Note: Descriptions are shown in the official language in which they were submitted.
PATENT
PC(MPM)7334
1 334560
- PRECIPITATED CALCIUM CARBONATE-CATIONIC STARCH
~ BINDER AS RETENTION AID SYSTEM FOR PAPE~MAXING
This invention relates to papermaking processes.
More specifically it relates to a binder system com-
prising a highly dispersible, high specific surface
area calcium carbonate (colloidal PCC) and a cationic
starch and the use of said system to produce paper
having improved levels of retention of fillers, and
improved strength and opacity.
In its efforts to produce high quality paper at
the lowest possible price and to minimize environmental
problems associated with disposal of the large volumes
of waste chemicals resulting from paper manufacture,
the paper industry has resorted to a variety of
approaches. These include the use of lower quality
pulps andJor the use of mineral fillers as substitutes
for cellulose fibers. However, such approaches tend to
reduce the strength, filler retention, dry brightness
and other properties of the resulting paper, often with
no beneficial effect upon environmental problems; and,
of course, no economic advantages.
Attention is directed to Volume 16 of the ~irk-
Othmer "Encyclopedia of Chemical Technology", pages
768-825 (1981), John Wiley & Sons, New York, for a
summary of paper making technology, including reference
to precipitated calcium carbonate as a filler in paper
making.
Fillers are added to the pulp slurry during the
paper making process to improve various properties such
as softness, smoothness, brightness and opacity.
Typical fillers are titanium dioxide, calcium
carbonate, talc, synthetic silicates and clays such as
. ~
1 334560
-2-
kaolin and bentonite. As regards fillers in general,
~ and calcium carbonate in particular, their retention;
i.e., filler retention, is a function of their particle
size, specific surface area, surface charge and
specific gravity. Calcium carbonate, more specifically
precipitated calcium carbonate (PCC), is enjoying
increasing use in the paper industry as a filler as a
result of the development of on-site PCC satellite
plants; i.e., plants which produce PCC at or close by
the paper mill in which it is to be used, making use of
the carbon dioxide produced by the paper mill to
convert calcium hydroxide to PCC.
In order to improve operation of the paper making
machine and to improve the quality of the paper,
various chemicals, generally referred to as processing
aids are added to the paper. These include retention
aids, flocculants, drainage aids, defoamers and
slimicides. Retention aids are used to improve filler
retention in the papermaking process by coflocculating
the filler with pulp fibers and fines. Typical
retention aids are amino or quaternary ammonium group
containing polymers such as condensation polymers of
diethylenetriamine-adipic acid polyamide which are
treated with epichlorohydrin and ammonia- or
dimethylamine-epichlorohydrin condensates and
polyacrylamides (PAM). However, PAM retention aids are
shear sensitive, can over flocculate a sheet causing
poor formation and reduced drainage on the paper
machine wire resulting in reduced productivity.
Starches are used in the papermaking process to
improve dry strength of the paper. Also used are
modified starches especially those having tertiary
~3~ 1 334560
amino or quaternary ammonium groups and which are
~ cationic in nature.
~- Recent developments in the paper industry which
tend to overcome the above-mentioned deficiencies
involve the use of binder systems such as combinations
of anionic retention agents, e.g. polyacrylamides,'and
cationic starch. The most recent development as
regards binder systems comprises a combination of
cationic s'arch and anionic colloidal silicic acid;
Such binder systems are described in U.S. patents
4,385,961 and 4,388,150, issued May'31, 1983 and
June 14, 1983, respectively. While these binder
systems, particularly those comprising cationic starch
and anionic colloidal silicic acid, result in paper
having improved filler retention and dry strength,
relative to paper made in the absence of said systems,
they do not enhance the opacity of the finished paper
because of over flocculation of tXe filler and
generally increase the cost of papermaking.
It has now been found that an anionic/cationic
system comprising colloidal PCC in combination with a
cationic starch forms an efficient binder system which
improves filler retention, permits a reduction in the
cellulosic fiber content of the paper and/or a
reduction in the quality of the cellulosic fiber used,
improves the opacity of the paper, and can be used in
smaller amounts than those required for an anionic
colloidal silicic acid/cationic starch system.
