What is Froth Flotation Process?
Froth flotation is one of the most popular operational processes for mineral beneficiation. In ore/mineral beneficiation, froth flotation is a method by which commercially important minerals are separated from impurities and other minerals by collecting them on the surface of a froth layer.
Flotation is the process of separation of beneficial minerals from a mixture by creating froth on which minerals separate out. This method of froth floatation is a method of mineral processing in which different minerals are separated selectively. Such ores containing multiple metals such as lead, copper and zinc can be selectively extracted by using froth floatation.
Method of Froth Floatation
The method of froth floatation can be classified into three steps:
1. True floatation – In this process minerals are selectively attached to froth. This process is very critical and important as the extraction of the valuable minerals is decided by this step only while the other two steps determine the separation efficiency between the mineral and the gangue.
2. Entrainment – Under this process of entrainment air bubbles are passed in liquid water which the froth leads to the generation of air bubbles.
3. Aggregation – In this process, the mineral particles are trapped by the froth.
An important criterion of separation of minerals by the froth floatation method is that the size of the particles of the ores must be very small equivalent to powder form. This is very important because the heavier and bigger particle would require a greater adhesive force without which they would no longer attach to the froth and settle down in the bottom. Thus separation will not be possible.
Froth flotation is mainly operated under two common techniques :
1. Direct floatation technique – In this method, the mineral sticks to the air bubble and the remaining gangue settles down at the bottom.
2. Reverse floatation technique – In this method, the gangue sticks to the air bubble and ore particle settle down in the bottom
Basic Principle of Froth Flotation Process
The process of froth floatation starts with the Comminution process in which the surface area of the ore is increased. First of all, the ores are crushed into very fine powder sized particles and mixed with water. The mixture obtained is called Slurry. A Collector which acts as a surfactant chemical is added to the slurry. This is done to enhance the hydrophobic nature of the mineral.
The slurry has now been converted into pulp. This pulp is added in the container filled with water and then air jets are forced into it to create bubbles. The required mineral is repelled by water and thus gets attached to the air bubbles. As these air bubbles rise up to the surface with mineral particles sticking to it, these are called froth. This Froth is separated and further taken for the next process of refining and extraction.
Mechanism of Froth Flotation Process
The basic principle applied in the process of Froth Flotation is the difference in the wetting ability of the ore and remaining impurities. The particles are categorised into two types on the basis of their wetting ability;
If the minerals are of Hydrophobic nature then only can get attracted toward froth and not with water. Once these minerals come to the surface, by the help of buoyant force applied on the froth, the particle-bubble contact will be intact only when there is the formation of a stabilized foam. The deciding factor of the stability of the froth is the strength of the attachment of the bubble to the mineral. This is calculated by the help of YOUNG-DUPRE EQUATION. This equation gives the relation between the strength of attachment and the interfacial energies.
Froth Floatation Column
The setup in which the froth floatation process is carried out is called the froth floatation column.
A common industrial column cell consists of a long cylindrical tank fitted with a feed inlet pipe in the upper portion of the cylinder. Two launders are also connected, one internally and one externally to collect and separate the foam. In the lower portion of the cylinder, an outlet pipe is also connected to remove the slurry and the non -floating material. Pipes for proper drainage and many nozzles for re-pulping are also fitted in the lower section of the column.
Many obstructing panels are also fitted in the column to ensure proper and uniform mixing inside the tank. The number of such panels depend on the geometry and size of the tank. A gas bubble generator system which is utilized for the generation of the bubbles is also fixed at the bottom of the column. A froth washing system, whose purpose is to separate the impurities from the froth, is attached on the top of the tank.
Fig- Froth floatation column.
These methods are extensively utilised for metals of low reactivity generally sulphur compounds. Sulphide ores can be easily wetted by the oils which will float on water. These minerals are first converted into a fine powder and then mixed with water. After that pine oil is poured into this slurry. Then Air bubbles are created by injecting high-pressure air. Thus the sulphide ore comes on the top with the froth and oil. The remaining gangue particles which did not dissolve in oil settle down. The foam is removed and taken for further processing. Thus the minerals are separated by the froth -flotation process. This method is extensively utilized for Copper sulphide, lead sulphide and Zinc sulphide.
List of Chemicals Used
In order to maintain uniform quality of froth and optimise the adhesive quality of the minerals different chemicals are required to be mixed in the slurry.some of such important chemicals are listed below.
- pH Modifier
A collector is such a type of organic compound that selectively attaches to the surface of the minerals and adds water repelling nature to the particles, a very critical factor for adhesion of mineral particles to the air bubble.
Collectors are commonly divided into three types – nonionic, anionic or cationic.
Non-Ionic collectors: These are simple hydrocarbon oils which are needed to increase the water-repelling nature of those minerals which have low hydrophobic strength such as coal. This is done by selective adsorption of oils by the minerals. Examples of non-ionic collectors are Fuel oil and Kerosene oil.
Anionic collectors: These collectors consist of a non-polar part and an ionic part in the anionic part of the compound while the cationic part has no important function enhancement of hydrophobic nature.
Examples: Carboxylates (Oxyhydryls) also known as soaps.
Examples of carboxylates are salts of oleic acid and linoleic acid. Soaps generally are more beneficial compared to other ionic collectors because they have a long chain of fatty acids and can easily dissolve in water. These anionic collectors can be used for the separation of ores of alkali metals and alkaline earth metals like calcium, magnesium, barium, strontium etc.
Cationic collectors: in such collectors, the cationic part of the compound plays a very important role in increasing the surface properties of the mineral. The ionic part is generally the nitrogen of the compound amines. They undergo physisorption and get bonded to the mineral through electrostatic force of attraction. Due to this reason these cationic collectors have low adhesive force.
Frothers These are the group of compounds which help to stabilize the foam. Apart from stabilizing the bubbles they also help in the effective removal of foam and separation of gangue. The desired properties of a typical frother are that it should be able to generate foam so that minerals can be separated. They must be easily soluble in water with a fair degree of homogeneity.
The most commonly used frothers are generally the compounds of carbonyl group, hydroxyl group, amino group and a carboxyl group. Alcohol-based frothers are the best type of frother.
Frothers can be divided into two parts
1. Natural frothers examples – Pine Oil, Cresol etc.
2. Synthetic frothers examples- MIBC [Methyl Isobutyl Carbinol], Cytec Oreprep 549 The natural frothers are generally used less often because of their lesser availability and less productivity.
These reagents activate the mineral surface towards the action of the Collectors, by enhancing their chemical properties. Therefore, they are often called friends of collectors. Generally, they are the easily ionisable soluble salts which react with the mineral surface. A very common example of an activator is in the case of the Sphalerite ore in which zinc is easily separated by the formation of zinc -Xanthate.
These reagents deactivate the mineral surface towards the action of Collectors, by changing their chemical properties. Hence, they are also called the enemies of the Collectors. They increase the Selectivity of flotation, by preventing one mineral from flotation while allowing another mineral to float unrestricted.
Depressants can be classified into two categories.
- Inorganic depressants such as Sodium cyanide, Zinc sulphate etc.
- Organic depressants such as Starch, tannin, Quebracho, Dextrin etc.
pH is also a very important factor in the process of floatation. Even a slight change in the pH of the slurry can result in loss of productivity and efficiency of the operation. Thus to ensure the optimum use of the resources and production is maximum pH modifiers are used. Lime, Sodium carbonate, Sodium hydroxide and Ammonia are often used to maintain the basic nature of the slurry whereas Sulphurous and Sulphuric acids are used to maintain the acidic medium.