Note: Descriptions are shown in the official language in which they were submitted.
CA 02208150 1997-06-18
MANUFACTURE OF PRECIPITATED CALCIUM CARBONATE
OF IMPROVED COLOUR
The present invention relates to a method for the
manufacture of precipitated calcium carbonate (PCC) from
lime using ethanolamine or an organic amine. In
particular, the method provides a precipitated calcium
carbonate with a high brightness.
Precipitated calcium carbonate is used in a wide
variety of end uses. In some end uses, the brightness of
the precipitated calcium carbonate is of minor
importance. However, in a number of end uses e.g. use of
precipitated calcium carbonate as a filler, as a coating
agent or as a pigment, it is important or critical that
the precipitated calcium carbonate have a high degree of
brightness. As used herein, brightness of calcium
carbonate is measured by the method of TAPPI T646 om-94.
Use of precipitated calcium carbonate as a filler,
coating agent or pigment frequently requires that the
brightness be not less than 950.
A number of techniques may be used to obtain
precipitated calcium carbonate. A typical method
involves forming a suspension of lime in water,
converting the lime into calcium hydroxide, and treating
the resultant suspension of calcium hydroxide with carbon
dioxide so as to form calcium carbonate. Such treatment
leaves grit, colored material and other insoluble matter
in the suspension, which contaminates the resulting PCC
that is formed. Thus, in order to obtain PCC with
acceptable properties, the lime used in the process must
be of a high purity.
Canadian application No. 2 203 210 of W.J. Wilson
and A.L. Porter, filed April 21, 1997, discloses admixing
impure calcium oxide with an aqueous solution of a salt
of an organic amine or alkanolamine with hydrochloric or
nitric acid, and separating the resultant solution and
recovering PCC. Such a method overcomes problems
associated with dissolution of lime with ammonium nitrate
or ammonium chloride in that, if carbon dioxide and air
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is used to precipitate the calcium carbonate, then a significant part of the
cost
of the process is the capital and operating cost of the scrubbing of ammonia
from the exit gas, which is a mixture of ammonia, carbon dioxide and air.
U.S. Patent 4 900 533 of P.J. Malden discloses slaking calcium oxide
in water, cooling the suspension to 45°C or lower, and then carbonating
the
suspension in water with carbon dioxide in the presence of a dithionite
bleaching agent.
A method for the manufacture of precipitated calcium carbonate using
an organic amine or alkanolamine of improved brightness has now been
found.
Accordingly, an aspect of the present invention provides for a method
for the manufacture of precipitated calcium carbonate with a high brightness
from impure calcium oxide, comprising:
(a) admixing said impure calcium oxide with an aqueous solution
consisting essentially of a salt of at least one compound selected from the
group consisting of organic amines of the formula RNH2 and alkanolamines of
the formula NH2(R'OH), where R and R' are alkyl groups of 1-4 carbon
atoms, and hydrochloric or nitric acid;
(b) adding a solution of a reducing agent to the solution of (a);
(c) separating the solution so obtained from insoluble matter
therein; and
(d) treating the solution with (i) carbon dioxide or (ii) the carbonate
of said amine or alkanolamine of step (a).
In a preferred embodiment of the method of the present invention, the
salt is added in step (i) in at least the stoichiometric amount to dissolve
the
lime.
In another embodiment, the reducing agent is added in an amount
effective to reduce the colour of the solution of (a).
In a further embodiment, the reducing agent is sodium hydrosulphite,
which is added in an amount to
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effect a reduction in colour of the solution subjected to
separation in step (c) and an improvement in the
brightness of the PCC obtained in step (d). In
embodiments, the sodium hydrosulphite is added to the
aqueous solution in step (a) prior to admixing of the
impure calcium oxide.
In a still further embodiment, the ratio of the salt
of organic amine or alkanolamine to water, on a weight
basis, is in the range of 1:1 to 1:3.
In yet another embodiment, the temperature of the
solutions is at least 70°C and especially at least 75°C.
In another embodiment, the solution of (a) is
additionally treated for separation of insoluble matter
therein prior to step (b).
In yet another embodiment, the method of the present
invention provides precipitated calcium carbonate having
a brightness of at least 950, and especially at least
97 0 .
