Decomposition of Aluminate Solution for Aluminum Hydroxide Precipitation by Carbonation: a Thermodynamic and Experimental Studies

Journal of ChemicalValeh Technology Aghazadeh, and Somayeh Metallurgy, Shayanfar 56, 1, 2021, 149-160

DECOMPOSITION OF ALUMINATE SOLUTION FOR ALUMINUM HYDROXIDE PRECIPITATION BY CARBONATION: A THERMODYNAMIC AND EXPERIMENTAL STUDIES

Valeh Aghazadeh, Somayeh Shayanfar

Department of Mineral Processing, Faculty of Mining Engineering Received 23 July 2019 Sahand University of Technology, Tabriz, Iran Accepted 21 January 2020 E-mail: [email protected]

ABSTRACT

Thermodynamic and experimental studies of the carbonation process of aluminate solution prepared by alkaline leaching of sintered nepheline syenite were performed. The effect of carbon dioxide injection in solution, pH, possible species stability and reactions and saturation index of the aluminate solution were thermodynamically investigated at temperatures of 70oC, 75oC and 80oC. The thermodynamic results showed that at studied temperatures, the precipitation of aluminum hydroxide occurred at the pH above 11. The amount of impurities such as silica and dawsonite increased with the increasing the carbon dioxide concentration. At low carbon dioxide concentration, by increasing the temperature, the formation of impurities is increasing too. Carbonation experiments were carried out using the thermodynamic results, at pH values of 9, 10 and 11 at every temperature studied. The major phase of aluminum hydroxide precipitate at pH 11 was bayerite while at pH 9 and 10 it bayerite was dawsonite. The silica content in the product was less than 0.9 wt. %. Keywords: carbonation, thermodynamic study, carbon dioxide, aluminum hydroxide, bayerite.

INTRODUCTION gas, liquid, and solid phases simultaneously present. Carbon dioxide injected into aluminate solution neutral- Nepheline syenite is used to produce alumina as the izes the free OH-, so that the mole ratio of total caustic most important aluminum none-bauxite source. In the soda to Al2O3, called as caustic ratio, decreases. After process, sodium and potassium carbonate and cement reaching a suitable pH, with further absorption of carbon are produced in addition to the production of alumina. dioxide, the aluminate solution becomes unstable and

During the process, nepheline syenite and limestone then aluminum hydroxide, Al(OH)3, precipitates. With are sintered at a temperature close to a melting point in further carbonation, at pH value below 10, aluminum the first step. The obtained product is porous aggregate carbonate alkali metals or dawsonite (NaAlCO3(OH)2) that ground and leached with alkaline liquor for separa- is formed thus contaminating the product. Depending on tion, then the soluble components are extracted from the concentration of the solution and gas injection rate, the sintered product to the leaching liquor. In the next the carbonation temperature varies but usually it is in the stage, the silica is removed from the aluminate solution range 70oC - 80oC. The quality of aluminum hydroxide and subsequently the amount of silica decreases in the is strongly affected by pH, temperature, stirring, anf the final product. The aluminate solution is a feed of the oC time of carbonation [1, 5, 6]. carbonation process[1, 2]. Carbonation is one of the The chemistry of electrolyte solutions is particu- methods used in this process for alumina production. larly complex and challenging to understand and predict Super-saturation by injecting carbon dioxide into the for real industrial systems containing many components aluminate solution at atmospheric pressure to change the and operating over wide ranges of temperature, pressure, pH is used. Therefore, aluminum hydroxide is precipi- and concentration. So, with comprehensively and ac- tating [3, 4]. Carbonation is a complex process wherein curately addressing the chemistry and thermodynamics

