Method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from calcium and magnesium containing leaching solution of chemical mineral separation byproduct

Abstract

The invention discloses a method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from a calcium and magnesium containing leaching solution of a chemical mineral separation byproduct. The method comprises the following steps: carrying out decalcification treatment on the calcium and magnesium containing leaching solution to obtain calcium sulfate and a decalcification solution containing dilute magnesium nitrate; evaporating the decalcified filtrate to obtain a solid-liquid mixture containing 72-76% of magnesium nitrate crystals; mixing dilute nitric acid with the solid-liquid mixture for reaction to obtain high-concentration nitric acid steam and a solid-liquid mixture of 52-56% magnesium nitrate crystals; separating the solid-liquid mixture to obtain magnesium nitrate crystals; dehydrating the magnesium nitrate crystal, and performing thermal decomposition to obtain magnesium oxide and a gas mixture; treating the nitric acid steam through a stripping section and a rectifying section of the concentration tower to obtain nitric acid steam with the concentration of 98% or above; nitric acid steam enters the bleaching tower to react to obtain finished nitric acid and concentrated nitric acid, and part of concentrated nitric acid flows back to the bleaching tower and the rectifying section of the concentration tower. According to the method, efficient utilization of chemical mineral separation byproducts is achieved, high-purity calcium sulfate and magnesium oxide products are prepared, and efficient recycling of nitric acid is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of chemical beneficiation by-product treatment, and in particular to a method for preparing calcium sulfate and magnesium oxide from a calcium and magnesium-containing leaching solution produced as a chemical beneficiation by-product and recovering nitric acid. Background Art

[0002] With the advancement of chemical beneficiation technology, the comprehensive utilization of calcium-magnesium leachate, a byproduct of the chemical beneficiation process, has attracted increasing attention. This leachate, typically obtained through filter press separation during the preparation of low-magnesium phosphate concentrate from medium- and low-grade phosphate rock, contains significant amounts of valuable elements such as calcium and magnesium. Efficiently recovering these valuable elements and achieving comprehensive resource utilization is a hot topic in current research.

[0003] Traditional treatment methods do not separate calcium and magnesium, resulting in a magnesium-containing calcium carbonate mixture with low utilization value and a significantly limited scope of application. Furthermore, actual production is difficult, the filtrate conversion rate is low, and the product content is unstable, resulting in the inability to effectively digest the magnesium-containing calcium carbonate currently produced. Furthermore, during the treatment process, the nitric acid in the magnesium nitrate solution is often not effectively recovered, resulting in a waste of resources and environmental pollution. Therefore, how to effectively treat the calcium-magnesium leachate, improve the product utilization value, and simultaneously recycle the nitric acid has become a technical problem that needs to be solved urgently. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the present invention provides a method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from calcium and magnesium-containing leaching liquid as a by-product of chemical mineral processing, which can effectively separate calcium and magnesium, improve the utilization value of products, reduce pollution and recover nitric acid.

[0005] The technical solution adopted by the present invention to solve the technical problem is to provide a method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from a calcium and magnesium-containing leaching solution produced as a by-product of chemical mineral processing, comprising the following steps:

[0006] S1, decalcifying the calcium-magnesium leaching solution, adding the calcium-magnesium leaching solution and magnesium sulfate solution into a decalcification reaction tank for reaction, and then filtering to obtain calcium sulfate and a decalcification solution containing dilute magnesium nitrate;

[0007] S2, placing the decalcified filtrate into an evaporator for evaporation to obtain a solid-liquid mixture containing 72-76% magnesium nitrate crystals;

[0008] S3, adding dilute nitric acid and the solid-liquid mixture into a mixer for mixing and reaction to obtain a solid-liquid mixture of nitric acid vapor having a nitric acid concentration of greater than 68.4% and 52-56% magnesium nitrate crystals;

[0009] S4, the solid-liquid mixture containing 52-56% magnesium nitrate crystals enters a concentration tower, passes through the bottom of the stripping section, and enters a centrifugal separation device for separation to obtain a mother liquor and magnesium nitrate crystals;

[0010] S5, dehydrating the magnesium nitrate crystals to obtain anhydrous magnesium nitrate, and then thermally decomposing the anhydrous magnesium nitrate to obtain magnesium oxide and a gas mixture;

[0011] S6, sending the nitric acid vapor obtained in step S3 to a concentrator, and further gasifying it in the stripping section of the concentrator to obtain nitric acid vapor with a nitric acid concentration of more than 80%, and then sending it to a rectifying section, and obtaining nitric acid vapor with a nitric acid concentration of more than 98% under the action of reflux acid in the rectifying section;

[0012] S7, nitric acid vapor with a nitric acid concentration of more than 98% enters a bleaching tower to react with cold nitric acid liquid to obtain hot nitric acid liquid and nitric acid vapor, wherein the hot nitric acid liquid is cooled to obtain finished nitric acid, and the nitric acid vapor is condensed to obtain concentrated nitric acid;

[0013] S8, 1 / 3 of the concentrated nitric acid is refluxed to the bleaching tower, and 2 / 3 of the concentrated nitric acid is refluxed to the distillation section of the concentrating tower as reflux acid.

