A method for purifying an ammonium chloride solution

Abstract

The present application belongs to the technical field of inorganic salt separation and purification, and discloses a method for purifying ammonium chloride solution, aiming to solve the technical problem that the prior art cannot effectively separate ammonium ions and sodium ions. The method comprises: S1, purifying agricultural-grade ammonium chloride solution containing sodium ions through a first membrane system to obtain a first permeate and a first retentate, wherein the separation layer surface of the first special membrane is electronegative; S2, secondly purifying the first permeate through a second membrane system; and S3, obtaining product ammonium chloride solution and reusable fresh water through a product recovery system from the second permeate. The present application utilizes the electronegativity of the membrane surface and the differential interface interaction of different monovalent cations to realize the preferential permeation of ammonium ions and the effective retention of sodium ions. The method does not need to adjust the pH value or heat the raw material solution, has a simple process, low energy consumption, and can obtain ammonium chloride products with a purity of more than 99.5%.

Description

TECHNICAL FIELD

[0001] The application relates to a method for purifying an ammonium chloride solution and belongs to the technical field of inorganic salt separation and purification. BACKGROUND

[0002] A large amount of salt wastewater is generated in industrial production. Generally, ammonium ions, sodium ions and chloride ions exist in the wastewater at the same time. If the ammonium ions and the sodium ions cannot be separated, the wastewater can only be treated as industrial solid waste. On the other hand, agricultural ammonium chloride contains impurity ions such as sodium ions and potassium ions. Enterprises usually remove the impurity ions to obtain high-purity ammonium chloride for use in the electroplating industry and the pharmaceutical industry.

[0003] In order to ensure the quality of ammonium chloride products, the ammonium chloride is refined by using a rinsing method and a recrystallization method in industrial production. The rinsing method needs to be washed for multiple times. In order to reduce the content of sodium, spraying water is needed. In order to stabilize the purity of ammonium chloride, barium chloride is added to introduce barium ions and heavy metals. The process is complicated and difficult to operate. The production efficiency is low by using the recrystallization method. Chinese patent 201310590527.X proposes a method for producing high-purity ammonium chloride by using a multi-stage flash evaporation and continuous crystallization. The purity of the ammonium chloride obtained is greater than or equal to 99.5%. However, the solution needs to be heated to 100-105 DEG C, and more than three evaporators are needed for flash evaporation. The required steam is expensive. The concentration of chloride ions in the ammonium chloride is high. The corrosion resistance requirement of the equipment is extremely high, which leads to high equipment cost.

[0004] At present, the membrane elements for commercial production and industrial application are mainly seawater reverse osmosis membranes and nanofiltration membranes. Since the compactness of the membrane, the retention rates of ammonium ions and sodium ions are the same, so the ammonium ions and the sodium ions cannot be separated. The nanofiltration membranes are mostly used to separate high-valence ions and low-valence ions. Similarly, the monovalent ammonium ions and the sodium ions cannot be separated. SUMMARY

[0005] The application provides a method for purifying an ammonium chloride solution, which mainly aims to solve the technical problems that the energy consumption is high and the process is long by using the recrystallization method in the prior art, and the conventional membrane separation technology cannot effectively separate the two kinds of monovalent ions of ammonium ions and sodium ions, and provides a method for purifying an ammonium chloride solution, which has the advantages of simple process, low energy consumption and good separation effect.

[0006] In order to achieve the above-mentioned purpose, the application provides a method for purifying an ammonium chloride solution, which comprises the following steps:

[0007] S1, purifying agricultural grade ammonium chloride solution containing sodium ions through a first membrane system to obtain a first permeate and a first retentate; wherein a first special membrane in the first membrane system has an electronegative surface on the separation layer.

[0008] S2, the first permeate is subjected to a secondary purification through a secondary membrane system to obtain a second permeate and a second retentate;

[0009] S3, the second permeate is subjected to a product recovery system to obtain a product ammonium chloride solution and reusable fresh water;

[0010] In step S1, the electronegativity of the surface of the primary special membrane separation layer is used to establish differential interfacial interactions with ammonium ions and sodium ions, and the separation mechanism is that the electronegative surface forms a weak hydrogen bond with the ammonium ions and uses the high polarizability effect of the ammonium ions to establish preferential permeation of the ammonium ions; at the same time, the electronegative surface produces strong electrostatic adsorption to the sodium ions as hard acid ions, thereby achieving the retention of sodium ions.

