A method for preparing industrial salt for chlor-alkali production by ion membrane method

Abstract

The present invention belongs to the technical field of industrial salt preparation, and discloses a method for preparing industrial salt for chlor-alkali production by ion membrane method. The method comprises the following steps: extracting brine; pre-treating the brine by centrifugation, and collecting the clear liquid; filtering the clear liquid after pre-treatment filtration through an ultrafiltration membrane, and collecting the ultrafiltration membrane clear liquid; filtering the clear liquid after ultrafiltration membrane filtration using a composite nanofiltration membrane, and collecting the nanofiltration membrane clear liquid; evaporating and drying the nanofiltration clear liquid to obtain industrial salt for chlor-alkali production by ion membrane method. The method for preparing industrial salt for chlor-alkali production by ion membrane method provided by the present invention does not need to add any chemicals to the brine, has low production cost, simple production steps, and is green and environmentally friendly.

Description

Technical Field

[0001] The invention relates to the technical field of industrial salt preparation methods, and in particular to a method for preparing industrial salt dedicated to chlor-alkali production using an ion membrane process. Background Art

[0002] As an important part of the basic chemical industry, the chlor-alkali industry occupies a key position in the chemical industry. Its main products include caustic soda (sodium hydroxide), chlorine and hydrogen. These products are widely used in many industries such as papermaking, textiles, printing and dyeing, chemicals, medicine, metallurgy, and petroleum, and play an indispensable role in the development of the national economy. In the long-term development of chlor-alkali production, the production process has undergone many changes and evolutions. In the early days, the traditional diaphragm method and mercury method were the main methods of chlor-alkali production. Although the diaphragm method has a certain production capacity, it has problems such as low product purity, high energy consumption, and serious environmental pollution. Although the mercury method can produce high-purity caustic soda and chlorine, the toxicity of mercury poses a serious threat to the environment and the health of operators. With the increasingly stringent environmental protection requirements, the mercury method has gradually been restricted and eliminated.

[0003] In this context, the ion membrane chlor-alkali production technology came into being. The ion membrane method uses the selective permeability of ion exchange membranes to anions and cations to separate the electrolytic cell into an anode chamber and a cathode chamber. Saturated salt water is introduced into the anode chamber to produce chlorine through electrolysis; pure water is introduced into the cathode chamber to generate hydrogen and sodium hydroxide solution through electrolysis. This method has significant advantages, such as high product purity. The purity of the produced caustic soda can reach more than 99%, and the purity of chlorine can also meet the production needs of various high-value-added downstream products; at the same time, the energy consumption is relatively low, which can reduce energy consumption by about 25% - 30% compared with the traditional diaphragm method, greatly improving energy utilization efficiency, which is in line with the current development trend of energy conservation and emission reduction.

[0004] As the main raw material for ion membrane chlor-alkali production, the quality and properties of industrial salt have a crucial impact on the stability of the entire production process, product quality and production cost. High-quality industrial salt should have the characteristics of high purity, low impurity content, suitable particle size distribution and stable chemical composition. Because industrial salt with excessive impurity content, such as excessive ions such as calcium, magnesium and sulfate, will react with the electrode during the electrolysis process to form precipitation or scaling, which will adhere to the surface of the ion membrane, reduce the conductivity and service life of the ion membrane, and thus affect the electrolysis efficiency and product quality, increase production costs, and may even cause the production equipment to be frequently shut down for cleaning and maintenance, seriously affecting the continuity and stability of production.

