A method for preparing sodium bicarbonate coproduction of potassium sulfate and ammonium chloride from sodium sulfate waste salt

Abstract

The application discloses a method for preparing sodium bicarbonate, co-producing potassium sulfate and ammonium chloride from sodium sulfate waste salt, which comprises the following steps: dissolving; purifying and carbonizing; separating and drying; secondary treatment; double decomposition and hydrolysis; and freezing and crystallizing. The process is short, the reaction condition is mild, no organic chemical raw material is selected, no high-temperature and high-pressure reaction is selected, the operation is easy to control, no flammable and explosive safety risk exists, all intermediate products are recycled, no sewage is generated, waste gas is discharged after being absorbed, a small amount of solid waste is from the purified sodium sulfate waste salt, no environmental protection pressure exists, no evaporation crystallization process is selected in the whole chemical reaction, energy consumption is low, secondary dissolution, low-temperature centrifugal separation and ammonium chloride hydrolysis technology are adopted, product conversion rate is high, equipment investment is small, economic benefits are good, and the method is suitable for industrialized production; the obtained product is stable in quality, the sodium sulfate waste salt is purified by using a chemical method, and the toxicity is removed, and the utilization problem of the by-produced industrial salt is effectively solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium sulfate resource utilization, and particularly relates to a method for preparing sodium bicarbonate, co-producing potassium sulfate and ammonium chloride from sodium sulfate waste salt. BACKGROUND

[0002] In the fields of vanadium product processing, petrochemical industry, insecticide, herbicide, smelting, new energy, etc., manufacturing enterprises use soda ash, industrial salt and sulfuric acid as raw materials in the production process of chemical reaction, flue gas desulfurization and wastewater treatment. A large amount of sodium sulfate waste salt is produced after concentration and salt extraction. At present, only a small amount of sodium sulfate waste salt in China is used to produce sodium sulfate and sulfidic alkali. Due to the limited capacity, a large amount of waste salt is stored, and the high waste salt disposal fee and environmental protection pressure make the industrial waste salt become a bottleneck for the survival and development of enterprises.

[0003] Sodium sulfate resource recycling is a better method to solve the storage problem. Domestic and foreign technical personnel have done a lot of research and development work on this. For example, on the basis of combined soda production, sodium sulfate is used to produce soda ash, which can alleviate the storage pressure of sodium sulfate to a certain extent. However, the existing technology still has some problems such as the following:

[0004] 1. It is impossible to effectively oxidize and separate organic matter and heavy metals to remove the toxicity of sodium sulfate waste salt. The conventional disposal methods in the prior art are: ① wet method: dissolving industrial waste salt in water, generally using advanced oxidation method, catalytic oxidation method, supercritical hydrothermal oxidation method. The waste salt disposal technology by wet method has harsh reaction conditions, involves high temperature and high pressure, requires high equipment, and has high operation cost, so it has limitations in industrial production; ② dry method: generally using high-temperature incineration, organic matter carbonization pyrolysis and high-temperature hot melting, which is to incinerate and melt the organic matter at 800-1200℃ to decompose the organic matter, and the effect of removing the organic matter is good, but high-temperature incineration will produce dioxin and toxic gas, and has high energy consumption and large equipment investment; ③ landfill method: high cost and risk of secondary pollution;

[0005] 2. It is impossible to effectively improve the conversion rate of raw materials and the quality of products. The methods adopted in the published invention patents are: ① near normal temperature carbonization reaction and solid-liquid separation; ② secondary addition of sodium sulfate in the carbonization solution to further react with unreacted ammonium bicarbonate; ③ adding sodium nitrate as a promoter to evaporate and crystallize ammonium sulfate; ④ adding sodium chloride powder to salt out ammonium chloride. The disadvantages of these process schemes include: low total conversion rate (87%-92%); incomplete separation of mixed salt, unstable product quality; addition of sodium chloride powder, salted-out mother liquor entering the next cycle of recovery and recycling, and limited dissolved sodium sulfate waste salt, which cannot achieve the purpose of resource utilization of sodium sulfate waste salt; high energy consumption of evaporation and crystallization, which will cause loss in industrial production, etc. SUMMARY

