EFFECT OF OPERATING CONDITIONS ON ELECTROFORCED SEDIMENTATION OF ZINC OXIDE

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22 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

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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
.