Potentially More Ecofriendly Chemical Pathway for Production of High

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A potentially more eco-friendly chemical pathway for production of high purity TiO2 from titanium slag Xiaofang Zhu, Shili Zheng, Ying Zhang, Zhigang Zak Fang, Min Zhang, Pei Sun, Qing Li, Yang Zhang, Ping Li, and Wei Jin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05102 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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ACS Sustainable Chemistry & Engineering

A potentially more eco-friendly chemical pathway for production of high purity TiO2 from titanium slag Xiaofang Zhu1,2, Shili Zheng1, Ying Zhang1*, Zhigang Zak Fang3, Min Zhang4, Pei Sun3, Qing Li1, Yang Zhang1, Ping Li1, Wei Jin1 1Key

Laboratory of Green Process and Engineering, National Engineering Laboratory

for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Bei er jie, Zhongguancun, Haidian District, Beijing 100190, China 2University

of Chinese Academy of Sciences, No. 19, Yuquan Road, Shijingshan

District, Beijing 100049, China 3Department

of Metallurgical Engineering, University of Utah, Room 412, 135 S 1460

E, Salt Lake City, Utah 84112, USA 4College

of Materials Science and Energy Engineering, Foshan University, No. 18,

Jiangwan 1# Road, Foshan, Guangdong 528000, China *Corresponding

author: [email protected]

ACS Paragon Plus Environment

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Abstract TiO2 is an important inorganic material which is commercially produced by either the chloride or the sulfate process. In general, the latter has lower cost and lower entry barrier than the former. However, the environmental impact associated with the sulfate process is more visible than the chloride process because the sulfuric acid with a low centration (~20 wt%) cannot be cost-effectively recycled; therefore, it has to be neutralized, generating a large amount of wet and useless red gypsum (RG). In this research, a potentially more eco-friendly chemical pathway for TiO2 production from concentrated titanium ore, aka titania slag, is presented. The new method consists of three critical steps including transformation of the titania slag to a lower valence titanium sub-oxide by aluminothermic reduction, digestion by using mild acid, and controlled hydrolysis accompanied by acid recycling. As a result of the phase transformation, the digestion of titanium from the titanium feedstock becomes easier such that it is feasible to use relatively mild acid to replace concentrated acid, reducing the environmental impact from the red gypsum because the need for neutralizing the waste acid can be eliminated. High purity hydrous TiO2 can be prepared after hydrolysis, and the spent liquor can be effectively recycled back to the digestion unit.

Keywords: Titanium dioxide, Titania slag, Aluminothermic reduction, Mild acid digestion, Controlled hydrolysis


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Introduction Titanium (Ti) is the 9th most abundant element and the 4th most abundant metal in the earth’s crust.1 Today Ti is mostly used in the form of titanium dioxide. TiO2 is a popular pigment used in paint, plastics, paper, and cosmetics.2,3 The production of TiO2 is the third largest inorganic materials segment in the chemical industry.4-8 The raw materials for TiO2 production include ilmenite, titania slag, upgraded titania slag, synthetic rutile and natural rutile. Traditionally, the commercial TiO2 is mainly produced by either the sulphate or the chloride processes.9-11 Industry statistical data shows that the chloride process has a bigger share of the production at 60% worldwide, while 98% manufacturers in China use the sulfate process.12 Some technical details and their respective challenges for the two processes are described below. Sulfate process: In the sulfate process, titanium-bearing minerals, mainly ilmenite with 45-65% TiO2 and titania slag with 75-90% TiO2, are digested by concentrated sulphuric acid to obtain sulphate salt mainly composed of titanyl sulfate (TiOSO4), iron sulfate, and free acid. The digested residue is leached by water and metallic iron scrap is introduced into the acid solution to reduce Fe3+ to Fe2+, followed by cooling crystallization to remove iron as FeSO4·7H2O. The purified titanyl sulfate solution undergoes hydrolysis to precipitate a hydrous titanium oxide, which is further washed, salt treated, calcined and surface treated to produce final pigment-grade TiO2 powders. The sulfate process is a mature method with low cost and simple devices. However, the environmental issues associated with this process are severe, especially in China. Strong sulphuric acid (90wt%) is used to destruct the strongly bonded structure of the feedstock. More water is added in the subsequent hydrolysis process which generates a



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large amount of mild sulphuric acid with a relatively lower concentration of ~20wt% (~4.7mol/L H+). This solution is difficult to be recycled to the digestion unit, and has to be treated by neutralization of adding lime, leaving a large amount of wet and useless red gypsum (RG). It has been reported that, 7-11 tons of spent acid, 3-4 tons of copperas, 200 tons of acidic water and 25000 m3 of exhaust gas are generated per ton of TiO2 produced, resulting in around 8.63 tons RG (solid content: 50~56%) per ton of TiO2.13 Chloride process: The chloride process has dominated the pigment industry outside of China due to its advantages over the sulfate process mostly in waste management. Unlike the sulfate process, the requirements for the feedstock for the chloride process are more stringent. It has to be natural or synthetic rutile, or upgraded slag (UGS) with high content of TiO2 and low contents of CaO and MgO. In the chloride process, the high-quality raw material is chlorinated at around 1000 C to obtain crude TiCl4, which will be further purified, and then oxidized to produce TiO2. Although the chloride process is the dominant process in the pigment industry globally, it is still an energy intensive process and also has a number of environmental challenges. First, the intermediate of TiCl4 is highly corrosive and volatile which is difficult to deal with, and any leak must be avoided. Second, based on the reaction (1), it is estimated that 550 Kg CO2 is directly generated per ton TiO2 produced, and approximately 1.9 million tons CO2 per year can be emitted from this reaction alone.14 Third, the residues after the chlorination of Ti slag, especially those residues after chlorination in molten salt medium instead of chlorination in a fluidized bed, which is necessary for some feedstock minerals, are detrimental to the environment .15, 16

