Refractory Metals & Hard Materials 11 (1992) 317-324 Production of Ultrapure Tungsten by Solvent Extraction With a D2EHPA/TOPO Mixture and Electron-Beam Zone Refining Hyo Shin Yu, Joon Soo Kim, Kang-In Rhee, Jae-Chun Lee Korea Institute of Geology, Mining and Materials, Daejeon, South Korea & H. Y. Sohn Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, USA (Received 11 May 1992; accepted 17 November 1992) Abstract: Laboratory and pilot-plant investigations have been carried out for the preparation of ultrapure tungsten. The starting material was commercially available ammonium paratungstate. Uranium- and thorium-free tungsten powder was prepared in the laboratory using solvent extraction with D2EHPA and TOPO. Optimum conditions of the purification were established. Based on the results of the laboratory work, a pilot plant of 2.4 tons per year capacity was constructed and operated to produce tungsten powder of 99"99% purity. From this powder, tungsten rods of 99"999% purity were prepared by electron-beam floating-zone melting. INTRODUCTION With the rapid growth of the microelectronics industry, demand for high-melting metals such as W, Mo, and Ta, of high purity has increased dra- matically. Ultrapure tungsten is used in the electronic industry as the preferred material for gate electrodes, interconnects, and barrier metal, because of its high resistance to electron migra- tion, high temperature stability, and tendency to form a stable silicide. To prepare tungsten as a sputtering target in semiconductor production, uranium and thorium contents must be very low, because these radioactive species induce soft errors in memory circuits due to the emission of a particles. 1 This investigation was concerned with the fol- lowing: (1) laboratory preparation of uranium- and thorium-free tungsten powder by purifying (2) (3) ammonium paratungstate (APT) from a commercial plant; the construction and operation of a pilot plant with a capacity of 2-4 metric tons per year of 99.99% pure tungsten powder; and the preparation of 99"999% pure tungsten rod from the above powder by electron- beam floating-zone melting. LABORATORY EXPERIMENTS Materials Ammonium paratungstate [APT, 5(NH4)20" 12WO3"xH20 , where x is usually 5] sample used in this work was a product of Korea Tungsten Mining Company. Its impurity contents are listed in Table 1. D2EHPA (di-2-ethylhexyl phosphoric acid) and TOPO (tri-n-octyl phosphine oxide) 317