Flotation. General characteristics of the method

There are several types of flotation reagents, differing in their operating principles:

• Gatherers- reagents that selectively adsorb on the surface of the mineral that needs to be converted into foam and impart hydrophobic properties to the particles. Substances whose molecules have a diphilic structure are used as collectors: a hydrophilic polar group, which is fixed on the surface of the particles, and a hydrophobic hydrocarbon radical. Most often, collectors are ionic compounds; Depending on which ion is active, collectors differentiate anionic And cationic types. Less commonly used are collectors that are nonpolar compounds that are not capable of dissociation. Typical collectors are: xanthates and dithiophosphates - for sulfide minerals, sodium soaps and amines - for non-sulfide minerals, kerosene - for coal preparation.
  Gatherers' consumption amounts to hundreds of grams per ton of ore;
• Regulators- reagents, as a result of selective sorption on the surface of the mineral, the latter becomes hydrophilic and incapable of flotation. Salts of inorganic acids and some polymers are used as regulators;
• Foaming agents- designed to improve air dispersion and impart stability to mineralized foams. Foaming agents are weak surfactants.
  The consumption of foaming agents is tens of grams per ton of ore.

Literature

• Meshcheryakov N.F., Flotation machines, M., 1972
• Glembotsky V. A., Klassen V. I., Flotation, M., 1973
• Handbook on ore dressing, M., 1974.
• Klassen V.I., Barsky V.I. Lectures by prof. Krivosheina V. R.

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See what “Flotation” is in other dictionaries:

flotation- and, f. flottation f., English. floatation of letters. ascent. A method of enriching minerals, based on the floating of crushed parts of a mineral onto the surface of a liquid located in an enrichment device. BAS 1. Flotation… … Historical Dictionary of Gallicisms of the Russian Language

- (French flottation, English flotation, lit. swimming on the surface of the water * a. flotation; n. Flotation, Flotatieren, Schaumschwimnaufereitung; f. flottation; i. flotacion) the process of separating small solid particles (main minerals) into … … Geological encyclopedia

Flotation- The process of enrichment of minerals, based on the difference in surface properties and selective contact of mineral particles to the phase interface: liquid gas, liquid liquid, etc. Source ... Dictionary-reference book of terms of normative and technical documentation

- (French flottation from flotter to float on the surface of water), the process of separating small solid particles (mainly minerals), based on their difference in wettability with water. Foam is widely used for the enrichment of mineral resources... ... Big Encyclopedic Dictionary

Extraction of various substances from water with the help of small air bubbles, which carry these substances to the surface of the water, which remain there in the form of foam. Used in wastewater treatment. Ecological Dictionary, 2001 Flotation extraction from water... ... Ecological dictionary

A beneficiation method in which crushed rock is treated with special solutions. In this case, particles of some minerals are wetted and sink, while others are not wetted and are carried away by foam, which makes it possible to get rid of waste rock. See also … Financial Dictionary

Separation, flotation Dictionary of Russian synonyms. flotation noun, number of synonyms: 2 separation (99) ... Synonym dictionary

flotation- A method of separating some minerals from others in a liquid medium, based on the ability of some minerals to stick to air bubbles and move with them into a foam layer, while others remain suspended [Terminological Dictionary... ... Technical Translator's Guide

- (French flottation, from flotter to float on the surface of water), the process of separating small solid particles (mainly minerals) based on differences in their wettability with water. Used for mineral processing... Modern encyclopedia

. (for cement production), magnesite, sand (for glass production), fluoride, etc.

Flotation can also be used to separate water-soluble salts suspended in their saturated solutions [for example, to separate sylvite (KCl) from halite (NaCl)]. Thanks to flotation, deposits of finely disseminated ores are involved in industrial production and the comprehensive use of minerals is ensured. Flotation is also used for cleaning organic matter(petroleum, oils, etc.), finely dispersed sediments of salts and sludge, for the isolation and separation of bacteria, etc.

In addition to the mining and processing industries, flotation is used in the chemical, food and other industries to accelerate settling, separate suspended solids and emulsify organic substances; for the separation of synthetic organics and the separation of ion exchangers loaded with various adsorbates from pulps; when processing paper waste to separate clean cellulose fibers from soiled ones; for cleaning from impurities; for extraction from coke oven gas cooling water; industrial wastewater treatment, etc.

Varieties of the process The widespread use of flotation has led to the emergence of a large number of varieties of the process.

Vacuum flotation. According to this method, proposed by F. Elmore (Great Britain, 1906), a liquid containing solid particles is saturated with gas, which, when reduced, is released from it in the form of small bubbles on the surface of hydrophobic particles.

Flotogravity is a combined mineral processing process that combines flotation and separation of small solid particles under the influence of gravity or in a field of centrifugal forces. The process is carried out in special devices (concentration tables, screw separators, belt sluices, concentrators, sedimentation machines). In them, due to the treatment of the pulp with flotation reagents and the introduction of bubbles into it, so-called aerofloccules of certain minerals are formed, which have a lower density than particles that do not interact with air bubbles. The difference in density created in this case contributes to a more efficient separation of mineral particles, including those of smaller size than with conventional gravitational enrichment. In industry, flotation gravity is used to separate sulfides from tungsten and tin concentrates, as well as to separate zircon from pyrochlore, scheelite from cassiterite, etc.

Ionic developed in the 50s. 20th century (F. Sebba, South Africa) for water purification, as well as the extraction of useful components from dilute solutions. Individual ions, molecules, finely dispersed sediments and colloidal particles interact with flotation reagents-collectors, usually of the cationic type, and are extracted by gas bubbles into a film on the surface of the solution. The method is promising for processing industrial wastewater, mineralized underground thermal and mine waters and sea water.

Electroflotation. To carry it out, the surface of hydrogen and oxygen bubbles released during the electrolytic decomposition of water is used.

A flotation method has also been proposed, according to which bubbles of CO 2 formed as a result of a chemical reaction are introduced into the pulp.

