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Title:
PROCESS IN ORDER TO IMPROVE OR MAINTAIN THE BRIGHTNESS OF PAPER CONTAINING MECHANICAL PULP
Document Type and Number:
WIPO Patent Application WO/2004/027147
Kind Code:
A1
Abstract:
The invention relates to a process to improve or maintain the brightness of paper made of a bleached mechanical pulp containing an inorganic filler where a suspension of the filler is treated with a chelating agent and/or an inorganic ion exchange product to thereby form complexes of and/or inactivate the undesirable metal ions present in the suspension.

Inventors:
KVITVANG ANDREAS (NO)
Application Number:
PCT/NO2003/000319
Publication Date:
April 01, 2004
Filing Date:
September 19, 2003
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
NORSKE SKOGINDUSTIER AS (NO)
KVITVANG ANDREAS (NO)
International Classes:
C09C1/42; D21H17/69; D21H21/14; (IPC1-7): D21H17/69
Other References:
DATABASE WPI Week 198320, Derwent World Patents Index; Class F09, AN 1983-47996K
Attorney, Agent or Firm:
Oslo, Patentkontor (Oslo, NO)
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Claims:
CLAIMS
1. Process to improve or maintain the brightness of paper manufactured partly or completely of mechanical pulp in which significant amounts of inorganic pig ments in the form of an aqueous pigment suspension and optional conventional additives are added to an aqueous suspension of the mechanical pulp, and there after converted to paper on a paper machine, characterized by the fact that the aqueous pigment suspension before being added to the suspended mecha nical pulp, is treated with waterinsoluble ion exchange agents or/and watersoluble chelating agents, in an amount sufficient to form complexes with and/or inactivate the metal impurities present in the pigment suspension.
2. Process according to claim 1, characterized by the fact that the waterinsoluble ion exchange product is aluminium silicate and/or synthetic or natural zeolite.
3. Process according to claim 2, characterized by the addition of the ion exchange product in an amount of 0,052, 5 % by weight of the weight of the dry pigment.
4. Process according to claim 1, characterized by the application of EDTA, DTPA or a similar product as the watersoluble chelating agent.
5. Process according to claim 4, characterized by the addition of the chelating agent in an amount of from 0,1 to 2,0 % by weight of the total weight of the pigment.
6. Process for the treatment of an inorganic pigment suspension, intended as pigment addition to mechanical pulp for the manufacture of paper, characterized by the addition of a complex builder and/or an ion exchange agent not soluble in water, in an amount sufficient to form complexes with and/or inactivate the metal ions present in the pigment suspension, in particular the iron ions.
Description:
PROCESS IN ORDER TO IMPROVE OR MAINTAIN THE BRIGHTNESS OF PAPER CONTAINING MECHANICAL PULP THE PURPOSE OF THE INVENTION The present invention relates to a method as described in the introduction to claim 1, in order to improve and/or maintain the brightness of paper when the raw materials comprise mechanical pulps and inorganic pigments. The invention relates particularly to paper containing pigments of a natural origin such as kaolin or clay, calcium carbo- nate, talc etc. used together with mechanical pulps.

BACKGROUND OF THE INVENTION Inorganic pigments, such as those mentioned above, are widely used in the paper industry, and they are particu- larly important in the production of printing paper where the pigments contribute to brightness and opacity and printhrough, the phenomenon where the print can be seen on the back side of the paper. The pigments comprise both clay and calcium carbonate (chalk) both of which are of a natural origin and therefore might be polluted by different metal compounds.

In this respect, the content of heavy metals such as iron, manganese, cobalt etc. is particularly important. Of these metals the iron compounds are clearly of the greatest importance since the iron ions have a negative catalytic influence on many chemical reactions that take place during the pulp and paper manufacturing process. These reactions will be discussed later.

NO 123259 discloses the manufacture of a white pigment containing titanium dioxide by letting the titanium dioxide react with sodium aluminium silicate.

