RU2221754C2 - Method and device for electroextraction of heavy metals from industrial waste solutions and waste waters - Google Patents

Abstract

FIELD: purification of electrochemical industry waste solutions and waste waters. SUBSTANCE: the invention presents a method and a device, that are used in purification of industrial waste solutions and waste waters for electroextruction of heavy metals such as Fe, Cr-3, Cu, Zn, Cd, etc. Such waste waters are usually formed at the enterprises of chromium compounds, nonferrous metallurgy and the enterprises of electroplatings. The treated solution, which is under effect of a variable three-phase electric power current, is periodically passed through the bipolar electroreactor made of a non-currentconducting material and containing six steel electrodes forming two three-electrode packs. At that electrodes at least of one pack are connected to three different phases of a three-phase current. The current may be supplied either to one of the three-electrode pack or connection of phases of a three-phase current may be executed in parallel to both packs. A fixed bipolar aluminum electrode is located in the interelectrode space. In one of its design versions it has a form of a perforated container made of the heat-resistant plastic filled in with a secondary aluminum or duralumin waste. According to other version the bipolar electrode made of a (duralumin) an aluminum waste may be manufactured without application of the perforated container. In this cars it is located in the interelectrode space by a pouring method. In the version to prevent a short-circuit each of the six steel electrodes is mounted in an insulating perforated plastic jacket interelectrode space a bulk image. Separation of non-ferrous metals is exercised in the form of sediments of chromites, ferrites and aluminates, and also hydroxy salts in the interelectrode space in absence of sediments of electrolysis on the electrodes. Then the sediments are separated from the waste solutions or waste waters using the known methods of filtration. Technical result an accelerated process of purification at the expense of increase of speed of separation of heavy metals, increase of productivity of the device without increase of non-productive loss of the electric power, decrease of the purification units sizes, increase of number of periods of their use in an operating mode. EFFECT: accelerated process of purification and speed of the heavy metals separation, increased productivity, decreased power consumption and the purification units sizes. 23 cl, 7 dwg, 7 tbl, 6 ex

Description

 The invention relates mainly to processes for treating industrial wastewater, in particular to electrochemical processes for their treatment to remove heavy metals such as Fe, Cr-3, Cu, Zn, Cd, etc. Such wastewaters are generated mainly on enterprises of chrome compounds, non-ferrous metallurgy and enterprises of electroplating coatings.

 Many reliable methods are known for purifying wastewater from chromium-6 and heavy metals, but not one of them is currently acceptable for treating large volumes (millions of cubic meters per year) of water.

The reason for this is either a low productivity, for example, of batch apparatuses with the need to withstand the water flow in the reaction zone for a sufficiently long time, or the inability to perform and operate these apparatuses larger than 100 m ³ , or the high cost of the reagents used, for example, in ion exchange technology, or the extreme complexity of the method, making it technologically vulnerable to continuity and trouble-free operation, for example in membrane ultrafiltration technology.

 Chemical methods of wastewater treatment today are unacceptable, in addition, due to secondary pollution of water, making it impossible to return it to production. As a rule, none of the methods of water purification offers the disposal of sludge contaminated from it.

There are a large number of electrocoagulation methods and devices for treating wastewater from chromium and heavy metals using direct current with a density on the electrodes of at least 2.5 A / dm ² in stationary electrolytic cells operating periodically (RU 2039710, 1995, 2045481, 1995).

The use of a constant electric current in electrocoagulation of wastewater has several disadvantages, the main of which are:
1. The need for the use of rectifier devices of high power and large losses of electric current for this rectification.

 2. The use of direct current in multielectrode packages with a small, as a rule, interelectrode (10 mm) distance leads to uneven wear of the electrodes, in which the anode material dissolves, and sticking of electroreduction products is observed on the cathodes. The consequence of this is frequent short circuits and emergency downtime of treatment systems for replacing electrode bags, as well as the conversion of replaced, partially worked, packages into secondary waste. The uneven wear of the electrodes when using direct current makes the methods of electrocoagulation of metals from wastewater with its help completely not technologically advanced even for small galvanic plants.

 In order to avoid the buildup of precipitation of electrocoagulation on the cathode, a number of patents propose the use of cathodes with a movable electrode or switching their polarity (US Pat.

 Known electrocoagulation methods and devices for purification, mainly from organic compounds, dyes and oils, using alternating current or alternating with superimposing it on direct current (SU 929582, 1982; SU 981240, 1982).

 The use of alternating current eliminates the unevenness of wear (dissolution) of the electrodes characteristic of methods and devices for direct current electrocoagulation, however, these methods do not eliminate the disadvantages of the latter caused by the periodicity of processes and small volumes of processed solutions that impede the treatment of wastewater in a continuous stream.

 A disadvantage of the known methods is also the use of non-industrial frequencies in them, i.e. either varying in the period of electrocoagulation at the frequency itself, or in the use of high frequencies (MHz) with a changing amplitude of positive and negative voltage pulses of different durations and shapes.

