Method for preparing high-purity calcium nitrate tetrahydrate by using fluorine-containing waste nitric acid

Abstract

The invention discloses a method for preparing high-purity calcium nitrate tetrahydrate by using fluorine-containing waste nitric acid, which comprises the following steps: S1, filtering the fluorine-containing waste nitric acid to remove impurities, and adding the fluorine-containing waste nitric acid into process water to enable the hydrogen ion concentration of the mixed solution to be 3-5mol / L; s2, removing impurities; s3, transferring the solution subjected to impurity removal into a reactor, adding lime milk, keeping the pH value at 6-9, continuously stirring, and then feeding into a filter pressing system to obtain a calcium nitrate tetrahydrate solution and calcium fluoride; s4, the calcium fluoride is rinsed and dried; s5, adding nitric acid into the calcium nitrate tetrahydrate solution to adjust the pH value, and sending the solution into an evaporation kettle for concentration; s6, after the density of the concentrated calcium nitrate tetrahydrate solution is greater than or equal to 1.65 g / mL, feeding the concentrated calcium nitrate tetrahydrate solution into a crystallization kettle to obtain calcium nitrate tetrahydrate crystals; s7, performing centrifugation; and S8, carrying out secondary crystallization. The lime milk is added into the fluorine-containing nitric acid to participate in the reaction, the nitrogen element and the fluorine element in the fluorine-containing nitric acid are recycled, the high-purity calcium nitrate tetrahydrate and calcium fluoride products are prepared, the process is simple, and the treatment cost is low.

Description

Technical Field

[0001] This invention relates to the field of chemical engineering, and in particular to a method for preparing high-purity calcium nitrate tetrahydrate using fluorine-containing waste nitric acid. Background Technology

[0002] In existing technologies, fluorine-containing waste nitric acid is typically treated using neutralization, concentration and recovery, chemical oxidation, or ion exchange. Neutralization typically involves using alkaline solutions (such as sodium hydroxide, sodium carbonate, or lime milk) to neutralize the nitrate solution, which is then precipitated and discharged in compliance with standards. However, the pH must be controlled within the range of 6-9 to avoid secondary pollution. Neutralization consumes a large amount of raw materials and generates no added value. Concentration and recovery is suitable for high-concentration waste acid, recovering nitric acid through evaporation and concentration processes. Vacuum distillation or thin-film evaporation technologies can be used, achieving a recovery rate of 60-80%, but it is energy-intensive and difficult to operate. Chemical oxidation is suitable for waste nitric acid containing organic matter, adding oxidants such as hydrogen peroxide to decompose the organic matter, achieving harmless treatment in conjunction with neutralization. However, chemical oxidation consumes a large amount of raw materials, is difficult to operate, and carries high risks. Ion exchange is suitable for waste acid with high metal ion content, recovering heavy metals through resin adsorption. However, it requires large initial investment, uses a single raw material, and the treated waste acid cannot be used directly. Therefore, a simple, low-cost, and economically valuable preparation method is needed. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing high-purity calcium nitrate tetrahydrate from fluorine-containing waste nitric acid. This invention recovers nitrogen and fluorine elements from the fluorine-containing nitric acid by adding lime milk to the fluorine-containing nitric acid to participate in the reaction, and prepares high-purity calcium nitrate tetrahydrate and calcium fluoride products. The process is simple, the disposal cost is low, and the economic value is high.

