FIELD OF THE INVENTION

Our present invention relates to a method for reducing the formation of incrustations during concentration of a black liquor obtained from a chemical pulping process, preferably a kraft or soda pulping process.

The invention also relates to an apparatus or plant for carrying out the method of the invention.

BACKGROUND OF THE INVENTION Multiple-effect evaporation systems are, known for the concentration of black liquors including black liquor as produced in the production of paper pulp. Even if these systems have been used for decades some problems relating to formation of incrustations and sedimentation in evaporation stages still occurs on a regular basis, requiring a temporary shut down or reduction in handling capacity during cleaning procedures.

One way of altering the susceptibility for formation of incrustations is the recycling of thick liquor back to earlier process positions in the multiple-effect evaporation system. An example of this technology could be seen in US 4.076.576, where a part of the concentrated black liquor is recycled back to the inlet of the evaporation stage in order to prevent frothing. This recycling effect corresponds to an increase in dry-matter content thus establishing a solution above the solubility limit for the crystallizing salt, but the maximum salt crystal mass to total dry-matter ratio could not be increased above the total crystallizing salt to dry-matter ratio of the original weak black liquor, which corresponds to about 140 kg of dissolved salts per ton of dry matter content. Another disadvantage is that large amounts of organic matter and alkali (NaOH) is recycled at the same time, which causes a disadvantageous increase of viscosity and increase of the boiling point of the black liquor, which results in reduced capacity and increased operating costs.

Another way to affect formation of incrustations on the interior wall of the evaporation stages is the method of mixing ashes, (see for example "Indunstning till 90% TS", by H., Jaakkola & C-G Berg at Sodahuskonferensen, Stockholm, 2007), typically obtained from electrostatic filters in the recovery boiler, into the black liquor, such that incrustations instead is formed on the ash particles added. Adding such ashes will introduce a large number of small crystal nucleis of sodiumsulfate (Na ₂ S0 ₄ ), and burkeit (2Na ₂ S0 ₄ ,Na ₂ C0 ₃ ),of about some μηι in size, but has the disadvantage of forming agglomerations that are disadvantageous for incrustations.

However, most of these systems add disadvantages in form of only partial success in reducing the formation of incrustations, and/or increased operational costs as well as increased risks for sedimentation (precipitation) that will clog up the evaporation stage and thus require built in cleaning equipment and/or regular shut down for cleaning.

When the dry matter content is increased in the multiple-effect evaporation process is the solubility limit exceeded for most salts, such as burkeit (2Na ₂ S0 ₄ ,Na ₂ C03), dicarbonate (Na ₂ S0 ₄ ,2Na ₂ C03), calcium carbonate (CaC0 ₃ ) as well as other salts of sodium (Na), silicon (Si) and aluminum (Al). The actual position when the solubility limit is exceeded for each individual salt is dependent on the original composition of the black liquor and the process flows.

OBJECTS OF THE INVENTION

It is an object of the invention to achieve a high final concentration in a manner that avoids disturbing formation of incrustations, while not increasing the operational costs or adds further disadvantages from clogging due to sedimentations.

A further object of this invention is to provide an improved method for concentrating black liquors and especially the black liquor product in the production of pulp using the soda or kraft process, whereby drawbacks of earlier processes are avoided. Still another object of this invention is to provide an improved apparatus or plant for carrying out the improved process and in which incrustation formation is reduced and plant economy increased.

Yet another object of this invention is to provide a method and apparatus which could be implemented in any kind of process position where the solubility limit of a salt is to be exceeded, and where a local recycling of salt crystals of this salt susceptible for forming incrustations is made. The range of recycled crystals will then form nucleus of crystals for efficient further crystal growth, avoiding formation of incrustations on the wall of the equipment. An effect of the recycling of the salt crystals is that the total amount of crystallizing salt is increased by at least 10%, preferably at least 50%, and in some cases more than 100%, calculated as kg of salts (dissolved + crystals) per kg dry matter content of the black liquor compared to not using the selective recycling of salt crystals. Alternatively, the number of the specific salt crystal particles, whose solubility limit is exceeded in the following evaporation stage, and within the particle range 30-500μηι, is increased at least 50%, preferably 100% or in some cases more than 200%, compared to not using the selective recycling of the salt crystals.

