- Email: [email protected]
Recycling of aluminum metal matrix composite using ionic liquids:
Electrochimica Acta 50 (2005) 3286–3295
Recycling of aluminum metal matrix composite using ionic liquids: Effect of process variables on current efficiency and deposit characteristics V. Kamavaram, D. Mantha, R.G. Reddy∗ Department of Metallurgical and Materials Engineering, Center for Green Manufacturing, The University of Alabama, Tuscaloosa, AL 35487, USA Received 30 August 2004; received in revised form 30 November 2004; accepted 4 December 2004 Available online 18 January 2005
Abstract Recycling of aluminum metal matrix composite via electrolysis in ionic liquids at low-temperature was investigated. The electrolytic melt comprised of 1-butyl-3-methylimidazolium chloride (BMIC) and anhydrous AlCl3 . Aluminum metal matrix composite (Duralcan® , Al-380, 20 vol.% SiC) was electrochemically dissolved at the anode, and pure aluminum (>98%) was deposited on a copper cathode. The influence of experimental parameters such as concentration of electrolyte and applied cell voltage on the efficiency of aluminum metal matrix composite recycling was studied at 103 ± 2 ◦ C. High applied voltages and concentration of AlCl3 yielded high current densities. Current densities obtained during this process were in the range of 200–500 A/m2 and current efficiencies in the range of 70–90%. The deposits were characterized by scanning electron microscope, X-ray diffractometer, mass spectrometer, and atomic absorption spectrophotometer. Characteristics of the deposited microstructure ranging from columnar to spherical were obtained. Energy consumption was in the range of 3.2–6.7 kWh/kg-Al for the experimental conditions studied. The optimum conditions obtained in the present investigation for maximum current efficiency and least energy consumption with uniform deposit microstructure were low applied voltage and intermediate electrolyte concentration. Low energy consumption and no emission of pollutants are the two main advantages of this process compared to the current recycling processes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Recycling; Aluminum metal matrix composite; Ionic liquids; 1-Butyl-3-methyl-imidazolium chloride
1. Introduction Metal matrix composites offer numerous beneficial properties such as high strength, stiffness, fatigue, and thermal properties. An enormous interest in the manufacture of metal matrix composites was triggered by their application in structural, automotive, and defense applications [1]. The advancement of metal matrix composites in the automotive market is still hampered by the low-volume usage of these materials due to their high cost in comparison with aluminum alloys. High demand and cost of production led to the recycling of these composites. Recycling, a significant factor in the supply of many of the metals used in daily life provides environmental ∗
Corresponding author. Tel.: +1 205 348 4246; fax: +1 205 348 2164. E-mail addresses: [email protected] (V. Kamavaram), [email protected] (R.G. Reddy). 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.12.002
benefits in terms of energy saving, reduced volumes of waste, and reduced pollutants emission [2]. Metal matrix composites are recycled by melting composite scrap in furnaces such as induction furnace, reverbatory melter, hearth furnace, and rotary barrel furnace. The scrap from aerospace and commercial applications is melted and cast into ingots. Only a slight change in the tensile properties of composite was observed after several recycling steps [3]. Salt fluxing technique is commonly used in the reclamation of ceramic-particulate-reinforced metal matrix composites [3,4]. This process is based on the principle of effective de-wetting of ceramic particles from aluminum matrix using molten salts. The molten salts are thermodynamically stable in the presence of liquid aluminum and can effectively remove ceramic particles by wetting them, but the salt itself is not wetted by the liquid aluminum. The salts used for separating reinforcement from aluminum matrix are a mixture
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
of halides of sodium and potassium, because surface energy of these salts is lower than that of aluminum [5]. This phenomenon occurs at high temperatures, normally above the melting temperature of aluminum. In the present study, aluminum metal matrix composite is reclaimed via electrorefining in ionic liquid at lowtemperature. Reddy and co-workers [6–9] have reported electrorefining of aluminum alloys using chloroaluminate ionic liquids, which yielded high-purity aluminum deposits with low energy consumption and no emission of pollutants. Ionic liquids are low melting salts with negligible vapor pressure, high electrical conductivity, and wide electrochemical window of about 4.0 V. These favorable properties render chloroaluminate ionic liquids as potential electrolytes for low-temperature aluminum production [10]. The concept of extraction and refining of aluminum using ionic liquids originated when ionic liquids were used for electrodeposition of metals [11–14]. Ionic liquids are non-volatile and recyclable which make them environmentally benign electrolytes for recycling metal matrix composites. Chloroaluminate ionic liquids are prepared by mixing alkyl-imidazolium chlorides with aluminum chlorides [15]. Chloroaluminate ionic liquids are liquids at roomtemperature and hence are also termed as room-temperature ionic liquids [16]. Here we take an example of 1-butyl-3methylimidazolium chloride (BMIC) and AlCl3 ionic liquid. In these melts several anionic species are often present in equilibrium, which depend on the ratio of the two components BMIC and AlCl3 given by Eq. (1): BMIC + AlCl3 ↔ [BMI]+ + [AlCl4 ]−
(1)
With an excess of the Lewis acid AlCl3 , additional anion species can be formed from further acid–base reactions given by Eqs. (2) and (3): [AlCl4 ]− + AlCl3 ↔ [Al2 Cl7 ]−
(2)
[Al2 Cl7 ]− + AlCl3 ↔ [Al3 Cl10 ]−
(3)
When BMIC is present in molar excess over AlCl3 , i.e., [AlCl3 :BMIC] < 1.0, the ionic liquid is termed as basic ionic liquid and only equilibrium equation (1) need to be considered. In an equimolar solution, i.e., [AlCl3 :BMIC] = 1.0, which is termed as neutral melt, AlCl4 − is the only anion present in the liquid. When AlCl3 is present in molar excess over BMIC, i.e., [AlCl3 :BMIC] > 1.0, the ionic liquid is termed as acidic ionic liquid and equilibria given by Eqs. (2) and (3) predominate in the solution [17]. This paper discusses the effect of process variables such as electrolyte concentration and applied voltage on current efficiency and deposit microstructure during the recycling of an aluminum metal matrix composite in a BMIC–AlCl3 ionic liquid electrolyte.
3287
2. Experimental procedure Experimental setup for the recycling of aluminum metal matrix composite in chloroaluminate ionic liquid electrolyte consisted of a 50 ml Pyrex® beaker fitted with a Teflon cap. Schematic of the experimental setup used in this study is shown in Fig. 1. Aluminum matrix composite, copper, and pure aluminum were used as anode, cathode, and reference electrodes, respectively. The composition of aluminum metal matrix composite (expressed in wt.%) used as anode in the present study is as follows: Al, 66.22; SiC, 23; Si, 5.78; Cu, 2.31; Fe, 1.05; Zn, 0.77; Mg, 0.38; and Ni, 0.10. Reference electrode was used to measure the electrode potentials of both anode and cathode individually, using a multimeter (Keithley Instruments Inc.® ). A constant potential was applied across the electrodes using a Kepco® power source. The experimental setup had provisions for introducing thermometer and inert gas into the electrolytic cell. The electrolyte was prepared by carefully mixing precise quantities of AlCl3 and BMIC in a dry 50 ml beaker. Anhydrous aluminum chloride (99.985% pure) obtained from Alfa Aesar® was used without further purification. BMIC was synthesized in our laboratory using 1-methylimidazole and 1-butylchloride [18]. Since the mixing process was exothermic, AlCl3 was added in small quantities to BMIC contained in the beaker. Components were weighed carefully to prepare an acidic ionic liquid composition with desired physical properties. Mixing and weighing of the components were done in a controlled atmosphere glove box filled with argon. The electrolyte was transferred to fume hood to conduct the experiment. The melt was heated to a predetermined experimental temperature before starting the electrolysis. For each experimental run the electrolyte was freshly prepared. Since the chloroaluminate ionic liquid electrolyte is moisture sensitive, argon was introduced into the cell at a flow rate 50 ml/min to maintain an inert atmosphere above the electrolyte throughout the experiment. The electrolyte was stirred at a constant speed using a magnetic stirrer and the
Fig. 1. Schematic of the experimental setup used for reclaiming aluminum from aluminum metal matrix composite anodes using BMIC–AlCl3 ionic liquids electrolyte.
