Comparison of ultrasound-assisted and traditional caustic leaching of spent cathode carbon (SCC) from aluminum electrolysis

Ultrasonics - Sonochemistry 40 (2018) 21–29

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Review

Comparison of ultrasound-assisted and traditional caustic leaching of spent cathode carbon (SCC) from aluminum electrolysis

MARK

⁎

Jin Xiao, Jie Yuan , Zhongliang Tian, Kai Yang, Zhen Yao, Bailie Yu, Liuyun Zhang School of Metallurgy and Environment, Central South University, Changsha, Hunan Province 410083, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Ultrasound-assisted leaching Traditional method Spent cathode carbon (SCC) Recycling

The spent cathode carbon (SCC) from aluminum electrolysis was subjected to caustic leaching to investigate the different effects of ultrasound-assisted and traditional methods on element fluorine (F) leaching rate and leaching residue carbon content. Sodium hydroxide (NaOH) dissolved in deionized water was used as the reaction system. Through single-factor experiments and a comparison of two leaching techniques, the optimum F leaching rate and residue carbon content for ultrasound-assisted leaching process were obtained at a temperature of 70 °C, residue time of 40 min, initial mass ratio of alkali to SCC (initial alkali-to-material ratio) of 0.6, liquid-to-solid ratio of 10 mL/g, and ultrasonic power of 400 W, respectively. Under the optimal conditions, the leaching residue carbon content was 94.72%, 2.19% larger than the carbon content of traditional leaching residue. Leaching wastewater was treated with calcium chloride (CaCl2) and bleaching powder and the treated wastewater was recycled caustic solution. All in all, benefiting from advantage of the ultrasonication effects, ultrasound-assisted caustic leaching on spent cathode carbon had 55.6% shorter residue time than the traditional process with a higher impurity removal rate.

1. Introduction Spent cathode carbon (SCC) is a hazardous waste generated from aluminum electrolysis [1]. During operation of the cells, cathode carbon blocks were continuously and inevitably subjected to corrosion by high-temperature electrolyte, molten aluminum, metal sodium and other substances [2,3]. Research papers show that after approximately 5–8 years [1] Hall–Héroult cells shutdown and produce massive amounts of spent pot lining (SPL), which consists of SCC, fire bricks, and insulating bricks [4]. In a large proportion of SPL, the SCC is composed of carbon fluorides, alumina, cryolite, aluminosilicate, and a trace of cyanide (0.2 wt%–1 wt%) [5,6]. Carbonaceous materials and fluorides cause the SCC to have high recovery potential and the components of greatest environmental concern are cyanides and soluble fluorides [7–9]. The untreated SCC seriously affects animal and plant health and ecological balance [10–12], therefore, many countries have classified it as a hazardous solid waste [13]. Considerable effort and studies on SCC treatment were performed and many processes explored its toxicity and its recoverability. SCC with high calorific value carbonaceous materials and chemical fluoride properties are used as raw material or as additive in some industries [14–16], and some processes for SCC treatment by thermal method were also investigated [17–19]. A variety of hydrometallurgical

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Corresponding author. E-mail address: [email protected] (J. Yuan).

http://dx.doi.org/10.1016/j.ultsonch.2017.06.024 Received 13 May 2017; Received in revised form 25 June 2017; Accepted 25 June 2017 Available online 27 June 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.

processes treating and recycling SCC has been tried, including flotation [20], acid leaching [21], soluble aluminum salt solution leaching [22], and caustic leaching [23,24]. As the main method of hydrometallurgy, caustic leaching has advantages of no toxic gases emission and impurities remove rapidly. Some achievements were obtained in the lab and plant by caustic leaching treatment of the SCC. However, everything has its drawback and these processes are no exception, including low leaching rate, long leaching time, special targeted method of cyanide treatment, and so on. Therefore, importing auxiliary treatment or pursuing new technology is necessary. Ultrasound is a mechanical wave with good directivity, strong penetrating ability, easy to obtain intensive sound energy, far transmission distance in water, etc. Ultrasound has received extensive attention in hydrometallurgy because of its special effects. Bese [25] studied the effect of ultrasound on copper recovery in acid leaching of copper converter slag and found that the leaching rate was enhanced. Chang et al. [26] compared ultrasound-augmented and conventional acidic thiourea leaching of silver from sintering dust and the leaching rates were 95% and 89.9%, respectively. Zhang et al. [27] discovered that the leaching rate of ultrasonic-assisted leaching of germanium from the by-product of zinc metallurgy increased by 3%–5% and the time reduced from 100 min to 40 min. Oh et al. [28] found that ultrasound substantially affected the fate of trace pollutants in industrial

