Note: Descriptions are shown in the official language in which they were submitted.
CA 02069107 2001-06-26
(a) TITLE OF THE INVENTION
PROCESS FOR THE PREPARATION OF COOKING LIQUORS HAVING HIGH
SULPHIDITY FOR SULPHATE PULP COOKING
(b) TECHNICAL FIELD TO WHICH THE INVENTIfON RELATES
The present invention relates to a process for the preparation of cooking
liquors
having high sulphidity for the sulphate pulp industry starting from chemicals
which are
available in the pulp mill or make-up chemicals which are common in the pulp
mill.
(c) BACKGROUND ART
Sodium sulphide and the closely-related chemical, sodium hydrogen sulphide,
are
often interchangeable with each other and the application thereof often
differs in order to
meet different sulphidity demands. In the aqueous phase, sodium sulphide
hydrolyses
completely or partly to sodium hydroxide and sodium hydrogen sulphide
according to the
equation:
Na2S + H20 ~J NaOH + NaSH (1)
The concept "sulphidity in the pulp industry" is usually expressed according
to the
equation:
sulphidity ( % ) = 2 x NaSH x 100
NaOH - NaSH
wherein NaSH and NaOH are expressed in molar units. This means, e.g., that an
aqueous
solution containing sodium hydrogen sulphide and sodium hydroxide having a
sulphidity of
40% contains 4 times more moles of sodium hydroxide than sodium hydrogen
sulphide. In
the same way the equilibrium equation (1) above expresses a solution having a
sulphidity of
100 % .
Large amounts of sodium sulphide are prepared in thc: sulphate pulp industry.
When
recovering the cooking chemicals, the so-called black liquor is burnt in a
soda recovery
unit, the lower part of which is reductive. In the lower zone of the soda
recovery unit, the
sulphur components of the black liquor are reduced to the sulphide state and
accordingly
1
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~ CA 02069107 2001-06-26
convert to sodium sulphide. The sulphate pulp mills often operate within the
sulphidity
range of 25 to 40% (white liquor sulphidity). A major part of the sodium
reacts with
carbon dioxide when burning black liquor to form sodium carbonate. The mixture
of
sodium sulphide and sodium carbonate forms a melt at the bottom of the soda
recovery
unit. This melt is withdrawn and is reacted with water to form so-called
"green liquor" . A
typical green liquor has the following composition:
Sodium carbonate 90 - 105 g/ 1
Sodium sulphide 20 - 50 g/1
Sodium hydroxide 15 - 25 g/ 1
(all substances calculated as sodium hydroxide).
Chemicals recovery according to the "sulphate process" (kraft) results in a
relatively
large amount of sulphur reaching the oxidative zone and being rejected from
the soda
recovery unit mainly as sodium sulphate (electrical filter ash) and sulphur
dioxide. If the
white liquor sulphidity exceeds 35%, problems begin to ariise, i.a., by high
emissions of
sulphur dioxide from the soda recovery unit. Scrubbing with an alkaline medium
is
therefore often used in order to eliminate or strongly reduce the sulphur
dioxide emissions.
The green liquor so-obtained is converted to white liquor according to the
known
causticizing process. The composition of a white liquor ma.y vary from mill to
mill but
approximate concentration values are as follows:
Sodium hydroxide, NaOH 80 - 120 g/ 1
Sodium sulphide, Na2S 20 - 50 g/1
Sodium carbonate, Na2,CO3 10 - 30 g/ 1
Sodium sulphate, NazS04 5 - 10 g/1
(all substances calculated as sodium hydroxide).
If NazS is estimated to be fully hydrolysed according to the equation:
Na2S + H20 ~ NaOH + NaSH
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CA 02069107 2001-06-26
this means that the amount of sodium, which is bound as c<~rbonate, often
amounts to more
than 20 % of the sodium present as hydroxide.
It is known that the presence of sodium hydrogen sulphide is advantageous in
pulp
cooking according to the sulphate process. The presence of sodium hydrogen
sulphide
increases the selectivity of the cooking towards higher lignin release. The
effect can also be
expressed by saying that an increased hydrogen sulphide content makes it
possible to reach
a lower kappa number at the same viscosity when the conditions otherwise are
comparable.
The kappa number is a measure of the lignin content and the viscosity is
considered
to be a measure of the strength of the cellulose fibre.
