A method for comprehensive utilization of magnesium resources in potassium sulfate brine

Abstract

The invention discloses a method for comprehensively utilizing magnesium resources in potassium sulfate brine, and belongs to the technical field of salt chemical industry. The invention filters the brine and uses bipolar membrane electrodialysis to concentrate the magnesium element in the clarified brine into a receiving solution, and produces a mixed acid solution of sulfuric acid and hydrochloric acid; citric acid and an initiator are added to the receiving solution enriched with magnesium elements to prepare a mixed solution, and a voltage is applied to the mixed solution to prepare a magnesium precursor, and then magnesium oxide is obtained by centrifugal separation, drying and calcination. The mixed acid solution is subjected to one-stage and two-stage nanofiltration, and evaporated and dried to obtain magnesium sulfate. The invention uses bipolar membrane electrodialysis to replace traditional electrolysis, and can safely separate magnesium resources in potassium sulfate brine, does not produce flammable and toxic gases such as hydrogen and chlorine, has high process safety and does not need to overcome the electrolysis potential of water, and greatly reduces power consumption, which is suitable for large-scale continuous industrial production.

Description

Technical Field

[0001] The invention belongs to the technical field of salt chemical industry, and particularly relates to a method for comprehensively utilizing magnesium resources in potassium sulfate brine. Background Art

[0002] Magnesium oxide is an inorganic material with wide applications. Due to its good adsorption characteristics, high fire resistance and insulation, it is widely used in building materials, water treatment, medical, chemical and steelmaking industries. In particular, high-purity magnesium oxide is widely used in high-end heat-resistant refractory materials, thermal conductive materials, magnetic device fillers, high-end ceramic raw materials and other fields, with high economic added value.

[0003] In some coastal areas of eastern my country, due to the rich brine resources, local salt chemical enterprises mostly use brine resources to produce potassium sulfate. Among them, the use of carnallite to produce potassium sulfate is currently the most common production process of potassium sulfate. This process will also produce old brine containing a large amount of magnesium chloride and magnesium sulfate as a by-product while producing potassium sulfate. Under the existing technical system, these old brines are mostly concentrated and dried by evaporation process to be sold as crude magnesium chloride products. The product quality is not high, the industrial added value is low, and the economic value is limited. In terms of composition, the old brine has high content of magnesium chloride and magnesium sulfate and low amount of impurity salt, which can be used as a magnesium source for high value-added magnesium oxide products. However, the relevant process routes and information are relatively lacking at present. In order to further improve the utilization rate of brine resources and increase the added value of the salt chemical industry, it is necessary to deeply develop the utilization methods of magnesium resources in the old brine.

[0004] The Chinese patent document with publication number CN114634193A discloses a method for obtaining magnesium oxide by directly electrolyzing seawater. The method uses seawater as a raw material, directly electrolyzes seawater to generate magnesium hydroxide precipitate from free magnesium in the seawater, and then converts the magnesium hydroxide precipitate into magnesium oxide by calcination or other means. The problem with this method is that during the implementation process, since the process of forming hydroxide by water electrolysis mainly occurs on the cathode of the electrode, the concentration of hydroxide near the cathode is relatively high. Therefore, during electrolysis, magnesium hydroxide precipitate is mainly generated near the cathode. These precipitates are easy to form a scale layer on the cathode, thereby preventing the electrolysis reaction from continuing, and are not suitable for continuous industrial production.

[0005] A Chinese patent document with publication number CN114717579A discloses a method for producing magnesium hydroxide by electrochemical deposition in an electrolytic cell using a magnesium source such as magnesium chloride. The method first dissolves the magnesium source into an aqueous solution, adds magnesium hydroxide seeds to prepare an electrolyte, and then feeds the electrolyte into an electrolytic cell for electrolysis, so that the hydroxide ions generated by electrolysis of free magnesium and water are deposited on the magnesium hydroxide seeds, thereby obtaining a larger magnesium hydroxide precipitate. In this method, it is still difficult to avoid the occurrence of scaling. Once scaling occurs in the electrolytic cell device, it can only be shut down for cleaning, which has an adverse effect on production efficiency, high cleaning costs, and is not conducive to continuous industrial production.

[0006] In addition, in conventional electrolysis devices or diaphragm electrolyzers, in order to produce the hydroxide necessary for the formation of magnesium hydroxide precipitation, the electrolysis potential of water needs to be overcome, which means that hydrogen will be produced during the electrolysis process, and the presence of easily electrolytic ions such as chloride in the electrolytic solution will produce electrolytic gases such as chlorine. Most of these electrolytic gases are flammable or toxic, posing a potential safety hazard in production. Summary of the invention

[0007] The purpose of the present invention is to provide a method for comprehensively utilizing magnesium resources in potassium sulfate brine. By means of bipolar membrane electrodialysis, electrodeposition crystallization and nanofiltration and other process means, high-value magnesium oxide materials and magnesium sulfate are prepared from brine containing magnesium chloride and magnesium sulfate as raw materials. The prepared magnesium oxide material has a spherical scaly structure, has good performance as a polymer thermal conductive filler, and has good market value. The method is suitable for continuous industrial production.