149 Journal of Chemical Technology and Metallurgy, 56, 1, 2021 of electrolyte solutions, the process can be designed THERMODYNAMIC FRAMEWORK and optimized effectively [7]. Due to the unknown and In this paper, the carbonation process of the alumi- complex mechanism of the carbonation process because nate solution is thermodynamically investigated. Param- of a multiphase reaction involving series of complicated eters such as pH changes, the speciation of solution and physicochemical processes, it is essential to understand saturation indices of possible components are calculated the effects of aqueous chemistry of aluminate solution at different conditions during the carbonation process. which includes the effects of the components, pH, The dominant aluminum species in high-pH alumi- temperature, and the other factors. Therefore, a reliable nate solutions are believed to be Al(OH)4 [15, 16]. Previ- thermodynamic investigation can help to understand ous investigations indicated that the separation of alu- the mechanism of this process [8]. So far, studies on minum hydroxide from aluminate solutions is based on - the carbonation of aluminate solutions are limited to the the hydrolysis of the Al(OH)4 ion [17]. The crystalline lime-soda sintering process[5, 9, 10]. Carbonation of forms of the aluminum hydroxide exist in four forms: the aluminate solutions obtained from nepheline syenite gibbsite, γ -Al(OH)3, bayerite, α-Al(OH)3, nordstrandite leaching is still insufficiently understood and required and doyleite. Gibbsite can be found mainly in natural more research and more data to understand this process. ores; bayerite is less found, but it can be synthesized [18, In most of the previous researches, aluminate solutions 19]. The gibbsite with a formation energy of -276 kcal were synthesized in the laboratory and the effect of pa- is more stable than bayerite with -275.5 kcal of forma- rameters such as the amount of gas injected, temperature, tion energy [20]. Wojcik and Pyzalski (1990) showed seed ratio, stirring rate, etc. have been experimentally that bayerite was the predominant species formed in the investigated. Aluminum hydroxide produced through carbonation process of potassium and sodium aluminate the carbonation process was almost contaminated with solutions in the range of 50°C - 80°C [21]. Shayanfar et dawsonite [3, 11 - 13]. al. [22] studied the carbonation of the aluminate solution The main purpose of this research is the inves- obtained from the leaching of sintered nepheline syen- tigation of the carbonation of the aluminate solution ite. The results showed that a single phase of bayerite thermodynamically. In an article published by the crystals was produced at 80°C and cooling to room tem- present authors, the thermodynamic modeling of the perature. A mixture of bayerite and gibbsite was formed carbonation process was investigated using the Aque- at a constant temperature of 80°C, however, bayerite was ous model of OLI Analyzer software. The effect of pH the major phase of aluminum hydroxide. Aghazadeh et and temperature on Bayerite precipitation was studied al. [23] investigated the effect of pH on the carbonation thermodynamically and experimentally. According to of the aluminate solution. The main phase of products at thermodynamic modeling and experimental results, pH pH 9, 10 and 11 was dawsonite, a mixture of dawsonite 11 and temperature of 80oC were suitable for bayerite and imogolite and bayerite, respectively. The reaction precipitation [14]. In this paper, the Visual MINTEQ of aluminate solution hydrolysis is expressed as follow: software has been used for the thermodynamic study. − − The experimental studies were done based on the re- Al(OH )4 (aq) ↔ Al(OH )3 (s) + OH (aq) (1) sult obtained from the thermodynamic studies. So, the amount of carbon dioxide for reduction of pH of the From the eq. (1) it can be deduced that neutralizing aluminate solution was predicted. The effect of carbon free hydroxide encourages an aluminate solution to form dioxide injection on the speciation of the solution, aluminum hydroxide [24]. In the carbonation process, pH and saturation index was investigated at the tem- the carbon dioxide reacts with water to form carbonic peratures of 70oC, 75oC and 80oC. Finally, carbonation acid, subsequently reacting with the hydroxide to form experiments were performed based on the predicted insoluble aluminum hydroxide and soluble salts such as amount of carbon dioxide and pH. In this study, the sodium and potassium carbonate. The equilibrium be- aluminate solution sample was prepared from industrial tween an aqueous solution and gaseous CO2 is described alkaline leaching of sintered nepheline syenite instead by Henry’s Law of gas solubility: of synthetic sample. Therefore the results of this study ↔ can be used with high precision in industry. CO2 (g) CO2 (aq) 150 Valeh Aghazadeh, Somayeh Shayanfar

The silica is the other undesirable impurity in the [CO ] K = 2(aq) = 2×10−3 (2) aluminate solution that at the stage of aluminum hydrox- P CO 2 ide crystallization will contaminate the product. The -2 First, the CO2(aq) reacts with water to form car- SiO3 ion is the dominant silicon species in high-pH bonic acid in the solution. The equilibrium constant for aluminate solutions. The silica may react with alumina this reaction is fairly small. and soda to precipitate as an insoluble sodium aluminum silicate. The solubility of silica in the aluminate solution

