Note: Descriptions are shown in the official language in which they were submitted.
t 177609
This invention refers to a process for the produc-
tion of pulps from vegetable, lignocellulose-containing
fibrous material, e.g. coniferous wood, straw and bagasse.
Traditional processes to produce pulps, and parti-
cularly the sulphate process which is used to produce almost
the total chemical pulp on the world market, have consider-
able drawbacks. Thus, there is required in order to bring
about in the cooking system an even distribution of reaction
agents in the starting or raw material and thereby good
delignification, plants with pressurized reaction devices
of big volume and circulation of remarkable quantities of
~ liquids heated by heat exchangers. Such plants are extremely
; expensive. The obtained pulp is difficult to bleach for
which reason very complex and highly expensive bleaching
plants become necessary for treatment of the pulp. Further-
more, the cooking and bleaching process causes pollution of
air and water which is unacceptable. Therefore, internal
treatment plants such as ~ustion and chemical recovery
installations, and external treatment plants such as water-
purifying basins, are necessary to bring the effluents to
acceptable level, which require high initial investment costs,
which in turn implies demand for large production units. Air
and water pollution are mainly due to the use of chemical
reactants which contain sulphur and chlorine and chlorine
derivates, the latter ones in large amounts as consequence
of the aforesaid low bleachability of the pulps. The pre-
sence of sulphur brings about i.a. the characteristic evil
smell from plants operating with the sulphate process.
The mentioned drawbacks involve also high energy
consumption due to the plants for impregnation of the raw
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1.177609
materials ~r cooking chemicals, ~or pulp bleaching, for
recovery of chemicals and $or enVironment protection.
;~, . .
~he main object o~ the invention is to provide a
process for production of pulps wherein the handling of the
chemicals is si~plified so that the aforesaid drawbacks are
~ avoided. A second object of the invention is by the simpli-
`; fied handling of chemicals to obtain lower investment costs
.
which results in that minor plants also become profitable
` 10 which permits installation of plants nearer the raw material
sources with consequent reduction of transport costs, which
also involves a generally widened range of raw or starting
materials.
,. j
~; 15 A third object of the invention is to render
possible in this connection a widened utilization of non-
traditional raw materials for produc~ion of pulps such as
green chips, i.e. twigs, branches, bushes and shrubs and low-
grade timber especially frcm coniferous wood as a result from
various thinning and cleaning operations, furthe.rmore agri-
cultural wastes like wheat, rice straw and bagasse, but also
e.g. banana stems, sovbeans and coconut shells, furthermore
also from other annual vegetables cultivated for their
fibre content more than with regard to their nutritive value,
such as cotton, flax, kenaf, but also wild annual vegetables
such as certain kinds of grass and reeds, and finally also
wastes from traditional woodworking industry in the form_of
~d sawmill waste and especially waste from tropical kinds o$
wood possessing very high density and therefore difficult to
treat with conventional processes of pulp production, and
waste from traditional pulp industry in the shape of knot
catcher rejects and some kind of fibre sludge.
,~
According to the present invention there is provided
a process to produce pulps from vegetable lignocellulose-
contain~ng fibrous materials in which the fibrous material is
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1 177609
sub~ected to a se~uence o~ treatment stages, of which in
stage 1 the fib~ous material is i~pregnated with a solution
of sod~um hydroxide Xn which the material is soaked and
partially disintegrated by mechanical workinq, in stage 2
excess o alkaline solution is squeezed out from the fibrous
material, but only so much that after the squeezing stage
the content of sodium hydroxide in the pulp amounts to at
least 15 kgs per ton of pulp calculated as dry substance (BD
ton of pulp~ in stage 3 the fibrous material is subjected to
additional disintegration while supplying a solution of
sodium carbonate so that a pumpable pulp suspension having a
sodium carbonate ~content of 150-400 kgs per BD ton of pulp
is obtained and in stage 4 the pulp suspension is subjected
to a cooking process at a temperature of at least 100C during
1-4 hours and wi-th stirring and a supply of oxygen, whereafter
the cooked pulp is washed.
- In one embodiment of the present invention
the fibrous material, prior to the impregnation with sodium
hydroxide, is washed, the material being disintegrated
at the same time. Desirably the material during the impreg-
nation in stage 1 is soaked for a time of from 15 to 45 minutes at a
temperature of between 50 and 110C. Preferably from the
fibrous material in stage 4 the excess of alkali solution is
squeezed out until the dry content of the pulp amounts to 20-
55%. Suitably in stage 4 during the cooking process 5-15 kgs
of magnesium carbonate per BD ton of pulp are supplied to
the pulp suspension.