[0008] In order to achieve the above object, the technical solution of the present invention is: a method for comprehensively utilizing magnesium resources in potassium sulfate brine, comprising the following steps:

[0009] (1) Using a bipolar membrane electrodialyzer, firstly, the clarified old brine is sent to the salt chamber of the bipolar membrane electrodialyzer, and pure water and a receiving solution containing seed crystals are simultaneously introduced into the alkali chamber, and pure water is introduced into the acid chamber. After bipolar membrane electrodialysis, a light magnesium brine is obtained from the salt chamber, a mixed acid solution of sulfuric acid and hydrochloric acid is obtained from the acid chamber, and a receiving solution enriched with magnesium is obtained from the alkali chamber;

[0010] (2) adding citric acid and an initiator to the magnesium-enriched receiving solution obtained in step (1) to form a mixed solution, applying voltage to the mixed solution to perform deposition and crystallization to obtain a turbid solution containing a magnesium precursor;

[0011] (3) centrifugally separating the turbid liquid containing the magnesium precursor obtained in step (2) to obtain the magnesium precursor and the receiving liquid containing the seed crystals, respectively, and returning the receiving liquid containing the seed crystals to step (1);

[0012] (4) drying and calcining the magnesium precursor obtained in step (3) to obtain magnesium oxide;

[0013] (5) subjecting the mixed acid solution of sulfuric acid and hydrochloric acid obtained in step (1) to a nanofiltration separation to obtain hydrochloric acid and sulfuric acid, respectively;

[0014] (6) adding the sulfuric acid obtained in step (5) to the light magnesium brine obtained in step (1), and performing two-stage nanofiltration separation to obtain hydrochloric acid and magnesium sulfate solution, respectively;

[0015] (7) The magnesium sulfate solution obtained in step (6) is evaporated and dried to obtain magnesium sulfate.

[0016] Furthermore, in step (1), the old brine contains 40wt%~45wt% of magnesium chloride and 3.0wt%~4.0wt% of magnesium sulfate; the membrane current density of the bipolar membrane electrodialyzer is 500A / m 2 ~800A / m 2 , the salt recovery rate is 94%~96%.

[0017] Furthermore, the seed-containing receiving solution described in step (1) is prepared from citric acid, magnesium hydroxide and pure water, wherein the magnesium content is 0.5wt%~0.6wt%; the amount of pure water entering the alkali chamber is adjusted to control the magnesium content in the receiving solution enriched with magnesium to be 2.5wt%~3.5wt%; the feed flow ratio of the old brine entering the salt chamber to the pure water entering the acid chamber is 1.0:3.0~4.0.

[0018] Furthermore, the concentration of citric acid in the mixed solution in step (2) is 0.5-0.7 mol / L.

[0019] Furthermore, in step (2), the initiator is one of aqueous ammonia, urea, and aqueous ammonia solution of ammonium sulfate; and the molar ratio of the nitrogen element in the initiator in the mixed solution to the magnesium content in the receiving solution enriched with magnesium element is 1.8-2.2:1.0.

[0020] Furthermore, in step (2), the voltage applied to the mixed solution is 16 kV to 20 kV, and the deposition and crystallization time is 2.0 h to 3.0 h.

[0021] Furthermore, the magnesium content in the receiving solution containing the seed crystals in step (3) is 0.5wt%~0.6wt%.

[0022] Furthermore, in step (4), the drying temperature is 80°C-90°C, and the water content of the magnesium precursor after drying is less than 1wt%; the calcination temperature is 850°C-950°C, and the calcination time is 3.0h-3.5h.

[0023] Furthermore, in step (5), the operating pressure of the first stage nanofiltration is 2.5 MPa to 3.0 MPa, and the recovery rate of chloride ions in the first stage nanofiltration produced water is controlled to be 90% to 92%.

[0024] Furthermore, in step (6), the operating pressure of the second stage nanofiltration is 3.5 Mpa to 4.0 Mpa, and the chloride ion recovery rate in the second stage nanofiltration produced water is controlled to be 98% to 99%.

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0026] (1) The present invention separates and processes potassium sulfate brine by using a bipolar membrane electrodialysis method. By utilizing the characteristic of bipolar membrane hydrolyzing water to generate hydroxide and hydrogen ions when powered, magnesium resources in potassium sulfate brine can be safely separated. Compared with the traditional electrolysis process, no flammable and toxic gases such as hydrogen and chlorine are produced. The process is highly safe and does not need to overcome the electrolysis potential of water. The power consumption is greatly reduced, which is conducive to reducing production costs and is suitable for large-scale continuous industrial production.

[0027] (2) The present invention applies an ultra-high voltage to a mixture of a receiving solution enriched with magnesium, citric acid, and an initiator, thereby generating a high-voltage electric field in the mixture. Under the action of the electric field, the hydration ability of magnesium ions in the mixture is enhanced, the solubility is increased, and a supersaturated magnesium solution is formed. At the same time, under the action of the initiator and citric acid, a solution system similar to that in the hydrothermal synthesis process is formed, so that the magnesium element is induced by the seed crystal to deposit and grow into a magnesium precursor with a certain microstructure, and after calcination, magnesium oxide with a spherical spherical structure can be formed. Compared with hydrothermal synthesis in traditional processes, this method has milder conditions and can be implemented in normal pressure or low pressure environments, which is more conducive to continuous industrial production.

[0028] (3) The present invention adopts a process of adding citric acid and an initiator to a receiving solution enriched with magnesium to form a mixed solution, and then applying voltage to the mixed solution for deposition and crystallization to form a magnesium precursor. The high-voltage electric field generated by the voltage applied in this process can be conducted back to the alkaline chamber of the bipolar membrane electrodialyzer through the receiving solution enriched with magnesium during the mixing process of the mixed solution. The high-voltage electric field is used to enhance the hydration of magnesium, improve the solubility of magnesium hydroxide generated in the alkaline chamber of the bipolar membrane electrodialyzer, further inhibit the deposition of the formed magnesium hydroxide on the surface of the bipolar membrane, and enhance the directional deposition of magnesium hydroxide on the seed crystal. Under the synergistic effect of the two, the operational stability of the electrodialyzer is improved, and the entire process has better production retention.

