Bleaching of Fibres

Question 1

What is the difference between bleaching and brightening?

Answer 1

Bleaching is the removal of lignin by breaking it down into smaller components so it can be washed out, whereas brightness is modification of lignin rather than removal. Conjugated double bonds within the lignin structure absorb light making the pulp look darker, oxidising the double bonds as done when brightening makes the pulp appear brighter as the lignin is not absorbing as much light.
Bleaching produces a more stable brightness that will not age as much.

Question 2

What is brightness and how do you measure it?

Answer 2

Brightness is the reflectance of visible light at a wavelength of 457nm. The absorbance of light which reduces this brightness is often caused by lignin.

Question 3

What are the advantages and disadvantages of the different bleaching chemicals?

Answer 3

Chlorine: +effective +Economical + good particle removal. -highly corrosive -environmental issue
Hypochlorite: +easy to make and use + low cost. – loss of pulp strength. – can cause chloroform
Chlorine dioxide: + high brightness without loss of strength. -costly, - environmental – corrosive
Oxygen: +low cost. +chloride free effluent. – large initial investment – potential loss in strength
Hydrogen peroxide: + easy + low capital cost. – high chemical cost – can have loss of strength
Ozone: +chlorine free effluent – poor particle bleaching and pulp strength

Question 4

What is oxidation, what is reduction?

Answer 4

OIL RIG oxidation is loss reduction is gain of electrons
Oxidation usually adding oxygen, reduction generally losing oxygen atom

Question 5

What are the two main reactions in chlorine bleaching?

Answer 5

Oxidation and Aromatic chlorine substitution.

Question 6

• Why can chlorine be used for delignification?

Answer 6

Chlorine dioxide is a powerful oxidant. Releases 5 oxidation equivalents when it is reduced to a chloride ion. These attack electron rich lignin sites, degrading the lignin making it soluble.

Question 7

• How does chlorine dioxide attack lignin compounds?

Answer 7

Chlorine dioxide is an electrophile and will therefore attack lignin in electron rich areas. There are 2 main reactions Oxidation and Aromatic chlorine substitution. These lead to increased water and alkali solubility of the lignin.

Question 8

What is the function of an alkaline extraction and how does it work?

Answer 8

The alkaline is added to activate the lignin. The alkali neutralises acidic groups within lignin, converting them to their anionic forms (phenolic and enolic hydroxyl groups converted to phenolate and enolate ions). These now have an increased solubility and it is easier to remove them from the pulp

Question 9

Why is lignin activated prior to oxygen delignification? How is this done?

Answer 9

Alkali is added prior to activate the lignin. This is because Oxygen is relatively weak and therefore a selective oxidising agent. The alkali converts phenolic and enolic hydroxyl groups into more reactive phenolate and enolate ions. The oxygen can then attack the now electron rich sites.

Question 10

What are the two main reactions during ozone delignification?

Answer 10

RUPTURE OF AROMATIC RINGS (Initial attack on aromatic rings leads to ozonide which is then hydrolysed to cleave the aromatic ring and a muconic acid derivative is formed.)
CLEAVAGE OF ETHER BONDS (ozone attack at alky or arly ends…)

Question 11

• What happens to carbohydrates during bleaching reactions?

Answer 11

Glycosidic bonds are ruptured in chlorination and ozone bleaching. This reduces the chain length, and therefore viscosity, decreasing the strength.

Question 12

What are the general oxygen delignification conditions at medium consistency?

Answer 12

Retention time ~50-60mins. Initial temp 85-105C. delignification% ~40%

Question 13

How does alkali charge and retention time affect oxygen delignification?

Answer 13

Increasing alkali charge increases delignification but also cellulose degradation. With time we get more delignification, it happens in stages first we see very rapid drop off, the second stage we see incremental reduction, this second stage is more influenced by concentration gradient of the fibre as it is a diffusion process so the alkalinity has less of an effect.

Question 14

What are the advantages and disadvantages of chlorine dioxide bleaching?

Answer 14

+Highly selective for lignin, it causes little damage to carbohydrate elements. – unstable chemical – corrosive – environmental impact.

Question 15

What is the relationship between chlorine and chlorine dioxide in delignification reaction of softwood kraft pulps?

Answer 15

Chlorine and chlorine dioxide are often used sequentially to maximise delignification efficiency. The chlorine often used first for aggressive delignification, followed by an extraction step and then chlorine dioxide to further delignify the pulp more selectively.

Question 16

What are the two phases in alkaline extractions?

Answer 16

First phase is rapid delignification. This is an extraction mechanism where the degree of alkalinity and high temperature provide the best extraction.
The second stage is much slower and assumed to be a diffusion mechanism rather than an extraction. The temperature and alkalinity are less important here, it is more the concentration gradient within the fibre that is the main driving factor.

Question 17

With which chemicals and how can the alkaline extraction be re-inforced?

Answer 17

Oxygen can be used to reinforce alkaline extraction. It helps to lower the kappa number, but not by removing more lignin, by oxidising remaining lignin so that it doesn’t consume as much potassium permanganate in the kappa test – this indicates the pulp will not consume as much chemicals in further stages
Can also reinforce with peroxide in the first extraction stage, this will decrease the amount of chlorine needed in the first bleaching stage, this is important for environmental reasons.

Question 18

How does pH, temperature and ozone charge influence the delignification process

Answer 18

pH effects the selectivity of ozone. At higher pH we get more carbohydrate degradation, ozone is more selective at lower pH (pH2). Additionally the Ozone is degraded when the pH is increased so it is important to be in acidic conditions.
Temperature is also a big factor – higher temperatures decompose the ozone, therefore low temperatures are more effective and give better selectivity. But this must be balanced with what is realistic.

Question 19

Why is hydrogen peroxide only usable for brightening reactions and how does the brightening work?

Answer 19

Hydrogen peroxide is not able to oxidise the henolic structures in lignin, therefore no delignification occurs. The hydrogen peroxide instead eliminates chromophoric groups, thus brightening the pulp. The hydrogen peroxide interrupts the conjugated chromophore system, shifting the maximum absorption from the near visible to the UV region.

Question 20

In which applications is sodium dithionite used?

Answer 20

Some chromophores formed during hydrogen peroxide bleaching can be reduced by dithionite.

Question 21

Explain alkali darkening during hydrogen peroxide bleaching, and how can it be avoided?

Answer 21

At high pH levels metal ions present in the pulp can catalyse the decomposition of hydrogen peroxide, this forms coloured complexes which appear to darken the pulp. Additionally in a highly alkaline environment the hydrogen peroxide can react with the lignin to form highly coloured quinones. To avoid this we need to add chelating agents prior, to remove metal ions, ensure we have efficient pH control and buffers to avoid very high alkalinity.

Question 22

What is the relationship between time, temperature and alkalinity during hydrogen peroxide bleaching?

Answer 22

At high alkalinity the pulp starts to darken, this is alkali darkening. But if the alkalinity is too low the bleaching efficiency is lowered, so we need to keep at optimal pH. High temperature is more of a factor than retention time, it is better to invest in higher temperature, pressurised towers rather than long ones that increase retention time.

Question 23

• What are the general bleaching conditions for dithionite bleaching? Discuss the following process variable on sodium dithionite bleaching: higher dithionite charge, low temperature, low pH, poor mixing?

Answer 23

Higher dithionite charge = higher brightness
high temperature = high brightness
high pH = high brightness
Good mixing = high brightness
General conditions = 1% charge. Temp 40-100C. pH 4-6.5.