H. S. Woon, K. P. Lim, C. K. Ho, C. L. Cheah and N. Riza / IJECCT 2011, Vol. 1 (2)
1
A Study on Waste-derived NiZn Soft Ferrites
As EMI Suppressor
H. S. Woon1, K. P. Lim2, C. K. Ho1, C. L. Cheah1 and N. Riza1
1
College of Engineering,
Universiti Tenaga Nasional (UNITEN),
Kampus Putrajaya,
Jalan Ikram-UNITEN,
43000 Kajang, Selangor, Malaysia.
2
Physics Department
Faculty of Science
Universiti Putra Malaysia
Serdang 43400
Selangor, Malaysia
E-mail: [email protected]
Abstract: Nickel-zinc soft ferrites with spinel structure are
important electronic components popularly used as EMI
suppressor, electromagnet core and transformer core. It
contains nickel, zinc or manganese, and the raw material
is mainly hematite. The most commonly use soft ferrites
are NiZn ferrites and MnZn ferrites. NiZn ferrites exhibit
higher resistivity than MnZn ferrites and are therefore
more suitable for frequencies above 1 MHz. In this work,
iron oxide waste generated from a local cold-rolling steel
mill was purified and converted into hematite. The wastederived hematite was used as the raw material in the
synthesis of NiZn ferrites. The magnetic properties such
as permeability, saturation magnetization and coercivity
of the waste-derived NiZn ferrites was analyzed and
compared to the industrial grade NiZn ferrites. Our results
show that the waste-derived ferrite possesses excellent
magnetic properties. The microstructure of the wastederived NiZn ferrite is also discussed.
Keywords – soft ferrite; hematite (Fe2O3); cold-roll steel
industry
I. INTRODUCTION
Ferrite cores are commonly used in electronic inductors,
transformers, electromagnets and EMI suppressor where
the high electrical resistance of the ferrite leads to very
low eddy current losses. They are usually seen as a lump
in a computer cable, called a ferrite bead. These ferrite
beads prevent high frequency electrical noise (radio
frequency interference) from exiting or entering the
equipment.
Nickel-zinc ferrite is produced using
conventional ceramic processing method (wet method) in
the industry. The process includes wet mixing the raw
materials, that are hematite, nickel oxide and zinc oxide,
heat treatment, forming and sintering. The NiZn ferrite
compound is produced shown by the chemical reaction:
NiO + ZnO + 2 Fe2O3 → 2 Ni0.5Zn0.5Fe2O4
Ferrite materials possess good magnetic properties such as
high permeability and saturation magnetization but low
coercivity[1,2,3].
The Malaysia steel-making industry consists of two major
streams; i.e. hot rolling and cold rolling industries. The
hot rolling process involves slab-heating (as well as billet
and bloom), rolling, and forming operations. Several types
of hot forming mills manufacture diverse steel products.
Long products are manufactured by hot rolling billets into
reinforcement bars, or for further rolling and drawing into
wire rods and sometimes coating. To prepare the steel for
cold rolling or drawing, acid pickling is performed. The
acid pickling chemically remove oxides and scale from
the surface of the steel. In continuous pickling processes,
the steel is immersed in acid cleaning tanks and then
cleaned in a series of water rinsing tanks. The spent acid
will normally be re-generated in the acid regeneration
plant (ARP) through fluidized-bed process where iron
oxide waste is produced.[4]
Colormetry is a simple but useful method to identify the
presence of hematite in the sample. The L*a*b* color
space (also referred to as CIELAB) is one of the most
popular color space for measuring object color and is
widely used in virtually all fields. In this color space, L*
indicate lightness and a* and b* are the chromaticity
coordinates. Figure 1 shows the a* and b* chromaticity
diagram. +a* is the red direction, -a* is the green
direction, +b* is the yellow direction and –b* is the blue
direction [5].
H. S. Woon, K. P. Lim, C. K. Ho, C. L. Cheah and N. Riza / IJECCT 2011, Vol. 1 (2)
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Mixing Fe2O3 : NiO:ZnO
Pre-Sintering at 1000°C
High Energy Milling
Forming
Sintering at 1150°C
Figure 1. The Color Ball and the L*a*b*
Colormetry System
Sample Characterizations
Figure 2. Schematic Diagram of
NiZn Soft Ferrites Synthesis Procedure
II. MATERIAL AND METHOD
Iron oxide waste produced by acid-regeneration-plant
(ARP) in a local cold-rolling steel mill was collected. The
factory waste then inspected, cleaned and dried in an
oven. A high-energy-blender was employed to mill the
waste into ultra-fine powder and high-gradient magnetic
separation was performed. The sub-micron powder was
then converted into hematite by means of heat treatment.
