Process for manufacturing bleached pulp including recycling

A process for bleaching wood pulp is provided comprising subjecting the wood pulp, after brown stock washing, to an oxygen delignification stage, a washing sequence, a first chlorine dioxide bleaching stage, an oxidative extraction stage, optionally at least one final chlorine dioxide bleaching stage and then recycling the filtrate from the oxidative extraction stage counter-currently through the bleaching plant and brown stock washing. Additionally, and quite beneficially, the filtrate from the first chlorine dioxide bleaching stage is also recycled counter-currently through the brown stock washing thereby significantly reducing the environmental impact associated with the manufacture of bleach wood pulp.

FIELD OF THE INVENTION 
This invention relates to an improved method for manufacturing bleached 
pulp. More particularly, this invention relates to improvements in 
processes for the manufacture of bleached pulp including a recycling 
process for reducing the environmental impact associated with the 
manufacture of bleached wood pulp. 
BACKGROUND OF THE INVENTION 
In the conventional Kraft process, wood pulp is produced by digestion of 
wood chips in a pulping liquor usually containing sodium hydroxide and 
sodium sulfide as the active pulping chemicals. Following the wood 
digestion process, pulp is separated from the spent pulping liquor. The 
spent pulping liquor is then recovered and regenerated for recycling. The 
Kraft process wood pulp is then bleached and purified in a bleach plant 
operation. In the bleach plant, pulp is subjected to at least one 
bleaching stage under acidic conditions with a bleaching agent such as 
ozone, chlorine, chlorine dioxide, mixtures of chlorine and chlorine 
dioxide and the like, followed by at least one bleaching stage under 
alkaline conditions with a bleaching agent such as hydrogen peroxide, 
oxygen in the presence of base or a combination thereof. Depending on the 
desired pulp brightness, additional acidic or alkaline bleaching stages 
are employed. Following each bleaching stage, spent bleaching chemicals 
are usually removed from the pulp by washing with a suitable source of 
water; as for example fresh water or previously used water from pulp 
washing or a combination of the two. Current state of the art requires 
that all wash water from the bleach plant along with spent bleaching 
chemicals be discharged to the sewer as effluents rather than being 
processed in the pulping liquor regeneration operation noted above. 
Various concerns have prevented the recovery of these bleach plant 
effluents in the pulping liquor regeneration operation. One concern is the 
possible build-up of chlorides usually as by-products of bleaching with 
chlorine based bleaching agents, as components of wood fiber entering the 
pulping process or the like, which causes corrosion and operational 
problems. Another concern is the lack of adequate systems for removing 
non-process metals, such as calcium, magnesium and manganese, which enter 
the bleach plant from the wood fiber and are typically removed with wash 
water and spent bleach chemicals. Further, the use of large volumes of 
water for pulp washing in the bleach plant has also prevented recovery of 
bleach plant effluents due to the resulting high evaporator load. The 
environmental impact of these practices has been widely noted and attempts 
have been made to reduce the impact. See for example U.S. Pat. Nos. 
3,698,995; 4,039,372; 5,127,992; 4,042,452; and 4,619,733 PCT/WO 91/18145; 
Pap. S. Afr., 10, No. 5; 32, 34 and 36 (September /October. 1990); and 
Swedish Patent No. 81-020828. Despite these serious concerns and attempts, 
the ability to overcome the problems associated with recovery as described 
above has not been developed. See for example, "Union camp Leads Ozone 
Pulping Drive in North American Mills", Pulp and Paper, p. 69, (September 
1944); "Union Camp Begins Ozone Era With New Kraft Bleaching Line at 
Franklin, Va.", Pulp and Paper, p. 42 (November 1992); Wells E. Nutt, 
"Union Camp's Mill Experience With Ozone Bleaching", Introductory Remarks 
to a Panel Discussion at the International Non-Chlorine Bleaching 
Conference, Amelia Island, Fla., March, 1994; and Paper ja Puu--Paper and 
Timber; May 1989. 
SUMMARY OF THE INVENTION 
One aspect of this invention relates to a process for manufacturing 
bleached wood pulp comprising subjecting wood pulp having a Kappa Number 
equal to or less than about 30 (preferably equal or less than about 25, 
more preferably equal to or less than about 20 and most preferably from 
about 5 to about 20) after one or more brown stock washing stages to the 
following steps conducted in sequence: 
a first acidic bleaching stage wherein the pH at some point during said 
bleaching stage is less than 7, (preferably equal to or less than about 6, 
more preferably equal to or less than about 5 and most preferably from 
about 1 to about 5) as for example, bleaching under acidic conditions with 
an active bleaching agent comprising ozone, elemental chlorine, hydrogen 
peroxide, chlorine dioxide, peracids such as peroxymono sulfuric acid, 
peroxyacetic acid, dimethyl dioxirane or mixtures thereof, preferably 
bleaching under acidic conditions with an active bleaching agent 
comprising not more than about 20% active elemental chlorine, more 
preferably not more than about 5% to about 10% active elemental chlorine 
and most preferably not more than about 5% active elemental chlorine, and 
in the embodiments of choice no or substantially no elemental chlorine 
(i.e. less than about 1% to about 5%); and 
a first alkaline bleaching stage wherein the pH at some point during said 
bleaching stage is greater than 7 (preferably equal to or greater than 
about 8.2, more preferably equal to or greater than about 9 and most 
preferably from about 9 to about 12) as for example bleaching under 
alkaline conditions with an active bleaching agent comprising hydrogen 
peroxide, oxygen in the presence of base, sodium hypochlorite or oxygen in 
the presence of base and peroxide, preferably an oxidative extraction with 
oxygen in the presence of base, more preferably such an extraction in the 
presence of an effective amount of hydrogen peroxide; and said process 
further comprising; 
subjecting said pulp to an acid wash wherein the pH at some point during 
said acid wash is less than 7 (preferably equal to or less than about 6, 
more preferably equal to or less than about 5 and most preferably from 
about 1 to about 5) prior to said first acidic bleaching stage and after 
brown stock washing to remove all or a portion of non-process metals 
contained in said pulp; 
recycling countercurrently all or a portion of the filtrate from said first 
acidic bleaching stage and all or a portion of the filtrate from said 
first alkaline bleaching stage comprising all or a portion chlorides 
entering said process with the wood fiber, chlorides generated in said 
first acidic bleaching stage or a combination thereof to said weak black 
liquor to form weak black liquor which comprises said chlorides; 
recycling said weak black liquor to a black liquor evaporation stage to 
produce a strong black liquor which comprises chlorides; 
combusting said strong black liquor to produce a smelt stream and a flue 
gas comprising particulates which comprise chlorides and sulfates and 
separating said smelt stream and said flue gas; 
treating said flue gas to separate said particulates as ash, and treating 
said ash to form a component relatively low in chlorides and relatively 
rich in sulfates and a component relatively rich in chlorides and 
relatively low in sulfates; 
recycling said component relatively low in chlorides and relatively rich in 
sulfates to said strong black liquor and discharging said component 
relatively rich in chlorides and relatively low in sulfates; and 
recycling said smelt to a causticizing system to regenerate pulping liquor. 
As used herein, "filtrate" is the aqueous phase collected or recovered from 
a mixture of said phase and pulp, said phase comprising water and 
dissolved and/or suspended materials, as for example the aqueous phase 
removed in a pulp thickening process on a decker, an aqueous phase removed 
in a conventional pulp washing process and the like. As used herein, 
"non-process metals" are calcium, magnesium and/or manganese which enter 
the pulping process with the wood fiber. As used herein, 
"countercurrently" means the direct or indirect flow of a filtrate in the 
pulp manufacturing process in a direction opposite to the flow of wood 
fiber or pulp in the process such that all or a portion of said filtrate 
at some juncture in the operation of the process flows to the weak black 
liquor, as for example flow of filtrate from a point in the process to the 
next preceding point or any preceding point in the process as for example 
to wash pulp, dilute pulp or a combination thereof. 
