LECTURE 16

EXTRACTION

16.1. EXTRACTION IN THE LIQUID-LIQUID SYSTEM

16.1.1. GENERAL INFORMATION

Extraction in liquid system - liquid is the process of extracting a dissolved substance or substances from a liquid with the help of a special other liquid that does not dissolve or almost does not dissolve in the first one, but dissolves the extracted components.

A schematic diagram of extraction is shown in Fig. 16.1.1.

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In many cases, extraction is used in combination with rectification. Since the heat consumption for rectification decreases with increasing concentration of the initial solution, preliminary concentration of the solution by extraction makes it possible to reduce the heat consumption for separating the initial mixture.

16.1.2. EQUILIBRIUM IN THE LIQUID-LIQUID SYSTEM

The transition of the distributed substance from one liquid phase (initial solution) to another (extractant) occurs until equilibrium is established, i.e., until the chemical potentials in the phases are equalized. The process involves three components (K=3) and two phases (F=2). According to the phase rule, system variation F=3 . However, the temperature and pressure during the extraction process are usually kept constant. Then the variability of the extraction system will be equal to one.

Consequently, a given concentration of a distributed substance in one phase in a state of equilibrium corresponds to a certain concentration in another.

Equilibrium in extraction processes is characterized by the distribution coefficient φ, which is equal to the ratio of the equilibrium concentrations of the extracted substance in both liquid phases - in the extract and raffinate.

In the simplest systems, dilute solutions are sufficient, obeying the Berthelot - Nernst law; at a constant temperature, the distribution coefficient does not depend on the concentration of the distributed substance and φ = ur/x, Where ur, x- equilibrium concentrations of the distributed substance in the extract and raffinate. In this case, the equilibrium line is straight:

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The distribution coefficient, as a rule, in industrial systems is determined experimentally.

If we consider both liquid phases to be insoluble in each other, then each phase will be a two-component solution. In this case, the extraction process, by analogy with other mass transfer processes, can be depicted in coordinates y- x.

If the liquid phases are partially mutually soluble, each of them will be a three-component solution during extraction. The compositions of three-component mixtures are presented in a triangular coordinate system (Fig. 16.1.2).

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When removing the substance to be distributed M from a mixture N and the points corresponding to the resulting compositions will lie on the straight line RM, and the more diluted the solution is, the closer to the side of the triangle L.E..

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R+E=N,

Where: R, E, N- mass of raffinate, extract, initial mixture, respectively, kg.

According to the rules of leverage we have

https://pandia.ru/text/78/416/images/image007_81.jpg" width="243" height="244 src=">

Rice. 16.1.4. Equilibrium line in a triangle diagram

Compositions of homogeneous two-component solutions M And L And M And E characterized by points on the sides of the diagram L.M. And EAT. Solvents L And E form homogeneous solutions only in small areas LR And EE. Solvent mixture on site RE separates into two homogeneous two-component saturated solutions R(saturated solution E V L) And E(saturated solution L V E). Moreover, the amount of saturated solutions in each of the two layers is determined by the position of the point N and is found according to the lever rule [(see equation (16.1.2)].

When adding a substance M into the composition mixture N a ternary mixture of composition characterized by a point is formed N1 lying on a straight line N.M.. Mixture of the composition https://pandia.ru/text/78/416/images/image009_88.gif" width="92" height="23 src=">. With further addition of the distributed substance to the mixture M2 , M3 , ...we get ternary mixtures of compositions N2 , N3 ..., which also separate into phases with equilibrium compositions R2 and E2, R3 and E3 etc. At the same time, the mass ratios of equilibrium flow rates also change until the moment when one of the phases disappears in the case under consideration with the composition N4. After this, when adding the substance to be distributed M homogeneous ternary solutions of the composition are formed N5 etc. If you connect R 1 and E1, R2 and E2... using straight lines, we obtain equilibrium chords R1 E1,R2 E2,..., corresponding to equilibrium compositions. The equilibrium chords converge at a point TO, called critical. The slope of the equilibrium chord is determined by the nature of the components and the composition of the phases. Connecting the dots characterizing the equilibrium compositions R, R1 R2 , ... and E, E1 E2, ..., a smooth curve, we obtain an equilibrium curve (binodal curve). Branch RK the equilibrium curve characterizes the equilibrium compositions of the solvent phase L, and the branch EC- equilibrium compositions of the solvent phase E.

The binodal curve in the triangular diagram demarcates the regions corresponding to two-phase mixtures (below the binodal curve) and single-phase solutions (outside the binodal curve).

Shown in Fig. 16.1.4 the equilibrium diagram is drawn up for a constant temperature and is called an isotherm.

In practice, we have to deal with components that have partial solubility in certain concentration ranges. According to the behavior of the components, triangular diagrams come with two and three zones of limited solubility.

Temperature also affects the equilibrium of a system. The mutual solubility of components, as a rule, increases with increasing temperature; therefore, the range of existence of heterogeneous systems decreases. With increasing temperature, the binodal curve in Fig. 16.1.4 will approach the axis L.E., while the area under the line RKE will decrease.

16.1.3. MASS TRANSFER DURING EXTRACTION

The kinetic laws of the extraction process are determined by the basic laws of mass transfer.

To increase the surface area of ​​phase contact, one of the phases is dispersed in the form of droplets in another continuous phase. The surface area of ​​the phase contact is determined by the retention of the dispersed phase in the extractor and the average surface-volume diameter of the droplets. The distributed substance diffuses from the continuous phase to the surface of the droplets, and then into the droplet, or, conversely, from the droplet through the phase interface into the continuous phase.

Mass transfer inside droplets is carried out by molecular and convective diffusion. Convection inside the droplets occurs due to the circulation of liquid. The shape and size of the droplets change many times during the extraction process due to dispersion and coalescence. In this case, the surface of the interfacial contact is renewed.

Fick's second law is used to describe mass transfer in extraction processes.

In the general case, when the diffusion resistance in the continuous and dispersed phases cannot be neglected, the mass transfer coefficient is determined by the expressions

https://pandia.ru/text/78/416/images/image005_121.gif" width="12" height="23 src=">.gif" width="17" height="24 src=">. gif" width="55" height="24">. Then the basic mass transfer equation will be rewritten as follows:

If the main diffusion resistance is concentrated in the dispersed phase, i.e..gif" width="113" height="25 src=">.

Mass transfer coefficients in phases are calculated using criterion equations, which are obtained on the basis of experimental data. Criterion equations are given below when describing extractor designs.

The average driving force is calculated taking into account the scale transition factor and introducing its value into the calculation equations.

16.1.4. DIAGRAMS AND CALCULATIONS OF EXTRACTION PROCESSES

In industry, periodic or continuous extraction is used according to the following schemes: single-stage, multi-stage countercurrent and multi-stage with cross-current extractant.

Single stage extraction used in cases where the separation coefficient is high. It can be carried out periodically and continuously according to the scheme shown in Fig. 16.1.5,a. The initial solution is loaded into the mixer apparatus F in quantity L kg solvent concentration Hn and extractant E, which are mixed with a stirrer and then separated into two layers: extract E and raffinate R.

https://pandia.ru/text/78/416/images/image025_42.gif" width="97" height="24 src="> (16.1.5)

Believing that y=φx and extraction module m= E/ L, we obtain the raffinate concentrations

https://pandia.ru/text/78/416/images/image028_36.gif" width="77" height="48 src="> (16.1.7)

At the same time, the degree of extraction

https://pandia.ru/text/78/416/images/image030_37.gif" width="80" height="21">

Let's consider the process of single-stage extraction on triangular and rectangular diagrams (Fig. 16.1.5, b, c). When the initial solution is mixed with the extractant, a ternary mixture is formed, the composition of which is characterized by the point N, located on the mixing line F.E.. After stratification of this mixture, an extract and raffinate are formed, the compositions of which are determined by the points R And E, lying on the equilibrium chord passing through the point N. The extractant module is determined by the lever rule: E/ F= FN/(EN)

Raffinate quantity R= https://pandia.ru/text/78/416/images/image032_34.gif" width="79" height="24">.

The composition of the raffinate is determined by the point RK, and the extract is the point Ek on the side of the triangle L.M..

Extreme values ​​of extractant modules determine points N1 And N2 on the binodal curve: And .

When the initial solution and the extractant are mutually insoluble in the diagram at-X the extraction process is represented by a straight line AB, for constructing from the point hn draw a line at an angle DIV_ADBLOCK13">

Extractant module for obtaining raffinate with a given concentration hk

The greater the module of the extractant, the less the tangent of the angle of inclination and the concentration of the extracted component in the raffinate and extract : And . However, as the module of the extractant increases, the cost of its regeneration increases. The optimal extraction factor values ​​are 1.2< mφ <2.

Multi-stage extraction carried out in multi-section extractors or extraction units, in which each unit represents an independent installation. Multi-stage extraction can be carried out with a countercurrent of the extractant, with a cross-flow of the original solution and the extractant, or a combined method in the presence of several extractants.

Countercurrent extraction can be carried out according to various schemes. For example, in spray, pack and plate extractors, the composition of both phases changes continuously along the length of the apparatus. In other extractors or installations, the composition of both or one phase changes abruptly when moving from section to section.

