High throughput preparation of magnesium hydroxide flame retardant via microreaction technology

Abstract

Magnesium hydroxide (MH) has attracted much attention as an environmentally friendly flame retardant. In this paper, high throughput preparation of high-grade MH flame retardant was successfully achieved via microreaction technology and hydrothermal treatment. The results indicated that the reactant molar ratio and reactant concentration had a significant effect on the properties of MH including the morphology, average particle size and BET specific surface area, while the flow velocity and reaction temperature had little effect. To obtain MH with desirable properties, a highly concentrated NaOH solution was added into the hydrothermal slurry. The properties of MH were remarkably influenced by hydrothermal parameters such as NaOH concentration, time, temperature and solid content. High-grade MH flame retardant can be obtained with a NaOH concentration ≥2.4 M, hydrothermal time ≥3 h, hydrothermal temperature ≥170 °C and solid content ≤6.0 wt%. On the basis of laboratory work, a microreaction system based on a stacked plate microchannel reactor with an external volume of 7 L was developed for the pilot scale preparation of MH. The production capacity of MH slurry was up to 12.6 m³ h⁻¹, and the pressure drop of the microreaction system was kept at 1.1 bar. The average particle size and BET specific surface area of as-prepared MH were 1.0 μm and 3.4 m² g⁻¹, respectively, which met the requirement of high-grade flame retardant.

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1. Introduction

With the improving requirements of environmental protection, non-halogenated flame retardants such as magnesium hydroxide (MH), aluminum hydroxide, red phosphorus and ammonium polyphosphate have attracted increasing attention in recent years.^1,2 Due to its high decomposition temperature (i.e. 300–330 °C, which is ca. 100 °C higher than the most widely used aluminum hydroxide), effective smoke suppression and no toxic gas release during combustion, MH has wide application prospects as an environmentally friendly flame retardant.^3,4 To get an outstanding flame retardant performance, MH with small particle size, narrow particle size distribution (PSD), low specific surface area and hexagonal plate/whisker-like morphology is preferred. It has been found that reducing the particle size can improve the flame retardant performance of MH. However, MH with particle size at nanoscale (several tens of nanometers) is rarely used in industry as a flame retardant because of difficult filtration during the washing process and easy aggregation during the doping process.^5–7 Currently, the average particle size and specific surface area of high-grade MH flame retardant on the market is about 0.5–1.5 μm and less than 10 m² g⁻¹, respectively.^5,8–10

In general, there are mainly two approaches for producing high-grade MH flame retardant in industry. One is hydration of MgO which is commonly produced from MgCl₂ solution by spray roasting.^10,11 The other is wet precipitation with sodium hydroxide, ammonia or lime as the precipitating agent and bittern as the magnesium source.^12–15 Conventionally, wet precipitation is always carried out in a stirred batch reactor. The precipitation process involves nucleation, growth, agglomeration and aging stages, which are significantly influenced by the process parameters (e.g. reaction temperature, reactant concentration, residence time) and in turn affect the final particle size, PSD and morphology, etc. Therefore, the precise control of the process parameters is a prerequisite for the production of high-grade MH flame retardant. Unfortunately, it is difficult for the conventional process to offer a uniform precipitation condition owing to the limitation of mixing in the stirred batch reactor, which inevitably leads to a wide PSD and poor reproducibility.^16,17

To overcome the disadvantages of conventional process mentioned above, several novel methods focusing on process intensification have been proposed. Tai et al. developed a continuous spinning disk reactor for preparing lamellar MH.¹⁸ The as-prepared lamellar MH was 50–80 nm in length and 10 nm in thickness. Another method which has shown great potential in the preparation of micro/nano materials is microreaction technology.^19–24 Due to the high heat and mass transfer rate resulting from its characteristic channel size of submillimeter, microchannel reactor can accurately control the process parameters, which guarantees narrow PSD as well as stable product quality in principle. Additionally, the continuous operation and numbering-up of microchannel reactor are both beneficial for the large-scale production of MH. Accordingly, Yoshiyuki et al. prepared MH with the particle size of 20–50 nm by using a special type of microreactor disclosed in their patent.²⁵ Shirure et al. used narrow channel reactor to prepare MH, and the particle size of the as-prepared MH was 5–30 μm (actually the secondary particle size).²⁶ Compared with the conventional stirred batch reactor, MH prepared in microchannel reactor owns smaller particle size, narrower PSD and better reproducibility, etc. However, the particle size of MH prepared via wet precipitation in microchannel reactor is commonly in several tens of nanometers, which cannot meet the requirement of high-grade flame retardant. In order to obtain MH with desirable average particle size, PSD, specific surface area and morphology, hydrothermal treatment has been extensively applied.^27,28 Ding et al. synthesized rod-, tube-, needle- and lamella-like MH with the particle size of 25–200 nm via a hydrothermal method.²⁹ The results showed that the characteristics of the products such as the particle size and morphology could be tuned by choosing different solvents and reaction conditions. By using agglomerate MH powder as raw material, Xiang et al. studied the growth behavior of MH in NaOH aqueous solution under hydrothermal condition.³⁰ They found the average particle size of MH increased and BET specific surface area decreased with the increase of NaOH concentration.

