This invention relates to processes of coagulating aqueous suspensions
of coagulatable material using water-soluble polymeric coagulants. The aqueous
suspensions can be true suspensions of suspended material or they can be liquors
containing colloidally dispersed material. The suspended or dispersed material
Aqueous suspensions are usually clarified by the addition of one or
more water-soluble organic polymers that are called coagulants or flocculants.
These terms tend to be used rather inaccurately. In this specification, we use
the term "flocculant" to denote a high molecular weight polymer that achieves its
effect (i.e. flocculation) primarily by a bridging mechanism, with the result that
they are sometimes referred to as bridging flocculants. We use the term "coagulant"
to indicate a lower molecular weight, highly ionic, material that achieves its
effect (i.e. coagulation) primarily by adsorbing onto the particles of the suspension
and changing the surface charge on them, with little or no bridging effect between
particles. As a result, the coagulants are sometimes referred to as charge neutralisation
The coagulants have low molecular weight and because of this, and
their high ionic content, they have low solution viscosity and dissolve easily
into water. Typically, the molecular weight of coagulants is never more than around
1.5 million and it is often below 1 million, and indeed when the polymer is anionic
it is usually below 0.5 million. In terms of intrinsic viscosity, IV is usually
below 3dl/g and generally below 2dl/g.
Throughout this specification, molecular weights are the values obtained
by gel permeation chromotag raphy and intrinsic viscosities are the values obtained
using a suspended level viscometer at 25°C in molar sodium chloride aqueous solution
buffered to pH 7.0.
Because of the low solution viscosity of these polymers, it is possible
to provide aqueous concentrates that have a satisfactory combination of viscosity
and polymer concentration. These concentrates are very easy to use since they can
be dosed direct into the suspension that is to be treated or can easily be diluted
in-line with dilution water to form a dilute solution that can then be dosed into
Many methods have been described in the literature for making solid
forms of water soluble polymers of various molecular weights. Solid forms do, of
course, have the advantage that they do not necessitate the transport and packaging
of large amounts of water and so can be more convenient, especially to the manufacturer,
than aqueous concentrates. However they have not been adopted widely and solid coagulants
only constitute a very small proportion of the total polymeric coagulant market.
For instance an important coagulant is polydiallyldimethyl ammonium chloride, and
large amounts of this are supplied worldwide. However only one grade is available
in solid form, namely Percol 368 and Magnafloc 368, from Allied Colloids Inc. and
Allied Colloids Limited respectively and this constitutes only a small proportion
of the total sales of this polymer.
US 3 288 770 describes production of polymers including polydiallyldimethylammonium
chloride in solid form, and their use for flocculating suspensions, but is silent
on the form in which they are added to the suspensions.
A solid grade coagulant must, of course, be in solution form before
it can function as a coagulant and, heretofore, this has necessitated the user
providing dissolution make-up equipment. The convenience and economy to the manufacturer
of supplying a solid, without the need to package and transport water, has therefore
tended to be outweighed by the user preferring to obtain what was considered to
be equivalent performance but without the inconvenience of having to provide make-up
An entirely different situation applies to flocculant polymers since
these are always much higher molecular weight and many of them tend to dissolve
only very slowly into water. For instance flocculant polymers generally have molecular
weights of at least 4 million and usually at least 5 million, and frequently above
10 million. Intrinsic viscosity is generally above 5dl/g and frequently above 10dl/g,
especially with the anionic and non-ionic polymers.
Because of the high molecular weights, flocculant polymers have very
high solution viscosities and so it is not possible to supply handlable concentrates
having adequate polymer concentrations. Accordingly, the flocculants generally
have to be supplied as powders or as dispersions in oil. The polymer then has to
be dissolved into water. The rate of dissolution depends upon ionic charge and
molecular weight. Some dissolution can occur with some polymers within a few minutes,
but many of the polymers take at least an hour to go to full solution. Such polymers
include, for instance, many of the polyacrylamides. Since the flocculant polymers
cannot exert their bridging effect until they are in proper solution, it is therefore
conventional for the user to install sufficient make-up apparatus to ensure that
the flocculant can be truly dissolved before it is dosed into the suspension that
is to be treated. Such make-up apparatus generally has to involve a storage vessel
that can hold the flocculant for at least an hour while it goes into true solution.
