Reduction of alkalinity in bauxite residue during Bayer digestion in high-ferrite diasporic bauxite

Hydrometallurgy 151 (2015) 98–106

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Reduction of alkalinity in bauxite residue during Bayer digestion in high-ferrite diasporic bauxite Xiaolin Pan ⁎, Haiyan Yu, Ganfeng Tu School of Materials and Metallurgy, Northeastern University, Shenyang 110819, PR China

a r t i c l e

i n f o

Article history: Received 9 July 2014 Received in revised form 25 November 2014 Accepted 27 November 2014 Available online 2 December 2014 Keywords: Bayer process Digestion Hydrogarnet Alkalinity Bauxite residue

a b s t r a c t Bauxite residue is a hazardous solid waste generated during the Bayer process of extracting alumina from bauxite ore, the high alkalinity of which limits its large-scale industrial applications. In order to reduce the alkalinity of bauxite residue, rich lime additions from 0 to 30 wt.% and high digestion temperatures from 250 °C to 300 °C were performed on a high-ferrite diasporic bauxite during the Bayer digestion. The alumina extraction efficiency from bauxite and the Na2O to SiO2 weight ratio of the bauxite residue were systemically investigated. The alkalinity in the bauxite residue decreases considerably with the increasing lime addition, digestion temperature and duration. The bauxite residues after digestion are mainly comprised of hematite, zeolite, grossular hydrogarnet, andradite–grossular hydrogarnet and perovskite, but their proportions vary greatly. A new method to accurately calculate the mineralogical compositions is proposed according to both the chemical compositions and XRD analyses of the bauxite residues. The mechanisms of alkalinity reduction in the bauxite residue and performance change in bauxite digestion are discussed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Bauxite residue, also known as Bayer red mud, a hazardous solid waste generated during the Bayer process of extracting alumina from bauxite ore, has been attracting more and more attention due to the environmental damage caused by it (Power et al., 2011; Liu et al., 2009; Samal et al., 2013). The global inventory of stored bauxite residue is currently estimated to be over 2.7 billion tonnes, with an annual growth rate of approximately 120 million tonnes. As a huge industrial waste, a variety of technologies have been proposed to utilize the bauxite residue (Klauber et al., 2011), though little evidence exists of any significant utilization due to the great barriers of volume, performance, cost and risk. However, high alkalinity is the primary reason for the lack of large-scale industrial applications (Gräfe et al., 2011). The pH in bauxite residue slurry (washer overflow) ranges 9.2–12.8 with an average value of 11.3 ± 1.0. The alkaline solids formed by the reaction of caustic soda with bauxite in the Bayer process are mainly comprised of various hydroxides, carbonates, aluminates and aluminosilicates. The usual chemical compositions of bauxite residue are Al2O3 18–25 wt.%, Na2O 6–12 wt.%, SiO2 15–20 wt.%, and TiO2 2–5 wt.% (Palmer et al., 2009). Fe2O3 and CaO are also present, but their proportions vary greatly depending on the bauxite type and the leaching conditions (Cao et al., 2013). A large amount of alumina and alkali is lost during the Bayer digestion process when they combine with the silica ⁎ Corresponding author at: School of Materials and Metallurgy, Northeastern University, No. 3-11, Wenhua Road, Heping District, Shenyang 110819, PR China. E-mail address: [email protected] (X. Pan).

http://dx.doi.org/10.1016/j.hydromet.2014.11.015 0304-386X/© 2014 Elsevier B.V. All rights reserved.

in bauxite to form desilication products (DSPs), an insoluble solid and the primary existing form of alkali in the bauxite residue. The mineralogy of DSPs formed during the conventional Bayer process is sodium aluminosilicate hydrate, which has several structures such as zeolite, sodalite and cancrinite (Barnes et al., 1999; Zheng et al., 1998). The general stoichiometry of sodium aluminosilicate hydrate is Na6[Al6Si6O24]·Na2X·nH2O, where X represents a variety of in− − 2− − organic anions, such as CO2− 3 , SO4 , 2Cl , 2OH or 2NO3 (Whittington et al., 1998; Radomirovic et al., 2013). The DSPs formed during the nonconventional Bayer process with lime addition are mainly comprised of Ca-cancrinite and hydrogarnet (Xu et al., 2010; Whittington and Fallows, 1997). Ca-cancrinite, formed due to the existence of carbonate, has the stoichiometry of Na6[Al6Si6O24]·CaCO3·nH2O. Hydrogamet, with the stoichiometry of X3Y2(SiO4)n(OH)(12–4n) where X and Y are cations, has a complex formation process. The crystal structure of hydrogamet has the space group Ia3d and all cation positions are fixed by symmetry (Hawthorne, 1981). X, Y and Si denote dodecahedral, octahedral and tetrahedral coordination relative to O respectively (Ballaran and Woodland, 2006). These site occupancies involve Ca, Mg and Na in X-site, and Al, Fe and Ti in Y-site (Locock, 2008). Obviously, of all the DSPs, hydrogamet is the ideal phase in terms of reduction of alkalinity in the bauxite residue. The recovery of valuable metals from bauxite residue has been studied by many researchers (Zhong et al., 2009; Liu et al., 2012). These metals are mainly iron, aluminum, calcium, sodium and titanium as well as some minor or trace components of gallium and scandium. Among all the major metals, the recovery of iron is the most attractive. The reason is that iron oxides usually accounts for half of the bauxite

