Carbonation of Sodium Aluminate/Sodium Carbonate Solutions for Precipitation of Alumina Hydrates—Avoiding Dawsonite Formation

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Article Carbonation of Sodium Aluminate/Sodium Carbonate Solutions for Precipitation of Alumina Hydrates—Avoiding Dawsonite Formation

Danai Marinos 1,* , Dimitrios Kotsanis 1, Alexandra Alexandri 1 , Efthymios Balomenos 1,2 and Dimitrios Panias 1

1 Laboratory of Metallurgy, School of Mining and Metallurgical Engineering, National Technical University of Athens (NTUA), Heroon Polytechniou 9, 15780 Zografou, Greece; dkotsanis@metal.ntua.gr (D.K.); [email protected] (A.A.); [email protected] (E.B.); [email protected] (D.P.) 2 MYTILINEOS S.A, METTALURGY BUSINESS, Agios Nikolaos, 32003 Voiotia, Greece * Correspondence: [email protected]; Tel.: +30-210-772-2299

Abstract: Experimental work has been performed to investigate the precipitation mechanism of aluminum hydroxide phases from sodium aluminate/sodium carbonate pregnant solutions by carbon dioxide gas purging. Such solutions result from leaching calcium aluminate slags with sodium carbonate solutions, in accordance with the Pedersen process, which is an alternative process for alumina production. The concentration of carbonate ions in the pregnant solution is revealed as a key factor in controlling the nature of the precipitating phase. Synthetic aluminate solutions of varying sodium carbonate concentrations, ranging from 20 to 160 g/L, were carbonated, and the resulting precipitating phases were characterized by X-ray diffraction analysis. Based on the results Citation: Marinos, D.; Kotsanis, D.; Alexandri, A.; Balomenos, E.; Panias, of the previous carbonation tests, a series of experiments were performed in which the duration of D. Carbonation of Sodium carbonation and the aging period of the precipitates varied. For this work, a synthetic aluminate Aluminate/Sodium Carbonate solution containing 20 g/L free Na2CO3 was used. The precipitates were characterized with X-ray Solutions for Precipitation of Alumina diffraction analysis and Fourier-transform infrared spectroscopy. Hydrates—Avoiding Dawsonite Formation. Crystals 2021, 11, 836. Keywords: precipitation; aluminum hydroxide; dawsonite; sodium aluminate solution; carbonation; https://doi.org/10.3390/cryst11070836 Pedersen process; bayerite; CO2 purging

Academic Editor: Béatrice Biscans 1. Introduction Received: 23 June 2021 Currently, the dominant process to produce metallurgical grade alumina is the Bayer Accepted: 16 July 2021 Published: 20 July 2021 process. With the Bayer Process, high-quality bauxites are digested in caustic solutions to selectively dissolve the aluminum hydroxide content of the ore. The resulting alumi-

Publisher’s Note: MDPI stays neutral nate solution is supersaturated in aluminum trihydroxide, which is precipitated through with regard to jurisdictional claims in seeding precipitation, which takes advantage of the temperature dependence of aluminum published maps and institutional afﬁl- trihydroxide solubility in alkaline solutions. A major drawback of the process is the ex- iations. cessive production of the solid residue of the leaching stage, the so-called bauxite residue, which contains all the unreacted components of the original ore, as well as phases that have precipitated during the caustic digestion step. Practically, the ratio of alumina to residue production is equal to one [1]. Alternatives to the Bayer process have been studied in the past, and the Pedersen process is one of them, which was applied in the Høyanger Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. plant in Norway from 1929 to 1969 with an annual production of about 17,000 tons of This article is an open access article alumina [2,3]. Although the Pedersen process was abandoned in 1969, studying the process distributed under the terms and from the perspective of sustainability and circular economy, several advantages stand conditions of the Creative Commons out [4]. Primarily, it promises a complete utilization of bauxite ores, even some rejected by Attribution (CC BY) license (https:// the Bayer process [5–7], since it aims at the production of iron and alumina. Of course, the creativecommons.org/licenses/by/ process also faces some limitations in terms of the type of raw material: if the raw material 4.0/). contains more than 44% of Al2O3, then the amount of SiO2 cant exceed 8% [5], and also,

Crystals 2021, 11, 836. https://doi.org/10.3390/cryst11070836 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 836 2 of 14

the process will not be economically viable if the iron content is very low. Moreover, a beneﬁt of the Pedersen process is that the hydrometallurgical conditions employed are moderate and, most importantly, the issue of bauxite residue is eliminated as, in its place, a calcium-based residue is produced in the leaching stage with potential applications. In the Pedersen process, prior to leaching, bauxite ore (of suitable Fe concentration) is reductively smelted in an Electric Arc Furnace (EAF) with ﬂuxes, mainly lime, to recover the metallic iron and produce a calcium aluminate slag [3,7–10]. In this stage, it is important to produce a slag of appropriate crystallinity and chemical composition, that will be easily soluble in the subsequent leaching step [7]. In the leaching step, the slag is leached with a di- lute sodium carbonate/sodium hydroxide solution to produce a calcium carbonate residue (called grey mud) and a pregnant sodium aluminate solution (PLS) [8–12]. Following a solid/liquid separation process, the pregnant solution enters the precipitation stage, where carbon dioxide gas is bubbled to lower the pH of the solution and precipitate aluminum as aluminum trihydroxide [8–10,12,13]. Carbon dioxide is dissolved in the sodium aluminate solution according to Reactions (1) and (2) [12,14–16]. With the increase in hydrogen ion concentration, resulting from the dissolution of CO2, the alkaline solution is gradually neutralized, reducing the pH and triggering the decomposition of the aluminate ion, resulting in the precipitation of aluminum hydroxide according to Reaction (3) [14–16].

+ = − + + CO2 H2O HCO3(aq) H(aq) (1)

− = 2− + + HCO3(aq) CO3(aq) H(aq) (2) ( )− = ( ) ↓ + − Al OH 4(aq) Al OH 3(s) OH(aq) (3) Concerning the Pedersen process, the pyrometallurgical stage has been extensively studied, and plenty of information is available in the literature. On the contrary, the literature concerning the hydrometallurgical stage and especially the precipitation stage of the process is quite limited. The patents of the Pedersen process [9,10] initially declared that carbon dioxide precipitation is feasible but not effective. Later, it was claimed that, by adding sodium hydroxide to increase alkalinity and alumina hydrate seed, precipitation can be performed at 80 ◦C according to the principles of the Bayer precipitation stage. The spent solution after precipitation is then recarbonated with carbon dioxide gas and recycled to the leaching stage. Additionally, knowledge on the Pedersen process provided by a report from the Bureau of Mines [12] indicates that the resulting PLS from the Pedersen process contained 15–20 g/L of Al2O3 and the precipitation lasted for 10–15 h without clarifying whether the duration mentioned only refers to the carbonation period or to both the carbonation and the aging period. Information is also available by some research groups that have attempted to modify the Pedersen process by changing the composition of the leaching solution or by using the process with alternate raw materials (e.g., clay instead of bauxite or other types of bauxites). These research groups applied carbonation temperatures ranging from 50 to 90 ◦C. Henry E. Blake et al. [17] ended the carbonation at a pH of 9.5, at 70 ◦C, precipitating 93% of aluminum (with 17–28% losses in the desilication step) as alumina hydrate. T. P. Hignett [8] presented a pilot plant operation, in which carbonation took place at 50–70 ◦C for 10 to 30 h, also applying a 2 h aging stage. Thomson et al. [12] mention that the sodium aluminate solutions resulting from leaching are unstable. Precipitation stopped at a pH of 10.8 at 80 ◦C with a duration of 4–8 h. Additionally, this patent [11] carried out the precipitation at 90 ◦C following the approach of the Pedersen patents. Besides being mentioned in the Pedersen process, precipitation by carbonation is highly attractive to alumina reﬁneries that operate based on sintering processes, since the neutralized product of sodium carbonate can be directly recycled. Such is the case in reﬁneries in Russia and China that have poor grade diasporic bauxites or nepheline [18–20]. ◦ The ore is mixed with Na2CO3 and is sintered in temperatures higher than 1000 C to Crystals 2021, 11, 836 3 of 14

