A method for preparing magnesium carbonate and co-producing ammonium sulfate by using magnesium sulfite as a by-product of desulfurization in a thermal power plant

Abstract

This invention discloses a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants, specifically relating to the fields of inorganic chemical engineering and secondary resource recycling. The invention involves pre-treating magnesium sulfite raw material through oxidation and purification to obtain a magnesium sulfate solution, which is then reacted with ammonium bicarbonate in the presence of a catalyst in a metathesis reaction. The resulting magnesium carbonate slurry is subjected to solid-liquid separation, washing, and drying to obtain the magnesium carbonate product. The reaction mother liquor and washing liquid are mixed and regenerated at room temperature using an ammonia medium to convert it into an ammonium sulfate solution. Finally, the ammonium sulfate product is obtained through evaporation and crystallization. The regenerated ammonia can be reused in the metathesis reaction process. This invention solves the problems of high secondary pollution risk, high energy consumption, low resource utilization, and insufficient product added value in existing magnesium sulfite treatment processes. The process of this invention is short, the reaction conditions are mild, the product purity is high, and the yield is stable, achieving the harmless disposal and high-value utilization of desulfurization solid waste.

Description

Technical Field

[0001] This invention relates to the field of inorganic chemical engineering and secondary resource recycling technology, and to a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants. Background Technology

[0002] Magnesium-based desulfurization technology is widely used in the flue gas desulfurization process of thermal power plants, which produces a large amount of magnesium sulfite as a byproduct. Statistics show that my country produces more than 5 million tons of magnesium sulfite annually. This substance is chemically unstable and easily decomposes. During storage, it releases sulfur dioxide when exposed to rainwater, causing secondary pollution. The small amounts of heavy metal impurities it contains can also enter the soil and water bodies through leachate, exacerbating environmental pollution. Currently, most thermal power plants treat it as solid waste and dump it in landfills, which not only occupies a large amount of land resources but also seriously threatens ecological and environmental security.

[0003] Current mainstream technologies for treating magnesium sulfite all have significant shortcomings, making it difficult to achieve both harmlessness and resource recovery. Direct calcination requires high temperatures, resulting in high energy consumption, large equipment investment, and low sulfur dioxide recovery efficiency, easily causing air pollution. Acid hydrolysis consumes high amounts of acid, has highly corrosive equipment, and is difficult to treat sulfur dioxide, posing significant environmental risks. While oxidation can convert magnesium sulfite into magnesium sulfate, magnesium sulfate has low market value and limited applications, failing to achieve high-value utilization of byproducts and resulting in extremely poor economic benefits.

[0004] Magnesium carbonate is an important inorganic chemical product widely used in rubber, plastics, coatings, pharmaceuticals, and other fields. Currently, its mainstream preparation process relies on magnesite calcination or seawater / brine precipitation, which suffers from high production costs and limited raw material resources. Ammonium sulfate is a major fertilizer and industrial raw material with strong market demand, and traditional production methods are highly dependent on its raw materials. If magnesium sulfite, a byproduct of desulfurization, can be efficiently converted into high-value-added magnesium carbonate and ammonium sulfate, it can not only completely solve the environmental problems it causes but also realize the resource value-added of industrial solid waste, possessing significant environmental and economic value. Summary of the Invention

[0005] To overcome the aforementioned deficiencies of the prior art, embodiments of the present invention provide a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants. This method addresses the industry pain points of storing and disposing of magnesium sulfite, a byproduct of magnesium-based desulfurization in thermal power plants, which easily causes secondary pollution; and the existing treatment processes suffer from high energy consumption, high pollution risk, low resource utilization, and insufficient product added value. Through a closed-loop wet process, the harmless disposal and high-value resource utilization of this industrial solid waste are achieved, simultaneously producing high-purity magnesium carbonate and ammonium sulfate products that meet national standards, thus balancing environmental and economic benefits.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants, specifically includes the following steps:

[0008] S1: Place magnesium sulfite slurry in a reactor, then add oxidant at a mass ratio of magnesium sulfite: oxidant = 0.5~2:1, and control the pH of the mixture to 3~6, ​​the system temperature to 50~90℃, and the reaction time to 1~4h to obtain magnesium sulfate solution;

[0009] S2: Add flocculant to the magnesium sulfate solution obtained in S1 at a mass ratio of flocculant:wastewater = 1~5:1. Then adjust the pH of the system to 6.0~9.0 and the temperature of the mixed system to 30~70℃. After settling for 30~180 minutes, collect the sediment and clear liquid respectively.

[0010] S3: Add ammonium bicarbonate to the clear liquid obtained in S2 at a molar ratio of ammonium ion: magnesium ion > 2.0. Then add catalyst to the solution at a molar ratio of sulfate ion: catalyst = 0.5~2:1 and react for 40~60 min at 20~50℃, pH = 7.0~9.0 and stirring speed of 200~500 r / min to obtain magnesium carbonate slurry.

