Method for preparing magnesium hydroxide from chlorine-alkalized brine

Abstract

The invention provides a method for preparing magnesium hydroxide from chlorine-alkalized brine, which comprises the following steps: taking a sodium hydroxide solution as a precipitator, enabling small-particle magnesium hydroxide as a seed crystal to participate in crystallization reaction of magnesium-containing crude brine again, increasing the particle size of magnesium hydroxide particles, and enabling a small part of large-particle magnesium hydroxide to return to a reaction mixer; according to the method disclosed by the invention, no auxiliaries or flocculants are added, under the condition that the concentration of magnesium ions in the raw material of the crude brine is extremely low, magnesium hydroxide particles with relatively large agglomerated particles are continuously generated, and the production cost of a brine refining process is effectively reduced; the particle size of magnesium hydroxide particles is increased from 1-5 microns to 15-20 microns, the sedimentation rate of the particles in a solution is greatly improved, the technical problem that magnesium hydroxide is difficult to settle in a chlor-alkali production system is fundamentally solved, and the moisture content of salty mud is reduced from about 50% to 35%-40%.

Description

Technical Field

[0001] The invention belongs to the technical field of brine refining, and particularly relates to a method for preparing magnesium hydroxide by chlorinating brine. Background Art

[0002] In industrial brine refining, the most widely used method to remove magnesium ions is alkali solution precipitation, which uses sodium hydroxide as a precipitant to react and generate magnesium hydroxide precipitate. However, the magnesium hydroxide particles in the obtained salt mud are small in size, easily forming colloidal precipitation, with low filtration efficiency and high salt content in the filter cake.

[0003] At present, there are many studies on the preparation of magnesium hydroxide. Chinese patent application 201010191112.1 discloses a method for preparing large-particle magnesium hydroxide using a strongly acidic quaternary ammonium salt as a crystal control agent; China University of Geosciences adds polyacrylamide to ultrafine magnesium hydroxide slurry for flocculation to increase the sedimentation rate of magnesium hydroxide to improve the filtration performance; Chinese patent application 200610042411.2 discloses a method for preparing high-purity magnesium hydroxide using atomized ammonia water as a precipitant, and large-particle magnesium hydroxide is obtained by adding crystal seeds; Tsinghua University's patent application with publication number CN104891534A adds a dispersant to calcium-containing agglomerated magnesium hydroxide to improve the filtration performance. However, the above method requires additional additives, and the chlor-alkali industrial electrolysis device requires that the saturated brine entering the electrolytic cell cannot introduce additives such as ammonia nitrogen, otherwise it will cause the electrolytic cell voltage to increase abnormally, resulting in safety hazards such as the inability of the equipment to operate stably.

[0004] Chinese patent application 202110553030.5 discloses the use of a room temperature reaction method to control the concentration of magnesium ions and hydroxide ions. Under the premise of accurately controlling the reaction pH value, the particle size of the magnesium hydroxide particles is controlled by controlling the reaction process. In this process, no chemical additives are added, no heating, and no pressurization are applied. Agglomerated large-particle magnesium hydroxide particles are directly synthesized in one step for rapid separation. However, the stirring speed in this method is very fast, which is difficult to implement in industry.

[0005] The above work has played a certain role in the growth and filtration of magnesium hydroxide. However, without adding any additives and flocculants, there are still problems such as high cost, difficulty in sedimentation and low efficiency when treating large flow of brine containing low concentration of magnesium ions in the chlor-alkali system at a specific temperature. It is necessary to further improve it. Summary of the invention

[0006] The invention provides a method for preparing magnesium hydroxide by chlorinating brine, which is used to solve the problems in the prior art of treating a large flow of brine containing low-concentration magnesium ions at a specific temperature, such as high cost, difficulty in sedimentation and low efficiency of by-product magnesium hydroxide.

