Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A method of recovering and reusing sulphur in a
sulphate cellulose plant
The invention relates to a method of recovering
and reusing sulphur in a sulphate cellulose plant, in
which method sulphur existing in liquor cycle is sep-
arated from it as gaseous compounds and oxidated into
sulphur trioxide, after which sulphuric acid is pre-
pared from the sulphur trio~ide and the sulphuric
acid obtained in this manner is reused in the cellu-
lose manufacturing process.
In a sulphate cellulose plant, cellulose pulp
is cooked of wood by means of alkaline liguor, after
which the cellulose pulp obtained is separated from
the liquor. The pulp is washed before bleaching in
order that the alkaline cooking liquor contained in
the pulp can be separated from it. The spent cooking
liquor contains plenty of soap, which is separated
from the liquor and passed to a tall oil plant. The
spent liquor, i.e. the black liquor, is concentrated
by evaporating and burned after the concentration in
a soda recovery boiler in order to provide sodium
carbonate and sodium sulph~de as well as to recover
the energy contained in the black liquor. Smelt pro-
duced in the soda recovery boiler is then dissolvedin water or in dilute white liquor, after which-it is
treated in a known manner by causticizing it chemi-
cally, in conseguence of which the final result of
the whole process is a new alkaline cooking liquor.
This process is fully known per se and it is there-
fore not described here in greater detail.
The brown cellulose pulp obtained is then
bleached in a known manner by means of various chlo-
rine compounds, which is also fully known per se and
obvious to one skilled in the art and it is not de-
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scribed as such in greater detail.
Among cooking and bleaching chemicals, as well
as among chemicals used in a tall oil plant there are
also sulphurous substances, such as sulphuric acid,
which is used both for preparing chlorine dioxide for
bleaching and for splitting tall oil in the tall oil
plant. The sulphur passes through the process and ty-
pically tends to concentrate in the alkaline cooking
liquor, which is rather harmful, and attempts are
made to prevent that in different ways. Similarly,
so-called odour gases are produced at different
stages of the process, which gases contain sulphur as
reduced gaseous compounds, which causes deposits of
sulphur and odour nuisances in the environment. At-
tempts have been made to eliminate the odour gasesfor instance by collecting the gases and by burning
them either in existing boilers, in a lime kiln or in
a separate combustion plant for odour gases. When
using a separate combustion plant, the energy con-
~0 tained in the gases is recovered by means of a waste-
heat boiler and flue gases are scrubbed by a scrub-
ber, which is typically a NaOH scrubber. In the
scrubber, SOz gases contained in the flue ~ases pro-
duce sulphate and sulphite salts, which pass together
with white liquor to a pulp treatment plant and are
thus recycled. Numerous methods have been presented
for desulphuration of the process. Finnish Patent
67243, for instance, discloses a method in which a
part of the fresh liquor is circulated as a parallel
desulphuration cycle in the cellulose cooking pro-
cess, whereby the liquor is carbonated and the hydro-
gen sulphide produced is separated by stripping,
after which it is burned and prepared into sulphuric
acid by means of an aqueous solution. This equipment
is very complicated and an expensive lnvestment.
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The object of the present invention is to pro-
vide such a method of separating sulphur from the
process of a sulphate cellulose plant by means of
which method the amount of sulphur can be regulated
in a simple and easy manner, due to which the sulphur
separated can be recycled in order to replace a sepa-
rate sulphuric acid needed for the bleaching stage
and for tall oil cooking, while the sulphur content
of the liquor can be maintained on a desired level.
10 The method of the invention is characterized in
that the gaseous sulphur compounds are recovered as
odour gases produced at different stages of the cel-
lulose manufacturing process and separable under the
influence of physical conditions only and the sul-
phuric acid is prepared from the odour gases by oxi-
dating them and by dissolving thus formed sulphur
trioxide in a liquid mainly consisting of water.
The substantial idea of the invention is that
the sulphur is recovered as odour gases produced at
the various stages of the process, which gases are
separated from the process and oxidated in such a way
that substantially all sulphur in the odour gases is
changed into sulphur trioxide, after which sulphuric
acid is prepared from the sulphur trioxide in a man-
ner known per se, which sulphuric acid again is re-
cycled to the process, to the stage of preparing
chlorine dioxide or of cooking tall oil. In this way
it is possible to replace a great deal of the sul-
phuric acid needed by the plant by the sulphuric acid
obtained, while the environmental drawbacks are re-
duced substantially. The definition in claim 1, viz.