CO2 (aq) + H 2O(l) ↔ H 2CO3 (aq) increases when the temperature increases [27, 28]. The pH value of the solution is one of the main fac- [H CO ] K = 2 3 =1.7×10−3 (3) tors determining the properties of the aluminum hydrox- [CO ] 2 ide. The pH of a solution is related to the concentration of However, as exhibited by the value of the equi- the hydrogen ion and the acid dissociation constant. The librium constant, only a small percentage of CO (aq) original definition of Sorensen relates pH to the negative 2 combines with water to from H CO . Operationally, it logarithm of hydrogen ion concentration. In higher ionic 2 3 is difficult to distinguish between them. So that concen- strength solutions and other non-ideal solutions, ions tration of H CO and CO (aq) are practically combined can participate in short and long distance interactions 2 3 2 and treated as H CO * that it is much weaker acid than reducing the reactivity of the ions compared to actual 2 3 true H2CO3. concentrations. The IUPAC definition relates pH to the * [H 2CO3 ](aq) = [H 2CO3 ](aq) + [CO2 ](aq) (4) negative logarithm of the activity of the hydrogen ion So, [29]. To demonstrate the influence of carbon dioxide on * CO2 (g) ↔ H 2CO3 (aq) (5) the pH of the solution, the equilibrium constant Eqs.(6) The carbonic acid subsequently dissociated to form and (7) can be written as a pH scale [26]: the bicarbonate ion: − − pH [HCO3 ] K = 10 * (9) * − + [H 2CO3 ] H 2CO3 (aq) ↔ HCO3 (aq) + H (aq) , + − 2− [H ][HCO ] − [CO ] = 3 = × −4 ′ = pH 3 (10) K * 2.00 10 (6) K 10 − [H 2CO3 ] [HCO3 ] Then, the bicarbonate ion dissociated to form car- One of the objectives of the thermodynamic calcula- bonate and hydronium ions, hence the pH decreases tion is the calculation of the concentration of a set of [25, 26]. related species or speciation. According to the IUPAC definition, speciation is “the distribution of an element HCO − (aq) ↔ H + (aq) + CO 2− (aq) 3 3 amongst defined chemical species” in a solution. Spe- + 2− ciation is difficult to determine experimentally, if not [H ][CO3 ] −11 K = − = 4.69×10 (7) impossible because it is not easy to separate the various [HCO3 ] forms of an element that are generally in kinetically fast The concentration of bicarbonate ion, with further chemical equilibrium [30, 31]. carbonation, increases and that of carbonate ion de- Saturation index (SI) can be used to estimate the creases which causes the precipitation of the dawsonite saturation of aluminate solution for a carbon dioxide (eq. (8)) that contaminate the final product which is a injection by estimating the relative saturation. The disadvantage from the technological point of view [6]. saturation index is determined by dividing the product Formation of dawsonite results in lost of some of the of the activities of the ions actually in the solution to the aluminum and sodium, so it has to be prevented. equilibrium constant of solubility. IAP +− SI = lg( ) (11) Al( OH )()33 s++ Na () aq HCO () aq ↔ (8) K sp ↔+NaAlCO32( OH )() s H 2 O () l where Ksp is the equilibrium constant for the solution of a 151 Journal of Chemical Technology and Metallurgy, 56, 1, 2021

solid, usually called solubility product, and IAP (ion ac- species are represented by Helgeson. MINTEQ sets a1 tivity product) is the product of the activities of the ions to 0.1 for all neutral aqueous species [37]. in solution. If SI < 1 then the solid is under-saturated, if SI > 1 then the solid is super-saturated and if SI = 1 lg10 γ i = a1I (14) then the solid is at saturation [32, 33]. Calculating the speciation of the solution, pH and SIT theory is used to calculate the activity coef- saturation indices require measuring the species concen- ficients when the concentration of solutes is too high to trations in the aluminate solution and then calculation be predicted accurately by the Debye-Hückel theory. SIT ionic activities by thermodynamic studies. In most of theory was first proposed by Brønsted (1922) and was the electrolytes, each element exists as more than one further developed by Guggenheim (1955). Scatchard ionic species. The activities of the ions are functions of (1936) extended it to allow the interaction coefficients the concentration and activity coefficient of the species: to vary with ionic strength. Interaction coefficients are determined from equilibrium constant values obtained ai = Ciγ i (12) with solutions at various ionic strengths. The activity coefficient of the ion in solution is written as γ i when where Ci is the concentration of ion i and γ is the activ- concentrations are on the molal concentration. The gen- ity coefficient that describes the degree of non-ideality eral equation of SIT theory can be expressed as [38, 39]: of the solution [34]. 2 0.51 I The Davies equation and Specific Ion Interaction lg10 γ i = Z i + ∑ε ik mk (15) Theory (SIT) which are combined in the Visual MINT- 1+1.5 I k EQ software are used to calculate the activities of the where ε and m are interaction coefficients and concen- ions, equilibrium aqueous speciation, solid phase satura- tration of ions on the molal scale, respectively. The first tion states, and precipitation. Visual MINTEQ uses three term in these expressions comes from the Debye-Hückel alternative formulations of an extended Debye–Hückel theory. The second term shows that the interactions are equation which contains two adjustable parameters, the dependent on concentration. Thus, the interaction coef- Davies equation for computing activity coefficients and ficients are used as corrections to the Debye-Hückel Specific Ion Interaction Theory to estimate single-ion theory when concentrations are higher than the region activity coefficients at relatively high electrolytes solu- of validity of that theory [35, 36, 40]. The SIT theory tion concentrations. The Davies equation is an empirical is accurate at 0.5 < I < 3 mol kg–1 [41]. MINTEQ uses extension of Debye–Hückel theory which can be used the equilibrium constants at a reference temperature of to calculate activity coefficients of electrolyte solutions 298.15K and for temperature other than 298.15K uses at relatively high concentrations at 25°C: the Van ‘t Hoff equation [42].

