Method for recovering sodium sulfate from salt-containing wastewater and preparing ammonium sulfate and sodium carbonate

Abstract

The invention provides a method for recovering sodium sulfate from salt-containing wastewater and preparing ammonium sulfate and sodium carbonate, and the method comprises the following steps: controlling sodium sulfate and sodium chloride to be close to a co-saturation point at an evaporation temperature, so that more impurities such as Cl <-> can be prevented from being mixed into solid sodium sulfate, and the solid sodium sulfate separated out from the salt-containing wastewater is higher in purity; the sodium carbonate and the ammonium sulfate are prepared from the separated high-purity sodium sulfate, the prepared sodium carbonate and the ammonium sulfate are fewer in impurities and higher in purity, and according to measurement, the purity of the obtained sodium sulfate is 99.0% or above, the conversion per pass of ammonium bicarbonate is 90% or above, the purity of the sodium carbonate is 98% or above, and the purity of the ammonium sulfate can meet the requirements of GB / T535-2020. The method not only can be used for preparing a product with higher purity, but also has better environmental benefits and economic benefits, and can better meet the requirements of current green metallurgy and solid waste recycling on clean production.

Description

Technical Field

[0001] The present application relates to the technical field of nonferrous metal smelting, and in particular to a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate. Background Art

[0002] In the process of industrial production such as metallurgy and chemical industry, a large amount of wastewater is generated. The organic matter in these wastewaters is reduced by biochemical treatment and other means to obtain high-salt wastewater containing sodium sulfate, sodium chloride and other salts. In traditional treatment methods, high-salt wastewater is usually treated by dilution and discharge or evaporation and crystallization to obtain mixed salt. However, the excessive sodium sulfate content in the salty wastewater makes it difficult to stack the separated sodium sulfate and difficult to recycle it.

[0003] Sodium sulfate, as a raw material, is mainly used to prepare sodium bicarbonate / sodium carbonate, which is in huge demand in industry. This is also an important way to utilize sodium sulfate in a high-value way. Therefore, sodium sulfate separated from saline wastewater can be used to prepare sodium bicarbonate. However, since the purity of the separated sodium sulfate is not high and it is doped with impurities such as chloride ions, the impurity content in the obtained ammonium sulfate and sodium carbonate is high, and industrial application has not been achieved. Therefore, it is necessary to propose a method for preparing higher-purity ammonium sulfate and sodium carbonate using sodium sulfate recovered from saline wastewater. Summary of the invention

[0004] To solve the above problems, the present application provides a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate, which can efficiently recover high-purity sodium sulfate from saline wastewater and prepare ammonium sulfate, sodium bicarbonate and sodium carbonate, thereby realizing industrial application.

[0005] In a first aspect, the present application provides a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate, the method comprising: subjecting the wastewater containing sodium sulfate and sodium chloride to a first evaporation crystallization to precipitate solid sodium sulfate until SO4 2+ and Cl - The concentration is close to the co-saturation point, completing the evaporation crystallization, filtering, and obtaining the solid sodium sulfate;

[0006] The solid sodium sulfate obtained by filtration is used to prepare a saturated sodium sulfate solution;

[0007] Adding solid ammonium bicarbonate to the saturated sodium sulfate solution to obtain a mixed system containing solid sodium bicarbonate through double decomposition reaction;

[0008] The mixed system is filtered, and the separated solid sodium bicarbonate is washed and calcined to obtain sodium carbonate;

[0009] Add sulfuric acid and / or sodium bisulfate to the liquid remaining after the filtration process to carry out a neutralization reaction, and after the reaction is completed, carry out a second evaporation crystallization at a temperature of 80-100° C. to precipitate the solid sodium sulfate;

[0010] After the solid sodium sulfate is precipitated, the remaining liquid is cooled and crystallized to further precipitate a sodium sulfate-ammonium sulfate double salt;

[0011] The remaining liquid after the sodium sulfate-ammonium sulfate double salt is precipitated is subjected to a third evaporation crystallization to obtain the ammonium sulfate.

[0012] Optionally, the temperature of the first evaporation crystallization is 60-95°C.

