Flotation
Flotation (formerly referred to as froth flotation) is a common method which is used for separating hydrophobic materials from hydrophilic ones. It is a highly versatile method which is used for physically separating particulate mixtures based on their inherent properties. One major property exploited by this technique is the difference in the ability of air bubbles to selectively adhere to specific material or mineral surfaces in a mineral/water slurry.
Flotation is a process which was first used in the mining industry. It is used to extract a certain type of useful mineral from its ore while simultaneously depressing the amount of useless or undesired minerals in the extracted concentrate. This is usually done by artificially and selectively rendering the useful mineral hydrophobic, using a combination of chemical reagents. This process involves reactions between mineral surfaces and dissolved ions including both sorption onto surfaces and displacement of surface cations (Scientific reports – X-ray spectroscopy, 2009).
The flotation process is important in several industries. Apart from the mining industry, is useful in waste water treatment plants, paper recycling industries, archaeology, biological laboratories, etc. In a lot of industrial waste water treatment plants where a lot of floating solids, fats, oils and grease need to be removed from water, this process is necessary. It is particularly used to remove oil flicks from waste water effluents originating from petrochemical and chemical plants, oil refineries, natural gas processing plants, plastic factories and other similar industrial facilities. Dissolved air flotation units are majorly used in most of these industrial facilities. Also, in other industries where paper is recycles, flotation is an important process which is used to recover recycled paper. This process is also referred to as ‘deinking’. The main essence of deinking is to free and wash off the hydrophobic contaminants from the just recycled paper. Most of the contaminants are oil products such as grease, printing ink and other sticky materials. In the paper recycling industry, the flotation units are in a two stage system with a combination in series of three to five flotation cells (Glembotskii et al., 1972).
The basis of flotation is the difference in hydrophobicity or hydrophilicity of different materials or minerals (Fuerstenau et al, 1985). Materials can naturally be hydrophobic, or the hydrophobicity can be artificially induced by the use of chemical reagents. Usually, the process of flotation starts with crushing and grinding of the mineral. This step, also known as comminution, is used to increase the surface area of the mineral ore. The ore is then grinded into fine powder and water is poured in to it. This mixture is referred to as slurry. The next step is the addition of a chemical reagent such as a surfactant or collector chemical. This chemical artificially induces hydrophobicity in the desired mineral and the type used depends on the characteristics of the desired mineral. Examples of chemicals used include pine oil (used to extract copper), xanthates, dethiophosphates, alcohols, cresylic acid (xylenol), etc. The mixture (slurry) of hydrophobic mineral and other hydrophobic particles is then introduced into a water tank and then aerated with the infusion of air bubbles. The hydrophilic portion remains in suspension because it does not adhere to the air bubbles and can be washed away. The hydrophobic mineral bearing portion of the slurring moves to the surface, forming a scum/foam (froth) which can then be filtered off as a separate product. The separated product can then be concentrated further and refined.
An important aspect of the flotation process is the use of chemical reagents called collectors. These reagents are used to selectively induce hydrophobicity on the surface of a non-hydrophobic molecule. They form a single layer on the particle’s surface that essentially makes a thin film of non-polar hydrophobic hydrocarbon. The collectors greatly increase the contact angle between the surface of the mineral and the air bubble so that the bubbles will adhere to the surface (Froth flotation – Fundamental Principles, 2009). Selection of the collector to be used will depend on the properties of the raw mineral mixture. Collectors are generally classified based on their ionic charges. The classes include anionic collectors, cationic collectors and non-ionic collectors.
Most anionic collectors are weak ionizers, that is, they are weak acids or weak bases which do not dissociate completely in water. When they dissociate in water, they produce a collector which has a negatively charged (anionic) end that attaches to the surface of the mineral and a hydrocarbon chain. The hydrocarbon chain extends into the liquid and imparts hydrophobic properties to the mineral. The use of anionic collectors depends on the minerals to be separated. There are anionic collectors for sulfide minerals and examples include dithiosulphates and various xanthates. Anionic collectors for oxide minerals include sodium oleate.
Cationic collectors attach to the mineral surface by using their positively charged amine groups. Just as opposite charges attract, the positive charge of the amine group influences its attachment to the negatively charged mineral surface, these class of collectors are basically used for the flotation of silicates and some rare metal oxides. They are also used for the separation of sylvite (potassium chloride) from halite (sodium chloride).
The third class of collectors, non-ionic collectors, do not have any charge. They have affinity for surfaces which are partially hydrophobic. They bind to these surfaces in order to increase their hydrophobicity. Examples include most hydrocarbon oils such as diesel, kerosene, fuel oil, etc.
All collectors can either bind chemically to the mineral surface, a process known as chemisorption, or bind to the surface by use of physical forces (physisorption) (Klassen & Mokrousov, 1963). The chemical bond formed by chemisorption is irreversible because the ions undergo a chemical reaction with the surface of the mineral. Chemisorption is highly selective because the chemical bonds formed are specific for a particular set of minerals or elements. There is reversible adsorption in physisorption. This occurs mainly by the use of electrostatic forces of attraction of van der Waals forces. This process can be reversed by altering factors such as pH or concentraton of the solution. Unlike chemisorption, physisorption is less selective.
