Obtaining Alumina from Kaolin Clay Via Aluminum Chloride

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Article Obtaining Alumina from Kaolin Clay via Aluminum Chloride

Vyacheslav I. Pak 1, Sergey S. Kirov 1, Anton Yu. Nalivaiko 2,* , Dmitriy Yu. Ozherelkov 2 and Alexander A. Gromov 2

1 Department of Non-ferrous Metals and Gold, National University of Science and Technology MISIS, Moscow 119991, Russia; [email protected] (V.I.P.); [email protected] (S.S.K.) 2 KINETICA Engineering Center, National University of Science and Technology MISIS, Moscow 119991, Russia; [email protected] (D.Y.O.); [email protected] (A.A.G.) * Correspondence: [email protected]; Tel.: +7-499-700-0306 (ext. 50703)

Received: 26 October 2019; Accepted: 25 November 2019; Published: 28 November 2019

Abstract: A method of alumina production based on hydrochloric acid processing of kaolin clays from the East Siberian deposits was studied. Hydrochloric acid leaching was carried out at 160 ◦C. The leaching solution was subjected to a two-stage crystallization of aluminum chloride hexahydrate (ACH). The precipitated crystals were calcinated in air at a temperature above 800 ◦C to produce alumina. The main part of water and chlorine during thermal decomposition of ACH was removed at 400 ◦C. The influence of temperature and duration of ACH calcination on the residual chlorine content in alumina was studied. The optimal temperature of ACH calcination was 900 ◦C with a duration of 90 min. It was shown that the increase in calcination temperature contributed to the decrease in chlorine content in the final product. However, an increase in calcination temperature above 900 ◦C led to the transition of the well-soluble γ-Al2O3 phase to the insoluble α-Al2O3, which negatively affected the further electrolysis of aluminum. The size of alumina particles was not affected by the calcination mode. The rate of dissolution of the prototype Al2O3 in Na3AlF6 was higher than for the alumina obtained by the classical method. Alumina content, particle morphology, and particle size distribution for the obtained alumina were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and laser diffraction methods. The obtained alumina is suitable for aluminum production according to the studied characteristics.

Keywords: alumina; aluminum chloride hexahydrate; hydrochloric acid process; leaching; physico-chemical properties

1. Introduction Nowadays, the Bayer process is the predominant method of alumina production in the world. This method is based on the leaching of pre-crushed bauxite in alkaline solutions. After leaching, aluminum hydroxide separates from the aluminate solution and is then calcinated to produce alumina. High-quality bauxites with an Al2O3 to SiO2 mass ratio of more than 8 are required as a raw material for alumina production by the Bayer process [1]. The world’s reserves of high-quality bauxite reduce every year [2–4]. In this regard, a lot of intensive research focused on the expanding of the raw material base with the involvement of the low-quality aluminum raw materials (high-silica bauxite, kaolin clay, nepheline, ash) has been carried out [5–8]. The development of highly-eﬃcient processing methods of low-quality aluminum raw materials with high silica content is an urgent task. One of the possible ways of high-silica raw materials processing is the sintering method. However, this method is multi-stage and characterized by high complexity as well as high fuel and reagents consumption.

Materials 2019, 12, 3938; doi:10.3390/ma12233938 www.mdpi.com/journal/materials Materials 2019, 12, 3938 2 of 12

One of the most promising methods of obtaining alumina from high-silica aluminum ores is a hydrochloric acid technology, followed by the selective separation of aluminum compounds from acidic solutions [9–11]. The main advantage of this technology is the removal of silicon oxide at the ﬁrst stage of the process. Silica separation helps to simplify the process and prevent the formation of harmful alumina production waste [12–14]. The advantage of hydrochloric acid as a leaching agent is the possibility of aluminum chloride hexahydrate (ACH) selective precipitation from leaching solutions. In addition, the hydrochloric acid method is the easiest in terms of acid regeneration compared to sulfuric and nitric acids. [15,16]. After the thermohydrolysis of aluminum chloride and iron crystals, gaseous HCl is captured by water and returned to leaching. The purpose of this research is the study of the physical and chemical properties of alumina obtained by the hydrochloric acid technology using kaolin clays from Eastern Siberia, as well as the assessment of the obtained alumina suitability for aluminum electrolysis.

2. Materials and Methods

2.1. Materials A sample of kaolin clay from the East Siberian deposits was used as the raw material for alumina production. Table1 shows the chemical composition of kaolin clay by the X-ray di ﬀraction (XRD) method.

Table 1. The chemical composition of kaolin clay.

Component SiO2 Al2O3 Fe2O3 TiO2 CaO Na2OK2O MgO Loss on Ignition Content, wt % 50.71 28.77 6.14 0.42 0.43 0.18 0.23 0.72 12.40

The initial samples of kaolin clay consisted of more than 50% silica, the content of aluminum oxide in the sample was 28.77%. Kaolin clay also contains oxides of iron, titanium, calcium, sodium, potassium, and magnesium. Experiments were carried out using hydrochloric acid (37 vol %). The required concentration of hydrochloric acid in the leaching process was achieved by diluting with distilled water. Hydrogen chloride gas was used in the crystallization process.