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106

residue, even as high as over 60 wt.%. The composition of iron oxides in bauxite ore can reach 20–40 wt.% in the forms of hematite, goethite and magnetite. These bauxite ores are generally called high-ferrite bauxite or Al–Fe associated ore. If both Al and Fe can be extracted, there is a considerable economic advantage. A new and clean technology to process the high-ferrite diasporic bauxite with zero waste has been proposed by the authors, and the processing schematic is shown in Fig. 1 (China Patent, 2012). The basic technical route is to: firstly extract alumina by the Bayer process, then produce iron from the bauxite residue with low alkalinity by smelting reduction based on using coal, and extract alumina from the iron smelting slag by the sinter process (Smith, 2009). The leached smelting slag after alumina extraction is mainly comprised of calcite and dicalcium silicate, which can be used to produce cement. However, the most difficult stage is to reduce the alkalinity of the bauxite residue in order to produce iron. The usual lime addition to the Bayer digestion process of diasporic bauxite is about 5–9 wt.% of the bauxite, and the corresponding DSPs formed during the high temperature digestion usually contain sodium aluminosilicate hydrate, Cacancrinite and hydrogarnet. Therefore, the alkali content in the bauxite residue is always over 6 wt.%. Since the addition of lime can promote the conversion of sodalite and cancrinite to hydrogarnet, the aim of this paper is to reduce the alkali content of bauxite residue to below 1 wt.% by adding excess lime and raising the digestion temperature. The corresponding reaction mechanisms are also discussed.

99

follows: caustic alkali 244.2 g/L (in the form of Na2O), total alkali 259.2 g/L (in the form of Na2O), alumina 140.5 g/L, silica 0.5 g/L, organic carbon 5.8 g/L, chloride 6.4 g/L, and sulfate 0.9 g/L. The initial concentrations of caustic Na2O and Al2O3 in the adjusted sodium aluminate liquor for all the digestion tests were 240.0 g/L and 136.1 g/L respectively. 2.2. Experimental procedures The digestion experiments were carried out in a 150 mL bomb reactor. The reactor was heated by molten salt with a temperature control accuracy of ± 1 °C and the bomb was rotated at 48 rpm. The slurry (28.57 g bauxite in 100 mL liquor) with different lime dosages was put into the bombs, sealed and rotated. The digestion experiments were performed at 250–300 °C for different durations. All the digestion experiments were repeated 2 or 3 times. The slurry was then cooled and separated by centrifuging. The solids were washed and dried. The alumina extraction efficiency from bauxite was calculated using the following formula: ηAl2 O3 ¼

ðA=SÞore −ðA=SÞresidue  100% ðA=SÞore

ð1Þ

(A/S)ore and (A/S)residue denote the weight ratios of Al2O3 to SiO2 in the bauxite ore and bauxite residue respectively. 2.3. Solid analyses

2. Materials and procedures 2.1. Materials The high-ferrite diasporic bauxite was ground to particle size less than 0.28 mm. Lime with 93.12 wt.% reactive CaO was produced by calcination of industrial limestone at 1050 °C for 3 h. Sodium aluminate liquor for the digestion experiments was prepared by dissolving industrial sodium hydroxide and aluminum hydroxide into industrial Bayer liquor. The concentrations of industrial Bayer liquor are as

The chemical compositions of the bauxite and bauxite residues were analyzed using an X-ray fluorescence spectrometer (XRF, ZSX100e). In order to identify the phase compositions of the bauxite and bauxite residues after digestion experiments, X-ray diffraction measurements (XRD, PANalytical PW3040/60) were performed using Cu-Kα Xradiation (all samples were ground to less than 45 μm in particle size). The scan rate was 3° 2θ/min. A semi-quantitative analysis using 10% crystalline MgO as an internal standard was performed to calculate the mineralogical composition of bauxite. Scanning electron microscopy

High-ferrite bauxite Crushing Excess lime

Evaporation

Raw slurry preparing Bayer digestion Settling

Sodium aluminate liquor

Seed precipitation

Calcination

Low-alkali red mud Pelletizing Pre-reduction Smelting reduction

Molten iron

Cacium aluminate slag Alumina extraction by the sinter process

Leached slag

Cement

Alumina Fig. 1. Processing schematic of high-ferrite diasporic bauxite to extract alumina and iron.

