The top ten factors in kraft pulp yield - The Kraft Pulping Course

Martin MacLeod
The top ten factors in kraft pulp yield
AbstrAct
Kraft pulp yield depends on a plethora of factors:
the nature of the wood and the quality
of the chips, the cooking recipe (especially
the key independent variables – alkali charge,
sulphidity, temperature, and kappa target),
the pulping equipment, and so on. Here, the
factors have been assembled into a “top ten”
list, and are assessed in terms of relative
importance, potential to influence yield values,
and contribution to practical knowledge
of how pulp yields can be improved. the ten
factors can be re-ordered at will, to rank the
magnitude of the yield changes they can produce,
for example, or to see which factors
have the highest potential for yield improvement
at modest cost.
What are the principal factors
affecting pulp yields in kraft mills? How
comprehensive is our understanding of
them? Are there practical ways to use existing
knowledge to improve yields?
To address these questions, here is a Top
Ten list (Fig. 1) of the key factors to consider,
followed by brief descriptions of why
each is important, what the size of the yield
gain might be, and how substantial and reliable
the information base is. The focus is
on practical opportunities for yield gains in
kraft mill operations, tying them to scientific
knowledge of cause-and-effect relationships.
The broad perspective is two-fold: how wood
and chemistry interact in the kraft pulping
process, and why uniformity of treatment
(whether chemical or mechanical) matters.
An anthology of papers on the subject of
kraft pulp yield is also available /1/.
The ten factors have been assembled in
the same order as fibrelines, i.e., from chips
through pulping to bleaching. The order can
be changed for different purposes, as will be
obvious later when they are ranked for magnitude
of potential yield gain and also for
what is practical to do at modest cost.
Wood species
Wood, an organic raw material, consists of
polysaccharides (cellulose and hemicelluloses),
lignin, and extractives. Their concentrations
vary substantially among commercial
wood species /2,3/: cellulose, approximately
1 Wood
species
( chemical
composition)
2 Wood
anatomy
( proportion
of
fibres)
3 Chip
size
distribution
4 Chip
quality
( other
than
size)
5 Pulping
chemistry
( conventional)
6 Modified/
advanced
pulping
chemistry
7 Mill
digester
systems
8 Beyond
pulping
9 Yield/
kappa
relationship
10 Wish
list
Fig. 1. These “top ten” factors in kraft pulp
yield are addressed in terms of their relative
importance, their potential magnitude, and their
reliability.
40–50% of wood; hemicelluloses, 25–35%;
lignin, 15–30%; extractives, 2–10%. The
higher the polysaccharide content (especially
cellulose) and the lower the amounts of
lignin and extractives, the higher will be the
yield of pulp from wood. Aspen is a leading
example – with lignin content often below
20% and (acetone) extractives below 3%,
it cooks rapidly to the highest bleachablegrade
kraft pulp yield in industrial practice,
typically about 55% at kappa 12. Western
red cedar, with an unusually high extractives
content, is at the low end of the spectrum,
providing a bleachable-grade pulp yield in
the low 40s at kappa 30 /4/.
In commercial kraft pulping practice
worldwide, the typical yield range
(unbleached pulp, in percent from wood)
is about mid-40s to mid-50s for
bleachable-grade hardwood pulps,
and about 40–50 with softwoods
(Fig. 2). We can widen the softwood
range to about 60% by including
linerboard basestock, the
high-kappa end of the kraft pulping
spectrum. It is also possible to
extend the lower limits of these
ranges by invoking the use of sawdust
or fines (or decayed wood of
any particle size).
Surprisingly for a worldwide
industry which has been in business
for many decades, there is
no simple, fast, and cheap way
Pulp Yield from Wood, %
60
55
50
45
40
15
0
Kraft
20
Pulp
Yie
40
Hardwoods
Softwoods
from
Wood,
%
to determine the gross chemical composition
of the wood in use.
The chemical composition of wood is
probably the primary variable in kraft pulp
yield. Fig. 3 shows normal yields in conventional
research-scale kraft pulping of species-pure
chips to bleachable-grade kappa
numbers versus their typical lignin contents
in the wood. This relationship makes reasonable
sense: the higher the lignin content
– which will be mostly removed in pulping
– the lower the pulp yield. It is remarkably
accurate over a yield range of 42–55%: Pulp
yield = - 0.69[Lignin] + 65.8 (r 2 = 0.95).
North American wood species are illustrated
in Fig. 3, but major commercial species
elsewhere in the world will conform to this
general picture.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
ld
60
80
100
Bleachable-grade
Fig. 2. On a global basis, bleachable-grade kraft
pulp yields from hardwoods and softwoods fall
into these ranges. The softwood range can be
extended to 60% by including unbleached kraft
paper and linerboard grades.
At
15
kappa/
HW
or
30
kappa/
SW
Aspen
Birch
20
Beech
Maple
Spruce
Jack
Pine
Loblolly
Pine
Balsam
Fir
E Larch
E Cedar
E Hemlock
25
30
1
1
35
Lignin
Content
in
Wood,
%
Fig. 3. There is a linear relationship between lignin content of
wood and probable yield of bleachable-grade kraft pulp.

