Stabilized calcium carbonate composition using sodium silicate and one or more weak acids or alum and uses therefor

An improved form of calcium carbonate which is acid resistant to enable its use as a filler material in the making of neutral to weakly acidic paper, and a process for producing this acid-resistant calcium carbonate is provided. This acid-resistant calcium carbonate composition comprises calcium carbonate and at least about 0.1% to about 25% based on the dry weight of the calcium carbonate, of sodium silicate, together with at least 0.1% to about 25 percent, based on the dry weight of the calcium carbonate, of a weak acid, such as phosphoric acid, formic acid, fluoroboric acid, polyacrylic acid, or alum, or a mixture of weak acids, such as phosphoric acid and polyacrylic acid.

BACKGROUND OF THE INVENTION 
This invention relates generally to calcium carbonate for use in 
papermaking, and related industries, and more particularly to a calcium 
carbonate having acid resistant properties. 
Titanium dioxide and calcined clay have traditionally been utilized as 
filler materials in the preparation of alkaline to weakly acidic paper in 
order to improve the optical properties, especially the brightness, of the 
resultant product. These materials, however, especially titanium dioxide, 
have the disadvantage of being very expensive, resulting in higher 
manufacturing costs and an uncompetitively priced paper product. 
Calcium carbonate, particularly precipitated calcium carbonate, has been 
used as a filler material in the making of alkaline paper. Such usage 
results in a paper with enhanced optical properties, without the expense 
incurred in using titanium dioxide fillers, resulting in a much less 
expensive product. Calcium carbonate, however, cannot generally be used as 
a filler in acidic paper because it has low acid-resistance, causing it to 
decompose in an acidic environment. Consequently, there has long been a 
need to develop a calcium carbonate composition which is acid stabilized 
and resistant to decomposition at low/neutral pH, so that it can be 
utilized as a filler material in the manufacture of acidic paper, such as 
groundwood paper, where the use of an alkaline filler would have a 
negative impact on the final paper properties, and where the pH of the 
process waters tends to increase, thereby increasing its foaming action. 
Paper made from mechanical pulps has been traditionally produced under 
acidic papermaking conditions because of "fiber alkaline darkening" that 
occurs as pH rises. This means that there is a reduction in brightness of 
the paper (brightness reversion) when the pH is raised from acid to 
alkaline in wood-containing systems. Alkaline darkening will occur to some 
degree in any wood pulps with significant lignin content. The degree of 
darkening generally depends factors, such as the particular pulps, the pH, 
and the water quality. In general, ground calcium carbonate and 
precipitated calcium carbonate fillers serve as a buffer in the 7.5-9.2 pH 
range when used in the wet end, which is that portion of the paper machine 
which includes the headbox, wire part, and first press section. 
Acid-resistant calcium carbonate compositions thus provide a means for 
reducing the degree of fiber alkaline darkening and brightness reversion 
due to their ability to maintain a stabilized pH. 
A variety of techniques to modify calcium carbonate to achieve acid 
resistance and to avoid the aforementioned problems are disclosed in the 
art. For instance, U.S. Pat. No. 5,043,017 discloses and claims an 
acid-stable calcium carbonate resistant to degradation in a mildly acidic 
environment which comprises a mixture of a calcium-chelating agent or a 
conjugate base, and a weak acid, such that calcium carbonate is coated by, 
and is in equilibrium with, the calcium-chelating agent or conjugate base 
and the weak acid. Preferred calcium carbonate compositions contain sodium 
hexametaphosphate and phosphoric acid. A disadvantage of this technique is 
that some mills are regulated on the amount of phosphates that can be in 
their effluents, and therefore, can not afford to have extra phosphates 
being introduced into their system. 
U.S. Pat. No. 5,000,791 discloses the preparation of an acid-resistant 
coating for calcium carbonate particles. This acid-resistant calcium 
carbonate is prepared by simultaneously mixing the calcium carbonate with 
a solution of a zinc compound and a solution of a silica-containing 
substance which is preferably sodium water glass. The zinc compound, 
preferably, is zinc chloride or zinc oxide. The addition of the zinc 
compound and the silica-containing substance is in combination with a 
strong acid. A disadvantage of this technique is that it uses zinc, which 
generally is an undesirable metal to have in the whitewaters, product, or 
effluents, since it does not meet EPA standards. 