So far, few studies have combined microreaction technology with hydrothermal treatment to prepare MH flame retardant. The combination of microreaction technology with hydrothermal treatment is expected to be beneficial for the preparation of high-grade MH flame retardant. In this paper, high throughput preparation of hexagonal plate-like MH with controllable particle size, narrow PSD and low specific surface area was achieved based on microreaction technology and hydrothermal treatment. A continuous flow T-type microchannel reactor with a special outlet structure was adopted to acquire relatively high Reynolds number. The effects of process parameters including reaction and hydrothermal conditions on the morphology, average particle size and specific surface area of MH were studied in detail. On the basis of laboratory work, a microreaction system involving a stacked plate microchannel reactor with the external volume of 7 L was fabricated. Furthermore, a pilot scale experiment with the production capacity of MH slurry of 12.6 m³ h⁻¹ was conducted and high-grade MH flame retardant was obtained.

2. Experimental section

2.1. Materials

Sodium hydroxide (NaOH) and magnesium chloride hexahydrate (MgCl₂·6H₂O) were purchased from Kemiou Chemical Reagent Co. (Tianjin, China). All reagents were analytical pure and used as received without further purification. Deionized water was used in all experiments.

2.2. Microchannel reactor

Fig. 1 shows the photos of the microchannel reactor and T-type microchannel plate. The microchannel reactor was comprised of a microchannel plate and a blind plate, which were sealed by two stainless steel plates. The microchannel was fabricated on a polyarylsulfone plate which was resistant to corrosion and high temperature. The size of two inlet channels and one reaction channel was 10 mm (length) × 0.8 mm (width) × 0.8 mm (depth) with a rectangular cross section. For the purpose of decreasing pressure drop of the entire reactor and preventing the reactor from blockage, the outlet at the end of the reaction channel was designed without any turnings.

Fig. 1 Photos of (a) microchannel reactor and (b) T-type microchannel plate.

2.3. MH preparation

MH was prepared by wet precipitation in a T-type microchannel reactor followed by hydrothermal treatment. The experimental setup for the preparation of MH is illustrated in Fig. 2. In a typical experiment, 2.0 M NaOH solution and 1.0 M MgCl₂ solution were pumped into microchannel reactor with the same volume flow rate of 75 mL min⁻¹. The molar ratio of Mg²⁺ to OH⁻ was 0.50 and the flow velocity in the reaction channel was 3.9 m s⁻¹. A water bath was used to control the reaction temperature at 70 °C. According to whether NaOH was used in hydrothermal treatment, MH was prepared by two routes. In route A, one part of the slurry obtained from microchannel reactor was washed with deionized water thoroughly and dried at 100 °C for 6 h. The other part was put into a Teflon-lined stainless steel autoclave (100 mL) and treated at 180 °C for 4 h. The autoclave was fixed on the arm of axle in the oven and rotated at 50 rpm. After cooling down to room temperature naturally, the slurry was centrifuged, washed with deionized water thoroughly, and dried at 100 °C for 6 h. In route B, the molar ratio of Mg²⁺ to OH⁻ was fixed at 0.60 while other reaction conditions were kept unchanged. Afterwards, the slurry obtained from microchannel reactor was centrifuged and the obtained cake was dispersed into a certain amount of NaOH solution. The concentration of NaOH was kept at 3.0 M and the solid content of MH was 4.0 wt% in the hydrothermal slurry. The suspension was put into a Teflon-lined stainless steel autoclave and treated at 180 °C for 4 h. The rest of the procedure was the same with route A.