In a preferred aspect of this embodiment of invention, this process further 
comprises: 
recycling countercurrently all or a portion of said filtrate from said 
first acidic bleaching stage and all or a portion of said filtrate from 
said first alkaline bleaching stage (preferably as a combined filtrate) 
through at least one of the brown stock washing stages as wash water then 
to said weak black liquor. 
In a more preferred aspect of this embodiment, the process of this 
invention further comprises: 
subjecting said pulp after said first alkaline bleaching stage to at least 
one post alkaline bleaching washing stage to remove all or a portion of 
alkaline bleaching stage solids and chlorides from said pulp to form a 
mixture comprising pulp and an alkaline bleach filtrate comprising solids 
and chlorides; and 
recovering all or a portion of said alkaline bleach filtrate and recycling 
countercurrently all or a portion of said recovered filtrate alone or 
incombination with all or a portion of said first acidic bleaching stage 
filtrate as wash water through at least one brown stock washing stage then 
to said weak black liquor. 
Another preferred embodiment of this invention further comprises: 
subjecting said pulp after said first acidic bleaching stage to at least 
one post acidic bleaching washing stage to remove all or a portion of 
acidic bleaching stage solids and chlorides from said pulp to form a 
mixture comprising said pulp and an acidic bleach filtrate which comprises 
said chlorides and said solids; and 
recovering all or a portion of said acidic bleach filtrate and recycling 
countercurrently all or a portion of said recovered filtrate alone or 
incombination with all or a portion of said alkaline bleach filtrate 
through at least one of said brown stock washing stages then to said weak 
black liquor. 
Yet another preferred embodiment of the invention comprises: 
treating said pulp after brown stock washing and prior to said acidic wash 
in an oxygen delignification stage to reduce the kappa number of the pulp 
to the desired level followed by at least one post oxygen delignification 
washing stage; and 
recycling countercurrently all or a portion of said first acidic bleach 
stage filtrate, said first alkaline bleach filtrate or a combination 
thereof as wash water through said at least one post oxygen 
delignification washing stage, at least one of said brown stock washing 
stage or a combination thereof. 
Yet another more preferred embodiment of this invention comprises: 
subjecting said pulp to a partial neutralization stage after the first 
acidic bleaching stage and prior to the alkaline bleaching stage in which 
the pH is raised, preferably to a value of from about 5 to 7 with said 
alkaline bleach filtrate. 
Still another more preferred embodiment of the invention further comprises: 
at least one additional bleaching stage after said at least one post 
alkaline bleaching washing stage and optionally treating all or a portion 
of the filtrate from said additional bleaching stage to reduce the amount 
of chlorides therein if a chlorine based bleaching agent is used. 
A most preferred embodiment of this invention further comprises: 
recycling countercurrently as wash water through said at least one post 
oxygen delignification washing stage, said at least one brown stock 
washing stage or a combination thereof all or a portion of said filtrates 
from said first acidic bleaching stage; from said partial neutralization 
stage; from said first alkaline bleaching stage; and from said at least 
one additional bleaching stage (preferably after treatment to reduce the 
amount of chlorides in the filtrate if a chlorine based bleaching agent is 
used). 
Yet another most preferred aspect of this embodiment of the invention 
further comprises treating all or a portion of the filtrate from the acid 
wash to remove all or a portion of non-process metals from said filtrate 
and recycling all or a portion of said treated filtrate as wash water to 
the at least one post oxygen delignification washing stage, to the at 
least one post acidic bleaching washing stage or a combination thereof. 
Another aspect of this invention relates to a process for manufacturing 
bleached wood pulp comprising subjecting wood pulp having a Kappa Number 
equal to or less than about 30 (preferably equal to or less than about 25, 
more preferably equal to or less than about 20 and most preferably from 
about 5 to about 20) after brown stock washing to the following steps 
conducted in sequence; 
a first acidic bleaching stage wherein the pH at some point during said 
bleaching stage is less than 7 (preferably equal to or less than about 6, 
more preferably equal to or less than about 5 and most preferably from 
about 1 to about 5) as for example bleaching under acidic conditions with 
an active bleaching agent as for example ozone, peracids such as 
peroxymono sulfuric acid and peroxy acetic acid, dimethyl dioxirane, 
chlorine dioxide, hydrogen peroxide or mixtures thereof, preferably 
bleaching under acidic conditions where the active bleaching agent 
comprises not more than about 30% active elemental chlorine, preferably 
not more than about 20% active elemental chlorine, more preferably not 
more than about 5% to about 10% active chlorine, and most preferably no or 
substantially no elemental chlorine (i.e. from less than about 1% to about 
5%); 
a first alkaline bleaching stage wherein the pH at some point during said 
bleaching stage is greater than 7 (preferably equal to or greater than 
about 8.2, more preferably equal to or greater than about 9 and most 
preferably from about 9 to about 12) as for example bleaching under 
alkaline conditions with an active bleaching agent comprising hydrogen 
peroxide, oxygen in the presence of base, sodium hypochlorite or oxygen in 
the presence of base and peroxide, and preferably oxidative extraction 
with oxygen in the presence of base, preferably such extraction in the 
presence of an effective amount of hydrogen peroxide; and said process 
further comprising: 
treating all or a portion of filtrate from said first acidic bleaching 
stage to remove all or a portion of non-process metals from said filtrate 
to form treated filtrate comprising all or a portion chlorides entering 
said process with the wood fiber, chlorides generated in said bleaching 
stage or a combination thereof; 
recycling countercurrently all or a portion of said treated filtrate from 
said first acidic bleaching stage and all or a portion of filtrate from 
said first alkaline bleaching stage comprising all or a portion of the 
chlorides entering the process with the wood fiber, chlorides generated in 
said first acidic bleaching stage or a combination thereof to said weak 
black liquor to form a weak black liquor which comprises said chlorides; 
recycling said weak black liquor to a black liquor evaporation stage to 
produce a strong black liquor which comprises chlorides; 
combusting said strong black liquor to produce a smelt stream and a flue 
gas comprising particulates which comprise chlorides and sulfates and 
separating said smelt stream and said flue gas; 
treating said flue gas to separate said particulates as ash and treating 
said ash to form a component relatively rich in chlorides and relatively 
low in sulfates and a component relatively low in chlorides and relatively 
rich in sulfates; 
recycling said component relatively low in chlorides and relatively rich in 
sulfates to said strong black liquor and discharging said component 
relatively rich in chlorides and relatively low in sulfates; and 
recycling said smelt to a causticizing system to regenerate pulping liquor. 
In the preferred embodiment of the invention, the process further 
comprises: 
recycling countercurrently all or a portion of said filtrate from said 
first acidic bleaching stage and all or a portion of said filtrate from 
said first alkaline bleaching stage (preferably as a combined filtrate) 
through at least one of said brown stock washing stages as wash water then 
to said weak black liquor. 
In a more preferred aspect of this embodiment, the process of this 
invention further comprises: 
subjecting said pulp after said first alkaline bleaching stage to at least 
one post alkaline bleaching washing stage to remove all or a portion of 
alkaline bleaching stage solids and chlorides from said pulp to form a 
mixture comprising pulp and an alkaline bleach filtrate comprising solids 
and chlorides; and 
recovering all or a portion of said alkaline bleach filtrate and recycling 
countercurrently all or a portion of said recovered filtrate alone or in 
combination with all or a portion of said acidic bleach filtrate as wash 
water through at least one brown stock washing stage then to said weak 
black liquor. 