 The consequence of this is the need to use special frequency generators, and the technique and technology of their application for large volumes of water is unknown both for batch electrocoagulation apparatuses and for cleaning solutions in a continuous stream. The use of frequency generators in such electrocoagulation devices does not improve the technical and commercial effectiveness of electrocoagulation against the use of rectifiers in direct current electrocardiograms due to comparable current losses, both for rectification and for generating frequencies of different asymmetries.

 The closest analogue adopted for the prototype is a method of treating wastewater from heavy metals using an electrocoagulator operating on alternating current (RF patent 2071449, C 01 F 1/463, op. 1997.01.10).

 The method includes processing solutions in an electric reactor containing fixed and moving electrodes using three-phase alternating current.

 The essence of the device in which the known method is carried out is the location of a movable electrode of aluminum, all-metal or bulk, of aluminum scrap, in a perforated container. The movable electrode is located in the interelectrode space formed by fixed steel electrodes, to which two phases of a three-phase current are supplied. The aluminum electrode is grounded and moves in the interelectrode space between the steel electrodes.

 In an electric reactor of any design using this principle of combining fixed steel electrodes with a moving aluminum electrode, an initial solution is poured that requires cleaning from heavy metals and, in part, from organic compounds, and include a two-phase power supply to the fixed steel electrodes. Due to the bipolarity of the movable aluminum electrode, a voltage is established between it and the steel electrodes, the drop of which by 1 cm of the interelectrode space is determined by the magnitude of the voltage applied to the steel electrodes and the interelectrode distances: phase-movable electrode. The process of electric processing of solutions in the electroreactor is carried out until the sampling from it reveals excesses in the separated substance (heavy metal or organic substance) in relation to the normatively required content. The solution from the electroreactor is then fed to a filter to separate the precipitated precipitate from the solution to be purified.

 Steel electrodes are located horizontally in plan, strictly opposite each other when using rectangular electric reactors. In the embodiment of the cylindrical reactor, all peripheral steel electrodes are also oriented parallel to the cross sections of the central steel electrode.

 Power supply for steel electrodes, i.e. the voltage drop between the central and peripheral electrodes is the same, i.e. equally so that all peripheral electrodes are connected in parallel to the same phase of the electric current. To conduct the process of electrocoagulation of 3 phases of alternating current, any two are simultaneously used, either 1-2, 2-3, 1-3. And all three phases of three-phase current are used, for example, for pumps and motors.

 The use of two phases of alternating current has the disadvantage of incomplete use of the electrical supply of the electric reactor with a three-phase current, as a result of which during the cleaning process there are unproductive electrical losses that reduce the performance of the device due to incomplete use of electrical supply.

 The objective of the present invention is to provide a method for the continuous separation of heavy metals from low concentration wastewater and technological solutions that have undergone preliminary purification from chromium-6 and high concentrations of heavy metals by other, less energy-intensive methods, which allows to process large volumes of wastewater by increasing the rate of release of heavy metals, which can be increased by creating large voltages (U) and current consumption in the solution to be cleaned, by using on the electrode x in the process of electrocoagulation of all three phases of a three-phase alternating current. Thus, it is possible to vary the amount of current over a very wide range without loss, directly increasing current consumption and reducing current loss to the environment. The current to the solution is used completely without loss.

 The objective of the invention is also to provide a device for electrowinning heavy metals, capable of processing large volumes of wastewater and industrial water without increasing non-production energy losses, which will lead to a reduction in the size of treatment plants and an increase in the number of periods of their use in operating mode, to increase the productivity of the device.

 To solve the problem in a method for electrowinning heavy metals from technological solutions and wastewater, comprising treating it in an electroreactor with an alternating three-phase electric current of industrial frequency, according to the invention, the treated solution is periodically passed through a bipolar electroreactor containing six fixed steel electrodes forming two three-electrode bags , with the ability to connect at least the electrodes of one three-electrode package to three different phases of the three-phase znogo current.

 To solve the problem in a device for implementing this method, containing an electroreactor with fixed steel electrodes connected to a three-phase electric current, in the interelectrode space of which there is a bipolar aluminum electrode, according to the invention, the electroreactor contains six steel electrodes forming two three-electrode packages, with the possibility of connection electrodes of at least one three-electrode packet to three different phases of alternating current. The bipolar aluminum electrode, in this case, is made stationary. According to one of the options, it can be made in the form of a removable perforated container filled with aluminum or dural scrap. In another embodiment, a bipolar electrode from (dyur) aluminum scrap can be made without using a perforated container and placed in an interelectrode space in bulk. In order to prevent electrical short circuits, each of the steel electrodes is coaxially placed in a perforated insulating casing with a perforation diameter not exceeding 5 mm. The distance between the walls of the electrode and the insulating casing is 10-20 mm.

 Fixed steel electrodes are uniformly located on the periphery from the inside of the electroreactor at a distance of not less than 40 mm from its wall and are made of secondary pipelines or all-metal cylindrical products.

 It is possible to make steel electrodes from other secondary products, for example, rails, T-beams.

 The electric reactor is made cylindrical with a conical bottom and a discharge pipe made of non-conductive material, which can be used, for example, textolite, heat-resistant fiberglass, concrete, brick. It is possible to use other materials.

