Process for producing coloured cardboard in a cardboard production apparatus, and coloured produced by such a process

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.