© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_60

Calciothermic Reduction and Electrolysis of Sulfides in CaCl2 Melt

Ryosuke O. Suzuki1  , Nobuyoshi Suzuki1, Yuta Yashima1, Shungo Natsui1 and Tatsuya Kikuchi1
(1)
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo Hokkaido, 060-8628, Japan
 
 
Ryosuke O. Suzuki
Email: rsuzuki@eng.hokudai.ac.jp

Abstract

A new metallurgical process via sulfide is proposed: a sulfide is produced at a high temperature from its metallic oxide using gaseous CS2, and this sulfide is electrochemically reduced to its metallic state using molten salt . This combined process via sulfide is effective to obtain high purity of metallic powder, even if the metal in its oxide is strongly combined with oxygen. For example, it is not easy to reduce stable oxide TiO2 , and only Ca can remove oxygen to form α-Ti. However, a fairly large amount of oxygen remains as Ti–O solid solution. Because the solubility of S in Ti is very small, this proposal was examined experimentally both on the conversion of TiO2 to TiS2 and on the successive reduction of TiS2 to Ti. TiO2 powder was exposed to CS2 gas flow at 1073 K, and the conversion to TiS2 was confirmed. TiS2 could be reduced to Ti powder either by calciothermic reduction or electrolysis in a CaCl2 melt. By Ca reduction at 1133 K in CaCl2 melt, sulfur concentration decreased to 0.03 mass%S when the amount greater than twice the stoichiometric calcium amount is added. By electrochemical reduction at 1173 K in CaCl2–CaS melt, S concentration significantly decreased to 0.01 mass%S when four times larger amount of electric charge was supplied.

Keywords

Titanium disulfideCalciothermic reductionMolten salt

Introduction

Direct reduction of oxide Metallic sulfides are commonly distributed as natural minerals and used as important resources for metal production (such as Zn and Cu). However, some elements do not form the stable sulfides but exist as the oxides in nature. For example, Ti, V, Nb and Ta have their stable oxides as the ore of metal production. These oxides are reduced by Al or Mg not by carbon because of their strong thermodynamical stability. In order to get highly pure metals (for example, <0.3 mass%O in Ti), their oxides are once converted to chloride and repeatedly refined by distillation to eliminate their impurities. The pure chlorides are then reduced by Mg to obtain the metallic state. The by-product MgCl2 after the Mg reduction is electrochemically decomposed in a MgCl2 melt to recycle to the reactant Mg. In the refining industries of the oxide-stable elements, the molten salt electrolysis of chloride is indispensable and the efficient electrolysis has been developed, for example, in Kroll process [1] for Ti production.

Recently the direct electrolysis of their oxides has been extensively studied as known as FFC Cambridge process [2–4], OS process [5–7], EMR [8], etc. Figure 1a illustrates the concept of OS process. These new refining processes intended to remove oxygen directly from the oxide as CaO using the CaCl2 melt, because of the wide solubility of CaO in CaCl2 [9]. However, these metals have the large solid solubility of oxygen [10], and a fairly large amount of oxygen remained after oxide reduction (For example, 1.0 mass%O). The optimization and industrial application of oxide reduction have been conducted over 20 years by many researchers, but there still remain the quality issues [11–16].
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Fig. 1

Concepts of a oxide reduction by molten salt electrolysis in CaCl2 (OS process) and b sulfide reduction to obtain the metallic Ti particles. Oxygen and sulfur anion react with carbon anode to form a CO/CO2 and b S2/CS2, respectively

Merit of reduction of sulfide In contrast, the solubilities of sulfur in metallic Ti, V and Nb are very small [10], and the serious effects of residual sulfur was not known in pure Ti. Sulfur locates just below oxygen in the periodic table, and many similarities may be found in thermochemical properties. If the reduction of oxide can be replaced by the reduction of sulfide , a higher quality of metal production can be expected from the sulfide . The thermochemical stability of oxides does not allow the reduction of hydrogen and carbon, and the strong reductant such as Ca is required. Therefore, the electrochemical technology of direct oxide reduction [5–7] can be applied to the reduction of sulfides, as shown in Fig. 1b.

