APPLICATION REPORT
Oxygen delignification
Picture 1. Two-stage oxygen delignification.
The process
Oxygen is reduced to water in reactions with the
organic components, and the organic components are oxidised. Oxygen in its normal state is
a weak oxidising agent, and is as such ineffective in delignification. Its oxidizing power can
be promoted by raising the temperature and by
alkaline conditions.
Picture 1 shows a simplified flowsheet of an
oxygen delignification stage. Alkali, oxygen and
steam are added to the pulp coming from the
washing stage. After a given retention time, the
substances formed in the reactions and any residual chemicals are washed out.
The most essential factor in oxygen delignification
is to bring oxygen gas into contact with the fibers
under alkaline conditions. This means that the pulp
suspension should have enough alkaline (OHions) to neutralize and dissolve the organic acids,
which are generated in oxygen – lignin reactions.
2611/03/10 EN • 12/2011
Introduction
Oxygen delignification is a process
between cooking and bleaching
sequences, where part of the residual
lignin left in pulp after cooking is
removed using oxygen and alkali.
Oxygen delignification is a direct
extension to delignification in cooking.
The targeted reactions are the oxidation
of lignin and breaking it down parts
which dissolve in alkali, as well as
destroying the coloured groups in
lignin and removal of impurities, such
as resin. Delignification with oxygen
is a more gentle way of reducing the
kappa number, than extended cooking.
It also lowers bleach plant effluents.
Therefore oxygen delignification system
is an environmental and an economic
investment.
2611/03/10 EN
O x y gen deli gni fic ation
Picture 2. Typical valves in oxygen delignification.
Oxygen delignification can be done in medium
or high consistency. A high consistency oxygen stage is more expensive and more difficult
to process than a medium consistency stage.
Therefore it is rarely used nowadays.
Medium consistency oxygen delignification can
be performed as a single- or two-stage system.
The number of stages depends on the kappa
number reduction required. A kappa number
reduction of less than 50% can be achieved for
softwood in a single reactor. Hardwood pulp
needs two reactors if a kappa number reduction
of over 35% is required.
Table 1 shows typical conditions in oxygen delignification.
Table 1. Conditions in oxygen delignification.
2 First stage
Second stage
temperature (°C)
80 - 85
90 - 105
pressure (bar)
7 - 10
3-5
retention time (min)
20 - 40
60 - 80
APPLICATION REPORT 12/11
Valves
Picture 2 shows typical valves in the oxygen delignification stage.
Oxygen delignification starts with a medium
consistency pump, which pumps the pulp to the
oxygen mixer. Medium consistency valve controls the pulp flow. Oxygen and pulp are mixed
in the mixer as a homogenous compound. That
compound is pumped to the first oxygen reactor
though a ball on-off valve.
Control valve controls the pulp flow from the first
reactor to the pump before second reactor. Sometimes steam is added in the system before that
pump. There is also on-off ball valve before and
control valve after the second oxygen reactor.
The pulp is pumped from the oxygen stage
discharge tank suitably diluted to the washing
equipment.
The washed pulp is then taken to the next process stage, which is usually a buffer storage
tower before final bleaching. The fraction of the
filtrate to be used for diluting the pulp going to the
oxygen stage is cooled to ensure the stock pump
operation. The rest of the filtrate is pumped as
wash water to the preceding brown stock washer.
2611/03/10 EN
O x ygen de li gni fic ation
Table 2 shows typical valve types and materials
(example materials are in brackets) in the oxygen delignification process.
Table 2. Valve types and materials.
Valve
Name
Type
Material
E.g.
1
MC control valve
V-port segment valve
CF8M (316 SS)
RE_A...AJJST
2
Bleaching agent control valve
V-port segment valve
CF8M
RAA...AS
3
Mixer on-off valve
Ball valve
CF8M
M1_A...AP
4
Control valve
V-port segment valve
CF8M
RE_A...AJJST
5
Pump on-off valve
Ball valve
CF8M
M1_A...AP
6
Control valve
V-port segment valve
CF8M
RE_A...AJJST
7
Discharge tank control valve
V-port segment valve
CF8M
RE_A...AJJST
8
Bleached and washed pulp
V-port segment or ball valve
CF8M
RE_A...AJJST
9
Filtrate valve
Triple eccentric disc valve
CF8M
L12A...AA
APPLICATION REPORT 12/11
3
APPLICATION REPORT
The information provided in this bulletin is advisory in nature, and is intended as a guideline only.
For specific circumstances and more detailed information, please consult with your local automation expert at Metso.
Metso Automation Inc.
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