Froth flotation

Froth flotation is a selective process for separating oxides prior to further refinement. Phosphates and coal are also processed by flotation technology.

The flotation process is also widely used in industrial waste water treatment plants, where it is applied to remove fats, oil, grease and suspended solids from waste water. These units are called natural gas processing plants and similar industrial facilities.

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 invented in 1901 by C.V Potter in Australia and in 1902 by G.D Delprat^[2] in Holland. In the early times, only naturally occurring chemicals such as surfactants and synthetic compounds have been adopted for various applications.

Principle of operation

Froth flotation commences by 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 (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 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:^[3]

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

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 (9.8 meters/second²).

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.^[3]

Flotation equipment

Flotation can be performed in mechanically agitated cells or tanks, in tall flotation columns and in several other units including the Jameson cell.

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. Mechanical flotation cell used for mineral concentration. 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 sometimes 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.

The Jameson cell uses neither impellers nor spargers, instead combining the slurry with air in a downcomer where high shear gives excellent bubble particle contacting.

In dissolved air flotation (DAF), which is used in wastewater treatment, air is dissolved into the flotation solution under high pressure. When the solution is released into the flotation chamber, the reduced pressure causes much of the dissolved air to come out of solution to form bubbles in the same way that dissolved carbon dioxide forms bubbles when a beer bottle is opened.

Mechanics of flotation

The following steps are followed:

Grinding to liberate the mineral particles
Reagent conditioning to achieve hydrophobic surface charges on the desired particles
Collection and upward transport by bubbles in an intimate contact with air or nitrogen
Formation of a stable froth on the surface of the flotation cell
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
Potassium Amyl Xanthate (PAX)	Potassium Isobutyl Xanthate (PIBX)	Sodium Isobutyl Xanthate (SIBX)
Sodium Isopropyl Xanthate (SIPX)	Sodium Ethyl Xanthate (SEX)

Dithiophosphates
Thiocarbamates	Xanthogen Formates	Thionocarbamates
Thiocarbanilide

Frothers

Pine oil	Alcohols (MIBC)	Polyglycols
Polyoxyparafins	Cresylic Acid (Xylenol)

Modifiers

pH modifiers such as:

Lime CaO
Soda ash Na₂CO₃
Caustic soda NaOH
Acid H₂SO₄, HCl

Cationic modifiers:

Ba²⁺, Ca²⁺, Cu⁺, Pb²⁺, Zn²⁺, Ag⁺

Anionic modifiers:

SiO₃^2-, PO₄^3-, CN^-, CO₃^2-, S^2-

Organic modifers:

starch, glue, CMC

Specific ore applications

Sulfide ores
Copper (see copper extraction)	Copper-Molybdenum	Lead-Zinc
Lead-Zinc-Iron	Copper-Lead-Zinc-Iron	Gold-Silver
Oxide Copper and Lead	Nickel	Nickel-Copper
Nonsulfide ores
Fluorite	Tungsten	Lithium
Tantalum	Tin	Coal

v • d • e Sublimation apparatus
Multiphase systems Eutectic

References

^ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants, 1st Edition, John Wiley & Sons Ltd.. LCCN 67019834.
^ Historical Note. Minerals Separation Ltd. Retrieved on 2007-12-30.
^ ^a ^b De Gennes, P. et al. (2004). Capillarity and Wetting Phenomena, 1st Edition, Springer-Verlag New York, Inc.. ISBN 0-387-00592-7.

Categories: Water treatment

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Froth_flotation". A list of authors is available in Wikipedia.