[0011] S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water to obtain clean magnesium carbonate filter cake.

[0012] S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer. After it is completely dried, place the magnesium carbonate in an electric resistance furnace and heat it to 450±5℃ at a heating rate of 5℃ / min and continue to calcine for 2 hours to obtain high-purity light magnesium carbonate.

[0013] S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature to obtain an ammonium sulfate solution.

[0014] S7: The ammonium sulfate solution obtained in S6 is heated and evaporated to crystallize ammonium sulfate.

[0015] Preferably, the magnesium sulfite slurry in S1 is made from magnesium sulfite obtained by desulfurization.

[0016] Preferably, the magnesium sulfite slurry contains ≥30% magnesium sulfite, and the magnesium sulfite obtained from desulfurization has a purity of ≥70%.

[0017] Preferably, the oxidant in S1 is any one or a combination of at least two of hydrogen peroxide, ozone, and oxygen.

[0018] Preferably, the flocculant in S2 includes any one or a combination of at least two of anionic polyacrylamide, nonionic polyacrylamide, polyaluminum chloride, and polyaluminum sulfate.

[0019] Preferably, the catalyst in S3 comprises any one or a mixture of at least two of diammonium hydrogen phosphate, magnesium dihydrogen phosphate, ammonium nitrate, magnesium nitrate, ammonium formate, and magnesium formate.

[0020] Preferably, the clean magnesium carbonate filter cake mentioned in S4 refers to a filter cake with an impurity ion content of ≤0.1% in the washing liquid after washing.

[0021] Preferably, the flash dryer in S5 is configured with an inlet air temperature of 220±10℃, an outlet air temperature of 95±5℃, and a drying air volume of 5000m³. 3 / h, dwell time is 30s.

[0022] Preferably, the reaction temperature of the ammonia medium regeneration reaction tank in S6 is 25~35℃ and the duration is 3~11h.

[0023] Preferably, the heating and evaporation temperature in S7 is 50~90℃.

[0024] The technical effects and advantages of the method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants, as described in this invention are as follows:

[0025] 1. This invention enables the harmless disposal and high-value resource utilization of magnesium sulfite, a byproduct of magnesium desulfurization in thermal power plants. It fundamentally solves the problems of easy decomposition of magnesium sulfite during storage and secondary pollution caused by leaching. The conversion rate of magnesium sulfite can reach over 99%, transforming low-value industrial solid waste into high-value-added magnesium carbonate and ammonium sulfate products. This breakthrough overcomes the industry bottleneck of low resource utilization rate and insufficient product added value of magnesium sulfite in existing technologies.

[0026] 2. This invention features a short process flow, mild reaction conditions, and significantly reduced energy consumption and operating costs. The core reactions are all carried out in the range of normal temperature and pressure. The metathesis reaction and ammonia medium regeneration do not require a high temperature and high pressure environment. Compared with the traditional 650℃ high-temperature calcination method, multiple evaporation, and cooling purification method, energy consumption is greatly reduced. At the same time, it avoids the defects of strong acid corrosion of equipment and high acid consumption in the acid hydrolysis method, and significantly reduces equipment investment and industrial operation costs.

[0027] 3. This invention produces high-quality, stable-yield products that combine atom economy and economic benefits. By precisely controlling the crystallization kinetics and supersaturation of the system through a catalyst, it can prepare high-purity light magnesium carbonate with a purity of ≥98.7%. The product has controllable particle size and good dispersibility, which can meet the application needs of high-end fields such as rubber and pharmaceuticals. The co-produced ammonium sulfate product meets the national first-class standard, with magnesium carbonate yield exceeding 96% and ammonium sulfate yield exceeding 95%. Compared with the traditional oxidation method, which can only produce low-value magnesium sulfate, the economic benefits are significantly improved.

[0028] 4. This invention achieves a closed-loop material circulation throughout the entire process, making it green and environmentally friendly with strong process adaptability. The ammonia medium can be reused in the metathesis reaction process after being regenerated at room temperature, with no wastewater or toxic and harmful exhaust gas discharged, completely avoiding the air pollution caused by SO2 emission in traditional processes. At the same time, the process has a wide operating window, strong resistance to impurity interference, and relaxed requirements on the purity of the raw material magnesium sulfite. It can be adapted to complex raw materials containing impurities such as chloride ions. Ammonium chloride is produced as a byproduct through graded crystallization, which is easy to scale up and continuously produce industrially. Attached Figure Description

[0029] Figure 1 This is a flowchart of a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as proposed in this invention. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0031] It should be noted that, in this document, relational terms such as "first" and "second" are used only 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 "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0032] Example 1:

[0033] This embodiment provides a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant. The specific implementation steps include:

[0034] Experimental materials:

[0035] Magnesium sulfite slurry (magnesium sulfite content 30%), hydrogen peroxide (H2O2 content 30%), polyaluminum chloride (Al2O3 mass fraction ≥28%), polyacrylamide (molecular weight 8 million~12 million), ammonium bicarbonate, diammonium hydrogen phosphate, and deionized water.

