Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1996-07-08
2000-10-03
Alvo, Steven
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 65, 162 76, 162 78, 162 88, D21C 914, D21C 9147, D21C 9153, D21C 916
Patent
active
061267821
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method for the manufacture of bleached cellulose pulp from any previously disclosed lignocellulose material using any previously disclosed alkaline pulping process and essentially environmentally friendly bleaching agents. A large number of lignocellulose materials is available in varying quantities throughout the world. One very common lignocellulose material is wood, which is usually reduced to the form of chips before the digesting or the pulping. The method in accordance with the invention is suitable for both hardwood and softwood. Examples of known alkaline pulping processes are the sulphate process, the polysulphide process, and processes of the soda (sodium hydroxide) process type in which catalyzers, such as some quinone compound, are used. The term sulphate process covers, for example, the use of high sulphidity, the use of counter-current digestion in which white liquor is also added at an advanced stage of the digestion process, and the use of a chemical treatment of the lignocellulose material prior to the actual sulphate digestion.
BACKGROUND ART
In the interests of protecting the environment, the use of bleaching agents such as oxygen (O), one or other per-compound (P) such as hydrogen peroxide and ozone (Z) has recently been suggested for the bleaching of, for instance, sulphate pulp. This has led to the introduction, including on a commercial scale, i.e. full scale, of the use of this type of bleaching agent, including those referred to above, and also in the sequence stated above. By avoiding the use of bleaching agents containing chlorine, which in the final analysis give rise to corrosive chloride, it has proved increasingly possible to close the bleaching plants. The expression closing is used to denote that the washing fluids are handled to an increasing extent within the bleaching plant. In traditional open bleaching plants, the washing fluids (waste liquors) appearing after the respective bleaching stage, including after extraction (E) stage, are allowed to flow directly out to the recipient or, where appropriate, to an external purification plant.
It has emerged from the use of the oxidative bleaching agents exemplified above, and in particular from the use of some per-compound, that the content of metals in the pulp and/or even the presence of metals in general leads to problems. The metals that cause the most problems are the transition metals, of which manganese is the most problematical due to the presence of manganese in such large amounts. Manganese, for example, occurs naturally in the raw material, i.e. in the lignocellulose material, for example in the form of wood. The process water that is used also contains manganese as a general rule, and manganese can also originate from the apparatus used in the pulp manufacturing chain. In an attempt to deal with this problem, a complex forming stage (Q) has been introduced into the pulp treatment chain, preferably directly ahead of the peroxide bleaching stage. The addition of complexers such as EDTA, DTPA and NTA, and others at a suitable pH value, ensures that any free manganese ions are collected and, in particular, the manganese is converted from a fixed form in the pulp to a water soluble complexed form. Manganese complexes of the type Mn(EDTA).sup.2- or Mn(DTPA).sup.3- occur in this case. It is important, after this treatment stage, for the pulp to be washed extremely thoroughly so that no significant quantities of manganese complexes and any free complexers accompany the pulp into the peroxide bleaching stage. The waste liquor generated at this position, i.e. the washing fluid from the complex forming stage, has attracted particular attention of experts and is the subject of more detailed comment below.
A gradually increased closing of the bleaching plant and the pulp manufacturing process in its entirety with regard to the liquid circuit has, as previously indicated, both been proposed and implemented in practice. Different counter-current washing processes for the pulp have be
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Abstract of Japanese Patent Kokai 21590/82--published Feb. 4, 1982.
Bryant P. et al. "Managanese Removal in Closed Kraft Mill Bleach Plants", Tappi Int. Bleaching Conf. Atlanta, Nov. 1-3, 1993, p. 43.
"Partial Closure in Modern Bleaching Sequences", Tappi Int. Bleaching Conf., Washinton, Apr. 14-18, 1996, p. 341.
"Kirk-Othmer Encycl.", the Fourth Edition, vol. 5, p. 779, line 11-15.
Ahlenius Lars .ANG.. G.
Liden Jan G.
Lindeberg Otto S. A. G.
Noreus Sture E. O.
Alvo Steven
Mo och Domsjo Aktiebolag
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