Froth flotation

Froth Flotation is a selective process for separating minerals from gangue by using surfactants and wetting agents. The selective separation of the minerals makes processing complex (that is, mixed) orebodies economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphate and coal are also processed by flotation technology.

William Haynes in 1869 patented a process for separating sulfide and gangue minerals using oil and called it bulk-oil flotation. The froth flotation process was invented in 1905 simultaneously by A.H. Higgins in England and by G.A. Chapman in Australia. In the early times, only naturally occurring chemicals such as fatty acids and oils were used as flotation reagents in a large quantity.

Froth flotation commences by comminution, which is used to increase the surface area of the ore for subsequent processing and break the rocks into the desired mineral and gangue (which then has to be separated from the desired mineral); the ore is ground into a fine powder. The desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical; the particular chemical depends on the mineral is being refined - as an example, pine oil is used to extract copper. This slurry 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 rises to the surface, forming a foam. The foam is removed and the concentrated mineral is further refined.

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.

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.

Chemistry of Flotation

Collectors

Xanthates:

Potassium Amyl Xanthate (PAX)
Sodium Isopropyl Xanthate (SIPX)
Sodium Ethyl Xanthate (SEX)

Dithiophosfates.

Thionocarbamates
Xanthogen Formates
Thionocarbamates
Thiocarbanilide

Frothers

Frothers include:

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:

Dextrin, starch, glue, CMC

Sulfide Flotation

Oxide and Silicate Flotation

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)

Flotation Operations

Sulfide Ores:

Copper
Copper-Molybdenum
Lead-Zinc
Lead-Zinc-Iron
Copper-Lead-Zinc-Iron
Gold-Silver
Oxide Copper and Lead
Nickel
Nickel-Copper

Non-Sulfide Ores:

External links

University of Pittsburgh School of Engineering Chemical and Petroleum Engineering Department, Froth Flotation Lab notes.

Metallurgy | Chemical engineering | Unit operations

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