Froth flotation is the most widely used way to separate valuable minerals from waste rock. It works because some mineral surfaces repel water while others are easily wetted by it. Air bubbles carry the water-repelling particles to the surface, where they are skimmed off as a froth. Flotation produces most of the world’s copper, lead, zinc and nickel concentrates, and it is also used for gold-bearing sulfides, molybdenum, phosphate, potash, lithium, graphite and coal.
This article explains the basics in plain terms. For the detailed chemistry, kinetics and circuit design, see our complete guide to froth flotation.
How Froth Flotation Works, Step by Step
- Grinding: the ore is crushed and ground fine enough to free the valuable mineral grains from the waste rock. Flotation feed is usually ground to a fraction of a millimetre.
- Making a slurry: the ground ore is mixed with water to form a slurry, also called pulp.
- Conditioning: chemicals called reagents are added and stirred in for a short time. They make the target mineral water-repellent and leave the waste rock wettable.
- Adding air: the slurry goes into a flotation cell, where air is blown or drawn in and broken into small bubbles, usually by a spinning impeller.
- Collecting the froth: water-repellent particles stick to the bubbles and rise to form a mineral-rich froth on top of the cell. The froth overflows into a launder and becomes the concentrate.
- Discarding the rest: wetted particles stay in the slurry and leave the bottom of the cell as tailings.
Usually the valuable mineral is floated and the waste sinks. Sometimes the reverse is easier: in many iron ore plants, the silica impurity is floated off and the iron minerals stay behind. This is called reverse flotation.
Why Some Particles Float and Others Sink
A particle floats only if it is hydrophobic, meaning water-repelling. A bubble can then push the water aside and attach to the particle, much as water beads up on a waxed car. A hydrophilic particle stays coated in water, so a bubble cannot grip it.
A few minerals are hydrophobic naturally, including coal, graphite, molybdenite, talc and sulfur. Most valuable minerals, including the copper, lead and zinc sulfides, are not water-repellent enough on their own. Quartz and other silicates are strongly hydrophilic. Flotation therefore relies on chemicals that make only the wanted mineral water-repellent.
Bubbles do not catch every target particle. Some waste particles are also carried into the froth with the water that rises between the bubbles, which is called entrainment. Very fine particles are the most affected. This is one reason a single flotation step is rarely enough.
The Three Main Types of Flotation Reagents
- Collectors coat the target mineral and make it water-repellent. Xanthates are the most common collectors for sulfide minerals. Fatty acids and amines are used for non-sulfide minerals such as phosphate and quartz. Only small amounts are needed, and too much collector starts to float unwanted minerals too.
- Frothers help create small bubbles and a froth that is stable enough to carry the mineral over the lip of the cell but breaks down easily afterwards. MIBC and polyglycol frothers are the most widely used.
- Regulators (also called modifiers) fine-tune the separation:
- Activators help a mineral respond to the collector. For example, copper sulfate makes zinc sulfide (sphalerite) floatable.
- Depressants stop unwanted minerals from floating. For example, lime and cyanide keep pyrite out of copper and lead concentrates.
- pH modifiers such as lime, soda ash and sulfuric acid set the acidity of the slurry, which strongly affects which minerals float. Most sulfide flotation runs in alkaline slurry.
- Dispersants such as sodium silicate keep fine particles, especially clays, from clumping together and coating other particles.
Roughers, Scavengers and Cleaners
A plant uses a series of flotation stages, called a circuit, because one stage cannot give both high recovery and a clean concentrate. Recovery is the share of the valuable mineral that reaches the concentrate. Grade is how pure the concentrate is. Pushing for one usually costs some of the other.
- Roughers treat the fresh feed and recover as much of the valuable mineral as possible.
- Scavengers treat the rougher tailings to recover what the roughers missed.
- Cleaners refloat the rougher concentrate, often two or three times, to reject waste and raise the grade to a saleable level.
- Regrind mills often grind the rougher concentrate or recycled streams finer, to free mineral grains that are still locked with waste.
Ores with more than one valuable mineral, such as copper–lead–zinc ores, use several circuits. They either float each mineral in turn or float a bulk concentrate and separate it afterwards. This is called differential or selective flotation.
Flotation Machines
- Mechanical cells are tanks with an impeller that keeps the solids suspended and breaks the air into bubbles. They are the most common type. Large modern tank cells hold several hundred cubic metres each and are arranged in banks.
- Flotation columns are tall cells with no impeller. Bubbles rise from spargers at the bottom while wash water rinses the froth at the top. They produce very clean concentrates and are often used as cleaners.
