Salt and nitrate separation method for high-salinity wastewater

Abstract

The invention discloses a high-salinity wastewater salt and nitrate separation method which comprises the following steps: high-salinity wastewater is pretreated to remove impurity ions, then enters an evaporation and concentration process, and is conveyed to a first-stage flash evaporation process through a conveying pump when sodium sulfate is saturated through evaporation and concentration; after the materials are subjected to primary flash evaporation crystallization, the materials are conveyed to a secondary flash evaporation process through a conveying pump, the materials are subjected to secondary flash evaporation crystallization, and generated mirabilite and sodium chloride mother liquid crystal slurry is conveyed to a centrifugal separation process through the conveying pump; the separated mirabilite is subjected to high-temperature redissolution and then subjected to evaporative crystallization, centrifugal separation, drying and packaging, and finally an anhydrous sodium sulfate product is obtained; evaporating and crystallizing mirabilite, wherein after impurity ions are enriched, the mirabilite is regularly returned to the front end for impurity removal; and carrying out evaporative crystallization and centrifugal separation on the separated sodium chloride mother liquor to finally obtain a sodium chloride product. Mirabilite is obtained through two-stage flash evaporation crystallization, so that the problems of blockage of heat exchange equipment, reduction of heat exchange efficiency and the like are avoided; the operation energy consumption is low; the technological process is short, and the equipment occupies small space.

Description

Technical Field

[0001] The invention relates to the technical field of salt-nitrate separation and treatment of high-salt wastewater, and in particular to a method for separating salt-nitrate from high-salt wastewater. Background Art

[0002] High-salinity wastewater refers to wastewater with a total salt content of at least 1%, which mainly comes from chemical plants and the collection and processing of oil and natural gas. This wastewater contains a variety of substances (including salt, oil, organic heavy metals and radioactive substances). Salty wastewater is produced in a wide range of ways, and the amount of water is increasing year by year. It is crucial to remove the impact of organic pollutants in salty wastewater on the environment.

[0003] Existing methods for separating salt and nitrate from high-salinity wastewater include:

[0004] Process 1: Thermal process, based on the principle of high temperature nitrate precipitation, low temperature salt precipitation, hot mother liquor circulation. Its advantages are compact process and low energy consumption. Its disadvantages are narrow process control range and high requirements for salt-nitrate ratio.

[0005] Process 2: Cold process, based on the principle of high-temperature salt precipitation, low-temperature nitrate precipitation, and cold mother liquor circulation. Its advantages are wide process control range and easy control. Its disadvantages are long process flow and high energy consumption, especially when the sodium sulfate content is low.

[0006] Process 3: The high brine is evaporated and concentrated until the sodium sulfate reaches saturation, and then frozen and crystallized to form mirabilite and sodium chloride mother liquor. The mirabilite is redissolved and evaporated to obtain anhydrous sodium sulfate, and the sodium chloride mother liquor is evaporated to obtain sodium chloride. Its advantages are that the purity and whiteness of anhydrous sodium sulfate are high. The disadvantages are: the heat exchange equipment is easily blocked or the heat exchange efficiency is significantly reduced during the freezing process, and the system has high energy consumption.

[0007] Therefore, we proposed a method for separating salt and nitrate from high-salt wastewater to solve the above-mentioned problems. Summary of the invention

[0008] The object of the present invention is to provide a method for separating salt and nitrate from high-salt wastewater to solve the problems raised in the above-mentioned background technology.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A method for separating salt and nitrate from high-salt wastewater comprises the following steps:

[0011] Step 1: The high-salinity wastewater is pre-treated to remove impurity ions and then enters the evaporation and concentration process. When the evaporation and concentration reaches saturation of sodium sulfate, it is sent to the primary flash evaporation process through a delivery pump;

[0012] Step 2: After the material is subjected to a flash crystallization, it is sent to a secondary flash crystallization process through a delivery pump, and the material is subjected to a secondary flash crystallization, and the resulting sodium sulfate and sodium chloride mother liquid crystal slurry is sent to a centrifugal separation process through a delivery pump;

