Short-process salt and nitrate separation process

Abstract

The invention belongs to the technical field of salt separation of high-concentration brine, and particularly relates to a short-process salt-nitrate separation process. The short-process salt and nitrate separation process comprises the following steps: sodium chloride side incoming water enters a sodium chloride evaporator, sodium sulfate side incoming water enters a sodium sulfate evaporator, regularly discharged mother liquor from the sodium chloride evaporator and the sodium sulfate evaporator is converged and enters a salt mixing evaporator, and mother liquor generated by the salt mixing evaporator is subjected to mother liquor curing treatment; the mixed salt produced by the mixed salt evaporator is dissolved after water is added, the dissolved water solution of the mixed salt enters the freezing crystallizer to be frozen, the freezing crystallizer produces two parts, one part is mirabilite, mirabilite and water from the sodium sulfate side are mixed and enter the sodium sulfate evaporator to be circulated, and the other part is frozen mother liquor. Freezing mother liquor and sodium chloride side incoming water are mixed and enter a sodium chloride evaporator for circulation; the sodium sulfate evaporator obtains a sodium sulfate dried product, and the sodium chloride evaporator obtains a sodium chloride dried product. The invention provides a short-process salt and nitrate separation process, which is low in investment, low in operation energy consumption, stable in system and relatively high in industrial cost performance.

Description

Technical Field

[0001] The invention belongs to the technical field of high-concentration brine salt separation, and specifically relates to a short-process salt-nitrate separation process. Background Art

[0002] At present, the requirements for wastewater treatment are gradually increasing, from the original harmless treatment to resource utilization and zero discharge. Especially in the fields of coal chemical industry, coking wastewater, mining, papermaking, metallurgy, etc., the wastewater discharge is large, the water quality is complex, contains a large number of organic pollutants, and may contain toxic pollutants such as biphenyl and pyrifos. Zero discharge of saline wastewater can effectively protect the ecological environment and avoid water and groundwater pollution. Common high-concentration brine salt separation technology can effectively solve the problem of impure salt disposal, separate and recycle sodium chloride and sodium sulfate to form products, and the water meets the relevant standards for discharge or recycling, realizing the resource utilization of mixed salt. There are two different process routes for salt separation and crystallization of saline wastewater, one is the membrane salt separation crystallization process, and the other is the thermal salt separation crystallization process. Because the water quality of each project is different, the actual process route of salt separation crystallization is different. Based on the multi-component water-salt system phase diagram, each unit is combined and sorted as needed, and the most suitable process route needs to be selected according to specific conditions.

[0003] Combined salt separation is a multi-stage salt separation process that combines nanofiltration (NF) salt separation process with thermal salt separation process and cold salt separation process. The combined salt separation process utilizes the advantages of various different salt separation processes and shows unprecedented advantages. The combined salt separation process will make great progress in the salt separation technology of high-concentration brine. However, in the current mainstream combined salt separation two-stage salt nitrate separation project, there are some problems that need to be solved by better process routes.

[0004] The current mainstream phase diagram salt-nitrate separation process relies on two to three stages of nanofiltration at the front end to reduce the sulfate in the incoming water on the sodium chloride side as much as possible to ensure that the sodium chloride evaporator can stably and continuously produce sodium chloride byproduct salt. However, in actual engineering, as a membrane device, the divalent ion retention rate of NF in a single cycle is not always high, and the large fluctuation of incoming water will greatly affect the purity of sodium chloride.

[0005] In order to maintain the TOC and whiteness of the by-product salt, the current mainstream two-stage salt separation process cannot evaporate and crystallize the water from the sodium chloride side to the eutectic point, so a large amount of mother liquor needs to be discharged. This part of the mother liquor evaporates and crystallizes crude salt. The amount of crude salt is large. In order to maintain the resource rate, the crude salt needs to be dissolved back to the front end of the NF system and rely on NF to separate sodium chloride and sodium sulfate. This practice increases the workload and risk of the NF system, and is also limited by the scale of NF in some transformation projects. In addition to the above problems, the current mainstream concentration and freezing process also has the problem that the scale of individual subsystems is very small and there is no cost-effectiveness. Usually, the process route of concentration and freezing requires sodium sulfate concentration evaporator, mirabilite freezing crystallizer, sodium sulfate melting crystallizer, sodium chloride evaporation crystallizer, and mixed salt evaporator, which are composed of five main subsystems. Among them, the scale of sodium sulfate melting crystallizer and mixed salt evaporator is very small, does not have industrial cost-effectiveness, and increases investment. The current mainstream concentration and freezing process cannot ensure that the freezing point of the freezing crystallization system is low enough, and it is often necessary to raise the freezing crystallization temperature to control the absence of ice phase precipitation in the system.