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wastewater sludge. Balakrishnan et al. [29] studied ultrasonic waterwashing coal and estimated the intensification of alkali removal process by sonication compared with agitation. Ambedkar et al. [30] studied the effect of ultrasound on coal-wash for de-sulfurization. Savić-Biserčić et al. [31] subjected anions and cations from fly ash to ultrasound and shaking-assisted water leaching and discovered that the ultrasoundassisted extraction lasted 60 min, far shorter than shaking-assisted extraction. Zhang et al. [32] subjected lead-rich and antimony-rich oxidizing slag to ultrasound-assisted HCl–NaCl leaching and found that leaching rates of Sb and Pb increased at a shorter time. Saterlay et al. [33] removed and destroyed cyanide and cryolite in used carbon cathodes from aluminum smelting using ultrasound assistance and discovered a faster leaching speed and higher leaching rate of cryolite than traditional leaching, destroying cyanide by the oxidative action of H2O2 generated in the sonication of water. Based on the specific mechanical effect cavitation and advantages of ultrasound-assisted leaching, and impurity removal complexity in SCC caustic leaching process, we choose ultrasound as an auxiliary method, compared the presence and absence of ultrasound, and finally achieved shorter residue time and higher carbon content of leaching residue. A small content of cyanide in SCC is found and most of the cyanide is water soluble, undergoing oxidative destruction by hydrogen peroxide and bleaching powder. Saterlay et al. [33] researched the effect of ultrasound on cyanide in the leaching process. In this paper, a detailed expression on cyanide destruction and removal is not presented.

Fig. 1. XRD pattern of spent cathode carbon.

combustion ash was weighed after cooling. Leaching experiments were performed under the same condition (no agitation) in the presence of ultrasound. Furthermore, the ultrasonic equipment used was an ultrasonic cleaner (KQ-400KDE, Kunshan Ultrasonic Equipment Company, China). Burning loss rate was the approximate calculated carbon content of leaching residue.

2. Experimental

2.2.2. Single factor experiment Based on the experimental results of particle size, optimum particle size could be determined for the leaching process. A 20-g optimum sized powder was taken, mixed with some alkali and deionized water in a 500-mL plastic beaker, and was subjected to ultrasound-assisted leaching under different temperatures, residue times, liquid to solid ratios, initial alkali-to-material ratios, and ultrasound power densities. After leaching, filtering, and multiple water-washing, the filter cake was dried in a vacuum oven at a temperature of 105 ± 1 °C for 4 h. Traditional leaching experiments were performed under the same conditions except for agitation instead of ultrasound and heat preservation in a thermostat water bath. According to the results, the optimum agitation speed was 500 r/min. Dried leaching residues were incinerated in a muffle stove at 800 °C for 4 h. After the experimental process, F leaching rate and carbon content of leaching residue were calculated. The effects of experimental factors on F leaching rate and leaching residue carbon content were studied.

2.1. Materials The SCC sample used in this study was collected from an aluminum smelter (Sichuan province, China). The elemental analysis of the SCC is listed in Table 1. The crystalline phases of the sample were investigated and Fig. 1 is the X-ray Diffraction (XRD) pattern. It is showed the main phases of the sample are carbon, NaF, Al2O3, Na3AlF6, and aluminosilicate. The raw material was crushed by a jaw crusher and a ball grinder and then passed through sieves of desired mesh sizes and separated into six groups, including −18 to +50 mesh, −50 to +100 mesh, −100 to +200 mesh, −200 to +300 mesh, −300 to +400 mesh and −400 mesh. Fig.2 is the scanning electron microscopy (SEM) images of the samples at different particle sizes. Fig.3 is the Thermogravimetry and differential scanning calorimetry (TGA-DSC) pattern of the sample. Chemicals calcium chloride (CaCl2), bleaching powder and sodium hydroxide (NaOH) were provided by Sinopharm Chemical Regent Co., Ltd. All chemical reagents were analytical reagent grade unless otherwise stated. The water was homemade deionized water.

2.2.3. Leaching wastewater treatment Ion concentrations of caustic leaching wastewater were measured by an ion meter (PXSJ-216). CaCl2 and bleaching powder were added to the solution and gradually formed CaF2 precipitate. CaF2 was filtered, and the remainder solution can be reused as caustic solution.

2.2. Experiment procedure 2.2.1. Particle size experiment A mass of 20 g of each particle size sample powders were supplied for particle size experiment. The experiments were under the same conditions, room temperature, leaching time of 60 min, liquid to solid ratio of 10:1 and initial alkali-to-material ratio of 1:1, agitation of 500r/ min, mixed uniformly with deionized water in 500 mL plastic beakers. The residue was filtered and underwent multiple water-washings after leaching, and dried in a vacuum oven at a temperature of 105 ± 1 °C for 4 h. A 5 g sample of each particle size leaching residue dried was taken and incinerated at 800 °C in a muffle stove for 4 h in air, and the

