Process for preparing magnesium carbonate trihydrate whiskers from sea brine

Abstract

The invention discloses a process for preparing magnesium carbonate trihydrate whiskers from sea brine. The process comprises the following steps: carrying out nanofiltration on seawater subjected to decalcification and carbon sequestration, introducing carbon dioxide, carrying out bipolar membrane electrodialysis, carrying out a mineralization crystallization process, filtering a product and recycling mother liquor, fresh seawater obtained after the nanofiltration step is a raw material solution of a bipolar membrane electrodialysis salt chamber, alkali liquor obtained after bipolar membrane electrodialysis reacts with concentrated seawater obtained after the nanofiltration step and bischofite for crystallization, and mother liquor obtained after filtration in the crystallization process is mixed with seawater obtained after decalcification and carbon sequestration for nanofiltration again. According to the process for preparing the magnesium carbonate trihydrate whiskers from the sea brine, provided by the invention, fine magnesium chemical products with high added values are prepared while mineralization and fixation of carbon dioxide are realized, and cyclic utilization of system resources is facilitated.

Description

Technical Field

[0001] This disclosure generally relates to the field of environmental protection technology, and specifically to the process of preparing magnesium carbonate trihydrate whiskers from sea brine. Background Technology

[0002] Magnesium is hailed as a "green engineering material of the 21st century" and is currently the lightest metallic material that can be directly used as a structural material. The world has relatively abundant magnesium resources, existing in both liquid and solid forms. Solid magnesium includes dolomite, magnesite, brucite, carnallite, etc.; liquid magnesium is mainly found in seawater and salt lake brines, existing as free magnesium ions. Seawater magnesium reserves alone can reach 2 × 10⁻⁶. 7 Hundreds of millions of tons. Currently, magnesium resources recovered from seawater mainly include magnesium hydroxide, magnesium sulfate, and magnesium chloride, with relatively low added value. Similarly, the main forms of magnesium resource development and utilization in brine are magnesium chloride, magnesium hydroxide, and magnesium oxide. Furthermore, with the development of the potash fertilizer industry around salt lakes, a large amount of high-magnesium old brine is generated annually and is discharged back into the salt lakes, causing serious resource waste and severe impact on the ecological environment. The aforementioned high-magnesium old brine can precipitate magnesium chloride hydrate through natural evaporation in salt fields, with a magnesium chloride hexahydrate content exceeding 96%. Simultaneously, magnesium chloride is an important industrial raw material, which can be used to produce high-purity magnesia, magnesium salt whiskers, and other magnesium-based products with high added value. Electrodialysis (BMED) produces acids and alkalis without the need for any additional chemical reagents; water dissociation is achieved solely under the influence of an electric field within a bipolar membrane, making it environmentally friendly. Therefore, utilizing the flexible acid-base adjustment of BMED to extract high-value-added magnesium resources from the trihydrate magnesium carbonate crystallization process is of great significance for the efficient development of magnesium resources in marine brine. Summary of the Invention

[0003] In view of the above-mentioned defects or deficiencies in the existing technology, it is desirable to provide a high-efficiency, energy-saving and environmentally friendly process for preparing magnesium carbonate trihydrate whiskers from sea brine.

[0004] Firstly, the process for preparing magnesium carbonate trihydrate whiskers from sea brine includes nanofiltration of seawater after decalcification and carbon fixation, carbon dioxide introduction, bipolar membrane electrodialysis, mineralization crystallization process, product filtration, and mother liquor recycling. The fresh seawater that comes out after the nanofiltration step is the raw material liquid for the bipolar membrane electrodialysis salt chamber. The alkaline solution from the bipolar membrane electrodialysis reacts with the concentrated seawater and magnesium chloride hydrate after the nanofiltration step to crystallize. The mother liquor filtered by the crystallization process is mixed with the decalcified and carbon-fixed seawater and then nanofiltration is performed again.

[0005] According to the technical solution provided in the embodiments of this application, the nanofiltration operation unit uses a laboratory-scale membrane assembly to conduct experiments. Seawater after decalcification and carbon fixation is pumped to the nanofiltration membrane inlet via a high-pressure pump. The pressure before and after the nanofiltration membrane is controlled by adjusting the concentrate valve and the frequency converter. After the pressure and flow rate of the nanofiltration system stabilize, concentrate and desalination samples are taken and their ionic composition is analyzed.