The method of the present invention relates to the
manufacture of precipitated calcium carbonate from impure
lime. Sources of impure lime are known and may be
obtained from the calcination of limestone in an
industrial kiln. Sources of limestone suitable for a
calcination process are known to persons skilled in the
art. It is understood that the limestone would be
contaminated with magnesium carbonate and that other
materials may also be present, some of which might be
colored. Examples of the latter could include iron
oxide, manganese oxide, iron carbonate, manganese
carbonate, sand and other silicates. Calcination of the
limestone may be carried out at temperatures in the range
of from about 950°C to about 1100°C, especially in the
range of from about 950°C to about 1050°C. Magnesium is
not solubilized in the process of the present invention
if the temperature of calcination is above 900°C.
Nonetheless, it is understood that any suitable source of
lime may be used for the process.
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The lime is slaked by admixing with a reagent
solution in an aqueous solution in which the calcium will
dissolve. The reagent solution is an aqueous solution of
a salt of at least one compound selected from the group
consisting of organic amines of the formula RNH2 and
alkanolamines of the formula NH2(R10H), where R and R1 are
alkyl groups of 1-4 carbon atoms. Such amines are
primary amines and the alkanolamines only have one -OH
moiety. The preferred alkyl groups are methyl and ethyl.
The organic amine is in the form of a salt with
hydrochloric acid or nitric acid.
In order to increase the concentration of PCC
ultimately obtained in the method, and thereby reduce the
amount of aqueous solution that must be handled during
operation of the method, it is preferred to increase the
concentration of the salt of the organic amine or
alkanolamine in the solution used to treat the lime i.e.
in step (a) of the method described above. However, it
is found, as least in some embodiments of lime that may
be used in the process, that there is a tendency for the
formation of colour in the solution of step (a). The
formation of such colour is detrimental to the brightness
of the PCC that is ultimately obtained. As an example,
if ethanolamine hydrochloride is used as the amine, then
with some sources of lime, colour may be formed as the
concentration of ethanolamine hydrochloride is increased
above 1 part ethanolamine hydrochloride to 3 parts of
water. The colour of the solution obtained is typically
brown in nature.
In the method of the present invention, the solution
of (a) is treated with a reducing agent. In particular,
the solution is treated with a reducing agent that does
not result in precipitation of a calcium compound in any
significant amount. The amount of reducing agent may be
varied, and is used in an amount effective to reduce the
colour of the solution of (a). In a preferred embodiment,
the reducing agent is present in the aqueous solution of
CA 02208150 1997-06-18
(a) prior to addition of the impure calcium oxide, which
tends to prevent formation of colour, especially brown
colour.
Examples of the reducing agent include sodium
5 hydrosulphite, also known as sodium dithionite (Na2S209),
and sodium sulphite. Sodium hydrosulphite is preferred.
The resultant solution is filtered so as to remove
impurities such as grit and other contaminants that have
not dissolved in the solution. While the solution is
filtered after the addition of reducing agent, the
solution may additionally be filtered prior to addition
of the reducing agent.
The filtered solution is then reacted in one of two
ways viz a) with carbon dioxide or b) with an amine
carbonate. In preferred embodiments, the amine of the
amine carbonate is the same as the amine in the aqueous
solution used to solubilize the lime. This results in
precipitation of calcium carbonate which, after
filtration, leaves a mother liquor that is suitable for
reuse. Use of carbon dioxide gas is also a preferred
method of effecting precipitation of calcium carbonate,
because a consequence of the use of organic amine or
alkanolamine in step (a) is the option to recover excess
carbon dioxide in step (d), which may readily be
accomplished. The precipitated calcium carbonate that is
filtered from the solution is washed, to purify the
calcium carbonate and wash off amine for recycling to the
process. In preferred embodiments of the method, the
calcium carbonate that is obtained is in the range of
97+o purity. ,
The ratio of the salt of organic amine or
alkanolamine, especially ethanolamine hydrochloride, to
water, on a weight basis may be at least 1:1, especially
in the range 1:l to 1:3, although ratios with higher
amounts of water may be used. Use of ratios in the range
of 1:1 to 1:3 results in a significant decrease in the
volume of the solution obtained on precipitation of the
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PCC, which must be processed for recovery of PCC. Ratios
of 1:1 or close to that ratio are preferred.
Heating of the solution containing PCC in the form
of a suspension prior to recovery of the PCC may be used
to effect a change in the crystal form of the PCC. For
instance, heating of the solution to 80°C tends to effect
a change of the crystals to, or facilitate formation of,
scalenohedral crystals. Heating to 100°C results in
aragonite crystals. In addition to the effects on the
type of crystal obtained, heating of the solution results
in a decrease in solution viscosity. The heated solution
is more readily filtered.
