Stepwise extraction of valuable components from red mud based on reductive roasting with sodium salts

Journal of Hazardous Materials 280 (2014) 774–780

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Stepwise extraction of valuable components from red mud based on reductive roasting with sodium salts Guanghui Li 2 , Mingxia Liu, Mingjun Rao ∗,1 , Tao Jiang ∗∗,1 , Jinqiang Zhuang, Yuanbo Zhang School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan 410083, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• An integrated route for comprehensive utilization of red mud was proposed. • Sodium salts are favorable to the magnetic separation and acid leaching efficiencies. • Metallic iron powder, TiO2 -rich residue, silica gel and Al(OH)3 were recovered.

a r t i c l e

i n f o

Article history: Received 27 May 2014 Received in revised form 6 August 2014 Accepted 3 September 2014 Available online 16 September 2014 Keywords: Red mud Comprehensive utilization Reductive roasting Sodium salts TiO2

a b s t r a c t The feasibility of an integrated technological route for comprehensive utilization of red mud was verified in this study. Valuable components in the mud, including Fe2 O3 , Al2 O3 and SiO2 were stepwise extracted by magnetic separation and sulfuric acid leaching from reduced red mud, and meanwhile TiO2 was enriched in the leaching residue. Sodium salts were proved to be favorable for the magnetic separation of metallic iron and the subsequent acid leaching of Al and Si, through facilitating the reduction of iron oxides and the growth of metallic iron grains, together with enhancing the activation of Al and Si components during the roasting process. After reductive roasting in the presence of 6% Na2 CO3 and 6% Na2 SO4 , a magnetic concentrate containing 90.2% iron with iron recovery of 95.0% was achieved from the red mud by magnetic separation. Subsequently, 94.7% Fe, 98.6% Al and 95.9% Si were extracted by dilute sulfuric acid leaching from the upper-stream non-magnetic material, yielding a TiO2 -rich material with 37.8% TiO2 . Furthermore, value-added products of silica gel and Al(OH)3 were prepared from the leachate by ripening and neutralizing. © 2014 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: Biology Building, RM 205, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 88830542. ∗∗ Corresponding author at: Biology Building, RM 320, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 88877656. E-mail addresses: [email protected] (G. Li), [email protected], [email protected] (M. Rao), [email protected] (T. Jiang). 1 These authors contributed equally to this work. 2 Address: Peace Building, RM 249, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 88830542. http://dx.doi.org/10.1016/j.jhazmat.2014.09.005 0304-3894/© 2014 Elsevier B.V. All rights reserved.

The pyrochemical extraction of alumina from bauxite ores produces a waste of red mud. Approximately, 0.8–1.5 tons of red mud are generated from per ton of alumina production depending on the properties of bauxite ores and operating conditions [1]. It is estimated that about 70 Mt of red mud is discharged to the environment annually in the world, and the amount of accumulated red mud will reach 350 Mt by 2015 in China [2,3]. Red mud is usually pumped away from alumina production factory and accumulated, resulting in high costs for land filling and heavy pollution to water, land and air [4–7].

G. Li et al. / Journal of Hazardous Materials 280 (2014) 774–780

2. Experimental 2.1. Materials 2.1.1. Red mud The red mud sample used in this research was taken from Shandong Aluminum Company, China. Particle size distribution of the

Table 1 Particle size distribution of the red mud. Particle size (␮m) Fraction (wt.%)

<45 14.41

45–75 11.86

75–150 22.89

150–400 50.84

Table 2 Chemical composition of red mud (wt.%). Fetotal

Al2 O3

SiO2

TiO2

Na2 O

CaO

S

P

LOIa

48.2

7.3

8.0

1.4

1.4

0.9

0.1

0.08

10.3

a

LOI: loss on ignition.

