Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
1995-03-15
2001-11-13
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S065000, C162S076000, C162S078000, C162S079000
Reexamination Certificate
active
06315862
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for bleaching pulp preferably comprising several bleaching stages, in which at a suitable stage after fiber liberation a first filtrate containing metals, principally in ionic form, is separated off from a pulp stream or flow and the pulp stream is subsequently bleached. In a preferred case, the metals and/or metal ions are reintroduced to the fiber stream from which they had been separated. Using the invention, a problem, which is otherwise difficult to solve, namely the problem of discharge of any released metals, is handled in a very simple manner.
BACKGROUND OF THE INVENTION
The environmental authorities are placing ever greater demands on the pulp industry to decrease the use of chlorine gas for bleaching. Permitted discharges of organic chlorine compounds (AOX) with the waste water from bleaching plants have been gradually decreased and are now at such a low level that the pulp factories have in many cases stopped using chlorine gas. Instead, only chlorine dioxide is used as a bleaching agent. Chlorine dioxide forms smaller quantities of AOX than chlorine gas while achieving the same bleaching effect.
However, even the use of chlorine dioxide has been questioned. On the one hand, the environmental authorities in certain countries demand that the discharges of organic chlorine compounds be reduced to such a low level that these demands can scarcely be met even if only chlorine dioxide is used for bleaching. On the other hand, in addition, customers in many countries have begun to demand paper products which are bleached entirely without using either chlorine gas or chlorine dioxide.
The pulp industry is therefore searching for methods which permit bleaching of pulp without using chlorine chemicals. Methods which have been successfully tested involve removing metals in an acid stage (A stage), or possibly by addition of chelating agents (Q stage), e.g. ethylenediamineteteacetic acid (EDTA), to an oxygen-delignified pulp. The pulp is washed and is further bleached using, for example, hydrogen peroxide (P) and/or ozone (Z) in different sequences. One example is the method which is described in European Patent 90850200 (Swedish Patent SE-A-8902058) of EKA NOBEL, the so-called Lignox method. Another known bleaching process includes the bleaching sequences AZ (EOP: alcaline extraction, oxygen, peroxide) where A is an acid stage without use of a chelating agent and Z is as described above.
It is a significant feature of these different methods that certain metal ions have a negative effect on the bleaching process in the form of impaired pulp quality and/or greater consumption of chemicals. In these methods, it has been the practice to wash out the metals by means of an open A/Q stage. A problem which arises in conjunction with these methods is that, as a result, a liquid flow is obtained from the washing stage after release of the metals which contains, on the one hand, a certain amount of released substances which may be toxic or cause pollution and, on the other hand, include dissolved metal ions, which situation is difficult to manage from the point of view of waste and recovery. According to conventional technology, this filtrate is treated by means of external purification, which can be complicated, time consuming and costly, after which the filtrate is released into a receiver.
SUMMARY OF THE INVENTION
The present invention is characterized in that the separated or released metals are subsequently supplied to a pulp fiber quantity, such as one that has been bleached, which adsorbs or readsorbs the metals and/or metal ions, in which connection the further treatment of the fiber quantity is to a considerable extent not negatively affected by the presence of metals. Using the process according to the invention, it has thus been possible to solve the problems associated with the metal-containing liquid flow, which problems, with currently known methods, cause both increased work and increased costs. With the aid of the invention, this somewhat troublesome problem has thus been solved in a very simple manner by supplying the separated metals to a fiber flow, the further treatment of which is at least not appreciably affected adversely by the presence of the metals.
In what should become the most usual application of the invention, there is only one pulp or fiber line, i.e. only one fiber flow line, and in this case the metals are therefore efficiently reintroduced to the same fiber flow line from which they had been separated at an earlier stage such as before bleaching. It can, however, be advantageous to have at least two parallel fiber flow lines in a pulp plant whereby the separated metals are preferably supplied to one and the same fiber flow, as a result of which it is possible to obtain as final products one or more lateral flows, which may be bleached or unbleached and which have not been supplied with metals, and at least one final product which then has an enriched quantity of metals.
The addition of the metals can take place in a pulp tank or the like or else mixed with a washing liquid for the washing apparatus of the last bleaching stage. In order to ensure that the metals are adsorbed, the pH in the pulp mixture is preferably modified in a suitable well known manner. In addition, supplementary chemicals can be used, for example a retention agent to assist readsorption of the metals.
In a preferred case, the filtrate, after giving up the metals, i.e. adsorption of the metals by the pulp, should be supplied to another part of the process, preferably to an earlier stage. In this way, the degree of adsorption can be significantly increased and the effluent quantity, and consequently the need for fresh water, decreased. This can conveniently be done by the pulp being thickened and washed in a washing apparatus, for example, a washing press, followed by further transport to an additional bleaching step (which is not negatively affected by the metals) with, for example, chlorine dioxide, or to a drying machine, storage tower or paper mill. The filtrate from this washing apparatus is then conveyed, in this preferred case, in a counter-current manner back into the pulp treating process, preferably to the washing apparatus of either the upstream Q stage or the A stage. In addition, the filtrate flow can be divided so that one part can be conveyed back to the washing apparatus of one of the bleaching stages.
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Liebergott et al., “Bleaching A Softwood Kraft Pulp Without Chlorine Compounds”, pp 323-332.
“Emerging Technologies Today and in the Future”, Copyright 1992, pp 1-6.
“1981 Oxidative Bleaching”, Denver Hilton, Denver, CO., Oct. 22, Copyright 1981, pp 75-78.
Anderson et al., Optimized Hydrogen Peroxide Bleaching in Closed White-Water Systems, pp 111-115.
Bergovist Anders
Dahllof Hakan
Alvo Steve
Kvaerner Pulping Technologies
Manelli Denison & Selter PLLC
Melcher Jeffrey S.
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