[0006] To solve the problems in the background art, the present application provides the following technical solution: a process for preparing sodium bicarbonate and co-producing potassium sulfate and ammonium chloride from sodium sulfate waste salt, comprising the following steps:

[0007] Step 1) Dissolution: Dissolve the sodium sulfate waste salt to prepare a saturated sodium sulfate solution, adjust the pH value with dilute sulfuric acid, add an appropriate amount of sodium sulfide after stirring, standing and pressure filtration to obtain filtrate I;

[0008] Step 2) Purification and carbonization: filtrate I is sequentially subjected to hydrogen peroxide oxidation, ozone oxidation and activated carbon adsorption, and then purified by pressure filtration to obtain filtrate II, ammonium bicarbonate is added to filtrate II for carbonization reaction to obtain a carbonization solution;

[0009] Step 3) Separation and drying: The carbonization solution is treated by cooling, centrifugal separation, water washing and drying to obtain mother liquor I and the first batch of sodium bicarbonate dry product;

[0010] Step 4) Secondary treatment: mother liquor I is used as a solvent to repeat steps 1) to 3) to obtain mother liquor II and the second batch of sodium bicarbonate dry product;

[0011] Step 5) Double decomposition: freeze out the salt from mother liquor II, filter to obtain mixed salt and frozen clear liquid, add potassium chloride to the frozen clear liquid for preheating and stirring, first hydrolyze by increasing the temperature and absorb the gas with ammonium chloride solution to obtain an absorption liquid, then cool and settle, centrifugally separate to collect mother liquor III and solid, and add saturated potassium sulfate solution to the solid for washing and drying to obtain potassium sulfate dry product;

[0012] Step 6) Freeze out the crystal: collect the absorption liquid for freezing, stirring, crystallization, centrifugal separation and drying to obtain ammonium chloride dry product.

[0013] By using the above technical solution, the saturated sodium sulfate solution containing sodium sulfate waste salt is treated by acidification, sulfidation, oxidation, decolorization and adsorption in steps 1) and 2), which can effectively separate and remove organic impurities and heavy metals in the sodium sulfate waste salt, so that there is no organic residue and heavy metals detected in the treated solution and products. At the same time, in step 4), mother liquor I is used to dissolve sodium sulfate waste salt again, realizing efficient utilization of sodium sulfate waste salt resources, and the total conversion rate of the final products sodium bicarbonate, potassium sulfate and ammonium chloride can reach 98.5%, 96% and 96%, respectively, and the quality indicators of the three products all meet the national standards. Moreover, the reaction conditions of the present technical solution are mild and easy to control, the equipment investment is low, the energy consumption is low, and it is suitable for industrial application.

[0014] Preferably, the pressure filtration in step 1) and step 2) is first performed by pumping a diatomite water solution into a filter as a pre-filter layer. The ratio of diatomite to solution in the diatomite water solution is preferably 1g:1000ml (m / v).

[0015] Preferably, the filtrate I in step 2) is placed in a reactor, stirring is started, and 25-30% concentration of hydrogen peroxide is added. After oxidation for 20-30 min, stirring is stopped, and static oxidation is performed for 20-30 min. Then ozone is introduced for oxidation for 30-60 min, and the temperature is kept at 40-45°C. Activated carbon is added, the solution is stirred, and decolorization adsorption is performed for 30-45 min, and static adsorption is performed for 20-30 min. The dosing ratio of 25-30% concentration of hydrogen peroxide is preferably hydrogen peroxide:solution = 2-3 ml:100 ml (v / v), the dosing ratio of ozone is preferably ozone:solution = 1 g:1 m 3 (v / m), and the dosing ratio of activated carbon is preferably activated carbon:solution = 5 g:1000 ml (m / v).

