The Selection of Flocculants and other Solid-Liquid Separation Aids

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE SPG PEG 307

The Selection of Flocculants and
other Solid-Liquid Separation Aids
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Process Engineering Guide:

The Selection of
Flocculants and other SolidLiquid Separation Aids

CONTENTS

0

FLOCCULANTS: BASIC PRINCIPLES

0.1
0.2
0.3

Coagulants and Flocculants
Surfactants
Dispersants

1

FLOCCULANTS: TEST PROCEDURES AND METHODS

1.1
1.2
1.3
1.4

Those Applicable to the Original Suspension
Those Applicable to the Flocculant Solution
General Tests Applicable to the Flocculation Process
Specific Tests Applicable to the Flocculation Process
and to the Flocculation System

2

EXAMPLES

2.1

Formation of M729 Vermiculite / Glass Fibre
Refractory Paper
Harvesting and Concentration of a Filamentous
Organism Suspension before Isolation of a
Pharmacologically-Active Compound

2.2

3

USE OF SURFACTANTS TO HELP REDUCE
FILTER-CAKE MOISTURE

4

REFERENCES


TABLES

1

A SUMMARY OF THE CHARACTERISTICS OF SYSTEMS
FLOCCULATED WITH THE AID OF POLYMERIC AND
SIMILAR AGENTS

2

THE IDEAL M729 FLOC STRUCTURE TO OPTIMIZE PARTICULAR
PAPER FORMING CHARACTERISTICS AND FINAL PAPER
PROPERTIES

3

SCHEMATIC REPRESENTATION OF THE EFFECTS ON PAPER
FORMING PROPERTIES OF THE PRINCIPAL VARIABLES IN THE
SYSTEM, INVOLVING ACID DISPERSION OF GLASS AND USE OF
NONIONIC OR ANIONIC FLOCCULANTS

4

THE MAJOR EFFECTS OF CHANGING PROCESS PARAMETERS ON
PAPER FORMING CHARACTERISTICS IN SYSTEMS INVOLVING USE
OF CATIONICFLOCCULANTS

FIGURES

1

A SUGGESTED SEQUENCE OF OPERATIONS IN THE SELECTION /
OPTIMIZATIONOF A FLOCCULANT

2

TYPICAL PROCEDURE FORMAKEUP OF AN M729/GF SLURRY
FORPAPER-MAKING


INTRODUCTION
The use of chemical additives, such as flocculants, is a common step in solidliquid separation operations. The correct selection of agent is an essential part of
the design of such processes. Many excellent reviews and guides deal with this
topic, and the interested reader is referred to works such as [l-4]. In particular the
Harwell-Warren Spring Report “The Use and Selection of Flocculants" provides a
good overview on the application of coagulants and flocculants. This section
does not attempt to reproduce a detailed treatment of that kind; instead it is our
intention to state a few general rules and principles concerning methods of
choosing an additive, and to illustrate briefly their application in practice.
The types of agents employed in solid-liquid separation fall into three principal
classes:
(i)

Coagulants and flocculants to facilitate the aggregation of fines to give
more easily separated species;

(ii)

Surfactants added to try to reduce filter-cake moisture content;

(iii)

Dispersants.

Generally, dispersing agents ((iii) above) are used not so much in the separation
operation itself but to help fluidize the concentrate obtained. We do not deal
further with this subject at this point but the theme is taken up again in GBHE
Suspension Processing Guides “Centrifugation” [GBHE SPG PEG 304],
“Sedimentation [GBHE SPG PEG 303], and “Filtration” [GBHE SPG PEG 305]
where examples of selection of dispersants to modify the rheological properties
of concentrated suspensions and sludge’s are described. Of the remainder,
coagulants and flocculants make up the preponderant part of processing aids
employed. They are dealt with in Sections 3 Surfactants added to help reduce
filter-cake moisture are less important but are considered in 3. A final class of
agents, solid filtration or sedimentation aids, are considered in GBHE SPG PEG
309 “Clarification”.


0

FLOCCULANTS: BASIC PRINCIPLES

There are two main types of flocculating agents:
(i)

Inorganic salt coagulants such as Ca2+, Al3+ compounds.

(ii)

Various organic, particularly polymeric, flocculating materials.

