WO2019159197A1

WO2019159197A1 - Method and apparatus for regenerating a working salt solution in salt purification

Info

Publication number: WO2019159197A1
Application number: PCT/IN2019/050120
Authority: WO
Inventors: Saravana Perumal SHANMUKHAM
Original assignee: S P Textile Processors Pvt. Ltd.
Priority date: 2018-02-15
Filing date: 2019-02-15
Publication date: 2019-08-22

Abstract

A method and apparatus for regenerating a working solution used in the purification of salt is provided. The salt is lixiviated in an extraction chamber using fresh working solution supplied from a reservoir. The contaminants in the salt are transferred to the working solution to purify the salt. The used working solution containing the contaminants undergoes sequential treatment steps in a hardness and/or heavy metal treatment chamber, an aeration chamber and an oxidative treatment chamber. The series of treatments enable removal of hardness impurities and degradation of the organic contaminants. The regenerated working solution is returned to the reservoir for reuse in salt purification. The disclosed apparatus and method provides for a highly efficient way to purify contaminated salt to >96% purity using the recycled working solution in a closed-loop with minimal make-up while avoiding liquid wastage.

Description

METHOD AND APPARATUS FOR REGENERATING A WORKING SALT SOLUTION IN SALT PURIFICATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Indian patent application No. 201841005818 dated February 15, 2018 entitled A METHOD AND APPARATUS FOR REGENERATING A WORKING SALT SOLUTION IN SALT PURIFICATION.

FIELD OF THE INVENTION

[0002] The disclosures relates generally to salt purification and specifically to a method and apparatus for regenerating a working solution used in salt purification.

DESCRIPTION OF THE RELATED ART

[0003] Advanced oxidation processes (AOP) are used for removal of contaminants from wastewater through production of intermediate free radicals and non-radical reactive species. Although various AOP processes are known in the art, they are either limited in their application, are expensive and/or ineffective for large scale use in waste water generating reactors. Ozone-based AOP processes, such as peroxide-ozone and UV-ozone technologies, require dissolution of ozone in the medium which is not always possible. UV-based processes, such as UV-titania and UV-peroxone technologies, require transmittance of UV which will be influenced by the color of effluents and are not suitable for all effluents. Fenton process requires strongly acidic conditions which is expensive, requires excess consumption of chemicals and leaves residues in the treated water. Aerobic/anaerobic processes require liquid environment conducive for growth and multiplication of microorganisms, and additionally require minimum dissolved oxygen in the case of aerobic organisms. Further the microorganisms also require nutrients in the medium for growth and multiplication particularly when the organics pollutants in the medium are not amenable to digestion. High salt and toxic effluents do not support such treatments. Enzymatic treatments require access to their reactive sites which may not be accessible in certain effluents such as toxic brine. Physico-chemical treatments require the addition of coagulants or flocculants, such as polymeric substances. The effectiveness of such treatments is limited in hypersaline solutions, they leave a residue in treated water, and cause secondary waste generation in the form of dissolved residues which require additional handling/disposal solutions. There is therefore a need for an improved process for purification of brine for use in large scale salt purification.

[0004] European patent EP0573400A1 discloses a salt treatment method for recycling contaminated salt brine in a tanning process. The US Patent No. 8273321B1 discloses a three-step process for the preparation of analytical grade sodium chloride from rock salt. The purification method disclosed in the‘321 patent includes washing entry salt in clarified brine to dislodge insoluble impurities adhering to the salt surfaces. The washed salt is then scrubbed with fresh water sprays to displace the wash brine from salt surfaces.

[0005] The US Application No. 4488958A discloses a preparation method for a kiln dried solar salt. The PCT application W02004024828A1 discloses a treatment of dye baths by a membrane process for reuse of the water and NaCl. Treatment of baths for the dyeing of cellulose fibers with reactive dyes including the steps pre-filtration, followed by neutralization, nanofiltration and then reverse osmosis is described. The US Application No. 20120201949 discloses a method of producing salt using solar energy. The Chinese patent CN102809516A discloses a treatment method and system for high- concentration mixed salt organic wastewater.

[0006] The major limitations of the available salt purification methods are generation of liquid effluents which are themselves environmental pollutants, wastage of the salt, and lower efficiency of purification. There is therefore a need for a purification method and system for clean purification of salt, which overcomes the aforementioned problems and provides for a cost-effective and integrated system for continuous reuse of working solutions essentially in a closed loop, with minimal make up of working solution and minimal or no liquid/secondary waste generated from the purification method. SUMMARY OF THE INVENTION

[0007] The disclosure relates to a method for the purification of a salt and regeneration of the working solution used for the purification of the salt. The working solution is continuously used for purifying contaminated salt and regenerated essentially in a closed loop process.

