Method and device for reducing salt in mine water

Abstract

The present invention provides a method and device for reducing the salt content of mine water. The method includes a pretreatment system, a hardness removal system, a membrane-based salt reduction system and a crystallization system for mine water. 1) The pretreatment system removes impurities such as suspended matter, colloids, and coal ash in the mine water to prepare for the subsequent membrane integration process; 2) The clarified liquid after pretreatment enters ultrafiltration and a first-stage nanofiltration to achieve coarse concentration of the mine water, and the first-stage nanofiltration water can be directly discharged; 3) The first-stage nanofiltration concentrated water enters the hardness removal system, which is mainly for hardness removal by chemicals, and then the concentrated water after the second-stage nanofiltration concentration enters the crystallization system to obtain anhydrous sodium sulfate, and the crystallization mother liquor is mixed with the nanofiltration water for discharge. Through the above method and device, the salt reduction of mine water and the output of high-purity anhydrous sodium sulfate can be achieved at the optimal investment cost and operating cost.

Description

Technical Field

[0001] The invention relates to a method and a device for reducing salt in mine water, belonging to the technical field of water treatment. Background Art

[0002] In the process of coal mining, high-mineralized mine water is produced, which is the main wastewater of coal mines. The total inorganic salt content in the mine water is greater than 1000 mg / L, containing Ca 2+ Mg 2+ , K + 、Na + 、SO4 2- , Cl - 、HCO3 - The water quality is neutral or alkaline, also known as brackish water. Highly mineralized mine water must be desalinated before it can be discharged and reused.

[0003] Traditional treatment methods mainly use coagulation, sedimentation, filtration, disinfection and other treatment processes. The treated mine water is reused for factory dust removal, ground coal washing water replenishment, circulating water, green irrigation, etc. However, traditional treatment methods can only remove suspended matter, but not soluble salts, so that when mine water is used for factory dust removal, green irrigation, etc., more salt will be precipitated, and it will eventually be collected underground. In addition, due to the large amount of mine water produced, it is impossible to consume the treated mine water, especially for coal mining enterprises far away from living areas; and directly discharging mine water into surrounding rivers, lakes and even reservoirs has caused serious pollution to the water environment. Therefore, the treatment and utilization of mine water has become a problem that needs to be solved urgently.

[0004] At present, there are also methods such as reverse osmosis and electrodialysis technology for treating mine water. Chinese patent CN109336323A discloses a method and system for treating high-mineralized mine water. It includes a mine water membrane concentration process and an evaporation crystallization salt separation process. Among them, the mine water membrane concentration process is concentrated in two stages, and then undergoes disc-tube reverse osmosis to make the salt content of concentrated water reach 5%-8%, reducing the cost of subsequent concentrated brine evaporation and crystallization. The process treatment cost reduction described in this patent is limited, and impurities will still be produced later. Summary of the invention

[0005] The object of the present invention is to provide a method for reducing the salt content of high-mineralized mine water, wherein the overall process package occupies a small area, has low investment cost and operation cost, and the crystallized product has high and stable quality and does not generate impurity salts.

[0006] A method for reducing salt in mine water comprises the following steps:

[0007] Step 1: Pre-filter the mine water;

[0008] Step 2, subjecting the filtrate obtained in step 1 to a first ultrafiltration treatment;

[0009] Step 3, performing a first nanofiltration treatment on the filtrate obtained in step 2;

[0010] Step 4, performing a precipitation reaction on the nanofiltration concentrate obtained in step 3 to remove impure cations;

[0011] Step 5, subjecting the wastewater obtained in step 4 to a second ultrafiltration treatment to filter out the precipitate;

[0012] Step 6, subjecting the filtrate obtained in step 5 to a second nanofiltration treatment;

[0013] In step 7, the nanofiltration concentrate obtained in step 6 is sequentially crystallized and dried to obtain recovered sodium sulfate.

[0014] In one embodiment, the pre-filtration treatment in step 1 includes the steps of coagulation, pre-filtration and activated carbon filtration.

[0015] In one embodiment, the coagulant used in the coagulation process includes polyaluminium chloride (PAC), polyacrylamide (PAM) and the like.

[0016] In one embodiment, superconducting magnetic materials may be added during the coagulation process to assist the coagulation, and the superconducting magnetic materials may be recovered by a cyclone separation method.

[0017] In one embodiment, the superconducting magnetic material is selected from iron oxide particles.

[0018] In one embodiment, removing impurity cations in step 4 refers to adding NaOH and Na2CO3 to perform a precipitation reaction.

[0019] In one embodiment, the molecular weight cutoff of the nanofiltration membrane in the first nanofiltration process is 150-1000 Da, more preferably 500-1000 Da.

[0020] In one embodiment, the molecular weight cutoff of the nanofiltration membrane in the second nanofiltration process is 150-1000 Da, more preferably 300-500 Da.

[0021] In one embodiment, in step 7, the nanofiltration concentrate obtained in step 6 needs to be sequentially subjected to cation exchange resin to remove impurity cations and a third nanofiltration treatment, and then the concentrate of the third nanofiltration is sequentially subjected to crystallization and drying treatments.

[0022] In one embodiment, the nanofiltration concentrate obtained in step 6 needs to be subjected to flocculation treatment; during the flocculation process, polyferric chloride is used as a flocculant and a nucleating agent is added at the same time.

[0023] In one embodiment, the nucleating agent is hydrophobically modified ferrosoferric oxide.

