Calciothermic Reduction of Zirconium Oxide in Molten CaCl2

Abstract

The reduction of zirconium dioxide using liquid calcium and molten calcium chloride was investigated. The study focusing on the influence of reductant amount, reaction time, CaCl 2 amount, and temperature on the reduction process. Zirconium powder with oxygen content less than 800 ppm was obtained at 1 100°C after 3 h and by using two times the theoretical amount of Ca and four times that of CaCl 2 . The reduction reaction of ZrO 2 was found to be multi steps process through the formation of intermediate phase CaZrO 3 . The morphology of the obtained zirconium was observed to be highly affected by the reaction temperature.KEY WORDS: zirconium; calciothermic reduction; calcium; calcium chloride. 25© 2007 ISIJ common reductant, was reported in their work. It is clearly seen that only calcium may be expected to produce pure zirconium from oxides.The third criterion is the ability to separate the pure metal from other reduction byproducts. For example, if magnesium used as a reductant the removal of the ignited magnesia (MgO) by acid leaching without attacking the formed zirconium would be a difficult task. 22,23) The fourth important requirement in the reducing agent is that it must be cheap and available commercially and that it must satisfy the environmental requirements. Rare earths are expensive and cannot be used as economic reductant. Radioactive or noxious elements are excluded as zirconium reductants.Calcium is the only candidate that satisfies all the above requirements. At 400°C, DG°of formation for ZrO 2 is Ϫ970.92 kJ/mol corresponding to an oxygen partial pressure P O 2 ϭ5ϫ10 Ϫ76 atm at the equilibrium of the reaction, ZrϩO 2 ↔ZrO 2 . The DG°of formation for 2CaO is Ϫ1062.6 kJ/mol. At the equilibrium of the reaction 2CaϩO 2 ↔2CaO, P O 2 is lowered to 3ϫ10 Ϫ83 atm i.e. by a factor of 2ϫ10 7 less than its value with zirconium. 24,25) From the O-Zr phase diagram, the solubility of O in Zr at equilibrium with ZrO 2 is 5 wt%. 26,27) Reducing the oxygen partial pressure by a factor of 2ϫ10 7 brings the dissolved oxygen below the ppm level. Calcium has no recognized intermetallic compounds with zirconium, and it has low solubility in b-Zr and a-Zr. Also it may be produced commercially through direct electrolysis of calcium chloride.Practically, Kroll 22,23) reported the presence of thousands ppm oxygen remained in the produced Zr after reducing with calcium. The slow mass transfer through the by-product CaO attached to the surface of Zr particles and hindered further reduction. Also, CaO that precipitated on the metal surface was trapped inside the particles when sintering took place. This CaO could not be eliminated during leaching by HCl and as a result, calcium and oxygen contents in the Ti powders could not be lowered to sufficient degree.Molten CaCl 2 can dissolve about 20 % mole CaO. [28][29][30] Threadgill 30) used this principle to precipitate electrolyticaly calcium metal from calcium oxide dissolved in molten calcium chloride bath. He also reported that the presence of CaO decrease the freezing point...

Fabrication of a micro-porous Ti–Zr alloy by electroless reduction with a calcium reductant for electrolytic capacitor applications

“…In the metallic zirconium reduced by calcium, the oxygen dissolved in the zirconium lattice interstitially and formed a Zr-O solid solution. A small amount of oxygen may have been captured as the byproduct CaO, unreacted ZrO 2 , and composite oxide CaZrO 3 in the secondary grains, although a thin native oxide film was formed on the materials after the investigations [17]. Metallic zirconium with low oxygen content can be successfully obtained by electroless reduction with a calcium reductant, and the lowest value of oxygen content is acceptable for industrial materials.…”

“…Titanium and zirconium are produced industrially by the Kroll process [15][16][17][18]. This process consists of three steps of operation: conversion from oxide to tetrachloride, subsequent reduction of tetrachloride to metal by liquid magnesium, and molten electrolysis of the byproduct, magnesium chloride.…”

Preparation of Zirconium Metal by the Electrochemical Reduction of Zirconium Oxide

“…Alkali fusion of zircon followed by hydrometallurgical treatments was preferred to produce ultra-high-pure zirconium oxide. The reduction of zirconium oxide with calcium in molten calcium chloride was investigated elsewhere [12] and succeeded in producing zirconium powder with less than 800 ppm oxygen content, but the large quantity needed from high-purity calcium and calcium chloride complicates the reduction step. Another problem in the preceding method is the vaporization of calcium due to its low partial pressure and its high affinity to oxygen, which means that an inert atmosphere and a very tight gas reaction vessel should be used.…”

Rapid reduction of titanium dioxide nano-particles by reduction with a calcium reductant

“…During oxide reduction, calcium chloride molten salt acts as a suitable solvent and has a high solubility for calcium and calcium oxide as a byproduct formed by reduction. Metallic titanium [1], zirconium [2], hafnium [3], niobium [4], tantalum [4], vanadium-titanium [5], titanium-aluminum-vanadium [6], titanium-chromium [7][8][9], samarium-iron [10], and niobium aluminide [11] are formed via reduction of these oxides with a calcium reductant in calcium chloride molten salt. The residual oxygen in the reduced metal decreases continuously by deoxidation with a calcium reductant [4].…”