[0004] To achieve the above objectives, the technical solution adopted by this invention is as follows: a method for preparing high-purity calcium nitrate tetrahydrate using fluoride-containing waste nitric acid, comprising: Step S1: filtration and homogenization, stirring the fluoride-containing waste nitric acid evenly and filtering to remove impurities, adding the impurity-removed fluoride-containing waste nitric acid to process water to make the hydrogen ion concentration of the mixture range 3-5 mol / L; Step S2: impurity removal, determining the impurity removal method based on the impurity composition in the mixture of Step S1 and performing impurity removal to obtain a solution after impurity removal; Step S3: neutralization and separation, transferring the solution after impurity removal in Step S2 to a reactor, stirring evenly and slowly adding lime milk to maintain the pH value at 6-9, and continuously stirring for 30 minutes before sending it to a pressure filtration system to obtain a calcium nitrate tetrahydrate solution and calcium fluoride; Step S4: rinsing and drying, rinsing and drying the calcium fluoride in Step S3. After washing and drying, calcium fluoride product is obtained; Step S5: Evaporation, add nitric acid to the calcium nitrate tetrahydrate solution in step S3 to adjust the pH to 4-5, and send the calcium nitrate tetrahydrate solution to an evaporator at 80-120℃ for concentration, continuously monitoring the density of the concentrated calcium nitrate tetrahydrate; Step S6: Crystallization, after the density of the concentrated calcium nitrate tetrahydrate solution is ≥1.65g / mL, send it to a crystallization kettle, maintain the temperature of the crystallization kettle at 10-20℃, and continue crystallizing for 1-3 hours to obtain calcium nitrate tetrahydrate crystals; Step S7: Centrifugation, send the calcium nitrate tetrahydrate crystals obtained in step S6 to a centrifuge to obtain high-purity calcium nitrate tetrahydrate crystals; Step S8: Secondary crystallization, dissolve the high-purity calcium nitrate tetrahydrate crystals in step S7 in water, filter them, and send them to a crystallization kettle for secondary crystallization to obtain high-purity calcium nitrate tetrahydrate product.

[0005] Preferably, the impurities in the mixture in step S2 include silicate and metal impurities, and the impurity removal method in step S2 includes evaporation and precipitation.

[0006] Preferably, the mass ratio of quicklime to water in the lime slurry in step S3 is 1:2.

[0007] Preferably, the drying in step S4 is performed by using an airflow dryer to dry calcium fluoride at a drying temperature of 100-120°C.

[0008] Preferably, the vacuum degree of the evaporation vessel in step S5 is >0.08 MPa, and the density of the concentrated calcium nitrate tetrahydrate is detected by a densitometer.

[0009] Preferably, the centrifugation rate of the centrifuge in step S7 is 3500 r / min.

[0010] Compared with the prior art, the present invention has the following advantages:

[0011] 1. This invention recovers nitrogen and fluorine elements from fluorinated nitric acid by adding lime milk to the fluorinated nitric acid to participate in the reaction, and prepares high-purity calcium nitrate tetrahydrate and calcium fluoride products. The process is simple, the disposal cost is low, and the economic value is high.

[0012] 2. This invention improves the crystallization efficiency of calcium nitrate tetrahydrate by adjusting the pH of the calcium nitrate solution to 4-5 before evaporation, ensuring the smooth precipitation of high-purity calcium nitrate tetrahydrate within the concentration range.

[0013] 3. The concentration of calcium nitrate tetrahydrate evaporation in this invention is selected to be at least 1.65 g / ml, which increases the concentration of the solution, reduces the amount of mother liquor, and lowers energy consumption.

[0014] 4. This invention employs secondary crystallization to further remove impurities and improve the purity of calcium nitrate tetrahydrate crystals.

[0015] 5. The fluorinated nitric acid used in this invention is fluorinated waste nitric acid from industrial production, which is hazardous waste. After treatment, a high-purity product is obtained, realizing resource recovery and turning waste into treasure.

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of the preparation method of the present invention. Detailed Implementation

[0018] The present invention will be further described below with reference to specific embodiments.

[0019] Example 1:

[0020] Step S1: Filtration and Homogenization

[0021] The recovered fluorine-containing waste nitric acid was stirred. After stirring evenly, a 400-mesh filter was used to filter out solid impurities generated during the transportation and storage of the fluorine-containing waste nitric acid. Then, the filtered fluorine-containing waste nitric acid was added to water and stirred evenly to make the concentration of hydrogen ions in the mixture 3 mol / L.

[0022] Step S2: Impurity Removal

[0023] The types of impurities in the mixture are detected. If the impurities are silicon nitric acid, evaporation is used to remove them, with the evaporation temperature maintained at 45-55℃. If the impurities are metallic, the pH value of the mixture is adjusted to allow the metallic impurities to precipitate before filtration. Alternatively, evaporation can be used to remove impurities. After removing the impurities, the mixture becomes a liquid containing only hydrofluoric acid and nitric acid, or a liquid containing only hydrofluoric acid, or a liquid containing only nitric acid.