[0014] Preferably, the concentration of the dilute nitric acid in step S2 is 45%-60%.

[0015] Further preferably, in step S2, the ratio of the dilute nitric acid to the solid-liquid mixture containing 72-76% magnesium nitrate crystals is 1:3-1:4.

[0016] Preferably, after filtering in step S1, a filter cake is obtained, and the filter cake is washed once and twice to obtain calcium sulfate, wherein the washing liquid after the first wash is used to dissolve magnesium sulfate, and the washing liquid after the second wash is used for washing in the first wash.

[0017] Preferably, the filtrate after filtration in step S1 is discharged to a dilute magnesium tank, the mother liquor obtained by centrifugal separation in step S4 is tested for calcium content, the mother liquor with a calcium content of less than 0.3% is sent to a dilute magnesium tank, and the mother liquor with a calcium content of greater than 0.3% is sent to a decalcification reaction tank.

[0018] Preferably, the gas mixture in step S5 is sent to a recovery chamber, and water is added to react to obtain 14-18% dilute nitric acid.

[0019] Preferably, the gas mixture in step S5 is sent to the distillation section of the concentration tower.

[0020] Preferably, the temperature for dehydrating magnesium nitrate in step S5 is 120-150°C.

[0021] Preferably, the temperature for thermal decomposition of anhydrous magnesium nitrate in step S5 is 380-420° C. and the time is 30-60 min.

[0022] Preferably, the concentrated nitric acid obtained in S7 is diluted to 60% dilute nitric acid and sent to the mixer.

[0023] The beneficial effects of the present invention are:

[0024] 1. By using magnesium nitrate as a dehydrating agent and setting up an indirect condenser to recover nitric acid vapor after the magnesium nitrate evaporates, the nitric acid contained in the magnesium nitrate solution is effectively recovered, which greatly saves water resources compared to direct water jet absorption;

[0025] 2. Compared with air bleaching (cold bleaching), nitric acid vapor not only improves product quality through thermal bleaching, but also reduces nitric acid tail gas pollution. Nitric acid can be recovered and recycled, thereby improving resource utilization.

[0026] 3. The present invention realizes the effective separation of calcium and magnesium to obtain calcium sulfate, magnesium oxide and nitric acid, thereby improving the utilization value of the products. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below with reference to the accompanying drawings and examples.

[0028] Figure 1 The present invention is a process flow chart of a method for preparing calcium sulfate and magnesium oxide from a calcium and magnesium-containing leaching solution produced as a by-product of chemical mineral processing and recovering nitric acid. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in conjunction with the embodiments and drawings, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without paying creative work are within the scope of protection of the present invention. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0030] Reference Figure 1 Example 1 of the present invention provides a method for preparing calcium sulfate and magnesium oxide from a calcium-magnesium-containing leaching solution produced as a by-product of chemical mineral processing and recovering nitric acid, comprising the following steps:

[0031] S1: decalcifying the calcium-magnesium leaching solution, adding the calcium-magnesium leaching solution and magnesium sulfate solution into a decalcification reaction tank for reaction, and then filtering to obtain calcium sulfate and a decalcification solution containing dilute magnesium nitrate.

[0032] It is understandable that the calcium-magnesium leaching solution is a by-product produced during the chemical beneficiation process, which contains a large amount of calcium ions and magnesium ions. The calcium-magnesium leaching solution is mixed with the magnesium sulfate solution in the decalcification reaction tank, and the sulfate ions react with the calcium ions to form insoluble calcium sulfate precipitates. The reaction equation is: Ca 2+ +SO42-→CaSO4↓. The reaction temperature is controlled at 25-30°C and the reaction time is 1-2 hours to ensure that the calcium ions react fully. After the reaction is completed, solid-liquid separation is performed through a filtration device (filter press) to obtain a calcium sulfate filter cake and a decalcified liquid containing dilute magnesium nitrate.