[0036] Experimental objective:

[0037] A method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in thermal power plants.

[0038] Experimental steps:

[0039] S1: Place magnesium sulfite slurry in a reactor, then add hydrogen peroxide in a mass ratio of MgSO3:H2O2=1:1, and control the pH of the mixture to 5, the system temperature to 70℃, and the reaction time to 2h to obtain magnesium sulfate solution.

[0040] S2: Add polyaluminum chloride to the magnesium sulfate solution obtained in S1 at a mass ratio of insoluble matter to polyaluminum chloride = 1:3 and stir continuously at 500 r / min for 2 min. Then add polyacrylamide at a mass ratio of polyaluminum chloride to polyacrylamide = 1000:1 and stir continuously at 100 r / min for 10 min. Then adjust the pH of the system to 7.0±0.2 and the temperature of the mixed system to 50±2℃. Finally, after settling for 120 min, collect the precipitate and the clear liquid separately.

[0041] S3: Add ammonium bicarbonate to the clear liquid obtained in S2 at a molar ratio of ammonium ions: magnesium ions = 2.2:1, then add diammonium hydrogen phosphate to the solution at a molar ratio of sulfate ions: diammonium hydrogen phosphate = 1:1, and react for 40 min at 35℃, pH = 8.0, and stirring speed of 200 r / min to obtain magnesium carbonate slurry;

[0042] S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water until the impurity ion content in the washing liquid is ≤0.1% to obtain clean magnesium carbonate filter cake.

[0043] S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer, setting the inlet air temperature to 220±10℃, the outlet air temperature to 95±5℃, and the drying air volume to 5000m³ / h. 3 / h, residence time is 30s, after which the magnesium carbonate is placed in an electric resistance furnace and heated to 450±5℃ at a heating rate of 5℃ / min and calcined for 2h to obtain high-purity light magnesium carbonate.

[0044] S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature. Set the reaction temperature of the ammonia medium regeneration reaction tank to 30℃ and the duration to 7h to obtain an ammonium sulfate solution.

[0045] S7: Heat the ammonium sulfate solution obtained in S6 to 70°C and evaporate to crystallize to obtain ammonium sulfate.

[0046] Experimental results: See Table 1 for details.

[0047] Table 1: Test Results of Example 1

[0048]

[0049] Example 2:

[0050] This embodiment provides a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant. The specific implementation steps include:

[0051] Experimental materials:

[0052] Magnesium sulfite slurry (magnesium sulfite content 30%, chloride ion content 1%), hydrogen peroxide (H2O2 content 30%), polyaluminum chloride (Al2O3 mass fraction ≥28%), polyacrylamide (molecular weight 8 million~12 million), ammonium bicarbonate, diammonium hydrogen phosphate, and deionized water.

[0053] Experimental objective:

[0054] To investigate the effect of chloride ions in magnesium sulfite slurry on the final production of ammonium sulfate.

[0055] Experimental steps:

[0056] S1: Place magnesium sulfite slurry in a reactor, then add hydrogen peroxide in a mass ratio of MgSO3:H2O2=1:1, and control the pH of the mixture to 5, the system temperature to 70℃, and the reaction time to 2h to obtain magnesium sulfate solution.

[0057] S2: Add polyaluminum chloride to the magnesium sulfate solution obtained in S1 at a mass ratio of insoluble matter to polyaluminum chloride = 1:3 and stir continuously at 500 r / min for 2 min. Then add polyacrylamide at a mass ratio of polyaluminum chloride to polyacrylamide = 1000:1 and stir continuously at 100 r / min for 10 min. Then adjust the pH of the system to 7.0±0.2 and the temperature of the mixed system to 50±2℃. Finally, after settling for 120 min, collect the precipitate and the clear liquid separately.

[0058] S3: Add ammonium bicarbonate to the clear liquid obtained in S2 at a molar ratio of ammonium ions: magnesium ions = 2.2:1, then add diammonium hydrogen phosphate to the solution at a molar ratio of sulfate ions: diammonium hydrogen phosphate = 1:1, and react for 40 min at 35℃, pH = 8.0, and stirring speed of 200 r / min to obtain magnesium carbonate slurry;

[0059] S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water until the impurity ion content in the washing liquid is ≤0.1% to obtain clean magnesium carbonate filter cake.

