Preparation of Synthetic Titania Slag Relevant to the Industrial Smelting Process Using an Induction Furnace

applied sciences

Article Preparation of Synthetic Titania Slag Relevant to the Industrial Smelting Process Using an Induction Furnace

Avishek Kumar Gupta 1,* , Matti Aula 1, Jouni Pihlasalo 2, Pasi Mäkelä 2, Marko Huttula 3 and Timo Fabritius 1

1 Process Metallurgy Research Unit, University of Oulu, P.O. Box 4300, FI-90014, 90570 Oulu, Finland; matti.aula@oulu.fi (M.A.); timo.fabritius@oulu.fi (T.F.) 2 Metso Outotec Research Center, Kuparitie 10, 28330 Pori, Finland; [email protected] (J.P.); [email protected] (P.M.) 3 Nano and Molecular Systems Research Unit, University of Oulu, P.O. Box 4300, FI-90014, 90570 Oulu, Finland; Marko.Huttula@oulu.fi * Correspondence: avishek.gupta@oulu.fi; Tel.: +358-469231085

Abstract: A high titania slag that is used as a feedstock for TiO2 manufacturing is obtained by ilmenite smelting (FeO.TiO2). The composition of the slag obtained by smelting is dependent on the composition of the mineral used for slag preparation, i.e., ilmenite in our study. At the laboratory scale, ilmenite slags are mostly obtained by using ilmenite as the raw material. An easy and simple way would be to prepare the synthetic slag using the individual components and heating them to high temperature in a furnace. The titania slag has a high oxidizing nature and requires an inert atmosphere to prevent oxidation of the slag as well as the molybdenum crucible. This paper describes the preparation of synthetic ilmenite slag using an induction furnace and the study of the composition and the phases formed in the slag. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were

used as analytical techniques for studying the slag. A comparison between obtained synthetic slag and industrial ilmenite slag was performed to test the possibility of preparing slags in the laboratory

Citation: Gupta, A.K.; Aula, M.; as per the required composition. The slags show similar phase formation as obtained in industrial Pihlasalo, J.; Mäkelä, P.; Huttula, M.; ilmenite slags, which means that the synthetic slags are identical to the industrial slags. Fabritius, T. Preparation of Synthetic Titania Slag Relevant to the Industrial Keywords: ilmenite; pyrometallurgy; titania slag; smelting; pseudobrookite Smelting Process Using an Induction Furnace. Appl. Sci. 2021, 11, 1153. https://doi.org/10.3390/app11031153 1. Introduction Received: 31 December 2020 Ilmenite with a composition of FeO.TiO2 is one of the major sources of titanium metal Accepted: 25 January 2021 and titanium dioxide, which is used as a pigment in paints, cosmetics, plastics, rubber, and Published: 27 January 2021 textiles [1–3]. To produce titanium dioxide, the ilmenite feed is first upgraded to a titanium oxide product [4,5]. From the upgraded product, TiO2 is commercially produced either by Publisher’s Note: MDPI stays neutral the sulfate or chloride route [6,7]. The sulfate route, though it can use lower grade ores, with regard to jurisdictional claims in is generally expensive and less environmentally friendly, whereas the chloride route is published maps and institutional affil- iations. limited to high-grade feedstocks as the impurities can affect the stability of the fluidized bed [8]. The up-gradation of ilmenite is performed by carbothermic reduction to remove iron oxide. It can be conducted either by solid-state reduction at around 1200 ◦C, where synthetic rutile is produced or in a fully liquid state above 1600 ◦C where a high titania slag is produced with pig iron as a by-product [9–11]. In the smelting of ilmenite, the iron Copyright: © 2021 by the authors. oxide in ilmenite is reduced by a suitable carbonaceous reductant to metallic iron, resulting Licensee MDPI, Basel, Switzerland. in a high titania slag containing 75–95% TiO2, of which slags with titania content above This article is an open access article 85% are used for the chloride route and others for the sulfate route [12]. The reductant distributed under the terms and conditions of the Creative Commons used is a carbonaceous material, such as anthracite, petroleum coke, metallurgical coke, Attribution (CC BY) license (https:// etc. [13]. The smelting process of ilmenite is an unusual case where slag is the primary creativecommons.org/licenses/by/ product, and the by-product is a valuable raw material to get pig iron and other metallic 4.0/). components [14,15].

Appl. Sci. 2021, 11, 1153. https://doi.org/10.3390/app11031153 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1153 2 of 13

