This is a method of synthesizing boron nitride by reacting borax and ammonium chloride as the main raw materials in an ammonia atmosphere. The two raw materials should be dehydrated and recrystallized separately before participating in the reaction. Borax is best dehydrated in a vacuum at 200~400°C. The recrystallization of ammonium chloride is to dissolve it into a saturated solution, filter to remove impurities, and recrystallize it. It can be repeated many times depending on the purity requirements. The crushed and dried borax is mixed with ammonium chloride in a mass ratio of 7:3, pressed into briquettes, and sent to the reaction furnace for synthesis. In order to speed up the reaction speed and improve the conversion rate, ammonia (NH3) needs to be introduced to make up for the deficiency of the reactants self-forming ammonia atmosphere. At low temperature, the feeding amount of ammonia is less than that in the high temperature stage. The specific feeding amount of ammonia should be adjusted according to the amount of reactants and the volume of the reaction furnace, so as to ensure that the reaction is fully carried out. The final heating temperature is 900~1000℃, and the temperature is kept for 6h. The reaction product is washed with water to remove remaining impurities such as boric acid and sodium chloride, and then dried and pulverized to obtain boron nitride powder with a mass fraction of 96%#98%. The main reactions of the compact during heating are:

Na2B4O7+2NH4CI+2NH3→4BN+2NaCl+7H2O