A method for producing sodium hydrosulfide using acid gas

Abstract

The present application belongs to the technical field of fine chemical industry, and particularly relates to a method for preparing sodium hydrosulfide by using acid gas. A multi-stage parallel flow absorption system with gradually decreasing total alkalinity concentration is constructed, the product discharge port is arranged at the stage with the lowest total alkalinity, CO2 absorption is inhibited by using low alkalinity medium, and the generation amount of sodium carbonate impurities is reduced by more than 67% at the source. In combination with normal-temperature chemical precipitation decarburization and fractional cooling crystallization impurity removal, the problem that soluble impurity salts such as CO2, NH3 and sodium thiosulfate are difficult to remove is solved. The present application uses industrial sulfur-containing tail gas as raw material, fully utilizes the heat release of the reaction of H2S and NaOH to maintain the absorption temperature, and uses the condensation waste heat in a step-by-step manner to supplement the vacuum evaporation, so that efficient recovery of sulfur resources and green production are realized. The present application has simple process and mature equipment, can switch to produce 43% liquid sodium hydrosulfide or more than 70% solid sodium hydrosulfide on the same production line, and is flexible and adaptive to tail gas with different H2S concentrations.

Description

Technical Field

[0001] This invention belongs to the field of fine chemical technology, specifically relating to a method for preparing sodium hydrosulfide using acidic gas. Background Technology

[0002] Sodium hydrosulfide (NaHS) is an important basic raw material for fine chemicals, widely used in non-ferrous metal beneficiation, pesticide intermediate synthesis, dye and rubber additive preparation, leather depilation, water treatment, and organic synthesis. Industrial sodium hydrosulfide products are available in both liquid and solid forms, and market demand has been growing steadily in recent years.

[0003] Traditional sodium hydrosulfide production processes suffer from numerous technical drawbacks. While the direct absorption of H2S by sodium hydroxide is simple in principle, it involves many side reactions, making it difficult to control the residual sodium sulfide (Na2S) content in the product, directly impacting product quality. The method of synthesizing sodium hydroxide using pure hydrogen sulfide gas as a source yields better product quality, but the production cost of high-purity H2S is high, and it fails to achieve resource utilization of sulfur-containing industrial tail gas, resulting in a waste of sulfur resources.

[0004] Industrial processes such as petroleum refining, coal chemical industry, and natural gas purification generate large amounts of acidic tail gas containing H2S. According to the requirements of GB / T 23937-2020 standard, the content of impurities such as iron, sodium sulfide, carbonates, and thiosulfates in industrial sodium hydrosulfide products must be strictly controlled. Some existing processes use a two-stage absorption reaction to produce sodium hydrosulfide, but this presents the following main technical challenges: Industrial sulfur-containing tail gas typically contains impurities such as CO2, NH3, and hydrocarbons. CO2 reacts with NaOH to produce sodium carbonate and sodium bicarbonate, which, when mixed into the product, severely affects the purity of sodium hydrosulfide; alkaline impurities such as NH3 enter the absorption liquid, accumulate and release during subsequent evaporation and concentration, causing product pollution and environmental problems; soluble impurity salts such as sodium thiosulfate, sodium sulfate, and sodium sulfite commonly found in sodium hydrosulfide solutions do not precipitate during evaporation and concentration, but accumulate continuously in the liquid product, leading to excessive impurities. The existing technology lacks an integrated process that can systematically solve the above-mentioned impurity problems while taking into account the resource utilization of acidic gas and the production of high-purity sodium hydrosulfide.

[0005] Therefore, developing a green process that can efficiently utilize industrial acidic gases to prepare high-purity sodium hydrosulfide while also solving the problems of CO2, NH3, and various soluble impurity salts has significant industrial value and environmental significance. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing sodium hydrosulfide using acidic gases, in order to solve the following technical problems existing in the prior art: (1) the utilization rate of industrial sulfur-containing tail gas is low, and sulfur resources are seriously wasted; (2) impurities such as CO2 and NH3 in acidic gases have an adverse effect on product quality; (3) soluble impurity salts such as sodium thiosulfate, sodium sulfate, and sodium sulfite in sodium hydrosulfide products are difficult to remove effectively; (4) traditional processes have problems of high energy consumption and complex processes. The ultimate goal of this invention is to achieve efficient resource utilization of industrial acidic gases, prepare high-purity sodium hydrosulfide products that meet the quality requirements of GB / T 23937-2020, and simultaneously meet green and environmentally friendly emission standards.

