FI104573B - Process for dithionite bleaching - Google Patents

Description

104573

Method for dithionite bleaching

The invention relates to a process for dithionite bleaching of the type set forth in the preamble of claim 1.

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The reducing effect of sodium dithionite is known to be utilized in bleaching prior to papermaking. The potency of sodium dithionite, also called sodium hydrosulfite, is based on the release of reactive hydrogen, which in turn reduces the color-forming molecular structures of the cellulose to a colorless form. Industrial bleaching processes for mechanical pulps have traditionally used either dithionite bleaching alone or, in order to improve bleaching efficiency, also a 2-step process in which oxygen bleaching at an alkaline pH with peroxide is followed by dithionite bleaching at an acid pH of 15.

European Patent Publication EP-B1-0187477 discloses a bleaching process in which wood pulp is bleached in a first step with a peroxygen compound, in a second step by a reducing compound such as sodium dithionite, and in a third step by a peroxygen compound. By way of example, the brightness of the resulting mass is increased by 5 to 10 percentage points (ISO) in the third stage, with a final brightness of more than 80% ISO.

The German patent application DE-A1-3809332, in turn, discloses a multi-step bleaching process in which the reducing dithionite bleaching is followed by treatment with gaseous oxygen and the final bleaching chemical being hydrogen peroxide.

It is an object of the invention to provide an improvement specifically in a bleaching process in which dithionite bleaching is the sole or final bleaching step.

It is known that the runnability of a papermaking machine is increasingly reduced as more dithionite is used to bleach mechanical pulp. This is typical of modern machines or machines whose water consumption has been reduced by closing the process. In water-intensive machines, water recycling is low and thus the runnability problems caused by the chemicals used and / or their degradation products are reduced. The increase in chips has been found to be related to the pH shock that occurs when the pulp stock is diluted in clear circulating water. Hereby dissolved or colloidal substances will precipitate into sticky stickies which will sooner or later cause a rupture. This is a problem for masses that have been bleached with dithionite. This may be a one-step process involving only dithionite bleaching or a two-step peroxide / dithionite bleaching process.

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Problems in papermaking following the above bleaching processes are eliminated by a process characterized in what is described in the characterizing part of claim 1 of the appended claims. The invention is based on the finding that the pH of clear circulatory waters is lowered by the oxidation of hydrogen sulphite ion formed in the dithionite decomposition reaction. The pulp from bleaching always contains dithionite residues and / or hydrogen sulphite residues produced by it, which cause problems when they are trapped in the pulp stock and in the recirculating water in the papermaking process. Oxygenation of hydrogen sulfite to sulfate is possible due to dissolved oxygen in the circulating water. The former part of the paper machine is experiencing strong aeration of the stock water. After filtration of the circulation water, the oxidation of the hydrogen sulfite as well as the simultaneous decrease of the pH occurs at a certain reaction rate in the circulation tower, whereby the pH can drop up to 1.5 pH-one to 25 lbs. Circulating water used for various purposes can easily lead to such pH shocks and machine runtime malfunctions. The circulating water is poorly buffered in the absence of the fiber phase, which makes the pH sensitive.

According to the invention, hydrogen sulphite ions derived from dithionite bleaching are controlled by a controlled oxygen-releasing compound, in particular 30 peroxygen compounds, without increasing the brightness of the pulp, and at the same time the pH lowered as a result of acidification is raised by the addition of base.

In contrast to the above-mentioned publications describing the multi-step bleaching processes, the pulp is no longer subjected to oxidative bleaching after di-thionite bleaching, but only the oxygen donor chemical is added for the controlled oxidation of hydrogen sulfite. Only a stoichiometric amount corresponding to the dithionite used up to the maximum amount of dithionite used is required, plus an amount which may possibly compensate for the excess sulfite remaining in the dithionite during the manufacture of the dithionite.

The oxygen donor compound and the base are preferably metered directly into the 5 pulp streams from dithionite bleaching. For dispensing, standard process automation aids may be used.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a flow chart of the process of the invention, and Figure 2 is a diagram illustrating the effect of the invention.

In the invention, the pH of the process is stabilized by oxidizing the oxidative decomposition product of dithionite, i.e. hydrogen sulfite, with a suitable oxygen donor compound. These are mainly peroxygen compounds, which readily cleave single-atom oxygen, such as organic and inorganic peroxides, in particular hydrogen peroxide. Also, ozone as a single atom oxygen donor may be contemplated. The drop in pH due to oxidation of hydrogen sulfite is compensated by the addition of a base to raise the pH to a suitable level. A suitable base for this purpose is sodium hydroxide. The oxygen donor compound and the base may be fed simultaneously.

