稀有金属(英文版) 2017,36(08),676-684

Extraction equilibria and kinetics of Ti(Ⅳ) from leached chloride liquors of ilmenite

Abd ElAziz A.Nayl Hisham F.Aly

Chemistry Department,College of Science, Al Jouf University

Hot Labs Centre, Atomic Energy Authority

收稿日期：11 May 2015

基金：financially supported by the Egyptian Atomic Energy Authority (No.141/2014);

Extraction equilibria and kinetics of Ti(Ⅳ) from leached chloride liquors of ilmenite

Abd ElAziz A.Nayl Hisham F.Aly

Chemistry Department,College of Science, Al Jouf University

Hot Labs Centre, Atomic Energy Authority

Abstract：

Extraction of titanium(Ⅳ)from real chloride leach liquor of ilmenite was carried out with 0.1 mol L^-1di-(2-ethylhexyl)phosphoric acid(HDEHP)in kerosene.Equilibrium and kinetics studies for Ti(Ⅳ)were carried out in the presence of impurities that were leached with Ti(Ⅳ).Parameters affecting extraction rate of Ti(Ⅳ)from chloride media in the presence of Fe(Ⅲ),Mg(Ⅱ),and Al(Ⅲ)were studied to evaluate the stoichiometry of extracted Ti(Ⅳ)species.It is found that the extraction rate of Ti(Ⅳ)is dependent on the extractant concentration and pH of solution.Under the optimum conditions,more than 95%Ti(Ⅳ)can be extracted.On the basis of slope analysis method,the extracted species of Ti(Ⅳ)appears to be[TiO(H₂A₂)₂]_org,where H₂A₂ refers to HDEHP.Further,the kinetic studies of the extraction process of Ti(Ⅳ)and other metal ion impurities were carried out by a Lewis cell with a constant interfacial area of 16.7 cm².Analysis of the experimental results suggests that TiO²⁺extraction rate by HDEHP is the first with respect to hydrogen ion concentration and HDEHP concentration.The results are interpreted by a reaction mechanism where the extraction process is controlled by a diffusion process at the interface rather than in the bulk phase.

Keyword：

Kinetics; Equilibrium; Titanium(Ⅳ); Ilmenite; Hydrochloric acid; Separation;

Author： Abd ElAziz A.Nayl,e-mail:aanayl@yahoo.com;

Received： 11 May 2015

1 Introduction

In the last decade,titanium metal and titanium isotopes are used in a wide range of industrial and research applications [ 1] .Titanium is regarded as an ideal target material for proton beam monitoring [ 2] .It has five stable isotopes and four radioactive isotopes made artificially.Ti-47 and Ti-48are used for the production of radioisotope V-48.Ti-49 is used for the production of radioisotope V-49,and Ti-50 is used for the production of super heavy elements.Titanium dioxide is one of the most important white pigments,and it is used in photo-and bio-activity catalysts with comprehensive application prospects [ 3] .Generally,titanium and titanium salts can be produced from its minerals by sulfate or chloride processes.Chloride process has some advantages over the sulfate process in cost and waste management [ 4] .However,this process generates large amounts of acidic waste chloride liquors containing different metals ions such as Mg(Ⅱ),Al(Ⅲ),Ti(Ⅳ),V(V),Cr(Ⅳ),Mn(Ⅱ),Fe(Ⅱ),and Fe(Ⅲ) [ 5] .Therefore,selective extraction of titanium from chloride leaching liquors represents a major extraction and environmental issue [ 5, 6] .

The speciation of titanium in HC1 solutions consists of different forms [ 4] .Recently,different processes to extract titanium from chloride media have been studied and developed by solvent extraction technique [ 4] .The recovery and the extraction of valuable metals from chloride solutions are widely investigated by this technique [ 3, 6, 7, 8, 9] .

Separation of Ti(Ⅳ) with other metal ions from acidic chloride media was studied using acidic or neutral organophosphorus extractants.Selective extraction of titanium from the other metal ions,such as Fe(Ⅲ),Fe(Ⅱ),Mn(Ⅱ),Al(Ⅲ),Zn(Ⅱ),V(V),and Cr(Ⅱ),was reported using acidic organophosphorus extractants [ 10, 11] .As reported in Refs. [ 7, 12, 13, 14] ,acidic organophosphorous derivative extractants,such as di-(2-ethylhexyl) phosphoric acid (HDEHP),mono-2-ethylhexylester (EHEHPA),and bis(2,4,4-trimethylpentyl) phosphinic acid (HBTMPP),were used for the recovery of Ti(Ⅳ),V(V),and Fe(Ⅲ)from acidic solutions.It was found that the extraction of Ti(Ⅲ) by acidic organophosphorus extractants was governed by cation exchange reactions at low acidities and solvating reactions at higher acidities [ 15] .

