FIELD OF THE INVENTION
[0001] The present invention relates to a process for fiber loading by creating calcium
carbonate
in situ from calcium hydroxide and carbon dioxide. The process provides a filler in a pulp
suspension by precipitating calcium carbonate in and on the fibers of the pulp suspension.
Calcium carbonate is a much used filler in the production of paper, board and pulp.
The filler improves the physical and optical properties of the finished product and
the printing properties of paper. By replacing part of the fibers with less expensive
inorganic material, the filler improves the economy of the papermaking.
BACKGROUND OF THE INVENTION
[0002] There are two different approaches which are used in the art for providing filler
within the fibers (or fibres) of a pulp suspension. These are generally called "lumen
loading" and "fiber loading". Lumen loading refers to the process of introducing minute
particles of filler, often previously precipitated calcium carbonate (PCC), into the
cavity or lumen of the fiber by vigorously mixing a pulp slurry containing an excess
of the filler particles. Fiber loading refers to the process of creating the calcium
carbonate particles in the fiber lumen and on the fiber cell walls by a chemical precipitation
reaction.
[0003] Two different reactions have been used to provide the precipitation reaction of the
fiber loading process, one typically comprising the reaction between sodium carbonate
and a calcium salt such as calcium chloride (1) and the other typically comprising
the reaction between calcium hydroxide and carbon dioxide (2):
Na
2CO
3 + CaCl
2 → CaCO
3 + 2 NaCl (1)
Ca(OH)
2 + CO
2 → CaCO
3 + H
2O (2)
[0004] The present invention relates to a fiber loading process utilizing the principle
of the second reaction (2) mentioned above, i.e. a reaction between calcium hydroxide
and carbon dioxide to precipitate calcium carbonate on and in the fibers of a pulp
suspension.
[0005] The principle of creating the calcium carbonate
in situ in the pulp by the chemical reaction between calcium hydroxide and carbon dioxide
was disclosed by
Klungness in US Patent 5,223,090. Calcium hydroxide or calcium oxide (which provides calcium hydroxide in water) was
mixed into a dewatered pulp suspension. Then carbon dioxide was added to the mixture
in a high shear pressurized refiner. In this manner about 50% of the precipitated
calcium carbonate could be retained on the fiber. The rest was removed with the white
water.
[0006] The fiber loading process of Klungness has since then been the subject of extensive
investigations in attempts to improve features such as the speed of the process, the
amount of precipitated filler, the crystal structure of the precipitate, the location
of the precipitate, etc. A vast number patent applications have been filed for such
improvements, some of which are mentioned below.
[0007] US-A1=2002/0088566 and
US-B1.6,471,825 describe continuous processes for fiber loading by reacting calcium oxide and/or
calcium hydroxide with a pressurized reactant gas selected from carbon dioxide and
ozone and mixtures thereof.
EP-A2-1 243 693 describes a fiber loading process, wherein the precipitation reaction is performed
in the presence of a bleaching agent.
US-A1-2003/0121624 describes a fiber loading process utilizing a plurality of reactors in series or
in parallel to split the loading process into several smaller processes.
US-A1-2004/0154770 describes the use of liquid carbon dioxide as a reactant in the fiber loading.
US-A1-2008/0210391 suggests an improvement in the fiber loading process by adding the calcium hydroxide
or calcium oxide at a very early stage of the papermaking, i.e. to the dry or moist
fibers prior to or during the initial pulping.
WO-A1-2011/151525 describes adding calcium hydroxide and carbon dioxide into water to provide an "acidic
water" having a pH below 8.3. The acidic water is used for dilution of the pulp and
calcium carbonate precipitation is triggered by raising the pH simultaneously with
dewatering of the pulp.
[0008] WO-A1-2008/131820 describes a fiber loading process based on the principles of equation (1) above and
wherein sodium carbonate reacts with a calcium salt to provide calcium carbonate.
In the described process, the typically used calcium salt, i.e. calcium chloride,
has been exchanged for the more alkaline calcium hydroxide. The pH may additionally
be adjusted by carbon dioxide or sodium hydroxide in order to cause the calcium carbonate
to precipitate primarily on the outer side of the fibers.
[0009] As noted above, the present invention relates to the Klungness reaction according
to equation (2) above and wherein the carbonate moiety of the precipitate is derived
from carbon dioxide. When calcium carbonate is loaded
in situ onto and into the fibers by precipitation in this manner, a frequent problem especially
in continuous papermaking processes is the presence of precipitated and non-precipitated
calcium species in the aqueous phase of the pulp suspension. As indicated in
US 5,223,090, the Klungness reaction resulted in only 50% of the precipitated calcium carbonate being retained
in the fibers. The rest of the calcium carbonate as well as any unreacted calcium
ended up in the white waters. With ever more closed circulations of papermaking process
waters, this calcium accumulates in the circulation, which in turn causes problems
in the papermaking. Among these problems there may be mentioned coagulation of sticky
particles, soap and ink particles, precipitation of inorganic calcium salts as a scaling,
precipitation of calcium oxalate and reprecipitation of calcium carbonate, a decrease
in the swelling ability of the fibers, interference with retention aids, dispersants
and other charged paper additives, etc. A further complication in the Klungness process
is caused by the need to use high pressure carbon dioxide in the precipitation reaction.
[0010] A mixture of fibre and calcium carbonate produced by fiber loading has better wet
strength values with a specific PCC filler amount in comparison to a mixture of fibre
and calcium carbonate produced in a conventional manner. Due to the improvement of
wet tensile strength it is possible to introduce into the pulp suspension substantially
larger amounts of fillers than previously, which typically has the effect of lowering
paper manufacturing costs. The scattering of light and opacity is also improved, because
there are more light scattering surfaces attached to the fibers.
[0011] In addition, there are advantageous effects on the wet tension properties of the
product being produced and it is possible to decrease the dusting of the final product
by the fiber loading method.
[0012] An object of the present invention is to provide a fiber loading process based on
the known Klungness reaction between calcium hydroxide and carbon dioxide and wherein
a high proportion of the calcium carbonate is retained in the pulp as filler in and
on the fibers.
[0013] Another object of the invention is to provide a process, wherein the amount of calcium
species, including free calcium ions, in the circulating process waters is low and
accumulation of calcium in the process is reduced or avoided.
[0014] A further object of the invention is to produce paper having a high level of calcium
carbonate filler produced
in situ without the need for a pressurized system.
[0015] An object of the inventions is also to produce paper having a high level of ash based
on calcium carbonate loading in the fibers during the papermaking process.
[0016] An object of the invention is further to provide paper having a high brightness.
This is especially valuable in case of deinked paper, DIP, wherein the precipitated
calcium carbonate will cover the ink particles, increase the brightness and reduce
the amount of sticky particles.
[0017] An object of the invention is to provide a fiber loading process, wherein the added
calcium compound is effectively utilized by substantially complete precipitation thereof
in and on the fibers.
[0018] An object is also to provide a fiber loading process with a low level of calcium
in the circulating process waters.
