WO2000078874A1

WO2000078874A1 - Pigment materials and their preparation and use

Info

Publication number: WO2000078874A1
Application number: PCT/US2000/016610
Authority: WO
Inventors: Jose Maria Rodriguez; Paul Rey
Original assignee: Imerys Pigments Inc
Current assignee: Imerys Pigments Inc
Priority date: 1999-06-18
Filing date: 2000-06-16
Publication date: 2000-12-28
Anticipated expiration: 2001-12-18
Also published as: AU5875700A

Abstract

A method of preparing and using a composite pigment material which incorporates a fine particulate material, which method comprises the steps of: (a) treating an aqueous medium containing dispersed particles of a fine particulate material by chemically reacting therein a first reactant and a second reactant to precipitate therein crystals of a white insoluble pigment compound thereby forming in the aqueous medium a composite pigment material comprising a composite matrix of precipitated crystals of the white pigment compound and particles of the fine particulate material dispersed and bonded within the matrix wherein the first and second reactants are such that they react together without producing a substantial amount of non-crystalline by-product; and (b) adding the composite material to a composition for forming or coating a fibrous sheet material. The method may thereafter include (c) forming or coating a fibrous sheet material using the composition incorporating the composite material. The said aqueous medium employed in step (a) may comprise an aqueous suspension or slurry.

Description

TITLE OF THE INVENTION

Pigment materials and their preparation and use

BACKGROUND OF THE INVENTION The present invention relates to pigment materials and their preparation and use. In particular, it relates to new pigment material comprising a composite containing a fine particulate material such as titanium dioxide and to the preparation of such material and its use in operations to make or coat paper and the like sheet materials .

Titanium dioxide, herein "Ti0₂", is an example of a fine particulate material which, unlike materials which are employed as bulk fillers, is employed for a specific function in paper making and paper coating operations. It offers excellent pigment opacity and brightness (which together result in excellent light scattering) and is therefore used to extend these properties in a paper making or coating composition. Ti0₂ is a very expensive material and is consequently used only sparingly in paper making or coating compositions, eg usually forming less than 5% by weight of the solids present in the composition. Because of its cost, Ti0₂ needs to be employed as efficiently as possible.

The optimum particle size for Ti0₂ pigment particles to give the best light scattering properties has been determined in the prior art to be about 0.2μm to 0.3μm. Ti0₂ pigment material is normally supplied commercially in a form wherein the particles have this optimum size. The Ti0₂ product may be supplied dry, in which case it requires dispersion in liquid media to wet and to deagglomerate or disperse the particles. Alternatively, Ti0₂ may be supplied commercially in a pre-dispersed slurry form which may incorporate a relatively large amount of anionic stabiliser.

Since the Ti0₂ particles employed in paper making are very fine and are usually dispersed with relatively high levels of dispersant to make slurries containing the material pumpable and to maximise the spacing between particles to give optimal light scatter, it is difficult to retain such particles when they are employed in a furnish or like composition in a paper or like sheet forming operation.

In order to improve Ti0₂ retention in such operations, in order to minimise Ti0₂ losses, various chemical retention aids are employed in the prior art. In general, such aids are expensive chemicals, eg water soluble polymers, and the extent of use of such chemicals employed for the conventional retention of Ti0₂ is considered to be very costly. Where the Ti0₂ is supplied with anionic stabiliser large amounts of cationic chemicals (which may also serve as or be delivered together with retention aids) may be required to reduce the anionic loading.

Ti0₂ particles (when used in a pigment-containing composition) have a tendency to agglomerate especially at higher loading levels, this effect being known was 'crowding' . Use of retention aid chemicals can increase crowding which has been demonstrated in the prior art to have an adverse effect on the light scattering efficiency of the particles. This can result in degradation of the expected optical properties of commercially available Ti0₂ material when used as a particulate pigment material in paper.

Furthermore, such added chemicals when used in substantial quantities to improve Ti0₂ retention, have an adverse effect on the formation of the paper or other sheet being produced and can result in sheets of less than ideal quality in which the constituents of the sheet are not uniformly distributed.

Attempts have been reported in the prior art to counteract the effects of crowding of fine particulate material, especially Ti0₂, by forming composites of the particles of the material to space the particles from one another. These attempts have been aimed primarily at improving the optical properties of the compositions to which the composite material is added.

One method of the prior art which involves titanium dioxide Ti0₂, is described in US Patent No 2,170,800 wherein a mixture of Ti0₂ and CaS0₄ is reacted with a solution of Na₂C0₃ to convert CaS0₄ to CaC0₃ and precipitate a composite pigment comprising Ti0₂ and CaC0₃. Another method for producing a Ti0₂- CaC0₃ composite pigment is described in US Patent No 3,528,838 in which a solution of Na₂C0₃ and a solution of CaCl₂ are employed. Pigmentary Ti0₂ is dispersed in one of these solutions and the two solutions are mixed to precipitate a Ti0₂-CaC0₃ composite pigment. Still another coalesced composite pigment is described in US Patent No 3,832,206 wherein pigmentary Ti0₂ is dispersed in a solution of Na₂C0₃. A slurry of Ca(OH)₂ is added to the Na₂CO₃-Ti0₂ mixture in order to react with the Na₂C0₃ and precipitate CaC0₃. The CaC0₃ formed in the presence of the Ti0₂ pigment particles results in a coalesced composite pigment of Ti0₂-CaC0₃. In each of these three prior art processes, the composite pigment product must, before it can be used in paper making, be separated from the aqueous phase in which it is formed and washed free of the by-product resulting from the chemical reaction involved in producing CaC0₃. This by-product is Na₂S0₄ in case of US Patent No

2,170,800, NaCl in case of US Patent No 3,528,838 and NaOH in case of US Patent No 3,832,206. Removing the by-product in each case is time consuming and costly. In US Patent No 4,028,173, Olson describes the use of a physical mixture of CaC0₃ and Ti0₂ in the manufacture of papers .

