Process for producing molten salt grade sodium nitrate

Abstract

A process for producing molten salt grade sodium nitrate comprises the following steps: step S01, evaporation and concentration of a raw material solution: evaporating and concentrating a chlorine-containing sodium nitrate raw material solution to obtain an evaporation concentrated solution; step S02, cooling and crystallizing: cooling and crystallizing the evaporation concentrated solution obtained in the step S01 to obtain a crystallized clear solution and underflow slurry; step S03, evaporation dechlorination: feeding all or part of the crystallization clear liquid obtained in the step S02 into an evaporation dechlorination system, removing sodium chloride through evaporation and salt separation, carrying out solid-liquid separation on the obtained sodium chloride slurry, and discharging solid sodium chloride waste salt; and step S04, centrifuging, washing and drying the underflow slurry, namely centrifugally dewatering the underflow slurry obtained in the step S02, washing the obtained wet salt with water, dewatering the washed slurry, and drying to obtain the molten salt grade sodium nitrate. According to the method, the content of sodium chloride in the molten salt grade sodium nitrate product can be reduced, and the purity of the obtained sodium nitrate can reach 99.9% or above.

Description

Technical Field

[0001] The invention relates to a process for producing molten salt-grade sodium nitrate, and in particular to a method for preparing high-quality large-particle molten salt-grade sodium nitrate by separating sodium chloride from chlorine-containing brine during the production of molten salt-grade sodium nitrate. Background Art

[0002] An important raw material required in solar thermal power generation projects is molten salt-grade sodium nitrate. The higher and more stable its quality is, the higher the selling price will be. Therefore, all manufacturers are committed to producing first-class molten salt-grade sodium nitrate (purity ≥ 99.7wt%).

[0003] The production methods of sodium nitrate at home and abroad mainly include neutralization method, absorption method, ion exchange method, double decomposition method and saltpeter method. In the first three production processes, sodium carbonate or sodium sulfate may be used as raw materials. The sodium chloride impurities in the sodium carbonate or sodium sulfate raw materials will enter the molten salt grade sodium nitrate production process; in the saltpeter production process, the sodium chloride, sodium sulfate and other impurities contained in the ore itself enter the sodium nitrate system through leaching. At present, most domestic manufacturers of sodium nitrate use a single-effect or double-effect evaporation plus a single-stage crystallization method to obtain the product from the chlorine-containing sodium nitrate solution. The impurity sodium chloride in the system has no other outlet. With the continuous circulation and concentration of the system, the content of impurity sodium chloride is getting higher and higher, and the product quality is affected. Therefore, sodium chloride must be separated from the system to ensure the quality of molten salt grade sodium nitrate products. At the same time, since the content of sodium chloride in the system is relatively low, it is difficult to reduce it again. It is necessary to find a suitable outlet in the system to achieve the purpose of removal. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art, provide a process for producing molten salt grade sodium nitrate, reduce the sodium chloride content in the molten salt grade sodium nitrate, and improve the purity of the molten salt grade sodium nitrate.

[0005] The technical solution adopted by the present invention to solve the technical problem is a process for producing molten salt grade sodium nitrate, comprising the following steps:

[0006] Step S01, evaporation and concentration of raw material liquid:

[0007] Evaporating and concentrating the chlorine-containing sodium nitrate raw material liquid (preferably sending it to a sodium nitrate evaporation system for evaporation and concentration) to obtain an evaporation concentrate;

[0008] Step S02, cooling crystallization:

[0009] Cooling and crystallizing the evaporated concentrated liquid obtained in step S01 to obtain a crystal clear liquid and an underflow slurry;

[0010] The obtained underflow slurry is sent to step S04 for centrifugation, washing and drying, and the obtained crystal clear liquid is sent to step S03 for evaporation and dechlorination system.

[0011] Furthermore, in step S02, cooling crystallization adopts a vacuum cooling crystallizer with two or more stages (preferably three stages) connected in series, and the vacuum degree of each stage is generally 91Kpa to 9Kpa.

[0012] Step S03, evaporation and dechlorination:

[0013] All or part of the crystallized clear liquid obtained in step S02 is sent to the evaporation and dechlorination system to remove sodium chloride by evaporation and salt precipitation, and the obtained sodium chloride slurry is separated into solid and liquid, and the solid sodium chloride waste salt is discharged; the mother liquor can be returned to the evaporation and concentration process of step S01;

[0014] Step S04, centrifuging, washing and drying the underflow slurry:

[0015] The underflow slurry (i.e., sodium nitrate slurry) obtained in step S02 is centrifuged and dehydrated, the obtained wet salt is washed with water, and the washed slurry is dehydrated and dried to obtain molten salt grade sodium nitrate.