[0005] Chinese patent publication number CN119059534A discloses a production method for producing industrial salt by chlor-alkali using an ion membrane method, comprising the following steps: extracting brine; filtering the brine, adding lime milk, sodium carbonate and a precipitating agent to the brine, removing the filter residue, and obtaining a primary filtrate; introducing boiler flue gas into the primary filtrate, removing the filter residue, and obtaining a secondary filtrate; pumping the secondary filtrate to an evaporation tank, adding waste from the ammonia-soda process for alkali production, evaporating the secondary filtrate to which the waste from the ammonia-soda process for alkali production is added, and drying the salt, wherein the calcium ion content, magnesium ion content and sulfate content of impurities in the industrial salt prepared by this method are low, so that the purity of the industrial salt prepared is improved. However, this method has complicated steps and requires the addition of chemicals such as lime milk, sodium carbonate and a precipitating agent, which is easy to form chemical pollution and has relatively high cost. Summary of the invention

[0006] In order to overcome the above problems of the prior art, the present invention provides a method for preparing industrial salt for chlor-alkali production by ion membrane method. The method for preparing industrial salt for chlor-alkali production by ion membrane method provided by the present invention does not require adding any chemicals to brine, has low production cost, simple production steps and is green and environmentally friendly.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0008] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0009] 1) Extracting brine;

[0010] 2) Pre-treating the brine by centrifugation and collecting the clear liquid;

[0011] 3) filtering the clear liquid after pretreatment filtration through an ultrafiltration membrane, and collecting the ultrafiltration membrane clear liquid;

[0012] 4) Filter the clear liquid after ultrafiltration through a composite nanofiltration membrane, and collect the clear liquid from the nanofiltration membrane;

[0013] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt for ion membrane chlor-alkali production.

[0014] Preferably, in step 2), the speed of the centrifuge is controlled at 3000-4000 r / min.

[0015] Preferably, the pore size of the ultrafiltration membrane in step 3) is 0.1-0.3 μm.

[0016] Preferably, in step 3), the operating pressure of the ultrafiltration membrane is 0.2-0.6 MPa.

[0017] Preferably, in step 4), the operating pressure of the nanofiltration membrane is 1.0-2.0 MPa.

[0018] Preferably, the method for preparing the composite nanofiltration membrane in step 4) comprises the following steps:

[0019] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving, to obtain an oil phase solution;

[0020] b) adding polyethyleneimine into water, stirring to dissolve, obtaining a polyethyleneimine solution, adding modified sodium alginate into the polyethyleneimine solution, and dispersing it evenly by ultrasonic oscillation, obtaining an aqueous phase solution;

[0021] c) soaking the polyethersulfone-based membrane in an aqueous solution, taking it out and soaking it in an organic solution to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0022] d) placing the nascent composite nanofiltration membrane in an oven for thermal curing and cross-linking to obtain a composite nanofiltration membrane.

[0023] Most of the commercial nanofiltration membranes on the market are negatively charged, and rely on the pore size of the nanofiltration membrane to separate two or more substances in the solution, but the negatively charged nanofiltration membrane cannot remove impurity ions such as magnesium and calcium in the brine NaCl. The composite nanofiltration membrane of the present invention is prepared by using polyethyleneimine as the aqueous phase monomer and trimesoyl chloride as the organic phase monomer, and the two undergo interfacial polymerization on the polyethersulfone-based membrane to obtain the composite nanofiltration membrane. Since the aqueous phase monomer polyethyleneimine is loaded with more amino groups, the amino groups that have not reacted with the acyl chloride group are protonated in water and are positively charged, so that the nanofiltration membrane is positively charged, and the positive charge on the surface of the nanofiltration membrane and the highly charged high-valent cations such as magnesium and calcium form an electrostatic repulsive force, thereby realizing the interception of ions such as magnesium and calcium, allowing NaCl to pass through the nanofiltration membrane, thereby removing impurity ions such as magnesium and calcium in the brine NaCl.

[0024] Although the positively charged nanofiltration membrane prepared by the present invention has a high removal rate for impurity ions such as magnesium and calcium, the flux of the nanofiltration membrane is low, which affects the filtration efficiency of the nanofiltration membrane for brine. To solve this problem, the present invention adds sodium alginate to the polyethyleneimine solution. Sodium alginate is a linear polymer formed by connecting β-D-mannuronic acid and α-L-guluronic acid through 1,4-glycosidic bonds. The molecule contains a large number of strong hydrophilic groups such as hydroxyl and carboxyl groups. Incorporating it into the nanofiltration membrane can greatly improve the hydrophilicity of the nanofiltration membrane, thereby increasing the water flux of the nanofiltration membrane and improving the filtration separation efficiency of the nanofiltration membrane.