Another avenue of producing carbonate agglomerates consists of utilizing organic chemicals or silicates to bind the aggregates. The following prior published patents disclose the use of such binders. In US Patent No 4,072,537, to F L Kurrle, a composite silicate pigment is prepared by a precipitation reaction employing an aqueous suspension of clay particles wherein spherical hydrous metal silicates particles are precipitated on the planar surface of the clay. In US Patent No 4,816,074 by Ravthatha et al, a process is described in which a structured aggregated kaolin pigment is prepared by mixing substantially dry kaolin in particulate form with an aqueous alkali metal silicate to deposit on the surface of the kaolin particles a substantially molecular level of said silicate without formation of silica gel, drying the treated kaolin without calcination and exposing it to an acidic gas. The product is useful as a pigment in the coating or filling of paper. In a further aspect of the invention, it is preferable to intermix with the kaolin feed, small quantities of an aggregation agent. Such agent is selected from one or more members of the group consisting of the alkaline earth metal carbonate, chlorides or hydroxides or lithium carbonate.

Polyacrylate alkali metal salts are known as dispersants for clay, eg kaolin. Sodium polyacrylate is a common dispersant, frequently used in the refining/working up the crude material. However, US Patent No 5,082,887 describes a pigment composition for the coating and filling of paper comprising a dispersion of an aqueous phase, a pigment or mixture of pigments such as kaolin, titanium dioxide and calcium carbonate, gypsum, mica and a dispersing agent comprised of carboxyl-containing polymers, eg an acrylic polymer, at least 60% of which is converted to the salt form with a polyvalent cation, calcium, and if desired the balance may be converted with a monovalent cation such as sodium, ammonium and quaternary amine cations. Gaseous carbon dioxide is added to the slurry, and reacts, with the remaining calcium ion to precipitate calcium carbonate onto the polymeric carboxyl calcium salt. The resultant composite aggregates are recovered and dried, as product .

The problem of maintaining a reasonable retention of Ti0₂ particles in a paper making operation, whilst at the same time avoiding agglomeration or crowding and an adverse effect on formation caused by the addition of chemicals to facilitate retention, has not satisfactorily been solved in the prior art. One purpose of the present invention is to provide a novel solution to this problem. Similar problems arise with the retention of other fine particulate materials, and it is another purpose of the present invention to provide a novel solution to the retention of such materials.

It is a further purpose of this invention to produce a novel pigment material which is useful in paper coating.

In the light of the foregoing, it may also be regarded as an object of the present invention, to provide a structural composite pigment, such as a composite titanium dioxide/calcium carbonate, kaolin/calcium carbonate, alumina/calcium carbonate, silica/calcium carbonate, mica/calcium carbonate, gypsum/calcium carbonate which possesses improved light scattering characteristics, and hence is useful as an opacifier and light scattering filler for paper and paper products, and which may similarly be used in other paper manufacturing applications, including in the coating of the same.

It is yet a further object of the present invention, to provide a method for producing structured composite mineral pigments as aforementioned, which consists of a minimal number of simply conducted steps, which utilize relatively simple and inexpensive apparatus.

It is a further object of the present invention, to provide a method for producing a pigment with good retention properties in the paper web of the expensive component of the aggregate pigment. Furthermore, the method provides a pigment without subjecting to any purification or calcination step. It is yet a further object of the present invention, to provide a pigment with a cationic zeta potential which decreases charge demand as compared to anionic or anionic dispersed pigments of the prior art .

SUMMARY OF THE INVENTION

The above purposes and objects and others which will become apparent from this specification are met by the present invention. The present invention is concerned with a method for the preparation and use of structured composite pigments, which can, amongst other things, improve the retention of fine particulate material such as Ti02 in paper making compositions without seriously affecting optical or other properties, in some cases beneficially improving such properties also. The resulting composite pigments can also be used beneficially in paper coating.

According to the present invention in a first aspect there is provided a method of preparing and using a composite pigment material which incorporates a fine particulate material, which comprises (a) treating an aqueous medium containing dispersed particles of a fine particulate material by chemically reacting therein a first reactant and a second reactant to precipitate therein crystals of a white insoluble pigment compound thereby forming in the aqueous medium a composite matrix of precipitated crystals of the white pigment compound and particles of the fine particulate material dispersed and bonded within the matrix wherein the first and second reactants are such that they react together without producing a substantial amount of non-crystalline byproduct; and (b) adding the composite material to a composition for forming or coating a fibrous sheet material. The method may thereafter include (c) forming or coating a fibrous sheet material using the composition incorporating the composite material. The said aqueous medium employed in step (a) may comprise an aqueous suspension or slurry. The fine particulate material and the first and second reactants employed to produce the precipitation reaction may be obtained from separate stocks and may be added together prior to step (a) to produce the composite pigment material. Either or both of these materials may be in dry or wet (eg slurry) form when they are added together. At least part of the fine particulate material may comprise fresh particles. By ^Λfresh' particles of fine particulate material is meant particles which have not previously been used in a sheet forming or coating or other operation.

The fibrous sheet material formed or coated in step (c) may comprise paper, paper board, card, cardboard, laminated paper and the like herein collectively called paper' , wherein the fibrous sheet material comprises organic, eg cellulosic fibres, and in many cases also inorganic filler comprising a particulate pigment material.

By "fine particulate material" is meant a particulate material wherein the particles have a size distribution such that at least 90% by weight have an esd (equivalent spherical diameter as measured in a known way by sedimentation) of less than 2μm. At least 50 per cent by weight may have an esd less than lμm. The mean particle size of the particles of the fine particulate material may for example be in the range 0. lμm to 1.5μm especially 0. lμm to 0.7μm. The fine particulate material will generally be one which is more expensive than bulk filler materials (eg conventional kaolin and/or calcium carbonate) and one which is used in paper for a specific function (other than bulk filling) and generally has a poor natural retention in paper making .

The present invention is especially beneficial where the fine particulate material comprises Ti0₂, although the particulate material may alternatively, or in addition, be selected from calcined kaolin, fine silica, eg so called fumed silica, sodium silicate, aluminium silicate, sodium aluminium silicate, talc, calcium sulfate, alumina, mica and so called plastic pigment materials, eg produced from melamine formaldehyde and mixtures of these materials. It should be noted that calcined material may contain aggregates of fine particles fused or sintered together but the fine particles which make up such aggregates may have the required particle size distribution properties if measured individually.

The said white insoluble pigment compound which is precipitated in the method according to the first aspect of the present invention may comprise a water insoluble salt, ie it may be formed from or regarded as formed from an acid and a base. It may comprise a simple salt formed from a single anion and a single cation. It may conveniently comprise an alkaline earth metal carbonate, eg calcium carbonate, produced by reacting carbon dioxide with a suitable hydroxide, eg calcium hydroxide. Reactants (eg providing acidic and basic species) to produce the white insoluble pigment compound may be added together or separately to the aqueous medium.

