Kinetics of the dissolution of scheelite in aqueous Na 4 EDTA solutions
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I.
INTRODUCTION
T U N G S T E N has been widely employed as a highly refractory metal in diverse areas of industry. Among the tungsten minerals, scheelite (CaWO,) as well as wolframite (Fe, Mn)WO4 are the main primary resources. Most scheelite ores have average WO3 contents less than 1 pct and are concentrated to 15 to 70 pct WO3 by ore-dressing processes. The concentrates are then treated by acid-leaching (HC1) or alkali-leaching (Na2CO3 or NaOH). Acid-leaching is suitable only for higher-grade concentrates. On the other hand, alkali-leaching is applicable to lower-grade concentrates. The Autoclave-Soda process I has the advantage that the low grade ores containing as little as 1 pct scheelite can be directly decomposed with the omission of ore-dressing processes. However, this leaching process operates at an expensive autoclave pressure (12 to 26 atmospheric pressure) and at high temperature (463 to 498 K), and it requires more reagent consumption in proportion to the degree of extracted scheelite. Instead of applying Autoclave-Soda process, the use of the chelating agent EDTA has been proposed. Payne 2 has carried out equilibrium experiments on the extraction of tungsten from synthetic scheelite and low grade ores (2 pct CaWO4) by aqueous Na4EDTA solutions. He found that tungsten could be selectively extracted even at atmospheric pressure and 373 K by using EDTA, and that, furthermore, the EDTA could be recovered for reuse by acidification of the solution. Marshall 3 also has performed the extraction experiments for lower grade scheelite ores (0.2 to 1.32 pct WO3) and showed that aqueous EDTA solution could efficiently extract tungsten at atmospheric pressure. In his work, further, the processes of tungsten recovery and EDTA regeneration have been considerably simplified by applying solvent extraction directly to the leach liquor, in comparison with the process designed by YASUHIRO KONISHI, Research Associate, HIROYUKI KATADA, Graduate Student, and SATORU ASAI, Professor, are with the Department of Chemical Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan. Manuscript submitted May 30, 1986. METALLURGICALTRANSACTIONS B
Payne. 2 It was concluded that this modified EDTA leaching process was a less expensive treatment of low grade (
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