A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride using magnesium oxide

Abstract

The application discloses a method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide. The method comprises the following steps: mixing magnesium oxide and ammonium chloride and roasting to obtain ammonia and roasting sand; dissolving the roasting sand in water and filtering to obtain a magnesium oxide residue phase and a magnesium chloride-containing solution; or adding magnesium oxide and ammonium chloride into a magnesium chloride-containing original solution to perform ammonia evaporation, obtaining ammonia; and performing solid-liquid separation on slurry after the reaction to obtain a magnesium oxide residue phase and a magnesium chloride-containing solution; cooling and crystallizing the magnesium chloride-containing solution to obtain magnesium chloride hydrate solid; pyrolyzing the magnesium chloride hydrate to obtain magnesium oxide solid and HCl gas; condensing and absorbing the HCl gas to obtain hydrochloric acid; and recycling the magnesium oxide residue phase and the magnesium oxide solid to the ammonia preparation process. The method can greatly improve the efficiency of preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide, and can improve the concentration and quality of ammonia water and hydrochloric acid.

Description

Technical Field

[0001] This invention belongs to the field of green chemistry and clean production, specifically relating to a method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide. Background Technology

[0002] Ammonium chloride is a byproduct of chemical and metallurgical processes. Its large quantity and limited market have led to severe stockpiling. When heated, ammonium chloride decomposes into a mixture of NH3 and HCl gases. Upon cooling, the NH3 and HCl recombine to form ammonium chloride crystals. Various methods have been attempted to effectively separate the NH3 and HCl products of the thermal decomposition of NH4Cl, but the separation results have been unsatisfactory. Among these, the magnesium oxide decomposition method is considered the most promising. There are two processes for the decomposition of ammonium chloride by magnesium oxide: a "wet process" and a "dry process." The "wet process" involves first adding MgO, Mg(OH)₂, or Mg(OH)Cl to an ammonium chloride solution, heating and stirring to decompose the ammonium chloride at 105-136°C, releasing ammonia gas and yielding a MgCl₂ solution. The MgCl₂ solution is then heated to 220-250°C, causing MgCl₂ to hydrolyze and release Mg(OH)Cl and HCl gas. Water is then added to the resulting slurry, and ammonium chloride is added while stirring. The ammonium chloride reacts with Mg(OH)Cl at 105-136°C, releasing ammonia gas and yielding another MgCl₂ solution. This solution is then heated to 220-250°C to hydrolyze MgCl₂, regenerating Mg(OH)Cl. This allows for the recycling of Mg(OH)Cl in the process of decomposing ammonium chloride to produce ammonia and hydrochloric acid. The "wet process" appears simple, but it involves two steps: ammonia and HCl removal. The ammonia removal process, in particular, requires continuous water replenishment as the NH3 concentration decreases to ensure complete ammonia evaporation before the solution temperature can be raised for HCl removal. Calculations show that the molar ratio of H2O to NH3 in the ammonia removal process is 44.6:1, resulting in high energy consumption and costs. Furthermore, the ammonia obtained is dilute ammonia solution with a concentration of only ~2 mol / L, requiring further concentration to obtain industrial-grade ammonia. Additionally, the concentration of HCl gas released from the MgCl2 solution heated to 220-250℃ is also low, making it unsuitable for directly producing qualified industrial hydrochloric acid. The tailing effect of both ammonia and HCl removal processes is a fatal flaw in the magnesium oxide-ammonium chloride decomposition "wet process," as it is difficult to prepare qualified industrial products using either dilute ammonia or dilute hydrochloric acid. When dilute hydrochloric acid with a mass fraction of <20% is evaporated and concentrated, an azeotropic reaction occurs at 108.6℃ when the HCl concentration reaches 20.2% under normal pressure. Conventional distillation methods cannot concentrate dilute hydrochloric acid to a concentration higher than the azeotropic composition. Furthermore, when the solution temperature rises to 136–220°C, ammonia and chlorine release occur simultaneously, and the gases evaporated in this temperature range contain both NH3 and HCl. Because the temperatures of ammonia and chlorine release overlap, the effective utilization rate of ammonium chloride in the process is only ~90% at most.

