Plasma arc process makes zirconia Ionarc, Inc., has commercialized three ultrahigh-temperature processes based on plasma arc technology. According to Merle Thorpe, chief executive officer of the Bow, N.H., firm, Ionarc stands ready to use a plasma arc reactor to make commercial amounts of high-purity zirconia, refractory metal compounds, and dense metal and ceramic microspheres. The firm plans to start construction in December of a second 1000-kw. plasma arc production unit to meet expected market demand for these initial products. The plasma arc reactor produces a streaming or elongated arc for creating ultrahigh temperatures. Reactions at temperatures well in excess of 3500° C. are possible in the arc. Details of the Ionarc process are proprietary, but the firm says in general that the process operates at atmospheric pressure. It will not reveal whether the process is based on an induction, a.c, or d.c. arc. The reactor consists of feed ports, electrodes, and arc reaction chamber. The balance of the unit is a collection chamber with water-cooled steel walls for product quenching. Mr. Thorpe says that the second reactor will be the firm's first system designed for continuous operation. Capable. "The initial engineering problems of lowering power consumption and increasing conversion per reactor pass are well behind us," Mr. Thorpe says. The present process, he explains, is capable of dissociating zircon at a power level of 0.6 kwh. per pound of feedstock. Intimate mixing of feed and arc results in nearly 100% conversion, he adds. Arc processes for production of acetylene are well known, a fact which caused Ionarc to concentrate on development of other promising inorganic and organic processes. Chemischewerke Huels operated an electric arc plant at Marl, West Germany, during World War II to produce acetylene, with excess electrical generating capacity available for other use during the night hours. In May 1963, Du Pont started up a rotating arc acetylene plant at Montague, Mich. Five years later, with no explanation for its action, the firm closed down the unit. Ionarc is most advanced with a process to produce high-purity zirconia in the plasma arc. High-purity zirconia (more than 99% pure Zr0 2 ) sells for about $1100 per ton. The world market for zirconia is about $19 million, with the U.S. share amounting to about $9.5 million. The largest use of high-purity zirconia is in production of 20
C&EN AUG. 30, 1971
tinted tiles, particularly peach and light yellow tints. About $3 million worth of zirconia is consumed annually in the U.S. for this use. Other markets for high-purity zirconia are abrasives, ceramic parts for electronics, and refractory ware. Production of high-purity zirconia with the Ionarc process occurs in two steps. In the first, zircon sands feed through the arc to yield dissociated zircon. Amorphous silica in the dissociated product is then easily leached out with dilute caustic to. produce better than 99% pure zirconia. Separate. In practice, Ionarc finds that Australian zircon (ZrSi0 4 ) sands imported at a cost of about $80 per
lonarc's Thorpe displays products made in its plasma arc reactor
ton are free of undesired alumina and possess the desired particle size range of 100 to 200 microns. Contaminating iron and titania (present as ilmenite) can be removed prior to reaction by using high-intensity magnetic separators. In passing through the plasma, zircon sand particles are wiped by the arc and probably reach temperatures of about 1775° C. True liquid zirconia and silica phases don't occur until temperatures of about 2700° C. are reached, but Ionarc scientists hypothesize that their reaction temperature is high enough to create a slush possessing sufficient interfacial tension to permit coalescence into 100-microndiameter spheres of dissociated product. Electron micrographs of these spheres indicate that the dissociated product consists of zirconia crystallites embedded in a highly stressed amorphous silica matrix. There is no evidence of zirconium silicate or crys-
talline silica in the spheres, according to the firm. Ionarc finds that the highly stressed and amorphous silica easily can be leached away from the zirconia to produce zirconia and sodium silicate of high purity. Zirconia is generally produced by either silica volatilization in a conventional carbon electrode furnace or by addition of a flux to zircon sands, followed by calcining. Ionarc claims an overall cost advantage in its process since silica in the dissociated spheres from the plasma arc readily can be leached with 50% caustic at about 120° C. By-product sodium silicate finds ready sale, the company says. Dissociated zircon product from the plasma reactor contains about 70% zirconia and 30% silica. This product can be sold directly for use in foundries that use investment casting techniques. In investment casting, intricate shapes composed of convolutions, protuberances, or eccentrics can be cast. The mold is removed by dissolution in caustic. At present, Ionarc has the capacity to produce about 20 tons per month of dissociated product and to process about 3 tons per month of this material to high-purity zirconia. About 1 pound of sodium silicate is produced per pound of zirconia. Screening. In addition to zirconia, the company is screening other minerals for possible commercial value. These include calamine, chrysocolla, thorite, rhodonite, sillimanite, garnierite, bauxite, and beryl. Ionarc is also investigating production of elemental metals such as silicon, titanium, boron, and iron from halides in the plasma arc (C&EN, July 5, page 39). In work on refractory metal compounds, the firm has developed a process that is adaptable to single-step reduction for carbides and nitrides, including titanium carbide, Mr. Thorpe says. Pilot-plant studies involve scaleup of processes based on chlorides of silicon, titanium, or boron. Ionarc is also continuing research on other chlorides such as aluminum, tantalum, niobium, and germanium as starting materials for elemental metal production. A third promising venture is development of a plasma arc process for producing dense metal microspheres for office copying machines. The spherodized powders are being evaluated for commercial use by a major office copier manufacturer. In organic chemicals, Ionarc finds it can produce liquid fluorocarbon polymer which has inertness properties similar to those of polyfluoroethylene (Teflon) using carbon tetrafluoride feed.