[0007] This invention is achieved through the following technical solution: A method for preparing sodium hydrosulfide using acidic gas includes the following steps: S1. Pretreatment of H2S-containing acidic gases: Collect H2S-containing acidic tail gas from industrial processes such as petroleum refining, coal chemical industry, or natural gas purification; The acidic exhaust gas is subjected to a series of treatments: dust removal to remove particulate matter, oil removal to remove hydrocarbons and organic sulfides, dehydration to reduce water vapor content, and ammonia removal to remove alkaline impurities such as ammonia, in order to obtain purified acidic gas. S2, Multi-gradient total alkalinity absorption reaction: The purified acidic gas is passed sequentially through two or more alkaline absorption zones connected in series. Each absorption zone uses an aqueous sodium hydroxide solution of different concentrations as the absorbent. The total alkalinity concentration of the absorbent in each absorption zone decreases sequentially along the gas flow direction. The total alkalinity is defined as the concentration of all molecules in the absorbent that can react with H+. + The components of the reaction (including OH) - HS - S 2- The sum of the molar concentrations of (etc.) is maintained within the design range of the total alkalinity of each absorption zone by periodically replenishing sodium hydroxide solution. The product sodium hydrosulfide solution is drawn out from the outlet of the absorption zone with the lowest total alkalinity, thereby achieving complete absorption of H2S and conversion into crude sodium hydrosulfide solution. S3, Room temperature chemical precipitation decarburization: The crude sodium hydrosulfide solution is passed into a decarbonation reactor at room temperature, and calcium hydroxide emulsion or calcium chloride solution with a stoichiometric ratio of 1.0 to 1.2 times is added to convert carbonate and bicarbonate ions into calcium carbonate precipitate. The reaction temperature is controlled at 20 to 40°C. After solid-liquid separation, a decarbonated sodium hydrosulfide solution is obtained. This process does not require heating, thus avoiding the thermal decomposition of sodium hydrosulfide, and the by-product calcium carbonate can be used as an industrial raw material. S4. Evaporation, crystallization, and product shaping: The decarbonized sodium hydrosulfide solution is evaporated and concentrated in a vacuum evaporator, using the exothermic reaction and the residual heat from evaporation and condensation as the main heat source. Based on heat calculation, when the volume fraction of H2S in the raw gas is less than 30% or the evaporation load is large, a small amount of low-grade steam (≤0.1MPa) or hot water circulation is added as an auxiliary agent. After concentration, the liquid is cooled, crystallized, and dried / sheet-made to obtain solid sodium hydrosulfide product; or liquid sodium hydrosulfide product is produced by directly cooling, crystallizing, filtering and separating the crude sodium hydrosulfide liquid.

[0008] Furthermore, the volume fraction of H2S in the industrial sulfur-containing tail gas described in step S1 is ≥15%.

[0009] Furthermore, the dust removal in step S1 adopts bag filter or electrostatic precipitator; The oil removal process employs activated carbon adsorption or coalescence separation, resulting in a total hydrocarbon content in the gas of ≤50 mg / m³. 3 ; The dehydration is performed by condensation dehydration or molecular sieve adsorption dehydration, and the water content in the gas after dehydration is ≤0.1% (volume fraction). The ammonia removal process employs water washing absorption or acid washing neutralization, and the NH3 content in the gas after ammonia removal is ≤50mg / m³. 3 .

[0010] Furthermore, the two or more alkaline absorption zones in step S2 adopt a three-stage absorption zone structure. The concentration of sodium hydroxide solution in the first-stage absorption zone is 20%~40% (mass fraction), so that the high total alkalinity absorption liquid comes into contact with the residual gas, ensuring that H2S in the tail gas is completely absorbed. The sodium hydroxide solution concentration in the second absorption zone is 10%~20% (mass fraction), which serves as a buffer and transition. The sodium hydroxide solution concentration in the third-stage absorption zone is 5%~10% (mass fraction), which utilizes the low total alkalinity environment to suppress side reactions and reduce the absorption of impurities such as CO2.

[0011] Furthermore, the reaction temperature of the multi-level gradient total alkalinity absorption reaction in step S2 is 50~90℃. The exothermic reaction of H2S and NaOH is used as the main heat source to maintain the system temperature, and the temperature is controlled by adjusting the circulation volume of the absorption liquid or the flow rate of the heat exchange medium.

[0012] Furthermore, in step S3, the amount of calcium hydroxide emulsion or calcium chloride solution added during room temperature chemical precipitation decarbonation is calculated to be 1.0 to 1.2 times the total amount of sodium bicarbonate and sodium carbonate in the crude liquid. After the reaction, the pH of the slurry is controlled at 8 to 10. The calcium carbonate precipitate is removed by plate and frame filtration or centrifugation, and the total residual carbonate content in the liquid phase is ≤0.3%.