2.2. Characterization Thermal analysis of the AlCl 6H O samples was performed using the STA 409 CD thermal 3· 2 analyzer (Netzsch Group, Selb, Germany) with a sample heating rate of 10 ◦C per minute in an argon gas atmosphere. The Vega LMH electron scanning microscope (Tescan, Brno, Czech Republic) with the Oxford Instruments Advanced AZtecEnergy analysis attachment (Asylum Research, NanoAnalysis, High Wycombe, UK) was used for the scanning electron microscopy (SEM) analysis of ACH crystals. Studies of the granulometric composition of alumina were carried out using the Microsizer-201S laser particle analyzer (Scientiﬁc instruments Jsc., Saint Petersburg, Russia). XRD of alumina was carried out using the D8 Advance equipment (Bruker Corp., Billerica, MA, USA).

2.3. Experimental Research The technological scheme of the hydrochloric acid method for alumina production from kaolin clays is shown on Figure1. Leaching of kaolin clay was carried out for 3 h at 160 ◦C in an autoclave. Hydrochloric acid (20 vol %) was used as a leaching agent. The pulp after leaching was used for the thickening and filtering of Si-stoff. Si-stoff with a residual moisture content of 40–50 wt % was washed with water, which aimed to absorb HCl from the calcination gases. This process was followed by the obtainment of circulating acid. After washing, Si-stoff was sent for drying, and it can be used as a product for cement manufacturing. Materials 2019, 12, x FOR PEER REVIEW 3 of 11 Materials 2019, 12, x FOR PEER REVIEW 3 of 11 temperature of 20 °C for 45 min in order to achieve a maximum degree of ACH crystallization (up to 97%).temperature The crystals of 20 °Cdeposited for 45 min at thein order first stageto achi wereeve arecrystallized maximum degree for 45 of min ACH to crystallizationobtain crystals (up with to low97%). impurity The crystals content. deposited AlCl 3at·6H the2O first crystals stage were recrystallizedcalcinated in fora 45tubular min to rotary obtain furnace crystals atwith a temperaturelow impurity of 900–1100content. °CAlCl for3·6H 90 2minO crystals [22,23]. were calcinated in a tubular rotary furnace at a Materialstemperature 2019, 12 of, 3938 900–1100 °C for 90 min [22,23]. 3 of 12

Figure 1. Technological scheme of alumina production from kaolin clays. Figure 1. Technological scheme of alumina productionproduction from kaolin clays.clays. An electrochemical cell was developed in order to determine the dissolution rate of Al2O3 in After Si-stoff separation, the aluminum chloride solution was crystallized [17–21]. The process Na3AlFAn6. electrochemicalThe scheme of this cell cell was is developedshown in Figure in order 2. Theto determine graphite cruciblethe dissolution was chosen rate asof theAl 2basisO3 in was carried out by saturating the aluminum chloride solution with gaseous HCl. Crystallization forNa 3theAlF electrochemical6. The scheme of cell this due cell tois shownthe stability in Figure in the 2. Theaggressive graphite molten crucible cryolite was chosen salts [24–26]. as the basis The of AlCl 6H O was conducted in two stages. The first stage of crystallization was carried out at a graphitefor the 3electrochemical ·crucible2 was enclosed cell due in toa steel the stabilitycasing an ind aluminathe aggressive backfill molten was located cryolite in the salts space [24–26]. between The thetemperaturegraphite crucible crucible and of 20 casing. was◦C forenclosed Graphite 45 min in in arods ordersteel (0.5 casing to cm achieve ×an 14d aluminacm) a maximum were backfill used degree as was electrodes. of located ACH crystallizationin The the measuring space between (up rod to was97%).the crucibleinstalled The crystals and in ordercasing. deposited to Graphite adjust at the the rods first interelectrode stage(0.5 cm were × 14 recrystallizeddistance cm) were between used for as 45 the electrodes. min electrode to obtain The holders. crystals measuring The with melt lowrod impurity content. AlCl 6H O crystals were calcinated in a tubular rotary furnace at a temperature of temperaturewas installed was in order controlled 3to· adjust2 by athe platinum–plati interelectrodenum distance rhodium between thermocouple the electrode. The holders.GW PSW7 The 30-72 melt power900–1100temperature source◦C forwas (GW 90 controlled mininstek, [22 ,23Taiwan) by]. a platinum–plati was used as numa direct rhodium current thermocouple (DC) power. supply.The GW The PSW7 cell 30-72 was heatedpowerAn usingsource electrochemical the (GW KC instek,2/15 cellmuffle Taiwan) was furnace developed was (Naberthermused in as order a direct toGmbH, determine current Lilienthal, (DC) the dissolutionpower Germany). supply. rate The of Alcell2O was3 in Naheated3AlFThe using6 .electrodes The the scheme KC were 2/15 of thissupplied muffle cell furnace is with shown an (Nabertherm electric in Figure curr2.ent TheGmbH, of graphite a given Lilienthal, crucibledensity Germany). (I-const.). was chosen The as change the basis in thefor voltage theThe electrochemical electrodes on the electrochemical were cell supplied due to cellwith the due stabilityan electricto the in addition curr theent aggressive of aalumina given molten density to the cryolite melt(I-const.). was salts recorded The [24 –change26]. until The in itgraphitethe reached voltage crucible a onconstant the was electrochemical value. enclosed As an in aelectrolyte cell steel due casing to comp the and additionosition alumina close of backfill alumina to the was indust to locatedtherial melt electrolyte in was the spacerecorded was between useduntil the crucible and casing. Graphite rods (0.5 cm 14 cm) were used as electrodes. The measuring rod (5it reachedwt.% of aAl constant2O3; 95 wt.%value. of As Na an3 AlFelectrolyte6 (2.7 cryolite comp× ositionratio)), close the melting to the indust pointrial of electrolytethis electrolyte was usedwas was installed in order to adjust the interelectrode distance between the electrode holders. The melt within(5 wt.% the of rangeAl2O3 ;of 95 945–960 wt.% of °C Na [27].3AlF To6 (2.7 avoid cryolite melt ratio)),freezing the due melting to alumina point additionof this electrolyte (5 wt.%), wasthe experimenttemperaturewithin the rangewas was carried controlledof 945–960 out at by°C an a[27]. electrolyte platinum–platinum To avoid temperature melt freezing rhodium of 1100 due thermocouple. °C. to alumina addition The GW (5 PSW7 wt.%), 30-72 the powerexperimentThe source experiment was (GW carried instek, was out carried Taiwan)at an outelectrolyte was at 0.5–1.0 used temperature asA/cm a direct2 current of current 1100 densities °C. (DC) and power at 4 supply. cm of interelectrode The cell was distance.heatedThe using experiment the KC 2was/15 mucarriedffle furnace out at 0.5–1.0 (Nabertherm A/cm2 current GmbH, densities Lilienthal, and Germany). at 4 cm of interelectrode distance.