[0033] The filter cake obtained after filtration undergoes primary and secondary washing to produce calcium sulfate. Specifically, the filter cake is initially washed with clean water at room temperature for 15 minutes. The washing liquid after the primary wash is collected and used to prepare the magnesium sulfate solution, improving resource utilization. The filter cake is also washed with clean water at room temperature for 10 minutes. The washing liquid after the secondary wash is collected and used for the primary wash of the next batch of filter cakes, forming a closed-loop utilization and reducing water resource consumption.

[0034] The filtrate after filtration is discharged into the dilute magnesium tank for subsequent treatment. The dilute magnesium tank is a storage container with a volume of about 5-10 cubic meters. It is made of corrosion-resistant stainless steel or polypropylene and is used to temporarily store solutions containing magnesium ions.

[0035] S2: The decalcified filtrate is placed in an evaporator for evaporation to obtain a solid-liquid mixture containing 72-76% magnesium nitrate crystals.

[0036] Specifically, the decalcification filtrate is sent from the dilute magnesium tank to an evaporator for evaporation and concentration. The evaporator adopts a forced circulation evaporator, the heating temperature is controlled at 105-110° C., and the vacuum degree is greater than 70 kPa. During the evaporation process, water evaporates continuously, and the concentration of magnesium nitrate in the solution gradually increases. When the concentration of magnesium nitrate in the solution reaches 72-76%, the evaporation process is stopped. At this time, part of the magnesium nitrate crystals have been precipitated in the solution to form a solid-liquid mixture. The evaporation process lasts for about 3-4 hours, and the specific time is adjusted according to the initial concentration of the decalcification filtrate and the efficiency of the evaporator.

[0037] The solid-liquid mixture containing 72-76% magnesium nitrate crystals after evaporation and concentration is discharged to the concentrated magnesium tank for subsequent treatment. The dilute magnesium tank is a storage container with a volume of about 5-10 cubic meters. It is made of corrosion-resistant stainless steel or polypropylene and is used to temporarily store solutions containing magnesium ions.

[0038] S3: adding dilute nitric acid having a concentration of 45% to 60% and the solid-liquid mixture containing 72-76% magnesium nitrate crystals obtained in step S2 to a mixer in a ratio of 1:3 to 1:4 for mixing and reacting to obtain a solid-liquid mixture of nitric acid vapor having a nitric acid concentration of greater than 68.4% and 52-56% magnesium nitrate crystals;

[0039] Specifically, the mixer is a stirred reactor made of corrosion-resistant material, equipped with a temperature control system and a stirring device. During the mixing reaction, the temperature is controlled at 140-150°C, the pressure is controlled at 0.3-0.4 MPa, and the stirring speed is 60-80 rpm. Under these conditions, dilute nitric acid reacts with magnesium nitrate crystals, partially vaporizing to form nitric acid vapor. Simultaneously, the concentration of the magnesium nitrate crystals is reduced to 52-56%. The reaction time is approximately 30-40 minutes, and the concentration of the nitric acid vapor generated during the reaction reaches above 68.4%.

[0040] The chemical nature of the mixing reaction is that the crystal water in the magnesium nitrate crystals interacts with the dilute nitric acid, resulting in the concentration and vaporization of the nitric acid, while the concentration of the magnesium nitrate crystals decreases.

[0041] S4: The solid-liquid mixture containing 52-56% magnesium nitrate crystals obtained in step S3 is transported to a concentration tower via a pipeline, and enters a centrifugal separation device through the bottom of the stripping section for separation to obtain a mother liquor and magnesium nitrate crystals;

[0042] It is understood that the concentrator is a distillation device consisting of a stripping section and a rectifying section. The solid-liquid mixture enters the stripping section of the concentrator and is directly discharged through a discharge port at the bottom of the stripping section. It then passes through a pipeline into a centrifugal separation device, which uses a horizontal spiral discharge centrifuge with a rotation speed of 1500-2000 rpm and a centrifugation time of 5-8 minutes. Under the action of centrifugal force, the solid and liquid phases are effectively separated, resulting in magnesium nitrate crystals and mother liquor.

[0043] The mother liquor obtained from centrifugation is then tested for calcium content. Mother liquors with a calcium content of less than 0.3% are sent to a dilute magnesium tank, while those with a calcium content greater than 0.3% are sent to a decalcification reaction tank for further decalcification. Calcium content is tested using EDTA titration or atomic absorption spectrometry, with an accuracy of no less than 0.01%. This diversion process ensures effective control of calcium ion content in the system, preventing calcium ion accumulation in subsequent processes, which can lead to equipment scaling and product quality degradation.