SUMMARY OF THE INVENTION
[0019] The present invention is defined in the appended claims. The invention provides a
process for fiber loading of a pulp suspension by a chemical reaction between calcium
hydroxide and carbon dioxide. The reaction is promoted by the use of an alkaline alkali
metal compound, which creates a catalyst in the suspension to provide a fast and substantially
complete precipitation of the reaction product on the fibers. After the precipitation,
the pulp suspension is dewatered and made into paper, board or pulp containing the
precipitated calcium carbonate as filler
[0020] More specifically, the invention comprises a process for fiber loading by creating calcium carbonate
in situ from calcium hydroxide and carbon dioxide, comprising the steps of
- a. providing an aqueous pulp suspension in a process for making paper, board or pulp;
- b. adding a calcium compound selected from the group consisting of calcium hydroxide
and calcium oxide into the pulp suspension;
- c. adding carbon dioxide to the pulp suspension in an amount corresponding to the
stoichiometric amount or more relative to the calcium compound;
- d. adding an alkaline alkali metal compound to the pulp suspension in a sub stoichiometric
catalytic amount relative to the calcium compound for promoting calcium carbonate
precipitation in said pulp suspension; and
- e. after calcium carbonate precipitation, dewatering the resultant fiber loaded suspension
to provide a fibrous web comprising calcium carbonate filler and an aqueous phase
depleted of calcium ions.
[0021] The use of
in situ precipitated calcium carbonate in paper improves the optical properties and the printing
properties of paper and, in addition, it typically reduces production costs per manufactured
ton of paper mainly due to the decreasing percentage of the amount of more expensive
fibre material.
[0022] The brightness and the wet tension properties of the paper is also improved and the
dusting of the paper can be reduced by the fiber loading method of the present invention.
[0023] The alkaline alkali metal compound advantageously comprises a hydroxide optionally
in combination with carbon dioxide. A preferred alkali metal compound comprises sodium
hydroxide or a non-stoichiometric combination of sodium hydroxide and carbon dioxide.
The alkali metal compound is used in a catalytic amount. The amount of the alkali
metal compound typically corresponds to 0.5 to 20 % of the stoichiometric reaction
amount of the calcium compound. The amount is preferably 2 to 10 % and most preferably
4 to 8 % of the stoichiometric reaction amount of the calcium compound. It is evident
that more than 20% alkali metal compound may be added if desired but for the purpose
of the invention a much smaller amount is generally enough.
[0024] The added alkali metal compound acts as a catalyst, since it is not consumed in the
precipitation reaction. The catalyst is regenerated at the precipitation of calcium
carbonate and recirculates in the precipitation system. Thus, the desired level of
catalyst can be maintained while keeping the consumption of chemicals at a low level,
which provides a cost effective process.
[0025] In addition to providing a reaction catalyst, the alkaline alkali metal compound
promotes the precipitation reaction by providing a high pH.
BRIEF DESCRPITON OF THE DRAWING
[0026] Figs. 1 to 10 show SEM images of pulps and sheets made with varying amounts of calcium
hydroxide. Figs. 1 to 8 relate to Example 1, while Figs. 9 and 10 relate to Reference
Example 2.
Fig. 1 shows a DIP pulp reference i.e. 0% Ca(OH)2
Fig. 2 shows a DIP pulp according to the invention with 4% Ca(OH)2
Fig. 3 shows a DIP pulp according to the invention with 6% Ca(OH)2
Fig. 4 shows a DIP pulp according to the invention with 8% Ca(OH)2
Fig. 5 shows a DIP sheet reference with 0% Ca(OH)2
Fig. 6 shows a DIP sheet according to the invention with 4% Ca(OH)2
Fig. 7 shows a DIP sheet according to the invention with6 % Ca(OH)2
Fig. 8 shows a DIP sheet according to the invention with 8% Ca(OH)2
Fig. 9 shows a DIP pulp reference
Fig. 10 shows a DIP pulp with 8% Ca(OH)2, without NaOH
Fig. 11 shows the pH dependent variation in the abundance of carbon dioxide, bicarbonate
and carbonate in water.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The present invention relates to a process for providing calcium carbonate filler
in paper, board or pulp by creating the calcium carbonate
in situ in the process itself. The calcium in the precipitated filler derives from calcium
hydroxide. The calcium compound may be added to the pulp suspension as calcium hydroxide
or in the form of calcium oxide, which in water forms calcium hydroxide. The compound
may be added as a powder or dissolved in water. A more uniform mixture is usually
obtained, if the compound is dissolved in water. A typical calcium hydroxide solution
used in the invention comprises about 5 to 15% calcium hydroxide.
[0028] The amount of calcium compound to be added to the pulp suspension is calculated based
on the desired amount of filler in the final dried product. A typical amount of calcium
hydroxide to add to the pulp suspension in the present invention is between 2 and
12 % calculated on the dry weight of the pulp. In an embodiment of the invention calcium
hydroxide solution is added in an amount of 4 to 8 % of the dry weight of the pulp.
[0029] The calcium compound may be added all at once, or it may be added in portions along
the continuous process. Calcium hydroxide is an alkaline compound and it typically
raises the pH of the pulp suspension. The pH may rise to pH 10 or more, typically
to 10-12 after the addition of calcium hydroxide.
[0030] The carbonate part of the filler created in the process of the invention derives
from carbon dioxide. The carbon dioxide may be added in gaseous form or predissolved
in water. The carbon dioxide may also be added in liquid or solid form. However, the
use of gaseous carbon dioxide is preferred. The carbon dioxide may be pure carbon
dioxide, but it is more economical to use a technical grade of carbon dioxide. It
is also possible to utilize the carbon dioxide in smoke gas or the like by-products
of the mill, as long as the impurities in the gas do not disturb the papermaking process.
[0031] In the prior art fiber loading processes pressurized carbon dioxide has been used
in order to ensure a proper reaction in and on the fibers. In the present invention
the carbon dioxide need not be pressurized. In a typical embodiment the carbon dioxide
is mixed into the pulp suspension at normal pressure, meaning that the pulp suspension
is at ambient pressure and the pressure of the gaseous carbon dioxide is no higher
than needed to make it flow into the suspension. The calcium hydroxide is reacted
with the carbon dioxide at ambient pressure.
[0032] Carbon dioxide dissolves in the aqueous phase of the pulp suspension forming carbonic
acid and/or bicarbonate ions according to the equation:
CO
2 + H
2O < - > H
2CO
3 < - > H
+ + HCO
3- <-> 2H
+ + CO
32- (3)
[0033] The amount of the above species in water varies depending on the pH in accordance
with the curves shown in Fig. 11. In the pH range of the present invention (pH 6 to
13), carbon dioxide has a pH lowering effect on the suspension.
[0034] The total amount of carbon dioxide added to the pulp suspension in accordance with
the invention is at least equal to the stoichiometric reaction amount of the calcium
compound. In order to ensure a complete precipitation of the calcium compound, the
amount of carbon dioxide typically corresponds to 100 to 150 %, preferably 100 to
125 %, most preferably 100 to 110 % of the stoichiometric reaction amount of the calcium
compound.
