Method for industrially producing sodium sulfite and co-producing sodium pyrosulfite by using sodium sulfate

Abstract

According to the method, sodium sulfite and sodium pyrosulfite products are directly prepared by adding quicklime into sodium sulfate and mixed salt or mirabilite and introducing SO2 for double decomposition reaction, so that the technical problems of high alkali consumption and high energy consumption in the prior art are solved. At present, only a liquid sodium hydrogen sulfite product containing more impurities can be produced in the prior art, but the water content of the liquid sodium hydrogen sulfite is up to 60-65%, the liquid sodium hydrogen sulfite is inconvenient to store and transport, the use amount of the liquid sodium hydrogen sulfite is small, and the market application is less; however, direct preparation of high-purity anhydrous sodium sulfite or sodium pyrosulfite products by using liquid sodium hydrogen sulfite has the cost problems of high alkali consumption and high energy consumption, so that the preparation cost of sodium sulfite and sodium pyrosulfite is difficult to be high. If the use amount of alkali neutralization is reduced, the barrier that impurity ions such as calcium, magnesium and iron in the sodium sulfite and sodium metabisulfite products do not exceed the standard is difficult to overcome; sodium hydrogen sulfite is neutralized until the pH value is greater than or equal to 11.0, more than 99.0% of foreign ions can be removed, but alkali for neutralization accounts for 70.0-75.0% of the production cost of anhydrous sodium sulfite; therefore, the invention provides a process technical scheme for preparing sodium sulfite and sodium pyrosulfite with low cost and low energy consumption.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate; in particular, the present application relates to a method for industrial production of sodium sulfite and co-production of sodium metabisulfite from by-product sodium sulfate, sodium sulfate mixed salt and its hydrate mixed salt in chemical industry. BACKGROUND

[0002] There are a large amount of by-product sodium sulfate discharged in chemical, pharmaceutical, new energy, environmental protection and other industries. Sodium sulfate (Na2SO4) is a salt composed of sulfate and sodium ion. Sodium sulfate is soluble in water and its aqueous solution is neutral. Sodium sulfate is soluble in glycerol but insoluble in ethanol. Sodium sulfate containing crystal water is generally sodium sulfate decahydrate, also known as mirabilite. Mirabilite has stable chemical properties and is insoluble in strong acid. With the rapid development of domestic chemical industry, more and more by-product sodium sulfate is produced, which leads to sodium sulfate becoming the most industrial waste, bringing pressure on enterprise production and environment. How to recycle and comprehensively utilize these sodium sulfate is a great challenge to clean production of chemical industry. In particular, a large amount of by-product sodium sulfate is produced in the production process of domestic dye intermediates, which is mostly anhydrous sodium sulfate evaporated from α-naphthol and β-naphthol production wastewater, and is a mixed salt containing 12.0-20.0% sodium sulfite component.

[0003] At present, the digestion and utilization direction of industrial by-product sodium sulfate is as follows: 1. adding coal powder to calcine sodium sulfate product; 2. using sodium sulfate and ammonium bicarbonate to prepare soda ash and ammonium sulfate; and 3. using by-product sodium sulfate to prepare sodium bisulfite.

