Optimization of Spent Cathode Carbon Purification Process under

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Optimization of spent cathode carbon purification process under ultrasonic action using Taguchi method Jie Yuan, Jin Xiao, Zhongliang Tian, Kai Yang, and Zhen Yao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05351 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Industrial & Engineering Chemistry Research

Optimization of spent cathode carbon purification process under ultrasonic action using Taguchi method Jie Yuan a, Jin Xiao a,b, *, Zhongliang Tian a, Kai Yang a , Zhen Yao a

a

School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China

b

National Engineering Laboratory of Efficient Utilization of Refractory Nonferrous Metal Resources, Central South University, Changsha, 410083, PR China

ABSTRACT: Optimum condition in caustic leaching test of spent cathode carbon (SCC) from aluminum electrolysis was investigated. The primary and secondary relationship of experimental factors was determined by Taguchi method, under traditional

mechanical

agitation

and

ultrasonic,

respectively.

Initial

alkali

concentration had significant influence both in ultrasonic-assisted and mechanical agitation leaching processes, and the effect was second only to that of ultrasonic power. Optimal parameters were temperature of 60℃, leaching time of 50 min, liquid-solid ratio of 10 mL/g, initial alkali concentration of 1 mol/L, particle size of -100 mesh, and ultrasonic power of 400 W. Purity of carbon powder obtained from treatment of spent cathode carbon at optimum conditions was 94.39%. Wastewater was reused after treated. It can be concluded that SCC purification conditions were optimized by synergistic effect of orthogonal and single-factor trails. Ultrasonic plays a notable effect in SCC caustic leaching process. Key words: Spent cathode carbon; Purification; Taguchi method; Process optimization; Thermodynamics; Ultrasonic 1. INTRODUCTION Spent Cathode Carbon (SCC) is a hazardous waste of electrolytic aluminum industry in Hall–Héroult process1. During electrolytic reduction process, cathode carbon blocks are subject to corrode by high-temperature electrolyte, molten aluminum, metallic sodium and other electrolyte salts, continuously and inevitably2, 3. 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The porous carbon cathode (open porosity: 10 - 25%)4 is gradually impregnated with bath components (Na3AlF6, AlF3, CaF2 and Al2O3, etc.)5. After a variable period of time (about 3-10 years)6-8, electrolytic cell is shutdown and decommissioning procedure would be initiated by the degenerated cathode. SCC is produced from decommissioned cell as an industrial waste and hundreds of thousands of tons SCC per year is the most frequently quoted amount by aluminum smelting enterprise. The waste is made of carbon, fluorides, alumina, cryolite, aluminosilicate, and a spot of cyanide. Carbon material and fluorides bring SCC high recovery potential9, and cyanides and soluble fluoride salts are the components of great environmental concern10. Untreated SCC causes pollution of surface water and groundwater when it be landfilled or piled up in the open air. SCC threatens the health of animal and plant seriously, and ecological balance11-13. As a worldwide concern about the environmental problem, spent cathode carbon has been classified as a hazardous solid waste in many countries14 and there are particular requirements in SCC treatment, storage, and transportation. Study on SCC treatment was started in the late 1970s, and over the years a variety of treatment technologies have been explored15. These processes include co-processing in third-party industries16, acid leaching17, flotation18, aluminum salt leaching15, 19, caustic leaching20, 21, and co-treatment with acid and alkali leaching22. The purpose of most treatment is to recover valuable components in SCC, and carbon is the most abundant recyclable material. However, purity of carbon power treated by the existing processes cannot meet high efficiency and wide application. And there is no technology widely accepted in the industry. It is imperative to optimize SCC treatment process or explore a perfect auxiliary method. Information of the primary and secondary influence relationship of reaction factors in SCC caustic leaching process was scarce23. To clarify the relationship, multivariate statistical techniques is employed necessarily to analyze and expound the purification process. Taguchi approach24, 25 is an efficient and widespread experimental design 2


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method for arranging multiple-factor and multiple-level experiment seeking optimal combination. Ultrasonic assisted leaching is an efficient and economical method and has been widely used to enhance chemical process rates26. Ultrasound requires only the presence of a liquid to transmit its energy, and the equipment can be

operated easily

in leaching process. Novel technique of ultrasound-assisted caustic leaching SCC has been shown in the work of Xiao27, researchers reduced processing time drastically, resulting at higher leaching efficiency. Also, such an effect was reported by Saterlay28, they removed and destroyed cyanide and cryolite in used carbon cathodes using ultrasound assistance, discovered a faster leaching speed and higher leaching rate of cryolite than that in traditional process. Tay29 found ultrasonic-assisted absorption would be one of the potential alternatives for CO2 capture with its advantages of high mass transfer coefficient and compact design. Effects of ultrasonic on desulfurization ratio from bauxite water slurry electrolysis in NaOH solution were examined under constant current by Ge30. M. Omran31 studied the effect of ultrasonic treatment on the efficiency of disintegration and removal of phosphorus, in their work the results of ultrasonic treatment were quite promising. To optimize SCC caustic leaching process and to improve the product carbon powder purity, in this study, Taguchi method was employed to determine the primary and secondary relationship of experimental factors under traditional mechanical agitation and ultrasonic agitation, respectively. The optimal parameters were optimized by synergistic effect of Taguchi method and single-factor experiments under ultrasonic assisted. 2. EXPERIMENTAL 2.1 Material SCC used in the study was provided by an aluminum smelter (Sichuan, China). The raw material was crushed and ball milled, and then sifted with sieves. Afterward, 3