[0035] The carbon dioxide is preferably added at multiple addition points. This way the
pH for the bulk of the precipitation reaction can be maintained at a high value even
though a lower pH is typically desired at the end of the papermaking.
[0036] The precipitation of the calcium carbonate in the present invention is promoted by
an alkaline alkali metal compound. The alkali metal compound is advantageously selected
from the group consisting of sodium or potassium hydroxide, sodium or potassium bicarbonate,
a mixture of sodium or potassium hydroxide and carbon dioxide, and combinations thereof.
Sodium hydroxide and a premixed non-stoichiometric combination of sodium hydroxide
and carbon dioxide are preferred alkali metal compounds. The sodium or potassium hydroxide
is typically added in the form of a solution at a concentration of 50% or less, most
typically 5 to 6%.
[0037] It is also possible to use solid sodium or potassium bicarbonate or carbonate as
the alkali metal compound. However, in practice these compounds are not preferred
since the handling and dissolving of these compounds in the mill is messy and provides
an extra manual operation while sodium hydroxide, for instance, is a compound which
is typically much used in the mill in any case and is readily available for use in
the fiber loading also.
[0038] The amount of alkaline alkali metal compound used in the process of the invention
is significantly less than the amount of calcium compound and carbon dioxide, since
the alkali metal compound is not consumed in the precipitation reaction. Therefore,
the alkali metal compound is added only in a catalytic amount. The catalytic amount
of the added alkali metal compound corresponds to 1 to 50 % of the stoichiometric
reaction amount of the calcium compound. In practice a very small amount of the alkali
metal compound, such as 5 to 10 % of the stoichiometric reaction amount of the calcium
compound is sufficient.
[0039] In case a pre-mixed combination of sodium or potassium hydroxide and carbon dioxide
is used as the alkaline alkali metal compound, the mixture is preferably produced
in an apparatus described in
EP-B1-1461499. The mixture can be produced as a non-stoichiometric combination with a desired alkaline
pH. This makes it possible to vary the pH of the alkaline compound and to control
the pH and the alkalinity of the process to a desired level. A typical pH of such
a non-stoichiometric combination is between 8.5 and 9.5.
[0040] The calcium carbonate precipitation reaction comprises a modification of the well
known chemical reaction between calcium hydroxide and carbon dioxide:
Ca(OH)
2 + CO
2 → CaCO
3 + H
2O (2)
[0041] In the present invention, the above reaction is promoted by an alkaline alkali metal
compound such as sodium hydroxide. The alkali metal compound is highly reactive and
is believed to provide alkali metal carbonate in the aqueous phase of the pulp suspension
by reacting with the carbonate ions produced by the added carbon dioxide. For sodium
hydroxide the reaction can be described as:
2NaOH + CO
2 → Na
2CO
3 + H
2O (4)
[0042] This alkali metal carbonate is believed to act in the manner of a catalyst and to
promote the formation of calcium carbonate within the lumen and in and on the cell
walls of the fibers.
[0043] When the calcium carbonate precipitates, the alkali metal compound is regenerated
and can act as a promoter in the next precipitation reaction. Although the chemistry
of the precipitation theoretically may comprise several steps with various ionic species
such as bicarbonates of alkali metal and Calcium, the promotion reaction can briefly
be described for sodium hydroxide by the following equation:
Ca(OH)
2 + 2NaOH + CO
2 → [Ca(OH)
2 + Na
2CO
3 + H
2O] →CaCO
3 + H
2O + 2NaOH (5)
[0044] Without wishing to be tied to any theory, it is believed that in the above equation,
the sodium hydroxide acts to produce a transient sodium carbonate moiety, which in
contacting a calcium ion of the dissolved calcium hydroxide provides an exchange of
cations so that calcium carbonate is formed. Since calcium carbonate precipitates,
there is a perpetual imbalance in the suspension between aqueous calcium carbonate
and aqueous sodium carbonate, which drives the reaction towards calcium carbonate.
This scenario is believed to create a "motor" for the formation and precipitation
of calcium carbonate.
[0045] With the precipitation of calcium carbonate the alkali metal catalyst is freed and
ready to act as a catalyst in the next precipitation reaction.
[0046] The alkali metal ions Na
+ and K
+ are more easily mobile in the aqueous phase and within the fibers than Ca
2+. The alkaline alkali metal compounds cause swelling of the fibers especially at high
pH and this swelling facilitates the absorption of the larger ions Ca
2+ and CO
3- into the lumen. The cations attach to the cell walls improving the retention of the
precipitated calcium carbonate on and in the fibers. The initial precipitate is minute
in size, but once some calcium carbonate has been formed on the fiber, the initial
precipitate acts as a seed for further crystal growth.
[0047] At the high pH (above pH 9.5 and typically pH 10-12) prevailing at the initial precipitation,
the reaction to form calcium carbonate from calcium hydroxide and carbon dioxide is
quick. Some calcium carbonate is formed in less than a minute. However, the initial
precipitate is very small and colloidal in consistency. In order to provide a good
coverage and a substantial loading of filler, there needs to be some crystal growth.
The crystals will also attach more securely to the fiber walls with time. It is the
crystals on the outside of the fibers which provide the coverage providing optical
benefits. However, the crystals especially on the inner side of the fiber wall provide
the mass of the loading, which gives physical benefits to the fibers compared to having
the same amount of filler on the outside of the fibers only. As has been ascertained
by SEM images taken of pulps and sheets treated with a fiber loading process according
to the invention, an exceptionally large amount of calcium carbonate crystals is found
on the inside of the fiber. This is believed to reduce the negative impact that the
addition of filler traditionally has on the tensile index of the fibers.
[0048] In an embodiment of the invention, the pulp suspension, after addition thereto of
calcium hydroxide, carbon dioxide, and alkaline alkali metal compound and/or alkali
metal catalyst, is retained as a reaction mixture in a vessel before dewatering for
allowing precipitation and crystal growth of calcium carbonate. Advantageously, the
retention time at least at one stage of the fiber loading process is at least 20 minutes
and typically between 0.5 and 20 hours. In an embodiment of the invention, the retention
time is 0.5 hours or more and preferably 1 to 8 hours, more preferably 1 to 4 hours
for good fiber loading results. The crystals grow and also change crystal form with
time. Thus, for best results, a retention time of at least half an hour is recommended
at a pH above 9.5 and preferably pH 10 to 12. This can best be obtained by adding
at least the carbon dioxide in multiple steps.
[0049] The precipitation reaction proceeds more quickly at a high pH than at a low pH. Thus,
the aim is to raise the pH of the pulp suspension above pH 8 and preferably above
pH 9.5 at least for the initial part of the reaction. The added calcium hydroxide
has a pH raising effect, while the added carbon dioxide has a pH lowering effect.