Reduction of sulfide Firstly the Ca reduction of sulfide [17, 18] is considered as analogy with reduction of oxides. A few mol%CaS can dissolve in molten CaCl2 [17] and ionize to Ca2+ and S2−. The theoretical decomposition voltage of CaS is smaller than that of CaCl2 [19], and CaS can deposit as Ca and S2 at cathode and anode, respectively, as shown in Fig. 1b. S2 gas on the carbon anode may react with C to form CS2 gas. These gases are exhausted out from the vessel. Ca droplets precipitated on the cathode quickly dissolves into the CaCl2 melt due to a large solubility of Ca (a few mol%Ca at 1173 K). The reducing environment containing the dilute metallic Ca reacts with TiS2 particles close to the cathode and sulfur will be removed as CaS, which dissolves into the melt. The reactions in the vessel are as,
$$ {\text{CaS}}\left( {{\text{in CaCl}}_{ 2} } \right) = {\text{Ca}}^{ 2+ } + {\text{S}}^{ 2- } $$
(1)
$$ 2 {\text{ S}}^{ 2- } = {\text{S}}_{ 2} \left( {\text{g}} \right) + 4{\text{e}}^{ - } ,{\text{and S}}_{ 2} \left( {\text{g}} \right) + {\text{C}} = {\text{CS}}_{ 2} \left( {\text{g}} \right) \left( {\text{at anode}} \right) $$
(2)
$$ {\text{Ca}}^{{ 2 { + }}} + 2 {\text{ e}}^{ - } = {\text{Ca}}\left( {{\text{liq}} .} \right)\left( {{\text{at}}\,{\text{cathode}}} \right) $$
(3)
$$ {\text{Ca}}\left( {{\text{liq}} .} \right) = {\text{Ca}}\left( {\text{near cathode}} \right) $$
(4)
$$ 2\underline{\text{Ca}} \left( {\text{near cathode}} \right) + {\text{TiS}}_{ 2} \left( {\text{near cathode}} \right) = {\text{Ti}} + 2 {\text{ CaS}}\left( {{\text{in CaCl}}_{ 2} } \right) $$
(5)
Formation of sulfide The second point of our proposal is the formation of sulfide of Ti, V and Nb, not from the metal but from the oxide. Because of strong thermodynamic stability of these oxides, the conversion to sulfide is not easy thermodynamically [19] by pure sulfur gas in an encapsulated tube. It was reported that CS2 gas flow can replace oxygen in oxide by sulfur [20–22]. In case of TiO2 it is as,
$$ {\text{TiO}}_{ 2} + {\text{CS}}_{ 2} \left( {\text{g}} \right) = {\text{TiS}}_{ 2} + {\text{CO}}\left( {\text{g}} \right)/{\text{CO}}_{ 2} \left( {\text{g}} \right) $$
(6)
Using CS2 gas, many complex sulfides (such as lanthanoids) were synthesized from various oxides and applied as thermoelectric materials [20–22]. Because CS2 liquid is usual solvent in organic film or fiber chemistry and has high vapor pressure at room temperature [19], it is handy and cost-affordable material for sulfidation. As shown in Fig. 2, CS2(g) produced by electrolysis at the second step may be recycled.
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Fig. 2

Concept of oxide sulfidation by CS2 gas and sulfide reduction to obtain the metal. The exhausted CS2 gas in the second step can be used in the first step

Purpose The purpose of this work is to show the applicability of our proposal consisting of two steps: sulfidation of TiO2 and reduction of TiS2. The conversion of TiO2 to TiS2 was shortly confirmed here. The successive reduction of TiS2 to Ti was mainly tested by two methods, either by calciothermic reduction or electrochemical reaction in the CaCl2 melt. The details were separately reported in [17, 18].

