阅读说明：本技术 一种亚微米级氢氧化镁的制备方法 (Preparation method of submicron magnesium hydroxide ) 是由 陈光文 杨梅 罗腊梅 于 2019-02-28 设计创作，主要内容包括：本发明提供一种亚微米级氢氧化镁的制备方法,采用氢氧化钠和氯化钠的混合水溶液为沉淀剂,与氯化镁水溶液于一定温度下在微反应器内发生沉淀反应,反应浆料从微反应器流出后直接进入水热合成釜,在一定温度下对其进行水热处理。水热处理后,浆料经过滤、洗涤、干燥,得到亚微米级氢氧化镁。与仅采用氢氧化钠做沉淀剂相比,在相同水热条件下本方法制备的氢氧化镁平均粒径更大,更易过滤与洗涤,显著提高了生产效率。此外,本发明因在微反应器内进行沉淀反应,制备过程半连续化且易放大,产品批次间重复性好。(The invention provides a preparation method of submicron magnesium hydroxide, which adopts mixed aqueous solution of sodium hydroxide and sodium chloride as a precipitator to perform precipitation reaction with aqueous solution of magnesium chloride in a microreactor at a certain temperature, and reaction slurry directly enters a hydrothermal synthesis kettle after flowing out of the microreactor and is subjected to hydrothermal treatment at a certain temperature. After hydrothermal treatment, filtering, washing and drying the slurry to obtain the submicron magnesium hydroxide. Compared with the method which only adopts sodium hydroxide as a precipitator, the magnesium hydroxide prepared by the method under the same hydrothermal condition has larger average particle size, is easier to filter and wash, and obviously improves the production efficiency. In addition, the invention carries out precipitation reaction in the microreactor, so that the preparation process is semi-continuous and easy to amplify, and the repeatability among product batches is good.)

1. A method for preparing submicron-sized magnesium hydroxide is characterized by comprising the following steps:

(1) preparing a magnesium chloride aqueous solution with the concentration of 0.2-1 mol/L;

(2) preparing a mixed aqueous solution of sodium hydroxide and sodium chloride, wherein the concentration of the sodium hydroxide in the mixed aqueous solution is 0.32-2.0mol/L, the concentration of the sodium chloride is not less than 1mol/L, and the total concentration of the sodium hydroxide and the sodium chloride is not more than 5 mol/L;

(3) continuously introducing the magnesium chloride aqueous solution and the mixed aqueous solution into a microreactor for reaction, and discharging from an outlet of the microreactor after the reaction; the reaction temperature is 20-80 ℃, and the reaction residence time is 0.2-12 ms;

(4) directly feeding the reaction slurry discharged from the outlet of the microreactor into a hydrothermal synthesis kettle for hydrothermal treatment, wherein the hydrothermal temperature is 150 ℃ and 220 ℃, and the hydrothermal time is 2-10 h;

(5) and after the hydrothermal treatment, filtering, washing and drying the hydrothermal product to obtain the submicron magnesium hydroxide.

2. The method of producing submicron magnesium hydroxide according to claim 1, wherein the microreactor has a molar ratio of magnesium ions to hydroxyl groups n (Mg)²⁺):n(OH^-) 1: 1.6-2.0.

3. The method for preparing submicron-sized magnesium hydroxide according to claim 1, wherein in the step (3), the flow rate of the magnesium chloride aqueous solution and the flow rate of the mixed aqueous solution introduced into the microreactor are the same, and the flow rate ranges from 50mL/min to 200 mL/min.

4. The method for preparing submicron magnesium hydroxide according to claim 1, wherein the hydraulic diameter of the channel in the microreactor is 0.5-1mm, and the channel length is 5-20 mm.

5. The method for producing submicron magnesium hydroxide according to claim 1, wherein the particle size of the submicron magnesium hydroxide is 0.5 to 1.0 μm.

Technical Field

The invention relates to a preparation method of submicron magnesium hydroxide, belonging to the field of inorganic materials and microchemical engineering.

Background

Magnesium hydroxide is an important inorganic chemical product, and is widely used in the fields of material processing (flame retardance, fine ceramics, coatings and the like), environmental protection (wastewater treatment, flue gas desulfurization, heavy metal removal and the like), food addition and the like due to the characteristics of high thermal stability, strong adsorption capacity, alkalescence, no toxicity, no harm and the like. The preparation method of magnesium hydroxide can be classified into a solid phase method, a liquid phase method and a gas phase method according to the state of a substance. Among them, the liquid phase precipitation method is the most commonly used preparation method, and the used precipitating agents include limestone, sodium hydroxide, ammonia water and the like. When sodium hydroxide is used as a precipitant, the precipitation reaction speed is very high, so that the generated magnesium hydroxide has small particle size and high surface energy. In addition, the crystal form of the generated magnesium hydroxide is not developed fully, the exposure proportion of the polar crystal face (101) is high, so that the interparticle acting force is strong, and the colloid is easy to form, so that the filtering and washing of the reaction slurry are very difficult, the production cost is high, and the efficiency is low.