TiO2  C  2Cl2  TiCl4  CO2


Reaction (1)

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Hydrochloride process: The hydrochloride process, developed by Berkovich,17 is to digest ilmenite using concentrated HCl, and 80% of titanium and iron are dissolved. A subsequent study by Thomas Lakshmanan et al18 reported that HCl medium was more desirable for digestion than other acids as it can be recovered from the downstream solution by pyro-hydrolysis. However, the real situation is more complex. On the one hand, concentrated hydrochloric acid is highly volatile and corrosive to the equipment; on the other hand, the concentration of HCl solution obtained by pyro-hydrolysis can only be 20 wt%, unsatisfactory for a closed circulation.19 Alkaline roasting process: Since the concentrations of the acids for digestion in both sulfate and hydrochloride processes are extraordinarily high (close to saturation) and the acid concentrations after titanium extraction in the sulfate process or regeneration in the hydrochloride process is only around 20 wt%, the effective reuse of the acids is very difficult. One way to overcome this difficulty is to weaken the chemical stability of the raw material, thus to moderate the conditions required for digestion. Alkaline roasting pretreatment was proposed,13,

20, 21

during which the original

crystalline phase is transformed into amorphous titanate and the titanate salt can be readily dissolved in a mild acid solution. Xue et al22 reported that when titania slag is roasted with molten NaOH or KOH at 500 C for 1 h, sodium or potassium titanate will form and they can be dissolved in dilute acid solution with pH 0.2. Reactions (2) and (3) are involved during this method.

TiO2  2 MOH  M 2TiO3  H 2O

Reaction (2)

M 2TiO3  4 H   TiO 2  2 M   2 H 2O

Reaction (3)

Since the pretreatment makes the mild acid digestion feasible, the effective



recycling of acid after titanium extraction now becomes achievable. The generation of RG solid waste can thus be largely eliminated. However, the recycling and the partial loss of the alkali medium are also costly which is hurdle that must be overcome with this pretreatment. In this work, a potentially more eco-friendly method for TiO2 production from titania slag is reported, which seeks to find a better balance by minimizing solid waste as well as energy consumptions. The new method is based on the route of first transforming Ti oxide in the feedstock to lower oxidation state, followed by titanium extraction using mild acid (moderate acid with H+ concentration less than 6mol/L) at low temperatures, and finally purifying the product by hydrolysis. In this paper details of the concept design will be described first, followed by the results and analysis for each critical step of the process.

Concept design of the new method Ti suboxide intermediates soluble in mild acid The existing methods for TiO2 production are processes of converting impure TiO2 into pure TiO2, which involves a series of structure-rebuilding processes during which a few intermediates are involved, including TiOSO4, TiCl4, TiOCl2, and Na2TiO3, and so forth. These intermediates from different processes as described above can be purified and transformed into TiO2. According to the Ti-O23 phase diagram, there are numerous forms of titanium suboxides including Magnéli phases (TinO2n-1, 4≤n≤9), Ti3O5 (TiO2·Ti2O3), Ti2O3 (TiO1.46-TiO1.56), and TiO (TiO0.7-TiO1.3), all of which possess different characteristics


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when dissolved in acid. Among them, Ti2O3 has the corundum alumina structure; it is, considered as a weak basic oxide that is insoluble in hydrochloric acid, sulfuric acid or nitric acid.19 However, it was also described as a favored suboxide which can enhance the leach-ability of titanium in slag in another study.24 Another oxide, TiO, is a basic oxide with a metallic nature.19 It is soluble in dilute hydrochloric acid and sulfuric acid. The dissolution of TiO in acid releases Ti3+ ion and hydrogen gas, as shown in reaction (4). 2TiO  6 H   2Ti 3  2 H 2O  H 2 

Reaction (4)

Therefore, if titania in titanium feedstock can be economically and effectively transformed into suboxides that are soluble in mild acid, TiOSO4 or TiOCl2 acid solutions can be obtained by dissolving the suboxides in either mild sulfuric or hydrochloride acid and oxidizing Ti3+ ion into Ti4+ ion by oxidant such as H2O2. Therefore, we hypothesize that if one can transform TiO2 in the feedstock material into titanium sub-oxides intermediates that are soluble in mild acid, there would be no need for the use of concentrated acid in pure TiO2 production. The challenge is to find a method to transform TiO2 in the Ti-slag to obtain the said sub-oxides.

The selection of the reductant Based on the Ellingham diagram for metals’ reaction with oxygen25 as shown in Fig. S1, there are a few metal reductants that are capable of reducing TiO2 to TiOx (x≤1.5), including Ca, Mg, Al, Li and Y. Due to the high chemical affinity of titanium to oxygen, Ca and Mg are frequently used for Ti metal production by reducing TiO2.26 They may also be used to produce



sub-oxides. However, it has been widely known that calcium titanate (or magnesium titanate) can coexist with intermediates during the reduction,27, 28 and they are relatively difficult to dissolve in mild acid at low temperatures. The formation of Mg or Ca titanate can lower the rate of extraction of titanium. In addition, the relatively high cost of Ca and Mg may also make the pretreatment of controlled reduction uneconomic. Al is another reductant. Compared to Ca and Mg, Al is preferred for the following reasons. (a) Al is the lowest cost metal among these metals; (b) the valence electron configuration of Al is 3S23P1, thus the mole dosage of Al needed is less than either Ca or Mg; (c) the alkalinity order of the three metal oxide is CaO＞MgO＞Al2O3, so Al2O3 is more stable in acid solutions, which suggests that Al2O3 won’t be dissolved by the mild acid. Thus, Al is selected as the reductant in this approach. The reduction of TiO2 to titanium suboxides in this research is based on the aluminothermic reduction of TiO2.