[0024] In one embodiment, the molecular weight cutoff of the nanofiltration membrane in the third nanofiltration process is 150-1000 Da, more preferably 150-300 Da.

[0025] A device for reducing salt in mine water, comprising:

[0026] Pre-filter, used for pre-filtering mine water;

[0027] A first ultrafiltration membrane is connected to the permeation side of the pre-filter and is used to perform ultrafiltration treatment on the filtrate of the pre-filter;

[0028] A first nanofiltration membrane is connected to the permeation side of the first ultrafiltration membrane and is used for performing nanofiltration concentration treatment on the filtrate of the first ultrafiltration membrane;

[0029] A precipitation reaction tank, connected to the concentration side of the first nanofiltration membrane, is used to perform precipitation reaction on the concentrated liquid of the first nanofiltration membrane to remove impurity cations;

[0030] NaOH dosing tank and Na2CO3 dosing tank are used to add NaOH and Na2CO3 to the precipitation reaction tank respectively;

[0031] The second ultrafiltration membrane is connected to the precipitation reaction tank and is used to filter the wastewater after the precipitation reaction to remove the precipitation;

[0032] A second nanofiltration membrane is connected to the permeation side of the second ultrafiltration membrane and is used to concentrate the filtrate of the second ultrafiltration membrane;

[0033] The crystallization device is connected to the concentration side of the second nanofiltration membrane and is used to perform crystallization treatment on the concentrated liquid of the second nanofiltration membrane to obtain recovered sodium sulfate.

[0034] In one embodiment, the pre-filter is one or a combination of a coagulation tank, a sand filter or an activated carbon filter.

[0035] In one embodiment, the material of the first ultrafiltration membrane and the second ultrafiltration membrane is selected from organic material or inorganic material; the organic material is PTFE, PVDF, PES, PS or acetate fiber; the inorganic material is aluminum oxide, zirconium oxide, titanium oxide or silicon carbide.

[0036] In one embodiment, the molecular weight cut-off of the first nanofiltration membrane is 150 to 1000 Da, more preferably 500 to 1000 Da.

[0037] In one embodiment, the molecular weight cut-off of the second nanofiltration membrane is 150-1000 Da, more preferably 300-500 Da.

[0038] In one embodiment, the crystallization apparatus is an evaporative crystallizer or a cryo-crystallizer.

[0039] In one embodiment, the concentrating side of the second nanofiltration membrane is connected to the ion exchange resin column connected to the third nanofiltration membrane, and the concentrating side of the third nanofiltration membrane is connected to the crystallization device.

[0040] In one embodiment, the ion exchange resin column is filled with cation exchange resin.

[0041] In one embodiment, the molecular weight cutoff of the three-stage nanofiltration membrane is 150 to 1000 Da, more preferably 150 to 300 Da.

[0042] In one embodiment, the concentration side of the second nanofiltration membrane is connected to a flocculation reactor, a flocculant addition tank and a nucleating agent addition tank are connected to the flocculation reactor, the flocculation reactor is connected to a third ultrafiltration membrane, and the permeation side of the third ultrafiltration membrane is connected to an ion exchange resin column.

[0043] In one embodiment, the flocculant addition tank is filled with aluminum sulfate, polyaluminum chloride (PAC), ferrous sulfate, ferric sulfate or ferric chloride; and the nucleating agent addition tank is filled with oleic acid-modified ferrosoferric oxide.

[0044] The above device is used for recovering sodium sulfate from mine water. Beneficial Effects

[0045] The present invention focuses on "multi-stage nanofiltration" salt reduction, which is different from "multi-stage reverse osmosis" and "reverse osmosis + nanofiltration". It aims to achieve salt reduction of mine water to meet emission standards and output of high-purity anhydrous sodium sulfate to achieve resource utilization of waste salt at the optimal investment cost and operating cost. It has obvious technical advantages and originality.

[0046] The present invention discloses a method and device for reducing the salt content of mine water. The present invention mainly utilizes a multi-stage nanofiltration process to reduce the salt content of high-mineralized mine water. Compared with multi-stage reverse osmosis, the investment cost is reduced by more than 30%, and the operating cost is reduced by more than 25%. The purpose of achieving standard discharge of mine water and resource utilization of waste salt is ultimately achieved without producing solid waste or hazardous waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a device diagram of the present invention;

[0048] Figure 2 is another device diagram of the present invention;

[0049] Figure 3 is another device diagram of the present invention;

[0050] Among them, 1. pre-filter; 2. first ultrafiltration membrane; 3. first nanofiltration membrane; 4. sedimentation reaction tank; 5. NaOH dosing tank; 6. Na2CO3 dosing tank; 7. second ultrafiltration membrane; 8. second nanofiltration membrane; 9. crystallization device; 10. drying device; 11. ion exchange resin column; 12. third nanofiltration membrane; 14. flocculation reactor; 15. flocculant dosing tank; 16. nucleating agent dosing tank; 17. third ultrafiltration membrane. DETAILED DESCRIPTION

[0051] The present invention is further described in detail below by specific embodiments. However, it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. If specific techniques or conditions are not specified in the examples, they are carried out according to the techniques or conditions described in the literature in this area or according to the product instructions. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially.

[0052] Approximations used herein can be used to modify any quantitative expression throughout the specification and claims, and can be allowed to change under the condition that does not cause its related basic function to change. Therefore, the value modified by terms such as "about" is not limited to the specified exact value. In at least some cases, approximations can correspond to the accuracy of the instrument used to measure the value. Unless otherwise indicated in the context or sentence, range limits can be combined and / or interchanged, and this scope is determined as and includes all sub-ranges included herein. Except indicating in the operating examples or elsewhere, the numerals or expressions of the amount, reaction conditions, etc. of all representation components used in the specification and claims should be understood to be modified by the word "about" in all cases.

