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Article Preparing High-Purity Anhydrous ScCl3 Molten Salt Using One-Step Rapid Heating Process

Junhui Xiao 1,2,3,4,*, Chao Chen 1,2, Wei Ding 1, Yang Peng 1, Kai Zou 1, Tao Chen 1 and Zhiwei Zou 1

1 Sichuan Provincial Engineering Lab of Non-Metallic Powder Modification and High-Value Utilization, Southwest University of Science and Technology, Mianyang 621010, ; [email protected] (C.C.); [email protected] (W.D.); [email protected] (Y.P.); [email protected] (K.Z.); [email protected] (T.C.); [email protected] (Z.Z.) 2 Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Chengdu 610041, China 3 Key Laboratory of Sichuan Province for Comprehensive Utilization of and Resources, Panzhihua University, Panzhihua 617000, China 4 Key Laboratory of Ministry of Education for Waste Treatment and Resource Recycle, Southwest University of Science and Technology, Mianyang 621010, China * Correspondence: [email protected]; Tel.: +86-139-9019-0544

 Received: 7 July 2020; Accepted: 27 July 2020; Published: 28 July 2020 

Abstract: In this study, a one-step rapid heating novel process was used to prepare high-purity anhydrous molten salt with low-purity scandium . High-purity anhydrous ScCl3 molten salt was used as the Sc-bearing raw material for preparing the Sc-bearing master alloy. Inert gas was used to enhance the purity of anhydrous and reduce the hydrolysis rate of scandium. The results show that high-purity scandium chloride (purity, 99.69%) with the scandium content of 29.61%, was obtained, and the hydrolysis rate of scandium was 1.19% under the conditions used: removing ammonium chloride; residual crystal water temperature of 400 ◦C; m(Sc2O3):m(NH4Cl) = 1:2.5; holding-time of 90 min; heating-rate of 12 ◦C/min; and flow of 7.5 L/min. XRD, SEM, and EPMA analyses further verified that anhydrous scandium chloride crystallization condition was relatively good and the purity of high-purity anhydrous scandium chloride approached the theory purity of anhydrous scandium chloride.

Keywords: scandium; scandium chloride;

1. Introduction Scandium (Sc)— 21—lies in the of the fourth cycle III subgroup. Along with and , scandium belongs to the rare earth family. It also has a particularity of mineralization with titanium, vanadium and that is not common with other rare earth elements. In the crust, the specific of scandium are very rare. Moreover, there are only small economic reserves of Sc-bearing minerals including only thortveitite (Sc,Y)2Si2O7, scandium phosphite ScPO 2H O, bazzite Be (Sc,Al) Si O , titanium silicate mineral Sc(Nb,Ti,Si) O and befanamite 4· 2 3 2 6 18 2 5 (Sc,Zr)2Si2O7, etc. In nature, scandium mostly occurs in ilmenite, zircon, bauxite, rare earth ore, ilmenite, V–Ti magnetite, ore, ore, ore, coal and other minerals. In recent years, the combination of mineral processing and hydrometallurgical separation technology has been widely used in the field of comprehensive recovery and extraction of scandium, which greatly improves the concentration ratio and recovery rate of scandium. In future research and production, the recovery

Appl. Sci. 2020, 10, 5174; doi:10.3390/app10155174 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5174 2 of 13 process of main and other valuable metals should be considered—with the recovery and extraction process of scandium simplified, and the production cost reduced as much as possible [1–3]. Scandium is not only rare in the crust (only 7 10 6 in the upper crust, equivalent to seven grams × − of scandium per ton of crust material), but also has a very low probability of forming scandium independent mineral in nature, which is only 0.4%. Current findings indicate that these rare independent minerals are mainly distributed in a few countries in Europe and Madagascar in Africa and have no real industrial value. In 1879, the Swedish chemist Neilson first discovered scandium in a – yttrium mine and a black gold mine in in Europe. Scandium is normally dispersed in a number of minerals that contain other elements, like salt or sugar dissolved in water, and is not directly visible to the naked eye. Therefore, it is very difficult to obtain pure scandium . It took half a century from the discovery of scandium to the first successful extraction [4–6]. Scandium products are mainly used in Al–Sc alloy materials, scandium high-intensity discharge (HID) lamps, SOFC, etc. Al–Sc alloys are mainly used in sporting goods production, automotive and aerospace industries. The higher power of scandium -based solid to the decrease of (SOFC) reaction temperature. Scandium lasers can be used in defense and medical fields. Scandium is a common tracer in the oil refining industry. The International Adamas intelligence research center divides scandium’s applications into three types: mature applications, emerging applications and future demand [7–10]. Scandium chloride can be used to make high melting-point alloys. The preparation of scandium intermediate alloys by doping scandium fluoride or scandium oxide metal thermal reduction and has been reported. Scandium metal is prepared by mixing scandium metal into aluminum alloy. Scandium metal is expensive and has a large burning-loss during smelting. In the preparation of scandium fluoride by thermal reduction method, corrosion and toxic chloride are used, and the thermal reduction temperature of the metal is also very high. The real yield of scandium by scandium oxide thermal reduction is not high. Organic solvent extraction is one of the main methods for separating and enriching scandium. In most cases, extraction is more efficient than precipitation or ion exchange, and can be used at all stages of scandium extraction. The solvent extraction method is characterized by a simple, easy-to-grasp method, rapid and good enrichment and separation effect and large processing capacity. However, these advantages require choosing a suitable extraction system and effectively considering all kinds of factors affecting the extraction separation in order to fully come into play. Extraction, washing and reverse extraction are three complementary processes in extraction and separation [11–14]. The deep research of scandium by Chinese scientists began with V–Ti magnetite in Panzhihua area of China. The value of scandium accounts for about 47% of Panzhihua V–Ti magnetite. The economic value of scandium is more than four hundred million USD, ranking first in the economic value of all elements of Panzhihua V–Ti magnetite. Because the grade of Panzhihua V–Ti magnetite is not high, it produces a large amount of tailings, which not only occupy much arable land, but also pollute the environment. There are rare scandium-containing minerals, making it difficult to separate and extract scandium. Consequently, scandium and its compounds become rather expensive. Scandium in V–Ti magnetite is mainly enriched in the tailings of titanium separation by beneficiation process, so it is necessary to extract scandium from the tailings of titanium separation. The tailings of V–Ti magnetite are pretreated by magnetic separation gravity separation electro separation process to enrich scandium to obtain scandium concentrate. To obtain scandium products with a high purity and recovery rate, it is necessary to combine multiple methods to improve the extraction process, to find more materials that can recover scandium—as well as improve the recovery and extraction methods of scandium, which will be of great significance to the effective use of the source [15–18]. Scandium oxide (Sc2O3) is the most common scandium compounds. Scandium oxide from high temperature burning is not soluble in dilute , but soluble in boiling concentrated . Scandium chloride (ScCl3) is an important raw material for the further preparation of scandium Appl. Sci. 2020, 10, 5174 3 of 13