 The perforated container and insulating casings of steel electrodes are made of conductive heat-resistant plastic, with a perforation diameter of not more than 5 mm. The perforated container is filled with aluminum or duralumin scrap not less than 10 mm in size and located at a distance of not less than 40 mm from the steel electrodes.

 Three-phase current is connected either to one of two three-electrode packets or in parallel to two three-electrode packets.

 Parallel connection of the phases of the three-phase current to two three-electrode packets is carried out to each of 2 oppositely placed electrodes.

 Thus, all six electrodes are connected to 3 different phases of a 3-phase electric current, forming two intersecting triangles inscribed in the inner cylinder of the electric reactor, in the corners of which are steel electrodes. The zero phase, in contrast to the prototype is not used. The voltage for all 3 phases (2 triangles) is supplied unchanged and amounts to the current-carrying value used by the enterprise (380 V in Russia, 480 V in the USA or others, smaller, with a small number of cleaned solutions and small volumes of electric reactors).

 It is possible to parallelly connect the phases of a three-phase current to each of 2 adjacent electrodes to increase the cross-sectional area of steel electrodes, for example, if necessary, increase the total current through the solution of the electric reactor.

 The bipolar electrode is not grounded and does not connect to any phase of a three-phase alternating current.

 A different-phase current supply to six electrodes creates a very complex configuration of electric fields that intersect in a solution in different directions, in contrast to the prototype, in which the electric field is unidirectional from fixed steel electrodes to a movable aluminum one. The crossing of electric fields in the solution from electrodes of different phases, as well as large three-phase current voltages, initially create “shock” electric currents of high strength in the solution, which, in turn, leads to a high speed of cleaning solutions from heavy metals and some organic substances. The cleaning process under such "shock" conditions of power supply is only a few minutes, in contrast to the cleaning time according to the well-known patent, 1-1.5 hours.

Thus, the main advantage of the invention over the prototype is a significant increase in the productivity of treatment facilities. This, in turn, leads to a decrease in the size of treatment plants and an increase in the number of periods of their use in operating mode. The use of initially large electric current capacities in the solution being cleaned does not lead to an increase in the electric current consumption for cleaning 1 m ^{3 of} solutions; even, on the contrary, a certain decrease will be observed as a result of a decrease in heat dissipation by the solution, due to a decrease in the time spent by the solutions heated by the electric current in the electric reactor.

 As a result, non-production losses of electric energy for heating the treated solutions are reduced. A three-electrode steel system with a bipolar electrode made of aluminum (duralumin) scrap continues to emit heavy metals from solutions even during power outages.

 The invention is illustrated by the following graphic images.

Figure 1 presents a diagram characterizing the method of electrowinning heavy metals from technological solutions and wastewater;
in FIG. 2 shows a device for electrowinning heavy metals from technological solutions and wastewater, a general view;
figure 3 is the same, view B;
figure 4 presents the symbol of the packages of electrodes with three-phase connection;
figure 5 - steel electrode in an insulating casing;
figure 6 is the same, a section aa, in figure 5;
Fig.7 is the same, section bB, in Fig.5.

 The method is as follows.

 Heavy metals from low-concentration solutions are removed from them in the form of ferrite chromites and aluminates, as well as hydrooxosols formed when solutions are placed in a three-phase alternating electric field of industrial frequency and voltage with a varying current strength across the solution volume in different directions.

 The method is carried out in a device containing a cylindrical electric reactor 1, the housing 2 of which is made of a non-conductive material with a conical bottom 3. As a non-conductive material, for example, textolite, heat-resistant fiberglass, concrete, brick can be used. The use of other non-conductive materials is not ruled out. On the periphery of the electroreactor 1, six steel, for example, cylindrical electrodes 4 are fixedly mounted on the inner side, in the interelectrode space of which a bipolar aluminum electrode is placed. Fixed steel electrodes 4 are placed in the corners 2 of the intersecting triangles 21, 22 (figure 4) so that each opposite two electrodes (4.1-4.4, 4.2-4.5, 4.3-4.6) are connected to the same phase of the three-phase current.

 Depending on the volumes of disinfected solutions and their electrical conductivity, which varies between 1-4 mS / cm, the diameter of the electrodes in the range is in the range of 20-180 mm. For the same reasons, the electrodes can be connected to the current-supplying phases of the power supply unit 10 in other ways, except for the main one above - in the corners of intersecting triangles (Fig. 4), in which two opposite electrodes are connected to the same phase.

 For example, if it is necessary to increase the total current through the electroreactor solution, it is advisable to increase the cross-sectional area of steel electrodes. For this purpose, pairwise connection to the same phase with two adjacent electrodes is possible, i.e. 4.1-4.2; 4.3-4.4; 4.5-4.6, according to figure 3, 4.

 On the contrary, if it is necessary to reduce the current consumption, you can turn off one of the triangles 21, 22 in figure 4, using only one of the two.

 The wastewater (or semi-industrial solutions) to be cleaned from heavy metals through a funnel 6 (Fig. 1) enters the receiving tank 7. The reagents necessary in this technology are supplied from the tanks 8 to the same tank 7, in particular, to normalize the pH (usually pH 7 -8 for wastewater). Mixing them with the source water is carried out with compressed air - valve 9. After normalizing the required pH in the receiving tank 7, the solution from it is poured into the electroreactor 1.

