WO2001070644A1

WO2001070644A1 - Kaolin products and their preparation and use in paper making and coating

Info

Publication number: WO2001070644A1
Application number: PCT/GB2001/001217
Authority: WO
Inventors: Minerals Limited Imerys
Original assignee: Husband, John, Claude
Priority date: 2000-03-20
Filing date: 2001-03-20
Publication date: 2001-09-27
Also published as: AU4667601A; GB0006623D0

Abstract

A method for the preparation of a koalin containing product for use in paper making or coating applications includes the steps of (a) preparing an aqueous suspension of a primary kaolin particulate material; (b) treating the suspension of primary kaolin particulate material by adding thereto an oxidative bleaching agent; (c) cleaning the suspension of primary kaolin particulate material by adding thereto a reductive bleaching agent. The kaolin particulate material produced by the method may be used in compositions for making or coating paper products in which an optical brightening agent is present giving enhanced fluorescence.

Description

KAOLIN PRODUCTS AMD THEIR PREPARATION AND USE IN PAPER MAKING AND COATING

The present invention relates to kaolin products and their preparation and use in paper making and coating. In particular, the invention is concerned with improved kaolin products to be used in compositions for paper making and especially for paper coating in which an optical brightening agent is employed.

Kaolin particulate materials are one of a number of kinds of mineral material employed as property enhancing fillers in compositions for making paper and like products and as property enhancing pigments in compositions for coating paper and like products.

Whiteness is a key factor in the appearance of more expensive printing and writing grades of paper and, in addition to pigments such as kaolin, optical brightening agents (OBAs) are frequently added to coating compositions and in some cases to paper making compositions. OBAs absorb ultraviolet radiation present in daylight and emit visible light as fluorescence making the paper appear whiter and brighter and bluer in shade .

It is believed that the fluorescence of OBAs in the presence of kaolin is quenched due to the presence of trace impurities in the kaolin. This causes an undesirable reduction in the effectiveness of the OBA when used in conjunction with a kaolin particulate material. This problem has not been solved satisfactorily in the prior art.

One purpose of the present invention is to provide a kaolin particulate material which shows enhanced fluorescence when used in compositions containing an OBA.

According to the present invention in a first aspect there is provided a method for the preparation of a kaolin containing product for use in paper making or coating applications which includes the steps of (a) preparing an aqueous suspension of a primary kaolin particulate material; (b) treating the suspension of primary kaolin particulate material by adding thereto an oxidative bleaching agent; (c) cleaning the suspension and (d) treating the suspension of primary kaolin particulate material by adding thereto a reductive bleaching agent. Step (b) is preferably applied prior to step (d) .

We have found that by preparing a kaolin particulate material by the method of the first aspect of the invention, surprisingly and beneficially, paper-making and/or coating compositions may be produced as demonstrated later which show improved fluorescence compared with those containing prior art kaolin products. In this specification the expression ^λpaper' embraces products which are of paper, board, card and the like. The kaolin particulate material treated by the method according to the first aspect of the invention may be a kaolin of the primary type, e.g. from one of the various parts of the world in which primary kaolin deposits exist, e.g. UK. The invention is particularly suitable for treating primary kaolins stained with surface iron and organic contaminants, especially humic acid and humic-iron complexes. In the method according to the first aspect of the invention, the oxidative bleaching step (b) is preferably applied using ozone, although it could alternatively be applied by using another bleaching agent known for the oxidative bleaching of kaolin or pulp, e.g. chlorine, chlorine dioxide or hydrogen peroxide.

The reductive bleaching step (d) is preferably applied using a dithionite such as sodium dithionite, although it could alternatively be applied by using another bleaching agent known for reductive bleaching, eg formamidine sulfinic acid.

Desirably, the suspension of primary kaolin particulate material in the method according to the first aspect of the invention is treated by an optional cleaning step (e) following treatment by reductive bleaching in step (d) .

Conveniently, the cleaning step (c) and the optional cleaning step (e) may each be carried out by filtering the kaolin particulate material and re-suspending it in clean water. Alternatively, the cleaning step may be carried out by diluting with water followed by re- concentration, e.g. by filtration. The pH of the suspension of kaolin particulate material may be adjusted prior to each such filtration to assist the filtration procedure by flocculating the kaolin.

