Silica-coated alkaline earth metal carbonate pigment

A method for forming a silica coating on at least a portion of the surface of alkaline earth metal carbonate particulates in aqueous suspension and the silica-coated alkaline earth metal carbonate so produced are claimed. The claimed method consists of allowing soluble silicate ions in the aqueous phase to slowly react with the alkaline earth metal carbonate surface to form silica micro-particulates on the surface. This silica coating confers upon the composition unique properties among which are pronounced thixotropic behavior when incorporated into an aqueous suspension and increased oil absorption when dry. The pigment produced by this method is especially suited for use as a filler in paper.

The present invention relates to a process for the preparation of alkaline 
earth metal carbonate particles, most particularly calcium carbonate 
particles, possessing at least a partial surface coating of silica or 
insoluble silicate micro-particulates. This at least partial coating 
substantially changes the rheological characteristics of an aqueous 
suspension of the particles, decreases the tapped bulk density of the 
particles in dry powder form, and increases the oil absorption of the 
particles most probably by creating a rougher particle surface containing 
voids between the silica micro-particulates on the surface. The thixotropy 
demonstrated by an aqueous suspension of the product of the present 
invention increases the utility of the material as a protective coating 
ingredient, that is, an ingredient of paints, or as a component of 
consumer cleaning products, such as the low-abrasion bathtub cleaners 
which utilize calcium carbonate as a significant component and which also 
contain organic polymers or other additives to make the composition highly 
thixotropic. As used herein, the term "pigment" is defined as a 
finely-divided particulate solid which may be employed to impart color, 
opacity, or volume to a filled system. The increased oil absorption of the 
product of the present invention increases its utility as a pigment 
incorporated into printing and writing papers which are designed to have 
specific ink absorption characteristics; the increased oil absorption of 
the pigment is a direct indication that it will impart improved ink 
absorption characteristics to the paper into which it is incorporated. 
BACKGROUND OF THE INVENTION 
This invention relates to a method for preparing alkaline earth metal 
carbonates, particularly calcium carbonates, with improved properties for 
use as fillers or pigments in paper, paints, plastics, and household 
cleaners. Most particularly, this invention relates to improving the 
properties of natural ground calcium carbonate suspensions for use as 
fillers in paper and paints. 
Many paper mills have converted to neutral or alkaline papermaking in the 
past two decades because this has offered them a number of advantages; 
among these is the opportunity to reduce pigment costs by replacing some 
of the expensive titanium dioxide used as a filler in papermaking with 
less expensive calcium carbonate pigment. The term "filler" is employed to 
refer to a pigment which is admixed with wood pulp in an aqueous 
suspension immediately prior to the dewatering of the pulp and pigment 
admixture to form a sheet of paper. Calcium carbonate, particularly 
precipitated calcium carbonate (PCC), is being used as a filler pigment in 
the making of paper; natural ground calcium carbonate (GCC) is being used 
in North America predominantly as a coating pigment while it is also used 
as a paper filler pigment in Europe. Both types of calcium carbonate 
pigments lack the physical properties which would allow papermakers to 
completely eliminate the use of more expensive pigments such as titanium 
dioxide or synthetic amorphous silicas and silicates. The GCC products are 
generally considered to be inferior to scalenohedral PCC for use as 
fillers for paper because their generally spherical particle morphologies 
do not scatter light as efficiently as the rosette-shaped PCC particles 
which contain many microvoids. 
The spherical GCC particles pack together very efficiently and can be 
dispersed to form a fluid suspension at relatively high solids 
concentrations. GCC is sold to the paper industry predominantly as an 
anionically-dispersed 75% to 76% solids suspension of fine particles 
because this form is free of dust, can be pumped, requires a small storage 
volume compared to other forms, and is less energy-intensive to produce 
and distribute than other forms. 
PCC is generally provided to the paper mill as a dry powder or as a 20% to 
30% aqueous suspension from a "satellite" PCC production facility located 
immediately adjacent to the paper mill. 
