Chapter 11 : Solid-Liquid Separation - Chapter Notes, Engineering Chemical Engineering Notes | EduRev

Chemical Engineering : Chapter 11 : Solid-Liquid Separation - Chapter Notes, Engineering Chemical Engineering Notes | EduRev

 Page 1


I1 
SOLID-LIQUID SEPARATION 
olid-liquid separation is concerned with mechanical 
processes for the separation of liquids and finely 
divided insoluble solids. 
S 
11.1. PROCESSES AND EQUIPMENT 
Much equipment for the separation of liquids and finely divided 
solids was invented independently in a number of industries and is 
of diverse character. These developments have occurred without 
benefit of any but the most general theoretical considerations. Even 
at present, the selection of equipment for specific solid-liquid 
separation applications is largely a process of scale-up based on 
direct experimentation with the process material. 
The nature and sizing of equipment depends on the economic 
values and proportions of the phases as well as certain physical 
properties that influence relative movements of liquids and 
particles. Pressure often is the main operating variable so its effect 
on physical properties should be known. Table 11.1 is a broad 
classification of mechanical processes of solid-liquid separation. 
Clarification is the removal of small contents of worthless solids 
from a valuable liquid. Filtration is applied to the recovery of 
valuable solids from slurries. Expression is the removal of relatively 
small contents of liquids from compressible sludges by mechanical 
means. 
Whenever feasible, solids are settled out by gravity or with the 
aid of centrifugation. In dense media separation, an essentially 
homogeneous liquid phase is made by mixing in finely divided solids 
(less than 100mesh) of high density; specific gravity of 2.5 can be 
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and 
coal are floated away from gangue by such means. In flotation, 
surface active agents induce valuable solids to adhere to gas bubbles 
which are skimmed off. Magnetic separation also is practiced when 
feasible. Thickeners are vessels that provide sufficient residence 
time for settling to take place. Classifiers incorporate a mild raking 
action to prevent the entrapment of fine particles by the coarser 
ones that are to be settled out. Classification also is accomplished in 
hydrocyclones with moderate centrifugal action. 
TABLE 11.1. Chief Mechanical Means of 
Solid-Liquid Separation 
1. Settling 
a. by gravi 
I. in thizeners 
ii. in classifiers 
b. by centrifugal force 
c. by air flotation 
d. by dense media flotation 
e. by magnetic properties 
a. on screens, by gravity 
b. on filters 
2. Filtration 
I. byvacuum 
ii. by pressure 
iii. by centrifugation 
a. with batch presses 
b. with continuous presses 
I. screw presses 
ii. rolls 
iii. discs 
3. Expression 
Freely draining solids may be filtered by gravity with horizontal 
screens, but often filtration requires a substantial pressure 
difference across a filtering surface. An indication of the kind of 
equipment that may be suitable can be obtained by observations of 
sedimentation behavior or of rates of filtration in laboratory vacuum 
equipment. Figure 11.1 illustrates typical progress of sedimentation. 
Such tests are particularly used to evaluate possible flocculating 
processes or agents. Table 11.2 is a classification of equipment 
based on laboratory tests; test rates of cake formation range from 
several cm/sec to fractions of a cm/hr. 
Characteristics of the performance of the main types of 
commercial SLS equipment are summarized in Table 11.3. The 
completeness of the removal of liquid from the solid and of solid 
from the liquid may be important factors. In some kinds of 
equipment residual liquid can be removed by blowing air or other 
gas through the cake. When the liquid contains dissolved substances 
that are undesirable in the filter cake, the slurry may be followed by 
I--. 
___--- 
** ___-- Zone 4 
Time 
~~~~~ i .:,.:.:. ... . .. ....,. ..._ :.:.:.:.:.:.:. 
I I 
Time 
Figure 11.1. Sedimentation behavior of a slurry, showing loose and 
compacted zones (Osborne, 1981). 
Page 2


I1 
SOLID-LIQUID SEPARATION 
olid-liquid separation is concerned with mechanical 
processes for the separation of liquids and finely 
divided insoluble solids. 
S 
11.1. PROCESSES AND EQUIPMENT 
Much equipment for the separation of liquids and finely divided 
solids was invented independently in a number of industries and is 
of diverse character. These developments have occurred without 
benefit of any but the most general theoretical considerations. Even 
at present, the selection of equipment for specific solid-liquid 
separation applications is largely a process of scale-up based on 
direct experimentation with the process material. 
The nature and sizing of equipment depends on the economic 
values and proportions of the phases as well as certain physical 
properties that influence relative movements of liquids and 
particles. Pressure often is the main operating variable so its effect 
on physical properties should be known. Table 11.1 is a broad 
classification of mechanical processes of solid-liquid separation. 
Clarification is the removal of small contents of worthless solids 
from a valuable liquid. Filtration is applied to the recovery of 
valuable solids from slurries. Expression is the removal of relatively 
small contents of liquids from compressible sludges by mechanical 
means. 
Whenever feasible, solids are settled out by gravity or with the 
aid of centrifugation. In dense media separation, an essentially 
homogeneous liquid phase is made by mixing in finely divided solids 
(less than 100mesh) of high density; specific gravity of 2.5 can be 
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and 
coal are floated away from gangue by such means. In flotation, 
surface active agents induce valuable solids to adhere to gas bubbles 
which are skimmed off. Magnetic separation also is practiced when 
feasible. Thickeners are vessels that provide sufficient residence 
time for settling to take place. Classifiers incorporate a mild raking 
action to prevent the entrapment of fine particles by the coarser 
ones that are to be settled out. Classification also is accomplished in 
hydrocyclones with moderate centrifugal action. 