Fig. 2 The experimental setup for the preparation of MH.

2.4. Characterization methods

X-ray diffraction (XRD) analysis was performed on a PANalytical X'Pert-Pro powder X-ray diffractometer using Cu Kα radiation source (λ = 0.1541 nm) with a scanning rate of 10° min⁻¹ in a 2θ range from 10° to 80°. Transmission electron microscope (TEM) images were obtained on a JEOL JEM-2000EX with the accelerating voltage of 120 kV. Scanning electron microscope (SEM) images were obtained on a Hitachi S-4800 with the accelerating voltage of 10 kV. The PSD of MH obtained from the pilot scale system was measured on a Winner2000B laser particle size analyzer by means of dynamic light scattering with the deionized water as dispersant. The free distribution analysis mode was adopted, and the size distribution by volume was calculated by Mie theory based on intensity distribution. The specific surface area was analyzed on a Quantachrome Quadrasorb SI instrument by Brunauer–Emmett–Teller equation based on the N₂ adsorption–desorption isotherm.

3. Results and discussion

3.1. Before and after hydrothermal treatment without NaOH

3.1.1. Effect of the molar ratio of Mg²⁺ to OH⁻. The molar ratio of Mg²⁺ to OH⁻ was varied by adjusting the flow rate of the two reactants while the reactant concentration was kept unchanged. Fig. 3 shows the XRD patterns of MH prepared at the molar ratio of Mg²⁺ to OH⁻ of 0.42 (20% excess of NaOH), 0.50 (stoichiometric ratio) and 0.60 (20% excess of MgCl₂). All diffraction peaks in the XRD patterns of the as-prepared MH before and after hydrothermal treatment can be assigned to hexagonal Mg(OH)₂ (JCPDS 07-0239). No other diffraction peaks are found, indicating a high purity of the as-prepared MH.

Fig. 3 The XRD patterns of MH prepared at different molar ratio of Mg²⁺ to OH⁻ before and after hydrothermal treatment.

As shown in Fig. 4a, the as-prepared MH exhibits a hexagonal plate-like morphology and the particle size is uniform at the molar ratio of Mg²⁺ to OH⁻ of 0.42 before hydrothermal treatment. As the molar ratio of Mg²⁺ to OH⁻ increases to 0.50, MH with quasi-circular plate-like morphology appears (Fig. 4b). When the molar ratio of Mg²⁺ to OH⁻ further increases to 0.60, the as-prepared MH is mainly composed of irregular plates (Fig. 4c). After hydrothermal treatment at 180 °C for 4 h, the morphology of MH (hexagonal plate) is well preserved at the molar ratio of Mg²⁺ to OH⁻ of 0.42 (Fig. 4d). Hexagonal plates and quasi-circular plates both exist at the molar ratio of Mg²⁺ to OH⁻ of 0.50 (Fig. 4e), while only quasi-circular plates are found at the molar ratio of Mg²⁺ to OH⁻ of 0.60 (Fig. 4f).

Fig. 4 The TEM images of MH prepared at different molar ratios of Mg²⁺ to OH⁻: (a, d) 0.42, (b, e) 0.50, (c, f) 0.60. (a–c) and (d–f) were MH prepared before and after hydrothermal treatment, respectively (reaction parameters: u = 3.9 m s⁻¹, C_MgCl2 = 1.0 M, C_NaOH = 2.0 M, T = 70 °C; hydrothermal parameters: T = 180 °C, t = 4 h).

