Ozone is a very powerful and reactive oxidant eliminating the need for large reactors with long retention times. In addition, the reaction products of ozone are harmless (water and oxygen). Ozone reacts very rapidly with the pulp constituents and the reaction takes place primarily in the mixers. The mixing is therefore a key issue in medium consistency ozone bleaching. Kempulp has both reliable and highly efficient mixers designed for the tough task of mixing ozone with the medium consistency pulp suspension.
The Ozone stage can be used on a variety of pulps, alone or in combination with Chlorine Dioxide in the first bleach stage. An ozone stage has been found to be particularly beneficial for the production of dissolving pulp where it possible to adjust the pulp viscosity. Ozone alters the cellulose crystalline structure, therefore, it improves the reactivity of the final bleached dissolving pulp.
Ozone reacts very rapidly with the pulp and the reaction takes place primarily in the mixers. Efficient mixing is, therefore, a key component in medium consistency ozone bleaching.
Ozone, when used in a mill scale, is a sensitive bleaching chemical. A key issue is that ozone spontaneously decomposes into oxygen at elevated temperatures. For this reason, it is of great importance to run an ozone stage at as low temperature as possible. Other important parameters include high pressure, high ozone concentration and a low pH.
The DUFLO® pump is used to feed the pulp to the first DualZmix® ozone mixer where the ozone will be injected and thoroughly mixed with the pulp. The pulp is discharged from the first ozone mixer and will immediately enter the second DualZmix® ozone mixer to remix the three-phase suspension. The pulp is discharged from the second ozone mixer and fed to the top of a new standpipe, where the pressure in the system is released.
The majority of the pulp will accumulate in the bottom of the standpipe, while any fiber is separated from the gas by a scrubber that is mounted on the top of the standpipe. The separated gas, containing mostly oxygen and any residual ozone, is sent to the ozone destruction unit by a dedicated blower. Pulp from the standpipe can be fed to the next stage.
Benefits compared to a conventional DO stage:
- Reduced AOX discharges to effluent
- Savings of chlorine dioxide
- Reduced yellowing of final pulpt