Another preferred embodiment of this invention further comprises: 
subjecting said pulp after said first acidic bleaching stage to at least 
one post acidic bleaching washing stage to remove all or a portion of 
acidic bleaching stage solids and chlorides from said pulp to form a 
mixture comprising said pulp and an acidic bleach filtrate which comprises 
said chlorides and said solids; and 
recovering all or a portion of said acidic bleach filtrate, and recycling 
countercurrently all or a portion of said recovered filtrate alone or in 
combination with all or a portion of said alkaline bleach filtrate as wash 
water through at least one brown stock washing stage then to said weak 
black liquor. 
Yet another preferred embodiment of the invention further comprises: 
subjecting said pulp after brown stock washing stage and prior to the first 
acidic bleaching stage to an oxygen delignification stage to reduce the 
Kappa Number of the pulp to the desired value followed by at least one 
post oxygen delignification washing stage; and 
recycling countercurrently all or a portion of said filtrates from said 
first alkaline bleaching stage, from said first acidic bleaching stage or 
a combination thereof as wash water through said at least one post oxygen 
delignification washing stage, said at least one brown stock washing stage 
or a combination thereof. 
In the more preferred embodiment of the invention, the process further 
comprises at least one additional bleaching stage after said post alkaline 
bleaching washing stage in which the filtrate from said additional 
bleaching stage is preferably treated to reduce the amount of chlorides 
therein when chlorine based bleaching agents are used. 
In the most preferred embodiments of the invention, the process further 
comprises recycling all or a portion of said filtrates from said at least 
one additional bleaching stage to said weak black liquor preferably 
through at least one post alkaline bleaching washing stage as wash water, 
through at least one of said at least one post oxygen delignification 
washing stage as wash water, through at least one of said at least one 
brown stock washing stage as wash water or a combination thereof, 
preferably in sequence. 
The process of the present invention provides for the bleaching of wood 
pulp and for the recycling and recovery of bleach filtrates in a manner 
that will allow for: 1) the production of pulp with acceptable brightness 
and quality; 2) the reduction of dissolved solids normally discharged with 
bleach effluents including chlorinated organic material; 3) minimal 
increase in the amount of water required to be evaporated; 4) no or 
substantially no increased build-up of chlorides in the pulping liquor 
regeneration cycle; and/or 5) removal of non-process metals to prevent 
scale formation. 
The present invention allows for a significant reduction in color, AOX, BOD 
and solids discharged in the effluent associated with bleaching of 
chemical pulp to a degree that represents a significant improvement 
relative to current practice. Furthermore, the present invention 
accomplishes this in a manner that represents a significant improvement 
over previously developed techniques that attempted various approaches for 
bleach filtrate recovery.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention overcomes one or more of the 
previously encountered problems in recycle of bleach plant filtrates 
employing a combination of various means. One essential feature of the 
present invention for overcoming these difficulties is to also use pulp in 
the first acidic bleaching stage having a Kappa number equal to or less 
than about 30, preferably equal to or less than about 25, more preferably 
from about 5 to about 20, and most preferably from about 6 to about 15 for 
hardwood pulp and from about 8 to about 18 for softwood pulp. Pulp having 
the desired Kappa number can be obtained from any convenient source or by 
any convenient means known to those of ordinary skill in the art. For 
example, the pulp can be obtained through use of modified continuous 
cooking (MCC), extended modified continuous cooking (EMCC), rapid 
displacement heating (RDH), super batch, anthraquinone, polysulfide, 
enzymes, high heat washing, oxygen delignification and the like, and 
combinations thereof. In the preferred embodiments of the invention 
suitable pulp is obtained by treating the pulp after brown stock washing 
and before the first acidic bleaching stage in an oxygen delignification 
stage in the presence of oxygen and preferably in the presence of base and 
preferably followed by at least one washing stage after oxygen 
delignification. 
Another essential feature of the present process is bleaching the pulp 
having an appropriate Kappa number sequentially in a first acidic 
bleaching stage and a first alkaline bleaching stage, and the recovery of 
the filtrates from the first acidic bleaching stage and the first alkaline 
bleaching stage and the countercurrent recycle of these filtrates to the 
weak black liquor and into the recovery system. All or a portion of these 
filtrates may be recycled. In a preferred embodiment of the invention, 
only a portion of these filtrates are recycled. For example, preferably a 
portion of the filtrates for these bleaching stages usually from a washer 
a decker or a combination thereof is recycled to the weak black liquor and 
the other portion is recycled to the bleaching stage, to the washer or 
decker or to a combination thereof to maintain water balance. Preferably 
from about 1 to about 50%, more preferably from about 5 to about 25% and 
most preferably from about 10 to about 20% of the filtrates are recycled 
to the weak black liquor, preferably as a combined filtrate. In the 
preferred embodiments of the invention, these filtrates are preferably 
recycled as wash water through at least one (preferably all) post oxygen 
delignification washing stages if such stages are present, brown stock 
washing stages or a combination thereof (preferably sequentially) and into 
the recovery stages as will be described in more detail below. In the 
preferred embodiments of this invention where the desired goal is to 
reduce effluent to the greatest possible extent filtrates from as many 
subsequent bleaching stages as possible are recovered, preferably all or 
substantially all of such filtrates and conveyed to the weak black liquor. 
Recovery of bleach plant filtrates can be accomplished for example by 
replacing the wash water used on at least one post alkaline bleaching 
washing stage with filtrate from a washing stage of one or more 
subsequent, preferably the next subsequent, additional bleaching stages 
such that wash water used on all or a portion of the washers of all 
bleaching stages except the final washer of the last bleaching stage is 
the filtrate from subsequent, preferably the next subsequent, bleaching 
stage. In the preferred embodiments of the invention, subsequent washing 
and bleach stage filtrates would be counter-currently recycled through all 
washing stages of all additional bleaching stages as for example by 
recycling the filtrate from the first washing stage of a subsequent 
bleaching stage to the last washing stage of the preceding bleaching 
stage. 
The bleaching agents employed in the bleaching sequence may vary widely 
provided that at least one stage is an acidic bleaching stage and at least 
one alkaline bleaching stage after the at least one acidic bleaching 
stage. Active bleaching agents used in the first acidic bleaching stage or 
any additional acidic bleaching stages may vary widely and any 
conventional bleaching agent operable under acidic conditions (pH less 
than 7, preferably equal to or less than about 6, more preferably equal to 
or less than about 5 and most preferably from about 1 to about 5) may be 
used. For example, useful agents include ozone, elemental chlorine, 
chlorine dioxide, peracids such as peroxymono sulfuric acid and 
peroxyacetic acid, dimethyl dioxirane or mixtures thereof. In the 
preferred embodiments of the invention, the bleaching agent in any one 
acidic bleaching stage or the total bleaching agent used in all acidic 
bleaching stages of the bleaching sequence, the amount of active elemental 
chlorine bleaching agent is less than about 30%, preferably less than 
about 20%, more preferably less than about 10%, and most preferably less 
than about 5% active elemental chlorine. In the embodiments of choice 
there is no or substantially no elemental chlorine employed in the acidic 
bleaching stages. By limiting the amount of elemental chlorine used in the 
bleach plant, which limits the chloride introduced into the pulping liquor 
cycle, and avoiding the cyclic flow of chloride contained in the boiler 
ash, the chloride concentration in the pulping liquor cycle and feed to 
the recovery boiler is controlled. In the more preferred embodiments of 
the invention, the bleaching agent employed in the first acidic bleaching 
stage is ozone or chlorine dioxide, more preferably chlorine dioxide, in 
which the amount of elemental chlorine can vary as described above. In the 
most preferred embodiments of the invention, the ozone or chlorine dioxide 
bleaching agent includes no or substantially no elemental chlorine. As 
shown in FIG. 1, it has been discovered that by reducing the amount of 
elemental chlorine in the bleaching agent employed in the first acidic 
bleaching stage that the increase in consumption of bleaching chemicals, 
if any, is much less than expected. Accordingly, the amount of elemental 
chlorine is minimized (less than about 5% to about 10%) and in the 
embodiments of choice no or substantially no elemental chlorine is used 
(less than about 1%). Bleach chemical consumption due to carryover of 
dissolved organic matter in the recycled combined filtrates from the first 
acidic and alkaline bleaching stages to the first acidic bleach stage is 
reduced by reducing the amount of elemental chlorine, preferably by 
complete substitution of chlorine dioxide for chlorine in the first acidic 
stage. 
