Digestion behavior and removal of sulfur in high-sulfur bauxite during bayer process

Minerals Engineering 149 (2020) 106237

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Digestion behavior and removal of sulfur in high-sulfur bauxite during bayer process

T

Zhanwei Liu , Hengwei Yan , Wenhui Ma , Keqiang Xie, Baoqiang Xu, Licong Zheng ⁎

⁎

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State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China Aluminum Industry Engineering Research Center of Yunnan Province, Kunming 650093, China

ARTICLE INFO

ABSTRACT

Keywords: High-sulfur bauxite Bayer process Digestion behavior Removal of sulfur

In this paper, digestion behavior and removal of sulfur in high-sulfur bauxite during Bayer process were studied. The thermodynamic analysis shows that pyrite reacts with alkali solution to generate different valence sulfur (S2−, S2O32−, SO32−, SO42−), Fe2O3, and Fe3O4; the standard Gibbs free energy of these reactions is negative and decreases with the increasing of temperature in range from 473 K to 573 K; the generation orders of different valence sulfur can be easily determined: S2− > SO42− > SO32− > S2O32−. The digestion experiment researches indicated that sulfur in high-sulfur bauxite entered solution mainly in the form of S2−, the digestion rates of sulfur increased with the increase of temperature, which was consistent with our thermodynamic calculation results. Based on the digestion behavior of sulfur, an appropriate method of sulfur removal was proposed, the S2− which is the main form of sulfur in liquor can be removed completely by adding ZnO during digestion process.

1. Introduction Rapid developments of alumina industry have made bauxite resources scarce. There are about 0.56 billion tons of diasporic high-sulfur bauxite in China (Li et al., 2017). These bauxite are mainly composed of aluminum of middle-high proportion, silicon of middle-low proportion, sulfur of high proportion (sulfur content is greater than 0.7%) and middle-high A/S (weight ratio of Al2O3 to SiO2) bauxite (Hu et al., 2011). High effective development and utilization of high-sulfur bauxite has an important realistic significance to ensure stable and secure supply of Chinese bauxite resources. Removal of sulfur is a crucial problem for high effective development and utilization of abundant high-sulfur bauxite resources. The sulfur-containing minerals in high-sulfur bauxite are pyrite and its isomers (marcasite and melnikovite), and sulfates such as CaSO4 (Liu et al., 2018a), the pyrite is the main sulfur-containing minerals. During the Bayer process, the pyrite goes into the solution in the form of S2−, then the S2− is gradually oxidized into S2O32−, SO32−, and SO42−, so, the removal of S2− is very important. The most negative effects of

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sulfur’s presence during the Bayer process are as follows (Abikenova et al., 2008; Caldeira et al., 2003; Han et al., 2018): (1) Increasing the sulfur and iron contents in aluminum hydroxide and alumina products. (2) Decreasing the digestibility of the alumina. (3) Accelerating the corrosion of steel equipment. (4) Increasing alkali consumption. (5) Decreasing the particle size of aluminum hydroxide and alumina products. (6) Scaling the evaporators and digesters. So, the sulfur content of bauxite is required to be less than 0.4% or at least 0.7% for the Bayer process. At present, there are a lots of sulfur removal methods, they can be divided into two categories. One is pretreatment desulfurization: roasting desulfurization (Eccleston and White, 2009; Lou et al., 2016; Yin et al., 2015), flotation desulfurization (Bulut et al., 2004; Chimonyo et al., 2017; Owusu et al., 2016; Taguta et al., 2017), electrolysis desulfurization (Awe et al., 2013; Gong et al., 2017a; Gong et al., 2017b; Helms et al., 1998; Marini et al., 2012), bioleaching desulfurization (Blight et al., 2000; Cheng et al., 2018; Gu et al., 2012; Sun et al., 2017) and the other is process desulfurization: desulfurization by precipitators (Kuznetsov et al., 1975; Liu et al., 2018a; Liu et al., 2018b; De Matos

Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (H. Yan), [email protected] (W. Ma).

https://doi.org/10.1016/j.mineng.2020.106237 Received 2 October 2018; Received in revised form 18 February 2019; Accepted 30 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.

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Table 1 Chemical compositions of high-sulfur bauxite.

Table 2 Chemical compositions of the alkali solution.

