| The purpose of bleaching is to remove remaining dark-colored lignin impurities in the pulp and thus meet certain quality criteria.
Bleaching of chemical pulp is carried out in several stages. In modern mills, the bleaching starts with oxygen delignification for a reduction of the consumption of more expensive bleaching chemicals and a reduced effluent load from the bleach plant.
The predominant bleaching methods are ECF (elemental chlorine free) and TCF (totally chlorine free). In classic chlorine bleaching, molecular chlorine or chlorine gas are used with other chlorine-containing chemicals such as hypochlorite. For environmental and quality reasons other bleaching chemicals are replacing molecular chlorine and hypochlorite.
In TCF bleaching available chemicals for TCF bleaching are oxygen, hydrogen peroxide, ozone and peracids. In ECF bleaching chlorine dioxide is used with chlorine-free chemicals. |