Additionally, the colloidal PCC/cationic starch system
of this invention is less sensitive to changes in
ratios of colloidal PCC and cationic starch than is a
colloidal silicic acid/cationic starch system. The
-4- 1 334560
herein described system permits the addition of up to
~ 50% more filler to paper than is possible using
polyacrylamide (PAM) with no reduction in strength. It
enables the papermaker to not only improve current
performance but to do so at a lower cost than is
possible with other two component systems.
The binder system of this invention comprises two
components; namely, a highly dispersible, high specific
area precipitated calcium carbonate (PCC) and a
cationic starch. The calcium carbonate component is
anionic and colloidal in nature. When used in a paper
making process in the presence of a cationic starch it
maximizes filler retention, improves drainage,
formation and optical properties while maintaining
acceptable strength characteristics in the finished
paper.
A PCC particularly useful in the present invention
is prepared by the process described herein. It
comprises introducing carbon dioxide into an aqueous
slurry of calcium hydroxide at a temperature of from
7C t~ 18C. The slurry contains from about 0.02 to
l.0 weight percent, based on the calcium carbonate
equivalent of the calcium hydroxide in the reaction
slurry, of an anionic organopolyphosphonate
polyelectrolyte and from 0 to lO weight percent of
aluminum sulfate octadecahydrate. The introduction is
continued until precipitation of the calcium carbonate
is substantially complete. The process to this point
is that of U.S. 4,367,207, issued Januar~y 4, 1983, but
differs in that the slurry may also contain aluminum
sulfate octadecahydrate. It is then heat aged to
s 1 334560
reduce the surface area and to increase dispersibility,
~ then treated with an anionic dispersant, preferably an
inorganic polyelectrolyte and more preferably sodium
S triphosphate or sodium hexametaphosphate. The calcium
carbonate thus prepared is a highly dispersible, finely
divided (colloidal) precipitated calcium carbonate. It
carries a negative charge (zeta potential - -10 to
-60 mv when cu6pended in deionized water at 1000 ppm
total 601ids).
The aging step is carried out by heating the
~ slurry at a temperature of from about 40C to about
90C, most preferably at 60C and at a pH of 9-11, most
preferably at pH 10, for a period of time sufficient to
reduce the surface area of the precipitated calcium
carbonate by about one-half and to increase
dispersibility. The time, of course, is dependent upon
the temperature. However, in general, periods of from
about 0.25 to about 4 hours, preferably 0.5 hour, are
adequate over the temperature range mentioned above.
To the aged slurry i6 then added, with ~tirring, the
anionic di~percant in quantity sufficient to achieve a
zeta potential of from -10 to -60 mV, preferably -20 to
-40 mV. In the case of the favored polyphosphate
dispersants thi~ amounts to from about 2% to 20% by
weight of precipitated calcium carbonate.
PCC having a surface area of from about 10 to
200 m2/g is useful in the binder system of this
invention. However, PCC having a surface area of from
about 50 to 150 m2/g is favored and that having a
surface area of from about 50 to 90 m2/g is preferred.
The highly dispersible state of the colloidal PCC
is demonstrated by the photon correlation spectrum of a
freshly sonicated suspension which indicates a narrow
-6- 1 334560
particle size distribution with a mean particle
-- diameter of 0.05-0.5 ~m in general, and 0.1-0.3 um for
the most preferred material.
The cationic starch can be derived from any of the
commonly available sources of starch producing
~aterials, such as potatoes, corn, wheat and rice. A
potato derived starch is favored, especially one in
which the degree of substitution is between 0.10% and
0.50%. The preferred cationic potato starch is one
made cationic by reaction with 3-chloro-2-hydroxypropyl
trimethylammonium chloride to a degree of substitution
of from 0.20~ to 0.40%.
The preferred ratio of PCC to cationic ~tarch
ranges from about 2:1 to 1:20, the preferred ratio of
cationic starch to pulp ranges from about 0.5 to 1.5~
(dry weight), and the most preferred ratio of cationic
6tarch to pulp ranges from 1.0% to 1.5%.
