Separation of solids from aqueous suspensions containing cellulosic fibers and fines

Cellulosic fines and cellulosic fibers are separated from a suspension containing the fines, the fibers and hydrophobic materials by screening the suspension to remove the fibers, and then subjecting the resultant filtrate to dispersed air flotation in the presence of calcium and generally a separation aid to form an upper Reject fraction in which the hydrophobic material is concentrated and a lower Accept fraction in which the cellulosic fines are concentrated.

This invention relates to the selective removal of suspended cellulosic
 materials from aqueous suspensions containing these together with
 suspended hydrophobic materials such as filler.
 BACKGROUND OF THE INVENTION
 It is well known to prepare waste paper for re-use in paper-making
 processes by subjecting it to de-inking. The waste paper is normally
 pulped with water in the presence of de-inking chemicals and then the pulp
 is subjected to a de-inking stage in which ink is separated from the pulp.
 The de-inking stage may involve for instance flotation and/or washing.
 Flotation generally takes place in a flotation cell, in which a foam is
 formed at the surface of the pulp. This foam is removed, since it contains
 large amounts of the ink. However, this foam may contain some desirable
 materials, for instance fibres and cellulosic fines. Significant amounts
 of fibres and cellulosic fines are also often carried through in the waste
 water which has been used for washing the deinked pulp (backwater).
 In general it is well known simply to dispose of the foam from the
 flotation stage. Backwater is often treated so as to clarify it. That is,
 solid materials are separated from the backwater to form a sludge. The
 clarified water can then be recycled to the de-inking process and the
 sludge sent for disposal. Clarification can be by for instance
 sedimentation, filtration or dissolved air flotation. All of these
 processes are designed to remove from the backwater substantially all
 suspended solids in a non-selective manner.
 Thus in such systems any fibres and cellulosic fines which have been
 carried into the foam or backwater are lost.
 It is known to screen backwater before it is subjected to clarification
 treatment so as to remove long cellulosic fibres. These can then be
 recycled to the de-inking or paper-making process. The filtrate is then
 treated as normal to remove all remaining suspended solids. This is often
 done by dissolved air flotation. Again, substantially all suspended solids
 are removed in a non-selective manner. It is also known to include
 flocculating or coagulating chemicals at this stage. These induce
 agglomeration of the suspended solids into flocs which can then be floated
 and removed. Systems of this general type are described by Krofta and
 Wang, in "Total Closing of Paper Mills with Reclamation and De-inking
 Installations", 43rd Purdue Industrial Waste Conference Proceedings, 10-12
 May 1988, pp. 673 to 687 (published 1989). These systems aim only to
 recover long fibres from backwater. After these have been recovered, the
 aim of the described system is to clarify and recycle the backwater.
 It is also known to clarify paper-making white waters in general by
 removing fibres using standard solid-liquid separation methods.
 In all of the above systems, even those which involve recovery of long
 fibres, valuable materials are lost, in particular cellulosic fines. Not
 only are these useful materials lost, but they form part of the foam or
 sludge which must be disposed of and contribute towards the significant
 disposal costs (either by landfill or burning) which are incurred by
 de-inking plants.
 Dorica and Simandl, in "Separation of Fibre and Ash in De-inking Effluents:
 A Case Study", Tappi Journal, Vol. 78, No. 5, pp 109 to 116, May 1995,
 have attempted to separate useful cellulosic materials from other
 materials such as clay present in de-inking backwaters. They aim to do
 this by using modified mechanical systems which include dispersed air
 flotation and screening. The flotation step is designed to float the
 fibres to form an upper fraction and to retain clay in the lower fraction.
 However, from the results given it appears that significant amounts of the
 fibre content of the suspension treated are lost in the lower, "clay"
 fraction. Dorica and Simandl also describe various two-stage processes.
 Those described however involve either two screening steps or a flotation
 step followed by screening of the floated materials.
 The systems described all rely on solely mechanical methods and do not give
 an efficient separation of cellulosic and hydrophobic materials.
