engleski

Application of high intensity ultrasound for obtaining magnesium hydroxide from seawater

Magnesium hydroxide in nano sized forms is a very important chemical with good physical and chemical properties and it has wide application in the paper industry, production of refractory materials, as a catalyst, additive. Various methods have been developed to synthesize nano magnesium hydroxide but the precipitation crystallization method is the most economic one for industrial continuous production, having a simple process and low energy consumption. In this paper magnesium hydroxide has been obtained from seawater by a well-known industrial process. The process involves the chemical reaction between magnesium ions of seawater with dolomite lime to produce the magnesium hydroxide precipitate. The purpose of this paper was to modify the precipitate of magnesium hydroxide by ultrasound of high intensity. The purpose was to obtain magnesium hydroxide with high purity, and reduced particle size without agglomeration. High intensity ultrasound was used during the precipitation of magnesium hydroxide (80% precipitation) and after the drying process to eliminate the agglomerates. The precipitated magnesium hydroxide is characterized by the particle size distribution determined by the laser scattering method in the range of 20 nm to 200 mm, and by the SEM/EDS analysis. The contents of MgO and CaO in the samples of the magnesium hydroxide product (80% precipitation) were: 65.28 wt % MgO, and 2.07 wt % CaO. The EDS analysis confirmed the chemical composition of the samples. It has been observed from the SEM analysis that particles of magnesium hydroxide can easily aggregate during the drying process which was carried out in an air dryer at 40 oC / 10h. The particle size of magnesium hydroxide in agglomerates is in the range of 130-250 nm. In order to eliminate agglomerates, ultrasound was used for 20, 40 60, 140, and 160 s after the drying process. The optimum time of using high intensity ultrasound is 140 s because after that time no changes were observed in samples by measuring the particle size distribution. After that the size of magnesium hydroxide agglomerates was in the range from 3 to 6 μm. The application of the ultrasonic generator at the examined operating conditions in the precipitation process is sufficient to reduce the particle size of precipitated magnesium hydroxide but is not sufficient to reduce the agglomerates after drying process.

magnesium hydroxide, seawater, ultrasound

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