TABLE 11.1. Chief Mechanical Means of 
Solid-Liquid Separation 
1. Settling 
a. by gravi 
I. in thizeners 
ii. in classifiers 
b. by centrifugal force 
c. by air flotation 
d. by dense media flotation 
e. by magnetic properties 
a. on screens, by gravity 
b. on filters 
2. Filtration 
I. byvacuum 
ii. by pressure 
iii. by centrifugation 
a. with batch presses 
b. with continuous presses 
I. screw presses 
ii. rolls 
iii. discs 
3. Expression 
Freely draining solids may be filtered by gravity with horizontal 
screens, but often filtration requires a substantial pressure 
difference across a filtering surface. An indication of the kind of 
equipment that may be suitable can be obtained by observations of 
sedimentation behavior or of rates of filtration in laboratory vacuum 
equipment. Figure 11.1 illustrates typical progress of sedimentation. 
Such tests are particularly used to evaluate possible flocculating 
processes or agents. Table 11.2 is a classification of equipment 
based on laboratory tests; test rates of cake formation range from 
several cm/sec to fractions of a cm/hr. 
Characteristics of the performance of the main types of 
commercial SLS equipment are summarized in Table 11.3. The 
completeness of the removal of liquid from the solid and of solid 
from the liquid may be important factors. In some kinds of 
equipment residual liquid can be removed by blowing air or other 
gas through the cake. When the liquid contains dissolved substances 
that are undesirable in the filter cake, the slurry may be followed by 
I--. 
___--- 
** ___-- Zone 4 
Time 
~~~~~ i .:,.:.:. ... . .. ....,. ..._ :.:.:.:.:.:.:. 
I I 
Time 
Figure 11.1. Sedimentation behavior of a slurry, showing loose and 
compacted zones (Osborne, 1981). 
306 SOLID-LIQUID SEPARATION 
TABLE 11.2. Equipment Selection on the Basis of Rate of 
Cake Buildup 
Process Type 
Rapid 
filtering 
Medium 
Slow 
filtering 
filtering 
Clarification 
Rate of 
Cake Buildup 
0.1-10 cm/sec 
0.1-10 cm/min 
0.1-lOcm/hr 
negligible 
cake 
Suitable Equipment 
gravity pans; horizontal belt or 
top feed drum; continuous 
pusher type centrifuge 
vacuum drum or disk or pan or 
belt; peeler type centrifuge 
pressure filters; disc and tubular 
centrifuges; sedimenting 
centrifuges 
cartridges; precoat drums; filter 
aid systems; sand deep bed 
filters 
(Tiller and Crump, 1977; Flood, Parker, and Rennie, 1966). 
pure water to displace the residual filtrate. Qualitative cost 
comparisons also are shown in this table. Similar comparisons of 
filtering and sedimentation types of centrifuges are in Table 11.19. 
Final selection of filtering equipment is inadvisable without 
some testing in the laboratory and pilot plant. A few details of such 
work are mentioned later in this chapter. Figure 11.2 is an outline 
of a procedure for the selection of filter types on the basis of 
appropriate test work. Vendors need a certain amount of in- 
formation before they can specify and price equipment; typical 
inquiry forms are in Appendix C. Briefly, the desirable information 
includes the following. 
1. Flowsketch of the process of which the filtration is a part, with 
the expected qualities and quantities of the filtrate and cake. 
2. Properties of the feed: amounts, size distribution, densities and 
chemical analyses. 
3. Laboratory observations of sedimentation and leaf filtering rates. 
4. Pretreatment options that may be used. 
5. Washing and blowing requirements. 
6. Materials of construction. 
A major aspect of an SLS process may be conditioning of the 
slurry to improve its filterability. Table 11.4 summarizes common 
pretreatment techniques, and Table 11.5 lists a number of 
flocculants and their applications. Some discussion of pretreatment 
is in Section 11.3. 
11.2. THEORY OF FILTRATION 
Filterability of slurries depends so markedly on small and 
unidentified differences in conditions of formation and aging that no 
correlations of this behavior have been made. In fact, the situation 
is so discouraging that some practitioners have dismissed existing 
filtration theory as virtually worthless for representing filtration 
behavior. Qualitatively, however, simple filtration theory is 
directionally valid for modest scale-up and it may provide a 
structure on which more complete theory and data can be 
assembled in the future. 
As filtration proceeds, a porous cake of solid particles is built 
up on a porous medium, usually a supported cloth. Because of the 
fineness of the pores the flow of liquid is laminar so it is represented 
by the equation 
dV AAP 
dt pR 
e=-=- 
(11.1) 
The resistance R is made up of those of the filter cloth Rf and that 
of the cake R, which may be assumed proportional to the weight of 
the cake. Accordingly, 
e--- dV - A AP 
dt p(Rf + R,) -p(Rf + acV/A) ' 
(11.2) 
a = specific resistance of the cake (m/kg), 
c = wt of solids/volume of liquid (kg/m3), 
p = viscosity (N sec/m') 
P = pressure difference (N/m2) 
A = filtering surface (m2) 
V = volume of filtrate (m') 
Q = rate of filtrate accumulation (m'/sec). 
R, and a are constants of the equipment and slurry and must be 
evaluated from experimental data. The simplest data to analyze are 
those obtained from constant pressure or constant rate tests for 
which the equations will be developed. At constant pressure Eq. 
(11.2) is integrated as 
LYC 
ALPt = RfV + - V2 
v 2A 
and is recast into linear form as 
t pac v 
R +-- 
VIA AP I 2APA 
__=_ 
(11.3) 
(11.4) 
The constants Rf and a are derivable from the intercept and slope 
of the plot of t/V against V. Example 11.1 does this. If the constant 
pressure period sets in when t = to and V = V,, Eq. (11.4) becomes 
A plot of the left hand side against V + V, should be linear. 