[0007] The present invention provides a method for preparing magnesium hydroxide by chlorinating brine, comprising the following steps: S1: respectively passing magnesium-containing crude brine and sodium hydroxide solution into a reaction mixer for crystallization to obtain a mixed crystal slurry; Wherein, the mass fraction of the sodium hydroxide solution is 1-20wt%; the magnesium ion concentration in the magnesium-containing crude brine is not higher than 300mg / L, and the concentration of sodium chloride is 300-310g / L; S2: passing the mixed slurry into the hydrocyclone through the feed port of the hydrocyclone for solid-liquid separation, obtaining small-particle magnesium hydroxide slurry at the top overflow port of the hydrocyclone, and obtaining large-particle magnesium hydroxide slurry at the bottom outlet of the hydrocyclone; S3: filtering the small-particle magnesium hydroxide slurry through a membrane filter to obtain a small-particle concentrated slurry and refined brine; S4: mixing all or part of the small particle concentrated slurry with the sodium hydroxide solution and returning the mixture to the reaction mixer to participate in the crystallization treatment again; At least part of the large-particle magnesium hydroxide slurry is mixed with the magnesium-containing crude brine and then returned to the reaction mixer to participate in the crystallization treatment again, and another part of the large-particle magnesium hydroxide slurry is output as a product.

[0008] Compared with the prior art, the invention is advantageous in that: sodium hydroxide solution is used as a precipitant, small-particle magnesium hydroxide is used as a seed to re-participate in the crystallization reaction of magnesium-containing crude brine, the particle size of the magnesium hydroxide particles is increased, a small portion of large-particle magnesium hydroxide is returned to the reaction mixer, and further used as a seed to promote the growth of magnesium hydroxide particles; the invention does not add any auxiliary agent or flocculant, and realizes continuous generation of larger-particle agglomerated magnesium hydroxide particles when the magnesium ion concentration in the crude brine raw material is extremely low, thereby effectively reducing the production cost of the brine refining process; the particle size of the magnesium hydroxide particles is increased from 1-5 microns to 15-20 microns, which greatly increases the sedimentation rate of the particles in the solution, fundamentally solves the technical problem that magnesium hydroxide is difficult to settle in the chlor-alkali production system, and reduces the moisture content of the salt mud from about 50% to 35%-40%.

[0009] Further, the D50 particle size of the magnesium hydroxide in the small-particle magnesium hydroxide slurry is 1-5 μm; and / or, The D50 particle size of the magnesium hydroxide in the large-particle magnesium hydroxide slurry is 10-15 μm.

[0010] Furthermore, the aspect ratio of the reaction mixer is (5-6):1.

[0011] Further, in step S4, the part of the small-particle magnesium hydroxide slurry returned to the reactor to participate in the crystallization treatment again accounts for 5-100% of the total amount; and / or, The portion of the large-particle magnesium hydroxide slurry that is returned to the reaction mixer and participates in the crystallization treatment again accounts for 1-5% of the total amount.

[0012] Furthermore, the pH value of the crystallization treatment is 9-10; and / or, The temperature of the crystallization treatment is 60-70°C; and / or, The stirring speed of the crystallization treatment is 30-150 r / min; and / or, The residence time of the crystallization treatment is 3 to 4 hours.

[0013] Furthermore, the mass fraction of the sodium hydroxide solution is 2.5-5wt%, and the excess amount of sodium hydroxide in the mixed slurry is 0.2-0.4g / l.

[0014] Further, the flow rate of the hydrocyclone is 0.328 g / s to 0.438 g / s; and / or, The solid-liquid separation treatment time is 3 to 4 hours.

[0015] Furthermore, the filter membrane of the membrane filter is made of ceramic. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of preparing magnesium hydroxide from chlor-alkali brine in one embodiment of the present invention; Figure 2 The figure is a comparison chart of the particle sizes of the magnesium hydroxide obtained in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of 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.

