Method for preparing sodium chloride and potassium chloride from brine

Abstract

The invention provides a method for preparing sodium chloride and potassium chloride from brine, and relates to the technical field of chemical separation, the method adopts a two-stage crystallization process for brine containing components such as NaCl, KCl, CaCl2, MgCl2, CaSO4 and the like, and the two-stage crystallization process comprises the following steps: firstly, crystallizing and separating out NaCl through an MVR (Mechanical Vapor Recompression) evaporative crystallization system at a relatively high temperature (such as 90 DEG C); then mother liquor of the MVR system is introduced into a flash evaporation crystallization system, and KCl is further crystallized and separated out under the low temperature (such as about 50 DEG C) and vacuum conditions. Through the steps of adding seed crystals (such as calcium sulfate seed crystals), using a seed crystal cyclone, controlling mother liquor circulation and discharge and the like, calcium, magnesium and sulfate radical impurities are effectively separated, and the yield and purity of NaCl and KCl are improved. The method is low in energy consumption, high in salt separation efficiency and suitable for industrial brine treatment and salt chemical industry production.

Description

Technical Field

[0001] This invention relates to the field of chemical separation technology, specifically a method for preparing sodium chloride and potassium chloride from brine. Background Technology

[0002] Traditional brine treatment processes often only produce mixed salt or primary salt products, resulting in low added value and causing resource waste and environmental pollution. With the development of evaporation crystallization technology, especially MVR (mechanical vapor recompression) technology, its high efficiency and energy saving advantages have led to its widespread application, making efficient brine separation possible. However, for complex brine systems containing both NaCl and KCl, especially when high concentrations of Ca are also present, efficient separation remains a challenge. 2+ Mg 2+ SO4 2- Achieving efficient and economical separation of NaCl and KCl while simultaneously treating impurities during the separation of impurity ions is a key technical challenge in industrial production. Existing technologies often suffer from low separation efficiency, limited product purity, high energy consumption, or complex process flows. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing sodium chloride and potassium chloride from brine, aiming to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: the brine contains at least sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), and calcium sulfate (CaSO4), and the method includes the following steps: S1. Raw material preparation; S2, MVR evaporation and crystallization of NaCl: The pretreated brine is transported to a crystallizer containing an MVR compressor, where it is evaporated and concentrated at high temperature. The crystallization conditions are controlled to allow NaCl to crystallize and precipitate preferentially, resulting in NaCl crystals and the first mother liquor. S3, flash evaporation crystallization to precipitate KCl: The first mother liquor obtained in step S2 is introduced into a flash crystallizer and flash evaporation and cooling crystallization are carried out under vacuum and below the MVR crystallization temperature to precipitate KCl crystals and obtain KCl crystals and a second mother liquor. S4. Solid-liquid separation and product collection: The NaCl crystals precipitated in step S2 and the KCl crystals precipitated in step S3 are separated by thickening and centrifugation to obtain solid NaCl product and solid KCl product, respectively. S5. Mother liquor treatment and recycling: The mother liquor generated after separating NaCl in step S4 is returned to the crystallization separator in step S2; part of the mother liquor generated after separating KCl in step S4 is returned to the flash crystallizer in step S3, and part of the mother liquor is discharged periodically as concentrated mother liquor.

[0005] Furthermore, before or during step S2, calcium sulfate seed crystals are added to the MVR evaporation crystallization system. The seed crystals have a particle size of approximately 20 μm, and the amount added is controlled at a certain ratio relative to the amount of brine treated.

[0006] Furthermore, it also includes a calcium sulfate seed separation and recycling step: the suspension containing calcium sulfate seeds is drawn out from the MVR evaporation crystallization system and separated into solid and liquid by a seed hydrocyclone. Part of the separated seeds are returned to the crystallization separator to maintain the seed concentration in the system, and the other part is discharged from the system as aged seeds. The separated supernatant is introduced into the flash crystallizer.

[0007] Furthermore, polyaluminum chloride (PAC) is selectively added after the seed hydrocyclone or in the seed circulation path to promote the flocculation and sedimentation separation of fine calcium sulfate particles.

[0008] Furthermore, in step S2, the operating temperature of the MVR evaporation crystallization is controlled at 85-95℃; in step S3, the operating temperature of the flash crystallization is controlled at 45-55℃.

[0009] Furthermore, during the evaporation and crystallization process in step S2 and / or step S3, an antifoaming agent is added to suppress foam formation.

[0010] Furthermore, during the flash crystallization process in step S3, a scale inhibitor is added to prevent or reduce scaling in the equipment.