[0003] The "dry process" involves mixing MgO and solid NH4Cl and heating the mixture to 350-550°C, causing it to decompose and release NH3, yielding a material containing MgCl2 (MgO + 2NH4Cl =Δ= MgCl2 + H2O + 2NH3↑). Water vapor is then introduced into the MgCl2 material, causing the MgCl2 to hydrolyze at 250-550°C, converting it back to MgO and releasing HCl gas. The HCl-containing gas is condensed to produce hydrochloric acid, and the hydrolysis product (MgO) is returned to the ammonia release process for reuse. However, with increasing reuse, the particle size of MgO increases, leading to decreased reactivity and ultimately necessitating waste disposal. Furthermore, because ammonium chloride produces water during ammonia release, MgCl2 hydrolyzes upon contact with water at high temperatures, and the hydrolysis temperature overlaps with the ammonia release temperature, resulting in a maximum utilization rate of only 90% for the "dry process" of ammonium chloride. Another thorny issue with the "dry process" is that it's difficult to achieve uniform contact between the MgCl2-containing material and water vapor during the chlorination release process. To completely hydrolyze the MgCl2 in the material, excessive water vapor must be introduced, severely reducing the concentration of HCl in the released chlorine gas. Consequently, the condensed chlorine gas yields substandard dilute hydrochloric acid. This demonstrates that traditional processes for decomposing ammonium chloride with magnesium oxide, whether "wet" or "dry," are inherently flawed and cannot produce qualified industrial ammonia and hydrochloric acid products. Therefore, there is an urgent need to develop a more efficient method for preparing ammonia and hydrochloric acid from ammonium chloride using magnesium oxide to improve industrial production efficiency. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide. This method can improve the effective utilization rate of ammonium chloride, completely separate the NH3 and HCl gases produced by the pyrolysis of ammonium chloride, and convert them all into qualified industrial ammonia and hydrochloric acid products. Furthermore, the method is simple, low-cost, and suitable for industrial production.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide. The method comprises: mixing and pressing magnesium oxide and ammonium chloride into a mold, followed by calcination to obtain calcined sand and ammonia-containing flue gas; condensing and absorbing the ammonia-containing flue gas to obtain ammonia water; dissolving the calcined sand in water and separating the solid and liquid phases to obtain magnesium oxide slag phase and magnesium chloride-containing solution; cooling and crystallizing the magnesium chloride-containing solution to obtain hydrated magnesium chloride solid and crystallization mother liquor; pyrolyzing the hydrated magnesium chloride solid to obtain magnesium oxide solid and HCl-containing gas; condensing and absorbing the HCl-containing gas to obtain hydrochloric acid; recycling the magnesium oxide slag phase and magnesium oxide solid during the calcination process; and recycling the crystallization mother liquor during the calcined sand dissolution process.

[0006] Alternatively, the original solution containing magnesium chloride is mixed with magnesium oxide and heated, and then ammonium chloride solid or ammonium chloride solution is added for ammonia stripping to obtain ammonia-containing gas and a reaction solution. The ammonia-containing gas is condensed and absorbed to obtain ammonia water. The reaction solution is subjected to solid-liquid separation to obtain magnesium oxide slag phase and magnesium chloride-containing solution. The magnesium chloride-containing solution is cooled and crystallized to obtain hydrated magnesium chloride solid and crystallization mother liquor. The hydrated magnesium chloride solid is pyrolyzed to obtain magnesium oxide solid and HCl-containing gas. The HCl-containing gas is condensed and absorbed to obtain hydrochloric acid. The magnesium oxide slag phase and crystallization mother liquor are returned to the ammonia stripping process for recycling.

