The problem of wastewater treatment has been 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 wastewater treatment methods is flotation. Depending on the characteristics of the pollutant, it only requires the replacement of reagents and the correction of process conditions.
Wastewater treatment
This method has been successfully used to treat wastewater containing fibers, oil products, oils and fats, and other substances that are poorly soluble in water. Wastewater is first transferred into suspension and emulsion using special substances.
The flotation process relies on the ability of gas bubbles to attach to particles, helping them float to the surface of a liquid.
General principles of the method
The simplest act of flotation is the attachmentinsoluble particles (for example, mineral, oil or any other) to air bubbles. The success of the purification depends on the rate at which a bond is formed between the particle and the bubbles, 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, the tendency to wetting with water, and the features of their interaction with reagents. Thus, flotation is a process that depends on many factors.
An elementary act can be carried out through one of the following mechanisms:
- bubbles are formed immediately in suspended particles;
- Suspension particles attach to a gas bubble when they collide with it;
- a small bubble is formed on the surface of the particle, which combines with another upon collision and increases in volume.
The complex, which is formed during the flotation process, in a practically immobile medium can only float under the condition that 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 bubbles and particles at the point of contact must be in a certain ratio. Adhesive 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 a cube). Thus, when a certain particle size is reached, the detachment forces exceed the sticking forces. So forSuccessful effluent treatment by flotation is important not only for the nature of the relationship of suspension with bubbles, but also their size.
Ways to saturate water with bubbles
There are many techniques that ensure the appearance of gas bubbles in wastewater. The main methods used in flotation are:
- Compression (or pressure) method based on increasing the solubility of air in water with increasing pressure.
- Mechanical method based on intensive mixing of liquid with air.
- Passing wastewater through porous materials causing them to disperse.
- Electric method based on the electrolysis of water, accompanied by the appearance of gas bubbles.
- A chemical process that causes the formation of bubbles during chemical reactions of certain reagents with wastewater components.
- Vacuum method characterized by pressure reduction.
Pressure flotation
It is the most effective for the extraction of fine and colloidal suspensions of low concentration. Purified water is saturated with air under pressure up to 7 MPa in a special reactor - saturator. After the release of water from it, the pressure drops sharply to normal (atmospheric), which provokes an intensive process of air bubbles.
In order to significantly increase the efficiency of water treatment, flotation is combined with coagulation and flocculation. Both of these approachescontribute to an increase in the size of undissolved particles. Coagulants are both inorganic compounds, usually s alts 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 flow charts
Installations 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 consumption is no more than 20 m3/h, while the latter have a much greater capacity. Combined placement of structures is often arranged, when large objects, for example, a saturator and a flotation cell, are outdoors, and pumps are indoors.
In the case of installations in conditions of a possible decrease in air temperature to negative values, it is necessary to provide a foam heating system. A classic compression flotation plant consists of the following equipment:
- Pump for liquid supply.
- Compressor for supplying air (or any gas) to the water treatment system.
- Saturator (its other name is a pressure tank), in which air is dissolved in wastewater.
- Flotation chambers, if the process provides for the stage of coarsening of suspended particles.
- Reagent device, including devices for dosing andmixing reagents with the liquid to be purified.
- The cleaning process control system.
Technological schemes involving wastewater treatment by pressure flotation can be:
- Cocurrent, when the full volume of the liquid to be purified 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 untreated water enters the saturator, and the rest is fed directly into the flotation chamber.
When choosing one of these schemes, the physical and chemical properties of the treated wastewater, 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 in increasing the intensity of flotation. This makes it possible to isolate water-insoluble petroleum products, lubricating oils, poorly soluble compounds of heavy and non-ferrous metals, which form stable emulsions in wastewater. But in addition to electrolysis gases, the removal of some impurities is influenced by an artificially created electric field in which charged particles move towards oppositely charged electrodes.
Significant disadvantages of electroflotation are low productivity, high cost of electrodes, their wear and contamination, as well as explosion hazard.
Foam fractionation method
It boils down to the adsorption of dissolved surfactants (surfactants) on gas bubbles rising up through the solution. In this case, foam is intensively formed, enriched with the adsorbed substance.
An important area of application for this type of flotation is the purification of water from detergents used in laundries. It is also suitable for separating activated sludge from biochemical treatment.
Ore dressing
The flotation process is successfully used in the primary processing of all kinds of ores, which makes it possible to separate a valuable fraction with a high content of metal or its compounds. It is based on differences in the properties of the surface of the separated minerals.
Ore flotation is a three-phase process:
- solid phase is a crushed mineral;
- liquid phase is pulp;
- the gas phase is formed by air bubbles passing through the pulp.
Flotation can be frothy, filmy or oily, depending on the shape of the product formed on the surface of the liquid phase.
