CN111875147A - Method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater - Google Patents
Method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater Download PDFInfo
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- 150000003839 salts Chemical class 0.000 title claims abstract description 80
- 239000002351 wastewater Substances 0.000 title claims abstract description 63
- 238000006477 desulfuration reaction Methods 0.000 title claims abstract description 37
- 238000000034 method Methods 0.000 title claims abstract description 34
- 230000023556 desulfurization Effects 0.000 title claims abstract description 33
- WRUGWIBCXHJTDG-UHFFFAOYSA-L magnesium sulfate heptahydrate Chemical compound O.O.O.O.O.O.O.[Mg+2].[O-]S([O-])(=O)=O WRUGWIBCXHJTDG-UHFFFAOYSA-L 0.000 title claims abstract description 32
- 229940061634 magnesium sulfate heptahydrate Drugs 0.000 title claims abstract description 32
- 239000013078 crystal Substances 0.000 title claims abstract description 26
- 235000002639 sodium chloride Nutrition 0.000 claims abstract description 107
- 238000002425 crystallisation Methods 0.000 claims abstract description 79
- 230000008025 crystallization Effects 0.000 claims abstract description 79
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 claims abstract description 56
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 42
- 239000012452 mother liquor Substances 0.000 claims abstract description 39
- 230000008014 freezing Effects 0.000 claims abstract description 38
- 238000007710 freezing Methods 0.000 claims abstract description 38
- 239000011780 sodium chloride Substances 0.000 claims abstract description 28
- 238000001704 evaporation Methods 0.000 claims abstract description 15
- 230000008020 evaporation Effects 0.000 claims abstract description 15
- 238000001728 nano-filtration Methods 0.000 claims abstract description 13
- 238000001816 cooling Methods 0.000 claims abstract description 12
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 claims abstract description 4
- 239000011575 calcium Substances 0.000 claims abstract description 4
- 229910052791 calcium Inorganic materials 0.000 claims abstract description 4
- CSNNHWWHGAXBCP-UHFFFAOYSA-L Magnesium sulfate Chemical compound [Mg+2].[O-][S+2]([O-])([O-])[O-] CSNNHWWHGAXBCP-UHFFFAOYSA-L 0.000 claims description 14
- 238000001035 drying Methods 0.000 claims description 14
- 230000001276 controlling effect Effects 0.000 claims description 9
- 229910052943 magnesium sulfate Inorganic materials 0.000 claims description 7
- 235000019341 magnesium sulphate Nutrition 0.000 claims description 7
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 claims description 4
- UXVMQQNJUSDDNG-UHFFFAOYSA-L Calcium chloride Chemical compound [Cl-].[Cl-].[Ca+2] UXVMQQNJUSDDNG-UHFFFAOYSA-L 0.000 claims description 3
- 238000007599 discharging Methods 0.000 claims description 3
- 238000010587 phase diagram Methods 0.000 claims description 3
- 239000003507 refrigerant Substances 0.000 claims description 3
- 230000033228 biological regulation Effects 0.000 claims description 2
- 239000000498 cooling water Substances 0.000 claims description 2
- 238000005261 decarburization Methods 0.000 claims description 2
- 238000001914 filtration Methods 0.000 claims description 2
- 238000000265 homogenisation Methods 0.000 claims description 2
- 230000001105 regulatory effect Effects 0.000 claims description 2
- 229910000029 sodium carbonate Inorganic materials 0.000 claims description 2
- 239000007921 spray Substances 0.000 claims description 2
- 238000011268 retreatment Methods 0.000 claims 1
- 239000012535 impurity Substances 0.000 abstract description 13
- 238000000926 separation method Methods 0.000 abstract description 10
- 238000005516 engineering process Methods 0.000 abstract description 4
- 238000011084 recovery Methods 0.000 abstract description 4
- 238000004519 manufacturing process Methods 0.000 abstract description 2
- 239000003814 drug Substances 0.000 abstract 1
- 239000010802 sludge Substances 0.000 abstract 1
- 208000028659 discharge Diseases 0.000 description 14
- 239000010703 silicon Substances 0.000 description 12
- 229910052710 silicon Inorganic materials 0.000 description 12
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 description 8
- 239000007788 liquid Substances 0.000 description 8
- 229910052938 sodium sulfate Inorganic materials 0.000 description 8
- 235000011152 sodium sulphate Nutrition 0.000 description 8
- 229910002651 NO3 Inorganic materials 0.000 description 6
- 239000000047 product Substances 0.000 description 6
- VWDWKYIASSYTQR-UHFFFAOYSA-N sodium nitrate Chemical compound [Na+].[O-][N+]([O-])=O VWDWKYIASSYTQR-UHFFFAOYSA-N 0.000 description 6
- 239000003344 environmental pollutant Substances 0.000 description 5
- 231100000719 pollutant Toxicity 0.000 description 5
- 238000005406 washing Methods 0.000 description 4
