At the end of this module, you will be able to:
- Define the purpose of extraction
- Describe the variables of extraction
- Describe the safety concerns of sodium hydroxide
- Identify equipment used for extraction
- Describe the role of oxygen and hydrogen peroxide in extraction
The following key questions are answered in this module:
What is an alkaline extraction stage?
An alkaline extraction stage follows a bleaching stage to make the lignin soluble so it can be removed by washing.
What chemicals are used for alkaline extraction?
The main chemical for is sodium hydroxide (NaOH). Also, oxygen and/or hydrogen peroxide are often added for better bleaching results.
Why are oxygen and/or hydrogen peroxide added to an alkaline extraction stage?
Alkaline extraction conditions are similar to those used for oxygen or peroxide. Adding these chemicals removes additional lignin and helps achieve a higher and more stable final brightness.
What is the retention time for alkaline extraction?
A typical retention time is 1-1.5 hours.
What are some of the hazards of sodium hydroxide?
Sodium hydroxide is a dangerous chemical that reacts violently with water and acid and it will cause severe chemical burns if it gets on your skin.
Below is a transcript of the video sample provided for this module:
Chlorination is the treatment of pulp with chlorine to solubilize lignin and ultimately bleach the pulp. Chlorine is an excellent chemical for pulp bleaching because of its low cost and lignin removal effectiveness. For these reasons, chlorine was traditionally used in the first bleaching stage, to remove up to 90% of the lignin remaining in unbleached pulp. Chlorine is a forgiving bleaching chemical. The reaction is fast and can occur over a wide range of pH, temperature, and consistency. Chlorine use is almost completely obsolete because it harms the environment. Only a few locations around the world still use elemental chlorine for bleaching. Chlorine makes lignin soluble in water or in alkali. So an alkaline extraction stage is required after chlorination to actually remove the lignin from pulp.
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