Note: Descriptions are shown in the official language in which they were submitted.
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- USE OF ACID-STAGE FILTRATE IN COOKING LIQUOR PREPARATION
BACKGROUND AND SUMMARY OF THE INVENTION
One of the most significant objectives of the pulp and paper
industry in attempting to limit the environmental impact of pulp and paper mills5 is the minimization of liquid discharge from the mill, in particular from the
bleach plant. Commonly referred to as "closing the bleach plant", this
objective usually entails some way of collecting and re-using all the liquid
effluents from the bleach plant with minimal discharge to the surrounding
environment. In the past, this objective was partially achieved in practice by
10 recirculating alkaline bleach plant effluent to the brownstock washing stage
and ultimately to the alkaline recovery system.
The recirculation of acidic bleach plant effluents is much more
difficult. These acidic effluents, for example, from an acid wash stage (A), an
acidic or neutral chelation stage (Q), an acidic ozone stage (Z), or other acidic
15 treatment stage, cannot be returned directly to the conventional recovery
system. [Throughout this application, for simplicity, the expression "acidic
treatment" or "acid treatment" should be interpreted to include a chelation
treatment which may be of approximately neutral pH, as well as an acid pH (6
or below) treatment.] These acidic streams can cause excessive foam
20 generation if mixed with the alkaline streams, can overload the recovery
system by the sheer volume of the liquid, and these streams contain
undesirable metal ions which can negatively impact the processes and
equipment.
One highly desirable destination for these acidic streams is
25 cooking liquor preparation, or what is commonly known as the recausticizationplant. This is the area in the pulp mill where cooking liquor is generated from
the chemicals recovered in the recovery process. For example, in kraft pulping,
white liquor is regenerated from the sodium salts in the recovery boiler smelt.
It is desirable to introduce the metal-laden acidic effluents to the
30 recausticization plant because this method provides the most efficient removal
of metals from the fiberline. For example, even metals that pass through the
recausticization plant and to the digester, with the white liquor, are eventually
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returned to the recovery system in black liquor and removed in the green liquor
dregs during green liquor clarification. Thus, the removal of metals in an acidic
stage of the bleach plant and passing them to the liquor recovery system
minimizes the introduction of any metals to the rest of the fiberline, e.g.
brownstock washing or oxygen delignification, where they are more difficult
to remove and can accumulate.
The production of white liquor requires various dilution and
washing steps that the acidic effluents can be used for. Typically, the water
used for dilution and washing in the recausticization plant eventually ends up
in the white liquor produced. However, as noted above, the acidic bleach plant
effluents contain dissolved metal ions which can also affect the efficiency of
the liquor manufacturing processes and the associated equipment. For
example, in the manufacture of kraft white liquor, metal ions, e.g., manganese,
iron, magnesium, etc., can accumulate within the lime cycle and detrimentally
impact the quality of the lime produced. Lime quality is typically indicated by
what is called its "activity", that is, its ability to effectively react with sodium
carbonate to form sodium hydroxide. The presence of metals reduces this
activity. Conventionally, the accumulation of metals in the lime is addressed
by purging some of the metal-laden lime and replacing it with fresh lime.
However, this has the disadvantage of introducing the additional
cost for the fresh chemical and also creates the potential for introducing to the
system further undesirable substances, such as silicon and aluminum, from the
purchased lime.
In addition to adversely affecting the activity of the lime, the
presence of metals in the lime mud can severely impair the sedimentation and
dewatering properties of the lime mud. In particular, the presence of
magnesium in the lime mud should be avoided for this reason.
Thus, acidic bleach plant effluents cannot be used in the
recausticization plant without some form of treatment or consideration of the
effect of the presence of metal ions upon the chemical processes involved.
According to the present invention a method and apparatus are
provided for overcoming problems created by the introduction of a metal-laden
acidic bleach plant effluent stream to the recausticization plant. According to
the invention the metal-laden acidic bleach plant effluent is properly treated
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and/or handled so that it can be effectively used in a manner that will facilitate
the closing of the bleach plant without adverse consequences on the recovery
system.
