U.S. patent application number 10/487071 was filed with the patent office on 2004-10-07 for method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam.
Invention is credited to Delin, Lennart.
Application Number | 20040194900 10/487071 |
Document ID | / |
Family ID | 20285486 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040194900 |
Kind Code |
A1 |
Delin, Lennart |
October 7, 2004 |
Method for recovery of pulping chemicals in an alkaline sulphite
pulping process and for production of steam
Abstract
A method of recovery of alkaline sulphite pulping chemicals and
for production of steam is disclosed. The method comprises
gasification of evaporated spent cooking liquor at conditions
resulting in a hydrogen sulphide containing gas and a solid
residue. The gas is combusted in a steam boiler, where the hydrogen
sulphide is converted into sulphur dioxide and steam is produced.
The solid residue is recovered in a leaching process, preferably a
two-stage leaching process, where process-foreign substances are
removed and the rest of the contents is divided into substantially
pure sodium carbonate and a mixture of sodium carbonate, sodium
sulphate and sodium sulphide. The substantially pure sodium
carbonate is used for absorption of sulphur dioxide from the steam
boiler. The mixture of sodium carbonate, sodium sulphate and sodium
sulphide is causticized and the resulting sodium hydroxide
containing solution can optionally be mixed with the substantially
pure sodium carbonate after the absorption of sulphur dioxide in
order to produce a fresh cooking liquor.
Inventors: |
Delin, Lennart; (Skarholmen,
SE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
20285486 |
Appl. No.: |
10/487071 |
Filed: |
February 19, 2004 |
PCT Filed: |
September 2, 2002 |
PCT NO: |
PCT/SE02/01556 |
Current U.S.
Class: |
162/29 ;
162/30.1; 162/36; 162/83 |
Current CPC
Class: |
D21C 11/02 20130101;
D21C 11/125 20130101 |
Class at
Publication: |
162/029 ;
162/030.1; 162/036; 162/083 |
International
Class: |
D21C 011/02; D21C
011/14; D21C 003/04; D21C 011/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2001 |
SE |
0103241-6 |
Claims
1. A method for recovery of pulping chemicals in an alkaline
sulphite pulping process and for production of steam, wherein a) an
evaporated spent liquor is fed into a gasification reactor in which
said liquor is decomposed into a hydrogen sulphide
(H.sub.2S)-containing gas and a solid residue containing sodium
carbonate (Na.sub.2CO.sub.3) and a controlled concentration of
sodium sulphide (Na.sub.2S); b) the solid residue from the
gasification reactor in step a) is leached in a counter-current
leaching process which involves at least two stages wherein in a
first leaching stage b') soluble process-foreign elements such as K
and Cl are dissolved and separated, and in a second leaching stage
b") sodium sulphide (Na.sub.2S), sodium sulphate (Na.sub.2SO.sub.4)
and a portion of the sodium carbonate (Na.sub.2CO.sub.3) present
are dissolved and separated; c) the remaining solid residue from
the leaching process in step b) is dissolved in water; d) the
solution resulting from step c) is filtered to remove non-soluble
process-foreign elements and any possible carbon residue and is
treated in an oxidizing stage to give a substantially pure sodium
carbonate (Na.sub.2CO.sub.3) solution; e) the solution containing
the sodium sulphide (Na.sub.2S), sodium sulphate (Na.sub.2SO.sub.4)
and sodium carbonate (Na.sub.2CO.sub.3) that was separated in the
second leaching stage of step b) is causticized, resulting in a
sodium hydroxide (NaOH)-containing solution; f) the hydrogen
sulphide (H.sub.2S)-containing gas from the gasification reactor in
step a) is led through a dust collector and is combusted in a steam
boiler, producing steam; g) the sulphur dioxide (SO.sub.2)
generated in the steam boiler in step f) is absorbed in the
substantially pure sodium carbonate (Na.sub.2CO.sub.3) solution
resulting from the oxidizing stage in step d), and h) carbon
dioxide is removed from the solution of step g) in a desorption
stage to form a substantially pure sodium sulphite
(Na.sub.2SO.sub.3) solution.
2. A method according to claim 1, wherein the leaching liquor of
the first leaching stage b') is a part of the saturated leaching
liquor that is separated from the second leaching stage b"), and
the leaching liquor of the second stage b") is a part of the sodium
carbonate (Na.sub.2CO.sub.3)-containing solution from step c).
3. A method according to claim 2, wherein the proportion of the
resulting leaching liquor from the second leaching stage of step b)
taken out for use as leaching liquor of the first leaching stage is
less than 5%.
4. A method according to claim 3, wherein the proportion taken out
is 1-2%.
5. A method according to claim 1, further comprising a step i) for
mixing the substantially pure sodium sulphite (Na.sub.2SO.sub.3)
solution resulting from step h) with a predetermined amount of the
sodium hydroxide (NaOH)-containing solution resulting from the
causticizing step d) thus giving a fresh sulphite cooking liquor
containing a controlled amount of sodium sulphide (Na.sub.2S).
6. A method according to claim 1, wherein the evaporated spent
liquor in step a) is gasified at a temperature of 650-750.degree.
C.