The extraction mechanism of Ti(Ⅲ) from chloride solution by acidic organophosphorus extractants,such as HDEHP,was studied,and the results obtained showed that the exchange of TiO²⁺or Ti(OH)2⁺in aqueous phase with H⁺of the extractants occurred when the acidity was relatively low [ 7, 14, 15, 16, 17, 18] .Further,the extraction efficiency of HDEHP for Ti(Ⅲ) was found to be significantly higher than that reported with other acidic extractants [ 19] .

Sole [ 11] ,and Saji and Reddy [ 7] observed that the extraction kinetics was very slow when using EHEHPA,where the equilibrium was reached at about 90 min.The extraction kinetics of titanium from H₂SO₄,HNO₃,and HCl media for this system was studied [ 20] .Biswas and Begum [ 18] studied the kinetics of extraction and stripping of Ti(Ⅳ) from HCl media by HDEHP in kerosene system using the single drop technique.They concluded that the investigated mechanisms included the chemical reaction of species at the interface or the diffusion of species from the interface,which was rate-determining.

Although many works were reported on extraction equilibrium and kinetics of Ti(Ⅳ) from simulated HCl media,there was very little work reported on the extraction equilibrium and kinetics of Ti(Ⅳ) from real chloride leaching liquor of ilmenite using a stirred Lewis cell.Therefore,this work discusses the extraction equilibrium and kinetics of Ti(Ⅳ) in the presence of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) from real chloride leaching liquor of ilmenite by 0.1 mol·L^-1HDEHP in kerosene using a stirred Lewis cell to learn about the parameters affecting the extraction rate.The obtained data were discussed,the mechanisms of extraction process were proposed,and the rate equations were deduced.

2 Experimental

2.1 Chemicals and reagents

HDEHP was obtained from Sigma and BDH Chemicals Companies.Odorless kerosene was obtained from Miser Petroleum Company,Egypt.All chemicals and reagents used were of analytical reagent (AR) grade and used without any purification.

HCl leaching solution resulted from the leaching processes investigated by Nayl and Aly [ 5] was firstly filtrated to remove any suspended matters.The resulted leaching liquor contained 2.10×10^-3 mol·L^-1 V,5.40×10^-3mol·L^-1 Cr,0.63 mol·L^-1 Ti,0.39 mol·L^-1 Fe,2.70×10^-2 mol·L^-1 Mg,1.70×10^-2 mol·L^-1 Al,and 6.40×10^-4 mol·L^-1 Ln,with fixing the ionic concentration of the solution at 0.1 mol·L^-1 with NaClO₄ [ 21] .

2.2 Equilibrium studies

Aqueous solutions were prepared from hydrochloric acid leaching solution of ilmenite with constant amounts of Ti(Ⅳ),Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) in each experiment.HDEHP dissolved in kerosene was used as the extractant(organic phase).

Equal volume (50 ml) of leaching solution and0.1 mol·L^-1 HDEHP were shaken at room temperature for30 min until reaching equilibrium,and then the phases were left to stand for 24 h for complete separation.The effect of Clconcentration on the extraction rate was studied by varying its concentration,and the ionic concentration was fixed at 1.0 mol·L^-1 with NaClO₄.Titanium was completely stripped from HDEHP with warm deionized water.

The distribution ratio (D) of the metal ions was calculated using the following expression:

where V_aq and V_Org refer to the volume of the aqueous(A) and the organic (O) phases,respectively,C_o is the original metal concentration in the aqueous phase before extraction,and C is the metal concentration in the aqueous phase after extraction.

The extraction rate (E) was calculated using the following equation (with O/A volume ratio of 1)

2.3 Kinetics studies

Kinetics of Ti(Ⅳ) extraction was studied with 0.1 mol·L^-1HDEHP in kerosene by means of a Lewis cell.The concentrations of the investigated metal ions were fixed,respectively,at 0.63 mol·L^-1 Ti,0.39 mol·L^-1 Fe,2.70×10^-2 mol·L^-1 Mg,and 1.70×10^-2 mol·L^-1 Al.