[0004] Based on the chemical reaction principle of generating sodium bisulfite and gypsum after passing excess SO2 into the suspension prepared by sodium sulfate and quicklime, the related patents and information have been published in the existing patents and documents, for example: the invention patent announcement No. CN110002472A discloses "industrialized method for continuously producing sodium bisulfite by byproduct sodium sulfate", but currently no related information of using sodium sulfate to directly prepare anhydrous sodium sulfite and sodium pyrosulfite has been realized industrialization. Patent application No. CN201710065330.2 discloses "a process for preparing magnesium hydroxide and sodium sulfite from magnesium sulfite", but the patent applicant abandoned the patent maintenance after the initial examination of the patent application, the main reason is that the purity of the anhydrous sodium sulfite product prepared by using sodium bisulfite solution in the research and development test process is not enough, and the consumption of the neutralizing alkali is too high, the impurities in the sodium sulfite product are too much, the cost is high, which restricts the industrialization of the sodium sulfite product prepared by using sodium sulfate; when the amount of neutralizing alkali is small, it will cause the impurities in the sodium sulfite product to exceed the standard and cause the equipment to be easily scaled and blocked. The main reason is that after passing excess SO2 into the suspension prepared by sodium sulfate and quicklime, the finally generated sodium bisulfite solution is acidic, and too much calcium bisulfite, magnesium bisulfite and other impurities are dissolved in it, so when directly evaporated and concentrated, it will cause the impurities in the sodium sulfite product to be too much and cause the equipment to be scaled and blocked; when the impurities are removed by neutralizing to alkaline with alkali, a large amount of soda ash or caustic soda will be consumed. It is known that quicklime contains 1.5-3.0% of magnesium oxide, and the pH value at which magnesium ions produce precipitation is 11.0-12.0, so a large amount of alkali is needed to neutralize the solution to pH≥11.0 to obtain a pure sodium sulfite solution, and the cost of the alkali used for neutralizing sodium bisulfite to sodium sulfite accounts for 74.0-76.0% of the production cost; if low-cost quicklime or calcium hydroxide is used to neutralize the acidic mixed solution, it will cause the gypsum to contain too much calcium sulfite, and the calcium sulfite entering the gypsum will cause excessive consumption of sulfur dioxide and affect the cost. The above factors lead to the fact that the cost and process conditions of the industrialization of sodium sulfite prepared by using sodium sulfate or its hydrate mixed salt are difficult to control, which is also the main technical barrier for the industrialization of sodium sulfite or sodium pyrosulfite prepared by using sodium sulfate, quicklime and SO2 complex decomposition reaction. SUMMARY

[0005] The purpose of the present application is to provide a method for industrialized production of sodium sulfite and co-production of sodium pyrosulfite by using sodium sulfate, which can greatly reduce the production cost of sodium sulfite and sodium pyrosulfite, consume a large amount of byproduct sodium sulfate and its hydrate mixed salt, make it get resource recycling, reduce environmental pollution and develop circular economy, and output high-purity sodium sulfite and sodium pyrosulfite products, the specific steps are realized by the following chemical reaction process.

[0006] The main reaction equation of the present application is as follows:

[0007] CaO + H2O → Ca(OH)2↓

[0008] Ca(OH)2 + SO2 → CaSO3↓ + H2O

[0009] CaSO3 + H2O + SO2 → Ca(HSO3)2

[0010] Na2SO4 + Ca(HSO3)2 + H2O → CaSO4·2H2O↓ + 2NaHSO3

[0011] Na2CO3 + 2NaHSO3 → 2Na2SO3 + CO2↑ + H2O, NaOH + NaHSO3 → Na2SO3 + H2O

[0012] Na2CO3 + Ca 2+ + OH - + HSO3 - → Na2SO3 + CaCO3↓ + H2O

[0013] NaOH + Mg 2+ + HSO3 - → Na2SO3 + Mg(OH)2↓ + H2O

[0014] Na2SO3 + SO2 + H2O → 2NaHSO3

[0015] 2NaHSO3 → Na2S2O5 + H2O

[0016] The sodium sulfite heptahydrate crystal involved in the present application is as follows:

[0017] Na2SO3 + H2O → Na2SO3·7H2O↓

[0018] The specific steps employed to achieve the present application are as follows:

[0019] Water, sodium sulfate or sodium sulfate hydrate mixed salt, CaO or Ca(OH)2 are added into the lime tank to stir and modulate into a suspension, the solution temperature is maintained at 30-50℃ until the sodium sulfate is completely dissolved, the molar ratio of sodium sulfate or its hydrate mixed salt to CaO or Ca(OH)2 is 1:0.20-0.8; the following processes are subdivided:

[0020] a) The SO2 absorption stirring reactor is closed, and is provided with 1, 2, 3 series counter current absorption. SO2 gas is introduced into the suspension to produce a double decomposition reaction. The reaction temperature is controlled by water cooling at 30.0-55.0℃. After the suspension absorbs SO2 supersaturation, the pH of the solution is measured at 2.8-3.5. The liquid phase is converted into a mixed solution containing sodium bisulfite, calcium bisulfite and calcium sulfate dihydrate. The introduction of SO2 gas is stopped, and the SO2 gas is switched to another 2-stage absorption reactor for continuous absorption. The suspension which stops absorbing SO2 is sent to the alkali neutralization reactor 1. After neutralization with alkali to pH 4.5-8.0, it is transferred to the solid-liquid separation A to remove the gypsum and other precipitates by filtration, to obtain a sodium sulfite solution with a concentration of 8.0-28.0%.