It is suggested that readers unfamiliar with the various mechanisms of
flocculation refer to “Centrifugation” [GBHE SPG PEG 304] for a synopsis of the
fundamentals of this topic.
The characteristics of inorganic salt flocculating systems are very straightforward:
Agents of this type (e.g. lime) can give a "cheap and cheerful" solution to
problems, there being little difficulty with steps such as flocculant/suspensions
mixing. For this reason such compounds ought to be given first consideration In
any screening process for flocculants (see Section 3 below). The main limitation
of inorganic coagulants is that, due to the particular mechanism of aggregate
growth, natural forces of attraction being relied upon to give interparticle
cohesion, the floes are usually rather open-textured and weak. This limits the
extent to which they can be handled conveniently.
As general rule coagulation efficiency increases rapidly with the charge on the
species used, i.e. Na+ and other monovalent species are less effective than
Ca2+, Mg2+ and so on. These, in turn, require higher dosages than Fe3+ or Al3+.
With the latter a complication arises since these agents are subject to hydrolysis;
iron or aluminium hydroxides are precipitated under certain conditions. In
practice this is usually an advantage as the precipitated material “sweeps up"
fines by entrainment or by surface adsorption. However, it does mean that
careful attention must be paid to pH and other factors to ensure that optimum
behavior is obtained. Further details on the use of Al3+ and other Inorganic salt
coagulants are contained in references [1, 2 ,4-6]. A description of settling, flow
and other characteristics of coagulated suspensions of this type are contained in
references [7-9] and in “Centrifugation” [GBHE SPG PEG 304], “Sedimentation
[GBHE SPG PEG 303].
With organic flocculants the position Is far more complicated. However, they offer
further degrees of control over suspension properties owing to the possibilities
they afford of differing flocculation mechanisms and differing floe structures from
those observed in simple coagulated systems. Table 1 summarizes the
characteristics of systems flocculated with various polymeric and similar agents,
including liquid bridging flocculants dealt with in “Sedimentation [GBHE SPG
PEG 303].

It should be noted that our nomenclature for agent type and flocculation
mechanism follows that defined in “Sedimentation [GBHE SPG PEG 303].
As should be evident from the Table, as a generality it is found that the higher
molecular weight (say > 50,000) polymeric flocculants are most useful since in
this size range phenomena such as polymer bridging between the surfaces of
particles can greatly change floc morphology and, probably more importantly,
robustness. Low molecular weight substances, in contrast, tend to give results
not too different from Inorganic coagulants, except in special cases.
More details on the fundamental properties of polymer flocculated systems are
given in references [10] and [11].

1

FLOCCULANTS: TEST PROCEDURES AND METHODS

Test methods for flocculants, for example to determine optimum dose, have been
dealt with in detail in many excellent texts and, indeed, in part elsewhere in this
work (see, for example, Section 3.5.6(b)). What we will attempt to describe In this
section are the kinds of techniques which should be used in an efficient
screening/optimization program, and, even more importantly, the sequence in
which they should be applied to arrive at a solution to a problem with minimum
effort. The tests which may be applied during a process of selection of a
flocculant divide conveniently into four classes:
(i) Those Applicable to the Original Suspension
This is to determine such factors as the chemical makeup of the particles,
their surface charge, particle size distribution, and background electrolyte
concentration. Usually certain of these data will already be known (e.g.
particle composition), or can be assumed with reasonable certainty (e.g.
the majority of colloidal suspensions contain negatively-charged particles.
However it is wise to have at least some idea of particle size and
background electrolyte concentration, whilst pH is an important variable if
polymer flocculants or hydrolyzing metal salt coagulants (e.g. Al3+ or Fe3+)
are to be used.


(ii) Those Applicable to the Flocculant Solution
Fortunately sufficient information on factors such as flocculant chemical
composition, charge or molecular weight will be available from sources
such as manufacturers' data sheets. However, on occasion it may be
necessary to characterize some aspect of the flocculant in detail if
sensible progress is to be made. Details of certain of the methods are
provided In reference [1].
(iii) General Tests Applicable to the Flocculation Process
These are methods such as the 'jar-test', flocculation kinetics and clarification
measurements, which give an idea of how quickly and completely a particular
agent aggregates the particles. These simple procedures also provide
information on dose range, and the influence of co-factors such as pH and
ambient electrolyte concentration. Tests for the concentration of (polymer)
flocculant residues in the supernatant may be necessary in some cases both
as a guide to efficiency of agent use and, in cases where a recycle loop is
Involved, to check that flocculant build-up problems are not going to arise.