[0060] S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer, setting the inlet air temperature to 220±10℃, the outlet air temperature to 95±5℃, and the drying air volume to 5000m³ / h. 3 / h, residence time is 30s, after which the magnesium carbonate is placed in an electric resistance furnace and heated to 450±5℃ at a heating rate of 5℃ / min and calcined for 2h to obtain high-purity light magnesium carbonate.

[0061] S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature. Set the reaction temperature of the ammonia medium regeneration reaction tank to 30℃ and the duration to 7h to obtain an ammonium sulfate solution.

[0062] S7: Heat the ammonium sulfate solution obtained in S6 to 70°C and evaporate to crystallize to obtain ammonium sulfate.

[0063] Experimental results: See Table 2 for details.

[0064] Table 2: Test Results of Example 2

[0065]

[0066] Example 3:

[0067] This embodiment provides a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant. The specific implementation steps include:

[0068] Experimental materials:

[0069] Magnesium sulfite slurry (magnesium sulfite content 30%), ozone, polyaluminum chloride (Al2O3 mass fraction ≥28%), polyacrylamide (molecular weight 8 million~12 million), ammonium bicarbonate, magnesium nitrate, and deionized water.

[0070] Experimental objective:

[0071] Ozone was used to oxidize magnesium sulfite slurry, with magnesium nitrate as the catalyst, to investigate the effect of ozone on the final ammonium sulfate.

[0072] Experimental steps:

[0073] S1: Place magnesium sulfite slurry in a reactor, then add ozone at a mass ratio of MgSO3:O3=1:1.5, and control the pH of the mixture to 4, the system temperature to 80℃, and the reaction time to 1.5h to obtain magnesium sulfate solution;

[0074] S2: Add polyaluminum chloride to the magnesium sulfate solution obtained in S1 at a mass ratio of insoluble matter to polyaluminum chloride = 1:3 and stir continuously at 500 r / min for 2 min. Then add polyacrylamide at a mass ratio of polyaluminum chloride to polyacrylamide = 1000:1 and stir continuously at 100 r / min for 10 min. Then adjust the pH of the system to 7.0±0.2 and the temperature of the mixed system to 50±2℃. Finally, after settling for 120 min, collect the precipitate and the clear liquid separately.

[0075] S3: Add ammonium bicarbonate to the clear liquid obtained in S2 at a molar ratio of ammonium ions: magnesium ions = 2.5:1, then add magnesium nitrate to the solution at a molar ratio of sulfate ions: magnesium nitrate = 1.5:1, and react for 40 min at 40℃, pH = 8.2, and stirring speed of 200 r / min to obtain magnesium carbonate slurry;

[0076] S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water until the impurity ion content in the washing liquid is ≤0.1% to obtain clean magnesium carbonate filter cake.

[0077] S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer, setting the inlet air temperature to 220±10℃, the outlet air temperature to 95±5℃, and the drying air volume to 5000m³ / h. 3 / h, residence time is 30s, after which the magnesium carbonate is placed in an electric resistance furnace and heated to 450±5℃ at a heating rate of 5℃ / min and calcined for 2h to obtain high-purity light magnesium carbonate.

[0078] S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature. Set the reaction temperature of the ammonia medium regeneration reaction tank to 30℃ and the duration to 7h to obtain an ammonium sulfate solution.

[0079] S7: Heat the ammonium sulfate solution obtained in S6 to 70°C and evaporate to crystallize to obtain ammonium sulfate.

[0080] Experimental results: See Table 3 for details.

[0081] Table 3: Test Results of Example 3

[0082]

[0083] Example 4:

[0084] This embodiment provides a method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant. The specific implementation steps include:

[0085] Experimental materials:

[0086] Magnesium sulfite slurry (magnesium sulfite content 30%), oxygen, polyaluminum chloride (Al2O3 mass fraction ≥28%), polyacrylamide (molecular weight 8 million~12 million), magnesium dihydrogen phosphate, magnesium nitrate, and deionized water.

[0087] Experimental objective:

[0088] To investigate the effects of the oxygen oxidation system and magnesium dihydrogen phosphate catalyst on the final performance of magnesium carbonate and ammonium sulfate products.

[0089] Experimental steps:

[0090] S1: Place magnesium sulfite slurry in a reactor, then introduce oxygen at a mass ratio of MgSO3:O2=0.8:1, and control the pH of the mixture to 5, the system temperature to 60℃, and the reaction time to 3h to obtain magnesium sulfate solution;

[0091] S2: Add polyaluminum chloride to the magnesium sulfate solution obtained in S1 at a mass ratio of insoluble matter to polyaluminum chloride = 1:3 and stir continuously at 500 r / min for 2 min. Then add polyacrylamide at a mass ratio of polyaluminum chloride to polyacrylamide = 1000:1 and stir continuously at 100 r / min for 10 min. Then adjust the pH of the system to 7.0±0.2 and the temperature of the mixed system to 50±2℃. Finally, after settling for 120 min, collect the precipitate and the clear liquid separately.