Figure 2. SchemeScheme of of the the electrochemical electrochemical cell. cell. Figure 2. Scheme of the electrochemical cell. The electrodes were supplied with an electric current of a given density (I-const.). The change in the voltage on the electrochemical cell due to the addition of alumina to the melt was recorded until it reached a constant value. As an electrolyte composition close to the industrial electrolyte was used (5 wt % of Al2O3; 95 wt % of Na3AlF6 (2.7 cryolite ratio)), the melting point of this electrolyte Materials 2019, 12, 3938 4 of 12

was within the range of 945–960 ◦C[27]. To avoid melt freezing due to alumina addition (5 wt %), the experiment was carried out at an electrolyte temperature of 1100 ◦C. The experiment was carried out at 0.5–1.0 A/cm2 current densities and at 4 cm of interelectrode distance.

3. Results and Discussion Materials 2019, 12, x FOR PEER REVIEW 4 of 11 The main reactions of kaolin clays during hydrochloric acid leaching were Equations (1) and (2): 3. Results and Discussion Al2O3 2SiO2 2H2O + 6HCl 2AlCl3 + 2SiO2 + 5H2O, (1) The main reactions of kaolin· clays· during hydrochloric→ acid leaching↓ were Equations (1) and (2): MexOy + 2yHCl xMeCl(2y/x) + yH2O, (2) Al2O3·2SiO2·2H2O + 6HCl→ → 2AlCl3 + 2SiO2↓ + 5H2O, (1) where Me is Fe, Ca, K, Na, Mg. MexOy + 2yHCl → xMeCl(2y/x) + yH2O, (2) As shown in Equations (1) and (2), aluminum dissolves in hydrochloric acid to form ACH. The amountwhere Me of is aluminum Fe, Ca, K, recovered Na, Mg. into the hydrochloric acid solution was 95%. In this case, silica (the mainAs component shown in of Equations kaolin clays) (1) and was (2), not aluminum dissolved disso in hydrochloriclves in hydrochloric acid and wasacid separated to form ACH. from The the solutionamount of by aluminum ﬁltration. recovered Other impurities into the alsohydrochloric transferred acid into solution the leaching was 95%. solution In this with case, a silica diﬀerent (the extractionmain component value. Theof kaolin composition clays) was of the not aluminum dissolved chloridein hydrochloric solution acid is presented and was separated in Table2. from the solution by filtration. Other impurities also transferred into the leaching solution with a different extraction value.Table The composition 2. Chemical composition of the aluminum of the hydrochloric chloride solution acid leaching is presented solution. in Table 2.

Component AlCl FeCl NaCl KCl CaCl MgCl HCl Table 2. Chemical3 composition3 of the hydrochloric acid leaching2 solution.2 Content, wt % 21.96 3.62 0.25 0.15 0.29 0.48 0.84 Component AlCl3 FeCl3 NaCl KCl CaCl2 MgCl2 HCl Content, wt.% 21.96 3.62 0.25 0.15 0.29 0.48 0.84 AfterAfter separation from from the the solid solid residue, residue, the the alum aluminuminum chloride chloride solution solution was was subjected subjected to a two- to a two-stage crystallization of AlCl 6H O. Selective deposition of AlCl 6H O crystals occurred during stage crystallization of AlCl3·6H3·2O.2 Selective deposition of AlCl3·6H3· 2O2 crystals occurred during crystallizationcrystallization andand most most of of the the metal metal chloride chloride impurities impurities remained remained in the solution.in the solution. Precipitated Precipitated crystals of AlCl 6H O were particles with a hexagonal form combined into agglomerates of diﬀerent sizes crystals3 ·of AlCl2 3·6H2O were particles with a hexagonal form combined into agglomerates of different (Figuresizes (Figure3) with 3) a with grain a sizegrain of size up toof 550up to microns. 550 microns.