At constant rate of filtration, Eq. (11.2) can be written 
V AAP 
t p(Rf + acV/A) 
e=-= 
and rearranged into the linear form 
_-_=_ Ap Ap p"f+qy, 
Q -V/t A A 
(11.5) 
(11.6) 
(11.7) 
The constants again are found from the intercept and slope of the 
linear plot of AP/Q against V. 
After the constants have been determined, Eq. (11.7) can be 
employed to predict filtration performance under a variety of 
constant rate conditions. For instance, the slurry may be charged 
with a centrifugal pump with a known characteristic curve of output 
pressure against flow rate. Such curves often may be represented by 
parabolic relations, as in Example 11.2, where the data are fitted by 
an equation of the form 
P= a - Q(b + cQ). (11.8) 
The time required for a specified amount of filtrate is found by 
integration of 
PV 
t=), dV/Q. (11.9) 
Page 3


I1 
SOLID-LIQUID SEPARATION 
olid-liquid separation is concerned with mechanical 
processes for the separation of liquids and finely 
divided insoluble solids. 
S 
11.1. PROCESSES AND EQUIPMENT 
Much equipment for the separation of liquids and finely divided 
solids was invented independently in a number of industries and is 
of diverse character. These developments have occurred without 
benefit of any but the most general theoretical considerations. Even 
at present, the selection of equipment for specific solid-liquid 
separation applications is largely a process of scale-up based on 
direct experimentation with the process material. 
The nature and sizing of equipment depends on the economic 
values and proportions of the phases as well as certain physical 
properties that influence relative movements of liquids and 
particles. Pressure often is the main operating variable so its effect 
on physical properties should be known. Table 11.1 is a broad 
classification of mechanical processes of solid-liquid separation. 
Clarification is the removal of small contents of worthless solids 
from a valuable liquid. Filtration is applied to the recovery of 
valuable solids from slurries. Expression is the removal of relatively 
small contents of liquids from compressible sludges by mechanical 
means. 
Whenever feasible, solids are settled out by gravity or with the 
aid of centrifugation. In dense media separation, an essentially 
homogeneous liquid phase is made by mixing in finely divided solids 
(less than 100mesh) of high density; specific gravity of 2.5 can be 
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and 
coal are floated away from gangue by such means. In flotation, 
surface active agents induce valuable solids to adhere to gas bubbles 
which are skimmed off. Magnetic separation also is practiced when 
feasible. Thickeners are vessels that provide sufficient residence 
time for settling to take place. Classifiers incorporate a mild raking 
action to prevent the entrapment of fine particles by the coarser 
ones that are to be settled out. Classification also is accomplished in 
hydrocyclones with moderate centrifugal action. 
TABLE 11.1. Chief Mechanical Means of 
Solid-Liquid Separation 
1. Settling 
a. by gravi 
I. in thizeners 
ii. in classifiers 
b. by centrifugal force 
c. by air flotation 
d. by dense media flotation 
e. by magnetic properties 
a. on screens, by gravity 
b. on filters 
2. Filtration 
I. byvacuum 
ii. by pressure 
iii. by centrifugation 
a. with batch presses 
b. with continuous presses 
I. screw presses 
ii. rolls 
iii. discs 
3. Expression 
Freely draining solids may be filtered by gravity with horizontal 
screens, but often filtration requires a substantial pressure 
difference across a filtering surface. An indication of the kind of 
equipment that may be suitable can be obtained by observations of 
sedimentation behavior or of rates of filtration in laboratory vacuum 
equipment. Figure 11.1 illustrates typical progress of sedimentation. 
Such tests are particularly used to evaluate possible flocculating 
processes or agents. Table 11.2 is a classification of equipment 
based on laboratory tests; test rates of cake formation range from 
several cm/sec to fractions of a cm/hr. 
Characteristics of the performance of the main types of 
commercial SLS equipment are summarized in Table 11.3. The 
completeness of the removal of liquid from the solid and of solid 
from the liquid may be important factors. In some kinds of 
equipment residual liquid can be removed by blowing air or other 
gas through the cake. When the liquid contains dissolved substances 
that are undesirable in the filter cake, the slurry may be followed by 
I--. 
___--- 
** ___-- Zone 4 
Time 
~~~~~ i .:,.:.:. ... . .. ....,. ..._ :.:.:.:.:.:.:. 
I I 
Time 
Figure 11.1. Sedimentation behavior of a slurry, showing loose and 
compacted zones (Osborne, 1981). 
306 SOLID-LIQUID SEPARATION 
TABLE 11.2. Equipment Selection on the Basis of Rate of 
Cake Buildup 
Process Type 
Rapid 
filtering 
Medium 
Slow 
filtering 
filtering 
Clarification 
Rate of 
Cake Buildup 
0.1-10 cm/sec 
0.1-10 cm/min 
0.1-lOcm/hr 
negligible 
cake 
Suitable Equipment 
gravity pans; horizontal belt or 
top feed drum; continuous 
pusher type centrifuge 
vacuum drum or disk or pan or 
belt; peeler type centrifuge 
pressure filters; disc and tubular 
centrifuges; sedimenting 
centrifuges 
cartridges; precoat drums; filter 
aid systems; sand deep bed 
filters 
(Tiller and Crump, 1977; Flood, Parker, and Rennie, 1966). 
pure water to displace the residual filtrate. Qualitative cost 
comparisons also are shown in this table. Similar comparisons of 
filtering and sedimentation types of centrifuges are in Table 11.19. 