Generally, the primary kaolin particulate material produced by the method according to the first aspect of the invention will have a relatively low surface iron content and a relatively low natural organic carbon content especially at the particle surfaces of the kaolin particulate material. It is demonstrated later how such a material unexpectedly and beneficially improves fluorescence when used in a composition containing an OBA.

According to the present invention in a second aspect a primary kaolin particulate material for use in compositions for making or coating paper products in which an OBA is present to give enhanced fluorescence has a soluble iron content of less than 20 parts per million (ppm) by weight measured as the concentration in an aqueous medium which has contacted the kaolin and into which the iron has passed in solution and a natural organic carbon content of less than 600 parts per million (ppm) by weight.

Desirably, the kaolin particulate material has a soluble iron content of less than lOpp , in some cases less than 8ppm, by weight measured as described above. The soluble iron content may be measured by forming a concentrate (ie. about 60% by weight solids) dispersed aqueous suspension of the kaolin at pH8, stirring the suspension formed, eg. ultrasonically, separating the kaolin from aqueous medium using pressure filtration through a membrane of pore size about 0.1 μm and measuring the dissolved iron content of the separated solution. Desirably, the kaolin particulate material has a natural organic carbon content of not greater than 500 ppm by weight, preferably of not greater than 400 ppm by weight. The expression natural' organic carbon content is used to distinguish organic substances naturally present on the kaolin, eg as humic acid contamination, from organic substances deliberately added to the kaolin, eg as dispersants.

The primary kaolin particulate material according to the second aspect of the invention may be prepared using the method according to the first aspect of the invention. The kaolin particulate material may provide a fluorescence response as defined herein of at least 4 ISO brightness units, desirably at least 5 ISO brightness units, preferably at least 6 ISO brightness units.

The fluorescence response' of a kaolin particulate material as defined herein is the difference in brightness at 457nm shown by the material with and without UV radiation present in incident light when the material is formed into an aqueous coating composition having a solids content of 40%-45% by weight and containing 5% by weight of a latex binder and 0.5% by weight of an OBA, the percentages by weight being based on the dry weight of the kaolin particulate material, and the composition is coated at a coat weight of 10 g.rrf² onto a non-fluorescent backsheet. The suspension of kaolin particulate material treated in step (b) of the method according to the first aspect of the invention may be alkaline or of neutral pH. For example, the pH may be from 7 to 12, preferably from 8 to 10. This may be reduced to a pH of from 2 to 5, preferably from 3 to 4, to assist flocculation and filtering of the kaolin particles in step (c) .

The suspension of kaolin particulate material treated in step (d) of the method according to the first aspect of the invention may be acidic or of neutral pH. For example, the pH may be from 1.5 to 7, e.g. from 2 to 3. The pH may be raised to assist flocculation and filtering during step (e) . The solids content of the aqueous kaolin suspension treated in steps (b) and (d) of the method according to the first aspect of the invention may be within a wide range of solids contents. For example, the suspension in each case may have independently a solids content of between 10% and 70% by weight, e.g. from 20% to 50%. Similarly, the temperature of the suspension may be within a wide range of temperatures, e.g. from 10°C to 80°C.

The kaolin particulate material treated in step (b) of the method according to the first aspect of the invention may have been treated prior to step (b) by one or more known physical and/or chemical processing steps to refine the material. For example, impurities such as quartz and mica may be removed by screening and/or hydrocyclone treatment.

The kaolin particulate material may beneficially be treated prior to the oxidative treatment in step (b) of the method of the first aspect by a particle size classification step to remove fine particles especially those containing impurities, e.g. particles having an average particle size of less than 1 μm, especially less than 0.5 μm. The particle size classification may be carried out in a manner known in the art, e.g. by using a known mechanical method, eg use of a centrifuge, or by using a known chemical method, e.g. by selective flocculation. The content of fine impurities, such as those containing iron and other transition metals, which can affect the subsequent efficiency of an added OBA as well as other impurities affecting product optical properties, e.g. iron stained titania, may be reduced in this way. The kaolin particulate material treated by the method according to the first aspect of the invention may be further treated by a magnetic separation process in a known manner. Such a process beneficially removes larger, magnetic impurities, eg. iron-containing impurity particles, especially those having a size greater than 1 μm. Such a process may beneficially be applied after a particle size classification step.