The titania slag forms a single phase with a stoichiometry of M3O5 [16] and is a molten mixture of Ti3O5, FeTi2O5, MnTi2O5, Al2TiO5, MgTi2O5,V2TiO5, and Cr2TiO5.M3O5 is a solid solution, which is called pseudobrookite, and is present in major quantities in the slag with a small amount of rutile and ilmenite [17–19]. Pseudobrookite is only formed when the reduction of ilmenite happens at a temperature f more than 1200 ◦C, below which rutile and ilmenite are the dominant phases [20,21]. The trivalent cations in the slag are easily oxidized if the slag comes into contact with oxygen or any oxidizer and the oxidation reaction is highly exothermic [22,23]. As the slag is very easily oxidized, an inert gas (such as Ar) is used when the slag is produced in the laboratory to prevent oxidation. The impurities in ilmenite, such as CaO, MgO, Al2O3, SiO2, and MnO2, are carried into the slag and can affect the slag if present in large quantities [24]. However, most of the ilmenite ores have a concentration of the impurities in quantities that do not affect the slag formation or its properties signiﬁcantly. Pesl and Eric [25] produced synthetic slag for preparing ternary diagrams using TiO2, Ti2O3, Fe, and Fe2O3 but without any other impurities. No further data were found in the literature where the liquid slag phase boundary was measured using saturation experiments. Pesl and Eric [25] investigated the slag model to assess the slag condition based on the ternary diagram. In the present work, several impurities will be used to replicate the industrial slag system. Such slag samples can be utilized to prepare calibration curves using various spectroscopic techniques, thus improving the accuracy of the system. In this paper, the phase study of synthetic ilmenite slags that were prepared in an induction furnace under a controlled argon atmosphere is reported. A phase study is carried out to determine the phases that are formed and to verify their similarity with the industrial slag. M3O5 slag composition is a characteristic for ilmenite slags, and the effect of the quantity of various components on the phases formed are studied. The studied method provides an alternative way to make synthetic slags, which is cheaper and easy to prepare.

2. Materials and Methods 2.1. Apparatus and Material An induction furnace (Type TNX 60, Plustherm Point GmBH, Wettingen, Switzerland) with a recrystallized alumina work tube was used. A schematic of the furnace is shown in Figure1. A molybdenum crucible was used as the inner crucible for the preparation of the slag. The furnace was water cooled and had openings to facilitate the introduction of an inert gas in the crucible and thermocouple. The temperature was measured by a B-type thermocouple, which was checked against a calibrated standard couple. The synthetic samples were made from mixtures of TiO2 (>99.5 pct), Ti2O3 (>99.9 pct), FeO (>99.5 pct), MgO (>99.9 pct), MnO (>99.9 pct), SiO2 (>99.9 pct), and Al2O3 (>99.5 pct). The concentration of components used for each test is described in Table1. The percentage composition of TiO2 and Ti2O3 selected for each test was comparable to the actual industrial slag. The presence of FeO can play an important role in the slag formation, so a wide range (1% to 15%) of composition was selected. The MgO composition in industrial slag is around 1–2%, but there can be an increase in composition because the ilmenite smelter operates with a freeze lining of a solidiﬁed slag against the furnace wall [15]. Any damage caused to the freeze lining may result in a higher concentration of MgO in the slag. To accommodate this variation, the percentage composition of MgO was from 0.6% to 10%. All the other impurities were within the range of industrial slags. ◦ ◦ A FeO-TiO2-Ti2O3 ternary phase diagram at 1600 C and 1500 C is shown in Figures2 and3, respectively, which were prepared with FactSage version 6.4 using FactPS and FToxid databases. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 15 Appl. Sci. 2021, 11, 1153 3 of 13

Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 15

Table 1. Sample weight of components (in grams) and the liquidus temperature for each test.

Weight in Grams Sample Temp (°C) TiO2 Ti2O3 FeO MgO MnO SiO2 Al2O3 1 55.7 34 5 0.6 1.5 2 1.2 1670 2 59.4 28 7 0.8 1.8 1.5 1.5 1690 3 59.9 30 3 1.2 2 1.9 2 1725 4 59.0 24 7 5.5 0.5 2 2 1720 5 52.3 32 9 1 2.2 1.7 1.8 1620 6 61.2 15 7 10 0.8 3 3 1745 7 68.8 18 1 5 1 4 2.2 1790 8 58.9 26 11 0.8 1.3 1 1 1660 9 55.4 20 13 6 0.3 2.5 2.8 1640 10 55.2 23 15 1 1.7 2.5 1.6 1620

A FeO-TiO2-Ti2O3 ternary phase diagram at 1600 °C and 1500 °C is shown in Figures Figure 1. Schematic diagram of the induction furnace used for the preparation of the slags. 2 and 3, respectively, which were prepared with FactSage version 6.4 using FactPS and FToxid databases. Figure 1. Schematic diagram of the induction furnace used for the preparation of the slags.

The synthetic samples were made from mixtures of TiO2 (>99.5 pct), Ti2O3 (>99.9 pct), FeO (>99.5 pct), MgO (>99.9 pct), MnO (>99.9 pct), SiO2 (>99.9 pct), and Al2O3 (>99.5 pct). The concentration of components used for each test is described in Table 1. The percentage composition of TiO2 and Ti2O3 selected for each test was comparable to the actual industrial slag. The presence of FeO can play an important role in the slag formation, so a wide range (1% to 15%) of composition was selected. The MgO composition in industrial slag is around 1–2%, but there can be an increase in composition because the ilmenite smelter operates with a freeze lining of a solidified slag against the furnace wall [15]. Any damage caused to the freeze lining may result in a higher concentration of MgO in the slag. To accommodate this variation, the percentage composition of MgO was from 0.6% to 10%. All the other impurities were within the range of industrial slags.

◦ Figure 2. FeO-TiO2-Ti2O3 ternary phase diagram at 1600 C using FactSage version 6.4. Figure 2. FeO-TiO2-Ti2O3 ternary phase diagram at 1600 °C using FactSage version 6.4.

The constitution of the ternary phases in the system was similar between 1500 °C and 1600 °C. Pseudobrookite formation was more prominent at 1500 °C with a higher concentration of TiO2 in the system [26]. However, the liquid slag phase was enlarged at 1600 °C, and the magneli phases were solidified, as shown in Figure 2. The bordering phase regions were shifted, accordingly, too. Appl. Sci. 2021, 11, 1153 4 of 13

Table 1. Sample weight of components (in grams) and the liquidus temperature for each test.

