The use of ozone as a bleaching agent has been contemplated for many years but because of perceived cost and quality problems has until recently been avoided in favor of conventional chlorine based chemicals. Today, with the focus on the environment and mill closure as well as a market driven pressure to produce ECF and TCF bleached products, ozone has resurfaced and demonstrated its feasibility as a viable bleaching chemical alternative. The introduction of ozone in bleach plants has to a great extent been associated with TCF bleaching. However, the use of ozone to achieve almost full bleach plant closure with ECF bleaching produces some significant cost and environmental benefits as demonstrated in the first full scale ozone installation for "bulk" delignification in an ECF OZ(EO)D bleach sequence at Union Camp's mill in Franklin, VA USA, which has now been in operation for over three years. For ECF bleaching, ozone is a viable replacement for the chlorination and/or first chlorine dioxide stage and as such eliminates the use of chlorine containing chemicals early in the sequence facilitaing closure eg. OZ(EO)D and the total elimination of all chlorine chemicals in TCF bleaching eg. OZQP. For ECF bleaching, the powerful delignification and brightening capabilities of a high consistency ozone delignification stage, as practiced at Union Camp's ozone bleach line, allows for a significant reduction in total chlorine dioxide use to a given brightness compared to current D-100 ECF bleaching. Perhaps the most important benefit of using ozone in an ECF sequence is the ability to recycle and recover the O, Z and EO filtrates from an OZ(EO)D bleach sequence. The open D-stage produces effluent of as little as 4-5 m~3/admt depending on the washing apparatus being used in the bleach plant, with greater than 90% reduction in effluent properties including AOX compared to conventional bleaching practices. Many bleach plants worldwide are facing the first big step towards a more environmentally friendly process, namely ECF. For most, to do so via 100% chlorine dioxide substitution, requires a significant investment in additional chlorine dioxide capacity. Considering a probable evolution toward a totally effluent free (TEF) mill via TCF bleaching, the implementation of ozone to avoid additional investment in chlorine dioxide for ECF is a sound economic strategy. The implementation of ozone delignification in existing mills as well as for greenfield installations and the criteria, economical and technical for justifying ozone in a modern flexible bleaching process for both ECF and TCF will be discussed.
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