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Effect of surfactants on the filtration properties of fine particles
PROCEEDINGS
OF THE FILTRATION
SOCIETY
Effect of Surfactants on the Filtration Properties Fine Particles
of
By G Stroh % W Stahl
lnstitut fur Mechanische This paper
Verfahrenstechnik was presented
und Mechanik,
to the filtration
der Universitat
Society at h/tech
Karlsruhe (TH), Postfach 6980, D-7500 Karlsruhe
89, Karlsruhe,
West Germany. September,
1, FRG
1989
The possibility for improved dewatering of filter cakes by influencing product properties with low foaming surfactants, is presented. The adsorption behaviour of surface active substances in aqueous suspensions is discussed with regard to the consequences for cake forming filtration. The influence of surfactants on wetting properties is demonstrated by measuring the capillary pressure of porous media with constant structure. According to these experiments, capillary pressure depends on dynamic surface tension only and not on receding contact angle. A model with regard to specific properties of surfactants is developed from that. Finally the surfactant’s influence on characteristic filtration results at laboratory pressure filtration is presented. MECHANICAL DEWATERING by applied pressure difference (vacuum, pressure filtration) is in widespread use in the beneficiation of raw materials. Upon the cake formation ceasing, dewatering of the porous filter cake usually takes place. Moisture contents reached by vacuum filtration are mostly not sufficient due to the limitation of pressure difference by the ambient pressure. As an alternative to expensive thermal drying, the applied pressure difference may be raised (pressure filtration) I), the capillary pressure of the porous filter cake may be lowered by surface active agents, or both measures may be taken. Due to the worldwide tendency towards fine-grained products, moisture contents will increase under the present operating conditions, and these measurements are of growing interest. In the following, the influence of new, low foaming biodegradable surfactants on the dewatering process of porous filter cakes are discussed.
Properties
surface layer, with maximum covering density built up. Beyond it, additional surfactant will form so-called micelles. a reversible aggregation of molecules in the solution. The concentration at which the surface is completely covered is called the critical concentration for micelle formation (cmc). When overcoming cmc, the monomer concentration in the solution and correspondingly the surface tension remains constant (cf Fig 1). Adding fine-grained solids to a surfactant’s solution, surface active molecules will adsorb at the solid’s interface. This leads to a decreasing monomer concentration in the solution. Due to this fact the surface tension of the suspension will become higher than that of the initial solution. Adsorption of ionogenic surfactants has an influence on the electric repulsion forces of suspended particles and hence on suspension stability”‘. The influence on wetting characteristics of plain surfaces can be discussed by Young’s equation as indicated in Fig 2:
of Surface Active Agents
Y\ - Y\L = yL cosb
Surfactants are chemical compositions, built up by a hydrophilic (ionic or polar) and a hydrophobic molecular part (mostly a hydrocarbon chain). Due to their molecular structure, surfactants behave surface-active, ie, they concentrate at phase boundaries coupled with the minimisation of free surface energy. The arrangements of surface active molecules shown in Fig 1 are possible in an aqueous suspension. At the liquid-gas interface the adsorption takes place with the hydrophilic. strongly hydrated molecular part drawn into the water. According to the lower interactions between the non-polar groups, the surface tension YL decreases. The thermodynamic equilibrium between the surface covering and the monomer concentration in solution is given by Gibbs’ equation:
(2)
The contact angle 6 increases, when a decrease of specific wetting energy (y, - y,J prevails as a consequence of adsorption (principle of flotation). On the other hand, the contact angle decreases when the surface tension YL decreases more than the specific wetting
p-IA&_ RT d(ln c) The decrease
in surface
tension
takes place up to a monomolecular
Arragement of surfactants in an aqueous suspension a) surface film c) aggregated to micelles b) molecular d) adsorbed at solids dissolved interface
Fig 2. Wetting of a plane surface
Reduction of surface tension at liquid air interface
cmc Fig 1. Surfactants Filtration
& Separation
May/June
1990
by adsorption
Inc
--+
in an aqueous suspension
197
PROCEEDINGS
OF THE FILTRATION
SOCIETY
energy is reduced by the surfactant’s adsorption (improvement of wetting). The determination of the force to enlargy the liquid-air interface by a completely wetted measuring plate (Wllhclmy plate) Icads to the surface tension */, In the case of slowly diffusing surface active agents, some hours are necessary to reach equilibrium surface tension. For technical applications this value is to be displaced br the dynamic surface tension. which is relevant for the prcscnt effect’ ).