Similarly, the bleaching agent employed in the first alkaline bleaching 
stage or any additional alkaline bleaching stages may vary widely and any 
conventional agent the is operable under alkaline conditions (pH at some 
point during said bleaching stage is greater than 7, preferably equal to 
or greater than about 8.2, more preferably equal to or greater than about 
9 and most preferably from about 9 to about 12) can be used. Illustrative 
of useful agents are hydrogen peroxide, oxygen in the presence of base, 
sodium hypochlorite or a combination thereof. The alkaline bleaching stage 
is preferably oxidative extraction with oxygen in the presence of base, 
preferably in the presence of hydrogen peroxide, which contributes to a 
relatively low bleach chemical consumption in the first acidic stage of 
bleaching due to dissolved organic matter in the combined filtrates from 
the acidic and alkaline bleaching stages because of the highly oxidized 
nature of the organic matter having been exposed to oxygen and/or peroxide 
in the extraction stage. This means that for a given extracted kappa 
number, carryover of organic matter from the combined recycled filtrate 
from the alkaline and acidic bleaching stages of the present process into 
the first acidic bleaching stage will not increase bleach chemical 
consumption unduly. 
The types of bleaching sequences and the number and type of bleaching 
stages comprising the sequences may vary widely, provided that the 
essential first acidic and alkaline bleaching stages are present. In the 
preferred embodiments of this invention described in this application the 
invention is described more particularly with respect to the D(EOP)D 
sequence wherein D represents bleaching with chlorine dioxide which 
contains no or substantially no elemental chlorine and more preferably no 
elemental chlorine and (EOP) represents oxidative extraction in the 
presence of base preferably sodium hydroxide and an effective amount of 
peroxide. Other possible sequences include: D(EOP)DD, D(EOP)(DD), 
D(EOP)(DED), DEDED, DEHD, ZED, EDED, EHDED, DEHED, DEDH/ED, DHED, ZEZ, 
DH/ED, DEDEDP, DEZP, Z(EOP), DEZ, ZEPP, DEZD, D(EOP)DO, D(EOP)(DED), 
D(EOP)D, D(EOP)DP, Z(EOP)D, Z(EOP)DP, Z(EOP)ZD, Z(EOP)PP, D(EOP)PP, and 
the like in which D is as described above and Z is ozone, E is extraction 
in the presence of base, O is oxygen and P is peroxide. 
Another essential requirement of the process of this invention is the 
collection of chlorides, as for example chlorides generated in the acidic 
bleaching stage(s), chlorides introduced into the bleaching process with 
the wood fiber or combination thereof, in the filtrates from the first 
acidic and alkaline bleaching stages, the countercurrent recycle of these 
filtrates to the weak black liquor, preferably as wash water through at 
least one post oxygen delignification washing stage (if oxygen 
delignification is used) and through one or more brown stock washing to 
the weak black liquor and the recovery system where the chlorides are 
removed. Several advantages flow from removal of the chlorides in the 
recovery system. For example, chlorides accumulate in the recovery system 
in a relatively concentrated solid form, and therefore are easier to 
remove than from the circulating filtrates and other liquors in the 
bleaching process to maintain the concentration of chlorides in the 
filtrates and other liquors at the desired level. Enrichment or 
concentration of chlorides in the ash also reduces the amount of raw 
materials and resources required to separate chlorides from the ash as 
compared to separating chlorides from a liquid as for example the white 
liquor. Moreover, removal of chlorides from the ash as compared to removal 
of chlorides from filtrates and liquors in the process avoids or reduces 
the cycle flow of chlorides, thereby reducing the amount of chlorides 
feeding the recovery boiler and reducing or avoiding problems associated 
with conventional systems as for example pluggage, corrosion, organo 
chlorides in boiler air emissions and the like. In the recovery system, 
the black liquor (which contains chlorides and which is formed from 
evaporation of the weak black liquor which in turn is formed from the 
filtrate from brown stock washing) is combusted in a boiler forming an ash 
and a smelt. The smelt is recycled to generate the pulping liquor. Sodium 
chloride has a significant vapor pressure at the operating temperatures of 
the boiler and as such chlorides feed the boiler, they are preferentially 
distributed in the boiler ash. Ash, when mixed in the black liquor and fed 
to the boiler, creates a significant cyclic flow of chlorides, which 
adversely affects the system. The subject process avoids or reduces this 
cyclic flow of chlorides by removing chlorides from the ash before it is 
re-introduced to the boiler. The process also provides significant benefit 
in the removal of potassium from the liquor cycle. The removal of chloride 
and potassium provides significant benefit to the recovery boiler 
operation. By removal of chlorides, chloride concentrations in the 
circulating liquors is therefor maintained and/or reduced to desired 
levels. Desired chloride concentration levels may vary widely depending on 
the nature of the mill, recovery boiler, evaporation system and the like. 
For example, desired chloride concentration levels can be as high as from 
50 to 100 g of chloride per liter of white liquor in "coastal mills" i.e., 
pulping mills where the wood supply is transported by sea water and as low 
as 5 g of chlorides per liter of white liquor in "inland mills" i.e. 
pulping mills where the wood supply is not transported via sea water. The 
process is especially useful in inland mills and desired chloride levels 
are maintained at levels desired for such mills. Such chloride 
concentration levels are preferably equal to or less than about 20 g/l, 
more preferably equal to or less than about 15 g/l, most preferably equal 
to or less than about 10 g/l. 
Typical chloride concentrations in white liquor and feed to the recovery 
boiler are contrasted in the following Table 1 for four mills: 
TABLE 1 
______________________________________ 
COMISON OF TYPICAL CHLORIDE CONCENTRATIONS 
FOR VARIOUS MILLS 
PRESENT 
INLAND COASTAL THUNDER INVEN- 
MILL MILL BAY MILL TION 
______________________________________ 
NaCl in White 
3.6 25-50 50 4.5 
liquor, grams 
NaCl/liter 
NaCl to Recovery 
0.2-0.5 6.1 6.0 0.45 
Boiler, % of 
B.L. Solids 
NaCl to Recovery 
1-3 11-36 115 10-12 
process, pounds 
NaCl per ton pulp 
______________________________________ 
Results from the Thunder Bay mill are reported literature values when that 
mill operated with the closed mill concept. The results for mills 
operating according to the present invention are calculated for an "inland 
mill" operating with the bleach filtrate recovery process shown in FIG. 1 
and FIG. 2. As shown in Table 1 in this embodiment of the process of the 
present invention, the chloride input to the liquor recovery operation is 
reduced by about 90% over that described by Reeve et al in U.S. Pat. No. 
4,039,372. 
The removed chlorides can be discharged from the process and disposed of by 
some suitable means as for example by discharge to the sewer, discharge to 
a holding/evaporation pond, and the like. The chlorides can also be 
treated to recover the chloride values employing any conventional 
procedure. For example, the chlorides, which may contain potassium salt 
contaminants, can be purified by some suitable conventional procedure such 
as ion exchange, evaporative recrystallization and the like, then 
subjected to electrochemical oxidation to form chlorate which is then 
chemically reduced to form chlorine dioxide which can be used as the 
bleaching agent in the acidic bleaching stage(s). The chlorides can also 
be used to regenerate ion exchange resin when ion exchange is used for 
metals removal. Such processes which may be used in the practice of the 
invention are conventional and will not be described in great detail. 
