Note: Descriptions are shown in the official language in which they were submitted.
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$ROCESS FOR THE PREPARATION OF DISCRETE PARTICLES
OF CALCIUM CARBONATE
Field of the Invention
The present invention concerns precipitated
calcium carbonate. More specifically, it concerr~.s a
novel method of preparing discrete particles of
prismatic calcium carbonate of varying particle size by
carbonating an aqueous lime slurry in the presence of
saccharides or polysaccharides and optionally other
metal ions.
Bac7cground of the Invention
Precipitated calcium carbonate tPCC) a.s
commonly prepared by carbonating an aqueous calcium
hydroxide slurry with a carbon dioxide containing gas
while controlling temperature to obtain calcium
carbonate particles of about 0.5 micron to 10 micron
average particle size. Heat aging is also used as a
mechanism to effect the size and size distribution of
the final product. Such calcium carbonate products are
useful as fillers in paper, as pigments for coated
paper, as pigments for paints and polymer applications,
and also find use in the pharmaceutical industry.
Y~Th,en attempting to produce precipitated
calcium carbonate much smaller than 0.5 micron average
particle size it is necessary to use low calcium
hydroxide concentrations, low temperatures, and
sometimes low levels of a size controlling agent.
However, most processes still remain difficult to
control and the calcium carbonate particle size a.s not
very predictable.
What has been found to be novel and
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unanticipated by the prior art is an improved process
for selectively producing discrete calcium carbonate
particles of from about 0.5 microns to about 0.018 =
microns.
It is therefore an object of the present
invention to provide a simple, predictable process for
the preparation of calcium carbonate particles in a
wide range of particle sizes. Another object of the
present invention is to provide calcium carbonate
products that are especially useful in the paint,
plastics, pharmaceutical and paper industries. These
and other objects of the present invention will become
apparent as further provided in the detailed
description which follows.
Prior Related Art
U.S. 5,332,564 discloses a process for
producing rhombic or barrel shaped PCC comprising
slaking quicklime in an aqueous sugar solution to form
a sla3ced lime slurry carbonating said lime slurry at
40 - 80°F. with carbon dioxide containing gas until
about neutral (pfi 7-8) to SSA 2-20 m2/g.
U.S. 4,237,147 discloses a process for
preparing a dry carbonated beverage concentrate for
preparing a carbonated beverage comprising (a.)
amorphous calcium carbonate and (b.) an anhydrous
nontoxic acid in an amount to completely evolve all the
carbon dioxide gas from the amorphous calcium
carbonate. Sugar is employed in the precipitation of
the calcium carbonate and results in fines on the edges
of irregular agglomerated particles.
U.S. 4,018,877 describes an improved process
for producing calcium carbonate by introducing a
complexing agent into the calcium carbonate slurry
either during or after the "primary nucleation stage."
The complexing agent is selected from a group which
includes sucrose or glucose.
U.5. 3,443,890 describes a process for
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producing precipitated calcium carbonate by carbonating an
aqueous calcium hydroxide slurry in the presence of
saccharides and a second active compound selected from the
group consisting of active Si02 compounds.
U.S. 2,467,082 discloses a process for producing
chalk by carbonating an aqueous calcium hydroxide slurry in
the presence of sugar beet residue extract.
U.S. 2,188,633 discloses the use of saccharides and
polyvinyl alcohol as an additive to an aqueous calcium
hydroxide slurry prior to carbonation to form a calcium
carbonate product.
In view of the above, there is nothing in the patent
literature which suggests that the use of sucrose alone or in
combination with aluminum sulfate (alum) and temperature
control of the calcium hydroxide slurry during carbonation
that would result in the present invention.
Summary of the Invention
In the novel process for producing discrete
prismatic calcium carbonate particles having a specific
surface area of from about 10 to about 120 m2/g, saccharides
or polysaccharides are used either alone or in combination
with other metal ions to effect the particle size.
The present invention provides a process for the
preparation of discrete particles of prismatic calcium
carbonate, the process consisting of preparing a first calcium
hydroxide slurry from calcium oxide and water; subsequently
adding to the first slurry from about 0.1 weight percent to
5.0 weight percent of a saccharide or polysaccharide to form a
second calcium hydroxide slurry, and while rapidly agitating,
carbonating the thus prepared second calcium hydroxide
slurry until the carbonation is substantially
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complete so as to obtain the discrete particles of
prismatic calcium carbonate, the carbonation being started
at a temperature of from about 8°C to about 64°C.
