Abstract: A method and apparatus for making an antimony trioxide powder having a specific particle size distribution from crude antimony trioxide which contains a small amount of impurities having a higher vaporization temperature by passing the crude antimony trioxide through a plasma arc tail gas reaction zone, vaporizing in a chamber communicating with the reaction zone the antimony trioxide at a temperature that liquifies the impurities, exiting the gases from the chamber through one port, draining the liquified impurities from the chamber through the port, contacting the exiting materials with a quench gas, mechanically clearing the exit port of solidified impurities and separating the antimony trioxide powder from the other exiting materials.