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2001/08/01: Eco-efficiency: Hydrosulfite well-suited for bleaching paper |
by Susanne Ackermann
Paper containing wood pulp has to be bleached in the production process to achieve a white finish. For this purpose paper manufacturers use special bleaching agents like hydrosulfite. BASF has a capacity of 76,000 tons per year available to make the chemical. Hydrosulfite, however, produces only the medium-quality bleaching effect needed for, say, newsprint. For real whiteness hydroperoxide works best. And yet, hydrosulfite bleach has the edge over hydroperoxide in terms of costs and environmental impact. For that reason paper manufactures should use hydrosulfite to the extent possible and use hydroperoxide in addition only to meet the highest bleaching standards.
That is the verdict of an Eco-Efficiency Analysis performed by BASF. The investigation was designed to examine and compare different chemicals used for bleaching paper for illustrated magazines. Dr. Peter Saling, GUP/C, observed: "Based on the Eco-Efficiency Analysis it is now possible to submit a clear-cut recommendation to paper manufacturers. They should dispense with the use of hydroperoxide for paper that requires less than absolute whiteness and pre-bleach with hydrosulfite in producing high-value papers.
In manufacturing pulp-containing paper, wood is first finely ground with water. Bleach is added to hot pulp. In a so-called bleaching tower the bleach removes the colorants from the pulp.
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Three bleaching processes examined
The analysis compared three paper bleaching processes: the two chemicals, hydrosulfite and hydroperoxide, as well as the Borol process. The latter requires a paper manufacturer to make his own hydrosulfite in a small reactor that yields a very diluted solution which, in turn, must be applied to the pulp immediately. The Borol process, too, achieves only a medium-grade bleach. The Eco-Efficiency Analysis compares the entire process and costs, starting with the effects on the environment in the production of the chemicals, their application by the papermaker through to the final stage - the environmental impact and costs of disposal.
The upshot of the analysis: Hydrosulfite evidences advantages in respect to its ecological effect and costs. In the "cradle to grave" examination the bleaching process with hydrosulfite had ecologically less of an impact than experienced in applying hydroperoxide. Another plus: Hydrosulfite, by contrast to hydroperoxide, requires no additives. Hydroperoxide develops its bleaching power only upon the addition of several auxiliaries, including caustic soda and a water glass solution.
These additives deteriorate the eco-efficiency of peroxide. They boost costs and energy consumption which for peroxide are twice as large during its life span than for hydrosulfite. Moreover, the additives cause additional emissions, both in their production as well as in the bleaching process.
Its gentle reductive bleaching process gives hydrosulfite another advantage. Hydroperoxide, an aggressive oxidation agent, destroys some of the wood fibers. As a result the paper manufacturer's effluent is burdened with released carbon (TOC) and the paper has less tensile strength.
In respect to eco-efficiency the Borol process performs better than peroxide bleach. But the process also has disadvantages: The paper manufacturer needs sulfur dioxide and caustic soda as a basis for making a hydrosulfite solution in order to achieve the needed bleaching power. Taking account of its entire life span hydrosulfite impacts the environment least and, in terms of costs, is a close second to the Borol process.
One caveat: Users of hydrosulfite have to take care because it self-ignites in contact with just a little water. Following a few precautions, make it, however, safe to handle: "Hydrosulfite has be carefully protected from humidity. That is merely a question of applying the correct technique. BASF makes the necessary know-how to master this problem available to its customers", advises Saling.
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