The pH may be maintained at a desired high level by adding less than the stoichiometric
amount of carbon dioxide at the initial stage. Adding the alkaline alkali metal compound
also counteracts the pH reduction caused by the carbon dioxide.
[0050] An ideal pH at the initial stage of the fiber loading reaction is pH 10 to 12. By
using two or more addition steps for the carbon dioxide and by adding alkaline alkali
metal compound, the pH of the suspension isretained at a high level until all or most
of the calcium hydroxide has been consumed. When carbon dioxide is added at multiple
addition points, it is preferred to make the final addition at a late point in the
process such as after refining, in the machine chest or in the short circulation so
that the carbon dioxide addition can be used to adjust the pH of the pulp suspension
to desired lower pH level.
[0051] In an embodiment of the invention, the pH of the pulp suspension is reduced with
carbon dioxide to a pH of 8.5 or less, preferably to a pH below 8 before the dewatering.
[0052] In an embodiment of the invention, at least one of the calcium compound, the carbon
dioxide and the alkali metal compound is/are added in at two or more addition steps.
In an embodiment, the carbon dioxide is added in two or more addition steps and the
final addition of carbon dioxide adjusts the pH of the pulp suspension to a desired
value between pH 6.5 and 8, preferably pH 7 to 7.5. In this embodiment of the invention
it is advantageous to add the alkaline alkali metal compound of the invention in an
amount to maintain the pH of the pulp suspension at pH 8 or more, preferably pH 10
or more until the final addition of carbon dioxide.
[0053] The temperature of the pulp suspension is not critical. However, calcium carbonate
has a tendency to precipitate more readily at higher temperatures. Thus, good precipitation
is ensured by maintaining the pulp suspension at a temperature of 40 to 60°C. Higher
temperatures up to about 90 °C increases the reaction rate but this is usually too
costly
[0054] The fiber loading reaction of the present invention can be performed at different
positions in the papermaking process. The chemicals can be added all at the same position
or at intervals along the continuous process. Calcium hydroxide may, for instance,
be mixed into the pulp suspension before a refiner, for example preceding the refiner.
In such a case, carbon dioxide is typically introduced after the refiner, for example
following the refiner. The alkaline alkali metal compound may be added with the calcium
hydroxide or with the carbon dioxide or both.
[0055] In an embodiment of the invention, the fiber loading process is performed in two
separate steps. One step is early in the papermaking process and at a relatively high
consistency. At least a part of the chemicals are advantageously added with the dilution
water. When the addition point is early in the process, the fiber loading chemicals
have a long retention time in the pulp suspension and thas time to form large crystals.
[0056] A second fiber loading step is performed later in the papermaking process at a lower
consistency to precipitate more calcium carbonate onto the previously formed crystals.
The second step may be performed in the short circulation or close to the short circulation
for ascertaining that substantially all of the solubilized calcium is precipitated
onto the fibers and that the aqueous phase of the suspension is depleted of the added
calcium. This cleans up the process waters and improves the overall process.
[0057] The chemicals of the present invention may be added to the pulp suspension in different
orders. However, at least some of the alkaline compounds (the calcium compound and/or
the alkaline alkali metal compound) should be added prior to the addition of carbon
dioxide since carbon dioxide will dissolve much more easily into an alkaline aqueous
phase. The alkaline alkali metal compound may be added prior to, simultaneously with
or after the addition of calcium hydroxide. It may also be added after some carbon
dioxide has already been added to the pulp suspension.
[0058] The aqueous pulp suspension which in the present invention is subjected to fiber
loading may be any kind of pulp suspension, which is suitable for having calcium carbonate
as filler. The present invention is also suitable for use in such processes, wherein
standard fiber loading (see e.g.
US 5,223,090) has typically been used. Thus, the fibers may be virgin fibers prepared by chemical,
mechanical or thermomechanical pulping, etc. The fibers may also be recycled fibers
such as deinked fibers (DIP), or mixtures of virgin fibers and recycled fibers.
[0059] Special benefits are provided by the present invention in connection with the use
of recycled fibers, since the high level of calcium carbonate on the fibers, which
is attainable with the invention, provides a good coverage on ink particles remaining
on the recycled fibers and increases the brightness of the pulp. This reduces the
need for cleaning of the fibers and/or the use of bleaching processes. Moreover, the
calcium carbonate which adheres to the fibers will also cover sticky particles on
the fibers and in the pulp suspension. When covered by the precipitate, the stickies
are rendered harmless. This improves the purity of the pulp and the waters. Also the
dewatering is improved. The improved process also results in improved quality of the
produced paper, board or pulp.
[0060] The consistency of the pulp suspension which is subjected to the fiber loading process
of the invention can vary within a broad range such as from 0.1 to 50%. High consistency
pulps are typically fiber loaded in the initial stages of the papermaking. Good results
have, for instance, been obtained by adding the calcium compound to a pulp having
a consistency of 30 to 40%. In such a case it is advantageous to add the compound
in a dilution screw to obtain a proper mixing of the calcium compound into the pulp.
The dilution is typically performed with circulating process water. The calcium hydroxide/calcium
oxide and, optionally, the alkali metal compound may be provided in the same dilution
water or as a separate stream. A dilution to about 5 to 15%, preferably 8 to 12% is
typically provided in the dilution screw.
[0061] In case the pulp which is subjected to the fiber loading process of the present invention
has a lower consistency, such as 4 to 10%, a dilution screw need not be used. However,
it is still important to mix the calcium compound and the alkali metal compound, if
added at this stage, well into the pulp to ensure a loading of calcium carbonate all
through the pulp suspension. The precipitation reaction may be performed at this consistency,
or the pulp may be further diluted before addition of the compounds of the reaction.
[0062] Very good results of the fiber loading have been obtained by retaining the pulp suspension
in a vessel such as a storage tank for a retention time between 0.5 and 20 hours at
a consistency of 2 to 10 %, preferably 4 to 8 % after addition thereto of calcium
compound and carbon dioxide as well as alkali metal compound. It was also found that
at the high pH (pH 10-11) provided by the calcium hydroxide and the alkali metal compound,
the gaseous carbon dioxide dissolved easily into the pulp suspension and there was
no need for increased pressure as in many prior art processes.
[0063] The pulp to be treated may also have a lower consistency such as 0.1 to 4%, which
is typical at the final process steps of the papermaking, such as in the short circulation.
The calcium compound and alkali metal compound may be added directly to the pulp stream
or to the dilution water. The carbon dioxide may be mixed into a separate water stream
or it may be added directly into the diluted pulp suspension.
[0064] In an embodiment of the invention at least one of the calcium compound, the carbon
dioxide and the alkali metal compound is/are added to the pulp suspension in two or
more addition steps. The first addition(s) may be made at a higher consistency and
later addition(s) at a lower consistency to build more calcium carbonate onto the
previously precipitated filler.
[0065] In an embodiment of the invention, fiber loading is performed at two or more locations
and the final one is performed in the short circulation of the process to build on
an extra layer of calcium carbonate on top of previously precipitated filler. After
the addition of calcium compound and alkali metal compound to raise the pH, carbon
dioxide is added to the alkaline pulp. In the final papermaking stage, carbon dioxide
is fed in an amount to bring down the pH of the pulp to a desired value of pH 6.5
to 8.
