Martin MacLeod
The top ten factors in kraft pulp yield
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
1
Chip quality (other than size)
5
Pulping chemistry (conventional)
6
Kraft Pulp Yield from Wood, %
Modified/advanced pulping chemistry
7
Mill digester systems
8
0
20
40
60
80
100
Beyond pulping
9
Yield/kappa relationship
H a rd wo ods
Bleachable-grade
10
So ft wo ods
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.
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.
40–50% of wood; hemicelluloses, 25–35%; to determine the gross chemical composilignin, 15–30%; extractives, 2–10%. The tion of the wood in use.
higher the polysaccharide content (especialThe chemical composition of wood is
ly cellulose) and the lower the amounts of probably the primary variable in kraft pulp
lignin and extractives, the higher will be the yield. Fig. 3 shows normal yields in convenyield of pulp from wood. Aspen is a leading tional research-scale kraft pulping of speexample – with lignin content often below cies-pure chips to bleachable-grade kappa
20% and (acetone) extractives below 3%, numbers versus their typical lignin contents
it cooks rapidly to the highest bleachable- in the wood. This relationship makes reagrade kraft pulp yield in industrial practice, sonable sense: the higher the lignin content
typically about 55% at kappa 12. Western – which will be mostly removed in pulping
red cedar, with an unusually high extractives – the lower the pulp yield. It is remarkably
content, is at the low end of the spectrum, accurate over a yield range of 42–55%: Pulp
providing a bleachable-grade pulp yield in yield = - 0.69[Lignin] + 65.8 (r2 = 0.95).
the low 40s at kappa 30 /4/.
North American wood species are illustratIn commercial kraft pulping prac- ed in Fig. 3, but major commercial species
tice worldwide, the typical yield range elsewhere in the world will conform to this
(unbleached pulp, in percent from wood) general picture.
is about mid-40s to mid-50s for
bleachable-grade hardwood pulps,
At 15 kappa/HW
1
or 30 kappa/SW
and about 40–50 with softwoods
6
0
(Fig. 2). We can widen the softAspen
wood range to about 60% by in55
cluding linerboard basestock, the
Birch
Beech
high-kappa end of the kraft pulpMaple
50
Spruce
ing spectrum. It is also possible to
Jack Pine
extend the lower limits of these
Balsam Fir
Loblolly Pine
45
E Hemlock
ranges by invoking the use of sawE Larch
E Cedar
dust or fines (or decayed wood of
40
any particle size).
15
20
25
30
35
Surprisingly for a worldwide
Lignin Content in Wood, %
industry which has been in business for many decades, there is Fig. 3. There is a linear relationship between lignin content of
no simple, fast, and cheap way wood and probable yield of bleachable-grade kraft pulp.
Pulp Yield from Wood, %
Abstract
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
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-
2
Papermaking Fibres, % of wood content
0
20
60
40
80
100
Spruces
D Fir, Pines
Fibres
only
White birch
Aspen
Sweetgum
Fibres and
vessel elements
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.
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
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
for calculation; it can also affect the
3
Chip Thickness, % of total mass
efficiency of pulping if the “recipe” changes (e.g., an unintentional
100
100
change in alkali charge due to an un80
80
seen change in wood moisture might
60
60
penalize pulp yield). Mechanical
40
40
damage to wood fibres can make
20
20
them more susceptible to chemi0
0
cal attack during pulping, lowering
>8mm
<2mm
<2mm
2-8mm
>8mm
2-8mm
yield. Biological decay, bark, or the
43.7
0.9
1.2
5.7
6.3
33.1
Total
Yield
presence of biological knots and
45.8
45.1
%
overthick chips in chip furnishes all
Yield = 0.7% = ~ 12,000 t/y more wood = ~ $1.2 million/y
impair pulp yield relative to fresh,
sound wood of suitable thickness
Fig. 5. Maximizing the 2–8 mm fraction of a chip
Any of these factors may represent
thickness distribution can significantly improve
only a small yield penalty; togethpulp yield.
er, they may reduce pulp yield by
2–4%.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
not a practical thing to do,
but decreasing the overthick fraction substantially
White Birch
55
would help. Since the trial,
53.8
+
5
.