Description of the new chemical route The new route to produce titanium dioxide involves three main steps: transformation of TiO2 in the raw material to obtain sub-oxides, mild acid digestion, and controlled hydrolysis. A flowchart of the process is shown in Fig. S2. Note that the raw material can be ilmenite, titanium slag, upgraded titanium slag, or a mixture thereof, but ilmenite alone with low grade of TiO2 and high Fe content is not preferred as iron oxide in the ilmenite will consume a large amount of Al reductant. As mentioned above, the transformation of the raw material aims at rebuilding the structure of chemically stable TiO2 into sub-oxides that are more active. This is


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accomplished by using aluminothermic reduction. Thermodynamically, the reaction between TiO2 and Al to form Ti2O3 or TiO is favorable according to Fig. S1. However, there are kinetic hurdles at low temperatures. Since molten Al metal has a very large surface tension, it may be difficult to wet the surface of TiO2 particles by the Al metal liquid. Based on the reported effects of molten salt on the magnesiothermic reduction and on the deoxygenation of Ti metal using calcium,29, 30 molten salt will also be applied in this research to kinetically facilitate the reduction and decrease the reaction temperature as much as possible. After the transformation step, it is anticipated that titanium suboxide can be effectively digested using a mild acid solution with a low free H+ concentration. Then a Ti3+-bearing solution is obtained. The concentrations of Ti3+ ion and free acid in the solution depend on the parameters of the digestion process including liquid-to-solid ratio and the initial acidity of the acid. Further, in a real situation of dealing with titanium-bearing feedstock, there are impurities therein, including silica, iron oxide, calcium oxide, magnesium oxide, and so forth, which may transfer into the solution simultaneously. The accumulations of the foreign ions in acid and their effects on the subsequent step of hydrolysis have to be evaluated, which is beyond the scope of this paper, but will be systematically studied in a future report. The trivalent titanium ion in the acid solution will then be oxidized to obtain oxidant  Ti 4 ), and the solution will be ready tetravalent titanium ion by oxidant ( Ti 3 

for hydrolysis to selectively precipitate hydrous titanium oxide at a desired temperature of around 100 ºC. The composition of the solution is expected to be right within the required range. This hydrolysis step shares the same principles as that in the sulfate



process. The hydrolysis can also be controlled to obtain the product powder with a certain particle size distribution and preferred morphology.26 The hydrous precipitates will be washed and calcined to produce pure TiO2 powder. One may note that the proposed procedures of this method has similarities to that of the Benilite process31-33 to produce synthetic rutile from ilmenite, because both processes have the steps of partial reduction, acid leaching, and acid regeneration. However, the two processes are fundamentally different for the following reasons. (1) The partial reduction in the Benilite process focuses on reduction of ferric iron into ferrous iron by a carbonaceous reducing agent, without change on Ti phase; but the partial reduction in this research uses a stronger reductant Al and aims to change tetravalent titanium into titanium sub-oxides. Iron would be reduced simultaneously. (2) The acid leaching in the Benilite process aims at dissolving iron into the solution selectively and keeping titanium in the solid residue; while the acid leaching in this method dissolves titanium and iron simultaneously. (3) The acid in the Benilite process is recycled by using costly pyrohydrolysis; while the acid regeneration in this method is the byproduct of extracting Ti from the acid solution via hydrolysis. (4) The product of the Benilite process is impure synthetic rutile for further TiO2 or Ti metal production, while the product of this method is pure TiO2.

Results and discussion Transforming the raw material Effect of the amount of the reductant on the phase transformation The phase transformation from rutile to various suboxides and other phases during


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aluminothermic reduction was studied by changing the Al-to-TiO2 molar ratio. The product after reduction was dissolved by HCl solution with 6 mol/L H+ to evaluate the efficiency of titanium extraction for various phases. According to the ternary phase diagram of Ti-O-Al34 shown in Fig. S3, the product of reducing TiO2 by Al can be a mixture of various phases which depends strongly on the dosage of the reductant, and theoretically, the formation of Al2TiO5 (difficult to dissolve with dilute acid) can be avoided because of the existence of the Magnéli phases. In addition, the formation of Ti-Al alloy should be limited which would otherwise increase the consumption of Al. Thus, various Al-to-TiO2 molar ratios based on the four reactions (5)-(8) below, 0.22, 0.33, 0.45, 0.67 and 0.89, were chosen and a series of reductions were carried out. Additionally, the experiments were conducted at 900 C for 12h with the addition of NaCl-KCl eutectic salt. TiO2  2 Al  1 Ti3O5  1 Al2O3 9 3 9

Reaction (5)

TiO2  1 Al  1 Ti2O3  1 Al2O3 3 2 6

Reaction (6)

TiO2  2 Al  TiO  1 Al2O3 3 3

Reaction (7)

TiO2  8 Al  1 Ti3O2  4 Al2O3 9 3 9

Reaction (8)

Prior to the experimental study of this work, a commercial software for predicting chemical reactions, HSC 6.0, was used to study the equilibrium phase compositions when reducing TiO2 by Al metal in 1 bar Ar atmosphere with different molars of TiO2, Al and Ar as a function of the temperature. The results are shown in Fig. S4. It shows that the equilibrium phases are mainly a function of the amount of Al. With increasing Al amount, the sub-oxides equilibrated with Al2O3 at 900 C were changing from Ti3O5 to Ti2O3, then to TiO.



