Author's Accepted Manuscript

MnZn ferrite synthesized by sol–gel auto-combustion and microwave digestion routes usingspent alkaline batteries

Li Yang, Guoxi Xi, Jinjin Liu

PII: S0272-8842(14)01752-0DOI: http://dx.doi.org/10.1016/j.ceramint.2014.11.019Reference: CERI9459

To appear in: Ceramics International

Received date: 4 October 2014Revised date: 1 November 2014Accepted date: 3 November 2014

Cite this article as: Li Yang, Guoxi Xi, Jinjin Liu, MnZn ferrite synthesized by sol–gelauto-combustion and microwave digestion routes using spent alkaline batteries,Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.11.019

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MnZn ferrite synthesized by sol–gel auto-combustion and

microwave digestion routes using spent alkaline batteries

Li Yang a, b, Guoxi Xi a*, Jinjin Liu b

aSchool of Environment, Henan Normal University, Key Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education,

Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan

453007, P. R. China

bDepartment of Experimental Center, Henan Institute of Science and Technology, Xinxiang Henan 453003, P. R. China

Xi Guoxi e-mail: syzxhome@163.com

Tel: 86-373-3325971

Fax: 86-373-3040015

Li Yang e-mail: manyouhome@aliyun.com

Jinjin Liu e-mail: 103093563@qq.com

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Abstract The combination of a sol–gel auto-combustion method and a microwave

digestion method was used to synthesize nanocrystalline MnZn ferrite powders using

spent alkaline batteries. The overall process involved four steps: dissolution of spent

batteries; the sol–gel formation; MnZn ferrite precursor powder formation; MnZn

ferrite powder formation. The products were characterized for phase composition,

morphology, and magnetic properties by inductively coupled plasma optical

emission spectroscopy (ICP-OES), fourier transform infrared spectroscopy (FT-IR),

thermogravimetric analysis/differential scanning calorimetry (TG/DSC), X-ray

diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy

(SEM-EDS), and vibrating sample magnetometer (VSM) techniques. IR spectra and

DTA/DSC studies revealed that the combustion process is an oxidation–reduction

reaction in which the NO3− ion is the oxidant and the citric acid is the reductant.

Nanocrystalline MnZn ferrite powders were successfully prepared by a microwave

digestion method using MnZn ferrite precursor powders at 120 ℃ for 15 min. The

results obtained showed the formation of single-phase MnZn ferrite powders with an

average particle size of about 50 nm. The results further revealed that microwave

digestion conditions had a greater influence on the magnetic properties of the MnZn

ferrite powders than the sol–gel auto combustion process alone.

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Keywords Nanocrystalline · Spent batteries · Sol-gel auto-combustion · Microwave

digestion · MnZn ferrite powders

1. Introduction

Compared with zinc–carbon batteries, alkaline batteries have a higher energy

density and longer shelf-life, with steady-state voltage and good low-temperature

performance [1-2]. Alkaline batteries are used in many household items such as MP3

players, digital cameras, and toys. However, improper disposal of these spent batteries

will lead to serious environmental problems in the future because they usually contain

heavy metals, such as Mn, Zn, and Fe.

At present, numerous studies have been carried out on recycling spent alkaline

batteries, such as chemical precipitation [3], ion exchange [4], leaching and extraction

[5, 6], and electrochemical methods [7], etc., which can be summarized into two

categories: pyrometallurgical and hydrometallurgical processes [8]. Pyrometallurgical

processes are neithor easy to control nor flexible, and they are high energy-consumers.

Hydrometallurgical processes, similar to those in the mining industry, have proven to

be quite expensive or inefficient [9]. Therefore, environmental pollution and

economic costs must be considered for the recycling of spent batteries. In this regard,

a microwave-assisted technique seems to be particularly promising, and it has recently

also been applied to the synthesis of ferrite nanoparticles [10-11].

MnZn ferrites are the most common soft ferrites. They have been widely applied in

the field of electronics as deflection yoke rings, antenna rods, electromagnetic

interference suppressors, core materials for electronic transformers, etc [12-16].