DESCRIPTION OF THE INVENTION

The composite pigment material according to the present invention comprises a fine particulate material dispersed and bonded within a matrix formed of crystals of a precipitated white insoluble inorganic compound, eg calcium carbonate. The parent application, US Serial No 08/957,280 describes such a composite material but in that case a substantial amount of fibers are also added to the aqueous suspension to be contained within the composite matrix. The production of material according to the present invention need not include deliberate addition of fibers, although in one embodiment of the invention the aqueous medium in which the precipitate crystals are formed may contain fibers already present, eg the aqueous medium may comprise white water from a paper making operation.

By the method of the invention, the particles of the fine particulate material, if suitably dispersed when the precipitation reaction is carried out, unexpectedly and beneficially remain dispersed in the solid composite matrix formed when the bonding crystals of the white pigment compound are precipitated and become cemented to the bonding crystals.

Where the precipitate compound formed is a basic compound, eg calcium carbonate, it can advantageously be effective in neutralizing any anionic charges present in a paper making or coating composition, eg due to the addition of anionic stabilisers, thereby allowing the amount of any added cationic chemicals to be reduced.

Particulate materials to be used as fillers in paper making usually carry a surface charge when dispersed in water. The surface charge is dependent on pH and chemical species at the crystal surface. The zeta potential is a convenient known way to measure the charge at the plane of shear near the crystal surface. Precipitated calcium carbonate is one of the few fillers having a positive zeta potential. It is well known, eg from "Retention of Fines and Fillers During Papermaking, Precipitated Calcium Carbonate", Chapter 14 by J M Gess (TAPPI Press, Atlanta, GA) that the charge on the surface of the filler particle has a dramatic influence on first pass retention. Positively charged fillers are retained better than negatively charged fillers. By forming a composite material comprising particles of fine particulate material such as Ti0₂, and/or one of the other materials referred to earlier, dispersed and bonded within a solid matrix of crystals of white pigment compound, a new pigment structure is provided which can show various unexpected benefits when used in paper making or paper coating operations as described hereinafter. According to the present invention in a second aspect there is provided a composite pigment material comprising a composite matrix of precipitated crystals of white pigment compound and particles of fine particulate material as defined hereinbefore, dispersed and bonded within the matrix which composite material is the product of step (a) of the method according to the first aspect.

Particles of the composite pigment material according to the second aspect may be employed in a known manner as a pigment filler ingredient in papermaking or as a pigment ingredient in paper coating. Such particles may be the sole filler or pigment source or they may optionally be blended with conventional or known fillers or pigment particles employed in such applications, eg selected from one or more of kaolin, calcined kaolin, calcium carbonate (derived from a natural or synthetic source or composited in a known manner with other materials, eg derived from waste streams), dolomite, talc, mica or untreated Ti0₂ or other fine particulate material (eg as used in the preparation of the composite pigment material) .

The composite pigment material according to the second aspect of the present invention may form from 1% to 100% by weight of the filler or pigment particles employed in such applications, eg in the making of a furnish employed in paper making or a coating composition employed in paper coating, or in specialist sheet coating operations, eg decorative laminate formation, although it may be blended with other, eg conventional filler or pigment, materials eg wherein it may form up to 50% by weight, in some cases up to 80% by weight of the blend. The filler (s) or pigment (s) employed in such applications will depend upon the use of the filler or pigment. Fillers in papermaking may form up to 40 per cent by weight (on a dry solids weight basis) of the paper making composition or furnish. The pigment (s) employed in coating compositions usually forms up to 80% by weight in some cases as high as about 95% (on a dry solids basis) by weight of the composition. The composite pigment material according to the second aspect of the present invention can allow fine particulate materials such as Ti0₂ and the other materials referred to earlier to be much more efficiently retained in a paper making operation.

For example, in contrast to the low retention levels obtained in the prior art for untreated Ti0₂ (without application of a retention aid) or for Ti0₂ treated with only a small amount of retention aid (eg retention levels of about 30% or less, even 20% or less, as illustrated hereinafter) , the retention level of Ti0₂ for a single pass in a paper sheet making operation by production and use of the composite material according to the second aspect of the present invention can advantageously be much greater as illustrated hereinafter (depending on the composition of the composite pigment material and the amount of the material loaded into the paper composition) . This allows a reduction in the quantity and cost of retention aid chemical (s) employed to retain fine particulate material such as Ti0₂, although some retention aid chemical (s) may optionally be added, (eg in the usual manner to the dilute pulp furnish stock from which the paper sheets are to be produced) .

Beneficially, reducing the amount of retention aid chemicals can also improve the burst strength (also known as burst index) of the sheet produced and the paper sheet formation, ie the overall quality of the paper sheet produced by providing more uniform distribution of the constituents of the sheet. In addition, the sheet so formed may beneficially have a stiffness or 'crackle' which is greater than that for a comparable sheet made from fine particulate material such as Ti0₂ as in the prior art. This allows, for example, paper of the same stiffness as comparable prior art sheets to be produced lighter in weight and (for sheets of the same weight as prior art sheets) more cheaply. Furthermore, because the fine particulate material is better retained in the paper making process, less fine particulate material is lost and it is not necessary to compensate for the usual loss which is expected to occur by addition of excessive quantities of the material, which, as in the case of Ti0₂, can be very expensive.

A further benefit obtained by preparation and use of composite pigment material according to the second aspect of the present invention in a paper making or coating operation is that the undesirable crowding (normally obtained as in the prior art) of particles of the fine particulate material caused or exacerbated by use of added chemicals is reduced and this allows the adverse effect on scattering of light from the particles caused by such crowding to be reduced. Since the crystals of the precipitated pigment compound may themselves be fine and highly scattering their presence contributes beneficially to the optical properties of the composite pigment material and may allow further reduction of the amount of fine particulate material, where pigment material such as Ti0₂, which may be employed as co pared with prior art compositions aimed at giving similar properties.

Thus, generally the present invention allows savings in the amount and cost of fine particulate material employed in paper making and paper coating as well as in the use of associated expensive chemicals as well as providing improvements in various properties of the sheet product.