(iv) Specific Tests Applicable to the Flocculation Process and to
the Flocculated Systems
These are the procedures necessary to find the correct flocculant type/dose
for a particular unit operation (or operations) under consideration. They
Include techniques which determine important fundamental characteristics
(such as modulus or viscosity – see Section 3.21, or which directly measure
the technological properties (e.g. filtration rate, sedimentation behaviour) in
question.
Probably the most effective manner of screening for a flocculating agent is
laid out in the flow diagram, Figure 1, We would make the following Important
points concerning the sequence of operations:
(a)

Checkout Coagulants First
Before searching for a suitable polymer flocculant, it is always worthwhile
looking to see If the problem may be solved by use of an Inorganic
coagulant, since agents of the latter type have such relative ease of use.


(b)

Pursue Simple Methods to Eliminate Options
To minimize the amount of work that has to be expended on fairly time
consuming tests of filtration, centrifugation, sedimentation or flotation
behavior, primary screening for candidate flocculants should be pursued
with simple techniques such as the jar test. Ideas of the appropriate dose
range and the effects of pH or added electrolyte (on polymer-particle
interaction) can also usefully be found in this way. Any additional
constraints (e.g. safety, toxicology) which could limit flocculant choice,
should also be considered at as early a stage as possible to minimize
experimental work.
The first "sifting" for suitable polymer flocculants Is best performed “on
paper" using the known characteristics of materials flocculated by different
mechanisms (Table 1), combined with such constraints as have been
introduced concerning the type of floes which must be obtained (e.g.
robust, open-textured flocs for filtration). This will give some idea of the
polymer characteristics, such as ionicity and molecular weight, required.
However this goes only some way towards cutting down the myriad choice
of synthetic and natural flocculant materials to manageable proportions for
testing. To go further in selection it is necessary to make such judgment
as to the likely strength of polymer-particle interaction. Ionicity is, of
course, an excellent guide in many cases - species of opposite charge will
tend to bind strongly - but in other instances one must simply use a
mixture of common sense and experience. For example, polymers
containing carboxyl groups may be expected, on the basis of simple
chemistry, to bind strongly to inorganic surfaces containing, say, Ca2+
Ions. For inorganic particulates, such as metal oxides or carbonates, it
should be borne in mind that polymer-particle adhesion Is usually quite
sensitive to pH as the character of the solid surface can often be changed
from predominantly negative to neutral and even positive on only limited
acidification.

(c)

Use the Approximate Test
Notwithstanding (b), final selection of flocculant type, dose and physical
conditions of application, must be made on the basis of the tests
appropriate to the unit operation (e.g. filtration, Section 3.5) for which
optimization is required. This is because the floc properties which are best
for one type of process will be entirely wrong for another separation
operation. For example, in sedimentation fairly compact flocs, which will
rapidly consolidate to a dense sediment, will usually be required.


In contrast, rapid filtration demands that the aggregates be robust, opentextured and resistant to compaction.
(d)

Optimize the Whole Process
It follows from (c) that if there are several different steps in the process
train, one has to consider the flocculant and conditions which will give the
optimum for the whole process. (NB this will be the case rare often than
not. For example, even in a simple, one-step dewatering process one will
be aiming to obtain a handleable concentrate. Thus factors such as
concentrate solids content and rate of dewatering have to be played off
against ease of handling of the final material.
This can only be done by screening for the separate stages then
determining the best compromise in a reconciliation stage. This matter will
be dealt with in more detail in Section 3.10. The first of the examples in
Section 3.7.3 deals with exactly this type of problem. In this instance it
was very easy to find flocculation conditions which would be entirely
suitable for a particular step In the process; finding ones which would
satisfy all of the constraints simultaneously proved, however, much more
difficult.

(e)

Remember Kinetics
So far in it has tended to be tacitly assumed that sufficient residence times
will be available for the equilibrium flocculated state to be obtained.
Generally this will be the case for the kinds of concentrated systems
typically encountered in solid isolation problems. However, this need not
always be so and, if necessary, flocculation kinetics may need to be
measured to determine optimum times of aggregation. This question will
be returned to in “Centrifugation” where we deal with clarification
operations. In the latter, which predominantly involve dilute suspensions,
kinetic factors are a major concern.

