CA2114437A1

CA2114437A1 - Dewatering of aqueous suspensions

Info

Publication number: CA2114437A1
Application number: CA 2114437
Authority: CA
Inventors: John G. Langley; Peter R. B. Lawrence
Original assignee: Individual
Current assignee: Ciba Specialty Chemicals Water Treatments Ltd
Priority date: 1991-08-02
Filing date: 1992-08-03
Publication date: 1993-02-18
Also published as: ZA925819B; NO940336D0; EP0597950A1; FI940436A7; JPH06509741A; FI940436A0; FI940436L; WO1993002966A1; GB9116702D0; BR9206330A; AU2379992A; MX9204516A; NO940336L

Abstract

2114437 9302966 PCTABScor01 A suspension which is preferably an ink-containing waste suspension formed during a paper deinking process is flocculated by a cationic polymeric flocculant, and the flocculated material is then aggregated by an anionic colloidal material mixed into the flocculated suspension, optionally further cationic flocculant is added, and the aggregated material is then separated from the suspension by flotation.

Description

.W093/02966 PCT/GB92/014~9 3 ~ ; ~
,~, ,, Dew~ ering of Aqueou~ ~uspensions This inver. on relates to a process for separating -~
suspended material from an aqueous suspension and which `:
includes a dewatering stage that is conducted by flotation. ~;
The invention is of particular applicability to processes in which the suspension contains hydrophobic material. Examples are oils and ink particles, and the 1nvention is useful when the suspension is a suspension of waste material in a paper deinking process. .`::
: It is well known to add a flocculant material to a :~
suspension so as to cause the suspended material to flocculate before dewatering. Suitahle flocculants for this puxpose include alum and other polyvalent metal salts, b~ntonite, and various natural or synthetic polymers that :;
ca~:be~non-ionic, anlonic or cationic~ In some lnstances combinations are k~nown~, for instance bentonite followed by a:~polymer, ~ Some~ of the flocculant materials might be

2~0 : considered, more accurately, to be coagulants rather tha~
flocculants.
: The choice of ~locculant is dictated by the particular :process steps that are in~olved, the materials that are being:~flocculated,~:and the economics of the process.: For ~ ~ Z5 instance a process that is designed to make a product that can be sold may justify a more exp~nsive flocculant than a pr:ocess designed:to make a waste product.: Similarly, a process designed to flocculate essPntially hydrophilic : materlals will generallly require!diferent floccula~rllts from a process designed to flocculate:essentially hydrophobic ~- ma~erials.
, ~
: ~ ~ A ~locculation process useful in the production of paper is commercialised under the trade name "Hydrocol'l and described in EP-A-2358~3. In this a substantially 35 linear syntheti~ cationic polymer haviny molecular weight ~bo~ 500, 000 is aclded to a cellulosic suspension to forrn fl~cs which are then broken down by shearing to form ~.

W093/02966 PCT/GB92/0l42 mi-crof 1QCS, and bentonite is then added to the sheared suspension, which is then drained through a screen to form a paper sheet. This sheet can then be dried over hot rolls or in an oven. It is essential in this process to apply sufficient shear to break the initial flocs down to microflocs as otherwise the paper that is for~ed by drainage through the screen will have bad formation and there will be inefficient drying of the paper in the oven or over the hot rolls because moisture will be trapped in the flocs.
. .
This process clearly offers no ass~istance to someone concerned with a flotation process since the dewatering ~-~
technique is entirely different ~flotation instead of .
filtration), the suspended solids are generally entir~ly .~
15 different (hydrophobic materi~ls instead of hydrophilic .~.
celluIose fibres) and the objective ia usually entirely different (separation of, usually, a waste hydropho~ic . ,:
:material instead of the production of valuable paper~
It is/ however, known to add various flocculating ~20 materials prior to: a:flotation stage so as to improve the efficiency of the separation that is achieved by the : flotation. Also, of cour~se, conventional flotation : chemicals may be~included.
: : Flocculants that have been proposed for improving flotation processes include phenol formaldehyde resin with polyethylen~ oxide, a single application of high molecular weight flocculant in v~ry hard water systems, or sequential appli~ations of an initial material (for instance polyamine, polydiallyl dimethyl ammo~nium chlor!ide ;or bentonite) followed by a high molecular weight anionic or non-ionic flocculan~
It would be desirable to be able to improve the ~loccula~ion in a flotation process so as to obtain better separation of suspended material from the aqueous

