DEWATERING OF AQUEOUS SUSPENSIONS

Abstract: 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: Dewatering of Aqueous Suspensions This invention 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 invention 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. Suitable flocculants for this purpose include alum and other polyvalent metal salts, bentonite, and various natural or synthetic polymers that can be non-ionic, anionic or cationic. In some instances combinations are known, for instance bentonite followed by a polymer. Some of the flocculant materials might be considered, more accurately, to be coagulants rather that, flocculants. The choice of flocculant is dictated by the particular process steps that are involved, the materials that are being flocculated, and the economics of the process. For instance a process that is designed to make a product that can be sold may justify a more expensive flocculant than a process designed to make a waste product. Similarly, a process designed to flocculate essentially hydrophilic materials will generally require different flocculants from a process designed to flocculate essentially hydrophobic materials. A flocculation process useful in the production of paper is commercialised under the trade name "Hydrocol" and is described in EP-A-235893. In this a substantially linear synthetic cationic polymer having molecular weight above 500,000 is added to a cellulosic suspension to form floes which are then broken down by shearing to form microflocs, 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 floes down to microflocs as otherwise the paper that is formed 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 floes. This process clearly offers no assistance to someone concerned with a flotation process since the dewatering technique is entirely different (flotation instead of filtration) , the suspended solids are generally entirely different (hydrophobic materials instead of hydrophilic cellulose fibres) and the objective is usually entirely different (separation of, usually, a waste hydrophobic material instead of the production of valuable paper) . It is, however, known to add various flocculating materials prior to a flotation stage so as to improve the efficiency of the separation that is achieved by the flotation. Also, of course, conventional flotation chemicals may be included. Flocculants that have been proposed for improving flotation processes include phenol formaldehyde resin with polyethylene oxide, a single application of high molecular weight flocculant in very hard water systems, or sequential applications of an initial material (for instance polyamine, polydiallyl dimethyl ammonium chloride or bentonite) followed by a high molecular weight anionic or non-ionic flocculant. It would be desirable to be able to improve the flocculation in a flotation process so as to obtain better separation of suspended material from the aqueous suspension. A process according to the invention for separating suspended material from an aqueous suspension comprises flocculating the suspended material by mixing a cationic polymeric flocculant into the suspension, aggregating the flocculated material by mixing an anionic colloidal material into the flocculated suspension, and separating the aggregated material from the suspension by flotation. The flotation may 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 under 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 to separate suspended 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 efficiency of the overall process for dewatering the suspended material. The suspension that is to be dewatered can be a suspension of 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 material that will act as a coagulant and another that will act as a bridging flocculant. The suspension can be a suspension of sewage solids, for instance a sewage sludge or raw sewage, or it can be an 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 instance for dumping. Alternatively, the suspension can be a suspension of industrially 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 material. This is 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. The invention is of particular value when it is part of a de-inking process in which event the suspended material may be insoluble ink particles that may initially have been water insoluble or that may have been insolubilised during the deinking process. 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 separate 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 this 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. Such a process is illustrated diagrammatically in the accompanying flow diagram. Cellulosic material that is contaminated with ink (e.g., printed newsprint or printed cardboard) is subjected to a pulping and deinking process that generally involves a series of steps including various bleaching and screening steps (that are not shown) to form an aqueous pulp that is passed along line 1 to a dewatering 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 the 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 filtered. The useful solids from these stages are taken through line 7 to a stock 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 invention, there is a sequential dosing system 12 by which cationic flocculant polymer is mixed into the liquor to flocculate the suspended material and anionic colloidal material is then added so as to aggregate this. If desired, a similar sequential dosing system 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 solids, may be fed 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, for 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 filtered typically has a solids content 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 of this slurry typically contains 10 to 70% (often 20 to 40%) by weight cellulosic fibres and/or fines, 0 to 80% (often 30 to 70%) pigment and other filler, and 5 to 50% (often 10 to 30%) insoluble ink. The content of suspended material in the deinking liquor, 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. The cationic polymer that is used in the invention can be a natural cationic polymer such as chitosan or a modified natural cationic polymer such as cationic starch. Preferably however the polymer is an organic synthetic polymer that is substantially water soluble and that is formed by polymerising one or more ethylenically unsaturated monomers, in generally acrylic monomers, that consist of or include cationic monomer. Suitable cationic monomers are dialkylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides, either as acid salts or preferably as quaternary ammonium salts. The alkyl groups may each contain 1 to 4 carbon atoms and the a inoalkyl group may contain 1 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-ionic monomer, preferably acrylamide. Cationic amphoteric polymers (including a minor amount of anionic groups) can also be used. Preferred polymers are particulate as in EP 202780. Various other cationic polymers that may be used include polyethylene i ines, dicyandiamide polymers, polyamine epichlorhydrin polymers and polymers of diallyl monomers such as diallyl methyl ammonium chloride, either as homopolymer or copolymer with acrylamide or other comonomer. The polymer can have sufficiently high molecular weight (e.g., intrinsic viscosity above 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 sufficiently low molecular weight that the flocculation process can more properly be described as coagulation. Preferably 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. Suitable low molecular weight polymers of this type are low molecular weight versions of polymers of one or more ethylenically unsaturated monomers including cationic monomer, as discussed above, and the polyethyleneimine dicyandiamide, polyamine epichlorhydrin polymers, and polymers of diallyl monomers, as discussed above. The cationic polymer generally has a