[0053] Values ​​expressed in range format should be understood in a flexible manner to include not only the values ​​explicitly listed as limits of the range, but also all individual values ​​or subranges encompassed within the range, as if each value and subrange were explicitly listed. For example, a concentration range of "about 0.1% to about 5%" should be understood to include not only the explicitly listed concentrations of about 0.1% to about 5%, but also individual concentrations (e.g., 1%, 2%, 3% and 4%) and subranges (e.g., 0.1% to 0.5%, 1% to 2.2%, 3.3% to 4.4%) within the indicated range.

[0054] The term "removal" in this specification includes not only the complete removal of the target substance but also the partial removal (reduction of the amount of the substance). The term "purification" in this specification includes the removal of any or specific impurities.

[0055] As used herein, the words "comprises," "comprising," "having," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that includes listed elements is not necessarily limited to those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0056] The present invention provides a method and device for reducing the salt content of mine water. The method includes a pretreatment system, a hardness removal system, a membrane-based salt reduction system and a crystallization system for mine water. 1) The pretreatment system removes impurities such as suspended matter, colloids, and coal ash in the mine water to prepare for the subsequent membrane integration process; 2) The clarified liquid after pretreatment enters ultrafiltration and a first-stage nanofiltration to achieve coarse concentration of the mine water, and the first-stage nanofiltration water can be directly discharged; 3) The first-stage nanofiltration concentrated water enters the hardness removal system, which is mainly for hardness removal by chemicals, and then the concentrated water after the second-stage nanofiltration concentration enters the crystallization system to obtain anhydrous sodium sulfate, and the crystallization mother liquor is mixed with the nanofiltration water for discharge. Through the above method and device, the salt reduction of mine water and the output of high-purity anhydrous sodium sulfate can be achieved at the optimal investment cost and operating cost.

[0057] In another embodiment, the crystallization mother liquor is mixed with the first and second stage nanofiltration produced water for discharge, and the TDS of the mixed discharged water is less than 1600 mg / L, and the sulfate ion is less than 650 mg / L.

[0058] In another embodiment, for the concentrated water after the second-stage nanofiltration concentration, an ion exchange resin column is used to remove the hardness, which can eliminate the divalent ions in the concentrated water, which is beneficial to improve the purity of the sodium sulfate after the concentrated water is crystallized. At the same time, after the divalent ions are eliminated, the concentrated nanofiltration concentration treatment can be carried out, which can effectively reduce the load of the three-stage nanofiltration membrane and improve the concentration multiple and flux of the nanofiltration membrane. When this method is adopted, the first stage of nanofiltration will reduce the volume of mine water and the floor space of the hardness removal sedimentation tank. The first and second stage nanofiltration systems select slightly loose nanofiltration membranes for salt separation, and the produced water leaks out monovalent salts, calcium ions, magnesium ions, etc. as much as possible, while meeting the discharge standards, reducing the operating pressure of the third stage of nanofiltration and improving its recovery rate; the salt concentration is increased to 10-20% through the third stage of nanofiltration, reducing steam consumption and reducing operating costs; and crystallization produces high-quality anhydrous sodium sulfate, with indicators better than the technical requirements of Class II first-class products in GB / T 6009-2014, and waste salt can be utilized as a resource; membrane produced water can meet the discharge standards, does not cause environmental pollution, and does not produce low-value sodium chloride and miscellaneous salts.

[0059] In another embodiment, the molecular weight cutoff of the three-stage nanofiltration membrane is 150-1000 Da, more preferably 150-300 Da; the three-stage nanofiltration membrane increases the salt concentration to 10-20%.

[0060] Since the water to be treated in the present invention is mine water, the water contains not only inorganic salts and particulate impurities, but also a large amount of organic COD impurities. These organic substances cannot be effectively removed in the pre-flocculation, pre-filtration and ultrafiltration systems, and will remain in the concentrated liquid of the subsequent nanofiltration membrane. As the nanofiltration membrane is concentrated, these COD substances will be further concentrated, affecting the flux and concentration multiple of the nanofiltration membrane, and also affecting the purity of the recovered sodium sulfate. In another embodiment, for the concentrated water obtained by the second-stage nanofiltration, after the COD substance therein is improved, it is flocculated by a deep flocculation method. It is best to use a polyferric chloride flocculant. Since the iron ions introduced and remaining in the flocculant can be removed by an ion exchange method through a subsequent ion exchange resin column, it will not affect the purity of the recovered concentrated sodium sulfate. At the same time, in order to further improve the flocculation effect of polyferric chloride, a nucleating agent can be used to assist flocculation, which can improve the flocculation effect of polyferric chloride. The nucleating agent used here can be magnetic ferroferric oxide, which has good compatibility with polyferric chloride and improves the flocculation effect. In order to further improve the flocculation effect, the surface of the magnetic ferroferric oxide is hydrophobicized, so that it can effectively adsorb organic impurities in the mine water and improve the flocculation effect.

[0061] In the above treatment, the water produced by nanofiltration can be discharged directly; the mother liquor after crystallization can also be discharged in compliance with the standards.