(f)

Do Not Forget That Specific Effects Can Occur
Specific Interactions between particle and flocculating agents occur with
both polymers and inorganics. Examples include the precipitation of
carboxylated biopolymers by Ca2+, and the tendency of Fe3+ to bind
strongly to biological surfaces and polymers (see Example 2 in 3.7.3
below). Thus even with today's degree of systematization in selection
of flocculants one must still be alive to special effects due to chemical
interactions.


Examples
Below are details of two examples, taken from a European client’s
experience, which illustrate selection of flocculants. One involves the
extensive screening of polymer flocculants in order to fabricate a paperlike product from suspended ingredients; the other concerns use of Fe3+
coagulant In a specialist biological separation.

2.1

Formation of M729 Vermiculite / Glass Fibre Refractory Paper
A good example of the manner in which flocculants can be broadly
selected, then screened in detail, is provided by the process of
manufacture of a refractory paper from M729 vermiculite and glass fibre.
The object was to develop a product, containing about a 50:50 ratio of clay
(i.e. vermiculite) and glass fibre, which could be produced on conventional
paper-making machinery. It had been shown that a material ("Fortress T"),
consisting of vermiculite coated onto (preformed) glass fibre tissue, could
retain its structural integrity at temperatures up to the order of 1000 oC.
This made it technically suitable for a number of flame barrier/fire
protection applications. However, preliminary studies indicated that a
product of equally desirable characteristics could probably be
manufactured at significantly lower cost by a paper-type route in which the
vermiculite was mixed with the glass fibre stock prior to tissue
manufacture. The clay was thus incorporated in the same operation as the
glass tissue was made. Not only would this improve the competitive
position in the then accessible markets but it would also probably open up
possibilities for new applications requiring a lower cost material than
'Fortress T". New technical procedures were needed as formation of such
highly-loaded papers in one operation was largely unknown territory.
The nature of the paper-making operation is not dissimilar to continuous
filtration in action - the dispersed glass, or other fibrous feedstock, falls
onto a moving wire mesh belt, the fibers forming a web as drainage
proceeds - and uptake of other ingredients, such as clay suspended with
the fibrous base, is dependent upon efficient capture in the interstices of
the mat. As M729 vermiculite Is colloidal (substantially ( 1 micron in
diameter) in scale, whereas typical glass had a diameter of ~ 10 microns
and a length of several mm, retention of the clay was extremely poor when
attempts were made to make paper from it and glass fibre alone.


Accordingly, a flocculant (retention aid) was required which would give
aggregates of vermiculite of sufficient size to be captured (with almost
100% efficiency) in the pores of the forming paper.
However, the problem was not simply one of producing large, robust flocs
of the clay: for example vermiculite aggregates of too large a size would
give sheet with such a “lumpy” appearance that it would have been
unsalable. Instead, a compromise had to be found which would best
satisfy a number of (partly contradictory) product and process constraints,
all of which were considered necessary for economic production of a
customer-acceptable material.
Details of these conditions, and of the paper structure likely to be best
suited to their fulfillment, are provided in Table 2. Consideration of the
relative Importance of the different factors suggested that the minimum
requirements of M729 aggregate structure would seem likely to be as
follows:
(i)

The flocs should be reasonably large and fairly robust so as to give
good retention.

(ii)

There must be high porosity in the forming M729/glass fibre we?
In order to give fast drainage and hence production rates. Ho:
probably this could best be achieved by having M729 aggregates
which have high individual porosity but structure having large
interfloc pores might also be acceptable.

(iii)

In order to give good paper texture and a manageable thermal
drying load the porosity of the floe structure in the formed (but not
necessarily dried) paper had to be low. For this requirement to be
compatible with (ii) either: (a) the flocs must be initially porous but
be sufficiently compressible that they consolidate to a considerably
denser structure during the last stages of drainage; or: (b) the flocs
must initially be fairly dense but the interfloc structure must be such
as to give enough large drainage pores.


Turning to the initial sifting process outlined in the decision tree In Figure
1, one was quickly led to the following conclusions:
(i)

As the simplest mechanism of flocculation, viz destabilization of the
colloid by limited addition of electrolyte, usually gives flocs which
are small, weak and often rather compact, the aggregate structure
would be expected to satisfy (iii) but not conditions (i) and (ii)
above. Experience supported this analysis and attempts to wetform M729/GF paper using Ca2+ as coagulant gave material of
excellent texture but with associated poor retention and drainage
during production [16].