Figure 1 illustrates an example of the use of the invention in the bleaching and papermaking process. It is known that dithionite is fed into a pulp feed tube (arrow A) at the lower end of the bleaching tower 1. From the upper end 1 of the bleaching tower, the pulp is led to another tower 2, in this case a storage tower, which is parallel to the unbleached pulp storage tower 30. According to the paper's lightness target, these towers dispense masses in a certain proportion. Warehouse Tower 2 -. the bleached pulp from the lower end is taken along a tube 3 and led to a pulp dispensing container 4, from where the pulp, after dilution with circulating water, ends up in a papermaking process on a papermaking machine.

In step B, the oxygen blowing compound and base are added to the bleaching tube 3 in an aqueous solution of suitable strength. In Fig. 1, the oxygen donor compound, in this case hydrogen peroxide, and the base, in this case sodium hydroxide, are dispensed from their own tanks 4 104573 5 and 6 as solutions of a certain strength into a common dosing tube 7 associated with tube 3.

% 5 Figure 1 shows the dashed automation associated with the method. Information on the amount of dithionite feed (as a percentage) is transmitted along line 9 as well as measurements of 10 flow (volume flow, consistency) of pipe 3 via line 11 to calculator 12, which calculates the appropriate dosing volume and provides 15. Thus, in a continuous process, the substances in question can be continuously dosed in the correct amount per unit of time, taking into account the amount of dithionite used in bleaching and the flow rate of filing pulp in tube 3. The amount of dithionite, expressed as a percentage of the dry matter content of the pulp, may be reported as a manual setting or may be automatically transmitted from the dionite feed, for example directly from the setpoint in the dithionite control.

Furthermore, it is apparent from Figure 1 that the pulp is not washed after dithionite bleaching 20 (single stage bleaching or the final step of multistage bleaching), but that the spent dithionite decomposition products and the residual material accompanying the dithionite enter the tube 3.

The following reaction diagrams illustrate the phenomena exploited in the process.

After bleaching, there are hydrogen sulphite ions in the pulp dithionite decomposition products. The following reactions take place at pH 5.0 to add 30 oxygen donor compounds and a base to the bleaching pulp.

s «HS03 + H202 = SO / - + H30 + / Hydrogen sulphite oxidation and drop in pH * 35 H30 + + NaOH = Na + + 2H2O / pH increase back to process pH i | 5 104573

If hydrogen peroxide were not added at this stage, oxidation would occur at a later stage analogous to the first reaction scheme in a circulating water tower, with the difference that the oxygen would then be derived from air oxygen dissolved in the circulating water.

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In dithionite bleaching, various reduction reactions take place in parallel, mainly resulting in hydrogen sulphite and sulphate. The prevailing reaction type depends on the process conditions. Assuming that in the bleaching dithionite decomposes to hydrogen sulfite and sulfate ions at a molar ratio of about 10, the dosing requirement can be calculated in advance, and the calculation can be carried out in the automation shown in Figure 1. to calculate the amount of sodium hydrogen sulphite produced (about 0.36%) consuming peroxide, calculated as 100%, about 15 0.12% of the dry matter of the pulp. The calculation is based on the assumption that one mole of dithionite produces an average of one mole of hydrogen sulfite, and accordingly requires one equivalent of an oxygen donor compound for complete oxidation. In this case, equivalent is the amount of substance capable of delivering one mole of single-atom oxygen. Correspondingly, 1 equivalent of base is required to bring the pH back to pre-oxidation level, i.e., in the case of sodium hydroxide, the amount is about 0.14% sodium hydroxide in the dry matter of the pulp. As the need for dithionite dosing changes, the dosing need for the oxygen donor compound and the base will change accordingly, and dosing may be continuously adjusted according to the principles outlined above. The dithionite used for bleaching will always be accompanied by excess sulfite present both in the commercial powder product and in the on-site dithionite solution. Given that one mole of dithionite, even in theory, produces up to two moles of hydrogen sulfite and some excess hydrogen sulfite initially comes with dithionite, three equivalents of oxygen donor compound per mole of dithionite is always sufficient.