[0018] The present invention provides a method for preparing magnesium hydroxide by chlorinating brine, comprising the following steps: S1: respectively passing magnesium-containing crude brine and sodium hydroxide solution into a reaction mixer for crystallization to obtain a mixed crystal slurry; The mass fraction of the sodium hydroxide solution is 1-20 wt %; the magnesium ion concentration in the magnesium-containing crude brine is not higher than 300 mg / L, and the concentration of sodium chloride is 300-310 g / L; S2: passing the mixed slurry into a hydrocyclone through the feed port of the hydrocyclone for solid-liquid separation, obtaining a small-particle magnesium hydroxide slurry at the top overflow port of the hydrocyclone, and obtaining a large-particle magnesium hydroxide slurry at the bottom outlet of the hydrocyclone; S3: filtering the small-particle magnesium hydroxide slurry through a membrane filter to obtain a small-particle concentrated slurry and refined brine; S4: mixing all or part of the small particle concentrated slurry with the sodium hydroxide solution and returning the mixture to the reaction mixer for crystallization treatment again; At least part of the large-particle magnesium hydroxide slurry is mixed with the magnesium-containing crude brine and then returned to the reaction mixer to participate in the crystallization treatment again, and the other part of the large-particle magnesium hydroxide slurry is output as a product.

[0019] Among them, the above-mentioned magnesium-containing crude brine comes from the chlor-alkali production system, and its main component is sodium chloride, as well as a small amount of calcium, magnesium, sulfate, iron, nickel, iodine, bromine and other impurities.

[0020] The present invention provides a method for preparing magnesium hydroxide from chlor-alkali brine. Sodium hydroxide solution is used as a precipitant, and small-particle magnesium hydroxide is used as a seed to re-participate in a crystallization reaction of a crude magnesium-containing brine, thereby small positively charged magnesium hydroxide particles are neutralized, repulsion between particles is reduced, and the particle size of magnesium hydroxide particles is increased. Meanwhile, a small portion of large-particle magnesium hydroxide is returned to a reaction mixer and further used as a seed to promote the growth of magnesium hydroxide particles, increase the proportion of large particles in a mixed slurry, and increase the sedimentation rate of large-particle magnesium hydroxide in a cyclone separator. The present invention does not add any auxiliary agent or flocculant, and when the magnesium ion concentration in the crude brine raw material is extremely low, continuous generation of large-particle agglomerated magnesium hydroxide particles is achieved, thereby reducing the moisture content of salt mud.

[0021] Among them, the D50 particle size of magnesium hydroxide in the small-particle magnesium hydroxide slurry is 1~5μm; The D50 particle size of magnesium hydroxide in the large-particle magnesium hydroxide slurry is 10~15μm.

[0022] The length-to-diameter ratio of the reaction mixer is (5-6):1, which can further reduce the moisture content of the salt mud filter cake and increase the particle size of the agglomerated product particles.

[0023] In a specific embodiment, in step S4, the part of the small-particle magnesium hydroxide slurry returned to the reactor to participate in the crystallization treatment again accounts for 5-100% of the total amount; the part of the large-particle magnesium hydroxide slurry returned to the reaction mixer to participate in the crystallization treatment again accounts for 1-5% of the total amount.

[0024] The pH value of the crystallization treatment is 9~10; the temperature of the crystallization treatment is 60~70°C; the stirring speed of the crystallization treatment is 30~150r / min; and the residence time of the crystallization treatment is 3~4h.

[0025] By further limiting the specific execution parameters of the crystallization treatment, the moisture content of the salt mud filter cake can be further reduced and the particle size of the magnesium hydroxide product particles can be increased.

[0026] Furthermore, the inventors found that when the mass fraction of the sodium hydroxide solution is 2.5-5wt%, the moisture content of the salt mud filter cake can be further reduced and the particle size of the magnesium hydroxide by-product can be increased.