FigureFigure 3.3. SEM images of aluminum chloridechloride hexahydratehexahydrate crystals.crystals.

TheThe mainmain impurityimpurity inin thethe ACHACH crystalscrystals waswas iron.iron. The main part of the iron was concentrated onon thethe surfacesurface ofof thethe crystalscrystals andand waswas representedrepresented byby thethe residualresidual mastermaster solutionsolution (see(see FigureFigure3 3).). At At the the samesame time,time, partpart ofof thethe ironiron impuritiesimpurities waswas distributeddistributed point-wisepoint-wise throughoutthroughout ACHACH crystalscrystals volumevolume (see(see FigureFigure4 4 and and Table Table3). 3).

Figure 4. SEM images of aluminum chloride hexahydrate (ACH) with marked micro-XRD spectral analysis points.

Materials 2019, 12, x FOR PEER REVIEW 4 of 11

3. Results and Discussion The main reactions of kaolin clays during hydrochloric acid leaching were Equations (1) and (2):

Al2O3·2SiO2·2H2O + 6HCl → 2AlCl3 + 2SiO2↓ + 5H2O, (1)

MexOy + 2yHCl → xMeCl(2y/x) + yH2O, (2) where Me is Fe, Ca, K, Na, Mg. As shown in Equations (1) and (2), aluminum dissolves in hydrochloric acid to form ACH. The amount of aluminum recovered into the hydrochloric acid solution was 95%. In this case, silica (the main component of kaolin clays) was not dissolved in hydrochloric acid and was separated from the solution by filtration. Other impurities also transferred into the leaching solution with a different extraction value. The composition of the aluminum chloride solution is presented in Table 2.

Table 2. Chemical composition of the hydrochloric acid leaching solution.

Component AlCl3 FeCl3 NaCl KCl CaCl2 MgCl2 HCl Content, wt.% 21.96 3.62 0.25 0.15 0.29 0.48 0.84 After separation from the solid residue, the aluminum chloride solution was subjected to a two- stage crystallization of AlCl3·6H2O. Selective deposition of AlCl3·6H2O crystals occurred during crystallization and most of the metal chloride impurities remained in the solution. Precipitated crystals of AlCl3·6H2O were particles with a hexagonal form combined into agglomerates of different sizes (Figure 3) with a grain size of up to 550 microns.

Figure 3. SEM images of aluminum chloride hexahydrate crystals.

The main impurity in the ACH crystals was iron. The main part of the iron was concentrated on the surface of the crystals and was represented by the residual master solution (see Figure 3). At the same time, part of the iron impurities was distributed point-wise throughout ACH crystals volume (seeMaterials Figure2019 ,412 and, 3938 Table 3). 5 of 12

Figure 4. SEM images of aluminum chloride hexahydrate (ACH) with marked micro-XRD spectral Figure 4. SEM images of aluminum chloride hexahydrate (ACH) with marked micro-XRD spectral analysis points. analysis points. Materials 2019, 12, x FOR PEERTable REVIEW 3. Micro-XRD spectral analysis results for ACH crystals. 5 of 11

Spectrum Label O Al Cl Fe Total

S1 50.66 31.89 17.03 0.42 100.00 S2Table 72.43 3. Micro-XRD spectral 12.58 analysis results 14.97 for ACH crystals. 0.02 100.00 S3Spectrum Label 69.69 O 11.22 Al 19.03 Cl 0.06 Fe Total 100.00 S4S1 66.74 50.66 13.83 31.89 19.3317.03 0.42 0.1 100.00 100.00 S2 72.43 12.58 14.97 0.02 100.00 S5 55.06 16.69 28.20 0.05 100.00 S3 69.69 11.22 19.03 0.06 100.00 S6S4 29.83 66.74 4.26 13.83 65.9119.33 0.000.1 100.00 100.00 S7S5 57.84 55.06 12.70 16.69 29.3728.20 0.05 0.09 100.00 100.00 S8S6 49.0829.83 18.46 4.26 32.4165.91 0.00 0.04 100.00 100.00 S7 57.84 12.70 29.37 0.09 100.00 S9 53.38 17.00 29.58 0.04 100.00 S8 49.08 18.46 32.41 0.04 100.00 S10S9 60.28 53.38 10.91 17.00 28.8129.58 0.04 0.00 100.00 100.00 S10 60.28 10.91 28.81 0.00 100.00

To determinedetermine the temperatures of dehydration, water and chlorine removal, as well as phase transitions, thethe obtainedobtained samples samples of of ACH ACH were were subjected subjected to thermalto thermal analysis analysis (see (see Figures Figures5 and 56 and). The 6). measurementsThe measurements were were performed performed on the on initial the initial ACH underACH under dynamic dynamic heating heating of the sample of the upsample to 1200 up ◦toC at1200 a rate °C at of a 10 rate degrees of 10 degrees per minute per inminute an argon in an atmosphere. argon atmosphere.

Figure 5. TG/DTATG/DTA of AlCl3·6H6H2OO sample sample in in an an argon argon atmosphere: atmosphere: TG mass loss curve, DTG mass loss 3· 2 rate curve, DSC didifferentialﬀerential scanningscanning calorimetrycalorimetry curve.curve.

