Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Reexamination Certificate
2000-05-19
2003-10-14
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
C162S090000
Reexamination Certificate
active
06632328
ABSTRACT:
FIELD OF INVENTION
This invention relates to the manufacture of paper and, more particularly, to the bleaching of mechanically-produced pulps containing lignin, including both softwood and hardwood, so as to reduce yellowness and improve brightness.
BACKGROUND OF INVENTION
Mechanically-produced pulps, softwood and thermomechanical pulps containing lignin, as opposed to chemically produced wood pulps, used for the production of paper, have traditionally been bleached to improve the whiteness thereof. Newsprint, which contains a relatively high lignin content, is either not bleached or only mildly bleached, with the result that it is usually of a darker quality than paper produced from fully bleached pulp, and tends to darken further when exposed to light, a phenomenon known as “reversion”.
Whiteness or “brightness” of paper is conventionally measured by brightness measurements based on the reflectance of light at a wavelength of 457 nm, using such instruments as an Elrepho brightness meter. There is, however, another measure of whiteness which is even more significant and that is the degree of yellowness (CIE yellow color coordinate b*). Bleached softwood pulps usually have a brightness of about 70-90% Elrepho and a yellowness b* of 8 or more (often 9-10 at a brightness of 76-78 points ISO), and there are several known methods for achieving these results, using either an oxidative process using hydrogen peroxide under strongly basic conditions or reductive processes using hydrosulfite (dithionite) or combinations thereof.
Currently, hydrogen peroxide bleaching of pulps is generally performed at an alkaline pH of ~10-11 in order to facilitate the ionization of hydrogen peroxide to HOO
−
. This is accomplished using a mixture of a water soluble alkali (usually sodium hydroxide) and a buffer (usually sodium silicate). This allows maintenance of the pH at a level that permits effective bleaching, and also buffers the solution to minimize peroxide decomposition, which increases with increasing alkalinity. Often a small amount of magnesium ion is added to form a colloidal suspension of magnesium silicate, which may help, by adsorption, to inactivate metallic catalysts of peroxide decomposition such as Mn and Fe ions.
Silicates, however, can result in buildup of silica scale, especially when bleach process water is recycled. The alkali (NaOH) is expensive and must be neutralized (soured) and/or washed out after the bleaching process. In addition, alkali is known to have a darkening effect on pulps. Alkali-induced darkening reactions in the pulp compete with bleaching and effectively limit the brightness to ISO ~84. Alkali-induced depolymerization and “peeling” reactions of cellulose are detrimental to fiber length and strength. Rates of the various alkali-induced reactions increase at higher temperatures and limit practical bleaching temperatures to ~80° C.
There is an ongoing need for improved but inexpensive mechanical and thermomechanical pulps having improved brightness of the order of 85 and decreased b* values of the order of less than 5; and with greater stability of the optical properties, i.e., decreased reversion.
OBJECT OF INVENTION
An object of the present invention is to provide improved mechanical and thermomechanical pulps having a brightness of 80 (ISO) or more and a yellowness (b*) value 6 or lower. A further object is to provide an improved process for producing such pulps, wherein added alkali and/or silicate is not employed but equivalent optical characteristics to processes employing alkali, optionally with silicate, are obtained. A still further object is to provide means to break the “brightness ceiling” of ~84-85 ISO common in alkali-based processes.
BRIEF DESCRIPTION OF INVENTION
By one aspect of this invention, a method for bleaching an unbleached softwood or hardwood pulp is provided, using hydrogen peroxide without added alkali for activation. The method comprises treating (digesting) said pulp in an aqueous hydrogen peroxide solution containing up to about 10% carbonate at a temperature in the range of 15 to 80° C., and preferably 15-60° C. The solution may be up to about 30% by weight of pulp. The carbonate is preferably magnesium or calcium carbonate. The pH range of the method is preferably about 6 to about 7.
By a second aspect of the invention, a method for bleaching an unbleached softwood or hardwood pulp is provided, comprising treating (digesting) said mechanical pulp in an aqueous hydrogen peroxide solution containing at least 4% H
2
O
2
and at least 6% alkaline earth metal carbonate by weight of pulp at a temperature of at least 15° C., more preferably at least 40° C., still more preferably at least 60° C. Still more preferably, the temperature range is 75° to 120° C., more preferably 80° to 115° C., still more preferably, 80° to 110° C. At such temperatures, the reaction may proceed for about one to three hours, with shorter times required at higher temperatures. No alkali (NaOH) is added to the reaction mixture. The alkaline earth metal carbonate is optionally calcium carbonate and preferably magnesium carbonate. Preferably the alkaline earth metal carbonate is at least 10% by weight of pulp, more preferably at least 20%, at least 25%, at least 30%, at least 35% or at least 40%. Preferably, the H
2
O
2
is at least 5%. But, the H
2
O
2
may be at least 10%, at least 15% or at least 20%. The invention provides the advantage of employing temperatures greater than the practical “ceiling” of ~80° C. for alkali bleaching reactions, without alkali induced darkening of the pulp.
According to this aspect of the invention, a b* value of less than 6 is preferably obtained. An ISO brightness value of at least 85 is preferably obtained, still more preferably an ISO brightness value of at least 90 obtained. In a particularly preferred embodiment, the wood pulp is a mechanical hardwood pulp, the ISO brightness value obtained is in the range of 90-96 and the b* value is less than 2.6.
If desired, the bleaching method of the present invention can be coupled with a formaldehyde/carbonate process as described in our U.S. application Ser. No. 08/936,200 (1997). In said process, a wood pulp containing lignin, such as a bleached hardwood or softwood pulp, is digested in an aqueous formaldehyde solution containing carbonate, preferably an alkali or alkaline earth metal carbonate, and preferably at ambient temperature. Yellowness b* is preferably reduced to less than 8, more preferably to less than 6, still more preferably to less than 5. This coupled method of a peroxide/carbonate treatment and a formaldehyde/carbonate treatment is also encompassed by the invention.
Preferably, the invention provides improved b* without significant delignification. Delignification commonly occurs under strongly acidic or strongly basic conditions and can result in significantly reduced yields.
The invention provides several advantages over conventional hydrogen peroxide bleaching:
1. Lower cost. An alkaline earth metal carbonate such as magnesium carbonate is very inexpensive as compared to sodium hydroxide and sodium silicate.
2. Increased brightness values and reduced b* values for both softwood and hardwood mechanical pulps over conventional bleaching. This increases the number and variety of applications for the product, significantly expanding its marketability.
3. Much less light sensitive pulps. This is indicated by their lower b* values, limiting light induced reversion processes.
4. Avoidance of problems with silica scale and alkaline corrosiveness on equipment.
5. Environmental benefits due to the avoidance of the use of soluble alkali, silicate and acid souring agents.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The inventors have found the inclusion of carbonate, particularly alkaline earth metal carbonate, in a hydrogen peroxide bleaching process to be beneficial and effective without need for added alkali (sodium hydroxide). The method of the invention is carried out at approximately neutral pH, and less than about pH 8, thus avoiding the disadvantages and dr
Depew M. Catherine
Wan Jeffrey K.-S.
Alvo Steve
Miernicki Steeg Carol
Queen's University at Kingston
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