Synthesis of Cryolite (Na3alf6) from Secondary Aluminum Dross Generated in the Aluminum Recycling Process

materials

Article Synthesis of Cryolite (Na3AlF6) from Secondary Aluminum Dross Generated in the Aluminum Recycling Process

Bingbing Wan 1,2,3, Wenfang Li 1,*, Wanting Sun 1,2, Fangfang Liu 4, Bin Chen 1, Shiyao Xu 3, Weiping Chen 3,* and Aihua Yi 1

1 School of Materials Science and Engineering, Dongguan University of Technology, Dongguan 523000, China; [email protected] (B.W.); [email protected] (W.S.); [email protected] (B.C.); [email protected] (A.Y.) 2 School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China 3 Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology, Guangzhou 510640, China; [email protected] 4 Department of Electromechanical Engineering, Guangdong University of Science and Technology, Dongguan 523083, China; [email protected] * Correspondence: mewﬂ[email protected] (W.L.); [email protected] (W.C.)

Received: 1 August 2020; Accepted: 27 August 2020; Published: 2 September 2020

Abstract: Secondary aluminum dross (SAD) is regarded as a solid waste of aluminum recycling process that creates serious environmental and health concerns. However, SAD can also be used as a good source of aluminum, so that utilizing the SAD for the production of valuable products is a promising approach of recycling such waste. In the present work, a novel eco-friendly three-step process was proposed for the synthesis of cryolite (Na3AlF6) from the SAD, and it consisted of (1) water-washing pretreatment of SAD, (2) extraction of Al component via pyro-hydrometallurgy, including low-temperature alkaline smelting, water leaching and purification of leachate in sequence, (3) precipitation of cryolite from the purified NaAlO2 solution using the carbonation method. By analysis of the parameter optimization for each procedure, it was found that the maximum hydrolysis efficiency of aluminum nitride (AlN) in the SAD was around 68.3% accompanied with an extraction efficiency of Al reaching 91.5%. On this basis, the cryolite of high quality was synthesized under the following optimal carbonation conditions: reaction temperature of 75 ◦C, NaAlO2 concentration of 0.11 mol/L, F/(6Al) molar ratio of 1.10, and 99.99% CO2 gas pressure, and flow rate of 0.2 MPa and 0.5 L/min respectively. The formation of Na3AlF6 phase can be detected by XRD. The morphological feature observed by SEM revealed that the as-synthesized cryolite had a polyhedral shape (~1 µm size) with obvious agglomeration. The chemical composition and ignition loss of the as-synthesized cryolite complied well with the requirements of the Chinese national standard (GB/T 4291-2017).

Keywords: secondary aluminum dross; recycling; extraction; synthetic cryolite

1. Introduction With the extensive development of secondary aluminum industry, secondary aluminum dross (SAD) is widely generated during aluminum recycling process through remelting with salt ﬂux. In general, the SAD primarily contains Al2O3, aluminum metal, AlN, NaCl, KCl, SiO2, Si, and MgAl2O4. As a rough estimation in a recent report [1], more than a million tons of the SAD are produced throughout the world each year. Nevertheless, it should be noted that nearly 95% of the SAD is dumped in landﬁlls, which imposes a severe menace to the ecosystem [2]. For example, several substances such as

Materials 2020, 13, 3871; doi:10.3390/ma13173871 www.mdpi.com/journal/materials Materials 2020, 13, 3871 2 of 19

F−, Cl−, and other toxic metal ions might leak into ground water consequently resulting in serious water pollution [3]. Inhalation of the SAD particles suspended in the air may cause some respiratory diseases [4]. In addition, when the AlN in the SAD comes in touch with water, water vapor, or moisture, hazardous ammonia gas (NH3) is released, which pollutes the atmosphere [5]. As mentioned above, dumping of the SAD will intensify environmental and health problems. At the same time, relatively higher amount of valuable aluminum element, which accounts for approximately 40–50 wt.% in the SAD [6,7], will be lost. In order to address these issues, the adoption of an appropriate recycling strategy for the SAD is of particular importance, and recently the synthesis of high value-added products (such as hydrotalcites, MgAl2O4 spinel, γ-alumina, α-alumina, zeolites, calcium aluminates, etc.,) from the SAD as a source of aluminum has attracted significant attention of researchers [8–13]. Cryolite (Na3AlF6) is an important industrial chemical which is extensively applied in industries, especially in the electrolytic process for aluminum production [14]. As for conventional synthesis of cryolite, relatively expensive industrial Al(OH)3 or Al2(SO4)3 tend to be chosen as the aluminum source, which increases the production cost of cryolite, and thus their practical use is limited to some extent. To our knowledge, the investigation on the use of cheap and readily available SAD as a potential resource for cryolite synthesis has not yet been reported. Currently, hydrometallurgical process is a common technique to manage the SAD. The extraction of aluminum from the SAD is achieved by leaching operations. Aluminum can be leached out of the SAD 3+ using acidic or alkaline media, making the media concentrated in Al or (Al(OH)4)− ions [6,9,15,16]. Aluminum leached into the solution can be used to synthesize valuable products. Numerous literatures have indicated that the extraction efficiency of Al from the aluminum dross in acidic leaching was 20–30% higher than that in alkaline leaching [17–20]. However, it is worth noting that higher purity of Al in the alkaline leachate can be achieved compared to acidic leaching, because of the fact that most of the base metals (Cu, Fe, Ca, Mg, etc.,) in the aluminum dross are insoluble in the alkaline media but soluble in the acidic media [15]. The presence of a large number of metal ions are not desirable for cyolite preparation, during which the contents of impurities are limited strictly. In view of the above, under the premise of further improving the extraction efficiency of Al, it is more suitable to use alkaline leaching route in the present work. In the studies by Guo et al. [21,22], a pyro-hydrometallurgical process, which referred to low-temperature (below 900 ◦C) alkaline smelting and subsequent water leaching under atmospheric pressure, was found to be a feasible method to realize a high efficient extraction of Al from the SAD, and the corresponding extraction efficiency of Al was as high as 87.52%, which was about 30% higher than that in conventional alkaline leaching [20] and, in other words, equivalent to that in conventional acidic leaching [17–19]. Nevertheless, interestingly, a higher amount of Si can be introduced into the alkaline leachate from aluminum dross as a result of the dissolution of amphoteric metal (Si) by the NaOH [15]. Kobayashi et al. [23] mentioned that the presence of Si impurity in cryolite used widely in electrolytic process can cause significant lowering of the current efficiency. Therefore, for the sake of obtaining pure leachate as a preparation for subsequent cryolite synthesis, the removal of Si impurity from the leachate was indispensable. In this study, a novel eco-friendly three-step process to synthesize cryolite from the SAD was developed. It consisted of water-washing pretreatment of SAD, extraction of Al component via pyro-hydrometallurgy (including low-temperature alkaline smelting, water leaching and Si removal from the leachate in sequence), and precipitation of cryolite by carbonation method. In order to realize comprehensive utilization of SAD efficiently, the influences of various processing parameters for each procedure were carefully investigated. In addition, as for the cryolite synthesized under optimal processing conditions, its chemical composition, ignition loss, phase, and morphology were characterized by reliable techniques. Materials 2020, 13, 3871 3 of 19 Materials 2020, 13, x FOR PEER REVIEW 3 of 19

2. Experimental Experimental

2.1. Materials Materials Secondary aluminum dross (SAD, −100100 + 150150 mesh mesh (referred (referred to to those powders which can pass − through the 100-mesh sieve rather than the 150-me 150-meshsh sieve)) was derived from a domestic secondary aluminum plant. NaOH, NaNO3,, CaO, NaNa2SiF66,, and and Na Na2COCO33 werewere analytical analytical grade grade and purchased from AladdinAladdin ChemistryChemistry Co.Co. Ltd. (Shanghai, (Shanghai, China). China). Carbon Carbon dioxide dioxide gas gas (CO (CO22)) with with a a purity of 99.99% was obtained from Dongguan Yuanbang GasGas Co.Co. Ltd.Ltd. (Dongguan, China). Deionized water was utilized at all stages of the experiments.

2.2. Synthesis Synthesis Processes of Cryolite The procedure ofof cryolitecryolite synthesissynthesis in in the the current current study study contains contains three three different different steps steps as as depicted depicted in inFigure Figure1: water-washing 1: water-washing pretreatment pretreatment of SAD (a); of extraction SAD (a); of extraction Al component of viaAl pyro-hydrometallurgicalcomponent via pyro- processhydrometallurgical (b); carbonation process (c). (b); carbonation (c).

Figure 1.1. FlowchartFlowchart for for the the synthesis synthesis of cryoliteof cryolite from from secondary secondary aluminum aluminum dross (SAD):dross water-washing(SAD): water- pretreatmentwashing pretreatment of SAD of (a ),SAD extraction (a), extraction of Al componentof Al component via pyro-hydrometallurgical via pyro-hydrometallurgical process process (b), (andb), and carbonation carbonation (c). (c).

2.2.1. Water-Washing Water-Washing PretreatmentPretreatment ofof SADSAD Water-washing pretreatmentpretreatment of of SAD SAD (see (see Figure Figure1a) 1a) was was carried carried out out in a in breaker a breaker ﬂask flask which which was placedwas placed on a constanton a constant temperature temperature magnetic magnetic stirrer stir usedrer for used heating for heating and stirring. and stirring. First, a certainFirst, a amountcertain amountof SAD wasof SAD loaded was intoloaded the into glass the reactor. glass reactor. Then a givenThen a amount given amount of deionized of deionized water waswater added was added into it. Theinto mainit. The chemical main chemical reaction reaction involved involved in the water-washing in the water-washing of SAD of was SAD as followswas as follows [5]: [5]: ↑ AlNAlN+ 3H + 3HO2=O =Al Al(OH)(OH) 3+ + NHNH3 (R1) 2 3 3 ↑ The hydrolyzing of AlN in the SAD began at a given temperature with a stirring speed of 300 rpm, and the NH3 generated from the reactor was absorbed in the H2SO4 solution. After a predetermined reaction time, the content of the reactor was filtered under vacuum. The resulting

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The hydrolyzing of AlN in the SAD began at a given temperature with a stirring speed of 300 rpm, and the NH3 generated from the reactor was absorbed in the H2SO4 solution. After a predetermined reaction time, the content of the reactor was ﬁltered under vacuum. The resulting washed solution was evaporated to obtain soluble salts while the remaining washed residue dried at 120 ◦C for 4 h and weighted. The following Equation (1) was used to calculate the hydrolysis eﬃciency of AlN:

ω(AlN)1W1 ω(AlN)2W2 E1 = − 100% (1) ω(AlN)1W1 × where E1 is the hydrolysis eﬃciency of AlN, W1 and W2 are the total mass of the SAD before and after water washing experiment, respectively, and ω(AlN)1 and ω(AlN)2 are the mass fraction of AlN in the SAD before and after water washing experiment, respectively.

