Qiao Wang,Qingshan Zhu*,Chuanlin FanZhen WangHongzhong Li

1State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2University of Chinese Academy of Sciences,Beijing 100049,China

1.Introduction

Titanium tetrachloride(TiCl4),as an intermediate material to prepare titanium white with high quality and titanium sponge,is mainly produced by the fluidized chlorination process at 1073–1273 K[1,2].However,such a process requires the raw minerals to have low level of impurities,especially low contents of MgO and CaO[3,4].Deriving from the operation empirical parameters,the total mass fractions of MgO and CaO in the feedstock are required to be lower than 1.0 wt%,preferably lower than 0.2 wt%for CaO[5,6],as the liquid MgCl2and CaCl2generated from CaO and MgO during the chlorination process cause the agglomeration of the solid particles,which could result in defluidization and cessation of the chlorination operation [7,8]. Nevertheless,abundant titaniumresources in China and Canadawith the contentof CaO higher than 0.2%, and the limited reserves of high gradetitanium resources cannot meet the increasing demand of the fluidizationchlorination process [9–11]. Therefore, attempts of ameliroatingthe agglomeration of the solid particles caused by the generated liquid MgCl2and CaCl2have attracted much research interests.

Various methods have been developed during the past several decades,including changing the bed structure[12,13],reducing the chlorination temperature or the content of alkaline earth oxides to avoid the generation of the liquid phase[7,14],and regulating the operation parameters to increase vaporization rate[15,16].Superiority of regulating the operation parameters may both guarantee the chlorination rate of feedstock in existing equipment and avoid complicatedpre-treated processes of the feedstock.Among them,promoting the vaporization rate to remove these generated liquid chlorides was found to be an effective tactic to avoid the agglomeration of the solid particles.Normally,vaporization operation is an efficient approach to recycle valuable elements or separate hazardous compound.However,due to the complexity of the chlorination of titanium ore systems during industrial operation,it is difficult to obtain valid vaporization kinetics of MgCl2–CaCl2melts.Therefore,investigation on the basic vaporization law of MgCl2–CaCl2melts could be beneficial for a deep understanding of its vaporization and accumulation performance in the bed.Furthermore,it could be advantageous to simulate vaporization rate and provide optimization design of process control,and consequently ameliorating the agglomeration of the particles.

Only a few reported literatures related to the vaporization of the chlorides,with the main focus of the hydrolysis and vaporization processes of their hydrates in electrothermal vaporizers lower than 1073 K[17,18],which is different from the vaporization of pure chlorides melt because of the hydrolyzation of chlorides hydrates.Besides,under industrial operation practicality,the generated MgCl2and CaCl2accumulated in the bed and form binary melt,which is different to the vaporization of pure chloride.Obviously,systematic investigation regarding the vaporization behavior of pure MgCl2,CaCl2and their binary melt is necessary.Furthermore,with simultaneous vaporization of each component,vaporization rate changes with the composition of the binary melts;a vaporization model would be meaningful for predicting the evolution behavior and optimizing the vaporization performance.However,investigations on the bi-components vaporization model were focused on the distribution of temperature or diffusion of liquid species inside the fuel droplets[19,20];while an equilibrium or a nonequilibrium model for distillation processes onlyconcentrated on the evolution of liquid composition with stage number,which does not involve vaporization rate [21,22]. Up to now, kineticsmodel of binary melt simultaneous vaporization has not beeninvestigated in detail in the literature, and no evolution law regardingthe vaporization rate with the composition of binary melts has everbeen reported. Therefore, systematic modeling study regarding the evolutionof vaporization rate with the composition of binary melts duringvaporization process is potentially attractive for simulating andpredicting the vaporization behavior with various initial compositions.

In order to describe the fluidization vaporization behavior relevantly,the present investigationwas conducted in a fluidized bed using titaniumslagparticles pre-adheredwithMgCl2andCaCl2.Asemi-empirical model for the vaporization of MgCl2–CaCl2binary melt was built from the obtained kinetics equations of both pure chlorides.The influences of the composition and temperature on the vaporization rate of the binary melts were elucidated.A correlation between the necessary operation temperature and the CaO/(CaO+MgO)of the feedstock was proposed.

