Effect of High Concentrations of Calcium ... - ACS Publications

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Effect of High Concentrations of Calcium Hydroxide in Neutralized Synthetic Supernatant Liquor - Implications for Alumina Refinery Residues Sara J. Palmer,† Matthew K. Smith,‡ and Ray L. Frost*,† †

Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, Queensland, Australia ‡ Rio Tinto Alcan, Business Development & Growth, Technology and R&D - QRDC, Queensland, Australia ABSTRACT: The presence of calcium hydroxide (Ca(OH)2) in Bayer residue slurry inhibits the effectiveness of the seawater neutralization process to reduce the pH and aluminum concentration in the residue. An increase in the slurry pH (reversion), after seawater neutralization, is caused by the dissolution of calcium hydroxide and hydrocalumite (solid components found in bauxite refinery residue). Reversion was not observed when the final solution pH was greater than 10.5, due to hydrocalumite being in a state of equilibrium at high pH. Hydrocalumite has been found to form during the neutralization process when high concentrations of calcium hydroxide are present in the residue liquor. The dissolution of hydrocalumite releases hydroxyl (OH-) and aluminum ions back into solution after the seawater neutralization (SWN) process, which causes pH and aluminum reversion to occur. This investigation looks at the effect of Ca(OH)2 and subsequently hydrocalumite on the pH and aluminum concentration in bauxite refinery residue liquors after the SWN process.

1. INTRODUCTION Bauxite refinery residues are derived from the Bayer process by the digestion of crushed bauxite in concentrated caustic (NaOH) at elevated temperatures and pressures. The process results in the dissolution of gibbsite (Al(OH)3) and boehmite (AlOOH) to form a solution of sodium aluminate ions, while the insoluble residue (red mud) is separated by means of flocculation and decantation.1-3 The composition of bauxite refinery residue is complex, with the concentration of compounds varying with the type of bauxite and refinery process used. The supernatant liquor from this residue is strongly alkaline4,5 and requires neutralization to a pH below 8.9, with an optimum pH value of 8.5-8.9,6 before environmental discharge can be considered. The aim of the neutralization process is to add sufficient seawater to permanently reduce the pH below 8.9; however, an increase in pH after the neutralization point (i.e., after all the seawater has been added) can occur, and this is referred to as “pH reversion”. Seawater neutralization is one such treatment that reduces both the pH and the dissolved metal concentrations of the residue, through the precipitation of Mg, Ca, and Al hydroxide and carbonate minerals.6 The formation of these hydrotalcite (HT)like compounds (also known as layered double hydroxide, LDH) also removes oxy-anions of transition metals through a combination of intercalation and adsorption mechanisms. The general formula for these structures is: [M1-x2þMx3þ(OH)2]xþAx/mm- 3 nH2O, where M2þ is a divalent cation, M3þ is trivalent cation, and A is an interlamellar anion with charge m-. LDH phases exist for 0.2 e x e 0.33.7-9 Because refineries are striving to become more environmentally accountable and sustainable, an increase in bauxite residue management practices is being investigated and employed.10-12 r 2011 American Chemical Society

A major objective for research in this particular field is understanding residue behavior before and after various treatments. Carter et al., who looked at characterizing bauxite residue using pH leaching tests and geochemical modeling methods, have done a good example of this.13 The solid residue consists of a variety of compounds including calcium aluminate species, organic material, and various oxides of iron, titanium, and silica. The most common type of calcium aluminate formed in the Bayer process is tricalcium aluminate (TCA - Ca3Al2(OH)12 3 6H2O). TCA is formed from the reaction of calcium hydroxide (Ca(OH)2), sodium aluminate (NaAl(OH4)), and sodium hydroxide (NaOH).14,15 The addition of lime (CaO-burnt lime or Ca(OH)2-slaked lime) at various stages of the Bayer process provides numerous benefits to the process including: (1) improving the dissolution of boehmite and diaspore during digestion, (2) helping to reduce liquor impurities, (3) assisting in phosphate control in pregnant liquor, and (4) reducing soda losses in red mud.14 A large amount of research has been published on Ca(OH)2 in the Bayer industry;14,15 however, little focus has been placed on the effects of high concentrations of Ca(OH)2 in residue liquors neutralized by seawater. Therefore, this investigation focuses on the effects of high Ca(OH)2 concentrations in residue liquor on the neutralization process, and the dissolution of Ca(OH)2 and hydrocalumite (Ca2Al(OH)6Cl 3 2H2O) in regards to pH and aluminum reversion. Received: September 15, 2010 Accepted: December 26, 2010 Revised: November 5, 2010 Published: January 13, 2011 1853

