CN111484178B - Comprehensive treatment method for seawater or strong brine - Google Patents

Abstract

The invention relates to a comprehensive treatment method of seawater or strong brine, which comprises the following steps: pretreatment, low-interception nanofiltration separation, low-interception nanofiltration permeate treatment, reverse osmosis concentrated water treatment, nanofiltration permeate treatment and nanofiltration concentrated water treatment. The method of the invention can completely recover fresh water, greatly reduce the yield of low-value magnesium chloride and calcium sulfate (gypsum) by-products, avoid the generation of low-value sodium sulfate, magnesium bromide and the like, and obviously improve the yield of high-purity high-value products such as sodium chloride, calcium chloride, potassium sulfate, magnesium sulfate and the like.

Description

A comprehensive treatment method for seawater or concentrated brine

technical field

The invention belongs to the fields of seawater desalination, comprehensive utilization of seawater resources, comprehensive utilization of salt lake or underground brine resources, and the like, in particular to a comprehensive treatment method of seawater or concentrated brine.

Background technique

China has abundant marine resources and brine resources. According to relevant statistics, the underground brine resources along the coast of the Bohai Sea alone can reach about 10 billion m ³ , which contains abundant and cheap water resources and chemical element resources. At present, the development and utilization of marine and brine resources in my country are mainly based on seawater desalination, salt production, and salt chemical production such as soda ash and chlor-alkali. Combining various industries to carry out comprehensive utilization of seawater and concentrated brine resources, a circular economy and sustainable The road of development is one of the inevitable trends in the current development of related industries.

At present, the methods for comprehensive utilization of marine resources and brine resources at home and abroad mainly include the following:

(1) Desalination of seawater to produce fresh water with membrane method (reverse osmosis) and thermal method (multi-effect evaporation or multi-stage flash evaporation) as the core technology. The thermal method needs to consume a lot of steam and is usually built near the power plant; the conventional seawater desalination treatment process has a fresh water yield of only about 40-55%. Due to the scaling problem of calcium sulfate, the concentrated brine produced by desalination cannot be directly processed and utilized in factories. Due to the high salinity of concentrated seawater (nearly twice that of ordinary seawater) and the presence of pretreatment chemicals, a large amount of direct discharge into the sea will cause marine ecological pollution; it also wastes freshwater resources and various salt resources in concentrated seawater.

(2) The combination of traditional salt pan drying and salt chemical industry is currently the mainstream way of comprehensive utilization of marine and brine resources, but there are shortcomings such as being seriously affected by seasons, relying on a large number of salt pans, and low resource utilization. In addition, in the traditional salt field drying process, the first precipitated salt is calcium sulfate, followed by sodium chloride. In the subsequent advanced treatment process of the brine crystallization mother liquor, since all calcium ions have been precipitated in the form of calcium sulfate, which makes the seawater It is impossible to produce other calcium salts such as high-value calcium chloride, and calcium sulfate precipitation also consumes 30-40% of sulfate ions in raw seawater, resulting in the production of other high-value sulfates (such as magnesium sulfate and potassium sulfate). reduce. Therefore, in the comprehensive treatment of seawater and concentrated brine, it is necessary to actively explore a kind of calcium sulfate that can limit the output of low-value calcium sulfate and increase other calcium salts (such as calcium chloride) and other sulfates with high market acceptance in seawater resources (such as calcium chloride). Magnesium sulfate, potassium sulfate, etc., are all chlorine-free fertilizers, and supplement the process and method of product yield such as the sulfur element required for the growth of crops.

(3) The comprehensive utilization technology of firstly extracting calcium and magnesium and then carrying out concentrated crystallization, that is, firstly remove calcium and magnesium in concentrated seawater by chemical precipitation method (adding _NaOH , _Na2CO3 and other chemical agents), ion exchange method or electrochemical technology , and then obtain various solid salt products through deep concentration and crystallization processes such as multi-effect evaporation or pressure steam distillation concentration. This technology effectively solves the problem of equipment fouling in the process, and has a high resource utilization rate, but it needs to consume a lot of chemicals or electric energy, and the by-products such as calcium carbonate and magnesium hydroxide are of low value, and the operating cost and investment cost are high. , the overall process economy needs to be improved.