[0066] The alkali metal catalyst together with a stoichiometric excess of carbon dioxide
ensures that the precipitation reaction is quick and complete. The aqueous phase of
the pulp suspension is thereby depleted of calcium ions. Here depletion of calcium
ions means that less than 20% and preferably less than 10% of the calcium ions added
to the pulp suspension remain in the process water.
[0067] In an embodiment of the invention 90 to 100% of the calcium added with the calcium
compound is consumed in the calcium carbonate precipitation reaction. In a typical
operation of the process according to the invention also calcium initially present
in the pulp suspension is precipitated so that the precipitation exceeds 100% of the
added calcium and the system is effectively cleaned by the fiber loading process.
In an embodiment of the invention, the precipitation reaction is continued until the
amount of calcium precipitated in the process exceeds the amount of calcium added
with the calcium compound.
[0068] Any minor amount of calcium ions that may remain in the water after completion of
the fiber loading of the invention will stay solubilized because of the excess of
carbonate in the circulating water. Due to the recirculation of the process waters,
any remaining calcium ions will take part in the subsequent round of fiber loading
reaction. Thus, accumulation of calcium ions in the circulating water is avoided.
In fact, in the present process alkali metal ions are circulated, while in the prior
art processes calcium ions circulate and accumulate in the process waters. Since alkali
metal salts are highly soluble, they do not cause problems in the papermaking process
in the manner that calcium ions do.
[0069] In an embodiment of the invention the process for making paper, board of pulp is
a continuous process and the dewatered fibrous web is processed to paper, board or
pulp containing precipitated calcium carbonate as filler within and on the outside
of the fibers.
[0070] The following examples illustrate some embodiments of the invention.
Example 1, fiber loading of DIP pulp using sodium compound catalyst
[0071] The pulps used were recycled pulp (DIP pulp) before a medium consistency (MC) refiner
and reductive bleaching. For fiber loading, 300g batches of the pulp at 33% consistency
were mixed with 940 g clear filtrate to 8% consistency. The samples were mixed for
a few minutes with a 10% solution of calcium hydroxide added at a rate of 0 (reference),
2, 4, 6 and 8%, respectively, in the different experiments.
[0072] To the pulp samples (except reference) was added 5.4 g of a 4% solution of NaOH.
This comprises a sub stoichiometric ratio compared to the amount of calcium hydroxide.
For the respective experiments with 2, 4, 6 and 8 % Ca(OH)
2, the added NaOH comprised 10, 5, 3 and 1 %, respectively of the stoichiometric amount.
The pulp mixtures were then reacted with carbon dioxide at normal pressure. Carbon
dioxide was added until the pH had been reduced to pH 7.0-7.8.
[0073] The carbon dioxide started reacting with the NaOH and calcium hydroxide and generated
insoluble calcium carbonate.
[0074] The total reaction is believed to be:
Ca(OH)
2 + 2NaOH + CO
2 = CaCO
3 + 2NaOH + H
2O
[0075] The consumption of NaOH was very low due to its role as an intermediary compound
in the calcium carbonate generation. The NaOH accelerated the calcium carbonate precipitation.
[0076] Each sample was stored in sealed plastic bags at 45°C for 6 hours in a heated water
bath. The above reactions continued until substantially all soluble calcium had precipitated
as calcium carbonate and had caused a crystallisation process in the storage plastic
bags.
[0077] After 6 hours storage the pulp was dewatered on a büchner funnel and then the pulp
was pressed to obtained paper handsheets. The pulp and filtrate was analysed and the
results are shown in Table 2.
[0078] The clear filtrate used for dilution of the pulp had the following content: pH 7.69,
alkalinity 5.6 mmol/l, hardness 21.2 °dH and 152 mg/l Ca
2+.
Table 1. The experimental data are
summarized in the
following table:
Samples |
Ca(OH)2 |
DIP pulp [g] |
Clear filtrate |
Ca(OH)2 [g] |
NaOH [g] |
pH after Ca(OH)2 |
pH after CO2 |
pH initial |
pH after 6h |
|
% |
33% |
[g] to 8% |
10% |
4% |
|
|
|
|
REF |
0 |
300 |
940 |
0 |
0 |
0 |
|
7.87 |
7.05 |
2. |
2% |
300 |
940 |
20 |
5.4 |
12.11 |
7.29 |
|
6.8 |
4. |
4% |
300 |
940 |
40 |
5.4 |
12.27 |
7.3 |
|
6.78 |
6. |
6% |
300 |
940 |
60 |
5.4 |
12.22 |
7.5 |
|
6.95 |
8. |
8% |
300 |
940 |
80 |
5.4 |
12.1 |
7.25 |
|
6.98 |
Table 2. The filtrate analysis data and pulp ash and calcium carbonate are summarized
in the following table:
Analys |
Initial |
6h |
With chemi-cals |
6h |
With chemicals |
6h |
With chemi cals |
6h |
With chemi-cals |
6h |
|
Reference |
2 % Ca(OH)2 |
4 % Ca(OH)2 |
6 % Ca(OH)2 |
8 % Ca(OH)2 |
°dH |
19.4 |
22 |
34.2 |
20.6 |
27.8 |
20 |
25.8 |
18.2 |
21.2 |
17.8 |
Ca [mg/l] |
138.7 |
157 |
244.5 |
147.3 |
198.8 |
143 |
184.5 |
130.1 |
151.6 |
127.3 |
Alk. |
7.8 |
|
|
|
|
|
|
|
|
|
Ash 525°% |
13.2 |
13.1 |
|
17.3 |
|
19.5 |
|
22.5 |
|
24.8 |
Ash 900°% |
10.6 |
10.6 |
|
13 |
|
14.3 |
|
15.9 |
|
17.2 |
CaCO3 % |
44 |
43 |
|
57 |
|
60 |
|
67 |
|
69 |
[0079] The paper hand sheets were caracterized by: calcium carbonate content, ash 525°C,
ash 900°C, optical and mechanical properties. SEM - Scanning electron microscopy images
(Figs. 1 to 8) were obtained to identify the localization of calcium carbonate crystals
into the fibre structure (lumen and fibre wall pores).
