Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02230315 1998-02-17
BACKGROUND OF THE INVENTION
22
In accordance with the invention ;3 bright mechanical pulp is produced from
lignocellulosic
24 material, for example wood chips , by subjecting the material to a process
in which it is
steamed, impregnated with a chelating agent, sodium perborate or peracetic
acid, preheated
26 and then subjected to mechanical force in a process called "refining" to
defibre the '
impregnated wood chip in the form of pulp and the pulp is discharged from the
refiner at a pH
28 above 5. 7'he process produces a bright pulp of at least 14 to 20 ISO
points gain in the refiner
CA 02230315 1998-02-17
stage only. Higher brightness has been achieved by carrying out the process
preferably by
2 passing the pulp successively through a primary refiner at elevated pressure
and then a bleach
tower, a solution containing oxidising chemicals being fed to each.
4 The usual way to prepare mechanical pulp is by defibrating wood chips in a
refiner
under steam pressure at high temperature without significant loss of chemical
components in
6 wood chips by optionally adding small quantity of hydrogen peroxide and
alkali. The
resulting pulps, while quite strong, are highly coloured probably due to the
colored
8 chromophores in lignin. Bright papers from this pulp is prepared by
modifying the colored
chromophores in a process called "tdeaching". This process when carried out in
a refiner is
called "refiner bleaching" and the same process when carried out in a tank is
called "tower
bleaching". Such bleaching process is usually carried out in a bleaching tower
after refining
12 by treatment with hydrogen peroxide: and /or other oxidative or reductive
chemicals, which
changes the chromophores to non-coloured products.
14 Recently alternative processes to improve brightness of pulp during the
refining and
subsequent tower bleaching by treating with oxidising and reducing agents have
been
16 suggested to reduce energy cost and improve optical properties of pulp. For
a number of
reasons, well known to those in the art, hydrogen peroxide and peracetic acid
have proven to
18 be of particular interest which are intended to brighten pulp efficiently
in presence of alkali.
In refining; process the usual temperature is very high (temperature greater
than 110°C for a
short exposure time) which favorisea decomposition of low temperature reactive
oxidants
such as peracetic acid and hydrogen peroxide and efficient brightening
reaction of generated
22 perhydroxyl ion with chromophore is difficult to attain without a
significant loss of these
chemicals by decomposition.
24 Addition of hydrogen peroxide and. sodium hydrosulfite during pulping
process in a refiner
for lignocellulosic material have been experimented with in the past but, as
far as is known,
26 none of those chemicals could show improved brightness efficiency over
conventional tower
bleaching. One of the reasons may be their fast decomposition reactions
catalyzed by the
28 presence of heavy metals such as manganese, iron and copper. For this
reason in all
2
CA 02230315 1998-02-17
conventional tower bleaching process the bleaching temperature used are
usually less than
2 90°C and even lower in case of mechanical pulp. In refiner bleaching
where the reaction
conditions such as temperature, time, chemical composition etc. are different
from that used
4 for conventional tower bleaching, it is not unexpected to have difference in
the efficiency of
the same hleaching chemicals in the two different processes. The prior art has
not recognised
6 the importance of addition of sodimm perborate in refiner. Sodium perborate
which is an
effective ,generator of hydrogen peroxide is very effective at higher
temperature (usually
8 above 80~~C and preferably above 100°C). Oxidative refiner bleaching
may, therefore, be
conducted) under the pulp refining condition which is usually at high
temperature and pressure
(usually above 120° C) for a shorter cycle than presently employed in
the conventional tower
bleaching..
12 As far a.s is known sodium perborate has never been used in refiner
bleaching of pulp. It
has two excellent advantages for application in refiner bleaching which are
the high
14 decomposition temperature of perb~orate and that it does not require alkali
to generate
hydrogen peroxide. It has been found that under the condition of refining
where the
16 temperature is above 100°C, incorporating sodium perborate without
alkali is effective in
brightening pulp and it improves pulp brightening efficiency over hydrogen
peroxide for an
18 equivalent: concentration of oxidant.
Peracetic acid generation using sodium perborate or hydrogen peroxide and
activators is
also a well known art and the process is in commercial operation in the
detergent industry.
Main adv;~ntage of using bleach activators is to reduce bleaching temperature
to as low as
22 20°C or even lower by generating active bleaching chemicals in a
solution containing sodium
perborate or hydrogen peroxide and an activator. This reduction in the
severity of the
24 conditions employed in bleaching reduces tendency towards cellulose
degradation and
increases efficiency of pulp brightening. Several such activators are known.
They are acetyl
26 derivatives of nitrogenous organic compounds such as
tetraacetylglycolurile,
diacetyldimethylurea, tetraacetylethelenediamine, triazine derivatives as well
as glucoacetyl
28 compound~.s such as pentaacetylglucos,e.
3
CA 02230315 1998-02-17
One wa.y to improve the pulp brightening efficiency of hydrogen peroxide or
sodium
2 perborate is the addition of these activators in the bleach liquor either in
the refining stage or
in a bleach tower. When an inorganic persalt or hydrogen peroxide is used
alone in pulp
4 bleaching it provides a satisfactory bleaching effect above 60°C, but
at a lower temperature,
such as a.t 40°C, its bleaching efficiency is extremely low. Among the
known bleaching
6 activators the TAED (tetraacetyl-ethylenediamine) is one of the most
efficient one for
detergent applications. It has been found however that under the normal
conditions of pulp
8 bleaching in bleach tower where the temperature is higher than 60°C,
a lowering of pulp
brightness has been obtained by adding TAED with hydrogen peroxide or sodium
perborate.
It has been discovered that a change in the bleaching conditions and more
particularly by
lowering the bleaching temperature enables improved pulp bleaching to high ISO
brightness
12 and this brightness gain is possible in a alkali-free bleach liquor when
sodium perborate is
used as oxidising chemical.