Final selection of filtering equipment is inadvisable without 
some testing in the laboratory and pilot plant. A few details of such 
work are mentioned later in this chapter. Figure 11.2 is an outline 
of a procedure for the selection of filter types on the basis of 
appropriate test work. Vendors need a certain amount of in- 
formation before they can specify and price equipment; typical 
inquiry forms are in Appendix C. Briefly, the desirable information 
includes the following. 
1. Flowsketch of the process of which the filtration is a part, with 
the expected qualities and quantities of the filtrate and cake. 
2. Properties of the feed: amounts, size distribution, densities and 
chemical analyses. 
3. Laboratory observations of sedimentation and leaf filtering rates. 
4. Pretreatment options that may be used. 
5. Washing and blowing requirements. 
6. Materials of construction. 
A major aspect of an SLS process may be conditioning of the 
slurry to improve its filterability. Table 11.4 summarizes common 
pretreatment techniques, and Table 11.5 lists a number of 
flocculants and their applications. Some discussion of pretreatment 
is in Section 11.3. 
11.2. THEORY OF FILTRATION 
Filterability of slurries depends so markedly on small and 
unidentified differences in conditions of formation and aging that no 
correlations of this behavior have been made. In fact, the situation 
is so discouraging that some practitioners have dismissed existing 
filtration theory as virtually worthless for representing filtration 
behavior. Qualitatively, however, simple filtration theory is 
directionally valid for modest scale-up and it may provide a 
structure on which more complete theory and data can be 
assembled in the future. 
As filtration proceeds, a porous cake of solid particles is built 
up on a porous medium, usually a supported cloth. Because of the 
fineness of the pores the flow of liquid is laminar so it is represented 
by the equation 
dV AAP 
dt pR 
e=-=- 
(11.1) 
The resistance R is made up of those of the filter cloth Rf and that 
of the cake R, which may be assumed proportional to the weight of 
the cake. Accordingly, 
e--- dV - A AP 
dt p(Rf + R,) -p(Rf + acV/A) ' 
(11.2) 
a = specific resistance of the cake (m/kg), 
c = wt of solids/volume of liquid (kg/m3), 
p = viscosity (N sec/m') 
P = pressure difference (N/m2) 
A = filtering surface (m2) 
V = volume of filtrate (m') 
Q = rate of filtrate accumulation (m'/sec). 
R, and a are constants of the equipment and slurry and must be 
evaluated from experimental data. The simplest data to analyze are 
those obtained from constant pressure or constant rate tests for 
which the equations will be developed. At constant pressure Eq. 
(11.2) is integrated as 
LYC 
ALPt = RfV + - V2 
v 2A 
and is recast into linear form as 
t pac v 
R +-- 
VIA AP I 2APA 
__=_ 
(11.3) 
(11.4) 
The constants Rf and a are derivable from the intercept and slope 
of the plot of t/V against V. Example 11.1 does this. If the constant 
pressure period sets in when t = to and V = V,, Eq. (11.4) becomes 
A plot of the left hand side against V + V, should be linear. 
At constant rate of filtration, Eq. (11.2) can be written 
V AAP 
t p(Rf + acV/A) 
e=-= 
and rearranged into the linear form 
_-_=_ Ap Ap p"f+qy, 
Q -V/t A A 
(11.5) 
(11.6) 
(11.7) 
The constants again are found from the intercept and slope of the 
linear plot of AP/Q against V. 
After the constants have been determined, Eq. (11.7) can be 
employed to predict filtration performance under a variety of 
constant rate conditions. For instance, the slurry may be charged 
with a centrifugal pump with a known characteristic curve of output 
pressure against flow rate. Such curves often may be represented by 
parabolic relations, as in Example 11.2, where the data are fitted by 
an equation of the form 
P= a - Q(b + cQ). (11.8) 
The time required for a specified amount of filtrate is found by 
integration of 
PV 
t=), dV/Q. (11.9) 
307 
Page 4


I1 
SOLID-LIQUID SEPARATION 
olid-liquid separation is concerned with mechanical 
processes for the separation of liquids and finely 
divided insoluble solids. 
S 
11.1. PROCESSES AND EQUIPMENT 
Much equipment for the separation of liquids and finely divided 
solids was invented independently in a number of industries and is 
of diverse character. These developments have occurred without 
benefit of any but the most general theoretical considerations. Even 
at present, the selection of equipment for specific solid-liquid 
separation applications is largely a process of scale-up based on 
direct experimentation with the process material. 
The nature and sizing of equipment depends on the economic 
values and proportions of the phases as well as certain physical 
properties that influence relative movements of liquids and 
particles. Pressure often is the main operating variable so its effect 
on physical properties should be known. Table 11.1 is a broad 
classification of mechanical processes of solid-liquid separation. 
Clarification is the removal of small contents of worthless solids 
from a valuable liquid. Filtration is applied to the recovery of 
valuable solids from slurries. Expression is the removal of relatively 
small contents of liquids from compressible sludges by mechanical 
means. 
Whenever feasible, solids are settled out by gravity or with the 
aid of centrifugation. In dense media separation, an essentially 
homogeneous liquid phase is made by mixing in finely divided solids 
(less than 100mesh) of high density; specific gravity of 2.5 can be 
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and 
coal are floated away from gangue by such means. In flotation, 
surface active agents induce valuable solids to adhere to gas bubbles 
which are skimmed off. Magnetic separation also is practiced when 
feasible. Thickeners are vessels that provide sufficient residence 
time for settling to take place. Classifiers incorporate a mild raking 
action to prevent the entrapment of fine particles by the coarser 
ones that are to be settled out. Classification also is accomplished in 
hydrocyclones with moderate centrifugal action. 