5
Wood Species +0.5
chip thickness screening and
53.3
53
Pilot-Plant Pulping +1.5
overthick chip crushing have
51.8
Best Mill Chips +0.5
51.3
been installed on the hard51
wood side at Espanola.
Reference Chips +3.0
49
• Pilot-Plant Pulping: Due
48.3
to good chip pre-steaming
PS-AQ Process Chemistry +1.5
47
practice, ideal temperature
46.8
KRAFT BASELINE
(Hanging baskets, mill chips)
control, and homogeneity of
45
impregnation and cooking
in small research digesters,
Fig. 6. Many aspects of chip quality and pulping
greater uniformity of pulping
practice offer substantial yield benefits,
resulted in a significant yield
including original wood quality, removal of
advantage (1.5%) regardless
fines and oversized particles, and uniformity of
of whether reference chips or
impregnation and cooking.
mill chips were cooked.
5
• Wood Species: Species
analysis of basket pulps from
Effect of Alkali Charge on Pulp Yield
mill “birch” chips showed
60
60
Mixed
Southern
that they actually contained
Hardwoods
Pi n e
24% maple on average.
55
55
Taking maple as one-quarEA, %
50
EA, %
50
5
.
1
0
ter of the mass, and assign15.0
17.5
17.5
45
45
20.0
ing this fraction a 2% yield
20.0
penalty from wood relative to
40
120
60
80
100
20
70
10
50
30
90
110
white birch /4/, a 0.5% yield
Kappa Number
Kappa Number
deficit was calculated.
Overall, the four factors
Fig. 7. Alkali charge plays a major role in pulp yield
illustrated here added up to a
– the higher the charge, the lower the yield, due to
potential yield gain of 5.5%,
increased susceptibility of the polysaccharides to
whether associated with the
alkaline degradation.
kraft baseline yield or with
A comprehensive examination of pulp the PS-AQ yield. Achieving best performyield with respect to chip quality was part ance in all of these factors significantly imof hanging basket experiments in a mill proves pulp yield.
trial to implement Paprilox® polysulphideanthraquinone pulping of hardwood in Conventional pulping chemistry
conventional batch digesters at Domtar’s Among the primary independent variables
Espanola, ON, kraft mill /8/: Four aspects of kraft pulping, high alkali charge, low
sulfidity, high maximum temperature, and
were measured (Fig. 6):
• Reference Chips: The removal of all bark, high lignin content in the wood are the
knots, decayed wood, and heartwood most dangerous for inferior yield, potenprovided ideal chips for kraft pilot-plant tially reducing the value by several perpulping, accounting for a 3% yield ad- centage points. By contrast, the higher the
vantage over the mill’s normal chips. cellulose-to-hemicellulose ratio in the wood,
The reference chips were made from the the better. Lower extractives content is also
stemwood of middle-aged white birch desirable. Liquor-to-wood ratio can affect
logs of uniform growth chosen at the yield in that it has a strong influence on
Espanola mill, and their thickness range pulping rate, and therefore the time during which the polysaccharides (especially
was 2–6 mm.
• Best Mill Chips: When only the 2–6 mm hemicelluloses) are degraded by alkaline
thick fraction of mill chips was used in attack. Hardwood lignin is chemically difpilot-plant experiments, a 0.5% yield ferent from softwood lignin, and accounts
gain was measured relative to whole mill for part of the reason why hardwoods often
chips, whether in kraft or PS-AQ pulp- have higher pulp yields (and faster deligniing. The mill chips had an average thick- fication rates).