[0092] S3: Add ammonium bicarbonate to the clear solution obtained in S2 at a molar ratio of ammonium ions: magnesium ions = 2.0:1, then add magnesium dihydrogen phosphate to the solution at a molar ratio of sulfate ions: magnesium dihydrogen phosphate = 0.8:1, and react for 40 min at 30℃, pH = 7.8, and stirring speed of 200 r / min to obtain magnesium carbonate slurry;

[0093] S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water until the impurity ion content in the washing liquid is ≤0.1% to obtain clean magnesium carbonate filter cake.

[0094] S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer, setting the inlet air temperature to 220±10℃, the outlet air temperature to 95±5℃, and the drying air volume to 5000m³ / h. 3 / h, residence time is 30s, after which the magnesium carbonate is placed in an electric resistance furnace and heated to 450±5℃ at a heating rate of 5℃ / min and calcined for 2h to obtain high-purity light magnesium carbonate.

[0095] S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature. Set the reaction temperature of the ammonia medium regeneration reaction tank to 30℃ and the duration to 7h to obtain an ammonium sulfate solution.

[0096] S7: Heat the ammonium sulfate solution obtained in S6 to 70°C and evaporate to crystallize to obtain ammonium sulfate.

[0097] Experimental results: See Table 4 for details.

[0098] Table 4: Test Results of Example 4

[0099]

[0100] Comparative Example 1:

[0101] This embodiment provides a method for removing magnesium sulfite using a conventional calcination method, the specific implementation steps of which include:

[0102] Experimental materials:

[0103] Magnesium sulfite slurry (magnesium sulfite content 35%, containing a small amount of Ca) 2+ Fe 3+ Impurities), sodium hydroxide solution (concentration 30%), hydrogen peroxide (H2O2 content 30%), deionized water.

[0104] Experimental objective:

[0105] The resource utilization effect of this invention is compared by decomposing magnesium sulfite through high-temperature calcination and recovering magnesium oxide and sulfur dioxide.

[0106] Experimental steps:

[0107] S1: Take magnesium sulfite slurry and place it in a high-temperature calcining furnace. Heat it to 650°C at a heating rate of 5°C / min. Keep it at this temperature for calcination for 3 hours. Collect the generated gas continuously during the calcination process. After the calcination is completed, cool it to room temperature with the furnace and collect the tail gas and solid residue respectively.

[0108] S2: The tail gas collected in S1 is passed into an alkaline absorption tower containing a 30% sodium hydroxide solution for absorption reaction to obtain a sodium sulfite solution. Then, the solution is reacted with... Hydrogen peroxide was added dropwise to H2O2 at a molar ratio of 1:1.1 to oxidize the product and obtain a dilute sulfuric acid solution. After concentration, sulfuric acid product was obtained, and the total conversion rate of SO2 was determined.

[0109] S3: Take out the solid residue obtained in S1, determine the residual MgSO3 content and MgO purity, and reuse the solid MgO as a desulfurizing agent in the flue gas desulfurization system.

[0110] Experimental results: See Table 5 for details.

[0111] Table 5: Test Results of Comparative Example 1

[0112]

[0113] Comparative Example 2:

[0114] This embodiment provides a method for removing magnesium sulfite by acid hydrolysis, the specific implementation steps of which include:

[0115] Experimental materials:

[0116] Magnesium sulfite slurry (magnesium sulfite content 35%, containing a small amount of Ca) 2+ Fe 3+ Impurities), concentrated sulfuric acid (98% concentration), and deionized water.

[0117] Experimental objective:

[0118] Magnesium sulfate heptahydrate was prepared by acid hydrolysis of magnesium sulfite with concentrated sulfuric acid and sulfur dioxide was recovered. The overall benefits of this invention are compared.

[0119] Experimental steps:

[0120] S1: Add magnesium sulfite slurry to an acid-resistant reactor, and slowly add 98% concentrated sulfuric acid dropwise while stirring. Control the molar ratio of sulfuric acid to MgSO3 to be 1.2:1. After the addition is complete, continue stirring at room temperature for 2 hours. Collect the SO2 gas generated during the reaction. After the reaction is completed, a magnesium sulfate mixture is obtained.

[0121] S2: The SO2 gas collected in S1 is passed into a two-stage water washing absorption tower and absorbed with deionized water to produce dilute sulfuric acid with a concentration of about 30%. The SO2 concentration in the tail gas after absorption is measured.

[0122] S3: Filter the magnesium sulfate mixture obtained in S1 to remove unreacted residues and insoluble matter, and obtain a clear magnesium sulfate solution;

[0123] S4: The magnesium sulfate solution obtained in S3 is pumped into an evaporator crystallizer, evaporated and concentrated at 80°C until crystals are formed, then cooled to 25°C to crystallize, and separated by centrifugation to obtain magnesium sulfate heptahydrate product. Its purity, yield and equipment corrosion are then determined.