[0062] In the above steps, the pretreatment system is mainly some sedimentation tanks, coarse filtration equipment, etc., which can also be defined as pre-filtration equipment to remove suspended matter, colloids, coal ash and other impurities in mine water; there is no special limitation on pre-filtration equipment. As specific solid-liquid separation treatment methods, centrifugal separation, pressing separation, filtering, floating separation, and sedimentation separation can be cited. As a centrifugal separation method, a horizontal continuous centrifugal separator (spiral decanter treatment), a separation plate centrifugal separator, a centrifugal filter, and a Sharples ultracentrifugal separator can be exemplified. As a filtering method, a belt filter, a belt press, a screw press, a pre-coating filter, and a filter press can be exemplified. As a floating separation method, a continuous floating separation device can be exemplified. As a sedimentation separation method, a coagulation sedimentation separator, a rapid sedimentation separator, etc. can be exemplified, but it is not particularly limited to any of the above. However, it is possible to reduce the load on the membrane during the treatment of the precision filtration membrane and / or ultrafiltration membrane by any of the above or a combination thereof.

[0063] In the above steps, the ultrafiltration membrane used is mainly used for precise separation. The "ultrafiltration membrane" in this specification refers to a filter membrane with a pore size of 0.001 to 0.01 μm and / or a filter membrane with a molecular weight cutoff of about 1000 to 300,000. The material of the ultrafiltration membrane can be roughly divided into inorganic membranes and organic membranes, and further divided into hydrophobic and hydrophilic. As a hydrophobic organic membrane, it is not limited to this, and polysulfone, polyethersulfone, polyether, polyvinylidene fluoride, polyethylene, polypropylene, etc. can be listed. As a hydrophilic organic membrane, it is not limited to this, and polyacrylonitrile, polyamide, polyimide, cellulose acetate, etc. can be listed. The filter element shape includes flat membrane, tubular membrane, spiral membrane, hollow fiber (hollow fiber) membrane, etc., all module forms. As a filtering method, it includes a full-volume filtration method and a cross-flow method. The full-volume filtration method refers to a method of filtering all the water supplied to the membrane. In contrast, the cross-flow method refers to a method of filtering by making the water flow parallel to the membrane surface, while suppressing the accumulation of suspended matter or colloids contained in the water supplied to the membrane on the membrane surface. The cross-flow method is not limited to this, and includes a once-through method, a backwash method, and a reflux method.

[0064] In the present invention, the hardness removal system mainly adds NaOH and Na2CO3 to carry out reagent precipitation reaction to remove the impurity cations therein. First, CO3 is added to the crude brine. 2- and OH - ions, after the reaction, CO3 2- and OH - Ions can make Ca 2+ Mg 2+ They are converted into CaCO3 and Mg(OH)2 respectively. When the crude brine also contains Cs+ and Ni+ ions, CO3 2- and OH - Ions can also be converted into Cs2CO3 and Ni(OH)2, and then sent to the ultrafiltration membrane for filtration, which can remove the CaCO3, Mg(OH)2, Cs2CO3 and Ni(OH)2 generated by these reactions to obtain a purified ultrafiltration clear liquid. 2+ Mg 2+ The concentration range of Cs+ and Ni+ ions is not particularly limited and can be in the range of 0.01 to 50 g / L. As long as a suitable precipitant CO3 is selected according to the concentration of impurity cations, 2- and OH - The amount of ions added can convert impurity cations into precipitation, CO3 2- and OH -The amount of ions added can be calculated by those skilled in the art according to the stoichiometric balance. In order to completely convert the impurity cations into precipitation, a precipitant is added selected from one or a mixture of NaOH, Na2CO3, KOH or K2CO3, and the amount of each precipitant added is greater than the amount required for complete precipitation of the impurity cations, for example: the amount of NaOH, Na2CO3, KOH or K2CO3 added is 0.2~0.3g / L more than the amount required for complete precipitation of the impurity cations. The "complete precipitation" described in the present invention refers to the required precipitation amount calculated according to the chemical reaction equilibrium formula, which can be calculated by those skilled in the art according to the chemical reaction molar ratio, and is not understood as the impurity ions being completely precipitated in the actual reaction.

[0065] The nanofiltration membrane described in the present invention is defined as a "pressure-driven membrane that blocks particles smaller than 2 nm and dissolved macromolecules". The effective nanofiltration membrane suitable for the present invention is preferably a membrane that has a charge on the membrane surface, thereby showing an improved separation efficiency through a combination of pore separation (particle size separation) and electrostatic separation due to the charge on the membrane surface. Therefore, it is necessary to use a nanofiltration membrane that can remove high-molecular substances by particle size separation while separating the alkali metal ions as the recovery target from other ions with different charge characteristics by means of charge. As the material of the nanofiltration membrane used in the present invention, polymer materials such as cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides and vinyl polymers can be used. The membrane is not limited to a membrane composed of only one material, and can be a membrane containing multiple materials. Regarding the membrane structure, the membrane can be an asymmetric membrane having a dense layer on at least one side of the membrane and having micropores with gradually increasing pore diameters from the dense layer to the inside of the membrane or the other side; or a composite membrane having a very thin functional layer formed of other materials on the dense layer of the asymmetric membrane.

[0066] The ion exchange resin in the present invention mainly refers to the cationic exchange resin, which can adsorb the impurity cations in the wastewater on the resin by ion exchange to achieve the effect of removing cationic impurities. The functional group of the cationic resin here is organic acid, and it is divided into strong acid and weak acid according to the strength of the acidity. The strong acid functional group is benzenesulfonic acid, and the weak acid functional group includes organic phosphoric acid, hydroxy acid and phenol.