Compositions

Al2O3

SiO2

Fe2O3

TiO2

K2O

Na2O

CaO

STotal

Compositions

Na2OT

Al2O3

Na2OK

Contents (wt.%)

65.63

8.31

6.78

2.80

1.19

0.008

0.24

2.26

Concentration (g/L)

237.48

150.34

235.63

Note: Na2OT—total soda (as Na2O), Na2Ok—caustic soda (as Na2O).

et al., 2018; Li et al., 2016; Perraud et al., 2012; Safarzadeh-Amiri et al., 2017), wet oxidation desulfurization (Dixon and Long, 2004; Liu et al., 2015; Podkrajsek et al., 2002), desulfurization by seed precipitation (Zhou et al., 2018). These methods have their own advantages and disadvantages, they have not been widely applied in industry. Pretreatment desulfurization prevents most of the sulfur in bauxite from entering the Bayer process, but sulfur in the pretreated bauxite still remains problematic, therefore, deep desulfurization is needed during the Bayer process. In order to find an applicable method of sulfur removal, especially for S2− removal, the first step is to study digestion behavior of sulfur in high-sulfur bauxite during the Bayer process. In this paper, the thermodynamic analysis of pyrite digestion was studied, the effects of temperature, time, lime dosage on digestion behavior of sulfur were also investigated, and an appropriate approach to S2− removal in the digestion process was proposed. This will provide a theoretical and technical basis for the effective utilization of high-sulfur bauxite.

It can be seen from Fig. 1, the main sulfur-containing mineral is pyrite. Fig. 1 also indicates that particle sizes of pyrite are 10–50 μm, and pyrite distributes in diaspore. The chemical compositions of the alkali solution are listed in Table 2. 2.2. Experimental method Digestion experiments for high-sulfur bauxite were carried out in a XYF-6 digester, it mainly consists of three parts: salt bath furnace, steel bomb, and temperature control box. The mineral sample and alkali solution were placed in a 100 ml steel bomb which was sealed throughout the experiment. The digester was heated to predetermined temperature, then the steel bomb was putted into it, the digestion process finished within the predetermined time. The digestion slurry was filtered, the Al2O3, Na2Ok, Na2OT, and different valence sulfur (S2−, S2O32−, SO32−, and SO42−) in the filtrate were chemically analyzed (Wang, 1992), where the concentrations of S2−, S2O32−, and SO32− were determined by iodimetry, the SO42− concentration was determined by barium sulfate weight method. The chemical compositions of red mud were analyzed by X-ray fluorescence analyzer and carbon-sulfur analyzer, the red mud phase was characterized by X-ray diffraction.

2. Experiment 2.1. Experiment materials The high-sulfur bauxite used in this experiment was obtained from certain mining area in China, it was treated by milling, screening, and drying. The chemical compositions of mineral sample are shown in Table 1. It can be seen from Table 1 that the total sulfur (STotal) of the bauxite is 2.26%, it belongs to high-sulfur bauxite. The A/S of the bauxite is 7.9, so the mineral sample belongs to high-sulfur bauxite with high grade. The QEMSCAN image of the mineral sample is shown in Fig. 1.

3. Thermodynamic analysis of pyrite digestion Different reactions between pyrite and sodium aluminate solution occur as shown in Table 3. The standard Gibbs free energy of these reactions was calculated using the thermodynamic software Factsage

Fig. 1. QEMSCAN image of the high sulfur bauxite.

2

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Table 3 Reactions of pyrite with sodium aluminate solution during digestion process. Chemical reaction equation