metal and Al–Sc alloy [19–22]. To date, the key point of preparing ScCl3 is how to reduce the hydrolysis rate of scandium chloride in ScCl3. For this reason, a new one-step rapid heating process is proposed to prepare high-purity anhydrous ScCl3 with the low-purity Sc2O3 (purity, 81.16%), along with the appropriate technological conditions. The high-purity anhydrous ScCl3 will deeded as the important scandium-bearing raw materials for the preparation of Sc-bearing master alloy by aluminum– thermal reduction.

2. Materials and Methods

2.1. Sampling The tailings samples of V–Ti magnetite from Panzhihua area of China were collected, which contained 38.61 g/t Sc2O3. The mineral composition of the tailings was complex and the content of scandium was very low. The scandium rough concentrate containing 76.98 g/t of Sc2O3 was further enriched by magnetic separation, gravity separation and electric separation from the tailing samples. The scandium oxide (Sc2O3, 81.16%) was obtained from the scandium rough concentrate by roasting, hydrolysis, leaching, extraction, purification, oxalic acid precipitation and roasting [23,24]. The main chemical composition analysis of scandium oxide is shown in Table1.

Table 1. Chemical composition analysis results of scandium oxide (%).

Sc2O3 Fe2O3 MnO SiO2 Al2O3 CaO MgO K2O Na2O 81.16 4.56 0.56 3.01 1.76 2.23 1.79 1.46 0.89

2.2. Chemical Reagent and Equipment The main chemical reagents used in this test were ammonium chloride, ammonia, glacial acetic acid, anhydrous sodium acetate, ascorbic acid, hydrochloric acid ethylene diamine tetraacetic acid, oxide, xylenol orange, Eriochrome Black T, and argon with analytical purity were Tianjin chemical reagent research institute Co., Ltd., Tianjin China. The main equipment used in the experiment included an electric atmosphere tube furnace (model: SX-6-16, Changsha Kehui Furnace Technology Co., Ltd., Changsha, China), high-temperature electric resistance furnace ( 1300 C, Shanghai Shiyan Electric Furnace Co., Ltd., Shanghai, China), ≤ ◦ low-temperature electric resistance furnace ( 600 C, Shanghai Shiyan Electric Furnace Co., Ltd. ≤ ◦ Shanghai, China), drying box (Shanghai Shiyan Yan Electric Furnace Co., Ltd., Shanghai, China), electro-thermostatic water bath, porcelain crucible (50 mL, 100 mL,150 mL, 200 mL), oscillator and vacuum filter (F 300, Southwest Chengdu Experimental Equipment Co., Ltd., Chengdu, China).

2.3. Experimental Design In the preparation of high-purity anhydrous scandium chloride in the process, the main objective is to prevent hydrolysis of the scandium chloride generated (ScOCl). Owing to the high and low back, the ScOCl content influences the of scandium recovery rate. The preparation process of aluminum among scandium also has adverse effect. Two kinds of dehydration methods to remove water contained in the crystallization of rare earth are vacuum dewatering and inert gases flow. The process of preparation and equation of scandium hydrolysis and combination reaction are shown in Equations (1) and (2):

ScCl + H O ScOCl + 2HCl (1) 3(s) 2 (g) → (s) (g)

∆ ScOCl + 2NH Cl ScCl + 2NH + H O (2) (s) 4 (s) → 3(g) 3(g) 2 (g) Scandium oxide is first dissolved by heating with concentrated hydrochloric acid. Then, the auxiliary salt and ammonium chloride are dissolved by heating with distilled water. Appl. Sci. 2020, 10, 5174 4 of 13

After mixing the two solutions, the mixture is filtered, and the fully stirred filtrate is put in an ordinary electric furnace for heating and evaporation. When a large number of crystals in the solution precipitate,Appl. Sci. 2020, the10, xsolution FOR PEERis REVIEW transferred to a constant temperature water pot for heating, so that4 of the 13 water can evaporate as completely as possible. The solution is moved to a vacuum drying oven for vacuumcompletely evaporation as possible. and The most solution of the iscrystallized moved to a watervacuum is removed.drying oven After for thevacuum water evaporation evaporation and of scandiummost of the chloride crystallized molten water salt is is remove complete,d. After the moltenthe water salt evaporation is taken of scandium out and ground chloride into molten powder. salt Finally,is complete, the molten the molten saltis salt put block into ais vacuumtaken out tube and atmosphere ground into furnace powder. to Finally, remove the ammonium molten salt chloride. is put Theinto remaininga vacuum tube crystal atmosphere and the prepared furnace to anhydrous remove ammonium scandium chloridechloride molten. The remaining salt is stored cryst inal and a dryer. the Byprepared analyzing anhydrous the content scandium of scandium chloride in moltenmolten saltsalt andis stored calculating in a dryer. the hydrolysis By analyzing rate ofthe scandium content of is determined.scandium in molten The calculation salt and calculat formulaing is the shown hydrolysis in Equation rate of (3). scandium The process is determined. of preparing The anhydrouscalculation scandiumformula is chlorideshown in is Equation shown in (3). Figure The 1process. of preparing anhydrous scandium chloride is shown in Figure 1.   Hydrolysis rate of scandium = ST ScS /ScT 100% (3) Hydrolysis rate of scandium = (−− )⁄×× 100% (3) wherewhere ScScSS is the solublesoluble scandiumscandium contentcontent ofof anhydrousanhydrous scandiumscandium chloridechloride moltenmolten saltsalt/%/% andand ScScTT isis thethe scandiumscandium contentcontent ofof anhydrousanhydrous scandiumscandium chloridechloride moltenmolten saltsalt/%./%.