[0027] In a specific embodiment, the excess amount of sodium hydroxide in the mixed slurry is 0.2-0.4 g / l, which can further ensure the full reaction of sodium hydroxide and magnesium ions and further increase the particle size of magnesium hydroxide particles.

[0028] By further limiting the flow rate of the hydrocyclone to 0.328 g / s to 0.438 g / s and the solid-liquid separation treatment time to 3 to 4 hours, the particle size of the magnesium hydroxide particles in the small-particle magnesium hydroxide slurry and the large-particle magnesium hydroxide crystal slurry obtained by the solid-liquid separation treatment can be further controlled, and the particle size of the magnesium hydroxide by-product can be further increased.

[0029] The membrane filter is made of ceramic, which can further separate and filter small particles of magnesium hydroxide in the brine, further improving the purity of the filtrate.

[0030] Hereinafter, a method for preparing magnesium hydroxide from chlor-alkali brine provided by the present invention is described in detail through specific examples. Example 1

[0031] S1: respectively passing magnesium-containing crude brine into a reaction mixer from a feed port, and passing a sodium hydroxide solution with a mass fraction of 2.5 wt% into the reaction mixer from a feed port, stirring and mixing to perform crystallization treatment, and obtaining a mixed slurry; The magnesium ion concentration in the magnesium-containing crude brine is 25 mg / L, and the sodium chloride concentration is 305 g / L; the amount of sodium hydroxide solution introduced is adjusted to maintain the pH value of the system at 9.78; the system temperature is 60°C, the residence time of the crystallization treatment is 3 hours, and the stirring speed is 150 rpm; the length-to-diameter ratio of the reaction mixer is 6:1; S2: The mixed slurry is introduced into the hydrocyclone through the feed port of the hydrocyclone for solid-liquid separation, and D is obtained at the top overflow port of the hydrocyclone. 50 Small-particle magnesium hydroxide slurry with a particle size of 3.1 μm is used to obtain large-particle magnesium hydroxide slurry with a D50 particle size of 12.6 μm at the bottom outlet of the hydrocyclone separator; S3: filtering the small-particle magnesium hydroxide slurry through a membrane filter to obtain a small-particle concentrated slurry and refined brine; S4: mixing the small particle concentrated slurry with the sodium hydroxide solution and returning it to the reaction mixer to participate in the crystallization treatment again; 1-5% of the large-particle magnesium hydroxide slurry is mixed with the magnesium-containing crude brine and then returned to the reaction mixer to participate in the crystallization treatment again; S5: The remaining large-particle magnesium hydroxide slurry discharged from the bottom outlet of the hydrocyclone separator is the magnesium hydroxide particle product, which is filtered through a plate and frame filter press and dried to obtain a salt mud filter cake.

[0032] The moisture content of the filter cake obtained by filtration is 37.02%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 19.3μm. Example 2

[0033] Compared with Example 1, the present embodiment is different in that: in step S1, the mass fraction of the sodium hydroxide solution is 3.5 wt %.

[0034] The moisture content of the filter cake obtained by filtration is 37.63%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 18.7μm. Example 3

[0035] Compared with Example 1, the present embodiment is different in that: in step S1, the mass fraction of the sodium hydroxide solution is 5 wt %.

[0036] The moisture content of the filter cake obtained by filtration is 38.08%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 18.3μm. Example 4

[0037] Compared with Example 1, the present embodiment is different in that: in step S1, the mass fraction of the sodium hydroxide solution is 6.5wt%.

[0038] The moisture content of the filter cake obtained by filtration is 39.21%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 16.3μm. Example 5

[0039] Compared with Example 1, the present embodiment is different in that: in step S1, the mass fraction of the sodium hydroxide solution is 8 wt %.

[0040] The moisture content of the filter cake obtained by filtration is 39.97%, and the magnesium hydroxide obtained is an agglomerated particle product. 50 is 15μm. Example 6

[0041] Compared with Example 1, the present embodiment is different in that: in step S1, the mass fraction of the sodium hydroxide solution is 1 wt %.

