Froth Flotation Process Explained

Patented in the early 1900s, froth flotation transformed mining by making low grade, finely disseminated ores economic. Today it processes billions of tonnes of ore each year and is the main concentration method for copper, lead, zinc, nickel and many other minerals.

The principle: hydrophobic and hydrophilic surfaces

Most minerals are naturally hydrophilic: water wets their surfaces. A few, such as talc, molybdenite and graphite, are naturally hydrophobic. Flotation chemistry selectively makes the target mineral hydrophobic, so that when an air bubble meets it, the water film thins and ruptures and the particle attaches to the bubble. The degree of hydrophobicity is described by the contact angle between the bubble, particle and water: the larger the angle, the stronger the attachment.

For a particle to be recovered it must:

1. Collide with a bubble.
2. Attach by thinning and rupturing the water film.
3. Stay attached while rising, despite turbulence.
4. Remain in the froth and overflow rather than draining back.

Flotation reagents

• Collectors: heteropolar molecules that adsorb onto the target mineral, with their hydrocarbon tail facing outward. Xanthates are the classic sulphide collectors; dithiophosphates, thionocarbamates and fatty acids (for oxide and salt minerals) are others.
• Frothers: surfactants such as MIBC (methyl isobutyl carbinol) and polyglycols that create small, stable bubbles and a froth that holds particles but breaks down after overflowing.
• Modifiers:
  • pH regulators such as lime and soda ash.
  • Depressants that keep unwanted minerals hydrophilic, for example lime or cyanide to depress pyrite, starch to depress iron oxides, or sodium silicate for silicates.
  • Activators that help a mineral accept collector, such as copper sulphate for sphalerite (zinc).
  • Dispersants to prevent slime coating.

Reagent dosages are small, often grams per tonne of ore, but must be carefully controlled.

Download diagram (WebP)

Flotation equipment

Mechanical cells

A rotating impeller inside a stator at the bottom of a tank disperses air into fine bubbles and keeps particles in suspension. Air is supplied by a blower (forced air) or drawn in by the impeller (self-aspirating). Modern tank cells exceed 300 m³ in volume.

Flotation columns

Tall columns, often 10 to 14 m high, with air spargers at the bottom and wash water sprayed on the froth to rinse out entrained gangue. Columns produce cleaner concentrates and are common in cleaner stages.

Other designs

Jameson cells and similar high intensity contactors mix air and slurry in a downcomer for fast kinetics; fluidised bed cells recover coarse particles.

Flotation circuits

A single flotation stage cannot achieve both high grade and high recovery, so cells are arranged in banks and circuits:

• Roughers: first stage, aim to recover most of the valuable mineral at moderate grade.
• Scavengers: treat rougher tailings to recover remaining values.
• Cleaners and recleaners: upgrade rougher concentrate to final grade, often after regrinding to improve liberation.
• Middlings and cleaner tailings are recycled to earlier stages.

Multi-metal ores use sequential or differential flotation, for example floating lead first while depressing zinc, then activating and floating zinc.

Grade and recovery

With feed grade f, concentrate grade c and tailings grade t, the two-product formula gives recovery R without needing to measure mass flows:

$$R = \frac{c\,(f - t)}{f\,(c - t)} \times 100\%$$

The two-product formula is simply a material balance on total mass and on copper.

Factors affecting froth flotation performance

• Particle size and liberation: coarse particles detach from bubbles and fine slimes float poorly or entrain; the target grind size is set by mineralogy.
• Reagent scheme and dosing points.
• Pulp density (percent solids), pH, air rate, froth depth and residence time.
• Water quality: dissolved ions from recycled water can affect chemistry.
• Ore variability from different parts of the mine.

Tailings and water

Flotation tailings are mostly finely ground gangue with residual reagents and minor sulphides. They are thickened to recover water for reuse in the plant, then pumped to tailings storage facilities or filtered and dry stacked. Sulphide rich tailings can generate acid drainage if exposed to air and water, so they may be kept saturated, covered or separately managed. Water recycling also brings residual reagents and dissolved salts back to the circuit, so water chemistry is monitored as carefully as ore feed.

Process control

Plants measure feed rate, slurry density, pH, levels in each cell, air flows and on-stream assays by X-ray fluorescence. Froth cameras analyse bubble size, colour and speed. Expert systems and advanced control adjust air, levels and reagents to stabilise grade and recovery. Reliable level measurement of the pulp and froth interface and flow measurement of slurry and air are central.

Where flotation fits

Flotation follows crushing and grinding in size reduction equipment and is followed by thickening and filtering of concentrate. Tailings are thickened and stored in engineered facilities, a key environmental responsibility of any mine. Ore for flotation comes from methods described in surface vs underground mining, after deposits are found through mineral exploration.