[0013] Furthermore, the operating conditions of the vacuum evaporator in step S4 are: vacuum degree -0.06~-0.08MPa, evaporation temperature 50~70℃, priority is given to utilizing the exothermic reaction and the residual heat of evaporation and condensation, and the insufficient part is supplemented by low-grade steam (≤0.1MPa) or hot water circulation.

[0014] Furthermore, the method also includes a cooling and crystallization process for the concentrated liquid after evaporation. By controlling the cooling temperature, impurity salts such as sodium thiosulfate, sodium sulfate, and sodium sulfite are precipitated in stages and removed by solid-liquid separation. The cooling and crystallization temperature is controlled at 20~30℃, and the mass fraction of NaHS in the concentrated liquid is controlled to be ≤45% to prevent the main product NaHS·2H2O from crystallizing and co-precipitating prematurely.

[0015] Furthermore, the temperature of the cooling crystallization process is controlled at 20~30℃.

[0016] The present invention has the following advantages over the prior art: 1. This invention constructs a multi-stage co-current absorption system with progressively decreasing total alkalinity concentration, breaking the traditional technical paradigm of equal concentration absorption of high-concentration alkaline solutions throughout the entire process. By placing the product outlet in the third absorption zone with the lowest total alkalinity, and utilizing the significantly reduced CO2 absorption capacity of the low-alkalinity medium, the generation of sodium carbonate impurities is reduced by more than 67% at the source. Combined with room-temperature chemical precipitation decarbonation (precipitating carbonate ions with calcium hydroxide or calcium chloride) and staged cooling crystallization for impurity removal, the risk of sodium hydrosulfide decomposition due to high-temperature heating is completely avoided, systematically solving the problem of difficult removal of soluble impurity salts such as CO2, NH3, and sodium thiosulfate from industrial acidic tail gas.

[0017] 2. This invention uses industrial sulfur-containing tail gas as raw material, achieving efficient recovery and harmless utilization of sulfur resources. Simultaneously, it fully utilizes the exothermic reaction between H2S and NaOH as the main heat source for evaporation and concentration. The exothermic reaction maintains the absorption system at 50-90℃, eliminating the need for external heating. The heat required by the vacuum evaporator is further supplemented by the waste heat from evaporation and condensation. When the calorific value of the raw gas is insufficient, only a small amount of low-grade steam needs to be added, resulting in significantly lower overall energy consumption than traditional processes. Furthermore, this invention operates under closed negative pressure throughout the entire process, controlling the H2S emission concentration in the tail gas to 4-8 mg / m³. 3 The emissions are far below the national emission limits, with no secondary pollution, achieving a balance between the resource utilization of acidic gases and green and clean production.

[0018] 3. The process route of this invention is simple and the unit equipment is mature. It can be flexibly adapted to industrial sulfur-containing tail gas with different H2S concentrations. By adjusting the concentration ratio of the three-stage absorbent and the cooling crystallization temperature, it is possible to switch between producing 43% liquid sodium hydrosulfide or more than 70% solid sodium hydrosulfide on the same production line to meet the needs of different markets for product form. Detailed Implementation

[0019] To further explain the present invention, the following specific embodiments are described.

[0020] In the following examples and comparative examples, product testing was performed according to the methods specified in GB / T 23937-2020 "Industrial Sodium Hydrosulfide". The removal rate of carbonate ions in calcium carbonate precipitate was determined by ion chromatography. The determination of NaHS content, Na2S content, and Na2CO3 content was carried out using the barium chloride precipitation-hydrochloric acid titration method (GB / T 23937-2020, Clause 6.3). Sodium thiosulfate (Na2S2O3) content determination: iodometric method was used; Iron (Fe) content determination: 1,10-phenanthroline spectrophotometric method (GB / T 3049). Determination of H2S concentration in exhaust gas: Iodometric method (HJ / T 60-2000). The residual total content of sodium carbonate and sodium bicarbonate was determined by barium chloride precipitation-acid-base titration.

[0021]

[0022] Example 1 A method for preparing sodium hydrosulfide using acidic gas includes the following steps: S1. Pretreatment of H2S-containing acidic gases: Collect acidic tail gas containing H2S from a coal chemical enterprise, with an H2S volume fraction of 20%, a CO2 volume fraction of approximately 10%, and an NH3 volume fraction of approximately 0.5%. Pass this acidic tail gas sequentially through a bag filter for dust removal and an activated carbon adsorption tower for oil removal (controlling the total hydrocarbon content at the outlet to ≤50 mg / m³). 3 ), condensation and dehydration (controlling the outlet water content ≤ 0.1% by volume), water washing and absorption for ammonia removal (controlling the outlet NH3 content ≤ 50 mg / m³). 3 The purified acidic gas is obtained, and the gas outlet temperature is about 35℃.