[0007] One approach of this invention involves first reacting magnesium oxide with ammonium chloride to prepare ammonia, then using the ammonia-producing material to prepare hydrated magnesium chloride, followed by pyrolysis of the hydrated magnesium chloride to prepare hydrochloric acid and obtain magnesium oxide. The magnesium oxide is then reused; specifically, the calcined ore obtained by calcining magnesium oxide and ammonium chloride contains anhydrous magnesium chloride. Dissolving the anhydrous magnesium chloride in water yields a high-concentration magnesium chloride solution. Cooling this high-concentration solution allows for the crystallization of hydrated magnesium chloride, which is then pyrolyzed to prepare magnesium oxide and hydrochloric acid. This process design, which involves transitioning from a pyrometallurgical method to a hydrometallurgical method and then back to a pyrometallurgical method, may seem unreasonable in terms of thermal efficiency, but it is actually the optimal choice for using magnesium oxide to decompose ammonium chloride to prepare ammonia and hydrochloric acid. Because the calcined ash obtained is anhydrous magnesium chloride, the process of preparing hydrated magnesium chloride from anhydrous magnesium chloride is simple. However, the existing process of using steam pyrolysis of anhydrous magnesium chloride to prepare magnesium oxide and hydrochloric acid requires a large amount of steam. Moreover, it is difficult to achieve sufficient and orderly contact between anhydrous magnesium chloride and steam during the pyrolysis process, resulting in a low concentration of HCl in the pyrolysis gas, which can only produce dilute hydrochloric acid. Most critically, the magnesium oxide formed by pyrolysis is in contact with high-temperature steam for a long time, causing its crystal particles to grow continuously and its reactivity to decrease, eventually making it unusable and discardable.

[0008] Option two is as follows: Use the original solution containing magnesium chloride as the reaction base liquid for the decomposition of magnesium oxide and ammonium chloride. First, heat the base liquid, stir and add ammonium chloride and magnesium oxide to evaporate ammonia, so that it is quickly converted into magnesium chloride solution and releases ammonia gas. The ammonia gas is absorbed by water to obtain ammonia water. The resulting reaction slurry is filtered while hot to obtain filtrate and filter residue. The filtrate is cooled and crystallized, and filtered to obtain hydrated magnesium chloride crystals and its crystallization mother liquor. Hydrated magnesium chloride is pyrolyzed to obtain magnesium oxide and hydrochloric acid.

[0009] As a preferred option, the prepared ammonia water has an NH3 content of ≥20wt%, and the prepared hydrochloric acid has an HCl content of ≥31wt%.

[0010] As a preferred option, the particle size of magnesium oxide is -250 mesh. The particle size of ammonium chloride is -250 mesh.

[0011] As a preferred embodiment, the pressing process is as follows: pressing the mineral material into a particle size of 0.5-5cm under a pressure of 0.25-25MPa.

[0012] As a preferred embodiment, during the roasting process, the molar ratio of magnesium oxide to ammonium chloride is 1~1.5:2.

[0013] As a preferred embodiment, the calcination conditions are: a temperature of 250~550℃ and a time of 0.5~2.5h.

[0014] As a preferred embodiment, the conditions for dissolving the calcined ore are: a solid-liquid ratio of 1:1~2 g / mL and a temperature of 100~150℃. The calcined ore is slowly added to the water while stirring, allowing the magnesium chloride to dissolve to or near saturation at a temperature of 100~150℃.

[0015] As a preferred embodiment, the mass concentration of magnesium chloride in the magnesium chloride-containing stock solution is 15-35%. Preferably, the magnesium chloride in the magnesium chloride-containing stock solution is controlled to reach saturation, which is beneficial for subsequent cooling and crystallization to precipitate hydrated magnesium chloride.

[0016] As a preferred embodiment, the magnesium chloride-containing stock solution also contains other water-soluble chlorides.

[0017] As a preferred embodiment, the other water-soluble chlorides include at least one of calcium chloride, potassium chloride, sodium chloride, lithium chloride, and ammonium chloride.