[0013] Step 3: The separated sodium sulfate is re-dissolved at high temperature, evaporated and crystallized, centrifuged, dried and packaged, and finally anhydrous sodium sulfate product is obtained;

[0014] Step 4: Glauber's salt evaporation and crystallization process, when the impurity ions are enriched, they are regularly returned to the front end for impurity removal;

[0015] Step 5: The separated sodium chloride mother liquor is evaporated, crystallized and centrifuged to finally obtain a sodium chloride product.

[0016] As a further solution of the present invention: the condition for the flash crystallization of the material in step 2 is an absolute pressure of 20 kPa.

[0017] As a further solution of the present invention: the condition for the secondary flash crystallization of the material in step 2 is 3kpa.

[0018] As a further solution of the present invention: the COD in the high-salt wastewater after pretreatment is not higher than 1000 mg / L, and the total hardness calculated as calcium carbonate is not higher than 50 mg / L.

[0019] As a further scheme of the present invention: in the step 1, the high-salt wastewater is pretreated to remove impurity ions, and the specific operations include: passing the high-salt wastewater into a first crystallization pool and adding sodium sulfate solution and calcium sulfate seeds, adjusting the molar concentration ratio of calcium ions and sulfate ions therein to 1: (2-4), and reacting for a first preset time; overflowing the effluent of the first crystallization pool into a sedimentation pool for sedimentation to obtain calcium sulfate dihydrate, recycling the calcium sulfate dihydrate to the first crystallization pool, and adjusting the solid content of the high-salt wastewater in the first crystallization pool to 5%-10%; overflowing the effluent of the sedimentation pool into a second crystallization pool and adding sodium hydroxide, and at the same time adjusting the solid content in the second crystallization pool to 3%-5%; passing the treated high-salt wastewater into a tubular membrane, returning the concentrated water obtained after the tubular membrane treatment to the second crystallization pool, and passing the produced water obtained after the tubular membrane treatment into an evaporation concentration process.

[0020] As a further solution of the present invention: the pH of the second crystallization pond is not less than 11.2.

[0021] As a further solution of the present invention: the molar concentration ratio of calcium ions to sulfate ions is adjusted to 1:(2-4), and the reaction is carried out for a first preset time, wherein the first preset time is 45min-75min.

[0022] As a further solution of the present invention: the effluent from the sedimentation tank overflows into the second crystallization tank and is added to sodium hydroxide, wherein the mass concentration of the sodium hydroxide is 18%-26%.

[0023] As a further solution of the present invention: the operating pressure of the tubular membrane is 0.3MPa-0.8MPa.

[0024] As a further solution of the present invention: the sodium chloride mother liquor separated in step 5 is evaporated, crystallized and centrifuged to finally obtain the sodium chloride product. In the sodium chloride mother liquor evaporation and crystallization process, when the impurity ions are enriched, it is regularly returned to the front end for impurity removal.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The method for separating salt and nitrate from high-salt wastewater obtains mirabilite through two-stage flash crystallization, thereby avoiding problems such as blockage of heat exchange equipment and reduced heat exchange efficiency. It does not require the use of a large amount of chilled water or a refrigeration unit for freezing and crystallization, and the operating energy consumption of the system is significantly reduced. The operating energy consumption is low, the process flow is short, and the equipment occupies a small area. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The figure is a schematic flow chart of the method for separating salt and nitrate from high-salt wastewater in the present invention. DETAILED DESCRIPTION

[0028] In one embodiment, Figure 1 As shown, a method for separating salt and nitrate from high-salinity wastewater comprises the following steps:

[0029] Step 1: The high-salinity wastewater is pre-treated to remove impurity ions and then enters the evaporation and concentration process. When the evaporation and concentration reaches saturation of sodium sulfate, it is sent to the primary flash evaporation process through a delivery pump;