Table 3. The paper hand sheets data and pulp mechanical and optical properties are
summarized in the following table:
Measured |
Analysis results |
Effect of fiber loading |
|
UM. |
Initial |
Referez |
DIP + 2% Ca(OH)2 |
DIP + 4% Ca(OH)2 |
DIP + 6% Ca(OH)2 |
DIP + 8% Ca(OH)2 |
DIP + 2% Ca(OH)2 |
DIP + 4% Ca(OH) 2 |
DIP + 6% Ca(OH)2 |
DIP + 8% Ca(OH) 2 |
Sheet |
EWD |
sec |
117.9 |
135.5 |
125.4 |
113.4 |
109.6 |
111.1 |
-10.1 |
-221 |
-25. |
-24.4 |
CSF |
ml |
156 |
166 |
176 |
169 |
173 |
195 |
10 |
3 |
9 |
29 |
Porosity Bendtsen |
ml/min |
123 |
121 |
127 |
171 |
185 |
196 |
6 |
50 |
64 |
75 |
PPS |
um |
8.87 |
8.8 |
8.58 |
8.45 |
8.08 |
8.22 |
-0.22 |
-0.35 |
-0.72 |
-0.58 |
kg/m3 |
597 |
593 |
617 |
605 |
602 |
589 |
24 |
12 |
9 |
-4 |
Tensile Index |
kNm/kg |
39.9 |
39.6 |
37.7 |
36.9 |
28.9 |
25.7 |
-1.9 |
-2.7 |
-10.7 |
-13.9 |
Asch Sheet 525°C |
% |
8.59 |
8.42 |
12.3 |
15.4 |
17.9 |
19.8 |
3.88 |
6.98 |
9.48 |
11.38 |
Asch Sheet 900°C |
% |
7.01 |
6.81 |
9.16 |
11 |
12.3 |
13.3 |
2.35 |
4.19 |
5.49 |
6.49 |
CaC03 Sheet |
% |
42 |
43 |
58 |
65 |
71 |
75 |
15 |
22 |
28 |
32 |
R457 420 |
|
57.53 |
57.685 |
59.33 |
61 |
62.445 |
63.015 |
1.645 |
3.315 |
4.76 |
5.33 |
R457 D65 |
|
60.1 |
60.3 |
61.86 |
63.5 |
64.83 |
65.385 |
1.56 |
3.2 |
4.53 |
5.085 |
ERIC |
|
232.92 |
225.69 |
219.91 |
199.55 |
189.31 |
183.31 |
-5.78 |
-26.14 |
-36.38 |
-42.38 |
Pulp |
|
|
Initial |
Ref |
2% |
4% |
6% |
8% |
2% |
4% |
6% |
8% |
Brightness |
|
59.19 |
58.85 |
61.06 |
62.48 |
63.61 |
65.1 |
2.21 |
3.63 |
4.76 |
6.25 |
Luminance |
|
64.37 |
63.52 |
65.77 |
66.77 |
67.73 |
69.12 |
2.25 |
3.25 |
4.21 |
5.6 |
DWL |
|
575 |
574.72 |
574.52 |
572.69 |
574.41 |
574.24 |
-0.2 |
-2.03 |
-0.31 |
-0.48 |
Purity |
|
5.45 |
4.94 |
4.82 |
4.32 |
4.08 |
3.89 |
-0.12 |
-0.62 |
-0.86 |
-1.05 |
ERIC |
|
294 |
326.9 |
274.46 |
163.13 |
248.6 |
227.31 |
-52.44 |
-163.8 |
-78.3 |
-99.59 |
[0080] The analyses were performed according to the respective standards indicated below:
Parameter |
Explanation |
Test method |
EWD |
Dewatering time (Entwässerungsdauer) |
|
CSF |
Freeness of paper |
ISO 5267-2 |
Porosity Bendtsen |
Porosity of paper |
SCAN-P 85 |
PPS |
Roughness of paper |
ISO 8791-4 |
Density |
Density of paper |
ISO 5270 |
Tensile index |
Strength of paper |
ISO 1924-3 |
Ash Sheet 525°C |
Residue on ignition at 575°C /900°C |
ISO 1762 and ISO 2144 |
Ash sheet 900°C |
|
R457 420 |
Paper brightness |
ISO brightness R457 |
R457 D65 |
Paper brightness |
ISO 11475 |
ERIC |
Effective Residual Ink Concentration of paper |
ISO 22754; TAPPI 567 Pm-97 at 950 nm |
Brightness, Luminance, DWL, Purity |
Optical measurements |
ISO 2470 |
[0081] SEM - Scanning electron microscopy images of the DIP pulp and sheets from the experiments
above are shown in Fig. 1,2,3,4,5,6,7,8 and showing localization of calcium carbonate
crystals into the fibre structure (lumen and fibre wall pores).
[0082] The images show a cross section of fibres. The white particles inside the fibres
and between fibres are calcium carbonate (CaCO
3). The main part of the CaCO
3 particles are between 0.3 and 2 µm. A small amount of particles are bigger, up to
5 µm (measured on the image).
[0083] The wavelength of light is around 0.5 µm. Particles with a size close to the wavelength
of light are optically active, light scattering. Particles precipitated inside and
between the fibres in the sample are big enough to scatter light.
Reference example 1, DIP pulp using traditional filler
[0084] Using the same DIP pulp as used in Example 1, paper hand sheets were produced in
the same manner as described in Example 1 with the exception that the filler was mixed
in the conventional manner into the pulp instead of being created
in situ. The filler was a standard calcium carbonate filler, Snowcal 45 from Omya UK Ltd.
The sheets were analysed as in Example 1 and the results are shown in Table 4.
Table 4. The paper hand sheets data and mechanical and optical properties with Snowcal
45.
Measured |
|
Analysis results |
Effect |
|
UM. |
Initial |
DIP + 2% of conventional filler Snowcal 45 |
DIP + 4% of conventional filler Snowcal 45 |
DIP + 2% of conventional filler Snowcal 45 |
DIP + 4% of conventional filler Snowcal 45 |
EWD |
sec. |
117.9 |
108.4 |
108.4 |
-9.5 |
-9.5 |
CSF |
ml |
156 |
185 |
185 |
29 |
29 |
Porosity Bendtsen |
ml/min |
123 |
163 |
187 |
40 |
64 |
PPS |
µm |
8.87 |
4.55 |
5.4 |
-4.32 |
-3.47 |
Density |
kg/m3 |
597 |
588 |
581 |
-95 |
-16 |
Tensile index |
kNm/kg |
39.9 |
36.1 |
33.3 |
-3.8 |
-6.6 |
Ash Sheet 525°C |
% |
8.59 |
7.97 |
12.3 |
1.81 |
3.71 |
Ash Sheet 900°C |
% |
7.01 |
10.4 |
9.04 |
0.96 |
2.03 |
CaC03 Sheet |
% |
42 |
53 |
60 |
11 |
18 |
R457 420 |
|
57.53 |
58.51 |
58.99 |
0.98 |
1.46 |
R457 D65 |
|
60.1 |
61.03 |
61.57 |
0.93 |
1.47 |
ERIC |
|
232.92 |
214.03 |
206.98 |
-18.89 |
-25.94 |
[0085] By comparing the data of Example 1 with those of Reference Example 1, it can be seen
that the papers made with fiber loading are superior to those made with a standard
filler.
[0086] For 2% Ca (OH)
2, 12.3% ash in the sheet (fiber loading, Table 3), the negative impact on fiber loading
on tensile index compared with conventional addition of filler (4% Snowcal 45, Table
4) on the same ash contained in the sheet, is 3.8 times lower.
