CN110683566A

CN110683566A - Preparation method of morphology-controllable magnesium hydroxide with low specific surface area

Info

Publication number: CN110683566A
Application number: CN201910148924.9A
Authority: CN
Inventors: 陈光文; 杨梅; 罗腊梅
Original assignee: Dalian Institute of Chemical Physics of CAS
Current assignee: Dalian Institute of Chemical Physics of CAS
Priority date: 2019-02-28
Filing date: 2019-02-28
Publication date: 2020-01-14
Anticipated expiration: 2039-02-28
Also published as: CN110683566B

Abstract

The invention provides a method for regulating and controlling the appearance of magnesium hydroxide with low specific surface area, namely, the magnesium hydroxide with controllable appearance and low specific surface area (the specific surface area is equal to or less than 5 m)²In terms of/g). The specific process is as follows: preparing magnesium chloride and sodium hydroxide aqueous solution respectively, continuously introducing the magnesium chloride and the sodium hydroxide aqueous solution into the microreactor for precipitation reaction, performing hydrothermal treatment twice after reaction slurry flows out of the microreactor, and centrifuging, washing and drying the slurry after the hydrothermal treatment is finished to obtain the magnesium hydroxide with the low specific surface area. Change of magnesium ion (Mg)²⁺) And hydroxide ion (OH)^‑) The molar ratio of (a) to (b) can control the morphology of the magnesium hydroxide with low specific surface area. The invention realizes the precipitation process in the microreactor, the reaction process is semi-continuous and easy to amplify, and the obtained product has the advantages of low specific surface area, controllable appearance and good repeatability among batches.

Description

Preparation method of morphology-controllable magnesium hydroxide with low specific surface area

Technical Field

The invention relates to a method for regulating and controlling the morphology of magnesium hydroxide with low specific surface area, belonging to the field of inorganic materials and material engineering.

Background

With the improvement of the environmental protection standard of China, the market demand of the halogen-free flame retardant is increased year by year. The magnesium hydroxide is used as an environment-friendly green inorganic flame retardant for plastic application due to the advantages of high decomposition temperature, good smoke suppression effect, safety, no toxicity, stable performance and the likeThe material, the rubber, the electric wire and the cable and other fields have wide application. The flame retardant property of magnesium hydroxide is obviously influenced by the particle size, particle size distribution, specific surface area, morphology and the like. Researchers can adjust and control the particle size, particle size distribution, specific surface area and morphology of the magnesium hydroxide to enable the magnesium hydroxide to have different flame retardant properties so as to adapt to different flame retardant applications. In order to reduce the influence of magnesium hydroxide addition on the performance of the high polymer material and improve the dispersity of the magnesium hydroxide flame retardant in the high polymer material, the specific surface area of the high-grade magnesium hydroxide flame retardant is usually less than 10m²/g。

Hitherto, published papers and patents have reported a plurality of methods for synthesizing magnesium hydroxide and modification strategies, and the rule of influence of process conditions on the particle size, particle size distribution, specific surface area and morphology of magnesium hydroxide under different methods is intensively studied. Shirute et al prepared magnesium hydroxide particles having an average particle size of 5-30 μm in a narrow channel reactor using a two-phase gas-liquid flow (Ind. Eng. chem. Res.,2005,44, 5500-. SEM photographs showed that any magnesium hydroxide particles were secondary particles, agglomerated from primary particles of smaller size, and the study did not relate to the specific surface area of magnesium hydroxide. CN201210222105.2 discloses a method for preparing submicron hexagonal sheet magnesium hydroxide flame retardant by using microchannel, which prepares magnesium hydroxide flame retardant with particle size of 100-1000nm under specific molar ratio of magnesium ion and hydroxyl ion, but specific surface area is more than 9m²(ii) in terms of/g. The morphology of the agglomerated magnesium hydroxide after hydrothermal modification by an aqueous sodium hydroxide solution is found to be regular and is hexagonal pieces with an average particle size of 0.36-0.87 mu m. When the NaOH concentration is equal to or greater than 4.0M, the magnesium hydroxide has a specific surface area of less than 10M²(ii) in terms of/g. CN201210330386.3 discloses a method for preparing magnesium hydroxide with low specific surface area in a microchannel reactor, and the specific surface area of the obtained magnesium hydroxide flame retardant is 3-9m²(ii) in terms of/g. Therefore, the research of the blue et al and the preparation of CN201210330386.3 both obtain samples with specific surface areas meeting the requirements of high-grade flame retardants, but both only pay attention to the regulation and control of the particle size and the specific surface area of magnesium hydroxide, and do not relate to the regulation and control of the morphology of the magnesium hydroxide. As mentioned above, the flame retardant property of magnesium hydroxide is influenced by the particle size, particle size distribution and specific surface areaBut also by topography. On the premise that the specific surface area meets the requirements of high-grade magnesium hydroxide flame retardants, the magnesium hydroxide is endowed with richer flame retardant performance by regulating the appearance, and the method has important research significance and practical value. However, no report on the regulation of the morphology of low specific surface area magnesium hydroxide has been found in the published papers and patents.

