Note: Descriptions are shown in the official language in which they were submitted.
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BLEi~C~lED KRZ~FT PULP MILL OPERATION
This invention relates to a bleached kraft pulp mill
operation.
This application is a division of copending Canadian
patent application Serial No. 279,885 filed June 6, 1977.
In a liquid effluent free bleached pulp mill, in
which bleached pulp is formed by digesting cellulosic
fibrous material and bleachin~ and purifying the pulp and in
which spent pulping li~uors are subjected to recovery and
regeneration to form fresh pulping liquor, liquid effluents
from the bleaching and purification operations (bleach plant
effluent~ are discharged into the recovery and regeneration
operation.
The organic materials content of the bleach plant
effluent is burned off in the recovery furnace of the
recovery and regeneration operation and the aqueous phase is
evaporated in the recovery and regeneration operation.
Owing to the high cost of évaporating water in a pulp
mill, in the interests of minimizing operating costs, it is
desirable to decrease the total volume of bleach plant
effluent which must be discharged into the pulp mill -
recovery and regeneration operation and hence minimize the
total evaporation load. It is also desirable that any
bleach plant effluent volume decrease not significantly
adversely affect the pulp quality obtained.
In accordance with-the invention-of the afore- - .
mentioned parent application, there is provided a bleach
plant operation in which water conservation is practised by
controlling the use of wash water in the bleach plant,
controlling the design and operation of washers, deckers
and other mechanical devices used in the ble~ch plant, and
controlling the inflow of water with chemicalsO
The present invention is concerned with decreasing
the BOD level of black liquor condensates resulting from the
35 evaporation of black li~uor prior to furnacing in a pulp -
mill of the type wherein bleach plant effluents are intro-
duced to the pulp mill recovery operation. The bleach plant
operation, such as that of the parent case, produces two
liquid effluents, one acid and the other alkaline. These
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effluents may bc passed to the recovery operation, preferably
: in accordance with ~he procedure of our Canadian Patent
No. 1,070,908.
In accordance with the present invention, there is
5 provided a bleached kraft pulp mill process, including the
steps of: (a~ digesting cellulosic fibrous material in a
pulping liquor containing sodium hydroxide and sodium
sulphide as the active pulping chemicals to form brown stock
and ailute black liquor, washing the resulting ~rown stock
10 with a first wash water, passing the washed pulp through
screens and cleaners and thence to a bleach plant; (b)
subjecting the pulp to bleaching, caustic extraction and
:. washing operations in the bleach plant to result in bleached
pulp of the required brightness and purity; Cc~ removing
15 two effluents from the bleach plant consisting of an alkaline
effluent and an acid effluent a~d utilizing part of the
alkaline effluent as part of the first wash water; (d)
passing the dilute black liquor from the digestion step to
~ sextuple effect black liquor evaporators to which heating
J 20 steam is fed and recovering condensates from the evaporators;
(el passing concentrated black liquor from the evaporators to
: a recovery furnace wherein organic materials present in the
, ~oncentrated ~lack liquor are burned and a smelt containing
sodium carbonate, sodium sulphide, sodium chloride and
25 sodium sulphate is formed; (f~ forming white liquor contain-
~ ing regenerated pulping chemicals from the smelt, separating
.: substantially pure solid sodium chloride from the ~hite
. liquor by an evaporation procedure, recovering condensed
- water from the evaporation, diluting the concentrated white
~: 30. liquor resulting from the evaporation procedure to the
required pulping liquor concentration usin~ the remainder
of the alkaline filtrate, recycling the latter pulping
~: liquor to the digestion step for use as at least part of
: the pulping li~uor therein and discharging the condensed
water; ~.gl subjecting~hotwell condensate, flash heat
~- double evaporator condensate and sextuple surface condenser
condensate from the sextuple effect evaporators along with
!~ ~ turpentine underflow from the digestion step to steam
~: stripping to remove therefrom BOD~causing volatile organic
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material, recovering the stripped volatile organic material
and utilizing the stripped condensate as a source
of water for the process; and (h) passing fifth and sixth --
evaporator condensate for use in the formation of white
liquor from the smelt; and (i~ passing the acid filtrate
for use in the formation of white liquor from the smelt,
In the procedure of this invention, therefore, the
most contaminated condensates are com~ined and steam
stripped to remove methanol, which may be used for its fuel
value or otherwise, while the less contaminated condensates
are used directly without stripping.
The invention is described further, by way of
illustration, with reference to the accompanying drawing,
which is a schematic flow sheet of a pulp mill recovery and
regeneration operation in accordance with one embodiment
of the invention.
Referring to the drawing, there is illustrated
therein a bleached kraft pulp mill operation in which t~e -~
overall material flow within an effluent-free pulp mill
is illustrated. With the elimination of the toxic effect
- of bleach plant effluent by introducing the same to the pulp
mill recovery operationj such as is descri~ed in the afore-
said parent application, ~lack liquor condensates ~ecome
the potential dominant effluent. While the total BOD of
the black liquor condensates is moderate, typically about
20 l~s of methanol APT of pulp, when compared with that of
bleach plant effluent, within the context of an "effluent~
free" pulp milI, the value is quite high.
~;~In the pulp mill operation illustrated in the -
drawing, the BOD level of the black li~uor condensates is
decreased to a very low level acceptable for discharge from ~ ;
the mill in the "effluent-free" environment. This is
achieved ~y combining the most contaminated condensates and
thRn steam stripping methanol from this mixture. The
methanol removed in this way then may be used for itS
fuel value or otherwise. The stripped condensate then may
e used in various locations in the mill.