[0044] S5: The magnesium nitrate crystals obtained in step S4 are sent to a dehydration device for dehydration treatment, and then the anhydrous magnesium nitrate is thermally decomposed to obtain a mixture of magnesium oxide and a gas. Specifically, the dehydration device is a rotary dryer, the temperature is controlled at 120-150° C., and the dehydration time is 60-90 minutes. Under these conditions, the crystal water in the magnesium nitrate crystals is evaporated and removed to obtain anhydrous magnesium nitrate.

[0045] The anhydrous magnesium nitrate is then fed into a thermal decomposition furnace for thermal decomposition. The thermal decomposition furnace is a rotary kiln structure with a temperature controlled at 380-420°C and a thermal decomposition time of 30-60 minutes. Under these conditions, the anhydrous magnesium nitrate undergoes a thermal decomposition reaction to produce magnesium oxide solid and a gaseous mixture of nitrogen dioxide and oxygen. The reaction formula is: 2Mg(NO3)2→2MgO+4NO2+O2.

[0046] The gas mixture is sent to a recovery chamber, where water is added to react and produce 14-18% dilute nitric acid. The recovery chamber is a gas-liquid contact device equipped with a packing layer to increase the gas-liquid contact area. In the recovery chamber, the water and gas mixture react as follows: 3NO2 + H2O → 2HNO3 + NO. The reaction temperature is controlled at 20-25°C, the pressure is atmospheric, and the reaction time is approximately 15-20 minutes. The resulting 14-18% dilute nitric acid can be recycled and reused in the production process.

[0047] S6: The nitric acid vapor with a nitric acid concentration of more than 68.4% obtained in step S3 is transported to the stripping section of the concentrator through a pipeline, and is further vaporized in the stripping section of the concentrator to obtain nitric acid vapor with a nitric acid concentration of more than 80%. The nitric acid vapor is then transported to the rectifying section, and is subjected to the action of reflux acid in the rectifying section to obtain nitric acid vapor with a nitric acid concentration of more than 98%.

[0048] In this embodiment, step S6 is performed simultaneously with step S4. The concentrator is a distillation device having a height of approximately 10-15 meters and is internally provided with multiple trays or packing to increase the gas-liquid contact area and mass transfer efficiency. In the stripping section, the nitric acid vapor is further vaporized and concentrated at a temperature of approximately 120-130°C and a pressure of approximately 0.2-0.3 MPa, increasing the nitric acid concentration to over 80%. Preferably, the stripping section has 10-15 trays, each of which is provided with an overflow weir and downcomer to ensure uniform distribution of the liquid within the tower.

[0049] The nitric acid vapor, reaching a concentration of over 80%, then continues to rise to the distillation section of the concentrator. In this distillation section, reflux acid initially added to the distillation section interacts with the rising nitric acid vapor in a countercurrent contact mass transfer process. The temperature of the distillation section is controlled at 80-90°C and the pressure is approximately 0.1-0.2 MPa. Under the action of the reflux acid, water in the nitric acid vapor is further removed, raising the nitric acid concentration to over 98%. Preferably, the distillation section has 8-12 trays and a mass transfer efficiency of no less than 95%.

[0050] S7: The nitric acid vapor with a nitric acid concentration of more than 98% obtained in step S6 is transported to a bleaching tower through a pipeline, reacts with the cold nitric acid liquid added to the bleaching tower, and produces hot nitric acid liquid and nitric acid vapor. The hot nitric acid liquid is cooled to obtain finished nitric acid, and the nitric acid vapor is condensed to obtain concentrated nitric acid.

[0051] The bleaching tower is a gas-liquid contact device with a height of about 5-8 meters. It is equipped with multiple layers of tower plates or packing inside. In the bleaching tower, nitric acid vapor contacts and reacts with cold nitric acid liquid to remove impurities such as NO2 in the nitric acid vapor to obtain pure nitric acid liquid. The hot nitric acid liquid is transported to the cooler through a pipeline for cooling. The cooler adopts a plate or shell and tube heat exchanger. The cooling medium is cooling water. The inlet temperature of the cooled nitric acid drops to 30-35℃, which is the finished nitric acid with a purity of not less than 98% and a transparent and colorless color.

[0052] In addition, part of the nitric acid vapor discharged from the top of the bleaching tower is transported to the condenser through a pipeline for condensation. The condenser adopts a shell and tube heat exchanger, and the cooling medium is cooling water. The inlet temperature is about 15°C and the outlet temperature is about 25°C. Concentrated nitric acid is obtained after condensation.

