A process for preparing concentrated nitric acid by efficient magnesium nitrate method

Abstract

The present invention relates to the technical field of concentrated nitric acid preparation, and in particular to a novel and efficient process for preparing concentrated nitric acid by a magnesium nitrate method. The technical key points are as follows: after distilling and concentrating a second magnesium nitrate solution, the second magnesium nitrate solution is returned to a stripping-rectification tower as a first magnesium nitrate solution for recycling; wherein, when the second magnesium nitrate solution meets at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated: in the second magnesium nitrate solution, Mg 2+ The concentration of Mg in the second magnesium nitrate solution is lower than the first preset value; 2+ The concentration of NO3 ‑ The ratio of the concentration of the second magnesium nitrate solution is less than or equal to the second preset value, and the concentration of other metal ions in the second magnesium nitrate solution is greater than or equal to the third preset value. 2+ Pretreatment is performed when the concentration or ion ratio reaches a certain standard, which effectively reduces the risk of magnesium nitrate crystals blocking the pipeline and improves the purity of the finished acid.

Description

Technical Field

[0001] The invention relates to the technical field of concentrated nitric acid preparation, and in particular to a novel and efficient process for preparing concentrated nitric acid by a magnesium nitrate method. Background Art

[0002] The production of concentrated nitric acid is to evaporate and concentrate dilute nitric acid (nitric acid mass fraction is 60%~68%) to remove water in the solution, so that the mass fraction of nitric acid reaches more than 98%. However, since nitric acid and water can form azeotropes, the mass fraction of dilute nitric acid can only be concentrated to 68.4% by conventional distillation. Magnesium nitrate has a strong ability to bind water. When it is added to a dilute nitric acid solution, it can destroy the azeotropic composition of nitric acid. Then, concentrated nitric acid with a mass fraction of ≥98% can be obtained by distillation. This process is called the magnesium nitrate process, also known as the salt distillation process; the magnesium nitrate process mainly includes dilute nitric acid distillation concentration, bleaching and magnesium nitrate evaporation concentration.

[0003] The process of preparing concentrated nitric acid by the magnesium nitrate method mainly has problems such as corrosion and leakage of the magnesium nitrate heating tube bundle, reduced efficiency in the distillation and concentration stage, and blockage of the concentrated magnesium nitrate pipeline. In the prior art, the above problems are usually solved by improving equipment materials and layout, but it is still impossible to fundamentally eliminate the risk of magnesium nitrate crystals blocking the pipeline and corroding the equipment, and it is also impossible to further improve the purity of the concentrated nitric acid. Summary of the invention

[0004] The purpose of the present invention is to provide a novel and efficient process for preparing concentrated nitric acid by using a magnesium nitrate method. By using a special design of a stripping-rectifying tower and rationally controlling the proportion of a mixed solution and operating conditions, nitric acid with a mass fraction greater than 98% is successfully prepared; at the same time, by recycling the second magnesium nitrate solution and adding Mg 2+ When the concentration or ion ratio reaches a certain standard, it is pretreated and then reused in the production process, effectively reducing the risk of magnesium nitrate crystals clogging the pipeline.

[0005] The above technical objectives of the present invention are achieved through the following technical solutions:

[0006] The present invention provides a novel and efficient magnesium nitrate method for preparing concentrated nitric acid, comprising the following steps:

[0007] Mixing the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:(2.6-3.2) to obtain a first mixed solution;

[0008] The first mixed liquid is sent to the middle part of the stripping-rectifying tower, nitric acid vapor with a mass fraction greater than or equal to 98% is obtained at the top of the tower, and a second magnesium nitrate solution is obtained at the bottom of the tower;

[0009] The nitric acid vapor is bleached in a bleaching tower and then condensed to obtain concentrated nitric acid;

[0010] After the concentrated nitric acid passes through the gas-liquid separator, part of it flows back to the stripping-rectification tower as reflux acid, and the other part enters the bleaching tower to remove nitrogen oxides and then condense to obtain finished acid;

[0011] After the second magnesium nitrate solution is distilled and concentrated, it is returned to the stripping-rectification tower as the first magnesium nitrate solution for recycling;

[0012] Wherein, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0013] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than a first preset value;

[0014] In the second magnesium nitrate solution, Mg 2+ The concentration of NO3 - The ratio of the concentrations is less than or equal to a second preset value;

[0015] In the second magnesium nitrate solution, the concentration of other metal ions is greater than or equal to the third preset value.

[0016] Furthermore, the first preset value is determined according to the following method:

[0017] Performing statistics on the distillation effect of the first concentrated nitric acid to obtain the first concentrated nitric acid concentration information corresponding to N second magnesium nitrate concentrations;

[0018] Obtaining a first distillation effect score according to the first concentrated nitric acid concentration information;

[0019] Performing a correlation analysis on the second magnesium nitrate concentration based on the first distillation effect score to obtain a correlation index between the second magnesium nitrate concentration and the distillation effect of the first concentrated nitric acid;

[0020] Based on the correlation index, obtain the correlation index inflection point;

[0021] The first preset value is determined according to the inflection point of the correlation index.

[0022] Furthermore, the second preset value is determined according to the following method:

[0023] The first equipment corrosion effect statistics and the first magnesium nitrate crystallization effect statistics are performed to obtain M Mg 2+ The concentration of NO3 - The first equipment corrosion rate information and the first magnesium nitrate crystallization rate information corresponding to the concentration ratio of ;

[0024] Obtaining a first equipment corrosion effect score and a first magnesium nitrate crystallization effect score according to the first equipment corrosion rate information and the first magnesium nitrate crystallization rate information;

[0025] Based on the first equipment corrosion effect score and the first magnesium nitrate crystallization effect score, Mg 2+ The concentration of NO3 - The correlation analysis was performed on the ratio of the concentration of Mg 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of Mg and the first equipment corrosion effect 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of and the first magnesium nitrate crystallization effect;

[0026] Based on the two correlation indexes, two correlation index inflection points are obtained;

[0027] The second preset value is determined according to the middle value of the two correlation index inflection points.