How the main independent variables of
ness classification of 11% < 2 mm, 59%
2-6 mm, and 30% >6 mm. Obviously, kraft pulping affect kraft pulp yield is clearly
removing 41% of the raw material is explained in Kleppe’s classic paper “Kraft
4
Total Yield, %
Total Yield, %
Screened Yield, %
Components of Pulp Yield Gain
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% sulphid5
Effect of Sulphidity on Pulp Yield
56
Birch (kappa 25)
54
1h
2h
52
3h
Pine (kappa 55)
1h
50
2h
3h
48
0
10
20
30
40
50
Sulphidity, %
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).
5
Effect of Temperature on Pulp Yield
Total yield, %
Digester A
Digester B
45.4
43.3
Difference in TY, %
Ascribed to chips
Ascribed to EA
Yield Loss due to Tmax, %
2 .1
- 0.3
- 0.1
1 .7
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.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
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
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
6
Yield Gain with Anthraquinone
1
2
3
Kraft baseline
Add AQ
Reduce H
Add AQ
Reduce AA
30
30
30
AA, %
AQ, %
H-factor
17.1
0
2350
17.1
0.10
2000
15.8
0.10
2350
Yield, %
42.6
43.5
44.8
0
0.9
2 .2
Mixed SW
Total Yield
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%.
Kappa
Yi el d Gain , %
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.
6
Yield Gain with Alkaline Sulphite-AQ
Digester equipment considerations can have a big influence on
yield in kraft pulping. Especially
HW
SW
6
10*
important are the chip pre-steam- Yield gain (brownstock), %
Kappa number
+10
+6
ing and liquor impregnation steps. Yield gain (bleached), %
5
6
Advanced batch and continuous Unbleached brightness
35
gain, % I SO
20
digesters do an effective job of chip
Bleaching chemical
pre-steaming by providing enough
consumption increase, %
~20
~30
contact time with atmospheric
steam (15+ minutes), but most di- * From aspen: total yield of 65% at kappa 18 a world record?
gester systems have either no delib- Fig. 11. Alkaline sulphite-AQ pulping offers
erate pre-steaming or not enough, astounding yield gains over kraft, but the
even when it is a combination process is burdened by slow a delignification
of atmospheric and low-pressure rate and chemical recovery is complex.
regimes. When air removal and
water saturation of the inner void spaces pregnation conditions can carry a significant
in wood chips are inadequate, the result is yield penalty.
a less-than-perfect liquid environment for
Pilot-plant experiments have also shown
pulping, leading to more heterogeneous that if chips are thoroughly pre-steamed and
delignification and inferior yield.
impregnation with white liquor is done with
Good impregnation is always a key to good temperature control, then bulk liquor
good kraft pulping. It needs to be long circulation through the cooking chip colenough (usually 30+ minutes) and at a low umn inside a steam-jacketed 20L digester
enough temperature (120° ± 5°C) to ensure is not vital in producing kraft pulp of high
that the liquid-phase chemistry is ready to yield and quality. Forced liquor circulation
begin everywhere inside the chips when they in mill digesters is a means to try to overare taken to delignification temperature. Fig. come temperature and chemical concen12 illustrates results from kraft pilot-plant tration gradients created during filling and
experiments on two softwood sawdust fur- impregnation. It is no surprise that the best
nishes from a mill operating M&D digesters liquor displacement batch digesters have
/16/. The M&D operations were simulated the lowest measured kappa variability inby combining the sawdust and cooking liq- side them /10/.
uor in bombs and driving the temperature
The era of modified kraft pulping has
to 185°C as fast as possible (~ 10 minutes). fostered longer and slower delignification
Even when starting with tiny sawdust-sized in continuous digesters and more effective
wood particles, plenty of rejects were gener- impregnation in liquor displacement batch
ated. But when we used conventional kraft digesters. Both provide an inherent advanconditions designed for chips, including tage in selectivity (although the main benefit
a 90-minute ramp of 1°C/min to cook- seems to be better preservation of cellulose
ing temperature for graceful impregnation, integrity).
