Note: Descriptions are shown in the official language in which they were submitted.
~47~s
~his invention relates to a process for recovery of
chemicals from a pulping waste liquor, and more particularly
to a process for recovery of chemicals from a smelt which
is obtained by combustion of concentrated pulp cooking waste
liquor containing a sulfite or a bisulfite.
Various processes for producing pulp from cellulosic
material, for example, wood chips have been practiced.
Among them, more interest has recently been dra~n to a
process in which the cooking chemicals are sodium sulfite or
sodium bisulfite in combination with sodium carbonate,
because cellulosic pulp is produced in high ~ield to make
the process economical. However, in this process no
effective commercial recovery ~ystem of chemicals from the
waste liquor has been established.
~ 15 In Japanese Patent Publication 14401/74, we proposed
;~ a system for recover~ of chemicals comprising conce~trating
C! a cooking waste liquor from sodium sulfite process, bur~ing
the concentrate to obtain a smelt mainly comprising sodium
j sulfide and sodium carbonate, oxidizing the smelt with air
`~' 20 to convert Rodium sulfide into sodium sulfite, di~solvingthe oxidized ~aterial in water and treating the resultin~
aqueous solution with a sulfur dioxide-containing gas to
convert sodium carbonate i~to sodium sulfite, thereby
regenerating an aqueous solution which can be used as a
cooking liquor in the pulping process.
Although this prior procsss deals with fundamental
technical co~cept for recovering chemicals from sulfite
pulping wa~te liquor, there are still many problems to be
solved. For example, the smelt contains, in addition to
sodium sulfide a~d sodium carbonate, a small amount of
-- 2 ~
.. . - .. .. .
~4 ~S
sodium sulfate and slight amounts of sodium thiosulfate t
sodium sulfite and sodium chloride some of which are hard
to convert into the desired chemicals, and the proportion
of various chemicals present in the smelt may var~ over a
wide range depending upon the cooking conditions employed
in the particular pulp mill; accordingly, it is difficult
to standardize the conditions under which the recover~ of
chemicals is practiced and to design an apparatus suitable
for carrying out the process.
Accordingly, an object of thi~ invention is to provide
-~; a proceæs for recovery of chemicals ~rom sulfite pulping
waste liquor without causing environmental pollution and
any appreciable 108s of chemicals.
; When a typical waste liquor from semichemical sulfite
process is concentrated and burned, the resulting smelt has
the following compositio~ r~nges (by weight):
Na2S 30 to 40%
Na2C3 45 to 60~o
Na2S4 5 t O l O~o
Na2S203 0 - 5~
others 2 - 4%
It ha~ alread~ been know~ that, when smelt is treated
with a molecular ox~gen-contai~ing gas in the presence of
water, the sodium ~ulfide and sodium sulfate are oxidized
a¢cording to the following reaction formulae:
2Na2S + 32 2Na2$3 ............................ (1)
Na2S + 202 ~ Na2S4 ........ (2)
347~:5
Na2SO~ + 1/2 2 ~ ~a2~4 ~ --- (3)
2 H20 + 202 ~ Na2S203 + 2NaOH (4)
~eaction (1) occurs at a relativel~ low temperature
a~d reactions (2) and (3) at relativel~ high temperature.
In addition to the reactions mentioned above, the
following side reactions concurrently occur:
Na2S203 + 2~aOH ~ 3/4 Na2S03 + 2/3 Na2S + H20
Na2~203 + 2NaOH ~ 2 ~ 2~a2$03 + E20 .................... (6)
On the other hand, the sodium carbonate which is one
component of the smelt is unchanged during the oxidation
treatment.
~hus, in general, such oxidation treatment i~volves
1 various reactio~s and the primar~ purpose is to convert
; 15 the sodium sulfide into sodium sulfite but to prevent the
formation of sodiu~ sulfate and sodium thiosulfate which
are inactive in the pulping process. Eowever, in practice,
it is difficult to control the oxidation treatment to such
an extent that onl~ the desired reaction (1) will occur.