[0028] Furthermore, the third preset value is determined according to the following method:

[0029] Perform statistics on the distillation effect of the second concentrated nitric acid to obtain the concentration information of the second concentrated nitric acid corresponding to the concentrations of P other metal ions;

[0030] Obtaining a second distillation effect score according to the second concentrated nitric acid concentration information;

[0031] Based on the second distillation effect score, correlation analysis is performed on the concentrations of other metal ions to obtain a correlation index between the concentrations of other metal ions and the second concentrated nitric acid distillation effect, and based on the correlation index, a first correlation index inflection point is obtained;

[0032] Performing statistics on the corrosion effect of the second device to obtain the corrosion rate information of the second device corresponding to P other metal ion concentrations;

[0033] Obtaining a corrosion effect score of the second equipment according to the corrosion rate information of the second equipment;

[0034] Performing correlation analysis on other metal ion concentrations based on the second equipment corrosion effect score to obtain a correlation index between other metal ion concentrations and the second equipment corrosion effect, and obtaining a second correlation index inflection point based on the correlation index;

[0035] Performing statistics on the crystallization effect of the second magnesium nitrate to obtain the crystallization rate information of the second magnesium nitrate corresponding to P other metal ion concentrations;

[0036] Obtaining a second magnesium nitrate crystallization effect score according to the second magnesium nitrate crystallization rate information;

[0037] Based on the second magnesium nitrate crystallization effect score, correlation analysis is performed on other metal ion concentrations to obtain a correlation index between other metal ion concentrations and the second magnesium nitrate crystallization effect, and based on the correlation index, a third correlation index inflection point is obtained;

[0038] A third preset value is determined according to the first correlation index inflection point, the second correlation index inflection point, and the third correlation index inflection point.

[0039] Further, when the second magnesium nitrate solution, Mg 2+ The concentration of Mg is lower than the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is greater than the second preset value and the concentration of other metal ions is less than the third preset value,

[0040] The pretreatment method is to add Mg(NO3)2 to the second magnesium nitrate solution so that Mg(NO3)2 2+ The concentration is higher than the first preset value.

[0041] Further, when the second magnesium nitrate solution, Mg 2+ The concentration of Mg is greater than or equal to the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is less than or equal to the second preset value, and the concentration of other metal ions is less than the third preset value,

[0042] The pretreatment method is to evaporate the nitric acid and water in the second magnesium nitrate solution in the form of an acidic mixed gas.

[0043] Further, when the second magnesium nitrate solution, Mg 2+ The concentration of Mg is greater than or equal to the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is greater than the second preset value and the concentration of other metal ions is greater than the third preset value,

[0044] The pretreatment method is to remove impurities from the second magnesium nitrate solution.

[0045] Furthermore, the tail gas obtained from the gas-liquid separator is distilled through the tower tail ejector to obtain an acidic aqueous solution, which enters the tower tail water tank.

[0046] Furthermore, after the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing vapor enters a distillation tower for distillation, and the recovered acidic aqueous solution enters a tower tail water tank.

[0047] Furthermore, the acidic mixed gas and the acid-containing steam enter the distillation tower for distillation.

[0048] Furthermore, the top temperature of the stripping-rectification tower is 80~90°C, and the top pressure is -4~-2KPa.

[0049] Furthermore, the top temperature of the bleaching tower is 72~78°C.

[0050] Furthermore, the concentration of the first magnesium nitrate solution is 66-70%.

[0051] In summary, the present invention has the following beneficial effects:

[0052] The novel and efficient magnesium nitrate method for preparing concentrated nitric acid provided by the present invention optimizes the design and operation of the stripping-rectification tower, reduces the magnesium nitrate feed ratio, ensures the purity of the finished acid, successfully avoids the problem of pipeline blockage, and ensures the smooth progress of the production process; the process provided by the present invention controls the impurity content through an effective pretreatment step to prevent impurities from accumulating in the equipment, thereby avoiding corrosion of the equipment and extending the service life of the equipment. At the same time, the process can stably produce high-purity concentrated nitric acid with a mass fraction greater than 98%, significantly improving product quality and production efficiency. DETAILED DESCRIPTION

[0053] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, a novel and efficient magnesium nitrate method for preparing concentrated nitric acid according to the present invention, its specific implementation method, characteristics and effects are described in detail as follows.

[0054] The raw materials of the nitric acid process are dilute nitric acid and a dehydrating agent. The dehydrating agent breaks the azeotropic equilibrium state of the solution and removes water from the nitric acid. The nitric acid solution is concentrated and the dilute nitric acid is distilled to produce concentrated nitric acid. The aqueous nitric acid solution is an azeotropic solution with a constant boiling point. When the constant boiling point of the mixture is 120°C, the corresponding nitric acid concentration is 67.9%, which cannot be further concentrated. By adding a dehydrating agent, the original azeotropic system is destroyed, and further distillation can produce 98% concentrated nitric acid. A dehydrating agent that can meet industrial application needs to have the following characteristics: (1) high dehydration efficiency, that is, it can greatly reduce the water vapor partial pressure on the surface of the nitric acid aqueous solution; (2) it will not react irreversibly with nitric acid; (3) its own vapor partial pressure is extremely low and will not mix into the nitric acid vapor; (4) it is easy to separate from water; (5) it has good thermal stability and will not decompose when heated; (6) it is cheap and easy to obtain or prepare; (7) it is less corrosive to equipment. According to the operating experience of nitric acid plants in various countries, the feasible dehydrating agents are concentrated sulfuric acid, Mg(NO3)2, Ca(NO3)2 and Zn(NO3)2. The ones that have been industrialized are concentrated sulfuric acid and Mg(NO3)2. The process of using concentrated sulfuric acid to remove water to produce concentrated nitric acid is referred to as the "sulfuric acid method" and the process of using magnesium nitrate solution to remove water is called the "magnesium nitrate method". The sulfuric acid method requires the construction of a complex dilute sulfuric acid concentration and reuse device. The device occupies a large area, has high infrastructure costs, high operating costs, and many emission indicators are difficult to meet environmental standards. There are trace amounts of sulfate ions in the product concentrated nitric acid that cannot be removed, which limits the use of the product. The magnesium nitrate adsorption method has a relatively low process investment, easy equipment selection and maintenance, low adsorbent price, good recycling effect, and the concentration of the finished nitric acid can reach 98%. It has fewer impurities and stable quality. It is currently the mainstream process for the preparation of concentrated nitric acid at home and abroad.

[0055] This specific implementation method analyzes the problems that arise in the operation of the existing magnesium nitrate method for preparing concentrated nitric acid, mainly involving the problems of poor operation of the nitric acid concentration tower, material corrosion, high consumption of circulating water and steam, and short operation cycle of the device, and proposes solutions as follows.