In an actual papermaking operation the filler
would be added to the system at the blend chest. The
colloidal anionic PCC would be added at the stuff box
and the cationic starch would be added before the fan
pump. However, total optimization would depend on the
approach flow system associated with each specific
papermaking machine.
In the 8ritt Jar retention and handsheet studies
presented herein a standard 75~ hardwood/25% softwood
pulp stock was used. As filler, 20% by weight of
precipitated calcium carbonate known as Albacar*5970 or
Albacar*HO, available from Pfizer Inc., assignee of
this invention, was used. The experiments described
were conducted in a Dynamic Drainage Jar (~Britt~ Jar)
available from Paper Research Materials, Inc. of
Syracuse, New York. The percent of fines, fi~ers which
* Trademarks
_7_ l 334560
pass through round holes 76 microns (125P) in diameter,
- and the percent filler retained when using various
~ retention aid components was determined.
Handsheet Studies
A handsheet experiment simulating high machine
speed and shear conditions was performed in the
laboratory by introducing turbulence and shear with a
Britt Dynamic Drainage Jar and then forming handsheets
on the Formax Sheet Former. This experiment determined
first pass fil-ler retention, pigment scattering
coefficients, pigment absorption coefficients,
corrected TAPPI opacity, TAPPI brightness, Scott
internal bond, Taber stiffness, and % CaCO3 values of
handsheets made with Albacar H0 Standard Filler Pigment
available from Pfizer Inc. as the only filler
pigment at theoretical filler loading levels of 8%, 24%
and 40%. The experiment compares the effects made upon
these sheet properties by the PCC/cationic starch
system of this invention to colloidal silicic
acid/cationic starch at ratios of 4 pàrts cationic
starch component to 1 part anionic colloidal silica or
colloidal PCC and at 10 parts cationic starch component
to 1 part anionic colloidal silica or colloidal PCC.
Comparison was against Percol*175, a typical high
molecular weight cationic polyacrylamide (available
from Allied Colloids, Fairfield, N.J.) system loaded at
llb/ton.
In the first phase of the experiment, colloidal
silicic acid was compared with colloidal PCC at ratios
of 4 and 10 parts cationic potato starch to 1 part
inorganic component (i.e., silicic acid or PCC). The
filler loading levels necessary to achieve 8%, 24% and
40% filler loading were held constant. The results
* Trademarks
-8- 1 334560
indicate that the colloidal PCC/cationic starch system
~ increases optical and stren~th properties of handsheets
~ when made under high shear conditions to a greater
S extent than does colloidal silicic acid/cationic
starch. Both systems exhibited e~ual sheet brightness.
Although the PCC system exhibited a slight decrease in
retention, this was offset by increased TAPPI opacity-,
pigment scattering, Scott internal bond, and Taber
stiffness values. The results indicate that the PCC
system is superior to typical polymeric retention aids
such as Percol 175 with respect to retention
properties, internal bonding strength and stiffness.
Percol 175 surpasses both the PCC/cationic starch and
silicic acid/cationic starch systems optically but does
not exhibit their strength or retaining potential.
General Procedure
Albacar HO Standard Filler was the only
filler used. It was made down to 20% pigment solids
and dispersed on a roll mill for 2 hours. Other
additives were a heat-aged colloidal PCC treated with
3% sodium triphosphate (Na5P3010) with a surface area
of 64 m /g, a mean particle diameter of 0.2 um and a
zeta potential of -25.1 mV and BMA, a 5 nm anionic
colloidal silica (Eka AB, S-445 01, Surte, Sweden).
Both of these materials were made down to 0.5%
concentration and dispersed on a roll mill for two
hours. The colloidal PCC was dispersed for 15 seconds
prior to its addition by means of an ultrasonic probe.