 It would be desirable to be able to reduce the costs of waste disposal by
 re-use of valuable materials, by separating these in an efficient and
 convenient manner from materials which are not required for re-use.
 It would also be desirable to avoid as far as possible the loss of
 suspended cellulosic materials from waste waters, especially those
 produced in the course of de-inking processes, and to recover these for
 re-use in an efficient manner.

DETAILED DESCRIPTION OF THE INVENTION
 The invention provides a method of separating cellulosic fines and
 cellulosic fibres from an aqueous suspension which contains cellulosic
 fibres, cellulosic fines and hydrophobic materials, comprising
 A) screening the aqueous suspension to remove cellulosic fibres and to
 provide a filtrate then
 B) subjecting the filtrate to a dispersed air flotation stage in the
 presence of a level of calcium ion of at least 50 ppm, preferably at least
 100 ppm, and at a pH of at least 6 and preferably in the presence of
 separation aid comprising one or more of:
 (a) fatty acid or salt thereof
 (b) cationic polymeric separation aid
 (c) anionic polymeric separation aid
 (d) amphoteric polymeric separation aid, thereby forming an upper Reject
 fraction and a lower Accept fraction whereby cellulosic fines concentrate
 in the Accept fraction hydrophobic material concentrates in the Reject
 fraction and
 C) separating the Reject and Accept fractions.
 We find that the central feature of the process of the invention is step
 (B). In this step hydrophobic materials are floated selectively to form
 the upper Reject fraction. Colloidal materials, if present, are also
 selectively floated into the Reject fraction. The hydrophobic and
 colloidal materials include ink, colloidal solids such as starch,
 partially dissolved cellulose, oils and adhesives, and in particular
 filler. We find that in de-inking processes in particular filler particles
 are rendered substantially hydrophobic as they pass through the de-inking
 and/or washing stages and are thus hydrophobic in the waste water which
 forms the aqueous suspension of the method of the invention. The invention
 can however be used to separate filler and other hydrophobic materials
 from any aqueous suspension in which the filler is substantially
 hydrophobic.
 In step (B) selective separation occurs such that hydrophobic and colloidal
 materials are predominantly floated into the Reject fraction whilst
 cellulosic fines are predominantly retained in the Accept fraction. By
 means of the process of the invention it is possible to achieve very good
 selectivity. Removal of filler and, in particular, residual ink leaves the
 cellulosic fines very clean. This means that if desired they can be
 recycled directly into the deinked pulp for transport to the paper-making
 process.
 The selectivity of this step is unexpected in view of the known action of
 the various flotation aid components used. Calcium ion is known to have a
 coagulating effect, whilst the soapy and polymeric materials which are
 preferably used in the invention tend normally to act as collectors,
 coagulants or flocculants. It would thus be expected that all of the solid
 materials suspended in the aqueous liquor would be agglomerated and
 floated in a non-selective manner. However, this does not occur and in the
 invention we can achieve high levels of selectivity.
 In step (A) the aqueous suspension is screened to remove cellulosic fibres.
 Screening may be by any suitable method of sorting by size and/or form,
 for instance by filtration. Fibres screened out in step (A) are normally
 of length such that they are retained on a screen having hole or slot size
 of less than 200 .mu.m or 100 .mu.m and preferably less than 70 .mu.m or
 often 80 .mu.m. Normally hole size is not more than 100 .mu.m. Particles
 of filler, colloidal solids and cellulosic fines are smaller and thus are
 not retained to any significant extent during screening.
 In this step it is possible to screen out as much as 70%, often 75% or 80%
 or more, even up to 90% or 95% or above, by weight of the cellulosic
 fibres present in the aqueous suspension.
 The solids fraction recovered can also be very rich in cellulosic fibres,
 for instance having a content of at least 70%, often at least 80 or 90% or
 even 95% by weight of solids screened out, of cellulosic fibre.
 Step (A) produces a solids (fibre) fraction and a liquid phase, which is
 the remaining aqueous suspension, in this specification termed the
 filtrate, whichever method is used for screening out the cellulosic
 fibres. The filtrate passes to step (B).
