Interest in applying ozone is growing

摘要

The first commercial ozone bleaching plants were installed at the beginning of the 90s. For several years the development of the use of ozone was rather slow. However, interest in applying ozone has grown considerably, of late, as improved pulp quality of ozone bleached pulp has been documented in many reports. Ozone delignification can be performed at both medium and high pulp consistency. High-consistency delignification in particular offers many advantages, thanks to the very low liquid amount associated with pulp, resulting in higher kappa reduction, lower operating costs and better recycling and recovery of filtrate. This also means flexibility in ozone concentration in the feed gas and lower pressure required in the reactor. Another important advantage is safe operation, as the low pressure, below atmospheric pressure, prevents leakages. Metso Paper (formerly Valmet) is one of the pioneers in developing both oxygen and ozone delignification technology for pulp bleaching. The company has now introduced a new improved high-consistency ozone delignification process called ZeTrac~(TM). Recently, the process was granted the "Prix Special du Jury" in the ATIP Innovation Contest in Bordeaux on October 17-19, 2000. Development work in the early 1990s concentrated mainly on finding TCF bleaching methods to produce good quality fully bleached pulp and ozone played an important role in this work. In addition, it was also found that in ECF sequences the use of ozone is very economical and the chemical costs are far lower than for a conventional ECF sequence. This has led to bleaching process development towards light ECF sequences, which enable minimized use of chlorine dioxide. Besides being very economical, these sequences also offer the best options for environmentally-sound bleaching. This article presents bleaching processes where ozone is used and where the design offers possibilities for further closure of the bleaching process. This is important to meet future environmental restrictions.