3288
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
temperature of electrolyte was controlled by a hot plate. All the experiments were carried out at temperature 103 ± 2 ◦ C for about 120 min. Electrolyte composition defined in terms of concentration ratio (hereafter referred to as CR in the text) of AlCl3 and BMIC, in the range of 1.3–2.0, and applied cell voltage in the range of 1.0–1.8 V were the experimental parameters studied. After electrolysis, aluminum deposit obtained on the copper cathode was mechanically separated from the substrate and powdered for X-ray diffraction studies. X-ray diffraction (XRD) studies were performed on a Philips® PW3830 Xray diffractometer which uses monochromatic Cu K␣ radiation. Compositional analysis of the anodes and deposits was performed using a Spectrolab® , Model LVO-M3 emission spectrometer (ES) and Perkin-Elmer® 560 semi-automated atomic absorption spectrophotometer (AAS). Morphology of the deposits was studied using Philips® XL30 scanning electron microscope (SEM).
Fig. 2. Variation of average current density with time during electrolysis of aluminum metal matrix composite in BMIC–AlCl3 ionic liquid of concentration ratio, CR = 2.0 and temperature 103 ± 2 ◦ C at different applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V.
3. Results and discussion Applying potential between the aluminum matrix composite anode and copper cathode, aluminum from the anode reacts with AlCl4 − ions present in melt to produce Al2 Cl7 − ions shown by the anodic reaction (Eq. (4)): At anode :
Al(anode) + 7AlCl4 − → 4Al2 Cl7 − + 3e−
(4)
Al2 Cl7 − ions traverse to the cathode either by diffusion or convection and get reduced to produce aluminum deposit. Cathodic reaction, which leads to aluminum deposition on copper electrode is shown in Eq. (5): At cathode :
4Al2 Cl7 − + 3e− → Al(cathode) + 7AlCl4 − (5)
At any given experimental condition, the total current output was noted at regular intervals and converted into current density by dividing with the aluminum deposited area of cathode. Fig. 2 shows the variation of current density as a function of time at different applied voltages at 103 ± 2 ◦ C and CR = 2.0 of the electrolyte. Similar behavior of current density with time was observed in electrolytes with different concentration ratios studied in the present research. Typical current density versus time curve is observed at all the applied voltages. Steep rise in current density for a short period of 10–15 min (region A), followed by a brief steady-state of 15–20 min (region B) and then a shallow decrease with time (region C) was observed for applied voltages of 1.3, 1.5, and 1.8 V. However, at applied voltage of 1.0 V, region A was observed upto 30 min, steady-state (region B) was observed for 60 min followed by a decrease in current density with time. The steep increase in current density in region A is due to activation polarization at cathode. It is confirmed by plotting the cathode potentials with current density during the initial period. Fig. 3 shows cathodic polarization plots for the curves during the period of activation polarization marked by
region A in Fig. 2. A linear relation between current density and cathode potential is an indication of activation polarization [19]. Approximate linear behavior was observed in all the cases and the slopes of these curves tend to decrease with increasing applied voltage. Generally the linear relation between over-voltage and current density is represented by Tafel equation, Eq. (6) [19]: η = a − b log(i)
(6)
a=
2.303RT log(i0 ) (1 − α)F
(7)
b=
2.303RT (1 − α)zF
(8)
Fig. 3. Cathodic polarization curve showing the variation of current density as a function of cathode potential in the region A shown in Fig. 2 for the following applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V, and at CR = 2.0 and temperature 103 ± 2 ◦ C.
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3289
Fig. 4. Optical micrographs of cross-sectional view of aluminum metal matrix composite anode (a) before and (b) after electrorefining process.
In Eq. (6), η is the cathode over-voltage, a and b are Tafel constants and i the current density. Expressions to evaluate the Tafel constants, a and b, in terms of charge transfer coefficient α, electrons transferred z, Faraday constant F and exchange current density i0 are given in Eqs. (7) and (8). The charge transfer coefficient α can be determined using Eq. (8) from the slopes b of plots in Fig. 3. Exchange current density i0 can be calculated from the intercepts of plots in Fig. 3 at η = 0. The charge transfer coefficient α obtained for the plots shown in Fig. 2 ranged from 0.79 to 0.9 indicating that the kinetics of aluminum deposition is a quasireversible process [19]. It has already been reported that aluminum deposition on copper in BMIC + AlCl3 melts is quasi-reversible process [20]. Aluminum deposition in AlCl3 –trimethylphenylammonium chloride [21], AlCl3 –1methyl-3-ethylimidazolium chloride [22] and AlCl3 –nbutylpyridinium chloride [23] melts was observed to be quasi-reversible. In region B, current density attains a steady-state as seen from Fig. 2. Steady-state region was smaller at higher voltages than at low voltages. After the steady-state (region B) the current density decreases steadily in region C. With the progress of time a thin layer is formed at anode, consisting of largely silicon and other impurities. Formation of the layer causes aluminum to diffuse through the layer to form Al2 Cl7 − ions. Hence concentration polarization at anode may be responsible for the decrease in current density in region C. The amount of metal deposited was determined by the weight gain W of cathode. Cathode current efficiency ηeff , expressed as percentage, is defined as the ratio of the actual amount of metal deposited to that expected theoretically. The theoretical amount of metal deposited for a fixed quantity of electricity passed through an electrochemical cell can be obtained using the Faraday law. The energy consumed (E) during the electrorefining was determined using Eq. (9): Q E=V ηeff
(9)
where V is the applied cell voltage, ηeff the cathode current efficiency, and Q the theoretical charge required for depositing a fixed amount of material according to the Faraday law.
The energy consumption for electrorefining aluminum metal matrix composite under the present experimental range of conditions was in the range of 3.2–6.7 kWh/kg-Al. Fig. 4(a) and (b) shows the cross-sectional optical micrographs of aluminum matrix composite anode with SiC before and after electrolysis. Fig. 4(a) shows that SiC particles are discontinuously reinforced in aluminum alloy matrix with a particle size of approximately 10–15 m. Fig. 4(b) shows the optical micrograph after electrorefining where SiC particles and other impurities remain with the anode. The composition of deposited aluminum was determined using atomic absorption spectrophotometer and emission spectrometer is shown in Table 1. A high-purity aluminum deposit was obtained. XRD patterns of anode, deposited aluminium, and anode residue are shown in Fig. 5. XRD pattern of deposit shows the purity of aluminum in the deposit and that of the anode after refining reveals relative increase in the intensities of other impurities. 3.1. Effect of cell voltage The effect of applied cell voltage on current density and cathode current efficiency of the recycling process was studied. The applied voltage was in the range of 1.0–1.8 V and temperature was 103 ± 2 ◦ C. Fig. 6 shows the variation of average current density with applied cell voltages at different concentration ratios (1.3–2.0). It can be seen that the current density increases approximately linearly with an increase in voltage from 1.0 to 1.8 V. Increase in voltage increases rate of reaction at cathode thereby increasing the current density.
Table 1 Composition of aluminum deposited on copper cathode Element
Weight (%)
Al Fe Zn Mg Cu Ni
98.15 0.04 0.31 0.13 1.26 0.10
3290
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 5. XRD pattern of anode before electrolysis, aluminum deposited on cathode and anode residue.