The micro chemical technology is the technology frontier field of multidisciplinary intersection which is started in the early 90 s of the 20 th century. The technology is mainly characterized in that various unit operations and reaction processes are realized by adopting microchannels with characteristic sizes of tens to hundreds of microns. Because the size of the channel is obviously reduced, the heat and mass transfer process is obviously strengthened, the utilization efficiency of energy in the reaction process and the production capacity of unit volume can be greatly improved, and the strengthening, the miniaturization and the greening of the chemical process are realized. In recent years, the preparation of micro-nano materials by utilizing the micro-chemical technology has attracted extensive attention in academia and industrial fields. Carrying out the precipitation process in a microreactor has the following advantages: (1) a good micro-mixing environment is provided, and different particles have the same nucleation rate and growth rate; (2) continuous operation and high production efficiency; (3) the reactor amplification is achieved by an increase in the number of channels, without amplification effect. Therefore, the micro-reactor is used for strengthening the precipitation process, the product granularity uniformity and the batch-to-batch repeatability can be enhanced, and the rapid amplification and continuous production can be realized.

In summary, aiming at the defects in the prior art, the micro-channel reactor is used for strengthening the precipitation process of magnesium chloride and sodium hydroxide, and the magnesium hydroxide with low specific surface area is prepared by twice hydrothermal treatments, and the shape of the magnesium hydroxide is regulated.

Disclosure of Invention

The invention aims to make up the defects of the prior art and provides a method for regulating and controlling the morphology of magnesium hydroxide with low specific surface area, which comprises the following specific process steps:

(1) respectively preparing a magnesium chloride aqueous solution with the concentration of 0.5-2mol/L and a sodium hydroxide aqueous solution with the concentration of 0.6-5 mol/L;

(2) continuously introducing the magnesium chloride aqueous solution and the sodium hydroxide aqueous solution into a microreactor, and carrying out precipitation reaction on magnesium chloride and sodium hydroxide in the microreactor to obtain reaction slurry I;

(3) directly feeding the reaction slurry I into a hydrothermal synthesis kettle for first hydrothermal treatment after flowing out of the outlet of the microreactor to obtain reaction slurry II, wherein the temperature of the first hydrothermal treatment is 180-;

(4) centrifuging the reaction slurry II obtained after the first hydrothermal treatment to obtain a filter cake, adding deionized water and solid sodium hydroxide into the filter cake, adjusting the solid content and the hydroxide concentration of the slurry, calculated by magnesium hydroxide, to certain values to obtain a reaction slurry III, and carrying out a second hydrothermal treatment on the reaction slurry III;

(5) centrifuging, washing and drying the reaction slurry subjected to the second hydrothermal treatment to obtain the magnesium hydroxide; the second hydrothermal temperature is 150-220 ℃, and the second hydrothermal time is 2-6 h.

Based on the above technical scheme, preferably, in the microreactor in the step (2), the molar ratio n (Mg) of magnesium ions to hydroxyl groups²⁺):n(OH^-) 1: 1.2-1.7 or 1: 2.0-2.5.