Other flotation methods. Among all the methods, oil flotation was the first to be proposed (1860) (W. Hines, UK). To carry it out, crushed ore is mixed with mineral oil and water; in this case, sulfide minerals are selectively wetted by the oil, float up with it and are removed from the surface of the water, and gangue rocks (quartz, feldspar, etc.) are deposited. In Russia, oil flotation was used for enrichment (Mariupol, 1904). This method was later improved: the oil was dispersed to an emulsion state, which made it possible to extract thin sludge, for example, manganese ores.

The ability of hydrophobic mineral particles to remain on the surface of water, while hydrophilic particles sink in it, was used by A. Nibelius (USA, 1892) and A. McCuisten (Great Britain, 1904) to develop film flotation. In this process, hydrophilic particles fall out from a thin layer of crushed ore located on the surface of a stream of water.

Currently, oil, film and some other flotation methods are practically not used.

Flotation reagents

Flotation reagents - chemical substances(surfactants are most often used), which are added to the pulp during flotation to create conditions for selective separation of minerals. Flotation reagents make it possible to regulate the interactions of mineral particles and gas bubbles, chemical reactions and physical and chemical processes in the liquid phase, at phase boundaries and in the foam layer by hydrophobizing the surface of some and the surface of other solid particles. Based on their purpose, there are three groups of flotation reagents: collectors, foam concentrates and modifiers. By chemical composition flotation reagents can be organic (mainly collectors and foaming agents) and inorganic (mainly modifiers); both of them can be nonionic, slightly or practically insoluble in water, and ionic, highly soluble substances in it.

Gatherers (collectors). The role of these reagents is to selectively hydrophobize (reduce wettability) the surface of certain mineral particles and thereby create conditions for gas bubbles to adhere to them. Hydrophobization is achieved by displacing the hydration film from the surface of the particles. Fixation on it can be due to van der Waals forces (physical adsorption) or the formation of a chemical bond (chemisorption). Based on their structural characteristics, collectors are divided into anionic, cationic, amphoteric and nonionic. Molecules of anionic and cationic reagents contain nonpolar (hydrocarbon) and polar (amino, carboxy, or other) groups. The latter are facing the mineral, are sorbed on the surface of the particles and hydrophobize it, and the non-polar groups are facing water, repel it and prevent hydration of the surface of the particles.

Anionic collectors include compounds that contain sulfhydryl (mercapto-) or hydroxyl groups, as well as their derivatives - the so-called sulfhydryl and oxhydryl reagents. Sulfhydryl reagents are intended for flotation of sulfide ores Cu, Pb, Zn, Ag, Au, Co, Ni, Fe and include xanthates (isopropyl, pentyl and ethyl derivatives), dithiophosphates (dicresyl and diethyl derivatives), mercaptans and their derivatives ( dialkylthionocarbamates). Oxhydryl reagents are used for flotation of carbonates, oxides, sulfates, phosphates, fluorides and some other minerals; These reagents include aliphatic (carboxylic) acids, monoalkyl sulfates, sulfosuccinates, alkane and alkylaryl sulfonates, alkylhydroxamic and alkyl arylphosphonic acids and their salts, alkylaryl esters of phosphoric acids and their salts, sulfonated alkyl monoglycerides.

Cationic collectors, among which the most common are aliphatic primary amines, as well as secondary amines (in kerosene), quaternary ammonium bases and short-chain amino esters, are used for the flotation of potassium salts (mainly KCl when separated from NaCl), quartz, silicates, sulfides, etc.

Amphoteric collectors contain amino and carboxyl groups, due to which they remain active in both acidic and alkaline environments. These collectors are especially effective for flotation of the oxide class in water of high hardness.

Nonionic collectors are represented by non-polar compounds - hydrocarbon liquids mainly of petroleum origin (gas oils, diesel oils, kerosene, etc.), as well as fats, etc. In the form of water, they serve for the flotation of diamonds, potassium salts, molybdenite, native sulfur, talc, coals, phosphates, etc. with a non-polar surface. The combined use of polar collectors with non-polar ones, as well as dispersion, for example using ultrasound, of the latter (which enhances their adhesive fixation on the surface due to physical adsorption) significantly improves the flotation of large particles; Moreover, along with adhesion, flotation is also accompanied by chemical reactions.

Foaming agents (foaming agents), adsorbed at the liquid-liquid interface, reduce surface tension, promote the formation of a stable hydration shell of air bubbles, reduce their size and prevent coalescence, and moderately stabilize mineralized foam. Monohydric aliphatic alcohols (for example, methylisobutylcarbinol), phenol homologues (cresols and xylenols), technical products (fir and pine oils) containing terpene alcohols, monomethyl and monobutyl ethers of polypropylene glycols, polyalkoxyalkanes (for example, 1,1, 1,3-tetraethoxybutane), etc. Some collectors (amines, carboxylic acids) have foaming properties.

Modifiers (regulators) make it possible, enhance, weaken or eliminate the adsorption of collectors on minerals. Thanks to the regulators, the consumption of collectors is reduced, separation with similar densities is achieved, and the enrichment of complex ores with the production of several concentrates is achieved. Modifiers that improve the fixation of collectors on certain surfaces and accelerate flotation are called activators; regulators that make it difficult to secure the collectors - suppressors or depressors.

For the class of oxides, the potential determining factors are H + and OH -; their concentrations are changed by supplying acids, alkalis and soda. For sulfides, the potential-determining metal cations and anions HS - and S 2 - are used. Therefore, a common activator in the flotation of sulfides with sulfhydryl collectors is, for example, Na 2 S. Liquid glass is used as a depressant for the flotation of silicate materials; lime and cyanides suppress the flotation of pyrite, Cu and Zn sulfides, etc. To reduce the negative impact on flotation of micron-sized particles (fine sludge), peptizing reagents (dispersants) are used to separate them; these include inorganic (for example, liquid glass) and organic (dextrin, carboxymethylcellulose, starch, lignosulfonates, etc.) compounds. In addition to those mentioned, there are also pH regulators.

In most cases, flotation reagents have a complex effect (which depends on the natural composition of the mineral surface, pH of the environment, pulp temperature, etc.) and their classification is very arbitrary.