Weight in Grams Temp Sample ◦ TiO2 Ti2O3 FeO MgO MnO SiO2 Al2O3 ( C) 1 55.7 34 5 0.6 1.5 2 1.2 1670 2 59.4 28 7 0.8 1.8 1.5 1.5 1690 3 59.9 30 3 1.2 2 1.9 2 1725 4 59.0 24 7 5.5 0.5 2 2 1720 5 52.3 32 9 1 2.2 1.7 1.8 1620 6 61.2 15 7 10 0.8 3 3 1745 7 68.8 18 1 5 1 4 2.2 1790 8 58.9 26 11 0.8 1.3 1 1 1660 9 55.4 20 13 6 0.3 2.5 2.8 1640 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 15 10 55.2 23 15 1 1.7 2.5 1.6 1620

◦ Figure 3. FeO-TiO2-Ti2O3 ternary phase diagram at 1500 C using FactSage version 6.4. Figure 3. FeO-TiO2-Ti2O3 ternary phase diagram at 1500 °C using FactSage version 6.4. The constitution of the ternary phases in the system was similar between 1500 ◦C andThe 1600 temperature◦C. Pseudobrookite (in °C) mentioned formation was was the more liquidus prominent temperature at 1500 for◦C each with sample a higher, whichconcentration was determined of TiO2 basedin the on system the theoretically [26]. However, calculated the liquid liquidus slag contour phase was diagram enlarged, as ◦ shownat 1600 in C,Figure and the4. The magneli calculation phases of were the solidiﬁed,diagram was as shown carried in out Figure with2. TheMTDATA bordering 6.0 softwarephase regions [27] using were a shifted,non-commercial accordingly, HTOX too. 9.0 database [28] for oxide and sulfide sys- ◦ tems. ItThe was temperature important to (in keepC) mentioneda check on the was liquidus the liquidus temperature temperature so that for the each complete sample, meltingwhich wasof the determined slag was achieved. based on This the theoreticallywas especially calculated so in the liquiduslaboratory contour furnace diagram, as the holdingas shown period in Figurewas just4. half The an calculation hour to achieve of the equilibrium. diagram was Samples carried with out low with FeO MTDATA started to6.0 melt software at lower [27 temperatures] using a non-commercial when compared HTOX to samples 9.0 database with [higher28] for FeO oxide content. and sulﬁde For thesystems. experiment, It was the important furnace towas keep heated a check to a on temperature the liquidus equal temperature to 20 °C soabove that the the liquidus complete temperaturemelting of theof the slag test was sampl achieved.e to make This sure was that especially the liquidus so intemperature the laboratory was achieved. furnace as Anthe S- holdingtype thermocouple period was was just halfplaced an near hour the to achievecrucible equilibrium. to measure the Samples temperature with low in the FeO furnace.started to melt at lower temperatures when compared to samples with higher FeO content. Appl. Sci. 2021, 11, 1153 5 of 13

For the experiment, the furnace was heated to a temperature equal to 20 ◦C above the liquidus temperature of the test sample to make sure that the liquidus temperature was Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 15 achieved. An S-type thermocouple was placed near the crucible to measure the temperature in the furnace.

FigureFigure 4.4. Liquidus contour diagram that waswas usedused forfor thethe calculationcalculation ofof liquidusliquidus temperature.temperature. The compositioncomposition of allall samplessamples isis shownshown inin thethe ﬁgure,figure, whichwhich waswas usedused toto determinedetermine thethe temperature.temperature.

2.2. Procedure 2.2. Procedure The reagents for the synthetic slag preparation were weighed and thoroughly mixed The reagents for the synthetic slag preparation were weighed and thoroughly mixed to form a uniform mixture weighing 100 g. The mixture was then charged in a 125 mL to form a uniform mixture weighing 100 g. The mixture was then charged in a 125 mL molybdenum crucible, which was placed in the induction furnace. Argon gas with a flow molybdenum crucible, which was placed in the induction furnace. Argon gas with a ﬂow rate of 1 L/min was purged above the insulating wool and inside the crucible to ensure an rate of 1 L/min was purged above the insulating wool and inside the crucible to ensure an inert gas atmosphere that prevented any oxidation of molybdenum crucible at a higher inert gas atmosphere that prevented any oxidation of molybdenum crucible at a higher temperature. The furnace was heated to the required temperature as mentioned above, temperature. The furnace was heated to the required temperature as mentioned above, and and a holding period of 30 min was given to attain equilibrium for fully liquid slags. A a holding period of 30 min was given to attain equilibrium for fully liquid slags. A quartz quartz tube was used to take a sample of the molten slag from the crucible. The molten tube was used to take a sample of the molten slag from the crucible. The molten slag, with slag, with the quartz tube, was quenched in a cold-water bucket. The quartz tube was the quartz tube, was quenched in a cold-water bucket. The quartz tube was broken to get broken to get the slag sample, which was a cylindrical slag sample. the slag sample, which was a cylindrical slag sample.

