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Equilibrium data of the NaOH−NaBO 2 −Na 2 CO 3 −H 2 O quaternary system at 30°C, 60°C, and 100°C were measured by an isothermal method. The phase diagrams at 30°C and 100°C were constructed, and the crystallization zones were discussed. In the phase diagram, the quaternary system has two invariant points, five univariant curves, and four crystallization zones corresponding to NaBO 2 ·4H 2 O (NaBO 2 ·2H 2 O at 100°C), NaBO 2 ·2H 2 O (NaBO 2 ·1/3H 2 O at 100°C), Na 2 CO 3 · H 2 O, and NaOH·H 2 O (NaOH at 100°C). In addition, the solubility isotherms of NaBO 2 in Na 2 CO 3 -saturated NaOH solution and the solubility isotherms of Na 2 CO 3 in NaBO 2 -saturated NaOH solution were plotted. The solubility of Na 2 CO 3 in NaOH solution was compared with that in NaBO 2 -saturated NaOH solution. On the basis of the solubility data, a strategy for the effective separation of NaBO 2 and Na 2 CO 3 from the NaOH−NaBO 2 −Na 2 CO 3 −H 2 O quaternary system was proposed.

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