sample powder was dried and characterized. Mineralogical analysis of sample was performed using X-ray diffraction (XRD) (Rigaku D/Max 2500, Japan, Cu Kal and scan speed of 10°/min), X-ray fluorescence (XRF) (XRF-1800, Japan), and scanning electron microscopy (SEM) (JSM-6360LV). Caustic solution in this study was prepared from a mixture of sodium hydroxide (NaOH, analytical grade) and homemade deionized water. Lime (CaO) used for wastewater treatment was analytical grade. 2.2 Procedure and apparatus Caustic leaching experiments were conducted in a 250 mL Teflon beaker filled with 10 g sample powder immersed in 100 mL caustic solution, stirred by magnetic stirrer and ultrasonic agitation (ultrasonic cleaner KQ-400KDE, China), respectively. After leaching, residue was washed and filtered repeatedly, and filter cake was dried. Caustic leaching wastewater was treated by lime addition to remove fluoride. Lime addition was at mole ratio of CaO/F 1.1:2 based on total fluoride in wastewater. Wastewater treatment was tested in the absence and presence of ultrasound at room temperature. At periodic intervals, 1 mL solution was extracted from wastewater to monitor F- ion concentration until it was less than 100 mg/L. The treated wastewater was reused as caustic solution. Ion concentration was measured by ion counter (PXSJ-216, China). SCC caustic leaching process was schematically shown in Fig. 1

Figure 1 Schematic process flow diagram of SCC purification

A variety of complex impurities were removed in caustic leaching process and it 4


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was difficult to calculate the removal rate of one or several impurities. Thus, to simplify the analysis course, SCC caustic-leaching results were expressed with F leaching rate and carbon content of leaching residue. F leaching rate was calculated by equation (1). η F = (1 −

C ×V ) × 100% m ×η 0

(1)

where X is ηF leaching rate, %; c is F concentration of filtrate, g/ml; V is volume of filtrate, ml; m is mass of sample for experiment, g; η0 is content of F in raw material; %. Carbon content of leaching residue was calculated by equation (2). η C = (1 −

ma ) ×100% ms

(2)

where is carbon content of leaching residue, %; is weight of residual ash for leaching residue incinerated at 800℃, g; is the weight of incinerated leaching residue at 800℃, g. 2.3 Experimental design By utilizing orthogonal experimental design, effects of experimental factors temperature (A), time (B), particle size (C), initial alkali concentration (D), stirring rate (E), and ultrasonic power (F), were investigated. Factors and the specific range of parameters in ‘4 level 5 factor’ L16(45) design were listed in Table 1. Carbon content of residues obtained from traditional mechanical agitation and ultrasonic assisted caustic leaching processes were compared to determine a better treatment method. To better optimize purification test parameters, univariate experiments were carried out subsequently. Orthogonal experimental results were processed by range analysis and there are

im , and Ri, in the analysis. Kim is defined as sum of three important parameters Kim, K im is the mean experimental indexes corresponding to factor column i and level m. K 5



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i [Eq. 3]. The value of Kim. Ri is the range of the maximum and minimum values of K larger the Ri value, the greater the influence of the factor on experimental index. According to the magnitude of Ri, the primary and secondary factors can be determined. R j = max ( K j1 , K j 2 ,..., K jm ) − min( K j1 , K j 2 ,..., K jm )

(3)

Table 1 Orthogonal experiment factors and levels Level Factor

Parameter 1

2

3

4

A

Temperature / ℃

20

40

60

80

B

Time / min

30

60

90

120

C

Particle size / mesh

-50~+100

-100~+200

-200~+300

-300

0.25

0.5

0.75

1.0

D

Initial alkali concentration / mol/L

E

Stirring rate / r/min

100

200

300

400

F

Ultrasonic power / W

100

200

300

400

3 RESULTS AND DISCUSSION 3.1 Characterization of SCC Proximate analysis and ultimate analysis of raw material were listed in Tables 2 and 3, separately. Carbon (about 63 wt.%) is the main phase, and there are small amount of water (1.62 wt.%) and volatile matter (1.14 wt.%) in the sample. After dried at 105℃ for 12h, effect of water and volatile matter can be disregarded during incineration. The sample XRD pattern was shown in Fig. 2, and phases of C, NaF, CaF2, Al2O3, NaAl11O17, Na3AlF6, and NaAlSiO4 could be confirmed on the graph. Some other impurity phases32 as AlN, Al4C3 not presented in XRD pattern may be trace impurities or react with steam in the air. Fig. 3 is SEM image of SCC sample, most of inorganic 6