[0016] Preferably, in step 2), the ammonium bicarbonate is slowly added to the filtrate II in multiple times, and the temperature is increased to 30-32°C while stirring. Carbonization reaction is performed by keeping the temperature for 1-1.5 h, and then the temperature is decreased to 10-15°C. After static settlement for 30-40 min, a carbonized solution is obtained. The dosing ratio of ammonium bicarbonate is preferably ammonium bicarbonate:sodium sulfate = 1.1 g:1 g (m / m).

[0017] Preferably, in step 5), the freeze-out of salts is performed by placing the mother liquor II in a freezing container, starting stirring, and freezing at 0-5°C for 3-4 h. After the mixed salts are separated out, the mixed salts are transferred to the next batch of step 2) before the addition of ammonium bicarbonate.

[0018] Preferably, in step 5), the temperature for preheating stirring is 30-35°C, and the temperature-keeping stirring time is 1-1.5 h. The dosing ratio of potassium chloride:ammonium sulfate for preheating stirring is preferably potassium chloride:ammonium sulfate = 1.13 g:1 g (m / m).

[0019] Preferably, in step 5), the temperature increase for hydrolysis is performed by first opening the air supply fan and the condenser coolant inlet and outlet valves, increasing the temperature to 70-80°C, and then keeping the temperature for 1.5-2 h.

[0020] Preferably, in step 5), the cooling and settlement is performed by cooling to 5-10°C, and then static settlement for 30-45 min.

[0021] The step 5) of the technical solution separates the ammonium chloride from the potassium sulfate solution by using the mother liquor II containing ammonium chloride to hydrolyze and release ammonia gas and hydrogen chloride gas, and the two gases are collected into the ammonium chloride absorption liquid to regenerate ammonium chloride after cooling.

[0022] Preferably, the freezing and stirring in the step 6) is to place the absorption liquid in a crystallizer, stir and freeze at 5-10 DEG C for 2.5-3.5 h, and then stand and settle for 30-40 min.

[0023] Preferably, the dissolving of the sodium sulfate waste salt in the step 1) is to use softened water or the mother liquor I or the mother liquor III as a solvent, uniformly add the sodium sulfate waste salt in the reactor while stirring and heating to 35-40 DEG C, and heat and stir for 0.5-1 h until the sodium sulfate is completely dissolved. The ratio of the sodium sulfate waste salt to the softened water is preferably 48 g:100 ml (m / v).

[0024] Preferably, the pH value adjustment in the step 1) is to slowly add a 30% sulfuric acid solution to adjust the pH value of the solution to 5.5-6.5, and stir for 20-30 min.

[0025] In summary, the present application has the following beneficial effects:

[0026] 1. The process of the present application is short, the reaction conditions are mild, no organic chemical raw materials are selected, there is no high temperature and high pressure reaction, it is easy to operate and control, and there is no risk of flammability and explosion safety;

[0027] 2. All intermediate products of the present application are recycled, no sewage is generated, the exhaust gas is discharged after absorption, and a small amount of solid waste is obtained from the purified sodium sulfate waste salt, and there is no environmental pressure;

[0028] 3. The present application does not use evaporation crystallization process in the whole chemical reaction, the energy consumption is low, and the product conversion rate is high by using the secondary dissolution, low temperature centrifugal separation and ammonium chloride hydrolysis technology, the equipment investment is less, the economic benefit is good, and it is suitable for large-scale industrial production;

[0029] 4. The present application can completely separate the potassium sulfate and the ammonium chloride, the quality of the obtained product is stable, the sodium bicarbonate product meets the quality indicators of GBT1606-2008, the potassium sulfate meets the quality indicators of GBT20406-2017, and the ammonium chloride meets the quality indicators of GBT2946-2018;

[0030] 5. This invention uses chemical methods to purify sodium sulfate waste salt and remove its toxicity, effectively solving the problem of utilizing by-product industrial salt and obtaining considerable economic benefits. It belongs to the category of comprehensive utilization of waste and circular economy, and is of great significance to the development of clean production and circular economy. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of the method for preparing sodium bicarbonate from sodium sulfate waste salt and co-producing potassium sulfate and ammonium chloride in Examples 1 and 2 of this application;

[0032] Figure 2 The diagram shows the equipment flow chart for the method of preparing sodium bicarbonate from sodium sulfate waste salt and co-producing potassium sulfate and ammonium chloride in Examples 1 and 2 of this application. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the embodiments of this application are all commercially available.