Alumina

100

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106

Table 1 Chemical composition of the diasporic bauxite (wt.%). Al2O3

Fe2O3

SiO2

TiO2

MgO

CaO

Na2O

LOI

52

25 ± 1

8.1 ± 0.2

2.7 ± 0.2

0.23 ± 0.03

0.10 ± 0.03

0.02 ± 0.03

12 ± 1

and energy dispersive spectroscopy (EDS) were performed on fresh bauxite residue samples using a scanning electron microscope (SEM, Shimadzu SSX-550), operating at 15 kV accelerating voltage. EDS was performed on selected points to obtain the local composition of the sample. 3. Results and discussion 3.1. Characteristics of bauxite The chemical composition of high-ferrite diasporic bauxite is presented in Table 1. The XRD pattern and the corresponding mineralogical composition of bauxite are given in Fig. 2 and Table 2, respectively. As presented in Table 1, the contents of Al2O3 and SiO2 in the bauxite are 52 wt.% and 8.1 wt.%, and the calculated weight ratio of Al2O3 to SiO2 is 6.4. The total ferrite content in bauxite (as Fe2O3) is 25 wt.%. The loss on ignition (LOI) is 12 wt.%. According to Fig. 2 and Table 2, the bauxite is comprised of diaspore (AlOOH), kaolinite (Al2O3·2SiO2·2H2O), hematite (Fe2O3), goethite (FeOOH), quartz (SiO2), and anatase (TiO2). The Al-containing minerals are mainly diaspore and kaolinite, though goethite present in the bauxite may contain some substitution of iron by aluminum. The Si-containing minerals are kaolinite and quartz. The ferrite minerals detected by XRD are hematite and goethite. The Ti-containing mineral is anatase. Aluminogoethite (Fe(1 − x)AlxOOH (x = 0–0.33)) present in the bauxite can be converted to hematite, which offers a possibility to extract the goethitic alumina (Suss et al., 2010). 3.2. Effect of lime addition on alkalinity reduction of bauxite residue In order to reduce the alkalinity in bauxite residue, different lime dosages from 0 to 30 wt.% of the bauxite were added during the Bayer digestion at 280 °C for 1 h. The digestion results are shown in Table 3, and the corresponding alumina extraction efficiency and the weight ratio of Na2O to SiO2 (N/S) in the bauxite residues are shown in Fig. 3. The alumina extraction efficiency without lime addition is

1,3

1-AlOOH 2-Fe2O3 3-SiO2 4-Al2O3·2SiO2·2H2O

Intensity

1 1

4,6 4

10

4

2

3

5

20

1

2 6

40

1

2

2

5

30

1

5- FeOOH 6-TiO2

5

very low, because most of the diaspore remains in the bauxite residue as seen from Fig. 4. Meanwhile, the N/S value is as high as 0.6 due to the high Na2O content, which is in a mineral form of zeolite (Na2O·Al2O3·1.68SiO2·1.73H2O, a kind of sodalite-type DSP). The ferrite minerals in the bauxite residue without lime addition do not differ from those in bauxite. The alumina extraction efficiency increases to 78% when the lime addition is 10 wt.%, and no diaspore is found in the bauxite residue (Fig. 5) indicating that all the diaspore in the bauxite is extracted to the liquor. The Na2O content in the bauxite residue decreases slightly compared to without lime addition, but the N/S value decreases to 0.4. The minerals in the bauxite residue with 10 wt.% lime addition are different from those without lime addition. Zhang et al. (2011) reported three kinds of hydrogarnet, i.e. grossular hydrogarnet, andradite hydrogarnet and andradite–grossular hydrogarnet, which have ideal chemical formulae of Ca3Al2(SiO4)n(OH)(12–4n), Ca3Fe2(SiO4)n(OH)(12–4n) and Ca3 [Al,Fe]2(SiO4)n(OH)(12–4n) respectively. Zoldi et al. (1987) and Li et al. (2010a) investigated the generation of different kinds of hydrogarnets during the digestion of bauxite. In this study, zeolite and two DSPs belonging to CaO-containing hydrogarnet were found to form in the bauxite residue. One was grossular hydrogarnet, and the other was andradite–grossular hydrogarnet. Owing to their isomorphic structure, it is difficult to calculate their compositions from the XRD patterns. No Ca-cancrinite was found to form in the bauxite residue. Goethite was found to disappear from the bauxite residue, indicating that it transforms to hematite during the digestion process. In addition, insoluble titanate perovskite (CaTiO3) instead of sodium titanate species formed in the bauxite residue, which is the main reason for a large increase of alumina extraction efficiency. The formation of sodium titanate species can generate an impervious nanometer thick layer covering the bauxite particles, which can seriously hinder further reaction of the alumina hydrates in the bauxite with the sodium aluminate liquor. As the lime addition is increased to 15 wt.%, the Na2O content in the bauxite residue decreases to almost half compared to that with 10 wt.% lime addition, and the N/S value decreases to 0.3. The transformation of zeolite to soda-free hydrogarnets minimizes the alkalinity of the bauxite residue. However, the alumina extraction efficiency is almost the same as for 10 wt.% lime addition. The alumina extraction efficiency from bauxite decreases with further increases of lime addition when over 10 wt.%, and the N/S value in the bauxite residue decreases with increasing lime addition from 0 to 30 wt.%. The Na2O content in the bauxite residue decreases to nearly 1 wt.% when the lime addition is 22.5 wt.%, and the corresponding N/S value is 0.1. Both the Na2O content and the N/S value are the lowest when the lime addition is 30 wt.%, being 0.2 wt.% and 0 respectively. The bauxite residues with different lime additions are all comprised of hematite, zeolite, grossular hydrogarnet, andradite– grossular hydrogarnet and perovskite, though their proportions are different. Fig. 6 shows the microstructure of the bauxite residue digested at 280 °C for 1 h with 30 wt.% lime addition. The elemental compositions