Wood anatomy
The physical nature of wood also plays an
important role in yield. Large differences
exist among wood species, especially in
percentage of “fibres” (the preferred cell
type for papermaking) versus that of less
desirable cells (e.g., ray parenchyma in softwoods,
vessel elements in hardwoods) /5/.
This is compounded by large ranges in the
principal wood fibre dimensions: length,
diameter, and cell wall thickness /6/. For
example, loblolly pine kraft pulp fibres
can be five times longer than sugar maple
fibres. Further, there are dimensional differences
between earlywood and latewood,
and between juvenile and mature wood. Of
all of these, only fibre length distribution
is routinely measured in the kraft pulping
world.
From a papermaker’s perspective, a more
appropriate concept might be the yield of
papermaking fibres from wood. In this sense,
different wood species offer very different
potential yields. If only long, narrow fibres
are sought, for example, then softwoods
have a large advantage over hardwoods, in
which wood anatomy is much more diverse
(Fig. 4). But by acknowledging that hardwoods
inevitably contain significant
amounts of vessel elements, we can add
them back since they are part of the pulp
yield, bringing the hardwood cases much
closer to the softwood ones. Still, there is
a substantial amount of cell material in all
woods that is not ideal from a papermaking
standpoint.
We can generalize with the following
observations:
• The higher the percentage of long, narrow
fibres (as opposed to any other cell
types) in the wood raw material, the
more uniform will be the pulping, en-
Papermaking
0
20
Fibres
only
Sweetgum
Fibres,
% of
wood
40
Aspen
60
White
birch
80
c
Spruces
D Fir,
Pines
Fibres
and
vessel
elements
ontent
100
Fig. 4. If yield is defined on the basis of suitable
papermaking fibres, softwoods have an
advantage due to wood anatomy. Whether vessel
elements are considered “suitable” makes a
large difference in the hardwood results.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
2
hancing the yield of pulp which is ideal
for papermaking.
• The greater the range of wood cell types,
the wider will be the dimensional ranges
of length, width, and cell wall thickness
in the raw material before pulping, and
hence in the kraft pulp which is produced.
• The anatomy of hardwoods is much more
complex and – in some papermaking
ways – adverse than that of softwoods.
Chip size distribution
In chip size, two things are clear – thickness
is the principal dimension of concern
in kraft pulping, and 2–8 mm thick chips
are ideal /7/. Thickness distributions are
routinely measured in chip classifiers, and
modern chip thickness screening systems in
mills are capable of controlling the thickness
range reasonably well. Sadly, they often
don’t. Greater precision in chip making
would help, whether during sawmilling
operations or in log chipping. Undersized
“chips”, although they pulp rapidly, carry
a substantial yield penalty. With oversized
chips, the danger is in generating rejects,
inherently a penalty in mills producing
bleachable-grade pulp whether the rejects
are re-processed or are removed from the
fibreline. If small wood particles can go to
a dedicated, separate production line, and
overthick chips are processed mechanically
to make them more amenable to pulping,
significant yield gains can be obtained when
pulping only the properly-sized chips, on the
order of 1–2%.
Fig. 5 illustrates two thickness distributions
of same-species softwood chips on
final delivery to two kraft digesters. The
mill on the left achieves excellent control
from a chip thickness screening plant with
100
80
60
40
20
0
Total
Yield
%
< 2mm
0.
9
Chip
T
2-8mm
43.
7
45.
8
hickness,
% of
> 8mm
1.
2
100
80
60
40
20
0
total
mass
< 2mm
5.
7
2-8mm
33.
1
45.
1
disc screens and slicers. From pilot-plant
pulping, these chips gave 46% pulp yield
at 25 kappa when only the 2–8 mm fraction
(containing 95% of the total mass) was
cooked. Using mass fractions and reasonable
assumptions to calculate the fractional yields
shown in Fig. 5, the actual total yield from
this chip furnish was 45.8%.
The older mill on the right had rudimentary
chip screening and therefore a
much broader thickness distribution. At
25 kappa, the penalties with the undersized
and oversized fractions were more serious,
bringing the total yield down to 45.1%.
Note that with significantly less 2–8 mm
material present, its fractional yield was ten
percentage points lower.
A yield difference of 0.7% may seem
rather small, but at a pulp production rate
of 1000 tpd the older mill requires 12,000
t more wood (on an oven-dry basis) annually.
That can easily translate into a cost
increase of a million dollars or more a year.
The penalty will be worse when accounting
for wasted volume in the digester occupied
by overthick chips, higher alkali consumption,
greater knotter rejects recycling costs,
more shives going forward, less uniform
pulp, and higher bleaching costs.
A chip thickness screening plant is a
necessary part of a modern kraft pulp mill.
But simply buying and installing such a
plant is not enough – it must also be maintained,
tested periodically, adjusted, and
improved.
Chip quality (other than chip size)
Many yield-related considerations fall into
this category. In mixed-species chip furnishes,
the proportions of the species, each
with its own yield potential, will affect overall
pulp yield. Moisture content can influence
yield values if green wood
(rather than dry wood) is the basis
> 8mm
6.
3
Yield
= 0.