7. A method according to claim 6, wherein the evaporated spent
liquor in step a) is gasified at a temperature of 700-750.degree.
C.
8. A method according to claim 1, wherein air is added to the
gasification reactor of step a) in an amount of 25-75% of the total
amount required for complete combustion of the evaporated spent
liquor.
9. A method according to claim 8, wherein the amount of air added
to the gasification reactor is 30-50% of the total amount required
for complete combustion of the evaporated spent liquor.
10. A method according to claim 1, wherein said evaporated spent
liquor is gasified in step a) at a pressure of at most 5 bar.
11. A method according to claim 10, wherein said liquor is gasified
at a pressure of at most 2 bar.
12. A method according to claim 1, wherein the gasification reactor
of step a) is a fluidized bed reactor.
13. A method according to claim 1 wherein the gas from the
gasification reactor in step a) is lead to the dust collector of
step f) without intermediate cooling.
14. A method according to claim 1, wherein the solution containing
sodium sulphide (Na.sub.2S), sodium sulphate (Na.sub.2SO.sub.4) and
sodium carbonate (Na.sub.2CO.sub.3) that is separated in the second
leaching stage of step b) contains 20-60% of the total amount of
the sodium salts that were fed into the second leaching stage.
15. A method according to claim 14, wherein the solution containing
sodium sulphide (Na.sub.2S), sodium sulphate (Na.sub.2SO.sub.4) and
sodium carbonate (Na.sub.2CO.sub.3) that is separated in the second
leaching stage of step b) contains 40-50% of the total amount of
the sodium salts that were fed to the second leaching stage.
16. A method according to claim 1, wherein the temperature of the
second leaching stage of step b) is 20-100.degree. C.
17. A method according to claim 16, wherein the temperature is
50-70.degree. C.
18. A method according to claim 1, wherein the steam boiler of step
f) has a working pressure of 60-120 bars and a super-heating
temperature of 450-560.degree. C.
19. A method according to claim 1, further comprising a step j) for
production of sulphuric acid (H.sub.2SO.sub.4) from a part of the
sulphur dioxide (SO.sub.2)-containing gas from the steam boiler of
step f).
20. A method according to claim 1, further comprising a
causticizing step k) for production of sodium hydroxide (NaOH) from
a part of the substantially pure sodium carbonate
(Na.sub.2CO.sub.3) solution from step d).
21. A method according to claim 19, wherein the sulphuric acid
(H.sub.2SO.sub.4) produced in step j) and/or the sodium hydroxide
(NaOH) produced in step k) and/or the sodium carbonate
(Na.sub.2CO.sub.3) from step d) are (is) used for bleaching of
pulp, resulting in a closed alkaline sulphite pulping and bleaching
process.
Description
[0001] The present invention relates to a method for recovery of
alkaline sulphite pulping chemicals and for production of
steam.
BACKGROUND OF THE INVENTION
[0002] The sulphate pulping process has hitherto been dominating in
the production of chemical pulps as well as semi-chemical pulps due
to both its ability of disclosing a wide variety of lignocellulosic
raw materials to a pulp of good quality and to its well-developed
chemical recovery system, which is tested in large scale and has a
high energy efficiency. Since the sixties, different alkaline
sulphite processes have been proposed for production of pulps
having the same or even superior strength qualities as compared to
corresponding sulphate pulps. Increased process flexibility has
also been noted concerning the adaptation to different technical
paper qualities and to the pulp yield. It has also been noted that
a proper choice of cooking conditions will result in substantially
improved bleaching properties of the unbleached pulp. These
alkaline sulphite processes, which are interesting from a fiber
point of view, have however been commercially impeded by the fact
that there has not been available any process for recovery of the
pulping chemicals, that has, in combination with the pulping
process, been able to compete in energy and economical aspects with
the well-trimmed recovery system of the sulphate process.
[0003] Even though the interest for alkaline sulphite processes got
a start at first during the sixties and later, pulping in neutral
or alkaline sulphite liquors has been studied and patented much
earlier, the first patent issued already in 1880 (Cross, C. F.,
British Patent No 4,984, 1880). The basic concept was the use of
concentrated sodium sulphite liquor to which was added small
amounts of other chemicals such as sodium hydroxide, carbonate,
bi-carbonate, sulphide or bi-sulphite. To achieve a defibration
within a reasonable time, rather high temperatures (170-190.degree.
C.) were required, also at a high concentrations of pulping
chemicals and this could easily lead to severe degradation of the
fiber material. The pulping technology, of that time, was thus
substantially limited to semi-chemical pulps, for which much milder
conditions could be used. Although several different methods for
chemical recovery have been proposed during the years none has been
technically successful (Rydholm, S. A., Pulping Processes,
Interscience Publishers 1965, s. 422). The field of semi-chemical
pulps has been to a high extent limited to so-called NSSC (Neutral
Sulphite Semi, Chemical) (yield about 80-85%) from hardwood for
production of fluting and, in cases with applicable chemical
recovery, as so-called cross-recovery, i.e. use of the spent liquor
as make-up in a neighboring sulphate plant.