Another essential component of the present invention is the removal of all 
or a portion of the non-process metals which enter the process with the 
wood fiber such as calcium, magnesium and manganese from the filtrates. 
The overall amount of metals removed may vary widely. Preferably at least 
about 50%, more preferably at least about 70% of the metals are removed 
based on the amount of non-process metals in the pulp entering the acidic 
bleaching stage. This enhances the recyclability of the filtrates 
especially when the filtrates are recycled as wash water in the alkaline 
bleaching stage(s) or in the acidic bleaching washing stage immediately 
preceding the alkaline bleaching stage because of the tendency of 
non-process metals to associate with pulp fibers under alkaline 
conditions. This association adversely affects the pulp manufacturing 
process as, for example, adversely affecting the bleaching process and 
pulp quality e.g., increased bleach chemical consumption, reduced 
brightness, brightness reversion and scaling and the like. 
Any method capable of removing non-process metals from filtrates can be 
used. In one preferred embodiment of the invention, the metals removal 
procedure relies on the disassociation of the non-process metals from wood 
fibers in aqueous acidic medium i.e., pH less than about 7, preferably 
equal to or less than about 6, more preferably equal to or less than about 
5 and most preferably from about 1 to about 5. In this embodiment of the 
invention, pulp is treated under aqueous acidic conditions at some point 
during the process, preferably during, before or after the first acidic 
bleaching stage and before the first alkaline bleaching stage to remove 
all or a portion of non-process metals. Any procedures known to those of 
skill in the art can be employed which takes advantage of this 
disassociation characteristic of the non-process metals. 
In one preferred embodiment of the present process the filtrate from the 
first acidic bleaching stage contains dissociated non-process metals. All 
or a portion of this filtrate can be discharged from the process as for 
example to the sewer. Alternatively, all or a portion of the filtrate can 
be treated to remove all or a portion of the non-process metals and all or 
a portion of the treated filtrate can be recycled to the process as for 
example as wash water in one or more of the washing stages, as water to 
dilute pulp at any suitable point in the process or the like. The 
untreated filtrate may also be recycled to another point in the process, 
preferably a point before the first alkaline bleaching stage. For example, 
all or a portion of the untreated filtrate can be recycled as wash water 
to the post oxygen delignification washing stage, brown stock washing 
stage or a combination thereof or recycled to some point prior to the 
first acidic bleaching stage to dilute relatively high consistency pulp 
prior to that stage. 
Another preferred embodiment of the invention uses an acid treatment step 
prior to the first acidic bleaching stage to remove non-process metals as 
illustrated in FIG. 2. The acidic wash filtrate can be disposed of or used 
employing any conventional procedure. For example, all or a portion of the 
filtrate can be discharged from the process, as for example to the sewer. 
Alternatively, all or a portion of the filtrate can be treated to remove 
all or a portion of the dissolved non-process metals employing 
conventional procedures, as for example ion exchange, precipitation by 
treatment with base to raise the pH to a value greater than seven and the 
separation of solid and aqueous phases and the like and all or a portion 
of the treated filtrate can be recycled to the process as for example as 
wash water in one or more of the washing stages, as water to dilute pulp 
on any suitable point in the process or the like. The untreated filtrate 
may also be recycled to another point in the process, preferably a point 
before the first alkaline bleaching stage. For example, all or a portion 
of the untreated filtrate can be recycled as wash water to the post oxygen 
delignification washing stage, brown stock washing stage or a combination 
thereof or recycled to some point prior to the first acidic bleaching 
stage to dilute relatively high consistency pulp prior to that stage. 
In the preferred embodiment of the invention which includes oxygen 
delignification, the location of the acid treatment stage after oxygen 
delignification and post oxygen washing is critical in enhancing the 
recovery of spent chemicals and dissolved organic matter from the wood 
pulp. This improved recovery is illustrated in the following Table 2: 
TABLE 2 
______________________________________ 
COMISON OF SEWER DISCHARGE FROM ACID 
TREATMENT OF SOFTWOOD PULP BEFORE AND 
AFTER OXYGEN DELIGNIFICATION 
BEFORE OXYGEN AFTER OXYGEN 
DELIGNIFICATION DELIGNIFICATION 
TO SEWER TO SEWER 
TOC Color TOC Color 
H.sub.2% SO.sub.4 
.sub.p H 
#/T #/T H.sub.2% SO.sub.4 
.sub.p H 
#/T #/T 
______________________________________ 
9.1 2.1 41.2 48.8 2.0 2.3 0.9 2.0 
______________________________________ 
Table 2 shows how sewer losses vary if the acid treatment process is 
located before oxygen delignification versus after oxygen delignification. 
Percent metal removal is the same regardless of location. As shown in 
Table 2, nearly 25 times as much color and 40 times as much total organic 
carbon (TOC) is lost to the sewer if the acid treatment is placed before 
oxygen delignification rather than after oxygen delignification. At high 
chlorine dioxide substitution the entire softwood bleach plant discharges 
about 50 to 60 #/T color. Placement of acid treatment before oxygen 
delignification would nearly negate the environmental benefit of 
recovering bleach filtrates. 
In the preferred embodiments of the invention, the overall objection of 
non-process metals removal is to achieve a lower non-process metal 
concentration in the washed pulp entering the first alkaline bleaching 
stage, relative to what is present in the pulp entering the first acidic 
bleaching stage. The extent of non-process metals removal is preferably as 
set forth in the following Table 3. 
TABLE 3 
______________________________________ 
Approximate Percent (%) Metal 
Remaining in Washed Acidic Stage 
Pulp ("168") 
Relative to Pulp Entering Acidic 
Bleach Stages Present in 
Stage ("122") 
Overall Bleach Sequence 
Ca Mg Mn 
______________________________________ 
No P Stage(s), No Z Stage(s) 
10-50% 10-50% 10-30% 
P Stage(s) and/or Z Stage(s) 
10-50% 10-50% 1-5% 
______________________________________ 
where "P" is peroxide and "Z" is ozone. 
There are two primary variables with which to achieve the desired degree of 
non-process metals removal. The first of the two variables is the volume 
of acidic filtrate that is "treated" in the metals removal process. This 
is referred to as the "volume treated". The requirement for the "volume 
treated" can vary considerably with each specific mill, but is typically 
of the order of about 10% to about 40% of the volume of total acidic 
filtrate. The second of the two variables is the efficiency of the metals 
removal process at removing non-process metals from solution. This is 
referred to as the "treatment efficiency". The "treatment efficiency" 
should be at least about 50%, preferably about 50% to about 75%, more 
preferably about 75% to about 90%, and most preferably greater than about 
90%. 
Any conventional process for removing dissolved metals from an aqueous 
solution can be used. In one preferred embodiment as, for example, the 
embodiment depicted in FIG. 7, non-process metals can be removed by a 
process which precipitates non-process metals as for example as carbonates 
and hydroxides. In this metal precipitation embodiment, all or a portion 
of the first acidic filtrate from the first acidic bleaching stage or from 
the acid wash is treated with base such as sodium hydroxide and/or sodium 
carbonate, or green liquor from the pulping process, sufficient to raise 
the pH above 7, preferably to between about 9 and 11, to provide an amount 
of carbonate ion such that insoluble non-process metal precipitates as, 
for example, carbonates and hydroxides will be formed. The aqueous 
solution is maintained for sufficient time and at a sufficient temperature 
as for example for at least about 10 minutes and at a temperature of up to 
about 160.degree. F. such that insoluble non-process metal compounds such 
as carbonates and hydroxides precipitate from the aqueous solution. A 
suitable separation device, preferably a precoat filter or a clarifier, is 
used to separate the aqueous phase from the solid precipitate. The solid 
precipitate can then be disposed of by landfill or introduced into the 
pulping liquor cycle either with heavy black liquor before combustion in 
the recovery boiler or with green liquor. If the precipitate is introduced 
into the pulping cycle, the non-process metals will be removed from the 
mill along with other non-process metals in the current practice of grits 
and dregs removal. 