[0056] This specific embodiment provides a novel and efficient magnesium nitrate method for preparing concentrated nitric acid, comprising the following steps:

[0057] Mixing the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:(2.6-3.2) to obtain a first mixed solution;

[0058] The first mixed liquid is sent to the middle part of the stripping-rectifying tower, nitric acid vapor with a mass fraction greater than or equal to 98% is obtained at the top of the tower, and a second magnesium nitrate solution is obtained at the bottom of the tower;

[0059] The nitric acid vapor is bleached in a bleaching tower and then condensed to obtain concentrated nitric acid;

[0060] After the concentrated nitric acid passes through the gas-liquid separator, part of it flows back to the stripping-rectification tower as reflux acid, and the other part enters the bleaching tower to remove nitrogen oxides and then condense to obtain finished acid;

[0061] After the second magnesium nitrate solution is distilled and concentrated, it is returned to the stripping-rectification tower as the first magnesium nitrate solution for recycling;

[0062] Wherein, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0063] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than a first preset value;

[0064] In the second magnesium nitrate solution, Mg 2+ The concentration of NO3 - The ratio of the concentrations is less than or equal to a second preset value;

[0065] In the second magnesium nitrate solution, the concentration of other metal ions is greater than or equal to the third preset value.

[0066] Dilute nitric acid solution exists in an azeotropic state. When the pressure is 0.1MPa(G), the boiling point of the azeotrope formed is 121.9℃, corresponding to a nitric acid mass fraction of 68.4%. Therefore, the 98% concentrated nitric acid required by industry cannot be obtained by simple distillation of dilute nitric acid, and other components must be added to destroy the azeotropic system. In the magnesium nitrate method, the liquid phase composition of the HNO3-H2O-Mg(NO3)2 ternary mixture at normal pressure and the nitric acid concentration in the gas phase can be calculated. In order to obtain more than 98% concentrated nitric acid, the theoretical ratio of Mg(NO3)2:HNO3 is 2.53:1, and the constant boiling point of the solution is 121℃. In actual operation, a lot of impurities will accumulate in the magnesium nitrate solution, mainly Ca 2+ , Fe 3+ Plasma reduces its water absorption performance. Therefore, in actual device operation, the feed ratio will be higher than the theoretical value, reaching 3.5~5.5:1. When the feed ratio is 3.5:1, the solution composition is 57.5% Mg(NO3)2, 13.3% HNO3, 29.2% H2O, and the boiling point of the solution is 124℃.

[0067] In this specific embodiment, the feed ratio of concentrated magnesium nitrate is reduced to 2.6-3.2:1, which is closer to the theoretical value, and the boiling point of the solution is reduced from 124°C and above to about 120°C. On the one hand, the risk of decomposition of nitric acid at high temperature is reduced and the concentration of concentrated nitric acid is increased, and on the other hand, energy consumption is greatly saved.

[0068] In order to make the feed ratio 1 of concentrated magnesium nitrate approach the theoretical value, the present specific embodiment effectively monitors and controls the metal ion impurities in the second magnesium nitrate solution in the preparation process system, that is, the magnesium nitrate solution before concentration, thereby ensuring the purity of the concentrated magnesium nitrate solution.

[0069] Exemplarily, the first preset value is determined according to the following method:

[0070] Performing statistics on the distillation effect of the first concentrated nitric acid to obtain the first concentrated nitric acid concentration information corresponding to N second magnesium nitrate concentrations;

[0071] Obtaining a first distillation effect score according to the first concentrated nitric acid concentration information;

[0072] Performing a correlation analysis on the second magnesium nitrate concentration based on the first distillation effect score to obtain a correlation index between the second magnesium nitrate concentration and the distillation effect of the first concentrated nitric acid;

[0073] Based on the correlation index, obtain the correlation index inflection point;

[0074] The first preset value is determined according to the inflection point of the correlation index.

[0075] Specifically, the first concentrated nitric acid distillation effect statistics are performed by gradually increasing the concentration of the second magnesium nitrate solution under different production conditions, and recording the concentration of the first concentrated nitric acid obtained after each distillation. Through multiple experiments, N groups of first concentrated nitric acid concentration data corresponding to different second magnesium nitrate concentrations are obtained. These data will serve as the basis for subsequent analysis.

[0076] The distillation effect score is to obtain the first distillation effect score based on the first concentrated nitric acid concentration information, that is, to evaluate each set of experimental data collected (that is, the concentrated nitric acid concentration at each second magnesium nitrate concentration). A variety of methods can be used to score each set of data, for example: scoring based on the degree to which the concentrated nitric acid concentration is close to 98%. If the concentration is too low, the score is low; if the concentration reaches or approaches 98%, the score is high; score quantification: the scoring results are quantified into numerical values ​​to form a set of distillation effect score data for subsequent correlation analysis.

[0077] The correlation analysis is to conduct correlation analysis on the second magnesium nitrate concentration based on the first distillation effect score: use statistical analysis tools (such as regression analysis, correlation analysis, etc.) to conduct correlation analysis on the collected score data and the corresponding second magnesium nitrate concentration. The purpose is to determine the influence of the second magnesium nitrate concentration on the first concentrated nitric acid distillation effect and obtain a set of correlation index data.

[0078] Construct a correlation model. According to the analysis results, construct a correlation model, such as a linear regression model, a polynomial regression model, etc., to describe the mathematical relationship between the second magnesium nitrate concentration and the distillation effect score.

[0079] Determine the inflection point of the correlation index. Through in-depth analysis of the correlation model, identify the inflection point in the model curve, that is, when the second magnesium nitrate concentration reaches a certain value, the distillation effect score changes significantly. The inflection point of the curve can be identified by calculating the first-order derivative or using other methods.

[0080] Determine the first preset value, determine the first preset value according to the inflection point of the correlation index, and set the second magnesium nitrate concentration corresponding to the identified inflection point of the correlation index as the first preset value. This value represents the key control point for maintaining the optimal concentrated nitric acid production effect in the process provided in this specific embodiment.

