Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
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The present invention relates to a process or
the recovery of pulping chemicals from kra~t black
liquor. More particularly, it relates to a process for
the reduction of sodium sulfate in a kraft recovery
process using a fluid bed combustion unit.
sACKGROUND OF THE INVENTION/PRIOR ART
The pulping of lignocellulosic material such as
wood chips usually entails the reaction and dissolution of
lignin by a pulping liquor, which in the case of kraft
pulping is called "white" liquor. This serves to liberate
the fibres which are separated from the residual cooking
liquor, which is known in the art of kraft pulping as
"black" liquor. The residual black liquor, which contains
inorganic compounds, primarily sodium carbonate and sodium
sulfate resulting from the cooking chemical, along with
the dissolved wood components, is first concentrated by
evaporation and then sprayed into a kraft recovery furnace
in which organic wood components are burnt with the evolu-
tion of heat, leaving behind à char bed containing inor-
ganic residue. Kraft recovery furnaces also include a
reducing zone in the lower portion where the sodium
sulfate is reduced to sodium sulfide. The resulting inor-
ganic residue which is called smelt is composed primarily
of sodium sulfide and sodium carbonate. The smelt is then
dissolved in water to provide a "green" liquor to which
lime is added to convert the sodium carbonate to sodium
hydroxide, while the resultant insoluble ~alcium carbonate
can be separated therefrom. This step called causticizing
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serves to convert the green liquor to the "white" liquorwhich can be employed in the kra~t pulping process.
It is also known (as taught in U.S. patent number
4,011,129) to burn a portion of the black liquor from the
kraft pulping in a fluidized bed under oxidizing condi-
tions, and to direct the pellets so formed onto the sur-
face of the char bed in a conventional kraft recovery
furnace, to reduce the sodium sulfate to sodium sulfide.
This process has proven to be generally satisfactory, how-
ever, the reduction efficiency of the furnace has beenfound, to be adversely affected by the addition of pellets
to the bed.
It has been proposed elsewhere (Soviet patent
220,962) to carry out a solid phase reduction of sodium
sulfate using a mixture of methane (35~), hydrogen (43%)
and carbon monoxide (22%). Canadian patent 321,240 also
teaches a similar solld phase reduction of sodium sulfate,
possibly in the presence of sizeable quantities of iron or
iron compound(s). A proposed kraft recovery process
including a solid phase reduction of sodium sulfate with
hydrogen is taught in Canadian patent 828,654. However,
none of these processes lends itself for use as a conveni-
ent, economlcal and efflcient reduction step following a
fluidized bed kraft black liquor oxidation unit.
Accordingly, it is an object of this invention to
provide a process for improving the reduction efficiency
of sodium sulfate to sodium salfide In a kraft recovery
process which employs a fluidized bed black liquor combus-
tion unit. ~ ~
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BRIEF DESCRIPTION OF THE INVENTION
In a process for the treatment of inorganic pulp-
ing chemicals ~ontained in a kraft black liquor (b.l.)
comprising the steps of burning said b.l. in a fluidized
bed under oxidizing conditions so as to form pellets com-
prising said inorganic chemicals, and reducing sodium
sulfate in said pellets to form sodium sulfide, wherein
the improvement additionally comprises: adding an iron-
containing material to said b.l. before said burning, said
material being chosen from the group consisting essen-
tially of iron and oxides, sulfides, sulfites, sulfates
and sodium compounds of iron, and being added in an amount
effective to improve reduction efficiency and constituting
at most about 3% of said inorganic pulping chemicals in
said b.l. and where said reducing of said pellets is
carried out by contacting said pellets with a reducing gas
so as to obtain reduced pellets comprising sodium sulfide,
sodium carbonate and residual iron containing compound.
BRIEF DESCRIPTION OF THE D~WINGS
Figure 1 illustrates schematically a preferred
embodiment of the process of the present invention.
Figures 2 and 3 show the reduction efficiency as
a function of the proportion of iron in the liquor ash
when natural gas is the reducing gas.
Figures 4 and 5 shows the same relationship, but
when the reducing gases are carbon monoxide and hydrogen,
respectively.
Figure 6 illustrates~the reduction efficiency as
a function of the reducing time.
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Figure 7 shows reduction efficiency as a function
of the temperature of the reduction furnace.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention provide a pro-
cess for treating kraft black liquor (b.l.) so as torecover the inorganic pulping chemicals contained there-
in. Broadly the process will comprise the steps of adding
iron-containing material to the concentrated b.l. before
it is fed to a fluidized bed combustion unit, firing this
b.l. in the fluidized bed to oxidize the organic compounds
contained therein and form pellets containing the
pulping-derived inorganic chemicals and the iron
compound(s), and reducing sodium sulfate in the pellets to
form sodium sulfide by passing a reducing gas through the
lS pellets. Following the reduction step, these pellets are
dissolved to form a green liquor and the insoluble iron
compound(s) which is (are) separated, may, if warranted,
be used in the further treatment of black liquor. The
green liquor from the precedlng step can be causticized in
the convential manner to form a white liquor.