According to the present invention in a third aspect there is provided a paper or like sheet material incorporating filler or coating pigment particles which comprise particles of the composite pigment material according to the second aspect defined earlier. Surprisingly and beneficially the formation and use of a composite pigment material in accordance with the invention allows certain optical properties, particularly scattering, of a paper sheet made or coated with a composition containing the composite pigment material according to the second aspect to be enhanced beyond the results obtained with simple admixtures of the two materials making up the composite, ie fine particulate material and white insoluble pigment compound (eg precipitated calcium carbonate) . The present invention therefore offers an inexpensive way of extending the effectiveness of the two materials making up the composite.

In the method according to the first aspect of the present invention if additional particulate solids are present, other than consumable reagent (s) employed to form the precipitate compound, in the aqueous medium in which the precipitation reaction is carried out the amount of such solids which may be present will depend on the kind of reactor employed which, in turn, will depend upon the process conditions required to be operated. These are discussed further below. Where step (a) of the method according to the first aspect is carried out in a conventional stirred batch reactor, eg for precipitating calcium carbonate, the said solids may form from 0% to 20% by weight of the aqueous suspension to be treated to give precipitation formation therein. Desirably, such solids constitute from 0% to 10% by weight, especially 0% to 7% by weight, of the treated suspension in the use of such a reactor.

In other reactors, eg to operate a continuous or semi-continuous process, the solids content may be higher, eg up to levels of 40% or even 50%.

In the composite pigment material according to the second aspect of the present invention the weight ratio of the said fine particulate material to precipitated white pigment compound present may be in the range 1:100 to 1:1, especially 1:20 to 1:2. The white pigment compound present in the composite pigment material (including any particles present in addition to the fine particulate material) may constitute at least 50% by weight, especially from 70% to 96% by weight of the material (on a dry solids basis) . The composite pigment material according to the second aspect of the present invention may comprise particles whose size will depend upon the size of the constituents used to form the particles. If desired, it is possible to treat the composite material particles produced by comminution, eg by wet stirring, milling or grinding, and optionally particle size classification as described hereinafter.

The individual precipitate crystals which make up the matrix of the composite pigment material will generally have a size comparable with such crystals formed in the prior art (when not part of a composite structure) . Generally, such crystals will be sub- micron size, eg having sizes in the range 0. lμm to lμm. Where the fine particulate material comprises Ti0₂, the Ti0₂ particles employed in the method according to the first aspect of the present invention may be of the rutile or anatase form. We prefer the rutile form. Commercially available Ti0₂ pigment material may be employed. However, because the problem of retention of Ti0₂ in paper making is considerably reduced by forming the composite pigment material according to the second aspect, it is possible although not essential in forming the composite material to use at least a portion of Ti0₂ particles which are finer than those conventionally used, eg having a size of about 0. lμm or even less. In the method according to the first aspect of the present invention, the fresh particles of the fine particulate material may be employed in an aqueous suspension to be treated to form a precipitate therein may be added in dry form to a reactor vessel containing water or in predispersed slurry form to the reactor vessel. In either case it is preferred that the mixed suspension so formed containing the fine particulate material is mechanically agitated, eg by vigorous stirring, preferably both before and during the precipitation reaction, to ensure that the particles of the fine particulate material are maintained in a dispersed state in the suspension whilst the crystal precipitation proceeds. If desired, a known dispersing agent, eg a polyelectrolyte such as one of the agents well known for the dispersion of particulate materials in an aqueous suspension, eg sodium polyacrylate, may also be present, preferably in a small amount, eg less than 0.5 per cent by weight .

The fine particulate material may be added with vigorous mixing to an aqueous lime suspension and the required precipitate may be produced by carbonating the mixed suspension so formed. It is not necessary to incorporate the fine particulate material, or all of that material, prior to carbonating. Some or all of the fine particulate material may be added after some carbonation, ie to produce some of the precipitate crystals. The fine particulate material may be added in more than one dose. Each addition may optionally be accompanied by an addition of lime and followed by a carbonation step. If the water employed to provide the aqueous medium (eg to provide the aqueous suspension employed in step (a) ) in the method according to the first aspect of the present invention contains soluble or insoluble species which will affect the process of precipitation of the white pigment compound it may be desirable to carry out a preliminary precipitation step in the water prior to introduction of the said fine particulate material which will form the composite pigment material together with the white pigment compound. For example, the employed water may be so-called white water from a paper making operation. The preliminary precipitation will entrain the species present in the white water in the manner described in EP 604,095. Up to 90% by weight, eg from 1% to 50%, or 10% to 50%, by weight of the white pigment compound to be formed, may be precipitated in the preliminary precipitation step(s) prior to addition of the fine particulate material. The resulting final product formed will consist of a mixture of particles of the white pigment compound, the fine particulate material and others containing only the precipitate and species originally present in the water used. The product may also contain aggregates of two or more of the various kinds of precipitate particles possible. As noted above, the precipitated white pigment compound may comprise an alkaline earth metal carbonate, especially calcium carbonate, which may conveniently be produced in a well known way by addition of a carbon dioxide-containing gas to an aqueous medium containing ions of the required alkaline earth metal (as well as the fine particles and the fibres when required) . Such production allows the required white pigment to be produced without substantial undesirable production of non- crystalline by-products as in the prior art. The aqueous medium may in this case comprise a medium containing a hydroxide of the required alkaline earth metal. Such a hydroxide may comprise a hydroxide of one or more of calcium, magnesium and barium.

Where the aqueous medium comprises a hydroxide, the hydroxide may be separately prepared and added to the aqueous medium or alternatively may be prepared in situ in the aqueous medium, in each case, for example, by slaking an alkaline earth metal oxide (eg calcium oxide or quicklime, when calcium hydroxide is desired) . Where an alkaline earth metal oxide, eg calcium oxide is to be slaked in an aqueous medium either separately or in situ in the reactor vessel, the oxide may previously be screened, eg using a 125μm screen, or a 53μm screen, to separate large particles so that these are not present in the slaking process.