Experimental

Figure 3a shows the reacting vessel used for Ca reduction . A stainless steel crucible (70 mm ID, 100 mm high) was filled with TiS2 powder, lumps of calcium, and CaCl2 powder. This sample mixture was dehydrated in vacuum and then heated at 1133 K for 30 min. Figure 3b shows the electrochemical cell used for reduction . A MgO crucible (90 mm ID, 200 mm high) was set up after being filled with precisely weighed CaCl2–CaS mixed powder (~600 g). The salt was dehydrated in vacuum and heated to 1173 K. The anode was a carbon bar (10 mm in diameter), and the basket-like cathode consisted of Ti net, in which about 1.5 g of TiS2 powder was packed. Both electrodes were attached to Ti rods.
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Fig. 3

Experimental setup for calciothermic reduction in stainless steel crucible (a), and electrochemical reduction in dense MgO crucible (b), using CaCl2 melt in Ar gas

Constant voltage of 3.0 V was applied between the two electrodes at 1173 K, and terminated after a certain amount of electricity was supplied. The solidified salt was removed by wet-chemical way and dried in vacuum. For sulfuration, about 1.5 g of TiO2 (rutile) powder was filled in Al2O3 boat, which was heated at 1073 K in Ar. The feeding Ar gas was firstly blown into the CS2 liquid kept at 273 K. After optimizing Ar gas flow rate, the gas mixture with a constant concentration of CS2 was fed to the furnace . After heating for a few hours, the sample was cooled in Ar. The sulfur and oxygen content were determined using LECO-CS600 and TC600 analyzers, and the phases and structure were characterized by X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM ).

Results and Discussion

Calciothermic Reduction

Figure 4 shows the relationship among the concentrations of sulfur , oxygen, and the charged amount of Ca [17, 18]. The stoichiometric Ca quantity was considered as r = 2 at the calciothermic reaction,
../images/468727_1_En_60_Chapter/468727_1_En_60_Fig4_HTML.gif
Fig. 4

Sulfur and oxygen concentration after Ca reduction using CaCl2 melt. The analytical error was within the size of makers. Molar ratio of Ca against TiS2, r, was taken to normalize the deviation of charge. r = 2 means that the stoichiometric amount of Ca reacts with the charged amount of TiS2. Phase identification by XRD was inserted

$$ {\text{TiS}}_{ 2} + 2\,{\text{Ca }} = {\text{Ti}} + 2 {\text{ CaS}}\left( {{\text{in}}\,{\text{CaCl}}_{ 2} } \right) $$
(7)
where the ratio, r, is taken to express the molar quantity of Ca charge. The concentration of sulfur decreased significantly in the Ca-poor region (r < 2), and 0.03 mass%S was obtained at r = 4. The formation of α-Ti was identified at r = 2 (stoichiometric ratio) and r = 4 by XRD. TiH was found at r = 4. This is because the excess amount of Ca reacted with washing water and generated hydrogen gas to form TiH. On the other hand, residual oxygen was observed over the entire range of r, and the minimum content was 0.5 mass%O. This residual oxygen may originate as a major impurity in raw materials or by contamination during wet chemical treatment for salt removal . It is noted that the oxygen content was analyzed as 20.9 mass%O at r = 0. Without Ca addition, TiS2 was easily oxidized to TiO2 at high temperatures although Ar gas was used. Covering by CaCl2 melt was slightly effective to suppress the oxidation .

Electrochemical Reduction

Figure 5 shows the concentrations of sulfur and oxygen with their identified phases for the samples. Here, Q represents the supplied charge in the experiment, and Q0 denotes the stoichiometric charge corresponding the reactions (1)–(5). α-Ti was identified at Q > Q0, and lower titanium oxides were detected at Q < Q0. No lower sulfide was found, similar to the case of the calciothermic reduction as above-mentioned. The concentration of sulfur decreased significantly; a very low level of sulfur such as 0.01 mass%S was achieved at Q = 4 Q0. However, the concentration of oxygen remained high, for example, 16.8 and 5.0 mass%O at Q/Q0 = 0.5 and 1.0, respectively. The concentration of oxygen did not greatly change, and it did not decrease below 1 mass%O for Q > Q0.
../images/468727_1_En_60_Chapter/468727_1_En_60_Fig5_HTML.gif
Fig. 5