7%
= ~ 12,
000
t/
y more
wood
= ~ $ 1.
2 million/
y
Fig. 5. Maximizing the 2–8 mm fraction of a chip
thickness distribution can significantly improve
pulp yield.
3
for calculation; it can also affect the
efficiency of pulping if the “recipe”
changes (e.g., an unintentional
change in alkali charge due to an unseen
change in wood moisture might
penalize pulp yield). Mechanical
damage to wood fibres can make
them more susceptible to chemical
attack during pulping, lowering
yield. Biological decay, bark, or the
presence of biological knots and
overthick chips in chip furnishes all
impair pulp yield relative to fresh,
sound wood of suitable thickness
Any of these factors may represent
only a small yield penalty; together,
they may reduce pulp yield by
2–4%.

White
Birch
Screened Yield, %
Components
55
53
51
49
47
45
of
Pulp
Reference
Chips
Yie
PS-AQ
Process
Chemistry
ld
+ 3.
0
+ 1.
5
KRAFT
BASELINE
( Hanging
baskets,
mill
chips)
Gain
53.
8
Wood Species
+ 0.
5 . 5
53.
3
Pilot-Plant
Pulping
Best
Mill
Chips
+ 1.
5
+ 0.
5
Fig. 6. Many aspects of chip quality and pulping
practice offer substantial yield benefits,
including original wood quality, removal of
fines and oversized particles, and uniformity of
impregnation and cooking.
Total Yield, %
60
55
50
45
Effect
of
Southern
Pine
Alk
EA,
%
15.
0
17.
5
20.
0
ali
20 40
60
80
100
120
Kappa
Number
Charge
on
Pulp
A comprehensive examination of pulp
yield with respect to chip quality was part
of hanging basket experiments in a mill
trial to implement Paprilox ® polysulphideanthraquinone
pulping of hardwood in
conventional batch digesters at Domtar’s
Espanola, ON, kraft mill /8/: Four aspects
were measured (Fig. 6):
• Reference Chips: The removal of all bark,
knots, decayed wood, and heartwood
provided ideal chips for kraft pilot-plant
pulping, accounting for a 3% yield advantage
over the mill’s normal chips.
The reference chips were made from the
stemwood of middle-aged white birch
logs of uniform growth chosen at the
Espanola mill, and their thickness range
was 2–6 mm.
• Best Mill Chips: When only the 2–6 mm
thick fraction of mill chips was used in
pilot-plant experiments, a 0.5% yield
gain was measured relative to whole mill
chips, whether in kraft or PS-AQ pulping.
The mill chips had an average thickness
classification of 11% < 2 mm, 59%
2-6 mm, and 30% >6 mm. Obviously,
removing 41% of the raw material is
51.
8
51.
3
48.
3
46.
8
Fig. 7. Alkali charge plays a major role in pulp yield
– the higher the charge, the lower the yield, due to
increased susceptibility of the polysaccharides to
alkaline degradation.
Total Yield, %
60
55
50
45
Y
Mixed
Hardwoods
+ 5
ield
10 30
50
70
90
110
Kappa
Number
4
EA,
%
15.
0
17.
5
20.
0
5
not a practical thing to do,
but decreasing the overthick
fraction substantially
would help. Since the trial,
chip thickness screening and
overthick chip crushing have
been installed on the hardwood
side at Espanola.
• Pilot-Plant Pulping: Due
to good chip pre-steaming
practice, ideal temperature
control, and homogeneity of
impregnation and cooking
in small research digesters,
greater uniformity of pulping
resulted in a significant yield
advantage (1.5%) regardless
of whether reference chips or
mill chips were cooked.
• Wood Species: Species
analysis of basket pulps from
mill “birch” chips showed
that they actually contained
24% maple on average.
Taking maple as one-quarter
of the mass, and assigning
this fraction a 2% yield
penalty from wood relative to
white birch /4/, a 0.5% yield
deficit was calculated.
Overall, the four factors
illustrated here added up to a
potential yield gain of 5.5%,
whether associated with the
kraft baseline yield or with
the PS-AQ yield. Achieving best performance
in all of these factors significantly improves
pulp yield.
Conventional pulping chemistry
Among the primary independent variables
of kraft pulping, high alkali charge, low
sulfidity, high maximum temperature, and
high lignin content in the wood are the
most dangerous for inferior yield, potentially
reducing the value by several percentage
points. By contrast, the higher the
cellulose-to-hemicellulose ratio in the wood,
the better. Lower extractives content is also
desirable. Liquor-to-wood ratio can affect
yield in that it has a strong influence on
pulping rate, and therefore the time during
which the polysaccharides (especially
hemicelluloses) are degraded by alkaline
attack. Hardwood lignin is chemically different
from softwood lignin, and accounts
for part of the reason why hardwoods often
have higher pulp yields (and faster delignification
rates).
How the main independent variables of
kraft pulping affect kraft pulp yield is clearly
explained in Kleppe’s classic paper “Kraft
Pulping” /9/. Higher alkali charge decreases
pulp yield at a given kappa number, all other
factors held constant, both with softwoods
and hardwoods (Fig. 7). For every 1% increase
in effective alkali charge (NaOH basis)
with softwoods, there is a 0.15% penalty
in yield. The problem is three times worse
with hardwoods, due mainly to the higher
proportion of hemicelluloses (especially
xylans) and their susceptibility to alkaline
attack.