350 G

300

G - Goethite H - Hematite Q - Quartz

H

250 HG

Intensity/CPS

The treatment and disposal of red mud has been a huge challenge for the alumina industry [8], due to large quantities and the fact that red mud is a corrosive and hazardous substance with a high alkalinity (pH 10–12.5) [9]. Prior to reuse for revegetation, red mud should be neutralized by seawater [10], gypsum [11], fungus [12] and sewage sludge [13], etc. to reduce its residual alkalinity. However, more attention should be paid to the knowledge gaps in the residual alkalinity and associated chemistry [5]. Red mud could be also regarded as a resource of multiple metals. Major constituents of red mud are Fe2 O3 , Al2 O3 , SiO2 , Na2 O, TiO2 , etc., and various minor/trace elements, especially rare earth and scattered metals, are also present [14–16]. Recovery and utilization of valuable components from red mud is of inherent economic value. In recent years, great efforts have been made to use red mud directly in multiple fields of environmental protection such as gas purifying [17], water treatment [18] or soil improvement [19] and in production of building materials such as cement [20], glass [21], etc. Moreover, catalyst, is another field in which red mud can be used [22]. These methods usually suffer new contamination or a difficulty in further treatments, and the economic value produced is relatively low [1]. Regarding recovery of valuable components from red mud, previous works mainly focused on recycling Al2 O3 , Na2 O and TiO2 from red mud, by using leaching method [23–25], hydrothermal process [26–29] and sintering process [30,31], etc. Moreover, prior to the recycling of Al2 O3 , Na2 O and TiO2 from red mud, iron recovery or removal was usually performed. Since hematite and goethite are the major constituents of red mud, magnetic separation is a basic method for iron recovery from red mud, in which iron minerals are separated from gangues by high gradient magnetic separation due to the magnetic susceptibility difference between iron/iron oxides and gangue minerals [32–34]. However, direct magnetic separation was thought to be inefficient because of the dissemination of fine iron oxides. In order to improve the efficiency of magnetic separation, some researchers adopted reductive roasting to convert hematite/goethite into magnetite [35,36] or metallic iron [2,37,38], which can be separated magnetically under more mild condition by enhancing the magnetic susceptibility of iron-bearing minerals. During reductive roasting of refractory iron ores, such as nickeliferous laterites [39], oolitic hematite ore [40], ludwigite ore [41] and alumina-rich limonite ore [42], sodium salts (Na2 SO4 and Na2 CO3 ) are proved to play an important role in the enhanced reduction of iron ores. Herein, in the present work, without dealkalization in advance, sodium salts were used to intensify the reduction of red mud and to facilitate the growth of metallic iron particles for improving magnetic separation efficiency. Effect of sodium salts on the subsequent acid leaching of non-magnetic material was also examined. An integrated technological route based on the solid-state carbothermic reduction of red mud in the presence of sodium salts was proposed, and stepwise extraction of valuable components involving: (1) the first step was reductive roasting followed by magnetic separation to recover Fe; (2) the second step was sulfuric acid leaching of the non-magnetic material to enrich TiO2 ; and (3) the last step was ripening and neutralizing the leaching leachate for the separation of Si, Al and Na.

775

200

Q

150

H H

100

G G

H G

H

40

50

H G G

50 0 10

20

30

H

G

60

Q

70

80

2-Theta/Degree Fig. 1. X-ray diffraction pattern of red mud.

red mud is shown in Table 1, and its chemical composition which was measured by chemical titration method is presented in Table 2. XRD pattern of the red mud sample is shown in Fig. 1, indicating that it mainly consists of goethite (FeO(OH)), hematite (Fe2 O3 ) and quartz (SiO2 ). 2.1.2. Reductant Lignite, which was crushed and screened to a size of 0.5–2.0 mm, served as the external reductant. Its proximate analysis and ash chemical composition is shown in Table 3, in which proximate analysis was performed in a proximate analyzer (HunanSundy, SDTGA5000, China) and chemical composition of the ash is performed by chemical titration. 2.1.3. Sodium salts Sodium carbonate (Na2 CO3 ) and sodium sulfate (Na2 SO4 ) were used as additives, and both of them were of analytical grade (Sinopharm, China). 2.2. Methods As depicted in Fig. 2, the experimental procedure mainly includes: (1) reductive roasting the red mud briquettes followed by magnetic separation to recover Fe; (2) sulfuric acid leaching of the non-magnetic material to concentrate TiO2 ; and (3) ripening and Table 3 Proximate analysis of lignite and its ash composition (wt.%). Proximate analysisa

Main chemical composition of ash

Mad

Ad

Vdaf

Fcad

CaO

MgO

Al2 O3

SiO2

6.9

5.3

35.2

52.6

3.4

0.6

32.2

45.5

a

Mad : moisture; Ad : ash content; Vdaf : volatile content; Fcad : fixed carbon content.