[0067] Etc. Weakly acidic cationic resins contain weakly acidic groups, such as carboxyl groups, which can dissociate H+ in water and become acidic. However, because the degree of dissociation is not high, it is generally only weakly acidic, so it belongs to weakly acidic cationic resins. The negatively charged groups remaining after the resin dissociates can be adsorbed and combined with other cations in the solution, thereby producing a cation exchange effect.

[0068] The flocculant used in the present invention can be a flocculant, as long as it is capable of flocculating the organic impurities in the wastewater, there is no particular limitation, and aluminum sulfate, polyaluminum chloride (PAC), ferrous sulfate, ferric sulfate, ferric chloride, organic polymer flocculants, etc. can be used. In addition, nucleating agents can also be added on the basis of flocculants to assist in achieving the effect of improving flocculation. As nucleating agents, there is no particular limitation, and for example, bentonite, powdered activated carbon, activated silica, etc. can be used. Preferably, a ferrous oxide nucleating agent treated by surface hydrophobic modification is used, and its preparation process can be Fe containing 5wt% O in anhydrous ethanol solution and oleic acid is added, and the addition amount of oleic acid is Fe O about 3 times, and after stirring, react at 80°C for about 1h, filter out precipitate and wash with ethanol and dry, and obtain the ferrous oxide nucleating agent treated by oleic acid modification.

[0069] In the following examples, the mine water used has a COD of 11 ppm, a TDS of 3450 ppm, a total hardness of 1920 ppm, and a sulfate content of 1300 ppm.

[0070] Based on the above method, the device provided by the present invention is as follows Figure 1-3 As shown:

[0071] In one embodiment, Figure 1 The device shown comprises:

[0072] A pre-filter 1, used for pre-filtering the mine water;

[0073] The first ultrafiltration membrane 2 is connected to the permeation side of the pre-filter 1 and is used to perform ultrafiltration treatment on the filtrate of the pre-filter 1;

[0074] The first nanofiltration membrane 3 is connected to the permeation side of the first ultrafiltration membrane 2 and is used for nanofiltration concentration treatment of the filtrate of the first ultrafiltration membrane 2;

[0075] The precipitation reaction tank 4 is connected to the concentration side of the first nanofiltration membrane 3 and is used to perform precipitation reaction on the concentrated liquid of the first nanofiltration membrane 3 to remove impurity cations;

[0076] NaOH adding tank 5 and Na2CO3 adding tank are used to add NaOH and Na2CO3 to the precipitation reaction tank 4 respectively;

[0077] The second ultrafiltration membrane 7 is connected to the precipitation reaction tank 4 and is used to filter and remove the precipitate from the wastewater after the precipitation reaction;

[0078] The second nanofiltration membrane 8 is connected to the permeation side of the second ultrafiltration membrane 7 and is used to concentrate the filtrate of the second ultrafiltration membrane 7;

[0079] The crystallization device 9 is connected to the concentration side of the second nanofiltration membrane 8 and is used to perform crystallization treatment on the concentrated liquid of the second nanofiltration membrane 8 to obtain recovered sodium sulfate.

[0080] In one embodiment, the pre-filter 1 is one or a combination of a coagulation tank, a sand filter or an activated carbon filter.

[0081] In one embodiment, the material of the first ultrafiltration membrane 2 and the second ultrafiltration membrane 7 is selected from organic material or inorganic material; the organic material is PTFE, PVDF, PES, PS or acetate fiber; the inorganic material is aluminum oxide, zirconium oxide, titanium oxide or silicon carbide.

[0082] In one embodiment, the molecular weight cut-off of the first nanofiltration membrane 3 is 150 to 1000 Da, more preferably 500 to 1000 Da.

[0083] In one embodiment, the molecular weight cut-off of the second nanofiltration membrane 8 is 150-1000 Da, more preferably 300-500 Da.

[0084] In one embodiment, the crystallization device 9 is an evaporative crystallizer or a cryo-crystallizer.

[0085] like Figure 2 In the device shown, in one embodiment, the concentrating side of the second nanofiltration membrane 8 is connected to the ion exchange resin column 11 connected to the third nanofiltration membrane 12, and the concentrating side of the third nanofiltration membrane 12 is connected to the crystallization device 9.

[0086] In one embodiment, the ion exchange resin column 11 is filled with cation exchange resin.

[0087] In one embodiment, the molecular weight cutoff of the three-stage nanofiltration membrane is 150 to 1000 Da, more preferably 150 to 300 Da.

[0088] like Figure 3 In the device shown, in one embodiment, the concentration side of the second nanofiltration membrane 8 is connected to the flocculation reactor 14, the flocculation reactor 14 is connected to a flocculant addition tank 15 and a nucleating agent addition tank 16, the flocculation reactor 14 is connected to the third ultrafiltration membrane 17, and the permeation side of the third ultrafiltration membrane 17 is connected to the ion exchange resin column 11.