In the method of the invention, the starting kaolin treated in step (a) preferably has a low content of iron, measured as ferric oxide equivalent. For example, the content by weight of iron, based on the dry weight of kaolinite present in the kaolin, is preferably not greater than 0.5% by weight, the iron content being measured as ferric oxide equivalent.

The kaolin particulate material may optionally be additionally treated by one or more further treatments following steps (d) and (e) . For example, addition of a treatment agent which comprises a complexing agent may be applied. As described in our copending patent application, the use of such an agent may give an enhancement in fluorescence response additional to that achieved by reductive bleaching. The complexing agent may comprise one or more known chelating agents for impurity metal ions, especially ions of transition metals such as iron. For example, the complexing agent may comprise one or more of triethanolamine (TEA) , ethylenediaminetetra- acetic acid (EDTA) or one of its salts, diethylenetriaminepentacetic acid (DTPA) or one of its salts, citric acid or one of its salts and sodium glucoheptonate. Following treatment using such a complexing agent the resulting suspension of kaolin particulate materials may be cleaned in one of the ways described earlier.

The primary kaolin particulate material treated in the method according to the first aspect of the invention may comprise a clay mineral, e.g. of the kaolin clay, ball clay, fire clay or China clay types. For example the clay mineral may contain at least 50% by weight kaolinite. Most commercially important kaolin clays contain greater than 75% by weight kaolinite and may contain greater than 85% by weight kaolinite and may be used in the method.

The kaolin particulate material according to the second aspect of the invention unexpectedly and beneficially shows enhanced fluorescence, measured as ^λfluorescence response' defined earlier, when used in a composition in which an OBA is present. The composition in which the OBA is present may comprise a paper making composition, e.g. for making a superior grade of uncoated paper, e.g. for making so-called supercalendered ("sc") papers. Typically, paper making compositions comprise a dilute aqueous suspension, e.g. having a solids concentration of about 1% by weight or less, of cellulose fibres obtained from wood pulp together with mineral filler particles, e.g. selected from kaolin, calcium carbonate, titanium dioxide and other known particulate filler materials. Generally, the cellulose fibres form at least 60% by weight of the solids present. When paper products are made from such compositions they are formed by dewatering and drying the composition.

Alternatively, or in addition, the OBA containing composition in which the kaolin particulate material according to the second aspect is present may be a coating composition for coating paper products. Examples of such compositions and their use are described later. A wide variety of organic molecules containing delocalised π electrons can be used as optical brightening agents, although for economic reasons the OBAs generally used in the paper industry are dye molecules which are derivatives of 4 : 4 -bistriazinyl aminostilbene-2 : 2 - disulfonic acid which has the generalised structure shown below:

The identity of the groups x, y, w and z vary and are known to affect the substantivity of the molecule to the substrate. In coating grades, molecules contain at least 4 and sometimes 6 sulfonic acid groups. The OBA used in the composition according to the third aspect of the invention defined later may though comprise one or more OBAs known in the art, for example it may contain between 2 and 6 sulfonate groups, e.g. it may be a substituted stilbene. In addition to the dye itself, solutions of commercial OBAs sometimes contain a stabiliser such as urea. An activator (or carrier) may be used in conjunction with the OBA. This interacts with the OBA molecules and allows greater levels of fluorescence to be achieved by preventing fluorescence quenching photoiso erism and excimer formation by the OBA molecules to take place. Self activating OBAs are also commercially available in which an activator has been combined with an OBA. Activators are well known in the art and include organic molecules such as polyvinyl alcohols, polyethylene glycols, carboxy methyl celluloses, and polysaccharides such as starch. These compounds may also be employed in the coating composition for other reasons, e.g. as a binder or as a thickening agent. According to the present invention in a third aspect there is provided a composition for coating paper or the like which composition comprises : (A) an aqueous suspension of pigment material; (B) a binder; and (C) an OBA; wherein the pigment material comprises primary kaolin particulate material according to the second aspect of the invention.