TABLE 11.1. Chief Mechanical Means of 
Solid-Liquid Separation 
1. Settling 
a. by gravi 
I. in thizeners 
ii. in classifiers 
b. by centrifugal force 
c. by air flotation 
d. by dense media flotation 
e. by magnetic properties 
a. on screens, by gravity 
b. on filters 
2. Filtration 
I. byvacuum 
ii. by pressure 
iii. by centrifugation 
a. with batch presses 
b. with continuous presses 
I. screw presses 
ii. rolls 
iii. discs 
3. Expression 
Freely draining solids may be filtered by gravity with horizontal 
screens, but often filtration requires a substantial pressure 
difference across a filtering surface. An indication of the kind of 
equipment that may be suitable can be obtained by observations of 
sedimentation behavior or of rates of filtration in laboratory vacuum 
equipment. Figure 11.1 illustrates typical progress of sedimentation. 
Such tests are particularly used to evaluate possible flocculating 
processes or agents. Table 11.2 is a classification of equipment 
based on laboratory tests; test rates of cake formation range from 
several cm/sec to fractions of a cm/hr. 
Characteristics of the performance of the main types of 
commercial SLS equipment are summarized in Table 11.3. The 
completeness of the removal of liquid from the solid and of solid 
from the liquid may be important factors. In some kinds of 
equipment residual liquid can be removed by blowing air or other 
gas through the cake. When the liquid contains dissolved substances 
that are undesirable in the filter cake, the slurry may be followed by 
I--. 
___--- 
** ___-- Zone 4 
Time 
~~~~~ i .:,.:.:. ... . .. ....,. ..._ :.:.:.:.:.:.:. 
I I 
Time 
Figure 11.1. Sedimentation behavior of a slurry, showing loose and 
compacted zones (Osborne, 1981). 
306 SOLID-LIQUID SEPARATION 
TABLE 11.2. Equipment Selection on the Basis of Rate of 
Cake Buildup 
Process Type 
Rapid 
filtering 
Medium 
Slow 
filtering 
filtering 
Clarification 
Rate of 
Cake Buildup 
0.1-10 cm/sec 
0.1-10 cm/min 
0.1-lOcm/hr 
negligible 
cake 
Suitable Equipment 
gravity pans; horizontal belt or 
top feed drum; continuous 
pusher type centrifuge 
vacuum drum or disk or pan or 
belt; peeler type centrifuge 
pressure filters; disc and tubular 
centrifuges; sedimenting 
centrifuges 
cartridges; precoat drums; filter 
aid systems; sand deep bed 
filters 
(Tiller and Crump, 1977; Flood, Parker, and Rennie, 1966). 
pure water to displace the residual filtrate. Qualitative cost 
comparisons also are shown in this table. Similar comparisons of 
filtering and sedimentation types of centrifuges are in Table 11.19. 
Final selection of filtering equipment is inadvisable without 
some testing in the laboratory and pilot plant. A few details of such 
work are mentioned later in this chapter. Figure 11.2 is an outline 
of a procedure for the selection of filter types on the basis of 
appropriate test work. Vendors need a certain amount of in- 
formation before they can specify and price equipment; typical 
inquiry forms are in Appendix C. Briefly, the desirable information 
includes the following. 
1. Flowsketch of the process of which the filtration is a part, with 
the expected qualities and quantities of the filtrate and cake. 
2. Properties of the feed: amounts, size distribution, densities and 
chemical analyses. 
3. Laboratory observations of sedimentation and leaf filtering rates. 
4. Pretreatment options that may be used. 
5. Washing and blowing requirements. 
6. Materials of construction. 
A major aspect of an SLS process may be conditioning of the 
slurry to improve its filterability. Table 11.4 summarizes common 
pretreatment techniques, and Table 11.5 lists a number of 
flocculants and their applications. Some discussion of pretreatment 
is in Section 11.3. 
11.2. THEORY OF FILTRATION 
Filterability of slurries depends so markedly on small and 
unidentified differences in conditions of formation and aging that no 
correlations of this behavior have been made. In fact, the situation 
is so discouraging that some practitioners have dismissed existing 
filtration theory as virtually worthless for representing filtration 
behavior. Qualitatively, however, simple filtration theory is 
directionally valid for modest scale-up and it may provide a 
structure on which more complete theory and data can be 
assembled in the future. 
As filtration proceeds, a porous cake of solid particles is built 
up on a porous medium, usually a supported cloth. Because of the 
fineness of the pores the flow of liquid is laminar so it is represented 
by the equation 
dV AAP 
dt pR 
e=-=- 
(11.1) 
The resistance R is made up of those of the filter cloth Rf and that 
of the cake R, which may be assumed proportional to the weight of 
the cake. Accordingly, 
e--- dV - A AP 
dt p(Rf + R,) -p(Rf + acV/A) ' 
(11.2) 
a = specific resistance of the cake (m/kg), 
c = wt of solids/volume of liquid (kg/m3), 
p = viscosity (N sec/m') 
P = pressure difference (N/m2) 
A = filtering surface (m2) 
V = volume of filtrate (m') 
Q = rate of filtrate accumulation (m'/sec). 
R, and a are constants of the equipment and slurry and must be 
evaluated from experimental data. The simplest data to analyze are 
those obtained from constant pressure or constant rate tests for 
which the equations will be developed. At constant pressure Eq. 