Yet another object of this invention is to provide an improved method and apparatus where the modification of the process is made by entirely using particles already naturally being part of the black liquor flow, thus avoiding additions of chemicals or particles that must be readily available for the process to the required extent. Thus no negative impact on the total chemical balance is introduced that may cause problems in other parts of the mill.

In following description is the expression "first predetermined particle size" used for the salt crystals in concern, by which is meant any practical minimum particle size possible to separate from the black liquor, while any content of the black liquor having particles below this size is considered and handled as liquid content. The expression "second predetermined particle size" is also used, by which is meant particles having an excessive size, which could accumulate in the system if recycled and instead is forwarded with the black liquor flow or alternatively is bled out at least in part. The "first predetermined particle size" is thus representing particles with a size smaller than particles of the "second predetermined particle size".

SUMMARY OF THE INVENTION

The objectives of the invention are accomplished in accordance with the inventive method for reducing the formation of incrustations during concentration of black liquor obtained from a chemical pulping process which comprises the steps of: subjecting said black liquor to a multiple-effect evaporation in a plurality of evaporation stages to partially concentrate the black liquor in each of said evaporation stages; and when the solubility level of salts in the black liquor, including salts from possible ash mixing, thick liquor recycling and salt crystals (particles) recycling, is exceeded in a specific evaporation stage, extracting at least a part of the concentrated black liquor after having passed at least said specific evaporation stage and subjecting this extracted part of the concentrated black liquor to a first separation process; and in said separation process separating crystals (particles) of said salt with a size above a predetermined particle size; and recycling at least a part of the salt crystals (particles) directly into or to a position before said specific evaporation stage and mixing these salt particles into the black liquor that is to be concentrated in said specific evaporation stage.

Using this technology would selectively recycle those crystals (particles) that are most effective for controlling crystal growth, thus avoiding formation of incrustations on the heat transfer surface of the evaporation equipment.

In a preferred embodiment of the inventive method outlined above, it further comprises the steps of in said first separation process obtaining at least one coarse fraction with a major part of the particles from the concentrated black liquor above the predetermined first particle size and at least one fine fraction with a major part of the particles from the concentrated black liquor below said predetermined first particle size; and in said recycling step recycling at least a part of the coarse fraction directly into or to a position before said specific evaporation stage and mixing this coarse fraction into the black liquor that is to be concentrated in said specific evaporation stage, and further recycling at least a part of the fine fraction to a position after said specific evaporation stage and mixing this fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler. Using this technology would avoid recycling excess organic matter and alkali to the evaporation stage as would be the case with recycling of thick liquor from the thick liquor storage. Thus reducing the negative impact that such recycled volumes would cause in the specific evaporation stage in aspects of viscosity changes, reduced capacity and increased steam consumption.

In a further embodiment of the inventive method outlined above, it further comprises the steps of subjecting the first coarse fraction to a second separation process; and in said second separation process obtaining at least one second coarse fraction with a major part of the particles from the first coarse fraction above a predetermined second particle size and at least one second fine fraction with a major part of the particles from the first coarse fraction below said predetermined second particle size; and recycling at least a part of the second fine fraction of salt crystals directly into or to a position before said specific evaporation stage and mixing this second fine fraction of salt crystals (particles) into the black liquor that is to be concentrated in said specific evaporation stage, and recycling at least a part of the second coarse fraction to a position after said specific evaporation stage and mixing this second coarse fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler. Using this technology with a 2-stage separation would avoid recycling excess organic particle matter such as fiber bundles and other coarser particle such as grit and gravel to the specific evaporation stage over time, which otherwise would accumulate in an increasing amount if all particles above a certain size is recycled. A certain amount of bleed-out of coarser particles could also be achieved by allowing a controlled by-pass of the first separation process.