Based on the technical scheme, the specific surface area of the magnesium hydroxide prepared by the method is preferably the specific surface area<10m²/g。

Based on the technical scheme, preferably, the magnesium hydroxide prepared by the method is polyhedral or hexagonal; when the precipitation reaction in the step (2) is carried out at the molar ratio of magnesium ions to hydroxyl ions n (Mg)²⁺):n(OH^-) When the method is carried out within the range of 1:2.0-1:2.5, the obtained magnesium hydroxide with low specific surface area is polyhedral in shape and has the average particle size of 200-400 nm; when the precipitation reaction is at n (Mg)²⁺):n(OH^-) When the ratio is within the range of 1:1.2-1:1.7, the obtained hexagonal tablets with the morphology of the magnesium hydroxide with the low specific surface area have the average grain diameter of 1.5-3.5 mu m and the ratio of the grain diameter to the thickness of 5-10. Based on the technical scheme, preferably, the magnesium chloride aqueous solution and the sodium hydroxide aqueous solution enter the microreactor at the same flow rate, and the flow rate range is 50-200 mL/min.

Based on the technical scheme, the preferable precipitation reaction temperature of the magnesium chloride and the sodium hydroxide is 20-80 ℃, and the reaction residence time is 0.2-12 ms.

Based on the technical scheme, the hydraulic diameter of the channel in the microreactor is preferably 0.5-1mm, and the length of the channel is preferably 5-20 mm.

Based on the technical scheme, preferably, the solid content of the reaction product III is 2-8 wt.% and the concentration of sodium hydroxide is 2-4mol/L based on magnesium hydroxide.

In another aspect, the present invention provides a magnesium hydroxide prepared by the preparation method described in any one of the above.

The invention has the following advantages:

(1) the magnesium hydroxide in the reaction slurry at the outlet of the microreactor has uniform particle size, and a precondition is provided for obtaining an expected product through subsequent hydrothermal treatment. Because the characteristic dimension of the internal channel of the microreactor is less than 1mm, the diffusion distance is greatly reduced, the phase interface area is obviously increased, and the heat and mass transfer rate is 1-3 orders of magnitude higher than that of the traditional batch reactor. Therefore, after the magnesium chloride aqueous solution and the sodium hydroxide aqueous solution enter the microchannel, the mixture can quickly reach the near molecular level, so that the precipitation reaction is carried out in a uniform reaction environment, and the morphology and the granularity uniformity of the obtained particles are ensured.

(2) The precipitation process is continuous, the traditional precipitation process is mostly carried out in an intermittent stirring kettle, the titration time of reaction materials is often tens of minutes to hours, the production efficiency is low, and the process controllability is poor. On the contrary, the micro-reactor is in a continuous operation mode, reaction raw materials continuously enter the micro-reactor, the production efficiency is high, and the process controllability is strong.

(3) The magnesium hydroxide with low surface area has controllable appearance, and the flame retardant property of the magnesium hydroxide can be expanded.

Drawings

Fig. 1 is a schematic structural diagram of a microreactor used in an embodiment of the present invention, wherein: 1.2 is a liquid inlet, 3 and 4 are liquid inlet channels, 5 is a reaction channel, and 6 is a slurry outlet;

FIG. 2 is a SEM photograph of a product of example 1 of the present invention;

FIG. 3 is a SEM photograph of a product of example 2 of the present invention;

FIG. 4 is a TEM photograph of a comparative example 1 product of the present invention;

FIG. 5 is a TEM photograph of a comparative example 2 product of the present invention;

FIG. 6 is an SEM photograph of a comparative example 3 product of the present invention.

Detailed Description

The present invention is described in detail below by way of examples, but the present invention is not limited to the following examples.

The microchannel reactor in the examples is briefly described:

as shown in fig. 1, the microchannel plate comprises two sealing plates and a microchannel plate, three microchannels are formed on the microchannel plate, namely two

liquid inlet channels

3 and 4 and a reaction channel 5 respectively connected with the inlet channels, the inlet channels are respectively connected with two inlets 1 and 2, and the reaction channel is connected with an outlet 6. The hydraulic diameters of the two inlet channels and the reaction channel are equal and are both 0.6 mm. The included angle between the two liquid inlet channels is 60 degrees, and the length of the reaction channel is 20 mm.

