Ozone-enhanced bleaching of softwood kraft pulp

Abstract

By using ozone in their bleaching processes, many hardwood pulp mills in various parts of the world have improved product quality and their environmental and process performance, and reduced operating costs to increase competitiveness. The challenge for softwood pulp is to rethink the use of ozone according to their specific requirements. This paper summarizes results obtained using ozone bleaching on softwood (Pinus radiata) kraft pulp, for which the brightening ability of limited ozone dosages can enhance the economic benefits without impairing pulp quality. This work evaluated the chemical justification to use ozone at the end of the bleaching sequence and the effect on pulp quality, and considered the practical consequences of this new option. Special attention was given to the economic and technical aspects, including investment cost, variable cost, and process implementation. Ozone was shown to be the ideal complement of chlorine dioxide for final pulp bleaching. Both chemicals can easily be combined at the end of the bleaching sequence. Taking into account pulp mill capacity, capital investment, and total operating cost, the financial savings were calculated to be in the range of EUR 5 million (USD 6 million) per year, with a payback period of about 1.5 years.