The Lewis cell is similar to that constructed by El-Hefny [ 22] .The temperature was fixed at (25.0±0.1)℃by a circulator (Julabo,Germany) connected to a stirred Lewis cell.The double-jacket Lewis cell modified by Daoud [ 23] was used in the kinetic studies.The effect of stirring speed on the extraction processes was studied by equally stirring the two phases without interrupting the interface using a Heidolph stirrer RZR 2020,and a plateau was obtained in the region of 350-500 r·min^-1.The two phases with the same volume were poured carefully into the apparatus,unless otherwise stated.The stirring rate was kept at 400r·min^-1 for the kinetic investigations carried out in this study.The first sample was drawn directly after filling the cells before the stirrers were started on.The concentration of the investigated metal ions in the organic phase was determined by atomic absorption spectroscopy (AAS) after stripping into the aqueous phase.After collecting a sample,l ml of each sample of the fresh organic and aqueous solution was added to the stirred cell.

2.4 Apparatus

The concentrations of Ti(Ⅳ),Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) in leaching chloride medium were determined by AAS(Model S series,Thermo-Electron Corporation).The metal ion concentrations in the organic phase were determined by the difference in their concentrations in the aqueous phase before and after extraction.

2.5 Titanium speciation in HCl

Extraction behavior of Ti(Ⅳ) from HCl solution by different types of extractants is affected by Ti(Ⅳ) speciation,where palent titanyl ion can exist either as TiO²⁺or as [ 24] .The Pourbaix diagram of the titaniumwater system is shown in Fig.1 [ 11] .

In high acidic solution ([H⁺]>4.0 mol·L^-1),as reported by Zhu et al. [ 4] ,the TiO²⁺species are stable and predominant at pH<0.3.This may explain why Ti(Ⅳ) is extracted by cation ion exchange mechanism.

2.6 Fe(Ⅲ) speciation in HC1

Extraction behavior of Fe(Ⅲ) from acidic HCl solution is affected by Fe(Ⅲ) speciation.At low HCl concentrations(below 2.0 mol·L^-1),the positive species (Fe³⁺,FeCl₂⁺,and FeCl²⁺) are the predominant Fe(Ⅲ) species in the aqueous solution (Fig.2, [ 25] ).For HCl concentration range of 2.0-7.0 mol·L^-1,neutral form (FeCl₃) is the predominant species and the anionic form (FeCl₄^-) begins to appear.When HCl concentration reaches>8.0 mol·L^-1,FeCl₄^-is the predominant Fe(Ⅲ) species in the aqueous solution [ 25] and this explains why these species are not extracted in this investigated system.

Fig.1 Potential (E_h)-pH diagram for Ti-H₂O system at 25℃

Fig.2 Species distribution diagram of Fe(Ⅲ) as a function of[HCl]

3 Results and discussion

3.1 Equilibrium investigations

The different parameters affecting the extraction equilibrium of Ti(Ⅲ),Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) from the chloride leaching liquors by 0.1 mol·L^-1 HDEHP in kerosene for 30 min with O/A ratio of 1 were studied at 25℃with an agitation rate of 350 r·min^-1,unless otherwise stated.

The effect of pH on the extraction rate of the investigated metal ions from the chloride leaching liquors of ilmenite by 0.1 mol.L^-1 HDEHP was investigated with a pH range of 0-3.0.The relationship between extraction rate and pH value is plotted,as shown in Fig.3.Extraction rate of Ti(Ⅳ) is found to sharply decrease from 95.2%to1.0%with pH value increasing to 2.0,while the extraction rate of Fe(Ⅲ) increases to 25.3%in this range.Extraction rates of both Mg(Ⅱ) and Al(Ⅲ) are nearly constant under these experimental conditions and do not exceed 30%.These results indicate the possible separation of Ti(Ⅳ)from Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) by 0.1 mol.L^-1 HDEHP in kerosene from leaching liquor at pH around 0.

Fig.3 Effect of pH on extraction rate (E) of investigated metals (ions from chloride leaching liquor by 0.1 mol·L^-1 HDEHP in kerosene; O/A ratio=1,temperature=25℃,agitation rate=350 r·min^-1, time=30 min)

The relation between lgDD and pH for extraction of Ti(Ⅳ),Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) by 0.1 mol·L^-1 HDEHP in kerosene is given in Fig.4.Straight lines with slopes of-1.7 for Ti(Ⅳ) and 0.14 for Fe(Ⅳ) are obtained,while slopes of almost zero,<<0.001,are obtained for both Mg(Ⅱ) and Al(Ⅲ).This indicates that in the case of Ti(Ⅳ),2 mol H⁺is released to the aqueous phase in the complex formation process between Ti(Ⅳ) and HDEHP and proves that in concentrated chloride leaching liquor of ilmenite,the dominating species of titanium is still present as TiO²⁺and the extraction process occurs in cation exchange mechanism [ 20] .On the other hand,in the cases of Fe(Ⅲ),Mg(Ⅱ),and Al(Ⅲ),the small slopes obtained indicate that the dependency of the distribution coefficient on pH is rather negligible.