[0011] Preferably, in step S1, the mass concentration of the agricultural grade ammonium chloride solution is not less than 6%, and the pH value is 5 to 6; and the agricultural grade ammonium chloride solution does not need to be adjusted in pH value and heated before entering the primary membrane system.

[0012] Preferably, the operating pressure of step S1 is 60 bar; and the purity of ammonium chloride in the first permeate reaches 98%.

[0013] Preferably, the secondary membrane system uses a secondary special membrane of the same type as the primary special membrane, and the operating pressure is 50 bar.

[0014] Preferably, the first retentate is reused to the main process section; and the second retentate is mixed with the agricultural grade ammonium chloride solution and then enters the primary membrane system for purification again.

[0015] Preferably, the product recovery system in step S3 is a reverse osmosis membrane system.

[0016] Preferably, the obtained product ammonium chloride solution has a purity of more than 99.5%; and the obtained reusable fresh water has a total dissolved solids (TDS) content of less than 300 mg / L.

[0017] Compared with the prior art, the present application has the following advantages: 1. The present application provides an ammonium chloride solution purification method, which uses a special membrane to purify ammonium chloride. The agricultural grade ammonium chloride solution (purity of 90-95%) does not need to be heated before entering the membrane, and the salt separation effect is good, which can realize the purification of ammonium chloride with a purity of more than 99.5%, and at the same time, fresh water is recovered, the reagent consumption is reduced, the process is short, the energy consumption is low, the operation is simple, and the method has considerable economic and social benefits, has a large-scale application prospect, and through the backmixing of the second retentate, the raw material utilization rate and the overall recovery efficiency of the system are further improved.

[0018] 2. By constructing a membrane separation interface with differentiated interactions for different monovalent cations, a novel ion separation pathway was established. This pathway does not rely on differences in ion size or charge number, but rather stems from the synergistic effect between the specific electronegativity of the membrane surface and the inherent physicochemical properties of ammonium and sodium ions. For ammonium ions, which have a unique structure and high polarizability, the interface provides an effective transmembrane transport channel through weak hydrogen bonding and electrostatic shielding. For sodium ions, which have a high charge density, the same interface effectively anchors and retains them through strong electrostatic adsorption. This separation method enables the selective separation of homovalent cations, which poses a challenge to conventional membrane technologies, to be carried out under mild physical conditions, avoiding high-energy-consuming and high-corrosion-risk pretreatment procedures such as heating or acid-base adjustment of the entire solution system to promote separation.

[0019] 3. The use of the same special membrane in both the primary and secondary membrane purification systems ensures that both systems follow the same ion-selective separation mechanism. This guarantees that the concentrated solution D produced by the secondary membrane system maintains the same ionic composition as the initial agricultural-grade ammonium chloride solution. This inherent homogeneity in the process allows concentrated solution D to be directly returned to the inlet of the primary membrane system without any chemical adjustment or additional purification, naturally merging with the new feed solution. This design transforms a potentially complex wastewater flow into a seamless internal circulating material flow. Without increasing system complexity or control burden, it improves the overall utilization efficiency of the feedstock and water recovery rate. Furthermore, the preferential retention of sodium ions by the first two membrane systems ensures that the solution C entering the final product recovery system is primarily composed of high-purity ammonium chloride. Based on this, the conventional reverse osmosis membrane used in the product recovery system no longer performs difficult ion-selective separation but returns to its most fundamental and stable function: efficient and indiscriminate retention of all ions, allowing only water molecules to pass through. This approach, which assigns selective separation and non-selective concentration to membrane elements with different mechanisms and optimal suitability for different stages, not only ensures the high purity of the final product and the high quality of the recycled freshwater, but also ensures that each link in the entire process operates within its most efficient and reliable operating range, thus forming a logically clear and robust overall purification solution. Attached Figure Description

[0020] Figure 1 A schematic diagram of the process flow for a method of purifying ammonium chloride solution provided in an embodiment of the present invention;

[0021] Figure 2 This is a graph showing the changes in the concentration of the main ions during the purification process of this invention.