[0023] S2, Multi-gradient total alkalinity absorption reaction: The purified acidic gas is sequentially passed through a three-stage alkaline absorption zone, with the gas flow direction being the same as the absorption liquid flow direction (parallel flow). The sodium hydroxide solution concentrations in the three absorption zones are as follows: First-stage absorption zone: Sodium hydroxide solution concentration 20% (mass fraction), solution circulation rate 6.0 m³ / min. 3 / h, volume 2.5m³ 3 ; Second-stage absorption zone: Sodium hydroxide solution concentration 10% (mass fraction), solution circulation rate 4.0 m³ / min. 3 / h, volume 2.0m³3 ; Third-stage absorption zone: Sodium hydroxide solution concentration 5% (mass fraction), solution circulation rate 3.0 m³ / min. 3 / h, volume 1.5m³ 3 .

[0024] The total alkalinity concentration of the absorbent in each absorption zone decreases sequentially along the gas flow direction. Online pH meters and conductivity meters monitor and automatically replenish concentrated alkaline solution to maintain the total alkalinity in each absorption zone within ±5% of the design value. The product sodium hydrosulfide solution is continuously drawn from the outlet of the third-stage (lowest total alkalinity) absorption zone. The absorption reaction utilizes the exothermic reaction of H₂S and NaOH as the main heat source. The system temperature is controlled at approximately 50°C by adjusting the absorbent circulation rate, requiring no external heating.

[0025] The main reaction and side reactions are as follows: Main reaction (absorption): NaOH + H₂S → NaHS + H₂O Side reaction (to be suppressed): 2NaOH + H₂S → Na₂S + 2H₂O Transformation reaction: Na₂S + H₂S → 2NaHS After this step is completed, approximately 6.5 ml of crude sodium hydrosulfide solution is obtained. 3 / h, of which NaHS mass fraction is about 28%, Na2S mass fraction is about 2.8%, and total carbonate content is about 2.0%.

[0026] S3, Room temperature chemical precipitation decarburization: The crude sodium hydrosulfide solution was fed into a decarbonation reactor, and a calcium hydroxide emulsion (the molar amount of calcium ions was 1.1 times the total molar amount of sodium bicarbonate and sodium carbonate in the crude solution) was added at 25°C. The mixture was stirred for 30 minutes to form calcium carbonate precipitate, and the pH of the slurry after the reaction was approximately 9. The solution was then filtered through a plate and frame filter press; the filter cake was calcium carbonate, and the filtrate was a decarbonated sodium hydrosulfide solution. After decarbonation treatment, the total residual carbonate content in the solution decreased to 0.25%.

[0027] S4. Product molding: Liquid products are produced by direct cooling, crystallization, filtration and separation of crude liquid: the decarbonized sodium hydrosulfide solution is cooled to 15°C, and impurities such as sodium thiosulfate and sodium sulfate are precipitated. After filtration and separation by plate and frame filter press, a clear liquid product is obtained, in which the mass fraction of NaHS is about 43%.

[0028] After secondary absorption by alkaline solution, the H2S emission concentration of the reaction tail gas was measured to be approximately 8 mg / m³. 3 The comprehensive utilization rate of sulfur resources is approximately 97.5%.

[0029] Example 2 A method for preparing sodium hydrosulfide using acidic gas includes the following steps: S1. Pretreatment of H2S-containing acidic gases: Acidic tail gas containing H2S was collected from an oil refinery. The volume fraction of H2S was 50%, the volume fraction of CO2 was approximately 18%, and the volume fraction of NH3 was approximately 1.5%. The purification treatment method was the same as in Example 1.

[0030] S2, Multi-gradient total alkalinity absorption reaction: The concentrations of sodium hydroxide solution in the tertiary absorption zone are as follows: First stage: Sodium hydroxide solution concentration 30% (mass fraction), solution circulation volume 8.0 m³ / min. 3 / h, volume 3.0m 3 ; Second stage: Sodium hydroxide solution concentration 15% (mass fraction), solution circulation volume 5.0 m³ / min. 3 / h, volume 2.5m³ 3 ; Third stage: Sodium hydroxide solution concentration 8% (mass fraction), solution circulation volume 4.0 m³. 3 / h, volume 2.0m³ 3 .