The selectivity of flotation is regulated, along with other factors, by the selection of reagents, the range of which reaches several hundred, and their consumption. As the surface of the floated increases, the consumption of collectors and activators increases. The consumption of foaming agents increases slightly with an increased content of the processed mineral and coarse grinding of the ore. The consumption of depressants increases with increased flotation of suppressed minerals, high concentrations of collectors in the pulp (for example, when separating collective concentrates), as well as when using low-selective collectors containing long-chain molecules. hydrocarbon radicals(eg higher fatty acids and soaps).

The floatable components are not completely extracted if there is a lack of foaming agents, and if there is an excess of them, the selectivity of flotation deteriorates. The average consumption of flotation reagents is low and usually ranges from several g to several kg per 1 ton of ore.

Flotation processes and equipment Ore beneficiation by flotation is carried out at flotation factories, the main equipment of which includes flotation machines, contact tanks and reagent feeders.

Flotation machines are intended for carrying out the actual flotation. They mix solid particles (slurry suspension) and maintain them in suspension; aeration of the pulp and dispersion of air in it; selective mineralization of bubbles by contact with particles treated with flotation reagents; creating a foam layer zone; separation of pulp and mineralization. foam; removal and transportation of enrichment products. The first patent for a flotation machine was issued in 1860; the first industrial models of machines were developed in 1910-14 (T. Hoover and D. Callow, USA).

The widespread use of flotation has led to the creation of different machine designs. Each machine consists of a number of chambers arranged in series with receiving and unloading devices for pulp; Each chamber is equipped with aerating and foam removal devices. There are single- and multi-chamber flotation machines. Single-chamber flotation columns include those in which the height of the chambers exceeds their width by more than 3 times; These devices are used for flotation enrichment of monomineral ores and flotation separation of sludge.

Multi-chamber machines make it possible to implement complex schemes for the enrichment of polymineral ores with the production of several concentrates.

According to the methods of pulp aeration, mechanical, pneumomechanical, pneumohydraulic and pneumatic machines are distinguished. In mechanical machines, particles are weighed (pulp mixing), suction and dispersion is carried out by an aerator or impeller. In contrast to these devices, in pneumomechanical machines (see Fig. for the chamber diagram), air is forced into the impeller zone using a blower. In pneumohydraulic machines, air is dispersed in special aerators. structures (for example, in ejectors) during the interaction of liquid and air jets. In pneumatic machines, air is dispersed when forced through porous partitions.

The operation of mechanical and pneumomechanical machines is largely determined by the design of the impeller, the option of supplying air to it, the features of pumping the pulp by the impeller and its circulation in the chamber. The characteristics of pulp aeration and the hydrodynamic regime in the chamber depend on the method of pumping pulp with an impeller. The latter is also determined by the size of the zone of intensive pulp circulation. Based on this feature, a distinction is made between machines with bottom circulation and circulation throughout the entire volume of the chamber.

The nature of the movement of flows of the pulp-air mixture in the chamber depends on the design of the machine stator (has the form of cylinders or plates), the device for removing mineralized foam from the surface of the pulp (usually a paddle skimmer is used), dampers (prevent the destruction of the foam layer), inter-chamber partitions, the presence of bumpers and the shape chamber (as a rule, it has side walls beveled from below, which eliminates the accumulation of solid particles in the corners and facilitates their movement at the bottom from the walls to the impeller).

The optimal degree of separation when changing the characteristics of the raw material is achieved by changing the amount of air supplied to the chamber, the thickness of the foam layer and pulp level, as well as the performance of the impeller. Average performance of modern mechanical and pneumomechanical machines: pulp flow capacity 0.2-130 m 3 /min; chamber volume from 12-40 m 3 (in Russia) to 30-100 m 3 (abroad). The use of large-volume chambers makes it possible to reduce capital costs by 20-30%, the metal consumption of machines, as well as their energy intensity (reaches 1.5-3.0 kW/m 3).

Compared to mechanical and pneumomechanical machines, pneumatic-hydraulic flotation machines are characterized by higher speed, low capital costs, high productivity, low metal and energy consumption, etc. However, due to the lack of a reliable and durable aerating device, these flotation machines are not yet widely used in the practice of mineral processing.

There are also known machines that are not yet widely used: vacuum and compression (aeration is achieved by releasing dissolved gases from the pulp); centrifugal and jet aerated; electroflotation (aeration of the pulp with bubbles released during electrolysis).

Other equipment. To treat the pulp with flotation reagents, contact vats (conditioners) are used, into which modifiers, as a rule, are first supplied, then collectors and then foaming agents. The contact time of the pulp with the reagents ranges from several seconds to tens of minutes. The reagent flotation regime is determined by the range of flotation reagents and the order of their introduction into the flotation tank. process. The supply of ingredients to the system in specified quantities is ensured by reagent feeders or reagent dispensers.

Main processes and auxiliary operations

Work of enterprises. Flotation processes are divided into direct and reverse. In direct flotation, a useful mineral is recovered into a froth product called a concentrate, a chamber product called waste or tailings, and gangue particles. The latter are recovered into a foam product during reverse flotation

There are also main, cleaning and control flotation operations. The main flotation produces the so-called rough concentrate, from which the finished concentrate is obtained as a result of cleaning flotation. The chamber product of the main flotation (unfloated particles) is subjected to one or more control flotation operations to obtain a waste product (waste).

The chambers of flotation machines are connected in a sequence that allows the above-mentioned operations to be carried out, the circulation of intermediate products and the production of concentrates of the required quality with a given extraction of the useful component. Flotation rates, especially for sulfide ores of non-ferrous metals, reach high level. Thus, from copper ore containing 1.5-1.7% Cu, copper concentrate (35% Cu) is obtained with the extraction of 93% Cu. From copper-molybdenum ore containing about 0.7% Cu and 0.05-0.06 Mo, copper concentrate (25% Cu) with a recovery of 80% Cu and molybdenum concentrate (over 50% Mo) with a recovery of over 70% are produced. Mo. From lead-zinc ore containing about 1% Pb and 3% Zn, lead concentrate containing over 70% Pb (recovery over 90%) and zinc concentrate containing 59% Zn (recovery over 90%), etc. are obtained.