14
CITATION OF THE RELEVANT ART
16
The most pertinent publications in this area of which applicants are aware are
two recent
18 publications, five US patents and one European patent. These are European
Pat. No. EP 0 437
329 A1 (1992); Foret, R., "Particulate Bleach composition" US patent no.
4283302; Dugenet,
Y., "Process for bleaching household laundry in a wash cycleBleach" US patent
no. 4775382;
Sanderson, W.R., "Particulate sodium perborate monohydrate containing adsorbed
activator"
22 US patent no. 4545784; Varennes S. et al., " Sodiurr~ perborate bleaching
of TMP", Tappi J.
vol. 79, no.3,p. 245 ( 1996) Sain, IVI. M. et al, "Activated bleaching of
thermomechanical
24 pulp"; J. Can. Chem. Eng., 1997, vol. 75 (1) pp. 62; Joachimides, T.;
"Process for bleaching
mechanical wood pulp with sodiurr~ hydrosulfite and sodium hydroxide in a
refiner", US
26 patent no. 5129987; Bengtsson, G.; "Method of manufacturing bleached
chemimechanical
and semichemical pulp by means of a one-stage impregnation process; US patent
no.
28 4756799.
4
CA 02230315 1998-02-17
In the first four of these documents use of an activator for increasing
bleaching efficiency
2 of sodium perborate in detergent industry is taught. Dugenet specifically
teaches that
perborate in combination with TAED activator is efficient at low temperature
for washing
4 clothes in a pH range 8 to 12. Dugenet did not work on bleaching of
mechanical wood pulp.
Thus no teaching or suggestion is provided by this author that perborate can
suitably bleach
6 pulp in refiner in presence or absence of TAED activator. What is probably
the reference of
most interest in this group is European pat. No. EP 0 437 329 A 1 ( 1992)
which teaches the
8 use of various peroxyacid bleach precursors in lowering the washing
temperature in detergent
applications. Tetraacetylethylenediarnine (TAED) is one of the bleach
precursors used to
improve llow temperature efficiency for washing. However, this reference does
not also
concern the refining and bleaching of mechanical wood pulp and in situ
generation of
12 peracetic .acid by adding TAED with hydrogen peroxide in a bleach tower.
Another article, which considered the use of different bleach activators on
the generation
14 of peracetic acid, is published in J. Prakt. Chem. Vol. 334 ( 1992) pp. 293-
297. Authors
reported that both sodium perborate; and hydrogen peroxide can generate
peracetic acid by
16 reacting with tetraacetyl at a relatively low temperature and the activator
efficiency is
maximum between 10 to 30 min of addition of the activator. This reference
strictly teaches
18 the gener;~tion of peracetic acid by reacting TAED with sodium perborate in
aqueous alkali.
No bleaching of wood pulp was suggested and the teaching does not concern with
refining of
wood puhp with peracetic acid.
One at~ticle published in Tenside Surf. Det. Vol 27, no.3, page 187 ( 1990)
considered the
22 use of TAED in peroxide solution to generate a strong oxidant such as
peracetic acid in an
alkaline medium. The article also reported that the peracetic acid can be
generated at a
24 temperature below 60°C. The teaching of which is incorporated in
this work as reference.
However, this reference strictly teaches the generation of peracetic acid. No
bleaching of pulp
26 in refiner or in bleach tower was suggested and the teaching does not
concern with refiner
pulp bleaching.
5
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In wood pulp bleaching and refining the most relevant documents are two papers
from our
2 group and two patents. An U.S. patent number 5129987 to Joachimides et al.
discloses an
addition of sodium hydrosulfite and sodium hydroxide in a refiner to improve
mechanical
4 wood pulp brightness. Another U.S. patent number 4756799 to Bengtsson et al.
disclosed that
an impregnation of wood chips with ;~ solution containing alkali and peroxide
and subsequent
6 preheatin~; and refining the chips below 100°C in two stages improved
brightness of pulp.
These references strictly concern with reductive and oxidising agents which
are now being
8 used industrially for refiner bleaching of pulp rather than process to use
sodium perborate in
refiner bleaching of mechanical wood pulp.
In one of the recent publication co-authored by S. Varnenes we have shown that
perborate
can bleach mechanical wood pulp in a conventional bleach tower. The teaching
of which is
12 taken here: as a reference. This paper did not recognise the use of
perborate in a refiner to
bleach wood pulp during refining. Tlms no proposition or recommendation was
made in this
14 paper to bleach pulp in-situ during rc;fining in a system containing
perborate with or without
the presence alkali. In our another p;~per coauthored by M. Parenteau we have
demonstrated
16 the use of TAED as an activator in mechanical pulp bleaching in a perborate
solution. It has
been found that perborate reacted with tetraacetylethylenediamine to generate
peracetic acid
18 which was an effective bleaching agent for thermomechanical pulp. This
paper did not
mention the use of TAED to increase the wood pulp bleaching efficiency of
hydrogen
peroxide.
Applicants consider that given substantial differences between perborate
promoted in-situ
22 refiner bleaching of pulp without addition of alkali followed by tower
bleaching of the pulp
by in-situ generated strong oxidant from perborate or peroxide and
tetraacetylethylenediamine
24 under alkaline condition and, conventional peroxide or perborate or
peracetic acid tower
bleaching of pulp that above references singly or in combination provide no
teaching enabling
26 one to carry out perborate bleaching of pulp in a refiner followed in
bleach tower with
hydrogen peroxide or sodium perborate and TAED added together in a temperature
range
28 between ~;5 to 60°C.
6
CA 02230315 1998-02-17
2 SUMMARY OF THE PRESENT INVENTION
It is accordingly an object of the present invention to provide a perborate
pulp bleaching
4 process which gives pulp of improved brightness without the need of addition
of alkali or the
need to increase either refining energy or the overall amount of oxidant used.
6 In accordance with the present invention bright mechanical wood pulps are
produced from
lignocellulosic material, for example wood chips, by subjecting the material
to a process in
8 which it is steamed for about 10 min., then pre-heated in a digester
(preheater) under pressure
at a tempE;rature range between 115 to 145°C for about 6 to 8 min.,
impregnated first with a
heavy metal chelating agent such as DTPA (diethylenetriaminepentaacetate) in
preheater and
then mixed with an oxidising agent selected from peracetic acid, generated in
a tank by
12 separately premixing an activator suc h as TAED with hydrogen peroxide or
sodium perborate
in 0.3: 1 molar ratio, and alkali-free: sodium perborate , a sodium silicate
stabiliser and a
14 cellulose protective agent such as magnesium sulphate at the eye of the
rotating refiner plates
and finally refined and discharged at a pulp consistency of about 20% on dry
(od) pulp basis.
16 It is also an object of this invention to improve brightness of above
refined pulp further by
subjecting it to tower bleaching process by adding a mixture of sodium
perborate or hydrogen
18 peroxide, TAED, sodium silicate, magnesium sulphate and alkali being used
only with
peroxide apt a temperature range 30 to 60°C for a time period between
30 to 90 min.