[0021] b) The sodium sulfite solution with a concentration of 8.0-26.0% is introduced into an evaporator for concentration under reduced pressure at ≤80℃ to near saturation state. The concentrated solution is transferred into the crystallizer C to precipitate sodium sulfite heptahydrate crystals and mother liquor I under water cooling at ≤24℃ for 1.0-4.0 hours. The sodium sulfite heptahydrate crystals are sent to a dryer for drying at ≤150℃ to obtain anhydrous sodium sulfite product. When the sulfate radical in the mother liquor I is ≥3.0% or the calcium and magnesium impurities are ≥2.0%, the mother liquor I is introduced into the alkali neutralization and filtration 2 to neutralize with alkali to pH 11.0-12.0. The calcium and magnesium residues are removed by filtration. The filtrate is sent to the second stage SO2 absorption stirring reactor for double decomposition reaction to generate gypsum precipitate to remove sulfate radical.

[0022] c) The saturated solution of water and sodium sulfite heptahydrate is introduced into the third stage sodium metabisulfite absorption reactor. SO2 is continuously introduced until the pH of the saturated solution is 3.8-4.1. Sodium metabisulfite crystals are precipitated in the crystallizer D under water cooling at ≤45℃ for 1.0-5.0 hours. The crystals are sent to the centrifugal solid-liquid separation E for dehydration to obtain sodium metabisulfite wet product and mother liquor II. The wet product is dried at ≤120℃ to obtain anhydrous sodium metabisulfite product. The mother liquor II is returned to the third stage SO2 absorption reactor for reuse. The SO2 tail gas is discharged into the absorption tower for purification treatment.

[0023] The mother liquor I is combined with the sodium sulfite solution with a concentration of 8.0-28.0% to continue concentration under reduced pressure at ≤80℃ to near saturation state, and then is continuously introduced into the crystallizer C for water cooling to precipitate sodium sulfite heptahydrate crystals.

[0024] The sodium sulfate refers to anhydrous sodium sulfate or sodium sulfate decahydrate. The sodium sulfate mixed salt refers to sodium sulfate containing 1.0-30.0% of anhydrous sodium sulfite, or sodium sulfate decahydrate containing 1.0-20.0% of sodium sulfite heptahydrate. The alkali refers to soda ash, caustic soda, quicklime or calcium hydroxide.

[0025] Any one of the following separation methods or a combination of at least two of the following separation methods: solid-liquid separation filtration A, centrifugal solid-liquid separation B; the solid-liquid separation equipment is preferably a centrifuge, a suction filter, a plate and frame filter press, a hydrocyclone, etc.

[0026] When it is detected that the impurities such as sulfate radicals and calcium and magnesium ions in the mother liquor II exceed the standard, the pump is used to punch into the second-stage SO2 stirring reactor for reuse.

[0027] The SO2 tail gas treatment is provided with two-stage absorption towers for countercurrent absorption, and the SO2 tail gas is absorbed and purified by spraying lime milk or dilute alkali solution.

[0028] The present application has the following advantages and beneficial effects:

[0029] 1) The present application directly prepares high-purity sodium sulfite and co-produces sodium metabisulfite product from industrial by-products sodium sulfate or sodium sulfate mixed salt, and simultaneously produces high-purity gypsum dihydrate; thus, the production cost of sodium sulfite or sodium metabisulfite can be greatly reduced, and a large amount of sodium sulfate by-product in the chemical industry can be consumed and turned into treasure.