776

G. Li et al. / Journal of Hazardous Materials 280 (2014) 774–780

Red Mud

Sodium Salts

Briquetting

Lignite

Reduction Roasting

Grinding & Magnetic separation Non-magnetic Material

Metallic Iron

Sulfuric Acid Leaching Leachate

TiO2-rich slag

Ripening & Filtrating Leachate

Silica Gel Fig. 3. Schematic diagram of the vertical resistance furnace.

Neutralizing & Filtrating Na2SO4 Solution

non-magnetic part was subjected to acid leaching subsequently. The yield of magnetic concentrate and recovery of iron were calculated according to Eqs. (2) and (3), respectively.

Al(OH)3

Fig. 2. Experimental procedure.

=

neutralizing the acid leaching leachate to achieve the separation of SiO2 , Al(OH)3 and Na2 O. 2.2.1. Reductive roasting of red mud briquettes Red mud was sufficiently mixed with a certain proportion of additives (6 wt.% Na2 CO3 and 6 wt.% Na2 SO4 ). The mixture was briquetted into cylinders with a diameter of 10 mm and height of 10 mm and then dried in a vacuum drying oven at 110 ◦ C for 4 h. After drying, 30 g dry red mud briquettes and 90 g lignite were mixed and then charged into a cylindrical heat-resistant stainless steel pot with an inner diameter of 60 mm and height of 200 mm. Subsequently, the pot was put into a vertical resistance furnace (see the schematic in Fig. 3) which has an inner diameter of 80 mm. Before the pot with sample is placed into the fixed high temperature zone of the vertical resistance furnace, the furnace was preheated to a signed high temperature firstly. Then, the briquettes in the pot were reduced at 1050 ◦ C for 60 min. The chosen roasting temperature and time was based on our previous work aiming at iron recovery from red mud [43]. After roasting, reduced briquettes were cooled to room temperature in the pot isolated from atmospheric oxygen. Thus the index of iron metallization ratio was calculated according to Eq. (1). =

ˇ × 100% ˛

(1)

Where:  is the iron metallization ratio, %; ˛ is the total iron content of the reduced sample, %; and ˇ is the metallic iron content of the reduced sample, %. 2.2.2. Magnetic separation of reduced briquettes Prior to separation, 20 g reduced samples was ground to about 90 wt.% passing 74 ␮m in an XMQ Ф240 × 90 wet grinding ball mill. The slurry was then separated in an XCGS-73 Davis Magnetic Tube using a magnetic field intensity of 0.1 Tesla. Metallic iron was enriched in the obtained magnetic concentrate, and the

m1 × 100% m0

(2)

where:  is the yield of magnetic concentrate, %; m0 is the feed mass of reduced sample subjected to magnetic separation, g; m1 is the mass of obtained magnetic concentrate, g. ε=×

 × 100% ˛

(3)

where: ε is the recovery of iron, %;  is the total iron content of magnetic concentrate, %. 2.2.3. Sulfuric acid leaching of non-magnetic material To extract residual species of Al2 O3 , SiO2 , Na2 O and TiO2 , the non-magnetic part obtained from magnetic separation was leached with sulfuric acid. All batch experiments were conducted in a 200 ml beaker, fitted with a plastic stirrer whose stirring speed was maintained constantly at 300 rpm. The beaker was placed in a water bath which controls the temperature. Finally, the solution was filtered for separating dissolved Fe, Al, Si constituents from titanium oxide which was enriched in the leaching residue. Then, the lixivium was collected for ripening and neutralization. 2.2.4. Ripening and neutralization of acid leaching leachate The lixivium rich in Fe, Al and Si was transferred to a beaker and heated in a water-bath. By controlling the bath temperature and time, the silicic acid polymerized and transformed into silica gel gradually by Ostwald ripening. Distilled water was then added into the beaker and stirred the mixture to break the silica gel. Then silica gel solution was filtered with a vacuum filter and separated the residue and filtrate. The residue was put into a vacuum drying oven for 4 h consistently at 110 ◦ C for chemical analysis and a filtrate rich in Al and Na constituents was obtained at the same time. Silica was obtained, washed and dried for chemical analysis. In the meanwhile, a filtrate rich in Al and Na constituents was obtained. The filtrate was neutralized with NaOH solution by controlling the solution alkalinity using a pH meter. Until Al3+ were precipitated completely, Al(OH)3 was separated from Na+ by filtering. The filtered cake was washed and dried in a vacuum drying oven for 4 h