[0006] CN215924417U discloses a separation and refining production system for a mixed waste salt solution of sodium chloride and sodium sulfate. After pre-treatment and impurity removal, the maximum concentration difference is that the concentration of sodium chloride is four times that of sodium sulfate, or the concentration of sodium sulfate is three times that of sodium chloride. Within a large range of the concentration difference between sodium chloride and sodium sulfate, sodium chloride and sodium sulfate can be separated and both can achieve the purpose of industrial refined salt quality for secondary utilization. The separation of sodium chloride and sodium sulfate is achieved by selectively utilizing a combination of evaporative crystallization and frozen crystallization to produce qualified sodium chloride and sodium sulfate products that meet the needs of industrial production recycling. A direct freezing method is adopted, which requires a larger freezing scale, and the system is prone to clogging due to its high freezing point, resulting in unstable operation.

[0007] CN110342544A discloses a salt resource recovery system and method for high-concentration organic wastewater. The treatment system includes a raw water pool, an organic membrane concentration unit, an evaporation concentration unit, a sodium chloride evaporator, a mixed salt evaporator and a mixed salt crystallizer; the salt resource recovery method includes membrane concentration, sodium sulfate evaporation crystallization, freezing crystallization, sodium chloride evaporation crystallization, and mixed salt evaporation crystallization. To achieve resource utilization of high-concentration organic wastewater, this technical route treats a single stream of incoming water. The resource rate that can be achieved in actual production is greatly affected by the salt-nitrate ratio of the incoming material, and its applicability is general. In pursuit of resource rate, the evaporator concentration ratio must be increased, and the quality of salt production cannot be maintained. Summary of the invention

[0008] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a short-process salt-nitrate separation process with low investment, low operating energy consumption, stable system and high industrial cost performance.

[0009] The short-process salt-nitrate separation process of the present invention comprises the following steps: water from the sodium chloride side enters a sodium chloride evaporator, water from the sodium sulfate side enters a sodium sulfate evaporator, fixed-discharge mother liquors from the sodium chloride evaporator and the sodium sulfate evaporator are combined and enter a mixed salt evaporator, mother liquor generated by the mixed salt evaporator is solidified, mixed salt generated by the mixed salt evaporator is dissolved by adding water, the dissolved aqueous solution of the mixed salt enters a freezing crystallizer for freezing, the freezing crystallizer outputs two parts, one part is sodium sulfate, the sodium sulfate is mixed with water from the sodium sulfate side and enters the sodium sulfate evaporator for reciprocating circulation, and the other part is frozen mother liquor, the frozen mother liquor is mixed with water from the sodium chloride side and enters the sodium chloride evaporator for reciprocating circulation; the above sodium sulfate evaporator obtains a sodium sulfate dry product, and the sodium chloride evaporator obtains a sodium chloride dry product.

[0010] The total salt content of the water from the sodium chloride side is 3wt% to 25wt%, and the remainder is water; the total salt content includes 85%-95% sodium chloride, 5%-15% sodium sulfate, and the remainder is inevitable impurities according to mass percentage.

[0011] The COD of the water from the sodium chloride side is 1-200 mg / L.

[0012] The total salt content of the water from the sodium sulfate side is 5wt% to 15wt%, and the balance is water; the total salt content includes 5%-25% sodium chloride, 75%-95% sodium sulfate, and the balance is inevitable impurities according to mass percentage.

[0013] The COD of the water from the sodium sulfate side is 1-500 mg / L.

[0014] The sodium sulfate evaporator and mixed salt evaporator use LWLZ model centrifuge for salt separation.

[0015] The RO concentrated water of the present invention is water from the sodium chloride side, and the NF concentrated water is water from the sodium sulfate side, and the two enter the sodium chloride evaporator and the sodium sulfate evaporator respectively. The mother liquors of the sodium chloride evaporator and the sodium sulfate evaporator are combined and then enter the mixed salt evaporator. The mother liquor produced by the mixed salt evaporator is subjected to mother liquor solidification treatment, and the mixed salt produced by the mixed salt evaporator is dissolved by adding water and then enters the freezing crystallizer for freezing. The mirabilite produced by the freezing crystallizer is dissolved by NF concentrated water and then enters the sodium sulfate evaporator together, and the frozen mother liquor is combined with RO concentrated water and enters the sodium chloride evaporator. The mother liquor of the mixed salt evaporator is discharged to the mother liquor solidification unit. The sodium chloride salt and the sodium sulfate salt enter their respective drying and packaging units.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The short-process salt-nitrate separation process of the present invention is equipped with a salt mixing and mixed salt redissolving freezing system at the back end. The high sulfate content in the water from the sodium chloride side does not affect the final sodium chloride recovery rate. In addition, since the concentration ratio of the sodium chloride evaporator can be lower than that of the traditional process, the quality of the salt produced is also better. Therefore, the sulfate content in the water from the sodium chloride side is low.