dx.doi.org/10.1021/ie101905b | Ind. Eng. Chem. Res. 2011, 50, 1853–1859

Industrial & Engineering Chemistry Research

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Table 1. Concentration of Ca(OH)2 (g/L) and the Concentration of Solid Ca(OH)2 Left in SNL and SWN-SNL 0.05 M

0.10 M

0.30 M

0.40 M

0.50 M

1.00 M

74.12 g/L

Concentration of Ca(OH)2 in the Initial SNL Solution (60 mL) g/L Ca(OH)2 in SNL

3.71 g/L

7.41 g/L

22.23 g/L

29.64 g/L

37.05 g/L

soluble Ca(OH)2 in SNL

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

remaining Ca(OH)2 in SNL

2.45 g/L

6.15 g/L

20.97 g/L

28.38 g/L

35.79 g/L

72.86 g/L

13.50 g/L

Concentration of Ca(OH)2 in Solution after SWN of SNL (330 mL) g/L Ca(OH)2 in SWN-SNL

0.67 g/L

1.35 g/L

4.04 g/L

5.39 g/L

6.70 g/L

Soluble Ca(OH)2 in SWN-SNL

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

1.26 g/L

Remaining Ca(OH)2 in SWN-SNL

0 g/L

0.09 g/L

2.78 g/L

4.13 g/L

5.44 g/L

12.24 g/L

2. METHODS 2.1. Experiments: Synthesis of Materials and Seawater Neutralization Reactions. 2.1.1. Materials. AR grade Ca-

(OH)2 was used in this investigation. Hydrocalumite was synthesized using the coprecipitation method, commonly used to prepare LDHs. The coprecipitation method involved the addition of two solutions, where solution 1 contained 2 M NaOH and a combination of Na2CO3 to give a concentration of 0.2 M, while solution 2 contained 0.66 M Ca2þ (CaCl2 3 2H2O) and 0.33 M Al3þ (AlCl3 3 6H2O). Solution 2 was added dropwise to solution 1, under vigorous stirring. The precipitated compound was then thoroughly washed to remove any residual salts and dried overnight in an oven (85 °C). 2.1.2. Seawater Neutralization of Synthetic Supernatant Liquor Spiked with Calcium Hydroxide and Hydrocalumite. The experiments involved the addition of different concentrations of calcium hydroxide (0.05-1 M) or hydrocalumite (0.01-0.5 M) to 60 mL of synthetic supernatant liquor (SNL), while being stirred for 5 min. The composition of synthetic SNL used in this investigation is 3.2 g/L Al2O3, 6.5 g/L Na2O (caustic), and 6.3 g/L Na2O (carbonate). The initial pH of SNL was maintained at around 12. The concentration of Ca(OH)2 used for each test and the theoretical concentration of solid Ca(OH)2 left in solution are given in Table 1. For 0.05 and 0.10 M, the complete dissolution of Ca(OH)2 occurs, while the other concentrations tested were above the solubility of Ca(OH)2. After 5 min, synthetic seawater (270 mL) was added to SNL at around 120 mL a minute and was left to stir for a further 2 h. The composition of synthetic seawater used in this investigation has been previously reported by the authors.16 Samples (20 mL) were taken every 30 min to monitor the ions in solution and the concentration of phases in the precipitate. Each sample was vacuum and syringe filtered (0.45 μm filters) for ICP analysis, while the precipitate was washed (after aqueous sample had been removed) and placed in an oven (85 °C) overnight to dry. The pH of the solution was monitored at 15 s intervals for a total of 2 h using a TPS-40 pH meter and general laboratory pH probe. The pH probe was calibrated using buffers 7 and 10. The experiment was repeated three times for each concentration of Ca(OH)2, and the results presented in this Article are an average of the three. 2.2. Methods for the Characterization of Solutions and Precipitate. 2.2.1. X-ray Diffraction (XRD). X-ray diffraction patterns were collected using a Philips X’pert wide angle X-ray diffractometer, operating in step scan mode, with Cu KR radiation (1.54052 Å). Patterns were collected in the range 3-75° 2θ with a step size of 0.02° and a rate of 90 s per step. Samples were

