The problem of wastewater treatment has remained relevant for many decades. The difficulty lies in the obsolescence of methods and equipment, as well as the emergence of new chemicals in household chemicals and in production, requiring completely new approaches to their removal from wastewater. One of the universal methods of wastewater treatment is flotation. Depending on the characteristics of the pollutant, it only requires the replacement of reagents and the correction of the process conditions.
Sewage treatment
This method has been successfully applied for the treatment of effluents containing fibers, oil products, oils and fats, as well as other substances which are poorly soluble in water. Pre-waste water is transferred into a suspension and emulsion using special substances.
The flotation process is based on the ability of gas bubbles to attach to particles, helping them float to the surface of the liquid.
General principles of the method
The simplest act of flotation is the attachment of insoluble particles (for example, mineral, oil or any other) to air bubbles. The success of the cleaning process depends on how fast the bond between the particle and the bubbles forms, on the strength of this bond and on the duration of the existence of this complex. Which, in turn, is determined by the nature of the particles, their tendency to wet with water and the peculiarities of their interaction with reagents. Thus, flotation is a process that depends on many factors.
An elementary act can be carried out by one of the following mechanisms:
- bubbles form immediately in suspended particles;
- suspended particles are attached to the gas bubble when they collide with it;
- a small bubble forms on the surface of the particle, which combines with another in a collision and increases in volume.
The complex, which is formed during the flotation process, can emerge in a practically stationary medium only if the lifting force of the gas bubble is greater than the weight of the particle. This will lead to the formation of a foam layer on the surface of the treated water.
In addition, the surface areas of the bubbles and particles at the point of contact should be in a certain ratio. The adhesion forces increase in proportion to the size of the particles squared, since the perimeter of their connection is limited by the size of the largest of their faces. And the separation force directly depends on the mass of the polluting particle (i.e., its linear dimensions in the cube). Thus, upon reaching a certain particle size, the separation forces exceed the adhesion forces. This means that for the successful treatment of wastewater by flotation, not only the nature of the relationship between the suspension and the bubbles, but also their size is important.
Ways to saturate water with bubbles
There are many methods that ensure the appearance of gas bubbles in wastewater. The main methods used during flotation are:
- Compression (or pressure) method based on increasing the solubility of air in water with increasing pressure.
- A mechanical method based on intensive mixing of a liquid with air.
- Passing wastewater through porous materials, which leads to their dispersion.
- An electrical method based on the process of electrolysis of water, accompanied by the appearance of gas bubbles.
- A chemical method that causes bubble formation during chemical reactions of certain reagents with wastewater components.
- Vacuum method, characterized by a decrease in pressure.
Pressure flotation
It is most effective for the extraction of fine and colloidal suspensions of low concentration. The purified water is saturated with air under pressure up to 7 MPa in a special reactor - a saturator. After the exit of water from it, the pressure drops sharply to normal (atmospheric), which provokes an intensive process of the release of air bubbles.
In order to significantly increase the efficiency of water treatment, flotation is combined with coagulation and flocculation. Both of these methods contribute to an increase in the size of undissolved particles. Coagulants are both inorganic compounds, usually salts of ferric iron or aluminum, and some organic substances. Flocculants are special polymers whose molecules in an aqueous medium form a charged network capable of attracting polluting particles, which leads to the appearance of flocculent aggregates.
Installations and technological schemes
Plants that carry out pressure flotation can be placed not only indoors but also outside them. So, the former are suitable for small volumes, if the water flow is not more than 20 m 3 / h, and the latter have much greater productivity. Combined placement of structures is often arranged when large objects, for example, a saturator and a flotation chamber, are in the open air, and pumps are indoors.
In the case of installation of installations in conditions of a possible decrease in air temperature to negative values, a foam heating system must be provided. A classic compression flotation plant consists of the following equipment:
- Pump for supplying liquids.
- A compressor for supplying air (or any gas) to the water treatment system.
- A saturator (its other name is a pressure tank), in which air is dissolved in wastewater.
- Flotamokara, if the process provides for the stage of enlargement of suspended particles.
- Reagent device, including devices for dispensing and mixing reagents with the liquid to be cleaned.
- A system for monitoring the cleaning process.
Technological schemes involving the treatment of wastewater by flotation with increasing pressure can be:
- Direct-flow, when the full volume of the cleaned liquid passes through the saturator.
- Recirculating, when only 20-50% of the clarified liquid passes through the saturator.
- Partially direct-flow, when about 30-70% of the untreated water enters the saturator, and the rest of them is fed directly to the flotamok.
When choosing one of these schemes, the physicochemical properties of the treated effluents, the requirements for the degree of treatment, local conditions and economic indicators are taken into account.
Electroflotation
This method began to be used in the second half of the 20th century. Then it was found that electrolysis gases are much more effective than inert gases or air, increase the intensity of flotation. This allows to isolate water-insoluble petroleum products, lubricating oils, sparingly soluble compounds of heavy and non-ferrous metals, which form stable emulsions in the effluents. But in addition to electrolysis gases, the removal of certain impurities is affected by an artificially created electric field in which charged particles move to oppositely charged electrodes.
A significant disadvantage of electroflotation is the low productivity, high cost of the electrodes, their wear and pollution, and explosion hazard.
Foam fractionation method
It is reduced to the adsorption of dissolved surface-active substances (surfactants) on gas bubbles rising up through the solution. In this case, a foam enriched in an adsorbed substance is intensively formed.
An important area of application of this type of flotation is the treatment of water from detergents used in laundries. It is also suitable for the release of activated sludge, which is formed during biochemical treatment.
Ore dressing
The flotation process is successfully used in the primary processing of all kinds of ores, which allows you to separate a valuable fraction with a high content of metal or its compounds. It is based on differences in the surface properties of the shared minerals.
Ore flotation is a three-phase process:
- the solid phase is a crushed mineral;
- the liquid phase is pulp;
- the gas phase is formed by air bubbles passing through the pulp.
Flotation can be foam, film or oil - depending on the shape of the product formed on the surface of the liquid phase.