[0018] (2) The short-process salt-nitrate separation process of the present invention does not rely on the re-dissolution of mixed salt to ensure the resource rate, and no longer re-dissolves to the front end of the NF, but is cooled with water, and self-circulates in the crystallization system, thereby reducing the investment in the NF.

[0019] (3) The short-process salt-nitrate separation process of the present invention only requires four main subsystems, namely, a sodium chloride evaporator, a sodium sulfate evaporator, a mixed salt evaporator and a freezing crystallizer, and the scales of the four subsystems are relatively appropriate and have industrial cost-effectiveness.

[0020] (4) The short-process salt-nitrate separation process of the present invention uses a frozen crystallizer feed of dissolved water whose main component is sodium chloride with adjustable concentration, which has a lower freezing point, and the incoming material temperature is also lower than that of the traditional process, and the material amount is also lower than that of the traditional process, which is more energy-saving. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of a short-process salt-nitrate separation process.

[0022] Figure 2 Schematic diagram of the traditional salt-nitrate separation process of Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with specific embodiments.

[0024] The process diagram used in the following embodiments is as follows: Figure 1 As shown, the short-process salt-salt separation process is as follows: water from the sodium chloride side enters the sodium chloride evaporator, water from the sodium sulfate side enters the sodium sulfate evaporator, the fixed-discharge mother liquors from the sodium chloride evaporator and the sodium sulfate evaporator are combined and enter the mixed salt evaporator, the mother liquor produced by the mixed salt evaporator is solidified, the mixed salt produced by the mixed salt evaporator is dissolved by adding water, the dissolved aqueous solution of the mixed salt enters the freezing crystallizer for freezing, the freezing crystallizer produces two parts, one part is Glauber's salt, Glauber's salt is mixed with water from the sodium sulfate side and enters the sodium sulfate evaporator for circulation, and the other part is frozen mother liquor, which is mixed with water from the sodium chloride side and enters the sodium chloride evaporator for circulation; the above sodium sulfate evaporator obtains a sodium sulfate dry product, and the sodium chloride evaporator obtains a sodium chloride dry product. Frozen crystallization is achieved using a frozen crystallizer.

[0025] Example 1

[0026] Using the above process, the water quality of the water from the sodium chloride side and the water from the sodium sulfate side is shown in Table 1.

[0027] Table 1 Water quality

[0028] project Sodium chloride side water Sodium sulfate side water Total mass (kg / h) 34450 39001 Total salt content (kg / h) 2513 2233 <![CDATA[H2O(kg / h)]]> 31937 36768 NaCl(kg / h) 2212 407 <![CDATA[Na2SO4(kg / h)]]> 169 1755 Impurities(kg / h) 132 71 COD(mg / L) 4.9 18.8

[0029] The water in Table 1 was treated according to the treatment process of the present invention, and the process data and result data are shown in Table 2.

[0030] Table 2 Salt balance data table of the process route of the present invention

[0031]

[0032]

[0033]

[0034] Comparative Example 1

[0035] like Figure 2 As shown, this comparative example uses the same water quality as Example 1, that is, the water quality of Table 1, and is processed according to the following traditional process, the steps are: the water from the sodium chloride side enters the sodium chloride evaporator crystallizer, the sodium chloride evaporator crystallizes and produces sodium chloride wet salt, which is dried and packaged as a product. The mother liquor discharged from the sodium chloride evaporator is sent to the mixed salt evaporator. The water from the sodium sulfate side first enters the sodium sulfate concentration evaporator, and the concentrated material enters the freezing crystallizer, and the Glauber's salt is precipitated in the freezing crystallizer. The precipitated Glauber's salt is evaporated by the sodium sulfate evaporator to produce sodium sulfate wet salt, which is dried and packaged as a product. The frozen mother liquor enters the mixed salt evaporator and evaporates together with the sodium chloride mother liquor discharged. The mixed salt produced by the mixed salt evaporator is dissolved and sent back to the front end.

[0036] The water in Table 1 was treated according to the treatment process of Comparative Example 1, and the process data and result data are shown in Table 3.

