船用低圧ボイラ水中におけるMg硬度の挙動

Abstract

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 PO43-, especially when Mg2+ and OH- 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- and/or PO43- 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.(1) In the reaction model developed so far, It has been thought so far that almost all Mg hardness reacts to form Mg (OH) 2 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 PO43-, and the mole ratio of reaction products Mg (PO4) 2 and Mg (OH) 2, 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 PO43- concentration and the reaction temperature in the range of 1MPa-3Mpa.(2) If SiO2 is mixed together with Mg hardness in the feed water, MgSiO3 is formed in boiler water. Its quantity increases with the increase of SiO2. The rest of Mg hardness unreacted with SiO2 is considered to react to form [Mg3 (PO4) 2] n⋅Mg (OH) 2 as mentioned above, and it is resulted in that pH and PO43- concentration are decreased.