[0016] The filtrate obtained by centrifugal dehydration of the underflow slurry can be returned to the previous process such as step S02 to improve the recovery rate of the system.

[0017] The order of step S03 and step S04 can be swapped, that is, step S03 can be processed first, and then step S04; or step S04 can be processed first, and then step S03.

[0018] Further, in step S01, the sodium nitrate content in the chlorine-containing sodium nitrate raw liquid is 20-60wt%, and the sodium chloride content is 0.2-5wt%. The mass ratio of sodium nitrate: sodium chloride in the raw liquid is ≥10:1. (If the content of raw sodium nitrate is too high or saturated, and the sodium chloride content is too low, dechlorination is not required; if the sodium chloride content is too high, dechlorination is required first or the circulation volume is increased to achieve the purpose of dechlorination and production of molten salt grade sodium nitrate.)

[0019] Further, in step S01, the chlorine-containing sodium nitrate raw material liquid can be evaporated and concentrated by a single-effect, multiple-effect or MVR evaporation process. A cocurrent or countercurrent evaporation process can be adopted. The evaporation and concentration temperature is 50-150° C. (preferably 100-140° C.) (if the evaporation and concentration temperature is too high, energy is wasted and it is not economical; if it is too low, the single recovery rate is not high, the circulation volume is increased, the energy consumption is increased, and the equipment investment becomes larger), and a saturated or nearly saturated evaporation concentrate of sodium nitrate is obtained.

[0020] Furthermore, in step S01, the concentration of sodium nitrate in the saturated or nearly saturated evaporation concentrate is 48-68 wt%.

[0021] Furthermore, in step S01, the concentration of sodium chloride in the saturated or nearly saturated evaporation concentrate of sodium nitrate is 1-6 wt %.

[0022] Furthermore, in step S01, the evaporation intensity is controlled to avoid over-evaporation to produce fine sodium nitrate crystals.

[0023] Further, in step S02, the evaporation concentrate obtained by evaporation and concentration in step S01 is sent to a cooling crystallization system for cooling and crystallization. By controlling the crystallization end temperature, mother liquor composition and water replenishment amount, the concentration of sodium nitrate slurry is controlled, and then the quality and particle diameter of the final sodium nitrate product are controlled.

[0024] Further, in step S02, a settling section or an external thickener, concentrator or other equipment is provided in the last stage cooling crystallizer to separate the crystal slurry to obtain a crystal clear liquid and an underflow slurry. All or part of the separated crystal clear liquid is sent (preferably by pumping) to an evaporation and dechlorination process (step S03). The underflow slurry (solid content 25wt% to 50wt%) is sent (preferably by pumping) to a centrifugal, washing and drying process.

[0025] Further, in step S02, the cooling crystallization can be carried out by a single-stage or multi-stage series cooling crystallization device, which can be operated continuously or intermittently. The crystallizer can be a jacketed and stirred stirring tank, a DTB crystallizer, an OSLO crystallizer or an evaporative crystallizer.

[0026] Furthermore, in step S02, the cooling crystallization terminal temperature is 40-60° C. The sodium nitrate content in the crystallization terminal mother liquor is 35-50 wt%, and the sodium chloride content is 5-12 wt%.

[0027] Further, in step S02, the amount of water added during the sodium nitrate crystallization process is 0-100% of the amount of flash water. The total residence time of sodium nitrate crystallization is 2-40 hours. If the amount of water added is too large, the energy consumption will increase; if the amount of water added is too small, the product purity will be affected; the total residence time refers to the total residence time of sodium nitrate crystallization. If the total residence time is too short, the particle diameter is small, and if the total residence time is too long, the investment is large and it is not economical.

[0028] Furthermore, in step S03, the purpose of removing sodium chloride is achieved by controlling the amount of evaporation water, temperature and terminal mother liquor composition of the evaporation dechlorination system, thereby reducing the sodium chloride content in the system.

