Novel Process for Titanium Dioxide Production from Titanium Slag

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Novel Process for Titanium Dioxide Production from Titanium Slag: NaOH-KOH Binary Molten Salt Roasting and Water Leaching Dong Wang,†,‡,§ Jinglong Chu,‡,§ Yahui Liu,‡,§ Jie Li,‡,§ Tianyan Xue,‡,§ Weijing Wang,†,‡,§ and Tao Qi*,‡,§ †

University of Chinese Academy of Science, Beijing 100039, PR China National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, No. 1 Bei Er Jie, Zhong Guan Cun, Beijing 100190, PR China § Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ‡

ABSTRACT: Titanium dioxide (TiO2) has been widely used as pigment in paints, fillers for plastic and paper, coatings, adsorbents, cosmetics, catalysts, and gas sensors. A novel process for TiO2 production has been developed by the Institute of Process Engineering. This work develops the novel process by using the NaOH/KOH binary molten salt (50 mol % NaOH, 50 mol % KOH) instead of NaOH molten salt. Under conditions of temperature 350 °C and mixed alkali-to-titanium slag ratio 1.4:1, the titanium conversion ratio obtained is >98% with reaction time around 90 min. The kinetics investigation indicates that the decomposition of titanium slag is controlled by mass diffusion in thed residual layer and the apparent activation energy is 43.1 kJ/mol. 98% of KOH and 86% of NaOH can be recycled by a water leaching process. On the basis of the experimental results, a flow sheet was developed and tested, and the content of TiO2 obtained in the product reached 99.5%. As the NaOH/KOH binary molten salt system provides a liquid as relatively low temperature, the energy consumption of this technology may probably be lower than that of the existing one.

1. INTRODUCTION Titanium slag (Ti-slag), which is prepared from ilmenite by carbothermal reduction, has been considered as a suitable feedstock for either the metal-producing or pigment industries. Of the various based products, titanium dioxide (TiO2) is most widely used. It is being used as pigment in paints, coatings, fillers for plastic and paper, adsorbents, cosmetics, catalysts, and gas sensors due to its high chemical stability and heat stability, and exceptional optical, mechanical, and electronic properties.1,2 There are two processes to manufacture titanium pigment: the sulfate process and the chloride process. In the sulfate process, ilmenite (40−60% TiO2) and Ti-slag (72−87% TiO2) are used as raw materials. In the chloride process, rutile (natural or synthetic) and high Ti-slag (85−90% TiO2) are used as raw materials.3 The chloride process has some advantages over the traditional sulfate process in cost and waste management, but unlike the sulfate process, in which low-grade titanium raw material is acceptable, the chloride process needs a high grade of rutile.1 As a result, a lot of work has been done to upgrade Ti-slag to high Ti-slag or synthetic rutile.4,5 However, the upgrading processes are usually expensive due to the involvement of multiple steps of energy sensitive thermo-reductive conversions and leaching to remove iron impurities. Recently, a novel process for TiO2 production has been developed by using Ti-slag as raw materials according to the principles of green chemistry.6,7 TiO2 rutile white pigment with good pigmentary properties was prepared via the new method.8 A demonstration plant has been built in Shandong Province, which has approached zero emission of pollution. In this process, the high Ti-slag (content of TiO2 is 91.8 wt %) or Ti-slag (content of TiO2 is 79.3 wt %) was decomposed in a molten NaOH system under atmosphere pressure. The NaOH © 2013 American Chemical Society

roasted product obtained can be converted into pigment grade titanium dioxide after further treatment. In comparison with the conventional process, this process achieves low emission of wastes and high decomposition of titanium slag can be fulfilled.3 Besides sodium hydroxide, several other alkaline solutions were used to decompose ilmenite or Ti-slag. For example, potassium hydroxide solutions,9,10 ammonium hydroxide,11 and lithium hydroxide.12 Compared with individual NaOH and KOH melts at 318 and 406 °C, respectively, the eutectic NaOH-KOH (51.5 mol % NaOH, 48.5 mol % KOH) mixture provides a liquid as relatively low temperature with a melting point at only 165 °C.13 Due to the low melting point, eutectic NaOH-KOH has been studied in fuel cell technology,14 corrosion engineering,15 synthesis of specific compounds,13,16−18 and some other important fields. However, although eutectic NaOH-KOH has great advantages over NaOH in saving energy, the use of eutectic NaOH-KOH to decompose Ti-slag is scare. In this article we first proposed an additional new process named NaOH-KOH binary molten salt method to produce titanium dioxide pigment from Ti-slag to lower the reaction temperature and reduce the energy consumption. Experiments with different alkali/slag mass ratios, temperatures, and particle sizes were conducted to elucidate the chemical parameters governing the decomposition process. Kinetics of decomposition of Ti-slag in NaOH-KOH, the phase indentification, and transformation of the products were also investigated. Received: Revised: Accepted: Published: 15756

