HO OH O

Fig. 7.139 Model of hexenuronic acid (HexA) on a xylan side chain.

wood kraft pulp, the level of “other compounds” is even higher, and can reach

almost 50%. (This explains why the degree of “delignification” achieved in an oxygen

stage of hardwood pulp seems more moderate compared with softwood pulp.

As removed “kappa number” it might reach just 30%, but as removed real “lignin”

it is typically better than 50%.)

The reason why hexenuronic acid remained undetected for decades is that the

normally applied analytical method for lignin preparation uses acidic conditions,

during which acid hydrolysis of the hexenuronic acid occurs.

The double bond of hexenuronic acid reacts with electrophilic bleaching agents,

such as chlorine dioxide and ozone. The question of how, logically, to remove hexenuronic

acid can be answered with the background of its analytical evasiveness.

At elevated temperature and low pH, hexenuronic acid is rapidly hydrolyzed. The

first report of the effect of hot acid treatment of hardwood pulp by Marechal in

1993 described the conditions required, namely a temperature above 90 °C, a pH

below 3.5, and extended time. The impact of time on the hot acid hydrolysis of an

oxygen-delignified eucalyptus kraft pulp is shown in Fig. 7.140. The time required

at 90 °C and pH 3 for complete hydrolysis of hexenuronic acid is rather long. The

sharp increase of the effluent load with prolonged time indicates an increasing

cellulose degradation. This becomes apparent from the parallel lowering of the

pulp’s viscosity. In mill applications, therefore, a “hot acid stage” will be conducted

for not more than about 2 h, in order to retain control of the pulp yield and

viscosity losses. From a practical standpoint, this also keeps the size of the towers

within reasonable limits. The process could be accelerated by using a higher temperature

or more acid, but typically the temperature is kept below 95 °C in order

to balance steam demand and to avoid pressurized conditions. A lower pH is similarly

unattractive because it would only accelerate cellulose depolymerization.

Hydrochloric acid may be a cost-attractive alternative to sulfuric acid, especially in

mills with an on-site electrolysis for chlorate. However, the potential problem of

using hydrochloric acid is corrosion of the equipment.

The high temperature and low pH used during the hydrolysis have an additional

effect, in that they allow a significant reduction in the manganese content

of the pulp [5,6]. It is speculated that manganese and iron might form complexes

with hexenuronic acid. In bleaching, hexenuronic acid destruction and metals

removal are equally important. A TCF bleaching sequence with only alkaline oxygen

884 7Pulp Bleaching

0 1 2 3 4

COD [kg/t]

Дата добавления: 2015-10-21