SE508239C2 - Treatment of pulp with complexing agents in at least one mixing step, without the use of the bleaching tower - Google Patents

Abstract

A method at chlorine-free bleaching of pulp where metals are removed by means of chelating agent. The chelating agent is mixed with the pulp in at least one effective mixing step without intermediate washing, whereafter the pulp without an intermediate tower passes directly to a pulp washer. The total retention time shall be at maximum 15 minutes, calculated after the chelating agent addition.

Description

508 239 In the factory, the QP bleaching is usually carried out in such a way that the pulp first passes through a pump which pumps the pulp suspension through the treatment steps. This is followed by a mixer where the complexing agent is mixed into the mass. The next step is a bleaching tower where the mass without further stirring slowly passes through and where all remaining reaction takes place. Finally, the pulp goes to a washing step where the exposed and complexed metal ions are washed out.

The exposed liquid that is enriched in metal ions usually goes to the drain.

The conditions normally used in a Q-step are a temperature of 70-90 ° C, a residence time of 1-2 hours, pulp concentration 10-14%, pH 5-7 and complexing agent charge 1-3 kg per tonne of pulp. With these conditions a very good metal separation can be achieved, which is necessary if subsequent bleaching steps bleach pulp with a low lignin content (the kappa number is below about 5) and if a larger amount of hydrogen peroxide is used in the step (more than about 15 kg / h). If, on the other hand, the pulp still contains a significant amount of lignin (kappa number above about 5) and the hydrogen peroxide charge in the subsequent step is limited to about 5-10 kg / h, the need for a complete metal release is less critical. Another case is when you can settle for a smaller metal release, ie less than 60% reduction, of the metal content in the step, which is for example the case when the metal content before the Q-step is so low relative to the metal content required after the Q-step . A somewhat simplified Q procedure would therefore be possible in such positions.

From a strict bleaching point of view, the Q-step is really an unnecessary treatment step because no actual bleaching takes place there. However, the Q-treatment is necessary for a successful P or PO bleaching as stated above. If the Q-step could be simplified without reducing the efficiency in the elimination of metals, large savings could be achieved because a complete bleaching step with bleaching tower is expensive.

The present invention is directed to a simplification of the Q-step, partly for pulps with kappa numbers above about 5, partly for 508 239 complexing treatments which are less than 60% metal reduction.

The simplification means that investment costs are reduced in relation to a conventional Q-step.

The invention is based on the fact that it has surprisingly been found that the efficiency of the Q-step is considerably less affected by increased reaction temperature than by increased retention time. According to classical chemical kinetics, this indicates that the Q-step is diffusion-controlled and not reaction-controlled. This means that the Q-step should be performed so that the diffusion is facilitated and if this is done efficiently, the total retention time can thus be considerably reduced.

The features of the invention appear from the claims.

The best way to reduce the diffusion resistance is to reduce the diffusion distance for active reactants and triggered reaction products and this is done on a technical scale by efficient mixing, for example in a mixer with a long residence time. Thus, by efficient mixing, the retention time in the Q step can be reduced. The number of mixing steps is not critical, but the total mixing time must be above a certain critical level, alternatively at least two mixing devices must be used. For economic reasons, however, the number of mixing steps must be kept at a reasonable level. For a mass with kappa numbers above about 5 and which is to be further bleached with a maximum of about 5-10 kg of hydrogen peroxide excluding any oxygen, a suitable number of mixing steps is 1-2.

The same applies if the metal reduction to be achieved in the complexing step is limited and does not have to reach 60%.

If two mixing steps are used, a certain retention time between the mixing steps is good, but this time does not have to be long, since the diffusion rate without mixing is still considerably slower than what can be achieved by mixing, for example in a mixer. The residence time between the mixing steps should thus be short, less than 10 minutes and preferably less than 5 minutes. Furthermore, the mixing steps must be performed one after the other without intermediate washing. The washing required to remove the metals from the pulp suspension 508 239 should be performed less than 15 minutes after the last mixing step.