[0053] S8: The concentrated nitric acid obtained in step S7 is distributed through a flow metering device, wherein 1 / 3 of the concentrated nitric acid is refluxed to the bleaching tower through a pipeline and can be used as cold nitric acid liquid in contact with the nitric acid vapor, thereby realizing the circulation of nitric acid, thereby reducing the amount of cold nitric acid liquid added to the bleaching tower and increasing the production of finished nitric acid.

[0054] In addition, 2 / 3 of the concentrated nitric acid is refluxed to the distillation section of the concentrator through a pipeline as reflux acid to reduce the amount of reflux acid added to the distillation section and realize the circulation of nitric acid. The reflux temperature is controlled at 20-25°C. The reflux acid undergoes reverse contact and mass transfer with the rising nitric acid vapor in the distillation section to improve the purity of the nitric acid vapor.

[0055] Through the processing of steps S1-S8 above, the entire process of producing calcium sulfate and magnesium oxide from the calcium-magnesium-containing leachate, a by-product of chemical beneficiation, and recovering nitric acid is achieved. This method not only separates calcium and magnesium from the calcium-magnesium-containing leachate, improving the comprehensive utilization of resources, but also achieves efficient recovery of nitric acid, with significant economic and environmental benefits.

[0056] Example 2, a method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from a calcium and magnesium-containing leachate produced as a by-product of chemical mineral processing, comprising the following steps:

[0057] S1: decalcifying the calcium-magnesium-containing leachate, adding the calcium-magnesium-containing leachate and magnesium sulfate solution to a decalcification reaction tank for reaction, and then filtering to obtain a filter cake (calcium sulfate) and a filtrate (decalcification solution containing dilute magnesium nitrate); specifically, the calcium-magnesium-containing leachate and the magnesium sulfate solution are mixed in a decalcification reaction tank, and sulfate ions react with calcium ions to generate insoluble calcium sulfate precipitates. The reaction temperature is controlled at 28° C. and the reaction time is 1.5 hours to ensure that the calcium ions react fully. After the reaction is completed, solid-liquid separation is performed by a filter press to obtain a calcium sulfate filter cake and a decalcification solution containing dilute magnesium nitrate.

[0058] The filtrate after filtration is discharged into the dilute magnesium tank for subsequent treatment. The filter cake obtained after filtration is washed once and twice to obtain calcium sulfate. The washing liquid after the first wash is used to dissolve magnesium sulfate, and the washing liquid after the second wash is used for washing in the first wash. Specifically, the filter cake is washed with clean water in the first wash, the washing temperature is 25°C, and the washing time is 15 minutes; the washing liquid after the first wash is collected and used to prepare the magnesium sulfate solution. The filter cake is also washed with clean water in the second wash, the washing temperature is 25°C, and the washing time is 10 minutes; the washing liquid after the second wash is collected and used for the first wash of the next batch of filter cakes.

[0059] S2: The decalcified filtrate obtained in step S1 is transferred to an evaporator for evaporation and concentration to obtain a solid-liquid mixture containing 75% magnesium nitrate crystals. Specifically, the evaporator is a forced circulation evaporator, the heating temperature is controlled at 108°C, and the vacuum is 75kPa. During the evaporation process, water evaporates continuously, and the magnesium nitrate concentration in the solution gradually increases. When the magnesium nitrate concentration in the solution reaches 75%, the evaporation process is stopped. At this point, some magnesium nitrate crystals have precipitated from the solution, forming a solid-liquid mixture. The evaporation process lasts for approximately 3.5 hours.

[0060] The water vapor generated during the evaporation process is recovered through condensation and can be recycled as process water.

[0061] The solid-liquid mixture containing 75% magnesium nitrate crystals after evaporation and concentration is discharged to the concentrated magnesium tank for subsequent treatment. The dilute magnesium tank is a storage container with a volume of about 5-10 cubic meters. It is made of corrosion-resistant stainless steel or polypropylene and is used to temporarily store solutions containing magnesium ions.

[0062] S3: Adding 50% dilute nitric acid and the solid-liquid mixture containing 74% magnesium nitrate crystals obtained in step S2 to a mixer in a ratio of 1:3.5 for mixing and reacting to obtain a solid-liquid mixture of nitric acid vapor having a nitric acid concentration of 70% and magnesium nitrate crystals having 54%; during the mixing reaction, the temperature is controlled at 145°C, the pressure is controlled at 0.35 MPa, and the stirring speed is 70 rpm. Under these conditions, the dilute nitric acid reacts with the magnesium nitrate crystals, partially vaporizing to form nitric acid vapor, while the concentration of the magnesium nitrate crystals is reduced to 54%. The reaction time is approximately 35 minutes, and the concentration of the nitric acid vapor generated during the reaction reaches 70%.

