Metallic dust clouds have proven to be explosive even when mixed with high percentages of inert solids. In this paper, fire hazard of metallic dust layers was investigated as a potential ignition source. The minimum ignition energy and flame spread velocity of layered Ti powder mixed with different percentages of TiO2 were determined using electric spark ignition. The results indicated that MIE of dust layers (MIEL) was far larger than that of dust clouds (MIEC) for Ti powder and its solid mixtures, likely due to a different ignition mechanism. As expected, MIEL of nano Ti powder was much lower than that of micro Ti powder. Once ignited, flame spread velocity of nano Ti powder layer reached 625 mm/s, a value far larger than the 5 mm/s for micro Ti powder. (C) 2017 Elsevier Ltd. All rights reserved.