[0042] The moisture content of the filter cake obtained by filtration is 37.12%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 19.8μm. Example 7

[0043] The present embodiment is different from the embodiment 1 in that: in step S1, the magnesium ion concentration in the magnesium-containing crude brine is 10 mg / L.

[0044] The moisture content of the filter cake obtained by filtration is 37.32%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 19.5μm. Example 8

[0045] The present embodiment is different from the embodiment 1 in that: in step S1, the magnesium ion concentration in the magnesium-containing crude brine is 5 mg / L.

[0046] The moisture content of the filter cake obtained by filtration is 37.01%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 19.7μm. Example 9

[0047] Compared with Example 1, the present embodiment is different in that the aspect ratio of the reaction mixer is 5:1.

[0048] The moisture content of the filter cake obtained by filtration is 37.18%, and the magnesium hydroxide obtained is an agglomerated large particle product with a D50 of 19.1 μm. Example 10

[0049] Compared with Example 1, the present embodiment is different in that the aspect ratio of the reaction mixer is 7:1.

[0050] The moisture content of the filter cake obtained by filtration is 36.68%, and the magnesium hydroxide obtained is an agglomerated large particle product with a D50 of 20.3 μm. Embodiment 11

[0051] Compared with Example 1, the present embodiment is different in that the aspect ratio of the reaction mixer is 4:1.

[0052] The moisture content of the filter cake obtained by filtration is 41.31%, and the magnesium hydroxide obtained is a mixed product of powder and granules. 50 It is 14.7μm.

[0053] Comparative Example 1 The difference between this comparative example and Example 1 is that in step S1, the mass fraction of the sodium hydroxide solution is 26wt%.

[0054] The moisture content of the filter cake thus obtained was 47.01%, and the magnesium hydroxide powder product obtained, D 50 It is 10.6μm.

[0055] Comparative Example 2 The difference between this comparative example and Example 1 is that in step S1, the mass fraction of the sodium hydroxide solution is 32wt%.

[0056] The moisture content of the filter cake obtained by filtration is 50.31%, and the magnesium hydroxide obtained is a powder product. 50 It is 8.3μm.

[0057] Comparative Example 3 Compared with Example 1, the difference between this comparative example and Example 1 is that: Step S4: only 1-5% of the large-particle magnesium hydroxide slurry is mixed with the magnesium-containing crude brine and then returned to the reaction mixer to participate in the crystallization treatment again.

[0058] The moisture content of the filter cake obtained by filtration is 38.33%, and the magnesium hydroxide obtained is a large agglomerated particle product. 50 It is 18.1μm.

[0059] Based on the above comparative examples and embodiments, it can be seen that: It can be seen from Examples 1-6 and Comparative Examples 1-2 that when the mass fraction of sodium hydroxide is higher than 20wt%, the magnesium hydroxide obtained is a powder, and the filter cake moisture content is high. Therefore, in order to obtain large agglomerated product particles and reduce the salt content of the filter cake, the mass fraction of sodium hydroxide should be between 1wt% and 20wt%. Comparative Example 1 and Comparative Example 3 show that adding small-particle magnesium hydroxide to the system has a greater effect on the growth of magnesium hydroxide than large particles as seeds. Comparative Example 1 and Examples 9 to 11 show that when the aspect ratio of the reaction mixer is less than 5:1, the moisture content of the filter cake obtained is high and the particles are small, while when the aspect ratio of the reaction mixer is greater than 6:1, the equipment volume is increased, and the improvement of the moisture content of the filter cake is not obvious. Therefore, in order to obtain large agglomerated product particles and reduce the salt content of the filter cake, the aspect ratio of the reaction mixer is preferably 5:1 to 6:1.