Where the aqueous medium comprises calcium hydroxide produced by slaking in an aqueous medium, the calcium hydroxide will itself be in the form of a suspension in the aqueous medium, ie so called

'slaked lime' . Calcium ions will sparingly enter solution and will be continuously consumed and replaced as the precipitation reaction proceeds, eg by addition of carbon dioxide. Preferably, a suspension formed in this way contains between from 5% to 50% by weight of the hydroxide particles. Desirably, there is between 0.5 mole and 3.0 moles, especially from 1.0 to 2.0 moles, of the alkaline earth metal hydroxide in the aqueous medium. For slaking of lime, the temperature of the aqueous medium may be from 0°C to 80 °C. The temperature will rise during the slaking process. If the aqueous medium is not at the appropriate temperature after slaking, it may be heated or cooled to achieve the desired temperature before further use. Efficient dispersion and dissolution of the alkaline earth metal hydroxide may also be assisted by agitation, eg by stirring of the aqueous suspension, to provide uniform distribution of the particulate solid material comprising the hydroxide. This agitation may be combined with that applied to the particles of the fine particulate material and the fibres.

The source of the alkaline earth metal ions, eg calcium oxide or calcium hydroxide (where the precipitated compound to be formed is calcium carbonate) , may be added to water employed to form the aqueous medium before, during or after the addition of the fine particulate material. Where a batch reactor vessel is employed, we prefer to add the fine particulate material to the reactor vessel after adding a suspension of slaked lime to the vessel. Water may be added after any of the individual addition stages.

Where an alkaline earth metal carbonate is to be precipitated by addition of C0₂ to an aqueous suspension containing a source of alkaline earth metal ions (and in at least part of the process also fine particulate material) the carbonation reaction may be carried out in a conventional vessel as is well known to those skilled in the art of precipitated carbonate production. Where the aqueous medium comprises slaked lime, the slaked lime suspension may be prepared in the vessel in which the carbonation is to be carried out, or in a separate vessel prior to introduction to the carbonation vessel.

The addition of a carbon dioxide-containing gas to an aqueous medium containing slaked lime (and, in at least part of the process, also fine particulate material and fibres) may be continued until the pH of the aqueous medium has fallen, eg to a pH less than 9.0 preferably to a pH less than 7.5, eg by monitoring the pH until it falls and then becomes stable. This indicates that all of the alkaline earth metal ions have been consumed, eg by consumption of all calcium hydroxide present.

The reactor vessel in which the precipitation reaction is carried out to produce the composite material according to the second aspect of the present invention may take various forms depending on the process conditions required to be operated as described herein. Reactor vessels known in the prior art for the production of precipitated calcium carbonate from slaked lime and carbon dioxide gas may be employed. The reaction may be carried out as a batch, continuous or semi-continuous process as appropriate . In its simplest form, the reaction vessel may be a container in which the various ingredients to be present in an aqueous suspension during the reaction may be added and mixed together in the vessel and C0₂ gas may be bubbled into the mixture.

The reaction may be carried out as a continuous or semi-continuous process in a cascade of reactor vessels. In such an arrangement an aqueous suspension containing lime, and where required fine particulate material such as Ti0₂, may be delivered to the various vessels in sequence and carbon dioxide may be applied to each vessel whereby the required reaction takes place progressively along the sequence . Another form of reactor which may be employed for a continuous or semi-continuous reaction is one in which the ingredients of the aqueous suspension and the final reactant, eg carbon dioxide, are mixed together via one or more static mixers of a known kind, eg in a known in-line arrangement. Doses of individual ingredients, eg lime, fine particulate material, or carbon dioxide, as appropriate may be delivered via two or more mixers to give a sequence of ingredient additions whereby the reaction required takes place progressively in stages.

Preferably, during the reaction, or each stage of the reaction, especially where fine particulate material is present, mixing is applied. Gas such as C0₂ which is applied may be pressurised. The aqueous medium in which the alkaline earth metal ions are contained and reacted with precipitate-forming reagent, eg carbon dioxide, in the method according to the first aspect of the present invention may (in addition to fine particulate material in at least part of the process) also contain one or more chemicals of a kind known for use in precipitate production, eg it may comprise a buffer solution to give the product so-called acid tolerance or a frothing agent to provide efficient reaction between C0₂ bubbles and lime, or an additive, eg an organic additive, eg citric acid, to facilitate crystal nucleation and growth.

As noted earlier, the aqueous medium in which the composite pigment material is to be produced in the method according to the first aspect may, (in addition to added fresh particles of a fine particulate material in at least part of the process) also contain other fine suspended solid material which may be in addition to any suspension of consumable particles employed to provide a source of alkaline earth metal ions. For example, there may be present other fresh or alternatively previously used pigment particles and/or fibres. For example, fines collected from a waste stream from a paper making process or a waste paper treatment or de-inking process as described in copending published patent applications EP-A-604,095 and EP-A-737,774 may also be present prior to any precipitation step involving the fine particulate material and, as noted hereinbefore, may be entrained by precipitation before the fine particulate material is present or, alternatively whilst the particulate material is present. Such fines may comprise used particles present in the waste stream such as organic particles such as ink, latex or polymer particles and/or inorganic particles such as mineral filler or coating particles as used in paper making or paper coating and/or fine fibres. Alternatively, or in addition, a selection of particles from a minerals refining or separation process or residue from an incineration process, eg incineration of paper de-inking waste, may be present as described in PCT/GB96/00884. Where such other solid materials are included in the treated aqueous medium they may be present in an amount of up to 50% by weight, eg 1% to 10% by weight, based on the dry weight of composite material product to be formed. Where such other solids are present in the suspension to be treated, such solids become entrained, together with the freshly added fine particulate material, and bond to the precipitate crystals formed and thereby form a multi- media mixed aggregate solid system. The aggregate product so formed is useful in the papermaking and paper coating applications described below.

Where a carbon dioxide-containing gas is employed to provide a carbonation reaction in the method according to the first aspect, the carbon dioxide-containing gas may be substantially pure carbon dioxide eg as commercially supplied in a compressed gas cylinder or may be present in a mixture with other gases. The supplied carbon dioxide gas may be diluted with other inert gases, eg air, nitrogen, etc. The carbon dioxide may be present as a mixture of spent gases such as flue gases, eg obtained from a lime calcination plant in which quicklime is produced for conversion into slaked lime (for use in the process according to the first aspect) . The gas may be applied under pressure, eg in the manner described hereinbefore. The temperature of the aqueous medium when the precipitate-containing composite material is formed therein, eg when a carbon dioxide-containing gas is added thereto, is preferably in the range of from 1°C to 80°C, especially 20°C to 60°C, more preferably 30°C to 55°C. Such an aqueous medium may be derived from used water from a paper making plant which may have an appropriate elevated temperature when delivered for use in the method of the present invention.