(4) Comprehensive utilization technology with multi-effect evaporation or steam distillation-seed crystal method (sometimes also called "crystal descaling method" or "on-line crystallization process") as the core. For example, the patent CN103253818A etc. adopts similar technology to realize the comprehensive utilization and zero discharge of seawater resources. The utilization rate of seawater resources using this comprehensive utilization technology is much higher than that of the traditional process, but the process still produces a large amount of gypsum (calcium sulfate), and the added value is low. The power consumption is very high. Generally, 8% of the concentrated brine is concentrated to about 26%, and the power consumption of each ton of fresh water is about 35-40 degrees (the steam consumption is about 15kg). The economy of the process needs to be further improved.

(5) Comprehensive utilization technology with electrodialysis concentration salt production as the core, as described in patent CN100999364B and so on. This technology mainly concentrates concentrated seawater by electrodialysis technology to replace brine production in salt fields (salt concentration is concentrated to more than 200g/L), and then through multi-effect evaporation or steam pressure distillation evaporation and crystallization to produce raw salt, the sodium chloride produced by this technical route Salt products are of high quality and are not affected by the season. However, there are problems such as high power consumption (currently, Japan's electrodialysis technology is the world leader, and the power consumption per ton of salt is still as high as 150 degrees), and the processing and treatment of by-product light brine is difficult.

(6) The technology combining ordinary nanofiltration and concentrated crystallization, such as patents CN 102849887B, CN102272053A, etc., adopt ordinary nanofiltration membranes (for SO ₄ ^2- rejection rate ≥ 90%, for Mg ²⁺ rejection rate ≥ 77%, for Ca ²⁺ Retention rate ≥ 70%, NaCl retention rate < 30%) to process seawater, the obtained nanofiltration permeate is a high-purity monovalent salt solution, and then high-purity NaCl is obtained by concentration and crystallization. Due to the relatively high retention rate of all divalent ions by ordinary nanofiltration membranes, sulfate ions and calcium ions that are easy to scale are mainly trapped in the concentrated water side of nanofiltration. The use of ordinary nanofiltration membranes in the patent can effectively reduce the amount of sodium The risk of fouling in the subsequent concentration equipment of filtrated permeate, improving the purity of solid salt, etc., but using this method to treat feed liquids with high calcium ion and sulfate ion content (such as by-product concentrated seawater in desalination plants, underground brine, etc.) , the yield of the nanofiltration permeate will be very limited, because the ordinary nanofiltration membrane does not have the ability to fundamentally selectively split the sulfate ions and calcium ions in it. If the concentration ratio is increased, it may cause ordinary nanofiltration. The scaling of calcium sulfate on the concentrated water side of the membrane cannot maximize the economy of the comprehensive seawater treatment process. At the same time, the ordinary nanofiltration concentrated water obtained by the patented process route is enriched with a large amount of divalent ions, especially sulfate ions and calcium ions that are easy to scale, even if the nanofiltration concentrated water is subjected to multi-effect evaporation or steam pressure Distillation-seed method concentrates the crystallization, the calcium ions in the concentrated water (that is, more than 68% of the calcium ions in the raw material solution) and most of the sulfate radicals will still be almost worthless calcium sulfate (instead of the more valuable CaCl ₂ , MgSO ₄ , etc.), and the equipment has the risk of scaling, so it is only piled up as waste (part of it is used as building materials or used for land reclamation), which is a serious waste of resources. Therefore, the introduction of ordinary nanofiltration process in the process of membrane or thermal seawater desalination can only solve the problem of calcium sulfate scaling when the nanofiltration permeate is deeply concentrated, and can obtain pure concentrated brine with higher concentration. The recovery rate has not increased, and the comprehensive utilization still loses the sulfate ions and most of the calcium ions in seawater, and the market acceptance of the main production of magnesium chloride is low rather than the main production of magnesium sulfate (chlorine-free fertilizer). high-quality products. In addition, for concentrated brine with high concentration such as concentrated seawater or underground brine (salt concentration ≥ 8%), ordinary nanofiltration cannot be used directly, and it is only suitable for low salt concentration.

To sum up, the current conventional treatment processes for seawater and concentrated brine cannot avoid the scaling problem of calcium sulfate, the problem of a large number of by-products of low-value calcium sulfate salts, and the inability to obtain high value-added products, potassium sulfate and magnesium sulfate; high value-added products The extraction technology of salt resources in seawater and concentrated brine is yet to be further developed. It is of great technical, economic and environmental significance to adopt technological innovation to overcome the shortcomings of traditional marine and brine resources comprehensive utilization equipment and process routes, and to improve economic competitiveness.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art and methods, and to provide a comprehensive treatment method of seawater or concentrated salt water with high efficiency and energy saving, low operation risk, high product profit, and environmental friendliness.