The degree of grinding of the raw material is important for sufficient complete separation, along with the ionic composition of the liquid phase of the pulp, the composition of the gases dissolved in it (the influence of air is especially strong), its temperature and density, the scheme and reagent mode of flotation. Particles with a particle size of 0.15-0.04 mm are best enriched. For the separation of particles finer than 40 microns, the most suitable are flotation columns in which the initial pulp, after mixing with flotation reagents, enters the middle or upper part (below the level of the foam layer), where it meets an upward flow of air bubbles introduced into the lower part.

Due to the counterflow of pulp and air, as well as greater secondary mineralization of the foam layer than in other flotation machines, high selectivity of the process is achieved. For flotation of particles larger than 0.15 mm, Russia has developed foam separation machines, in which the pulp is fed onto a layer of foam that retains only hydrophobized particles, as well as fluidized bed machines with ascending flows of aerated liquid.

In flotation machines, a side process very often occurs, which consists in the deposition of hydrophobic particles on the walls of the chamber. This process, called solid wall flotation, is based on the separation of thin sludges (10 microns or less) using a carrier - hydrophobic particles of flotation size, selectively interacting with the recovered sludge; the resulting aggregates are subjected to conventional froth flotation

In flotation technology, much attention is paid to water quality, which is characterized by the limits of suspended particles, cations and anions, pH, hardness, etc. To achieve the required quality, water is subjected to special conditions. preparation, including removal of suspended particles using coagulants and flocculants, electrochemical. processing, adjustment of the ionic composition by supplying lime, acids, alkalis, etc. (see also Water treatment).

The perfection of flotation, in addition to the quality of the resulting concentrates, the level of extraction of useful components, the consumption of flotation reagents, etc., is also determined by the degree of use of recycled water. For example, in US flotation factories that enrich phosphate ores, with a flow rate of 11.2-84.2 m 3 per 1 ton, the share of water circulation is 66-95%; at phosphate factories former USSR 13.8-35.7 m 3 per 1 ton is consumed with a water rotation of 80-100%.

The target flotation products are sent for dehydration to continuously operating settling tanks-thickeners, hydroseparators and hydrocyclones (40-60% moisture in the condensed product), filters (10-15%) and dryers (1-3% moisture). To accelerate thickening, the pulp is treated with flocculant reagents (polyacrylamide, polysaccharides, etc.) and magnesium. methods.

Flotation at processing plants is carried out as a mechanized, automated continuous process - from receipt to release of concentrates and tailings. Regulation of particle size during grinding, supply of flotation reagents based on their residual concentration in the pulp, continuous analysis of its density, temperature and pH form the basis of automated control of the operation of flotation factories. An important place in them is occupied by internal transport of raw materials and finished products, water and energy supply, labor and environmental protection, etc. The capacity of the largest modern factories for rock mass reaches 50-55 thousand tons per day. One of the first flotation factories in the world was launched in Russia (1904).

Main directions for process improvement

1. Development of drainless systems based on the use of selective flotation reagents that provide separation in water with increased hardness.

2. Wider use of methods of electrochemical activation of flotation through targeted changes in the flotation properties of minerals, regulation of the redox potential and ionic composition of the liquid phase of the pulp.

3. The use of flotation-chemical technologies for processing poor and difficult-to-process ores for the purpose of integrated use of raw materials and environmental protection.

4. Further improvement of the designs of flotation machines with large-capacity chambers, ensuring a reduction in capital and energy costs, by improving the aeration characteristics of the machines, using wear-resistant materials, and automating the main components.

In addition, the improvement of flotation follows the path of synthesizing new flotation reagents, replacing them with other gases (nitrogen, oxygen), as well as introducing systems for controlling the parameters of the liquid phase of the flotation pulp.

Our company produces pressure and electroflotators with insoluble electrodes, under the brand name. The flotators are made of chemically resistant polypropylene and are completely ready for connection and operation. The connection of flotation equipment can be carried out by our company’s personnel or by the customer himself.

We will always promptly help you with the selection of flotation equipment and tell you about the advantages and disadvantages of the model you have chosen.

You can view prices for flotation equipment by clicking on the link

Video of wastewater flotation using our equipment.

Flotation− this is the process of molecular adhesion of particles of floated material to the interface of two phases, usually gas (usually air) and liquid, caused by an excess of free energy of surface boundary layers, as well as surface wetting phenomena. Flotation is used to remove dispersed impurities from wastewater, which spontaneously do not settle well. The process of purifying industrial wastewater containing surfactants (surfactants), oil, petroleum products, oils, fibrous materials by flotation involves the formation of “bubble-particle” complexes, the floating of these complexes and the removal of the resulting foam layer from the surface of the treated liquid. The compaction and destruction of the foam layer can be intensified by heating or using special spray devices. The adhesion of particles contained in it to the surface of a gas bubble is possible only when non-wetting or poor wetting of the particle by the liquid is observed. The formation of a “bubble-particle” complex depends on the intensity of their collision with each other, the chemical interaction of substances, excess air pressure in wastewater, etc. When flotation is used to remove dissolved substances, such as surfactants, the process is called froth separation or froth concentration. The possibility of forming a “bubble-particle” flotation complex, the speed of the process and the strength of the bond, the duration of existence of the complex depend on the nature of the particles, as well as on the nature of the interaction of the reagents with their surface and the ability of the particles to be wetted by water. When a bubble is attached, a three-phase perimeter is formed - a line that limits the area of ​​adhesion of the bubble and is the boundary of three phases: solid, liquid and gaseous. The tangent to the surface of the bubble at the point of the three-phase perimeter and the surface of the solid body form an angle θ facing the liquid, called the contact angle.