[0124] Experimental results: See Table 6 for details.

[0125] Table 6: Test Results of Comparative Example 2

[0126]

[0127] Comparative Example 3:

[0128] This embodiment provides a method for preparing magnesium carbonate using a multiple evaporation-cooling process, the specific implementation steps of which include:

[0129] Experimental materials:

[0130] Magnesium sulfate heptahydrate (MgSO4·7H2O, purity ≥99%), ammonium carbonate ((NH4)2CO3, purity ≥99%), and deionized water.

[0131] Experimental objective:

[0132] Magnesium carbonate was prepared by purifying pure reagents through multiple evaporation-cooling cycles, which has advantages over the route of this invention, which uses desulfurization byproducts to directly prepare magnesium carbonate and co-produce ammonium sulfate.

[0133] Experimental steps:

[0134] S1: Dissolve magnesium sulfate heptahydrate in deionized water to prepare a 1.5 mol / L magnesium sulfate solution, and dissolve ammonium carbonate in deionized water to prepare a 2.0 mol / L ammonium carbonate solution. Slowly add the ammonium carbonate solution to the magnesium sulfate solution while stirring, controlling the Mg content... 2+ and The molar ratio is 1:1.1. The pH of the system is adjusted to 8.5 with ammonia water, the temperature is raised to 60℃ and stirred for 1 hour to produce magnesium carbonate slurry.

[0135] S2: Filter the magnesium carbonate slurry obtained in S1, and wash the filter cake with deionized water until the conductivity of the washing liquid is <50 μS / cm to obtain a wet magnesium carbonate filter cake.

[0136] S3: Add deionized water to the wet filter cake obtained in S2 to make a slurry, put it into an evaporation kettle, evaporate and concentrate it to 1 / 3 of the original volume at 80℃, then transfer it to a crystallization kettle and cool it to 25℃ for crystallization, and filter to obtain crude magnesium carbonate that has been purified once.

[0137] S4: Add deionized water to the crude magnesium carbonate purified once and slurry it again. Repeat the operation of S3 to evaporate, concentrate, cool, crystallize and filter. Repeat this operation twice to obtain purified magnesium carbonate.

[0138] S5: The purified magnesium carbonate obtained in S4 is dried at 105℃ for 2 hours to obtain the magnesium carbonate product. Its purity, particle size and total energy consumption are determined.

[0139] Experimental results: See Table 7 for details.

[0140] Table 7: Test Results of Comparative Example 3

[0141]

[0142] Example 1 employs a double decomposition reaction route catalyzed by hydrogen peroxide oxidation and diammonium hydrogen phosphate catalysis. The mechanism involves hydrogen peroxide efficiently oxidizing sulfite to sulfate. Diammonium hydrogen phosphate, acting as a catalyst, precisely controls the supersaturation of the system through the common ion effect and buffering action, allowing magnesium carbonate to grow at a slower nucleation rate under lower supersaturation. Furthermore, its phosphate component selectively complexes trace amounts of iron, calcium, and other impurity ions in the solution, preventing them from entering the magnesium carbonate lattice. This simultaneously achieves the orderly growth of magnesium carbonate grains and impurity blocking. Meanwhile, excess ammonium bicarbonate in the mother liquor is regenerated into ammonium sulfate at room temperature via ammonia medium, realizing a closed-loop cycle of ammonium and carbon dioxide throughout the process. Under oxidation conditions of 70°C and pH=5, and double decomposition conditions of 35°C and pH=8.0, the magnesium sulfite conversion rate reaches 99.2%, the product magnesium carbonate purity reaches 98.7%, and the ammonium sulfate recovery rate reaches 95.5%, achieving an optimal balance among the three.

[0143] Example 2 uses the same process route as Example 1, but an additional 1% chloride ions are introduced into the raw materials. From the perspective of phase equilibrium of the water-salt system, during the evaporation and crystallization process, as water is continuously removed, ammonium sulfate preferentially precipitates in large quantities due to its relatively small solubility temperature coefficient, while chloride ions gradually accumulate in the residual liquid, changing the composition of the remaining liquid phase. / / Cl -When the enrichment level exceeds the saturation limit of ammonium chloride at that temperature, part of the ammonium sulfate mother liquor is cooled to 30°C. Utilizing the cooling crystallization principle that the solubility of ammonium chloride drops sharply with decreasing temperature, it precipitates as ammonium chloride, achieving directional separation of chloride ions and ammonium sulfate. This avoids contamination of the ammonium sulfate product by chloride ions. The purity of the main product, magnesium carbonate, remains as high as 98.5%, and the ammonium sulfate meets the first-class product standard. At the same time, an additional ammonium chloride byproduct with a purity of 24.0% is obtained, proving that this process has good adaptability to chlorine-containing industrial solid waste.