prepared as a finely pressed powder into aluminum sample holders. The Profile Fitting option of the software uses a model that employs 12 intrinsic parameters to describe the profile, the instrumental aberration, and wavelength-dependent contributions to the profile. 2.2.2. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Samples of the initial synthetic SNL and resulting solution after the SWN process were analyzed using a Varian ICP-OES instrument. Standards containing aluminum, magnesium, and calcium were prepared to establish a calibration curve. Results were obtained using an integration time of 3 s with three replications. The relative amounts of each element were recorded on a Varian Liberty 2000 ICP-OES at wavelengths of 394.400, 279.553, and 393.366 for aluminum, magnesium, and calcium, respectively. 2.2.3. Thermogravimetric Analysis (TGA). Thermal decomposition of the hydrotalcite was carried out in a TA Instrument incorporated with a high-resolution thermogravimetric analyzer (series Q500) in a flowing nitrogen atmosphere (80 cm3/min). Approximately 50 mg of sample was heated in an open platinum crucible at a rate of 2.0 °C/min up to 1000 °C. The synthesized hydrotalcites were kept in an oven at 85 °C for 24 h before TG analysis. Thus, the mass losses are calculated as a percentage on a dry basis.

3. RESULTS AND DISCUSSION 3.1. Synthetic SNL with Ca(OH)2. Different amounts of Ca(OH)2 were mixed (5 min) with the same volume of SNL before the same volume of seawater was added. The solution pH was monitored at 15 s intervals for a 2 h period. 3.1.1. pH. All concentrations of Ca(OH)2 investigated observed an increase in pH after the addition of synthetic seawater, Figure 1. The aim of the neutralization process is to add sufficient seawater to permanently reduce the pH below 8.9. However, an increase in pH after the neutralization point (i.e., after all the seawater has been added) can occur, and this is referred to as “pH reversion”. The required seawater volumes to neutralize Bayer residue are dictated by the dissolved levels of caustic and alumina in the liquor. Therefore, the presence of solid-phase compounds that can increase this neutralization requirement, such as Ca(OH)2, will increase the seawater requirement. In the presence of Ca(OH)2, reversion occurs almost instantaneously after the final volume of seawater is added to SNL. At low concentrations (0.05 and 0.10 M), an increase of less than 0.5 pH units occurs. It is also apparent that an increase in Ca(OH)2 increases the neutralization point (minimum pH reached after the addition of seawater). This increase in neutralization point is 1854

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Figure 1. Combined pH plots for the SWN of synthetic SW and SNL with varying concentrations of Ca(OH)2.