[0081] It can be understood that the first preset value provided in this specific embodiment is the minimum value of the magnesium nitrate concentration in the second magnesium nitrate solution. When the magnesium nitrate concentration in the second magnesium nitrate solution is lower than this value, even if the concentration of the first magnesium nitrate solution obtained by concentration is still low, within the range of the feed ratio of magnesium nitrate to dilute nitric acid specified in this specific embodiment, the concentration of the first magnesium nitrate is too low, the hygroscopicity decreases, resulting in a decrease in the concentration of concentrated nitric acid, while affecting the concentration of the finished acid.

[0082] Exemplarily, the second preset value is determined according to the following method:

[0083] The first equipment corrosion effect statistics and the first magnesium nitrate crystallization effect statistics are performed to obtain M Mg 2+ The concentration of NO3 - The first equipment corrosion rate information and the first magnesium nitrate crystallization rate information corresponding to the concentration ratio of ;

[0084] Obtaining a first equipment corrosion effect score and a first magnesium nitrate crystallization effect score according to the first equipment corrosion rate information and the first magnesium nitrate crystallization rate information;

[0085] Based on the first equipment corrosion effect score and the first magnesium nitrate crystallization effect score, Mg 2+ The concentration of NO3 - The correlation analysis was performed on the ratio of the concentration of Mg 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of Mg and the first equipment corrosion effect 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of and the first magnesium nitrate crystallization effect;

[0086] Based on the two correlation indexes, two correlation index inflection points are obtained;

[0087] The second preset value is determined according to the middle value of the two correlation index inflection points.

[0088] Specifically, in order to determine the second preset value, this specific method provides the following detailed steps, involving comprehensive analysis of equipment corrosion and magnesium nitrate crystallization and construction of a correlation model.

[0089] Data collection, equipment corrosion effect statistics: Under different Mg 2+ Concentration and NO3 - The equipment corrosion experiment was carried out under the conditions of the ratio of concentration and the corrosion rate of the equipment under these conditions was recorded. The collected data formed the M group Mg 2+ With NO3 - Equipment corrosion rate information corresponding to the ratio.

[0090] The crystallization effect of magnesium nitrate is statistically analyzed. 2+ Concentration and NO3 - Under the conditions of the ratio of concentration, the magnesium nitrate crystallization experiment was carried out and the crystallization rate was recorded. The collected data formed the M group Mg 2+ With NO3 - The magnesium nitrate crystallization rate information corresponding to the ratio.

[0091] Effect rating, equipment corrosion effect rating, according to different Mg 2+ With NO3 - The equipment corrosion rate under the ratio is used to establish a scoring system. The lower the corrosion rate, the higher the score, and vice versa. The score can be quantified in a linear or nonlinear way to form the first equipment corrosion effect scoring data set.

[0092] Magnesium nitrate crystallization effect score, according to different Mg 2+ With NO3 - A scoring system is established based on the crystallization rate under the ratio. The lower the crystallization rate, the higher the score, and vice versa. This forms the first magnesium nitrate crystallization effect scoring data set.

[0093] Correlation analysis, equipment corrosion correlation analysis, using regression analysis or other statistical methods, the first equipment corrosion effect score and Mg 2+ With NO3 - The correlation model was obtained by correlation analysis.

[0094] The model can be linear regression, polynomial regression or other models suitable for data characteristics to generate Mg 2+ With NO3 - The correlation index between the ratio and the equipment corrosion effect score; magnesium nitrate crystallization correlation analysis, the same method, the first magnesium nitrate crystallization effect score and Mg 2+ With NO3 - The correlation analysis of the ratios was performed to obtain the correlation index of the crystallization effect score.

[0095] Inflection point analysis, the inflection point of equipment corrosion effect, by calculating the first-order derivative of the equipment corrosion effect score correlation model, determine the inflection point of the score change, which represents the inflection point of the equipment corrosion effect score correlation model. 2+ With NO3 - The inflection point of the crystallization effect of magnesium nitrate is determined by calculating the first-order derivative of the crystallization effect score correlation model, which represents the inflection point of the Mg nitrate crystallization effect. 2+ With NO3 - The point at which the crystallization rate or crystal quality changes significantly under the ratio.

[0096] Determine the second preset value, calculate the inflection point middle value, and convert the Mg corresponding to the two inflection points 2+ With NO3 - The ratios are compared and the middle value of the two ratios is taken as the second preset value.

[0097] This middle value represents a balance point. Under this ratio, the corrosion of equipment can be effectively reduced, and the crystallization rate of magnesium nitrate can be controlled, making the process operation more stable.

[0098] It is understandable that this specific embodiment provides a second preset value, that is, in the second magnesium nitrate solution, when Mg 2+ With NO3 - When the ratio of the concentration of the ions is less than the second preset value, it indicates that the amount of nitric acid in the second magnesium nitrate solution is too high, which is easy to corrode the equipment and cause magnesium nitrate crystallization. Therefore, in this specific embodiment, the equipment corrosion rate and the magnesium nitrate crystallization rate are combined with the Mg in the second magnesium nitrate solution. 2+ With NO3 - The ratio of the concentration of the ions is associated with each other, and its threshold is defined, so that the nitric acid content in the second magnesium nitrate solution can be monitored and controlled in time, the risk of equipment corrosion and magnesium nitrate crystallization is greatly reduced, and the smooth operation of the equipment is ensured. At the same time, the concentration of magnesium nitrate in the recycled first magnesium nitrate solution is ensured, so as to achieve the purpose of reducing the feed ratio of magnesium nitrate and dilute nitric acid while ensuring the concentration of concentrated nitric acid as desired in this specific implementation method.

[0099] Exemplarily, the third preset value is determined according to the following method:

[0100] Perform statistics on the distillation effect of the second concentrated nitric acid to obtain the concentration information of the second concentrated nitric acid corresponding to the concentrations of P other metal ions;

[0101] Obtaining a second distillation effect score according to the second concentrated nitric acid concentration information;

[0102] Based on the second distillation effect score, correlation analysis is performed on the concentrations of other metal ions to obtain a correlation index between the concentrations of other metal ions and the second concentrated nitric acid distillation effect, and based on the correlation index, a first correlation index inflection point is obtained;

[0103] Performing statistics on the corrosion effect of the second device to obtain the corrosion rate information of the second device corresponding to P other metal ion concentrations;

[0104] Obtaining a corrosion effect score of the second equipment according to the corrosion rate information of the second equipment;

[0105] Performing correlation analysis on other metal ion concentrations based on the second equipment corrosion effect score to obtain a correlation index between other metal ion concentrations and the second equipment corrosion effect, and obtaining a second correlation index inflection point based on the correlation index;

[0106] Performing statistics on the crystallization effect of the second magnesium nitrate to obtain the crystallization rate information of the second magnesium nitrate corresponding to P other metal ion concentrations;

[0107] Obtaining a second magnesium nitrate crystallization effect score according to the second magnesium nitrate crystallization rate information;

[0108] Based on the second magnesium nitrate crystallization effect score, correlation analysis is performed on other metal ion concentrations to obtain a correlation index between other metal ion concentrations and the second magnesium nitrate crystallization effect, and based on the correlation index, a third correlation index inflection point is obtained;

[0109] A third preset value is determined according to the first correlation index inflection point, the second correlation index inflection point, and the third correlation index inflection point.