6

50

2θ Fig. 2. XRD pattern of the diasporic bauxite.

60

70

Table 2 Mineralogical composition of the diasporic bauxite (wt.%). Diaspore

Hematite

Goethite

Kaolinite

Quartz

Anatase

56 ± 3

19 ± 2

10 ± 2

6±1

5±1

3±1

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106 Table 3 Chemical compositions of bauxite residues with different lime additions at 280 °C for 1 h. Lime addition

Al2O3 (wt.%)

SiO2 (wt.%)

Fe2O3 (wt.%)

Na2O (wt.%)

Total weight (g)

0 10 15 20 22.5 25 30

42 (9.9) 17 (3.4) 16 (3.5) 16 (3.7) 16 (3.8) 16 (4.1) 17 (4.6)

10 (2.3) 13 (2.5) 11 (2.4) 10 (2.4) 10 (2.5) 10 (2.5) 10 (2.6)

30 (7.0) 35 (7.0) 33 (6.9) 30 (7.1) 30 (7.3) 29 (7.3) 27 (7.3)

6.1 5.7 3.1 1.6 1.0 0.6

23.6 19.9 21.2 23.4 24.3 25.5 27.4

0.2 (1.4) 0.2 (1.1) 0.1 (0.7) 0.1 (0.4) 0.1 (0.2) 0.1 (0.2) 0.2 (0)

± ± ± ± ± ± ±

0.3 0.1 0.2 0.3 0.2 0.5 0.3

1-AlOOH 2-Fe2O3

1

3-Na2O·Al 2O3 ·1.68SiO 2 ·1.73H 2O 4-FeOOH

2,3

Intensity

± ± ± ± ± ±

101

2 2 1

3.3. Effect of digestion conditions on alkalinity reduction of bauxite residue The results from different digestion temperatures and durations with 22.5 wt.% lime addition are shown in Table 5. The alumina extraction efficiency from bauxite and the N/S value of the bauxite residues are given in Figs. 7 and 8 respectively. The alumina extraction efficiency from bauxite is greater at greater digestion temperature. It is lowest at 250 °C and is still less than 69% even when the duration increases to 90 min. In contrast, the N/S value is lower at greater digestion temperature. At a specific digestion temperature, the alumina extraction efficiency increases, and the N/S value decreases with increasing digestion duration. As seen in Figs. 7 and 8, both the greatest alumina extraction efficiency from the diasporic bauxite and the lowest N/S value of the bauxite residue are obtained at 300 °C for 90 min. The XRD patterns of the bauxite residues at different digestion temperatures for 60 min are presented in Fig. 9. As for the bauxite residues with different lime additions, the main minerals are hematite, zeolite, grossular hydrogarnet, andradite–grossular and perovskite. However, some diaspore remains undigested at lower digestion temperatures, especially at 250 °C, which is the reason for low alumina extraction efficiency at these temperatures.

10

0.2 40

Al2O3 extraction efficiency Na2O to SiO2 ratio

0.0 -0.1

0

5

10

15

20

25

60

70

The reactions of bauxite ore during the Bayer digestion process include the dissolution of Al-containing and Si-containing minerals and the formation of DSPs. For diasporic bauxite the reactions become more complicated because of the high digestion temperature and the extra lime addition. Lime performs many useful functions in the digestion process of diasporic bauxite (Whittington, 1996). It can enhance the digestion process, minimize the inhibiting effect of sodium titanates on alumina extraction (Li et al., 2010b), purify the sodium aluminate liquor by controlling or removing the carbonate, silica, and phosphorous impurities, and promote the aluminogoethite/hematite transformation to minimize the soda loss in the bauxite residue (Suss et al., 2010). Hydration of lime in caustic liquor proceeds according to Eq. (2). CaOðsÞ þ H2 O→CaðOHÞ2 ðaqÞ:

ð2Þ

The possible dissolution reactions of minerals in diasporic bauxite during digestion are listed as Eqs. (3) to (6). AlOOHðsÞ þ NaOHðaqÞ þ H2 O→NaAlðOHÞ4 ðaqÞ

1,5

2,3

4

5

2,3

2,3

2,3

2,3

2,3

1,5 5 4

5

10

1 1 2,3,4 5 2,3,4 1 1

1,5 4 5

5

20

ð3Þ

1 1

2,3,4 2,3 5 5

1,5 2,3 4 5

0.1

30

50

4. Discussion

Intensity

Alumina extraction efficiency (%)

0.3 50

Na2O to SiO2 ratio

0.4 60

40

Fig. 4. XRD pattern of the bauxite residue without lime addition digested at 280 °C for 1 h.

