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
  • C02F1/26—Treatment of water, waste water, or sewage by extraction
• • 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
  • C02F1/66—Treatment of water, waste water, or sewage by neutralisation; pH adjustment
• • 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
  • 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
  • C02F2001/46152—Electrodes characterised by the shape or form
  • C02F2001/46157—Perforated or foraminous electrodes
• • 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
  • C02F2101/20—Heavy metals or heavy metal compounds
  • C02F2101/22—Chromium or chromium compounds, e.g. chromates
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
  • C02F2103/22—Nature of the water, waste water, sewage or sludge to be treated from the processing of animals, e.g. poultry, fish, or parts thereof
  • C02F2103/24—Nature of the water, waste water, sewage or sludge to be treated from the processing of animals, e.g. poultry, fish, or parts thereof from tanneries
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F9/00—Multistage treatment of water, waste water or sewage

Definitions

• the present invention relates to a process for treating effluents, in particular tanneries effluents, comprising chromium salts, allowing good recovery of chromium and destruction of organic pollutants contained in said effluents.
• It also relates to a treatment process implementing a subsequent step of recovering chromium for possible recycling in the industry, in particular for the tanning of leathers.
• Trivalent chromium salts are used to tan animal skins and transform them into leather. Trivalent chromium is inserted between the collagen fibers and crosslinks by forming complexes with the anionic sites of the polypeptide chains. Tanning with chromium salts leads to a leather having excellent physicochemical characteristics, in particular flexibility, resistance to tearing and great thermal resistance (denaturation of the skin above 1 00 ° C only). Only a few special leathers are still made with natural or synthetic organic tannins.
• the tanning baths are fairly highly concentrated in chromium, typically 20 g / liter. During a tanning operation, the leather absorbs about 60%. The effluents can therefore contain up to 8 g / liter of chromium, more usually around 2 g / liter.
• the used bath can be recycled and recharged with chromium salt, either for a new tanning operation or during a pickling pre-treatment.
• the quality of the leather is not as good if the bath contains a recycled effluent. In fact, during recycling operations, the bath accumulates mineral salts and organic compounds, the content of which must be limited and controlled in order to obtain leather of satisfactory quality.
• the spent bath, the purge and the rinsing water therefore constitute the effluent from the tanneries, which always contains a significant amount of chromium, mineral salts and organic compounds such as fats and proteins.
• the most common method of removing chromium from effluents is to precipitate it by raising the pH to around 8-9. This increase is obtained by addition of lime or magnesia, chromium trihydroxide Cr (OH) 3 is formed , magnesium chromite MgCr 2 0 4 or calcium chromite CaCr 2 0 4 very little soluble and calcium sulphate poorly soluble.
• the precipitate can be filtered and washed then treated with H 2 S0 4 to reform a solution of chromium sulphate. It is also possible to simply leave it to settle. For this more economical operation, lime is better.
• the most interesting precipitant is magnesia, because its use prevents the formation of a large amount of precipitated calcium sulphate.
• the precipitate treated with dilute sulfuric acid, provides a chromium sulfate solution containing 20 - 90 g / liter which can be reused for tanning.
• the chromium sulphate solution thus recovered always contains other ions present in the precipitate or in the interstitial liquid and organic impurities. It does not allow tanning of as good a quality as that obtained from basic pure chromium sulphate.
• the Cr (III) of the effluent is first adsorbed on a cation exchange resin, then oxidized to Cr (VI) by ammonium persulfate at 1 00 ° C or by sodium hypochorite (bleach) at room temperature.
• This process has the advantage of destroying certain troublesome ions such as formate ions.
• Cr (VI) can be converted to Cr (III) by reduction, the authors propose methanol as a reducing agent but do not indicate how to isolate pure chromium sulfate.
• the process requires a large excess of oxidant, takes a long time and probably consumes a lot of cation exchange resin, especially if the resin is degraded during oxidation.
• an ion exchange membrane should be of expensive perfluorinated material to resist oxidation. "Nation" type cation exchange membranes risk being quickly “clogged” by Cr 3 + ions. Anion exchange membranes are less selective and probably also not very durable.
• concentration conditions corresponding to the treatment of tanning effluents treatment of precipitates or raw effluents
• the object of the present invention is to propose an original method for the electrochemical treatment of tanning effluents with a view to quantitatively oxidizing the trivalent chromium to hexavalent chromium and simultaneously destroying the oxidizable organic compounds.
• the proposed process is equally applicable to the raw effluent or to the precipitate obtained using lime or magnesia, provided that the pH of the medium has been adjusted to a value which is not too basic at the start (pH ⁇ 5) if that is necessary.
• the hexavalent chromium obtained can be used as such in chromium plating baths or transformed into a derivative of trivalent chromium which can be used in tanning.