In multi-section countercurrent installations (Fig. 16.1.6, a) the initial solution F and extractant E come from opposite ends of the installation. An extract with a concentration of the extracted component close to saturation interacts in the first stage with the initial solution F concentration hn. After separating the ternary mixture in the first stage, an extract with a concentration of = yTo and raffinate concentration x1 . Raffinate composition x1 in the second stage it interacts with the extract of composition E3. After separation, a raffinate of the composition is obtained R2 and composition extract E2. In the last nth stage, the raffinate depleted in the extracted component Rn-1 concentration interacts with fresh extractant E concentration DIV_ADBLOCK14">

Let us depict the process of multi-stage countercurrent extraction in a diagram at- X(Fig. 16.1.6, b). To do this, we will create an equation for the working line of the process.

We write the material balance for the entire installation for the extracted component, neglecting the mutual solubility of the solution and the extractant, in concentrations per 1 kg of extractant:

https://pandia.ru/text/78/416/images/image043_26.gif" width="169" height="24 src=">

https://pandia.ru/text/78/416/images/image045_26.gif" width="151" height="41 src=">

which is the equation of a straight line with a tangent of slope

https://pandia.ru/text/78/416/images/image047_21.gif" width="57" height="24"> and to the point.

The position of the kinetic line is determined by the extraction coefficient and the hydrodynamic situation in the apparatus.

The triangular diagram process is shown in Fig. 16.1.6 V.

In the first section of the extraction installation, on the flow of the initial solution of the latter F interacts with the extract from the previous second stage E2 with the formation of a ternary point mixture N1 , after separation of which an extract is obtained in a separator E1 and raffinate Rl in the general case of nonequilibrium composition.

In the second stage, raffinate Rl interacts with the extract from the third stage E3, forming a ternary mixture N2 , which is divided into R2 and E2.

By connecting two points corresponding to the phase compositions at the inlet and outlet of each section with lines FE1,RE2,R2 E3 etc. and continuing them, we get the intersection point R.

Similar processes occur in the remaining sections of the extractor. As a result, the initial solution becomes depleted in the extracted component and leaves the latter nth concentration sections hk, and the extractant is saturated with the component to the final concentration uk.

Extraction with cross-flow extractant can be carried out in several sections continuously (Fig. 16.1.7 ,A) or in one section periodically (Fig. 16.1.7 ,b).

When the process is carried out continuously, the initial solution F is introduced into the first section, in which it is processed with an extractant E, after separation, the raffinate is obtained R1 and extract . Raffinate R1 is introduced into the second section, in which it is again processed with fresh extractant E. Extracts E1 And E2 are removed from the installation, and the raffinate composition R2 enters the next section, where the process is repeated again. As a result, a raffinate of a given composition is obtained Rn and extract of variable composition E1, E2,..., Ep.

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https://pandia.ru/text/78/416/images/image035_26.gif" width="16" height="15 src=">, the tangent of which is determined by the extraction module.

Multi-stage countercurrent extraction is a more efficient process than cross-flow extraction. With countercurrent extraction, a higher average driving force of the process is achieved. Due to the equalization of the driving force at the beginning and end of the installation, a more complete extraction of the component from the solution occurs, while the extraction module is reduced compared to extraction in cross flow, but the required number of contact stages to achieve the same degree of purification.

16.1.5. DESIGNS AND CALCULATIONS OF EXTRACTORS

The efficiency of mass transfer in extraction processes is proportional to the area of ​​the mass transfer surface and the average driving force of the process. In order to increase the mass transfer surface area in extractors, one of the liquid phases is dispersed and distributed into the other in the form of drops. The mass transfer process occurs between the dispersion and continuous phases. To carry out the process with the greatest driving force, the interaction of flows is organized in the extractors under conditions approaching ideal displacement. This is achieved by carrying out the process in a thin layer in packed, centrifugal extractors, by sectioning the extractors or using multi-stage sectional extraction units.

According to the principle of process organization, extractors can be either continuous or periodic.

Depending on the method of phase contact, extractors can be divided into three groups: step or sectional, differential contact and mixing and settling.

Stepped (sectional) extractors consist of separate sections in which the change in concentration in the phases occurs abruptly. In a number of cases, each section approaches the concentration field of an ideal mixing apparatus. An extractor, consisting of several such sections, approaches an ideal displacement apparatus in terms of the concentration field.

The need for phase separation after each extraction section in the case of poorly separated emulsions can lead to a significant increase in the size of the extractor.

Differential contact extractors provide continuous contact between phases and a smooth continuous change in concentrations in the phases. Due to the longitudinal mixing of phases in such devices, there can be a significant reduction in the average driving force compared to ideal displacement devices.

Energy is required to disperse the liquid phase. Depending on the type of energy expended, extractors can be without or with external energy supply. External energy can be introduced into the interacting phases by mixing devices, vibrators and pulsators, for example in vibration pulsation extractors, in the form of centrifugal force in centrifugal extractors, kinetic energy of the jet in injection and ejector extractors.

Mixing and settling extractors consist of several stages, each of which includes a mixer and a separator. In the mixer, due to the supply of external energy, one of the liquid phases is dispersed to form a dispersive phase, which is distributed in the other, the continuous phase. The dispersed phase can be either a light or a heavy phase.

In the separator, which is a settling tank, and in modern installations a separator, the emulsion is separated into raffinate and extract. The diagram of the simplest mixing and settling extractor is shown in Fig. 16.1.9.

https://pandia.ru/text/78/416/images/image055_12.jpg" width="197" height="253 src=">

Rice. 16.1.10. Disc extractor:

1 – cylindrical body; 2 – overflow device; 3 – sieve plates

The dispersed phase (light or heavy) passes through the holes in the plates and is crushed into droplets. The continuous phase moves along the plate from overflow to overflow. Droplets on the plates coalesce and form a continuous layer of liquid above the plate (heavy liquid) or below the plate (light liquid). The support layer sections the extractor in height and provides support for dispersing liquid through the holes of the plates. Sectioning the extractor reduces back mixing of the phases and leads to an increase in the average driving force of the process.

The speed of the dispersed phase in the holes of the plate is determined from the conditions for creating the jet mode. The critical speed corresponding to the transition from the drip mode to the jet mode depends on the diameter of the holes:

vTop=4,4/ d0 .

To operate the extractor in a stable jet mode, the speed is increased by approximately 20% compared to the critical one.

To determine the mass transfer coefficients in the dispersed phase, we can recommend the expression

https://pandia.ru/text/78/416/images/image057_24.gif" width="116" height="25 src="> - diffusion Nusselt number (here (βd - mass transfer coefficient in the dispersed phase; duh- equivalent drop diameter; Dd - diffusion coefficient in the dispersed phase); - Reynolds criterion for a drop (here - the relative velocity of the drop in the continuous phase; vc - kinematic viscosity of the continuous phase); - Prandtl diffusion criterion for the dispersed phase (here v- kinematic viscosity of the dispersed phase).

Rotary disc extractor(Fig. 16.1.11) refers to extractors with mechanical mixing of phases. It is a vertical multi-sectional apparatus, in a cylindrical body of which a rotor with round horizontal disks is installed along the axis. The disks rotate in the middle plane of the extractor section and are separated by annular partitions, which prevents longitudinal mixing of the flows and helps to increase the driving force of the process. When the rotor rotates, the disks create axial flows of the continuous phase directed from the rotor axis to the walls of the extractor.

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Rotor disc diameter Dp is 0.5...0.7 of the extractor diameter, and the diameter of the holes in the annular partitions D =(0,6...0,8) Duh(Where Duh- extractor diameter), section height H=(0,15...0,3) Duh.

In other designs, open turbine mixers are located on the rotor in the middle plane of each section. Sectioning is achieved using ring partitions. In such extractors, mixing and separation zones alternate.

Instead of annular partitions, the mixing zones can be separated by a layer of packing, for example Raschig rings, in which the ternary mixture is separated into light and heavy liquid. In Fig. Figure 16.1.12 shows an extractor with turbine mixers and settling zones filled with Raschig rings.

https://pandia.ru/text/78/416/images/image063_17.gif" width="157" height="27 src=">which determines high mass transfer coefficients and surface area of ​​interphase contact; division of the reaction volume into sections, which leads to an increase in the average driving force to values ​​close to those for an ideal displacement apparatus; the ability to regulate the rotor speed, which allows you to change the productivity and efficiency of the extractor.

To calculate and model rotary extractors, it is necessary to know the size of the droplets formed, the duration of retention of the dispersed phase in the extractor, mass transfer coefficients, the maximum load of the extractor for the continuous and dispersed phases, longitudinal and transverse mixing of the phases.

If the diffusion resistance is concentrated in the continuous phase, then the mass transfer coefficient can be determined from the equation

https://pandia.ru/text/78/416/images/image065_18.gif" width="116" height="25 src="> - Nusselt diffusion criterion; βс- mass transfer coefficient in the continuous phase; https://pandia.ru/text/78/416/images/image067_19.gif" width="119" height="25 src="> (16.1.11)

where: A = 6.58 and 17.9, respectively, for stationary drops and for drops with internal circulation, i.e. βd is inversely proportional to the average volume diameter of the drop.

For drops with internal liquid circulation

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The size of the holes in the extractor plates is 3...5 mm, the area of ​​all holes is assumed to be equal to 20...25% of the cross-sectional area of ​​the column; the distance between the plates is 50 mm.

Better distribution and dispersion are achieved on trays with rectangular holes and guide vanes.