After filling the electric reactor 1 to the required volume, it is powered by a three-phase current from the power supply unit 10. In some cases, it is advisable to power through a periodic circuit breaker. The energy consumption for cleaning 1 m ^{3 of the} solution can be reduced by at least 2 times, if the pulse duration and interruption are the same in time.

The electrical treatment in the electroreactor of 1 solution (especially wastewater) from heavy metals, and partly (for low volatility) from organic compounds, for example oil and dyes, is performed in the time interval of 2-10 minutes, depending on the volume of the solution to be neutralized. In the presence of an organic phase in the solution, scale foam is formed, which rises to the top of the electric reactor by the end of the electric treatment. The processing process is completed as soon as the solution reaches a temperature of 100 ^o C.

 The process of cleaning solutions from heavy metals is controlled by sampling through valve 11 (Fig. 1) from the side branch of the drain pipe 12. Samples are filtered before analysis. As aluminum is consumed in the electroreactor 1, the interelectrode space is replenished with a new portion of aluminum scrap. Old portions of aluminum settle, making room for new loading. Aluminum (duralumin) is consumed in the process of cleaning solutions from heavy metals, completely, to the end.

At the end of the neutralization process, the electroreactor 1 is released from the pulp through the valve 13. The pulp is sent to the filtration, preferably through a vacuum filter 14 (Fig. 1), and then to the intermediate storage tank 15. Precipitate 16, from the surface of the filter 14, aluminates, ferrites, hydroxides and hydrosols of heavy metals are sent for processing as a semi-finished product for non-ferrous and ferrous metallurgy, as well as in chemical production. Purified water 17 is reused in the main production or sent to an intermediate artificial reservoir containing flora, assimilating the salt content of alkali and alkaline earth metals and acid anions (sodium, SO ₄ ^2- , NO ₃ ^1- , potassium, calcium, etc.).

 In the presence of organic phase, for example, dyes in the cleaned solutions, it is often necessary to oxidize them or discolor the solutions after filtration 14. For this purpose, kits (tanks 8) must have the appropriate oxidizing agents: ozone, oxygen, hydroperoxides, pyrolusite, sodium ferrates and etc. Tanks 8 with oxidizing agents in this case are connected not only with a receiving tank 7, but also with a storage 15 having a mixing device. If the oxidation of the organic phase is accompanied by an additional precipitation, then from the tank 15 the solution with the precipitate is again wrapped on the filter 14.

 Technological design of the device does not have to be carried out strictly according to FIG. 1. Devices 1-15 can be located, for example, horizontally, i.e. on one mark or in any other way. In this case, transportation of the solution between the devices using pumps should be considered.

 The device of the electric reactor 1 is illustrated in more detail in Fig.2-7.

The electroreactor 1 is a cylinder with an outer casing 2 and a bottom 3. The casing 2 is made of fiberglass, textolite or other plastic, non-flowing, i.e. structurally stable, at temperatures up to 120 ^o C, as well as brick or concrete.

 Along the perimeter of the inner wall at a distance of no closer than 40 mm there are six replaceable, steel electrodes 4 (4.1, 4.2, 4.3, 4.4, 4.5, 4.6). It is possible to manufacture them from waste pipelines or all-metal cylindrical products or secondary products, for example, rails, T-beams.

 According to one of the options (figure 2, 3) in the interelectrode space at a distance not closer than 40 mm from the electrodes 4 is a plastic heat-resistant container 5 to the entire depth of the electroreactor 1 to the conical bottom 3. The container 5 is perforated with a hole diameter of 5 mm The container is filled with aluminum (or duralumin) scrap with pieces of at least 10 mm in size.

 In another embodiment, a bipolar electrode from (dure) aluminum scrap can be made without using a perforated container 5, while it is placed in an interelectrode space in bulk. In this case, to prevent electrical short circuits, each of the six steel electrodes 4 is installed in an insulating perforated plastic, for example, tubular casing 23 with a hole diameter of 5 mm (Figs. 5-7). The distance between the walls of the electrode 4 and the specified casing 23 is 10-20 mm

 The lower part of the electric reactor - the bottom 3 - is made in the form of a cone, in the center of which there is a nozzle 12 for the release of purified pulp with precipitation of heavy metals, iron and aluminum, replacing the allocated (cemented) heavy metals. The pipe 12 is equipped with a discharge pipe 11 for sampling while controlling the degree of purification of the solutions. Inside, a steel mesh 18 with a hole diameter of not more than 3 mm is placed inside the pipe in order to prevent pieces of residual aluminum from getting into the further transport cleaning system according to the technology shown in FIG. 1.

In addition to the electrodes 4 and the container 5, an industrial pH meter sensor (pH 2-10), a temperature sensor (up to 110 ^o С), a compressed air bubbler tube should be placed in the interelectrode space of the electroreactor. The electroreactor 1 is also equipped with a level gauge to control the volume of filled and electrically processed water. These technology control devices are not shown in FIGS.

