Specific Energy Characteristics of the Plasma-Electrolyte Process for Producing Hollow SiO2 Particles

The energy parameters of plasma-electrolyte spraying leading to the formation of hollow SiO2 particles from an aqueous solution of Na2SiO3 (5 wt % by SiO2) in a 300-mL reactor were studied. The discharge was excited at the end of a hollow steel cathode tube by rectangular pulses with a duration of 50 μs and an amplitude of up to 300 V. Modes with a frequency of 5 and 20 kHz were compared. Oscillograms U(t) and I(t), the change in the mass of the solution, and the heating and thermal field of the wall, as well as the yield and morphology of the solid product, were recorded. At 5 kHz, the average power obtained was 75 ± 5 W and the yield was 5.5 ± 0.2 g SiO2 in 10 min; at 20 kHz, 270 ± 10 W and 11.4 ± 0.3 g in 10 min. The specific energy of synthesis was 8.1 ± 0.6 and 14.2 ± 1.2 kJ/g, respectively. The energy balance showed that, at 5 kHz, the main expenses are due to heating the solution and evaporation of water (a total of ~95%), while at 20 kHz a significant share is taken up by heat removal through the wall and electrode. According to morphological analysis, hollow spherical particles with a diameter of 3–15 µm predominate; an increase in frequency decreases the median diameter and increases the proportion of hollow particles. The obtained relationships between the supplied energy, phase transitions, and product yield were used to formulate ways for optimizing the pulse mode.