Note: Descriptions are shown in the official language in which they were submitted.
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Process for the delignification of cellulose containin~ raw ~aterials
The invention relates to a process for the delignification of cellulose
- containing raw materlals using an extraction solution of concentrated
aqueous acetlc acid at elevated temperatures and pressures.
Sulphite and sulphate processes are the classical, commercially used
processes for the extraction of cellulose from the typical raw materials,
such as suitable leaf wood and coniferou~ wood, annual or perennial
fibrous plants, for example bagasse, reeds, or Miscanthus siniensis, as
well as ~rain straw. The negative environmental impact of the extraction
solutions used in these processes is known. The disadvantages of these
processes are on one hand the use of sulphur containing extraction
solutions and the complicated disposal of the resulting spent liquor which
contains sulphur and the extracted lignin, and on the other hand the use
of chlorine containing bleaching agents which is also problematic.
An approach other than the sulphite or sulphate processes is taken in
those processes which use organic solvents as extraction solutions instead
of the environmentally potentially harmful chemicals. Acetic acid has
been shown to b~ especially advantageous in this respect. Processes at
ambient temperature are known for the extraction of cellulose from
ligno-cellulose containing starting materials with acetic acid in the
presence of catalytically active amount~ of mineral acids. Also known are
non-catalytic processes for the extraction of cellulose from
ligno-cellulose containing starting material~ with acetic acids at
temperatures of 150-205C (see US 3,553,076). Under optimum conditions
and if, for ex~mple, coniferous wood is used as the startlng material,
residual lignin contents of 3.4 to 6.8 wt.% may be achieved with these
processes, which corxesponds to a cappa number of 20-40.
A further decrease in the lignin content was either not desired or not
achievable with conventional chlorine containing bleaching chemicals.
In another process for the extraction of wood with acetic acid disclosed
35 in German Published Application 2445132, the extraction liquid contains in
addition to concentrated acetic acid a small portion of a mineral acid
used as catalyst, for example, hydrochloric acid. Disclosed is an
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Pxtraction with diluted aqueous soda lye wh~ch would require a technically
costly recycling of alkali components. Fu~hermore, the continuous
percolation of wood described in this published application is linked with
a circulation of high amounts of acetic acid, w~ereby a weight ratio of
wood to solvent of about 1-20 is typical. Acetic acid is used both in th~
boiling step for the extraction of the starting material such as
coniferous wood or yearling plants and in a subsequent bleaching step
using hydrogen peroxide. Ozone in acetic acid has also been used
successfully as bleaching agen~ as disclosed in European Patent
10 Application 0 325 891. However, on one hand, the catalytically active
amounts o~ mineral acids are practically almost non- recyclable from the
generated waste waters or from the disolved wood decomposition products.
On the other hand, these mineral acids, for example, hydrogçn chloride or
hydrogen bromide, are volatile which among other things leads to an
annoying smell. Also, the salts which are already contalned in the
inorganic components of the extracted material or which may be produced by
neutralization wi~h diluted aqueous soda lye solutions would increase the
waste water load and would limit the fur~her processing or disposal of the
separated li~nin because of the resulting content of halogen containing
compounds. Furthermore, it is known that the presence of hydrogenated
halogens in a liquid phase is linked to significant corrosion problems
with the container materials generally used.
These deficiencies have been shown to have i~lpeded the industrial
application of the processes according to German Published Application 34
45 132 or European Application 0 325 891.
Furthermore, these processes are characterized by a high consumption of
bleaching chemicals at comparatively low resulting white grades.
According to the process of the German Published Application, a hydrogen
peroY.ide consumption of about 5% must be expected for conifero~s wood
relative to absolutely dry cellulose (in the following referred to as
atro) with a final white grade of 48.6% ISO, or, in a process as disclosed
in the European patent application, a consumption of about 2. 6~o per weight
of ozone and 1% per welght of hydrogen peroxide must be expected relative
to atro cellulose wlth a final white grade of 62% ISO.
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In view of these deflciencies, it is an ob~ect to provlde a process for
the ex~raction o~ cellulose suitable for paper produc~ion or for the
manufacture of products from regenerated or chemically modified cellulose
as described above, which process includes a serles of process steps that
permit complete delignifica~ion of the wood as well as bleaching without
the use of chlorine containing chemicals.
