PURPOSE: Provided is a method for environmentally friendly isolating magnesium oxide, which is produced from manufacturing of CB4 powder using B2O3, C and Mg by a self-propagation high temperature synthesis method. Thereby, MgO is effectively separated and can be recycled. CONSTITUTION: The method comprises steps of: (i) washing reaction products of B2O3, C and Mg with distilled water or alcohol to remove unreacted B2O3; (ii) after dispersing the resultant products of the step (i) in dilute sulfuric acid, stirring them to react the unreacted Mg and MgO of the reaction product with the sulfuric acid; (iii) filtering the resultant products of the step (ii) to separate CB4; and (iv) concentrating the filtrate of the step (iii) to obtain crystalline MgSO4nH2O.
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