It is of interest to be able to cook the pulp to as low kappa number as
possible. This
especially applies if it shall be bleached to high, e.g., 90 (ISO) brightness.
For this purpose
bleaching is required with chlorine-containing bleaching chemicals resulting
in synthetic
chlorine-carbon bonds (TOCI - total organic chlorine bound) which are a great
environmental load. In order to reduce the proportion of lil;nin bleached-off
with chlorine-
containing bleaching chemicals, bleaching with an oxygen ;gas has also been
developed.
This technique is in the process of development in, i. a. , Sweden and Japan.
In order to reduce the bleaching with chlorine chemicals it is known further
to
improve the selectivity of the cooking by so-called "modified cooking"
resulting in still
lower kappa numbers. Modified cooking according to the present technique of
the prior art
is based upon the following process conditions:
1. The alkali concentration shall be as constant as possible during the course
of the
cooking.
2. The hydrogen sulphide concentration shall be as high as possible,
particularly
in the beginning of the bulk delignification phase. The hydrogen sulphide
concentration can be low at the end phase of the cooking.
3. The concentrations of lignin released and sodiwn ions shall be as low as
possible, particularly in the last stage of the cooking.
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CA 02069107 2001-06-26
4. The temperature shall be low, particularly in the beginning and at the end
of the
cooking.
Of the above-cited items, item 2 is of special interest with regard to the
present
invention. So far, one has been allowed to contend with the; concentration of
hydrogen
sulphide ions provided by the 40% sulphidity in the white liquor.
From, i.a., S. Norden et al, Tappi, Vol 62, No. 7, July 1979, p. 49; B.
Johansson et
al, Svensk Papperstidning No. 10, 87 (1984), p. 30; and D. Tormund et al,
Tapi, Vol. 72,
No. 5, May 1989, p. 205, it is evident that a further increased content of
hydrogen
sulphide ions beyond 40 % sulphidity is very advantageous in the initial stage
of the
cooking.
In modified cooking the cooking liquor is added at two or several places. An
extra
high sulphidity in the cooking liquor added in the beginning of the cooking is
of greatest
use while the sulphidity in cooking liquors added in the final phase of the
cooking may be
low.
(d) DESCRIPTION OF THE INVENTION
An object of a first aspect of the present invention is to provide the
important pulp
mill balance between sodium and sulphur.
An object of a second aspect of the present invention is to provide such
balance in
cooking with high sulphidity, particularly in the so-called "modified cooking"
.
The present invention provides, in a first aspect, an vnprovement in a process
for
preparing, under reducing conditions, cooking liquors for sulphate pulp
cooking, wherein
the black liquor which is formed in the cooking process is i:ed, after
evaporation,
completely or partly to a reactor operating at increased temperature, the
increased
temperature being obtained by energy which is supplied from an external heat
source
and/or release of energy from the black liquor, thereby to form a melt of
sodium sulphide,
the melt then being withdrawn to be further processed to cooking liquor. The
improvement
comprises feeding, to the reactor, the melt and the whole or part of at least
one of sulphur-
containing, sulphur- and sodium-containing materials which are present in the
pulp mill
including sulphur-containing, and sodium- and sulphur-containing make-up
chemicals used
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CA 02069107 2001-06-26
for the total chemicals balance of the pulp mill, in such a way that the mole
ratio of sodium
to sulphur in the total mixture which is fed to the reactor is within the
range of from 1.5 to
4.
The present invention provides, in a second aspect, aai improved process for
preparing, under reducing conditions, cooking liquors for sulphate pulp
cooking, wherein
black liquor which is formed in the cooking process is fed, after evaporation,
completely or
partly to a black liquor gasifier operating at an elevated temperature of
700° to 950°, the
elevated temperature being obtained by energy which is supply from an external
heat
source and/or release of energy from the black liquor, to form a melt having a
low sodium
carbonate content, the melt being withdrawn to be further processed to cooking
liquor. The
improvement comprises feeding, to the black liquor gasifier, the melt and the
whole or a
part of at least one of sulphur-containing and sulphur- and ;>odium- and
sulphur-containing
materials which are present in connection with pulp production in such amounts
that the
mole ratio between sodium and sulphur in the total mixture which is fed to the
gasifier is
within the range of 1.5 to 4.
