Method for preparing nano hexagonal lamellar magnesium hydroxide from seawater bittern

Abstract

The invention relates to a method for preparing nano hexagonal lamellar magnesium hydroxide from seawater bittern. The method comprises the following steps: (1) adding a magnesium hydroxide seed crystal into bittern in a reactor, adding a sodium hydroxide solid, and stirring for reaction to obtain a magnesium hydroxide precursor; (2) adding a magnesium hydroxide precursor into an aqueous solution of dimethylimidazole, stirring, putting into a reaction kettle with a polytetrafluoroethylene lining, and stirring and reacting at 130-160 DEG C for 2-8 hours to obtain slurry; and filtering, washing and drying to obtain the hexagonal flaky magnesium hydroxide crystal. The nano hexagonal flaky magnesium hydroxide is prepared through hydrothermal treatment under the condition of high solid-to-liquid ratio, so that the production efficiency is improved, and the production cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of comprehensive utilization of liquid waste, and specifically relates to a method for preparing a high-value-added magnesium hydroxide flame retardant by adopting a seawater brine-sodium hydroxide precipitation hydrothermal two-step method. Background Art

[0002] my country ranks first in the world in sea salt production, with an annual output of about 23 million tons, accounting for 40% of the country's salt production. The byproduct mother liquor after sea salting is called sea bittern, also known as bittern. Its main chemical composition and content are shown in Table 1 (Mohamed A, Maraqa M, Handhaly J. Impact of land disposal of reject brine from desalination plants on soil and groundwater [J]. Desalination, 2005, 182 (1-3): 411-433.). Brine is rich in chemical resources such as magnesium, sodium and potassium, and has extremely high extraction and utilization value. Sea bittern is Mg 2+ ,Na + ,K + / / SO 4 2- ,Cl - ,Br - -H 2 O complex water-salt system, in which many salts have similar solubility and are easy to form magnesium-containing complex salts, and the bromide content is low, resulting in technical and economic bottlenecks in the efficient separation and extraction of inorganic products such as potassium, magnesium, sodium salts and bromine from bittern. At present, my country's annual bittern production is more than 20 million m 3 , but the comprehensive utilization rate is less than 20%. The large amount of bittern discharged not only causes a huge waste of resources, but also seriously pollutes the nearshore marine environment. The main production method of bittern chemical industry is the brine mixing process, which uses evaporation crystallization salt separation and air blowing method to extract bromine. The products are primary chemical products such as magnesium chloride, potassium chloride, sodium chloride, sodium sulfate and bromine. There are major defects such as low recovery rate of valuable resources, large steam consumption in the potassium chloride production process, large energy consumption and equipment investment in air blowing method for bromine extraction and limited bromine yield, low value of industrial magnesium chloride products and lack of market.

[0003] Table 1 Main components of bittern

[0004]