[0022] Figure 3The detailed process and separation principle diagram of the purification method of the present application are shown.

[0023] The object implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0024] The present application is further described below. In the embodiments, the purity of ammonium chloride is calculated as follows: ammonium chloride purity = (ammonium ion concentration x ammonium chloride relative molecular mass / ammonium ion relative molecular mass) / (ammonium ion concentration x ammonium chloride relative molecular mass / ammonium ion relative molecular mass + sodium ion concentration x sodium chloride relative molecular mass / sodium ion relative atomic mass) x 100%. However, the following examples are only simple examples of the present application and do not represent or limit the protection scope of the present application.

[0025] The present embodiment provides a purification method of an ammonium chloride solution, which comprises the following steps:

[0026] Table 1 Composition of a 90% purity ammonium chloride solution

[0027] Component Sodium ion Ammonium ion Chloride ion Mass concentration Content, mg / L 2300 19000 41000 6.2%

[0028] S1, primary membrane system purification: a 90% purity ammonium chloride solution, the composition of which is shown in Table 1. After purification by the primary membrane system, solution A (i.e. the first retentate) and solution B (i.e. the first permeate) are obtained; solution A is reused to the main process section. The composition of solution B is shown in Table 2, the purity of ammonium chloride is 98%, the operating pressure is 60 bar, and the temperature is room temperature.

[0029] Table 2 Composition of solution B

[0030] Component Sodium ion Ammonium ion Chloride ion Mass concentration Content, mg / L 262 14377 28599 4.4%

[0031] S2, secondary membrane system purification: solution B is directly purified by the secondary membrane system without treatment, to obtain solution C (i.e. the second permeate) and solution D (i.e. the second retentate); solution D is mixed with an agricultural grade ammonium chloride solution (purity of 90-95%) and continues to be purified by the primary membrane system, the operating pressure is 50 bar, and the temperature is room temperature.

[0032] Table 3 Composition of solution D

[0033] Component Sodium ion Ammonium ion Chloride ion Mass concentration Content, mg / L 552 19539 39167 6.2%

[0034] S3, product recovery system: solution C is directly subjected to the product recovery system without treatment, to obtain product solution E and solution F; the purity of ammonium chloride in product solution E is 99.6%, and the TDS of solution F is ≤300 mg / L. The operating pressure is 65 bar, and the temperature is room temperature.

[0035] Table 4 Product solution E ingredient composition.

[0036] Component Sodium ion Ammonium ion Chloride ion Mass concentration Content, mg / L 104 21225 41788 6.2%

[0037] Table 5 Solution F ingredient composition.

[0038] Component Sodium ion Ammonium ion Chloride ion TDS Content, mg / L 0.26 53.2 104 157.4

[0039] The core of the method of the present application is that the primary special membrane used in step S1 and the secondary special membrane of the same kind used in step S2 can achieve high-efficiency selective separation of ammonium ions and sodium ions, both of which are monovalent cations. The fundamental reason for this selective separation function is that the separation layer surface of the special membrane exhibits a specific electronegativity, which is derived from the negative charge functional groups inherent in the polymer material constituting the membrane separation layer or introduced by later modification. This unique surface property enables the membrane to establish two different interface interaction mechanisms with the two ions in terms of strength and nature.

[0040] Specifically, in the primary membrane system purification process of step S1, when the agricultural-grade ammonium chloride solution containing sodium ions contacts the surface of the special membrane under pressure driving, the following two different ion-membrane interface interaction mechanisms are simultaneously established and continuously occur:

[0041] The preferential transport mechanism for ammonium ions: Ammonium ions are not simple spherical ions. Their unique tetrahedral structure makes the hydrogen atoms connected to the nitrogen atom have a certain positive charge, which can act as a hydrogen bond donor. When ammonium ions approach the negatively charged membrane surface, in addition to the basic electrostatic interaction, a weak hydrogen bond is formed between the hydrogen atoms of the ammonium ions and the negatively charged atoms on the membrane surface, which has a fast binding and dissociation kinetics. At the same time, the high polarizability of ammonium ions makes their electron cloud easily deformed under the action of the membrane surface electric field, thereby instantaneously shielding the electrostatic repulsion of the membrane surface negative charge to a certain extent. The synergistic effect of these two effects provides an effective, low-energy barrier transport path for ammonium ions, allowing them to preferentially permeate the membrane at a higher rate and enter the first permeate.