Serial number

−

2−

FeS2(s) + 3OH (aq) = 1/2Fe2O3(s) + S (aq) + 1/2S22−(aq) + 3/2H2O(l) FeS2(s) + 8/3OH−(aq) = 1/3Fe3O4(s) + 2/3S2−(aq) + 2/3S22−(aq) + 4/3H2O(l) FeS2(s) + 3OH−(aq) = 1/2Fe2O3(s) + 3/2S2−(aq) + 1/2S(s, l) + 3/2H2O(l) FeS2(s) + 8/3OH−(aq) = 1/3Fe3O4(s) + 4/3S2−(aq) + 2/3S(s, l) + 4/3H2O(l) FeS2(s) + 15/4OH−(aq) = 1/2Fe2O3(s) + 7/4S2−(aq) + 1/8S2O32−(aq) + 15/8H2O(l) FeS2(s) + 11/3OH(aq)− = 1/3Fe3O4(s) + 5/3S2−(aq) + 1/6S2O32−(aq) + 11/6H2O(l) FeS2(s) + 4OH−(aq) = 1/2Fe2O3(s) + 11/6S2−(aq) + 1/6SO32−(aq) + 2H2O(l) FeS2(s) + 4OH−(aq) = 1/3Fe3O4(s) + 16/9S2−(aq) + 2/9SO32−(aq) + 2H2O(l) FeS2(s) + 4OH−(aq) = 1/2Fe2O3(s) + 15/8S2−(aq) + 1/8SO42−(aq) + 2H2O(l) FeS2(s) + 4OH−(aq) = 1/3Fe3O4(s) + 11/6S2−(aq) + 1/6SO42−(aq) + 2H2O(l) S22−(aq) + 3/2OH−(aq) = 3/2S2−(aq) + 1/4S2O32−(aq) + 3/4H2O(l) S(s, l) + 2OH−(aq) = 1/3SO32−(aq) + 2/3S2−(aq) + H2O(l) S2O32−(aq) + 2OH−(aq) = 2/3S2−(aq) + 4/3SO32−(aq) + H2O(l) S2O32−(aq) + 2OH−(aq) = S2−(aq) + SO42−(aq) + H2O(l) FeS2(s) + 2OH−(aq) = Fe(OH)2(s) + S22−(aq) Fe(s) + S2O32−(aq) + 2NaOH−(aq) = S2−(aq) + SO42−(aq) + Fe(OH)2(s)

(a)

20

20

0

0

-20 -40 -60

-

2-

22

FeS2(s)+3OH (aq)=1/2Fe2O3(s)+S (aq)+1/2S (aq)+3/2H2O(l) -

-80

-40 -60 -80

FeS2(s)+15/4OH-(aq)=1/2Fe2O3(s)+7/4S2-(aq)+1/8S2O32-(aq)+15/8H2O(l) 2-

-100

FeS2(s)+4OH-(aq)=1/2Fe2O3(s)+15/8S2-(aq)+1/8SO42-(aq)+2H2O(l) 400

450

500

2-

FeS2(s)+8/3OH (aq)=1/3Fe3O4(s)+4/3S (aq)+2/3S(aq)+4/3H2O(l) FeS2(s)+11/3OH-(aq)=1/3Fe3O4(s)+5/3S2-(aq)+1/6S2O32-(aq)+11/6H2O(l) -

2-

FeS2(s)+4OH (aq)=1/2Fe2O3(s)+11/6S (aq)+1/6SO3 (aq)+2H2O(l) 350

FeS2(s)+8/3OH-(aq)=1/3Fe3O4(s)+2/3S2-(aq)+2/3S22-(aq)+4/3H2O(l) -

FeS2(s)+3OH (aq)=1/2Fe2O3(s)+3/2S (aq)+1/2S(s, l)+3/2H2O(l)

300

-20

2-

-

-100

(b)

40

G (kJ·mol -1)

G (kJ·mol -1)

40

(1) (1′) (2) (2′) (3) (3′) (4) (4′) (5) (5′) (6) (7) (8) (9) (10) (11)

550

600

2-

2-

FeS2(s)+4OH (aq)=1/3Fe3O4(s)+16/9S (aq)+2/9SO3 (aq)+2H2O(l) FeS2(s)+4OH-(aq)=1/3Fe3O4(s)+11/6S2-(aq)+1/6SO42-(aq)+2H2O(l) 300

350

400

T/K

450

500

550

600

T/K

-25

(c)

-50

(d)

50

0

-100

G (kJ·mol )

-125

-1

G (kJ·mol-1)

-75

-150 -175 -200 -225 -250

2-

FeS 2(s)+2OH -(aq)=Fe(OH) 2(s)+S 22-(aq) Fe(s)+S 2O 32-(aq)+2NaOH -(aq)=S 2-(aq)+SO 42-(aq)+Fe(OH) 2(s)

-100

S22-(aq)+3/2OH -(aq)=3/2S 2-(aq)+1/4S 2O32-(aq)+3/4H 2O(l) -

-50

2-

S(s, l)+2OH (aq)=1/3SO 3 (aq)+2/3S (aq)+H 2O(l) S2O32-(aq)+2OH -(aq)=2/3S 2-(aq)+4/3SO 32-(aq)+H 2O(l)

-150

S2O32-(aq)+2OH -(aq)=S 2-(aq)+SO 42-(aq)+H 2O(l) 300

350

400

450

T/K

500

550

300

600

350

400

450

500

550

600

T/K

Fig. 2. The diagram of standard Gibbs free energy with temperature of the reactions: (a) pyrite react with sodium aluminate solution to generate Fe2O3, (b) pyrite react with sodium aluminate solution to generate Fe3O4, (c) reactions of different valence sulfur with sodium aluminate solution, (d) other reactions.