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2. EXPERIMENTAL SECTION 2.1. Materials. The solid NaOH and KOH are both analytical grade, and deionized water was used throughout. Water for analysis was super-purified by a water superpurification machine (Milli-Q, Millipore). The Ti-slag used in the present work was obtained from Shandong Dongjia Group (Shandong, PR of China). The detailed chemical composition of the Ti-slag was examined by ICP-OES and the analytic results are listed in Table 1. The X-ray diffraction (XRD) analysis of the Ti-slag (Figure 1) indicates that the main Table 1. Chemical Compositions of the Titanium Slag (wt %) titanium slag

TiO2

∑Fe

MgO

MnO

Al2O3

CaO

SiO2

78.37

7.45

2.14

0.99

2.00

1.01

4.89

Figure 2. XRD patterns of products at different reaction temperatures.

The extent of titanium conversion was calculated by dissolving the alkali roasted product in 4.7 vol % HCl, and the dissolution took place according to the following reaction:7 NaKTiO3 + 4HCl = TiO2 + + Na + + K+ + 4Cl− + 2H 2O

After the complete dissolution of the sample (the unreacted Ti-slag would not be dissolute), the concentration of titanium in the filtrate was analyzed by ICP-OES, and the extent of titanium conversion ratio was calculated. Leaching was conducted in glass round-bottom flasks in a temperature controlled water bath with precise temperature (±0.1 °C) controls. A schematic graph of the water washing apparatus is shown in Figure 3. The calculated amount of water was added to the flasks and heated to the selected temperature. Once the desired temperature was reached, the molten salt roasting sample was added and the leaching began. The slurry was filtered once the desired time was reached. Sodium and potassium of the washed residue was analyzed via ICP-OES. The leaching of sodium and potassium (%) were calculated using the following formula.

Figure 1. XRD patterns of the titanium slag.

compositions are pseudobrookite (Me3O5) and rutile (TiO2). The Me3O5 can be viewd as a solide solution of the end members FeTi2O5, Ti3O5, MnTi2O5, MgTi2O5, Cr2TiO5, Al2TiO5, and V2TiO5.19 2.2. Experimental Procedure. The Ti-slag was dried, ground, and dry-sieved to different narrow size fractions. Mixed alkali was prepared by milling NaOH and KOH with a mole ratio of 1:1 and mixed thoroughly. Experiments were performed in a muffle furnace using nickel crucibles with the volume of 30 mL separately. Mixed alkali and Ti-slag were homogeneously mixed in the nickel crucibles and put into the muffle furnace. The temperature of the muffle furnace was controlled by a programmable temperature controller. All experimental data were the average of two parallel experiments. The results were repeated until the error was less than 1%. In Figure 2, the XRD pattern of the hydrothermal decomposition product shows potassium sodium titanium oxide (NaKTiO3) as the main product. The reaction of Ti-slag with the mixed alkali system yields potassium sodium titanium oxide (NaKTiO3) and could be described as follows:

XL =

(3) +

+

where XL is the leaching ratio of Na or K , M1 is the total mass of Na+ or K+ in the leaching water, and M2+ is the total mass of Na+ or K+ in solid before leaching. 2.3. Characterization. The chemical composition of samples was examined by ICP-AES (Optima 5300DV, PerkinElmer, USA). The structure of the potassium sodium titanium oxide and Ti-slag were characterized by XRD (performed using Cu Kα radiation (λ=1.5408), X’Pert PRO MPD, PANalytical, The Netherlands).

3. RESULTS AND DISCUSSION 3.1. Decomposition of Titanium Slag. 3.1.1. Effect of Particle Size. The dependence of particle size on titanium conversion ratio was carried out in the NaOH-KOH system at 350 °C with alkali-to-slag mass ratio of 1.4:1 by using the three particle size fractions of Ti-slag: 106−150, 75−106, and 63−75 μm.