[0013] Optionally, SO4 in the wastewater 2+ and Cl - When the concentration approaches the co-saturation point, the SO4 2+ The concentration is 3-6wt%, the Cl - The concentration is 20-30wt%.

[0014] Optionally, before the first evaporation and crystallization, the mass ratio of sodium sulfate to sodium chloride in the wastewater is greater than 0.2.

[0015] Optionally, the mass ratio of the solid ammonium bicarbonate to the saturated sodium sulfate solution is (0.9-1.5):1.

[0016] Optionally, the temperature of the metathesis reaction is 30-40° C., and the reaction time is 0.5-3 h.

[0017] Optionally, when the solid sodium bicarbonate is washed and calcined, it is washed with a saturated sodium bicarbonate solution and / or anhydrous ethanol, and then calcined at a temperature of 200-250° C. for 40-90 minutes to obtain the sodium carbonate.

[0018] Optionally, the temperature of the cooling crystallization is 10-25°C.

[0019] Optionally, the temperature of the second evaporative crystallization is 80-100°C.

[0020] In a second aspect, the present application provides ammonium sulfate and sodium carbonate obtained by the above-mentioned method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate.

[0021] In summary, the present application includes at least one of the following beneficial technical effects:

[0022] 1. The present application provides a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate, which can avoid Cl -Various impurities such as sodium sulfate are crystallized and precipitated together with sodium sulfate, so that the obtained solid sodium sulfate has a higher purity, and then the high-purity sodium sulfate is used to prepare ammonium sulfate and sodium carbonate. The prepared ammonium sulfate and sodium carbonate also have fewer impurities and higher purity, which effectively solves the problem of low purity of the recovered sodium sulfate and the problem that the purity of the ammonium sulfate and sodium carbonate prepared from the sodium sulfate cannot meet the needs of industrial production;

[0023] 2. The present application discloses a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate. The recovered sodium sulfate has higher purity. When producing ammonium sulfate and sodium carbonate, post-processing steps such as impurity removal, multiple evaporation and filtration are avoided, which greatly reduces energy consumption, achieves better environmental and economic benefits, and meets the requirements of green metallurgy and solid waste resource utilization for clean production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the description of the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0025] Figure 1 A flow chart of a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate provided in an embodiment of the present application is shown;

[0026] Figure 2 The phase diagram of the sodium sulfate-sodium chloride-water system and its crystallization operation process when the salt-containing wastewater provided in the embodiment of the present application is subjected to the first evaporation crystallization to recover sodium sulfate are shown. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0028] In the drawings, the size of the constituent elements, the thickness of the layer or the area may be exaggerated for the sake of clarity. Therefore, any implementation of the present disclosure is not necessarily limited to the size shown in the drawings, and the shapes and sizes of the components in the drawings do not reflect the true proportions. In addition, the drawings schematically show ideal examples, and any implementation of the present disclosure is not limited to the shapes or values ​​shown in the drawings.

[0029] According to statistics, the annual output of saline wastewater in my country is about 1×10 9t, the annual production of waste salt that can be recorded is 2.1×10 7 t, it is worth noting that the sodium sulfate content in saline wastewater is very high, and a lot of sodium sulfate will be precipitated through evaporation and crystallization, which leads to the problem that the separated sodium sulfate is difficult to stack and difficult to recycle. Pure sodium sulfate and sodium chloride are commonly used chemicals in industry. If the sodium sulfate and sodium chloride separated from saline wastewater can be recycled, it can not only increase the factory's profits, but also meet environmental protection needs. It is the research focus in the field of wastewater recycling.

[0030] The technical principle of using sodium sulfate to prepare sodium bicarbonate is based on Hou's alkali production process. However, since the purity of sodium sulfate separated from saline wastewater is not high, the operation steps of preparing sodium bicarbonate and / or sodium carbonate are more complicated and the production energy consumption is high. Moreover, the impurity content in the obtained ammonium sulfate product is also very high, which has not been realized in industrial application.

[0031] Based on the above problems, the present application proposes a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate. By controlling the temperature and crystallization point, the purity of the recovered sodium sulfate is improved, and the recovered sodium sulfate is used to prepare sodium carbonate and ammonium sulfate. This method can not only simplify the operating steps, but also further improve the quality of the obtained products, providing strong guidance for wastewater treatment and resource recycling.