MnZn ferrites have been successfully synthesized by various chemical methods,

ranging from co-precipitation [17-19] to hydrothermal [20, 21] and sol–gel [22-24] or

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sol–gel auto-combustion [25-27]. Co-precipitation is particularly suitable for

large-scale production on biological applications due to the formation of nanoparticles

in aqueous media. Hydrotherma and sol–gel methods are good alternatives in terms of

precise chemical stoichiometry. It is imperative to develop an energy-efficient

technique that presents more environmental and economic merit for the preparation of

ultrafine powders with a narrow size distribution.

Iron, zinc and manganese are not only the main components of MnZn ferrites, but

they are also the main components of spent alkaline batteries. In this regard, the

authors developed a novel way to recycle spent alkaline batteries using a combination

of sol–gel auto-combustion and microwave digestion methods to produce

ferromagnetic MnZn ferrite powders. This study focused on not only the study of the

suitable process conditions, the optimization of microwave digestion conditions and

the synthesis mechanism, but also on the composition and morphology of the

resulting MnZn ferrite powders.

2. Experimental

2.1 Materials

Spent alkaline batteries were collected from Henan Normal University campus in

Xinxiang, China. These batteries were discarded as useless in the battery product line

due to low voltage, zero voltage, or irregular appearance. The substances contained in

these spent batteries were similar to those in the same type of batteries commercially

available. The analysis indicated that a single cell weighed an average of 23.4 g,

including approximately 4.6 g iron, 10.5 g MnO2, 5.1 g zinc, 1.5 g KOH, 0.8 g

organic materimals (separator, label, and plastic cap), and 0.9 g of other materials,

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such as copper, carbon, and binders [28]. Nitric acid, hydrogen peroxide, manganese

nitrate, zinc nitrate, iron nitrate, citric acid, and ammonia were all of analytical purity.

2.2 Sample preparation and experimental method

A series of mechanical processes for the recycling of spent alkaline batteries was

conducted in the following sequence to yield Mn, Zn, and Fe: the separation of

packaging materials and splitting of negative and positive pole materials; copper can

be directly removed by cutting using a saw. The preparation process of MnZn ferrite

powders is shown in Fig. 1.

Fig. 1

Manganese and zinc oxides are the main components of the negative and positive

pole materials. After splitting, the negative and positive pole materials were dissolved

with a 6 mol L−1 nitric acid solution containing 2.5 wt% hydrogen peroxide to prepare

ferric nitrate, zinc nitrate and manganese nitrate. The reactions can be represented as

follows:

Zn (s) + 2HNO3(aq) = Zn(NO3)2(aq) + H2 (g) ↑

ZnO(s) + 2HNO3(aq) = Zn(NO3)2(aq) + H2O(aq)

MnO(s) + 2HNO3(aq)= Mn (NO3)2(aq) + H2O(aq)

Mn2O3(s) + 2HNO3(aq) = Mn (NO3)2(aq) + MnO2 + H2O(aq)

Mn3O4(s) + 4HNO3(aq) = 2Mn (NO3)2 + MnO2 +2 H2O(aq)

MnO2(s) + 2HNO3(aq) + H2O2(aq) = Mn (NO3)2(aq) + 2H2O(aq) + O2(g) ↑

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Fe(s) + 2HNO3(aq) = Fe(NO3)2 + H2(g) ↑

Fe2O3(s) + 6HNO3(aq) = 2Fe(NO3)3(aq) + 3H2O(aq)

After the reaction, the solution was filtered and the filtrate was analyzed for the

concentration of Fe, Zn, and Mn by inductively coupled plasma optical emission

spectrometry (ICP-OES). The result indicated that the concentration of Fe, Zn and Mn

was 15.3 mg mL-1, 30.8 mg mL-1 and 25 mg mL-1, respectively. In order to prepare

ferrite (Mn0.6Zn0.4Fe2O4), suitable amounts of analytical-purity iron nitrate, zinc

nitrate, and manganese nitrate were added to adjust the concentrations of Fe, Zn, and

Mn. Then, an appropriate amount of citric acid was added to the solution in order to

adjust the molar ratio of the total concentration of metal ions to citric acid to 1:1.