[0011] Furthermore, in step S5, the mother liquor circulation volume returning to the flash crystallizer is controlled, and the concentrated second mother liquor is periodically discharged. At the same time, condensate or low-concentration brine is added to maintain the ion concentration balance and KCl product purity in the flash crystallizer.

[0012] Furthermore, the process flow system used in the method includes an MVR evaporation crystallization unit and a flash crystallization unit, as well as corresponding seed crystal addition and separation units, solid-liquid separation units, and mother liquor circulation pipelines.

[0013] The beneficial effects of this invention are: This invention employs a two-stage stepwise crystallization process of "MVR high-temperature crystallization of NaCl + flash evaporation low-temperature crystallization of KCl", which fully utilizes the solubility difference between NaCl and KCl at different temperatures to achieve efficient separation of the two. The process flow is clear and easy to operate.

[0014] By introducing calcium sulfate seed crystals and combining them with seed crystal hydrocyclone separation and circulation technology, the calcium and sulfate ions in the brine are effectively controlled, allowing them to grow and be discharged in the form of calcium sulfate crystals under controlled conditions. This significantly reduces the scaling problem on the heat exchange surface of the evaporator and improves the system's operational stability and heat transfer efficiency.

[0015] The combination of reasonable mother liquor circulation and regular discharge not only improves the total salt recovery rate but also avoids the vicious cycle of impurity ions, ensuring the purity of the final product (especially KCl).

[0016] The entire method combines the energy-saving advantages of MVR with specific impurity treatment processes, achieving comprehensive resource utilization while reducing overall energy consumption and production costs, and is particularly suitable for large-scale brine resource utilization projects. Attached Figure Description

[0017] Figure 1 This is a process flow diagram for preparing sodium chloride and potassium chloride.

[0018] In the diagram: 1. MVR compressor; 2. Crystallizer separator; 3. Seed tank; 4. Hot water preheater; 5. Non-condensable steam preheater; 6. Tubular heater; 7. No. 1 thickener; 8. No. 1 centrifuge; 9. Seed hydrocyclone; 10. Flash crystallizer; 11. No. 2 thickener; 12. No. 2 centrifuge; 13. Electric steam generator. Detailed Implementation

[0019] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0020] Example: After adjustment, the main components of the brine are approximately: NaCl: 209 kg / h, KCl: 84 kg / h, CaCl2: 12.9 kg / h, MgCl2: 5.4 kg / h, CaSO4: 1.2 kg / h (based on a brine processing capacity of 0.6-1 t / h).

[0021] The specific steps are as follows: Raw material preparation: According to the target concentration, mix the brine with the supplementary raw material salt (NaCl, KCl, etc.) and water in the mixing tank, stir and dissolve evenly to prepare brine that meets the feeding requirements.

[0022] Operation of MVR evaporation crystallization system and NaCl precipitation: a. Pump the prepared brine into the MVR evaporation and crystallization system. The system mainly consists of an MVR compressor 1, a main heater, a crystallizer separator 2, and a forced circulation pump (not shown in the figure).

[0023] b. Before starting the system, prepare a calcium sulfate seed suspension (particle size approximately 20 μm) in seed tank 3. After the system is running, add the seeds to the system and maintain a certain seed concentration (e.g., an initial addition of approximately 30 kg, which is then maintained through circulation).

[0024] c. Start the electric steam generator 13 to preheat the system, including the hot water preheater 4, the non-condensable steam preheater 5, and the tubular heater 6, until the feed temperature reaches 90°C. Then start the MVR compressor 1, and the system enters continuous evaporation and crystallization mode. Control the evaporation temperature at around 90°C. Under these conditions, a large amount of NaCl crystallizes out.

[0025] d. The NaCl slurry discharged from the salt leg of crystallizer 2 enters thickener 7 for concentration, and is then separated using centrifuge 8 to obtain wet NaCl salt, which is dried to obtain the product. The separated mother liquor is returned to crystallizer 2.

[0026] e. A suspension containing calcium sulfate seed crystals is drawn from the MVR crystallization system and enters the seed hydrocyclone 9. A portion of the separated underflow (concentrated seed slurry) is returned to the crystallization separator 2, and the remainder is discharged as aged seed crystals. If necessary, a small amount of PAC (polyaluminum chloride) is added to the seed circulation line to promote fine crystal flocculation. The overflow liquid (supernatant) from the seed hydrocyclone 9 serves as the first mother liquor, which is mainly rich in KCl, while the calcium and sulfate content is reduced.

[0027] Operation of the flash crystallization system and KCl precipitation: a. The first mother liquor from the MVR system is introduced into the flash crystallizer 10. The flash crystallizer 10 is maintained at a low temperature of about 50°C by a vacuum pump (not shown in the figure).