Using this method, fresh water and various salt resources in seawater can be completely extracted and fully utilized, and high value-added products different from traditional salt fields can be produced on the basis of greatly improving the freshwater yield, completely avoiding the back discharge of concentrated seawater, and realizing The high-capacity three-dimensional compact modern salt factory replaces the traditional low-capacity salt farm that relies on the sky for food.

In order to achieve the above object, the comprehensive treatment method of seawater or concentrated brine of the present invention comprises the following steps:

1) Pretreatment: the seawater or concentrated brine to be treated is treated by microfiltration and ultrafiltration with low molecular weight cut-off to remove bacteria, algae, sediment, suspended solids and organic matter, etc., to obtain a pretreated feed liquid;

2) low-cut-off nanofiltration separation: the pretreated feed liquid is separated by a low-cut-off nanofiltration membrane device with a high rejection rate for magnesium sulfate, sodium sulfate and magnesium chloride and a low rejection rate for calcium chloride, respectively to obtain rich SO _{4 .} ^2- , Mg ²⁺ nanofiltration concentrated water, and nanofiltration permeate containing Na ⁺ , Cl ^- , Br ^- , NO ₃ ^- , K ⁺ , Ca ²⁺ ;

3) Low-cut-off nanofiltration permeate treatment: the nanofiltration permeate in the step 2) is subjected to a reverse osmosis process to produce fresh water, and the fresh water yield is 60 to 80%, and a reverse osmosis of 8 to 15° Bé can be obtained at the same time. infiltration of concentrated water;

4) Reverse osmosis concentrated water treatment: the concentrated water in the step 3) is treated by nanofiltration to obtain nanofiltration concentrated water rich in divalent and high-valent ions, and nanofiltration rich in monovalent ions at 7～14°Bé permeate;

5) Nanofiltration permeate liquid treatment: described step 4) Nanofiltration permeate liquid obtains refined NaCl by concentrating crystallization, then removes silicon, and then successively extracts bromine, magnesium (magnesium chloride), potassium, and calcium (chloride). Calcium) and lithium extraction; the fresh water produced in this process is used directly or after post-treatment such as low pressure reverse osmosis;

6) Nanofiltration concentrated water treatment: the step 2) nanofiltration concentrated water is mixed with the nanofiltration concentrated water described in step 4), and evaporation-crystallization obtains calcium sulfate, strontium sulfate, sodium chloride, etc. successively, and then sequentially removes silicon, Bromine extraction, magnesium extraction (a large amount of magnesium sulfate, a small amount of magnesium chloride), and lithium extraction; the fresh water produced in this process is used directly or after post-treatment such as low-pressure reverse osmosis.

Further, the concentrated brine of the present invention can be the concentrated water by-product of the reverse osmosis method seawater desalination process, the by-product concentrated water of the multi-effect evaporation or multi-stage flash method seawater desalination process, the by-product concentrated water of the pressure distillation method seawater desalination process, Membrane distillation seawater desalination process by-product concentrated water, concentrated seawater produced by electrodialysis of seawater, and concentrated seawater produced by any industrialized seawater or brackish water desalination process, and concentrated seawater produced by partial concentration of salt pan drying, It can also be the external drainage of the seawater medium circulating cooling system of the thermal power plant, the waste water produced by the seawater flue gas desulfurization in the thermal power plant, salt lake water or underground brine, etc.

Further, step 1) adopts microfiltration and low molecular weight cut-off ultrafiltration process to replace traditional medicament sterilization and algae killing, flocculation and sedimentation and other processes to remove bacteria, algae, sediment, suspended matter and organic matter in the feed liquid; The organic concentrated water produced by the molecular weight ultrafiltration process is biochemically treated with salt-tolerant bacteria, and the generated biological sludge is recycled with the substances retained by the microfiltration.