During this procedure, the solution was continuously stirred with a magnetic agitator,

and the temperature was controlled at 60 ℃. When the citric acid was dissolved

completely, the solution was kept at 75 ℃ until the solution became almost saturated.

After that, a certain amount of ammonia was added to the solution, which became a

brown-red sol under continuous stirring. Then, the sol was put into a dish and dried in

an oven at 135 ℃ for 2 h to transform it into dried gels. The gels were then ignited to

begin the sol-gel auto-combustion reaction. The resulting products were burnt

powders. The loose powders, namely the MnZn ferrite precursor powders, were

obtained by the auto-combustion of the dried gels. Finally, nanocrystalline MnZn

ferrite powders were prepared using the MnZn ferrite precursor powders, which were

digested at 120 ℃ for 15 min in a microwave-assisted digestion system. Microwave

digestion is a technique used to synthesize materials in solution through the

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application of radiation.

2.3 Characterization

In order to analyze and adjust the concentration of the Fe, Mn, and Zn metal ions, a

Perkin Elmer ICP-OES (model Optima 2100DV, USA) was used. A microwave

sample preparation system (model MARS 5, CEM Corp., USA) equipped with Teflon

vessels was used for the sample microwave-assisted digestion. X-ray diffraction

(XRD) patterns of the dried gels, the MnZn ferrite precursor powders and MnZn

ferrite powders were collected on a Bruker X-ray diffraction with Cu Ka radiation

(model BRUKER.axs, Germany). Evaluation of the thermal properties of the products

were carried out by thermogravimetric analysis/differential scanning calorimetry

(TG-DSC) (model STA-449 F3, Germany) at a heating rate of 10 ℃/min in static air.

Fourier transform infrared spectroscopy (FT-IR) was recorded on a Bruker

spectrometer in the range of 400–4000 cm−1 using the KBr pellet method (model

Tensor 27, Bruker). The microstructure of the ferrite powder was observed using a

scanning electron microscope (SEM) (model Quanta-200, FEI, Netherlands) with

energy dispersive X-ray spectrometry (EDS) (model INCA 250 x, Oxford Instruments,

Oxford, UK). Magnetic measurements were carried out on a vibrating sample

magnetometer (VSM) (model LDJ9600, USA) with a maximum field of 7.0 KOe at

room temperature.

3. Results and discussion

3.1 Optimization of microwave digestion procedure

Fig. 2

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In order to optimize the microwave digestion procedure, investigation of the

influence of microwave digestion temperatures and digestion times was carried out by

FT-IR spectra and XRD pattern analysis. Fig. 2(A)(B) shows the FT-IR spectra and

XRD patterns of the MnZn ferrite precursor powders digested at different microwave

digestion times and temperatures. The FT-IR spectra curves in Fig. 2(A) show the

obvious characteristic bands of MnZn ferrite powders at about 569 cm−1. When the

digestion times were less than 15 min, the obvious characteristic band appeared as a

weak band. It was also found that the characteristic absorption bands of the samples

showed almost no change when the digestion times were more than 15 min. The phase

and crystallinity of the MnZn ferrite powders were investigated by XRD, as shown in

Fig. 2(B). When the digestion temperatures were less than 120 ℃, it is obvious that

there is an amorphous structure. When the digestion temperatures were more than

120 ℃, the resulting product is highly crystallized. These results further confirm that

as an energy efficient technique, MnZn ferrite powders can be formatted at 120 ℃

with a 15 min microwave digestion.