G. Li et al. / Journal of Hazardous Materials 280 (2014) 774–780

777

Reduction and seperation indexes/%

100

95 92.1 90.2

90

90.2

83.7 80

78.2

76.9

70

67.1

60

None

6%Na2CO3+6%Na2SO4 Addition of sodium salts

Fe metallization of reduced briquettes Fe grade of magnetic concentrate

Yield of magnetic concentrate Fe recovery of magnetic concentrate

Fig. 4. Effect of addition of sodium salts on the iron metallization ratio and magnetic separation efficiency.

consistently at 110 ◦ C for quantitative analysis by chemical titration method. 3. Results and discussion 3.1. Recovering Fe from red mud by reductive roasting followed by magnetic separation

Fig. 5. Microstructure of reduced briquettes: (a) in the absence of sodium salts and (b) in the presence of sodium salts.

It is evident that diffractions of quartz and hercynite disappeared and those of sodium aluminosilicate (Na1.75 Al1.75 Si0.25 O4 and NaAlSiO4 ) were intensified, and perovskite (CaTiO3 ) emerged. This suggests that in the presence of sodium salts, hercynite and quartz were converted into sodium aluminosilicate according to Eq. (7). Meanwhile, iron within the hercynite was also reduced into metallic iron, which can be likewise confirmed by the increased metallization ratio of iron (Fig. 4). Fe2 O3 + CO → 2FeO + CO2

(4)

Fe

Intensity/CPS

1000

(a)

900 200

Fe 100

Quartz

0

Intensity/CPS

As the reductive roasting and magnetic separation parameters of complex iron ores have been intensively investigated in our previous work [39–41,43], effect of addition of sodium salts on iron metallization ratio and magnetic separation efficiency was primarily investigated in this work, under the fixed reductive roasting temperature of 1050 ◦ C, roasting time of 60 min, magnetic field intensity of 0.1 Tesla and magnetic separation feed size of 90 wt.% passing through a 200 mesh sieve (74 ␮m). For comparison, the results in the absence and presence of sodium salts (6 wt.% Na2 SO4 and 6 wt.% Na2 CO3 ) are illustrated in Fig. 4. The results show that the presence of sodium salts has a favorable effect on the reduction and magnetic separation of red mud briquettes. The quality of magnetic concentrate was significantly improved with the addition of sodium salts, because the Fe grade of the magnetic concentrate increased from 83.7% to 90.2%, and the corresponding Fe recovery increased from 92.1% to 95.0%. Improved magnetic separation efficiency can be explained by the microstructure of reduced briquettes in the absence and presence of sodium salts (see Fig. 5). It indicates that metallic iron particles remain fine and dispersive when the briquettes are reduced in the absence of additives (Fig. 5(a)). In contrast, in the presence of sodium salts (Fig. 5(b)), the metallic iron particles are gathered with a larger size, which is favorable for the particle liberation during grinding and also for the downstream magnetic separation of metallic iron particles from non-magnetic part. XRD patterns of magnetic separation products obtained from reduced briquettes in the absence and presence of sodium salts are presented in Figs. 6 and 7, respectively. When reduced in the absence of sodium salts additives, the mineral phases were quite complex, including quartz, hercynite (Fe0.899 Al0.101 )O(Al0.899 Fe1.101 )O3 , sodium aluminosilicate (NaAlSiO4 ) and metallic iron (see Figs. 6(a) and 7(a)). The formation of hercynite could be illustrated in Eqs. (4)–(6). When sodium salts were added during the roasting process, the existing mineral phases were much different (see Fig. 6(b) and (b)).

(b)

Fe

900 800 200

Fe

100 0 10

20

30

40

50

60

70

80

2-Theta/Degree Fig. 6. XRD patterns of magnetic concentrates obtained from reduced briquettes: (a) in the absence of sodium salts and (b) in the presence of sodium salts.

