Chemical formula: ZrSiO4
Synonyms: Zircon, zirconium (IV) orthosilicate, zirconium (+4) orthosilicate.
Amount: 200 g
Appearance and odour: fine, white powder without appreciable odour.
Description and uses: it is often used in pottery glazes as a colouring opacifier.
It is one of the main zirconium ores and thus a source of zirconium for chemists and industry. As this is a natural material it hasn’t been de-Hafnified and contains a few percent of hafnium silicate (HfSiO4). Hafnium and zirconium are chemically almost indistinguishable and thus very hard to separate.
Extracting the zirconium from this mineral is fairly easy and can be done with the following procedure, the so-called wet fusion process.
Weigh up a 20 % excess of NaOH with respect to the following stoichiometry: ZrSiO4 + 4 NaOH === > Na2ZrO3 + Na2SiO3
Dissolve the caustic soda carefully in the same amount of water (by weight), obtaining a 50 % NaOH solution. Mix all of this solution carefully with the zirconium silicate, obtaining a smooth paste. Heat this paste in a suitable crucible at 200 C for 2 h, then heat it for a further 2 h at 600 C.
After the heating cycle, allow to cool completely to room temperature and add enough water to cover the fusion product. Allow to soak overnight.
Carefully prize out the caustic fusion product, crush it and neutralise hydrolyse it carefully with an excess of strong hydrochloric acid (HCl). The zirconium will enter solution as zirconyl chloride (ZrOCl2), leaving behind fine, quite pure silica. Separate by decantation/filtration. The silica, washed and dried, is a valuable chemical in its own right.
The crude ZrOCl2 can be recrystallised by firstly concentrating the solution by partial solvent evaporation (boiling in), followed by adding concentrated HCl to the cooled concentrated ZrOCl2 solution which causes ZrOCl2.8H2O (zirconyl chloride octahydrate) to precipitate while most impurities (often iron) will remain in solution. This process can be repeated for extra purity if desired.
Highly pure zirconia (ZrO2) can be obtained from ZrOCl2 by hydrolysing it with NaOH, Na2CO3 or better strong ammonia solution. Freshly precipitated Zr(OH)4 is thus formed, which can be filtered off and used as a precursor for other zirconium salts or can be calcined to pure zirconia (ZrO2).
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