[0042] The high-efficiency retention mechanism for sodium ions: Sodium ions are typical hard acid ions with small ionic radius, high charge density, and low polarizability. According to the principle of strong binding between hard acid and hard base, sodium ions will form a stable ion pair with the negatively charged functional group sites on the membrane surface, which has high binding energy and slow dissociation rate. This strong, non-specific ion-electrostatic adsorption effectively anchors sodium ions on the membrane surface or at the entrance of the membrane pores, greatly increasing the energy barrier of their transmembrane migration and resulting in a very low membrane permeation rate, thereby being efficiently retained and enriched in the first retentate.

[0043] Therefore, the essence of the separation of the present application is to use the electronegativity of the membrane surface to promote the transmission of ions by establishing weak hydrogen bonds and polarizing shielding, and to cause the interception of the two different interface interactions of strong static adsorption, so as to realize the differential control of the ion transmission rate across the membrane.

[0044] Based on the above principle, in this embodiment, without any pretreatment of the agricultural grade ammonium chloride solution with a pH value of 5 to 6 and a purity of 90% to 95%, the first permeate liquid with a purity of ammonium chloride significantly improved to 98% can be obtained at room temperature and an operating pressure of 60 bar. Subsequently, the first permeate liquid is further purified by a secondary membrane system with the same mechanism at a pressure of 50 bar in step S2. Finally, in step S3, the obtained second permeate liquid enters a conventional reverse osmosis membrane system, which uses the dense physical structure of the reverse osmosis membrane to intercept all ions, thereby obtaining a product ammonium chloride solution with a purity of up to 99.5% or more and reusable fresh water with a TDS of less than 300 mg / L. The whole process has a short process flow, low energy consumption and no consumption of chemical agents, showing significant economic and social benefits. In this embodiment, a special concentration membrane of CCT-8040-RC type from Guangdong Osbo Membrane Material Technology Co., Ltd. is used to stably achieve the above technical effects.

[0045] Based on the above technical principle, the present application provides a specific purification method, which combines Figures 1 to 3 , a kind of ammonium chloride solution purification method is described, as Figure 1 Indicated, the agricultural grade ammonium chloride solution with a purity of 90%-95% first enters the first membrane system for purification, the first retention liquid generated, i.e., solution A returns to the main process section, and the first permeate liquid, i.e., solution B, enters the secondary membrane system for purification. The second retention liquid generated by the secondary membrane system purification, i.e., solution D, is mixed with the initial agricultural grade ammonium chloride solution and then enters the first membrane system for purification again, and the second permeate liquid, i.e., solution C, enters the product recovery system, which finally produces product solutions E and F; as Figure 2 Indicated, the concentration of ammonium ion, sodium ion and chloride ion in the solution at the four key stages of raw material liquid, first membrane permeate liquid, second membrane permeate liquid and final product is shown, wherein the vertical coordinate is the concentration, and the unit is mg / L; as Figure 3As shown, the agricultural grade ammonium chloride solution enters the primary membrane system for purification as a sodium-containing raw material solution. The system uses the electronegativity of the special membrane surface to preferentially permeate ammonium ions, producing a first retentate solution A that returns to the main process section. The first permeate solution B enters the secondary membrane system for purification. The secondary membrane system uses the same special membrane to further improve the purity. The second retentate solution D produced by the secondary membrane system returns to the inlet of the primary membrane system. The second permeate solution C enters the product recovery system, which uses a reverse osmosis membrane to concentrate the product and recover fresh water. Finally, a product ammonium chloride solution with a purity of ≥ 99.5% is obtained as product solution E, and a reusable fresh water solution F with a TDS content of ≤ 300 mg / L is obtained.