 In step (B) selective removal of hydrophobic and colloidal material is
 obtained. In this process the filtrate produced in step (A) is subjected
 to dispersed air flotation (also known as induced air flotation). This
 process is distinct from dissolved air flotation. In the dispersed air
 flotation step (B) bubbles of relatively large diameter are produced.
 These may have size from around 0.1 to 1.0 mm. Any known equipment for
 dispersed air flotation may be used. In general, known flotation cell
 systems produce air bubbles of the desired size either by rapid stirring
 of the filtrate which is being subjected to flotation or otherwise pulling
 air into the flotation cell, for instance through an air hole. The air may
 be introduced by suction or pressure.
 Generally the presence of turbulence in the flotation cell is desirable
 during the dispersed air flotation stage.
 Various other conditions for the flotation stage (B) are important in the
 invention, as follows.
 The filtrate is at a pH of at least about 6 and is preferably at alkaline
 pH for instance at least about 9 or 10. Generally it is not more than
 about 11 or 12. If the aqueous suspension is washing stage backwater or
 rejected foam from a de-inking process it will normally have a pH of at
 least 6. If the aqueous suspension does not have appropriate pH this can
 be provided by the addition of suitable buffer materials, in standard
 manner.
 In flotation stage (B) the content of calcium ion is also important. It
 must be at least about 50, and preferably at least about 100, ppm calcium
 ion based on volume of filtrate being subjected to flotation. Often it is
 at least about 120 ppm, often at least 150 ppm and may be as high as 200
 ppm or more. Generally it is not more than about 400 or 500 ppm.
 The filtrate may contain sufficient levels of calcium ion as it enters step
 (B). For instance, it may comprise very hard water or may comprise
 residual calcium ion from processes to which it has previously been
 subjected. Generally however the levels of calcium ion are not sufficient
 to achieve the desired results. In such cases calcium ion is added to the
 filtrate in step (B), normally before flotation begins, although it may be
 added before or during step (A). It can be added in the form of any
 convenient calcium salt, for instance CaCl.sub.2. Around 50 to 150 ppm of
 the calcium ion content is normally derived from added calcium salt, often
 70 to 100 ppm, in particular around 90 ppm.
 Other polyvalent salts can also be used in these amounts, for instance to
 provide aluminium, ferric or ferrous ion, instead of or in addition to
 calcium ion.
 The process is greatly improved if a separation aid is included in step B.
 This may comprise one or more of various types of suitable material.
 It may comprise material of class (a), selected from fatty acids and salts
 thereof. Sulphonated or alkali metal salts of fatty acids can be used.
 Sodium salts are preferred. We believe that in systems in which a fatty
 acid or derivative thereof is used as a component of the separation aid,
 it interacts with calcium ion to form a calcium salt of the fatty acid,
 which then acts on the suspended solids to aid in giving the desired
 effects. Thus any salt of fatty acid may be used which will give the
 corresponding calcium salt in the presence of calcium ion.
 Suitable fatty acids for use in acid or salt form include C.sub.10-20 fatty
 acids, preferably C.sub.12-18 and more preferably C.sub.16-18 fatty acids.
 The separation aid may comprise material (b), selected from cationic
 polymeric materials generally having intrinsic viscosity of not more than
 20 dl/g. Any suitable polymeric material may be used, for instance
 cationised polymeric emulsions. The material may be a naturally occurring
 cationic polymer or a modified naturally occurring polymer but preferably
 it is a synthetic polymer which has been formed from ethylenically
 unsaturated monomer or monomer blend.
 The cationic polymeric material may have relatively high intrinsic
 viscosity of at least about 5 dl/g, often not more than 15 dl/g, for
 instance 6 or 7 to 10 or 12 dl/g. Polymers having too high an intrinsic
 viscosity can in some systems give reduced selectively. Preferably the
 relatively high intrinsic viscosity materials have a relatively low
 cationic charge density. For instance, if the material is a synthetic
 polymer produced from a blend of monomers, preferably this blend comprises
 not more than 70 wt %, preferably 50 wt %, often not more than 30 wt %,
 cationic monomer.