Sulfur and oxygen concentration after electrochemical reduction applying 3.0 V at 1173 K using 0.5 mol%CaS–CaCl2 melt [17]. Supplied charge Q is normalized by the stoichiometric amount of charge Q0 for reduction. Q/Q0 = 1 means that the stoichiometric amount of charge to reduce the charged amount of TiS2

When CaS was added, the current during electrolysis increased and the electrolysis time necessary for TiS2 reduction became shorter [17, 18]. For example, this time can decrease to a quarter of that at 0.5 mol% CaS. However, the residual sulfur concentration in the samples also increased, and the concentration of oxygen remained as high as 1 mass%O.

Figure 6 shows the SEM image of the sample surfaces. The plate-like morphology of TiS2 maintained even after the reduction at Q/Q0 = 2, and the thin plates of metallic Ti have many small pores. This curious morphology disappeared due to sintering when the longer time was taken for electrolysis.
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Fig. 6

SEM image of the samples before and after electrolysis of TiS2

Sulfuration of TiO2

By controlling Ar gas flow rate, evaporation rate of CS2 could be set constant. Gas mixture of Ar and CS2 was continuously supplied to the furnace . After heating for 7.2 ks at 1073 K, TiO2 was mostly converted to TiS2, as shown in Figs. 7 and 8. About 10 times larger amount of CS2 than stoichiometric amount was blown to TiO2 for 14.4 ks, and the complete conversion was obtained. However, judging from the mass change of whole sample, 50–80% of exposed TiO2 powder was converted to TiS2. Some parts in the Al2O3 boat still remain white, which is characteristic of TiO2 . This inhomogeneity in the Al2O3 boat will be solved by rising temperature or by using fluidized bed method, as seen with the chlorination of TiO2 in Kroll process [1].
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Fig. 7

XRD patterns of the samples converted from TiO2 powder. The power on alumina boat was exposed under constant flow of CS2 and Ar gas mixture at 1073 K for 7.2 and 14.4 ks. The black specimens were taken from the upper part of powder at the central part of the boat

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Fig. 8

SEM image of the sample converted from TiO2 powder. The power on alumina boat was exposed under constant flow of CS2 and Ar gas mixture at 1073 K for 14.4 ks. A crystal growth with hexagonal symmetry is a characteristic for TiS2 crystal

Thus the optimal conditions for sulfidation still remain as future’s work, but the new concept of sulfide usage was well established from our experimental evidences. The pursuit of higher quality titanium via sulfide will be studied soon.

Conclusion

The commercial TiO2 with rutile structure was favorably sulfidated to TiS2 at 1073 K by CS2 gas carried by Ar. In addition, we examined the reduction of TiS2 both by metallic calcium in molten CaCl2 and by electrolysis in CaCl2–CaS molten salt . The residual concentrations of sulfur and oxygen in the obtained samples were analyzed, and the phase constitutions were identified by XRD . With the use of calciothermic reduction , a single phase of metallic titanium was obtained with 0.03 mass%S. With the use of electrolysis, a very low impurity such as 0.01 mass%S was obtained when the supplied charge was Q = 4 Q0. A single phase of metallic titanium was obtained when the supplied charge Q was greater than 2 Q0. The concentration of oxygen decreased to 0.5 mass%O with calciothermic reduction and to 1.0 mass%O with electrolysis. The further addition of CaS in the melt can decrease the electrolysis time for reduction .

We believe that our experimental results significantly contribute to improve the proposal to obtain titanium via TiS2.

Acknowledgements

This work was partly supported by the “Innovative Structural Materials Project (ISMA, Future Pioneering Projects)” and KOBE STEEL, LTD. under contact No. 14100139-b, and by JSPS KAKENHI (Grant Number 17H0343407).