[0032] First, see Figure 1 , Figure 1 The flowchart of the method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate provided in the embodiment of the present application is shown, and the specific operation steps are as follows:

[0033] First evaporation crystallization: Specifically, the wastewater containing sodium sulfate and sodium chloride undergoes first evaporation crystallization to precipitate solid sodium sulfate until SO4 2+ and Cl - The concentration is close to the co-saturation point, and evaporation crystallization is completed;

[0034] Filtration: Specifically, filtering the liquid after evaporation and crystallization to obtain solid sodium sulfate, and using the solid sodium sulfate obtained by filtration to prepare a saturated sodium sulfate solution;

[0035] Synthesis reaction: Specifically, solid ammonium bicarbonate is added to a saturated sodium sulfate solution, and a mixed system containing solid sodium bicarbonate is obtained through double decomposition reaction;

[0036] Solid-liquid separation: Specifically, the mixed system is filtered, and the separated solid sodium bicarbonate is washed and calcined to obtain sodium carbonate;

[0037] Sulfuric acid neutralization: Specifically, sulfuric acid and / or sodium bisulfate are added to the liquid remaining after the filtration process to carry out a neutralization reaction;

[0038] Second evaporation crystallization: Specifically, after the neutralization reaction is completed, a second evaporation crystallization is performed at a temperature of 80-100° C. to precipitate solid sodium sulfate;

[0039] Cooling crystallization: Specifically, the remaining liquid after solid sodium sulfate is precipitated is cooled and crystallized to further precipitate sodium sulfate-ammonium sulfate double salt;

[0040] Third evaporation crystallization: Specifically, the remaining liquid after precipitation of sodium sulfate-ammonium sulfate double salt is subjected to third evaporation crystallization to obtain ammonium sulfate.

[0041] It should be noted that the co-saturation point refers to the state point in a multicomponent system where each component reaches its solubility limit at the same time, that is, each solute in the solution has reached its maximum solubility under the condition and no more solute can be dissolved. At this time, the solution is in a saturated state.

[0042] Close to the co-saturation point means that it is infinitely close to the co-saturation point but has not reached the co-saturation point.

[0043] By adopting the above method, when sodium sulfate is obtained by evaporation and crystallization from wastewater, as evaporation proceeds, SO4 2+ and Cl - When the concentration of sodium sulfate approaches the co-saturation point, the evaporation and crystallization operation is terminated. At this time, the solubility of sodium sulfate and sodium chloride in the wastewater reaches a relative balance, and sodium sulfate can be crystallized in a purer form, reducing the mixing of other impurities such as chloride ions into the sodium sulfate crystals, thereby improving the purity of the recovered solid sodium sulfate.

[0044] The recovered high-purity sodium sulfate is then used to prepare a saturated sodium sulfate solution. After the saturated sodium sulfate solution is subjected to a double decomposition reaction with ammonium bicarbonate, the obtained solid sodium bicarbonate can be directly precipitated due to the low solubility of sodium bicarbonate, thereby avoiding subsequent separation and other operation steps and reducing the difficulty of operation. The obtained solid sodium bicarbonate is washed to remove other ions attached to the surface to keep the surface clean, and then calcined to perform a thermal decomposition reaction to remove a molecule of carbon dioxide and water to obtain sodium carbonate, wherein the obtained sodium bicarbonate can also be used as a product;

[0045] After filtration, some unreacted ammonium bicarbonate and sodium sulfate remain in the remaining liquid, which are evaporated and crystallized to precipitate solid sodium sulfate. The precipitated solid sodium sulfate is added back into the saturated sodium sulfate solution for reuse, thereby realizing resource recycling.

[0046] Sodium sulfate-ammonium sulfate double salt is generated in the liquid undergoing the neutralization reaction, and after being precipitated by cooling and crystallization, the double salt is added to the neutralized liquid again to repeat the reaction;

[0047] The remaining liquid undergoes a third evaporation crystallization to obtain ammonium sulfate product.