Example 1

Preparing 1.0mol/L MgCl₂1L each of the aqueous solution and 1.7mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact, mix and react at the flow rate of 100mL/min, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 1.7. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and first hydrothermal reaction is carried out at 180 ℃ for 4 hours. After the first hydrothermal treatment, the slurry was centrifuged to obtain a filter cake having a solids content of 18.49 wt.% based on magnesium hydroxide. 15g of filter cake were taken, 26.62g of water and 4.65g of NaOH were added thereto, the solids content in the slurry, calculated as magnesium hydroxide, and the sodium hydroxide concentration were adjusted to 6 wt.% and 3M, and a second hydrothermal treatment was carried out at 180 ℃ for 4 hours. After the second hydrothermal treatment, the slurry is centrifuged, washed and dried to obtain magnesium hydroxide in the shape of hexagonal plate with average particle diameter of 2.1 μm, average thickness of 300nm, particle diameter-to-thickness ratio of 7 and specific surface area of 3.2m²The SEM photograph is shown in FIG. 2.

Example 2

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 2.0mol/L aqueous NaOH solution. The two aqueous solutions are 100mThe flow of L/min enters a micro-reactor for contact, mixing and reaction, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 2.0. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and first hydrothermal reaction is carried out at 180 ℃ for 4 hours. After the first hydrothermal treatment, the slurry was centrifuged to obtain a filter cake having a solids content of 36.33 wt.% based on magnesium hydroxide. 15g of filter cake were taken, 66.67g of water and 9.14g of NaOH were added thereto, the solids content in the slurry, calculated as magnesium hydroxide, and the sodium hydroxide concentration were adjusted to 6 wt.% and 3M, and a second hydrothermal treatment was carried out at 180 ℃ for 4 hours. After the second hydrothermal treatment, the slurry is centrifuged, washed and dried to obtain polyhedral magnesium hydroxide with the average particle size of 250nm and the specific surface area of 5.0m²The SEM photograph is shown in FIG. 3.

Example 3

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 1.2mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact, mix and react at the flow rate of 100mL/min, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 1.7. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and first hydrothermal reaction is carried out at 200 ℃ for 4 hours. After the first hydrothermal treatment, the slurry was centrifuged to obtain a filter cake having a solids content of 18.49 wt.% based on magnesium hydroxide. 15g of filter cake were taken, 30.22g of water and 5.05g of NaOH were added thereto, the solids content in the slurry, calculated as magnesium hydroxide, and the sodium hydroxide concentration were adjusted to 6 wt.% and 3M, and a second hydrothermal treatment was carried out at 220 ℃ for 6 hours. After the second hydrothermal treatment, the slurry is centrifuged, washed and dried to obtain magnesium hydroxide which is hexagonal flake, the average particle size is 3.5 mu m, the average thickness is 350nm, the ratio of the particle size to the thickness is 10, and the specific surface area is 2.8m²/g。

Example 4

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 2.2mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact, mix and react at the flow rate of 100mL/min, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 2.2. Reaction slurryDirectly enters a hydrothermal synthesis kettle after flowing out of the outlet of the microreactor, and carries out first hydrothermal at 180 ℃ for 4 h. After the first hydrothermal treatment, the slurry was centrifuged to obtain a filter cake having a solids content of 37.83 wt.% based on magnesium hydroxide. 70.06g of water and 9.52g of NaOH were added to 15g of the filter cake, the solids content in the slurry, calculated as magnesium hydroxide, and the sodium hydroxide concentration were adjusted to 6% by weight and 3M, and a second hydrothermal treatment was carried out at 200 ℃ for 4 h. After the second hydrothermal treatment, the slurry is centrifuged, washed and dried to obtain the magnesium hydroxide which is hexagonal flake, the average particle size is 320nm, and the specific surface area is 3.1m²/g。

Comparative example 1

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 1.7mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact, mix and react at the flow rate of 100mL/min, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 1.7. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and is subjected to hydrothermal treatment at 180 ℃ for 4 hours. After the hydrothermal treatment is finished, the slurry is centrifuged, washed and dried, and the obtained magnesium hydroxide is hexagonal flake, has the average particle size of 350nm and the specific surface area of 20m²The SEM photograph is shown in FIG. 4.

Comparative example 2

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 2.0mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact, mix and react at the flow rate of 100mL/min, and n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 2.0. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and first hydrothermal reaction is carried out at 180 ℃ for 4 hours. After the hydrothermal treatment is finished, the slurry is centrifuged, washed and dried, and the obtained magnesium hydroxide is hexagonal flake, has the average particle size of 190nm and the specific surface area of 22m²The TEM photographs are shown in FIG. 5.