In vibrating extractors, vibration of the plate block occurs at higher frequencies and lower amplitudes than the pulsation of liquid in pulsating extractors. The energy consumption for vibration of a block of plates is much less than in pulsation extractors for moving the entire column of liquid.

The advantage of pulsation and vibration extractors is effective mass transfer, which is achieved by increasing mass transfer coefficients, the average driving force of the process and the developed phase contact surface. VETS in such extractors is 5...6 times lower than in plate sieve extractors.

High specific loads exceed the permissible loads in rotary-disc extractors.

The high efficiency of mass transfer made it possible to significantly reduce the metal consumption of extraction equipment, which led to a reduction in capital costs.

At the same time, pulsation and vibration extractors require more powerful foundations that can withstand significant dynamic loads. The operating costs for such extractors are slightly higher than for conventional disc extractors.

In centrifugal extractors (Fig. 16.1.14), extraction occurs with continuous contact of phases moving in countercurrent with a minimum interaction time.

In the machine body, which consists of two casings: upper and lower, there is a shaft with a rotor attached to it. The shaft is hollow at both ends and is made of the “pipe-in-pipe” type, and in the central part it is solid, with channels for draining light liquid. The shaft together with the rotor rotates at a frequency of about 4500 rpm.

The solution being processed and the extractant enter the extractor from opposite ends of the hollow shaft, as shown in Fig. 16.1.14. Light liquid is supplied from the drive side, and heavy liquid is supplied from the opposite end of the shaft. The shaft is sealed using double mechanical seals. The sealing fluid is the liquid processed in the extractor.

Inside the rotor there is a package of concentric V-shaped rings. The rotor has channels for the passage of light and heavy liquids. Heavy liquid enters the rotor package, into its central part, while light liquid enters the peripheral part of the rotor. When the rotor rotates together with the ring package, the heavy liquid, under the influence of centrifugal force, rushes towards the outer perimeter of the rotor, and the light liquid moves towards the rotor shaft. Thus, the liquids come into contact in a countercurrent manner. Due to repeated dispersion of the liquid into droplets and coalescence of the droplets, high extraction efficiency is achieved.

After the ternary mixture is separated, the liquids are discharged through channels in the rotor into the hollow shaft: heavy fluid is discharged from the drive side, and light fluid is discharged from the opposite end of the shaft, from the input side of the heavy fluid.

Phase inversion occurs inside the rotor. If in the peripheral part of the rotor the dispersed phase of a light liquid interacts with the continuous phase of a heavy liquid, then in the area adjacent to the rotor axis, on the contrary, the dispersed phase of a heavy liquid comes into contact with the continuous phase of a light liquid.

https://pandia.ru/text/78/416/images/image071_6.jpg" width="335" height="224 src=">

Rice. 16.1.15. Scheme of a continuous extraction unit :

1,2 - pumps ; 3,4,5,6 - containers ; 5 - extractor

The performance of extractors is determined from the maximum load corresponding to the “flooding” of the extractor. At the “flooding” point, the load is calculated based on the maximum holding capacity of the apparatus and the characteristic droplet velocity, equal to the average settling velocity of droplets in the stationary continuous phase.

Let's use the Thornton-Pratt equation

https://pandia.ru/text/78/416/images/image073_19.gif" width="115" height="37 src=">.gif" width="163" height="25 src="> ( 16.1.15)

and find the holding capacity of the extractor

https://pandia.ru/text/78/416/images/image078_15.gif" width="41" height="24">. Characteristic speed of drops v0 determined by the corresponding equations for each type of extractor.

The operating speed of the continuous phase is taken to be 20...40% below the limit:

https://pandia.ru/text/78/416/images/image080_16.gif" width="143" height="25 src="> (16.1.18)

Where: hWithAndh d is the height of transfer units in the continuous and dispersed phases, respectively; - extraction factor.

Values hWithAndh d is determined depending on the value of the mass transfer coefficients:

https://pandia.ru/text/78/416/images/image083_12.gif" width="17" height="24 src="> and h d - mass transfer coefficients in the continuous and dispersed phases, respectively, kmol/(m2*s*kmol/kmol); - specific surface area, m2/m3.

The values ​​of βc, βD and are calculated using criterion and empirical equations obtained for extractors of a certain type. For example, for packed and plate extractors with sieve trays, the equation can be used to calculate the mass transfer coefficient in the dispersed phase

https://pandia.ru/text/78/416/images/image088_15.gif" width="26 height=31" height="31"> drops.

Mass transfer coefficients in the continuous phase can be approximately determined by the equation

https://pandia.ru/text/78/416/images/image088_15.gif" width="26" height="31 src=">.gif" width="16" height="17 src=">c - density of the continuous phase, kg/m3; µс - dynamic viscosity of the continuous phase, Pa s); Рrc=µs/s DC- Prandtl criterion for the continuous phase (here DC- diffusion coefficient in the continuous phase, m2/s).

For rotary-disk extractors, the mass transfer coefficient in the dispersed phase is determined by equation (16.1.12), and in the continuous phase by (16.1.10).

In the case where there is no data on mass transfer coefficients or the height of EEP transfer units, the height of the extractor is calculated by determining the number of theoretical steps of concentration change.

Control questions

1. What is the essence of the extraction process? What components are involved in the extraction process? 2. What factors determine the equilibrium during the extraction process? What does the distribution coefficient depend on? 3. Under what conditions is equilibrium in the extraction process described by a straight line? 4. What diagrams depict extraction processes? 5. In what cases can the extraction process be depicted on a rectangular y-x diagram? 6. What diagrams of extraction processes are used in the food industry? 7. What is an extraction module and how does it affect the position of the process working line on the y-x diagram? 8 How is the process of countercurrent extraction depicted on a triangular diagram and in y-x coordinates? 9. In what apparatus are extraction processes carried out? 10. What laws of mass transfer do extraction processes obey? 11. How to calculate the mass transfer coefficient during extraction in general and special cases? 12. What advantages do extractors with mixing devices have compared to gravity extractors? 13. What is the principle of operation of centrifugal extractors? What advantages do centrifugal extractors have over other types of extractors? 14. What is the kinetic calculation of extractors? 15. What values ​​determine the height of the column extractor?

16.2. EXTRACTION IN A SOLID BODY SYSTEM-LIQUID

16.2.1. GENERAL INFORMATION

Leaching(a special case of extraction) is the extraction of one or more substances from a solid using a solvent with selective ability.

In the food industry, capillary-porous bodies of plant or animal origin are treated by leaching.

The following solvents are used: water - for extracting sugar from beets, coffee, chicory, tea; alcohol and water-alcohol mixture - for obtaining infusions in alcoholic beverage and beer-non-alcoholic production; gasoline, trichlorethylene, dichloroethane - in oil extraction and essential oil production, etc. Leaching is the main process in beet sugar production; it is used to extract sugar from sugar beets. Vegetable oil is extracted from sunflower seeds using gasoline.

Leaching is often followed in a process flow by filtration, evaporation and crystallization processes.

16.2.2. STATICS AND KINETICS OF LEACHING

The leaching process involves the penetration of a solvent into the pores of a solid and dissolution of the extracted substances.

Leaching equilibrium is established when the chemical potential of the solute and its chemical potential in the solid material are equalized. The achieved concentration of a solution corresponding to its saturation is called solubility.

Near the surface of a solid body, equilibrium is established within a short period of time. Therefore, when analyzing the mass transfer process, it is assumed that the concentration at the solid-solvent interface is equal to the concentration of the saturated solution we have.

The main task of leaching kinetics is to determine the duration of contact of the interacting phases necessary to achieve a given degree of extraction of the extracted substance. The duration of phase contact determines the size of the extraction apparatus.

Mass transfer during leaching is greatly influenced by the internal structure of the solid: the size and shape of the capillaries, and the chemical composition of the particles. The rate of mass transfer depends on the internal structure of the solid. As was indicated in Chapter 4.1, the complexity of the internal structure of a porous body makes it difficult to analytically describe the process of mass transfer inside a capillary-porous body.

Leaching is a complex multi-stage process that consists of the diffusion of a solvent into the pores of a solid, the dissolution of the extracted substances or substances, the diffusion of the extracted substances in capillaries inside the solid to the phase interface and the mass transfer of the extractable substances in the liquid solvent from the phase interface into the core of the extractant flow .

Of the four stages of the process listed, the ones that limit the overall rate of mass transfer are, as a rule, the last two, since the rate of mass transfer in the first two stages is usually significantly higher compared to the rate of the two subsequent stages.

Thus, the total diffusion resistance of mass transfer consists of the diffusion resistances inside the solid and in the solvent.

The rate of diffusion of a substance inside a capillary-porous body is described, as is known, by the mass conductivity equation (12.30).

The rate of mass transfer from the phase interface into the core of the extractant flow is described by the mass transfer equation (12.15).

To assess the relationship between the rates of mass conductivity and mass transfer, the Biot criterion is used [see. equation (12.32)].

A particularly low rate of mass conductivity occurs in capillary-porous bodies of plant and animal origin.

In Fig. Figure 16.2.1 shows a diagram of the structure of a plant cell.

https://pandia.ru/text/78/416/images/image095_13.gif" width="200" height="24 src=">, and in the area of ​​low concentrations -

In the case when the main diffusion resistance is concentrated in the liquid phase, the mass transfer equation (12.15) can be used to describe the process.

The driving force of the leaching process is the difference between the concentration of the extracted substance at the surface of the solid and its average concentration in the extractant mass usr.

