Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
2000-08-31
2001-09-18
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S189000, C162SDIG008, C210S723000, C210S912000, C210S928000
Reexamination Certificate
active
06290812
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for treating process water. More specifically, it concerns a method for treating process water in connection with bleaching of lignocellulosic pulp.
BACKGROUND OF THE INVENTION
Owing to the increasing interest in environmental matters there is a great wish to reduce the emissions of pollutants from human activities. The pulp and paper producers are considered culprits in this context. In recent years, however, great resources have been used to reduce the emissions caused by our pulp and paper mills, resulting in great progress.
An important goal that has been strived for is to provide the closed pulp mill, that is to say a pulp mill which minimises emissions by regenerating as much as possible existing chemicals in the process and reusing the resulting spent liquors. One stage is to try to return spent bleach liquors counter-currently to the pulp in the process. A problem arising in connection with this procedure is that certain process-foreign substances, for instance ions of transition metals and alkaline earth metals, which are supplied to the process with, for instance, the wood raw material, may be enriched in the system when spent liquors are being returned.
An increasing quantity of the papermaking pulp is today bleached by means of hydrogen peroxide, peracetic acid or ozone. These bleaching processes are disturbed in different ways by present ions of transition metals. One therefore tries as much as possible to complex these ions of transition metals before bleaching. The chemicals currently used as complexing agents are expensive. There is thus a great need for a method, in which these chemicals can be regenerated in an economical manner.
Large amounts of compounds of transition metals and/or alkaline earth metals can separately, or in combination with each other, cause precipitations on the pulp.
With a view to minimising the enrichment of ions of transition metals and alkaline earth metals, a large number of methods have been presented.
SE 504,424 discloses a method for precipitating transition metals and alkaline earth metals from spent bleach liquors by adding an alkaline liquid. In this method, a green and/or white liquor is added to the spent bleach liquor which is then evaporated with the obtained precipitate remaining in the liquor.
WO 94/232122 discloses a further method for treating process water. An alkaline liquid is added to the process water, whereby the metal ions are precipitated, and then the precipitated metal compounds are separated from the process water.
WO 94/21857 discloses one more method for treating spent liquors from bleach plants. Also in this method, an alkaline liquid is added to precipitate metals. The alkaline liquid is first treated with carbon dioxide to reduce the sulphur content and increase the carbonate content thereof.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide an alternative, efficient method for treating process water from bleach plants in pulp mills, said method being advantageous compared with the methods described above.
Further objects, features and advantages of the present invention will appear from the following description.
The present invention relates to a method for treating process water containing ions of transition metals and alkaline earth metals, in connection with bleaching of lignocellulosic pulp. The method comprises the following steps:
a) adjusting the Mg
2+
content of the process water to form a flocculation base for metals and an organic substance,
b) supplying a carbonate source to the process water to precipitate calcium as calcium carbonate,
c) increasing the pH of the process water by adding white liquor to precipitate metals such as metal hydroxides,
d) adding a flocculant to the process water, and
e) separating the precipitated, flocculated metal compounds from the process water by flotation.
An advantage of the method according to the present invention is that there is no stoichiometric locking between the amount of added carbonate source and the amount of added white liquor, like in WO 94/21857, where the amount of added alkaline solution controls the amount of added carbon dioxide. According to the present invention, the carbonate source and the white liquor are each added separately to the process water which is to be purified, i.e. one amount added does not control the other. A further advantage of the present invention is that the complexing agents are regenerated and are dissolved in the outgoing clarified filtrate which can be returned to the complexing step without further treatment.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 5607549 (1997-03-01), Koskiniemi
patent: 5639347 (1997-06-01), Lindberg
patent: 1522334 (1993-01-01), None
patent: 1564443 (1993-10-01), None
patent: B098226 (1997-05-01), None
patent: 9300798 (1994-09-01), None
patent: C2-9403779 (1997-02-01), None
patent: 1-9421857 (1994-09-01), None
patent: 1-9423122 (1994-10-01), None
patent: 1-9614467 (1996-05-01), None
Hansson Jonny
Nilsson Margareta
Rampotas Christos
Svensson Viveka
Birch, Stewaret, Kolasch & Birch, LLP
Kemira Kemi AB
Nguyen Dean T.
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