[0024] Step S3: Neutralization and Separation

[0025] The impurity-removed mixture from step S2 is sent to the reactor and stirred until homogeneous. Then, lime slurry is slowly added to the reactor and stirred until homogeneous. The pH value of the solution in the reactor is continuously monitored and adjusted to 6. After stirring continuously for 30 minutes, the solution is sent to the pressure filtration system for separation to obtain calcium nitrate tetrahydrate solution and calcium fluoride.

[0026] The mass ratio of quicklime to water in lime slurry is 1:2.

[0027] The filter press system is a centrifuge or filter press, which separates the mixed solution.

[0028] The main reaction equations involved are as follows:

[0029] 2HF + Ca(OH)₂ → CaF₂↓ + 2H₂O

[0030] Ca(OH)2+2HNO3→Ca(NO3)2↓+2H2O

[0031] Ca(NO3)2+4H2O→Ca(NO3)2·4H2O

[0032] Step S4: Rinse and dry

[0033] The calcium fluoride separated in step S3 is rinsed with water and then sent to an airflow dryer. The drying temperature of the airflow dryer is selected as 100℃ to obtain the calcium fluoride product.

[0034] Step S5: Evaporation

[0035] The filtrate obtained after separation in step S3 is a calcium nitrate tetrahydrate solution. Pure nitric acid is added to the calcium nitrate tetrahydrate solution to adjust its pH to 4. Then, the calcium nitrate tetrahydrate solution is sent to an evaporator at 80-120℃ for concentration. The vacuum degree of the evaporator is >0.08MPa. During the evaporation process, samples are taken from the evaporator at fixed intervals, and the density of the sampled calcium nitrate tetrahydrate solution is detected by a hydrometer. The evaporator stops working when the density of the calcium nitrate tetrahydrate solution reaches 1.65g / mL. This fixed time is a preset time based on experience. The lower the evaporation temperature of the evaporator, the lower the density of the concentrate obtained and the greater the energy waste.

[0036] Step S6: Crystallization

[0037] The concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 has a density ≥1.65 g / mL. This concentrated calcium nitrate tetrahydrate solution is then sent to a crystallization kettle. The temperature of the crystallization kettle is maintained at 10°C during the crystallization process of calcium nitrate tetrahydrate, and crystallization is continued for 1 hour to obtain calcium nitrate tetrahydrate crystals. If the temperature of the concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 is lower than 40°C, a small amount of calcium nitrate crystals are added to the concentrated calcium nitrate tetrahydrate solution to facilitate crystallization.

[0038] Step S7: Centrifugation

[0039] The calcium nitrate tetrahydrate crystals obtained in step S6 are sent to a centrifuge. The impurities in the calcium nitrate tetrahydrate crystals are removed by the centrifuge to obtain high-purity calcium nitrate tetrahydrate crystals. The centrifugation rate of the centrifuge is adjusted to 3500 r / min.

[0040] Step S8: Secondary crystallization

[0041] The high-purity calcium nitrate tetrahydrate crystals obtained in step S7 are dissolved in water, filtered through a 600-mesh filter, and then sent to a crystallization kettle for secondary crystallization to obtain the high-purity calcium nitrate tetrahydrate product. The secondary crystallization further removes calcium fluoride from the calcium nitrate tetrahydrate crystals, thereby improving the purity of the calcium nitrate tetrahydrate.

[0042] The obtained high-purity calcium nitrate tetrahydrate and calcium fluoride products were tested separately. The content of calcium nitrate tetrahydrate in the high-purity calcium nitrate tetrahydrate product was ≥99.0%, and the content of calcium fluoride in the calcium fluoride product was ≥99.0%.

