Phase And Microstructure Changes Of Titanium Slag During The Oxidation-reduction Pretreatment And Its Influence On Leaching Process

The Panxi titanium slag in China is a kind of intractable low-grade slag,which is rich in magnesium and calcium and difficult to be industrially upgraded to the feedstock suitable for the chloride process,making it difficult to meet the great demand for the future development of the chloride process in China.The oxidation-reduction-acid leaching process is the most hopeful method to upgrade the Panxi titanium slag to the chloride process feedstock.The upgrading process is greatly dependent on the chemical composition of the titanium slag.Since the influence of the slag chemical composition on the oxidation-reduction-acid leaching process has not yet been fully clarified,unable for people to obtain a guidance for upgrading the Panxi and other low-grade titanium slags,it is therefore very important to study the phase and micro structure evolutions during the oxidation-reduction-acid leaching process.To solve this problem,microstructure evolutions during the oxidation and phase transition during the reduction were systematically investigated based on the Panxi titanium slag in the present study,where the influence law of the slag chemical composition on the oxidation-reduction-acid leaching process was inspected,with the main focus on providing guidance for determining the optimum upgrading conditions and controlling the upgrading results of various titanium slags.The main findings of this thesis are summarized as follows:i)Microstructure evolutions of titanium slag particle oxidized at low?800??and high?1000??temperature were systematically investigated,and two different oxidation mechanisms were found.At 800?,the oxidation of slag particle was achieved by the outward migration of Fe²⁺ ion and inward migration of O^2-,without the migration of Ti³⁺.Fe²⁺ migrated to the surface of the particle and was oxidized to whisker Fe₂O₃.The inward migration of O^2- led to the formation of a Fe-rich belt in the reaction interface.The reaction interface kept shrinking towards the particle center,leading to the oxidatioI of Fe²⁺ and Ti³⁺.Due to the slow ion migration at low temperature and in the dense particle structure,the shrinking of the reaction interface slowed down drastically,resulting in that the slag particle could not be completely oxidized even in a long duration.At 1000?,the oxidation of slag particle was achieved by outward migration of Ti³⁺?Fe²⁺ and inward migration of O^2-.Ti³⁺?Fe²⁺ migrated to the surface of the particle and was oxidized to the inner Fe₂TiO₅ and outer TiO₂ layers separately.Due to the fast ion migration at high temperature and the porous particle structure,the reaction interface fastly shrank to the particle center,resulting in the fastly complete oxidization of the slag particle.ii)Phase transition and microstructure evolution of titanium slag particles oxidized under different oxygen volume percent?vO₂?%??were investigated.It was found that vO₂?%?mainly affected the microstructure evolution of the slag particle and further changed its acid solubility.At low vO₂?%??3 vol.%O₂?,no obvious Ti³⁺?Fe²⁺ outward migration happened,resulting in a dense particle structure and the inward diffusion of hydrochloric acid became the rate controlling step during the following leaching process.Increasing vO₂?%?promoted Ti³⁺,Fe²⁺ outward migration,which led to two microstructure changes.On one hand,the slag particle became more porous,on the other hand,Fe/Mg molar ratio of the?M₃O₅?₂ phase in the particle center decreased.As a result,at moderate vO₂?%??6 vol.%O₂?,the particle structure became porous,which ensured the fastly inward diffusion of hydrochloric acid.And the bad acid soluble?M₃O₅?₂ phase still had a high enough Fe/Mg molar ratio to be transformed into the good acid soluble M₂O₃ phase.So most impurities in the slag could be acid leached.But at high vO₂?%??21 vol.%O₂?,the?M₃O₅?₂ phase in the particle center could be hardly transformed into the M₂O₃ phase due to its low Fe/Mg molar ratio.Therefore,although the particle presented a dense structure,the impurities could be hardly acid leached.iii)Oxidation kinetics and microstructure evolutions of the oxidized titanium slags with different Fe/Mg molar ratio were investigated.And the influence laws of Fe/Mg molar ratio on the oxidation temperature of titanium slag was clarified.For the slag with Fe/Mg<2,Fe²⁺ outward migration was restricted at 800 0C.And O^2-has a slow migration due to the low temperature and dense particle structure.So the titanium slag could not be completely oxidized even in a long duration.Raising temperature to 1000? promoted the outward migration of Ti³⁺ and a small quantity of Fe²⁺.O^2-has a fast migration due to the high temperature and the formed porous particle structure.Therefore,the slag with Fe/Mg<2 needed high temperature?1000??to complete the oxidation.For the slag with Fe/Mg?2,since increasing Fe/Mg molar ratio promoted Fe²⁺ outward migration,and the formed porous particle structure released the inner diffusion resistance of O^2-.So this titanium slag could be completely oxidized at both high and low temperature?800-1000??.iv)Phase transition process during the reduction of?M₃O₅?₂??Fe₂TiO₅?d?MgTi₂O₅?e?with different Fe/Mg molar ratio was investigated.And the influence laws of Fe/Mg molar ratio on this process was clarified.The reduction process of?M₃O₅?₂ underwent the formation of the intermediate?M₃O₅?₃?FeTi₂O₅?h?MgTi₂O₅?i in the M₃O₅-type and subsequently the?M₃O₅?₃ further decomposed into the M₂O₃ phase?FeTiO₃?f?MgTiO₃?g.Fe/Mg molar ratio did not affect the reduction process of?M₃O₅?₂??M₃O₅?₃,but affect the thermal decomposition of?M₃O₅?₃ to the good acid soluble M₂O₃ phase.As Fe/Mg?1.8,?M₃O₅?₃ had a poor stability and could completely decompose into M₂O₃.Decreasing the Fe/Mg molar ratio enhanced the thermostability of?M₃O₅?₃.Therefore,as Fe/Mg<1.8,?M₃O₅?₃ could just partially decompose into M₂O₃,remaining some Mg-rich?M₃O₅?₃ even after a long reduction.v)Based on the above results,it was experimentally demonstrated that Fe/Mg molar ratio was a core factor that obviously affected the upgrading results of titanium slag.And a guiding principle of upgrading titanium slags with different Fe/Mg molar ratio was established accordingly.For the titanium slag with Fe/Mg<1,the oxidation-reduction-acid leaching process cannot upgrade it to the chloride process feedstock.Increasing Fe/Mg to?1 could realize the successful upgrading.For the titanium slag with 1?Fe/Mg<1.8,high temperature?1000??oxidation and pressure acid leaching were necessary,the titanium slag with TiO₂ 72 wt.%,CaO + MgO₅ wt.%could be upgraded to the high-grade slag with TiO₂ 89 wt.%,CaO + MgO 0.9 wt.%.Further increasing Fe/Mg could lower the oxidation temperature and realize the atmospheric acid leaching.For the titanium slag with Fe/Mg>1.8,low/moderate temperature?800 ?-900??oxidation and atmospheric acid leaching were enough,the titanium slag with TiO₂>68 wt.%,CaO + MgO 5 wt.%could be upgraded to the high-grade slag with TiO₂>85 wt.%,CaO + MgO<0.8 wt.%.