[0060] Further comparison of Examples 1-3 with Examples 4-6 shows that in order to make the obtained product as large as possible, increase the filtration speed, reduce the salt content of the filter cake, and in order to avoid unnecessary waste of reaction mixer space, the further preferred range of the mass fraction of sodium hydroxide is 2.5% to 5%. Further comparison of Examples 1, 7 and 8 shows that when the mass fraction of magnesium ions in the brine is lower than 2.5% (low magnesium ion concentration), agglomerated large product particles can also be obtained, and the salt content of the filter cake is effectively reduced.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein by equivalents. If these modifications and variations fall within the scope of the claims of the present invention and their equivalent technologies, they should be considered to be within the protection scope of the present invention.

Claims

1. A method for preparing magnesium hydroxide by chlor-alkali salt water, characterized in that: The following steps are involved: S1: respectively passing magnesium-containing crude brine and sodium hydroxide solution into a reaction mixer for crystallization to obtain a mixed crystal slurry; Wherein, the mass fraction of the sodium hydroxide solution is 1-20wt%; the magnesium ion concentration in the magnesium-containing crude brine is not higher than 300mg / L, and the concentration of sodium chloride is 300-310g / L; S2: passing the mixed slurry into the hydrocyclone through the feed port of the hydrocyclone for solid-liquid separation, obtaining small-particle magnesium hydroxide slurry at the top overflow port of the hydrocyclone, and obtaining large-particle magnesium hydroxide slurry at the bottom outlet of the hydrocyclone; S3: filtering the small-particle magnesium hydroxide slurry through a membrane filter to obtain a small-particle concentrated slurry and refined brine; S4: mixing all or part of the small particle concentrated slurry with the sodium hydroxide solution and returning the mixture to the reaction mixer to participate in the crystallization treatment again; At least part of the large-particle magnesium hydroxide slurry is mixed with the magnesium-containing crude brine and then returned to the reaction mixer to participate in the crystallization treatment again, and another part of the large-particle magnesium hydroxide slurry is output as a product.

2. The method for preparing magnesium hydroxide from chlor-alkali salt water according to claim 1, characterized in that: The D50 particle size of the magnesium hydroxide in the small-particle magnesium hydroxide slurry is 1-5 μm; and / or, The D50 particle size of the magnesium hydroxide in the large-particle magnesium hydroxide slurry is 10-15 μm.

3. The method for preparing magnesium hydroxide from chlor-alkali brine according to claim 1, characterized in that: The length-to-diameter ratio of the reaction mixer is (5-6):

1.

4. The method for preparing magnesium hydroxide from chlor-alkali brine according to claim 1, characterized in that: In step S4, the portion of the small-particle magnesium hydroxide slurry that is returned to the reactor and participates in the crystallization treatment again accounts for 5 to 100% of the total amount; and / or, The portion of the large-particle magnesium hydroxide slurry that is returned to the reaction mixer and participates in the crystallization treatment again accounts for 1-5% of the total amount.

5. The method for preparing magnesium hydroxide from chlor-alkali brine according to claim 1, characterized in that: The pH value of the crystallization treatment is 9-10; and / or, The temperature of the crystallization treatment is 60-70°C; and / or, The stirring speed of the crystallization treatment is 30-150 r / min; and / or, The residence time of the crystallization treatment is 3 to 4 hours.

6. The method for preparing magnesium hydroxide from chlor-alkali brine according to claim 1, characterized in that: The mass fraction of the sodium hydroxide solution is 2.5-5wt%, and the excess value of the sodium hydroxide in the mixed slurry is 0.2-0.4g / l.

7. The method for preparing magnesium hydroxide from chlor-alkali brine according to claim 1, characterized in that: The flow rate of the hydrocyclone is 0.328 g / s to 0.438 g / s; and / or, The solid-liquid separation treatment time is 3 to 4 hours.

8. The method for preparing magnesium hydroxide from chlor-alkali brine according to any one of claims 1 to 6, characterized in that: The filter membrane of the membrane filter is made of ceramic.

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