[0030] 2) The present application is based on the chemical reaction principle that excessive SO2 is introduced into the suspension liquid prepared from sodium sulfate and quicklime to generate sodium bisulfite, and realizes the purpose of preparing sodium sulfite product and co-producing sodium metabisulfite product under the conditions of less raw material consumption, low energy consumption and low production cost.

[0031] 3) The present application aims to solve the technical problems of high alkali consumption and high cost in directly preparing sodium sulfite from sodium sulfate or its hydrate mixed salt, and the existing technology generally only produces liquid sodium bisulfite product with many impurities, and the concentration of NaHSO3 is 35.0-40.0%, if the production of anhydrous sodium sulfite or sodium metabisulfite is continued, there are problems of high alkali consumption, high cost and low economic benefit, and it is difficult to solve the technical problems of excessive calcium, magnesium and iron impurity ions in the prepared sodium sulfite or sodium metabisulfite product and reducing the alkali neutralization consumption; if only liquid sodium bisulfite product is produced without alkali neutralization, although the production cost can be greatly reduced, the water content of liquid sodium bisulfite is as high as 60-65%, which is not convenient for storage and transportation, the product is limited by the purpose and small amount of users, and the radiation range of the sales market network is also relatively low.

[0032] 4) After excessive SO2 is introduced into the suspension liquid, the solution is acidified, calcium and magnesium ions brought in by the quicklime also generate water-soluble calcium bisulfite and magnesium sulfite, sodium ions in the sodium sulfate combine with bisulfite ions in the solution to form sodium bisulfite, and sulfate radicals combine with calcium ions to form calcium sulfate. When the acidic sodium bisulfite solution is neutralized by alkali, with the increase of pH value, part of the calcium ions combine with carbonate radicals to form calcium carbonate precipitate, and then the newly generated solid precipitate easily causes equipment scaling, so it is necessary to control the amount of soda ash, and maintain the reasonable pH value range of the neutralization liquid at 4.5-9.0.

[0033] 5) The present application is to improve the concentration of sodium sulfite solution, using a closed screw feeder to add dry powder alkali and online pH instrument control, to achieve precise adjustment and control of the amount of alkali used for neutralization.

[0034] 6) The present application is to use the property of sodium sulfite to generate heptahydrate at room temperature, to ensure the purity of the precipitated sodium sulfite heptahydrate crystals, and to achieve efficient separation of the impurities contained in the mother liquor, such as other inorganic salts and organic matter. At the same time, the generated heptahydrate crystals also bring out a large amount of pure water from the solution, reducing the heat energy consumption of sodium sulfite solution evaporation.

[0035] 7) The present application is to neutralize the 2, 3 times mother liquor after the precipitation of sodium sulfite heptahydrate to ≥11.0 magnesium, reducing the alkali consumption by 45-50% compared with the conventional process.

[0036] 8) The present application is to adopt low-temperature and reduced-pressure evaporation concentration for sodium sulfite solution, and water-cooled precipitation of pure sodium sulfite heptahydrate crystals, solving the problem of purification of sodium sulfite and sodium pyrosulfite products. The present application is to directly dry and dehydrate the sodium sulfite heptahydrate crystals to obtain anhydrous sodium sulfite product, solving the problem of easy oxidation to sodium sulfate during high-temperature evaporation concentration of sodium sulfite solution. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 Process flow chart of the present application DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only for the purpose of explaining the present application and do not limit the present application.

[0039] The main purpose of the process route of the present application is to use the double decomposition reaction of sulfur dioxide, sodium sulfate and quicklime to produce liquid sodium bisulfite with high impurity content into high-purity sodium sulfite and sodium pyrosulfite products. It solves the current situation of small market capacity, few users and inconvenient storage and transportation of liquid sodium bisulfite, and solves the problems of high alkali consumption and large evaporation heat energy consumption in the production process. At the same time, it takes advantage of the property of cooling to precipitate high-purity sodium sulfite heptahydrate crystals at room temperature, greatly reducing the impurity content in sodium sulfite and sodium pyrosulfite products.

[0040] Example 1

[0041] a) Add water and sodium sulfate to the SO2 stirring reactor, keep the solution temperature at 40-45℃ until all the sodium sulfate is dissolved, add CaO to become a suspension; the molar ratio of sodium sulfate to calcium oxide CaO is 1:0.40.