Although this is true of normal flocculation processes, a few exceptions
to this general rule have been proposed in the literature.
For instance in JP-A-48084776 and JP-A-49049802 high molecular weight
flocculant is added as powder direct into a sludge that is being transported or
pumped into a pit. In each instance the flocculant is partially hydrolysed polyacrylamide
of molecular weight 5 million or more, and so presumably is not highly anionic
(depending on the degree of hydrolysis). Also, it is known to add flocculant powder
to a slurry of mine tailings that is being pumped down through a mine to form a
backfill. In all these processes, the flocculant powder can be in contact with
the slurry for a considerable time before it needs to complete its flocculation
effect, and so there is time for the powder to dissolve. Similarly, in JP-A-60282787
a mixture or high molecular weight powdered flocculants is added to an emulsion
but again the separation process appears to be sufficiently slow to permit dissolution.
The main difficulty with such methods is that high molecular weight
flocculants only dissolve slowly. It has been proposed to treat the polymer particles
in various ways, presumably with the intention of accelerating their rate of dissolution.
Disclosures of such processes are in, for instance, JP-A-50003974, JP-A-49121309,
JP-A-58070807 and JP-A-58089915 and US 4089831. It is also known to dissolve high
molecular weight polymeric flocculant from within bags and shaped articles that
are immersed for prolonged periods in a flowing suspension, for instance as described
in JP-A-53091072 and JP-A-56115605 and ER-A-255283.
All these methods tend to utilise polymer at a rather uncontrollable
rate and so may suffer from underdosing or overdosing.
Although it is normally required for the polymer to be in true solution
to function effectively there have been some disclosures (e.g., U.S. 3,235,490
and 3,021,269) where apparently cross linked and potentrally insoluble polymers
were homogenised before use as flocculants so as to permit them to form what appears
to be a true solution of relatively low molecular weight polymer. Other disclosures
of increasing the suitability of polymers by shearing are in U.S. 4,705,640 and
Whereas true solubility is normally required before contact with the
aqueous suspension, in U.S. 4,720,346 the polymer is a high molecular weight synthetic
polymeric flocculant and performs its flocculating function on the suspended solids
while the polymer is still in the form of polymeric particles having a size of
below 10µm. These particles can, if left long enough, be truly soluble in water
but preferably the polymer is cross linked so that they cannot dissolve fully into
The very small particle size, of below 10µm, is essential. Normally
it is provided by introducing the polymer particles as a dispersion that has been
made by reverse phase polymerisation but in EP-A-326382 it is provided by introducing
the polymer in the form of friable aggregates that have been made by bonding the
substantially dry polymer particles having a size of below 10µm with an aqueous
liquid and drying the aggregates, whereby the aggregates disintegrate upon addition
to water to release the individual polymer particles.
In all coagulation and flocculation processes, there is always the
desire to achieve better performance, leading to better clarity or reduced consumption
of polymer or both, and/or to achieve simpler techniques of operation.
It is accordingly an object of the invention to provide improved processes
for the separation of dispersed or suspended material from aqueous dispersions
and suspensions. It is a further object to provide such processes that use polymer
in the form of a powder but which avoids the need for normal solids make-up apparatus
and eliminates the need for normal concentrate handling equipment such as pumps
and in-line dilution equipment. It is a further object of the invention to provide
improved separation of solids from the suspension when treated by the treatment
In particular, it is an object of the invention to provide an improved
way of supplying and utilising ionic coagulant polymer having a sufficiently low
molecular weight that it can normally be supplied as a aqueous concentrate but
which, for convenience, is supplied as a powder. A further object is to obtain improved
The invention provides a process of solid coagulating an aqueous suspension
of solid coagulatable material by using a water-soluble, low molecular weight,
ionic polymeric coagulant. This results in coagulation of the coagulatable material
and the coagulated material is then separated from the suspension.
In particular, the invention is defined in claim 1 and claim 2 and
comprises providing the defined particulate, water soluble, ionic, polymeric coagulant
that is in the form of solid particles that have a size of at least 90% by weight
above 30µm, mixing the said solid particles into the suspension and thereby dissolving
the particles in the suspension and coagulating the suspension, and subjecting
the coagulated suspension to a separation process and thereby separating said coagulatable
material from said liquor.