The variation of cathodic current efficiency with applied cell voltage at a range of concentration ratios (1.3–2.0) is shown in Fig. 7. Current efficiencies under all experimental conditions were found to fall within a range of 70–90%. Intermediate voltages (1.3 and 1.5 V) yield low cathode current efficiencies as compared with lowest (1.0 V) and highest (1.8 V) voltages used in this study. Current efficiency calculation is based on the weight difference of cathode before and after electrolysis and the theoretical amount that can deposit under the given conditions. As the applied voltage increases the total charge passed through the cell increases, hence the theoretical amount deposited also increases. However, the actual weight deposited does not follow a similar trend as that of current density with respect to applied voltage due to the current losses (polarization effects) involved in electrolysis processes. The calculated current efficiencies directly
Fig. 6. Variation of average current density with the applied cell voltage, at concentration ratios: (䊉) 1.3, () 1.5, (+) 1.8, and () 2.0 and at temperature of 103 ± 2 ◦ C.
Fig. 7. Variation of cathode current efficiency with applied cell voltage at concentration ratios: (䊉) 1.3, () 1.5, (+) 1.8, and () 2.0 and at temperature of 103 ± 2 ◦ C.
relate to the net effect of driving force and resistance to aluminum deposition. Therefore, lower current efficiencies at intermediate potentials (Fig. 7) can be attributed to the above phenomena involved in the electrodeposition process. Deposits were characterized for microstructure by scanning electron microscopy (SEM). Fig. 8(a)–(d) depicts the SEM images of the deposits obtained in an electrolyte of CR = 1.3 as a function of applied voltage. Fig. 8(a) and (b) displays almost similar microstructure except that at higher voltages (1.3 V) growth of particles is seen. Fig. 8(c) shows that the microstructure has a more uniform deposition of particles with an average size range of 10–20 m. Fig. 8(d) depicts the microstructure with a more open deposition where cluster of particles is seen with lot of porosity. Cluster size varied from 20 to 100 m. From the analysis of the deposit microstructure at concentration ratio CR = 1.3, it appears that the morphology of deposits was relatively independent of applied voltage except that at high voltage the spherical particles tend to agglomerate. However, deposit microstructures obtained at highest concentration ratio (CR = 2.0) of the electrolyte revealed interesting features as a function of applied voltage. Fig. 9(a)–(d) depicts the SEM images of aluminum deposits at different applied voltages showing the varied morphological patterns obtained in this study. Fig. 9(a) refers to the deposit microstructure obtained at applied voltage 1.0 which shows a dendritelike deposition with dendrites of length 100 m. Fig. 9(b) and (c) refers to deposit microstructures obtained at applied voltages of 1.3 and 1.5 V, respectively, which reveal a uniform deposition of fine particles in the range of 5–25 m. However, at applied voltage of 1.5 V the microstructure appears to be an agglomeration of particles into small clusters with an average size 20 m as can be seen in Fig. 9(c). At the highest applied voltage (1.8 V) of this study a compact layer of deposit microstructure was observed as can be seen in Fig. 9(d). It can be seen from the above microstructures that at CR = 2.0, the
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3291
Fig. 8. SEM micrographs showing the effect of voltage on the microstructure of the aluminum deposits: (a) 1.0 V, (b) 1.3 V, (c) 1.5 V, and (d) 1.8 V obtained at constant CR = 1.3 and temperature 103 ± 2 ◦ C.
Fig. 9. SEM micrographs showing the effect of voltage on the microstructure of the aluminum deposits: (a) 1.0 V, (b) 1.3 V, (c) 1.5 V, and (d) 1.8 V obtained at constant CR = 2.0 and temperature 103 ± 2 ◦ C.
3292
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 10. Variation of average current density with the concentration ratio (CR = AlCl3 :BMIC) of the electrolyte, at applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V, and T = 103 ± 2 ◦ C.
Fig. 11. Variation of cathode current efficiency with concentration ratio (AlCl3 :BMIC) at applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V, and () 1.8 V and at temperature 103 ± 2 ◦ C.
type of deposit microstructure changed rapidly as a function of applied voltage. Unlike the case of low concentration ratio (CR = 1.3) where the deposit microstructure is relatively independent of applied voltage, at CR = 2.0, the influence of applied voltage is clearly seen on the deposit microstructure. The combined influence of increased acidity of melts and applied voltage seems to have led to a nucleation-controlled microstructure.
efficiency with concentration ratio (CR) is due to the increase in amount of aluminum deposited at the cathode. As mentioned earlier, concentration ratio of the electrolyte determines the type of anions present in solution. It can be seen from cathodic reaction equation (5) and anodic reaction equation (4) that the concentrations of Al2 Cl7 − and AlCl4 − affect their rate. It was observed that as concentration ratio of the chloroaluminate ionic liquid increases from 1.3 to 2.0 the concentration of AlCl4 − decreases and that of Al2 Cl7 − increases [14]. It was reported from mass spectrometer analysis [24] that there exists an ion-pair interaction between the chloroaluminate anion AlCl4 − and the organic cation R+ but not between Al2 Cl7 − and R+ . In neutral melts (1:1 molar ratio of AlCl3 :RCl) the ions in the solution may be visualized as follows:
3.2. Effect of concentration of electrolyte Concentration of electrolyte is an important parameter that influences the reactions taking place at the electrodes. Concentration ratio of electrolyte used in the present study was in the acidic range, i.e., the concentration ratio or molar ratio of AlCl3 to BMIC varied from 1.3 to 2.0. Fig. 10 shows the variation of average current density as a function of concentration ratio of electrolyte at a range of applied voltages 1.0–1.8 V and temperature of 103 ± 2 ◦ C. Current density was found to increase slightly with increasing concentration ratio (1.3–2.0) at all the voltages shown in Fig. 10. The slight increase in the current density with concentration ratio is due to the increase in the number of ions of the electrolyte. Fig. 11 shows the effect of concentration ratio of the electrolyte on cathode current efficiency at a range of applied voltages 1.0–1.8 V and temperature of 103 ± 2 ◦ C. Although the current efficiencies at all the voltages were in the range of 70–90%, there is a distinct dip in current efficiencies at concentration ratio of CR = 2.0. The variation in current efficiency is essentially affected by the amount deposited at the cathode and the theoretical amount of aluminum that can get deposited for a given condition. Since the theoretical amount deposited is related to the total charge passed through the cell, which is fixed at a constant applied potential, the theoretical amount of aluminum deposited does not change with concentration ratio of the electrolyte. Increase in cathode current
AlCl4 − · · · R+ · · · AlCl4 − · · · R+ · · · AlCl4 − which means that there is no free R+ and Al2 Cl7 − in the neutral melt. However, in basic melts (molar ratio of AlCl3 :RCl < 1:1 and R+ is in excess) aluminum was found to be oxidized spontaneously [23,25]. A similar reaction can be expected in acidic melts when free organic cation R+ is present to serve as electron acceptor. Thus, as the acidity of the melts is increased, the concentration of free R+ increases and a higher oxidation of aluminum can be expected. This clearly explains the decrease in cathode current efficiency with increasing concentration ratio of electrolyte. Fig. 12(a)–(d) shows the effect of concentration ratio on the morphology of aluminum deposits at a fixed applied voltage of 1.0 V and temperature 103 ± 2 ◦ C. Fig. 12(a) depicts the microstructure of deposit obtained at the lowest applied voltage (1.0 V) and at the lowest concentration ratio (CR = 1.3) of the electrolyte used in this study. A uniform deposit of spherical particles in a close size range of 20–50 m can be observed where some amount of agglomeration of particles appears to have taken place. Fig. 12(b) shows the
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3293
Fig. 12. SEM micrographs showing the effect of concentration ratio of electrolyte (a) 1.3, (b) 1.5, (c) 1.8, and (d) 2.0 on the microstructure of the aluminum deposits obtained at constant voltage 1.0 V and temperature 103 ± 2 ◦ C.
