Thermal Plasma Processes for Production of Hollow Spherical Powders: Theory and Experiment

Abstract

Present paper considers processes of production and treatment of hollow particles in plasma flows. Specifics of agglomerated powders treatment and expansion of hollow droplets are analyzed theoretically. Experimental results on production of hollow micro particles of ZrO 2 , Ni, CoNiCrAlY alloy, SiO 2 are shown here.

Experience in plasma production of hollow ceramic microspheres with required wall thickness

“…The volumetric density of all the fractions of obtained microspheres is shown in the same figure; this density monotonically decreases from 5.3 g/cm 3 for fine spheres down to 1 g/ cm 3 and even to lower values for particles coarser than 100 μm. The volumetric density is an important characteristics of microballoons intimately related to their wall thickness by mass balance [18,20]…”

“…Plasma spheroidization of powders was carried out in a nitrogen plasma jet produced by a linear DC plasma gun that was designed at ITAM SB RAS, Novosibirsk [20]. The process was held under atmospheric pressure using a standard regime ensuring complete particle spheroidization; this regime was as follows: plasma-forming nitrogen flow rate 30 slpm, arc current 150-200 A (voltage 180-190 V), thermal efficiency 60%, anode diameter 10 mm, feed rate of powder 6 kg/h.…”

“…This provided average gas temperature at nozzle exit 6-8 kK, velocity about 300-400 m/s, and typical length of the plasma plume 200-250 mm. The powder was injected into the plasma jet using two radial injectors into the region under the plasmagun nozzle exit [20]. The powder was processed in a plasmochemical water-cooled reactor whose volume was 30 l [21].…”

“…Hollow microspheres of refractory ceramics can be obtained by spheroidization of a powder in a low-temperature (4-20 kK) plasma with throughput in excess of 20 kg/h [20]. The authors of [20,21] have attempted prediction of characteristics (density and shell thickness) of produced spheres on the assumption that the change of particle sizes in plasma processing is negligible, not exceeding 2-5%.…”

“…The authors of [20,21] have attempted prediction of characteristics (density and shell thickness) of produced spheres on the assumption that the change of particle sizes in plasma processing is negligible, not exceeding 2-5%. However, later in [22] it was shown that the approach of [20,21] is inadequate, and the obtained hollow microspheres show a wide spread of shell thicknesses even if taken from narrow granulometric fractions.…”