U.S. Pat. No. 5,164,006 discloses and claims an acid resistant calcium 
carbonate which is prepared by preparing an aqueous slurry of calcium 
carbonate, heating the slurry to about 75-80 degree Celsius, slowly adding 
sodium silicate solution in an about 5-10% by weight, adding gaseous 
carbon dioxide, cooling the slurry and adding zinc chloride to the slurry 
to bring the pH to a range of 7.5 to 8.0. This technique has the same 
disadvantage as the aforesaid U.S. Pat. No. 5,000,791 since zinc in the 
whitewaters, product, or effluents does not meet EPA standards. 
Other techniques to surface treat or coat calcium carbonate to achieve 
higher acid-resistance are disclosed in U.S. Pat. Nos. 5,531,821; 
5,593,488; 5,593,489; 5,599,388; and 5,647,902 and in U.S. patent 
application 08/546,145 owned by the same assignee as this present 
application. 
SUMMARY OF THE INVENTION 
The present invention relates to improved calcium carbonate compositions 
which are stabilized relative to acid environments and, which are 
therefore, acid resistant. These compositions are useful as a filler 
material in the making of neutral to weakly acid paper. The instant 
invention is also directed to a process for producing this acid resistant 
calcium carbonate. 
More particularly, this invention is directed to an acid resistant calcium 
carbonate composition comprising: a) calcium carbonate; b) at least about 
0.1 weight percent based on the dry weight of the calcium carbonate, of a 
suitable silicate; and c) at least about 0.1 weight percent, based on the 
dry weight of the calcium carbonate, of i) at least one weak acid, on an 
active basis; or ii) alum, on an active basis. It has surprisingly been 
found that the inclusion of a silicate and either at least one weak acid 
or alum confers a higher degree of stability and acid resistance for 
calcium carbonate in the presence of fiber slurry, and a longer term of pH 
stability, than known acid-stabilized calcium carbonate compositions. 
It is an object of the present invention to provide a stabilized and acid 
resistant calcium carbonate composition especially suitable for use in 
acid papermaking applications. 
It is a further object of the present invention to provide a process for 
the preparation of the aforesaid calcium carbonate compositions. 
A still further object of the present invention is to provide an improved 
paper product having enhanced optical qualities prepared using the calcium 
carbonate compositions of the present invention. 
A still further object of the present invention is to provide improved 
calcium carbonate compositions used as a filler in wood containing 
papermaking systems.

DETAILED DESCRIPTION OF THE INVENTION 
The improved calcium carbonate compositions of the instant invention are 
stabilized relative to acidic environments. This acid resistance or 
tolerance enables their use as filler materials in the making of neutral 
to weakly acid paper, rubber, and plastics, but for purposes of 
illustration will be discussed herein with reference as a filler material 
in the making of neutral to weakly acid paper. While not wishing to be 
bound by any particular theory as to the operability of the present 
invention, it is believed that the acid resistance of the improved calcium 
carbonate compositions of the present invention is a result of the 
inactivation of the surface of the calcium carbonate by the addition of a 
silicate. i.e. sodium silicate, in combination with at least one weak 
acid, e.g. polymeric acid, phosphoric acid, formic acid, or fluoroboric 
acid, or alum. 
The instant invention is directed to an acid resistant calcium carbonate 
composition, comprising: 
a) calcium carbonate; 
b) at least about 0.1%, based on the dry weight of said calcium carbonate, 
of a silicate, preferably sodium silicate; and 
c) at least about 0.1%, based on the dry weight of calcium carbonate, of: 
i) at least one weak acid, on an active basis, or 
ii) alum, on an active basis. 
The instant invention is further directed to a method for preparing an acid 
resistant calcium carbonate composition, comprising: 
a) adding to a calcium carbonate composition at least about 0.1%, based on 
the dry weight of calcium carbonate in said composition, of a silicate, 
preferably sodium silicate; 
b) adding to said calcium carbonate composition, at least about 0.1%, based 
on the dry weight of the calcium carbonate, of: 
i) at least one weak acid, on an active basis, or 
ii) alum, on an active basis. 