It is known, eg as described in US-A-715, 832, that the reaction conditions employed to produce a precipitated calcium carbonate product can be selected to aim for a predominant precipitate crystal form, eg scalenohedral, aragonite or rhombohedral, which will give desired properties, eg brightness, from the crystals when used in paper. Such reaction conditions may be selected and applied in operation of the method according to the first aspect of the present invention. However, since the reaction medium in which the crystals will precipitate in the method of the invention is not a homogeneous one, the crystal form of calcium carbonate achieved in practice in producing the composite pigment material is unlikely to be near to 100% of a selected form. It is quite usual for one crystal form even when predominant to be mixed with other forms . Such mixed forms will however generally give suitable product properties because the optical properties of the product will not result from the precipitate crystals alone.

The addition of a carbon dioxide-containing gas to an aqueous lime-containing suspension (with or without fine particulate material, may be continued until the pH of the aqueous medium has fallen, eg to a pH less than 9.0 preferably to a pH of 7.5 or less, eg by monitoring the pH until it becomes stable. This indicates that all of the alkaline earth metal ions have been consumed, eg by consumption of all calcium hydroxide present. The composite pigment material according to the second aspect of the present invention when produced is preferably obtained in the form of an aqueous suspension which preferably has a viscosity of not more than 500mPa.s (as measured by a Brookfield Viscometer using a spindle speed of 100 rpm) and is preferably a pumpable and flowable slurry.

The aqueous suspension comprising the composite pigment material formed as a product in the method according to the first aspect of the present invention may be further processed for example by dewatering and/or by subjecting the composite- containing suspension to comminution, eg rapid stirring, milling or grinding by one of several known procedures . As described in EP-A-768,344 such a comminution step may be carried out by attrition grinding. In such grinding, the grinding medium employed in the comminution step may comprise one of the hard, inorganic materials well known in the grinding of particulate materials. For example, silica sand having a median particle diameter in the range from about 0.1mm to 4mm, eg 0.2mm to 2mm, is a preferred grinding medium. The grinding medium could alternatively be aluminium oxide, zirconium oxide, hard steel or a mixture of any of these materials. When the product comprising the composite pigment material is subjected to a comminution step the pH of the aqueous suspension containing the product being treated may rise, as described in EP 768,344A, eg to pH 11 or more, as free basic material, eg unconverted lime (where lime is used as the source of alkaline earth metal ions), entrapped in the crystalline structure comprising the composite material is released by the comminution. Such a pH level may be undesirable in the applications in which the material may be employed, as described hereinafter, because it is potentially harmful to machinery and to operators who have to process the suspension.

An additional step to reduce the pH of the aqueous suspension containing the composite pigment material may be applied after a comminution step. The additional step may be applied until the pH falls to a suitable level, eg pH 10 or below, eg pH 9 or below, preferably pH 7.5 or below. The additional step may comprise further treatment by carbonation of the precipitate-containing suspension. Alternatively, or in addition, a material known for use in reducing the pH of a mineral suspension may be added. Such a material may, for example, comprise a mild mineral acid such as phosphoric acid. The aqueous suspension containing composite pigment material product formed in the method according to the first aspect of the present invention may be treated so as to separate partially or fully the aqueous host medium from the composite material solids using one or more separation processes which may be known processes. For example, processes such as filtration, sedimentation, centrifugation or evaporation may be used. Filtration using a filter press is usually preferred. The separated aqueous medium, eg water, may, optionally with further purification or clarification by one or more chemical, biochemical or mechanical processes which may be known per se, may be recycled for reuse, eg in a paper mill, eg for use in diluting the paper making stock or for use as showers for washing machinery. The separated solids may be assessed for quality control by measurements taken on samples and subsequently delivered to a storage tank and thereafter supplied as necessary for use in a user application, eg as described hereinbefore. The solids-containing suspension may be re-diluted for use at the user plant.

It is not necessary for an aqueous suspension containing a composite pigment material according to the second aspect of the present invention to be dewatered prior to supply for use in a user application, eg for use in paper making in a paper mill. The aqueous suspension or slurry may be delivered to a storage tank or directly to the user plant without substantial dewatering. Where the composite pigment material is to be used as a filler in a paper making composition, the composite material may be supplied to the papermaking mill in one of various concentrations in water. The concentration may range from dilute suspension form to dry particulate solids. The composite pigment material after preparation in the method according to the first aspect of the present invention may or may not be treated as appropriate, eg by dewatering or not, so that it may be delivered to the user plant, eg paper making mill, in the required concentration. The extent of dilution or concentration of the form in which the composite pigment material product is added to the user composition, eg paper making furnish composition, does not critically affect the properties of the resulting product, eg paper sheet. It may, however, for economic and practical reasons be more suitable to supply the composite pigment material product in the form of a concentrated pumpable aqueous slurry. Where this product is supplied for use in a process at another location it may be preferable or desirable to dry the product prior to transport. Where the product is supplied to a nearby plant it is likely to be delivered in slurry form. Where this product has been concentrated or dried prior to delivery and subsequently re-dispersed in or diluted with clean water prior to re-use, the concentration and dilution steps do not materially affect the usefulness of the product.

In any event, where the composite pigment material product is to be used as a pigment filler material in paper making, this product may, as will be clearly evident to those familiar with the paper making art, be used in a well known manner. It may be blended in various proportions with conventional filler materials, eg precipitated or natural, eg ground, calcium carbonate, kaolin or other clay, calcined kaolin, talc, calcium sulfate etc, the ingredients and composition as well as the host cellulosic fibres being selected according to the quality of paper required to be produced. In general, these materials are likely to be in slurry form when they are mixed. The host cellulosic fibres may be any one or more of the forms of fibers employed in paper making. The fibers may be fresh or previously unused fibers, and/or recycled fibers obtained from a used source, eg broke.