[0018] As a preferred embodiment, the mass concentration of the other water-soluble chlorides does not exceed 35%. This invention adds chlorides such as calcium chloride and potassium chloride to a magnesium chloride solution, which can significantly increase the boiling point of the solution, accelerate the reaction of ammonium chloride and magnesium oxide to release ammonia, and simultaneously reduce the solubility of magnesium chloride, increase the crystallization rate of magnesium chloride in the reaction solution, and reduce the amount of water of crystallization in the resulting hydrated magnesium chloride crystals. The molecular formula of conventional hydrated magnesium chloride crystals is MgCl2•6H2O. However, in mixed solutions, especially in solutions containing a high concentration of calcium chloride, the amount of water of crystallization in the precipitated hydrated magnesium chloride crystals is reduced, resulting in hydrated magnesium chloride crystals with the molecular formula MgCl2•nH2O, where n = 2~6. The reduced water of crystallization in magnesium chloride crystals also lowers the energy consumption of its thermal dechlorination process.

[0019] As a preferred embodiment, during the ammonia stripping process, the molar ratio of magnesium oxide to ammonium chloride is 1~1.5:2.

[0020] As a preferred embodiment, the temperature for ammonia stripping is 95~155℃. In this invention, magnesium oxide and ammonium chloride, either solid or in solution, are periodically or continuously stirred and added to the reaction substrate, causing them to rapidly convert into a magnesium chloride solution at a temperature of 95~155℃, releasing a high concentration of ammonia gas. The ammonia gas is condensed and absorbed to obtain qualified industrial ammonia water. Once the magnesium chloride in the reaction slurry reaches or is nearly saturated, it is partially or completely discharged. The slurry is then filtered while still hot to obtain filtrate and filter residue. Partial discharge of the reaction slurry is a continuous operation, while complete discharge is an intermittent operation.

[0021] As a preferred embodiment, the temperature for cooling and crystallizing the magnesium chloride solution is -10℃ to 60℃.

[0022] As a preferred embodiment, the chemical formula of the hydrated magnesium chloride solid is MgCl2•nH2O, where n = 2~6. The amount of water of crystallization in the hydrated magnesium chloride crystals obtained using the method of this invention is reduced, which helps to lower the energy consumption of the chlorine release process.

[0023] Because chloride solutions absorb ammonia, the filtrate obtained by hot filtration of the reaction slurry after ammonia stripping is an ammonia-containing solution. Completely evaporating the ammonia from this solution requires a large amount of heat and involves significant water evaporation. This invention cools the filtrate, allowing magnesium chloride to selectively crystallize out as MgCl2•nH2O. This completely separates magnesium chloride from the ammonia in the solution. The filtration yields MgCl2•nH2O crystals and its mother liquor, which is then recycled back to the ammonium chloride ammonia release process. This ensures complete separation of chlorine and ammonia (ammonium) in the ammonium chloride solution, avoids the ammonia stripping process of dilute solutions and the generation of low-concentration ammonia gas, and maximizes the efficiency of ammonia stripping. Using a high-concentration magnesium chloride solution as the reaction base liquid can significantly increase the temperature of the reaction solution. A higher solution temperature results in a faster ammonia stripping rate. Furthermore, the high magnesium chloride concentration slows down the evaporation rate of water in the solution, leading to a higher ammonia concentration. Consequently, the molar ratio of H2O to NH3 in the ammonia stripping gas is reduced from 44.6:1 in the traditional process to 4~6:1, significantly reducing the amount of water evaporated during the ammonia stripping process. This results in a very significant energy-saving and consumption-reducing effect.

[0024] As a preferred embodiment, the pyrolysis method of the hydrated magnesium chloride solid includes at least one of roasting pyrolysis, molten salt pyrolysis, and spray pyrolysis.