This object is achieved with a process in accordance with the invention
for the delignification of cellulose containing raw materials including a
first delignifica~ion step, wherein a cellulose containing starting
material is treated in an aqeous acetic acid at a weight ratio of raw
material to aqueous acetic acid extraction solution of between 0.08 to 8
and 0.5 to 1, a water content in the extraction solution of 5 to 50 wt.%,
a temperature of 140-230C, a pressure of 3 to 30 bar and a residence time
between 0.5 and 8 haurs including a subsequent extraction or washing and a
second delignification step, wherein the acetic acid wet cellulose of a
consistency of 3 to 60 wt.% is treated with an ozone containing gas in a
liquid phase which corresponds in composition to the extraction solution,
at a pressure of 1 to 12 bar and a temperature of 15-50C, the amount of
ozone bein8 0.1 to 2.5 wt.%/atro raw material.
Thus, the process in accordance with the invention includes a
delignification in two successive delignification steps with an ozone
delignification in the second step, whereby the total lignin content of
the cellulose obtained may be reduced to less than 1 percent per weight.
Although a lignin content of about 1 ~/wt may be achieved with the process
according to German published application DE 24 45 132, this i8 only
possible in the presence of hydrogenated halogens and at a high bleaching
agent consumption.
Depending on the desired quality of the cellulose end product, one or more
additional bleaching steps including the use of hydrogen peroxide,
peracetic acid solutions, sodlum borohydrate or chlorodioxide may be added
subsequent to the bleaching steps provided in the prvcess in accordance
with the invention.
The preferred Cl-C4 aliphatic monocarbonic acid for use in the process in
acrordance with the invention is acetic acid, however9 in place thereof or
in addition thereto other Cl-C4 aliphatic monocarbonic acids may be used
for the separation of the cellulose containing raw materials, preferablg
propionic acid and less preferably formic acid or butyric acid.
In the present two stage process, the water content of the raw material
added, which was previously mechanically chopped or degraded to chips, is
preferably reduced ~o a desired level by carefully carried out contact or
convection drying processe~. If required, the raw material may be
pre-impregnated with acetic acid.
Wood or other ligno-cellulose con~aining materials which may be
pre-impregnated, if required, a~e mlxed with a sol~ent consisting of a
mixture of acetic acid and water, whereby the aceti& acid portion in the
extraction solution is at least 50 wt.% including the wat r contained in
the added material, and treated at a temperature of 140-230~C, preferably
170-200C and at an elevated pressure of 3 - 30, preferably 5 - 12 bar.
The reaction times are between 0.5 and 8, preferably 1 - 4 hours depending
on the temperature and the starting materials used.
During the reaction, the weight ratlo of atro starting material to solYent
may be varied between 0.08:1 and 0.5:1, preferably 0.2:1 and up to
0.33:1. During continuous operation, a relatively higher weight portion
of wood may be used relative to the amount of solvent~ When fibrous
plant~ are used 9 the weight ratio may be var~ed and adapted to the
respective process requirements. After the reaction9 the chips or fibrous
plant portions are mechanically pulped for example by using a mixlng
apparatus and cleaned of decomposition products by extraction with an
aqueous acetlc acid solution.
The extracted cellulose obtained in this first step is subsequently
sub~ected to a further delignification with ozone containing gases at an
ozone content in the carrier gas of about 5 - 10 wt.%. This reaction may
be carried out in a consistency range between a solid ~atter content of 3
~t.% and up to or o~er 60 wt.% preferably 30 - 50 wt.%. A&etic acid
solution may be squeeæed off or added depending on the desired
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consistency range of the cellulose. The reaction is g~nerally carried out
at a pressure of 1 - 2 bar, however, especially in the intermediate
consistency range of 8 - 16 wt.% solid content, reaction pressures of 1 to
12 bar may be used. The ozone treatment is preferably carried out at a
temperature of 0 to 70C, more preferably 20 to 40C. Suitable ozone
carrier gases are, for example, air, nitrogen, and oxygen. The production
of the ozone gases is carried out in commercially available apparatus.
The reaction times are 0.01 to 2, preferably 0.05 to l hours.
The transport of th~ ozone to the cellulose fiber takes placeJ depending
on the consistency, elther through dlffusion across the liquid film
adhering to the fiber, or when the cellulose is suspended in a continuous
liquid phase, from the ozone portion which has been brought into solution
in the liquid phase by way of appropriate valve arrangements. The type of
the subsequent treatment depends on the desired use of the obtained
cellulose and may include further steps such as extraction9 bleaching or
washing under acidic, neutral or alkaline conditions. Water as well as
organlc liquids may be used as solvents.