Both of these materials are anionic and function as a
part of a co-binder system with BMB cationic starch
having a degree of substitution of 0.35% (available
from Eka AB, S-445 01, Surte, Sweden). The cationic
starch solution (0.5~ concentration) was prepared by
9 1 334550
boiling it in dimineralized water for 30 minutes,
- cooling to room temperature, and adjusting the
~ concentration with dimineralized water. Percol 175, a
high molecular weight cationic polyacrylamide was also
evaluated for its retaining properties. A 0.05%
concentration of it was prepared by dispersing it, in
dimineralized water, for two hours. In this
experiment, the cationic addition level utilizing
either the starch or Percol 175 was held constant at
1.0% (20 lbs/ton) or 0.05% (1 lb/ton), respectively,
based on O.D. fiber weight. Two sets (5 sheets/set) of
blanks (sheets without filler) were made, one set with
the cationic starch as the only additive and the other
with Percol 175 as the only additive.
Pulp was added to the Britt Jar under agitation
(75% bleached hardwood kraft:25% bleached softwood
kraft; 0.5% consistency, 400 Canadian Standard
Freeness; pH 7.0-7.2) for 15 seconds at 1000 RPM. The
weight of pulp added to the Britt Jar was adjusted to
each loading level to result in a 2.5 gram unfilled or
filled sheet. The Britt Jar was modified by replacing
the screen with a solid plastic disk, so that when
placed under agitation all the components in the jar
remained contained in the jar. Albacar HO PCC
(~x-482) was loaded into the modified vaned Britt
Dynamic Drainage Jar to achieve theoretical loading
levels of 8%, 24% and 40%. After 30 seconds of
agitating pulp and filler, if an inorganic anionic
component was to be added, it was introduced at this
point. At 45 seconds, this addition was followed by
the addition of 1% cationic starch. If Percol 175, a
cationic polyacrylamide, was the retention system of
choice, then it alone was added to the agitating pulp
-lO- 1 334560
and filler at 45 seconds into agitation. For the
- series of handsheets made with Percol 175, filler
~~ loading levels were adjusted to account for poorer
retention. After a total of 60 seconds of agitation at
1000 RPM in the Britt Jar the contents were transferred
to the deckle box of a Formax Sheet Mold where
handsheets were-formed using tap water. After forming,
the sheets were removed from the Formax's screen and
0 pressed once at 25 psi. The sheet was then dried using
a drum dryer set at 125C. 81ank sheets (sheets
without filler) were made by adding cationic starch or
Percol 175 only to the modified Britt Jar at 45 seconds
into agitation.
The handsheets were conditioned at constant
temperature and humidity for 24 hours at 23F and 50%
relative humidity per TAPPI procedure before testing.
Testing
The following tests were performed on all of the
sheets:
Optical Phsical Other
TAPPI Opacity Grammage % CaCO3
(Texas Nuclear)
TAPPI Roo Scott Bond
Taber Stiffness
334560
From the test results, the following values were
- calculated:
.
Optical Other
Pigment Scattering First Pass
Coefficient Retention
- Pigment Absorption
Coefficient
Corrected TAPPI
Opacity
TABLE I
Ratlo( ) Pigment Plgment Corrected(b) Scott Taber
Retention Starch: Ba~ls ~'ir~t Pass Scatterlng Absorption TAPPI T~PPI Internal Stlffne~s
U d Inorganlc Weight Retention Coefficlent Coefficlent Opaclty brlght- Bond *(lOOmN_
System Component (OD g/m2) % CaC03 (FPR) (c 2/g) (cm /g) ( ) ness (%) (ft-lbs) m2/g)
PCC/Cat~ ~c 4:1 62.0 9.27 73.5 2524 5.27 86.3 84.8 106 4.39
Starch ~ 4:1 62.2 23.77 69.8 2109 5.48 90.7 87.5 70 3.10
4:1 65.3 41.58 69.4 1866 4.64 93.2 90.4 54 1.43
10:1 61.9 9.43 74.7 2614 6.42 ` 86.6 84.7 109 4.07
10:1 62.6 24.07 71.2 2027 5.67 90.5 87.5 66 3.11
10:1 64.9 42.61 70.7 1827 4.43 93.2 90.3 41 1.47
~iliclc acld/ 4:1 62.0 8.95 70.9 2450 -1.10 85.7 85.8 100 4.09
catlonlc 4:1 62.9 24.35 72.3 1937 3.11 90.0 88.2 64 3.10
~tarch 4:1 67.1 43.64 74.9 1751 4.07 92.9 90.4 44 1.45
10:1 62.4 9.39 74.5 2513 5.10 86.3 84.9 104 4.23
10:1 63.4 24.60 73.7 1965 4.41 90.3 87.6 71 3.23
10:1 66.3 43.20 73.2 1785 ~4.40 93.0 90.3 46 1.41 ---
Percol 175 (c) hl.1 8.95 63.7 2707 7.49 87.0 85.4 85 3.46 ~,
63.6 26.63 62.6 2087 5.96 91.5 88.3 52 2.5266.3 45.05 64.8 1881 5.40 93.8 90.8 44 1.04
(a) catlonlc starch content held constant at 1.0% (20 lb~/ton)
(b) corrected to 60 g/m
(c) 0.05% (1.0 lb/ton)
(d) materlal of Preparatlon A.