Fig. 5a shows the average particle size of MH prepared at different molar ratios of Mg²⁺ to OH⁻ after hydrothermal treatment. It can be seen that the average particle size of MH increases from 150 nm to 370 nm as the molar ratio of Mg²⁺ to OH⁻ increases from 0.42 to 0.60. This may be caused by the fact that the nucleation and growth stages were conducted at different pH values. When the molar ratio of Mg²⁺ to OH⁻ was 0.42, the pH value of the reacting mixture was higher than the isoelectric point of MH (ca. pH 12). Therefore, negative electric charge was expected on the surface of MH plates and Na⁺ ions were sorbed around MH plates, which hindered the incoming Mg²⁺ ions and resulted in smaller particle size.³¹ On the contrary, larger particle size was obtained at the molar ratio of Mg²⁺ to OH⁻ of 0.60 because positive electric charge was expected on the surface of MH plates. In addition, the different morphologies of MH prepared at different molar ratios of Mg²⁺ to OH⁻ after hydrothermal treatment may also be ascribed to the altered pH value of hydrothermal slurry. In other words, the strong or weak alkaline hydrothermal environment led to a difference in the growth rate of different crystal facets, thus making the dissimilar morphology appear. Fig. 5b depicts the BET specific surface area of MH before and after hydrothermal treatment. Evidently, hydrothermal treatment reduces the BET specific surface area of MH remarkably. Taking MH prepared at the molar ratio of Mg²⁺ to OH⁻ of 0.42 as an example, the BET specific surface area before and after hydrothermal treatment is 80.2 m² g⁻¹ and 19.1 m² g⁻¹, respectively. Furthermore, the molar ratio of Mg²⁺ to OH⁻ has little effect on the BET specific surface area. This phenomenon can be attributed to the fact that the BET specific surface area of plate-like MH is determined not only by the average particle size but also the particle thickness. It can be predicted that the particle thickness decreases with the increase in the molar ratio of Mg²⁺ to OH⁻, thus maintaining the BET specific surface area at a constant value. The PSD of MH after hydrothermal treatment is shown in Fig. 5c–e. The histogram of PSD was obtained by measuring at least two hundred particles in each sample. The particle size are mainly in the range of 80–300 nm, 100–400 nm and 100–700 nm when the molar ratio of Mg²⁺ to OH⁻ is 0.42, 0.50 and 0.60, respectively. The range of PSD becomes wide with the increase of average particle size, while the PSD are all quite narrow with the corresponding average particle size.

Fig. 5 Effect of the molar ratio of Mg²⁺ to OH⁻ on the (a) average particle size of MH after hydrothermal treatment (b) BET specific surface area of MH before and after hydrothermal treatment; (c–e) PSD of MH after hydrothermal treatment (reaction parameters: u = 3.9 m s⁻¹, C_MgCl2 = 1.0 M, C_NaOH = 2.0 M, T = 70 °C; hydrothermal parameters: T = 180 °C, t = 4 h).

3.1.2. Effect of the flow velocity. In this section, the flow velocity varied from 2.0 m s⁻¹ to 10.0 m s⁻¹, and the corresponding Reynolds numbers changed from 1500 to 8000. Evidently, the precipitation process was carried out at turbulent flow. The theoretical mixing time was calculated to be 0.3–0.8 ms in the above Reynolds number range according to the previous study of Falk et al.³² The residence time of the reacting mixture in the reaction channel was 1–5 ms, which was much longer than the mixing time. This ensured the precipitation process proceeded sufficiently in the microchannel reactor. The morphologies of MH prepared at different flow velocities after hydrothermal treatment were quite similar with that in Fig. 4e. Fig. 6 shows the average particle size and BET specific surface area of MH prepared at different flow velocities. As the flow velocity increases from 2.0 m s⁻¹ to 10.0 m s⁻¹, the average particle size of MH after hydrothermal treatment is maintained in the range of 200–240 nm. The BET specific surface areas of MH before and after hydrothermal treatment are ca. 85 m² g⁻¹ and 20 m² g⁻¹, respectively. It can be seen that the morphology, average particle size and BET specific surface area are unaffected by the flow velocity because of sufficient mixing in the range investigated.

Fig. 6 Effect of the flow velocity on the (a) average particle size of MH after hydrothermal treatment and (b) BET specific surface area of MH before and after hydrothermal treatment (reaction parameters: n(Mg²⁺/OH⁻) = 0.50, C_MgCl2 = 1.0 M, C_NaOH = 2.0 M, T = 70 °C; hydrothermal parameters: T = 180 °C, t = 4 h).