If more than one mixing step is used, the whole addition of complexing agents should be done in the first mixing step, the subsequent mixing steps only providing a re-mixing. However, a division of the investment so that a maximum of one third of the complexing investment takes place in the last step is not harmful if the subsequent mixing is sufficiently effective.

The complexing agent is suitably of the type EDTA (ethylenediaminetetraacetic acid) or DTPA (diethylenetriaminepentaacetic acid), but other types of complexing agents can also be used, for example NTA (nitrilotriacetic acid) or DTPMP (diethylenetriaminepentamethylenephosphonic acid). The total amount of complexing agent added should be 0.5 - 3 kg per tonne of pulp (calculated as pure complexing agent), preferably 1-2 kg per tonne of pulp.

The temperature in the Q stage should be above 75T, preferably above 80 ° C and more preferably above 85 ° C. A preferred temperature range is 85-110 ° C. The pH range sought is pH 5 to pH II. The pulp concentration should be between 2 and 16%.

Particularly efficient mixing can be achieved by using high-intensity mixers or with another type of mixer where the mixing is efficient and where the residence time is sufficient so that a homogeneous state between fibers and complexing agents is achieved.

The table below shows some results from a one-step Q-treatment carried out with a high-intensity mixer for 3 seconds and with a subsequent retention time without mixing of 40 seconds before the mass wash and where the treatment was carried out at 95 ° C.

For comparison, data are also given from a conventional Q treatment with conventional lab mixing of the complexing agent and a subsequent retention time of 60 minutes where the step is performed at 90 ° C.

Experiment: 508 239 The starting material was oxygen bleached and of softwood sulphate type with kappa number 10.1, viscosity 975 dmP / h 43.0% ISO. The bleaching was done according to Q (OP) with 10 kg Hg »/ h. and brightness pH 8.6 in the Q-stage Q according to Q according to the invention conventional technique Capacity 5.7 5.7 Viscosity dnP / kg 771 807 Brightness% ISO 63.4 64.9 pH 10.6 in the Q-stage Q according to Without Q-stage before the invention (OP) -stage.

Capacity 5.7 6.1 Viscosity dmP / kg 771 790 Brightness% ISO 62.2 59.1 The invention is of course not limited to the above embodiments but can be varied within the scope of the claims.

Claims (10)

about ua 508 259 Patent claims 1. In the case of chlorine-free bleaching of pulp where metals are removed by means of a complexing agent, characterized in that the complexing agent is mixed with the pulp without the use of bleaching towers in at least one effective mixing step at a pH between 5 and 11 and that the pulp after less than 15 minutes are washed after the last mixing step. 2. A method according to the preceding claim, characterized in that the kappa number of the pulp before the complexing step is over 5 or that the desired metal reduction in the complexing step is less than 60%. 3. A method according to any one of the preceding claims, characterized in that the entire addition of complexing agents amounts to 0.5 - 3 kg per tonne of pulp, preferably 1 - 2 kg per tonne of pulp. A method according to any one of the preceding claims, characterized in that a maximum of 110 ° C. 5. A method according to any one of the preceding claims, characterized in that the pulp concentration in the complexing step is 2-16%. 6. A method according to any one of the preceding claims when at least two mixing steps are used, characterized in that the entire addition of complexing agent is made in the first mixing step, the subsequent mixing steps only bringing about a renewed mixing. that the temperature in the complexing step is above 7i J 508 239 7. A method according to claim 6, characterized in that the complexing agent is added in several mixing steps. 8. A method according to any one of the preceding claims, characterized in that the complexing agent is of the EDTA or DTPA type. 9. A method according to any one of the preceding claims, characterized in that the complexing agent is of the type NTA or DTPMP. 10. A method according to any one of the preceding claims, characterized in that the mixing of the complexing agent takes place by means of high-intensity mixers which break up the fiber network in the pulp and provide a close contact between fibers and complexing agents.