[0042] b) In the suspension of SO2 stirring reactor, SO2 is introduced to carry out the metathesis reaction, and the reaction temperature is controlled at 33.0-48°C by water cooling. After the solution is saturated with SO2, the pH of the conversion solution is measured to be 3.0. After the suspension is saturated with SO2, the pH of the solution is measured to be 2.8-3.5. The liquid phase is converted into a mixed solution containing sodium bisulfite, calcium bisulfite and calcium sulfate dihydrate. The introduction of SO2 is stopped. The mixed solution in which SO2 is stopped is transferred into an alkali neutralization reactor ① to be neutralized with soda ash, and the pH of the solution is adjusted to 7.5. The solution is filtered by a solid-liquid separator A to remove insoluble substances such as gypsum, and a sodium sulfite solution with a concentration of about 18.0-20.0% is obtained.

[0043] c) The sodium sulfite solution is concentrated to a near-saturated state in an evaporator at ≤65°C under reduced pressure. The concentrated solution is transferred into a cooling crystallizer C, and sodium sulfite is crystallized in the form of heptahydrate by stirring at ≤20°C for 3.0 hr. The sodium sulfite heptahydrate is separated by a centrifugal solid-liquid separator B. The mother liquor I is returned to the evaporator for reuse.

[0044] d) The sodium sulfite heptahydrate crystals are dried by a gas flow dryer at ≤150°C to obtain anhydrous sodium sulfite product. The conversion rate of sodium sulfite is 96.0%, and the purity of the product is ≥95.0%.

[0045] e) In the SO2 3-stage absorption reactor, water and sodium sulfite heptahydrate are mixed to form a saturated solution. SO2 is continuously introduced until the pH of the solution is 4.0. Sodium metabisulfite crystals are precipitated from the saturated solution by water cooling at 40°C for 3.0 hr in a crystallizer D. The sodium metabisulfite crystals are separated by a centrifugal solid-liquid separator E. The product is dried by a gas flow dryer at ≤120°C to obtain anhydrous sodium metabisulfite product with a purity of ≥96.0%. The mother liquor II is returned to the SO2 3-stage absorption reactor for continuous absorption of SO2.

[0046] Example 2

[0047] a) In the SO2 stirring reactor, water and sodium sulfate mixed salt (containing 23.0% sodium sulfite) are added to keep the solution temperature at 30-45°C until the sodium sulfate is completely dissolved. CaO is added to form a suspension. The molar ratio of sodium sulfate to calcium oxide CaO is 1:0.45.

[0048] b) In the SO2 stirring reactor, SO2 is bubbled into the suspension to carry out the metathesis reaction. The reaction temperature is controlled at 35.0-46°C by water cooling. When the pH of the conversion solution is 2.9, the liquid phase has been converted into a mixture of sodium bisulfite and calcium sulfate dihydrate, and the bubbling of SO2 is stopped. The SO2 gas is switched to another stirring reactor for continuous absorption. The mixture in which the absorption of SO2 is stopped is moved into the alkali neutralization reactor ① and neutralized with soda ash. The pH of the solution is adjusted to 6.0. The gypsum and other insoluble substances are removed by filtration through the solid-liquid separation filter A, and a sodium sulfite solution with a concentration of about 15.0-16.0% is obtained.

[0049] c) The above sodium sulfite solution is concentrated to a near-saturated state under reduced pressure at ≤75°C. The concentrated solution is moved into the cooling crystallizer C and stirred at ≤18°C for 2.5 hr to make the sodium sulfite heptahydrate form crystallize. The sodium sulfite heptahydrate is obtained by centrifugal solid-liquid separation B. The product is dried and dehydrated in an air flow dryer at ≤150°C to obtain anhydrous sodium sulfite. The conversion rate of sodium sulfite is 95.0%, and the purity of the product is ≥93.0%.