[0110] Specifically, to determine the third preset value, this specific implementation provides the following specific steps, which involves detailed data analysis and correlation model construction in three aspects: distillation effect, equipment corrosion, and magnesium nitrate crystallization.

[0111] Data collection and statistics, statistics on the distillation effect of the second concentrated nitric acid, under different other metal ion concentration conditions (such as Ca 2+ and Fe 3+ ), conduct a distillation experiment of concentrated nitric acid, record the concentration of concentrated nitric acid obtained in each experiment, collect the second concentrated nitric acid concentration data corresponding to the concentrations of other metal ions in group P, and form a data set.

[0112] The second equipment corrosion effect statistics, under the same other metal ion concentration conditions, measure the corrosion rate of the equipment, collect the second equipment corrosion rate data corresponding to the other metal ion concentrations of group P, and form an equipment corrosion effect data set.

[0113] The second magnesium nitrate crystallization effect statistics, under the same other metal ion concentration conditions, the magnesium nitrate crystallization experiment was carried out, the crystallization rate data was recorded, and the second magnesium nitrate crystallization rate data corresponding to the other metal ion concentrations of group P were collected to form a magnesium nitrate crystallization effect data set.

[0114] The second distillation effect scoring system is established based on the collected concentrated nitric acid concentration data. For example, the concentrated acid concentration close to the target concentration (such as 98%) has a higher score, and the score with a large deviation is lower; thus forming the second distillation effect scoring data set.

[0115] The second equipment corrosion effect scoring is to establish a scoring system based on the equipment corrosion rate. For example, the lower the rate, the higher the score, and vice versa; forming the second equipment corrosion effect scoring data set.

[0116] The second magnesium nitrate crystallization effect scoring system is established according to the magnesium nitrate crystallization rate. The scoring is performed according to whether the crystallization rate is moderate, and the higher the rate, the lower the score, forming the second magnesium nitrate crystallization effect scoring data set.

[0117] The correlation analysis of the second concentrated nitric acid distillation effect is to use regression analysis or other appropriate statistical methods to correlate the second distillation effect score with other metal ion concentrations to establish a correlation model; and to generate a correlation index between the other metal ion concentrations and the second concentrated nitric acid distillation effect score.

[0118] By analyzing the derivative of the correlation model, the inflection point of the correlation index is determined. This inflection point reflects the point at which the distillation effect changes significantly at a certain metal ion concentration.

[0119] The second equipment corrosion effect correlation analysis is similar to the analysis of distillation effect. The equipment corrosion effect score is correlated with other metal ion concentrations to establish a correlation model for corrosion effect; and a correlation index between other metal ion concentrations and equipment corrosion effect scores is generated.

[0120] By analyzing the derivative of the correlation model, the inflection point of the correlation index of the equipment corrosion effect is determined. This inflection point reflects the point at which the equipment corrosion rate changes significantly under a certain metal ion concentration.

[0121] The second magnesium nitrate crystallization effect correlation analysis is to correlate the magnesium nitrate crystallization effect score with other metal ion concentrations to establish a correlation model for the crystallization effect; and to generate a correlation index between other metal ion concentrations and the magnesium nitrate crystallization effect score.

[0122] By analyzing the derivative of the correlation model, the inflection point of the correlation index of the crystallization effect is determined. This inflection point reflects the point at which the crystallization rate or quality of magnesium nitrate changes significantly at a certain metal ion concentration.

[0123] The inflection point intermediate value calculation compares the correlation index inflection points obtained in the above three analyses, determines the other metal ion concentration values ​​corresponding to the three inflection points, and calculates the intermediate value of the three inflection point concentration values ​​as the final third preset value.

[0124] It is understandable that this third preset value reflects the optimal concentration level of other metal ions under the condition of comprehensive consideration of three key factors: concentrated nitric acid distillation effect, equipment corrosion and magnesium nitrate crystallization. In the actual process, the concentration of other metal ions is controlled according to this preset value, and the concentration of concentrated nitric acid can be guaranteed under the condition of low magnesium nitrate solution feed ratio.

[0125] Exemplarily, when the second magnesium nitrate solution contains Mg 2+ The concentration of Mg is lower than the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is greater than the second preset value and the concentration of other metal ions is less than the third preset value,

[0126] The pretreatment method is to add Mg(NO3)2 to the second magnesium nitrate solution so that Mg 2+ The concentration is higher than the first preset value.

[0127] Exemplarily, when the second magnesium nitrate solution contains Mg 2+ The concentration of Mg is greater than or equal to the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is less than or equal to the second preset value, and the concentration of other metal ions is less than the third preset value,

[0128] The pretreatment method is to evaporate the nitric acid and water in the second magnesium nitrate solution in the form of an acidic mixed gas.

[0129] It can be understood that in the above case, it can be determined that the nitric acid content in the second magnesium nitrate solution is too high, which will cause equipment corrosion or magnesium nitrate crystallization. Even on the basis of the original concentration, it is still impossible to obtain a magnesium nitrate solution of appropriate concentration. Therefore, in this specific embodiment, evaporation is used to remove excess nitric acid.

[0130] Exemplarily, when the second magnesium nitrate solution contains Mg 2+ The concentration of Mg is greater than or equal to the first preset value, 2+ The concentration of NO3 - When the ratio of the concentration of is greater than the second preset value and the concentration of other metal ions is greater than the third preset value,

[0131] The pretreatment method is to remove impurities from the second magnesium nitrate solution.

[0132] Exemplarily, the tail gas obtained from the gas-liquid separator is distilled through a tower tail ejector to obtain an acidic aqueous solution, which enters a tower tail water tank.