The b.l. resulting from a kraft cook will
contain; in addition to the organics which comprise
lignin, wood sugars, etc., the residual inorganlcs which
comprise primarily sodium salfate and sodium carbonate.
The b.l. which is initially at a consistency of about 16%
is concentrated in a conventional manner to a consistency
of about 40 to 65~ before further treatment. Iron-
containing materlal chosen from the group consisting of
iron, oxides, sulfides, sulfites and sulfates of iron, is
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added to the concentrated b.l., preferably in a slurry or
solution, so as to facilitate their distribution through-
out the b.l. Sodium-iron compounds such as ferrites can
also be used for this purpose. Most such iron compounds
can be used in this invention. Such compounds must be
process-compatible which means that they are non cor-
rosive, do not decrease the melting point of the mixture
of sodium and iron compounds, are relatively non toxic,
etc. Particularly preferred for use herein will be oxides
of iron. In the following, however, for the sake of con-
venience the iron compound will be referred to as iron
oxide, although it should be noted that a significantly
broader class of iron compounds is included thereunder.
Iron oxide will be added to the b.l. in a quanti-
ty so as to ensure an iron concentration of at least 0.25%
in the pellets following the liquor combustion. In the
case when the reducing gas is composed primarily of natu-
ral gas or carbon monoxide, it is preferrable that the
added iron constitute at most 0.75% of the pellets formed
following the combustion step. Should hydrogen be used as
the reducing gas, the added iron compound can constitute
up to about 3~ or even higher of the pellets without sig-
nificant losses of reduction efficiency in the following
stage. However further addition above of about 2.5% does
not improve the reduction efficiency, and the extra iron
will only be wasted. Finally, it is preferred that the
iron compound if added to the b.l. in an insoluble form,
be in a slurry composed of relatively small particles
e.g. size about ~, - 20 micro~s or less.
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Following the mixing of the iron compound slurry
into the concentrated b.l., it is sprayed into a fluid
bedcombustion unit where the organic chemicals are burnt
under oxidizing conditions to leave behind a substantially
inorganic residue in the form of pellets which are com-
posed primarily of sodium sulfate, sodium carbonate and
traces of added iron. The pellets which are usually about
0.8mm. in diameter are then fed to a reduction unit where
the sodium sulfate in the pellets is reduced to sodium
sulfide by the passage of a reducing gas through the pel-
lets, with the iron compound functioning as a catalyst.
The reduction unit can be either of a fixed or fluidized
bed type. The reducing gas can be any conventional reduc-
ing gas such as, for example hydrogen, carbon monoxide,
methane or natural gas, or a mixture thereof. Preferably,
it is a mixture of hydrogen and carbon monoxide (synthesis
gas) prepared by an incomplete combustion of hydrocarbons
or biomass which can be carried out in combustion units
specially designed for that purpose. The reduction will
usually be carrled out at a temperature in the range 600
to 700C, at as high a temperature as possible below the
eutectic temperature of the~mixtureO The reduced pellets
which are composed of sodlum sulfide, sodium carbonate and
traces of the residual iron compound are then dissolved in
water to prepare the green liquor, while the residual iron
compound can be separated therefrom. The green~llquor so
obtained can be causticized in the conventional manner to
obtain the white liquor. The separated iron compound(s)
can be recycled for use in the treatment ~;~` add1tional
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b.l., if warranted by the economics o~ the operation.
Referring now to Figure 1 which illustrates a
preferred embodiment of the present invention, we note
that iron oxide slurry stream 8 is mixed with a black
liquor stream 10 and sprayed into a fluidized bed recovery
furnace 14 to which combustion air and spent reducing gas
from the reduction furnace 24 are supplied to support
combustion and provide a fluidizing medium. Following
combustion of the organics in the fluidized bed recovery
furnace which results in fluidized bed pellets composed
primarily of sodium sulfate, sodium carbonate, and traces
of iron compound, are fed to the reduction furnace 24
which is supplied by a reducing gas stream 28 preferably
composed of a mixture of hydrogen and carbon monoxide,
formed by an incomplete oxidation of natural gas in burner
26 which has been specifically designed for this purpose.
The reduction furnace will usually be operated in a tem-
perature range of 600 to 700C, depending on the melting
characteristics of the sodium sulfate/sulfide/carbonate
and iron oxide mixture. The time period of this reduction
which will be 2 to 10 hours depending on the particle size
and additionally varies inversely with the temperature at
which the reduction is carried out. The reduced pellets
which comprise sodium sulfide, sodium carbonate and traces
of iron are then dissolved in water to provide a green
liquor. Following the settling and separating of iron
oxide ln the settling~step 32, the iron oxide, which is
the principal component of the dregs from the settling
step 32, can be r-~cvcled for use in the treatment of
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further black liquor as shown in 34. The green liquor
then proceeds to the white liquor system shown at 40 where
it is causticized by treatment with lime to convert the
sodium carbonate in the liquor to sodium hydroxide and
complete the formation of white liquor for use as pulping
chemical in the kraft pulping process. The iron oxide
containing dregs from the settling step 32 will occasion-
ally have to be bled off so as to prevent an accumulation
in the recovery system of cations such as calcium, etc.,
which are often found in the water used to dissolve the
reduced pellets. With the exception of this bleed-off,
the ferric oxide can be substantially completely re-used
if warranted by the economies of the operation. Another
of the advantages accruing from the use of this process
results from the combination of an efficient combustion
unit such as provided by the fluidized bed recovery
furnace, followed by a reduction step. This provides
reduction efficiencies hitherto unobtained by the combina-
tion of a fluidized bed recovery furnace~the pellets of
which were injected into the reducing zone of the conven-
tial kraft recovery furnace. Since the product of the
reduction urnace 24 is a reduced~pellet, which is
dissolved in the water to prepare green liquor, this elim-
inates the molten smelt obtained from the conventional
kraft recovery furnace and the explosion hazard and the
loss of energy that accompanied its handling.