Basics of Dewatering
sr
0
1 Saturation
by Pressure
Difference
The capillary pressure curve is of fundamental importance for the dcwatering of filter cakes. It describes the connection between cake saturation (or moisture content) and corresponding capillary function pressure (“. as shown in Fig 3. As a product characterising the capillary pressure curve has to be established by measuring. It is determined directly from filtration tests with applied constant pressure difference. thus having regard to cake forming conditions on the port structure. Characteristic values in this function arc the residual saturation S,. the mechanical limit of dewatering by applied pressure difference and the capillary inlet prcssurc pKb, which has to be established first to achieve an initial considerable dewatering of the cake. The capillary pressure in a porous media can be descrlbcd as a function of product propertics.
S
Fig 3. Schematic graph of a capillary pressure curve
I&F F---
pK =y, cash
‘f
P,~=L\P-
r
2 .
dynamic measurement
0,8
plgh
q
/ E’
0
//tatic
measuremer
$4$E’ lza’
? 0,2 .3 it??
o,o
/
030
/
/
/
/
/
/
/
on2
/
nonionic 1 a nonionic 2 0 nonionic 3 A nonionic 4 i7 anionic 1 0,6
= 0
y1 yw
on capillary
concentration
t2
E rll (h, + h,,)-
Influence of Wetting Properties
1 ,o
038
relative surface tension of surfactants
- 2P, Pc.,c,, I 6) (AP - P&))
dt2
(4)
0
/’
0,4
Fig 4. Influence
Is-
The driving force for the dewatering kinetic is given by the difference between the capillary pressure curve and the constantly applied filtration pressure. cf Fig 3. Hence the driving potential is reduced with advanced dewatcring of the filter cake. To increase the driving pressure difference, an enhanced mechanical expenditure can be applied. on the one hand, by continuous pressure filtration, while. on the other hand. lowering the capillary retention forces by surfactants is possible (cf Eq (3)). Finally the combination of both measures can be taken at the dcwatering of very fine-grained products. Besides accelerated dcwatering, both methods lead to decreased moisture content in the filter cake.
A0
’
(3)
In addition to the dependence of wetting characteristics which can be modified by surfactants, it is to be seen that the capillary pressure is an inverse function of the fineness of the pores to dewater. In technical processes the dcwatering kinetic is of special interest, as on real continuous filters finite dewatering times are given. The decrease of saturation degree S with dewatering time t, is described by’%:
AP
180
K x1.2
pressure
c < cmc
12
concentration
>o
on Capillary
Pressure
To elaborate the influence of wetting properties on capillary pressure, the pore structure has to be constant (cf Eq (3)). For basic research a packed bed of crushed glass splinters with a nominal pore diameter of 13pm was used. As in beneficiation practice, filtrate is often recycled in closed water circuits. some special low foaming, biodegradable non-ionic alkyl-polyalkylenoxides were chosen. The surfactants had different hydrophobic character and wetting properties. A high foaming anionic sulfosuccinate which is already industrially applied was used for comparison. The measured value was the capillary entry pressure at the porous medium being saturated with the surfactant solution. The results, given in Fig 4. show that static capillary pressure corresponds strongly with surface tension. Referring to Eq (3), no influence could thcrcfore be proved on receding contact angle in the
12
c 2 cmc
=o
t2
rstat
rdyn
<
%tat
l-stat
rdyn
=
‘Istat
rdyn
’
*I’stat
ystat
*Idyn
= Ystat
l-stat
Fig 5. Model of pore dewatering
199
May/June
1990 Filtration & Separation
PROCEEDINGS
OF THE FILTRATION
SOCIETY
Preliminary flot. coal 1 Ap =0.8
bar
cv =30 vol.-%
hK =9 mm nonionic 100
t2 =18Os
surfactant
1 22.5
r
20,o
175
Results from Laboratory
t, =
o-100 0
200
400
600
surfactant
800
dosage
mT
1000
-1 12,5
1200’
[g/t]