[0005] In order to solve the current difficulties in bittern chemical production, the present invention starts from the efficient utilization of bittern magnesium salts and the complete demagnesization of bittern to optimize and improve the separation process of sodium, potassium and bromine resources, and invents a method for completely removing magnesium by solid phase precipitation of sodium hydroxide from seawater bittern-hydrothermal two-step precipitation reaction to prepare nano-hexagonal lamellar magnesium hydroxide crystals. At present, the preparation and application of magnesium hydroxide flame retardants have been widely studied at home and abroad, and magnesium hydroxide with various microscopic morphologies such as hexagonal lamellar, cubic, needle-shaped, lamellar, and spherical shapes has been prepared. Among them, the surface polarity and surface energy of magnesium hydroxide crystals with hexagonal lamellar morphology are low, the compatibility with polymers is good, and the flame retardant and mechanical properties are excellent, which is the focus of current research. For example, Yang et al. (Yang Q, Gao X, Fang L, et al. Controllable crystal growth of Mg(OH) 2hexagonal flakes and their surface modification using graft polymerization [J]. Advanced Powder Technology, 2021, 32 (7): 2634-2644.) Hexagonal flake magnesium hydroxide was synthesized by hydrothermal method using magnesium chloride solution as raw material, ammonia water as precipitant and polyethylene glycol as modifier. Chen et al. prepared hexagonal flake nano-magnesium hydroxide by adding monoethanolamine, diethanolamine or triethanolamine to affect the hydrothermal reaction of magnesium chloride, citric acid and ammonia water during the ammonia hydrothermal method. Wu Jiansong (Wu Jiansong, Huang Xueqing, Wu Siyun, et al. Preparation of flame-retardant magnesium hydroxide from bittern of Xuwen salt field [J]. Inorganic Salt Industry, 2016, 48 (3): 56-58.) et al. used bittern of Xuwen salt field as raw material, first added sodium carbonate-sodium bicarbonate buffer solution into the system, then used 0.40 mol / L NaOH solution as precipitant, and prepared magnesium hydroxide flame retardant with high purity and high crystallinity under the conditions of reaction temperature of 75°C and crystallization time of 24h. At present, the preparation of hexagonal flake magnesium hydroxide products is mainly based on low-concentration magnesium chloride solution and low-concentration alkaline substances as raw materials, and is obtained by reaction under the action of crystal control agent. In the preparation of nano hexagonal magnesium hydroxide from a high-concentration complex salt solution of bittern, Sun Baishuo et al. (Sun B, Li Y, Guo H, et al. Fast and complete recovery of magnesium from sea bittern to synthesize magnesium hydroxide hexagonal nanosheet for enhanced flame retardancy and mechanical properties of epoxy resin [J]. Desalination, 2024, 583: 117716.) used sodium hydroxide powder as a precipitant, added a small amount of magnesium hydroxide seeds to saturated bittern, first reacted to obtain a magnesium hydroxide intermediate with a mesh structure that is easy to filter, and then separated it and added it to an aqueous solution containing polyethylene glycol to prepare nano hexagonal magnesium hydroxide through a hydrothermal reaction. Although this method solves the problem of difficult separation of magnesium hydroxide precipitation from sodium hydroxide and saturated bittern, and conversion of magnesium hydroxide precipitation intermediates to prepare nano hexagonal magnesium hydroxide, when the magnesium hydroxide precipitation intermediate is hydrothermally converted to prepare nano hexagonal magnesium hydroxide, the solid-liquid ratio is small, the content of magnesium hydroxide in the system is only 1% (wt), the production efficiency is low, and the cost is high.

[0006] In order to improve the efficiency of converting magnesium hydroxide precipitation intermediates to prepare nano hexagonal flaky magnesium hydroxide, reduce production costs, and improve the morphology of nano hexagonal flaky magnesium hydroxide, this patent application invents a method for preparing nano hexagonal flaky magnesium hydroxide by hydrothermal conversion of an ionic liquid crystal form controller. Summary of the invention

[0007] The purpose of the present invention is to solve the problems of low liquid-solid ratio, low magnesium hydroxide content in the system, high production cost, etc. when preparing nano hexagonal flaky magnesium hydroxide by solid phase precipitation and hydrothermal conversion of high concentration solution of seawater bittern, and propose a method for preparing magnesium hydroxide intermediate by completely solid phase precipitation of bittern magnesium salt with sodium hydroxide, and preparing nano hexagonal flaky magnesium hydroxide by hydrothermal conversion reaction of magnesium hydroxide intermediate in the presence of specific crystal form control agent ionic liquid dimethylimidazole. The method uses sodium hydroxide particles as a precipitant and magnesium hydroxide seeds as a crystal inducing agent, firstly precipitates all magnesium ions in bittern to generate a magnesium hydroxide intermediate that is easy to filter; then the prepared magnesium hydroxide intermediate uses an ionic liquid as a crystal form control agent, and is hydrothermally treated under high solid-liquid ratio to prepare nano hexagonal flaky magnesium hydroxide, thereby improving production efficiency and reducing production cost.