[0048] In the above reaction process, since the solid sodium sulfate recovered from the wastewater has higher purity and fewer impurities, the purity of the sodium carbonate and ammonium sulfate products prepared using this sodium sulfate is also higher, which can meet the requirements of industrial production. There are few impurities in the entire preparation process, and the waste generated by the reaction is very little. Part of the product can also be recycled and reused, which not only saves resources but also has better environmental protection.

[0049] In some embodiments, the initial concentration of sodium sulfate in the wastewater is 5-20 g / L, and the initial concentration of sodium chloride is 1-5 g / L. The initial concentrations of sodium sulfate and sodium chloride in the wastewater will affect the crystallization rate of the subsequent first evaporation crystallization and the purity of the precipitated solid sodium sulfate. The concentrations of the two salts are controlled within the above range so that the two salts reach a dissolution equilibrium in the wastewater to increase the crystallization rate.

[0050] In some embodiments, the temperature of the first evaporation crystallization is 60-95° C. Preferably, the temperature of the first evaporation crystallization is 90-95° C.

[0051] Since the solubility of sodium sulfate decreases with increasing temperature, the temperature of the first evaporation crystallization is controlled to make SO4 2+ and Cl - The concentration is close to the co-saturation point to obtain high-purity sodium sulfate.

[0052] In some embodiments, before the first evaporation crystallization, the mass ratio of sodium sulfate to sodium chloride in the wastewater is greater than 0.2. If the mass ratio of sodium sulfate to sodium chloride is less than 0.2, the solid precipitated by evaporation crystallization before reaching the co-saturation point is sodium chloride, so it is necessary to reasonably control the mass ratio of sodium sulfate to sodium chloride.

[0053] In some embodiments, when the solid sodium sulfate obtained by filtration is used to prepare a saturated sodium sulfate solution, the preparation process is carried out at a temperature of 30-40°C.

[0054] Preferably, when preparing the saturated sodium sulfate solution, the preparation process is carried out at a temperature of 30-35° C. In this temperature range, sodium sulfate can be well dissolved in water to form a relatively stable saturated sodium sulfate solution, providing a stable liquid environment for subsequent reactions.

[0055] In some embodiments, the mass ratio of solid ammonium bicarbonate to saturated sodium sulfate solution is (0.9-1.5): 1. Preferably, the mass ratio of solid ammonium bicarbonate to saturated sodium sulfate solution is (1-1.2): 1. If the mass ratio is less than the above range, the conversion rate of sodium sulfate is low; if the mass ratio is greater than the above range, the conversion rate of ammonium bicarbonate is low, the reaction effect is poor, and ammonium bicarbonate is wasted.

[0056] In some embodiments, the temperature of the metathesis reaction is 30-40° C., and the reaction time is 0.5-3 h.

[0057] Preferably, the double decomposition reaction is carried out at a temperature of 30-35° C. for 0.5-1 h, which helps the reaction proceed smoothly and is more conducive to the generation and collection of sodium bicarbonate.

[0058] In some embodiments, when solid sodium bicarbonate is washed and calcined, it is washed with a saturated sodium bicarbonate solution and / or anhydrous ethanol, and then calcined at a temperature of 200-250° C. for 40-90 min to obtain sodium carbonate.

[0059] Preferably, the washed sodium bicarbonate is calcined at a temperature of 220-240° C. for 60-80 min to obtain sodium carbonate. The calcination temperature can ensure that the sodium bicarbonate is completely decomposed, thereby increasing the yield of sodium carbonate.

[0060] In some embodiments, the solid sodium sulfate precipitated by the second evaporation crystallization can continue to participate in the double decomposition reaction, thereby achieving resource recycling and reducing resource waste.

[0061] Preferably, the temperature of the second evaporation crystallization is 90-95°C.

[0062] In some embodiments, the temperature of cooling crystallization is 10-25° C. The sodium sulfate-ammonium sulfate double salt precipitated by cooling crystallization is added to the solution after the neutralization reaction, and is continuously recycled to participate in subsequent reactions.

[0063] In some embodiments, the temperature of the third evaporation crystallization is 80-100° C. Preferably, the temperature of the third evaporation crystallization is 90-95° C.