microstructure of the deposit obtained in an electrolyte of CR = 1.5 at an applied voltage of 1.0 V. The deposit morphology reveals a non-uniform deposit containing non-spherical particles formed as small clusters with an average cluster size of 30 m. As the concentration ratio of the electrolyte increases further to 1.8, the deposit obtained was similar to that obtained at the lowest concentration ratio (CR = 1.3) but with finer particles (average size of 20 m). At the highest concentration ratio (CR = 2.0) of the electrolyte the deposit depicted dendritic-like microstructure. It can be seen that, at an applied voltage of 1.0 V the aluminum deposit microstructure has strong dependence on the electrolyte composition. The effect of increase in the concentration ratio (CR) of the electrolyte, at a fixed applied voltage of 1.8 V, on the morphology of aluminum deposit is depicted in Fig. 13(a)–(d). Fig. 13(a) shows SEM image of the deposit microstructure at CR = 1.3 which reveals a uniform distribution of clusters of fine particles with an average size of 15 m. At a slightly higher concentration ratio (CR = 1.5), the deposit microstructure changes to discreet clusters of small particles with an average particle size of 5 m. The cluster size is bigger than in the previous case. Fig. 13(c) shows a similar microstructure as that of Fig. 13(b) but with a slight increase in the average particle size. At CR = 2.0 of the electrolyte, SEM image of the deposit microstructure shown in Fig. 13(d) depicts a dense compact layer of aluminum. As the concentration ratio of electrolyte increased, the morphology of aluminum deposits changed from clusters of large spherical particles
to a dense compact layer of small particles. A nucleationcontrolled deposit growth is evident from the analysis of deposit microstructures.
3.3. Effect of anode composition Current efficiencies of electrorefining process depend not only on the process parameters such as concentration of the electrolyte, applied cell voltage, temperature, but also on the composition of the electrode that is being electrorefined. Aluminum metal matrix composite studied in this research contained 23 wt.% SiC among other metallic impurities such as Si, Cu, Zn, Fe, etc. The effect of SiC content of the anode on current efficiencies at different experimental conditions can be understood by the comparison of the results with those obtained for an aluminum alloy (A360) in an earlier study [7]. Current efficiencies of electrorefining of A360 alloy, where the major metallic impurities were the same as those in the present aluminum metal matrix composite with different relative amounts of individual metal, were in the range of 90–99%. Comparing the results of electrorefining for the two anodes (aluminum metal matrix composite and A360) it can be said that lower current efficiencies in the present case can be attributed partly to the presence of SiC in the anode. Additionally, since the current efficiency is calculated based exclusively on the weight difference of the cathode before and after experiment, experimental errors could play a vital role in the current efficiency calculations.
3294
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 13. SEM micrographs showing the effect of concentration ratio of electrolyte (a) 1.3, (b) 1.5, (c) 1.8, and (d) 2.0 on the microstructure of the aluminum deposits obtained at constant voltage 1.8 V and temperature 103 ± 2 ◦ C.
4. Conclusions Aluminum was successfully reclaimed from aluminum metal matrix composite by electrolysis in ionic liquids at 103 ± 2 ◦ C. High-purity aluminum (>98%) deposits were obtained. High current densities were obtained at high applied voltages and concentration ratios of electrolyte. Current densities in the range 200–500 A/m2 and cathode current efficiencies in the range 70–90% were obtained. From the present study, it appears that high applied cell voltage and intermediate concentration ratios are optimum conditions for obtaining high cathode current efficiencies. From the microstructure analysis, within the range of experimental parameters of this study, it appears that at low concentration ratios of electrolyte the morphology of aluminum deposit is relatively independent of applied voltage. However, concentration ratio had a significant influence on the deposit microstructures at any fixed applied voltage. Energy consumption for the present process was estimated to be in the range of 3.2–6.7 kWh/kgAl.
Acknowledgements The authors are pleased to acknowledge the financial support for this research by US Department of Energy (DOE) under Award number DE-FC07-02ID14397, National Science Foundation (NSF) under grant number NSF-EPS-9977239, and the University of Alabama, Tuscaloosa (USA). We are
also thankful to Mr. Ravinder N. Reddy for his assistance in synthesizing and characterizing the ionic liquids used in this research.
References [1] S. Rawal, JOM 4 (2001) 14. [2] J.F. Papp, U.S. Geological Survey Minerals Yearbook, U.S. Geological Survey, 1999, p. 62. [3] D.M. Schuster, et al., JOM 45 (1993) 26. [4] Y. Nishida, N. Izawa, Y. Kuramasu, Met. Mater. Trans. A 30 (1999) 839. [5] J.R. Davis, et al., Metals Handbook, vol. 2, American Society for Metals, Ohio, 1991. [6] B. Wu, R.G. Reddy, R.D. Rogers, in: D.L. Stewart Jr., J.C. Daley, R.L. Stephens (Eds.), Proceedings of the Fourth International Symposium on Recycling of Metals and Engineered Materials, TMS, Warrendale, PA, 2000, p. 845. [7] V. Kamavaram, D. Mantha, R.G. Reddy, J. Min. Metall. B 39 (1–2) (2003) 43. [8] V. Kamavaram, R.G. Reddy, in: B. Bjorkman, C. Samuelsson, J.O. Wikstrom (Eds.), Recycling and Waste Treatment in Mineral and Metal Processing: Technical and Economic Aspects, Lulea, Sweden, The Mineral, Metals & Materials Society, Warrendale, PA, 2002, p. 517. [9] V. Kamavaram, R.G. Reddy, in: D. Hui (Ed.), Proceedings of the Ninth Annual International Conference on Composite Engineering, New Orleans, LA, 2002, p. 359. [10] M. Zhang, V. Kamavaram, R.G. Reddy, JOM 11 (2003) 54. [11] G.A. Capuano, W.G. Davenport, J. Electrochem. Soc. 118 (1971) 1688. [12] T.P. Wier, F.H. Hurley, J. Electrochem. Soc. 98 (1951) 203.
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295 [13] Q. Liao, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 144 (1997) 936. [14] C.J. Smit, T.P.J. Peters, in: R.B. Miller, W.S. Peterson (Eds.), Light Metals, TMS, Warrendale, PA, 1986, p. 253. [15] Z.J. Karpinski, R.A. Osteryoung, Inorg. Chem. 23 (1984) 1491. [16] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co., KGaA, Weinheim, 2002. [17] J.S. Wilkes, J.A. Levisky, C.L. Hussey, Inorg. Chem. 21 (1982) 1263. [18] V. Kamavaram, Ph.D. Dissertation, The University of Alabama, Tuscaloosa, AL, USA, 2004.
3295
[19] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2000. [20] V. Kamavaram, R.G. Reddy, in: W.A. Schneider (Ed.), Light Metals, TMS, Warrendale, PA, 2002, p. 253. [21] Y. Zhao, T.J. Vandernoot, Electrochim. Acta 42 (1997) 1639. [22] P.K. Lai, M. Skyllas-kazacos, J. Electrochem. Soc. 248 (1988) 431. [23] P.K. Lai, M. Skyllas-kazacos, Electrochim. Acta 32 (1987) 1443. [24] B.L. Ackermann, A. Tsarbopoulos, J. Allison, Anal. Chem. 57 (1985) 1766. [25] J. Gale, R.A. Osteryoung, Inorg. Chem. 18 (1979) 1603.