[0008] The technical solution provided by the present invention is as follows:

[0009] (1) adding magnesium hydroxide seed crystals to bittern in a reactor and stirring to obtain a mixture A, then adding sodium hydroxide solid thereto and stirring to react, after reacting for 10-30 minutes, filtering the slurry to obtain a filtrate and a filter cake, washing the filter cake with deionized water to obtain a magnesium hydroxide precursor;

[0010] Wherein, the bittern composition includes MgCl 2 :150~180g·L -1 ; NaCl: 60~110g·L -1 ; KCl: 18~22g·L -1 MgSO 4 :60~75g·L -1 ; KBr: 8~12g·L -1 ;

[0011] Wherein, in the step (1), the mass ratio of magnesium ions in bittern:seed crystals:sodium hydroxide = 1:(0.024-0.072):3.29;

[0012] The mixed solution is rotated at 300-500 rpm in an open container;

[0013] The magnesium hydroxide seed crystals are in the shape of hexagonal nanosheets, with an average particle size of about 40 to 60 nm and a purity of more than 99%;

[0014] The stirring time after the seed crystals are added is 10 to 30 minutes;

[0015] (2) adding the magnesium hydroxide precursor obtained in step (1) to an aqueous solution of dimethylimidazole, stirring, and placing in a reaction kettle with a polytetrafluoroethylene liner, stirring and reacting at 130 to 160° C. for 2 to 8 hours to obtain a slurry; filtering, washing, and drying to obtain hexagonal magnesium hydroxide crystals;

[0016] Wherein, the mass ratio in step (2) is dimethylimidazole: magnesium hydroxide intermediate: water = (0.01-0.03): 0.1: (1-10);

[0017] The drying in step (2) is carried out at 60 to 100° C. for 8 to 10 hours.

[0018] The beneficial effects of the present invention are:

[0019] The present invention uses seawater bittern as raw material, solid sodium hydroxide as precipitant, and magnesium hydroxide seed crystals to prepare magnesium hydroxide nano hexagonal sheets through a two-step precipitation-hydrothermal reaction method. The complete removal and transformation of bittern magnesium ions and the rapid sedimentation and separation of intermediate magnesium hydroxide are achieved, and other impurities are not introduced, and the concentration of bittern solution is not diluted, which greatly reduces the difficulty of separating resources such as potassium, sodium, and bromine in the de-magnesiumized bittern. Dimethyl imidazole crystal form control agent is used in the hydrothermal reaction process of the magnesium hydroxide intermediate, and nano hexagonal flaky magnesium hydroxide products are successfully synthesized and prepared in a hydrothermal reaction system with a high magnesium hydroxide content (1 to 10% (wt)), thereby improving production efficiency. The technology of the present invention solves the problem that the synthesis of nano hexagonal flaky magnesium hydroxide prepared from bittern high concentration solution is difficult to control, and the production process is simple, the production cost is low, and there is no discharge of three wastes, which creates conditions for the later processing and utilization of de-magnesiumized bittern, has good economic benefits, social benefits and environmental benefits, and is easy to realize industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the XRD spectrum of magnesium hydroxide seed crystals.

[0021] Figure 2 This is the SEM spectrum of magnesium hydroxide seed crystals.

[0022] Figure 3 The XRD spectrum of the magnesium hydroxide precursor obtained in Example 1 is shown in FIG.

[0023] Figure 4 This is the SEM spectrum of the magnesium hydroxide seed crystals obtained in Example 1.

[0024] Figure 5 The following are SEM images of the products synthesized in Examples 1-6 and the comparative example.

[0025] Figure 6 The XRD spectra of the products synthesized in Examples 1-6 and the comparative example are shown. DETAILED DESCRIPTION

[0026] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0027] The morphology of magnesium hydroxide was observed using a scanning electron microscope (SEM, Nova Nano SEM 450, FEI, USA).

[0028] X-ray diffraction (XRD test was carried out using a German Bruker company, model D8 Focus, and the relevant parameters of the instrument were: Cu Kα radiation, tube current 200mA, tube voltage 40kV, scanning speed 6° / min, scanning range 10–70°) was used to characterize the phase composition of the product.

[0029] The EDTA titration method was used to determine the content of magnesium ions in the solution.