The electric reactor is equipped with an electrically insulating decorative cover 19 with all the necessary holes for connecting the electrodes 4 to the power supply unit 10 (Fig. 1) and for connecting the above control sensors (pH, T, bubbler, circulation pipe, level gauge) to the recording devices. An inspection window 20 is made on the decorative cover 19. The diameter and height of the electroreactor are calculated from the data of the required productivity of neutralizing (cleaning) the source water from heavy metals, including chromium - 3 (copper, zinc, nickel, tin, etc.), except manganese-2. (The latter can also be eliminated by raising the pH of the solution to a pH of 10). The following figures can serve as the basis for calculations. The voltage drop in the interelectrode space per 1 cm of the path should be at least 3.1 V / cm. The current density per 1 cm ^{2 of the} transverse dimension of any steel electrode should be at least 0.027 A / cm ² . In addition, the dimensions of the electric reactor and their number are calculated not only on the basis of the required productivity for the treated water, but also taking into account the power supply for voltage U and current strength I provided by enterprises for the separation of heavy metals from treated water. For the successful separation of heavy metals within a few (2-10) minutes, it is necessary to take into account that the voltage drop per 1 cm of interelectrode distances is 4.1-4.2; 4.1-4.6; 4.2-4.3; (Figs. 3, 4), etc. should not be less than 3.1 V / cm.

The current density in the cross section of each electrode should also not be lower than 270 A / m ² . Using these data, as well as the available maximum voltage and current provided by this enterprise, the dimensions of each electric reactor and their number are calculated, taking into account the performance of water neutralized from heavy metals.

 The connection of six electrodes to the power supply unit 10 (Fig. 1) is made so that only three electrodes (4.1, 4.3, 4.5, 4.2, 4.4, 4.6) are fed in parallel with a three-phase current. The six-electrode system of FIG. 3, 4 therefore represents two inverted intersecting triangles 21, 22. In this case, opposing electrodes are connected to the same phase of the three-phase current, i.e. according to FIG. 3, 4, electrodes 4.1-4.4 are connected to the same phases; 4.2-4.5; 4.3-4.6. This does not exclude other connection methods. If necessary, it is possible, for example, a connection in which adjacent electrodes are connected to the same phase of a three-phase current: 4.1-4.2, 4.3-4.4, 4.5-4.6.

 With this inclusion, it is possible to increase the current strength in the interelectrode space between opposite pairs of electrodes.

With a large current developing in the electric reactor on six electrodes, it can be reduced if necessary, if a three-phase electric current is connected only to three electrodes of one triangle out of two. Aluminum (duralumin) scrap placed in a container (or in bulk) of the interelectrode space serves as a bipolar electrode in the electric reactor, on which heavy metals with negative redox potentials, such as Cr3, Zn ²⁺ , Ni ²⁺ , Cd ^{2+, are released} etc., i.e. approaching in sign and size to aluminum. The selection of metals with E> 0, i.e. electropositive, like copper, is largely provided by steel active electrodes.

 Chemical-theoretical background and justification of the technology for the separation of heavy metals from wastewater.

где Me=Al, Cr, Fe³⁺, и для переходов

;
где Me=Mg, Zn, Cu, Cd, Co, Fe²⁺, Ni и др., сопровождающие цветную металлургию. Эти окислительно-восстановительные потенциалы для указанных металлов приведены в табл. 1.The main premise is the difference in the values of redox potentials necessary for the restoration of the ionic form of different metals into metal, i.e., for transitions

where Me = Al, Cr, Fe ³⁺ , and for transitions

;
where Me = Mg, Zn, Cu, Cd, Co, Fe ²⁺ , Ni, etc., accompanying non-ferrous metallurgy. These redox potentials for these metals are given in table. 1.

 The vast majority of wastewater requiring treatment before being reused or released into the environment is represented by the metals listed in the redox potentials in Table 1. Any other metals, as a rule, are contained in wastewater (in dissolved form) either in immeasurably lower quantities than those listed in Table 1, or have redox potentials in the range from Zn to Cu. Thus, the electrochemical separation of Zn and Cu will always be accompanied by the simultaneous release of these unspecified metals, for example, Mo, V, W, coprecipitating with Cu and Zn under the conditions of electrochemical treatment of the solutions containing them.

 According to the series of redox potentials given in Table 1, it is expected that each previous element in metallic form will displace the next, which is in ionic form, into hydroxide at not too low pH> 3. For example, metallic aluminum will displace iron-2 from its sulfate solutions, and metallic iron will displace copper-2 from solutions of its salts.

Все без исключения металлы в табл. 1 с Е_0,v<0 будут разрушать дихромат-ионы, способствуя превращению хрома-6 в хром-3 согласно реакции
Cr₂O₇ ^2-+Fe+H₂SO₂--> Cr₂(SO₄)₃+Fe₂(SO₄)₃+H₂O (3)
От хрома-3 далее можно освободиться уже с помощью металлического алюминия.

All metals without exception in the table. 1 s E _{0, v} <0 will destroy dichromate ions, contributing to the conversion of chromium-6 to chromium-3 according to the reaction
Cr ₂ O ₇ ^2- + Fe + H ₂ SO ₂ -> Cr ₂ (SO ₄ ) ₃ + Fe ₂ (SO ₄ ) ₃ + H ₂ O (3)
From chromium-3, you can then free yourself with the help of aluminum metal.