form sodium aluminate, which is highly soluble in water. Out of such aluminate solutions, alumina hydrates are precipitated by carbonation [21]. The carbonation process is carried out in temperatures of room temperature (RT) to 80 ◦C[14–16,22–24] and duration times ranging from 5–8 h [25]. Lee et al. [26] showed that higher temperatures (70 ◦C) favor the formation of gibbsite, while lower temperatures (RT) favor the formation of bayerite. Padilla et al. [22] carried out the carbonation at RT, precipitating 97% of Al as boehmite and 20% of the silicon. Shayanfar and Aghazadeh et al. [14–16] carried out extensive research on the carbonation of aluminate solutions resulting from the nepheline lime-sinter process. Both thermodynamically and experimentally, they have shown that at an ending pH of 9, 10 and 11, dawsonite, a mixture of dawsonite and imogolite and bayerite, correspondingly, are precipitated. Apparently, the carbonation of sodium aluminate solutions is a process not sufﬁciently understood, as suggested by the literature review that was presented, concerning: the composition of the PLS undergoing precipitation is not always mentioned, temperature can ◦ range from RT to 90 C, ending pH from 9.5 to 11, gas composition (% of CO2 in gas), the gas ﬂow rate, stirring, reactor design, the phases precipitating mentioned are not always the same, etc. [9–12,14–19,23,24,27]. The focus of the current work is to understand the effect of the concentration of unreacted sodium carbonate that remains from the leaching step and how it affects the type/composition of the precipitates and at the same time to maximize the recovery of aluminum in the form of alumina hydrates in synthetic sodium aluminate/sodium carbonate solutions.

2. Materials and Methods Synthetic solution preparation: To study the precipitation mechanism, synthetic solutions resembling the solutions generated by leaching the calcium aluminate slags with sodium carbonate solution had to be prepared. The reagents used to prepare the aluminate solution were: NaOH pellets, Na2CO3 anhydrous for analysis, technical grade sodium aluminate (NaAlO2) and technical grade anhydrous sodium metasilicate (Na2SiO3). Depending on the desired final concentration of the solution, the reagents were mixed in deionized water and the solution was added in a 1 L volumetric flask. The solution was then filtered to remove any impurities and the resulting solution was analyzed with Atomic Absorption Spectroscopy (AAS), (PinAAcle 900T, Perkin Elmer, Waltham, MA, USA) to determine the concentration of Al and Si. The composition of the starting aluminate solution is given in Table1. The NaOH concentration is always 18.75 g/L, while the concentrations of free Na2CO3, Al and Si varied between 20 and 160 g/L, 8 and 8.5 g/L and 0.2 and 0.24 g/L, respectively. The initial and final solutions in all experiments were analyzed for their Al and Si content.

Table 1. Concentration of synthetic sodium aluminate solution.

Compound Starting Synthetic Solution to Precipitation (g/L)

Na2CO3 20, 40, 60, 80, 160 NaOH 18.75 Al 8–8.5 Si 0.2–0.24

Carbonation experiments: The experiments were conducted with the experimental setup shown in Figure1, which is a custom-made autoclave system by AMAR. The reactor (1) is made of Inconel 625 to resist highly alkaline solutions and has a 1.8 L working volume. 1 It uses an electrical ceramic band for heating and a 4 hp AC motor for stirring controlled by the controller (2). Additionally, the vessel is equipped with baffles and a serpentine cooling coil. The reactor has a flush-bottom valve to unload the final pulp. The head of the reactor has a CO2 gas inlet, an outlet, a motorhead stirrer, an opening for sampling or for the pH electrode, a cooling coil inlet and outlet, a thermocouple inlet, and a funnel to insert solid or liquid samples. The pH is monitored and recorded by an Endress and Hauser measuring Crystals 2021, 11, x FOR PEER REVIEW 4 of 14

head of the reactor has a CO2 gas inlet, an outlet, a motorhead stirrer, an opening for sampling or for the pH electrode, a cooling coil inlet and outlet, a thermocouple inlet, and a funnel to insert solid or liquid samples. The pH is monitored and recorded by an Endress and Hauser measuring system (3). The gas flow rate is controlled with a 0–500 mLn/min mass flowmeter by Bronkhorst controlled by computer software (4). Apart from the resistance heating, cooling or heating can be also controlled by a chiller (5) recirculating liquid through the cooling coil. Pure carbon dioxide gas (99.995% purity) was used throughout all the experiments. The pH was monitored, through a pH electrode by En- dress and Hauser, inserted into the solution, equipped with a pH recorder, using a 1 min step. A volume of 800 mL of the starting synthetic solution was introduced through the reactor’s built-in funnel and heated to 40 °C. When the desired temperature was reached, CO2 gas was injected at a constant flow rate of 160 mLn/min. The stirring rate was set at Crystals 2021, 11, 836 200 rpm. At the end of the experiment (predesigned time and aging period), the solution4 of 14 was un-loaded and filtered. The filtrate was analyzed by AAS to determine the concentration of Al and Si after precipitation. The solid precipitate was dried at 110 °C for 24 h andsystem then (3).it was The analyzed gas ﬂow by rate X-ray is controlled diffraction with (XRD) a 0–500 on mLn/minan X’Pert Pro mass diffractometer ﬂowmeter by (PANalytical)Bronkhorst controlled with CuKa by computerradiation software(diffraction (4). patterns Apart from were the recorded resistance between heating, 10 cooling and 70°or 2 heatingθ, in 0.02° can steps be also and controlled 2 s per step) by a and chiller Fourier-transform (5) recirculating infrared liquid through spectroscopy the cooling (FT- IR)coil. onPure a PerkinElmer carbon dioxide Spectrum gas (99.995% 100 spectrometer, purity) was equipped used throughout with a diamond all the experiments. attenuated totalThe reflectance pH was monitored, (ATR) accessory through (spectra a pH electrode were acquired by Endress in transmittance and Hauser, mode, inserted from into 4000 the cmsolution,−1 to 650 equipped cm−1 with with a resolution a pH recorder, of 4 cm using−1 and a 16 1 minscans step. per spectrum).