2.2.2. Extraction of Al Component via Pyro-Hydrometallurgical Process As illustrated in the Figure1b, at the beginning, aluminous constituents in the washed residue (washed dross) obtained in Section 2.2.1 were converted into the soluble salts by low-temperature alkaline smelting process. A muffle furnace was used for smelting operation. The smelting was conducted by mixing the washed dross (10 g) with NaOH and NaNO3 in specific proportions at a given temperature for a certain time. The NaOH and NaNO3 acted as the absorbent and oxidant, respectively. During the smelting, different reactions related to aluminous constituents and main impurities (such as Si and SiO2 particles) in the washed dross were taking place as follows:

Al(OH)3 + NaOH = NaAlO2 + 2H2O (R2)

Al2O3 + 2NaOH = 2NaAlO2 + H2O (R3) 10Al + 4NaOH + 6NaNO = 10NaAlO + 3N +2H O (R4) 3 2 2 ↑ 2 5AlN + 2NaOH + 3NaNO = 5NaAlO + 4N +H O (R5) 3 2 2 ↑ 2 Si + SiO2 + 4NaOH + O2 = 2Na2SiO3 + 2H2O (R6) Subsequently, the smelting products were subjected to leaching with deionized water at a given temperature. After a certain leaching time, the resulting solution was separated from solid residue by vacuum ﬁltration, and then analyzed for the recovery percent of Al and Si components. Extraction eﬃciency of Al and Si from washed dross could be calculated according to the following Equation (2):

cV E = 100% (2) 2 mω× where E2 is the extraction eﬃciency of each element, c is the mass concentration of each element in the leaching solution, V is the volume of solution, m is the mass of washed dross, and ω is the mass fraction of each element in the washed dross. For the removal of Si impurity from the leaching solution, a certain amount of CaO was added into 100 mL of leachate in a 200-mL glass beaker. The experiment was carried out at 90 ◦C with a stirring speed of 300 rpm for 60 min. The Si element in the leachate, existing as Na2SiO3, could react with Ca(OH)2 and NaAlO2 to generate calcium silicate slags [24,25], which were then separated from the solution by vacuum filtration. Eventually, the purified NaAlO2 solution was obtained.

2.2.3. Carbonation As shown in the Figure1c, prior to the synthetic reaction of cryolite, ﬂuorine source (NaF) was gained by the desilication process of low-cost Na2SiF6 as a by-product of the production of phosphatic Materials 2020, 13, 3871 5 of 19

fertilizer. The following desilication reaction occurred upon the mixing of the Na2SiF6 solution with Na2CO3 solution at 100 ◦C under proper agitation (500 rpm):

Na SiF + 2Na CO = 6NaF + SiO + 2CO (R7) 2 6 2 3 2 2 ↑ After the reaction (Equation R7) completed, the slurry was filtered under vacuum to remove the suspended SiO2 particles, and then the supernatant (NaF solution) was collected. Afterwards, the as-prepared NaF solution used as a precursor for fluorine was added into the purified NaAlO2 solution obtained in Section 2.2.2. Meanwhile, the CO2 (99.99% purity) under a pressure of 0.2 MPa was injected into the solution mixture with the gas flow rate of 0.5 L/min. The following carbonation reaction began at the temperatures ranged from 25 ◦C to 90 ◦C with a stirring speed of 300 rpm:

6NaF + NaAlO2 + 2CO2 = Na3AlF6 + 2Na2CO3 (R8)

Finally, the precipitated cryolite was separated from Na2CO3 solution by vacuum ﬁltration, washed and dried at 120 ◦C for 4 h. Interestingly, as seen from the Equations R7 and R8, the Na2CO3 and CO2 can be recovered for recycling.

2.3. Methods of Characterization The crystalline phases of the samples were identified by X-ray poly-crystalline diffractometer (XRD, Bruker D8 Advance) with Cu kα (λ = 1.5418 Å) radiation at an operating voltage of 40 kV with current of 40 mA. The chemical compositions of the SAD and as-synthesized cryolite were examined by X-ray fluorescence spectrometer (XRF, Axios Pw4400, Axios, Alemlo, The Netherlands), respectively. The Al and Si elements extracted in the leaching solution were detected by inductively coupled plasma atomic emission spectrometer (ICP-AES, JY ULTIMA-2, HORIBA JY, Paris, France). The variation in content of AlN in the SAD before and after the water-washing was measured by the Kjeldahl method. The contents of Na2O and Al2O3 in the leaching solution were analyzed by chemical titration method. The content of SiO2 in the leaching solution was determined according to the molybdenum blue photometric method (YS/T 575.3-2007 [26]). The evaluation of ignition loss of as-synthesized cryolite was carried out based on the YS/T 273.2-2006 standard [27]. After both the as-synthesized cryolite and the commercial cryolite were coated with a thin gold layer, their morphologies were observed using a scanning electron microscope (SEM, ZEISS Sigma 500, ZEISS, Jena, Germany) at an operating voltage of 3 kV. Besides, this microscope was equipped with an energy dispersive X-ray spectrometer (EDX, Oxford Instrument, Oxford, UK) for elemental analysis of samples.

3. Results and Discussion

3.1. Characterization of As-Received SAD Table1 presents the chemical composition of the as-received SAD. It can be seen that, nearly 75 wt.% of the as-received SAD consisted of Al and O elements, and the rest of them were composed of salts constituents like Na, K, F, and Cl, and other alloying elements (Mg, Si, Ca, Fe, Ti, Mn, Cu, etc.,). In addition, the as-received SAD also contained some trace elements with content below 0.10 wt.% such as S, Cr, Sr, Zn, and Hg. The corresponding XRD pattern of the as-received SAD is depicted in Figure2. According to the diffraction peaks, it can be confirmed that the as-received SAD was mainly constituted of nine crystalline phases: sodium chloride (NaCl), potassium chloride (KCl), pure aluminum (Al), aluminum nitride (AlN), corundum (Al2O3), silicon (Si), silica (SiO2), calcium fluoride (CaF2), and spinel (MgAl2O4). It is worth mentioning that certain other phases containing the elements as shown in Table1 were not detected by XRD, which is attributed to the fact that they may be in an amorphous state or exist as crystalline substances but in very low amounts [8,28]. Materials 2020, 13, 3871 6 of 19

Table 1. Chemical composition of the as-received SAD (wt.%).

Element O Al Si Na Mg F Cl K Ca Content 31.68 42.87 3.54 5.71 1.68 2.06 7.56 2.39 1.22 Element Fe Ti Cu Mn S Sr Zn Cr Hg Content 0.19 0.41 0.35 0.17 0.09 0.02 0.02 0.03 0.01 Materials 2020, 13, x FOR PEER REVIEW 6 of 19

Figure 2. The XRD pattern of the as-received SAD. 3.2. Water-Washing Pretreatment of SAD 3.2. Water-Washing Pretreatment of SAD Prior to the Al extraction, it is essential for the SAD to be subjected to the water-washing Prior to the Al extraction, it is essential for the SAD to be subjected to the water-washing pretreatment. On the one hand, the amount of AlN constituent possessing high reactivity with water pretreatment. On the one hand, the amount of AlN constituent possessing high reactivity with water can be decreased remarkably, thereby enhancing the chemical stability of the SAD as a raw material for can be decreased remarkably, thereby enhancing the chemical stability of the SAD as a raw material cyolite production. Meanwhile, the NH3 generated from the hydrolysis of AlN can be absorbed by for cyolite production. Meanwhile, the NH3 generated from the hydrolysis of AlN can be absorbed acid to generate ammonium salt fertilizer, thus eliminating the hazard of NH3 and simultaneously by acid to generate ammonium salt fertilizer, thus eliminating the hazard of NH3 and simultaneously achieving the recovery of nitrogen in the SAD. On the other hand, water-soluble salts constituents achieving the recovery of nitrogen in the SAD. On the other hand, water-soluble salts constituents like NaCl and KCl in the SAD can be recovered back by washing and evaporation. More importantly, like NaCl and KCl in the SAD can be recovered back by washing and evaporation. More importantly, the hydrolysis of AlN and the dissolution of salts contribute to increasing the efficiency of the subsequent the hydrolysis of AlN and the dissolution of salts contribute to increasing the efficiency of the leaching process [16,17]. subsequent leaching process [16,17]. 3.2.1. Hydrolysis of AlN in the SAD 3.2.1. Hydrolysis of AlN in the SAD Regarding the SAD, in order to maximize the removal of the AlN constituent and simultaneously Regarding the SAD, in order to maximize the removal of the AlN constituent and realize the centralized recovery of nitrogen as much as possible, one-factor-at-a time (OFAT) method simultaneously realize the centralized recovery of nitrogen as much as possible, one-factor-at-a time was adopted to examine the effects of water-washing processing parameters such as temperature, (OFAT) method was adopted to examine the effects of water-washing processing parameters such as reaction time and liquid-to-solid ratio on the hydrolysis efficiency of AlN in the SAD calculated by temperature, reaction time and liquid-to-solid ratio on the hydrolysis efficiency of AlN in the SAD Equation (1), eventually optimizing the water-washing process. As for the OFAT method, single calculated by Equation (1), eventually optimizing the water-washing process. As for the OFAT factor experiments are repeated until an optimal combination of factors is found. Here, the ranges of method, single factor experiments are repeated until an optimal combination of factors is found. influencing factors were set as follows: temperature (25–90 ◦C), reaction time (2–6 h), and liquid-to-solid Here, the ranges of influencing factors were set as follows: temperature (25–90 °C), reaction time (2– ratio (5–13 mL/g). In each experiment, the dosage of SAD was fixed at 10 g. 6 h), and liquid-to-solid ratio (5–13 mL/g). In each experiment, the dosage of SAD was fixed at 10 g. Figure3 illustrates the effects of the water-washing conditions on the hydrolysis efficiency of AlN. Figure 3 illustrates the effects of the water-washing conditions on the hydrolysis efficiency of It can be observed from the Figure3a that when the temperature varied from 25 ◦C to 90 ◦C, the hydrolysis AlN. It can be observed from the Figure 3a that when the temperature varied from 25 °C to 90 °C, the efficiency of AlN was increased continuously. A higher temperature can promote the dissolution of salts hydrolysis efficiency of AlN was increased continuously. A higher temperature can promote the on the surface of the AlN particles and make the water penetrate quickly into the AlN [5], thereby ensuring dissolution of salts on the surface of the AlN particles and make the water penetrate quickly into the the proceeding of the hydrolysis process more adequately. Besides, with the increase in the temperature, AlN [5], thereby ensuring the proceeding of the hydrolysis process more adequately. Besides, with the precipitation of NH3 occurred more rapidly because of the decreasing of the solubility of NH3, thus the increase in the temperature, the precipitation of NH3 occurred more rapidly because of the promoting the proceeding of forward reaction of hydrolysis, i.e., AlN + 3H2O Al(OH)3 + NH3 . decreasing of the solubility of NH3, thus promoting the proceeding of forward reaction→ of hydrolysis,↑ i.e., AlN + 3H2O → Al(OH)3 + NH3↑. Therefore, 90 °C was selected as the optimal water-washing temperature. Figure 3b shows the relationship between reaction time and hydrolysis efficiency of AlN. As the reaction time was extended from 2 h to 6 h, the hydrolysis efficiency of AlN remained almost constant, indicating the completion of hydrolysis reaction within 2 h. Hence, 2 h was chosen as the optimal water-washing time. By taking the temperature of 90 °C and the reaction time of 2 h for the water-washing process, a variation of liquid-to-solid ratio on the hydrolysis efficiency of AlN was investigated to determine the optimal liquid-to-solid ratio, and the corresponding results are described in Figure 3c. Apparently, the hydrolysis efficiency of AlN exhibited a decreasing trend with