2.Experimental

2.1.Materials

CommercialPanzhihuatitaniumslagwithanaveragesizefrom200to 300 μm(Panzhihua Iron and Steel Group Corporation,China)was used.The as-received titanium slag contained 71.3%TiO2,11.2%Fe2O3,6.6%SiO2,4.2%MgO,2.5%Al2O3,1.8%MnO and 1.1%CaO in mass,which was determined by X-ray fluorescence spectroscopy(XRF,PW4400/40;PANalytical,theNetherlands).Toinvestigatetheintrinsicvaporizationkinetics,the analytical reagent grade of MgCl2(99%,Alfa Aesar)and CaCl2(99%,AlfaAesar)wasused.N2gas(99.99%purity)was supplied by Beijing Beiwen Gas Chemical Industry Co.Ltd.,Beijing,China.It is necessary to removethe easily chlorinated substances on the surface of the titanium slagparticles, suchas Fe2O3and MnO,to eliminate the chemical reaction betweenchloridesandtheslag[23].The effect of the easily chlorinated substances can be eliminated by the pretreatment of the mixture of MgCl2and the titanium slag at 1173 K in N2atmosphere.

2.2.Adhesion of chlorides

The adhesion of MgCl2and CaCl2on the surface of the pretreated titanium slag particles was conducted in a horizontal tube electric furnace in N2atmosphere.The mixture of the titanium slag and the chlorides(with the constant content of the chlorides was 3.0 wt%)was put into a corundum boat(width 18 mm,depth 12 mm,and length 92 mm)and was heated at the desired temperature(1053 K for MgCl2,and 1073 K for CaCl2and MgCl2–CaCl2compounds)for 5 min,and then the samples for fluidized vaporization were obtained.

The microstructure of slag particles before and after adhesion wasexamined by scanning electron microscope (SEM, JSM-7001F; JEOL).Fig. 1(a) displays the surface and cross-sectional morphology of particlebefore adhesion. The surface of the particle is rough and the inner regionof the particle is dense except some defects. Due to this dense structure,the chloride melt can only be adhered on the particle interface, and theinternal diffusion can be ignored during the vaporization process.Fig. 1(b) presents the surfacemorphology of slag particle after adhesion.The rough surfaces of the particles were covered by a thin layer of chlorides.Fig. 1(c) provides the cross-sectional morphology of the particlesafter adhesion. The inner region still maintained dense character, combinedwith the energy dispersive spectrometer (EDS) measurement, itcould be concluded that the chlorides were adhered on the surface ofparticles without penetrating into the inner regions. Due to the watersolubility of chlorides, the sample was pretreated by dry grinding thatcauses tenuous scratch. Fig. 1(d) shows the chlorides were completelyadhered on the surface of particles without any sporadic chloride particles scattered among the slag particles through the adhesion technologyat temperatures above the melting points of chlorides.

Fig.1.Morphology of slag particles before and after adhesion:(a),surface and cross-section morphology of particle before adhesion;(b),surface morphology of particle after adhesion;(c),cross-section morphology of particle after adhesion;(d),particles after adhesion.

2.3.Vaporization of chlorides

The vaporization kinetics of the chlorides was conducted in a fluidizedbed reactor(14mmin inner diameter of the quartz tube)at1073–1273K in N2atmosphere,the flow rate of the N2gas was controlled by a mass flow meter.In order to obtain the intrinsic vaporization kinetics parameters,the elimination of the mass-transfer effects could be achieved by operatingin a shallow bed (3.4 g titanium slag for each run) using a high gas velocity[24].To determine the optimum gas velocity,experiments were conducted by using nitrogen gas ranging from1.5to2.0L·min-1.The vaporization ratio increases with increasing N2flow rate until 1.8 L·min-1(0.19 m·s-1),beyond which the vaporization rate is independent on the gas flowrate.1.8L·min-1was there fore deter minedas the minimumgas flow rate free of mass transfer resistance and was used as the constant carrier gas flow rate in the following experiments.After specified vaporized time,the quartz reactor was taken out and cooled quickly by water,and then the sample was removed to the drying oven after being cooled to room temperature.It is needed to emphasize that water vapor should be avoided to come into contact with chlorides in the whole procedure due to the high hygroscopicity of MgCl2and CaCl2.The obtained samples were subjected to analysis.

