Titanium Production via Titanium Sulfide

A new metallurgical process via titanium sulfide from ilmenite is proposed and experimentally approved: It consists of several stages; 1) The ilmenite ore is exposed to gaseous CS 2 to selectively sulfurize to FeS, which is wet-chemically removed. 2) The residual oxide is again exposed to CS 2 to form TiS 2 . 3) TiS 2 is electrochemically reduced to metallic Ti using molten CaCl 2 -CaS as an application of OS process. TiFeO 3 was exposed to Ar-CS 2 mixed gas flow at 1173 K to form the mixture of FeS+TiO 2 . FeS was easily separated by immersing in H 2 SO 4 solution at 313 K. After recovery of TiO 2 , it was converted completely to TiS 2 by the second sulfurization with CS 2 . TiS 2 could be reduced to Ti powder by calciothermic reduction and simulteneous electrolysis in a CaS-CaCl 2 melt for about 6 hours at 1173 K and 3.0 V. The impurity decreased to a low level such as 0.021 mass%S due to very small solubility of S in a-Ti. However, 1.06 mass%O remained because of wide solubility of oxygen in a-Ti and water contamination in initial CaCl 2 .


Introduction
Titanium dioxide converted from ilmenite (mainly FeTiO 3 ) has been normally taken as the starting material both for Kroll process [1] or for newly developing refining processes such as FFC Cambridge process [2] and OS process [3].In case of direct reduction from the oxide, the complete removal of oxygen from the obtained metallic Ti has not yet achieved although many researchers have challenged.The reasons of this oxygen contamination are thermodynamically wide solubility of oxygen in a-Ti and the technically insufficient oxygen removal from the reactors.Once the oxide is taken as the starting material of reduction, a fairly amount of oxygen still remains in the obtained Ti particles, although many operating parameters were optimized such as reaction temperature, time, applying voltage, gas environment, crucible, salt constitutions and their compositions, particle size, oxide phases, electrode materials and cell arrangements [4].
Kroll process removes oxygen from TiO 2 once by conversion to TiCl 4 and CO/CO 2 gas, and highly pure titanium metal is formed, because Ti does not dissolve Mg and Cl, and because the distillation of TiCl 4 is available to get the higher purity [1].In analogy with Kroll process, the authors proposed utilization of sulfur: Unfortunately there is no natural resource of Ti sulfides, but TiO 2 can be easily converted to TiS 2 and CO/CO 2 gas when CS 2 gas is used [5].The reduction of TiS 2 is expected as an application of FFC Cambridge process or OS process; the molten salt electrolysis of TiS 2 in CaCl 2 melt is applied to remove sulfur from the cathode as CaS, which is exhausted out as S 2 gas from the carbon anode [6,7].The extracted S 2 gas may be used to synthesis CS 2 for TiO 2 -TiS 2 conversion, but the simple mixture of carbon and S 2 gas can be thermodymically used for sulfization of TiO 2 [8].This may correspond to chlorination process in Kroll process.Fig. 1 shows the combinations of various reactions on Ti refining.Fig. 2 illustrates the reactions in case of reduction of TiS 2 in the molten CaCl 2 -CaS mixture [6,7].About 1.9 mol%CaS can dissolve in the CaCl 2 melt at 1173 K [9], and it decomposes to Ca 2+ and S 2-as shown in eq.(1).By applying a higher cell voltage than the theoretical decomposition voltage (2.1-2.2V at 1173 K [8]), metallic Ca and S 2 gas can be electrochemically precipitated on the cathode and anode surfaces, respectively, as expressed in eq.( 2) and (3).
CaS => Ca 2+ + S 2- (1) Ca 2+ + 2 e -=> Ca (at cathode) (2) S 2-=> 1/2 S 2 (g) + 2 e -(at anode) As considered in OS process [3,4], the deposited Ca from CaCl 2 melt again dissolves into the bulk due to the metallic solubility of Ca (about a few mol%).The dissolved Ca metal will be written here as Ca.It exists in the close vicinity of cathode, and thermochemically reacts with TiS 2 particles that were filled inside the cathode basket.
Ca (at cathode) => Ca (near cathode) (4) 2 Ca (near cathode) + TiS 2 (in cathodic basket) => 2 CaS + Ti (5) The product Ti precipitates as solid particles inside the cathodic basket.The another product, CaS, will dissolve by following eq.( 1).In this sequence, Ca circulates in the melt and does not go out from the reaction vessel.The deposited S 2 (g) in eq.( 3) will leave from the anodic surface to the melt surface and diffuses to the gas phase over the melt.Finally it deposits on the wall of reaction vessel at the cooled parts, and it will be detected as amorphous sulfur or crystalline S 8 (s).
For synthesis of TiCl 4 in Kroll process, chlorine gas should be safely circulated from the molten salt electrolysis without any leakage to the chlorination furnace.Because the melting point of TiS 2 is higher than 1273 K, the electrolysis and reduction can be operated as solid state of TiS 2 .A stockyard of solid TiS 2 seems safer and more stable than a tank of liquid TiCl 4 , and the circulation of solid sulfur seems not so serious in factory processing.It is also noted that a plenty amount of sulfur can be served from copper smelting industries.Once sulfur can be set in the material cycle, it is not necessary to charge the additional sulfur as the principle of this proposal.Although the reduction of Ti sulfide by the other alkaline earth metals such as Mg is applicable, the recycling of Mg may cause another issue, while Ca recycling is ready in OS process [3,4].
The purpose of this work is to confirm the experimental quality of Ti obtained by our proposed process when FeTiO 3 is taken as the starting material, although the fundamental reactions in molten salt electrolysis, (1)- (5), have already been studied by starting from the commercial TiS 2 powder [6,7].Conversion from TiO 2 to TiS 2 was reported using CS 2 gas, and this study takes this idea, although the starting oxide is FeTiO 3 .