• the invention therefore relates firstly to a process for treating effluents, in particular tanning effluents, comprising chromium of oxidation state III, characterized in that said effluents are subjected, brought or being at a pH below 6, in a compartment comprising an anode and a cathode, to an electrochemical reaction in such a way that the chromium of degree of oxidation III is transformed into chromium of degree of oxidation VI and in that said treated effluents are recovered.
• a cathode with a small surface is used relative to the anode in a relatively acid medium (pH ⁇ 4) so that the reduction of water is preferably carried out over the reduction of chromium VI.
• a relatively acid medium pH ⁇ 4
• the electrochemical reactor capable of transforming the trivalent chromium derivatives into hexavalent chromium is not provided with ion exchange membranes or other separators. It is therefore a single compartment.
• the electrochemical reaction is carried out at a temperature between approximately 50 and 100 ° C., advantageously between approximately 80 and 95 ° C.
• the reaction medium preferably has a pH of less than 4 and greater than 2.
• the ratio between the active surface of the cathode and the active surface of the anode is between 1/1 00 and 20/1 00.
• the anode can be in the form of a cylinder made of, for example, platinum-plated titanium, and the cathode can be in the form of a rolled out cylinder, for example of titanium.
• These anodes and cathodes are of suitable meshes which can be determined by a person skilled in the art.
• the anode can be of mesh F and the cathode of mesh N.
• the chromium concentration usually varies between 1 g to 8 g of chromium per liter.
• the invention is not limited to this variant and other concentrations of chromium can be envisaged depending on the nature and the origin of the effluents.
• the current applied is partly a function of the duration of the electrolysis. Those skilled in the art can adapt these parameters according to the nature of the effluent. It should nevertheless be noted that a short-term electrolysis with weak currents makes it possible to obtain a better faradic yield but to the detriment of the chemical yield. It will be necessary in an industrial application to adapt the parameters of this process according to the amount of residual chromium present in the effluent.
• the intensity of the current is generally between 2 and 10 A per liter and the duration of the electrochemical reaction is a few hours.
• the process comprises a preliminary stage in which the effluents are subjected to a stage of precipitation of chromium of oxidation state III, recovery of the precipitate which is redissolved in an acid medium for subsequent electrolysis.
• the most common method for effecting this precipitation is to increase the pH in the region by about 8-9. This increase is obtained by adding lime or magnesia, chromium trioxide Cr (OH) 3 is formed from the magnesium chromite MgCr 2 0 4 or from the calcium chromite CaCr 2 O 4 very poorly soluble and from calcium sulphate little soluble.
• the precipitate can be filtered and washed then treated with H 2 S0 4 to reform a solution of chromium sulphate. It is also possible to simply leave it to settle.
• We prefer for this precipitation use CaO lime or MgO magnesia. However, according to a preferred variant, magnesia is used which prevents the formation of a large amount of precipitated calcium sulphate.
• the precipitate, treated with dilute sulfuric acid provides a solution of chromium sulphate which is then subjected to the process according to the invention as described above.
• the invention is not limited to this aspect as just described. Indeed, it also relates to a treatment process which comprises a subsequent stage of recovery of the trivalent chromium from the hexavalent chromium formed following the eiectrochemical reaction.
• the invention therefore relates to a process for treating effluents by an electrochemical reaction as described above followed by a step of recovery of hexavalent chromium.
• the precipitation takes place in the form of insoluble chromates; only barium and lead chromates are very insoluble. It is possible to quantitatively remove Cr (VI) by precipitation of PbCr0 4 using Pb (N0 3 ) 2 , but such a procedure risks leaving lead in the solution, which is not a route. satisfactory for the environment.
• Another preferred variant in the context of the present invention consists in carrying out a selective extraction of the chromic acid obtained (H 2 Cr0 4 ) by suitable organic solvents in an acid medium.
• the pH being less than or equal to 3, for example close to 1.
• the electrolysis solution is acidified to the appropriate pH with sulfuric acid and it is placed in the presence of an organic solvent which allows an almost total extraction of the chromic acid in the organic phase.
• suitable organic solvents mention is made of basic solvents in the Lewis sense and poorly soluble in water.
• trioctylamine, tributylphosphate, tetrabutylammonium hydroxide optionally with a co-solvent of the volatile hydrocarbon type.
• these co-solvents petroleum hydrocarbons are preferred.
• it also comprises a step of recovering trivalent chromium from hexavalent chromium formed following the electrolysis reaction. It is first possible to bring the organic solution into contact with an acid solution (for example dilute sulfuric acid), in the presence of a reducing agent, so as to reduce the hexavalent chromium to trivalent chromium which passes into aqueous solution in the form of chromium salt for example, chromium sulfate.
• an acid solution for example dilute sulfuric acid
• a reducing agent so as to reduce the hexavalent chromium to trivalent chromium which passes into aqueous solution in the form of chromium salt for example, chromium sulfate.