Further, the low cut-off nanofiltration membrane described in step 2) has a good cut-off performance for SO ₄ ^2- , and a good permeation performance for Ca ²⁺ , preferably for divalent anions (such as SO ₄ ^2- , HPO ₄ ^2- , CO ₃ ^2- , etc.) rejection rate is very high, while the rejection rate of monovalent anions, monovalent cations and some divalent cations (especially Ca ²⁺ , Sr ²⁺ , Ba ²⁺ ) is very low Very low nanofiltration membrane; preferably low cut-off nanofiltration membrane for SO ₄ ^2- rejection ≥ 95% (more preferably greater than 98%), Mg ²⁺ rejection ≥ 75%, Ca ²⁺ rejection < 35% (more preferably <20%), monovalent ion rejection <20% (more preferably <5%).

Further, the low-cut-off nanofiltration process described in step 2) has the following characteristics: under the condition of high recovery rate of produced water and high membrane flux, the effective interaction between divalent anion SO ₄ ^2- and divalent cations Ca ²⁺ and Sr ²⁺ is realized. Resolution and concentration of SO ₄ ^2- ions in the feed liquid.

Further, suitable operating conditions for the low cut-off nanofiltration membrane described in step 2) are: operating pressure of 0.5-4.1 MPa, operating temperature of 5-45°C, and pH of 3-12.

Further, the reverse osmosis described in step 3) adopts the coupling of one or more processes such as low pressure reverse osmosis, medium pressure reverse osmosis, and high pressure reverse osmosis.

Further, the ordinary nanofiltration described in step 4) has a higher retention rate for divalent ions or high-valent ions such as SO ₄ ^2- , Ca ²⁺ , Mg ²⁺ and Sr ²⁺ , while for Na ⁺ , Cl ^- , Br ^- and other monovalent ions have a lower retention rate.

Further, the concentration-crystallization process in steps 5) and 6) is one of simple evaporation-crystallization, multi-effect evaporation-crystallization, membrane distillation-crystallization, steam pressure distillation-crystallization and other processes.

Further, the silicon removal process in steps 5) and 6) is: adjusting the pH value of the feed liquid to 3-4 with hydrochloric acid, and separating out the silicon in the feed liquid in the form of solid silicon dioxide.

Further, the process of extracting bromine in steps 5) and 6) is: feeding chlorine into the acid feed liquid after desiliconization, and extracting bromine by traditional stripping, rectification or gaseous membrane method.

Further, the magnesium extraction process in steps 5) and 6): the pH value of the feed liquid after the bromine extraction is adjusted to 7.0-8.5 with sodium hydroxide, and then the magnesium salt is precipitated by evaporation and crystallization. Wherein step 5) obtains magnesium chloride and carnallite; step 6) obtains a large amount of magnesium sulfate and a small amount of magnesium chloride.

Further, the potassium extraction process of step 5) is: taking carnallite as raw material to recover potassium salt, and utilizing carnallite and magnesium sulfate produced in step 6) to produce potassium sulfate.

Further, the calcium extraction process of step 5) is: recovering calcium salt in the form of calcium chloride.

Further, the lithium extraction process of steps 5) and 6) is: recovering potassium salt in the form of lithium chloride and lithium sulfate.

Further, the present invention greatly avoids the mass production of low-value by-product gypsum, and the output of high-value products such as high-purity sodium chloride, calcium chloride, potassium sulfate, magnesium sulfate, bromine is significantly increased, and sodium sulfate is completely avoided. Low value product; and no effluent or wastewater discharge.

Compared with the comprehensive utilization process of traditional seawater, concentrated seawater, salt lake water or underground brine, the present invention has the following positive effects and advantages:

1) The permeate of the low-cut-off nanofiltration process is mainly monovalent ions, some calcium ions and a small amount of magnesium ions, which can be deeply concentrated through the reverse osmosis process, and the fresh water yield reaches 60-80%, which is higher than that of conventional membrane desalination Industrial increase by 30% to 80%;

2) For special solution systems such as seawater and concentrated seawater, the low-rejection nanofiltration process objectively separates SO ₄ ^2- and Ca ²⁺ in seawater or concentrated brine effectively, and combines with reverse osmosis and ordinary nanofiltration processes. , not only greatly reduces the production of low-value products such as calcium sulfate, but also significantly increases the output of high-value products such as calcium chloride, magnesium sulfate, potassium sulfate and refined sodium chloride, and reduces the risk of scaling in subsequent concentration equipment;

3) The biomass-containing seawater or concentrated brine produced by the pretreatment can be biochemically treated by salt-tolerant bacteria, and a small amount of biological sludge formed after treatment can be further treated as a resource, without any waste water, sewage or concentrated water being discharged out. secondary pollution;

