Research On Hydrothermal Synthesis And Conversion Mechanism Of Titanate Species In The System Of Na₂O-Al₂O₃-SiO₂-CaO-TiO₂-H₂O

In the world,more than 95%of alumina is produced by bauxite.As one of the main compositions of bauxite,TiO2 has much bad effect on the Bayer digestion process despite its low content(0.5-8%).TiO2 in the Bayer process has great harm,because in the alumina digestion process,especially of diaspore,the presence of TiO2 will make the alumina digestion rate decrease,increase the consumption of alkali and cause scaling.How to avoid or reduce the harm of TiO2 is always the hot-button issue in the alumina industry..Therefore,it is significant to discuss the reaction behavior and conversion mechanism of titanate species.In this paper,the hydrothermal synthesis and conversion mechanism of titanate species in Na2O-Al2O3-SiO2-CaO-TiO2-H2O System were studied.First,thermodynamic analysis on conversion process of titanate species were conducted.Then the interaction between each component were investigated.Finally,reaction behaviors and the conversion mechanisms in Na2O-Al2O3-CaO-TiO2-H2O and Na2O-Al2O3-SiO2-CaO-TiO2-H2O System of titanate species were studied.Firstly,the thermodynamic stability of titanium bearing minerals synthesized was calculated after the estimation of the standard Gibbs free energy of Al4Ti2SiO12 and Ca3TiSi2(Al2Si0.5Ti0.5)O14.Under specific reaction conditions,mNa2O·nTiO2(Na2O·TiO2,Na2O·2Ti02,Na2O 3Ti02,Na20·6Ti02),Al4Ti2Si012,Ca3TiSi2(Al2Si0.5Ti0.5)O14,CaTiO3 can exist steadily.Secondly,the interaction and reaction between the components of the Na2O-Al2O3-SiO2-CaO-TiO2-H2O system were investigated by the combination of different compon ents.The results show that TiO2 reacting in NaOH and sodium aluminate solution can form amorphous sodium titanate mNa2O·nTiO2,the cryptocrystalline minerals(or qua si minerals)has large specific surface area,high surface energy and has a certain deg ree of adsorption.When the thermodynamic factors varies,it will gradually convert to phaneritic minerals to reduce the surface energy.The activation energy and the reacti on order of the different temperature ranges in Na2O-Al2O3-CaO-TiO2-H2O and Na2O-Al2O3-SiO2-CaO-TiO2-H2O System were calculated by non-isothermal dynamics analysi s,and the reaction control types were determined.Third,the conversion route of titanate species in the system of Na2O-Al2O3-CaO-TiO2-H2O was established after the analysis on the reaction of different components above and hydrothermal synthesis experiments under the digestion conditions of diaspore and gibbsite.With the increase of temperature,titanate species convert along the path:TiO2?mNa2O nTiO2?CaTiO3.The chemical equations of conversion process are:·2mNaOH+nTiO2= mNa20·nTiO2+mH2O mNa2O·nTiO2+nCa(OH)2=nCaTiO3+2mNaOH+(n-m)H2O The total chemical equation is:TiO2+ Ca(OH)2 NaOH?CaTiO3+ H2OThe increase of temperature and the concentration of sodium aluminate solution can promote the formation of CaTiO3.Finally,the conversion route of titanate species in the system of Na2O-Al2O3-SiO2-CaO-TiO2-H2O was established after the analysis on the reaction of different components above and hydrothermal synthesis experiments under the digestion conditions of diaspore and gibbsite in silicon containing and silicon excluding systems.With the increase of temperature,titanate species convert along the path:TiO2?Al4Ti2SiO12(60?)?Ca3TiSi2(Al2Si0.5Ti0.5)014(200?)?CaTi03(260?)TiO2,Al4Ti2Si012,Ca3TiSi2(Al2Si.5Ti.5)O14 and CaTiO3 are all phaneritic minerals containing titanium.Among these,CaTiO3 has the most stable chemical properties in which the ions stack most closely in the unit cell.In addition,mNa2O·nTiO2 still exists in the system of Na2O-Al2O3-SiO2-CaO-TiO2-H2O in company with the dissolution and recrystallization of phaneritic minerals.