In another embodiment of the present invention the
process is applied to a fibrous material which has been dis-
integrated mechanically into piece-shape, such as chips in
which process the disintegrated fibrous material is impreg-
nated by becoming steamed, compressed and thereafter allowed
to expand in the solution of sodium hydroxide and to soak up
part thereof.
- 2a -
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l 177609
~n another embodiment of the present invention the
; cooking p~oces$ of stage 4 is performed in two separate cook-
~ng steps, the first of wh~ch is ca~r~ed out at a temperature
of 140-160C fox between 60 and 120 minutes on a pulp sus-
pension coming from stage 3 and containing 150-200 kgs sodium
; carbonate per BD ton of pulp, the cooked pulp thereby ob-
tained is washed and solution is squeezed off and thereafter
A~ additional 150-200 kgs sodium carbonate per BD ton of pulp
are supplied, whereupon the pulp is subjected to a second
` 10 cooking step at a temperature of 130-150C for between 60 and
120 minutes with stirring and a supply of oxygen so that the
total pressure amounts to 10-20 Bar at 110-150C. Desirably
the pulp after the first cooking step is liberated from
solution by squeezing to a dry content of 30~ and thereafter
subjected to additional disintegration under simultaneous addi-
tion of sodium carbonate.
.~
In a further embodiment of the present invention
the finally treated pulp is bleached by means of ozone,
the ozone being produced from oxygen gas in ozonisers and
after the bleaching the oxygen gas is purified from residual
p ozone by being passed through a coal bed in the presence
of water and thereafter returned to the process.
;,
, 25 The use according to the invention of other chemical
reactants than sulphur and its compounds and compositions
with sodium in an alkaline surrounding, molecular chlorine
; and, in addition, a significant reduction of the use of
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I 177609
chloro-derivates result in a reduction of environment pollu-
tion. Thus, i.a. the problem of evil smell can-practically
be eliminated by the process of the present invention. The
use of the proposed chemical reactants allows simplifica-
tion of the plants for recovery of chemicals and heat andassures also the production of chemical pulps which are
easily bleachable, making bleaching plants less complex
and less capital-intensive. According to the invention, it
is possible to produce, in addition to chemical pulps,
also pulps of the so-called chemi-mechanical and semi-che-
mical types.
The invention will in the following be described in
; more detail with reference to the embodiment shown in the
'~ accompanying drawing.
; 15 The figure shows diagrammatically a complete plant
for utilization of straw, the process being subdivided into
' blocks (A, B, C, D, E, F, H, I) and units, each one re-
presenting a section of the plant. Following the direction
' of the main flow represented by double drawn arrows, one
will observe:
Block A represents the washing in that section of
the plant where the fibrous material is washed in several
steps with water and waste liquor from the pulp washing
after the first cooking step to remove heavier particles
' 25 and sand, but also some fine substance of partly colloidal
nature, from the fibrous material. The washing can be
' carried out under some simultaneous disintegration of the
fibrous material.
Block B represents that part of the plant where the
fibrous material is imbibed with make-up-chemicals'("make-
up" = losses in the washing and recovery systems) in the
form of sodium hydroxide under continued disintegration,
squeezing out of excess of chemicalsand thereby adjustment
to the concentrati'on of chemicals to the correct one for
the process. The whole process stage is carried out at
temperatures between 50 and 110C and results in addition
to impregnation in a first defibration and delignification.
1 1776~9
The impregnated fibrous material is dehydrated in
a press to a dry content between 20 and 55%. The dry content
in the outlet from the press and the concentration of the
circulating liquid in block B renders possible control of
the concentration of chemicals in the outlet from the system
so that the amount is the necessary one to counterbalance
losses in the process and to activate the carbonate accord-
ing to the principles of the invention.
Block C represents that section of the plant where
a continued delignification in a pressurized reactor is
effected after supply of recycled sodium carbonate (Na2C03)
from a chemical recovery system. Block C represents also
that section of the plant where the delignification is
completed by means of oxygen gas. The vegetable raw material
coming from block B is added with the cooking chemical
Na2C03 in that defibration stage which constitutes the
beginning of block C. The delignificating reaction is com-
pleted in a pressurized oxygen gas reactor. The oxygen gas
is added to the fibrous material in known manner in a
mixer especially constructed for this purpose.
