Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
1993-06-29
2001-04-24
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S076000, C162S078000, C162S088000
Reexamination Certificate
active
06221209
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for bleaching cellulose paper pulps belonging to the chemical pulps class.
TECHNOLOGY REVIEW
It is known to treat unbleached chemical paper pulps obtained by cooking lignocellulose materials by means of a sequence of delignifying and/or bleaching treatment stages involving the use of oxidising chemical products. The first stage of a conventional sequence of chemical pulp bleaching has the object of perfecting the delignification of the unbleached pulp as it exists after the cooking operation. This first delignifying stage is traditionally carried out by treating the unbleached pulp with chlorine in acidic medium or with a chlorine/chlorine dioxide combination, as a mixture or in sequence, so as to react with the residual lignin of the pulp and to give rise to chlorolignins which could be extracted from the pulp by solubilisation of these chlorolignins in alkaline medium in a subsequent treatment stage.
For various reasons, it proves useful, in certain situations, to be able to replace this first delignifying stage by a treatment which no longer requires a chlorinated reactant or which uses a decreased amount of chlorinated reactants.
For about ten years, it has been proposed to replace the first treatment stage by means of chlorine or the chlorine/chlorine dioxide combination, at least partially, by a stage with gaseous oxygen in alkaline medium (Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 19, New York 1982, page 415, 3rd paragraph and page 416, 1st and 2nd paragraphs). The degree of delignification which is obtained by this treatment with oxygen is not, however, sufficient if the aim is to produce chemical pulps of high brightness.
It has been proposed to bleach sulphite or sulphate pulps by means of high-strength hydrogen peroxide in the presence of sodium silicate (J. Kappel, HC-Peroxidbleiche für Zellstoff, Wochenblatt für Papier-fabrikation, 120, May 1992, No. 9, pages 328-334). It is, however, difficult with this process to produce a final brightness greater than 85° ISO, even at the price of significant amounts of hydrogen peroxide greater than 3 g/100 g of dry pulp.
The invention overcomes these disadvantages of the known processes, by providing a new delignification and/or bleaching process for chemical paper pulps which makes it possible to achieve high levels of brightness without excessive damage to the cellulose.
SUMMARY OF THE INVENTION
To this end, the invention relates to a process for bleaching a chemical paper pulp which makes it possible to obtain high levels of brightness, of at least 89° ISO, according to which the pulp is subjected to a sequence of treatment stages including a final stage with hydrogen peroxide in alkaline medium, the final stage with hydrogen peroxide being carried out in the presence of at least one stabilising agent and at a consistency of at least 25%, the pulp which is subjected to the final stage with hydrogen peroxide having been purified in the preceding stages so that its manganese content does not exceed 3 ppm by weight with respect to the solids and having been delignified to a kappa number (measured according to the SCAN standard C1-59) not exceeding 5.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, chemical paper pulp is understood to denote the pulps which have already been subjected to a delignifying treatment in the presence of chemical reactants such as sodium sulphide in alkaline medium (kraft cooking or sulphate cooking), sulphur dioxide or a metal salt of sulphurous acid in acidic medium (sulphite cooking or bisulphite cooking). According to the invention, chemical paper pulp is also understood to denote the pulps called in the literature “semi-chemical pulps”, such as those where the cooking was carried out using a salt of sulphurous acid in neutral medium (neutral sulphite cooking, also called NSSC
cooking), as well as the pulps obtained by processes using solvents, such as the Organosolv, Alcell®, Organocell® and Asam pulps described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A18, 1991, pages 568 and 569.
This is particularly aimed at the pulps which have been subjected to a kraft cooking. All the types of wood used for the production of chemical pulps are suitable for the use of the process of the invention, and in particular those used for kraft pulps, namely soft-woods such as, for example, the various species of pine and fir, and the hardwoods such as, for example, beech, oak, eucalyptus and hornbeam.
According to the invention, the bleaching process includes a final bleaching stage with hydrogen peroxide in alkaline medium which is carried out at the end of the bleaching sequence. Preferably, the final bleaching stage with hydrogen peroxide ends this sequence.
This final stage with hydrogen peroxide is carried out, in accordance with the invention, in the presence of at least one stabilising agent. The known stabilising agents of peroxygenated products are well suited. Examples of such stabilising agents are alkaline-earth metal salts, in particular soluble magnesium salts, inorganic silicates, phosphates and polyphosphates such as the silicates, pyrophosphates and metaphosphates of alkali metals, organic polycarboxylates and aminopoly-carboxylates such as tartaric, citric, gluconic, ethylenediaminetetraacetic, diethylenetriaminepentaacetic or cyclohexanediaminetetraacetic acid and their salts, poly-&agr;-hydroxyacrylic acids and their salts and the phosphonic acids such as ethylenediaminetetra(methylenephosphonic), diethylenetriaminepenta(methylenephosphonic) or cyclohexanediaminetetra(methylenephosphonic) acid and their salts. It is also possible to combine a number of these stabilising agents as a mixture. As a general rule, the silicates, polycarboxylates or phosphonic acids are well suited, in particular when they are combined with at least one magnesium salt. Sodium silicate has given good results.
The amount of stabilizing agent to be used in the final stage with hydrogen peroxide varies according to the type of wood used for the manufacture of the pulp as well as the operating conditions of the cooking which have prevailed during the pulping of the type of wood used and the effectiveness of the bleaching stages which have preceded the final stage with hydrogen peroxide. In certain cases, it is possible to use only a reduced amount of stabilising agent. Preferably, however, the amount of stabilising agent will be at least 0.1% by weight expressed with respect to the dry pulp. Most often, it will not exceed 5% and, preferably, not 4% of the dry pulp.
In accordance with the invention, the final stage with hydrogen peroxide is carried out at a consistency of at least 25% solids. Consistency is understood to denote the percentage by weight of the solids with respect to the total weight of the solids and of the aqueous solution of reactants. Preferably, the final stage with hydrogen peroxide will be carried out at a consistency of at least 30% solids.
Generally, the consistency of the final stage with hydrogen peroxide does not exceed 45%. A consistency of 30% has given very good results.
According to the invention, the pulp subjected to the final stage with hydrogen peroxide is selected from the pulps which have been delignified, in the stages which have preceded the final stage, to a high delignification level corresponding to a kappa number not exceeding 5. Preferably, the pulp subjected to the final stage will have a kappa number not exceeding 3. Generally, the pulp subjected to the final stage has a kappa number of at least 0.1. Any treatment sequence which makes it possible to delignify the pulp to such a value of the kappa number is in accordance with the invention. Examples of such sequences comprise sequences involving stages using at least one chlorinated reactant such as chlorine in acidic medium, chlorine dioxide, the combination of chlorine and of chlorine dioxide as a mixture or in sequence, the hypochlorite of an alkali metal or alkaline-earth metal or also, and
Desprez Francois
Devenyns Johan
Essemaeker Paul
Troughton Nicholas
Alvo Steve
Solvay Interox (Societe Anonyme)
Spencer George H.
Venable
Wells Ashley J.
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