3~ suspension.
A process according to the invention for separating su pended material from an aqueous suspension comprises W093/02966 PCT/GB92/01429 ::
2 ~ 7 flocculating the suspended material by mixing a cationic polymeric flocculant into the suspension, aqgregating the flocculated material by mixing an anionic colloidal material into the flocculated suspension, and separating the aggregated material ~rom the suspension by flotation.
The flotation ~ay be conducted as a dispersed air flotation process in which air at atmospheric pressure is forced into the liquor, or it may be conducted as a dissolved air flotation processes in which the in-flowing liquor is pressurised in the presence of air so as to dissolve air in the liquor, and the pressure is released in the flotation cell (or alternatively the inflowing liquor .. ..
may be under atmospheric pressure and the flotation cell may be und~r reduced pressure).
The flotation process will normally result in the initially suspended (generally hydrophobic~ material being floated to form a: Reject fraction with the cleaned liquor remaining as the~Accept fraction. .:~
The process can be used ~o separate suspendP.d material ~;:
that is industrially useful (for instance a pigment~ which , : may then be recycled or recovered for use, but it is often .
preferred to apply the invention to the separation of waste material from an aqueous liquor in which event the reject liquor may be dumped or the suspended material in it may be further dewatered, e.g., by filtration, before dumping.
The invention has the advantage that it promotes the separation of the suspended material into the Reject liquor from the aqueous liquor and, in general, contributes to the overall efflclenc,y,of the overall process for dewatering ~he susp~nded material.
The suspension that is to be dewatered can be a suspension o~ organic and/or inorganic particles, and the particles may initially be colloidal or dispersed.
Depending on the nature of the particles, the flocculant may either act as a conventional bridging flocculant or it may act more as a coagulant than as a flocculant, and in some instances it is desirable to add one polymeric ' f W093/02966 PCTtGB92/01429 ~ 4 !~ 1 materlal that wlll act as a coagulant and another that will act as a bridging flocGulant.
The suspension can be a suspension of sewage solids, ;~`~
for instance a sewage sludge or raw sewage, or it can be an s industrial effluent or an industrial liquor that is being used to recover waste material. For instance the suspension can be an effluent that is to be discharged after dewatering or it can be a suspension that is to be recycled after removing most of its suspended solids, for lO instance for dumping. Alternatively, the suspension can .
be a suspensi:on of industriaIly useful material, for :
instance a pigment suspension from which pigment is to be ~:
separated.
The invention is of particular value when the ~:~
suspended material comprises hydrophobic material. This , suspension may be a suspension of oil in an aqueous medium ~-, : since the oil particles can be flocculated by the addition of the flocculant and the flocculated particles can then be aggregated by the addition of the anionic colloidal materialO This i~s of value in, for instance, the recovery : - of drilling~mud liquors. : ~
It should be noted that we are using the term ~"~flocculation" (and flocculate and flocculant) as a generic : ~ term to include both:those proceses that might technically more accurately be considered to be coagulation (for instance charge neutralisation) and those processes that : would traditionally be regarded as true flocculation (for instance involving bridging by high molecular weight polymers~
Although the suspended material that is to be dewatered by the flotation can be liquid, it is generally solid, in particular hydrophobic solid ~he invention is ~,~ o~ par~icular value when it is part of a de-inking process ~ in which event the suspended material may be insoluble ink : 35 particlPs that may initially have been water insoluble or :~hat may have been insolubilised during the deinking process.

. ~W093/02966 PCT~GB92/0l429 2 1 ~ 3 7 In such a process, the suspension that is to be subjected to flotation is frequently made by subjecting a deinked pulp to a first flotation or other dewatering process to s2parate a relatively hydrophobic waste from relatively hydrophilic solids material, and then washing and dewatering the hydrophilic solids to produce, as the washings, the suspension that is to be subjected to flotation in the invention. The Reject fraction that is obtained by ~his flotation typically contains at least 5 by weight (based on the total solids) insoluble ink particles and this suspension may then be dewatered by filtration, generally by a belt press or other pressure filter, to produce a cake that may be dumped.
Suoh a process is illustrated diagrammatically in the accompanying flow diagram.
Cellulosic material that is contaminated with i~k (e.g., printed newsprint or printed cardboaxd) is subjected to a pulping and deinking process that generally involves a~series of steps including various bleachins and screening steps (that are not shown) to form an ~queous pulp that is passed along line 1 to dewaterin~ stage 2. This dewatering stage is generally an air flotation stage in which the ink-containing solids are floated off as a Reject slurry, that is~:removed along line 3 to a collection vessel