Recycling of aluminum metal matrix composite using ionic liquids: Effect of process variables on current efficiency and deposit characteristics V. Kamavaram, D. Mantha, R.G. Reddy∗ Department of Metallurgical and Materials Engineering, Center for Green Manufacturing, The University of Alabama, Tuscaloosa, AL 35487, USA Received 30 August 2004; received in revised form 30 November 2004; accepted 4 December 2004 Available online 18 January 2005
Abstract Recycling of aluminum metal matrix composite via electrolysis in ionic liquids at low-temperature was investigated. The electrolytic melt comprised of 1-butyl-3-methylimidazolium chloride (BMIC) and anhydrous AlCl3 . Aluminum metal matrix composite (Duralcan® , Al-380, 20 vol.% SiC) was electrochemically dissolved at the anode, and pure aluminum (>98%) was deposited on a copper cathode. The influence of experimental parameters such as concentration of electrolyte and applied cell voltage on the efficiency of aluminum metal matrix composite recycling was studied at 103 ± 2 ◦ C. High applied voltages and concentration of AlCl3 yielded high current densities. Current densities obtained during this process were in the range of 200–500 A/m2 and current efficiencies in the range of 70–90%. The deposits were characterized by scanning electron microscope, X-ray diffractometer, mass spectrometer, and atomic absorption spectrophotometer. Characteristics of the deposited microstructure ranging from columnar to spherical were obtained. Energy consumption was in the range of 3.2–6.7 kWh/kg-Al for the experimental conditions studied. The optimum conditions obtained in the present investigation for maximum current efficiency and least energy consumption with uniform deposit microstructure were low applied voltage and intermediate electrolyte concentration. Low energy consumption and no emission of pollutants are the two main advantages of this process compared to the current recycling processes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Recycling; Aluminum metal matrix composite; Ionic liquids; 1-Butyl-3-methyl-imidazolium chloride
1. Introduction Metal matrix composites offer numerous beneficial properties such as high strength, stiffness, fatigue, and thermal properties. An enormous interest in the manufacture of metal matrix composites was triggered by their application in structural, automotive, and defense applications [1]. The advancement of metal matrix composites in the automotive market is still hampered by the low-volume usage of these materials due to their high cost in comparison with aluminum alloys. High demand and cost of production led to the recycling of these composites. Recycling, a significant factor in the supply of many of the metals used in daily life provides environmental ∗
Corresponding author. Tel.: +1 205 348 4246; fax: +1 205 348 2164. E-mail addresses: [email protected] (V. Kamavaram), [email protected] (R.G. Reddy). 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.12.002
benefits in terms of energy saving, reduced volumes of waste, and reduced pollutants emission [2]. Metal matrix composites are recycled by melting composite scrap in furnaces such as induction furnace, reverbatory melter, hearth furnace, and rotary barrel furnace. The scrap from aerospace and commercial applications is melted and cast into ingots. Only a slight change in the tensile properties of composite was observed after several recycling steps [3]. Salt fluxing technique is commonly used in the reclamation of ceramic-particulate-reinforced metal matrix composites [3,4]. This process is based on the principle of effective de-wetting of ceramic particles from aluminum matrix using molten salts. The molten salts are thermodynamically stable in the presence of liquid aluminum and can effectively remove ceramic particles by wetting them, but the salt itself is not wetted by the liquid aluminum. The salts used for separating reinforcement from aluminum matrix are a mixture
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
of halides of sodium and potassium, because surface energy of these salts is lower than that of aluminum [5]. This phenomenon occurs at high temperatures, normally above the melting temperature of aluminum. In the present study, aluminum metal matrix composite is reclaimed via electrorefining in ionic liquid at lowtemperature. Reddy and co-workers [6–9] have reported electrorefining of aluminum alloys using chloroaluminate ionic liquids, which yielded high-purity aluminum deposits with low energy consumption and no emission of pollutants. Ionic liquids are low melting salts with negligible vapor pressure, high electrical conductivity, and wide electrochemical window of about 4.0 V. These favorable properties render chloroaluminate ionic liquids as potential electrolytes for low-temperature aluminum production [10]. The concept of extraction and refining of aluminum using ionic liquids originated when ionic liquids were used for electrodeposition of metals [11–14]. Ionic liquids are non-volatile and recyclable which make them environmentally benign electrolytes for recycling metal matrix composites. Chloroaluminate ionic liquids are prepared by mixing alkyl-imidazolium chlorides with aluminum chlorides [15]. Chloroaluminate ionic liquids are liquids at roomtemperature and hence are also termed as room-temperature ionic liquids [16]. Here we take an example of 1-butyl-3methylimidazolium chloride (BMIC) and AlCl3 ionic liquid. In these melts several anionic species are often present in equilibrium, which depend on the ratio of the two components BMIC and AlCl3 given by Eq. (1): BMIC + AlCl3 ↔ [BMI]+ + [AlCl4 ]−
(1)
With an excess of the Lewis acid AlCl3 , additional anion species can be formed from further acid–base reactions given by Eqs. (2) and (3): [AlCl4 ]− + AlCl3 ↔ [Al2 Cl7 ]−
(2)
[Al2 Cl7 ]− + AlCl3 ↔ [Al3 Cl10 ]−
(3)
When BMIC is present in molar excess over AlCl3 , i.e., [AlCl3 :BMIC] < 1.0, the ionic liquid is termed as basic ionic liquid and only equilibrium equation (1) need to be considered. In an equimolar solution, i.e., [AlCl3 :BMIC] = 1.0, which is termed as neutral melt, AlCl4 − is the only anion present in the liquid. When AlCl3 is present in molar excess over BMIC, i.e., [AlCl3 :BMIC] > 1.0, the ionic liquid is termed as acidic ionic liquid and equilibria given by Eqs. (2) and (3) predominate in the solution [17]. This paper discusses the effect of process variables such as electrolyte concentration and applied voltage on current efficiency and deposit microstructure during the recycling of an aluminum metal matrix composite in a BMIC–AlCl3 ionic liquid electrolyte.
3287
2. Experimental procedure Experimental setup for the recycling of aluminum metal matrix composite in chloroaluminate ionic liquid electrolyte consisted of a 50 ml Pyrex® beaker fitted with a Teflon cap. Schematic of the experimental setup used in this study is shown in Fig. 1. Aluminum matrix composite, copper, and pure aluminum were used as anode, cathode, and reference electrodes, respectively. The composition of aluminum metal matrix composite (expressed in wt.%) used as anode in the present study is as follows: Al, 66.22; SiC, 23; Si, 5.78; Cu, 2.31; Fe, 1.05; Zn, 0.77; Mg, 0.38; and Ni, 0.10. Reference electrode was used to measure the electrode potentials of both anode and cathode individually, using a multimeter (Keithley Instruments Inc.® ). A constant potential was applied across the electrodes using a Kepco® power source. The experimental setup had provisions for introducing thermometer and inert gas into the electrolytic cell. The electrolyte was prepared by carefully mixing precise quantities of AlCl3 and BMIC in a dry 50 ml beaker. Anhydrous aluminum chloride (99.985% pure) obtained from Alfa Aesar® was used without further purification. BMIC was synthesized in our laboratory using 1-methylimidazole and 1-butylchloride [18]. Since the mixing process was exothermic, AlCl3 was added in small quantities to BMIC contained in the beaker. Components were weighed carefully to prepare an acidic ionic liquid composition with desired physical properties. Mixing and weighing of the components were done in a controlled atmosphere glove box filled with argon. The electrolyte was transferred to fume hood to conduct the experiment. The melt was heated to a predetermined experimental temperature before starting the electrolysis. For each experimental run the electrolyte was freshly prepared. Since the chloroaluminate ionic liquid electrolyte is moisture sensitive, argon was introduced into the cell at a flow rate 50 ml/min to maintain an inert atmosphere above the electrolyte throughout the experiment. The electrolyte was stirred at a constant speed using a magnetic stirrer and the
Fig. 1. Schematic of the experimental setup used for reclaiming aluminum from aluminum metal matrix composite anodes using BMIC–AlCl3 ionic liquids electrolyte.
