Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Chemical treatment after start or completion of mechanical...
Reexamination Certificate
2000-12-04
2002-08-06
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Chemical treatment after start or completion of mechanical...
C162S071000, C162S076000, C162S078000, C162S080000, C162S083000
Reexamination Certificate
active
06428653
ABSTRACT:
TECHNICAL FIELD
A process for bleaching cellulose-containing pulp which has been subjected to previous bleaching with an oxidizing agent such as a peroxide compound.
BACKGROUND OF THE INVENTION
The bleaching of cellulose pulps has been conducted for many years. The purpose of bleaching pulp is to create a resultant pulp product which is suitable for use in producing a wide range of derivative products such as newsprint, printing papers, molded articles, corrugated paper, cardboard, paperboard etc.
As a general rule, the value of the resultant pulp product increases with the effectiveness of the bleaching process that the pulp is subjected to.
The term “cellulose-containing pulp” includes any pulp which comprises an amount of cellulose, which is the chief component of the cell walls of plants. Representative forms of cellulose-containing pulp include wood pulps, straw pulps, hemp pulps, cotton pulps, etc. The pulp most commonly used in the manufacture of paper products is wood pulp.
Wood pulps are classified according to the manner in which they are created. Mechanical wood pulps are created solely by mechanical action which is imparted to wood particles to reduce them to fibers. Chemical wood pulps are created solely by using chemical action to strip wood particles of their non-cellulosic materials such as lignins and impurities. Chemi-mechanical wood pulps are created by the use of chemical softeners to pre-treat wood particles, followed by mechanical action to reduce the softened wood particles to fibers.
Pulps produced by mechanical and chemi-mechanical pulping techniques are often referred to collectively as “mechanical pulps” to distinguish them from “chemical pulps” which are produced purely by chemical pulping techniques.
As a result of the mechanism of action of chemical pulping techniques, chemical pulps tend to contain high proportions of cellulose, typically in the order of about 80% to about 95% by weight of pulp. Chemical pulps are sometimes referred to as “high cellulose” or “low yield” pulps.
Mechanical pulps resulting from mechanical pulping techniques continue to contain significant amounts of non-cellulosic materials, with the result that mechanical pulps are referred to as “low cellulose” or “high yield” pulps. The cellulose content of mechanical pulps is typically in the order of about 40% to about 60% by weight of pulp.
Bleaching processes for cellulose-containing pulps such as wood pulps can be roughly divided into three categories. The first category is bleaching with oxidizing agents such as peroxide compounds. The second category is bleaching with oxidizing agents containing chlorine. The third category is bleaching with reducing agents.
Peroxide bleaching is commonly used for bleaching pulps derived from mechanical pulping techniques because of demonstrated effectiveness in bleaching pulps containing significant amounts of lignins and other impurities.
Chlorine bleaching is commonly used for bleaching pulps derived from chemical pulping techniques in which the lignin content is minimized.
Bleaching with reducing agents has been used for pulps derived from both mechanical pulping techniques and chemical pulping techniques.
Current and evolving technologies for bleaching both mechanical and chemical pulps often involve “multi-stage bleaching”, in which combinations of bleaching processes are used in an effort to achieve a higher level of pulp brightness.
A second issue which is sometimes addressed by these technologies relates to minimizing the amount of colour reversion which is experienced by the pulp following the bleaching process. Colour reversion is a particular concern with mechanical pulps, which contain a relatively high amount of lignin and other impurities. It is believed that it is the lignin and these other impurities which may be largely responsible for colour reversion in mechanical pulps.
U.S. Pat. No. 3,100,732 (Smedberg) relates to a bleaching process involving the combined and simultaneous action of peroxide and borohydride. The stated purpose of the process is to eliminate the need to neutralize the pulp using sulfur dioxide after peroxide bleaching. Smedberg also discusses the trend toward multi-stage bleaching processes for both mechanical and chemical pulps. In particular, Smedberg describes a prior art two stage bleaching process for mechanical pulps in which the pulp is initially bleached with peroxide, is neutralized with sulfur dioxide to prevent colour reversion, and is then subjected to a second bleaching stage with a chelating agent and a reducing agent such as a hydrosulfite.
The issue of colour reversion relating to peroxide bleached pulps is addressed further in U.S. Pat. No. 3,709,778 (Lincoln et al) and Canadian Patent No. 916,367 (Meyers et al), in which mechanical pulps which have been bleached with a peroxide compound are thereafter contacted with a sulfite compound as an after-treatment to avoid colour reversion caused by active oxygen compounds remaining in the pulp. In other words, it is theorized in these references that it is residual peroxide and not lignin which is responsible for colour reversion in mechanical pulps.
The use of thiourea dioxide (also known as formamidine sulfinic acid or FAS) and related compounds in connection with the bleaching of both mechanical and chemical pulps is known.
U.S. Pat. No. 3,384,534 (Kindron et al) describes a single stage bleaching process for mechanical pulps using thiourea dioxide. U.S. Pat. No. 3,481,828 (Turner et al) describes a single stage bleaching process for mechanical pulps using a bleaching composition which contains both thiourea dioxide and zinc sulphate. U.S. Pat. No. 3,507,743 (Cartsunis et al) describes a single stage bleaching process for mechanical pulps in which thiourea is added to reductive bleaching agents such as alkali metal borohydride or thiourea dioxide in order to minimize the colour reversion of the bleached pulp.
U.S. Pat. No. 4,244,780 (Rende et al) describes the addition of thiourea dioxide during chlorine bleaching of chemical pulps for the purpose of creating bleached pulps having improved fiber strength. Although it is stated that the Rende invention may be used in conjunction with multi-stage bleaching processes, the thiourea dioxide must be present during the initial chlorination stage in order to be effective.
U.S. Pat. No. 5,073,301 (Suess et al) describes a process for stabilizing the viscosity of wood pulps which have been treated with ozone or ozone/oxygen (but not with chlorine). The process involves treating the pulp with formamidine sulfinic acid during an alkali extraction step which takes place after the ozone or ozone/oxygen treatment. The alkali extraction step occurs at a pH of between 8 and 12 and at a temperature of between 40° Celsius and 90° Celsius. Additional bleaching steps may be performed after the alkali extraction step.
The use of thiourea dioxide in multi-stage bleaching processes involving peroxide compounds is also known.
U.S. Pat. No. 4,675,076 (Darlington) describes a method for additional bleaching of mechanical pulps following peroxide bleaching, in which thiourea is added to the pulp at a pH of between 4 and 7 in the presence of residual peroxide. The pulp is then maintained at a pH of between 8 and 11 to achieve further bleaching of the pulp as the thiourea reacts with the residual peroxide to form thiourea dioxide. Optionally, the pH of the pulp may thereafter be lowered to between 5 and 6 to achieve further additional bleaching of the pulp. Finally, the pulp may optionally be subjected to a final peroxide bleaching step to gain yet additional pulp brightness.
Similarly, U.S. Pat. No. 5,958,184 (Kanada et al) describes an “on-site” process for economically producing thiourea dioxide for the purpose of pulp bleaching, which process involves the mixing of thiourea, a peroxide and optionally at least one reaction catalyst. The mixing step may occur either in the presence or in the absence of pulp. The process is expressed to be a substitute for post-bleaching of pulps with other reducing agents.
U.S. Pat
Akerlund Gorgen
Denton Richard C.
Alvo Steve
Kuharchuk Terrence N.
Rodman & Rodman
West Fraser Timber Co. Ltd.
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