[0025] Preferably, in step b), the method for preparing modified sodium alginate comprises the following steps:

[0026] The epoxy silane coupling agent KH-550 and nano-silica are added into deionized water, heated and stirred for reaction, and then centrifuged, washed and dried to obtain epoxy silane coupling agent modified nano-silica;

[0027] The p-hydroxybenzenesulfonic acid catalyst is added to the sodium alginate solution, and the mixture is stirred evenly. Then, the epoxy silane coupling agent is added to modify the nano-silica, and the mixture is heated and stirred for reaction. After centrifugal separation, washing and drying, the modified sodium alginate is obtained.

[0028] In the technical solution of the present invention, the hydrophilicity of the nanofiltration membrane is improved by sodium alginate, thereby improving the water flux of the nanofiltration membrane. However, sodium alginate is negatively charged after ionization, which will neutralize the positive charge on the surface of the nanofiltration membrane, causing the positive charge on the surface of the nanofiltration membrane to weaken, thereby causing the electrostatic repulsion of impurity ions such as magnesium and calcium to weaken, thereby causing the composite nanofiltration membrane to reduce the retention rate of calcium and magnesium ions. In order to solve this technical problem, the present invention modifies sodium alginate and grafts sodium alginate on the surface of nano-silicon dioxide through epoxy silane coupling agent KH-550 to obtain modified sodium alginate. The polyethersulfone-based membrane impregnated with modified sodium alginate and polyethyleneimine is immersed in a solution of trimesoyl chloride to cause an interfacial polymerization reaction. During the polymerization reaction, the relatively light polyethyleneimine moves and spreads faster, while the mass of the sodium alginate grafted on the silica surface increases significantly compared to before grafting. During the polymerization reaction, it diffuses slowly on the surface of the polyethersulfone-based membrane and is closer to the surface of the polyethersulfone-based membrane, thereby forming a sodium alginate embedding structure after the interfacial polymerization reaction, that is, the sodium alginate is located inside the nanofiltration membrane, not distributed on the surface of the nanofiltration membrane, thereby avoiding the influence of sodium alginate on the surface potential of the nanofiltration membrane.

[0029] Preferably, in step b), the amount of modified sodium alginate added is 6.0-9.0 wt % of polyethyleneimine.

[0030] In order to improve the hydrophilicity of the nanofiltration membrane (increase the flux of the nanofiltration membrane), the present invention requires the addition of a sufficient amount of modified sodium alginate in the nanofiltration membrane. Experiments have confirmed that the amount of modified sodium alginate added is at least 6.0% of the mass of polyethyleneimine, and sodium alginate has a strong effect on improving the hydrophilicity of the nanofiltration membrane. However, the present invention team found in research that the pressure resistance (operating pressure) of the nanofiltration membrane prepared in some experiments is weak, and the weak pressure resistance easily shortens the service life of the composite nanofiltration membrane. The research found that this problem is closely related to modified sodium alginate. When the modified sodium alginate is relatively excessive in the aqueous solution, that is, the amount of modified sodium alginate added exceeds 9.0% of the mass of polyethyleneimine, the hydroxyl groups in the sodium alginate molecules will transitionally participate in the reaction with the acyl chloride groups on the trimesoyl chloride groups, which will seriously affect the reaction between the amino groups in the polyethyleneimine and the acyl chloride groups on the trimesoyl chloride groups, resulting in a loose cross-linked network formed by the reaction between polyethyleneimine and trimesoyl chloride, thereby causing the compression resistance of the nanofiltration membrane to decrease. Therefore, the present invention also strictly controls the amount of modified sodium alginate added to not exceed 9.0% of the mass of polyethyleneimine.