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impurities (bright fractions) are adhering to carbon surface and some are wrapped in the holes. It can be inferred that impurities are separated from carbon easily15, 18. Table 2 Proximate analysis of SCC/% Sample

water

fixed carbon

volatile matter

ash

Content

1.62

61.37

1.14

35.87

Table 3 Ultimate analysis of SCC/% Element

C

F

Na

Al

O

Si

Ca

K

Fe

Others

Content

63.06

12.94

8.75

6.85

5.43

0.47

1.22

0.61

0.39

0.28

Figure 2 SCC sample XRD pattern

Figure 3 SCC sample SEM image

7



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3.2 Thermodynamic analysis The major mineral compositions of SCC are shown in Fig. 2. It is deduced that the main reactions between inorganic impurities and NaOH or water occurring vigorously in caustic solution are Eqs. (4) - (7). Al2O3 + 2 NaOH + 3H 2O = 2 NaAl (OH ) 4

(4)

Al (OH ) 3 + NaOH = NaAl (OH ) 4

(5)

Na3 AlF6 + 4 NaOH = NaAl (OH ) 4 + 6 NaF

(6)

AlN + 3H 2O = NH 3 ↑ + Al (OH ) 3

(7)

∆ r G Θm = ∑ γ∆ f G Θm (Pr oduct ) − ∑ γ∆ f GmΘ(Re ac tan t )

(8)

∆ r G Θm = − RT ln K

(9)

θ

θ

Where ∆r Gm is standard Gibbs free energy of reaction, KJ/mol; ∆f Gm is standard

Gibbs free energy of formation, KJ/mol; γ is stoichiometric parameter of matter in reaction; T is temperature, K; R is molar gas constant, J/(mol·K); K is chemical equilibrium constant. Based on Eqs. (8) and (9), the relationship between ∆r Gθm and temperature for reactions (4) - (7) is illustrated in Fig. 4 (left), and relationship between chemical equilibrium constants and temperature is in Fig. 4 (right). Thermodynamic data of reactions (4) - (7) was referenced from Software HSC Chemistry 6.0. Values of reactions ∆r Gθm are much less than 0 when impurities Al2O3, Al(OH)3, Na3AlF6 and AlN react in NaOH solution at 20 - 100℃, what means the four reactions can proceed smoothly under this condition. These are four spontaneous reactions at the experimental conditions. In Fig. 4 (right), chemical equilibrium constants (K) of Eqs. (4) - (7) decline with the increasing temperature. Elevated temperature is not 8


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conducive to all four reactions. Therefore, SCC caustic leaching process may not be need high temperature.

Figure 4 Diagrams of ∆r Gθm -T and lg K - T

Al is one of the major impurity elements, and there are reaction equations and equilibrium constants of Al-H2O system listed in table 4. Thermodynamic data in Table 4 was referenced from work of Yang33. Relationship between species concentration fraction and pH in Al-H2O system was illustrated in Fig. 5. Cryolite (Na3AlF6) could react with alkaline in solution, and this process includes multiple hydrolytic reactions. Reaction equations were presented in Table 534,

35

. D. F.

Lisbona34 investigated the effect of pH on Al-F species equilibrium distribution in Al-F-H2O system at 25℃, the diagram was presented in Fig. 6. As the increase of pH value, in Fig. 5, concentration of ion Al3+ declines near to 0 as pH value is higher than 6. Concentrations of Al(OH)x3-x present the same variation tendency, a peak-shape curve, with increasing pH value. During SCC caustic leaching process the pH value of alkali solution was higher than 7 absolutely. There were only two main aluminiferous components Al(OH)3(aq) and Al(OH)4-, in Fig.5. The two components express an inverse variation tendency with increased pH value. In this process the species of aluminium compounds reduce, and Al(OH)4- is the major one when pH value is higher than 10. In Fig. 6, molar fraction of each of the soluble Aluminum compounds, Al(OH)4- except, decreases when pH value is high enough. 9



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Ion Al(OH)4- replaces the others to be the main one with the increasing pH. It is concluded that high concentration of alkali in SCC caustic leaching process will be propitious to impurities in sample removed. Table 4 Reaction equations and equilibrium constants of Al-H2O system NO.