3.2 FT-IR analysis of samples

Figure 2(C) shows the FT-IR spectra of the MnZn ferrite precursor powders, MnZn

ferrite powders, and the dried gels in the range 400–4000 cm−1. The dried gels show

characteristic bands at about 3420, 1618, and 1387 cm−1, which are ascribed to the

stretching mode of the O–H group in the free and absorbed water and citrate acid, the

H–O–H bending vibration of the residual water, and the anti-symmetric NO3−

stretching vibration, respectively. The FT-IR spectra of the MnZn ferrite precursor

powders show that the characteristic bands of the MnZn ferrite powders were not fully

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formed, which revealed that the auto-combustion process was a heat-induced

oxidation–reduction reaction between nitrate ions and citrate acid. The FT-IR spectra

of the MnZn ferrite powders show that the intensities of the bands corresponding to

the O–H group and NO3− decrease significantly, which is associated with the loss of

the residual water and the decomposition of citrate acid and NO3− in the dried gels.

This proved that single-phase MnZn ferrite precursor powders can be achieved at 569

cm−1.

The formation process of the MnZn ferrite precursor powders could be explained as

follows, with the amount of ammonia addition and the solution continuously stirring

the following reaction occurs:

C6H8O7 (aq) + Mn2+ + Zn2+ + Fe2+ + NO3− + NH3·H2O (aq) Mn0.6Zn0.4Fe2O4

(precursor) + CO2(g) ↑+ NH3 (g) ↑ + NO2 (g) ↑ + H2O (g) ↑

3.3 XRD pattern of MnZn ferrite powders

The XRD pattern analysis was performed to determine the chemical purity and

phase homogeneity of the prepared MnZn ferrite powders and the results are shown in

Fig. 2(D). The intensity and d values of the observed diffraction peaks (2 2 0), (3 1 1),

(4 0 0), (4 2 2), (5 1 1), and (4 4 0) confirm the formation of the single-crystalline

cubic spinel structure. The dried gels were amorphous in nature and the MnZn ferrite

precursor powders had a relatively weak spinel structure. When the MnZn ferrite

precursor powders were digested at 120 ℃ for 15 min or when the digestion times

were more than 15 min, a single-phase MnZn ferrite with a strong spinel structure was

formed. This indicates that the MnZn ferrite powders can be prepared by sol–gel

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auto-combustion and microwave digestion methods using spent batteries. The mean

particle diameter was calculated from the XRD pattern according to the line width of

the (3 1 1) plane refraction peak using the following Debye–Scherrer equation [29]:

The equation uses the reference peak width at angle θ, where λ is the X-ray

wavelength (1.5418 Å), β1/2 is the width of the XRD peak at half height, and K is the

shape factor, about 0.9 for spherical-shaped particles. The average crystallite size

calculated from the peak width is about 21 nm.

3.3 TG/DSC curves of the dried gels

Fig. 3

Thermal analysis was performed to understand the auto-combustion process used

for the synthesis of the MnZn ferrite powders. Simultaneous TG/DSC curves of the

dried gels are shown in Fig. 3. The TG curve shows a multistep weight loss, and there

is also an endothermic trough and three exothermic peaks in the DSC curve. The

weight loss from room temperature to 105 ℃ is attributed to the loss of residual

water in the gels, which appears on the DSC curve as an endothermic peak at 65 ℃.

Thereafter, a continuous two-step weight loss is noticed at temperatures ranging from

145 to 380 ℃, showing a maximum weight loss, which also appears as two broad

endothermic peaks at 188 and 298 ℃ on the DSC curve. This first small weight loss

is attributed to the decomposition of the citric acid. The last strong exothermic peak

with large weight loss is due to the decomposition of the dried gels occurring

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suddenly in a single step, which was induced by an autocatalytic oxidation–reduction

reaction between the nitrate and citrate acid. The last exothermic peak is due to

decomposition of the residual organic matter combustion in the temperature ranging

from 500 to 700 ℃.