[0031] Preferably, in step c), the interfacial polymerization reaction time is 1-5 min.

[0032] Preferably, in step d), the heat curing cross-linking temperature is 50-70° C. and the time is 15-30 min.

[0033] The present invention has the following beneficial effects:

[0034] 1) The composite nanofiltration membrane is prepared by using polyethyleneimine as the aqueous phase monomer and trimesoyl chloride as the organic phase monomer. The two undergo interfacial polymerization on the polyethersulfone-based membrane to obtain a composite nanofiltration membrane. Since the aqueous phase monomer polyethyleneimine is loaded with a large number of amino groups, the amino groups that have not reacted with the acyl chloride groups are protonated in water and are positively charged, so that the nanofiltration membrane is positively charged. The positive charge on the surface of the nanofiltration membrane and the highly charged high-valent cations such as magnesium and calcium will form an electrostatic repulsion force, thereby achieving the interception of magnesium, calcium and other ions, allowing NaCl to pass through the nanofiltration membrane, thereby removing impurity ions such as magnesium and calcium in the brine NaCl;

[0035] 2) By adding sodium alginate to the polyethyleneimine solution, sodium alginate is a linear polymer formed by β-D-mannuronic acid and α-L-guluronic acid connected by 1,4-glycosidic bonds. The molecule contains a large number of strong hydrophilic groups such as hydroxyl and carboxyl groups. Incorporating it into the nanofiltration membrane can greatly improve the hydrophilicity of the nanofiltration membrane, thereby increasing the water flux of the nanofiltration membrane and improving the filtration separation efficiency of the nanofiltration membrane. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0037] Unless otherwise specified, the raw materials used in the examples of the present invention are all commercially available or available to those skilled in the art; unless otherwise specified, the methods used in the examples of the present invention are all methods known to those skilled in the art. Example 1

[0038] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0039] 1) Extracting brine from salt lakes;

[0040] 2) Pre-treat the salt lake brine by centrifugation, control the centrifuge speed at 3500r / min, and collect the clear liquid;

[0041] 3) The clear liquid after pre-treatment filtration is filtered through an ultrafiltration membrane with a pore size of 0.2 μm and an operating pressure of 0.5 MPa, and the clear liquid permeated through the ultrafiltration membrane is collected;

[0042] 4) The clear liquid after the ultrafiltration membrane filtration is filtered through a composite nanofiltration membrane, the operating pressure of the composite nanofiltration membrane is 1.5MPa, and the clear liquid permeated through the nanofiltration membrane is collected;

[0043] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt for ion membrane chlor-alkali production.

[0044] Preparation of composite nanofiltration membrane:

[0045] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving to obtain an oil phase solution with a concentration of 0.5 wt%;

[0046] b) adding polyethyleneimine into water and stirring to dissolve to obtain a polyethyleneimine solution with a concentration of 1.5 wt %, adding modified sodium alginate into the polyethyleneimine solution in an amount of 8.0 wt % of the polyethyleneimine, and dispersing the modified sodium alginate uniformly by ultrasonic oscillation to obtain an aqueous phase solution;

[0047] c) soaking the polyethersulfone-based membrane in an aqueous solution for 10 minutes, taking it out and soaking it in an organic solution for 4 minutes to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0048] d) placing the nascent composite nanofiltration membrane in an oven for heat curing and cross-linking at a temperature of 60° C. for 25 min to obtain a composite nanofiltration membrane.