Further, an agueous solution of the oxidized product
is treated with a sulfur dioxide-containing gas to convert
the sodiu~ c~rbonate into sodium sulfite. ~he source of
said sulfur dioxide is, in ge~eral, an exhaust gas from the
recovery boiler and substantiall~ all of the sulfur dioxide
released durin~ the combustion of concentrated waste liquor
is recovered b~ being absorbed in the aqueous solution to
re~enerate a cooking liquor whereby the overall process
can be operated as a closed system.
Accordingl~, in order to success~ully carry out
~0 the process, it is essential that, in the mi~ture to be
.... . .
47~5
oxidized, the molar ratio of ~/Na20 be maintained sub-
stantially equal to that of the smelt and the water content
be kept at an appropriate level.
In general, the temperature at which the oxidation
is effected is controlled by adjusting the amount of water
co~tained in the reaction mixture, consequently, by adjusting
the amount of heat removed from the mixture by evaporation
of water, so that the reaction temperature is maintained
within a range within which sodiu~ sulfide i8 effectively
converted into sodium sulfite.
According to this invention7 the ~olar ratio of
S/~a20 in the reaction mixture is readily controlled by
adjusting the amount of aqueous slurry to be supplied to
a solid-liquid separation step and the water content of
the wet cake to be oxidized and by establishing the balance
between the ~ake up water to be supplied to the aqueOu~
slurr~ and the a~ount of water lost from the s~stem, ~ai~l~
b~ evaporation in the s~elt hopper and the oxidizers.
According to this invention, there is provided a
process for recover~ of chemicals from sodium sulfite pulping
waste liquor compri~ing steps of:
(1) introducing a smelt obtained by burning a
concentrated waste liquor into an aqueous slurr~
while suppl~ing make up water and a weak aqueous
slurr~ rec~cled fro~ step (2) to effect inco~plete
dissolving of the smelt into the aqueous slurry and
suppl~ing a part of the resulting aqueous slurr~ to
step (2) thereby ~aintaining the aqueous slurry to
a total solid content of from about 35 to about 70%
b~ weight, a proportion of sodiu~ carbonate in the
-- 5 --
7~5
total solid material lower than that of the s~elt
and a te~perature of from about 55 to about 90C,
(2) separating the slurr~ formed in step (1) into
a wet cake containing water in a proportion of from
about 10 to about 50% b~ weight and having a molar
ratio of S/Na20 substantially equal to that of the
smelt and a weak aqueous slurry, and recycling the
weak slurr~ to step (1), and
(3) mi~ing the wet cake with hot particles containing
sodiu~ sulfite and sodium carbonate while supplying
~imultaneousl7 a mole~-ular ox~gen-contai~ing gas to
effect oxidation of sodium sulfide present i~ the wet
cake into sodium sulfite.
~his invention will be explained in detail.
~irst steP
A smelt which is formed in conventional recovery
boiler by co~bustion of a concentrated pulping cooking waste
liquor i8 discharged to smelt receiving means comprising
a smelt hopper and a screen. ~gainst the s~elt strea~ directed
into the hopper, a steam or air jet iæ impinged to cool and
divide out the smelt into solid particles. An aqueous slurry
rec~cled from a circulating tank is flowed downwardly on the
internal surface of the hopper to re~ove ths particles
attached thereto. ~he solid particles drop onto a conveying
~5 screen provided below the hopper to separate lumps or large
particles and particles, the former being supplied to a
~reen liquor tank in which the lumps are dissolved in an
aqueous medium introduced from a gas washer and the latter
bein~ supplied to a slurry circulating tank.
The atmosphere of the smelt hopper is maintained
_
``
. . .
47~S
under a reduced pressure b~ means of air fan which discharges
the air in the hopper to the atmosphere after being treated
in a gas washer, and the air passinæ through the hopper
effects oxidation of a part of the sodium sulfide present
in the smelt and the rec~cled slurr~ into sodiu~ thiosulfate.