 The cationic polymer is preferably formed from water soluble ethylenically
 unsaturated monomer or monomer blend. Suitable cationic monomers include
 dialkylaminoalkyl(meth) acrylates and -acrylamides, as acid addition or,
 preferably, quaternary ammonium salts, and diallyldialkyl ammonium
 halides. Preferred (meth) acrylates are di-C.sub.1-4 alkylaminoethyl
 (meth) acrylates and preferred (meth) acrylamides are di-C.sub.1-4
 alkylaminopropyl (meth) acrylamides, in particular dimethylaminoethyl
 (meth) acrylates [DMAE(M)A] and dimethylaminopropyl (meth) acrylamide
 [DMAP(M)A], as acid addition and, preferably, quaternary ammonium salts.
 The preferred diallyldialkyl ammonium halide is diallyldimethyl ammonium
 chloride (DADMAC).
 Preferred cationic polymers of relatively high IV are copolymers of the
 above monomers with non-ionic ethylenically unsaturated monomer. Suitable
 non-ionic monomers include (meth) acrylamide, in particular acrylamide.
 Other cationic polymeric materials which are useful as separation aid (b)
 are of the lower molecular weight coagulant type and have intrinsic
 viscosity of not more than about 3 dl/g and preferably have a relatively
 high cationic charge density. In particular, preferred cationic polymeric
 materials in this class are formed from water-soluble ethylenically
 unsaturated monomer or monomer blend in which at least 50 wt %, generally
 at least 80 wt %, of the monomers are cationic. Polymers in which 100% of
 the monomers are cationic are preferred.
 Although any of the cationic monomers listed above can be used,
 polydiallyldimethyl ammonium chloride (polyDADMAC) is preferred.
 Copolymers of DADMAC which contain up to 30 wt % acrylamide are also
 useful.
 The low molecular weight coagulant type cationic polymeric materials
 preferably have IV below about 2 dl/g, often below 1.5 or 1 dl/g, for
 instance down to 0.1 to 0.2 dl/g.
 Generally the cationic polymeric material is a linear synthetic polymer.
 However, it is possible to use polymers which have been produced in the
 presence of cross-linking or branching agent. For instance, the cationic
 polymer can be of the type described in EP-A-202,780.
 The separation aid may also comprise materials (c) selected from anionic
 polymeric materials generally having intrinsic viscosity of not more than
 25 dl/g. The anionic polymeric material may be a natural polymeric
 material such as a polysaccharide or a modified natural polymeric
 material. Preferably however it is a synthetic polymer, in particular one
 formed from water-soluble ethylenically unsaturated monomer or monomer
 blend.
 Anionic polymeric material of low intrinsic viscosity, for instance below 4
 dl/g, may be used. Preferably however the intrinsic viscosity is at least
 about 5, preferably at least 8, for instance from 8 to 12 or 15 dl/g.
 Preferably the intrinsic viscosity is not especially high, since in some
 systems this can reduce the selectivity achieved. Preferably then the
 intrinsic viscosity is not more than 15 or 12 dl/g.
 Generally at least 5, 10 or 15 wt % up to 100 wt %, preferably 20 to 80 wt
 %, of the monomers used to produce the polymer are anionic, with any other
 monomers being non-ionic.
 Preferred anionic monomers are ethylenically unsaturated carboxylic or
 sulphonic acids, generally as their water-soluble alkali metal salts.
 Examples are 2-acrylamido-2-methyl propane sulphonic acid (AMPS, US Trade
 Mark), methacrylic acid and acrylic acid (as sodium or other alkali metal
 salts). Sodium acrylate is usually preferred.
 Suitable water-soluble ethylenically unsaturated non-ionic comonomers
 include (meth) acrylamide, especially acrylamide.
 Preferred anionic polymers are copolymers of acrylamide and, usually, 50 to
 80% by weight sodium acrylate. Alternatives include homopolymers of sodium
 acrylate and copolymers of acrylamide and AMPS, in particular copolymers
 of AMPS and up to 97 wt %, often up to 95 wt %, (meth) acrylamide.
