Treatment of cellulose material with additives while...

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...

Reexamination Certificate

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C162S041000, C162S076000, C162S090000

Reexamination Certificate

active

06241851

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to various methods and apparatuses for treating comminuted cellulosic fibrous material during the pulping process with a solution containing additives for improving the efficiency of the pulping process or for improving the quality of the pulp produced. Typical additives include, but are not limited to, polysulfide, sulfur and sulfur-containing compounds (e.g. hydrogen sulfide), surfactants, and anthraquinone and their equivalents and derivatives. In the following discussion it is to be understood that use of the term “anthraquinone” is meant to encompass all anthraquinone-based chemicals, their equivalents and derivatives.
Paper products today are manufactured from cellulose pulps produced by a variety of methods. For example, newsprint is made from a high-yield mechanical process in which the wood is ground to produce a pulp which retains 80% or more of the original constituents of the wood, including the undesirable, color-degrading and strength-diminishing constituents, for example, lignin. Fine papers of high brightness and cleanliness used for writing papers or food containers, for example, are typically made by chemical treatment in which the undesirable non-cellulose constituents of the wood, for example, lignin, are dissolved through chemical action typically under pressure and temperature, to produce a relatively pure form of cellulose fibers from which, for example, fine papers can be made. However, because the cellulose and non-cellulose constituents are not segregated in the wood and are typically intermingled with each other, it is difficult to dissolve the non-cellulose constituents without dissolving some of the cellulose. As a result, in the chemical treatment of wood, though the original wood may typically comprise or consist of 70 to 80% of the desirable cellulose and hemicellulose—(that is, the usable carbohydrates), typically only about 60 to 70% of the usable carbohydrates are retained in the final product. Some of the desirable carbohydrates are dissolved at the same time as the undesirable non-carbohydrate material. The percentage, by weight, of the amount of cellulose (and some non-cellulose) retained, excluding moisture, compared to the amount of wood introduced to the process is referred to as the “yield” of the process. Where mechanical pulping methods may have yields greater than 80%, chemical pulping processes typically have yields of about 50%. Of course, the paper manufacturer desires the highest yield possible.
In addition to yield, another important property of cellulosic pulps is the relative strength of the paper produced from the pulp. Typically, the strength of a paper is a function of two features of the cellulose fibers from which the paper is produced: the intrinsic strength of the fibers and the strength of the bonds between the fibers. The strength of individual fibers is typically characterized as the amount of load that the fiber can withstand while under axial tension and also the amount of load that the fiber can withstand when exposed to a transaxial force, that is, shear. The strength of individual fibers is typically associated with what is termed the “tear strength” of a sample of paper produced from the fiber. The strength of the bonds between fibers is a function of the relative surface area and the flexibility of the fiber, among other things. The strength of these bonds is typically indicated by what is called the “tensile strength” of a sample of paper produced from the fibers. The tear and tensile strength of a paper sample are typically inversely proportional: as the tear strength increases, the tensile strength decreases, and vice versa.
The kraft chemical pulping process (also known as the sulfate process) is typical of a chemical pulp process that produces pulps of high strength and yields of around 50%. In the kraft process the wood is chemically treated under temperature and pressure with an aqueous solution of sodium hydroxide [NaOH] and sodium sulfide [Na
2
S]. However, it is sometimes possible to incrementally increase the yield of the kraft process by introducing additives or chemical treatments to the process, typically before treatment with the sulfide and hydroxide. Note that a 1% increase in yield for a typical 1000 ton-per-day pulp mill, which sells pulp at approximately $500.00 per ton, can mean over 3 million dollars in added revenue per year, with no increase in wood usage. Thus, single-digit increases in yield can have significant impact upon the profitability of a pulp mill. If a pulp mill is capacity limited due to limitations in increasing the capacity of its recovery boiler, an increase in the yield of a pulping process can increase the capacity of the mill while avoiding the limitations of the recovery system.
As described in Pulping Processes (1965) by Rydholm [pp. 1003-1004] and elsewhere, it is generally understood that cellulose degradation under alkaline conditions is governed by what are referred to as “peeling” reactions and “stopping” reactions. Peeling reactions are the reactions that occur at the ends of cellulose molecules in which individual carbohydrate units, or monomers, are detached or “peeled” from the end of the carbohydrate chain. In this reaction, the aldehydic end groups of the cellulose chains are cleaved from the chain exposing a new aldehydic end group. This newly-exposed end groups can continue to be cleaved until a carboxyl end group is formed and the peeling reaction is terminated. This formation of a carboxyl end group is referred to as the “stopping” reaction. This stopping reaction stabilizes the carbohydrate chain against further degradation by “peeling”. As described by Rydholm, typically 50 or more monomers are “peeled” from a newly-exposed end of a carbohydrate chain during alkaline chemical treatment. This degradation of the cellulose molecular chains can be manifest as a reduction in yield (that is, “peeling” causes the dissolution and loss of cellulose).
Conventional mechanisms for increasing the yield of chemical pulping process are directed toward limiting the amount of cellulose lost through alkaline peeling by promoting the stabilization of the end groups against this peeling reaction, that is, they promote the formation of a carboxylic end group.
As explained, for example, in Pulp and Paper Manufacture, Volume 5: Alkaline Pulping”, edited by Grace, et al. [pp. 114-122], several recognized additives can be used to stabilize the alkaline peeling reaction and incrementally increase the yield of chemical pulp mills. These include sodium borohydride [NaBH
4
], sodium polysulfide [Na
2
S
n
] (known simply as “polysulfide”), and anthraquinone (AQ). Smook (1989) in his Handbook of Pulp and Paper Technologists also mentions that hydrogen sulfide [H
2
S] gas pretreatment of chips can be used to increase yield.
U.S. Pat. No. 4,012,280 discloses that improved yield of an alkaline chemical pulping process can be obtained by adding cyclic keto compounds, including anthraquinone, to the cooking liquor and treating cellulose material with the cooking liquor-AQ solution at pulping temperatures. However, in such a process the AQ additive is not recovered and is simply lost to the pulping process, even though it is known that AQ is a catalyst. U.S. Pat. No. 4,127,439 improved on the earlier AQ treatment process by limiting the exposure of cellulose material to AQ only in a pretreatment stage prior to digestion. In this process, the pretreatment liquor is separated from the cellulose material prior to digestion and the separated pretreatment liquor containing residual AQ is re-used for pretreatment. U.S. Pat. No. 4,127,439 includes the option of pretreating cellulose in a continuous process in which the treatment liquid counter-currently displaces the pretreatment liquor in a single treatment zone. However, the removal and recovery of the pretreatment liquor is limited due to the treatment in one treatment zone.
U.S. Pat. No. 4,310,383 d

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