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RESEARCH PRODUCT

Study of the hydrolysis of lithium hydride

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The hydrolysis of LiH at room temperature and under low water vapor pressure (PH2O < 10 hPa) is investigated by thermogravimetry and FTIR spectroscopy with low sample mass. Then, to be closer to industrial conditions, hydrolysis of LiH is studied by manometry either in closed (adjustable PH2O) or open (constant PH2O) system using larger amounts of sample and heavy water. Products of the reaction are characterized by X-ray diffraction and FTIR spectroscopy. The first set of experiments show that the mechanism of hydrolysis starts with the formation of lithium oxide Li2O. Then, when the oxide layer is sufficiently thick, the hydrolysis reaction is followed by the formation of lithium hydroxide LiOH and afterwards with the formation of lithium hydroxide monohydrate LiOH, H2O. Besides, the Li2O/LiOH outer layer forms a protective barrier on the surface of LiH. The second set of experiments clearly highlights for the first time that the hydrolysis reaction occurs in two steps: first water is adsorbed on the LiH surface and then the hydrolysis reaction starts. The reaction rate is however extremely low and only a very small fraction of LiH is hydrolysed. The kinetic can be well predicted by the shrinking-core model limited by the diffusion through the external ash layer (Li2O and/or LiOH). For practical application, it is concluded that if the LiH powder is stored for several years under a controlled atmosphere or in a sealed container where the vapor water pressure is less than 0.04 hPa, there is no major risk of LiOH formation.