[0004] The use of anthraquinone as an accelerator of the
delignification and as a protection of the carbohydrates was
proposed in 1972-1973. Its usefulness also for neutral and alkaline
sulphite pulping is disclosed in the Japanese patent No JP 112,903
(1976) (Nomura, Y., Wakai, M. and Sato, H) and in a modified
version in the Canadian patent CA 1 079 906 (1980). By the addition
of anthraquinone, cooking time was considerably reduced and thus
the field of chemical pulp became available under reasonable
conditions: A few alternative methods, working at different
pH-ranges were proposed (Ingruber, O. V., Pulp and Paper
Manufacture, Volume 4, Sulphite Science & Technology, Joint
Textbook Committee of the Paper Industry, TAPPI/CPPA 1985, s.41).
However, commercial introduction was impeded by the absence of a
competitive chemical recovery system. The at that time most
approved chemical recovery systems in sulphite processes were based
on a processing smelt from a soda recovery boiler for production of
sulphite from the sulphide in the smelt. The complexity of the
process and the inherent energy losses were a too heavy drawback of
the process. This type of chemical recovery in the soda recovery
boiler therefore has a commercial application only in occasional
cases due to the drawbacks discussed above.
[0005] An interesting version of alkaline pulping is the MSSAQ
(Mini Sulphide Sulphite Ahthraquinone) process (Olm, Teder, Wikn,
Swedish patent application No 8405061-6; Olm, Teder, Svensk
Papperstidning, No 16-1986 89 s.20-22, 25-26; Dahlbom Olm, Teder,
Tappi Journal, Vol 73, No 3, March 1990, s 257), which allows the
presence of sulphide in the pulping liquor. The sulphide interacts
with the anthraquinone to speed up the cooking process thus
providing for increased degrees of delignification, especially if a
final stronger alkaline delignification stage is added to the
process. However, each amount of anthraquinone charged to the
process has a corresponding optimal sulphide charge (at a higher
anthraquinone charge the optimal amount of sulphide is lower).
However, the positive effect of the sulphide is relatively soon is
lost at smaller or larger than optimal sulphide charges.
[0006] A further development of alkaline sulphite processes has
been to, in addition to a so-called redox-catalyst such as
anthraquinone, to add a low boiling point organic solvent, such as
methanol (R. Patt och 0. Kordsachia, EP-A1-205778,
Sulfitaufschlussverfahren zur Herstellung von Zellstoff aus
lignozellulosehaltigen Materialen mit Ruckgewinnung der
Aufschlusschemikalien). A process called ASAM (Alkaline Sulphite
Anthraquinone Methanol) has accordingly been developed and tested
in a pilot plant in Germany (Ahonen und Lehner, "Umweltsvertrgliche
Holzaufschlussverfahren". Schriftenserie "Nachwachsende Riksstoffe"
13 und 8, Bundesministerium fur Ernhrung, Landwirtschaft und
Forsten, Bonn 1997).
[0007] A chemical recovery system for the ASAM process has also
been developed (M. Bobik, D. Chybin, A. Glasner och K. Taferner,
WO-A1-9423124, Process for converting sodium sulphate). This system
involves combustion in a conventional recovery boiler and a
multi-stage carbonization using a part of the purified flue gas to
drive off the sulphide as hydrogen sulfide, leaving sodium
carbonate in the solution. The hydrogen sulphide is combusted to
sulphur dioxide (SO.sub.2) and used for production of sodium
sulphite (Na.sub.2SO.sub.3) from the more or less pure sodium
carbonate. There is no intention of keeping any substantial amount
of sulphide in the fresh cooking liquor produced, but small amounts
of sulphate and thiosulphate might remain and are not considered to
interfere with alkaline pulping processes. This recovery process
has been standard-forming to the contemporary version of the
above-mentioned ASAM process (Ahonen und Lehner,
"Umweltsvertrgliche Holzaufschlussverfahren"). Recovery of methanol
is also included into the system, however without mentioning how it
is supposed to co-operate with the pulping technology. The system
further involves causticizing of the separated sodium carbonate
according to a normal sulphate process type, in order to produce
sodium hydroxide (NaOH) required for the cooking.
[0008] Pyrolysis of the spent liquor has also been discussed in
EP-A1-205778 (R. Patt and O. Kordsachia, Sulfitaufschlussverfahren
zur Herstellung von Zellstoff aus lignozellulosehaltigen Materialen
mit Ruckgewinnung der Aufschlusschemikalien) where the intention is
to produce a more or less pure sodium carbonate Na.sub.2CO.sub.3,
where carbon is a contaminant, and a pyrolytic gas, containing
sulphur in the form of hydrogen sulphide. The gas is combusted and
sulphur dioxide is absorbed. The document EP-A1-205778 also refers
to E. Horntvedt, Tappi 53(1.1): 2147 (1970), which discloses a
pyrolysis that, contrary to the process of EP-A1-205778, was
commercialized, but at the time of the publication of EP-A1-205778
still was fighting against serious drawbacks concerning disturbing
amounts of carbon in the pyrolytic residue and a poor energy
balance.