An independent example with kraft
pulping of aspen to 15 kappa showed these
results: total yield of 55.6% at 11% effective
alkali, 54.4 % yield at 13.5% EA, and
52.8% yield at 17% EA. Thus, an increase
of 6% effective alkali led to a yield loss of
2.7%, just as predicted (i.e., 6 x 0.45%).
Although not particularly important
in industrial kraft pulping (the majority of
which is done at or above 30% sulphidity),
how sulphidity affects yield is informative.
Again from Kleppe /9/, with birch (at a kappa
target of 25), the yield plateau at 54%
comes at 30% sulphidity. At 0% sulphidity,
pulp yield is about 50% instead, a deficit of
4%; note that pulping rate is much slower
as well. With pine at 55 kappa number, the
51% pulp yield plateau is at ~40% sulphid-
56
54
52
50
48
Effect
of
0
2h
3h
20
Sulphi
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
3h
10
Effect
of
Sulphidity
T
Total
yield,
%
2h
1h
Difference
in
TY,
%
Ascribed
to
chips
Ascribed
to
EA
30
dity,
%
on
Pulp
Birch
( kappa
25)
1h
40
Pine
( kappa
55)
50
Y
emperature
on
Pulp
ield
Fig. 8. Sulphidity has a minor effect, providing
that it is at the plateau level of 30% or above
(this is true for the majority of kraft mills).
Y
5
ield
Digester A Digester
B
45.
4
2.
1
- 0.
3
- 0.
1
Yield Loss
due
to
Tmax,
% 1.
7
43.
3
Fig. 9. Maximum temperature of cooking has a
major effect on pulp yield – although it speeds
up the delignification rate, it accelerates
polysaccharides degradation even more.
5

ity. At 0% sulphidity, pulp yield is 48%, a
deficit of 3%. Again, the pulping rate decreases
significantly with lower sulphidity.
In both cases, then, pulp yield is directly
related to sulphidity, but not in a linear
manner. Sulphidity needs to be at or above
30% for optimum yield and rate reasons.
The maximum temperature of pulping
is also important for yield. In the case shown
in Fig. 9, two chip furnishes from the same
wood species were being delivered to two
continuous digesters. They were pulped in
a pilot-plant digester at process conditions
taken from the two mill digesters (A: 18.5%
effective alkali, 163°C maximum; B: 19.1%
EA, 175°C max.). Case A had 86% 2-8 mm
chips and 7% > 8 mm chips; Case B, 79%
2–8 mm chips and 14% > 8 mm chips.
The difference in total yield at kappa
number 30 was 2.1%. When adjusted for
the differences attributable to chip thickness
distribution and applied effective alkali, the
yield deficit due to the 12°C higher maximum
temperature in Digester B was 1.7%.
Modified pulping chemistry
The era of modified kraft pulping (originally
called extended delignification) which
began in the 1980s was founded on chemical
principles intended to make kraft pulping
more selective for delignification over
polysaccharide degradation. Combined with
appropriate changes in mill digesters, some
yield benefits have accrued. Liquor displacement
batch systems can improve yield over
conventional batch systems (as measured by
hanging baskets) by 1–2% /10/. Continuous
digesters with multiple white liquor inputs
and black liquor extractions appear to offer
a yield advantage – particularly with
hardwoods – of up to 4% /11/. In general,
however, evidence for a universal yield benefit
with modified kraft pulping equipment
is scanty.
Modifying kraft pulping with additives
(e.g., anthraquinone, or polysulfide,
or both) can improve pulp yields by about
1–3%. The research knowledge is extensive
and deep /12/, and both additives have been
used for the past 30 years in mills scattered
around the world. An obvious advantage
with AQ is that it can work in all types of
kraft digesters – no equipment changes are
required. To achieve maximum benefits with
AQ, its strategy of use needs to be based
on optimizing all the key factors in kraft
delignification, including alkali charge, sulphidity,
and kappa target. Fig. 10 shows an
example /13/.
A recent implementation of PS-AQ
pulping of hardwoods demonstrated that
the change from kraft resulted in a yield gain
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
of about 2% whether measured by
hanging baskets in the mill or in
pilot-plant pulping using the chips
and cooking liquors from the mill
/8/ (see also Fig. 6).
Occasionally, an astonishing
possibility emerges, such as alkali
sulphite-AQ pulping /14,15/.
Although not in use industrially
because of its slow delignification
rate and complex chemical
recovery issues, AS-AQ pulping
can provide yield gains of 5–10%
(Fig. 11), depending on the scenario.
No other industrially-feasible
process chemistry change can
do better.
Mill digester systems
Digester equipment considerations
can have a big influence on
yield in kraft pulping. Especially
important are the chip pre-steaming
and liquor impregnation steps.