FigureFigure 1. 1. PrecipitationPrecipitation reactor-experimental reactor-experimental setup. setup. A volume of 800 mL of the starting synthetic solution was introduced through the 3. Results reactor’s built-in funnel and heated to 40 ◦C. When the desired temperature was reached, 3.1. Effect of Initial Sodium Carbonate Concentration CO2 gas was injected at a constant flow rate of 160 mLn/min. The stirring rate was set atThe 200 examined rpm. At concentrations the end of the of experiment Na2CO3 in (predesigned the starting solution time and were aging 20, period), 40, 60, 80, the 100,solution and 160 was g/L, un-loaded as shown and in filtered.Table 1. The carbonation filtrate was analyzedtime was byset AASarbitrarily to determine to 70 min the andconcentration no aging period of Al and was Si employed. after precipitation. Figure 2 Thedepicts solid the precipitate XRD graphs was dried obtained at 110 by◦ Cthe for 24 h and then it was analyzed by X-ray diffraction (XRD) on an X’Pert Pro diffractometer (PANalytical) with CuKa radiation (diffraction patterns were recorded between 10 and 70◦ 2θ, in 0.02◦ steps and 2 s per step) and Fourier-transform infrared spectroscopy (FT-IR) on a PerkinElmer Spectrum 100 spectrometer, equipped with a diamond attenuated total reflectance (ATR) accessory (spectra were acquired in transmittance mode, from 4000 cm−1 to 650 cm−1 with a resolution of 4 cm−1 and 16 scans per spectrum).

3. Results 3.1. Effect of Initial Sodium Carbonate Concentration

The examined concentrations of Na2CO3 in the starting solution were 20, 40, 60, 80, 100, and 160 g/L, as shown in Table1. The carbonation time was set arbitrarily to 70 min and no aging period was employed. Figure2 depicts the XRD graphs obtained by the powder XRD examinations of the precipitates from each experiment. At 20 g/L of sodium carbonate in the starting solution, boehmite is the only precipitating phase; the broad peaks indicate a poorly crystalline phase with small particle size [28]. A better- crystallized bayerite is coprecipitated along with boehmite at 40 g/L of sodium carbonate in the starting solution Crystal structures and X-ray powder diffraction data can be found Crystals 2021, 11, x FOR PEER REVIEW 5 of 14

powder XRD examinations of the precipitates from each experiment. At 20 g/L of sodium carbonate in the starting solution, boehmite is the only precipitating phase; the broad Crystals 2021, 11, 836 5 of 14 peaks indicate a poorly crystalline phase with small particle size [28]. A better-crystallized bayerite is coprecipitated along with boehmite at 40 g/L of sodium carbonate in the starting solution Crystal structures and X-ray powder diffraction data can be found in Supple- mentaryin Supplementary Materials: Materials:Figure S1, Table Figure S1, S1, Figure Table S2 S1, and Figure Table S2 S2. and At Table 60 g/L S2. of Atsodium 60 g/L car- of bonatesodium in carbonate the starting in thesolution, starting boehmite solution, and boehmite bayerite and are bayerite precipitated, are precipitated, but the main but peak the ofmain dawsonite peak of (at dawsonite about 15.5) (at can about also 15.5) be observed can also in be the observed XRD graph. in the At XRD 80 and graph. 100 g/L At 80of sodiumand 100 carbonate g/L of sodium in the carbonate starting solution, in the starting dawsonite solution, is precipitated dawsonite isalong precipitated with bayerite along andwith boehmite. bayerite and At 160 boehmite. g/L of sodium At 160 g/Lcarbonate of sodium in the carbonate starting solution, in the starting only dawsonite solution, onlycan bedawsonite observed can in the be observedXRD graph. in the XRD graph.

3 3

amount of Na2CO3 3 3 3 3 3 3 3 160 g/L

100 g/L

2 80 g/L 2

Intensity (a.u.) Intensity 2 2 2 60 g/L 2

40 g/L

1 1 1 1 20 g/L 1

10 20 30 40 50 60 70 2θ (degrees)

Figure 2. XRD graphs of precipitates of carbonation in sodium aluminate solutions of different Figure 2. XRD graphs of precipitates of carbonation in sodium aluminate solutions of different start- starting concentration of Na CO [carbonated at 40 ◦C, 160 mLn/min CO ﬂow rate, 70 min carbona- ing concentration of Na2CO32 [carbonated3 at 40 °C, 160 mLn/min CO2 flow2 rate, 70 min carbonation timetion timeand 200 and rpm 200 rpmstirring. stirring. Νο aging No aging period]. period]. 1—Boehmite 1—Boehmite [AlOOH], [AlOOH], 2—Bayerite 2—Bayerite [Al(OH) [Al(OH)3], 3—3], Dawsonite3—Dawsonite [NaAlCO [NaAlCO3(OH)3(OH)2]. 2].

TheThe results presented above,above, showshow that that the the initial initial free free sodium sodium carbonate carbonate concentration concentra- tionaffects affects substantially substantially the the type type of of phases phases that that are are precipitated. precipitated. DawsoniteDawsonite is gradually increasedincreased until until it it becomes becomes the the dominating dominating precipitated precipitated phase phase by increasing by increasing the initial the initial free sodiumfree sodium carbonate carbonate concentration concentration in the insoluti theon. solution. Dawsonite Dawsonite is not present is not at present the precipi- at the tatingprecipitating phases phasesup until up the until 60 g/L the amount 60 g/L of amount sodium of carbonate sodium carbonate concentration concentration in the starting in the solution.starting solution.As the goal As of the this goal research of this researchis to produce is to producepure alumina pure aluminahydroxides, hydroxides, the concen- the concentration of 20 g/L of sodium carbonate in the starting solution was chosen as the tration of 20 g/L of sodium carbonate in the starting solution was chosen as the starting starting condition for further optimization. condition for further optimization. 3.2. Effect of Carbonation Duration 3.2. Effect of Carbonation Duration To elucidate the precipitation mechanism during carbonation of sodium aluminate solutionsTo elucidate with low the free precipitation sodium carbonate mechanism content during (20 g/L), carbonation experiments of sodium were performedaluminate solutions◦ with low free sodium carbonate content (20 g/L), experiments were performed at 40 C, with a 200 rpm stirring rate and 160 mL/min CO2 ﬂow rate varying in duration atwithout 40 °C, with an aging a 200 period rpm stirring at the rate end and of carbonation. 160 mL/min CO The2 valuesflow rate of varying duration inthat duration were withoutchosen arean aging as follows: period 30, at 45, the 60, end 70, of 80, carbonat 90, 110,ion. and The 205 values min. The of duration values of that the were pH of cho- the sensolution/pulp are as follows: along 30, with 45, the60, corresponding70, 80, 90, 110, Al and recovery 205 min. against The thevalues duration of the carbonation pH of the solution/pulpcan be seen in along Figure with3. In the Figure corresponding4, the XRD graphsAl recovery of the against precipitates the duration obtained carbonation from each canexperiment be seen in are Figure shown. 3. In Figure 4, the XRD graphs of the precipitates obtained from each experimentIn region are Ishown. (30 min of carbonation), the system exhibits high buffering capacity, and the pHIn region value graduallyI (30 min of decreases. carbonation), In this the region, system the exhibits free hydroxide high buffering ions concentrationcapacity, and theis high, pH value and therefore,gradually thedecreases. acidity producedIn this region, by the the dissolved free hydroxide CO2 through ions concentration Reactions (1)is and (2) is consumed, slightly lowering the pH value of the solution. In region I, not enough mass of precipitate was obtained to perform XRD or FTIR analysis. In region II (30–45 min of carbonation), the solution buffer capacity is substantially lower, and therefore, the acidity produced by the added CO2 sharply decreases the solution pH. In this region, aluminum precipitation has already started, precipitating boehmite (22% Crystals 2021, 11, x FOR PEER REVIEW 6 of 14