In another preferred embodiment of this invention, all or a portion of the 
non-process metals can be removed by treating the acidic filtrate from the 
acidic wash or from the first acidic bleaching stage with an ion exchange 
resin where the dissolved non-process metals are removed by the resin to 
provide a filtrate containing a reduced amount of dissolved non-process 
metals. For example, in one preferred embodiment of this invention, as for 
example the embodiment depicted in FIG. 8, all or a portion of the 
non-process metals can be removed by passing all or some of the acidic 
bleach filtrate or acidic wash filtrate through an ion exchange system 
containing a cation exchange resin as, for example, the system available 
from Advanced Separation Technologies, Inc. under the Tradename "ISEP" and 
the system from Eco-Tec, Inc. and/or PROSEP Technologies, Inc., under the 
Tradename "RecoFlo", such that the dissolved non-process metals in the 
acidic bleach or acidic wash filtrate are preferentially removed from the 
filtrate relative to other metals. Prior to passage through the ion 
exchange resin the filtrate can be treated to remove suspended solids by 
conventional means such as by dissolved air flotation, filtration, 
sedimentation, precoat, centrifugation and the like. In the preferred ion 
exchange system, the cation exchange resin in the ion exchange process 
removes non-process metals until its capacity to continue to do so is 
exhausted. At such time the cation exchange resin can be "regenerated" 
with regenerant which serves to remove the non-process metals and produce 
a highly concentrated waste regenerant containing the non-process metals. 
Sources of regenerant include sodium chloride, and/or by-product 
sesquisulfate or sodium sulfate produced during the manufacture of 
chlorine dioxide, and/or sodium chloride solution removed in the recovery 
system. 
The waste regenerant can subsequently be discharged from the process so as 
to satisfy the need for purging the non-process metals, without taking 
away from any of the benefit achieved by the process. Alternatively, the 
waste regenerant can be treated with a base, such as sodium hydroxide 
and/or sodium carbonate, sufficient to raise the pH to provide an amount 
of carbonate ion such that insoluble metal carbonates and hydroxides will 
be formed. Insoluble carbonates and hydroxides of calcium, magnesium, 
manganese and iron precipitate from solution upon treatment with sodium 
hydroxide and/or sodium carbonate. The precipitate can be separated from 
the regenerant using any conventional means for separating as precipitated 
solid from a liquid phase such as precoat filter or a clarifier. The solid 
precipitate can then be disposed of in some suitable fashion or, for 
example, it can be combined with the green liquor dregs for disposal, 
returned to Kraft recovering into the pulping liquor cycle, or disposed of 
on its own to a landfill. The aqueous stream produced by separation of the 
metal precipitate is suitable to be re-used in the process as for example 
for regenerating the exhausted cation exchange resin or it can be 
discharged from the process. 
The aqueous phase from the metal removal system containing reduced 
concentrations of non-process metals is then preferably reused at some 
point in the pulp manufacturing process. This aqueous phase contains a 
reduced amount of metals and is preferably relatively free of non-process 
metals and can be used for example in the first acidic bleach stage washer 
to displace entrained liquor containing metals. This method of washing 
pulp with filtrate from which all or a portion of the metals have been 
removed reduces the quantity of metals carried into the first alkaline 
bleaching stage without the use of fresh water. Use of filtrate from the 
metal removal system is not limited to washing after the first acidic 
bleach stage. It can also be used for pulp dilution prior to the high 
density storage or for washing pulp prior to acidic bleaching. 
The following examples are presented to describe the invention in greater 
detail but should not be construed as limitations thereto. 
EXAMPLE I 
Referring to FIG. 1, Southern Pine Softwood (hardwood) Chips 10 are 
screened (not shown) to remove oversized and undersized chips, then cooked 
in a digester 12 using the Kraft pulping process with a white liquor 14 
charge of from about 15 to 22%, and preferably 18%, active alkali and 
about 20 to 40%, preferably about 30%, sulfidity under cooking conditions 
of about 310.degree. to about 350.degree. F., and preferably about 
344.degree. F., and from about 80 psi to about 140 psi, and preferably 
about 110 psi. 
The resulting pulp having a Kappa number ranging from about 25 to about 35, 
and preferably about 30, is discharged under pressure into a blow tank 
(not shown), then screened to remove uncooked knots (not shown). After 
removal of knots, the brown stock 16 is washed successively with from two 
to four stages, and preferably three stages of washing. After washing, 
pulp is screened 20 to remove rejects and the resulting pulp 22 is then 
charged to the oxygen delignification stage 32. 
As best shown in FIG. 2, the pulp, after brown stock washing and screening, 
is shown entering the last wash stage 26 prior to entering the oxygen 
delignification stage 32, wherein it is admixed with oxygen 28 and 
oxidized white liquor 30 and then further delignified in an oxygen 
delignification stage 32 to a Kappa number ranging from about 14 to about 
20, and preferably about 16, corresponding to about 30 to about 55%, and 
preferably about 45%, delignification. The oxygen delignification stage 32 
is run under typical conditions of temperature and pressure with alkali 
as, for example, alkali supplied in the form of the oxidized white liquor 
30. 
Following oxygen delignification, the pulp is again washed successively 
with from 2 to 4 stages of washing (shown as 34 and 38) to remove the 
lignin and inorganic materials. The wash water 35 is derived from the 
washings from the next subsequent wash stage 38. The final wash is derived 
from filtrate 40 obtained from the decker 42 after the first bleach stage 
as described hereinbelow. The resulting washed pulp is then stored in a 
high density (10-12%) storage vessel 44. 
The filtrates from the post oxygen delignification washers, 34 and 38, are 
recycled back counter currently as wash water through the pre-oxygen 
delignification washer 26 as best seen in FIG. 2. All of the organic and 
inorganic materials are eventually concentrated through a train of black 
liquor evaporators 46 and sent to the recovery boiler 48 for combustion, 
as shown in FIG. 1. 
As best seen in FIG. 1, the inorganic salts, recovered as smelt 50, are 
dissolved in water to form green liquor in liquid preparations 52, which 
in turn is treated with calcium oxide to regenerate the white liquor 14 
fed to the digester 12. The calcium carbonate by product is burned in a 
kiln (not shown) expelling carbon dioxide and regenerating calcium oxide 
which is used to regenerate the white liquor. 
Throughout the pulping (including oxygen delignification), washing and 
recovery processes, no organic or inorganic materials are intentionally 
discharged and/or disposed of, with the exception of the small amount of 
dregs that settle out in the green liquor clarifier and grits from the 
green liquor shaker (not shown). 
Returning now to FIG. 2, the pulp from high density storage 44 is then 
subjected to an acidification treatment in tank 54 equipped with mixer 57. 
The pulp can be treated with acid to a desired pH as, for example, a pH 
within the range of from about 2 to about 3, entering tank 54 through a 
line 59. The primary function of the acidification treatment is to 
solubilize the non process metals that come in with the wood supply and 
which must be purged from the process. In the past, this typically was 
done in the first acidic bleaching stage where the filtrate was discharged 
to the sewer. By adding an acidic stage with no ability to delignify, the 
metals can be removed and thereby allow the first acidic bleaching stage 
with its content of solids (color, BOD, etc.,) to be recovered. By acid 
treating the pulp from the high density storage not only are the metals 
removed but most of the dissolved organics associated with the pulp slurry 
will be precipitated onto the fibers and will be carried into the first 
bleaching stage rather than discharged with the metals to the sewer. This 
is preferred to significantly reduce the discharge of dissolved organics. 
