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Diagram of a cylindrical froth flotation cell with camera and light used in image analysis of the froth surface.

Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several process industries. Historically this was first used in the mining industry.

Contents

History

William Haynes in 1869 patented a process for separating sulfide and gangue minerals using oil and called it bulk-oil flotation. The modern froth flotation process was independently invented the early 1900s in Australia by C.V Potter and around the same time by G.D Delprat1. Initially, naturally occurring chemicals such as fatty acids and oils were used as flotation reagents in a large quantity to increase the hydrophobicity of the valuable minerals. Since then, the process has been adapted and applied to a wide variety of materials to be separated, and additional collector agents, including surfactants and synthetic compounds have been adopted for various applications.

In the 1960s the froth flotation technique was adapted for deinking recycled paper.

Industries

Mining

Froth flotation is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and waste gangue are increased through the use of surfactants and wetting agents. The selective separation of the minerals makes processing complex (that is, mixed) ores economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphates and coal are also processed upgraded by flotation technology.

Waste water treatment

The flotation process is also widely used in industrial waste water treatment plants, where it is removes fats, oil, grease and suspended solids from waste water. These units are called Dissolved air flotation (DAF) units.2 In particular, dissolved air flotation units are used in removing oil from the wastewater effluents of oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industrial facilities.

Paper recycling

Froth flotation is one of the processes used to recover recycled paper. In the paper industry this step is called deinking or just flotation. The target is to release and remove the hydrophobic contaminants from the recycled paper. The contaminants are mostly printing ink and stickies. Normally the setup is a two stage system with 3,4 or 5 flotation cells in series3.

Principle of operation

Froth flotation commences by comminution (that is, crushing and grinding), which is used to increase the surface area of the ore for subsequent processing and break the rocks into the desired mineral and gangue in a process known as liberation, which then has to be separated from the desired mineral. The ore is ground into a fine powder and mixed with water to form a slurry. The desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical. The particular chemical depends on which mineral is being refined. As an example, pine oil is used to extract copper (see copper extraction). This slurry (more properly called the pulp) of hydrophobic mineral-bearing ore and hydrophilic gangue is then introduced to a water bath which is aerated, creating bubbles. The hydrophobic grains of mineral-bearing ore escape the water by attaching to the air bubbles, which rise to the surface, forming a foam or a scum (more properly called a froth). The froth is removed and the concentrated mineral is further refined.

Science of flotation

To be effective on a given ore slurry, the surfactants are chosen based upon their selective wetting of the types of particles to be separated. A good surfactant candidate will completely wet one of the types of particles, while partially wetting the other type, which allows bubbles to attach to them and lift them into a froth. The wetting activity of a surfactant on a particle can be quantitated by measuring the contact angles that the liquid/bubble interface makes with it. For complete wetting the contact angle is zero.

Another consideration, especially important for heavy particles, is to balance the weight of the particle with the surfactant adhesion and buoyant forces of the bubbles that would lift it.

For typical values of metal densities and surface tensions, if the bubbles are larger than the ore particles, and the particles are or less that 1 mm radius, then particles will rise into the froth layer if:4

\bar{\gamma} R> \rho g R^3

where \scriptstyle R is the radius of the particles, \scriptstyle \bar{\gamma} is the average surface tension between the three pairs of phases (particle, flotation solution, air), \scriptstyle \rho is the mass density of the particles, and \scriptstyle g is the acceleration of gravity (9.81 m/s2).

For particles larger than the bubbles, they too can rise into the froth, each buoyed by a swarm of bubbles, under similar conditions as those expressed in the inequality.4

Flotation equipment

Diagram of froth flotation cell. Numbered triangles show direction of stream flow. A mixture of ore and water called pulp [1] enters the cell from a conditioner, and flows to the bottom of the cell. Air [2] or nitrogen is passed down a vertical impeller where shearing forces break the air stream into small bubbles. The mineral concentrate froth is collected from the top of the cell [3], while the pulp [4] flows to another cell.

Flotation can be performed in rectangular or cylindrical mechanically agitated cells or tanks, flotation columns, Jameson cells or deinking flotation machines.

Mechanical cells use a large mixer and diffuser mechanism at the bottom of the mixing tank to introduce air and provide mixing action. Flotation columns use air spargers to introduce air at the bottom of a tall column while introducing slurry above. The countercurrent motion of the slurry flowing down and the air flowing up provides mixing action. Mechanical cells generally have a higher throughput rate, but produce material that is of lower quality, while flotation columns generally have a low throughput rate but produce higher quality material.

The Jameson cell uses neither impellers nor spargers, instead combining the slurry with air in a downcomer where high shear creates the turbulent conditions required for bubble particle contacting.

Mechanics of flotation

The following steps are followed:

  1. Grinding to liberate the mineral particles
  2. Reagent conditioning to achieve hydrophobic surface charges on the desired particles
  3. Collection and upward transport by bubbles in an intimate contact with air or nitrogen
  4. Formation of a stable froth on the surface of the flotation cell
  5. Separation of the mineral laden froth from the bath (flotation cell)

Simple flotation circuit for mineral concentration. Numbered triangles show direction of stream flow, Various flotation reagents are added to a mixture of ore and water (called pulp) in a conditioning tank. The flow rate and tank size are designed to give the minerals enough time to be activated. The conditioner pulp [1] is fed to a bank of rougher cells which remove most of the desired minerals as a concentrate. The rougher pulp [2] passes to a bank of scavenger cells where additional reagents may be added. The scavenger cell froth [3] is usually returned to the rougher cells for additional treatment, but in some cases may be sent to special cleaner cells. The scavenger pulp is usually barren enough to be discarded as tails. More complex flotation circuits have several sets of cleaner and re-cleaner cells, and intermediate re-grinding of pulp or concentrate.

Chemicals of flotation

Collectors

Xanthates
 
Dithiophosphates

Frothers

Modifiers

pH modifiers such as:

Cationic modifiers:

  • Ba2+, Ca2+, Cu+, Pb2+, Zn2+, Ag+

Anionic modifiers:

  • SiO32-, PO43-, CN-, CO32-, S2-

Organic modifers:

Chemicals for deinking of recycled paper

Specific ore applications

Sulfide ores
  • Copper-Molybdenum  
  • Lead-Zinc  
  • Lead-Zinc-Iron  
  • Copper-Lead-Zinc-Iron  
  • Gold-Silver  
  • Oxide Copper and Lead  
  • Nickel  
  • Nickel-Copper  
Nonsulfide ores
v • d • e
Separation processes
Processes Separation process schematic
Devices
Multiphase systems
Concepts

See also

References

  1. ^ "Historical Note". Minerals Separation Ltd. Retrieved on 2007-12-30.
  2. ^ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st Edition ed.). John Wiley & Sons Ltd.. LCCN 67019834. 
  3. ^ Voith EcoCell flotation plant http://www.voithpaper.com/applications/productsearch/files/594_VPR-PB-07-0001-GB-07.pdf
  4. ^ a b De Gennes, P. et al. (2004). Capillarity and Wetting Phenomena (1st Edition ed.). Springer-Verlag New York, Inc.. ISBN 0-387-00592-7. 
  5. ^ Nellesen, Bernhard & Christina Northfleet, "METHOD OF DEINKING", WO 011717, published 05.02.2004, issued 2004
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