2.3. Calculation of F leaching rate and C content of residue F leaching rate is calculated by the following equation:

cv ⎞ X= ⎜⎛1− ⎟ × 100% ⎝ mη ⎠

(1)

where X is F leaching rate (%), c is F content of the filtrate (g/mL), V is

Table 1 Ultimate analysis of the spent cathode carbon sample. Element

C

F

Na

Al

O

Si

Ca

K

Fe

Others

Content/%

64.93

12.94

7.85

6.38

4.93

0.47

1.22

0.61

0.39

0.28

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Fig. 2. The SCC sample SEM images at different sizes A: −18–+50 mesh, B: −50–+100 mesh, C: −100–+200 mesh, D: −200–+300 mesh, E: −300–+400 mesh, F: −400 mesh.

the constant volume of the filtrate (mL), m is the mass of the SCC sample for experiments (g), η is the F content of the SCC sample (%). Carbon content of leaching residue is calculated by the following equation:

m ηc = ⎛1− a ⎞ × 100% ⎝ ms ⎠ ⎜

2.4. Characterization The elemental analysis was analyzed by X-ray fluorescence (XRF) (XRF-1800, Shimadzu Corporation, Japan) and burning loss rate for C. The XRD patterns were recorded on a Rigaku MiniFlex 600 diffractometer using Cu Kα1 radiation (40 kV, 40 mA, 10°/min from 10°–80°). SEM images were recorded on the JSM-6360LV (JEOL. Ltd, Japan) at 20 kV and samples were deposited on Si wafers. TGA-DSC was characterized on the SDTQ600 thermogravimetric analysis apparatus (TA Co. Ltd. USA) in air and in N2 at a flow rate of 100 mL/min and heated from room temperature to 1200 °C. F ion concentration was

⎟

(2)

where ηc is carbon content of leaching residue (%), ma is the ash weight of the leaching residue incinerated at 800°Cfor 4 h (g), ms is the leaching residue weight ashed at 800 °C for 4 h (g).

Fig. 3. TGA-DSC plot of spent cathode carbon a: in air, b: in N2.

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structure with good lubricating and ductility properties. With decreased particle size, graphite rotates with the balls during ball mill process, and relative motion between the two is smaller than that between balls and inorganic impurities; therefore, graphite is harder to grind to a smaller size. Moreover, graphite has a Mohs rating of 1–2. Inorganic impurities; alumina and cryolite with poor ductility are harder than graphite and can be ground to a smaller-sized power much more easily. Consequently, carbon content of raw material decreases with decreased powder particle size, which has a good agreement with the conclusions of Li [13] and Lisbona et al. [34]. Li [13] discovered that the carbon content of the SCC powder at particle size +200 mesh was significantly higher than that in −200 mesh powder. In the study of Lisbona et al. [34], the inorganic fraction in spent potlining (SPL) was concentrated in the smallest size reflecting the fact that inorganic fractions were present in SPL as smaller chunks. Similar carbon content residue variation tendencies were known between ultrasound-assisted and traditional caustic leaching processes of the SCC. Carbon content of leaching residue increased with the decreasing powder particle size when particle size was greater than 100 mesh; however, a reverse effect was observed when powder particle size was −100 mesh. When at +100 mesh, carbon and inorganic electrolyte fractions of the SCC powder with large particle were not completely separated, with a bad leaching result in the same residue time. With decreased powder particle size, inorganic impurities were dispersed in caustic solution much more uniformly and impurities reaction dynamics in caustic solution became better, resulting in a better leaching process. When particle size was −100 mesh, carbon content of leaching residue decreased with the increasing SCC powder particle size. Dispersion in caustic solution is no longer a leaching limiting factor and carbon content of leaching residue was affected by the carbon content of the raw material. In ultrasound-assisted and traditional caustic leaching processes of the SCC, by contrast, the carbon content of the former leaching residue was higher than the latter. This phenomenon is because the cavitation and mechanical effect of ultrasound created shock wave and micro-jet. Shock wave and micro-jet with desorption and cleaning effects on solid surface can clean surface intermediate products or reaction product and passivation layer. As seen in Fig.2, inorganic fractions adhere to the SCC surface and are wrapped in carbon holes. Under ultrasound effect, more electrolytes can be exposed and evenly dispersed in caustic solution. The leaching time of 1 h was too short to remove all of the soluble and reactive impurities in traditional leaching process and as a result the residue carbon content was lower than ultrasound-assisted leaching process. Based on the experimental results illustrated in Fig.4 and in consideration of ball milling energy consumption and grinding difficulty level, a particle size of −100 mesh was chosen as the optimum size in traditional and ultrasound-assisted leaching process.

measured by ion meter (PXSJ-216ion meter, Shanghai Hengci Co. Ltd. China). Sample particle size was characterized by laser particle size analyzer Mastersizer 2000 (Malvern Instruments Ltd., United Kingdom). 3. Results and discussion Five reactions are involved in the SCC caustic leaching process as follows:

AlN + 3H2 O= NH3 ↑ + Al(OH)3

(3)

Al 4 C3 + 12H2 O= 3CH 4 ↑ + 4Al(OH)3

(4)

Al2O3 + 2NaOH + 3H2 O= 2NaAl(OH)4

(5)

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

(6)

Na3AlF6 + 4NaOH = NaAl(OH)4 + 6NaF

(7)

3.1. Effect of particle size Effects of different SCC powder particle sizes on the leaching were investigated by ultrasound-assisted and traditional caustic leaching processes. Determination of the optimum particle size of leaching residue was represented by leaching residue burning loss rate under 800 °C for 4 h. The experimental results are illustrated in Fig.4. As illustrated on the graph, impurities in the SCC were dissolved and removed effectively in the caustic solution. Carbon content of raw materials decreased with decreasing particle sizes. Carbon content of leaching residue increased gradually from 84.49% at a particle size of −18–+50 mesh to 89.68% at a particle size of -100–+200 mesh and then decreased to 87.84% at a particle size of −400 mesh; the particle size of −100–+200 mesh was the inflection point of raw materials carbon content variation tendency curve. In ultrasound-assisted leaching process, carbon content variation tendency coincided with traditional leaching. Carbon content of leaching residue was 90.57% at a particle size −18–+50 mesh and increased to 93.71% at a particle size of −100–+200 mesh gradually, and then decreased to 92.47% at −400 mesh. Leaching residue carbon content variation tendency curves of ultrasound-assisted and traditional caustic leaching processes had the same inflection point at raw material particle size of −100–+200 mesh. The SCC is composed of carbonaceous materials and inorganic electrolytes, and carbonaceous materials consist of graphite and amorphous carbon. Graphite is in flake form possessing a layered

3.2. Effect of reaction conditions 3.2.1. Leaching time The effects of leaching time on F leaching rate and residue carbon content were investigated in the traditional and ultrasound-assisted leaching process, and leaching results were illustrated in Fig.5. The conditions for leaching process designed with the univariate method by evaluating the effect of different times were: temperature of 60 °C, initial alkali-to-material ratio of 0.6, liquid-to-solid ratio of 10:1, stirring rate of 500 r/min in traditional leaching process and a power of 400 W in ultrasound-assisted leaching process. As shown in Fig.5, the outcomes reveal the effect of residue time on caustic leaching of the SCC. During ultrasound-assisted leaching, the carbon content of leaching residue increases from 78.26% to 93.87% when residue time lasts from 10 min to 40 min. Extending leaching time results in a minor change on leaching residue carbon content, and with time of 60 min the carbon content only increases by 0.03%. By

Fig. 4. Loss rate of leaching residue by combustion at: A: −18–+50 mesh, B: −50–+100 mesh, C: −100–+200 mesh, D: −200–+300 mesh, E: −300–+400 mesh, F: −400 mesh.

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Fig. 5. Effect of residue time on leaching.

Fig. 6. Effect of temperature on caustic leaching.

contrast, leaching residue carbon contents are 73.66% and 80.54% at times of 10 and 30 min, respectively. Traditional leaching produces lesser carbon content compared with ultrasound-assisted leaching in the same leaching time. During traditional leaching process, the inflection point when residue carbon content changes with time is 90 min and the residue carbon content is 92.18%, 1.69% less than that of ultrasound-assisted leaching inflection point. The curves of residue carbon content are flat after the inflection points and only a slight effect on leaching results was observed as time passes. F leaching rates of ultrasound-assisted and traditional caustic leaching increase with prolongation of aging time on the graph, and the change trends tend to be gentle after the inflection point. F leaching rate of ultrasound-assisted leaching increases from 75.68% at 10 min to 91.39% at 30 min. As time reaches 30 min, the F leaching rate only increases by 0.09%. F leaching rate of traditional leaching increases from 72.06% at 10 min to 90.26% at inflection point of 90 min, slightly changing with the prolongation of residue time and suggesting that the residue time of 90 min is enough to achieve the greatest degree of F leaching rate under the existing conditions. In the picture, F leaching rate and residue carbon content variation curves of traditional leaching have the same inflection point at 90 min; however, two different inflection points are observed at 30 and 40 min on the ultrasound-assisted leaching variation curve, respectively. The difference is because under ultrasound cavitation and shaking, fluorides dissolve in caustic solution easily and quickly and other impurities react at a slower speed, resulting in different times and achieving the end of leaching process. A time span of 30 min in traditional leaching is large and conceals as light time difference on F leaching rate and residue carbon content variation curves inflection point. From the leaching results illustrated in Fig.5, leaching time under ultrasound-assisted leaching is significantly shortened and leaching results are better than traditional leaching. The optimal leaching times of ultrasound-assisted and traditional leaching are 40 and 90 min, respectively. Ultrasound-assisted leaching has been widely used in hydrometallurgical processes. When the cavitation bubble collapses, instantaneous high temperature and high pressure produced will make solid reactants melt and disintegrate under local high temperature and intensive shaking, increasing the dispersion in caustic solution. In the leaching process, inorganic impurities, solid particles in the SCC were separated with carbon under ultrasound condition much more quickly and completely. With smaller particle size, larger specific surface area, and more opportunities to come in contact with caustic solution, the leaching efficiency improved greatly. Cavitation and mechanical effect of ultrasound also reduce the thickness of the boundary layer between solid and liquid reactants surface, increasing the particle diffusion rate and optimizing the reaction kinetics in solution. The reaction rate

