The self-propagation high-temperature synthesis of ultrafine high purity tungsten powder from scheelite
- PDF / 378,493 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 89 Downloads / 199 Views
MATERIALS RESEARCH
Welcome
Comments
Help
The self-propagation high-temperature synthesis of ultrafine high purity tungsten powder from scheelite J. C. Jung, S. G. Ko, C. W. Won, S. S. Cho, and B. S. Chun Engineering Research Center for Rapidly Solidified Materials, Chungnam National University, Daejeon 305-764, Korea (Received 3 July 1995; accepted 20 February 1996)
High-purity tungsten was prepared by the self-propagating high-temperature synthesis (SHS) process from a mixture of CaO ? WO3 and Mg. The complete reduction of CaO ? WO3 required a 33% excess of magnesium over the stoichiometric molar ratio MgyCaO ? WO3 of 3 : 1. The MgO and CaO in the product were leached with an HCl solution. The product tungsten had a purity of 99.980% which was higher than that of the reactants. The high purity results because the nontungsten reactants and products are volatilized by the high temperatures generated during the rapid exothermic SHS reaction and are dissolved during HCl leaching of the product.
I. INTRODUCTION
Tungsten has useful characteristics of high melting point (3683 K) as well as high-temperature strength and electrical conductivity. Thus, W finds wide use in illumination, electronics, electrical contact, and heatresistant structures. W is also the main raw material for the altrahard alloys used in cutting tools, antifriction tools, and high-speed steels.1 Generally, tungsten powder has been prepared by the following conventional procedures.2 Scheelite (or CaO ? WO3 produced from wolframite) is decomposed to tungstic acid by hydrochloric acid. Calcium tungstate is obtained from wolframite by (i) roasting with sodium carbonate and sodium nitrate, leaching, and filtering to remove Fe2 O3 and Mn3 O4 ; (ii) digestion with MOH solution (M Na or K); and (iii) precipitation of CaO ? WO3 from the tungstate solutions by CaCl2 . The tungstic acid may be purified by dissolution in sodium hydroxide solution and reprecipitation, or by extraction with ammonium hydroxide solution from which ammonium paratungstate crystallizes. The tungstic oxide (or ammonium paratungstate) is calcined to WO3 which is reduced to metal with hydrogen. The powdered tungsten is pressed into bars that are sintered (in hydrogen) to produce massive tungsten. Bars so produced are mechanically processed further by hot swaging and hot drawing. Arc melting may be used for producing larger ingots. Currently tungsten is produced by the reduction of WO3 at 1273 to 1373 K with carbon, H2 , or calcium, or by the hydrogen reduction of WF6 at about 873 K. However, the SHS (self-propagating high-temperature synthesis) method has not been applied to the preparation of tungsten powder from raw ore. Lately, many pure substances have been prepared by the SHS process.3–5 This process can be used to prepare J. Mater. Res., Vol. 11, No. 7, Jul 1996
http://journals.cambridge.org
Downloaded: 26 Mar 2015
fine particles of high-temperature materials at 1773 to 4273 K, using their high exothermic heats of formation. The process is relatively simple and rapid wit
Data Loading...