(Hereunder this is referred to a preliminary oxidation~)
The circulating tank to which solidified smelt
particles are supplied, contains a large amount of the
aqueous slurr~ to which simultaneousl~ a weak green liquor
and/or a weak aqueous slurry rec~cled from the second step
are supplied thereb~ effecting partial or incomplete di~solving
of the chemicals in the aqueous slurry. By "partial or
inco~plete dissolving", we mean a state in which the final
mixture of the chemi~als and the aqueous medium contains some
undissolved solid ~aterial, so that the for~ation of a slurr~
by precipitati~g solid particle fro~ an aqueous solution
with cooling or from an aqueous supersaturated solution is
also within this meanlng.
The temperature of the aqueous slurr~ is ~aintai~ed
at fro~ 55 to 90C, preferabl~ 75 to 83C b~ a cooling
means, for exa~ple, a cooling coil or jacket provided in
the tank.
~he substantial portion of the slurr~ is recycled to
the smelt hopper and the remainder is supplied to the subse-
~uent step of solid-liquid separation after which the separated
liquid phase is recycled to the slurr~ circulating tank.
The a~ount of slurry to be recycled to the smelt
hopper is fro~ 20 to 200 times, preferably about 100 times
based on the weight o~ smelt introduced from the recovery
boiler. Thus, the ~ount o~ a~ueous slurry hold in the
7~S
circulating tank is extremely large amou~t in c~mparison
with the amount of the smelt introduced and the resident
time of the aqueous slurry in the tank is usually ~ore
than 20 hours.
The slurr~ i~ composed of a solid phase the main
component of which is sodium carbonate and a liquid phase
the main component of which is an aqueous sodium sulfide.
In consequence, the liquid phase rec~cled from the second
step contains mainly sodium sulfide. ~herefore, at ~tead~
state, the proportion of sodium carbonate present in the
slurry is ~aintained at a co~stant level lower than that
; of the smelt and the content of total solid in the slurry
is from 35 to 70% preferably 45 to ~P~, by weight, the
total solid being the sum of che~icals present in the
slurry as solute and solid particles undissolved as well
as in soluble materials.
Second E;teP
q'he slurr~ is pl~ped from the circulating tank to a
solid-liquid separator, for example, a screw decantor at a
constant flow rate and a constant feed pressure through a
head tank. In the separator, the slurry is separated into
a wet cake which comprises solid sodium carbonate and an
aqueous sodiu~ sulfide and contains water of from 10 to 50%,
pre~erably 15 to 3gh by weight, and a weak aqueous slurry
which comprises the aqueous sodium sulfide and microparticles
not separated. ~he wet cake i5 supplied to the subsequent
oxidation step and the weak aqueous slurry is recycled to
the circulati~g tank.
In order to carr~ out the overall process according
to this invention satisfactorily, it is essential to control
`76$
the molar ratio of ~/Na2~ in the wet cake to be substantially
equal to that of the smelt, and this ca~ be done b~ adJusting
appropriatel~ the a~ount of aqueous sodium sulfide to be
present in the cake.
However, in some cases, depending upon the cooking
conditions and properties of the pulp to be produced, the
proportion of sodium carbonate in the cooking che~icals is
; small, for exa~ple, less than 10% b~ weight; then the
sodiu~ sulfide content of the smelt increases to abo~e
40% b~ weight. Thus, it is necessar~ that the wet cake
contains a higher proportio~ of ~odium sulfide in order to
achieve the required level of the molar ratio of 8/~a20;
this naturall~ means that the liquid phase in the wet cake
must a high concentration of sodium ~ulfide if the amount
f the aqueou~ sodium sulfide i~ relatively small or that
the proportion of the liquid phase must be increa~ed, if
the concentration is relatively low.