[0029] (4) The magnesium oxide separated by the present invention is higher than 99.91% by EDTA titration method, which is higher than the requirement of ≥95% for superior magnesium oxide in HG / T2573-2012 "Determination of Industrial Light Magnesium Oxide". The magnesium oxide obtained is high-purity magnesium oxide, which has a submicron scale-like spherical structure and a specific surface area of ​​117.8 m 2 / g or more, when used as a thermal conductive filler for PVC polymer, it has higher thermal conductivity than the current market products. The purity of the obtained magnesium sulfate is not less than 99.16% in terms of magnesium sulfate heptahydrate. According to the provisions of HG / T 2680-2017 "Industrial Magnesium Sulfate" that the purity of first-class magnesium sulfate heptahydrate is ≥99.0, this magnesium sulfate is a first-class product. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a process flow chart of a specific implementation mode of the present invention.

[0031] In the figure: 1 is a calcining furnace, 2 is a drying bed, 3 is a rotary drum centrifuge, 4 is an electrical aging tank, 5 is a citric acid storage tank, 6 is an initiator storage tank, 7 is an evaporation crystallizer, 8 is a nanofiltration two-stage device, 9 is a nanofiltration one-stage device, 10 is a brine buffer tank, 11 is a mixed acid buffer tank, 12 is a bipolar membrane electrodialyzer, 13 is a pure water storage tank, 14 is an old brine buffer tank, 15 is a pretreatment filter, and 16 is a receiving liquid buffer tank.

[0032] Figure 2 This is the SEM image of the magnesium oxide sample in Example 1.

[0033] Figure 3 This is the XRD diagram of the magnesium oxide sample in Example 1. DETAILED DESCRIPTION

[0034] In order to more clearly understand the purpose, technical features and effects of the present invention, the following is a detailed description in conjunction with the accompanying drawings and embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] In a specific embodiment of the present invention, the method for comprehensively utilizing magnesium resources in potassium sulfate brine is as follows Figure 1 As shown, the device operates according to the following steps:

[0036] (1) The old brine containing a large amount of magnesium chloride and magnesium sulfate comes from the potassium sulfate plant, is collected in the old brine buffer tank 14, and then sent to the pretreatment filter 15 for filtration. In this device, the pretreatment filter 15 is a composite filter composed of a sand filter and PP filter cotton. The purpose of filtration is mainly to eliminate large solid particles such as sand and stone in the magnesium brine to prevent them from clogging or damaging the bipolar membrane electrodialyzer 12. The old brine is filtered to become clarified old brine, which is sent to the bipolar membrane electrodialyzer 12 for electrodialysis treatment.

[0037] (2) The two ends of the bipolar membrane electrodialyzer 12 are electrodes, and between the electrodes is a membrane stack composed of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane arranged in sequence. The arrangement order from positive electrode to negative electrode is bipolar membrane-anion exchange membrane-cation exchange membrane-bipolar membrane. There is a chamber for liquid to pass through between the membranes, wherein the chamber composed of the bipolar membrane and the anion exchange membrane is called the acid chamber, the chamber composed of the anion exchange membrane and the cation exchange membrane is called the salt chamber, and the chamber composed of the cation exchange membrane and the bipolar membrane is called the alkali chamber. The clarified brine enters the bipolar membrane electrodialyzer 12 from one end of the salt chamber, and at the same time, pure water from the pure water storage tank 13 is passed into the acid chamber of the bipolar membrane, and the receiving liquid containing crystal seeds from the receiving liquid buffer tank 16 and pure water from the pure water storage tank 13 are passed into the alkali chamber. After the liquid is passed in, the bipolar membrane electrodialyzer is energized. Under the action of the current, the bipolar membrane dissociates water into hydrogen ions and hydroxides and supplies them to the acid chamber and the alkali chamber respectively. Various ions in the clarified old brine move in a directional manner under the action of electric current, among which chloride and sulfate ions enter the acid chamber through the anion exchange membrane, and form a mixed acid solution of sulfuric acid and hydrochloric acid with hydrogen ions and pure water in the acid chamber, and leave from the other end of the acid chamber. Magnesium ions enter the alkali chamber through the cation exchange membrane, and combine with hydroxide in the alkali chamber to form magnesium hydroxide precipitation. At this time, since the receiving solution entering the alkali chamber contains crystal seeds, which are magnesium hydroxide crystals, according to the principle that crystal precipitation preferentially proceeds on similar surfaces, magnesium hydroxide precipitation first proceeds on the crystal seeds under the induction of the crystal seeds, so that the membrane is not easy to scale due to the accumulation of magnesium hydroxide, so that the electrodialysis process continues. The amount of pure water introduced into the alkali chamber is determined according to the magnesium content in the receiving solution enriched with magnesium after electrodialysis, in order to control the magnesium content in the receiving solution from being too high and rapidly scaling in the salt chamber, thereby affecting the normal operation of the electrodialyzer. After electrodialysis, magnesium is continuously concentrated in the alkali chamber, and the receiving solution enriched with magnesium leaves the bipolar membrane electrodialyzer 12 from the other end of the alkali chamber. The magnesium ion concentration of the old brine flowing through the salt chamber is continuously reduced during the electrodialysis process, and the light magnesium brine leaves the bipolar membrane electrodialyzer 12 from one end of the salt chamber.

