FI122840B - Procedure for treating pulp - Google Patents

Description

METHOD FOR HANDLING MASS

The present invention relates to a process for treating pulp. In particular, the invention relates to a pulp bleaching process for the recovery of oxygen which acts as an ozone carrier gas in the context of ozone bleaching. In the previously disclosed methods, this has not always been possible when chlorine and chlorine dioxide have been used in addition to ozone.

10 Fl patent application 894028 describes a bleaching process in which ozone and chlorine are used together to bleach kraft pulp. An advantage of the invention is to describe a more efficient lignin removal than using chlorine alone. In other words, ozone has been found to enhance bleaching with chlorine-based bleaching chemicals, even to the extent that the reasonably high investment costs required for the introduction of ozone are considered acceptable.

A similar result, i.e. the chlorine-based bleaching effect of ozone use, has also been found in Fl patent applications 906400 and 906401, which describe a somewhat more environmentally friendly bleaching process according to Fl patent application 894028. In this process, ozone and chlorine dioxide are added to the pulp in two successive blenders so that the actual bleaching takes place in the presence of both chemicals. After bleaching, the process involves two devices for separating residual chemicals, one for separating gaseous chemicals and the other for liquid chemicals. Although these separators or scrubbers effectively remove chemicals from the pulp, the separated gas contains gas components containing various forms of chlorine, which make it difficult to recover, further use or treat the residual gas. This residual gas is mainly oxygen used as a carrier gas for ozone bleaching that could otherwise be utilized.

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A similar type of bleaching method, where the pulp is first blended? chlorine dioxide, followed by ozone before the dioxide has run out, is also disclosed, for example, in FR patent application 2,663,348. Of course, it is also difficult to recover and reuse the residual gases from the process according to this method.

US Patent No. 5,607,545 further describes a similar bleaching process in which the pulp is first bleached with ozone, and when the ozone reaction is nearing completion or complete, the mixture is mixed with, for example, chlorine dioxide, followed by introducing the pulp into a reaction vessel.

The solution described in U.S. Patent No. 5,645,687 separates gas from an aberrant ozone reactor at a pressure of 10-12 bar. The gas separation from the pressurized process is not effective due to the compression of the gas. Effective gas separation above-10 requires that the gas is first allowed to expand by lowering the pressure of the pulp and only then is the gas separated. In practice, when gas is separated in the manner disclosed, surprisingly much gas remains in the pulp.

Using conventional methods, exemplified by the prior art methods described above, wherein chlorine dioxide and ozone are present in the pulp simultaneously, it has been surprisingly found that the chlorine dioxide and / or chlorine is able to evaporate into the carrier gas present in the bleaching step before reacting with chlorine chemicals. It is also surprising that, according to our research, the chlorine chemical transported into the carrier gas does not re-dissolve in the water in the pulp and thus also does not react with the lignin in the pulp. Thus, two problems arise simultaneously. First, chlorine bleaching chemicals are drowned in the gas phase and are of no use in pulp bleaching. Secondly, the carrier gas accumulates so much chlorine that it cannot easily be reused without, for example, corrosion problems. The present invention eliminates these problems.

For example, thinking of the aforementioned prior art, one might think that the chlorine dioxide step-followed scrubber filtrates leading upstream of the ozone phase could contain, 1, some active chlorine, which would then easily be consumed during ozone bleaching and would not cause problems in the ozone step. Surprisingly, however, in our studies, the problem with ozone whitening has been that active chlorine-containing filtrates cannot be introduced into the pulp prior to the ozone phase. The small amounts of actin-2 chlorine contained in these filtrates do not run out during ozone bleaching, but also contaminate the main-

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§ the residual gas formed by the carrier gas as described above. So it is ha-

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the importance of having an active chlorine content in the filtrates of less than 50 mg / l was claimed. It is more preferred 3 that the filtrates have an active chlorine content of less than 10 mg / l, most preferably about 0 mg / l when the mixture of pulp and water / filtrate enters the ozone stage.