Equation

∆GθT / KJ/mol

lgK

i

Al3+ + H2O = Al(OH)2+ + H+

21.89

-3.84

ii

Al3+ + 2H2O = Al(OH)2+ + 2H+

52.37

-9.18

iii

Al3+ + 3H2O = Al(OH)3 + 3H+

85.88

-15.05

iv

Al3+ + 4H2O = Al(OH)4- + 4H+

136.49

-23.92

Figure 5 Relationship between species concentration fraction and pH in Al-H2O system Table 5 Equilibrium reactions in Al-F-H2O system No. Ⅰ

Equation Na3 AlF6 ⇌3Na

No. +

+AlF36

3+

Equation

Ⅰ

Al3+ +2F-+OH-⇌AlF2 (OH) Al3+ +2F-+2OH- ⇌AlF2 (OH)-2

Ⅰ

AlF36 ⇌Al +6F

Ⅰ

Ⅰ

3+ AlF25 ⇌Al +5F

XIII

Al3+ +3F-+OH-⇌AlF3 (OH)-

Ⅰ

AlF-4⇌Al3+ +4F-

XIV

Al OH ⇌ AlOH

3+

Ⅰ

AlF3⇌Al +3F-

XV

Al3+ + 2OH-⇌ Al(OH)+2

Ⅰ

AlF+2 ⇌Al3+ +2F-

XVI

Al3+ + 3OH-⇌ Al(OH)3

Ⅰ

AlF2+ ⇌Al3+ +F-

XVII

Al3+ + 4OH-⇌ Al(OH)-4

Ⅰ

Al3+ +F- +OH- ⇌AlF(OH)+

XVIII

HF⇌Hl+ +F-

10


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Ⅰ

Al3+ +F- +2OH-⇌AlF(OH)2

Ⅰ

Al3+ +F- +3OH-⇌AlF(OH)-3

XIX

HF- ⇌H+ +2F

Figure 6 Modelled pH influence on the equilibrium distribution diagram of Al-F species

3.3 Orthogonal experiment According to L16 (45) matrix, SCC caustic leaching experiments were carried out under traditional mechanical agitation and ultrasonic agitation, respectively. The im and Rj have been corresponding carbon content of leaching residue, values of K listed in Tables 6 and 7. Table 6 Orthogonal experiment result (mechanical agitation) Sequence

A

B

C

D

E

Carbon content of leaching residue/%

1

1

1

1

1

1

78.26

2

1

2

2

2

2

84.19

3

1

3

3

3

3

88.97

4

1

4

4

4

4

91.08

5

2

1

2

3

4

87.84

6

2

2

1

4

3

87.67

7

2

3

4

1

2

82.38

8

2

4

3

2

1

87.03

9

3

1

3

4

2

87.95

10

3

2

4

3

1

88.27

11

3

3

1

2

4

84.46

12

3

4

2

1

3

84.34

13

4

1

4

2

3

82.96

11



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14

4

2

3

1

4

82.6

15

4

3

2

4

1

90.13

16

4

4

1

3

2

89.21

1 K 2 K

85.63

84.25

84.9

81.9

85.92

86.23

85.68

86.63

84.66

85.93

3 K

86.26

86.49

86.64

88.57

85.99

4 K

86.23

87.92

86.17

89.21

86.5

R

0.63

3.67

1.74

7.31

0.58

Table 7 Orthogonal experiment result (ultrasonic) Sequence

A

B

C

D

F

Carbon content of leaching residue/%

1

1

1

1

1

1

77.64

2

1

2

2

2

2

84.47

3

1

3

3

3

3

91.63

4

1

4

4

4

4

94.35

5

2

1

2

3

4

94.02

6

2

2

1

4

3

80.66

7

2

3

4

1

2

84.39

8

2

4

3

2

1

86.43

9

3

1

3

4

2

88.13

10

3

2

4

3

1

86.81

11

3

3

1

2

4

89.82

12

3

4

2

1

3

87.61

13

4

1

4

2

3

88.62

14

4

2

3

1

4

88.35

15

4

3

2

4

1

88.72

16

4

4

1

3

2

89.61

1 K 2 K

87.02

87.1

86.93

84.5

84.9

88.88

87.57

88.71

87.34

86.65

3 K

88.09

88.64

88.64

90.52

89.63

4 K

88.83

89.5

88.54

90.47

91.64

R

1.86

2.4

1.77

6.02

6.74

As was remarked above, a larger Ri value implies a greater effect of the factor on experimental index36. In Table 6, range value of initial alkali concentration was the maximum one, and the minimum one was range value of stirring rate. The order of range values was as follow: R(D) > R(B) > R(C) > R(A) > R(E). Experimental factor 12


