1 Screening

Mechanical separation using apertures to sort particles by size:

• Grizzly Screens: Coarse screening (>50mm) using parallel bars or rails
• Trommel Screens: Rotating cylindrical screens for wet/dry materials (5-100mm)
• Vibrating Screens: Most common for 0.5-50mm, with various motion types:
  • Linear motion - for precise cuts
  • Circular motion - for high capacity
  • Elliptical motion - for sticky materials
• Flip-Flow Screens: For difficult-to-screen materials <1mm

Screening Efficiency (%) = [(O_u × (F_f - O_o)) / (F_f × (O_u - O_o))] × 100

Where O_u is undersize in overflow, F_f is undersize in feed, O_o is undersize in underflow

2.2 Hydroclassification

Separation based on particle settling rates in fluid (usually water):

• Hydrocyclones: Centrifugal separation (10-300µm)
  • Feed pressure: 30-70 kPa
  • Cut size range: 10-300µm
  • Capacity: Up to 1000 m³/hr
• Spiral Classifiers: Gravity-based settling (100-1000µm)
  • Single or double pitch designs
  • Slope: 12-18 degrees
• Sedimentation: For very fine particles (<10µm)
• Centrifugal Classifiers: For fine classification (5-100µm)

Cut Size (d₅₀)

The particle size at which 50% reports to overflow and 50% to underflow. Critical parameter for classifier efficiency.

d₅₀ = K × (μ × D_c³ / (Δρ × Q))^0.5

Where:

• K = Cyclone design constant
• μ = Fluid viscosity (Pa·s)
• D_c = Cyclone diameter (m)
• Δρ = Density difference between particle and fluid (kg/m³)
• Q = Volumetric feed rate (m³/s)

Which classification method is most suitable for particles smaller than 300µm?

Grizzly screens

Hydrocyclones

Trommel screens

Vibrating screens

Correct Answer: Hydrocyclones

Hydrocyclones are effective for particle sizes between 10-300µm, while screens are generally used for coarser separations (>0.5mm). Hydrocyclones use centrifugal force to separate particles based on size and density differences.

What happens to the cut size (d₅₀) of a hydrocyclone when feed density increases?

Increases proportionally

Remains unchanged

Increases (coarser separation)

Decreases (finer separation)

Correct Answer: Increases (coarser separation)

As feed density increases, the cut size (d₅₀) increases because the higher solids content reduces the effective separation efficiency, resulting in coarser particles reporting to the overflow.

3. Concentration Methods

Processes that separate valuable minerals from gangue based on physical or chemical properties.

3.1 Froth Flotation

Process that selectively separates hydrophobic minerals from hydrophilic gangue using air bubbles (particle size range: 10-300µm).

Conditioning

Add reagents to modify surface properties:

• Collectors: Make minerals hydrophobic (e.g., xanthates for sulfides)
• Frothers: Stabilize bubbles (e.g., MIBC, pine oil)
• Modifiers: Adjust pH (lime, sulfuric acid) or depress gangue

Flotation

Key parameters:

• Air flow rate: 0.5-2 m³/min/m²
• Impeller speed: 5-10 m/s tip speed
• Residence time: 5-15 minutes
• Pulp density: 25-45% solids

Concentrate Collection

Skim froth to recover mineral concentrate (typically 20-50% grade improvement)

Recovery (%) = (C × c) / (F × f) × 100

Where:

• C = Mass of concentrate
• c = Grade of concentrate
• F = Mass of feed
• f = Grade of feed

Flotation Kinetics

R = R_∞(1 - e^-kt)

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

3.2 Gravity Separation

Uses density differences between minerals (effective when ΔSG >1.0):

• Jigs: Pulsating water flow (particle size: 0.5-50mm)
  • Pulsation rate: 50-300 strokes/min
  • Applications: Coal, tin, tungsten
• Spirals: For 1-3mm particles
  • Slope: 10-20 degrees
  • Capacity: 1-3 tph per start
• Shaking Tables: Precise separation (50µm-3mm)
  • Deck slope: 2-5 degrees
  • Stroke: 10-25mm
  • Frequency: 250-300 rpm
• Centrifugal Concentrators: For fine gold (10-100µm)
  • G-force: 50-200G
  • Feed rate: 1-2 tph

E = (ρ_h - ρ_f) / (ρ_l - ρ_f)

Where E is separation efficiency, ρ_h and ρ_l are heavy/light mineral densities, ρ_f is fluid density

3.3 Magnetic Separation

Separates minerals based on magnetic susceptibility:

• Low Intensity (0.1-0.3T): For ferromagnetic minerals (magnetite, pyrrhotite)
• High Intensity (0.5-2T): For paramagnetic minerals (hematite, ilmenite)
• High Gradient (>2T): For weakly magnetic minerals

Magnetic Force: F_m = μ₀χVH(dH/dx)

Where μ₀ is permeability of free space, χ is susceptibility, V is particle volume, H is field intensity

3.4 Leaching

Chemical dissolution of valuable minerals:

• Cyanidation: For gold/silver (0.05-0.3% NaCN, pH 10-11)
• Acid Leaching: For copper, uranium (pH 1.5-3.5)
• Bacterial Leaching: For sulfide ores (Thiobacillus ferrooxidans)

Gold Dissolution: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH

Which chemical is commonly used as a collector in sulfide mineral flotation?

