MX2008006877A - Electrochemical treatment of solutions containing hexavalent chromium - Google Patents

Abstract

It is described a process of electrochemical reduction, optionally coupled to a final stage of chemical finishing, of solutions containing hexavalent chromium. The electrochemical reduction is carried out making use of a cell of cylindrical geometry with tangential solution inlet and outlet, which establish and maintain a spiral flow across the whole electrolysis bulk, achieving effective mass transport conditions.

Description

ELECTROCHEMICAL TREATMENT OF SOLUTIONS CONTAINING HEXAVALENTE CHROME DESCRIPTION OF THE INVENTION DESCRIPTION OF THE PREVIOUS TECHNIQUE Hexavalent chromium, in the form of chromic acid and its derived salts, has a long history of industrial applications, for example in the tanning, water treatment and galvanic industries, characterized by increasing difficulties associated with the high toxicity. Sodium chromates for example have been used at the level of tens of ppm as anti-corrosion agents in cooling water with tower circuits: such circuits are characterized by two types of emissions, the first constituted by the liquid purges normally carried out to maintain constants. the levels of salinity in the circulating water, and the second constituted by the dragging of micro-drops in the airflow of the tower While the former are neutralized for example by the addition of chemical reducing agents followed by the filtration of the precipitated trivalent chromium , the latter elude any reasonable possibility of treatment and therefore constitute a serious source of contamination for the environment. For this reason, chromates have been abandoned for a long time in the case of tower cooling circuits, and their use is nowadays limited to watertight cooling systems characterized by the only optional presence of liquid purges. On the contrary, the use of hexavalent chromium in the galvanic industry, in the form of a solution of chromic anhydride and sulfuric acid, in particular in hard chrome plating for mechanical applications, is still practiced and at the moment it does not seem to have any appreciable competitor. Chromium plant waste, which generally consists of washing water for the finished parts and exhausted baths, usually contain sulfuric acid and chromates, where the latter term indicates the family of ions originated by the complex polymerization equilibriums established depending on the pH. These solutions also contain the trivalent chromium ion, which is in effect a byproduct of the deposition reaction of chromium metal, and other metal ions, in particular iron ions released by the parts that must be plated. The presence of trivalent chromium negatively affects the efficiency of the chrome and the quality of the final product, therefore its accumulation is allowed up to certain critical levels, beyond which it is necessary to effect an expurgation of the solution. These solutions must be treated to comply with direct discharge regulations or a sewage consortium, according to which the permitted concentrations of hexavalent chromium are in the order of fractions of parts per million (ppm), typically 0.05 - 0.25 ppm. The processes adopted are in most cases of chemical type and provide for the addition of reducing agents such as sulphite or sodium bisulfate, ferrous sulfate, dispersed metallic iron particles, coupled with a neutralization of the acidity with a final filtration of the precipitated hydroxides: among the mentioned reducing agents, the most common is sodium sulfite (or metabisulfite). Sodium sulfite, Na2S03, is able to reduce the concentration of hexavalent chromium (chromate) below the limits imposed by the discharge regulations according to the reaction: 2 H2Cr04 + 3 Na2S03 + 3 H2S04? Cr2 (S04) 3 + 3 Na2S04 + 5 H20 The reaction indicates that the use of sodium sulfite determines a strong increase in the total salt concentration, thereby creating obvious problems in the final disposal or in the possible recovery of trivalent chromium by crystallization] of chromium sulfate. In the technical literature, different classes of electrochemical processes are also described, these are distinguished between two types respectively characterized by the direct reduction of hexavalent chromium to the cathode of the electrolysis cell and by the indirect reduction by means of a reducing agent generated inside the the same cell The first type of process, of which some embodiments are described in EP 074936, US 3,679,557, US 4,256,557, is characterized by the overall reaction 2 H2Cr04 f 3 H ^ S04? Cr2 (S04) 3 + 3/2 02 + 5 H20 In each embodiment, it is invariably provided that the cathode has a high surface area, for example constituted by a conductive bed of carbon particles through which the solution to be treated is transported. The objective of this complex electrode structure is to achieve a high mass transport capacity, also in the case of low final concentrations of hexavalent chromium, thus containing the cell size within acceptable limits The anode may have a structure equivalent to that of the cathode carbon, as subject to the corrosion caused by the anodic evolution of oxygen, is in any case capable of preventing the reoxidation of chromium from trivalent to hexavalent. The process is not satisfactory from a practical point of view due to the complexity of manufacturing very large electrodes constituted by beds. of particles, and the need to intervene periodically to rebuild the anode corroded The second type of process disclosed in the technical literature provides that the anode of the electrolytic cell is an iron anode that releases ferrous ions, or a tin anode that releases stannous ions, as proposed in JP 54110147, both ions being able to react with hexavalent chromium: therefore the reduction of hexavalent chromium is no longer carried out directly on the cathode surface, but is indirectly conducted in a homogeneous phase into the volume of the solution. The indirect process overcomes the problems associated with mass transport, but is nevertheless very impractical due to the need for periodic intervention when the anode is consumed beyond a certain limit. It is an object of the present invention to provide an electrochemical method for reducing hexavalent chromium (chromate) characterized by the use of an electrolysis cell of simplified structure, devoid of cathodes constituted by particle beds as in the electrochemical processes of the prior art.