[0031] The absorption reaction temperature was controlled at 70℃, and temperature stability was achieved by adjusting the circulation rate of the absorbent liquid and the cooling water flow rate of the heat exchange coil. The crude sodium hydrosulfide solution contained approximately 32% NaHS by mass, approximately 1.5% Na2S by mass, and approximately 1.2% total carbonate content.

[0032] S3, Room temperature chemical precipitation decarburization: Calcium chloride solution (calcium ion molar amount 1.05 times the total molar amount of sodium bicarbonate and sodium carbonate in the crude solution) was added at 30℃. After the reaction, the pH of the slurry was adjusted to 9 with dilute NaOH, and the calcium carbonate precipitate was removed by centrifugation. After decarbonation, the total residual carbonate content in the solution decreased to 0.2%.

[0033] S4. Evaporation, crystallization, and product shaping: The decarbonized sodium hydrosulfide solution was concentrated in a vacuum evaporator (vacuum degree -0.07 MPa, evaporation temperature 60°C), primarily utilizing the exothermic absorption reaction and the residual heat from evaporation and condensation. Due to the high H2S concentration in this embodiment, the heat of reaction was sufficient, eliminating the need for an external heat source. After evaporation, the NaHS mass fraction increased to approximately 42% (controlled not to exceed 45% to prevent co-precipitation of the main product during cooling crystallization). The concentrated solution was then cooled and crystallized (staged cooling: first cooling to 40°C to separate sodium sulfate, then cooling to 25°C to separate sodium thiosulfate). The purified solution entered the drying and tableting unit, where it was vacuum dried at 65°C for 2.5 hours to obtain a solid product with a NaHS mass fraction of 70%.

[0034] The H2S emission concentration in the exhaust gas is approximately 6 mg / m³. 3The comprehensive utilization rate of sulfur resources is approximately 98.5%.

[0035] Example 3 A method for preparing sodium hydrosulfide using acidic gas includes the following steps: S1. Pretreatment of H2S-containing acidic gases: Acidic tail gas containing H2S was collected from a natural gas purification plant. The volume fraction of H2S was 80%, the volume fraction of CO2 was approximately 5%, and the volume fraction of NH3 was approximately 2.5%. The purification treatment method was the same as in Example 1.

[0036] S2, Multi-gradient total alkalinity absorption reaction: The concentrations of sodium hydroxide solution in the tertiary absorption zone are as follows: First stage: Sodium hydroxide solution concentration 40% (mass fraction), solution circulation volume 10.0 m³. 3 / h, volume 3.5m³ 3 ; Second stage: Sodium hydroxide solution concentration 20% (mass fraction), solution circulation volume 6.0 m³. 3 / h, volume 3.0m 3 ; Third stage: Sodium hydroxide solution concentration 10% (mass fraction), solution circulation volume 5.0 m³ / min. 3 / h, volume 2.5m³ 3 .

[0037] The absorption reaction temperature was controlled at 90℃. The crude sodium hydrosulfide solution contained approximately 35% NaHS by mass, approximately 0.8% Na2S by mass, and approximately 0.8% total carbonate content.

[0038] S3, Room temperature chemical precipitation decarburization: A calcium hydroxide emulsion (calcium ion molar amount 1.0 times the total amount of carbonate) was added at 35℃, and the mixture was filtered after the reaction. After decarbonation, the residual total carbonate content decreased to 0.15%.

[0039] S4. Product molding: The solid product was prepared according to the method in Example 2, and the NaHS mass fraction was 72% after drying.

[0040] The H2S emission concentration in the exhaust gas is approximately 4 mg / m³. 3 The comprehensive utilization rate of sulfur resources is approximately 98.8%.

[0041] Comparative Example 1: No decarbonization treatment S1. Pretreatment of H2S-containing acidic gases: The exhaust gas source and pretreatment method are the same as in Example 2.

[0042] S2, Multi-gradient total alkalinity absorption reaction: The absorption reaction parameters were the same as in Example 2, and the crude sodium hydrosulfide solution contained approximately 32% NaHS, approximately 1.5% Na2S, and approximately 1.2% total carbonates.

[0043] S3, Product Forming: The crude sodium hydrosulfide solution was directly fed into a vacuum evaporator for concentration (evaporation conditions as in Example 2), without undergoing decarbonization treatment. After concentration, the solution was cooled and crystallized (as in Example 2), then dried and tableted to obtain solid sodium hydrosulfide product.

[0044] Test results The product contains approximately 68% NaHS by mass and approximately 1.5% total carbonates, exceeding the limits for carbonate content in GB / T 23937-2020 (≤0.8% for liquid products and ≤1.0% for solid products).

