Abstract:
Methods for the reductive post-treatment of NaCl-containing solutions, wherein such methods comprise: providing a NaCl-containing solution obtained from an anode side of an NaCl electrolysis cell, the solution comprising reducible components; and subjecting the solution to cathodic electrochemical reduction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Conventionally, membrane electrolysis methods are used, for example, for electrolyzing sodium chloride-containing solutions (see, e.g., Peter Schmittinger, CHLORINE, Wiley-VCH Verlag, 2000). A divided electrolysis cell can be used in this case, which divided cell consists of an anode compartment with an anode and a cathode compartment with a cathode. The anode and cathode compartments can be separated by an ion-exchange membrane. A sodium chloride-containing solution, also referred to hereinafter as brine, having a sodium chloride concentration of conventionally approx. 300 g/l, is introduced into the anode compartment of such a cell. The chloride ion in the brine is oxidized to yield chlorine on the anode, and the chlorine can then be conveyed out of the cell with the depleted sodium chloride-containing solution, also referred to herein as the anolyte brine, which can have a remaining sodiumchloride concentration of approx. 200 g/l. 
         [0002]    So that the sodium chloride, which passes out of the cell in the depleted NaCl-containing solution, does not have to be discarded, this solution can be concentrated again with solid sodium chloride. In so doing, impurities, such as calcium, iron, aluminium or magnesium compounds or sulfates pass from the added sodium chloride into the brine, such that purification has to be performed. It is conventionally attempted, for example, to remove the iron and aluminium impurities by precipitation and subsequent filtration. The calcium and magnesium ions are conventionally removed by ion exchange resins. To protect the precipitation/filtration apparatus and ion exchange resins from chlorine and chlorine compounds, hypochlorites and chlorates, these strong oxidizing agents generally have to be removed. To accomplish such removal, the chlorine/hypochlorite concentration of the chlorine-containing anolyte brine is first lowered by lowering the pH, and then, for example, by stripping with steam. In this way, a chlorine content of less than 100 ppm may be achieved in the NaCl-containing solution. Moreover, in order to remove the residual remaining chlorine, which is in hydrolysis equilibrium with hypochlorite, chemical reduction is conventionally performed, e.g. with sodium bisulfite. The NaCl solution is then concentrated with solid sodium chloride and fed to the precipitation/filtration process, calcium and magnesium ions are removed by ion exchange resins and the solution is fed once again to the anode part of the electrolysis cell. 
         [0003]    One disadvantage of using sodium bisulfite and similar sulfur-containing compounds for chemical dechlorination is that sulfate arises in the NaCl solution as a reaction product of the chemical reduction with bisulfite or sulfur-containing compounds. However, the sulfate cannot be electrochemically degraded, such that it gradually accumulates in the NaCl-containing solution. Modern high performance ion exchange membranes are damaged by relatively high concentrations of sulfates, however. An excessively high concentration of sulfate in the brine brings about the formation of oxygen on the anode and reduces current efficiency, so impairing the economic viability of the electrolysis method. Damage to the coating of the anode is likewise possible. Manufacturers therefore state maximum limit values for sulfate concentration in the NaCl solution. As a result of recirculating the brine, some of the NaCl solution must be removed and discarded so as to prevent damage to the anode coating and membrane etc. In this way, large amounts of sodium chloride are lost, which has a negative effect on the economic viability and environmental compatibility of the electrolysis method. 
         [0004]    To prevent or reduce brine removal due to sulfate build-up, it is possible, for example, to use the nanofiltration method or the DSR method, the latter being a chromatographic method using amphoteric resins, with which methods sulfate may be removed from the brine (see, e.g., WINNACKER KÜCHLER, Chemische Technik Prozesse and Produkte, 5th edition, Vol. 3 Anorganische Grundstoffe, Zwischenprodukte (inorganic primary materials, intermediates), 2005, page 438, et seq.). One disadvantage of such methods is that sulfate formation is not prevented, but rather an additional method step is necessary in order to remove sulfate from the brine. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention relates, in general, to methods of removing chlorine from a solution containing NaCl, which solution can originate from the anode half-cell of an NaCl electrolysis cell. The chlorine contained in the NaCl-containing solution is subjected to electrochemical treatment on a negatively polarized electrode. 
         [0006]    The various embodiments of the present invention provide post-treatment methods for NaCl solutions which can achieve dechlorination with a markedly smaller addition of sulfur-containing reducing agents or even without such addition as compared with known methods. The various embodiments of the present invention provide dechlorination methods which exhibit a significant improvement over methods known in the art. 