The effect of Clconcentration (9.0-11.0 mol·L^-1) on the extraction of Ti(Ⅳ) from leaching chloride liquor was investigated by 0.1 mol·L^-1 HDEHP,and the relation between lg[Cl^-]and lgD is plotted,as shown in Fig.5.The data obtained show that as the chloride concentration increases within the investigated range,the extractions of the investigated metal ions remain unchanged.This indicates that Cl^-do not participate in the formation of extracted species of titanium,TiO(HA₂)2 [ 20] .

Fig.4 Plot of lgD vs pH with 0.1 mol·L^-1 HDEHP (O/A ratio=1,temperature=25℃,agitation rate=350 r rnin^-1,time=30 min)

From the above results,the behavior of Ti(Ⅳ) extraction under these conditions can be explained depending on the fact that TiCl₄ in acidic HCl aqueous solution exists as TiO2⁺due to the formation and ionization of TiOCl₂according to the following reactions [ 18]

From Reaction (4),there is an equilibrium between TiO²⁺and T1OCl₂.However,since the rate of extraction and complex formation between TiO²⁺and HDEHP is faster than that of ionization [ 18] ,the extraction of Ti(Ⅳ)from leaching chloride liquor by HDEHP is not affected by chloride ion concentration.

The relation between E of the investigated metal ions and[HDEHP]in Fig.6 shows that as[HDEHP]increases from 0.01 to 0.10 mol·L^-1,the extraction rate of Ti(Ⅳ)increases to 98.2%,while the extraction rates of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) slightly increase to 31.7%,22.4%,and17.9%,respectively.These indicate that the extraction of Ti(Ⅳ) is greatly dependent on[HDEHP].However,as the[HDEHP]increases from 0.1 to 0.2 mol·L^-1,the extraction rate of Ti(Ⅳ) increases slightly from 95.2%to 98.2%.This may be attributed to the fact that nearly quantitative extraction is achieved and the viscosity of HDEHP increases [ 26] .In addition,with high viscosity of HDEHP,mixing between the aqueous medium and the extractant is difficult and the extraction process needs more time to attain equilibrium [ 26] .

The relation between lgD and lg[HDEHP]gives straight lines with slopes of 2.01,0.05,0.07,and 0.158 for Ti(Ⅳ),Mg(Ⅱ),Al(ⅢI),and Fe(Ⅲ),respectively,as shown in Fig.7.This can be attributed to the fact that one molecule of Ti(IV) is extracted by two dimeric molecules of HDEHP.Under these investigated conditions,the extraction rates of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) are slightly affected by the change in[HDEHP].

Fig.5 Plot of lgD vs lg[Cl^-]with 0.1 mol·L^-1 HDEHP (O/A ratio=1,temperature=25℃,agitation rate=350 r·min^-1,pH=0,time=30 min)

Fig.6 Effect of[HDEHP]on extraction rate (E) of investigated metals ions from HCl leaching liquor (O/A ratio=1,tempera-ture=25℃,agitation rate=350 r·min^-1,time=30 min)

Fig.7 Plot of lgD vs lg[HDEHP]in kerosene (O/A ratio=1,temperature=25℃,agitation rate=350 r·min^-1,time=30 min)

Based on the preceding data,at low acid concentration,the extraction equilibrium of Ti() by HDEHP from leaching chloride liquor of ilmenite is governed by the cation exchange of TiO²⁺or Ti(OH) in aqueous phase with H⁺in organic phase[HDEHP]as in Reactions (5) and(6);

where (HA)2 refers to the dimeric form of HDEHP and Ti(Ⅳ) is extracted as Ti(OH) .This was proposed by Ma et al. [ 15] .Another form of the extraction equilibrium is given by the following extraction reaction [ 27] :

According to the extraction mechanisms of cation exchange shown in Reactions (5) and (6),the Ti(Ⅳ)extraction equilibrium constant (K_ex) with[HDEHP]could be expressed as Eq.(7):

The experimental results for the various organic solvents employed were plotted by taking the logarithm of Eq.(7)and rearranging,where pH=-lg[H⁺]

Slope analysis from Figs.3-7 supports these mechanisms.From the data obtained in Figs.3-7,it can be concluded that the extraction rate of TiO²⁺from real acidic chloride leaching liquor of ilmenite by 0.1 mol·L^-1 HDEHP under the investigated conditions is dependent on[HDEHP]and the pH value of leaching liquor.Under the investigated conditions in this work,Ti(Ⅳ) can be removed from the other metal ions of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ).