4) The process of the present invention adopts the reverse osmosis technology with low investment and low operating cost, which is coupled with low-retention nanofiltration technology and compressed gas distillation, which greatly reduces the equipment investment and energy consumption of seawater desalination and deep concentration process, thus making the overall The process investment, operation risk and operation cost are significantly reduced;

5) The present invention can completely extract and fully utilize fresh water and various salt resources in seawater, produce high value-added products different from traditional salt fields on the basis of completely reclaiming fresh water, completely avoid seawater back discharge, and have high resource utilization rate, With low operation risk and no secondary pollution, it can replace the traditional flat-spread low-capacity salt field that relies on the sky for food with a high-capacity three-dimensional and compact modern salt factory.

Description of drawings

Fig. 1 is the process flow schematic diagram of the comprehensive treatment method of seawater or concentrated brine of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific implementation cases. Elements and features described in one figure or embodiment of the present invention may be combined with elements and features shown in one or more other figures or embodiments. It should be noted that representations and descriptions of components or processes known to those of ordinary skill in the art that are unrelated to the present invention are omitted from the drawings and description for the purpose of clarity.

All components used in this example are commercially available.

As shown in Figure 1, the present invention is mainly aimed at by-product concentrated water in seawater, concentrated seawater (including but not limited to reverse osmosis seawater desalination process by-product concentrated water, multi-effect evaporation or multi-stage flash method seawater desalination process by-product concentrated water, compressed gas distillation Concentrated water by-product from the desalination process of French seawater, concentrated water from the membrane distillation process, concentrated seawater produced by electrodialysis of seawater, and concentrated water from any industrialized seawater or brackish water desalination process, salt pan drying The generated concentrated seawater, the external drainage of the seawater medium circulating cooling system of the thermal power plant, the waste water produced by the seawater flue gas desulfurization in the thermal power plant, the salt lake water and the underground brine are used as raw materials for comprehensive treatment and utilization of resources.

The specific steps are:

1) Preprocessing

First, use conventional sand filtration, microfiltration and ultrafiltration with low molecular weight cut-off to remove seawater, concentrated seawater, salt lake water or underground brine and other feed liquids to remove bacteria, colloids, algae, sediment and molecular weight greater than 1000Da or even greater than 400Da organic pollutants, etc. The organic concentrated water produced by the low molecular weight cut-off ultrafiltration can be biochemically treated with salt-tolerant bacteria, and the generated biological sludge and the substances retained by the microfiltration can be recycled as resources.

2) Low cut-off nanofiltration separation

Pretreated seawater, concentrated seawater, salt lake water or underground brine are treated with high flux, high selectivity and low cut-off nanofiltration membranes. The separation and interception performance of the nanofiltration membrane adopted in the present invention is shown in Table 1:

Table 1. Retention performance of low-cut-off nanofiltration membranes

After pretreatment, the feed liquid undergoes a low cut-off nanofiltration process, and the permeate contains monovalent ions such as Na ⁺ , Cl ^- , NO ₃ ^- , Br ^- , K ⁺ and some Ca ²⁺ , a small amount of Mg ²⁺ , and a small amount of SO ₄ ^{2 -} ; Concentrated water is rich in SO ₄ ^2- , Mg ²⁺ and a certain amount of Ca ²⁺ and other monovalent ions. The recovery rate of the permeate during the nanofiltration process with low interception can reach 75-85%.

3) Treatment of low cut-off nanofiltration permeate: reverse osmosis, ordinary nanofiltration

The low-retention nanofiltration permeate is concentrated through one or more processes coupled with low-pressure reverse osmosis, medium-pressure reverse osmosis, and high-pressure reverse osmosis to produce fresh water, and at the same time, concentrated water of about 8-15°Bé can be obtained.

In the reverse osmosis process, the concentrated water enters the ordinary nanofiltration process, and the main ions in the feed liquid are intercepted as shown in Table 2; the nanofiltration concentrated water rich in divalent and high-valent ions is obtained, and the 7-14°Bé rich in mono Nanofiltration permeate of valence ions.

