New bleaching technology is helping pulp mills reduce costs - a vital development in today's increasingly cut-throat pulp sector

by Mark Lockie

Pulp producers hunt for lower cost bleaching

To operate in today's pulp market, a mill's bleaching technology must be able to compete on a global scale. This means getting the right quality at a competitive cost, while meeting increasingly demanding environmental requirements. Such demands are hitting totally chlorine-free (TCF) pulp producers, who can sometimes see bleaching chemical costs 40-50% higher than elemental chlorine-free (ECF) producers.

Part of the problem for TCF pulp producers is that there is no longer a price premium for their pulp. According to the technical director at Södra Cell in Sweden, Steve Moldenius, "We used to command a premium of up to $50/ton. Now we have a lot of customers using TCF pulp and our main challenge is to produce TCF at the same cost and quality as ECF producers."

In order to cut costs, one of Södra's pulp mills, Varö, is installing a second oxygen bleaching stage and has decided to pressurize its first peroxide stage. The mill believes this will lead to more efficient use of bleaching chemicals and after installation, Moldenius reckons that the mill will be able to compete with ECF on cost. "We might even be cheaper," he hints. One advantage of the new mill process is that it should prove less damaging to the fiber than extra cooking, "We can back off on our cooking, which enables us to retain the strength of the pulp," he points out.

The second oxygen stage is based on Sunds Defibrator's Oxytrac process, which was specifically developed to increase the effectiveness of oxygen delignification. According to Sunds, delignification can be increased by 20% without any adverse effect on pulp quality.

Good insurance

Another area where Södra Cell has been active is in the use of peracetic acid. Peracetic acid selectively removes lignin and can give high brightness and better physical properties than more traditional bleaching chemicals. According to Moldenius, "At present, we are able to reach full brightness (88% ISO) and quality, but our margin for error is narrow. At our Varö mill, we use hydrogen peroxide with some peracetic acid bleaching. It ensures our pulp has good properties, it is a kind of insurance policy."

But there is a problem with using peracetic acid - cost. As Moldenius points out, "The chemical can cost two to three times more than hydrogen peroxide. If our new technology additions are successful we will hopefully be able to eliminate the peracetic acid line."

A different path is being taken by Enso's Veitsiluoto mill in Finland. On the basis of testing at the mill since early 1997, peracetic acid is set to become a permanent feature. Enso wants peracetic acid to help the mill reach its environmental targets. According to the head of the mill's business unit, Lasse Ilves, mill trials ran smoothly and his company will be the first in Finland to implement the technology needed for peracetic acid bleaching later this year. Ilves also believes that peracetic acid could support the closed water systems at the mill and reduce harmful discharges.

This view was given a boost recently by a combined study between the Helsinki University of Technology and Kemira Chemicals, the world's largest supplier of distilled peracetic acid. The team investigated the chemical composition of bleaching effluent and as part of the study sorption tests were used to evaluate the behaviour of bleaching filtrates in biological treatment.

Apart from chemical decomposition and evaporation, the sorption mechanism (the adhesion of organic matter to sludge) is one of the ways in which the active sludge process removes organic material. Measured as chemical oxygen demand (COD), the bond between organic material and the sludge was greatest in filtrates from a peracetic acid stage. This was despite the fact that the peracetic acid and mP (acidic peroxide activated by molybdate) filtrates contained a great deal of micromolecular acids, formic acid and acetic acid.

Significantly, chlorine dioxide and ozone filtrates, which contained large amounts of carbohydrates, were not adsorbed very efficiently.

Can't cook?

While the use of peracetic acid is one way of improving pulp strength, the cooking stage in the pulp mill is also being developed to improve performance. In papermaking, softwood pulp is usually used to provide the necessary strength for a paper sheet, while hardwood pulp is used to deliver formation and optical properties. This means that the strength of the softwood pulp is one of the most important pulp properties for papermakers.

Extensive laboratory studies have been carried out in cooperation with the Helsinki University of Technology and Åbo Akademi and supported by the Technology Development Center. The work showed that alternating of alkali concentrations in various kraft cooking stages is a useful tool for controlling pulp properties.
With softwood, a higher effective alkali (EA) concentration toward the end of the cook resulted in higher strength, bulk and brightness. It also gave a higher bleachability, although brown pulp yield was lower.

These findings helped in the development of a new cooking method called the EAPC (Enhanced Alkali Profile Cooking) by Ahlstrom. It is a further advance of company's Lo-Solids pulping process (PPI October 1996, p.27), but is more tailored toward softwood pulp (Figure 1).

EAPC cooking involves pumping steamed chips via a high-pressure feeder to the top of the digester. The digester's first zone is a concurrent pre-treatment stage where most of the alkali introduced in the feed system is consumed. After pre-treatment, the consumed alkali is displaced and extracted to recovery, while the chips are heated and saturated with fresh alkali in the countercurrent impregnation zone. Countercurrent heating and impregnation gives uniform temperature and alkali profiles over the digester cross-section, even at lower circulation rates. One EAPC system has already been sold to Zellstoff und Papierfabrik Rosenthal in Germany, which will upgrade its existing sulfite process to a kraft process. Startup is expected in October next year.

Since the demise of chlorine bleaching and the general acceptance of ECF and TCF bleaching, producers are now interested in getting back to the business of driving down costs and improving pulp quality. Today's new technology is addressing these needs.