 The synthetic polymers of class (c) are generally substantially linear,
 although they may be produced in the presence of cross-linking or
 branching agent.
 Suitable separation aids include those of class (d), amphoteric polymeric
 materials. These are generally produced from a monomer blend, normally of
 water-soluble ethylenically unsaturated monomer, comprising 5 to 50 wt %
 cationic monomer and 5 to 50 wt % anionic monomer, optionally with
 non-ionic monomer. Either cationic monomer or anionic monomer may be in
 excess. Suitable cationic, anionic and non-ionic monomers are as discussed
 above.
 All of the above polymeric materials of types (b), (c) and (d) are
 preferably water-soluble, although they can be emulsions provided the
 emulsion polymer gives suitable flotation and separation performance.
 In this specification intrinsic viscosity is measured by suspended level
 viscometer in buffered pH 7 1M NaCl at 25.degree. C.
 Blends of all of the above materials can be used as components of the
 separation aid. Specific amounts and combinations of material are chosen
 for the particular suspension to be treated by flotation so as to obtain
 the best selectivity results.
 In some systems it is desirable to use materials from class (a) as the sole
 component of the separation aid, or to use materials from classes (b), (c)
 and (d) or combinations thereof as the separation aid. In particular, use
 of components (b) and/or (c) is preferred.
 It is often preferred however to use a combination of components from class
 (a) with components from any or all of classes (b), (c) and (d), in
 particular from classes (b) and/or (c). In such systems it is particularly
 unexpected that selectivity is obtained rather than non-selective
 agglomeration and flotation of all solid materials, since in these systems
 a collector type material, material (a), is being used in combination with
 a flocculant or coagulant type material.
 Materials (a) are often used in amounts of from 50 to 150 ppm based on
 volume of filtrate, preferably 80 to 120 ppm, for instance around 100 ppm.
 Materials (b), (c) and (d) are often used in amounts of from 0.1 to 15 ppm,
 often below 10 ppm, preferably 0.3 to 7 ppm, more preferably 0.5 to 5 ppm,
 based on volume of filtrate.
 In the invention aqueous suspensions containing significant amounts of
 cellulosic fibre are treated by subjecting them to step (A) and then step
 (B). It is possible to treat such suspensions using step (B) only,
 although use of step (A) before step (B) is preferred. Aqueous suspensions
 which do not contain significant amounts of cellulosic fibre can be
 treated using steps (B) and (C) only.
 The Reject and Accept fractions are separated in step C, but if convenient
 this separation can be regarded as part of step B. This separation can be
 done in any standard manner, for instance by skimming the upper foam which
 forms the Reject fraction. This material comprises filler and other
 hydrophobic materials and can then be disposed of, for instance by burning
 or to landfill.
 The resultant Accept fraction then contains a high proportion of cellulosic
 fines as suspended solids. The entire Accept fraction may be recycled to
 the relevant process, for instance a de-inking process. Preferably however
 the cellulosic fines are separated from the Accept fraction, for instance
 in a subsequent step. This may be done by any suitable solid-liquid
 separation method, for instance filtration, sedimentation or centrifuging.
 The preferred method is flotation, usually dissolved air flotation, if
 necessary in the presence of coagulating or flocculating agents.
 The cellulosic fines collected are often very clean and can be reused as
 desired. For instance, if the aqueous suspension is a waste water produced
 by a de-inking process, the fines can be incorporated into the de-inked
 pulp and sent to the paper making process or recycled directly to a paper
 making process. Alternatively, in particular for less clean batches of
 cellulosic fines, they may be recycled to the de-inking process.
 In this specification, cellulosic fines are insoluble cellulosic materials
 derived from cellulosic fibres, for instance extremely short cellulosic
 fibres having a length usually below 200 .mu.m or 100 .mu.m and preferably
 below about 70 or 80 .mu.m but generally above 10 or 20 .mu.m.