 If an electric current is passed through such a heterogeneous system with metals placed in solutions, then all processes of displacement from a solution by metals with high redox potential | -E | ions with less | -E | will accelerate with increasing current density on the displacing metal electrodes.

 When using alternating current for this, many processes will occur on the electrodes, the main of which will be the restoration of displaced ions and dissolution of the metal electrodes (their transition to the ionic state). Alternating current also contributes to the destruction in the solution of molecular formations having a polar-dipole structure, for example, organic compounds.

Suppose there is a certain sulfate solution containing all the elements of Table 1 in ionic form, i.e. heavy metals, except, naturally, light metals - Mg and AL. Solutions of sulfates of these heavy metals and Cr ₂ O _{7 are} preferable to consider, since the vast majority of wastewater is formed by them. If electrodes made of aluminum or its alloys with magnesium (duralumin) are placed in solutions of such sulfates and an alternating current is passed through them through the solution, then the electrodes, in the first place, will be the process of cementation of heavy metal ions, according to the reaction,
Me ²⁺ + A1 (Mg) -> Me ^о + A1 ³⁺ (Mg ²⁺ ). (4)
The displaced metals formed in a subatomic form, due to alternating current, do not stand out, however, in a metallic form, but form hydroxosalts up to hydroxides such as (MeOH) SO ₄ ; [Me (OH) ₂ ] ₂ SO ₄ and Meun; Me (OH) ₃ , where Me-3.

At the same time, the free acid contained in the solutions is spent on interaction with the subatomic metal, which is why the pH of the solutions increases over the period of this process. Aluminum and magnesium of duralumin bipolar electrodes also do not actually form sulfates in ionic (dissolved) form by reaction (1; 2), but are released in the form of hydroxosulphates of variable composition. An increase in pH, in these cases, further leads to the hydrolysis of all sulfates in the initial ionic form by reaction
MeSO ₄ + HOH -> (MeOH) ₂ SO ₄ + HOH -> variable hydroxosulphates (5)
those. with the formation of the same hydroxosulfates (Me-2).

Thus, the result of such an electrochemical treatment only with the use of (dyur) aluminum electrodes will result in precipitation of very voluminous, flocculent aqua-complexes of hydroxosulphates with the general formula
(Me ²⁺ ) _x (O) ^2- _y (H ₂ O) _z (OH) _u (Me ³⁺ ) _v (SO ₄ ) _p ^2- (6),
as products of coprecipitation of aluminum hydroxy sulfates with hydroxyl sulfates of heavy metals, including chromium and iron. The coprecipitation process is accompanied by adsorption on the surface of highly charged hydroxosulphates (due to AL-3, Cr-3, and Fe-3) of the directly dissolved salts of heavy metals themselves, which form a diffuse adsorption layer on this surface, oriented by its polar dipoles to the surface mainly by negatively charged sulfate anions. For this reason, electrocoagulation with alternating current, simultaneously with the release of heavy metals into the precipitate, leads to the removal of sulfur from solutions. Precipitation from solutions is separated by conventional filtration, for example on vacuum filters and / or gravel and sand.

In the present invention, a three-phase alternating current is used, which implies a three-electrode electrolytic cell, in which steel cylindrical rods or secondary scrap of steel pipelines are used as the main electrodes. An aluminum bipolar electrode is placed in the interelectrode space in the form of a bulk (aluminum scrap) perforated container or in bulk between steel electrodes without a container. As a result, only Cr-3 and Zn will not be cemented on the surface of steel electrodes, but all subsequent iron will be cemented in Table. 1 metals. This process will be accompanied by the release of Fe-2 ions into the solution by analogy with reaction (2). Since iron-3 and chromium-3 are contained in the wastewater, in the presence of the Fe-2 ion, ferrites and iron chromites Fe ₃ O ₄ and FeСr ₂ O _{4 are formed} , incapable of aquacomplex formation of hydrophobic sediments, regular regular crystal structure with very low solubility. Together with a large amount of precipitate of magnetite FeFe ₂ O ₄ and chromite in this case, co-crystallization of ferrite chromites of other metals with oxidation state-2 occurs with the general formula Me Fe ₂ O ₄ and MeCr ₂ O ₄ , where heavy metal ions act as metal oxidation state-2: Zn, Cu, Ni, etc., as well as Mg and Ca. Zn and Cr-3 are cemented at the bipolar electrode, but the formation of ferrite chromites favors a preferential shift of the reaction of the formation of hydroxosulfates towards heavy metal aluminates, which also co-crystallize with ferrite chromites.

 If iron-3 is absent in the initial cleaned solutions, then it is obvious that during electrocoagulation treatment it is necessary to subject aeration solutions (air bubbling) or to add iron-3 salts to them intentionally.

 It should be noted one more feature characteristic of solutions in contrast to water. When an alternating electric current is passed through them, solutions containing electrically conductive solutes heat up faster than pure water.

According to the Joule-Lenz law, Q = 1 ² Rt W • hour (7)
According to the law of heat absorption by a solution for heating it at ΔT = T _end -T _beg
Q = mcΔT (8).