[0009] Further development of gasification technology for sulphite
pulping processes has been impeded by the weak position of the
traditional sulphite industry and the doubts regarding commercial
development of new sulphite pulping techniques. The gasification
technology development has therefore been directed to chemical
recovery in sulphate pulping processes. Gasification technology has
thus been of use in smaller reactors, working in parallel to soda
recovery boilers, to give a slight increase in the chemical
recovery capacity in sulphate processes having inadequate capacity
in the recovery boiler. A more general application of gasification
technology in sulphate processes, as an alternative to soda
recovery boiler technology, has however not been achieved.
[0010] Document SE-B-462106 (A. Andersson and B. Warnqvist)
discloses a process for recovery of energy and process chemicals,
primarily intended for sulphate processes. In this process, spent
liquor is thermally decomposed under an elevated pressure (10-50
bar) and an oxygen supply that is insufficient for complete
combustion. The decomposition temperature is below the temperature
that gives a melt (i.e. 700-850.degree. C.). The gas formed during
the thermal decomposition is led to a first scrubber where hydrogen
sulphide and other at sulphur containing compounds are absorbed in
a sodium hydroxide solution. Thereafter the gas is led to a second
scrubber for washing the gas with water. The thus washed and cooled
gas, which is still under high pressure (16-20 bar), is
subsequently led to a gas turbine, where energy is produced. The
exhaust gas from the gas turbine is finally combusted in a steam
boiler, where steam is generated. The sodium carbonate-containing
solid residue of the pyrolysis is dissolved in water and the
remaining solid phase is separated, while the liquid sodium
carbonate containing phase is lead to a causticizing plant.
[0011] Document SE-C2-503455 (L. Stigsson and J.-E. Kignell)
discloses a method for preparation of a sulphite containing cooking
liquor comprising chemical recovery of a sulphite process spent
liquor wherein the spent liquor is decomposed into a hot gas and a
melt, in a reactor working at a high temperature. The aim of this
method is mainly to concentrate hydrogen sulphide by a
sorption/desorption operation. Chemical recovery of sulphite
process spent liquor according to this process has turned out to be
complicated and expensive.
[0012] Another development of the gasification technology is to use
a fluidized bed reactor for the gasification process (E. Dahlquist,
R. Jacobs, "Development of a Dry Black Liquor Gasification
Process", 1992 International Chemical Recovery Conference,
Proceedings TAPPI/CPPA), which advantageously gives an opportunity
to a more exact control of the temperature and the reaction
process. The main problem related to the use of a fluidized bed
reactor for gasification of sulphate process spent liquors is to
prevent absorption of carbon dioxide (CO.sub.2) during the
absorption of hydrogen sulphide (H.sub.2S) from the pyrolysis gas.
Gasification of sulphite process spent liquors in a fluidized bed
reactor according to the present invention will not be afflicted
with this problem.
[0013] Document U.S. Pat. No. 3,711,593, (P. E. Schick and W. H.
Flood) discloses a process for regeneration of chemicals from a
sulphite pulping process. In this process the reaction is performed
in two-stage or multi-stage fluidized bed treatment. The reason for
this is that at the very temperature needed to obtain a complete
removal of sulphur a considerable part of the carbon remains as a
solid carbon residue. The second stage is operated at a higher
temperature to allow combustion of remaining carbon to yield a
substantially pure sodium carbonate. This process is not applicable
in cases where a sodium sulphide-containing residue is desired.
[0014] Gasification technology in connection with alkaline sulphite
processes has been suffering from the difficulty to avoid remaining
sulphide in the non-volatile residue at reasonable pyrolysis
conditions. A solution of the sodium salts of the residue cannot be
directly used for absorption of sulphur dioxide, for production of
sulphite, since sulphide will react with sulphite and cause a
significant loss of active chemicals. In processes where an
essentially sulphide free cooking liquor is required, which has
normally been the case, an additional separation by leaching
regarding the different solubility of sodium sulphide (Na.sub.2S)
and sodium carbonate (Na.sub.2CO.sub.3) or an evaporation of
hydrogen sulphide (H.sub.2S) by the use of a carbon dioxide
containing flue gas would be required. Such processes would thus be
unreasonably complicated and expensive, also in comparison with the
rather complicated and expensive soda recovery boiler method of the
above mentioned WO-A1-9423124. Document U.S. Pat. No. 5,507,912 (P.
P. H Lownertz) describes a method where dissolved smelt from a soda
recovery boiler is divided into a sulphide-rich and a sulphide-poor
flow. This method is based upon the recovery of a smelt from a soda
recovery boiler. It does further not yield a totally sulphide free
cooking liquor for use in sulphite pulp production and it does not
include any means for leaching of soluble process-foreign
substances, such as K or Cl.
[0015] As the pulp production systems get more and more closed, the
need for controlled ejection of process-foreign substances, which
particularly enter the process with the wood raw material is
accentuated. No satisfying gasification process that is able to
deal with this problem has so far been proposed. Document
SE-B-448007 (S. Santen, R. Bernhard and S.-E. Malmeblad) describes
removal of NaCl, which is sparingly soluble in a concentrated
NaOH-solution that can be exclusively produced according to this
method. The spent liquor is subjected to a low temperature
pyrolysis to a Na.sub.2CO.sub.3--C-blend, which is further
subjected to pyrolysis in a reactor at 600-800.degree. C. Since the
primary aim of this process is, by use of external heating (e.g. a
plasma generator), to convert the components of the pulping spent
liquor to a blend mainly consisting of sodium sulphide, sodium
hydroxide, monatomic sodium, hydrogen and carbon monoxide (at
1000-1300.degree. C. in the reactor), this method hardly has any
practical use in this context, especially since the externally
heating energy required has turned out to be prohibitively large.
