Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 3 S ~ ~ PC7750
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RHOMBOHEDRAL CALCIUM CARBONATE AND
ACCELERATED HEAT-AGING PROCESS
FOR THE PRODUCTION THEREOF
This invention relates to calcium carbonate having a
specified crystal morphology, average particle size,
particle size distribution and surface area, a process for
producing this form of calcium carbonate product, and a
method of use of the product as a filler material, for
example, in papermaking, to improve the optical properties
of the resulting paper.
The present trend in papermaking is toward the
manufacture of sheets with higher brightness and opacity.
This type of paper is now made mostly through the use of
extenders such as calcined clay and titanium dioxide. These
materials present a number of disadvantages, however, in
that they are rather expensive and unavailable in sufficient
supply to satisfy the needs of large scale paper
manufacturers. The competition for the limited quantities
available further causes the price of these materials to
remain high or to further increase. Consequently, their use
is effectively limited to high grade, expensive papers which
are usually produced in more limited quantities.
The papermaking industry has been clamoring for the
development of a filler-extender material which is
inexpensive, available in large quantities and which
provides the desired properties in the final paper.
Precipitated calcium carbonate has been used as a
filler material in paper for many years. Precipitated
calcium carbonate has the advantage of being able to be
produced in large quantity at relatively low cost.
Therefore, it was decided to develop a form of precipitated
calcium carbonate having a morphology which when used as a
filler or extender in paper, affords the high level of
brightness and opacity in the final paper product comparable
to that achieved with the more expensive calcined clay and
titanium dioxide filled papers.
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One method of altering the morphology of a crystalline
substance, such as precipitated calcium carbonate, in order
to change its properties, is by Ostwald ripening or
heat-aging.
Conventional heat-aging, also known as Ostwald
ripening, is a process whereby crystals, such as of calcium
carbonate, initially at a higher internal energy state, and
having a relatively small average particle size and
relatively high phase solubilities, undergo a phase
transformation by dissolving and redepositing on crystals at
a lower internal energy state. The process results in a
final crystal product characterized by greater perfection of
its crystal lattice structure, a narrower particle size
distribution, greater degreee of particle discreteness and
lower surface energy.
The procedure for conventional heat-aging of
precipitated calcium carbonate produced by the reaction of
calcium hydroxide and carbon dioxide is to endpoint the
precipitated calcium carbonate synthesis at pH 8.0, screen
the material to remove the impurities, and heat a 10% by
weight solids slurry to the aging temperature (usually
80C). The pH of the system rises to approximately 10.5 due
to the unreacted Ca(OH) 2 in the slurry. The aging reaction
can be monitored by measuring the surface area of the
calcium carbonate at hourly intervals.
Unfortunately, conventional heat-aging is a slow, time
consuming and highly capital intensive process. Heat-aging
of calcium carbonate with an initial morphology having a
higher surface area and smaller average particle size to a
final morphology having a surface area of from about 3 to
about 15 m2/g and an average discrete particle size of from
about 0.2 to about 0.9 microns typically takes extended
periods of time ranging up to several hundred hours,
depending in part on the degree of purity of the starting
material and the aging temperature. The time required for
heat-aging is inversely dependent on the purity of the
starting material, the purer the material, the shorter the
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aging time required. Calcium carbonates containing
impurities such as magnesium carbonate, at levels as low as
several weight percent, require considerably longer
heat-aging time to rearrange their morphologies. The time
required for heat-aging is also inversely dependent on the
aging temperature; a longer time is required at lower
temperatures, and a shorter time is required at higher
temperatures. In order to produce commercial scale
quantities of product obtained by the conventional
heat-aging process, large equipment volumes are needed, thus
making this process uneconomical.
The primary objective of the present invention is a
high opacifying, high brightness material that is applicable
at a paper machine's wet end, size press, or coating stage
and is competitive with current expensive fillers such as
calcined clay and titanium dioxide, and a process for the
rapid, cost effective production of such material in large
quantity.