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Therefore, 90 ◦C was selected as the optimal water-washing temperature. Figure3b shows the relationship between reaction time and hydrolysis efficiency of AlN. As the reaction time was extended from 2 h to 6 h, the hydrolysis efficiency of AlN remained almost constant, indicating the completion of hydrolysis reaction within 2 h. Hence, 2 h was chosen as the optimal water-washing time. By taking the temperature of 90 ◦C and the reaction time of 2 h for the water-washing process, a variation of liquid-to-solid ratio on the hydrolysis efficiency of AlN was investigated to determine the optimal liquid-to-solid ratio, andMaterials the 2020 corresponding, 13, x FOR PEER results REVIEW are described in Figure3c. Apparently, the hydrolysis efficiency of7 AlN of 19 exhibited a decreasing trend with increasing the liquid-to-solid ratio. According to the study of Li et al. [5], increasing the liquid-to-solid ratio. According to the study of Li et al. [5], they found that the they found that the hydrolysis of AlN was an exothermic reaction using thermodynamic calculation. hydrolysis of AlN was an exothermic reaction using thermodynamic calculation. Based on this fact, Based on this fact, a higher temperature of the reaction system appears at a lower liquid-to-solid ratio, a higher temperature of the reaction system appears at a lower liquid-to-solid ratio, thereby resulting thereby resulting in the adequate hydrolysis of AlN in the SAD. Consequently, the liquid-to-solid ratio of in the adequate hydrolysis of AlN in the SAD. Consequently, the liquid-to-solid ratio of 5 mL/g was 5 mL/g was taken as the optimal value for the water-washing process. taken as the optimal value for the water-washing process.

Figure 3.3. EEffectsﬀects of temperaturetemperature (a),), reactionreaction timetime ((b),), andand liquid-to-solidliquid-to-solid ratioratio ((c)) onon thethe hydrolysishydrolysis eefficiencyﬃciency ofof AlN.AlN.

InIn summary,summary, the the optimal optimal water-washing water-washing conditions conditions were were obtained obtained as follows: as follows: temperature temperature of 90 ◦ C,of reaction90 °C, reaction time of time 2 h, andof 2 liquid-to-solidh, and liquid-to-solid ratio of 5ratio mL/ g.of Under5 mL/g. these Under conditions, these conditions, the hydrolysis the hydrolysis efficiency ofefficiency AlN could of reach AlN 68.3%, could and reach through 68.3%, a rough and calculation, through ita wasrough found calculation, that approximately it was 38found kg of NHthat3 canapproximately be produced 38 after kg of the NH water-washing3 can be produced pretreatment after the of water-wash one metricing ton pretreatment of the SAD used of one in this metric study. ton of the SAD used in this study. 3.2.2. Recovery of Salts in the SAD 3.2.2.Under Recovery the optimalof Salts in water-washing the SAD conditions mentioned above, 100 g of SAD was subjected to the washingUnder the process. optimal Figure water-washing4 shows the conditions XRD patterns ment ofioned the SADabove, before 100 g and of SAD after was water subjected washing. to Asthecompared washing process. to the original Figure SAD, 4 shows the peaksthe XRD of Al, patte AlN,rns Al of2O the3, Si,SAD SiO before2, CaF 2and, and after MgAl water2O4 remainedwashing. inAs the compared XRD pattern to the of original washed SAD, residue the whilepeaks theof Al, peaks AlN, of Al soluble2O3, Si, salts SiO like2, CaF NaCl2, and and MgAl KCl2 disappeared.O4 remained Thus,in the itXRD may pattern be concluded of washed that resi bothdue NaCl while and the KCl peaks have of been soluble eﬀectively salts like removed NaCl and by the KCl water-washing disappeared. ofThus, the SAD.it may In be addition, concluded another that both major NaCl phase and of Al(OH)KCl have3 in been the washedeffectively residue removed was by also the detected, water- whichwashing was of derived the SAD. from In theaddition, hydrolysis another process major of AlNphase in of the Al(OH) SAD. As3 in seen the fromwashed the residue areas marked was also by bluedetected, dotted which squares was in derived Figure4 ,from the peak the hydrolysis intensity of proc the AlNess of was AlN remarkably in the SAD. weakened As seen afterfrom the the water areas washingmarked by of blue the SAD, dotted due squares to the in hydrolyzing Figure 4, the depletion peak intensity of AlN. of As the for AlN the was obtained remarkably washed weakened solution, after the water washing of the SAD, due to the hydrolyzing depletion of AlN. As for the obtained washed solution, it was evaporated to get the mixed NaCl and KCl crystals. As expected, it would be economically beneficial to reuse the recovered salts in the secondary aluminum smelter.

Materials 2020, 13, 3871 8 of 19 it was evaporated to get the mixed NaCl and KCl crystals. As expected, it would be economically beneﬁcial to reuse the recovered salts in the secondary aluminum smelter. Materials 2020, 13, x FOR PEER REVIEW 8 of 19

Figure 4.4. The XRDXRD patterns patterns of of the the SAD SAD before before and and after after water wa washing:ter washing: (1) original (1) original SAD, (2) SAD, washed (2) washed residue. residue. 3.3. Extraction of Al Component via Pyro-Hydrometallurgical Process 3.3. Extraction of Al Component via Pyro-Hydrometallurgical Process 3.3.1. Effect of Different Factors on the Extraction Efficiency of Al 3.3.1.As Effect mentioned of Different in Section Factors 2.2.2 on the, the Extraction Al component Efficiency in theof Al washed dross can be extracted via pyro-hydrometallurgicalAs mentioned in Section process. 2.2.2, Meanwhile, the Al component the main in impurity the washed element dross (Si) can will be extracted also be introduced via pyro- intohydrometallurgical the leachate. Under process. the Meanwhile, premise of the the obtained main impu optimalrity element water-washing (Si) will also process, be introduced in order to into get maximumthe leachate. extraction Under the rate premise of Al and ofsimultaneously the obtained optimal suppress water-washing the leaching ofprocess, Si efficiently, in order the to OFAT get approachmaximum wasextraction employed rate toof optimizeAl and simultaneously the pyro-hydrometallurgical suppress the leaching process. of Si The efficiently, ranges of the di ffOFATerent factorsapproach influencing was employed the extraction to optimize efficiency the pyro-hydro of Al and Simetallurgical calculated by process. Equation The (2) wereranges set of as different follows: alkalifactors (NaOH)-to-dross influencing the extraction mass ratio efficiency (0.9–1.7), of salt Al (NaNOand Si calculated3)-to-dross by mass Equation ratio (0.1–0.9), (2) were smeltingset as follows: time (40–120 min), leaching temperature (50–90 C), and leaching time (20–100 min). In each experiment, alkali (NaOH)-to-dross mass ratio (0.9–1.7),◦ salt (NaNO3)-to-dross mass ratio (0.1–0.9), smelting time the(40–120 smelting min), temperature leaching temperature was specified (50–90 to be °C), 500 and◦C, leaching which was time found (20–100 to be min). theoptimal In each temperatureexperiment, tothe extract smelting Al fromtemperature aluminum was dross specified in the to low-temperature be 500 °C, which alkaline was found smelting to be the on theoptimal basis temperature of the study byto extract Guo et Al al. from [21]. Besides,aluminum for dross the sake in the of makinglow-temp theerature smelting alkaline products smelting fully dissolveon the basis in water of the during study leaching,by Guo et theal. [21]. ratio Besides, of deionized for the water sake toof smeltingmaking the products smelting was products fixed at fully 6 mL dissolve/g. in water during leaching, the ratio of deionized water to smelting products was fixed at 6 mL/g. Effect of alkali-to-dross mass ratio • • Effect of alkali-to-dross mass ratio To evaluate the effect of alkali-to-dross mass ratio on the extraction efficiency of Al and Si, salt-to-drossTo evaluate mass the ratio, effect smelting of alkali-to-dross time, leaching mass temperature, ratio on the extraction and leaching efficiency time were of Al considered and Si, salt- at to-dross mass ratio, smelting time, leaching temperature, and leaching time were considered at specified values of 0.5, 80 min, 70 ◦C, and 60 min, respectively. Figure5 shows the variation of extraction especifiedfficiency versusvalues alkali-to-drossof 0.5, 80 min, mass 70 °C, ratio. and As 60 the mi alkali-to-drossn, respectively. mass Figure ratio 5 was shows increased the variation from 0.9 toof 1.3,extraction the extraction efficiency effi versusciency alkali of Al-to-dross and Si was mass increased ratio. As accordingly. the alkali-to-dross This may mass be ratio ascribed was toincreased the fact from 0.9 to 1.3, the extraction– efficiency of Al and Si was increased accordingly. This may be ascribed that the increasing of OH ion activity in the reaction system is beneficial for the formation of NaAlO2 to the fact that the increasing of OH– ion activity in the reaction system is beneficial for the formation and Na2SiO3 (see Equations (R2)–(R6)). However, further increase of alkali-to-dross mass ratio resulted inof NaAlO a decrease2 and in Na leaching2SiO3 (see effi Equationsciency of Al(R2)–(R6)). and Si. ThisHowever, can be fu duerther to increase the fact of that alkali-to-dross excessive dosage mass ofratio NaOH resulted increases in a decrease the viscosity in leaching of reaction efficiency system of Al and and thus Si. This decreases can be the due rate to the of massfact that transfer excessive [21]. Therefore,dosage of consideringNaOH increases the principle the viscosity of achieving of reacti a higheron system Al concentration and thus decreases and lower the Si concentrationrate of mass simultaneouslytransfer [21]. Therefore, in the leachate, considering the alkali-to-dross the principle massof achieving ratio of 1.3a higher was selected Al concentration as the optimal and lower value forSi concentration the extraction simultaneously process and the in subsequentthe leachate, single the alkali-to-dross factor experiments mass ratio will of be 1.3 conducted was selected at this as alkali-to-drossthe optimal value mass for ratio. the extraction process and the subsequent single factor experiments will be conducted at this alkali-to-dross mass ratio.

