Alkali-fusion processes for the recovery of zirconia and zirconium chemicals from zircon sand

Abstract

There are two industrial sources of zirconia: zircon and baddeleyite [1-5]. The baddeleyite reserves in Phalaborwa (the world’s major baddeleyite source) are expected to be depleted by the year 2005 [1-3]. This leaves the Russian Baddleyite (Kola Peninsula) and zircon as the only industrial sources of zirconia. The major drawback to zircon use is the large amounts of impurities it is found concentrated with, especially radioactive impurities (Uranium and Thorium) [2-3]. Acid leaching of zircon does not remove these impurities [4-5]. The impurities are usually included in the zircon lattice. The tetragonal structure of zircon with the high coordinated bisdisphenoids ZrO8 and low coordinated tetrahedra SiO4 create a safe (inaccessible and stable) habitat for these impurities [7]. Processes for the recovery of zirconia and zirconium chemicals rely heavily on precipitation or cyrstallisation techniques for purification [8-16]. Precipitation techniques need to be repeated to obtain the required purity. The purity of products from such methods is still suspect, as there still remains a high radioactivity content after purification [2]. The long process time is another disadvantage of these precipitation processes. These factors together are the reason for the high cost of zirconia and zirconium chemicals. Zirconium and its compounds are regarded to be of low toxicity [1-6]. This implies that they have a great potential of replacing numerous high toxic chemicals. Prominent examples are seen in leather tanning and paints. In leather tanning chromium chemicals can be replaced. In paints lead driers and chromium chemicals for corrosion resistance can be replaced. The objective of this study was to characterise and optimise the De Wet’s zirconium extraction processes for the beneficiation of zircon sand into high purity zirconia and zirconium chemicals. However, at each process step some factors were varied e.g. fusion temperature, reactant mole ratios and composition of leach solutions. Attention was also paid to reducing the total number of process steps. The products produced at each step were analysed. Particular attention was given to the fate of the radioactive impurities. Characterisation of the decomposition step, showed that within the zircon tetragonal structure, the SiO4 bisdisphenoids linkages. This was shown by the preference of sodium for the SiO4 tetrahedra. Fusion for 336 hours with periodic intermediate milling proved the preference of sodium for attacking the SiO4 tetrahedra linkages. This selectivity was clearly demonstrated when decomposing zircon in sodium poor(<4 moles NaOH per mol of zircon) and low temperature (e.g. 650°C) reaction conditions. The advantage of fusing at 650°C with a mole (or even two moles) of sodium hydroxide is that it leads to minimal (<5% m/m Na2O) sodium in the insoluble solids after the removal of soluble silicates. This is a solution to alkali fusion processes, as high amounts of water are usually required to wash out the neutralised sodium salt e.g. 50g of NAC1 usually requires a litre of distilled water to reach levels below 600 ppm NA2O. This reaction condition can be employed when synthesising products where low amounts of sodium are required in the final products e.g. when synthesising zirconia for the ceramic industry. When fusing for two hours without the intermediate milling step the following results were observed. The reaction at 850°C when fusing a mole of zircon with two moles of sodium hydroxide, was the most efficient in consuming sodium hydroxide. Near complete zircon decomposition was at 850°C when fusing a mole of zircon with six moles of sodium hydroxide. Characterisation with XRD, Raman and IR spectroscopy was misleading as complex spectra were measured, indicating many different phases present. The inconsistency was partly attributed to non-homogeneity in the samples due to NaOH migration. When fusing for 336 hours with the intermediate milling step the following results were observed. The reaction at 850°C when fusing a mole of zircon with a mole of sodium hydroxide was the most efficient in consuming sodium hydroxide. This reaction condition was able to liberate 0.58 moles of zirconia per mole of sodium hydroxide. The highly improved efficiency was attributed to the formation of phases Na2ZrSiO5, Na4Zr2Si3O12 and SrO2. The process is pseudo-catalytic as it liberates zirconium while showing minimal sodium consumption. Decomposition at 650°C also showed improved efficiency but not as efficient as the 850°C sub-stoichiometric fusion. The improved decomposition was attributed to the polymerisation of the orthosilicate monomers Na4SiO4 to the metasilicate chains Na2SiO3.