Fig 6. Filtration results at vacuum pressure difference 0 moisture content mc [wt.-%] 0 surface tension yL. 1O3 [N/m] A cake formation time t, [set)
characteristic pore. Hence complete wetting was preset for dynamic capillary pressure measurements with more rapidly increasing pressure difference. All surfactants under consideration indicated a typical relationship. For high surface tensions (ie low concentrations) capillary pressures as for pure water were measured. With increasing surfactant concentration (c-+cmc) the dynamic capillary pressure approaches the static results for low surface tension. This effect may be explained by the following model. The surface area of a pore is stretched during dewatering as indicated in Fig 5. The consequence is a decreasing number of molecules per unit of surface thus increasing dynamic surface tension (cf Fig 1). if the transport of molecules from the subsurface is not sufficient. As to the results elaborated, the transport of molecules is improved for increasing surfactant concentrations. The sharp decrease of dynamic capillary pressure for concentrations near cmc, indicates that the spontaneous dissolution of these aggregates seems to be responsible for holding up a constant rate of molecules at the surface area during dynamic dewatering. Hence, the greatest efficiency in the dewatering of filter cakes is expected when molecular aggregates are present in suspension.
L iA
4
4 I
r \,
\ \
I \ \ \ \ 1 \
3
0 \
2
1
‘I~
r,W-4 AP c,
15,0
LA-A.-A_
Pressure
Filtration
To elaborate the influence of surfactants on cake structure and dewatering results, two flotation coal concentrates of different fineness were used. They were given as suspension free of polymer flocculants. Due to the coal surface having contact with water it is presumed that the surface was largely oxidised having a hydrophilic character. In general, all surfactants showed nearly the same influence on filtration characteristics. Typical results of the finer concentrate using a hydrophobic, weak wetting non-ionic surfactant for filtration m a pressure filter cell(‘) at vacuum pressure difference are given in Fig 6. Cake resistance rcr medium resistance R, and cake porosity e were not significantly influenced by surfactant addition. Accordingly to Eq (5), derived from Darcy’s equation, they were summarised in cake forming time t,: h: 2+-
hKRM rc
I
(5)
As given in Fig 6, cake forming time remains constant for the given cake height at all surfactant dosages. This leads to constant mass throughputs in continuous filtration. The moisture content at constant dewatering conditions shows a significant decrease for surfactant concentrations approaching cmc as indicated by the suspension’s surface tension in Fig 6. This result is comparable with those of dynamic capillary pressure tests. For the description of the influence of surfactants on the results of pressure filtration the measured moisture contents for the coarse coal are presented in Fig 7. The use of the non-ionic surfactant and the anionic sulphosuccinate at the surface tension of 33 10P’Nim lead to the same moisture contents. The experimental results show perfect agreement with the calculated values according to Eq (3). Up to these results it is possible to halve capillary pressures in practice. This means a reduction of mechanical expenditure of pressure filtration, or an improved dewatering at given dewatering potential.
Conclusions Basic investigations as well as first laboratory filtration tests with beneficiation products, have shown that the addition of surfactants lowers the suspension surface tension essentially which allows improved dewatering. The occurrence of molecular aggregates in the suspension supports the effect of surface active agents in dynamic technical dewatering processes. Hence a reduction of mechanical expenditure at dewatering, particularly with fine-grained beneficiation products, in addition to an improved dewatering with present filters is imaginable.