The instant invention is further directed to improved paper products 
containing an effective amount of the instant acid resistant calcium 
carbonate composition and to a method for preparing the same comprising 
adding to a papermaking stream an effective amount of an acid resistant 
calcium carbonate composition, comprising: 
a) calcium carbonate; 
b) at least about 0.1%, based on the dry weight of said calcium carbonate, 
of a silicate, preferably, sodium silicate; and 
c) at least about 0.1%, based on the dry weight of said calcium carbonate, 
of: 
i) at least one weak acid, on an active basis, or 
ii) alum, on an active basis. 
As used herein, the term "calcium carbonate" refers to a ground calcium 
carbonate (GCC), which is a marble which has been crushed and ground to 
between 30 and 100% finer than 2 microns, or to a precipitated calcium 
carbonate (PCC), which is made by bubbling CO.sub.2 through a lime slurry. 
As used herein, the term "silicate" refers to any suitable silicate, which 
is water soluble, and broadly defined as a salt derived from silica or the 
silicic acids. Alkali silicates are preferred, with the most preferred 
silicate being sodium silicate. 
As used herein, the term "weak acid" refers to acids that are not 100% 
ionized in in a given solvent, preferably, water. 
As used herein, the term "effective amount" refers to that quantity of the 
instant acid resistant calcium carbonate composition necessary to provide 
sufficient acid resistance to the paper stream, slurry, or product being 
treated. Generally, at least about 0.1 ppm of such composition are added 
to the paper stream, slurry, or product being treated with preferred 
dosages ranging from about 0.1% to about 30% based on the fiber weight of 
the groundwood. 
In the practice of the present invention, the calcium carbonate 
compositions are rendered acid resistant by the addition of at least about 
0.1 percent, based on the dry weight of the calcium carbonate, of a 
suitable silicate together with at least about 0.1%, based on the dry 
weight of the calcium carbonate, of at least one weak acid or a mixture of 
two or more weak acids, on an active basis, or alum, on an active basis. 
Preferred weak acids are selected from the group consisting of organic 
acids containing one or more carboxyl radicals. More preferred are 
polymeric weak acids, such as polymeric acids prepared from 
ethylenetically unsaturated carboxylic monomers, such as acrylic acid, 
methacrylic acid, fumaric acid, and maleic acid. These polymers preferably 
have weight average molecular weights of less than about 1,000,000, and 
preferably less than 50,000, as determined by light scattering techniques. 
Other weak acids that are preferred are selected from the group consisting 
of phosphoric acid, metaphosphoric acid, hexametaphosphoric acid, 
ethylenediaminetetraacetic acid (EDTA), sulfurous acid, acetic acid, boric 
acid, gallic acid, glutanic acid, benzoic acid, oxybenzoic acid, 
salicyclic acid, stearic acid, citic acid, formic acid, or fluoroboric 
acid. More preferred from this group of weak acids is phosphoric acid. 
Mixtures of such acids can also be used. If only one weak acid is used, it 
is most preferably, selected from the group consisting of fluoroboric 
acid, formic acid, or polyacrylic acid. Alternately, alum, which is 
aluminum sulphate (Al.sub.2 (SO.sub.4).sub.3. 18H.sub.2 O) can be used 
instead of the weak acid(s) in conjunction with the silicate for the 
surface treatment of the calcium carbonate. The preferred range for the 
sodium silicate and the weak acid or mixture of weak acids, or alum is 
from about 0.1% to about 25%, based on the dry weight of the calcium 
carbonate. Most preferably, the range for the sodium silicate is about 
1.0% to about 5.0%. 
While not wishing to be bound by any theory, it is believed that the 
capability of the acid-stabilized calcium carbonate of the present 
invention to resist dissociation in an acidic environment is due to the 
formation of ionic bonds between the calcium and the silicate. This 
mechanism of ionic bonding is distinct from the reaction of the prior art 
chelating agent or silica-containing substance on the surface of calcium 
carbonate. Ionic bonding can provide an insoluble calcium silicate surface 
which reduces the dissolution reaction of calcium carbonate; whereas, a 
chelating agent or silica-containing substance acts as a coordinating 
compound in which a single ligand occupies more than one coordinating 
position or precipitated silica on the surface. 
As indicated above, the preferred silicate is sodium silicate. Sodium 
silicate utilized in the compositions of the present invention is 
commercially available in forms suitable for direct inclusion into the 
calcium carbonate mixture. The amount of the sodium silicate utilized is 
at least 0.1%, based on the dry weight of the calcium carbonate, and is 
preferably about 0.1% to about 25%, based on the dry weight of calcium 
carbonate. 