According to one aspect of the present invention a method of
5 handling acidic waste liquids from a bleach plant of a sodium-based (e.g. kraft,
sodium-based sulfite, or soda) cellulose pulp mill having a recausticization plant
in which lime mud is removed from causticized green liquor to produce white
liquor and then reutilized to causticize green liquor, is provided. The method
comprises the following steps: (a) Acid treating cellulose pulp produced by a
10 sodium based cooking process. (b) Washing or thickening the pulp from step
(a) to produce a filtrate containing dissolved metals. (c) Treating at least part
of the filtrate from step (b) with an alkaline stream to precipitate solids
containing metals from the filtrate, producing a reduced metal content filtrate.(d) Using the reduced metal content filtrate from step (c) to wash or dilute lime
15 mud. And, (e) using the washed or diluted lime mud from step (d) to produce
lime used to causticize green liquor to produce white liquor.
Typically the pulp mill has a recovery boiler which produces
smelt used to make green liquor. The method then comprises the further steps
of: (f) using part of the filtrate from step (b) that is not treated in step (c) to
20 dissolve smelt from the recovery boiler to make green liquor, and then (9)
clarifying the green liquor to produce clarified green liquor and dregs containing
metals. Preferably at least part of the alkaline stream used to practice step (c)
is clarified or unclarified green liquor from step (g) or clarified or unclarified
white liquor from step (e), although an alkaline filtrate from an alkaline
25 bleaching or delignification stage of the bleach plant may also be used. As is
conventional step (e) is typically practiced by dewatering the washed or dilutedlime mud, burning the dewatered lime mud in a lime kiln to produce lime, and
using the lime to causticize clarified green liquor to produce white liquor.
The pulp mill may also include an alkaline bleaching stage. The
30 above method may further comprise the following further steps: (a1 ) treatingthe pulp in an alkaline stage; (a2) washing or thickening the pulp from step (a1 )
to produce an alkaline filtrate; and (a3) combining at least some of the alkaline
filtrate with the filtrate from step (b) prior to step (c).
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The invention also relates to a method comprising the following
steps: (a) Acid treating cellulose pulp produced by a sodium based cooking
process. (b) Washing or thickening the pulp from step (a) to produce a filtrate
containing dissolved metals. (c) Treating at least part of the filtrate from step
5 (b) with an alkaline stream to precipitate solids containing metals from the
filtrate, producing a reduced metal content filtrate. (d) Using part of the filtrate
from step (b) that is not treated in step (c) to dissolve smelt from the recovery
boiler to make green liquor, and then (e) clarifying the green liquor to produceclarified green liquor and dregs containing metals.
Apparatus is also provided according to the present invention in
the form of a pulp mill system for producing cellulose pulp produced from a
sodium-based cooking process. The pulp mill system comprises the following
components: A digester including at least one white liquor inlet, at least one
black liquor outlet, and a pulp outlet. A pulp acid treatment stage downstream
15 of the pulp outlet from said digester. A wash or thickening stage after the acid
treatment stage, the wash or thickening stage comprising a filtrate outlet.
Means for removing metals from at least some of the filtrate passing out of the
wash stage filtrate outlet to produce a reduced metal content filtrate. A
recovery boiler connected to the digester black liquor outlet, and including a
20 smelt discharge. A smelt dissolver connected to the smelt discharge. A green
liquor clarifier connected to the smelt dissolver. A causticizer connected to the
green liquor clarifier. A lime mud separator connected to the causticizer.
A lime mud washing or dilution stage connected to the lime mud
separator to produced washed or diluted lime mud. Means for directing the
25 reduced metal content filtrate to the lime mud washing or diluting stage. And,
means for using the washed or diluted lime mud to produce lime used in the
causticizer to produce white liquor from green liquor.