"as odour gases separable under the influence of phy-
sical conditions only", means that the odour gases
are separated from the solutions of the process on
the basis of the temperature and other physical con-
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ditions without a separate treatm~nt carried out by
chemical substances, the main purpose of which treat-
ment is only to prepare sulphurous gases and to sepa-
rate them from the process solution at some stage of
the cellulose process. Moreover, ~he sulphur content
of the liquor can be maintained sufficiently low by
means of the method of the in~ention, due to which
the drawbacks in the use and the service life of the
equipment caused by the sulphur can be largely avoid-
ed. The method of the invention especially improvesthe economical profitability of the cellulose plant,
because distinct savings are achieved by this method
in chemical costs.
The invention will be described in greater de-
tail by way of example by means of the enclosed draw-
ings, in which
Figure 1 shows schematically a cellulose cook-
ing and recovery process and a method according to
the invention,
2~ Figure 2 shows schematically one manner of pre-
paring sulphuric acid in the method of the invention
and
Figure 3 shows schematically another manner of
preparing sulphuric acid in the method of the inven-
tion.
Figure l shows a cellulose cooking process con-
ventional per se, in which process wood 1 to be cook-
ed is brought into a digester 2. In the digester it
is cooked together with alkaline cooking liquor into
cellulose pulp, after which the spent alkaline cook-
ing liquor, i.e. the black liquor, is washed off from
the cellulose in a washing department 3 and the wash-
ed cellulose pulp is brought into a bleaching plant
40 The black liquor is led further to an evaporation
plant 5 and 6, where water is separated from it un-
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til it is finally concentrated to a dry content as
high as possible. Also a separation of soap takes
place in a tank 16 before the evaporation 5. The soap
obtained is passed to a tall oil plant 13. At present
it is known that the alkaline liquor is at the ini-
tial stage 5 at first concentrated under atmospheric
pressure, whereby it can be heated with steam at a
low temperature, after which it is further concentra-
ted 6 under a pressure higher than atmospheric pres-
sure and by heating directly or indirectly with a
steam under a high pressure and at a high temperatu-
t re, as a conse~uence of which the final result is
alkaline liquor with a dry content of up till 75-80 %
under such a pressure and at such a temperature that
it can b~ displaced further by means of a pump and
fed into a soda recovery boiler 7. After the normal
evaporation plant 5 the figure shows a concentration
stage 6 of this kind, from which the liquor is fed
into a separate separating reactor ~, in which steam
and various sulphurous odour gases, such as H2S,
CH3HS, ~CH3)2S, (CH3)2S2 etc., are still separated from
it when the pressure falls slightly. Sulphurous odour
gases are produced both in the digester 2, in the
evaporation plant 5 and in the sulphur separating re-
actor 8 after the concentration as well as at the
washing stage 3 of the liquor. All these are passed
to an oxidation stage 9 of odour gases, in which the
sulphur compounds are oxidated in such a way that the
final result is sulphur trioxide, which is scrubbed
free from flue gases into sulphuric acid at a scrub-
bing stage 10. Finally, the flue gases are still
scrubbed at another scrubbing stage 11 with an alka-
line solution, e.g. sodium hydroxide, with which
slight residues of sulphur produce a solution con-
taining sodium sulphite~ sodium bisulphite and sodium
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sulphate, which solution is fed into the desulphura- -
tion reactor 8 or a dissolving tank of the soda re-
covery boiler, and after this mainly carbon dioxide
goes into a chimney. The sulphuric acid produced is
concentrated at a concentration stage 12, whereby the
impure dilute sulphuric acid can be led to the tall
oil cooking plant 13 and the strong sulphuric acid is
led to a preparation 14 of bleaching chemicals to re-
place the sulphuric acid to be purchased and brought
from outside. The waste acid produced at the prepara-
tion 14 of bleaching chemicals is also led to the
tall oil plant 13, from whicH is as waste obtained
sodium sulphate, which is led together with the black
liquor to the evaporation plant 5. The washed cellu-
lose pulp is led to the cellulose pulp bleachingplant 4, into which the produced bleaching chemicals,
i.e. mainly chlorine dioxide, are led. Respectively,
the steam obtained from the sulphur separating reac-
tor 8 can be led to the last concentration stage 6
for heating the black liquor. From the sulphur sepa-
rating reactor the black liquor is led into the soda
recovery boiler 7, from which the green liquor pro-
duced by smelt and water is passed to a causticizing
stage 15 and after the causticizing further to the
digester 2 as the alkaline cooking liquor.
