Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/467—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis by electrochemical disinfection; by electrooxydation or by electroreduction
  • C02F1/4672—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis by electrochemical disinfection; by electrooxydation or by electroreduction by electrooxydation
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/72—Treatment of water, waste water, or sewage by oxidation
  • C02F1/722—Oxidation by peroxides
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/46104—Devices therefor; Their operating or servicing
  • C02F1/46109—Electrodes
  • C02F2001/46133—Electrodes characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2101/00—Nature of the contaminant
  • C02F2101/006—Radioactive compounds
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2101/00—Nature of the contaminant
  • C02F2101/10—Inorganic compounds
  • C02F2101/16—Nitrogen compounds, e.g. ammonia
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2101/00—Nature of the contaminant
  • C02F2101/30—Organic compounds
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2201/00—Apparatus for treatment of water, waste water or sewage
  • C02F2201/46—Apparatus for electrochemical processes
  • C02F2201/461—Electrolysis apparatus
  • C02F2201/46105—Details relating to the electrolytic devices
  • C02F2201/46115—Electrolytic cell with membranes or diaphragms
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2305/00—Use of specific compounds during water treatment
  • C02F2305/02—Specific form of oxidant
  • C02F2305/026—Fenton's reagent

Definitions

• the present invention relates to an improved method for the reduction of an electrolyte containing nitric acid More specifically, the present invention relates to a method for avoiding accumulation of nitrous acid and/or for suppressing production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid
• the present invention further relates to an improved method for the decomposition of organic matter when added to an aqueous electrolyte containing nitric acid and silver ions Additionally the present invention relates to an apparatus designed for performing the said improved method
• Nitric acid is a powerful oxidizing agent, the use of which generally involves reduction of nitric acid and results in the production of nitrogen oxides which can create serious pollution problems
• the nitrogen oxides most commonly present in the gaseous effluents from a nit ⁇ c acid oxidizing system are the colorless nitric oxide NO and the brown nitrogen dioxide NO 2 Since nitric oxide reacts easily and almost quantitatively with atmospheric oxygen to mainly produce nitrogen peroxide NO 2 or N,O 4 , with additional traces of N : O 3 and N,O 5 due to thermodvnamic instability under standard conditions, this mixture of oxides being generally considered as a single NO toxic pollutant Generation of concentrated NO x fumes where concentrated acid solutions containing nitric acid are used as an oxidizing agent necessitates control and treatment, such as by water scrubbing, to prevent it from becoming an intolerable health hazard
• electrochemical techniques either to produce molecules or to oxidise or reduce selectively different chemical species for specific
• nitric acid may be recovered by scrubbing NO from the catholyte off-gas, using water or dilute acid, then concentrating it in an evaporator.
• NO may be scrubbed by the catholyte liquor continuously withdrawn from the cathode and returned through a packed column.
• US-A-3,945,865 discloses preventing the evolution of NO fumes in the dissolution of metals in a nitric acid-containing solution by adding hydrogen peroxide to said solution and maintaining the hydrogen peroxide concentration during dissolution between 1 and 20 g/1.
• Metals considered in this procedure are copper, zinc, titanium, zirconium and molybdenum. This document further states that the stability of hydrogen peroxide is adversely affected by the presence of certain metals such as iron, copper and lead, which catalyze its decomposition, in which case a hydrogen peroxide stabilizer must be added.
• US-A-4,938,838 discloses reducing the emission of NO gas in a bath for stainless steel, copper or brass containing nitric acid by measuring the redox potential in the said bath and automatically supplying hydrogen peroxide to the bath in an amount such that the redox potential is at about its maximum value.
• a first objective of the present invention is to provide a safe and efficient means for avoiding accumulation of nitrous acid and/or for suppressing production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid.
• a second objective of the present invention is to provide an improved method for the decomposition of organic matter when added to an aqueous electrolyte containing nitric acid and silver ions.