[0045] Comparative analysis After omitting the decarbonation step, carbonate impurities accumulated to 1.5% in the product, exceeding the national standard limit. This is because sodium bicarbonate and sodium carbonate salts in the crude sodium hydrosulfide solution were not removed during the evaporation and concentration process; instead, they accumulated with the evaporation of water and ultimately entered the product, leading to excessive carbonate levels. This demonstrates that the decarbonation and impurity removal step is essential for controlling the carbonate content in the product.

[0046] Comparative Example 2: Impurity Removal via Cooling Crystallization S1. Pretreatment of H2S-containing acidic gases: The exhaust gas source and pretreatment method are the same as in Example 2.

[0047] S2, Multi-gradient total alkalinity absorption reaction: The absorption reaction parameters were the same as in Example 2, and crude sodium hydrosulfide solution was obtained.

[0048] S3, Decarbonization and impurity removal treatment: The decarbonization treatment was the same as in Example 2, resulting in a decarbonized sodium hydrosulfide solution.

[0049] S4. Product molding: The decarbonized sodium hydrosulfide solution was concentrated in a vacuum evaporator (evaporation conditions were the same as in Example 2), but without undergoing a cooling and crystallization process, the concentrate was directly dried and sheeted to obtain solid sodium hydrosulfide product.

[0050] Comparative analysis After omitting the cooling crystallization process, the sodium thiosulfate content in the product is approximately 0.8%, exceeding the limit in GB / T 23937-2020 (thiosulfate limit ≤0.5%); the NaHS mass fraction is approximately 65%. This is because soluble impurity salts such as sodium thiosulfate continuously accumulate during the evaporation and concentration process. Without cooling crystallization and solid-liquid separation, these impurity salts cannot be effectively removed, ultimately entering the product and causing excessive thiosulfate levels and a decrease in the main content. This demonstrates that the cooling crystallization process is crucial for removing soluble impurity salts.

[0051] Comparative Example 3: Traditional isoconcentration absorption, without gradient design S1. Pretreatment of H2S-containing acidic gases: The exhaust gas source and pretreatment method are the same as in Example 2.

[0052] S2, Equal Concentration Absorption Reaction (Comparison): Traditional process: Absorption is performed using only a single concentration of NaOH solution (20%), without employing a multi-stage gradient total alkalinity design, and the absorbent circulation volume is 15m³. 3 / h, absorption temperature 50℃.

[0053] S3, Decarburization and impurity removal treatment and product molding: Same as Example 2.

[0054] Test results The product contains approximately 65% ​​NaHS by mass, approximately 1.5% Na2S residue, and approximately 1.2% total carbonate content, all of which are inferior to Example 2.

[0055] Comparative analysis In traditional isoconcentration absorption processes, the absorbent concentration is uniform throughout the entire tower, resulting in poor absorption efficiency and selectivity for H2S. The initial absorption phase is characterized by vigorous reactions, with some H2S reacting with NaOH to form Na2S, leading to a high residual Na2S content in the product. Simultaneously, impurity gases such as CO2 are also largely absorbed under high-concentration alkaline conditions, generating sodium carbonate as a byproduct. In contrast, the stepped absorption design of this invention, through a progressively decreasing alkaline concentration distribution, creates a "CO2 suppression zone" in the third-stage low-alkalinity region. This not only inhibits the absorption of impurities but also promotes the formation of the target product, NaHS.

[0056] Comparative Example 4: Low-purity acid gas, H2S 12% S1. Pretreatment of H2S-containing acidic gases: Low-concentration H2S-containing tail gas was collected from a chemical plant. The H2S volume fraction was 12%, the CO2 volume fraction was approximately 35%, and the water vapor content was relatively high. The purification treatment method was the same as in Example 2.

[0057] S2, Multi-gradient total alkalinity absorption reaction: The absorption reaction parameters are the same as in Example 2, but the gas throughput is reduced by 30% to maintain the absorption effect.

[0058] S3, Decarburization and impurity removal treatment and product molding: Same as Example 2.

[0059] Test results The product contains approximately 38% NaHS (liquid) or 60% NaHS (solid), approximately 2.0% Na2S residue, and approximately 1.5% total carbonate content, all of which fail to meet national standards.

[0060] Comparative analysis When the volume fraction of H2S is below 20%, the H2S concentration in the feed gas is low, while the relative content of impurity gases such as CO2 increases. On the one hand, the low concentration of H2S leads to insufficient driving force for the absorption reaction, resulting in a reduced absorption rate, and some H2S escapes without being absorbed. On the other hand, the high concentration of CO2 is absorbed in the alkaline solution to generate a large amount of sodium carbonate impurities, which are difficult to completely remove in the decarbonization process. Therefore, the method of this invention preferably uses industrial sulfur-containing tail gas with an H2S volume fraction ≥ 20%.