Nomenclature C concentration of surfactant volumetric concentration of the suspension C” gravitational acceleration cake height equivalent cake height (hKE = l&/r,) 2E constant, including particle characteristics moisture content mc surfactant dosaae mT specific cake permeability 2 re,. L relative liquid permeability applied pressure difference AP capillary pressure PK caoillarv entrv oressure PKE universal gas-constant R specific cake resistance rC filter medium resistance R4 cake saturation z residual saturation T‘ absolute temperature cake formation time t1 dewatering time tz mean particle diameter of surface distribution T’,’ molar concentration per surface area surface tension (liquid-air) YL surface tension (solid-air) Y. interfacial tension (solid-liquid) YSL surface tension (water-air) YW contact angle b E porosity dynamic fluid viscosity ‘1L liquid density PL
z
n 1
5,5
10,o
12,5
moisture
15,o
content
mc
17,5 [w-t.-%]
Fig 7. Influence of surfactants on capillary pressure curve 0 without surfactant yL = 73.1 0m3Nlm 0 non-ionic surfactant 1 YL = 33. 10e3N/m A anionic surfactant 1 YL = 33.1 0e3N/m --- calculated value due to surface tension Filtration
& Separation
May/June
1990
20,o
REFERENCES I Bott. R, Zur kontmuierlichen Drucktiltratlon. GDMB-Vcrlag. Clawthai Zcllcrtcld (1986). 2 Lanpc. H. EintluR van Tcnsidcn und andercn St&en au! die Sedimentanon VCIschiedener Pulver I” Wasser: Kolloid-Z. u. Z. flir Polymcrc 211 (1966) Nr I-2. 106-113 3 Kowwg. K. Tcnsidc: Ullmanns Enzyklopadle der techni\chen Chcmx. Band 22: Vcrlap Chemie. W&ham (1982). 4 Schuhcrt. H. Kapillar~tat in pororen Feststoff\y\tcmcn. Sprmgcr Vcrlag. BcrhnHeidelberg-New York (1986). 5. Anlauf, H. Enrteuchtung van Filterkuchcn hci dcr Vakuum-, Druck-und Druck:Vdkuumfiltrimun. VDI Vcrlag. Dusxldorf (1986)
199
OF THE FILTRATION
SOCIETY
Effect of Surfactants on the Filtration Properties Fine Particles
of
By G Stroh % W Stahl
lnstitut fur Mechanische This paper
Verfahrenstechnik was presented
und Mechanik,
to the filtration
der Universitat
Society at h/tech
Karlsruhe (TH), Postfach 6980, D-7500 Karlsruhe
89, Karlsruhe,
West Germany. September,
1, FRG
1989
The possibility for improved dewatering of filter cakes by influencing product properties with low foaming surfactants, is presented. The adsorption behaviour of surface active substances in aqueous suspensions is discussed with regard to the consequences for cake forming filtration. The influence of surfactants on wetting properties is demonstrated by measuring the capillary pressure of porous media with constant structure. According to these experiments, capillary pressure depends on dynamic surface tension only and not on receding contact angle. A model with regard to specific properties of surfactants is developed from that. Finally the surfactant’s influence on characteristic filtration results at laboratory pressure filtration is presented. MECHANICAL DEWATERING by applied pressure difference (vacuum, pressure filtration) is in widespread use in the beneficiation of raw materials. Upon the cake formation ceasing, dewatering of the porous filter cake usually takes place. Moisture contents reached by vacuum filtration are mostly not sufficient due to the limitation of pressure difference by the ambient pressure. As an alternative to expensive thermal drying, the applied pressure difference may be raised (pressure filtration) I), the capillary pressure of the porous filter cake may be lowered by surface active agents, or both measures may be taken. Due to the worldwide tendency towards fine-grained products, moisture contents will increase under the present operating conditions, and these measurements are of growing interest. In the following, the influence of new, low foaming biodegradable surfactants on the dewatering process of porous filter cakes are discussed.