Figure 6. ACH synchronous thermal analysis: mass spectral ion current versus temperature.

The first stage of mass loss occurred between 105 °C and 156 °C and was associated with the removal of adsorbed water. Further heating of the sample (up to 1200 °С) removed water and chlorine from AlCl3·6H2O, with a mass loss of 76.66%. The main release of water and chlorine occurred up to 400 °C. At high temperatures, the DSC curve showed an exothermic peak associated with the beginning of γ-Al2O3 to α-Al2O3 transition. The thermal decomposition reaction of ACH crystals can be represented by the following Equation (3):

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Table 3. Micro-XRD spectral analysis results for ACH crystals.

Spectrum Label O Al Cl Fe Total S1 50.66 31.89 17.03 0.42 100.00 S2 72.43 12.58 14.97 0.02 100.00 S3 69.69 11.22 19.03 0.06 100.00 S4 66.74 13.83 19.33 0.1 100.00 S5 55.06 16.69 28.20 0.05 100.00 S6 29.83 4.26 65.91 0.00 100.00 S7 57.84 12.70 29.37 0.09 100.00 S8 49.08 18.46 32.41 0.04 100.00 S9 53.38 17.00 29.58 0.04 100.00 S10 60.28 10.91 28.81 0.00 100.00

To determine the temperatures of dehydration, water and chlorine removal, as well as phase transitions, the obtained samples of ACH were subjected to thermal analysis (see Figures 5 and 6). The measurements were performed on the initial ACH under dynamic heating of the sample up to 1200 °C at a rate of 10 degrees per minute in an argon atmosphere.

MaterialsFigure2019 ,5.12 TG/DTA, 3938 of AlCl3·6H2O sample in an argon atmosphere: TG mass loss curve, DTG mass loss6 of 12 rate curve, DSC differential scanning calorimetry curve.

Figure 6. ACH synchronous thermal analysis: mass sp spectralectral ion current versus temperature.

The ﬁrstfirst stage of mass loss occurredoccurred betweenbetween 105105 ◦°CC and 156 ◦°CC and was associated with the removal ofof adsorbed water.water. Further heatingheating ofof the sample (up to 1200 °◦СC)) removed water and chlorine from AlCl3·6H6H22O,O, with with a a mass mass loss loss of of 76.66%. 76.66%. The The main main re releaselease of ofwater water and and chlorine chlorine occurred occurred upup to · to400 400 °C.◦ C.At Athigh high temperatures, temperatures, the the DSC DSC curve curve showed showed an an exotherm exothermicic peak peak associated associated with the beginning ofof γγ-Al-Al22O3 toto α-Al-Al22OO33 transition.transition. The The thermal thermal decompositi decompositionon reaction reaction of of ACH ACH crystals can be represented by thethe followingfollowing EquationEquation (3):(3):

2AlCl 6H O Al O + 6HCl + 9H O . (3) 3· 2 → 2 3 ↑ 2 ↑ To determine the residual chlorine content in the products of ACH calcination, ACH thermal decomposition in a stationary mode at 450–1250 ◦C and a duration of 30–90 min was made. Calcination (in air) was carried out in the RT 50-250/13 tube furnace (Nabertherm GmbH, Lilienthal, Germany). The weight of the ACH sample was 10 g. The results of the study are presented in Table4. The calcination temperature had the most influence on the residual chlorine content in rough alumina. The chlorine content decreased from 7.20% to 0.12% with increasing temperature from 450 ◦C to 1250 ◦C during the 30 min duration of the calcination process. With increasing duration from 30 to 90 min, the chlorine content reduced from 7.20% to 4.26% at the temperature of 450 ◦C. The minimum chlorine content in alumina (0.05 wt %) was obtained at 1250 ◦C and 90 min of calcination. The alumina obtained at 1250 ◦C was characterized by an increase in α-Al2O3 content, which adversely affected the subsequent electrolysis of aluminum. The calcination of ACH below 900 ◦C contributed to an increase of chlorine content in the final product.

Table 4. The dependence of residual chlorine content from the parameters of the calcination.

Experiment Code Temperature, ºC Duration, min Chlorine Content, wt % 1 450 30 7.20 2 450 60 6.00 3 450 90 4.26 4 900 30 1.70 5 900 60 0.91 6 900 90 0.22 7 1250 30 0.12 8 1250 60 0.08 9 1250 90 0.05

According to the results of the thermal analysis of AlCl 6H O (see Figure5), studies on the 3· 2 calcination of aluminum chloride hexahydrate crystals were carried out. The studies were conducted Materials 2019, 12, 3938 7 of 12 at three diﬀerent calcination temperatures for 90 min each. The chemical compositions of alumina obtained by calcination are presented in Table5.

Table 5. Chemical composition of alumina obtained in diﬀerent modes.