Block D-Dl represents the different bleaching stages
used on the unbleached, easily bleachable pulp which is pro-
duced by the process of the invention. Of particular inte-
rest is the possibility to use ozone (block Dl) which
will be described below. -
Block E represents the plant section where dehydra-
tion, drying and baling following traditional techniques
are carried out.
Blocks F, G, H, I, L, M represent auxiliary plants
of the main line. The continuous lines coupled with the
dotted lines represent alternative flow lines.
Block F represents a section where in case of using
raw materials with high silica or silica-compound content,
a first treatment of the effluents is obtained to reduce
silica. This treatment will also be described nearer more
below.
l 177609
Block G represents the section of the plant for use
as storage or for production of chemicals necessary for
the process and particularly:
Gl corresponds to storage of reactants and auxiliary
~ 5 additives for the various cooking and bleaching stages,
! G2 corresponds to the section for production of
chlorine dioxide (C102),
G3 corresponds to storage of sodium hydroxide (NaOH),
j G4 corresponds to storage of hydrogen peroxide (H202),
- G5 and G6 correspond respectively to the sections
for production of, respectively, oxygen gas (2) and ozone
(3)
It is obvious that not all these sections and/or
storages of the plant are co-existing; some of them are
alternatives to the other sections depending on the
bleaching sequency used.
; - Block H- represents the section for the treatment of
the gases consisting of a mixture of 2 + C2 + U20 coming
from the relief valve of the oxygen gas reactor (block C).
. .
The aim is to separate and recycle 2 following a technique
which will be described hereinafter.
Block I represents the section to remove traces of
03 from the mixture 2 + 3 coming from the bleaching system
in case the bleaching is based on ozone (block Dl). The
corresponding technique is described`nearer in the following.
B10ck L represents the section for secondary treat-
ment of effluents coming from the individual sections of the
plant: at 1) admission of the effluent from the raw material
washing (see block A); at 2) admission of the effluent from
the bleaching when the alternative technique chosen origi-
nates an effluent containing chlorine dioxide; at 3)
admission of effluents from various mill services.
Block M finally represents the plant section for
recovery of chemicals and heat as follows:
Ml represents the evaporating system for concentrat-
ing black liquors coming from the main pulp production line~
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1 1~7~09
M2 represents the system for concentrated black
liquor combustion and heat recovery from the dissolved
organic matter of the raw material and recovery of chemicals
such as Na2C03.
M3 represents dissolution and storage of the recovered
- Na2C03 solution which is recycled to the delignification
stages (blocks B and C).
In accordance with the invention sodium carbonate is
used as reactant. The reactivity of the mentioned salt is
known to be low towards lignine but also towards the cellu-
losic components and the cellulose and in any case inferior
to the reactivity of the correspondent hydrate. Therefore
the carbonate is activated, in the two stages where the
delignification is accomplished by means of sodium hydrate
which is added in that stage, where chips imbibition takes
place, in quantities corresponding to the necessary ones
to make up the loss of process reactants.
The quantity varies, according to raw materials and
final products, between 15 and 30 kgs of hydrate per BDT
(bone dry ton) of processed fibrous material.
For the activation 2 also and possibly other oxida-
tive agents (peroxides) are used in the last phase of de-
lignification where working preassure, temperature and
amount of 2 are varied within very wide limits (10-20 Bar,
110-150C and 6-14% of 2) according to raw material and
desired final products. The process according to the inven-
tion is therefore characterized by the use of carbonate as
alkaline salt directly in the process of delignification
which develops in one or several stages.
Very good delignification (kappa no between 15 and 8)
has been obtained whith the aforedescribed process simultane-
ously with viscosity levels high enough after the subsequent
bleaching process to ensure good characteristics of resis-
tance of the paper pulp together with a brightness of about
50% Scan impossible to be obtained by any known alkaline
process.
- ` I 177609
; Characteristic for the process according to the in-
vention is also that it yields pulps with low extract;ve
content and with high cleanliness inspite of utilization
of low quality raw material conta';ning bark, for example.
This is due to the fact that oxygen gas in alkaline medi'um
reacts specifically with bark and fibre bundles which are
difficult to become del;gn;f;ed by usual processes.
These propert;es of the pulps and particularly the
high brightness permit very simplified bleaching treatments
` 10 in comparison with usual processes. Particularly, chlorine
treatment of the pulp is not required.