Figure 1. Scheme of preparing anhydrous scandium chloride molten salt.

The EDTA standard standard solution solution was was prepared prepared at at a concentration a concentration of aboutof about 0.04mol/L 0.04mol. T/L.hen Then,, the thescandium scandium content content in scandiumin scandium chloride chloride salt salt was was dete determinedrmined by by EDTA EDTA complexometric complexometric titration. Scandium chloride oxide (ScOCl) formed by the hydrolysishydrolysis ofof scandiumscandium chloridechloride containingcontaining crystal waterwater is insoluble in water, whilewhile scandiumscandium chloridechloride isis soluble in water. Therefore, the total scandium and soluble scandiumscandium contentscontents inin moltenmolten saltsalt couldcould bebe determineddetermined first.first. The method method for for determination determination of the of content the content of soluble of soluble scandium scandium was: 0.5 was g anhydrous: 0.5 g anhydrous scandium chloridescandium molten chloride salt samplesmolten salt were samples put in 100-mL were put beaker, in 10 400- mL wasbeaker, added 40 ofmL distilled was added water of to dissolvedistilled andwater fully to dissolve mixedafter and filtration.fully mixed Then, after 10filtration mL pH. Then, material 10 mL 4 drips pH material of HAc—NaAc 4 drips of bu HAcffer solution—NaAc wasbuffer added solution the filtrate.was add Then,ed the a filtrate small. amount Then, a ofsmall ascorbic amount acid of wasascorbic added, acid with was xylenol added, orangewith xylenol as an orange as an indicator. Titration was completed with EDTA standard solution; the titration end solution changed from red to yellow. The content of soluble scandium in molten salt was calculated according to the consumption and concentration of EDTA standard solution and the weight of sample. The total content of scandium was determined as follows: Another sample of 0.5 g molten salt was added to a 100-mL beaker. Then, 20 mL of melted concentrated hydrochloric acid was added. Appl. Sci. 2020, 10, 5174 5 of 13 indicator. Titration was completed with EDTA standard solution; the titration end solution changed from red to yellow. The content of soluble scandium in molten salt was calculated according to the consumption and concentration of EDTA standard solution and the weight of sample. The total content of scandium was determined as follows: Another sample of 0.5 g molten salt was added to a 100-mL beaker. Then, 20 mL of melted concentrated hydrochloric acid was added. The remaining molten salt dissolved; all heating was stopped. Then, 30 mL of distilled water in a beaker was added with 1:1 ammonia to neutralize the acid solution. When a small amount of precipitation was seen, we stopped adding ammonia, and added 15 MLPH material 4 drips of HAc—NaAc buffer solution. A small amount of ascorbic acid was added, along with xylenol orange as an indicator. Titration was completed with an EDTA standard solution. The titration end solution changed from red to yellow. The total scandium content in molten salt was calculated according to the consumption and concentration of EDTA standard solution and the weight of sample.

2.4. Analysis and Characterization The chemical composition of raw materials and anhydrous scandium chloride molten salt was analyzed by a Z-2000 atomic absorption spectrophotometer (Hitachi Co., Ltd. Tokyo, Japan).The composition of the anhydrous scandium chloride molten salt was analyzed by X-ray diffraction (XRD, X Pert pro, Panaco, The Netherlands). The microstructure of the anhydrous scandium chloride molten salt was observed by SEM (S440, Hirschmann Laborgeräte GmbH & Co. KG, Eberstadt, Germany) equipped with an energy dispersive X-ray spectroscopy (EDS) detector (UItra55, Carl Zeiss NTS GmbH, Hirschmann Laborgeräte GmbH & Co. KG, Eberstadt, Germany). The micro area composition of anhydrous scandium chloride molten salt was analyzed by electron probe microanalysis (EPMA-1720, Electron probe microanalyzer, Shimazu Instrument Company of Japan, Chengdu, China).

3. Results and Discussion

3.1. Effect of Temperature Temperature is one of the key factors affecting the rate of scandium hydrolysis. If ammonium chloride and residual crystal water temperature is too slowly removed, ScOCl is formed—a hydrolytic product of scandium chloride that does not react well with ammonium chloride. If the temperature is too high, the sublimation speed of ammonium chloride is accelerated, and the degree of chlorination of ScOCl is reduced [25–28]. The results in Figure2 show that the removing ammonium chloride and residual crystal temperature has an obvious hydrolysis rate of scandium. When the temperature was 400 ◦C, the hydrolysis rate of scandium presented a minimum value of 3.68% and scandium content of 23.44%. When the temperature was lower than 400 ◦C, the temperature increased and the hydrolysis rate of scandium decreased. When the temperature increased to 450 ◦C, the hydrolysis rate of scandium increased obviously. When the temperature was lower than 400 ◦C, the hydrolysis product ScOCl did not react completely with ammonium chloride, resulting in the increase of the hydrolysis rate of scandium. When the temperature was higher than 400 ◦C, the sublimation speed of ammonium chloride was accelerated, and the corresponding ScOCl could not be chlorinated completely. Therefore, considering the removing ammonium chloride and surplus crystal water temperature of 400 ◦C was optimal. The anhydrous scandium chloride with scandium content of 23.44% was obtained, and the hydrolysis rate of scandium was 3.68%. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13

The remaining molten salt dissolved; all heating was stopped. Then, 30 mL of distilled water in a beaker was added with 1:1 ammonia to neutralize the acid solution. When a small amount of precipitation was seen, we stopped adding ammonia, and added 15 MLPH material 4 drips of HAc— NaAc buffer solution. A small amount of ascorbic acid was added, along with xylenol orange as an indicator. Titration was completed with an EDTA standard solution. The titration end solution changed from red to yellow. The total scandium content in molten salt was calculated according to the consumption and concentration of EDTA standard solution and the weight of sample.