In a further alternative to the 2-stage separation process of the inventive method outlined above, it instead also comprises the steps of from the salt particles separated in said first separation process obtaining at least one coarse fraction with a major part of the particles from the concentrated black liquor above a second predetermined particle size and at least one fine fraction with a major part of the particles from the concentrated black liquor below said second predetermined particle size; and recycling at least a part of the coarse fraction to a position after said specific evaporation stage and mixing this coarse fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler, or alternatively bleeding out at least a part of the coarse fraction, subjecting the fine fraction to a second separation process; and in said second separation process obtaining at least one second coarse fraction with a major part of the particles from the first fine fraction above the first predetermined particle size and at least one second fine fraction with a major part of the particles from the first fine fraction below the first predetermined particle size; and recycling at least a part of the second coarse fraction directly into or to a position before said specific evaporation stage and mixing this second fine fraction into the black liquor that is to be concentrated in said specific evaporation stage, and recycling at least a part of the second fine fraction to a position after said specific evaporation stage and mixing this second fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler.

Using this alternative technology with a 2-stage separation would first extract any particles with excessive size and avoid recycling excess organic particle matter such as fiber bundles and other coarser particles such a grit and gravel to the evaporation stage over time, which otherwise would accumulate in an increasing amount if all particles above a certain size is recycled. A certain amount of bleed-out of coarser particles could also be allowing a controlled by-pass of the first separation process. Most often is a separation of the excessive particles sizes easier to implement as a first separation step.

In yet an alternative embodiment of the inventive method it further comprises the steps of performing the extraction and first separation from two different positions in the multiple- effect evaporation process. This alternative is preferably implemented in evaporation processes reaching higher dry matter content in the concentrated black liquor, typically above 80%. Different crystals form in different process positions when the solubility limit is exceeded, and typically is burkeit (2Na ₂ S0 ₄ ,Na ₂ C0 ₃ ) formed earlier in the multiple-effect evaporation plant, while dicarbonate (Na ₂ S0 ₄ ,2Na ₂ C0 ₃ ) is formed later.

In a further embodiment of the inventive method it further comprises the steps of performing the extraction and first separation in a position of the multiple-effect evaporation process where the black liquor is still pressurized, and before any pressure reduction of the black liquor to a pressure below 1 ,0 bar (e) is made. In such a process position the water content of the liquor is still slightly higher, the temperature is still high and the separation process of the specific particles to be recycled is easier to implement.

The inventive apparatus for reducing the formation of incrustations during concentration of a black liquor obtained from a chemical pulping process comprises: a plurality of evaporation stages in a multiple-effect evaporation plant receiving black liquor from a feed pipe, said evaporation stages treating the black liquor in succession and increasing the concentration of the black liquor further in each of said evaporation stages; and an extraction pipe connected to the flow of partially concentrated black liquor after having passed at least one specific evaporation stage said extraction pipe being connected to first separation means; and in said separation means separating particles of a salt formed in said specific evaporation stage; and a recycling pipe for at least a part of said separated salt particles connected from the separation means and directly into or to a position before said specific evaporation stage and mixing these separated salt particles into the black liquor that is to be concentrated in said specific evaporation stage.

In a preferred embodiment of the apparatus it further comprises in said first separation means obtaining at least one flow of a first coarse fraction with a major part of the salt particles from the concentrated black liquor above a first predetermined particle size and at least one first fine fraction with a major part of the particles from the concentrated black liquor below said first predetermined particle size; and a recycling pipe for at least a part of the first coarse fraction connected from the separation means and directly into or to a position before said specific evaporation stage and mixing this coarse fraction into the black liquor that is to be concentrated in said specific evaporation stage, and a recycling pipe for at least a part of the first fine fraction to a position connected from the separation means to a position after said specific evaporation stage and mixing this fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler (REC.B)

In yet a preferred embodiment of the apparatus it further comprises a second separation means (SEP B) connected via a pipe to first separation means (SEP A); said pipe forwarding the first coarse fraction to the second separation means; and in said second separation means obtaining at least one second coarse fraction with a major part of the particles from the first coarse fraction above a second predetermined particle size and at least one second fine fraction with a major part of the particles from the first coarse fraction below said second predetermined particle size; and a recycling pipe for at least a part of the second fine fraction connected from the second separation means (SEP B) directly into or to a position before said specific evaporation stage and mixing this second fine fraction into the black liquor that is to be concentrated in said specific evaporation stage, and a recycling pipe for at least a part of the second coarse fraction connected from the second separation means (SEP B) to a position after said specific evaporation stage and mixing this second coarse fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler (REC.B).