         [0007]    The present inventors have found that it is possible to dispense completely, or for the most part, with the removal, of chlorine by acidification and/or stripping with steam, and subsequent chemical reduction of the chlorine in the anolyte brine with sulfur-containing compounds, if the anolyte brine is subjected to electrochemical dechlorination. 
         [0008]    The present invention provides a method for the reductive post-treatment of NaCl-containing solutions obtainable from the anode side of an NaCl electrolysis, characterized in that the reducible components in the NaCl-containing solution are reduced by cathodic electrochemical reduction. 
         [0009]    One embodiment of the present invention includes a method comprising: providing a NaCl-containing solution obtained from an anode side of an NaCl electrolysis cell, the solution comprising reducible components; and subjecting the solution to cathodic electrochemical reduction 
         [0010]    The NaCl-containing solution, which can be treated according to the various embodiments of methods of the present invention, may originate in particular from NaCl electrolysis using membrane methods, NaCl electrolysis using membrane methods in which a gas diffusion electrode is used on the cathode side, or from diaphragm methods. 
         [0011]    The methods according to the invention may be used to remove dissolved chlorine, hypochlorite which is still present, chlorate and other reducible compounds such as for example nitrogen trichloride, without reaction products passing into the NaCl-containing solutions, whereby markedly smaller quantities of the NaCl-containing solution have to be worked-up, or removed and discarded. The economic viability and environmental compatibility of NaCl electrolysis performed using the methods according to the various embodiments of the invention are markedly improved. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    As used herein, the singular terms “a” and “the” are synonymous and used interchangeably with “one or more” and “at least one,” unless the language and/or context cleary indicates otherwise. Accordingly, for example, reference to “a material” herein or in the appended claims can refer to a single material or more than one material. Additionally, all numerical values, unless otherwise specifically noted, are understood to be modified by the word “about.” 
         [0013]    Certain preferred embodiments of the present invention include methods wherein the NaCl-containing solution is additionally treated before or after the electrochemical reduction by chemical reduction via treatment with hydrogen peroxide, and more preferably, an aqueous hydrogen peroxide solution. 
         [0014]    Generally, hypochlorite and chlorate are, inter alia, also present in addition to chlorine in the chlorine-containing anolyte brine passing out of the anode chamber. These compounds or ions can be reduced on a cathode under cathodic potential, for example, according to the following reaction equations: 
         [0000]      cathode: 2Cl 2 +2 e   − →2Cl − 
 
         [0000]      2OCl − +4H + +2 e   − →Cl 2 +2H 2 O
 
         [0000]      2ClO 3   − +6H + +4 e   − →Cl 2 +3H 2 O
 
         [0015]    At the counter-electrode, the cell&#39;s anode, chlorine may for example be produced from sodium chloride-containing solution. It is likewise conceivable for oxygen to evolve on the anode or for iron(II) chloride to be oxidised to yield iron(III) chloride. 
         [0016]    The electrode compartments may for example be separated by an ion-exchange membrane, including, e.g., conventional commercial membranes of the type DUPONT NX 982 or 324 made by DuPont de Nemours. In this way, mixing of anolytes and catholytes and the components contained therein and mixing of the gases formed at the respective electrodes can be prevented. 
         [0017]    Diaphragms may also be used to separate the electrode compartments. If a diaphragm, for example, is used to separate the electrode compartments, the electrode compartments should be rendered inert, in order to prevent the formation of explosive mixtures such as for example mixtures of chlorine and hydrogen. 
         [0018]    Hydrogen may be formed in membrane or diaphragm methods during the cathodic reaction at elevated current densities or in the case of excessive residence times of the electrolytes in the cathode chamber in the event of galvanostatic operation of the cell. Galvanostatic operation means that a current intensity is established, and this is maintained by adjusting the voltage. Thus, in the event of galvanostatic operation, electrolysis is performed at a generally constant current intensity, the cell voltage being adjusted accordingly. In this case, where the residence time of the electrolytes in the cell is too long, the secondary reaction of water electrolysis may take place, during which hydrogen is formed at the cathode. To avoid this, the surface area of the cathode may be enlarged, e.g., by using three-dimensional cathodes. A three-dimensional cathode can include, for example, a graphite bed or carbon nonwovens. 
         [0019]    However, the electrolysis cell may also be potentiostatically operated, i.e., at a constant potential corresponding to a constant cell voltage. Operation at constant potential has the advantage that, at a sufficiently low selected potential, the above-stated compounds may be reduced without hydrogen formation. One disadvantage is that very high current densities cannot be selected, such that a long residence time is necessary and/or a large anode surface area should be provided. 
