Froth flotation collectors definition
The reagents used to change a hydrophilic surface to a hydrophobic one are called froth flotation collectors. This change is accomplished by froth flotation collectors accumulating at the mineral/water interface, a process called adsorption. Froth flotation collectors usually are organic compounds which render selected minerals water-repellent by adsorption of molecules or ions on to the mineral surface. It is also reducing the stability of the hydrated layer separating the mineral surface from the air bubble to such a level that attachment of the particle to the bubble can be on contact.
How froth flotation collectors work?
Hydrophobicity has to impart to most minerals in order to float them. In order to achieve this, surfactants known as collectors are added to the pulp and time is allowed for adsorption during agitation This process known as conditioning period. Froth flotation collectors molecules are hydrocarbons with a polar group of atoms, one end known as a reactive head group. The head group reacts with the particle surface and leaves the hydrocarbon chain pointed away from the surface. This means the particle becomes coated with hydrocarbon. Since hydrocarbons are non-polar and do not form hydrogen bonds with water, the particle surface becomes hydrophobic.
Froth flotation collectors classification
Froth flotation collectors molecules may be ionising compounds, which dissociate into ions in water. Froth flotation collectors can also be non-ionising compounds, which are practically insoluble, and render the mineral water-repellent by coveting its surface with a thin film. This classification as in the figure below.
Ionising froth flotation collectors have found very wide application in flotation. They have complex molecules which are asymmetric in structure and are heteropolar. In short, the molecule contains a non-polar hydrocarbon group and a polar group which may be one of a number of types. The non-polar hydrocarbon radical has pronounced water repellent properties, whereas the polar group reacts with water. Ionising froth flotation collectors are classed in accordance with the type of ion, anion or cation that produces the water-repellent effect in water.
Amphoteric froth flotation collectors possess a cationic and an anionic function, depending on the working pH. This kind of froth flotation collectors have been in use to treat sedimentary phosphate.
Collectors effect and reaction mechanism to the froth flotation
An excessive concentration of a froth flotation collectors can also have an adverse effect on the recovery of valuable minerals. It is possibly due to the development of collector multi-layers on the particles. Those can cause reducing the proportion of hydrocarbon radicals orientated into the bulk solution. The hydrophobicity of the particles is thus minimise, and hence their floatability. The flotation limit may spread without loss of selectivity by using a collector with a longer hydrocarbon chain. In order to aim producing greater water- repulsion, rather than by increasing the concentration of a shorter chain collector.
However, chain length is usually modest to two to five carbon atoms, since the solubility of the collector in water rapidly diminishes with increasing chain length. Although there is a corresponding decrease in solubility of the froth flotation collectors products, which therefore adsorb very readily on the mineral surfaces, it is, of course, necessary for the collector to ionise in water for chemisorption to take place on the mineral surfaces. Not only the chain length but also the chain structure, affects solubility and adsorption (Smith, 1989); branched chains have higher solubility than straight chains.
It is common to add more than one froth flotation collectors to a flotation system. A selective collector may be use at the head of the circuit, to float the highly hydrophobic minerals. Furthermore after, a more powerful, but less selective one, is add on to promote recovery of the slower floating minerals.
Froth flotation collectors for sulphide minerals
For sulphide minerals, the reaction with froth flotation collectors is chemical nature. There are two mechanism by which this can occur. The first is an ion exchange reaction. Example, Xanthate ions may adsorb onto a particle by replacing sulphate ions on the surface. The second and more important mechanism is electrochemical. Electrochemical reactions involve the transfer of electrons from one substance to another.