Flotation reagents-collectors

In flotation beneficiation plant, main euipments are important such as ball mill, flotation machine, crushing machine etc. And the flotation reagents are also important for flotation process, the reagents used in flotation are usually divided into three classes:

Collectors: organic chemicals which make the surface hydrophobic and hence the mineral is capable of being collected in the process.

Frothers: organic chemicals which reduce the surface tension of the water to stabilize the bubbles into a froth layer at the top of the flotation cell to make concentrate removal

easier.

Modifiers: organic or inorganic chemicals used to modify the slurry conditions to enhance the difference in surface chemistry between the valuable and gangue minerals.

Let’s introduce collectors in this article

Collectors

Collectors are generally heteropolar organics with a charged polar group and an uncharged non-polar group. The non-polar group is typically a hydrocarbon chain and in the beginning of the flotation process this hydrocarbon chain was provided in the form of oil. This oil flotation was phased out in the early 1920's by the development of more selective organics such as the xanthates and dithiophosphates. Oil in the form of kerosene or diesel is still used as the principal collecting agent in the coal and other industries. The process of collection can be illustrated by taking a clean glass plate and placing a few drops of water onto the surface as shown in Fig. 1.

Fig1. Spreading of water over a hydrophilic clean glass surface.

The water spreads out over the plate because glass is hydrophilic. In Fig. 2, a thin film of oil or grease is smeared over the plate surface and a few drops of water are again placed on the plate. In this case the water shrinks up into droplets as the oil film is hydrophobic. The thin film of oil has converted the hydrophilic glass surface to an hydrophobic surface.

Fig.2. Beading of water on an oiled hydrophilic surface

Minimisation of the system free energy is the driving force for the reduction of the amount of high energy oil/water interface. Thus the water will shrink to minimize the contact area between the water and oil. When an air interface is available as shown in Fig.3, the oil molecule, represented by a stick figure will concentrate at the air interface and extend into the air phase as much as possible. The molecule is represented as aligning perpendicular to the air/water interface. The collectors used in flotation contain a non-polar hydrocarbon chair with a polar group on the chain. The polar group may be ionised and hydrophilic so that the organic may be water soluble. The polar group is also modified to be specifically attracted to certain minerals. At an air/water interface the hydrocarbon chain of the collector will stick out into the air phase at right angles to the interface while the hydrophilic polar group remains in the water phase (Fig.2). If a suitable mineral surface is present and the polar group is attracted to the solid surface, a raft of collector ions will be adsorbed onto the mineral surface, effectively forming a thin film of oil on the mineral surface and hence making the surface hydrophobic, through adsorption rather than a physical smearing (Fig.3B). If an air interface is now provided, in the form of an air bubble, the hydrocarbon chain will extend into the air phase and if the bond strength between the polar group and the mineral surface is strong enough, the particle will be lifted to the surface by the buoyancy of the air bubble (Fig.3C).

Fig3. A: Collector dissolved in the aqueous phase, B: Adsorption onto a mineral surface and C: attachment of an air bubble onto the hydrophobic surface.

Collectors may be non-ionic (hydrocarbon oils) or ionised. The ionised collectors are either cationic, (amines) or anionic (fatty acids or sulphydril compounds such as xanthates or dithiophosphates).
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