EP-317921 relates to additives for alkaline bleaching agents containing peroxide for pulp, including pulp containing lignin. Both inorganic silicate ion exchange products insoluble in water and complex-forming chemicals are mentioned as additives for the bleaching agent SU-1548301 describes a mixture of sizing agents obtained by adding a filler with ion exchange properties to a clay- suspension in an amount of 10-25 % by weight, based on dry clay, followed by the addition of the sizing agent itself.

The purpose of this invention is to reduce the consumption of sizing agent.

JP 58060097 describes among other matters that a component with ion exchange properties may be added to a paper pulp simultaneously with the filler. It is stated that a printing paper with good filler retention is obtained.

It is general knowledge that the problems associated with heavy metals are not significant as long as the work is restricted to chemical pulps. This may be due to the pulps being exposed to a comprehensive cooking-and bleaching process where the heavy metals are separated from the pulp in several washing stages. It is therefore not normal to be concerned with the content of iron when the paper is mainly composed of bleached, chemical pulps.

The issue is quite different with mechanical pulps which are bleached in a so-called"lignin preserving bleaching process"as opposed to the chemical pulps where the lignin is first removed in the cooking process and subsequently in several bleaching stages. The intention of bleaching mechanical pulp is instead to increase the brightness of the pulp as much as possible without a loss of yield, which would give a direct economic loss and would also give a greater content of organic substance in the effluent, and associated environmental problems.

The bleaching can either be carried out under reducing conditions with dithionite or in an oxidizing bleaching process with peroxide. If a higher brightness is required of the pulp, it is common to combine the two processes, but the peroxide treatment will then always take place in the last stage. Bleaching with peroxide alone can also be carried out in two stages. Numerous scientific studies have shown that ions of heavy metals will have a strong negative effect on the bleaching process, for instance, because these ions may break down the peroxide to a form which will have no significant bleaching effect. It has therefore been accepted practice to add so-called complex builders or chelating agents such as EDTA, DTPA and others to the pulp.

In order to save peroxide it has also been standard practice to thicken the pulp, press out the bleaching liquor still containing unreacted peroxide and recirculate this liquid as make-up for new peroxide solution. This has undoubtedly provided a more efficient use of the bleaching chemicals, but the cost of the peroxide is still a very significant cost factor in the process of making mechanical pulps with high brightness. These pulps are required in modern magazine paper and similar grades because of the demands for better reproduction of the advertisements which are of vital importance to the economy of the different periodicals.

Despite the enormous workload allocated to the optimization of the bleaching process for mechanical pulps and the fact that a reduction of the brightness level frequently has been observed, no one has previously thought that something could be done about the other main component of the fur- nish, namely the pigments. Up to 30% pigment might be used in modern magazine paper, the pigments most frequently being clay or calcium carbonate with an addition of other pigments such as talc, in order to achieve special effects.

It was therefore a total surprise that a significant improvement in brightness of the paper was achieved by adding the corresponding complex builders used in the mechanical pulps, to the pigments used.

SUMMARY OF THE INVENTION A detailed description of how the process is carried out in a paper mill will follow later. It has furthermore been shown that a similar positive result might be obtained by adding inorganic products with some ion exchange capacity.

Without being bound by any special theory, it is assumed that the advantages are associated with the content of the heavy metal ions, specifically the iron ions in the pig- ments. Studies performed after achieving the positive results, have shown that the pigment suppliers have, in certain circumstances, treated the crude pigment with a reducing bleaching agent which may have reduced ferric iron (Fe+++) to ferro iron (Fe++). These Fe++-compounds will generally be less coloured and frequently even colourless or white. We will not speculate as to the background for this treatment, but it is reasonable to believe that the increased competition between the different pigment suppliers may have contributed more to higher demands on brightness levels.

It is also possible that the suppliers have been forced to use sedimentary sources of raw materials with a greater content of heavy metals. Whatever the reason, it has been found that the brightness of the pulp became higher and the consumption of peroxide to obtain a certain brightness le- vel lower, when an ion exchange agent or a complex builder was added to the pigment before the pigment was brought into contact with the mechanical pulp. It has thus been assumed that the ferro-ions, with a valence of two, in the pigment will be oxidized by the remaining peroxide in the mechanical pulp to regenerate ferric iron, with a valence of three. It is consequently these ferric ions which have given the reduction in the brightness which has been previously has observed, but has been unexplainable.

