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Zirconium oxide occurs in combination with silica as zircon, which is itself obtained as a by-product of heavy minerals separated from beach sand. Zirconia is an important material that has wide applications in ceramics, refractories, electronics and other related industries. Work has been published 1-33 on the alkali fusion of zircon to recover ZrO 2 , but few papers have been concerned with the mechanism and kinetics of the fusion process.The present authors are engaged in a series of investigations with the objective of producing zirconia by reactions of local zircon with alkaline earth metal oxides (CaO and MgO-CaO) and chlorination. [34][35][36] Investigations of the treatment of zircon by two different routes are reported here. The first of the two studies was concerned with the alkali fusion of local zircon sand with soda ash at temperatures between 900 and 1200°C for various periods, followed by acid leaching of the fused product to dissolve the zirconium compound and precipitate silica in the form of silica gel. The zirconium salt solution is treated chemically to produce zirconium oxide. The second route examined is based on the fusion of zircon by NaOH in the temperature range 350-700°C. The fused products are treated with water to extract the soluble impurities (i.e. sodium silicate), leaving a precipitate of hydrous zirconia, which is then treated further either by calcination or with HCl to produce a high-purity zirconium oxide. The kinetics and mechanism of the overall fusion process are also investigated. Raw materialZircon concentrate of grain size 125 µm was separated from the heavy-mineral fraction of beach sand obtained from El-Bardaweel Lake, Egypt, by electrostatic and magnetic separation techniques. The zircon used for the experiments analysed 66% ZrO 2 , 32.13% SiO 2 , 0.29% Fe 2 O 3 , 0.17% TiO 2 , 0.23% Al 2 O 3 and 0.25% moisture and (RE) 2 O 3 . The other materials employed were chemically pure anhydrous Na 2 CO 3 , NaOH and HCl. (1) Fusion of zircon by soda ash Experimental techniquesMixtures of zircon (mean particle size, 63 µm) and sodium carbonate in proportions ranging from stoichiometric to two times stoichiometric were weighed, blended and mixed thoroughly, charged into a nickel crucible and inserted in an electric furnace. The stoichiometry of Na 2 CO 3 is the theoretical amount required to achieve complete formation of sodium zirconium silicate, referred to hereinafter as 1´:ZrSiO 4 + Na 2 CO 3 ® Na 2 ZrSiO 5 + CO 2 (1)The mixtures were heated for several hours at different temperatures in the range 900-1200°C, cooled at the end of the experiments, weighed and subjected to leaching with HCl to dissolve the alkali zirconium compound and precipitate the silica in the form of silica gel. The filtrate, which contained a soluble zirconium salt, was treated with ammonia to produce zirconium hydroxide, which was calcined to obtain ZrO 2 . Another series of experiments was performed with a continuously recording thermobalance system, from which the extent of the fusion reaction is cal...

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