Preferred combinations of sodium silicate and weak acids for use in the 
present invention include sodium silicate/polyacrylic acid/phosphoric 
acid. Preferred combinations of sodium silicate and a weak acid for use in 
the present invention include sodium silicate/phosphoric acid, sodium 
silicate/formic acid, and sodium silicate/fluorobic acid. A further 
preferred combination includes sodium silicate/alum. 
The calcium carbonate utilized is preferably finely divided and it can be 
either a precipitated calcium carbonate or a natural ground limestone. 
As an example exemplifying the best mode, the process for producing an acid 
resistant calcium carbonate involves first forming a mixture of calcium 
carbonate with at least about 0.1%, based on the dry weight of the calcium 
carbonate, of the sodium silicate. Then, at least about 0.1%, based on the 
dry weight of the calcium carbonate, of a weak polymeric acid, such as a 
polyacrylic acid having a molecular weight of less than about 50,000 is 
added to this resultant mixture. Finally, the resultant mixture is blended 
for a sufficiently long period of time to ensure uniform mixing of the 
ingredients. 
The calcium carbonate can be utilized in the above-described process either 
as a dry powder or an aqueous slurry with up to about 70% by weight solids 
content. 
The silicate can be utilized in the instant process either as a dry solid 
or as an aqueous solution. When the calcium carbonate is used in dry 
powder form, it is preferable to utilize an aqueous solution of the sodium 
silicate in order to facilitate homogeneous mixing. Where a slurry of the 
calcium carbonate is utilized, the solid form of the sodium silicate 
readily dissolves therein so that an aqueous solution is unnecessary. 
The acids or alum can be utilized in the process of preparation in either a 
concentrated form or a diluted aqueous solution. 
In further preferred embodiments of the instant process, sodium silicate is 
first added to a calcium carbonate slurry followed by addition of a weak 
acid, such as a polymeric or a phosphoric acid, and then finally, the 
second acid, if two weak acids are utilized. If fluoroboric acid, or 
formic acid, or alum, or only one weak acid is utilized, then preferably, 
the sodium silicate is first added to the calcium carbonate slurry 
followed by either the fluoroboric acid, formic acid, alum, or weak acid. 
These components can be added by conventional means well-known in the art. 
The compositions of the present invention can be utilized to improve the 
optical properties of neutral to weakly acidic paper by the addition of an 
effective amount of such a composition to the paper during standard 
manufacturing processes. Typically, the calcium carbonate composition of 
the present invention is added to a first paper furnish containing 
components necessary for making acidic paper to thereby form a second 
paper furnish. 
The invention will be further illustrated by the following Examples, which 
are to be considered illustrative of the invention, and not limited to the 
precise embodiments shown. Examples 1 through 6 below involve the 
preparation of acid stabilized calcium carbonate slurries that are stable 
at pH's lower than 7.5. 
EXAMPLE 1 
Scalenohedral Precipitated Calcium Carbonate 
Acid stabilized scalenohedral precipitated calcium carbonate slurry can be 
obtained by the addition of sodium silicate, followed by the addition of a 
weak acid such as phosphoric acid and a polymeric acid such as polyacrylic 
acid. Initially, 1% sodium silicate, based on the dry weight of calcium 
carbonate, was added into 18.5% solids slurry of scalenohedral 
precipitated calcium carbonate and mixed for about a minute. After mixing, 
this original slurry was transferred into several aliquots and 1% 
polyacrylic acid and amounts varying from 1 to 6% of phosphoric acid, 
based on the dry weight of calcium carbonate, were added to the aliquots. 
A plot of the pH was measured for each sample after 24 hours ageing as 
shown in FIG. 1. A composition containing 1% sodium silicate, based on the 
dry weight of calcium carbonate, and 6% phosphoric acid and 1% of 
polyacrylic acid, based on the dry weight of calcium carbonate was found 
to have an initial pH 5.28, and a pH of 5.75 after 24 hours ageing. 