[0008] In various embodiments, a method of purifying a contaminated salt while recycling a working solution used in the purification is provided. The method includes the steps of: contacting the working solution with the contaminated salt to obtain a salt slurry, wherein the contaminated salt comprises one or more contaminants and the working solution comprises saturating concentrations of the salt dissolved in one or more solvents; separating purified salt from the salt slurry thereby obtaining a used working solution comprising the one or more contaminants; contacting the used working solution with one or more reagents for removing hardness, heavy metals, or a combination thereof to obtain a reagent-treated used working solution and a precipitate; separating the precipitate from the reagent-treated used working solution; adjusting pH of the reagent- treated used working solution to a value in the range of 4 to 8; aerating the reagent- treated used working solution while maintaining the pH in the range of 4 to 8; contacting the reagent-treated used working solution with one or more oxidants to remove organic contaminants, thereby regenerating the working solution; and supplying the regenerated working solution for further purification of contaminated salt.

[0009] The one or more reagents may include sodium hydroxide, calcium oxide, calcium carbonate, calcium bi-carbonate, sodium carbonate, sodium bicarbonate, calcium hydroxide, or a combination thereof. The salt may include Na₂S0₄, NaCl or a combination thereof. The solvent may include water. In some embodiments, the used working solution is filtered prior to contacting with said one or more reagents. In some embodiments, the one or more oxidants includes oxygen, hydrogen peroxide, ozone, chlorine, a hypochlorite, a peroxide, a permanganate, a persulfate, a ferrate, peracetic acid, a peroxysulfate, hydroxyl radical, sulphate radical, superoxide ion, or a combination thereof. In some embodiments, the one or more oxidants degrade organic contaminants contributing to color, total organic carbon (TOC), or both in the used working solution. In some embodiments, the one or more oxidants are generated in catalytic system. In some embodiments, the catalytic system is a chemical catalytic system or a multiphase electrochemical system. In some embodiments, the one or more oxidants are generated by application of electrical potential in the multiphase electrochemical system. In some embodiments, the multiphase electrode system is a two- phase or a three-phase electrochemical system. In some embodiments, the one or more oxidants are generated in the presence of a homogenous or a heterogeneous catalyst. In some embodiments, the catalyst comprises Mn0_2. In some embodiments, the contaminated salt is mixed with the working solution at 1-75 w/w%, or in the range of 0.5 to 100 L/kg. In some embodiments, the contaminated salt in supplied in counter- current flow of the working solution. In some embodiments, the reactor is operated in a batch mode or continuous mode.

[0010] In various embodiments, an apparatus for purifying contaminated salt while recycling a working solution used in the purification is provided. The apparatus includes: a reservoir configured to store and supply the working solution; one or more extraction chambers configured to mix the contaminated salt and the working solution to form a salt slurry, wherein the contaminated salt comprises one or more contaminants and the working solution comprises saturating concentrations of the salt dissolved in one or more solvents; a separator configured to filter purified salt from the salt slurry to obtain a used working solution comprising the one or more contaminants; one or more first treatment chambers configured to contact the used working solution with one or more reagents for removing hardness, heavy metals, or a combination thereof; one or more second treatment chamber for pH adjustment and aeration of the used working solution; and one or more third treatment chambers configured to contact the reagent-treated used working solution with one or more oxidants to remove organic contaminants, and regenerate the working solution. [0011] The one or more extraction chambers may be configured to mix the contaminated salt in a counter-current flow of the working solution. In some embodiments, a plurality of extraction chambers connected in a serial or parallel configuration is provided. The process scheme may additionally include one or more filters configured to remove suspended solids in the used working solution. The treatment chambers may include a two phase or a three phase electrochemical system. The treatment chambers may also include a heterogeneous or homogenous chemical catalytic system. The apparatus may form a closed-loop system for purification of the contaminated salt.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0013] FIG. 1A illustrates a method for the purification of contaminated salt using a working solution.

[0014] FIG. 1B shows a method of regenerating a working solution used in the purification of contaminated salt.

[0015] FIG. 2A shows apparatus for purification of a contaminated salt and regeneration of a working solution.

[0016] FIG. 2B is schematic of a counter-current setup for reuse of working solution in a continuous extraction process.

[0017] FIG. 2C is schematic of a counter-current setup for reuse of working solution in a batch extraction process.

[0018] FIG. 3A shows a three-phase electrode system for removal of organic contaminants.

[0019] FIG. 3B shows a two-phase electrode system for removal of organic contaminants. DETAILED DESCRIPTION

[0020] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0021] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0022] The invention in its various embodiments provides for a method 100 for purifying a salt using a working solution, as shown in FIG. 1A. In step 101, the contaminated salt is optionally screened for the removal of particulate impurities. In step 103, the contaminated salt comprising one or more contaminants is contacted with a fresh working solution to extract the impurities into the working solution. The fresh working solution is made up of saturating concentrations of the salt dissolved in one or more solvents. In step 105, the slurry is separated into purified salt and used working solution containing the one or more contaminants transferred from the contaminated salt. In some embodiments, the purified salt after separation from the slurry is optionally washed with fresh working solution to displace residual used working solution associated with the salt. This results in additional purification of the salt. In step 107, the used working solution is separately collected for treatment and regeneration. In step 109, the salt is further dried and packaged. The packaged purified salt may be reused or sent to a storage unit. [0023] In some embodiments, the lixiviation step 103 may be repeated more than once to obtain the desired purification level of the salt. In some embodiments, the lixiviation step 103 is performed in a batch mode or a continuous mode. In some embodiments, the lixiviation step 103 is carried out at a temperature in the range of 10 to 70 C and under a pH in the range of 4 to 11. In some embodiments, the purity of the salt obtained from step 103 is greater than 96%. In some embodiments, one kilogram of the contaminated salt is contacted with the working solution in the range of 0.3 to 50 liters. In some embodiments, the contaminated salt is mixed with the working solution at a range of 1- 75 (w/w) weight/weight %, or in the range of 0.5 to 100 L/kg.

