Manufacture of Al-Zn-Mg Alloys Using Spent Alkaline Batteries and Cans

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 2 (2015) 4971 – 4977

Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015

Manufacture of Al-Zn-Mg alloys using spent alkaline batteries and cans R. Ochoaa, A. Floresa*, J. Torresa, J. Escobedoa a

Cinvestav-Saltillo, Industria Metalurgica 1062, Ramos Arizpe 25900,Mexico

Abstract

Aluminum and its alloys are important materials in contemporary life, due to the high specific strength, corrosion resistance, light weight and recyclability. Alkaline batteries are the highest consumption and each one of the spend batteries get 30% of high purity ZnO. These two materials can be processed through the metallothermic reduction of oxides. This paper analyzes the effect of temperature (1023, 1048 and 1073 K), treatment time (0-80 min) to determine the variation of the concentration of Zn in the alloy. The results obtained showed that it is possible the preparation of Al-Zn-Mg alloys containing of up to 2.47 wt-% Zn. © 2014 Elsevier ElsevierLtd. Ltd.All Allrights rights reserved. © 2015 reserved. Selection andPeer-review Peer-review under responsibility of Conference Committee of Aluminium Two Thousand World Congress and Selection and under responsibility of Conference Committee of Aluminium Two Thousand World Congress and International Conference onConference Extrusion and ICEBBenchmark 2015 International onBenchmark Extrusion and ICEB 2015. Keywords: Recycling; Batteries; Aluminum Cans, Metallothermic reduction; Al-Zn-Mg Alloys.

1. Introduction Batteries are a great source of environmental pollution due to the toxic elements they contain (i.e. Hg, Cd, Mn and salts of Ag, Zn, Li), and the high amount of humans consumption. Today it is seeking alternatives for handling these wastes. It is well documented that for every discharged alkaline battery, 40% of Mn 2O3 and 30% of high purity ZnO are obtained [1]. The zinc oxide has several applications, for example, is used in fertilizers, anti-fouling paints,

* A. Flores. Tel.: +1-528-444-389-600; fax:+1-528-444-389-650. E-mail address :[email protected]

2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee of Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015 doi:10.1016/j.matpr.2015.10.076

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cosmetics, lubricants and colorants. On the other hand, 50% of aluminum alloys are made of alloys of the 3000, 5000, 6000 and 7000 series. Therefore, the aim is to produce alloys of the Al-Zn-Mg type (7075) by aluminothermic reduction of ZnO, because it is thermodynamically feasible according to Eq. 1:

2 Al  Mg  4ZnO MgAl2O4

0 'G1073 K

209.801kcal

(1)

The most important alloy of this group is the 7075 (Al-5.5Zn-2.5Mg-1.5Cu, in wt-%) and is used primarily for corrosion protection and high mechanical resistance, being applied in aerospace and automotive industries, molds for plastic injection, blowing, drawing and extrusion dies, etc. [2]. 1.1 Aluminothermic reduction The aluminothermic reduction process refers to the extraction of metals by reducing their metal oxides by aluminum [3-5]. Therefore, the production of alloys using ZnO powders is quite feasible. The aluminothermic reduction is dependent on surface properties (wettability), because this process involves interaction between the solid oxide and molten metal at a specific interface. These conditions indicate that without a good wettability, the velocities of chemical reaction and transport are largely reduced. In an extreme case where the wettability between the solid oxide and the molten metal does not exist, the reaction simply will not occur [6]. Fig. 1 shows a schematic representation in which the zinc oxide layer is reduced by molten aluminum.

Fig. 1 Diagram of the mechanism of zinc oxide reduction by molten aluminum.

1.2 Influence of magnesium in aluminothermic reduction. Magnesium has an important effect on the recovery of zinc metal in the melt, due to its natural reactivity. Moreover, also due to the lower surface tension value of this element in comparison to that of pure aluminum (γAl = 914 dyn/cm, and γMg = 559 dyn/cm). Because of this, the addition of magnesium reduces the surface tension of molten aluminum which leads to improve the wettability between the solid and liquid, thereby increasing the kinetics of the reactions taking place at the solid-liquid interface. That is, without a good wettability, the chemical reaction rate and transport of reagents are reduced. From the thermodynamic point of view, dissolved magnesium in the fluid also has high affinity for oxygen, as it can be seen by the Gibbs free energy values of Eq. 2:

2Mg  O2

2MgO

0 'G1073 K

231kcal

(2)

According to thermodynamics (Ellingham diagrams), magnesium dissolved in the molten aluminum reduces the ZnO by metallothermic reaction, as shown in equation 1 [7,8]. Magnesium dissolved in the bath is due to the reuse of aluminum beverage cans. Also the thermodynamic conditions indicate that increasing the concentration of

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magnesium increases the reactivity of the aluminothermic process. However, with the use of cans increases the content of other elements, for example, the iron content of 0.4 to 0.8 wt-%, which contributes to the highly faceted plaque formation of the intermetallic compound β-Al6FeSi type, which affects the mechanical properties. But there are experimental studies showing that one can alter the morphology of the intermetallic β-Al8Fe2Si form other compounds such as α-less harmful or α-Al15(Fe, Mn)3Si2 with the addition of alloying elements such as Mn, Mg, Sr, Cr , Be, Ni, V and TiB2. Therefore, this study was conducted to study the ZnO reduction process in molten aluminum alloys produced from aluminum cans scrap, with the aim to introduce a method for the production of Al-Zn-Mg alloys using alkaline spend batteries. Helping to minimize problems of environmental pollution and the effects on humans, because there is no control over the disposal of discharged batteries. In addition, reducing the costs of raw materials to prepare these alloys. 2. Experimental procedure Electrodes of discharged alkaline batteries were obtained by recycling a quantity of batteries of different brands. These batteries were dismantled by hand, using a copper cutter, making a cut on the top to remove the cap on one end and remove the cathode (Mn2O3). Then, it was removed the tissue in the anode which contains zinc oxide and at the bottom of the stack to release the vacuum seal which subjected the electrodes, removing the bag containing the ZnO. Treatment consisted of washing the powder with de-ionized water in a rotating reactor for one hour, then filtering and drying in an oven at 373 K for 5 hours for removing moisture, and finally to break ground by biker lumps formed during this treatment. The product was characterized by X-ray diffraction (XRD) in order to identify the species present in the dust, using samples of 3g of powder with a size of 100 mesh. Scanning electron microscopy (SEM) was also used to determine the morphology of ZnO particles. Once characterized the ZnO powder, it were carried out ZnO aluminothermic reduction trials, for which an induction furnace with silicon carbide crucible with a capacity of 13 kilograms of molten aluminum was used. This furnace is equipped with an automatic temperature control system, as well as a stirring system consisting of an alumina refractory impeller and a lid where insulation was provided by introducing an inert gas (ultra high purity Ar). The initial charge was 10 kg (20% Al of commercial purity and 80% of aluminum cans), using a common practice for the melting of the alloys. Once molten the alloy, the temperature of treatment was reached and maintained during the experiment (1023, 1048 and 1073 K). Later on, it was added in a single event 100 g. of ZnO powder, at time intervals of 20 minutes (up to complete 80 minutes). Agitation speed was fixed on 180 rpm, in order to reduce splashing of the surface of the bath. The aggregated amount of ZnO was determined according to the stoichiometry of reaction (1) and related to the amount of molten metal. Samples from the melt were taken every 10 minutes and poured into a metal mold. The solidified samples were chemically analyzed by spark emission spectrometry technique to determine the contents of Zn and Mg. Part of the samples were prepared metallographycally to be observed in both the scanning electron microscope (SEM) and optical microscope. The slag produced in every experiment was also analyzed by x ray diffraction to determine the species formed during the reduction process of zinc oxide. 3. Results and Discussion 3.1 Alkaline spend batteries Table 1 shows the average chemical composition of the anode of spends alkaline batteries after washing treatment. Fig. 2 shows the diffraction pattern of the material obtained from the anodes after washing treatment, where it can be seen that this is composed mainly of ZnO.

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Table 1. Average chemical composition of spends alkaline batteries after washing treatment.

Element

C

Mn

Fe

Si

Zn

K

Na

Pb

Al

Cd

wt-%

0.61

0.015

0.020

<0.01

97.00

0.11

0.03

<0.001

<0.05

<0.0002

Fig. 2. X-Ray diffraction pattern of the material of the anode after washing (ZnO).

Additionally, the material from the anodes after washing was examined by scanning electron microscopy (SEM). The micrograph of Fig. 3 shows ZnO particles, having bar-type morphology, and the corresponding EDS spectrum.