EXAMPLE 2 
Scalenohedral Precipitated Calcium Carbonate 
Acid stabilized scalenohedral precipitated calcium carbonate slurry can be 
obtained by addition of sodium silicate, followed by the addition of a 
weak acid, such as fluoroboric acid (HBF.sub.4). Initially, 1% sodium 
silicate, based on the dry weight of calcium carbonate, was added into 
18.5% solids slurry of scalenohedral precipitated calcium carbonate, and 
mixed for about 1 minute. The initial pH of this untreated calcium 
carbonate was 8.57. After mixing, 4% of fluoroboric acid was added to the 
slurry. A plot of the pH was measured after 66 hours ageing as shown in 
FIG. 2. A composition containing 1% sodium silicate, based on the dry 
weight of calcium carbonate, and 4% fluoroboric acid, based on the dry 
weight of calcium carbonate, was found to have an initial pH of 6.11, and 
a pH of 6.54 after 66 hours ageing, as shown in FIG. 2. 
EXAMPLE 3 
Scalenohedral Precipitated Calcium Carbonate 
Acid stabilized scalenohedral precipitated calcium carbonate slurry can be 
obtained by the addition of sodium silicate, followed by the addition of 
alum, such as aluminum sulphate (Al.sub.2 (SO.sub.4).sub.3.18 H.sub.2 O). 
1% sodium silicate, based on the dry weight of calcium carbonate, was 
added into 18.5% solids slurry of scalenohedral precipitated calcium 
carbonate, and mixed for one minute. The pH of this untreated 
scalenohedral precipitated calcium carbonate was 8.57. To this mixture, 4% 
alum was added. Another mixture was mixed for another minute. The initial 
pH of this mixture treated with 1% sodium silicate/4% aluminum sulphate 
was 6.42. The pH of this treated mixture after 65 hours ageing was 6.61. A 
plot of the pH is shown in FIG. 3. 
EXAMPLE 4 
Rhombic Precipitated Calcium Carbonate 
Acid stabilized rhombic precipitated calcium carbonate slurry can be 
obtained by the addition of sodium silicate, followed by the addition of a 
weak acid such as phosphoric acid and a polymeric acid such as polyacrylic 
acid. First, 0.5% sodium silicate, based on the dry weight of calcium 
carbonate, was added into 18.2% solids slurry of rhombic precipitated 
calcium carbonate, and blended. From this slurry, several aliquots were 
prepared by adding 1% polyacrylic acid and 1%-6% phosphoric acid, based on 
the dry weight of calcium carbonate. The pH measurement was monitored for 
24 hours ageing and 47 hours ageing. These results are shown in FIG. 4. 
One of the examples showed that the initial pH of rhombic precipitated 
calcium carbonate slurry treated with 0.5% sodium silicate/6% phosphoric 
acid/1% polyacrylic acid was 5.16; after 24 hours ageing, the pH was found 
to be about 5.5; and after 47 hours, the pH was found to be 5.98. 
EXAMPLE 5 
Rhombic Precipitated Calcium Carbonate 
Acid stabilized precipitated calcium carbonate slurry can be obtained by 
the addition of sodium silicate, followed by the addition of phosphoric 
acid and polyacrylic acid. First, 1% sodium silicate, based on the dry 
weight of calcium carbonate, was added to 18.2% solids slurry of rhombic 
precipitated calcium carbonate and mixed for one minute The pH of the 
slurry was 8.79. The slurry was separated into the aliquots. To one of 
these aliquots, 1% phosphoric acid and 4% polyacrylic acid was added. To 
the second sample, 1% phosphoric acid and 6% polyacrylic acid was added. 
The pH's were taken after 40 hours ageing, and the results are shown in 
FIG. 5. One of the examples showed that the initial pH of the rhombic 
precipitated calcium carbonate slurry treated with 1% sodium silicate/1% 
phosphoric acid/4% polyacrylic acid was 6.03; another pH of the slurry was 
found to be 6.44 after 40 hours ageing as shown in FIG. 5. 
EXAMPLE 6 
Ground Calcium Carbonate 
The initial pH of a ground calcium carbonate was 8.01. Two 20% solid 
slurries of ground calcium carbonate were prepared; one slurry containing 
3% sodium- silicate based on the dry weight of calcium carbonate, and a 
second slurry containing 5% sodium silicate based on the dry weight of 
calcium carbonate. Blended to each of these slurry mixtures were 6% 
phosphoric acid and 1% polyacrylic acid. The pH of these two slurries was 
checked periodically with the results appearing in FIG. 6. The initial pH 
of the slurry containing 3% sodium silicate/6% phosphoric acid/1% 
polyacrylic acid was measured and found to be 5.08, and after 48 hours 
ageing was found to be 6.59, as shown graphically in FIG. 6. In 
comparison, the initial pH of the slurry with 5% sodium silicate/6% 
phosphoric acid/1% polyacrylic acid was measured and found to be about 
5.5, and after 48 hours ageing was measured and found to be about 6.8. 