[0029] Furthermore, in step S03, a settling section or an external thickener or concentrator is added to the evaporator or crystallizer of the evaporation and dechlorination system to further increase the residence time and the size of the sodium chloride particles. The particle diameter of the crystallized sodium chloride is increased to facilitate separation.

[0030] Further, in step S03, the mother liquor (mother liquor with a solid content of <0.5wt%, i.e., the dechlorinated clear liquor) obtained after solid-liquid separation and desalination in the evaporative dechlorination system can be returned to the sodium nitrate raw material liquid evaporation process in step S01 for recycling. The mother liquor obtained after evaporation and solid-liquid separation and desalination in the evaporative dechlorination system can be returned to the sodium nitrate raw material liquid evaporation process in step S01 for recycling after clarification.

[0031] Furthermore, in step S03, the evaporation and dechlorination process can adopt single-effect, multi-effect evaporation or MVR evaporation crystallization process, and vacuum cooling crystallization (preferably single-stage or two-stage vacuum cooling crystallization) can also be added after evaporation.

[0032] Furthermore, in step S03, the composition of the mother liquor at the end point of evaporation is close to the co-saturation point of sodium nitrate and sodium chloride, specifically, the sodium nitrate content is 40-65wt%, and the sodium chloride content is 4-9wt%.

[0033] Further, in step S03, the temperature of the evaporation and dechlorination process is controlled to be 80-140° C., and the amount of evaporated water in the evaporation and dechlorination process is controlled to account for 15-30wt% of the total amount of the crystallization clear liquid (i.e., the dechlorination mother liquor). Too much evaporated water will cause over-evaporation and loss of sodium nitrate product; too low a temperature will cause too little sodium chloride to be evaporated in a single time, and the amount of mother liquor entering the dechlorination system needs to be increased, otherwise the amount of evaporated sodium chloride will be too little, and the product quality cannot be guaranteed.

[0034] Further, in step S03, the amount of sodium chloride solids in the mother liquor (i.e., the chlorine-free clear liquid, i.e., the filtrate) obtained after solid-liquid separation and desalination in the evaporative dechlorination system is controlled to be less than 0.5wt%. (The solid content in the mother liquor is controlled by means of a settling section of an evaporator or crystallizer or an external thickener, concentrator, etc. A high solid content will cause fine sodium chloride crystals to be brought into the front-end system, which will affect the product quality.)

[0035] Further, in step S03, the sodium chloride slurry (solid content 20-50wt%) obtained after thickening by the salt leg of the evaporator, the salt discharge section of the crystallizer or the external thickener, thickener and other equipment is continuously or intermittently sent to the waste salt (sodium chloride) centrifuge for centrifugal separation, and the waste salt (sodium chloride) is discharged from the system, and the centrifuge filtrate is returned to the dechlorination evaporation system through a pump. The size of the sodium chloride particles can be increased by increasing the sedimentation area and increasing the residence time.

[0036] Furthermore, in step S03, the particle size of the waste salt sodium chloride discharged after solid-liquid separation is ≥0.1 mm.

[0037] Further, in step S04, the underflow slurry obtained in step S02 is dehydrated, and the crude product after dehydration falls into a washing tank. According to the composition of the wet salt, clean water is added to the washing tank for washing. The washed slurry is dehydrated and dried to obtain a molten salt grade sodium nitrate product.

[0038] Further, in step S04, the centrifuge filtrate is pumped back to step S02 for vacuum cooling and crystallization.

[0039] Furthermore, in step S04, the mass ratio of the amount of clean water for washing: sodium nitrate is 0-0.35:1.

[0040] The present invention utilizes chlorine-containing brine, reduces the sodium chloride content in the sodium nitrate system through an evaporation desalination process, separates the sodium chloride, and prepares high-quality large-particle molten salt-grade sodium nitrate. The present invention proposes a method for preparing high-quality molten salt-grade sodium nitrate from sodium chloride in a separation system, solves the problem that sodium chloride cannot be separated in the sodium nitrate production process, and provides a scientific and feasible solution, thereby improving product quality and improving economic benefits.

[0041] Compared with the prior art, the method for producing high-quality molten salt-grade sodium nitrate of the present invention has the following advantages: the present invention combines the sodium nitrate evaporation and crystallization process, further evaporates and desalts the crystallization mother liquor, and can reduce the content of sodium chloride in the molten salt-grade sodium nitrate product, which has very good industrial value. Compared with the prior art, its process operation method is simple and reliable, the equipment investment is low, there is no waste of raw materials, no additional steam consumption, the production cost is low, and it can be flexibly adjusted according to the sodium chloride content in the raw material liquid composition, the applicability is strong, it has high economic benefits, and it is energy-saving and environmentally friendly.