If a high co~centration of sodium sulfide is
available, the temperature of the aqueous slurry must be
raised in order to undesirable precipitation of sodiu~
sulfide on various parts, especially on the cooling means,
since the solubilit~ of sodium sulfide in water lowers
toward to lower temperature. At a high temperature,
especiall~ above 85C, aqueous sodium sulfide is extremely
corrosive and at such a high te~perature even 18-8 stainless
steel cPnnot resist and more expensive non-corrosive material
will be required. On the other ha~d, if the liquid phase
is of low co~centration, the wet cake must contain a
relativel~ large a~ount of liquid phase in order to achieve
re~uired ~olar ratio of S/~a20. However, the screw decantor
7~5
which is usuall~ employed in this invention can not perform
~3uccessful in the case where a relatively dilute wet cake
is intended, accordingly, at a solid content at which a
wet cake is readil~ available, the amount of sodium sulfide
contained in the wet cake becomes shortage.
~ or the foregoing reasons, when the smelt has a
higher proportion of sodium sulfide, it is difficult to
increase the amount of liquid phase in the wet cake in
order to achieve the required molar ratio of S/~a20 for
smoothl~ proceeding the process and additio~al sodium
sulfide must be supplied to the oxidation reaction mixture.
One feature of the process according to this invention
is that solid material obtained b~ contacting a part of the
aqueous slurr~ discharged from the first step and/or a part
of the weak aqueous slurr~ recovered from the solid-liquid
separator wi~h a cooling surface, is supplied to the
oxidation step thereb~ ad~usting the molar ratio of S/~a20
of the reaction mixture to be oxidized to that of the smelt.
Such cooling surface is con~eniently a drum flaker
comprising an aqueous slurr~ receiving vat, a rotary drum
positioned in the vat and so arranged that the lower part
thereof is immer~ed in the slurr~, a cooling mea~s to maintain
the dru~ at a temperature below the solidif~ing temperature of
the aqueous slurr~ &nd a doctor means for re~oving the
solidified material from the drum surface.
Third SteP
The filter cake recovered in the 3econd step a~d, if
necessar~, additional solid material obtained from the drum
flaker are continuousl~ supplied to an oxidation reactor in
which the feed is thoroughl~ admixed with hot particles of
-- 10 --
7~5
~odium carbo~ate and sodiu~ sulfide at a~ elevated te~perature
lmder agitation while a ~olecular oxygen-contai~ing gas is
passed therethrough to oxidi~e sodium sulfide present in the
feed to sodium sulfite. ~he oxidation reactor is conveniently
a doùble-s aft Z type kneader having a feeding port and an
overflow chute for discharging the oxidized product.
In the kneader, the sha~t adjacent to the overflow
chute is rotated at a speed of 10 to 50 r.p.m. and higher
by 10 to 20h than that of the other shaft to facilitate
the discharge of the oxidized product. ~he oxidized product
i5 obtained in the for~ of particles having a dia~eter of,
for example, from 200 to 400~ depending upon the resident
ti~e.
Depending upon the oxidation conditions and the
composition of the feed to be oxidized, a relatively small
amount of unoxidized sodium sulfide ma~ be present in the
oxidation product. In such a case, the product is subjected
to additio~al oxidation in which the product is passed
through a ~onfined space in a piston flow while a molecular
ox~gen-co~taining gas i~ supplied æimultaneousl~.
During the ~ain oxldation and the suboxidation,
sodiu~ sulfide and sodium thiosulfate are oxidized to
sodiu~ sulfite. The molecular oxygen-containing gas is
o~ygen or air, the latter being preferred.