Material and experiments
0.2 -1.0 g FeTiO 3 (Furuuchi Chemicals, >99.9%) was placed on the alumina or mullite boats and heated at 573 -1473 K in Ar-CS 2 gas flow, as illustrated in Fig. 3. Gas flow rate (20-100mL/min Ar) and reaction time (>1.8 ks) were chosen so that an excess amount of CS 2 (>99.0%)gas was carried into the furnace.The reduction experiments using 600 g CaCl 2 (Kanto Chemicals, >99.0%) -0.5 mol%CaS mixed salt in MgO crucible were conducted at 1173 K, 3.0 V using graphite anode (10 mm in diameter) and Ti basket-like cathode, as reported separately [6,7].The morphology, phases and metallic impurities were measured by SEM, XRD and XRF, respectively.The residual oxygen and sulfur were studied using LECO TC-600 and CS-600 analyzers, respectively.

Sulfurization of FeTiO 3
Powdery FeTiO 3 contained a small amount of TiO 2 , as confirmed by XRD measurement (Fig. 4).Volatile CS 2 in ice-water was carried by Ar gas bubbling into the furnace, where the FeTiO 3 powder was set on the ceramic boat.Fig. 4 shows also that the sample after sulfurization at 1073 K for 3.6 ks consisted of Fe 1-x S and TiO 2 .Selectively Fe was sulfurized by CS 2 and no trace of Ti sulfides was found by XRD measurements.Fig. 5 shows the mass change during sulfurization by varying the holding temperature.When we assume that whole parts of initial sample consist of pure FeTiO 3 and that this sample is converted into the stoichiometric FeS and TiO 2 , the conversion ratio of FeTiO 3 to FeS is defined as "yield of Fe 1-x S", y, as shown in Fig. 5. y becomes 100% at DW/W i = 14.93%,where the non-stoichiometry of Fe 1-x S, x = 0.14.Note that x = 0 at DW/W i = 10.59%.The sulfurization started at the temperature as low as 773 K, and a steady value of 100% conversion was obtained above 1173 K.The sample at 1473 K was partially melted because the melting point of FeS is 1465 K [8].By changing the Ar gas flow rate, the concentration of CS 2 in the mixed gas could be varied in the range of 25-34 vol%.However, DW/W i was in a small variation of 11.5-15% for 3.6 ks at 1173 K and Ti sulfides were never found by XRD.Even after a long sulfurization such as 14.4 ks, no Ti sulfides were formed.The deposition of solid sulfur was found at the low temperature area of downstream.Therefore, the corresponding reaction of sulfurization can be shown as eq.( 6) and/or (7).
Thermodynamically these three reactions should proceed forward, although some data on the lower sulfides in Ti-S system are missing [8].Therefore, the kinetic effect such as core-shell model is suspected that the converted FeS might cover the particle surface and prohibit further sulfurization.