[0042] The purity (dry basis) of the sodium nitrate obtained by the invention can reach above 99.9%. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below in conjunction with specific embodiments.

[0044] In this specification, unless otherwise specified, the percentages are percentages by mass.

[0045] Example 1

[0046] The process for producing molten salt grade sodium nitrate in this embodiment comprises the following steps:

[0047] Step S01, evaporation and concentration of raw material liquid:

[0048] This process uses chlorine-containing sodium nitrate raw material liquid as raw material, in which the mass percentage of sodium nitrate is 47% and the mass percentage of sodium chloride is 2%.

[0049] The chlorine-containing sodium nitrate raw material liquid is sent to a three-effect countercurrent evaporation system. The raw material liquid first enters the three-effect evaporator, and then is successively sent to the second-effect evaporator and the first-effect evaporator after evaporation. The clear liquid after dechlorination in the dechlorination evaporation process returns to the second-effect evaporation. The temperature of the concentrated liquid at the outlet of the first-effect evaporation is 130-140°C. The sodium nitrate in the evaporated concentrated liquid at the outlet is nearly saturated, with a sodium nitrate concentration of 67wt% and a sodium chloride concentration of 3.3wt%.

[0050] Step S02, cooling and crystallizing: cooling and crystallizing the evaporated concentrated liquid obtained in step S01 to obtain a crystal clear liquid and an underflow slurry; the specific operation is:

[0051] The evaporated concentrated liquid at the outlet of the first-effect evaporator flows by gravity through the pressure difference into the four-stage series vacuum cooling crystallization system. The type of crystallizer is a modified DTB crystallizer. The four-stage series vacuum cooling crystallizer is used to ensure that the evaporated concentrated liquid has sufficient residence time (21h) and temperature reduction gradient to obtain sodium nitrate products with larger particle diameters. In the step-by-step cooling crystallization, the amount of water added at each stage is proportionally controlled (less than or equal to the amount of flash water in the crystallization process) so that sodium chloride at each stage does not precipitate, thereby improving the purity of the product. The vacuum cooling crystallization endpoint temperature of 50°C and the endpoint pressure of 9kPaA (absolute pressure) are controlled by the vacuum system to obtain a four-stage crystal slurry.

[0052] In step S02, a settling section is set inside the last stage cooling crystallizer, and the upper part of the settling section is mainly pumped to extract the crystallization clear liquid. After heat exchange, part of the crystallization clear liquid is sent to the evaporation and dechlorination process (step S03), and part is sent to the front-end process. The bottom flow slurry (solid content 25wt%) of the four-stage crystallizer is pumped to the downstream centrifugal washing and drying process. The concentration of sodium nitrate in the obtained clear liquid is 43wt%, and the concentration of sodium chloride is 8.6wt%.

[0053] In step S02, the amount of water replenished during the sodium nitrate crystallization process is 50% of the amount of flash water.

[0054] Step S03, evaporation and dechlorination:

[0055] The crystallized clear liquid obtained in step S02 is sent to an evaporation and dechlorination system. This embodiment adopts single-effect evaporation. By controlling the amount of evaporated water, the mother liquor composition at the evaporation end point is obtained to have a sodium chloride content of 4.8wt% and a sodium nitrate content of 63.8wt%, thereby achieving the purpose of removing sodium chloride. The amount of sodium chloride removed is substantially equivalent to the amount introduced into the raw material liquid, while reducing the sodium chloride content in the system.

[0056] The dechlorination evaporator in this embodiment is equipped with a settling section and a salt leg in the main body of the equipment. The upper part of the settling section is mainly pumped to extract the dechlorination clear liquid, which is sent to the sodium nitrate evaporation process (step S01) to further recover the sodium nitrate. The slurry in the salt leg (solid content 25wt% to 50wt%) is sent to the downstream sodium chloride centrifugal separation process by gravity.

[0057] In step S03, the temperature of the evaporation and dechlorination process is controlled to be 125°C, and the amount of evaporated water in the evaporation and dechlorination process is controlled to account for 27wt% of the total amount of the crystallization clear liquid (ie, the dechlorination mother liquid).