Economic benefits would be realized by the paper manufacturer if 
scalenohedral PCC could be inexpensively modified to allow it to be used 
as a greater proportion of the overall filler pigment admixture within 
speciality papers. Economic and environmental benefits would be realized 
if GCC could be inexpensively modified to make it an acceptable 
paper-filling pigment when used at high filler levels in printing and 
writing papers; an evaluation conducted as a result of EC Directive 
1836/93, the EC Eco-Audit Directive, found GCC to be less energy intensive 
in its production and distribution to the paper mill than PCC. 
Improvement in the specific pigment physical properties of ink absorption, 
opacity, and first pass retention (retention of the pigment in the paper 
sheet when it is formed from the pulp suspension) would allow papermakers 
to use a greater proportion of calcium carbonate filler in paper thus 
reducing their use of other, more expensive fillers. 
In addition to prior art concerning the production of both GCC and PCC 
pigments, several patents teach modification of the surface of calcium 
carbonate particles to make them more acceptable for use as filler 
pigments under weakly acidic papermaking conditions. 
A variety of techniques to modify calcium carbonate to achieve a degree of 
acid resistance have been patented. U.S. Pat. No. 5,037,477 discloses 
precipitating a coating on calcium carbonate particles by simultaneously 
adding to an aqueous suspension of calcium carbonate particles, at an 
elevated temperature, a solution of a water-soluble silicate and a 
solution of a water soluble zinc compound so as to precipitate a coating 
onto the particle surfaces very rapidly; U.S. Pat. No. 5,043,017 discloses 
adding a calcium chelating agent or a conjugate base and a weak acid into 
an aqueous suspension of calcium carbonate particles such that the calcium 
carbonate particles are coated by and in equilibrium with the additives; 
U.S. Pat. No. 5,531,821; 5,584,923; 5,593,488; 5,593,489; 5,599,388; and 
5,647,902 also disclose processes for rendering calcium carbonate 
particles more acid-resistant but do not employ silica. 
U.S. Pat. No. 5,514,212 teaches the use of a calcium carbonate pigment 
which has a starch-soap complex precipitated onto the particle surface so 
as to improve the efficiency of sizing agents added to the wood pulp and 
pigment admixture during the papermaking process. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a process for the 
preparation of a ground calcium carbonate (GCC) which is especially 
suitable for use as a filler in the papermaking process. 
It is a further object of the present invention to provide a process for 
the preparation of an alkaline earth metal carbonate composition which 
demonstrates substantial thixotropy when incorporated into an aqueous 
suspension for use as a paint or household cleaning composition. 
A still further object of the present invention is to provide a process for 
the preparation of a precipitated calcium carbonate (PCC) with enhanced 
properties for use as a filler in the papermaking process. 
A still further object of the present invention is to provide a process for 
the improvement of commercial GCC slurry products for paper filling 
applications which process can be practiced at the location where the 
paper is manufactured and subsequent to the transportation of an aqueous 
suspension of fine calcium carbonate particles containing about 75% to 76% 
solids by weight from its place of manufacture to the paper manufacturing 
facility so as to maintain the economic and ecological advantages of these 
GCC slurry products while increasing their utility. 
Yet another object of the present invention is to provide a process for the 
improvement of the suitability of commercial PCC products produced at 
"satellite" facilities for paper filling applications such that the 
process can be practiced after the PCC product has been supplied to the 
paper mill. 
