Bleaching Basics
After wood chips are pulped, either by chemical or mechanical means, the resulting pulp has a brownish color. In order to create a white or lighter colored final product, the pulp much be bleached. This module provides a brief background on wood fibers and chemical pulping, and then provides information on what bleaching is and how it fits into the papermaking process.
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Learning Objectives
• Identify and describe key equipment used in bleaching operations
• Describe the purpose and application of each piece of equipment
• Identify and describe typical bleach stage layouts
• Identify key considerations in selection of pump types
• Describe advantages and disadvantages of different tower types
• Identify and describe safety hazards associated with equipment used in bleaching
Specs
| Course Level | Intermediate |
| Languages | English, Portuguese, French, Polish, Russian |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is pulp bleaching?
Bleaching is a series of chemical processes that increase the brightness of cellulosic pulps.
What is pulp brightness and how is it measured?
Brightness is a measure used to tell how white the pulp appears. Brightness is measured by calculating the amount of blue light that reflects off the surface of a pulp handsheet sample. The higher the brightness, the whiter the pulp.
What are the chemicals used for pulp bleaching?
Today, the most common bleaching chemicals are chlorine dioxide, oxygen, hydrogen peroxide, and ozone.
What is the difference between “bleaching” and “brightening”?
Bleaching increases the brightness of pulp by removing lignin from the pulp. Brightening preserves and modifies lignin so it no longer appears brown. Brightening is usually temporary.
What does “Selectivity” of a bleaching chemical mean?
Selectivity is used to measure how well a bleaching chemical will remove lignin without affecting the cellulose and hemicelluloses in the pulp. A more selective chemical will preferentially react with lignin. Chemicals with low selectivity will attack cellulose and reduce the strength of the bleached pulp.
Sample Video Transcript
Lignin refers to a group of organic compounds, which are present in the outer layers of the cell walls and between the fibers. It is the glue that holds the fibers together to make wood. To convert wood into paper the lignin that holds the fibers together must be broken down. The lignin content of soft woods is 26 to 32% by weight and for hard woods it is 20 to 28%. Lignin is dark brown in color. Even after most of the lignin has been removed from wood in chemical pulping processes, the pulp fibers are still brown. Lignin is the compound which must be removed or lightened by the bleaching process.
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