The present invention also provides a process for
S the preparation of discrete particles of prismatic calcium
carbonate consisting of preparing a first calcium hydroxide
slurry from calcium oxide and water, subsequently adding to
the first slurry from about 0.1 weight percent to 0.5
weight percent of a saccharide or polysaccharide and from
about 0.1 weight percent to about 5.0 weight percent of a
metal salt to form a second calcium hydroxide slurry and
while rapidly agitating, carbonating the thus prepared
second calcium hydroxide slurry until the carbonation is
substantially complete so as to obtain the discrete
IS particles of prismatic calcium.carbonate, the carbonation
being started at a temperature of from about 8°C to about
64°C.
The calcium carbonate product is useful as
pigment in the paint and plastics industry, as pigment in
paper coating, and as filler in papermaking.
Detailed Description of the Invention
We have now found that precipitation of calcium
carbonate by carbonation of aqueous lime slurries
containing a saccharide or polysaccharide or a saccharide
or a polysaccharide and a metal ion at temperatures of from
about 8°C. to about 64°C. consistently produces the desired
calcite product with specific surface areas (SSA) in the
range of from about 10 m2/g to about 120 m2/g.
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Accordingly, the present invention entails a
process for the preparation of discrete particles of a
prismatic calcium carbonate which comprises introducing '
carbon dioxide into an aqueous slurry of from about 1
weight percent to about 30 weight percent calcium
hydroxide containing a saccharide or polysaccharide and
optionally a metal ion. The preferred process entails
the use of from about 0.1 weight percent to about 5.0
weight percent saccharide or polysaccharide and
optionally from about 0.1 weight percent to about 5.0
weight percent metal ion, starting the introduction of
carbon dioxide at a temperature of from about 8°C. to
about 64°C. and continuing the introduction until the
calcium carbonate precipitation is substantially
complete.
Preferably the calcium hydroxide slurry
concentration is from about 5 weight percent to about
weight percent. The amount of sucrose present in
the calcium hydroxide is more preferably about 0.5
20 weight percent or the amount of sucrose and alum
present in the calcium hydroxide is about 0.5 weight
percent and 4.0 weight percent respectively. Vigorous
agitation is used during the carbonation process, which
process should take no more than about 80 or 90 minutes
25 to complete.
The saccharide or polysaccharide useful in
the present invention is selected from the group
consisting of sucrose, glucose, fructose, raw sugar,
molasses, gums, starches, and other organic compounds
similar in nature. Preferably the saccharide or
polysaccharide is sucrose. Various inorganic metal
oxides, especially gel forming metals such as Si02,
_MnO, ZnO, Zr02, and A1203 are useful in the process of
the present invention. Preferably the metal ion is
A1203 (Alum) .
All percentages used herein are weight
percent, and when they describe the amount of additive
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used they are weight percent based on the calcium
carbonate equivalent of the available lime.
The nature of the carbon dioxide containing
gas is not particularly critical in that pure carbon
dioxide gas can be employed or standard mixtures of
carbon dioxide in either air or nitrogen. Liquid
carbon dioxide as well, can be used in accordance with
the present invention.
While the present process is applicable to
all concentrations of lime slurries which can be
carbonated, it is practically limited to those slurries
in which the calcium hydroxide concentration of the
starting slurry is greater than about 5 weight percent.
This is because precipitated calcite of the desired
particle size can be realized under the present
carbonation conditions with slurries having an initial
calcium hydroxide concentration of about 5 percent or
less even in the absence of the additives. Such low
concentrations, however, are not economical. For most
economical operation, the concentration of the calcium
hydroxide in the slurry to be carbonated is preferably
from about 10 to 20 percent by weight.