The X-ray diffraction patterns of the reduced products after washing by water are shown in Fig. 1. It shows that the valence state of Ti in the Ti-O compounds was being lowered with the increasing of Al-to-TiO2 molar ratio. Even though the phase compositions were not exactly the same with those predicted by HSC, the sequence of phase transformation was quite similar. There was still unreduced TiO2 in the powder when the Al-to-TiO2 molar ratio was 0.22. Single Ti2O3 titanium phase was found when the Al-to-TiO2 molar ratio was 0.33. TiO appeared when the ratio reached 0.45, and it dominated in the sample with the Al-to-TiO2 molar ratio at 0.67. Furthermore, Ti3Al was observed when the ratio was 0.89.


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Fig. 1 XRD patterns of reduced products with Al-to-TiO2 molar ratio of 0.22 (A), 0.33 (B), 0.45 (C), 0.67 (D), 0.89 (E), and the comparison of XRD patterns of the products before and after acid leaching when Al-to-TiO2 molar ratio was 0.45 (F).

To confirm the phases that were supposed to be reactive in dilute acid solutions, a certain amount of the reduced powders after washing by water was leached by 6 mol/L HCl solution, and the absolute leaching rate (ALR) of titanium was calculated with the results shown in Fig. S5. The method of calculating the ALR was described in supporting information. It is observed that the leaching rate increased with the increasing Al dosage. Even though it was reported that Ti2O3 may not be dissolved in mild acid,19 a leaching rate of 91.27 wt%, which is rather high, was observed when the main phase was Ti2O3 in the samples, demonstrating that the Ti2O3 formed in this reaction was reactive. An ALR of 99.59 wt% was achieved when the molar ratio of Al to TiO2 was 0.45. It is still unclear why there was a gradual decrease of the leaching rate of Ti when the Al-to-TiO2 molar ratio was higher than 0.45. One possible reason for this is that the oxide film on the metallic phases of Ti6O and Ti3Al protected them from corrosion by acid. Even though it is generally thought that Al2O3 belongs to the category of alkaline oxides and is soluble in acid solution, the XRD pattern demonstrates that the corundum phase Al2O3 is inert to some extent,35 and almost pure Al2O3 phase was found in the leaching residue (Fig. 1(F)). The inertness of alumina after the transformation step is important because the byproduct won’t consume acid during leaching and the contamination the Ti3+-bearing solution by Al3+ ions can thus be prevented.



Thus a molar ratio of Al to TiO2 of 0.45:1 was found to be the optimum in this study, which corresponds to the mass ratio of 0.152:1 of Al to TiO2.

Effect of temperature and time on the phase transformations The melting point of Al metal is 660 C, thus a temperature higher than 660 C was applied in this study to investigate the solid-to-liquid reaction between TiO2 and Al. Since temperature is a critical factor affecting the kinetics of mass transfer, the effect of temperature on the phase transformation was studied at 700 C, 800 C and 900 C respectively. At first, reductions at 700 C, 800 C and 900 C for 12 hours with varying molar ratios of Al to TiO2 were done, and the extent of phase transformations was evaluated by using the leaching rate of titanium as an indicator. The results are shown in Fig. 2(A). In general, the extraction of titanium was more efficient with samples processed at elevated temperatures, but the trend of leaching rate with the molar ratio of Al to TiO2 was the same at all temperatures. Specifically, the leaching rate increased initially with the increase of Al-to-TiO2 molar ratio and then it decreased. The XRD patterns of the reduced powders with the highest extraction rate of titanium corresponding to different temperatures are compared in Fig. 2(B). Except for the differences in peak intensities, the peak positions for the three samples were similar. It is noted that the peaks for alumina are more visible for 800 C and 900 C samples than for the 700 C sample. This partially explains why the leaching rate of aluminum was much higher for the samples prepared at 700 C, since an incomplete crystallization of alumina can happen at lower temperatures.


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Further, the effect of reduction time on titanium extraction at various temperatures was investigated. As indicated above, a molar ratio of Al/TiO2 of 0.45 was chosen. The duration of the reduction experiments was controlled to be 4h, 8h and 12h. The leaching rates of titanium versus reduction time and temperature are shown in Fig. S6. Except for at 700 C, the effect of time on the phase transformation was not significant within the ranges of this study, after taking the minor fluctuations into consideration.

Effect of low-melting-point eutectic salt on the phase transformations It has been reported that the surface tension of molten Al is 1.007 N/m at 680 C,36 almost twice of that of molten Mg (0.579 N/m at 680 C).37 The relatively high surface tension may lead to the difficulty of wetting the surface of TiO2 particles by metallic Al liquid. Some reports studied the relationship of  (contact angle) and temperature in Al/TiO2 system, showing that the wetting angle   90 at 900 C,38 which is unsatisfactory. To solve the kinetic hurdle that might be caused by poor wetting during reduction, a typical strategy is to use molten salt. It is well-known that molten salts (single or eutectic) are excellent solvents and can be used as reaction media making use of their high thermal and chemical stability, high thermal and ionic conductivities, and low surface tension and viscosity.29, 39 The role of molten salts has been reported widely in various applications. Possible roles include transporting metal reductant via dissolution and diffusion within the molten salt,40 promoting the reaction by dissolving the byproduct41 or by reacting with the byproduct,42 and lowering the reaction temperature.43 In this research, the reductions with or without the presence of NaCl-KCl eutectic



salt were compared. The reason to choose this salt is that (1) its surface tension is as small as 0.1 N/m at 700 C44 and (2) its hygroscopicity and volatility are much weaker than those common salts containing MgCl2, CaCl2, and AlCl3. The leaching rates of titanium after phase transformation with or without salt were shown in Fig. 2(C). The amount of salt added was 80 wt% mass of that of TiO2. The data verified that the use of molten salt significantly improved titanium extraction efficiency under the same molar ratio of Al to TiO2. The XRD pattern of the reduced powder after reduction without salt under the molar ratio of 0.45 was shown in Fig. 2(D), and it is obvious that the phase transformation in this case was incomplete since the peaks for residual Al and Ti9O17 can still be observed. Even though the role of NaCl-KCl salt in this research is still not fully understood, the improved wettability bridged by the molten salt on the TiO2 surface and even slight dissolution of Al metal in the salt may have played a positive role.


