Note: Descriptions are shown in the official language in which they were submitted.
~S5656
The present invention relates to the multis~age bleach-
ing of pulps.
Pulps which are obtained by means of various hydrolyz-
ing processes of an alkaline or acid kind still contain consid- ~
erable amounts of lignin, hemicelluloses and other si~ilar ~. . .
components, which must be removed by subsequent bleaching pro-
cesses in order to obtain products having high whiteness, white-
ness stability or specific serviclability, as for example, suit- :
ability for use in the viscose rayon process. sleaching chemi~
cals such as chlorine (C), chlorine dioxide (D), sodium or .~
calcium hypochlorite ~H) are used in a variety of stages with ~.
intermediate alkaline extractions (E). MGreover, it is known
to use hydrogen peroxide and sodium peroxide and organic per-
compounds (P) for bleaching pulps, ei~her in the rorm of single-
stage bleaching or as the final stage in multistage bleaching
particularly in order -to obtain good whiteness stability :i.e.
resistance to yellowing.
The full bleaching of pulps (whiteness degree of 88%
of MgO or better 90~ of MgO) requires one or several chlorina-
tion stages and additional hypochlorite stages and selectively ;
also chlorine dioxide stages. Because of -these processes sub-
stantial amounts of hydrochloric acid, or when neutralization ..
is effected, substantial amounts of sodium chloride get into the
effluent water and thus into the main canal and into the waters
which receive the effluent water. The multistage bleaching pro-
cesses C/E/H, C/E/H/H, C/E/D/E/D, C/E/H/D/P, C/E/H/D/E/D and
C/E/H/D/~ are given as examples of the bleaching processes used :
today. Thus, all these processes start with a chlorination
stage and subsequent extraction stage.
In connection with above waste water problems and the
loading of the main canal with salts and dissolved organic sub-
stances bleaching sequences such as PDPDP or even P/PES/P/PES/P,
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0556S6
where P is peroxide and PES is pex-acid which allow a major por-
tion of the bleaching waste waters to be charged to cooking-
liquor regeneration processes are disclosed in German Patent
Applications Nos. P 22 19 504.6-45 and P 22 19 505.7-45 published
- 15/11/1973 and 8/11/73 respectively.
The attempt to replace, in the sulphate pulping pro-
cess, the chlorine treatr.lent by a treatment with molecular oxygen _l-
is referred to. How~ver, the ~reatment must be carried out under
pressure. Moreover, it is still open to question whether the
losses in strength which are caused per se by the alkaline treat- ¦
ment with oxygen under pressure can be prevented by the addi-
tions proposed in the literature such as magnesium carbonate or !`
magnes.ium oxide (German Published SpeciEication 2,:lO9,542,
published September 16, 1971, Rowlandson, Tappi 5~, 962-967
(1971), No. 6).
However, this technology was used heretofore only
tentatively on a large industrial scale and the fact that
nothing is known about the feasability of applying it to sul-
phite pulps constitutes a further limitation.
The present invention provides a multistage process
for fully bleaching pulps which can be carried out in existing
bleaching apparatus of a pulp mill with substantially lower ~¦
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amounts of chlorine than heretofore.
I It has now been discovered that pulps can be fully .
bleached in sevexal stages without the use of pressure when
the bleaching sequence ~if required in co~,lbination with a
slight partial chlorination) starts with a stronyly alkaline ~;
peroxide bleach hydrolysis.
This strongly alkaline peroxide bleaching stage (P) .
which simultaneously acts as a hydrolysis replaces the conven-
tional acid chlorination with subsequent alkaline extraction
stage, i.e., the stages C-E of these processes mentioned herein-
L[D55656 1~ ~
before. This makes it possible to operate with substantia]ly . ~ -
lower amounts of chlorine than heretofore.
As in the known bleaching processes, the bleaching : :~
effect
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depends on the kind of wood, i.er conifers (soft wood) or hard
wood or annual plants, as well as on the manner in which the
pulping process is carried out and thus on the degree of hydroly- `
sis .