Preferably, the aqueous suspension of pigment material contains a soluble iron content of less than 20ppm by weight measured as described earlier.

The kaolin particulate material according to the first aspect of the present invention may be the sole component of the pigment material used in the composition. Alternatively the kaolin particulate material may be blended with one or more additional pigment materials well known to those skilled in the art. The other pigment material (s) may for example comprise a pigment selected from hydrous kaolin, calcined kaolin, titania, talc, so called plastic pigment, calcium sulfate, silica, mica and calcium carbonate. If calcium carbonate is used in the additional pigment material it may be either a natural ground calcium carbonate or a synthetic precipitated calcium carbonate. If the treated kaolin of the second aspect of the present invention is blended with one or more other pigment materials to form the pigment material used in the suspension of the composition according to the third aspect of this invention then it may be present in an amount of between 1% and 99% by weight in the blended pigment material. For example the blended pigment material may contain between 10% and 90% by weight of the primary kaolin according to the second aspect, especially between 25% and 75% by weight.

The solids content of the composition according to the third aspect may be from 30% to 90%, e.g. 50% to 80%, in many cases from 60% to 75% by weight. Usually the highest solids content giving suitable runnability on a coating machine is required. The binder may be added, on an active basis, in an amount of up to 30% by weight based on the dry weight of the pigment material. In many cases, the binder content will be not greater than 15%, e.g. from 4% to 7% by weight, based on the dry weight of the pigment material. The OBA may be added in an active dose of up to 3% by weight based on the dry weight of the pigment material. Although higher OBA doses may be employed, the fluorescence falls with such higher doses owing to increased excimer formation and, of course, the use of higher doses is more costly. Preferably, the OBA dose is between 0.5% and 2.0% by weight on an active basis based on the dry weight of pigment material present. The composition according to the third aspect of the invention may be prepared by any person skilled in the coating art in a known manner and may also contain a known activator (or carrier) to allow greater levels of fluorescence to be achieved which has been separately added. Alternatively the OBA may be self activating. The activator may be present in an amount of up to 6% by weight based on the dry pigment weight. The preferred activator is polyvinyl alcohol which may be present in an amount of up to 3%, by weight, preferably from 1.5% to 2.5% by weight, based on the dry weight of kaolin.

The coating composition may be prepared in a known manner by first forming an aqueous kaolin suspension. This may be done by dispersing the dry treated kaolin into a solution of water and dispersant. Alternatively, dispersant and if necessary additional water may be added to wet kaolin produced by the method according to the first aspect of the invention.

The dispersant may be present, on an active basis, in an amount of up to 3% by weight e.g. from 0.01% to 2% by weight, e.g. from 0.05% to 1%, especially 0.1% to 0.5% based on the dry weight of the pigment material. The dispersant may be selected from any well known in the art, for example it may be an inorganic dispersant such as a polyphosphate, or an organic dispersant such as a polyelectrolyte, e.g. a polyacrylate dispersant, e.g. sodium polyacrylate. A small amount of a base, e.g in the form of an aqueous solution, e.g. a dilute solution such as a 5% to 15% solution, such as sodium hydroxide or sodium carbonate, may optionally be added to adjust the pH of the composition to greater than 7. The solids content of this suspension may be determined and then a binder may be added. The binder may be used, on an active basis, in an amount of from 0.1% to 30% by weight, e.g. from 1% to 25%, especially from 4% to 15% based on the dry weight of the pigment material. The binder employed in the composition invention may comprise one or more binders known in the art, for example selected from commonly used waterborne polymeric latices which may be natural or synthetic substances, polysaccharides such as starch derivatives, proteinaceous adhesives or polyvinyl alcohols. An activator as described earlier may be added to the kaolin suspension and may be used, on an active basis, in an amount of 0.1% to 6% by weight, e.g. 1% to 4%, especially 1.5% to 2.5% based on the dry weight of the pigment material. The activator may be added before or after, preferably after, the binder. The OBA may be added before or after, preferably after, the activator. The OBA may comprise one or more OBAs known in the art as previously described. The pH of this coating composition may then be adjusted as described earlier. The coating composition may also optionally contain other additional and well known additives to those herein before described to control other properties of the composition, in a well known manner.