The wetting ability of a liquid depends on its polarity, with increasing polarity the liquid’s ability to wet solids weakens. An external manifestation of a liquid’s ability to wet is the value of its surface tension at the boundary with a gaseous medium, as well as the difference in polarities at the boundary of the liquid and solid phases. The flotation process is effective when the surface tension of water is no more than 60 – 65 mN/m. The degree of water wettability of solid or gas particles suspended in water is characterized by the contact angle θ. The larger the angle θ, the more hydrophobic the surface of the particle, thus increasing the likelihood of adhesion to it and the strength of retention of air bubbles on its surface. Such particles have low wettability and float easily. The size, quantity and uniformity of distribution of air bubbles in wastewater are of great importance in flotation. The optimal size of air bubbles is 15–30 microns, and the maximum is 100–200 microns. Thus, the flotation process is as follows: when a rising air bubble in water approaches a solid hydrophobic particle, the layer of water separating them breaks through at a certain critical thickness and the bubble sticks to the particle. The bubble-particle complex then rises to the surface of the water, where the bubbles collect and form a foam layer with a higher concentration of particles than in the original wastewater. Adhesion occurs when a bubble collides with a particle or when a bubble forms from a solution on the surface of a particle. The wettability of the surface of suspended particles is influenced by adsorption phenomena and the presence of surfactant impurities, electrolytes, etc. in water. Surfactants - collecting reagents, adsorbed on particles, reduce their wettability, i.e. make them hydrophobic. Oils, fatty acids and their salts, mercaptans, xanthogenates, dithiocarbonates, alkyl sulfates, amines and other substances are used as collecting reagents. The hydrophobicity of particles can be increased by sorption of molecules of dissolved gases onto their surface. Energy of formation of the “bubble-particle” complex

where σ is the surface tension of water at the interface with air. For particles that are well wetted by water, θ tends to zero; therefore, Cosθ tends to unity, which means that the adhesion strength is minimal. For non-wettable particles, on the contrary, the energy of formation of the “bubble-particle” complex will be maximum. The effect of flotation separation depends on the size and quantity of air bubbles. In this case, a high degree of saturation of water with bubbles or a high gas content in it is necessary. Specific air flow rate decreases with increasing impurity concentration, as the likelihood of collision and sticking increases. Stabilization of bubble sizes during flotation is of great importance. For this purpose, various foaming agents are introduced, which reduce the surface energy of the phase interface. These include pine oil, cresol, phenols. The weight of the particle should not exceed the force of its adhesion to the bubble and the lifting force of the bubbles. The size of particles that float well depends on the density of the material and is equal to 0.2 - 1.5 mm. In the practice of industrial wastewater treatment, various design schemes, techniques and flotation methods have been developed. Flotation is used to treat wastewater from many industries: oil refining, pulp and paper industry, as well as tanning, engineering, food and chemical industries. Flotation is used to separate activated sludge after biochemical treatment. The advantages of flotation are:

• continuity of the process;
• wide range of applications;
• low capital and operating costs;
• simplicity of equipment;
• selectivity for the release of impurities;
• higher process speed compared to settling;
• the possibility of obtaining sludge with lower humidity (90 - 95%);
• high degree of purification (95 - 98%);
• possibility of recovery of removed substances.

Flotation is accompanied by aeration of wastewater, a decrease in the concentration of surfactants and easily oxidized substances, bacteria and microorganisms. This contributes to the success of the following cleaning stages. The most significant fundamental differences between flotation methods are associated with the saturation of the liquid with air bubbles of a certain size. Based on this principle, the following methods for treating industrial wastewater can be distinguished:

— flotation with the release of air from the solution;

- flotation with mechanical air dispersion (impeller, gravity and pneumatic flotation units);

- flotation with air supply through porous materials;

- electroflotation;

- biological and chemical flotation.

Flotation units can consist of one or two compartments (chambers). In single-chamber installations, in the same compartment, the liquid is simultaneously saturated with air bubbles and floating contaminants float to the surface. In two-chamber installations, consisting of a receiving and settling compartment, the formation of air bubbles and “bubble-particle” aggregates occurs in the first compartment, and the floating of sludge (foam) and clarification of the liquid occurs in the second.

Flotation with release of air from solution

It is used for the purification of industrial wastewater containing very small particles of contaminants, since it allows you to obtain the smallest air bubbles. The essence of the method is to create a supersaturated solution of air in the waste liquid. The air released from such a solution forms microbubbles, which float the contaminants contained in the wastewater. The amount of air that must be released from the supersaturated solution and provide the required flotation efficiency is usually 1 - 5% of the volume of wastewater being processed. Depending on the method of creating bubbles, a distinction is made between vacuum, pressure and airlift flotation.

Vacuum flotation(Fig. 2.2).

The advantage of vacuum flotation is that the formation of gas bubbles, their adhesion to contaminant particles and the floating of the resulting “bubble-particle” aggregates occur in a calm environment and the likelihood of their destruction is minimized. The energy consumption for saturating the liquid with air and the formation and grinding of air bubbles are also minimal.

Disadvantages of the method:

— the need to construct sealed tanks;

- complexity of operation of vacuum flotation units;

— limited range of application of vacuum flotation units (the concentration of contaminants in wastewater should not exceed 250 mg/l).

The waste liquid entering the flotation is pre-saturated with air for 1 - 2 minutes in the aeration chamber 1, from where it enters the deaerator 2 to remove undissolved air. Then, under the influence of vacuum (0.02 - 0.03 MPa), the wastewater enters the flotation chamber 3, in which the dissolved air atmospheric pressure is released in the form of bubbles and carries contaminant particles into the foam layer. The residence time of wastewater in the flotation chamber is 20 minutes, and the load per 1 m2 of surface area is about 200 m3/day. The accumulated foam is removed by rotating scrapers into the foam collector. To remove the treated wastewater, the required level difference between the flotation chamber and the receiving tank is ensured, or pumps are installed.

Pressure flotation(Fig. 2.3).