 The method of the invention allows highly efficient separation of
 cellulosic fines from hydrophobic and any colloidal materials. In
 particular, in step (B) the Reject fraction preferably contains at least
 55%, more preferably at least 75% or 80%, of the hydrophobic and any
 colloidal materials, in particular filler, originally present in the
 filtrate. It is even possible in the method of the invention to recover in
 step (B) 90 or 95% and even up to 98% of the hydrophobic and any colloidal
 content of the filtrate.
 The invention also allows highly selective separation, in that the Reject
 fraction has a high content of hydrophobic materials such as filler and a
 low content of cellulosic fines. In particular, the solids content of the
 Reject fraction is made up of at least 60%, preferably at least 75%,
 filler.
 The highly selective nature of the method of the invention also allows
 efficient recovery of cellulosic fines. It is possible to carry out
 methods such that the Accept fraction contains at least 60%, preferably at
 least 70% of the cellulosic fines originally present in the filtrate.
 Preferably also the selectivity for cellulosic fines in step (B) is high.
 In particular the solids content of the Accept fraction is at least 60%,
 often at least 70 or 80% and even up to 90% or more, cellulosic fines,
 with a corresponding low content of hydrophobics and colloidal materials.
 The method of the invention may be used for the treatment of any aqueous
 suspension containing cellulosic fibres, fines and hydrophobic materials.
 Suitable suspensions are some paper-making white waters and mill wastes.
 It is preferred however that the aqueous suspension is a waste water from a
 de-inking process. In such a de-inking process waste paper is provided and
 pulped with water and de-inking chemicals. It is then subjected to a
 de-inking stage, which comprises flotation de-inking and/or washing
 de-inking. Flotation de-inking produces a foam at the surface of the pulp
 which is skimmed off and sent for disposal. The aqueous suspension used in
 the method of the invention may be produced by adding water to and
 reconstituting such a foam. Preferably however the aqueous suspension is
 or is derived from spent wash water. This wash water contains some
 cellulosic fibres and significant amounts of cellulosic fines.
 In a de-inking process recovered cellulosic fibres and fines can be
 recycled to the de-inked pulp if they are especially clean or to the
 de-inking process.
 The following is an example of the invention.
 EXAMPLE
 The tests described below are carried out on a laboratory prepared
 simulated back-water from the washing stage of a de-inking process.
 The following raw materials were used:
 60% old newsprint (ONP) consisting of minimum 7 different offset printed
 newspapers,
 40% old magazines (OMG) consisting of 20% supercalendered (SC) paper and
 20% light weight coated (LWC) paper
 Age of the paper:
 newsprint 1-3 months SC and LWC max 12 months
 Storage:
 In black plastic bags at room temperature.
 Back-water was produced from a simulated de-inking loop. The water
 simulating the de-inking loop was prepared according to the scheme shown
 in FIG. 1.
 The standard pulping and de-inking condition included in the water
 preparation process were as follows:
 Raw material:
 pieces of size about 2.times.2 cm.sup.2, cut in a guillotine.

Pulping Alternative 1. Lamort laboratory
 pulper, capacity 1.5 kg
 Alternative 2. Hobart laboratory
 pulper, capcity 100 g
 consistency 10%
 temperature 45-50.degree. C.
 time 20 min
 (Hobart 5 min
 speed 1 + 20 min
 speed 2).
 Chemicals
 NaOH 1.0% (active on
 dry fibre)
 Na-silicate 2.0% (active on
 dry fibre)
 collector SERFAX DB 0.8% to pulper +
 2.0% to the 2nd
 cycle and 0.2% to
 the 3rd cycle.
 H.sub.2 O.sub.2 1.0% (active on
 dry fibre)
 CaCl.sub.2 390 mg/l (ca
 20 .degree. dH, 141 mg/l
 Ca.sup.2+)
 pH in pulper 10.0-10.5
 Holding time consistency 5%
 temperature 45-50.degree. C.
 time 30 min
 Flotation Alternative 1. Swemac laboratory cell
 De-Inking Alternative 2. Voith laboratory cell
 Alternative 3. Degussa laboratory cell
 pH at start 8.5-9.0
 consistency 1%
 temperature 45-50.degree. C.
 time 15 min
 Hot tap water corrected to 390 mg/l CaCl.sub.2 (141 mg Ca.sup.2+) was used
 for dilution of the pulp to 5% before holding and further to 1% before
 flotation in the first cycle.