The speed of the process in this case

https://pandia.ru/text/78/416/images/image099_11.gif" width="21" height="25 src="> - mass transfer coefficient in the liquid phase.

The rate of molecular diffusion in a boundary layer of thickness δ is determined by the Fick equation (12.9)

https://pandia.ru/text/78/416/images/image101_10.gif" width="319" height="25 src="> (16.2.2)

where: https://pandia.ru/text/78/416/images/image103_11.gif" width="59" height="21"> . Then from (16.2.1) it follows that it is proportional to D2/3. By generalizing the experimental data taking into account the indicated dependence, an equation was obtained for calculating the mass transfer coefficient https://pandia.ru/text/78/416/images/image104_11.gif" width="143" height="27 src="> (16.2. 3)

where: https://pandia.ru/text/78/416/images/image106_10.gif" width="85" height="21"> - Reynolds criterion (here v- extractant speed; µ - dynamic viscosity of the extractant); Pr= v/ D- Prandtl criterion.

From expression (16.2.2) it is clear that β increases with decreasing thickness of the diffusion layer δ. From the theory of the boundary layer it is known that the thickness of the diffusion layer decreases with increasing Reynolds criterion, i.e. with increasing relative speed of the extractant (relative to solid particles). Consequently, the leaching process can be intensified by creating an effective hydrodynamic environment, including by grinding solid material.

Grinding leads to an increase in the mass transfer surface, as well as a decrease in the diffusion path of the extracted material from the depths of the capillaries to the surface of the material. Due to the fact that the mass conductivity coefficient increases with increasing temperature, leaching is carried out at temperatures close to the boiling point of the extractant. At the same time, the concentration of the saturated solution of unas also increases, which leads to an increase in the driving force of leaching and dissolution.

The rate of mass conductivity can also be increased by special processing of food raw materials, leading to a decrease in diffusion resistance in the cell.

In practice, process intensification can be achieved in extractors with efficient hydrodynamic conditions, for example in fluidized bed extractors, as well as in vibrating and pulsating extractors.

As noted, carrying out processes in a fluidized bed with crushed materials leads to a sharp increase in the mass transfer surface and a decrease in diffusion resistance.

In Chapter 4.4 it was indicated that low-frequency vibrations of interacting phases lead to a significant intensification of the extraction process.

16.2.3. CALCULATION OF EXTRACTION DEVICES

The zonal method for calculating extraction processes from a solid, developed in recent years, is based on solving the problem of non-stationary mass conductivity. To calculate the duration of the process in bodies of regular geometric shape, equation (18.11) can be used. However, due to the lack of experimental data on mass conductivity coefficients, the use of this method in calculation practice is difficult. Therefore, to calculate extractors, a method is used that is based on determining the number of theoretical steps of concentration change. The introduction of efficiency coefficients into calculations makes it possible to determine the number of real stages of multi-stage devices or the length of the device.

Let's consider a graphical method for determining the number of theoretical steps using a triangular diagram (Fig. 16.2.2). For ease of calculation, let us imagine the diagram in the form of a right triangle instead of an equilateral triangle.

Let the initial solid material to be extracted consist of an insoluble component L and a soluble component M, which is extracted with a liquid extractant E. As a result of the process, an extract is obtained consisting of an extractant E and the substance dissolved in it M, and raffinate, consisting of an insoluble substance L, in the pores of which there is a certain amount of substance M, dissolved in extractant E.

https://pandia.ru/text/78/416/images/image108_0.jpg" width="438" height="129 src=">

Rice. 16.2.3. Scheme of multi-section countercurrent solid-liquid extraction

Solution E of the extracted substance M in extractant E let's call it the upper flow, and the flow R solid mixtures L with extractable substance M- bottom flow.

The material balance equations will be written as follows:

F+ E= R y and

The percolator (Fig. 16.2.5) is a vertical cylindrical apparatus with a conical bottom and a lid. At the bottom there is a grate onto which a layer of crushed solid material is loaded through the top hatch. After leaching, the material is discharged through the lower hinged hatch.

Bunker" href="/text/category/bunker/" rel="bookmark">a bunker for loading beet chips and augers for removing pulp from the apparatus.

Inside the apparatus, the chips are moved by two parallel screws from bottom to top. Augers are formed by blades located along a helical line. The blades of each auger enter the inter-blade space of the other. This arrangement of augers promotes uniform movement of chips along the length of the apparatus and prevents the possibility of rotation of beet chips along with the blades. For the same purpose, counterblades and partitions are installed on the lower part of the covers.

https://pandia.ru/text/78/416/images/image138_3.jpg" width="241" height="257 src=">

Rice. 16.2.7. Double column diffusion apparatus:

1.5 - fittings; 2 - rotary thrower; 3 - drum; 4 - body; 6 - chain; 7 - frame

A rake conveyor and a chip thrower are designed to feed chips into the device. Heated juice is supplied through the nozzles into the machine.

Diffusion juice is taken from the apparatus through self-regenerating sieves with conical holes installed in the chamber and a pipe. Barometric water enters the apparatus through the upper row of nozzles, and pulp water enters through the lower row.

The chips entering the apparatus move to the place where they are unloaded from the apparatus. Barometric and pulp water are supplied to the upper part of the second column in countercurrent to the beet chips. Diffusion juice is sent to production, and pulp is sent to presses or to pulp storage. In some plants, barometric and pulp water are first fed into one large mixing tank and then into a heater to heat the mixture.

In the design of the apparatus under consideration, beet chips are scalded inside the apparatus and no additional installation of a scalder is required. The juice intended for scalding is heated to a certain temperature in heaters.

There are designs of apparatus in which solid material is moved by buckets.

The use of chain conveying devices with frames or buckets leads to compaction of the mass of solid material on the frames or in the buckets, which impairs the extraction process. In diffusion apparatuses with bladed shafts and counterblades, significant grinding of chips occurs, which makes it difficult to filter the diffusion juice in the apparatus and thereby reduces the extraction rate. As a result of the use of large beet chips, the extraction rate also decreases due to an increase in intra-diffusion resistance.

Suspended bed diffusion devices do not have these disadvantages. In a two-column apparatus (Fig. 16.2.8), developed by prof. , beet chips are in suspension. The driving force for moving the contents in the apparatus is the pressure difference above the material in the first and second columns. When the piston transport device moves upward, a vacuum is created underneath it. Beet chips enter the upper part of the first column, which is filled to a certain level with diffusion juice. The juice level is maintained using a level gauge. Thus, beet chips enter the diffusion juice and are evenly distributed throughout the apparatus.

https://pandia.ru/text/78/416/images/image140_5.gif" width="128" height="43 src="> (16.2.11)

Where: VP- useful volume of the apparatus, m3; q- mass of chips per unit of useful volume of the apparatus, kg/m3 (for column apparatus q=600...700 kg/m3); τ - duration of the extraction process, s.

Belt extractors (Fig. 16.2.9) are used to extract oil from sunflower seeds. The solid phase - crushed seeds are moved along the belt in a thin layer, and the extractant - gasoline is supplied from above using pumps and irrigates the material located on the belt. The process is carried out according to a complex combined flow pattern of solid material and extractant: cross-flow in each section and counter-flow as a whole in the extractor. The design of the extractor does not ensure effective interaction of the solid phase with the extractant; extraction proceeds at a low speed. Several extraction steps are required to completely extract the oil.

Rice. 16.2.9. Belt extractor:

1 - body; 2 - nozzles; 3 - loading shaft; 4 - transport device; 5 – pumps

Control questions

1. What is the essence of the leaching process? What components are involved in the leaching process? 2. What factors determine the rate of leaching? 3. In what case is the leaching rate described by the Shchukarev equation? 4. What is the calculation of extractors? 5. How is the countercurrent extraction process represented in a triangle diagram? 6. How is the number of steps of concentration change in a triangular diagram determined? 7. Name the schemes by which leaching processes are carried out. 8. What designs of extractors are used in the food industry?

Introduction

Extraction in the “liquid-liquid” system. Basic concepts and indicators

Organic solvents used in extraction

Chemical and mass transfer processes occurring during extraction

Main methods of extraction

Modern extraction equipment

Calculation part

Cleaning of drains

Conclusion

Bibliography

Introduction

Extraction in a broad sense refers to the processes of extracting one or more components from solutions or solids using selective solvents. Therefore, in principle, extraction can be carried out in solid-liquid systems (for example, extraction of gold from ores with cyanide solutions) or liquid-liquid. In hydrometallurgy, extraction, or extraction, usually refers to the process of liquid extraction, which consists in extracting a substance dissolved in one solvent using another solvent that is immiscible with the first. An aqueous metal-containing solution of chemical reagents and an organic liquid are used as two such liquid media.

The main advantages of the extraction process, compared to other processes for separating liquid mixtures, are:

low operating temperature (the process is usually carried out at room temperature);

high rate of mass transfer between two contacting phases (due to the very large area of ​​their contact during emulsification of the organic phase in an aqueous solution);

high selectivity of extractants, allowing the separation of related, difficult-to-separate elements;

ease of separation of two phases (immiscible liquids with different densities);

the ability to extract metals from highly dilute solutions;

as deep cleaning of the resulting metal as desired;

the possibility of regenerating spent reagents;

the possibility of complete mechanization and automation of the process.

These circumstances determine the widespread use of extraction processes in modern hydrometallurgy.