Settings pressure flotation simple and reliable in operation. This method has a wider range of applications, since it allows you to adjust the degree of supersaturation in accordance with the required efficiency of wastewater treatment with an initial concentration of contaminants of up to 4 - 5 g/l or more. To increase the degree of purification, coagulants are added to wastewater. Pressure flotation devices provide 5–10 times less residual contaminant content compared to oil traps and have 5–10 times smaller dimensions. The process is carried out in two stages: saturation of wastewater with air under high pressure and release of dissolved gas under atmospheric pressure. Pressure flotation units have a capacity from 5 to 2000 m3/h. The residence time of water in the pressure tank is 10 - 15 minutes, and in the flotation chamber - 10 - 20 minutes. During pressure flotation (Fig. 2.3), wastewater is supplied through a pipeline by pump 2 to pressure tank 3 (saturator) from receiving tank 1. The suction pipeline has a pipe for air suction. A saturator or pressure tank is used to uniformly dissolve air in wastewater. The volume of the saturator is calculated for the required duration of saturation with air (usually 1 - 3 min) at an excess pressure of 0.15 - 0.4 MPa. The amount of air dissolved in the saturator should be 3 - 5% of the volume of wastewater being treated. Water saturated with air is supplied to the flotation chamber 4, where, at atmospheric pressure, dissolved air is released in the form of bubbles and suspended particles float. The floating mass is continuously removed by mechanisms for raking foam into foam containers. Foam is removed via line III in the upper part of the flotator. Clarified water is discharged from the bottom of the flotator - line IV. The area of ​​the flotation chamber should be taken based on a hydraulic load of 6 - 10 m3/h per 1 m2 of chamber surface area. Flotation duration is 20 minutes. The volume of sucked air is 1.5 - 5% of the volume of purified water. The parameter values ​​depend on the concentration and properties of the contaminants. When designing flotators for wastewater treatment with a flow rate of up to 100 m3/h, rectangular flotators in terms of a chamber with a depth of 1 - 1.5 m are accepted, with a flow rate of more than 100 m3/h - radial flotators with a depth of at least 3 m. The depth of the flotation and settling zones is assigned at least 1.5 m, and the residence time of wastewater in them is 5 and 15 minutes, respectively. According to the schemes (Fig. 2.2 and 2.3), all wastewater entering the flotation is saturated with air. Schemes with recirculation (Fig. 2.4a) and with partial water supply by a pump (Fig. 2.4b) are recommended to be used if preliminary coagulation of wastewater is carried out in order to prevent or reduce the destruction of flocs in the pump. In these schemes, only part of the wastewater is pumped and saturated with air. The working fluid scheme (Fig. 2.4c) is used when there is a high concentration of contaminants in wastewater, when the operation of the flotation unit according to the scheme (Fig. 2.2) is ineffective. Already purified or natural water is used as the working fluid. In this case, the volume of working fluid exceeds the volume of wastewater being treated. Improved flotation in this case occurs due to the preservation of contaminant flakes and their faster floating. The disadvantage of the scheme is the high energy consumption for pumping the working fluid.

Wastewater (Fig. 2.5), saturated with air, enters the flotation chamber 3 from below through a rotating water distributor 2. Air bubbles released from the water float up along with contaminant particles. Using a rotating mechanism 4, the foam is raked into the tray and removed - line IV. The treated water is removed from the bottom of the flotator 1 and poured through vertical channels into the discharge ring tray 5.

Bandwidth of one radial flotator should not exceed 1000 m3/h.

Apply cylindrical flotators, having different diameters, therefore, different performance. Floaters differ in the design of wastewater input and output and the mechanism for collecting foam and its removal. Multi-chamber flotation units are also used. In a multi-chamber installation (Fig. 2.6), contaminated wastewater, accumulating in container 1, is first supplied by pump 2 to hydrocyclone 4, where some of the suspended particles are removed. Then it is sent to the first chamber of the flotator 3, where the waste water is mixed with circulating water from the pressure tank 6, saturated with air, entering through the aerators 7. In the first chamber of the flotator, air bubbles are released, which float the contaminants. After this, the wastewater enters the second chamber and subsequent ones, in which the flotation process also occurs, after mixing the wastewater with purified water. Thus, multi-stage wastewater treatment occurs. Having passed the last chamber of the flotator, the purified water is removed from the installation - line II. The foam is removed by foam skimmers 5. Part of the purified water is supplied by pump 8 to the pressure tank 6, where the air entering the suction line of the pump is dissolved.

If it is necessary to carry out simultaneous processes of flotation and oxidation of contaminants, wastewater is saturated with air enriched with oxygen or ozone. To eliminate the oxidation process, inert gases should be supplied to the flotation instead of air. Pressure flotation is used to purify wastewater from oil, petroleum products, fats, oils, fibrous substances and others.

(Fig. 2.7).

Airlift installations are used for wastewater treatment in the chemical industry. The simplicity of the device and the reduction in energy consumption during airlift flotation to carry out the process by 2 - 4 times, compared with pressure flotation, are the advantages of the method. But the design of the installation requires a significant difference in elevation between the feed tank and waste water and a flotation chamber, which significantly narrows the scope of this method. Waste water from tank 1, located at a height of 20 - 30 m, enters aerator 3 through pipeline 2. Compressed air is also supplied there - line II, which dissolves in water under increased pressure. Rising through the airlift pipeline 4, the liquid is enriched with air bubbles, which are released in the flotator 5. The resulting foam with particles of contaminants is removed by gravity or by scrapers - line III. The clarified water is sent for further purification - line IV.

Flotation with mechanical air dispersion

When a stream of air moves in water, an intense vortex motion is created, under the influence of which air jet breaks up into separate bubbles. There are impeller, gravity and pneumatic flotation. Impeller flotation (Fig. 2.8). Vigorous mixing of wastewater in flotation impeller units creates in it big number small vortex flows, which makes it possible to obtain bubbles of a certain size. The main element of such an installation is the impeller - a small pump-type turbine, which is a disk with radial upward-facing blades. Waste water from receiving pocket 1 flows to impeller 6, into which air is sucked through tube 4. The impeller rotates at the lower end of the shaft, enclosed in a tube through which air is sucked through pipe 4, since when it rotates, a zone of low pressure is formed. Above the impeller there is a stator 3 in the form of a disk with holes for internal water circulation. The water and air mixed by the impeller are discharged through the stator. Grates 7 located around the stator promote finer mixing of air in water. Air bubbles settle over the grate. Foam containing floatable particles is removed by a paddle skimmer. Typically, a flotation unit consists of several chambers connected in series. Impeller diameter 600 - 700 mm. From the first chamber, water flows into the second chamber of the same design, where additional wastewater treatment occurs.

The degree of air dispersion and cleaning efficiency depend on the impeller rotation speed. The higher the impeller speed, the smaller the bubbles and the higher the efficiency of the process. However, at high speeds, flow turbulence increases sharply and flocculent particles can be destroyed, which, on the contrary, will lead to a decrease in the efficiency of the cleaning process. The impeller diameter should be no more than 750 mm. The impeller service area should not exceed the size of a square with a side equal to six impeller diameters. The height of the flotation chamber Hf is assumed to be 1.5 - 3 m, the flotation duration is 15 - 20 minutes.