7.0. The diagram of the standard Gibbs free energy with temperature (ΔGθ-T) was drawn, as shown in Fig. 2. As can be seen from Fig. 2(a), (b), pyrite reacts with sodium aluminate solution to generate different valence sulfur (S2−, S2O32−, SO32−, SO42−), Fe2O3, and Fe3O4; the standard Gibbs free energy of these reactions is negative and decreases with the increasing of temperature in range from 473 K to 573 K; the more negative the ΔGθ

value, the more favorable the reaction is, so the orders of generating different valence sulfur can be obtained: S2− > SO42− > SO32− > S2O32−. It can be seen from Fig. 2(c) that the standard Gibbs free energy of these reactions is negative, so these reactions all can happen, the S22− and S can further react with sodium aluminate solution to generate S2−, S2O32−, SO32−; the standard Gibbs free energy of reaction (9) is much 3

Minerals Engineering 149 (2020) 106237

(b)

500

510

520

Temperature

530

540

(%) S

2-

S 2S2O3

6

Sulfur concentration(g/L)

490

5

SO3

2-

SO4

2-

6

4 3 2 1 0

470

46 44 (a) 42 40 38 36 34 32 30 28 26 24 22 20 10 20

Sulfur concentration(g/L)

7

88 86 84 82 80 78 76 74 S 72 70 A 68 66 64 62 60 58 56 54 52 550 560

480

490

500

510

520

530

540

550

560

84 80 76 72 68 64 60 56 52 30

40

50

60

Time(min)

70

80

48 100

90

(b)

5 2-

S 2S2O3

4

3

SO3

2-

SO4

2-

2

1

0

Temperature

88

A(%)

48 46 (a) 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 470 480

A(%)

S(%)

Z. Liu, et al.

10

20

30

40

50

60

70

80

90

100

Time(min)

Fig. 3. (a) Effects of temperature on digestion rates of sulfur and alumina. (b) Effects of temperature on different valence sulfur concentrations in digestion liquor.

Fig. 4. (a) Effects of time on digestion rates of sulfur and alumina. (b) Effects of time on different valence sulfur concentrations in digestion liquor.

lower than the standard Gibbs free energy of reaction (8), so S2O32− is unstable in sodium aluminate solution, and S2O32− is finally decomposed into S2− and SO42−. It can be seen from Fig. 2(d) that the ΔGθ of the reaction of pyrite with sodium aluminate solution to generate Fe(OH)2 maintains a positive value between 298 K and 573 K, therefore this reaction is not happen; the ΔGθ of the reaction of S2O32− with Fe is negative and decreases with the increasing of temperature in range from 298 K to 573 K, therefore this reaction is more likely to occur. During digestion process, the S2O32− reacts with Fe to corrode equipment, it is extremely dangerous in Bayer process. When the sulfur content of bauxite is over 0.5%, the equipment (such as sleeve pipe preheater, flow pipes, and valves etc.) can be corroded seriously after one month. So the S2O32− must be removed in the production of alumina.

As can be seen from Fig. 3(a), when temperature is below 533 K, the digestion rates of alumina increase as temperature increases, the digestion rates of alumina remain stable when temperature is above 533 K; the digestion rates of sulfur increase as temperature increases, which is consistent with our thermodynamic calculation results (as shown in Fig. 2). It can be seen from Fig. 3(b) that the sulfur in high-sulfur bauxite enters solution mainly in the form of S2−, the orders of different valence sulfur concentration are as follows: S2− > SO42− > SO32− > S2O32−, which is consistent with our thermodynamic calculation results (as shown in Fig. 2). 4.2. Effects of time on sulfur digestion behavior

4. Results and discussion

The effects of time on sulfur digestion behavior were studied under the conditions of 533 K, lime dosage of 13%, time from 15 min to 90 min, and the results are shown in Fig. 4. As can be seen from Fig. 4(a), when time is below 60 min, the digestion rates of alumina increase greatly as time increases, the digestion rates of alumina remain stable when time is above 60 min; the digestion rates of sulfur increase as time increases.