80

0.5

4

2θ

0.7

70

4

30

2

2

3

3

20

5

0.6

1

3

4

of point 1 and point 2 corresponding to Fig. 6a by EDS are listed in Table 4. The EDS results confirm the existence of grossular hydrogarnet and andradite–grossular hydrogarnet, which are much close to their formulae of Ca2.93Al1.97Si0.64O2.56(OH)9.44 and Ca3AlFeSiO4(OH)8 obtained from the XRD results. As shown in Fig. 6b, the minerals of grossular hydrogarnet and andradite–grossular hydrogarnet can be distinguished from the backscattered electron (BSE) image, with the former being much darker than the latter.

1

1

3

( ) weight of each oxide in the bauxite residue in gram.

1 1

5

1

1

1 2,3 1 2,3 5 4

1

15% lime

2,31 2,3 1 5 4

1

22.5% lime

1

30% lime

1 2,3 2,3 5

2,3,4 1 1 2,3 5

30

1

40

4

2,3 5

4

1

50

10% lime

60

70

2θ

30

Lime addition (%) Fig. 3. Effect of lime addition on the digestion of bauxite.

Fig. 5. XRD patterns of bauxite residues digested with different lime additions. 1— Fe 2O 3; 2—Ca2.93 Al1.97 Si0.64 O 2.56 (OH) 9.44 ; 3—Ca 3 AlFeSiO 4(OH) 8 ; 4—CaTiO 3; and 5— Na2O·Al2O3·1.68SiO2·1.73H2O.

102

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106

(a)

(b) 1

2

Fig. 6. SEM images of bauxite residue digested at 280 °C for 1 h with 30.0 wt.% lime addition: (a) secondary electron image and (b) BSE image.

Al2 O3  2SiO2  2H2 OðsÞ þ 6NaOHðaqÞ→2NaAlðOHÞ4 ðaqÞ þ 2Na2 SiO3 ðaqÞ þ H2 O

ð4Þ

SiO2 ðsÞ þ 2NaOHðaqÞ→Na2 SiO3 ðaqÞ þ H2 O

ð5Þ

TiO2 ðsÞ þ 2NaOHðaqÞ→Na2 TiO3 ðaqÞ þ H2 O

ð6Þ

The desilication reactions resulting in the products indicated by XRD in this paper may occur per Eqs. (7) to (9). The DSPs are possibly formed by one or more steps. 2NaAlðOHÞ4 ðaqÞ þ 1:68Na2 SiO3 ðaqÞ→ Na2 O  Al2 O3  1:68SiO2  1:73H2 OðsÞ þ 3:36NaOHðaqÞ þ 0:59H2 O ð7Þ 1:97NaAlðOHÞ4 ðaqÞ þ 0:64Na2 SiO3 ðaqÞ þ 2:93CaðOHÞ2 ðaqÞ→ 2:93CaO  0:985Al2 O3  0:64SiO2  4:72H2 OðsÞ þ 3:25NaOHðaqÞ ð8Þ

bauxite residues can be calculated. The bauxite residues in this research are mainly comprised of zeolite (Na2O·Al2O3·1.68SiO2·1.73H2O), grossular hydrogarnet (Ca2.93Al1.97Si0.64O2.56(OH)9.44), andradite–grossular hydrogarnet (Ca3AlFeSiO4(OH)8), hematite (Fe2O3), and perovskite (CaTiO3). As Na2O only exists in zeolite, the mineralogical composition of zeolite (wzeolite) is easy to calculate using Eq. (12): wzeolite ¼ wNa2 O−m 

Mzeolite MNa2 O

ð12Þ

where wNa2 O−m represents the Na2O composition in the bauxite residue, Mzeolite and MNa2 O denote the molar masses of zeolite and Na2O respectively. The remaining concentrations of Al2O3 (wAl2 O3 −r) and SiO2 (wSiO2 −r) minus zeolite can be calculated by Eqs. (13) and (14): wAl2 O3 −r ¼ wAl2 O3 −m −wzeolite 

MAl2 O3

ð13Þ

Mzeolite

þ0:525H2 O NaAlðOHÞ4 ðaqÞ þ Na2 SiO3 ðaqÞ þ 3CaðOHÞ2 ðaqÞ þ 0:5Fe2 O3 ðsÞ→ 3CaO  0:5Fe2 O3  0:5Al2 O3  SiO2  4H2 OðsÞ þ NaOHðaqÞ þ 0:5H2 O ð9Þ Titanate perovskite is formed via Eq. (10). Goethite transforms to hematite by the catalytic action of Ca(OH)2 as indicated in Eq. (11), which synchronously promotes the dissolution of goethitic Al2O3 into the caustic liquor. Na2 TiO3 ðaqÞ þ CaðOHÞ2 ðaqÞ→CaTiO3 ðsÞ þ 2NaOHðaqÞ