SUMMARY OF THE INVENTION 
A method is disclosed for preparing an alkaline earth metal carbonate 
composition consisting of alkaline earth metal carbonate particles with an 
average particle size between about 0.8 microns and about 20 microns and 
having at least a portion of the surface of said alkaline earth metal 
carbonate particles covered by a silica species which has been 
precipitated thereon so as to substantially alter the physical properties 
of the particles so as to make them more suitable for use in paints, 
cleaning compositions, and as a filler pigment in paper. Surprisingly, it 
has been discovered that an insoluble silicate species will be 
precipitated onto the surface of alkaline earth metal carbonate particles 
slowly over a period of about 24 hours at ambient temperature from a 
concentrated aqueous suspension of insoluble alkaline earth metal 
carbonate particles admixed with soluble alkali metal silicate; this is 
accomplished without the addition of any foreign cations or acid to the 
admixture. The present invention yields an alkaline earth metal carbonate 
composition which has a lower tapped bulk density and an increased oil 
absorption capability, compared to the alkaline earth metal carbonate 
powder prior to the practice of the present invention. When a 
commercially-available sodium silicate solution, such as CHEM-SILATE 41A 
produced by Chemical Products Corporation, is admixed with an aqueous 
suspension of alkaline earth metal carbonate particles at ambient 
temperature, an increase in the low-shear viscosity of the suspension 
becomes apparent beginning about 30 minutes after the addition of the 
soluble silicate solution and becomes more pronounced with the passage of 
time with marked thixotropy developing to such an extent that the 
suspension may assume the appearance of a solid under conditions of zero 
shear depending upon the amount of water present in the admixture. The 
development of this pronounced thixotropic behavior of the aqueous 
suspension is highly desirable for the incorporation of this composition 
into paints and household cleaning products. This marked thixotropic 
rheological behavior is believed to be the result of a high degree of 
interparticle attraction resulting from hydrogen bonding between silica 
particles attached to the surfaces of different alkaline earth metal 
carbonate particles. Hydrogen bonding capability can be employed to aid in 
the retention of these particles in a paper sheet during the papermaking 
process through the formation of hydrogen bonds between the particles and 
wood pulp fibers. Commercially-available anionically-dispersed natural 
ground calcium carbonate suspensions, typically used by the paper industry 
as paper coating pigments, are improved through the practice of the 
present invention such that they can be employed as pigments for paper 
filling because of their increased retention in the paper sheet when it is 
formed on the paper machine. 
DETAILED DESCRIPTION OF THE INVENTION 
The improved form of alkaline earth metal carbonate prepared by the present 
invention is composed of alkaline earth metal carbonate particles at least 
partially covered by a silica or silicate surface coating formed over a 
period of at least about 30 minutes and preferably at least about 4 hours 
after a water-soluble silicate solution is added with stirring to a 
concentrated aqueous suspension of alkaline earth metal carbonate 
particles at ambient temperature. While not wishing to be bound by any 
particular theory as to the operability of the present invention, it is 
believed that the increased oil absorption conferred upon the alkaline 
earth metal carbonate compositions of the present invention can be 
explained as the result of the chemisorption of soluble silica species 
onto the carbonate surface followed by polymerization of the silica 
present in the solution to form nanometer-sized silica particles attached 
to the alkaline earth metal carbonate surface which may grow further over 
time depending upon the amount of silicate in solution. In many 
applications the presence of soluble silicate is undesirable, so 
subsequent to the growth of silica particles on the surface of alkaline 
earth metal carbonate particles, any remaining soluble silicate would 
preferably be precipitated by any of the prior art methods well known to 
those skilled in the art. The hydrogen bonding that occurs between silica 
particles results in the phenomenon of three dimensional silica gel 
networks; this gel formation occurs only between silica particles and not 
water-soluble silicate species, therefore the onset of thixotropy during 
the practice of the present invention represents the appearance of silica 
particles on the surface of the alkaline earth metal carbonate particles 
due to precipitation or polymerization of the water-soluble silicate 
species into silica particles. This precipitation or polymerization is 
unexpected because it occurs without the addition of any foreign cations 
or acid species, and is believed to result in a surface coating of highly 
structured silica particles because it proceeds slowly over a period of at 
least many minutes and preferably over a period of many hours. 