[0025] As a preferred embodiment, the calcination pyrolysis method involves directly calcining hydrated magnesium chloride solid. The calcination temperature is 250~550℃. The direct calcination of the hydrated magnesium chloride solid yields magnesium oxide and releases HCl gas. The HCl gas is absorbed by water to form hydrochloric acid, while the magnesium oxide is recycled back into the ammonia preparation process. The ammonia preparation process includes a mixed calcination process of magnesium oxide and ammonium chloride and / or an ammonia stripping process.

[0026] As a preferred embodiment, the molten salt pyrolysis method involves adding hydrated magnesium chloride, either in solid or molten form, to a molten salt at a temperature of 350–750°C for pyrolysis. The molten salt is a magnesium chloride melt, or a melt comprising magnesium chloride and at least one of calcium chloride, potassium chloride, sodium chloride, lithium chloride, and ammonium chloride. The mass ratio of hydrated magnesium chloride to molten salt is 1:8–24.

[0027] This invention uses magnesium chloride and at least one of calcium chloride, potassium chloride, sodium chloride, lithium chloride, and ammonium chloride to form a molten salt as the pyrolysis medium for hydrated magnesium chloride. The molten salt is first heated to 350-750°C, and then solid or solution of hydrated magnesium chloride is slowly added to the molten salt, causing it to rapidly convert into magnesium oxide and release HCl gas. The HCl gas is absorbed by water to obtain hydrochloric acid. The resulting reaction slurry is kept hot and then separated into liquid and solid, yielding molten salt and magnesium oxide precipitate. The molten salt is returned to the hydrochloric acid preparation process for continued use, and the magnesium oxide precipitate is washed and returned to the ammonia preparation process for recycling.

[0028] As a preferred embodiment, the spray pyrolysis method involves heating and melting hydrated magnesium chloride, then spraying it into a heat treatment device for pyrolysis. The spray pyrolysis temperature is not lower than 600℃. In this invention, hydrated magnesium chloride is first heated and dissolved, then the resulting magnesium chloride solution is sprayed into a fluidized bed furnace at a temperature ≥600℃, causing it to rapidly decompose into magnesium oxide and HCl gas. Gas-solid separation yields HCl gas and magnesium oxide powder. The HCl gas is absorbed by water to obtain hydrochloric acid, and the magnesium oxide powder is recycled back into the ammonia preparation process.

[0029] Magnesium oxide can be prepared by molten salt pyrolysis or spray pyrolysis of hydrated magnesium chloride. This not only simplifies the traditional process of preparing magnesium oxide and HCl gas by humidifying magnesium chloride pyrolysis, but also solves the technical problem of reduced reactivity of magnesium oxide when reused in the traditional process. This is because the magnesium oxide obtained by molten salt pyrolysis or spray pyrolysis of hydrated magnesium chloride has a large specific surface area and good reactivity.

[0030] Compared with the prior art, the present invention has the following advantages and effects:

[0031] (1) This invention ingeniously adds a process for preparing hydrated magnesium chloride in the process of preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide, so that the NH3 and HCl gases generated in the process are completely separated. The NH3 and HCl gases obtained are condensed to directly obtain qualified industrial ammonia water and hydrochloric acid products. This invention successfully solves the technical problem that has long troubled engineers in the process of preparing ammonia and hydrochloric acid by pyrolysis of ammonium chloride, which is unable to effectively separate NH3 and HCl gases. It effectively avoids the generation of dilute ammonia water and dilute hydrochloric acid, greatly improves the production efficiency and product quality of preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide, and makes it truly have application value for large-scale industrial production. At the same time, this also paves the way for clean production of chlorination processes in many chemical and metallurgical industries.

[0032] (2) The method is simple, low-cost, easy to operate, economical and environmentally friendly, clean and efficient, and suitable for industrial-scale production. Detailed Implementation

[0033] The present invention will be further described below with reference to embodiments. These embodiments are intended to illustrate the present invention and not to further limit the present invention.