The use of impeller units is advisable when treating wastewater with a high concentration of undissolved contaminants (more than 2 - 3 g/l) and containing oil, petroleum products and fats. The disadvantage of impeller flotation is the relatively high water content of the foam. This drawback becomes especially significant in cases where the main purpose of flotation is to extract dissolved surfactants, since a large volume of water in the foam forces the creation of additional installations for its processing, which increases the cost of purification as a whole. Impeller flotation units are widely used in the beneficiation of minerals, and are also used for the treatment of wastewater with a high content of suspended particles (at a concentration of more than 2 g/l).

Gravity flotation.

Air dispersion in non-pressure installations occurs due to vortex flows created by the impeller of a centrifugal pump. The flotation scheme is similar to pressure flotation, but it does not have a saturator, which is an advantage of gravity flotation. The bubbles formed in the chamber of the free-flow installation are larger, and therefore the effect of flotation of small particles is reduced. Gravity flotation units are usually used to treat wastewater from grease and wool.

Pneumatic flotation.

Pneumatic flotation units are used for treating wastewater containing dissolved impurities that are aggressive to mechanisms (pumps, impellers, etc.) that have moving parts. Grinding of air bubbles is achieved by introducing air into the flotation chamber through nozzles, which are located on air distribution tubes laid on the bottom of the flotation chamber at a distance of 0.25 - 0.3 m from each other. The diameter of the nozzle holes is 1 - 1.2 mm, the working pressure in front of them is 0.3 - 0.5 MPa, the flotator depth is assumed to be 3 - 4 m. The jet speed at the nozzle exit is 100 - 200 m/s. The required air flow depends on the intensity of aeration, which lies in the range of 15 - 20 m3/h per m2 of flotator flow area.

Flotation with air supply through porous materials

The advantages of this method include relatively low energy consumption, since there are no pumps and impellers, and the simplicity of the design of the flotation chamber. Air is supplied to the flotation chamber through finely porous plates, pipes, and nozzles placed at the bottom of the chamber. The efficiency of flotation depends on the size of the material holes, air flow, flotation duration, and water level in the flotator. The diameter of the holes should be 4 - 20 microns, air flow within 40 - 70 m3/h per 1 m2 of flotator flow area, air pressure 0.1 - 0.2 MPa, flotation duration 20 - 30 minutes, air flow is determined experimentally. The working level of the treated wastewater before flotation is 1.5 - 2 m. The duration of flotation is 20 - 30 minutes. The disadvantage of this method is the possibility of overgrowing and clogging of pores, as well as the difficulty of selecting finely porous materials with holes of the same diameter, ensuring the release of small, similar-sized air bubbles. When air is passed through porous ceramic plates and caps, bubbles are obtained, the size of which is determined by the formula

where R is the radius of the bubbles; r is the radius of holes in the porous material; σ is the surface tension of water.

Pressure to overcome forces surface tension, determined by Laplace's formula

To treat small volumes of wastewater, flotation chambers with porous caps are used (Fig. 2.9a) wastewater is supplied to the upper part of the flotation chamber 1, and air enters through the porous caps 2. The foam is poured through the annular chute 3 and removed from it. Clarified water is discharged through level regulator 4. Installations can have one or several stages. For large volumes of processed wastewater, filter plates are used (Fig. 2.9b), the flotation scheme is similar to the previous one.

Electroflotation

The essence of the electroflotation method of wastewater treatment is the transfer of polluting particles from the liquid to its surface using gas bubbles formed during the electrolysis of wastewater. During the electrolysis of wastewater, hydrogen is released at the cathode and oxygen at the anode. The main role in the flotation process is played by bubbles released at the cathode. The size of the bubbles coming off the electrode surface depends on the contact angle, the curvature of the electrode surface, and its design features. Replacing a plate cathode with a wire cathode leads to a decrease in the size of the bubbles, and, consequently, to an increase in the efficiency of the electroflotator. When soluble electrodes (usually iron or aluminum) are used at the anode, anodic dissolution of the metal occurs, as a result of which iron or aluminum cations pass into the water, leading to the formation of hydroxide flakes. The simultaneous formation of coagulant flakes and gas bubbles in the cramped conditions of the interelectrode space creates favorable conditions for reliable fixation of gas bubbles on the flakes and intensive coagulation of contaminants, which ensures the efficiency of the flotation process. Such installations are called electrocoagulation-flotation installations. With a throughput of up to 10 - 15 m3/h, the installations can be single-chamber, and with a higher throughput - two-chamber horizontal (Fig. 2.10) or vertical type. The calculation of installations for electroflotation and electrocoagulation comes down to determining the total volume Wу of the installation, the volumes WE of the electrode compartment and the flotation chamber Wф:

We will not provide detailed calculations, because... The purpose of this review is to familiarize yourself with flotation technology, and not to specifically calculate installations.

Electrochemical redox processes occurring during electroflotation provide additional disinfection of wastewater. The use of aluminum and iron electrodes causes the transition of aluminum and iron ions into solution, which contributes to the coagulation of the smallest particles of contaminants contained in wastewater.

Biological and chemical flotation

Used to compact sewage sludge. During the flotation of wastewater, foam is formed, which usually has a film-structured structure. Such foam contains a significant amount of water, especially in the lower layers, and its stability and mobility vary depending on the nature of the floated materials. The process of compaction of floating sludge occurs most intensively in the first two hours, then it slows down, and after four hours it practically stops completely. General patterns of compaction of foamy sludge for wastewater of different compositions were derived based on an analysis of compaction graphs. If we take the volume of sludge as a unit at the time when all air bubbles have risen into the foam layer, which in flow-through installations corresponds to a flotation duration of 30 minutes, then the relative volume of sludge after 1; 2; 3 and 4 hours are 0.6, respectively; 0.33; 0.24 and 0.21.