Heat-aged calcium carbonate of rhombohedral morphology,
having a surface area of from about 3 to about 15 m2/g, an
average discrete particle size of from about 0.2 to about
o.9 microns, wherein the discrete particles have an aspect
ratio less than 2, and a particle size distribution such
that at least about 60 weight percent of the particles lie
2S within 50 percent of the equivalent discrete particle averge
spherical diameter, when utilized as a filler in the
manufacture of paper, results in a paper product that
demonstrates a significant improvement in performance with
respect to the optical properties of the paper over any
previously made paper utilizing other forms of calcium
carbonate as the filler material.
In order to meet the demand for large quantities of
heat-aged calcium carbonate having these properties, it was
desired to determine a way to significantly shorten the
amount of time required for heat-aging. Accordingly, this
invention teaches an accelerated process for heat-aging
which is particularly adapted to producing large amounts of
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heat-aged calcium carbonate having properties which make it
an unexpectedly superior filler material in papermaking.
The product produced by the accelerated heat-aging process
of the present invention is also useful as a filler for
rubber, plastics, paint, food products, synthetic resins and
the like.
The accelerated heat-aging process of the present
invention involves the adjustment of the pH= of the calcium
carbonate at the aging temperature to about 6.5, followed by
the addition of an alkali metal hydroxide, preferably sodium
hydroxide, to the calcium carbonate, to raise the pH to from
about 9.5 to about 12Ø
Fig. 1 shows the specific surface area over time of
samples of ultrafine precipitated calcium carbonate prepared
according to both the accelerated heat-aging process and the
conventional heat-aging process.
Fig. 2 is a particle size distribution curve of
rhombohedral calcium carbonate showing the narrow size
distribution.
Fig. 3 is a photomicrograph of the crystal morpholoqy
of particles of precipitated calcium carbonate heat-aged by
the accelerated heat-aging process.
Fig. 4 is a photomicrograph of the industry standard
non-heat-aged precipitated calcium carbonate having a
scalenohedral rosette structure.
Fig. 5 is a comparison of pigment scattering
coefficient in handsheets versus the percent filler content
for various forms of heat-aged ultrafine precipitated
calcium carbonate and non-heat-aged precipitated calcium
carbonate fillers.
Fig. 6 is a comparison of brightness in handsheets
versus the percent filler content for various forms of
heat-aged ultrafine precipitated calcium carbonate and
non-heat-aged precipitated calcium carbonate fillers.
Fig. 7 is a comparison of handsheet opacity versus the
percent filler content for various forms of heat-aged
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ultrafine precipitated calcium carbonate and non-heat-aged
precipitated calcium carbonate fillers.
Fig. 8 shows handsheet opacity versus specific surface
area for various heat-aged ultrafine precipitated calcium
carbonate and non-heat-aged precipitated calcium carbonate
fillers.
Fig. 9 shows pigment scattering coefficient versus
specific surface area for various heat-aged ultrafine
precipitated calcium carbonate and non-heat-aged
precipitated calcium carbonate fillers.
Fig. 10 is a transmission electron micrograph of
non-heat-aged ultrafine precipitated calcium carbonate
showing initial crystal morphology before heat-aging.
Fig. 11 is a transmission electron micrograph of
heat-aged ultrafine precipitated calcium carbonate showing
the rhombohedral morphology.
Fig. 12 is a transmission electron micrograph of
non-heat-aged scalenohedral precipitated calcium carbonate.
Fig. 13 is a transmission electron micrograph of
heat-aged scalenohedral precipitated calcium carbonate
showing the prismatic morphology.
Fig. 14 shows pigment scattering coefficient versus
percent filler content in handsheets for various forms of
heat-aged precipitated calcium carbonate and non-heat-aged
precipitated calcium carbonate fillers.
Fig. 15 shows opacity versus percent filler content in
handsheets for various forms of heat-aged precipitated
calcium carbonate and non-heat-aged precipitated calcium
carbonate fillers.
Fig. 16 shows brightness versus percent filler content
in handsheets for various forms of heat-aged precipitated
calcium carbonate and non-heat-aged precipitated calcium
carbonate fillers.