[0034] Example 1

[0035] (a) Take a sample of sodium sulfate waste salt:

[0036] Sampling unit: China Nonferrous Metals Group Daye Smelting Plant.

[0037] Source of waste salt: Wet desulfurization of copper smelting flue gas (soda ash solution)

[0038] Test results:

[0039] Main components Sodium sulfate Moisture Pb Ash Sodium sulfite Content (%) 90 4 0.07 4 1.95

[0040] (II) Process Steps:

[0041] Step 1) Dissolve:

[0042] 1.1 Pretreatment: Put 80L of softened water into glass mixing tank No. 1, start stirring, add 43kg of sodium sulfate waste salt, heat to 40℃ while stirring, keep warm and stir for 1 hour until completely dissolved, adjust the pH value to 6.0 with 30% dilute sulfuric acid, then add 110g of sodium sulfide, keep warm and stir for 30 minutes to make a saturated sodium sulfate solution.

[0043] 1.2, filtration: 100 g of diatomite was weighed and dissolved in water to form a solution, which was pumped into a stainless steel pilot plate frame as a pre-filter layer, and the moisture was blown dry with compressed air; the above saturated sodium sulfate solution was pumped into the stainless steel pilot plate frame and pressure filtration was carried out. After filtration, the moisture was blown dry with compressed air, and 1.3 kg of filter residue and 88 L of filtrate I were obtained; step 2) purification and carbonization:

[0044] 2.1, purification: the above filtrate I was placed in a No. 1 glass stirring tank, the stirring was started, 2.8 L of hydrogen peroxide was added, and stirring was carried out at 35°C for 30 min, and static oxidation was carried out for 30 min; the stirring was started, 16 g of ozone was introduced, and stirring was carried out for 1 h; static oxidation was carried out for 30 min, 440 g of powdered activated carbon was added, and stirring was carried out at 45°C for 45 min, and static adsorption was carried out for 30 min;

[0045] 2.2, filtration: 100 g of diatomite was weighed and dissolved in water to form a solution, which was pumped into a stainless steel pilot plate frame as a pre-filter layer, and the moisture was blown dry with compressed air; the above filtrate I was pumped into the stainless steel pilot plate frame and pressure filtration was carried out. After filtration, the moisture was blown dry with compressed air, and 1.2 kg of filter residue and 87.6 L of filtrate II were obtained;

[0046] 2.3, carbonization: the above filtrate II was placed in a No. 2 glass stirring tank, the stirring was started, the temperature was controlled at 32°C, and 43 kg of ammonium bicarbonate was added in four portions, heated to 32°C, and stirred for 1.5 h, then cooled to 13°C, and static settlement was carried out for 40 min, and the supernatant was removed to obtain a carbonization solution;

[0047] Step 3) separation and drying: the stirring was started, the carbonization solution was placed in a stainless steel centrifuge, and the mother liquor was spun dry; 5 L of softened water was used to wash the water, and the moisture was spun dry, and the sodium bicarbonate semi-finished product was taken out, and the carbonization solution was centrifuged twice;

[0048] The first product obtained was the first batch of sodium bicarbonate wet product 31.5 kg and the mother liquor I 96 L;

[0049] Step 4) secondary treatment:

[0050] The above mother liquor I was placed in a No. 1 glass stirring tank, 21.5 kg of waste sodium sulfate salt was added, and secondary dissolution, pretreatment, purification and carbonization were carried out, and the above steps 1)-3) were repeated;

[0051] The second time the amount of the second batch of sodium bicarbonate wet product 25.3 kg and the mother liquor II 103 L was obtained.

[0052] The second time the amount of the second batch of sodium bicarbonate wet product 25.3 kg and the mother liquor II 103 L was obtained.