The tangible embodiments produced by the process aspect of the present
invention
possess the inherent physical characteristics of being relatively bright pulps
when tested by
22 standard brightness methods, and having superior pulp brightness to pulps
bleached by
hydrogen peroxide in refiner and in bleach tower under the conditions employed
in prior art
24 processes, thus being useable for all standard uses of lignocellulosic pulp
based paper.
In yet another embodiments of this invention the bleaching process provides a
mean to
26 brighten pulp with an oxidant such as sodium perborate in absence of
alkali.
7
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Special mention is made of embodiments of the invention wherein the method has
2 advantage over peroxide bleaching process for better optical property
achievement.
Special mention was also made of the embodiments of the invention wherein the
tower
4 bleaching with perborate or peroxide and tetraethylenediaminetetraacetate
reduces the
bleaching temperature from about 70''C to about 40°C and the bleaching
time from about 120
6 min to about 60 min. thus saving energy cost for pulp bleaching.
The bleaching of mechanical wood pulp earned out by refining softwood chips
first with
8 sodium pe:rborate in refiner in a concentration range between 2 to 15 parts
per 100 parts of dry
wood chips, 3 parts of sodium silicate: per 100 parts of wood chips within a
temperature range
115 to 14'i°C for the refining followed by tower bleaching of refined
pulp with 2 to 5 parts of
hydrogen peroxide or 6 to 15 parts of sodium perborate, with a peroxide/alkali
ratio between
12 0 to 1.5 b:y weight and TAED/ peroxide molar ratio 0.1 to 1 within a
temperature range 30 to
70°C and time range 30 to 120 min. is. within the scope of this
invention.
14 DESCRIPTION OF THE PREFERRED EMBODIMENTS
The wood chips used in the present study was softwood origin. The process of
the
16 invention is an unique process for brightening pulp by adding an efficient
oxidising agent in
refiner and by adding a bleach activator in bleach tower. To practice the
process of the
18 invention, the lignocellulosic material, conveniently softwood
thermomechanical pulp
prepared from wood chips having two different ageing times, conveniently one
month and
six months.
IN-REFINER BLEACHING
22 For in-refiner bleaching wood chips were pre-steamed under atmospheric
pressure for few
minutes, conveniently 10 min, may be introduced into a pressurized pre-heated
digester for 6
24 to 8 min. conveniently with a plug screw where the wood chips were heated
to a temperature
conveniently about 130"C where the said preheated wood chips were impregnated
with a
8
CA 02230315 1998-02-17
chelating agent such as DTPA in small amount, conveniently 0.2% by weight of
dry wood
2 chips before being fed directly to a refiner eye wherein the chips are
treated with a bleaching
solution having an effective amount of oxidising agent selected 10.5% by
weight of dry
4 wood chips, in absence of sodium hydroxide and to which also have been added
a stabiliser
such as sodium silicate, conveniently about 3 % by weight of dry wood chips to
prevent
6 degradation of bleach oxidant generated in the process and a cellulose
protector such as
magnesium sulphate in trace amount.. conveniently 0.05% by weight of dry wood
chips. The
8 pulping and brightening of pulp cm be obtained by passing the impregnated
wood chips
through a refiner which contains rotating plates, preferably flat disc and
conical disc types
with a pre-adjusted plate gap, conveniently between 0.3 to 1.65 mm to defibre
the wood chips
and thereby facilitating defibered wood chips to react with impregnated
bleaching solution.
12 After imI>regnation with chelating agent, oxidising agent, cauticising
agent, if any, and
stabilizing chemicals in standard fashion, the mixture of wood chips and added
chemicals
14 may be refined in a conventional CD 300 Sund-defibrator pilot-scale
refiner, conveniently at
a production rate 0.55 kg/min with a measured discharge, conveniently about
20% of dry
16 pulp, the resulting pulp being stored in a plastic bag for further reaction
conveniently for 30 to
45 min. to produce a pulp conveniently with a freeness of about 200 ml and
brightness about
18 68% ISO. The resulting pulp, may be. further bleached in a bleach tower or
it may be formed
directly to paper. The resulting paper obtained from refiner has improved
mechanical
strength over conventional thermomechanical pulp.
POST BLEACHING
22 In a preferred embodiment the refined pulp subjected to further bleaching
to practice the
process of the invention, is treated with a solution containing bleach
effective amount of
24 hydrogen peroxide, conveniently 3.5% by weight of pulp, sodium silicate ,
conveniently 3%
by weight, sodium hydroxide, conveniently 2.7 % by weight and to which has
been added a
26 bleach activator, conveniently tetraacetylethylenediamine in a molar ratio
of
activator/peroxide 0.3, a chelating agent, conveniently DTPA and a cellulose
protective
9
CA 02230315 1998-02-17
agent, conveniently magnesium sulphate. Refined pulp, conveniently 12% by
weight ,
2 impregnated with bleach solution, has been subjected to heat in a bleach
tower conveniently
at 40°C for a short time period , conveniently about 60 min. to obtain
a bright pulp of ISO
4 brightness above 75.
In another preferred embodiment the refined pulp subjected to further
bleaching to
6 practice the process of the invention, is treated with a solution containing
bleach effective
amount of sodium perborate, conveniently 10.5% by weight of pulp, sodium
silicate ,
8 conveniently 3% by weight, and to which has been added a bleach activator,
conveniently
tetraacetylethylenediamine in a molar ratio of activator/peroxide 0.3, a
chelating agent,
conveniently DTPA and a cellulose protective agent, conveniently magnesium
sulphate.
Refined pulp, conveniently 12% by weight , impregnated with bleach solution,
has been
12 subjected to heat in a bleach tower conveniently at 40°C for a short
time period , conveniently
about 60 min. to obtain a bright pulp of ISO brightness above 75.
14 A series of trials were carried out both for in-refiner bleaching and post
bleaching
operation to establish improved process conditions of the invention. More
details of these
16 experimental trials are as follows:
As used herein and in the appended claims the term "aging" means physical and
chemical
18 transformation of components in wood chip due to longer storage time under
ambient
condition.
As used herein and in the appended claims the term "refining" means
defibrating wood
chips into individual fibres and to develop good mechanical properties of
these fibres by
22 applying heat, pressure and mechanical force.