[0038] (3) The magnesium-enriched receiving solution obtained in the alkali chamber is sampled and analyzed to determine the magnesium content and the required amount of citric acid and initiator, and then the citric acid and initiator from the citric acid storage tank 5 and the initiator storage tank 6 are added to prepare a mixed solution and sent to the electrical aging tank 4, which is composed of two aging tank units of the same specifications, each of which is equipped with a high-voltage electrode unit. After the configured receiving solution enters the aging tank, the high-voltage electrode unit immediately applies a high voltage to the liquid in the aging tank and forms an electric field. Under the action of the electric field, the hydration capacity of the magnesium element in the aging tank is enhanced, the solubility increases, and a supersaturated state similar to the hydrothermal crystallization method environment is formed. Under the participation of citric acid and initiator and the guidance of the seed crystal, further deposition and crystallization are carried out to form a turbid solution containing a magnesium precursor. In order to ensure the crystallization effect, the embodiment specifies that the receiving solution needs to stay in the aging tank for a certain time after entering the aging tank, which is called the deposition and crystallization time. This time is counted from the time the receiving solution enters the aging tank. The two electrical aging tanks alternately perform feed aging and discharge to continuously perform deposition and crystallization operations. In this state, the feed flow is stable and the conductivity of the flow itself is formed by the continuity of the feed. The high-voltage electric field generated by the high-voltage electrode unit in the aging tank can be transmitted to the alkali chamber of the bipolar membrane electrodialyzer through the mixed liquid feed flow and the receiving liquid enriched with magnesium, so that the hydration capacity of the magnesium in the alkali chamber is enhanced and the solubility is increased, playing a role similar to that of an electric descaler. At this time, the deposition of magnesium on the membrane surface is further suppressed, and the tendency to deposit on the surface of the seed crystal is enhanced. The synergistic effect of the two further enhances the operational stability of the bipolar membrane electrodialyzer.

[0039] (4) When a sufficient amount of magnesium precursor is formed in the electrical aging tank 4, the magnesium precursor is poured out into the rotary drum centrifuge 3 together with the remaining liquid in the aging tank. Under the centrifugal action of the centrifuge, the magnesium precursor is separated from the liquid. By controlling the rotation speed of the centrifuge, the remaining part of the magnesium precursor in the liquid is used as a seed crystal, and the receiving liquid containing the seed crystal is obtained and stored in the receiving liquid buffer tank 16, and then sent back to step (1) for reuse. The magnesium precursor after separating the liquid is sent to the drying bed 2 for drying, and then sent to the calcining furnace to be calcined at a specified temperature to become a magnesium oxide product.

[0040] (5) The mixed acid solution of sulfuric acid and hydrochloric acid generated in the acid chamber of the bipolar membrane electrodialyzer 12 is first stored in the mixed acid buffer tank 11 for temporary storage, and then passed into the nanofiltration first stage device 8 to separate part of the hydrochloric acid from the water production side. In order to prevent the hydrochloric acid concentration generated on the water production side from being too high to damage the nanofiltration membrane, the production process needs to pass pure water to the water production side to receive and dilute the hydrochloric acid. The light magnesium brine generated from the salt chamber of the electrodialyzer first enters the light magnesium brine buffer tank 10 for temporary storage, and then is mixed with the concentrated water generated after the hydrochloric acid is separated by the nanofiltration first stage device 8, and is sent to the nanofiltration second stage device 9 to separate most of the chloride ions and produce part of the hydrochloric acid on the nanofiltration second stage water production side. The hydrochloric acid produced by the first and second stage nanofiltration devices is sent out of the device after being combined, and can be used for the production of high-concentration hydrochloric acid in the chlor-alkali plant. After the mixed solution of the light magnesium brine and the first stage concentrated water is concentrated and separated by the nanofiltration second stage device, a solution containing high-concentration magnesium sulfate is formed, and after evaporation and crystallization by the evaporation crystallizer 7, magnesium sulfate is produced. Example 1

[0041] This example uses old brine containing 40wt% magnesium chloride and 3.5wt% magnesium sulfate for testing, and the feed amount for the entire test period is 1.5t. The following steps are followed during implementation:

[0042] (1) Add enough old brine to the brine buffer tank 14 according to the feeding amount of the whole test cycle. When the test process system is started for the first time, first take high-purity magnesium hydroxide (purity>99.9%) as the starting raw material, add it to pure water to prepare a magnesium-enriched receiving solution with a magnesium content of 2.5wt%, then add citric acid and an ammonia solution of ammonium sulfate as an initiator (wherein the molar ratio of ammonium sulfate to ammonia is 1:1) to the magnesium-enriched receiving solution to prepare a mixed solution, adjust the amount of citric acid and initiator added, control the citric acid concentration in the mixed solution to 0.53 mol / L~0.55 mol / L, and the molar ratio of nitrogen element in the initiator in the mixed solution: magnesium element in the magnesium-enriched receiving solution is 1.85~1.90:1.0. After preparation, send the mixed solution to one of the aging tank units of the electrical aging tank 4. At the same time, a voltage of 16kV~16.5kV is applied to the liquid in the aging tank unit through the high-voltage electrode unit of the unit, so that the mixed liquid in the tank undergoes deposition and crystallization reaction to form magnesium hydroxide crystals. When the device is first started, in order to obtain good crystal seeds, the deposition and crystallization time should be 3 times the residence time of the liquid in the tank in the electrical aging tank 4 when the device is normally operated. In this embodiment, the time is 2.2h during normal operation, so 6.6h is taken here. After reaching the reaction time, pour out the unit contents and send them into the rotary drum centrifuge 3 to separate the magnesium hydroxide crystals and the receiving liquid containing the crystal seeds, and control the centrifuge speed so that the magnesium content in the separated receiving liquid containing the crystal seeds is 0.5wt%. The receiving liquid containing the crystal seeds obtained after separation is used for the device to start, and the magnesium hydroxide crystals are recovered and fully washed with pure water, which can be used as crystal seeds for the next start of the device.