G. Li et al. / Journal of Hazardous Materials 280 (2014) 774–780

Intensity/CPS

H

800 300

NN

200

(a)

S - Na1.55Al1.55Si0.45O4 P - CaTiO3 I - Fe I

Q

1000

NNN

Q

N

H

100

Intensity/CPS

0 2000

S

1800 1600

N

600 400

N P N N

S T

Q - SiO2 (b) H - (Fe0.899Al0.101)O(Al1.899Fe0.101)O3 N - NaAlSiO4 T - Fe

N

200

T

P

20

30

40

80

50

98.6

95.9

60

51.7 40

29.2

25.1

20.8

20

6.3 0

60

70

80

None

2-Theta/Degree Fig. 7. X-ray patterns of non-magnetic products obtained from reduced briquettes: (a) in the absence sodium salts and (b) in the presence of sodium salts.

Fig. 8. Effect of addition of sodium salts during the reductive roasting on the sulfuric acid leaching of Fe, Al, Ti and Si from the non-magnetic material.

3400

Fetotal

Al2 O3

SiO2

TiO2

Magnetic part

Absence Presence Absence Presence

74.3 90.2 23.6 26.5

5.5 1.7 26.0 28.4

4.5 1.8 5.0 5.5

1.2 0.4 25.9 6.3

FeO + Al2 O3 → FeO·Al2 O3

(5)

(2FeO·SiO2 ) + Al2 O3 + CO → Fe + FeO·Al2 O3 + SiO2 + CO2

(6)

FeO·Al2 O3 + Na2 O + SiO2 + CO → Fe + NaAlSiO4 + CO2

(7)

Table 4 shows the main chemical composition of magnetic separation products of reduced red mud briquettes. It indicates that the quality of iron concentrate in the absence of sodium salts has overwhelming superiority over that in the absence of sodium salts. The total iron content reached 90.2% with only a tiny amount of impurities. 3.2. Enriching TiO2 from non-magnetic material by dilute sulfuric acid leaching Effect of addition of sodium salts during reductive roasting on the downstream sulfuric acid leaching of Fe, Al, Ti and Si from the non-magnetic material was further examined, and the results are shown in Fig. 8. The leaching conditions were sulfuric acid concentration of 20%, leaching temperature of 30 ◦ C, time of 30 min, liquid to solid ratio of 10:1. XRD patterns and main chemical composition of acid leaching residues are presented in Fig. 9 and Table 5, respectively. These two kinds of non-magnetic materials, derived from magnetic separation of reduced red mud in the absence and presence of sodium salts, present a great difference in the acid leaching. In comparison, in the presence of sodium salts, leaching ratios of Fe, Al and Si increased from 51.9% to 94.7%, 29.2% to 98.6%, and 20.8% to 95.9%, respectively (Fig. 8). The improved leaching efficiency of non-magnetic material is a result of the simultaneous activation of Al2 O3 and SiO2 constituents during the reductive roasting in the presence of sodium salts, as hercynite and quartz had been converted into sodium aluminosilicate (Na1.75 Al1.75 Si0.25 O4 and NaAlSiO4 ) (Eq. (7)). Owing to the good solubility of sodium aluminosilicate in the sulfuric acid, Al2 O3 , Na2 O and SiO2 components were leached (Fig. 9). Correspondingly, TiO2 was enriched more

Q - Quartz H - Hercynite(Fe0.899Al0.101)O(Al1.899Fe0.101)O3) S - Sodium Iron Titanium Oxide Na0.6Fe0.53Ti1.55O3.995 P - perovskite(CaTiO3)

Q

(a)

3000 2800 600

Q

400

H H

200

QQ Q H Q

Q Q

Q

Q

0 1500

Intensity/CPS

Absence/presence of sodium salts

Intensity/CPS

3200

Products

6%Na2CO3+6%Na2SO4 Addition of sodium salts

Table 4 Main chemical composition of magnetic separation products of reduced red mud briquettes (wt.%).