[0133] Exemplarily, after the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing vapor enters a distillation tower for distillation, and the recovered acidic aqueous solution enters a tail water tank of the tower.

[0134] Exemplarily, the acidic aqueous solution in the tail water tank of the tower is subsequently treated or reused.

[0135] Exemplarily, the acidic mixed gas and the acid-containing steam enter into a distillation tower for distillation.

[0136] Exemplarily, the top temperature of the stripping-rectification tower is 80-90° C., and the top pressure is -4-2 KPa.

[0137] Exemplarily, the top temperature of the bleaching tower is 72-78°C.

[0138] Exemplarily, the concentration of the first magnesium nitrate solution is 66-70%.

[0139] Exemplarily, the mixing temperature of the dilute nitric acid and the first magnesium nitrate solution is maintained in the range of 20-40° C. to ensure the uniformity of the solution; and the mixing is performed under normal pressure.

[0140] Exemplarily, the bottom temperature of the stripping-rectifying tower is controlled at 120-140° C. to promote the formation of the second magnesium nitrate solution; and the reflux ratio of the stripping-rectifying tower is 1:3-5.

[0141] Exemplarily, in the bleaching tower, the nitric acid vapor is preheated to 90-100°C before entering the bleaching tower to ensure that the removal rate of nitrogen oxides reaches more than 99%.

[0142] Exemplarily, the nitric acid vapor after bleaching is condensed to 20-30° C. to obtain concentrated nitric acid.

[0143] Exemplarily, the operating temperature of the gas-liquid separator is between 15°C and 25°C to ensure complete separation of the gas.

[0144] Exemplarily, the amount of reflux acid is controlled at 30-50% to maintain the balance of the stripping-rectification tower.

[0145] Exemplarily, the second magnesium nitrate solution is concentrated by distillation, and the distillation temperature is within the range of 130-160° C. to concentrate the second magnesium nitrate solution.

[0146] Example 1

[0147] This embodiment provides a novel and efficient process for preparing concentrated nitric acid by a magnesium nitrate method, comprising the following steps:

[0148] S1. Mix the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:2.6-3.2.

[0149] Wherein, in the first magnesium nitrate solution, the mass fraction of magnesium nitrate is 68%, the mixing temperature is controlled at 20° C., and the mixing process is carried out under normal pressure.

[0150] S2, sending the first mixed liquid into the middle part of a stripping-rectifying tower for distillation.

[0151] Among them, the top temperature of the tower is controlled at 89°C; the bottom temperature of the tower is maintained at 122°C, the operating pressure is maintained at -4Kpa, and the reflux ratio is set at 1:3.

[0152] S3. The nitric acid vapor is drawn out from the top of the stripping-rectification tower and enters the bleaching tower for bleaching.

[0153] The nitric acid vapor is preheated to 90°C before bleaching, and the nitric acid vapor after bleaching is condensed to 20°C to obtain concentrated nitric acid.

[0154] S4. The condensed concentrated nitric acid is sent to a gas-liquid separator, and the operating temperature is controlled at 15°C. A part of the separated concentrated nitric acid is refluxed to the stripping-rectification tower as reflux acid, and the reflux rate is controlled at 30%. The other part of the concentrated nitric acid enters the bleaching tower, and is condensed again after the nitrogen oxides are removed to obtain the final finished acid.

[0155] S5. The tail gas obtained from the gas-liquid separator is distilled by the tower tail ejector to obtain an acidic aqueous solution, which enters the tower tail water tank. After the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing steam enters the distillation tower for distillation, and the recovered acidic aqueous solution enters the tower tail water tank for subsequent treatment.

[0156] S6. The second magnesium nitrate solution drawn from the bottom of the stripping-rectifying tower is concentrated by distillation, and the concentrated magnesium nitrate solution is circulated back to the stripping-rectifying tower for continued use as the first magnesium nitrate solution.

[0157] The distillation temperature is controlled at 130°C to ensure effective concentration.

[0158] Finished acid processing and storage: The final finished acid is stored at low temperature (<25°C) to prevent it from decomposing or volatilizing.

[0159] In this embodiment, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0160] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than the first preset value, i.e. 0.5 mol / L;

[0161] In the second magnesium nitrate solution, Mg 2+The concentration of NO 3- The ratio of the concentrations is less than or equal to a second preset value, i.e., 0.45;

[0162] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L.

[0163] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.5 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.5 mol / L;

[0164] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, i.e., 0.45, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The concentration ratio is greater than 0.45;

[0165] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0166] Example 2

[0167] This embodiment provides a novel and efficient process for preparing concentrated nitric acid by a magnesium nitrate method, comprising the following steps:

[0168] S1. Mix the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:2.8.

[0169] Wherein, in the first magnesium nitrate solution, the mass fraction of magnesium nitrate is 68%, the mixing temperature is controlled at 30° C., and the mixing process is carried out under normal pressure.

[0170] S2, sending the first mixed liquid into the middle part of a stripping-rectifying tower for distillation.

[0171] Among them, the top temperature of the tower is controlled at 85℃; the bottom temperature of the tower is maintained at 122℃, the operating pressure is maintained at -4Kpa, and the reflux ratio is set at 1:3.

[0172] S3. The nitric acid vapor is drawn out from the top of the stripping-rectification tower and enters the bleaching tower for bleaching.

[0173] The nitric acid vapor is preheated to 95°C before bleaching, and the nitric acid vapor after bleaching is condensed to 20°C to obtain concentrated nitric acid.

[0174] S4. The condensed concentrated nitric acid is sent to a gas-liquid separator, and the operating temperature is controlled at 25°C. A part of the separated concentrated nitric acid is refluxed to the stripping-rectification tower as reflux acid, and the reflux rate is controlled at 30%. The other part of the concentrated nitric acid enters the bleaching tower, and is condensed again after the nitrogen oxides are removed to obtain the final finished acid.

[0175] S5. The tail gas obtained from the gas-liquid separator is distilled by the tower tail ejector to obtain an acidic aqueous solution, which enters the tower tail water tank. After the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing steam enters the distillation tower for distillation, and the recovered acidic aqueous solution enters the tower tail water tank for subsequent treatment.

[0176] S6. The second magnesium nitrate solution drawn from the bottom of the stripping-rectifying tower is concentrated by distillation, and the concentrated magnesium nitrate solution is circulated back to the stripping-rectifying tower for continued use as the first magnesium nitrate solution.