[0050] d) When it is detected that the sulfate in the mother liquor I is ≥3.0% or the calcium and magnesium is ≥2.0%, the mother liquor I is sent to the alkali neutralization ② for treatment. Sodium hydroxide is added to neutralize the pH value to 12.0. The calcium and magnesium residues in the mother liquor I are removed by filtration, and then the filtrate is transferred to the second SO2 stirring reactor to make the accumulated sulfate and calcium ions react to form gypsum precipitate and be removed.

[0051] e) In the SO2 absorption reactor, water and sodium sulfite heptahydrate are added to form a saturated solution. SO2 is continuously bubbled into the solution until the pH value is 3.9. The saturated solution is transferred to the crystallizer D and stirred at 37-39°C for 2.5 hr to make the sodium metabisulfite crystallize in the saturated liquid phase. The sodium metabisulfite product is obtained by centrifugal dehydration in the centrifugal solid-liquid separation E. The product is dried in an air flow dryer at ≤120°C to obtain anhydrous sodium metabisulfite product with a purity of ≥95.0%. The mother liquor II is returned to the SO2 absorption reactor for continuous absorption of SO2.

[0052] Example 3

[0053] a) In the SO2 absorption stirring reactor, water and sodium sulfate decahydrate are added to keep the solution temperature at 30-45°C until the sodium sulfate is completely dissolved. Ca(OH)2 is added to become a suspension. The molar ratio of sodium sulfate decahydrate mixed salt to calcium hydroxide Ca(OH)2 is 1:0.22.

[0054] b) In the SO2 stirring reactor, SO2 is bubbled into the suspension to carry out the metathesis reaction. The reaction temperature is controlled at 30.0-50.0°C, and the stirring reaction time is 2.5 hours. After the conversion solution absorbs SO2 supersaturation, the pH of the conversion solution is measured to be 3.0-3.1, and the liquid phase has been converted into a mixed solution containing sodium bisulfite, calcium bisulfite and calcium sulfate dihydrate. The SO2 gas is stopped, and is switched to another stage of absorption reactor for continuous absorption. The mixed solution after stopping the absorption of SO2 is transferred into the alkali neutralization reactor ① for neutralization with soda ash. The pH of the solution is adjusted to 6.5-7.0. The gypsum and other insoluble substances are removed by filtration and washing in the solid-liquid separation A to obtain a 20.0% concentrated sodium sulfite solution.

[0055] c) The above sodium sulfite solution is concentrated under reduced pressure at ≤60°C to near saturation. The concentrated solution is transferred into the cooling crystallizer C for water cooling and stirring at ≤23°C for 2.5 hours. Sodium sulfite is crystallized in the form of heptahydrate, which is centrifuged and dewatered in the solid-liquid separation B to obtain sodium sulfite heptahydrate. The mother liquor I is combined with the low-concentration sodium sulfite solution without concentration, and is continuously evaporated and concentrated at low temperature to near saturation.

[0056] d) When the sulfate in the mother liquor I is ≥3.0% or the calcium and magnesium impurities are ≥2.0%, the mother liquor I is sent to the alkali neutralization filter ② for neutralization with sodium hydroxide at pH 13.0. The calcium and magnesium residues in the mother liquor are precipitated and removed, and then the filtrate is transferred into the second stage SO2 absorption reactor to react the accumulated sulfate and calcium ions to generate gypsum precipitate for removal.

[0057] e) Several batches of sodium sulfite heptahydrate crystals are mixed and homogenized, and are sent to a roller dryer or an air flow dryer for drying and dewatering at ≤150°C to obtain anhydrous sodium sulfite product. The conversion rate of sodium sulfite is 97.0%, and the product purity is ≥95.0%.

[0058] f) In the SO2 3 absorption reactor, water and sodium sulfite heptahydrate are mixed to form a saturated solution. SO2 is continuously bubbled into the saturated solution until the pH value is 3.9. After water cooling at 36-38°C for 2.0 hours in the crystallizer D, the saturated solution phase precipitates sodium metabisulfite crystals, which are transferred into the solid-liquid separation E for centrifugal dewatering to obtain sodium metabisulfite product. After drying at ≤120°C, anhydrous sodium metabisulfite product is obtained, and the product purity is ≥94.0%. The mother liquor II is returned to the SO2 3 absorption reactor for continuous SO2 absorption.