the rejects decreased by about two-thirds,
Together, all of these factors can improve
meaning that the screened pulp yield rose pulp yields by several percentage points.
by 2%. This case shows that extreme im-
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
terials need to be minimized Yield/kappa relationship
– they are proof of inadequate The typical yield/kappa relationship for kraft
upstream process conditions, pulping (as illustrated in Fig. 13) requires
4.0
they add to processing costs, some caveats. There is, of course, a yield inSW sawdust
3.5
and they make the pulp less tercept which is strongly related to wood spe3.0
uniform. Common examples cies, chip size, and pulping conditions. The
2.5
are knotter rejects (especially straight line represents the bulk delignifica2.0
+1.8% SY
+2.0% SY
from biological knots) being tion phase, which covers almost the whole
1.5
A
B
recycled to digesters /17/, and kappa range of commercial kraft pulping
1.0
0.5
final screen rejects being re- from high-kappa linerboard base stock to
0
fined and recycled in bleach- bleachable-grades.
Kraft-AQ
Kraft
Kraft
Kraft-AQ
Conventional
M&D
Conventional
M&D
able-grade mills.
Fig. 14 amplifies the meaning of a speVersion
Version
Version
Version
It is instructive to examine cific yield/kappa relationship. This is a
the yield/kappa relationships spruce/pine/fir case in which pilot-plant
Fig. 12. Even with sawdust-sized wood particles,
of pulping, oxygen deligni- kraft pulping of 2–8 mm thick chips was
inferior impregnation conditions lead to
fication, and ECF bleaching done at five H-factors (the highest one was
excessive rejects; good pre-steaming and
together. Fig. 13 provides a duplicated). Because the fibre liberation
conventional impregnation significantly reduce
generic softwood case.
point with softwoods is at about kappa 40,
rejects generation, translating it into higher
For kraft pulping, the slope screened yield equals total yield at all but the
pulp yield.
of a softwood line to ~30 ka- highest kappa level. Three linear regressions
ppa is 0.15 ± ~0.05; for hard- can be calculated:
8
woods to ~15 kappa, the slope • For all six total yield values, total yield =
Yield/Kappa Relationship Beyond Pulping
is the same. Both are straight
0.12(kappa) + 41.3
r2 = 0.95
50
Theoretical (lignin only)
lines. With softwoods, the line • For the highest four yields, total yield =
LR for 5 points
TY=0.06kappa + 45.2 r =0.99
represents the bulk delignifica0.11(kappa) + 42.0
r2 = 0.94
48
tion phase starting from about • For the lowest three yields, total yield =
46
100 kappa (the high-yield end
0.22(kappa) + 38.9
r2 = 0.98
Kraft Pulping
of
kraft
pulping),
and
is
a
fact
This
demonstrates
that
where
you stop
44
LR for highest 4 points
TY=0.23kappa + 40.1 r =0.99
which can’t be changed eas- kraft pulping has a significant effect on pulp
Oxygen Delignification
42
ily. The kraft case in Figure 13 yield. For bleachable grades, the idea is to
LR for highest 5 points
TY=0.09kappa + 44.2 r =0.99
is for a softwood with a pulp aim for the end of the bulk delignification
40
yield of 47% at kappa 30.
phase without falling into the residual phase.
30
40
20
10
With oxygen delignifica- Being seduced by ever lower kappa numbers
Kappa Number
tion, the slope is about 0.10, prior to oxygen delignification or bleaching
and extends down to perhaps kappa 15 be- has its price!