In the process of the invention the mixing comprises flowing the
suspension along duct means for a period of not more than about 5 minutes from
a position at which the particulate coagulant is added to the suspension to a
position at which the said separation process is initiated, and the coagulant particles
substantially fully dissolve before the suspension reaches the said position at
which the said separation process is initiated.
In the preferred process a low molecular weight ionic coagulant polymer,
preferably of diallyl dimethyl ammonium chloride, is added as solid particles to
a suspension that is to be coagulated and dissolves rapidly in the suspension and
coagulates it, and the coagulated material is separated from the suspension. Generally
this separation is facilitated by adding a flocculant to the coagulated suspension,
for instance by adding a conventional counterionic high molecular weight flocculant
solution to the suspension.
We have surprisingly found that the addition of solid, low molecular
weight, ionic coagulant to the suspension generally gives a significant, and often
very highly significant, improvement in coagulation performance relative to when
the equivalent polymer is added to the suspension as a solution in conventional
By the invention, the performance properties are always as good as
those obtained using a polymer solution and, as indicated, they are usually very
much better, and yet the invention has the further advantage that it eliminates
the need for equipment for handling aqueous concentrates (e.g., pumps and in-line
dilution equipment) and it avoids the equipment traditionally associated with solid
grade polymers, namely the make-up equipment that is traditionally installed to
allow the solid grade polymer to be put into the form of a solution prior to dosage
into the suspension.
We are unclear as to why it is possible, by this technique, to obtain
better results than are obtainable using a preformed solution of the same polymer,
but it may be associated in some way with the polymer being released continuously
over a useful period into the suspension. Thus it seems that as polymer molecules
dissolve from the solid polymer particles, these molecules are absorbed substantially
immediately on to the coagulatable material to form the desired ionic surface characteristics
on the particles of coagulatable material. If agitation or other effects cause
a deficiency of polymer (and therefore of ionic charge) on part of the surfaces
of suspension particles, this deficiency is probably made up by fresh polymer that
is still dissolving from the polymer particles. This is in contrast to conventional
processes where all the polymer is made available instantaneously as a solution.
Once that polymer has been adsorbed onto the particles there will probably be a
much lower tendancy for fresh polymer to be available to adsorb onto any surfaces
where there is a deficiency of polymer, for instance as a result of breakdown of
the coagulated particles.
However, irrespective of the reason or the mechanism, the process
definitely has the advantage that it is possible to obtain greatly improved performance
at the same time as using a solid material by a simplified process and apparatus
and without a make-up or dilution stage.
The term "coagulatable material" is used herein to embrace both materials
that are already present as suspended solids and materials that are present as
a colloidal dispersion. Thus it embraces particulate solids such as pigment, clay,
coal and sewage solids, and it also embraces colloidally dispersed materials such
as colouring matter in water and humates. The process of the invention can bring
more of this colloidally dispersed material out of colloidal solution as a particulate
solid than when the same polymer is added as a solution.
Flocculation processes involving bridging flocculants can be relatively
slow and indeed bridging may occur during, for instance, sedimentation after addition
of a flocculant. However coagulation processes depend primarily upon charge effects
and coagulation processes therefore tend to be very fast. For instance, coagulation
often precedes the addition of a bridging flocculant, and this is generally added
very soon after the coagulant.
In the invention, the polymeric coagulant starts dissolving immediately
upon contact with the suspension, because the coagulant is water soluble. Accordingly
it immediately makes coagulant polymer molecules available for coagulation. The
polymer particles go into substantially complete solution in less than about five
minutes and preferably much quicker, generally within about one minute and preferably
within about thirty seconds, often within about fifteen seconds.
Whether or not a polymer particle has dissolved can be determined
by filtration in that a screen that retains a dry or gelled polymer particle will
not retain a solution of the polymer particle. A convenient way of testing the
solubility therefore is to select a screen that retains a known proportion, for
instance substantially all, of the polymer particles when dry and then to filter
the aqueous suspension into which the polymer particles have been mixed and observe
the proportion of polymer particles retained on the screen. The retained proportion
at the end of the mixing period (i.e., when they should be substantially fully dissolved)
must be very much less than the retained proportion of dry polymer particles, for
instance having a dry weight below a quarter and preferably below one tenth of
the weight of the retained dry particles.
Generally the polymer particles are substantially individual particles
but if they are aggregates than break down into smaller particles before dissolution
(for instance as in EP 326382) then the size of screen that should be used for
testing whether or not dissolution has occurred should be the size that retains
the individual component particles within each aggregate.