Materials 2020, 13, 3871 9 of 19

Materials 2020, 13, x FOR PEER REVIEW 9 of 19 Materials 2020, 13, x FOR PEER REVIEW 9 of 19

Figure 5. Effect of alkali-to-dross mass ratio on the extraction efficiency of Al and Si. FigureFigure 5. 5. EffectEﬀect of of alkali-to-dross alkali-to-dross mass mass ratio on th thee extraction efficiency eﬃciency of Al and Si.

• EffectEffect of of salt-to-dross salt-to-dross mass mass ratio ratio • Effect of salt-to-dross mass ratio TheThe investigation investigation on on the the effect effect of salt-to-dross mass ratio was performed under the following experimentalexperimental conditions: conditions: alkali-to- alkali-to-drossdross mass mass ration ration of of 1.3, 1.3, smelting smelting time time of of 80 80 min, leaching temperaturetemperaturetemperature ofof of 7070 70 °C,°C,◦C, andand and leachingleaching leaching timetime time ofof of6060 60min.min. min. FigureFigure Figure 66 presentspresents6 presents thethe effecteffect the e ofofffect salt-to-drosssalt-to-dross of salt-to-dross massmass ratiomass on ratio the onextraction the extraction efficiency effi ofciency Al and of Si. Al Apparently, and Si. Apparently, with the salt-to-dross with the salt-to-dross mass ratio increasing, mass ratio thetheincreasing, extractionextraction the efficiencyefficiency extraction ofof e ffi SiSiciency waswas nearlynearly of Si was constaconsta nearlynt, constant,whereas the whereas extraction the extraction efficiency e ffiofciency Al was of increasedincreasedAl was increased initiallyinitially andand initially thereafterthereafter and thereafter keptkept stable.stable. kept AsAs fofo stable.r the Si As extraction, for the Si the extraction, formation theof water-soluble formation of Na2SiO3, represented by Equation (R6), cannot be affected by the addition of NaNO3. During the Nawater-soluble2SiO3, represented Na2SiO 3by, represented Equation (R6), by Equation cannot be (R6), affected cannot by be the aff ectedaddition by theof NaNO addition3. During of NaNO the3. progress of Al extraction, the NaNO3, acted as a strong oxidizer, can promote the oxidation of Al and progressDuring the of progressAl extraction, of Al the extraction, NaNO3 the, acted NaNO as a3 ,strong acted asoxidizer, a strong can oxidizer, promote can the promote oxidation the of oxidation Al and AlNof Al constituents and AlN constituents in the washed in the washed dross (see dross Equati (see Equationsons (R4) (R4)and and(R5)), (R5)), and and then then more more oxidation oxidation products (Al2O3) might react with alkali to form the water-soluble NaAlO2 (see Equation (R3)). When products (Al (Al22OO3)3 )might might react react with with alkali alkali to form to form the water-soluble the water-soluble NaAlO NaAlO2 (see 2Equation(see Equation (R3)). When (R3)). thetheWhen salt-to-drosssalt-to-dross the salt-to-dross massmass ratioratio mass reachedreached ratio reached 0.3,0.3, reactantsreactants 0.3, reactants of Al ofand Al AlN and AlNwere were exhausted. exhausted. According According to the to above-mentionedthe above-mentioned analysis, analysis, the salt-to-dross the salt-to-dross mass mass ratio ratio of 0.3 of was 0.3 was considered considered as the as optimal the optimal value value for thethefor theextractionextraction extraction processprocess process andand and thethe the influenceinfluence influence ofof ofotherother other factfact factorsors on on the the extraction extraction efficiency efficiency of of Al Al and and Si Si will will bebe studied studied at at this this salt-to-dross salt-to-dross mass ratio.

Figure 6. Effect of salt-to-dross mass ratio on the extraction efficiency of Al and Si. FigureFigure 6. 6. EffectEffect of of salt-to-dross salt-to-dross mass mass ratio ratio on th thee extraction efficiency efficiency of Al and Si. • • EffectEffect of of smelting smelting time time • The following experimental conditions were applied to investigate the effect of smelting time on The following experimental conditions were applied to investigate the effect effect of smelting time on the extraction efficiency of Al and Si: alkali-to-dross mass ratio of 1.3, salt-to-dross mass ratio of 0.3, thethe extraction efficiency efficiency of Al an andd Si: alkali-to-dross alkali-to-dross mass mass ratio ratio of of 1.3, 1.3, salt-to-dr salt-to-drossoss mass mass ratio ratio of of 0.3, 0.3, leachingleaching temperaturetemperature ofof 7070 °C,°C, andand leachingleaching timetime ofof 6060 min.min. TheThe resultsresults areare illustratedillustrated inin thethe FigureFigure 7. As the smelting time was extended from 40 min toto 6060 min,min, thethe extractionextraction efficiencyefficiency ofof AlAl waswas increasedincreased accordingly.accordingly. AtAt aa shortershorter smeltingsmelting timetime,, inadequateinadequate leachingleaching reactionsreactions ledled toto aa lowerlower

Materials 2020, 13, 3871 10 of 19

leaching temperature of 70 ◦C, and leaching time of 60 min. The results are illustrated in the Figure7. As the smelting time was extended from 40 min to 60 min, the extraction efficiency of Al was increased accordingly.Materials 2020, 13 At, x aFOR shorter PEER REVIEW smelting time, inadequate leaching reactions led to a lower extraction10 rateof 19 of Al. Nevertheless, when the smelting time was longer than 60 min, the extraction efficiency of Al extraction rate of Al. Nevertheless, when the smelting time was longer than 60 min, the extraction remained at a nearly constant value, suggesting the completion of leaching reactions. As for the Si efficiency of Al remained at a nearly constant value, suggesting the completion of leaching reactions. extraction, when the smelting time was shorter than 80 min, the leaching efficiency of Si was increased As for the Si extraction, when the smelting time was shorter than 80 min, the leaching efficiency of Si with the proceeding of leaching reaction. Moreover, increasing smelting time beyond 80 min had was increased with the proceeding of leaching reaction. Moreover, increasing smelting time beyond an adverse effect on the Si extraction. This observation is explained by the possibility that SiO2 can 80 min had an adverse effect on the Si extraction. This observation is explained by the possibility that be combined with certain substances in the reaction system to generate water-insoluble composite SiO2 can be combined with certain substances in the reaction system to generate water-insoluble oxide particles (such as 3Al2O3 2SiO2 and 2CaO SiO2) under longer time of smelting. Consequently, composite oxide particles (such· as 3Al2O3·2SiO· 2 and 2CaO·SiO2) under longer time of smelting. the smelting time of 120 min, as an optimal value for the extraction process, was chosen to study other Consequently, the smelting time of 120 min, as an optimal value for the extraction process, was factors affecting the extraction efficiency of Al and Si. chosen to study other factors affecting the extraction efficiency of Al and Si.

Figure 7. EffectEﬀect of smelting time on the ex extractiontraction efficiency eﬃciency of Al and Si.

• EffectEffect of of leaching leaching temperature temperature • After the smelting operation, as-produced NaAlO2 and Na2SiO3 were dissolved in water by After the smelting operation, as-produced NaAlO2 and Na2SiO3 were dissolved in water by water leaching,water leaching, realizing realizing the separation the separation of Al and of Si Al with and other Si with impurities. other impurities. To investigate To investigate the effect of the leaching effect temperatureof leaching temperature on the extraction on the effi extractionciency of Aleffici andency Si, theof followingAl and Si, experimental the following conditions experimental were considered:conditions were alkali-to-dross considered: mass alkali-t ratioo-dross of 1.3, mass salt-to-dross ratio of 1.3, mass salt-to- ratiodross of 0.3, mass smelting ratio timeof 0.3, of smelting 120 min, andtime leachingof 120 min, time and of leaching 60 min. time Figure of8 60 shows min. theFigure e ff ect8 shows of leaching the effect temperature of leaching on temperature the extraction on etheffi ciencyextraction of Al efficiency and Si. It of is Al apparent and Si. that,It is appare with increasingnt that, with the increasing leaching temperature, the leaching the temperature, extraction ethefficiency extraction of Al efficiency remained of almost Al remained the same almost level, the whereas same level, the extraction whereas ethefficiency extraction of Si efficiency was decreased of Si firstwas decreased and then keptfirst and at a then stable kept trend. at a stable The decline trend. The in the decline extraction in the eextractionfficiency of efficiency Si may beof Si due may to thebe due fact to that the thefact elevatedthat the elevated temperature temperature of solution of solution system system promotes promotes silicon silicon suboxide suboxide to react to react with otherwith other substances substances to produce to produce precipitates precipitates readily read separatedily separated from the from leachate. the leachate. According According to theresults to the results shown in Figure 8, the leaching temperature of 80 °C was chosen for the subsequent extraction shown in Figure8, the leaching temperature of 80 ◦C was chosen for the subsequent extraction eefficiencyfficiency experiments.

Materials 2020, 13, x FOR PEER REVIEW 10 of 19 extraction rate of Al. Nevertheless, when the smelting time was longer than 60 min, the extraction efficiency of Al remained at a nearly constant value, suggesting the completion of leaching reactions. As for the Si extraction, when the smelting time was shorter than 80 min, the leaching efficiency of Si was increased with the proceeding of leaching reaction. Moreover, increasing smelting time beyond 80 min had an adverse effect on the Si extraction. This observation is explained by the possibility that SiO2 can be combined with certain substances in the reaction system to generate water-insoluble composite oxide particles (such as 3Al2O3·2SiO2 and 2CaO·SiO2) under longer time of smelting. Consequently, the smelting time of 120 min, as an optimal value for the extraction process, was chosen to study other factors affecting the extraction efficiency of Al and Si.

Figure 7. Effect of smelting time on the extraction efficiency of Al and Si.