[0177] The distillation temperature is controlled at 135°C to ensure effective concentration.

[0178] Finished acid processing and storage: The final finished acid is stored at low temperature (<25°C) to prevent it from decomposing or volatilizing.

[0179] In this embodiment, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0180] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than the first preset value, i.e. 0.8 mol / L;

[0181] In the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- The ratio of the concentrations is less than or equal to the second preset value, i.e., 0.43;

[0182] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.02 mol / L.

[0183] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.8 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.8 mol / L;

[0184] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3-When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, i.e., 0.43, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The ratio of the concentrations is greater than 0.43;

[0185] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.02 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0186] Example 3

[0187] This embodiment provides a novel and efficient process for preparing concentrated nitric acid by a magnesium nitrate method, comprising the following steps:

[0188] S1. Mix the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:2.9.

[0189] Wherein, in the first magnesium nitrate solution, the mass fraction of magnesium nitrate is 68%, the mixing temperature is controlled at 40° C., and the mixing process is carried out under normal pressure.

[0190] S2, sending the first mixed liquid into the middle part of a stripping-rectifying tower for distillation.

[0191] Among them, the top temperature of the tower is controlled at 90℃; the bottom temperature of the tower is maintained at 130℃, the operating pressure is maintained at -2Kpa, and the reflux ratio is set at 1:3.

[0192] S3. The nitric acid vapor is drawn out from the top of the stripping-rectification tower and enters the bleaching tower for bleaching.

[0193] The nitric acid vapor is preheated to 90°C before bleaching, and the nitric acid vapor after bleaching is condensed to 20°C to obtain concentrated nitric acid.

[0194] S4. The condensed concentrated nitric acid is sent to a gas-liquid separator, and the operating temperature is controlled at 15°C. A part of the separated concentrated nitric acid is refluxed to the stripping-rectification tower as reflux acid, and the reflux rate is controlled at 30%. The other part of the concentrated nitric acid enters the bleaching tower, and is condensed again after the nitrogen oxides are removed to obtain the final finished acid.

[0195] S5. The tail gas obtained from the gas-liquid separator is distilled by the tower tail ejector to obtain an acidic aqueous solution, which enters the tower tail water tank. After the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing steam enters the distillation tower for distillation, and the recovered acidic aqueous solution enters the tower tail water tank for subsequent treatment.

[0196] S6. The second magnesium nitrate solution drawn from the bottom of the stripping-rectifying tower is concentrated by distillation, and the concentrated magnesium nitrate solution is circulated back to the stripping-rectifying tower for continued use as the first magnesium nitrate solution.

[0197] The distillation temperature is controlled at 130°C to ensure effective concentration.

[0198] Finished acid processing and storage: The final finished acid is stored at low temperature (<25°C) to prevent it from decomposing or volatilizing.

[0199] In this embodiment, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0200] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than the first preset value, i.e., 0.9 mol / L;

[0201] In the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- The ratio of the concentrations is less than or equal to the second preset value, i.e., 0.48;

[0202] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.03 mol / L.

[0203] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.9 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.9 mol / L;

[0204] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, i.e., 0.48, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The concentration ratio is greater than 0.48;

[0205] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.03 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0206] Comparative Example 1

[0207] This comparative example is substantially the same as Example 1, except that, in Comparative Example 1, the second magnesium nitrate solution is not pretreated.

[0208] Comparative Example 2

[0209] This comparative example is substantially the same as Example 1, except that, in this comparative example, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0210] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than the first preset value, i.e. 0.1 mol / L;

[0211] In the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- The ratio of the concentrations is less than or equal to a second preset value, i.e., 0.45;

[0212] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L.

[0213] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.1 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.1 mol / L;

[0214] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, i.e., 0.45, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The concentration ratio is greater than 0.45;

[0215] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0216] Comparative Example 3

[0217] This comparative example is substantially the same as Example 3, except that, in this comparative example, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0218] In the second magnesium nitrate solution, Mg 2+The concentration is lower than the first preset value, i.e. 0.5 mol / L;

[0219] In the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- The ratio of the concentrations is less than or equal to a second preset value, i.e., 0.3;

[0220] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L.

[0221] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.5 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.5 mol / L;

[0222] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, that is, 0.3, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The concentration ratio is greater than 0.3;

[0223] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.01 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0224] Comparative Example 4

[0225] This embodiment is substantially the same as Example 1, except that, in this comparative example, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated:

[0226] In the second magnesium nitrate solution, Mg 2+ The concentration is lower than the first preset value, i.e. 0.5 mol / L;

[0227] In the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- The ratio of the concentrations is less than or equal to a second preset value, i.e., 0.45;

[0228] In the second magnesium nitrate solution, Ca 2+ or Fe 3+ The concentration is greater than or equal to the third preset value, i.e., 0.1 mol / L.

[0229] The pretreatment means is that when the second magnesium nitrate solution contains Mg 2+ When the concentration is lower than the first preset value, i.e., 0.5 mol / L, Mg(NO3)2 is added to make the concentration of Mg(NO3)2 equal to 0.5 mol / L;

[0230] When the second magnesium nitrate solution, Mg 2+ The concentration of NO 3- When the ratio of the concentration of Mg 2 O 2 is less than or equal to the second preset value, i.e., 0.45, the second magnesium nitrate solution is evaporated to make Mg 2+ The concentration of NO 3- The concentration ratio is greater than 0.45;

[0231] When the second magnesium nitrate solution, Ca 2+ or Fe 3+ When the concentration is greater than or equal to the third preset value, i.e., 0.1 mol / L, ion exchange resin or membrane technology is used to remove excess metal ions to improve the purity of the second magnesium nitrate solution.

[0232] Performance Testing

[0233] After the equipment was in operation for 3 months, the purity of the finished acid obtained in Examples 1 to 3 and Comparative Examples 1 to 4 was tested, and the decrease in the discharge amount of the second magnesium nitrate solution in Examples 1 to 3 and Comparative Examples 1 to 4 was tested. At the same time, the corrosion of the equipment was checked. The results are shown in the table below.