Previous combined chlorine dioxide-ozone steps have generally employed only chlorine dioxide. However, according to our research, this is not as effective as having some chlorine in the bleaching step. A suitable amount of chlorine is introduced into the chlorine dioxide solution by running the chlorine dioxide plant so that about 10-5% of chlorine in the amount of active chlorine is introduced into the solution. In this way, the chlorine dioxide step following the ozone step can be greatly enhanced. Although chlorine 10 may also be added as chlorine gas, this arrangement may prove to be somewhat difficult in practice.

Thus, the prior art solutions, as described above, have many disadvantages. Firstly, one problem is the residual gas containing chlorine compounds alone, which cannot be reused directly or even readily. In addition, the chlorine-containing residual gas in question causes corrosion problems in the gas pickup system and its equipment. It has also been found, as stated above, that the combination of chlorine dioxide and ozone alone does not achieve optimum results also in bleaching. It is an object of our present invention to provide a process for both the separation of pure oxygen gas from the ozone phase when bleaching with a mixture of chlorine and chlorine dioxide and the enhanced bleaching step so that the pulp brightness can be easily raised to ISO 88%.

Thus, the presently invented process is a more efficient bleaching process than previous similar-type processes, while the oxygen-containing residual gas can be recovered and reused. According to a preferred embodiment, the lightening filtrates can be recycled inside the bleaching plant and thus reduce the waste water load.

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Among other things, the above-mentioned problem of reuse of the ozone-phase residual gas can be solved by the method 2 according to the present invention, preferably using two bleaching steps and a number of separators or scrubbers. The first bleaching step is ozone bleaching and the second chlorine dioxide-chlorine bleaching. inventions

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The concept is based on the idea that low oxygen, which is mainly used as an ozone carrier gas, is separated from the pulp at low pressure, between 0.5 and 2.0 bar, before blending 4 chlorine dioxide or chlorine into the pulp, after which residual gas is easy to purify and reuse with the pulp come active chlorine into the ozone phase.

The advantage of the present invention is e.g. the absence of chlorine or chlorine dioxide during the ozone phase, which makes it possible to assemble gases which can be reused without corrosion problems. Any residual ozone contained in the collected gas can be removed from the residual gas if desired, for example by a scrubber.

Other features of the present invention will be more fully apparent from the appended claims.

In the following, the process of the present invention will be described in more detail with reference to the accompanying figure, which shows an apparatus for carrying out ozone-chlorine dioxide bleaching according to a preferred embodiment of the invention.

In the embodiment shown in the figure, pulp is introduced along line 10 to a bleaching step with ozone, chlorine dioxide and chlorine. The bleaching step begins by washing or diluting the pulp along lines 12 and 14 with available water or filtrates 20 in a washing apparatus 16. One purpose of the washing is to ensure that the pulp does not contain the specified amounts of active chlorine when pumped to pump ozone stage 20. , that the filtrate used for the washing liquid also contains little chlorine residue. According to our studies, the residual chlorine content of the filtrate used as washing liquid should be less than 50 mg / l calculated as active chlorine, more preferably less than 10 mg / l, and, of course, most preferably 0 g / l.

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Treatment with ozone is carried out under pressure at a pressure of 5 to 15 bar, preferably 5 to 35% consistency, more preferably 7 to 18%, such that one or more mixers 22 are introduced into the mass o at one or more points and 22 ', an ozone-containing gas having an ozone content of 8 to 18% by weight, preferably 10 to 15%. The gas in question is a mixture of ozone and carrier oxygen 2. The ozone is dosed to the mass in the range of 2 to 8 kg / admt. The ozone bleaching section is carried out either in mixers that allow long delay only or by

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the mixture of pulp, ozone and carrier gas is introduced into reaction vessel 24 to provide the desired reaction delay.