The most preferable way of preparing sulphuric
acid is to use a catalyst. In the method according to
Figure 2, the odour gases are burned in a waste-heat
boiler 21, after which the produced gases containing
S02 are oxidated into S03 gases in a catalyst reactor
22 by means of a catalyst. It is possible to provide
steam in the waste-heat boiler by combustion, which
steam can be used suitably at various stages of the
cooking process. As a catalyst can be used for in-
stance vanadium pentaoxide (V20s) or other catalyst
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suitable for the purpose and known per se. After thecatalyst reactor 22 the gases are led to a scrubber
23, in which a two-stage scrubbing takes place. At
the first stage the gases are scrubbed either with
water or a dilute H2SO4 solution, whereby the sulphur
trioxide reacts and produces sulphuric acid together
with water. In order to get the sulphur trioxide en-
tirely removed, the remaining gases are scrubbed at
the second stage with a NaOH solution, after which
the gases can be led out and the produced solution
containing sodium sulphide or sodium hydrogen sul-
phide is removed e.g. to the dissolving tank of the
soda recovery boiler. Instead of the separate waste-
heat boiler 21 and the catalyst reactor 22, catalytic
combustion can also be used, due to which sulphur
trioxide is directly produced in the combustion cham-
ber and the gases are scrubbed as described above.
The embodiment of Figure 3 shows how the flue
gases of the waste-heat boiler according to the fore-
going example are changed into sulphuric acid byusing an active carbon reactor known per se instead
of a conventional catalyst reactor. The figure does
not show the waste-heat boiler but the combustion
gases coming from the waste-heat boiler through a
channel 31, which gases are led into separate active
carbon reactor chambers 32. The gases are led below
an active carbon layer 33, from which they are con-
veyed through the active carbon layer 33 and go fur-
ther into the chimney through a channel 34. Washing
li~uid, mainly water, is brought above the active
carbon layer through a channel 35 and fed by pumps 36
above the active carbon layer 33. The gases contain-
ing SO2 are oxidated in the reactor 32 under the in-
fluence of the active carbon layer 33, due to which
the produced S03 iS combined with the washing liquid,
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i.e. water, which flows through the active carbon
bed, and the sulphuric acid goes through a channel 37
into a collecting tank 38, from which it is pumped by
a pump 39 to be further concentrated according to the
diagram presented in Figure 1 and to be further re-
used.
An example of process savings to be achieved
according to the method of the invention is described
in the following. The volume flow of the odour gases
produced in the process is about 5 Nm3/second, the SO2
content being about 3000 ppm. The mass flow of sul-
phur dioxide is 42,9 g/sec. and about 90 ~ of the
sulphur dioxide can be oxidated into sulphur trioxide
by means of a catalyst. 30,9 g sulphur trio~ide is
then obtained per second and, washed into sulphuric
acid, 33,5 g sulphuric acid is obtained per second.
When an odour gas plant is used 8000 hours per year,
the final result can be 1022 tons of sulphuric acid.
2050 tons of sulphuric acid is consumed in the
tall oil plant per year, whereby about half of the
sulphuric acid needed is obtained from odour gases.
Then the sulphuric acid to be brought from outside
constitutes only 50 ~ of what would be needed normal-
ly. Further, the NaOH scrubbing required after the
odour gas plant consumes considerably less chemicals,
because most of the sulphur has been removed before
the scrubbing stage and hardly any reaction products
demanding a removal of the scrubber liquid are then
produced, but the scrubber liquid can be recycled for
a considerably longer time than previously.
The method of the invention can still be inten-
sified considerably in such a way that sulphurous
gases are separately removed from the finally concen-
trated black liquor. This can be carried out by heat-
ing it to a temperature higher than the cooking tem-
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perature and by keeping it there for a sufficient
time, whereby almost all the sulphur is separated as
sulphurous odour gases with water vapour from the
alkaline liquor when the liquor is permitted to ex-
pand to a lower pressure, and thus only a very insig-
nificant amount of sulphur comes into the soda recov-
ery boiler. This procedure can be carried out for in-
stance in connection with the last concentration
stage, when the liquor is kept at a sufficiently high
temperature during the last concentration stage and
it is then permitted to expand. Above in the specifi-
cation a~d the drawings, the invention has been de-
scribed by way of example and it is by no means re-
stricted to it. By pplying the method of the inven-
tion by means of the above procedure, it is possibleto provide an almost closed sulphur circulation,
whereby it is hardly necessary to add sulphuric acid
to the process at all. Especially by separating sul-
phur from concentrated black liquor in the above man-
ner, the greatest part of the sulphuric acid neededfor the process can be obtained by preparing it from
the sulphur separated, and thus the sulphur emissions
and the drawbacks caused by them can be greatly mini-
mized.
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