3.1.3. Effect of the reactant concentration. Fig. 7 shows the TEM images of MH prepared at different MgCl₂ concentrations before and after hydrothermal treatment. Before hydrothermal treatment, the as-prepared MH is composed of irregular fragments and the size of original precipitate is lower than 20 nm at the MgCl₂ concentration of 0.1 M. When the MgCl₂ concentration increases to 1.0 M and 3.0 M, the as-prepared MH exhibit quasi-circular plate-like morphology with average particle size ca. 50 nm and hexagonal plate accompanied by irregular fragment-like morphology with average particle size ca. 40 nm, respectively. This may be explained by the positive correlation between reactant concentration and supersaturation. The supersaturation is low when the MgCl₂ concentration is 0.1 M. Consequently, the nucleation process is slow and the generated nuclei are difficult to grow up, thus making the tiny nuclei coalesce with each other to form irregular agglomerate. When the MgCl₂ concentration increases to 1.0 M and 3.0 M, the nucleation process is rapid and the generated nuclei are easy to grow up due to the high supersaturation. After hydrothermal treatment, the morphology of MH turns into quasi-circular plate at the MgCl₂ concentration of 0.1 M. Hexagonal plates and quasi-circular plates both exist at the MgCl₂ concentration of 1.0 M, while MH prepared at the MgCl₂ concentration of 3.0 M only consist of hexagonal plates. With the increase in MgCl₂ concentration, the average particle size of MH after hydrothermal treatment firstly increases from 40 nm to 200 nm and then keeps at ca. 200 nm, while the BET specific surface area decreases significantly from 95.5 m² g⁻¹ to 22.1 m² g⁻¹ and then is maintained at ca. 20 m² g⁻¹ (Fig. 8). The aforementioned results suggest the MgCl₂ concentration is supposed to be not lower than 1.0 M to prepare MH flame retardant with relatively large particle size and low BET specific surface area.

Fig. 7 The TEM images of MH prepared at different MgCl₂ concentration (a, d) 0.1 M (b, e) 1.0 M (c, f) 3.0 M. (a–c) and (d–f) were MH prepared before and after hydrothermal treatment, respectively (reaction parameters: n(Mg²⁺/OH⁻) = 0.50, u = 3.9 m s⁻¹, T = 70 °C; hydrothermal parameters: T = 180 °C, t = 4 h).

Fig. 8 Effect of MgCl₂ concentration on the (a) average particle size of MH after hydrothermal treatment and (b) BET specific surface area of MH before and after hydrothermal treatment (reaction parameters: n(Mg²⁺/OH⁻) = 0.50, u = 3.9 m s⁻¹, T = 70 °C; hydrothermal parameters: T = 180 °C, t = 4 h).

3.1.4. Effect of reaction temperature. Fig. 9a shows the average particle size of MH prepared at different reaction temperatures after hydrothermal treatment. As the reaction temperature increases from 25 °C to 95 °C, the average particle size of MH is maintained in the range of 200–230 nm. The BET specific surface area decreases slightly from 102.0 m² g⁻¹ to 84.0 m² g⁻¹ before hydrothermal treatment and keeps around 20 m² g⁻¹ after hydrothermal treatment (Fig. 9b). In addition, MH prepared at different reaction temperatures after hydrothermal treatment show similar morphology with Fig. 4e. Apparently, the reaction temperature has little influence on the average particle size, BET specific surface area and morphology. As is well known, the reaction temperature can affect product properties through controlling the nucleation and growth process. However, the supersaturation in the nucleation process is extremely high because of the high concentration of the reactant. Furthermore, owing to the extremely short residence time (1–5 ms) in the microchannel reactor, the growth process is mainly conducted in the Teflon-lined stainless steel autoclave with the same temperature. Consequently, the influence of reaction temperature may be covered up due to the reasons mentioned above.

Fig. 9 Effect of the reaction temperature on the (a) average particle size of MH after hydrothermal treatment and (b) BET specific surface area of MH before and after hydrothermal treatment (reaction parameters: n(Mg²⁺/OH⁻) = 0.50, u = 3.9 m s⁻¹, C_MgCl2 = 1.0 M, C_NaOH = 2.0 M; hydrothermal parameters: T = 180 °C, t = 4 h).