4.1. Effects of temperature on sulfur digestion behavior The effects of temperature on sulfur digestion behavior were studied under the conditions of 60 min, lime dosage of 13%, temperature from 473 K to 553 K, and the results are shown in Fig. 3.

4

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44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14

(a)

88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56 54 52

enters solution mainly in the form of S2−, the concentration of S2− increases as lime dosage increases, when lime dosage is above 13%, the concentration of SO42−decrease as lime dosage increases, the reason is that the excess lime reacts with SO42− to form calcium sulfate, we also find CaSO4·0.5H2O in red mud, as shown in Fig. 6(b). 4.4. XRD analysis of red mud

6

7

7

8

9

10 11 12 13 14

Lime dosage(%)

15 16 17 18

(%)

A

S(%)

Z. Liu, et al.

Taking into account the recovery of alumina, digestion rate of sulfur and alkali consumption, the optimum digestion conditions in this experiment are as follows: 533 K, 60 min, lime dosage of 13%. The X-ray diffraction pattern of red mud obtained under these conditions is shown in Fig. 6(a). When lime dosage is 15%, the X-ray diffraction pattern of red mud obtained under the same other conditions is shown in Fig. 6(b). The Fe2O3 and Fe3O4 can be found in red mud (as shown in Fig. 6(a) and Fig. 6(b)), which is consistent with our thermodynamic calculation results (as shown in Fig. 2). The Ca3Al2SiO4(OH)8 and CaSO4·0.5H2O are found in red mud when lime dosage is above 15%.

(b)

Sulfur concentration(g/L)

6

4.5. Sulfur removal by adding ZnO during digestion process

5

Based on the digestion behavior of sulfur, we proposed an appropriate method of sulfur removal, that is to say, the S2− which is the main form of sulfur in solution can be removed completely by adding ZnO in digestion process. The effects of ZnO dosages on the concentrations of different valence sulfur were studied under the conditions of 533 K, 60 min, lime dosage of 13%, ZnO dosage from 0% to 5%, and the results are shown in Fig. 7. The X-ray diffraction pattern of red mud is shown in Fig. 8, where ZnO dosage is 3%. It can be seen from Fig. 7 that when ZnO dosage is below 3%, the concentration of S2− in solution decreases significantly with the increase of ZnO dosage; when ZnO dosage is 3%, the S2− is almost completely removed; as ZnO dosage increases, the concentration of SO32− in liquor decreases slightly, while the concentration of SO42− increases slightly throughout the experiment, the reason is that the SO32− is easily oxidized to SO42−, the SO42− is the stable state in sodium aluminate solution; the S2O32− remains stable during the whole experiment process. So, the S2− which is the main form of sulfur in solution can be removed completely by adding ZnO in digestion process. It can be seen from Fig. 8 that the sulfur in solution goes into red mud in the form of the ZnS by adding ZnO during digestion process. The effects of ZnO dosages on the Zn concentrations in digestion liquor were also studied, and the results are shown in Fig. 9. It can be seen from Fig. 9 that the Zn concentration is 3.8 mg/L in digestion liquor when there is no addition of ZnO; when ZnO dosage is 3%, the Zn concentration is 4.0 mg/L, so ZnO has no effect on the Zn concentration in digestion liquor; when ZnO dosages are 4% and 5%, the Zn concentrations are 4.8 mg/L and 6.0 mg/L respectively, so the ZnO will remain in aqueous solution when excessive ZnO is added. Comprehensive consideration suggests that the optimum ZnO dosage is 3% in this experiment. The cost of sulfur removal was calculated, where the ZnO dosage is 3%, and the result is shown in Table 4. It can be seen from Table 4 that the alumina cost increases 34 RMB ¥/t-Al2O3, it is worth because the rich high-sulfur bauxite resources can be effectively used for alumina production.

2-

S 2S2O3

4 3

SO3

2-

SO4

2-

2 1 0 6

7

8

9

10

11

12

13

14

15

16

17

18

Lime dosage() Fig. 5. (a) Effects of lime dosage on digestion rates of alumina and sulfur. (b) Effects of lime dosage on different valence sulfur concentrations in digestion liquor.