[0063] S4: The solid-liquid mixture of 54% magnesium nitrate crystals obtained in step S3 is transported to a concentration tower via a pipeline. The mixture treated at the bottom of the stripping section enters a centrifugal separation device via a pipeline. The centrifugal separation device uses a horizontal spiral unloading centrifuge with a rotation speed of 1800 rpm and a centrifugation time of 6 minutes. Under the action of centrifugal force, the solid and liquid phases are effectively separated to obtain magnesium nitrate crystals and mother liquor.

[0064] The mother liquor obtained by centrifugation is tested for calcium content. The mother liquor with a calcium content of less than 0.3% is sent to a dilute magnesium tank, and the mother liquor with a calcium content of more than 0.3% is sent to a decalcification reaction tank.

[0065] S5: The magnesium nitrate crystals obtained in step S4 are dehydrated in a rotary dryer at 135°C for 75 minutes. Under these conditions, the water of crystallization in the magnesium nitrate crystals is evaporated and removed to produce anhydrous magnesium nitrate. The anhydrous magnesium nitrate is then decomposed in a thermal decomposition furnace at 400°C for 45 minutes. The anhydrous magnesium nitrate undergoes a thermal decomposition reaction to produce magnesium oxide solid and a gaseous mixture (NO2 and O2).

[0066] The gas mixture is sent to a recovery chamber, where water is added to react to produce 18% dilute nitric acid. The 18% dilute nitric acid can be recycled and used in the production process, such as phosphate rock leaching.

[0067] S6: The nitric acid vapor with a nitric acid concentration of 70% obtained in step S3 is transported via a pipeline to the stripping section of the concentrator. In the stripping section, the nitric acid vapor is further vaporized and concentrated at a temperature of approximately 125°C and a pressure of approximately 0.25 MPa, raising the nitric acid concentration to 85%. The nitric acid vapor with a concentration of 85% continues to rise to the rectifying section of the concentrator. In the rectifying section, reflux acid and the rising nitric acid vapor undergo countercurrent contact and mass transfer. The temperature of the rectifying section is controlled at 85°C and the pressure is approximately 0.15 MPa. Under the action of the reflux acid, water in the nitric acid vapor is further removed, raising the nitric acid concentration to 99%.

[0068] S7: The nitric acid vapor with a nitric acid concentration of more than 99% obtained in step S6 is transported to a bleaching tower through a pipeline, reacts with the cold nitric acid liquid added to the bleaching tower, and obtains hot nitric acid liquid and nitric acid vapor. The hot nitric acid liquid is cooled in a cooler to obtain finished nitric acid. The nitric acid vapor discharged from the top of the bleaching tower is transported to a condenser through a pipeline for condensation, and concentrated nitric acid is obtained after condensation.

[0069] S8: dilute the concentrated nitric acid obtained in step S7 into 50% diluted nitric acid and send it to the mixer for the mixing reaction process in step S3. The dilution process adopts a quantitative water addition method, the dilution temperature is controlled at 25°C, and the concentration of the diluted nitric acid is accurately controlled at 60±0.5%.

[0070] Example 3, a method for preparing calcium sulfate and magnesium oxide and recovering nitric acid from a calcium and magnesium-containing leachate produced as a by-product of chemical mineral processing, comprising the following steps:

[0071] S1: decalcifying the calcium-magnesium-containing leachate, adding the calcium-magnesium-containing leachate and magnesium sulfate solution to a decalcification reaction tank for reaction, and then filtering to obtain a filter cake (calcium sulfate) and a filtrate (decalcification solution containing dilute magnesium nitrate); specifically, the calcium-magnesium-containing leachate and the magnesium sulfate solution are mixed in a decalcification reaction tank, and sulfate ions react with calcium ions to generate insoluble calcium sulfate precipitates. The reaction temperature is controlled at 26° C. and the reaction time is 1.8 hours to ensure that the calcium ions react fully. After the reaction is completed, solid-liquid separation is performed by a filter press to obtain a calcium sulfate filter cake and a decalcification solution containing dilute magnesium nitrate.

[0072] The filtrate after filtration is discharged into the dilute magnesium tank for subsequent treatment. The filter cake obtained after filtration is washed once and twice to obtain calcium sulfate, wherein the washing liquid after the first wash is used to dissolve magnesium sulfate, and the washing liquid after the second wash is used for washing in the first wash. Specifically, the filter cake is washed with clean water in the first wash, the washing temperature is 26°C, and the washing time is 12 minutes; the washing liquid after the first wash is collected and used to prepare the magnesium sulfate solution. The filter cake is also washed with clean water in the second wash, the washing temperature is 26°C, and the washing time is 8 minutes; the washing liquid after the second wash is collected and used for the first wash of the next batch of filter cakes.