3.2. Hydrothermal treatment with NaOH

3.2.1. Effect of the molar ratio of Mg²⁺ to OH⁻. Due to the small particle size and high specific surface area of MH prepared without additive in the hydrothermal process, high concentrated NaOH solution was added into the hydrothermal slurry. The effect of the molar ratio of Mg²⁺ to OH⁻ was first investigated because of its significant effect on the morphology of MH before hydrothermal treatment (Section 3.1.1). As shown in Fig. 10a–c, the as-prepared MH all exhibit a hexagonal plate-like morphology when the molar ratio of Mg²⁺ to OH⁻ varies from 0.42 to 0.60. The average particle size of MH synthesized at the molar ratio of Mg²⁺ to OH⁻ of 0.42, 0.50 and 0.60 was 140 nm, 210 nm and 610 nm, respectively (Fig. 10d). Interestingly, the average particle size of MH synthesized at the molar ratio of Mg²⁺ to OH⁻ of 0.42 and 0.50 were similar with those hydrothermally treated without NaOH, while the average particle size of MH prepared at the molar ratio of Mg²⁺ to OH⁻ of 0.60 was much larger. This result may be caused by the faster growth rate of irregular plates during the dissolution–recrystallization process in high concentrated NaOH solution. This inference can also be supported by the fact that the specific surface area of MH remarkably declines from 18.6 m² g⁻¹ to 5.1 m² g⁻¹ when the molar ratio of Mg²⁺ to OH⁻ increases from 0.42 to 0.60 (Fig. 10e). In addition, the PSD of MH prepared at the molar ratio of Mg²⁺ to OH⁻ of 0.60 is mainly in the range of 200–1400 nm (Fig. 10f), which displays a narrow feature with the average particle size of 610 nm. For the purpose of preparing MH with relatively large size and low specific surface area, the reactant molar ratio of Mg²⁺ to OH⁻ was fixed at 0.60 as described in Section 2.3 (route B).

Fig. 10 TEM and SEM images of MH prepared under different molar ratio of Mg²⁺ to OH⁻: (a) 0.42, (b) 0.50, (c) 0.60 after hydrothermal treatment with NaOH; effect of molar ratio of Mg²⁺ to OH⁻ on the (d) average particle size and (e) BET specific surface area of MH after hydrothermal treatment with NaOH; (f) PSD of MH sample with molar ratio of Mg²⁺ to OH⁻ of 0.60 after hydrothermal treatment with NaOH (hydrothermal parameters: C_NaOH = 3.0 M, T = 180 °C, t = 4 h, ω = 4.0 wt%).

3.2.2. Effect of the hydrothermal parameters. The effect of hydrothermal parameters including NaOH concentration, time, temperature and solid content were investigated in detail. Fig. 11 shows the effect of NaOH concentration in the hydrothermal slurry on the average particle size and BET specific surface area of MH. It can be seen that the average particle size increases approximately linearly from 220 nm to 1200 nm, while the BET specific surface area decreases from 14.0 m² g⁻¹ to 3.3 m² g⁻¹ as the NaOH concentration increases from 0.5 M to 5.0 M. This trend can be attributed to the promotion effect of OH⁻ on the growth rate of the precursor in the hydrothermal slurry. By the linear interpolation, the NaOH concentration should be higher than 2.4 M in order to obtain high-grade MH flame retardant with the average particle size larger than 500 nm and the BET specific surface lower than 10 m² g⁻¹.

Fig. 11 Effect of NaOH concentration on the (a) average particle size and (b) BET specific surface area of MH after hydrothermal treatment with NaOH (reaction parameter: n(Mg²⁺/OH⁻) = 0.60; hydrothermal parameters: T = 180 °C, t = 4 h, ω = 4.0 wt%).

Fig. 12a and b show the effects of hydrothermal time on the average particle size and BET specific surface area of MH. When the hydrothermal time rises from 0 h to 10 h, the average particle size of MH increases approximately linearly from 100 nm to 1100 nm, while the BET specific surface area decreases dramatically from 36.0 m² g⁻¹ to 7.0 m² g⁻¹ within the first two hours and gradually to ca. 3 m² g⁻¹ in the subsequent hours. The as-prepared MH all exhibit a hexagonal plate-like morphology and narrow PSD. The representative SEM image and PSD of MH hydrothermally treated for 10 h are shown in Fig. 12c and d. It can be seen that the PSD is mainly in the range of 300–2800 nm, which is quite narrow with the average particle size of 1100 nm. Consequently, the hydrothermal time should be not less than 3 h for the purpose of preparing high-grade MH flame retardant. In addition, it is worth pointing out that 0 h means the time consumed by heating process from room temperature to 180 °C is not counted in the hydrothermal time. From the red line in Fig. 12a it can be seen the average particle size of MH increases from 50 nm to 100 nm during heating process.