[0061] The test results are shown in Table 1 below.

[0062] Table 1 Performance test results of each embodiment and comparative example

[0063] As shown in Table 1 above, compared with the original process, the improved decarbonization step completely avoids the 90~100℃ heating operation, fundamentally eliminating the risk of NaHS thermal decomposition to generate H2S. Meanwhile, the room-temperature chemical precipitation method has high decarbonization efficiency (carbonate residue ≤0.3%), and the by-product calcium carbonate can be used as an industrial raw material without secondary pollution. Heat calculations show that when the H2S volume fraction is ≥30%, the exothermic reaction fully meets the evaporation requirements; when the H2S volume fraction is between 15% and 30%, a small amount of low-grade steam (≤0.1MPa) needs to be added, but the overall energy consumption is still more than 40% lower than the traditional process. Controlling the cooling crystallization temperature at 20~30℃ and ensuring the NaHS concentration after evaporation is ≤45% effectively prevents premature precipitation of the main product and improves impurity separation efficiency.

[0064] Compared to Example 2, Comparative Example 1, which omitted the decarbonation treatment, had a total carbonate content of 1.5% in its product, exceeding the limit of GB / T 23937-2020 (the limit for carbonates in solid products is ≤1.0%), making the product unqualified. Example 2, under the same conditions, added a decarbonation treatment (heating to 95°C to convert sodium bicarbonate to sodium carbonate, then cooling to 20°C for crystallization separation), reducing the total carbonate content to 0.4%. This comparison fully demonstrates that decarbonation treatment is a crucial step in reducing carbonate impurities in products and ensuring compliance with national standards. Its principle is based on the difference in solubility between sodium carbonate and sodium bicarbonate at different temperatures, achieving the separation and removal of sodium carbonate through cooling crystallization.

[0065] In Comparative Example 2, omitting the cooling crystallization process resulted in a sodium thiosulfate content in the product increasing to 0.8%, exceeding the limit set by GB / T23937-2020 (thiosulfate ≤0.5%), rendering the product unqualified. In Example 2, through staged cooling crystallization (separating sodium sulfate at 40°C and sodium thiosulfate at 25°C), the sodium thiosulfate content decreased to 0.15%. This comparison demonstrates that the cooling crystallization process is crucial for removing soluble impurities such as sodium thiosulfate and sodium sulfate. Without this process, these impurities accumulate in the product, severely impacting product quality.

[0066] Comparative Example 3, using a traditional isoconcentration absorption process (20% isoconcentration absorption), resulted in a Na2S residue of up to 1.5% and a total carbonate content of 1.2% in the product, significantly higher than the gradient absorption process of this invention (in the isoconcentration process, CO2 is largely absorbed in high-concentration NaOH to generate sodium carbonate, while the high alkalinity environment first produces Na2S intermediates and subsequent conversion is insufficient). This invention, through a three-stage alkaline concentration gradient (30% / 15% / 8%) design, creates a "CO2 inhibition zone" in the third-stage low-alkalinity region—the absorption and conversion capacity of NaHS solution for CO2 is significantly reduced at a total alkalinity of approximately 8%. Combined with a progressively decreasing total alkalinity gradient, this design inhibits the absorption of impurity gases such as CO2 at the source, while simultaneously promoting the complete conversion of Na2S to NaHS. The final product contains ≤0.3% Na2S residue and ≤0.4% total carbonate content.

[0067] A comparison between Comparative Example 4 and Example 2 shows that when the volume fraction of H2S in the raw gas is below 20%, even with the same absorption and purification processes, the product's various indicators are difficult to meet national standards: the NaHS mass fraction is significantly low (only 60% in solid products), the Na2S residue is as high as 2.0%, and the total carbonate content is 1.5%. This is because under low-concentration H2S conditions, the driving force for the absorption reaction is insufficient, and the reaction rate decreases; at the same time, the relative concentration of CO2 in the system increases, increasing the probability of being absorbed and forming sodium carbonate, making it difficult for the decarbonization process to completely remove it. Therefore, the method of this invention preferably uses industrial sulfur-containing tail gas with an H2S volume fraction ≥ 20% as the raw material.

