Method for forming aggregated kaolin pigment

A method for forming an aggregated kaolin clay pigment, comprising: preparing an aqueous 6-30% solids slurry of a fine particle size feed kaolin; intermixing with said slurry from about 10 to 20% of alum, by weight of the dry kaolin; adding to the mix of step (b) from about 15 to 30% of sodium silicate, by weight of the dry kaolin; mixing the reactants of step (c); and filtering and drying the slurry to recover aggregates of adherent platelets.

FIELD OF THE INVENTION 
This invention relates to a method for preparing aggregated pigments from 
kaolin clays by chemical aggregation at substantially ambient, i.e. room 
temperature. The resulting aggregated pigments display high porosity 
compared to the feed clay, rendering them highly useful in paper filling 
and coating compositions. 
BACKGROUND OF THE INVENTION 
Hydrothermal treatment of kaolin clays to produce modified products of 
various types, has been known for many years, and is widely described in 
the prior art technical and patent literature. The reaction of sodium 
silicate with kaolin clays, for example, has been studied under various 
hydrothermal conditions, as reported by Kurbus et al, Z. Anogr. Allg. 
Chem., 1977, Volume 429, pages 156-161. These reactions were studied under 
hydrothermal conditions using essentially equivalent molar ratios of the 
kaolin and sodium silicate with the reaction being carried out in an 
autoclave. The products of the reactions, as identified by x-ray, electron 
microscope, and infrared methods, showed that sodium silicate reacts with 
kaolin to form an alumino-silica gel or a crystallized zeolite mineral 
analcime of the formula: 
EQU Na.sub.2 O:Al.sub.2 O.sub.3 :4SiO.sub.2 2H.sub.2 O 
In the reaction, the kaolin dissolves and alpha-quartz simultaneously 
appears in the product of reaction. 
U.S. Pat. No. 4,812,299 issued Mar. 14, 1989 to S.K. Wason (assigned to 
J.M. Huber Corp.) describes compositions designated as synthetic alkali 
metal alumino-silicates, or simply SAMS, prepared by the hydrothermal 
reaction of an alkali metal silicate and kaolin clay. By the term 
"hydrothermal" it is meant that the reaction is carried out under aqueous 
conditions of elevated temperatures and pressures of greater than 
atmospheric. The reaction is conducted under conditions of agitation. In a 
preferred operation of the said process, the aqueous slurry of the 
starting clay material and the alkali metal silicate is formed, the system 
is closed and heat applied to gradually raise the temperature. In general, 
the pressure in the system will range from about 50 to 360 psig at 
temperatures ranging from about 140 to 250 degrees C. A specifically 
preferred range of conditions is to operate the process at pressures of 
100 to 200 psig and temperatures of 164.degree. to 194.degree. degrees C. 
The temperatures are preferably correlated to the pressure such as that 
provided by steam. The reaction time is about 0.25 to 4 hours. After 
completion of the reaction, heat is removed and the mixture is allowed to 
cool, after which the system is opened, the product separated by 
filtration or centrifugation, washed with water, and dried. Spray drying 
is preferred at inlet temperatures of 1000.degree. F. (538.degree. C.) and 
outlet temperature of 250.degree. F. (121.degree. C ). The products are 
defined as structured agglomerates wherein the primary particles comprise 
altered kaolin clay platelets integrated with one or more adjacent areas 
of essentially amorphous alkali metal silicate base-kaolin clay reaction 
products. More specifically, they are described as altered kaolin 
platelets having an integrated rimmed area of amorphous, non-diffracting 
alkali metal silicate-kaolin reaction product. The products are said to be 
useful as reinforcing agents or fillers for papers, paints, plastics and 
rubber and to have increased opacity and brightness as compared with the 
starting clay material. 
U.S. Pat. No. 4,816,074 issued Mar. 28, 1989 to Rasik H. Raythatha et al, 
assigned to E.C.C. America Inc., describes a non-hydrothermal process 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. The 
process serves to aggregate the very fine particles (i.e. the sub 0.25 
micrometer particles) present in very fine feeds. Thus, the otherwise 
large percentages of troublesome extremely fine particles are effectively 
removed as separate entities, but without the need for separation steps, 
together with the costly equipment required for such operations. 
U.S. Pat. No. 3,765,825 issued Oct. 16, 1973 to V.J. Hurst teaches reducing 
the viscosity of kaolin clay slurries by hydrothermal treatment. In U.S. 
Pat. No. 3,769,383 issued Oct. 30, 1973, he teaches hydrothermal treatment 
of kaolin to produce a variety of alumino-silicate products. 
U.S. Pat. No. 4,499,062 issued Feb. 12, 1985 to P. Christophliemk et al 
(assigned to Henkel) teaches hydrothermal decomposition of industrial 
waste products such as filter sludge containing aluminum silicate and/or 
alkali aluminum silicate, by addition of aqueous NaOH solution and sand. 