As used herein and in the appended claim the term "bleaching" means
modification of
24 coloured chromophores present in pulp lignin in order to improve the
brightness of
mechanical pulp.
CA 02230315 1998-02-17
The term "bleach effective amount" of perborate oxidant means a concentration
of oxidant
2 such as perborate in solution of from about 6% to about 15% by weight and
"bleach effective
amount" of peroxide means from about 2% to about 5% by weight.
4 As used herein and in the appended claims, the term "activator" contemplates
water
soluble tetraacetylethylenediamine.
6 The term "chelation" as used herein and in the appended claim means a
chemical capable
of reactin;; with heavy metals such as~ iron, manganese, copper to reduce
their freed amount in
8 pulp, thereby, reducing the degradation of oxidising agent in pulp.
The term "stabiliser" contemplates water soluble sodium silicate compound and
the term
"cauticiser" contemplates a solution of sodium hydroxide in a concentration
range 1.0 to 5.0%
by weight. The term "cellulose protective compound" contemplates water soluble
magnesium
12 sulphate.
The :presence of trace metals in wood catalytically decompose the bleaching
chemicals.
14 TMP from hardwood and softwood reveals itself to be effective for
brightening, it is
pretreated with a chelating agent such as DTPA. In refining and bleaching, the
pretreatment
16 process, i.e., a process according to which the pulp is treated with a
chelating agent DTPA,
and consequently, a chelate complex with free metal in pulp is formed. In
practice of the
18 invention, just prior to the addition of bleaching chemicals in refiner,
the pulp is treated with
0.2% by weight of DTPA. In refiner bleaching DTPA is added just after the plug
screw in
digester prior to addition of bleaching chemicals at the refiner eye at the
point of rotating disc
while the pulp consistency is maintained at about 20%.
22 In one: of the preferred embodiments of the present invention, the chemical
composition
used during refining to brighten the pulp is either sodium perborate or
peracetic acid obtained
24 by mixing; TAED and hydrogen peroxide in a tank at 0.3:1 molar proportion,
sodium silicate,
magnesium sulphate and alkali, if any. The balance was water to make up the
desired pulp
26 consistency. The pre-heating temperature in digester is varied between 115
to 145°C. Once
11
CA 02230315 1998-02-17
the refining is completed pulps have been stored for 30 to 45 additional
minutes in a plastic
2 bag, then diluted to 1 % consistency before being produced in different
freeness levels. The
pulp is then diluted and neutralized at pH 5.5 with sodium metabisulfite,
thickened, washed
4 and pressed to a consistency of about 20% by weight and handsheets are made
for brightness
and mechanical testing according to CPPA E.1 and CPPA D.12.
6 The initial pH of the refiner bleach liquor may vary from 10 to 13 . For
refining with
perborate the preferred pH range is bcaween 10 and 11 and for peracetic acid
the prefereed pH
8 range is between 10.5 and 11.5. The final pH of the spent liquor after
refining may be in the
range between 5 to 7.5. The preferred pH range in between 5.5 to 6.5.
A preferred chelating agent is DTPA. Other chelating agents such as EDTA and
phosphonates may be used. The preferred concentration of chelating agent is
between 0.2 to
12 0.4% by weight of dry pulp. A higher concentration may be used but without
additional
advantage in pulp brightness gain. A lower concentration is less effective and
is not included
14 in the present invention.
In refiner bleaching it is advantageous to add the chelating agent prior to
addition of
16 bleaching chemicals. The benefit is c;helation of trace metals prior to
addition of oxidants
which could render oxidant more stable during the chromophore modification
reaction with
18 lignin in pulp. Chelating agent can be added together with other chemical
in the refiner eye,
but generally efficiency would be less.
A preferred alkali is sodium hydroxide, but other alkali metal salts could
also be used
which could maintain a pH range of about 10 to 11 during refining and
bleaching, a preferred
22 alkali/oxidant ratio being 0.8 to 1Ø Lower concentrations may be used,
but the efficiency of
process will depend on the nature of oxidant. Usually for peroxide and
peracetic acid a higher
24 concentration is preferred and for perborate a bleach liquor without alkali
could be used to
obtain a significant gain in pulp brightness . A higher concentration of
alkali/oxidant ratio of
26 about 1.5/1 is not worthwhile for hydrogen peroxide or peracetic acid
because generally it
12
CA 02230315 1998-02-17
reduces the brightness gain and increases cost. Perborate without alkali is
preferred because it
2 reduces the chemical cost without sacrificing pulp brightness. More
preferred weight ratio of
alkali/peracetic acid is 0.8/1 for peroxide and TAED based system wherein the
refined pulp
4 has improved brightness and good mechanical properties.
In refiner bleaching of pulp, the preferred pulp of about 20% consistency is
bleached in-
6 situ during refining by sodium perborate or peracetic acid. During bleaching
at least two
intermediate products are formed: perhydroxyl ion and hydrogen peroxide. The
mechanism
8 by which perborate and peracetic acid is not known in detail but it is
expected that perborate
and perac:etic acid could generate an unknown intermediate strong oxidant
which then reacts
with colored chromophores in lignin of pulp to chemically modify them and
improve pulp
brightness. Independent of the correct explanation of the pulp brightening
mechanism it has
12 been found that a surprising increase in pulp brightness could be achieved
by adding
perborate in refiner even in the absence of alkali.
14 The temperature of the preheated digester prior to introduction of pulp to
refiner was varied
between 115 to 145°C. For optimum pulp brightness, the refiner pre-
heating temperature is an
16 important factor and should be kept low. The extent of pulp brightening was
good in a
temperature range between 115 - 130°C. A higher temperature has no
practical advantage
18 because it decomposes the oxidants and the brightness gain is low. Below
115°C. the
efficiency of the process is generally too low to be worthwhile.
The sodium perborate concentration may range from about 5 % by weight of dry
pulp to
about 15 % by weight of dry pulp, although a lower concentration down to about
2% by
22 weight of dry pulp may be used with reduced beneficial effect. Similarly,
improvements of
the brightness of the pulp are not very significant with sodium perborate
concentration above
24 12% by weight of dry pulp, while additional cost is not justified by the
small additional
improvement. Generally, therefore, a sodium perborate concentration range
between about
26 6% by weight to 12% by weight on dry pulp, preferably around 11 % by weight
of dry pulp is
used.