[0068] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing sodium hydrosulfide using acidic gas, characterized in that, Includes the following steps: S1. Pretreatment of H2S-containing acidic gases: Industrial sulfur-containing tail gas is collected and pretreated by dust removal, oil removal, dehydration, and ammonia removal to obtain purified acidic gas; The volume fraction of H2S in the industrial sulfur-containing tail gas is ≥15%; The dust removal method is either bag filter or electrostatic precipitator; The oil removal process employs activated carbon adsorption or coalescence separation, resulting in a total hydrocarbon content in the gas of ≤50 mg / m³. 3 ; The dehydration is performed by condensation dehydration or molecular sieve adsorption dehydration, and the water content in the gas after dehydration is ≤0.1%; The ammonia removal process employs water washing absorption or acid washing neutralization, and the NH3 content in the gas after ammonia removal is ≤50mg / m³. 3 ; S2, Multi-gradient total alkalinity absorption reaction: The purified acidic gas is passed sequentially through two or more alkaline absorption zones connected in series. Each absorption zone uses sodium hydroxide aqueous solution of different concentrations as the absorbent. The total alkalinity concentration of the absorbent in each absorption zone decreases sequentially along the gas flow direction. The product sodium hydrosulfide solution is drawn out from the outlet of the absorption zone with the lowest total alkalinity, thereby achieving complete absorption of H2S and conversion into crude sodium hydrosulfide solution. The two or more alkali absorption zones connected in series adopt a three-stage absorption zone structure. The sodium hydroxide solution concentration in the first-stage absorption zone is 20%~40%, the sodium hydroxide solution concentration in the second-stage absorption zone is 10%~20%, and the sodium hydroxide solution concentration in the third-stage absorption zone is 5%~10%. S3, Room temperature chemical precipitation decarburization: The crude sodium hydrosulfide solution is passed into a decarbonation reactor, and calcium hydroxide emulsion or calcium chloride solution is added to convert carbonate and bicarbonate ions into calcium carbonate precipitate. After solid-liquid separation, a decarbonated sodium hydrosulfide solution is obtained. S4. Evaporation, crystallization, and product shaping: The decarbonized sodium hydrosulfide solution is concentrated by evaporation in a vacuum evaporator, using the exothermic absorption reaction and the residual heat from evaporation and condensation as heat sources. When the volume fraction of H2S in the raw gas is less than 30% or the evaporation load is too high, low-grade steam is added as an auxiliary agent. After concentration, the liquid is cooled, crystallized, and dried / sheet-made to obtain solid sodium hydrosulfide product; or liquid sodium hydrosulfide product is produced by directly cooling, crystallizing, filtering and separating the crude sodium hydrosulfide liquid.

2. The method for preparing sodium hydrosulfide using acidic gas according to claim 1, characterized in that, The reaction temperature of the multi-level gradient total alkalinity absorption reaction in step S2 is 50~90℃. The exothermic reaction of H2S and NaOH is used as the main heat source to maintain the system temperature, and the temperature is controlled by adjusting the circulation volume of the absorption liquid or the flow rate of the heat exchange medium.

3. The method for preparing sodium hydrosulfide using acidic gas according to claim 1, characterized in that, The total alkalinity mentioned in step S2 is defined as the total alkalinity of the absorbent solution that can react with H+. + The sum of the molar concentrations of the reacting components is maintained within the designed range for the total alkalinity of each absorption zone by periodically replenishing sodium hydroxide solution.

4. The method for preparing sodium hydrosulfide using acidic gas according to claim 1, characterized in that, In step S3, the amount of sodium hydroxide emulsion or calcium chloride solution added during room temperature chemical precipitation decarbonation is calculated to be 1.0 to 1.2 times the total amount of sodium bicarbonate and sodium carbonate in the crude liquid. After the reaction, the pH of the slurry is controlled at 8 to 10. The calcium carbonate precipitate is removed by plate and frame filtration or centrifugation, and the total residual amount of carbonate ions in the liquid phase is ≤0.3%.

5. The method for preparing sodium hydrosulfide using acidic gas according to claim 1, characterized in that, The operating conditions of the vacuum evaporator in step S4 are: vacuum degree -0.06~-0.08MPa, evaporation temperature 50~70℃, using the heat released by absorption reaction and the waste heat from evaporation and condensation as the heat source, and supplementing the insufficient part with low-grade steam or hot water circulation.

6. The method for preparing sodium hydrosulfide using acidic gas according to claim 1, characterized in that, The method also includes a cooling and crystallization process for the concentrated liquid after evaporation, in which impurity salts are precipitated in stages by controlling the cooling temperature and removed by solid-liquid separation.

7. The method for preparing sodium hydrosulfide using acidic gas according to claim 6, characterized in that, The temperature of the cooling crystallization process is controlled at 20~30℃.

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