Fig. 17 shows specific surface area versus time for
various heat-aged ultrafines, heat-aged at various
temperatures.
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Fig. 18 shows a comparison of the aging rates between
80C and 200C for the accelerated heat-aging process using
hydrothermally aged and non-hydrothermally aged starting
materials.
Fig. 19 is a photomicrograph of hydrothermally
heat-aged ultrafine precipitated calcium carbonate showing
the rhombohedral morphology.
While not wishing to be limited to a particular theory
to explain the basis for the significant improvement in the
time required for heat-aging according to the present
process, we believe that the reduction in heat-aging time is
attributable to the presence of a high concentration of
hydroxide ions produced by the dissociation of the alkali
metal hydroxide added to raise the pH. The hydroxide ions
supplied by the dissociated alkali metal hydroxide act to
suppress the phenomenon, known to those skilled in the art
as "flashback". Flashback is the dissociation of calcium
hydroxide, which is entrained in all forms of calcium
carbonate at some level of concentration, even in high
purity calcium carbonate. Calcium ions produced by the
dissociation of calcium hydroxide are responsible for
interfering with and inhibiting the rate of the conventional
calcium carbonate heat-aging recrystallization process,
thereby causing the lengthy time required for conventional
heat-aging. The calcium ions prevent the dissociation of
calcium carbonate which in turn hinders the
recrystallization phase of the heat-aging process. In the
accelerated heat-aging process of the present invention, the
presence of the high concentration of hydroxide ions
supplied by the dissociating alkali metal hydroxide added to
the calcium carbonate causes an equilibrium shift in the
reverse direction to suppress disso-ciation of the entrained
calcium hydroxide in the calcium carbonate, resulting in a
more equal concentration of calcium and carbonate ions in
the recrystallization mixture, which greatly speeds up the
heat-aging recrystallization process.
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According to the accelerated heat-aging process of the
present invention, the crystal morphology of calcium
carbonate is rearranged from an initial morphology having a
surface area of greater than about 15 m2/g and an average
discrete particle size of from about 0.01 to about 0.8
microns, to a final morphology having a surface area of from
about 3 to about 15 m2/g, and an average discrete particle
size of from about 0.2 to about 0.9 microns.
As defined herein, average discrete particle size
refers to the equivalent spherical diameter of an individual
particle which can exist as an individual particle or as
part of a cluster or agglomerate, as opposed to the
equivalent spherical diameter of the cluster or agglomerate
itself.
The process comprises the steps of initiating heat-
aging of the calcium carbonate by heating it to an aging
temperature of from about 40 to about 100C; adjusting the
pH of the calcium carbonate at the aging temperature to
about 6.5, such as by addition of carbon dioxide; adding an
alkali metal hydroxide to the calcium carbonate at the aging
temperature to raise the pH to from about 9.5 to about 12.0;
maintaining the calcium carbonate at the aging temperature-
for a sufficient time to cause the morphology of the calcium
carbonate to rearrange to the final form; and terminating
heat-aging to fix the morphology of the calcium carbonate in
the final form.
The process of the present invention can be applied to
calcium carbonate either as precipitated calcium carbonate
or as fine ground natural limestone.
The calcium carbonate can be in a form either as a dry
powder, which is subsequently slurried, or as an aqueous
slurry. When the calcium carbonate is used in the form of
an aqueous slurry, it has been found that a slurry having
about a 10 weight percent calcium carbonate solids content
is preferred.
The alkali-metal hydroxide can be a hydroxide of any
metal from group IA of the periodic table. Sodium hydroxide
21~3S69
is preferred. It has been found that calcium hydroxide is
not effective for use in adjusting the pH of the calcium
carbonate at the aging temperature. This is believed to be
due to the presence of the calcium ions formed on
dissociation of the calcium hydroxide inhibiting the
dissolution of calcium carbonate, thus slowing the aging
process.
