papr11 - Bleaching Lecture 11 PAPR 1000 Introduction to...

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1 Bleaching Lecture 11 PAPR 1000 Introduction to Pulp and Paper Manufacturer
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2 Bonus Questions ± What is the difference between weak black liquor and strong black liquor? ± How is green liquor formed? ± What special feature about the brown stock washers makes it more economical?
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3 Bleaching Bleaching ± Cellulose and hemicellulose are inherently white and do not contribute to pulp color ± It is “chromophoric groups” on the lignin that are responsible for color ± Oxidative mechanisms are believed to convert part of the lignin’s phenolic groups to quinone- like substance that are known to absorb light. ± The colored groups are highly conjugated systems containing carbon-carbon double bonds and carbonyl groups.
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4 Bleaching Bleaching ± Bleaching is the process of chemically treating wood pulp fibers before papermaking to increase their brightness by reducing or removing lignin and resin. – Improve appearance- whiter, brighter & cleaner (customer demand) – Improve print contrast
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5 Bleaching Mechanisms Bleaching Mechanisms ± ± Two mechanisms Two mechanisms Lignin removal Lignin removal reduction in residual lignin reduction in residual lignin (Kappa number) (Kappa number) Lignin preserving or brightening Lignin preserving or brightening - - removal of removal of colored compounds colored compounds
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6 Bleaching Chemicals Bleaching Chemicals ± ± Bleaching is a multi Bleaching is a multi - - stage process stage process ± ± Most common bleaching chemicals are Most common bleaching chemicals are oxidizers oxidizers In most cases, these oxidizers are strong In most cases, these oxidizers are strong electrophiles electrophiles they steal electrons from lignin
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papr11 - Bleaching Lecture 11 PAPR 1000 Introduction to...

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