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Dyeing of paper
Coloured papers are obtained by dyeing the paper stock or the paper surface (size press, paper coating). Optically brightened papers can be produced in the same manner. Stock dyeing is the most widely used type of paper dyeing. Dyes, pigments, and optical brighteners are added either batch-wise in the pulper or mixing chest or introduced continuously into the stock flow. Continuous addition has the advantage of a shorter zone in the stock line that must be cleaned when the colour is changed. However, because of the lower contact time compared to batch addition, a lower colour yield is obtained for intensely coloured papers and more complex equipment is required for this dyeing process. When the surface of the paper is coloured in the size press, the dyes are added to the size press liquor. Surface dyeing has gained acceptance only in individual cases because uniform dyeing of the paper is difficult to achieve. However, this process has the advantage of the absence of dyes in the water circuits.
Surfaces of papers can also be coloured by coating. In normal coating the surface of the paper or board is covered with a pigment coating. In the case of coloured coatings, the starting material is the white coating mixture, and the desired shade is attained by adding a dispersion of an organic or inorganic pigment. Depending on the fibrous material to be dyed and the intended purpose of the paper, different types of pigments and dyes are used as basic dyes (cationic dyes), direct dyes, and acid dyes. Additionally, fixing agents and other additives are used to improve dye fixation and to obtain better dyeing results. Inorganic pigments or organic pigments (e.g. azo and phthalocyanine types) and carbon black are pigments used for paper dyeing.
The potential environmental impact of dyeing is mainly the releases to water. Especially in mills with several changes of tints or shades per day, the water circuits have to be cleaned after a certain time. Usually, the paper mills work in campaigns producing first the paler tints, changing step by step to the deeper tints. The colouration of the water then just has to be readjusted. However, when for instance deep green is reached, the water system has to be washed. The coloured waste water is sent to the external recipient via the waste water treatment plant. Several times per month, the piping is subjected to a chemical treatment to remove deposits and colour in the piping. In some mills, elemental chlorine and hypochlorite are used as cleaning agents.
Source: Best Available Techniques (BAT), Reference Document for the Production of Pulp, Paper and Board (2015)
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