13
CA 02230315 1998-02-17
For refiner pulp brightening with peracetic acid generated in a separate tank
by reacting
2 hydrogen peroxide and TAED in a molar ratio 1:0.3, the concentration of
hydrogen peroxide
may vary from 2% to 5% by weight of dry chips. It is preferred to carry out
refiner bleaching
4 with peracetic acid generated from 4 wt% of hydrogen peroxide and 0.3:1
molar ratio of
TAED and hydrogen peroxide. A lower concentration of generated peracetic acid
may be
6 used, but the efficiency of the process reaches a plateau at this
concentration and no
advantage is usually gained by the use of higher concentration of generated
peracetic acid.
8 Wood species from softwood sources may be used. Both fresh and aged wood
chips
may be used. The preferred wood chips for refiner bleaching is aged chips. The
reason for
this improved brightness gain with aged chips is not known. A possible
explanation may be
modification of colored chromophorea in lignin to easily oxidisable states
during aging.
12 Whatever may be correct mechanism of aged wood chips brightening process, a
high
brightness gain has been achieved with aged chips over fresh chips.
14 The refiner plate design and plate gap are also important to obtain
homogeneous reaction
and optimum defibration of wood chips during refiner bleaching process. Flat
disc plate and
16 conical disc plate are preferred plate configurations. Other design also
can be used but with
less efficiency. The preferred plate gap in refiner for flat disc is between
0.3 to 1.0 mm, more
18 preferred range is between 0.4 to 0.8 mm. A higher plate gap may be used
but, the pulp
freeness will be very high. A low gap will decrease the pulp freeness to an
unacceptable level.
For conical disc type refiner plate the preferred gap is between 1.2 to 2.4
mm. However, for
optimum results a suitable combination of nip gaps of flat type and conical
type is preferred
22 which may be any suitable gap betwc;en 0.4 to 0.8 mm for flat type and 1.2
to 2.4 mm for
conical type plates.
24 Surprisingly and unpredictably it has been found that the specific refining
energy of
pulp refined with perborate has not been increased over standard
thermomechanical pulp,
26 TMP, process or pulp developed with alkaline peroxide in refiner although
the brightness
14
CA 02230315 1998-02-17
gain and mechanical strength of perborate treated pulp is superior to both TMP
and alkaline
2 peroxide treated mechanical pulp, APMP.
The in-refiner bleaching with perborate has additional advantage over
conventional tower
4 bleaching in that it reduces the amount of bleach effluent because of the
high consistency of
refiner pulp which is usually 20% and above compared to about 10 to 12%
consistency for the
6 tower bleaching.
It is also an object of the present invention to provide a tower bleaching
method of the
8 aforesaid refined pulp of the present invention with peroxide and perborate
which avoids the
disadvantages of the prior art processes and which provides advantageous
conditions of pulp
brightening. Other additional objects and advantages will become apparent from
the following
description.
12 In a I>referred embodiment of the present invention the further brightening
performance
of various kind refined pulps including TMP and APMP is being compared with
pulp refined
14 with perborate. It has been found that perborate and peracetic acid in-
refiner bleached pulp
may be bleached further with the same efficiency as compared to APMP under a
standard
16 bleaching condition which is usually 70-80° C. and 2-3 hours of
residence time in a bleach
tower.
18 Also, surprisingly it has been found that pulp refined with perborate or
peracetic acid
may be fizrther brightened to a ISO brightness point above 75 in a single
conventional
bleaching stage by treating the pulp together with alkaline peroxide or
perborate and TAED
activator in a bleach tower at a relatively low temperature ranging between 20
to 60°C. for a
22 suitable time period eg., 0.25-1.5 hours, but other chemicals and
conditions are variable as
described in a prior art in the form c>f publication by Daneault and Varennes
and included
24 herein as a reference. Successful bleaching in the process of the invention
was obtained by
using 6 to 15% by weight of perborate or 1 to 5% by weight of peroxide and a
26 TAED/perborate or TAED/peroxide molar ratio between 0.05 to 1. An
alkali/perborate
CA 02230315 1998-02-17
weight ratio 0 to 0.3 has been found to be optimum with no practical advantage
in using a
2 higher weight ratio. The alkali/peroxide weight ratio for TAED/peroxide
combination was
between 0.8 to 1.5. Surprisingly, the brightness gain after the treatment of
the refined pulp of
4 the present invention with hydrogen peroxide or sodium perborate and TAED
under the above
low temperature and short bleaching time conditions was superior to the
brightness gain
6 obtained after bleaching the same pulp under a standard industrial bleaching
condition, which
is usually 70°C and 120 min. at an equal concentration of hydrogen
peroxide dosage.
8 The concentration of perborate in bleach tower may vary from 2 to 15% by
weight of dry
pulp. The preferred concentration is between 8 to 12% by weight. A lower
concentration is
not preferred because the brightening efficiency is very low. The optimum
concentration of
peroxide for TAED/peroxide bleaching system is between 2 to 5% of peroxide. A
higher
12 concentration is not useful because the brightness gain is small because a
plateau is reached
near this concentration level. A lower concentration below 1 % is also not
efficient for
14 achieving; high pulp brightness.
The efficiency of the process is increased by increasing TAED/perborate and
16 TAED/peroxide molar ratio, and a molar ratio of 0.02 to 1 may be used, and
a molar ratio
between 0.05 to 0.3 has been found to be optimum for TAED-peroxide combination
and a
18 molar ratiio of 0.1 to 0.5 was found to be optimum for TAED-perborate
combination. A higher
molar radio may be used, but the brighntess gain is generally very marginal to
be worthwhile.
The ratio of alkali/perborate is an important factor for achieving optimum
optical and
mechanical properties during the tower bleaching process. Bleaching without
alkali is
22 preferred for maximising cost reduction. However, an alkali/perborate
weight ratio of 0 to 0.5
may be used to achieve higher brightness gain and improved mechanical
properties. The
24 optimum alkali/perborate weight ratio is between 0 and 0.3. A ratio above
0.7 is less efficient
because it results in pulp yellowing: and reduces brightness of pulp. The
optimum ratio of
26 alkali/peroxide in TAED/peroxide bleaching is between 0.8 to 1.5. A ratio
lower than 0.6
16
CA 02230315 1998-02-17
significantly decreases the bleaching efficiency and a higher ratio above 1.5
increases
2 chemical cost and develops pulp yellowness.
Temperature is the most important factor in the present invention of tower
bleaching.