2.3. Analytical Techniques A Thermo ScientiﬁcScientific iCAP 7000 inductively coupled plasma analyzeranalyzer equippedequipped withwith optical emission spectroscopy (ICP-OES)(ICP-OES) was used to measure the elemental componentscomponents of the slag after the total dissolutiondissolution of the samplesample to anan acidacid mixturemixture consistingconsisting ofof nitricnitric acid, hydrochloric hydrochloric acid acid,, a andnd perchloric perchloric acid acid.. The The metallic metallic iron iron was was analy analyzedzed with with ICP- ICP-OES OESafter afterdissolution dissolution to bromine to bromine methanol methanol,, and trivalent and trivalent titanium titanium content content was analy was analyzedzed by a bytitration a titration-based-based method method from a from precipitate a precipitate of dissolution of dissolution of the sample of the sample to bromine to bromine metha- methanolnol because because metallic metallic iron is disturb iron ised disturbed in this titration in this method. titration In method. addition, In the addition, SiO2 content the of the samples was analyzed colorimetrically as siliconmolydene blue with a Skalar Seg- mented Flow Analyzer (SFA San++) via dissolution of the sample to NaOH melt, diluted with hydrochloric acid, and then reduction with ascorbic acid. Skalar is an automated spectrophotometric analyzer used at the UV/Vis region in this analysis method. A Rigaku Smartlab 9 kW X-ray powder diffraction (XRD) was used for sample phase identification. Appl. Sci. 2021, 11, 1153 6 of 13

SiO2 content of the samples was analyzed colorimetrically as siliconmolydene blue with a Skalar Segmented Flow Analyzer (SFA San++) via dissolution of the sample to NaOH melt, diluted with hydrochloric acid, and then reduction with ascorbic acid. Skalar is an automated spectrophotometric analyzer used at the UV/Vis region in this analysis method. A Rigaku Smartlab 9 kW X-ray powder diffraction (XRD) was used for sample phase identiﬁcation. The PDXL2 software suite with integrated available PDF-4 2018 database was used to analyze the weight percentage of the phases present in the samples. Scanning electron microscopy (SEM) with backscattered electron imaging was used for chemical characterization of the slag samples. Backscattered electron imaging was used to obtain the atomic number contrast in the microstructural image. All the phase diagrams and calculations were made with FactSage version 6.4 using FactPS, FToxid databases, and MTDATA software.

3. Results and Discussion Each slag sample was quenched and sent for laboratory tests. The results of the ICP-OES analysis, wet chemical analysis, and coulometric analysis are given in Table2. The experiments are given as test numbers, which represent the sample number also. The elemental composition obtained from ICP-OES was converted into weight percentage. The composition of each component obtained in Table2 was slightly different than the raw materials that were used, as shown in Table1. An increase in the quantity of TiO 2 was observed in all samples compared to the values in Table1, which indicates the oxidation of titanium from Ti3+ to a Ti4+ oxidation state. The amount of FeO and MgO remained almost the same, whereas the other impurities had some variation. Another reason for the difference can be attributed to the small sample size that was used for ICP-OES. Later, these values were normalized to represent a 100% composition value in the ﬁnal slag composition values.

Table 2. Weight (in grams) of each component in the synthetic slag after smelting.

Weight Percentage of Each Component Sample TiO2 Ti2O3 FeO MgO MnO SiO2 Al2O3 1 62.47 26.76 5.02 0.53 1.48 1.50 2.25 2 67.16 21.40 6.48 0.71 1.63 1.11 1.51 3 66.39 24.38 2.98 1.12 1.84 1.29 1.99 4 60.68 22.54 7.37 4.93 0.57 1.87 2.04 5 62.07 22.90 9.07 0.82 2.37 0.92 1.85 6 67.40 11.46 7.03 8.96 0.84 1.49 2.83 7 68.83 18.33 1.17 5.01 0.97 3.37 2.32 8 68.58 16.42 10.84 0.86 1.27 0.97 1.06 9 59.45 15.75 13.98 5.36 0.34 2.19 2.92 10 62.93 16.12 14.62 0.99 1.67 1.73 1.95

Energy dispersive X-ray microscopy (EDS) was performed to analyze the elemental composition of each phase and then compared with XRD to get a better idea about the phases present as XRD is not an accurate measuring technique for quantitative analysis. Figure5 shows the XRD spectra of the cooled synthetic slag samples. Three samples were selected to show the peaks corresponding to the pseudobrookite, rutile, and silicate phases, as shown on the XRD pattern in Figure5. The major and minor phases observed in each sample are listed in Table3. It was observed that the pseudobrookite phase, having an M3O5 composition of divalent and trivalent ions, was consistent in all samples. The major phases were seen as a solid solution of various titanates, such as pseudobrookite, armalcolite, and Mg2TiO5, all having pseudobrookite–karrooite structure. Similar phases and structures were observed in a high titania industrial slag study [29]. The karrooite structure can accommodate a wide variety of mixed-valence metal cations. Other authors also reported that the solidiﬁed titania slag consisted mainly of a uniform M3O5 solid solution with minor phases consisting of rutile, silicate glass phase, metallic iron, and Appl. Sci. 2021, 11, 1153 7 of 13

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magneli phases [30,31]. The minor phases found in our samples consisted of rutile, magneli phases (titanium oxide), silicate phase, and anatase in a few samples.

FigureFigure 5. X-ray 5. X-ray powder powder diffraction diffraction (XRD (XRD)) patterns patterns of three of threesynthetic synthetic slags slags..