In an alternative preferred embodiment of the apparatus it instead further comprises from the salt particles separated in said first separation process obtaining at least one coarse fraction with a major part of the particles from the concentrated black liquor above a second predetermined particle size and at least one fine fraction with a major part of the particles from the concentrated black liquor below said second predetermined particle size; and recycling at least a part of the coarse fraction to a position after said specific evaporation stage and mixing this coarse fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler, or alternatively bleeding out at least a part of the coarse fraction, a second separation means (SEP _B ) connected via a pipe (20) to first separation means (SEP _A ); said pipe forwarding the first fine fraction (FFi) to the second separation means; and in said second separation means obtaining at least one second coarse fraction (CF ₂ ) with a major part of the particles from the first fine fraction above a first predetermined particle size and at least one second fine fraction (FF ₂ ) with a major part of the particles from the first fine fraction below said first predetermined particle size; and a recycling pipe (12) for at least a part of the second coarse fraction (CF ₂ ) connected from the second separation means (SEP _B ) to a position directly into or before said specific evaporation stage and mixing this second coarse fraction into the black liquor that is to be concentrated in said specific evaporation stage, and a recycling pipe (1 1 ) for at least a part of the second fine fraction (FF ₂ ) connected from the second separation means (SEP _B ) to a position after said specific evaporation stage and mixing this second fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler (REC.B).

In a further embodiment of the inventive apparatus it further comprises at least two different extraction pipes being connected to the flow of partially concentrated black liquor after having passed different evaporation stages, said extraction pipes each being connected to an individual first separation means (SEP A and SEP B respectively) and wherein recycling pipes from each individual separation means being connected at different positions in the multiple-effect evaporation process.

In yet a further embodiment of the inventive apparatus it further comprises that the extraction pipes are connected to the flow of black liquor trough the multiple-effect evaporation process before reducing the pressure of the black liquor in a flash tank, and before the thick liquor storage tank.

The invention could be used in the recovery process in any kind of pulping systems. It could be used for pulping hardwood, softwood or annual plants, using different pulping processes such as kraft, sulphite or AQ/PS based delignification processes. The type of separation process used for implementing the selective separation of the salt crystals is not critical to the invention. Any kind of separation process could be implemented, using hydrocylones, filters, centrifuges or sedimentation/settling tanks.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying highly diagrammatic drawing in which:

FIG. 1 shows a flow-diagram of a conventional multiple-effect evaporation process according to prior art ; and

FIG. 2 shows a flow-diagram of a multiple-effect evaporation process according to a first embodiment of the invention, and FIG. 3 shows a flow-diagram of a multiple-effect evaporation process according to a second embodiment of the invention, and

FIG. 4a, 4b, 4c, 4d and 4e shows five examples of a two-stage separation process used in order to separate out the selected particles for recycling according to the invention, and FIG. 5 shows an example of the distribution of particles obtained in the separation process according to the invention, and

FIG. 6 shows a flow-diagram of a multiple-effect evaporation process according to a third embodiment of the invention, and

FIG. 7a and 7b shows two examples of different separation principles possibly used in the invention.

SPECIFIC DESCRIPTION

In prior art systems, schematically shown in figure 1 , is a black liquor BL, which has been concentrated in part in preceding stages of the evaporation process, supplied in line 1 to final stages 1 C/1 B/1 A. The final stages 1 C/1 B/1 A are typically of a conventional design with falling-film evaporator stages A, B and C, either built as a plate or tube evaporator stage. Fresh low-pressure steam ST _LP , which heats the black liquor indirectly in said tube or plate evaporators, is supplied in parallel to the last stages (concentrators) 1 A, 1 B and 1 C , as seen in the flow direction of black liquor, and with steam driven off is collected and led to the next effect 2 via line 2a. The black liquor is thus exposed to heating which boils off liquid and increase the concentration of the black liquor. Most often is also ash added to the flow of black liquor, which ash A _E SP conventionally is obtained from an electrostatic filter in the flue passage from a recovery boiler. Thus as indicated in figure 1 , at least a part of the black liquor flow BL is led to an ash mixer AM and subsequently reintroduced into the flow of black liquor in front of the first evaporation stage 1 C. The black liquor BL _E T having passed the final stage 1 A of the evaporation train is typically having a temperature of about 130 ^Q C, and after a pressure drop in a flash tank FT the temperature could be about 100-1 10 ^Q C and with a dry matter concentration of about 75%. After this final treatment the concentrated black liquor is sent to a thick liquor storage tank TLS, before it is pumped to the liquor guns in the recovery boiler (not shown). In some systems a recycling of the thick black liquor is done from this thick liquor storage tank as indicated with the dotted flow arrow R.

