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.
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