In pending patent application Ser. No. 416,936 of Dunaway et al. filed Oct. 
4, 1989, which application is assigned to the assignee of the instant 
application, there is disclosed a method which enables relatively low 
temperature aggregation of kaolin, which can be hydrothermal. In the 
practice of the Dunaway invention an aqueous slurry of particulate kaolin 
clay and sodium aluminate is prepared, having a total solids concentration 
of from about 2 to 25% by weight, the weight ratio between kaolin and 
aluminate being greater than 1. The slurry is then treated in a closed 
system to form aggregates of adherent kaolin platelets. The treatment is 
generally effected at temperatures of from about 50.degree. to 300.degree. 
C., with a more preferable temperature being in the range of from about 
90.degree. to 200.degree. C. Pressure conditions in the closed system 
correspond to saturated steam at the temperature utilized. The slurry is 
subjected to mixing during the hydrothermal treatment, with such treatment 
being carried out for from about 0.1 to 8 hours; and preferably for from 
about 0.1 to 2 hours. In a further aspect of the Dunaway et al invention, 
it has been found that generally lower temperatures can be used during the 
hydrothermal reaction, where the slurry further includes sodium silicate. 
Where so used, the molar ratio between the sodium aluminate (expressed as 
equivalent Al.sub.2 O.sub.3) and sodium silicate (expressed as equivalent 
SiO.sub.2) is from about 1:2 to 2:1. Following the hydrothermal reaction, 
the resultant aggregates are recovered and dried. Wide varieties of clays 
can be processed including inferior, low grade clays which may be 
unsuitable for calcination. Discolored clays having low brightnesses can 
also be used as feed for the process, and unexpectedly brightened pigments 
are thereby yielded. 
In Swift, U.S. Pat. No. 3,849,149, a method is disclosed for coating 
mineral particles which can include alumino silicates. The purpose of the 
coating on the mineral particles is to render the surface acidic so that 
the coating materials enter into a polymerization reaction with basic 
organic materials. Since the objective is not aggregation, the quantity of 
additives is relatively low, and it is important that the particles remain 
separate so that each particle can be coated in the subsequent 
polymerization reaction. 
In Hanahan, U.S. Pat. No. 2,296,637, a high-surface hiding pigment material 
and process of making same is disclosed. In one aspect of the method of 
preparation, a sodium silicate solution is added to a clay pigment slurry 
and is precipitated upon the clay surface by creating an acid environment 
such as by adding sulfuric acid. 
In Hanahan, U.S. Pat. No, 2,296,639, similar materials to those described 
in the 2,296,637 patent are precipitated, especially upon lithopone. The 
patent also indicates that the process can be used with other extender 
materials such as aluminum silicates. The reaction product formed can be 
that of sodium silicate and aluminum sulfate. The preferred amounts are 
very low (0.35%-1.5% silicate). Percentages higher than 10% are said by 
the patentee to result in decreased surface hiding power when the pigments 
are used in paints. 
In Kurrle, U.S. Pat. No. 4,026,721, a composite silicate pigment is 
prepared by a precipitation reaction wherein spherical hydrous metal 
silicate particles are precipitated on the planar surfaces of clay 
particles having a platelet-type structure. An aqueous suspension of the 
clay pigment is formed, and a water soluble salt of an alkaline earth 
metal is blended into the clay slurry. Under high shear, a water soluble 
alkali metal silicate is then metered into the slurry to precipitate the 
alkaline earth metal silicate (preferably calcium silicate) in the form of 
spherical particles on the clay platelets. The composite pigment is then 
filtered and washed. The clay component may comprise kaolinite. The small 
spherical particles on the clay platelets are stated by the patentee to 
not significantly alter the overall particle size distribution of the base 
material. 
In accordance with the foregoing, it may be regarded as an object of the 
present invention to provide a process for aggregating a kaolin feed, 
which process can be effectively practiced at substantially room 
temperature (and pressure) and which result in an aggregated pigment 
displaying good brightness and high pore volume, thereby rendering such 
pigment highly suitable for paper coating and paper filling applications. 
