EFFECT OF OPERATING CONDITIONS ON ELECTROFORCED
SEDIMENTATION OF ZINC OXIDE
Mohammed Saedi1, Masashi Iwata2, Suleyman A. Muyibi1, Mohd Ismail Abd. Karim1
Department of Biotechnology Engineering, International Islamic University Malaysia
2
Department of Chemical Engineering, Graduate School of Engineering Osaka Prefectural University, Japan
1
Abstract
Electroforced sedimentation experiments under constant electric current density of zinc oxide
suspended in aqueous media were carried out and comparison is made 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 Epg and a modified
consolidation coefficient Ce 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 electroforced sedimentation
can be represented by an average consolidation ratio Uc 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.
Electrokinetic effects play a large role in such
separations. In this study, constant-voltage
electroforced sedimentation (EFS) is analyzed
by use of the Terzaghi model assuming that the
electroosmotic properties of the material are
constant.
Electroforced
sedimentation
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 initial and final void
ratios) of sedimentation were examined.
2. Materials and Methods
A sedimentation column 45 cm high and 4
cm internal diameter made of acryl was used
(Saedi et al., 2008). As experimental material,
zinc oxide was used. The slurry was kept until
gravitational sedimentation is completed and
equilibrium height is achieved. Then, D.C.
voltage was applied between the electrodes,
both made 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 continuously.
3. Results and Discussion
The analytical solution of the basic
differential
equation
for
electroforced
sedimentation, which represents the solid
compressive pressure ps distribution in the
material as a function of time and position, can
be represented as


p s  ,   E pg   s   g 0     p 0



n 1

 2n  1
n 1
 80 E pg  1 sin 
2


2 2



2
n

1




 
1 

 0 


2 2


 2n  1  C e 
exp 
 
2
4
 0 



(1)
where Epg 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; p0 the value of ps corresponding to the
initial concentration of suspension and can be
determined from the C-P data; Ce the modified
consolidation coefficientandtime.
Figure 1 shows the progress of sedimentation
as the height of the sediment versus time
calculated from thickness 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
agreement with the experimentally obtained
results.
Uc [–]
0
0.5
Theoretical
1
0
500
1000
1/2
 [s

]
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 constant 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
sedimentation increases with voltage density.
Acknowledgement
This work is supported by Ministry of Higher
Education Malaysia Fundamental Research
Grant Scheme (Grant no FRGS0308-88) and we
gratefully acknowledge the support.
References
Jami, M. S. & Iwata, M. (2008). A New Method
for the Theoretical Analysis of Electroforced
Sedimentation Using Terzaghi-Voigt Combined
Model, Sep. Sci. Technol., 43(5), 979-995.