[0024] In some embodiments, the one or more salt contaminants include a single contaminant. In other embodiments, the one or more salt contaminants include at least 2 different contaminants. The contaminant may be an organic contaminant, a recalcitrant, a partially degraded compound, a partially degraded dye, a hydroquinone, a phenol, an aromatic compound, an aliphatic compound, an unsaturated ketone, an aldehyde, an ether, an alcohol, a surfactant, an organic acid, or a heavy weight polymer. Inorganic contaminants may be multivalent cations such as magnesium, calcium, aluminium, iron, chromium, mercury, lead, bismuth, barium, arsenic, cadmium, thallium, copper, selenium, zinc or multivalent anions such as carbonates, bi-carbonates, phosphates or non-charged contaminants such as silica in colloidal, suspended and dissolved forms.

[0025] The contaminated salt may be sodium chloride (NaCl), sodium sulphate (Na₂S0₄), or a mixture of both. The contaminated salt may also include any other salts known in the art. The solvent may be a polar solvent, including water, ammonia, methanol, ethanol, acetic acid, acetone, or a mixture thereof. Any suitable solvent may be used.

[0026] In various embodiments, a method 200 for recycling a working solution used in the purification of a contaminated salt is provided, as shown in FIG. 1B. The working solution regenerated is supplied back for purification of salt in method 100 forming essentially a closed loop for purification of contaminated salt in a reactor. The working solution may be any brine solution known in the art. The contaminants in the used working solution may be dissolved contaminants, suspended contaminants, or both. In step 201, the used working solution obtained as a byproduct from step 107 is optionally passed through at least one filter to remove suspended particles. Any suitable filtration process known in the art may be used include nano- or ultra-filtration. In step 203, the used working solution is contacted with one or more reagents to remove any hardness, heavy metals and alkalinity that is present in the used working solution. In step 205, the reagent-treated used working solution is separated from wet sludge containing hardness precipitate formed as a result of the treatment. In step 207, residual alkalinity in used working solution is removed by pH correction and/or aeration at a pH in the range of 4 to 8. At this pH, the reaction will progress rapidly in the reactor reducing the overall process time. The pH is maintained by the addition of an acid or a base as required. The alkalinity reduction by aeration removes the excess carbonates and bicarbonates in the used working solution. In step 209, the used working solution is subjected to catalytic chemical oxidation or electro oxidative treatment in the presence of one or more oxidants to remove organic contaminants from the used working solution. The organic contaminants may be pollutants causing color and total organic carbon (TOC). In step 211, the oxidant treated working solution is supplied for reuse in purification of contaminated salt, as already discussed with reference to step 103 in FIG. 1A.

[0027] In some embodiments, step 203, 205 and 207 may be conducted in a single chamber. In other embodiments, step 203 and step 205 are conducted in a single chamber. In one embodiment, step 203 and 205 are conducted in separate chambers. In yet other embodiments, step 203 or 207 is conducted in a plurality of chambers each.

[0028] The one or more reagents for removing hardness, heavy metals, or both in step 203 may be any suitable reagent known to a skilled artisan. In various embodiments, precipitation softening agents may be employed to remove hardness and heavy metals. A basic salt may be used as a hardness remover. The basic salt is selected from the group consisting of sodium hydroxide, calcium oxide, calcium carbonate, calcium bi-carbonate, sodium carbonate, sodium bicarbonate, calcium hydroxide or a combination thereof. In various embodiments, the hardness producing minerals includes calcium, magnesium or a combination thereof. In some embodiments, the chamber pH in step 203 was maintained in the range of 8 to 14.

[0029] In some embodiments, alkalinity reduction by aeration in step 207 may be carried out at a pH in the range of 4 to 8. In other embodiments, aeration is carried out a pH in the range of 5 to 6.5. The pH correction is done by adding an appropriate acid-base pair. HC1 or H₂S0₄ may be used as acids to adjust the pH from the initial pH. In some embodiments, the alkalinity reduction is carried out at any suitable pH. The initial pH at which the hardness precipitates are removed may be in the range of pH 8 to 11. C0₂ is liberated from the solution by aeration at a pH in the range of 4 to 8 which removes carbonate and/or bicarbonate containing minerals, or other contaminants present in the used working solution.