4, while ~he hydrophilic:Accept liquor is passed through line 5 to one or more stages 6 in which the liquor is : concentrated ~e.g., by drum or other filtration~ and the solids in it are washed and fi~tered. The useful solids from.~these s~ag,es are taken through line 7 to a stQck tower for recycling while ink-contaminated wash water is taken by line 8 towards a Krofta clarifier or other flotation cell 9. The hydrophobic Reject slurry that is floated in this cell is taken by line 10 to the vessel 4, while water can be removed by line 11 and recycled.
In accordance with the inven~ion, thexe is a sequerltial dosing system 12 by which cationic 10cculant polymer is mixed into the 17 quor to ~locculate the WO 93/02966 r PC~/GB92/1:11429 suspended material and anionic colloidal material is then added so as to aggregate this~ If desired, a similar sequential dosing syste~ can be provided before other flotation stages, for instance stage 2.
In addition to feeding the reject slurries from flotation cells 2 and 9 to the vessel 4, frequently other reject solids, for instance general mill effluent solid~, may be f~d to .it through line 13.
The slurry accumulated in vessel 4 is passed through line 14 and dosing system 15 to an Adritz or other belt press 16 where it is subjected to pressure filtration to produce a water effluent removed through pipe 17 and a cake that can be removed,:~or instance as shown at 18, and can then~-be dumped, for instance as landfill.
The slurry that is obtained as the Reject fraction and : that may then be ~iltered~typically has a solids con~ent of :below 5%, usually below 3% and often below 1%, for instance :below 0.5;%. The solids content is usually at least 0.05%, : ~and is often at least 0.1%. The solids content o~ this : 2:0 ;~ slurry typic:ally contains 10 to 70% (often 20 to 40%) by weight cellulosic fibres and/or fines, 0 to 80% (often 30 to 70%) pigment a~nd other filler, and 5 to 5V% (often 10 ~o 3~) insoluble i~k.
The content of suspended material :in the deinking 25 : llquor, drilling~mud liquor or other :liquor that is to be subjected to flotation is generally below 2% (by weight of the suspension) and usually below 0.5%, but is generally above lOppm and usually above lOOppm.
i i The ~a~i~nic polymer that~ is used~,in the invPntion can be a natural cationic polymer such as chitosan or a modified natural cationic polymer such as cationic starch, Preferably however the pol~ner is an organic synthetic polymer that is substantially water solu~le and that is ~ormed by polymer:ising one or more ethylenically unsa~urated monomers, in generally acrylic monomers, that consist of or include cationic monomer.
Suita~le cationic monomers are dialkylaminoalkyl (meth) ~ W093/02966 21 1 ~ !i 3 7 PCr/GB92/01429 ~

