Two-Stage Neutral Magnefite (Bisulfite-MgO)

The magnefite cooking process provides a number of advantages over the regular

acid sulfite process, including better pulp strength properties, provided that the

kappa number is kept above a certain level (e.g., 30). This, as has been outlined

above, requires a high subsequent bleaching effort for obtaining pulps of full

brightness (88% ISO and higher). A modification to the magnefite process – the

two-stage neutral magnefite – has been introduced in which the pH is increased

to about 6.0–6.5 during the latter part of the cook by the addition of magnesium

hydroxide [13]. There, the first stage is carried out according to the classical magnefite

process (pH 3.7) and after a period of 90 min at approximately 165 °C,

(hydrated) MgO is injected into the digester until a pH of 6.5 is reached. The

amount of magnesium hydroxide slurry required to reach the target pH level is

equivalent to about 4.0–5.0% on o.d. wood. It is worth mentioning that the best

pulp strength properties are obtained when the MgO addition is carried out when

the cook is in the 60–65% yield range. The introduction of magnesium hydroxide

slurry into the digester reduces the temperature of the acid by up to 15 °C. Subsequently,

the temperature must be raised to 170–175 °C, which is the cooking temperature

of the second stage. This temperature is maintained for about 120–

150 min. The solubility of MgO can be increased by adding sodium salts, or even

better by introducing it into a sulfite spent liquor from a previous cook [10]. The

overall cooking time is about 1 h longer than for a one-stage magnefite cook at the

same kappa number. In the case of softwoods, the yield is quite comparable to

that of a regular magnefite pulp. Pulps were stronger in all categories than from

single-stage magnefite, and were much stronger than from conventional acid sulfite

pulping. They were said to be comparable in tensile and burst, but inferior in

tear strength to kraft pulps from the same wood furnish.

4.3 Sulfite Chemical Pulping 467

Even though the unbleached brightness of the two-stage neutral pulps is lower

as compared to the magnefite pulps, the bleachability of both pulps remains

equally good. The combination of elevated pH levels close to neutral conditions

and the high temperature causes the extraction of hemicelluloses. In the case of

hardwood pulps, a very low pentosan content (6.4%) and a rather high alpha-cellulose

content (90.3%) were observed [13]. This concept of two-stage magnesiumbase

pulping has been installed at the Weyerhaeuser mill at Cosmopolis,

Washington, to produce high reactive dissolving pulps, termed “HO”. In addition,

the two-stage neutral sulfite pulps show slow hydrating properties and good opacity,

which permits the production of pulps of a wide range of properties [14].

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