TableTable 3. Phases 3. Phases in weight in weight (percentage) (percentage) detected detected by by X X-ray-ray powderpowder diffraction diffraction (XRD) (XRD for) for each each sample sample using using the Rietfeld the Rietfeld refinement . refinement. Phases (wt%) Slag Phases (wt%) Slag Pseudobrookite Silicate Rutile Ilmenite Titanium Oxide Iron Titanium Oxide Pseudobrookite Silicate Rutile Ilmenite Titanium Oxide Iron Titanium Oxide 1 51.2 36.0 12.9 1 51.2 36.0 12.9 2 64.0 4.1 5.9 26.0 2 364.0 57.84.1 7.0 5.9 26.0 35.2 3 457.8 84.87.0 12.3 2.9 35.2 4 584.8 68.212.3 14.62.9 10.3 5 668.2 95.214.6 1.010.3 3.0 6 795.2 85.11.0 7.03.0 4.6 7 885.1 51.17.0 1.94.6 29.0 18.0 8 951.1 88.21.9 11.529.0 0.3 18.0 10 78.0 13.0 8.6 9 88.2 11.5 0.3 10 78.0 13.0 8.6 3.1. Slag Microstructure 3.1. Slag MicrostructureThe microstructure of the samples was studied by EDS-SEM analysis. Different phases were,The microstructure observed under of backscattered the samples electron was studied imaging, by EDSand- theySEM were analysis. compared Different with the phasesphases were, obtained observed by under XRD, asbackscattered shown in Table electron3. All imaging the samples, and contained they were pseudobrookite compared with asthe the phases major obtained phase. The by XRD M3O,5 asphase shown formed in Table in the3. All slag the samples samples consisted contained of pseu- a mixture dobrookiteof Ti2FeO as 5the, Mg major0.5Ti 2.5phase.O5, and The Mg M32.5OTi5 phase0.5O5, formedaccording in the to the slag XRD samples analysis. consisted As all of of a them mixturewere of of Ti the2FeO M53, OMg5 composition,0.5Ti2.5O5, and theyMg2.5 areTi0.5 representedO5, according as to pseudobrookite the XRD analysis. in this As discussion all of themfor were simplicity. of the M The3O5 othercomposition common, they phases are represented observed were as pseudobrookite the ilmenite, rutile, in this and dis- silicate cussionglass for phases, simplicity. as discussed The other using common the ﬁgures phases below. observed were the ilmenite, rutile, and silicate glassFigure phases6 shows, as discussed the ilmenite using slag the sample figures after below. it was quenched in water. The cross- sectionFigure 6 of shows the sample the ilmenite could beslag seen sample to have after a it hollow was quenched tube-like in area, water. which The might cross- have sectionformed of the during sample the could quenching be seen for to airhave passage. a hollow Three tube- regionslike area of, which the cross-section might have were formedanalyzed during using the quenching EDS-SEM for to understandair passage. the Three phase regions composition of the cross and-section the differences were an- as we alyzedmove using from EDS the-SEM outer to surfaceunderstand towards the thephase hollow composition inner surface. and the In differences this study, theas we region near the outside surface was called as outer area, and the region near the hollow part, as the move from the outer surface towards the hollow inner surface. In this study, the region inner area. The region in between these two was referred to as the center, which represents near the outside surface was called as outer area, and the region near the hollow part, as the bulk material. the inner area. The region in between these two was referred to as the center, which represents the bulk material. Appl. Sci. 2021, 11, 1153 8 of 13 Appl. Sci.Appl. 20 21Sci., 11 20, 21x FOR, 11, x PE FORER REVIEWPEER REVIEW 9 of 159 of 15

(a) (a) (b) (b) FigureFigure 6. (a) Water6.Figure (a) Water quenched 6. (a quenched) Water slag quenched sample. slag sample. ( slagb) The sample.(b )cross The - (crosssectionb) The-section cross-sectionof the of slag the sample slag of the sample showing slag sample showing the showing the the inner inner innerand outer andand outersurface outer surface regions surface regions. regions..

FigureFigure 7 shows 7Figure shows the7 microstructure showsthe microstructure the microstructure of sample of sample 1, of where sample 1, where three 1, wherethree different different three phases different phases can be phasescan be can be observed.observed. Similarobserved. Similar phase Similarphase formation formation phase is formation observed is observed isin observed sample in sample 3 in (Figure sample 3 (Figure 8 3) (Figureand 8) theand8 )other andthe other thesam- other sam- samples. ples. In Insome some samples, samples, two two types types of of silicates silicates were were observed observed,, as as shown shown in in Figure Figure 88::S S1 ap-appeared ples. In some samples, two types of silicates were observed, as shown in Figure 8: S1 ap- 1 peared lighter thanthan thethe SS2 inin thethe backscatteredbackscattered electronelectron image, image, indicating indicating the the presence presence of of a higher peared lighter than the S2 in the2 backscattered electron image, indicating the presence of a highera higher averageaverage average atomic atomic atomic number number number within within within the the silicate the silicate silicate phase. phase. phase. The The amountThe amount amount of ofsilicon siliconof silicon di dioxideoxide dioxide present in present thein the darker darker silicate silicate (S (S) phase2) phase was was higher higher than, than the, the lighter lighter silicate silicate (S (S)1 phase.) phase. All All the slag present in the darker silicate (S2) phase2 was higher than, the lighter silicate (S1) phase.1 All the slagthe samplesslagsamples samples consisted consisted consisted of three ofof threethree to four toto fourphasesfour phasesphases where wherewhere the pseudobrookite thethe pseudobrookitepseudobrookite and silicate andand silicatesilicate phases phasesphases were were were consistent consistent consistent with with with the the other the other other phase phase phase consisting consisting consisting of of ilmenite, ilmenite, of ilmenite, rutile rutile, rutile, or magneli magneli, or magneli phases. This can be observed throughout the microstructure images for samples. phases.phases. This canThis be can observed be observed throughout throughout the microstructure the microstructure images images for samples. for samples.

