Hebei Messi Biology Co., Ltd. stated that magnesium hydroxide, as an important chemical product and intermediate, has a wide range of applications. In the field of environmental protection, it is called “green and safe water treatment agent” and can be used for neutralization of acidic wastewater, decolorization of printing and dyeing wastewater, removal of heavy metals, flue gas desulfurization, etc. It can also be used as an antacid, antibacterial bactericide, paper protector and a precursor for the preparation of magnesium oxide. As an environmentally friendly inorganic flame retardant, magnesium hydroxide has the advantages of green environmental protection, high thermal stability, flame retardancy, smoke elimination, and drip resistance, and has become a hot topic for scholars. However, magnesium hydroxide synthesized under room temperature conditions is mostly agglomerated solids, with a small average particle size of a single particle, a large specific surface area (BET), large surface polarity and microscopic strain of the (101) and (110) planes, and the grains are easily reunited. The dispersion in high molecular polymer materials is poor, which affects the processing performance of composite materials.
Hydrothermal modification technology is an effective method to reduce the surface polarity and surface microstrain of magnesium hydroxide, improve crystallinity, and improve particle dispersibility. However, direct hydrothermal treatment of magnesium hydroxide slurry, due to the high hydration characteristics of magnesium hydroxide, causes a large amount of crystal water to exist on the surface of the new nucleus, which hinders the superposition of growth units on the surface of the new nucleus and inhibits crystal growth, so hydrothermal treatment takes a long time and consumes a lot of energy. Researchers have also explored adding magnesium hydroxide filter cake to hydrothermal mineralizers to reduce hydrothermal time and improve the dispersibility of magnesium hydroxide.
Hydrothermal treatment in CaCl2 solution obtains flaky magnesium hydroxide with an average particle size of about 0.3~0.7μm and a BET value of 10.5m2·g-1, but the purity of magnesium hydroxide will inevitably decrease due to the addition of Ca2+. Hydrothermal treatment in monoethanolamine solution can prepare flaky magnesium hydroxide with an average particle size of 0.3~0.4μm. By hydrothermal treatment in KOH solution, flaky magnesium hydroxide with a purity of 98.88%, a BET value of 10.11m2·g-1, and a (101) surface microstrain of 2.57×10-3 can be obtained. Although the introduction of the above hydrothermal mineralizer can significantly reduce the hydrothermal time, the recycling of the hydrothermal mineralizer in the system has not been achieved.
Using 4moL·L-1NaOH as a hydrothermal mineralizer, hydrothermal treatment at 200℃ for 8h can prepare hexagonal flaky magnesium hydroxide with an average particle size of about 0.25μm, a thickness of about 61nm, and an agglomeration index of 10.95. In order to shorten the hydrothermal time, reduce the hydrothermal temperature, and efficiently prepare flame-retardant magnesium hydroxide that meets the first-level flame retardant magnesium hydroxide standard, this paper systematically studies the modification effect of high-concentration sodium hydroxide hydrothermal mineralizer on the basis of the previous work. After dilution, sodium hydroxide can be used as a precipitant for preparing a magnesium hydroxide precursor; the sodium chloride filtrate obtained by the reaction crystallization of magnesium hydroxide can be used as a lining liquid for the reaction to reduce the supersaturation of the system and improve the filtration performance of the product at room temperature, and can also produce sodium hydroxide through electrolysis to achieve the recycling of resources.