2.4.Analysis

0.5g of each sample was dissolved in 50ml0.25mol·L-1ammonium nitrate solution and agitated for 2 h to ensure the chlorides dissolved completely.The content of Cl-was analyzed by precipitation titration method.The contents of Ca2+and Mg2+were determined by EDTA comp lexometry.The mass content of chlorides in the residues can be obtained from these two comparable results in condition that deduct the background of the sample.The ratio of the vaporization of chlorides was calculated by using the following equation,

whereXis the ratio of the vaporized chlorides,mis the mass fraction of the chlorides in the residues by analyses,m0is the initial mass fraction of the chlorides adhered on the titanium slag particles.

3.Results and Discussion

3.1.Vaporization kinetics of pure MgCl2and CaCl2melts

The effect of temperature on the vaporization of molten MgCl2was conducted at 1073–1273 K with a constant mass content of MgCl2in the slag.Fig.2 shows the vaporization results of MgCl2,where both the vaporization rate and ratio increase significantly with increasing temperature.More than 90%MgCl2vaporized in the first 2 min at 1173–1273 K,which indicates the rapid vaporization of pure molten MgCl2.Besides,the vaporization ratio enhanced obviously with the increase of temperature,especially in the first minute,the vaporization ratio at 1273 K is more than three times than that at 1073 K.

Fig.2.Vaporization ratio of MgCl2at different temperatures.

The MgCl2melt was considered to shrink along with the vaporization process,and the shrinking core model was used to obtain the intrinsic vaporization kinetics[25].The liner relationships of the conversion function,1-(1-X)1/3,versusthe vaporized timet,is presented in Fig.3(a),whereXis the vaporization ratio of the MgCl2melt,while the slopes represent the apparent rate constantsN.All the fitting curves have excellent correlation with the data at different temperatures,which clarify the MgCl2melt shrinks along with the vaporization process.Furthermore,Arrhenius plot of the rate constants from Fig.3(a)is provided in Fig.3(b),and the obtained apparent activation energy is 100.6 kJ·mol-1.The apparent rate constant can be expressed as follows:

Meanwhile,the effect of the temperature on the vaporization of molten CaCl2was also conducted at 1073–1273 K.Fig.4 shows that both the vaporization rate and ratio of CaCl2increase significantly with increasing temperature.In comparison with the vaporization of MgCl2,only 16.6%CaCl2vaporized in the first 2 min at 1173 K,while the vaporization ratio of MgCl2approaches to 90%.The relationships of the conversion function of CaCl2,1-(1-X)1/3,versustime,are shown in Fig.5(a).Though with some deviations,the vaporization processofCaCl2melt canal so be described by the shrinking core model.The obtained apparent activation energy is 109.7 kJ·mol-1according to the data in Fig.5(b),a little higher than that of MgCl2.The apparent rate constant can be expressed as follows:

Fig.4.Vaporization ratio of CaCl2at different temperatures.

Fig.3.Intrinsic kinetics of the vaporization of MgCl2from Fig.2:(a)Plot of 1-(1-X)1/3vs.vaporization time;(b)Arrhenius plot of the rate constants of MgCl2(E=100.6 kJ·mol-1).

Fig.5.Intrinsic kinetics of the vaporization of CaCl2from Fig.4:(a)Plot of 1-(1-X)1/3vs.vaporization time;(b)Arrhenius plot of the rate constants of CaCl2(E=109.7 kJ·mol-1).

According to Eqs.(2)and(3),the corresponding apparent vaporization rate constants of MgCl2and CaCl2at different temperatures are provided in Table 1.The different vaporization rate constants between molten MgCl2and CaCl2should be ascribed to the difference in saturated vapor pressure.The saturated vapor pressure of MgCl2and CaCl2calculated by the Clausius–Clapeyron equation is also shown in Table 1.At 1173 K,the saturated vapor pressure of MgCl2is more than one hundred times than that of CaCl2,which leads to the vaporization rate of MgCl2much quicker than that of CaCl2.Therefore,during industrial chlorination process,the restriction on the content of MgCl2is not as strict as that of CaCl2in the bed.