[0030] The raw material used in the present invention is the liquid after seawater is dried and salted, and the composition of the liquid is: MgCl 2 :150~180g·L -1 ; NaCl: 60~110g·L -1 ; KCl: 18~22g·L -1 MgSO 4 :60-75g·L -1 ; KBr: 8-12 g·L -1

[0031] The crystal seeds involved in the present invention are well-known materials, and their preparation methods are specifically described in the literature (Sun B, Li Y, Guo H, et al. Fast and complete recovery of magnesium from sea bittern to synthesize magnesium hydroxide hexagonal nanosheet for enhanced flame retardancy and mechanical properties of epoxy resin [J]. Desalination, 2024, 583: 117716.), and the magnesium hydroxide crystal seeds are uniform hexagonal nanosheets, with an average particle size of about 50 nm and a purity of 99.1%. Its XRD spectrum is shown in Figure 1 , SEM images are shown in Figure 2 .

[0032] Example 1: The specific operation is as follows

[0033] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L -1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1 . The total concentration of magnesium ions is about 2.29 mol / L.

[0034] (1) 100 ml of bittern (containing 5.57 g of magnesium ions) was placed in a 250 ml three-necked round-bottom flask, 0.1336 g of seed crystals was added, and the flask was placed in a 25°C water bath, and magnetically stirred at 300 rpm for 10 min. Then 18.32 g of sodium hydroxide particles was added, and the reaction was stirred for 15 min to obtain a slurry. The slurry was filtered to obtain a filtrate and a solid, and the solid was washed with deionized water.

[0035] The XRD spectrum of the magnesium hydroxide precursor in step (1) is shown in Figure 3 , SEM images are shown in Figure 4 .Depend on Figure 3 It can be seen that the precursor is magnesium hydroxide crystals. Figure 4 It can be seen that the precursor magnesium hydroxide has a honeycomb flake structure.

[0036] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethylimidazole, stirred evenly, and then placed in a reaction kettle with a polytetrafluoroethylene liner, sealed and reacted at 160° C. for 8 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0037] Wherein, the mass ratio in step (2) is dimethylimidazole: magnesium hydroxide intermediate: water = 0.01: 0.1: 1. The drying in step (2) is drying at 80° C. for 8 hours.

[0038] Example 2: The specific operation is as follows

[0039] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L -1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1. The total concentration of magnesium ions is about 2.29 mol / L.

[0040] (1) The preparation of magnesium hydroxide precursor is the same as in Example 1.

[0041] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethylimidazole, stirred evenly, and then placed in a reaction kettle with a polytetrafluoroethylene liner, sealed and reacted at 140° C. for 8 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0042] Wherein, the mass ratio in step (2) is dimethyl imidazole: magnesium hydroxide intermediate: water = 0.03: 0.1: 1. The drying in step (2) is drying at 100° C. for 8 hours.

[0043] Example 3: The specific operation is as follows

[0044] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L -1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1 . The total concentration of magnesium ions is about 2.29 mol / L.

[0045] (1) The preparation of magnesium hydroxide precursor is the same as in Example 1.

[0046] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethyl imidazole, stirred evenly, and then placed in a reaction kettle with a polytetrafluoroethylene liner, sealed and reacted at 130° C. for 2 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0047] Wherein, the mass ratio of dimethylimidazole: magnesium hydroxide intermediate: water is 0.02: 0.1: 7 in step (2). The drying in step (2) is carried out at 70° C. for 9 hours.

[0048] Example 4: The specific operation is as follows

[0049] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L-1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1 . The total concentration of magnesium ions is about 2.29 mol / L.

[0050] (1) The preparation of magnesium hydroxide precursor is the same as in Example 1.

[0051] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethyl imidazole, stirred evenly, and then placed in a reaction kettle with a polytetrafluoroethylene liner, sealed and reacted at 130° C. for 3 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0052] Wherein, the mass ratio of dimethyl imidazole: magnesium hydroxide intermediate: water is 0.01:0.1:10 in step (2). The drying in step (2) is carried out at 60°C for 10 hours.

[0053] Example 5: The specific operation is as follows

[0054] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L -1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1 . The total concentration of magnesium ions is about 2.29 mol / L.

[0055] (1) The preparation of magnesium hydroxide precursor is the same as in Example 1.

[0056] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethylimidazole, stirred evenly, and then placed in a reaction kettle with a polytetrafluoroethylene liner, sealed and reacted at 130° C. for 8 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0057] Wherein, the mass ratio of dimethylimidazole: magnesium hydroxide intermediate: water in step (2) is 0.02: 0.1: 5. The drying in step (2) is carried out at 80° C. for 10 hours.