Non-magnetic part

94.7

P

0 10

Fe Al Ti Si

100

Extraction of elements/%

778

P

(b) 1400

P 600 400

S

200 0 10

20

P

P S S

P 30

S

40

S

P

S

50

60

2-Theta / Degree

70

80

Fig. 9. XRD patterns of acid leaching residues: (a) in the absence sodium salts and (b) in the presence of sodium salts. Table 5 Main chemical composition of sulfuric acid leaching residues (wt.%). Absence/presence of sodium salts

Fetotal

Al2 O3

SiO2

TiO2

Absence Presence

12.3 2.5

24.1 3.4

43.3 9.7

7.0 37.8

Table 6 Chemical compositions of ripening and neutralization products (wt.%). Products

Fetotal

SiO2

Al2 O3

TiO2

LOI

Silica gel Al(OH)3 precipitate

0.4 6.0

64.2 1.1

2.2 35.3

0.4 1.5

32.6 53.6

significantly in the leaching residue, and a TiO2 -rich material with 37.8% TiO2 was obtained (Table 6). 3.3. Recovering Al, Si and Na from leachate by ripening and neutralization After reductive roasting in the presence of sodium salts, 98.6% Al and 95.9% Si were leached from the non-magnetic material (Fig. 8). Aiming to recover Al, Si, and Na from the leachate, ripening and neutralization was further conducted. After separating Si by ripening at 90 ◦ C for 4 h, the filtrate was neutralized with NaOH solution

G. Li et al. / Journal of Hazardous Materials 280 (2014) 774–780

(3) The ripening and neutralization process results in a silica gel containing 64.2% SiO2 and an Al(OH)3 precipitate containing 35.3% Al2 O3 . The value-added products such as magnetic concentrate which can be used for steelmaking and Al(OH)3 precipitate used for alumina extraction, as well as the reuse of Na2 O may compensate the cost of the whole process.

400

Intensity/CPS

(a) 300 200 100

Acknowledgements

0 3000

(b)

Intensity/CPS

2500 2000 1500 1000 500 0 10

779

20

30

40

50

60

70

80

All the authors hope to thank the National Natural Science Foundation of China (51174230 and 51234008) and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-11-0515) for the financial support of this work. Special thanks go to Dr. Zhiwei Peng (Department of Materials Science and Engineering, Michigan Technological University) for helpful comments and suggestions on paper writing.

2-Theta/degree Fig. 10. X-ray diffraction patterns of silica gel (a) and Al(OH)3 precipitate (b).

Al3+ ,

until the pH reached 5.5 for the precipitation of and then the recycle of Na2 SO4 can be achieved. The main chemical composition and X-ray diffraction patterns of ripening and neutralization solid products are listed in Table 6 and Fig. 10, respectively. As can be seen from Fig. 10, both of the X-ray diffraction patterns were amorphous, verifying the components were silica gel and Al(OH)3 respectively. Table 6 shows that the content of SiO2 of silica gel reached 64.2%, and also a limited content of Al(OH)3 and TiO2 was remained in the silica gel. Besides, due to the polymerization with a large amount of water during ripening, the mass loss on ignition was high. As for the obtained Al(OH)3 by precipitating, Al2 O3 content reached 35.3%. Therefore, value-added products of silica gel and Al(OH)3 precipitate and the re-use of Na2 O may compensate the cost of the whole process. 4. Conclusions The feasibility of an integrated technological route for stepwise extraction of valuable components from reduced red mud in the presence of sodium salts was investigated, and the following conclusions were obtained: (1) A magnetic concentrate containing 90.2% iron with the corresponding iron recovery of 95.0% was obtained from a red mud containing 48.2% Fetotal , 7.3% Al2 O3 and 8.0% SiO2 , from the red mud reduced at 1050 ◦ C for 60 min in the presence of 6% Na2 CO3 and 6% Na2 SO4 by magnetic separation. Subsequently, 94.7% Fe, 98.6% Al and 95.9% Si were extracted by sulfuric acid leaching at 30 ◦ C for 30 min with a liquid to solid ratio of 10:1 and H2 SO4 concentration of 25% from the upper-stream nonmagnetic material, yielding a TiO2 -rich material with 37.8% TiO2 . (2) During the reductive roasting of red mud, sodium salts play an important role not only in the reductive roasting and magnetic separation of Fe, but also in the subsequent acid leaching of Al, Si and Na constituents. The addition of sodium salts is capable of intensifying the reduction of iron oxides and promoting the growth of metallic iron grains. Thus, the magnetic separation of metallic iron from non-magnetic materials is improved. Moreover, as hercynite and quartz were transformed into metallic iron and sodium aluminosilicate in the presence of sodium salts, the subsequent sulfuric acid leaching efficiencies of Al and Si were enhanced significantly, which is favorable for the enrichment of TiO2 .

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