Temperature, ◦C Al2O3 MgO Na2OK2O CaO SiO2 Fe2O3 Cl 800 99.362 0.043 0.003 0.010 0.028 0.005 0.015 0.534 900 99.678 0.047 0.003 0.007 0.029 0.007 0.020 0.209 1000 99.799 0.048 0.002 0.006 0.031 0.008 0.025 0.081

Various modifications of alumina were formed depending on the calcination temperature (see Figure7). Alumina obtained at 800 ◦C is mainly represented by γ-Al2O3 with a small fraction of δ-Al2O3. Identical peaks were received at 900 ◦C. At 1000 ◦C there was a transition to θ-Al2O3 and α-Al2O3. These results were comparable to previous studies [28,29]. As can be seen in Table5 and Figure7, an increase in the calcination temperature above 900 ◦C led to a decrease in the chlorine content of the final product, but at the same time promoted the transition of γ-Al2O3 to α-Al2O3, which negatively affected the dissolution of the resulting alumina during electrolysis of aluminum. Materials 2019, 12, x FOR PEER REVIEW 7 of 11

Figure 7. XRD patterns of alumina obtained at 800 ◦°C,C, 900 ◦°C,C, 1000 °C.◦C.

To study the eeffectﬀect ofof temperaturetemperature conditionsconditions on the particleparticle size of alumina,alumina, experiments on calcination of ACH were carried out in two technological modes:modes: a. With smooth heating of the material to the operating temperature for 2 h, followed by 2 h of a. With smooth heating of the material to the operating temperature for 2 h, followed by 2 h of exposure at operating temperature; exposure at operating temperature; b. Thermal shock-placing of the material into a hot furnace with a 4 hr exposure at operating b. Thermal shock-placing of the material into a hot furnace with a 4 hr exposure at temperature. operating temperature. The operating temperature of the calcination was 900 °C. The ACH crystal size subjected to calcinationThe operating was more temperature than 450 microns. of the calcinationThe size distribution was 900 ◦ C.of Thealumina ACH obtained crystal sizeby calcination subjected tois showncalcination in Figure was more8. than 450 microns. The size distribution of alumina obtained by calcination is shown in Figure8. The average size of alumina particles obtained at both temperature modes of calcination was 150 microns. Thus, the particle size decreased by more than three times during ACH calcination, which can be explained both by the destruction of agglomerates of the initial ACH and primary crystals. According to Figure8, when the ACH was placed in a hot furnace, there was a slight over-grinding of the material compared to the smooth heating. This was due to the more intense release of the gas phase from the ACH.

Figure 8. The size distribution of alumina (calcination process at 900 °C).

The average size of alumina particles obtained at both temperature modes of calcination was 150 microns. Thus, the particle size decreased by more than three times during ACH calcination, which can be explained both by the destruction of agglomerates of the initial ACH and primary crystals. According to Figure 8, when the ACH was placed in a hot furnace, there was a slight over-grinding of the material compared to the smooth heating. This was due to the more intense release of the gas phase from the ACH. Figure 9 shows SEM images of alumina obtained by hydrochloric acid technology. The samples were obtained by the calcination of aluminum chloride hexahydrate at 900 °C. As can be seen on the figure, the surface of alumina obtained from ACH was covered by the net of thin cracks emerging from the grain body. These cracks promote the removal process of hydrogen chloride and water vapor. At the same time, crack development was observed in certain directions according to the structural anisotropy of the AlCl3·6H2O crystal, without significant destruction of the ACH particle’s

Materials 2019, 12, x FOR PEER REVIEW 7 of 11

Figure 7. XRD patterns of alumina obtained at 800 °C, 900 °C, 1000 °C.

To study the effect of temperature conditions on the particle size of alumina, experiments on calcination of ACH were carried out in two technological modes: a. With smooth heating of the material to the operating temperature for 2 h, followed by 2 h of exposure at operating temperature; b. Thermal shock-placing of the material into a hot furnace with a 4 hr exposure at operating temperature. The operating temperature of the calcination was 900 °C. The ACH crystal size subjected to calcinationMaterials 2019 ,was12, 3938 more than 450 microns. The size distribution of alumina obtained by calcination8 of is 12 shown in Figure 8.

FigureFigure 8. 8. TheThe size size distribution distribution of of alumina alumina (calcination (calcination process process at at 900 900 °C).◦C).

TheFigure average9 shows size SEM of alumina images particles of alumina obtained obtained at both by hydrochloric temperature acid modes technology. of calcination The sampleswas 150 Materials 2019, 12, x FOR PEER REVIEW 8 of 11 microns.were obtained Thus, bythe the particle calcination size decreased of aluminum by more chloride than hexahydrate three times during at 900 ◦ ACHC. As calcination, can be seen which on the figure, the surface of alumina obtained from ACH was covered by the net of thin cracks emerging canoriginal be explained shape. The both destruction by the destruction of ACH ofparticles agglomerates during of calcination the initial occurredACH and mainly primary due crystals. to the from the grain body. These cracks promote the removal process of hydrogen chloride and water Accordingdestruction to of Figure agglomerates 8, when consistingthe ACH was of a placedlarge nu inmber a hot of furnace, fine fractions. there was In addition, a slight over-grinding the developed vapor. At the same time, crack development was observed in certain directions according to the ofporous the material surface compared of alumina to obtainedthe smooth from heating. ACH hadThis awas high due active to the surface more andintense promoted release adsorptionof the gas phasestructural from anisotropy the ACH. of the AlCl3 6H2O crystal, without significant destruction of the ACH particle’s of fluorine-containing waste gase· s of aluminum electrolysis and increased the rate of alumina original shape. The destruction of ACH particles during calcination occurred mainly due to the dissolutionFigure 9 in shows the electrolyte. SEM images of alumina obtained by hydrochloric acid technology. The samples destruction of agglomerates consisting of a large number of fine fractions. In addition, the developed were obtainedTable 6 shows by the the calcination compliance of aluminumof the impurity chloride cont hexahydrateent in the test at samples 900 °C. Aswith can the be requirements seen on the porous surface of alumina obtained from ACH had a high active surface and promoted adsorption of figure,for metallurgical the surface alumina. of alumina The obtained content fromof controlled ACH was impurities covered inby test the samplesnet of thin of aluminacracks emerging obtained fluorine-containing waste gases of aluminum electrolysis and increased the rate of alumina dissolution fromby hydrochloric the grain body. acid These technology cracks metpromote the requirements,the removal process which ofdemonstrated hydrogen chloride the possibility and water of in the electrolyte. vapor.aluminum At the electrolysis. same time, crack development was observed in certain directions according to the structural anisotropy of the AlCl3·6H2O crystal, without significant destruction of the ACH particle’s