In figure 2 is a first simplest form of the inventive apparatus by which the inventive method could be used. In most parts it resembles the system as shown in figure 1 but for one modification. In order to reduce incrustations in the stages 1A-1 C is an extraction pipe 10 connected to the flow of partially concentrated black liquor BL _E T after having passed at least one specific evaporation stage, here all of said stages 1 A-1 C, said extraction pipe being connected to first separation means SEP A. Said first separation means SEP A obtaining at least one flow of a first coarse fraction R _F R with a major part of the particles from the concentrated black liquor above a predetermined first particle size and at least one first fine fraction sent via line 1 1 with a major part of the particles from the concentrated black liquor below said predetermined first particle size. The recycling pipe 12 for at least a part of the first coarse fraction connected from the separation means and directly into or to a position before said specific evaporation stage, here before all stages 1A-1 C, and mixing this coarse fraction R _F R into the black liquor that is to be concentrated in said specific evaporation stage. The recycling pipe 1 1 for at least a part of the first fine fraction is connected from the separation means SEP A to a position after said specific evaporation stage and mixing this fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler (REC.B). The separation means could preferably be a conventional hydrocyclone, which could separate out coarser particles from the flow of black liquor.

By this embodiment shown in figure 2 could coarser particles/crystals be recycled that could form nucleus of crystals for further crystal growth, avoiding formation of incrustations on the wall of the equipment for those salts that during the passage of stages 1A-1 C, due to increased concentration of the black liquor, will exceed its solubility limit. In figure 3 is a second form of the inventive apparatus by which the inventive method could be used. In most parts it resembles the system as shown in figure 2 but for one modification as of an additional superconcentrator SC, driven by medium pressure steam ST _M p, and thus capable of reaching a higher concentration. In order to reduce incrustations in the stages 1 A- 1 C and SC is an extraction pipe 10 connected to the flow of partially concentrated black liquor BL _E T after having passed at least one specific evaporation stage, here after the superconcentrator SC, said extraction pipe being connected to first separation means SEP A. Said first separation means SEP A obtaining at least one flow of a first coarse fraction R _F R with a major part of the particles from the concentrated black liquor above a predetermined first particle size and at least one first fine fraction sent via line 1 1 with a major part of the particles from the concentrated black liquor below said predetermined first particle size. The recycling pipe 12 for at least a part of the first coarse fraction connected from the separation means and to a position before said specific evaporation stage, here before all stages 1 A- 1 C, and mixing this coarse fraction R _F R into the black liquor that is to be concentrated in said specific evaporation stage. The recycling pipe 1 1 for at least a part of the first fine fraction is connected from the separation means SEP A to a position after said specific evaporation stage and mixing this fine fraction into the black liquor that is to be forwarded to final treatment and combustion in a boiler (REC. B). The separation means could preferably be a conventional hydrocyclone, which could separate out coarser particles from the flow of black liquor.

Also by this embodiment shown in figure 3 could coarser particles/crystals be recycled that could form nucleus of crystals for further crystal growth, avoiding formation of incrustations on the wall of the equipment for those salts that during the passage of stages 1 A-1 C and SC, due to increased concentration of the black liquor, will exceed its solubility limit.

