Froth flotation air supply influences bubble generation and mineral recovery, so airflow must be matched to ore and cell conditions. It relies on differences in particle-surface chemistry rather than particle density alone.
Finely ground ore is mixed with water and flotation reagents to form a slurry. Air is then introduced into the flotation cell, producing bubbles that interact with the suspended particles. Hydrophobic mineral particles attach to the bubbles, rise to the surface and form a mineral-rich froth that can be recovered.
Forced-air flotation cells require a controlled external air supply. A roots blower can provide this air when the required volume and pressure lie within the operating range of positive-displacement rotary-lobe technology.
Roots blowers are not required for every flotation machine. Some mechanically agitated cells are self-aspirating and draw air through the rotor’s pumping action. Large forced-air plants may also use rotary-screw, multistage centrifugal or turbo blowers. Technology selection should be based on airflow, pressure, turndown, efficiency and plant configuration.
Role of Air in Froth Flotation
Air performs several connected functions:
- Creates bubbles for particle attachment
- Provides gas-liquid interfacial area
- Transports attached mineral particles upward
- Supports froth formation
- Influences froth depth and stability
- Affects mineral recovery and concentrate grade
- Influences water and gangue entrainment
Increasing airflow does not always improve flotation. Insufficient air limits the number and surface area of bubbles available for particle capture. Excessive air can cause bubble coalescence, unstable froth, increased entrainment and reduced selectivity.
The optimum airflow must be determined for the specific ore, cell geometry, particle-size distribution and reagent system.
Forced-Air and Self-Aspirating Cells
Forced-Air Flotation Cells
Forced-air cells receive air from an external blower through a header, control valve and cell injection point.
The external air system provides:
- Independent airflow control
- Stable air supply across several cells
- Ability to compensate for pressure changes
- Measurement of individual cell airflow
- Centralized blower operation
- Flexible control of flotation banks
Roots blowers are most relevant to this configuration.
Self-Aspirating Flotation Cells
Self-aspirating cells use the rotating impeller to create a local pressure reduction that draws atmospheric air into the cell.
These cells may not require a separate flotation-air blower during normal operation. However, they can have less independent control of airflow because aeration is linked to rotor speed, slurry conditions and air-path resistance.
Before specifying a roots blower, the flotation-cell aeration principle must be confirmed.
How a Roots Blower Supplies Flotation Air
A roots blower transports an approximately fixed gas volume during each revolution. Two synchronized rotors move air from the inlet toward the discharge without requiring lubricant in the air chamber.
The blower supplies air to a distribution header. Individual branch lines then deliver controlled airflow to each flotation cell or flotation-bank section.
A typical system may include:
- Atmospheric inlet filter
- Roots Blower
- Inlet and discharge silencers
- Non-return valve
- Pressure-relief valve
- Main air header
- Branch isolation valves
- Cell flow-control valves
- Flowmeters
- Pressure transmitters
- Flexible connections
- Control system
The blower handles clean air. Slurry, froth or process liquid must not be allowed to flow backward into the machine.
Superficial Gas Velocity
Flotation airflow is frequently related to cell cross-sectional area using superficial gas velocity:
Superficial Gas Velocity
- Jg = superficial gas velocity
- Qg,cell = actual gas volume entering the cell
- Acell = effective cell cross-sectional area
If Qg,cell is expressed in m3/s and Acell in m2, Jg is expressed in m/s.
The gas volume must correspond to the pressure and temperature at the cell injection point. A normal or standard airflow should not be used directly without conversion.
Superficial gas velocity allows airflow to be compared across cells of different sizes, but it does not fully describe bubble dispersion or flotation performance.
Bubble Size and Surface-Area Flux
Particle recovery depends strongly on the available bubble surface area. A high volume of large bubbles may provide less total surface area than a lower volume of smaller bubbles.
For approximately spherical bubbles, bubble surface-area flux can be estimated as:
Bubble Surface-Area Flux
- Sb = bubble surface-area flux
- Jg = superficial gas velocity
- d32 = Sauter mean bubble diameter
Bubble surface-area flux increases when gas velocity rises or bubble diameter decreases.
Bubble size depends on:
- Frother type and concentration
- Impeller or rotor design
- Air-injection method
- Slurry viscosity
- Solids concentration
- Surface-active contaminants
- Dissolved salts
- Cell hydrodynamics
- Gas rate
The blower controls airflow but does not independently determine bubble size. Cell aeration equipment and slurry chemistry are equally important.
Gas Hold-Up
Gas hold-up represents the fraction of the gas-slurry dispersion occupied by gas:
Gas Hold-Up
- εg = gas hold-up
- Vgas = gas volume within the dispersion
- Vslurry = slurry volume within the same region
Gas hold-up is influenced by airflow, bubble size, slurry properties and cell hydrodynamics.
A higher gas hold-up can increase available bubble surface, but excessive gas can interfere with mixing, promote coalescence or destabilize the froth.
Hydrostatic Pressure at the Air-Injection Point
The roots blower must overcome the hydrostatic pressure at the submerged air-entry point:
Static Pressure at the Air Injection Point
- ΔPstatic = hydrostatic pressure
- ρslurry = slurry density
- g = gravitational acceleration
- hsubmergence = vertical distance below the slurry surface
Slurry density should be used rather than water density.
The actual pressure can change with:
- Slurry solids concentration
- Cell liquid level
- Froth depth
- Feed-density variation
- Mineral type
- Process-water composition
The design should consider the maximum credible combination of slurry density and submergence.
Total Blower Pressure Requirement
The flotation-air blower must overcome the complete system resistance:
Total Blower Pressure Requirement
- ΔPtotal = total pressure requirement
- ΔPstatic = slurry hydrostatic pressure
- ΔPinjection = loss through cell injection equipment
- ΔPpiping = friction and fitting losses
- ΔPvalves = control and isolation-valve losses
- ΔPheader = main header and distribution losses
A reasonable allowance may be included for fouling and operating variation. Excessive pressure margin should be avoided because it increases motor power and can force control valves to dissipate unnecessary pressure continuously.
Airflow at Actual Conditions
Blower capacity must be evaluated at actual inlet conditions:
Convert Standard Flow to Actual Blower Inlet Flow
- Qactual = actual blower inlet volume
- Qstandard = flow at the stated standard conditions
- Tactual = actual absolute inlet temperature
- Tstandard = standard absolute temperature
- Pactual = actual absolute inlet pressure
- Pstandard = standard absolute pressure
- Zactual = actual compressibility factor
- Zstandard = standard compressibility factor
Use absolute temperatures and absolute pressures in the calculation.
All pressures must be absolute and all temperatures must be in kelvin.
The airflow should also be converted to cell-injection conditions when superficial gas velocity is calculated.
Glossary
Bubble surface-area flux: Bubble interfacial area passing through a unit cell area per unit time.
Cell submergence: Depth of the air-injection point below the flotation-cell liquid surface.
Froth depth: Vertical depth of the froth layer above the pulp in a flotation cell.
Froth layer: Upper bubble-rich region that carries floated mineral particles.
Gas hold-up: Fraction of a flotation cell volume occupied by gas bubbles.
Gas rate per cell volume: Air or gas flow supplied per unit volume of a flotation cell.
Self-aspirating cell: Flotation cell that draws air through its own hydrodynamic action.
Superficial gas velocity: Gas flow divided by the empty cross-sectional area of the flotation cell.