1 334560
-13-
The first pass retention (FPR) values at all
- filler levels using colloidal PCC/cationic starch or
~ colloidal silicic acid/cationic starch were superior to
those achieved by the polyacrylamide system. At the 8%
filler level in the sheet both ratios of the colloidal
PCC/cationic starch system (1:4 and 1:10) achieved FP~
values similar to those of the colloidal silicic
acid/cationic starch at the 1:10 ratio, and a superior
FPR value at the 1:4 ratio.
The colloidal PCC/cationic starch system is
superior to colloidal silicic acid/cationic starch in
pigment scattering power. Both systems are inferior to
the polyacrylamide system in this property; and in
opacity values.
Each of the colloidal PCC/starch and colloidal
silicic acid/starch systems produces a stronger and
stiffer sheet than does the polyacrylamide retention
aid.
In general colloidal PCC/cationic starch (1:4) when
introduced to a furnish composed of pulp and filler
under high shear conditions improves the optical,
strength and retention properties of the s~stem.
A further series of experiments was run using the
Britt Jar in which the shear rate, the ratio of
catiGnic starch to aged PCC or colloidal silicic acid,
and the amount of cationic starch were varied. ~iller
and fines retention were calculated.
1 334560
GENERAL EXPERIMENTAL PROCEDURE FOR BRITT JAR RETENTION
- The pulp stock used 75% Hardwood, 25% Softwood,
- had a Canadian Standard Freeness (CSF) of approximately
400, a consistency of 0.5%, and was adjusted to pH 7.2.
The Albacar 5970 filler pigment was dispersed in
deionized water at 10% solids. The anionic retention
aid components were dispersed in deionized water at
0.25% solids. The Britt Jar was equipped with a 125P
screen, having 76 micron openings, and agitation was
provided by a-2 inch diameter propeller rotated at 750
RPM (shear).
Test Procedure:
1. Add 500 g pulp stock, start timer
2. At 15 seconds, add filler (Albacar 5970,
etc.)
