EFFECT OF OPERATING CONDITIONS ON ELECTROFORCED
SEDIMENTATION OF ZINC OXIDE
Mohammed Saedi
1
,
Masashi Iwata
2
, Suleyman A. Muyibi
1
, Mohd Ismail Abd. Karim
1
1
Department of Biotechnology Engineering, International Islamic University Malaysi
a
2
Department of C
hemical Engineering, Graduate School of Engineering
Osaka Prefectural University, Japan
Abstract
Electroforced sedimentation experiments under constant electric current density of zinc oxide
suspended in aqueous media were carried out and comparison is m
ade between the experimental
results and model predictions. The basic differential equation based on the model is solved
analytically
by assuming that both an electro

osmotic pressure gradient
E
pg
and a modified
consolidation coefficient
C
e
of the material
are constant. The solution can explain the time course of
changes in a solid compressive pressure distribution. The predicted final moisture distribution of the
material agreed with our experimental observations. The progress of electrofor
ced sed
imentatio
n
can be represented by an average consolidation ratio
U
c
as in mechanical expression. The effect of
current density on the settling behavior is also studied.
Keywords
: Electroforced Sedimentation, Consolidation Ratio, Terzaghi Model, Zinc oxide,
Current
density
1.
Introduction
Sedimentation is
one of the various
solid

liquid separation tools widely used in both
laboratory and industrial scales
.
Application of
electric field, to enhance the separation, is one
of the techniques studied recently.
Electrok
inetic effects play a large role in such
separations.
In this study,
constant

voltage
electro
forced sedimentation (EFS)
is analyzed
by use of the Terzaghi model
assuming that the
electroosmotic properties of the material are
constant
.
Electroforced
sedimen
tation
experiments under constant electric
voltage
density
of
zinc oxide
suspended in aqueous
media were carried out.
The effects of
voltage
density and the total
solid volume per unit
cross

sectional area
on the effectiveness
(
relative difference of the i
nitial and final void
ratios
) of sedimentation were examined.
2.
Materials and Methods
A sedimentation column 45 cm high and 4
cm internal dia
meter made of acryl was used
(Saedi et al., 2008)
. As exp
erimental material,
zinc oxide
was used. The slurry was ke
pt until
gravitational sedimentation is completed and
equilibrium
height
is achieved. Then, D.C.
voltage was applied between the electrodes,
both ma
de of stainless steel.
Experiments were
conducted under various
voltage
densities and
total
solid volume per
unit cross

sectional area
.
The change in height of sedimentation was
visually monitored c
ontinuously
.
3.
Results and Discussion
The analytical solution of the basic
differential equation for electroforced
sedimentation,
which represents the solid
compress
ive pressure
p
s
distribution in the
material as a function of time and position,
can
be represented as
0
2
e
2
2
2
2
0
1
n
pg
0
1
n
0
0
s
pg
s
C
4
1
n
2
exp
1
n
2
1
2
1
n
2
sin
1
E
8
p
g
E
,
p
(1)
where
E
pg
is the
electro

osmotic pressure
gradient
;
s
and
density of solid and liquid
respectively;
0
the total
solid volume per unit
cross

sectional area;
the net solid volume per
unit cross

sectional area extending from the
bottom
up to an arbitrary position in the
material
;
p
0
the value of
p
s
corr
esponding to the
initial concentration of suspension and can be
determined from the C

P data;
C
e
the modified
consolidation
coefficient
and
time
.
Figure 1
shows the progress of sedimentation
as the height of the sediment versus time
calculated from thic
kness decrease measured.
On the other hand, it was found that the total
solid volume per unit cross

sectional area
has
little effect on the effectiveness of EFS.
The
theoretical predictions
of the void ratio
distribution of the final cake are
in good
agree
ment with
the
experimental
ly
obtained
results
.
0
500
1000
1
0.5
0
U
c
[–]
[s
1/2
]
Theoretical
Figure 1 Average Consolidation Ratio
4.
Conclusions
In this study, constant
voltage
electroforced
sedimentation properties of zinc oxide sediment
were investigated. The basic differential
equation for const
ant
voltage
electroforced
sedimentation is solved analytically under the
assumption that the electro

osmotic property of
the material does not change during the
process
.
The
effectiveness of
electro

forced
sedim
entation increases with
voltage
density.
Ac
knowledgement
This work is supported by
Ministry of Higher
Education Malaysia Fundamental Research
Grant Scheme (Grant no
FRGS0308

88)
and
we
gratefully
acknowledge the support.
Reference
s
Jami,
M.
S
.
&
Iwata,
M.
(2008)
.
A New Method
for the Theoretical Analysis of Electroforced
Sedimentation Using Terzaghi

Voigt Combine
d
Model,
Sep. Sci. Technol.
, 43
(5), 979

995
.
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