• Effect of leaching temperature

After the smelting operation, as-produced NaAlO2 and Na2SiO3 were dissolved in water by water leaching, realizing the separation of Al and Si with other impurities. To investigate the effect of leaching temperature on the extraction efficiency of Al and Si, the following experimental conditions were considered: alkali-to-dross mass ratio of 1.3, salt-to-dross mass ratio of 0.3, smelting time of 120 min, and leaching time of 60 min. Figure 8 shows the effect of leaching temperature on the extraction efficiency of Al and Si. It is apparent that, with increasing the leaching temperature, the extraction efficiency of Al remained almost the same level, whereas the extraction efficiency of Si was decreased first and then kept at a stable trend. The decline in the extraction efficiency of Si may be due to the fact that the elevated temperature of solution system promotes silicon suboxide to react with other substances to produce precipitates readily separated from the leachate. According to the Materialsresults shown2020, 13, in 3871 Figure 8, the leaching temperature of 80 °C was chosen for the subsequent extraction11 of 19 efficiency experiments.

Materials 2020, 13, x FOR PEER REVIEW 11 of 19 Figure 8. Effect of leaching temperature on the extraction efficiency of Al and Si. Figure 8. Effect of leaching temperature on the extraction efficiency of Al and Si. Effect of leaching time • Effect of leaching time In view of the water leaching, besides the leaching temperature, an appropriate leaching time also In view of the water leaching, besides the leaching temperature, an appropriate leaching time plays a significant role in the extraction of Al and Si. The research on the effect of leaching time on also plays a significant role in the extraction of Al and Si. The research on the effect of leaching time the extraction of Al and Si was conducted under the following optimal conditions based on previous on the extraction of Al and Si was conducted under the following optimal conditions based on experimental results: alkali-to-dross mass ratio of 1.3, salt-to-dross mass ratio of 0.3, smelting time of previous experimental results: alkali-to-dross mass ratio of 1.3, salt-to-dross mass ratio of 0.3, 120 min, and leaching temperature of 80 C. Figure9 illustrates the variation of extraction e fficiency smelting time of 120 min, and leaching ◦temperature of 80 °C. Figure 9 illustrates the variation of versus leaching time. Under the condition of leaching time varied from 20 min to 40 min, the extraction extraction efficiency versus leaching time. Under the condition of leaching time varied from 20 min efficiency of Al and Si exhibited an increasing trend, due to the proceeding of dissolution of NaAlO to 40 min, the extraction efficiency of Al and Si exhibited an increasing trend, due to the proceeding2 and Na2SiO3. With further increasing the leaching time beyond 40 min, the extraction efficiency of of dissolution of NaAlO2 and Na2SiO3. With further increasing the leaching time beyond 40 min, the Al remained almost constant while the extraction efficiency of Si was increased continuously. But it extraction efficiency of Al remained almost constant while the extraction efficiency of Si was should be noticed that the extraction efficiency of Si began to decrease when the leaching time exceeded increased continuously. But it should be noticed that the extraction efficiency of Si began to decrease 60 min. This phenomenon might be ascribed to the possibility that SiO reacts with Na O and Al O when the leaching time exceeded 60 min. This phenomenon might be ascribed2 to the possibility2 that2 3 to form sodium-silicon residue in the solution system. As a result, the leaching time of 40 min was SiO2 reacts with Na2O and Al2O3 to form sodium-silicon residue in the solution system. As a result, taken as the optimal value for the extraction process. the leaching time of 40 min was taken as the optimal value for the extraction process.

Figure 9. EffectEﬀect of leaching time on the extraction effi eﬃciency of Al and Si.

In summary,summary, the the optimal optimal conditions conditions of pyro-hydrometallurgicalof pyro-hydrometallurgical process process were obtainedwere obtained as follows: as follows:alkali-to-dross alkali-to-dross mass ratio ma ofss 1.3, ratio salt-to-dross of 1.3, salt-to-dross mass ratio of mass 0.3, smelting ratio of temperature 0.3, smelting of 500temperature◦C, smelting of 500time °C, of 120smelting min, deionizedtime of 120 water min, to deionized smelting productswater to ratio smelting of 6 mL products/g, leaching ratiotemperature of 6 mL/g, leaching of 80 ◦C, temperature of 80 °C, and leaching time of 40 min. Under these conditions, the extraction efficiency of Al reached up to 91.5%, which was 34% higher than that obtained in conventional alkaline leaching [20]. At the same time, the extraction efficiency of Si was limited to 53.8%.

3.3.2. The Removal of Si Impurity from NaAlO2 Solution Under optimal conditions of pyro-hydrometallurgical process, the resulting leaching solution was obtained. To acquire the pure NaAlO2 solution, the deep desilication approach by adding CaO [29] was implemented to effectively remove Si impurity from the leachate. The corresponding reaction was taken place as below [30]:

3Ca(OH)2 + Na2SiO3 + 2NaAl(OH)4 = 3CaO·Al2O3·SiO2·4H2O + 4NaOH + H2O (R9) After the completion of the reaction (Equation (R9)), the calcium silicate slag, i.e., 3CaO·Al2O3·SiO2·4H2O, was separated from the NaAlO2 solution, thereby realizing the removal of Si impurity. Table 2 shows the mass concentrations of major components of NaAlO2 solution before and after the purification. As can be seen, the silica module, i.e., the mass ratio of Al2O3 to SiO2, was increased from 20 to 449, and the desilication rate reached up to nearly 96%. These results indicated

Materials 2020, 13, 3871 12 of 19 and leaching time of 40 min. Under these conditions, the extraction eﬃciency of Al reached up to 91.5%, which was 34% higher than that obtained in conventional alkaline leaching [20]. At the same time, the extraction eﬃciency of Si was limited to 53.8%.

3.3.2. The Removal of Si Impurity from NaAlO2 Solution Under optimal conditions of pyro-hydrometallurgical process, the resulting leaching solution was obtained. To acquire the pure NaAlO2 solution, the deep desilication approach by adding CaO [29] was implemented to eﬀectively remove Si impurity from the leachate. The corresponding reaction was taken place as below [30]:

3Ca(OH) + Na SiO + 2NaAl(OH) = 3CaO Al O SiO 4H O + 4NaOH + H O (R9) 2 2 3 4 · 2 3· 2· 2 2 After the completion of the reaction (Equation (R9)), the calcium silicate slag, i.e., 3CaO Al O SiO 4H O, was separated from the NaAlO solution, thereby realizing the removal of · 2 3· 2· 2 2 Si impurity. Table2 shows the mass concentrations of major components of NaAlO 2 solution before and after the purification. As can be seen, the silica module, i.e., the mass ratio of Al2O3 to SiO2, was increased from 20 to 449, and the desilication rate reached up to nearly 96%. These results indicated a better purifying effect of NaAlO2 solution by CaO addition, which will be very beneficial for the subsequent synthesis of high-quality cryolite.

Table 2. The mass concentrations of major components of NaAlO2 solution before and after the purification.

Major Components (g/L) Stages Al2O3 Na2O SiO2 Before the puriﬁcation 48.16 73.84 2.41 After the puriﬁcation 43.52 71.59 0.097

3.4. Synthesis of Cryolite (Na3AlF6) by Carbonation Method

3.4.1. Effect of Different Factors on the Quality of Synthetic Cryolite The Chinese national standard of GB/T 4291-2017 has already put forward stringent requirements on the quality of synthetic cryolite, mainly involving the chemical composition and ignition loss of the cryolite [31]. Thereinto, the level of crystal water and volatile impurity phases in the cryolite can be denoted by the ignition loss. Apparently, the higher the ignition loss, the lower the utilization of cryolite. In general, during the reaction process of carbonation represented by Equation (R8), in addition to the principal phase of Na3AlF6, the cryolite products can also contain several impurity phases that will directly influence the purity of main product. Under the premise of the obtained optimal water-washing and Al extraction processes, in order to effectively search for the optimal conditions of carbonation process to precipitate high-quality synthetic cryolite, a comprehensive study was carried out to reveal the effects of reaction temperature, NaAlO2 concentration and F/(6Al) molar ratio on the aluminum content, fluorine content, and ignition loss of synthetic cryolite. To optimize the quality of synthetic cryolite, the OFAT method was adopted. The ranges of above-mentioned factors were set as follows: reaction temperature (30–90 ◦C), NaAlO2 concentration (0.02–0.14 mol/L), and F/(6Al) molar ratio (0.90–1.10). In each experiment, the pH value of reaction system was adjusted to 9.5 which has been confirmed to be the optimal pH value to synthesize cryolite in our preliminary experiments of the present study.

Eﬀect of reaction temperature • The investigation on the eﬀect of reaction temperature was carried out under the following experimental conditions: NaAlO2 concentration of 0.08 mol/L, F/(6Al) molar ratio of 1.00. Figure 10 Materials 2020, 13, x FOR PEER REVIEW 12 of 19

a better purifying effect of NaAlO2 solution by CaO addition, which will be very beneficial for the subsequent synthesis of high-quality cryolite.

Table 2. The mass concentrations of major components of NaAlO2 solution before and after the purification.

Major Components (g/L) Stages Al2O3 Na2O SiO2 Before the purification 48.16 73.84 2.41 After the purification 43.52 71.59 0.097

3.4. Synthesis of Cryolite (Na3AlF6) by Carbonation Method

3.4.1. Effect of Different Factors on the Quality of Synthetic Cryolite The Chinese national standard of GB/T 4291-2017 has already put forward stringent requirements on the quality of synthetic cryolite, mainly involving the chemical composition and ignition loss of the cryolite [31]. Thereinto, the level of crystal water and volatile impurity phases in the cryolite can be denoted by the ignition loss. Apparently, the higher the ignition loss, the lower the utilization of cryolite. In general, during the reaction process of carbonation represented by Equation (R8), in addition to the principal phase of Na3AlF6, the cryolite products can also contain several impurity phases that will directly influence the purity of main product. Under the premise of the obtained optimal water-washing and Al extraction processes, in order to effectively search for the optimal conditions of carbonation process to precipitate high-quality synthetic cryolite, a comprehensive study was carried out to reveal the effects of reaction temperature, NaAlO2 concentration and F/(6Al) molar ratio on the aluminum content, fluorine content, and ignition loss of synthetic cryolite. To optimize the quality of synthetic cryolite, the OFAT method was adopted. The ranges of above-mentioned factors were set as follows: reaction temperature (30–90 °C), NaAlO2 concentration (0.02–0.14 mol/L), and F/(6Al) molar ratio (0.90–1.10). In each experiment, the pH value of reaction system was adjusted to 9.5 which has been confirmed to be the optimal pH value to synthesize cryolite in our preliminary experiments of the present study. • Materials Effect2020 of, 13 reaction, 3871 temperature 13 of 19 The investigation on the effect of reaction temperature was carried out under the following experimental conditions: NaAlO2 concentration of 0.08 mol/L, F/(6Al) molar ratio of 1.00. Figure 10 presents the eﬀect of reaction temperature on the aluminum content, ﬂuorine content, and ignition loss presents the effect of reaction temperature on the aluminum content, fluorine content, and ignition of synthetic cryolite. loss of synthetic cryolite.