Table 2 Main ion interception in ordinary nanofiltration process

*The ratio of the total amount of ions in the concentrate to the total amount of ions in the permeate and concentrate

4) Treatment of ordinary nanofiltration permeate

The ordinary nanofiltration permeate is successively obtained by concentrating and crystallization to obtain refined NaCl and calcium sulfate. When most of the sodium chloride in the feed liquid has been precipitated, the pH value of the feed liquid is adjusted to 3 to 4 with hydrochloric acid to remove the silicon therein, and then Chlorine gas is introduced, and bromine is extracted by traditional stripping, rectification or gaseous membrane method. Thus, the mixed crystallization of magnesium bromide and magnesium chloride is avoided.

After the bromine extraction, the pH value of the feed liquid is adjusted to 7.0-8.5 with sodium hydroxide, and then the magnesium salt (mainly in the form of magnesium chloride) is precipitated by evaporation and crystallization, and then potassium (mainly in the form of carnallite) and calcium (calcium chloride) are extracted. form, efficient snow melting agent), lithium extraction.

The fresh water produced in this process is used directly or after post-treatment such as low pressure reverse osmosis.

5) Treatment of nanofiltration concentrated water

Mix the concentrated water of the ordinary nanofiltration process with the concentrated water of the low-cut-off nanofiltration process, adjust the pH value of the feed liquid with hydrochloric acid, and concentrate the feed liquid by the concentrated evaporation-on-line crystallization technology to obtain calcium sulfate, strontium sulfate, sodium chloride, etc. .

When most of the sodium chloride in the feed liquid crystallizes out, add hydrochloric acid, adjust the pH value of the feed liquid to 3 to 4 to remove silicon, and then introduce chlorine gas, and extract it by traditional stripping, rectification or gaseous membrane method. bromine.

Then use sodium hydroxide to adjust the pH back to 7.0-8.5, and continue to concentrate and crystallize to obtain magnesium sulfate and a small amount of magnesium chloride; the mother liquor is used for potassium and lithium extraction.

The fresh water produced in this process is used directly or after post-treatment such as low pressure reverse osmosis.

The carnallite obtained in step 4) can also be added to the concentrated water of nanofiltration after the bromine extraction, redissolving magnesium chloride and potassium chloride, and further evaporating and concentrating to obtain jarite (magnesium sulfate and potassium sulfate). After further evaporation and crystallization, magnesium chloride is obtained, and the remaining mother liquor is used to extract lithium salts.

The present invention will be further described below through specific examples.

Example 1

Feed liquid: seawater in a certain area, 3.4°Bé, pH=8.1, the composition is shown in Table 3 below:

table 3

Pretreatment: use a polypropylene melt spray filter with an accuracy of 5 μm and an ultrafiltration membrane with a molecular weight cut-off of 6000 and 2000, respectively. The overall operating pressure of the microfiltration and ultrafiltration processes is 0.25MPa. The microfiltration and ultrafiltration concentrated water enters the sludge resource recovery device for treatment, and the produced clear liquid is returned to the pretreatment stage.

Low Cutoff Nanofiltration Process:

The pretreated feed liquid enters the low cut-off nanofiltration device (the core membrane element is NF8040 roll membrane, see Table 1 for performance, effective area 30.7m ² ), operating pressure 1.45MPa, temperature 35°C, pH=8.1. The volume ratio of the nanofiltration permeate 1 obtained after the treatment to the nanofiltration concentrated water 1 is about 4:1, and the specific composition is as follows in Table 4:

Table 4

Comprehensive utilization of nanofiltration permeate:

Nanofiltration permeate 1 is concentrated to about 9°Bé by using a reverse osmosis membrane of type SUPER RO, and the yield of fresh water in the reverse osmosis process is about 73%, which can be reused; Treatment, operating pressure 3.5MPa, temperature 35 ℃. The volume ratio of the nanofiltration permeate 2 obtained after the treatment to the nanofiltration concentrated water 2 is about 0.85:0.15, and the specific composition is as follows in Table 5:

table 5

The pH of the nanofiltration permeate 2 is adjusted to about 6.0 using hydrochloric acid, and then concentrated to about 25°Bé by steam pressure distillation, and the brine at about 25°Bé is prepared by using a two-effect evaporation-crystallization device to prepare sodium chloride crystals. The purity of the sodium chloride product can reach more than 99%, and the total content of calcium and magnesium ions carried out in the crystallization process is only about 150ppm. The obtained crystallization mother liquor is acidified and oxidized to extract the bromine in it by stripping method or gaseous membrane method. The acidification process is to use hydrochloric acid to adjust the pH to about 3.5. At this time, silicon element in the mother liquor will also be precipitated in the form of silica solid to be precipitated by the solid. remove. The feed liquid after the bromine extraction was adjusted to pH 8.5 with NaOH, and the evaporative concentration-crystallization method was used to obtain products such as magnesium chloride, carnallite and calcium chloride in turn; after the mother liquor was further concentrated, tributyl phosphate was used as the extractant. , using solvent extraction method to extract lithium chloride from mother liquor. After the carnallite is hydrolyzed and crystallized to remove magnesium, the potassium chloride crystal product is directly obtained by adding water and washing.