Fig. 13. The yield/kappa lines of kraft
fore beginning a steeper fall /18/. With final
With hardwoods, only bleachable-grade
pulping, oxygen delignification, and ECF
lignin removal, in theory the slope is about pulp is made, and the entire kappa range is
bleaching have progressively lower slopes,
0.05; this is chemically close to what ECF about 12–18, so there is much less room
hence greater selectivity for lignin removal
bleaching actually does. In all three cases, for unintentional overpulping. The use of
over polysaccharide degradation. The kraft
lower slope means better selectivity during an excessive alkali charge is the greater risk.
pulping and oxygen delignification lines enter
lignin removal, the right direction for yield
At the low-kappa end, the onset of the
danger zones below about kappa 20 and 15,
enhancement.
residual delignification phase will begin to
respectively.
Several aspects of yield/kappa relation- increase the slope rapidly, sacrificing yield
ships need to be remembered:
Yield beyond pulping
• There are non-linear conse9
Slope of Yield/Kappa Line
quences for yield when either
Three main considerations apply here: the
pulping or oxygen delignifichemical selectivity of oxygen delignification
LR for highest 4 points
50
T Y = 0 . 1 1 k a p p a + 4 2 . 0 r 2 = 0 .9 4
cation is taken below its pracand chlorine dioxide bleaching, the uniLR for lowest 3 points
tical kappa limit where the
formity of the fibrous pulp passing through
T Y = 0 . 2 2 k a p p a + 3 8 . 9 r 2 = 0 .9 8
48
selectivity for lignin removal
the chemical operations, and any physical
46
is lost.
losses of fibres in the progression of operaTotal
• The yield gap widens in fations along a fibreline.
Screened
Yield
44
Yield
vour of oxygen delignificaThe yield losses accompanying oxygen
LR for all 6 TY points
tion over pulping as kappa
delignification and ECF bleaching are much
42
T Y = 0 . 1 2 k a p p a + 4 1 . 3 r 2 = 0 .9 5
number decreases.
smaller than those in pulping, offering less
40
opportunity to improve yield substantially • Raising the kappa target of
15
25
35
45
55
pulping lifts the whole picby process changes. But attention is required
Kappa Number
ture to higher yield, notwithto avoid unnecessary mechanical degradastanding the higher cost of Fig. 14. When a typical yield/kappa line for kraft pulping of
tion of pulp fibres through these areas of a
removing residual lignin later a softwood is separated into parts, it becomes clear that
mill’s fibreline so as not to lose yield solely
in the process line.
due to “leakage” of fibrous debris. Also,
seeking kappa targets below the high 20s inevitably sacrifices
any recycles of unacceptable fibrous mayield by entering the residual delignification phase.
7
Screen Rejects, %
Lower Rejects with Better Impregnation
Pulp Yield, %
2
2
Pulp Yield, %
2
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
10
A Short Wish List
Lignin-free trees
Extractives-free trees
Hardwoods with no vessel elements
CTS plants which perform to specifications
(and receive regular audits)
Practical working knowledge of kraft pulping
chemistry a qualification for digester operators
Fig. 15. Substantial yield improvements would
come from all of these items. While the first
three remain intractable, the last two are
possible today.
Magnitude of Change
Wood species
SW to HW
S W to SW
H W t o HW
AS-AQ vs. conventional kraft
SW
HW
PS-AQ vs. conventional kraft
SW
HW
A dd o xy ge n de l ign ifi c ati on
Improve impregnation
a n d c ook ing uni for mi ty
Factors
1
2
14%
8%
7%
6
6%
10%
6
3%
3%
2%
2%
8
7
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%.