[0058] In this example, by providing a sedimentation section, the amount of sodium chloride solids in the dechlorination clear liquid can be reduced to <0.5wt%, thereby reducing the effect of a small amount of sodium chloride particles entrained in the clear liquid on the crystallization of sodium nitrate.

[0059] The particle size of sodium chloride in the waste salt obtained in this embodiment (i.e. the particle size of sodium chloride in the waste salt discharged after centrifugal separation) is ≥0.15mm.

[0060] Step S04, centrifuging, washing and drying the sodium nitrate slurry:

[0061] In step S04, the sodium nitrate slurry obtained in step S02 is centrifuged, and the crude product after dehydration in the centrifuge falls into the washing tank. Clean water is added to the washing tank for washing, and the washed slurry is sent to a fine centrifuge for dehydration and drying to obtain a high-quality molten salt grade sodium nitrate product.

[0062] The centrifuge filtrate is collected and returned to step S02 of the sodium nitrate crystallization process through a pump to improve the system recovery rate.

[0063] In step S04, the mass ratio of the amount of clean water for washing: sodium nitrate is 0.2:1.

[0064] The product obtained in this embodiment can meet the requirements of first-class molten salt grade sodium nitrate, the average particle diameter of the product is 0.4mm (the standard particle size range of the product is 0.3-0.6mm), the water content of the product is 0.09%, and the dry basis mass content is as follows: 99.9% (dry basis) of sodium nitrate, 0.01% (dry basis) of sodium nitrite, 0.04% (dry basis) of sodium chloride, 0.01% (dry basis) of sodium carbonate, 0.003% (dry basis) of calcium nitrate, 0.003% (dry basis) of magnesium nitrate, and 0.001% (dry basis) of iron.

[0065] Example 2

[0066] The process for producing molten salt grade sodium nitrate in this embodiment comprises the following steps:

[0067] Step S01, evaporation and concentration of raw material liquid:

[0068] In this embodiment, the sodium nitrate content of the sodium chlorine-containing sodium nitrate raw material liquid is 35wt%, and the sodium chloride content is 1.25wt%.

[0069] This embodiment adopts a three-effect downstream sodium nitrate evaporation process. After preheating, the raw liquid first enters the first-effect evaporation, and then is pumped downstream to the second and third-effect evaporation. After chlorination, the clear liquid returns to the sodium nitrate first-effect evaporation tank, the energy is fully utilized, and a small amount of fine particles of sodium chloride entrained in the clear liquid can be further dissolved.

[0070] In this embodiment, the temperature of the triple-effect evaporation is 100° C., and the composition of the evaporation concentrate is controlled to be close to the co-saturation point of sodium nitrate and sodium chloride at 100° C. The concentration of sodium nitrate in the outlet evaporation concentrate is 59wt%. The concentration of sodium chloride at the outlet is 4.2wt%.

[0071] Step S02, cooling and crystallizing the evaporated concentrated liquid obtained in step S01:

[0072] In this embodiment, the concentrated evaporation liquid obtained by evaporation is sent to a cooling crystallization tank with an outer jacket and stirring in intermittent operation. The residence time is controlled at 30 hours, and the crystallization end temperature is controlled at 40°C. Large-particle sodium nitrate products are obtained by slow crystallization. The sodium nitrate content in the end mother liquor is 45wt%, and the sodium chloride content is 5.7wt%.

[0073] After reaching the residence time, the solid content of the slurry is between 15wt% and 40wt%, and is pumped to the next process for centrifugal separation.

[0074] Since this crystallization process uses indirect heat exchange, there is no water flash and no additional water is required.

[0075] Step S03, centrifuging, washing and drying the sodium nitrate slurry:

[0076] The sodium nitrate slurry obtained in step S02 is centrifuged, and the crude product after dehydration in the centrifuge falls into the washing tank. Clean water is added to the washing tank for washing, and the washed slurry is sent to a fine centrifuge for dehydration and drying to obtain a high-quality molten salt grade sodium nitrate product.

[0077] The crude centrifuge filtrate is collected and pumped into a thickener for secondary clarification, and the clear liquid portion is returned to step S02 for sodium nitrate crystallization, and the rest is sent to step S04 for evaporation and dechlorination. The obtained filtrate has a sodium nitrate content of 45wt% and a sodium chloride content of 5.7wt%.

[0078] In step S03, the mass ratio of the amount of clean water for washing: sodium nitrate is 0.15:1.