The use of halogenous additives or bleaching agents is no longer required
in the process in accordance with the invention which provides for a
residual lignin content of less than 1% per weight even if the yielded
cellulose was extrac~ed from coniferous wood which is hard to delignify.
The two step process in accordance with the invention ~s further described
in detail by way of the following example and the results obtained are
summarized in the table further below. Furthermore, the regulations
applied in determi~ing the characteristic values of the sollds content,
the cappa number, the white grade and the rigidity are listed below.
ExamDle
250 g atro spruce wood chips with a water content of 32.5 wt.% were
admixed in a 1.6 l autoclave with 42.1 g of water and 1087.5 g pure acetic
acid. The autoclave was heated for one hour until a reaction temperature
of 170C and a pressure of 9 bar was reached, which heating was achieved
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by way of a heat transfer liquid maintained at 202~C and flowing through a
double wall of the autoclave. After 240 minute~ at constant temperature
and pressure, the circulation of the heat transfer liquid was
interrupted. The temperature in the autoclave decreased until a
temperature of 105C was reached after one hour. The extracted chips were
remo~ed from the autoclave and separated from the dark colored acetic acid
solution by way of a Buchner funnel. The cellulose is .subsequently washed
by beating it with a mixer 3 times in 1.5 1 87% pure acetic acid at 70C
and drying by way of the Buchner funnel. The cellulose was cleaned from
further solvent in a centrifuge.
370.1 g of wet cellulose were obtained with a dry matter content of 35.6
wt.%, which corresponds to a yield of 52.7 wt.%. 10 g atro cellulose were
washed with water and the cappa number was determined.
Second Deli~nification Step
215.1 g acetic acid wet cellulose (80 g atro cellulose with a dry matter
content of 37.2 wt.%) obtained in the first delignification step were
transferred to a 4 1 flask and connected to a rotation evaporator adapted
for the ozone bleaching step. The rotating flask was held in a water bath
maintained at 20C
The ozone was produced from oxygèn in a commercially available ozone
generator. The concentration was 105 g ozone/m3 gas under normal
25 conditions of 0C and 1013 mbar.
22.86 1 of the ozone/oxygen mixture were fed to the slowly rotating flask
for a duration of 22 minutes and 51 seconds. The gases which escaped from
the flask were fed into an acidic potassium iodine solution and 0.33 g
unused ozone was determined by titration with sodium thiosulphate using
starch as the indicator. After completion of the reaction, the cellulose
was extensi~ely washed with water and cleaned from splinters in a lab
sorter (slot wid~h 0.15 mm).
209.7 g wet cellulose were obtained, which corresponds to a celluloqe
yield of 47.3wt.% of atro wood. The corresponding dry matter content is
therefore 35.1wt%. The cappa number was determined.
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Final bleachin~ steP
170.9 g wet cellulose (60 g atro cellulose with a dry matter content of
3501wt.%) from the second delignification step was well mixed with 389.1 g
water, 40 mm 1 M sodium carbonate solution and 0.65 mm 40% pure per acetic
acid solution (PS-40~, Peroxidchemie GmbH). The wet cellulose was
~ransferred into a polyethylene bag and submerged for one hour in a water
bath heated to 70C. The cellulose was subsequently extensively washed
with water and dried in a centrifuge.
151.2 g wet cellulose with a dry matter content of 38.1wt.% was obtained,
which corresponds to a yield of 45.4wt.%, and tested with respect to its
op~ical and mechanical properties.
Delignification
step Parameter Results
1 Cappa number 21,3
Yield (weight %) 51,4
2 Cappa number <1,0
Yield (weight %) 47,3
25 Final bleaching step White grade (%IS0) 80,1
Yield (Gew.-%) 45,4
Mill grade (SR) 18 27 42
Rupture length (m) 8310 12530 12610
Burst area (m2) 46,6 72,5 74,4
Ripping resiqtance (cN) 88,0 63,9 61,0
* Trade-mark
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The following regulations were used for the determination of the listed
parameters and material characteristics:
Solids conten~ according to Zellcheming IV/42/62
Cappa number according to Zellcheming IV/37/80
White grade according to Zellcheming V/l9/S3
Rigidlty according to Zellcheming V/4/61, V/5/60, V/8/76J V/7/61,
V/11/~7,
V/3/62, V/12/57