[0043] Example 2

[0044] Step S1: Filtration and Homogenization

[0045] The recovered fluorine-containing waste nitric acid was stirred. After stirring evenly, a 400-mesh filter was used to filter out solid impurities generated during the transportation and storage of the fluorine-containing waste nitric acid. Then, the filtered fluorine-containing waste nitric acid was added to water and stirred evenly to make the concentration of hydrogen ions in the mixture 4 mol / L.

[0046] Step S2: Impurity removal, the same as the impurity removal method in Example 1;

[0047] Step S3: Neutralization and Separation

[0048] The impurity-removed mixture from step S2 is sent to the reactor and stirred evenly. Then, lime milk is slowly added to the reactor and stirred evenly. The pH value of the solution in the reactor is continuously monitored and kept at 7. After stirring for 30 minutes, the mixture is sent to the filter press system for separation to obtain calcium nitrate tetrahydrate solution and calcium fluoride.

[0049] Step S4: Rinse and dry

[0050] The calcium fluoride separated in step S3 is rinsed with water and then sent to an airflow dryer. The drying temperature of the airflow dryer is selected as 110℃ to obtain the calcium fluoride product.

[0051] Step S5: Evaporation

[0052] The filtrate obtained after separation in step S3 is a calcium nitrate tetrahydrate solution. Pure nitric acid is added to the calcium nitrate tetrahydrate solution to adjust its pH to 4.5. Then, the calcium nitrate tetrahydrate solution is sent to an evaporator at 80-120℃ for concentration. The vacuum degree of the evaporator is >0.08MPa. During the evaporation process, samples are taken from the evaporator at fixed intervals, and the density of the sampled calcium nitrate tetrahydrate solution is detected by a hydrometer. The evaporator stops working when the density of the calcium nitrate tetrahydrate solution reaches 1.68g / mL. This fixed time is preset based on experience. The lower the evaporation temperature of the evaporator, the lower the density of the concentrate obtained and the greater the energy waste.

[0053] Step S6: Crystallization

[0054] The concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 has a density ≥1.65 g / mL. This concentrated calcium nitrate tetrahydrate solution is then sent to a crystallization vessel. The temperature of the crystallization vessel is maintained at 15°C during the crystallization process of calcium nitrate tetrahydrate, and crystallization continues for 2 hours to obtain calcium nitrate tetrahydrate crystals. If the temperature of the concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 is lower than 40°C, a small amount of calcium nitrate crystals are added to the concentrated calcium nitrate tetrahydrate solution to facilitate crystallization.

[0055] Step S7: Centrifugation, the same centrifugation method as in Example 1;

[0056] Step S8: Secondary crystallization

[0057] The high-purity calcium nitrate tetrahydrate crystals obtained in step S7 are dissolved in water, filtered through a 600-mesh filter, and then sent to a crystallization kettle for secondary crystallization to obtain the high-purity calcium nitrate tetrahydrate product. The secondary crystallization further removes calcium fluoride from the calcium nitrate tetrahydrate crystals, thereby improving the purity of the calcium nitrate tetrahydrate.

[0058] The obtained high-purity calcium nitrate tetrahydrate and calcium fluoride products were tested separately. The content of calcium nitrate tetrahydrate in the high-purity calcium nitrate tetrahydrate product was >99.0%, and the content of calcium fluoride in the calcium fluoride product was >99.0%.

[0059] Example 3

[0060] Step S1: Filtration and Homogenization

[0061] The recovered fluorine-containing waste nitric acid was stirred. After stirring evenly, a 400-mesh filter was used to filter out solid impurities generated during the transportation and storage of the fluorine-containing waste nitric acid. Then, the filtered fluorine-containing waste nitric acid was added to water and stirred evenly to make the concentration of hydrogen ions in the mixture 5 mol / L.

[0062] Step S2: Impurity removal, the same as the impurity removal method in Example 1;

[0063] Step S3: Neutralization and Separation

[0064] The impurity-removed mixture from step S2 is sent to the reactor and stirred evenly. Then, lime milk is slowly added to the reactor and stirred evenly. The pH value of the solution in the reactor is continuously monitored and kept at 9. After stirring for 30 minutes, the solution is sent to the filter press system for separation to obtain calcium nitrate tetrahydrate solution and calcium fluoride.