The amount of alkali metal hydroxide added to the
calcium carbonate is in an amount of from about 0.1 to about
15 weight percent, based on the dry weight of calcium
carbonate. The alkali metal hydroxide added to adjust the
pH can be pure or an aqueous alkali metal hydroxide
solution. Sodium hydroxide is the preferred alkali metal
hydroxide.
The length of time the calcium carbonate must be
maintained at the aging temperature in order to
recrystallize to the new morphology is determined by both
the initial morphology of the calcium carbonate and the
nature and extent of any impurities present in the calcium
carbonate.
Where the calcium carbonate starting material has an
initial average particle size of from about 0.01 to about
0.5 microns and has a high purity, the aging time is as
short as about 60 minutes. For calcium carbonate starting
material having a larger initial agglomerated particle size
of from about 0.5 to about 2 microns, and/or containing
impurities, particularly such as magnesium carbonate, of up
to about 5 weight percent, the heat-aging time can be as
long as 24 hours.
When the calcium carbonate has been heat-aged
sufficiently long so that transformation of the calcium
carbonate crystal morphology is complete, the heat-aging
process must be terminated to fix the morphology in the
desired state, and prevent further recrystallization.
Heat-aging can be terminated by either reducing the
temperature to below about 40C; by reduction of the pH; or
by a combination of temperature and pH reduction.
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It has been found that heat-aging does not occur or
proceeds at such à slow rate as to be insignificant when the
temperature is reduced to about 40C or below. The
heat-aging process is also very pH sensitive and a reduction
of the pH to below about 8.5 effectively halts heat-aging
and further recrystallization of the calcium carbonate.
Termination of heat-aging by temperature reduction can
be effected rapidly by quenching the calcium carbonate, such
as by immersion in an ice bath.
Termination of heat-aging by pH reduction is effected
by addition of C02 or of a polybasic acid to the calcium
carbonate. The polybasic acid is preferably phosphoric
acid.
Calcium carbonate can also be heat-aged according to
further embodiments of the process of the present invention
which utilize hydrothermal techniques.
It has been found that heat-aging in a hydrothermal
bomb under hydrothermal conditions of high temperature, up
to about 300C, and under elevated pressure, up to about 7so
psi, causes an acceleration of the aging process and
reduction in the required aging time from the time required
for conventional heat-aging, and even a further reduction in
aging time from the time required for non-hydrothermal
accelerated heat-aging at conditions of lower temperature
and ambient pressure.
Hydrothermal-aging alone, with no adjustment of the
initial pH of the starting calcium carbonate, causes faster
rearrangement of the calcium carbonate crystal morphology
than under the initial pH-adjusted, lower temperature,
ambient pressure accelerted heat-aging conditions of the
other embodiment of the process of this invention.
It has been further found that the heat-aging crystal
morphology rearrangement process is still further
accelerated in another embodiment of the process of the
present invention wherein heat-aging is initiated at a
temperature of from about 40 to about 100C, and the initial
pH of the starting calcium carbonate is adjusted to about
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6.5, followed by addition of an alkali metal hydroxide,
preferably sodium hydroxide, to the calcium carbonate at the
heat-aging temperature to raise the pH to from about 9.5 to
about 12.0, as in the non-hydrothermal accelerated
heat-aging process of the present invention, and the calcium
carbonate is then hydrothermally-aged in a hydrothermal bomb
under hydrothermal conditions in which the bomb is
pressurized to a pressure up to about 750 psi and the
temperature in the bomb is then raised to the hydrothermal
temperature at the bomb pressure, of up to about 300C, at
which conditions even more rapid rearrangement of the
calcium carbonate crystal morphology occurs, resulting in an
even shorter overall heat-aging time.
It has been found that the optical properties of paper,
particularly brightness, are greatly improved by utilizing
heat-aged calcium carbonate having a surface area of from
about 3 to about 15 m2/g, an average discrete particle size
of from about 0.2 to about 0.9 microns, wherein the discrete
particles have an aspect ratio less than 2, and a particle
size distribution such that at least about 60 weight percent
of the particles lie within 50 percent of the equivalent
discrete particle averge spherical diameter, as a filler
material during papermaking. While any heat-aged calcium
carbonate having the desired physical properties is
effective, that produced according to the accelerated
heat-aging process and the hydrothermal-aging process of the
present invention, is highly desirable in that it is fast
and economical to produce on a large scale in the quantities
required for papermaking.
