CA1171608A - Recovery process for kraft black liquor - Google Patents
Recovery process for kraft black liquorInfo
- Publication number
- CA1171608A CA1171608A CA000405594A CA405594A CA1171608A CA 1171608 A CA1171608 A CA 1171608A CA 000405594 A CA000405594 A CA 000405594A CA 405594 A CA405594 A CA 405594A CA 1171608 A CA1171608 A CA 1171608A
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- CA
- Canada
- Prior art keywords
- iron
- pellets
- reducing
- sodium
- liquor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
- D21C11/0092—Substances modifying the evaporation, combustion, or thermal decomposition processes of black liquor
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- Paper (AREA)
Abstract
RECOVERY PROCESS FOR KRAFT BLACK LIQUOR
ABSTRACT
The present invention provides a process for the recovery of inorganic compounds in kraft black liquor.
The invention comprises the addition of small amounts of iron containing material to kraft black liquor before it is burnt in a fluidized bed to form pellets of substan-tially inorganic content. The sodium sulfate fraction of the pellets is reduced to sodium sulfide by contacting the pellets with a reducing gas stream. The reduced pellets are dissolved in water and the residual iron-containing compound separated to form a green liquor.
ABSTRACT
The present invention provides a process for the recovery of inorganic compounds in kraft black liquor.
The invention comprises the addition of small amounts of iron containing material to kraft black liquor before it is burnt in a fluidized bed to form pellets of substan-tially inorganic content. The sodium sulfate fraction of the pellets is reduced to sodium sulfide by contacting the pellets with a reducing gas stream. The reduced pellets are dissolved in water and the residual iron-containing compound separated to form a green liquor.
Description
~7~6~3 FIELD OF THE INVENTION
_ 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
_ 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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~7~8 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 ~ g _ .
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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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iQ~3 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 - .
- . . .
- ', ~ .." ' ' : , 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.
.__ _ . .
Reduction Efficiency (~) .__ .
Particle Size Commercial Sodium Fluid Bed Pellets (Mesh) Sulfate (l) Containing Fe No Fe 0.4% Fe 0.02~ Fe 0.7% Fe 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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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. ~ ~
~7~
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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~7~8 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 ~ g _ .
:
. .
.
- . . ;~;
, 7~
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 . ~
: . . : . .. ., ~:
- . , : ~ ,: ., ..
iQ~3 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 - .
- . . .
- ', ~ .." ' ' : , 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.
.__ _ . .
Reduction Efficiency (~) .__ .
Particle Size Commercial Sodium Fluid Bed Pellets (Mesh) Sulfate (l) Containing Fe No Fe 0.4% Fe 0.02~ Fe 0.7% Fe 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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Claims (11)
1. In a process for the treatment of inorganic pulp-ing chemicals comprising sodium sulfate and sodium carbon-ate contained 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 comprising said inorganic chemicals, and reducing said sodium sulfate in said pellets to form sodium sulfide, wherein the improve-ment additionally comprises:
adding iron-containing material to said b.l.
before said burning, said iron-containing materi-al being chosen from the group consisting essen-tially of iron; oxides, sulfides, sulfites, sulfates, and sodium compounds of iron; and added in an amount effective to improve reduction efficiency and constituting at most about 3% of said inorganic pulping chemicals in said b.l., 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.
adding iron-containing material to said b.l.
before said burning, said iron-containing materi-al being chosen from the group consisting essen-tially of iron; oxides, sulfides, sulfites, sulfates, and sodium compounds of iron; and added in an amount effective to improve reduction efficiency and constituting at most about 3% of said inorganic pulping chemicals in said b.l., 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.
2. A process as defined in Claim 1, comprising the additional steps of:
dissolving said reduced pellets in water, and separating said residual iron-containing compound from solution of said dissolved pellets, so as to obtain green liquor.
dissolving said reduced pellets in water, and separating said residual iron-containing compound from solution of said dissolved pellets, so as to obtain green liquor.
3. A process as defined in Claim 2, where said residual iron-containing compound is recycled for use as a constituent of said iron-containing material added to said b.l. before said burning.
4. A process as defined in Claim 2, additionally comprising causticizing said green liquor to form a kraft pulping liquor.
5. A process as defined in Claims 1, 2 or 3 wherein said iron-containing material is added to said b.l. in the form of slurry.
6. A process as defined in Claims 1, 2 or 3, wherein said reducing gas is chosen from the group comprising carbon monoxide, hydrogen and methane.
7. A process as defined in Claims 1, 2 or 3, wherein said reducing gas is hydrogen and said iron-containing compound constitutes at least about 0.25% of said pellets before said reducing step.
8. A process as defined in Claims 1, 2 or 3, wherein said reducing gas comprises at least one of carbon monox-ide and methane and said iron-containing compound consti-tutes between 0.25 to about 0.75% of said pellets before said reducing step.
9. A process as defined in Claims 1, 2 or 3, wherein said reducing of said pellets is carried out at a tempera-ture in the range of about 600 to 700°C.
10. A process as defined in Claims 1, 2 or 3 wherein said reducing gas is hydrogen, said iron-containing mate-rial constitutes at least about 0.25% of said pellets before said reducing step, and said reducing of said pellets is carried out at a temperature in the range of about 600 to 700°C.
11. A process as defined in Claims 1, 2 or 3 wherein said reducing gas comprises at least one of carbon monox-ide and methane, said iron-containing compound constitutes between about 0.25 to about 0.75% of said pellets before the reducing step, and said reduction is carried out at a temperature in the range of about 600 to 700°C.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000405594A CA1171608A (en) | 1982-06-21 | 1982-06-21 | Recovery process for kraft black liquor |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000405594A CA1171608A (en) | 1982-06-21 | 1982-06-21 | Recovery process for kraft black liquor |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1171608A true CA1171608A (en) | 1984-07-31 |
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ID=4123057
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000405594A Expired CA1171608A (en) | 1982-06-21 | 1982-06-21 | Recovery process for kraft black liquor |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1171608A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1994007797A1 (en) * | 1992-09-30 | 1994-04-14 | North Broken Hill Limited | A process for recovering alkali metal hydroxide from organic liquors |
| CN114875241A (en) * | 2022-04-07 | 2022-08-09 | 江西理工大学 | Method for comprehensively recovering valuable metals from neodymium iron boron waste acid leaching residues under sulfuric acid system |
| CN115074530A (en) * | 2022-04-07 | 2022-09-20 | 江西理工大学 | Method for comprehensively recovering valuable metals from neodymium iron boron waste acid leaching residues under hydrochloric acid system |
-
1982
- 1982-06-21 CA CA000405594A patent/CA1171608A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1994007797A1 (en) * | 1992-09-30 | 1994-04-14 | North Broken Hill Limited | A process for recovering alkali metal hydroxide from organic liquors |
| US5545385A (en) * | 1992-09-30 | 1996-08-13 | Amcor Limited | Process for recovering alkali metal hydroxide from organic liquors |
| CN114875241A (en) * | 2022-04-07 | 2022-08-09 | 江西理工大学 | Method for comprehensively recovering valuable metals from neodymium iron boron waste acid leaching residues under sulfuric acid system |
| CN115074530A (en) * | 2022-04-07 | 2022-09-20 | 江西理工大学 | Method for comprehensively recovering valuable metals from neodymium iron boron waste acid leaching residues under hydrochloric acid system |
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