Comprehensive utilization of nanofiltration concentrated water:

The nanofiltration concentrated water 1 obtained in the above-mentioned steps is mixed with the nanofiltration concentrated water 2, and the mixed solution uses hydrochloric acid to adjust the pH to about 6, then enters the steam distillation-calcium sulfate online crystallization device, is concentrated to about 24 ° of Bé, and precipitated. Calcium sulfate, strontium sulfate, etc. are separated by a hydraulic separator; the feed liquid at about 24°Bé is then simply evaporated and crystallized to obtain a sodium chloride product (containing a small amount of calcium and magnesium sulfate radicals); the crystallization mother liquor is acidified and oxidized using the stripping method or the gaseous membrane method. The bromine in it is extracted, and the acidification process is to use hydrochloric acid to adjust the pH to about 3.5. At this time, the silicon element in the mother liquor will also settle out in the form of silicon dioxide and be removed. The feed liquid after the bromine extraction is adjusted back to pH 8.5 with NaOH, and the evaporative concentration-crystallization method is continued to obtain products such as magnesium sulfate, kaleite, magnesium chloride, etc. respectively; Na ₂ CO ₃ precipitating agent is added to the remaining mother liquor to obtain lithium carbonate .

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Furthermore, the scope of the present application is not limited to the specific embodiments of the processes, devices, means, methods and steps described in the specification. Those of ordinary skill in the art will readily appreciate from the present disclosure that existing and future future projects that perform substantially the same function or achieve substantially the same results as the corresponding embodiments described herein can be used in accordance with the present invention. A developed process, apparatus, means, method or step. Accordingly, the appended claims are intended to include within their scope such processes, apparatus, means, methods, or steps.

Claims (10)