DESCRIPTION OF THE INVENTION The invention consists of an electrochemical process that makes it possible to carry out the cathodic reduction of the hexavalent chromium contained in a crude solution to trivalent chromium in an electrolysis cell devoid of separator and equipped with a stainless steel cathode and an anode suitable for evolution of oxygen, capable of establishing and maintaining conditions of high turbulence through the entire volume at reduced solution flows, preferably not exceeding 10 m3 / n per m2 of cathodic surface. In a preferred embodiment of the invention, the process is carried out in a cell characterized by a cylindrical geometry with the cathode constituting the external wall and the anode installed in a coaxial configuration; the cell is provided with tangential entry and exit respectively for the gross and reduced solution. The process of the invention is preferably carried out using an anode suitable for the evolution of oxygen to potentials at which the reoxidation of the trivalent chromium to hexavalent chromium does not occur at all or takes place at a rate that does not interfere significantly with the cathodic reduction; In one embodiment of the invention, the cathodic reduction of hexavalent chromium is carried out with simultaneous formation of trivalent and metallic chromium. In a preferred embodiment, the anode suitable for the evolution of oxygen is provided with a porous, catalytically inert layer capable of acting as a diffusive barrier. In a preferred embodiment of the invention, the cathodic reduction is prolonged until obtaining a residual concentration of hexavalent chromium according to the norms applicable to the discharge of liquid wastes of industrial origin; the treated solution can then be neutralized, precipitating and separating the trivalent chromium hydroxide by filtration, or it can be concentrated by evaporation, separating the trivalent chromium as chromium sulfate by crystallization. In an alternative embodiment, the cathodic reduction is on the contrary arrested at a final concentration of hexavalent chromium higher than the limits established by the current regulations relating to liquid wastes of industrial origin, and the resulting solution is subjected to a final treatment with a chemical reducing agent that renders it conforming to these standards, for example sulf to or sodium metabisulfite. For a better understanding of the present invention, reference will be made to the accompanying drawings, which are merely provided as examples without constituting in any way a limitation of the invention BRIEF DESCRIPTION OF THE FIGURES The invention will now be described with the help of the following figures: Figure 1: circuit comprising an electrolysis cell of vertical cylindrical geometry suitable for a first embodiment of the invention. • Figure 2: circuit comprising an electrolysis cell of vertical cylindrical geometry suitable for a second embodiment of the invention.

