Modification of ADP extinguishing powder by siliconization in spray drying

Superfine spherical fire-extinguishing powder, ammonium dihydrogen phosphate (ADP, NH₄H₂PO₄), was prepared by spray drying and modified in situ with methyl hydrogen silicone oil (MHSO) emulsion and the fluorinated surfactant FK-510. The influences of the MHSO mass ratio on the hydrophobicity, surface composition, surface morphology, dispersion and particle-size distribution of the NH₄H₂PO₄ were studied, and the influence of the drying air temperature on the decomposition of the NH₄H₂PO₄ was also researched. The results indicate that the MHSO and FK-510 congregate on the particle surfaces and then form a hydrophobic shell. This shell improves the particle hydrophobicity and leads to a fine dispersion of the particles. During the process of preparing the precursor solution, 3 wt% (based on the weight of NH₄H₂PO₄) was chosen as the optimum value of the MHSO mass ratio. During the spray drying, a low absolute humidity of the air should be maintained, and it is very important to keep the exit-air temperature below 100 °C to avoid decomposition.