high, and therefore, the acidity produced by the dissolved CO2 through Reactions (1) and (2) is consumed, slightly lowering the pH value of the solution. In region I, not enough mass of precipitate was obtained to perform XRD or FTIR analysis. In region II (30–45 min of carbonation), the solution buffer capacity is substantially lower, and therefore, the acidity produced by the added CO2 sharply decreases the solution pH. In this region, aluminum precipitation has already started, precipitating boehmite (22% recovery, at 45 min of carbonation). In region III (45–60 min of carbonation), the solution exhibits remarkably high buffering capacity again, which can be attributed to the massive aluminum precipitation, according to Reaction (4), that again liberates free hydroxide ions consuming the protons generated by the added CO2. In this region, aluminum is precipitated as boehmite again (41% recovery, at 60 min of carbonation). Finally, in region IV (>60 min of carbona- Crystals 2021, 11, 836 tion), the value of the pH is gradually decreased again until the end of precipitation.6 ofIn 14 general, aluminum recovery expressed as % of Al precipitated out of the solution, which increases as more carbon dioxide is added (Figure 3), showing a sigmoid curve with negligible aluminum precipitation in region I, very low in region II, low to medium in region IIIrecovery, and medium at 45 to min high of in carbonation). region IV. In region III (45–60 min of carbonation), the solution exhibits remarkably high buffering capacity again, which can be attributed to the massive 𝐴𝑙(𝑂𝐻) = 𝐴𝑙𝑂𝑂𝐻 +𝑂𝐻 +𝐻 𝑂 aluminum precipitation, according() to Reaction() (4),() that again () liberates free hydroxide(4) ionsIn consumingmore detail, the as seen protons in Figure generated 4, between by the 45 added and 70 CO min2. Inof thiscarbonation region, aluminum(pH 10.96– is 10.73),precipitated boehmite as boehmiteis the only again precipitating (41% recovery, phase atcorresponding 60 min of carbonation). to recovery Finally, up to in57.28% region ofIV Al. (>60 From min 70 ofto carbonation),90 min of carbonation the value (pH of the 10.73–10.38), pH is gradually boehmite decreased along with again bayerite until the areend precipitated, of precipitation. corresponding In general, to aluminum a recovery recovery of Al up expressed to 77%. At as 110 % ofmin Al of precipitated carbonation out andof afterward, the solution, dawsonite which increases is the major as more precipit carbonate, dioxidewith bayerite is added and (Figure boehmite3), showingas minor a phases,sigmoid achieving curve with an almost negligible complete aluminum recove precipitationry of aluminum in regionin the precipitate I, very low (~100%). in region II, low to medium in region III and medium to high in region IV.

13 100

Region III Region IV 90 12.5 Region I 80 12 70 pH 11.5 60 Al recovery (%) 11 50 pH

40 10.5 Region II 30 Aluminum Recovery (%) Recovery Aluminum 10 20 9.5 10

9 0 0 30 60 90 120 150 180

carbonation duration (min)

Figure 3. Graph representing the pH and the aluminum recovery (%) in the precipitate vs. the Figure 3. Graph representing the pH and the aluminum recovery (%) in the precipitate vs. the car- Crystals 2021, 11, x FOR PEER REVIEWcarbonation duration. [Sodium aluminate solution that contains 20 g/L sodium carbonate, carbonated7 of 14 bonation duration. [Sodium aluminate solution that contains 20 g/L sodium carbonate, carbonated at 40 ◦C, 160 mLn/min CO ﬂow rate and 200 rpm stirring. No aging period.]. at 40 °C, 160 mLn/min CO2 flow2 rate and 200 rpm stirring. Νο aging period.].

2 3 3 3 3 3 3 205 min 3 2 2 2 110 min 2 2 90 min

80 min Intensity (a.u.)

70 min

60 min

1 1 1 1 45 min 1

10 15 20 25 30 35 40 45 50 55 60 65 70 75 2 theta (deg) FigureFigure 4. XRDXRD graphs graphs of of precipitates precipitates [Sodium [Sodium aluminate aluminate solution solution that that contains contains 20 20g/L g/L sodium sodium car- ◦ carbonate,bonate, carbonated carbonated at at40 40 °C,C, 160 160 mLn/min mLn/min CO CO2 flow2 ﬂow rate rate and and 200 200 rpm rpm stirring. stirring. Νο No aging aging period.]. 1—Boehmite [AlOOH], 2—Bayerite [Al(OH)3], 3—Dawsonite [NaAlCO3(OH)2]. 1—Boehmite [AlOOH], 2—Bayerite [Al(OH)3], 3—Dawsonite [NaAlCO3(OH)2].

3.3. Effect of Aging The tests conducted in Section 3.2 were reproduced, this time with the inclusion of aging as a parameter. Aging durations of 2, 4, and 24 h was tested. Upon completion of the carbonation process, CO2 gas purging stopped, and the pulp formed during precipitation was kept in agitation under temperature with predefined aging duration. At the end of the aging period, the precipitates were characterized with XRD, and the solution was analyzed for its Al and Si content. Al recoveries for different aging durations in the precipitate can be observed in Figure 5a, while the corresponding Si recoveries are observed in Figure 5b. Additionally, the XRD diffractograms are presented in Figure 6.

100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 Si recovery in precipitate (%) precipitate in recovery Si Al recovery in precipitate (%) precipitate in recovery Al 0 0 0 50 100 150 200 050100150200 Carbonation duration (min) Carbonation duration (min) no aging 2 hour aging 4 hour aging 24 hour aging (a) (b) Figure 5. (a) Aluminum recovery in precipitate vs carbonation time. (b) Silicon recovery in precipitate vs carbonation duration. Aging period of 0, 2, 4 or 24 h. [Sodium aluminate solution that contains 20 g/L sodium carbonate, carbonated at 40 °C, 160 mLn/min CO2 flow rate and 200 rpm stirring].