Flotation is an industrial process which has been used extensively for the separation of materials. It is a highly versatile process which has revolutionalized the face of ore extraction, waste water management and many other industrial processes. It is an important method which is still going to be used over the next thirty years because it is relatively simple and fast. It is also economically cheap when compared with other methods of separation.
Flotation is a process which was first used in the mining industry. It is used to extract a certain type of useful mineral from its ore while simultaneously depressing the amount of useless or undesired minerals in the extracted concentrate. This is usually done by artificially and selectively rendering the useful mineral hydrophobic, using a combination of chemical reagents. This process involves reactions between mineral surfaces and dissolved ions including both sorption onto surfaces and displacement of surface cations (Scientific reports – X-ray spectroscopy, 2009).
The flotation process is important in several industries. Apart from the mining industry, is useful in waste water treatment plants, paper recycling industries, archaeology, biological laboratories, etc. In a lot of industrial waste water treatment plants where a lot of floating solids, fats, oils and grease need to be removed from water, this process is necessary. It is particularly used to remove oil flicks from waste water effluents originating from petrochemical and chemical plants, oil refineries, natural gas processing plants, plastic factories and other similar industrial facilities. Dissolved air flotation units are majorly used in most of these industrial facilities. Also, in other industries where paper is recycles, flotation is an important process which is used to recover recycled paper. This process is also referred to as ‘deinking’. The main essence of deinking is to free and wash off the hydrophobic contaminants from the just recycled paper. Most of the contaminants are oil products such as grease, printing ink and other sticky materials. In the paper recycling industry, the flotation units are in a two stage system with a combination in series of three to five flotation cells (Glembotskii et al., 1972).
The basis of flotation is the difference in hydrophobicity or hydrophilicity of different materials or minerals (Fuerstenau et al, 1985). Materials can naturally be hydrophobic, or the hydrophobicity can be artificially induced by the use of chemical reagents. Usually, the process of flotation starts with crushing and grinding of the mineral. This step, also known as comminution, is used to increase the surface area of the mineral ore. The ore is then grinded into fine powder and water is poured in to it. This mixture is referred to as slurry. The next step is the addition of a chemical reagent such as a surfactant or collector chemical. This chemical artificially induces hydrophobicity in the desired mineral and the type used depends on the characteristics of the desired mineral. Examples of chemicals used include pine oil (used to extract copper), xanthates, dethiophosphates, alcohols, cresylic acid (xylenol), etc. The mixture (slurry) of hydrophobic mineral and other hydrophobic particles is then introduced into a water tank and then aerated with the infusion of air bubbles. The hydrophilic portion remains in suspension because it does not adhere to the air bubbles and can be washed away. The hydrophobic mineral bearing portion of the slurring moves to the surface, forming a scum/foam (froth) which can then be filtered off as a separate product. The separated product can then be concentrated further and refined.
An important aspect of the flotation process is the use of chemical reagents called collectors. These reagents are used to selectively induce hydrophobicity on the surface of a non-hydrophobic molecule. They form a single layer on the particle’s surface that essentially makes a thin film of non-polar hydrophobic hydrocarbon. The collectors greatly increase the contact angle between the surface of the mineral and the air bubble so that the bubbles will adhere to the surface (Froth flotation – Fundamental Principles, 2009). Selection of the collector to be used will depend on the properties of the raw mineral mixture. Collectors are generally classified based on their ionic charges. The classes include anionic collectors, cationic collectors and non-ionic collectors.
Most anionic collectors are weak ionizers, that is, they are weak acids or weak bases which do not dissociate completely in water. When they dissociate in water, they produce a collector which has a negatively charged (anionic) end that attaches to the surface of the mineral and a hydrocarbon chain. The hydrocarbon chain extends into the liquid and imparts hydrophobic properties to the mineral. The use of anionic collectors depends on the minerals to be separated. There are anionic collectors for sulfide minerals and examples include dithiosulphates and various xanthates. Anionic collectors for oxide minerals include sodium oleate.
Cationic collectors attach to the mineral surface by using their positively charged amine groups. Just as opposite charges attract, the positive charge of the amine group influences its attachment to the negatively charged mineral surface, these class of collectors are basically used for the flotation of silicates and some rare metal oxides. They are also used for the separation of sylvite (potassium chloride) from halite (sodium chloride).
The third class of collectors, non-ionic collectors, do not have any charge. They have affinity for surfaces which are partially hydrophobic. They bind to these surfaces in order to increase their hydrophobicity. Examples include most hydrocarbon oils such as diesel, kerosene, fuel oil, etc.
All collectors can either bind chemically to the mineral surface, a process known as chemisorption, or bind to the surface by use of physical forces (physisorption) (Klassen & Mokrousov, 1963). The chemical bond formed by chemisorption is irreversible because the ions undergo a chemical reaction with the surface of the mineral. Chemisorption is highly selective because the chemical bonds formed are specific for a particular set of minerals or elements. There is reversible adsorption in physisorption. This occurs mainly by the use of electrostatic forces of attraction of van der Waals forces. This process can be reversed by altering factors such as pH or concentraton of the solution. Unlike chemisorption, physisorption is less selective.
Flotation is an industrial process which has been used extensively for the separation of materials. It is a highly versatile process which has revolutionalized the face of ore extraction, waste water management and many other industrial processes. It is an important method which is still going to be used over the next thirty years because it is relatively simple and fast. It is also economically cheap when compared with other methods of separation.
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