Properties
surface layer, with maximum covering density built up. Beyond it, additional surfactant will form so-called micelles. a reversible aggregation of molecules in the solution. The concentration at which the surface is completely covered is called the critical concentration for micelle formation (cmc). When overcoming cmc, the monomer concentration in the solution and correspondingly the surface tension remains constant (cf Fig 1). Adding fine-grained solids to a surfactant’s solution, surface active molecules will adsorb at the solid’s interface. This leads to a decreasing monomer concentration in the solution. Due to this fact the surface tension of the suspension will become higher than that of the initial solution. Adsorption of ionogenic surfactants has an influence on the electric repulsion forces of suspended particles and hence on suspension stability”‘. The influence on wetting characteristics of plain surfaces can be discussed by Young’s equation as indicated in Fig 2:
of Surface Active Agents
Y\ - Y\L = yL cosb
Surfactants are chemical compositions, built up by a hydrophilic (ionic or polar) and a hydrophobic molecular part (mostly a hydrocarbon chain). Due to their molecular structure, surfactants behave surface-active, ie, they concentrate at phase boundaries coupled with the minimisation of free surface energy. The arrangements of surface active molecules shown in Fig 1 are possible in an aqueous suspension. At the liquid-gas interface the adsorption takes place with the hydrophilic. strongly hydrated molecular part drawn into the water. According to the lower interactions between the non-polar groups, the surface tension YL decreases. The thermodynamic equilibrium between the surface covering and the monomer concentration in solution is given by Gibbs’ equation:
(2)
The contact angle 6 increases, when a decrease of specific wetting energy (y, - y,J prevails as a consequence of adsorption (principle of flotation). On the other hand, the contact angle decreases when the surface tension YL decreases more than the specific wetting
p-IA&_ RT d(ln c) The decrease
in surface
tension
takes place up to a monomolecular
Arragement of surfactants in an aqueous suspension a) surface film c) aggregated to micelles b) molecular d) adsorbed at solids dissolved interface
Fig 2. Wetting of a plane surface
Reduction of surface tension at liquid air interface
cmc Fig 1. Surfactants Filtration
& Separation
May/June
1990
by adsorption
Inc
--+
in an aqueous suspension
197
PROCEEDINGS
OF THE FILTRATION
SOCIETY
energy is reduced by the surfactant’s adsorption (improvement of wetting). The determination of the force to enlargy the liquid-air interface by a completely wetted measuring plate (Wllhclmy plate) Icads to the surface tension */, In the case of slowly diffusing surface active agents, some hours are necessary to reach equilibrium surface tension. For technical applications this value is to be displaced br the dynamic surface tension. which is relevant for the prcscnt effect’ ).
Basics of Dewatering
sr
0
1 Saturation
by Pressure
Difference
The capillary pressure curve is of fundamental importance for the dcwatering of filter cakes. It describes the connection between cake saturation (or moisture content) and corresponding capillary function pressure (“. as shown in Fig 3. As a product characterising the capillary pressure curve has to be established by measuring. It is determined directly from filtration tests with applied constant pressure difference. thus having regard to cake forming conditions on the port structure. Characteristic values in this function arc the residual saturation S,. the mechanical limit of dewatering by applied pressure difference and the capillary inlet prcssurc pKb, which has to be established first to achieve an initial considerable dewatering of the cake. The capillary pressure in a porous media can be descrlbcd as a function of product propertics.
S
Fig 3. Schematic graph of a capillary pressure curve
I&F F---
pK =y, cash
‘f
P,~=L\P-
r
2 .
dynamic measurement
0,8
plgh
q
/ E’
0
//tatic
measuremer
$4$E’ lza’
? 0,2 .3 it??
o,o
/
030
/
/
/
/
/
/
/
on2
/
nonionic 1 a nonionic 2 0 nonionic 3 A nonionic 4 i7 anionic 1 0,6
= 0
y1 yw
on capillary
concentration
t2
E rll (h, + h,,)-
Influence of Wetting Properties
1 ,o
038
relative surface tension of surfactants
- 2P, Pc.,c,, I 6) (AP - P&))
dt2
(4)
0
/’
0,4
Fig 4. Influence
Is-
The driving force for the dewatering kinetic is given by the difference between the capillary pressure curve and the constantly applied filtration pressure. cf Fig 3. Hence the driving potential is reduced with advanced dewatcring of the filter cake. To increase the driving pressure difference, an enhanced mechanical expenditure can be applied. on the one hand, by continuous pressure filtration, while. on the other hand. lowering the capillary retention forces by surfactants is possible (cf Eq (3)). Finally the combination of both measures can be taken at the dcwatering of very fine-grained products. Besides accelerated dcwatering, both methods lead to decreased moisture content in the filter cake.