Figure 7. Microstructures of slag sample 1 analyzed by scanning electron microscopy (SEM). The Figure 7. MicrostructuresFigure 7. Microstructures of slag sample of 1 analyzedslag sample by scanning1 analyzed electron by scanning microscopy electron (SEM). microscopy The rutile (SEM phase). The is represented rutile rutilephase phase is represented is represented as R, theas R, silicate the silicate phase phase as S, andas S the, and pseudobrookite the pseudobrookite phase phase as psb. as psb. as R, the silicate phase as S, and the pseudobrookite phase as psb. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15 Appl. Sci. 2021, 11, 1153 9 of 13 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15

Figure 8. Microstructures of slag sample 3 analyzed by SEM. The silicate phase consisted of two Figure 8. MicrostructuresFigure 8. Microstructures ofdifferent slag sample silicate 3of analyzed slag phases sample, by as SEM. represented3 analy Thezed silicate byby SEM.S1 phase and The S consisted2, silicatethe lighter phase of two and consist different darkered silicate silicateof two phases phases,, respec- as different silicatetively. phases , as represented by S1 and S2, the lighter and darker silicate phases, respec- represented by S1 and S2, the lighter and darker silicate phases, respectively. tively. Among the impurities present in the slag, nearly all the SiO2 and a majority of Al2O3 Among the impurities present in the slag, nearly all the SiO2 and a majority of Al2O3 Amongwere the impuritiesin the the glass glass present phase phase,, whereasin whereas the slag, the thenearly other other impuritiesall impuritiesthe SiO oxides2 and oxides, a, majoritysuch such as MnO of as Al MnO and2O3 MgO and, MgO, were present majorly in the M3O5 solid solution and also in the silicate phase [23]. The amount were in the glasswere phase present, whereas majorly the in other the M impurities3O5 solid solutionoxides, such and as also MnO in theand silicate MgO, were phase [23]. The of Al2O3 that 3went5 to the silicate phase was almost equal to a third of the amount of SiO2 present majorlyamount in the of AlM2OO3 solidthat wentsolution to the and silicate also in phase the silicate was almost phase equal [23]. The to a thirdamount of the amount 2+ 2+2+ 2+ 2+ of Al2O3 thatof[3 went2 SiO]. Within2 to[32 the]. the silicate Within slag, thephase the divalent slag, was the almost impurities divalent equal impurities(Mg to a thirdand Mn (Mgof the) substitute andamount Mn ofd )the SiO substituted Fe2 , whereas the [32]. WithinFethe the2+ trivalentslag,, whereas the impuritiesdivalent the trivalent impurities (Al3+ impurities) substitute (Mg2+ (Alandd 3+the Mn) substitutedTi2+3+) substitute[31]. Pistorius thed Tithe3+ explained Fe[312+]., whereas Pistorius the relation explained be- 3+ 3+ the trivalentthetween impurities relation the composition (Al between) substitute the of compositionFeOd the and Ti Ti 2[O31 of3 ]in. FeO Pistorius an andilmenite Ti explained2O slag3 in anthat the ilmenite fol relationlows slagM be-3O that5 stoichiom- follows tween the compositionMetry3O 5[31stoichiometry]. Figure of FeO 9 andshows [31 Ti].2 FigureO the3 in relationan9 shows ilmenite between the slag relation that the betweenfol equivalentlows M the3O equivalent5 %Ti stoichiom-2O3 and %Ti equivalent2O3 and etry [31]. Figureequivalent%FeO. 9 In shows Figure %FeO. the 9, Inthe relation Figure long dash9 between, the line long shows the dash equivalent the line trend shows for %Ti thethe2O 3 trendCanadian and for equivalent the and Canadian South African and %FeO. In FigureSouthilmenite 9, African the slags long ilmenitewith dash the line slagsexpected shows with the relationship the trend expected for thein relationship a Canadian solid solution inand a solidSouth (M3O solution African5) [31], whereas (M 3O5)[ 31the], ilmenite slagswhereasshort with dash the the line expected short shows dash relationship the line trend shows for in the the a trendsolid synthetic solution for the slags. synthetic (M 3O5) [31 slags.], whereas the short dash line shows the trend for the synthetic slags.