 The dewatering apparatus was a type of enlarged "Draining Jar", the volume
 of which was about 15 liters. The bottom wire was a Gemini plus 8 shaft
 duplex wire of polyester, supplied by UNAFORM LTD. The mesh size for
 efficient particle removal corresponds to the 200 mesh (76 .mu.m) standard
 wire in the Britt Dynamic Drainage Jar. The stirring speed was about 100
 rpm and at the end of the dewatering the pulp was pressed to make the
 amount of water removed match the amount of water needed in the following
 loop.
 A typical composition of back-water produced in this way was:

Solids 2.5 g/l
 Fibre 0.15 g/l
 Filler 50% of defibred water
 Cellulosic Fines 50% of defibred water
 Step (A), screening, was carried out by passing the back-water through a
 laboratory TRENNER device, available from Meri Anlagentechnik, Germany, to
 produce a filtrate.
 Step (B) : the filtrate produced in step (A) was subjected to dispersed air
 flotation in a Denver laboratory flotation cell or a Voith laboratory
 flotation cell.
 The flotation resulted in formation of an upper Reject fraction which was
 removed and a lower Accept fraction which was retained.
 Step (C): the Accept fraction was subjected to dissolved air flotation
 using a laboratory dissolved air flotation cell available from Meri
 Anlagentechnik, Germany.
 In step (B), various materials were used as flotation aid. The filtrate
 entering stage (B) had a calcium ion content of approximately 80 ppm. The
 flotation aids used were as follows:
 E: calcium carbonate.
 F: sodium salt of C.sub.16-18 fatty acid.
 G: polydiallyldimethyl ammonium chloride, intrinsic viscosity below 1 dl/g.
 H: copolymer of 75 wt % acrylamide and 25 wt % dimethylaminoethyl acrylate
 quaternary ammonium salt, intrinsic viscosity 7 dl/g.
 J: copolymer of 30 wt % acrylamide and 70 wt % sodium acrylate, intrinsic
 viscosity 8 dl/g.
 Various combinations of these materials were used, as follows:
 Run 1: 90 ppm E, 100 ppm F, 1 ppm J
 Run 2: 90 ppm E, 100 ppm F, 5 ppm G
 Run 3: 90 ppm E, 5 ppm G
 Run 4: 90 ppm E, 100 ppm F, 0.5 ppm H
 Run 5: 90 ppm E, 100 ppm F, 0.5 ppm J
 Run 6: 90 ppm E, 100 ppm F
 Run 7: 90 ppm E, 100 ppm F, 1 ppm H.
 Results are given below in Table 1 for step (B) Filler selectivity is the
 weight percentage of filler in the solids content of the Reject fraction.
 Filler yield is the weight percentage of total filler in the filtrate
 which is in the Reject fraction.
 TABLE 1
 Run No. % Filler Selectivity % Filler Yield
 1 81.9 69.3
 2 82.4 59.2
 3 77.4 67.3
 4 82.2 71.4
 5 72.9 96.1
 6 73.4 79.4
 7 72.8 100
 In the processes carried out in the examples the recovery of fibre in step
 (A) was generally at least 82% (ie at least 82 wt % of the total fibre in
 the back-water was recovered).
 In step (C) over 70% of the cellulosic fines present in the Accept fraction
 were recovered to give a cellulosic fraction of which around 97% by weight
 of solids was made up of cellulosic fines.
 The above results demonstrate the surprisingly efficient and selective
 separation which can be achieved by the use of the method of the
 invention, in particular incorporating selective dispersed air flotation
 step (B) The amounts of materials to be dumped are greatly reduced, thus
 reducing cost, and loss of valuable cellulosic materials is alleviated.