2 I − lg10 γ i = AZ i ( − 0.3I) (13) EXPERIMENTAL 1+ I where A is the Debay-Hückel constant which depends The SIT theory was used to calculate the activity of on the dielectric constant and temperature (A ≈ 0.5 for aluminate solution, equilibrium aqueous speciation, and water at 25°C), Zi is the charge on species i and I is the saturation indices. In this study, the ionic strengths of ionic strength. The second term, 0.3I, goes to zero as the aluminate solutions at a CO2 concentration range of 0 g ionic strength goes to zero, so the equation reduces to the dm-3 - 50 g dm-3 were about 0.3 mol kg-1 - 2 mol kg-1. Alu- Debye–Hückel equation at low concentration. However, minate solution was provided from Azarshahr alumina as concentration increases, the second term becomes pilot in Eastern Azerbaijan province, Iran. In this pilot, increasingly important, so the Davies equation can be the aluminate solution is prepared by alkaline leaching used for solutions too concentrated to allow the use of of sintered nepheline syenite. The chemical composition the Debye–Hückel equation [35, 36]. The Davies equa- of the prepared aluminate solution is shown in Table 1. tion is usable for electrolytes up to with ionic strength I The effect of CO2 concentration on the pH, speciation ≈ 0.5 mol kg–1. Activity coefficients for neutral aqueous and saturation of the solution at the temperatures of 152 Valeh Aghazadeh, Somayeh Shayanfar

70oC, 75oC and 80oC were investigated to determine the H+ ions is increased. Also with increasing temperature,

amount of CO2 and pH to produce aluminum hydroxide. less carbon dioxide is needed to reduce pH. The carbonation process was performed based on the The changes caused due to the injection of carbon results of the thermodynamic studies. dioxide on the solution investigated by calculating the The carbonation experiments were carried out at equilibrium constant of carbon dioxide species reac- the desired temperature range. Carbonation test was tions (Table 2). As seen, with increasing temperature, conducted in a 2.5 L glass reactor filled with 1.5 L of the K-value decreases. The equilibrium constant for the * solution. Carbon dioxide was injected through a bub- formation of H2CO3 (eq. 5) is larger than that for the bler ring with many holes at a certain flow rate into the equilibrium constant of dissociation of bicarbonate ions. aluminate solution and stirred at 400 rpm. After reach- Therefore, this reaction can be considered instantane- ing the desired pH, gas injection operation stopped. ously at equilibrium than the eq. (6). The K-value for eq. The aluminate solutions were allowed to precipitate (7) is very low, indicating that this reaction occurs slowly aluminum hydroxide for 8 hours at room temperature. and controlling the reaction of carbon dioxide speciation. After the separation of the liquid phase and the particles

formed during the decomposition of solutions, the so- Effect of CO2 concentration on the species of the lutions filtered and the solid phases were washed with solution hot distilled water to remove the alkaline. The washed In the carbonation process by injecting carbon precipitates dried at 50oC for 48 h. The products were dioxide into the aluminate solution, different species characterized by X-ray diffraction (XRD) to determine are formed. The species before carbonation and after the phase of products, X-ray fluorescence (XRF) and carbonation are predicted by Visual MINTEQ software Laser diffraction spectrometry (LDS) to determine the and the results are presented in Table 3. As seen, new elemental composition and distribution of the products species are formed after gas injection. The concentration particle size, respectively. The morphology and el- of all species varies with the concentration of carbon ementary composition and purity of the products were dioxide. Before carbonation, aluminate ion is a domi- observed by scanning electron microscopy (SEM) and nant ion among aluminum species, and the amount of Energy-Dispersive spectroscopy (EDS). other species is negligible. Sodium and potassium ions are soluble and their concentration changes slightly at RESULTS AND DISCUSSION various concentrations of carbon dioxide. At the begin- ning of carbonation, the carbonate ion is the dominant of CO speciation while bicarbonate ion being somewhat Effect of CO2 concentration on the solution pH 2 less abundant and carbonic acid usually the least. By The effect of CO2 concentration on the pH of the solution was investigated. A titration curve, pH versus increasing the concentration of carbon dioxide in solution, bicarbonate ion predominates. CO2 concentration at three temperatures, is given in Fig. Changes in the concentration of ions such as alumi- 1. Injecting CO2 to the solution reduces the pH initially. The pH does begin to slowly decrease about one unit at a -3 CO2 concentration of 30 g dm (pH ≈ 11) because more hydroxide ions are produced that they are neutralized + -3 by the H ions. At CO2 concentration ≥ 40 g dm , the pH increases sharply because the amount of OH- ions in the solution is decreased while the concentration of Table 1. Chemical composition of aluminate solution. Component Concentration (g dm-3)

Al2O3 20.2

Na2O 21.6

K2O 42.2

SiO2 0.01 Fig. 1. A titration curve of pH versus CO2 concentration.