The amou~t of the molecular oxygen--containin~ gas
to be supplied to the main oxidatio~ step is from 2 to 20
times, preferabl~ 10 times, in ter~s of oxygen required for
co~pletely oxidize the sodium sulfide the a~ount of which
i~ based on the a~ount in the smelt, since so~e of the
sodium sulfide in the s~elt has been converted into other
P~L~476S
c~mpounds, for example, such as sodium thiosulfate, and the
exact amount thereof in the reaction mixture is hardl~
determined. ~he temperature of the reaction mixture rises
by the heat generated in the oxidation reaction. At a hlgh
temperature, for example, above about 300C, undesirably
large amount of sodiu~ sulfate will be formed; on the other
hand, at a lower te~perature, for e~ample, below 100C,
sodium sulfide is ~ot completel~ converted into sodiu~
sulfite rather the formation of sodium thiosulfate increases.
The heat of reaction in converting sodium sulfide
into sodiu~ sulfite is 171 E cal. per one ~ole of sodium
sulfide and that of into sodium thiosulfate is 112 E cal.
per one mole of ~odium sulfide. ~hus, it is beneficial to
effect the preoxidation of the smelt in order to prevent
the temperature from rising undul~ during the oxidation
reaction. ~he temperature of the reaction mixture to be
oxidized is held to an appropriate level by adjusting the
water content of the wet cake, and additional water ma~ be
supplied to the oxidation reactor, if necessary.
~he oxidation reaction is carried out at a temperature
of from 100 to 300C, preferabl~ 150 to 250C, for 2 to 15
houx~, preferably 3 to 6 hours with the supply of a molecular
ox~gen-containing gas at 100 to 200C, preferabl~ 150 to
180C.
In the case where the suboxidation reaction is effected,
the molecular ox~gen-co~taini~g gas i8 supplied i~ an a~ount
of from 10 to 30~ used for the mai~ oxidation.
In the reactions (5) and (6) ~entioned above, there
are sodium h~droxide is required; this will be supplied from
the reaction (4~ in the stoichiometric quantit~. If any
- 12 -
7~5
shortage of sodium hydroxide is observed, a~ additional
amount may be supplemented. Residual sodiu~ sulfide is
often observed in the oxidation product due to the shortage
of water, especially in the suboxidation reactor; in this
case water may be added to the reaction mixture.
~ine particles which are entrained in the exhaust
gas fro~ the main oxidation and the suboxidation reactors
are collected in a dust collector æuch as cyclone and
recycled to any of the reactors. The exhaust gas thus
treated is further cleaned in a scrubber, if necessary,
to completely remove the fine particles entrai~ed.
Fourth Step
~he oxidized product is continuously supplied to a
dissolving tank to which washed water discharged from the
scrubber is simult~neously supplied or fresh water to for~
an aqueous solution containing sodium carbonate and sodium
sulfite at a co~centration of from about 15 to about 25%
by weight and ha~ing a temperature of above about 30C.
The aqueous solution is allowed to stand to effect
sedi~entation of insoluble materials including carbon and
iron compound, which are then removed b~, for example, a
thickener. ~ince such insoluble materials catalytically
promote the oxidation of sodium sulfite into undesirable
sodium sulfate in the subsequent step.
Into the aqueous solution thus clarified, the
exhaust gas discharged from the recovery boiler which
contains sulfur dioxide formed by combustion of the black
liquor is blown to form sodium sulfite by the reaction of
sodium carbonate and sulfur dioxide. If necessary, the
exhaust gas from an auxiliary boiler is used together with
- 13 - ~
s
the recover~ boiler exhaust gas. Such exhaust gas often
contains a relatively large amount of sulfur trioxide which
reacts with sodium carbonate to form undesirable sodium
sulfate and therefore, in such a case, the precaution must
be taken to remove sulfur trioxide b~ washing with water.
~ he resulting aqueous solution contains sodium
sulfite and sodium carbonate in a proportion and at
concentrations suitable for use in a pulp making process.
Fifth ~tep
Since the aqueous solution obtained contains solid
particles which are accompanied with the exhaust gas, the
aqueous solution is treated with a thickener to remove sludge.