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Figure 2 (A) Absolute leaching rates of titanium and aluminum at different reduction temperatures; (B) XRD patterns of the reduced products obtained at 700 C, 800C, 900C when with the highest extraction rate of titanium; (C) Comparison of the ALR of titanium after structure rebuilding with and without eutectic salt; (D) XRD pattern of the reduced powder without salt added with Al-toTiO2 molar ratio of 0.45 Transformation of upgraded titanium slag Based on the results above, the relatively optimized parameters for the phase transformation from TiO2 to titanium suboxides by partial thermal reduction are as follows: molar ratio of Al to TiO2 at 0.45, temperature at 800-900 C, and the duration of 4h in Ar gas and molten salt. Theoretically, the dosage of the reductant should be adjusted when the consumption of the reductant by the impurities in the slag is considered. According to Fig. S1, some



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impurity oxides can be reduced by Al simultaneously with the reduction of TiO2. Since the contents of these purities (Fe, Si and Mn) are relatively low in the upgraded titanium slag (UGS), the process parameters for transforming pure TiO2 as described earlier were applied to the partial reduction of of UGS directly. The mass ratio of Al to UGS for partial reduction is 0.152:1. The XRD pattern of the reduced slag is shown in Fig. S7. It shows that the transformation of UGS was thorough. Only peaks for -Al2O3 and Ti2O3 were detected.

Mild acid digestion After reduction and water washing, the solid sample was further digested by acid to dissolve titanium into the aqueous acid solution. The reduced product, titanium suboxides, will react with acid via the following reaction (9). Note that the acid here can be either sulfuric acid or hydrochloride acid. In this study, HCl solution with 6 mol/L H+ was used.

TiOx (1  x  1.5)  3H   Ti 3  xH 2O  (1.5  x) H 2 

Reaction (9)

The leaching was done on the reduced powders derived from upgraded titanium slag to optimize the leaching parameters including temperature and time. For the purpose of preventing the loss due to volatilization of HCl, a sealed leaching container was equipped with a reflux unit. Since constant sampling will reduce the total volume of the slurry, the leaching was evaluated by the change of the concentration of titanium ions in the solution vs. time at different temperatures. The leaching curves are shown in Fig. 3(a). Figure 3(a) shows that the kinetics of dissolving titanium was faster at higher


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temperature from the slopes of the leaching curves in the first 200 minutes. The variation of the highest titanium concentrations in the plateaus at different temperatures was due to the frequent samplings which had carried out some solids containing titanium simultaneously. The durations to reach the leaching limit were approximately 200 minutes at 90 C, 300 minutes at 80 C and 400 minutes at 70 C. However, the temperature has to be carefully controlled to avoid it being too high which could cause Ti3+ ions to be oxidized while it is being dissolved, giving rise to the hydrolysis of Ti4+. Leaching experiments were further conducted at the three temperatures for the length of time that yield most efficiency respectively. The compositions of the leaching solutions were analyzed using ICP in order to determine the titanium leaching ratio. The titanium ion concentrations in different leaching solutions at 70C for 400 minutes, 80 C for 300 minutes, and 90 C for 200 minutes, were compared, which were 41.03 g/L, 44.08 g/L, and 44.50 g/L respectively, corresponding to leaching ratios of 87.02%, 93.49% and 94.38%. Clearly the titanium extraction at 90 C for 200 minutes was the highest. Controlled hydrolysis Hydrolysis is a way to selectively transform Ti ions into solid titanic acid, and the general chemical reaction for hydrolysis in HCl medium is as follows.

TiOCl2  (1  x) H 2O  TiO2  xH 2O  2 HCl

Reaction (10)

The conventional way to produce high-quality pigment-grade TiO2 via hydrolysis from TiOSO4 solution is a well-established process. It is thus easier to use mild sulfuric acid instead of hydrochloride acid to digest the structure-transformed UGS and to have the equivalent TiO2 concentration and F value (mass ratio of valid acid to TiO2) match



that of process parameters in the industry. This process is expected to have no difficulties. However, it is one of the authors’ objectives in this research to prepare coarse and near spherical TiO2 powders as the precursor for Ti metal production. The replacement of SO42- with Cl- would change the features of the hydrolyzed product, including the phase composition, particle size, and morphology. These characteristics are also functions of temperature, equivalent TiO2 concentration, acidity, time, and so forth. In this work, the solution after acid digestion and filtration of insoluble leaching residue is a Ti3+-bearing solution, i.e. TiCl3 acid solution. There were a few reports in the literature that are related to the preparation of fine TiO2 from TiCl3 solution.45,46 Since oxidation is necessary to obtain titanic acid, the purple solution was oxidized chemically here by using H2O2 to yield TiOCl2 acid solution. It has been demonstrated by the present authors in their previous research26 that coarse particles can be obtained when hydrolyzing TiOCl2 solution with an equivalent TiO2 concentration of ~1 mol/L and a free H+ concentration of ~3 mol/L at 95~100 C for 16~24h, which was also verified in this research. The near spherical morphology of the particles and the particle size distribution ranging from 20-60 microns are shown in Fig. 3(b). It is also a unique feature of this process that the parameters for acid digestion and hydrolysis can be specifically designed and adjusted according to the requirements for titanic acid to meet various applications.