The possible partial chlorination is necessary only when
hard pulps, i.e., pulps rich in lignin and hemicellulose are
bleached to whiteness contents of 90% of MgO. A partial chlorina-
tion can be carried out either prior to the start of the actual
bleaching operation, i.e., prior to the strongly alkaline peroxide
stage, or the strongly alkaline peroxide stage is Eollowed by a
partial chlorination and e~ctraction stage. ~pproximately one-flfth
o the amount of chlorine used heretofore and approximately one-
half of the amount of alkali used heretofore are required.
The amounts of chlorine used in these chlorination
stages are only a fraction of the amounts normally used in the
chlorination, as shown above. The peroxide stage is carried out ~;
with a large excess of alkali, as mentioned above. This partial
chlorination can also be dispensed with the usual oxidation
stages which follow the hydrolysis bleaching, such as hypochlorite, ;`
chlorine dioxide or peroxide, are efEected. This partial chlorina-
tion may be omitted altogether when whiteness contents above 90%
of MgO are not required and when whiteness con-tents between 85 and i~
90% of ~gO are suficient. ~ -
Bleaching with chlorine and alkali (C-E stage) dissolves
out organic substances. Therefore, oxygen-consuming substances ',!~
and compounds having a high chlorine content get into the effluent
water of the pulp mill,iwhereupon the waste water cannot be any ~
longer discharged into the main canals. ~ ~`
It is a known fact that bleaching waste waters which con-
tain chlorine or chloride ions cause intense corrosion in theregenerative devices. The strongly alkaline bleach hydrolysis
described permits the evaporation and concentrat:ion of the sub-
.. . ~, .
stances dissolved out, their combustion and the regeneration of
alkali in conventional apparatus. Moreover, the strongly alkaline
peroxide treatment surprisingly showed that 85 to 90% of the sub-
stances loading the waste waters are dissolved out in this stage
and thus are not discharged into the main canal.
~ydrogen peroxide, sodium peroxide and other inorganic
or organic peroxides or hydrogen peroxides, preferably hydrogen
peroxide, sodium peroxide or t-butyl hydrogen peroxide or mixtures
of these components in aqueous solutions are suitable as the per-
compounds. Hydrogen peroxide and sodium peroxide are particularly
suitable. The amount of peroxide used may be between 0.2 and 10%
by weight, preferably 0.6 to 6% by weight, computed as a 100%
hydrogen peroxide and relative to atro pulp (atro=absolutely dry).
Sodium hydroxide or even calcium hydroxide or ammonium
hydroxide (N~13 or Ntl40~1) are preferably used as alkalis, i.e.,
alkali or alkaline earth hydroxides, required for the strongly
alkaline peroxideic bleach hydrolysis described hereinbefore.
Said hydroxides are used in amounts from 2 to 15% by weight, pre-
ferably from 4 to 8% by weight, computed as a 100% substance and
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relative to absolutely dry pulp.
The alkaline-peroxidic bleach hydrolysis is conventionally
, carried out at temperatures ranging Erom 20C to the boiling point
; of the bleaching liquor.
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~;- It is of course possible to carry out the bleaching
operation under pressure, but the advantage obtained when using
-the percompounds and the bleaching sequence mentioned hereinbefore
lies particularly in the fact that the operation is at atmospheric
~, pressure, i.e., without the use of additional pressure and appara-
tus.
As mentioned hereinbefore, the bleaching process is car-
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ried out in existing bleacheries; the pulp consistencies prefer-
ably are between 10 and 20%. By pulp consistency is meant percent
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by weight of pulp in the pulp suspension. However, it is also
possible to carry out the strongly alkaline peroxidic stage at
higher solids contents, i.e., up to 35%, using suitable technical
aids, as for example, thick-pulp pumps, kneaders, and high- `
pressure filters.
he absence of sodium tetrasilicate, which is normally
used in peroxidic bleaching stages, is a fu;rther advantage.