The invention therefore relates to"the inactivation"of present ferro ions by enclosing these ions in complexes by means of, for instance, EDTA, DTPA or similar products or being bound by the application of ion exchange agents which are insoluble in water, such as aluminium silicates and among these, natural and synthetic zeolites.

The manipulation of the iron ions has the effect that the brightness requirement for the mechanical pulp is signi- ficantly lowered when the brightnes level required in the paper is unchanged. This means that a significant amount of bleaching chemicals will be saved. Alternatively a bonus may be obtained in the shape of greater paper brightness at the same consumption level of bleaching chemicals.

When water-soluble chelates or complex builders are used, the metal impurities will be enclosed and removed from the production chain with the surplus water. When non-soluble ion exchange agents are used, the metal impurities will be carried out of the production chain with the paper product.

The ion exchange agent will in this case make up a small fraction of the paper pigments.

DETAILED DESCRIPTION OF THE INVENTION The pigments may either be delivered to the mill in a dry form or as a slurry which can be pumped. In the former case the pigments will be dispersed in water with the appropri- ate dispersing agents and thereafter pumped to storage tanks where they will be slowly agitated continuously. If the mill receives the pigment in a slurry, only adjustment of the consistency of the slurry remains before the pigment is passed on to the storage tanks as previously mentioned.

The following paragraphs will describe how the present process might be implemented in practice.

General description During the preparation of the pigment suspension, the different chemicals (chelating agents or ion exchange mate- rial) are added under vigorous agitation. The consistency of the pigment suspension is from 30 to 60 %. The temperature during the process is the normal system temperature which may vary from 30 to 60 °C, so that any adjustment of the temperature is not necessary. The effect of the different additives is improved when the suspension is given a certain residence time after processing.

Example 1 Trials have shown that by adding a chelating agent, such as EDTA or a similar compound, to the pigment suspension, the required brightness of the mechanical pulp that has been bleached with peroxide can be reduced by 2,5-3, 0 % in ISO- units. Despite this reduction, the brightness of the paper produced will be the same, or in other words, it would have been necessary to use a pulp bleached to 2,5-3, 0 % ISO higher in brightness, if the pigments had not been treated with EDTA to achieve the desired brightness.

This means that a limited effort using a chelating agent gives a 25-30 % saving in the total bleaching costs during the production of the pulp.

Example of the application of an ion exchange product By adding 0, 4% by weight (based upon the weight of the pigment) of an ion exchange product (for instance a zeo- lite) to the pigment during the dispersion, the impurities of the coloured metal compounds in the pigment will be masked and thereby be inaccessible for the oxidation of the remaining peroxide. This means that the required brightness of the mechanical pulp may be reduced by 1,5-2, 5 ISO-units, and the desired paper brightness may still be retained, such as was the case with a chelating agent.

By means of the present invention the consumption of peroxide, which is a relatively expensive bleaching agent, will be significantly reduced during the bleaching of the mechanical pulp, and the intended brightness of the finis- hed paper product may still be attained.

When the chelating agents (EDTA, DTPA, etc. ) are used, they should be added straight to the liquid into which the pigment is dispersed. The amount of the chelating agent to be used may be from 0,1 to 1,0 % by weight of the dry substance of the pigment.

The chelating agent might also be added to the finished pigment slurry. This might be accomplished by adding the chelating agent into the suction pipe ahead of a transport pump. The transport through the pump will in this case give adequate mixing of the chelating agent.

When an ion exchange agent (such as several natural and synthetic aluminium silicates, zeolites etc. ) is used, the ion exchange agent is added together with the pigment to the liquid used to disperse the pigment.

The amount of ion exchange agent to be used should be in the order of magnitude from 0,1 to 2 % by weight of the total dry substance of the pigment. A vigorous agitation and an adequate storage time after the addition will improve the result with the ion exchange agent.