[0043] (2) The old brine is drawn out from the old brine buffer tank 14 and sent to the pretreatment filter 15 for filtration. After filtration, the clarified old brine is input into the bipolar membrane electrodialyzer 12 from one end of the salt chamber of the bipolar membrane electrodialyzer. At the same time, the receiving liquid containing the seed crystals and the pure water from the pure water storage tank 10 are introduced into the alkali chamber of the electrodialyzer, and the pure water from the pure water storage tank 10 is introduced into the acid chamber. Then the electrodialyzer is started, and the operating voltage and feed flow rate are adjusted to control the membrane current density of the bipolar membrane electrodialyzer 12 to 680A / m 2 ~700A / m 2 The feed flow ratio of the old brine entering the salt chamber to the pure water entering the acid chamber is 1.0:3.0, and the salt recovery rate is 94%~94.5%. The amount of pure water entering the alkali chamber is adjusted to control the magnesium content in the magnesium-enriched receiving solution produced by the alkali chamber to be 2.5wt%~2.7wt%.

[0044] (3) The magnesium-enriched receiving solution leaving the bipolar membrane electrodialyzer 12 is extracted, and citric acid from the citric acid storage tank 5 and an ammonia solution of ammonium sulfate as an initiator from the initiator storage tank 6 (wherein the molar ratio of ammonium sulfate to ammonia is 1:1) are added to prepare a mixed solution and sent to a certain unit of the electrical aging tank 4. By controlling the amount of citric acid and initiator added, the citric acid concentration in the mixed solution is controlled to be 0.50 mol / L ~ 0.55 mol / L, and the molar ratio of the nitrogen element of the initiator in the mixed solution to the magnesium content in the magnesium-enriched receiving solution is between 1.80~1.90:1.0. While the mixed solution is introduced, the high-voltage electrode unit of the aging tank is started to apply a voltage of 16kV~16.5kV to the mixed solution, so that the magnesium element in the mixed solution is deposited and crystallized to form a solid magnesium precursor. The deposition and crystallization time is controlled to be 2.2h, and then the turbid solution containing the magnesium precursor formed in the tank is poured out of the aging tank. When the aging tank unit pours out the material, another unit of the electrical aging tank 4 continues to carry out the feeding, deposition and crystallization operations.

[0045] (4) The turbid liquid containing the magnesium precursor poured out of the electrical aging tank 4 is sent to the rotary drum centrifuge 3 for centrifugal separation to separate the magnesium precursor and the receiving liquid containing the seed crystals. By controlling the speed of the centrifuge, the magnesium content in the receiving liquid containing the seed crystals is controlled to be 0.5wt%~0.52wt%. The obtained receiving liquid is stored in the receiving liquid buffer tank 16, and then returned to step (2) for reuse. The separated magnesium precursor is sent to the drying bed, dried at 80°C to remove free water, and the drying time is controlled so that the water content of the magnesium precursor after drying is less than 1wt%. It is then sent to the calcining furnace and calcined at 850°C for 3.5h to produce magnesium oxide.

[0046] (5) The mixed acid solution of sulfuric acid and hydrochloric acid generated in the acid chamber of the bipolar membrane electrodialyzer 12 and the light magnesium brine generated in the salt chamber enter the mixed acid buffer tank 11 and the light magnesium brine buffer tank 10 for temporary storage respectively, and then the mixed acid solution of sulfuric acid and hydrochloric acid is extracted from the mixed acid buffer tank 11 and sent to the first stage nanofiltration device 9. At the same time, pure water from the pure water storage tank 10 is introduced into the water production side of the first stage nanofiltration device 9, and the operating pressure of the first stage nanofiltration device 8 is controlled at 2.5 MPa. ~2.6MPa, the feed flow rate ratio of pure water and mixed acid solution is controlled to be 1.0:1.0, so that the recovery rate of chloride ions in the first stage of nanofiltration water is maintained between 90% and 92%, thereby separating part of the hydrochloric acid on the water production side, and then the concentrated water produced by the first stage of nanofiltration device 9 is mixed with the light magnesium brine extracted from the light magnesium brine buffer tank 10, and sent to the second stage of nanofiltration device 8, and the operating pressure of the second stage of nanofiltration device 8 is controlled between 3.5MPa and 3.6MPa, so that the recovery rate of chloride ions in the second stage of nanofiltration water is maintained between 98% and 99%, and hydrochloric acid is separated again on the water production side, and the hydrochloric acid produced by the two stages of nanofiltration devices is combined and sent out of the device. The concentrated water produced by the second stage of nanofiltration device 8 is sent to the evaporation crystallizer 7, and the evaporation crystallization is magnesium sulfate.

[0047] The specific surface area of ​​the main product magnesium oxide was measured by nitrogen physical adsorption and desorption method, which was 126.9m 2 / g. Its purity is 99.93% as determined by EDTA titration method in accordance with HG / T 2573-2012 "Determination of Industrial Light Magnesium Oxide", which is high-purity magnesium oxide. The purity of the obtained magnesium sulfate is determined by EDTA titration method as described in HG / T 2680-2017 "Industrial Magnesium Sulfate", and its purity is 99.16% as magnesium sulfate heptahydrate. Example 2

[0048] This example uses old brine containing 42wt% magnesium chloride and 3.0wt% magnesium sulfate for testing, and the feed amount for the entire test period is 1.0t. The following steps are followed during implementation:

[0049] (1) Sufficient amount of old brine is added to the brine buffer tank 14 according to the feeding amount of the full test cycle for standby use. When the test process system is started for the first time, the magnesium hydroxide crystals obtained in step (1) of Example 1 are taken, pure water is added to prepare a receiving solution containing seeds with a magnesium content of 0.56wt%, and the solution is stored in a receiving solution storage tank. In order to enhance the conductivity of the receiving solution, citric acid is added to the receiving solution containing seeds in advance when the system is started for the first time, and the concentration of citric acid in the receiving solution containing seeds is controlled to be between 0.60mol / L and 0.62mol / L.