[0064] In some embodiments, the remaining liquid after precipitation of sodium sulfate-ammonium sulfate double salt is subjected to a third evaporation crystallization to obtain ammonium sulfate, which is removed by filtration, and the remaining filtrate is mixed with the neutralized liquid and recycled to continue to participate in subsequent reactions.

[0065] In the second aspect, the present application provides sodium carbonate and ammonium sulfate obtained by the above-mentioned method of recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate. The purity of the obtained sodium carbonate is above 98%, and the purity of the ammonium sulfate can meet the requirements of the national standard GB / T535-2020.

[0066] In order to enable those skilled in the art to better understand the present invention, a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate provided by the present invention is described below through multiple specific embodiments.

[0067] Example

[0068] Example 1

[0069] A method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate:

[0070] 1. Take 10L of saline wastewater (sodium sulfate concentration is 10.19g / L, sodium chloride concentration is 1.21g / L) and perform the first evaporation crystallization at 90℃ to control SO4 2+ and Cl - The concentration is close to the co-saturation point at 90°C, i.e., 25.48wt% NaCl, 4.41wt% Na2SO4, and 99g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0071] 2. Prepare a saturated sodium sulfate solution by mixing the obtained solid sodium sulfate at 30°C;

[0072] 3. Add 100 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 30° C., and continue the reaction for 0.5 h after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0073] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 220° C. for 60 min to obtain a sodium carbonate product;

[0074] 5. Add 5 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, perform a second evaporation crystallization at 90° C. to precipitate solid sodium sulfate, filter to obtain solid sodium sulfate and the remaining liquid, and return the obtained solid sodium sulfate to step 3 to continue to participate in the double decomposition reaction;

[0075] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 10° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0076] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a third evaporation crystallization at 90° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0077] Example 2

[0078] A method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate:

[0079] 1. Take 10L of saline wastewater (sodium sulfate concentration is 10.19g / L, sodium chloride concentration is 1.21g / L) and perform the first evaporation crystallization at 90℃ to control SO4 2+ and Cl - The concentration is close to the co-saturation point at 90°C, i.e., 25.48wt% NaCl, 4.41wt% Na2SO4, and 99g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0080] 2. Prepare a saturated sodium sulfate solution by mixing the obtained solid sodium sulfate at 32°C;

[0081] 3. Add 110 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 32 ° C, and continue the reaction for 0.5 h after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0082] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 230° C. for 60 min to obtain a sodium carbonate product;

[0083] 5. Add 5 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, evaporate and crystallize at 90° C. to precipitate solid sodium sulfate. Filter to obtain solid sodium sulfate and the remaining liquid. The obtained solid sodium sulfate is returned to step 3 to continue to participate in the double decomposition reaction;

[0084] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 15° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0085] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a third evaporation crystallization at 90° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0086] Example 3

[0087] A method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate:

[0088] 1. Take 10L of saline wastewater (sodium sulfate concentration is 19.35g / L, sodium chloride concentration is 4.29g / L) and perform the first evaporation crystallization at 90℃ to control SO4 2+ and Cl -The concentration is close to the co-saturation point at 90°C, i.e., 25.48wt% NaCl, 4.41wt% Na2SO4, and 185g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0089] 2. Prepare the obtained solid sodium sulfate into a saturated sodium sulfate solution at 34°C;

[0090] 3. Add 222 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 35 ° C, and continue the reaction for 1 hour after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0091] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 230° C. for 70 min to obtain a sodium carbonate product;

[0092] 5. Add 10 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, perform a second evaporation crystallization at 95 ° C. to precipitate solid sodium sulfate, filter to obtain solid sodium sulfate and the remaining liquid, and return the obtained solid sodium sulfate to step 3 to continue to participate in the double decomposition reaction;

[0093] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 20° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0094] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a third evaporation crystallization at 95° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0095] Example 4

[0096] A method for preparing ammonium sulfate and sodium carbonate using sodium sulfate recovered from saline wastewater:

[0097] 1. Take 10L of saline wastewater (sodium sulfate concentration is 19.35g / L, sodium chloride concentration is 4.29g / L) and perform the first evaporation crystallization at 90℃ to control SO4 2+ and Cl - The concentration is close to the co-saturation point at 90°C, i.e., 25.48wt% NaCl, 4.41wt% Na2SO4, and 185g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0098] 2. Prepare a saturated sodium sulfate solution by mixing the obtained solid sodium sulfate at 35°C;

[0099] 3. Add 222 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 35 ° C, and continue the reaction for 1 hour after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0100] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 250° C. for 90 min to obtain a sodium carbonate product;

[0101] 5. Add 10 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, perform a second evaporation crystallization at 95 ° C. to precipitate solid sodium sulfate, filter to obtain solid sodium sulfate and the remaining liquid, and return the obtained solid sodium sulfate to step 3 to continue to participate in the double decomposition reaction;

[0102] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 25° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0103] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a third evaporation crystallization at 95° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0104] Example 5

[0105] The difference between Example 5 and Example 4 is that the temperature of the first evaporation crystallization is different.

[0106] A method for preparing ammonium sulfate and sodium carbonate using sodium sulfate recovered from saline wastewater:

[0107] 1. Take 10L of saline wastewater (sodium sulfate concentration is 19.35g / L, sodium chloride concentration is 4.29g / L) and perform the first evaporation crystallization at 60℃ to control SO4 2+ and Cl - The concentration is close to the co-saturation point at 60°C, i.e., 24.5wt% NaCl, 5.1wt% Na2SO4, and 184g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0108] 2. Prepare a saturated sodium sulfate solution by mixing the obtained solid sodium sulfate at 35°C;

[0109] 3. Add 220 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 35 ° C, and continue the reaction for 1 hour after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0110] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 250° C. for 90 min to obtain a sodium carbonate product;

[0111] 5. Add 10 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, perform a second evaporation crystallization at 95 ° C. to precipitate solid sodium sulfate, filter to obtain solid sodium sulfate and the remaining liquid, and return the obtained solid sodium sulfate to step 3 to continue to participate in the double decomposition reaction;

[0112] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 25° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0113] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a second evaporation crystallization at 95° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0114] Example 6

[0115] The difference between Example 6 and Example 4 is that the temperature of the first evaporation crystallization is different.

[0116] A method for preparing ammonium sulfate and sodium carbonate using sodium sulfate recovered from saline wastewater:

[0117] 1. Take 10L of saline wastewater (sodium sulfate concentration is 19.35g / L, sodium chloride concentration is 4.29g / L) and perform the first evaporation crystallization at 95℃ to control SO4 2+ and Cl - The concentration is close to the co-saturation point at 95°C, i.e., 25.8wt% NaCl, 4.4wt% Na2SO4, and 186g of solid sodium sulfate is precipitated after the reaction (the solid sodium sulfate obtained is anhydrous sodium sulfate);

[0118] 2. Prepare a saturated sodium sulfate solution by mixing the obtained solid sodium sulfate at 35°C;

[0119] 3. Add 223 g of solid ammonium bicarbonate to the saturated sodium sulfate solution, control the double decomposition reaction temperature at 35 ° C, and continue the reaction for 1 hour after the addition is completed to obtain a mixed system containing solid sodium bicarbonate (this solid sodium bicarbonate is crude sodium bicarbonate);

[0120] 4. Filter the mixed system, wash the separated sodium bicarbonate with a saturated sodium bicarbonate solution, and calcine the sodium bicarbonate at 250° C. for 90 min to obtain a sodium carbonate product;

[0121] 5. Add 10 mL of sulfuric acid to the remaining liquid filtered in step 4 for neutralization reaction. After the reaction is completed, perform a second evaporation crystallization at 95 ° C. to precipitate solid sodium sulfate, filter to obtain solid sodium sulfate and the remaining liquid, and return the obtained solid sodium sulfate to step 3 to continue to participate in the double decomposition reaction;

[0122] 6. After filtering the solid sodium sulfate, the remaining liquid is cooled and crystallized, cooled to 25° C., filtered to obtain a sodium sulfate-ammonium sulfate double salt, and the sodium sulfate-ammonium sulfate double salt is mixed with the liquid after the neutralization reaction in step 5 to enter the next cycle;

[0123] 7. After the sodium sulfate-ammonium sulfate double salt is precipitated, the remaining liquid is subjected to a second evaporation crystallization at 95° C., and the ammonium sulfate product is obtained by filtration. The remaining filtrate is mixed with the liquid after the neutralization reaction in step 5 and enters the next cycle.