[0087] Also the optical properties are superior in the fiber loading process (Table 3) compared
with conventional addition of filler Snowcal 45 (Table 4).
[0088] The tests show that by the fiber loading of the invention more of the added calcium
filler is retained in the fibers than by a conventional method and still the fiber
loaded filler has less of a negative impact on the tensile strength of the paper than
the filler added by conventional means. Moreover, the optical properties of the fiber
loaded papers are improved over those made with conventional filler.
Reference Example 2, DIP pulp using fiber loading without alkaline sodium compound
[0089] A DIP pulp was fiber loaded as described in Example 1 but without the use of a sodium
compound as catalyst. The pulp used was a DIP pulp before the MC refiner and reductive
bleaching.
[0090] For fiber loading, two 300g batches of pulp at 33% consistency were mixed with 940g
clear filtrate to 8% consistency. One of the samples was mixed for a few minutes with
a 10% solution of calcium hydroxide added at a rate of 8%. The other sample was mixed
without calcium hydroxide (0% addition).
[0091] To the pulp sample containing calcium hydroxide, carbon dioxide at normal pressure
was added until the pH was 7.15. No NaOH was added. Carbon dioxide started reacting
with Ca(OH)
2 and generated CaCO
3 based on the following reaction:
Ca(OH)
2 + CO
2 = CaCO
3 + H
2O
[0092] Both samples were stored in sealed plastic bags at 45°C for 6 hours in a heated water
bath. The reaction continued until substantially all soluble calcium had precipitated
as calcium carbonate and caused crystallisation in the storage plastic bags.
[0093] After 6 hours storage the pulp was dewatered on a büchner funnel and then the pulp
was pressed to obtain paper handsheets.
[0094] The clear filtrate used for dilution had the following content: pH 7.4 9, alkalinity
4.7 mmol/l, hardness 22 °dH and 157 mg/l Ca
2+.
Table 5. The experimental data are
summarized in_ the
following table:
Sample |
Ca(OH)2 |
DIP pulp [g] |
Clear filtrate |
Ca(OH)2 [g] |
pH after Ca(OH)2 |
pH after CO2 |
pH initial |
pH after 6h |
% |
33% |
[g] to 8% |
10% |
REF |
0 |
300 |
940 |
0 |
0 |
|
7.87 |
7.05 |
1. |
8% |
300 |
940 |
80 |
12.1 |
7.15 |
|
7.08 |
Table 6. The filtrate analysis data and pulp ash and calcium carbonate are summarized
in the_following table:
Analysis |
Initial |
6h |
With chemical s |
6h |
|
Reference |
8 % Ca(OH)2 |
°dH |
19.7 |
23 |
37 |
25 |
Ca [mg/l] |
140.86 |
164.45 |
264.55 |
178.75 |
Alk. |
5.4 |
|
|
|
Ash 525°% |
11.0 |
11.21 |
|
23.8 |
Ash 900°% |
9.21 |
9.22 |
|
16.3 |
CaCO3 % |
37 |
37.5 |
|
71 |
[0095] The paper hand sheets were caracterized by: calcium carbonate content, ash 525°C,
ash 900°C, optical and mechanical properties. SEM - Scanning electron microscopy images
were obtained to identify the localization of calcium carbonate crystals into the
fibre structure (lumen and fibre wall pores).
Table 7. The paper hand sheets data and pulp mechanical and optical properties are
summarized in the following table:
Measured |
Analysis results |
Effect |
UM. |
Referent |
DIP + 8% Ca(OH)2 |
DIP + 8% Ca(OH)2 |
Sheet |
EWD |
sec. |
146.4 |
127.0 |
-19.355 |
CSF |
ml |
149 |
152 |
3.5 |
Porosity Bendtsen |
ml/min |
121 |
134 |
13 |
PPS |
(jm |
8.10 |
7.58 |
-0.52 |
kg/m3 |
573 |
611 |
37.77 |
Tensile index |
kNm/kg |
40.5 |
24.5 |
-15.96 |
Ash Sheet 525°C |
% |
7.93 |
20.7 |
12.77 |
R457 420 |
|
60.11 |
63.42 |
3.31 |
R457 D65 |
|
62.34 |
65.12 |
2.78 |
Pulp |
Brightness |
|
61.28 |
65.4 |
4.12 |
Luminance |
|
66.545 |
71.1 |
4.6 |
DWL |
|
574.07 |
572.91 |
-1.16 |
Purity |
|
5.29 |
3.75 |
-1.54 |
ERIC |
|
283.17 |
217.285 |
-65.89 |
[0096] For 8% Ca(OH)
2, The fiber loading without addition of NaOH provided 20.7% ash in the sheet, which
gave a negative impact on the tensile index of -15.96 kNm/kg compared to the reference
(Table 7). At the same Ca(OH)
2 level, the fiber loading with NaOH provided 19.8 % ash which gave a negative impact
on the tensile index of only -13.9 kNm/kg compared to the reference (Table 3). Thus,
the fiber loading of the present invention reduced the tensile index significantly
less (by 2,06 kNm/kg lower) than a fiber loading without addition of NaOH.
[0097] The optical properties of the paper produced by fiber loading according to the invention
(Table 3) are clearly better than those provided by the fiber loading without NaOH
(Table 7).
[0098] The SEM images taken of the pulps produced in Reference Example 2 (Figs 9 and 10)
clearly show that less calcium carbonate has crystallized on the inner walls of the
fibers when no NaOH catalyst was present.
[0099] Thus, the fiber loading of the present invention provides more calcium carbonate
inside the fibers and therefore has a lesser impact on the tensile index of the paper.
Example 2, fiber loading of TGW pulp using sodium compound as catalyst
[0100] The pulps used were a TGW (Thermo Ground Wood) pulp after peroxide bleaching before
the high-consistency refiner. For fiber loading, 400g batches of pulp at 33% consistency
were mixed with clear filtrate to 8% consistency. The samples were mixed for a few
minutes with a 10% solution of calcium hydroxide added at a rate of 0 (reference),
2, 4, 6 and 8%, respectively, in the different experiments.
[0101] To the pulp samples (except the 0% reference) was added a 4% solution of NaOH in
a sub stoichiometric ratio of 5 % of the stoichiometric reaction amount of calcium
hydroxide. The pulp mixtures were then reacted with carbon dioxide at normal pressure.
The pH was 7.0-7.8.
[0102] The samples were stored in sealed plastic bags at 65°C for 6 hours in a heated water
bath.
[0103] The reaction between NaOH, CO
2 and Ca(OH)
2 continued until substantially all soluble calcium had precipitated as calcium carbonate.
Crystal growth took place inside and outside the fibers in the storage plastic bags.
[0104] After 6 hours storage the pulp was dewatered on a büchner funnel and then the pulp
was pressed to obtained paper hand sheets.
[0105] The produced papers had good ash and tensile index parameters corresponding to those
of the paper produced in Example 1.