3288
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
temperature of electrolyte was controlled by a hot plate. All the experiments were carried out at temperature 103 ± 2 ◦ C for about 120 min. Electrolyte composition defined in terms of concentration ratio (hereafter referred to as CR in the text) of AlCl3 and BMIC, in the range of 1.3–2.0, and applied cell voltage in the range of 1.0–1.8 V were the experimental parameters studied. After electrolysis, aluminum deposit obtained on the copper cathode was mechanically separated from the substrate and powdered for X-ray diffraction studies. X-ray diffraction (XRD) studies were performed on a Philips® PW3830 Xray diffractometer which uses monochromatic Cu K␣ radiation. Compositional analysis of the anodes and deposits was performed using a Spectrolab® , Model LVO-M3 emission spectrometer (ES) and Perkin-Elmer® 560 semi-automated atomic absorption spectrophotometer (AAS). Morphology of the deposits was studied using Philips® XL30 scanning electron microscope (SEM).
Fig. 2. Variation of average current density with time during electrolysis of aluminum metal matrix composite in BMIC–AlCl3 ionic liquid of concentration ratio, CR = 2.0 and temperature 103 ± 2 ◦ C at different applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V.
3. Results and discussion Applying potential between the aluminum matrix composite anode and copper cathode, aluminum from the anode reacts with AlCl4 − ions present in melt to produce Al2 Cl7 − ions shown by the anodic reaction (Eq. (4)): At anode :
Al(anode) + 7AlCl4 − → 4Al2 Cl7 − + 3e−
(4)
Al2 Cl7 − ions traverse to the cathode either by diffusion or convection and get reduced to produce aluminum deposit. Cathodic reaction, which leads to aluminum deposition on copper electrode is shown in Eq. (5): At cathode :
4Al2 Cl7 − + 3e− → Al(cathode) + 7AlCl4 − (5)
At any given experimental condition, the total current output was noted at regular intervals and converted into current density by dividing with the aluminum deposited area of cathode. Fig. 2 shows the variation of current density as a function of time at different applied voltages at 103 ± 2 ◦ C and CR = 2.0 of the electrolyte. Similar behavior of current density with time was observed in electrolytes with different concentration ratios studied in the present research. Typical current density versus time curve is observed at all the applied voltages. Steep rise in current density for a short period of 10–15 min (region A), followed by a brief steady-state of 15–20 min (region B) and then a shallow decrease with time (region C) was observed for applied voltages of 1.3, 1.5, and 1.8 V. However, at applied voltage of 1.0 V, region A was observed upto 30 min, steady-state (region B) was observed for 60 min followed by a decrease in current density with time. The steep increase in current density in region A is due to activation polarization at cathode. It is confirmed by plotting the cathode potentials with current density during the initial period. Fig. 3 shows cathodic polarization plots for the curves during the period of activation polarization marked by
region A in Fig. 2. A linear relation between current density and cathode potential is an indication of activation polarization [19]. Approximate linear behavior was observed in all the cases and the slopes of these curves tend to decrease with increasing applied voltage. Generally the linear relation between over-voltage and current density is represented by Tafel equation, Eq. (6) [19]: η = a − b log(i)
(6)
a=
2.303RT log(i0 ) (1 − α)F
(7)
b=
2.303RT (1 − α)zF
(8)
Fig. 3. Cathodic polarization curve showing the variation of current density as a function of cathode potential in the region A shown in Fig. 2 for the following applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V, and at CR = 2.0 and temperature 103 ± 2 ◦ C.
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3289
Fig. 4. Optical micrographs of cross-sectional view of aluminum metal matrix composite anode (a) before and (b) after electrorefining process.
In Eq. (6), η is the cathode over-voltage, a and b are Tafel constants and i the current density. Expressions to evaluate the Tafel constants, a and b, in terms of charge transfer coefficient α, electrons transferred z, Faraday constant F and exchange current density i0 are given in Eqs. (7) and (8). The charge transfer coefficient α can be determined using Eq. (8) from the slopes b of plots in Fig. 3. Exchange current density i0 can be calculated from the intercepts of plots in Fig. 3 at η = 0. The charge transfer coefficient α obtained for the plots shown in Fig. 2 ranged from 0.79 to 0.9 indicating that the kinetics of aluminum deposition is a quasireversible process [19]. It has already been reported that aluminum deposition on copper in BMIC + AlCl3 melts is quasi-reversible process [20]. Aluminum deposition in AlCl3 –trimethylphenylammonium chloride [21], AlCl3 –1methyl-3-ethylimidazolium chloride [22] and AlCl3 –nbutylpyridinium chloride [23] melts was observed to be quasi-reversible. In region B, current density attains a steady-state as seen from Fig. 2. Steady-state region was smaller at higher voltages than at low voltages. After the steady-state (region B) the current density decreases steadily in region C. With the progress of time a thin layer is formed at anode, consisting of largely silicon and other impurities. Formation of the layer causes aluminum to diffuse through the layer to form Al2 Cl7 − ions. Hence concentration polarization at anode may be responsible for the decrease in current density in region C. The amount of metal deposited was determined by the weight gain W of cathode. Cathode current efficiency ηeff , expressed as percentage, is defined as the ratio of the actual amount of metal deposited to that expected theoretically. The theoretical amount of metal deposited for a fixed quantity of electricity passed through an electrochemical cell can be obtained using the Faraday law. The energy consumed (E) during the electrorefining was determined using Eq. (9): Q E=V ηeff
(9)
where V is the applied cell voltage, ηeff the cathode current efficiency, and Q the theoretical charge required for depositing a fixed amount of material according to the Faraday law.
The energy consumption for electrorefining aluminum metal matrix composite under the present experimental range of conditions was in the range of 3.2–6.7 kWh/kg-Al. Fig. 4(a) and (b) shows the cross-sectional optical micrographs of aluminum matrix composite anode with SiC before and after electrolysis. Fig. 4(a) shows that SiC particles are discontinuously reinforced in aluminum alloy matrix with a particle size of approximately 10–15 m. Fig. 4(b) shows the optical micrograph after electrorefining where SiC particles and other impurities remain with the anode. The composition of deposited aluminum was determined using atomic absorption spectrophotometer and emission spectrometer is shown in Table 1. A high-purity aluminum deposit was obtained. XRD patterns of anode, deposited aluminium, and anode residue are shown in Fig. 5. XRD pattern of deposit shows the purity of aluminum in the deposit and that of the anode after refining reveals relative increase in the intensities of other impurities. 3.1. Effect of cell voltage The effect of applied cell voltage on current density and cathode current efficiency of the recycling process was studied. The applied voltage was in the range of 1.0–1.8 V and temperature was 103 ± 2 ◦ C. Fig. 6 shows the variation of average current density with applied cell voltages at different concentration ratios (1.3–2.0). It can be seen that the current density increases approximately linearly with an increase in voltage from 1.0 to 1.8 V. Increase in voltage increases rate of reaction at cathode thereby increasing the current density.
Table 1 Composition of aluminum deposited on copper cathode Element
Weight (%)
Al Fe Zn Mg Cu Ni
98.15 0.04 0.31 0.13 1.26 0.10
3290
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 5. XRD pattern of anode before electrolysis, aluminum deposited on cathode and anode residue.