Advanced batch and continuous
digesters do an effective job of chip
pre-steaming by providing enough
contact time with atmospheric
steam (15+ minutes), but most digester
systems have either no deliberate
pre-steaming or not enough,
even when it is a combination
of atmospheric and low-pressure
regimes. When air removal and
Mixed
SW
water saturation of the inner void spaces
in wood chips are inadequate, the result is
a less-than-perfect liquid environment for
pulping, leading to more heterogeneous
delignification and inferior yield.
Good impregnation is always a key to
good kraft pulping. It needs to be long
enough (usually 30+ minutes) and at a low
enough temperature (120° ± 5°C) to ensure
that the liquid-phase chemistry is ready to
begin everywhere inside the chips when they
are taken to delignification temperature. Fig.
12 illustrates results from kraft pilot-plant
experiments on two softwood sawdust furnishes
from a mill operating M&D digesters
/16/. The M&D operations were simulated
by combining the sawdust and cooking liquor
in bombs and driving the temperature
to 185°C as fast as possible (~ 10 minutes).
Even when starting with tiny sawdust-sized
wood particles, plenty of rejects were generated.
But when we used conventional kraft
conditions designed for chips, including
a 90-minute ramp of 1°C/min to cooking
temperature for graceful impregnation,
the rejects decreased by about two-thirds,
meaning that the screened pulp yield rose
by 2%. This case shows that extreme im-
Yie
AA,
%
AQ,
%
H-factor
ld
Total Yield
Yield,
%
Gain
Yield
Gain,
%
with
Kraft
baseline
30
Kappa
17.
1
0
2350
42.
6
0
Anthraquinone
1 2
Add
AQ
Reduce
H
Fig. 10. Optimal anthraquinone’s effectiveness
as a kraft pulping additive depends on the
strategy of use vis-à-vis other primary variables
such as alkali charge and sulphidity.
Yie
ld
Gain
30
17.
1
0.
10
2000
43.
5
0.
9
with
Alkaline
pregnation conditions can carry a significant
yield penalty.
Pilot-plant experiments have also shown
that if chips are thoroughly pre-steamed and
impregnation with white liquor is done with
good temperature control, then bulk liquor
circulation through the cooking chip column
inside a steam-jacketed 20L digester
is not vital in producing kraft pulp of high
yield and quality. Forced liquor circulation
in mill digesters is a means to try to overcome
temperature and chemical concentration
gradients created during filling and
impregnation. It is no surprise that the best
liquor displacement batch digesters have
the lowest measured kappa variability inside
them /10/.
The era of modified kraft pulping has
fostered longer and slower delignification
in continuous digesters and more effective
impregnation in liquor displacement batch
digesters. Both provide an inherent advantage
in selectivity (although the main benefit
seems to be better preservation of cellulose
integrity).
Together, all of these factors can improve
pulp yields by several percentage points.
3
Add
AQ
Reduce
AA
30
15.
8
0.
10
2350
44.
8
2.
2
Sulphite-AQ
S W HW
Yield gain
( brownstock)
, %
6 10*
Kappa
number
+ 10
+ 6
Yield
gain
( bleached)
, %
Unbleached
brightness
5
6
gain,
% ISO
Bleaching
chemical
20
35
consumption
increase,
% ~ 30
~ 20
* From
aspen:
total
yield
of
65%
at
kappa
18
a world
record?
Fig. 11. Alkaline sulphite-AQ pulping offers
astounding yield gains over kraft, but the
process is burdened by slow a delignification
rate and chemical recovery is complex.
6
6

Screen Rejects, %
Pulp Yield, %
Lower
4.
0
3.
5
3.
0
2.
5
2.
0
1.
5
1.
0
0.
5
0
50
48
46
44
42
40
10
R
Kraft
M&D
Version
ejects
Yield beyond pulping
with
Better
Impregnation
+ 2.
0%
SY
A B
Yield/
Kappa
Kraft
Conventional
Version
Relatio
Theoretical
( lignin
only)
LR
for
5 points
TY= 0.
06kappa
+ 45.
2 r2=
0.
99
Oxygen
Delignification
LR
for
highest
5 points
TY= 0.
09kappa
+ 44.
2 r2=
0.
99
20
Kraft-AQ
M&D
Version
Fig. 12. Even with sawdust-sized wood particles,
inferior impregnation conditions lead to
excessive rejects; good pre-steaming and
conventional impregnation significantly reduce
rejects generation, translating it into higher
pulp yield.
nship
SW
sawdust
Beyond
Three main considerations apply here: the
chemical selectivity of oxygen delignification
and chlorine dioxide bleaching, the uniformity
of the fibrous pulp passing through
the chemical operations, and any physical
losses of fibres in the progression of operations
along a fibreline.
The yield losses accompanying oxygen
delignification and ECF bleaching are much
smaller than those in pulping, offering less
opportunity to improve yield substantially
by process changes. But attention is required
to avoid unnecessary mechanical degradation
of pulp fibres through these areas of a
mill’s fibreline so as not to lose yield solely
due to “leakage” of fibrous debris. Also,
any recycles of unacceptable fibrous ma-
7
+ 1.
8%
SY
Kraft-AQ
Conventional
Version
Pulping
Kraft
Pulping
LR
for
highest
4 points
TY= 0.
23kappa
+ 40.
1 r2=
0.