Conventional ceramic preparation process was employed
in this work in the synthesis of NiZn soft ferrites. The
process includes mixing, pre-sintering, granulation,
forming and sintering. The experimental procedure is
summarized in Figure 2.
III. RESULTS
Steel industry waste is generally a rich iron source (> 70%
Fe) and consists of three distinct layers of iron oxides;
namely wuistite (FeO), magnetite (Fe3O4) and hematite
(Fe2O3). During the milling process in this work, the
factory waste was grinded into ultra-fine particles and in
the form of single-phase-particle. Using magnetic
separation, we obtained defective FeO from the factory
waste. This defective FeO was further oxidized into useful
Fe2O3. The following chemical equation illustrates the
process.
4 FeO
For sample characterization, a chromameter (Model:
Minolta CR-10) was used to study the color property of
the waste-derived hematite. The L*, a*, b*values were
read by the chromameter and recorded. The
microstructure of the sintered samples was analyzed by
using a Scanning Electron Microscope (Model: Philips
LEO-VSEM). Finally, B-H tracer (Model: Linkjoin
MATS-2010S) was employed to analyze the magnetic
properties of the samples. Magnetic properties such as
permeability, saturation magnetization and coercivity
were obtained.
O2
2 Fe 2 O 3
Figure 3 shows the raw waste material and the treated
waste-derived hematite. The color change is obvious. It
changes from light blue to red. The color property was
analyzed by a chromameter (Model: Minolta CR-10) and
the results is shown in Table 1. After several treatments,
the value of a* improved from 1.3 to as high as 9.5. This
value is an indication of the reddishness of the samples.
Pure hematite always possesses high a* value. Also, the
value of b* shows the blue property but it is not our
concern in this study. We can conclude that high purity
hematite was obtained and the treatments were successful.
H. S. Woon, K. P. Lim, C. K. Ho, C. L. Cheah and N. Riza / IJECCT 2011, Vol. 1 (2)
Figure 3. Cold-rolling Waste (Up) and Waste-derived Hematite
(Down)
Table 1. Color Property of Cold-rolling Waste
and Waste-derived Hematite
Color
Property
L*
a*
b*
Coldrolling
Waste
36.2
1.3
-1.6
Wastederived
Hematite
38.9
9.5
3.1
Figure 4 reveals the magnetic properties of the wastederived NiZn ferrite compared to an industrial grade NiZn
ferrite. The hysteresis graph shows the magnetic induction
of the samples to the external field provided.
3
Figure 4. Hysteresis-graph of Waste-derived NiZn Ferrite (Up)
and Industrial Grade NiZn Ferrite (Down)
Permeability is an important information to show the
ability of the sample to acquire high magnetization in
relatively weak magnetic fields. Both the samples possess
high permeability (μ) that is 7.731 mH/m and 7.882
mH/m respectively. These high permeability can be
explained by the fact that atomic magnetic moments are
randomly oriented on the inter-atomic scale and the field
gradually aligns them as in the case of paramagnets.
Saturation magnetization or saturation remanence of the
samples is 2246 Gauss and 2328 Gauss respectively
which are high enough to be used as EMI suppressor.
Saturation magnetization is the highest magnetic
induction possessed by the samples when high external
field is supplied. However the coercivity (Hc) of the
waste-derived samples that is 0.1024 Oe is considered
slightly too high to be categorized as a high-end product.
This is because high coercivity may cause high lost in the
signal transmission system. Eddy current generated may
lead to heat agitation and signal quality will be distorted.
The target value for coercivity is usually 0.06 Oe or
lower.
H. S. Woon, K. P. Lim, C. K. Ho, C. L. Cheah and N. Riza / IJECCT 2011, Vol. 1 (2)
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from the impurities content, which resided at the grain
boundaries.
IV. CONCLUSIONS
The purifying process which includes milling, magnetic
separation and heat treatment is proven successful to
convert the cold-rolling waste material into useful
hematite. The waste-derived hematite performs the color
with high a* value and thus it is an acid test for the
presence of hematite. The magnetic properties such as
magnetic permeability and saturation magnetization are
high and meet the requirement for the sample to be used
as EMI suppressor. The only drawback seems to be the
high coercivity possessed by the waste-derive NiZn
ferrite. The high coercivity may lead to high lost and may
eventually have adverse effect to the signal quality.
REFERENCES
[1]
[2]
Figure 5. Microstructure of Waste-derived NiZn Ferrite (Up)
and Industrial Grade NiZn Ferrite (Down)
Figure 5 unveiled the microstructure of the waste-derived
NiZn ferrite and the industrial grade NiZn ferrite. From
our observation of all the SEM micrographs, the higher
impurity in the waste-derived NiZn ferrite sample is
detrimental to the reaction between the grains after
sintering. This is due to the formation of secondary phases
[3]
[4]
[5]
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