The variation of cathodic current efficiency with applied cell voltage at a range of concentration ratios (1.3–2.0) is shown in Fig. 7. Current efficiencies under all experimental conditions were found to fall within a range of 70–90%. Intermediate voltages (1.3 and 1.5 V) yield low cathode current efficiencies as compared with lowest (1.0 V) and highest (1.8 V) voltages used in this study. Current efficiency calculation is based on the weight difference of cathode before and after electrolysis and the theoretical amount that can deposit under the given conditions. As the applied voltage increases the total charge passed through the cell increases, hence the theoretical amount deposited also increases. However, the actual weight deposited does not follow a similar trend as that of current density with respect to applied voltage due to the current losses (polarization effects) involved in electrolysis processes. The calculated current efficiencies directly
Fig. 6. Variation of average current density with the applied cell voltage, at concentration ratios: (䊉) 1.3, () 1.5, (+) 1.8, and () 2.0 and at temperature of 103 ± 2 ◦ C.
Fig. 7. Variation of cathode current efficiency with applied cell voltage at concentration ratios: (䊉) 1.3, () 1.5, (+) 1.8, and () 2.0 and at temperature of 103 ± 2 ◦ C.
relate to the net effect of driving force and resistance to aluminum deposition. Therefore, lower current efficiencies at intermediate potentials (Fig. 7) can be attributed to the above phenomena involved in the electrodeposition process. Deposits were characterized for microstructure by scanning electron microscopy (SEM). Fig. 8(a)–(d) depicts the SEM images of the deposits obtained in an electrolyte of CR = 1.3 as a function of applied voltage. Fig. 8(a) and (b) displays almost similar microstructure except that at higher voltages (1.3 V) growth of particles is seen. Fig. 8(c) shows that the microstructure has a more uniform deposition of particles with an average size range of 10–20 m. Fig. 8(d) depicts the microstructure with a more open deposition where cluster of particles is seen with lot of porosity. Cluster size varied from 20 to 100 m. From the analysis of the deposit microstructure at concentration ratio CR = 1.3, it appears that the morphology of deposits was relatively independent of applied voltage except that at high voltage the spherical particles tend to agglomerate. However, deposit microstructures obtained at highest concentration ratio (CR = 2.0) of the electrolyte revealed interesting features as a function of applied voltage. Fig. 9(a)–(d) depicts the SEM images of aluminum deposits at different applied voltages showing the varied morphological patterns obtained in this study. Fig. 9(a) refers to the deposit microstructure obtained at applied voltage 1.0 which shows a dendritelike deposition with dendrites of length 100 m. Fig. 9(b) and (c) refers to deposit microstructures obtained at applied voltages of 1.3 and 1.5 V, respectively, which reveal a uniform deposition of fine particles in the range of 5–25 m. However, at applied voltage of 1.5 V the microstructure appears to be an agglomeration of particles into small clusters with an average size 20 m as can be seen in Fig. 9(c). At the highest applied voltage (1.8 V) of this study a compact layer of deposit microstructure was observed as can be seen in Fig. 9(d). It can be seen from the above microstructures that at CR = 2.0, the
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3291
Fig. 8. SEM micrographs showing the effect of voltage on the microstructure of the aluminum deposits: (a) 1.0 V, (b) 1.3 V, (c) 1.5 V, and (d) 1.8 V obtained at constant CR = 1.3 and temperature 103 ± 2 ◦ C.
Fig. 9. SEM micrographs showing the effect of voltage on the microstructure of the aluminum deposits: (a) 1.0 V, (b) 1.3 V, (c) 1.5 V, and (d) 1.8 V obtained at constant CR = 2.0 and temperature 103 ± 2 ◦ C.
3292
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 10. Variation of average current density with the concentration ratio (CR = AlCl3 :BMIC) of the electrolyte, at applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V and () 1.8 V, and T = 103 ± 2 ◦ C.
Fig. 11. Variation of cathode current efficiency with concentration ratio (AlCl3 :BMIC) at applied voltages (䊉) 1.0 V, () 1.3 V, (+) 1.5 V, and () 1.8 V and at temperature 103 ± 2 ◦ C.
type of deposit microstructure changed rapidly as a function of applied voltage. Unlike the case of low concentration ratio (CR = 1.3) where the deposit microstructure is relatively independent of applied voltage, at CR = 2.0, the influence of applied voltage is clearly seen on the deposit microstructure. The combined influence of increased acidity of melts and applied voltage seems to have led to a nucleation-controlled microstructure.
efficiency with concentration ratio (CR) is due to the increase in amount of aluminum deposited at the cathode. As mentioned earlier, concentration ratio of the electrolyte determines the type of anions present in solution. It can be seen from cathodic reaction equation (5) and anodic reaction equation (4) that the concentrations of Al2 Cl7 − and AlCl4 − affect their rate. It was observed that as concentration ratio of the chloroaluminate ionic liquid increases from 1.3 to 2.0 the concentration of AlCl4 − decreases and that of Al2 Cl7 − increases [14]. It was reported from mass spectrometer analysis [24] that there exists an ion-pair interaction between the chloroaluminate anion AlCl4 − and the organic cation R+ but not between Al2 Cl7 − and R+ . In neutral melts (1:1 molar ratio of AlCl3 :RCl) the ions in the solution may be visualized as follows:
3.2. Effect of concentration of electrolyte Concentration of electrolyte is an important parameter that influences the reactions taking place at the electrodes. Concentration ratio of electrolyte used in the present study was in the acidic range, i.e., the concentration ratio or molar ratio of AlCl3 to BMIC varied from 1.3 to 2.0. Fig. 10 shows the variation of average current density as a function of concentration ratio of electrolyte at a range of applied voltages 1.0–1.8 V and temperature of 103 ± 2 ◦ C. Current density was found to increase slightly with increasing concentration ratio (1.3–2.0) at all the voltages shown in Fig. 10. The slight increase in the current density with concentration ratio is due to the increase in the number of ions of the electrolyte. Fig. 11 shows the effect of concentration ratio of the electrolyte on cathode current efficiency at a range of applied voltages 1.0–1.8 V and temperature of 103 ± 2 ◦ C. Although the current efficiencies at all the voltages were in the range of 70–90%, there is a distinct dip in current efficiencies at concentration ratio of CR = 2.0. The variation in current efficiency is essentially affected by the amount deposited at the cathode and the theoretical amount of aluminum that can get deposited for a given condition. Since the theoretical amount deposited is related to the total charge passed through the cell, which is fixed at a constant applied potential, the theoretical amount of aluminum deposited does not change with concentration ratio of the electrolyte. Increase in cathode current
AlCl4 − · · · R+ · · · AlCl4 − · · · R+ · · · AlCl4 − which means that there is no free R+ and Al2 Cl7 − in the neutral melt. However, in basic melts (molar ratio of AlCl3 :RCl < 1:1 and R+ is in excess) aluminum was found to be oxidized spontaneously [23,25]. A similar reaction can be expected in acidic melts when free organic cation R+ is present to serve as electron acceptor. Thus, as the acidity of the melts is increased, the concentration of free R+ increases and a higher oxidation of aluminum can be expected. This clearly explains the decrease in cathode current efficiency with increasing concentration ratio of electrolyte. Fig. 12(a)–(d) shows the effect of concentration ratio on the morphology of aluminum deposits at a fixed applied voltage of 1.0 V and temperature 103 ± 2 ◦ C. Fig. 12(a) depicts the microstructure of deposit obtained at the lowest applied voltage (1.0 V) and at the lowest concentration ratio (CR = 1.3) of the electrolyte used in this study. A uniform deposit of spherical particles in a close size range of 20–50 m can be observed where some amount of agglomeration of particles appears to have taken place. Fig. 12(b) shows the
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
3293
Fig. 12. SEM micrographs showing the effect of concentration ratio of electrolyte (a) 1.3, (b) 1.5, (c) 1.8, and (d) 2.0 on the microstructure of the aluminum deposits obtained at constant voltage 1.0 V and temperature 103 ± 2 ◦ C.
