Preparation of magnesium hydroxide from nitrate aqueous solution

摘要

Nucleation of Mg(OH)_2 was investigated by measuring the electrical conductivity and pH of the Mg(NO_3)_2 reaction solution to which ammonia containing different amounts of NH_4NO_3 was added. NH_4NO_3 increases solubility and slows down precipitation of Mg(OH)_2 in the system. Data are presented on the influence of NH_4NO_3 on the solubility of Mg(OH)_2 at 25℃. The phenomena observed can be explained by the solvation effect of nitrate ions brought to the system with the addition of ammonium nitrate, which was proved by NMR spectroscopy. When the mass fraction of NH_4NO_3 exceeds 15 %, homogeneous nucleation does not proceed. It was found that seeding of the system with Mg(OH)_2 crystals only influenced the rate of Mg(OH)_2 crystallisation, not the size and shape of the crystals. Primary crystals are smaller than 0.1 μm. The large difference in the surface energy of individual crystal planes leads to oriented agglomeration. This process is accelerated in a pressure reactor at 130℃. The resulting polycrystals are hexagonal plates 0.2 μm thin with a diameter of 1-2 μm. Under variable reaction conditions, agglomerates as big as 30μm can be prepared.