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
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%,
far higher than any kraft pulp yield cur- are a lot of opportunities which can derently obtained commercially.
liver 1–3% yield gains: additives such as
• Extractives-free trees: The same general anthraquinone and polysulphide, moving
argument applies. Because there is no to advanced modes of digester operation,
great business in by-products from ex- oxygen delignification (especially with a
tractives any more, it would be nice to higher kappa target after pulping), and close
avoid dealing with extractives at all.
attention to the quality of chips being fed
• Hardwoods without vessel elements: to a digester. It is also good to have a strong
The wood would be denser, providing command of existing knowledge and apply
higher pulp yield per unit volume of it to the technical details of good kraft pulpdigester space, and the pulp would be ing practice.
more uniform, allowing improvements
Enhanced yields can also come from betin stock refining, papermaking, coating, ter chip making and dimensional control,
and printing.
improved pre-steaming and impregnation
• Chip thickness screening: Most CTS practices, cooking at lower temperatures
plants don’t come close to their origi- for longer times wherever possible, mininal specifications for segregating and mization of rejects from pulping (and the
controlling chip dimensions, nor work re-processing of them), efficient fibre spill
consistently well in cold-weather loca- collection, and tight process control of oxytions. Overthick chip processing spans gen delignification and bleaching. Research
the range from very good to abysmal demonstrates that impressive, cumulative
/17/.
yield gains are possible.
• Working knowledge: Training of digester
Finally, Fig. 17 is an attempt at reality
operators is not as good as it should be – what can you do in a kraft mill to improve
(especially in North America). There pulp yield at modest cost with the equipis usually no certification of personal ment you have today? The items are listed
knowledge of the chemistry of pulping, in order of increasing cost:
so digesters tend to be treated foremost • Get out – and stay out – of the residual
as mechanical entities. Is this satisfactory
delignification phase.
for the operation of chemically complex • Make your CTS plant perform to maxisystems worth upwards of $100 million
mize the 2–8 (or 9 or 10) mm thick
that produce tens of billions of dollars
fraction. Minimize the fines going to
worth of pulp per year? Standards are
pulping, and deal effectively with the
much stricter in many other lines of
(small) fraction of overthick material.
work, including regular continuing edBuy or make chips with a narrower disucation plus re-testing. Why not in our
tribution of thickness.
business?
• Push continually to increase your best
Having assembled this Top Ten list for
species for yield. Know the real numkraft pulping yield, it is possible to rank
bers by species from R&D work done
the factors in a variety of ways. Fig. 16 does
on your wood sources.
this based on magnitude of yield gain. For • Make sure that your alkali charge and
example, a bleachable-grade kraft swing
maximum temperature of cooking don’t
mill could gain 14% going from the lowest
creep too high, or your sulfidity too low.
softwood yield to the highest hardwood one
Process creep can occur over the long
(Figs. 2 and 3). No mill has the wood basket
term, and current process targets may
to do this. But in the northern boreal forlose their connections to the original
est zone, a 7–8% yield gain is routine when
reasons for change.
going from spruces to aspen. The same
Practical To Do At Modest Cost
is true in hardwood mills going from
Factors
maples to aspen.
9
Alkaline sulphite-AQ pulping has Stay out of residual delignification phase
been done industrially, but only briefly Get full performance from CTS plant
3
and confined to two mills. In the right
1
2
circumstances, its use in linerboard pro- Optimize for best species in a mixture
duction could be interesting from a yield Optimize pulping recipe for EA, S, Tmax
5
perspective. Unfortunately, slow pulping
6
rate and complex chemical recovery are Add AQ
serious hurdles to overcome.
7
Improve pre-steaming, impregnation regimes
Most of the opportunities in Fig. 16
provide yield gains of 3% or less – not Fig. 17. When ranked according to what is practical
so exciting, perhaps, but feasible and to do at a modest cost, the top ten factors offer
operating in some mills. In fact, there plenty of opportunities for improvement.
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007
• 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, 3rd 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. 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, 3rd 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 (LoSolids® 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 consultant on kraft pulping. He can be reached at: 150 Sawmill
Private, Ottawa, ON K1V 2E1 Canada; phone + 1 613 5264798; e-mail [email protected]. This paper was
adapted from a presentation at the TAPPI Growing Pulp
Yield from the Ground Up Symposium, Atlanta, GA, May
17, 2006.
184x133mm
Paperi ja Puu – Paper and Timber Vol.89/No. 4/2007