It is believed that under the circumstances existing in the practice of the 
present invention, the distinction becomes blurred between silica, a 
compound composed solely of the elements silicon and oxygen, and silicate, 
an ionic compound containing an anionic component which is composed of 
silicon and oxygen. The term "silica" as used herein is meant to encompass 
both when describing the solid phase on the surface of alkaline earth 
metal particles. 
U.S. Pat. No. 5,037,477, discussed previously, teaches in Examples 3 and 6 
that the acid resistance of the claimed calcium carbonate product is 
reduced if the soluble silicate is added prior to a foreign cation which 
precipitates the silicate; it is believed that the present invention 
yields a silica-coated surface which is not uniform as required for 
optimum acid resistance, but rather is microscopically rough because of 
the formation of silica particulates around chemisorbed silicate species 
on the surface. Chapter 12, "High-Structure Amorphous Silica Pigments in 
Paper", in Pigments for Paper, TAPPI Press, 1997, presents the 
polymerization behavior of silica; it states, "High-structure amorphous 
silicas have unique structures, and there still exists a degree of art in 
synthesizing the various structures that are commercially available." It 
is believed that the means of synthesizing yet another unique silica 
structure is herein disclosed. 
In a preferred embodiment of the present invention, an 
anionically-dispersed GCC slurry containing 76% calcium carbonate and 24% 
water is agitated at ambient temperature while a solution of an alkali 
metal silicate is added such that 0.5% to 20%, preferably 3% to 10%, 
silica is added based on the dry weight of the calcium carbonate. At least 
24 hours after admixing the GCC slurry and the alkali metal silicate, any 
remaining water-soluble silicate may be removed from solution by either 
washing of the particulates or, more preferably, by addition of a foreign 
cation or an acid to precipitate said silicate. 
The invention will be further illustrated by the following examples which 
are to be considered illustrative but not limiting in their presentation 
of precise embodiments of the present invention.

EXAMPLE 1 
Demonstration of the Permanence of the Silica Coating Formed by the 
Practice of the Present Invention 
A sample of HYDROCARB.RTM. 90 (H90) anionically-dispersed natural ground 
calcium carbonate aqueous suspension containing 76% calcium carbonate by 
weight, produced by Omya Incorporated, was admixed with 10% by weight 
CHEM-SILATE.RTM. 42A sodium silicate solution produced by Chemical 
Products Corporation, based on the weight of the HYDROCARB.RTM. 
suspension, this is 3.8% silica based on the dry weight of calcium 
carbonate. The admixture was diluted with 10% water based on the weight of 
the H90 suspension after one hour, and with 20% water after 24 hours, to 
maintain a workable consistency; the objective was to maintain the 
fluidity of the suspension under conditions of moderate shear. No foreign 
ions were added to the admixture. After 76 hours the aforesaid admixture 
was diluted with water to form a very dilute suspension containing less 
than 1% suspended solids. After 2 days equilibration time, the elemental 
composition of the material on and immediately under the surface of the 
particles was determined by energy dispersive spectroscopy. The aforesaid 
admixture which is the product of the present invention was found to 
contain 8.5% silica by weight in the material composing the surface layer 
on the particles, whereas the initial H90 sample was analyzed by the same 
means and found to have no detectable silica in its surface layer. 
EXAMPLE 2 
Oil Absorption Determination by the Spatula Rub-Out Method 
Samples of HYDROCARB.RTM. 90 (H90) anionically-dispersed calcium carbonate 
slurry produced by Omya Incorporated and CARBITAL.RTM. 90 (C90) 
anionically-dispersed calcium carbonate slurry produced by ECC Americas, 
Inc. were each admixed with 9% by weight CHEM-SILATE.RTM. 41A (41A) sodium 
silicate solution produced by Chemical Products Corporation, based on the 
weights of the calcium carbonate slurries, according to the practice of 
the present invention; this is about 3.5% by weight of silica based on the 
dry weight of the calcium carbonate in each case. After the admixtures 
were allowed to react for about 50 hours, each sample was further admixed 
with a dilute magnesium sulfate solution such that about 1.5% magnesium 
was added by weight based on the weights of the initial calcium carbonate 
slurries; this was done to insure that no soluble silicate remained in the 
aqueous phase which might act to cement the calcium carbonate particles 
together during drying. The products of the present invention as well as 
samples of the initial calcium carbonate slurries were then oven dried and 
the dried samples were crushed to fine powders in a mortar and pestle. 
