Cleaner Production Process Of Titanium Dioxide By Decomposition Of Titanium Slag With Molten Sodium Hydroxide

Tititanium reserves in China are 965 million tons(TiO₂),accounting for 38.85%of the total in the world,ranked the first place,of which 90.5%occurs in Panzhihua-Xichang region, present as multi-component associated magnetite containing vanadium and titanium.Due to the complexity and lower grade of resource,the comprehensive utilization of titanium is only 14.5%.Titanium dioxide pigment white,used as coatings,fillers for plastic and paper,has been the third most important raw material in chemical industry,following ammonia and phosphoric acid.Industrial production methods of titanium dioxide pigments include the sulfate process and the chloride process,which discharge a large amount of toxic waste,resulting in serious environmental problems.According to the characteristic of titanium resources in China,a novel metallurgical clean process for production of titanium dioxide by decomposing titanium slag with NaOH molten salt was proposed in the present paper.The aim of this process is to eliminate the pollution at the source and to fully utilize valuable compounds in titanium slag. The main points are as follows:(1) A novel reaction system of molten NaOH and titanium slag was established.By thermodynamic calculation,the possible products of the main compositions existed in titanium slag were investigated in the decomposition process with molten NaOH.Effects of reaction temperature,time,NaOH-to-slag mass ratio and particle size on titanium fractional conversion were studied.Approximately 98%of titanium in the slag could be converted under the optimal reaction conditions,and the product was characterized to be Na₂TiO₃ by XRD. The decomposition kinetics of titanium slag with molten NaOH under optimal reaction conditions indicates that the shrinking core model with chemical reaction controlled process is most applicable for the decomposition of titanium slag,with the apparent activation energy of 40.84kJ mol^-1.(2) The recycle of NaOH reaction medium could be carried out by multi-stage countercurrent wash of molten reaction product.Approximately 90%of the Na⁺ in molten product could be leached at the optimal conditions by multi-stage countercurrent washing, and the lye obtained could be recycled by further purification and evaporation.The impurities of Cr,Al,Mn and Si were leached to the alkaline solution,while titanium and impurities of Fe,Ca,Mg remained in the solid intermediate,thus,titanium was initially separated from impurities effectively and selectively.The chemical composition of solid intermediate after wash is TiO₂ 65%～70%,Na 10%～12%.(3) Preparation of hydrous titanium oxides by ion-exchange and separation of impurities were studied.The solid intermediate with high content of titanium was transferred to spherical and needle-like hydrous titanium oxides particles by ion-exchange at boiling point in diluted H₂SO₄ and HCl solutions,respectively.The impurities of Fe,Ca and Mg were leached to the acid solution,while titanium precipitated in the form of hydrous titanium oxides and subsequently effectively separated from impurities by filtration.The leaching rate of impurities can be improved by adding reductants during ion-exchange process.Experimental results show that the purity of product obtained in HCl medium is superior to that in H₂SO₄ medium.(4) Prepartation of hydrous titanium oxides by thermal hydrolysis of titanium sulfate solution was investigated.The solid intermediate was first digested in diluted sulfuric acid, forming titanium sulfate solution,which was subsequently hydrolyzed at boiling point, yielding hydrous titanium oxides precipitate.The unreacted titanium slag and impurities such as Ca,Mg and Si can be removed by filtration from titanium sulfate solution before its thermal hydrolysis.The data of thermal hydrolysis of titanium(â…£) ions show that well dispersed spherical titanium dioxide particles with average size of 150 nm can be obtained under optimal reaciton condition.The results analyzed by XRF,XRD and whiteness show that chemical composition of the product is TiO₂ 98.6%,Fe₂O₃ 0.002%with Hunter white (W_h) 97.84 and rutile content(R%) 97.64%.After further treatment of coating and crush,the product can be used as pigment.