Extraction in the “liquid-liquid” system. Basic concepts and indicators

The following extraction terminology has been adopted. The two solvents involved in the process (aqueous and organic) in the initial state are called “source solution” and “extractant”. At the moment of contact (during extraction) they are called “aqueous” and “organic” phases, and after extraction (settling and separation) they are called “raffinate” and “extract”.

The extraction process consists of the following stages:

preparation of the initial solution and extractant (Fig. 1, a);

contacting these solutions with emulsification of the organic and aqueous phases (Fig. 1, b, c);

settling and separation of these phases (well observed visually) (Fig. 1, d);

separation of raffinate and extract (Fig. 1, e).

Fig.1. Scheme of the liquid-liquid extraction process. 1 - initial solution; 2 - extractant; 3 - raffinate; 4 - extract.

From an extractant saturated with the extracted element (extract), metals are extracted by the re-extraction method, which consists of treating the extract with an aqueous solution of some chemical reagent, creating favorable conditions for the reverse transfer of metals from the organic phase to the aqueous phase. The flowchart for the re-extraction process is similar to the extraction steps. In this case, the reagent used to extract a substance from the organic phase is called a re-extractant, and the resulting product is called a re-extract. Consequently, the extractant and the extract are an organic phase, and the re-extractant and the re-extract are an aqueous phase. Almost always, after re-extraction, the extractant is regenerated to its original state, which is why it is called a regenerated extractant.

Thus, during extraction and re-extraction, the following product designations are used as the process progresses:

Extraction:

extractant ® organic phase ® extract

initial solution ® aqueous phase ® raffinate

Re-extraction:

extract ® organic phase ® regenerated extractant

re-extractant ® aqueous phase ® re-extract.

The final product of the “extraction - re-extraction” cycle is again an aqueous solution - re-extract. But the aqueous solution obtained as a result of strip extraction differs from the original one in that it does not contain or contains only a small amount of impurities, the separation of the valuable component from which is the main difficulty in extracting it from the solution. In this case, the re-extract, unlike the original solution, is often enriched in metal.

Organic solvents used in extraction

Organic compounds are used as extractants.

An ideal extractant should have the following properties:

be sufficiently selective (that is, selectively extract only the components of interest to us from aqueous solutions containing a sum of metals);

have a high extraction capacity (absorb a significant amount of the extracted component per unit volume);

provide fairly easy regeneration of the extractant with the extraction of metal from the organic phase;

be safe to operate (non-toxic, non-volatile, non-flammable);

remain stable during storage or in contact with acids and alkalis;

be cheap enough.

Finding such an ideal extractant is almost impossible, so a compromise solution is usually made.

Taking into account the fact that mass transfer plays an important role in the mechanism of extraction separation, one of the main physical properties of the organic phase is viscosity. Knowledge of the characteristics of viscosity, energy of the interphase boundary, and density of media is extremely necessary for judging the kinetics of the extraction process, not only in the sense of mass transfer, but also from the point of view of phase dispersion and the rate of settling of liquid phases brought into equilibrium. However, organic extractants are usually quite viscous media. In this case, the viscosity of the organic phase increases sharply with increasing saturation of it with metal ions. An increase in the viscosity of the organic phase above a certain limit can sharply slow down the extraction process. Therefore, it is sometimes impractical to achieve significant saturation of the extractant with metals. But even if the possible saturation of the extractant is limited, in some cases it is necessary to artificially reduce the viscosity of the organic phase.

In addition, for good phase separation after extraction, there must be a sufficient difference in the densities of these phases, that is, the extractant must be much lighter than the aqueous solution. Therefore, in practice, the extractant is rarely used in its pure form; it is usually diluted with a cheap organic solvent to reduce viscosity and density. This auxiliary solvent is usually inert and does not participate in the extraction process. In such a system of two organic solvents, the organic compound involved in the chemical reactions of extraction is called the extraction reagent, and the solvent of the extraction reagent is called the diluent. The entire organic solution is an extractant. It should be noted that the diluent is used not only to reduce the viscosity and density of the organic phase, but also to dissolve the resulting products during the extraction reaction.

The most widely used types of organic solvents are:

hydrocarbons and their halogen derivatives;

Hydrocarbons and their chlorine derivatives most often used as diluents for extraction reagents. Due to the fact that hydrocarbons are highly volatile, flammable and toxic substances, only a limited number of them are suitable for industrial use. The most commonly used are: benzene C 6 H 6; toluene, or methylbenzene CH 3 C 5 H 5; kerosene; diesel fuel; hexane (C 6 H 4), octane (C 8 H | 8), gasoline. Of the chlorine derivatives of hydrocarbons, the most commonly used are carbon tetrachloride CCl 4, chloroform CHC1 3 and dichloromethane CH 2 C1 2. Chlorine derivatives are sometimes used as extractants for inorganic compounds (for example, CCl 4 or CHCl 3 are extracted with GeCl 4).

Oxygen-containing extractants are divided into compounds that do not contain and those containing salt-forming groups. Oxygen-containing organic solvents that do not have salt-forming groups are used as extractants for the extraction of halides, nitrates, thiocyanates and other metal salts. These include alcohols ROH, esters ROR, esters R-OCO-R, ketones R-COR, d-ketones RCOCH 2 COR (where R is an organic radical). Extraction proceeds successfully in strongly acidic solutions, in which the formation of oxonium salts is possible, or in solutions with low acidity, but in the presence of salting out agents. When using alcohols, ethers, ketones, the formation of a solvate is observed, for example, according to the scheme: mROR + nMeCl 3 + pHCl = mROR × nMeCl 3 × pHCl. Moreover, the acidity level greatly influences the course of this process.

Of the ethers, the most commonly used are diethyl ether C 2 H 5 OS 2 H 5 and its chlorine derivative - chlorex ClC 2 H 4 OS 2 H 4 Cl, or (C 2 H 4 Cl) 2 O. Chlorex is an extremely weak base and extracts only very strong acids. It is used, for example, in the extraction of chloroauric acid from aqua regia solutions in the precious metals refining cycle.

Among aliphatic (acyclic) alcohols (ROH, where R is C n H (2n + 1)), butyl (C 4 H 9 OH), amyl (C 5 H 11 OH), isoamyl, hexyl (C 6 H 13 OH) are used. , caprylic (C 7 H 15 OH), octyl (C 8 H 17 OH), nonyl (C 9 H 19 OH), a mixture of alcohols C 7 - C 9 and decyl (C 10 H 21 OH). Of the acyclic alcohols (containing cycles in molecules - rings of three or more carbon atoms), cyclohexanol C 11 H 11 OH is most often used. Of the aromatic alcohols (containing rings in their molecules - benzene rings), a-naphthol is used and a, a’-naphthols .

When using oxygen-containing organic solvents with salt-forming groups (carboxylic acids RCOOH), water-insoluble compounds - soaps - are formed due to the extraction not of salts or their acid complexes, but of metal cations. Carboxylic acids are dimerized in structure .

This dimerization persists during extraction, that is, the organic salt M(HR 2) 2 is formed. Extraction with carboxylic acids is usually carried out at a pH 0.5 less than the hydrolysis pH of the starting inorganic metal salt. A similar type of extraction using fatty acids C n H 2 n +1 COOH is used, for example, in cobalt hydrometallurgy to purify cobalt-containing solutions from impurities.

or directly (P-C bond, organophosphorus compounds):

where R is an alkyl (CnH 2 n +), cycloalkyl or aryl (monovalent residue of aromatic hydrocarbons) radical.

The extraction ability of medium esters of phosphoric, phosphonic and phosphinic acids, as well as oxides of substituted phosphines, has been most studied. Extraction with all these reagents proceeds on the basis of the donor-acceptor ability of phosphoryl oxygen - P=O, which increases in the series:

Consequently, the extraction ability of these compounds increases in the same direction. Of the medium esters of phosphoric acid, the most widely used extractant is tributyl phosphate TBP ((C 4 H 9 O) 3 PO), used in the hydrometallurgy of radioactive metals (for example, in the production of nuclear fuel, in particular, in the extraction of uranyl nitrate), in the hydrometallurgy of rare metals (niobium, tantalum, zirconium, etc.). Dialkyl alkyl phosphinates DAAF (R 1 P(O)(OR 2) 2) are used in the extraction of scandium from hydrochloric acid, niobium, tantalum and other rare earth elements.

From nitrogen-containing extractants The most widely used for extraction purposes are amines of varying degrees of substitution (obtained by replacing ammonia protons with an organic radical): primary, secondary , tertiary and quaternary ammonium bases (QAB): R 4 NOH. Many salts of primary, secondary and tertiary amines having normal alkyl radicals C n H 2 n +1 (alkylamines) are sparingly soluble in liquid hydrocarbons, preferably in aromatic ones (> 0.1 mol/l).

For the extraction of copper, nickel and cobalt, mixtures of a-hydroxymes with the general formula are proposed, where R and R’ are radicals; R'' is a radical or hydrogen atom.

Sulfur-containing extractants. Due to the lower electron-donating ability of the sulfur atom compared to the oxygen atom, replacing oxygen with sulfur in the corresponding oxygen-containing organic compounds (ethers, alcohols, etc.) leads to a decrease in the extraction properties of sulfur-containing organic compounds (thioethers R 2 S; thioalcohols RSH; thioacids , ; dithioacids, etc.).