• the present invention is based on the principle of entirely oxidizing nitrous acid before any formation of nitrous oxides during electrolytic reduction. Therefore, in a first aspect the present invention provides a method to avoid accumulation of nitrous acid and/or suppress production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid, by adding an efficient amount of a substance reactive with nitrous acid to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid.
• the substance added is an oxidant, preferably hydrogen peroxide or a material which acts like or generates hydrogen peroxide in situ when in contact with aqueous solutions, e.g. ozone or even oxygen.
• aqueous solutions e.g. ozone or even oxygen.
• the present invention provides the use of the above method specifically for the decomposition of organic matter by electrolytic reduction, in which an aqueous electrolyte containing nitric acid and silver ions is subjected to an electric potential and the organic matter is added to the said electrolyte.
• the present invention provides a method of treating waste matter, more particularly organic waste matter or radioactively contaminated waste matter.
• the present invention provides an apparatus for performing the above-mentioned method, comprising a first container for storing and feeding an oxidable species such as an organic matter, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, and further comprising a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment.
• the single attached figure is a schematic representation of an electrolytic apparatus for performing the reduction of an electrolyte containing a nitric acid aqueous solution by means of the improved method of this invention when hydrogen peroxide is used as the substance reactive with nitrous oxide.
• the present invention relates to a method to avoid accumulation of nitrous acid and/or suppress production of nitrogen oxides during an electrochemical reduction process of an aqueous electrolyte containing nitric acid and simultaneous oxidation of oxidable species, by adding an efficient amount of a substance reactive with nitrous acid to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid.
• a preferred substance reactive with nitrous acid for use in this invention is a strong oxidant, most preferably hydrogen peroxide or any substance (such ad oxygen or ozone) able to act like or generate hydrogen peroxide in situ under the conditions of the said electrolytic reduction.
• nitrous acid cannot decompose anymore into the polluting nitrogen oxides.
• nitrous acid reacts rapidly and easily with hydrogen peroxide according to the following equation: HNO 2 + H 2 O 2 ⁇ HNO 3 + H 2 O
• This equation shows a further advantage of the present invention, i.e. nitric acid is directly and rapidly regenerated in the electrolytic cell itself. Therefore, as long as the amount of the substance reactive with nitrous acid, e.g. hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ, injected into the electrolytic cell is sufficient, nitrous acid cannot accumulate in the aqueous electrolyte and consequently the massive production of polluting gases NO x which conventionally occurs in the absence of the substance reactive with nitrous acid, is necessarily suppressed.
• the substance reactive with nitrous acid e.g. hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ
• the cathode used for electrolytic reduction consists of a material which is not susceptible to be corroded by the mixture of nitric acid and of the substance reactive with nitrous acid.
• the substance reactive with nitrous acid is hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ, a slight excess of hydrogen peroxide in the catholyte can be tolerated provided that the cathode is suitably selected.
• suitable selected mean that the cathode must be compatible with both high concentrations of nitric acid and the presence of hydrogen peroxide.
• the cathode may be made for instance from any material such as zirconium, niobium, tantalum, graphite, glassy carbon, gold, SS316 stainless steel or another class of steel specifically designed for the technology of highly concentrated nitric acid solutions. On the contrary, it is not desirable that the cathode be made from titanium since the latter easily forms complexes with hydrogen peroxide.
• the amount of the substance reactive with nitrous acid to be used in the method according to the invention is rather critical, i.e. the said substance should be used at least in stoechiometric amount with respect to the nitrous acid susceptible to be formed during electrolytic reduction, and preferably in excess of the amount sufficient for entirely oxidizing nitrous acid to nitric acid at the cathode.
• the continuous or discontinuous addition of the substance reactive with nitrous acid is preferably controlled on the basis of a measurement of the concentration of the said substance, such as hydrogen peroxide, in the catholyte and, most preferably (for reasons related to process control technology such as the dead time resulting from the duration of the measurement), on the basis of both said hydrogen peroxide concentration in the catholyte and the electrolytic current.
• the measurement of said substance (such as hydrogen peroxide) in the nitric acid solution can be achieved by any suitable means already known to those skilled in the art.