[0079] Step S04, evaporation and dechlorination:

[0080] The crystallized clear liquid obtained in step S03 is sent to an evaporation and dechlorination system, using single-effect evaporation. In this embodiment, by controlling the amount of evaporated water, the mother liquor composition at the evaporation end point has a sodium nitrate content of 60.5wt% and a sodium chloride content of 6wt%, which can achieve the purpose of removing sodium chloride. The amount of sodium chloride removed is substantially equivalent to the amount brought into the raw material liquid, while reducing the sodium chloride content in the system.

[0081] In step S03, the temperature of the evaporation and dechlorination process is controlled to be 125°C, and the amount of evaporated water in the evaporation and dechlorination process is controlled to account for 25wt% of the total amount of the crystallization clear liquid (ie, the dechlorination mother liquid).

[0082] The dechlorination evaporator in this embodiment is equipped with a settling section and a salt leg in the main body of the equipment. The upper part of the settling section is mainly pumped to extract the dechlorination clear liquid, which is sent to the sodium nitrate evaporation process (step S01) to further recover the sodium nitrate. The slurry in the salt leg (solid content 25wt% to 50wt%) is sent to the downstream sodium chloride centrifugal separation process by gravity.

[0083] In this example, by providing a sedimentation section, the amount of sodium chloride solids in the dechlorination clear liquid can be reduced to <0.5wt%, thereby reducing the effect of a small amount of sodium chloride particles entrained in the clear liquid on the crystallization of sodium nitrate.

[0084] The particle size of sodium chloride in the waste salt obtained in this embodiment is ≥0.15 mm.

[0085] The product obtained in this embodiment can meet the requirements of first-class molten salt grade sodium nitrate, with an average particle diameter of 0.6 mm (standard particle size range of the product is 0.4-0.8 mm), a moisture content of 0.08%, and a dry basis mass content of 99.9% (dry basis) of sodium nitrate, 0.01% (dry basis) of sodium nitrite, 0.04% (dry basis) of sodium chloride, 0.01% (dry basis) of sodium carbonate, 0.003% (dry basis) of calcium nitrate, 0.003% (dry basis) of magnesium nitrate, and 0.001% (dry basis) of iron.

Claims

1. A process for producing molten salt grade sodium nitrate, characterized in that: The following steps are involved: Step S01, evaporation and concentration of raw material liquid: The chlorine-containing sodium nitrate raw material liquid is evaporated and concentrated to obtain an evaporated concentrated liquid; Step S02, cooling crystallization: Cooling and crystallizing the evaporated concentrated liquid obtained in step S01 to obtain a crystal clear liquid and an underflow slurry; The obtained underflow slurry is sent to step S04 for centrifugation, washing and drying, and the obtained crystal clear liquid is sent to step S03 for evaporation and dechlorination system; Step S03, evaporation and dechlorination: Sending all or part of the crystallized clear liquid obtained in step S02 to an evaporation and dechlorination system to remove sodium chloride by evaporation and salt precipitation, separating the obtained sodium chloride slurry into solid and liquid, and discharging solid sodium chloride waste salt; Step S04, centrifuging, washing and drying the underflow slurry: The underflow slurry obtained in step S02 is centrifuged and dehydrated, the obtained wet salt is washed with water, and the washed slurry is dehydrated and dried to obtain molten salt grade sodium nitrate; The order of step S03 and step S04 can be swapped, that is, step S03 can be processed first, and then step S04; or step S04 can be processed first, and then step S03.

2. The process for producing molten salt grade sodium nitrate according to claim 1, characterized in that: In step S01, the content of sodium nitrate in the chlorine-containing sodium nitrate raw material liquid is 20-60wt%, and the content of sodium chloride is 0.2-5wt%; the mass ratio of sodium nitrate:sodium chloride in the raw material liquid is ≥10:1; and / or, in step S01, the chlorine-containing sodium nitrate raw material liquid is evaporated and concentrated by a single-effect, multi-effect or MVR evaporation process; and / or, a cocurrent or countercurrent evaporation process is adopted; and / or, the evaporation and concentration temperature is 50-150°C.

3. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S01, the concentration of sodium nitrate in the evaporation concentrate saturated or nearly saturated with sodium nitrate is 48-68wt%; and / or, in step S01, the concentration of sodium chloride in the evaporation concentrate saturated or nearly saturated with sodium nitrate is 1-6wt%.

4. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S02, cooling crystallization adopts more than two stages of vacuum cooling crystallizers connected in series, and the vacuum degree of each stage is 91Kpa~9Kpa; and / or, in step S02, the evaporation concentrate obtained by evaporation concentration in step S01 is sent to the cooling crystallization system for cooling crystallization, and the concentration of sodium nitrate slurry is controlled by controlling the crystallization end temperature, mother liquor composition and water replenishment amount, thereby controlling the quality and particle diameter of the final sodium nitrate product; and / or, in step S02, a sedimentation section is set in the last stage cooling crystallizer or an external thickener or thickener is set to separate the crystal slurry to obtain a crystal clear liquid and an underflow slurry; and / or, all or part of the separated crystal clear liquid is sent to the evaporation and dechlorination process; and / or, the underflow slurry is sent to the centrifugation, washing and drying process.

5. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S02, cooling crystallization adopts single-stage or multi-stage cooling crystallization equipment in series, and is operated continuously or intermittently; the crystallizer adopts a stirred tank with a jacket and stirring, a DTB crystallizer, an OSLO crystallizer or an evaporative crystallizer; and / or, in step S02, the cooling crystallization terminal temperature is 40-60° C.; and / or, the sodium nitrate content in the mother liquor composition at the crystallization terminal is 35-50wt%, and the sodium chloride content is 5-12wt%; and / or, in step S02, the water replenishment amount in the sodium nitrate crystallization process is 0-100% of the flash water amount; and / or, the total residence time of sodium nitrate crystallization is 2-40h; and / or, in step S03, the mother liquor obtained by solid-liquid separation is returned to the evaporation and concentration process of step S01.

6. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S03, the purpose of removing sodium chloride is achieved by controlling the amount of evaporated water, temperature and terminal mother liquor composition of the evaporative dechlorination system, thereby reducing the sodium chloride content in the system; and / or, in step S03, a sedimentation section or an external thickener or concentrator is added to the evaporator or crystallizer equipment body of the evaporative dechlorination system; and / or, in step S03, the mother liquor obtained after solid-liquid separation and desalination in the evaporative dechlorination system is returned to the sodium nitrate raw material liquid evaporation process of step S01 for recycling; and / or the mother liquor obtained after evaporation, solid-liquid separation and desalination in the evaporative dechlorination system is returned to the sodium nitrate raw material liquid evaporation process of step S01 for recycling after clarification.

7. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S03, the evaporation and dechlorination process adopts a single-effect, multi-effect evaporation or MVR evaporation crystallization process, or vacuum cooling crystallization is added after evaporation; and / or, in step S03, the composition of the mother liquor at the evaporation end point is close to the co-saturation point of sodium nitrate and sodium chloride, specifically, the sodium nitrate content is 40-65wt% and the sodium chloride content is 4-9wt%.

8. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S03, the temperature of the evaporation dechlorination process is controlled to be 80-140° C.; and / or, the amount of evaporated water in the evaporation dechlorination process is controlled to account for 15-30wt% of the total amount of the crystallization clear liquid; and / or in step S03, the amount of sodium chloride solid in the mother liquor obtained after solid-liquid separation and desalination in the evaporation dechlorination system is controlled to be less than 0.5wt%.

9. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S03, the sodium chloride slurry obtained after thickening by the salt leg of the evaporator, the salt discharge section of the crystallizer or the external thickener and concentrator is continuously or intermittently sent to the waste salt centrifuge for centrifugal separation, and the waste salt is discharged out of the system, and the centrifuge filtrate is returned to the dechlorination evaporation system through a pump; and / or, in step S03, the particle size of the waste salt sodium chloride discharged after solid-liquid separation is ≥0.1mm.

10. The process for producing molten salt grade sodium nitrate according to claim 1 or 2, characterized in that: In step S04, the underflow slurry obtained in step S02 is dehydrated, and the crude product after dehydration falls into a washing tank; according to the composition of the wet salt, clean water is added to the washing tank for washing; the washed slurry is dehydrated and dried to obtain a molten salt grade sodium nitrate product; and / or, in step S04, the centrifuge filtrate is returned to the vacuum cooling crystallization process of step S02 by a pump; and / or, in step S04, the mass ratio of the amount of clean water for washing: sodium nitrate is 0-0.35:1.