DETAILED DESCRIPTION OF THE FIGURES In figure 1 are shown, without reference to the relative dimensions, the main components of the circuit used in the process of complete reduction of hexavalent chromium by exclusively electrochemical route: in particular, (1) indicates the global circuit , (2) the electrolysis cell equipped with the cylindrical cathode (3) and with the coaxial central anode (4), (5) the storage tank of the crude solution containing the hexavalent chromium that must be reduced to trivalent chromium, (6) the pump for feeding the crude solution to the cell, (7) the hydrogen and oxygen gases respectively evolved to the cathode and the anode of the cell, (8) the biphasic mixture comprising the electrolyzed solution and the gases, (9) ) the gas / solution separator, (10) the dilution air necessary to maintain the concentration of hydrogen within the flammability threshold, (11) the dilution air containing the hydrogen oy the oxygen separated from the solution, (12) the recycle of electrolyzed solution to the storage tank maintained until reaching the desired final concentration of hexavalent chromium, (13) the separator for the micro-drops of water entrained by the dilution air, equipped with a mist eliminator (14), (15) the vent for the dilution air containing hydrogen and oxygen but which is free of entrained solution, (16) the recycle of the liquid phase formed by the separated micro-droplets, (17) the pump that is operated at the end of electrolysis to transfer the reduced solution contained in the storage tank to the neutralization and final filtration of the chromium sulphate, or to the evaporation and crystallization treatment (not shown in the figure). According to the construction described for example in AU 2003204240, the cell is equipped with a lower and an upper nozzle, both horizontally and tangentially engaged, respectively to feed the crude solution containing the hexavalent chromium that must be reduced and to extract the mixture consisting of gases (hydrogen and oxygen produced in the cell) and electrolyzed solution depleted of hexavalent chromium. With this arrangement of nozzles, the solution flow assumes a spiral configuration, which is substantially maintained along the entire cell body: such a flow ensures a high mass transport with a construction much simpler and easier to perform with respect to to that of the prior art based on the use of cathodes constituted by particle beds. The design of the cell is further simplified by the fact that the process of the invention does not require the presence of a separator, for example of a porous diaphragm or of an ion exchange membrane, to separate the cathode from the anode. The figure ? shows a circuit used in a second embodiment of the process according to the invention, wherein (5) identifies, as in figure 1, the storage tank of the reduced solution obtained by arresting the electrolysis in correspondence of residual concentrations of hexavalent chromium higher to those allowed for the discharge in the environment, (17) as in figure 1 the pump to circulate the reduced solution, which is activated only after the electrolysis is finished, (18) a reactor in which the reduced solution sent the pump (17) is reacted with a chemical reducing agent (19) in order to obtain the final reduction of the hexavalent chromium concentration, (20) a stirrer that ensures the mixing of the reduced solution with the reducing agent , (21) a potentiometric element to determine the redox potential of the solution as described in the notorious electroanalytical techniques, (22) a valve that must be open at the end of the chemical reduction process, (23) the pump aimed at transferring the completely reduced solution to the neutralization and final filtration of the chromium sulphate or to the evaporation and crystallization treatment EXAMPLE 1 The circuit of Figure 1 was used to test a first embodiment of the process of the invention Cell (1) consisted of a stainless steel cylindrical body of type AISI 316-L connected to the negative pole of a rectifier and which It acted as a cathode, with an anode also cylindrical and centrally installed and coaxial with the cathode. On the cathode the reduction of hexavalent chromium to trivalent with a contemporary marginal deposition of metallic chromium and evolution of hydrogen took place, the anodic reaction consisted of the evolution of Oxygen The circuit of Figure 1 and the cell described above were used to effect the treatment of a crude solution from a chromium plant containing 125 g / 1 hexavalent chromium, 2.6 g / 1 trivalent chromium, 5 g / 1 of ferrous ion, and free sulfuric acid in a concentration such that it established a pH of 1.1. The solution was subdivided into five equivalent parts of 5 liters each, used in the tests described below. The cell used included a vertical cylindrical cathode of stainless steel with a thickness of 2 millimeters, an internal diameter of 48 millimeters and a length of 265 millimeters, which corresponded to a surface of 400 cm2. As an anode, a titanium tube of 10 mm external diameter and 1 mm thick was used, installed in a central position and coaxial with respect to the cathode: the tube was provided with an electrocatalytic coating for evolution of oxygen. The prior art suggests the use of coatings of platinum metal, platinum-iridium alloys, oxides of metals of the platinum group, such as for example the mixed oxide of iridium and tantalum; it is also notorious that these coatings may be provided with an additional porous oxide-only oxide layer, for example tantalum oxide, on the external surface in contact with the solution to be electrolyzed, as disclosed in WO / 0100905: in the course of the experimental campaign, different coated titanium anode formulations were used, as will be specified below. The cell was also equipped with two nozzles, upper and lower, respectively, to feed the crude solution with a regulated flow rate around 400 1 / h and to extract the mixed phase constituted by the electrolyzed solution and the evolved hydrogen and oxygen to the cathode and the anode , both with a horizontal and tangential orientation to produce a spiral upward flow to the interior of the cell. A constant current of 20 A was applied to the cell, which corresponded to a cathodic current density of 2400 A / m2. The voltage was between 4 and 5 volts. During electrolysis, sulfuric acid was injected for the purpose of restoring the spent acid and maintaining the pH at the value indicated above 1.1. In the first test, the anodic catalytic coating consisted of a commercial formulation of mixed oxide of iridium and tantalum in a molar ratio of 1.7: 1. Analysis of the hexavalent chromium content indicated an approximately linear decrease in time over a period of about 160 hours with a final concentration of 0.26 g / 1 (260 ppm), corresponding to an average current efficiency of about 30%. The product of the electrolysis was essentially trivalent chromium, with only a marginal portion constituted by metallic chromium, corresponding to approximately 1-2% of the generated trivalent chromium. Prolonging the test, it was observed that the decrease in the content of hexavalent chromium no longer followed a linear dependence over time, which indicates the establishment of a mass transport control of diffusive type. In particular, it was noted that the hexavalent chromium content decreased to about 0.4 ppm after a subsequent electrolysis period of 10 hours, then remaining constant. This result is undoubtedly interesting because it is remarkably close to the limits of 0.05-0.2 ppm foreseen by the regulations in force for industrial waste water. The reason for the lack of a further decrease in the residual concentration of hexavalent chromium can probably be attributed to the ability of the anode provided with a mixed oxide iridium and tantalum coating to re-oxidize, although at a reduced speed, the trivalent chromium generated to the cathode to hexavalent chromium. Appropriate determinations in effect indicated that the anodic electrochemical potential was around 1.5 V / SHE, while the minimum potential needed to allow the oxidation of trivalent to hexavalent chromium is approximately 1.35-1.4 V / SHE: the fact that the potential of oxidation of trivalent chromium was below the anodic working potential indicates in fact that oxidation was possible. To reduce the already satisfactory residual concentration of hexavalent chromium, a second and a third test were carried out, using respectively the same anode of the first test with the addition of an additional porous coating of tantalum oxide, totally inert under the operating conditions of electrolysis. , capable of acting as a diffusive barrier without appreciably affecting the evolution of oxygen, and an anode provided with an experimental electrocatalytic coating of mixed oxide of iridium and tantalum with the two elements in a molar ratio of 4: 1, characterized by a working potential of 1.4 volts, lower than that of commercial type due to the best electrocatalytic activity for the evolution of oxygen associated with the higher content of iridium. The second test showed a decrease in the time of the hexavalent chromium concentration analogous to that of the first test, with a practically constant final value of 0.3 ppm reached after 180 hours of electrolysis. An even more interesting result was achieved in the third test, where the constant final value of the residual concentration of hexavalent chromium was positioned around 0.15 ppm, demonstrating the importance of the level of catalytic activity of the anode.