[0030] Step 2: After the material is subjected to a flash crystallization, it is sent to a secondary flash crystallization process through a delivery pump, and the material is subjected to a secondary flash crystallization, and the resulting sodium sulfate and sodium chloride mother liquid crystal slurry is sent to a centrifugal separation process through a delivery pump;

[0031] Step 3: The separated sodium sulfate is re-dissolved at high temperature, evaporated and crystallized, centrifuged, dried and packaged, and finally anhydrous sodium sulfate product is obtained;

[0032] Step 4: Glauber's salt evaporation and crystallization process, when the impurity ions are enriched, they are regularly returned to the front end for impurity removal;

[0033] Step 5, the separated sodium chloride mother liquor is evaporated, crystallized, and centrifuged to finally obtain a sodium chloride product;

[0034] Equipment and components involved: pretreatment, centrifugation, evaporation crystallization, flash evaporation, evaporation concentration;

[0035] The condition for the first flash crystallization of the material in step 2 is an absolute pressure of 20 kPa; the condition for the second flash crystallization of the material in step 2 is 3 kPa; the COD in the high-salinity wastewater after pretreatment is not higher than 1000 mg / L, and the total hardness in terms of calcium carbonate is not higher than 50 mg / L;

[0036] In step 1, the high-salt wastewater is pretreated to remove impurity ions, and the specific operations include: passing the high-salt wastewater into a first crystallization pool and adding sodium sulfate solution and calcium sulfate seeds, adjusting the molar concentration ratio of calcium ions and sulfate ions therein to 1: (2-4), and reacting for a first preset time; overflowing the effluent of the first crystallization pool into a sedimentation pool for sedimentation to obtain calcium sulfate dihydrate, recycling the calcium sulfate dihydrate to the first crystallization pool, and adjusting the solid content of the high-salt wastewater in the first crystallization pool to 5%-10%; overflowing the effluent of the sedimentation pool into a second crystallization pool and adding sodium hydroxide, while adjusting the solid content in the second crystallization pool to 3%-5%; passing the treated high-salt wastewater into Tubular membrane, the concentrated water obtained after the tubular membrane treatment is returned to the second crystallization pool, and the produced water obtained after the tubular membrane treatment is passed to the evaporation and concentration process; the calcium ions and magnesium ions in the high-salt wastewater are removed to prevent these divalent ions from scaling in the subsequent membrane system; the calcium in the wastewater is removed first and then the magnesium in the wastewater, and magnesium hydroxide is used to crystallize on the surface of calcium sulfate to prevent calcium sulfate from contaminating the tubular membrane and reduce the cleaning cost of the tubular membrane; sodium sulfate is used instead of sodium carbonate to reduce the system's dosing cost, and normal temperature crystallization technology is used to induce calcium ion and sulfate crystallization at room temperature to promote the resource recovery and utilization of high-quality calcium sulfate; tubular membranes are used to replace coagulation, flocculation, clarification and ultrafiltration, and the process flow is simple.

[0037] The pH of the second crystallization pool is not less than 11.2; the molar concentration ratio of calcium ions to sulfate ions is adjusted to 1:(2-4), and the reaction is carried out for a first preset time, which is 45min-75min; the effluent from the sedimentation pool overflows into the second crystallization pool and is added to sodium hydroxide, the mass concentration of sodium hydroxide is 18%-26% solution; the operating pressure of the tubular membrane is 0.3MPa-0.8MPa;

[0038] Step 5: The separated sodium chloride mother liquor is evaporated, crystallized and centrifuged to finally obtain the sodium chloride product. In the sodium chloride mother liquor evaporation and crystallization process, when the impurity ions are enriched, it is regularly returned to the front end for impurity removal;

[0039] Step 5: The separated sodium chloride mother liquor is evaporated, crystallized, and centrifuged to finally obtain a sodium chloride product, which specifically includes the following steps:

[0040] S1, passing the sodium chloride mother liquor into the Oslo crystal separator, and using an external falling film heat exchanger to heat and evaporate the sodium chloride wastewater mother liquor in the Oslo crystal separator, evaporate part of the solvent to make the solution supersaturated until crystals are precipitated, and perform evaporation and crystallization operation of the sodium chloride solution;

[0041] S2, discharging the mother liquor evaporated by the Oslo crystal separator into a thickener for separation of low-concentration semi-finished product slurry;

[0042] S3, discharging the sodium chloride crystals in the Oslo crystal separator after the mother liquor is separated into a centrifuge for centrifugal dehydration operation;

[0043] S4, the mother liquor separated from the thickener is transferred to the Oslo crystallizer for secondary flash crystallization operation to separate the sodium chloride solid again;

[0044] S5, discharging the sodium chloride crystals after preliminary separation in the thickener into a centrifuge for centrifugal dehydration operation;

[0045] S6, then continue to discharge the mother liquor entering the Oslo crystallizer for the second time into the thickener for a second low-concentration crystallization separation operation;

[0046] S7, introducing the sodium chloride crystals dehydrated in the centrifuge into a vibrating drying fluidized bed for drying, and packaging the dried finished product to obtain a sodium chloride product;

[0047] The sodium chloride mother liquor is transported to an external liquid storage tank through a sewage pump, and the sodium chloride wastewater mother liquor in the liquid storage tank is transported to the Oslo crystal separator by the pump body for temporary storage and transportation of the sodium chloride mother liquor, so that it is not easy to have a large amount of wastewater discharged at a time, and it is difficult to completely pass it into the Oslo crystal separator for treatment at one time, which is conducive to the temporary storage of waste liquid in the liquid storage tank and subsequent batch processing operations;

[0048] Specifically, the hot steam generated by evaporation and crystallization in the Oslo crystal separator is introduced into the steam compressor through the steam exhaust pipe for heating and compression, and then returned to the falling film heat exchanger through the steam exhaust pipe, which is conducive to timely recovery of heat in the Oslo crystal separator, improves the utilization rate of thermal energy, and reduces the waste of thermal energy.

[0049] The mother liquor in the Oslo crystal separator is transported to the thickener through the pump body. The water inlet and water outlet of the pump body are respectively fixedly installed on the mother liquor discharge pipe of the Oslo crystal separator and the mother liquor inlet pipe of the thickener through pipes, so that the mother liquor can be transported more smoothly.

[0050] In addition, the mother liquor in the thickener is returned to the Oslo crystallizer through the pump body. The water inlet and outlet ends of the pump body are fixedly installed on the reflux pipe of the Oslo crystallizer and the outlet pipe of the thickener through pipes, respectively, to facilitate the return operation of the mother liquor.

[0051] The mother liquor after secondary crystallization separation in the thickener has basically crystallized and separated the salt, and the remaining residual salt is small. The mother liquor after secondary treatment is passed into the external Glauber's salt pool for system replacement circulation operation. After the secondary crystallization separation treatment, the remaining salt content in the wastewater is small and difficult to separate. The wastewater after secondary treatment can be directly discharged into the Glauber's salt pool for subsequent treatment operations;

[0052] In addition, a small amount of water introduced into the external Glauber's salt pool is connected to the water pump in the steam compressor through a pipe, so that the water can be transported to the steam compressor for heating to generate secondary steam for recycling, so that there is sufficient water in the steam compressor to generate sufficient steam.

[0053] The present invention obtains sodium sulfate through two-stage flash crystallization, thereby avoiding the problems of blockage of heat exchange equipment and reduced heat exchange efficiency; does not need to use a large amount of chilled water or a refrigeration unit for freezing crystallization, and the operating energy consumption of the system is significantly reduced; the operating energy consumption is low; the process flow is short and the equipment occupies a small area.