7 .;:
acrylates and dialkylaminoalkyl ~meth) acrylamides, either as acid salts or preferably as quaternary ammoniu~ salts.
The alkyl groups may each contain l to 4 carbon atoms and the aminoalkyl group may contain l to 8 carbon atoms.
Particularly preferred are dialkylaminoethyl (meth) acrylates, dialkylaminoethyl (meth) acrylamides and dialkylaminopropyl (meth) acrylamides. These cationic :
monomers are preferably copolymerised with a non-io~ic monomer, preferably acrylamide. Cationic amphoteric polymers (including a ~inor amount of anionic groups) can also be used. Preferred polymers are particulate as in EP
202780. : `
Various other cati~nic polymers that may be used include polyethylene imines, dicyandiamide polymers, pol~amine epichlorhydrin polymers and polymers of diallyl monomers such-as diallyl methyl ammonium chloride, either ~:
as ~homopolymer or copolymer with acrylamide or other ,.
comonomer. :~
The p~lymer~ can have sufficiently high molecular weight ~e.y., intrinsic viscosity a~ove~:4dl/g) that it is ~ a bridging flocculant, However it is often preferred that : the cationic polymer that is added to the suspension has a :suffi~ciently low mo1ecular weight that the flocculation : ~ process can more properly be ~described as coagulation. :~:
Pre~erably the molecular weight of the polymer is such that : IV is not above 3dl/g, e.g., 0.2 to 3dl/g or molecular .
weight 50,000 to 2 million. Suita~le Iow molecular weight polymers of this type are low molecular weight versions of `;~
p~ly~ers of ~ne,or more ethylenically unsaturated mqnomers including c~tionic monomer, as discussed above, and the : polyethyleneimine dicyandiamide, polyamine epichlorhydrin ;~
polymers, and polymers of diallyl mon~mers, as discussed above.
-The c~tionic polymer generally has a relatively high charge density, for instance above 0.2 and preferably 0.4 ~o Z~5 equivalents of catonic nitrogen per kilogram of polymer. When ~he cationlc polymer has IV below about t.~3 1 ;; ~
,~ 8 3dl/g the cationic content is preferably relatively high.
For instance the polymer may be a substantial homopolymer or formed from a monomer blend at least S0% and generally at least 80%, by weight cationic monomer, any rem~ining monomer generally being acrylamide or other non-ionic monomer. At higher molecular weight it cah be satisfactory for the amount of cationic monomer to be, for ins1:ance, 8 to 40, of ten around lO to 20, mole percent.
In som instances it is desirable to include a relatively low molecular weight cationic polymer (for instance a relatively highly charged cationic polymer having molecular weight 50,000 to 2 million~ as a coagulant followed by a higher molecular weight CatiOniG polymer, for instance to act as a bridging flocculant, followed by the lS anionic colloidal material. For instance the coagulant can remain from an earlier stage in the process and the flocculant can be added to the slurry prior to flotation.
Although the process can be conducted with relati~ely low degrees of mixing, with the result that there is little or no degradation of the initial flocs before the anionic colloidal material is added, it is o~ten preferred to break the flocs down into smaller flocs before adding the anionic .
colloidal material. This reduction in floc size can be achlevPd by applyiny stirring or other agitation to the ~5 flocculated suspension~ Naturally the extent of agitation must not be so :great that the initial solids are resuspended in the aqueous suspension but it is possible, particularly if relatively large amounts of the cationic polymer flocculant are used, to choose a degree of agitàtion that breaks the initial flocs down to microflocs - ; that are st~ble in the system against further reduction in si~e, and which are then aggregated by the anionic ~oll~idal material. An advan~age of reduciny the floc size in ~his manner is that it can facilitate dewatering to a hiyher solids content csmpared to the solids content than is conveniently availa~le if the f locs do not undergo size reduction be~ore adding the anionic colloidal material.

9 2 1 1 g ~ 3 ~ :

Size reduction of the f locs by agitation is particularly desirable when the polymer has IV above 4dl/g.
The addition o~ cationic polymer is made at some position ahead of the flotation cell, and anionic colloldal material is added after the polymer addition but be~ore the flotation pressure filtration stage. There must be sufficient interval between the two points of addition to allow floculation to occur and, as indicated above, it can be dssirable to apply sufficient mixing or other shear between the two points of addition so as to break down the initial flocs that are formed into microflocs.
~ The anionic colloidal material can be an emulsion of water-insoluble anionic or~anic polymer, or it can be an anlonic inorganic polymer such ~s colloidal silicic acid, but preferably it is an anionic swelling clay. S~ch clays :are known to swell;to a very large extent when the dry clay : is contacted with water. They are generally smectites and : are frequently montmorillonites. Suitable materials are referred~to as bentonites such as Wyomi~g bentonite, or 20 :Fullers Earth. The bentonite or other~ clay may have been ~hemically modified,:e-.g., by alkali treatment to convert calcium bentonite to alkali metàl bentonite. The :: bentonite or other clay preferably swells by at least 10 or :~ 20 times its own;volume and the dry clay is contacted with water and the surface area of the anionic colloid~before ~swelling:is preferably at least 30m2tg and the surface a~ter swelling ;i5 preferably at least 400, e.g, up to ~OOm2/g The bentonite or othPr clay may be provided as a powder that is mixed with water to f~rm a slurry for ~ur,venient addition to the aq~eous suspen~ion, or may initially be provided as a slurry, and this in turn may be pr~vided as a :concentrated slurry that contains low ~ molecular weight sodium polyacrylate or other dispersing agent or other material that wlll render the concentrate sufficiently fluid for e~se of handlin~.