[0124] Comparative Example

[0125] Comparative Example 1

[0126] The difference between Comparative Example 1 and Example 4 is that the temperature of the first evaporation crystallization is 50° C., and an equal mass of sodium sulfate precipitated from the crystals is used to prepare a saturated sodium sulfate solution for subsequent reaction.

[0127] Comparative Example 2

[0128] The difference between Comparative Example 2 and Example 4 is that the concentration of sodium sulfate in the wastewater is 30 g / L and the concentration of sodium chloride is 15 g / L.

[0129] Comparative Example 3

[0130] The difference between Comparative Example 3 and Example 4 is that the mass of ammonium bicarbonate is 278 g, and the mass of sodium sulfate in the saturated sodium sulfate solution is 185 g.

[0131] Comparative Example 4

[0132] The difference between Comparative Example 4 and Example 4 is that the calcination temperature is 180°C.

[0133] First, in the embodiments of the present application, Figure 2 The phase diagram of the sodium sulfate-sodium chloride-water system and its crystallization operation process when the salt-containing wastewater provided in the embodiment of the present application is subjected to the first evaporation crystallization to recover sodium sulfate are shown.

[0134] exist Figure 2In the figure, points C, D, E and F are the co-saturation points of sodium sulfate and sodium chloride at 90°C, 50°C, 18°C ​​and 10°C respectively. Evaporation and crystallization of saline wastewater is carried out at 90°C (operation line is AB). When the solution evaporates to point B, the crystals precipitated at this time are anhydrous sodium sulfate. As the evaporation proceeds, the solution gradually reaches the co-saturation point along BC. The evaporation and crystallization operation is terminated when approaching the co-saturation point, and the solid obtained is anhydrous sodium sulfate crystals.

[0135] When the crystallization temperature is 50℃, 18℃ and 10℃ respectively, the corresponding sodium sulfate and sodium chloride reach different co-saturation points. Sodium sulfate decahydrate is crystallized at 10-20℃; mixed crystals of anhydrous sodium sulfate and sodium sulfate decahydrate are obtained at 20-30℃; solid phase anhydrous sodium sulfate is crystallized at 40℃-90℃.

[0136] The purity of sodium sulfate recovered from the wastewater, the single-pass conversion rate of ammonium bicarbonate, the purity of the product sodium carbonate, and the purity of ammonium sulfate in Examples 1-6 and Comparative Examples 1-4 were tested to see whether they met the requirements of GB / T535-2020. The test results are shown in Table 1 below.

[0137] Table 1

[0138]

[0139] According to Table 1, the sodium sulfate prepared in Examples 1-6 has high purity, and the sodium carbonate and ammonium sulfate prepared therefrom have few impurities, and the corresponding purity is further improved, which effectively solves the problem of low purity of products prepared from sodium sulfate recovered from wastewater.

[0140] According to Table 1, the single-pass conversion rate of ammonium bicarbonate reaches more than 91%, the reactant almost completely reacts with sodium sulfate, and the remaining small amount of ammonium bicarbonate also generates double salt and ammonium sulfate products through subsequent steps, and its utilization rate is extremely high, avoiding waste of resources.

[0141] According to Table 1, in Comparative Example 1, sodium sulfate was precipitated by ordinary crystallization. Due to the low crystallization temperature, the crystallization efficiency was low. - The embodiment of the present application limits the temperature of the first evaporation crystallization, and then controls the precipitation of sodium sulfate according to the co-saturation point of sodium sulfate and sodium chloride at this temperature, thus solving the problem of a small amount of Cl - The problem of sodium sulfate precipitation will be solved, and the obtained sodium sulfate will be of higher purity.

[0142] According to Table 1, the concentrations of sodium sulfate and sodium chloride in Comparative Example 2 are too high, and it is difficult to reach a dissolution equilibrium in the wastewater, resulting in a low purity of the precipitated sodium sulfate.