At 30 min of carbonation and a 24 h aging period, 10% of Al and 63% of Si was precipitated, but only a very small amount of precipitate could be obtained, and thus XRD analysis could not be performed. Instead, FTIR analysis was performed, which mainly identified a sodium zeolite phase.

Crystals 2021, 11, x FOR PEER REVIEW 7 of 14

2 3 3 3 3 3 3 205 min 3 2 2 2 110 min 2 2 90 min

80 min Intensity (a.u.) Crystals 2021, 11, 836 7 of 14

70 min

60 min 1 ( )− = + − + Al OH1 4(aq) AlOOH1 (s)1 OH(aq) H2O(l) (4) 45 min 1

In more10 detail, 15 as 20 seen 25 in Figure 30 354, between 40 45 45 and 50 70 55 min 60 of carbonation 65 70 ( 75pH 10.96–10.73), boehmite is the only precipitating phase2 theta corresponding (deg) to recovery up to 57.28% of Al. From 70 to 90 min of carbonation (pH 10.73–10.38), boehmite along with bayerite are Figure 4. XRD graphs of precipitates [Sodium aluminate solution that contains 20 g/L sodium car- precipitated, corresponding to a recovery of Al up to 77%. At 110 min of carbonation and bonate, carbonated at 40 °C, 160 mLn/min CO2 flow rate and 200 rpm stirring. Νο aging period.]. afterward, dawsonite is the major precipitate, with bayerite and boehmite as minor phases, 1—Boehmite [AlOOH], 2—Bayerite [Al(OH)3], 3—Dawsonite [NaAlCO3(OH)2]. achieving an almost complete recovery of aluminum in the precipitate (~100%). 3.3. Effect of Aging 3.3. Effect of Aging The tests conducted in Section 3.2 were reproduced, this time with the inclusion of The tests conducted in Section 3.2 were reproduced, this time with the inclusion of agingaging as as a parameter. AgingAging durationsdurations of of 2, 2, 4, 4, and and 24 24 h wash was tested. tested. Upon Upon completion completion of theof the carbonation process, CO2 gas purging stopped, and the pulp formed during precipi- carbonation process, CO2 gas purging stopped, and the pulp formed during precipitation tationwas kept was in kept agitation in agitation under temperatureunder temperat withure predeﬁned with predefined aging duration. aging duration. At the end At of the the endaging of period,the aging the period, precipitates the precipitates were characterized were characterized with XRD, and with the XRD, solution and was the analyzedsolution wasfor itsanalyzed Al and Sifor content. its Al and Al recoveriesSi content. forAl differentrecoveries aging for different durations aging in the durations precipitate in the can precipitatebe observed can in Figurebe observed5a, while in Figure the corresponding 5a, while the Si recoveriescorresponding are observed Si recoveries in Figure are 5ob-b. servedAdditionally, in Figure the 5b. XRD Additionally, diffractograms the XRD are presented diffractograms in Figure are6 presented. in Figure 6.

FigureFigure 5. 5. ((aa)) Aluminum Aluminum recovery recovery in in precipitate precipitate vs vs carbonation carbonation time. time. ( (bb)) Silicon Silicon recovery recovery in in precipitate precipitate vs vs carbonation carbonation duration.duration. Aging Aging period of 0, 2,2, 44 oror 2424 h.h. [Sodium[Sodiumaluminate aluminate solution solution that that contains contains 20 20 g/L g/L sodium sodium carbonate, carbonate, carbonated carbonated at at 40◦ °C, 160 mLn/min CO2 flow rate and 200 rpm stirring]. 40 C, 160 mLn/min CO2 ﬂow rate and 200 rpm stirring]. AtAt 30 30 min min of of carbonation carbonation and and a 24 a 24h aging h aging period, period, 10% 10% of Al of and Al and63% 63%of Si ofwas Si pre- was cipitated,precipitated, but butonly only a very a very small small amount amount of ofprecipitate precipitate could could be be obtained, obtained, and and thus thus XRD XRD analysisanalysis could could not not be be performed. performed. Instead, Instead, FTIR FTIR analysis analysis was was performed, performed, which which mainly mainly identifiedidentiﬁed a a sodium sodium zeolite zeolite phase. phase. The experiment at 45 min resulted in an aluminum recovery increase from 22% to 44% at 2 h aging and 58% at 24 h aging. At 60 min of carbonation, the aluminum recovery increased from 41% to 72% at 2 h of aging and 84% at a 24 h aging period. From 45 to 60 min

of carbonation, boehmite was initially precipitated, which during aging was transformed to bayerite. At 70 min of carbonation, aluminum recovery boosted from 57% to 82% at a 2 h aging period and 89% at a 4 h aging period. Though, at 4 h of aging, dawsonite was also precipitated. The following experiments showed an increase in aluminum recovery during aging again, but the precipitating phase was always dawsonite after aging, as depicted in Figure6. Crystals 2021, 11, x FOR PEER REVIEW 8 of 14

The experiment at 45 min resulted in an aluminum recovery increase from 22% to 44% at 2 h aging and 58% at 24 h aging. At 60 min of carbonation, the aluminum recovery increased from 41% to 72% at 2 h of aging and 84% at a 24 h aging period. From 45 to 60 min of carbonation, boehmite was initially precipitated, which during aging was transformed to bayerite. At 70 min of carbonation, aluminum recovery boosted from 57% to 82% at a 2 h aging period and 89% at a 4 h aging period. Though, at 4 h of aging, dawsonite was also precipitated. The following experiments showed an increase in aluminum recovery during aging again, but the precipitating phase was always dawsonite after aging, as depicted in Figure 6. Overall, silicon has a higher precipitation rate than aluminum, as seen in Figure 5b. Silicon precipitation starts earlier than aluminum precipitation, which serves as a desilication step. Almost all the silicon was co-precipitated, a result that is in accordance with the work of previous authors [2,9,10,17]. In general, in all cases after the end of the aging period, the recovery of aluminum was higher than the corresponding recovery obtained without aging. The results show slow alumina hydrate formation kinetics during carbonation, indicating that the system Crystals 2021, 11, 836 under investigation requires a sufficiently long aging period in order to precipitate8 of 14as much of the aluminum as possible. This shows the necessity of applying aging for the process designed.