microstructure of the deposit obtained in an electrolyte of CR = 1.5 at an applied voltage of 1.0 V. The deposit morphology reveals a non-uniform deposit containing non-spherical particles formed as small clusters with an average cluster size of 30 m. As the concentration ratio of the electrolyte increases further to 1.8, the deposit obtained was similar to that obtained at the lowest concentration ratio (CR = 1.3) but with finer particles (average size of 20 m). At the highest concentration ratio (CR = 2.0) of the electrolyte the deposit depicted dendritic-like microstructure. It can be seen that, at an applied voltage of 1.0 V the aluminum deposit microstructure has strong dependence on the electrolyte composition. The effect of increase in the concentration ratio (CR) of the electrolyte, at a fixed applied voltage of 1.8 V, on the morphology of aluminum deposit is depicted in Fig. 13(a)–(d). Fig. 13(a) shows SEM image of the deposit microstructure at CR = 1.3 which reveals a uniform distribution of clusters of fine particles with an average size of 15 m. At a slightly higher concentration ratio (CR = 1.5), the deposit microstructure changes to discreet clusters of small particles with an average particle size of 5 m. The cluster size is bigger than in the previous case. Fig. 13(c) shows a similar microstructure as that of Fig. 13(b) but with a slight increase in the average particle size. At CR = 2.0 of the electrolyte, SEM image of the deposit microstructure shown in Fig. 13(d) depicts a dense compact layer of aluminum. As the concentration ratio of electrolyte increased, the morphology of aluminum deposits changed from clusters of large spherical particles
to a dense compact layer of small particles. A nucleationcontrolled deposit growth is evident from the analysis of deposit microstructures.
3.3. Effect of anode composition Current efficiencies of electrorefining process depend not only on the process parameters such as concentration of the electrolyte, applied cell voltage, temperature, but also on the composition of the electrode that is being electrorefined. Aluminum metal matrix composite studied in this research contained 23 wt.% SiC among other metallic impurities such as Si, Cu, Zn, Fe, etc. The effect of SiC content of the anode on current efficiencies at different experimental conditions can be understood by the comparison of the results with those obtained for an aluminum alloy (A360) in an earlier study [7]. Current efficiencies of electrorefining of A360 alloy, where the major metallic impurities were the same as those in the present aluminum metal matrix composite with different relative amounts of individual metal, were in the range of 90–99%. Comparing the results of electrorefining for the two anodes (aluminum metal matrix composite and A360) it can be said that lower current efficiencies in the present case can be attributed partly to the presence of SiC in the anode. Additionally, since the current efficiency is calculated based exclusively on the weight difference of the cathode before and after experiment, experimental errors could play a vital role in the current efficiency calculations.
3294
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295
Fig. 13. SEM micrographs showing the effect of concentration ratio of electrolyte (a) 1.3, (b) 1.5, (c) 1.8, and (d) 2.0 on the microstructure of the aluminum deposits obtained at constant voltage 1.8 V and temperature 103 ± 2 ◦ C.
4. Conclusions Aluminum was successfully reclaimed from aluminum metal matrix composite by electrolysis in ionic liquids at 103 ± 2 ◦ C. High-purity aluminum (>98%) deposits were obtained. High current densities were obtained at high applied voltages and concentration ratios of electrolyte. Current densities in the range 200–500 A/m2 and cathode current efficiencies in the range 70–90% were obtained. From the present study, it appears that high applied cell voltage and intermediate concentration ratios are optimum conditions for obtaining high cathode current efficiencies. From the microstructure analysis, within the range of experimental parameters of this study, it appears that at low concentration ratios of electrolyte the morphology of aluminum deposit is relatively independent of applied voltage. However, concentration ratio had a significant influence on the deposit microstructures at any fixed applied voltage. Energy consumption for the present process was estimated to be in the range of 3.2–6.7 kWh/kgAl.
Acknowledgements The authors are pleased to acknowledge the financial support for this research by US Department of Energy (DOE) under Award number DE-FC07-02ID14397, National Science Foundation (NSF) under grant number NSF-EPS-9977239, and the University of Alabama, Tuscaloosa (USA). We are
also thankful to Mr. Ravinder N. Reddy for his assistance in synthesizing and characterizing the ionic liquids used in this research.
References [1] S. Rawal, JOM 4 (2001) 14. [2] J.F. Papp, U.S. Geological Survey Minerals Yearbook, U.S. Geological Survey, 1999, p. 62. [3] D.M. Schuster, et al., JOM 45 (1993) 26. [4] Y. Nishida, N. Izawa, Y. Kuramasu, Met. Mater. Trans. A 30 (1999) 839. [5] J.R. Davis, et al., Metals Handbook, vol. 2, American Society for Metals, Ohio, 1991. [6] B. Wu, R.G. Reddy, R.D. Rogers, in: D.L. Stewart Jr., J.C. Daley, R.L. Stephens (Eds.), Proceedings of the Fourth International Symposium on Recycling of Metals and Engineered Materials, TMS, Warrendale, PA, 2000, p. 845. [7] V. Kamavaram, D. Mantha, R.G. Reddy, J. Min. Metall. B 39 (1–2) (2003) 43. [8] V. Kamavaram, R.G. Reddy, in: B. Bjorkman, C. Samuelsson, J.O. Wikstrom (Eds.), Recycling and Waste Treatment in Mineral and Metal Processing: Technical and Economic Aspects, Lulea, Sweden, The Mineral, Metals & Materials Society, Warrendale, PA, 2002, p. 517. [9] V. Kamavaram, R.G. Reddy, in: D. Hui (Ed.), Proceedings of the Ninth Annual International Conference on Composite Engineering, New Orleans, LA, 2002, p. 359. [10] M. Zhang, V. Kamavaram, R.G. Reddy, JOM 11 (2003) 54. [11] G.A. Capuano, W.G. Davenport, J. Electrochem. Soc. 118 (1971) 1688. [12] T.P. Wier, F.H. Hurley, J. Electrochem. Soc. 98 (1951) 203.
V. Kamavaram et al. / Electrochimica Acta 50 (2005) 3286–3295 [13] Q. Liao, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 144 (1997) 936. [14] C.J. Smit, T.P.J. Peters, in: R.B. Miller, W.S. Peterson (Eds.), Light Metals, TMS, Warrendale, PA, 1986, p. 253. [15] Z.J. Karpinski, R.A. Osteryoung, Inorg. Chem. 23 (1984) 1491. [16] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co., KGaA, Weinheim, 2002. [17] J.S. Wilkes, J.A. Levisky, C.L. Hussey, Inorg. Chem. 21 (1982) 1263. [18] V. Kamavaram, Ph.D. Dissertation, The University of Alabama, Tuscaloosa, AL, USA, 2004.
3295
[19] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2000. [20] V. Kamavaram, R.G. Reddy, in: W.A. Schneider (Ed.), Light Metals, TMS, Warrendale, PA, 2002, p. 253. [21] Y. Zhao, T.J. Vandernoot, Electrochim. Acta 42 (1997) 1639. [22] P.K. Lai, M. Skyllas-kazacos, J. Electrochem. Soc. 248 (1988) 431. [23] P.K. Lai, M. Skyllas-kazacos, Electrochim. Acta 32 (1987) 1443. [24] B.L. Ackermann, A. Tsarbopoulos, J. Allison, Anal. Chem. 57 (1985) 1766. [25] J. Gale, R.A. Osteryoung, Inorg. Chem. 18 (1979) 1603.