[0034] Example 1

[0035] 45g of magnesium oxide (MgO) powder and 110g of ammonium chloride (NH4Cl) were weighed, mixed and ground to -250 mesh. The powder was then loaded, pressed and demolded to obtain a round cake with a thickness of 1cm and a diameter of 2cm. The forming pressure of the round cake was 2.5MPa. The obtained round cake was placed in a crucible and sent to a tube furnace for roasting at 420℃ for 2h. The roasting flue gas was condensed and absorbed in multiple stages to obtain ammonia water with a w% concentration of 27.5% ammonia (NH3). The weight of the obtained roasted sand was 98.53g. The effective utilization rate of ammonium chloride during the roasting process was 98.82%. The above-mentioned calcined sand was dissolved by adding 130 mL of water and stirring and heating to 125°C. The mixture was then filtered while hot to obtain filtrate and residue. The residue was returned to the calcination process for reuse. The filtrate was cooled and crystallized, and filtered to obtain MgCl2•6H2O crystals and its crystallization mother liquor. The crystallization mother liquor was returned to the calcined sand dissolution process for recycling. The hydrated magnesium chloride crystals were heated and melted, then sprayed to pyrolyze at 680°C. Gas-solid separation yielded MgO powder and HCl-containing pyrolysis gas. The MgO powder was returned to the calcination process for recycling. The HCl-containing pyrolysis gas was condensed and absorbed to obtain hydrochloric acid with a concentration of 37.4 wt%. The effective utilization rate of ammonium chloride decomposition to prepare ammonia and hydrochloric acid in the entire process was 98.01%.

[0036] Example 2

[0037] First, magnesium chloride hexahydrate crystals were added to water and stirred to dissolve, preparing 500 mL of a 35% magnesium chloride solution. This solution was then added to a 1000 mL three-necked flask as the base solution for the decomposition of ammonium chloride by magnesium oxide. The base solution was heated, and 50 g of magnesium oxide powder was added while stirring. When the solution temperature reached 105°C, a 25% ammonium chloride solution obtained by dissolving 107 g of ammonium chloride in water was slowly added dropwise. During the addition process, the solution was kept boiling to evaporate ammonia. The evaporated ammonia-containing gas was absorbed by multi-stage countercurrent condensation to obtain ammonia water with a concentration of 25.78 wt%. After the ammonium chloride solution was completely added, heating and stirring continued until the reaction slurry temperature reached 122°C and the number of bubbles on the liquid surface significantly decreased. The mixture was then filtered while hot to obtain the filtrate and filter residue. The filter residue was returned to the distillation flask. The ammonia process utilizes the filtrate, which is cooled to room temperature to crystallize. Filtering yields MgCl₂•6H₂O crystals and its mother liquor. The mother liquor is returned to the ammonia stripping process for continued use. The MgCl₂•6H₂O crystals are washed with a saturated magnesium chloride solution and dried at 60°C, weighing 206.78 g. During the ammonia stripping process, the molar ratio of H₂O to NH₃ volatilization is 5.3:1. The resulting hydrated magnesium chloride crystals are added to a boat and placed in a tube furnace for calcination at 600°C for 1 hour. Nitrogen gas is introduced as a carrier gas during calcination. The HCl gas generated during calcination is absorbed by a three-stage condenser using dilute hydrochloric acid to obtain hydrochloric acid with a concentration of 35.8 wt%. The calcined slag, cooled to room temperature, weighs 41.01 g and is returned to the ammonia stripping process for recycling. The recovery rates of NH₃ and HCl in the process are 98.7% and 99.5%, respectively.