4 Surprisingly, a high brightness gain has been achieved by using a low
temperature range
between 20 to 50°C. and optimum temperature range was between 30 and
50°C. A
6 temperature lower than 20°C. is also possible but the efficiency is
low. A higher temperature
above 60°C is not useful because the efficiency of peroxide-TAED or
perborate-TEAD
8 oxidation potential towards pulp decreases significantly.
Preferred bleaching time in tower with TAED-peroxide and TAED-perborate
systems is
between :30 to 75 minutes. A shorter time than 25 minutes may be used with a
marginal loss
in brightness gain. A higher time above 90 minutes is not suitable because it
decreases the
12 pulp brightness and increase energy cost.
In yet another preferred embodiment of the present invention the refined pulp
obtained
14 from the aforesaid refining process may be pre-treated with a chelating
agent. A preferred
chelating agent is DTPA, but other chelating agents may be used, a preferred
concentration
16 being 0.2 to 0.4% by weight of dry pulp. A lower concentration may be used,
but efficiency is
less and no advantage is usually gained by the use of more concentrated
alkali.
18 The process of invention provides a number of advantages over prior art
processes as
well as the possibility to improve the quality of product discussed above. The
in-situ
perborate bleaching during refining is cheap because it eliminates partly
conventional
bleaching stage in bleach tower or reduce bleach chemical requirements in
conventional
22 bleaching stage and presents no undue problem of handling and treating a
large quantity of
bleach effluent. The product gives significant brightness gain and better
mechanical properties
24 over all prior art in-refiner bleaching and conventional bleaching
processes. TAED activated
peroxide bleaching of perborate-refined pulp gives an unique possibility to
reach high
17
CA 02230315 1998-02-17
brightness for a mechanical pulp in a single after-refiner bleaching stage at
a low temperature
2 and with a very short reaction time as discussed above.
Although described herein with reference to softwood species it is expected
that the
4 process of the invention will be applicable to in-refiner bleaching and
tower bleaching of a
wide range of natural products including hardwood and nonwood species to yield
useful
6 bright pulp for papermaking and nonwoven applications.
The following examples further illustrate the best mode comtemplated by the
inventors for
8 the practice of their invention.
Example 1
Aged softwood chips from spruce (75%) and Balsam(25%) were first pre-steamed
for 10
min. under atmospheric pressure and then impregnated with 0.2% of DTPA in
digester for
12 about 6 minutes to 130°C and conveyed to the refiner eye where it
was refined with or
without injection of bleaching solution. All bleaching solutions contained
3.0% sodium
14 silicate, 0.05% magnesium sulphate, 3.0% sodium hydroxide except where
sodium perborate
is used and other oxidising chemicals as given in Table below. The pulp after
refiner was
16 stored for 30 min. in a plastic bag and then diluted and neutralized at pH
5.5 with sodium
metabisulfite. The pulp freeness range is between 100 to 200 ml. The
handsheets made from
18 those pulps show optical properties:
22
24
18
CA 02230315 1998-02-17
In-refiner oxidisingWt%(dry wood Brightness, Opacity, %
agent %ISO
chi
Sodium perborate 11.7 69.8 90.5
Peroxide a uivalence)(4.0)
Peracetic acid 8.9 60.7 90.8
from
eroxide*
Peracetic acid 8.9 63.4 91.6
from
erborate*
H dra en eroxide 4.0 63.4 90.2
None 0 49.6 97.5
*Prepared in a separate tank by reacting 4.0 g of hydrogen peroxide with 8.05
g of TAED; **
2 Prepared in a separate tank by reacting 11.7 g of perborate with 8.05 g of
TAED.
It is evident that in-refiner bleaching with sodium perborate gives maximum
brightness gain
4 and optical properties are superior to in-refiner hydrogen peroxide
brightened pulp. About 20
points gain with perborate was obtained compared to about 14 points with
hydrogen peroxide.
6 It is further evident that peracetic acid generated from a reaction mixture
of perborate and
TAED is more effective in pulp brightening during refining compared to that
generated from
S peroxide and TAED.
Example 2
The example shows the effect of different in-refiner oxidising agents on
mechanical
properties and their energy consumption during refining. All conditions are
the same as in
12 Example 1.
14
16
19
CA 02230315 1998-02-17
In-refiner oxidisingWt%(dry wood Specific refiningTensile index,
agent Nm/g
chi ener , MJ/k
Sodium perborate 11.7 9.0 40.0
(Peroxide a uivalence)(4.0)
Peracetic acid 8.9 7.3 38.4
from
eroxide*
Peracetic acid 8.9 7.3 35.2
from
erborate*
Hydrogen peroxide 4.0 7.3 36.1
APMP
None (TMP) 0 7.3 34.7
*Prepared. in a separate tank by reacting 4.0 g of hydrogen peroxide with 8.05
g of TAED; **
2 Prepared i.n a separate tank by reacting 11.7 g of perborate with 8.05 g of
TAED.
It is evident that specific refining energy does not change very much during
in-refiner
4 bleaching with perborate and peracetic acid compared to hydrogen peroxide.
The tensile
index of in-refiner perborate bleached pulp has improved properties over TMP
and the one
6 obtained by peroxide in-refiner bleaching.
Example 3
8
This example compares the in-refiner bleaching of unaged or fresh wood chips.
Fresh softwood chips from spruce (75%) and Balsam(25%) were first pre-steamed
for 10
min. under atmospheric pressure and then impregnated with 0.2% of DTPA in
digester for
12 about 6 minutes to 130°C and conveyed to the refiner eye where it
was refined with or
without injection of bleaching solution. All bleaching solutions contained
3.0% sodium
14 silicate, 0..05% magnesium sulphate, :3.0% sodium hydroxide and other
oxidising chemicals as
given in Table below. The pulp after refiner was stored for 30 min. in a
plastic bag and then
16 diluted, neutralized at pH5.5 with sodium metabisulfite and obtained in a
freeness level of 200
ml. The handsheets made from those pulps show optical properties:
CA 02230315 1998-02-17
2
In-refiner oxidantSodium perborateHydrogen peroxideNone
(Peroxide (APMP) (TMP)
a uivalence
Wt% (dry wood 10.5 3.5 0
chip)
3.5
Bri htness, %ISO 67.9 65.0 53.6
O Tacit , % 90.0 89.3 94.6
Sp. Refining energy,9.7 11.4 11.7
MJ/k
Tensile index, 37.2 38.4 35.2
Nml
From results showed in above table it is evident that perborate in-refiner
bleaching is
4 superior to peroxide in-refiner bleaching with respect to optical properties
and refining energy
requirement. The mechanical properties of perborate pulp is comparable to
peroxide pulp and
6 are superior to TMP. As mentioned before a chelating agent was used before
addition of
bleach liquor. Chelating agent, DTPA, was added after plug screw at higher
concentration
8 than usual for tower bleaching because of the alkaline pulp condition.
