Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Waste paper or textile waste
Reexamination Certificate
2000-01-03
2003-04-01
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Waste paper or textile waste
C162S009000, C162S158000, C162S181200, C162S181400, C162S183000
Reexamination Certificate
active
06540870
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application is the national stage under 35 U.S.C. 371 of PCT/FI98/00558, filed Jun. 25, 1998.
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 the papermaking system. The invention also provides processes for the production of paper or board in papermaking systems wherein solid calcium carbonate is present as a filler and/or pigment.
The pulp suspension used according to the present invention may comprise recycled pulps or virgin mechanical or chemical pulps, or mixtures thereof. Special benefits of the present invention are obtained when calcium carbonate containing recycled fibers are processed into paper in accordance with the invention.
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. 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 no t a suitable filler for paper making 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. This applies especially to paper produced from bleached chemical and mechanical pulps. 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.
In the 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.
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 fibrilation 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. Eklund, T. Lindström, Paper Chemistry—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 acidic or neutral papermaking.
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.
Consequently, the present invention relates to a process for reducing or substantially retarding the dissolution and/or dissociation of calcium carbonate in a papermaking system. The process comprises the steps of providing an aqueous pulp suspension in said papermaking system; causing solid calcium carbonate to be present in said pulp suspension; and introducing carbon dioxide to said pulp suspension in connection with subjecting said calcium carbonate in said papermaking system to pH conditions of about 8 or lower.
The carbon dioxide is introduced in an amount sufficient to significantly retard dissolution of said calcium carbonate in said pulp suspension and reduce the amount of free calcium ions in said papermaking system compared to a similar system operating without carbon dio
Laurila-Lumme Auli
Leino Hannu Juhani
Pakarinen Heikki
AGA Aktiebolag
Alvo Steve
Browdy and Neimark , P.L.L.C.
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