Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Patent
1998-02-19
2000-05-02
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
162 79, 162 89, D21C 916
Patent
active
060568534
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
THIS INVENTION relates to a process for peroxide bleaching of pulp. Pulps which may be bleached in the process of the invention include lignocellulose pulp which may be produced mechanically and chemi-mechanically with yields in the region of greater than 75% which are otherwise known as high yield pulps.
BACKGROUND OF THE INVENTION
In a conventional peroxide bleaching process, sodium hydroxide is used as an alkali source. To achieve a desired brightness with maximum efficiency, auxiliary substances are also used. Such auxiliary substances include sodium silicate, magnesium sulphate and chelating agents inclusive of DTPA (sodium salt of diethylene triamino pentaacetic acid).
Reference may be made to a prior art article by Soteland et al., 1988, TAPPI Proceedings 231-236, which describes a peroxide bleaching process which utilises magnesium oxide as a sole alkaline source. The pulp was pretreated with DTPA and magnesium oxide particles were utilised in a size range of 1.00 mm-0.25 mm or smaller. The magnesium oxide were also used in a concentration of 2-3% based on the dry weight of the pulp. The MgO used in the process was light-burnt MgO and finely crushed. It was found that brightness levels obtained were very close to that which was achieved by conventional bleaching using NaOH. In the bleaching process, the pulp was diluted to form a pulp suspension and the amount of MgO was added to the suspension under vigorous stirring. Hydrogen peroxide was subsequently added to the suspension at a concentration of 3% based on the weight of the pulp. This reference also made the observation that coarse particles are less effective as an alkaline source during peroxide bleaching.
Having regard to the abovementioned reference, an observation was also made in the corresponding patent specification DE3617942 that use of MgO as sole alkaline source considerably simplified the bleaching process since sodium hydroxide as alkaline source and auxiliary chemicals such as sodium silicate could be omitted.
Another advantage of using MgO as sole alkaline source was that only a small amount of waste is produced in the bleaching plant. Thus, for example, in integrated mills which produce magnesium sulphite pulp and peroxide bleached high-yield pulps, the used bleaching liquor is combusted and the MgO may be recovered for re-use.
However, the use of MgO as sole alkaline source in a peroxide bleaching process has not as yet achieved widespread commercial acceptance because although the principle of utilising MgO as sole alkaline source was described in the Soteland at al. references, the means of reducing the principle to practice on a commercial scale has not yet been fully elucidated.
SUMMARY OF THE INVENTION
Surprisingly, it has now been discovered that commercial usage of magnesium oxide as a sole alkaline source in peroxide bleaching of wood pulp may be achieved by employing MgO particles with a particle size of less than 500 micron and more preferably less than 75 micron and having particle surface area (PSA) of between 20-60 m.sup.2 /g and more preferably between 30-50 m.sup.2 /g. By using such parameters, an efficient peroxide bleaching process may be carried out most efficiently on a commercial scale which may be achieved within a maximum bleaching time of 180 minutes and achievement of a maximum target of ISO brightness of 65 in regard to freshly prepared pulp.
Utilizing MgO with parameters outside those stated above will result in a less efficient bleaching process leading to higher usage of chemicals and therefore higher operating costs.
The dosages of MgO that may be utilised in the process of the invention is 0.3-2% based on the weight of the pulp.
The amount of hydrogen peroxide that may be utilised in the process of the invention is from 1-5% based on the weight of the pulp.
To achieve maximum efficiency, the MgO particles are preferably added to the pulp in the form of a powder or slurry prepared in situ.
Preferably the MgO is added to the pulp simultaneously with the peroxide or prior
REFERENCES:
Abstract: Paperchem No.: AB6608917, May 1995.
McLean Ian Alexander
Vincent Adam Hayden
Nguyen Dean T.
Orica Australia Pty. Ltd.
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