2 2 2 2 2 2 2 2 2 2 Intensity (a.u.) Intensity Intensity (a.u.) Intensity 1 1 1 1 1 1 1 1 1 1 10 20 30 40 50 60 70 10 20 30 40 50 60 70 2 theta (deg) 2 theta (deg) (a) (b)

2 2 2 3 2 2 2 2 3 2 2 3 2 3

Intensity (a.u) Intensity 1 Intensity (a.u.) Intensity 1 1 1 1 1 1 1 1

10 20 30 40 50 60 70 10 20 30 40 50 60 70 2 theta (deg) 2 theta (deg) (c) (d)

3 2 3 3 3 2 2 3

Intensity (a.u.) Intensity 3 3 3 Intensity (a.u.) Intensity 2 2 2 2 1 1 2 1 1 1 1 1 1 2

10 20 30 40 50 60 70 10 20 30 40 50 60 70 2 theta (deg) 2 theta (deg) (e) (f)

No aging 2 h aging 4 h aging 24 h aging

Figure 6. XRD graphs of precipitates. Carbonation period of (a) 45 min, (b) 60 min, (c) 70 min, (d) 80 min, (e) 90 min, Figure 6. XRD graphs of precipitates. Carbonation period of (a) 45 min, (b) 60 min, (c) 70 min, (d) 80 min, (e) 90 min, (f) (f) 110 min Aging period of 0. 2, 4 or 24 h. [Sodium aluminate solution that contains 20 g/L sodium carbonate, carbonated 110 min Aging period of 0. 2, 4 or 24 h. [Sodium aluminate solution that contains 20 g/L sodium carbonate, carbonated at at 40 ◦C, 160 mLn/min CO flow rate and 200 rpm stirring]. 1—Boehmite [AlOOH], 2—Bayerite [Al(OH) ], 3—Dawsonite 40 °C, 160 mLn/min CO2 flow2 rate and 200 rpm stirring]. 1—Boehmite [AlOOH], 2—Bayerite [Al(OH)33], 3—Dawsonite [NaAlCO[NaAlCO33(OH)(OH)22]. Overall, silicon has a higher precipitation rate than aluminum, as seen in Figure5b . Silicon precipitation starts earlier than aluminum precipitation, which serves as a desilication step. Almost all the silicon was co-precipitated, a result that is in accordance with the work of previous authors [2,9,10,17]. In general, in all cases after the end of the aging period, the recovery of aluminum was higher than the corresponding recovery obtained without aging. The results show slow alumina hydrate formation kinetics during carbonation, indicating that the system under investigation requires a sufficiently long aging period in order to precipitate as much of the aluminum as possible. This shows the necessity of applying aging for the process designed. Furthermore, all precipitates were analyzed with FTIR spectroscopy. A typical graph of the precipitate obtained with an initial concentration of 20 g/L sodium carbonate, ◦ carbonation time of 60 min at 40 C, 160 mLn/min CO2 flow rate, and 200 rpm stirring is shown in Figure7 and the FTIR data in Table2. It can be observed that the major phase is bayerite (identified in wavenumbers: 3656, 3550, 3462, 3438, 3429, 1021, 981 and 718 cm−1) along with boehmite (identified in wavenumbers: 3316 and 3096 cm−1) as a minor phase, as was expected from the XRD analysis. Apart from these phases, a phase termed aluminum hydroxide carbonate gel (identified in wavenumbers: 1523 and 1406 cm−1), which is an amorphous phase, was also present in the precipitate. According to the literature [29,30], Crystals 2021, 11, x FOR PEER REVIEW 9 of 14

Furthermore, all precipitates were analyzed with FTIR spectroscopy. A typical graph of the precipitate obtained with an initial concentration of 20 g/L sodium carbonate, carbonation time of 60 min at 40 °C, 160 mLn/min CO2 flow rate, and 200 rpm stirring is shown in Figure 7 and the FTIR data in Table 2. It can be observed that the major phase is bayerite (identified in wavenumbers: 3656, 3550, 3462, 3438, 3429, 1021, 981 and 718 cm−1) Crystals 2021, 11, 836 along with boehmite (identified in wavenumbers: 3316 and 3096 cm−1) as a minor phase,9 of 14 as was expected from the XRD analysis. Apart from these phases, a phase termed aluminum hydroxide carbonate gel (identified in wavenumbers: 1523 and 1406 cm−1), which is an amorphous phase, was also present in the precipitate. According to the literature aluminum hydroxide carbonate gel presence is usually accompanied by adsorbed water, [29,30], aluminum hydroxide carbonate gel presence is usually accompanied by adsorbed which was also observed on the FTIR analysis. Gel formation could be a result of the rapid water, which was also observed on the FTIR analysis. Gel formation could be a result of precipitation, as it is well known from the literature [31,32] that carbonates contribute to the rapid precipitation, as it is well known from the literature [31,32] that carbonates con- maintaining aluminum hydroxide gel in the amorphous form. When aluminum hydrolysis tribute to maintaining aluminum hydroxide gel in the amorphous form. When aluminum is performed in the presence of CO2, the resulting carbonates are speciﬁcally adsorbed hydrolysis is performed in the presence of CO2, the resulting carbonates are specifically on aluminum hydroxide gel, stabilizing it in the form of aluminum hydroxycarbonate gel adsorbed on aluminum hydroxide gel, stabilizing it in the form of aluminum hydroxycar- (Reaction (5)) which retains its gelatinous nature even upon aging and does not contain bonate gel (Reaction (5)) which retains its gelatinous nature even upon aging and does not other cations except those of aluminum [30]. contain other cations except those of aluminum [30]. 2− − Al(OH) + xCO = Al(OH) (CO3) ( ) + 2xOH (5) 𝐴𝑙(𝑂𝐻)3(gel()) +𝑥𝐶𝑂3 = 𝐴𝑙(𝑂𝐻)(3()−2x) (𝐶𝑂)x()gel +2𝑥𝑂𝐻 (5) It is suggested that aluminum hydroxycarbonatehydroxycarbonate gel is the amorphous form of scar- bröite, which is a crystallinecrystalline phasephase ofof aluminumaluminum hydroxycarbonate,hydroxycarbonate, Al(OH)COAl(OH)CO3 [29].[29]. Therefore, the ﬁrstfirst precipitateprecipitate in thethe presencepresence ofof COCO22 gas willwill alwaysalways containcontain aa smallsmall amount ofof aluminumaluminum hydroxycarbonatehydroxycarbonate gel, gel, as as it it is is seen seen also also in in this this work work under under all all studied stud- −1 experimentalied experimental conditions. conditions. Additionally, Additionally, adsorbed adsorbed water water was was observed observed at 1651 at 1651 cm cm. −1.

Figure 7. FTIR graph of precipitate obtainedobtained withwith initialinitial concentrationconcentration ofof 2020 g/Lg/L sodium carbonate, carbonated at 40 ◦°C, 160 mLn/min CO2 flow rate, 200 rpm stirring and 24 h aging period]. carbonated at 40 C, 160 mLn/min CO2 ﬂow rate, 200 rpm stirring and 24 h aging period].