improves effectively under optimized kinetic conditions in ultrasoundassisted leaching. 3.2.2. Temperature The effects of leaching temperature on F leaching rate and residue carbon content were investigated in traditional and ultrasound-assisted leaching processes, and the results are illustrated in Fig.6. The conditions for leaching process designed with the univariate method by evaluating the effect of temperature were: time of 40 and 90 min for ultrasound-assisted and traditional leaching, respectively, initial alkalito-material ratio of 0.6, liquid-to-solid ratio of 10:1, stirring rate of 500r/min in traditional leaching process, and power of 400 W in ultrasound-assisted leaching process. Variation curves of F leaching rate and residue carbon content were observed in the SCC ultrasound-assisted and traditional caustic leaching processes in Fig.6. As can be seen on the graph, F leaching rate and residue carbon content both increased dramatically with increased temperature and then the curves gradually decelerate after the inflection points appeared. At 25 °C, F leaching rate and residue carbon content in ultrasound-assisted leaching process are 82.71% and 81.73%, respectively. With increasing temperature, the two experimental results increase to 91.55% and 94.38% at 60 °C with the increase stopping at a higher leaching temperature (80 °C). F leaching rate slightly decreases with temperature increases above 70 °C. The two dependent variables have similar variation curves in traditional leaching and in ultrasound-assisted leaching process. F leaching rate and residue carbon content are 79.27% and 80.43%, respectively, at 25 °C, and 90.26% and 92.35% at inflection point 70 °C. Accordingly, temperature of 70 °C is the optimized parameter of both ultrasoundassisted and traditional caustic leaching processes. At 25 °C, the ΔG values of formulas (3) and (4) are −156.76 and −1658.09 kJ/mol, respectively, both lesser than 0, indicate that formula (3) and (4) are spontaneous reactions under water condition. Light yellow powders are easily found on the SCC generated from new shut-down cells and the powder is Al4C3. As time passes, color of the light-yellow powder gradually faded until white at the final stage. This phenomenon suggests that at room temperature and atmospheric pressure, Al4C3 can react with water in air to form aluminum hydroxide. AlN and Al4C3 with high water reactivity are both small in the SCC sample, with slight influence on the caustic leaching process. According to results of thermodynamics calculation, ΔG values of formula (5)(7) are far less than 0 and three spontaneous reactions at temperature of 0–100 °C are found. On the graph in Fig.6, temperature increase is beneficial to the dissolution of inorganic impurities in the SCC sample when temperature is below 60 °C, and a further increase in temperature leads to the formation of aluminum hydroxyfluoride hydrate. When above 60 °C elevated temperature promoted production of 25

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AlF2(OH) [22]. In this study, Al content of the SCC sample is small and the formation of aluminum hydroxyfluoride hydrate has little influence on leaching results. NaAl(OH)4 is the same final reaction product of formula (5)(7) and these are the three competitive reactions. When at low temperature, ions in solution with weak activity move slowly and the chemical equilibrium constant K is small, production of NaAl(OH)4 limits dissolution of aluminum compounds, and at the same time fluoride has a higher solubility than aluminum compounds. With increasing temperature, chemical equilibrium constant K increases and reactions are much more exhaustive. Under this condition, restrictive effect of fluoride on the dissolving aluminum compounds gradually weakens when the leaching rate of fluoride reaches a certain degree. Furthermore, F leaching rate curve reaches a peak value much more quickly than carbon content of residue curve. Experimental results show that F leaching rate and residue carbon content in ultrasound-assisted leaching process are both higher than traditional leaching process and the difference is more pronounced at 30 °C–60 °C. The reason is that ultrasonic thermal effect improves the solution temperature and higher leaching temperature increases ionic kinetic energy. Also, cavitation and mechanical effect aggravates particle oscillation, decreasing interaction force among particles and solution viscosity. Zhang [32] found that diffusion was related with concentration of thickness of diffusion boundary layer, temperature, leaching agent, solution viscosity, and particle size. The diffusion rate of diffusion boundary layer is expressed as (8) [35]:

dn D = −A (C−Cs) dt x

Fig. 7. Effect of initial alkali-to-material ratio on leaching.