[0049] Preparation of modified sodium alginate:

[0050] 2 g of epoxy silane coupling agent KH-550 and 10 g of nano-silica were added to 500 mL of deionized water, heated to 50° C., stirred for reaction for 2 h, and centrifuged, washed and dried to obtain epoxy silane coupling agent-modified nano-silica;

[0051] Add 1 g of sodium alginate into 200 mL of water and stir to dissolve to obtain a sodium alginate solution, then add 0.01 g of hydroxybenzenesulfonic acid catalyst and stir evenly, then add 2 g of epoxy silane coupling agent modified nano-silica, heat to 100°C, stir to react for 3 hours, and obtain modified sodium alginate after centrifugal separation, washing and drying. Example 2

[0052] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0053] 1) Extracting brine from salt lakes;

[0054] 2) Pre-treat the salt lake brine by centrifugation, control the centrifuge speed at 3500 r / min, and collect the clear liquid;

[0055] 3) The clear liquid after pre-treatment filtration is filtered through an ultrafiltration membrane with a pore size of 0.2 μm and an operating pressure of 0.3 MPa, and the clear liquid permeated through the ultrafiltration membrane is collected;

[0056] 4) The clear liquid after ultrafiltration is filtered through a composite nanofiltration membrane. The operating pressure of the composite nanofiltration membrane is 1.5 MPa, and the clear liquid permeated through the nanofiltration membrane is collected;

[0057] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt specially used for chlor-alkali production by ion membrane method.

[0058] Preparation of composite nanofiltration membrane:

[0059] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving to obtain an oil phase solution with a concentration of 0.5 wt%;

[0060] b) adding polyethyleneimine into water and stirring to dissolve to obtain a polyethyleneimine solution with a concentration of 1.5 wt %, adding modified sodium alginate into the polyethyleneimine solution, wherein the modified sodium alginate addition amount is 6.5 wt % of the polyethyleneimine, and uniformly dispersing by ultrasonic oscillation to obtain an aqueous phase solution;

[0061] c) soaking the polyethersulfone-based membrane in an aqueous solution for 10 minutes, taking it out and soaking it in an organic solution for 2 minutes to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0062] d) placing the nascent composite nanofiltration membrane in an oven for heat curing and cross-linking at a temperature of 60° C. for 20 min to obtain a composite nanofiltration membrane.

[0063] Preparation of modified sodium alginate:

[0064] 2 g of epoxy silane coupling agent KH-550 and 10 g of nano-silica were added to 500 mL of deionized water, heated to 50° C., stirred for reaction for 2 h, and centrifuged, washed and dried to obtain epoxy silane coupling agent-modified nano-silica;

[0065] Add 1 g of sodium alginate into 200 mL of water and stir to dissolve to obtain a sodium alginate solution, then add 0.01 g of hydroxybenzenesulfonic acid catalyst and stir evenly, then add 2 g of epoxy silane coupling agent modified nano-silica, heat to 100°C, stir to react for 3 hours, and obtain modified sodium alginate after centrifugal separation, washing and drying. Example 3

[0066] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0067] 1) Extracting brine from salt lakes;

[0068] 2) Pre-treat the salt lake brine by centrifugation, control the centrifuge speed at 3500 r / min, and collect the clear liquid;

[0069] 3) The clear liquid after pretreatment filtration is filtered through an ultrafiltration membrane with a pore size of 0.2 μm and an operating pressure of 0.4 MPa, and the clear liquid permeated through the ultrafiltration membrane is collected;

[0070] 4) The clear liquid after ultrafiltration is filtered through a composite nanofiltration membrane. The operating pressure of the composite nanofiltration membrane is 1.5 MPa, and the clear liquid permeated through the nanofiltration membrane is collected;

[0071] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt specially used for chlor-alkali production by ion membrane method.

[0072] Preparation of composite nanofiltration membrane:

[0073] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving to obtain an oil phase solution with a concentration of 0.5 wt%;

[0074] b) adding polyethyleneimine into water and stirring to dissolve to obtain a polyethyleneimine solution with a concentration of 1.5 wt %, adding modified sodium alginate into the polyethyleneimine solution in an amount of 7.0 wt % of the polyethyleneimine, and dispersing the modified sodium alginate uniformly by ultrasonic oscillation to obtain an aqueous phase solution;

[0075] c) soaking the polyethersulfone-based membrane in an aqueous solution for 10 minutes, taking it out and soaking it in an organic solution for 3 minutes to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0076] d) placing the nascent composite nanofiltration membrane in an oven for heat curing and cross-linking at a temperature of 60° C. for 20 min to obtain a composite nanofiltration membrane.