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Fig. 3 (a) The change of titanium concentration in the solution vs. time at different temperatures and (b) Morphology and particle size distribution of the hydrolyzed titanic acid particles

Regarding the purity of the powders, a typical ion concentration in the pre- and post-hydrolyzed solutions were analyzed, seen Table S2. These values show Si is the only impurity that would contaminate the hydrous titanium oxide. If ultrahigh purity TiO2 is required, a desilication step can be incorporated to remove Si before hydrolysis. It is well-known that the requirement on Si content for TiO2 pigment is rather flexible. However, if the powder is to be used to produce Ti metal, Si content has to be minimized. The strategy to remove Si impurity from highly acidic TiOCl2 solution by adsorption has been demonstrated to be very effective by the present authors.47

Assessment of the impact of the new method With the growing global concern over the situation of energy supply, efficiency, and environmental degradation, the need for the development of sustainable technologies are urgent to improve the efficiency of the utilization of resources and to reduce waste. The goal of the new method in this research is to produce high purity TiO2 with less solid waste, less carbon emission, and less energy consumption



compared to the conventional processes. As described in a literature report,2 there are four steps for evaluation of the environment impact of any process: (1) defining the scope of the analysis, (2) defining the material and energy requirements, (3) impact assessment of emission and byproducts, and (4) interpretation of the impacts and the potential improvement. Here, a “feed-to-gate” model instead of “cradle-to-gate” model is adopted to exclude the consideration of energy requirements for mining, transporting and preparation of the raw minerals for use as the feed stock. To further simplify the comparison, titanium slag is chosen as the “feed” for the three processes, i.e. sulfate method, chloride method, and the new method developed in this research (hereafter referred to as “mild acid method”, or MILD for short). The step for iron purification from the solutions is considered a minor and omitted in this analysis. In addition, the processes of producing pigment products including surface treatment, milling and packaging are not considered because they are the same for the production of pigment product regardless which chemical route is used to produce pure TiO2. Figure 4 shows the general flow chart for the three different methods starting from the same raw material and ending with the same product. Regarding the energy consumption, an LCA study done by Tioxide Industries48 reported that the sulfate process and the chloride process consumed 103 MJ/kg TiO2 and 109 MJ/kg TiO2 respectively when titanium slag was used as the feed material. It should be noted here that the estimated high energy demand for the sulfate process is due to the inclusion of sulphric acid recycling by reconcentration, which is much more energy-intensive than the neutralization treatment. Generally speaking, an ilmenite-


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based sulfate process combined with acid neutralization consumes significantly less energy than the chloride process based on slag, or the sulfate process based on slag and acid reconcentration. Regarding the energy needed for the MILD process, the embodied energy of Al reductant has to be considered first. Based on the research results above, to produce 1 kg of TiO2 from the slag, 1.114 kg of upgraded titanium slag is needed (=1/(94.5%×95%), 94.5% TiO2 in UGS and 95% leaching rate), thus Al required is 0.169 kg/kg of TiO2. It is known that the primary Al production consists of two separate stages, the Bayer process for Al2O3 production from bauxite and the Hall–Héroult process for the electrolytic reduction of Al2O3 to Al metal,49 and it has also been reported that the embodied energy for primary Al is 200-220 MJ/kg50 which is relatively high compared to metals such as iron and steel. However, it is widely known that more than a third of Al produced each year originates from recycled scrap metal, and the energy needed for recycling is only 22-30 MJ/(kg Al).50 Therefore, the embodied energy from Al during partial reduction is 33.8-37.2 MJ/(kg TiO2) when primary Al is used, which would contribute a significant portion to the total energy consumption of this process, making it less competitive compared to the conventional chloride and sulfate processes. However, if recycled Al is used, the embodied energy for prereduction would be only 3.72-5.07 MJ/(kg TiO2). Further, in comparison to the conventional chloride and sulfate processes, the energy needed for pre-reduction in the MILD process at relatively moderate conditions also has to be included, while no acid re-concentration is required for MILD process as it is for the sulfate process. In summary, when all factors are considered, it is believed that the total energy



consumption for the proposed MILD method is comparable to that of the chloride and sulfate processes when starting with the same raw material, and using recycled Al instead of primary Al for for pre-reduciton. However, exact energy consumptions of the present MILD process cannot be estimated accurately until it is industrialized. With regard to the generation of solid waste and the emission of polluting gases, there are large differences for the three different methods. As shown in Fig. 4, in the sulfate process, when 8~10 tons spent acid per ton TiO2 production with a H2SO4 concentration of 20 wt% is not recycled but neutralized, a large amount of wet red gypsum (7 kg/kg TiO2) is generated. RG is usually disposed as landfill, posing a potential environmental issue. In addition, with stringent digestion, there is usually an acidic exhaust fume. In the chloride process, the amount of slag after chlorination is much less of 0.14 kg/kg TiO2, but the emission of CO2 (0.62 kg/kg TiO2) and the handling of the toxic chlorine gas are challenges. In the MILD process, red gypsum and acidic exhaust fume can be effectively eliminated. Some residue mainly composed of chemically-stable Al2O3 solid will be generated (0.375 kg/kg TiO2) which can be used as a feedstock for the alumina industry. For CO2, even though there is not CO2 emission as a result of the chlorination, the CO2 emission as a result of the use of Al metal has to be considered, which is around 11-13 kg/(kg Al) for primary Al and 1.9-2.3 kg/(kg Al) for recycled Al.50 Therefore, in the MILD process, 1.86-2.20 kg CO2/(kg TiO2) is generated when primary Al is used, and 0.32-0.39 kg CO2/(kg TiO2) is generated when recycled Al is used. From the aspect of chemical reclaiming, a closed-loop system for the reagents is an important way for “dematerialization” to reduce cost and protect the environment.