SUMMARY OF THE INVENTION 
Now in accordance with the present invention, it has been found that a fine 
particle size kaolin can be aggregated at a low pH by acid-induced 
polymerization with alum (aluminum sulfate) and sodium silicate. The 
desired reaction takes place at room temperature and at a low pH, in the 
range of about 3.5 to 5.0 and preferably below about 4.0. However, higher 
temperatures can be utilized without detrimentally affecting the product 
quality. The preferable feed utilized in preparing the aggregates is a 
kaolin which has a particle size distribution wherein substantially all 
particles are, below 2 micrometers in equivalent spherical diameter (ESD), 
and the average particle size in the feed is below one micrometer. A 
preferable feed has an average particle size below 0 5 .mu.m. So-called 
cream kaolins or grey kaolins can be effectively aggregated by the method 
of the invention. The degree of aggregation can be monitored by the change 
in the subquarter micron fraction. In accordance with the invention this 
fraction is reduced by at least 50%. This subsequent decrease in the 
sub-quarter micrometer fraction is accompanied by a corresponding increase 
in pore volume. The product has a typically pore volume of at least 0.55 
cm.sup.3 /g. 
It is preferable that the kaolin feed has prior thereto been processed 
without the use of organic dispersants. Sodium silicate/alum content (i.e. 
the sum of the two additives) in the reaction mix may range from about 20% 
to 50% by weight and preferably is in the range of from about 25 to 40% by 
weight on a dry basis. The ratio of Si/Al can range from 1-2. Where the 
product of the invention is used as a coating pigment in preparation of 
coated papers, the resulting paper products show excellent improvements in 
sheet brightness, sheet gloss, dry/litho print gloss and smoothness as 
compared to a conventional heat aggregated or prior art aggregated coating 
pigments. The products are similarly useful in the filling of papers.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In a typical procedure in accordance with the invention, an aqueous 6-30% 
solids slurry of a fine particle size feed kaolin is prepared. The slurry 
is intermixed with from about 10 to 20% of alum, by weight of the dry 
kaolin. From about 15 to 30% of sodium silicate, by weight of the dry 
kaolin is then added to the mix. The reactants are mixed for 5 to 10 
minutes and slurry is filtered and dried to recover aggregates of adherent 
platelets. 
Practice of the present invention will now be illustrated by examples, 
which are deemed illustrative of both the process taught by the present 
invention and of the products yielded in accordance with the invention: 
EXAMPLE 1 
A 40 gram sample of kaolin having a particle size distribution such that 
about 97 by weight is less than 1 micrometer ESD was slurried at 10% 
solids in 160 grams of water. To this solution was added 8 grams of alum. 
The pH of the solution was about 3.8. 90 grams of a 10% sodium silicate 
solution was added with vigorous stirring. The resulting reaction mixture 
contained 30% by weight of the sodium silicate/alum additives. The mixture 
was stirred for ten minutes, filtered and dried, by oven drying at 
100.degree. C. Spray drying was also used on other samples. 
EXAMPLE 2 
The above procedure was repeated. In this instance the sodium silicate/alum 
addition was to the extent of about 12.5% by weight on a dry basis in the 
reaction mixture. 
EXAMPLE 3 
The procedures of Example 1 and 2 were repeated, using 22.5% by weight of 
the sodium silicate/alum additives. 
EXAMPLE 4 
The products of Examples 2-3 were redispersed at 1O% solids and blended for 
two minutes in a Waring blendor. Particle size, surface area and pore 
volume measurements were made on the products before and after blending, 
with the object of determining if the aggregate strengths could withstand 
the paper making process. Of interest was whether the aggregates could 
withstand the type of agitation occurring in paper making, and were 
sufficiently resistant to break up by resolublization of the silicate 
polymer. The results are shown in Table 1 below. 
TABLE 1 
______________________________________ 
Alum/Na Silicate Low Temperature/ 
Low pH Aggregation Using a Beneficiated Fine 
Particle Size Gray Kaolin 
Mercury Average 
% Na G.E. Surace Porosimetry 
PSD 
Silicate/Alum 
Brightness.sup.1 
Area cm.sup.3 /g 
.mu.m 
______________________________________ 
0% (control) 
78.0 24.0 0.39 0.25 
12.5% 
as reacted 
78.0 32.8 0.58 1.3 
after redis- 
78.1 32.8 0.54 1.0 
persion 
22.5% 
as reacted 
78.9 30.5 0.60 0.98 
after redis- 
78.4 34.5 0.54 0.97 
persion 
______________________________________ 
.sup.1 TAPPI Procedure T646, OS75 
FNT .sup.1 TAPPI Procedure T-646 OS-75 
In Table 1 characteristics of the resulting pigments are set forth with 
respect to brightness, surface area, mercury porosimetry and average 
particle size. It is seen from Table 1 that there was no change in the 
particle size distribution curve of the 22.5% or the 12.5% sodium 
silicate/alum aggregated product with blending in a Waring blendor at 10% 
solids for two minutes. In FIGS. 1 and 2 PSD curves appear for the feed 
kaolin and the resulting aggregated product. This illustrates the 
significant aggregation achieved by the invention. 