[0030] In various embodiments, catalytic chemical or electrochemical oxidative treatment is utilized to reduce or degrade organic contaminants present in the used working solution in step 209. In some embodiments, a catalyst is utilized to excite the oxidant to form reactive species with a high oxidation capacity. The catalyst may be a solid, a liquid, a homogenous catalyst, a heterogeneous catalyst, or any suitable catalyst known in the art. In some embodiments, the catalyst includes Mg, Ca, Sr, Ba, Y, La, W, Mn, Mo, Nd, Sm, Eu, Pr, Zr, Ti0₂, Sm₂0₃, V₂Os, Mo0₃, BeO, Mn0₂, MgO, La₂0₃, Nd₂0₃, EU₂0₃, Zr0₂, SrO, Na₂W0₄, Mn/W0₄, BaO, Mn₂0₃, Mn₃0₄, Mg₆Mn0₈, Na/B/Mg₆Mn0₈, Li/B/Mg₆Mn0₈, NaMn0₄, CaO, MgO, La₂0₃, Nd₂0₃, Na₂W0₄, Mn/W0₄, Mn₂0₃, Mn₃0₄, Mg₆Mn0₈, Na/B/Mg₆Mn0₈, Li/B/Mg₆Mn0₈ or combinations thereof. In some embodiments, the catalyst is Mn0₂. In other embodiments, the catalyst includes metal oxides, metal hydroxides, metal ions, metals, meial chelates, monodentate or multidentate ligands complexed with metal ions and monodentate or multidentate macrocyclic ligands complexed with metal ions. In some embodiments, the one or more oxidants are directly added to the chamber. The one or more oxidants may also be formed in situ from a precursor agent upon contact with the catalyst. The one or more oxidants may further generate additional free radicals in the chamber.

[0031] In some embodiments, the one or more oxidants may include ozone, oxygen, chlorine, a hypochlorite, hypochlorous acid, a peroxide, acetic acid, hydrogen peroxide, a permanganate, ferrate (with iron in +4, +5, +6 oxidation states), peracetic acid, a persulfate, a peroxysulfate, hydroxyl radical, sulphate radical, singlet oxygen, superoxide ion, hydroperoxyl radical, other free radicals or any combination thereof. These oxidants may be generated in-situ in the working solution itself or ex-situ in another solution and applied to the working solution. As an example, ferrate may be generated in-situ in the working solution by the use of iron electrodes. In some embodiments, the initial TOC of the working solution is in the range of 2500 to 50000 mg/L and the final TOC after treatment is in the range of 10 mg/L to 1000 mg/L. In various embodiments, the regenerated working solution is re-used for the purification of salt as shown in step 211.

[0032] The invention in its various embodiments proposes an apparatus 300 for purifying a contaminated salt and recycling the working solution used in purification of the contaminated salt as shown in FIG. 2A. The apparatus is configured for continuously supplying fresh working solution stream 350 for purification of the contaminated salt by regenerating the used working solution stream 360 with minimal or zero discharge of liquid effluents in the purification method forming a closed loop.

[0033] In some embodiments, the apparatus 300 includes a reservoir 301 for storing the working solution. The working solution is a solution of saturating concentrations of the salt and one or more solvents. The reservoir initially receives freshly prepared working solution from a source. In one embodiment, the working solution requires minimal or no additional make-up over several purification cycles. The reservoir 301 supplies an extraction or lixiviation chamber 303 with the fresh working solution via a working solution inlet 305. The first chamber 303 receives the salt 370 through a salt inlet 306 from a salt supply line. In various embodiments, the salt entering the extraction chamber 303 may be prescreened using an appropriate filter 317 to remove suspended impurities. The prescreening results in removal of large particulate impurities which are predominantly organic in nature. The contaminated salt is lixiviated in 303 using the fresh working solution 350 supplied from the reservoir to form salt slurry. In one embodiment, a single extraction chamber 303 is sufficient for salt purification. In other embodiments, multiple extraction chambers 303 may be serially connected for salt purification in batch or continuous modes. In some embodiments, the operating pressure of the chamber 303 is at least 3 bar. In other embodiments, the operation pressure can be varied between 0 to 10 bar. In some embodiments, the operating temperature is in the range of 20 to 50 C. In other embodiments, the operation temperature can be varied outside the specified range.

[0034] The salt slurry exits the first chamber through an outlet 307 connected to a second chamber or a separator unit 309. The separator unit 309 is configured to filter the salt slurry separating it into purified salt and used working solution containing one or more contaminants transferred from the contaminated salt. The purified salt may be further dried and packaged in one or more additional drying and packaging units 319.

[0035] One or more first treatment chambers 311-1,..., 311-n is configured to receive the used working solution from the second chamber 309 after optionally screening in filter unit 315. The used working solution in the first treatment chamber 311 is contacted with one or more reagents to remove any hardness and/or heavy metal. In some embodiments, the treatments may be performed in a single first treatment chamber 311- 1. In some embodiments, the treatments are performed in a plurality of first treatment chambers 311-1, ...311-n. [0036] The used working solution free of wet sludge containing the hardness precipitate undergoes further processing in one or more second treatment chamber 325-1, ....,325-n. The second treatment chamber 325 is configured to receive the used working solution after removal of heavy metals and/or hardness. The used working solution may be acidified and aerated under a constant pH range to remove excess alkalinity. The pH is maintained at the required level by the addition of suitable acid or base as necessary. The aeration may be done by supplying ambient air to displace carbon dioxide formed thereby reducing carbonate and bi-carbonate alkalinity.

