After the pulp is washed in the Brown Stock Washers, it can be bleached.  There are two types of bleaching systems:

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• Elemental Chlorine Free (ECF) - using ClO2 and Peroxide

• Total Chlorine Free (TCF) - using Oxygen and Ozone

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ECF is more popular as it is less expensive.  It comes with a ClO2 plant that generates ClO2 from sodium chlorate and sulfuric acid in the presence of methanol.  

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TCF uses oxygen and ozone.  It is more expensive but it suitable for producing pulp and supporting end uses that need to be devoid of elemental chlorine.