• organic reducing agents such as formic acid, methanol or inorganic reducing agents such as sulfur dioxide, sodium bisulfite in an aqueous medium to directly form sodium sulfate.
• chromium Cr 2 (S0 4 ) 3 in solution or a basic chromium sulphate, for possible recycling. It is advantageous to use a reducing agent the oxidation product of which is gaseous such as carbon dioxide or compatible such as the SO 4 2 " anions.
• the reactions can be diagrammed according to the following equations:
• the resulting solution comprises chromium sulphate
• it can be directly recycled in the leather tanning process for example.
• the single figure appended to the present description describes a schematic view of an electrochemical reactor making it possible to carry out the treatment process according to the invention.
• the reactor 1 with a double jacket 2 inside which circulates a cooling fluid 3 and provided with an agitator 4 actuated by a motor 5.
• the anode 6 is an expanded metal cylinder arranged coaxially around of the agitator and the cathode (not shown), is placed coaxially with the agitator around it so that the surface of the cathode is of small surface compared to that of the anode.
• the anode is connected to a generator 7.
• the electrochemical reactor is shown diagrammatically in the figure, it is constituted by a double jacket thermostatic reactor of the commercial "Grignard" type, fitted with a stirrer with Teflon propellers.
• the volume treated is 1 liter and initially contains a solution of chromium (III) sulfate Cr 2 (S0 4 ) 3 at a rate of 2 g to 8 g of chromium per liter.
• the pH is adjusted to 3 by addition of sulfuric acid.
• the anode is an expanded cylinder of platinum titanium "Degussa" of mesh F with a radius of 47.7 mm, height 1 00 mm, with an apparent surface
• the cathode placed in the center, surrounding the side rod.
• the agitator is an expanded cylinder of titanium "Degussa" with mesh N, radius 6 mm, height 80 mm, apparent surface 0.41 dm 2 .
• the initial solution is used as supplied, it is cloudy and blue-green in color.
• the amount of chromium present is 1.954 g / l.
• the rest of the chromium is contained in a deposit adhering to the cathode (0.94 g of deposit) containing approximately 20% of chromium, this deposit (chromite of magnesium or calcium very probably) dissolves in sulfuric acid and the solution obtained can be added to the next electrolysis.
• the solution at the end of the electrolysis is light yellow, perfectly transparent with a pH of 4.14 (this pH is a little high and could advantageously be reduced by adding a little H 2 S0 4 ).
• the chemical yield of oxidation of chromium III to chromium VI reaches in this example 89% (without counting chromium III recoverable in the deposit), the faradic yield however is low (1 1%). This is due to the simultaneous oxidation of the organic materials present (formates, oxalates, fats and soluble proteins) and chloride ions.
• Example 5 demonstrates the feasibility of extracting chromic acid and then reducing it.
• Example 5 An aqueous solution of chromic acid containing 4.33 g of chromium per liter (50 ml) is acidified with 1 ml of concentrated sulfuric acid. The solution is extracted twice with 30 ml of tributylphosphate, the organic solution turns yellow.
• the chromium (VI) residual in the aqueous phase is determined by potentiometry using a solution of ferrous salt. 0.086 g of chromium is found per liter. The extraction therefore extracted more than 98% of the chromium (VI) present in the solution.
• the organic solution thus obtained (a little more than 60 ml) is treated with 100 ml of a 0.1 M solution of NaHS0 3 , the organic solution discolours.
• the aqueous phase turns green, a sign of the presence of chromium (III).
• the determination of chromium in the aqueous phase provides 0.205 g of chromium, ie 94% of the chromium contained in the initial aqueous solution.
• a solution containing 4 g per liter of Cr (VI) is acidified with sulfuric acid. 100 g per liter of sodium chloride are added thereto. 30 ml of this solution are extracted with a mixture of 15 ml of tributylphosphate and 15 ml of petroleum ether (40-70 ° C). There is no longer any dosable chromium in the aqueous solution.
• the 30 ml of organic phase are treated with 3 ml of 7M sodium hydroxide. The chromium passes entirely into aqueous solution in the form of sodium chromate.
• the organic phase is reused for a further extraction of 30 ml of the initial aqueous solution, then treated with a stoichiometric amount of NaHSO 3 (80 mg) dissolved in 6 ml of sulfuric acid solution of pH 1.
• the chromium passes entirely back into aqueous solution in the form of chromium (III) sulphate at a concentration close to 20 g per liter, as used in tanning.

Landscapes

• 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)
• Treatment Of Water By Oxidation Or Reduction (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=9543900

Family Applications (1)

Country Status (8)

Families Citing this family (11)

Family Cites Families (7)

• 1999
  • 1999-04-01 FR FR9904085A patent/FR2791662B1/fr not_active Expired - Fee Related
• 2000
  • 2000-03-29 TN TNTNSN00066A patent/TNSN00066A1/fr unknown
  • 2000-03-31 SK SK1391-2001A patent/SK13912001A3/sk unknown
  • 2000-03-31 US US09/937,603 patent/US20020185382A1/en not_active Abandoned
  • 2000-03-31 EP EP00915269A patent/EP1171388A1/de not_active Withdrawn
  • 2000-03-31 WO PCT/FR2000/000821 patent/WO2000059833A1/fr not_active Application Discontinuation
• 2001
  • 2001-09-25 BG BG105948A patent/BG105948A/xx unknown
  • 2001-09-28 MA MA26341A patent/MA25348A1/fr unknown

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events