Thermo-mechanical pulping (TMP) is a kind of mechanical pulping process without using any chemicals. This process is applied for making a high-yield pulp using mechanical means such as a refiner. Refining stage in the thermo-mechanical pulping process easily defiberates wood fibers as the lignin is softened during presteaming. Chemical thermo-mechanical pulp process (CTMP) pretreats wood chips with Na2CO3, NaOH, Na2SO3, and other chemicals prior to mechanical treatment like refining. In this process, 80-95% of pulp is produced. The properties of high yield chemical pulp and mechanical pulp steps can be accomplished in a variety of locations. Preprocessing step is the most commonly applied method in CTMP process because of the potential improving pulp properties and defiberation. In CTMP process, the wood properties are changed through the reactions between sulfite chemical and wood chips for softening of the wood chips, defiberation and refining entangled complex. The softening of the wood chip is an important process due to impacts on energy consumption in mechanical steps.
Pinus densiflora was used as raw materials in this study for manufacturing of CTMP. The species generally contains 7-12% of the resin, which may lead to pitch trouble in CTMP. Therefore pitch must be suitably controlled by using a certain chemicals.
This study focused to investigate the softening effect of pine chips using various pulping conditions for CTMP. The amount of NaOH and Na2SO3 plays a vital role in softening of wood chips. Pulping temperature is also a key factor to influence the penetration of pulping liquor into wood chips. In order to evaluate the softening effect of pine chips, shives contents, sulfur contents in wood chips and freeness were measured. If CTMP has a low freeness, it can be assigned that wood chips are severely softened by a certain treatment.
The softening experiments were conducted by varying the mixing ratio of NaOH and Na2SO3 in order to soften the wood chips under different impregnation temperature.
Under the same refining condition, addition ratios of NaOH and Na2SO3 did rarely affect the softening of wood chips. On the other hand, soaking temperature played a vital role in decreasing freeness. That is, irrespective of chemical treatment, high temperature over 100℃ made wood chips softened more than low temperature This means that refining of wood chips soaked at higher temperature can be done within a much shorter period. This means that higher soaking temperature can save more refining energy than low soaking temperature.
For sulfur contents in wood chips, 10% addition of Na2SO3 on oven-dried wood weight did have a significant impact on the increase of sulfur. In particular, higher impregnation temperature like 120℃ and 140℃ contributed to the higher sulfur contents compared to 100℃. However, addition ratios of NaOH did not have a important effect on the sulfur contents. It can be confirmed that soaking temperature and the addition ratio of Na2SO3 had a great influence on the sulfonation of lignin.
Shives contents were closely related to the softening of wood chips. More softened chips might generate lower contents of shives in CTMP than less softened ones. Under the soaking temperature 100℃ and 120℃ respectively, the amount of Na2SO3 played a key role in decreasing shives contents. It is interesting to note that the amount of NaOH had a little effect on the decrease of shives content. However, at the soaking temperature over 140℃, any factor like NaOH and Na2SO3 could not contribute to the decrease in shives contents.
In conclusions, in order to soften wood chips as raw materials for CTMP, soaking temperature must be maintained at over 100℃, and the amount of Na2SO3 played a more important role than that of NaOH. More softened chips may contribute to the reduction of refining energy.