A0
’
(3)
In addition to the dependence of wetting characteristics which can be modified by surfactants, it is to be seen that the capillary pressure is an inverse function of the fineness of the pores to dewater. In technical processes the dcwatering kinetic is of special interest, as on real continuous filters finite dewatering times are given. The decrease of saturation degree S with dewatering time t, is described by’%:
AP
180
K x1.2
pressure
c < cmc
12
concentration
>o
on Capillary
Pressure
To elaborate the influence of wetting properties on capillary pressure, the pore structure has to be constant (cf Eq (3)). For basic research a packed bed of crushed glass splinters with a nominal pore diameter of 13pm was used. As in beneficiation practice, filtrate is often recycled in closed water circuits. some special low foaming, biodegradable non-ionic alkyl-polyalkylenoxides were chosen. The surfactants had different hydrophobic character and wetting properties. A high foaming anionic sulfosuccinate which is already industrially applied was used for comparison. The measured value was the capillary entry pressure at the porous medium being saturated with the surfactant solution. The results, given in Fig 4. show that static capillary pressure corresponds strongly with surface tension. Referring to Eq (3), no influence could thcrcfore be proved on receding contact angle in the
12
c 2 cmc
=o
t2
rstat
rdyn
<
%tat
l-stat
rdyn
=
‘Istat
rdyn
’
*I’stat
ystat
*Idyn
= Ystat
l-stat
Fig 5. Model of pore dewatering
199
May/June
1990 Filtration & Separation
PROCEEDINGS
OF THE FILTRATION
SOCIETY
Preliminary flot. coal 1 Ap =0.8
bar
cv =30 vol.-%
hK =9 mm nonionic 100
t2 =18Os
surfactant
1 22.5
r
20,o
175
Results from Laboratory
t, =
o-100 0
200
400
600
surfactant
800
dosage
mT
1000
-1 12,5
1200’
[g/t]
Fig 6. Filtration results at vacuum pressure difference 0 moisture content mc [wt.-%] 0 surface tension yL. 1O3 [N/m] A cake formation time t, [set)
characteristic pore. Hence complete wetting was preset for dynamic capillary pressure measurements with more rapidly increasing pressure difference. All surfactants under consideration indicated a typical relationship. For high surface tensions (ie low concentrations) capillary pressures as for pure water were measured. With increasing surfactant concentration (c-+cmc) the dynamic capillary pressure approaches the static results for low surface tension. This effect may be explained by the following model. The surface area of a pore is stretched during dewatering as indicated in Fig 5. The consequence is a decreasing number of molecules per unit of surface thus increasing dynamic surface tension (cf Fig 1). if the transport of molecules from the subsurface is not sufficient. As to the results elaborated, the transport of molecules is improved for increasing surfactant concentrations. The sharp decrease of dynamic capillary pressure for concentrations near cmc, indicates that the spontaneous dissolution of these aggregates seems to be responsible for holding up a constant rate of molecules at the surface area during dynamic dewatering. Hence, the greatest efficiency in the dewatering of filter cakes is expected when molecular aggregates are present in suspension.
