Clean ion-exchange technologies. 2. Recovery of high-purity magnesium compounds from seawater by an ion-exchange isothermal supersaturation technique

This paper reports the results obtained by studying the ion-exchange isothermal supersaturation (IXISS) of magnesium carbonate solutions which is observed during elution of Mg2+ from carboxylic resin equilibrated with decalcinated seawater (artificial or natural) with either 2 M NH4HCO3 or with the mixture of 1.5 M Na2CO3 and 0.6 M NaHCO3. Crystallization of supersaturated solution in the first case has been shown to lead to the formation of previously unknown complex ammonium-magnesium carbonate. The results of X-ray, chemical, and differential thermal analyses have allowed the proposal of the following formula for the new magnesium compound: (NH4)2Mg(CO3)2.5H2O. In the second case, crystallization of magnesium carbonate leads to the formation of a pure phase of nesquegonite: MgCO3.3H2O of 99.9% purity. Samples of MgO obtained after annealing ammonium-magnesium precipitates at 773 K contain less than 10-3% of impurities.