Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With heat recovery
Patent
1997-12-16
2000-10-24
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With heat recovery
162 67, 162 68, 162 78, D21C 1106, D21C 910
Patent
active
061361453
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION.
The present invention relates to a method of treating pulp and to an apparatus for applying the method. The primary object of the invention is to develop a method and an apparatus for treating pulp at a temperature which as precisely as possible meets the requirements of the treatment, and thereby to optimize the process with respect to also the temperature. A particular object of the invention is a method of bleaching pulp with a sequence using chlorine dioxide, in as environmentally friendly a way as possible. The method of the invention is especially suitable for small-scale rebuilds of existing pulp mills, which are based on use of elemental chlorine and chlorine dioxide. A second object of the invention is an elemental chlorine free bleaching sequence, which is suitable for places where chlorine dioxide is, however, desired to be used. Use of chlorine dioxide is, however, minimized in the sequence by arranging for optimal process conditions for chlorine dioxide in the dioxide bleaching, whereby the AOX number of effluents may be maintained very at a very low level. The invention also relates to an apparatus used, e.g., in bleaching, which apparatus enables a significant reduction of steam consumption at the mill. Especially, reduction in use of high-pressure steam is concerned because the mills often have more than enough low-pressure steam, but in some cases the mill is not even capable of producing as much high-pressure steam as the heating requires. The invention also relates to treatments which are effected with other bleaching and treatment chemicals, and in which the treatment temperature needs to be changed between different stages.
Pulp mills are nowadays trying to get rid of use of elemental chlorine and partly also of chlorine dioxide. Reasons for this are both environmental protection and market factors. Drawbacks caused by elemental chlorine are clearly noticeable malodorous gaseous emissions as well as liquid discharges from pulp mills to water-courses. Liquid chlorine dioxide is not so much of a nuisance as for its odour, but it has drawbacks which mainly affect watercourses. However, when these chlorine chemicals are compared with each other by means of the AOX number, which indicates their load on the water-courses, it can be seen that the harmfulness of elemental chlorine is multiple in comparison with that of chlorine dioxide. The AOX number of chlorine is about 4 to 7 and that of chlorine dioxide about 1 to 1.5 or even slightly below 1.
However, sequences using chlorine dioxide are still favoured and they are feasible as for environmental aspects also. There are many reasons for that. In comparison with other chemicals, the price for chlorine dioxide is very competitive; it is today approx. half of the price for, e.g., competing hydrogen peroxide or its various derivatives. Moreover, the strength and brightness values of the pulp obtained by dioxide bleaching are high. In fact, they are at least of the same standard as the values obtained by using peroxide, with the same chemical consumption (kg/adt).
An object of the invention is to apply, in an economical. and pro-environmental manner, at least one chlorine dioxide stage (D) to a bleaching sequence, which uses modern bleaching chemicals such as ozone (Z) or peroxide (P), in other words, hydrogen peroxide or some other chemical behaving like a hydrogen peroxide in bleaching. Instead of using chlorine dioxide as the main bleaching chemical as it has been used earlier, its role has been changed in our invention. Here it is in the first place used as an additional or auxiliary chemical for activating and subjecting pulp to the influence of the main chemical. In this case, when chlorine dioxide is used together with peroxide and ozone in accordance with the present invention, the AOX number of chlorine dioxide may be even less than 0.5. Furthermore, the filtrates from the chlorine dioxide stage may be treated together with the filtrates from the chelating stage in a separate evaporating unit, s
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McDonald, R. G. "The Pulping of Wood", vol. 1, 2nd Edition, pp. 586-588, 1969.
Stromberg, M. "1994 International Pulp Bleaching Conference", pp. 199-202, Fig. 1-14.
Allen et al, "Spiral Heat Exchangers Preheat High Solids Black Liquor Without Plugging," Pulp & Paper, Feb., 1991, pp. 60-63.
Henricson Kaj O.
Pikka Olavi E.
Andritz-Ahlstrom Oy
Nguyen Dean T.
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