[0037] Table 3 Salt balance data of traditional process route

[0038]

[0039]

[0040]

[0041] By comparing the above Example 1 of the present invention with the conventional process Comparative Example 1, under the condition that the resource rate is close and the salt production can reach the standard. The total evaporation capacity of the conventional process is 40+5.4+43.6+8.4=97.4t / h, while the total evaporation capacity of the process of the present invention is 34.5+5.7+35.4=75.6t / h, and the total evaporation capacity is reduced by 22.4%. The freezing processing capacity of the conventional process requires 12t / h, and the processing capacity of the process of the present invention is 10.6t / h, which is reduced by 12%. One subsystem is reduced.

[0042] Example 2

[0043] Using the above process, the water quality of the water from the sodium chloride side and the water from the sodium sulfate side is shown in Table 4.

[0044] Table 4 Water quality

[0045] project Sodium chloride side water Sodium sulfate side water Total mass (kg / h) 19529 6996 Total salt content (kg / h) 1183 526 <![CDATA[H2O(kg / h)]]> 18346 6470 NaCl(kg / h) 1042 101 <![CDATA[Na2SO4(kg / h)]]> 64 416 Impurities(kg / h) 77 9 COD(mg / L) 2.8 3.3

[0046] The water in Table 4 was treated according to the treatment process of the present invention, and the process data and result data are shown in Table 2.

[0047] Table 5 Salt balance data table of the process route of the present invention

[0048]

[0049]

[0050] Comparative Example 2

[0051] like Figure 2 As shown, this comparative example uses the same water quality as Example 2, that is, the water quality in Table 4, and is processed according to the following traditional process, and the steps are the same as those of Comparative Example 1.

[0052] The water in Table 4 was treated according to the treatment process of Comparative Example 1, and the process data and result data are shown in Table 6.

[0053] Table 6 Salt balance data of traditional process route

[0054]

[0055]

[0056] By comparing Example 2 of the present invention with Comparative Example 2 of the conventional process, the resource rates are similar and the salt production can reach the standard. The total evaporation capacity of the conventional process is 5.6+1.4+23.4+4.3=34.7t / h, while the total evaporation capacity of the process of the present invention is 18.9+1.7+6.2=26.1t / h, and the total evaporation capacity is reduced by 25%. The freezing processing capacity of the conventional process requires 5t / h, and the processing capacity of the process of the present invention is 3.1t / h, which is reduced by 38%. One subsystem is reduced.

Claims

1. A short-process salt-nitrate separation process, characterized in that: Water from the sodium chloride side enters the sodium chloride evaporator, and water from the sodium sulfate side enters the sodium sulfate evaporator. The mother liquors discharged from the sodium chloride evaporator and the sodium sulfate evaporator are combined and enter the mixed salt evaporator. The mother liquor produced by the mixed salt evaporator is solidified. The mixed salt produced by the mixed salt evaporator is dissolved by adding water. The dissolved aqueous solution of the mixed salt enters the freezing crystallizer for freezing. The freezing crystallizer produces two parts, one part is Glauber's salt, which is mixed with water from the sodium sulfate side and enters the sodium sulfate evaporator for reciprocating circulation, and the other part is frozen mother liquor, which is mixed with water from the sodium chloride side and enters the sodium chloride evaporator for reciprocating circulation. The above sodium sulfate evaporator obtains a sodium sulfate dry product, and the sodium chloride evaporator obtains a sodium chloride dry product.

2. The short-process salt-nitrate separation process according to claim 1, characterized in that: The total salt content of the water from the sodium chloride side is 3wt% to 25wt%, and the remainder is water; the total salt content includes 85%-95% sodium chloride, 5%-15% sodium sulfate, and the remainder is inevitable impurities according to mass percentage.

3. The short-process salt-nitrate separation process according to claim 2, characterized in that: The COD of the water from the sodium chloride side is 1-200 mg / L.

4. The short-process salt-nitrate separation process according to claim 2, characterized in that: The total salt content of the water from the sodium sulfate side is 5wt% to 15wt%, and the balance is water; the total salt content includes 5%-25% sodium chloride, 75%-95% sodium sulfate, and the balance is inevitable impurities according to mass percentage.

5. The short-process salt-nitrate separation process according to claim 4, characterized in that: The COD of the water from the sodium sulfate side is 1-500 mg / L.

6. The short-process salt-nitrate separation process according to claim 1, characterized in that: The sodium sulfate evaporator and mixed salt evaporator use LWLZ model centrifuge for salt separation.

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