[0077] Preparation of modified sodium alginate:

[0078] 2 g of epoxy silane coupling agent KH-550 and 10 g of nano-silica were added to 500 mL of deionized water, heated to 50° C., stirred for reaction for 2 h, and centrifuged, washed and dried to obtain epoxy silane coupling agent-modified nano-silica;

[0079] Add 1 g of sodium alginate into 200 mL of water and stir to dissolve to obtain a sodium alginate solution, then add 0.01 g of hydroxybenzenesulfonic acid catalyst and stir evenly, then add 2 g of epoxy silane coupling agent modified nano-silica, heat to 100°C, stir to react for 3 hours, and obtain modified sodium alginate after centrifugal separation, washing and drying. Example 4

[0080] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0081] 1) Extracting brine from salt lakes;

[0082] 2) Pre-treat the salt lake brine by centrifugation, control the centrifuge speed at 4000 r / min, and collect the clear liquid;

[0083] 3) The clear liquid after pretreatment filtration is filtered through an ultrafiltration membrane with a pore size of 0.3 μm and an operating pressure of 0.6 MPa, and the clear liquid permeated through the ultrafiltration membrane is collected;

[0084] 4) The clear liquid after the ultrafiltration membrane filtration is filtered through a composite nanofiltration membrane, the operating pressure of the composite nanofiltration membrane is 2.0 MPa, and the clear liquid permeated through the nanofiltration membrane is collected;

[0085] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt specially used for chlor-alkali production by ion membrane method.

[0086] Preparation of composite nanofiltration membrane:

[0087] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving to obtain an oil phase solution with a concentration of 0.5 wt%;

[0088] b) adding polyethyleneimine into water and stirring to dissolve to obtain a polyethyleneimine solution with a concentration of 1.5 wt %, adding modified sodium alginate into the polyethyleneimine solution in an amount of 9.0 wt % of the polyethyleneimine, and dispersing the modified sodium alginate uniformly by ultrasonic oscillation to obtain an aqueous solution;

[0089] c) soaking the polyethersulfone-based membrane in an aqueous solution for 10 minutes, taking it out and soaking it in an organic solution for 5 minutes to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0090] d) placing the nascent composite nanofiltration membrane in an oven for heat curing and cross-linking at a temperature of 70° C. for 30 min to obtain a composite nanofiltration membrane.

[0091] Preparation of modified sodium alginate:

[0092] 2 g of epoxy silane coupling agent KH-550 and 10 g of nano-silica were added to 500 mL of deionized water, heated to 50° C., stirred for reaction for 2 h, and centrifuged, washed and dried to obtain epoxy silane coupling agent-modified nano-silica;

[0093] Add 1 g of sodium alginate into 200 mL of water and stir to dissolve to obtain a sodium alginate solution, then add 0.01 g of hydroxybenzenesulfonic acid catalyst and stir evenly, then add 2 g of epoxy silane coupling agent modified nano-silica, heat to 100°C, stir to react for 3 hours, and obtain modified sodium alginate after centrifugal separation, washing and drying. Example 5

[0094] A method for preparing industrial salt for chlor-alkali production by an ion membrane process comprises the following steps:

[0095] 1) Extracting brine from salt lakes;

[0096] 2) Pre-treat the salt lake brine by centrifugation, control the centrifuge speed at 3000 r / min, and collect the clear liquid;

[0097] 3) The clear liquid after pre-treatment filtration is filtered through an ultrafiltration membrane with a pore size of 0.1 μm and an operating pressure of 0.2 MPa, and the clear liquid permeated through the ultrafiltration membrane is collected;

[0098] 4) The clear liquid after ultrafiltration is filtered through a composite nanofiltration membrane. The operating pressure of the composite nanofiltration membrane is 1.0 MPa, and the clear liquid permeated through the nanofiltration membrane is collected;

[0099] 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt for ion membrane chlor-alkali production.