due to the dissolution of Ca(OH)2 during the addition of seawater, which releases a greater amount of OH- ions into solution. Because of a constant volume of seawater being used in these experiments, the amount of neutralizing cations remains constant, and therefore there are no additional cations available to remove the newly released OH- ions from solution. The primary mechanism for the removal of OH- ions from solution is through the formation of two hydrotalcite structures: 6MgCl2ðaqÞ þ 2NaAlðOHÞ4ðaqÞ þ 8NaOHðaqÞ þ Na2 CO3ðaqÞ þ xH2 OðlÞ f Mg6 Al2 ðOHÞ16 ðCO3 Þ 3 xH2 OðsÞ þ 12NaClðsÞ ð1Þ 8MgCl2ðaqÞ þ 2NaAlðOHÞ4ðaqÞ þ 12NaOHðaqÞ þ Na2 CO3ðaqÞ þ xH2 OðlÞ f Mg8 Al2 ðOHÞ20 ðCO3 Þ 3 xH2 OðsÞ þ 16NaClðsÞ ð2Þ The formation of hydrotalcite is pH dependent, and as such a mixture of 3:1 and 4:1 hydrotalcite structures form.17 High concentrations of Ca(OH)2 (0.50 and 1.00 M) prevented a reduction in pH, with the final pH remaining at values greater than 11. The neutralization of high Ca(OH)2 suspensions still showed a small reduction in pH before an increase occurred. This reduction is due to the formation of hydrotalcite, hydrocalumite (Ca2Al(OH)6Cl 3 2H2O), and brucite (Mg(OH)2). The formation of hydrotalcite and brucite can be observed by the significant decrease in Mg2þ concentration, Figure 2. Hydrotalcite forms over a large pH range (7-12), while hydrocalumite and brucite formation is favored at high pH (greater than 10). A significant reduction in pH is not observed for high concentrations of Ca(OH)2 due to an influx of OH- ions caused by the dissolution of Ca(OH)2. As hydrotalcite, hydrocalumite, and brucite form, and the pH begins to fall, the dissolution of Ca(OH)2 is facilitated until insufficient amounts of magnesium, aluminum, and calcium remain in solution. A state of equilibrium for Ca(OH)2 has been observed to occur between pH 11 and 11.5 when insufficient cations remain in solution. The dissolution of Ca(OH)2 releases 2 mol of OH- ions into solution, which causes the pH to rise: CaðOHÞ2ðsÞ T Ca2þ ðaqÞ þ 2OH - ðaqÞ

ð3Þ

Figure 2. Concentration of magnesium cations in solution for varying concentrations of Ca(OH)2 in SWN-SNL over 2 h.

There are three shifts in equilibrium observed for the dissolution of Ca(OH)2 during the seawater neutralization process: (1) At low pH, the equilibrium reaction shifts to the right (dissolution of Ca(OH)2) due to the use of OH- ions in the formation of hydrotalcite. (2) The use of Ca2þ in the formation of CaCO3 and CaCl2 shifts the equilibrium to the right. (3) At high concentrations of Ca(OH)2, the release of OHions is readily used up in the formation of brucite, hydrotalcite, and hydrocalumite. The solubility of Ca(OH)2 in pure water is 1.26 g/L at 50 °C.18 It should be noted that the solubility of Ca(OH)2 increases at lower temperatures. Therefore, the presence of Ca(OH)2 in the residue (disposed of in tailings dam) will show a continual pH increase as the residue cools. At 0.05 and 0.10 M, it is also proposed that the dissolution of hydrocalumite occurs. This is shown in Figure 3 by an increase in Ca2þ ions in the first 30 min. An increase in Al3þ concentration is also expected to occur; however, due to excess Mg2þ ions in solution, hydrotalcite forms immediately removing Al3þ from solution. A full discussion on the effect of hydrocalumite in SNL will be discussed in section 3.2. It is proposed that along with the dissolution of Ca(OH)2, the following reactions may also contribute to the rise in pH, albeit at a much slower rate: CaCO3ðsÞ þ 2NaClðaqÞ T CaCl2ðaqÞ þ Na2 CO3ðaqÞ 18