~ his sludge and the sludge recovered in the fourth
step contain a relatively large amount of aqueous solution
of sodium salts~ Both sludges are combined and mixed with
water. From the resulting mixture, the aqueous solution
containing useful chemicals is separated b~ a thickener
and a filter and is recycled to the first step and/or the
fourth step to use for dissolving the smelt and/or the
oxidation product.
As mentioned above, according to this invention
the overall process is operated as a closed system in which
the sulfur dioxide generated in the recover~ boiler and
present in the exhaust gas is absorbed in the aqueous
solution which contains substantiall~ all of the sodium
component and sulfur component present in the smelt as
soaium carbonate and sodium sulfite, and little or no loss
of chemicals will occur.
~igure 1 illustrates the process according to this
invention.
- 14 -
76,5
Concentrated black liquor is burned in recovery
boiler 1 to form a smelt which is supplied continuously in
the form of stream 2 to smelt hopper 3. ~team or high
pressure air stream 4 is directed to the smelt stream to
effect cooling and dividing out of the smelt into fine
particles. An aqueous ~lurry recycled from circulating
ta~k 6 is flowed downwardl~ on the inner surface of the
hopper to prevent accumulation of smelt thereon. ~he rate
of the slurr~ to be supplied to the hopper is 100 times
tha~ of the smelt, by weight~ l'he smelt particles drop on
conveying screen 5 by which fine particles and large parti-
cles are separated, the former being supplied to the slurry
circulating tank 6 and the latter to green liquor tank 7.
In the green liquor tank, the large particles are
dissolved under agitation i~ water, which is supplied from
scrubber 8 for washing exhaust ~as from the hopper, to form
a weak gree~ liquor which is pumped to the slurr~ circulatin~
tank.
Though the concentration of the green liquor produced
varies depending upon the amount of water to be supplied,
which is determined taking in account the total water balance
throughout the overall operation, the concentration is a
factor in determi~ing the concentration of slurry to be
formed in the slurr~ circulating tank and is usually
maintained at about 10~ by weight.
Under stead~ operation conditionæ, a well established
balance of water between (1) the sum of the supply to oxi-
dation step and the water di~charged together with air from
the smelt hopper ~nd (2) water supplied ~rom the green liquor
tank is maintained to make the solid content in the green
15 -
3L1~34~S
liquor tank at a constant level.
~he green liquor is mixed in the slurr~ circulating
tPnk with the aqueous slurry, the solidified smelt particles
and a weak aqueous slurry recycled from the second step
under agitation, to form a slurry comprising a multi-
component aqueous phase co~taining mainly sodium sul~ide
as well as sodium h~droxide, sodium polysulfides and other
sodium salts such as sodium thiosulfate, sodium sulfate,
sodium carbonate and sodium chloride and a solid phase
cont~i~ing mainly sodium carbonate and other undissolved
components above and various derivatives therefrom.
~he molar ratio of S/~a20 of total solid in the
slurry is considerably higher than that of the smelt, for
example, the ratio of ~melt being about 0.5 the ratio of
slurry being from 0.8 to 1Ø ~he temperature of the slurry
is maintained at from 55 to 90C, preferably 75 to 83C,
for example, by means of cooling water passing through a
cooling coil or jacket and maintained at a temperature from
5 to 40C, preferably 10 to 20C, lower than that of the
slurry.
The use of cooling water of too low temperature
results in the precipitation of solid on the cooling surface
to impede cooling efficiency. lf desired, cooling by passing
cold air through the slurry may be used in addition to such
water cooling. In this case, some of the sodium sulfide is
o~idi~ed.
'~he total content of the chemicals in the slurry is
maintained within a ra~ge of from 35 to 70%, preferably
45 to 60~o by weight.
At a concentration below 35yo, thoug~ cooling efficiency
- 16 -
' 47~5
is improved, the wet cake obtained in the second step
contains more sodium carbonate than required for maintaining
the desired molar ratio of S/Na20. Further, if the slurr~
or the filtrate is cooled on a drum flaker, there is
insufficient solidification, or the resulted flakes contain
much water which lowers the oxidation temperature in the
oxidation reactor to which the flakes are supplied.