[0065] Step S4: Rinse and dry

[0066] The calcium fluoride separated in step S3 is rinsed with water and then sent to an airflow dryer. The drying temperature of the airflow dryer is selected as 120℃ to obtain the calcium fluoride product.

[0067] Step S5: Evaporation

[0068] The filtrate obtained after separation in step S3 is a calcium nitrate tetrahydrate solution. Pure nitric acid is added to the calcium nitrate tetrahydrate solution to adjust its pH to 5. Then, the calcium nitrate tetrahydrate solution is sent to an evaporator at 80-120℃ for concentration. The vacuum degree of the evaporator is >0.08MPa. During the evaporation process, samples are taken from the evaporator at fixed intervals, and the density of the sampled calcium nitrate tetrahydrate solution is detected by a hydrometer. The evaporator stops working when the density of the calcium nitrate tetrahydrate solution reaches 1.7g / mL. This fixed time is preset based on experience. The lower the evaporation temperature of the evaporator, the lower the density of the concentrate obtained and the greater the energy waste.

[0069] Step S6: Crystallization

[0070] The concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 has a density ≥1.65 g / mL. This concentrated calcium nitrate tetrahydrate solution is then sent to a crystallization vessel. The temperature of the crystallization vessel is maintained at 20°C during the crystallization process of calcium nitrate tetrahydrate, and crystallization continues for 3 hours to obtain calcium nitrate tetrahydrate crystals. If the temperature of the concentrated calcium nitrate tetrahydrate solution obtained after evaporation in step S5 is lower than 40°C, a small amount of calcium nitrate crystals are added to the concentrated calcium nitrate tetrahydrate solution to facilitate crystallization.

[0071] Step S7: Centrifugation, the same centrifugation method as in Example 1;

[0072] Step S8: Secondary crystallization

[0073] The high-purity calcium nitrate tetrahydrate crystals obtained in step S7 are dissolved in water, filtered through a 600-mesh filter, and then sent to a crystallization kettle for secondary crystallization to obtain the high-purity calcium nitrate tetrahydrate product. The secondary crystallization further removes calcium fluoride from the calcium nitrate tetrahydrate crystals, thereby improving the purity of the calcium nitrate tetrahydrate.

[0074] The obtained high-purity calcium nitrate tetrahydrate and calcium fluoride products were tested separately. The content of calcium nitrate tetrahydrate in the high-purity calcium nitrate tetrahydrate product was >99.0%, and the content of calcium fluoride in the calcium fluoride product was >99.0%.

[0075] Preparation of high-purity calcium nitrate tetrahydrate and calcium fluoride products

[0076] Preparation: Dissolve 300g of calcium oxide in 600mL of water and stir continuously for later use; dilute 300mL of fluoride-containing nitric acid to 1000mL and test for nitric acid 20% and hydrofluoric acid 3%;

[0077] Neutralization: Add lime milk to nitric acid to adjust the pH to 7.5, consuming 675g of lime milk;

[0078] Separation, rinsing and drying: The neutralized solution was filtered (filtrate density 1.45 g / mL), and the filter residue was collected, rinsed and dried to obtain calcium fluoride product;

[0079] Evaporation: Divide the filtrate into three equal portions, labeled ①, ②, and ③ respectively. Add nitric acid to ① to adjust the pH to 4. Then, add ①, ②, and ③ to flasks and evaporate. Concentrate filtrates ① and ② to 1.65 g / mL and filtrate ③ to 1.55 g / mL. Transfer the concentrated solutions ①, ②, and ③ to crystallization kettles for cooling.

[0080] Crystallization: The cooling temperature of the crystallization vessel is controlled at 10-12.5℃ for cooling crystallization, and crystallization is continued for 1 hour; a large number of transparent rod-shaped crystals precipitate in ①, only a small amount of crystals precipitate in ②, and no precipitation in ③;

[0081] Centrifugation and secondary crystallization: The transparent rod-shaped crystals precipitated in ① are sent to a centrifuge for centrifugation to obtain calcium nitrate tetrahydrate crystals. Water is added to dissolve and filter the crystals. Secondary crystallization is then carried out to precipitate crystals again, which are high-purity calcium nitrate tetrahydrate crystals.