It is emphasized that the removal of NaCl is far from covering the
concept of process-foreign substances, which includes both normally
water soluble and non-soluble compounds.
[0016] In summary it can be noted that the alkaline sulphite
processes up till now have been lacking a pulping chemical recovery
system that is able to compete with those of the sulphate process
(P. Axeg.ang.rd, B. Backlund, B. Warnqvist, "The Eco-cyclic Pulp
Mill--With Focus on Closure, Energy-Efficiency and Chemical
Recovery Development", Pre-print, 2001 Int. Chemical Recovery
Conf., Whistler, B.C., Canada). Recovery systems based on a
traditional soda recovery boiler have turned out to be too
complicated and expensive. A gasification process thus seems to be
the best starting point for further development. However there is a
problem to completely expel the sulphur and directly produce a
sufficiently pure Na.sub.2CO.sub.3 for the sulphite production
without using impossible temperatures, which would result in large
amounts of carbon in the residue. By a subsequent leaching stage it
would be possible though to isolate the sodium carbonate, but in
that case it would be necessary to expel H.sub.2S from the sulphide
containing part or to accept an amount of sulphide in the cooking
liquor. The former leads to complicated and expensive systems. The
latter has surprisingly turned out to be feasible and even
desirable, but only if the sulphide content is adjusted to the
cooking process requirements. The existing gasification processes
have suffered from the problem of achieving a good energy balance
due to the reduction heat required for the pyrolysis of sulphite to
sulphide, the take out of hot pyrolysis residue (solid or as a
melt) for subsequent dissolution in water and/or extensive
dust-cleaning of the pyrolysis gases in a scrubber before final
combustion. The existing gasification processes have so far also
lacked a settled removal of process-foreign substances, soluble as
well as non-soluble. It would thus be desirable to develop a
chemical recovery process, which involves a good energy balance and
means for handling the presence of sulphide in the solid phase of a
gasification reactor.
SHORT DESCRIPTION OF THE INVENTION
[0017] The method of the present invention provides a simple,
energy effective and flexible method of recovery of alkaline
sulphite pulping chemicals, which method will render alkaline
sulphite processes competitive in production of high quality pulps
also in comparison to a modern sulphate pulping process. The method
comprises gasification of the evaporated spent cooking liquor,
resulting in a hydrogen sulphide containing gas and a solid
residue. The gas is combusted in a steam boiler, where the hydrogen
sulphide is converted into sulphur dioxide and steam is produced.
The solid residue is recovered in a leaching process, where
process-foreign substances are removed and the rest of the contents
is divided into substantially pure sodium carbonate and a mixture
of sodium carbonate, sodium sulphate and sodium sulphide. The
substantially pure sodium carbonate is used for absorption of
sulphur dioxide from the steam boiler and the mixture of sodium
carbonate, sodium sulphate and sodium sulphide is causticized and
the resulting sodium hydroxide containing solution can optionally
be mixed with the substantially pure sodium carbonate after the
absorption of sulphur dioxide in order to produce a fresh cooking
liquor.
SHORT DESCRIPTION OF THE DRAWING
[0018] FIG. 1 shows the alkaline sulphite chemical recovery process
schematically in a block diagram. The reference letters a)-k) in
the diagram represent the process steps of the present method and
correspond to the process steps as described in the claims. The
process steps are:
[0019] a) gasification, b) leaching, c) dissolving, d) filtration
and oxidation, e) causticizing, f) dust cleaning and combustion in
steam boiler, g) SO.sub.2-absorption, h) CO.sub.2-desorption, i)
preparation of fresh cooking liquor, j) production of
H.sub.2SO.sub.4 and k) separate causticizing FIG. 1 also includes
the Total Dry Substance (TS) of evaporated spent liquor that is fed
into the gasification stage and the amounts of certain chemicals in
the different stages of the process expressed in kg/ADt (Air Dry
tonne) chips charged in the cook. The steam boiler effect is given
in GJ/ADt.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention thus provides a method for recovery of
pulping chemicals in an alkaline sulphite pulping process and for
production of steam.
[0021] The method will here be described in detail with reference
to FIG. 1, which illustrates an example of the process adapted to a
standard ASAM-process, with the exception that sulphide is accepted
and desired in the fresh cooking liquor. The recovery process of
the present invention can of course also be adapted to other
alkaline sulphite processes.
[0022] In the recovery method of the present invention, evaporated
spent cooking liquor is fed into a gasification reactor (step a),
in which the evaporated liquor is decomposed into a hydrogen
sulphide (H.sub.2S)-containing gas and a solid residue. The
application of gasification technology provides for a simple
regeneration of cooking liquor from spent liquor in one stage. The
choice of gasification technology is a response to the need of
achieving a simple and robust system that can be scaled to the very
large capacities, as required by the pulp mills of today.