On the other hand, at a concentration above 70%,
there are disadvantages in that it becomes more difficult
to maint~in the slurry temperature below 90C and there is
clogging of the pipe lines.
~he slurry is pumped via head tank 9 to screw
decantor 10. With the provision of the head tank, the
slurr~ can be supplied continuously at a constant pressure
to the decantor. ~y the decantor, a part of the solid
phase is separated and removed as a wet cake from the
aqueous slurr~ and the wet cake i8 supplied to oxidation
reactor 11 while the reminder is recycled to the slurry
circulating tank as a weak aqueous slurry.
In the case where the smelt has a proportion of
sodium sulfite less tha~ 40~, especially less than 30%,
b~ weight, it is easy to adjust the molar ratio of S/Na20
of the wet cake to that of the smelt.
On the other hand, the proportion of sodium sulfide
i~ the smelt increases to more than 30~, especiall~ more
than 4~h by weight, it is naturall~ necessar~ to increase
the proportion of sulfur component in the aqueous slurry
in order to adjust the ~/Na20 ratio of the wet cake to
that of the smelt. If the concentration of sodium sulfide
; 30 in the aqueous slurry increases, the cooling efficiency of
- 17 -
4 ~ ~5
the slurr~ lowers due to the precipitation of sodium sulfide
on the cooling surface, then, it is difficult to lower the
temperature of the aqueous slurry to the required level.
In such a case, part of the slurry discharged from the head
tank is divided out a~d directed to drum flaker 12; alterna-
tively a part of the filtrate from the decantor i8 directed
to the drum flaker.
~ y the drum flaker, the chemicals present in the
slurry or the filtrate are solidified on the drum which iæ
usually cooled to a temperature below 70C, preferably 30
to 50C and the flakes formed are removed and supplied to
the main oxidation reactor.
The flakes have a composition similar to that of
the a~ueous slurry supplied. ~or easy operation of the
drum flaker, it is preferred to solidify only a part of
the aqueous slurr~ and to rec~cle the remaining aqueous
slurr~ to the circulating tank. Thus, by controlling the
amou~t of flakes æupplied to the reactor, the molar ratio
of S/Na20 of the combined filter cake a~d flake is readily
adjusted to that of the smelt.
~ he main oxidation reactor is a double-shaft Z type
kneader the lower portion of which contains a large amount
of solid particles of the oxidized product containing a
small am~u~t of water of, for example, a few percents and 25 up to 5% by weight. As the wet cake a~d the flake are
supplied to the reactor, the~ are immediately mixed with
the oxidized product particles, while hot air (lO0 to 200C,
preferabl~ 150 - 185C) is simulta~eousl~ supplied. Si~ce
the exhaust gas from the reactor contains a considerable
amount of moisture, if too small an amount of air i~ supplied,
- 18 -
471~5
the exhaust gas becomes to have a higher moisture content
hich results in water droplets condensing on the surface
of various parts, for example, cyclone separator and duct.
Such droplets catch fine particles entrained in the exhaust
gas a~d cause clo~ging. The supplied air also facilitates
agitation and mixing of the reaction mixture in the
reactor; therefore, an adequate air suppl~ is desirable.
However, too much air requires more energy for heating and
supplying air, and in additio~ causes the escape of a large
amount of particles from the reactor. ~hus, the amOuQt of
air to be supplied is from 2 to 20 times, preferably 10
times th~t required to completely o~idize the total sodium
sulfide present in the smelt.
The exhaust gas from the reactor i~ fed to cyclone
14 in which particles e~trai~ed are recovered and recycled
to the reactor.
The oxidized product which contains uno~ dized
chemicals and oxmdation intermediates is discharged from
the reactor via the overflow chute a~d is supplied to
suboxidation reactor 13.
