Separation Techniques in Mineral Processing

Terminal Velocity

• A modified Newton's law estimates the terminal velocity of a particle during hindered settling: v = kd(D s − D p 1/ 2

Separation: Classification

• Classifiers separate particles based on their settling velocity.
• Particles with a settling velocity higher than the upward velocity of the fluid reach the bottom of a sorting column as underflow.
• Particles with a settling velocity lower than the upward velocity of the fluid are carried into the overflow.

Separation: Dense Medium Separation (DMS)

• Minerals with different densities are placed in a liquid with an intermediate density.
• Grains with densities less than the liquid float, while grains with densities greater than the liquid sink.

Separation: Flotation

• Flotation involves three phases: solid (ore), liquid (water), and gas (air).
• The success of flotation depends on the interaction of physical, chemical, and mechanical factors.
• Flotation reagents modify the physical-chemical characteristics of the system by adsorbing at interfaces.
• Hydrophilic substances have a greater affinity for water, while hydrophobic substances have a greater affinity for air.
• Many minerals can be made hydrophobic by the adsorption of collectors or frothers.

Separation: Classification (cont.)

• Classification separates minerals based on their size and density.
• Coarse particles settle faster than fine particles of the same density.
• High-density particles settle faster than lower-density particles of the same size.
• Increased density or viscosity of the carrier fluid reduces settling rates of particles.

Separation: Free Settling

• Particles fall through a stationary fluid without influencing each other's rate of fall.
• Occurs when the percentage of solids is less than 15%.
• Forces acting on particles: gravitational force, buoyancy, and viscous drag.
• Stokes's Law applies to particles less than 50 µm in diameter: v = gd2 (D s − D f ) / 18
• Newton's Law applies to particles greater than 0.5 cm in diameter: v =  3gd(D s − D f )  /  D f 1/ 2

Separation: Hindered Settling

• Occurs when the percentage of solids exceeds 15%.
• Crowding causes a decrease in particle falling rate.
• The system behaves as if the particles are falling through a fluid with the density of the pulp.
• Pulp density and viscosity are generally higher than those of the carrier fluid, slowing particles down.

Crushing: Jaw Crusher, Cone Crusher, and Rock Crusher

• Jaw crushers, cone crushers, and rock crushers are used for crushing rocks and ores.

Comminution: Grinding

• Grinding combines impact and abrasion, typically in tumbling mills.
• Optimal mineral recovery and energy consumption require the correct degree of grinding.
• Grinding uses the most energy at a mill.
• Grinding can be done wet or dry.

Grinding: Ball Mill

• Ball mills use a grinding medium of steel rods, steel balls, hard rock, or the ore itself.
• Examples of tumbling mills include rod mills, center peripheral discharge mills, end peripheral discharge mills, overflow mills, ball mills, and autogenous mills.

Separation: Overall Process

• The overall process involves:
  • Comminution (crushing and grinding)
  • Separation (classification, flotation, electrostatic concentration, magnetic concentration, dewatering)
  • Reduction and Refining (pyrometallurgy, hydrometallurgy, electrometallurgy)
• The purpose of separation is to concentrate valuable minerals from gangue.

Separation: Physical Methods

• Physical methods utilize differences in physical properties of minerals.
• Methods include:
  • Dense Medium Separation (based on density)
  • Froth Flotation (based on surface tension)
  • Electrostatic Concentration (based on electrical conductivity)
  • Magnetic Concentration (based on magnetic properties)

Separation: Classification (cont.)

• The velocity of a particle falling through a fluid depends upon its shape, size, and density.