FigureFigure 10.10. EEffectﬀect of reaction temperature on the aluminum content,content, ﬂuorinefluorine content,content, andand ignitionignition lossloss ofof syntheticsynthetic cryolite.cryolite.

According to Figure 10, as the temperature was increased from 30 ◦C to 75 ◦C, the ﬂuorine content in the cryolite was increased while the ignition loss of cryolite was decreased. However, when the temperature exceeded 75 ◦C, the ﬂuorine content in the cryolite began to decrease while the ignition loss of cryolite remained almost the same level. Regarding the aluminum content of cryolite, it was increased continuously when the temperature varied within the range from 30 ◦C to 90 ◦C. These evolution trends can be attributed to the following two reasons. On the one hand, the dawsonite (NaAlCO (OH) 2H O) will be formed during the carbonation process according to 3 2· 2 the reaction proposed by Bénézeth et al. [32]:

NaAlO + CO + 3H O = NaAlCO (OH) 2H O (R10) 2 2 2 3 2· 2 In addition, they also found that the solubility of dawsonite was increased notably with increasing temperature [32]. Namely, the higher the reaction temperature, the less the amount of dawsonite precipitates. With the change in temperature lying in the range from 30 ◦C to 75 ◦C, the relative amount of ﬂuorine-free dawsonite was decreased, thus resulting in an increase of ﬂuorine content in the cryolite products. Meanwhile, the reduction in the content of crystal waters relevant to the dawsonite would lead to a decline in the ignition loss of cryolite products. On the other hand, the following reaction is induced during the process of synthesizing cryolite under alkaline environment [33]:

3+ 3OH− + Al = Al(OH)3 (R11)

Through thermodynamic calculation, the formation of Al(OH)3 precipitate belonged to an endothermic reaction (Equation (R11)) [33]. Theoretically, the higher the reaction temperature, the more the amount of Al(OH)3 precipitates. Furthermore, it should be noted that the aluminum element in Al(OH)3 (34.6 wt.%) was significantly higher than that in Na3AlF6 (12.9 wt.%). Thus, the continuously rising trend of aluminum content in the cryolite products appeared with the temperature raising. Besides, owing to the increment in relative amount of Al(OH)3, the fluorine content in the cryolite products exhibited a declining trend when the temperature varied from 75 ◦C to 90 ◦C. Eventually, based on the above-mentioned results, 75 ◦C was selected as the optimal reaction temperature to investigate other factors affecting the quality of synthetic cryolite.

Eﬀect of NaAlO concentration • 2 Materials 2020, 13, x FOR PEER REVIEW 13 of 19

According to Figure 10, as the temperature was increased from 30 °C to 75 °C, the fluorine content in the cryolite was increased while the ignition loss of cryolite was decreased. However, when the temperature exceeded 75 °C, the fluorine content in the cryolite began to decrease while the ignition loss of cryolite remained almost the same level. Regarding the aluminum content of cryolite, it was increased continuously when the temperature varied within the range from 30 °C to 90 °C. These evolution trends can be attributed to the following two reasons. On the one hand, the dawsonite (NaAlCO3(OH)2·2H2O) will be formed during the carbonation process according to the reaction proposed by Bénézeth et al. [32]:

NaAlO2 + CO2 + 3H2O = NaAlCO3(OH)2·2H2O (R10) In addition, they also found that the solubility of dawsonite was increased notably with increasing temperature [32]. Namely, the higher the reaction temperature, the less the amount of dawsonite precipitates. With the change in temperature lying in the range from 30 °C to 75 °C, the relative amount of fluorine-free dawsonite was decreased, thus resulting in an increase of fluorine content in the cryolite products. Meanwhile, the reduction in the content of crystal waters relevant to the dawsonite would lead to a decline in the ignition loss of cryolite products. On the other hand, the following reaction is induced during the process of synthesizing cryolite under alkaline environment [33]:

3OH- + Al3+ = Al(OH)3 (R11)

Through thermodynamic calculation, the formation of Al(OH)3 precipitate belonged to an endothermic reaction (Equation (R11)) [33]. Theoretically, the higher the reaction temperature, the more the amount of Al(OH)3 precipitates. Furthermore, it should be noted that the aluminum element in Al(OH)3 (34.6 wt.%) was significantly higher than that in Na3AlF6 (12.9 wt.%). Thus, the continuously rising trend of aluminum content in the cryolite products appeared with the temperature raising. Besides, owing to the increment in relative amount of Al(OH)3, the fluorine content in the cryolite products exhibited a declining trend when the temperature varied from 75 °C to 90 °C. Eventually, based on the above-mentioned results, 75 °C was selected as the optimal reaction Materialstemperature2020, 13 to, 3871 investigate other factors affecting the quality of synthetic cryolite. 14 of 19

• Effect of NaAlO2 concentration The following experimental conditions were adopted to study the eﬀect of NaAlO2 concentration The following experimental conditions were adopted to study the effect of NaAlO2 on the aluminum content, ﬂuorine content, and ignition loss of synthetic cryolite: reaction temperature concentration on the aluminum content, fluorine content, and ignition loss of synthetic cryolite: of 75 C, F/(6Al) molar ratio of 1.00. The results are depicted in Figure 11. reaction◦ temperature of 75 °C, F/(6Al) molar ratio of 1.00. The results are depicted in Figure 11.

Figure 11.11. EEffectﬀect ofof NaAlO NaAlO2 2concentration concentration on on the the aluminum aluminum content, conten ﬂuorinet, fluorine content, content, and and ignition ignition loss ofloss synthetic of synthetic cryolite. cryolite.

It cancan bebe clearlyclearly seenseen fromfrom FigureFigure 1111 thatthat thethe aluminumaluminum content,content, fluorinefluorine content,content, andand ignitionignition lossloss ofof synthetic synthetic cryolite cryolite were were decreased decreased first and first then and kept then unchanged kept unchanged with the NaAlO with2 concentrationthe NaAlO2 increasing.concentration It is increasing. worthy to mentionIt is worthy that to the mention falling trendthat the of thefalling aluminum trend of content the aluminum was more content significant was than that of the fluorine content. A possible explanation for this phenomenon is that, the increasing + of Na ion concentration can promote the transformation of NaAlF4 into Na3AlF6 according to the reaction [34]: + Na3AlF6 = NaAlF4 + 2Na + 2F− (R12) Herein, after the phase transformation of equimolar cryolite completed, it was calculated that the extent of decline of the aluminum content was almost four times greater than that of the fluorine content. Furthermore, Veryasov et al. [35] pointed out that the roles of NaAlF4 and Na3AlF6 were respectively equivalent to NaF AlF 0.83H O and 3NaF AlF 0.167H O in the system of NaF-AlF -H O. · 3· 2 · 3· 2 3 2 Therefore, when the NaAlF4 phase was transformed into Na3AlF6 phase, the decreasing of crystal water content would cause the ignition loss of cryolite products decreasing. Interestingly, further increase of NaAlO2 concentration beyond 0.11 mol/L had little impact on the chemical composition of cryolite products. According to the above-mentioned findings, the NaAlO2 concentration of 0.11 mol/L was chosen for the subsequent single factor experiments.

Eﬀect of F/(6Al) molar ratio •

By taking the temperature of 75 ◦C and the NaAlO2 solution of 0.11 mol/L for the carbonation reaction, a variation of F/(6Al) molar ratio on the aluminum content, fluorine content, and ignition loss of synthetic cryolite was investigated to determine the optimal F/(6Al) molar ratio. The results are shown in Figure 12. Materials 2020, 13, x FOR PEER REVIEW 14 of 19

more significant than that of the fluorine content. A possible explanation for this phenomenon is that, the increasing of Na+ ion concentration can promote the transformation of NaAlF4 into Na3AlF6 according to the reaction [34]:

Na3AlF6 = NaAlF4 + 2Na+ + 2F- (R12) Herein, after the phase transformation of equimolar cryolite completed, it was calculated that the extent of decline of the aluminum content was almost four times greater than that of the fluorine content. Furthermore, Veryasov et al. [35] pointed out that the roles of NaAlF4 and Na3AlF6 were respectively equivalent to NaF·AlF3·0.83H2O and 3NaF·AlF3·0.167H2O in the system of NaF-AlF3-H2O. Therefore, when the NaAlF4 phase was transformed into Na3AlF6 phase, the decreasing of crystal water content would cause the ignition loss of cryolite products decreasing. Interestingly, further increase of NaAlO2 concentration beyond 0.11 mol/L had little impact on the chemical composition of cryolite products. According to the above-mentioned findings, the NaAlO2 concentration of 0.11 mol/L was chosen for the subsequent single factor experiments. • Effect of F/(6Al) molar ratio

By taking the temperature of 75 °C and the NaAlO2 solution of 0.11 mol/L for the carbonation reaction, a variation of F/(6Al) molar ratio on the aluminum content, fluorine content, and ignition Materialsloss of synthetic2020, 13, 3871 cryolite was investigated to determine the optimal F/(6Al) molar ratio. The results15 of 19 are shown in Figure 12.

Figure 12. EffectEﬀect of F/(6Al) F/(6Al) molar ratio on the aluminum content,content, ﬂuorinefluorine content and ignition loss of synthetic cryolite.