[0028] b. The mother liquor is further concentrated and cooled in flash crystallizer 10. Due to the characteristic that the solubility of KCl decreases significantly with decreasing temperature, a large amount of KCl crystallizes out, while the solubility of NaCl does not change much under these conditions, and the amount of NaCl precipitated is very small.

[0029] c. The KCl slurry discharged from flash crystallizer 10 enters thickener 11 (No. 2), and is then pumped into centrifuge 12 (No. 2) for separation to obtain wet KCl salt, which is then dried to obtain the product. Testing shows that the purity of the KCl product (based on potassium content) is ≥95%.

[0030] d. The second mother liquor produced after KCl separation is partially returned to flash crystallizer 10 to continue the crystallization cycle, while the other part is concentrated to a certain factor (e.g., 5-6 times) and then periodically discharged to prevent impurity accumulation. Simultaneously, condensate is added to the system to maintain liquid level and concentration balance. A portion of the second mother liquor is also returned to crystallizer separator 2 after being heated by a steam heater.

[0031] Auxiliary measures: During MVR and flash crystallization processes, defoamers can be added continuously or intermittently depending on the foaming situation. A trace amount of scale inhibitor can be added to the feed to the flash system.

[0032] System control: The system automatically controls each pump, compressor, and valve through a DCS or PLC system, and monitors key parameters such as temperature, pressure, flow rate, and liquid level to ensure continuous and stable operation of the system.

[0033] The above process enables the continuous and stable separation and production of qualified NaCl and KCl products from complex brine, while effectively managing impurities such as calcium, magnesium, and sulfate, and the system operates with low energy consumption.

Claims

1. A method for preparing sodium chloride and potassium chloride from brine, characterized in that, The brine contains at least sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), and calcium sulfate (CaSO4), and the method includes the following steps: S1. Raw material preparation; S2, MVR evaporation and crystallization of NaCl: The pretreated brine is transported to a crystallizer containing an MVR compressor, where it is evaporated and concentrated at high temperature. The crystallization conditions are controlled to allow NaCl to crystallize and precipitate preferentially, resulting in NaCl crystals and the first mother liquor. S3, flash evaporation crystallization to precipitate KCl: The first mother liquor obtained in step S2 is introduced into a flash crystallizer and flash evaporation and cooling crystallization are carried out under vacuum and below the MVR crystallization temperature to precipitate KCl crystals and obtain KCl crystals and a second mother liquor. S4. Solid-liquid separation and product collection: The NaCl crystals precipitated in step S2 and the KCl crystals precipitated in step S3 are separated by thickening and centrifugation to obtain solid NaCl product and solid KCl product, respectively. S5. Mother liquor treatment and recycling: The mother liquor generated after separating NaCl in step S4 is returned to the crystallization separator in step S2; part of the mother liquor generated after separating KCl in step S4 is returned to the flash crystallizer in step S3, and part of the mother liquor is discharged periodically as concentrated mother liquor.

2. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, Before or during step S2, calcium sulfate seed crystals are added to the MVR evaporation crystallization system. The seed crystals have a particle size of about 20 μm, and the amount added is controlled at a certain ratio relative to the amount of brine treated.

3. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, It also includes a calcium sulfate seed separation and recycling step: the suspension containing calcium sulfate seeds is drawn out from the MVR evaporation crystallization system and separated into solid and liquid by a seed hydrocyclone. Part of the separated seeds are returned to the crystallization separator to maintain the seed concentration in the system, and the other part is discharged from the system as aged seeds. The separated supernatant is introduced into the flash crystallizer.

4. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, in After the seed hydrocyclone or in the seed circulation path, polyaluminum chloride (PAC) is selectively added to promote the flocculation and sedimentation separation of fine calcium sulfate particles.

5. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, In step S2, the operating temperature of the MVR evaporation crystallization is controlled at 85-95℃; in step S3, the operating temperature of the flash crystallization is controlled at 45-55℃.

6. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, During the evaporation and crystallization process in step S2 and / or step S3, an antifoaming agent is added to suppress foam formation.

7. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, During the flash crystallization process in step S3, a scale inhibitor is added to prevent or reduce scaling on the equipment.

8. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, In step S5, the mother liquor circulation rate returning to the flash crystallizer is controlled, and the concentrated second mother liquor is periodically discharged. At the same time, condensate or low-concentration brine is added to maintain the ion concentration balance and KCl product purity in the flash crystallizer.

9. The method for preparing sodium chloride and potassium chloride from brine according to claim 1, characterized in that, The process system used in the method includes an MVR evaporation crystallization unit and a flash crystallization unit, as well as corresponding seed crystal addition and separation units, solid-liquid separation units, and mother liquor circulation pipelines.

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