[0089] In one embodiment, the flocculant addition tank 15 is filled with aluminum sulfate, polyaluminum chloride (PAC), ferrous sulfate, ferric sulfate or ferric chloride; the nucleating agent addition tank 16 is filled with oleic acid-modified ferrosoferric oxide. Example 1

[0090] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 15ppm PAC and 10ppm PAM. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 7mg / L, the TDS is 3100mg / L, the sulfate is 1320mg / L, and the SS is 26 mg / L. It then enters the ultrafiltration and first-stage nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.3Mpa and a cross-flow velocity of 3m / s. The stable flux of the ceramic membrane is maintained at 171L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0091] The water produced by the first stage of nanofiltration is directly discharged; the concentrated water from the first stage of nanofiltration enters the hardness removal system, which mainly uses chemicals to remove hardness. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.5. The total hardness of the softened mine water is 120 ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.3Mpa and a cross-flow velocity of 3m / s to remove the generated precipitate. The filtrate from the ultrafiltration is then concentrated through a two-stage nanofiltration process. The molecular weight cutoff of the second-stage nanofiltration is 400Da and the operating pressure is 1.5Mpa. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 7.2 times, and its produced water is directly discharged; the concentrated water from the second-stage nanofiltration enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals at a crystallization temperature of -5°C. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate, with a product whiteness of 89%, a sodium sulfate content of 97.3%, and a moisture content of 0.1%; the crystallization mother liquor and the produced water from the first and second stages of nanofiltration are mixed and discharged, with a TDS of 1130mg / L and a sulfate ion of 72mg / L. Example 2

[0092] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 20ppm Fe2O3, 15ppm PAC and 10ppm PAM. The concentrated liquid is recovered by the cyclone separator and reused in the coagulation system. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 7mg / L, TDS is 3200mg / L, sulfate is 1300mg / L, SS is 22 mg / L, and enters the ultrafiltration and nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.3Mpa and a cross-flow velocity of 3m / s. The stable flux of the ceramic membrane is maintained at 213L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0093] The water produced by the first stage of nanofiltration is directly discharged; the concentrated water from the first stage of nanofiltration enters the hardness removal system, which mainly uses chemicals to remove hardness. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.5. The total hardness of the softened mine water is 110 ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.3Mpa and a cross-flow velocity of 3m / s to remove the generated precipitate. The filtrate from the ultrafiltration is then concentrated through a two-stage nanofiltration process with a molecular weight cutoff of 400Da and an operating pressure of 1.5Mpa. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 8 times, and the produced water is directly discharged. The concentrated water from the second-stage nanofiltration enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals at a crystallization temperature of -5°C. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate with a product whiteness of 90%, a sodium sulfate content of 97.8%, and a moisture content of 0.1%. The crystallization mother liquor and the produced water from the first and second stages of nanofiltration are mixed and discharged with a TDS of 1100mg / L and a sulfate ion of 70mg / L.

[0094] By comparing Example 1 and Example 2, it can be seen that in the process of flocculating mine water, when magnetic Fe2O3 is used as an auxiliary coagulant, the coagulation effect can be effectively improved, the stable flux in the subsequent ceramic membrane filtration process can be improved, and the flocculation effect can be effectively improved, the COD of the treated wastewater can be reduced, and the purity of the recovered sodium sulfate can be improved. Example 3

[0095] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 15ppm Fe2O3, 20ppm PAC and 10ppm PAM. The concentrated liquid is recovered by the cyclone separator and reused in the coagulation system. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 8mg / L, TDS is 3100mg / L, sulfate is 1300mg / L, SS is 20 mg / L, and enters the ultrafiltration and nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s. The stable flux of the ceramic membrane is maintained at 242L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0096] The water produced by the first stage of nanofiltration is directly discharged; the concentrated water from the first stage of nanofiltration enters the hardness removal system, which mainly uses chemicals to remove hardness. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.0. The total hardness of the softened mine water is 100 ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s to remove the generated precipitate. The filtrate from the ultrafiltration is then concentrated through a two-stage nanofiltration process. The molecular weight cutoff of the second-stage nanofiltration is 400Da and the operating pressure is 1.5Mpa. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 8.4 times, and its produced water is directly discharged; the concentrated water from the second-stage nanofiltration enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals at a crystallization temperature of -5°C. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate, with a product whiteness of 90%, a sodium sulfate content of 97.6%, and a moisture content of 0.1%; the crystallization mother liquor and the produced water from the first and second stages of nanofiltration are mixed and discharged, with a TDS of 1050mg / L and a sulfate ion of 65mg / L. Example 4

[0097] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 15ppm Fe2O3, 20ppm PAC and 10ppm PAM. The concentrated liquid is recovered by the cyclone separator and reused in the coagulation system. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 8mg / L, TDS is 3100mg / L, sulfate is 1300mg / L, SS is 20 mg / L, and enters the ultrafiltration and nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s. The stable flux of the ceramic membrane is maintained at 242L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0098] The first stage of nanofiltration produced water is directly discharged; the first stage of nanofiltration concentrated water enters the hardness removal system, which mainly removes hardness with chemicals. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.0. The total hardness of the softened mine water is 100 ppm. It is filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s to remove the generated precipitation. The ultrafiltration filtrate is then concentrated through the second stage of nanofiltration. The molecular weight cutoff of the second stage of nanofiltration is 400Da, and the operating pressure is 1.5Mpa. When the flux of the second stage of nanofiltration membrane decreases by 70%, the concentration multiple reaches 8.6 times, and its produced water is directly discharged; the COD of the concentrated water of the second stage of nanofiltration is 14mg / L. It is first softened by a cation exchanger to remove residual calcium and magnesium ions. The ion exchanger is equipped with weakly acidic cation exchange resin to reduce the total hardness to 35ppm. The softened mine water then enters the third stage of nanofiltration for further treatment. The salt concentration was further increased to 120 g / L. The three-stage nanofiltration used a molecular weight cutoff of 300 Da and was operated at 2.0 MPa. The concentration was 2.3 times, and the flux of the three-stage nanofiltration membrane decreased by 60%. When the three-stage nanofiltration concentrated water was crystallized, the three-stage nanofiltration produced water was mixed with the first and second stage nanofiltration produced water and discharged. The three-stage nanofiltration concentrated water entered the crystallization system and obtained sodium sulfate crystals after frozen crystallization. The crystallization temperature was -5 °C. The sodium sulfate crystals were dried to obtain anhydrous sodium sulfate. The product had a whiteness of 91%, a sodium sulfate content of 98.2%, and a moisture content of 0.1%. The crystallization mother liquor and the three-stage nanofiltration produced water were mixed with the first and second stage nanofiltration produced water and discharged. The TDS was 960 mg / L and the sulfate ion was 55 mg / L.