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initial alkali concentration had the most dominant effect on purity of carbon powder obtained in caustic leaching trial under traditional mechanical agitation, followed by temperature and time. Range values of particle size and stirring rate were little difference and the two factors showed negligible effects on carbon content of caustic leaching residue. In Table 7, in ultrasonic assisted leaching test, there were some different influences of the same factors on residue carbon content and the range value order was inconsistent, compared with traditional process in Table 6. Order of range values was as follow: R(F) > R(D) > R(B) > R(A) > R(C). Range value of ultrasonic power was larger than the others. Accordingly, ultrasonic power had a chief effect on carbon content of leaching residue. Initial alkali concentration still had a significant influence in caustic leaching trial and the effect was second only to ultrasonic power, followed by factors temperature, particle size, and time. In Tables 6 and 7, Taguchi experimental results showed that the best parameters group of SCC caustic-leaching process under traditional mechanical agitation was A3B4C3D4E4. The best group under ultrasonic assisted was A2B4C2D3E4, wherein particle size of -100 ~ +200 mesh, 80℃, 60 min, initial alkali concentration of 0.75 mol/L, and ultrasonic power of 400 W. Fig. 7 shows caustic leaching results obtained at mechanical agitation and ultrasonic assisted, respectively. Residue purity was higher under ultrasonic than that in traditional process, except for results at ultrasonic power 100 W. Therefore, ultrasonic is an effective assistant method in SCC caustic leaching process. To better optimize caustic leaching parameters under ultrasonic assisted, univariate experiments were carried out subsequently.

13



Figure 7 Carbon content of leaching residue under ultrasonic and mechanical agitation

3.4 Effect of Single factor experiment Based on orthogonal experimental results in Table 7, follow-up single variable test was designed to investigate effect of reaction factors particle size, temperature, time, initial alkali concentration, and ultrasonic power on leaching residue carbon content. 3.4.1 Ultrasonic power

Figure 8 Effects of ultrasonic power and time in leaching process (80℃, initial alkali concentration 1 mol/L, liquid-solid ratio 10:1, and particle size -100 mesh)

In Fig. 8, ultrasonic power and time both had positive effects on F leaching rate and carbon content of caustic leaching residue. In the resultant picture, leaching rates of inorganic impurities improved remarkably with time prolonging and ultrasonic power increased. Purity of carbon powder increased in experimental duration. There were 14


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different impurities reacting with NaOH and water, and also the soluble compounds dissolved in caustic solution. Eq. (6) is a composite equation consisting of multiple hydrolysis reactions of cryolite (Table 5). Kinds of aluminium hydroxyfluoride hydrate are formed in metastable zone37, in Fig. 6. These reactions signify that leaching efficiency of Na3AlF6 is improved with increasing time. By comparison of the two pictures in Fig. 8, it is known that time corresponding to inflection points on F leaching rate curves was shorter than time corresponding to turn points on carbon content curves. The main reason is that some water-soluble fluoride compounds could dissolve in shorter time than impurities reaction in caustic solution. Time plays significantly effect on fluorides and oxide dissolution20. At the same condition, carbon content of residue had the tendency increased with ultrasonic power increasing. Ultrasonic assisted leaching is an efficient and widely used method in separation and purification38, 39. In Fig. 8, on the optimal leaching result curve at ultrasonic power of 400 W, F leaching rate increased from 75.8% to 93.28%, and carbon powder purity increased from 83.26% to 94.54%, accompanied by time from 10 min to 50 min. It is known in the work of Xiao27 that residue carbon content under ultrasound-assisted leaching was higher than that of traditional leaching process, and reaction time was shorter than the latter one. Higher impurity leaching rate and shorter time could be contributed to the specific effects of ultrasonic. As a feature of ultrasonic, it brings instantaneous high temperature and high pressure when cavitation bubble collapses40. The duration of local high temperature and high pressure is very short, accompanied by strong shock wave and micro jet. Material structure can be destroyed by shock wave, micro jet, and local high temperature and high pressure, resulting in physical and chemical properties changed. These phenomena are difficult to occur under traditional conditions41. Liquid macroscopic turbulence and high velocity collisions between solid particles in the leaching system were produced by ultrasonic effects. Effects of surface, spot energy, turbulent and micro-disturbing caused by cavitation, enhance 15



mass transfer in solid-liquid system. Mass transfer rate can be increased by ways of increasing mass transfer coefficient, increasing driving force, and reducing contact area. Turbulent effect impels particles high-speed oscillation and collision, and micro jet and shock wave produce holes and plaques on particle surface. Mass transfer rate was accelerated because particle boundary layer was thinned, and diffusion in boundary layers was strengthened. Weak perturbation enhanced diffusion rate of materials in micropores, which was hard to achieve by traditional mechanical agitation42. Under local high temperature and intensive shaking, solid particles could be disintegrated and the dispersion in solid-liquid system was elevated. Interfaces were impacted, eroded, and peeled, by the strengthened eddy diffusion. As a result of synergistic effects of cavitation and high speed acoustic stream, inorganic impurities in SCC were separated with carbon much more quickly and completely than in traditional leaching process. In solid-liquid system, ultrasonic has effects of accelerating chemical reaction rate and mass transfer rate. Ultrasonic mechanical and cavitation effects could enhance particles diffusion rate and Brown motion in solution effectively28. Strength of these effects is determined by ultrasonic sound density. Ultrasound sound density is expressed by Eq. (10). As one of the major factors affecting on ultrasonic cavitation, in general, the increscent sound energy density could reinforce cavitation effect. In this study, experimental equipment ultrasonic cleaner and reaction vessel Teflon beaker were both constant sizes. Therefore, ultrasonic sound energy density was replaced by power. Generally, cavitation shows weak steady state and has little effect on mass transfer when ultrasonic power is low42. As power is high, it is manifested as a strong transient cavitation process, with a significant role on mass transfer and interfacial reaction. Bubble density, mass transfer, and heat transfer, increase in heterogeneous solid–liquid system with ultrasonic power increased. The emulsification of ultrasonic increased sample wettability, making impurities solubility increasing. And cavitation caused SCC a large number of inorganic debris. Under cavitation, efficiency of 16