Table 1The rate constants and saturated vapor pressure of MgCl2and CaCl2at different temperatures

3.2.Vaporization kinetics model correlate pure liquids with their binary mixture

There are essential differences between pure chlorides and their binary mixture on the vaporization kinetics,as the mole fraction of each component changes with the vaporization proceeding.However,there is few investigation on the vaporization kinetics between pure chlorides and their binarymixture. Therefore, the present study devotesto investigate the evolution law of the overall vaporization rate with thechange of themelts composition, and establishes the relationship of thevaporization rate between the pure chlorides and their binary melt.

The partial pressure of component A can be expressed as:

whererepresents the vapor pressure of the pure liquid A,xArepresents the mole fraction of component A in the mixture,γx,Arepresents the activity coefficient of component A.In general,vaporization rateVis proportional to vapor pressurePby Langmuir's equation[26].It could be obtained that the vaporization rate constant of component A equals to the rate constant of pure liquid A multiply its mole fractionxA:

whereis the vaporization rate constant of pure liquid A.The apparent rate constantsNcan be obtained experimentally in Section3.1.According to the shrinking core model,there exists relationship betweenandNA,which can be expressed as:

NAis the obtained apparent vaporization rate constant of pure liquid A,ρAis the density of pure liquid A,andr0is the initial radius of the melt.Supposing each component evenly spreading in the binary melt which constituted by innumerable spherical droplets,the radius of the droplet shrinks with the vaporization process.The vaporization occurs exclusively at the surface of droplets at a rate proportional to the receding surface area and the mole fraction of each component,and then the changing mass of each component can be calculated as:

The changing mass of component A is proportional to its volume fraction νA:

wherenAandnBare the mole contents of components A(MgCl2)and B(CaCl2),MAandMBare the molecular weights of components A and B,respectively.

Then νAand νBare obtained as:

The changing surface area of component A is proportional to its mole fractionxA:

And then the changing weight of each component can be expressed as:

Simplifying Eqs.(15)and(16)as:

To the binary melt,there exists:

And then could be obtained the relationship betweenxA,randt:

As magnesium and calcium both belong to alkaline earth metal and have similar physical properties in the size and shape of the molecules,it is therefore the binary melt is assumed to be an ideal solution and the activity coefficient of each component in the solution roughly equals to 1[27].Substituting the value ofγx,A,γx,B,MA,MB,ρAandρBintoEqs.(20)and(21),

Given the initial mole composition of the binary melt and the apparent vaporization rate constant of pure liquid,numerical algorithm for the partial differential Eqs.(22)and(23)could be calculated by MATLAB with the fourth order Runge–Kutta method.The weight of each component is obtained by substitutingxAandrinto Eq.(9),and then we could calculate the vaporization fraction of the binary melt with different times.

3.3.Vaporization of binary melt:Numerical simulation versus experimental data

According to the mass ratio of MgO and CaO in the titanium resources,the ratio of 1:0.2 was selected to investigate the relationship between vaporization ratio and temperature.The theoretical value and experimental value of overall vaporization ratios at different temperatures are provided in Fig.6.The comparison between simulation results and experimental data shows that the proposed semi-empirical model well represents the evolution of the vaporization for the considered range of temperature.Obviously,rising temperature is still the effective way to increase the vaporization ratio of the binary melts,even though the effect is not as significant as pure chlorides.The experimental value at 1073 K is slightly higher than the calculated value in 2 min because the melting point of the binary melt is approximate 950 K from Fact Sage salt databases,which means that the vaporization starts at a temperature lower than the melting point of the pure MgCl2(987 K)and CaCl2(1055 K).Due to the slow intrinsic vaporization rate of each component at 1073 K,the effect from the low melting point on the elevation of the rate is remarkable.While at1173K and 1273 K,the effect is minor because of the rapid intrinsic vaporization rate of the melt.Subsequently,as the vaporization proceeded,the effects caused by the decrease of the melting point on the vaporization become insignificant.

Fig.6.Overall vaporization ratio at different temperatures with the mass ratio of MgCl2to CaCl2is 1:0.2.