[0006] According to the technical solution provided in the embodiments of this application, the bipolar membrane electrodialysis experiment uses the continuously fed-out apparatus shown. The nanofiltration-produced desalinated water is fed into the salt chamber solution tank at a feed rate of 10 mL / min. The initial solution in the acid chamber is a 0.5 mol / L HCl solution, and the initial solution in the alkali chamber is a 0.4 mol / L Na₂CO₃ solution. The rate of water replenishment to the acid and alkali chambers is 5 mL / min. Simultaneously, CO₂ gas is introduced into the alkali chamber solution via a glass rotor flowmeter, and the pH value of the alkali chamber solution is controlled by adjusting the current density and CO₂ gas flow rate.

[0007] According to the technical solution provided in the embodiments of this application, the reaction crystallization process involves an online crystallization device. First, a 0.7 mol / L MgCl2 solution is prepared. A certain volume of the alkaline solution is placed in the crystallizer. After the temperature and rotation speed reach preset values, different CO3... 2- and Mg 2+ Different volumes of 0.7 mol / L MgCl₂ solution were added to the crystallizer at a rate of 5 mL / min to maintain a total reaction volume of 600 mL. During the experiment, the chord length and image of the crystal particles were monitored online using FBRM and PVM. After the reaction was completed, the experiment was quickly stopped, filtration was performed, and samples were taken for analysis of the Mg in the mother liquor. 2+ CO3 2- and HCO3 - Ion concentration; solid products were dried in a 40°C forced-air oven for 24 hours.

[0008] Secondly, the process for preparing magnesium carbonate trihydrate whiskers from sea brine includes the following steps:

[0009] S1: After decalcification and carbon fixation, seawater is subjected to nanofiltration to separate monovalent and divalent ions, resulting in concentrated seawater and fresh seawater.

[0010] S2: The fresh seawater after nanofiltration in S1 is fed into a bipolar membrane electrodialysis operation, and carbon dioxide gas is introduced to prepare an alkaline solution.

[0011] S3: The alkaline solution prepared in S2 is reacted with the concentrated seawater after nanofiltration in S1 and magnesium chloride in water to produce magnesium carbonate trihydrate product through a crystallization reaction.

[0012] S4: The solution prepared in S3 is filtered, and the resulting magnesium carbonate trihydrate is washed and dried.

[0013] S5: The mother liquor obtained after S4 filtration is recovered and mixed with decalcified nanofiltration seawater for nanofiltration to achieve resource recycling.

[0014] In summary, this application discloses a process for preparing magnesium carbonate trihydrate whiskers from sea brine. Based on the specific technical solutions described above, the process of this application is geared towards the comprehensive utilization of sodium and magnesium resources in seawater after decalcification and carbon fixation. It combines with seawater "blue carbon" and proposes a process for preparing magnesium carbonate trihydrate whiskers from sea brine based on nanofiltration-bipolar membrane electrodialysis-reaction crystallization, thus realizing the high-value application of magnesium resources in sea brine.

[0015] This application proposes a nanofiltration operation to separate NaCl and MgCl2 from seawater, and investigates the effects of stable operating time, operating pressure, and feed flow rate. The freshwater produced by the nanofiltration process is used as feedstock in the brine chamber and fed into a bipolar membrane electrodialysis system to prepare an alkaline solution. Based on the unit operations of nanofiltration, bipolar membrane electrodialysis, and reaction crystallization, this application presents a nanofiltration-bipolar membrane electrodialysis-reaction crystallization process. After ten closed-loop cycle experiments, the composition of the produced material is basically stable, and the system reaches equilibrium.

[0016] This application's recycling process achieves efficient utilization of seawater desalination and resource recovery through a combination of nanofiltration, bipolar membrane electrodialysis, and reactive crystallization. The desalinated seawater after nanofiltration undergoes bipolar membrane electrodialysis, simultaneously producing acids and alkalis, which can be used as acid-base regulators in other processes. Pre-treatment of seawater with nanofiltration technology increases the operating pressure and recovery rate of subsequent reverse osmosis (SWRO), effectively removes hardness ions from seawater, reduces scaling problems in the SWRO process, and ensures the safe and long-term stable operation of the membrane modules. Compared with thermal desalination technology, bipolar membrane electrodialysis has lower energy consumption. Throughout the process, the salt content of the electrodialysis concentrate is approximately 2%, and the treatment of this concentrate is crucial for the resource recovery of the entire process. The mother liquor from crystallization separation is returned to the nanofiltration step, achieving recycling, reducing waste generation, and meeting the requirements of green economy and circular production. The combination of nanofiltration and bipolar membrane electrodialysis improves the efficiency of the entire desalination system, achieving higher freshwater production and lower energy consumption. Compared to distillation, bipolar membrane electrodialysis offers advantages such as simple operation and ease of control. This process, which combines nanofiltration, bipolar membrane electrodialysis, and reactive crystallization, not only improves water resource utilization efficiency but also enables the recovery and reuse of chemicals, resulting in significant economic and environmental benefits. Attached Figure Description