3.4 SEM photographs of MnZn ferrite precursor powders and MnZn ferrite powders

Fig. 4

Figure 4 shows SEM photographs of the MnZn ferrite precursor powders and MnZn

ferrite powders. The influence of microwave digestion on the microstructure of the

MnZn ferrites is illustrated. The average grain size of the MnZn ferrite precursor

powders is larger and much more inhomogeneous than the MnZn ferrite powders. We

also can see in Fig. 2(B) that the shape of the MnZn ferrite particles is basically

globular and the particle diameters are about 50–60 nm, exhibiting a finer-grained

microstructure. At the same time, in order to confirm the chemical composition, MnZn

ferrite powders were detected by EDS analysis. The results were almost equal to the

initial stoichiometry (Mn0.6Zn0.4Fe2O4) in the solution after adjusting the metal ions.

3.5 VSM curves of samples

Fig. 5

Since microwave digestion conditions have a significant effect on the morphology

and the phase constitution of MnZn ferrite powders, magnetic hysteresis loops of the

MnZn ferrite powders and MnZn ferrite precursor powders were measured at room

temperature shown in Fig. 5. These magnetization curves show that MnZn ferrite

powders were more ferromagnetic than MnZn ferrite precursor powders in terms of

saturation magnetization and retentivity. The observed results for the MnZn ferrite

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powders fabricated by microwave digestion conditions are believed to be due to the

more complete crystallization of the ferrite phase and the enhancement in crystallinity

of the ferrite-phase nanoparticles.

4. Conclusion

Nanocrystalline MnZn ferrite powders were fabricated by the combination of a

sol–gel auto-combustion method and a microwave digestion method using spent

batteries. The results revealed that the auto-combustion process was a heat-induced

oxidation–reduction reaction between nitrate ions and citrate acid and that

single-phase MnZn ferrite powders can be achieved. When MnZn ferrite precursor

powders were microwave-digested at 120 ℃ for 15 min, the saturation

magnetization and retentivity of the MnZn ferrite powders synthesized by further

microwave digestion conditions were more ferromagnetic than MnZn ferrite precursor

powders prepared by the sol–gel auto-combustion process alone.

Acknowledgements The authors are thankful to financial supports obtained from the

National Natural Science Foundation of China (Grant No. 51174083), (Grant No. 51304064) and

the Doctoral Fund of the Ministry of Education of China (20114104110004) and Henan Institute

of Science and Technology students' innovative training project (2013CX061).

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Fig. 1 The process diagram for preparing MnZn ferrite powders

Fig. 2 (A) IR spectra of MnZn ferrite precursor powders digested at 120 ℃ for

different digestion times (B) XRD patterns of MnZn ferrite precursor powders

digested at different microwave digestion temperatures for 15min (C) IR spectra

and (D) XRD patterns of MnZn ferrite precursor powders, MnZn ferrite powders and

the dried gels

Fig. 3 TG/DSC curves of the dried gels

Fig. 4 SEM photographs of (A) MnZn ferrite precursor powders and (B) MnZn

ferrite powders

Fig. 5 Magnetic hysteresis loops of (A) MnZn ferrite powders and (B) MnZn ferrite

precursor powders.

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Fig. 1 The process diagram for preparing MnZn ferrite powders

The sol-gel formation

MnZn ferrite precursor powders formation

Microwave digestion

MnZn ferrite powders

Adjustment the concentration of metal

Dissolving and filtering

Separation

Spent alkaline batteries

Separation

Negative and positive pole materials Packaging materials

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Fig. 2 (A) IR spectra of MnZn ferrite precursor powders digested at 120 ℃

for different digestion times (B) XRD patterns of MnZn ferrite precursor

powders digested at different microwave digestion temperatures for 15min (C)

IR spectra and (D) XRD patterns of MnZn ferrite precursor powders, MnZn

ferrite powders and the dried gels

18

Fig. 3 TG/DSC curves of the dried gels

Fig. 4 SEM photographs of (A) MnZn ferrite precursor powders and (B) MnZn

ferrite powders

19

Fig. 5 Magnetic hysteresis loops of (A) MnZn ferrite powders

and (B) MnZn ferrite precursor powders