It can be seen from Fig. 4(b) that the sulfur in high-sulfur bauxite enters solution mainly in the form of S2−, the concentration of S2− increases as time increases, the orders of different valence sulfur concentration are as follows: S2− > SO42− > SO32− > S2O32−. 4.3. Effects of lime dosage on sulfur digestion behavior The effects of lime dosage on sulfur digestion behavior were studied under the conditions of 533 K, 60 min, lime dosage from 7% to 17%, and the results are shown in Fig. 5. As can be seen from Fig. 5(a), when lime dosage is below 13%, the digestion rates of alumina and sulfur increase as lime dosage increases; when lime dosage is above 13%, the digestion rates of alumina decrease, the reason is that the excess lime can react with alumina to form calcium aluminosilicate, we also find Ca3Al2SiO4(OH)8 in red mud, as shown in Fig. 6(b), while the digestion rates of sulfur increase slightly with the increase of lime dosage. It can be seen from Fig. 5(b) that the sulfur in high-sulfur bauxite

5

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400

(a)

: KAl 2 Si 3 AlO 10 (OH)

350

Intensity(counts)

300

:

Na 8(Al 6 Si 6O 24 )(OH) 2.04 (H 2 O) 2.66

:

Fe 3O 4

:

250

2

Fe 2O 3

200

150

100

50

0

10

20

30

40

50

60

70

2 θ (°) 400

:KAl 2 Si 3 AlO 10 (OH) 2 :Ca 3 Al 2 (SiO 4 )(OH) 8 : Na 8 (Al 6 Si 6 O 24 )(OH) 2.04 (H 2 O) 2.66 : Fe 3 O 4 : Fe 2 O 3 : CaSO 4 ·0.5H 2 O

(b) 350

Intensity(counts)

300 250 200 150 100 50 0

10

20

30

40

2 θ (°)

50

60

70

Fig. 6. X-ray diffraction pattern of red mud: (a) lime dosage is 13%, (b) lime dosage is 15%.

The mechanism of S2− removal was proposed. Based the above results and discussion, we propose the following mechanism of sulfur removal (as shown in Fig. 10). ZnO exists mostly in the form of ZnO2− in sodium aluminate solution, the S2− promotes ZnO2− to generate Zn2+ (Pandey et al., 2014; Liu et al., 2017; Bendikov et al., 2002), and then the S2− reacts with Zn2+ to produce ZnS entering red mud, the reactions are as follows: ZnO22− + 2H2O = Zn2+ + 4OH− Zn

2+

2−

+S

= ZnS

5. Conclusion The digestion behavior of sulfur has been clarified by means of thermodynamic analysis and experiments, the main sulfur-containing mineral of high-sulfur bauxite is pyrite, pyrite reacts with alkali solution to generate different valence sulfur (S2−, S2O32−, SO32−, SO42−), Fe2O3, and Fe3O4, the orders of different valence sulfur concentrations in digestion liquor are as follows: S2− > SO42− > SO32− > S2O32−, experiment results are consistent with thermodynamic calculation results. The digestion temperature, time, and lime dosage have important

(1) (2)

6

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0.60

6

Concentrations of S 2 O 3 (g/L)

2-

0.50

5

2-

Concentrations of S (g/L)

0.55

0.35 0.30

3

0.25 0.20

2

0.15 0.10

1

0.05

0

0.00 0

1

0.7

2

3

4

5

0

1

2

1.8

ZnO dosage (%)

Concentrations of SO4 (g/L)

0.6

3

4

5

4

5

ZnO dosage (%)

1.7

2-

2-

Concentrations of SO 3 (g/L)

0.45 0.40

4

0.5 0.4 0.3 0.2 0.1 0.0

0

1

2

3

4

1.6 1.5 1.4 1.3 1.2 1.1

5

0

1

2

ZnO dosage (%)

3

ZnO dosage (%)

Fig. 7. Effects of ZnO dosages on the concentrations of different valence sulfur.

Fig. 8. X-ray diffraction pattern of red mud. Fig. 9. Effects of ZnO dosages on the Zn concentrations in digestion liquor.

effect on the digestion rates of sulfur. Taking into account the recovery of alumina, digestion rate of sulfur and alkali consumption, the optimum digestion conditions in this experiment are as follows: 533 K, 60 min, lime dosage of 13%. Based on the digestion behavior of sulfur, an appropriate method of sulfur removal was proposed, the S2− which is the main form of sulfur in liquor can be removed completely by adding ZnO during digestion process.

Table 4 Sulfur removal cost.

7

Addtive agent

Unit price (RMB ¥/t)

Amount (kg/tAl2O3)

Cost (RMB¥/tAl2O3)

ZnO

11,000

3

34

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Fig. 10. Schematic illustration of sulfur removal mechanism by adding ZnO during digestion process.