In the following table examples are given for pro-
ceedings and results in the bleaching of a pulp produced
from wheat straw by treatment according to the invention':
' 15 Sequence ' Consumption of chemicals Brightness
as % of BD product _ % Scan
D 0.5 - 0.75 70 - 75
P 2 70 - 72
D/E/P 0.5/1, 5/1 + 1.5 80
D/E/D 1/1; 5/0, 5 85 - 88
03 0.3 - 0.4 85 - 90
03/E/P 0.3/1, 5/1, 5 90 - 92
D = Chlorine dioxide
P = Hydrogen peroxide
E = Alkaline washing treatment (consumption expressed as
NaOH equivalent)
3 = zone
As is apparent from this table, pulps produced accord-
ing to the invention are highly bleachable by ozone which
affords a number of advantages. Ozone bleaching is carried
out in a single stage at atmospheric pressure. Investment
'and energy costs of the ozone bleaching are lower than for
other comparable processes. The production costs for ozone
and chlorine dioxide are on the same level as today's
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^` I 1776~9
technology, the energy consumption is 10 kWh per kg ozone provided that
ozonisers are fed with oxygen, in comparison to 12-14 kWh-per kg of
chlorine dioxide starting from sodium chlorate. The effluent from the
ozone bleaching stage can be conveyed to black liquor evaporation and
combustion, for which reason environmental pollution is reduced to a
minimum.
The process according to the invention is characterized also
by the possibility of utilizin~ 2 (moist and not pure) resulting from
the ozone bleaching treatment. Industrial ozonisers when fed with
oxygen yield a gas mixture where the ozone content is 2.5-2.7 %.
After exhaustion by reaction of ozone in the bleaching process oxygen
may be recycled to the second delignification stage. The gas mixture
coming from the bleaching stage (block D) is here passed through an
ozone purifier in the shape of a coal bed (block I) in the presence
of water in order to prevent carbohydrate degradation which is par-
ticularly severe at the used high reactor temperature even when traces
only of ozone are present.
Pressure control in the oxygen gas reactor (block C) and
providing security that any explosive gas mixture of carbon monoxide
(C0) and oxygen gas (2) cannot be formed are made by continuous
venting of the reactor for treatment of the mixture of steam, carbon
dioxide and oxygen gas. The treatment is carried out by washing the
mixture with a carbonate solution. Sufficiently pure oxygen gas is
obtained which is separated and recycled to the reactor after re-
compression whereas the carbonate is turned into bicarbonate. By
heating the bicarbonate solution the carbon dioxide is expelled and
the obtained carbonate solution is concentrated and can be used again
for washing (block H).
Every process for production of cellulose pulp requires both
for economic reasons and environment protection reasons destruction or
recovery in some shape of used inorganic chemicals and organic material
released in the process.
Every process for recovery of chemicals and heat involves
necessarily combustion, mostly after concentration by evaporation, to
a dry content of 50-65 % of black liquors collected from the pulp
washing during and/or after the delignification process. Black liquors
contain those inorganic
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1 1 77609
chemicals which have been used for the dellgnification, chemi-
cals to a small extent as free reactants and for the most
part in combination with salts of organic acids, alkali
lignin and other compounds of sometimes very complex struc-
ture and forming non-cellulosic wood components.
Combustion of black liquors liberates carbon dioxide
(CO2) and for this reason the alkaline salts are recovered
as carbonates in a sulphur-free process. Conventionally, the
carbonate is turned to hydrate in order to restore the re-
activity and delignifying capacity of the alkali, which pro-
cedure is carried out in so-called causticizing plants which
involve high investment costs, high energy consumption and
cause considerable injury to the environment. The direct
utilization of carbonate in the delignification process
according to the invention thus eliminates the causticizing
step andthe disadvantageous effects thereof (block M in the
figure).
Straw and other annual vegetables normally have a
high silica content which causes troubles in connection with
evaporation and combustion and even results in that the re-
covered chemicals are contaminated. The process according
to the invention renders possible a treatment for reducing
the silica content in black liquors. The treatment is based
on the fact that the solubility of silica is a function of the
pH of the alkaline solutions and that the silica content is
higher in the so-called "O" fibre fraction that in other
fibre fractions. The treatment is carried out as separation
of that fraction of the black liquor from the straw washing
stage which has a high content of "O" fibres. The pH reduc-
tion of the liquor by means of that CO2 which is present in
the combustion gas enables silica to be precipitated out of
the system (block F in the figure).
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