L iA
4
4 I
r \,
\ \
I \ \ \ \ 1 \
3
0 \
2
1
‘I~
r,W-4 AP c,
15,0
LA-A.-A_
Pressure
Filtration
To elaborate the influence of surfactants on cake structure and dewatering results, two flotation coal concentrates of different fineness were used. They were given as suspension free of polymer flocculants. Due to the coal surface having contact with water it is presumed that the surface was largely oxidised having a hydrophilic character. In general, all surfactants showed nearly the same influence on filtration characteristics. Typical results of the finer concentrate using a hydrophobic, weak wetting non-ionic surfactant for filtration m a pressure filter cell(‘) at vacuum pressure difference are given in Fig 6. Cake resistance rcr medium resistance R, and cake porosity e were not significantly influenced by surfactant addition. Accordingly to Eq (5), derived from Darcy’s equation, they were summarised in cake forming time t,: h: 2+-
hKRM rc
I
(5)
As given in Fig 6, cake forming time remains constant for the given cake height at all surfactant dosages. This leads to constant mass throughputs in continuous filtration. The moisture content at constant dewatering conditions shows a significant decrease for surfactant concentrations approaching cmc as indicated by the suspension’s surface tension in Fig 6. This result is comparable with those of dynamic capillary pressure tests. For the description of the influence of surfactants on the results of pressure filtration the measured moisture contents for the coarse coal are presented in Fig 7. The use of the non-ionic surfactant and the anionic sulphosuccinate at the surface tension of 33 10P’Nim lead to the same moisture contents. The experimental results show perfect agreement with the calculated values according to Eq (3). Up to these results it is possible to halve capillary pressures in practice. This means a reduction of mechanical expenditure of pressure filtration, or an improved dewatering at given dewatering potential.
Conclusions Basic investigations as well as first laboratory filtration tests with beneficiation products, have shown that the addition of surfactants lowers the suspension surface tension essentially which allows improved dewatering. The occurrence of molecular aggregates in the suspension supports the effect of surface active agents in dynamic technical dewatering processes. Hence a reduction of mechanical expenditure at dewatering, particularly with fine-grained beneficiation products, in addition to an improved dewatering with present filters is imaginable.
Nomenclature C concentration of surfactant volumetric concentration of the suspension C” gravitational acceleration cake height equivalent cake height (hKE = l&/r,) 2E constant, including particle characteristics moisture content mc surfactant dosaae mT specific cake permeability 2 re,. L relative liquid permeability applied pressure difference AP capillary pressure PK caoillarv entrv oressure PKE universal gas-constant R specific cake resistance rC filter medium resistance R4 cake saturation z residual saturation T‘ absolute temperature cake formation time t1 dewatering time tz mean particle diameter of surface distribution T’,’ molar concentration per surface area surface tension (liquid-air) YL surface tension (solid-air) Y. interfacial tension (solid-liquid) YSL surface tension (water-air) YW contact angle b E porosity dynamic fluid viscosity ‘1L liquid density PL
z
n 1
5,5
10,o
12,5
moisture
15,o
content
mc
17,5 [w-t.-%]
Fig 7. Influence of surfactants on capillary pressure curve 0 without surfactant yL = 73.1 0m3Nlm 0 non-ionic surfactant 1 YL = 33. 10e3N/m A anionic surfactant 1 YL = 33.1 0e3N/m --- calculated value due to surface tension Filtration
& Separation
May/June
1990
20,o
REFERENCES I Bott. R, Zur kontmuierlichen Drucktiltratlon. GDMB-Vcrlag. Clawthai Zcllcrtcld (1986). 2 Lanpc. H. EintluR van Tcnsidcn und andercn St&en au! die Sedimentanon VCIschiedener Pulver I” Wasser: Kolloid-Z. u. Z. flir Polymcrc 211 (1966) Nr I-2. 106-113 3 Kowwg. K. Tcnsidc: Ullmanns Enzyklopadle der techni\chen Chcmx. Band 22: Vcrlap Chemie. W&ham (1982). 4 Schuhcrt. H. Kapillar~tat in pororen Feststoff\y\tcmcn. Sprmgcr Vcrlag. BcrhnHeidelberg-New York (1986). 5. Anlauf, H. Enrteuchtung van Filterkuchcn hci dcr Vakuum-, Druck-und Druck:Vdkuumfiltrimun. VDI Vcrlag. Dusxldorf (1986)
199