(II)

Many of the mechanisms of flocculation (e.g. addition of oppositely
charged surfactants of low-medium molecular weight polymers)
involve random aggregation of the particles to give relative open,
voluminous structures. If these aggregates were sufficiently large
and strong enough to give good retention, though draining well,
they would tend to give high moisture retentions and (probably) a
poor paper texture.
In this mechanism the particles are joined by flexible polymer
bridges rather than there being Intimate contact between the
surfaces of the particulate species. This (given appropriate process
conditions) helps the formation of large, reasonably tough, open
flocs. However, such aggregates are fairly compressible and will
eventually settle or compact (as in the last stages of drainage
during filtration) to quite a dense bed,

(iii)

(IV)

Optimum conditions for flocculation by polymer “bridging” require
that the Interaction between particle and polymers is not too strong.
If particle-polymer Interaction is very strong an alternative
mechanism of aggregation occurs. This is” heterogel” flocculation in
which the particles initially thickly coat droplets of the added
flocculant solution; subsequent agitation folds and stretches the
initially-formed globular species to give highly characteristic
“stringy” flocs (see Chapter 2 and reference [10]). Though these
aggregates are large, and often remarkably tough, they are usually
less porous, immediately after formation, than optimum polymer
bridged species. In addition, beds of the flocs are much less
compressible than those of aggregates induced by polymer
bridging. Thus it was expected that such “heterogel” species would
perform somewhat less well in practice than purely bridged
aggregates owing to poorer drainage and less satisfactory paper
texture. Subsequent studies largely bore out this prediction though,


as shall be seen, it was possible to make efficient use of highlyinteracting high molecular weight flocculants under special
conditions when the basic aggregation mechanism was probably
somewhat modified.
One final problem with “heterogel” flocculation is that the floc
structure is notoriously sensitive to agitation history. This tends to
present considerable problems as agitation regime usually proved
to be one of the more difficult variables to control closely in any
practical flocculation and pumping regime.
(V)

Due to the likely crucial dependence of papermaking on M729 floc
structure it was evident that the key process parameters for
M729/GF paper production would probably be those which
governed the basic mechanisms of flocculation.

The next stage in the research and development program was to confirm
(or otherwise) the conclusions reached in the above paper exercise. It was
known from early (empirical) scouting studies that (I) was certainly true,
this lack of sufficient floc strength also extending to systems flocculated with
medium molecular weight (~ 50,000) cationic flocculants (cf (II)). Bow the M729
vermiculite particles were known to be negatively-charged under most conditions
and, accordingly, added high molecular weight cationic polymers were expected
to give very tough, large aggregates. Simple screening of a few agents
corroborated this supposition. As anticipated, such suspensions gave rise to
lumpy, uneven papers of completely unacceptable quality (cf (IV)). Not only were
the flocs generally too large but also mixing difficulties with the strongly
interacting flocculant solution and particulate suspension gave rise to
inhomgeneities which contributed to the poor texture. However, high molecular
weight anionic or nonionic agents, acting probably by a bridging mechanism,
showed promise. Thus, quite rapidly, an apparently unlimited choice of
flocculation aids had been reduced to quite a modest selection of compounds to
be screened.
Final selection of polymers, and investigation of the effects of physical variables,
such as mixing conditions, were now performed using a laboratory paper former
(see reference [19] for details), with flocculated M729/glass fibre stock made up
as shown in Figure 2. The flocculants examined were a range of commerciallyavailable polyacrylamide-based agents of molecular weight greater than 1
million. Fortunately the basic rules which are known concerning the relationships
between basic parameters and floc properties (Table 1) proved to hold and this
rapidly guided the experimental program, especially in respect of isolation of key
variables.