153 Journal of Chemical Technology and Metallurgy, 56, 1, 2021

Table 2. K-value of carbon dioxide dissociation. Temperature (oC) 70 75 80 Eq. 5 0.014 0.013 0.012 Eq. 6 4.91E-7 4.76E-7 4.59E-7 Eq. 7 7.44E-11 7.48E-11 7.49E-11

nate, hydroxide, bicarbonate and, carbonate lead to the Effect of CO2 concentration on the formation of the production of aluminum hydroxide and other carbona- products tion products (according to eqs. (1), (6) and (7)). The The tendencies to precipitate is estimated by cal- concentration variation of aluminate, hydroxide, carbon- culating the saturation indices and are given in Tables 4 ate and bicarbonate ions versus pH is illustrated in Fig. to 6. At all range of carbon dioxide and temperature, the 2. By injecting the carbon dioxide into the aluminate solution is supersaturated with aluminum species (e.g. solution, the concentration of carbonate ions is increased Al(OH)3 and AlO(OH)), hence the aluminum species as a result of the eq. (7), but the concentration of alumi- is expected to precipitate. The nucleation of boehmite nate ions remains constant. In pH about 11, the rate of is the thermodynamically least-stable phase and as the - the HCO3 was equal to the rate of the free hydroxide kinetically favored, boehmite undergoes rapid trans- ions in the solution. Further carbonation by increasing formation to bayerite rather than gibbsite owing to the - the CO2 concentration causes the concentration of OH structural similarity [43]. At CO2 concentration above ions almost zero, therefore aluminum hydroxide can 30 g dm-3, the solution is supersaturated with silica as continuously precipitate out of the solution according to well as aluminum and precipitation of species such as the eq. (1) which is indicated by the color change of the imogolite (Al2SiO3(OH)4) and dawsonite are expected solution from the clear to opaque in the laboratory. At to contaminate the product. As can be seen, silica com- - pH ≤ 10, with increasing the amount of HCO3 , the rate pounds and dawsonite at high concentrations of CO2 are - o of the HCO3 concentration is higher than the rate of free more expected at 70 C. It is necessary to mention that the hydroxide ion, at the same time, dawsonite according to thermodynamic data predicts the probability of species eq. (8) will be formed that contaminates the final prod- formation and some species may not be formed under uct. The concentration of carbonate ion also is reduced experimental conditions because of kinetic reasons. because carbonate ion reacts with water and bicarbonate From the thermodynamic study results, it could ion is formed. Since the aim of the carbonation process, be concluded that at a concentration less than 30 g in addition to the aluminum hydroxide precipitation, dm-3 of carbon dioxide or pH about 11, pure aluminum is the production of carbonate solution suitable for the hydroxide can be produced. At high concentration of carbonate salts, it seems the pH ≥ 11 is appropriate for carbon dioxide, with decreasing the temperature, the the carbonation process. formation of silica compounds and dawsonite increases.

Table 3. Species in aluminate solution before and after the carbonation process.

Before the carbonation process After the carbonation process - + Al(OH)4 K K+ Na+ + -2 Na CO3 - - OH HCO3 - - H3SiO4 OH -2 - H2SiO4 H3SiO4 -2 H2SiO4

154 Valeh Aghazadeh, Somayeh Shayanfar

So, to avoid the formation of impurities, the carbonation process should be done in the value of less than 30 g -3 dm of CO2 gas.

Experimental results To verify the thermodynamic study, carbonation experiments were carried out at pH 9, 10 and 11 at the temperatures of 70oC, 75oC and 80oC. The XRD analysis was performed to determine the phase and the chemical

composition of aluminum hydroxide obtained from the carbonation process. The XRD pattern of product, shown in Fig. 3, indicates the bayerite as a major phase of the product at pH 11 and dawsonite as the dominant phase at pH 9 at all temperatures studied. The main phase of the product at pH 10 is dawsonite, however, the product contains an amount of imogolite. At 80oC, the peaks were very sharp, indicating good crystallinity of the compound. The results of XRF analysis of bayerite prod-

ucts are shown in Table 7. They showed that the Al2O3 percentage is higher at 70oC than at other temperatures.

By increasing the temperature, the amount of Al2O3 in the products decreases because of the presence of other elements in the product. The amount of silica in products was reported by less than 0.9 wt. %. The particle size distribution of products is shown in Fig. 4. The bayerite obtained from all experiments had the bimodal particle size distribution and 100 % of the particles are below 53 μm. The percentage of particles larger than 10 μm in aluminum hydroxide produced at temperatures of 70 oC, 75 oC, and 80 oC, are 29 %, 30 % and 37 %, respectively. Fig. 2. The concentration variation of species versus pH: Under the precipitation temperature ranging from A - at 70oC, B - at 75oC, C - at 80oC. 70oC to 80oC, the variation of the mean particle sizes

o Table 4. The saturation index of the solution at 70 C and different CO2 concentration (g dm-3). CO2 Al(OH)3 AlO(OH) Al2SiO3(OH)4 NaAlCO3(OH)2 0 -0.01 0.01 -5.52 0.00 5 0.09 0.11 -5.11 0.72 10 0.21 0.24 -4.60 0.76 15 0.38 0.41 -3.92 0.79 20 0.64 0.67 -2.90 0.82 25 1.24 1.27 -0.72 0.88 30 2.40 2.44 2.10 0.98 35 2.95 2.97 4.43 0.99 40 3.30 3.32 5.39 1.00 45 3.66 3.68 6.30 1.02 50 4.16 4.18 7.46 1.03

155 Journal of Chemical Technology and Metallurgy, 56, 1, 2021

o -3 Table 5. The saturation index of the solution at 75 C and different CO2 concentration (g dm ).