[0143] According to Table 1, the ammonium bicarbonate in Comparative Example 3 is excessive, which makes the one-way conversion rate of ammonium bicarbonate relatively low. The excess ammonium bicarbonate causes a certain waste of resources and needs to be reasonably allocated.

[0144] According to Table 1, the calcination temperature in Comparative Example 4 is relatively low, so that the sodium bicarbonate cannot be completely decomposed, resulting in a relatively slightly lower purity of the obtained sodium carbonate. Therefore, it is necessary to control the calcination temperature within the range of 200-250° C. so that the sodium bicarbonate is fully decomposed to obtain sodium carbonate with higher purity.

[0145] In summary, the method proposed in the present application for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate can achieve efficient recovery of sodium sulfate and co-produce ammonium sulfate and sodium carbonate. The purity of sodium sulfate is above 99.0%, the single-pass conversion rate of ammonium bicarbonate is above 90%, and the purity of sodium carbonate is above 98%. The purity of ammonium sulfate meets the requirements of the national standard GB / T535-2020, which greatly improves the recovery rate and utilization rate of resources. The whole process is simple and efficient to operate, without the generation of waste gas and waste liquid, has high economic and environmental benefits, and can meet the current requirements of green metallurgy for clean production.

[0146] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0147] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the embodiments of the present application.

[0148] Finally, it should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or terminal device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or terminal device. In the absence of further restrictions, the elements defined by the sentence "including one..." do not exclude the existence of other identical elements in the process, method, article or terminal device including the elements.

[0149] The above is a detailed introduction to a method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate provided by the present application. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea; at the same time, for general technicians in this field, according to the ideas of the present application, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate, characterized in that: The method comprises: The wastewater containing sodium sulfate and sodium chloride is first evaporated and crystallized to precipitate solid sodium sulfate until the SO4 2 + and Cl - The concentration is close to the co-saturation point, the evaporation crystallization is completed, and the solid sodium sulfate is obtained by filtering; The solid sodium sulfate obtained by filtration is used to prepare a saturated sodium sulfate solution; Adding solid ammonium bicarbonate to the saturated sodium sulfate solution to obtain a mixed system containing solid sodium bicarbonate through double decomposition reaction; The mixed system is filtered, and the separated solid sodium bicarbonate is washed and calcined to obtain sodium carbonate; Add sulfuric acid and / or sodium bisulfate to the liquid remaining after the filtration process to carry out a neutralization reaction, and after the reaction is completed, carry out a second evaporation crystallization at a temperature of 80-100° C. to precipitate the solid sodium sulfate; After the solid sodium sulfate is precipitated, the remaining liquid is cooled and crystallized to further precipitate a sodium sulfate-ammonium sulfate double salt; The remaining liquid after the sodium sulfate-ammonium sulfate double salt is precipitated is subjected to a third evaporation crystallization to obtain the ammonium sulfate.

2. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The temperature of the first evaporation crystallization is 60-95°C.

3. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The SO4 2+ and Cl - When the concentration approaches the co-saturation point, the SO4 2+ The concentration is 3-6wt%, the Cl - The concentration is 20-30wt%.

4. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: Before the first evaporation crystallization, the mass ratio of sodium sulfate to sodium chloride in the wastewater is greater than 0.

2.

5. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The mass ratio of the solid ammonium bicarbonate to the saturated sodium sulfate solution is (0.9-1.5):

1.

6. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The temperature of the double decomposition reaction is 30-40° C., and the reaction time is 0.5-3 h.

7. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: When the solid sodium bicarbonate is washed and calcined, it is washed with a saturated sodium bicarbonate solution and / or anhydrous ethanol, and then calcined at a temperature of 200-250° C. for 40-90 minutes to obtain the sodium carbonate.

8. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The temperature of the cooling crystallization is 10-25°C.

9. The method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to claim 1, characterized in that: The temperature of the third evaporation crystallization is 80-100°C.

10. Ammonium sulfate and sodium carbonate obtained by the method for recovering sodium sulfate from saline wastewater and preparing ammonium sulfate and sodium carbonate according to any one of claims 1 to 9.

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