= 2y NaKTiO3 (s) + 2x MeO(s) + (6 + 2x)H 2O(g) Me = Mg, Fe, Mn, Al)

M1+ × 100% M 2+ +

MexTi yO5(s) + 6NaOH(l) + 6KOH(l) + O2 (g)

(x + y = 3,

(2)

(1) 15757

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necessary to maintain the liquidity of the reactants and ensure sufficient reactions while the extra sodium hydroxide could not significantly affect the economy of the whole process because of the recycling procedure in the overall process.20 Mixed alkali-to-Ti-slag mass ratios of 1.2:1, 1.3:1, 1.4:1 were adopted to investigate the influence of alkali-to-slag mass ratio on titanium conversion at 350 °C by using 75−106 μm size fraction of Ti-slag. The results in Figure 5 show that the titanium

Figure 3. Schematic diagram of experimental installation for water leaching.

The results presented in Figure 4 show that titanium conversion ratio increases with the decrease of the particle

Figure 5. Effect of alkali-to-slag mass ratio on titanium conversion.

conversion ratio is improved with the increase of mixed alkali-toslag mass ratio. The ratio of 1.4:1 is recommended, at which the titanium conversion ratio of 98.1% could be achieved. 3.1.3. Effect of Temperature. The effect of temperature on the titanium conversion was carried out in the temperature range of 250−400 °C by using 75−106 μm size fraction of Tislag with alkali-to-slag mass ratio of 1.4:1. The results in Figure 6

Figure 4. Effect of particle size on titanium conversion.

size, which is attributed to the fact that the decrease of particle size increases specific surface area of the slag, as well as its reactivity. On the other hand, when using particle size fraction of 63−75, there is a very slight increase of conversion ratio over particle size fraction of 75−106. Thus, subsequent experiments were performed by using 75−106 μm size fraction of Ti-slag. 3.1.2. Effect of NaOH-to-Slag Mass Ratio. The amount of mixed alkali used during reaction is an important factor, influenceing mass transportation and its recycling amount in the whole process. According to reaction 1 and the chemical analysis results of Ti-slag, the theoretical mass ratio of Ti-slag and mixed alkali for complete reaction is about 1:1.2 (assumed MexTiyO5 is Ti3O5). Mixed alkali acts as a fluidizing and fluxing agent in the reaction mixture. Excess of sodium hydroxide was

Figure 6. Effect of temperature on titanium conversion.

indicate that the temperature has significant influence on the titanium conversion. The titanium conversion ratio increases with the increase of reaction temperature, especially in the initial stage of reaction. For example, the titanium conversion ratio reached 85% within 20 min at 400 °C. The titanium conversion ratio increased from 78.4% at 250 °C to 98.1% at 350 °C after 90 min. On the other hand, the conversion ratio increased only 0.2% at 15758

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400 °C from 350 °C after 90 min. Temperature above 350 °C has an insignificant effect on the conversion of titanium from Ti-slag, so we choose 350 °C as the optimal reaction temperature. 3.2. Macrokinetics of Titanium Slag Decomposition. The temperature dependence of Ti-slag decomposition can be used to estimate the apparent activation energy and elucidate the macrokinetics of the process, both helpful for reactor scaleup and project excise. During the decomposition of Ti-slag particle in NaOH-KOH system, the following steps can be included: (1) the reactant cluster from NaOH-KOH diffuses from the melt to the particle surface through the liquid boundary layer; (2) the reactant cluster diffuses from the particle surface to the reaction interface through the residue layer; (3) Reaction 6 happens in the interface; (4) the reaction product NaKTiO3 from Reaction 6 diffuses from the interface to the particle surface; and (5) the NaKTiO3 diffuses from the particle surface to the melt. The decomposition of Ti-slag and high Ti-slag in the molten NaOH system can be theoretically described by unreacted shrinking core mode.7,21 As alkali is consumed in the reaction process, the system viscosity increased greatly and the system finally became completely solid. Therefore, steps 4 and 5 can be neglected. The unreacted shrinking core mode is illuminated in Figure 7. The Ti-slag particles can be seen as spheres. The radius of the

concentration of the reactant cluster on the surface of the particle. Step 2: The reactant cluster diffuses from the particle surface to the reaction interface through the residual layer. The mass flux rP is ⎛ dC ⎞ R 0R i rp = 4πR i 2De⎜ (Cs − C i) ⎟ = 4πDe R0 − Ri ⎝ dR i ⎠

(5)

where Ri is the radius of the unreacted core, De is the mass transfer coefficient of the cluster in the residue layer, and Ci is the reactant cluster concentration on the surface of the unreacted core. Step 3: Reaction 1 happens on the interface. The reaction is considered irreversible and estimated as first order, where krea is the reaction rate constant reaction.7,20 The reaction rate vi is vi = 4πR i 2k reaC i