ð4Þ

CaðOHÞ2ðaqÞ þ Na2 CO3ðaqÞ T CaCO3ðsÞ þ 2NaOHðaqÞ 19 ð5Þ 3.1.2. ICP. The Mg2þ ion concentration decreases significantly with increased Ca(OH)2 concentrations, Figure 2. The increase in OH- ions in solution, due to Ca(OH)2 dissociating, results in the additional formation of hydrotalcite and brucite. At 0.05 and 0.10 M, the pH of solution is below pH 9, not favorable for brucite formation, and thus the removal of Mg2þ is dependent on the concentration of Al3þ ions for the formation of hydrotalcite. Analysis has shown that essentially all Al3þ ions (representing Al(OH)4- anions) are removed from solution for all concentrations of Ca(OH)2 tested. Therefore, excess Mg2þ ions remain in solution for Ca(OH)2 concentrations of 0.05 and 0.10 M as hydrotalcite is unable to form. The absence of Al3þ ions at low pH allows for pH reversion to occur as hydrotalcite formation is the primary mechanism for the reduction of pH during neutralization. High Ca(OH)2 concentrations result in a 1855



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Figure 3. Concentration of calcium cations in solution for varying concentrations of Ca(OH)2 in SWN-SNL over 2 h.

Figure 5. DTG curves of 1.00 M Ca(OH)2 before SWN (Ca(OH)2 þ SNL) and after SWN (neutralization point (Ca(OH)2 þ SNL þ SW) and 30 min after SWN (Ca(OH)2 þ SNL þ SW (30 min))).

Figure 4. XRD patterns of calcium aluminate species tested as triggers and the corresponding reference patterns.

solution pH greater than 10, which results in all Mg2þ ions being used up in the formation of brucite. ICP results have shown a significant increase in calcium in solution as the concentration of Ca(OH)2 increases, Figure 3, but this relationship is not linear. Therefore, the increase in Ca2þ is believed to be due to soluble calcium salt CaCl2 (eq 4), the dissolution of hydrocalumite (eq 6), and Ca(OH)2 (eq 3). 3.1.3. XRD. XRD identified six mineralogical components in the precipitate: hydrotalcite, calcium hydroxide, hydrocalumite, calcite, aragonite, and minor quantities of sodium chloride. Ca(OH)2 and hydrocalumite are stable at high pH, while

hydrotalcite, calcite, and aragonite are stable in all alkaline solutions. The XRD pattern highlights the absence of Ca(OH)2 peaks for 0.05 and 0.10 M, confirming the complete dissolution of Ca(OH)2, Figure 4. Very small peaks are observed for 0.50 M, indicating that only a small portion of Ca(OH)2 is present in the solid phase, while a large quantity of Ca(OH)2 is still present in the precipitate for 1.00 M Ca(OH)2. XRD confirmed that hydrotalcite forms at all concentrations of Ca(OH)2; however, different Mg:Al ratios are predicted to form due to the range of pH values in which these structures form. It is also observed that calcite (CaCO3) predominantly forms, with small amounts of aragonite forming at lower pH (smaller concentrations of Ca(OH)2). The absence of CaCl2 in the precipitate indicates that the formation of calcium carbonate species is the predominant mechanism for the removal of Ca 2þ ions. CaCO3 is a more stable structure than CaCl 2, and hence its formation is favored. 3.1.4. TGA. The interpretation of the DTG curves is based on previous work by the authors in the investigations on synthetic hydrotalcite and hydrotalcite prepared under seawater neutralization conditions.20-22 DTG curves of the SWN solids of SNL1.00 M Ca(OH)2 confirmed the formation of hydrocalumite, observed as a shoulder on the hydrotalcite peak at 300 °C, Figure 5. However, hydrotalcite still remained the predominant species. The decomposition of the solid residue from seawater neutralization containing excess Ca(OH)2 occurs in four steps: (i) the removal of adsorbed water to the external surface of the precipitate (70 °C), (ii) the dehydroxylation and decarbonation 1856



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Figure 6. Combined pH plots for the SWN of synthetic SW and SNL with varying concentrations of hydrocalumite.

Figure 7. Aluminum concentration in solution after the SWN of SNL with varying concentrations of hydrocalumite.