2.4. Analysis and Characterization The chemical composition of raw materials and anhydrous scandium chloride molten salt was analyzed by a Z-2000 atomic absorption spectrophotometer (Hitachi Co., Ltd. Tokyo, Japan).The phase composition of the anhydrous scandium chloride molten salt was analyzed by X-ray diffraction (XRD, X Pert pro, Panaco, The Netherlands). The microstructure of the anhydrous scandium chloride molten salt was observed by SEM (S440, Hirschmann Laborgeräte GmbH & Co. KG, Eberstadt, Germany) equipped with an energy dispersive X-ray spectroscopy (EDS) detector (UItra55, Carl Zeiss NTS GmbH, Hirschmann Laborgeräte GmbH & Co. KG, Eberstadt, Germany). The micro area composition of anhydrous scandium chloride molten salt was analyzed by electron probe microanalysis (EPMA-1720, Electron probe microanalyzer, Shimazu Instrument Company of Japan, Chengdu, China).

3. Results and Discussion

3.1. Effect of Temperature Temperature is one of the key factors affecting the rate of scandium hydrolysis. If ammonium chloride and residual crystal water temperature is too slowly removed, ScOCl is formed—a hydrolytic product of scandium chloride that does not react well with ammonium chloride. If the temperatureAppl. Sci. 2020, 10is, too 5174 high, the sublimation speed of ammonium chloride is accelerated, and the degree6 of 13 of chlorination of ScOCl is reduced [25–28].

30 20

28 Scandium content Hydrolysis rate of Scandium 18

(%) 26 3 16 ) 24 % 14 ( 22 20 12 18 10

16 8 14 6 12 4 Appl. Sci. 2020, 10, x FOR PEER REVIEW10 6 of 13 8 2 m(Sc2O3)︰m(NH4Cl)=1︰2 6 ℃ 0 increased obviously. When the temperature was lowerHeating thanrate: 1 0400 /m °Cin , the hydrolysis Scandium rate of Hydrolysis product ScOCl did 4 Holding time: 60 min -2 not react completely with ScCl anhydrous content of Scandium ammonium2 chloride, Inert resulting gases flow in (Ar): the 7.0 increaseL/min of the hydrolysis rate of -4 scandium. When the temperature0 was higher than 400 °C, the sublimation speed of ammonium 200 250 300 350 400 450 500 550 chloride was accelerated, and the corresponding ScOCl could not be chlorinated completely. Temperature (℃) Therefore, considering the removing ammonium chloride and surplus crystal water temperature of 400FigureFigure °C was 2. 2. EffectE ff optimal.ect of of temperature temperature The anhydrous on on scandium scandium scandium content content chlorideof of anhydrous anhydrous with scandium scandium scandium chloride chloride content and and ofhydrolysis hydrolysis 23.44% was obtained,raterate of ofand scandium scandium. the hydrolysis. rate of scandium was 3.68%. 3.2. Effect of Ammonium Chloride Dosage 3.2. EffectThe of results Ammonium in Figure Chloride 2 show Dosage that the removing ammonium chloride and residual crystal temperatureThe effect has of an di ff obviouserent ammonium hydrolysis chloride rate of dosage scandium. on scandium When the content temperature of anhydrous was 400 scandium °C, the The effect of different ammonium chloride dosage on scandium content of anhydrous scandium hydrolysischloride and rate hydrolysis of scandium rate present of scandiumed a minimum is shown value in Figure of 3.68%3. The and amount scandium of ammonium content of chloride 23.44%. chloride and hydrolysis rate of scandium is shown in Figure 3. The amount of ammonium chloride Whenalso has the a temperature great influence was onlower the than hydrolysis 400 °C rate, the oftemperature scandium. increase Too littled and or toothe muchhydrolysis ammonium rate of also has a great influence on the hydrolysis rate of scandium. Too little or too much ammonium scandiumchloride may decrease tod. incompleteWhen the chlorinationtemperature ofincreased ScOCl or to lead 450 to °C increased, the hydrolysis reagent cost—whichrate of scandium is not chloride may lead to incomplete chlorination of ScOCl or lead to increased reagent cost—which is conducive to specific implementation [29–32]. not conducive to specific implementation [29–32].

30 10 Hydrolysis rate of Scandium 9 (%) 28 3

Scandium content ) 8 %

26 ( 7 24 6

22 5

20 4

18 3 2 16 Temperature : 400 ℃ 1 14 Heating rate: 10 ℃/min 0

Holding time: 60 min Hydrolysis rate of Scandium 12 Inert gases flow (Ar): 7.0 L/min -1

Scandium contentScandium ofScCl anhydrous 10 -2 1∶1 1∶1.5 1∶2 1∶2.5 1∶3 1∶3.5 m (Sc O )︰m (NH Cl) 2 3 4

FigureFigure 3. 3. EffectEffect of of ammonium ammonium chloride chloride dosage dosage on on scandium scandium content content of of anhydrous anhydrous scandium scandium chloride chloride andand hydrolysis hydrolysis rate rate of of scandium scandium..

ItIt can can be be seen seen from from Figure Figure 3 3that that with with increas increasinging ammonium ammonium chloride chloride dosage, dosage, the hydrolysis the hydrolysis rate ofrate scandium of scandium decreased decreased significantly, significantly, which which is condu is conducivecive to toanan increase increase of of scandium scandium content content of of anhydrousanhydrous chlorinated chlorinated molten salt. When When m(Sc2OO33):):mm(NH(NH44Cl)Cl) == 11:2.5,:2.5, the the hydrolysis hydrolysis rate rate of of scandium scandium decreaseddecreased to to 2.61%. 2.61%. When When the the dosage dosage is is increased increased to to 1 1:3,:3, the the hydrolysis hydrolysis rate rate of of scandium scandium remain remaineded unchanged.unchanged. Th Therefore,erefore, the the weight weight ratio ratio of of ammonium ammonium chloride chloride to to scandium scandium oxide oxide in in the the raw raw material material waswas 1 1:2.5.:2.5. The The anhydrous anhydrous scandium scandium chloride chloride with with scandium scandium content content of of 25.03% 25.03% was was obtained, obtained, and and the the hydrolysishydrolysis rate rate of of scandium scandium was was 2.61%. 2.61%.