3.2 Kinetics of extraction

The extraction kinetics of Ti(Ⅳ) in the presence of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) from acidic chloride media with0.1 mol·L^-1 HDEHP was investigated using Lewis cell.

3.2.1 Data treatment by‘F’method

The extraction rate of metal ions depends on the interfacial area and the concentration of the reactants involved.The average mass flux (F) through the liquid-liquid interface can be measured by the following equation [ 28, 29] :

where F is mass flux (kmol·m^-2·s^-1),m_tv is the mass (kmol)of titanium transferred into the organic phase HDEHP,A is interfacial area (m²),and t is phase contact time (s).

The value of F is proportional to[Ti0^2-]^a,[HDEHP]^b,and[H⁺]^c as shown below:

where kf is forward extraction rate constant and a,b,and c are reaction orders with respect to TiO^2-,HDEHP,and H⁺,respectively.

By taking logarithms of Eq.(10):

Equation (11) indicates that by keeping the concentration of two variables at constant values,the order with respect to the third variable concentration can be obtained from the slope of the lg-lg plot of F to the concentration of the third variable.The value of kf can be calculated from the intercept of the plots [ 28] .

In this system,the effect of the temperature on the extraction rate can be treated by Arrhenius equation from which the value of apparent activation energy (E_a) can be calculated:

where E_a is activation energy (kJ·mol^-1),A is pre-exponential factor,R is molar gas constant (8.314×10^-3kJ·mol^-1·K^-1),and T is absolute temperature (K).

By substituting Eq.(12) into Eq.(11),the following equation can be obtained [ 28] :

The plot of lgF versus l/T should yield a straight line whose slope multiplied by-2.303R is the activation energy (E_a).

Further,in the‘F'method,by using Arrhenius and activated complex theories,the values of changes in enthalpy (ΔH^±) and entropy (ΔS^±) can be calculated as described before [ 29, 30] .

3.2.2 Effect of stirring speed on extraction rate

The effect of stirring speed on the extraction rate of the investigated metal ions was studied using a stirred Lewis cell in stirring speed range of 100-500 r·min^-1 at different time periods.The data obtained in Fig.8 show that the extraction rate increases by increasing the stirring speed till350 r·min^-l,and then a plateau region occurs after higher than 350 r·min^-l.However,the stirring speed is controlled at 400 r·min^-1 in order not to disturb the interface.Therefore,stirring speed is fixed at 400 r·min^-1 in order to control the extraction process by chemical reactions rather than by diffusion.In this process,the extraction rate is governed by the stirring speed.As reported before,when the extraction process is governed by diffusion reaction,the extraction rate increases by increasing the stirring speed till it reaches a plateau where the extraction rate remains constant with the increase in the stirring speed (kinetic regime) [ 30] .In this regime,the diffusion control is minimized and the extraction rate is governed by chemical reaction in the bulk phase or at the interface [ 30] .

3.2.3 Effect of interfacial area on extraction rate

As reported in Refs. [ 29, 30] ,the effect of interfacial area is one of the most important parameters to differentiate chemical reactions that occur in the bulk phase or at the interface.When the chemical reactions occur in the bulk phase,the extraction rate is independent on the interfacial area.In contrast,when the chemical reaction occurs at the interface,the extraction rate increases with the interfacial area increasing.Therefore,in this work,the effect of the interfacial area was studied using cells with different interfacial areas ranging from 3.6 to 16.7 cm² with keeping the volume of both phases constant (50.0 ml of each).

Fig.8 Effect of stirring speed on extraction rate (E) of Ti(IV)(temperature=25℃,[HDEHP]=0.1 mol·L^-1 in kerosene,pH=0)

As shown in Fig.9,straight line passing through the origin is obtained in the plot of the extraction rate against the interfacial area.It is observed that the extraction rate increases linearly with the increase in interfacial area (A),where E of titanium increases from 19.7%at 3.6 cm² to97.8%at 16.7 cm².It is indicated that the rate-controlling reaction takes place at the interface rather than in the bulk phase and the extraction rate is dependent on the interfacial area (Fig.9).Therefore,further experiments were carried out at 16.7 cm².

3.2.4 Effect of HDEHP and pH value on extraction rate

The extraction rate of TiO²⁺from chloride leaching liquor of ilmenite at various HDEHP concentrations(0.02-0.10 mol·L^-1) and at pH (0-0.8) was separately investigated and studied.Under these conditions,high extraction rates of Ti(IV) and low extraction rats of other investigated metal ions are obtained.The results obtained are explained and represented in Figs.10 and 11,respectively.