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 3
- 230000014759 maintenance of location Effects 0.000 description 3
- 239000012528 membrane Substances 0.000 description 3
- 239000002957 persistent organic pollutant Substances 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 235000010344 sodium nitrate Nutrition 0.000 description 3
- 239000004317 sodium nitrate Substances 0.000 description 3
- 239000012267 brine Substances 0.000 description 2
- OSGAYBCDTDRGGQ-UHFFFAOYSA-L calcium sulfate Chemical compound [Ca+2].[O-]S([O-])(=O)=O OSGAYBCDTDRGGQ-UHFFFAOYSA-L 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 230000018044 dehydration Effects 0.000 description 2
- 238000006297 dehydration reaction Methods 0.000 description 2
- 239000012153 distilled water Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000004886 process control Methods 0.000 description 2
- 238000001223 reverse osmosis Methods 0.000 description 2
- 238000004062 sedimentation Methods 0.000 description 2
- HPALAKNZSZLMCH-UHFFFAOYSA-M sodium;chloride;hydrate Chemical compound O.[Na+].[Cl-] HPALAKNZSZLMCH-UHFFFAOYSA-M 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 239000002956 ash Substances 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000001112 coagulating effect Effects 0.000 description 1
- 238000005345 coagulation Methods 0.000 description 1
- 230000015271 coagulation Effects 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000010881 fly ash Substances 0.000 description 1
- 239000002920 hazardous waste Substances 0.000 description 1
- 229910001385 heavy metal Inorganic materials 0.000 description 1
- 239000010842 industrial wastewater Substances 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 238000001471 micro-filtration Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000004806 packaging method and process Methods 0.000 description 1
- 238000002203 pretreatment Methods 0.000 description 1
- 238000004064 recycling Methods 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 239000013535 sea water Substances 0.000 description 1
- 239000010865 sewage Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 238000001694 spray drying Methods 0.000 description 1
- 238000000108 ultra-filtration Methods 0.000 description 1
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-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F9/00—Multistage treatment of water, waste water or sewage
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D3/00—Halides of sodium, potassium or alkali metals in general
- C01D3/04—Chlorides
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D3/00—Halides of sodium, potassium or alkali metals in general
- C01D3/14—Purification
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/40—Magnesium sulfates
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/80—Compositional purity
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/001—Processes for the treatment of water whereby the filtration technique is of importance
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/02—Treatment of water, waste water, or sewage by heating
- C02F1/04—Treatment of water, waste water, or sewage by heating by distillation or evaporation
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F5/00—Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents
- C02F5/02—Softening water by precipitation of the hardness
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Inorganic Chemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Materials Engineering (AREA)
- Hydrology & Water Resources (AREA)
- Environmental & Geological Engineering (AREA)
- Water Supply & Treatment (AREA)
- Geology (AREA)
- Heat Treatment Of Water, Waste Water Or Sewage (AREA)
Abstract
The invention discloses a method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater. The desulfurization wastewater is pretreated to remove impurities such as calcium hardness and alkalinity, and then enters an MVR evaporation crystallization system, concentrated water obtained after MVR evaporation crystallization reduction enters a freezing crystallization system after flash evaporation cooling treatment, magnesium sulfate heptahydrate crystal salt is generated by freezing crystallization, and sodium chloride crystallization system is used for treatment after freezing mother liquor exchanges heat. Magnesium sulfate heptahydrate is dried to finally produce magnesium sulfate heptahydrate crystalline salt. Compared with the nanofiltration salt separation technology or other salt separation technologies, the process flow of the invention is simpler, the adding amount of the medicament and the sludge production amount in the pretreatment stage can be greatly reduced, the magnesium sulfate heptahydrate product has high salt purity and can be sold, the salt recovery rate is high, and the yield of the miscellaneous salt can be reduced to the maximum extent.