Equations 1 and 2 equalize when passing an electric current through liquid conductors, so that
1 ² Rt = mcΔT, (9)
where I is the current strength;
R is the resistance of the solution;
m is the mass of the solution;
t is the time of electric heating;
C is the heat capacity of the solution;
T _nach T _finite - initial and final temperature of the solution.

From equality (9) it follows that
t = mc (T _end -T _beg ) / I ² R, (10),
where I ₂ R = IU is the power of the electric reactor.

From equality (10) it follows that when the same power I ₂ = IU is applied to water and solutions, it is possible to heat solutions with ΔT faster with a shorter electric heating time than water, only if the heat capacity of the solution is lower than heat capacity of water C = 4.180 J / gram.

 The study of practical solutions of leather production (chrome-3), galvanic production (chrome-6), as well as washing water of printed circuit boards (Cu) shows in the general case a decrease in their heat capacity to 1.5 J / gram. As a result, the rate of their heating can be reduced in comparison with the rate of heating of water under comparable conditions by 1.5-2.5 times.

Examples
Example 1 (table 2).

 In an electric reactor constructed structurally according to FIGS. 2, 3, a 12 liter volume was poured into the tannery waste water containing chromium-3, as well as organic reagents and dyes for treating genuine leather (Table 2).

The operating voltage of the three-phase current was 380 V. In this case, only one triangle was read (Figs. 3, 4), i.e. three out of six electrodes. Before turning on the electroreactor, 10 ml of ammonia water with a content of 28% NH ₃ are added to it (to the solution) to neutralize the excess acid (with a change in pH from pH _nach to pH _kon in Table 2). The process of electric processing was completed when the temperature of the solution reached 100 ^o C. Due to the presence of an organic phase in the solution, it forms a head of foam rising as it is processed, which, when cooled, settles in the form of a precipitate that precipitates with chromium-3.

 The analysis of the final solutions shown in table 2, as in all other examples, was carried out in parallel in independent laboratories. In all cases (examples 1-5), parallel independent tests showed results that are 1-2 orders of magnitude lower than those given in Tables 2-7.

 Example 2 (table 3).

The object of the test is tannery wastewater containing Cr- (III). The tests were carried out on a laboratory electroreactor with a capacity of 25 dm ³ . As the electrodes, steel pipes with a diameter of 34 mm and a length of 480 mm were used. The number of electrodes - 6 pcs. Electrodes are inserted into perforated plastic pipes. The voltage is 380 V. The conductivity of the solution is 2.66 mSm / cm. In the table. 3 presents the results of testing the wastewater of leather production at the laboratory electric reactor - 0.025.

 Example 3

 The test object is wastewater of leather production containing Cr- (III), as in example 2, but with a lower content of Cr- (III) and with lower electrical conductivity than in example 2.

The tests were carried out on a laboratory electroreactor with a capacity of 25 dm ³ . As the electrodes, steel pipes with a diameter of 34 mm and a length of 480 mm were used. The number of electrodes - 3 pcs. Electrodes are inserted into perforated plastic pipes. The interelectrode distance (side of an equilateral triangle) is 195 mm. The voltage is 380 V. The conductivity of the solution is 2.1 mSm / cm., PH 7.74. The content of Cr- (III) after electrocoagulation decreased from 0.06 mg / dm ³ to 0.02 mg / dm ³ .

 Example 4 (tables 4, 5).

 The test object is wastewater after etching printed circuit boards. the chemical composition of the source water is given in table. 4.

The tests were carried out on a laboratory electroreactor with a capacity of 25 dm ³ . As the electrodes, steel pipes with a diameter of 34 mm and a length of 480 mm were used. The number of electrodes - 6 pcs. Electrodes are inserted into perforated plastic pipes.

The voltage was 380 V, the time of electrocoagulation was 7 minutes to a temperature of 85 ^o C. The current increased from 36 A to 53 A. The pH decreased from 8.99 to 8.62 units. pH Electrical conductivity - 2.3 mSm / cm. The initial concentration of copper is 11.4 mg / dm ³ , after electrocoagulation and filtration - 0.30 mg / dm ³ . The test results are presented in table. 5.

 Example 5 (table 6).

The same as in example 4, the time of electrocoagulation was 10 minutes to a temperature of 85 ^o C. The current increased from 30 A to 80 A. The pH decreased from 9.5 to 9.3 units. pH, initial copper content - 120 mg / dm ³ , electrodes - 3 pcs.

 Example 6 (table 7).

 The test object is wastewater after etching printed circuit boards.

The tests were carried out on an industrial electric reactor with a capacity of 450 dm ³ . Steel pipes with a diameter of 34 mm and a length of 830 mm were used as electrodes. The number of electrodes - 3 and 6 pcs. Electrodes are inserted into perforated plastic pipes. The interelectrode distance (side of an equilateral triangle) is 200 mm. The length of the electrodes is 830 mm, the working length of the electrodes, taking into account the volume of water used for the tests, is 500 mm.

