A new process for the production of granular boric oxide by the calcination of ammonium pentaborate in a fluid bed calcinator

This paper describes the production of pure and granular boric oxide by calcination of ammonium pentaborate octahydrate [(NH4)2O • 5B2O3 • 8H2O)]in a fluid bed calcinator (FBC). Such calcination is incomplete since agglomeration starts at 623 K, and to avoid agglomeration the boric oxide particles have been covered with a material with a higher melting point than boric oxide itself. In order to achieve this high melting cover, a mixture of ammonium pentaborate and alkali borate has been fed into the FBC. During calcination, the proper percentage of alkali borate has been determined by examining the behaviour of different samples. As the percentage of alkali borate increases, a more homogeneous cover is formed on the surface of particles. This allows the agglomeration temperature of the end-product to shift to 811 K. As a result, granular boric oxide of 99.55%–99.80% purity and containing 0.20%–0.35% Na2O has been produced.