The synthesis of boron nitride by nitridation with boron anhydride (B4O3) is one of the important methods for industrial production of boron nitride. Due to the low melting point of boron anhydride (glass state is 294°C, crystalline state is 450-600°C), it becomes a high-viscosity melt at the nitriding temperature, which hinders the flow of ammonia and makes the reaction slow and extremely incomplete. In order to overcome this shortcoming, high melting point material can be used as filler to reduce the viscosity of boron anhydride melt. The filler itself does not participate in the reaction and can be easily removed at the end. Commonly used fillers are magnesium oxide (MgO), calcium carbonate (CaCO3), tricalcium phosphate [Ca3(PO4)2], boron nitride (BN), etc. Among them, tricalcium phosphate is the best, because tricalcium phosphate and boron The mixture of anhydrides has the highest nitridation reaction rate.
Boric anhydride and tricalcium phosphate are uniformly mixed in a mass ratio of 5:3. After processing according to this process, the nitridation reaction is carried out in a quartz tube or a corundum tubular electric furnace. The furnace body device is slightly inclined to facilitate drainage. The nitridation reaction starts at about 300°C, proceeds to 800°C and is initially nearly complete, and the final temperature is 900-1000°C for 4-24 hours. The main reactions of the entire nitridation process are:

B2O3+2NH3→2BN+3H2O

After the reaction is completed, the tricalcium phosphate filler can be washed with hydrochloric acid, and the residual boron anhydride can be washed with hot alcohol at 70 °C to obtain crude boron nitride, called "900 °C" boron nitride, with a conversion rate of 75%~ 85%, the mass fraction is 80%~90%, and the main impurity is the intermediate compound of boron. Such impurities can be purified by continuing nitridation in ammonia at 1400°C, or by heating to above 1800°C in nitrogen or argon to volatilize them.