General
Impact on quality
Chlorine dioxide bleaching, ECF
Oxygen chemical bleaching, TCF

 

 

 

 

 

 

 

 

 

 

 

General

Kraft pulp is bleached in several separate bleaching stages with washing in between. The combination of these many individual bleaching stages and intermediate washing is known as a bleaching sequence. In this way, higher brightness is achieved in bleaching without sacrificing pulp strength. In addition, chemical charges are smaller than if the pulp was bleached in a single stage. Multi-stage bleaching produces the best quality and a cost-effective result.

Sequences where there is at least one chlorine dioxide stage are called chlorine dioxide bleaching (ECF) . Correspondingly, sequences where only oxygen-based chemicals are used are called oxygen chemical bleaching (TCF) .

In the second half of the 1990s it became evident that new bleaching sequences installed in mills sometimes were neither pure ECF nor pure TCF sequences but a mix of these two. These mixes are called light ECF and TCF bleaching sequences. The reason for this mix was that some mills did not want to totally replace chlorine dioxide as it was considered to be very efficient bleaching chemical. However, the amount of chlorine dioxide used in these ECF sequences is usually kept at such a low level that the problematic emission with respect to chlorinated organic compounds (AOX) is very low . This trend has resulted in a number of alternatives when choosing a bleaching sequence.

When selecting the bleaching sequence the raw material to be used must be taken into account. Softwood and hardwood pulps differ from each other in their chemical structure. The same raw material from a different species may behave differently in bleaching depending on its origin and age.

Environment protection factors may partly affect the selection of the bleaching sequence because the majority of the effluent and COD discharge from the pulp mill originates from the bleach plant.

Bleaching consists of initial bleaching and final bleaching. In initial bleaching, the residual lignin content is reduced and in final bleaching the brightness is increased by reducing the colorized groups.

Until the 1980’s the most common chemicals used were chlorine, chlorine dioxide and hypochlorite, but since then oxygen–based chemicals (oxygen, ozone, peroxide) have become more prevalent. In addition, bleaching sequences have been shortened, i.e. the number of bleaching stages in the bleach plant has been reduced. Constructions of bleach plants give more flexibility, i.e. the same equipment can be used for various sequences .

Some examples of bleaching sequences in different years:

1990

D(EO)DED
D(EO)D(EP)D
1994

D(EOP)DD
D(EOP)D(PO)
Q(EOP)Q(PO)
ZQ(PO)

2000

D(EOP)D
Q(PO)DD
Q(PO)(DQ)(PO)
(Z(EO))DD
mP(EOP)ZP
Z(PO)(PaaQ)(PO)
A/D(EOP)DP

2015

D(EOP)D
Dht(EOP)D
A/D(EOP)D
Z/D-EOP-D
D(EOP)DP
Dht(EOP)DP
A/D(EOP)DP
Z/D-EOP-D-P
D(EOP)DD
Dht(EOP)DD
Aht(OP)D(PO)
Q(OP)D(PO)
(Ze)DP
(Ze)DD
A-EOP-Z/Q-P
Z/Q-EOP-Z/Q-P
Z/Q-EOP-Q-P

 

Impact on quality

It is worth remembering that in pulp making, pulp strength once lost cannot be regained. The strength of pulp deteriorates in bleaching if the bleaching chemicals attack the carbohydrate chains, i.e. the cellulose and hemicellulose. These destructive reactions can be controlled by regulating the conditions of the sequence stages.

Uniformity of pulp quality is crucial; a momentary peak in quality is of no value.

In general, it can be said that chlorine dioxide bleaching achieves slightly stronger pulp. Of the oxygen-based chemicals, ozone and peroxide are critical in relation to bleaching conditions.

 

Chlorine dioxide bleaching, ECF

Often chlorine dioxide bleaching is preceded by an oxygen delignification stage. The bleaching sequence comprises three or four stages . Short sequences are more suitable for hardwood than softwood pulps .

Chemical charges vary depending on the wood species and the background of the pulp such as cooking conditions. The charge depends particularly on the kappa number.

In ECF bleaching sequences, a certain brightness level can be attained with additional stages with a smaller amount of chemicals, or a higher brightness can be reached, or both.

The amount of chemicals (kappa factor) in the first chlorine dioxide stage (D0) also affects the brightness level to be reached. The higher the brightness target, the greater the kappa factor is. The chlorine dioxide stage (D) is followed by an alkaline extraction stage (E), which is often reinforced with oxygen and peroxide (e.g. EO or EOP).

Sometimes it may be required to reduce the amount of chlorine dioxide in bleaching when the mill wants to reduce its AOX emissions or when ClO2 cannot be produced in sufficient quantities. This can be done:

 

Oxygen chemical bleaching, TCF

The following oxygen-based stages are used in oxygen chemical bleaching :

The metals contained in the pulp are removed first with a chelating stage (Q). Alternatively metals can also be removed by acid hydrolysis.

The chemical charge and process conditions vary depending on the wood species, the degree of closure of the bleach plant, etc.

Ozone and peracetic acid are more effective than oxygen and peroxide at delignifying the pulp at lower kappa numbers. They can also be used in initial bleaching, i.e. to reduce the kappa number of the pulp. In final bleaching brightness is increased typically with a (PO) stage .

In TCF bleaching with hydrogen peroxide and oxygen as the only bleaching agents, e.g. the sequence Q(PO) or Q(OP)Q(PO), the maximum brightness that can be reached increases with decreasing kappa number prior to bleaching. Even if the kappa number after the oxygen delignification is low, for example about 10, it is usually not possible to reach a brightness higher than about 85% ISO for softwood pulp.

An effective solution for higher brightness is to introduce an ozone bleaching stage between the oxygen delignification stage and the (PO) stage, e.g. the sequence becomes (Zq)(PO) or Q(OP)(Zq)(PO) .