Fig. 3. SEM micrograph of (a) ZnO powder particles; (b) EDS spectrum corresponding to the ZnO particles.

3.2 Aluminothermic reduction of ZnO from spend alkaline batteries Once it was determined that zinc oxide batteries obtained has high purity, it was continued to the preparation of the Al-Zn-Mg alloys, based on the reduction of ZnO by both aluminum and magnesium contained in aluminum cans. Table 2 shows the chemical composition of the base alloy (initial), and (final) shows the result of metallothermic reduction treatment at 1073 K, after 80 minutes of treatment. Fig. 4 shows the effect of temperature on Zn concentration increase in molten aluminum alloy, as a function of reaction time. The results show that the

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ZnO is reduced by molten aluminum alloy. As can be expected for typical diffusion governed reactions, at 1073 K, it is achieved the highest increase in zinc concentration (2.47 wt-%). The magnesium content also have a significant effect in reducing zinc oxide during the process, as it can be seen in Fig. 5 which shows that the concentration of magnesium decreases with time, at the indicated temperatures. It is believed that this effect is due to the fact that magnesium lowers the surface tension of the molten bath. Table 2. Chemical composition of the starting alloy (initial) and the composition of the alloy after 80 minutes of treatment at 1073 K (final).

Element wt-% Initial Final

Al

Si

Fe

Cu

Mn

Mg

Zn

Ti

0.071

0.329

0.084

0.453

0.920

0.061

------

97.94

0.057

.0361

0.094

.0477

0.477

2.47

------

97.89

Fig. 4. Effect of temperature on zinc concentration in molten aluminum alloy as a function of time, at the indicated temperatures.

Fig. 5. Magnesium content decrease during the reduction of ZnO powders by molten aluminum metal as a function of time, at the indicated temperatures.

These results shows that there is the possibility of manufacturing Al-Zn-Mg alloys with the reuse of materials such as beverage cans and zinc oxide obtained from the alkaline battery, using the thermite method. On the other hand, Figure 6 shows a SEM micrograph of the microstructure obtained after 80 minutes of treatment at 1073 K, and the corresponding EDS spectrum. Apart of the Al6(Fe,Mn) intermetallic particles, common in the alloys produced, a rich Zn matrix is observed. Therefore, it can be deduced that the zinc incorporated remains upon solidification in the solid solution. Finally, Figure 7 shows the X-ray diffraction pattern of a typical slag obtained from the test at 1073 K, showing that the slag contains MgO, MgAl2O4, ZnO and aluminum. Al comes from droplets trapped during sampling.

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Fig. 6. (a) SEM micrograph of the Al-Zn-Mg alloy; (b) EDS spectrum corresponding to the matrix and of the intermetallic Al6(Fe,Mn).

The presence of ZnO is attributed to powder particles that did not react during the process. MgAl2O4 and MgO are the reaction products expected, according to Eq. 1 and the following additional Eqs. 3 and 4:

2Zn  Mg Zn  MgO

0 'G1073 K

58.626kcal

(3)

2Al2O3  MgO MgAl2O4

0 'G1073 K

3.292kcal

(4)

These equations indicate that ZnO is also reduced by the dissolved Mg in the alloy. It is worth mentioning that the reaction products involved in the metallothermic reduction process causes the formation of spinel (MgAl2O4).

Fig. 7. X-ray diffraction pattern of the slag obtained at 1073 K.

4. Conclusions One of the important factors for the realization of this project was the use of recycling materials, such as spent alkaline batteries and aluminum cans, for the preparation of Al-Zn-Mg alloys by the method of metallothermic reduction of oxides. The results obtained after performing investigation trials about the metallothermic reduction of ZnO by Al-Mg molten alloys shown that it is possible the preparation of Al-Zn-Mg alloys containing of up to 2.47 wt-% Zn, at 1073 K, and of up to 0.47 wt-% Mg. From analysis of slags and samples taken from the molten bath as a function of temperature and treatment time, the sequence of reaction was determined. It can be concluded that magnesium plays an important role on reaction kinetics, as the formation of MgO and MgAl 2O4 as reaction products was determined.

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Acknowledgments Authors acknowledge the support provided for the development of this research to CINVESTAV-IPN, as well as to CONACYT, project CB 81251. References [1] [2] [3] [4] [5]

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