[0059] Example 4

[0060] a) In the SO2 absorption stirring reactor, water and sodium sulfate mixed salt (containing 15.0% sodium sulfite heptahydrate) are mixed. The solution temperature is maintained at 30-45°C until the sodium sulfate mixed salt is completely dissolved. Ca(OH)2 is added to form a suspension. The molar ratio of sodium sulfate mixed salt to calcium hydroxide Ca(OH)2 is 1:0.25.

[0061] b) In the SO2 stirring reactor, the suspension is bubbled with SO2 to carry out the metathesis reaction, the reaction temperature is controlled at 35.0-50.0℃, the stirring reaction time is 2.5 hr, after the SO2 supersaturation is absorbed by the conversion solution, the pH of the conversion solution is measured at 2.9-3.0, the liquid phase has been converted into a mixed solution containing sodium bisulfite, calcium bisulfite and calcium sulfate dihydrate; stop bubbling SO2; switch the SO2 gas to another stage of absorption reactor for continuous absorption. In the mixed solution of stopping SO2 absorption, the pure lye is added into the filter ① to neutralize the solution and adjust the pH to 7.0-7.5, the gypsum and other insoluble substances are removed by solid-liquid separation A to obtain a 23.0% concentration of sodium sulfite solution.

[0062] c) The above sodium sulfite solution is concentrated to near saturation state at ≤65℃ under reduced pressure, the concentrated solution is moved into the cooling crystallizer C, water-cooling stirring is carried out at 20℃ for 4.0 hr, sodium sulfite heptahydrate is precipitated, then into the solid-liquid separation B centrifugal dewatering to obtain sodium sulfite heptahydrate; the mother liquor I and the un-concentrated sodium sulfite solution are combined to continue low-temperature evaporation concentration to near saturation state, the above-mentioned same way is continued to carry out water-cooling salt precipitation.

[0063] d) When the sulfate in the mother liquor I is ≥3.0% or the calcium and magnesium impurities are ≥2.0%, the mother liquor I is sent to the lye neutralization filter ② to remove the magnesium ions in the mother liquor by neutralizing with caustic lye to control the pH value at 12.0, then the mother liquor is transferred into the second stage SO2 absorption reactor to make the accumulated sulfate in the mother liquor react with calcium ions to generate gypsum precipitate to be removed.

[0064] e) The sodium sulfite heptahydrate crystal is mixed and homogenized, sent to the roller dryer ≤150℃ for drying and dewatering to obtain the anhydrous sodium sulfite product, the conversion rate of sodium sulfite is 95.0%, the product purity is ≥95.0%.

[0065] f) In the sodium metabisulfite 3-stage absorption conversion tank, the sodium sulfite heptahydrate is added to form a saturated solution, SO2 is continuously bubbled until the pH value of the saturated solution is 4.1, the sodium metabisulfite crystal is precipitated in the crystallizer D after water-cooling at 40-42℃ for 2.0 hr, the sodium metabisulfite product is obtained by centrifugal solid-liquid separation E dewatering, after drying at ≤120℃, the anhydrous sodium metabisulfite product is obtained, the product purity is ≥95.0%; when the sodium sulfate concentration in the mother liquor exceeds 2.0%, the mother liquor II is pumped into the second stage SO2 stirring reactor to continue the metathesis reaction with SO2 to remove the accumulated sulfate in the mother liquor.

[0066] The above examples are only one of the various embodiments, for those skilled in the art, on the basis of the above description, other different forms of changes or changes can also be made, and these belong to the essential spirit of the present application and the other changes derived from the present application still belong to the protection scope of the present application.

