In order to establish the theortetical basis and optimize the production conditions for the separation process of the H3BO3-MgSO4 mixed solution by using Mg（NO3）2 as the salting out agent.The phase diagrams of H3BO3─MgSO4─Mg（NO3）2─H2O system at 25℃ and 100℃ were studied.The results show that there appear MgSO4·7H2O,MgSO4·6H2O,Mg（NO3）2·6H2O and H3BO3 three crystallization regions in the phase diagram at 25℃,and three crystallization regions of MgSO4·H2O,Mg（NO3）2·2H2O,and H3BO3 at l00℃.When the temperatures rise from 25℃ to 100℃,the crystallization regions for magnesium sulfate hydlates grow larger and while that of H3BO3 get smaller.Only MgSO4·4H2O could be crystallized out alone,if evaporating the H3BO3─MgSO4 solution in the presence of Mg（NO3）2.On cooling the MgSO4 mother liquor,H3BO3 can be separated alone without any addition of water.
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