Magnefite Process

The magnefite process belongs to the bisulfite processes using magnesium as

base to ensure an effective and simple recovery of the cooking chemicals. The process,

including a description of the magnefite chemical recovery system, was proposed

by Tomlinson et al. for the pulping of conifers [11,12]. The cooking liquor

consists of a more or less pure hydrogen sulfite solution comprising an initial pH

value of 3.6–4.0 (25 °C) and a total SO2 concentration of 4–5%, corresponding to a

MgO concentration of about 1.3–1.6%. With a typical liquor-to-wood ratio of 4:1,

the total SO2-charge based on wood calculates to 16–20%. Due to the low hydrogen-

ion concentration, sulfonation of lignin structures is favored over the reaction

with reactive phenols from extractives. Thus, pine, fir, Douglas fir, larch and other

extractive-rich wood species can be satisfactorily used to produce high-grade pulp.

The rate of heating is not critical, and provided that enough steam is available,

digesters can be brought to cooking temperature within 1 h. The cooking temperature

is about 155 °C for hardwoods and about 165 °C for softwoods. The higher

reaction temperatures compared to the acid sulfite cook compensates for the

lower delignification rate attributable to the significantly lower ion product,

[H+]·[HSO3

– ](see Tab. 4.66, acid sulfite pulping). Depending on the wood source

and the target pulp properties, cooking is completed within only 2–3 h. In total,

the cover-to-cover cycle is only 6–8 h, and allows 25–80% higher cooking capacity

466 4 Chemical Pulping Processes

as compared to conventional acid sulfite pulping. As the cook proceeds, acids that

are released from the wood components tend to reduce the liquor pH. If the pH

falls below 3.0 during the heating period, it is necessary to relieve some of the gas.

Since the cooking liquor contains no free SO2, hydrolysis of the carbohydrates proceeds

at a slower rate even when considering the higher temperature. Hence, pulps

cooked according to the magnefite process contain a higher hemicellulose content,

and yields are approximately 2–4% higher as compared to acid sulfite pulps at the

same kappa number. Magnefite pulps have better strength and optical properties

(brightness), and in somecases are better suited to papermaking than the corresponding

acid sulfite pulps. However, the lower acidity of the cooking liquor limits the

extent of delignification. Indeed, this turned out to be a serious disadvantage when

the need for chlorine-free bleached grades emerged. During the course of developing

ECF and TCF bleaching grades, the kappa number had to be reduced from about 30–

35 to the lower 20s in order to keep the demand on bleaching chemicals and hence

the effluent charges within acceptable limits. Consequently, some of the unquestionable

advantages of the original magnefite process – such as the high strength

properties and short cooking cycles – have been markedly diminished.

Дата добавления: 2015-10-21