[0054] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for separating salt and nitrate from high-salt wastewater, characterized in that: The following steps are involved: Step 1: The high-salinity wastewater is pre-treated to remove impurity ions and then enters the evaporation and concentration process. When the evaporation and concentration reaches saturation of sodium sulfate, it is sent to the primary flash evaporation process through a delivery pump; Step 2: After the material is subjected to a flash crystallization, it is sent to a secondary flash crystallization process through a delivery pump, and the material is subjected to a secondary flash crystallization, and the resulting sodium sulfate and sodium chloride mother liquid crystal slurry is sent to a centrifugal separation process through a delivery pump; Step 3: The separated sodium sulfate is re-dissolved at high temperature, evaporated and crystallized, centrifuged, dried and packaged, and finally anhydrous sodium sulfate product is obtained; Step 4: Glauber's salt evaporation and crystallization process, when the impurity ions are enriched, they are regularly returned to the front end for impurity removal; Step 5: The separated sodium chloride mother liquor is evaporated, crystallized and centrifuged to finally obtain a sodium chloride product.

2. The method for separating salt and nitrate from high-salt wastewater according to claim 1, characterized in that: The condition for the material to undergo a flash crystallization in step 2 is an absolute pressure of 20 kPa.

3. The method for separating salt and nitrate from high-salt wastewater according to claim 1, characterized in that: The condition for the secondary flash crystallization of the material in step 2 is 3kpa.

4. The method for separating salt and nitrate from high-salt wastewater according to claim 1, characterized in that: The COD in the pretreated high-salt wastewater is not higher than 1000 mg / L, and the total hardness in terms of calcium carbonate is not higher than 50 mg / L.

5. The method for separating salt and nitrate from high-salt wastewater according to claim 1, characterized in that: In the step 1, the high-salt wastewater is pretreated to remove impurity ions, and the specific operations include: passing the high-salt wastewater into a first crystallization pool and adding sodium sulfate solution and calcium sulfate seeds, adjusting the molar concentration ratio of calcium ions and sulfate ions therein to 1: (2-4), and reacting for a first preset time; overflowing the effluent of the first crystallization pool into a sedimentation pool for sedimentation to obtain calcium sulfate dihydrate, recycling the calcium sulfate dihydrate to the first crystallization pool, and adjusting the solid content of the high-salt wastewater in the first crystallization pool to 5%-10%; overflowing the effluent of the sedimentation pool into a second crystallization pool and adding sodium hydroxide, and adjusting the solid content in the second crystallization pool to 3%-5%; passing the treated high-salt wastewater into a tubular membrane, returning the concentrated water obtained after the tubular membrane treatment to the second crystallization pool, and passing the produced water obtained after the tubular membrane treatment into an evaporation concentration process.

6. The method for separating salt and nitrate from high-salt wastewater according to claim 5, characterized in that: The pH of the second crystallization pool is not less than 11.

2.

7. The method for separating salt and nitrate from high-salt wastewater according to claim 5, characterized in that: The molar concentration ratio of calcium ions to sulfate ions is adjusted to 1:(2-4), and the reaction is carried out for a first preset time, wherein the first preset time is 45 min-75 min.

8. The method for separating salt and nitrate from high-salt wastewater according to claim 5, characterized in that: The effluent from the sedimentation tank overflows into the second crystallization tank and is added to sodium hydroxide, wherein the mass concentration of the sodium hydroxide is 18%-26% of the solution.

9. The method for separating salt and nitrate from high-salt wastewater according to claim 5, characterized in that: The operating pressure of the tubular membrane is 0.3MPa-0.8MPa.

10. The method for separating salt and nitrate from high-salt wastewater according to claim 1, characterized in that: In the step 5, the separated sodium chloride mother liquor is evaporated, crystallized and centrifuged to finally obtain the sodium chloride product. In the sodium chloride mother liquor evaporation and crystallization process, when the impurity ions are enriched, it is regularly returned to the front end for impurity removal.

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