Passing of carbon dioxide into the solution obtained
with a 1:1 mixture tends to result in formation of a gel
of PCC, which may be converted to a crystalline form.
During such conversion, the particle size of the crystals
may be controlled e.g. particle sizes in the range of 0.5
to 1 micron may be obtained.
The product obtained has a high brightness, as
measured by the method of TAPPI T646 om-94. In preferred
embodiments, the PCC has a brightness of at least 970.
In addition, the PCC obtained from the process is
essentially free of grit and insolubles, in contrast to
the conventional method of preparing PCC, and is
therefore believed to be a PCC of high quality. The
absence of grit means that the product can be used
without being abrasive to calendering rolls in paper
making processes.
It has also been found that under the conditions of
preparation, only traces of magnesium carbonate are found
in the resulting product, primarily because the magnesium
tends not to dissolve in the slaking process and
therefore is filtered out prior to carbonation.
The PCC obtained by the process of the present
invention may be prepared in a variety of crystalline
forms, depending on the temperature of crystallization,
as is well known in the art. For example, at 10 to 15°C,
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the product is obtained as rhombohedral crystals, while
at 35 to 40°C vaterite crystals are obtained. This
metastable form of calcium carbonate may be converted to
aragonite crystals, which are obtained in the form of
needles, by heating to about 95°C.
While processing with COz in general tends to give
coarse crystals, i.e. greater than 1 micron in size,
mixing of solutions of ethanolamine carbonate with the
reaction mixture can give gelatinous products which with
stirring causes crystals to grow. After a period of
time, which in embodiments and depending on the
particular solution might be about 30 minutes, the
crystals grow to an appropriate size, e.g. in the range
of where the majority of the crystals are less than 1
micron.
Thus by varying the reaction conditions,
temperatures and concentrations, it is possible to obtain
PCC products in a wide range of particle sizes. This
provides versatility to the process and extends the
potential application of the invention.
The present invention is illustrated by the
following examples.
EXAMPLE I
In a comparative experiment, 100 grams of a sample
of lime were slaked in 1000 ml of water, and the
resultant suspension was passed through a 325 mesh screen
to remove solid matter. The temperature of the resultant
solution was raised to 40°C, and then carbon dioxide gas
was passed through the solution.
The precipitated calcium carbonate that was obtained
was examined by electron microscopy, and found to be
scalenohedral crystals with a size of about 1 micron.
The brightness was measured, using an elrepho brightness
meter according to the method of TAPPI T646 om-94, and
found to be 82.2.
The procedure was repeated, with sodium
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hydrosulphite added to the solution at 40°C prior to
passing carbon dioxide through the solution. The
brightness of the resultant precipitated calcium
carbonate was found to be 91.4.
EXAMPLE II
Ethanolamine chloride was prepared by mixing 2207
grams of ethanolamine with 3819 grams of concentrated
hydrochloric acid, to give a 1:2 solution of ethanolamine
hydrochloride in water. 1125 grams of the lime of
Example I were added to the solution, which was then
stirred for a period of one hour. The solution was brown
in colour.
500 ml of the solution that was obtained were heated
to 80°C, following which 2 gm of sodium hydrosulphite were
added. The solution was stirred for five minutes, and
filtered to remove residue. The resultant solution was
very slightly yellow in colour.
200 ml of the solution obtained was maintained at
80°C, and carbon dioxide was passed through, with
continuous stirring, until a pH of 6.35 was reached. The
solution was then filtered, and the filtrate was washed
with water containing carbon dioxide, at a temperature of
80°C.
The crystals of the calcium carbonate obtained were
rhombohedral, and had a brightness of 97.8.
This example shows that the method of the present
invention provides PCC with substantially improved
brightness.
EXAMPLE III
In separate experiments, the above procedure was
repeated with and without the addition of sodium
hydrosulphite. It was found that when the sodium
hydrosulphite was added, the PCC obtained had a
brightness of 97.6. In contrast, when the sodium
hydrosulphite was omitted, the PCC obtained had a
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brightness of only 91.1.
This example illustrates the substantial improvement
in brightness of the PCC obtained at high concentrations
of ethanolamine hydrochloride, when sodium hydrosulphite
is added at high temperatures (80°C) to the brown solution
of the slurry of lime, and filtered.