FigureFigure 9.9. SEM images of alumina obtained by hydrochloric acidacid technology.technology.

Table6 shows the complianceTable 6. Comparative of the impurity evaluation content of in alumina the test impurities. samples with the requirements for metallurgical alumina. The content of controlled impurities in test samples of alumina obtained Component Average Content, wt. % Industrial Requirements, wt.% by hydrochloric acid technology met the requirements, which demonstrated the possibility of SiO2 0.006 ≤0.020 aluminum electrolysis. Fe2O3 0.020 ≤0.030 Na2O 0.003 ≤0.400 K2O 0.007 (Na2O + K2O)

ТiO2 <0.001 ≤0.010 CaO 0.029 Not specified MgO 0.045 Not specified Cl- 0.21 Not specified

Alumina was produced according to the technological scheme (Figure 1) of kaolin clay processing by the hydrochloric acid technology. The following parameters were used to obtain alumina: leaching temperature was 160 °C with 3 h duration. Then, two-stage crystallization of ACH was carried out: at a temperature of 20 and 80 °C. The duration of both stages was 45 min. In accordance with the studies of ACH heat treatment, the optimal calcination mode was at 900 °C and 90 min duration. The alumina produced satisfied the following properties: Al2O3 content was 99.7 wt.%, the content of impurities didn’t exceed maximum allowed values. Alumina was represented by γ-modification, the average particle size was 150 microns. To assess the applicability of alumina obtained by the hydrochloric acid method, studies of its solubility in comparison with alumina produced by the classical Bayer process were made. The results of experimental studies are presented in Figure 10.

Materials 2019, 12, 3938 9 of 12

Table 6. Comparative evaluation of alumina impurities.

Component Average Content, wt % Industrial Requirements, wt % SiO 0.006 0.020 2 ≤ Fe O 0.020 0.030 2 3 ≤ Na O 0.003 0.400 2 ≤ (Na2O + K2O) K2O 0.007 ТiO <0.001 0.010 2 ≤ CaO 0.029 Not speciﬁed MgO 0.045 Not speciﬁed

Cl− 0.21 Not speciﬁed

Alumina was produced according to the technological scheme (Figure1) of kaolin clay processing by the hydrochloric acid technology. The following parameters were used to obtain alumina: leaching temperature was 160 ◦C with 3 h duration. Then, two-stage crystallization of ACH was carried out: at a temperature of 20 and 80 ◦C. The duration of both stages was 45 min. In accordance with the studies of ACH heat treatment, the optimal calcination mode was at 900 ◦C and 90 min duration. The alumina produced satisﬁed the following properties: Al2O3 content was 99.7 wt %, the content of impurities didn’t exceed maximum allowed values. Alumina was represented by γ-modiﬁcation, the average particle size was 150 microns. To assess the applicability of alumina obtained by the hydrochloric acid method, studies of its solubility in comparison with alumina produced by the classical Bayer process were made. The results ofMaterials experimental 2019, 12, x studiesFOR PEER are REVIEW presented in Figure 10. 9 of 11

FigureFigure 10.10. Voltage changechange inin thethe electrochemicalelectrochemical cell.cell.

TheThe achievementachievement of of a a constant constant voltage voltage on on the the electrochemical electrochemical cell cell using using alumina alumina produced produced by acid by technologyacid technology occurred occurred on average on average 4 s faster, 4 s whichfaster, indicateswhich indicates a better a solubility better solubility of the alumina of the alumina in cryolite. in cryolite.The electrochemical voltage consists of the voltage drop in the electrolyte (U1), the voltage drop in theThe contacts electrochemical and conductors voltage (U 2consists), decomposition of the voltage voltage drop (U in3), the and electrolyte polarization (U1 voltage), the voltage (U4) (4): drop in the contacts and conductors (U2), decomposition voltage (U3), and polarization voltage (U4) (4): U = U1 + U2 + U3 + U4. (4) U = U1 + U2 + U3 + U4. (4) TakingTaking intointo accountaccount thethe smallsmall contactcontact areaarea ofof thethe current-carrying elementselements and small amperage, thethe voltagevoltage dropdrop inin thethe contactscontacts andand conductorsconductors (U(U22)) waswas assumedassumed toto bebe zero.zero. WhenWhen aluminaalumina waswas addedadded toto thethe cryolite,cryolite, thethe electrolysiselectrolysis reactionreaction tooktook thethe formform (5):(5):