99
30
Kappa
Number
Fig. 13. The yield/kappa lines of kraft
pulping, oxygen delignification, and ECF
bleaching have progressively lower slopes,
hence greater selectivity for lignin removal
over polysaccharide degradation. The kraft
pulping and oxygen delignification lines enter
danger zones below about kappa 20 and 15,
respectively.
8
40
terials need to be minimized
– they are proof of inadequate
upstream process conditions,
they add to processing costs,
and they make the pulp less
uniform. Common examples
are knotter rejects (especially
from biological knots) being
recycled to digesters /17/, and
final screen rejects being refined
and recycled in bleachable-grade
mills.
It is instructive to examine
the yield/kappa relationships
of pulping, oxygen delignification,
and ECF bleaching
together. Fig. 13 provides a
generic softwood case.
For kraft pulping, the slope
of a softwood line to ~30 kappa
is 0.15 ± ~0.05; for hardwoods
to ~15 kappa, the slope
is the same. Both are straight
lines. With softwoods, the line
represents the bulk delignification
phase starting from about
100 kappa (the high-yield end
of kraft pulping), and is a fact
which can’t be changed easily.
The kraft case in Figure 13
is for a softwood with a pulp
yield of 47% at kappa 30.
With oxygen delignification,
the slope is about 0.10,
and extends down to perhaps kappa 15 before
beginning a steeper fall /18/. With final
lignin removal, in theory the slope is about
0.05; this is chemically close to what ECF
bleaching actually does. In all three cases,
lower slope means better selectivity during
lignin removal, the right direction for yield
enhancement.
Several aspects of yield/kappa relationships
need to be remembered:
• There are non-linear consequences
for yield when either
pulping or oxygen delignification
is taken below its practical
kappa limit where the
selectivity for lignin removal
is lost.
• The yield gap widens in favour
of oxygen delignification
over pulping as kappa
number decreases.
• Raising the kappa target of
pulping lifts the whole picture
to higher yield, notwithstanding
the higher cost of
removing residual lignin later
in the process line.
Pulp Yield, %
50
48
46
44
42
Yield/kappa relationship
The typical yield/kappa relationship for kraft
pulping (as illustrated in Fig. 13) requires
some caveats. There is, of course, a yield intercept
which is strongly related to wood species,
chip size, and pulping conditions. The
straight line represents the bulk delignification
phase, which covers almost the whole
kappa range of commercial kraft pulping
from high-kappa linerboard base stock to
bleachable-grades.
Fig. 14 amplifies the meaning of a specific
yield/kappa relationship. This is a
spruce/pine/fir case in which pilot-plant
kraft pulping of 2–8 mm thick chips was
done at five H-factors (the highest one was
duplicated). Because the fibre liberation
point with softwoods is at about kappa 40,
screened yield equals total yield at all but the
highest kappa level. Three linear regressions
can be calculated:
• For all six total yield values, total yield =
0.12(kappa) + 41.3 r 2 = 0.95
• For the highest four yields, total yield =
0.11(kappa) + 42.0 r 2 = 0.94
• For the lowest three yields, total yield =
0.22(kappa) + 38.9 r 2 = 0.98
This demonstrates that where you stop
kraft pulping has a significant effect on pulp
yield. For bleachable grades, the idea is to
aim for the end of the bulk delignification
phase without falling into the residual phase.
Being seduced by ever lower kappa numbers
prior to oxygen delignification or bleaching
has its price!
With hardwoods, only bleachable-grade
pulp is made, and the entire kappa range is
about 12–18, so there is much less room
for unintentional overpulping. The use of
an excessive alkali charge is the greater risk.
At the low-kappa end, the onset of the
residual delignification phase will begin to
increase the slope rapidly, sacrificing yield
Slope
of
ield/
Kappa
Line
LR
for
highest
4 points
TY= 0.
11kappa
+ 42.
0 r2=
0.
94
LR
for
lowest
3 points
TY= 0.
22kappa
+ 38.
9 r2=
0.
98
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
Y
Screened
Yield
LR
for
all
6 TY
points
TY= 0.
12kappa
+ 41.
3 r2=
0.
95
9
Total
Yield
40
15 25
35
45
55
Kappa
Number
Fig. 14. When a typical yield/kappa line for kraft pulping of
a softwood is separated into parts, it becomes clear that
seeking kappa targets below the high 20s inevitably sacrifices
yield by entering the residual delignification phase.

Lignin-free
trees
Extractives-free
trees
Hardwoods
Fig. 15. Substantial yield improvements would
come from all of these items. While the first
three remain intractable, the last two are
possible today.
despite the further slow decrease in kappa
number. Because the residual lignin is more
resistant to delignification while the polysaccharides
continue to degrade, the selectivity
of kraft pulping becomes progressively worse
– the slope of the line becomes steeper.
This relationship is a crucial aspect of
every kraft pulping scenario, and it should
be known for every mill operation. Often,
that is not the case. To obtain accurate numbers,
such information is determined in
research-scale pulping. It should be done
routinely when any significant changes are
made in chip furnishes and cooking recipes,
including any proposed use of pulping
additives.