Leaching of FeS in H 2 SO4 solution
In order to separate Fe from the metallic Ti in this proposal, it is necessary to remove Fe before electrolysis in the molten salt.Because the mixture of FeS and TiO 2 was obtained after CS 2 sulfurization, FeS removal was conducted in an acidic solution.FeS can dissolve quickly either in 1M HCl or 1 M H 2 SO 4 solutions, while FeTiO 3 was hardly dissolved in these solutions at room temperature.The sample sulfurized at 1073 K for 14.4 ks was leached in 1 M HCl solution for a day, and a fairly large amount of FeS remained.However, 1 M H 2 SO 4 solution could remove most of FeS phase at the same conditions as identified by XRD measurement (Fig. 6).In addition, the background intensity of XRD pattern was very high before leaching because of Fe fluorescence X-ray under Cu-Ka radiation.The residual Fe in the leached samples in 1 M H 2 SO 4 solution was therefore examined by XRF analysis as shown in Fig. 7.  Fe removal in 1 M H 2 SO 4 solution could be conducted at room temperature and the heating did not enhance the Fe dissolution.It is noteworthy that a wet chemical process to remove Fe from FeTiO 3 is normally operated above 473 K in a conc.H 2 SO 4 for a several days.FeS removal in this study was effectively quick to promote Fe separation.As seen in Fig. 7, 0.28 mass%Fe leached for 172.8 ks at 313 K satisfied with the regulation for 3rd class of Japanese Industrial Standard (JIS) titanium.The residue in 1 M H 2 SO 4 solution was coarse TiO 2 particles with about 10 mm in diameter, although its surface was rough suitable for successive gas reaction.

Secondary sulfurization by CS 2 gas
The FeTiO 3 samples sulfurized at a certain temperature by CS 2 gas flow for 14.4 ks were leached to form TiO 2 , which were again sulfurized at the same temperature in CS 2 gas flow.Ti 1.083 S 2 , Ti 2.45 S 4 and Ti 3 S 4 were identified at 1073 K, 1173-1273 K and 1373-1473K, respectively, by XRD.
Fig. 8 shows the mass gain of the samples during sulfurization.At the higher temperatures, the lower sulfides in the Ti-S binary system appeared, and the mass change became the smaller.A small amount of SiO 2 and Al 2 O 3 were detected at the high temperature samples, which may originate from the impurity in the initial FeTiO 3 , or from the unexpected reaction with the boats.Fig. 9 shows the analysis of oxygen and sulfur of the samples reacted for 14.4 ks.At the higher temperatures, the sulfur content decreased, and oxygen content increased significantly due to the reaction with the ceramic boats.Therefore, the conditions at 1173 K for 14.4 ks were taken as an optimal condition for the subsequent study.The sample at these conditions contained 3.58 mass%O and 46.3 mass%S, although the lowest oxygen content is desired.It is also noteworthy that the greater concentration of sulfur needs the larger amount of electricity in electrochemical reduction of molten salt.
After optimizing the other sulfurization parameters, two samples were prepared with about 1.5 g from FeTiO 3 at 1173 K for 43.2 ks by feeding CS 2 at 293 K in Ar 20mL/min flow.Their phase identification and impurity analysis are given in Table 1.They were used in the reduction experiments as described later.It is noted that the sulfurized powder from the commercial TiS 2 powder contained 0.43 mass%O and 58.4 mass%S in the same conditions [6,7].The oxygen level below 0.4 mass%O were easily achieved at these conditions if TiO 2 powder was taken as the starting material.Fig. 10 shows the SEM morphology of the sample obtained after second sulfurization at 1073 K for 14.4 ks.The secondary grain size was about 10 mm, and the primary particles were hexagonal plates of 100-200 nm size.This hexagonal thin plate-like character is common in transition metal chalcogenides (Ch) with formula of MCh 2 .The "polytype" of Ti sulfides exist in Ti-S binary system, and they consist of multi-stacking layers of closed hexagonal packing and fcc packing.Therefore, the sulfur concentration is varied in a wide range.