[0050] (2) The old brine is drawn out from the old brine buffer tank 14 and sent to the pretreatment filter 15 for filtration. After filtration, the clarified old brine is formed and is input into the bipolar membrane electrodialyzer 12 from one end of the salt chamber of the bipolar membrane electrodialyzer. At the same time, the receiving liquid containing the seed crystals and the pure water from the pure water storage tank 10 are introduced into the alkali chamber of the electrodialyzer, and the pure water from the pure water storage tank 10 is introduced into the acid chamber. Then the electrodialyzer is started, and the operating voltage and feed flow rate are adjusted to control the membrane current density of the bipolar membrane electrodialyzer 12 to 500A / m 2 ~520A / m 2 The feed flow ratio of the old brine entering the salt chamber to the pure water entering the acid chamber is 1.0:3.7, and the salt recovery rate is 95%~95.5%. The amount of pure water entering the alkali chamber is adjusted to control the magnesium content in the magnesium-enriched receiving solution produced by the alkali chamber to be 2.9wt%~3.1wt%.

[0051] (3) The magnesium-enriched receiving solution leaving the bipolar membrane electrodialyzer 12 is extracted, and citric acid from the citric acid storage tank 5 and urea as an initiator from the initiator storage tank 6 are added to prepare a mixed solution, which is then sent to a unit of the electrical aging tank 4. By controlling the amount of citric acid and initiator added, the citric acid concentration in the mixed solution is controlled to be between 0.60 mol / L and 0.62 mol / L, and the molar ratio of the nitrogen element in the initiator in the mixed solution to the magnesium content in the magnesium-enriched receiving solution is between 2.15 and 2.20:1.0. While the mixed solution is introduced, the high-voltage electrode unit of the aging tank is started to apply a voltage of 19.5 kV to 20 kV to the mixed solution, so that the magnesium element in the mixed solution is deposited and crystallized to form a solid magnesium precursor. The deposition and crystallization time is controlled to be 3.0 hours, and then the turbid solution containing the magnesium precursor formed in the tank is poured out of the aging tank. When the aging tank unit pours out the material, another unit of the electrical aging tank 4 continues to carry out the feeding, deposition and crystallization operations.

[0052] (4) The turbid liquid containing magnesium precursor poured out of the electrical aging tank 4 is sent to the rotary drum centrifuge 3 for centrifugal separation to separate the magnesium precursor and the receiving liquid containing the seed crystals. By controlling the speed of the centrifuge, the magnesium content in the receiving liquid containing the seed crystals is controlled to be 0.57wt%~0.60wt%. The obtained receiving liquid containing the seed crystals is stored in the receiving liquid buffer tank 16, and then returned to step (2) for reuse. The separated magnesium precursor is sent to the drying bed, dried at 90°C to remove free water, and the drying time is controlled so that the water content of the magnesium precursor after drying is less than 1wt%. It is then sent to the calcining furnace and calcined at 900°C for 3.0h to produce magnesium oxide.

[0053] (5) The mixed acid solution of sulfuric acid and hydrochloric acid and the light magnesium brine generated by the acid chamber and the salt chamber of the bipolar membrane electrodialyzer 12 are respectively temporarily stored in the mixed acid buffer tank 11 and the light magnesium brine buffer tank 10, and then the mixed acid solution of sulfuric acid and hydrochloric acid is extracted from the mixed acid buffer tank 11 and sent to the nanofiltration device 9. At the same time, pure water from the pure water storage tank 10 is introduced into the water production side of the nanofiltration device 9, and the operating pressure of the nanofiltration device 8 is controlled at 2.7 MPa. ~2.9MPa, the feed flow rate ratio of pure water and mixed acid solution is controlled to be 1.12:1.0, so that the recovery rate of chloride ions in the first stage nanofiltration water is maintained between 90% and 92%, thereby separating part of the hydrochloric acid on the water production side, and then the concentrated water produced by the first stage nanofiltration device 9 is mixed with the light magnesium brine extracted from the light magnesium brine buffer tank 10, and sent to the second stage nanofiltration device 8, and the operating pressure of the second stage nanofiltration device 8 is controlled between 3.7MPa and 3.8MPa, so that the recovery rate of chloride ions in the second stage nanofiltration water is maintained between 98% and 99%, and hydrochloric acid is separated again on the water production side, and the hydrochloric acid produced by the two nanofiltration devices is combined and sent out of the device. The concentrated water produced by the second stage nanofiltration device 8 is sent to the evaporation crystallizer 7, and the evaporation crystallization is magnesium sulfate.

[0054] The specific surface area and purity of the main product magnesium oxide were measured according to Example 1, and its specific surface area was 136.3 m 2 / g. The purity is 99.96%, which is high-purity magnesium oxide. The purity of the obtained magnesium sulfate is determined by the EDTA titration method described in HG / T 2680-2017 "Industrial Magnesium Sulfate", and its purity is 99.31% based on magnesium sulfate heptahydrate. Example 3

[0055] This example uses old brine containing 45wt% magnesium chloride and 4.0wt% magnesium sulfate for testing, and the feed amount for the entire test period is 1.2t. The following steps are followed during implementation:

[0056] (1) Sufficient amount of old brine is added to the old brine buffer tank 14 according to the feeding amount of the full test cycle for standby use. When the test process system is started for the first time, the magnesium hydroxide crystals obtained in step (1) of Example 1 are taken, pure water is added to prepare a receiving solution containing seeds with a magnesium content of 0.6wt%, and the solution is stored in a receiving solution storage tank. In order to enhance the conductivity of the receiving solution, citric acid is added to the receiving solution containing seeds in advance when the system is started for the first time, and the concentration of citric acid in the receiving solution containing seeds is controlled to be between 0.68mol / L and 0.7mol / L.