Wish list
A Short
with
no
vessel
Wish
List
CTS
plants
which
perform
to
( and
receive
regular
audits)
lements
Although industrial kraft pulping practice
has changed slowly and incrementally over
the years, it is always useful to imagine how
it could be made better, and by how much.
Figure 15 lists some possibilities, from the
far-fetched to the practical:
• Lignin-free trees: In Factor 1, Fig. 3,
the linear regression suggests that the
lignin-free case has a Y-intercept of 66%,
e
specifications
Practical
working
knowledge
of
kraft
pulping
chemistry
a qualification
for
digester
operators
Magnitude
of
Change
Wood
species
SW
to
HW
14%
SW
to
SW
8%
HW
to
HW
AS-AQ
vs.
conventional
kraft
7%
SW
6%
HW
PS-AQ
vs.
conventional
kraft
10%
SW
3%
HW
3%
A dd
oxygen
delignification
2%
Improve
impregnation
a nd
cooking
uniformity
2%
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
1
10
Factors
Fig. 16. When ranked according to magnitude of
potential yield gain, the top ten factors emerge
in this order. Very few options offer individual
gains above 3%.
6
6
8
7
2
far higher than any kraft pulp yield currently
obtained commercially.
• Extractives-free trees: The same general
argument applies. Because there is no
great business in by-products from extractives
any more, it would be nice to
avoid dealing with extractives at all.
• Hardwoods without vessel elements:
The wood would be denser, providing
higher pulp yield per unit volume of
digester space, and the pulp would be
more uniform, allowing improvements
in stock refining, papermaking, coating,
and printing.
• Chip thickness screening: Most CTS
plants don’t come close to their original
specifications for segregating and
controlling chip dimensions, nor work
consistently well in cold-weather locations.
Overthick chip processing spans
the range from very good to abysmal
/17/.
• Working knowledge: Training of digester
operators is not as good as it should be
(especially in North America). There
is usually no certification of personal
knowledge of the chemistry of pulping,
so digesters tend to be treated foremost
as mechanical entities. Is this satisfactory
for the operation of chemically complex
systems worth upwards of $100 million
that produce tens of billions of dollars
worth of pulp per year? Standards are
much stricter in many other lines of
work, including regular continuing education
plus re-testing. Why not in our
business?
Having assembled this Top Ten list for
kraft pulping yield, it is possible to rank
the factors in a variety of ways. Fig. 16 does
this based on magnitude of yield gain. For
example, a bleachable-grade kraft swing
mill could gain 14% going from the lowest
softwood yield to the highest hardwood one
(Figs. 2 and 3). No mill has the wood basket
to do this. But in the northern boreal forest
zone, a 7–8% yield gain is routine when
going from spruces to aspen. The same
is true in hardwood mills going from
maples to aspen.
Alkaline sulphite-AQ pulping has
been done industrially, but only briefly
and confined to two mills. In the right
circumstances, its use in linerboard production
could be interesting from a yield
perspective. Unfortunately, slow pulping
rate and complex chemical recovery are
serious hurdles to overcome.
Most of the opportunities in Fig. 16
provide yield gains of 3% or less – not
so exciting, perhaps, but feasible and
operating in some mills. In fact, there
are a lot of opportunities which can deliver
1–3% yield gains: additives such as
anthraquinone and polysulphide, moving
to advanced modes of digester operation,
oxygen delignification (especially with a
higher kappa target after pulping), and close
attention to the quality of chips being fed
to a digester. It is also good to have a strong
command of existing knowledge and apply
it to the technical details of good kraft pulping
practice.
Enhanced yields can also come from better
chip making and dimensional control,
improved pre-steaming and impregnation
practices, cooking at lower temperatures
for longer times wherever possible, minimization
of rejects from pulping (and the
re-processing of them), efficient fibre spill
collection, and tight process control of oxygen
delignification and bleaching. Research
demonstrates that impressive, cumulative
yield gains are possible.
Finally, Fig. 17 is an attempt at reality
– what can you do in a kraft mill to improve
pulp yield at modest cost with the equipment
you have today? The items are listed
in order of increasing cost:
• Get out – and stay out – of the residual
delignification phase.
• Make your CTS plant perform to maximize
the 2–8 (or 9 or 10) mm thick
fraction. Minimize the fines going to
pulping, and deal effectively with the
(small) fraction of overthick material.
Buy or make chips with a narrower distribution
of thickness.
• Push continually to increase your best
species for yield. Know the real numbers
by species from R&D work done
on your wood sources.
• Make sure that your alkali charge and
maximum temperature of cooking don’t
creep too high, or your sulfidity too low.
Process creep can occur over the long
term, and current process targets may
lose their connections to the original
reasons for change.
Stay
out
of
Get
full
p
residual
d
elignification
phase
erformance
from
CTS
Optimize
for
best
Optimize
pulping
Add
AQ
Improve
Practical
To
Do
At
Modest
Cost
p
pecies
in
a
s
re-steaming,
p
m
lant
ixture
recipe
for
EA,
S,
Tmax
impregnation
regimes
Factors
Fig. 17. When ranked according to what is practical
to do at a modest cost, the top ten factors offer
plenty of opportunities for improvement.