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impurities reacting with alkali in solution was sped up. Ultrasound played a constructive role in SCC caustic leaching test. Ultrasonic sound energy density is expressed as Eq. (10). p=

W S

(10)

where P is ultrasonic energy density, W/cm2; W is ultrasonic output power, W; S is ultrasonic emission area, cm2. Based on experimental results illustrated in Fig. 8, ultrasonic power of 400W and residence time of 50 min were determined as optimal leaching parameters. 3.4.2 Initial alkali concentration

Figure 9 Effects of initial alkali concentration and time in leaching process (80℃, ultrasonic power 400 W, liquid-solid ratio 10:1, and particle size -100 mesh)

Results of effects of initial alkali concentration and time were illustrated in Fig. 9. On the graph, initial alkali concentration had a notable effect on F leaching rate and carbon content of residue. The two experimental indexes both increased with increasing initial alkali concentration. In the best result curve corresponding to initial alkali concentration of 1 mol/L, F leaching rate increased from 80.22% to 93.2%, and carbon powder purity increased from 83.13% to 94.49%, accompanied by leaching time extended from 10 min to 50 min. As initial alkali concentration increased from 0.25 mol/L to 1 mol/L, F leaching 17



rate increased and the two curves corresponding to 0.75 mol/L and 1 mol/L were almost coincident as time was longer than 40 min, and also carbon content of leaching residue increased and the increase scope was diminishing, determined by curves position in Fig. 9. Such an experimental phenomenon implied that initial alkali concentration had a positive effect on SCC purification. When initial alkali concentration increased to 0.5 - 0.75 mol/L, sufficient caustic emerged for reactions between NaOH and impurities, and caustic extraction effect was at high benefit. Experimental results at 1 mol/L were better than results obtained at 0.75mol/L, in 40 min, time played a dominant role. As can be seen in Figs. 5 and 6, alkali concentration has notable effect on reactions of NaOH and impurities Al2O3, Al(OH)3, and Na3AlF6 to produce NaAl(OH)4, and higher alkali concentration means higher pH value resulting in higher impurities removal rate. Also, in Fig. 6, pH value of Al-F compounds produced is lower than that of Al(OH)4-, what may be the main reason of high F leaching rate at low alkali concentration. Increasing NaOH concentration could promote impurities dissolving21 and also buildup of ions F- and Na+ to precipitate. There is a figure1 showing effect of caustic concentration on fluorides and alumina solubility. On the graph, fluoride ions solubility decreases as caustic concentration increases, whereas aluminate ion solubility increases with caustic concentration. The inverse solubility behavior leads to sodium fluoride (NaF) precipitated in a high alkali concentration solution27. Therefore, an appropriate caustic concentration should be determined for the test. And initial alkali concentration of 1 mol/L was the optimal parameter in the leaching process. 3.4.3 Temperature Effects of temperature and time on F leaching rate and carbon content of leaching residue were shown in Fig. 10. It can be seen temperature could improve purification effect of SCC in leaching process. Position and tendency of the two carbon content variation curves corresponding to 20℃ and 40℃ were similar. Reason of this observation was that because of ultrasonic thermal effect, final reaction temperature 18


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was elevated to close to 40℃, when initial system temperature was at room temperature 20℃. Position and variation tendency of the two carbon content curves corresponding to 60℃ and 80℃ were extremely similar, especially at time longer than 40 min. Moreover, F leaching rate curves at 60℃ and 80℃ were almost coincident as residence time was longer than 30 min. These observations signified that at 60℃ leaching process reached a fantastic result under conditions listed in Fig. 10.