The carbonation of the lime slurry is
continued until calcite precipitation is substantially
complete, preferably being terminated when the pH of
the carbonated slurry is about 7. Such carbonation is
usually accomplished in a period of about one hour and
one half. Normal care is exercised to neutralize any
unreacted calcium hydroxide still present in the
carbonated slurry. Various techniques known to those
skilled in the art can be used to accomplish this
neutralization. These include, for example,
monitoring the slurry pH with the introduction of
additional carbon dioxide gas as necessary as well as
treating the carbonated slurry with a sufficient amount
of an organic or inorganic polybasic acid such as
citric, malefic, malic, malonic, phthalic, tartaric,
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boric, phosphoric, sulfurous or sulfuric acid. The
calcium carbonate in the final slurry may be utilized
as such, or may be filtered, dried and milled for use
as a dry product.
Provided the reactants are of sufficient
purity, the product resulting from the process of the
instant invention is sufficiently pure to readily meet
the USP specification for precipitated calcium
carbonate. The product will typically analyze, when
using methods specified by USP, to greater than 98.0
percent calcium carbonate.
The product is useful as a pigment in
plastics and paint applications. It is especially
useful as a pigment for paper coating and as a filler
or retention aid in papermaking.
The size of the discrete particles of
prismatic calcium carbonate being extremely small (0.5p
to 0.018p) is more accurately determined and expressed
by the specific surface area measurement. The specific
surface area (SSA) of the products are determined using
a Micromeritics FLOWCARB II 2300 which employs BET
theory with nitrogen as the absorbing gas. By
prismatic calcium carbonate we mean that the particles
have a generally prismatic shape and the aspect ratio
(L/W) averages 2.0 or less as described in U.S.
3,320,026 to Waldeck and in Pigments for Paper (TAPPI
Press) edited by Robert W. Hagemeyer.
The following examples are merely
illustrative of the process of the present invention
and are not to be construed as limiting the invention,
the scope of which is defined by the appended claims.
Examt~le 1
A 4-liter jacketed, baffled, cylindrical
stainless steel reactor, having an internal diameter of
13.5 em, a height of 38 cm and a hemispherical bottom,
equipped with a high-speed agitator having two 5 cm
diameter flat blade turbine impellers positioned about
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1.5 cm and 5.5 cm from the bottom and driven by a 1/15
hp variable speed motor, and a 0.3 em inside diameter
stainless steel tube curved under the center of the
bottom blade for the introduction of a carbon
dioxide/air stream, was used for preparation and
reaction of calcium hydroxide (slake) to make
precipitated calcium carbonate (PCC).
A 13.4 weight percent (0.1443 g/ec) aqueous
calcium hydroxide slurry was prepared by adding 200
ZO grams of granular active lime from Specialty Minerals
Inc., hereinafter referred to as Adams lime, having an
available calcium oxide content of about 94 or more
weight percent as determined by ASTM procedure C-25-72,
to 1000 ml of water in the above described 4-Liter
reactor at 25C. and stirred at 1000 RPM for 10
minutes. The slurry was diluted to about 10.2 weight
percent (0.2080 g/ec), screened through a 60 mesh
screen to remove grit and cooled in the reactor to
25C. The agitator was adjusted to 1250 RPM and 0.1
percent sucrose by weight, based on the calcium
carbonate equivalent of the available lime, was added
to the slurry. The calcium hydroxide slurry was
carbonated to precipitate calcium carbonate by
introducing a gas mixture of 28 volume percent carbon
dioxide in air at 4.4 standard liters per minute (SLM)
into the slurry. The carbonation continues until the
pH value was less than 7.4. The slurry was passed
through U.S. Standard No. 325 (44 microns) sieve to
remove grit.
The slurried product was shown by SEM to be
well dispersed and quite discrete. A portion was then
vacuum filtered on a Buchner funnel, acetone washed,
and the subsequent filter cake dried at 120C. for at
least one hour to give a PCC product having a specific
surface area (SSA) of 34.6 m2/g. This run is
identified as Example 1 in Table 1.
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Example 2
Two additional discrete precipitated calcium
carbonate particles of varying surface areas were
prepared by using the process described in Example 1
but having higher sucrose levels as designated in
Examples 2 and 2A in Table 1.
Example 3
Three more experiments were conducted by
using the same process but in these cases lime from
Germany Valley (G. V.) Limestone Company was carbonated
at 35°C. instead of 25°C. Sucrose was added at 0.5
percent, 1.0 percent, and 4.5 percent as shown in Table
1 Examples 3, 3A and 3B.
The particle size of PCC has been shown to be
affected by the addition of various reagents. The
level of sucrose, at a specific temperature determines
the particle size (surface area) of the synthesized PCC
as shown in Table 1. The table also shows effects of
sucrose levels at two temperatures 25°C. and 35°C. and
comparison of two limes - Adams and Germany Valley.