The final pH difference of the slurry containing the 5% sodium silicate was 
thus 1.72 units greater compared to the slurry containing the 3% sodium 
silicate after 48 hours ageing. 
The above six examples show that acid resistance slurries of treated 
precipitated and ground calcium carbonate can be made by using sodium 
silicate in conjunction with a weak acid, such as phosphoric acid, 
polyacrylic acid, formic acid, fluoroboric acid, or with alum, and that 
these treated slurries survive after several hours below the 7.5 
composition pH of calcium carbonate. 
In a second part of the experiment of the present invention, the inventors 
were able to determine that a treated calcium carbonate slurry according 
to the teachings of the present invention does, in fact, resist 
decomposition at pH 7.0. This was proven by showing the residual calcium 
carbonate after a set time, the results of which are shown in FIG. 7, 
which will be discussed hereinbelow in the following example. 
EXAMPLE 7 
Treated Scalenohedral Precipitated Calcium Carbonate 
A 19.5% solids slurry of scalenohedral precipitated calcium carbonate was 
treated with 2% sodium silicate based on the dry weight of calcium 
carbonate. This mixture was mixed for 5 minutes. After mixing, 4% 
polyacrylic acid, based on the dry weight of calcium carbonate, was added 
to this blend. This resultant slurry was mixed at 300 RPM for another 5 
minutes. 
The pH of 500 ml of deionized water was adjusted to 6.0 with 1% sulfuric 
acid and/or 25% caustic solution. To this 500 ml of deionized water, 2 ml 
of scalenohedral precipitated calcium carbonate slurry which was treated 
in accordance with the preceding paragraph was added. The pH of this 
resultant slurry was kept constant at 7.0 for three minutes by adding 1% 
sulfuric acid solution. This sulfuric acid solution was filtered out of 
the slurry through an ashless filter paper. The residue slurry was heated 
at 500.degree. C. for 1 hour in a crucible which was weighed prior to the 
slurry being added therein. The amount of ash recovered for the treated 
slurry is shown in FIG. 7 as being 95%, which shows that the treated PCC 
did not decompose since if it had, this percentage value would have been 
lower than 95%. 
These results can be compared to a second example slurry which was not 
treated with sodium silicate, but which was prepared with the same 
parameters as the treated precipitated calcium carbonate hereinabove. The 
amount of ash for this untreated slurry was only 65%, proving that the 
uncoated or untreated calcium carbonate decomposes at a pH of 7.0, whereas 
the coated or treated calcium carbonate only slightly decomposes at a pH 
of 7.0, thereby showing that the treated PCC survives at a pH of 7.0. 
In a third part of the experiment involving the present invention, 
brightness pads with wood containing furnishes and calcium carbonate 
fillers treated in accordance with the teachings of the present invention 
were formed. 
EXAMPLE 8 
Scalenohedral Precipitated Calcium Carbonate 
A 19.5% (active with 80.5% water) solids slurry of scalenohedral 
precipitated calcium carbonate was treated with 2% sodium silicate, based 
on the dry weight of calcium carbonate. This resultant slurry was mixed 
for 5 minutes at 300 RPM. After mixing, 4% formic acid, based on the dry 
weight of calcium carbonate was added to the slurry, and the resultant 
slurry was mixed for another 5 minutes. 
Forming Of Brightness Pads 
The pH of a pulp slurry was adjusted to 6.0 or 7.0 with 1% sulfuric acid 
solution on a 25% caustic solution. The pulp slurry contained groundwood 
stock obtained from Crown Vatague Company, St. Francisville, La. The 
calcium carbonate treated as put forth in the previous paragraph and 
untreated calcium carbonate were added to the pulp in different additions 
of 0.2, 0.4, 1.0 and 2.0 ml each. The pH of this resultant mixture for the 
different pulp samples containing treated and untreated calcium carbonate 
was kept constant at 6.0 or 7.0 for 30 minutes. Afterwards, pulp pads were 
made by filtering this resultant mixture through a Buchner filter equipped 
with a WHATMAN.RTM. 41 filter paper. The pads were pressed twice in order 
to squeeze out as much water as possible, and dried in a drier drum. The 
brightness of each of the pads for both the treated and untreated calcium 
carbonate was measured with a BRIGHTIMETER.TM. Micro 5-5 device 
manufactured by Technidyne. 