Acknowledgements

of alkaline sulfide solutions on platinum electrodes. Monatshefte Fur Chemie 129 (6–7), 617–623. Hu, X.L., Chen, W.M., Xie, Q.L., 2011. Sulfur phase and sulfur removal in high sulfurcontaining bauxite. Trans. Nonferrous Metals Soc. China 21 (7), 1641–1647. Kuznetsov, S.I., Grachev, V.V., Tyurin, N.G., 1975. Interaction of iron and sulfur in alkaline aluminate solutions. Zh Prikl Khim 48 (4), 748–750. Li, X.B., Niu, F., Tan, J., Liu, G.H., Qi, T.G., Peng, Z.H., Zhou, Q.S., 2016. Removal of S2− ion from sodium aluminate solutions with sodium ferrite. Trans. Nonferrous Metals Soc. China 26 (5), 1419–1424. Li, X.B., Niu, F., Liu, G.H., Qi, T.G., Zhou, Q.S., Peng, Z.H., 2017. Effects of iron-containing phases on transformation of sulfur-bearing ions in sodium aluminate solution. Trans. Nonferrous Metals Soc. China 27 (4), 908–916. Liu, Z.W., Li, W.X., Ma, W.H., Yin, Z.L., Wu, G.B., 2015. Conversion of sulfur by wet oxidation in the bayer process. Metall. Mater. Trans. B 46 (4), 1702–1708. Liu, Z.W., Ma, W.H., Yan, H.W., Xie, K.Q., Li, D.Y., Zheng, L.C., Li, P.F., 2017. Removal of sulfur by adding zinc during the digestion process of high-sulfur bauxite. Sci. Rep. 7, 1–9. Liu, Z.W., Li, D.Y., Ma, W.H., Yan, H.W., Xie, K.Q., Zheng, L.C., Li, P.F., 2018a. Sulfur removal by adding aluminum in the bayer process of high-sulfur bauxite. Miner. Eng. 119, 76–81. Liu, Z.W., Ma, W.H., Yan, H.W., Xie, K.Q., Li, D.Y., Zheng, L.C., Li, P.F., 2018b. Sulfur removal with active carbon supplementation in digestion process. Hydrometallurgy 179, 118–124. Lou, Z.N., Xiong, Y., Feng, X.D., Shan, W.J., Zhai, Y.C., 2016. Study on the roasting and leaching behavior of high-sulfur bauxite using ammonium bisulfate. Hydrometallurgy 165 (SI), 306–311. Marini, S., Salvi, P., Nelli, P., Pesenti, R., Villa, M., Berrettoni, M., Zangari, G., Kiros, Y., 2012. Advanced alkaline water electrolysis. Electrochim. Acta 82 (SI), 384–391. Owusu, C., Quast, K., Addai-Mensah, J., 2016. The use of canola oil as an environmentally friendly flotation collector in sulphide mineral processing. Miner. Eng. 98, 127–136. Pandey, S.K., Pandey, S., Parashar, V., Yadav, R.S., Mehrotra, G.K., Pandey, A.C., 2014. Bandgap engineering of colloidal zinc oxysulfide via lattice substitution with sulfur. Nanoscale 6 (3), 1602–1606. Perraud, I., Ayral, R.M., Rouessac, F., Ayral, A., 2012. Combustion synthesis of porous ZnO monoliths for sulfur removal. Chem. Eng. J. 200, 1–9. Podkrajsek, B., Grgic, I., Tursic, J., 2002. Determination of sulfur oxides formed during the S(IV) oxidation in the presence of iron. Chemosphere 49 (3), 271–277. Safarzadeh-Amiri, A., Walton, J., Mahmoud, I., Sharifi, N., 2017. Iron(III) - polyphosphates as catalysts for the liquid redox sulfur recovery process. Appl. Catal. BEnviron. 207, 424–428. Sun, J., Dai, X.H., Liu, Y.W., Peng, L., Ni, B.J., 2017. Sulfide removal and sulfur production in a membrane aerated biofilm reactor: Model evaluation. Chem. Eng. J. 309, 454–462. Taguta, J., O'Connor, C.T., McFadzean, B., 2017. The effect of the alkyl chain length and ligand type of thiol collectors on the heat of adsorption and floatability of sulphide minerals. Miner. Eng. 110, 145–152. Wang, D.L., 1992. Light Metal Metallurgical Analysis. Metallurgical Industry Press, pp. 67–69. Yin, J.G., Han, M.R., Yang, W.Q., An, J., Zhou, X.J., Xia, W.T., Huang, L.W., 2015. Roasting pretreatment of high-sulfur bauxite with low-median grade in Chongqing China. Light Metals 11–14. Zhou, X., Yin, J., Chen, Y., Xia, W., Xiang, X., Yuan, X., 2018. Simultaneous removal of sulfur and iron by the seed precipitation of digestion solution for high-sulfur bauxite. Hydrometallurgy 181, 7–15.