NaOH in solution reacted with reactants as cryolite and alumina, and leaching rates of inorganic impurities in the SCC increase with the increasing caustic in solution. At initial alkali-to-material ratio of 0.5–0.7, F leaching rate and residue carbon content in the two different leaching processes both change little and the latter increases from 91.76% at initial alkali-to-material ratio of 0.5–92.35% at 0.7 in traditional leaching process. The reason is that chemical equation of whole cryolite reacting process with caustic is formula (7) and formula (5)(7) are three competitive reactions with the same reaction product, NaAl(OH)4. Formula (7) with complicated reactions takes longer time to achieve reaction equilibrium. With increased initial alkali-to-material ratio from 0.5 to 0.6, the two values’ variation curves tend to stabilize. Therefore, 0.6 was chosen as the optimum initial alkali-to-material ratio. Compared with traditional leaching process, the researchers found that the F leaching rate and residue carbon content variation curves reach a peak value at lower initial alkali-to-material ratio in ultrasoundassisted leaching process. The reason is that under ultrasonic cavitation and other effects, SCC is broken into small pieces and dispersed in solution more uniformly, with better reaction kinetics for caustic leaching process. The effect of ultrasound also decreases the SCC powder particle size, resulting in weakening gravity effect and Brown movement, whereas the slurry stability gradually increases, which is helpful in reacting with caustic solution. The fluorides of SCC are more soluble in basic solutions, and when leached at a pH of 12 are much closer to the actual fluorides content [18].

(8)

where A is the solid reactant surface area (m ), △x is the thickness of boundary layer (m), C is the bulk concentration of reactant (mol/L), Cs D is the concentration at the solid reactant interface (mol/L), x = kd is the diffusion rate constant, D is diffusion coefficient (m2/s) related with temperature, concentration of solution, the nature of substance, and solvent properties. Based on Newton’s viscosity law, the researchers found that the reason for solution viscosity is the attractive force between molecules, and increasing temperature and agitation are both helpful to reduce solution viscosity. It is an important factor in improving the soluble reactant dissolving in solution when temperature increases. Ultrasonic cavitation bubble can result in instantaneous local high temperature when collapsing. The residue carbon content resulting from the two different leaching processes at 94.43% and 92.38%, respectively, means that ultrasound-assisted leaching is helpful in the removal of impurities in the SCC caustic leaching process. 2

3.2.4. Liquid-to-solid ratio The effects of liquid-to-solid ratio on F leaching rate and residue carbon content were investigated in traditional and ultrasound-assisted leaching processes, and the results are illustrated in Fig.8. The conditions for leaching process designed with the univariate method by evaluating effect of liquid-to-solid ratio were: temperature of 70 °C, time of 40 and 90 min for ultrasound-assisted and traditional leaching processes, respectively, initial alkali-to-material ratio of 0.6, stirring rate of 500 r/min in traditional leaching process, and power of 400 W in ultrasound-assisted leaching process. As shown in Fig.8, when liquid-to-solid ratio is below 7.5 mL/g, the F leaching rate and residue carbon content both increase with liquid-tosolid ratio increasing in traditional and ultrasound-assisted caustic leaching processes of the SCC. Moreover, when above 7.5 mL/g, the two values change little with increased liquid-to-solid ratio in ultrasound-assisted leaching. By contrast, F leaching rate and residue carbon content show a slight increase in traditional leaching process. The inflection points of ultrasound-assisted and traditional leaching processes are 7.5 and 10 mL/g, respectively. The difference of two inflection points contributed to the ultrasonic special effect on the SCC particle.

3.2.3. Initial alkali-to-material ratio The effects of initial alkali-to-material ratio on F leaching rate and residue carbon content were investigated in traditional and ultrasoundassisted leaching processes and the results are illustrated in Fig.7. The conditions for leaching processes designed with the univariate method by evaluating effect of initial alkali-to-material ratio were: temperature of 70 °C, time of 40 and 90 min for ultrasound-assisted and traditional leaching processes, respectively, liquid-to-solid ratio of 10:1, stirring rate of 500 r/min in traditional leaching process, and power of 400 W in ultrasound-assisted leaching process. F leaching rate and residue carbon content variation curves of ultrasound-assisted and traditional caustic leaching processes of the SCC were observed. On the graph, when initial alkali-to-material ratio of 0.2–0.5 was observed, F leaching rates and residue carbon contents all increase with initial alkali-to-material ratio. The two values increase from 82.75% to 80.39% at initial alkali-to-material ratio of 0.2–91.52% and 94.42%, respectively, at 0.5 in ultrasound-assisted leaching process; meanwhile, the two values increase from 82.49% and 78.25% to 90.31% and 91.76%, respectively, in traditional leaching process. These phenomena means that in this range of initial alkali-to-material ratio, 26

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Fig. 8. Effect of liquid-to-solid ratio on caustic leaching.

