Abstract:
Oxygen delignification can be defined as the use of oxygen and alkali to remove a substantial fraction of lignin in unbleached pulp. The process is usually conducted under pressure and delignification is normally in the range of 35-50 %. Oxygen is usually applied to Kraft wood pulps but can be used for sulfite, secondary fiber, nonwood, and other types, of pulp.
The main benefits of oxygen bleaching are environmental. These derive from the fact that both the chemicals applied to the pulp and the materials removed from the pulp are compatible with the Kraft chemical recovery system. This enables the recycling of oxygen stage effluent to the recovery system by way of the brown stock washers, decreasing the potential environmental impact of the bleach plant. The decrease is roughly proportional to the amount of delignification achieved in the oxygen stage. This applies not only to chlorinated organic by-products, but also to other environmental parameters associated with the bleach plant effluents, including BOD, COD and color. The decrease in color, however, is larger than expected on the basis of the lignin removed in the oxygen stage.
The industrial application of oxygen bleaching has expanded very rapidly in the recent years. The first commercial system was started up until 1970 and the world capacity in 1980 was only about 10,000 tons per day. By 1992 the number of operating systems stood at 155, with a total capacity of about 85,000 -tons per day as illustrated in Figure 1 (1). Furthermore, this rapid growth is likely to continue. As trends away from the use of chlorine and chlorine containing compounds intensify, both technical and economic considerations will mandate the use of oxygen. Ozone, for example, though it possesses certain advantage over oxygen, will probably see application only in conjunction with oxygen. Oxygen will be used to predelignify pulp to the point where the necessary ozone charge becomes small enough to be economical and selective..................