[0058] Example 6: The specific operation is as follows

[0059] The bittern raw material used in the present invention is basically composed of: MgCl 2 :165g·L -1 ;NaCl: 85g·L -1 ; KCl: 20 g·L -1 MgSO 4 :67.5g·L -1 ; KBr: 10.87 g·L -1 . The total concentration of magnesium ions is about 2.29 mol / L.

[0060] (1) The preparation of magnesium hydroxide precursor is the same as in Example 1.

[0061] (2) The obtained magnesium hydroxide precursor is added to an aqueous solution of dimethylimidazole, stirred evenly, and then placed in a reaction kettle lined with polytetrafluoroethylene. After sealing, the reaction is carried out at 150° C. for 6 hours to obtain a slurry; after filtering, washing, and drying, hexagonal flaky magnesium hydroxide crystals are obtained; the SEM image of the product is shown in FIG. Figure 5 , XRD spectrum see Figure 6 , in the shape of hexagonal flakes.

[0062] Wherein, the mass ratio of dimethyl imidazole: magnesium hydroxide intermediate: water is 0.03: 0.1: 1 in step (2). The drying in step (2) is carried out at 90° C. for 8 hours.

[0063] The SEM images of the products obtained in Examples 1-6 are as follows: Figure 5 As shown, the synthesized products all exhibited a uniform hexagonal flake morphology, a thickness of 10-20 nm, and a particle size of about 200 nm, indicating that dimethylimidazole has a certain modification effect on the morphology of magnesium hydroxide and the synthesized particle size is uniform.

[0064] The XRD patterns of the products obtained in Examples 1-6 are as follows: Figure 6 As shown, the synthesized products all exhibit characteristic diffraction peaks belonging to magnesium hydroxide, proving that the synthesized samples are all pure phase magnesium hydroxide.

[0065] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing nano hexagonal flaky magnesium hydroxide from seawater bittern, characterized in that the method comprises the following steps: (1) adding magnesium hydroxide seed crystals to bittern in a reactor and stirring to obtain a mixture A, then adding sodium hydroxide solid thereto and stirring to react, after reacting for 10-30 minutes, filtering the slurry to obtain a filtrate and a filter cake, washing the filter cake with deionized water to obtain a magnesium hydroxide precursor; in, The bittern composition includes MgCl2: 150-180 g·L -1 ; NaCl:60~110g·L -1 ;KCl:18~22g·L -1 ;MgSO4:60~75g·L -1 ;KBr:8~12g·L -1 ; Calculated by mass ratio, magnesium ion in bittern: seed crystal: sodium hydroxide = 1: (0.024-0.072): 3.29; (2) adding the magnesium hydroxide precursor obtained in step (1) to an aqueous solution of dimethylimidazole, stirring, and placing in a reaction kettle with a polytetrafluoroethylene liner, stirring and reacting at 130 to 160° C. for 2 to 8 hours to obtain a slurry; filtering, washing, and drying to obtain hexagonal magnesium hydroxide crystals; Wherein, the mass ratio in the step (2) is dimethylimidazole: magnesium hydroxide intermediate: water = (0.01-0.03): 0.1: (1-10).

2. The method for preparing nano hexagonal flaky magnesium hydroxide from seawater bittern as claimed in claim 1, characterized in that In step (1), the mixed solution is rotated at 300 to 500 rpm in an open container.

3. The method for preparing nano hexagonal flaky magnesium hydroxide from seawater bittern as claimed in claim 1, characterized in that In step (1), the stirring time after the seed crystals are added is 10 to 30 minutes.

4. The method for preparing nano hexagonal flaky magnesium hydroxide from seawater bittern as claimed in claim 1, characterized in that In step (1), the magnesium hydroxide seed crystals are in the shape of hexagonal nanosheets, with an average particle size of about 40 to 60 nm and a purity of more than 99%.

5. The method for preparing nano hexagonal flaky magnesium hydroxide from seawater bittern as claimed in claim 1, characterized in that The drying in step (2) is carried out at 60 to 100° C. for 8 to 10 hours.

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