CO2 Al(OH)3 AlO(OH) Al2SiO3(OH)4 NaAlCO3(OH)2 0 -0.06 -0.01 -5.60 0.00 5 0.04 0.09 -5.18 0.73 10 0.17 0.22 -4.67 0.77 15 0.34 0.39 -3.99 0.80 20 0.60 0.65 -2.97 0.83 25 1.17 1.23 -0.86 0.89 30 2.23 2.29 2.53 0.98 35 2.78 2.83 3.99 0.99 40 3.13 3.18 4.96 1.00 45 3.48 3.53 5.88 1.02 50 3.98 4.03 7.05 1.03

o -3 Table 6. The saturation index of the solution at 80 C and different CO2 concentration (g dm ).

CO2 Al(OH)3 AlO(OH) Al2SiO3(OH)4 NaAlCO3(OH)2 0 -0.11 -0.03 -5.67 0.00 5 -0.01 0.07 -5.24 0.74 10 0.12 0.20 -4.73 0.78 15 0.29 0.37 -4.04 0.81 20 0.55 0.63 -3.03 0.84 25 1.10 1.19 -0.99 0.90 30 2.06 2.14 2.08 0.98 35 2.61 2.69 3.57 0.99 40 2.96 3.03 4.54 1.00 45 3.31 3.39 5.47 1.02 50 3.81 3.88 6.66 1.03

(d50) as a function of precipitation temperature is shown products have uniform morphology and consist of small in Fig. 5. It was found that the d50 increased slowly at spherical particles and large ovoid shaped agglomerates. the precipitation temperature ranging from 70oC to 75oC, It means the crystal agglomeration can be the predomi- and then increased rapidly at 80oC, e.g., from 6.8 μm to nant mechanism which is controlling the size and the 8.1 μm because of a crystal growth and agglomeration. morphology of the bayerite products. The precipitation The SEM images and elementary composition of temperature which improves the bayerite agglomeration the particle of the typical bayerite precipitates in this in the aluminate solution will also enlarge the size of research are shown in Fig. 6. It is indicated that the bayerite product. Thus, the particle size of aluminum

Table 7. The XRF analysis of bayerite products.

Compounds Al2O3 SiO2 CaO Na2O K2O SO3 P2O5 Cl L.O.I % % % % % % % % % g-70 ̊C 64.19 0.34 <0.1 0.23 0.36 <0.1 <0.1 <0.1 34.82 h-75 ̊C 63.44 0.50 <0.1 0.29 0.50 <0.1 <0.1 <0.1 35.21 j-80 ̊C 63.36 0.83 <0.1 0.24 0.30 <0.1 <0.1 <0.1 35.12

156 Valeh Aghazadeh, Somayeh Shayanfar

Fig. 4. The particle size distribution of produced bayerite: A - 70 oC, B - 75 oC, C - 80 oC.

Fig. 5. Effect of temperature on the mean particle size (d ) 50 of bayerite precipitates.

in the product were below the detection level. The EDS results confirmed the purity of the products.

CONCLUSIONS Thermodynamic results showed that the hydrolysis of aluminate solution occurs at pH above 11. The amount of impurities such as silica and dawsonite increases by increasing the concentration of carbon dioxide. The carbonate ion in solution decreases by increasing Fig. 3. XRD pattern of the products of the carbonation the concentration of carbon dioxide, so the remaining processes: A - pH 9, (a) 70 oC, (b) 75 oC and (c) 80 oC; B - solution is not suitable for the production of carbonate pH 10, (d) 70 oC, (e) 75 oC and (f) 80 oC; C - pH 11, (g) 70 salts. The solution is supersaturated with aluminum at all oC, (h) 75 oC and (j) 80 oC. ranges of carbon dioxide and temperature. At high carbon dioxide concentration, the solution is supersaturated hydroxide precipitated from the aluminate solution at with silica and dawsonite and contaminate the products. 80oC increased. EDS analysis indicated that the pre- At high carbon dioxide concentration, the formation of dominant constituent was Al at the three temperatures. silica compounds and dawsonite are more expected at The products also contain a small amount of Na and K. 70oC. XRD Analysis of carbonation products showed From the EDS results, it can be seen that the percent- that bayerite and dawsonite are the dominant phases at ages of Al, O at the temperature of 80oC were lower the pH value above 11 and 10 at the temperatures of than other temperatures but the amount of Na, K and 70oC, 75oC and 80oC, respectively. XRF analysis showed

Ca were slightly higher. The percentages of Si present that by increasing the temperature, the amount of Al2O3 157 Journal of Chemical Technology and Metallurgy, 56, 1, 2021

the products consisted mainly of Al, O and very small amounts of Na and K and silica was not detected.

