Flame phenomena in nanogrinding process for titanium and iron

Minimum ignition energies of nano-iron and nano-titanium, both of which are extremely sensitive materials, are less than 1 mJ. A factory nanogrinder was operated for 15 min; the temperature increased from 44.25 °C to 46.25 °C and the relative humidity decreased from 47.5% to 36.1%. No combustion occurred at air speeds of 14.7 m/s and 23.5 m/s when 20 kg of micro-sized iron powder was mixed with 200 g of 35-nm iron powder particles in the grinding machine for 40 min. Grinding a mixture of 20 kg micro-titanium powder and 200 g of 35-nm titanium powder particles at an air speed of 14.7 m/s for 40 min did not generate a combustion reaction. However, combustion did occur 5 min after the initiation of a 23.5-m/s airflow, suggesting that the occurrence of combustion is related to the airflow speed. Combustion occurred when the electrostatic volts in the plastic hose of the grinder reached 2.3 kV. The combustion process lasted for approximately 0.1798 s and the temperature at this location increased from 25 °C to 130 °C. The pre-exponential factor (min−1) was 3.18 × 1015 and the activation energy was 185.32 kJ/mol.