By a first variant of these first and second aspects of the present invention,
the melt
contains 0.25 moles/litre or less of sodium carbonate.
By a second variant of these first and second aspects of the present
invention, and/or
the above-recited variant thereof, the mole ratio between sodium and sulphur
in the total
mixture which is fed to the reactor is within the range of 2 to 3. Preferably,
the mole ratio
between sodium and sulphur in the total mixture which is fc~d to the reactor
is within the
range of 2 to 2.8 '
By a third variant of these first and second aspects of the present invention,
and/or
the above-recited variants thereof, the sulphur-containing and sulphur- and
sodium-
containing materials which are present in the pulp mill and/or are present in
connection
with pulp production and which are fed to the reactor partly or completely,
consists of at
least one of electrical filter ash, residual product from chlorine dioxide
production, sodium
hydrogen sulphite-containing solutions from scrubbing of sulphur dioxide,
waste liquors
from CTMP, NSSC or other sulphite pulp process, sulphur dioxide from roasting
of copper
sulphide as well as hydrogen sulphide-containing condensat:es or air streams.
CA 02069107 2001-06-26
By a fourth variant of these first and second aspects of the present
invention, and/or
the above-recited variants thereof, the sulphur-containing and sulphur- and,
sodium-
containing materials which are present in connection with pulp production and
which are
fed to the reactor partly or completely consist of sulphur-containing andlor
sulphur- and
sodium-containing make-up chemicals which consist of at least one of sulphur,
sulphur
dioxide, sulphuric acid, sodium sulphite, sodium hydrogen sulphate, sodium
thiosulphate
and sodium sulphate.
By a fifth variant of these first and second aspects of the present invention,
and/or the
above-recited variants thereof, the melt or an aqueous solution thereof is
mixed with white
liquor, thereby to obtain a white liquor having increased sulphidity.
By a sixth variant of these first and second aspects of the present invention,
and/or
the above-recited variants thereof, an aqueous solution of the melt is used in
the so-called
modified sulphate cooking in which cooking liquors are ch~~rged to the reactor
in two steps.
Thus, it is seen that, by passing, together with black liquor, other sodium-
and
sulphur-containing materials which are present in the pulp mill to a reactor
in such a way
that the mole ratio of sodium to sulphur falls within the range 1.5 to 4, it
is possible, under
reducing conditions, to prepare a melt of sodium sulphide (NazS) having a
lower content of
sodium carbonate than in a conventional soda recovery unit melt. From a
solution of this
melt a cooking liquor having very high sulphidity may be prepared. At a sodium
to sulphur
ratio of 2 to 3, the carbonate content is so low that the solution may be used
directly for
cooking purposes.
In other words, it has now surprisingly been found that a process for
preparing a
cooking liquor having high sulphidity which is particularly suited for
modified cooking
according to the sulphate process is one wherein cooking liquor having high
sulphidity is
added in such way that, when cooking according to prior art, a pulp may be
prepared with
lower kappa number than normally is obtained.
More particularly, to recapitulate, the invention in a broad aspect provides a
process
for preparing, under reducing conditions, cooking liquors having high
sulphidity for
sulphate pulp cooking wherein the black liquor formed in the cooking process
is passed,
after evaporation, completely or partly to a reactor operating at an increased
temperature
which is obtained by energy supply from an external heat source and/or release
of energy
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CA 02069107 2001-06-26
from the black liquor, a melt essentially consisting of sodium sulphide being
formed and
withdrawn to be further processed to cooking liquor. The improved process of
this broad
aspect of the invention comprises adding to the reactor, in .addition to the
melt, the whole
or part of the sulphur-containing and/or sulphur- and sodium-containing
materials which
are present in the pulp mill, including sulphur-containing a:nd/or sodium- and
sulphur-
containing make-up chemicals which are used for the total chemicals balance of
the pulp
mill, in such a way that the mole ratio of sodium to sulphur in the total
mixture which is
fed to the reactor is within the range from 1.5 to 4.
Preferably, mole ratios of sodium to sulphur in the total mixture fed to the
reactor
should be within the range of from 2 to 3, and most preferably should be
within the range
of from 2 to 2. 8. Further, it is preferred to feed; to the reactor, up to 30
% of the black
liquor stream which is formed in the pulp mill.