(6)

In the case of stability, eqs 4−6 are equal, and the main reaction rate r of Ti-slag particle decomposition is obtained r=

4πR 0 2C0 1 kM

+

R 0(R 0 − R i) DeR i

+

R 02 k reaR i 2

(7)

The radius of the unreacted core Ri is hard to measure, so the conversion ratio X is used to replace the radius as X=

4 4 πR 0 3 − 3 πR i 3 3 4 πR 0 3 3

⎛ R ⎞3 = 1 − ⎜ i⎟ ⎝ R0 ⎠

(8)

In the span time of dt, the amount of titanium conversion amount is ⎛ρ⎞ r dt = −x⎜ ⎟4πR i 2 dR i ⎝M⎠

(9)

where ρ is the density of Ti-slag particle, t is the reaction time, M is the molar weight of Ti-slag, and x is the product stoichiometric coefficient which is 1 in the chemical equation. By integrating eq 8 and eq 9, the relationship between the conversion ratio and the main reaction rate can be obtained as dX 3M = dt ρR 0

C0 1 kM

+

R0 ((1 De

−1/3

− X)

− 1) +

1 (1 k res

− X )−2/3

Figure 7. Schematic diagram of shrinking core model.

(10)

By integrating eq 10, the follow equation is deduced, Ti-slag particle is R0. After a certain time of reaction, the outer sphere is covered by reaction product (NaKTiO3) and the inner sphere is Ti-slag (unreacted core). As the reaction proceeds, the ratio of unreacted core, Ri, is decreasing continually. The volume of the sphere is assumed as constant. Steps 1−3 can be derived by the following. Step 1: The reactant cluster from molten NaOH-KOH diffused from the melt to the particle surface through the liquid boundary layer. The mass flux rL can be obtained by rL = 4πR 0 2kM(C0 − Cs)

R X + 0 [1 − 3(1 − X )2/3 + 2(1 − X )] 3kM 6De MC0 1 [1 − (1 − X )1/3 ] = t + k res xρR 0

(11)

Equation 11 is the overall kinetic equation of Ti-slag decomposing in NaOH-KOH controlled by diffusion through the liquid boundary layer, chemical reaction, and diffusion through residual layer. While in practice the reaction maybe only be controlled by one step or two steps. Under different process control, the kinetics equations can be simplified as: Diffusion through the liquid boundary layer process

(4)

where kM is the mass transfer coefficient of the reactant cluster from NaOH-KOH in liquid boundary layer, R0 is the radius of the particle (the thickness of the liquid boundary layer can be neglected compared with the size of the particle), C0 is the concentration of the reactant cluster at t = 0, and Cs is the

X= 15759

3kMMC0 t = k1t xρR 0

(12)

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Chemical reaction controlled process 1 − (1 − X )1/3 =

Table 2. Reaction Rate Constant at Various Temperatures

k resMC0 t = k 2t xρR 0

(13)

Diffusion through the residual layer process 1 − 3(1 − X )2/3 + 2(1 − X ) =

6DeMC0 xρR 0 2

t = k 3t

T, °C

T, K

1000/T, K−1

k3

ln k3

r

350 325 300 275 250

623 598 573 548 523

1.605136 1.672241 1.745201 1.824818 1.912046

0.01746 0.01246 0.00857 0.00568 0.00355

−4.04784 −4.38523 −4.75949 −5.1708 −5.64081

0.996 0.999 0.999 0.999 0.998

(14)

To evaluate the process, the conversion ratios are substituted into different empirical macrokinetics equations according to eqs 12−14. As shown in Figure 8, the result shows that eq 14

Figure 10. Natural logarithm of reaction rate constant versus reciprocal temperature.

inner mass transfer in the product layer would enhance the decomposition, such as increase of temperature and decrease of particle size.22 Compared with the decomposition of Ti-slag in molten NaOH which is in a chemical reaction controlled process,7 the decomposition of Ti-slag has smaller resistance in Step 3. Then the specific apparent activation energy can be calculated based on Arrhenius equation as shown in eq 15:

Figure 8. Titanium extraction rate versus time at 300 °C fitted by three kinetics equations.