Figure 10 represents the dependence of the extraction rate of TiO²⁺on[HDEHP]with a plot of lgF against lg[HDEHP]at two different sets of experimental parameters.The slopes of the straight lines are 0.90 and 0.92 with intercepts of—2.66 and-2.68 at pH of 0 and 0.2,respectively.Under all experimental conditions,the slopes obtained are almost equals to 1.0,which indicates that the extraction rate of TiO²⁺pulp to organic phase is directly proportional to[HDEHP]and indicates that the order with respect to[HDEHP]almost equals to 1.0.

Fig.9 Effect of interfacial area (A) on extraction rate (E) of Ti(Ⅳ)in Lewis cell (temperature=25℃,time=30 min,[HDEHP]=0.1 mol·L^-1 in kerosene,pH=0,O/A ratio=1)

Fig.10 Effect of[HDEHP]on mass transfer flux of TiO²⁺(stirring speed=400 r·min^-1,time=30 min,temperature=25℃,A=16.7 cm²)

The dependence of extraction rate of TiO²⁺on pH value is shown in Fig.11 at two different sets of experimental conditions,where lgF plots against pH.The obtained lines with slopes of-1.02 and-0.89 and intercepts of-3.48and-3.70 are obtained at HDEHP concentrations of 0.10and 0.06 mol·L^-1,respectively.Under these experimental conditions,the slopes of the lines indicate that the rate of TiO²⁺extraction from leaching liquor phase by HDEHP phase is inversely proportional to bulk pH and indicates that the order with respect to hydrogen ion concentration is almost 1.0.

Fig.11 Effect of pH value on mass transfer flux of TiO²⁺(stirring speed=400 r·min^-1,time=30 min,temperature=25℃,A=16.7 cm²)

Moreover,under these conditions,it is noted that chloride ion concentration has almost no effect on the extraction rate of Ti(Ⅳ).

From the values of intercepts obtained in Figs.10 and11,the k_t value is 1×10^-2.28 with a standard deviation of0.02.This means that the diffusions of both HDEHP and H⁺from the interface are rate-determining [ 17] .Depending on results obtained in this study,the rate of mass flux of TiO²⁺from leaching liquor to HDEHP at a constant temperature can be expressed as follows:

3.2.5 Effect off temperature on extraction rate

The effect of temperature on the extraction rate of TiO²⁺from chloride leaching liquor of ilmenite by 0.1 mol·L^-1HDEHP in kerosene at pH=0 for 30.0 min was investigated in the temperature range of 293-323 K,and the data obtained are presented in Fig.12 as a relation between lgF and 1000/T,which yields a straight line with slope of-0.296 and intercept of-2.58.The activation energy (E_a)can be estimated using Arrhenius equation in Eq.(12).

The value of E_a was calculated from the slope and found to be 5.67 kJ·mol^-1.This means that the extraction rate of Ti(Ⅳ) under these investigated conditions is controlled by diffusion process.The low value of E_a(<20.9 kJ·mol^-1)indicates that the extraction of Ti(Ⅳ) in this work is controlled by diffusion process,and the rate constant of the reaction is independent on temperature [ 28] .

According to transition state theory [ 29] ,the following relation can be used:

Fig.12 Effect of temperature on mass flux of TiO²⁺(stirring speed=400 r·min^-1,time=30 min,pH=0,A=16.7 cm²)

Fig.13 McCabe-Thiele plot for Ti(Ⅳ) extraction

where h is Planck constant (6.6252×10^-37 kJ·s),k is Boltzmann constant (1.38×10^-26 kJ.K^-1),ΔH is the enthalpy of activation (kJ·mol^-1),and AS is the entropy of activation (kJ·mol^-1.K^-1).

The relation between lg[Fh/(kT)]and 1000/T is plotted,as shown in Fig.12,with slope of-0.42 and intercept of-10.01.The values ofΔH and AS were calculated from the slope and intercept of the straight line obtained,and they are found to be about 8.0 kJ·mol^-1 and about-191.9 J·mol^-1·K^-1,respectively.Also,the value of change in the Gibbs free energy (AG) is calculated to be65.2 kJ·mol^-1.

From the calculated thermodynamic parameters,it is clear that the extraction of Ti(IV) from chloride leaching liquor of ilmenite with 0.1 mol·L^-1 HDEHP is endothermic and the structure of the extracted Ti(Ⅳ) species is more or less ordered.