[0073] S2: The decalcified filtrate obtained in step S1 is transferred to an evaporator for evaporation and concentration to obtain a solid-liquid mixture containing 72% magnesium nitrate crystals. Specifically, the evaporator is a forced circulation evaporator, the heating temperature is controlled at 106°C, and the vacuum is 80 kPa. During the evaporation process, water evaporates continuously, and the magnesium nitrate concentration in the solution gradually increases. When the magnesium nitrate concentration in the solution reaches 72%, the evaporation process is stopped. At this point, some magnesium nitrate crystals have precipitated from the solution, forming a solid-liquid mixture. The evaporation process lasts for approximately 3.2 hours.

[0074] The solid-liquid mixture containing 72% magnesium nitrate crystals after evaporation and concentration is discharged to the concentrated magnesium tank for subsequent treatment. The dilute magnesium tank is a storage container with a volume of about 5-10 cubic meters. It is made of corrosion-resistant stainless steel or polypropylene and is used to temporarily store solutions containing magnesium ions.

[0075] The water vapor generated during the evaporation process is recovered through condensation and can be recycled as process water.

[0076] S3: Adding 60% dilute nitric acid and the solid-liquid mixture containing 72% magnesium nitrate crystals obtained in step S2 to a mixer in a ratio of 1:4 for mixing and reacting to obtain a solid-liquid mixture of nitric acid vapor having a nitric acid concentration of 68.4% and magnesium nitrate crystals having a 52% concentration; during the mixing reaction, the temperature is controlled at 142° C., the pressure is controlled at 0.32 MPa, and the stirring speed is 65 rpm. Under these conditions, the dilute nitric acid reacts with the magnesium nitrate crystals, partially vaporizing to form nitric acid vapor, while the concentration of the magnesium nitrate crystals is reduced to 52%. The reaction time is approximately 32 minutes, and the concentration of the nitric acid vapor generated during the reaction reaches 69%.

[0077] S4: The solid-liquid mixture of 52% magnesium nitrate crystals obtained in step S3 is transported to a concentration tower through a pipeline. The mixture after being treated at the bottom of the stripping section enters a centrifugal separation device through a pipeline. The centrifugal separation device uses a horizontal spiral unloading centrifuge with a speed of 1800 rpm and a centrifugation time of 6 minutes. Under the action of centrifugal force, the solid and liquid phases are effectively separated to obtain magnesium nitrate crystals and mother liquor.

[0078] The mother liquor obtained by centrifugation is tested for calcium content. The mother liquor with a calcium content of less than 0.3% is sent to a dilute magnesium tank, and the mother liquor with a calcium content of more than 0.3% is sent to a decalcification reaction tank.

[0079] S5: The magnesium nitrate crystals obtained in step S4 are dehydrated in a rotary dryer at 130°C for 80 minutes. Under these conditions, the water of crystallization in the magnesium nitrate crystals is evaporated and removed to produce anhydrous magnesium nitrate. The anhydrous magnesium nitrate is then decomposed in a thermal decomposition furnace at 390°C for 40 minutes. The anhydrous magnesium nitrate undergoes a thermal decomposition reaction to produce magnesium oxide solid and a gaseous mixture (NO2 and O2).

[0080] The gas mixture is sent to the distillation section of the concentration tower to participate in the concentration process of nitric acid, thereby improving the energy utilization efficiency and nitric acid recovery rate of the system.

[0081] S6: The nitric acid vapor with a nitric acid concentration of 69% obtained in step S3 is transported via a pipeline to the stripping section of the concentrator. In the stripping section, the nitric acid vapor is further vaporized and concentrated at a temperature of approximately 123° C. and a pressure of approximately 0.22 MPa, raising the nitric acid concentration to 82%. The nitric acid vapor with a concentration of 82% continues to rise to the rectifying section of the concentrator. In the rectifying section, reflux acid and the rising nitric acid vapor undergo countercurrent contact mass transfer. The temperature of the rectifying section is controlled at 82° C. and the pressure is approximately 0.12 MPa. Under the action of the reflux acid, water in the nitric acid vapor is further removed, raising the nitric acid concentration to 98.5%.

