Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2l7q~¢~
PRODUCTION OF HIGH BRIGHTNESS
CEDAR MECHANICAL PULP
Field of Invention
The present invention relates to a method of 1";~1,'. ., ,g a m~h~n~ pulp
made from a dark wood species using peroxide as the primary pulp brightening agent.
Ba.l~., a of the Invention
Dark wood species is intended to mean species having a high extractive content
of at least l O% and in pal Li~,ulally cedar, most ~a~ ula~ly western red cedar.Cedar fibers and pulps are known to have UU~ allllillg paper making and
strength properties. However, each suffer from a major di~ad~a~ ; in that it is
difficult to brighten cedar mechanical pulp to a brightness equivalent to that required
for the production of newsprint. This is primarily caused by the exceptionally high
content of wood extractives, usually over 10% of the wood, and which in the heart
15 wood, can be as high as 15 to 25%.
Both the ~,luu~l~u~llulic water soluble lignans and the colored water insoluble
native lignans in the cedar mechanical pulp consume a large amount of bleaching
chemicals and make cedar pulp extremely difficult to brighten. Many attempts have
been made to enhance the bl~rh~hili~y of cedar mechanical pulps by using various20 chemical treatments, however, none of them has been particularly successful.
A paper entitled "The Optical Properties of Eight Eastern Canadian Wood
Species" by Lee, J. et al. published in TAPPI Proceedings 1990 Pulping Conference, pp
545 549, outlines some of the problems incurred in attempting to bleach pulp of various
species and shows cedar to be the most difficult to brighten. This is partially caused by
25 the fact that the absorption coefficient (Sk) varies s45..;r.~a l~ly for cedar so that the
effectiveness of a bleaching process will vary ~i~l ir~ ly for pulps from different
trees of the same species.
European patent application 120132 A1 published October 3, 1984 discloses a
process of bleaching mechanical pulp using peroxide by pretreating the pulp with an
30 alkaline aqueous solution containing f"",.,.,,,;.l;"~ sulfinic acid (FAS) followed by a
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2 ~ 7 ~ ~ ~ 3
peroxide hri~hf~nin~ stage. In this case, pulp (mechanical pulp) is also treated with a
chelating agent such as dietaylene triamine penta acetic acid (DTPA).
Obviously, the use of FAS as a bleaching or ~u~l.,~,llillg stage followed by theperoxide stage adds to the cost of the bleaching operation.
Other techniques have been applied to the bleaching of cedar. However, none
have proved to be particularly ~aLi~r~ul~.
Brief Description of the Pre~ent Invention
It is an object of the present invention to provide a system for bleaching or
hri~htPnin~ of m~h:~nir~l pulp derived from a dark species using peroxide as theprincipall";~ "",~:agent.
Broadly, the present invention relates to a process for hri~ht~nin~ mPf h~mirsllpulps from dark species comprising extracting said pulp in an extraction stage (E stage)
consisting essentially of treatment in an aqueous medium containing 0.5 to 10% caustic
(measured as NaOH) based on the dr~v weight of said pulp, at a ~iull~ ,y of 0.5 to
40%, ~ c~a~ in the range of 40 to 100C and for a time period of 10 min to 3
hours to provide an extracted pulp, and then peroxide hri~ht~nine said extracted pulp in
a peroxide hri~ht~nin~ stage (P stage) thereby to increase brightness of said pulp.
Preferably said process comprises a bleaching sequence consisting essentially ofan extraction stage (E stage) followed by a chelating stage (Q stage) and then by a
20 peroxide stage (P stage).
Preferably, said process comprises a bleaching sequence consisting essentially
of an extraction stage (E stage), followed by a washing stage (W stage), followed by a
chelating stage (Q stage) and then followed by a peroxide stage (P stage).
Preferably, said chelating stage comprises applying D~rPA to said pulp.
Preferably, said aqueous medium in said extraction stage contains 1.5 to 4% of
said caustic.
Preferably, said peroxide stage will comprise a first peroxide stage (P,) and a
second peroxide stage (Pl) in sequence.
