Cubic Zirconia from Zircon Sand by Firing with CaO/MgO Mixture

The effects of firing temperature and molar ratio of CaO/MgO on the formation of cubic Zr02 from zircon were determined. Experimental results indicate that a thermal reaction completed by firing at 1500°C for 2 h using a charge of zircon and 0.4CaO/0.6MgO molar ratio is favorable for the production of mainly pure cubic Zr02 ss containing 1.29%MgO and 0.22%CaO after separating the silica by acid leaching. Different kinetic models have been tried, but the mechanism of the overall firing reaction obeys the diffusion model in the form of: X+ (1-X) In (1 -X) =kt, where X is the extent of solubilized silica in time t and k is the rate constant. The apparent activation energy of the decomposition process is calculated and found to be 120 kJ/mol. [Received November 25, 1997; Accepted November 6, 1998] Key-words : Zircon, Sintering, CaO/MgO mixture, Recovery, Zirconia, Kinetics and mechanism, Structure 1. Introduction Zirconia is one of the important materials which find wide usage depending on its purity and crystal structure in several scope of applications such as ceramics, refractories, electronics and others. Cubic zirconia forms also the nucleus of. a new class of ceramic materials which has great potential engineering future for structural applications. A little number of investigations')-6) have been reported in the literature on the decomposition of zircon using CaO/MgO mixture. This paper is one of a series of investigations carried out by the authors aiming to produce zirconia by reactions of local zircon sand with alkaline earth metal oxides (CaO), alkali (Na2CO3 & NaOH) and with C12 gas in presence of carbon. ')-1') It deals with the preparation of cubic zirconia by the pyrochemical displacement of zirconia from local zircon by CaO/MgO mixture. The final product is evaluated physically, chemically and microscopically. The sequence of reaction mechanism and kinetics for the decomposition process of zircon by CaO/MgO mixture is also considered. 2. Experimental 2.1 Raw material Zircon concentrate sample of grain size 125 ,um is obtained as a by-product from the beneficiation of Egyptian black sands (El Bardaweel lake) which are present in different localities along Egypt's Mediterranean sea coast and Nile delta. Analysis of zircon concentrate is found to contain 66% Zr02, 32.13% SiO2, 0.29%Fe203i 0.17%Ti02, 0.23% A1203, 0.25% moisture and (RE)203. The other materials used are Analar grade of HCl and chemically CaO and MgO prepared from the decomposition of pure CaC03 and MgC03 at -900'C till constant weight. 2.2 Experimental techniques Mixtures of zircon (mean particle size : 63,um) and CaO/MgO ranging from x=0.5 to 1(x=molar ratio of (Ca0+Mg0)/zircon) are blended and mixed thoroughly, charged in a platinum crucible and inserted in an electric tube furnace at a fixed temperature. The molar ratio (x) is the theoretical amount of CaO/MgO required to achieve a complete disintegration of zircon to produce Zr02 and calcium magnesium silicate. The firing experiments are carried out in the temperature range 1200°-1500°C for various periods up to 4 h. The temperature of the furnace was controlled within ±5°C. At the end of each experiment, the fired charge was drawn gradually and left inside the furnace to cool down, and then it was taken out for grinding andanalyzing. Leaching of the powdered specimens with dilute HC1 acid (s/l ratio= l g/50 ml, 0.72 M at 50°C for 1 h) is performed to separate the solubilized silica (calcium magnesium silicate) and leaving behind Zr02 as the main product in the residue. After leaching, the residues were calcined at 900°C for 1 h and the extent of the solid-state reaction is determined from the amount of solubilized silica by atomic absorption spectrometry") with respect to the initial Si02 content in the zircon sample. To confirm the results, the remnant silica in the zirconia residues of some samples has been analyzed by HF. X-ray diffraction (XRD) using Cu radiation is used to identify the phases present and selected specimens are investigated microscopically. 3. Results and discussion 3.1 Effect of CaO/MgO mixture Charges containing different percentage of CaO/MgO (1:1) ranging from 20.78%, 28.2% and 34.43% to zircon concentrate are fired at 1400°C for 2 h. They correspond to x=0.5, 0.75 and 1.0. in case of x=1.0 ZrSiO4+CaO+MgO -f Zr02 + CaMgSi04 (1) The results in Fig. 1 demonstrate that the intensities of lines representing the cubic structure of zirconium oxide in the fired products at 1500°C for 2 h increase as CaO/MgO percentage increases tending to be predominant at 34.43% (1x). The monoclinic zirconia tends to be minority as CaO/MgO percentage increases. Hence, it is pointed out that one molar ratio (x=1.0) CaO/MgO (equivalent to 34.43% by weight to zircon) is the proper percentage of basic oxides added to obtain cubic zirconia (other percentages such as 38.6% and 42.3% have been applied, which are equivalent to x=1.2 and x=1.4) . Regarding to the formation of orthosilicate during firing, the compound CaMgSi206 is identified in the fired products containing 0.5x CaO/MgO mixture (20.78%), while both CaMgSi2O6 and Ca2MgSi2O7 are identified at 0.75x and 1x CaO/MgO mixture (28.2% and 34.43%). Leaching of the fired product with HCl acid (0.72 M at 50°C for 1 h, s/l= 1 g/50 ml, higher conc. of HCl has no effect on the improvement in the recovery of solubilized silicate in the fired product) gives solubilized Si02 of 70%, 75% and 96% for zircon samples containing CaO/MgO mixture of 20.78%, 28.2% and 34.43%, respectively. 3.2 Effect of temperature and time Zircon sample containing CaO/MgO (1 : 1) mixture ofpercentage 34.43%(x=1)by weight to zircon are fired at