[0023] At a low temperature, 700.degree. C., the equilibrium of
sulphur gives that most of the sulphur will turn into the gas
phase. When gasifying of conventional kraft black liquor, it has in
practice been found that the amount of sulphur transferred to the
gas phase is in the region of 50-75%. In the current ASAM-case the
spent liquor has an initial sulphur content about twice as high as
for kraft black liquor, which in the least should lead to an amount
of sulphur transferred to the gas phase of at least the same size.
The energy balance of the gasification stage is crucial to the
competitive strength of the process. The solid residue contains
sodium carbonate (Na.sub.2CO.sub.3) and a controlled concentration
of sodium sulphide (Na.sub.2S). It has been discovered that the
allowance of Na.sub.2S in the solid residue is essential to the
construction of this system, and it is necessary to be able to
control the sulphur content, so as to achieve a level adequate to
the digestion process. The gasification process is adjusted so that
the content of sulphur remaining in the solid residue is in
accordance with the cooking process claims, by a proper selection
of temperature and air supply, so that the amount of sulphide in
the solid residue corresponds with the amount desired in the fresh
cooking liquor. The temperature must be below the melting point of
the salts in the non-volatile residue.
[0024] The evaporated spent liquor fed into a gasification reactor
has a dry content of 60-85%. In order to achieve a proper
decomposition, the gasification is carried out at a temperature of
650-750.degree. C., preferably 700-750.degree. C. and at a pressure
of at most 5 bar, preferably at most 2 bar. Air is added to the
gasification reaction in an amount of 25-75%, preferably 30-50% of
the total amount required for complete combustion of the evaporated
spent liquor. Since each reactor is individual and requires
different conditions, these parameters have to be experimentally
determined in each case. The gasification reaction is
advantageously carried out in a fluidized bed reactor, but other
types of gasification equipment could also be appropriate.
[0025] The chemicals remaining in the reactor after the
gasification process in step a), mainly comprises Na.sub.2CO.sub.3
and Na.sub.2S and are taken out from the gasification reactor as a
solid phase. This solid residue is transferred via an optional heat
exchanger to a series of treatment stages (steps b-d) where
non-process elements are separated and the remaining quantity of
sodium salts is divided into two separate cooking liquor lines, one
of which is a substantially pure sodium carbonate
(Na.sub.2CO.sub.3) stream and the other a mixture of sodium
sulphide (Na.sub.2S), sodium sulphate (Na.sub.2SO.sub.4) and sodium
carbonate (Na.sub.2CO.sub.3). The pure sodium carbonate
(Na.sub.2CO.sub.3) stream is used for absorption of SO.sub.2 and
the mixed stream containing all sodium salts is led to
causticizing. This measure gives a possibility to allow sulphide in
the solid phase after gasification, since the sulphide is bypassed
from the absorption of SO.sub.2. Moreover, since SO.sub.2 in not
absorbed in NaOH but in Na.sub.2CO.sub.3, the causticizing need
will be much lower than in a conventional process. Division into
two cooking liquor lines also gives the opportunity to both
conventional cooking where all chemicals are simultaneously
charged, and specialized cooking processes where chemicals are
charged in two or more steps.
[0026] The solid residue is led into to a leaching stage (step b),
where any soluble process-foreign substances (such as K and Cl) and
sodium sulphate (Na.sub.2SO.sub.4) and a portion of the sodium
carbonate (Na.sub.2CO.sub.3) present and the sodium sulphide
(Na.sub.2S) are dissolved and separated. The solid residue, mainly
consisting of sodium carbonate (Na.sub.2CO.sub.3), remaining after
the leaching process is then led to a dissolving stage (step c),
where it is dissolved in water, after which it is filtered and
oxidized (step d).
[0027] The leaching process of step b), which constitutes a central
section of the recovery process, involves at least two leaching
stages b') and b"), wherein the solid residue is leached,
preferably in a counter-current process. A counter-current leaching
process involves a transport direction of the leaching liquor that
is opposite to that of the solid residue. Each leaching stage is
carried out in a separate vessel. In some cases, e g when a higher
purity of the fractions is desired, it may be advantageous to
involve more than two leaching stages.
[0028] In the first leaching stage b'), soluble process-foreign
substances such as K and Cl are dissolved and separated. In a
second leaching stage b"), sodium sulphide (Na.sub.2S), sodium
sulphate (Na.sub.2SO.sub.4) and a portion of the sodium carbonate
(Na.sub.2CO.sub.3) present are dissolved and separated. This
separated stream will be further treated in a causticizing stage
e).
[0029] The solid residue remaining after the leaching process in
step b) consists mainly of sodium carbonate (Na.sub.2CO.sub.3) and
is now substantially free from Na.sub.2S, but any non-soluble
process-foreign substances are still present. The residue is
dissolved in water in a dissolving stage (step c), and the
resulting solution is filtered (step d) to remove non-soluble
process-foreign substances and any possible remaining carbon. The
solution is then treated in an oxidizing stage (step d) to remove
even small amounts of disturbing Na.sub.2S remnants in order to
give a substantially pure sodium carbonate (Na.sub.2CO.sub.3)
solution. If desired the filtration can be carried out after the
oxidizing stage. The resulting pure Na.sub.2CO.sub.3 solution will
subsequently be used for absorption of SO.sub.2 from the steam
boiler flue gas and for production of sulphite as required.
