(70a) Reducing Energy Consumption in Thermomechanical Pulp Production Using Chlorine Dioxide

There is an increasing demand for wood fibre products that could be met, in part, by greater use of high yield pulps such as thermomechanical pulp (TMP). However, compared to bleached chemical pulp, TMP generally has lower strength properties, lower brightness and exhibits brightness reversion. This limits further utilization of TMP in the production of high-grade products. In addition, the TMP process consumes a large amount of electrical energy. A promising strategy to overcome these drawbacks could be conducting chemical treatment on the high-consistency (HC) refined TMP prior to low-consistency (LC) refining.

In prior work, we have shown that the treatment of TMP with oxidizing agents such as, highly alkaline hydrogen peroxide, oxygen and ozone, can improve the tensile strength of TMP and protect the fibre from cutting during subsequent LC refining. The combination of oxidation and LC refining can result in total energy savings, to a given tensile, of around 500 kWh/t. The improved tensile is a result of the formation of acid groups in the fibres and fines.

Chlorine dioxide is a strong oxidizing agent commonly used in kraft pulp bleaching. In this work we report the effects on pulp properties of treating high-freeness softwood TMP with chlorine dioxide under various conditions. Hemlock and spruce/pine/fir (SPF) high-freeness pulps have been treated with chlorine dioxide over a range of temperatures (25-65 oC), times (10-120 min) and chlorine dioxide charges (0.5-4.5% w/w). These chlorine dioxide treatments were followed by alkali soaking or reaction with alkaline peroxide. The resulting changes in pulp properties were determined.

A positive correlation was found between tensile gains and acid group content of the pulp, implying that the formation of carboxylic acid in the fines and fibres creates increased bonding. The gain in tensile on alkaline soaking is attributed to ionization of the acid groups and swelling of the fibres and fines. A rapid increase in tensile strength occurs under mild conditions. Varying the treatment temperature, treatment time, and chlorine dioxide charge had little effect on further changes in tensile strength. However, a positive relationship was then established between alkali charge and tensile strength of chlorine dioxide treated pulp. A positive relationship was also found between alkali charge and pH of the pulp. After chlorine dioxide treatment, the pH of the pulp dropped to a range of 3.2-1.5 pH. Soaking the chlorine dioxide treated pulp in alkali brought the pH back to neutral and even higher, depending on the alkali charge.