Phosphonate type
chelant may be used in lesser amount because of their improved chelation
stability at higher
pH condition.
Example 4
12 This example illustrates the effect of sodium perborate concentration and
refining
temperature during in- refiner bleaching stage.
14 Fresh softwood chips from spruce (75%) and Balsam(25%) were first pre-
steamed for 10
min. under atmospheric pressure and then impregnated with 0.2070 of DTPA in
digester for
16 about 6 minutes to 130°C and conveyed to the refiner eye where it
was refined with or
without injection of bleaching solution. All bleaching solutions contained
3.0% sodium
18 silicate, 0.05% magnesium sulphate. The pulp after refiner was stored for
30 min. in a plastic
21
CA 02230315 1998-02-17
bag and then diluted, neutralized at pH 5.5 with sodium metabisulfite and
obtained in a
2 freeness level of 200 ml. The handsheets made from those pulps show optical
and mechanical
properties:
Perborate. % 6.0 15.0 6.0 15.0
Tem erature,oC 115 115 145 145
Bri htness,% 67.1 72.2 63.2 65.0
ISO
b* 14.1 12.4 15.7 15.4
O acit , % 90.6 88.6 91.3 89.8
Tensile index.,30.1 33.8 40.5 39.0
Nm/
Burst index., 2.30 2.49 2.52 2.44
kPa* m2/
Tear index, 9.78 9.79 11.07 9.55
mN*m2/
Sp. Refining 7.65 11.18 12.96 11.58
energy,
MJ/k
Fiber len th, 1.69 1.76 1.87 1.77
mm
Perborate 4.42 8.89 5.11 12.58
consum tion,
%
4
It is evident from above results that increasing perborate concentration has a
positive effect
6 on pulp brightness and mechanical properties as well. A in-refiner perborate
bleached pulp
with ISO brightness more than 72 points has been obtained at higher
concentration. Increasing
8 temperature to 145°C from 115°C. ha.s a detrimental effect on
pulp brightness. Pulp brightness
decreased by about 7 points ISO by increasing the refining temperature from
115°C to 145°C.
for a perborate concentration of 15% by weight. The same trend was found at a
lower
concentration of perborate. The b* value further indicates that pulp is more
yellow at higher
12 refiner temperature.
Example 5
14 This example illustrates the effect of alkali-free in-refiner bleaching
with hydrogen
peroxide, sodium perborate and peracetic acid where peracetic acid has been
generated by
16 reacting sodium perborate and TAED ex-situ and then added to the refiner
during in-refiner
bleaching on optical and mechanical properties. Aged softwood wood chips from
spruce
18 (75%) and Balsam(25%) were first pre-steamed for 10 min. under atmospheric
pressure and
then impregnated with 0.2% of DTPA in digester for about 6 minutes to
130°C and conveyed
to the refiner eye where it was refined with or without injection of bleaching
solution. All
22
CA 02230315 1998-02-17
bleaching solutions contained 3.0% sodium silicate, 0.05% magnesium sulphate,
and 11.7%
2 perborate and peracetic acid was obtained by reacting 11.8 g of perborate
with 8.05 g of
TAED separately in a tank and then injecting it at the refiner eye . The pulp
after refiner was
4 stored for 30 min. in a plastic bag and then diluted, neutralized at pH 5.5
with sodium
metabisulfite and obtained in a freeness level of 200 ml. The handsheets made
from those
6 pulps show optical properties:
In-refiner oxidisingISO Brightness,Opacity,Burst index,Tensile
agent % % kPa.m2/ index,
Nm/
Sodium erborate 69.8 90.5 2.5 40.0
Peracc~tic acid 63.4 91.6 2.1 35.2
from
erborate**
Peroxide 63.4 90.2 2.6 36.1
No bleachin (TMP) 49.6 ~ 97.5 2.1 ~ 34.7
~
8 ** Prepared in a separate tank by reacting
Evidenl:ly, in-refiner perborate bleaching gives best optical properties with
about 20 points
increase in the ISO brightness points. The mechanical properties of in-refiner
perborate
bleached pulp was superior to TMP. In-refiner peroxide bleaching, however, did
not brighten
12 the pulp very much. Therefore, it is clear that perborate is unique in in-
refiner bleaching of
woodchips even without the addition of free alkali.
14 Example 6
This example compares the efficiency of perborate in-refiner bleaching with
perborate
16 tower bleaching at same concentration of oxidant.
18
23
CA 02230315 1998-02-17
In-refiner Tower
bleachin
Perborate, % TMP 6.0 15.0 Max. TMP 6.0 15.0 Max.
Gain/ initial Gain/
loss loss
Bri htness,% 56.3 67.1 72.2 15.9 59.1 71.5 73.6 14.5
ISO
b* 14.8 14.1 12.4 2.4 - - _ _
O acit , ~0 94.6 90.6 88.6 -6.0 - - - _
Tensile index.,35.2 30.1 33.8 -5.1 42.8 - 45.3 2.5
Nm/
Burst index., 2.18 2.30 2.49 0.31 3.30 - 3.09 0.06
kPa*m2/
Tear index, 10.3 9.78 9.79 -0.53 12.02 - 11.85 -0.17
mN*m2/
Sp. Refining 11.69 7.65 11.18 0.51 - - _ _
energy,
MJ/k
Perborate - 4.42 8.89 - - 3.1 5.8 -
2 consum tion,
%
Constant conditions:
4 Refiner: Steaming 10 min
Discharge consistency = 20%
6 Preheater temperature= 115°C.
Pulp freeness: 200 ml.
8 Bleach tower:
Consistency = 12%
Temperature = 70°C.
Time = 120 min.