Figure 9. Comparison of of an an actual actual trend trend of of ‘equivalent ‘equivalent FeO’ FeO’ and and ‘equivalent ‘equivalent Ti Ti2O23O’for3’for slags slags from from Figure 9. ComparisonCanadian of and an Southactual Africantrend of ilmenites ‘equivalent (Pistorius, FeO’ and 2002) ‘equivalent and the slagsTi2O3 ’ preparedpreparedfor slags from synthetically.synthetically. Canadian and South African ilmenites (Pistorius, 2002) and the slags prepared synthetically. Appl. Sci. 2021, 11, 1153 10 of 13

It is observed that the trend for the synthetic slags deviated from the trend observed in the industrial slag. The deviation can be attributed to the lower amount of pseudobrookite formed in the synthetic slag and the composition of the slag. The linear relation for industrial slag is observed if the M3O5 solid solution is simply a mixture of Ti3O5 and FeTi2O5 and the amount of rutile and silicate is small, which was not true for our slag composition. There were a few samples for which the silicate and rutile formation was less, and the trend was similar to the industrial slag. For samples with the highest pseudobrookite content (samples 6 and 9), the points on the trendline lay closer to the industrial slag trendline. The percentage content of pseudobrookite in all the samples was in the range of 51% to 95%. Most of the impurities added in the slag, such as Al2O3, MnO, and SiO2, were found in the silicate phase that consisted mostly of SiO2. The silicate phases were observed between the boundaries of the pseudobrookite phase. It was found that the amount of pseudobrookite formed in the slag was proportional to the amount of impurities (FeO, MgO, MnO, SiO, and Al2O3) present in the raw material. An increase in the amount of impurity resulted in an increase in pseudobrookite formed. Samples 1, 2, and 3 had the least impurity content and the lowest amount of pseudobrookite content in the samples, whereas samples 6 and 9 had the largest amount of impurity and pseudobrookite (Tables2 and3). Two of the most important impurities in the ilmenite slags were FeO and MgO, as they affected the slag properties and freeze lining conditions, respectively. The other impurities were present in the oxide phase and did not have a signiﬁcant effect on the smelting process as FeO or MgO. MgO had the most pronounced effect on the pseudobrookite formation than any other impurity. A higher amount of MgO in the samples resulted in higher pseudobrookite formation. The samples having a pseudobrookite amount around 85% or more were samples 4, 6, 7, and 9; these samples had the highest amount of MgO among the ten prepared samples. It should also be noted that the nominal composition of MgO in the slag varied from 1 to 2% in the industrial slags. Other impurities can have a similar effect as MgO, but no such trend was observed in this study.

3.2. Micro-Analyses The results of the EDS analysis for sample 7 and sample 10 are summarized in Tables4 and5, respectively. These two samples were selected for comparison due to them having the lowest and the highest amount of FeO present among all samples. The analysis was conducted in three different regions on the sample cross-section. The outer surface undergoes the fastest cooling, and toward the inner surface, the cooling rate decreases. The silicate and the pseudobrookite phase present near the outer surface had a smaller amount of FeO as compared to the center region. This was consistent with the expected solidification behavior that when the pseudobrookite phase solidified first, it will have a higher TiO2 content and lower iron content than the phases that were solidified later. Similar trends were observed in the industrial slag samples and the pilot plant slag samples studied by Pistorius [23]. The difference was more pronounced in their work as the sample size was large, and thus, the cooling rate would vary significantly.

Table 4. Average composition of different regions in the slag sample 7 obtained with energy dispersive X-ray microscopy (EDS).

Position TiO2 SiO2 FeO MgO MnO Al2O3 Pseudobrookite Outer surface 91.04 0.79 5.74 0.64 1.79 Center 90.77 1.04 6.22 1.97 Inner Surface 91.19 0.88 5.90 0.21 1.83 Silicate Outer surface 24.14 55.77 1.44 5.52 4.88 8.25 Center 19.76 53.92 2.90 7.55 7.39 8.48 Inner Surface 20.06 55.16 2.20 7.60 6.52 8.46 Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 15

Table 4. Average composition of different regions in the slag sample 7 obtained with energy dispersive X-ray microscopy (EDS).

Position TiO2 SiO2 FeO MgO MnO Al2O3

Pseudobrookite Outer surface 91.04 0.79 5.74 0.64 1.79 Center 90.77 1.04 6.22 1.97 Inner Surface 91.19 0.88 5.90 0.21 1.83 Appl. Sci. 2021, 11, 1153 11 of 13 Silicate Outer surface 24.14 55.77 1.44 5.52 4.88 8.25 Center 19.76 53.92 2.90 7.55 7.39 8.48 Table 5. AverageInner Surface composition 20.06 of different 55.16 regions 2.20 in the slag7.60 sample 106.52 obtained 8.46 with EDS.

Table 5. Average composition of different regions in the slag sample 10 obtained with EDS. Position TiO2 SiO2 FeO MgO MnO Al2O3 Position TiO2 SiO2 FeO MgO MnO Al2O3 Pseudobrookite Outer surface 82.14 14.07 1.05 1.36 1.39 PseudobrookiteCenter 83.94 12.31 1.19 1.11 1.44 Outer surface 82.14 14.07 1.05 1.36 1.39 Inner Surface 85.10 11.96 1.37 0.00 1.57 Center 83.94 12.31 1.19 1.11 1.44 Silicate Inner Surface 85.10 11.96 1.37 0.00 1.57 Outer surface 23.63 61.36 7.08 0.00 2.67 5.25