ð10Þ

2FeOOHðsÞ→Fe2 O3 ðsÞ þ H2 O

ð11Þ

Because of the various and complex minerals formed in the bauxite residues, it is very difficult to accurately calculate the mineralogical compositions of the bauxite residues by Rietveld analysis of XRD patterns. XRD cannot be used to distinguish grossular hydrogarnet or andradite– grossular hydrogarnet from the total hydrogarnets. According to the reactions from Eqs. (6) to (11), the mass fractions of minerals in the

wSiO2 −r ¼ wSiO2 −m −wzeolite 

Point

O

Ca

Al

Si

Fe

Na

Ti

1 2

54 56

23 20

14 7

5 7

1 7

1 1

1 1

ð14Þ

Mzeolite

where wAl2 O3 −m and wSiO2 −m represent the Al2O3 and SiO2 compositions in the bauxite residue, and MAl2 O3 and MSiO2 denote the molar masses of Al2O3 and SiO2 respectively. Table 5 Chemical compositions of bauxite residues digested at different temperatures for different times (wt%). Temperature Time Al2O3 (°C) (min) (wt.%)

SiO2 (wt.%)

Fe2O3 (wt.%)

Na2O (wt.%)

Total weight (g)

300

10 (2.4) 10 (2.4) 10 (2.3) 10 (2.4) 9 (2.4) 10 (2.4) 10 (2.5) 10 (2.5) 9 (2.4) 10 (2.5) 10 (2.5) 10 (2.4) 9 (2.5) 9 (2.5) 9 (2.4) 10 (2.4)

30 (7.3) 30 (7.2) 31 (7.2) 31 (7.3) 30 (7.4) 30 (7.3) 30 (7.3) 31 (7.5) 29 (7.5) 29 (7.5) 30 (7.4) 30 (7.3) 27 (7.5) 28 (7.5) 28 (7.4) 29 (7.5)

1.2 ± 0.1 (0.3) 0.8 ± 0.1 (0.2) 0.6 (0.1) 0.6 (0.1) 1.7 ± 0.1 (0.4) 1.4 ± 0.1 (0.3) 1.0 ± 0.1 (0.2) 0.7 (0.2) 2.2 ± 0.2 (0.6) 1.8 (0.5) 1.6 ± 0.1 (0.4) 1.3 ± 0.1 (0.3) 2.7 ± 0.1 (0.8) 2.2 ± 0.1 (0.6) 1.9 ± 0.1 (0.5) 1.5 ± 0.1 (0.4)

24.5 23.7 23.5 23.4 25.0 24.6 24.3 24.1 26.1 25.6 25.0 24.7 27.5 27.0 26.1 25.7

280

265

Table 4 Elemental compositions of point 1 and point 2 corresponding to Fig. 6(a) by EDS (at.%).

1:68  MSiO2

250

20 40 60 90 20 40 60 90 20 40 60 90 20 40 60 90

15 (3.8) 15 (3.6) 14 (3.4) 14 (3.4) 17 (4.3) 16 (4.0) 16 (3.8) 16 (3.8) 18 (4.8) 17 (4.4) 16 (4.1) 16 (3.9) 22 (6.1) 20 (5.5) 20 (5.1) 19 (4.8)

( ) weight of each oxide in the bauxite residue in gram.

± ± ± ± ±

0.2 0.3 0.1 0.4 0.2

± 0.2 ± 0.4 ± ± ± ± ± ± ±

0.1 0.2 0.4 0.2 0.1 0.1 0.1

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106

2,3,4 1 1 1,5 2,3 5 6 4 2,3,4 1 2,3 1,5 5 64 2,3,4 1 1 1,5 2,3 5 4 2,3,4 1 1

2,3 5 75

2,3 5

Intensity

Alumina extraction efficiency (%)

80

70

2,3 5

250O C 265O C 280O C 300O C

65

2,3

1,5 2,3

5

4

60 20

40

60

80

103

2,3 2,3 1 1 4 5

1

2,3 2,3 1 1 5 4

1

2,3 2,3 1 1 4 5

1

2,3 2,3 1 1 4 5

1

6

o

250 C

o

265 C

6

5

o

280 C

o

300 C

100

10

Digestion time (min)

20

30

40

50

60

70

2θ Fig. 7. Alumina extraction efficiency from bauxite digested at different digestion temperatures for different durations.