[0144] Example 3 employs an ozone oxidation-magnesium nitrate catalyzed metathesis reaction system. In the oxidation stage, ozone replaces hydrogen peroxide. Ozone, with its higher redox potential, can rapidly and irreversibly oxidize sulfite to sulfate under acidic conditions of 80°C and pH=4, without introducing additional salts into the system. This reduces the types and concentrations of impurities in the subsequent mother liquor from the source. In the metathesis stage, magnesium nitrate is used as a catalyst. Its nitrate and magnesium ions jointly participate in the regulation of the ionic strength and charge balance of the solution. In a weakly alkaline environment of 40°C and pH=8.2, a stable supersaturation window required for magnesium carbonate crystallization is maintained, ensuring uniform generation and growth of crystal nuclei. Therefore, even if the reaction temperature and pH deviate from the optimal central range, the purity of magnesium carbonate remains stable at 98.5%. The conversion rate of magnesium sulfite, the yield of magnesium carbonate, and the yield of ammonium sulfate are highly consistent with those of Example 1, indicating that this route has a wide process window and high operational flexibility.

[0145] Example 4 employs a metathesis reaction system catalyzed by oxygen oxidation and magnesium dihydrogen phosphate. In the oxidation stage, inexpensive oxygen replaces the liquid oxidant. Under mild conditions of 60°C and pH=5, the deep conversion of sulfite to sulfate is completed within 3 hours through the coupling of gas-liquid mass transfer and oxidation, significantly reducing the cost of the oxidant. In the metathesis stage, magnesium dihydrogen phosphate replaces part of the ammonium phosphate as a catalyst, directly supplementing the system with magnesium. While maintaining the same crystallization control capability as in Example 1, this reduces the dependence on external ammonium salts, helping to optimize the production ratio of magnesium carbonate to ammonium sulfate in the product structure. Under relatively low and near-neutral conditions of 30°C and pH=7.8, the purity of magnesium carbonate still reaches 98.5%, and the conversion rate of magnesium sulfite is 99.0%, demonstrating that using inexpensive air or oxygen as an oxidant and magnesium-containing phosphate as a catalyst is an effective way to reduce industrial operating costs.

[0146] Comparative Example 1 uses the traditional high-temperature calcination method, which essentially utilizes the thermal decomposition reaction of magnesium sulfite at high temperatures. This process is a strongly endothermic reaction, requiring a continuous input of a large amount of heat energy to maintain a high temperature of 650°C, resulting in an energy consumption as high as 3.2 × 10⁻⁶. 6kJ / t; at the same time, the release of SO2 gas exhibits a nonlinear rate characteristic and is prone to secondary reaction with solid magnesium oxide, resulting in an SO2 recovery rate of only 68%, an SO2 concentration in the tail gas exceeding the standard by 30%, and causing 12% undecomposed residue. This route faces the contradiction of high energy consumption, low conversion, and secondary pollution from a thermodynamic perspective, and its resource recovery efficiency is far lower than that of the wet chemical conversion route of this invention.

[0147] Comparative Example 2 uses the traditional sulfuric acid acidolysis method, which utilizes the proton displacement reaction of strong acid on magnesium sulfite. Although the reaction can drive SO2 to escape at room temperature, the acid consumption per unit of magnesium sulfite is as high as 0.8 t / t, and the high concentration of sulfuric acid causes severe corrosion to the equipment at a rate of 0.5 mm / year. The escaped SO2 requires multi-stage water washing for partial absorption, and the tail gas concentration is still as high as 110 mg / m³. 3 The environmental risks are significant, and the final product is only magnesium sulfate heptahydrate with low market value. Furthermore, there is no nitrogen resource recovery. From the perspectives of atom economy and total life cycle cost, it cannot compete with the closed-loop recycling process of this invention, which uses waste to treat waste and co-produce high-value nitrogen fertilizer.

[0148] Comparative Example 3 employed a pure reagent multiple evaporation-cooling recrystallization method. Its separation principle relies on the difference in solubility of magnesium carbonate and coexisting soluble salts at different temperatures. Purification is achieved through repeated cycles of three evaporation concentrations and cooling crystallizations. Each cycle requires the vaporization and re-condensation of a large amount of solvent water, resulting in a total energy consumption of 1.8 × 10⁻⁶. 6 The product yield is 1.5 times that of Example 1. The repeated dissolution-nucleation-growth process resulted in a crystal particle size of only 20 μm. The fine particles are prone to agglomeration, and the product morphology is inferior to the one-crystallization product of Example 1. More importantly, this route uses commercial pure reagents as raw materials, which is costly. It also completely abandons the resource recovery of ammonium salts in the mother liquor and does not produce ammonium sulfate. Its technical and economic efficiency and resource recycling concept cannot be compared with the present invention.

