IT202100017015A1 - PROCEDURE AND ELECTROCHEMICAL CELL FOR THE RECOVERY OF PURE CHROME (III) FROM USED BATHS IN THE TANNING INDUSTRY - Google Patents

Description

DESCRIPTION

of the industrial invention entitled:

Electrochemical cell for the recovery of high purity heavy metals from exhausted baths of industrial processes

Summary

The present invention refers to an electrochemical cell with differentiated compartments obtained in the space between the anode and cathode through an innovative arrangement of anionic and cationic membranes such as to create a zone between two paired anionic membranes which lends itself to being progressively basified in order to obtain a controlled electroprecipitation as hydroxide of the heavy metal(s) present in industrial wastewater. The metal hydroxides cos? obtained are characterized by high purity as co-precipitation of any organic compounds present is avoided. The correct and efficient functioning of the cell? favored by the treatment of the membranes with antifouling polyelectrolytes.

Sector of application of the invention

The inventive cell described above can find application in all those industrial processes that generate liquid waste containing residual heavy metals that need to be separated for compliance with environmental regulations and/or recovered in conditions of maximum purity in relation to their traded value.

State of art

The main electrochemical techniques applied to the treatment of waste water are represented by electrolysis and electrodialysis. In both cases the electrolytic process takes place inside d? a cell, in the most? simple consisting of a cathode, an anode, an electrolyte and a separating element (a porous septum or an ion exchange membrane) which divides the cathodic compartment from the anionic one.

In particular, the electrodialysis technology is widely used when it comes to creating through the application of an electric field, such as in the processes of desalination, a selective migration of the ions present in solution towards the electrodes, with separation in the cathodic and anodic compartments of the membranes permeable to cations and anions respectively and consequent production of two output streams, one diluted and the other concentrated.

Electrodialysis therefore lends itself well to the separation of metal ions from the respective solutions when working with simple water-based matrices; the reality? it changes when it is necessary to operate on wastewater from industrial processes, made up of complex matrices and almost always rich in undesirable organic substances.

In these cases the direct contact between the cathode and the wastewater to be treated leads to the deposition on the electrode of an encrusting precipitate which progressively reduces its functionality, the water molecules have less availability? to the dissociation with consequent extension of working times and necessity? of raising the applied electric potentials, there is an anomalous rise in the temperature at which the system operates, a possible damage to the membranes, the irreversible deposition of the ions involved and of the hydroxides on the membranes themselves, further drawbacks which actually prevent the application of this technology as well.

The purpose of the present invention ? to overcome the drawbacks complained of above in order to fully seize all the opportunities? that the electrochemical deposition as hydroxides of metal ions present in industrial waste entails in terms of functionality, quantitative recovery and above all purity of the recovered metal.

Therefore, always with a view to making full use of electrodialysis technology, yes? proceeded to develop a new electrochemical cell, no more? made up of two compartments, like those commercially usable, but made with three compartments.

Detailed description of the invention

The innovative electrochemical cell (IC), as represented in fig. 1, ? consisting of three compartments which originate from the arrangement between the cathode (1) and the anode (7) of two anion exchange membranes (3, 5) and three tortuous path spacers (2, 4, 6) arranged between the individual electrodes and the adjacent anionic membranes and between the two anionic membranes themselves. The cathode and the anode are both in contact with their own aqueous flow, while the electrolyte to be treated circulates in the central compartment delimited by the two anionic membranes. In the compartment delimited by the cathode (1) and by the anion exchange membrane (3) a flow of water recirculates (8); in the compartment delimited by the anionic membrane (3) and by the anonic membrane (5) the electrolyte to be treated recirculates (9); in the compartment delimited by the anionic membrane (5) and by the anode (7) a flow of water recirculates (10).

The configuration described above, which differs from the normal structuring of two-compartment electrochemical cells, allows the many problems associated with these to be overcome. In particular the direct contact between the cathode and the? anode with? respective aqueous flows placed in recirculation (8, 9) guarantees the protection of the electrodes from the deposition of unwanted ions and therefore avoids the consequent encrustations which lead to the reduction of the surface available for the water dissociation reaction and consequently an increasingly decrease? sensitive in the efficiency of the process. Furthermore (fig. 2) the direct contact of the cathode (-) with water free from pollutants favors the rapid and abundant production of hydroxyl ions (OH-) which easily migrate into the central compartment crossing the adjacent anion exchange membrane (3) under the ?influence of the applied electric field. At the same time the metal ions with a positive sign (M<+>) cannot migrate towards the anode (+) being blocked by the anion exchange membrane (5) and therefore remain in the central compartment whose solution (9) is slowly basing itself. On the other hand, do the anions (A ) possibly present in the solution to be treated migrate towards the anode, causing them to move away from the matrix being treated thus? to contribute to the high purity of the electroprecipitated hydroxide.