relatively high charge density, for instance above 0.2 and preferably 0.4 to 2.5 equivalents of catonic nitrogen per kilogram of polymer. When the cationic polymer has IV below about 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 50% and generally at least 80%, by weight cationic monomer, any remaining monomer generally being acrylamide or other non-ionic monomer. At higher molecular weight it can be satisfactory for the amount of cationic monomer to be, for instance, 8 to 40, often around 10 to 20, mole percent. In some 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 cationic polymer, for instance to act as a bridging flocculant, followed by the 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 relatively low degrees of mixing, with the result that there is little or no degradation of the initial floes before the anionic colloidal material is added, it is often preferred to break the floes down into smaller floes before adding the anionic colloidal material. This reduction in floe size can be achieved by applying stirring or other agitation to the 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 " agitation that breaks the initial floes down to microflocs that are stable in the system against further reduction in size, and which are then aggregated by the anionic colloidal material. An advantage of reducing the floe size in this manner is that it can facilitate dewatering to a higher solids content compared to the solids content than is conveniently available if the floes do not undergo size reduction before adding the anionic colloidal material. Size reduction of the floes by agitation is particularly desirable when the polymer has IV above 4dl/g. The addition of cationic polymer is made at some position ahead of the flotation cell, and anionic colloidal material is added after the polymer addition but before 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 desirable to apply sufficient mixing or other shear between the two points of addition so as to break down the initial floes that are formed into microflocs. The anionic colloidal material can be an emulsion of water-insoluble anionic organic polymer, or it can be an anionic inorganic polymer such as colloidal silicic acid, but preferably it is an anionic swelling clay. Such 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 Wyoming bentonite, or Fullers Earth. The bentonite or other clay may have been chemically modified, e.g. , by alkali treatment to convert calcium bentonite to alkali metal 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 30m /g and the surface after swelling is preferably at least 400, e.g, up to 800m/g. The bentonite or other clay may be provided as a • powder that is mixed with water to form a slurry for convenient addition to the aqueous suspension, or may initially be provided as a slurry, and this in turn may be provided as a concentrated slurry that contains low molecular weight sodium polyacrylate or other dispersing agent or other material that will render the concentrate sufficiently fluid for ease of handling. 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 colloidal material, it is desirable to include sufficient cationic polymer to ensure that the microflocs that are present after the agitation all carry a relatively heavy cationic charge due to 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 frequently below 50ppm. Values of 5 to 50, often around 20 to 30, ppm are often preferred. These are all based on the total weight of suspension. Based on the solids content of the 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 dispersion. The amount needed for optimum results may be reduced if the suspension already contains cationic polymer from a previous stage, for instance from a previous flotation stage. If the amount of cationic polymer results in polymer being carried through to a subsequent stage, such as pressure filtration, this may make it possible to reduce the amount of cationic polymeric flocculant that might " otherwise be required for that subsequent stage. Flocculant for subsequent filtration or other dewatering stages may be any of those described above. In all instances, the flocculant is generally water soluble and substantially linear, but it can be useful for the polymer to flocculate the suspended material while the polymer is particulate, as in EP 202780. This is particularly desirable in respect of polymer that is being added to flocculate suspension in a subsequent pressure filtration stage. The amount of anionic colloidal material is generally at least lOppm and usually at least 50ppm and preferably at least lOOppm, based on the weight of the dispersion. The amount is generally below 500ppm and frequently it is below 250ppm. Amounts 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 suspension, 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 flocculant in the suspension and then aggregating it by adding anionic colloidal material prior to pressure filtration, it can be desirable to add further cationic polymeric flocculant to the suspension after adding the anionic colloidal material and before the flotation. This added cationic material is generally of high molecular weight (for instance intrinsic viscosity above 4dl/g at 25°C in IN sodium chloride solution) and can have relatively low charge density, for instance being formed from 3 to 25 mole percent, often 8 to 15 mole percent, cationic monomer with the balance acrylamide. When the overall process includes a subsequent dewatering stage to produce a cake of solids, this is generally dumped without further drying (for instance by heating or combustion) but if desired it can be subjected ' to further treatment before use or dumping. As an example of the invention, waste inked paper is pulped in the presence of alkali silicate and chelating agent, bleached and filtered and soap is then added to the suspension. Referring to the drawing, the suspension is then subjected to air flotation at 2 to form a first Reject slurry (line 3) and an Accept liquor which is thickened and washed 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 of which about 50% is filler, 30% fibre fines and 20% ink. The addition of the second Reject is desirable as it 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 dosing system 12, with 25ppm homopolymer of diallyldimethyl ammonium chloride having IV 2dl/g being mixed into the slurry followed by lOOppm bentonite. In a comparative process (not in accordance with the invention) the two-stage dosing consists of adding the bentonite followed by a conventional high molecular weight cationic polymer. Less effective separation is obtained. Preferably a cationic polymer of 10 mole % quaternised dimethyla inoethyl aerylate and 90 mole % acrylamide and having IV above 8dl/g is added at a dosage of lOOg/t after the addition of bentonite and before the flotation. In a particularly advantageous process, the Reject suspension obtained from the flotation process of the invention is subjected to pressure filtration after -flocculating the suspended material in that suspension by mixing a cationic polymeric flocculant into the suspension and then aggregating the flocculated material by mixing anionic colloidal material into the flocculated suspension, and then separating the aggregated material from the suspension by belt pressing or other pressure filtration. Thus, in a preferred process, the dosing system 15 may also be a sequential dosing system for adding cationic polymer followed by anionic colloidal material. The materials and amounts may be similar to those described above.

Claims: 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, polya ines, 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.

Inventors: LANGLEY JOHN GRAHAM, LAWRENCE PETER ROBIN BROUGHTON, LANGLEY, JOHN, GRAHAM, LAWRENCE, PETER, ROBIN, BROUGHTON
Applicants: ALLIED COLLOIDS LTD, ALLIED COLLOIDS LIMITED
Application year: 1992
Main classification: D21C5
IPC codes: B03D1, C02F1, C02F9, D21C5