         [0020]    If chlorine is produced on the anode from a sodium chloride-containing solution, the chlorine may be fed into the already existing substance circuits of the NaCl electrolysis. Likewise, oxygen could be evolved anodically and further utilized. 
         [0021]    If a brine is used as anolyte and chlorine is produced, and if at the same time the electrolysis cell is provided with a diaphragm, the pressure in the cathode chamber should preferably be higher than that in the anode chamber, so that the catholyte passes into the anode chamber and chlorine-containing anolyte does not pass into the cathode chamber. If the reverse were the case, chlorine-containing anolyte would be forced into the catholyte, which would be undesirable since chlorine needs to be removed from the catholyte. 
         [0022]    The anode material used is, for example, a standard material for NaCl electrolysis anodes, such as titanium provided with a coating containing a noble metal or a noble metal oxide. This material could likewise be used as the cathode material. Generally, materials resistant to chlorine and sodium chloride-containing solutions may be used. Noble metals are here understood in particular to be metals from the series comprising osmium, iridium, platinum, ruthenium, rhodium and palladium. 
         [0023]    Carbon, graphitized carbon and/or graphite may likewise be used as the cathode material. Various shapes of electrode may be produced therefrom. 
         [0024]    The pH value of the electrolytes may preferably be so selected that sufficient material strength is achieved if the chlorine is present for the most part as hypochlorite. This may be the case preferably at a pH value greater than 7. However, it is likewise feasible for the pH value to be lower than 7, such that less chlorine is present in dissolved form as hypochlorite. 
         [0025]    Since each pH adjustment constitutes additional expense, the chlorine-containing anolyte is preferably introduced untreated from the NaCl electrolysis anode chamber directly into the cathode chamber for electrochemical chlorine reduction. 
         [0026]    Since very low concentrations of compounds such as chlorine, hypochlorite, chlorate or nitrogen trichloride have to be reduced, it is advantageous for the local current density at the cathode to be selected to be very low. To this end, electrodes which have a large surface area may be used as the cathode. Cathodes with a large surface area may be understood to include those in which the internal surface area is larger than the external, geometric surface area, preferably at least twice as large. 
         [0027]    For example, use may be made of metal electrodes with a foam structure, e.g., of titanium sponge, sintered titanium electrodes, or spherical metals, graphite or foam-like graphite, graphite coated with a noble metal, woven carbon fabrics, carbon cloth and carbon nonwovens. 
         [0028]    Metal electrodes may preferably be coated with noble metal(s), noble metal oxide(s) or noble metal compound(s) or mixtures thereof. Graphite electrodes may likewise preferably contain noble metal(s), noble metal oxide(s) or noble metal compound(s) or mixtures thereof. 
         [0029]    The residence time of the NaCl-containing solution can preferably be adjusted in such a way that, as far as possible, all reducible compounds may be reduced, without there being any onset of water reduction according to 
         [0000]      2H 2 O+2 e   − →H 2 +2OH − 
 
         [0030]    The residence time in the cathode compartment is preferably about 1 to 30 minutes, and more preferably 1 to 10 minutes. 
         [0031]    The current density, calculated in relation to the true surface area, is preferably about 5 to 100 A/m 2 . The amount of charge to be introduced for a chlorine content of anolyte brine of 100 mg/l amounts to 0.05 to 0.5 Ali/1 of anolyte brine. Markedly higher amounts of charge are needed if hydrogen evolution is permitted as a parallel reaction. 
         [0032]    A further preferred alternative embodiment of the new method is the additional treatment of NaCl-containing solution from an NaCl electrolysis anode chamber by addition of hydrogen peroxide. One advantage of the addition of hydrogen peroxide over the addition of for example sodium sulfite or sodium bisulfite is that no sulfate forms in the NaCl-containing solution during chemical reduction, but instead only water. When employing the addition of hydrogen peroxide, unreacted chlorine, chlorate or hypochlorite and/or optionally excess hydrogen peroxide is reduced in an electrochemical cell in the cathode chamber connected downstream of NaCl hydrolysis. The added quantity of hydrogen peroxide should preferably correspond as far as possible to the redox equivalent of the compounds to be reduced. Either a deficit or an excess of, for example, 0.95 parts or 1.2 parts, respectively, relative to the redox equivalent, may be used. Preferably, a deficit is used. 
         [0033]    The hydrogen peroxide may be apportioned to the chlorine-containing anolytes for example by means of a pump, and mixing may take place for example by means of a static mixer in a pipe. The solution treated in this way may then be reduced electrochemically. Excess hydrogen peroxide is then preferably reduced, as are other reducible compounds still present. 