(11.2) is integrated as 
LYC 
ALPt = RfV + - V2 
v 2A 
and is recast into linear form as 
t pac v 
R +-- 
VIA AP I 2APA 
__=_ 
(11.3) 
(11.4) 
The constants Rf and a are derivable from the intercept and slope 
of the plot of t/V against V. Example 11.1 does this. If the constant 
pressure period sets in when t = to and V = V,, Eq. (11.4) becomes 
A plot of the left hand side against V + V, should be linear. 
At constant rate of filtration, Eq. (11.2) can be written 
V AAP 
t p(Rf + acV/A) 
e=-= 
and rearranged into the linear form 
_-_=_ Ap Ap p"f+qy, 
Q -V/t A A 
(11.5) 
(11.6) 
(11.7) 
The constants again are found from the intercept and slope of the 
linear plot of AP/Q against V. 
After the constants have been determined, Eq. (11.7) can be 
employed to predict filtration performance under a variety of 
constant rate conditions. For instance, the slurry may be charged 
with a centrifugal pump with a known characteristic curve of output 
pressure against flow rate. Such curves often may be represented by 
parabolic relations, as in Example 11.2, where the data are fitted by 
an equation of the form 
P= a - Q(b + cQ). (11.8) 
The time required for a specified amount of filtrate is found by 
integration of 
PV 
t=), dV/Q. (11.9) 
307 
308 SOLID-LIQUID SEPARATION 
Laboratory routine 
FIIIOI sizing ana 
process costing 
Flnal lest work 
I 
. Hydracyclone test 
lube centrifuge Sedimentat ion I 
test test 
I 
Mognet test 
1 I 
Select filter medium from those 
with suitable chemical resistance 
rl another medium 
Qs clarity sotislactory? 
Try grade either side of chosen medium 
and choose fostest permissible prode 
Plot total flow 
1s filter aid required? 
Precoat vacuum 
leaf test 
Is form rate>’/16mch in 3min 
Perforated basket 
centrifuge test 
t 
Vacuum Ieof test 
Is pickup satisfactory? 
Complete the 
investigation 
I 
Thicken 
the slurry 
I 
. Mognetic seporotor 
. Sedimentat ion cent rif uges -- 
Continuous nozzle 
Batch tubular bowl 
Batch disc bowl 
Batch disc bowl. 
self -opening 
. Continuous rotory 
prccaat filter 
. Batch centrifugal filters - 
-Continuous rotary 
vacuum filter 
. Centrifugal filters 
Continuous pusher 
Continuous worm discharge 
Continuous oscillating screer 
- Helical conveyor decanter - 
- Continuous table filter - 
centrifuge 
- Various pressure filters - 
Continuous arum 
Batch plate 
Balch tubular element 
Batch cartridge 
Batch piate and frame 
Botch leaf 
Figure 11.2. Experimental routine for aiding the selection of solid-liquid separation equipment (Dauies, 1965). 
Page 5


I1 
SOLID-LIQUID SEPARATION 
olid-liquid separation is concerned with mechanical 
processes for the separation of liquids and finely 
divided insoluble solids. 
S 
11.1. PROCESSES AND EQUIPMENT 
Much equipment for the separation of liquids and finely divided 
solids was invented independently in a number of industries and is 
of diverse character. These developments have occurred without 
benefit of any but the most general theoretical considerations. Even 
at present, the selection of equipment for specific solid-liquid 
separation applications is largely a process of scale-up based on 
direct experimentation with the process material. 
The nature and sizing of equipment depends on the economic 
values and proportions of the phases as well as certain physical 
properties that influence relative movements of liquids and 
particles. Pressure often is the main operating variable so its effect 
on physical properties should be known. Table 11.1 is a broad 
classification of mechanical processes of solid-liquid separation. 
Clarification is the removal of small contents of worthless solids 
from a valuable liquid. Filtration is applied to the recovery of 
valuable solids from slurries. Expression is the removal of relatively 
small contents of liquids from compressible sludges by mechanical 
means. 
Whenever feasible, solids are settled out by gravity or with the 
aid of centrifugation. In dense media separation, an essentially 
homogeneous liquid phase is made by mixing in finely divided solids 
(less than 100mesh) of high density; specific gravity of 2.5 can be 
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and 
coal are floated away from gangue by such means. In flotation, 
surface active agents induce valuable solids to adhere to gas bubbles 
which are skimmed off. Magnetic separation also is practiced when 
feasible. Thickeners are vessels that provide sufficient residence 
time for settling to take place. Classifiers incorporate a mild raking 
action to prevent the entrapment of fine particles by the coarser 
ones that are to be settled out. Classification also is accomplished in 
hydrocyclones with moderate centrifugal action. 
TABLE 11.1. Chief Mechanical Means of 
Solid-Liquid Separation 
1. Settling 
a. by gravi 
I. in thizeners 
ii. in classifiers 
b. by centrifugal force 
c. by air flotation 
d. by dense media flotation 
e. by magnetic properties 
a. on screens, by gravity 
b. on filters 
2. Filtration 
I. byvacuum 
ii. by pressure 
iii. by centrifugation 
a. with batch presses 
b. with continuous presses 
I. screw presses 
ii. rolls 
iii. discs 
3. Expression 
Freely draining solids may be filtered by gravity with horizontal 
screens, but often filtration requires a substantial pressure 
difference across a filtering surface. An indication of the kind of 
equipment that may be suitable can be obtained by observations of 
sedimentation behavior or of rates of filtration in laboratory vacuum 
equipment. Figure 11.1 illustrates typical progress of sedimentation. 
Such tests are particularly used to evaluate possible flocculating 
processes or agents. Table 11.2 is a classification of equipment 
based on laboratory tests; test rates of cake formation range from 
several cm/sec to fractions of a cm/hr. 
