Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Waste paper or textile waste
Reexamination Certificate
2000-09-01
2003-09-23
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Waste paper or textile waste
C162S009000, C162S083000, C162S158000, C162S181200, C162S183000
Reexamination Certificate
active
06623599
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application is the national stage under 35 U.S.C. 371 of PCT/FI99/00156, filed Mar. 1, 1999.
The present invention relates to the art of papermaking. In particular the invention relates to papermaking processes wherein calcium carbonate is included in a pulp suspension. Specifically, the present invention provides a process for preventing or substantially retarding the dissolution of calcium carbonate in a papermaking system containing mechanical pulp. The invention also provides processes for the production of paper or board in papermaking systems containing mechanical pulp and wherein solid calcium carbonate is present as a filler and/or pigment. Finally, the invention relates to the use of carbon dioxide for retarding the dissolution of calcium carbonate in a papermaking system.
The pulp suspension used according to the present invention comprises mechanical pulps or mixtures thereof with recycled pulps and/or chemical pulps. Special benefits of the present invention are obtained when thermomechanical pulp (TMP) is processed into paper having calcium carbonate as a filler. Such pulps and pulp combinations are currently used largely for the manufacture of supercalendered (SC) and light weight coated (LWC) papers, for newsprint and the like.
Calcium carbonate CaCO
3
is commonly used in papermaking as a filler or pigment because it has a high brightness and it is the whitest filler in the price range in question. The calcium carbonate may be naturally occurring chalk or calcite or it may be synthetically produced precipitated calcium carbonate (PCC). Calcium carbonate is sparingly soluble in alkaline conditions above a pH of about 8, but it is attacked by acids such as sulfuric acid and alum. as a result of which it is solubilized. Consequently, normal calcium carbonate is not a suitable filler for papermaking at an acidic pH.
In an attempt to solve the problem with solubilization of calcium carbonate at acidic pH an acid resistant precipitated calcium carbonate has been provided. However, production of this calcium carbonate is technically complicated making the use thereof expensive and even so this product is not either totally acid resistant.
Some papermakers have converted their processes from acidic to neutral pH, partly in order to be able to use calcium carbonate as a filler and/or pigment. The expression “neutral pH” corresponds in these processes to a pH in the short circulation of approximately 7-8.5, most preferably 7-8. The expression “pseudoneutral pH” refers to a pH below that at which calcium carbonate dissolves and it generally refers to a pH of 7 or lower.
Papermaking processes utilizing mechanical pulps, especially TMP, should not be run at pH levels above about 7.2, as a high pH will cause a decrease in the brightness of the paper so produced.
In papermaking processes calcium carbonate is added as a filler to the stock prior to paper formation and consequently a part of the filler particles will enter the process waters circulating in the papermaking system. When calcium carbonate is used as a pigment in coated papers, a part of the calcium carbonate will be recirculated to the process with the broke.
Recycled waste paper as well as broke (herein generally referred to as recycled fibers) may contain calcium carbonate as filler and/or pigment. The repulping of recycled fibers is generally performed at an alkaline pH wherein the calcium carbonate remains essentially in solid form. However, if the paper machine is run at an acidic, neutral or pseudoneutral pH range, the calcium carbonate deriving from the recycled fibers will start to dissolve.
Also under alkaline conditions the stock preparation and the short circulation includes addition of a number of paper chemicals and dilution waters, some of which are acidic and therefore decrease the pH of the pulp. At each acidic addition calcium carbonate may be lost and there may be foaming problems due to a decomposition of dissolved calcium carbonate.
Solubilized calcium carbonate dissociates in water 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
At a pH below about 8 the dissolution of calcium carbonate and consequently the concentration of free calcium ions increases and foaming is observed as carbon dioxide gas is released. With use of closed circulating waters in the papermaking system, the solubilization of calcium carbonate accumulates high concentrations of calcium ions which cause complex 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.
Consequently, there exists a need to improve the use of calcium carbonate in paper making processes, especially processes wherein mechanical pulps such as TMP are included.
Carbon dioxide is a gas, which dissolves in water or a pulp suspension forming carbonic acid and/or bicarbonate ions according to the reaction:
CO
2
+H
2
O⇄H
2
CO
3
⇄H
+
÷HCO
3
−
Use of carbon dioxide in paper making has been suggested in the prior art for various reasons. According to U.S. Pat. No. 1,993,265 carbon dioxide is used for inhibiting the destructive action of calcium carbonate on a rosin size precipitated with alum.
According to U.S. Pat. No. 2,114,809 a calcium carbonate containing stock is sized using alum, whereby carbon dioxide is created in the reaction between alum and carbonate filler.
According to U.S. Pat. No. 5,378,322 bicarbonate ions required for catalyzing non-acidic sizing with alkylketene dimers may be generated by dissociation of carbon dioxide in the aqueous pulp. If calcium carbonate is added as a filler, the catalytic bicarbonate ions may be produced by a reaction between dissolved carbon dioxide and calcium carbonate. However, this combination of carbon dioxide and calcium carbonate is proposed only for a pH down to 8.6.
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 the calcium as calcium carbonate providing PCC in the processing system.
According to EP Patent 0 296 198 the washing of alkaline pulps may be improved by adding carbon dioxide to the washing water.
According to EP Patent 0 281 273 carbon dioxide may be used for adjusting the pH of alkaline pulps upstream of the fibrillation step.
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 well known in the art that the solubility of calcium carbonate increases with decreasing pH. According to literature (D. Ekiund, T. Lindstrom, Paper Chemistryo—an introduction, DT Paper Science Publications, Grankulla, Finland 1991, p. 253) an increase in the carbon dioxide partial pressure increases the solubility of calcium carbonate. Consequently, the prior art papermaking processes have recommended the use of calcium carbonate as a filler for alkaline but not for the acidic or neutral papermaking which is typical when TMP is used in the process.
It has now surprisingly been found that carbon dioxide may be used to improve the function of calcium carbonate in paper making processes operating at pH levels below those at which calcium carbonate traditionally has been used.
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 process for preventing or subs
Laurila-Lumme Auli
Leino Hannu Juhani
Pakarinen Heikki
AGA Aktiebolag
Alvo Steve
Browdy and Neimark , P.L.L.C.
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