In figures 4a, 4b, 4c, 4d and 4e are examples of alternative 2-stage separation processes shown that could be used in figure 2 or 3. In figure 4a is schematically shown a 2-stage particle separation process using screens SCi and SC2 as separation means. In general the input flow is led to an upper chamber with a lower chamber beneath a screen SCi/SC2, and a fine fraction is passing the screen and accumulates in the lower chamber. Here a second separation means SEP B is connected via a pipe 20 to first separation means SEP A, said pipe forwarding the first fine fraction FFi to the second separation means. In said second separation means is at least one second coarse fraction CF ₂ obtained with a major part of the particles from the first fine fraction above a predetermined third particle size and at least one second fine fraction FF ₂ with a major part of the particles from the first fine fraction below said predetermined third particle size. A recycling pipe 12 for at least a part of the second coarse fraction CF ₂ is connected from the second separation means SEP B to a position R _F R before said specific evaporation stage and mixing this second coarse fraction into the black liquor that is to be concentrated in said specific evaporation stage. Further a recycling pipe 1 1 for at least a part of the second fine fraction FF ₂ is connected from the second separation means SEP B to a position after said specific evaporation stage and mixing this second fine fraction into the black liquor BL _E T that is to be forwarded to final treatment and combustion in a boiler REC. B.

In figure 4b is schematically shown a 2-stage separation process using instead hydrocyclones as separation means. In general the input flow is led to a top or mid part of the hydrocyclone, and by centrifugal force coarse particles are separated into the lower part of the cyclone. Here the second separation means SEP _B is connected via a pipe 20 to first separation means SEP _A , said pipe forwarding the first fine fraction FFi to the second separation means. In said second separation means is at least one second coarse fraction CF ₂ obtained with a major part of the particles from the first fine fraction above a predetermined third particle size and at least one second fine fraction FF ₂ with a major part of the particles from the first fine fraction below said predetermined third particle size. A recycling pipe 12 for at least a part of the second coarse fraction CF ₂ is connected from the second separation means SEP _B to a position R _F R before said specific evaporation stage and mixing this second coarse fraction into the black liquor that is to be concentrated in said specific evaporation stage. Further a recycling pipe 1 1 for at least a part of the second fine fraction FF ₂ is connected from the second separation means SEP _B to a position after said specific evaporation stage and mixing this second fine fraction into the black liquor BL _E T that is to be forwarded to final treatment and combustion in a boiler REC.B.

In figure 4c is schematically shown a 2-stage separation process similar to that of figure 4a but having the screens so designed that the interesting particle content instead follows the first coarse fraction CFi and instead is sifted out in the second separation means SEP _B . In figure 4d is schematically shown a 2-stage separation process similar to that of figure 4b but having the hydrocyclones so connected that the interesting particle content instead follows the first coarse fraction CFi and instead is separated in the second separation means SEP _B .

Finally, in figure 4e is schematically shown the principles for an alternative separation process using centrifugal technique with only one separator. The entire separator is typically a vertically oriented cylinder vessel that is rotated around its vertical center axis at considerable speed, typically well above 1000 rpm, and often in the range 2000-5000 rpm. With this separation technique could a first coarse fraction CF be separated, due to higher specific weight, at an outer layer and the finest fraction FF be separated, due to lowest specific weight, at the center of the rotating separator, both of them merged and sent to further evaporation and to the recovery boiler in flow BL _E T- The interesting particle content could be obtained in an intermediate layer, if the specific weight of the salt crystals is in an intermediate range, and extracted for further use according to the invention. Irrespective of the actual specific weight of the salt crystals, it could be separated similarly in any outlet adapted for the relative volume flows in each respective outlet.

As should be understood could each separator means be of any kind of type, i.e. a screening type of separator such as that shown schematically in figure 4a or 4c, or a hydrocyclone such as that shown schematically in figure 4b or 4d or a centrifuge as shown in figure 4e. However, other types of separating means could be used, for example decanter centrifuges, sedimentations vessels, as well as separators capable of separating more than 2 fractions in one separator unit.

