We obtaining cellulose pulp with lower DP than expected after bleaching process.
We are using following bleaching sequence,
Chlorination – O2+Hypo -ClO2-SO2
This sequence I don't understand.
Why chlorine and clo2 +O2 simultaneously in this sequence
While Chlorine and ClO2 are both highly selective, hypochlorite is much less. Where is your extraction step? I assume you are using the SO2 as anti-chlor? Are you controlling the incoming pH of your chlorine based stages? What temperature are you bleaching at? Lower the temperature if you can in the subsequent stages and you'll preserve viscosity. What is your incoming Kappa#?
There are many variables that you can control to preserve the dp of your fiber - not one answer. If your % NaOH in your oxygen stage is too high, you can see a large drop in viscosity as well.
It has been a long time since I've worked in a bleach plant with a Chlorine stage - they have all been eliminated in N. America, so I don't recall all of the conditions that we used. If you replace that stage with Chlorine Dioxide, you can extend the time as long as you can maintain the final pH. You will lower the lignin content while not appreciably harming the viscosity.
Thank you Sanjay Naithani and Kurt Haunreiter...
We are using this sequence to remove lignin selectively.
We are maintaining acidic pH in chlorination, ClO2 & SO2 treatment and basic pH in O2. Bleaching temperature is 60-80ºC. chlorination is at RT.
Incoming kappa is in 10-15.
Kurt Haunreiter I am curious to know how ClO2 and Chlorine selectively removes lignin only, as there is hemicellulose and cellulose also,,,, and all are organics...
What is brightness target
Chlorination cum alkali extraction degrade cellulose drastically and not recommend for environmental point of view
You can use other sequence depending upon target of final product
Dhiraj - there is no brief answer that I can provide you regarding the selectivity of these chemical species. Briefly, the first stage of bleaching is a continuation of the delignification process. While these chemicals can certainly attack cellulose, if you control the process conditions, you will favor the reactions with lignin. Also, the reactions with carbohydrates are much slower than with lignin. For a thorough discussion of the chemistry and application, I would recommend the book by Carlton W. Dence; "Pulp Bleaching, Principles and Practice". It devotes several chapters to this discussion.
https://imisrise.tappi.org/TAPPI/Products/01/B/0102B061.aspx
I agree with the answer of Kurt Haunreiter
You can only control the degradation of carbohydrates by optimising conditions and sequence of bleaching for selective removal of lignin during extended delignification
Thank you Sanjay Naithani and Kurt Haunreiter for the valuable inputs and article.
- Badges
- Science topic
- Similar topics
- Chemistry
- Nanotechnology
- Nanostructures
- Thin Films