[0037] One or more third treatment chambers 313-1, ....313-n is configured to receive the reagent-treated used working solution from the second treatment chamber 325. The used working solution entering the third treatment chamber 313 is subjected to catalytic chemical oxidation or electrooxidation by generated oxidants or both. In various embodiments, the oxidants enable the removal of organic pollutants causing color and total organic carbon (TOC). In some embodiments, a liquid catalyst or a heterogeneous catalyst is used to excite the oxidant to form reactive species.

[0038] In some embodiments, the precipitate containing wet sludge from the chemical treatment chamber 311 is further separated at a separator unit 321 into residual used working solution and the hardness precipitate. The used working solution is sent back to the chemical treatment chamber 311 from the separator 321.

[0039] In some embodiments, the used working solution exiting the separator unit 309 as the used working solution stream 360 may be optionally screened using at least one filter unit 315. The screening chambers 315, 317 comprises of at least one appropriately rated filter. The ratings of the filter depend on the particle size distribution of contaminants in the raw salt. The ratings may be in the range between nanometers to millimeters, and may include membranes or screens, depending on the size of contaminants. The precipitate containing hardness causing minerals from 321 may be sent to a drying and packaging chamber 323 for drying and packing. Similarly, the screened particulate contaminants from the filter units 315, 317 may be sent for drying and packaging to 323. Any suitable filtration process known in the art may be used for the filters mentioned herein including nano- or ultra-filtration.

[0040] Part of the retentate from the filter unit 315 is recycled to micron filter inlet, while a portion of it is sent to a separate treatment for removal of rejected contaminants and subsequently blended with the filtrate stream. This treatment could be a separate precipitation treatment like hardness removal or coagulation and flocculation treatment. This practice of bleeding a part of the retentate, treating it by precipitation or coagulation is configured to prevent buildup of undesirable contaminants such as suspended solids and colloids before the membrane.

[0041] In various embodiments, the purified salt obtained from the separator unit 309 is further dried to obtain the dried form. The purified salt is further sent to drying and packaging unit 319. The purified salt obtained may be used for most of the industrial processes requiring salt of purity greater than 96%. The purified salt obtained finds its application for dyeing in textile industry, for tanning in leather industry or the like.

[0042] In various embodiments, the one or more extraction chambers 303-1, -2, -3,.., -n is configured to allow counter-current flow of the incoming contaminated salt 371 and incoming fresh working solution stream 351 for optimal use of the fresh working solution in purification of the salt, as shown in FIG. 2B and FIG. 2C. In counter-current flow, the salt of highest purity contacts the working solution of highest quality. In a continuous mode of the extraction process, the contaminated salt from the supply line 371 is sent through one or more serially connected extraction chambers 303-1, 303-2, 303-3 and the exiting purified salt 372, 373, 374 from the chambers 303-1, 303-2, 303-3 is of increasing purity after passing through each extraction chamber. The exiting salt from continuous extraction mode 374 is of highest purity and can be sent for drying and packaging. The fresh working solution stream 351 enters on the opposite side of the serially connected extraction chambers 303-1, 303-2, 303-3 such that it is contacted in the extraction chamber with the salt of highest purity first. The used working solution exiting the chamber 352, 353, 354 in the reverse direction to salt is of decreasing purity after passing through each extraction chamber. The exiting used working solution stream 354 is of the lowest purity and may be sent for regeneration. In a batch mode of extraction, the contaminated salt 371 enters extraction chamber 303-1 and exits as partly purified salt 372 against the counter current flow of partly used working solution entering in opposite direction 353 and leaving as used working solution 354 to complete a first batch. The used working solution 354 resulting from the first batch is of lowest purity. The exiting partly purified salt 372 from first batch may be used in subsequent batches for further purification thereby obtaining purified salt of increasing purity 373, 374 or working solution of increasing purity 353, 352. The one or more batch extraction chambers 303-1, 303-2, ...303-n may be set up such that salt exiting the first batch of a first extraction chamber 303-1 may enter subsequent batches of any of the other extraction chambers 303-2, 303-3,.. 303-n. Similarly, working solution exiting the last batch of a first extraction chamber 303-1 may enter earlier batches of any of the other extraction chambers 303-2, ..303-n for optimal use of the working solution.

[0043] For chemical catalytic oxidation, a homogenous or heterogeneous catalyst in granular or powder form is utilized in a chemical treatment chamber (not shown). The chamber is configured to contact the used working solution with the catalyst.

[0044] In some embodiments, as shown in FIG. 3 A and 3B, a 3 phase or a 2 phase electrode system 500, 500a is used to treat the used working solution for oxidative removal of organic contaminants. In one embodiment, system 500 includes at least one active electrode or cathode 401, a counter electrode or anode 403, and a gas diffuser or sparger 405. The system may use solid electrodes with working liquids as electrically conducting medium and gas sparging at either one or more electrodes. The gas diffuser 405 is preferably located below either or both the electrodes and connected to an external source of gas supply. The gas may be air or any other gas. Electrode 401 is optimized for multiple phase reaction and may be a porous electrode. Example materials for electrode 401 may include porous carbon-based electrodes such as of carbon nanotubes, graphene, graphene oxide, reduced graphene oxide, porous conductive carbon structures such as carbon foam, mesh, screen, cloth, or fiber or graphite, or layered coatings thereof, or composites thereof, films thereof, beads, or any combination thereof. Additional suitable materials include any dimensionally stable electrode materials. In other embodiments, the electrode 401 could be the anode and the counter electrode 403 could be the cathode. The active electrode 411 in a two-phase system 500a may include any dimensionally stable electrode.