In figure 5 is shown an example of the distribution of particles obtained in the separation process according to the invention if a two stage separation process is used, and typically used for recycling nucleus of burkeit crystals for further crystal growth. If a system according to figure 4a is used then the first screen member SCi is equipped with slots that allows all particles with a size below 500μηι to pass into a the lower chamber. Thus all particles in the area III and IV will thus pass out in the first coarse fraction CFi . The second screen member SC ₂ is equipped with slots that allows all particles with a size below 30μηι to pass into a the lower chamber. Thus all particles in the area II will thus be recycled in pipe 12, while the second finer fraction, area I in figure, will pass out in pipe 1 1 and merged with the first coarse fraction CFi . By a selective separation of the particles in this range 30-500μηι could the burkeit crystals that promotes further crystal growth be recycled, while the remaining content of liquids and smaller particles is sent on further to the recovery boiler. The 2-stage separation processes as shown in figure 4a and 4b would also separate out all larger particles, which may be grit or gravel and larger fiber bundles that may cause sedimentation and build up in volume of these fractions over time in the specific evaporation stage. However, the larger particles could in some cases be drained from the system by other means, and the important feature is to be able to avoid recycling of smaller particles and chemicals that may overload the evaporation stage in concern. The particle sizes sifted out could also be modified to the salt crystals being formed in the specific evaporation stage. If for example the crystal growth is fast and the typical crystal size is larger than 30 μηι, then the lower and upper predefined particle sizes could be adjusted accordingly to any lower and upper predefined particle sizes covering the bulk volume of the salt crystals in concern. The ranges 30-500μηι is representative for covering the bulk volume of burkeit crystals in a typical evaporation process for black liquor obtained from a kraft cooking process.

In figure 6 is a third form of the inventive apparatus shown by which the inventive method could be used. In most parts it resembles the system as shown in figure 3 but for two modifications as of an additional evaporation stage 2 located before the evaporation stages 1 A-1 C, and a second extraction and separation process. Here at least two different extraction pipes 10a/10b being connected to the flow of partially concentrated black liquor after having passed different evaporation stages 1 A and SC respectively. Said extraction pipes 10a/10b each being connected to an individual first separation means, SEPi and SEP ₂ respectively and wherein recycling pipes 12a/12b from each individual separation means being connected at different positions in the multiple-effect evaporation process. In these systems with multiple evaporation stages and where the final dry matter content reaches 80% or more could the solubility limit of a burkeit be exceeded already in stage 2, and in a first extraction and recycling loop could such burkeit crystals be recycled to this stage. As another salt, for example dicarbonate, could the solubility limit of a dicarbonate be exceeded later in the superconcentrator SC, and in a second extraction and recycling loop could such dicarbonate crystals be recycled to this superconcentrator. In figure 6, as could also be the case for other embodiments, are also shown regulating valves \ and V ₂ for controlling the amount of flow that is to be treated in the separation means, and the part volume treated in this separation equipment could adjustably be set to any range from part flows to the entire output flow volume from the specific evaporation stage, depending upon the efficiency of separating the salt crystals in concern and the required amount of nucleus of crystals needed in the specific evaporation stage avoiding incrustations. This is heavily dependent upon the specific salt in concern.

In figure 6, as could also be the case for other embodiments, are also shown regulating valves V ₃ and V ₄ for controlling the amount of thick liquor recycled into each separation loop. In the shown preferred embodiments are the extraction pipes for extraction and further separation connected to the flow of black liquor trough the multiple-effect evaporation process before reducing the pressure of the black liquor in a flash tank, and before the thick liquor storage tank. This will maintain some of the easily evaporable liquids in the concentrated black liquor that could escape in the flash tank, which content may be favorable for the separation process. Once the concentrated black liquor have passed the flash tank and is sent to the thick liquor storage, is the viscosity increased and further handling is made more difficult. However, in some cases could the separation process also be implemented on the final thick liquor, but at more difficult conditions for a successful separation process.

In figure 7a and 7b are two examples shown of different separation principles possibly used in the invention. In figure 7a is schematically the separation process outlined as implemented in the method according to claim 4 and the apparatus according to claim 10, and also shown in the two stage separation process according to figure 4a and 4b. In figure 7b is schematically the separation process outlined as implemented in the method according to claim 3 and the apparatus according to claim 9. From a separation point of view the difference in approach is that the oversized material is directly sifted out from the bulk flow in figure 7a, and the useful salt particles are sifted out from the remaining flow containing also the undersized material and the bulk volume of liquids. In figure 7b is instead the undersized material directly sifted out from the bulk flow, i.e. the bulk volume of liquids, and the useful salt particles are sifted out from the remaining flow containing also the oversized material. What kind of separation process to use is dependent on the type of separation means used.