Accordingly, it can be seen that the acidification treatment step prevents 
the build-up of metals within the brownstock washing/bleach plant cycle 
which would otherwise limit the viability of the process. 
After acidification, the pulp is washed in washer 58 with fresh water to 
remove the acidic wash and the non-process metals. This wash, which is low 
in environmental impact since no bleaching or delignification is effected, 
is discharged to the sewer at 60. 
After the acid treatment and wash, chlorine dioxide or a mixture of 
chlorine dioxide and chlorine is added to the pulp at an application rate 
of about 2.0 to 3.4%, preferably about 2.6%, calculated as active 
chlorine. The pulp is treated with the chlorine dioxide or a mixture of 
chlorine dioxide and chlorine solution in the first bleaching stage 62 
under conditions of 10-12% consistency, a reaction time ranging from about 
30 to about 90 minutes, preferably about 60 minutes, and a temperature 
ranging from about 100 to about 160.degree. F., preferably about 
140.degree. F. Although chlorine can be added along with the chlorine 
dioxide in the first bleaching stage, it is considered important in 
achieving a bleaching stage with minimal chloride residuals to limit the 
use of chlorine to less than about 10%, or completely eliminate the use of 
molecular chlorine. 
After the first acidic bleaching stage, the pulp is then discharged into a 
mixing tank 64 for partial neutralization to a pH ranging from about 5 to 
about 7, and preferably to about 7, using the alkaline filtrate 66 from 
the first oxidative extraction stage 68 with some caustic make-up in the 
form of oxidized white liquor 65, generally less than 1% on pulp. The pulp 
is then thickened by passage over decker 42 and oxidized white liquor is 
added at an application rate of about 1 to about 2%. Hydrogen peroxide is 
then introduced to the stream from the decker and oxygen is added to the 
pulp through high intensity mixer 70. Additional delignification is thus 
accomplished in oxidative extraction stage 68 thereby producing a pulp 
with a Kappa number ranging from about 2 to about 6, and preferably about 
4. The pulp is then diluted at the oxidative extraction tower discharge 
using a portion of the filtrate 66 from the first post extraction stage 
washer 72. Following the oxidative extraction stage 68, the pulp is washed 
successively on at least one post-extraction stage washer and preferably 
on two washers 72 and 73 as depicted in the drawings. The pulp is then 
heated by steam in a steam mixer 74 and chlorine dioxide is added in a 
high intensity mixer 76 and allowed to react in the chlorine dioxide 
bleaching tower 78 at a temperature ranging from about 150.degree. to 
about 180.degree. F., and preferably about 160.degree. F., for from about 
two to about four hours, and preferably for about three hours. 
The bleached pulp is then washed using paper machine white water with a 
portion of the filtrate being sewered at 80 and not recovered within the 
process. The remaining portion of the filtrate can be recycled to the 
chlorine dioxide bleaching tower 78 for pulp dilution. Thus, from the 
bleach plant 24, the only intentional discharges are flows from the acid 
pretreatment stage at 60 and from the final wash applied to the fully 
bleached pulp at 80. 
Filtrate, from washing pulp exiting the oxidative extraction stage 68, is 
used, in part, for discharge dilution of the pulp from the oxidative 
extraction tower 68 by way of line 66, and in part, is used for 
neutralization of the pulp exiting the first bleaching stage 62 in 
neutralization stage 64. Addition of fresh water to wash the pulp after 
the extraction stage is reduced by using two stages of washing and as such 
is preferred to reduce, if not eliminate, additional evaporator 
requirements. Thus two stages of washing after the oxidative extraction 
stage is an important element to the viability of this process. 
As shown in FIG. 2, filtrate 40 is produced when decker 42 thickens the 
pulp received from the neutralization stage 64. Filtrate 40, which is a 
combination of filtrate from the first chlorine dioxide stage and 
extraction stage, is used as wash water in the last post-oxygen 
delignification washer 38. The filtrate 35 from the last post oxygen 
delignification washer 38 is, as discussed, hereinabove, recycled 
counter-current to pulp 71 through washers 34 and 26 via lines 35 and 33. 
As shown in FIG. 1, filtrate 31 from washer 26 flows counter-current to 
pulp through screening 20 and brown stock washing 18. In this way, 
chlorides, spent bleaching chemicals and organic matter removed during 
bleaching are combined with spent pulping liquor and evaporated producing 
strong black liquor 84. 
Chloride from the bleach plant, contained in black liquor 84 feeding the 
recovery boiler 48, is partially volatilized in the recovery boiler and 
carried to the electrostatic precipitator 86 where it is collected and 
removed with salt cake in the precipitator ash. This ash is dissolved with 
water in the salt leaching process at a temperature ranging from about 
80.degree. to about 120.degree. F., and preferably about 100.degree. F., 
to separate out sodium sulfate from sodium chloride. The aqueous sodium 
chloride solution is sewered at 90 and the solid sodium sulfate 88, with 
some residual chloride is recycled back to the mix tank 89. The leaching 
process prevents the build up of chloride concentration in the liquor 
cycle and maintains the concentration of sodium chloride in the white 
liquor at less than 5 grams per liter. The leaching process as described 
herein operates at greater than 95% efficiency in terms of salt cake 
recovery. 
EXAMPLE II 
A series of experiments were carried out to evaluate the effect of using 
different bleaching agents in the first acidic bleaching stage and the 
first alkaline bleaching stage and the effect of countercurrently 
recycling the combined filtrates from these bleaching stages to the first 
acidic bleaching stage. Effects studied were bleach consumption, and 
color, AOX and BOD in the bleach plant effluent. The results are set forth 
in the following Tables 4 and 5. 
TABLE 4 
______________________________________ 
EFFECT ON CHLORINATION AND EXTRACTED KAPPA 
NUMBERS WHEN ORGANIC MATTER IS ADDED TO THE 
CHLORINATION STATE (Combined Filtrate (D100 + EO) 
From Bleach Filtrate Recovery Added To First D Stage Of 
OD (EO)D Sequence) 
Added organic 
Matter 
% TOC On Pulp D100 Kappa 
Extracted Kappa 
______________________________________ 
0 6.3 4.1 
0.145 6.6 4.0 
0.29 7.0 4.3 
0.44 7.1 4.2 
______________________________________ 
TABLE 5 
______________________________________ 
BLEACH PLANT EFFLUENT CHARACTERISTICS 
BLEACH SEQUENCE 
COLOR AOX BOD.sub.5 
______________________________________ 
O(C + D.sub.10) (EO)D 
120 8 30 
O(D.sub.50 /.sub.c) (EO)D 
90 4 25 
OD(EO)D 50 2 20 
Recycle OD(EO)D 
10 1 10 
OD(EO)D 1 0.1 2 
______________________________________ 
All values in lbs/ton 
The data show that the process significantly reduces the color, AOX and BOD 
in the bleach plant effluent as shown in Table 5 wherein the effects of 
chlorine dioxide substitution for molecular chlorine and the effect of the 
filtrate recycle in accordance with the present invention are shown. The 
data also shows that the use of chlorine dioxide and extraction in the 
presence of oxygen and peroxide results in more lower consumption of 
bleaching chemicals than was expected. 
EXAMPLE III 
A laboratory simulation of the bleaching sequence with filtrate recycle as 
depicted in FIG. 2 was conducted. A mill softwood pulp (Kappa 14.2) taken 
from the second stage of post oxygen washing, similar to washer 38 in FIG. 
2, was bleached using the scheme in FIG. 2. In these runs sodium hydroxide 
was substituted for oxidized white liquor. Fresh mill pulp from the second 
post oxygen washer was bleached in repeated runs. Filtrates from each run 
were saved and applied in the manner shown in FIG. 2 during the succeeding 
run. This was continued until equilibrium dissolved solids were 
established in the recycling filtrates. Chlorine dioxide was completely 
substituted for molecular chlorine in these runs. 