3.3. Effect of Holding Time The effect of different holding times on the scandium content of anhydrous scandium chloride and hydrolysis rate of scandium is shown in Figure 4. Holding time is also one of the main factors affecting the scandium hydrolysis rate and scandium content of scandium chloride molten salt. If the insulation time is too short, ScOCl chloride is not complete, the hydrolysis rate of scandium is high, and ammonium chloride is not easy to be completely removed—seriously affecting the quality of scandium chloride molten salt and reducing the production efficiency [33–37]. The hydrolysis rate of scandium decreased significantly when the time was less than 75 min. When the time was extended to 90 min, the hydrolysis rate of scandium showed the lowest value of 2.05%. Because the time was too short, the chlorination of ScOCl was not complete; On the contrary, if the time was too long, the water vapor in the inert gas tended to hydrolyze scandium chloride due to the sublimation of ammonium chloride, leading to an increase of the hydrolysis rate of scandium. Appl. Sci. 2020, 10, 5174 7 of 13

3.3. Effect of Holding Time The effect of different holding times on the scandium content of anhydrous scandium chloride and hydrolysis rate of scandium is shown in Figure4. Holding time is also one of the main factors Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13 affecting the scandium hydrolysis rate and scandium content of scandium chloride molten salt. If the insulation time is too short, ScOCl chloride is not complete, the hydrolysis rate of scandium is high, Therefore, 90 min was more suitable for the holding time of removing ammonium chloride and the and ammonium chloride is not easy to be completely removed—seriously affecting the quality of residual crystal water. The anhydrous scandium chloride with scandium content of 26.01% was scandium chloride molten salt and reducing the production efficiency [33–37]. obtained, and the hydrolysis rate of scandium was 2.05%.

30 6.0 Hydrolysis rate of Scandium 29 5.5 Scandium content

(%) 28 3

5.0 ) 27 % ( 26 4.5 25 4.0

24 3.5 23 3.0 22 21 2.5 20 2.0 19 Temperature : 400 ℃ 1.5 18 m(Sc2O3)︰m(NH4Cl)=1︰2.5 1.0 of Scandium rate Hydrolysis 17 Heating rate: 10 ℃/min

Scandium content of anhydrous ScCl of anhydrous content Scandium 16 Inert gases flow (Ar): 7.0 L/min 0.5 15 0.0 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 Holding time (min)

FigureFigure 4. 4. EffectEffect of of holding holding time time on on scandium scandium content content of of anhydrous anhydrous scandium scandium chloride chloride and and hydrolysis hydrolysis raterate of of scandium. scandium.

3.4. EffectThe of hydrolysis Heating Rate rate of scandium decreased significantly when the time was less than 75 min. When the time was extended to 90 min, the hydrolysis rate of scandium showed the lowest value of Tests of the effect of different heating rate tests were carried out; the results are shown in Figure 2.05%. Because the time was too short, the chlorination of ScOCl was not complete; On the contrary, 5. Since the crystallization water in scandium chloride is removed under the protection of inert gases if the time was too long, the water vapor in the inert gas tended to hydrolyze scandium chloride due flow, there are strict requirements on the heating speed. The purpose is to make the crystal water to the sublimation of ammonium chloride, leading to an increase of the hydrolysis rate of scandium. rapidly gasify while the water vapor is quickly taken away by the inert gas—and to minimize the Therefore, 90 min was more suitable for the holding time of removing ammonium chloride and the probability of water vapor hydrolyzing with scandium chloride. residual crystal water. The anhydrous scandium chloride with scandium content of 26.01% was obtained, and the hydrolysis30 rate of scandium was 2.05%. 6.0 Hydrolysis rate of Scandium 29 5.5 Scandium content 28

3.4. Effect of Heating Rate (%) 3 5.0 ) 27 % ( Tests of the effect of di26fferent heating rate tests were carried out; the results4.5 are shown in Figure5. Since the crystallization water25 in scandium chloride is removed under the4.0 protection of inert gases 24 flow, there are strict requirements on the heating speed. The purpose is3.5 to make the crystal water 23 3.0 rapidly gasify while the water22 vapor is quickly taken away by the inert gas—and to minimize the probability of water vapor21 hydrolyzing with scandium chloride. 2.5 20 2.0 When the heating rate was less than 12 ◦C/min, increasing the heating rate was conducive to 19 Temperature : 400 ℃ 1.5 reducing the hydrolysis rate18 of scandium. The rate of heating was 12 ◦C/min and the hydrolysis rate of m(Sc2O3)︰m(NH4Cl)=1︰2.5 1.0 Scandium of rate Hydrolysis scandium decreased to the17 minimum value of 1.86%Holding andtime: 90 scandium min content of 27.46% in anhydrous Scandium content of anhydrous ScCl of content anhydrous Scandium 16 0.5 scandium chloride. When the heating rate increasedInert gases to flow 16 (Ar):◦C/ 7.0min, L/min the hydrolysis rate of scandium 15 0.0 increased to 2.34% instead.4 This5 was6 7 related8 9 to10 the11 factors12 13 14 such15 16 as the17 18 slow sublimation rate of the crystal water and the long action time of theHeating water rate vapor (℃/m andin) scandium chloride. Therefore, anhydrous scandiumFigure chloride5. Effect of with heating scandium rate on content scandium of 27.46%content wasof anhydrous produced scandium and the hydrolysischloride and rate hydrolysis of scandium wasrate 1.86% of scandium. under the condition used a heating rate of 12 ◦C/min.

When the heating rate was less than 12 °C/min, increasing the heating rate was conducive to reducing the hydrolysis rate of scandium. The rate of heating was 12 °C/min and the hydrolysis rate of scandium decreased to the minimum value of 1.86% and scandium content of 27.46% in anhydrous scandium chloride. When the heating rate increased to 16 °C/min, the hydrolysis rate of scandium increased to 2.34% instead. This was related to the factors such as the slow sublimation rate of the crystal water and the long action time of the water vapor and scandium chloride. Therefore, Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13

Therefore, 90 min was more suitable for the holding time of removing ammonium chloride and the residual crystal water. The anhydrous scandium chloride with scandium content of 26.01% was obtained, and the hydrolysis rate of scandium was 2.05%.