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The reaction medium of sulphuric acid is typically not recycled in the sulfate process, while chlorine is reused in the chloride process. In the MILD process, the pretreatment reagent Al powder is not reused since it is in the form of Al2O3, while the main medium acid is almost fully reused without the need for either reconcentration or dilution. To sum up, no single process is a global solution. The selection of the optimum production route at a particular location should balance the consideration of a few factors including the type of feedstocks, availability of acid and chlorine, local environmental sensitivities, and so forth. From the environmental standpoint, whether the concept of the new MILD method is eco-friendly or not, it depends on which kind of Al reductant is used and it can be considered to be potentially more eco-friendly when recycled Al is used instead of primary Al.

Fig. 4 The feed-to-gate system of TiO2 production by different methods (per kg TiO2)

Conclusions A new chemical pathway for production of titanium dioxide from titanium slag is



described in this work. Three critical steps in this new route include phase transformation of the titanium slag, mild acid digestion, and controlled hydrolysis. The process also includes the recycling of acid. The phase transformation is based on structure rebuilding of the stable rutile phase to form titanium suboxides (Ti2O3 or TiO) by partial aluminothermic reduction. The suboxides can be dissolved by mild acid with a H+ concentration no higher than 6mol/L. The hydrolysis of hydrous titanium oxide from the acid solution can be strictly controlled, and the spent liquor with its acid concentration almost equal to that for acid digestion can be effectively recycled. The factors that affect the phase transformation include the amount of reductant, temperature, the use of salt with low melting point, and time. Titanium in the upgraded titanium slag can be transformed into single Ti2O3 phase. The use of mild acid medium instead of concentrated acid leads to the feasibility of recycling the acid, thus eliminating the generation of solid waste, namely red gypsum, and reducing the environmental impact of TiO2 pigment production.

Supporting Information 1) Experiment section: materials, experimental procedures, analysis methods. 2) Supplementary Figures and tables: Ellingham diagram, flowchart, phase diagram, HSC calculation results, ICP and XRD result, including 7 figures and 2 tables.

Acknowledgement This work was financially supported by the Natural Science Foundation of China under Grant No. 51771179 and the Beijing Municipal Natural Science Foundation


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under Grant No. 2192056. The financial support from the CAS Interdisciplinary Innovation Team and the State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization were also acknowledged. In addition, this work was supported in part by the ARPA-E of the US DOE under contract number DEAR0000420.

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For Table of Contents Use Only

Synopsis The sustainable three-step MILD process to produce TiO2 from Ti-slag is featured of reaction medium recycling, solid waste and carbon footprint reductions.


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Step1: structure rebuilding Ti slag

Step2: Mild acid digestion

Step3: Oxidation＆Controlled hydrolysis

Al powder Eutectic salt mild acid

≤6mol/L H+

Oxidation TiOCl2 solution

Corundum Al2O3

Hydrolysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


TiCl3 solution

Ti suboxide Acid recycling or Ti2O3

TiO

𝑻𝑻𝑻𝑻𝑻𝑻𝟐𝟐 + 𝑨𝑨𝑨𝑨 → 𝑻𝑻𝑻𝑻𝑻𝑻𝒙𝒙 𝟏𝟏 ≤ 𝒙𝒙 ≤ 𝟏𝟏. 𝟓𝟓 + 𝑨𝑨𝑨𝑨𝟐𝟐 𝑶𝑶𝟑𝟑

Mother liquid +

𝑻𝑻𝑻𝑻𝑻𝑻𝒙𝒙 𝟏𝟏 ≤ 𝒙𝒙 ≤ 𝟏𝟏. 𝟓𝟓 + 𝟑𝟑𝟑𝟑 → 𝑻𝑻𝑻𝑻𝟑𝟑

++

𝒙𝒙𝑯𝑯𝟐𝟐 𝑶𝑶 + 𝟏𝟏. 𝟓𝟓 − 𝒙𝒙 𝑯𝑯𝟐𝟐 ↑ ACS Paragon Plus Environment

Pure TiO2

𝟐𝟐𝑻𝑻𝑻𝑻𝑻𝑻𝑻𝑻𝟑𝟑 + 𝑯𝑯𝟐𝟐 𝑶𝑶𝟐𝟐 → 𝟐𝟐𝑻𝑻𝑻𝑻𝑶𝑶𝑪𝑪𝑪𝑪𝟐𝟐 + 𝟐𝟐𝑯𝑯𝑯𝑯𝑯𝑯

𝟐𝟐𝐓𝐓𝐓𝐓𝐎𝐎𝑪𝑪𝑪𝑪𝟐𝟐 + 𝟐𝟐 + 𝟐𝟐𝒙𝒙 𝑯𝑯𝟐𝟐 𝑶𝑶 = 𝟐𝟐𝑻𝑻𝑻𝑻𝑻𝑻𝟐𝟐 ∙ 𝒙𝒙𝑯𝑯𝟐𝟐 𝑶𝑶 + 𝟒𝟒𝑯𝑯𝑯𝑯𝑯𝑯

(A)  

−Ti3O5 -Ti2O3 −Al2O3



           

 

     

PDF#03-065-1118







 

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-Ti2O3 -Al2O3





Intensity, cps

 

ACS Sustainable Chemistry &(B)Engineering  −TiO2

  

 

  

   

PDF#01-076-166 PDF#01-085-0868

PDF#01-085-0868

PDF#01-088-0826

PDF#01-088-0826

20

40

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80

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60



(C)

80

2 (°)

2 (°) 

 

(D)

-Ti2O3

-TiO -Ti6O

-TiO 



  

−Al2O3

−Al2O3



 





 



 



Intensity, cps

Intensity, cps



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

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(F)

-TiO

•-AlTi3  







A



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A

Intensity，cps

    • • • 

80

before acid leaching after acid leaching

-Ti6O −Al2O3

•



2 (°)

2 (°) (E)



PDF#01-072-0020

PDF#01-072-0020

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Intensity, cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