Voltage - 380 V, the time of electrocoagulation - from 8 to 22 minutes (up to a temperature of 60 ^o C). The current changed from 50 A to 95 A. The pH decreased from 8.88 to 8.52 units. pH The electrical conductivity of wastewater was 2.18 mSm / cm. The initial concentration of copper is 21.4 mg / dm ³ , the copper content after electrocoagulation and filtration is from 0.36 to 2.4 mg / dm ³ . The test results are presented in table.7.

Thus, the use of the proposed method and device for electrowinning heavy metals from technological solutions and wastewater allows to achieve the following technical result:
1. Acceleration of the cleaning process by increasing the rate of release of heavy metals, due to the creation of high voltages and current consumption in the solution being cleaned, due to the use of all three phases of a three-phase alternating current of industrial frequency on the electrodes in the process of electrocoagulation.

 2. Increasing the productivity of the device, reducing the size of the treatment apparatus and increasing the number of periods of their use in the operating mode.

 3. Decrease in non-production losses of electric energy for heating the processed solutions.

 4. The ability to process large volumes of wastewater and industrial water without increasing energy losses.

 5. The three-electrode system (one-pack or two-pack) of steel with a bipolar electrode made of aluminum (duralumin) scrap continues to emit heavy metals from solutions during power outages.

 6. The method allows you to cyclically wrap the treated water for re-treatment in the same electric reactor.

 7. Allows you to receive precipitation of ferritic chromites, aluminates and hydroxides of heavy metals with a high filterability.

8. The secondary pollution of water by treatment reagents is excluded;
9. The precipitate of the electrochemical treatment forms a concentrate of non-ferrous and ferrous metals, suitable for secondary processing in metallurgy and chemistry.

Claims (23)

1. A method for electrowinning heavy metals from technological solutions and wastewater, including processing solutions in an electric reactor with an alternating three-phase electric current of industrial frequency, characterized in that the solution to be treated is periodically passed through a bipolar electric reactor containing six fixed steel electrodes forming two three-electrode packages with the possibility of connection at least the electrodes of one three-electrode stack to three different phases of a three-phase current. 2. The method according to claim 1, characterized in that the current is supplied to only one three-electrode package. 3. The method according to claim 1, characterized in that the phases of the three-phase current are connected in parallel to two three-electrode packets. 4. The method according to claim 3, characterized in that the parallel connection of the phases of the three-phase current is carried out to each of two oppositely placed electrodes. 5. The method according to claim 3, characterized in that the parallel connection of the phases of the three-phase current is carried out to each of two adjacent electrodes. 6. A device for electrowinning heavy metals from technological solutions and wastewater, containing an electroreactor with fixed steel electrodes installed with the ability to connect to a three-phase electric current, in the interelectrode space of which there is a bipolar aluminum electrode, characterized in that the electroreactor contains six steel electrodes, uniformly located on the periphery from the inside of the electric reactor, forming two three-electrode packages, with the ability to connect at least m Here, the electrodes of one three-electrode package to three different phases of a three-phase current, while the bipolar aluminum electrode is made stationary. 7. The device according to claim 6, characterized in that the fixed steel electrodes are installed with the ability to connect the phases of a three-phase alternating current to only one three-electrode package. 8. The device according to claim 6, characterized in that the fixed steel electrodes are installed with the possibility of connecting the phases of a three-phase alternating current in parallel to two three-electrode packages. 9. The device according to claim 8, characterized in that the three phases of the three-phase current are connected in parallel to each of the two oppositely placed electrodes. 10. The device according to claim 8, characterized in that the three phases of the three-phase current are connected in parallel to each of two adjacent electrodes. 11. The device according to claim 6, characterized in that the aluminum bipolar electrode is made in the form of a removable perforated container filled with aluminum or duralumin scrap, while the steel electrodes are made without insulating casings. 12. The device according to claim 6, characterized in that the aluminum bipolar electrode is made bulk in the interelectrode space without a container, with each steel stationary electrode coaxially placed in a perforated insulating casing of a non-conductive material. 13. The device according to p. 12, characterized in that the distance between the walls of the steel electrode and the insulating casing is 10-20 mm 14. The device according to PP.11 and 12, characterized in that the size of the pieces of aluminum or aluminum scrap is at least 10 mm. 15. The device according to claim 11, characterized in that the perforated container is made of conductive heat-resistant plastic. 16. The device according to claim 11, characterized in that the perforated container is located at a distance of at least 40 mm from the steel electrodes. 17. The device according to PP.11 and 12, characterized in that the perforated container and the insulating casing are made with a perforation diameter of not higher than 5 mm. 18. The device according to claim 6, characterized in that all the steel stationary electrodes are uniformly located on the periphery from the inside of the electroreactor at a distance of not less than 40 mm from its wall. 19. The device according to claim 6, characterized in that the steel electrodes are made of secondary pipelines or all-metal cylindrical products. 20. The device according to claims 6 and 19, characterized in that the diameter of the steel electrodes varies between 20-180 mm. 21. The device according to claim 6, characterized in that the steel electrodes are made of secondary products, such as rails, T-beams. 22. The device according to claim 6, characterized in that the electric reactor is cylindrical with a conical bottom and a drain pipe. 23. The device according to claim 6, characterized in that the electroreactor is made of conductive material, such as textolite, heat-resistant fiberglass, concrete, brick.

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