[0045] In both of the above cases 500 or 500a solid electrodes may refer to electrodes directly connected to power supply or indirectly connected through a feeder electrode. In directly connected electrodes, the oxidative removal of contaminants predominantly occurs in the same electrode connected to power supply. In an indirectly connected electrode, the current supply may be separated from the electrode where oxidative processes take place.

[0046] In either embodiment illustrated in FIG. 3 A or 3B, the reagent treated working solution entering the chamber is used as electrolyte 407. The electrolyte 407 may further include one or more additional agents. A power supply 409 is connected across the active electrodes 401 or 411 and the counter electrode 403. The materials used in electrodes and additional agents added to electrolyte may be any substance known in the art for purposes of effecting oxidation or catalysis. Suitable materials for cathode and anode may include a dimensionally stable electrode such as a boron-doped diamond electrode, electrodes from metals such as ruthenium, iridium, lead, bismuth, cadmium, tantalum, titanium, niobium, mercury, chromium, molybdenum, silver, vanadium, copper, platinum, gold, cobalt, rhodium, nickel, indium, copper, zirconium, tin, metal oxides thereof, alloys thereof, stainless steel. [0047] The disclosed method, system and devices allows for continuously supplying fresh working solution used in the purification of the contaminated salt by regenerating the used working solution with minimal make-up and minimal or zero discharge of liquid effluents in the purification method. The method, system and devices are suitable for degrading contaminants in the saturated or highly concentrated brine where normal oxidative treatment of contaminants becomes ineffective. The method, system and devices regenerate working solution without any salt or working solution wastage.

[0048] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation without departing from its scope. Further, the examples to follow are not to be construed as limiting the scope of the invention which will be as delineated in the claims appended hereto.

[0049] EXAMPLES

[0050] Example 1: Purification of contaminated sodium chloride salt and regeneration of the working solution using catalytic chemical oxidation

[0051] Contaminated sodium chloride salt containing total hardness and partially degraded dyes was considered for purification in batch mode of operation. It contained particulate impurities of 3 mm mesh particle size. Size of salt crystals was much finer being less than 1 mm. The particulate impurities were first removed in a pre-screening filter of 3 mm or finer rating. The contaminated sodium chloride salt was contacted in counter-current sequence with two baths of working solution, which contains saturating concentrations of sodium chloride in water. About 15 L of working solution, which has been used once in a previous purification batch was made to contact one kg of the salt in first bath. A second bath of regenerated fresh working solution was used. Same ratio of working solution to salt was used. Both baths were carried out at a temperature of 25°C for 1 hour each. The used solution from second bath and the purified salt were separated. The used solution from the second bath was used as first bath in the next purification batch while the used solution from the first bath was sent for regeneration. The purified salt (-96% or greater purity) was sent for drying and packaging.

[0052] The used solution was filtered through a 1 micron filter to remove suspended particles. The used solution was then passed into a first treatment chamber where sodium hydroxide was used as a hardness remover. A pH of 11 was maintained. The hardness and associated alkalinity were reduced. The presence of hardness minerals is reduced from about 1500 mg/L to about 200 mg/L.

[0053] In a consecutive treatment chamber, pH of the working solution was corrected and maintained between 6 to 7 while it was aerated. At this point the presence of alkalinity (carbonates/bicarbonates) was reduced significantly from 3000 mg/L to 150 mg/L. In the next step, organic pollutants (indicated by parameters such as total organic carbon or chemical oxygen demand) were removed by catalytic chemical oxidation from initial 2500 to 5000 mg/L TOC to final values of 10 mg/L to 1000 mg/L. Hydrogen peroxide was used as an oxidant in the presence of a Mn0₂ catalyst to remove organic pollutants. The oxidation treatment was carried out for about 5 hours.

[0054] Example 2- Purification of contaminated sodium chloride and regeneration of the working solution using electro-oxidative method

[0055] Contaminated sodium chloride salt was considered for purification in batch mode of operation. It contained particulate impurities of 1 mm mesh particle size. These were first removed in a pre-screening filter of 1 mm or finer rating. The contaminated sodium chloride salt was contacted in counter-current sequence with five baths of working solution, which contains saturating concentrations of sodium chloride in water. About 3 L of working solution was made to contact at least one kg of the salt in all baths. The method was carried out at a temperature of 55° C for about 15 minutes for each bath. The used solution from last bath and the purified salt were separated. The purified salt (~ 96% or greater) was sent for drying and packaging. The used solution from the first bath was sent for regeneration. While the used solution from the last and intermediate baths were subsequently used in next salt batch.