[0038] Example 3

[0039] First, slowly add calcium chloride dihydrate to a 20% magnesium chloride solution while stirring until the magnesium chloride is saturated. Then, add 500 mL of the mixed solution of magnesium chloride and calcium chloride to a 1000 mL three-necked flask, followed by stirring and adding 50 g of magnesium oxide powder. Heat the solution until the temperature reaches 108°C, then slowly add a 30% ammonium chloride solution prepared with 100 g of NH4Cl to evaporate ammonia. The addition process is maintained at boiling. The evaporated ammonia-containing gas is absorbed by multi-stage countercurrent condensation to obtain ammonia water with a concentration of 23.03 w%. After the ammonium chloride solution is completely added, continue heating and stirring until the temperature of the reaction slurry reaches 138°C. The number of bubbles on the slurry surface significantly decreases. Filter while hot, obtaining filtrate and residue. The residue is returned to the ammonia evaporation process for continued use. The filtrate is cooled to room temperature to crystallize and precipitate hydrated magnesium chloride. Filter to obtain MgCl2•nH2O crystals and its crystallization mother liquor. The crystallization mother liquor is returned to the ammonia evaporation process for continued use. The nH2O crystals were washed with a saturated magnesium chloride solution, filtered, and dried at 70°C, yielding 179.49 g. The resulting MgCl2•nH2O crystals had an n of 5.17, and the average molar ratio of H2O to NH3 volatilized simultaneously during the ammonia stripping process was 4.6:1. The resulting hydrated magnesium chloride crystals were then heated and dissolved, and slowly added with stirring to a molten salt composed of MgCl2-NaCl-KCl-CaCl2 with a minimum melting temperature of 385°C. The mixture was then pyrolyzed at 500°C. The HCl gas produced during pyrolysis was absorbed by condensation to obtain hydrochloric acid with a concentration of 34.03%. After the reaction slurry settled naturally at a constant temperature, solid-liquid separation was achieved, yielding molten salt and precipitate slurry. The molten salt was returned to the hydrated magnesium chloride pyrolysis process for continued use, while the precipitate slurry was diluted with water, filtered, and washed to obtain regenerated magnesium oxide. The regenerated magnesium oxide was returned to the ammonia stripping process for recycling. The recovery rates of NH3 and HCl in the process were 97.8% and 98.9%, respectively.

[0040] Comparative Example 1

[0041] 44.31g of magnesium oxide (MgO) and 109.04g of ammonium chloride (NH4Cl) were weighed, mixed, and ground to -250 mesh. The mixture was then pressed into cakes, placed in a crucible, and calcined in a muffle furnace at 470℃ for 2 hours, yielding 98.49g of calcined ore. The effective utilization rate of ammonium chloride during the calcination process was 96.73%. The obtained calcined ore was further ground, added to a boat, and placed in a tube furnace. It was then pyrolyzed with steam at 450℃ for 1 hour, causing hydrolysis, releasing HCl gas, and yielding 40.16g of regenerated MgO. The 40.16g of regenerated MgO was then mixed with 108.92g of ammonium chloride (NH4Cl) and ground to -250 mesh. The mixture was pressed into briquettes and calcined at 470℃ for 2 hours, yielding 87.36g of calcined ore. The effective utilization rate of ammonium chloride during the calcination process decreased to 91.91%. With the increasing number of times magnesium oxide was reused, the particle size of magnesium oxide increased, and the reactivity decreased.

[0042] Comparative Example 2

[0043] 107g of ammonium chloride and 50g of magnesium oxide powder were weighed and added to a three-necked flask containing 600mL of water. The mixture was heated and stirred until boiling, and ammonia was evaporated. The evaporated ammonia gas was absorbed by water after cooling to obtain ammonia water. After boiling and evaporating ammonia for 3 hours, the turbidity of the solution decreased significantly. After 4 hours, the evaporated gas was still strongly alkaline when tested with moistened pH paper, and the solution temperature rose to 125℃, indicating that the concentration of MgCl2 in the ammonia evaporation residue was close to 36%. To remove residual ammonia from the slurry, water was added and ammonia evaporation continued until the ammonia evaporated to the endpoint. The molar ratio of H2O to NH3 volatilized during the ammonia evaporation process was 44.8:1. The total evaporation time was 7.5 hours, and the concentration of the resulting ammonia water was only 2%, with an NH3 recovery rate of 92.3%. It is very difficult to completely evaporate ammonia from the magnesium chloride solution.