[0082] Submission for testing: High-purity calcium nitrate tetrahydrate crystals and calcium fluoride products were submitted for testing.

[0083] Experimental conclusions: ① After cooling, 34.7g of calcium nitrate tetrahydrate was obtained by filtration, and the density of the filtrate was 1.46g / mL.

[0084] Test results: In the calcium fluoride product: calcium fluoride content: 99.06%; In the high-purity calcium nitrate tetrahydrate crystals: calcium nitrate tetrahydrate content (calculated as calcium nitrate tetrahydrate): 99.13%.

[0085] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing high purity calcium nitrate tetrahydrate using fluorine-containing spent nitric acid, characterized by, The application relates to a method for preparing high-purity calcium nitrate tetrahydrate. Step S1: filtering and homogenizing, stirring the fluorine-containing waste nitric acid to be uniform, filtering out impurities, adding the fluorine-containing waste nitric acid into process water, and making the hydrogen ion concentration of the mixed solution range from 3 to 5 mol / L; Step S2: impurity removal, determining the impurity removal method based on the impurity components in the mixed solution in step S1, and removing the impurities to obtain a solution after impurity removal; Step S3: neutralization and separation, transferring the solution after impurity removal in step S2 to a reactor, stirring uniformly, slowly adding lime milk, keeping the PH value at 6-9, continuously stirring for 30 min, and then sending to a filter pressing system to obtain calcium nitrate tetrahydrate solution and calcium fluoride; Step S4: rinsing and drying, rinsing and drying the calcium fluoride in step S3 to obtain calcium fluoride products; Step S5: evaporation, adding nitric acid to the calcium nitrate tetrahydrate solution in step S3 to adjust the PH value to 4-5, sending the calcium nitrate tetrahydrate solution to an evaporation kettle at 80-120 DEG C for concentration, and continuously monitoring the density of the concentrated calcium nitrate tetrahydrate solution; Step S6: crystallization, sending the concentrated calcium nitrate tetrahydrate solution with a density >=1.65 g / mL to a crystallization kettle, keeping the temperature of the crystallization kettle at 10-20 DEG C, continuously crystallizing for 1-3 hours, and obtaining calcium nitrate tetrahydrate crystals; Step S7: centrifugation, sending the calcium nitrate tetrahydrate crystals obtained in step S6 to a centrifuge to obtain high-purity calcium nitrate tetrahydrate crystals; Step S8: secondary crystallization, dissolving the high-purity calcium nitrate tetrahydrate crystals in step S7 in water, filtering through a filter, and then sending to a crystallization kettle for secondary crystallization to obtain high-purity calcium nitrate tetrahydrate products.

2. A process for the preparation of high purity calcium nitrate tetrahydrate using fluorine-containing spent nitric acid according to claim 1, characterized in that, The impurities in the mixed solution in step S2 include silicates and metal impurities, and the impurity removal method in step S2 includes evaporation impurity removal and precipitation impurity removal.

3. A method for preparing high-purity calcium nitrate tetrahydrate from fluorine-containing waste nitric acid according to claim 1, characterized in that, The mass ratio of quicklime to water in the lime milk in step S3 is 1:

2.

4. A method for preparing high-purity calcium nitrate tetrahydrate from fluorine-containing waste nitric acid according to claim 1, characterized in that, The drying in step S4 is carried out by using an air flow dryer to dry the calcium fluoride, and the drying temperature is 100-120 DEG C.

5. A method for preparing high-purity calcium nitrate tetrahydrate from fluorine-containing waste nitric acid according to claim 1, characterized in that, The vacuum degree of the evaporation kettle in step S5 is >0.08 MPa, and the density of the concentrated calcium nitrate tetrahydrate solution is detected by a density meter.

6. A method for preparing high-purity calcium nitrate tetrahydrate from fluorine-containing waste nitric acid according to claim 1, characterized in that, The centrifugal rate of the centrifuge in step S7 is 3500 r / min.

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