Chapter Ten - Hexenuronic Acids and Their Removal

Abstract

Hexenuronic acid groups (HexA) have attracted great attention in recent years because of their effect on bleaching operations. Polysaccharides undergo some undesirable reactions during kraft pulping and lead to the formation of HexA. These are harmful to kraft bleaching systems in the papermaking process as they reduce bleaching efficiency by consuming a disproportionate amount of bleaching chemicals. When the bleaching chemicals are being consumed in nonessential reactions, both the efficiency of the chemicals and the cost efficiency of an operation decrease. Studies have shown that the consumption of bleaching chemicals correlates with the amount of hexenuronic acid groups in hardwood kraft pulps. These groups also influence the retention of nonprocess elements, which results in loss of efficiency and speeds up deposit buildup on process equipment. It also binds transition metal ions, intensifying the use of expensive complexing agents in hydrogen peroxide bleaching. In addition, the presence of HexA increases the difficulty of reaching a high degree of brightness and also increases brightness reversion. HexA groups contribute to the pulp kappa number and linearly correlates with kappa reduction, as these groups consume part of the potassium permanganate used in this determination. The above-mentioned problems are more pronounced in the case of hardwood pulps because of their higher xylan content. Degradation of HexA dominates over its formation only in the last part of the hardwood pulping and not in the beginning as is the case for softwoods. Because of these problems encountered in bleaching operations due to the presence of HexA, removal of HexA is very important. Low HexA pulps are likely to provide a cleaner production opportunity and to be cost effective due to lesser consumption of bleach chemicals and pollution. Removal of HexA can be carried out by bleaching at acidic conditions or by acidic treatment at high temperature without bleaching chemicals. The reactive double bond in the hexenuronic acid reacts with several bleaching chemicals such as chlorine, chlorine dioxide, ozone, and peracid but not with alkaline oxygen and hydrogen. Enzymes are also found to be effective in removing hexenuronic acid from kraft pulp. Other ways of reducing hexenuronic acids are either by modifying the pulping conditions or by using mild acid hydrolysis. A number of hot acid or hot D (dioxide) are already in operation or are coming into operation in BEKP mills.