Fig. 9. Effect of ultrasonic power density on caustic leaching.

Cavitation and mechanical effect separate inorganic impurities with carbonaceous materials more easily and completely, and soluble inorganic fractions disperse in caustic solution more uniformly. F leaching rate and residue carbon content achieve peak values at lower liquid-to-solid ratio with better reaction kinetics in ultrasound-assisted leaching process. At bellow 7.5 mL/g liquid-to-solid ratio, the F leaching rate variation curves in ultrasound-assisted and traditional leaching processes have the same change tendency and the two curves possess approximately straight lines. The reason is that at this liquid-to-solid ratio, water in caustic solution is too little to dissolve soluble fluoride fractions in the SCC, NaF solubility is 4.68 g at 60 °C, and F leaching rate is limited by water in solution. The slope of F leaching rate change curves in liquid-to-solid ratio of 2.5–5 mL/g greater than which in 5–7.5 mL/g, produce a strong evidence for the explanation. When liquid-to-solid ratio is above 7.5 mL/g, water for soluble fluoride fractions dissolving is enough, resulting in the slope decrease of F leaching rate change curves. In the comprehensive consideration of liquid-to-solid ratio effect on F leaching rate and residue carbon content in ultrasound-assisted and traditional leaching processes, 10 mL/g is the optimum leaching liquidto-solid ratio.

ultrasonic transmitting power is below 200 W. Furthermore, investigating ultrasound advantages on leaching is difficult because of lower transmitting power. Values of F leaching rate and residue carbon content increase sharply at transmitting power of 200 W–280 W, and a better leaching result contributes to increased transmitting power. Under this condition, cavitation begins to affect the SCC particles effectively, and cavitation intensity is big enough to strip impurities from carbon resulting in more uniform dispersibility and better leaching reaction kinetics. Above 360 W, many cavitation bubbles in solution affects the reactants, and leaching result reaches the peak value. 3.3. Treatment of waste water Wastewater would is produced in the SCC caustic leaching process containing a large amount of various inorganic ions OH−, F−, Al, Na+ and a small amount of CN−. Al is in the form of Al(OH)4−. In Table 2, OH−, F−, Na+, and Al contents of wastewater in the two leaching processes are little difference, and CN− content of ultrasoundassisted leaching wastewater is less than the content of traditional leaching wastewater. This phenomenon is caused by the hydrogen peroxide generated in ultrasound-assisted leaching solution, and the formula is expressed as: Table 3

3.2.5. Effect of ultrasonic power density The effects of ultrasonic power density on F leaching rate and residue carbon content were investigated. The conditions for leaching process were: temperature of 70 °C, time of 40 min, initial alkali-tomaterial ratio of 0.6 and liquid-to-solid ratio of 10 mL/g. One of the two important parameters for ultrasound is power density and the formula is expressed as:

P=

W S

CN− + H2 O2 = CNO− + H2 O

(10)

CaCl2 added into wastewater can result in a precipitate of CaF2, and solidification of F. The chemical equation is shown in formula (11) and the precipitate XRD pattern is shown in Fig.10. On the graph in Fig.10, only two phases CaF2 and Ca(OH)2 are found, and the element Al still remains in the solution and can be precipitated using the Bayer process.

Ca2 + + 2F− = CaF2 ↓

(9)

(11)

A considerable amount of NaOH was found in wastewater, and after F was removed, the wastewater can be reused as caustic solution. CN− in wastewater can be destroyed and removed by the method of pH adjustment and bleaching powder, which also achieves solidification of F. The purification mechanism is shown in formula (11) and (12).

where P is the ultrasonic power density (W/cm2), W is ultrasonic transmitting power (W), S is ultrasonic emission area (cm2). In this study, the experimental equipment ultrasonic cleaner and reaction vessel plastic beakers are both constant sizes; therefore, ultrasonic power density can be used instead of ultrasonic transmitting power. The results are illustrated in Fig.9. With increased ultrasonic transmitting power, F leaching rate increases from 79.35% to 91.67% and then stabilizes, and residue carbon content increases from 81.26% to 94.54% and then changes little; the two curves both have a turning point at ultrasonic transmitting power of 360 W. Based on the experimental results illustrated in Fig.9, we choose 400 W as the optimum ultrasonic transmitting power. Cavitation intensity increases with the increasing of ultrasonic transmitting power. On the graph, ultrasonic power density is too low to produce enough cavitation intensity affecting the SCC when

5ClO− + 2CN− + 2OH− = 2CO32 − + N2 ↑ + 5Cl− + H2 O

(12)

Table 2 Ion species and contents of leaching waste water. Sample

OH−/ mol·L

F−/g·L

Al/g·L

Na+/g·L

CN−mg/L

A B

1.13 1.24

12.87 12.41

3.48 2.93

37.02 35.64

1354 3867

A: Ultrasound-assisted leaching waste water, B: Traditional leaching waste water

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Table 3 Results under optimum leaching parameters. Sample

Carbon content/%

True density/g·cm3

BET surface area/ m2·g

A B C

64.93 92.53 94.72

2.3867 2.4083 2.4479

7.73 10.63 11.47

A: Raw material, B: Traditional leaching residue, C: Ultrasound-assisted leaching residue.