The effect of the initial compositions on the vaporization of the binary melt was investigated at 1173 K while maintaining the overall chloride content as a constant.Fig.7 provides the theoretical vaporization curves and the experimental values with different initial compositions.The overall vaporization ratio of the melt declined distinctly with in creasing CaCl2content.When the initial massratio of MgCl2toCaCl2is 1:0.5,the vaporization ratio in 4 min is equal to the ratio of 1:0.1 in 1 min,which reveals that the rate of the vaporization of the binary melt significantly was affected by the composition.Actually,a small quantity of CaCl2has double influence to deteriorate the overall vaporization rate.From the constructed semi-empirical model,low mole fraction of CaCl2together with slow intrinsic vaporization rate constant could result in its lower vaporization rate on one hand,it could also decrease the vaporization rate of MgCl2viadecreasing its mole fraction on the other hand.Therefore,CaCl2decreases the vaporizationrate and ratio of the binary melts, and the effect increases with increasingthe mole fraction of CaCl2.The theoretical values are consistent well with the experimental values when the vaporization ratios are lowerthan60%,beyond which,there exist salittle deviation,as the nonuniform distribution of each component in the melt after various vaporization time.However,the deviation has no significant effect on theconsistencyof the vaporiz ationtrend.In conclusion,the constructed semi-empirical model is valid for predicting the vaporization of the MgCl2–CaCl2binary melt with various initial compositions.

Fig.7.Overall vaporization ratio with different initial mass ratios of MgCl2to CaCl2at 1173 K.

In comparisonwith other vaporizationmodels for bi-component liquid,there is fewattention that has been paid to evolution law regardingthe vaporization rate with the composition of binary melts. Sathin [19]developed a simplified model for bi-component droplet heating andevaporation, which has been applied to the analysis of the observedaverage droplet temperature. Strotos [20] reported a numerical investigationof the evaporation of two-component droplet, the evaporationrate model employed is stemmed from Fick's law and uses the vaporconcentration gradient at the liquid–gas interface as the driving forceof vaporization. Harbou [21] used an equilibrium stage model to describethe distillation process, it could accurately predict the evolutionof liquid compositionwith stage number, but does not involve vaporizationrate. The objective of this model is to connect the vaporization ratebetween pure liquid and their binary melt, and to investigate theevolution law regarding the vaporization rate with the composition ofbinary melts. The change of the mole fraction and vaporization ratiowith different initial compositions could be calculated if the apparentvaporization rate constants of each pure liquid were predetermined.Moreover, this approach is not only applicable to the MgCl2–CaCl2binary melt system,but also can be extended to other ideal binary mixture systems.

3.4.Process prediction and optimization for vaporization operation conditions

The vaporization kinetics model can simulate the evolution of overall vaporization rate with the change of melts composition.Therefore,the necessary operation temperature could be simulated by the model for various initial compositions that achieve the same vaporization rate.The composition of the melts depends on the initial titanium resource composition.Chlorination reactions to produce titanium tetrachloride are generally carried out at a temperature of approximately 1273 K,as chlorination process of alkaline earth metal oxides in the feedstock is not the rate controlling step above 1273 K[28,29],it is therefore supposed that compositions of the generated MgCl2–CaCl2binary melts are similar to initial compositions of the alkaline earth metal oxides.The ratio of CaO/(CaO+MgO)in the industrial feedstock preferably required to be lower than 0.2[5],numerical simulation by the model indicates that almost 86.6%melts vaporized within 2 min at 1273 K when the ratio of CaO/(CaO+MgO)in the feedstock is 0.2.It would be advantageous to utilize the relationship between initial oxide composition and necessary operation temperature to facilitate rapid vaporization for higher CaO content feedstock by increasing the operation temperature.

Fig.8.The necessary operation temperature with different initial mass ratios of CaO/(CaO+MgO)in the feedstock while 86.6%melts vaporized in 2 min.

Fig.8 displays the necessary operation temperature predicted by the model for different initial ratios of CaO/(CaO+MgO)while86.6%melts vaporized within 2 min.The temperature increases with increasing mass fraction of CaO,and the evolution law bet ween operation temperature and composition of feedstock can be expressed by the following equation whereTrepresents the necessary operation temperature in K,andyrepresents the ratio of CaO/(CaO+MgO)in the feed stock.According to the equation,when the CaO/(CaO+MgO)in the feedstock increases to 0.3 and 0.4,the corresponding operation temperature should be raised to 1341 K and 1365 K,which are 68 K and 92 K greater than that of the feedstock with CaO/(CaO+MgO)=0.2,respectively.The above an alysesrevealed that en hancedva porizationr ates to keep low level of MgCl2and CaCl2during fluidized bed chlorination process may be achievable through raising the operation temperature for titanium feedstock with higher CaO contents.Although increasing temperature has been suggested to enhance vaporization rate for high alkaline earth oxide feedstock[15,16],no scheme has ever been established to correlate the operation temperature with the change of initial oxide composition.The present study could provide a guideline to determine the operation temperature,as correlated by Eq.(24).Detailed vaporization process analysis is a fundamental step toward understanding the vaporization mechanism,it would be crucial for establishing the best possible route for the vaporization of high CaO content resources to ameliorate the agglomeration of particles during chlorination operation.

