Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Reexamination Certificate
1998-01-30
2001-08-14
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
C162S080000, C162S081000, C162S082000, C162S087000, C162S238000, C162S252000, C162S263000, C162SDIG004
Reexamination Certificate
active
06273994
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for measuring the bleach requirement, bleachability, and effectiveness of hemicellulase enzyme treatment of pulp, and in particular, to a method and a device for rapidly making these measurements to enable pulp mill operators to determine more accurately the amount of bleaching chemicals required to bleach the pulp. The present invention also includes an optical device for use in the device.
2. Background of the Invention
Paper is made from wood and nonwood fibers. Wood is the predominant fiber source for paper in North America and Europe while straw predominates in Asia. The paper manufacturing processes based on wood and nonwood fibers are similar. The processes described below are for paper made from wood but can be directly related to paper made from nonwood fibers. This description is found in more detail in “Pulping Processes” by S. Rydholm, 1965.
The fibers used to make paper consist primarily of cellulose, hemicellulose, and lignin. The amounts of these three constituents present in the final paper product depends on the manufacturing process used. Pulp made from virgin fiber can be described in terms of the three primary classes of pulping processes: mechanical, semichemical, and chemical. Recycled pulp, also known as secondary fiber, consists of a combination of one or more of the three classes of pulp made from virgin fiber.
In mechanical pulping, the wood is broken up into a mat of pulp fibers by mechanical action, such as grinding with stones or blades or passing the pulp through refiners. When high temperatures are used to aid in the mechanical pulping, the process is known as thermomechanical pulping. Mechanical pulp has almost all of the original cellulose, hemicellulose, and lignin present and is used for lower-grade, disposable papers such as newsprint.
Paper made from mechanical pulp often must be of at least a minimum degree of whiteness to satisfy the end use. The degree of whiteness is known as the brightness of the pulp and is generally measured in terms of ISO brightness. One of ordinary skill in the art will recognize that there are various methods for measuring ISO brightness, such as TAPPI method T452. Mechanical pulp is sometimes bleached with perhaps 5 Kg/t hydrogen peroxide and/or 2 Kg/t sodium hydrosulfite to increase its brightness, although seldom to brightness levels obtained with other pulping methods. The bleaching chemicals are typically added in one operation for the hydrogen peroxide and a second operation for the sodium hydrosulphite. These bleaching operations are called stages.
Semichemical pulping refers to the combination of mechanical action and chemicals used to make pulp. The mechanical action is similar to that used in mechanical pulp. The chemicals added are typically neutral salts such as sodium sulphate. The presence of the salts aids in the pulping process and thereby decreases the intensity of mechanical action required. This decreases the damage to the fibers and results in pulp of higher strength than mechanical pulp. Semichemical pulps are used for products where some degree of strength is required, such as packaging. Like mechanical pulp, semichemical pulps retain almost all of the initial cellulose, hemicellulose, and lignin.
Semichemical pulps are sometimes bleached to improve their brightness. As with mechanical pulps, the most important bleaching chemicals are hydrogen peroxide and sodium hydrosulfite. The amount of these chemicals used is often 5-fold greater than with mechanical pulps, as a higher brightness is often desired. The bleaching of semichemical pulp is carried out typically with two or three hydrogen peroxide stages and then a sodium hydrosulfite stage.
The highest quality paper is made by chemical pulping processes. The manufacture of high quality, bright white paper largely depends on removing the lignin from the wood pulp with minimal degradation to the cellulose and hemicellulose. Complete lignin removal is essential for the production of fine paper, because lignin weakens and imparts color onto the pulp. The two principle chemical pulping processes are sulfite pulping, in which the pulping liquor is acidic sodium sulfite, and Kraft pulping, in which the cooking liquor is alkaline sodium sulfide. Kraft pulping is the more common process, with about 37 million tonnes of Kraft pulp and 3 million tonnes of sulfite pulp produced annually in North America.
In chemical pulping, 80% to 95% of the lignin is removed from the wood by cooking it in the chemical liquor in a batch reactor or a tall flow through tower, either of which is known as a “digester.” After being washed with water, the cooked material contains 1.5% to 5% residual lignin and is known as brownstock. In some mills, almost half of the residual lignin is removed using an oxygen delignification reactor. Regardless of whether a mill carries out oxygen delignification or not, the remaining lignin is removed by a multistage bleaching process to obtain a bright, stable final product.
A general reference for bleaching pulp is “Pulp Bleaching: Principles and Practice” edited by D. Reeve and C. Dence, 1996 (hereafter “Reeve and Dence”).
The first stage of a conventional bleaching process involves treating the brownstock with chlorine or chlorine dioxide or a mixture thereof. This is known as the “first stage” or the “chlorination stage”, and the terms are used interchangeably herein. The chlorination stage is the most important bleaching stage, as 50% to 80% of the total bleaching chemicals are used in this stage.
In the second bleaching stage, the pulp is extracted with sodium hydroxide. This is known as the “second stage” or the “extraction stage”, and the terms are used interchangeably herein. The chlorination and extraction stages reduce the lignin concentration in the pulp to less than 1%. The pulp requires 1 to 4 hours to pass through these two stages.
After extraction, the final lignin remaining in the pulp is removed in three additional stages. The “third” or “D1” stage consists of treating the pulp with chlorine dioxide. The “fourth” or “E2” stage is an extraction with sodium hydroxide. The “fifth” or “D2” stage is another chlorine dioxide stage. The final product of the D2 stage is the desired bright white pulp. Typically, 5 to 8 hours are required for the final three stages, and therefore 6 to 12 hours is required for pulp to pass through the bleach plant.
Secondary fiber can consist of paper made from mechanical, semichemical, or chemical pulping processes, or a mixture of paper made by any two or all three of these processes. The secondary fiber is repulped by processes analogous to, but milder than, the mechanical, semichemical, and chemical pulping processes used for virgin fiber. The secondary pulp is then bleached with hydrogen peroxide, sodium hydrosulfite, or chlorine-based oxidizing chemicals as is done with virgin pulps. A somewhat lesser amount of bleaching chemicals are required for bleaching secondary fiber than virgin fiber.
Whether the pulp consists of virgin or secondary fiber, and whether it is from a wood or nonwood fiber source, the most important specification for the bleached pulp is its brightness. The pulp customer specifies the minimum brightness that they can or will tolerate in their use of the pulp. Pulp that fails to achieve the minimum brightness, i.e. pulp that is underbleached, is known as “off-grade” pulp. This must be repulped and run through the bleach plant a second time, at great expense and loss of production time. Pulp whose brightness greatly exceeds the minimum for the customer is acceptable for sale, but the excess brightness is achieved at a cost of additional bleaching chemicals that is, in effect, wasted. This pulp is overbleached.
Pulp mill operators therefore balance between using a sufficient amount of bleaching chemicals to avoid off-grade pulp, but not overbleaching at large cost and waste of bleaching chemicals. Several methods and instruments have evolved to help the operators contr
Creber Brian
Foody Brian
Foody Patrick J.
Tolan Jeffrey S.
Chin Peter
Fitzpatrick ,Cella, Harper & Scinto
Iogen Corporation
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