Description
Technical Field
The invention relates to the technical field of sewage treatment, in particular to a method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater.
Background
The desulfurization wastewater is tail-end wastewater generated by wet desulfurization treatment in a production process of a power plant, has the characteristics of high salt content and high corrosivity, and therefore needs to be safely and stably treated. At present, domestic desulfurization wastewater is directly discharged after being treated by dosing coagulation sedimentation, and the discharged wastewater still contains a large amount of soluble salts, so that the method not only can possibly cause secondary pollution, but also cannot meet the current environmental protection concept requirement. Based on this, the concept of zero discharge of desulfurization waste water will be generally applied to the treatment of desulfurization waste water.
Patent No. CN201310555063.9 discloses a desulfurization wastewater zero-discharge treatment device and method, which utilizes the combination of dosing, precipitate film concentration, a seawater film and a pollution-resistant high-pressure film in a reverse osmosis strong brine pretreatment method to react and crystallize strong brine with salt content of about 15-20% from the high-pressure film and fly ash in an ash yard to form a complete desulfurization wastewater zero-discharge treatment process system.
Patent No. CN201510529034.4 discloses a desulfurization waste water zero discharge treatment system, and the system adopts the processes of coagulating sedimentation, ultrafiltration, reverse osmosis and evaporative crystallization to realize the zero discharge of desulfurization waste water.
The patent CN201610072782.9 discloses a high-desulfurization industrial waste water salt-separation zero-discharge system, in the method, after high-desulfurization waste water is treated by a precision pretreatment unit, a membrane separation concentration unit and a crystallization resource unit, produced water can be recycled, and sodium chloride, sodium sulfate, sodium nitrate crystal salt and miscellaneous salt are obtained.
Patent No. CN2018100732782.9 discloses a zero discharge process for recovering crystallized salt from high desulfurization wastewater and a treatment system thereof, wherein the treatment system comprises a tubular microfiltration system, a weak acid resin hardness removal system, a nanofiltration membrane salt separation system, a nanofiltration concentrated water oxidation system, a nanofiltration concentrated water sodium sulfate evaporation crystallization system, a sodium sulfate freezing crystallization system and the like. On the premise of ensuring the quality, the integral recovery rate of salt is more than 90%; and finally, a small amount of mother liquor is solidified through spray drying, and zero discharge of waste water is realized.
The patent with the application number of CN2018110732782.8 discloses a complete salt separation crystallization technology for high-desulfurization wastewater. The high-desulfurization wastewater enters an NF salt separation system after passing through a pretreatment system: NF produced water enters an evaporative crystallization system after being concentrated and purified, and sodium chloride crystal salt is finally produced; the NF concentrated water enters a freezing crystallization and melting crystallization system after being purified and concentrated, and finally sodium sulfate crystallized salt is produced; and (3) allowing part of mother liquor of the frozen crystals to enter a frozen NF system for further salt and nitrate separation, returning frozen NF concentrated water to a frozen crystallization unit, and conveying the frozen NF produced water to be mixed with the NF produced water.
In the technical scheme, the separation of sodium chloride and sodium sulfate crystal salt can be realized by a thermal method or a nanofiltration membrane method, and according to the operation condition of the existing practical project, the problems that a zero-emission salt separation system cannot continuously and stably operate, the investment and operation cost is high, the amount of generated miscellaneous salt is large, the salt recovery rate is low, the purity of product salt is low, the product salt cannot be sold outside, the hazardous waste disposal amount of miscellaneous salt is high and the like exist.