[0017] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0018] Figure 1The diagram shown is a process flow chart for preparing magnesium carbonate trihydrate whiskers from marine brine according to the present invention. Detailed Implementation

[0019] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] Please refer to Figure 1 The diagram shown is a process flow chart for preparing magnesium carbonate trihydrate whiskers from sea brine provided in this application.

[0022] Figure 1 The process for preparing magnesium carbonate trihydrate whiskers from sea brine is characterized by including nanofiltration of seawater after decalcification and carbon fixation, carbon dioxide introduction, bipolar membrane electrodialysis, mineralization crystallization process, product filtration, and mother liquor recycling.

[0023] Figure 1 Seawater after decalcification and carbon fixation enters the nanofiltration unit. The decalcified and carbon-fixed seawater is pumped to the nanofiltration membrane inlet via a high-pressure pump. The pressure before and after the nanofiltration membrane is regulated by adjusting the concentrate valve and a variable frequency pump. After the pressure and flow rate of the nanofiltration system stabilize, concentrate and desalination samples are taken and their ionic composition is analyzed. Monovalent and divalent ions are separated by nanofiltration. Under operating pressure of 2 MPa and feed flow rate of 300 L / h, the effect of nanofiltration membrane operating time on permeate water quality was investigated. Under feed flow rate of 420 L / h, the effect of operating pressure variation on the separation of decalcified and carbon-fixed seawater was investigated. Under operating pressure of 2.8 MPa, the treatment energy of different feed flow rates (180, 240, 300, 360, 420, and 480 L / h) on the decalcified and carbon-fixed seawater was explored.

[0024] Figure 1After nanofiltration, seawater is separated into concentrated water and fresh water. The fresh water serves as the feed solution for the salt chamber of the bipolar membrane electrodialysis system. The fresh water produced by nanofiltration is fed into the salt chamber solution tank at a feed rate of 10 mL / min. The initial solution for the acid chamber is a 0.5 mol / L HCl solution, and the initial solution for the alkali chamber is a 0.4 mol / L Na₂CO₃ solution. The feed rate to the acid and alkali chambers is 5 mL / min. The electrode chamber solution is a 0.2 mol / L sodium sulfate solution. The bipolar membrane electrodialysis device comprises four chambers: an acid chamber, an alkali chamber, a salt chamber, and an electrode chamber. The bipolar membrane electrodialysis membrane stack consists of a positive electrode, a negative electrode, and at least one set of compartment electrodialysis units disposed between the positive and negative electrodes. Each compartment electrodialysis unit consists of a bipolar membrane, an anion membrane, a cation membrane, and another bipolar membrane. When the power switch is turned on, ions migrate under the influence of an electric field, producing corresponding solutions in the acid, alkali, and salt chambers, respectively. Meanwhile, CO2 gas is introduced into the alkali chamber solution at a rate of 20 ml / min via a glass rotor flow meter, and the pH value of the alkali chamber solution is controlled by adjusting the current density and the CO2 gas flow rate.

[0025] Figure 1 A portion of the alkaline solution produced after the bipolar membrane electrodialysis unit is fed into the crystallizer to prepare a 0.7 mol / L MgCl2 solution. The prepared magnesium chloride solution is then mixed with the concentrated water from nanofiltration. Once the temperature and rotation speed in the crystallizer reach preset values, different CO3... 2- and Mg 2+ Different volumes of magnesium-containing solutions were added to the crystallizer at a rate of 5 mL / min to maintain a total reaction volume of 600 mL, and the reaction time was 2 h. After the reaction was complete, the experiment was immediately stopped, filtration was performed, and samples were taken for analysis of the Mg content in the mother liquor. 2+ CO3 2- and HCO3 - Ion concentration; solid magnesium carbonate trihydrate product was dried in a 40℃ forced-air oven for 24 hours.

