I work on Hydrothermal synthesis of spinel(Zn-substituded magnetite) from hematite in 175 °C and 15 hour but after 15hour in 175° C spinel was converted to Hematite. Do you know why spinel is converted to hematite after 15 hour in constant temperature? Please, Introduce articles about this topic to me. Thanks
The Zn analogue of Magnetite is Franklinite. So I would approach the problem from here. The other Zn-members of the Spinel group are Gahnite and Zincochromite (sometimes wrongly called Zn-Spinel). There is one article that discusses the Zn distribution in Franklinite: O'Neill, H. St. C.(1992) Temperature dependence of the cation distribution in zinc ferrite (ZnFe2O4) from powder XRD structural refinements. European Journal of Mineralogy: 4: 571-580. There is one article that discusses sythesis of Franklinite from a couple of perspectives. You can download it here: http://www.famaf.unc.edu.ar/~bercoff/papers/Synthesis%20and.pdf. The synthesis of Franklinite involves Zincite and Hematite both of which are hexagonal minerals. Since Spinel-group minerals are cubic, if the material has not properly crystalised giving cubic ZnFe2O4 then the reaction was not successful and you will have a mixture of the two minerals. The procedure you using seems to be producing Hematite, but as you state that you are using a hydrothermal method that has failed. This would mean that possible hydrated oxides of zinc and/or Fe2+ should result as well. It would seem temperature and pressure are the issues but looking at the articles (as a start) could indicate where to look.
As you know, magnetite is a mixture of FeO and Fe2O3. This spinel is not very stable in air and specially in basic solution, because Fe2+ ions are oxidized to Fe3+ by oxygen and some of the magnetite is converted to hematite (Fe2O3). So, try to do the reaction under an inert atmosphere.
Tanks Mr.de Fourestier and Mr Kooti
Mr.Kooti, This means that: Oxygen has been increased in my solution after 15 hour?Do you Know any article about this topic?
Probably your jar for the hydrothermal treatment is not sealed hermetically. You can try to close it in a dry box under argon atmosphere and also insert argon as inert in the jar, to limit the contact of the solution with oxygen.
When magnetite is oxidized to hematite, Fe2+ is oxidized to Fe3+. Another species has to be reduced in the whole reaction and O2 is reduced to either O2- or OH- and therefore no increase of oxygen is expected.I haven't come across to any article discussing this process.
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