1
9
3
5
6
7
2

• Anthraquinone? It is probably the simplest
quick fix for yield gain if you can
afford it. Don’t waste it by adding too
much, losing some of it in an early black
liquor extraction, or failing to recognize
trade-offs with other primary factors
such as alkali charge, sulphidity, and
kappa target.
• Do anything you can to improve chip
pre-steaming. Optimize impregnation
by ensuring that the ingredients you
put in your digester are the best you can
provide. Don’t exceed what the chemistry
can actually do.
• And if the opportunity comes, go to an
advanced batch or continuous digester
system and advanced oxygen delignification.
Happy kraft pulping!
References
1. Kraft Pulp Yield Anthology (CD-ROM), 100 published
papers, 1990–2001, TAPPI, Atlanta, GA.
2. Gullichsen, J.: Fiber Line Operations, in Chemical
Pulping, Volume 6A, Papermaking Science and
Technology, J. Gullichsen and H. Paulapuro, eds.,
TAPPI/Finnish Paper Engineers’ Association, Atlanta/Helsinki,
1999, Chapter 2, p. A27–28.
3. Process Variables, in Alkaline Pulping, Volume 5,
Pulp & Paper Manufacture Series, 3 rd edition,
184x133mm
Grace, T.M., Leopold, B., and Malcolm, E.W.,
eds., Joint Textbook Committee of the Paper
Industry, CPPA-TAPPI, Montreal/Atlanta, 1989,
Chapter 5, p. 82.
4. MacLeod, J.M.: Kraft Pulping: Connecting Theory to
Industrial Practice, Notes of PAPTAC Kraft Pulping
Course, Session 1, Pointe-Claire, QC, October
23–25, 2006 (Typical Yields of Kraft Pulps).
5. Hakkila, P.: Structure and Properties of Wood and
Woody Biomass, Volume 2, Papermaking Science
and Technology, J. Gullichsen and H. Paulapuro,
eds., TAPPI/Finnish Paper Engineers’ Association,
Atlanta/Helsinki, 1998, Chapter 4, p.143.
6. ibid., p.141–150.
7. Process Variables, in Alkaline Pulping, Volume 5,
Pulp & Paper Manufacture Series, 3 rd edition,
Grace, T.M., Leopold, B., and Malcolm, E.W.,
eds., Joint Textbook Committee of the Paper
Industry, CPPA-TAPPI, Montreal/Atlanta, 1989,
Chapter 5, p. 90–96.
8. MacLeod, J.M., Radiotis, T., Uloth, V.C., Munro,
F.C., Tench, L.: Basket cases IV: Higher yield with
Paprilox ® polysulphide-AQ pulping of hardwoods,
new Tappi J 1(8):3 (2002).
9. Kleppe, P.J.: Kraft Pulping, Tappi J 53(1):35
(1970).
10. Tikka, P.O., Kovasin, K.K.: Displacement vs. conventional
batch kraft pulping: delignification
patterns and pulp strength delivery, Paperi ja Puu
72(8):773 (1990).
11. Lebel, D.J.: Continuous Digester Operations,
Notes of PAPTAC Kraft Pulping Course, Session
3, Pointe-Claire, QC, October 23-25, 2006 (Lo-
Solids ® Pulping).
12. Anthraquinone Pulping: a TAPPI PRESS Anthol-
ogy of Published Papers, G. Goyal, ed., TAPPI,
Atlanta, GA, 1997, 600 pages.
13. MacLeod, J.M.: Improving kraft pulp yield with
anthraquinone and polysulphide: science and strategy,
2002 Kraft Pulp Yield Workshop Preprints,
TAPPI, Atlanta, GA, Session 6, Paper 6-1.
14. MacLeod, J.M.: Alkaline Sulphite-Anthraquinone
Pulps from Softwoods, J Pulp Paper Sci 13(2):J44
(1987).
15. MacLeod, J.M.: Alkaline sulphite-anthraquinone
pulps from aspen, Tappi J 69(8):106 (1986).
16. MacLeod, J.M., Kingsland, K.A.: Kraft-AQ pulping
of sawdust, Tappi J 73(1):191 (1990).
17. MacLeod, J.M., Dort, A., Young, J., Smith, D., Kreft,
K., Tremblay, M.-A., Bissette, P.-A.: Crushing: Is
this any way to treat overthick softwood chips for
kraft pulping? Pulp Paper Can 106(2):44 (2005).
18. Gullichsen, J.: Fibre Line Operations, in Chemical
Pulping, Volume 6A, Papermaking Science and
Technology, J. Gullichsen and H. Paulapuro, eds.,
TAPPI/Finnish Paper Engineers’ Association, Atlanta/Helsinki,
1999, Chapter 2, p. A146.
Martin MacLeod is a teacher, writer, and technical consult-
ant on kraft pulping. He can be reached at: 150 sawmill
Private, Ottawa, ON K1V 2E1 canada; phone + 1 613 526-
4798; e-mail the.macleods@sympatico.ca. this paper was
adapted from a presentation at the tAPPI Growing Pulp
Yield from the Ground Up symposium, Atlanta, GA, May
17, 2006.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007