However, a decrease in the basicity of thio compounds can lead to increased selectivity of extraction, as a result of which sulfur-containing organic extractants are of certain interest. Organic sulfides (thioesters) are quite effective extractants. For example, diisobutyl sulfide (iC 4 H 9) 2 S extracts ferric chloride well from solutions of hydrochloric acid in the form of НFeСl 4, like ordinary oxygen-containing dibutyl ether (C 4 H 9) 2 O. In relation to inorganic acids and uranium salts, oxides have been tested as extractants dialkyl sulfides, obtained by oxidation of the corresponding dialkyl sulfides with hydrogen peroxide in acetic acid CH 3 COOH. Sulfonic acids R-SO 3 H (or ), which are cation-exchange extractants, are of practical interest in hydrometallurgy. Sulfonated hydrocarbons are recommended for the industrial extraction of nickel and cobalt from aqueous solutions with metal concentrations from 0.5 to 10 g/l.

Chemical and mass transfer processes occurring during extraction

The separation of substances during the extraction process is based on the difference in distribution between two immiscible liquids. In the simplest case, when the extracted substance is in the same form in both phases (the so-called physical distribution), Nernst’s law applies:

,

where K d is the distribution constant. The distribution constant Kd does not depend on the concentration of the extracted substance in the aqueous phase and, with an established constant ratio of the volumes of the contacting phases (P:E) at a given temperature, remains a constant value for both rich and poor solutions. Therefore, in several successive cycles of the process it is possible to achieve an arbitrarily deep degree of extraction or purification.

However, the distribution law in its classical form is not applicable to most real extraction systems, since in both phases the interaction of the substance with the solvent can occur; extraction of the substance in the form of several types of compounds is also possible, a change in the mutual solubility of the phases under the influence of the extracted substances, etc. Therefore, to characterize the distribution of a substance, the distribution coefficient is usually used

where C x O and C x B are, respectively, the total analytical concentrations of the extracted substance in all compounds in the organic and aqueous phases.

Since extraction is carried out not so much to extract metal from pure solutions, but to selectively isolate a valuable element from solutions containing a sum of impurities, another indicator is used, called the separation factor:

.

That is, it represents the ratio of the distribution coefficients of two substances. For separation conditions it is necessary to have the inequality D Me1 ¹ D Me 2. The best separation occurs when D Ме1 >> D Ме2. Moreover, the closer S is to unity, the greater the number of extraction stages required. When calculating the value of the separation factor, it is customary to place the larger distribution coefficient D Me in the numerator, therefore S ³ 1 always.

As in any hydrometallurgical process, an important indicator of extraction is the amount of metal recovered (or percentage of extraction):

,

where V 0 and V B are the volume of the organic phase and aqueous solution, respectively. The distribution coefficient D and the degree of extraction E are interrelated quantities:

.

Most often, the extraction of metals from the aqueous to the organic phase is carried out in three ways:

Cation exchange extraction - extraction of metals found in iodine solutions in the form of cations with organic acids or their salts. The extraction mechanism consists of the exchange of the extracted cation for H + or another extractant cation.

Anion exchange extraction - extraction of metals found in aqueous solutions in the form of anions, with salts of organic bases. Extraction occurs due to the exchange of a metal-containing anion with an extractant anion.

Coordination extraction, in which the extracted compound is formed as a result of the coordination of a molecule or ion of the extractant directly to the atom (ion) of the extracted metal, as a result of which the metal and the extractant find themselves in the same sphere of the extracted complex.

Coordination or complex compounds are those that have a central atom or ion surrounded by a specific number of ions or molecules called ligands.

The number of chemical (coordination) bonds between the central atom or ion (complexing agent) and the ligands is called the coordination number. Coordination bonds often have a donor-acceptor character, i.e., they are formed when the donor atom has a lone (free) pair of electrons that binds to the acceptor atom. When, for example, a complex ion (NH 4) + is formed:

,

nitrogen, which has a lone pair of electrons in the NH 3 molecule, is a donor, and the hydrogen ion is an acceptor.

The ligands are anions of inorganic acids, organic acids and neutral molecules (for example, H 2 O), and the formation of complex ions can be represented as the displacement of water molecules surrounding (hydrating) the ion by another ligand. Ligands, depending on the number of atoms forming a coordination bond, can be monodentate, bidentate, etc.

Polydentate ligands (bidentate and more) form cyclic complexes, i.e., the extracted ion is surrounded by several molecules of an organic extractant.

The central atom and coordinated groups (ligands) form the internal coordination sphere of the complex - a complex ion. Positive or negative ions compensating for the charge of the complex ion form the outer sphere of the complex compound.

Cation exchange extraction

This type of extraction can be generally described by the equation

where Me is a metal with valence z;

R is the acidic residue of an organic acid. Common cation exchange extractants are fatty acids of the RCOOH type (for example, carboxylic acids) with the number of carbon atoms in the R radical from seven to nine (C 7 - C 9) and naphthenic acids:

Naphthenic acids are obtained from crude oil; their molecular weight ranges from 170 to 330. Alkyl phosphoric acids are often used, in particular derivatives of orthophosphoric acid - alkyl orthophosphates. If two hydrogen ions in orthophosphoric acid (H 3 PO 4) are replaced by organic radicals, products called dialkyl orthophosphates are obtained, for example di-(2-ethylhexyl)-phosphoric acid (D2EHPA).

A type of cation exchange extraction is extraction with complexing (chelating) mono-, bi- and polydentate extractants such as oximes - compounds containing the (=N-OH) group. In this case, extraction occurs as a result of ion exchange and coordination of the extractant to the atom (ion) of the extracted metal with the formation of intracomplex compounds.

Anion exchange extraction

Anion exchange extractants belong to the class of amines, which are derivatives of ammonia NH 3. Depending on the number of hydrogen atoms replaced in ammonia by hydrocarbon radicals, primary, secondary or tertiary amines are obtained:

R is a hydrocarbon radical containing from 7 to 9 (sometimes up to 16) carbon atoms.

In amines, nitrogen has a lone pair of electrons, which determines the ability of these extractants to form coordination compounds

Amine salts formed during acid treatment can exchange the acid anion for metal-containing anions, for example

In an alkaline environment, amines can be found not in the form of salts capable of exchanging anions, but in the form of neutral molecules, so they are used only in acidic environments.

The most common amines are the ANP collector - primary amine, dilaurylamine (secondary amine) and trioctylamine (tertiary amine).

In addition to extraction by the type of anion exchange, extraction with amines sometimes leads to the introduction of an amine into the internal coordination sphere of the extracted complex annon with the formation of strong metal-nitrogen bonds (which is typical, for example, for platinum metals). The intracomplex compounds formed in this case are very strong, as a result of which the process of reverse transfer of the metal from the organic phase to the aqueous phase - re-extraction - is difficult.

Another class of anion exchange extractants are quaternary ammonium bases (QAB) and their salts (QAB). QAO are derivatives of ammonium ion (NH 4) +:

,

where R is a hydrocarbon radical.

The most commonly used QACs are trialkylbenzylammonium chloride - abbreviated TABAC, trialkylmethylammonium chloride (CH 3 R 3 N)Cl - TAMAC, tetraalkylammonium chloride (R 4 N)Cl - TAAX. R - C n H 2 n +1, where n = 8 - 10.

QAS extract metals only by the type of anion exchange reaction:

where z is the charge of the metal-containing anion MeX;

m is the charge of the HOUR anion;

Y - anion HOUR.

QAS are capable of extracting metal-containing salts not only from acidic, but also from alkaline solutions.

Amine and QAS salts in some cases have limited solubility in commonly used diluents (kerosene, hydrocarbons). To improve solubility, organic alcohols (for example, decyl alcohol) are added to the organic phase, however, large (over 10%) alcohol concentrations usually impair extraction due to interaction with the extractant.

Main methods of extraction

The following extraction methods are mainly used: single extraction, multiple extraction with cross and countercurrent movement of the solvent, continuous countercurrent extraction. Extraction with one solvent is most widely used in industry, although extraction with two extractants is also used.

Single (single-stage) extraction. This method of extraction consists in the fact that the initial solution F and the extractant S are mixed in a mixer, after which they are separated into two layers in a settling tank: extract E and raffinate R. It is usually believed that phase equilibrium is established in the mixer due to intensive mixing and sufficient contact time , i.e., a single extraction allows one to achieve an efficiency corresponding to the theoretical level of concentration change. The degree of extraction with this extraction method can be increased by increasing the supply of extractant to the apparatus, but this will lead to a decrease in the concentration of the extract and an increase in the cost of the process.

The process can be carried out either periodically or continuously. When organizing the process periodically, the stage of separating the extract and raffinate can be carried out in a mixer. In this case, there is no need for a sump.

Multiple extraction with cross flow of solvent. When carrying out extraction using this method (Fig. 2), the initial solution F and the corresponding raffinates are treated with a portion of fresh extractant S1, S2, etc. at each extraction stage, consisting of a mixer and a settling tank (settling tanks are not shown in Fig. 2), and the raffinates are sent sequentially to the next stages, and extracts E 1, E 2 of each stage are removed from the system. With this extraction method, the initial solution F enters the first stage, and the final raffinate Rn is taken from the last, nth stage.

Rice. 2. Scheme of multiple extraction with a cross flow of solvent (1, 2,3, ..., n - stages).

Using this method, it is possible to almost completely extract the distributed component from the initial solution and obtain a pure raffinate. However, in this case, losses of the solvent contained in the initial solution are inevitable, since at each stage there is a partial removal of this solvent with the extract.