The optimum duration for dissolution will depend upon the particular
suspension and process that is being coagulated. If the rate of dissolution is
too slow, some of the polymer may remain undissolved during the period when a coagulation
effect is required, for instance during passage in a flow line to a flocculation
stage, and so may involve the use of more polymer than would be required if it
dissolved faster. Preferably the polymer is substantially fully dissolved before
the suspension reaches the next treatment stage, and in particular before it reaches
the next point of addition of treatment chemical, for instance bridging flocculant.
Although the polymer particles (or the component particles within
aggregates that disintegrate into particles in the suspension) can have a size
as small as 30µm it is generally preferred for the average size to be at least
100µm and generally at least 200µm. If the polymer particles have an average size
that is too large then this can be undesirable, for instance because it can reduce
the time taken to achieve dissolution. Thus although it can be above 1mm, preferably
it is below 1mm and usually below 800µm. It is generally preferred for the polymer
particles to have a size of at least 90% by weight in the range 100µm to 1,000µm,
often in the range 200µm to 800µm. Although it is normally preferred for polymers
to have a reasonably narrow range of particle sizes, in the invention it can be
advantageous to have a relatively wide spread of particle sizes so as to spread
the time over which polymer dissolves.
The ionic coagulant polymers with which this invention is concerned
are the materials that have, heretofore, usually been supplied as aqueous solutions
because of their low solution viscosities, and therefore the particles will inevitably
tend to start dissolving very quickly, and generally will be substantially fully
dissolved within about a minute, often within about half a minute. These low solution
viscosity and high dissolution rates are associated with the charge density and
the molecular weight (intrinsic viscosity) of the polymer. If the molecular weight
is too high then the polymer will not be an ionic coagulant but will instead tend
to be a bridging flocculant and, in any event, will have a solution viscosity such
that it is not commercially convenient to supply it as an aqueous concentrate.
The optimum intrinsic viscosity depends upon the ionic charge and the particular
polymer. When the polymer is a cationic polymer, intrinsic viscosity is generally
at least about 0.2, preferably in the range of about 0.5 to 3, most preferably around
0.8 to 2.4dl/g. Expressed in terms of molecular weight, it is generally preferred
for the molecular weight to be below 2 million, most preferably below 1.5 and,
preferably, below 1 million, although it should generally be above 100,000 and
preferably above 500,000.
When the polymer is anionic, lower molecular weights are usually desirable,
and in particular the anionic polymer should have a molecular weight below 1,000,000,
most usually below 500,000. It must not be too low as otherwise it will act as
a dispersant rather than a coagulant and so normally the molecular weight is above
50,000, often above 100,000. Expressed in terms of intrinsic viscosity, this should
normally be at least 0.2dl/g and is preferably not more than about 2dl/g. Preferably
intrinsic viscosity is in the range about 0.5 to 2dl/g, most preferably about 0.8
The polymer can be a low molecular weight, rapidly dissolving, highly
ionic, naturally occurring polymer but generally it is a synthetic polymer formed
by polymerisation of a water soluble ethylenically unsaturated monomer or blend
of monomers and has a high charge density. Accordingly it is necessary for at least
80% of the monomers from which it is formed to have an ionic charge. Although the
polymer can be amphoteric, preferably all the ionic monomers have the same charge.
Although polymers in which 100% of the monomers are ionic are preferred, if non-ionic
monomer is present its amount is below 20% by weight. Any of the non-ionic, ethylenically
unsaturated, water soluble monomers that are conventionally present in polymeric
ionic coagulants can be used, but acrylamide is preferred. It is generally best
for the polymer to be a homopolymer of ionic monomer.
Preferred anionic monomers are ethylenically unsaturated carboxylic
or sulphonic acids (including their water soluble alkali metal or other salts).