[0100] Preparation of composite nanofiltration membrane:

[0101] a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving to obtain an oil phase solution with a concentration of 0.5 wt%;

[0102] b) adding polyethyleneimine into water and stirring to dissolve to obtain a polyethyleneimine solution with a concentration of 1.5 wt %, adding modified sodium alginate into the polyethyleneimine solution in an amount of 6.0 wt % of the polyethyleneimine, and dispersing the modified sodium alginate uniformly by ultrasonic oscillation to obtain an aqueous solution;

[0103] c) soaking the polyethersulfone-based membrane in an aqueous solution for 10 minutes, taking it out and soaking it in an organic solution for 1 minute to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane;

[0104] d) placing the nascent composite nanofiltration membrane in an oven for heat curing and cross-linking at a temperature of 50° C. for 15 min to obtain a composite nanofiltration membrane.

[0105] Preparation of modified sodium alginate:

[0106] 2 g of epoxy silane coupling agent KH-550 and 10 g of nano-silica were added to 500 mL of deionized water, heated to 50° C., stirred for reaction for 2 h, and centrifuged, washed and dried to obtain epoxy silane coupling agent-modified nano-silica;

[0107] Add 1 g of sodium alginate into 200 mL of water and stir to dissolve to obtain a sodium alginate solution, then add 0.01 g of hydroxybenzenesulfonic acid catalyst and stir evenly, then add 2 g of epoxy silane coupling agent modified nano-silica, heat to 100°C, stir to react for 3 hours, and obtain modified sodium alginate after centrifugal separation, washing and drying. Comparative Example 1

[0108] The difference between Comparative Example 1 and Example 1 is that:

[0109] Sodium alginate was not modified during the preparation of the composite nanofiltration membrane;

[0110] The remaining steps are the same as those in Example 1. Comparative Example 2

[0111] The difference between Comparative Example 2 and Example 5 is:

[0112] The amount of modified sodium alginate added was 5.0wt% of polyethyleneimine;

[0113] The remaining steps are the same as those in Example 5. Comparative Example 3

[0114] The difference between Comparative Example 3 and Example 4 is that:

[0115] The amount of modified sodium alginate added was 10wt% of polyethyleneimine;

[0116] The remaining steps are the same as those in Example 4. Comparative Example 4

[0117] The difference between Comparative Example 4 and Example 4 is that:

[0118] The amount of modified sodium alginate added was 11wt% of polyethyleneimine;

[0119] The remaining steps are the same as those in Example 4. Comparative Example 5

[0120] The difference between Comparative Example 5 and Example 1 is that:

[0121] No sodium alginate was added during the preparation of the composite nanofiltration membrane;

[0122] The remaining steps are the same as those in Example 1.

[0123] Performance Testing

[0124] 1. The industrial salts prepared in the examples and comparative examples were subjected to determination of calcium and magnesium ion impurity components. The calcium and magnesium ion contents were determined in accordance with GB / T13025.6-91.

[0125] 2. Install the composite nanofiltration membranes prepared in the examples and comparative examples into a membrane performance evaluation device to measure the separation performance of the membranes. The water flux and the retention rate are calculated as follows:

[0126] J = V / A × t;

[0127] Where J is the water flux (L / m 2 h); V is the permeate volume (L); A is the membrane effective area (m 2 );t is the penetration time.

[0128] R=(1-C p / C f )×100%;

[0129] Where, R is the ion retention rate (%); C p is the mass concentration of the permeate (g / L); C f is the mass concentration of the feed solution (g / L).