Multiple extractions with countercurrent movement of solvent. This method of extraction is characterized by repeated contact in stages 1, 2, etc. with countercurrent flows of raffinate R and extract E (Fig. 3), provided that the initial solution F and extractant S are supplied from opposite ends of the installation. Since the method of extraction with countercurrent movement of the solvent makes it possible to obtain products of a given quality with a sufficiently high productivity of the installation, this extraction method is quite widely used in industry.

Rice. 3. Scheme of multiple extraction with countercurrent movement of the solvent (1,2, ..., n-1. n - steps).

Continuous countercurrent extraction. This extraction method is carried out in column-type devices (for example, packed ones). A heavier solution (for example, the original solution) is continuously fed to the upper part of the column (Fig. 4), from where it flows down.

A light liquid (in our case, a solvent) enters the bottom of the column, which rises up the column. As a result of the contact of these solutions, the distributed substance is transferred from the original solution to the extractant. This extraction method is often used in industry.

Countercurrent extraction with reflux. If it is necessary to more completely separate the initial solution, extraction can be carried out with reflux by analogy with the rectification process (Fig. 5). In this case, the initial mixture F is supplied to the middle part of the apparatus (to the feed stage). After regeneration of the extract in regenerator 2, part of the resulting product R0 is returned in the form of reflux to apparatus 1, and the other part is taken in the form of component B extracted from the original solution. Obviously, the compositions of the solutions R0 and B are the same. Thus, node 2 of the extraction unit is an analogue of the reflux unit of the distillation unit.

Rice. 5. a) (left) diagram of countercurrent extraction with reflux: 1 - extraction apparatus; 2 - apparatus for extract regeneration; b) extraction scheme with two solvents: 1 - extraction apparatus; 2 - apparatus for extract regeneration.

The reflux flow R0, upon contact with the extract flow, washes out from the latter the initial solvent A, partially or completely dissolved in it, which ultimately passes into the raffinate, as a result of which the degree of separation and the yield of the raffinate increase.

It should be noted that extraction with reflux, while improving the separation of the initial solution, leads to an increase in the consumption of the extractant and the volume of equipment, which makes this process more expensive. Therefore, the choice of the amount of reflux should be made on the basis of a technical and economic calculation.

Extraction with two solvents. If the initial solution contains two or more components that need to be extracted separately or in groups of several components, then extraction with two immiscible solvents is used (Fig. 5b). Solvents are selected in such a way that each of them preferentially dissolves one component or group of components. The initial mixture F, consisting of components A and B, is fed into the middle part of the apparatus 1. The extractant S, (heavier than S 1), selectively dissolving component A, enters the upper part of the apparatus 1, and the extractant S 1, selectively dissolving the component B, - in its lower part.

Two-solvent extraction is usually used to separate substances with similar solubility. This method requires a relatively large consumption of extractants, which significantly increases the cost of the process.

extraction solvent organic cation exchange

Modern extraction equipment

According to the principle of interaction or method of phase contact, extractors are divided into two groups: step and differential contact. Within these groups, extractors are often divided into gravitational (the speed of the phases in them is determined by the difference in the densities of these phases) and mechanical (when energy is added to flows from the outside by mechanical mixing, centrifugal force, a piston pulsator, etc.). In almost any of the devices of the named groups, to increase the contact surface of the phases, one of the phases is dispersed in various ways and distributed in another, continuous phase in the form of drops. After each mixing of the phases in the apparatus, separation of these phases follows, which is necessary primarily for the regeneration of the extractant (under the influence of gravitational or centrifugal forces). We also note that in industry, continuous extractors are usually used.

Step extractors. Extractors of this group consist of discrete stages, in each of which phase contact occurs, after which they separate and move in countercurrent to subsequent stages. In Fig. Figure 6 shows a diagram of single-stage (a) and multi-stage (b and c) installations of one of the most common types of stage extractors - mixing and settling.

Rice. 6. Schemes of single-stage (a) and multi-stage (b, c) installations of mixing-settlement extractors: 1 - mixers; 2 - settling tank; 3 - pumps.

The advantages of mixing-settling extractors include their high efficiency (the efficiency of each stage can approach one theoretical separation stage), the ability to quickly change the number of stages, suitability for operation over wide ranges of changes in physical properties and volumetric phase ratios, relatively easy scaling, etc. Disadvantages These extractors are characterized by a large production area, the presence of mixers with individual drives, and large volumes of gravity settling chambers.

High-capacity mixing-settlement extractors (up to 1500 m 3 /h) are used in hydrometallurgy, uranium technology and in various other large-tonnage industries.

Differential contact extractors. Extractors of this group are distinguished by continuous contact between phases and a smooth change in concentration along the height of the apparatus. In such extractors (unlike step ones), equilibrium between the phases across the cross section of the apparatus is not achieved. Differential contact extractors are more compact than step extractors and occupy a smaller production area.

In gravitational extractors, the movement of phases occurs due to the difference in their densities. Gravity extractors include spray, packed and tray columns.

Rice. 7. Hollow (spray) column extractors: a - with heavy phase spraying; b - with light phase spraying; 1 - extractors; 2 - sprinklers; 3 - water seals; 4 - phase interface.

The simplest representatives of gravitational extractors in design are spray columns (Fig. 7). An important advantage of spray extractors is the ability to process contaminated liquids in them. Sometimes these devices are used for extraction from pulps.

Packed extractors (Fig. 8), which are similar in design to packed absorbers, have become quite widespread in industry.

Rice. 8. Attachment extractor: 1 - nozzle; 2 - distributor; 3 - settling tanks; 4 - water seal; 5 - phase interface.

Raschig rings are often used as nozzle 1, as well as other types of nozzles. The packing is placed on support grids in sections, between which the phases are mixed. One of the phases (extractant in Fig. 8) is dispersed using distribution device 2 in the flow of the continuous phase (initial solution). In the packing layer, droplets can coalesce multiple times and then break up, which increases the efficiency of the process. The choice of nozzle material is very important. It should preferably be wetted by the continuous phase, since this eliminates the possibility of unwanted coalescence of droplets and the formation of a film on the surface of the nozzle, which leads to a sharp decrease in the phase contact surface. Note that ceramic and porcelain nozzles are better wetted by the aqueous phase than by the organic phase, and a plastic nozzle is usually better wetted by the organic phase. Phase separation in packed columns occurs in settling zones 3, often having a larger diameter than the extractor diameter for better phase separation.

Mechanical extractors include differential contact extractors with the supply of external energy to the contacting phases.

One of the most common mechanical extractors in technology is the rotary disk extractor. Rotary extractors differ mainly in the design of the mixing devices. So, instead of smooth disks, various types of mixers are used, sometimes sections are filled with a nozzle, etc. The main advantages of rotary extractors include high efficiency of mass transfer, low sensitivity to solid impurities in phases, the possibility of creating devices with high unit power, etc.

At the same time, rotary extractors have a serious drawback - the so-called scale effect, i.e. a significant increase in EEP with increasing diameter of the apparatus. The reason for this phenomenon is the unevenness of the velocity field along the height and cross-section of the apparatus, the formation of stagnant zones, bypassing, which contribute to increased longitudinal mixing and disruption of the uniform structure of flows in the apparatus.

The efficiency of the mass transfer process during extraction can be increased by pulsating the phases. Pulsation extractors use two main methods of imparting pulsations to liquids. According to the first method, pulsations in the column extractor are generated hydraulically by an external mechanism (pulsator); according to the second, through vibration of perforated plates mounted on a common rod, which is subjected to reciprocating motion.

The use of pulsations during the extraction process promotes better dispersion of the liquid, intensive renewal of the phase contact surface, and an increase in the residence time of the dispersed liquid in the extractor. The most widely used in technology are sieve plate and packed pulsation extractors.

The pulsating extractor (Fig. 9.) is a column with sieve trays without branch pipes for the flow of the continuous phase. In the column, with the help of a special mechanism (pulsator), pulsations are transmitted to the liquid - vibrations of small amplitude (10-25 mm) and a certain frequency. A valveless piston pump is most often used as a pulsator, connected to the bottom of the column (Fig. 9, a) or to the light liquid supply line (Fig. 9, b). When pulsations are imparted to the liquid, repeated fine dispersion of one of the phases occurs, which causes intense mass transfer. In addition to sieve extractors, packed pulsation columns are also used.

An effective method of intensifying the extraction process by imparting pulsations to the liquid can also be used in other types of extraction apparatus.

To reliably separate the pulsator mechanism from the working environment when processing chemically aggressive and radioactive substances, a membrane (Fig. 9, c), a bellows (Fig. 9, d) or a pneumatic device (Fig. 9, e) is used. In the latter case, a buffer layer of air is placed between the pulsator piston and the column, which alternately expands and contracts, imparting vibrations to the liquid in the column.

Rice. 9. Pulsating sieve extractors (A - heavy phase, B - light phase): a - the pulsator is attached to the bottom of the column; b - the pulsator is connected to the pipeline for supplying light liquid; c - pulsations are transmitted through the membrane; d - pulsations are transmitted through a bellows; e - pulsations are transmitted through a buffer layer of air (air cushion).

Pulsation extractors are highly efficient; they allow extraction without contact of operating personnel with the liquids being processed, which is very important if the liquids are radioactive or toxic.