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One of the essential objectives of the process of the present invention is to ensure that the oxygen supplied as carrier gas for ozone bleaching does not come into contact with the chlorine and / or chlorine dioxide used in the next bleaching step, but remains free from chlorine or chlorine compounds. Only then can oxygen be reused efficiently. In order to ensure the purity of the residual gas, the mixture of carrier gas and residual ozone is separated in a separator 26 after the ozone bleaching reactor 24 before contacting the pulp with chlorine and / or chlorine dioxide, preferably over 80%, more preferably over 90%. In order to separate the residual gas 10 as closely as possible from the pulp, according to an advantageous embodiment of the invention, its separation is preferably treated at two points, at least one of which is at a substantially lower pressure than the ozone phase. By lowering the pressure to 0.5 - 2.0 bar, the gas volume increases and gas separation is significantly enhanced. One is the aforementioned separator 26 and the other is a separator 28 whose pressure level is preferably 15 near or even below atmospheric pressure. By providing a low pressure level of at least one of the separators in the range of 0.5 to 2.0 bar, it is ensured that the gas volume during the separation process is high and thus the residual gas can be easily separated. In practice, this means that the pulp is preferably conducted from a gas separator 26 disposed at the outlet end of the reaction vessel 24 to a second separating device 28, which may be a silent gas separator pump or a gas pump of the type separating device.

The waste gas removed from the pulp by separators 26 and 28 can, if desired, be washed with a scrubber 32 to remove residual ozone. The washing water 34 used in the scrubber 32 may be any suitable filtrate or fresh water. The scrubbed scrubbing gas is then re-routed along the pipeline 36, at least in part, for reuse, for example in another bleaching step, in ozone production, in combustion or in oxidation of brown pulp filtrates. The ozone-containing water 38 £ 30, in turn, is preferably supplied to a suitable location in the bleaching plant.

co σ> o ^ After separation of the carrier gas, the pulp is introduced either by a gas separating pump 28, as shown in the figure, or by its own special pump into the dioxygen bleaching staircase.

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40. First, before the bleaching tower 44, chlorine dioxide and preferably a small amount of chlorine is mixed with pulp 42 in the mixer. Chlorine incorporation ensures 6 better final brightness. Adding chlorine to chlorine dioxide bleach easily reaches a final brightness above 88 ISO. Chlorine dioxide alone can make it difficult to achieve such high final brightnesses, or more bleaching steps may be required compared to when chlorine is present. The starting point for the administration of either chlorine dioxide alone or in combination with chlorine can be a value of more than 3 kg / admt, preferably more than 7 kg / admt of active chlorine. The ratio of chlorine dioxide to chlorine calculated as active chlorine is between 5/1 and 100/1, preferably about 10/1 to 50/1.

From the mixer 42, the pulp flows to the chlorine bleaching reactor, which, after preferably over 80%, more preferably over 90% of the active veil, is removed, transferred to a washer 46 and washed in known manner with either water or an appropriate filtrate. The filtrate separated from the pulp by means of a scrubber 46 can be passed along the pipeline 48 for use, for example, in washing or dilution of pulp entering the ozone stage if its active chlorine content is low enough, i.e. less than 50 mg / l, preferably less than 10 mg / l. The active chlorine content of the pulp and / or filtrate can be treated, for example, by destroying the residual chlorine, e.g. SO 2, with water before or after the pulp washing treatment in the washer 46.

The method and apparatus described above are somewhat more expensive than the known methods of the ai-20 em as an investment. However, there are benefits to improved operating economy, reduced water and gas emissions and reduced risk of corrosion. When the specific aim of the invention is to keep as close as possible to the ozone and its carrier gas, the corrosion problems can almost be forgotten compared to the prior art.