In particular knowledge of the underlying principles enabled water hardness to be
quickly identified as an important parameter (due to the sensitivity of polymerparticle interaction to added electrolyte). A relatively modest amount of
experimental work allowed a picture to be sketched out of the effects on paperforming properties of changing the main variables (Table 3). From this optimum
flocculation "recipes" were determined to deal with various possible process
conditions (e.g. hard and soft make-up water; changes In M729 feedstock type).
Although In most circumstances anionic or nonionic (these materials tend to
hydrolyze to give weakly anionic polymers on making up the flocculant solution)
flocculants gave excellent results there was one version of the process in which
they could not be used and alternatives had to be sought.
This was when a cationic surfactant was employed to aid dispersion of (certain)
glass fibre stocks. Coacervation (i.e. co-precipitation) of anionic or nonionic
retention aid with the surfactant was observed, the problem being particularly
acute under recycle conditions when process "white water" was employed for
M729 suspension make-up:
The surfactant would tend to strip out flocculant from solution with deleterious
effects on process efficiency. Though It had been proved earlier that high
molecular weight polymers of significant cationicity could not be used (owing to
floc strength being excessive) it was found that weakly cationic materials could
be employed, presumably because polymer-particle binding was sufficiently
reduced to allow (as before) the formation of easily compressible flocs. A limited
experimental program enabled key variables and trends to be Identified (Table
4), and workable formulations to be derived. The Important process parameters
were CARL all the same as for the anionic /nonionic retention aid case. However,
these changes were all readily predicted from the known characteristics of the
flocculation mechanism (Table l), eliminating the need for much tedious, and
ultimately pointless, empirical screening.
Following the research phase the correctness of the conclusions reached was
proved in two pilot-scale production campaigns In Germany [17, 18].
Subsequently it was found that near optimum process 'recipes" for paper-making
plants of potential licensees, in several parts of the world, could be derived, with
modest effort, based upon the principles determined in the work. Further
technical details on the project are contained in references [19] and [20].


2.2

Harvesting and Concentration of a Filamentous Organism Suspension
before Isolation of a Pharmacologically-Active Compound

Our second example concerns one step in the Isolation of a fermentation
product. The object of the process in question was to make a 25-30% solids
paste from a fermenter broth (Initial concentration ~ 1-2% solids). This
concentrate would be the intended feedstock for a solvent extraction operation by
which the final product, a pharmacologically-active substance, would be isolated.
Constraints on process options were at least three-fold viz:

(i)

As the organisms in the broth were of colloidal size some kind of
flocculation procedure was required for convenience of separation.

But
(ii)

As the organism in question was filamentous (i.e. acicular) in
nature, care had to be taken not to induce too strong a degree of
flocculation otherwise the flocculated network in suspension would
not readily consolidate to the required concentration. (This is a
result of simple geometrical considerations which govern network
strength - see Basic Principles & Test Methods [GBHE SPG PEG
302] for further details.)

(iii)

Owing to the product use there were clear limitations on the kinds
of flocculant which could be employed.

And

There were also likely to be the usual difficulties which attend separation of a
biological (as opposed to inanimate) system – for example the propensity of
flocculants to react with biopolymers in solution rather than flocculate the
particles with optimum efficiency (see Section 3 for a discussion of this and other
questions concerning bio-separations).
All of the above Indicated that it was desirable to find a relatively simple
flocculation aid, which would not give rise to safety concerns, and which would
not give rise to too strong flocculation and a difficult-to-compress sludge.
Screening of some possible agents indicated that species such as Ca2+ or
cationic polymers were relatively ineffective.

However Fe3+ gave much more promising results and the material could be
thickened to the desired degree by centrifugation then filter-pressing. The high
efficiency of the iron was probably not only attributable to its high charge, but
also to some specific interaction between the Fe3+ and the biological species.
Sufficient floc robustness was obtained to allow "balling up" of the aggregates
into more compact structures, This Illustrates the important principle that one
may expect specific (and not readily predicted) metal-polymer interactions when
highly-charged ions and/or biological polymers are concerned.
Whilst concentration could be carried out by the above process the position was
not entirely satisfactory; there tended to be significant 'bleeding" of fines during
mechanical dewatering, indicating either Insufficient floe robustness, or
incomplete flocculation, with excess fines being only weakly entrained in (rather
than strongly incorporated Into) the aggregates.
The opposite trap, of producing an over-strong network had, though, been
avoided. In the initial investigations flocculation had been performed at acid pH
(due to the use of acidic FeCl3 as the source of iron) and attention now turned to
control of this parameter as a means of modifying the flocculation behavior.
Raising the pH in the process yielded some precipitation of Fe(OH)3, and a great
improvement in fines retention owing to the "sweeping" action of the hydroxide.
This clearly demonstrated the Importance of pH adjustment in the effective use of
trivalent coagulants. It also showed what extra benefits, in terms of “sweeping” as
well as coagulating action, may be obtained with such agents.
As a final stage in development a small amount of high molecular weight cationic
polymer flocculant was added after pH modification and Fe hydroxide
precipitation. This helped to improve the aggregate robustness sufficiently that
the problem of fines loss was completely eliminated. It was now also possible to
simplify the process by removing the centrifugation step, concentration being
achieved in one operation In the filter press. All that now remained was "finetuning" of conditions to give an overall optimum compromise between efficient
harvesting and concentration, and ease of redispersion of the concentrate (at the
later solvent extraction stage). Tests of cake strength, using the Shearometer (cf
Section 3), provided an Important technique in the attempt to achieve maximum
efficiency in filtration.


