Sewerage

Method and process of flotation. Pressure flotation. Flotation is...

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 correction of the process conditions.

Sewage treatment

This method is successfully used for containing fibers, oil products, oils and fats, as well as other substances that are poorly soluble in water. Previously, wastewater is 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 considered to be the attachment of insoluble 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 by one of the following mechanisms:

bubbles are formed immediately in suspended particles;
suspension particles are attached to the 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 that is formed in the process of flotation can float in a practically immobile medium only 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. This means that for successful wastewater treatment by the flotation method, not only the nature of the relationship between the suspended matter and bubbles is important, 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.
The passage of wastewater through porous materials, which leads to their dispersion.
An electric method based on the process of electrolysis of water, accompanied by the appearance of gas bubbles.
A chemical method that causes the formation of bubbles during the chemical reactions of certain reagents with wastewater components.
Vacuum method, characterized by a decrease in pressure.

Pressure flotation

It is the most effective for extracting 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 intense process of release of air bubbles.

In order to significantly increase the efficiency of water treatment, flotation is combined with coagulation and flocculation. Both of these techniques 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.

Plants and technological schemes

Installations that carry out pressure flotation can be located not only indoors, but also outside them. So, the first ones are suitable for small volumes, if the water consumption is not more than 20 m 3 / h, and the second ones have a much higher productivity. Combined placement of structures is often arranged, when large objects, for example, a saturator and a flotation cell, are outdoors, and pumps are indoors.

If the units are located 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 supplying liquids.
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.
Flotation chambers, if the process provides for the stage of coarsening of suspended particles.
Reagent device, including devices for dosing and mixing reagents with the liquid to be purified.
Cleaning process control system.

Technological schemes that provide for wastewater treatment by flotation with pressure increase can be:

Direct-flow, when the full volume of the liquid to be purified passes through the saturator.
Recirculation, 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 cell.

When choosing one of these schemes, the physicochemical properties of the treated effluents, the requirements for the degree of purification, local conditions and economic indicators are taken into account.

Electroflotation

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.

A significant disadvantage of electroflotation is the low productivity, high cost of electrodes, their wear and tear and contamination, as well as the risk of explosion.

Foam fractionation method

It comes down to the adsorption of dissolved surfactants (surfactants) on gas bubbles rising up through the solution. In this case, a foam enriched with the adsorbed substance is intensively formed.

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 the separation of activated sludge, which is formed during biochemical treatment.

Ore beneficiation

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.