The process of compaction and destruction of the foam layer can be intensified by heating or using special spray devices. In most cases, disposal of foam condensate is not economically feasible. Wastewater treatment by chemical flotation is based on the properties of certain substances, when introduced into wastewater, to release gases (O2, CO2, Cl2, etc.) as a result of a chemical reaction. Bubbles of these gases can stick to undissolved suspended particles and carry them into the foam layer. This phenomenon, for example, is observed when treating wastewater with bleach with the introduction of coagulants. Biological flotation is used to compact sludge from primary settling tanks in the treatment of domestic wastewater. For this purpose, the sediment is heated with steam in a special container to 35 - 55 ° C and kept under these conditions for several days. As a result of the activity of microorganisms, gas bubbles are released, which carry sediment particles into the foam layer, where they are compacted and neutralized. In this way, in 5–6 days the moisture content of the sludge can be reduced to 80% and thereby simplify its further processing. Ion flotation is a process that is carried out as follows: air is introduced into wastewater, breaking it into bubbles in some way, and a collector (surfactant). The collector forms ions in water that have a charge opposite in sign to the charge of the extracted ion. Collector and contaminant ions are concentrated on the surface of gas bubbles and carried into the foam. The foam is removed from the flotation cell and destroyed, and concentrated ions of the substance being removed are extracted from it. This process can be used to extract metals (molybdenum, tungsten, vanadium, platinum and others) from wastewater.

Solution Sheatera Prepared by heating 500 ml of water and 750 g of beet sugar. Thus, a saturated sugar solution is obtained. The solution prepared in this way can be stored in the refrigerator for a long time. The required amount is diluted with water, mixed well and at the same time a hydrometer is used to achieve the required specific density, that is, 1.15 g/cm 3 . To the solution prepared in this way, add 0.7 ml of phenol per 100 ml of solution to prevent mold growth. The solution is poured into a bottle and stored at room temperature or in the refrigerator.

Another commonly used flotation solution is Brez's solution, the specific density of which is 1.25 - 1.30 g/cm3. Its use can contribute to the deformation of thin membranes, especially in protozoa. Therefore, the prepared samples are examined as quickly as possible, because over time the deformation of the shells increases and makes a correct diagnosis impossible.

For cooking Brez solution prepare a saturated solution of magnesium sulfate, which is obtained by dissolving 1 kg of MgSO 4 in 1 liter of hot water and leaving a small excess to crystallize overnight. A saturated solution of sodium thiosulfate (Na 2 S 2 O 3) is obtained by diluting 2 kg of salt in 1 liter of hot water. To prepare the flotation solution itself, mix 3 parts of a saturated magnesium sulfate solution with three parts of a sodium thiosulfate solution and 1 part of water. You can also use another method: 725 g of MgSO 4 are dissolved in 1 liter of water, and 1425 g of Na 2 S 2 O 3 are dissolved in 1 liter of water. The solutions are heated to boiling and left to cool. The next day, the solutions are filtered. After mixing the solutions in a 1:1 ratio, dilute with water to obtain the required specific density of 1.25 - 1.30 g/cm 3 .

To study feces using the flotation method, a sample the size of a walnut is taken, filled with water in a mortar and ground to a pasty consistency. Strain through cheesecloth into a beaker, trying to filter out impurities as much as possible. Pour into centrifuge tubes and centrifuge for 2 - 3 minutes at 1500 - 2000 rpm. Then the supernatant is drained and the selected flotation solution is added to the sediment. The contents of the test tube are thoroughly mixed and shaken. Centrifuge again for 2–3 minutes at 1500–2000 rpm. The test tube is placed in a stand for 10–15 minutes, after which the surface layer is carefully transferred with a loop onto a glass slide and examined under a microscope. During testing, the sample should not dry out.

is the process of molecular “sticking” of particles to the interface between phases, most often gas and water, caused by an excess of free surface energy of surface boundary layers, as well as wetting phenomena. Flotation is used to purify water from suspended solids, petroleum products, oils, fats, and surfactants.

Flotation process

Flotation method(purification) consists of saturating the water with gas (air) bubbles and the formation of particle-gas bubble complexes, the floating of these complexes to the surface of the treated water and the removal of the resulting foam layer from this surface. The formation of the particle-bubble complex, which is the basis of the flotation process, is due to wetting phenomena.

If a drop of water applied to a surface spreads over that surface, then the surface is said to be wetted. If this drop does not spread, but retains an approximately spherical shape, then the surface is considered non-wettable. An example of a wetted surface is the surface of clean glass, while a non-wettable surface is the surface of wax or paraffin. Can the degree of surface wettability be assessed by the contact angle? (Fig. 1.)

Rice. 1. Contact angle

If the contact angle is zero, then the surface is considered absolutely wettable; if it is 180°C, then it is absolutely non-wettable. Absolutely wettable and absolutely non-wettable surfaces do not exist in nature. Therefore, they conditionally accept what at?<90°C, поверхность смачиваема; при?>90°C – non-wettable.

The reasons for wettability and non-wettability of a surface lie in the polar structure of molecules. It is known that water molecules have a polar structure, i.e. have a certain dipole moment. In addition, the molecules of many substances are polar: acids, bases, salts, etc.

If a particle of a substance whose molecules have a polar structure is placed in water, then due to the interaction of polar molecules, this particle will be surrounded by a so-called hydration layer, consisting of water molecules strictly oriented in space (Fig. 2.). Such a particle is called hydrophilic.

Fig.2. Structure of the hydration layer

A more strict orientation of water molecules is observed at the interface. With distance, due to the thermal movement of molecules, this orientation is constantly disrupted. The mobility of water molecules in the hydration layer is highly limited, so it has a number of properties that differ from the properties of water in the bulk. These include increased strength, lower freezing point, such water does not dissolve gases and other substances well. These properties become more pronounced the greater the polarity of the particle molecules.

If the particle consists of molecules with a non-polar structure, then hydration layers are not formed, and the particle is called hydrophobic.

Most important property hydration layers for flotation is their strength. Along with the polarity of molecules, the strength of hydration layers is influenced by the presence of irregularities (protrusions, depressions) on the surface of particles, as well as the adsorption of certain substances (surfactants) that weakly interact with water molecules. Due to the fact that irregularities are a significant obstacle to the interaction of water molecules in the surface layer, rather weak hydration layers can form on particles of substances even with high polarity of molecules, but having a developed surface.

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