[0099] It can be seen from Examples 3 and 4 that by treating the concentrated water from the second-stage nanofiltration with an ion exchange resin, the hardness of the concentrated water can be effectively reduced, and the concentration multiple of the nanofiltration can be further increased, which is beneficial to the subsequent crystallization process and can improve the purity of the recovered sodium sulfate. Example 5

[0100] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 15ppm Fe2O3, 20ppm PAC and 10ppm PAM. The concentrated liquid is recovered by the cyclone separator and reused in the coagulation system. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 7mg / L, TDS is 3100mg / L, sulfate is 1300mg / L, SS is 20 mg / L, and enters the ultrafiltration and nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s. The stable flux of the ceramic membrane is maintained at 242L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0101] The water produced by the first stage of nanofiltration is directly discharged; the concentrated water from the first stage of nanofiltration enters the hardness removal system, which mainly uses chemicals to remove hardness. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.0. The total hardness of the softened mine water is 100 ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s to remove the generated precipitate. The filtrate of the ultrafiltration is then concentrated through a second-stage nanofiltration with a molecular weight cutoff of 400Da and an operating pressure of 1.5Mpa. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 8.6 times, and the produced water is directly discharged. The COD of the concentrated water from the second-stage nanofiltration is 14mg / L. 10ppm polyferric chloride flocculant and 5ppm ferroferric oxide nucleating agent are added to the concentrated water for flocculation reaction. After the reaction, the COD of the produced water is reduced to 5 mg / L. The flocculation product is filtered out through a 50nm ceramic ultrafiltration membrane. The ultrafiltration adopts a pressure of 0.2Mpa and a membrane surface velocity of 3m / s, with a stable flux of 293 L / m 2 •h, the raw water from the ultrafiltration membrane is softened by a cation exchanger to remove residual calcium and magnesium ions. The ion exchanger is equipped with weakly acidic cation exchange resin to reduce the total hardness to 24ppm. The softened mine water enters the third-stage nanofiltration to further increase the salt concentration to 130g / L. The third-stage nanofiltration adopts a molecular weight cutoff of 300Da and operates under 2.0MPa conditions. The concentration multiple is 2.8 times, and the flux of the third-stage nanofiltration membrane decreases by 60%. When the third-stage nanofiltration concentrated water is crystallized, the third-stage nanofiltration produced water is mixed with the first and second stage nanofiltration produced water and discharged. The third-stage nanofiltration concentrated water enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals with a crystallization temperature of -5℃. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate with a product whiteness of 93%, a sodium sulfate content of 98.8%, and a water content of 0.1%. The crystallization mother liquor and the third-stage nanofiltration produced water are mixed with the first and second stage nanofiltration produced water and discharged, with a TDS of 940mg / L and a sulfate ion of 50mg / L.

[0102] It can be seen from Examples 4 and 5 that by using polyferric chloride flocculation on the concentrated water of the second-stage nanofiltration, the COD in the nanofiltration concentrate can be effectively reduced, the subsequent nanofiltration process can be concentrated, the concentration multiple is increased, and the load of the crystallization process is reduced. Example 6

[0103] The mine water first enters the coagulation system after the flow and water quality are buffered in the regulating tank. The doser adds 15ppm Fe2O3, 20ppm PAC and 10ppm PAM. The concentrated liquid is recovered by the cyclone separator and reused in the coagulation system. The supernatant is then filtered by sand and activated carbon to obtain the clarified liquid of the pretreatment system. The COD is 8mg / L, TDS is 3100mg / L, sulfate is 1300mg / L, SS is 20 mg / L, and enters the ultrafiltration and nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane, which is operated at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s. The stable flux of the ceramic membrane is maintained at 242L / m 2 •h, the molecular weight cut-off of the first stage nanofiltration is 800Da, the operating pressure is 1.0Mpa, and concentration is carried out.