Table 2. FTIRTable data 2. (FunctionalFTIR data (Functional group/vibration group/vibration type, attributed type, phasesattributed and phases references). and references). Attributed Phase Wavenumber (cm−1) Functional Group/Vibration References Attributed Phase Wavenumber (cm−1) Functional Group/Vibration References 3656, 3550, 3462, 3438, 3429 O–H stretching band [33–39] Bayerite 3656, 3550,1021, 3462, 981 3438, 3429 O–H O–H stretchingbending band band [33,35–37] [33–39] Bayerite 1021,718 981 O–HAl–OH bending vibration band [33[33,36],35–37 ] 718 Al–OH vibration [33,36] Aluminum hydroxide carbonate gel 1523, 1406 CO3−2 asymmetric stretching band [32,40,41] Aluminum hydroxide carbonate gel 1523, 1406 CO −2 asymmetric stretching band [32,40,41] Boehmite 3316, 3096 3 O–H stretching band [32,35,37,42–44] Boehmite 3316, 3096 O–H stretching band [32,35,37,42–44] Adsorbed water 1651 H–O–H bending [30,37,38,44,45][30,37,38,44,45]

4. Discussion Based on the author’s knowledge of the alumina hydrate precipitation in aluminate solutions [46], the following hypothesis is proposed as an explanation for the experimental observations: as the bubbles of CO2 gas are added in the highly alkaline sodium aluminate/sodium carbonate solution, the neutralization of hydroxide ions is performed, and thus the solution pH is gradually decreased. Locally, in the area around the CO2 gas bubble Crystals 2021, 11, x FOR PEER REVIEW 10 of 14

4. Discussion Based on the author’s knowledge of the alumina hydrate precipitation in aluminate Crystals 2021, 11, 836 solutions [46], the following hypothesis is proposed as an explanation for the experimental10 of 14 observations: as the bubbles of CO2 gas are added in the highly alkaline sodium aluminate/sodium carbonate solution, the neutralization of hydroxide ions is performed, and thus the solution pH is gradually decreased. Locally, in the area around the CO2 gas bub- (Figure8), a low pH area due to acidity imposed by the CO 2 gas dissolution is established, ble (Figure 8), a low pH area due to acidity imposed by the CO2 gas dissolution is estab- instantly causing the formation of boehmite (Reaction (6)), as it has been shown in previous lished, instantly causing the formation of boehmite (Reaction (6)), as it has been shown in work [47]. In addition, the vigorous agitation transfers the protons through convection to previous work [47]. In addition, the vigorous agitation transfers the protons through con- the bulk solution, which is strongly alkaline and thus promotes a bayerite formation [22] vection to the bulk solution, which is strongly alkaline and thus promotes a bayerite for- according to Reaction (7). Boehmite precipitation has also been observed by only two mation [22] according to Reaction (7). Boehmite precipitation has also been observed by research groups [6,48]. only two research groups [6,48]. − + Al(OH) + H = AlOOH + 2H O (6) 𝐴𝑙(𝑂𝐻)4(aq()) +𝐻(aq)() = 𝐴𝑙𝑂𝑂𝐻(am()) +2𝐻2 𝑂(l()) (6)

Low pH area

CO 2 Al+3  AlOOH bubble (gel)

FigureFigure 8. 8.Schematic Schematic representation representation of of boehmite boehmite precipitation. precipitation.

Then,Then, during during aging, aging, as as the the CO CO22bubbling bubblingstops, stops,the theabove-stated above-stated low low pH pH area area cannot cannot bebe established,established, andand thus boehmite cannot cannot be be precipitated. precipitated. On On the the contrary, contrary, and and as asthe the so- solutionlution is is still still oversaturated, oversaturated, bayerite bayerite starts starts precipitating precipitating [[46]46] according toto ReactionReaction (3)(3) as as underunder those those conditions, conditions, (mild (mild to to strong strong alkaline alkaline environment) environment) is is the the most most stable stable phase. phase. Therefore,Therefore, thethe initial precipitates always always contain contain boehmite boehmite and and bayerite. bayerite. During During the theaging aging period, period, bayerite bayerite is precipitated is precipitated as the assolution the solution is still oversaturated, is still oversaturated, while boehmite, while boehmite,formed during formed carbonation, during carbonation, is gradually is transformed gradually transformed to the stable toalumina the stable hydrate alumina (bay- ◦ hydrateerite), which (bayerite), is in the which thermodynamically is in the thermodynamically stable phase at stable 40 °C phase under at the 40 alkalineC under condi- the alkalinetions prevailing conditions in prevailingthe solution in [49–51] the solution following [49–51 Reaction] following (7). Reaction (7). 𝐴𝑙𝑂𝑂𝐻 +𝐻 𝑂=𝐴𝑙(𝑂𝐻) AlOOH(am()) + H2O = Al(OH)3(()s) (7)(7) On the other hand, the hydrolysis of aluminum in the presence of at least stoichio- On the other hand, the hydrolysis of aluminum in the presence of at least stoichio- metricmetric amounts amounts of of sodium sodium bicarbonate bicarbonate (pH (pH less less than than around around 10) 10) leads leads to to the the formation formation of of dawsonitedawsonite [ 15[15,32],32] according according to to Reaction Reaction (8). (8). This This is is also also observed observed in in this this work work for for the the first first precipitatesprecipitates with with no no aging aging period period at at 110 110 min min of of carbonation, carbonation, where where dawsonite dawsonite is is initially initially precipitatedprecipitated and and the the pH pH value value is is remarkably remarkably close close to to 10. 10. 𝐴𝑙(𝑂𝐻)− + 𝑁𝑎𝐻𝐶𝑂() =𝑁𝑎𝐴𝑙(𝑂𝐻)()(𝐶𝑂)() +𝑂𝐻− +𝐻𝑂 (8) Al(OH)4 + NaHCO3(aq) = NaAl(OH)(2)(CO3)(s) + OH + H2O (8) In summation, the aluminum recoveries, the pH values, and the time derivative of pH (In summation,, which denotes the aluminum as ΔpH) are recoveries, shown in theFigure pH values,9 and Table and the3, where, time derivative based on the of dpH pHprecipitating ( dt , which phases, denotes now, as ∆fivepH) broad are shown regions in Figurecan be9 distinguished. and Table3, where, Region based I is onwhere the a precipitatingsodium zeolite phases, phase now,starts five to precipitate broad regions with canAl recovery be distinguished. up to 10%, Region while the I is derivative where a sodiumof pH ranges zeolite from phase 0.01 starts to to−0.04, precipitate showing with a slow Al recovery decrease up in to pH. 10%, This while is the the region derivative that ofrelates pH ranges to the fromneutralization 0.01 to − 0.04,of hydroxide showing ions. a slow In decreaseregion II, in alumina pH. This hydrate is the regionphases thatstart relatesto precipitate to the neutralization with Al recovery of hydroxide up to 58%, ions. along In with region a pH II, aluminagradient hydrate as high phasesas 0.15 startand a todecrease precipitate in pH. with In Althis recovery region, upmost to of 58%, the alonghydroxide with aions pH have gradient been as neutralized. high as 0.15 Moving and a decreaseon, region in pH.III is In where this region, alumina most hydrates of the hydroxideare precipitated, ions have with been recovery neutralized. up to 84% Moving after on,aging region (first III boehmite is where is alumina precipitated hydrates which are transforms precipitated, to withbayerite recovery during up aging) to 84% and after the aging (first boehmite is precipitated which transforms to bayerite during aging) and the ∆pH ranges from 0 to −0.01 showing an almost steady pH. In this region, the acidity produced by the addition of CO2 gas is neutralized by the alkalinity produced from the massive precipitation of aluminum. The following region, region IV, represents that stage of carbonation where metastable alumina hydrates are precipitated (first, boehmite is precipitated which is then transformed to bayerite during aging, and finally, a sufficient Crystals 2021, 11, x FOR PEER REVIEW 11 of 14

ΔpH ranges from 0 to −0.01 showing an almost steady pH. In this region, the acidity produced by the addition of CO2 gas is neutralized by the alkalinity produced from the mas- Crystals 2021, 11, 836 sive precipitation of aluminum. The following region, region IV, represents that stage11 ofof 14 carbonation where metastable alumina hydrates are precipitated (first, boehmite is precipitated which is then transformed to bayerite during aging, and finally, a sufficient period of aging is induced) with a recovery of Al up to 94%. Finally, in region V, dawsonite period of aging is induced) with a recovery of Al up to 94%. Finally, in region V, dawsonite is isprecipitated, precipitated, initially initially reaching reaching a recovery a recovery of Al of up Al to up 100%. to 100%. In this In region, this region, the pH the de- pH creasesdecreases with with a ΔpH a ∆ ofpH −0.01 of − to0.01 −0.02 to − initially,0.02 initially, and of and 0 to of − 00.01 to −after0.01 158 after min. 158 min.