Example 3, fiber loading pulp in continuous process
[0106] In a continuous papermaking process producing paper from a 50/50 mixture of DIP and
virgin fibers, calcium hydroxide is mixed into the clear filtrate used to dilute the
pulp suspension before the refiner which is used in the papermaking. The amount of
added calcium hydroxide is 6% calculated on the dry weight of the pulp.
[0107] After refining, the pulp suspension is directed into a tank and a non-stoichiometric
mixture of sodium hydroxide and carbon dioxide having its pH adjusted to pH 9 is added
in an amount of 8% of the stoichiometric amount of the calcium hydroxide. Thereafter,
gaseous carbon dioxide is introduced into the mixture at ambient pressure until the
pH has been reduced to 8.5.
[0108] The average retention time for the pulp in the non-pressurized tank is 0.5 hours
during which time most of the dissolved calcium precipitates as calcium carbonate
within and on the outside of the fibers.
[0109] As the pulp passes on to the machine chest, the pulp is further diluted with water
and carbon dioxide is added into the dilution water so that the pH of the pulp suspension
is reduced to 7.2 due to the added carbon dioxide. Solubilized calcium hydroxide remaining
in the pulp suspension is precipitated as calcium carbonate and the aqueous phase
of the suspension is depleted of calcium ions. The precipitation takes place preferentially
onto previously formed calcium carbonate crystals thus increasing the crystal size.
[0110] Due to the abundance of carbonate anions in the pulp suspension, the precipitated
calcium carbonate stays in solid form and is retained as fiber loaded filler in the
web which is subsequently produced in the wire section. The back water has a calcium
ion content which is lower than that of the aqueous phase of the pulp subjected to
the fiber loading process.
Example 4, fiber loading after refining and in the short circulation
[0111] In a continuous papermaking process producing paper from a mixture of DIP and virgin
fibers, a 10 % solution of calcium hydroxide is mixed into the pulp suspension after
refining. The amount of calcium hydroxide is 5% calculated on the dry weight of the
pulp.
[0112] Simultaneously with the calcium hydroxide addition, a 5 % solution of sodium hydroxide
is added to the pulp so that the amount of sodium hydroxide comprises 3 % of the stoichiometric
amount of the calcium hydroxide. Gaseous carbon dioxide is added to the pulp at ambient
pressure in an amount of 75 % of the stoichiometric amount of the calcium hydroxide.
The pulp is vigorously mixed to provide contact of the reagents within the pulp suspension.
[0113] Due to the presence of sodium hydroxide and the low amount of carbon dioxide relative
to the calcium hydroxide, the pH of the pulp suspension remains at a high level (above
pH 9). The high pH and the action of the sodium hydroxide catalyst ensure that the
calcium carbonate precipitation reaction is very fast.
[0114] Just before the pulp suspension enters the short circulation, additional sodium hydroxide
is added in an amount of a further 2 % of the stoichiometric amount of calcium hydroxide.
Then carbon dioxide is added in the short circulation in an amount which comprises
the remaining 25% of the stoichiometric amount of calcium hydroxide and an additional
amount to bring down the pH of the pulp suspension to pH 7.3.
[0115] Because of the presence of catalysing sodium hydroxide and because of the relatively
high pH at the initial stage of the reaction, the precipitation is fast and substantially
complete. The excess of carbon dioxide effectively precipitates the solubilized calcium
and retains the calcium carbonate in solid form. The system is thereby cleaned from
solubilized calcium ions and the fibrous web leaving the wire section is loaded with
filler produced
in situ both in and on the fibers.
1. A process for fiber loading by creating calcium carbonate
in situ from calcium hydroxide and carbon dioxide, comprising the steps of
a. providing an aqueous pulp suspension in a process for making paper, board or pulp;
b. adding a calcium compound selected from the group consisting of calcium hydroxide
and calcium oxide into the pulp suspension;
c. adding carbon dioxide to the pulp suspension in an amount corresponding to the
stoichiometric amount or more relative to the calcium compound;
d. adding an alkaline alkali metal compound to the pulp suspension in a sub stoichiometric
catalytic amount relative to the calcium compound for promoting calcium carbonate
precipitation in said pulp suspension; and
e. after calcium carbonate precipitation, dewatering the resultant fiber loaded suspension
to provide a fibrous web comprising calcium carbonate filler and an aqueous phase
depleted of calcium ions.
2. The process according to claim 1, wherein the alkaline alkali metal compound comprises
a hydroxide optionally in combination with carbon dioxide.
3. The process according to claim 2, wherein the alkali metal compound comprises a non-stoichiometric
combination of sodium hydroxide and carbon dioxide.
4. The process according to claim 1, wherein the amount of the alkali metal compound
corresponds to 1 to 50 %, preferably 3 to 20 %, most preferably 4 to 10 % of the stoichiometric
reaction amount of the calcium compound.
5. The process according to claim 1, wherein the pulp suspension, after addition thereto
of calcium hydroxide, carbon dioxide and alkaline alkali metal compound is retained
as a reaction mixture in a vessel before dewatering for allowing precipitation and
crystal growth of calcium carbonate.
6. The process according to claim 5, wherein the retention time is 0.5 hours or more,
preferably 1 to 8 hours, more preferably 1 to 4 hours.
7. The process according to claim 5 or 6, wherein the pH of the pulp suspension is reduced
with carbon dioxide to a pH of 8.5 or less, preferably to a pH below 8, before the
dewatering.
8. The process according to claim 1, wherein the amount of carbon dioxide corresponds
to 100 to 150 %, preferably 100 to 125 %, most preferably 100 to 110 % of the amount
of the calcium compound.
9. The process according to claim 1, wherein calcium hydroxide is reacted
with carbon dioxide at ambient pressure.
10. The process according to claim 1, wherein at least one of the calcium
compound, the carbon dioxide and the alkali metal compound is/are added to the pulp
suspension in two or more addition steps.
11. The process according to claim 10, wherein the carbon dioxide is added in
two or more addition steps and the final addition of carbon dioxide adjusts the pH
of the pulp suspension to a desired value between pH 6.5 and 8, preferably pH 7 to
7.5.
12. The process according to claim 11, wherein the alkali metal compound is
added in an amount to maintain the pH of the pulp suspension at pH 8 or more, preferably
pH 10 or more until the final addition of carbon dioxide.
13. The process according to claim 1, wherein 90 to 100% of the calcium
added with the calcium compound is consumed in the calcium carbonate precipitation
reaction.
14. The process according to claim 13, wherein the precipitation reaction is
continued until the amount of calcium precipitated in the process exceeds the amount
of calcium added with the calcium compound.
15. The process according to claim 1, wherein the pulp suspension is retained
in a vessel at a consistency of 2 to 10 %, preferably 4 to 8 % after addition thereto
of calcium compound and carbon dioxide as well as alkali metal compound, and the retention
time is between 0.5 and 20 hours .
16. The process of any one of the preceding claims, wherein the process for
making paper, board or pulp is a continuous process and the dewatered fibrous web
is processed to paper, board or pulp containing precipitated calcium carbonate as
filler within and on the outside of the fibers.