[0130] A magnesium sulfate solution with a concentration of 1 g / L was prepared. Under the conditions of an operating pressure of 1 MPa and an operating temperature of 25°C, after running for 30 min, the magnesium sulfate retention rate and pure water flux of the nanofiltration membrane were tested respectively.

[0131]

[0132] 3. Install the composite nanofiltration membranes prepared in Examples 1-5 and Comparative Examples 3-4 into a membrane performance evaluation device, prepare a magnesium sulfate solution with a concentration of 1 g / L, operate under a high pressure of 2 MPa for 3 days, and then test the retention rate of the composite nanofiltration membrane for magnesium sulfate.

[0133]

[0134] After running for 3 days under high pressure of 2MPa, the retention rate of magnesium sulfate by the composite nanofiltration membranes of Examples 1-5 remained basically unchanged, while the retention rate of magnesium sulfate by Comparative Examples 3 and 4 dropped significantly, indicating that the composite nanofiltration membranes of the comparative examples were damaged under high pressure operation conditions, and the pressure resistance of the composite nanofiltration membranes of Comparative Examples 3 and 4 was lower than that of the composite nanofiltration membranes of Examples 1-5.

[0135] The above are only preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be regarded as limiting the present invention, and the protection scope of the present invention should be based on the scope defined by the claims. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing industrial salt for chlor-alkali production by ion membrane method, characterized in that: The following steps are involved: 1) Extracting brine; 2) Pre-treating the brine by centrifugation and collecting the clear liquid; 3) filtering the clear liquid after pretreatment filtration through an ultrafiltration membrane, and collecting the ultrafiltration membrane clear liquid; 4) Filter the clear liquid after ultrafiltration through a composite nanofiltration membrane, and collect the clear liquid from the nanofiltration membrane; The preparation method of the composite nanofiltration membrane comprises the following steps: a) adding trimesoyl chloride into n-hexane solvent, stirring and dissolving, to obtain an oil phase solution; b) adding polyethyleneimine into water, stirring to dissolve, obtaining a polyethyleneimine solution, adding modified sodium alginate into the polyethyleneimine solution, and dispersing it evenly by ultrasonic oscillation, obtaining an aqueous phase solution; The preparation method of modified sodium alginate comprises the following steps: The epoxy silane coupling agent KH-550 and nano-silica are added into deionized water, heated and stirred for reaction, and then centrifuged, washed and dried to obtain epoxy silane coupling agent modified nano-silica; Adding p-hydroxybenzenesulfonic acid catalyst to sodium alginate solution, stirring evenly, then adding epoxy silane coupling agent to modify nano-silica, heating and stirring to react, centrifugation, washing and drying to obtain modified sodium alginate; c) soaking the polyethersulfone-based membrane in an aqueous solution, taking it out and soaking it in an organic solution to perform an interfacial polymerization reaction, thereby obtaining a primary composite nanofiltration membrane; d) placing the nascent composite nanofiltration membrane in an oven for thermal curing and cross-linking to obtain a composite nanofiltration membrane; 5) Evaporate the nanofiltration clear liquid and dry the salt to obtain industrial salt specially used for chlor-alkali production by ion membrane method.

2. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In the step 2), the speed of the centrifuge is controlled at 3000-4000 r / min.

3. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: The pore size of the ultrafiltration membrane in step 3) is 0.1-0.3 μm.

4. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In step 3), the operating pressure of the ultrafiltration membrane is 0.2-0.6 MPa.

5. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In step 4), the operating pressure of the nanofiltration membrane is 1.0-2.0 MPa.

6. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In the step b), the amount of modified sodium alginate added is 6.0-9.0 wt % of the polyethyleneimine.

7. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In the step c), the interfacial polymerization reaction time is 1-5 minutes.

8. The method for preparing industrial salt for chlor-alkali production by ion membrane method according to claim 1, characterized in that: In the step d), the heat curing cross-linking temperature is 50-70° C. and the time is 15-30 minutes.

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