Therefore, it is an urgent need to solve the problem of providing a reliable, economical and feasible desulfurization waste water quality-divided crystallization technology.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater, which realizes quality-divided crystallization of the high-salinity wastewater of the desulfurization wastewater so as to meet the external requirement on protecting the ecological environment and the self requirement on resource utilization.
The technical scheme of the invention is as follows: a method for extracting magnesium sulfate heptahydrate crystal salt from desulfurization wastewater specifically comprises the following steps:
s1), inputting the wastewater into a regulating tank to realize water quantity regulation and water quality homogenization on the wastewater;
s2), inputting the wastewater treated in the step S1) into a high-efficiency softening and clarifying unit, and adding sodium carbonate to remove most calcium hardness in the wastewater;
s3), inputting the wastewater treated in the step S2) into a filter tank filtering unit to remove suspended matters in the wastewater;
s4), inputting the wastewater treated in the step S3) into a decarburization unit to further remove the total alkalinity in the wastewater;
s5), inputting the wastewater treated in the step S4) into an MVR evaporative crystallization system to perform maximum concentration treatment on the wastewater;
s6), inputting the dehydrator mother liquor concentrated in the step S5) into a cooling system to reduce the temperature of the concentrated wastewater to 40-45 ℃;
s7), inputting the concentrated water after heat exchange in the step S6) into a freezing crystallization system, and freezing and crystallizing to separate out magnesium sulfate heptahydrate crystal salt;
s8), conveying the magnesium sulfate heptahydrate crystal salt precipitated in the step S7) through freezing crystallization into a drying system for drying treatment;
s9), dividing the mother liquor discharged by the freezing crystallization in the step S7) into two parts, wherein one part of the mother liquor I is conveyed to a mother liquor drying system for miscellaneous salt crystallization treatment, and the other part is sent to a sodium chloride crystallization system after heat exchange and temperature rise through a heat exchanger to precipitate sodium chloride crystal salt.
Preferably, in step S5), the concentration of magnesium sulfate in the wastewater after concentration by the MVR evaporative crystallization system is 30% to 35%.
Preferably, in step S5), a small amount of sodium chloride and calcium sulfate is separated out after the MVR evaporative crystallization system is evaporated and concentrated, the water produced by evaporation is sent to a recycling system, and the concentrated dehydrator mother liquor is sent to step 6) for cooling.
Preferably, in step S5), in order to ensure the purity of the magnesium sulfate crystallization salt, the MVR crystallization process of the MVR evaporative crystallization system is implemented by periodically discharging a certain amount of mother liquor to step S7), and combining with the concentrated water of MVR evaporative concentration, and processing again to recover the salt.
Preferably, in step S6), the cooling system adopts a form of flash cooling and chilled water cooling, wherein the condenser is cooled by circulating cooling water.
Preferably, in step S7), the freezing crystallization control temperature of the freezing crystallization system is 10 to 15 ℃, and the refrigerant is calcium chloride solution.
Preferably, in step S9), the mother liquor drying system precipitates the miscellaneous salt in the form of spray crystallization.
Preferably, in step S9), the sodium chloride crystallization system adopts multi-effect evaporative crystallization, or MVR evaporative crystallization, or TVR evaporative crystallization, and the maximum crystallization yield and purity of sodium chloride are achieved by controlling the evaporation amount, residence time, and amount of discharged mother liquor according to the balance of the feeding amount, feeding components and concentration, and the multi-phase diagram.
Preferably, in step S9), the mother liquor discharged from the freezing crystallization is subjected to two-stage nanofiltration before entering a sodium chloride crystallization system, so that the salt-nitrate ratio is increased, the crystallization yield of sodium chloride is greatly increased, and the impurity concentration (CODCr, silicon, etc.) of the sodium chloride crystallization inlet water is greatly reduced, thereby improving the purity and quality of the sodium chloride crystal salt.
Preferably, in step S9), the concentrated water after the two-stage nanofiltration treatment is discharged into a freezing and crystallizing system.
Preferably, in step S9), the mother liquor discharged from the sodium chloride crystallization system is input into a mother liquor drying system.