As seen in the drawing, wood chips are fed by line
51Q to a digester 512 to which white liquor is fed by line
4~ 514 and steaming vessel steam is fed by line 516. Pulp
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'-`` 108~
wash water also is fed to the digester 512 by line 518. The
brown stock pulp is fed from the digester 512 by line
520 to a brown stock washer 522 to which wash water is fed
by line 524. The washed pulp passes by line 526 through
cleaners and screens 528 and line 530 to a bleach plant 532,
such as that described in the aforesaid parent application
and including an unbleached decker.
Wash water from the bleach plant 532 passes by line
534 to the cleaners and screens 528 and El.stage effluent
from the ~leach plant 532 passes by line 536 to join the .
wash water in line 524 passing to the brown stock washer
522. Wire cleaning water also passes to the cleaners and ~.
screens 528 by line 538.
Chlorine chemical preparation 540 provides chlorine .-
15 .dioxide and chlorine solutions to the bleach plant 532 by .
line 542 and sodium hypochlorite solution by line 544. The
chemical preparation is fed by water in line 545 and all
may be produced from a single chlorine dioxide and chlorine
generator, for example, using the procedure outlined in
2Q U.S. Patent No. 4,010,112. The latter procedure enables
the volume of water entering the bleach plant ~ith the
bleaching chemicals to be minimized.
Sodium hydroxide for the bleach plant 532 is fed by
line 546 while wash water in the form of pulp machine dryer
white water is fed ~y line 548 to the bleach plant. Other
inputs for the bleach plant 532 are heating steam by line
550 and ~as~er screen cleaner ~ater by line 552.
The bleached pulp passing out of the bleach plant 532
passes by line 554 to the pulp machine dryer 556. Bleached
pulp exits the dryer by line 558 while any excess white
water not required in line 546 is passed to sewer by line
560, while some moisture passes to atmosphere through the
dryer stack 562~ ~ater for a variety of purposes enters to
pu~p machine dryer 556 by line 563, including vacuum pump :. :
35 seaI ~ater, condensate cooler water, trim jet ~ater and ~ ~.
; steam shower ste~m. .
The dilute black liquor and .fla-sh steam ~rom the
digester 512 pass by lines 564 and 566 to black liquor
evaporators 568. Additional heating steam is fed to the
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evaporators 568 by line 570. The black liquor evaporators
takes the form of sextuple effect evaporators which produce
concentrated black liquor which passes by line 572 to the
recovery furnace 574. Various other liquid effluents are
produced and these will ~e described further ~elow.
Moisture is lost through weak black liquor oxidation stack
576.
In the recovery furnace 574 all the organic materials
are burned and there is formed a smelt in line 578 containing
10 sodium car~onate, sodium sulphide, sodium chloride and ~-
sodium sulphate. Stack gases are vented by line 580. Steam
is generated in the furnace and the blow down is passed by
line 582 to the evaporators 568.
The smelt in line 578 then is passed to liquor
preparation 584 wherein white liquor is regenerated. D/C
effluent from the bleach plant 532 passes by line 586 to
liquor preparation 584 for kiln scrubbing therein. Smelt
spray water is fed to the liquor preparation 584 by line
588. Solid green liquor dregs are removed from the liquor
2a preparation 584 by line 590 as are dregs from the causti-
cization ~y line 5~2.
The white liquor resulting from c~emical pr~paration
passes to a salt reco~ery process 5q4 by line 596. In the
salt recovery process, which typically may be that outlined --
in U.S. Patent No. 3,q50,217, solid.sodium chloride is
i removed from the white l;quor by an evaporative procedure
and recovered by line 598. The concentrated white liquor
i is diluted by El filtrate from the bleach plant 532 fed by
line 600 to the desired concentration.to the digester 512 by ~:
line 514, as descri~ed.in our a~orementioned Canadian Patent
No. 1,070,908. . ~ :
Burkeite ~lso deposited in the salt recovery process
594 passes by line 602 to the liquor preparation 584, while
excess condensate from the salt recovery process 5~4 is
: : 35 passed to sewer by line 604. Water for salt leaching in
the salt recovery process 59.4 is fed by line 605.
: The only liquid effluents being sewered from the
system are excess white water in line 56Q and excess conden- ~:
sate from the salt recovery process in line 6Q4. Both of
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these liquors are pure water and hence their discharge is
not harmful.
As mentioned above, there are a number of condensates
from the black liquor evaporators 568. Those most con-
taminated with methanol from the black liquor, the hotwell
condensate, the flash heat double evaporator condensate and
the sextuple surface condenser condensate pass b~ lines 606, 608 a~d
610 respectively to a methanol stripper 612 along with
turpentine underflow from the digester 512 in line 614.
10 In the methanol stripper 612, steam, fed by line 616,
strips methanol from the contaminated condensate. The
methanol is recovered by line 618 while the purified
condensate passes by line 620 to the brown stock washer 522
for use as wash water therein.
Part or all of the purified condensate may be used
in a variety of other locations within the mill, for
example, in wire cleaning in the cleaners and screens 528,
for chlorine dioxide adsorption in chemical preparation 540,
or as wash water or make-up water in the bleach plant.
The condensate from the fifth and sixth effect evapor-
ators in the black liquor evaporators 568 is passed by line
622 to liquor preparation 584, while condensate rom the
second, third and fourth effect evaporators being
relatively free of contaminants may be discharged or may
25 join with the purified condensate in line 620 ~y line 624.
The present invention, therefore, provides a bleached
kraft mill water utilization system which eliminates
noxious a~ueous effluents. Modifications are possible
within the scope of the invention. ~ -
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