The nature and scope of the present invention may be
more fully understood in view of the following non-limiting
examples.
All calcium carbonate referred to in the following
examples was precipitated from lime produced at the Pfizer
quarry in Adams, MA. All reactions were conducted on either
a 4 or 30 liter scale.
EXAMPLE 1: Accelerated heat-aging.
2~35~9
Typically, when precipitated calcium carbonate is made,
the reaction is complete when the pH of the system reaches
8Ø After the endpoint, the pH of the system rises to
approximately 9.5 due in part to the unreacted Ca(OH)2 in the
system. However, solubility product calculations done to
determine the effect of Ca(OH) 2 on the solubilization and
precipitation of calcium carbonate at elevated pH indicate
that Ca(OH) 2 in solution retards the aging. Additionally,
solubility product calculations also indicate that the
optimum pH for aging calcium carbonate is 10.2 - 11.5
although aging occurs at other basic pH's.
Precipitated calcium carbonate having less than 0.1
micron particle size, referred to herein as "ultrafine", and
a surface area of from about 25 to about 40 m2/g, was
endpointed at pH of 6.7 to eliminate as much Ca(OH) 2 as
possible. The product was screened to eliminate impurities,
and 2400g of the product in a 10% by weight slurry was
heated to 80C. At 80C, the pH of the slurry was again
brought down to 6.7 with carbon dioxide, and 48g of sodium
hydroxide was added to the slurry (2% by weight) to bring
the pH up to an optimum aging pH of 10.9 - 11.1. Within one
hour, the viscosity increased approximately 40-fold.
Samples were taken every hour for surface area
determination. The aging reaction was observed to proceed
at an accelerated rate. By this process, a product surface
area in the range of 8 m2/g was achieved within 4-8 hours as
compared to over 22 hours by a normal aging process. Figure
1 shows the comparison between ultrafine that was aged by
the accelerated process and a conventional aging process.
EXAMPLE 2: Uniqueness of particle morphology by
accelerated heat-aging.
Fresh 30 liter batches of ultrafine precipitated
calcium carbonate in which no additives were used during
slaking, carbonation, or post-carbonation were aged by the
accelerated heat-aging process described in Example 1. The
aging temperature was 80C and the pH was 10.5. The aged
products, with surface areas ranging from 18.2 to 7.4 */g,
.
21 ~35~g
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were evaluated in hand sheets. The final particle
morphology of ultrafine precipitated calcium carbonate
particles was rhombohedral, with a final particle size of
about 0.3 - 0.5 microns. The particle size distribution of
this material, showing its narrow size distribution, wherein
at least about 60 weight percent of the particles lie within
50 percent of the equivalent discrete particle average
spherical diameter, is shown in Figure 2. Figure 3 is a
photomicrograph of the crystal morphology of particles
heat-aged by the accelerated heat-aging process of this
invention. This figure should be compared to Figure 4 which
is a photomicrograph of the crystal morphology of
non-heat-aged precipitated calcium carbonate having a
scalenohedral rosette structure, which was heretofore the
best known filler for achieving enhanced optical properties
in paper.
The results of the hand sheet study indicated that the
heat-aged products had superior optical properties than the
industry's best non-heat-aged scalenohedral precipitated
calcium carbonate. This is shown in Figures 5, 6, and 7.
The study also indicated that for a heat-aged precipitated
ultrafine calcium carbonate made from Adams, MA lime, the
best surface area for optical performance is 10 - 5 */g as
shown in Figures 8 and 9.
EXAMPLE 3: Comparison of particle morphologies for
different precursors.
Numerous materials such as ultrafine and scalenohedral
precipitated calcium carbonates were heat-aged to surface
areas between 8 and 12 m2/g with particle sizes of from
about 0.40 to about 0.55 microns by the accelerated heat-
aging process of Example 1. Table 1 lists the properties of
the starting materials and the aged materials.