30 6.0 Hydrolysis rate of Scandium 29 5.5 Scandium content

(%) 28 3

5.0 ) 27 % ( 26 4.5 25 4.0

24 3.5 23 3.0 22 21 2.5 20 2.0 19 Temperature : 400 ℃ 1.5 18 m(Sc2O3)︰m(NH4Cl)=1︰2.5 1.0 of Scandium rate Hydrolysis 17 Heating rate: 10 ℃/min

Scandium content of anhydrous ScCl of anhydrous content Scandium 16 Inert gases flow (Ar): 7.0 L/min 0.5 15 0.0 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 Holding time (min) Figure 4. Effect of holding time on scandium content of anhydrous scandium chloride and hydrolysis rate of scandium.

3.4. Effect of Heating Rate Tests of the effect of different heating rate tests were carried out; the results are shown in Figure 5. Since the crystallization water in scandium chloride is removed under the protection of inert gases flow, there are strict requirements on the heating speed. The purpose is to make the crystal water rapidlyAppl. Sci. gasify2020, 10 while, 5174 the water vapor is quickly taken away by the inert gas—and to minimize8 the of 13 probability of water vapor hydrolyzing with scandium chloride.

30 6.0 Hydrolysis rate of Scandium 29 5.5 Scandium content 28 (%) 3 5.0 ) 27 % ( 26 4.5 25 4.0

24 3.5 23 3.0 22 21 2.5 20 2.0 19 Temperature : 400 ℃ 1.5 18 m(Sc2O3)︰m(NH4Cl)=1︰2.5 1.0 Scandium of rate Hydrolysis 17 Holding time: 90 min Scandium content of anhydrous ScCl of content anhydrous Scandium 0.5 16 Inert gases flow (Ar): 7.0 L/min 15 0.0 Appl. Sci. 2020, 10, x FOR PEER REVIEW4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 8 of 13 Heating rate (℃/min) anhydrous scandium chloride with scandium content of 27.46% was produced and the hydrolysis Figure 5. Effect of heating rate on scandium content of anhydrous scandium chloride and hydrolysis rate ofFigure scandium 5. Eff wasect of 1.86% heating under rate on the scandium condition content used of a anhydrousheating rate scandium of 12 °C chloride/min. and hydrolysis raterate of of scandium. scandium. 3.5. Effect of Inert Gases Flow 3.5.When Effect ofthe Inert heating Gases rate Flow was less than 12 °C/min, increasing the heating rate was conducive to reducingThe mainthe hydrolysis function rate of inert of scandium. gas in the The process rate of heating of salt preparationwas 12 °C/min is toand protect the hydrolysis the process rate The main function of inert gas in the process of salt preparation is to protect the process atmosphere atmosphereof scandium and decreased the discharge to the minimum of water valuevapor. of If 1.86% the flow and rate scandium of inert content gas is oftoo 27.46% small, inwater anhydrous vapor and the discharge of water vapor. If the flow rate of inert gas is too small, water vapor cannot be cannotscandium be chloride. discharged When in time. the heating Too much rate flowincreased will resultto 16 °C in /min, the waste the hydrolysis of inert gas. rate The of scandium effect of discharged in time. Too much flow will result in the waste of inert gas. The effect of different inert differentincreased inert to 2.34% gases instead. (argon andThis nitrogen was related) flow to rate the testsfactors were such conducted as the slow and sublimation the results aratere shown of the gases (argon and nitrogen) flow rate tests were conducted and the results are shown in Figure6. incrystal Figure water 6. and the long action time of the water vapor and scandium chloride. Therefore,

3.5 35 3.5 34 (a) Hydrolysis rate of Scandium (b) Hydrolysis rate of Scandium 3.0 34 3.0 Scandium content Scandium content 33 (%)

32 (%) 3 3 2.5 ) 2.5 )

% 32 % ( 30 2.0 ( 31 2.0

28 1.5 30 1.5 1.0 29 1.0 26 28 0.5 0.5 24 27 0.0 26 0.0 22 -0.5 25 -0.5 Temperature : 400 ℃ Temperature : 400 ℃ 20 -1.0 24 -1.0 m(Sc O )︰m(NH Cl)=1︰2.5 m(Sc O )︰m(NH Cl)=1︰2.5 2 3 4 23 2 3 4 Hydrolysis rate of Scandium ofScandium rate Hydrolysis Holding time: 90 min -1.5 Holding time: 90 min -1.5 Scandium of rate Hydrolysis 18 22 Heating rate: 12℃/min ℃ Scandium content of anhydrous ScCl content of anhydrous Scandium Heating rate: 12 /min -2.0 content of anhydrous ScCl Scandium -2.0 16 21 -2.5 20 -2.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Argon flow (L/min) Nitrogen flow (L/min)

Figure 6. Effect of inert gases flow (a) argon; (b) nitrogen on scandium content of anhydrous scandium Figure 6. Effect of inert gases flow (a) argon; (b) nitrogen on scandium content of anhydrous scandium chloride and hydrolysis rate of scandium. chloride and hydrolysis rate of scandium. The test results in Figure6 further confirm that under the same inert gas flow condition, anhydrous scandiumThe test chloride results moltenin Figure salt 6 producedfurther confirm by argon that as u inertnder gas the was same superior inert gas to nitrogen flow condition, as inert. anhydrousThe reason scandium for this waschloride that argonmolten was salt heavierproduced than by airargon and as could inert carrygas was away superior water to vapor nitrogen more aseasily, inert. whichThe reason helps for to improvethis was that the qualityargon was of molten heavier salt. than With air and the increasecould carry of inert away gases water flow vapor rate, morethe hydrolysis easily, which rate helps of scandium to improve first decreasedthe quality and of thenmolten increased. salt. With When the theincrease inert gasesof inert flow gases increased flow rate,to 7.5 the L hydrolysis/min, the hydrolysis rate of scandium rate of scandiumfirst decrease decreasedd and then to 1.23% increase andd. theWhen scandium the inert of gases anhydrous flow increasedscandium to chloride 7.5 L/min, increased the hydrolysis to 29.56%. rate When of scandium the inert decreased gases flow to continued 1.23% and to the increase, scandium and ofthe anhydrousscandium scandium hydrolysis chloride rate changes increased little, to 29.56% which. indicated When the that inert the gases inert flow gases continue flow wasd to increase, too small, andthe the water scandium vapor could hydrolysis not be rate discharged changes in little, time andwhich the indicate interactiond that time the between inert gases scandium flow was chloride too small,andwater the water vapor vapor increased, could whichnot be willdischarged lead to thein time increase and the of the interaction scandium time hydrolysis between rate scandium and the chlorideunnecessary and water argon vapor was consumed. increased, Therefore,which willargon lead to flow the rateincrease of 7.5 of L /themin scandium was more hydr suitable.olysis rate and the unnecessary argon was consumed. Therefore, argon flow rate of 7.5 L/min was more suitable.