Titanium resources with high contents of calcium and magnesium are abundant in the world,like in the area of Quebec Canada and Panzhihua China.To be applicable to the chlorination process,this type of titanium resource needs to be upgraded to high grade synthetic rutile(SR)with the CaO+MgO less than 1.0 wt%and CaO less than 0.2 wt%,by the oxidation–reduction–hydrochloric acid pressure leaching process developed by Quebec Iron and Titanium Ltd.(QIT).It was however found that the process,was quite effective to remove MgO while less effective for CaO removal[30].For the Panzhihua ilmenite with higher CaO content than the Quebec ilmenite,it has been demonstrated that only CaO contents of 0.2 wt%–0.4 wt%are achievable by the oxidation–reduction–leaching process[31,32].It is for this reason that billions of tons of the Panzhihua titanium resource cannot be used by the fluidized chlorination process up to the present.The present study provides the perspective on how to relax the restriction of CaO and to utilize titanium resources with high contents of calcium.

4.Conclusions

A kinetic model study was developed to provide guidelines for ameliorating the agglomeration of titanium particles with CaO content higher than 0.2 wt%.The focus of the present study was to construct a vaporization kinetics model of MgCl2–CaCl2binary melts to provide optimization design between operation temperature and initialcomposition to promote vaporization rate.

The vaporization rate constant of MgCl2is more than seven times than that of CaCl2at 1273 K,the vaporization apparent activation energies of MgCl2and CaCl2are 100.6 kJ·mol-1and 109.7 kJ·mol-1,respectively.The overall vaporization ratio of the MgCl2–CaCl2binary melts declined distinctly with in creasing content of CaCl2.The vaporization model devotes to simulate the evolution of overall vaporization rate with the change of melts composition,and establish the relationship of vaporization rate between the pure chlorides and their binary melt.

Optimal operation condition of chlorides vaporization is determined by correlating the ratio of CaO/(CaO + MgO) in the feedstock with necessary operation temperature. The necessary operation temperatureincreases with the mass fraction of CaCl2in the binary melts.Predictions by the model revealed that CaO content of 0.2 wt%–0.4 wt%could reach the equivalent vaporization rate with CaO content0.2wt%resources at a temperature lower than 1365 K.The constructed correlation provides the perspective on how to relax the restriction of CaO and to utilize titanium resources with high contents of CaO.

Nomenclature

Eactivity energy,kJ·mol-1

kAvaporization rate constant of component A,g·cm-2

vaporization rate constant of pure liquid A,g·cm-2

MAmolecular weight of component A,g·mol-1

MBmolecular weight of component B,g·mol-1

mmass fraction of the chlorides in the residues by analyses

m0initial mass fraction of the chlorides adhered on the titanium slag particles

NAapparent vaporization rate constant of pure liquid A,min-1

NBapparent vaporization rate constant of pure liquid B,min-1

nAmole content of component A(MgCl2),mol

nBmole content of component B(CaCl2),mol

PApartial pressure of component A,Pa

PA?vapor pressure of the pure liquid A,Pa

rradius of the droplet,cm

r0initial radius of the melt,cm

SAsurface area of the droplet,cm2

Ttemperature,K

ttime,min

WAweight of component A,g

Xratio of the vaporized chlorides

xAmole fraction of component A in the mixture

xBmole fraction of component B in the mixture

yratio of CaO/(CaO+MgO)in the feedstock

γx,Aactivity coefficient of component A

γx,Bactivity coefficient of component B

νAvolume fraction of component A

ρAdensity of pure liquid A,g·cm-3

ρBdensity of pure liquid B,g·cm-3

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