The invention has the beneficial effects that:
1. the method utilizes the MVR evaporative crystallization, the freezing crystallization and the mother liquor drying mode, the process route is mature and reliable, and the precipitation amount and the purity of the magnesium sulfate heptahydrate are ensured by controlling the evaporation amount, the retention time and the amount of the discharged mother liquor;
2. the invention controls the freezing crystallization temperature, the impurity concentration of the freezing crystallization circulation feed liquid, the external discharge capacity of the mother liquid and the evaporation capacity according to the water quality;
3. for SO of the invention4 2-The retention rate is more than or equal to 99 percent, the salt-nitrate ratio is greatly improved, and nitrate cannot be separated out due to high solubility, so that the two-stage nanofiltration well ensures that sodium chloride cannot be separated out and sodium sulfate and sodium nitrate cannot be separated out simultaneously;
4. the two-stage nanofiltration of the invention intercepts impurities such as organic pollutants, silicon and the like, and greatly improves the quality of sodium chloride crystal inlet water, thereby greatly improving the quality of sodium chloride crystal salt and reducing the discharge amount of mother liquor;
drawings
FIG. 1 is a schematic flow diagram of the process of the present invention.
Detailed Description
The following further describes embodiments of the present invention with reference to the accompanying drawings:
example 1
The desulfurization wastewater treated by the embodiment is wet desulfurization wastewater of a certain plant area, and the average concentration of each main pollutant is as follows: CODCr205mg/L, Ca2+606Mg/l, Mg2+10838mg/L, total alkalinity (as CaCO)3Calculated) is 500mg/L, total silicon is 30mg/L, TDS is 74262mg/L, Cl-16442mg/L, SO4 2-37496mg/L, Na+Was 8880 mg/L. The water inflow is designed to be 30 t/h.
The method disclosed by the invention is used for treating the wastewater as shown in figure 1, wherein the average concentration of each pollutant of the high-efficiency softened and clarified effluent is as follows: CODCr1213.8mg/l, Ca2+40Mg/l, Mg2+9212.3mg/l, total alkalinity (as CaCO)3Calculated) is 500mg/l, total silicon is 30mg/l, TDS is 74262mg/l, Cl-16442mg/l, SO4 2-37496.1mg/l, Na+8880.3mg/l and SS 10 mg/l.
The average concentration of each pollutant in the filtered water of the filter tank is as follows: CODCr1213.8mg/l, Ca2+40Mg/l, Mg2+9212.3mg/l, total alkalinity (as CaCO)3Calculated) is 500mg/l, total silicon is 30mg/l, TDS is 74262mg/l, Cl-16442mg/l, SO4 2-37496.1mg/l, Na+8880.3mg/l and SS 2 mg/l.
The average concentration of each pollutant of the decarbonized effluent is as follows: CODCr1213.8mg/l, Ca2+40Mg/l, Mg2+9212.3mg/l, total alkalinity (as CaCO)3Calculated) is 50mg/l, total silicon is 30mg/l, TDS is 74262mg/l, Cl-16442mg/l, SO4 2-37796.1mg/l, Na+8880.3mg/l and SS 10 mg/l.
The desulfurization waste water after pretreatment has a little change of desulfurization amount, most calcium hardness alkalinity is removed, simultaneously impurity components such as organic pollutants, silicon, heavy metal and the like are reduced, and the operation stability of the subsequent process is greatly improved.
The MVR vapor compressor and forced circulation evaporation mode is adopted, and the average concentration of each pollutant of the concentrated water after MVR evaporation is as follows: CODCr617.3mg/l, 68.6mg/l total silicon, 238929.3mg/l TDS, Cl-54765.7mg/l, SO4 2-95225.6mg/l, Na+It was 81651.6 mg/l. The water amount of the concentrated water after MVR evaporation is 2t/h, the treatment scale of the subsequent process is greatly reduced, and the operation cost is greatly reduced.