[0056] The used solution was filtered through a 50 micron filter followed immediately by an ultrafiltration membrane with a molecular weight cut-off of 1,000 kilo Daltons. The ultra-filtered used solution was then passed into a first treatment chamber where sodium hydroxide and calcium oxide were used to remove hardness. A pH of 10.5 was maintained. Thus, the hardness and associated alkalinity was removed precipitation. In the consecutive second regeneration chamber after separation of the hardness sludge, pH was then reduced using hydrochloric acid and the solution was aerated for about 2 hours to remove excess alkalinity. During aeration pH was maintained in the range of 4 to 8 by adding appropriate amounts of acid.

[0057] Subsequently, organic pollutants (indicated by parameters such as total organic carbon or chemical oxygen demand) were reduced by electro oxidative method. Ozone was used as an oxidant in the presence of a three- phase electrode as shown in FIG. 3A to remove organic pollutants. The electrode setup consisted of a porous reduced graphene oxide cathode and a dimensionally stable anode. The used working solution was used as the electrolyte. The electrode setup was followed by catalytic chemical oxidation system using a homogenous liquid catalyst. Hydrogen peroxide was used as an oxidant in the presence of a multidentate macrocyclic ligand complexed with a metal ion. The combined oxidation treatment was carried out for about 4 hours. Treatment duration was varied depending on the pollutant type, oxidant concentration, degradation kinetics, the pollutant concentrations and required purification. The solution was considered regenerated and suitable for use in subsequent salt purification.

[0058] Example 3 - Purification of sodium sulphate salt and regeneration of the working solution [0059] Sodium sulphate salt containing the contaminants, total hardness, aromatic compounds (such as naphthalene, diazo compounds) and surfactants was considered for purification. It contained particulate impurities of size in the range of 3 mm to 6 mm mesh particle size. These were removed in a pre-screening filter of 3 mm or finer rating. The contaminated sodium sulphate salt was contacted with three baths of working solution in a counter flow manner in batch mode of operation. The working solution contains saturating concentrations of sodium sulphate in water. About 25 L of working solution was made to contact at least one kg of the salt in all baths. The method was carried out at a temperature of 40°C for 45 mins for each bath. The used solution from last bath and the purified salt were separated. The purified salt was sent for drying and packaging. The used solution from the first bath was sent for regeneration. While the used solution from the last and intermediate baths were subsequently used in next salt batch.

[0060] The used solution was filtered through a 25 micron filter followed immediately by an ultrafiltration membrane with a molecular weight cut-off of 10 kilo Daltons. The used ultra filtered solution was then passed into a first regeneration chamber where sodium hydroxide and sodium bicarbonate was used for removing hardness by precipitation. A pH of 10 was maintained. Thus, the hardness and associated alkalinity was removed. In a consecutive second regeneration chamber after separation of the hardness sludge pH was then reduced using H₂S0₄ and the solution was aerated for 4 hours to remove excess alkalinity. During aeration pH was maintained in the range of 4 to 8 by adding appropriate amount of acid or base.

[0061] Subsequently, organic pollutants (indicated by parameters such as total organic carbon or chemical oxygen demand) were degraded by electro oxidation. Air was used an oxidant in the presence of a multiple- phase electrode to remove organic pollutants. A three phase electrode setup as shown in FIG. 3A was utilized to degrade the contaminants. The electrode setup consisted of a porous carbon felt cathode and a mixed metal oxide DSA anode. The working solution was used as the electrolyte. In addition, ferrate or KMn04 may be added to the electrolyte as additional mediated oxidant or oxidant. The combined oxidation treatment was carried out for about 2 hours. Treatment duration was varied depending on the pollutant type, oxidant dosage, degradation kinetics, the pollutant concentrations and required purification. The solution was considered regenerated and suitable for use in subsequent salt purification.

[0062] Example 4 - Purification of mixed sodium chloride and sodium sulphate waste salt and regeneration of the working solution

[0063] Mixed sodium chloride and sodium sulphate salt containing the contaminants, total hardness, aromatic compounds (such as naphthalene, diazo compounds) and surfactants was considered for purification. It contained particulate impurities of size in the range of 1 mm to 4 mm mesh particle size. The maximum dimension of salt crystals was less than 2 mm. The particulate impurities were removed in a pre-screening filter of 3 mm or larger rating. The contaminated mixed salt was contacted with three baths of working solution, which contains saturating concentrations of sodium chloride and sulphate in water such a ratio so that there is no net exchange of salts between the solid and liquid phases. About 8 L of working solution was made to contact at least one kg of the salt in all baths. The method was carried out at a temperature of 25°C for 30 mins for each bath. The last used solution and the purified salt were separated. The purified salt was sent for drying and packaging. The used solution from the first bath was sent for regeneration. While the used solution from the last and intermediate baths were subsequently used in next salt batch.

[0064] The used solution was filtered through a 10 micron filter followed immediately by a nanofiltration membrane. The nano-filtered used solution was then passed into a first regeneration chamber where sodium hydroxide and sodium carbonate was used for removing hardness by precipitation. A pH of 10.5 was maintained. Thus, the hardness and associated alkalinity was removed. In a consecutive second regeneration chamber after separation of the hardness sludge, pH was then reduced using H₂S0₄ and the solution was aerated for 3 hours to remove excess alkalinity. During aeration pH was maintained in the range of 4 to 8 by dosing appropriate amounts of acid.

