engleski

The effect of the pH of the rinsing solution on the properties of magnesium oxide from seawater

We have investigated the of the pH of the rinsing agent on the content of B_2O_3, CaO and MgO in magnesium oxide obtained from seawater by substoichiometric precipitation (where the precipitation of magnesium hydroxide took place with 80 percentage of the stoichiometric quantity of dolomite lime as the precipitation agent). In such a case, i.e. when this precipitation method is employed, the boron content adsorbed onto the magnesium hydroxide during the precipitation process is somewhat higher than during stoichiometric precipitation, and should therefore be reduced. The purpose of the study was to ensure high-purity magnesium oxide, particularly with respect to the B_2O_3 content, because boron causes problems in refractory magnesia for specialized refractory applications where a high hot-strength is required. The rinsing agents were alkalized distilled water with pH = 11.0 and 12.5, which were alkalized by the addition of concentrated NaOH prior to each use, and non-alkalized distilled water with pH = 5.95. It was established that the content of B_2O_3 in magnesium oxide samples is significantly reduces when the pH value of the agent used for rinsing the magnesium hydroxide precipitate increases. The magnesium oxide samples obtained were then sintered with the addition of TiO_2 (1, 2 and 5 mass percentage) in order to determine the properties of the sintered samples (density, porosity, the B_2O_3 content) relative to the method used for rinsing the magnesium hydroxide precipitate and to the quantity of added TiO_2. The results indicate that all the samples, sintered at 1650 oC for 3 hours, had very low porosity, and densities up to 95 percentage of the theoretical density. Also, the B_2O_3 content in the products was much lower than in the samples prior to sintering.

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