3. At 30 seconds, add anionic component
(colloidal PCC, silica)
4. At 45 seconds, add cationic component
(starch, PAM)
5. At 60 seconds, start draining: drain off
first 100 ml only
6. Filter through a tared Whatman ~40 filter
paper
7. Dry and weigh filter paper to determine
solids in effluent
8. Ash filter paper for 4 hours at 525C to
determine amount of filler in effluent
9. Calculate % retention
1 334560
-15-
Calculations:
% fines retained = (AxBxR)
A
% filler retained = (DxE) [(JD E) x ~ l x 100
where
A = total weight of stock
B = stock~consistency
D = weight of PCC slurry added
E = solids of PCC slurry
F = weight of effluent
G = tare weight of filter paper
H = weight of filter paper ~ solids
I = crucible tare weight
J = crucible + ash weight
K = amount of fines in pulp
1 334560
-16-
BRITT JAR STUDY
- A. Heat aqed PCC (treated with sodium hexameta-
~ phosphate)/cationic starch system (material of
-- Preparation B)
Conditions (see key) % Filler % Fines
f(x,y,z) Retention Retention
f(0.5, 10:1, 750) 60.32 63.78
f(1.5, 10:1, 750) 86.08 85.22
lOf(0.5, 1:1, 750) 72.48 78.36
f(1.5, 1:1, 750) 78.16 82.72
f(0.5, 10:1, lO00) 54.52 62.22
f(1.5, 10:1, 1000) 70.14 75.32
f(0.5, 1:1, lO00) 63.76 ` ~2.02
f(1.5, 1:1, 1000) 59.30 68.04
1~f(0.5, 10:1, 1250) 48.72 60.66
f(l.5, 10:1, 1250) 54.20 65.. 42
f(0.5, l:l, 1250) 55.04 65.68
f(l.5, 1:1, 1250) 40.44 53.36
Maximum
Conditions for % Filler
Given Shear Rate Retention Key
f(1.35, 10:1, 750) 87.0 X = % Cationic
Starch
f(l.2, 10:1, 1000) 73.3 Y = Ratio
Starch:
Colloidal
Component
f(l.l, 10:1, 1250) 61.0 Z = rpm
-17- l 334560
BRITT JAR STUDY
~eat a~ed PCC (treated with sodium triphosphate)/
-- cationic starch ~ystem (material of Preparation B)
5 Conditions (see ~ey) % Filler % Fines
f(x,y,z) Retention Retention
f(0.5, 10:1, 750) ' 75.11 72.33
f(1.5, 10:1, 750) 89.01 91.13
f(0.5, 1:1, 750) 73.11 75.17
f(1.5, 1:1, 750) 89.57 89.77
f(0.5, 10:1, 1000) 63.39 69.73
f(1.5, 10:1, 1000) 67.79 77.93
f(0.5, 1:1, 1000) 63.17 71.91
f(1.5, 1:1, 1000) 70.13 75.91
l~ f(0-5, 10:1, 1250) 51.69 69.38
f(1.5, 10:1, 1250) 46.59 62.48
f(0.5, 1:1, 1250) 53.25 . 70.90
f(1.5, 1:1, 1250) 50.71 59.80
Maximum
Conditions for % Filler
Given Shear Rate Retention Key
f(1.3, 10:1, 750) 89.90 X = % Cationic
Starch
. f(1.15, 1:1, 1000) 72.90 Y = Ratio
Starch:
Colloidal
Component
f(0.95, 1:1, 1250) 57.80 Z = rpm
1 334~60
-18-
BRITT JAR STUDY
C. Colloidal Silicic Acid/Cationic Starch System
5 Conditions (see key) % Filler . ~ Fines
f~x,v,z) Retention Retention
f(0.5, 10~ 50) 84.05 88.01
f(l.5, 10:1, 750) 79.89 83.05
f(0.5, 1:1, 750) 68.87 86.19
f(l.5, 1:1, 750) 79 43 88.17
f(0.5, 10:1, 1000) . 68.74 75.81
f(l.5, 10:1, 1000) 74.88 78.53
f(0.5, 1:1, 1000) 60.20 69.01
f(l.5, 1:1, 1000) 81.06 79.37
f(0.5, 10:1, 1250) 53.43 64.97
f(l.5, 10:1, 1250) 69.87 75.37
f(0.5, 1:.1, 1250). 51.53 53.19
f(1.5, 1:1, 1250) 82.69 71.23
Maximum
Conditions for % F.ller
Given Shear Rate Retention Key
f(0.95, 10:1, 750) 89.9 X = %
Cationic
Starch
f(l.3, 4:1, 1000) 82.6 Y = Ratio
Starch:
Colloidal
Component
f(l.5, 1:1, 1250) 82.7 Z = rpm
1 334560
--19--
The superiority of the PCC/cationic starch system
- over Percol 175 for improving filler and fines re-
- tention was demonstrated by this experiment conducted
in a Britt Jar at a shear rate of 750 rpm. Albacar
5970 was used as filler at the 20% level.