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

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A A A

A

A

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PDF#01-088-0826


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2 (°)

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



     

80

900C,12h,Ti 800C,12h,Ti 700C,12h,Ti 900C,12h,Al 800C,12h,Al 700C,12h,Al

60

Intensity, cps

Absolute leaching rate，wt%



(B)

(A)

100

40

       

900C,12h

800C,12h 700C,12h −Ti2O3 −Al2O3

20

−TiO −Ti6O

0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

20

40

2 (°)

Mole ratio of Al to Ti 100

(C)

900C, 12h

(D)

 

80 

With NaCl-KCl

Intensity, cps

Absolute leaching rate of Ti, wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

-Ti9O17 -Ti2O3 -Al2O3 -Al

          



PDF#01-088-0826 PDF#01-065-2869

0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

20

40

60

2 (°)

Mole ratio of Al to Ti ACS Paragon Plus Environment



PDF#01-085-1061 PDF#01-085-0868

W/O NaCl-KCl

20

 

80

Al/Ti=0.45 900C, 12h



      

60

80


60

(b)

(a)

50 70 C 80 C 90 C

40 30

30

Volume (%)

Ti concentration, g/L

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 46

20

10

0 0.1

1

10 100 Particle Size ( m)

1000

(L/S)initial=10:1

20 10 0 0

200

400

600

800

1000

Time, min


100 μm

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sulfate Process

The New Process

Ti Slag Chloride Process

~90wt% H2SO4

Al powder: 0.169 kg

Coke

Digestion

Pre-reduction Chlorination & Purification

Water

Preparation of TiOSO4 solution

CO2: 0.62 kg Slag: 0.14 kg

Cl2

Water & Lime

Hydrolysis & Calcination

Digestion ~18wt% HCl

Hydrolysis & Calcination

Oxidation

Wet red gypsum: 7 kg (Water content: 50%) Oxygen

H2O2 or air Water

TiO2 1 kg TiO2 ACS Paragon Plus Environment

α-Al2O3: 0.375 kg


-200

Oxygen potential, KJ/mol O2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 40 of 46

-400

FeO

-600

TiO

Ti2O3

Ti3O5

TiO2

SiO2

V2O3

MnO

Cr2O3

-800 -1000 Al2O3

-1200 0

200

400

Li2O

600

MgO CaO Y2O3

800

1000 1200 1400 1600

Temperature, qC


Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ti-bearing Raw Material

Al Powder

Aluminothermic Reduction

Salt (optionally)

Mixture of Ti sub-oxides, Al2O3 and optionally salt Mild Acid Digestion  Separation

Acid

Ti3+-bearing acid solution Oxidation  Hydrolysis

Hydrous TiO2


Solid Residue (mainly α-Al2O3)

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 42 of 46

O

TiO2

Al2TiO5

Magnéli phases

Al2O3

Ti2O3 β-TiO α-TiO Ti3O2

Ti2O

I

II A

III B

α-Ti[O,Al]

C

β-Ti

Ti3Al[O]

TiAl[O] TiAl2 TiAl3


Al

Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


kmol

0.50

File: C:\Users\u0956256\Dropbox (Heavystone)\Share-with-Xiaofang\HSC calculation\0.OGI kmol 0.50 Ar(g) Ti2O3 0.45

Ar(g)

0.45

Mole ratio of Al to TiO2=0.22

0.40 0.35


0.40 0.35

Ti3O5

0.30

0.30

0.25 0.20

File: C:\Users\u0956256\Dropbox (Heavystone)\Share-with-Xiaofang\HSC calculation\0.OGI

0.25 0.20

Ti4O7

0.15 Ti2O3 Al2O3 0.10

Al2O3

0.15 0.10

0.05

0.05

Ti3O5

Ti4O7

0.00 0 kmol

0.50

200

400

600

800

1000

Temperature C

File: C:\Users\u0956256\Dropbox (Heavystone)\Share-with-Xiaofang\HSC kmol calculation\0.OGI 1.0 TiO

Ar(g)

0.45

0.00 0

200

400


1000

Temperature C

File: C:\Users\u0956256\Dropbox (Heavystone)\Share-with-Xiaofang\HSC calculation\0.OGI

TiO

0.8


0.8

800

File: C:\Users\u0956256\Dropbox (Heavystone)\Share-with-Xiaofang\HSC kmolcalculation\0.OGI 0.9

0.9

0.40

600


0.7

TiO

0.35

0.7 Ti2O3

0.30

0.6

0.6 0.5

Ar(g)

0.25

Ar(g)

0.5 0.4

Al2O3

0.20

0.4

0.15

0.3

0.10

0.2

0.05

0.1

0.00 0

200

400

600

800

1000

Temperature 0.0 C 0

Al2O3

Al2O3

0.3 0.2 TiAl

0.1

200

400

600

800


1000

Temperature 0.0 C 0

200

400

600

800

1000

Temperature C


100

Absolute leaching rate, wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 44 of 46

80 60

Ti Al Reduction condition: 900C, 12h Al/TiO2 mole ratio: 0.22, 0.33, 0.45, 0.67, 0.89

40 20 0 0.2

0.3

0.4

0.5

0.6

0.7

Mole ratio of Al to TiO2 ACS Paragon Plus Environment

0.8

0.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Absolute leaching rate of Ti, wt%

Page 45 of 46

100

95

90

700 C 800 C 900 C

85

80 4

6

8

Time, h


10

12


'

'

'

Intensity, cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 46 of 46

'

'

' ' j h

j jh

''

''

UGS

Reduced slag j h j h j h h jj hh

' TiO2 h Al2O3 j Ti2O3

20

40

60

2T

q


80

Potentially More Ecofriendly Chemical Pathway for Production of High

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