A further demonstration of the importance of the anodic work potential was obtained with a fourth test, in which the cylindrical cell was equipped with a titanium coaxial anode provided with a thick pure 5 micron platinum coating, deposited galvanically according to the technique previous. In this case it was noted how the concentration of hexavalent chromium decreased with a trend in time substantially analogous to that of the previous tests up to a substantially constant final value of 15 ppm. Anodic work potential was centered around 1.7 volts.

EXAMPLE 2 A fifth test was carried out using the circuit of Figure 2 where the cylindrical cell run, configured as in the first test, was arrested after 150 hours at a hexavalent chromium concentration of about 10 g / 1: this solution was reacted in the stirred reactor (18) with a solution containing 50 g / 1 of sodium bisulfite added in such amount as to cause a displacement of the redox potential of the solution, according to the value measured with the probe (21). ), up to a value close to 0 V / SHE, which corresponds to the presence of a small residue of unreacted free bisulfite. The value of 10 g / 1 was chosen arbitrarily, however prolonging the electrolytic treatment up to concentrations between 5 and 25 g / 1 is particularly advantageous for an ideal coupling with a subsequent treatment with bisulfite or another chemical reducing agent. Under the indicated conditions, the residual concentration of hexavalent chromium after the subsequent treatment with bisulfite turned out to be 0.05-0.1 ppm, thus allowing the elimination of the solution according to the regulations in force. The advantage of the second embodiment of the process of the invention is in the reduction of the operating time in the electrochemical section and in the speed with which the solution is brought to minimum levels of hexavalent chromium, with consequent increase in the capacity of treatment for a given size of apparatus against the small penalty of a marginal increase in sulfate concentration, negligible for what concerns the processes of elimination or crystallization mentioned above. As will be apparent to a person skilled in the art, the invention can be practiced by introducing other variations or modifications in the cited examples. For example, in the process of the invention the applied current can be reduced as a function of the electrolysis time according to a pre-established program; the 1-cell cathodes can also be provided with catalytic coating, in this case a coating for evolution of hydrogen, for example a coating of galvanically deposited metallic ruthenium, whose catalytic activity makes it possible to arrest the reduction of hexavalent chromium to trivalent chromium without generating the above mentioned small amounts of metallic chromium. The above description will not be understood as a limitation of the invention, which can be practiced according to different embodiments without departing from its objectives, and whose scope is uniquely defined by the appended claims. In the description and claims of the present application, the word "understand" and its variations such as "comprises" and "understood" are not intended to exclude the presence of other accessory elements or components.