With this new provision there is no need? to raise the electric potential over time, there are no temperature rises inside the system and not s? occur damage to the membranes even for continuous use, the anionic membrane (3) on the cathode side (-) does not undergo any type of encrustation, while some extra attention? deserves the anionic membrane (5) on the anode side (+) for the possible accumulation during the treatment of the anions, when present in quantity? substantial in the original matrix.

A different approach? necessary instead as regards the fouling of the anionic membrane (3) on the anode side (+) and of the anionic membrane (5) on the cathode side (-) that can? occur and tend to advance in the course of? activity? for the deposition on the indicated surfaces of positive hydroxyl ions and organic substances almost always present in a consistent form in the wastewater.

This inconvenience, extremely limiting on functionality? of any electrochemical application, ? eliminated by introducing specific up-grading characteristics into the innovative cell dictated by the long experimental process conducted.

In particular, the arrangement inside the cell of three spacers with a tortuous path, d? of which one represented at a specific level in fig. 3, inserted between the anionic membrane (3) on the anode side and the anionic membrane (5) on the cathode side, also identified in fig. 1 with the number 4, and the other two arranged as represented in fig.1 with the numbers 2 and 6. L? use of this type of spacers, together with the staggered arrangement between the inputs and outputs of the matrix to be treated and the adoption of a speed? laminar not less than 30 cm / sec, it allows on the one hand to put the matrix itself in contact with the totality of the surface of the membrane thus ensuring? the full functionality? and from the other to avoid the formation of undesired preferential routes and phenomena of parking all the more? possible in the hydroxyl precipitation processes of the positively charged ions of interest.

Different considerations must instead be made regarding the presence in the wastewater of organic matter inserted or derived from the industrial process. These macromolecules, whether they are hydrophilic or hydrophobic, are characterized by an overall charge of a negative sign so that, by migrating under the action of an electric field, once they come close to of an anionic membrane, they will not be rejected in consideration of the electrostatic attraction exerted by the fixed positive charges present on the surface of the membrane, but at the same time they will not be able to penetrate and cross it due to their size and the high negative charge . A very high percentage of the organic molecules present in the solution to be treated therefore tends to stop near the? of the membrane causing the formation of a deposit which causes the reduction of the active surface area, the passage of the other ions and consequently influencing the performance up to generating increases in the electrical resistance and often irreversible deterioration of the membrane itself.

The complex problems of organic fouling are overcome in the innovative cell proposed through the use of anion exchange membranes pretreated with polyacrylsulfonamide? or other polyelectrolytes of the same family, consisting of polymeric chains containing sulphonic and amide functional groups (11), as shown in fig. 4.

These polymers go sterically to form a thin negative layer on the surface of the membrane which, without modifying the selectivity? overall ? still able to change the permeability? in the vicinity? of the solution-membrane interface.

The polyelectrolyte used as modifier develops in length towards the solution and perpendicularly to the membrane (12) as in fig. 5; remains anchored to it through its anionic functional groups and at the same time coordinates around itself a sufficient number of water molecules such as to prevent the hydrophobic organic molecules from approaching the surface of the membrane itself, avoiding its contamination,

Finally, the geometric structure of the innovative cell, as a result of the device symmetries, lends itself excellently to the periodic reversal of the current flow, through the application of reverse electrodialysis techniques (EDR) based precisely on the inversion of the reactions at the electrodes, the system .

The application of a reverse current allows numerous advantages, such as the detachment of the polarization films, the elimination of any residual encrustations, the reduction of the sludge deposited on the surface of the membranes, the elimination of the chemical products normally used for cleaning the electrodes . In other words, EDR technology allows more? in general, a significant increase in functionality? of the system and operating times, which is accompanied by a significant reduction in management.

Yes ? so far a general description of the invention has been given. With the help of the following example, of an explanatory and non-limiting nature of the invention, an application is illustrated in a notoriously polluting industrial sector such as that of the leather tanning industry, aimed at a better understanding of its purposes, characteristics and obtainable results.