In world practice, sieve pulsation columns with a diameter of up to 3 m and packed columns with a diameter of up to 2 m are used.

The disadvantages of pulsation columns include large dynamic loads on the foundation, increased operating costs, and the difficulty of processing easily emulsified systems.

Calculation part

Task 1. Calculation of the required extractant consumption in a continuously operating countercurrent extractor of the “mixer-settler” type.

Determine: volumetric (V E, m 3 /s) and mass (G, kg/s) consumption of the extractant.

We compose the extraction material balance equation:

Determination of volumetric flow rate of extractant:

3. Determination of extractant mass flow:

Task 2. Calculation of the required number of extraction steps when extracting Molybdenum from a solution with a 0.3 M solution of D2EHPA.

5. Calculation of the required theoretical number of extraction stages:

The result is rounded up to whole numbers.

(steps)

Task 3. Calculation of the efficiency of the Me salt extraction process (in a “mixer-settler” type extractor).

Volumetric flow rate of extractant
Diameter of 6 blade turbine mixer
Mixer rotation speed
Viscosity of aqueous solution
Extractant viscosity
Interfacial tension
Distribution coefficient
Extractant volume
Delay of extractant in extractor
Stirrer power function
Extractant density
Density of aqueous solution

Determine: extraction efficiency.

Determination of mixture density:

Calculation of droplet diameter:

m

Calculation of the average duration of phase contact:

With

Calculation of extraction efficiency:

Cleaning of drains

An example of extraction treatment of wastewater is the treatment of phenols in the coke, oil shale and coal processing industries; from aniline; from acetic acid; from epichlorohydrin in the chemical industry with organic solvents (benzene, ethers and esters).

When extracting phenolic wastewater, butyl acetate, diisopropyl ether, benzene, etc. are used as extractants. To increase the efficiency of phenol extraction, it is proposed to use mixed solvents: butyl acetate mixed with butyl alcohol, with diisopropyl ether, etc. However, butyl acetate or a mixture of butyl acetate is most often used with isobutyl acetate (phenosolvan), which have a high extracting ability with respect to phenols.

Installations for extraction purification of wastewater from phenols include four sections: 1) preparation of phenolic wastewater for extraction - separation of resins by settling and filtration, cooling of wastewater, capture of solvent vapors and, if necessary, carbonization; 2) extraction; 3) regeneration of the extractant from water; 4) solvent regeneration from the extract and production of commercial phenols.

Various solvents (benzene, esters, absorption oil, etc.) can be used for extraction treatment of wastewater from coke plants, but benzene, obtained from coal coking, is most widely used. Due to the fact that the distribution coefficient of benzene relative to phenol is small (about 2.2 at 20 °C), significant volumes of benzene are used and the concentration of phenols in the extract is low. Therefore, to regenerate benzene, not distillation methods are used, but the absorption method with an aqueous solution of alkali (benzene-phenolate method).

The benzene-phenolate purification method includes the following stages: 1) de-tarring of water by settling, filtering and washing with circulating benzene; 2) extraction of phenols from wastewater with benzene; 3) purification of benzene from acid gases soluble in it by washing with an alkaline-phenolate solution; 4) extraction of phenols from benzene with an alkaline solution; 5) separation of dissolved benzene from dephenolized wastewater. The resulting solutions of phenolates after preliminary evaporation are sent for processing.

Some coke plants use butyl acetate, phenosolvan, coal oil, etc. as extractants.

Extraction methods for dephenolization of wastewater have great advantages: high purification efficiency, the ability to extract non-volatile phenols, etc.

Conclusion

The main advantage of the extraction process compared to other processes for separating liquid mixtures (distillation, evaporation, etc.) is the low operating temperature of the process, which is most often carried out at normal (room) temperature. In this case, there is no need to expend heat to evaporate the solution. At the same time, the use of an additional component - an extractant - and the need for its regeneration leads to some complication of the equipment and an increase in the cost of the extraction process.

When extracting volatile substances, extraction can successfully compete with rectification in cases where separation by rectification is either difficult and sometimes practically impossible (separation of mixtures consisting of close-boiling components and azeotropic mixtures) or is associated with excessively high costs (extraction of harmful impurities or valuable substances from highly dilute solutions).

Extraction is indispensable for the separation of mixtures of substances that are sensitive to elevated temperatures, such as antibiotics, which can decompose when separated by rectification or evaporation. The use of extraction can often effectively replace processes such as the separation of high-boiling substances using high vacuum, such as molecular distillation, or the separation of mixtures by fractional crystallization.

The use of extraction for the separation of mixtures of inorganic substances when other separation methods are not applicable is very promising. Liquid extraction processes are currently successfully used for processing nuclear fuel, obtaining zirconium and hafnium and many other rare metals. Using extraction, high-purity non-ferrous and precious metals can be obtained.

In some cases, a significant effect is achieved by combining extraction with other separation processes. Examples of such combined processes are: separation of low-boiling and azeotropic mixtures using extractive rectification, pre-concentration of dilute solutions by extraction before evaporation and rectification, which are carried out with less heat consumption.

Bibliography

1. Einstein V.G. General course of processes and apparatus of chemical technology. - M.: Chemistry, 2002 - 1758 pp.

Dytnersky Yu.I. Processes and apparatus of chemical technology. Part 2. - M.: Chemistry, 2002 - 368 pages.

Zyulkovsky Z. Liquid extraction in the chemical industry. - L.; State Chemical Publishing House, 1963 - 479 pp.

Karpacheva S.M., Zakharov E.I. Pulsating extractors. - M.: Atomizdat, 1964 - 299 pages.

Kasatkin A.G. Basic processes and apparatuses of chemical technology. - M.: Chemistry, 1973 - 750 pp.

Leonov S.B. Hydrometallurgy. Part 2. Isolation of metals from solutions and environmental issues. - 2000 - 491 pages.

Meretukov M.A. Processes of liquid extraction and ion exchange sorption in non-ferrous metallurgy. - M.: Metallurgy, 1978 - 120 pages.

Planovsky A.N., Ramm V.M. Processes and apparatus of chemical technology. - M., Chemistry Publishing House, 1966 - 848 pp.

Proskuryakov V.A. Shmidt L.I. Wastewater treatment in the chemical industry. - L. Chemistry, 1977 - 464 pages.

Yagodin G.A., Kagan S.Z. Basics of liquid-liquid extraction. - M.: Chemistry, 1981 - 400 pages.

Goal of the work: Acquiring skills in calculating processes and apparatus for extraction wastewater treatment.

Introductory part.

Extraction is the selective extraction of a component from a liquid (refined sugar) using a liquid solvent (extractant). The phase that is enriched with the pollutant substance is called an extractant - before contact, and an extract - after contact.

One of the conditions for the extraction process is mutual insolubility and a sufficient difference in the densities of the phases (refined sugar and extractant).

Liquid extraction consists of a number of technological operations:

Contacting the liquid to be purified with the extractant;

Transfer of a component from one phase to another;

Phase separation;

Extractant regeneration.

Extractors can be horizontal and vertical, continuous and periodic, single-stage and multi-stage, cross-flow and counter-flow, with the supply of mechanical energy (for phase contact) and without the supply of mechanical energy, etc.

The simplest type of extractor is a vertical spray column with continuous phase contact (Figure 2.1). Waste water is fed into a hollow vertical cylindrical column from above, and an extractant whose density is less than the density of water is sprayed from below using a dispersant (in the form of drops). The countercurrent movement of the phases is ensured by gravity, i.e. difference in phase density (driving force). The resulting droplets pass through the work area, extract the contaminant and are collected in the upper settling tank.

Removing the light phase from the upper settling zone does not cause any difficulties; excess liquid is drained through the pipe. The removal of the heavy phase requires special adjustment, otherwise all the liquid may spill out from the bottom. The simplest device is the Florentine vessel, the operating principle of which is based on balancing columns of liquids (communicating vessels) discharged by flows of light and heavy phases

Requirements for the extractant:

Minimum mutual solubility with refined sugar;

High selectivity;

High distribution coefficient and large capacity;

Sufficient difference in density compared to refined sugar;

Availability, low cost, ease of regeneration;

Non-toxic, explosion-proof, minimal corrosive effects.

Extraction is effective when there is a high content of dissolved organic substances of technical value in industrial wastewater. It is most widely used for the purification of wastewater from enterprises for the thermal treatment of solid fuels (coal, shale, peat) containing a significant amount of phenols.

Calculation method

1. Extraction factor:

where Cin and Cout are the input and required output (MPC) concentrations of the pollutant in wastewater.

2. Volumetric flow rate of extractant:

, m 3 / h, (2.2)

where Q SW is wastewater flow rate, m 3 /h;

m - distribution coefficient.

3. Concentration of the extracted substance in the extract (with the original pure extractant):

, mg/l. (2.3)

4. Required extraction degree:

. (2.4)

5. Cross section of the apparatus:

, m 2 , (2.5)

where w is the flow speed, m/s. In calculations w=0.02 m/s.

6. Column diameter:

, m. (2.6)

7. Column height: H=(5-7)D, m. (2.7)

8. Height of TF outlet (from the equation of communicating vessels):

, m, (2.8)

Where And – density of LF and TF (water), =1000 kg/m3;

And – heights of LF and TF (Figure 2.1). Accepting that
, you can set or (For example, =H/7) and calculate the height of the TF output.

Table 2.1 - Initial data (options).