TABLE 1
Example Lime Carbonation Sucrose SSA
Temperature
No. Type (C) Level { o) {m2/g)
1 Adams 25 0.1 34.6
2 Adams 25 0.5 42.4
2A Adams 25 2.0 53.2
3 G.V. 25 0.5 46.2
3A G.V. 35 0.5 36.4
3B G.V. 35 1.0 41.3
3C G.V. 35 4.5 61.1
Examples 3 and 3A show a decrease in SSA with
an increase in temperature. Examples 2 and 3 indicate
a small difference in SSA between products made with
different limes. The increase in sucrose gives an
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obvious increase in SSA (decrease in particle size).
Although the particle size can be determined
' by setting the level of sucrose between from about 0.1
percent and about 5 percent, in many cases a.t is more
' 5 desirable to maintain a low sucrose level to avoid
browning upon drying. The preferred range of sucrose
a.s 0.1 to 1.0 percent and more preferably about 0.5
percent.
Example 4
To demonstrate the effect of changing
temperature while carbonating at a sucrose level of 0.5
percent, a series of six experiments, Example 4 through
4E, were carried out from 10°C. to 55°C. by using the
process as described in Example 1 and the carbonation
temperatures are indicated for each respective
experiment. Germany Valley lime was used for Example
4E while Adams lime was used for the others. The
results are tabulated in Table 2.
Example 5
A 70-gallon modified mortar mixer was used to
prepare calcium hydroxide slurry (slake) for a scaled-
up carbonation. The carbonation was carried out in a
30-liter jacketed, baffled cylindrical stainless steel
reactor having an internal diameter of 11.5 inches, a
height of 20 inches, and having a hemispherical bottom.
The reactor was equipped with a high-speed agitator
having two 4.5 inch diameter flat blade turbine
impellers positioned about 4 inches and 8 inches from
the bottom and driven by a 5 HP variable speed motor.
It was also equipped with an 0.25 inch inside diameter
stair3.less steel tube curved under the center of the
A
bottom of the blade for the introduction of a carbon
dioxide/air stream. A 20.1 weight percent (0.2257
g/cc) aqueous calcium hydroxide slurry was prepared by
adding 2000 g of granular active lime from Specialty
Minerals Inc. having an available calcium oxide content
of about 94 or more weight percent as determined by
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ASTM procedure C-25-72, to 10.0 liters of water a.n the
above described 70 gallon mortar mixer, at 50°C. and
stirred for 10 minutes. The slurry was diluted to '
about 10.2 weight percent (0.1080 g/cc), screened
through a 60 mesh screen to remove grit and heated in '
the reactor to 60°C. The agitator was adjusted to 615
RPM and 0.5 percent sucrose by weight, based on the
calcium carbonate equivalent of the available lime, was
added to the slurry. The calcium hydroxide slurry was
carbonated to precipitate calcium carbonate by
introducing a gas mixture of 28 volume percent carbon
dioxide in air at 1.47 standard cubic feet per minute
SCFM into the slurry. The carbonation was continued
until the pH was less than 7.4. The slurry was passed
through U.S. Standard No. 325 (44 microns) sieve to
remove grit. The slurried product was shown by SEM to
be well dispersed and quite discrete. A portion was
then vacuum filtered on a Buchner funnel, acetone
washed, and the subsequent filter cake dried at 120°G.
for at least one hour to give a PCC product having a
specific surface area (SSA) of 11.1 m2/g (see Table 2,
Example 5).
The reaction temperature is proportional to
the reaction rate, meaning, the higher the temperature
the faster the reaction and it is also related to the
crystal size. Higher reaction temperatures produce
larger PCC crystals. By adding a reagent that
interacts with the Ca(OH)2 the crystal size is
modified. In Table 2 0.5 percent sucrose allows the
synthesis of a broad range of particles without
exhibiting the typical quantum change. In this case
,.