Al2O3 and SiO2 coexist in zeolite, grossular hydrogarnet and andradite–grossular hydrogarnet, but fortunately the indexes of SiO2 in these minerals are different. As the zeolite fraction has already been calculated, the mineralogical compositions of grossular hydrogarnet and andradite–grossular hydrogarnet can be calculated from the residual Al2O3 and SiO2 contents according to their formulae using two linear equations:

wAl2 O3 −r

1 1:97  MAl2 O3  MAl2 O3 2 ¼ wandradite  þ wgrossular  2 Mandradite Mgrossular

wSiO2 −r ¼ wandradite 

MSiO2 Mandradite

þ wgrossular 

0:64  MSiO2 Mgrossular

ð15Þ

ð16Þ

Na2O to SiO2 ratio

 wgrossular ¼

wandradite

 2wAl2 O3 −r  MSiO2 −wSiO2 −r  MAl2 O3  Mgrossular 1:33  MAl2 O3  MSiO2

ð17Þ

  1:97 1:28 wSiO2 −r  MAl2 O3 − wAl2 O3 −r  MSiO2  Mandradite 1:33 1:33 ¼ : MAl2 O3  MSiO2

Fe2O3 exists in both hematite and andradite–grossular hydrogarnet. Since the mineralogical composition of andradite–grossular hydrogarnet is already obtained, the mineralogical composition of hematite can be calculated using Eq. (19): whematite ¼ wFe2 O3 −m −wandradite 

0:5  MFe2 O3 Mandradite

ð19Þ

where wFe2 O3 −m and MFe2 O3 denote the Fe2O3 composition in the bauxite residue and the molar mass of Fe2O3 respectively. The concentration of perovskite (wperovskite) can be obtained using Eq. (20) according to the mass conservation of TiO2 and Fe2O3 both in the bauxite and bauxite residue.

0.35

250OC 265OC 280OC 300OC

0.25

Hence the compositions of grossular hydrogarnet and andradite– grossular hydrogarnet can be calculated using Eqs. (17) and (18):

ð18Þ

where wandradite and wgrossular represent the compositions of andradite– grossular hydrogarnet and grossular hydrogarnet in the bauxite residue, and Mandradite and Mgrossular denote the molar masses of andradite–grossular hydrogarnet and grossular hydrogarnet respectively.

0.30

Fig. 9. XRD patterns of bauxite residues digested at different temperatures for 1 h. 1— Fe2O3; 2—Ca2.93Al1.97Si0.64O2.56(OH)9.44; 3—Ca3AlFeSiO4(OH)8; 4—CaTiO3; 5— Na2O·Al2O3·1.68SiO2·1.73H2O; and 6—AlO(OH).

wperovskite ¼ wTiO2 −b 

wFe2 O3 −m wFe2 O3 −b



Mperovskite MTiO2

ð20Þ

0.20

0.15

0.10

0.05

0.00 20

40

60

80

100

Digestion time (min) Fig. 8. Na2O to SiO2 ratio of bauxite residues with different digestion temperatures and durations.

wTiO2 −b and wFe2 O3 −b represent the TiO2 and Fe2O3 compositions in the bauxite, and Mperovskite and MTiO2 denote the molar masses of perovskite and TiO2 respectively. The calculated results with different lime additions and digestion conditions are listed in Tables 6 and 7, and the actual weights of minerals in the bauxite residues are shown in Figs. 10 and 11 respectively. The tests without lime addition and when digested at 250 °C are not included because of the obvious existence of undigested diaspore in the bauxite residue. Although the minerals in the bauxite residues are the same, their compositions are obviously different, especially for those with different lime additions. As listed in Fig. 10, the zeolite content in the bauxite residue decreases gradually with the increase of lime addition, which corresponds to the decrease of Na2O content. Meanwhile,

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X. Pan et al. / Hydrometallurgy 151 (2015) 98–106 15

Lime addition

Zeolite

Grossular hydrogarnet

Andradite–grossular hydrogarnet

Hematite

Perovskite

10.0 15.0 20.0 22.5 25.0 30.0

27 ± 1 15 8±1 5 3 1

28 33 38 39 43 49

4±3 22 ± 1 29 33 ± 1 33 ± 1 32 ± 2

33 ± 2 24 20 18 ± 1 16 14

6 6 6 5 5 5

±1 ±1 ±1 ±1

the content of grossular hydrogarnet increases gradually with increasing lime addition, resulting in the corresponding decrease of alumina extraction efficiency because of the Al2O3 loss into the bauxite residue. However, the content of andradite–grossular hydrogarnet varies with lime addition. It is only 1.0 g (4 wt.%) when the lime addition is 10 wt.%, but it increases to 4.7 g (22 wt.%) as the lime addition increases to 15 wt.%. This is the reason that the alumina extraction efficiency with 10 wt.% lime addition is almost the same as that of 15 wt.% lime addition. Then the content of andradite–grossular hydrogarnet increases with lime addition, but it is almost constant when the lime addition is over 22.5 wt.%. Correspondingly, the hematite content decreases with increasing lime addition. The perovskite content remains the same as lime addition increases. As for gibbsitic bauxites, the alumina extraction efficiency decreases approximately by 0.7–0.9% as the lime addition increases by 1 wt.% when digested at lower temperatures (Pan et al., 2013). However, the alumina extraction efficiency from the diasporic bauxite in this study only decreases by 0.3% in average as the lime addition increases by 1 wt.% when added over 10 wt.%. The formation of andradite–grossular hydrogarnet, in partial replacement of grossular hydrogarnet, digested at higher temperatures contributes to the reduction of alumina extraction efficiency on increasing lime addition. As shown in Fig. 11, at a typical digestion temperature, the contents of zeolite (Fig. 11a), grossular garnet (Fig. 11b) and hematite (Fig. 11c) in the bauxite residue decrease gradually with the increasing digestion time; in contrast, the content of andradite–grossular hydrogarnet (Fig. 11d) increases with duration of digestion. For a typical digestion duration, the contents of zeolite, grossular hydrogarnet and hematite decrease gradually, while the content of andradite–grossular hydrogarnet increases with increasing digestion temperature. At all digestion temperatures and times, the perovskite content (Fig. 11e) changes only slightly. By combining the calculated mineralogical results with the digestion results, we can conclude that the alkalinity of the bauxite residue mainly depends on the zeolite composition, and the alumina extraction efficiency from the diasporic bauxite mainly depends on the proportion of andradite–grossular hydrogarnet. Therefore, with greater zeolite,