• measuring the concentration of hydrogen peroxide in a nitric acid aqueous solution can be readily achieved by the detection of its coloured complexes by means of a spectrophotometer or colorimeter.
• the electrolytic current intensity is not a parameter critical to the present invention and may conveniently be comprised in a range between about 50 amps and about 200,000 amps, as already known to those skilled in the art, although a current intensity below or above this range is not believed to be detrimental to the efficiency of the method of the invention.
• the concentration of nitric acid in the aqueous electrolyte is not a parameter critical to the present invention and may conveniently be comprised in a range between about 4 M and about 15 M.
• the present invention relates to a method incorporating the above-described inventive feature and further comprising the decomposition of organic matter by electrolytic oxidation, in which an aqueous electrolyte containing nitric acid and silver ions as an electrochemically regenerable oxidising species is subjected to an electric current and the organic matter is added to the electrolyte.
• the organic matter involved in this embodiment of the invention usually is waste matter which may be in polymeric or monomeric form.
• organic matter refers both to a matter consisting essentially of organic moieties and to a matter comprising organic moieties together with inorganic moieties or elements such as metallic species either covalently or ionically linked to said organic moieties.
• the organic matter may comprise natural polymers such as cellulose and rubber, or synthetic polymers and resins such as, for instance, synthetic rubbers, thermoplastic and thermosetting resins and formulations, ion exchange resins (including, without limitation, anionic and cationic polymers such as divinylbenzene/styrene copolymers, phenol/formaldehyde copolymers and the like) and ion exchange resins loaded with cations such as heavy metals or radioactive species.
• the organic matter involved in this embodiment of the invention may also be or comprise a radioactively contaminated waste matter.
• the organic waste matter may comprise a solvent derived from the reprocessing of nuclear fuel or a contaminated hydraulic fluid, oil or grease or kerosene.
• silver ions used in this embodiment of the invention is not a parameter critical to the present invention.
• Silver ions may be present in the anolyte in the form of, for instance, silver nitrate and their concentration may conveniently be comprised in a range between about 0.02 M and about 1 M, as already known to those skilled in the art, although a concentration below or above this range is not believed to be detrimental to the efficiency of the method.
• nitric acid be the only component of the aqueous electrolyte.
• the said electrolyte may further comprise one or more other powerful oxidizing agent(s) such as another mineral acid easily miscible with nitric acid, for instance perchloric acid.
• the method according to the present invention provides numerous advantages: first of all, it provides an efficient and easily implementable means for avoiding the presence of nitrous acid and/or polluting nitrogen oxides in the course of any industrial process comprising a step of electrolytic reduction of an aqueous electrolyte containing nitric acid. Secondly it provides a safe and cost-effective means for treating waste matter containing organic species optionally mixed together with contaminating species. Thirdly it allows to reduce the size of a regeneration device of nitric acid in the cathode compartment of the electrolytic cell, as explained hereinbelow, up to the point where the whole device becomes compact and moveable.
• the present invention therefore relates to an apparatus for performing the above-described method, comprising a first container for storing and feeding an oxidable species such as an organic matter, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, and further comprising a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment.
• the mixture of water and of the substance reactive with nitrous acid should be as homogeneous as possible at the time when it is injected into and admixed with the nitric acid aqueous solution. Homogeneity of the said mixture will be obtained in different ways depending on the chemical and/or physical nature of the substance reactive with nitrous acid. When the latter is a liquid easily soluble in water, such as hydrogen peroxide, the mixture of water and hydrogen peroxide will readily be provided by a third container for storing the said mixture.
• the substance reactive with nitrous acid is a gas which generates hydrogen peroxide in situ when in contact with aqueous solutions, e.g.
• the device for providing a mixture of water and the substance reactive with nitrous acid may be a high efficiency static gas-liquid mixer.
• the apparatus of the present invention may comprise, upstream of the said high efficiency static gas-liquid mixer, an ozoniser for producing ozone from oxygen and it may also include a device for destroying a possible excess of ozone before release.