Comparative example 3

Preparing 1mol/L MgCl₂1L each of the aqueous solution and 1.7mol/L aqueous NaOH solution. The two aqueous solutions enter a micro-reactor to contact at the flow rate of 100mL/minMixing and reacting, n (Mg)²⁺):n(OH^-) The reaction temperature was 60 ℃ at 1: 1.7. And the reaction slurry flows out of the outlet of the microreactor and then directly enters a hydrothermal synthesis kettle, and first hydrothermal reaction is carried out at 160 ℃, wherein the hydrothermal time is 4 h. After the first hydrothermal treatment, the slurry was centrifuged to obtain a filter cake having a solids content of 18.49 wt.% based on magnesium hydroxide. 15g of filter cake were taken, 26.62g of water and 4.65g of NaOH were added thereto, the solids content in the slurry, calculated as magnesium hydroxide, and the sodium hydroxide concentration were adjusted to 6 wt.% and 3M, and a second hydrothermal treatment was carried out at 180 ℃ for 4 hours. After the second hydrothermal treatment, the slurry was centrifuged, washed and dried to obtain magnesium hydroxide as a mixture of hexagonal plates with an average particle size of 700nm and polyhedrons with an average particle size of 1.4 μm (very irregular in morphology compared to the sample obtained in example 2), and the SEM photograph is shown in fig. 6.

Claims

1. A method for preparing magnesium hydroxide, which is characterized by comprising the following steps:

(1) respectively preparing a magnesium chloride aqueous solution and a sodium hydroxide aqueous solution; the concentration of the magnesium chloride aqueous solution is 0.5-2 mol/L; the concentration of the sodium hydroxide aqueous solution is 0.6-5 mol/L;

(2) continuously introducing the magnesium chloride aqueous solution and the sodium hydroxide aqueous solution into a microreactor to react to obtain reaction slurry I;

(3) discharging the reaction slurry I from the outlet of the microreactor, and then feeding the reaction slurry I into a hydrothermal synthesis kettle to perform a first hydrothermal reaction to obtain reaction slurry II; the temperature of the first hydrothermal reaction is 180-220 ℃, and the reaction time is 2-6 h;

(4) centrifuging the reaction slurry II, and adding deionized water and solid sodium hydroxide into a centrifugal product to obtain a reaction product slurry III;

(5) carrying out a second hydrothermal reaction on the reaction slurry III, and centrifuging, washing and drying after the second hydrothermal reaction to obtain the magnesium hydroxide; the second hydrothermal temperature is 150-220 ℃, and the second hydrothermal time is 2-6 h.

2. Such as rightThe process for producing magnesium hydroxide according to claim 1, wherein in the step (2), the molar ratio n (Mg) of magnesium ions to hydroxyl groups²⁺):n(OH^-) 1:2.0-1:2.5, the magnesium hydroxide is polyhedral in shape, has an average particle size of 200-400nm and a specific surface area<10m^2/g; molar ratio of magnesium ion to hydroxyl n (Mg)²⁺):n(OH^-) When the ratio of the particle size to the thickness is 1:1.2-1:1.7, the magnesium hydroxide is hexagonal flake-shaped, the average particle size is 1.5-3.5 mu m, the ratio of the particle size to the thickness is 5-10, and the specific surface area is<10m²/g。

3. The method for preparing magnesium hydroxide according to claim 1, wherein in the step (2), the magnesium chloride aqueous solution and the sodium hydroxide aqueous solution are fed into the microreactor at the same flow rate, and the flow rate is 50-200 mL/min.

4. The method for preparing magnesium hydroxide according to claim 1, wherein in the step (2), the reaction temperature in the microreactor is 20 to 80 ℃ and the reaction residence time is 0.2 to 12 ms.

5. The method of claim 1, wherein the channels in the microreactor have a hydraulic diameter of 0.5 to 1mm and a channel length of 5 to 20 mm.

6. The method for producing magnesium hydroxide according to claim 1, wherein the reaction slurry iii has a solid content of 2 to 8 wt.% based on the magnesium hydroxide; the concentration of the sodium hydroxide in the reaction slurry III is 2-4 mol/L.