Fig. 12 Effect of hydrothermal time on the (a) average particle size and (b) BET specific surface area of MH after hydrothermal treatment with NaOH; (c) SEM and (d) PSD of MH sample with hydrothermal time of 10 h (reaction parameter: n(Mg²⁺/OH⁻) = 0.60; hydrothermal parameters: C_NaOH = 3.0 M, T = 180 °C, ω = 4.0 wt%).

Fig. 13a and b show the effects of hydrothermal temperature on the average particle size and BET specific surface area. As the hydrothermal temperature increases from 140 °C to 200 °C, the average particle size of MH rises from 220 nm to 880 nm, while the BET specific surface area decreases from 11.0 m² g⁻¹ to 3.1 m² g⁻¹. Taking MH prepared at 200 °C as an example, the as-prepared MH is composed of hexagonal plates and the PSD is mainly in the range of 300–1700 nm, which is quite uniform with the average particle size of 880 nm (Fig. 13c and d). Accordingly, the hydrothermal temperature needs to be not less than 170 °C for preparing high-grade MH flame retardant.

Fig. 13 Effect of hydrothermal temperature on the (a) average particle size and (b) BET specific surface area of MH after hydrothermal treatment with NaOH; (c) SEM and (d) PSD of MH sample with hydrothermal temperature of 200 °C (reaction parameter: n(Mg²⁺/OH⁻) = 0.60; hydrothermal parameters: C_NaOH = 3.0 M, t = 4 h, ω = 4.0 wt%).

Fig. 14a and b show the effects of the solid content on the average particle size on the average particle size and BET specific surface area. With the increase of the solid content from 2.0 wt% to 6.0 wt%, the average particle size and BET specific surface area of MH are ca. 600 nm and 4 m² g⁻¹, respectively. However, the average particle size decreases to 280 nm and the BET specific surface area rises to 7.4 m² g⁻¹ when the solid content increases to 8.0 wt%. This may be caused by the reduced amount of OH⁻ for each growth unit of MH. Additionally, the PSD of hexagonal plate-like MH is mainly in the range of 100–800 nm when the solid content is 8.0 wt%, which is not narrow with the average particle size of 280 nm. This result may be caused by the partial uneven distribution of MH in the hydrothermal slurry (Fig. 14c and d). As a result, the solid content of MH in the hydrothermal slurry should not be higher than 6.0% in order to prepare high-grade MH flame retardant.

Fig. 14 Effect of solid content on the (a) average particle size and (b) BET specific surface area of MH after hydrothermal treatment with NaOH; (c) SEM and (d) PSD of MH sample with solid content of 8.0 wt% (reaction parameter: n(Mg²⁺/OH⁻) = 0.60; hydrothermal parameters: C_NaOH = 3.0 M, T = 180 °C, t = 4 h).

3.3. Scaling up

In the past decade, researchers in academia and industry have made great effort to realize the industrial application of microreaction technology.^33–35 For example, Krtschil et al. reported a pilot plant comprised of three microstructured reactors which were operated in parallel for the Kolbe–Schmitt synthesis of 2,4-dihydroxybenzoic acid.³⁶ Wolf et al. developed a valve-assisted micromixer with a throughput up to 300 L h⁻¹, and the micromixer was used for the preparation of catalysts from laboratory to industrial scale.³⁷ Ghaini et al. reported the modular microstructured reactors produced by new manufacturing techniques for pilot and production scale chemistry.³⁸ Despite the progress in the industrial application of microreaction technology, the production of high-grade MH flame retardant via microreaction technology in industrial scale has not been reported up to now.

Based on the aforementioned results of single microchannel reactor, a stacked plate microchannel reactor made of stainless steel was fabricated for the scaling up experiment. The stacked plate microchannel reactor is composed of 66 units and each unit has 64 reaction channels with the size of 40 mm (length) × 0.7 mm (width) × 2.0 mm (depth). The external dimension of stacked plate microchannel reactor is 218 mm (length) × 215 mm (width) × 150 mm (height) and the corresponding volume is equal to 7 L. Fig. 15a shows the schematic diagram of microreaction system for the production of MH flame retardant based on the stacked plate microchannel reactor. Before the experiment begins, the flow rate of the two raw materials were adjusted. Taking the brine as an example, line 1 was designed for shunting the flow from the main line and line 2 was for making the brine flow back to the MgCl₂ tank instead of entering the microchannel reactor. In addition, the pipe filter and check valve in the main line were installed for filtering impurities and preventing return flow, respectively. Similarly, the flowmeter and pressure gage were for monitoring the flow rate of the raw material and pressure drop of the microchannel reactor, respectively. Fig. 15b shows the photo of microreaction system.