The sodium sulphide melt which is obtained in the process according to aspects
of the
present invention may be dissolved in water and may be further processed to
cooking
liquor in a way which is know per se. According to a preferred embodiment of
an aspect of
the present invention, a solution of the melt is fed directly i:o the digester
for optimum use
of its high sulphidity in modified cooking. In an alternative process of an
aspect of the
present invention, a solution of the melt is mixed with part of the white
liquor which has
been prepared in the usual way.
In order to allow the reduction reactions in the reactor to proceed quickly,
and
accordingly to obtain shorter residence times and smaller rf:actor volumes,
additional
energy, in addition to the energy which is released from thf; black liquor in
partial
oxidation can be supplied to the mixing zone of the reactor by a hot gas, the
heat content
and oxidation potential of which are adjusted to the reduction work required.
The heat
energy may, e.g.; be supplied by a gas which is heated by a plasma generator.
The very
hot gas or gas mixture can also be formed directly or indire>ctly with an oxy-
fuel burner.
As gas or gas mixture, use can be made of air, recirculating process gas,
hydrogen
gas, natural gas, carbon monoxide, etc. When using an oxy-fuel burner, the gas
or gas
mixture is obtained by combustion of, e.g., acetylene or liq~uified petroleum
gas with
oxygen-enriched air or pure oxygen gas.
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CA 02069107 2001-06-26
A preferred process according to an aspect of the present invention is that
the hot gas
is fed to the reactor close to the material which is fed, which in turn must
be finely divided
which can be obtained by different kinds of atomization techniques known by a
person
skilled in the art. The reactor must be designed to be sufficiently large in
order for the
reaction to have time to take place, i.e., the reactor volume must ensure a
certain minimum
residence time.
The reactor is preferably a closed reaction vessel and the temperature in the
reactor
shall be at least the temperature at which the sodium sulphide is formed under
otherwise
prevailing conditions. A person skilled in the art may establish such
temperature from case
to case, e.g., by routine experiments. The temperature is preferably not below
700°C.
The pressure in the reactor is preferably atmospheric pressure. However, the
process
may be effected at an increased pressure, e.g., in order to reduce the reactor
volume.
In Swedish Patent SE 8501465-2 there is disclosed a process to relieve the
soda
recovery unit by means of plasma gasification of a partial stream of the black
liquor. This
makes it possible to increase the pulp production in a mill having too small a
soda recovery
unit capacity, or, e.g., to introduce oxygen gas bleaching a:nd/or modified
cooking in a mill
restricted in soda recovery unit capacity without losing production capacity.
Of essential importance for the process of aspects of the present invention is
that the
mole ratio of sodium to sulphur in the total mixture fed to the reactor be
below 4 and be
within the range from 1.5 to 4, preferably from 2 to 3. This adjustment of the
sodium to
sulphur ratio is effected by means of sulphur-containing and/or sulphur- and
sodium-
containing materials which are present in the pulp mill, including sulphur-
containing and/or
sodium- and sulphur-containing make-up chemicals which acre used for the total
chemicals
balance of the pulp mill.
The make-up chemicals which are used in order to adjust the mole ratio of
sodium to
sulphur correctly may consist of sulphur, sulphur dioxide, sulphuric acid,
sodium hydrogen
sulphate, sodium sulphate, sodium sulphite, sodium hydrogen sulphite and
sodium
thiosulphate.
Among sulphur-containing and/or sulphur- and sodium-containing materials which
are present in the pulp mill, the following can be mentionedl:
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" CA 02069107 2001-06-26
a. Residual acid from chlorine dioxide production, A mixture of sulphuric acid
and sodium sulphate having NaIS ratio of <_ l may be obtained from so-called
Mathieson
plants. In other common processes of so-called R-8 type, sodium sesquisulphate
(Na3H(S04)2) having a Na/S ratio of 1.5 is formed. The deposition of the
residual acid is
generally a problem. It often has to be dumped.
b. So-called electrical filter ash which mainly consists of sodium sulphate.
There
are normally formed from 60 to 125 kg electrical filter ash per ton of pulp
which are
recirculated to the combustion zone of the soda recovery unit. The Na/S ratio
is __<2.
c. Sulphate-containing solutions from soda recovery unit scrubber. The Na/S
ratio
is 2.
d. A process is disclosed in EP 87850238.4 wherein a partial stream of the
sodium
hydrogen sulphide of the white liquor is reacted with copper oxide, sodium
hydroxide and
copper sulphide being formed. The copper sulphide is roasted to form sulphur
dioxide and
copper oxide. The sulphur dioxide formed is a sodium-free sulphur source.