Sodium silicate

Potassium amyl xanthate

Calcium oxide

Polyacrylamide

Correct Answer: Potassium amyl xanthate

Xanthates are the most common collectors for sulfide minerals, making them hydrophobic for flotation. Sodium silicate is a dispersant, calcium oxide is a pH modifier, and polyacrylamide is a flocculant.

Which gravity separator is most effective for recovering fine gold particles?

Jig

Shaking table

Centrifugal concentrator

Spiral classifier

Correct Answer: Centrifugal concentrator

Centrifugal concentrators use enhanced gravitational forces (50-200G) to recover fine gold particles (10-100µm) that are too small for conventional gravity methods.

4. Agglomeration Techniques

Processes that bind fine particles into larger masses for improved handling and processing.

4.1 Sintering

Blending

Mix iron ore fines (0-10mm) with:

• Coke breeze (5-8%) as fuel
• Flux (10-15% limestone/dolomite)
• Return fines (20-30%)
• Moisture (6-8%)

Ignition

Heat to 1300-1480°C to form partial melts:

• Ignition hood temperature: 1100-1300°C
• Burn-through point: 300-400°C
• Sintering time: 15-30 minutes

Cooling

Produce hard, porous lumps (5-50mm) with:

• Cold crushing strength: 200-300 kg/cm²
• Porosity: 40-50%
• Reducibility index: 60-70%

4.2 Pelletizing

Stage	Process Parameters	Equipment
Balling	Green balls: 9-16mm diameter Moisture: 8-10% Bentonite binder: 0.5-1.5%	Disc or drum pelletizers
Induration	Drying: 100-400°C Firing: 1250-1350°C Cooling: 100-200°C	Grate-kiln or straight grate

Cold Crushing Strength (kg/pellet) = F / (π × r²)

Where F is crushing force (kg), r is pellet radius (cm)

4.3 Briquetting

Process that compacts fines into defined shapes using binders:

• Binder Types:
  • Organic: Pitch, tar, starch (2-10%)
  • Inorganic: Cement, lime (5-15%)
• Pressure: 50-200 MPa
• Applications: DRI, ferroalloys, coal

Agglomeration Benefits

• Improved permeability in blast furnaces
• Reduced dust losses
• Better gas-solid contact
• Increased production rates
• Lower energy consumption compared to direct use of fines

What is the primary purpose of agglomeration in mineral processing?

To chemically purify the ore

To improve handling and furnace performance

To increase the ore's mineral content

To reduce energy consumption in grinding

Correct Answer: To improve handling and furnace performance

Agglomeration converts fines into larger, stronger masses that resist breakdown during handling and allow better gas flow in smelting furnaces, improving reduction efficiency.

Which binder is typically used in iron ore pelletizing?

Portland cement

Bentonite clay

Coal tar

Lime

Correct Answer: Bentonite clay

Bentonite clay (0.5-1.5%) is the standard binder in iron ore pelletizing due to its excellent binding properties and minimal effect on iron content. Other binders are used in different applications.

5. Key Formulas

Comminution

Bond Work Index: W_i = 1.1 × 44.5 × P₁^0.23 × G_bp^0.82 × (10/√P₈₀ - 10/√F₈₀)

Where P₁ is test sieve size (µm), G_bp is grams per revolution

Classification

Screen Efficiency: E = [(O_u × (F_f - O_o)) / (F_f × (O_u - O_o))] × 100

Where O_u is undersize in overflow, F_f is undersize in feed, O_o is undersize in underflow

Flotation

Selectivity Index: SI = (R_a/R_b) × (G_b/G_a)

Where R is recovery, G is grade, a and b are different minerals

Gravity Separation

Concentration Criterion: CC = (ρ_h - ρ_f)/(ρ_l - ρ_f)

Where ρ_h and ρ_l are heavy/light mineral densities, ρ_f is fluid density

6. Interactive Process Diagram

Crushing

Grinding

Classification

Flotation

Agglomeration

Crushing Process

Reduces ore from 150-250mm to 10-30mm using:

• Primary: Jaw or gyratory crushers
• Secondary: Cone or impact crushers
• Energy: 0.5-1.5 kWh/ton

Grinding Process

Further reduces size to 75-250µm using:

• Ball mills (10-20 kWh/ton)
• Rod mills (8-15 kWh/ton)
• SAG mills (15-25 kWh/ton)

Classification

Separates particles by size:

• Screens (>0.5mm)
• Hydrocyclones (10-300µm)
• Spiral classifiers (100-1000µm)

Flotation Process

Separates minerals based on surface chemistry:

• Particle size: 10-300µm
• pH range: 6-11
• Recovery: 80-95%

Agglomeration

Converts fines into usable forms:

• Sintering (5-50mm lumps)
• Pelletizing (9-16mm balls)
• Briquetting (defined shapes)