~ Retention
Addition
Retention Aid Level Filler Fiber F$nes
Percol 175 0.02% 34.1 57.4
~ 0.04% S0.0 64.6
0.06% 56.8 69.5
Collo~dal PCC
(62 m /g)/
IS cationic 0.13%/ 89.9 89.0
potato starch 1.3%
1 334560
--20--
PREPARATION A
- The following calcium carbonate precipitation was
~ conducted in a 30 liter stainless steel reactor
e~uipped with a cooling jacket, an agitator having two
pitched-blade turbine impellers, a stainless steel
carbonation tube to direct a carbon dioxide gas stream
to the impeller and probes for monitoring the pH and
temperature of the suspension.
A calcium hydroxide slurry was prepared by rapidly
adding 1,550 g of pulverized reactive lime having an
approximately 93% available calcium oxide content to
7.75 liters of water at 50C contained in the 30 liter
reactor while agitating the contents at 400 RPM. After
10 minutes the slaked lime slurry reached a temperatue
of 64C. It was then diluted with 15.50 liters of
water to give a final calcium hydroxide conentration of
8.8% by weight, then cooled to lower the temperature to
10C.
An amount of active ~2-hydroxyethylimino)
bis(methylene)bisphosphonic acid (e.g Wayplex*61-A,
Philip A. Hunt Chemical Corp.) equivalent to 0.15% and
an amount of A12(SOq)3 18H2O equivalent to 3.1% by
weight of the calcium carbonate equivalent of the
calcium hydroxide slurry were added to the slaked lime
slurry and mixed over a period of about one minute, the
agitation rate having been set at 800 RPM. The slurry
was then carbonated by passing a 28 volume percent
carbon dioxide in air mixture at 29 liters/minute
through the slurry. The batch was carbonated over a 25
minute period to a pH of 10Ø The batch was cooled
throughout the carbonation during which time the
temperature increased to 15C. The slurry was then
* Trademark
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removed from the reactor, passed through a 325 mesh
screen to remove the grit present in the lime and then
returned to the reactor, a small sample being retained
-5 for surface area analysis.
The screened slurry was then restabilized at pH 10.0
(by C02 gas addition) and heated to 60-C over a period of
about lS minutes with gentle agitation. It was then
maintained at pH 10.0 and 60-C for 30 minutes after which
the batch was quickly cooled to 24-C. An amount of
sodium triphosphate equivalent to 3.0% based on calcium
carbonate was then added and the mixture gently agitated
for 10 minutes.
The above product, a highly dispersible, colloidal
lS precipitated calcium carbonate, was characterized as
follows. A portion of the heat-aged material before
addition of the sodium triphosphate was treated with 3.0%
(by weight of calcium carbonate) of phosphoric acid, the
material dewatered, dried and subjected to surface area
analysis (single-point BET N2 adsorption on a Micromentics
Flowsorb II instrument). Its specific surface area was 62
m2/g vs llS m2/g for the material prior to heat-aging.
The specific surface area of the final (triphosphate-
treated) product was 64 m2/g. The final product was also
analyzed for particle size distribution by photon
correlation spectroscopy. In this analysis, the
triphosphate-treated product was diluted to 1% solids and
then subjected to ultrasonic energy with a Heat Systems
Model W-380 horn at an ou~u~ setting of S for S minutes.
This material was further diluted to about 100 ppm solids
and the photon correlat~on spectrum measured at 90-
scattering angle on a Coulter Electronics Model N4
instrument. The mean particle
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diameter computed from the photon correlation spectrum
was 0.185 um, the distribution width being 0.065 ~m.
The final product was also analyzed for urface charge
- S by measurement of its electrophoretic mobiiity (zeta
potential). In this analysis, the triphosphate-
treated product was diluted to 1000 ppm solid6 with
deionized water. Under these conditions the mean
zeta potential was -25.1 mV as measured by the Coulter
Electronics Model 440 DELSA instrument.
PRE~ARATION B
The equipment and process conditions were the same
as Preparation A except for the following. The final
slaking temperature was 73C and carbonation was
started at 8C. The final carbonation temperature was
17C.
The final product (as treated with either sodium
tri- or hexametaphosphate) had a ~pecific ~urface area of
62 m2/g (vs 112 m2/g for the unaged product) and a zeta
potential of -24 mV.
2S