As can be seen from Figure 12,12, both the aluminum content and the fluorine fluorine content in the cryolite exhibited a rising trend when the FF/(6Al)/(6Al) molarmolar ratioratio variedvaried fromfrom 0.900.90 toto 1.00.1.00. Under a lowerlower molarmolar ratio of FF/(6Al),/(6Al), thethe excessiveexcessive NaAlONaAlO22 is likely to react with CO2 toto form form impurity impurity phase phase (dawsonite). (dawsonite). - 3+ Moreover, thethe FF- ionion withwith lowerlower concentrationconcentration couldcould bebe combinedcombined withwith AlAl3+ ion to generate mainly - - AlF4- ion.ion. When When the the concentration concentration of of F F- ionion was was increased, the formation of fluorine-freefluorine-free dawsonitedawsonite containing high crystal water was inhibitedinhibited andand therebythereby thethe generationgeneration ofof cryolitecryolite (NaAlF(NaAlF44) waswas promoted, resultingresulting in in a significanta significant decline decline of ignition of ignition loss andloss remarkable and remarkab enhancementle enhancement in the fluorine in the contentfluorine for content cryolite for products. cryolite products. Meanwhile, Meanwhile, because of because the aluminum of the aluminum content in content the dawsonite in the dawsonite (15 wt.%) being(15 wt.%) lower being than lower that in than the NaAlFthat in4 (21.4the NaAlF wt.%),4 the(21.4 aluminum wt.%), the content aluminum in the content cryolite in products the cryolite was increasedproducts was to some increased extent. to However, some extent. when However, the F/(6Al) wh molaren the ratioF/(6Al) exceeded molar ratio 1.00, exceeded all of the aluminum1.00, all of content,the aluminum fluorine content, content, fluorine and ignition content, loss and of syntheticignition loss cryolite of synthetic were decreased cryolite continuously,were decreased as indicatedcontinuously, in Figure as indicated 12. Under in aFigure higher 12. molar Under ratio a ofhigher F/(6Al), molar F- ion ratio with of higher F/(6Al), concentration F- ion with seemed higher 3+ 3- toconcentration be combined seemed with Alto beion combined to generate with mainlyAl3+ ion AlFto generate6 ion. Themainly formation AlF63- ion. of NaThe3AlF formation6 led to of a diminutionNa3AlF6 ledin tothe a diminution aluminum in content, the aluminum fluorine content, andfluorine ignition content, loss. and As aignition result, the loss. F/ (6Al)As a result, molar ratiothe F/(6Al) of 1.10 molar was taken ratio asof the1.10 optimal was taken value as the for theoptimal carbonation value for reaction. the carbonation reaction. In summary,summary, the the optimal optimal conditions conditions of carbonationof carbonation reaction reaction (Equation (Equation (R8)) to(R8)) produce to produce high quality high syntheticquality synthetic cryolite werecryolite obtained were asobtained follows: reactionas follows: temperature reaction oftemperature 75 ◦C, NaAlO of2 concentration75 °C, NaAlO of2 0.11 mol/L, and F/(6Al) molar ratio of 1.10. Under these conditions, the aluminum content, fluorine content, and ignition loss of the synthetic cryolite could reach 12.6 wt.%, 53.2 wt.%, and 2.1 wt.%, respectively. In addition, further measurement by XRF was also performed, and its sodium and SiO2 impurity content were 32.18 wt.% and 0.125 wt.%, respectively.

3.4.2. Characterization of As-Synthesized Cryolite Figure 13 illustrates the XRD patterns of commercial cryolite and as-synthesized cryolite. Apparently, the XRD characteristic peaks of as-synthesized cryolite matched very well with those of commercial cryolite, which confirmed that the main phase precipitated from the carbonation reaction under optimal conditions was Na3AlF6. Besides, it can also be observed that the characteristic peak intensity of the as-synthesized cryolite was relatively weaker than that of the commercial one. This may be ascribed to the fact that there exist a certain amount of impurities in the washed dross leachate influencing the purity of cryolite product. Figure 14 represents the SEM images of the commercial cryolite and as-synthesized cryolite. As seen from Figure 14a,c, the morphology of as-synthesized cryolite powders was similar with that of commercial cryolite powders. The powder consisted of several agglomerates of different sizes. Figure 14b,d revealed that the agglomerates were composed of fine particles (~1 µm size) in polyhedral shape which exhibited a strong tendency toward agglomeration. Figure 15 shows the EDX analysis of commercial cryolite and as-synthesized cryolite. The EDX Materials 2020, 13, x FOR PEER REVIEW 15 of 19 concentration of 0.11 mol/L, and F/(6Al) molar ratio of 1.10. Under these conditions, the aluminum content, fluorine content, and ignition loss of the synthetic cryolite could reach 12.6 wt.%, 53.2 wt.%, and 2.1 wt.%, respectively. In addition, further measurement by XRF was also performed, and its sodium and SiO2 impurity content were 32.18 wt.% and 0.125 wt.%, respectively.

Materialsseveral 2020agglomerates, 13, 3871 of different sizes. Figure 14b,d revealed that the agglomerates were composed16 of 19 of fine particles (~1 μm size) in polyhedral shape which exhibited a strong tendency toward agglomeration. Figure 15 shows the EDX analysis of commercial cryolite and as-synthesized cryolite. resultsThe EDX disclosed results adisclosed major chemistry a major ofchemistry Al, F, and of Na Al, as F, the and main Na elementsas the main of cryolite. elements Interestingly, of cryolite. aInterestingly, trace amount a oftrace Si andamount O were of Si detected and O were as impurities detected inas the impurities as-synthesized in the as-synthesized cryolite rather cryolite than in therather commercial than in the cryolite. commercial In addition, cryoli thete. In contents addition, of major the contents components of major (Al, components F, and Na) of (Al, as-synthesized F, and Na) cryoliteof as-synthesized as represented cryolite in as Figure repr esented15b were in almostFigure consistent15b were almost with the consistent XRF results with as the mentioned XRF results in Sectionas mentioned 3.4.1. It in was Section calculated 3.4.1. that It thewas molecule calculated ratio that of as-synthesizedthe molecule ratio cryolite of (deﬁnedas-synthesized as the molecule cryolite ratio(defined ofNaF as the/AlF molecu3 or thele molarratio of ratio NaF/AlF of Na3 /Al)or the was molar approximately ratio of Na/Al) three, was which approximately belonged to three, high molecularwhich belonged ratio. to high molecular ratio.

Materials 2020, 13,Figure x FOR PEER 13. The REVIEW XRD patterns of commercial cryolite and as-synthesized cryolite. 16 of 19

Figure 14. SEM micrographs ofof (a(a)) commercial commercial cryolite, cryolite, (b ()b enlarged) enlarged image image of of the the area area A markedA marked in ( ain), (ca)), as-synthesized (c) as-synthesized cryolite, cryolite, and and (d) enlarged(d) enlarged image image of the of areathe area B marked B marked in (c in). (c).

As mentioned above, it is clear that the chemical composition and physical property of the as-synthesized cryolite with high molecular ratio (such as aluminum content, ﬂuorine content, sodium content, SiO2 impurity content, and ignition loss) are well in line with the requirements of the Chinese national standard (GB/T 4291-2017 [31]). Besides, the obtained results of XRD and SEM/EDX analyses, as depicted in Figure 13, Figure 14, and Figure 15, indicated that the as-synthesized cryolite had nearly the same phase composition and morphological feature with the commercial one. Based on the above comprehensive analysis, we can ﬁnally conclude that the as-synthesized cryolite can be a promising candidate for industrial applications.

Figure 15. EDX analysis of (a) commercial cryolite and (b) as-synthesized cryolite.

As mentioned above, it is clear that the chemical composition and physical property of the as- synthesized cryolite with high molecular ratio (such as aluminum content, fluorine content, sodium content, SiO2 impurity content, and ignition loss) are well in line with the requirements of the Chinese national standard (GB/T 4291-2017 [31]). Besides, the obtained results of XRD and SEM/EDX analyses, as depicted in Figure 13, Figure 14, and Figure 15, indicated that the as-synthesized cryolite had nearly the same phase composition and morphological feature with the commercial one. Based on the above comprehensive analysis, we can finally conclude that the as-synthesized cryolite can be a promising candidate for industrial applications.

4. Conclusions In this paper, a novel eco-friendly method to synthesize cryolite from the SAD was developed to realize comprehensive utilization of SAD efficiently. This method consisted of three different procedures: water-washing pretreatment of SAD, extraction of Al component via pyro- hydrometallurgy, and precipitation of cryolite by carbonation method. After systematic investigations, the following conclusions were drawn: (1) The maximum hydrolysis efficiency of AlN in the SAD, i.e., 68.3%, was achieved when the water-washing temperature, reaction time, stirring speed, and liquid-to-solid ratio were 90 °C,

Materials 2020, 13, x FOR PEER REVIEW 16 of 19

MaterialsFigure2020 ,14.13, SEM 3871 micrographs of (a) commercial cryolite, (b) enlarged image of the area A marked in17 of 19 (a), (c) as-synthesized cryolite, and (d) enlarged image of the area B marked in (c).

Figure 15. EDXEDX analysis analysis of ( a)) commercial commercial cryolite and ( b) as-synthesized cryolite.

4. ConclusionsAs mentioned above, it is clear that the chemical composition and physical property of the as- synthesized cryolite with high molecular ratio (such as aluminum content, fluorine content, sodium In this paper, a novel eco-friendly method to synthesize cryolite from the SAD was developed content, SiO2 impurity content, and ignition loss) are well in line with the requirements of the Chinese to realize comprehensive utilization of SAD efficiently. This method consisted of three different national standard (GB/T 4291-2017 [31]). Besides, the obtained results of XRD and SEM/EDX analyses, procedures: water-washing pretreatment of SAD, extraction of Al component via pyro-hydrometallurgy, as depicted in Figure 13, Figure 14, and Figure 15, indicated that the as-synthesized cryolite had and precipitation of cryolite by carbonation method. After systematic investigations, the following nearly the same phase composition and morphological feature with the commercial one. Based on conclusions were drawn: the above comprehensive analysis, we can finally conclude that the as-synthesized cryolite can be a (1)promisingThe maximumcandidate for hydrolysis industrial e ffiapplications.ciency of AlN in the SAD, i.e., 68.3%, was achieved when the water-washing temperature, reaction time, stirring speed, and liquid-to-solid ratio were 90 ◦C, 4. Conclusions2 h, 300 rpm, and 5 mL/g, respectively. Meanwhile, approximately 38 kg of NH3 can be produced Inafter this the paper, water-washing a novel eco-friendly pretreatment method of one to metric synthesize ton of thecryolite SAD. from In addition, the SAD the was water-soluble developed to realizesalts constituentscomprehensive such utilization as NaCl and of KClSAD were effici recoveredently. This by method washing consisted and evaporation. of three different (2)procedures:The optimal water-washing conditions ofpret pyro-hydrometallurgicalreatment of SAD, extraction process wereof Al obtained component as follows: via alkalipyro- hydrometallurgy,(NaOH)-to-dross and mass precipitation ratio of 1.3, saltof (NaNOcryolite3 )-to-drossby carbonation mass ratio method. of 0.3, smelting After temperaturesystematic investigations,of 500 ◦C, the smelting following time conclusions of 120 min, were deionized drawn: water to smelting products ratio of 6 mL/g, leaching temperature of 80 ◦C, and leaching time of 40 min. Under these conditions, the extraction (1) Theefficiency maximum of Al hydrolysis reached up efficien to 91.5%,cy whichof AlN was in 34%the SAD, higher i.e., than 68.3%, that achievedwas achieved in conventional when the water-washingalkaline leaching. temperature, reaction time, stirring speed, and liquid-to-solid ratio were 90 °C, (3) The synthetic cryolite of high quality was precipitated under the following optimal conditions of carbonation: synthesis temperature of 75 ◦C, NaAlO2 concentration of 0.11 mol/L, F/(6Al) molar ratio of 1.10, and 99.99% CO2 gas pressure and flow rate of 0.2 MPa and 0.5 L/min respectively. The XRD and SEM/EDX analyses suggested that, the as-synthesized cryolite mainly consisted of the Na3AlF6 phase, and despite the occurrence of agglomeration, its morphology still exhibited a polyhedral shape with ~1 µm size. Moreover, aluminum content (12.6 wt.%), fluorine content (53.2 wt.%), sodium content (32.18 wt.%), SiO2 impurity content (0.125 wt.%), and ignition loss (2.1 wt.%) of the as-synthesized cryolite with high molecular ratio complied well with the requirement of the Chinese national standard (GB/T 4291-2017). Besides, from the angle of aluminum source and fluorine source used to synthesize cryolite, this novel eco-friendly three-step process was of lower cost and higher efficiency compared to conventional synthetic methods, suggesting it had the potential to be applied to the cryolite industry. Materials 2020, 13, 3871 18 of 19