The paper maker will normally select the concentration of the composite pigment material (produced in accordance with the present invention) used in aqueous suspension form and the delivery rate of the suspension at the point of addition to the paper making composition, eg furnish. As noted above, this may require re-dilution of a suspension which has been delivered to the paper mill in concentrated form. Generally, the composite pigment material may form up to about 40%, usually up to about 30%, by weight of the solids content of the paper making composition on a dry weight basis. Where other fillers also form part of the filler content of the paper making composition a total filler composition of up to 40% by weight of the solids content of the paper composition may be employed. The composite pigment material according to the second aspect may form from 1% to 100% by weight of the added filler on a dry weight basis. Production of a paper sheet using the paper making furnish is, of course, carried out in a well known manner .

Where the composite pigment material according to the second aspect of the present invention is to be employed as a pigment material in a paper coating composition, the composition will generally comprise an aqueous suspension of pigment, including the composite pigment material according to the second aspect and optionally other known ingredients, mixed together with a hydrophilic adhesive and optionally other known ingredients. The composite pigment material employed in the composition may be mixed with one or more conventional pigments, eg as described above. The adhesive may form from 4% to 30%, especially 4% to 15%, by weight based on the total dry weight of pigment or pigments present. The adhesive may be one of the known paper coating adhesives employed in the art, eg chosen from the group consisting of starches, proteinaceous adhesives such as casein and latices of, for example, styrene butadiene rubbers and acrylic polymers. The paper coating composition may also include one or more of the various well known optional additives conventionally used in paper coating compositions, eg a thickener, eg in an amount of up to 2% by weight based upon the total dry weight of pigment or pigments present, up to 1% by weight of a soluble binder, a lubricant, eg sodium stearate forming 0.5% of the weight of pigment present, and/or soluble binder and/or an insolubiliser forming up to 1% by weight of binder present.

The paper coating composition may be formed by mixing together an aqueous dispersed suspension of the composite pigment material optionally with one or more further aqueous dispersed suspensions containing other pigments, with the adhesive and any other optional constituents, eg thickener and/or lubricant and/or soluble binder and/or insolubiliser, in a manner familiar to those skilled in the art. Use of the coating composition formed to coat a paper or other sheet material is of course carried out in a well known manner, eg using one of the many coating machines employed in the prior art.

Precipitation of calcium carbonate in a fines- containing aqueous waste suspension, eg obtained from a paper making waste stream, is described in EP-B- 658,606. The aqueous suspension in the process described therein may contain waste fines which include fine fibers and fine inorganic materials which may incidentally include a very small concentration of Ti0₂ particles or other fine particulate materials. However, generally, since these fines are used materials, they may have been produced from a variety of waste streams and therefore may be of variable composition and concentration, and they are not suitable to provide (on their own) sources of the fine particulate material of the quality required of the fresh particles of the fine particulate material as used in the present invention. Fine particulate material such as Ti0₂ derived from such waste streams will generally be agglomerated and coagulated because of the presence of a variety of polymeric chemical additives in the waste stream and will not therefore show the benefits obtained by forming the novel composite pigment material (according to the second aspect of the present invention) from fresh particles of fine particulate material and bonded precipitated white pigment compound such as CaC0₃ as described hereinbefore. However, as noted above, it is possible that the aqueous medium employed in the method according to the first aspect of the present invention may optionally contain such fines as an additional solids component which will then constitute part of the composite pigment material produced. Embodiments of the present invention will now be described by way of example only with reference to the following illustrative Examples and with reference to the accompanying drawings, wherein: BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing, for three different samples of composite pigment material embodying the invention, scattering coefficient of paper sheets incorporating each sample as filler plotted against the percentage by weight of filler (pigment) in the sheets .

Figure 2 is a graph showing, for a sample of composite pigment embodying the invention and in comparison a sample of untreated Ti0₂ in admixture with calcium carbonate, % retention of Ti0₂ in a paper sheet incorporating each sample as filler plotted against the percentage by weight of filler (pigment) in the sheets (obtained by incineration as described later) .

In the following Examples, the properties of samples of composite pigment material embodying the invention were investigated. The samples, designated Samples A, B and C, were prepared as follows.

SAMPLE A

To a stirred 5 gallons reactor, 11.4 liters of water were added. The stirrer speed was 500rpm. 1400 grams of lime were slowly added and the temperature was kept constant at a temperature of about 38°C throughout the addition. The lime was slaked for 1 hour and 10 minutes. After slaking the reactor stirrer speed was increased to 700rpm and 6.24 grams of citric acid were added followed by the addition of 266.67 grams of titanium dioxide. To this agitated slurry, we introduced C0₂ at a rate of 2.521/min and compressed air at a rate of 8.01/min. This started the carbonation of the slurry. The reaction was carried to completion.

SAMPLE B

To a 5 gallon reactor stirred at 700rpm, we added 2841ml of a 17.6% by weight precipitated calcium carbonate slurry. This slurry was carbonated for a few seconds and then we added 100 grams of dispersed titanium dioxide and 2758ml of a slaked lime slurry. The slaked lime slurry had a solids concentration of 12.5% by weight. The mixture was then further carbonated by adding C0₂ at a rate of 6.0 litres per minute. The carbonation was carried to completion.

SAMPLE C

To a stirred (500rpm) 5 gallon reactor, we added 11.4 liters of water. Lime (1400 grams) was added very slowly in order to maintain the reactor temperature at about 38°C throughout the addition of the lime. The total slaking time was 1 hour and 10 minutes. After slaking was completed, the stirrer speed was increased to 700rpm and 6.24 grams of citric acid were added. Carbonation was started by bubbling C0₂ at a rate of 2.521/min and compressed air at a rate of 2.521/min through the stirred slurry in the reactor. Half way through carbonation, we added 266.67 grams of titanium dioxide. The reaction was carried to completion by continuing the addition of carbon dioxide and compressed air. In addition, for comparison purposes, the following Sample D was prepared using a procedure known in the prior art.

SAMPLE D

A pec product was prepared in the manner of Sample B, except that no Ti0₂ was employed during the course of the pec production procedure. Subsequently, Tiθ₂ was admixed with the Tiθ2 to give a mixture of Ti0₂ and pec in a weight ratio of 10:90, the same target weight ratio in the composite product, Sample B.