From the slopes of these lines obtained in Figs.8-12,it can be concluded that the extraction of TiO²⁺from acidic chloride leaching liquor of ilmenite by 0.1 mol·L^-1HDEHP under the investigated conditions is first-order dependent on[HDEHP](slope=1),inverse first-order dependent on pH (slope=-1),and nearly zero order with respect to the other investigated parameters such as chloride ion concentrations.

Numbers 1 and 2 in Fig.13 are the numbers of the theoretical counter-current stages required for the quantitative extraction of Ti(Ⅳ) from real chloride leaching liquor containing 0.627 mol·L^-1 titanium by 0.1 mol·L^-1HDEHP in kerosene at O/A ratio=1.0,they can be predicted using McCabeThiele diagram,as shown in Fig.13.It shows that only two extraction stages are quite sufficient stages for extraction of Ti(Ⅳ) from this real leaching solution under the investigated parameters.

Table 1 shows the conditions and data obtained in some recent works on the extraction of Ti(Ⅳ) from chloride solution by HDEHP reported before [ 17, 21] and the results obtained in this work.The main difference between this work and the others is the used technique and that the extraction process was carried out from real leaching liquor in this work.As reported above,the extraction equilibrium and kinetics of Ti(Ⅳ) from real chloride leaching liquor of ilmenite is more or less similar with that reported in the literature.

4 Conclusion

The extraction rate of TiO²⁺from acidic chloride leaching liquor by 0.1 mol·L^-1 HDEHP is found to increase by increasing concentration of HDEHP,inversely dependent on the pH value,and not affected by chloride ion concentration.Under the optimum conditions,0.1 mol.L^-1HDEHP in kerosene with O/A ratio of 1 for 30 min at25℃with agitation rate of 350 r·min^-1 and pH value of 0,the extraction rate of titanium reaches more than 95%while the extraction rates of Mg(Ⅱ),Al(Ⅲ),and Fe(Ⅲ) do not exceed 30.5%.The kinetic results indicate that the extraction rate of TiO⁺² by 0.1 mol·L^-1 HDEHP in kerosene is controlled by the diffusion reaction at the interface rather than in the bulk phase.The extraction rate constant(k_f) is found to be 1×10^-2.29 with a standard deviation of0.02.Based on the results obtained in the present investigation,the rate of mass flux of TiO²⁺from leaching liquor to HDEHP at a constant temperature can be expressed by the following relation of F=l×10^-2.29[TiO^2-][HDEHP][H⁺].

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Table 1 Comparison of conditions and data from references and present work

参考文献

[1] Takacs S,Sonck M,Scholten B,Hermanne A,Tarkanyi F.Excitation functions of deuteron induced nuclear reactions on~(nat)Ti up to 20 MeV for monitoring deuteron beams.Appl Radiat Isot.1997;48(5):657.

[2] Szelecsenyi F,Tarkanyi F,Takacs S,Hermanne A,Sonck M,Shubin Yu,Mustafa MG,Zhuang Y.Excitation function for the~(nat)Ti(p,x)48V nuclear process:evaluation and new measurements for practical applications.Nucl Instrum Methods Phys Res B.2001;174(1-2):47.

[3] Wang X,Liang B,Lii L,Wu P,Li C,Ma J.Simultaneous oxidation and extraction of iron from simulated ilmenite hydrochloric acid leachate.Hydrometallurgy.2012;129-130:105.

[4] Zhu Z,Zhang W,Cheng CY.A literature review of titanium solvent extraction in chloride media.Hydrometallurgy.2011;105(3-4):304.

[5] Nayl AA,Aly HF.Acid leaching of ilmenite decomposed by KOH.Hydrometallurgy.2009;97(1-2):86.

[6] Remya PN,Reddy ML.Solvent extraction separation of titanium(Ⅳ),vanadium(Ⅴ)and iron(Ⅲ)from simulated waste chloride liquors of titanium minerals processing industry by the trialkylphosphine oxide Cyanex 923.J Chem Technol Biotechnol.2004;79(7):734.

[7] Saji J,Reddy MLP.Selective extraction and separation of titanium(Ⅳ)from multivalent metal chloride solutions using2-ethylhexylphosphinic acid mono-2-ethylhexyl ester.Sep Sci Technol.2003;38(2):427.

[8] Toyabe KT,Kirishima KK,Shibayama HT,Hanawa HK.Process for recovering valuable metal from waste catalyst.US Patent;5431892.1995.

[9] Gupta CK.Extractive metallurgy of niobium,tantalum and vanadium.Int Met Rev.1984;29(1):405.

