PROCESS FOR PRODUCING COLOURED CARDBOARD IN A CARDBOARD PRODUCTION APPARATUS, AND COLOURED PRODUCED BY SUCH A PROCESS

Abstract

According to the invention, in producing coloured cardboard, carbon dioxide is injected and dissolved in a mixture of pulp and at least a sulphur dye, while the mixture is pumped with a delivery pressure higher than 200 kPa out of a first pulp reservoir (11) towards a second pulp reservoir (12), and in such manner that the mixture is acidified by the carbon dioxide to become an acidified coloured pulp mixture (14) with a pH-value between 6.5 and 7.0. The acidified coloured pulp is injected under high pressure out of a pulp duct structure (7) into the second pulp reservoir (12) at a distance of at least 2 meter below a level (15) of the acidified coloured pulp mixture (14) in the second pulp reservoir (12). Accordingly, the invention provides a high-quality colouring of cardboard, wherein environmental problems are prevented.

Claims

1. A process for producing coloured cardboard in a cardboard production apparatus, wherein the cardboard production apparatus comprises a pulp colouring apparatus for colouring a pulp with at least a sulphur dye, wherein: the pulp colouring apparatus comprises a first pulp reservoir, a second pulp reservoir, a pulp duct structure, pulp pumping means, carbon dioxide injection means, and pulp injection means, and the process comprises: mixing the pulp and at least a sulphur dye in the first pulp reservoir, pumping, by the pulp pumping means via the pulp duct structure, the pulp, and the sulphur dye mixed with the pulp, out of the first pulp reservoir towards the second pulp reservoir, wherein the pulp pumping means has a delivery pressure higher than 200 kPa, injecting, by the carbon dioxide injection means, carbon dioxide into the pulp duct structure, in such manner that in the pulp duct structure the carbon dioxide is mixed with the pulp and the sulphur dye mixed with the pulp which are being pumped out of the first pulp reservoir towards the second pulp reservoir, wherein a residence time of the carbon dioxide in the pulp duct structure is longer than 20 seconds, so that the pulp and the sulphur dye mixed with the pulp, by the carbon dioxide injected and dissolved therein, in the pulp duct structure is being acidified to become an acidified coloured pulp mixture having a pH-value between 6.5 and 7.0, injecting the acidified coloured pulp mixture, by the pulp injection means, out of the pulp duct structure into the second pulp reservoir, wherein, at the location of injecting the acidified coloured pulp mixture, the acidified coloured pulp mixture to be injected has a static pressure in the pulp duct structure which is higher than 150 kPa, and wherein said injecting the acidified coloured pulp mixture takes place at a distance of at least 2 meters below a level of the acidified coloured pulp mixture in the second pulp reservoir, pumping the acidified coloured pulp mixture out of the second pulp reservoir towards cardboard production parts of the cardboard production apparatus, and producing, by said cardboard production parts, coloured cardboard from the acidified coloured pulp mixture.

2. A coloured cardboard produced by the process according to claim 1.

Description

[0020] In the following the invention is further elucidated with reference to a non-limiting embodiment, with reference to the schematical FIG. 1 in the appended drawing.

[0021] FIG. 1 schematically shows in side-view an example of a pulp colouring apparatus during the execution of an example of an embodiment of a process according to the invention for producing coloured cardboard in a cardboard production apparatus.

[0022] The cardboard production apparatus 1 of FIG. 1 comprises the pulp colouring apparatus 2 and the cardboard production parts 3. These cardboard production parts 3 are depicted in FIG. 1 only highly schematically.

[0023] In the shown example the first pulp reservoir 11 of the pulp colouring apparatus 2 is a mixing tank having therein a mixture of paper pulp, with a fiber concentration of more than 2%, and a sulphur dye. In this example the sulphur dye is black sulphur. The pH-value of the mixture in this mixing tank lies between 10 and 11. The pulp duct structure comprises ducts 4 and 5, in which the pump 8 is placed with a working head. The pulp duct structure further comprises a curvy duct 6. The duct 6 is relatively thin (DN150) in order to obtain a relatively high velocity of the pulp mixture being pumped therein.

[0024] The carbon dioxide injection means 9 is connected to the duct 6, that is to say relatively shortly after the pump 8, where the pressure is relatively high and where the velocity of the pulp mixture is higher than 2.5 m/sec. This promotes turbulence, which is favourable for good mixing.

[0025] Downstream of the duct 6 the pulp duct structure further comprises a duct 7, which is relatively long and relatively thick (DN250) for obtaining a long residence time of the pulp mixture being acidified in the duct 7. In the shown example the duct 7 comprises curvy sections 7A to improve the mixing action.

[0026] At its downstream end the duct 7 is connected to the pulp injection means 10 in the form of the shown nozzle structure 10. Via this nozzle structure 10 the acidified coloured pulp mixture 14 is injected out of the duct 7 into the second pulp reservoir 12. The second pulp reservoir 12 is a mixing reservoir in which the coloured pulp mixture 14 is further mixed.

[0027] The injection into the second pulp reservoir 12 takes place at a distance of at least 2 meter below the level 15 of the coloured pulp mixture 14 in the second pulp reservoir 12. Preferably this injection takes place as low as possible below in the second pulp reservoir 12. This injection as low as possible below in the second pulp reservoir 12 is important to keep the pressure drop along the nozzle structure 10 low, in order to thereby avoid excessive stripping of carbon dioxide from the vena contracta of the nozzle structure 10. For, excessive stripping of the carbon dioxide could disturb the pulp flow in the second pulp reservoir 12. Furthermore, the nozzle structure 10 does not have only one, but a plurality of exit flow openings, whereby the pulp flow is not or hardly blocked in case of stripping.

[0028] By the shown pump 17 the coloured pulp mixture 14 is pumped, via the shown ducts 16, 18 and 19, out of the second pulp reservoir 12 towards the cardboard production parts 3 of the cardboard production apparatus 1 for producing coloured cardboard.

[0029] Keeping the coloured pulp mixture 14 in the second pulp reservoir 12 at a sufficiently high level 15 can be realized via frequency control of the pump 17.