3.6. Repeated Tests In order to further validate test repeatability, repeated tests were carried out under the conditions employed as follows: a removing ammonium chloride and residual crystal water temperature of 400 °C; m(Sc2O3):m(NH4Cl) = 1:2.5; a holding time of 90 min; a heating rate of 12 °C/min; and an argon flow of 7.5 L/min. The repeated results are shown in Tables 2 and 3. These results in Table 2 show that the anhydrous scandium chloride molten salt index (scandium content and hydrolysis rate of scandium) of repeated experiments was superior to the conditions of resulted. Scandium chloride with the scandium content of 29.61% was obtained, and hydrolysis rate of scandium was 1.19%.This indicated that it was feasible to prepare high-purity scandium chloride using non-high-purity scandium chloride raw materials by the process of rapid temperature desorption of ammonium chloride to a certain temperature and residual crystal water, and the hydrolysis rate of scandium chloride in scandium chloride was low. Compared with the pure scandium chloride molten salt, the scandium content in the prepared scandium chloride was 29.61%, Appl. Sci. 2020, 10, 5174 9 of 13

3.6. Repeated Tests In order to further validate test repeatability, repeated tests were carried out under the conditions employed as follows: a removing ammonium chloride and residual crystal water temperature of 400 ◦C; m(Sc2O3):m(NH4Cl) = 1:2.5; a holding time of 90 min; a heating rate of 12 ◦C/min; and an argon flow of 7.5 L/min. The repeated results are shown in Tables2 and3.

Table 2. Repeated tests results of preparing anhydrous scandium chloride (%).

Scandium Content in Anhydrous Hydrolysis Rate Repeat ScCl3 Molten Salt of Scandium 1 29.56 1.19 2 29.62 1.18 3 29.54 1.26 4 29.66 1.21 5 29.64 1.19 6 29.59 1.24 7 29.67 1.16 8 29.59 1.16 Average 29.61 1.19

Range (R = Emax E ) 0.13 0.100 − min PN di i=1| | 0.039 0.027 Arithmetic mean error (δ = N ) N N 2 = P 2 = P ( ) Sum square variation (SS di Ei E ) 0.015 0.01 i=1 i=1 − SS Average deviation (MS = f ) 0.002 0.001 r P 2 (Ei E) √ 0.045 0.032 Standard deviation (s = N −1 = MS) −

Table 3. Main chemical composition analysis results of anhydrous scandium chloride (%).

Sc Cl− Fe2O3 SiO2 Al2O3 CaO MgO K2O Na2O 29.61 70.12 0.001 0.04 0.008 0.006 0.008 0.006 0.002

These results in Table2 show that the anhydrous scandium chloride molten salt index (scandium content and hydrolysis rate of scandium) of repeated experiments was superior to the conditions of resulted. Scandium chloride with the scandium content of 29.61% was obtained, and hydrolysis rate of scandium was 1.19%.This indicated that it was feasible to prepare high-purity scandium chloride using non-high-purity scandium chloride raw materials by the process of rapid temperature desorption of ammonium chloride to a certain temperature and residual crystal water, and the hydrolysis rate of scandium chloride in scandium chloride was low. Compared with the pure scandium chloride molten salt, the scandium content in the prepared scandium chloride was 29.61%, which was close to the theoretical scandium content in scandium chloride was 45/(45 + 35.5 3) 100% = 29.703%, and the × × purity was 29.61%/29.703 = 99.69%. Meanwhile, it is known from the results in Table3 reveal that the content of other impurities in the anhydrous scandium chloride molten salt also lower and provide high quality scandium-bearing raw materials for the subsequent preparation of aluminum–scandium alloy. Appl. Sci. 2020, 10, 5174 10 of 13

3.7. Analysis and Characterization of Anhydrous Scandium Chloride Molten Salt A novel one-step rapid heating process was used to prepare preparing high-purity anhydrous scandium chloride with non-high-purity scandium oxide, and the product index is ideal. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electronic probe microanalyzer (EPMA) were used to analyze micro area composition of the high-purity anhydrous scandium chloride molten salt. The XRD analysis results of high-purity anhydrous Appl.scandiumAppl. Sci. Sci. 20 202020 chloride, 10, 10, x, xFOR FOR werePEER PEER REVIEW show REVIEW in Figure7. The SEM-EDS images analysis results of the high-purity1010 of of 13 13 anhydrous scandium chloride are shown in Figure8. The micro area composition analysis results of washigh-puritywas feasible feasible anhydroususing using the the process process scandium of of inert chlorideinert gase gase ares sflow flow shown step step in in in Table quick quick4. heating heating and and removal removal of of ammonium ammonium chloridechloride and and residual residual crystal crystal water. water.

60006000 55005500 ScClScCl3 3 50005000 45004500 40004000 35003500 30003000 25002500 20002000 Intensity Intensity (a.u.) Intensity Intensity (a.u.) 15001500 10001000 500500 0 0 -500-500 1515 2020 2525 3030 3535 4040 4545 5050 5555 6060 6565 7070 7575 2θ2 θ(°) (°)

FigureFigure 7. 7.7. XRD XRD diffractograms didiffractogramsffractograms of ofof high high-purityhigh-purity-purity anhydrous anhydrousanhydrous scandium scandiumscandium chloride chloridechloride molten moltenmolten salt. salt.salt.