[0053] Analysis shows that:

[0054] (1) Two times total input of sodium sulfate waste salt 64.5 kg, refined sodium sulfate 58 kg;

[0055] (2) Two times total obtained sodium bicarbonate wet product 56.8 kg, dried product 49.3 kg after drying in oven;

[0056] (3) 103 L x 0.076 = 7.8 kg of sodium bicarbonate solution remained in second carbonization, which can be recycled to next batch; (4) Conversion rate of sodium bicarbonate single pass is 49.3 / 58 x 100% = 85%;

[0057] (5) Sampling test results of sodium bicarbonate: total alkali (w / %) 98.6%, moisture 0.18%, pH 8.6, chloride 0.3%, sulfate 0.04%, arsenic (As) and heavy metal (Pb) not detected, which meets industrial class III standard.

[0058] Step 5) Double decomposition:

[0059] 5.1, Freeze out salt: Put mother liquor II in No. 2 tank, start stirring, control 2-6°C freezing for 3 h, then pressurized filtration with stainless steel plate frame, after filtration, blow dry moisture with compressed air, obtain mixed salt crystal 3.9 kg and frozen clear liquid 99 L;

[0060] 5.2 Preheat stirring:

[0061] Preparation: ① Send sample of mother liquor II for testing, ammonium sulfate content is 54.5%; ② Thoroughly clean No. 1 and No. 2 glass stirring tanks; ③ Put 70 L of softened water in No. 2 glass stirring tank, use hose to introduce exhaust gas of No. 1 glass stirring tank into No. 2 glass stirring tank, insert hose to 150 mm below liquid level.

[0062] Firstly, put above-mentioned frozen clear liquid into No. 1 glass stirring tank, start stirring, heat to 33°C, then add 61 kg of potassium chloride, keep stirring for 1.5 h;

[0063] 5.3, Hydrolysis: Stir No. 1 tank while heating, start exhaust fan when temperature rises to 55°C, gas passes through exhaust hose of No. 1 tank into water in No. 2 tank, stop heating when temperature of No. 1 tank rises to 72°C, at this time, temperature of No. 2 tank slowly rises, start cooling, keep between 30-35°C. Keep temperature of No. 1 and No. 2 tanks for 2 h;

[0064] 5.4 Cooling and centrifugation: Cool double decomposition liquid in No. 1 tank to 8°C, stand for 40 min, draw out supernatant; start stirring, put lower settled liquid into stainless steel centrifuge, centrifuge and dry mother liquor; wash with 5 L of saturated potassium sulfate solution, centrifuge and dry mother liquor, take out semi-finished product of potassium sulfate, centrifuge for four times in this batch;

[0065] The product obtained is: potassium sulfate semi-product 76.3 kg and mother liquor III 94 L, which can be used as waste salt for dissolving sodium sulfate in the next batch.

[0066] Step 6) Freeze crystallization:

[0067] The absorption solution in No. 2 tank is stirred and cooled to 5°C, and then left to freeze for 2 h, and the supernatant is drawn out. The stirring is started, and the lower sediment is centrifuged in a pilot stainless steel centrifuge. The mother liquor is spun dry, and the ammonium chloride wet product is taken out. This batch is centrifuged twice.

[0068] The product obtained is: ammonium chloride wet product 22 kg and mother liquor 82 L, which is reused in the next batch.

[0069] Analysis shows that:

[0070] (1) The potassium sulfate wet product is 76.3 kg, and 63.6 kg is obtained after drying. The theoretical amount of potassium sulfate is 74.06 kg, and the single-pass conversion rate is 85.88%. There are 9.46 kg in the mother liquor.

[0071] (2) The ammonium chloride wet product is 22 kg, and 18.4 kg of product is obtained after drying. The theoretical amount of ammonium chloride is 45.47 kg, and the single-pass conversion rate is 40.53%. There are 25.32 kg in the mother liquor.

[0072] (3) The potassium sulfate product is in powder form. After sample testing, the K2O content is 46%, the sulfur (S) content is 14.9%, which is close to the standard, and other indicators are not over standard, reaching the qualified product standard.