Figure 10 Effects of temperature and time in leaching process (initial alkali concentration 1 mol/L, ultrasonic power 400 W, liquid-solid ratio 10:1, and particle size -100 mesh)

Elevated temperature helps soluble matters dissolving in solution. Impurities Al2O3 and Al(OH)3 react with alkali easily, as shown by Eqs. (4) - (6). Water-soluble fluorides43 are produced in these reactions. According to thermodynamics calculation result, formulas (4) - (6) are three spontaneous reactions at 25 - 100℃. NaAl(OH)4 is the same product of formulas (4) - (6), they are competitive reactions. At low temperature, ions in solution with weak activity move slowly and chemical equilibrium constant (K) is small. With elevated temperature, ion activity and value of reaction equilibrium constant K both rise, promoting reactions much more exhaustive. High temperature facilitates dissolution of inorganic impurities in SCC caustic leaching process27. Temperature, surface tension coefficient, and liquid viscosity coefficient, are the main factors affecting on ultrasonic cavitation. Low liquid viscosity coefficient and 19



surface tension coefficient are propitious to cavitation production. There is a significant effect of temperature on liquid viscosity coefficient and surface tension coefficient36. Liquid viscosity coefficient and surface tension elevate with the increscent temperature36. System temperature can be increased by thermal effect of ultrasound. Compared with solution heat convection velocity, the acoustic flow velocity in solution under ultrasonic radiation is much faster. Acoustic flow effect can speed up the circulation velocity and makes solution vibrate violently31. This is the chief reason that ultrasonic power plays a much more considerable effect than temperature in Table 7. So, 60℃ was chose as the optimal parameter in SCC caustic leaching test. 3.4.4 Particle size

Figure 11 Effects of particle size and time in leaching process (60℃, initial alkali concentration 1 mol/L, liquid-solid ratio 10:1, and ultrasonic power 400 W)

20


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Figure 12 Carbon content of sample at different particle sizes

Effects of particle size and time on carbon content of caustic leaching residue were illustrated in Fig. 11. Effect of particle size on F leaching rate was not covered in this part because ultimate analysis of samples at different size were not equivalent. Carbon content of SCC samples at different particle sizes were illustrated in Fig. 12. Carbon content of sample at size of -100 ~ +200 mesh was the biggest one. In Fig. 11, as time was longer than 20 min, the best and the worst purification results were at size -100 ~ +200 mesh and at size -50 ~ +100 mesh, respectively. Inflection point of the four curves corresponded to the same time. Thus, sample at particle size of -100 mesh was the optimal parameter. Carbon materials in SCC consist of graphite and amorphous carbon. Graphite has good lubricity and ductility, and low hardness. Impurities in sample with poor ductility are harder than graphite, therefore, impurities can be grounded to smaller-sized power more easily. So, carbon content of sample decreases with decreased powder particle size, which has good agreement with the conclusions of Li18 and D.F. Lisbona15. Under ultrasonic effect, more electrolytes can be exposed and evenly dispersed in caustic solution. Impurities adhered to carbon in smaller particle size sample were separated much more lightly than the bigger one, and this experimental observation explains the reasons for the higher leaching rate of -100 mesh sample as time was longer than 20 min. As particle size of SCC sample was -100 mesh, most inorganic impurities were uncovered and contacted with solution. At this state, particle size of raw material played a little effect in this process. 3.4.5 Stability of purification process Based on results of orthogonal and single-factor experiments, optimum parameters of SCC caustic leaching test under ultrasonic-assisted were as follow: temperature of 60℃, time of 50 min, liquid-solid ratio of 10 mL/g, initial alkali concentration of 1 mol/L, particle size of -100 mesh, and ultrasonic power of 400 W. Repeated experiments were conducted to validate the stability of leaching process under 21



Page 22 of 30

optimal parameters, and results were listed in Fig.13. Results of four repeated experiments and the standard deviation signified that SCC caustic leaching process had good stability at optimized conditions. Table 8 is ultimate analysis of repeated test product. Fig.14 is XRD pattern of caustic leaching residue. Purity of carbon powder obtained from SCC purification process at optimized conditions was high and carbon powder could be purified further in acid leaching process. Table 8 Ultimate analysis of leaching residue under optimal conditions/% Element Content

C

F

Na

Al

O

Si

Ca

K

Fe

Others

94.39

1.06

0.83

0.97

0.39

0.61

0.98

0.22

0.29

0.26

Figure 13 Result of repeated trials at optimal parameters

3.5 Wastewater treatment High concentration of various inorganic ions, including OH-, F-, Al, Na+ and a small amount of CN-, were in wastewater produced in SCC caustic leaching process (Table 9). Al was in the form of Al(OH)4−. Table 9 Ion species and contents of leaching waste water/g·L-1 Ion

OH- / mol·L-1

F-

Al

Na+

CN-

Content

0.73

12.07

6.39

34.52

0.138

22


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Figure 14 XRD pattern of caustic leaching residue

Ion CN− could be treated and destroyed by the hydrogen peroxide generated by the sonication of solution28. In a separate experiment, ultrasonic had no effect on the concentration of ions F- and Na+ in model aqueous solutions. Lime was added into wastewater and calcium fluoride (CaF2) precipitated. The equation is shown in formula (11) and XRD pattern of precipitate is shown in Fig.15. There are only two phases CaF2 and Ca(OH)2. Al still remained in solution and could be precipitated in Bayer process, or be formulated by carbonation method in Eq. (12)44. Ca 2+ + 2 F − = CaF2 ↓