Conventional inorganic complexing agents or stabilizers, > -
as for example, magnesium sulphate, or organic nitrogen- or
phosphorus-containing complexing agents, as for example, ethylene ;~ -~diamine tetraacetic acid, diethylene triamine pentaacetic acid
and nitrolo triacetic acid, can be used in the bleaching liquor.
It is also possible to use nitrogen-and phosphorus-free complex-
ing agents. For example, polyoxy-carboxylic acids can be corres- ;~
pondingly used (German Published SpeciEications Nos. 1,904,940
published Aug. 6, 1970, 1,904,941 published August 6, 1970 and
1,942,556 published August 11, 1971. ~
The advance in the art oE the process according to the `;
- present invention lies in that pulps, including hard pulps, can
be fully bleached in existing bleaching apparatus of the pulp
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, mills~ A further important advantagç lies in the recovery of ~
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waste waters in which substances loading the main canal are dis-
solved out in the first stage. The waste waters thus obtained
can be evaporated and regenerated such that they are noncorrosive.
This includes the effluent waters obtained in full bleaching of
; hard pulps. ~
The examples hereafter serve to illustrate the process ;
according to the invention. The percentages always are percent
by weight, relative unbleached dry pulp. In all the tests tap ~`
water of 5 German hardness was used and the operations were
carried out in batches of 100 to 500 g of pulp (dry weight) in
enamelled containers. The pH values were measured at the start
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~ of the bleaching operation. After each stage the pulp was washed
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with tap water and after the last stage it was acidified with ;
sulphur dioxide. The whiteness degrees~luminance factors) were
determined according to German standard methods (Zellscheming
Merkblatter) with the Zeiss Elrepho whiteness degree meter (filter
R46).
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i~ Example 1
,
A beech-rayon sulphite pulp (kappa number 9.6, whiteness
degree unbleached 51.0% of MgO) which had been produced according
to the calcium bisulphite process, was bleached in a four-stage
sequence under the following conditions: -
1st stage 1~3% of H2O2l+ 8% of NaOH, 12V,~O~`pulplconsils~enc~r65C,2h
2nd stage 0.36% of NaOCl 12% pulp consistency, 50C,l~h
3rd stage 0.13% of ClO2 14% pulp consistency, 65C,l~h
4th stage 0.57~/O of NaOCl 12% pulp consistency, 55C,2h
Ater ~he fourth stage the pu;lp had a whiteness content
of 93.3% of MgO. A slightly higher whiteness content of 94.~/O of
, MgO could be obtained with the following sequence:
1st stage 1.3% of H2O2 - 8% of NaOH, 12% pulp consistency, 65C,2h
, 2nd stage 0.13% of ClO2 14% pulp consistency, 65C,l~h --`
3rd stage 0.36% of NaOCl 12% pulp consistency, 50C,l~h
~th stage 0.57% of NaOCl 12% pulp consistency, 55C,2h
The kappa number could be reduced from approximately 9.6
in the unbleached pulp to below 1. The lignin content decreased ~
; from 1.2% to 0.18%. ~ -
`s The other chemical characteristic values such as hemi-
'~; cellulose~d~cel~ ose~ R18 value, ash and methanol-benzene extract
; also correspond to conventional methods. More specifically, the
pulp can be readily processed into viscose rayon. The loss of
substance due to this bleaching process (P-H-D-H) is approximately
7.5% relative to unbleached beech-rayon pulp.
Example 2
A beech-paper sulphite pulp (kappa number 22.3, whiteness
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degree unbleached 56.7% of MgO) was bleached in a four-stage
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sequence under the following conditions:
1st stage 1.3% of H2O2 - 8.9% of NaOH, 12% pulp consistency, 65C,2h ~
2nd stage 0.5% of NaOCl 12% pulp consistency, 40C,l~h `;~ -
3rd stage 0.22V/o of ClO2 l~/o pulp consistency, 65C,2h
r~, 4th stage 0.17% of NaOCl 12% pulp consistency, 50C,2h
The final whiteness content was 88.0% of MgO in this case.