[0057] (2) The old brine is drawn out from the old brine buffer tank 14 and sent to the pretreatment filter 15 for filtration. After filtration, the clarified old brine is input into the bipolar membrane electrodialyzer 12 from one end of the salt chamber of the bipolar membrane electrodialyzer. At the same time, the receiving liquid containing the seed crystals and the pure water from the pure water storage tank 10 are introduced into the alkali chamber of the electrodialyzer, and the pure water from the pure water storage tank 10 is introduced into the acid chamber. Then the electrodialyzer is started, and the operating voltage and feed flow rate are adjusted to control the membrane current density of the bipolar membrane electrodialyzer 12 to 770A / m 2 ~800A / m 2 The feed flow ratio of the old brine entering the salt chamber to the pure water entering the acid chamber is 1.0:4.0, and the salt recovery rate is 95.7%~96%. The amount of pure water entering the alkali chamber is adjusted to control the magnesium content in the magnesium-enriched receiving solution produced by the alkali chamber to be 3.3wt%~3.5wt%.

[0058] (3) The magnesium-enriched receiving solution leaving the bipolar membrane electrodialyzer 12 is extracted, and citric acid from the citric acid storage tank 5 and ammonia water as an initiator from the initiator storage tank 6 are added to prepare a mixed solution, and then sent to a certain unit of the electrical aging tank 4. By controlling the amount of citric acid and initiator added, the citric acid concentration in the mixed solution is controlled to be between 0.68 mol / L and 0.7 mol / L, and the molar ratio of the nitrogen element of the initiator in the mixed solution to the magnesium content in the magnesium-enriched receiving solution is between 2.05 and 2.10:1.0. While the mixed solution is introduced, the high-voltage electrode unit of the aging tank is started to apply a voltage of 18 kV to 18.5 kV to the mixed solution, so that the magnesium element in the mixed solution is deposited and crystallized to form a solid magnesium precursor. The deposition and crystallization time is controlled to be 2.5 hours, and then the turbid solution containing the magnesium precursor formed in the tank is poured out of the aging tank. When the aging tank unit pours out the material, another unit of the electrical aging tank 4 continues to carry out the feeding, deposition and crystallization operations.

[0059] (4) The turbid liquid containing magnesium precursor poured out of the electrical aging tank 4 is sent to the rotary drum centrifuge 3 for centrifugal separation to separate the magnesium precursor and the receiving liquid containing the seed crystals. By controlling the speed of the centrifuge, the magnesium content in the receiving liquid containing the seed crystals is controlled to be 0.53wt%~0.55wt%. The obtained receiving liquid containing the seed crystals is stored in the receiving liquid buffer tank 16, and then returned to step (2) for reuse. The separated magnesium precursor is sent to the drying bed, dried at 85°C to remove free water, and the drying time is controlled so that the water content of the magnesium precursor after drying is less than 1wt%. It is then sent to the calcining furnace and calcined at 950°C for 3.3h to produce magnesium oxide.

[0060] (5) The mixed acid solution of sulfuric acid and hydrochloric acid and the light magnesium brine generated by the acid chamber and the salt chamber of the bipolar membrane electrodialyzer 12 are respectively temporarily stored in the mixed acid buffer tank 11 and the light magnesium brine buffer tank 10, and then the mixed acid solution of sulfuric acid and hydrochloric acid is extracted from the mixed acid buffer tank 11 and sent to the first stage nanofiltration device 9. At the same time, pure water from the pure water storage tank 10 is introduced into the water production side of the first stage nanofiltration device 9, and the operating pressure of the first stage nanofiltration device 8 is controlled at 2.9 MPa. ~3.0MPa, the feed flow rate ratio of pure water and mixed acid solution is controlled to be 1.2:1.0, so that the recovery rate of chloride ions in the first stage of nanofiltration water is maintained between 90% and 92%, thereby separating part of the hydrochloric acid on the water production side, and then the concentrated water produced by the first stage of nanofiltration device 9 is mixed with the light magnesium brine extracted from the light magnesium brine buffer tank 10, and sent to the second stage of nanofiltration device 8, and the operating pressure of the second stage of nanofiltration device 8 is controlled between 3.9MPa and 4.0MPa, so that the recovery rate of chloride ions in the second stage of nanofiltration water is maintained between 98% and 99%, and hydrochloric acid is separated again on the water production side, and the hydrochloric acid produced by the two nanofiltration devices is combined and sent out of the device. The concentrated water produced by the second stage of nanofiltration device 8 is sent to the evaporation crystallizer 7, and the evaporation crystallization is magnesium sulfate.

[0061] The specific surface area and purity of the main product magnesium oxide were measured according to Example 1, and its specific surface area was 117.8 m 2 / g. The purity is 99.91%, which is high-purity magnesium oxide. The purity of the obtained magnesium sulfate is determined by the EDTA titration method described in HG / T 2680-2017 "Industrial Magnesium Sulfate", and its purity is 99.19% based on magnesium sulfate heptahydrate. Comparative Example 1

[0062] The comparative example uses PVC (commercially available product, injection molding grade, thermal conductivity 0.17 W / m·K) as the base material, and the magnesium oxide products obtained in Examples 1, 2 and 3 and the nano-spherical magnesium oxide (particle size 500 nm, specific surface area 69.2 m 2 / g, purity ≥99.9%) was mixed as filler to investigate the change in thermal conductivity of PVC after adding filler.