EXAMPLE 1 Handsheets were made with different filler levels using the standard procedure described in TAPPI Test Method 205. The fillers used in preparing the handsheets were separately Samples A, B and C. The furnish used in making the handsheets was a 50/50 blend (by weight) of soft and hard wood fibers from Weyerhaeuser, Prince Albert, Canada. The sheet formation was kept constant at around 130, as measured by a Paprican Micro-Scanner made by OpTest Equipment Inc. The sheet grammage was kept at around 75gsm^~2. The optical properties of the sheet were measured by an instrument produced and supplied under the tradename Technibrite Micro TB-IC by Technidyne Corporation. The scattering coefficients were calculated using Kubelka-Munk equations as in the manner described in EP-A-60 , 095. The results obtained for Samples A, B and C are plotted as curves labelled respectively A, B and C in Figure 1.

It can be clearly seen from Figure 1 that the composite pigment material made by introducing the titanium dioxide during the carbonation step had higher scattering coefficients than the others although all three materials show beneficial scattering curves.

EXAMPLE 2

The zeta potentials of composite Sample B and separately titanium dioxide were measured at different pH values utilizing a charge analyzer manufactured by SKS associates. The results are given in Table 1 as follows.

TABLE 1: Zeta Potential

Table 1 shows that in contrast to Tiθ₂ untreated, the Ti0₂-CaC0₃ composite of the invention, Sample A, has a positive charge potential in its surface. This quality makes it easier to the papermaker to retain the pigment in the sheet.

EXAMPLE 3

Handsheets were prepared using the same procedure as in Example 1. The filler pigments used in this Example 3 were separately Sample B and Sample D. Furthermore, headbox pads were obtained during the experiments, so that the retention of the fillers could be measured in a known manner. The sheets were incinerated at 500°C and 950°C and the weight of ash remaining was measured. This allowed us to calculate the weight percent titanium dioxide in the sheets.

(The loss in weight between 500°C and 950°C allows the weight of CaC0₃ present to be determined. ) The results are plotted in Figure 2 in which a curve labelled B represents Sample B and a curve labelled D represents Sample D.

As can be readily seen from Figure 2, the titanium dioxide retention in the composite pigment embodying the invention, Sample B, is higher than in the mixture Sample D. In fact an increase of approximately 200% can be observed.

Claims

1. A method of preparing and using a composite pigment material which incorporates a fine particulate material, which method comprises the steps of: (a) treating an aqueous medium containing dispersed particles of a fine particulate material by chemically reacting therein a first reactant and a second reactant to precipitate therein crystals of a white insoluble pigment compound thereby forming in the aqueous medium a composite pigment material comprising a composite matrix of precipitated crystals of the white pigment compound and particles of the fine particulate material dispersed and bonded within the matrix wherein the first and second reactants are such that they react together without producing a substantial amount of non-crystalline byproduct; and (b) adding the composite material to a composition for forming or coating a fibrous sheet material.

2. A method as claimed in claim 1 and wherein the fine particulate material and the first and second reactants employed to produce the precipitation reaction are obtained from separate stocks.

3. method as claimed in claim 1 and wherein at least part of the fine particulate material is fresh material which has not previously been used in an industrial operation.

4. A method as claimed in claim 1 and which includes the step (c) of making or coating a fibrous sheet material using the composition produced in step (b) .

5. A method as claimed in claim 4 and wherein the sheet material comprises paper, paper board, card, cardboard or the like.

6. A method as claimed in claim 1 and wherein the white pigment compound comprises one or more alkaline earth metal compounds.

7. A method as claimed in claim 6 and wherein the white pigment compound comprises calcium carbonate.

8. A method as claimed in claim 1 and wherein the precipitation reaction is carried out in a batch reactor .

9. A method as claimed in claim 1 and wherein the method is carried out as a continuous or semi- continuous process.

10. A method as claimed in claim 1 and wherein the aqueous suspension is mechanically agitated before and during the precipitation reaction.

11. A method as claimed in claim 1 and wherein at least one preliminary precipitation reaction is carried out in the aqueous medium from which the aqueous suspension employed in step (a) is produced, prior to the fine particulate material being present therein.

12. A method as claimed in claim 11 and wherein the fine particulate material is added to the aqueous medium in multiple doses, a precipitate reaction stage following each such addition.

13. A method as claimed in claim 1 and wherein the white pigment compound comprises calcium carbonate and is obtained in the composite pigment material by including in the aqueous suspension, together with the fine particulate material in at least a part of the process, calcium hydroxide, and passing a carbon dioxide-containing gas into the aqueous medium to provide precipitation of the calcium carbonate by reaction of the calcium hydroxide and carbon dioxide.

14. A method as claimed in claim 13 and wherein the carbon dioxide-containing gas is added until the pH of the aqueous suspension falls to a value of 7.5 or less .

15. A method as claimed in claim 1 and wherein the aqueous medium which is treated to form the white pigment compound by the precipitation reaction comprises, in addition to the fine particulate material, any consumable solids required as reactant to produce the precipitate, additional solid fines derived from a waste stream, the fines optionally being treated by a preliminary precipitation step prior to treatment in step (a) .

16. A method as claimed in claim 1 and wherein the temperature of the aqueous suspension when the composite pigment material is being formed by the precipitation reaction therein in step (a) is in the range 20°C to 60°C.

17. A method as claimed in claim 1 and wherein the composite pigment material is further treated before addition to the paper making or paper coating composition, the further treatment comprising one or more of dewatering, comminution, pH adjustment and re-dilution.

18. A method as claimed in claim 1 and wherein in step (b) the composite pigment material is added to other pigment materials during the production of the paper making or paper coating composition.

19. A method as claimed in claim 1 and wherein the composite pigment material is added to the paper making or paper coating composition in an amount which will provide from 1% to 100% by weight of the pigment present in the paper making or paper coating composition.

20. Composite material for use as a pigment material in paper making or paper coating which composite material comprises a composite matrix of precipitated crystals of an insoluble white pigment compound and particles of a fine particulate material, other than the precipitated crystals, dispersed and bonded within the matrix, the composite material being a product of treating an aqueous suspension of dispersed, fresh particles of a fine particulate material by chemically precipitating the white pigment compound in the suspension in a reaction substantially free of by-products.

21. A composite material as claimed in claim 20 and wherein the fine particulate material comprises Tiθ₂ and the white pigment compound comprises calcium carbonate .

22. A composite material as claimed in claim 21 and wherein the weight ratio of Ti0₂ to calcium carbonate in the material is in the range of from 1:100 to 1:1.