2~7~a
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Brief Description of the Drawings
Further features, objects and advantages will be evident from the following
detailed description of the preferred embodiments of the present invention taken in
conjumction ~vith the d~,~,ullluallyillg drawings in which,
Figure I is a schematic flow diagram illustrating the various stages in the
bleaching sequence of the present invention.
Description ~f the Preferred ~
Referring to Figure 1, the sequence of the present invention is clearly
,1. .,,,",~1,,.1~l1 In this sequence, the mechanical pulp from a dark species enters, as
indicated at 10, and passes to the first to an optional chelating stage 12 (Ql stage),
where metals may be chelated. Preferably, this stdge will add appropriate amounts of
DPTA or other known chelates to the pulp.
As above indicated, the chelating stdge 12 is optional. However, the extraction
stdge 14 (E stdge) is imperative and requires that the pulp from the dark species (cedar)
be subjected to an alkaline extraction using 0.5 to 10% caustic measured as NaOHbased on the dry weight of the pulp. The Cv~ y within this E stdge 14 will be
within the normal range of 0.5 to 40%, the Lclll,ucla~ c between 40 and 100C and the
time in this stage will be from 10 min to 3 hours. Preferably, the amount of caustic
used will be in the range of 1.5 to 4%, CUI15; ~t~ y l to 10%, t~lllpclaLulc so - 70C and
time 15 - 60 min.
The pulp is then chelated in a conventional manner as indicated by the chelatingstage (Q2 stdge) 16 which is not absolutely essential but depends on the metdl content of
the pulp since the metal content may be detrimentdl to the peroxide bleaching stages to
follow. Again, the chelating stage 16 will preferably employ DPTA as the chelating
agent.
The pulp may then be washed in a washing stage (W stdge) as indicated at 18
but again, the washing step 1~ is not essential. If washing is preferred, any
uullvcllLiullal washing stdge may be used.
The pulp then passes into a peroxide stdge (P stage) indicated at 20, which may
be in tbe form of a single peroxide stage, P, or a sequence of peroxide stages such as Pl,
P~ (P1 being indicated by the box 22), i.e. the peroxide may be applied to the pulp in a
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single dose or in two or more doses to produce pulp of the required brightness. The
brightened pulp leaves the system as indicated at 24.
Conventionally used ~ and times and ~ may be used in
the P stage, thus the peroxide added will be in the range 2 to 10% based on dry weight
5 of fne fibers, caustic (NaOEI) to peroxide ratio of 0.5 to 1 to I to I (optimized for each
stage), t~ about 50 to 100C, ~ ll."y up to about 40% and time up to
about 3 hours, i.e. as required to attain the desired or target brightness. When a
sequence of P stages is used, each of the peroxide stages is carried out in a c,ollv~ iulldl
manner amd will normally apply between 2 and 10% peroxide. The first stage (Pl) will
10 apply an amount of peroxide generally at least equal that applied in the second stdge
(P~) and normally less will be applied in the second stage.
Examples
20 kg of samples of ~ pulp (TMP) made from Canadian
western red cedar and refined to 165 ml Canadian standard freeness (CSF) and having a
15 brightness of 33.1% ISO was used as a starting material for some of the bleaching
sequences and a chemi-th~ pulp made by applying a sodium sulfite
solution, pH 9.1 to apply a chemical charge of 4.1% NalSO~ oven dried on the wood
amd the chips refined to 340 CSF to provide a second pulp of Canadian red cedar as a
starting material for other of the bleaching ~ T_is CTMP pulp had a
20 brightness of 35% ISO.
These pulps were treated in a variety of different stages and a variety of
different sequences. The stages used are defined as follows:
- E = caustic extraction
- F = FAS (f~ - 7' ' sulfinic acid) bleaching
25 - P = peroxide bleaching
- Q = chelation which in the current example apply DPTA
- R = Borol bleaching (solution of sodium b~ in alkaline aqueous
medium)
The conditions used in these bleaching stages are defined in Table I.