';

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W093l02966 PCT/GB92/01429 A~ 3 1 i' 1 0 The amounts of cationic polymeric flocculant and anionic colloidal material used in the invention will depend upon the suspension that is being treated and the degree of agitation that is applied and the nature of the flotation process. The optimum can be determined by routine screening. If the flocculated suspension is to be agitated significantly prior to adding the anionic colluidal material, it is desirable to include sufficient cationic polymer to ensure ~hat the microflocs that are present after the agitation all carry a relatively heavy cativnic charge due ~o the flocculant polymer. The amount of polymer that is included in the suspension is generally at least 2ppm (based on the weight of suspension) and is generally at least 5 or lOppm. It can be up to, for instance, 500ppm but the amount is generally below lOOppm and is frequ~ntly below 50ppm. Values of 5 to 50, often around 20 to 30, ppm are often preferred. These are all - based on the ~otal weight of suspension. Based on the solids content of~he suspension, the amounts typically are at least 0.1% and often at least 0.5~, but generally below

5% and~ often below 2%, by weight of the solids content of ;, , the dispersîon. ~
- ~ The amount needed for optimum results may be reduced - if the suspension already contains cationic polymer from a ~: 25 previous stage, for instance from a prPvious ~lotation ~ stage. If the amount of cationic polymer results in : polymer being carried through to a subsequent stage, such ~s pressure filtration, this may make it possible to reduce the amount of cationic polymeric flocculant that might ~0 otherwise be required for that subsequent stage.
~ locculant ~or subsequent filtration ~r other dewa~ering stages may be any of those described above.
~ n all in~tances, the flocculant is generally water ~ soluble and substantially linear, but it can be useful for 35 ~he polymer to f locculate the suspended material while the polymer i5 particulate, as in EP 20Z780. This is particularly desirable in respect of polymer that is being ,,, . - . . . ~ - :

2 1 ~ ' 7 added ~o flocculate suspension in a subsequent pressure filtra~ion stage.
The amount of ar;ionic colloidal material is generally at least lOppm and usually at least 50ppm and prefera~ly at least lOOppm, based on the weight of the dispersion. The amount is generally below 500ppm and frequently it is below 250ppm. ~mounts around 50 to 150ppm are often preferred. ;~
These amounts are~based on the weight of the aqueous suspension that is to be subjected to the flotation. `
Based on the solids content of the suspensio~, the amounts typically are above 0.1% and generally above 0.3% and often above 1%, but the amount is generally below 5%, and preferably below 3% by weight. -~ In addition to providing the cationic polymeric f 1OCGU1ant in the suspension and then aggregating it by adding anionic colloidal material prior to pressure filtration, it ~an be desirable to add further cationic polymeric flocculan~ to the suspension after adding the ~ anionic colloidal~material and bef~r~e the ~lotation. This ~added catio~ic material is genera}ly~ of high molecular weight (for instance intrinsic viscosity above 4dl~g at -2~C in lN sodium chloride solution) and can have ~
.
relatively low charge density, for instance being formed ~rom 3 to 25 mole percent, often 8 to 15 mole p~rcent, cationic monomer with the balance acrylamide.
When the overall process ~includes a subsequent dewatering stage~ to produce a cake of solids, this is genera1ly dumped without further drying (for instanc by heating or combustion) but if desired it can be subje~ted to further treatment before use or dumping.
` A an example o~ the invention,~waste inked paper is pulped in the presence of alkali silicate and chelating agent, b~eached and filtered and soap is then added to the ~ .
s~spension.
~5 ~erring to the drawing, the suspension is then ~;
sub~ec~d to air flotation at 2 to form a first Reject slurry ~line 3) and an Accept liquor which is thickened and ~ rl 12 was ~ at 6 by suction drainage or passage over a drum thickener or screw press, and the filtrate is taken to a clarifier 9 while the solids are rewashed and again filtered, with the washings being taken to the clarifier.
This clarifier 9 is a Krofta flotation cell in which the liquor is subjected to air flotation with the clarified Accept liquor being recycled to the pulping stage and a second Reject slurry being collected.
. The first and second Reject slurries are mixed together at 4 to form a slurry having a solids content o~
which about 50~ is filler, 30% fibre fines and 20% ink~ :
The addition of the ~second Reject is desirable as it l:.:
increases the fibre content of the combined slurry and so improves its handling. ;
In the process of the invention, two-stage dosing is :~
.
conducted at the do;sing system i2, with 25ppm homopolymer of diallyldimethyl ~ammonium chloride having IV 2dl/g being :~
- mlxe~ into the slurry~ followed by lOO~ppm bentonite.
: ~ ~ In a comparative process (nvt in ~accordance with the :
invention~: the two-stage dosi~g consists of adding the . ~ .
. ~entonite followed by a conventional high~molecular~weight .-~
::: cationic polymer. :Less effective separation is obtained. :~
.
: Preferably a cationic polymer of lO mole % quaternised : : ~ dimethylaminoethyl acrylate and 90 mole % acrylamide and havinq IV above 8dl/g is added a~ a dosage of lOOg/t after the addition of bentonite and be~ore the:flotation.
In a partlcularly advantageous process, the Reject suspension obtalned from the flotation process of the in~ention is subjected to pressure filtration after flocculating the suspended material in~ that suspension by : mixing a cationic polymeric flocculant i~to the ~uspension and th~n aggregating the flocculated material by mixing anionic colloidal material into the floccula~ed suspension, ~:
~nd then separating ~he aggregated material from the suspension by belt pressing or other pressure filtration.
Thus, in ~ pre~erred process, ~he dosing system 15 m~y also : be a sequential dosing system ~or adding cationic polymer "

