船用低圧ボイラ水中におけるMg硬度の挙動 Behavior of Mg hardness in Boiler Water of Marine Auxiliary Boilers
It is very important to investigate accurately the behavior of sea water component, especially of Mg hardness, for the boiler water qualtiy control of marine boilers. And also, important to investigate the behavior of PO<SUB>4</SUB><SUP>3-</SUP>, especially when Mg<SUP>2+</SUP> and OH<SUP>-</SUP> coexist with it, for making the automatic control method proposed by the authors more feasible and reliable. Accordingly, this paper studies experimentally the behavior of Mg hardness when OH<SUP>-</SUP> and/or PO<SUB>4</SUB><SUP>3-</SUP> coexist, and when silica is mixed. Experiments were carried out at the condition of the same solution level with the water quality criterions adopted for actual boilers, by the use of an autoclave of which temperature and pressure adjusted as the same level with actual auxiliary boilers. The obtained results are summarized as follows.<BR>(1) In the reaction model developed so far, It has been thought so far that almost all Mg hardness reacts to form Mg (OH) <SUB>2</SUB> in the boiler water when the phosphate concentration is controlled appropriately by the alkali treatment. In this experiment, however, it was found that Mg reacts not only with OH-but also with PO<SUB>4</SUB><SUP>3-</SUP>, and the mole ratio of reaction products Mg (PO<SUB>4</SUB>) <SUB>2</SUB> and Mg (OH) <SUB>2</SUB>, increases with the decrease of pH, that is, 0 at pH=12.0, and around 2 on pH 10.5.The mole ratio is hardly affected by the PO<SUB>4</SUB><SUP>3-</SUP> concentration and the reaction temperature in the range of 1MPa-3Mpa.<BR>(2) If SiO<SUB>2</SUB> is mixed together with Mg hardness in the feed water, MgSiO<SUB>3</SUB> is formed in boiler water. Its quantity increases with the increase of SiO<SUB>2</SUB>. The rest of Mg hardness unreacted with SiO<SUB>2</SUB> is considered to react to form [Mg<SUB>3</SUB> (PO<SUB>4</SUB>) <SUB>2</SUB>] <SUB>n</SUB>⋅Mg (OH) <SUB>2</SUB> as mentioned above, and it is resulted in that pH and PO<SUB>4</SUB><SUP>3-</SUP> concentration are decreased.
日本舶用機関学会誌 33(10), 759-763, 1998-10-01
The Japan Institute of Marine Engineering