The freezing crystallization temperature is controlled to be 10 ℃, and the refrigerant adopts calcium chloride solution. The freezing inlet water can be approximately regarded as NaCl-MgSO according to the characteristics of the feed water quality4-H2An O system ". According to the phase diagram data of a water salt system, the quality of inlet concentrated water and the like, the impurity concentration of the freezing and crystallizing circulation feed liquid is controlled not to exceed the limit by adjusting the amount of the outlet mother liquor, so that the precipitation amount and the purity of the magnesium sulfate heptahydrate are ensured. The final amount of discharged mother liquor (mother liquor 1) is 0.5t/h, and the amount of magnesium sulfate heptahydrate produced by freezing crystallization is 3.04t/h (calculated according to the solid content of 95 percent after dehydration) through comprehensive calculation. At this time, the main impurity indexes of the freezing and crystallizing circulation feed liquid are as follows: CODCr1276.6mg/l, Ca2+25.4Mg/l, Mg2+16.9mg/l for total silicon and 55.7mg/l for total silicon.
The frozen and crystallized magnesium sulfate heptahydrate crystalline salt can meet the standard requirements of class I first-class products in the GB/T6009-2014 industrial magnesium sulfate heptahydrate standard through dehydration, drying and packaging. The guarantee measures for the purity of the magnesium sulfate heptahydrate crystal salt mainly comprise the following steps:
1. the process route is mature and reliable: MVR evaporative crystallization, freezing crystallization and mother liquor drying;
2. and (3) process control: controlling the freezing and crystallizing temperature, controlling the impurity concentration of the freezing and crystallizing circulation feed liquid, controlling the external discharge capacity of mother liquid, and adjusting the evaporation capacity according to the water quality;
3. equipment type selection: the crystallizer has a salt leg structure with the functions of salt collection, classification, elutriation, redissolution, cooling and the like, so that the quality of salt is greatly improved; and (4) further washing and purifying the crystallized salt by using distilled water by using a dehydrator with a washing function.
And determining the water inlet amount of the freezing crystallization to be 2t/h according to the calculation of the freezing crystallization cycle. The temperature of mother liquor discharged from the freezing crystallization is controlled to be 12 ℃, so that the area of a heat exchanger and the water inlet temperature of the freezing crystallization are determined. And returning the concentrated water (15% magnesium sulfate) of the frozen mother liquor to the MVR evaporative crystallization system to continuously recover magnesium sulfate, and discharging the accumulated organic matters, silicon, chloride and other impurities out of the mother liquor discharge system through freezing crystallization.
In this embodiment, an MVR crystallization and a freezing crystallization mode are adopted, and the precipitation amount and purity of magnesium sulfate heptahydrate are ensured by controlling the evaporation amount, the residence time, and the amount of discharged mother liquor. The final salt content of the magnesium sulfate finished product is 1.20 t/h. The amount of the discharged mother liquor (mother liquor 2) was 0.5 t/h.
The magnesium sulfate heptahydrate crystalline salt of the embodiment can meet the primary standard requirement of refined industrial salt in the GB/T5462-. The main measures for ensuring the purity of the sodium chloride crystal salt are as follows:
1. by the reaction of SO4 2-The retention rate is more than or equal to 99 percent, the salt-nitrate ratio is greatly improved, and nitrate cannot be separated out due to high solubility, so that the two-stage nanofiltration well ensures that sodium chloride cannot be separated out and sodium sulfate and sodium nitrate cannot be separated out simultaneously; in addition, the interception of impurities such as organic pollutants, silicon and the like by the two-stage nanofiltration greatly improves the quality of sodium chloride crystal inlet water, thereby greatly improving the quality of sodium chloride crystal salt and reducing the discharge amount of mother liquor.
2. And (3) process control: controlling the impurity concentration of the crystallization circulating feed liquid, controlling the external discharge capacity of the mother liquid, and adjusting the evaporation capacity according to the water quality;
3. equipment type selection: the crystallizer has a salt leg structure with the functions of salt collection, classification, elutriation, redissolution, cooling and the like, so that the quality of salt is greatly improved; and (4) further washing and purifying the crystallized salt by using distilled water by using a dehydrator with a washing function.
In conclusion, the finished product of the sodium chloride crystal salt is 0.60t/h, the finished product of the sodium sulfate crystal salt is 1.29t/h, the impurity salt yield is 0.29t/h, the recovery rate of the system salt is 86.70%, and the impurity salt rate of the system is 13.30%.