[10] Reddy MLP,Saji J.Solvent extraction of tetravalent titanium with organophosphorus extractants.Min Process Extr Metall Rev.2002;23(3-4):199.

[11] Sole KC.Recovery of titanium from the leach liquors of titaniferrous magnetites by solvent extraction.Part 1.Review of the literature and aqueous thermodynamics.Hydrometallurgy.1999;51(2):239.

[12] Islam MF,Biswas RK.The solvent extraction of Ti(Ⅳ),Fe(Ⅲ)and Mn(Ⅱ)from acidic sulfate—acetate medium with bis-(2-ethylhexyl)-phosphonic acid in benzene.J Inorg Nucl Chem.1981;43(8):1929.

[13] Islam F,Rahman H,Ali M.Solvent extraction separation study of Ti(Ⅳ),Fe(Ⅲ)and Fe(Ⅱ)from aqueous solutions with di-(2-ethylhexyl)-phosphoric acid in benzene.J Inorg Nucl Chem.1971;41(2):217.

[14] Saji J,John KS,Reddy MLP.Liquid-liquid extraction of tetravalent titanium from acidic chloride solutions by bis-(2,4,4-trimethylpentyl)-phosphinic acid.Solvent Extr Ion Exch.2000;18(5):877.

[15] Ma C,Guo X,Bai P.Solvent extraction of titanium(Ⅳ)with organophosphorus extractant from chloride solutions.Asian J Chem.2014;26(8):2277.

[16] Thomas J.Investigations on the recovery of titanium,vanadium and iron values from the waste chloride liquors of titania industry.Kerala:Cochin University;2004.9.

[17] Biswas RK,Begum DA.Kinetics of extraction and stripping of Ti(Ⅳ)in HCl-D2EHPA-kerosene system using the single drop technique.Hydrometallurgy.2000;55(1):57.

[18] Biswas RK,Begum DA.Solvent extraction of tetravalent titanium from chloride solution by di-2-ethylhexyl phosphoric acid in kerosene.Hydrometallurgy.1998;49(3):263.

[19] Saji J.Studies on liquid-liquid extraction separation of valuable metals from titania wastes.Kerala:Cochin University;2002.55.

[20] Fontana D,Kulkarni P,Pietrelli L.Extraction of titanium(Ⅳ)from acidic media by 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester.Hydrometallurgy.2005;77(3-4):219.

[21] Aw wad NS,Ibrahium HA.Kinetic extraction of titanium(Ⅳ)from chloride solution containing Fe(Ⅲ),Cr(Ⅲ)and V(Ⅴ)using the single drop technique.J Env Chem Eng.2013;1(1-2):65.

[22] El-Hefny NE.Kinetics and mechanism of extraction and stripping of neodymium using a Lewis cell.Chem Eng Process.2007;46(7):623.

[23] Daoud JA.Overview on the liquid-liquid extraction mechanisms of uranium from aqueous solutions.In:Proceedings of the Eighth Conference of Nuclear Sciences&Applications,Cairo,Egypt;2004.1.

[24] Gotton FA,Wilkinson G.Advanced Inorganic Chemistry—A Comprehensive Text.4th ed.New York:Wiley;1980.692.

[25] Navarro R,Gallardo V,Saucedo I,Guibal E.Extraction of Fe(Ⅲ)from hydrochloric acid solutions using Amberlite XAD-7resin impregnated with trioctylphosphine oxide(Cyanex 921).Hydrometallurgy.2009;98(3-4):257.

[26] Hu G,Chen D,Wang L,Liu JC,Zhao H,Liu Y,Qi T,Zhang C,Yu P.Extraction of vanadium from chloride solution with high concentration of iron by solvent extraction using D2EHPA.Sep Purif Technol.2014;125:59.

[27] Sato T.Liquid-liquid extraction of tetravalent titanium(IV)from hydrochloric acid solutions by di-(2-ethlyhexyl)-phosphoric acid.Sigen-to-Sozai.2003;119:175.

[28] Javanshir S,Abdollahy M,Abolghasemi H,Khodadadi Darban A.Kinetics of Au(Ⅲ)extraction by DBC from hydrochloric solution using Lewis cell.Int J Min Process.2011;98(1-2):42.

[29] Yang X,Wang X,Wei C,Zheng S,Sun Q,Wang D.Extraction kinetics of tantalum by MIBK from pulp using Lewis cell.Hydrometallurgy.2013;131-132:34.

[30] Yang X,Zhang J,Fang X.Extraction kinetics of niobium by tertiary amine N235 using Lewis cell.Hydrometallurgy.2015;151:56.