Spatula rub-out oil absorption measurements on the four samples using 
linseed oil were as follows: 
HYDROCARB.RTM. 90 39 cc/100 grams 
Silica-coated HYDROCARB.RTM. 90 56 cc/100 grams 
CARBITAL.RTM. 90 33 cc/100 grams 
Silica-coated CARBITAL.RTM. 90 45 cc/100 grams 
If the increased oil absorption were attributed solely to internal void 
volume or porosity of the silica coating on the calcium carbonate particle 
surfaces, that silica would be exhibiting an oil absorption of about 500 
cc/100 grams of silica and about 350 cc/100 grams of silica, respectively, 
in the two above-described cases. "Synthetic Amorphous Silicas and 
Silicates in the Paper Industry" in the 1995 TAPPI Dyes, Fillers, and 
Pigments Short Course--Course Notes (TAPPI Press, 1995) reports that oil 
absorption values for highly structured commercial precipitated silica and 
silicate products are on the order of about 200 cc/100 grams of silica. It 
is speculated that the remarkable increase in the oil absorption of the 
products of the present invention can be attributed to areas between fixed 
silica micro-particulates on the surface of the calcium carbonate 
particles acting as voids which can contain oil. 
EXAMPLE 3 
Tapped Bulk Density Determinations 
The four powder samples described in Example 2 were each weighed and placed 
in graduated cylinders which were each dropped from a height of one inch 
onto their bases 100 times to compact the powder within; after this 
compaction by "tapping" the volume occupied by the powder was recorded. 
The tapped bulk densities of the four samples were calculated to be: 
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HYDROCARB .RTM. 90 1.09 g/cc 
Silica-coated HYDROCARB .RTM. 90 
0.828 g/cc 
CARBITAL .RTM. 90 0.981 g/cc 
Silica-coated CARBITAL .RTM. 90 
0.791 g/cc 
______________________________________ 
The products of the present invention are "bulkier" than the calcium 
carbonate powders from which they were formed. The volume occupied by one 
gram of each powder is calculated as the inverse of the tapped bulk 
density; the volume occupied by the same weight of powder is increased by 
31.5% and 23.5%, respectively, in the above two cases. This "bulkiness" 
would be expected to enhance a filler's performance in a paper sheet. 
EXAMPLE 4 
Retention of Fillers in Laboratory-Prepared Paper Sheets 
Admixtures were prepared as described in Example 2 from H90 and C90. In 
addition, a sample of H90 was admixed with 25% CHEM-SILATE.RTM. 41A sodium 
silicate solution based on the weight of the H90 suspension; after about 
50 hours reaction time and dilution with water to maintain fluidity, 
carbon dioxide gas was bubbled into the admixture with stirring until the 
pH of the admixture dropped to about 9 as a means of removing all 
remaining silicate ions from solution. All samples were submitted as 
aqueous suspensions to a specialized paper testing laboratory. 