[0234] Table 1 Purity of finished acid in each embodiment and each comparative example

[0235]

[0236] According to the comparison of the above data, within the range of the feed ratio of concentrated magnesium nitrate and dilute nitric acid defined in the present invention, if the dilute magnesium nitrate solution, i.e., the second magnesium nitrate solution in the present invention, is not treated as in Comparative Example 1, or is not treated in time, such as in Comparative Examples 2 to 4, the purity of the finished acid will decrease.

[0237] According to the degree of decrease in the discharge amount of the second magnesium nitrate solution, it can be seen that if the dilute magnesium nitrate solution, i.e., the second magnesium nitrate solution in the present invention, is not treated as in Comparative Example 1, or is not treated in time, such as in Comparative Examples 2 to 4, the discharge amount of the second magnesium nitrate solution will decrease, i.e., the pipeline will be blocked by magnesium nitrate crystals, or the equipment will be corroded.

[0238] In summary, the preparation process provided by the present invention effectively monitors and controls the Mg²⁺ concentration in the second magnesium nitrate solution, that is, other specific indicators. In the case of a low feed ratio of magnesium nitrate, the purity and component balance of the solution are ensured through a pretreatment step, the stability of the process and the consistency of the product are maintained, and the high purity of the final product is guaranteed.

[0239] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been presented as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method, characterized in that: The steps include: Mixing the dilute nitric acid solution and the first magnesium nitrate solution in a ratio of 1:(2.6-3.2) to obtain a first mixed solution; The first mixed liquid is sent to the middle part of the stripping-rectifying tower, nitric acid vapor with a mass fraction greater than or equal to 98% is obtained at the top of the tower, and a second magnesium nitrate solution is obtained at the bottom of the tower; The nitric acid vapor is bleached in a bleaching tower and then condensed to obtain concentrated nitric acid; After the concentrated nitric acid passes through the gas-liquid separator, part of it flows back to the stripping-rectification tower as reflux acid, and the other part enters the bleaching tower to remove nitrogen oxides and then condense to obtain finished acid; After the second magnesium nitrate solution is distilled and concentrated, it is returned to the stripping-rectification tower as the first magnesium nitrate solution for recycling; Wherein, when the second magnesium nitrate solution satisfies at least one of the following conditions, the second magnesium nitrate solution is pretreated and then distilled and concentrated: In the second magnesium nitrate solution, Mg 2+ The concentration is lower than a first preset value; In the second magnesium nitrate solution, Mg 2+ The concentration of NO3 - The ratio of the concentrations is less than or equal to a second preset value; In the second magnesium nitrate solution, the concentration of other metal ions is greater than or equal to a third preset value; The first preset value is determined according to the following method: Performing statistics on the distillation effect of the first concentrated nitric acid to obtain the first concentrated nitric acid concentration information corresponding to N second magnesium nitrate concentrations; Obtaining a first distillation effect score according to the first concentrated nitric acid concentration information; Performing a correlation analysis on the second magnesium nitrate concentration based on the first distillation effect score to obtain a correlation index between the second magnesium nitrate concentration and the distillation effect of the first concentrated nitric acid; Based on the correlation index, obtain the correlation index inflection point; The first preset value and the second preset value are determined according to the inflection point of the correlation index, and are determined according to the following method: The first equipment corrosion effect statistics and the first magnesium nitrate crystallization effect statistics are performed to obtain M Mg 2+ The concentration of NO3 - The first equipment corrosion rate information and the first magnesium nitrate crystallization rate information corresponding to the concentration ratio of ; Obtaining a first equipment corrosion effect score and a first magnesium nitrate crystallization effect score according to the first equipment corrosion rate information and the first magnesium nitrate crystallization rate information; Based on the first equipment corrosion effect score and the first magnesium nitrate crystallization effect score, Mg 2+ The concentration of NO3 - The correlation analysis was performed on the ratio of the concentration of Mg 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of Mg and the first equipment corrosion effect 2+ The concentration of NO3 - The correlation index of the ratio of the concentration of and the first magnesium nitrate crystallization effect; Based on the two correlation indexes, two correlation index inflection points are obtained; The second preset value is determined according to the middle value of the two correlation index inflection points. The third preset value is determined according to the following method: Perform statistics on the distillation effect of the second concentrated nitric acid to obtain the concentration information of the second concentrated nitric acid corresponding to the concentrations of P other metal ions; Obtaining a second distillation effect score according to the second concentrated nitric acid concentration information; Based on the second distillation effect score, correlation analysis is performed on the concentrations of other metal ions to obtain a correlation index between the concentrations of other metal ions and the second concentrated nitric acid distillation effect, and based on the correlation index, a first correlation index inflection point is obtained; Performing statistics on the corrosion effect of the second device to obtain the corrosion rate information of the second device corresponding to P other metal ion concentrations; Obtaining a corrosion effect score of the second equipment according to the corrosion rate information of the second equipment; Performing correlation analysis on other metal ion concentrations based on the second equipment corrosion effect score to obtain a correlation index between other metal ion concentrations and the second equipment corrosion effect, and obtaining a second correlation index inflection point based on the correlation index; Performing statistics on the crystallization effect of the second magnesium nitrate to obtain the crystallization rate information of the second magnesium nitrate corresponding to P other metal ion concentrations; Obtaining a second magnesium nitrate crystallization effect score according to the second magnesium nitrate crystallization rate information; Based on the second magnesium nitrate crystallization effect score, correlation analysis is performed on other metal ion concentrations to obtain a correlation index between other metal ion concentrations and the second magnesium nitrate crystallization effect, and based on the correlation index, a third correlation index inflection point is obtained; The third preset value is determined according to the first correlation index inflection point, the second correlation index inflection point, and the third correlation index inflection point.

2. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: The top temperature of the stripping-rectifying tower is 80-90° C., and the top pressure is -4-2 KPa.

3. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: The top temperature of the bleaching tower is 72-78°C.

4. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: The concentration of the first magnesium nitrate solution is 66-70%.

5. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: The bottom temperature of the stripping-rectifying tower is controlled at 120-140°C.

6. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: The tail gas obtained by the gas-liquid separator is distilled by the tower tail ejector to obtain an acidic aqueous solution, which enters the tower tail water tank.

7. A process for preparing concentrated nitric acid by an efficient magnesium nitrate method according to claim 1, characterized in that: After the second magnesium nitrate solution is distilled and concentrated, the obtained acid-containing steam enters a distillation tower for distillation, and the recovered acidic aqueous solution enters a water tank at the tail of the tower.

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