Acknowledgements This work was supported financially by the Iranian Mines & Mining Industries Development & Renovation (IMIDRO) (No. 494/7087).

REFERENCES 1. E. Jorjani, M. Amirhosseini, Alumina Production Process from Nepheline Ore in Razgah (Iran), Miner. Process. Tech., 2007. 2. B. Arlyuk, A. Pivnev, Efficiency of nepheline ore processing for alumina production, Ess. Read. in Light Metal., 2016, 983-997. 3. YJ. Li, T. Lei, DJ. Yang, Radionuclide of Process of Carbon Decomposition and Anneal of Liquor after Desilication from Nepheline, Applied. Mechanic. Mater., 2013. 4. I. Yeboah, EK. Addai, F. Acquah, SK. Tulashie, A comparative study of the super cooling and carbonization processes of the gibbsitic Ghanaian Bauxite, Int. J. Eng. Sci. Inno. Tech., 2014. 5. Q. Zhou, D. Peng, Z. Peng, G. Liu, X. Li, Agglomeration of gibbsite particles from carbonation process of sodium aluminate solution, Hydrometallurgy, 99, 2009, 163-169. 6. A. Klimenko, V.V. Shapovalov, T.V. Kolesnik, T.V. Shapovalova, A.A. Osovska, The question of the mechanism of allocation aluminum hydroxide from solutions of sodium aluminate, J. Sci. Donetsk. Inter. Tech. Uni., 2013, 158-166, (in Russian). 7. D. Linz, M. Rafal, J. Berthold, Introduction to OLI Fig. 6. SEM images and EDS analysis of the products: Electrolytes. OLI Systems Inc., 2003, 1-21. A - at 70 oC, B - at 75 oC, C - at 80 oC. 8. Xb. Li, Xm. Liu, Zr. Gou, Zh. Peng, Gh. Liu, Qs. Zhou, Ap. Ding, M. Li, Yx. Liu, The thermody- in the products decreases whereas the amount of SiO2 namics of carbonization precipitation of sodium increases. The products had less than 0.9 wt. % silica aluminate solution, Chin. J. Non-Ferrous Met., 14, as an impurity. LDS showed that all products have the 2003, 1006-1007. bimodal particle size distribution and mean particle size 9. B. Peng, W. Gui, C. Yang, Z. Hu, X. Wang, An o (d50) of bayerite precipitate at 80 C was greater than at Intelligent Control System for Continual Carbonation other temperatures. SEM showed that the products have Decomposition Process of Sodium Aluminate uniform morphology and consist of small spherical Solutions Based on Expert and Prediction Strategy, particles and coarse ovoid shaped agglomerates so that in 2006 6th World Congress on Intelligent Control it means the crystal agglomeration can be the predomi- and Automation, IEEE., 2, 2006, 7775-7779. nant mechanism which is controlling the size and the 10. X. Wang, C. Yang, W. Gui, B.R. Young, X.D. Chen, morphology of the bayerite products. EDS indicated that CSTR-based modelling for the continuous carbona- 158 Valeh Aghazadeh, Somayeh Shayanfar

tion of sodium aluminate solution, Can. J. Chem. solution using carbon dioxide gas: effect of pH and Eng., 89, 2011, 617-624. seeding, Miner Process Extr M., 2019, 1-7. 11. B. Chen, Xb. Li, Gh. Liu, Zh. Peng, Qs. Zhou, Xh. 24. JG. Reynolds, DA. Reynolds, A Modern Interpretation Xu, Xm. Liu, Investigation on the rate of reaction of the Barney Diagram for Aluminum Solubility in