40004000 P1P1 35003500 ClCl

30003000

25002500

20002000 ScSc

15001500 Intensity (cps) Intensity (cps) 10001000

500500

0 0

-500-500 -1.0-1.0-0.5-0.5 0.00.0 0.50.5 1.01.0 1.51.5 2.02.0 2.52.5 3.03.0 3.53.5 4.04.0 4.54.5 5.05.0 BindingBinding energy energy (keV) (keV)

Figure 8. SEM-EDS images analysis results of high-purity anhydrous scandium chloride molten salt. FigureFigure 8 .8 SEM. SEM-EDS-EDS images images analysis analysis results results of of high high-purity-purity anhydrous anhydrous scandium scandium chloride chloride molten molten salt. salt. The XDR, SEM-EDS and EPMA analysis resulted of anhydrous scandium chloride confirm that the crystallizationTableTable 4. 4. E Electronlectron condition probe probe microanalysisof microanalysis high-purity (EPMA (EPMA anhydrous) )micro micro scandium area area composition composition chloride of of washigh high relatively-purity-purity anhydrous anhydrous good, di ff erent location of the electron probe micro elementsscandiumscandium also chloride chloride show that(%). (%). the impurity element content was lower and close to the theoretical purity of scandium chloride molten salt. Furthermore, further illustrate that preparing high-purityPositionPosition anhydrous Sc scandiumSc ClCl chloride SiSi withCaCa high-purity AlAl scandiumTotalsTotals oxide was feasible using the process of inertPP1 gases1 29.589 flow29.589 step 70.15670.156 in quick 0.0050.005 heating 0.0020.002 and removal0.0120.012 99.764 of99.764 ammonium chloride and residual crystal water. PP2 2 29.66229.662 70.23170.231 0.0060.006 0.0030.003 0.0130.013 99.91599.915 PP3 3 29.61729.617 70.11970.119 0.0030.003 0.0040.004 0.0240.024 99.76799.767 PP4 4 29.62229.622 70.11770.117 0.0060.006 0.0030.003 0.0310.031 99.77999.779 PP5 5 29.59929.599 70.20570.205 0.0090.009 0.0060.006 0.0430.043 99.86299.862 PP6 6 29.66929.669 70.09870.098 0.0080.008 0.0050.005 0.0390.039 99.81999.819 AverageAverage 29.62629.626 70.15470.154 0.0060.006 0.0040.004 0.0270.027 99.81899.818

4.4. Conclusions Conclusions

BasedBased on on the the test test results results of of preparing preparing high high-purity-purity anhydrous anhydrous sccl sccl3 3molten molten salt salt obtained obtained in in this this study,study, we we drew drew the the main main following following conclusions: conclusions: (1)(1) Hi Highgh-purity-purity anhydrous anhydrous ScCl ScCl3 3molten molten salt salt was was prepared prepared using using one one-step-step rapid rapid heating heating process process toto a a certain certain temperature temperature in in an an inert inert gas gas stream stream to to remove remove ammonium ammonium chloride chloride and and residual residual crystal crystal water.water. High High-purity-purity scandium scandium chloride chloride molten molten salt salt (purity, (purity, 99.69%) 99.69%) with with the the scandium scandium content content of of Appl. Sci. 2020, 10, 5174 11 of 13

Table 4. Electron probe microanalysis (EPMA) micro area composition of high-purity anhydrous scandium chloride (%).

Position Sc Cl Si Ca Al Totals

P1 29.589 70.156 0.005 0.002 0.012 99.764

P2 29.662 70.231 0.006 0.003 0.013 99.915

P3 29.617 70.119 0.003 0.004 0.024 99.767

P4 29.622 70.117 0.006 0.003 0.031 99.779

P5 29.599 70.205 0.009 0.006 0.043 99.862

P6 29.669 70.098 0.008 0.005 0.039 99.819 Average 29.626 70.154 0.006 0.004 0.027 99.818

4. Conclusions

Based on the test results of preparing high-purity anhydrous sccl3 molten salt obtained in this study, we drew the main following conclusions: (1) High-purity anhydrous ScCl3 molten salt was prepared using one-step rapid heating process to a certain temperature in an inert gas stream to remove ammonium chloride and residual crystal water. High-purity scandium chloride molten salt (purity, 99.69%) with the scandium content of 29.61%, was obtained and the hydrolysis rate of scandium was 1.19% under the conditions used: a removing ammonium chloride and residual crystal water temperature of 400 ◦C; m(Sc2O3):m(NH4Cl) = 1:2.5; a holding time of 90 min; a heating rate of 12 ◦C/min; and an argon flow of 7.5 L/min. The index of high-purity scandium chloride molten salt was ideal. (2) XRD, SEM and EPMA analysis results also further showed that anhydrous scandium chloride crystallization condition is relatively good. Impurity elements are lower in high-purity anhydrous ScCl3 molten salt. The purity of scandium chloride are close to the theory purity of scandium chloride. Therefore, it is feasible to prepare the high-purity anhydrous scandium chloride by the quick heating a certain temperature in inert gases flow process with non-high-purity scandium oxide. The high-purity scandium chloride molten salt can be used as an important raw material for preparing Al–Sc master alloy using Al–Mg thermoreduction method.

Author Contributions: This is a joint work of the seven authors; each author was in charge of their expertise and capability: J.X.: writing, formal analysis and original draft preparation; W.D. and C.C.: conceptualization; Y.P.: validation; T.C. and K.Z.: methodology; Z.Z.: investigation. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Sichuan Science and Technology Program (Grant Nos.2018FZ0092, Nos.201FS0451 and Nos.201FS0452); China Geological Big Survey (Grant No. DD20190694); Key Laboratory of Sichuan Province for Comprehensive Utilization of Vanadium and Titanium Resources Foundation (2018FTSZ35). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design, analyses and interpretation of any data of the study.

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