Direct Hydrothermal Precipitation of Pyrochlore-Type Tungsten Trioxide Hemihydrate from Alkaline Sodium Tungstate Soluti
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THE extractive metallurgical process of tungsten trioxide using tungsten concentrates such as wolframite, scheelite, and the mixture of wolframite and scheelite as starting minerals is still long and complicated, although many technological progresses such as the direct extraction of tungsten species from the alkaline tungstate solution,[1,2] low-grade and complicated tungsten concentrate treatment,[3,4] and techniques of various impurity removal from the solution[5–7] have been achieved in the past two decades. The typical process employed in the current tungsten extractive metallurgy industry consists mainly of (1) the preparation of sodium tungstate solution through the reaction of tungsten concentrates and sodium hydroxide or sodium carbonate, (2) the conversion of sodium tungstate to ammonium tungstate, (3) the evaporation of ammonium tungstate solution, and (4) the crystallization of ammonium paratungstate (APT) followed by the calcination of APT. As for the treatment of tungsten concentrates, almost all of the sodium hydroxide or sodium carbonate consumed in the whole process could impractically be recycled economically, resulting in the formation of a large amount of salts such as sodium chloride and XIAOBIN LI, ZHIHONG PENG, and GUIHUA LIU, Professors, JIANPU LI, Graduate Student, QIUSHENG ZHOU, Associate Professor, and TIANGUI QI, Lecturer, are with School of Metallurgical Science and Engineering, Central South University, Changsha, 410083 Hunan Province, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 5, 2010. Article published online December 10, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
subsequent discharge to the environment. Furthermore, a considerable amount of ammonia-nitrogen wastewater and ammonia-bearing gases, which are difficult to deal with, are produced in the APT extractive process. This leads to not only more energy consumption, more raw materials demanding, and high production cost but also serious environmental pollution. If direct precipitation of tungsten trioxide or its hydrates with the absence of sodium from the alkaline or weak alkaline sodium tungstate solution was accomplished, then soda recycling could be realized and many unit operations such as the solution conversion, solvent extraction, ion exchange, and the evaporation crystallization needed in the current industrial tungsten extractive metallurgical process could be omitted. In addition, ultrafine tungsten trioxide and their alkali-metal-intercalated compounds have attracted considerable attention because of the variety of crystalline structures they can take up[8,9] and their many exciting electronics and electrochemical properties such as photochromism,[10] electrochromism,[11] catalytic,[12] pressure–gas sensitivity,[13–15] and strong near-infrared absorption.[16] They have great potential applications in many industrial sectors. Unfortunately, because of the variety of the precipitates and the difficulty in the removal of the impurity of sodium, almost all the industrial-scale micrometer o
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