• the said apparatus further comprises at least an electrodialyser for maintaining the concentration of nitric acid in the cathodic circuit of the electrolyser constant.
• the said apparatus further comprises at least a reverse osmose unit for maintaining the concentration of nitric acid in the anodic and cathodic circuits of the electrolyser constant.
• a means for injecting a substance reactive with nitrous acid, for instance hydrogen peroxide, into the said cathode loop.
• nitrous acid for instance hydrogen peroxide
• hydrogen peroxide oxidises nitrous acid to nitric acid.
• Each electrodialyser (12,19) comprises a concentrate loop and a diluate loop, allowing a fraction of the catholyte to be enriched in acid and the other fraction being diluted in acid.
• the enriched fraction from the first electrodialyser (12) is returned to the cathode loop of the electrolyser (4) while the diluted fraction from the first electrodialyser (12) is sent to a second electrodialyser (19) where again a fraction is enriched in acid and sent back to the cathode loop of the electrolyser (4) while the other fraction is diluted again and sent to a reverse osmose unit (27).
• the said reverse osmose unit (27) then withdraws water from the diluate loop of the second electrodialyser (19).
• an example of an apparatus for performing the second embodiment of the method according to the present invention is schematically represented on the enclosed single figure and comprises the following elements : - a storing and feeding container (1) for the organic matter to be treated / decomposed, a storing and feeding container (2) for a concentrated aqueous solution comprising nitric acid (and optionally one or more other mineral acid(s) in miscible proportions with nitric acid), a storing and feeding container (3) for an aqueous solution of a substance reactive with nitrous acid, e.g.
• an electrolyser (4) comprising an anode compartment (5) connected to the storing and feeding container (1) and a cathode compartment (6) connected to the storing and feeding containers (2) and (3), - a first electrodialyser (12) comprising a concentrate compartment (13) connected with the cathode compartment (6) of the electrolyser (4) and a diluate compartment (14), a second electrodialyser (19) comprising a concentrate compartment (20) and a diluate compartment (21), the diluate compartment (14) of the first electrodialyser (12) being connected to the second electrodialyser (19), preferably to both compartments thereof, and - a reverse osmose unit (27) comprising a low pressure compartment (28) connected to the anode compartment (5) of the electrolyser (4) and a high pressure compartment (29) connected to the diluate compartment (21) of the second electrodialyser (19).
• the anode compartment (5) of the electrolyser (4) constitutes a part, together with container (1), of a circulation loop further comprising a circulation pump (7) and a filter (8).
• the cathode compartment (6) of the electrolyser (4) is also included in a circulation loop constituted by a buffer container (9), a circulation pump (10) and a filter (11).
• the concentrate compartment (13) of the first electrodialyser (12) is part, together with the buffer container (9), of a circulation loop further comprising a circulation pump (15) and a filter (16).
• the diluate compartment (14) of the first electrodialyser (12) is connected:
• the high pressure compartment (29) of the reverse osmose unit (27) is connected to the buffer container (24) upstream of the diluate compartment (21) of the second electrodialyser (19), whereas the low pressure compartment (29) of the reverse osmose unit (27) is connected, through a circulation pump (30) and a filter (31), to the storing and feeding container (1) and, hence, to the anode compartment (5) of the electrolyser (4).
• the storing and feeding container (3) is connected, through a circulation pump (32) and a filter (33), to the buffer container (9) which additionally receives nitric acid from the storing and feeding container (2).

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Hydrology & Water Resources (AREA)
• Engineering & Computer Science (AREA)
• Environmental & Geological Engineering (AREA)
• Water Supply & Treatment (AREA)
• Organic Chemistry (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)

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• 2000
  • 2000-03-28 WO PCT/EP2000/002725 patent/WO2001072641A1/en not_active Application Discontinuation
  • 2000-03-28 AU AU2000238151A patent/AU2000238151A1/en not_active Abandoned
  • 2000-03-28 EP EP00917006A patent/EP1284931A1/de not_active Withdrawn

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