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TABLE 1
Sample Initial Initial Final Aged
Number Morph. S.A.(I) MolPh. SSA(l) PSD(2)
Sample 1 Ultrafine 42.2Rho~l~h~d~al 8.6 .473
Sample 2 Scsl~ .h~-~.,.l 19.4Prismatic 9.6 .455
Plu~lies of samples subn~itted for ~ t evaluation to compare the optical properties of aged
calcium c~b~ lJoh~ morphology vs. prismatic morphology.
(1) SSA: Specific Surface Area in m2/g.
(2) PSD: 509~ Particle Size Distribution in microns.
Aging a precursor such as ultrafine resulted in a final
product having a rhombohedral morphology. Aging a
scalenohedral precursor resulted in a final product having
20 a prismatic morphology. This can be clearly seen in Figures
10, 11, 12 and 13, which show the comparison between the
precursors and the aged products. All the materials listed
in Table 1 were evaluated in laboratory hand sheets. The
results of the study indicated that even though the surface
25 areas and particle sizes of the products were similar
regardless of the precursor, the product morphology has a
significant effect upon the optical properties of the
particle. Rhombohedral particles exhibited superior optical
properties over prismatic particles. The optical
performance of the prismatic particles was comparable to or
lower than that of the current industry standard
scalenohedral calcium carbonate. However, in all cases,
aging improved brightness. The results of the handsheet
study are shown in Figures 14, 15, and 16.
EXAMPLE 4: Optimum accelerated heat-aging tempera-
ture.
Three different heat-aging temperatures 40, 60, and
80C were compared to determine the effect of temperature on
the heat-aging rate of calcium carbonate. The pH of the
aging reaction was 10. 5. As was expected, the rate of
aging, as determined by the surface area of the aged
material, was positively correlated with the aging
21~3~69
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temperature; that is, the rate of aging was faster at a
higher aging temperature. The sample that was aged at 60C
aged similarly to the sample aged at 80C, with respect to
the final surface area of the product, except that the
reaction lagged approximately 10-15 hours behind the 80C
reaction. The surface area of the products from.the 40C
aging did not change significantly even after 45 hours of
aging. The surface area of heat-aged ultrafine versus the
heat-aging time for each of the three temperatures is shown
in Figure 17.
EXAMPLE 5: Hydrothermal processing.
Since the aging reaction proceeds faster at high
temperature, it was assumed that a sample that is aged
hydrothermally attains a low surface area at a much faster
rate than a sample that is aged at 80C at ambient pressure.
A batch of ultrafine precipitated calcium carbonate
(surface area 38 m2/g) was synthesized without additives and
endpointed at pH 8Ø A sample of this material was
slurried to a 10% by weight solids content and 1800 ml of
the slurry was placed into a 4 liter PARR hydrothermal bomb.
The temperature was raised to 200C at 500 psi, within one
hour, held at 200C for one hour and cooled for a total
aging time of 2.5 hours. The surface area of the product
was 15.7 */g, which would have taken 7 hours to attain under
conventional heat-aging conditions.
EXAMPLE 6: Hydrothermal-aging via accelerated aging
process.
A batch of ultrafine precipitated calcium carbonate
(surface area 38 m2/g) was divided into two portions, A and
B. Portion A was aged by the method described in Example 1.
Portion B was treated in a similar manner as portion A, as
described in Example 1, up to and including the point were
the sample was heated to 80C and NaOH was added. At this
point, portion B was placed into a 4 liter PARR hydrothermal
bomb where the temperature was raised to 200C at 500 psi.
Portion B was kept at these conditions for 1 - 1.5 hours at
which point the material was quenched to room temperature to
Z~356~
prevent further aging. Figure 18 shows the comparison of
the aging rates between 80C and 200C where it can be seen
that a decrease in surface area occurs at a much faster rate
when the starting material is hydrothermally-aged.
S The morphology of the hydrothermally-aged product is
shown in Figure 19.