Further, elemental sulphur may be used or any other sulphur-containing
chemical
having a Na/S ratio being equal to or below 4.
By suitable combinations of the whole or part of the lblack liquor stxeam and
one or
more of the above-stated products, an indefinite amount of .cooking liquor
having high
sulphidity can be prepared.
(e) AT LEAST ONE MODE FOR CARRYING OUT 7CHE INVENTION
The invention is further illustrated by means of the following working
examples.
Example 1
The following material streams were fed continuously per hour to a reactor
operating
at atmospheric pressure.
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CA 02069107 2001-06-26
620 kg black liquor (65 % dry substance contenl;) containing 129 kg sodium
(Na) and 35 kg sulphur (S) per ton of black liquor.
A residual acid mixture from chlorine dioxide production according to the
Mathieson process containing 80 kg H2S04 and 62 kg NazS04.
800 kg Na2S04 in the form of electrical filter ash.
The above material streams were mixed with an oxygen-containing gas and passed
to
a reaction chamber. The oxygen-containing gas was heated to 750°C in a
plasma generator.
Energy was released by partial oxidation of the black liquor, the temperature
in the
reaction chamber being maintained at 950°C.
The process gas evolved at the partial oxidation was cooled. After any final
oxidation, heat recovery as well as scrubbing, the gas may be released to the
atmosphere.
Alternatively, a major part of the energy content of the liquor may be
released by
partial oxidation, whereby it is not necessary to preheat the oxygen-
containing gas in a
plasma generator.
Incoming sulphur compounds are reduced in the reaction chamber essentially to
sodium sulphide (NazS), thereby forming a melt phase which is withdrawn from
the
system.
Owing to the high partial pressure of sulphur in the reaction chamber and the
higher
affinity of sulphur to sodium compared to carbon dioxide at the prevailing
reaction
conditions, the formation of sodium carbonate is suppressed in the inorganic
melt phase.
A 4.0 molar solution with regard to the sodium is prepared containing 1.85
moles of
NaOH, 1.85 moles NaSH and 0.15 moles NaZC03 per litre from the melt prepared.
In preparation of experiments with modified two-step pulp cooking where 70 %
of the
cooking chemicals were charged in step 1 and the residual 3~0 % in step 2, the
following
cooking liquors were prepared.
One (1) part of the liquor obtained above according to aspects of the present
invention was mixed with 4.63 parts of ordinary cooking liquor (white liquor)
containing
2.8 moles of NaOH and 0.7 moles of NaSH per litre solution. The cooking liquor
thus
prepared having a sulphidity amounting to 51 % was charged in the first
cooking step,
whereas the normal white liquor having a sulphidity of 40 % was charge in step
2.
CA 02069107 2001-06-26
Example 2
The following material streams were fed continuously per hour to a reactor
operating
at atmospheric pressure, namely:
566 kg black liquor (65 % dry substance content:) containing
129 kg sodium (Na) per ton and 35 kg sulphur (S) per ton,
48 kg sulphur dioxide,
80 kg NazS04 in the form of electrical filter ash and
25 kg of Na2S04 as make-up.
One proceeded in exactly the same way as in Example 1 and obtained a melt
phase
which was withdrawn from the system.
The sulphur dioxide which was added had been generated by roasting according
to
the process for preparation of sulphide-free liquor disclosedl in EP
87850238.4. A 4.0
molar solution was prepared with regard to sodium containiing 1.75 moles of
NaOH, 1.75
moles of NaSH and 0.25 moles of Na2C03 from the melt.
In preparation of experiments with modified two-step pulp cooking where 70 %
of the
cooking chemicals were charged in step 1 and the residual .30 % in step 2, the
following
cooking liquors were prepared.
According to the same charging process as in Example l, in mixing 1.0 part of
the
liquor prepared above with 1.14 parts of ordinary cooking liquor (white
liquor) (40
sulphidity) one obtained a cooking liquor having a sulphidity of 68 % . This
liquor was
charged in the first cooking step.
In the other step, use was made of the sulphide-free liquor prepared according
to EP
87850238.4.
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