Claims (23)

1 Process of abatement of the hexavalent chromium content in a crude solution with production of a reduced solution comprising an electrolytic reduction carried out in an electrolysis cell without separator, provided with inlet and outlet nozzles of the crude solution, capable of maintaining a high level transport of mass through the entire volume of the cell, and equipped with a cathode of stainless steel and an anode suitable for the evolution of oxygen, wherein said anode is a titanium anode provided with a catalytic coating for evolution of oxygen suitable for work at a potential lower than 1 7 V / SHE 2 Process of abatement of the hexavalent chromium content in a crude solution with production of a reduced solution comprising an electrolytic reduction carried out in an electrolysis cell without separator, provided with inlet nozzles and output of the gross solution, capable of maintaining a high transport of mass through the ntero volume of the cell, and equipped with a cathode of stainless steel and an anode suitable for the evolution of oxygen, said cell having a vertical cylindrical geometry and comprising a cathode constituting the external wall and a cylindrical anode installed in central position coaxial with said cathode, and said inlet and outlet nozzles being able to maintain a high mass transport by being respectively positioned in correspondence of the lower and upper ends of said cell with a horizontal and tangential orientation.

The process according to claim 1 wherein said cell has a vertical cylindrical geometry and comprises a cathode constituting the outer wall and a cylindrical anode installed in a central position coaxial with said cathode, and said intake and outlet nozzles are apt to maintain a high transport of mass to be respectively positioned in correspondence of the lower and upper ends of said cell with a horizontal and tangential orientation.

4. The process according to claim 2 or 3 wherein said high mass transport is established by a spiral flow.

5. The process according to any one of claims 2 to 4 wherein said solution has a flow rate not exceeding 10 m3 / h per m2 of cathodic surface.

The process according to any one of the preceding claims wherein said electrolytic reduction of hexavalent chromium produces trivalent chromium and metallic chromium.

7. The process according to any one of claims 1 to 6 wherein said stainless steel cathode is further provided with a catalytic coating for evolution of hydrogen.

8. The process according to claim 7 wherein said catalytic coating is a metallic ruthenium coating.

9. The process according to claim 7 or 8 wherein said electrolytic reduction of hexavalent chromium produces only trivalent chromium.

10. The process according to any one of claims 2 to 9 wherein said anode is a titanium anode.

The process according to claim 10 wherein said catalytic coating consists of mixed oxide of iridium and tantalum.

The process according to claim 10 or 11 wherein an additional porous coating of catalytically inert material is applied over said catalytic coating.

The process according to claim 12 wherein said additional porous coating comprises tantalum oxide.

14. The process according to any one of the preceding claims wherein said electrolytic reduction is prolonged to a final concentration of hexavalent chromium not exceeding 0.2 parts per million.

15. The process according to any one of claims 1 to 13 wherein said electrolytic reduction is arrested at a residual concentration of hexavalent chromium in the reduced solution exceeding the value prescribed by the discharge regulations of industrial waters and is followed by a final treatment of said reduced solution comprising the addition of a chemical reducing agent.

16. The process according to claim 15 wherein said final treatment is carried out in a reactor provided with a potentiometric element.

The process according to claim 15 or 16 wherein said final treatment reduces the concentration of hexavalent chromium to a value not greater than 0 2 parts per million.

18. The process according to any one of claims 15 to 17 wherein said reduced solution subjected to said final treatment has a hexavalent chromium concentration comprised between 5 and 25 g / 1.

The process according to any one of claims 15 to 18 wherein said chemical reducing agent is selected from the group consisting of sodium sulfite, sodium bisulfite, ferrous salts, iron particles

20. The process according to any one of claims 15 to 19 wherein said potentiometic element detects the redox potential of the reduced solution.

21. The process according to claim 19 or 20 wherein said chemical reducing agent is sodium bisulfite and said addition is arrested when the potentiometic element detects a redox potential around 0 V / SHE.

22. The process according to any one of the preceding claims wherein said reduced solution is further neutralized with precipitation of trivalent chromium hydroxide, and said chromium hydroxide is successively removed by filtration.

23. The process according to any one of claims 1 to 21 wherein said reduced solution is subsequently evaporated with successive separation of the trivalent chromium by crystallization as chromium sulfate.