The system preferably further comprises a conduit directing
some of the filtrate from the filtrate outlet to the smelt dissolver to dissolve30 smelt to produce green liquor. The means for removing metals from at least
some of the filtrate passing out of the washing stage filtrate outlet preferablycomprises means for adding an alkaline stream of liquor from the pulp mill to
the filtrate. The means for adding the alkaline stream preferably comprises
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means for adding at least one of white liquor and green liquor to the filtrate,
although a bleach plant alkaline stage alkaline filtrate may alternatively be used.
The system may also include a pulp alkaline treatment stage, a
washing or thickening stage after the alkaline treatment stage which includes
5 an alkaline filtrate outlet, and a means for combining at least some of the
alkaline filtrate with the acidic filtrate prior to passing the filtrate to the metal
removing means.
It is the primary object of the present invention to facilitate
closing of a bleach plant of a cellulose pulp mill utilizing a sodium-based
10 cooking process by proper treatment and handling of acidic bleach plant
effluents, such as from the acid wash stage, acidic or neutral chelation stage,
or an acidic ozone stage. This and other objects of the invention will become
clear from an inspection of the detailed description of the invention and from
the appended claims.
15 BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of an exemplary cellulose pulp mill
system according to the invention for practicing exemplary methods of
handling acidic waste liquids from a bleach plant, according to the present
invention; and
20 FIGURE 2 is a schematic detail view of one embodiment of the metals
removal stage of FIGURE 1.
DETAILED DESCRIPTION
The system illustrated generally by reference numeral 10 in
FIGURE 1 includes a digester 11 ~continuous digesters are preferred although
25 batch digesters may be utilized) which produces cellulose pulp utilizing a
sodium-based cooking process, e.g. kraft (sulfate), sodium-based sulfite, or
soda cooking processes. Slurried wood chips or other cellulose material is fed
into the digester 1 1 as indicated by line 12, and white liquor (used
substantially generically herein to cover sodium-based cooking liquors) is added30 to the digester by line 13, white liquor typically also being added at other
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points in the digester 11 and with the wood chips introduced in line 12. The
pulp produced in the digester 11 is discharged in line 14 typically to a
brownstock washing stage 15, and then to a bleach plant, shown generally by
reference numeral 16. The bleach plant typically includes a number of different
5 stages, such as an alkaline bleaching or delignification stage or stages 17 (e.g.
an oxygen delignification stage and/or alkaline extraction stage, or other
conventional alkaline stages) followed by a wash stage 18, and one or more
acid bleach/treatment stages 19 followed by a wash thickening stage 20.
Typical processes utilized for the stage 19 comprise acid washing (A), acidic
10 or neutral chelation (Q), acidic ozone (Z), acidic peroxide (Pa), or peracetic acid.
Other bleaching stages are indicated schematically at 21. Bleached pulp is
produced as indicated in line 22.
Filtrate from the acidic wash or thickening stage 20 is
represented by line 23 in FIGURES 1 and 2. As shown in the international
15 patent application PCT/US94/012373, the disclosures of which are hereby
incorporated by reference herein, metals are removed from the filtrate in line
23 utilizing a metals removal stage 24. The metals removal stage
24 may include any of the configurations illustrated in the international
application, or the like, one exemplary such system being illustrated in FIGURE
20 2. For example as indicated in FIGURE 2, the filtrate in line 23 may be oxidized
as indicated at 25, and passed to a reactor 26 where an alkaline stream of
material (typically liquid) is added as indicated at 27. The alkaline stream
causes solids containing metals to precipitate from the filtrate, the filtrate with
precipitated solids then passing first to clarification and then to the stage 28,
25 or directly to stage 28. At stage 28 the solids that have been precipitated are
removed, as by filtration, sedimentation, flotation, or any other suitable
conventional technique. The metal-laden stream may take the form of solids
and be discharged as sludge, as indicated at line 29 or a metal-laden liquid
stream, the retentate 29', may be used, for example, for smelt dissolution 37,
30 or burned in the recovery boiler 35. The metal depleted stream is discharged
in line 30.