12 In another trial, the improvement of optical and mechanical properties of a
in-refiner
bleached pulp was compared with that of the tower bleached pulp. In both cases
a
14 thermomechanical pulp (TMP) made in the same refiner under similar
condition as used for
this trial in in-refiner and tower bleaching were used as a reference. It has
been clear from
16 above Table that in-refiner bleaching has better efficiency in achieving
brightness gain over
in-tower bleaching. This demonstrates that oxidative bleaching earned out in
the refining
18 zone with sodium perborate is more efficient than conventional medium
consistency
bleaching, suggesting that continuous fracturing of wood chips exposes
chromophores that are
readily accessible to oxidation by perhydroxyl ion. Again because sodium
perborate has
24
CA 02230315 1998-02-17
higher decompostion temperature it is expected to be more efficient at higher
temperature
2 which is in this case again can be achieved during refining.
However, it has been found that mechanical property improvement was more in
tower
4 bleaching which is not unexpected because higher refining temperature could
have a more
severe degradative effect on lignocellulosics than that at a lower tower
bleaching temperature.
6
EXAMPLE 7
8 This example compares the optical and mechanical properties of four
different types of
pulps prepared in the refiner. For chemithermomechanical pulp the
concentration of sodium
sulfite was 3% on od pulp, for peroxide process (APMP) peroxide was 3.5% and
for perborate
process sodium perborate (SPBMP) was 10.5% which is equivalent to 3.5% of
hydrogen
12 peroxide. 'lfie refiner temperature was 130°C.
Pro erties TMP CTMP APMP SPBMP
Brightness, 56.3 58.3 65.0 67.9
ISO
b* 14.8 13.7 15.0 14.1
Opacity, 94.6 92.8 89.3 90.0
Tensile index..35.2 41.1 38.4 37.2
Nm/
Burst index.. 2.18 2.70 2.54 2.41
kPa*m2/
Tear index. 10.3111.35 10.80 10.08
mN*m2/
Energy. 11.698.70 11.41 9.74
MJ/k
Fiber len th. 1.73 1.96 1.84 1.73
mm
Consum lion. --- --- 2.98 8.49
%
14 From above results it is evident that brightness gain is much superior for
SPBMP
compared to CTMP but only with a marginal less mechanical strength. Again.
SPBMP has
16 superior optical properties than APMP at a comparable mechanical properties
and also has
superior optical and mechanical properties to TMP. In refiner bleaching with
perborate is an
18 unique way to make bright pulp.
The following examples illustrate the effect of post-refiner bleaching of
perborate pulp
using TAEl~ as an activator.
CA 02230315 1998-02-17
EXAMPLE 8
2 Example 8 illustrates the effect of low temperature post-bleaching of TMP
obtaining by
above refining process with two different oxidising systems. which are
peroxide and TAED
4 acivated peroxide. During the bleaching process DTPA chelated pulp of about
10 g was
placed in a plastic bag where bleach chemicals were added. Beside peroxide or
TAED and
6 peroxide mixtures. other bleach chemicals were 3% Na-silicate. NaOH in an
alkali peroxide
ratio between 0.8 to 1Ø The pulps were bleached for 60 min at 40°C.
Hydrogen peroxide. Brightness.Peroxide consumed.% ISO gain
wt% ISO% wt%
0 59.1 - 0
2 71.3 0.85 12.2
3 72.1 1.59 13.0
4 73.1 2.12 14.0
73.8 2.67 14.7
Activated peroxide* Brightness.Peroxide consumed.% ISO gain
wt% of ISO% wt%
peroxide(molar ratio
TAED :
peroxide = 0.3 :1)
2.0 71.9 - 12.8
3.0 ?4.2 - 15.1
4.0 75.3 - 16.2
5.0 75.7 I - 16.6
8
Results from the above Table indicates that refined pulp can achieve a
brightness of
75% and above at 40°C. only by using activated bleaching process. This
results show the
unique advantage of using activated bleaching with a peroxide and TAED at a
low bleaching
12 temperature. A clear two points gain in the brightness level indicates the
advantage of using
activated bleaching. In pulping industry it has a direct implication on the
energy cost. The
14 improved bleaching efficiency of peroxide-TAED mixture can be attributed to
in-situ
generation of a strong oxidant during the bleaching process which improved the
bleaching
16 efficiency.
EXAMPLE 9
18 Example 9 illustrates the effect of low temperature post-bleaching of in-
refiner perborate
bleached pulp with two different oxidising systems. which are peroxide and
TAED acivated
peroxide. During the bleaching process DTPA chelated pulp of about 10 g was
placed in a
26
CA 02230315 1998-02-17
plastic bag where bleach chemicals were added. Beside peroxide or TAED and
peroxide
2 mixtures. other bleach chemicals were 3% Na-silicate. NaOH in an alkali
peroxide ratio
between 0.8 to 1Ø The pulps were bleached for 60 min at 40°C.
Hydragen peroxide.Brightness% ISO Perborate, wt% Brightness,% ISO
gain
wt% ISO% gain %ISO
0 66.6 0 - - -
3.5 70.1 3.5 10.8 68.7 2.1
Activated peroxide*Brightness% ISO Activated perborate*Brightness,% ISO
gain
wt% of peroxide(molarISO% gain wt% of
ISO%
ratio TAED : perborate(molar
peroxide = ratio
0.3 :1 ) TAED : peroxide
=
0.3 :1)
3.5% 72.1 5.5 10.8 71.2 4.6
4
Results from the above Table indicates that in-refiner perborate bleached pulp
can achieve a
6 brightness of 72% and above at 40°C. only by using activated
bleaching process. This results
show the. unique advantage of using activated bleaching with a peroxide and
TAED at a low
8 bleaching temperature. A clear two points gain in the brightness level
indicates the advantage
of using activated bleaching. In pulping industry it has a direct implication
on the energy cost.
The improved bleaching efficiency of peroxide-TAED mixture can be attributed
to in-situ
generatian of a strong oxidant during the bleaching process which improved the
bleaching
12 efficiency.
claim:
14 1. A process simultaneously refining wood pulp in a refiner to produce pulp
with
improved brightness mechanical properties comprising concurently introducing
16 chelating agent. bleach liquor an ood chips or pulp in one or more stages
including a
primary refiner at high temperatures achiev by increasing steam pressures and
where
18 the bleach liquor comprises a solution of sodium per~e without any alkali
or ex-situ
generated peracetic acid from peroxide or perborate and TAE~th NaOH including
other chemicals such as sodium silicate and magnesium suphate. said
solut~having an
alkaline pH of from pH about 10 to about 12 and wherein not more than about 1
27