Electrolysis and Reduction in CaCl 2 -CaS melt
The samples doubly sulfurized in CS 2 gas (#T and #U) were reduced at 1173 K, at 3.0 V in CaCl 2 -0.5mol%CaS.The electric charge Q was served until Q/Q 0 = 400 %, where Q 0 is defined as the theoretical charge to form the necessary amount of Ca by considering the reduction of pure and stoichiometric TiS 2 .As shown in Fig. 11, the current during the electrolysis with a constant voltage dropped at the initial stage, and approached to a constant value at Q/Q 0 = 100 %.The measured current vibrated a little probably because of the gas bubbling at the anode.After cooling, yellow sulfur was deposited on the inner surface of cooled parts of reaction vessel.The solidified salt and the sample were removed from the Ti basket in the dilute acetic acid with a slight smell of sulfur.After several rinsing with water, ethanol and acetone, the sample was dried and analyzed as listed in Table 1.Very low sulfur concentration reflects the non-soluble nature of sulfur in Ti.It proved that the present proposal can work from ilmenite, and that sulfur acts as the media to produce a high quality of metallic titanium with a very low sulfur concentration.Carbon contamination might come from the graphite anode through the electrolysis of CO32- [3,4].It is also possible that CS2 gas may be formed at the anode from the reaction of anodic graphite and S2-.In the used alignment for electrolysis, this CS2 gas can react with the obtained Ti to form TiC. These carbon contaminations can be improved by the cell modification for electrolysis.
Oxygen contamination may come from the residual oxygen in sulfurization and partially from the water in CaCl2 [4].Thus some extra-tuning in operating conditions of sulfurization, iron removal and electrolysis is needed.Fig. 12 shows the comparison of titanium metallic particles obtained via TiS2.Fig. 12(a) and (b) show the powders starting from the commercial TiO2 and from ilmenite, respectively, at the same electrochemical conditions.Reflecting the hexagonal plates of Ti sulfides, these Ti powders showed the porous morphology that some nodules jointed with a central spherical ball.This characteristic feature of Ti powder is expected suitable for 3D printer application.

Conclusion
Using CS2 gas evaporated at room temperature, ilmenite was easily sulfurized to the mixture of Fe1-xS and TiO2 (rutile).Fe1-xS was selectively removed by 1 M H2SO4 solution even at room temperature to the suitable level of 0.5 mass%Fe in Ti.Heating and violent stirring were not needed.After second sulfurization by CS2 gas, Ti sulfides lower than TiS2 were obtained and this mixture was electrochemically reduced in the molten CaCl2-CaS at 1173 K. Successfully -Ti was obtained and the residual sulfur in Ti was as low as 0.021 mass%S.The by-product S2(gas) in this work should be served to synthesis CS2, which has already utilized in the chemical industry.

Fig. 4
Fig. 4 XRD patterns of starting sample and the sample sulfurized in CS 2 gas flow at 1073 K.

Fig. 9
Fig. 9 Sulfur and oxygen analysis of secondary sulfurized samples on the ceramic boats.

Fig. 10
Fig. 10 SEM observation of the sample doubly sulfurized in CS 2 gas flow at 1073 K for 14.4 ks.

Fig. 12
Fig. 12 SEM observation of the obtained titanium powder via TiS2 (a) from TiO2 and (b) from ilmenite.