[0073] (4) The ammonium chloride product is in crystalline powder form. After sample testing, the ammonium chloride (NH4Cl) content is 99.4%, and other indicators are not over standard, reaching the industrial qualified product standard.

[0074] Example 2

[0075] (I) Take sodium sulfate waste salt sample: same as example 1;

[0076] (II) Process steps:

[0077] Steps 1) to 6) are operated according to the process steps of example 1, and the mother liquor involved in example 1 is recycled and used, as follows:

[0078] ① The mother liquor III in the double decomposition in step 5) of example 1 is used in the pretreatment process in step 1) of this example for dissolving, instead of soft water for dissolving sodium sulfate waste salt;

[0079] ② The hydrolysis mother liquor in step 5) of example 1 is used in the hydrolysis process of step 5) of this example, instead of soft water in No. 2 tank as absorption solution;

[0080] (3) The mixed salt separated from mother liquor II after freezing in the double decomposition in step 5) of Example 1 was used in the carbonation process in step 2) of the present example, and was redissolved before adding ammonium bicarbonate, and was recycled. Analysis showed that:

[0081] (1) The amount of sodium sulfate waste salt added:

[0082] Substep Weighed amount (kg) Content (%) Purified amount (kg) First time 40 90 36 Second time 20 90 18 Total 60 54

[0083] (2) The amount of sodium bicarbonate produced:

[0084] Substep Wet weight (kg) Dry weight (%) First time 50.3 41.8 Second time 25.3 21 Total 75.6 62.8

[0085] Data analysis: The theoretical amount of sodium bicarbonate: 54 x 1.18 = 63.72 kg;

[0086] The actual amount of dry product obtained: 62.8 kg;

[0087] The single-pass conversion rate: 62.8 / 63.72 x 100% = 98.56%.

[0088] The above data show that after recycling the mixed salt separated from mother liquor II and the double decomposition mother liquor III produced in Example 1, the single-pass conversion rate of sodium bicarbonate is increased from 85% in Example 1 to 98.56%, reaching the designed conversion rate of the present application. The main reason is that the sodium bicarbonate and sodium sulfate in the mixed salt and the double decomposition mother liquor III are effectively recycled and utilized, and it is verified that the sodium bicarbonate in the double decomposition mother liquor III does not decompose significantly at 70-80°C.

[0089] (3) The amount of sodium bicarbonate and potassium chloride added:

[0090]

[0091] (4) The volume of each step mother liquor:

[0092]

[0093] The above table shows that after recycling the mother liquor, the total water balance can be maintained, and the amount of double decomposition mother liquor is not increased compared with Example 1;

[0094] (5) The amount of potassium sulfate and ammonium chloride produced:

[0095] Material name Wet weight (kg) Dry weight (kg) Theoretical weight (kg) Potassium sulfate 76.4 63.7 66.2 Ammonium chloride 46.7 39 40.6

[0096] Data analysis: The single-pass conversion rate of potassium sulfate: 63.7 / 66.2 x 100% = 96.22%;

[0097] The single-pass conversion rate of ammonium chloride: 39 / 40.6 x 100% = 96.06%;

[0098] The potassium sulfate and ammonium chloride in the mother liquor are effectively utilized by recycling the complex decomposition mother liquor III and the ammonium chloride absorption liquor of the previous batch, the single-pass conversion rate of potassium sulfate is increased from 85.88% in Example 1 to 96.22%, the single-pass conversion rate of ammonium chloride is increased from 40.53% in Example 1 to 96.06%, and the product share in the mother liquor in Example 1 is offset.

[0099] (6) Product quality

[0100] In this batch, only the main content and heavy metals of three products are detected, and other items are not detected:

[0101]

[0102] The product quality detection results show that the separation and removal of organic matter and heavy metals can realize efficient resource utilization of the sodium sulfate waste salt generated by the flue gas desulfurization of a smelter.