Center 14.48 69.00 7.24 0.00 2.65 6.64 Silicate Inner Surface 14.17 64.60 11.05 0.00 3.27 6.91 Outer surface 23.63 61.36 7.08 0.00 2.67 5.25 Center 14.48 69.00 7.24 0.00 2.65 6.64 TheInner effect Surface of the cooling14.17speed 64.60 on the grain11.05 size can0.00 be observed3.27 in6.91 Figure 10, which presents electron microscopy images of different slag regions for slag samples 7 and 10. As The effect of the cooling speed on the grain size can be observed in Figure 10, which mentionedpresent earlier,s electron the microscopy cooling wasimages fastest of different for the slag outer regions surface; for slagthis samples is reﬂected 7 and 10. in the grain size. AsAs wementioned move earlier, from the cooling outer was surface fastest to for the the innerouter surface surface,; this the is reflected grain gotin the larger as the coolinggrain slowed size. As down. we move from the outer surface to the inner surface, the grain got larger as the cooling slowed down.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 15

Figure 10. Microstructure for samples 7 and 10 obtained with energy dispersive X-ray microscopy (EDS) at the same mag- Figure 10. Microstructurenification of 2000×.for (a) Outer samples surface 7 andof sample 10 obtained 7, (b) Outer with surface energy of sample dispersive 10, (c) Cent X-rayer part microscopyof sample 7, (d (EDS)) Center at the same magniﬁcationpart of of 2000 sample×.( 10,a) ( Outere) Inner surface surface of of sample sample 7, ( 7,f) Inner (b) Outer surface surface of sample of 10. sample 10, (c) Center part of sample 7, (d) Center part of sample 10, (e) Inner surface of sample 7, (f) Inner surface of sample 10. The silicate phases were trapped between the pseudobrookite grains and could be observed around the larger grains in the center and inner surface regions. The grain size near the outer surface was relatively small, representing more nucleation due to faster cooling. It is interesting to note that cracks were present in the pseudobrookite phase in slag 7, having the lowest amount of FeO but not in sample 10, having the highest amount. The cracks were more often present within the rutile and silicate phases in sample 10. In the center, the pseudobrookite grain was more elongated and with larger segregation of silicate phases. The silicate phases had more defined boundaries, as observed in sample 10. The synthetic slags showed similar characteristics to industrial slags. The impurities were in the silicate phase, thus giving an expected pseudobrookite phase with M3O5 composition along with rutile. This method of slag preparation gives the user the freedom to select the composition of the slag. The amount of pseudobrookite in the slag can be im- proved by controlling the amount of Ti2O3 in the slag mixture. The key features observed in the synthetic slags that are found in the industrial slags were the elongated pseudobrookite grains, the silicate phase between the pseudobrookite grain, and microcracking throughout the cross-section.

The silicate phases were trapped between the pseudobrookite grains and could be observed around the larger grains in the center and inner surface regions. The grain size near the outer surface was relatively small, representing more nucleation due to faster cooling. It is interesting to note that cracks were present in the pseudobrookite phase in slag 7, having the lowest amount of FeO but not in sample 10, having the highest amount. The cracks were more often present within the rutile and silicate phases in sample 10. In the center, the pseudobrookite grain was more elongated and with larger segregation of silicate phases. The silicate phases had more deﬁned boundaries, as observed in sample 10. The synthetic slags showed similar characteristics to industrial slags. The impurities were in the silicate phase, thus giving an expected pseudobrookite phase with M3O5 composition along with rutile. This method of slag preparation gives the user the freedom to select the composition of the slag. The amount of pseudobrookite in the slag can be im- proved by controlling the amount of Ti2O3 in the slag mixture. The key features observed in the synthetic slags that are found in the industrial slags were the elongated pseudobrookite grains, the silicate phase between the pseudobrookite grain, and microcracking throughout the cross-section.

4. Conclusions The preparation of the synthetic ilmenite slag was successfully carried out using a molybdenum crucible placed inside an induction furnace. The slag samples were quenched in water. Most of the slags consisted of three phases, namely pseudobrookite, rutile, and silicate phases. The quantity of the pseudobrookite formed in the synthetic slag was less than that found in industrial slags, but for a few samples, it was similar to an industrial slag. The samples having a lesser amount of Ti2O3 in the slag mixture resulted in a larger percentage of pseudobrookite in the ﬁnal slag. The solidiﬁed titanium slag consisted of a microstructure that is similar to that observed in the industrial slags. The silicate phases were formed between the pseudobrookite grains, and the pseudobrookite grains had elongated shapes. The results suggest that the induction furnace with a controlled atmosphere can be used to prepare synthetic slags with the desired chemical composition. The synthetic slags can be helpful when preparing a calibration model, which requires samples with varying chemical compositions. For example, quantitative analysis of ilmenite slags with laser-induced breakdown spectroscopy (LIBS) requires such calibration.

Author Contributions: Conceptualization, A.K.G. and M.A.; Data curation, A.K.G.; Formal analysis, A.K.G.; Funding acquisition, M.H. and T.F.; Investigation, A.K.G. and M.A.; Methodology, J.P.; Project administration, M.H. and T.F.; Resources, P.M.; Software, A.K.G.; Supervision, M.A., P.M., and T.F.; Validation, M.A. and T.F.; Visualization, A.K.G. and M.A.; Writing—original draft, A.K.G.; Writing— review & editing, M.A. and J.P. All authors have read and agreed to the published version of the manuscript. Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 713606. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article and supplementary material. Acknowledgments: The authors are grateful for the financial support from the I4Future doctoral programme—Imaging for the Future. Furthermore, the authors acknowledge the support of the Academy of Finland projects (Academy of Finland, No. 311934 and No. 326291) and Metso Outotec Research Center for their participation in the project. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Appl. Sci. 2021, 11, 1153 13 of 13

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