Patent Publication Number: US-2023141227-A1

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

Description:
The invention relates to 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. The invention further relates to coloured cardboard produced by such a process. 
     It is known to colour pulp, which is used in producing cardboard, with sulphur dyes. Sulphur dyes provide high-quality colouring of cardboard and have certain advantages in producing cardboard. After all, sulphur dyes applied in cardboard do not discolour, do not bleed, provide practically colourless flowback water and having a low cost price. 
     Due to adding a sulphur dye to the pulp, the pH-value of the pulp increases (typically to pH=10 to 11 as an order of magnitude). During the production process this high pH-value needs to be decreased (to pH=6.5 to 7.0), because otherwise the dye would not precipitate in the right form on and in the fibers of the pulp. 
     In producing coloured cardboard it is known to acidify the pulp, to which a sulphur dye has been added, with hydrochloric acid, alum or acetic acid. However, this kind of acidification causes substantial environmental problems. For, in acidification with hydrochloric acid (a strong acid), at the location of an injected jet of hydrochloric acid there is released very much H 2 S, which is a toxic and explosive chemical. And acidification with alum or acetic acid (weak acids) undesirably results into leaving behind of acid radicals in the waste water, which causes various problems and overloading in the waste water purification. Acidification with alum for example leads to an excess of sulphate in the effluent. 
     Because of these environmental problems, in producing coloured cardboard, with its open processes, sulphur dyes are used less and less. 
     It is an object of the invention to provide a solution for producing cardboard coloured with a sulphur dye, wherein the above-mentioned environmental problems are prevented. 
     For that purpose the invention provides a process according to the appended claim  1 , as well as coloured cardboard according to the appended claim  2 . 
     Hence, the invention provides 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,
         characterized in that:   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.   
               

     It appears that, thanks to thus applying carbon dioxide to the pulp and the sulphur dye mixed with the pulp, the sulphur dye precipitates in the right form on and in the fibers of the pulp, without the above-mentioned environmental problems that occur in the above-mentioned known manners of acidification of pulp to which a sulphur dye is added. 
     This is explained as follows. Carbon dioxide is one of the weakest acids. The pKa-value of carbon dioxide is 6.3 as an order of magnitude, whereby the pH-value of the acidified coloured pulp mixture does not easily get below 6.8. Because of that, the equilibrium H 2 ↔HS − ↔S −  stays as much as possible away from the undesirable H 2 S form. In addition, according to the invention the carbon dioxide is applied under pressure, whereby relatively much carbon dioxide dissolves and the acidified coloured pulp mixture acquires a pH-value lying between 6.5 and 7.0, and in preferable embodiments lying between 6.6 en 6.8. Hereby the sulphur dye precipitates in the right form on and in the fibers of the pulp, whereby an excellent colouring result is obtained. The ‘under pressure’ application has the additional advantage that the H 2 S remainder stays dissolved. At pressure release, the pH-value increases again, whereby the H 2 S remainder converts again in HS − . The acid radical is HCO 3   − , which is not environmentally troublesome. 
    
    
     
       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. 
         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. 
     
    
    
     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. 
     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. 
     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. 
     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  7 A to improve the mixing action. 
     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. 
     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. 
     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. 
     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 .