[0030] The leaching liquor used in the leaching process is
preferably a portion of the substantially pure sodium carbonate
(Na.sub.2CO.sub.3) stream, taken from the filtrate stream after the
filter of step d). Water is fed into the dissolving tank c) and is
led through the dissolving stage of step c) and the filtration
stage of step d). After the filter, the stream is divided and one
part is led to the oxidizing stage of step d) and the other is led
to the leaching stages b") and b'). The leaching liquor taken out
after the filtration stage of step d), is first fed into the second
leaching stage b'). After having passed the second leaching stage
b"), the leaching liquor is divided into two streams. One of those
streams is led to the first leaching stage b') and the other to a
causticizing step e). The stream that is led to the first leaching
stage b') amounts to less than 5%, preferably 1-2% of the total
amount of the leaching liquor taken out from the second leaching
stage b").
[0031] The solid residue that is transferred to the first leaching
stage b') is thus leached with a solution, which represents a
portion of the saturated liquid phase of leaching stage b"). The
separated stream taken out from leaching stage b'), containing the
soluble process-foreign substances (such as K and Cl) consequently
also contains sodium carbonate (Na.sub.2CO.sub.3), sodium sulphate
(Na.sub.2SO.sub.4) and Na.sub.2S, but since the stream is so small
the chemical loss is of little consequence. The remains of the
solid residue are then transferred to the second leaching stage
b"), where sodium sulphide (Na.sub.2S) and sodium sulphate
(Na.sub.2SO.sub.4) are dissolved in the filtrate from step d). The
saturated leaching liquor taken out from the second leaching stage
b") contains 20-60%, preferably 40-59% of the total amount of the
sodium salts that were fed into the second leaching stage. Both
leaching stages are carried out at a temperature of 20-100.degree.
C., preferably 50-70.degree. C. The conditions of the leaching
process should advantageously be adjusted so that the amount of
pure Na.sub.2CO.sub.3 resulting from the filtration/oxidation stage
(step d) will be sufficient for the absorption of SO.sub.2 from the
steam boiler.
[0032] If desired a portion of the pure Na.sub.2CO.sub.3-solution
can optionally be utilized in a separate causticizing stage for
production of additional NaOH, for use e.g. in a bleaching plant.
In that case the amount of pure Na.sub.2CO.sub.3-solution resulting
from the filtration/oxidation stage must be increased in relation
to the amount of NaOH desired.
[0033] The solution that was separated in the second leaching stage
of step b), which contains the sodium sulphide (Na.sub.2S), sodium
sulphate (Na.sub.2SO.sub.4) and the portion of sodium carbonate
(Na.sub.2CO.sub.3) that is not required for absorption of SO.sub.2,
is led to a causticizing stage (step e), where it is causticized.
The resulting sodium hydroxide (NaOH)-containing solution may be
use for preparation of fresh cooking liquor. Since the absorption
of SO.sub.2 utilizes the pure sodium carbonate resulting from the
filtration/oxidation stage, no NaOH will be required for that
purpose and it will thus be sufficient if the causticizing stage
produces the amount of NaOH required for production of fresh
cooking liquor. The causticized liquor, containing mainly
Na.sub.2S, Na.sub.2CO.sub.3 and NaOH, is then optionally mixed with
the Na.sub.2SO.sub.3 solution that has passed through the
SO.sub.2-absorption stage, so as to form the required fresh cooking
liquor. The causticized liquor or a part of it may also be charged
into the cooking process in a later stage if that would be
desired.
[0034] The hydrogen sulphide (H.sub.2S)-containing gas from the
gasification reactor in step a) is led to a dust cleaning stage
(step f). In the dust cleaning stage all possible particles are
separated e g in a cyclone or a ceramic filter. The resulting clean
gas is then combusted in a steam boiler (step f), for production of
steam. Since the gas does not contain any substantial amount of
chemicals it would be possible to choose steam conditions
considerably higher, than in a conventional recovery boiler. The
gas has preferably not been subjected to any cooling before it is
led to final combustion, but if desired the gasification air may be
heated by heat exchange with the gas before combustion. During the
combustion, the hydrogen sulphide (H.sub.2S) is converted to
sulphur dioxide (SO.sub.2). The steam boiler of step f) has a
working pressure of 60-120 bar and a super-heating temperature of
450-560.degree. C.
[0035] The thus generated sulphur dioxide (SO.sub.2) is led to an
absorption stage (step g) where it is absorbed in the substantially
pure sodium carbonate (Na.sub.2CO.sub.3) solution that results from
the oxidizing stage of step d). In order to yield a high quality
cooking liquor, the resulting solution is then led through an
additional carbon dioxide-desorption stage (step h), where carbon
dioxide corresponding to the bicarbonate formed, is removed. The
result will be a substantially pure sodium sulphite
(Na.sub.2SO.sub.3) solution, which may be mixed with a desired
portion of NaOH.
