Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-11-09
2003-02-04
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S484000
Reexamination Certificate
active
06514333
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a platey, fines deficient delaminated kaolin composition for particular use as a lightweight coating (LWC) for groundwood paper and comprising platelets produced by grinding a coarse fraction or a whole fraction of the kaolin crudes to a shape factor ranging from about 13 to about 18 where there is a substantial amount of platelets and an insubstantial amount of ultrafines.
BACKGROUND OF THE INVENTION
It is well known in the paper industry that a wide variety of pigments, such as titanium dioxide, calcium carbonate, talc, synthetic silicates, and clays such as bentonite and kaolin, are suitable for use as paper fillers and/or coatings. Kaolin, a natural occurring hydrated aluminate silicate, is presently the most widely utilized and is available in a range of particle sizes and brightness, as well as being either delaminated or non-delaminated.
Kaolin clay pigments are obtained from kaolin crudes. The crudes contain kaolin particles, oversize (grit) and fine and coarse particle size impurities. Some of the impurities (e.g., fine ferruginous or titaniferous impurities) impart undesirable color to the clay. Other impurities have an undesirable effect on the rheology of the kaolin.
The kaolin portion of kaolin crudes is “polydisperse” in the sense that the particles occur over a range of sizes and shapes. Thus, a kaolin crude will not contain particles of a single size such as, for example, particles all of which are 2 micrometers. Typically a degritted kaolin crude will contain particles ranging in size from submicron or colloidal to particles 20 micrometers or larger.
Kaolins from different deposits, or even from different zones in the same deposit, can vary widely in the content of impurities, particle size distribution as well as the morphology of the kaolin particles. In general, crude kaolin particles are composed of individual platelets, and stacks or booklets concentrated in the greater than 2 micron faction. Particle sizes of kaolins are conventionally determined by sedimentation using Stokes law to convert settling rates to particle size distribution, and assume a spherical particle shape for the kaolin particles. Hence, the use of the conventional term “equivalent spherical diameters (e.s.d.)” to designate particle size.
Hydrous kaolin is white in color, has a fine particle size, is relatively chemically inert, and makes an ideal low cost paper filler. Calcined (anhydrous) kaolin is also available for use as a paper filler and can impart greater opacity to paper than the hydrous kaolin, but has the serious disadvantage of being more abrasive.
Prior art kaolin paper fillers and coatings are typically produced by a beneficiation process which typically consists of fractionating in a continuous centrifuge to remove oversize material followed by leaching to remove iron-based colored compounds. In the leaching process the kaolin is acidified with H
S
SO
4
to a pH of 3.0 to solubilize the iron. Sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The flocculated clay, generally at approximately 30% solids by weight, is then filtered, such as by dewatering on a rotary vacuum filter to a solids level approximately 60% by weight. The filter cake is then dried or redispersed with additional dry clay if it is to be sold as approximately 70% by weight solids slurry. To produce high brightness products, i.e., fillers having a brightness index greater than 90, impurities may be removed from the kaolin clay through flotation or magnetic separation. To produce a delaminated product, the coarse fraction from the initial centrifugation is ground in sand, grinders to shear the stacks of platelets normally found in kaolin and thereby produce individual particles having an equivalent spherical diameter less than 2 microns.
It is well appreciated in the art that kaolin clay pigments must have certain Theological and optical properties to be suitable for use in paper manufacture as paper coatings or paper fillers. The kaolin clay pigment must be available as a high solids suspension typically having a clay solids content of about 50% to about 70% by weight, but still exhibiting a viscosity low enough to permit efficient and economical pumping, mixability with other filler or coating components, and application to the paper. Additionally, it is of utmost importance that the kaolin pigment exhibit certain optical properties, namely high brightness, high gloss, and high opacity.
The influence of particle size distribution upon the optical properties of kaolin pigments, has long been appreciated in the art. For example, in U.S. Pat. No. 2,992,936, Rowland discloses that a kaolin clay product having the following particle size distribution (in terms of equivalent spherical diameter, e.s.d.) will consistently show improved brightness, gloss and opacity when used as a paper coating clay:
99-100% by wt. less than 5 microns e.s.d.
98-100% by wt. less than 4 microns e.s.d.
88-100% by wt. less than 1.7 microns e.s.d.
85-97% by wt. less than 1.5 microns e.s.d.
70-84% by wt. less than 1.0 microns e.s.d.
25-37% by wt. less than 0.5 microns e.s.d.
10-15% by wt. less than 0.3 microns e.s.d.
Rowland further discloses producing such a controlled particle size kaolin product by first degritting a kaolin clay slurry, thence passing the degritted kaolin clay slurry at 21% solids by weight through a Sharples centrifuge at 400 cc per minute at 6300 r.p.m. and then recentrifuging the overflow effluent at the same rate and r.p.m. The final overflow effluent represented a cut taken off the fine end of the degritted clay slurry and amounted to 22% by weight of the degritted clay slurry. The degritted clay slurry remaining after removal of this 22% fine cut, i.e., the combined underflows from the two centrifugation steps, was reslurried to about 20% solids with 0.15% sodium hexametaphosphate and allowed to settle. The sedimented coarse clay which amounted to about 48% of the degritted clay slurry, was discarded leaving about 30% by weight of the original degritted kaolin clay slurry as an intermediate product to be subjected to further treatment via bleaching, filtering and drying to yield a commercial coating clay product.
In a paper entitled “Chemically Induced Kaolin Floc Structures for Improved Paper Coating”, presented at the 1983 TAPPI Coating Conference, W. H. Bundy et al. disclosed an improved high bulking paper coating pigment, referred to as 1089, which comprises a chemically modified kaolin produced by the Georgia Kaolin Company, Inc. and marketed under the trade name Astra-Lite. Structures of optimum functionality are said to be derived by chemically treating a base kaolin clay having a particle size distribution wherein from about 80% to 93% by weight of the kaolin particles are less than 2 microns e.s.d. to selectively flocculate a portion of the submicron fines therein thereby aggregating a portion of these fines on the surface of larger kaolin platelets and effectively inactivating large portion of colloidal particles. Such a chemically modified kaolin coating pigment derived from a base kaolin wherein 92% by weight particles under 2 microns is presented by Bundy et al. having a particle size distribution as follows:
99% by wt. less than 5 microns e.s.d.
97% by wt. less than 3 microns e.s.d.
90.5% by wt. less than 2 microns e.s.d.
65.5% by wt. less than 1 microns e.s.d.
31.5% by wt. less than 0.5 microns e.s.d.
12.5% by wt. less than 0.3 microns e.s.d.
5.5% by wt. less than 0.17 microns e.s.d.
Such a chemically flocculated kaolin coating pigment may be produced, for example, as disclosed,in U.S. Pat. Nos. 4,075,030; 4,076,548; or 4,078,941, by selectively flocculating a base kaolin clay with the addition: of either a low molecular weight (less than 1,000,000) organic flocculant such as a polyfunctional amine, e.g., ethylene diamine or hexamethylene diamine, or long carbon chain amine, with or without citric acid and, optionally, in the presence of fine mica below 150 mesh in s
Brown Vince H.
Garforth William L.
Pruett Robert J.
Yuan Jun
Bell Mark L.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Imerys Pigments, Inc.
Manlove Shalie
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