[0149] Reference Appendix Figure 1This invention presents a closed-loop process from the input of magnesium sulfite, a byproduct of desulfurization in a thermal power plant, to the final output of magnesium carbonate and ammonium sulfate. It sequentially demonstrates the serial layout of six core functional units: a pretreatment unit, a metathesis reactor, a solid-liquid separation unit, a drying unit, an ammonia regeneration unit, and a crystallization unit. The input nodes for materials such as magnesium sulfite, oxidant, flocculant, ammonium bicarbonate, and catalyst are clearly marked, as well as the material transfer paths between units, mapping the core process logic of oxidation purification, catalytic metathesis, solid-liquid separation, product drying, ammonia medium regeneration, and evaporation crystallization. Furthermore, the accompanying diagrams highlight the recycling path of regenerated ammonia from the ammonia regeneration unit back to the metathesis reactor, intuitively demonstrating the invention's closed-loop material recycling and zero secondary pollution design concept. This clearly showcases the invention's advantages of a short process flow, high resource utilization, and green, low-carbon operation, providing a clear and intuitive process guide for those in the relevant technical field to understand the invention's technical solution and carry out industrial implementation.

[0150] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of protection of the claims.

[0151] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, characterized in that, Specifically, the following steps are included: S1: Place magnesium sulfite slurry in a reactor, then add oxidant at a mass ratio of magnesium sulfite: oxidant = 0.5~2:1, and control the pH of the mixture to 3~6, ​​the system temperature to 50~90℃, and the reaction time to 1~4h to obtain magnesium sulfate solution; S2: Add flocculant to the magnesium sulfate solution obtained in S1 at a mass ratio of flocculant:wastewater = 1~5:

1. Then adjust the pH of the system to 6.0~9.0 and the temperature of the mixed system to 30~70℃. After settling for 30~180 minutes, collect the sediment and clear liquid respectively. S3: Add ammonium bicarbonate to the clear liquid obtained in S2 at a molar ratio of ammonium ion: magnesium ion > 2.

0. Then add catalyst to the solution at a molar ratio of sulfate ion: catalyst = 0.5~2:1 and react for 40~60 min at 20~50℃, pH = 7.0~9.0 and stirring speed of 200~500 r / min to obtain magnesium carbonate slurry. S4: The magnesium carbonate slurry obtained in S3 is subjected to solid-liquid separation to obtain magnesium carbonate filter cake and mother liquor. Then the magnesium carbonate filter cake is washed with deionized water to obtain clean magnesium carbonate filter cake. S5: Place the clean magnesium carbonate filter cake obtained in S4 into a flash dryer. After it is completely dried, place the magnesium carbonate in an electric resistance furnace and heat it to 450±5℃ at a heating rate of 5℃ / min and continue to calcine for 2 hours to obtain high-purity light magnesium carbonate. S6: Combine the mother liquor obtained in S4 and the washing liquid in S4, and then place the mixture in an ammonia medium regeneration reaction tank for ammonia medium regeneration at room temperature to obtain an ammonium sulfate solution. S7: The ammonium sulfate solution obtained in S6 is heated and evaporated to crystallize ammonium sulfate.

2. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The magnesium sulfite slurry described in S1 is made from magnesium sulfite obtained through desulfurization.

3. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 2, is characterized in that... The magnesium sulfite slurry contains ≥30% magnesium sulfite, and the magnesium sulfite obtained from desulfurization has a purity of ≥70%.

4. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The oxidant mentioned in S1 is any one or a combination of at least two of hydrogen peroxide, ozone, and oxygen.

5. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The flocculant described in S2 includes any one or a combination of at least two of anionic polyacrylamide, nonionic polyacrylamide, polyaluminum chloride, and polyaluminum sulfate.

6. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The catalyst described in S3 includes any one or a mixture of at least two of the following: diammonium hydrogen phosphate, magnesium dihydrogen phosphate, ammonium nitrate, magnesium nitrate, ammonium formate, and magnesium formate.

7. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The clean magnesium carbonate filter cake mentioned in S4 refers to the filter cake in which the impurity ion content in the washing liquid is ≤0.1% after washing.

8. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The S5 flash dryer is configured with an inlet air temperature of 220±10℃, an outlet air temperature of 95±5℃, and a drying air volume of 5000m³ / h. 3 / h, dwell time is 30s.

9. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, is characterized in that... The reaction temperature of the ammonia medium regeneration reaction tank in S6 is 25~35℃ and the duration is 3~11h.

10. The method for preparing magnesium carbonate and co-producing ammonium sulfate from magnesium sulfite, a byproduct of desulfurization in a thermal power plant, as described in claim 1, characterized in that... The heating and evaporation temperature described in S7 is 50~90℃.

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