Fig. 15 (a) Schematic diagram of microreaction system, (b) photo of microreaction system and (c) microreaction system in the production of MH slurry.

Fig. 15c shows the microreaction system in production. The operating process of microreaction system was implemented as follows. Firstly, the refined brine was pumped into the MgCl₂ tank and diluted to 1.4 M. Meanwhile, the concentrated NaOH solution was diluted from 11.3 M to 5.4 M and then pumped into the NaOH tank. Secondly, the two kinds of raw material were both heated to 60 °C and then pumped to the stacked plate microchannel reactor. The reactant molar ratio of Mg²⁺ to OH⁻ was kept at 0.60 and the flow rate of the brine and NaOH solution was 8.8 m³ h⁻¹ and 3.8 m³ h⁻¹, respectively. Correspondingly, the production capacity of MH slurry was 12.6 m³ h⁻¹. The precipitation process proceeded continuously for 30 min and 6.3 m³ of MH slurry was collected. Thirdly, the slurry was pumped into the slurry storage tank and then fed into the pressure filter. The obtained filter cake was added into a slurry tank together with 2.8 m³ of 5.3 M NaOH solution. The deionized water was added until the total volume of the slurry in the tank reached to 6.3 m³. The concentration of NaOH and solid content in the slurry were measured to be 2.2 M and 4.6 wt%, respectively. After that, the slurry in the tank was stirred sufficiently and pumped into the autoclave with the volume of 8 m³. The hydrothermal process was performed at 200 °C for 6 h with the stirring speed of 80 rpm. Finally, MH flame retardant was achieved after filtrating, washing and drying.

As shown in Fig. 16a, the as-prepared MH shows hexagonal plate-like morphology. The average particle size and BET specific surface area are 1.0 μm (determined by SEM) and 3.4 m² g⁻¹, respectively. The PSD of as-prepared MH measured by a laser particle size analyzer exhibits a unimodal distribution with the D₁₀ of 0.78 μm and D₉₀ of 5.42 μm (Fig. 16b). In summary, high-grade MH flame retardant was successfully prepared in the microreaction system with the throughput of 12.6 m³ h⁻¹. The microreaction system was stable and no clogging problem was observed in the production process, which could be supported by the fact that the pressure drop of microchannel reactor kept at 1.1 bar all the time. In addition, the reaction process can be easily controlled due to the small volume of microchannel reactor. However, the recycling of NaOH solution after hydrothermal treatment needs to be studied in detail. Furthermore, the combination of hydrothermal parameters needs to be optimized for the purpose of preparing high-grade MH flame retardant with minimum cost.

Fig. 16 (a) SEM image and (b) PSD of the pilot scale product of MH.

4. Conclusions

High throughput preparation of MH slurry in a T-type microchannel reactor was conducted continuously. Combined with hydrothermal process, the hexagonal platelike MH flame retardant with controllable average particle size and narrow PSD were prepared. The results indicated that the reactant molar ratio and reactant concentration had significant effect on the properties of MH including morphology, average particle size and BET specific surface area, while the flow velocity and reaction temperature had little effect. To obtain MH with desirable properties, a high concentrated NaOH solution was added into the hydrothermal slurry. The properties of MH were remarkably influenced by hydrothermal parameters such as NaOH concentration, time, temperature and solid content. High-grade MH flame retardant can be obtained with the NaOH concentration ≥2.4 M, hydrothermal time ≥3 h, hydrothermal temperature ≥170 °C and the solid content ≤6.0 wt%. Based on the laboratory work, a pilot scale experiment was carried out with the production capacity of MH slurry up to 12.6 m³ h⁻¹. Accordingly, high-grade MH flame retardant was obtained with large average particle size (1.0 μm), narrow PSD and low specific surface area (3.4 m² g⁻¹).

Acknowledgements

We gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 21225627, 21406226).

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