Claims

1. A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide, characterized in that: Magnesium oxide and ammonium chloride are mixed, pressed into shape, and then calcined to obtain calcined sand and ammonia-containing flue gas. The ammonia-containing flue gas is condensed and absorbed to obtain ammonia water. The calcined sand is dissolved in water and then separated into solid and liquid phases to obtain magnesium oxide slag phase and magnesium chloride-containing solution. The magnesium chloride-containing solution is cooled and crystallized to obtain hydrated magnesium chloride solid and crystallization mother liquor. The hydrated magnesium chloride solid is pyrolyzed to obtain magnesium oxide solid and HCl-containing gas. The HCl-containing gas is condensed and absorbed to obtain hydrochloric acid. The magnesium oxide slag phase and magnesium oxide solid are recycled back to the calcination process, and the crystallization mother liquor is recycled back to the calcined sand dissolution process. Alternatively, the original solution containing magnesium chloride is mixed with magnesium oxide and heated, and then ammonium chloride solid or ammonium chloride solution is added for ammonia stripping to obtain ammonia-containing gas and a reaction solution. The ammonia-containing gas is condensed and absorbed to obtain ammonia water. The reaction solution is subjected to solid-liquid separation to obtain magnesium oxide slag phase and magnesium chloride-containing solution. The magnesium chloride-containing solution is cooled and crystallized to obtain hydrated magnesium chloride solid and crystallization mother liquor. The hydrated magnesium chloride solid is pyrolyzed to obtain magnesium oxide solid and HCl-containing gas. The HCl-containing gas is condensed and absorbed to obtain hydrochloric acid. The magnesium oxide slag phase and crystallization mother liquor are returned to the ammonia stripping process for recycling.

2. The method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1, characterized in that: During the roasting process, the molar ratio of magnesium oxide to ammonium chloride is 1~1.5:

2.

3. A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1 or 2, characterized in that: The calcination conditions are: temperature of 250~550℃ and time of 0.5~2.5h.

4. A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1 or 2, characterized in that: The conditions for dissolving the calcined sand are: a solid-liquid ratio of 1:1~2 g / mL and a temperature of 100~150℃.

5. The method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1, characterized in that: The mass concentration of magnesium chloride in the stock solution containing magnesium chloride is 15-35%.

6. A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1 or 5, characterized in that: The stock solution containing magnesium chloride also contains other water-soluble chlorides; The other water-soluble chlorides include at least one of calcium chloride, potassium chloride, sodium chloride, lithium chloride, and ammonium chloride; The mass concentration of the other water-soluble chlorides shall not exceed 35%.

7. The method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1, characterized in that: During the ammonia stripping process, the molar ratio of magnesium oxide to ammonium chloride is 1~1.5:

2.

8. A method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1 or 7, characterized in that: The temperature for ammonia stripping is 95~155℃.

9. The method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1, characterized in that: The chemical formula of the hydrated magnesium chloride solid is MgCl2•nH2O, where n = 2~6.

10. The method for preparing ammonia and hydrochloric acid by decomposing ammonium chloride with magnesium oxide according to claim 1, characterized in that: The pyrolysis method of the hydrated magnesium chloride solid includes at least one of roasting pyrolysis, molten salt pyrolysis, and spray pyrolysis; The roasting and pyrolysis method is as follows: hydrated magnesium chloride solid is directly roasted at a temperature of 250~550℃. The method of molten salt pyrolysis is as follows: hydrated magnesium chloride is added in solid or molten form to molten salt at a temperature of 350~750℃ for pyrolysis; the molten salt is magnesium chloride melt, or includes a melt formed by magnesium chloride and at least one of calcium chloride, potassium chloride, sodium chloride, lithium chloride, and ammonium chloride; The spray pyrolysis method is as follows: hydrated magnesium chloride is heated and melted, then sprayed into a heat treatment device for pyrolysis; the spray pyrolysis temperature is not lower than 600℃.