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Published: Kem. Ind. 54 (10) (2005) 419–423
Paper reference number: KUI-33/2004
Paper type: Original scientific paper
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The Effect of the Rinsing Method on the B2O3 Content in Magnesium Oxide Obtained From Seawater

V. Martinac, M. Labor and N. Petric


The effect of the method of rinsing the magnesium hydroxide precipitate, obtained from seawater by substoichiometric precipitation with the addition of 80 % of the stoichiometric quantity of dolomite lime as the precipitation reagent has been examined. The substoichiometric precipitation of magnesium hydroxide from seawater is a much more economical precipitation method in the so-called “wet phase” than the stoichiometric precipitation method, mainly due to the increase in the rate of sedimentation of the precipitate formed, and thereby in the thickening capacity. However, with the 80 % precipitation, the B2O3 fraction in the product, i.e. magnesium oxide obtained from seawater increases. The aim of this study was to determine the efficiency of potable water (decarbonated) as the agent for rinsing the magnesium hydroxide precipitate in order to reduce the B2O3 content in magnesium oxide from seawater as much as possible. Specifically, boron causes disturbances in electrolysis of magnesium chloride when metal magnesium is produced, if magnesium chloride obtained from seawater is used as cell batch, and it also adversely affects the quality of refractory material produced from sintered magnesium oxide obtained from seawater. Examinations were carried out at different conditions, i.e. the effect of pH of the rinsing agent was examined, as well as duration of stirring and contact of the suspension with the rinsing agent and the effect of the addition of mannitol to seawater before precipitation, and/or to the rinsing solution. Examination results indicate that rinsing by alkalized decarbonated potable water with pH value of 12.50 reduces the B2O3 mass fraction in MgO samples (80 % precipitation) down to w = 0.238, which is by 65 % less than the B2O3 content in samples prepared by rinsing the magnesium hydroxide by non-alkalized decarbonated tap water (w = 0.679). This is due not only to pH but also to inter-ionic effects. The addition of mannitol to seawater before precipitation also has a favorable effect and reduces the B2O3 fraction by additionally 9 % (w = 0.216), while the addition of mannitol to the rinsing solution has no significant effect. Mannitol binds the orthoborate acid, H3BO3, present in seawater, in a weakly dissociated complex acid HB(OC)4, thereby significantly reducing the quantity of boron adsorbed during reaction precipitation. The duration of the contact of magnesium hydroxide precipitate with the rinsing solution is very important, but the quantity of the rinsing agent in these examinations has no major effect, indicating that the rinsing solution can be recycled, its use multiplied with appropriate refreshing, which is also very favorable. The optimum duration of the contact of the suspension with the rinsing solution is 1.5 h.

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substoichiometrical precipitation, precipitate rinsing method, B2O3 fraction, seawater magnesium oxide