Examples are 2-acrylamido-2-methyl propane sulphonic acid, methacrylic acid and,
especially, acrylic acid. Thus a suitable anionic polymer is a homopolymer of sodium
Preferred cationic monomers are dialkylaminoalkyl (meth) -acrylates
and -acrylamides, generally as acid addition or quaternary ammonium salts, and
diallyl dialkyl ammonium halides. The preferred acrylates and methacrylates are
preferably di-C1-4 alkylaminoethyl (meth) acrylates and the preferred
acrylamides are di-C1-4 alkylaminopropyl (meth) acrylamides, in particular
dimethylaminoethyl (meth) acrylate and dimethylaminopropyl (meth) acrylamide (with
the respective methacrylate and methacrylamide compounds being particularly preferred)
as acid addition and quaternary ammonium salts. For most purposes the most suitable
cationic monomer is diallyl dimethyl ammonium chloride. Generally a single cationic
monomer is used, but if desired a copolymer may be formed, for instance from diallyl
dimethyl ammonium chloride and dimethylaminopropyl methacrylamide salt, generally
with the latter in a minor proportion.
Instead of forming the polymer by addition polymerisation of ethylenically
unsaturated monomers, any other known ionic coagulant polymers can be used. For
instance suitable polymers are polyethylene imine and polyamines, e.g., as made
by condensation of epichlorhydrin with an amine. Other polymers include aminomethylolated
polyacrylamide (free base or quaternary or acid salt), poly (2-hydroxypropyl-1-N-methylammonium
chloride), poly (2-hydroxy-propyl-1, 1-N-dimethylammonium chloride, and poly (2-vinylimidazolinum
Particularly preferred polymers for use in the invention are polymers
of diallyl dimethyl ammonium chloride, generally as homopolymers but optionally
with up to 20% of other monomer, generally acrylamide, having IV of about 0.6 to
3, most preferably around 0.8 to 2.5dl/g. Preferably the polymer is in the form
The particles of polymer are preferably substantially bead shaped,
for instance as made by reverse phase suspension polymerisation. Reverse phase
suspension polymerisation typically comprises dispersing beads of aqueous monomer
in a water immiscible liquid, generally in the presence of amphiphilic polymeric
stabiliser, polymerisation within the beads, drying by azeotropic distillation
and then separation of the beads from the liquid, often followed by further drying.
Suitable methods and materials are well known and are described in, for instance,
US 2,982,749, 4,506,062 and 4,528,321.
Other solid particulate forms can be used. For instance a solution
of the polymer can be drum dried or otherwise dried as a film which can then be
converted into flakes. If the polymer itself has characteristics such that it is
difficult to provide it in the form of solid particles that can be handled without,
for instance, caking, then the particles can be formed of a mixture of a carrier
(that promotes formation of particles) and the polymer. For instance the polymer
can be distributed throughout beads of a matrix material that will ' liberate the
polymer rapidly upon contact with water, for instance as a result of disintegration
of the matrix in water. A suitable matrix material is a carbohydrate, for instance
starch, or an inorganic carrier material. The blend can be made as in, for instance,
EP-A-188489. Preferably however the polymer particles consist solely of the desired
Suitable polymers are available in bead form from Allied Colloids
Inc. and Allied Colloids Limited under the trade names Magnafloc 368, Percol 368
(cationic polydiallyldimethyl ammonium chloride) and Versicol Sll (anionic polyacrylic
The mixing of the particulate polymeric coagulant into the suspension
is normally accompanied by some degree of agitation of the suspension, although
with highly soluble particles adequate mixing may be achieved merely by adding
the particles to the suspension. Generally however some degree of turbulence should
be applied to the suspension to promote dissolution of the particles. The turbulence
that inevitably arises during flow of a stream that is being coagulated in conventional
manner can be sufficient and excessive shear (eg as provided in a homogeniser)
is undesirable. Accordingly the preferred mixing consists essentially of the flow
along duct means leading to the separation stage, although some initial mixing
may be applied to promote uniform distribution of the polymer particles as they
are scattered, flowed or injected into the flowing suspension.
The process may be carried out batchwise, but generally the particles
are added to a flowing stream of the suspension and the suspension is caused to
flow turbulently along duct means from the position at which the particles are
added to the position at which the separation process is initiated. For instance
this flow can be along a simple duct (optionally as a launder provided with baffles
to create extra turbulence) or it can be along a series of ducts, for instance
including some substantially downwardly extending ducts so as to promote extra
In conventional coagulation and flocculation processes, the respective
polymers are added as fully dissolved aqueous solutions and it is well known that
it is then undesirable to continue applying shear (i.e., to continue turbulence)
for too long since this tends to result in resuspension of the originally suspended
particles and reduction in performance. However in the invention excess shear is
not so undesirable, and indeed can be positively advantageous, since fresh polymer
molecules can continuously be dissolved from the polymer particles and so even
if the distribution of polymer on the particles of the original suspension is damaged
fresh polymer molecules are available to recoagulate the surfaces of the suspension
particles. Nevertheless, it is generally desirable that the entire coagulation
process, and in particular the mixing or application of shear that occurs during
the process, should be relatively short and preferably mixing does not continue
for significant periods after the polymer has dissolved.