[0104] The water produced by the first stage of nanofiltration is directly discharged; the concentrated water from the first stage of nanofiltration enters the hardness removal system, which mainly uses chemicals to remove hardness. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions, and the pH is adjusted to 12.0. The total hardness of the softened mine water is 100 ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s to remove the generated precipitate. The filtrate of the ultrafiltration is then concentrated through a second-stage nanofiltration with a molecular weight cutoff of 400Da and an operating pressure of 1.5Mpa. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 8.6 times, and the produced water is directly discharged. The COD of the concentrated water from the second-stage nanofiltration is 14mg / L. 10ppm polyferric chloride flocculant and 5ppm oleic acid-modified ferroferric oxide nucleating agent are added to the concentrated water for flocculation reaction. After the reaction, the COD of the produced water is reduced to 1 mg / L. The flocculation product is filtered out through a 50nm ceramic ultrafiltration membrane. The ultrafiltration adopts a pressure of 0.2Mpa and a membrane surface velocity of 3m / s, with a stable flux of 315 L / m 2 •h, the raw water from the ultrafiltration membrane is softened by a cation exchanger to remove residual calcium and magnesium ions. The ion exchanger is equipped with weakly acidic cation exchange resin to reduce the total hardness to 21ppm. The softened mine water enters the third-stage nanofiltration to further increase the salt concentration to 130g / L. The third-stage nanofiltration adopts a molecular weight cutoff of 300Da and operates under 2.0MPa conditions. The concentration multiple is 3.0 times, and the flux of the third-stage nanofiltration membrane decreases by 60%. When the third-stage nanofiltration concentrated water is crystallized, the third-stage nanofiltration produced water is mixed with the first and second stage nanofiltration produced water and discharged. The third-stage nanofiltration concentrated water enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals with a crystallization temperature of -5℃. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate with a product whiteness of 94%, a sodium sulfate content of 99.1%, and a water content of 0.1%; the crystallization mother liquor and the third-stage nanofiltration produced water are mixed with the first and second stage nanofiltration produced water and discharged, with a TDS of 920mg / L and a sulfate ion of 45mg / L.

[0105] It can be seen from Examples 5 and 6 that the oleic acid-modified ferric chloride nucleating agent can effectively adsorb organic pollutants in the nanofiltration concentrate, improve the flocculation effect, reduce the COD in the nanofiltration concentrate, and increase the concentration multiple of the subsequent concentration nanofiltration concentration process, and avoid COD substances remaining in the concentrate, and also improve the purity of the recovered sodium sulfate.

Claims

1. A method for reducing COD in two-stage nanofiltration concentrated water, characterized in that: The steps include: The mine water first passes through the regulating tank to buffer the flow and water quality, and then enters the coagulation system, where 15ppmFe2O3, 20ppmPAC and 10ppmPAM are added. The concentrated liquid is filtered through a cyclone separator to recover Fe2O3 and reused in the coagulation system; the supernatant is then filtered through sand and activated carbon to obtain the clarified liquid of the pretreatment system, with a COD of 8mg / L, a TDS of 3100mg / L, a sulfate concentration of 1300mg / L, and a SS of 20mg / L. It enters the ultrafiltration and first-stage nanofiltration systems. The ultrafiltration uses a 50nm ceramic ultrafiltration membrane and is operated at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s. The stable flux of the ceramic membrane is maintained at 242L / m 2 h, the molecular weight cut-off of the first nanofiltration is 800Da, the operating pressure is 1.0Mpa, and the first nanofiltration is used for concentration; the water produced by the first nanofiltration is directly discharged; The concentrated water from the first stage of nanofiltration enters the hardness removal system for hardness removal. Sodium carbonate and sodium hydroxide are added to remove calcium and magnesium ions. The pH is adjusted to 12.

0. The total hardness of the softened mine water is 100ppm. It is then filtered through a 50nm ceramic ultrafiltration membrane at a pressure of 0.4Mpa and a cross-flow velocity of 4m / s to remove the generated precipitate. The filtrate from the ultrafiltration is then concentrated through the second stage of nanofiltration. The molecular weight cutoff of the second stage of nanofiltration is 400Da and the operating pressure is 1.5Mpa for concentration. When the flux of the second-stage nanofiltration membrane drops by 70%, the concentration multiple reaches 8.6 times, and its produced water is directly discharged; the COD of the concentrated water of the second-stage nanofiltration is 14mg / L, and 10ppm polyferric chloride flocculant and 5ppm oleic acid modified ferroferric oxide nucleating agent are added to it for flocculation reaction. After the reaction, the COD of the concentrated water drops to 1mg / L, and the flocculation product is filtered out by a 50nm ceramic ultrafiltration membrane. The ceramic ultrafiltration membrane adopts 0.2Mpa pressure and 3m / s membrane surface flow rate, and the stable flux is 315L / m 2 h, the water produced by the ceramic ultrafiltration membrane is softened by the cation exchanger to remove residual calcium and magnesium ions. The ion exchanger is equipped with weakly acidic cation exchange resin to reduce the total hardness to 21ppm. The softened mine water enters the third-stage nanofiltration to further increase the salt concentration to 130g / L. The molecular weight cutoff of the third-stage nanofiltration is 300Da. It is operated under 2.0MPa conditions, the concentration multiple is 3.0 times, and the flux of the third-stage nanofiltration membrane decreases by 60%. When the third-stage nanofiltration concentrated water is crystallized, the third-stage nanofiltration produced water is mixed with the first and second stage nanofiltration produced water for discharge. The third-stage nanofiltration concentrated water enters the crystallization system and is frozen and crystallized to obtain sodium sulfate crystals. The crystallization temperature is -5℃. The sodium sulfate crystals are dried to obtain anhydrous sodium sulfate. The product whiteness is 94%, the sodium sulfate content is 99.1%, and the water content is 0.1%; the crystallization mother liquor and the third-stage nanofiltration produced water are mixed with the first and second stage nanofiltration produced water for discharge, with a TDS of 920mg / L and a sulfate ion of 45mg / L; The preparation steps of the oleic acid modified ferroferric oxide nucleating agent are as follows: oleic acid is added to an anhydrous ethanol solution containing 5wt% of Fe3O4, the amount of oleic acid added is 3 times that of Fe3O4, after stirring evenly, reacting at 80°C for 1h, filtering out the precipitate, washing it with ethanol and drying it to obtain the oleic acid modified ferroferric oxide nucleating agent.

Images