Ind. Eng. Chem. Res. 2009, 48, 763–768
763
Synthesis and Characterization of Magnesium Hydroxide Using a Bubbling Setup X. Li, G. B. Ma, and Y. Y. Liu* College of Chemical Engineering, Shenyang Institute of Chemical Technology, Shenyang 110142, China
Magnesium hydroxide [MH] with porous nanoplates were synthesized from boric magnesium fertilizer (MgSO4 · 7H2O), using a large bubbling setup designed by the authors. The regular hexagonal plates of MH with a smaller specific area were formed after hydrothermal treatment. The effects of various operating variables, including the ammonia flow rate, reaction temperature, magnesium source, particle sizes, and dispersion properties of the synthesis products were investigated. X-ray diffraction, scanning electron microscopy, laser particle size analyzer, and thermogravimetric analyzer have been employed to characterize these products. The results showed that the mean particle size (D50) of the MH was about 2 µm, and products were loose enough for filtering. The diffraction intensity of the (001) direction was higher than the (101) direction. The Mg2+ conversion in the continuous setup reached 95% and the decomposition temperature of products was 370 °C. 1. Introduction In recent years, more and more environmental legislations have limited the application of halogen-containing flame retardants. Inorganic hydroxide fillers have replaced the halogenbased flame retardants in many applications such as polypropylene, polyethylene, ethylene vinyl acetate, and polyamide.1 There is a growing interest in magnesium hydroxide (MH) as flame retardant filler that does not evolve toxic substances upon combustion. MH was decomposited into MgO and H2O at a relatively higher temperature (300 °C) than the most widely used aluminum trihydrate fillers. In the flame phase, water vapor forms an envelope around the flame, which tends to exclude the air and dilute the flammable gases. The MgO residue forms a thermally inert layer that could create a protective barrier on the polymer substrate.2,3 It has to be noted that the inorganic MH filler can affect the mechanical properties of the polymer matrix. The mechanical strength of composites can be enhanced by dispersing nanometer-sized inorganic platelets of the filler in the polymer.4-7 Many studies have been recently focusing on the morphology control and filter improvement of the nano-MH.8-10 Utamapanya and co-workers prepared MH nanoparticles via a sol-gel technique followed by a hypercritical drying procedure using metal-organic precursors in the sol-gel procedure.11 Ding et al. employed hydrothermal reaction using different precursors and solvents as reactants to prepare MH with controlled morphologies.12 Booster synthesized MH from magnesium fluoride as part of a process to bleed magnesium from zinc sulfate electrolyte.13 Xiang synthesized MH via MgCl2-NaOH route and hydrothermal modification route in a large-scale experimental setup.14 Li and their co-workers proposed a route to synthesize MH nanorods, based on the hydrothermal reaction of pure magnesium powder in an autoclave at higher temperature using long reaction time and low reactant concentration.15 Similarly, Yu and co-workers produced MH nanoplates by the hydrothermal reaction of commercial bulk magnesium oxide crystals.16 All these chemical synthesis were, however, characterized by problems such as relatively low yields and difficulty filtering and were economically unacceptable. As a result, it is still desired to develop a high-yield method capable of generat* To whom correspondence should be addressed. Tel.: +86-24-89383902. Fax: +86-24-8938-3760. E-mail:
[email protected].
ing nanosized MH in a moderate quantity with a well-controlled dimension and morphology. On an industrial scale it is necessary to produce large quantities of uniform precipitate with low cost. For such a complex system, the use of ammonia gas bubbling reactors is very interesting because of the relative ease of using them to produce uniform and consistent precipitates in a continuous mode. There is no published literature available on precipitation of MH in ammonia gas bubbling reactor.17-19 In addition, China has a plentiful magnesium resource and the boric magnesium fertilizer from Liaoning Province is used as a raw material for MH manufacture after simple purification pretreatment. Ammonia is also relatively cheaper than NaOH as precipitant. All mentioned above will reduce the preparation cost of MH. In the present work, we extended this bubbling method to synthesize MH using boric magnesium fertilizer as the starting material on a large experiment setup. In comparison with conventional methods, the new approach was simple and more suitable for industrial scale production. The objective of this paper was to investigate how to influence the MH particle size, shape, and agglomeration level, during and after synthesis by ammonia precipitation in ammonia gas bubbling reactors. In addition, the obtained MH was analyzed with XRD, SEM, TGDTA, and laser particle size analyzer to determine its morphology and crystal structure. 2. Experimental Section 2.1. Preparation of MH. The samples were obtained by precipitation, at a controlled temperature 40-60 °C, of a magnesium salt solution of concentration 1.2 mol/L with the help of peristaltic pumps working at a discharge of 25 L/h, by addition of NH3 flux 2.1 m3/h. The composition of the raw material boric magnesium fertilizer is given in Table 1. A 40 L volume of magnesium salt solution with impurities removed was present initially in the large experiment setup, and vigorous stirring 90 rpm was applied during the bubbling of the NH3. 8 wt % sodium hydroxide solution was finally added to improve the conversion of magnesium ion. The solid phase obtained was Table 1. Composites of Boric Magnesium Fertilizer composite
MgSO4 · 7H2O
FeSO4
H3BO3
H2SO4
H2O
W/%
>85