Fig. 10. XRD pattern of waste water precipitate.

Fig. 13. Particle sizes of raw material and leaching residues A: Raw material, B: Ultrasound-assisted leaching residue, C: Traditional leaching residue.

3.4. Experiment under optimum leaching parameters The SCC was treated under optimum parameters in ultrasound-assisted and traditional leaching processes. The leaching conditions were: temperature of 70 °C, time of 40 and 90 min for ultrasound-assisted and traditional leaching, respectively, initial alkali-to-material ratio of 0.6, liquid-to-solid ratio of 10 mL/g, stirring rate of 500r/min in traditional leaching process, and power of 400 W in ultrasound-assisted leaching process. Comparing the properties of ultrasound-assisted and traditional leaching residues under optimum leaching parameters, it is observed that carbon content of ultrasound-assisted leaching residue is 94.72%, 2.19% larger than the carbon content of traditional leaching residue, and the true density and BET surface area of the former are both bigger than that of the latter. In Fig.11, little difference is found between the two XRD patterns of leaching residues in ultrasound-assisted and

Fig. 11. XRD pattern of leaching residue in different processes A: Ultrasound-assisted leaching residue, B: Traditional leaching residue.

Fig. 12. SEM images of leaching residue A: Ultrasound-assisted leaching residue, B: Traditional leaching residue.

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traditional leaching processes. This phenomenon is because impurity phases remaining in two residues is nearly the same and the carbon contents of two residues possesses a difference of only 2.19%. On the graphs in Figs. 12 and 13, particle size of ultrasound-assisted leaching residue is 2.989 μm smaller than traditional leaching residue with smooth surface. The properties of leaching residue under ultrasound effect are superior to the properties of traditional leaching residue. Moreover, caustic leaching the SCC from aluminum electrolysis under ultrasound is an important method. 4. Conclusion (1) Soluble impurities in the SCC are removed efficiently in ultrasoundassisted caustic leaching process, and the optimum parameters were determined as temperature of 70 °C, residue time of 40 min, initial alkali-to-material ratio of 0.6, liquid-to-solid ratio of 10 mL/g, and ultrasonic power of 400 W. Under these conditions, leaching residue carbon content was 94.72%. (2) Compared with traditional leaching, the researchers found that the electrolyte fractions can be separated with carbonaceous materials more easily and completely in ultrasound-assisted leaching process. The residue of medium particle size of ultrasound-assisted leaching reduces by 8.17% compared with the SCC sample size, and by 8.51% than the medium particle size of the traditional leaching residue. Ultrasound-assisted leaching can considerably shorten residue time and the optimum leaching times for two processes are 40 and 90 min, respectively. (3) Hydrogen peroxide is produced in ultrasound-assisted solution and cyanides in the solution could be destroyed and removed. Cyanide content of ultrasound-assisted leaching filtrate is sharply lower than that of traditional leaching filtrate. Acknowledgements The authors are grateful supported by Hunan Provincial Innovation Foundation for Postgraduate (NO. cx2017B061) and the National Natural Science Foundation of China (NO. 51374253). References [1] R. Breault, S. Poirier, G. Hamel, A. Pucci, A ‘green’ way to deal with spent pot lining, Aluminium Int. Today J. Aluminium Prod. Process. 23 (2011) 22–24. [2] M. Sørlie, H.A. Øye, Cathodes in Aluminium Elyctrolysis, third ed., (2010). [3] K. Tschöpe, C. Schøning, J. Rutlin, T. Grande, Chemical degradation of cathode linings in hall-héroult cells—an autopsy study of three spent pot linings, Metall. Mater. Trans. B 43 (2011) 290–301. [4] K. Tschöpe, C. Schøning, T. Grande, Autopsies of spent potlinings–a revised view, Light Met. 2009 (2009) 1085–1090. [5] S.B. Sleap, B.D. Turner, S.W. Sloan, Kinetics of fluoride removal from spent pot liner leachate (SPLL) contaminated groundwater, J. Environ. Chem. Eng. 3 (2015) 2580–2587. [6] W. Li, X. Chen, Chemical stability of fluorides related to spent potlining, Light Met. (2008) 855–858. [7] B. Mazumder, S.R. Devi, Adsoraption of oils heavy metals and dyes by recovered carbon powder from spent pot liner of aluminum smelter plant, J. Environ. Sci. Eng. 50 (2008) 203–206. [8] S. Huang, The treatment of SPL and its technical analysis, Light Met. (China) (2009) 29–34.

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