Embodiments of the present invention as well as comparative prior art examples will now be described by way of example with reference to the following Examples and the accompanying drawings in which: Figure 1 is a graph of ISO brightness versus dithionite dose for kaolins treated by reductive dithionite addition.

Figure 2 is a graph of fluorescence response versus dithionite dose for kaolins treated by reductive dithionite addition.

Figure 3 is a graph of iron amount removed versus dithionite dose for kaolins treated by reductive dithionite addition. Figure 4 is a graph of brightness versus dithionite dose for kaolin treated by ozone followed by dithionite addition.

Figure 5 is a graph of fluorescence versus dithionite dose for kaolins treated by ozone followed by dithionite addition.

Figure 6 is a graph of fluorescence versus coat weight for kaolin treated by ozone followed by dithionite addition.

EXAMPLE 1 (PRIOR ART)

A primary English kaolin Kl having the following properties was obtained: 1) ISO brightness: 77.1 2) ISO yellowness: 9.7

3) Surface area 16.9 m.².g^_1

4) Particle size properties ( measured by Sedigraph 5100) weight % < 2μm 69% weight % < Iμm 47% weight % < 0.5 μm 25% weight % < 0.25 μm 10%

5) Kaolinite content 89% by weight

6) Fe₂0₃ content by weight (measured by X-ray fluorescence spectroscopy ^λXRF"). 0.76% 7) Ti0₂ content by weight (measured by XRF) 0.02%. 150g of the kaolin Kl in dry form was slurried with 450 cm³ water and the pH was lowered to 2.3 using 10% sulphuric acid. Sodium dithionite was then added to give a required dose (from 0.5 to 5 kg.t^"1* followed by 1.0 kg diammonium phosphate per dry tonne of kaolin. After 20 minutes stirring, the pH was raised to 4 and the slurry was filtered. A sample of the filtrate was retained for analysis. The filtered solid was resuspended in clean water and refiltered to remove any soluble iron, followed by drying at 80°C.

The powder brightness of the product, ie diothinite processed kaolin Kl, was measured using standard procedures. The product was formed into a coating composition and the fluorescence of such a composition was measured in the following manner. The treated kaolin was dispersed in clean water at high solids with 0.3 wt% sodium polyacrylate and made into a coating composition using the following formulation:

100 parts by weight of kaolin 5 parts by weight (active) latex (Dow

950 (TM) ) 2 parts by weight as (active) polyvinyl alcohol (Polyviol LL603 (TM) )

0.5 parts by weight as received Blankophor P (TM) OBA NaOH to pH 8.0.

Blankophor P (Bayer) is a typical commercially available coating grade of OBA based on a stilbene-derivative. A dose of 2 pph polyvinyl alcohol was chosen to give maximum activation of the OBA. The composition was diluted to around 40 - 45 weight per cent solids before coating onto non-fluorescent non-absorbent polyester film (Synteape) using wire wound bars. Coat weights obtained were between 9 and 14 gπf². The latex level was kept low to assist in removing excess coating from the polyester film when measuring the coat weight. In any case, latex does not affect fluorescence. The coated sample was placed over a stack of the polyester films uncoated and the reflectance was measured at 457 nm using an Elephro 3000 (TM) machine with and without UV in the light source using a UV filter. The difference (ISO units) between the two brightness readings was taken as the fluorescence response. The results were plotted against coat weight and were interpolated to a coat weight of 10 g.nf².

The results of the powder brightness measurements and the fluorescence response measurements in the coating composition obtained are plotted as curves Ela and Elb in Figures 1 and 2 respectively.