FIG. 8 shows a graph of the brightness vs. the addition rate for both the 
treated calcium carbonate and untreated calcium carbonate. As is shown in 
FIG. 8, the pads which had calcium carbonate treated with 2% sodium 
silicate/4% formic acid, based on the dry weight of calcium carbonate were 
brighter than those pads which contained an untreated calcium carbonate 
slurry. 
EXAMPLE 9 
Scalenohedral Precipitated Calcium Carbonate 
A 19.5% (active with 80.5% water)solids slurry of scalenohedral 
precipitated calcium carbonate was treated with 2% sodium silicate, based 
on the dry weight of calcium carbonate, and 4% polyacrylic acid, based on 
the dry weight of calcium carbonate. Four pads were made by the process 
described in Example 8 for forming brightness pads. 
The brightness of these pads which were treated according to the preceding 
paragraph is also shown in FIG. 8, where, again, these pads containing the 
treated calcium carbonate were brighter than those pads containing the 
untreated calcium carbonate. 
EXAMPLE 10 
A 19.5% solids slurry of scalenohedral precipitated calcium carbonate was 
treated with 1% sodium silicate, based on the dry weight of calcium 
carbonate and 4% phosphoric acid, based on the dry weight of calcium 
carbonate. Four pads were made by the process described in Example 8 for 
forming the brightness pads, except that 1% phosphoric acid solution was 
used instead of the sulfuric acid to keep the pH constant at 7.0 during 
the pad making process. The results of these pads are shown in FIG. 9, 
where the brightness vs. the addition rate is shown for the several pads 
containing the untreated calcium carbonate and those pads containing the 
untreated calcium carbonate. From this graph of FIG. 9, it is apparent 
that the pads containing the treated calcium carbonate were brighter than 
those pads containing the untreated calcium carbonate. 
EXAMPLE 11 
Pilot Paper Machine Trial 
Paper was made on a laboratory scale custom papermaking machine which is 
manufactured by Eastern Machine Builders Company. This machine includes a 
clear plastic headbox; a foundrinier; a press section; a Yankee dryer; and 
two dryer sections. The stock consisted of a 100% high consistency 
groundwood obtained from Champion Paper Company, Deferiet, N.Y. The pH of 
the stock was maintained at 6.0 to 7.0 at the headbox by adding a solution 
of 10% phosphoric acid or of 10% NaOH to the solution via the inlet to the 
fan pump located near the headbox. A precipitated calcium carbonate slurry 
was mixed and added to the thin stock just before the stock enters the 
headbox. 
Samples 0 through 25% solids slurries of precipitated calcium carbonate 
were left untreated and added to the stock as set forth in the preceding 
paragraph, while several samples of 0 through 25% solids slurries of 
precipitated calcium carbonate were treated with 1% sodium silicate based 
on the dry weight of calcium carbonate, and 4% phosphoric acid based on 
the dry weight of calcium carbonate. 
The brightness results for the several types of paper are shown in FIG. 10. 
The paper which was not coated with a calcium carbonate filler is 
represented as a solid triangle in FIG. 10. The paper coated with the 
calcium carbonate which was untreated is represented by an asterisk. The 
paper coated with the calcium carbonate which was treated in accordance 
with the teachings of the present invention is represented by an open 
triangle. As shown in FIG. 10, the brightness values for the paper 
containing the treated calcium carbonate are higher than the paper 
containing the untreated calcium carbonate or no calcium carbonate filler. 
Examples 8, 9, 10 and 11 are represented herein to prove that calcium 
carbonates treated in accordance with the teachings of the present 
invention can be used at lower pH values in conjunction with fiber stock 
which is mechanically produced, thereby demonstrating the feasibility of 
using these calcium carbonates as fillers in wood containing papermaking 
systems. 
While the present invention has been particularly set forth in terms of 
specific embodiments thereof, it will be understood in view of the instant 
disclosure, that numerous variations upon the invention are now enabled to 
those skilled in the art, which variations yet reside within the scope of 
the present invention. Accordingly, the invention is to be broadly 
construed, and limited only by the scope and spirit of the claims now 
appended hereto.