Example 1

In the tanning industry and more specifically in chrome tanning, compared to the amount of chromium (IN) that is offered, only two thirds (2/3) are fixed to the leather, while the residual part remains in the exhausted bath with all the consequent problems ranging from loss of a consistent value attributable to the metal, up to the presence of the same chromium in the sludge deriving from the treatment of waste water purification.

The abatement and recovery of chromium from such wastes constitutes a problem of primary importance which risks constituting an essential limit not only to development but also and above all to continuity? operating across the industry.

In the literature, various applications are reported in this regard ranging from absorption on activated carbon to membrane processes (micro-nano-ultra filtration and reverse osmosis), to the use of ion exchange resins, finally to technologies that make use of the of electric current such as electrocoagulation (EC), electroflotation (EF), electrooxidation (EO).

In particular, the electrodialysis technique (ED-EDR) makes it possible to predict good results, especially when it is necessary to carry out the simultaneous removal of more? metals present in the exhausted solution, but in any case the two-compartment cell always has the disadvantages already? cited to which, in the case of chromium, it is necessary to add the facility? of the transition from the state of oxidation (III) to that (VI) with all the problems determined by?acclarata concerogeneit? of the latter.

Therefore, the limits of the known art can be traced back to the fact that currently the only recovery technology that is actually applied? the precipitation of the chromium present in the exhausted baths by means of basification and separation/concentration of the chromium hydroxide generally formed by filter-pressing.

This technology, quite obsolete, presents the defect of an inevitable co-precipitation of organic substances and other pollutants present in the tanning bath together with the chromium, such that not even the polishing carried out on the base chromium with diatomaceous earth manages to reduce its presence in the final product to values acceptable for reuse as such in tanning baths.

Selective precipitation by electrodialysis eliminates all the problems previously mentioned in relation to the purity of the! chromium hydroxide and moreover does not require ancillary operations such as concentration and ennobling.

In view of the industrial application and according to the design and construction principles of a plant sized for the purpose, yes? then proceeded to put in the series pi? cells identical to that previously described innovative base, thus building a ?pile? of units of electrodialysis, according to a sequence such as: cathode - Anionic - Anionic - Cationic - ( Anionic - Anionic - Cationic n-times) - Anionic - Anionic - anode.

With this configuration, limited in the representation in fig. 6 to n? 3 units? stacked, with the cationic membranes (C) introduced that prevent the passage of ions from a unit? of electroflocculation to? the other, ? possible to recirculate the water (8) in contact with the cathode, thus containing the hydroxyl ions {OH ) produced by dissociation of the same water molecules, simultaneously in the first, in the fourth, ne! seventh compartment and so on. The same occurs for the tanning bath (9) and for the water in the anodic compartment (10); consequently, in the specific case, the surface area of the anodic membranes in contact with the matrix to be treated is tripled, with an evident reduction in working times.

The precipitation of chromium Cr<3+ >(13) ? exclusive of the compartments delimited by the two anionic membranes paired from time to time, while in the flow of water (10) the anions A<- >(14) will move, essentially consisting of chloride and sulphate ions, while in the flow of water (8 ) we will have the formation of OH- ions.

mode performing a meaningful test

The composition of the residual bath of the ion object of the treatment has the following characteristics:

The cell pack used has the following characteristics:

external dimensions 100 x 49 cm, internal 82 x 39 cm; n? 9 cationic polystyrene-based membranes functionalized with chlorosulphonic acid supported on polyethylene mesh spacers n? 22 polystyrene-based anionic membranes functionalized with trimethylammonium groups supported on polyethylene mesh spacers for a total equivalent volume of about 9600 cm<3>; n? 2 titanium electrodes 0.8 mm thick

The working conditions are as follows:

feed flow rate 1000 l / day including 20 recirculations (1.45 l / min); voltage 10 -12 vdc; current 40 -70A

under these conditions, with regard to the feeding of one (1) ml of liquid as described above, it is possible to recover a quota greater than 99% of the chromium present in the exhausted bath, divided into 3380 g present in the precipitate (concentrated phase) and 210 g in the supernatant (diluted phase), the supernatant cos? obtained has such characteristics (presence of trivalent chromium < 6%) that it can be directly reused in the nickel phase prior to tanning, together with the other waters circulating in the subsidiary compartments which assume characteristics of high acidity.

The characterization of the precipitate, reported below, highlights that the adopted system allows to obtain an extremely compact precipitate, rich in chromium hydroxide, at the same time very poor in co-precipitated organic substances, and practically free from other metals.