Characteristics of the performance of the main types of 
commercial SLS equipment are summarized in Table 11.3. The 
completeness of the removal of liquid from the solid and of solid 
from the liquid may be important factors. In some kinds of 
equipment residual liquid can be removed by blowing air or other 
gas through the cake. When the liquid contains dissolved substances 
that are undesirable in the filter cake, the slurry may be followed by 
I--. 
___--- 
** ___-- Zone 4 
Time 
~~~~~ i .:,.:.:. ... . .. ....,. ..._ :.:.:.:.:.:.:. 
I I 
Time 
Figure 11.1. Sedimentation behavior of a slurry, showing loose and 
compacted zones (Osborne, 1981). 
306 SOLID-LIQUID SEPARATION 
TABLE 11.2. Equipment Selection on the Basis of Rate of 
Cake Buildup 
Process Type 
Rapid 
filtering 
Medium 
Slow 
filtering 
filtering 
Clarification 
Rate of 
Cake Buildup 
0.1-10 cm/sec 
0.1-10 cm/min 
0.1-lOcm/hr 
negligible 
cake 
Suitable Equipment 
gravity pans; horizontal belt or 
top feed drum; continuous 
pusher type centrifuge 
vacuum drum or disk or pan or 
belt; peeler type centrifuge 
pressure filters; disc and tubular 
centrifuges; sedimenting 
centrifuges 
cartridges; precoat drums; filter 
aid systems; sand deep bed 
filters 
(Tiller and Crump, 1977; Flood, Parker, and Rennie, 1966). 
pure water to displace the residual filtrate. Qualitative cost 
comparisons also are shown in this table. Similar comparisons of 
filtering and sedimentation types of centrifuges are in Table 11.19. 
Final selection of filtering equipment is inadvisable without 
some testing in the laboratory and pilot plant. A few details of such 
work are mentioned later in this chapter. Figure 11.2 is an outline 
of a procedure for the selection of filter types on the basis of 
appropriate test work. Vendors need a certain amount of in- 
formation before they can specify and price equipment; typical 
inquiry forms are in Appendix C. Briefly, the desirable information 
includes the following. 
1. Flowsketch of the process of which the filtration is a part, with 
the expected qualities and quantities of the filtrate and cake. 
2. Properties of the feed: amounts, size distribution, densities and 
chemical analyses. 
3. Laboratory observations of sedimentation and leaf filtering rates. 
4. Pretreatment options that may be used. 
5. Washing and blowing requirements. 
6. Materials of construction. 
A major aspect of an SLS process may be conditioning of the 
slurry to improve its filterability. Table 11.4 summarizes common 
pretreatment techniques, and Table 11.5 lists a number of 
flocculants and their applications. Some discussion of pretreatment 
is in Section 11.3. 
11.2. THEORY OF FILTRATION 
Filterability of slurries depends so markedly on small and 
unidentified differences in conditions of formation and aging that no 
correlations of this behavior have been made. In fact, the situation 
is so discouraging that some practitioners have dismissed existing 
filtration theory as virtually worthless for representing filtration 
behavior. Qualitatively, however, simple filtration theory is 
directionally valid for modest scale-up and it may provide a 
structure on which more complete theory and data can be 
assembled in the future. 
As filtration proceeds, a porous cake of solid particles is built 
up on a porous medium, usually a supported cloth. Because of the 
fineness of the pores the flow of liquid is laminar so it is represented 
by the equation 
dV AAP 
dt pR 
e=-=- 
(11.1) 
The resistance R is made up of those of the filter cloth Rf and that 
of the cake R, which may be assumed proportional to the weight of 
the cake. Accordingly, 
e--- dV - A AP 
dt p(Rf + R,) -p(Rf + acV/A) ' 
(11.2) 
a = specific resistance of the cake (m/kg), 
c = wt of solids/volume of liquid (kg/m3), 
p = viscosity (N sec/m') 
P = pressure difference (N/m2) 
A = filtering surface (m2) 
V = volume of filtrate (m') 
Q = rate of filtrate accumulation (m'/sec). 
R, and a are constants of the equipment and slurry and must be 
evaluated from experimental data. The simplest data to analyze are 
those obtained from constant pressure or constant rate tests for 
which the equations will be developed. At constant pressure Eq. 
(11.2) is integrated as 
LYC 
ALPt = RfV + - V2 
v 2A 
and is recast into linear form as 
t pac v 
R +-- 
VIA AP I 2APA 
__=_ 
(11.3) 
(11.4) 
The constants Rf and a are derivable from the intercept and slope 
of the plot of t/V against V. Example 11.1 does this. If the constant 
pressure period sets in when t = to and V = V,, Eq. (11.4) becomes 
A plot of the left hand side against V + V, should be linear. 
At constant rate of filtration, Eq. (11.2) can be written 
V AAP 
t p(Rf + acV/A) 
e=-= 
and rearranged into the linear form 
_-_=_ Ap Ap p"f+qy, 
Q -V/t A A 
(11.5) 
(11.6) 
(11.7) 
The constants again are found from the intercept and slope of the 
linear plot of AP/Q against V. 