In preferred processes, the total time that elapses between initially
adding the polymer particles and introducing the coagulated suspension to the separation
process apparatus (.e.g, a sedimentation vessel) is not more than about one minute,
preferably not more than about thirty seconds, and most preferably not more than
about fifteen seconds, and during this time the polymer particles should have been
substantially fully dissolved.
As is known, there are some instances where a coagulant polymer by
itself (conventionally added as a solution) will give adequate coagulation and
separation, without any subsequent chemical treatment. Accordingly, this is also
possible in the invention.
However, it is more normal to follow the addition of a coagulant solution
with the addition of a flocculant solution, and in the invention it is usually
preferred to incorporate a polymeric flocculant into the suspension after the coagulant
polymer has dissolved and coagulated the suspension. The incorporation of the flocculant
may be by conventional means, such as the addition of flocculant solution followed
by brief (e.g., up to 15 seconds and often less than 5 seconds) agitation to achieve
When flocculant is used, it is usually counterionic to the coagulant
polymer that is added in particulate form. The coagulant polymer is preferably
cationic, and so the flocculant is preferably anionic. However in some instances
anionic coagulant followed by anionic flocculant can be useful, e.g., on some coal
washery slurries. A surprising advantage of the invention is that the amount of
flocculant that is required to achieve ' any particular level of clarification
is generally less, when the coagulant polymer is added in particulate form, than
when the coagulant polymer is added as a solution. For instance the amount of counterionic
flocculant may be one fifth to three quarters, typically around half, of the amount
The flocculant may be any of the conventional bridging flocculants,
and thus should have a sufficiently high molecular weight to give a bridging, as
opposed to a coagulating, mechanism. Typically therefore the molecular weight is
at least 5,000,000 and/or intrinsic viscosity is preferably at least 6dl/g. The
polymeric flocculant can be a natural or modified natural polymer but is generally
a synthetic polymer formed from the monomers listed above. Since the coagulant
is preferably cationic, the flocculant is preferably anionic, often sodium polyacrylate
or a copolymer with acrylamide.
The suspended material is solid (i.e., suspended or colloidally dispersed
solids) and the separation process preferably comprises a sedimentation process.
It can consist solely of sedimentation but generally it involves a further dewatering
step. Any such further dewatering step is generally conducted on the sediment obtained
by the sedimentation. Such further processes include centrifugation and filtration,
e.g., on a filter press or belt press.
One particularly preferred process of the invention involves applying
the process to tailings from a coal washer or from an iron ore concentrator. Thus
coal or iron ore is washed and the desired product (coal or iron ore) is separated
by sedimentation or other appropriate separation technique to leave a liquor, termed
tailings, that is a suspension of the waste products. These waste products often
comprise clay. Most of the solids may be suspended solids but some can be colloidally
dispersed, as colourants. Conventional processes involve adding a solution of coagulant
polymer to the tailings as they flow along a duct towards a sedimentation tank,
with flocculant solution generally being added immediately before the sedimentation
tank . The liquor resulting from the sedimentation generally still contains some
suspended solids and can be coloured.
By the invention, it is possible to obtain much better separation
of solids, and in particular to remove not only substantially all the suspended
solids but also most of the colourant. Thus the process of the invention can result
in a very clear, less coloured, liquor than is obtainable by conventional techniques,
and yet avoids the need for a make-up unit or for the pumps or in-line dilution
apparatus normally associated with the use of aqueous concentrates.
The preferred polymer for use in this technique is a homopolymer of
diallyl dimethyl ammonium chloride (or occasionally a copolymer with up to 20%
acrylamide) having intrinsic viscosity 0.5 to 3 (preferably 0.8 to 2.5) dl/g and
that preferably fully dissolves within about one minute, preferably within about
thirty seconds and most preferably within about fifteen seconds. The polymer is
preferably added in the form of beads which typically mainly have a size in the
range 200 to 1000µm. A solution of flocculant (e.g., sodium polyacrylate
molecular weight above 5 million) can be added immediately prior to the sedimentation
tank. The total time between adding the particles and introducing the suspension
into the sedimentation tank is generally less than one minute, preferably less
than about thirty seconds and most preferably is less than about fifteen seconds.