FigureFigure 9. 9.GraphGraph representing representing the thepH pHduring during carbonatio carbonation,n, the aluminum the aluminum recovery, recovery, and the and time the de- time rivativederivative of pH of (dpH/dt) pH (dpH/dt) vs carbonation vs carbonation duration duration (min). (min). [Sodium [Sodium aluminate aluminate solution solution that contains that contains 20 ◦ g/L20 sodium g/L sodium carbonate, carbonate, carbonated carbonated at 40 °C, at 40 160C, mLn/min 160 mLn/min CO2 flow CO 2rateﬂow and rate 200 and rpm 200 stirring]. rpm stirring].

TableTable 3. 3.DistinguishedDistinguished regions, regions, aluminum aluminum recovery recovery (%) (%) and and ΔpH.∆pH.

MaximumMaximum Al Recovery Al Recovery (%) in (%) RegionRegion Phase Phase Precipitating Precipitating RangeRange of of Dph/Dt Dph/Dt Precipitatein Precipitate After afterAging Aging I Sodium zeolite 10% 0 to −0.03 I Sodium zeolite 10% 0 to −0.03 II IIAlumina Alumina hydrates hydrates 58% 58% −−0.040.04 to to−0.15−0.15 III III Alumina Alumina hydrates hydrates 84% 84% 0 0to to −0.01−0.01 IV MetastableMetastable alumina hydrates alumina 94% 0.01 to 0.02 IV 94% 0.01 to 0.02 V Dawsonitehydrates 100% 0 to −0.02 V Dawsonite 100% 0 to −0.02 5. Conclusions 5. ConclusionsThe concentration of sodium carbonate in the sodium aluminate pregnant solution is an importantThe concentration factor for the of precipitation sodium carbonate stage into theobtain sodium Al(OH) aluminate3 through pregnant the carbonation solution is process.an important At concentrations factor for the lower precipitation than 40 g/L stage of Na to obtain2CO3, alumina Al(OH) 3hydratesthrough (boehmite the carbonation and bayerite)process. are At precipitated, concentrations whereas lower at than concentrations 40 g/L of Na higher2CO3 than, alumina 60 g/L hydrates of Na2CO (boehmite3, daw- soniteand bayerite)is co-precipitated, are precipitated, and as the whereas free so atdium concentrations carbonate concentration higher than 60 increases, g/L of Na it2 be-CO3, comesdawsonite the predominant is co-precipitated, precipitated and as phase. the free sodium carbonate concentration increases, it becomesIn general, the predominant a solution with precipitated 20 g/L of phase. sodium carbonate can precipitate alumina hydrates Inin general,the form aof solution crystalline with bayerite 20 g/L with of sodium a maximum carbonate recove canry precipitateof 84%. This alumina was achievedhydrates under in the carbonation form of crystalline at 40 °C bayeritewith 60 min with CO a maximum2 purging with recovery pure of CO 84%.2 gas This and was a ◦ flowachieved rate of under160 mLn/min carbonation after a at 24 40 h agingC with pe 60riod. min It COwas2 purgingshown that with first, pure boehmite CO2 gas pre- and cipitates,a flow rateduring of 160 aging, mLn/min transform after to abayerite 24 h aging and period. if the carbonates It was shown content that increases, first, boehmite into dawsonite.precipitates, Silicon during is aging,always transform co-precipitated to bayerite with andaluminum if the carbonates and should content be carefully increases, into dawsonite. Silicon is always co-precipitated with aluminum and should be carefully monitored in the leaching step. Desilication can happen at the first stages of solution carbonation but leads to some alumina losses in the order of 10%. Additionally, five broad areas were recognized based on the pH trendline and results of region I, relating to the neutralization of hydroxide ions, where silicon has started a bulk precipitation. In region II, where most of the hydroxide ions have been neutralized, alumina hydrate phases start to precipitate with Al recovery up to 58%. Moreover, region Crystals 2021, 11, 836 12 of 14

III is where alumina hydrates are precipitated with recovery up to 84%. The following region, region IV, represents that stage of carbonation where metastable alumina hydrates are precipitated (first, boehmite is precipitated, which is then transformed to bayerite during aging and finally to dawsonite if a sufficient period of aging is induced), with a recovery of Al up to 94%. Finally, in region V, dawsonite is precipitated, initially reaching a recovery of Al up to 100%. Overall, key parameters were determined and helped improve the general knowledge of precipitation of sodium aluminate solutions with carbon dioxide. Unreacted sodium carbonate content from the leaching step was one of the key parameters tested and the precipitation pathway was revealed, showing that boehmite is first precipitated, followed by bayerite and finally dawsonite if the carbonate content is high enough. A lot of future work is needed in order to more carefully specify the exact consistency of the solution when dawsonite starts to form, and of course to produce metallurgical grade alumina, other properties such as purity, appropriate particle size, etc., need to be investigated.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/cryst11070836/s1, Figure S1: Crystal structure of boehmite. PDF 04-016-2858, Table S1: X-ray Powder Diffraction data for boehmite. PDF 04-016-2858, Figure S2: Crystal structure of bayerite. PDF 01-074-1119, Table S2: X-ray Powder Diffraction data for bayerite. PDF 01-074-1119. Reference [52] is cited in supplementary materials. Author Contributions: Conceptualization, D.M., D.P. and E.B.; methodology, D.M., D.P. and E.B.; software, D.M. and D.K., data curation, D.M.; D.K. and A.A.; writing—original draft preparation, D.M.; writing—review and editing, D.M., D.P., D.K. and E.B.; visualization, D.M., D.P. and E.B.; supervision, D.P. and E.B.; project administration, D.P. All authors have read and agreed to the published version of the manuscript. Funding: The research leading to these results has received funding from the European Community’s Horizon 2020 Programme (H2020/2014–2020) under grant agreement no. 767533. This publication reflects only the author’s view, exempting the Community from any liability. Project web site: https://www.ensureal.com/ (accessed on 1 October 2017). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available within the article and in supplementary materials. Conflicts of Interest: The authors declare no conflict of interest and the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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