The digester 11 has a black liquor (waste liquor containing
dissolved lignin) outlet conduit 32 associated therewith. The black liquor is
evaporated and/or concentrated as is conventional and as indicated by box 33
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in FIGURE 1, and may be subjected to other treatments ~such as pressure
heated treatment to reduce the viscosity and produce organic sulfur containing
gases) as indicated at 34 in FIGURE 1. Ultimately the black liquor passes to
a conventional recovery boiler 35 where it is burned producing a smelt which
is discharged from the recovery boiler in line 36. The smelt in line 36 passes
to a smelt dissolver 37 where liquid is added to the smelt to produce green
liquor discharged in line 38. The green liquor is clarified in a clarifier or filter
39, to separate solids, i.e., dregs (containing metals), which dregs are
discharged in line 40 for environmentally suitable disposal, while clarified green
liquor passes in line 41 to a causticizer 42. In the causticizer 42, lime is added
in line 43, the lime typically being from a lime kiln 44. From the causticizer 42
the causticized green liquor passes to a lime mud separation stage 45 which
is typically a filtration or sedimentation means. The end product produced in
line 46 is white liquor (or like cooking liquor) which of course is ultimately used
in line 13 associated with the digester 11. The lime mud that is separated in
stage 45 passes to a washing or dilution 48, and then typically is dewatered
at stage 49 before being burned in the lime kiln 44 to regenerate lime from
calcium carbonate ~a suspension of calcium carbonate is what is known as
"lime mud").
According to the present invention the acidic filtrate in line 23
is used in an optimum manner in the recausticization plant -- the entire
structure 50 illustrated in FIGURE 1. The metal depleted stream in line 30 is
ideal for washing or diluting the lime mud in stage 48, as illustrated in FIGURE1. The liquid in line 30 has a minimum potential to introduce undesirable
metals into the lime cycle and therefore the activity of the lime is not adversely
affected. Also sedimentation and dewatering are optimized because they can
be hindered by the presence of metals, few of which are present in the metal
depleted stream 30.
Though not as desirable as using the metal-laden stream for
smelt dissolution, stream 23 may also be used for lime mud washing in 48.
However, if this is done, some accommodation must be made for the potential
to accumulate metals in the lime cycle.
In the international patent application PCT/US94/012373 it was
suggested that the metal depleted stream could be utilized for green liquor
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production, that is by utilizing it as the liquor in the smelt dissolver 37 to
produce green liquor in line 38. However it has been recognized according to
the present invention that it is not necessary to effect removal of the metals
from the acidic stream 23 for this purpose. Rather a part of the acidic bleach
5 plant effluent
- that part in line 52 -- may be fed directly to the smelt dissolver 37 to be used
alone (or with other liquid from conventional sources) to produce a green
liquor. This use of the acidic bleach plant effluent is innocuous because after
10 dissolution of the smelt and the formation of the green liquor in line 38 thegreen liquor is clarified in clarifier 39. Thus metal containing solids are
removed in the clarifier 39 in the form of the dregs 40 which can be disposed
of in a conventional environmentally friendly manner without introducing
metals to the processes in the liquor manufacturing cycle which they could be
15 detrimental to.
It will thus be seen that according to the present invention a
novel and desirable process and apparatus are provided for closing a bleach
plant. Metal-laden acidic bleach plant effluent is reused in the preparation of
cooking liquor without adversely affecting the efficiency of the chemical
20 processes or the operation of the equipment. Again these techniques are
applicable to any sodium-based cooking process including kraft, sodium-based
sulfite, and soda cooking processes.
While the invention has been herein shown and described in
what is presently conceived to be the most practical and preferred embodiment
25 thereof, it will be apparent to those of ordinary skill in the art that many
modifications may be made thereof within the scope of the invention, which
scope is to be accorded the broadest interpretation of the appended claims so
as to encompass all equivalent systems and processes.