[0046] Comparative Example 1

[0047] This comparative example provides a method for treating an ammonium chloride-containing solution using a conventional membrane. The difference compared to the example is that a conventional seawater reverse osmosis membrane without selective separation ability for monovalent ions is used instead of the special membrane in the example. The specific steps and operating parameters are as follows:

[0048] S1, primary membrane system treatment: The 90% purity ammonium chloride solution with the same composition as in Table 1 of the example is treated by a primary conventional seawater reverse osmosis membrane system. The operating pressure is 80 bar and the temperature is room temperature. Since the membrane has a very high retention rate for both ammonium ions and sodium ions, it cannot achieve selective separation. The main experimental data are shown in Table 6.

[0049] Table 6 Experimental data of Comparative Example 1

[0050] Solution name Sodium ion (mg / L) Ammonium ion (mg / L) Chloride ion (mg / L) Mass concentration Purity of ammonium chloride Raw material solution (same as Table 1) 2300 19000 41000 6.2% 90.0% First permeate solution (solution B) 23 190 410 0.06% / First retentate solution (solution A) 2869 23702 51147 7.8% 90.0%

[0051] As can be seen from the data in Table 6, after treatment with a conventional seawater reverse osmosis membrane, the first permeate (solution B) has a purity of 90.0% ammonium chloride, which has not improved compared to the raw material solution, and the ion concentration of the permeate is extremely low, losing its value as a product for further purification. At the same time, in the first retentate (solution A), although the ion concentration has increased, the purity of ammonium chloride is also 90.0%, indicating that the membrane almost has no difference in retention between ammonium ions and sodium ions, and completely lacks the ability of selective separation.

[0052] The results of this comparative example clearly show that even under the same operating conditions, using conventional membrane elements in the art cannot achieve effective separation of ammonium ions and sodium ions, and cannot achieve the purpose of the present application.

[0053] It is obvious to those skilled in the art that the present application is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application.

[0054] Finally, it should be noted that the above examples are merely intended to illustrate the technical solutions of the present application and not to limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for purifying ammonium chloride solution, characterized in that, Includes the following steps: S1. The agricultural-grade ammonium chloride solution containing sodium ions is purified through a primary membrane system to obtain a first permeate and a first retentate; wherein, the primary special membrane in the primary membrane system has an electronegative separation layer surface. S2. The first permeate is purified a second time through a two-stage membrane system to obtain the second permeate and the second retentate. S3. Pass the second permeate through the product recovery system to obtain ammonium chloride solution and reusable fresh water; In step S1, the electronegativity of the surface of the primary special membrane separation layer is used to establish differentiated interfacial interactions with ammonium ions and sodium ions. The separation mechanism is as follows: weak hydrogen bonds are formed between the electronegative surface and ammonium ions, and the high polarizability of ammonium ions is utilized to establish preferential permeation of ammonium ions; at the same time, the electronegative surface generates strong electrostatic adsorption for sodium ions, which are hard acid ions, thereby achieving the retention of sodium ions. Furthermore, the secondary membrane system uses a secondary special membrane of the same type as the primary special membrane; the product recovery system in step S3 is a reverse osmosis membrane system.

2. The method for purifying ammonium chloride solution according to claim 1, characterized in that, In step S1, the agricultural-grade ammonium chloride solution has a mass concentration of not less than 6% and a pH value of 5 to 6; and there is no need to adjust the pH value or heat the agricultural-grade ammonium chloride solution before it enters the primary membrane system.

3. The method for purifying ammonium chloride solution according to claim 1, characterized in that, The operating pressure in step S1 is 60 bar; the purity of ammonium chloride in the first permeate reaches 98%.

4. The method for purifying ammonium chloride solution according to claim 1, characterized in that, The operating pressure in step S2 is 50 bar.

5. The method for purifying ammonium chloride solution according to claim 1, characterized in that, The first retentate is recycled to the main process section; the second retentate is mixed with the agricultural-grade ammonium chloride solution and then re-enters the primary membrane system for purification.

6. The method for purifying ammonium chloride solution according to claim 1, characterized in that, The obtained ammonium chloride solution has a purity of over 99.5%; the obtained reusable fresh water has a total dissolved solids (TDS) content of less than 300 mg / L.

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