, 217q~
TABLE I
Shndard Treatment Conditions
Stage Chemical Dosage T: , IUI . pH Consistency Time
ENaOH 2% 60C #12 1% 30 min
FFAS1% 80C 10 10% 30 mi~
Pl Peroxidet 2-10% 80C 10-12 15, 25% 3 hr
NaOH 0.5~ 1$
P2 Peroxide~ 2-6% 80C 10-12 2s% 3 hr
NaOH o .5 :1 -
0.9:1$
QDTPA 0.15% 50C 4.0 3% 30 min
RBorol 1% 50C ~12 10% 30 min
(as NaBH4)
Notes:
tAuxiliary chemicals used: 5% silicate, 0.05% MgSO4
~Caustic to peroxide ratio
EYample 1
Table II shows the results obtained when applying the various stages as defined
in Table I in the selected sequences as defined in Table 1I. In treating TMP, a single
peroxide stage was used at 6% peroxide and 4.8% NaOH using a ;Ulli~ lUy of 15%.It can be seen from Table II that the use of caustic extraction (E stage) alone
~vith or without an extra wash, i.e. the W stage 18, resulted in a brightness after the
15 peroxide stage of at least as good as treatment with FAS or Borol and almost as
effective as treatment by extrætion followed by FAS or Borol, thereby indicating that
the use of an extraction stage (E stage) substantially free of other hri~hf~nin~ agents
such as Borol or FAS prior to a peroxide stage (P stage) is extremely effective in
increasing the brightness, of the TMP pulps. It is clear that the alkaline extraction stage
20 (E stage) prior to the P stage does not change the initial brightness of the pulp
sigluficantly but increases substantially the peroxide hri~ht.-nin~ response by about 10
points (percent ISO).
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TABLE II
Brightness (% ISO)
Sequence Pre-Treatment Afler Afler
Pre-Treatment Peroxide
QPNone 33.5 54.5
QFQP FAS 46.3 63.3
QRQP Borol 35.4 65.3
QEQP Extraction 33.9 64.3
QEQP Extraction at extreme 36.7 64.7
conditions (5% NaOH, 80C)
QEWQP Extraction + extra wash stage 63.6
QEFQP Extraction + FAS 45.9 67.0
QERQP Extraction + Borol 31.9 66.1
All experiments done with the cedar TMP, using standard conditions (Table I).
All peroxide stages done with 6% peroxide, 4.8% NaOH, at 15% Cs.
To test the effect of the change in caustic, l~ Luu~ amd extraction time,
further ~ were conducted using an E stage with a caustic charge of 5%
NaOH based on the dried weight of the pulp, a Icil~ Lulc of 80C and an extraction
tirne of 60 minutes. The resultant pulp had essentially the same brightness after a P
stage as that obtained when using the standard E stage as defined in Table 1.
E~amPle 2
A plurality of sequences were applied to the CTMP pulps which were then
brightened at 25% ~UU~;~t~ y using the standing conditions defined in Table I but
applying different amounts of peroxide in the different P stages.
When the amount of peroxide applied was split between a P, and a P2 stage, a
slight advantage was seen comparing a single stage application of a given percentage of
peroxide to applying the same arnount of peroxide but in two stages and a further
UII~ U~,uL was found when at least half of the peroxide was applied in the first stage.
The use of a Q~ stage as a ~ LI~,a~lllcillL to the extraction stage (E stage) is found
to contribute little, if amytbing, to the end result. However, in most cases, a Q~ stage
will be employed to remove metals before the peroxide stage (P stage), preferably
between the extraction stage (E stage) and the peroxide stage (P stage).
2 ~ ~7 ~
TABLE III
Pl P2
Sequence [H202] [NaOH] Brightness [H202] [NaOH] Brightness
% % % ISO % % % ISO
EQP 6 3.6 65.5
EQP 8 4.8 68.6
EQP10 5 0 69.4
EQPP 2 1.8 53.0 6 3.6 69.5
EQPP 6 3.6 66.5 2 1.6 70.7
EQPP 4 2.8 59.0 6 4.2 72.7
EQPP 5 3.0 61.4 5 3.0 73.4
EQPP 6 3.0 62.4 4 3.2 72.5
EQPP 8 4.0 67.1 2 1.4 72.7
All ~ JI ' done with the cedar CTMP, using standard conditions (Table 1).
All peroxide brightening done at 25% Cs.
Having described the invention, rn~\difi,~tion~ will be evident to those skilled in
the art with~ut departing from the scope of the invention as defined in the appended
claims.