Claims

1. A process for the industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate, characterized in that: In the lime tank, water, sodium sulfate or sodium sulfate hydrate mixed salt, CaO or Ca(OH)2 are added to prepare a suspension under stirring, and the solution temperature is maintained at 30-50℃ until the sodium sulfate is completely dissolved, and the molar ratio of sodium sulfate or its hydrate mixed salt to CaO or Ca(OH)2 is 1:0.20-0.8; the following processes are subdivided: a) The SO2 absorption and stirring reactor is airtight, and is provided with 1, 2, 3 series of countercurrent absorption. SO2 gas is introduced into the suspension to cause a double decomposition reaction, and the reaction temperature is controlled by water cooling at 30.0-55.0℃. After the suspension absorbs SO2 to supersaturation, the pH of the solution is measured to be 2.8-3.5, and the liquid phase is converted into a mixed solution containing sodium bisulfite, calcium bisulfite and calcium sulfate dihydrate. The introduction of SO2 is stopped, and the SO2 gas is switched to another 2-stage absorption reactor for continuous absorption. The suspension that stops absorbing SO2 is sent to the alkali neutralization reactor ①, and after neutralization to a pH of 4.5-8.0, it is transferred to the solid-liquid separation A for filtration to remove gypsum and other precipitates, and a sodium sulfite solution with a concentration of 8.0-28.0% is obtained; b) The sodium sulfite solution with a concentration of 8.0-28.0% is introduced into an evaporator for concentration under reduced pressure at ≤80℃ to a near-saturated state, and the concentrated solution is moved to the crystallizer C for water cooling at ≤24℃ for 1.0-4.0 hours to precipitate sodium sulfite heptahydrate crystals and mother liquor I. The sodium sulfite heptahydrate crystals are sent to a dryer for drying at ≤150℃ to obtain anhydrous sodium sulfite product. When the sulfate in the mother liquor I is ≥3.0% or the calcium and magnesium impurities are ≥2.0%, the mother liquor I is introduced into the alkali neutralization and filtration ②, and is neutralized with alkali to a pH of 11.0-12.

0. The calcium and magnesium residues are removed by filtration, and the filtrate is sent to the second-stage SO2 absorption and stirring reactor for double decomposition reaction to generate gypsum precipitates to remove sulfate; c) Water and sodium sulfite heptahydrate are added to the third-stage sodium metabisulfite absorption reactor to prepare a saturated solution, and SO2 is continuously introduced until the pH of the saturated solution is 3.8-4.

1. Sodium metabisulfite crystals are precipitated in the crystallizer D under water cooling at ≤45℃ for 1.0-5.0 hours, and are sent to the centrifugal solid-liquid separation E for dehydration to obtain sodium metabisulfite wet product and mother liquor II. The wet product is dried at ≤120℃ to obtain anhydrous sodium metabisulfite product; The mother liquor II is returned to the third-stage SO2 absorption reactor for reuse, and the SO2 tail gas is discharged into an absorption tower for purification treatment.

2. The industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate according to claim 1, characterized by: The mother liquor I and the sodium sulfite solution with a concentration of 8.0-28.0% are combined and continuously concentrated under reduced pressure at ≤80℃ to a near-saturated state, and are then introduced into the crystallizer C for water cooling to precipitate sodium sulfite heptahydrate crystals.

3. The industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate according to claim 1, characterized by: Sodium sulfate refers to anhydrous sodium sulfate or sodium sulfate decahydrate; the sodium sulfate mixed salt refers to sodium sulfate containing 1.0-30.0% of anhydrous sodium sulfite, or sodium sulfate decahydrate containing 1.0-20.0% of sodium sulfite heptahydrate; the alkali refers to soda ash, caustic soda, quicklime or calcium hydroxide.

4. The industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate according to claim 1, characterized by: The separation mode of the solid-liquid separation filtration A and the centrifugal solid-liquid separation B is any one or a combination of at least two; the solid-liquid separation equipment is preferably a centrifuge, a suction filter, a plate and frame filter press, a hydrocyclone, etc.

5. The industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate according to claim 1, characterized by: When the impurities such as sulfate, calcium and magnesium ions in mother liquor II exceed the standard, the pump is used to pump into the second-stage SO2 stirring reactor for reuse.

6. The industrial production of sodium sulfite and co-production of sodium metabisulfite from sodium sulfate according to claim 1, characterized by: The SO2 tail gas treatment is provided with two-stage absorption tower countercurrent absorption, and the spray of lime milk or dilute lye is used for absorption purification treatment.

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