As noted above, a new and previously unknown process for treating pulp in combined ozone-chlorine dioxide bleaching has been developed. However, with reference to the foregoing, it should be understood that the foregoing description of the invention is to be understood as only a preferred example of the process of the invention, which is not intended to limit the invention as set forth in the appended claims which alone define the scope of scale, o δ oo

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Claims (11)

A process for treating pulp in a continuous process wherein the pulp is added to a combined ozone-chlorine dioxide stage, where a) the pulp is washed, b) the pulp is fed to an ozone stage (20), where b1) ozone is mixed into the pulp at a pressure of 5-15 bar, b2) the ozone is allowed to react with the pulp, b3) the pulp is removed from the ozone stage (20), b4) the pulp is removed from the ozone stage for the separation of gas before chlorine dioxide is mixed into the pulp, c) the pulp is added to a chlorine dioxide stage ( 40), c1) chlorine dioxide is mixed into the pulp, c2) the dioxide reacts with the pulp in a treatment vessel, c3) the pulp is transferred from the treatment vessel to a washing device, c4) the pulp is washed so that at least one filtrate is obtained from the washing device, characterized in that the pulp is washed or diluted in step a) in a washing device (16) with water or filtrate 20 which does not contain substantially any active chlorine, such that the mixture of pulp and water after washing and / or the dilution in step b1) contains less than 50 kg of active chlorine / l, preferably below 10 mg / l and most preferably about 0 mg / l, and that in step b5) the residual gas is separated from the mass, which residual gas contains mainly ozone and in addition, possibly including ozone in some men, such that the separation of gas occurs at least partially at a pressure of 0.5 - 2.0 bar, and the pressure of the pulp is increased by a pump after the separation of gas. i OJ A process according to claim 1, characterized in that as the carrier gas in the ozone stage o (20) is oxygen. £ 30 3. Process according to claim 1, characterized in that chlorine is used in addition to chlorine dioxide as a bleaching chemical in the chlorine dioxide stage (40). o o (M 11 Process according to claim 1, characterized in that the active chlorine content of the water or filtrate used in the washing device is below 50 mg / l, preferably below 10 mg / l, most preferably about 0 g / l. Process according to claim 4, characterized in that the pulp is bleached in step c) within the consistency range 5 - 35%, preferably 7-18%, with ozone using an ozone / oxygen gas mixture, where the ozone content is 8 - 18 weight percent, preferably 10-15%, the remainder being essentially oxygen which acts as a carrier gas. 10 Process according to claim 1, characterized in that step b4) is started when more than 80%, preferably more than 90%, of the ozone, the dose of which is 2 - 8 kg 03 / adt, is consumed and the residual gas, which is in the mainly of oxygen used as carrier gas for ozone, is separated from the pulp. 15 Process according to Claim 1, characterized in that the residual gas is purified in a washing or separation device (32) preferably with water or filtrate (34), whereby an oxygen gas (36) which is almost pure from ozone residues is obtained to be re-used at least partially. another bleaching step. 20 8. A process according to claim 1, characterized in that the ozone-containing water (38) obtained from the washing or separating device (32) is fed back to the ozone bleaching stage for recovery of bleaching chemicals, for example, by diluting the pulp before the pump (18). 25 Process according to claim 1, characterized in that the pulp is bleached in step d) 3 with chlorine dioxide and chlorine by adding the chemicals and bleaching the pulp in a treatment vessel (44) such that the ratio of chlorine dioxide to chlorine as active chlorine is between 5/1, 1 and 100/1, preferably about 10/1 - 50/1. o X £ 30 Process according to claim 1, characterized in that when more than 80%, preferably more than 98%, of the chlorine and chlorine dioxide, the total dose of which is more than 3 kg / adt, 2 preferably 7 kg / adt, active chlorine has been consumed, the mass is transferred. to a washing device section (46), where the filtrate is separated by constriction or pressing and passed to a pipe CM (48). 35 12 Method according to Claim 1, characterized in that the filtrate from the washing device (46) is preferably used at least partially in step b) and its active chlorine content is maintained sufficiently, if necessary, by destroying residual chlorine, for example with SO2 before or after the washing device (46). ). 5 o (M C \ l O x cc CL CD O) O δ o o (M