Author Contributions: Conceptualization, B.W., W.L., and W.C.; methodology, B.W., B.C., and A.Y.; validation, B.W., W.L., and W.C.; investigation, B.W. and S.X.; data curation, W.S. and F.L.; writing—original draft preparation, B.W., B.C., and A.Y.; writing—review and editing, W.S., F.L., B.C., and A.Y.; supervision, W.L. and W.C.; project administration, W.L. and W.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515110726), the Project funded by China Postdoctoral Science Foundation (No. 2020M673388), the Opening Project of National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials (South China University of Technology) (No. 2019010), and the Major Scientific Research Program of Higher Education of Guangdong (No. 2019KTSCX254). Acknowledgments: The authors would like to thank Guangdong Research Center of High Performance Light Alloys Forming Technology and Novel Light Alloy and its Process Technology Key Laboratory of Dongguan City for providing equipments for preparation and characterization of materials. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Mahinroosta, M.; Allahverdi, A.; Dong, P.; Bassim, N. Green template-free synthesis and characterization of mesoporous alumina as a high value-added product in aluminum black dross recycling strategy. J. Alloy. Compd. 2019, 792, 161–169. [CrossRef] 2. Bonet-Martínez, E.; Pérez-Villarejo, L.; Eliche-Quesada, D.; Castro, E. Manufacture of Sustainable Clay Bricks Using Waste from Secondary Aluminum Recycling as Raw Material. Materials 2018, 11, 2439. [CrossRef] [PubMed] 3. Meshram, A.; Singh, K.K. Recovery of valuable products from hazardous aluminum dross: A review. Recour. Conserv. Recycl. 2018, 130, 95–108. [CrossRef] 4. Adeosun, S.O.; Sekunowo, O.I.; Taiwo, O.O.; Ayoola, W.A.; Machado, A. Physical and mechanical properties of aluminum dross. Adv. Mater. 2014, 3, 6–10. [CrossRef] 5. Li, Q.; Yang, Q.; Zhang, G.F.; Shi, Q. Investigations on the hydrolysis behavior of AlN in the leaching process of secondary aluminum dross. Hydrometallurgy 2018, 182, 121–127. [CrossRef] 6. David, E.; Kopac, J. Aluminum recovery as a product with high added value using aluminum hazardous waste. J. Hazard. Mater. 2013, 261, 316–324. [CrossRef][PubMed] 7. Yang, Q.; Li, Q.; Zhang, G.F.; Shi, Q.; Feng, H.G. Investigation of leaching kinetics of aluminum extraction from secondary aluminum dross with use of hydrochloric acid. Hydrometallurgy 2019, 187, 158–167. [CrossRef]

8. Gil, A.; Arrieta, E.; Vicente, M.A.; Korili, S.A. Synthesis and CO2 adsorption properties of hydrotalcite-like compounds prepared from aluminum saline slag wastes. Chem. Eng. J. 2018, 334, 1341–1350. [CrossRef]

9. Zhang, Y.; Guo, Z.H.; Han, Z.Y.; Xiao, X.Y.; Peng, C. Feasibility of aluminum recovery and MgAl2O4 spinel synthesis from secondary aluminum dross. Int. J. Miner. Metall. Mater. 2019, 26, 309–318. [CrossRef] 10. Mahinroosta, M.; Allahverdi, A. Enhanced alumina recovery from secondary aluminum dross for high purity nanostructured γ-alumina powder production: Kinetic study. J. Environ. Manag. 2018, 212, 278–291. [CrossRef] 11. Tripathy, A.K.; Mahalik, S.; Sarangi, C.K.; Tripathy, B.C.; Sanjay, K.; Bhattacharya, I.N. A pyro-hydrometallurgical process for the recovery of alumina from waste aluminium dross. Miner. Eng. 2019, 137, 181–186. [CrossRef] 12. López-Delgado, A.; Robla, J.I.; Padilla, I.; López-Andrés, S.; Romero, M. Zero-waste process for the transformation of a hazardous aluminum waste into a raw material to obtain zeolites. J. Clean. Prod. 2020, 255, 120178. [CrossRef] 13. López, F.A.; Martín, M.I.; Alguacil, F.J.; Sergio Ramírez, M.; González, J.R. Synthesis of Calcium Aluminates from Non-Saline Aluminum Dross. Materials 2019, 12, 1837. [CrossRef][PubMed] 14. Wang, L.S.; Wang, C.M.; Yu, Y.; Huang, X.W.; Long, Z.Q.; Hou, Y.K.; Cui, D.L. Recovery of ﬂuorine from bastnasite as synthetic cryolite by-product. J. Hazard. Mater. 2012, 209, 77–83. [CrossRef] 15. Murayama, N.; Maekawa, I.; Ushiro, H.; Miyoshi, T.; Shibata, J.; Valix, M. Synthesis of various layered double hydroxides using aluminum dross generated in aluminum recycling process. Int. J. Miner. Process. 2012, 110, 46–52. [CrossRef] 16. Meshram, A.; Jain, A.; Gautam, D.; Singh, K.K. Synthesis and characterization of tamarugite from aluminium dross: Part, I. J. Environ. Manag. 2019, 232, 978–984. [CrossRef] 17. Das, B.R.; Dash, B.; Tripathy, B.C.; Bhattacharya, I.N.; Das, S.C. Production of η-alumina from waste aluminium dross. Miner. Eng. 2007, 20, 252–258. [CrossRef] Materials 2020, 13, 3871 19 of 19

18. Dash, B.; Das, B.R.; Tripathy, B.C.; Bhattacharya, I.N.; Das, S.C. Acid dissolution of alumina from waste luminium dross. Hydrometallurgy 2008, 92, 48–53. [CrossRef] 19. Amer, A.M. Aluminium extraction from aluminum industrial wastes. J. Miner. Met. Mater. Soc. 2010, 62, 60–63. [CrossRef] 20. Sakiridis, P.E.; Oustadakis, P.; Agatzini-Leonardou, S. Aluminium recovery during black dross hydrothermal treatment. J. Environ. Chem. Eng. 2013, 1, 23–32. [CrossRef] 21. Guo, X.Y.; Li, F.; Tian, Q.H.; Ji, K. Recovery of aluminium from secondary aluminum dross using low-temperature alkaline smelting. J. Cent. South Univ. (Sci. Technol.) 2012, 43, 809–814. 22. Guo, X.Y.; Liu, J.X. Optimization of low-temperature alkaline smelting process of crushed metal enrichment originated from waste printed circuit boards. J. Cent. South Univ. 2015, 22, 1643–1650. [CrossRef] 23. Kobayashi, Y.; Nakamura, T. Process of Preparing High Purity Cryolite from Fluoride Solution Containing Dissolved Silica. U.S. Patent Application 64,363, 17 February 1979. 24. Barzgar, S.; Lothenbach, B.; Tarik, M.; Di Giacomo, A.; Ludwig, C. The Eﬀect of Sodium Hydroxide on Al uptake by Calcium Silicate Hydrates (C-S-H). J. Colloid Interface Sci. 2020, in press. [CrossRef][PubMed]

25. Martín-Garrido, M.; Martínez-Ramírez, S. CO2 adsorption on calcium silicate hydrate gel synthesized by double decomposition method. J. Therm. Anal. Calorim. 2020, in press. 26. Methods for Chemical Analysis of Bauxite–Part 3: Determination of Silicon Dioxide Content–Molybdenum Blue Photometric Method; YS/T 575.3–2007; National Development and Reform Commission: Beijing, China, 2007. 27. Chemical Analysis Methods and Physical Properties of Cryolite–Part 2: Determination of Ignition Loss; YS/T 273.2–2006; National Development and Reform Commission: Beijing, China, 2006. 28. Yoldi, M.; Fuentes-Ordoñez, E.G.; Korili, S.A.; Gil, A. Eﬃcient recovery of aluminum from saline slag wastes. Miner. Eng. 2019, 140, 105884. [CrossRef]

29. Ma, J.Y.; Li, Z.B.; Zhang, Y.; Demopoulos, G.P. Desilication of sodium aluminate solution by Friedel0s salt (FS: 3CaO A1 O CaCl 10H O). Hydrometallurgy 2009, 99, 225–230. [CrossRef] · 2 3· 2· 2 30. Sun, H.L.; Wang, B.; Zhang, J.X.; Zong, S.F.; Liu, J.J. Secondary reaction mechanism of leaching process of calcium aluminate slag. Trans. Nonferr. Met. Soc. China 2015, 25, 1334–1340. [CrossRef] 31. Synthetic Cryolite; GB/T 4291–2017; General Administration of Quality Supervision, Inspection & Quarantine of the People’s Republic of China (AQSIQ), Standardization Administration of the People’s Republic of China (SAC): Beijing, China, 2017. 32. Bénézeth, P.; Palmer, D.A.; Anovitz, L.M.; Horita, J. Dawsonite synthesis and reevaluation of its

thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2. Geochim. Cosmochim. Acta 2007, 71, 4438–4455. [CrossRef] 33. Chen, J.Y.; Lin, C.W.; Lin, P.H.; Li, C.W.; Liang, Y.M.; Liu, J.C.; Chen, S.S. Fluoride recovery from spent

fluoride etching solution through crystallization of Na3AlF6 (synthetic cryolite). Sep. Purif. Technol. 2014, 137, 53–58. [CrossRef] 34. Wan, B.B.; Li, W.F.; Liu, F.F.; Lu, T.W.; Jin, S.X.; Wang, K.; Yi, A.H.; Tian, J.; Chen, W.P. Determination of fluoride component in the multifunctional refining flux used for recycling aluminum scrap. J. Mater. Res. Technol. 2020, 9, 3447–3459. [CrossRef] 2 35. Veryasov, G.; Matsumoto, K.; Hagiwara, R. The discrete AlF5 − fluoroaluminate anion in the structure of [Tetraethylammonium]2[AlF5](H2O)2. Eur. J. Inorg. Chem. 2015, 32, 5306–5310. [CrossRef]

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