EXAMPLE 2 (PRIOR ART)

A secondary kaolin K2 from mid-Georgia, USA was obtained having the following properties:

1) ISO brightness: 82.7

2) ISO yellowness: 8.0 3) Surface area (BET) 15.8 m.².g^_1

4) Particle size properties (Sedigraph 5100) % by weight smaller than < 2μm: 90%

% by weight smaller than < Iμm: 77% % by weight smaller than < 0.5μm: 59% % by weight smaller than < 0.25μm: 34%

5) Percentage by weight Ti0₂ (XRF) : 1.38%

6) Percentage by weight Fe₂0₃ (XRF): 0.52%

The kaolin K2 was processed with sodium dithionite at low pH in the manner described in Example 1. The powder brightness of the product obtained was measured. The product was formed into the same coating composition as in Example 1

(replacing Kl with K2) and the fluorescence was measured as described in Example 1. The results obtained of the powder brightness measurements, curves Ela and E2a for Kl and K2 respectively and fluorescence response measurements curves Elb and E2b for Kl and K2 respectively are plotted in Figures 1 and 2 respectively. The results shown in Figures 1 and 2 show that despite powder brightness of Kl and K2 increasing by 6 ISO units and 3 ISO units respectively by the addition of dithionite, the fluorescence response provided by the bleached kaolins using a maximum dithionite dose is, within error, the same as that obtained without dithionite bleaching of the kaolins, i.e. about 2.6 units for Kl and about 4.0 units for K2. This is surprising since, as shown in Figure 3, appreciable removal of iron from both Kl and K2 was obtained. Curves Elc and E2c in Figure 3 show respectively the amount of iron removed from Kl and K2 in Example 1 and 2 respectively. This shows that removal of iron alone is not sufficient to give suitably enhanced fluorescence.

EXAMPLE 3 (INVENTION)

Kaolin Kl was treated by the following oxidative ozone processing. 50g of kaolin Kl in dry form was slurried with 450 cm³ water and the pH was adjusted to 9 with NaOH. The slurry produced was transferred to a gas flow bottle and treated by addition of ozone at a flow rate of 80 litres per hour for a period of 60 minutes.

The pH of the resulting slurry was reduced to 3.5 - 4.0 with H₂S0₄ and the kaolin was filtered and reslurried in clean water followed by purging with nitrogen for a further 30 minutes. The resulting slurry was then treated by dithionite bleaching in the manner described in Example 1. The treated kaolin product was dried and its powder brightness was measured. The product was formed into a coating composition to measure fluorescence response as described in Example 1. The results obtained for powder brightness and fluorescence response are shown in Figures 4 and 5. Curves E3a and E3c show the results obtained for powder brightness and fluorescence response resectively with dithionite treatment but without ozone treatment. Curves E3b and E3d show the results obtained for powder brightness and fluorescence response respectively with both ozone treatment and dithionite treatment. As seen in Figures 4 and 5, although processing with dithionite only benefits brightness it does not in itself benefit fluorescence. However, when oxidative bleaching using ozone is first applied, curves E3b and E3d show that both brightness and fluorescence are significantly enhanced. As seen in Figure 5, the fluorescence can be raised up to about 5 ISO units .

We found that the total amount of iron removed by ozone and dithionite treatments in Example 3 was little different from that removed using dithionite treatment only as in Example 1.

However, in a further experiment, we measured the natural organic carbon content as well as fluorescence response of kaolin Kl treated as described in Example 1 and in this Example 3. This was done in a standard manner by heating the material to oxidise any carbon present to form C0₂ and measuring the concentration of C0₂ formed. The carbon content results obtained are shown in Table 1 as follows:

TABLE 1

Table 1 shows that a combination of lowering of natural organic carbon content as brought about by oxidative ozone treatment as well as reductive dithionite treatment to lower soluble and surface iron content is needed give the enhancements in fluorescence response shown in Figure 5.

The result for ozone treatment but no dithionite treatment of Kl given in Table 1 shows that ozone treatment alone does not give the required fluorescence enhancement.

The requirement for removal of both soluble iron and natural organic carbon was confirmed by deliberately adding soluble iron and humic acid to a particulate mineral, i.e. calcium carbonate for which these contaminants were low. The fluorescence response in each case was correspondingly made inferior by the presence of the added contaminants. Similar enhancements in fluorescence are obtained when the coating composition containing kaolin prepared according to this Example is coated onto a paper product in a known manner.