If the amount of peroxide used in the first stage is in-
creased to 2.6% of H2O2, then a whiteness degree of 91~4V/o of MgO
. . . ~
; 10 can be obtained with a total of only three stages (P-D-H).
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1st stage 2.6% of H2O2 - 8.0% of NaOH, 12% pulp consistency, 65C,2h ~`
2nd stage 0.22% of C102 12% pulp consistency, 65C,2h
3rd stage 0.5% of NaOCl 14% pulp consistency, 45C,2h
Example 3
In order to attain a maximum bleaching efEect on a beech-
calcium bisulphite paper pulp (see Example 2) a weak chlorination
stage is inserted prior to the strongly alkaline-peroxidic bleach
hydrolysis.
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1st stage 0.5% of C12 4% pulp consistency, cold, ~h
2nd stage 1.3% of H2O 12% pulp consistency, 65C, 2h
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3rd stage 0.66% of NaOCl 12% pulp consistency, 50C, 2h
~, 4th stage 0.23% of ClO2 14% pulp consistency, 65C, l~h
5th stage 0.27% of NaOCl 12% pulp consistency, 52C, 2h ;
0.2% of NaQH
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~ ~fter this stage sequence a whiteness content of 95.4% ~;
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of MgO was attained. The pulp yield was 93.2% relative totthe
unbleached pulp. In the first two stages (CP) 87.1% oE the sub-
s
stances loading the waste waters were dissolved and were then pas-
sed to a regeneration process. ;
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Example 4
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A spruce paper pulp (kappa number 19.55, whiteness un-
` bleached 50.2% of MgO) produced according to the calcium bisulphite
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process is bleached in a four-stage process according to the fol-
lowing sequence:
1st stage 2.6% of H202 l~/o pulp consistency, 65C, 2h
8.0% of NaOH
2nd stage 0.2% of C102 12% pulp consistency, ~5-50Cg lh
3rd stage 0.3% of C102 14~/o pulp consistency, 57C, lh
4th stage 0.63% of NaOCl 12% pulp consistency, 43C, 2h `
By means of this bleaching process a whiteness content
of 87.4% of MgO is attained.
This Example clearly shows that for obtaining whiteness
degrees between 85 and 90% of MgO a chl-orination stage can be
dispensed with even in hard spruce paper pulps. ~-
For whiteness contents higher than 90% the Eollowing
sequences with subsequent partial chlorination are suLtable:
1st stage 1.3% of H22 12% pulp consistency, 65C, 2h
6.0% o NaOH
2nd stage 1.3% of C12 4% pulp consistency, cold, ~h
3rd stage 1.2% of NaOH 12% pulp consistency, 60C, lh
4th stage 0.62% of NaOCl 12% pulp consistency, 50C, 2h
5th stage 0.34% of C102 14% pulp consistency, 70C, 2h
, 6th stage 0.30% of NaCl 12% pulp consistency, 50C, 2h
This whiteness content was 92.5% of MgO in this case.
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With a preceding partLal chlorination a whiteness content
of 94.1% of MgO is attained.
;...................................................................... .
1st stage 1% o~ C12 4% pulp consistency, cold, ~h
2nd stage 1.3% of H22 12V/o pulp consistency, 65C, 2h
6% of NaOH
, 3rd stage 0.66% of NaOCl 12% pulp consistency, 50C, 2h
4th stage 0.30% of C102 1~% pulp consistency, 65C, l~h
5th stage 0.66% of NaOCl l~/o pulp consistency, 53C, 2h
`; 0.2% of NaOH
The abo~e Examples show that the whiteness degree required
for high bleaching of pulps can be attained by means of the bleach
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hydrolysis of the present invention. The yield corresponded to
the order known from conventional strongly chlorine-containing :
multistage bleaching processes. Moreover, since in the alkaline~
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peroxidic bleach hydrolysis 7/8 of the organic substances which . :~
can be dissolved out are no longer discharged~into the main canal
but are fed to the regeneration process, the load on the waste :
water is substantially reduced.
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