Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Reexamination Certificate
1996-06-05
2002-11-05
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
C162S039000, C162S068000, C162S076000, C162S082000
Reexamination Certificate
active
06475338
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
In response to market trends and government regulations, the Pulp and Paper Industry throughout the world is presently experiencing a transition from chlorine-based bleaching methods to non-chlorine or reduced-chlorine-based bleaching methods. The technical literature is presently replete with technical advancements extolling the performance of Totally Chlorine Free (TCF) bleaching processes or Elemental Chlorine Free (ECF) bleaching processes. The former processes employ no chlorine-containing chemicals at all, while the latter processes use chlorine dioxide as the only chlorine containing bleaching chemical along with other non-chlorine bleaching agents.
This trend toward eliminating or minimizing the presence of chlorine-containing compounds from the bleaching process also introduces the potential of re-using the non-chlorine containing liquid streams generated in the bleach plant of a pulp mill. In the trade and technical literature such “closed” mills are now described by the expressions Effluent Free Mill (EFM) or Closed-Cycle Mill. In the past, the collection and re-use of these liquids was uneconomical or technically impossible due to the potential corrosion damage or to interference with the process chemistry caused by the chlorine-containing chemicals in the systems and equipment used to recover and re-use these liquids. These chlorine-containing effluents were typically “sewered” and then, with or without treatment, discharged to the local “recipient”, that is, to a lake, river or other body of water However, as disclosed in U.S. Pat. Nos. 5,374,333; 5,300,19, 5,302,246, 5,439,555 and copending application Ser. No. 08/113,642 filed on Aug. 31, 1993, now U.S. Pat. No. 5,549,788 and marketed under the trademark MIM by Ahlstrom Machinery of Glens Falls, N.Y., several processes and systems have been developed for reducing or eliminating chlorine-containing compounds in pulp mill liquid streams and effluent streams, or for recovering and re-using these valuable chemical-containing resources with no or minimal effect upon the surrounding environment.
However, current examination of the process chemistry, either theoretical or in mill trials, has revealed that this reduction or elimination of chlorine-based bleaching compounds is hindered by the presence of metal ions in the process streams. These metal ions, for example, ions of iron, copper, manganese, and magnesium, among others, interfere with process chemistry, for example pulping and bleaching, and can accumulate in the pulp mill and become manifest as precipitation deposits, that is “scale”, on equipment.
These metals can be introduced to the pulp mill from several sources. For example, the wood supply may contain metal ions that naturally exist in the trees or other fiber source used. Metals may also be introduced as impurities in the chemicals introduced to the pulp mill, for example, in purchased acids or make-up chemicals. Metals ion may also originate from corrosion in the equipments itself.
In the past, when chlorine-based bleaching processes were the norm, the presence of metal ions in the pulp or process streams was not significant. For example, chlorine in an acidic environment solubilized the metal ions and they were subsequently removed from the system with the chlorine bleach effluent. As a result, the metal ions were simply purged from the system when the chlorine-containing bleach effluent was sewered. However, present non-chlorine-based bleaching chemicals, such as ozone and peroxide, do not react with the metal ions in a beneficial fashion but tend to be consumed by metal ions and thus these ions negatively effect the bleaching reaction. It is now believed that in addition to consuming, for example, peroxide, metal ions may catalyze reactions which produce by-products that undesirably also consume bleaching chemical. Thus the presence of dissolved metal ions in the bleaching stages reduces the efficiency, and hence increases the cost, of the bleaching process. To address this problem, typical conventional TCF or ECF bleaching sequences incorporate some form of metal-removing treatment, for example, a treatment with chelating agents (also known as sequestering agents or metal-complex forming agents) or an acid wash, or a “hot acid” treatment as disclosed in pending, U.S. application Ser. No. 08/542,646 filed on Oct. 13, 1995, and now abandoned.
In this specification and claims the term “chelating agent” or “chelant” is used to refer to any chemical compound having strong affinity for transition metal ions, including Mn, Fe and Cu ions, and tending to combine with the metal ions. This process is also referred to the “sequestering” of metal ions; thus these compounds are also referred to as “sequestering agents”. Furthermore, the chelant-metal ion compounds that are formed are often referred to as “complexes”; thus chelating agents are also referred to as “metal complexing agents” or “complex forming agents”. The term “chelating agent” as used herein encompasses all these terms, and the released transition metals combined with chelating agent are referred to as “complexes”.
The presence of dissolved transition metal ions also hampers the process of mill “closure”, that is, the recovering and re-using effluents. As discussed above, the presence of metals in a bleach plant is typically minimized by some form of metal-removal treatment. However, if the effluents resulting from such treatment, which contain dissolved metal ions, are to be reused the metal ions must be removed. U.S. Pat. No. 5,401,362 and co-pending applications Ser. No. 08/113,645 filed on Aug. 31, 1993 and Ser. No. 08/195,139 filed on Feb. 14, 1994 illustrate several methods of treating metal-containing effluents to remove the metals prior to re-use.
In addition to these treatments of the effluent streams, the presence of transition metal ions within the pulp mill can also be minimized by purifying the chemicals introduced to the mill to eliminate their introduction of metals. Also, non-corrosive metallurgy can also be used to minimize or eliminate the potential for introducing dissolved metals from corrosion. However, regardless of these and other remedies for reducing the introduction of metal ions to the pulp mill, metal ions can still enter the mill with the original wood supply, or other source of cellulose.
Published PCT application WO 95/02726 discloses one method that attempts to reduce the metal ion concentration prior to digestion in a digester. The disclosed process includes a treatment of cellulose material, for example, softwood chips, with a liquid containing a chelating agent prior to formal digestion of the material. Though this treatment reduces the concentration of various dissolved metals, it does not treat the material with chelating agents in the most advantageous stage. Since metals-containing substances are released from the cellulose in essentially all phases of the cook, treating and removing the metals prior to the cook as disclosed in the PCT application is not the most advantageous treatment. There are certain stages of the cooking process where more metal ions are released from the cellulose and can be more effectively removed. Furthermore, some metals are inherently removed during the kraft cooking process. Chelating chemicals need not be wasted removing metals that would be removed from the process anyway. In addition, the PCT publication does not recognize other significant features of the present invention.
Though the chemical mechanism is not yet completely understood, it is believed that the naturally-occurring metal ions that are present in cellulose material , for example, wood chips, are strongly bound to the wood material. As shown in the article “The Behavior of Certain Inorganic in the Wood/White Liquor System” by Hartler, et al. (Svensk Papperstidning, No. 12, 1973), these metal ions are typically not released from the cellulose until well into the pulping process, for example, not until the pulping temperature reaches at least 50° C., preferably at least ab
Henricson Kaj O.
Jiang Jian Er
Alvo Steve
Andritz Inc.
Nixon & Vanderhye P.C.
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