Five samples were evaluated as paper fillers by preparing paper handsheets 
in the laboratory incorporating each sample separately as the filler in 
the paper; each sample was added to the furnish at the 15% and 20% 
loadings based on the dry weight of the pulp. The ash contents of the 
resulting handsheets were measured to determine how much of the filler 
remained in the paper with the wood pulp. The products of the present 
invention were retained in the sheets to a greater extent than the calcium 
carbonate slurries from which they were prepared when hadnsheets were 
prepared under identical conditions in the laboratory. The following ash 
contents were reported: 
Percent ash based on the weight of the dry paper 
______________________________________ 
15% added filler 
20% added filler 
______________________________________ 
H90 2.11 3.43 
H90 + 9% 41A 4.83 7.67 
H90 + 25% 41A 5.68 9.06 
C90 3.12 4.24 
C90 + 9% 41A 5.84 6.19 
______________________________________ 
EXAMPLE 5 
TAPPI Opacity (89% Reflectance) of the Laboratory-Prepared Papers 
Papers containing C90 and the product of the present invention prepared 
from C90 as described in Example 4 were tested according to Trade 
Association of the Pulp and Paper Industry (TAPPI) Test Method T-425 to 
determine their opaqueness. Five paper samples were prepared and tested 
for each filler sample with the filler loading at 15% by weight dry filler 
based on the dry weight of the pulp. The average opacities and standard 
deviations over the five paper samples tested for each filler sample were: 
______________________________________ 
Average opacity 
Standard deviation 
______________________________________ 
C90 78.7 0.32 
C90 + 9% 41A 81.8 0.27 
______________________________________ 
This is a significant improvement in the opacity of the paper sheets. 
EXAMPLE 6 
Brookfield Heliopath Viscosity Measurements 
To demonstrate the unique and surprising thixotropic behavior resulting 
from the practice of the present invention, two admixtures were prepared 
from a sample of H90 calcium carbonate suspension. While the two 
admixtures were prepared utilizing exactly the same ingredients in exactly 
the same amounts, Admixture A is the product of the present invention and 
Admixture B is intended to be a simple mixture of silica gel and calcium 
carbonate particles. 
Admixture A was prepared by admixing 408 grams of H90 slurry and 38 grams 
of CHEM-SILATE.RTM. 41A sodium silicate solution at ambient temperature, 
adding 41 grams of water with stirring after about one hour as the 
viscosity of the admixture began to increase significantly, and finally 
adding 6.5 grams of anhydrous magnesium sulfate dissolved in 144 grams of 
water with stirring after 26 hours. Admixture B was prepared by admixing 
38 grams of CHEM-SILATE.RTM. 41A sodium silicate solution with 6.5 grams 
of anhydrous magnesium sulfate dissolved in 185 grams of water, and 
finally, after about 26 hours, adding 408 grams of H90 slurry with 
stirring to the semi-solid, gelatinous silica suspension which had formed. 
The low-shear viscosities of these admixtures were measured utilizing a 
Brookfield viscometer equipped with a T-bar spindle and a Heliopath 
attachment after the compositions had been allowed to stand without being 
stirred for 5 minutes. The viscosities were as follows: 
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Admixture A 0.5 rpm 139,600 
centipoise 
2.5 rpm 30,800 
centipoise 
10 rpm 6600 centipoise 
Admixture B 0.5 rpm 53,200 
centipoise 
2.5 rpm 15,200 
centipoise 
10 rpm 4880 centipoise 
______________________________________ 
After the abovesaid admixtures had been allowed to stand for 30 minutes 
without being stirred, the viscosity of Admixture A was measured at 0.5 
rpm to be 230,000 centipoise and the viscosity of Admixture B was measured 
at 0.5 rpm to be 58,000 centipoise. 
The HYDROCARB.RTM. 90 sample used to prepare the abovesaid admixtures was 
measured in the same manner as above and found to have viscosities of 2000 
centipoise, 1120 centipoise, and 800 centipoise at 0.5 rpm, 2.5 rpm, and 
10 rpm, respectively. 
The foregoing description and examples have been set forth merely to 
illustrate the invention and are not intended to be limiting. Since 
modifications of the described embodiments incorporating the spirit and 
substance of the invention may occur to persons skilled in the art, the 
invention should be construed broadly to include all variations falling 
within the scope of the appended claims and equivalents thereof.