Although the polymers of diallyl dimethyl ammonium chloride are particularly
suitable for this process, other low molecular weight cationic polymers can be
used, for instance polymers of cationic (meth) acrylates or cationic (meth) acrylamides,
as discussed above.
Although the process is of particular value when applied to suspensions
that are mineral washery tailings, such as iron ore and coal washery tailings,
it has surprisingly been found that it is also of value on a range of other industrial
waste waters and other suspensions.
One important aspect of the invention arises when the aqueous suspension
is a china clay effluent, for instance in the winning of such clays.
Another important aspect of the invention is in the treatment of waste
water from the feldspar and alumina industries. In particular, red mud washery
liquors contaminated with colloidal humate can be decolourised by the addition
of the chosen polymer in particulate form, instead of adding it in the conventional
solution form as proposed in U.S. 4,578,255.
In addition to being of use in the treatment of mineral suspensions,
the invention is also of value in the treatment of organic suspensions and in the
production of potable water.
Another important aspect of the invention is when the suspension is
a cellulosic suspension, for instance a white water effluent from a paper mill
or some other suspension associated with paper production and that needs to have
solids removed and to be decolourised, for instance by the removal of fibres, resinous
materials and lignins.
Another important aspect of the invention includes the treatment of
suspensions comprising sewage solids, for instance sewage sludge.
Another important aspect of the invention involves the removal of
colouring matter and optionally suspended solids from potable water, typically
prior to sedimentation or filtration through a sand bed.
Another important type of suspension is a coloured effluent from the
textile industry, where the process is designed to decolourise the effluent.
The solids content of the suspension is generally below 10%, typically
0.02% to 8%, but another important aspect of the invention involves applying the
process to suspensions having a higher solids content, typically 10 to 30%, for
instance suspensions obtained by sedimentation in a previous separation process.
Thus another process comprises dewatering a suspension (often after the addition
of particulate coagulant as proposed in the invention) and then adding particulate
polymer to the dewatered product and subjecting it to further dewatering after
the polymer has dissolved, in accordance with the invention.
The optimum dosage of particulate coagulant polymer can be selected
by routine experiments and is generally within or below conventional amounts of
dissolved coagulant polymer. For instance it is often 0.1 to 20, frequently 0.5
to 5 mg/l suspension.
Example 1 (illustrative, outside scope of invention)
A homopolymer is formed from diallyl dimethyl ammonium chloride by
reverse phase bead polymerisation by the general technique shown in U.S. 4,506,062.
The beads have a particle size of 90% by weight between 200 and 800µm and they
have intrinsic viscosity of 1.4dl/g. Suitable beads are available from Allied Colloids
Inc. under the trade name Percol 368.
Coal washings are subjected to froth flotation and the tailings from
the flotation cells drop around 30 metres from the cells down a duct to the ground
and to a pump by which they are pumped about 100 metres vertically upwards to a
feed launder leading to a feed well at the inlet to a sedimentation (or thickener)
tank. A solution of anionic polymeric flocculant (copolymer of 60% acrylamide 40%
sodium acrylate, IV about 16 dl/g) is added at the feed well, so as to promote
settling in the tank.
The tailings have pH 7.4, around 7% solids material of which around
75% is below 100 mesh and is mainly clay.
In one process the cationic polymer beads are screw fed into the tailings
from the flotation cells as they leave the cells.
In another process an aqueous solution of a similar polymer is added
into the feed launder to the sedimentation tank.
It is found that addition of about 80g/min (dry weight) of the solution
polymer gives clarified water containing 80 to 90NTU, whereas dosage of only 36g/m
of the polymer beads gives clarified water of 60 to 70NTU and the filtration of
the sediment is performed more easily.
Example 2 (illustrative, outside scope of invention)
The polymer of example 1 is fed into a stream of coal tailings discharging
from a primary settling pond to a secondary settling pond. Flow is by gravity down
a pipe with a passage time of less than thirty seconds. The suspended solids of
the overflow from the secondary settling pond is determined. With polymer dosages
ranging from 0.0025 to 0.0125 g/l, the suspended solids range from around 0.18%
to 0.16% respectively.