Mineralogical weight (g)

Table 6 Mineralogical compositions of bauxite residues with different lime additions at 280 °C for 1 h (wt.%).

Zeolite Grossular hydrogarnet Andradite–grossular hydrogarnet Hematite Perovskite

12

9

6

3

0 10

15

20

25

30

Lime addition (%) Fig. 10. Mineralogical weights of bauxite residues with different lime additions at 280 °C for 1 h.

the greater the alkalinity of bauxite residue; the greater the andradite–grossular hydrogarnet, the greater the alumina extraction efficiency will be.

5. Conclusions The alumina extraction efficiency from a high-ferrite diasporic bauxite and the Na2O to SiO2 weight ratio of the bauxite residues were investigated during the Bayer digestion process by adding excess lime additions from 0 to 30 wt.% and raising digestion temperatures from 250 °C to 300 °C. The alkalinity in the bauxite residue decreases with the increase of lime addition, digestion temperature and digestion duration. The alumina extraction efficiency without lime addition is very low, but decreases with the increase of lime addition over 10 wt.%. Raising the digestion temperature and increasing the digestion duration help to increase the alumina extraction efficiency. The alkalinity of the bauxite residue can be controlled below 1 wt.% when the lime addition is over 22.5 wt.% and the digestion temperature is over 280 °C. The bauxite residues with different lime additions and digestion conditions are all mainly comprised of hematite, zeolite, grossular hydrogarnet, andradite–grossular hydrogarnet and perovskite, but their mineralogical compositions calculated according to both the chemical compositions and the XRD results are different. The formation of grossular hydrogarnet and andradite–grossular hydrogarnet, instead of zeolite, contributes to the decrease of alkalinity, while the proportion of andradite–grossular hydrogarnet determines the alumina extraction efficiency during the Bayer digestion.

Table 7 Mineralogical compositions of bauxite residues digested at different temperatures for different digestion times (wt.%). Temperature (°C)

Time (min)

Zeolite

Grossular hydrogarnet

Andradite–grossular hydrogarnet

Hematite

Perovskite

300

20 40 60 90 20 40 60 90 20 40 60 90

6 4 3 2 8 7 5 4 10 ± 1 9 7 6

39 37 34 33 49 42 39 38 56 49 44 40

28 ± 1 35 ± 1 39 ± 1 43 14 25 ± 1 33 ± 2 39 ± 5 2±2 13 ± 1 21 ± 1 29

19 18 18 17 23 20 18 ± 1 17 ± 1 27 ± 1 23 21 19

5 6 6 6 5 5 5 6 5 5 5 6

280

265

± ± ± ± ± ± ± ±

1 1 1 1 1 3 1 1

±1

X. Pan et al. / Hydrometallurgy 151 (2015) 98–106 3.2

16

(a) Zeolite

O

265 C 280 OC 300 OC

14

Mineralogical weight (g)

2.4

10

(b) Grossular hydrogarnet

265 OC 280 OC 300 OC

2.8

Mineralogical weight (g)

105

2.0 1.6 1.2 0.8

8

12

6

10

4

8

2

0.4

6 20

40

60

80

100

20

40

Digestion time (min)

60

7.5

(c) Andradite–grossular hydrogarnet

(d) Hematite 265 OC 280 OC 300 OC

7.0

Mineralogical weight (g)

Mineralogical weight (g)

8

6

4

265 OC 280 OC 300 OC

0 40

60

80

100

6.0 5.5 5.0 4.5 4.0

(e) Perovskite

20

40

60

80

100

Digestion time (min)

Digestion time (min)

265 OC 280 OC 300 OC

2.0

Mineralogical weight (g)

6.5

3.5 20

2.5

0 100

Digestion time (min)

10

2

80

1.5

1.0

0.5

0.0 20

40

60

80

100

Digestion time (min) Fig. 11. Mineralogical weights of bauxite residues digested at different temperatures for different digestion times.

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