EXAMPLE 4 (INVENTION)

A primary Kaolin K3, having a particle size distribution such that 92% by weight of particles have a particle size less than 2μm and 80% by weight of particles have a particle size less than lμ , and having an ISO brightness of 85.2, was treated by the following oxidative ozone processing. 50g of the kaolin K3 in dry form was slurried with 450 cm³ water and the pH was adjusted to 9 with NaOH. The slurry produced was transferred to a gas flow bottle and treated by addition of ozone at a flow rate of 80 litres per hour for a period of 60 minutes. The pH of the resulting slurry was reduced to 3.5 - 4.0 with H₂S0 and the kaolin was filtered and reslurried in clean water followed by purging with nitrogen for a further 30 minutes. The resulting slurry was then treated by dithionite bleaching in the manner described in Example 1. The treated kaolin product was dried and its ISO (powder) brightness was measured at 90.7. The product was formed into a coating composition to measure fluorescence response as described in Example 1. The results obtained for fluorescence response are shown in Figure 6. Curve E4a shows the results obtained for fluorescence response with dithionite treatment but without ozone treatment. Curve E4b shows the results obtained for fluorescence response with ozone treatment but without dithionite treatment. Curve E4c shows the results obtained for fluorescence response with both ozone treatment and dithionite treatment. As seen in Figure 6, applying ozone and dithionite treatment to the kaolin K3 produces significantly enhanced fluorescence response compared to either ozone or dithionite treatment applied alone. The fluorescence can be raised up to about 8.4 ISO units .

Claims

1. A method for the preparation of a kaolin containing product for use in paper making or coating applications which includes the steps of (a) preparing an aqueous suspension of a primary kaolin particulate material; (b) treating the suspension of primary kaolin particulate material by adding thereto an oxidative bleaching agent; (c) cleaning the suspension following step (b) ; and (d) treating the suspension of primary kaolin particulate material by adding thereto a reductive bleaching agent.

2. A method according to claim 1 and which includes the additional step (e) of cleaning the suspension following step (d) .

3. A method according to claim 1 or claim 2 and wherein steps (b) and (c) precede step (d) .

4. A method according to claim 1, claim 2 or claim 3 and wherein the oxidative bleaching agent comprises ozone.

5. A method according to any one of the preceding claims and wherein the reductive bleaching agent comprises a dithionite.

6. A method according to any one of the preceding claims and which includes following step (d) the additional step (f) of adding to the suspension a complexing agent.

7. A method according to claim 6 and which includes following step (f) the additional step (g) of cleaning the suspension.

8. A method according to any one of the preceding claims and wherein the suspension is subjected to one or more additional treatment steps prior to step (b) .

9. A method according to any one of the preceding claims and wherein the suspension is treated by a particle size classification step prior to step (b) .

10. A method according to any one of the preceding claims and which includes the additional step of magnetic particle separation.

11. A method according to any one of the preceding claims and wherein the primary kaolin in step (a) has an iron content measured as Fe₂0₃ of not greater than 0.5 per cent by weight.

12. A primary kaolin particulate material for use in compositions for making or coating paper products in which an OBA is present to give enhanced fluorescence the primary kaolin particulate material having a soluble iron content of less than 20 parts per million (ppm) by weight and measured as the concentration in an aqueous medium which has contacted the kaolin and into which the iron has passed in solution and a natural organic carbon content of less than 600 parts per million (ppm) by weight.

13. A kaolin particulate material according to claim 12 and which is produced by the method claimed in any one of claims 1 to 11.

14. A composition for coating paper or the like which composition comprises: (A) an aqueous suspension of pigment material;

(B) a binder; and

(C) an an optical brightening agent; wherein the pigment material comprises primary kaolin particulate material according to claim 11 or claim 12.

15. A composition for coating paper according to claim 14 and which includes an activator for the optical brightening agent.

16. A composition according to claim 14 or claim 15 and which has a fluorescence of at least about 4 ISO units.