Table 2. Summary of pH Results for Hydrocalumite Concentrations That Showed pH Reversion neutralization

delay time

final

% increase

concentration

point

(min)

pH

(pH)

0.10 M

8.20

8.04

N/A

0.20 M 0.30 M

8.32 8.46

32.5 15.0

10.23 10.23

23.0% 20.9%

0.40 M

8.53

5.5

10.17

19.2%

0.50 M

10.17

2.75

10.38

2.1%

N/A

of the brucite-like hydroxyl layers of Bayer hydrotalcite and hydrocalumite (270-300 °C), (iii) the dehydroxylation of calcium hydroxide (360-380 °C), and (iv) decarbonation of calcium carbonate (600-620 °C). 3.2. Synthetic SNL with Hydrocalumite. Different amounts of synthetic hydrocalumite were mixed (5 min) with the same volume of SNL and seawater used for the Ca(OH)2 experiments. The solution pH was monitored at 15 s intervals for a 2 h period. 3.2.1. pH. Reversion (pH) is observed when the concentration of hydrocalumite in SNL is greater than 0.10 M, Figure 6. At concentrations below this, the final pH remains between 8.0 and 8.5. At low concentrations (0.10 M and less), the release of OHions from the dissolution of hydrocalumite (eq 6) is used up by the formation of hydrotalcite, thus preventing the pH to increase. Reversion only occurs when the concentration of Mg2þ ions in solution is insignificant, and hydrotalcite is unable to form. The rate at which reversion occurs is dependent on the concentration of hydrocalumite in solution, Table 2. Increasing the concentration of hydrocalumite releases more OH- ions into solution, therefore causing the pH to rise at a faster rate until a state of equilibrium is reached. Hydrocalumite (2Ca2Al(OH)6Cl 3 2H2O) can be rewritten in the oxide phases that it is formed from: 3CaO 3 Al2O3 3 CaCl2 3 10H2O. The synthesis of hydrocalumite in a sulfate-rich environment would form ettringite (3CaO 3 Al2O3 3 CaSO4 3 10H2O), common to the cement industry.23 Hydrocalumite and ettringite are chemically similar; therefore, it is proposed that they possess similar stabilities and reactivities. It has been reported that the stability of ettringite decreases in solutions with a pH below 10.5.23 As the pH falls below 10.5, the dissolution of ettringite occurs. Therefore, the same is proposed to be true for hydrocalumite. The following equilibrium reaction

Figure 8. Magnesium concentration in solution after the SWN of SNL with varying concentrations of hydrocalumite.

is proposed: Ca2 AlðOHÞ6 Cl 3 2H2 OðsÞ þ NaClðaqÞ T CaCl2ðaqÞ þ NaAlðOHÞ4ðaqÞ þ CaðOHÞ2ðaqÞ þ 2H2 OðaqÞ

ð6Þ

As the pH falls below 10.5 during neutralization, the dissolution of hydrocalumite becomes favored and thus releases aluminate (Al(OH)4-) and hydroxyl ions into solution, causing both pH and aluminum reversion. This corresponds well with the dissolution pH reported for ettringite. It is observed that the neutralization point increases with elevated hydrocalumite concentrations. The final pH of solution, for all concentrations of hydrocalumite that showed pH reversion, is between pH 10 and 10.5. In this pH range, the remaining hydrocalumite in solution is in equilibrium, and therefore the dissolution of hydrocalumite is no longer favored. 3.2.2. ICP. An increase in Al3þ concentration after the neutralization point is referred to as “Al3þ reversion”. ICP confirmed that Al3þ reversion occurred for hydrocalumite when present in concentrations above 0.10 M in SNL, Figure 7. The reversion of aluminum appeared to correspond well with pH reversion; therefore, it is believed pH and aluminum reversion are directly related. There is an inverse relationship between the Al3þ (Figure 7) and Mg2þ (Figure 8) concentrations in solution. It can be clearly seen from these charts that Al3þ reversion is prevalent when all Mg2þ ions are removed from solution, shown for 0.50 M hydrocalumite. The concentration of magnesium 1857