(11)

2 Al (OH ) 4− + CO2 = 2 Al (OH ) 3 ↓ + H 2O + CO32 −

(12)

Figure 15 XRD pattern of wastewater precipitate 23



Figure 16 Results of fluoride removal from wastewater

A considerable amount of NaOH remained in caustic leaching wastewater, therefore, wastewater can be reused as alkaline solution after F removed. The trial was carried out to remove F- with CaO addition. F removal was experimented in the absence and presence of ultrasound at room temperature 25℃. Traditional treatment test was at agitation speed of 400 r/min and ultrasonic-assisted treatment test was at ultrasonic power of 400 W. The experimental results are represented in Fig.16. In Fig.16, result of ultrasound-assisted treating on wastewater was better than traditional agitation treatment result. Time of F- concentration declined to less than 100 mg/L under ultrasound-assisted was shorter than that of traditional treatment, and ultrasound-assisted treatment result was better than traditional result in the same time. CaO reacts with water producing slightly soluble calcium hydroxide Ca(OH)2. Ultrasound could promote solid Ca(OH)2 dissolving in solution and improve reaction rate of ions Ca2+ and F-. Also, thermal effect, mechanical effect, and activation effect, accompanied by ultrasonic cavitation, play significant roles in mass transfer process in the solid-liquid system, which is helpful for solid CaO dissolving and CaF2 precipitate generation. However, in the picture it is shown that times of experiment ending are 210 min and 330 min, respectively. There is not marked difference between the two methods and ultrasound has no considerable advantage in SCC caustic leaching wastewater treatment. 24


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In summary, effect of ultrasonic in wastewater treatment process isn’t notable when ion F- was removed by CaO addition. Taking into account the equipment and energy consumption, room temperature and appropriate processing time are employed to clean the wastewater without ultrasound. 4. CONCLUSION In this study, SCC purification with caustic leaching under ultrasonic-assisted was investigated, and process parameters were optimized by Taguchi approach and single variable experiment. The work concludes as follow. (1) Parametric optimization of caustic-leaching process was investigated by orthogonal experiment under traditional mechanical agitation and ultrasonic, separately. Through range analysis, order of the primary and secondary factors in mechanical agitation run was initial alkali concentration, temperature, time, particle size, and stirring rate, from big to small. And the order in ultrasonic action test was ultrasonic power, initial alkali concentration, temperature, particle size, and time. Initial alkali concentration had a significant influence both in the two trials. Ultrasonic played a notable role in SCC caustic leaching test. Contrast analysis on the two experimental results indicated that ultrasonic assisted leaching could improve carbon powder purity. (2) Based on orthogonal experimental result, parameters were further optimized in univariate experiments. The optimum parameters of SCC caustic leaching process under ultrasonic-assisted were temperature of 60℃, time of 50 min, liquid-solid ratio of 10 mL/g, initial alkali concentration of 1 mol/L, particle size of -100 mesh, and ultrasonic power of 400 W. Four repeated trials were conducted to verify the stability of leaching process under optimal parameters, the average purity of purified product was 94.39%. (3) SCC caustic leaching wastewater was treated and lime was added to remove ion F-. By contrasting results of F- removal experiments in the absence and presence of 25



ultrasonic, it was known that ultrasonic had no considerable advantage in wastewater treatment. Room temperature and appropriate time were employed to treat the wastewater without ultrasound. Wastewater was reused as caustic solution after fluoride removed. AUTHOR INFORMATION Corresponding author E-mail addresses: [email protected] (J. Xiao).

ORCID Jie Yuan: http://orcid.org/0000-0002-8166-6132 Jin Xiao: http://orcid.org/0000-0002-9244-7859 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors are grateful supported by National Key R&D Program of China (NO. 2017YFC0210402) and Hunan Provincial Innovation Foundation for Postgraduate (NO.CX2017B061). REFERENCES (1) Holywell, G.; Breault, R. An overview of useful methods to treat, recover, or recycle spent potlining. JOM. 2013, 65(11), 1441-1451. (2) Tschöpe, K.; Schøning, C.; Grande, T. Autopsies of spent pot linings - a revised view. TMS. 2009, 1085-1090. (3) Silveira, B. I.; Dantas, A. E.; Blasquez, J. E.; Santos, R. K. P. Characterization of inorganic fraction of spent potliners evaluation of the cyanides and fluorides content. J. Hazard. Mater. 2002, B89, 177-183. (4) Øye, H. A. Treatment of spent potlining in Aluminum electrolysis, a major engineering and environmental challenge. Energeia. 1994, 2. (5) Sturm, E.; Prepeneit, J.; Sahling, M. Economic and environmental aspects of an effective 26


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Optimization of Spent Cathode Carbon Purification Process under

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