- High-intensity cells, such as the Jameson cell, mix the slurry and air hard in a small space and then let the froth separate in a short tank.
- Specialized cells target particles that ordinary cells handle poorly: flash flotation cells in the grinding circuit for coarse, fast-floating minerals such as gold-bearing sulfides, and fluidized-bed cells for coarse particles.
What Affects Flotation Performance
- Grind size and liberation: grains that are not freed from waste cannot be separated cleanly. Very fine particles float slowly and are easily entrained. Very coarse particles tend to fall off the bubbles. Flotation works best on intermediate sizes.
- Reagent type and dose: too little collector loses recovery. Too much costs money and floats unwanted minerals.
- pH: each mineral floats best in a particular pH range, so pH is one of the main levers for selectivity.
- Air rate and froth depth: more air and a shallower froth raise recovery but carry more waste. Less air and a deeper froth give a cleaner concentrate.
- Flotation time: slow-floating particles need more time, which is why plants use banks of cells and scavenger stages.
- Water quality: dissolved salts and leftover reagents in recycled water can change how reagents behave.
- Clays and slimes: fine clays coat other particles and thicken the slurry, which hurts both grade and recovery.
Where Froth Flotation Is Used
- Copper and copper–molybdenum ores: the largest use of flotation, especially at large porphyry mines.
- Lead–zinc and copper–zinc ores: differential flotation produces a separate concentrate for each metal.
- Nickel and platinum ores: sulfide flotation, with depressants to reject talc and other floatable gangue.
- Gold ores: flotation concentrates the gold-bearing sulfides, which are then leached or treated further.
- Iron ore: reverse flotation removes silica to make a higher-grade concentrate.
- Industrial minerals: phosphate, potash, lithium (spodumene), graphite, fluorite and others.
- Coal: fine coal is floated to separate it from ash-forming minerals.
Flotation Circuits at Real Plants
These Global Circuits profiles show how flotation is used at operating mines and planned projects. Each summary is based on the operation’s public technical report.
- Highland Valley Copper (Canada, operating): Bulk copper-molybdenum flotation with rougher and scavenger cells, then regrinding and column cleaning.
- Thompson Creek (United States, restart under construction): Molybdenite floated with fuel oil as the collector, then reground and cleaned.
- Perkoa (Burkina Faso): Zinc flotation, with lime, copper sulfate, xanthate and frother added in a conditioner ahead of the rougher cells.
- Ntaka Hill (Tanzania, project): Nickel-copper flotation, with pre-aeration to suppress pyrrhotite and three cleaner stages.
- North American Lithium (Canada, operating): Spodumene recovered by grinding, desliming, magnetic separation and flotation.
- Vanscoy Potash (Canada): The VAULT expansion design floats potash in separate coarse and fine rougher circuits, then cleans and re-cleans the concentrate.
- Conda (United States): Fine phosphate floated in CavTube flotation columns.
- Lake Giles Iron (Australia, project): Reverse flotation removes silica from a magnetite concentrate.
Browse all profiles tagged Flotation in Global Circuits.
Key Terms
- Concentrate: the mineral-rich froth product.
- Tailings: the waste slurry that does not float.
- Recovery: the percentage of the valuable mineral in the feed that ends up in the concentrate.
- Grade: the metal or mineral content of the concentrate, for example 28% copper.
- Liberation: how completely the valuable mineral grains are freed from waste by grinding.
- Entrainment: unwanted particles carried into the froth with water, rather than attached to bubbles.
- Conditioning: mixing the slurry with reagents before flotation so they have time to act.
Conclusion
Froth flotation separates minerals by making the wanted ones water-repellent and letting air bubbles lift them out of the slurry. Reagents control which minerals float, and grind size, pH, air and froth set how well the separation works. A circuit of roughers, scavengers and cleaners turns a rough first cut into a saleable concentrate. To go deeper into reagent chemistry, flotation kinetics, circuit design and plant control, continue with our complete guide to froth flotation.
Sources and Further Reading
- Wills, B.A. and Finch, J.A. (2016). Wills’ Mineral Processing Technology, 8th edition. Butterworth-Heinemann/Elsevier, Oxford.
- Fuerstenau, M.C., Jameson, G.J. and Yoon, R.-H. (Eds.) (2007). Froth Flotation: A Century of Innovation. SME, Littleton, CO.
- Lynch, A.J. et al. (2010). History of Flotation. Spectrum Series 18. AusIMM, Carlton, Victoria.
- Froth flotation, Wikipedia
- Flotation (ore dressing), Encyclopaedia Britannica
- Froth Flotation 101, 911Metallurgist