[0026] Figure 1 After solid-liquid separation, the solution in the crystallizer is found to contain sodium chloride. The mother liquor is then mixed with decalcified and carbon-fixed seawater and fed into the nanofiltration unit, realizing a cyclical process of nanofiltration-bipolar membrane electrodialysis-reactive crystallization, thus achieving resource recycling.

[0027] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with, but not limited to, technical features disclosed in this application that have similar functions.

Claims

1. A process for the preparation of magnesium carbonate trihydrate whiskers from sea brine, characterized in that, The process comprises the steps of nanofiltration of seawater after decalcification and carbon fixation, carbon dioxide injection, bipolar membrane electrodialysis, mineralization and crystallization, product filtration and mother liquor recycling. The dilute seawater after the nanofiltration step is used as the raw material liquid for the salt chamber of the bipolar membrane electrodialysis, the alkali solution produced by the bipolar membrane electrodialysis is reacted with the concentrated seawater after the nanofiltration step and bischofite to crystallize, and the mother liquor after the crystallization process is mixed with the seawater after decalcification and carbon fixation and then subjected to nanofiltration again.

2. The process for preparing magnesium carbonate trihydrate whiskers from sea brine according to claim 1, characterized in that: The seawater after decalcification and carbon fixation is pumped to the nanofiltration membrane inlet by a high-pressure pump, the pressure before and after the nanofiltration membrane is adjusted by adjusting the concentrated water valve and the frequency conversion pump, and after the pressure and flow of the nanofiltration system are stabilized, the concentrated water and dilute water samples are taken and analyzed for ion composition, and the single-valent and divalent ions are separated by nanofiltration operation.

3. The process for preparing magnesium carbonate trihydrate whiskers from sea brine according to claim 2, characterized in that: The dilute seawater after the nanofiltration step is used as the raw material liquid for the salt chamber of the bipolar membrane electrodialysis, the alkali chamber raw material liquid for the bipolar membrane electrodialysis is a 0.4 mol / L sodium carbonate solution, the acid chamber raw material liquid is a 0.5 mol / L hydrochloric acid solution, and the electrode chamber solution is a 0.2 mol / L sodium sulfate solution; The bipolar membrane electrodialysis adopts continuous feeding and output operation, the dilute water produced by nanofiltration is fed into the salt chamber solution tank at a feeding rate of 10 mL / min, and the water feeding rate to the acid chamber and the alkali chamber is 5 mL / min; The bipolar membrane electrodialysis device comprises four chambers: an acid chamber, an alkali chamber, a salt chamber and an electrode chamber, the bipolar membrane electrodialysis membrane stack is composed of a positive electrode, a negative electrode and at least one set of compartment electrodialysis units arranged between the positive electrode and the negative electrode, and the compartment electrodialysis unit is composed of a bipolar membrane, a negative membrane, a positive membrane and a bipolar membrane.

4. The process for preparing magnesium carbonate trihydrate whiskers from sea brine according to claim 3, characterized in that: The bipolar membrane electrodialysis generates ion migration under the action of an electric field when the power switch is turned on, and corresponding solutions are generated in the acid chamber, the alkali chamber and the salt chamber, wherein the alkali chamber solution produced by the bipolar membrane electrodialysis is used as the raw material liquid for the crystallization unit; A 0.7 mol / L MgCl2 solution was prepared. A certain volume of the base chamber solution was placed in the crystallizer, and after the temperature and rotation speed reached the preset values, different volumes of the 0.7 mol / L MgCl2 solution were added to the crystallizer at a speed of 5 mL / min, so that the total volume of the reaction solution remained 600 mL. 2- and Mg 2+ The molar ratio of the 0.7 mol / L MgCl2 solution was added to the crystallizer at a speed of 5 mL / min, so that the total volume of the reaction solution remained 600 mL. The alkali chamber solution is a sodium carbonate solution which is reacted with the concentrated seawater after nanofiltration and bischofite to prepare magnesium carbonate trihydrate whiskers.

5. The process for preparing magnesium carbonate trihydrate whiskers from sea brine according to claim 4, characterized in that: The solution after crystallization is subjected to suction filtration to separate the solid and the liquid, the solid after suction filtration is magnesium carbonate trihydrate whiskers, and the liquid after suction filtration is a sodium chloride solution, the sodium chloride solution is mixed with the seawater before decalcification and nanofiltration according to claim 2 to perform nanofiltration, and resource recycling operation is realized.

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