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of hydrocalumite. Hydrocalumite forms during the seawater neutralization process (reaction of Ca2þ ions in seawater and Al(OH)4- ions in liquor); however, it is unstable at pH values below 10.5. The dissolution of hydrocalumite not only causes pH reversion, but also Al3þ reversion. Therefore, the concentration of calcium hydroxide impacts the efficiency of the seawater neutralization process for alumina refinery residues both directly and indirectly.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Figure 9. Calcium concentration in solution after the SWN of SNL with varying concentrations of hydrocalumite.

steadily decreases after the SWN process due to the simultaneous dissolution of hydrocalumite and hydrotalcite formation. The formation of additional hydrotalcite, stimulated by the release of aluminate ions, causes both the Mg2þ concentration and the pH to decrease. The Al3þ concentration in solution does not appear to increase significantly after 30 min, because the pH of solution has reached equilibrium pH within this time period. Therefore, the dissolution of hydrocalumite no longer occurs. The calcium concentration varied significantly with the concentration of hydrocalumite, Figure 9. Increasing the concentration of hydrocalumite in solution increased the concentration of calcium in solution, as expected. However, once the concentration of hydrocalumite reached 0.50 M, the calcium levels in solution decreased significantly. The calcium concentration is dependent on the concentration of hydrocalumite and pH. At low hydrocalumite concentrations, 0.01 M, the amount of calcium in solution (from hydrocalumite dissolution) is negligible, because the calcium ions were immediately consumed via the formation of calcium carbonate. However, as the concentration of hydrocalumite increased (0.02, 0.03, and 0.04 M), the dissolution of hydrocalumite became more noticeable. In these instances, the excess Ca2þ ions remained dissolved because the carbonate anions had been depleted. The same trend was observed for hydrocalumite concentrations of 0.20, 0.30, and 0.40 M. However, when the hydrocalumite levels are above 0.5 M, the corresponding pH increase appears to cause a secondary increase in carbonate levels, mainly due to an elevated rate of CO2 absorption (and so CO32-). The pH for 0.50 M hydrocalumite remained between pH 10 and 10.5. As a result, more CaCO3 precipitated out of solution, thus decreasing dissolved Ca2þ levels when normal reaction kinetics suggested it should increase. It is also proposed a small amount of Ca(OH)2 formed.

4. CONCLUSIONS This investigation has shown that the presence of solid calcium hydroxide in supernatant liquor results in a pH rise after seawater neutralization. It is believed this pH increase is due to (1) the dissolution of Ca(OH)2 and (2) the dissolution of hydrocalumite. The dissolution of Ca(OH)2 in the solution continued until a pH above 11 is obtained. At this pH value, a state of equilibrium is reached. The presence of carbonate also promoted calcium hydroxide dissolution, through the precipitation of calcite, resulting in the additional release of hydroxide ions into solution. The presence of Ca(OH)2 in the liquor promotes the formation

