Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-12-18
2003-04-01
Griffin, Steven P. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S185000, C106S464000, C423S430000
Reexamination Certificate
active
06540878
ABSTRACT:
The present invention relates to a stabilized calcium carbonate filler, a process for stabilizing calcium carbonate, its use in a paper making system and to an improved paper making process wherein stabilized calcium carbonate is used as a filler. Specifically the invention relates to a process wherein the dissolution of calcium carbonate filler is retarded compared to conventional paper making processes.
Calcium carbonate CaCO
3
is commonly used in paper making as a filler or pigment because it has a high brightness and it is the whitest filler in the price range in question. There are many products under the name calcium carbonate with the same chemical composition but with different physical properties. The products fall into two general classifications:
the natural products made directly by physically grinding chalk, limestone and marble, and
the precipitated products manufactured by chemically reacting various raw materials.
Precipitated calcium carbonate, PCC, can be produced as a filler to a host paper mill through a so called on-site plant concept. A small plant is built within the confines of the paper mill to manufacture PCC. Carbon dioxide containing gas such as smoke as is introduced into a slurry of slaked lime. At a pH above about 10.3 calcium carbonate is precipitated. The resulting PCC is then sent via pipeline directly to the paper mill at typically 20% solids. Due to the short run and the low solids viscosity of the PCC slurry, no dispersant is necessary as with natural products. Therefore, the system has proven to be advantageous for better wet-end paper machine chemistry. Another benefit of this system is that, since each on-site plant is associated with a particular mill, customized PCC fillers can be made for a mill's needs.
PCC is manufactured with high chemical purity which provides a consistent “clean” particle surface. This surface characteristic along with uniform particle size and unique morphologies are felt to be the predominate reasons for the substantial difference observed in the paper filling performance between PCC and natural products. PCC fillers tend to provide higher opacity and bulk, while natural products are less determined on sheet strength and internal sizing.
Calcium carbonate, irrespective of whether it is of natural origin or precipitated, is partially soluble in water e.g. according to the following equations:
CaCO
3
<->Ca
2+
+CO
3
2−
CO
3
2−
+H
2
O<->HCO
3
−
+OH
−
HCO
3
−
+H
2
O<->H
2
CO
3
+OH
−
H
2
CO
3
<->CO
2
+H
2
O
Calcium carbonate is fairly stable in alkaline conditions above a pH of about 7.5, but it is attacked by acids such as sulfuric acid and alum, as a result of which carbon dioxide is released. Therefore, it is well known to those skilled in the art that calcium carbonate is not generally a suitable filler for paper making at an acidic pH.
In attempts to improve the use of calcium carbonate at lower pH ranges so called acid tolerant PCC products have been developed, as exemplified by U.S. Pat. No. 5,043,017, wherein a slurry of calcium carbonate (PCC) is mixed with a calcium chelating agent or a conjugate base such as sodium hexametaphosphate and a weak acid such as phosphoric acid to provide an acid stabilized PCC. In the laboratory tests described in said US Patent it seems that the PCC slurry had been treated with carbon dioxide to obtain a stable initial pH of 8.0.
There are a number of draw-backs associated with the use of acid-tolerant PCC, such as environmental problems with the phosphorous compounds used in the process. The use of acid-tolerant PCC also requires technically complicated handling of chemicals which are foreign to the paper making processes.
Consequently, there exists a need to improve the use of calcium carbonate as a filler in paper making processes.
The present invention is based on the realization that carbon dioxide may be used to stabilize the calcium carbonate prior to its entering the paper making process.
Carbon dioxide is a gas, which dissolves in water forming carbonic acid and bicarbonate ions according to the reaction:
CO
2
+H
2
O<->H
2
CO
3
<->H
+
+HCO
3
−
Carbon dioxide or smoke gas containing carbon dioxide is used to produce precipitated calcium carbonate, PCC. Carbon dioxide has also been used (as mentioned e.g, in U.S. Pat. No. 5,043,017) for lowering the pH of PCC slurries to 8.0 prior to the acid stabilization treatment.
According to U.S. Pat. No. 5,262,006 precipitation of gypsum in an alkaline recycle or broke derived pulp may be prevented by adding carbon dioxide to form bicarbonate ions in the alkaline pulp and to precipitate calcium as carbonate.
According to EP Patent 0 281 273 carbon dioxide may be used for adjusting the pH of alkaline pulps upstream of the fibrilation step.
It is well known in the art that the solubility of calcium carbonate increases with decreasing pH. According to GB Patent Application 2 008 562 carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.
It is also known that increasing the carbon dioxide partial pressure increases the solubility of calcium carbonate (D. Eklund, T. Lindstrom, Paper Chemistry—an introduction, DT Paper Science Publications, Grankulla, Finland 1991, p. 253). Consequently, the prior art paper making processes have recommended the use of calcium carbonate as a filler for alkaline but not for acidic pulps.
It has now surprisingly been found that carbon dioxide may be used to improve the function of calcium carbonate as a filler in paper making processes operating at pH below 8.0 and preferably a pH range of 6.5 to 7.5, more preferably at 7.0 to 7.2.
Contrary to common belief, it has been found that calcium carbonate can be used as a filler also in slightly acidic and neutral pulps provided that carbon dioxide is used to protect the calcium carbonate from dissolving by the acid attack.
The invention according to the present application is defined in the appended claims, the contents of which are included herein by reference.
Consequently, the present invention relates to a stabilized calcium carbonate filler for use in a paper making system. The filler comprises finely divided ground natural calcium carbonate or precipitated calcium carbonate in an aqueous slurry to which has been added an amount of carbon dioxide sufficient to lower the pH of the slurry below 8.0 and preferably to a pH below 7.5 thereby retarding the dissolution of said calcium carbonate in said paper making system.
The present invention also relates to a process for producing a stabilized calcium carbonate filler for use in a paper making system, wherein an aqueous slurry of finely divided ground natural calcium carbonate or precipitated calcium carbonate is treated with an amount of carbon dioxide sufficient to lower the pH of the slurry below 8.0 and preferably to a pH below 7.5 for retarding the dissolution of said calcium carbonate in said paper making system.
The invention further relates to a process for producing paper using stabilized calcium carbonate as a filler, comprising providing a pulp suspension in a paper making system; providing an aqueous slurry containing finely divided ground natural calcium carbonate or precipitated calcium carbonate; adding to said aqueous calcium carbonate slurry an amount of carbon dioxide sufficient to lower the pH of said slurry below 8.0 and preferably to a pH below 7.5; immediately after said addition, feeding said carbon dioxide containing calcium carbonate slurry to said pulp suspension; feeding said calcium carbonate containing pulp suspension to a web forming device of a paper machine, and processing said web into paper or board having a major portion of said calcium carbonate as a solid filler.
For an optimum effect to be achieved, the carbon dioxide should be introduced into the slurry just prior to its being fed into the paper making system. In order to ac
AGA Aktiebolag
Griffin Steven P.
Hug Eric
Pearne & Gordon LLP
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