,WO 93/02~66 21 1 ~ 4 3 7 PCr~GB92/0l429 followed by anionic colloidal material. The materials and amounts may be similar to those described above.

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Claims

1. A process for separating suspended material from an aqueous suspension by mixing a cationic polymeric flocculant into the suspension and then separating the flocculated material from the suspension by flotation to provide a Reject fraction containing the flocculated material characterised in that the flocculated material is aggregated by mixing an anionic colloidal material into the flocculated suspension and the suspension containing the aggregated solids is subjected to the flotation.

2. A process according to claim 1 in which the suspended material is a hydrophobic liquid or hydrophobic solid.

3. A process according to claim 1 which is part of a paper deinking process and in which the suspended material comprises insoluble ink particles.

4. A process according to claim 3 in which the Reject fraction is a suspension containing at least 5% by weight (based on total solids) insoluble ink particles.

5. A process according to claim 3 in which the Reject fraction is a suspension: containing 5 to 50% by weight insoluble ink particles, 10 to 70% by weight cellulosic fibres and fines, and 0 to 80% by weight filler, the percentages being based on total solids.

6. A process according to any of claims 3 to 5 in which the suspension of ink particles is made by a deinking process comprising subjecting a deinked pulp to a first flotation process to separate hydrophobic waste including the ink particles from hydrophilic solids material, and washing and dewatering the hydrophilic solids to produce the suspension containing ink particles.

7. A process according to any of claims 3 to 6 in which the Reject fraction is subsequently dewatered by pressure filtration to produce a cake, and the cake is dumped.

8. A process according to any preceding claim in which the cationic polymer is a water soluble organic synthetic polymer selected from polymers of cationic ethylenically unsaturated monomer, polyethylene imines, polyamines, dicyandiamide polymers, and polyamine epichlorhydrin polymers,

9. A process according to any preceding claim in which the cationic polymer comprises polymer that has IV 0.2 to 3dl/g and is formed from monomer of which at least 50% by weight is cationic.

10. A process according to claim 9 in which the polymer is a homopolymer of diallyl dimethyl ammonium chloride or a copolymer with up to 50% acrylamide.

11. A process according to claim 8 in which the cationic polymer comprises polymer having IV at least 4dl/g.

12. A process according to any preceding claim in which the flocculated solids are broken down to microflocs by stirring before adding the anionic colloidal material.

13. A process according to any preceding claim in which the anionic colloidal material is an anionic swelling clay.

14. A process according to any of claims 1 to 12 in which the anionic colloidal material is selected from anionic inorganic polymers and anionic swelling clays.