gives better linear tendency (from 0 to 45 min) at each reaction temperature, taking the temperature of 300 °C, for example. Plots of eq 14 vs time shown in Figure 9 indicate that the extraction of titanium at 250−400 °C gives a good correlation

ln k 3 = ln A −

E 1 × R T

(15)

where E is the apparent activation energy, A is the pre-exponential factor, and R is the molar gas constant. The apparent activation energy of titanium conversion in NaOH-KOH is calculated to be E = 43.1 kJ/mol, which is much lower than that of Ti-slag decomposed in molten NaOH system (62.4 kJ/mol).7 3.3. Water Leaching Process. Leaching of the roasted product was performed to recycle the NaOH and KOH reaction medium. NaKTiO3 reacts with water to form mixed alkali (NaOH and KOH) solution as shown in the following dissolution reaction. NaKTiO3 + 2H 2O = NaOH + KOH + H 2TiO3

(16)

At the same time, manganese, aluminum, and silicon follow similar dissolution reactions. After the removal of these impurities,23 mixed alkali can be recycled by evaporation of water. A leaching test was performed using a three-stage counter-current leaching process at a liquid to solids ratio of 5:1. The leaching was conducted at 50 °C for 30 min. The leaching results are listed in Table 3. As shown in Table 3, 98% of K+ and 86% of Na+ can be recycled by ion exchange with H+ via the three-stage counter-current leaching process. As KOH is more expensive than NaOH, the recycle content of KOH is the key factor that determines whether this novel process is economic. Furthermore, the recycled alkali has little effect on the decomposition of titanium slag.24

Figure 9. Plot of extraction kinetics under various reaction temperatures.

with the diffusion through the residual layer process. The data are shown in Table 2 and Figure 10. Therefore, decomposition of Ti-slag in the NaOH-KOH system is controlled by mass transfer in the product layer. Any method that can improve 15760

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NaOH-KOH. The new system has great advantages over the NaOH molten salt system in both thermodynamics and macrokinetics. • Different parameters such as mixed alkali-to-Ti-slag, particle size, and temperature can influence the decomposition and Ti-slag conversion ratio. Among them, temperature and mixed alkali-to-Ti-slag are the most important factors influencing content of titanium conversion. Under conditions of temperature 350 °C, mixed alkali-to-Ti-slag ratio 1.4:1, and particle sizes between 76−105 μm, >98% of titanium conversion ratio can be obtained with reaction time around 90 min. • The decomposition of Ti-slag in NaOH-KOH system is a typical process of solid−liquid reaction with a decrease of the reaction surface area and thickening of product layer of solid. From macrokinetics results, a shinking core model can be used to describe the decomposition process. Compared with the decomposition of Ti-slag in molten NaOH which is a chemical reaction controlled process,7 the decomposition of Ti-slag in the NaOHKOH system is a diffusion through the residual layer process. Apparent activation energy of Ti-slag decomposition in the temperature range from 250 to 350 °C is 43.1 kJ/mol. • Compared with NaOH molten salt system, the decomposition temperature can decrease from 550 °C7 to 350 °C, which may probably be an energy savings process. • Compared with Na+, K+ can recycled more thoroughly by the water leaching process (98%). As almost no K+ is lost, the novel process proves good economically.

Table 3. Leaching Recovery of Na and K Na K

first stage

second stage

third stage

sum

70.1% 82.7%

11.5% 12%

4.7% 3.3%

86.3% 98%

3.4. Proposed Flow Diagram. The flow diagram shown in Figure 11 summarizes all the steps and conditions required to

Figure 11. Principle flow sheet of titania production with NaOHKOH binary molten salts.

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produce synthetic rutile that was followed in this investigation with the Ti-slag. The X-ray diffraction pattern of the product is shown in Figure 12, which indicates well crystallized rutile

AUTHOR INFORMATION

Corresponding Author

*T.Q.: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology Institute of Process, Engineering, Chinese Academy of Sciences, No. 1 Bei Er Jie, Zhong Guan Cun, Beijing, P. R. China 100190. E-mail: tqgreen@ home.ipe.ac.cn. Telephone: +86-10-62631710. Fax: +86-1062631710. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge supports from the project supported by the Major Program of the National Natural Science Foundation of China (Grant No. 51090380), National Science Foundation for Distinguished Young Scholars of China (Grant No. 51125018), and National Natural Science Foundation of China (Grant Nos. 51004091, 21006115, 51104139).

■

REFERENCES

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Figure 12. XRD pattern of rutile TiO2.

TiO2. The chemical composition of product is TiO2 99.5%, SiO2 0.111%, ZrO2 0.114%, Fe2O3 0.014%, and SO3 0.0804%. It could be used as pigment after further treatment.

4. CONCLUSION A novel titanium dioxide pigment process is proposed and proved feasible in the work with a new reaction system of 15761

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