In order to improve the filtration performance of magnesium hydroxide, the skilled person has developed various strategies, mainly: (1) adding a surfactant. For example, modification of magnesium hydroxide by liquid precipitation with polyethylene glycol (PEG6000) has been studied by Lulixan et al. The particle size of the prepared magnesium hydroxide is reduced and then increased with the increase of the addition amount of PEG6000, and the particle size of the magnesium hydroxide obtained by the reaction is the smallest and about 0.2 μm when the addition amount of PEG6000 is 4%. With the increase of the addition amount of PEG6000, the dispersibility of magnesium hydroxide in organisms is better and better (inorganic salt process, 2018,50, 29-32); (2) adding an inorganic modifier. For example, CN201810969825.2 discloses a method for improving the filtration performance of magnesium hydroxide by using ammonia water or a strong acid weak base salt containing ammonium ions as an improving agent; (3) and (4) carrying out hydrothermal treatment. During the hydrothermal treatment, the magnesium hydroxide is dissolved and recrystallized, the average particle size is increased, and the filtering performance is improved. In order to increase the growth rate of magnesium hydroxide, the hydrothermal treatment is usually carried out in 3-6mol/L aqueous sodium hydroxide solution. However, the presence of sodium hydroxide complicates the post-treatment of the filtered supernatant and the washing wastewater. The sodium hydroxide in the filtered clear liquid and the washing wastewater can be treated by two modes, namely recycling and acid-base neutralization. Obviously, both approaches increase the complexity of the production process and the production cost. Therefore, it is desirable to provide a method for producing magnesium hydroxide slurry with improved filtration performance without increasing the complexity of the production process and the production cost.

Disclosure of Invention

The invention aims to make up the defects of the prior art and provides a preparation method of submicron magnesium hydroxide, namely, a mixed aqueous solution of sodium hydroxide and sodium chloride is used as a precipitator to strengthen the growth process of magnesium hydroxide so as to improve the filtering performance of magnesium hydroxide slurry. The invention discovers that the average particle size of magnesium hydroxide prepared by adopting the mixed aqueous solution of sodium hydroxide and sodium chloride as a precipitator is obviously larger than that of magnesium hydroxide prepared by adopting the mixed aqueous solution of sodium hydroxide and sodium chloride as a precipitator and the filtering performance of slurry is obviously improved compared with the conventional method of only adopting sodium hydroxide as a precipitator after hydrothermal treatment under the same condition. The method provided by the invention does not introduce compounds which are not involved in the reaction process of magnesium chloride and sodium hydroxide. The sodium chloride in the precipitant can adopt the same post-treatment mode (salt drying or evaporative crystallization) as the sodium chloride generated in the reaction process, so that the process steps are not increased, and the influence on the production cost is not great. The method comprises the following specific steps:

(1) preparing a magnesium chloride aqueous solution with the concentration of 0.2-1 mol/L;

(2) preparing a mixed aqueous solution of sodium hydroxide and sodium chloride, wherein the concentration of the sodium hydroxide in the mixed aqueous solution is 0.32-2.0mol/L, the concentration of the sodium chloride is not less than 1mol/L, and the total concentration of the sodium hydroxide and the sodium chloride is not more than 5 mol/L;

(3) continuously introducing the magnesium chloride aqueous solution and the mixed aqueous solution of sodium hydroxide and sodium chloride into a microreactor, discharging reaction slurry of the magnesium chloride aqueous solution and the mixed aqueous solution of sodium hydroxide and sodium chloride after precipitation reaction in the microreactor from an outlet of the microreactor, wherein the reaction temperature is 20-80 ℃; the reaction residence time is 0.2-12 ms;

(4) the reaction slurry directly enters a hydrothermal synthesis kettle after flowing out of the outlet of the microreactor, and is subjected to hydrothermal treatment, wherein the hydrothermal temperature is 150-220 ℃, and the hydrothermal time is 2-10 h;

(5) after hydrothermal treatment, the slurry is centrifuged, washed and dried to obtain submicron magnesium hydroxide.

Based on the technical scheme, the preferable molar ratio n (Mg) of magnesium ions to hydroxide radicals²⁺):n(OH^-) Is 1:1.6-1: 2.0.

Based on the technical scheme, preferably, the magnesium chloride aqueous solution and the mixed aqueous solution of sodium hydroxide and sodium chloride enter the microreactor at the same flow rate, and the flow rate range is 50-200 mL/min.

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, the average particle size of the magnesium hydroxide prepared by the method is 0.5-1 μm.

The invention has the following advantages:

(1) the method adopts the mixed aqueous solution of sodium hydroxide and sodium chloride as a precipitator, improves the growth rate of magnesium hydroxide in the subsequent hydrothermal treatment process, and does not introduce compounds which are not involved in the reaction process of the magnesium chloride and the sodium hydroxide. The sodium chloride in the precipitant can adopt the same post-treatment mode (salt drying or evaporative crystallization) as the sodium chloride generated in the reaction process, so that the process steps are not increased, and the influence on the production cost is not great.

(2) Because the heat mass transfer rate of the micro-reactor is 1-3 orders of magnitude higher than that of the traditional batch reactor, the magnesium chloride aqueous solution and the mixed aqueous solution of sodium hydroxide and sodium chloride can be quickly mixed to 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.

(3) The precipitation process is continuous, the production efficiency is high, and the process controllability is strong.

Drawings

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

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

FIG. 3 is an SEM photograph of a comparative example 1 product of the present invention;

FIG. 4 is an SEM photograph of a comparative example 2 product of the present invention.

Detailed Description

The present invention will be described in detail below by way of examples, but the present invention is not limited to the following examples. The microreactor used in the examples is described in simple terms as: consists of two closing plates and a microchannel plate. The microchannel plate is provided with three microchannels, namely two liquid inlet channels and a reaction channel which is respectively connected with the inlet channels, wherein the inlet channels are respectively connected with two inlets, and the reaction channel is connected with an outlet. The hydraulic diameters of the two inlet channels and the reaction channel are equal and are both 0.8 mm. The included angle between the two liquid inlet channels is 180 degrees, and the length of the reaction channel is 10 mm.