[0082] S7: The nitric acid vapor with a nitric acid concentration of 98.5% obtained in step S6 is transported to a bleaching tower through a pipeline, reacts with the cold nitric acid liquid added to the bleaching tower, and obtains hot nitric acid liquid and nitric acid vapor. The hot nitric acid liquid is cooled in a cooler to obtain finished nitric acid. The nitric acid vapor discharged from the top of the bleaching tower is transported to a condenser through a pipeline for condensation, and concentrated nitric acid is obtained after condensation.

[0083] S8: dilute the concentrated nitric acid obtained in step S7 into 60% dilute nitric acid and send it to the mixer for the mixing reaction process in step S3. The dilution process adopts a quantitative water addition method, the dilution temperature is controlled at 25°C, and the concentration of the diluted nitric acid is accurately controlled at 60±0.5%.

[0084] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A method for preparing calcium sulfate and magnesium oxide from a calcium and magnesium-containing leaching solution produced as a by-product of chemical mineral processing and recovering nitric acid, characterized in that: The following steps are involved: S1, decalcifying the calcium-magnesium leaching solution, adding the calcium-magnesium leaching solution and magnesium sulfate solution into a decalcification reaction tank for reaction, and then filtering to obtain calcium sulfate and a decalcification solution containing dilute magnesium nitrate; S2, placing the decalcified filtrate into an evaporator for evaporation to obtain a solid-liquid mixture containing 72-76% magnesium nitrate crystals; S3, adding dilute nitric acid and the solid-liquid mixture into a mixer for mixing and reaction to obtain a solid-liquid mixture of nitric acid vapor having a nitric acid concentration of greater than 68.4% and 52-56% magnesium nitrate crystals; S4, the solid-liquid mixture containing 52-56% magnesium nitrate crystals enters a concentration tower, passes through the bottom of the stripping section, and enters a centrifugal separation device for separation to obtain a mother liquor and magnesium nitrate crystals; S5, dehydrating the magnesium nitrate crystals to obtain anhydrous magnesium nitrate, and then thermally decomposing the anhydrous magnesium nitrate to obtain magnesium oxide and a gas mixture; S6, sending the nitric acid vapor obtained in step S3 to a concentrator, and further gasifying it in the stripping section of the concentrator to obtain nitric acid vapor with a nitric acid concentration of more than 80%, and then sending it to a rectifying section, and obtaining nitric acid vapor with a nitric acid concentration of more than 98% under the action of reflux acid in the rectifying section; S7, nitric acid vapor with a nitric acid concentration of more than 98% enters a bleaching tower to react with cold nitric acid liquid to obtain hot nitric acid liquid and nitric acid vapor, wherein the hot nitric acid liquid is cooled to obtain finished nitric acid, and the nitric acid vapor is condensed to obtain concentrated nitric acid; S8, 1 / 3 of the concentrated nitric acid is refluxed to the bleaching tower, and 2 / 3 of the concentrated nitric acid is refluxed to the distillation section of the concentrating tower as reflux acid.

2. The method according to claim 1, wherein: The concentration of the dilute nitric acid in step S2 is 45%-60%.

3. The method according to claim 2, wherein: In step S2, the ratio of the dilute nitric acid to the solid-liquid mixture containing 72-76% magnesium nitrate crystals is 1:3-1:

4.

4. The method according to claim 1, wherein: After filtering in step S1, a filter cake is obtained, and the filter cake is washed once and twice to obtain calcium sulfate, wherein the washing liquid after the first wash is used to dissolve magnesium sulfate, and the washing liquid after the second wash is used for washing in the first wash.

5. The method according to claim 1, wherein: The filtrate after filtration in step S1 is discharged to a dilute magnesium tank, and the mother liquor obtained by centrifugal separation in step S4 is tested for calcium content. The mother liquor with a calcium content of less than 0.3% is sent to a dilute magnesium tank, and the mother liquor with a calcium content of more than 0.3% is sent to a decalcification reaction tank.

6. The method according to claim 1, wherein: The gas mixture in step S5 is sent to a recovery chamber, and water is added to react to obtain 14-18% dilute nitric acid.

7. The method according to claim 1, wherein: The gas mixture in step S5 is sent to the distillation section of the concentration tower.

8. The method according to claim 1, wherein: The temperature for dehydrating magnesium nitrate in step S5 is 120-150°C.

9. The method according to claim 1, wherein: The temperature for thermal decomposition of anhydrous magnesium nitrate in step S5 is 380-420° C. and the time is 30-60 min.

10. The method according to claim 1, wherein: The concentrated nitric acid obtained in S7 is diluted to 60% dilute nitric acid and sent to the mixer.

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