When the same polymer is added as a 1% solution at polymer dosages
of 0.0125 to 0.024g/l, the suspended solids range from 0.3% to 0.25% respectively.
This again demonstrates the dramatic reduction in suspended solids obtainable by
the use of the solid form of polymer in contrast to the solution form.
Example 3 (illustrative, outside scope of invention)
In this example, the polymer is similar to example 1 except that it
has higher molecular weight, intrinsic viscosity about 2dl/g.
12.5 litres of an 8% aqueous dispersion that is an iron ore tailings
is agitated in a large bucket and 0.0027 grams polymer is added, with agitation
continuing for 15, 30 and 60 seconds. Samples are taken after each period of agitation
and the settling rate in inches (2.54cm) per minute is recorded. The highest value
is desirable. A clarity wedge value is recorded after 10 minute settling, and again
the highest result is desirable. The results are shown in the following table.
These results clearly demonstrate that the best results are obtained
with the dry polymer, and that the results are better at 15 seconds mixing than
at 60 seconds mixing.
Example 4 (illustrative, outside scope of invention)
A slurry of 1% kaolin in water containing 1g/l sodium chloride is
prepared. 1 litre of the slurry is placed in a cylinder provided with a perforated
steel plunger, depression of which causes mixing of the slurry. The test polymer
is added, at dosages ranging from 1 to 5 mg/l and the slurry is subjected to 3,
6 or 9 plunges of the plunger.
The process is repeated using polymers of diallyl dimethyl ammonium
chloride having, respectively, intrinsic viscosity 0.4, 0.9 and 2.0 dl/g. The settlement
rate and clarity is recorded.
In every instance, the settlement rate is better, at a given number
of plunges, for the solid polymer than the solution polymer and decreases with
an increase in the number of plunges. In every instance the settlement rate of
the polymer having IV 2 is better than the settlement rate for the polymer having
IV 0.9 which, in turn, is better than the settlement rate for the polymer having
Similarly, the compaction deteriorates (increases) with an increase
in the number of plunges and with decreasing IV. In general, the compaction is
always better with the solid polymer than with the corresponding solution polymer.
As regards clarity, there is little difference between the results
obtained with the solid and solution polymers having IV 0.4 but the solid form
of the other two polymers is always better than the solution form, with best results
being obtained with the polymer having IV 2 and about 6 plunges.
The settlement rate and clarity each tend to improve as the dosage
increases from 1 to 5 mg/l, except that with the polymer of IV 0.9dl/g the best
settlement rate is achieved at a dosage of about 2.5 mg/l.
Example 5 (illustrative, outside scope of invention)
A slurry is prepared of 1% china clay, 150mg/l sodium humate and 200g/l
sodium hydroxide. 500ml samples of this slurry are stirred at 150rpm and dosed
with various amounts of polymeric coagulant, either as solid or as aqueous solution.
Stirring is continued for 11 minutes and 2mg/l sodium polyacrylate (molecular weight
above 5 million) is added and stirred in for the last minute.
A sample of supernatant is removed and the humate concentration in
parts per million calculated from its UV absorbents. At 30 and 90 mg/l polydiallyl
dimethyl ammonium chloride (IV 0.9dl/g) the humate concentrations are substantially
the same both when introduced as solid and as liquid, but at 60mg/l dosage the
humate concentration is much lower (better) using the solid than the solution polymer
(36ppm compared to 44ppm).
Example 6 (illustrative, outside scope of invention)
A slurry is made up as in Example 4 and then tested as in Example
4 using, as the coagulant polymer, powdered (or solution) sodium polyacrylate having
molecular weight about 250,000. Turbidity and settlement rates are recorded. In
every instance, the solid gives better settlement rate than the solution at 3 and
6 plunges, and although the trend continues to be true at higher numbers of plunges
(i.e., continued turbulence, it is less distinct). For instance the solid grade
polymer at 1mg/l gives settlement rates of 7.6, 5.7 and 4.5cm/min at 3, 6 and 9
plunges respectively, whereas the corresponding values when the polymer is added
as a solution are 4.7, 4.3 and 4.0cm/min. There is no signficant difference in turbidity
obtained using solid or solution polymers in this example.