We gratefully acknowledge the support received from the National Natural Science Foundation of China (No. 51764032 and No. 51404121) and the Project of State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province (KKPT201652009). References Abikenova, G.K., Kovzalenko, V.A., Ambarnikova, G.A., Ibragimov, A.T., 2008. Investigation of the effect and behavior of sulfur compounds on the technological cycle of alumina production. Metall. Non-ferrous Metals 49 (2), 91–96. Awe, S.A., Sundlcvist, J.E., Sandstrom, A., 2013. Formation of sulphur oxyanions and their influence on antimony electrowinning from sulphide electrolytes. Miner. Eng. 53, 39–47. Bendikov, T.A., Yarnitzky, C., Licht, S., 2002. Energetics of a zinc-sulfur fuel cell. J. Phys. Chem. B 106 (11), 2989–2995. Blight, K., Ralph, D.E., Thurgate, S., 2000. Pyrite surfaces after bio-leaching: a mechanism for bio-oxidation. Hydrometallurgy 58 (3), 227–237. Bulut, G., Arslan, F., Atak, S., 2004. Flotation behaviors of pyrites with different chemical compositions. Miner. Metall. Process. 21 (2), 86–92. Caldeira, C.L., Ciminelli, V.S.T., Dias, A., Osseo-Asare, K., 2003. Pyrite oxidation in alkaline solutions: nature of the product layer. Int. J. Miner. Process. 72 (1–4), 373–386. Cheng, Y.Q., Chen, Y.L., Lu, J.C., Nie, J.X., Liu, Y., 2018. Fenton treatment of bio-treated fermentation-based pharmaceutical wastewater: removal and conversion of organic pollutants as well as estimation of operational costs. Environ. Sci. Pollut. Res. 25 (12), 12083–12095. Chimonyo, W., Corin, K.C., Wiese, J.G., O'Connor, C.T., 2017. Redox potential control during flotation of a sulphide mineral ore. Miner. Eng. 110, 57–64. De Matos, L.P., Costa, P.F., Moreira, M., Gomes, P.C.S., Silva, S.D., Gurgel, L.V.A., Teixeira, M.C., 2018. Simultaneous removal of sulfate and arsenic using immobilized non-traditional SRB mixed culture and alternative low-cost carbon sources. Chem. Eng. J. 334, 1630–1641. Dixon, D.G., Long, H., 2004. Pressure oxidation of pyrite in sulfuric acid media: a kinetic study. Hydrometallurgy 73 (3–4), 335–349. Eccleston, E., White, J., 2009. Development of roasting parameters for the ConRoast process with low-sulphur feedstock. J. S. Afr. Inst. Min. Metall. 109 (1), 65–69. Gong, X.Z., Wang, Z., Zhuang, S.Y., Wang, D., Wang, Y.H., Wang, M.Y., 2017a. Roles of electrolyte characterization on bauxite electrolysis desulfurization with regeneration and recycling. Metall. Mater. Trans. B 48 (1), 726–732. Gong, X.Z., Wang, Z., Zhao, L.X., Zhang, S., Wang, D., Wang, M.Y., 2017b. Competition of oxygen evolution and desulfurization for bauxite electrolysis. Ind. Eng. Chem. Res. 56 (21), 6136–6144. Gu, G.H., Sun, X.J., Hu, K.T., Li, J.H., Qiu, G.Z., 2012. Electrochemical oxidation behavior of pyrite bioleaching by acidthiobacillus ferrooxidans. Trans. Nonferrous Metals Soc. China 22 (5), 1250–1254. Han, G.H., Su, S.P., Huang, Y.F., Peng, W.J., Cao, Y.J., Liu, J.T., 2018. An insight into flotation chemistry of pyrite with isomeric xanthates: a combined experimental and computational study. Minerals 8 (4), 1–16. Helms, H., Schlomer, E., Jansen, W., 1998. Oscillation phenomena during the electrolysis

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