AlAlO2O3 ++ 1.5C →2Al 2Al+ +1.5CO 1.5CO2.. (5) (5) 2 3 → 2 Decomposition voltage (U3) in the temperature range of 1000–1100 °C was 1.15 V [30]. According to the results [31], the polarization voltage in the system С–СO2|Na3AlF6 (2.7–3.1 cryolite ratio) + Al2O3|Al-C at the current density of up to 1 A/cm2 is in the range of 0.03–0.06 V, with accepted 0.04 V. The voltage drop in the electrolyte depends on the current density, conductivity of the electrolyte, and interelectrode distance, according to formula (6):

U1 = ρ · i · l, (6)

where ρ is the resistivity of the electrolyte, Оm·cm; i is the current density, А/cm2; and l is the interelectrode distance, cm. The resistivity of the electrolyte at the points of voltage stabilization on the electrochemical cell was determined (Table 7). According to Table 7, the resistivity of the used electrolyte, measured using the developed design of the electrochemical cell, was in the range of 0.623–0.637 Om·cm. This value was consistent with the literature data [32,33], which indicates the adequacy of the used method.

Table 7. Calculation of the electrolyte resistivity.

Current Density, А/сm2 Parameter 0.50 0.75 1.00 Decomposition voltage, V 1.15 1.15 1.15 Polarization voltage, V 0.04 0.04 0.04 The voltage drop in the electrolyte, V 1.25 1.87 2.55 Total 2.44 3.06 3.74 Resistivity of the electrolyte, Оm·сm 0.625 0.623 0.637 The parameters of electrolysis: interelectrode distance = 4 cm; temperature = 1200 °С; electrolyte composition = 5 wt. % Al2O3; 95 wt. % Na3AlF6 (2.7 cryolite ratio)

Materials 2019, 12, 3938 10 of 12

Decomposition voltage (U3) in the temperature range of 1000–1100 ◦C was 1.15 V [30]. According to the results [31], the polarization voltage in the system C–CO2|Na3AlF6 (2.7–3.1 cryolite ratio) + 2 Al2O3|Al-C at the current density of up to 1 A/cm is in the range of 0.03–0.06 V, with accepted 0.04 V. The voltage drop in the electrolyte depends on the current density, conductivity of the electrolyte, and interelectrode distance, according to Formula (6):

U = ρ i l, (6) 1 · · where ρ is the resistivity of the electrolyte, Om cm; i is the current density, A/cm2; and l is the · interelectrode distance, cm. The resistivity of the electrolyte at the points of voltage stabilization on the electrochemical cell was determined (Table7).

Table 7. Calculation of the electrolyte resistivity.

Current Density, A/cm2 Parameter 0.50 0.75 1.00 Decomposition voltage, V 1.15 1.15 1.15 Polarization voltage, V 0.04 0.04 0.04 The voltage drop in the electrolyte, V 1.25 1.87 2.55 Total 2.44 3.06 3.74 Resistivity of the electrolyte, Om cm 0.625 0.623 0.637 · The parameters of electrolysis: interelectrode distance = 4 cm; temperature = 1200 ◦C; electrolyte composition = 5 wt % Al2O3; 95 wt % Na3AlF6 (2.7 cryolite ratio)

According to Table7, the resistivity of the used electrolyte, measured using the developed design of the electrochemical cell, was in the range of 0.623–0.637 Om cm. This value was consistent with the · literature data [32,33], which indicates the adequacy of the used method.

4. Conclusions

1. In this study, the residual chlorine content at calcination temperatures between 450 ◦C and 1250 ◦C with a duration between 30 min and 90 min was studied. The minimum chlorine content in alumina of 0.05 wt % was obtained at a temperature of 1250 ◦C and a calcination duration of 90 min, but the samples were characterized by an increase in the α-phase content, which adversely affected the subsequent electrolysis of aluminum. The calcination of ACH below 900 ◦C contributed to an increased chlorine content in the final product. The optimal mode of aluminum chloride hexahydrate calcination was determined as 90 min at 900 ◦C; 2. During thermal decomposition of ACH crystals, the main part of water and chlorine precipitated during the heating up to 400 ◦C; 3. The mode of calcination did not significantly affect the size of the obtained alumina. At the same time, the particle size during the calcination of ACH in both modes of calcination decreased by three times, from 450 microns for ACH to 150 microns for alumina;

4. The rate of Al2O3 in Na3AlF6 dissolution was studied. It was revealed that the dissolution of the alumina test sample in cryolite was faster than alumina obtained by the classical method (Bayer process).

Author Contributions: Conceptualization, V.I.P.; Data curation, V.I.P. and A.Y.N.; Formal analysis, S.S.K.; Investigation, S.S.K. and A.Y.N.; Methodology, S.S.K.; Supervision, A.A.G.; Visualization, D.Y.O.; Writing—original draft, V.I.P. and A.Y.N.; Writing—review and editing, D.Y.O. and A.A.G. Funding: The work is ﬁnancially supported by the Russian Science Foundation (RSF), grant NO. 19–79–30025. Materials 2019, 12, 3938 11 of 12

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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