Figures 2 and 3 show the reduction efficiency as
a function of the iron concentration in the liquor ash
when naturaL gas is used. It w~ e readily seen ~rom
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these graphs that the reduction efficiency is strongly
dependent on the temperature with reduction efficiencies
varying directly with the temperature. The reduction
efficiency also varies with the iron concentration in the
pellets, the optimuTn ~or this combination of sulfur con-
tent, reducing gas, temperature and iron are found to be
at approximately 0.3 - 0.7% iron in the liquor ash.
~ igures 4 and 5 show the reduction efficiency as
a function of the iron concentration of liquor ash when
the gases used are carbon monoxide and hydrogen. By
comparison with the preceding graphs, it will be seen that
the variation of the reduction efficiency when carbon
monoxide is used is less strongly dependent upon the iron
concentration in the liquor ash, with a similar optimum
for reduction effiency being found at about 0.45 - 0.55%
concentration of iron in the liquor ash.
In the case of hydrogen/ however, no such optimum
amount is found to exist. Concentrations of iron greater
than 0.25~ in the liquor ash result in an increased reduc-
tion efficiency with maximum reduction efficiencies beingfound at iron concentrations of over about 1.0~.
A combination of hydrogen and carbon monoxide
(e.g. synthesis gas) such as would be produced by an
incomplete combustion of hydrocarbons or natural gas will
result in a reduction efeiciencies intermediate between
those o hydrogen and carbon monoxide as shown in Figure 4
and it wouLd conceivably yield a reduction efEiciency
curve intermediate between those shown in this graph with
tlie exact position of the optlmum if any, being de~e,ld?nt
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upon the respective concentrations of hydrogen and carbon
monoxide in the reducing gas. Carbon monoxide for use as
a reducing gas can alternatively be produced by the incom-
plete combustion of coal in burners which are specifically
designed for this purpose.
Turning now to figure 7 which shows the reduction
efficiency as a function of temperature for a given iron
concentration, reduction pellets and where hydrogen is the
reducing gas, it is readily seen that the reduction effi-
ciency varies directly with the temperature.
Figure 8 shows the reduction efficiency varyingdirectly with time, at two different concentrations of
iron, with hydrogen as the reducing gas and a reduction
temperature of 620.
Table 1 below illustrates the difference between
reduetion efficiencies using hydrogen, of sodium sulfate
particles having iron intimately mixed-with them and
equivalently sized fluidized bed pellets containing iron,
prepared in aecordance with this invention-when they were
reduced using hydrogen. For the purposes of comparison,
reduction results of equivalently sized sodium sulfate
partieles without any added iron, are included. It will
be readily seen that fluidized bed pellets containing far
lower amounts of iron resulted in far higher reduction
efficieneies then equivalently sized sodium sulfate parti-
cles with iron admixed. The seemingly low reduction effi-
ciencies of the fluidized bed pellets can be attributed to
the nonoptimization of the iron content, as well as the
low reduction tonperature (a~out 600C) combilled with a
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relatively short duration ~a~ou~ 2 hours) of this reduc-
tion. In practice, however, the combination of a properly
adjusted iron content and temperature duration will result
in far higher reduction efficiencies.
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Reduction Efficiency (~)
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Particle Size Commercial Sodium Fluid Bed Pellets
(Mesh) Sulfate (l) Containing Fe
No Fe 0.4% Fe 0.02~ Fe 0.7% Fe
10 - 20 0 26 39 68
60 - 80 l 32 60
(l) From "Catalytic Reduction of Sodium Sulfate" -
C.J. Nyman and T.D. O'Brien - Industrial and Engineering
Chemistry, Vol. 39, NoO 8, p. lOl9 (1947).
2Q It will be readily evident to a person skilled in
the art that the present process can be readily integrated
into a kraft recovery installation which employs a fluid-
ized bed combustion unit for burning the-black 1iquor such
as those used to burn b.l. in excess of the capacity of
the conventional kraft recovery furnace. The ad~ed cost
of a reduction furnace is nominal and permits higher
reduction efficiencies to be obtained than were hitherto
possible for pellets obtained from a fluidized bed combus-
tion unit.
3Q Having described the invention, modifications
will be evident to those skilled in the art without
departing from the spirit of the invention as defined in
the appended claims.
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