The efficiency of vertical zone-melting technique as a function of molten-zone length, number of zone passes, molten-zone velocity and phosphoric acid concentration on the purification process of phosphoric acid is studied. The results showed that adopting a relatively long molten-zone length in the initial passes to effect a rapid movement of impurities and a short molten-zone length in the later passes to obtain greater purification; additional zone-passes cannot be made as impurities redistribution approaches limiting distribution; for stirring mechanism is not adopted in the present study and as diffusion is the only mixing process, slow molten-zone velocity 3–6 mm h−1 is better; the purification efficiency increases with phosphoric acid concentration, and 94–99.8% concentration of phosphoric acid is better. © 2012 Canadian Society for Chemical Engineering
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