         [0034]    A similarly feasible preferred embodiment of the method according to the invention consists firstly in electrochemically reducing only the majority, i.e. at least 80%, preferably at least 90%, particularly preferably at least 95%, of the chlorine present, and then in treating the remainder of the chlorine, chlorate and hypochlorite for example by means of conventional chemical reduction by the addition of for example sodium sulfite or hydrogen peroxide. In this way, the purge quantity of brine may likewise be markedly reduced over the prior art. 
         [0035]    The invention will now be described in further detail with reference to the following non-limiting examples. 
       EXAMPLES 
       [0036]    The electrolysis cell used in the Examples below for the reduction of chlorine, chlorate and hypochlorite consists of an anode compartment with an anode and a cathode compartment with a cathode, which is formed of a bed of graphite and a current distributor. The material of the anode in the anode compartment and of the current distributor in the cathode compartment consists of a titanium expanded metal coated with noble metal oxide, a so-called Standard DSA® Coating made by Denora. The volume of the anode or cathode compartment with anode or current distributor amounts to 230 ml. 
         [0037]    The electrolyte was introduced both into the anode and into the cathode compartment from below and removed again from above. 
         [0038]    Anode compartment and cathode compartment are separated by a commercially available ion-exchange membrane from DuPont de Nemours: DUPONT 324 or Nafion 982. The membrane area amounts to 100 cm 2 . 
         [0039]    An NaCl-containing solution with an NaCl concentration of 204 g/l was introduced into the anode compartment at a volumetric flow rate of 1.01/h. 
         [0040]    The brine to be treated, as may conventionally be removed from the anode compartment of an NaCl electrolysis, was introduced into the cathode compartment. The composition was as follows: the NaCl content was approx. 200 g/l, pH value approx. 4, the chlorine content approx. 400-450 mg/l. 
         [0041]    In the Examples given, no hydrogen was evolved in the cathode compartment on the cathode, consisting of a graphite bed of graphite balls with an average diameter of 2 mm, this being monitored by measurement. 
       Example 1 
       [0042]    In the above-described cell, provided with a Nation 982 ion-exchange membrane from DuPont de Nemours, the chlorine-containing, NaCl-depleted anolyte brine was passed at a volumetric flow rate of 1.0 l/h out of the NaCl electrolysis into the cathode compartment with a chlorine content of 422 mg/l. The cathode compartment was filled with graphite balls, the residual volume of the cathode compartment after deduction of the volume of graphite balls amounting to 160 ml. The residence time of the brine to be treated in the cathode compartment was 5.6 min. The voltage amounted to 1.72 V, and the current intensity to 0.8 A. The concentration of chlorine in the outflow of the cathode compartment was approx. 89 mg/l. The pH value of the anolyte brine was pH 4. A charge of 0.48 Ah/l of brine was introduced. The current density relative to the total surface area of the graphite balls used was 8.5 A/m 2 . 
       Example 2 
       [0043]    The chlorine-containing anolyte brine from another NaCl electrolysis with a corresponding chlorine content of 1522 mg/l and a pH value of 10 was introduced at 1.0 l/h into the above-described cell, provided with a Nafion 324 ion-exchange membrane from DuPont de Nemours. The cathode compartment was filled with graphite balls, the residual volume of the cathode compartment after deduction of the volume of graphite balls amounting to just 95 ml. The residence time of the NaCl brine to be reduced was 5.7 min. The cell voltage amounted to 2.33 V, and the current intensity to 1.5 A. The concentration of chlorine in the outflow of the cathode compartment was approx. 113 mg/l. The current density relative to the total surface area of the graphite balls used was 9.7 A/m 2 . 
       Example 3 
       [0044]    The chlorine-containing anolyte brine from an NaCl electrolysis with a corresponding chlorine content of 422 mg/l and a pH value of 4 was introduced at 1.1 l/h into the above-described cell, provided with a DUPONT Nation 982 ion-exchange membrane. The cathode compartment was filled with graphite balls, the residual volume of the cathode compartment after deduction of the volume of graphite balls amounting to 95 ml. The residence time of the brine to be reduced in the cathode compartment was 5.3 min. The cell voltage amounted to 1.72 V, and the current intensity to 0.8 A. The concentration of chlorine in the outflow of the cathode compartment was less than 1 mg/l. The current density relative to the total surface area of the graphite balls used was 5.2 A/m 2 . 
         [0045]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined, by the appended claims.