1. a comprehensive treatment method of seawater or concentrated brine, is characterized in that, comprises the steps: 1) Pretreatment: The seawater or concentrated brine to be treated is treated by microfiltration and ultrafiltration with low molecular weight cut-off to remove bacteria, algae, sediment, suspended solids and organic matter to obtain pretreated feed liquid; 2) Low-cut-off nanofiltration separation: The pretreated feed liquid is separated by a low-cut-off nanofiltration membrane device with a high rejection rate of magnesium sulfate, sodium sulfate and magnesium chloride, and a low rejection rate of calcium chloride, respectively, to obtain rich SO _{4 .} ^2- , Mg ²⁺ nanofiltration concentrated water, and nanofiltration permeate containing Na ⁺ , Cl ^- , Br ^- , NO ₃ ^- , K ⁺ , Ca ²⁺ ; 3) Low cut-off nanofiltration permeate treatment: the nanofiltration permeate of the step 2) is subjected to a reverse osmosis process to produce fresh water, the fresh water yield is 60-80%, and a reverse osmosis of 8-15°Bé is obtained at the same time Concentrated water; 4) Reverse osmosis concentrated water treatment: the reverse osmosis concentrated water in the step 3) is treated by nanofiltration to obtain nanofiltration concentrated water rich in divalent and high-valent ions, and 7-14°Bé rich in monovalent ions. Nanofiltration permeate; the nanofiltration treatment has a higher retention rate for SO ₄ ^2- , Ca ²⁺ , Mg ²⁺ divalent ions or higher ion ions, while Na ⁺ , Cl ^- , Br ^- a The retention rate of valence ions is low; 5) Nanofiltration permeate treatment: the nanofiltration permeate in step 4) is concentrated and crystallized to obtain refined NaCl, then silicon is removed, and then bromine, magnesium, potassium, calcium, and lithium are extracted in sequence; Described calcium extraction reclaims calcium salt in the form of calcium chloride; 6) Nanofiltration concentrated water treatment: the nanofiltration concentrated water in the step 2) is mixed with the nanofiltration concentrated water in the step 4), and the evaporation-crystallization obtains calcium sulfate, strontium sulfate, and sodium chloride in turn, and then removes silicon in sequence. , extract bromine, extract magnesium, extract lithium; The magnesium extraction in the steps 5) and 6) adopts the feed liquid after the extraction of bromine, adjusts the pH value of the feed liquid to 7.0~8.5 with sodium hydroxide, and then evaporates and crystallizes out magnesium salts; what is obtained in step 5) is magnesium chloride, photohalogen Stone; what step 6) obtains is a large amount of magnesium sulfate, a small amount of magnesium chloride; The low-cut-off nanofiltration membrane device described in the step 2) adopts a low-cut-off nanofiltration membrane, and the low-cut-off nanofiltration membrane has a good interception performance for SO ₄ ^2- and a good permeability for Ca ²⁺ performance, the low cut-off nanofiltration membrane is a nanofiltration membrane with a high rejection rate for divalent anions, but a very low rejection rate for monovalent anions, monovalent cations and some divalent cations; The filter membrane has a rejection rate of SO ₄ ^2- ≥95%, a rejection rate of Mg ²⁺ ≥75%, a rejection rate of Ca ²⁺ <35%, and a rejection rate of monovalent ions <20%; the low-cut-off nanofiltration membrane The operating pressure is 0.5～4.1MPa, the operating temperature is 5～45℃, and the pH is 3～12; The concentrated brine is the by-product concentrated water of the reverse osmosis seawater desalination process, the by-product concentrated water of the multi-effect evaporation or multi-stage flash method seawater desalination process, the by-product concentrated water of the compressed gas distillation seawater desalination process, and the membrane distillation method of seawater desalination. Process by-product concentrated water, concentrated seawater produced by electrodialysis of seawater, concentrated water produced by any industrialized seawater or brackish water desalination process, and concentrated seawater produced by drying in salt fields, and also for thermal power plant seawater medium circulation External drainage of cooling system, waste water produced by seawater flue gas desulfurization in thermal power plants, salt lake water or underground brine. 2. The comprehensive treatment method of seawater or concentrated brine according to claim 1, characterized in that, the organic concentrated water produced by the low molecular weight cut-off ultrafiltration process described in step 1) is biochemically treated with salt-tolerant bacteria, and the resulting biological Sludge and the substances retained by microfiltration are recycled for recycling. 3. The comprehensive treatment method of seawater or concentrated brine according to claim 1, characterized in that, the low-rejection nanofiltration membrane described in step 2) has a rejection rate of SO ₄ ^2- greater than 98%, and rejection rate of Mg ²⁺ ≥75%, the retention rate for Ca ²⁺ is <20%, and the retention rate for monovalent ions is <5%. 4 . The comprehensive treatment method for seawater or concentrated brine according to claim 3 , wherein the low cut-off nanofiltration membrane has an operating pressure of 1.45 MPa, an operating temperature of 35° C., and a pH of 8.1. 5 . 5. The comprehensive treatment method of seawater or concentrated brine according to claim 1, characterized in that, the reverse osmosis described in step 3) adopts one or more processes of low pressure reverse osmosis, medium pressure reverse osmosis, and high pressure reverse osmosis. coupling. 6. The comprehensive treatment method of seawater or concentrated brine according to claim 1, wherein the concentrated crystallization of step 5) is simple evaporation-crystallization, multi-effect evaporation-crystallization, membrane distillation-crystallization, steam pressure distillation-crystallization one of the. 7. The comprehensive treatment method of seawater or concentrated brine according to claim 1, characterized in that, in the desiliconization of the steps 5) and 6), hydrochloric acid is used to adjust the pH value of the feed liquid to 3~4, and the Silicon is precipitated as solid silica. 8. the comprehensive treatment method of seawater or concentrated brine according to claim 1, is characterized in that, the bromine extraction of described steps 5) and 6) adopts to pass chlorine gas into the acid feed liquid after desiliconization, with traditional blowing Bromine removal, rectification or gaseous membrane extraction. 9 . The comprehensive treatment method of seawater or concentrated brine according to claim 1 , wherein the potassium extraction in step 5) takes carnallite as a raw material to recover potassium salt, and further produces potassium sulfate. 10 . 10 . The comprehensive treatment method of seawater or concentrated brine according to claim 1 , wherein the lithium extraction in steps 5) and 6) recovers lithium salts in the form of lithium chloride and lithium sulfate. 11 .

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