Analytical characterization

I : inlet wastewater, Fpp : final precipitate, Fsur : final supernatant

The application of the innovative cell, so? as proposed, it allows the precipitation of Chromium (III) in the form of practically pure hydroxide and therefore offers clear advantages over the classic methods which are based on the chemical precipitation of chromium with basic agents, given that in these cases there is no ? possible to avoid the co-precipitation and the? ing? obamento of considerable quantities? of organic products still present in the exhausted bathrooms.

The obtained precipitate has a dryness at 105 ?C of around 22-25% against 5-7% of traditional methods.

The precipitated chromium hydroxide does not require further finishing operations and can be used directly for the preparation of basic chromium sulphate by means of a rapid reaction with sulfuric acid.

The dissolution is in fact completed in about fifteen minutes, resulting therefore extremely more? fast compared to the classic method gi? quoted.

The innovative cell, repeatedly tested, has not shown signs of decay in the working times or fouling phenomena on both surfaces of the anionic membranes used.

In many applications yes ? resorted to the technique of reverse electrodialysis (EDR) by simply reversing the polarity? of the electrodes, with obvious management advantages.

DRAWINGS

A brief description of the figures shown below is provided:

Figure 1 - Three-compartment model cell with the arrangement of the membranes and separator partitions in the area between the anode and cathode. In particular, the cathode (1) and the anode (7), the two anion exchange membranes (3, 5), the three spacers (2, 4, 6), the recirculating water flow (8), the electrolyte to be treated in recirculation (9), the water flow (10).

Figure 2 - Simplified diagram (Cathode/Anionic/Anionic/Anode) of the three basic compartments and migration of the various ionic species as the current passes.

Figure 3 - Tortuous separator and schematic flows.

Figure 4 - Molecular structure of polyacrylsulfonamide.

Figure 5 - Scheme of the polyelectrolytic macromolecule bound to the anion exchange membrane and its spatial configuration.

Figure 6 - Representation of a ?stack? formed by three cells, spaced between them by cationic membranes (C) introduced in order to prevent the passage of ions from a unit? of electroflocculation to the other. Example of migration of the various ionic species as the current passes.

Claims (4)

1) Electrochemical cell consisting of three compartments which originate from the arrangement between the cathode and the anode of two anionic membranes with three spacers arranged one in each compartment, precisely between each electrode and the adjacent anion membrane and between the two membranes anionic compounds, and provided with a containment structure defining inside: a) a service chamber, delimited by the negative electrode and by an anionic membrane, provided with at least one inlet opening and one outlet opening, through which a first operating fluid (water) is made to flow, b) a precipitation chamber, delimited by the two anionic membranes, provided with at least one inlet opening and one outlet opening, through which the bath containing metal cations to be treated is made to flow; c) a service chamber, delimited by the positive electrode and by an anionic membrane, provided with at least one inlet opening and one outlet opening, through which a second operating fluid (water) is made to flow; d) a positive and a negative electrode, separated from each other with at least a first service chamber interposed, a chamber with the fluid being treated and a second service chamber, which can be fed by an electric power source with opposite polarities? to generate an operating electric field within the service chambers and the treatment chamber where, due to the effect of the migration of hydroxyl anions (OH ) with a negative charge through the anion exchange membrane, hydroxide precipitation occurs; e) n? 2 electrodes both in titanium on which ? dispersed a layer of powdered platinum with a thickness of 5 nm.; f) n?3 tortuous spacers with channel length/width ratio greater than 100:1 and with sections with a non-uniform structure such as to make the line of flow of the fluid variable and not continuous which operates at speed? not less than 50 cm/sec. 2) Electrochemical cell according to claim 1, characterized in that anion exchange membranes modified in their permeability are used in the cell. by surface treatment with aqueous solutions with a concentration of not less than 10% of polyelectrolytes containing sulphonic and amide functional groups (polyacrylsulfonamides) or other polyelectrolytes of the same family; 3) Electrochemical cell according to any one of the preceding claims, characterized in that the electric power source is capable of periodically inverting the polarity? of the two electrodes. 4) Series d? electrochemical cells according to the preceding claims, characterized by the insertion of a cation exchange membrane supported by its own spacer between cell and cell, where it is necessary to operate with more? cells in series in order to increase the exchange surfaces of the anionic membranes involved in the precipitation of the metal hydroxide.