’ ACKNOWLEDGMENT The financial and infra-structure support of the Queensland Research and Development Centre (RioTintoAlcan) and the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. ’ REFERENCES (1) Chvedov, D.; Ostap, S.; Le, T. Surface properties of red mud particles from potentiometric titration. Colloids Surf., A 2001, 182, 131– 141. (2) Hind, A. R.; Bhargava, S. K.; Grocott, S. C. The surface chemistry of Bayer process solids: a review. Colloids Surf., A 1999, 146, 359–374. (3) Jamialahmadi, M.; Muller-Steinhagen, H. Determining silica solubility in Bayer process liquor. JOM 1998, 50, 44–49. (4) Lin, C.; Maddocks, G.; Lin, J.; Lancaster, G.; Chu, C. Acid neutralising capacity of two different bauxite residues (red mud) and their potential applications for treating acid sulfate water and soils. Aust. J. Soil Res. 2004, 42, 649–657. (5) Menzies, N. W.; Fulton, I. M.; Morrell, W. J. Seawater neutralization of alkaline bauxite residue and implications for revegetation. J. Environ. Qual. 2004, 33, 1877–1884. (6) Hanahan, C.; McConchie, D.; Pohl, J.; Creelman, R.; Clark, M.; Stocksiek, C. Chemistry of seawater neutralization of bauxite refinery residues (red mud). Environ. Eng. Sci. 2004, 21, 125–138. (7) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. New synthetic routes to hydrotalcite-like compounds. Characterization and properties of the obtained materials. Eur. J. Inorg. Chem. 1998, 50, 1439–1446. (8) Frost, R. L.; Ding, Z.; Kloprogge, J. T. The application of nearinfrared spectroscopy to the study of brucite and hydrotalcite structure. Can. J. Anal. Sci. Spectrosc. 2000, 45, 96–102. (9) Frost, R. L.; Erickson, K. L. Thermal decomposition of synthetic hydrotalcites reevesite and pyroaurite. J. Therm. Anal. Calorim. 2004, 76, 217–225. (10) McConchie, D.; Clark, M.; Hanahan, C.; Fawkes, R. The use of seawater-neutralised bauxite refinery residues (red mud) in environmental remediation programs. REWAS; Minerals, Metals & Materials Society: Warrendale, 1999. (11) Graham, G. A.; Fawkes, R. Red mud disposal management at QAL. Proceedings of the International Bauxsite Tailings Workshop; Allied Colloids: Perth, Australia, 1992. (12) Paradis, M.; Duchesne, J. Long-term neutralisation potential of red mud bauxite with brine amendment for the neutralisation of acidic mine tailings. Appl. Geochem. 2007, 22, 2326–2333. (13) Carter, C. M.; Van Der Sloot, H. A.; Cooling, D.; Van Zomeren, D.; Matheson, T. Characterization of untreated and neutralized bauxite residue for improved waste management. Environ. Eng. Sci. 2008, 25, 475–488. (14) Whittington, B. I. The chemistry of CaO and Ca(OH)2 relating to the Bayer process. Hydrometallurgy 1996, 43, 13–35. 1858



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(15) Whittington, B. I.; Fallows, T. Formation of lime-containing desilication product (DSP) in the Bayer process: factors influencing the laboratory modelling of DSP formation. Hydrometallurgy 1997, 45, 289– 303. (16) Palmer, S. J.; Grand, L. M.; Frost, R. L. Thermal analysis of hydrotalcite synthesised from aluminate solutions. J. Therm. Anal. Calorim. 2010, in press. (17) Smith, H. D.; Parkinson, G. M.; Hart, R. D. In situ absorption of molybdate and vanadate during precipitation of hydrotalcite from sodium aluminate solutions. J. Cryst. Growth 2005, 275, 1665–1671. (18) Durrant, P. J. General and Inorganic Chemistry; Longmans: London, 1960. (19) Heslop, R. B.; Robinson, P. L. Inorganic Chemistry; Elsevier: London, 1961. (20) Palmer, S. J.; Frost, R. L. The effect of synthesis temperature on the formation of hydrotalcites in Bayer liquor - a vibrational spectroscopic analysis. Appl. Spectrosc. 2009, 63, 748–752. (21) Palmer, S. J.; Soisonard, A.; Frost, R. L. Determination of the mechanism(s) for the inclusion of arsenate, vanadate, or molybdate anions into hydrotalcites with variable cationic ratio. J. Colloid Interface Sci. 2009, 329, 404–409. (22) Palmer, S. J.; Frost, R. L.; Nguyen, T. Thermal decomposition of hydrotalcite with molybdate and vanadate anions in the interlayer. J. Therm. Anal. Calorim. 2008, 92, 879–886. (23) Chrysochoou, M.; Dermatas, D. Evaluation of ettringite and hydrocalumite formation for heavy metal immobilization: Literature review and experimental study. J. Hazard. Mater. 2006, 136, 20–33.

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