[0036] The use of substantially pure Na.sub.2CO.sub.3 for
absorption of SO.sub.2 from the steam boiler flue gas leads to that
the causticizing need of the present process will be much lower
than in a sulphate process. With a, in this case presumed, NaOH
charge of 10% based upon the wood, the causticizing need will
amount to only 2/3 of a sulphate process causticizing need.
[0037] The chemical recovery plant can easily be supplemented with
a separate plant for production of H.sub.2SO.sub.4 from
SO.sub.2-rich flue gas from the combustion in the steam boiler, in
order to support the bleaching plant with H.sub.2SO.sub.4. By
including the production of sulphuric acid (step j) and/or the
separate causticizing (step k) to the pulping chemical recovery
process, a closed alkaline sulphite pulping process can be
achieved.
[0038] The recovery process can, as mentioned above, optionally be
completed by mixing (step i) the resulting substantially pure
sodium sulphite (Na.sub.2SO.sub.3) solution with a predetermined
amount of the sodium hydroxide (NaOH)-containing solution resulting
from the causticizing stage (step d) thus giving a fresh sulphite
cooking liquor containing a controlled amount of sodium sulphide
(Na.sub.2S). The predetermined amount is adjusted in relation to
the desired amount of sodium sulphite in the fresh cooking
liquor.
[0039] Comparison of the energy balances for a modern sulphate
process, an ASAM (Alkaline Sulphite Anthraquinone Methanol)-process
utilizing the chemical recovery method according to the present
invention and a conventional ASAM-process. The examples relate to
plants where the lime sludge reburning kiln uses bark as fuel. The
surplus of falling bark is burned in a bark boiler and the steam
excess, if any, is used for production of electricity by means of
condensing power.
[0040] Chemical charges into the ASAM cooking process in the
example according to the present invention are expressed as kg per
tonne of dry wood:
1 Na.sub.2SO.sub.3 260 Na.sub.2S 51 Na.sub.2CO.sub.3 30 NaOH
(excluding NaOH 100 from hydrolysis of Na.sub.2S) Na.sub.2SO.sub.4
(as ballast) 5 Anthraquinone 0.7 Methanol 15 vol-% of total
liquid
[0041] A comparison of the energy balances shows that an
ASAM-process with chemical recovery according to the present
invention is much more energy efficient than the conventional ASAM
process, and even slightly better than the sulphate process
2 Steam consumption GJ/ADt ASAM with chemical Conventional Sulphate
recovery according to ASAM- process the present invention process
Soot blowing 1.0 0.0 Chemical recovery 4.2 5.2 Fibre line 3.5 3.6
Pulp dryer 2.2 2.2 Misc.process 0.4 0.4 consumption Sum process
11.3 11.4 .about.15 Condensing turbine/ 5.5 6.4 .about.0 steam
surplus Back pressure turbine 3.1 2.9 .about.3 Total consumption
19.8 20.7 .about.18
[0042]
3 Steam production GJ/ADt ASAM with chemical Conventional Sulphate
recovery according to ASAM- process the present invention process
Recovery boiler 17.7 18.2 .about.16 Bark boiler 1.5 1.9 .about.1.5
Recycled secondary 0.6 0.6 0.6 heat Total production 19.8 20.7
.about.18
[0043]
4 Power balance kWh/ADt ASAM with chemical Conventional Sulphate
recovery according to ASAM- process the present invention process
Mill consumption 712 689 .about.750 Power surplus, sold 656 711
.about.0 Total 1368 1401 .about.750 Back pressure 831 771
.about.750 generation Condensing power 537 630 .about.0 generation
Total power 1368 1401 750 generation
[0044] Some Advantages of the Invention
[0045] A difficult problem related to the closing of pulp mills
where also the bleaching plant is to be included into the chemical
recovery, is normally related to the Na/S balance. In the system of
the present invention, this problem is easily solved, due to its
good capacity of internal generation of the bleaching chemicals.
This leads accordingly to excellent opportunities of closing the
bleaching plant together with the rest of the pulp mill.
[0046] In comparison to a sulphate process the recovery method of
the present invention achieves a number of savings. The
gasification and combustion of the gas in a separate boiler gives
two large energy savings. Firstly, the need for soot blowing steam
will vanish since the combusted gas is clean. Conventional recovery
boilers use 5-10% of the steam produced for soot blowing. Secondly,
the energy loss related to the smelt in a conventional recovery
boiler of a sulphate process is omitted. This is due to that the
non-gaseous chemicals taken out of the gasification reactor are not
in the form of a smelt, but in the form of a solid residue, and
thus contain less heat.
[0047] The gas boiler of the present recovery method will give
about 7% or 1.2 GJ/ADt more useful energy than can be obtained from
a conventional recovery boiler. Due to the higher steam data that
can be used for the gas boiler, in comparison to a conventional
recovery boiler, the electricity production will increase even
more.
[0048] Overall the chemical recovery method of the present
invention leads to a more favorable steam balance than in the
sulphate process. The decreased causticizing need also results in a
decreased need of lime and fuel. The saving amounts to 80 kg
lime/ADt.
* * * * *