~ he purpose of the suboxidation reactor is to effect
as complete an oxidation reaction as possible, if such
u~oxidized chemicals and intermediates are introduced in
the ~ubsequent step, they react with sulfur dioxide to form
h~drogen sulfide which is a pollutant gas and sodium thio-
sulfate which is undesirable for pulp making.
To the suboxidation reactor, air is introduced to
effect additio~al oxidation at a temperature of from 100
to 300C, preferably 150 to 250C. The water, up to about
5% by weight, pre~ent in the ~olid particles discharged
- 19 -
7~5
from the main reactor is enough for performing the conversion
of residual sodium sulfide i~to sodium sulfite. However, if
less amount of water is presentl an additional water may b~
fed in order to facilitate the oxidation reaction. ~he
e~haust gas from the suboxidation reactor is supplied to
the cyclone 14 in which e~trained solid particles are
recovered.
If the e~trained particles are not completel~ separated
in the cyclone, then the exhaust gas is further treated in
scrubber 14 to which fresh water or a dilute aqueous chemical
solution is supplied to dissolve solid particles a~ completely
as possible.
The oxidized product is discharged from the suboxi-
dation reactor and supplied to dissolving tank 15 to which
the aqueous solution discharged from the scrubber 16 is
supplied to form an aqueous solution containing sodium
carbonate and sodium sulfite~
The amount of aqueous solution to be supplied to the
dissolving tank is controlled so that the con¢entration of
the resulting aqueous solution is about 20~o by weight. ~t
a concentration above 20yo~ there is encountered difficulty
in treating the aqueous solution with sulfur dioxide for
producing a pulp cooking liquor; on the other hand too dilute
a~ aqueous solution cannot give a liquor having a co~centra-
tion suitable for cooking.
The aqueous solution is supplied to ediment tank17 in which insoluble material is separated as sludge, and
the clarified liquor is supplied to absorber 18.
In the absorber, the clarified liquor and the exhaust
gas discharged from the recovery boiler are intimatedly
- 20 -
Lr7~ 5
contacted to effect conversion of sodium carbonate into
~odium sulfite to the extent required for the desired
cooking liquor composition. The exhaust gas from the
absorber does not contain sulfur dioxide and is vented
to atmosphere.
The cooking liquor thus produced is clari~ied in
thickener 19 and used ~or pulp making. The sludge recovered
from the thickener is combi~ed with the sludge from the
sediment tank, washed with water, filtered and removed from
the processing æystem. The washing and the filtrate are
recycled to the scru~ber and/or the dissol~ing tank.
~ his invention will be explained by means of Examples.
However, it should be understood that this invention is in
uo way limited b~ these Examples.
Example 1:
Smelt recovered from a recovery furna~e was treated
according to the procedures explained above ~nd using the
apparatus illustrated b~ referring to ~ig. l to produce a
cooking liquor. ~he temperature and the content o~ total
chemicals in the slurry circulating tank 6 were maintained
at 83.5C and 59.6~ b~ weight, respectivel~. In the seco~d
step, the screw decantor 9 wa~ used but ~ot the drum flaker
11. The temperatures of the reaction mixtures in the main
oxidation reactor 10 and the ~uboxidation reactor 1~ were
maintained at 190C a~d 150a, respectivelyO
The smelt was introduced in the process at a rate
of 2.0 tons per hour and fresh water was supplied at
24 tons per hour.
The composition in each step is given in Table 1.
- 21 _
7~5
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-- 22 --
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1 Example 2:
Procedures similar to those of Example 1 were repeated.
The slurry in the circulating tank had a temperature
of 78C and total chemical of 55.1%. In the second step, the
scre~w decantor and the drum flaker were used. The temperature
of the main oxidation reactor and the suboxidation reactor
were 210 C and 160C, respectively. The amount of water
introduced to the suboxidation reactor was 50 liters per hour.
The compositions in each step is given in Table 2.
- 23 -
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_ 24 --
r~