After the constants have been determined, Eq. (11.7) can be 
employed to predict filtration performance under a variety of 
constant rate conditions. For instance, the slurry may be charged 
with a centrifugal pump with a known characteristic curve of output 
pressure against flow rate. Such curves often may be represented by 
parabolic relations, as in Example 11.2, where the data are fitted by 
an equation of the form 
P= a - Q(b + cQ). (11.8) 
The time required for a specified amount of filtrate is found by 
integration of 
PV 
t=), dV/Q. (11.9) 
307 
308 SOLID-LIQUID SEPARATION 
Laboratory routine 
FIIIOI sizing ana 
process costing 
Flnal lest work 
I 
. Hydracyclone test 
lube centrifuge Sedimentat ion I 
test test 
I 
Mognet test 
1 I 
Select filter medium from those 
with suitable chemical resistance 
rl another medium 
Qs clarity sotislactory? 
Try grade either side of chosen medium 
and choose fostest permissible prode 
Plot total flow 
1s filter aid required? 
Precoat vacuum 
leaf test 
Is form rate>’/16mch in 3min 
Perforated basket 
centrifuge test 
t 
Vacuum Ieof test 
Is pickup satisfactory? 
Complete the 
investigation 
I 
Thicken 
the slurry 
I 
. Mognetic seporotor 
. Sedimentat ion cent rif uges -- 
Continuous nozzle 
Batch tubular bowl 
Batch disc bowl 
Batch disc bowl. 
self -opening 
. Continuous rotory 
prccaat filter 
. Batch centrifugal filters - 
-Continuous rotary 
vacuum filter 
. Centrifugal filters 
Continuous pusher 
Continuous worm discharge 
Continuous oscillating screer 
- Helical conveyor decanter - 
- Continuous table filter - 
centrifuge 
- Various pressure filters - 
Continuous arum 
Batch plate 
Balch tubular element 
Batch cartridge 
Batch piate and frame 
Botch leaf 
Figure 11.2. Experimental routine for aiding the selection of solid-liquid separation equipment (Dauies, 1965). 
TABLE 11.4. Action and Effects of Slurry Pretreatments 
2. Solid 
particles 
4. Solid/liquid 
interaction 
Action On Technique EffeaS 
reduction of viscosity, thereby speeding 
filtration and settling rates and reducing 
cake moisture content 
medium or cake and impeding filtration 
ing particles to agglomerate into 
microflocs 
each other to permit further agglomera- 
tion into large flocs 
size of individual particles increases, e.g., 
by crystal growth 
rate of filtration increased, especially 
prevents gas bubbles forming within the 
destabilizes colloidal suspensions, allow- 
I 
1. Liquid 1. heating 
2. dilution with solvent 
3. degassing and stripping 
1. coagulation by chemical 
additives 
2. flocculation by natural 
or forced convection 
microflocs are brought into contact with 
3. aging 
1. increase by appropriate 
as settling tank, cyclone 
flotation cell or 
filter/thickener 
fines, using sedimentation 
or cyclone 
3. add filter powder (e.g., 
diatomite) or other solids 
to act as 'body aid' 
1. heat treatment, e.g., 
Porteus process 
involving pressure 
cooking 
2. freeze/thaw 
3. ultrasonics 
4. ionized radiation 
5. addition of wetting 
3. Concentration 
of solids first-stage device such if initial concentration <2% 
2. classifv to eliminate rate of filtration increased and cake 
moisture content reduced 
rate of filtration increased by more 
porous cake and possibly by high total 
solid concentration 
physical methods which condition sludge 
and induce coagulation and/or 
flocculation 
reduces the interfacial surface tension, 
improves the draining characteristics of 
the cake, and decreases the residual 
moisture content 
1 
agents 
(Purchas, 1981). 
TABLE 11.5. Natures and Applications of Typical Flocculants 
Normal Normal Approx. 
Ty eor Typical Range of pH Effective Price 
Trade Name Composition MecEanism Application Effectiveness Concentration per Ib" Manufacturer 
Alum 
Ferric 
sulfate 
Sodium 
CMC 
Kelgin W 
Separan 
Fi brefloc 
Corn 
starch 
Polynox 
Silica sol 
Sodium 
Guar gum 
Sulfuric 
acid 
aluminate 
AIZ(SO,),.XH2O 
Fe,(SO,)XH,O 
sodium carboxy- 
algins 
acrylamide 
polymer 
animal glue 
corn starch 
methylcellulose 
polyethylene 
oxide 
activated 
silica sol 
sodium 
guar gum 
HZSO4 
aluminate 
electrolytic and 
coagulation 
electrolytic 
coagulation 
coagulation and 
coagulation and 
bridging 
electrolytic 
bridging 
bridging 
electrolytic 
bridging 
bridging 
coagulation 
water treatment 
water treatment 
and chemical 
processing 
mineral 
processing 
water treatment 
chemical 
processing 
waste treatment 
mineral 
processing 
chemical 
processing 
waste treatment 
coagulation water treatment 
bridging mineral 
electrolytic waste treatment 
processing 
5-10 
any 
3-9 
4-1 1 
2-10 
1-9 
2-10 
2-10 
4-6 
3-12 
2-12 
1-5 
15 PPm 2e 
5-100 ppm 2e 
0.03-0.5 Ib/ton 5Oe 
up to 5 ppm $1.50 
0.2-1 0 ppm $1 .OO-$2.00 
5-30 ppm 1W 
10 Ib/ton 7e 
1-50 ppm $2.00 
1-20 ppm 1.5q as 
sodium 
silicate 
2-10 ppm 1 oe 
0.02-0.3 Ib/ton 35e 
highly variable le 
inorganic chemical 
manufacturers 
inorganic chemical 
manufacturers 
Hercules, DuPont 
Kelco Co. 
Dow Chemical Co. 
Armour and Co. 
- 
Union Carbide 
inorganic chemical 
manufacturers 
National Aluminate 
General Mills 
inorganic chemical 
manufacturers 
a 1966 prices, for comparison only. 
(Purchas, 1981). 
309 
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