The product of the Example was evaluated in a sulfide paper system for 
scatter properties and compared with the product produced via the Kurrle, 
U.S. Pat. No. 4,026,721 cited above. The 22.5% sodium silicate/alum 
products show an appreciable increase in comparison to the product 
produced by the methodology of U.S. Pat. No. 4,026,721, i.e. 523 v. 577, 
with a reduced level of additives. 
TABLE 2 
______________________________________ 
10% Normalized Scatter.sup.2 in a Sulfide Paper System 
______________________________________ 
Product of the 
577 Feed from Example 1 
Invention (Example 3) 
Treated with 22.5% Na 
Silicate/alum at 
room temperature 
Product of U.S. 
523 Feed from Example 1 treated 
Pat. No. 4,026,721 with 40% CaCl.sub.2 /Na Silicate 
at room temperature 
______________________________________ 
.sup.2 See U.S. Pat. No. 4,818,294 
FNT .sup.2 See U.S. Pat. No. 4,818,294 
EXAMPLE 5 
To demonstrate that a high degree of aggregation occurs with a wide variety 
of feed materials, several different feed kaolins and blends were 
aggregated by the method described in Example 3. The results are set forth 
in Table 3 below: 
TABLE 3 
______________________________________ 
% sub 0.25.mu. Content 
Feed Materials 
Before Reaction 
After Aggregating 
______________________________________ 
A Platey 68 5 
South Carolina 
Hard Kaolin 
B Grey Washington 
63 4 
County GA. (WC) 
Kaolin 
Cream WK Kaolin 
63 0 
Delaminated Sand 
17 1 
Ground WC Cream 
Kaolin 
Blend of A and B 
65 3 
______________________________________ 
EXAMPLE 6 
The aggregated product described in Example 3 was evaluated in a low weight 
coating offset application. This room temperature aggregate was utilized 
at 40 parts replacement in a 50/50 KCS.RTM./Alphaplate.RTM. pigmented 
coating formulation and compared to 40 parts Deltagloss.RTM. replacement, 
or 10 parts Alphatex.RTM. replacement in the same 50/50 
KCS.RTM./Alphaplate.RTM. pigmented coating formula. Deltagloss is 
described in U.S. Pat. No. 4,818,294. Alphatex.RTM. is described in U.S. 
Pat. No. 4,381,948. Alphaplate.RTM. is a delaminated kaolin product of 
ECCA, having an average particle size of about 0.5 .mu.m. KCS.RTM. is a 
No. 2 coating clay. The evaluation data is listed in Table 4. It can be 
seen from the data that the product of this invention outperformed the 
other pigment systems in sheet brightness, sheet gloss, dry/litho 
printgloss, and smoothness..sup.3 
FNT .sup.3 Hollingsworth, Jones, Bonny, The Effect of Calcined clays on the 
Printability of Coated Rotogravure and Offset Printing Papers, Tappi 
Proceedings, page 9-16, 1983 Coating Conference. 
TABLE 4 
__________________________________________________________________________ 
Parts by Weight 
30 ALPHAPLATE .RTM. 
45 ALPHAPLATE .RTM. 
30 ALPHAPLATE .RTM. 
30 KCS .RTM. 
50 ALPHAPLATE .RTM. 
45 KCS .RTM. 
30 KCS .RTM. 
40 Product of 
Sample: 50 KCS .RTM. 
10 ALPHATEX .RTM. 
40 DELTAGLOSS .RTM. 
Invention 
__________________________________________________________________________ 
Brightness, ISO 
66.3 67.3 66.8 68.0 
Opacity, Printing 
93.1 93.4 93.3 93.7 
Gloss 56.0 56.5 60.7 62.5 
Hunter L 85.14 85.55 85.36 85.69 
a 0.27 0.28 0.21 0.28 
b 5.73 5.32 5.57 4.94 
Print Gloss 
74.0 74.3 75.0 77.6 
Litho Print Gloss 
71.6 73.0 72.6 75.9 
Delta Print Gloss 
18.0 17.8 14.3 15.1 
Print Density 
1.51 1.47 1.47 1.49 
Litho Print Density 
1.48 1.46 1.44 1.48 
Print Density Ratio 
0.98 0.99 0.98 0.99 
Dry Pick, VVP 
28.4 24.3 25.2 25.8 
Parker Print Surf: 
1.02 0.97 0.94 0.92 
Roughness, Microns 
Backing S10 
Paperscape 
4.96 5.04 5.17 3.85 
au2 (128) 
Bulk 0.65 0.62 0.72 0.84 
__________________________________________________________________________ 
While the present invention has been particularly set forth in terms of 
specific embodiments thereof it will understood in view of the instant 
disclosure that numerous variations upon the invention are now enabled to 
those skil in the art, which variations yet reside within the scope of the 
present teaching. Accordingly, the invention is to be broadly construed 
and limited only by the scope and spirit the claims now appended hereto.