50° change in temperature produced about 60 m2/g change
in specific surface areas. Example 5 indicates that
scale-up of the reaction gives results that agree with
the 4-liter laboratory trials.
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TABLE 2
' Example Reaction Carbonation SSA
Temperature
No. Size ( C) (m2/g)
4 4L 10 69.6
4A 4L 25 42.4
4B 4L 35 36.4
4C 4L 38 29.2
4D 4L 50 25.2
4E 4L 55 21.7
5 30L 62 11.1
Example 5
A discrete precipitated calcium carbonate was
prepared using a 12.4 weight percent (0.1325 g/cc)
aqueous calcium hydroxide slurry prepared by adding
150 g of granular active lime from Specialty Minerals
Inc., to 1200 ml of water in the 4-liter reactor
described in Example 1, at 65°C. and stirred at 1000
RPM for 10 minutes, the slurry was diluted to about 7.6
weight percent (0.0799 g/ec), screened through a 60
mesh screen to remove grit, cooled in the reactor to
12.0°C. and the agitator was adjusted to 1250 RPM.
During the cooling process, 0.5 percent sucrose by
weight based on the theoretical PCC, was added to the
slurry a.n addition to 3.15 percent aluminum sulfate
based on theoretical PCC, solubilized to a 10 percent
by weight solution in water. The calcium hydroxide
slurry was carbonated to precipitated calcium carbonate
by introducing a gas mixture of 28 volume percent
r
carbon dioxide in air at 7.3 standard liters per minute
(SLM) into the slurry while holding the reaction
temperature close to isothermal conditions by running
chilled water through the vessel jacket.
The carbonation was continued until a pH
value of less than 7.4. The slurry was passed through
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U.S. Standard No. 325 (44 micron) sieve to remove grit
and a portion was then vacuum filtered on a Buchner
funnel, acetone washed, the subsequent filter cake was '
dried at 120°C. for more than 1 hour to give a PCC
product having a specific surface area (SSA) of 111.7
m2/g. See Example 5 a.n Table 3.
Examr~le 6
Two experiments were run in the 4-liter
equipment as described in Example 5 using 0.5 percent
sucrose and 3.15 percent alum but using carbonation
temperatures of 20° and 25° instead of 12°C. These
experiments produced products with specific surface
area values of 78.4 and 92.3 m2/g respectively, as
shown in Table 3, as Examples 6 and 6A respectively.
Examr~le 7
An experiment was conducted in the 30-liter
equipment as described a.n Example 4 but using 0.5
percent sucrose and 3.15 percent alum rather than
sucrose alone as was described in Example 5. Tn this
experiment 1700 grams of lime was used and was
carbonated at 38°C. at a rate of 16 SLM C02 to give a
specific surface area of 67.5 m2/g.
The four sucrose/alum experiments (Examples
5, 6, 6A, and 7) are compared with Examples 4, 4A, 4B,
4C which were synthesized using only sucrose and
temperature selection (from Table 2).
By adding 3.15 percent alum to 0.5 percent
sucrose prior to carbonation, the particle size can be
further reduced over use of sucrose alone. The surface
area is increased about 35 m2/g for the same reaction
temperature to about 113 m2/g at 10°C. By using
n
sucrose and a combination of sucrose and alum, the
specific surface area is extended to a range of about
10 to 115 m2/g (compare data from Tables 2 and 3).
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TABLE 3
., Example Additive Carbonation SSA
Temperature
No. Level (~) (C) (m2/g)
4 0.5~ sucrose 10 69.6
4A 0.5~ sucrose 25 42.4
4B 0.5~ sucrose 35 36.4
4C 0.5~s sucrose 38 29.2
5 0.5~ sucrose/ 12 112
3.15 alum
6 0.5~ sucrose/ 20 92.3
3.15 alum
6A 0.5~ sucrose/ 25 78.4
3.15 alum
7 0.5~ sucrose/ 38 67.5
3.15 alum
Precipitated calcium carbonate finds many
applications in a variety of markets. The breadth of
the range of particle sizes of the PCCs of this
invention allows even broader application. The direct
synthesis approach which can minimize or eliminate the
need for aging will allow a more economical
substitution for standard fine products used in many
applications. The elimination of chilling required for
some standard products will further reduce the cost of
these novel products. Table 4 shows typical examples
of applications for these products.
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