between gas (CO2) and liquor (NaOH) phases, J. Tank Waste-10075, Waste Management, 2010. Guangdong Non-Ferrous Met., 3, 2006. 25. J.J. Carroll, A.E. Mather, The system carbon dioxide- 12. Y. Li, Y. Zhang, C. Yang, Y. Zhang . Precipitating water and the Krichevsky-Kasarnovsky equation, J. sandy aluminium hydroxide from sodium aluminate Solution Chem., 21, 1992, 607-621. solution by the neutralization of sodium bicarbonate, 26. K.A. Hunter, Acid-base Chemistry of Aquatic Hydrometallurgy, 98, 2009, 52-57. Systems, Dunedin, New Zealand: University of 13. R. Bassett, DC. Melchior, Chemical modeling of Otago, 1998. aqueous systems: An overview, ACS Publications, 27. V. Babushkin, G. Matveyev, O. Mchedlov-Petrossyan, 1, 1990, 1-14. Thermodynamics of silicates, Springer-Verlag, 14. S. Shayanfar, V. Aghazadeh, A. Samiee Beyragh, Berlin, Translation of Thermodinamika Silikatov by Thermodynamic Modeling and Experimental Studies BN Frenkel & VA Terentyev, 1985. of Bayerite Precipitation from Aluminate Solution: 28. L. Zeng, Z. Li, Solubility and modeling of sodium Temperature and pH Effect, Iranian Journal of aluminosilicate in NaOH–NaAl(OH)4 solutions and Chemistry and Chemical Engineering (IJCCE), its application to desilication, Ind. Eng. Chem. Res., 2018, in press. 51, 2012, 15193-15206. 15. TW. Swaddle, J. Salerno, PA. Tregloan, Aqueous 29. R.P. Buck, S. Rondinini, A.K. Covington, F.G.K. aluminates, silicates, and aluminosilicates, Chem. Baucke, C.M. Brett, M.F. Camoes, P. Spitzer, Soc. Rev., 23, 1994, 319-325. Measurement of pH. Definition, standards, and 16. P. Sipos, The structure of Al (III) in strongly alkaline procedures (IUPAC Recommendations 2002), Pure aluminate solutions-A review, J. Mol. Liq., 146, Appl. Chem., 74, 2002, 2169-2200. 2009, 1-14. 30. R. Smith, A. Martell, R. Motekaitis, NIST standard 17. A. Czajkowski, A. Noworyta, M. Krótki, Studies and reference database 46, in NIST Critically Selected modelling of the process of decomposition of alu- Stability Constants of Metal Complexes Database minate solutions by carbonation, Hydrometallurgy, Version 2, 2004, Available via the Internet at: http:// 7, 1981, 253-261. www.nist.gov/srd/nist46.cfm. 18. A.K. Karamalidis, D.A. Dzombak, Surface complex- 31. J.P. Simonin, Thermodynamic consistency in the ation modeling: gibbsite, John Wiley & Sons, 2011. modeling of speciation in self-complexing electro- 19. K. Wefers, C. Misra, Oxides and Hydroxides of lytes, Ind. Eng. Chem. Res., 56, 2017, 9721-9733. Aluminum, Alcoa Technical Paper, Alcoa Lab, 1987. 32. A. Hounslow, Water quality data: analysis and inter- 20. J.A. Apps, J. Neill, C.H. Jun, Thermochemical pretation, 1st Ed. CRC press, 1995. Properties of Gibbsite, Bayerite, Boehmite, Diaspore, 33. J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, and the Aluminate Ion between 0 and 3500oC, in Molecular thermodynamics of fluid-phase equilibria, Lawrence Berkeley National Laboratory, 1988. Pearson Education, 1998. 21. M. Wojcik, M. Pyzalski, Factors effecting on the 34. P. Atkins, Jd. Paula, Atkins’s physical chemistry, 8th properties of Bayerite obtained in the carbonization ed. W.H. Freeman and Company, 2006. process af alkaline aluminate solutions. Light Metals, 35. C. Davies, Ion Association London, Butterworths, 1990, 161-165. 1962. 22. S. Shayanfar, V. Aghazadeh, A. Saravari, P.Hasanpour, 36. J. Gustafsson, Visual MINTEQ ver. 3.0., 2010, Aluminum hydroxide crystallization from aluminate Available at http://www2.lwr.kth. se/English/ solution using carbon dioxide gas: Effect of tem- OurSoftware/vminteq/index. htm [Verified 7 July perature and time, J. Cryst. Growth, 496, 2018, 1-9. 2011]. 23. V. Aghazadeh, S. Shayanfar, P. Hassanpour, 37. H.C. Helgeson, T.H. Brown, R.H. Leeper, Handbook Aluminum hydroxide crystallization from aluminate of theoretical activity diagrams depicting chemical 159 Journal of Chemical Technology and Metallurgy, 56, 1, 2021

equilibria in geologic systems involving an aqueous ionic strength of activity coefficients. Application phase at one Atm and 0 to 300 oC. Freeman Cooper to some chloride salts of interest in the speciation & Co., 1969. of natural fluids, Chem. Speciation Bioavailability, 38. G. Scatchard, Concentrated solutions of strong elec- 16, 2004, 105-110. trolytes, Chem. Rev, 19, 1936, 309-327. 42. A. Felmy, D. Girvin, E. Jenne, MINTEQ - A 39. E. Guggenheim, J. Turgeon, Specific interaction of computer program for calculating aqueous geo- ions, Trans. Faraday Soc, 51, 1955, 747-761. chemical equilibria, US EPA Office of Research and 40. Ingmar G, Federico M, Kastriot S, et al. TDB-2 Development, 1984. Guidelines for extrapolation to zero ionic strength. 43. Y.F. Jiang, C.L. Liu, J. Xue, P. Li, J.G. Yu, Insights OECD Nuclear Energy Agency, Data Bank Issy-les- into the polymorphic transformation mechanism of Moulineaux (France), 2013. aluminum hydroxide during carbonation of potas- 41. C. Bretti, C. Foti, S. Sammartano, A new approach sium aluminate solution, Cryst. Eng. Comm., 20, in the use of SIT in determining the dependence on 2018, 1431-1442.

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