Although the present invention has been described with reference to preferred embodiments, it is not intended to limit the scope of the invention. It will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined by the appended claims, including any equivalents thereof.
The foregoing embodiments and description have been presented only to illustrate the principles and preferred embodiments of the invention, and various changes and modifications may be made therein without departing from the spirit and scope of the invention as hereinafter claimed.
Claims (10)
1. The method for extracting the magnesium sulfate heptahydrate crystalline salt from the desulfurization wastewater is characterized by comprising the following steps:
s1), inputting the wastewater into a regulating tank to realize water quantity regulation and water quality homogenization treatment on the wastewater;
s2), inputting the wastewater treated in the step S1) into a high-efficiency softening and clarifying unit, and adding sodium carbonate to remove most calcium hardness in the wastewater;
s3), inputting the wastewater treated in the step S2) into a filter tank filtering unit to remove suspended matters in the wastewater;
s4), inputting the wastewater treated in the step S3) into a decarburization unit to further reduce the total alkalinity in the wastewater;
s5), inputting the wastewater treated in the step S4) into an MVR evaporative crystallization system to perform maximum concentration treatment on the wastewater;
s6), inputting the dehydrator mother liquor concentrated in the step S5) into a cooling system to reduce the temperature of the concentrated wastewater to 40-45 ℃;
s7), inputting the concentrated water after heat exchange in the step S6) into a freezing crystallization system, and freezing and crystallizing to separate out magnesium sulfate heptahydrate crystal salt;
s8), conveying the magnesium sulfate heptahydrate crystal salt precipitated in the step S7) through freezing crystallization into a drying system for drying treatment;
s9), dividing the mother liquor discharged by the freezing crystallization in the step S7) into two parts, wherein one part of the mother liquor I is conveyed to a mother liquor drying system for miscellaneous salt crystallization treatment, and the other part is sent to a sodium chloride crystallization system after heat exchange and temperature rise through a heat exchanger to precipitate sodium chloride crystal salt.
2. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in the step S5), the concentration of magnesium sulfate in the wastewater after concentration by the MVR evaporative crystallization system is 30-35%.
3. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in the step S5), in order to ensure the purity of magnesium sulfate crystallization salt, the MVR crystallization process of the MVR evaporative crystallization system is implemented by periodically discharging a certain amount of mother liquor to the step S7), and the mother liquor is combined with concentrated water obtained by MVR evaporative concentration, and the salt is recycled by retreatment.
4. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in the step S6), the cooling system adopts a flash cooling and chilled water cooling mode, wherein the condenser is cooled by circulating cooling water.
5. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in the step S7), the freezing crystallization control temperature of the freezing crystallization system is 10-15 ℃, and the refrigerant adopts calcium chloride solution.
6. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in the step S9), the mother liquor drying system adopts a spray crystallization mode to separate out the miscellaneous salt.
7. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: in step S9), the sodium chloride crystallization system adopts multi-effect evaporative crystallization, or MVR evaporative crystallization, or TVR evaporative crystallization, and realizes the maximum crystallization yield and purity of sodium chloride by controlling the evaporation amount, residence time, and amount of discharged mother liquor according to the balance of the feeding amount, feeding components and concentration, and the multi-phase diagram.
8. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization wastewater as set forth in claim 1, characterized in that: and step S9), the mother liquor discharged by the freezing crystallization is treated by two-stage nanofiltration before entering a sodium chloride crystallization system.
9. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization waste water according to claim 8, characterized in that: in step S9), the concentrated water after the two-stage nanofiltration treatment is discharged into a freezing crystallization system.
10. The method for extracting magnesium sulfate heptahydrate crystalline salt from desulfurization waste water according to claim 7, characterized in that: in the step S9), the mother liquor discharged by the sodium chloride crystallization system is input into a mother liquor drying system for drying treatment.
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| CN115072753B (en) * | 2022-07-02 | 2023-05-19 | 山东海化集团有限公司 | Method for preparing magnesium sulfate heptahydrate and sodium chloride |
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