[0063] The implementation method is to put the PVC pellets into a twin-screw mixer and melt them at 180°C, add a magnesium oxide sample at a mass fraction of 50%, fully mix and extrude it into a composite PVC material, and then prepare the material into a 2mm thick sheet. The sheet is placed in a flat vulcanizer, molded at 170°C and 15MPa for 10min, and then naturally cooled to room temperature and the sample is taken out and cut into a disc sample with a diameter of 12mm. Referring to GB / T42919-2023 "Determination of Thermal Conductivity and Thermal Diffusivity of Plastics", the thermal conductivity of the sample is measured using the laser flash method, and the results are shown in the table below.

[0064]

[0065] Figure 2 The SEM image of the magnesium oxide sample of Example 1 shows that Example 1 is mainly spherical with flaky structure, with high sphericity, mixed with a small amount of debris crystals, and its particle size distribution is about 510nm~390nm, which is submicron magnesium oxide. The flaky structure forms a large number of grooves on the surface of the magnesium oxide ball, which is conducive to the improvement of specific surface area.

[0066] Figure 3 This is the XRD diagram of the magnesium oxide sample of Example 1. The main diffraction peaks are located at 36.947°, 42.909°, 62.306°, 74.679° and 78.613°, which corresponds well to the magnesium oxide PDF card and can be confirmed as magnesium oxide. There are few miscellaneous peaks and amorphous waves in the XRD diagram, and the peak shape is sharp, indicating that Example 1 has high purity and crystallinity, good crystal consistency, and high thermal conductivity potential.

[0067] Compared with the comparative example, the magnesium oxide product prepared by the embodiment of the present invention has a higher specific surface area. The higher specific surface area can provide a larger material contact surface, which is beneficial to material filling, thereby further enhancing heat transfer. After being added to PVC at a mass fraction of 50wt%, the thermal conductivity coefficients of the PVC samples of Examples 1, 2 and 3 are 129%, 133% and 126% of the PVC sample of the comparative example, respectively, indicating that the magnesium oxide prepared by the method of the present invention as a polymer filler has a better thermal conductivity enhancement ability than similar products on the market.

Claims

1. A method for comprehensive utilization of magnesium resources in potassium sulfate brine, characterized in that: The steps include: (1) Using a bipolar membrane electrodialyzer, firstly, the clarified old brine is sent to the salt chamber of the bipolar membrane electrodialyzer, and pure water and a receiving solution containing seed crystals are simultaneously introduced into the alkali chamber, and pure water is introduced into the acid chamber. After bipolar membrane electrodialysis, a light magnesium brine is obtained from the salt chamber, a mixed acid solution of sulfuric acid and hydrochloric acid is obtained from the acid chamber, and a receiving solution enriched with magnesium is obtained from the alkali chamber; (2) adding citric acid and an initiator to the magnesium-enriched receiving solution obtained in step (1) to form a mixed solution, applying voltage to the mixed solution to perform deposition and crystallization to obtain a turbid solution containing a magnesium precursor; (3) centrifugally separating the turbid liquid containing the magnesium precursor obtained in step (2) to obtain the magnesium precursor and the receiving liquid containing the seed crystals, respectively, and returning the receiving liquid containing the seed crystals to step (1); (4) drying and calcining the magnesium precursor obtained in step (3) to obtain magnesium oxide; (5) subjecting the mixed acid solution of sulfuric acid and hydrochloric acid obtained in step (1) to a nanofiltration separation to obtain hydrochloric acid and sulfuric acid, respectively; (6) adding the sulfuric acid obtained in step (5) to the light magnesium brine obtained in step (1), and performing two-stage nanofiltration separation to obtain hydrochloric acid and magnesium sulfate solution, respectively; (7) evaporating and drying the magnesium sulfate solution obtained in step (6) to obtain magnesium sulfate; In the step (2), the voltage applied to the mixed solution is 16 kV to 20 kV, and the deposition and crystallization time is 2.0 h to 3.0 h.

2. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: In the step (1), the old brine contains 40wt%~45wt% of magnesium chloride and 3.0wt%~4.0wt% of magnesium sulfate; the membrane current density of the bipolar membrane electrodialyzer is 500A / m 2 ~800A / m 2 , the salt recovery rate is 94%~96%.

3. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: The receiving solution containing seed crystals in step (1) is prepared from citric acid, magnesium hydroxide and pure water, wherein the magnesium content is 0.5wt% to 0.6wt%; the amount of pure water entering the alkali chamber is adjusted to control the magnesium content in the receiving solution enriched with magnesium to be 2.5wt% to 3.5wt%; The feed flow ratio of old brine into the salt chamber and pure water into the acid chamber is 1.0:3.0~4.

0.

4. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: The concentration of citric acid in the mixed solution in step (2) is 0.5-0.7 mol / L.

5. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: In the step (2), the initiator is one of aqueous ammonia, urea, and aqueous ammonia solution of ammonium sulfate; the molar ratio of the nitrogen element in the initiator in the mixed solution to the magnesium content in the receiving solution enriched with magnesium element is 1.8-2.2:1.

0.

6. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: The magnesium content in the receiving solution containing the seed crystals in step (3) is 0.5wt% to 0.6wt%.

7. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: In the step (4), the drying temperature is 80°C-90°C, and the water content of the magnesium precursor after drying is less than 1wt%; the calcination temperature is 850°C-950°C, and the calcination time is 3.0h-3.5h.

8. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: The operating pressure of the first stage nanofiltration in step (5) is 2.5 MPa to 3.0 MPa, and the recovery rate of chloride ions in the first stage nanofiltration produced water is controlled to be 90% to 92%.

9. The method for comprehensive utilization of magnesium resources in potassium sulfate brine according to claim 1, characterized in that: The operating pressure of the second stage nanofiltration in step (6) is 3.5 MPa to 4.0 MPa, and the chloride ion recovery rate in the second stage nanofiltration produced water is controlled to be 98% to 99%.

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