3

USE OF SURFACTANTS TO HELP REDUCE FILTER-CAKE MOISTURE

In addition to surface-active agents with overt flocculating action, on occasion
other compounds are employed to try to reduce the moisture content of filter
cakes obtained in various solid-liquid separation operations. Ostensibly such
materials function by reduction of filtrate surface tension and/or by modifying the
liquid/solid contact angle, allowing filter cake capillaries to drain more readily,
thereby reducing cake moisture content. In the current stage of knowledge,
however, there are no good rules for choosing such aids. Indeed, for most fine
particle separations there must be some doubt whether the effects obtainable are
worth the effort of screening and the cost of the agents. At present, probably the
best guidance available is to refrain from trying to use such compounds except
(perhaps) in special circumstances via:
(a)

When the primary particles are large (i.e. of the order of tens of microns in
size); and/or

(b)

When pressure blowing is being used to reduce filter cake water content.

The theoretical background to the phenomenon to be exploited is very simple. It
follows the analysis related in Section 3 for pressure required to cause expulsion
of liquid from filter-cake pores by a blowing mechanism.
Recapping:


Thus If ƔL.V. can be reduced significantly, in principle the cake will drain at much
lower applied pressures, perhaps even just on standing. The same applied, of
course, to the COS ƟL.S. term. Note that these two factors may, or may not; move
in concert with one another - ideally they should do so of course [12]. This makes
surfactant selection even more of a problem since ƟL.S. depends specifically on
the molecular interaction between the particular surfactant and the particular
solids being filtered. In practice ƔL.V. reduction seems to be the main
phenomenon exploited [14]. In reality, it appears fairly difficult to obtain effects of
useful magnitude. As was shown in Sections 3), ΔP tends to be very high for
colloidal-scale species and hence even if the interfacial tension is diminished by
a perceptible factor the pressure required to give drainage may still be
inaccessible. However, If the primary particles, and hence the pore size is
relatively large, helpful results may be obtained:
(i)

Pearse and Allen [12], in a paper written principally from the
perspective of one European manufacturer ( who manufacturers
surfactants for use as filter-cake deliquoring aids), have reported
significant moisture content reductions with cakes of largish-sized
materials such as sand, fine coal, and 11-12 micron
silica spheres.

(ii)

Significant reductions in filter-cake moisture content have been
noted in several instances (see, for example references [21] and
[22]) on “oiling of fine coals”. Typically only ~1% of oil is used, far
below that normally needed to give oil-bridged agglomerates (see
Basic Principles & Test Methods [GBHE SPG PEG 302]).


(iii)

Drainage aids are often used to reduce moisture content in newlylaid paper (fibre size: typically mm x 10 micron or more diameters).
Probably these agents mainly function by flocculating action but
surface-active effects may also matter [13].

(iv)

Moir and Read, in their review on reduction of the liquid content of
filter cakes [41, have reported variable results from surfactant use.
Beneficial reductions in moisture content of up to 10% were
observed in certain cases, but negligible effects in many others.
Virtually all significant findings have been for coal filtration systems,
and even in these cases the data suggests that in many instances
positive effects can be attributed to flocculation rather than surface
tension or contact angle phenomena (however cf (ii) above).

Thus, the balance of evidence is that for filter cakes composed of colloidal scale
particles, surfactant addition will probably not be all that useful. For cakes
containing larger size primaries there might be some beneficial effects, and
screening of commercially available products (see e.g. reference [12]) may have
merit.
Two other points which we would make with respect to this topic are:
(I)

Understanding of best practice in this area is somewhat confused
due to lack of systematic research, or GBHE experience, on use of
surfactants as aids to filter cake deliquoring. Better guidelines can
only come from some thorough work In the area.

(II)

Agents which are effective in reducing filter cake moisture content
will also tend to reduce (by the same mechanism) the excessive
compaction, due to capillary forces, which sometimes attends
drying of a filter cake or other porous structure. Use of processing
aids of this type might be helpful where it is necessary to retain
maximum porosity In a dried product (see reference [15] and Basic
Principles & Test Methods [GBHE SPG PEG 302]).


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