[0065] Subsequently, organic pollutants (indicated by parameters such as total organic carbon or chemical oxygen demand) were degraded by electro oxidation. The electrode setup consisted of a dimensionally stable anode and a stainless steel cathode. The reagent treated working solution was used as the electrolyte. In addition, ferrate or hydrogen peroxide may be added to the electrolyte as additional mediated oxidant or oxidant. Oxidants were generated in- situ with the supply of electricity and without bubbling gases into the solution to remove organic pollutants. The oxidation treatment was carried out for about 5 hours.

Claims

I CLAIM:

1. A method of purifying a contaminated salt while recycling a working solution used in the purification, the method comprising the steps of:

a) contacting the working solution with the contaminated salt to obtain a salt slurry, wherein the contaminated salt comprises one or more contaminants and the working solution comprises saturating concentrations of the salt dissolved in one or more solvents;

b) separating purified salt from the salt slurry thereby obtaining a used working solution comprising the one or more contaminants;

c) contacting the used working solution with one or more reagents for removing hardness, heavy metals, or a combination thereof to obtain a reagent-treated used working solution and a precipitate;

d) separating the precipitate from the reagent-treated used working solution; e) adjusting pH of the reagent-treated used working solution to a value in the range of 4 to 8;

f) aerating the reagent-treated used working solution while maintaining the pH in the range of 4 to 8;

g) contacting the reagent-treated used working solution with one or more oxidants to remove organic contaminants, thereby regenerating the working solution; and h) supplying the regenerated working solution for further purification of contaminated salt.

2. The method of claim 1, wherein said one or more reagents comprises sodium hydroxide, calcium oxide, calcium carbonate, calcium bi-carbonate, sodium carbonate, sodium bicarbonate, calcium hydroxide, or a combination thereof.

3. The method of claim 1, wherein said salt is Na₂S0₄, NaCl or a combination thereof.

4. The method of claim 1, wherein said solvent is water.

5. The method of claim 1, wherein the used working solution is filtered prior to contacting with said one or more reagents.

6. The method of claim 1, wherein said one or more oxidants comprises oxygen, air, hydrogen peroxide, ozone, chlorine, a hypochlorite, a peroxide, a permanganate, a persulfate, a ferrate, peracetic acid, a peroxysulfate, hydroxyl radical, sulphate radical, superoxide ion, or a combination thereof.

7. The method of claim 1, wherein said one or more oxidants degrade organic contaminants contributing to color, total organic carbon (TOC), or both in the used working solution.

8. The method of claim 1, wherein said one or more oxidants is generated by application of electrical potential in a multiphase electrochemical system.

9. The method of claim 8, wherein the multiphase electrochemical system is a two- phase or a three-phase electrode system.

10. The method of claim 1, wherein said one or more oxidants are generated in the presence of a homogenous or a heterogeneous catalyst.

11. The method of claim 10, wherein the catalyst comprises Mn0₂

12. The method of claim 1, wherein said contacting the working solution with the contaminated salt comprises: mixing the contaminated salt with the working solution in the range of 1-75 w/w % or in the range of 0.5 to 100 L/kg.

13. The method of claim 1, wherein said contacting the working solution with the contaminated salt comprises: supplying the contaminated salt in counter-current flow to the working solution.

14. The method of claim 1, wherein said contacting the working solution with the contaminated salt is performed in a batch mode or continuous mode.

15. An apparatus (300) for purifying contaminated salt while recycling a working solution used in the purification, the apparatus comprising:

a) a reservoir (301) configured to store and supply the working solution;

b) one or more extraction chambers (303-1, 303-2, ...303-n) configured to mix the contaminated salt and the working solution to form a salt slurry, wherein the contaminated salt comprises one or more contaminants and the working solution comprises saturating concentrations of the salt dissolved in one or more solvents;

c) a separator (309) configured to filter purified salt from the salt slurry to obtain a used working solution comprising the one or more contaminants;

d) one or more first treatment chambers (311-1, 3l l-2,...3l l-n) configured to contact the used working solution with one or more reagents for removing hardness, heavy metals, or a combination thereof;

e) one or more second treatment chamber (325-1, 325-2, ..325-n) for pH adjustment and aeration of the used working solution; and

f) one or more third treatment chambers configured to contact the reagent-treated used working solution with one or more oxidants in a multiphase electrochemical system (500, 500a) and/or a homogeneous or heterogeneous chemical catalytic system (313) to remove organic contaminants, and regenerate the working solution.

16. The apparatus of claim 15, wherein said one or more extraction chambers (303-

1....303-n) is configured to mix the contaminated salt in a counter-current flow with the working solution.

17. The apparatus of claim 15, comprising a plurality of extraction chambers (303-

1....303-n), wherein the plurality of extraction chambers are connected in a serial or parallel configuration.

18. The apparatus of claim 15, further comprising a filter (315) configured to remove suspended solids in the used working solution.

19. The apparatus of claim 15, wherein the treatment chambers (500a) comprise a two phase electrode system.

20. The apparatus of claim 15, wherein the treatment chambers (500) comprise a three phase electrode system.

21. The apparatus of claim 15, wherein the apparatus forms a closed-loop system for purification of the contaminated salt.