[0103] The contents not described in detail in the specification belong to the prior art known to those skilled in the art, although the embodiments of the present application have been shown and described, it can be understood by those skilled in the art that various changes, modifications, replacements and variations can be made to these embodiments without departing from the principles and spirits of the present application, the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A process for the preparation of sodium bicarbonate co-producing potassium sulfate and ammonium chloride from sodium sulfate waste salt, characterized by The method comprises the following steps: Step 1) dissolving: dissolving sodium sulfate waste salt to prepare a saturated sodium sulfate solution, adjusting the pH value with dilute sulfuric acid, adding an appropriate amount of sodium sulfide, stirring, standing, and pressure filtration to obtain filtrate I; Step 2) purification and carbonization: filtrate I is sequentially subjected to hydrogen peroxide oxidation, ozone oxidation and activated carbon adsorption, and then pressure filtration purification to obtain filtrate II, ammonium bicarbonate is added to filtrate II to perform carbonization reaction to obtain a carbonized solution; Step 3) separation and drying: the carbonized solution is treated by cooling, centrifugal separation, water washing and drying to obtain mother liquor I and the first batch of sodium bicarbonate dry product; Step 4) secondary treatment: mother liquor I is used as a solvent to repeat steps 1) to 3) to obtain mother liquor II and the second batch of sodium bicarbonate dry product; Step 5) double decomposition: mother liquor II is subjected to freezing salt separation and filtration to obtain mixed salt and frozen clear liquid, potassium chloride is added to the frozen clear liquid for preheating and stirring, first hydrolysis by increasing the temperature and absorption of gas with ammonium chloride solution to obtain an absorption liquid, then cooling, settling, centrifugal separation, and collection to obtain mother liquor III and solid, and saturated potassium sulfate solution is added to the solid for washing and drying to obtain potassium sulfate dry product; Step 6) freezing crystallization: the absorption liquid is collected for freezing, stirring, crystallization, centrifugal separation and drying to obtain ammonium chloride dry product.

2. A process for the preparation of sodium bicarbonate co-producing potassium sulfate and ammonium chloride from sodium sulfate waste salt as claimed in claim 1, wherein: The pressure filtration in step 2) and step 5) is first to pump a diatomite water solution into a filter as a pre-filter layer, and then to filter.

3. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: In step 2), the filtrate I is placed in a reactor, stirring is started, and then 25-30% concentration hydrogen peroxide is added, oxidation is performed for 20-30 min, stirring is stopped, and static oxidation is performed for 20-30 min, then ozone is introduced for oxidation for 30-60 min, the temperature is kept at 40-45℃, activated carbon is then added, the solution is stirred, decolorization and adsorption are performed for 30-45 min, and static adsorption is performed for 20-30 min.

4. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The freezing salt separation in step 5) is to place mother liquor II into a freezing container, start stirring, and freeze at 0-5℃ for 3-4 h, and then the mixed salt is transferred to the next batch of step 2) for redissolution before adding ammonium bicarbonate.

5. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The preheating and stirring temperature in step 5) is 30-35℃, and the temperature is kept for 1-1.5 h.

6. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The hydrolysis by increasing the temperature in step 5) is to first open the air blower and the inlet and outlet valves of the condenser coolant water, increase the temperature to 70-80℃, and keep the temperature for 1.5-2 h.

7. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The cooling and settling in step 5) is to cool to 5-10℃, and stand for 30-45 min.

8. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The freezing and stirring in step 6) is to place the absorption liquid in a crystallizer, stir and freeze at 5-10℃ for 2.5-3.5 h, and then stand for 30-40 min.

9. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The dissolving of sodium sulfate waste salt in step 1) is to use softened water or mother liquor I or mother liquor III as a solvent, stir in a reactor, increase the temperature to 35-40℃, uniformly add sodium sulfate waste salt, keep the temperature and stir for 0.5-1 h, and then completely dissolve the sodium sulfate.

10. A process for the preparation of sodium bicarbonate co-producing potassium sulphate and ammonium chloride from sodium sulphate waste salt as claimed in claim 1, wherein: The pH value adjustment with dilute sulfuric acid in step 1) is to slowly add 30% sulfuric acid solution to adjust the pH value of the solution to 5.5-6.5, and stir for 20-30 min.

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