* 35 Figure 2 shows the pH of the water discharged from the circulation tank as a function of time. The dosage amounts were as follows by weight based on the dry weight of the pulp: 6 104573 period dithionite peroxide lye I 0.3 0.06 0.07 II 0.8 0.16 0.19 III 0.3 0.06 0.07 IV 0.8 0 , 16 0.19-0.21 V 0.8 0.0 0.0 Using the addition of peroxide and sodium hydroxide, the pH has been stabilized to about 5 (section I). The dosing of hydrogen peroxide and NaOH (lye) relative to dithionite has been constant. The higher rise of dithionite-5 causes a decrease in the curve in section II due to the change in the bulk of the pulp (the addition of a pH-lowering opacity pigment), but here again stabilization to about 4.6-4.7 is observed. In addition, a lower dithionite dosage in the period and a change in the supplementation material to Section I is again reflected in a clear increase in pH. In section 10, IV dithionite dosing has risen again and the additive material has changed to section II. For the well (arrow) in the curve, the dosing of the lye has been increased to 0.21, which again raised the pH. Section V describes driving without dosages of substances, and it is clearly seen that without dosages of peroxide compound and base, the pH of circulating water drops below 4.5.

To further improve runnability, pH stabilization has the further advantage of eliminating the other disadvantages of lowering the pH caused by high dithionite administration. For example, a paper machine cab. The 20 roses in the splash areas are reduced, and the pH values of the press section drains, which may be up to 1.0 to 1.5, return to normal. Other base feed and monitoring systems installed for process pH control may be omitted.

Due to the invention, it is also possible to use calcium carbonate as a filler without foaming problems in the production of wood-containing paper under neutral conditions when the mechanical pulp is bleached with dithionite. With the addition of an oxygen donor compound and a base, the uncontrolled pH drop caused by the oxidation of hydrogen sulfite and its subsequent decomposition of carbonate to carbon dioxide! (foaming) are prevented. Foaming problems have been found to be directly related to dithionite dosing, and foaming causes flotation of 7,104,573 colloidal materials, and the susceptibility to breakage is thus directly related to dithionite dosing.

The oxygen donor compound may be dispensed at any position 5 before the headbox of the papermaking machine. Dispensing is performed on the pulp, which, upon leaving the storage tower, gradually dilutes as it flows toward the headbox, without again increasing the consistency of the pulp prior to the application point. For example, adding directly to the flow in the tube or adding to a mixing tank between the bleaching step and the headbox may be an issue. The base may be introduced at the same site or at a different site from the oxygen donor compound and may be introduced prior to any pH measurement point normally prior to the wire section of the papermaking machine for adjusting the pH so that its input may be combined with a corresponding pH control, e.g. . In this case, the base can be dosed on the basis of pH measurement and its dosing is not calculated according to dithionite. Of course, the pH of the pulp can be adjusted to any desired pH, taking into account the requirements of the papermaking process and the raw materials of the pulp, for example the aforementioned requirements for carbonate fillers. It is also possible for such a combination that the dosing of the base is initially roughly proportional to the amount of dithionite, and the pH is adjusted to the desired value by subsequent separate addition of the base or acid combined with the pH adjustment prior to the wire section.

The invention is not limited to the embodiments described above, but may be modified within the scope of the claimed invention. The dosage need not necessarily be exactly the molar amount of dithionite, but may also be lower. In addition, the method does not need to be continuously operable in connection with dithionite casting, but dosing may be initiated, for example, when the amount of dithionite is raised above a certain level. In the preceding description, *. · Dithionite also means any compound containing a dithionite anion.

% 35 The process of the invention for dithionite bleaching can be used for the following pulps for which dithionite bleaching has previously been commonly used: 104573 Grind (SGW), pressure groundwood (PMP), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) 5 - chemical heat pulp (CTMP, chemical TMP) and deinking pulp (DIP, recycled fiber).

The use of the invention is, of course, not limited to the abovementioned pulps, but can be used in all dithionite bleaching processes.

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Claims (7)

104573. A process for dithionite bleaching wherein after bleaching with dithionite, the bleached pulp is fed to a papermaking machine 5 for papermaking, characterized in that an oxygen donor compound is introduced into the pulp which causes oxidation of the dithionite decomposition products without increasing the pH of the pulp. to compensate. 10 Process according to Claim 1, characterized in that the oxygen donor compound is fed up to three equivalents per mole of dithionite. Process according to Claim 1 or 2, characterized in that the oxygen donor compound is fed into a pulp flow tube (3) or mixing tank between the dithionite bleaching step and the papermaking machine. Method according to one of the preceding claims, characterized in that the compound and the base are fed together in a common flow to the pulp. 5. A method according to any one of the preceding claims, characterized in that the proportion of dithionite in the dry matter of the pulp fed to the bleaching step is continuously monitored and automatically calculated based on the data obtained, at least the amount of oxygen donor compound being fed. 30 A method according to any one of the preceding claims, known as -; in that the feed of the base is combined with a pH adjustment prior to the wire section of the papermaking machine. A process according to any one of the preceding claims, characterized in that the oxygen donor compound is a peroxygen compound, preferably hydrogen peroxide, and the base is hydroxide, preferably sodium hydroxide. . 104573