Results of metal removal in the acid treatment stage are shown in the 
following Table 6: 
TABLE 6 
______________________________________ 
ACID LEACHING OF OXYGEN DELIGNIFIED PULPS 
Na K Mg Ca Mn Fe Al 
______________________________________ 
metals in Hardwood Pulp, ppm 
As Is: 2765 273 572 1358 66 11 26 
After 124 32 29 66 4 6 9 
leaching: 
% 95 88 94 95 93 54 65 
Removed: 
Metals in Softwood Pulp, ppm 
As Is: 6498 667 382 2529 88 16 22 
After 180 30 15 97 4 7 5 
Leaching: 
% 97 96 96 96 95 56 77 
Removed: 
______________________________________ 
The acid treatment stage was conducted at pH 2. More than 90% of the 
sodium, magnesium, calcium, and manganese present in the oxygen 
delignified and washed pulp was removed during acid treatment. Removal of 
potassium, iron and aluminum was lower, about 88% for potassium, 54% for 
iron and 65% for aluminum. 
Bleach chemicals demand and final brightness achieved during these recycle 
procedures are shown in the following Table 7 and are compared to lab 
bleaching of the same softwood pulp without filtrate recycle. 
TABLE 7 
______________________________________ 
Laboratory Bleaching of Oxygen Delignified Softwood Pulp 
With and Without Bleach Plant Filtrate Recycle 
Kappa number after oxygen delignification 14.2; 
Viscosity 15.1 cp. 
Without Recycle 
With Recycle 
______________________________________ 
First Chlorine Dioxide Stage: 
Chlorine dioxide, 
3.13 3.13 
% as active chlorine 
Chlorine dioxide, 
1.19 1.19 
% as chlorine dioxide 
Final pH 1.9 1.8 
Oxidative Extraction Stage: 
Sodium Hydroxide 
1.3 1.8 
applied, % 
Final pH 11.2 11.3 
CEK No. 2.1 2.1 
Brightness 52.4 51.1 
Viscosity, cp 14.4 14.0 
Second Chlorine Dioxide Stage: 
Chlorine Dioxide, 
1.0 1.25 
% as chlorine dioxide 
Final pH 3.2 3.1 
Brightness.sup.1 
86 86 
Viscosity, cp 12.7 13.1 
______________________________________ 
.sup.1 Chlorine dioxide requirement to achieve 86 brightness interpolated 
from several bleaching experiments. 
A modest increase in both sodium hydroxide, 0.5%, and chlorine dioxide, 
0.25%, was observed for the case of filtrate recycle compared to no 
filtrate recycle. Targeted brightness of 86 was achieved in both cases 
viscosity was similar. 
EXAMPLE IV 
Referring to FIG. 5 of the drawings, wood chips or other comminuted 
cellulosic fibrous material is fed via line 90 to a continuous digester 92 
where the materials are subjected to pulping action of a pulping liquor 
generated in causticizer 94 and conveyed to digester 92 via line 96. A 
variety of pulping procedures may be used such as the Kraft, soda, 
soda-oxygen, H.sub.2 S-pretreatment Kraft, alkaline, polysulfide and 
alkaline sulfite processes. In FIG. 5, the process will be described with 
particular reference to the preferred Kraft process in which the active 
pulping chemicals are contained in a white liquor comprising sodium 
hydroxide and sodium sulfide. The liquor will preferably comprise from 
about 15 to 22% sodium hydroxide and from about 20 to 40% sulfide and the 
digesting temperature and pressure are preferably from about 310.degree. 
F. to about 350.degree. F. and from about 80 psi to 110 psi, respectively. 
The resulting pulp, usually having a Kappa number of greater than about 
30, is fed via line 98 to brown stock washing stage 100 for at least one 
stage of washing, preferably from two to about four stages of washing and 
more preferably about three stages of washing to remove all or a portion 
of entrained pulping liquor. After brown stock washing the washed pulp is 
charged via line 102 to oxygen delignification stage 104. In the oxygen 
delignification stage 104, the pulp is treated with oxygen, preferably in 
the presence of base such as sodium hydroxide, and at elevated temperature 
and at elevated pressure to reduce the Kappa number of the pulp to less 
than 30, preferably less than about 25, more preferably from about 5 to 
about 20 and most preferably from about 14 to about 20 for hardwood pulp 
and from about 8 to about 18 for pulp. While oxygen delignification is 
used in the preferred embodiments of this invention depicted in the 
drawing to reduce the Kappa Number of the pulp to the desired value other 
conventional procedures can be used. The oxygen delignified pulp is then 
subjected to at least one stage of post oxygen delignification washing, 
preferably from 2 to about 4 stages, where the oxygen delignified pulp is 
washed. While the washer employed in the preferred embodiments of the 
invention is a press washer, the type of washer used may vary widely and 
other conventional washers, as for example a CB washer, drum washer, belt 
washer, press washer, diffusion washer, two stage washer, and the like may 
be used. In FIG. 6, the pulp is conveyed to two post oxygen 
delignification washers 106 and 108 via lines 110 and 112, respectively, 
to further remove lignin and inorganic materials. Wash water from at least 
one of the post oxygen delignification washers, preferably, the last 
washer 108 is obtained by countercurrent recycle of filtrate from at least 
one post alkaline bleaching washer 118. As depicted in FIG. 6, the wash 
water for each post oxygen delignification washer is derived by 
countercurrent recycle of the wash water from a subsequent, preferably the 
next subsequent, post oxygen delignification washer. For example as shown 
in FIG. 5, the wash water for washer 106 is derived by countercurrently 
recycling the wash water from washer 108 via line 114. The wash water for 
the last post oxygen delignification washer 108 is derived from filtrate 
from post alkaline bleaching washing stage 118 which is recycled counter 
currently to washer 108 via line 116, and as will be described above is 
itself a combination of filtrates from first acid bleaching stage 120 and 
post alkaline bleaching washing stage 118. If the filtrates from any 
subsequent bleaching stages are recycled, the recycled filtrate also 
includes such filtrates. As depicted in the figure, the filtrate from post 
alkaline bleaching washing stage 118 is directly recycled to washer 108, 
however, it should be appreciated that the filtrate can be stored in some 
holding tank (not depicted) prior to use, or can be recycled to washer 106 
rather than washer 108 or directly to brown stock washing 100. The only 
requirement is that all or a portion of these filtrates ultimately reach 
the weak black liquor flowing to evaporator 126 thus entering the recovery 
system. As depicted in the figure, the combined filtrates from the post 
oxygen delignification washers 106 and 108 are recycled countercurrently 
via line 115 to brown stock washing stage 100 to form a weak black liquor 
which is conveyed via line 124 to evaporator 126 where the organic and 
inorganic materials contained in the counter currently recycled filtrates 
and the spent pulping liquor are concentrated into a strong black liquor. 
Strong black liquor is conveyed via line 128 to recovery boiler 130 where 
the strong black liquor is combusted to produce a smelt and a flue gas 
comprising particles of chlorides from recycled bleach plant filtrates 
from wood entering the process or from combination thereof and sulfates 
which may have been present in the strong black liquor. The flue gas is 
conveyed via line 132 to electrostatic precipitator 134 where the 
particles comprising chlorides and sulfates are collected as ash. The 
stack gas is vented via line 136 and the ash is conveyed via line 138 to 
chloride and sulfate separator 140. The ash is treated in separator 140 to 
form a component rich in chlorides and if sulfates are present a component 
rich in sulfates. 
Any conventional procedure known to those of skill in the art to separate 
chlorides and sulfates can be used. In the preferred embodiments of the 
invention because of the differing solubilities of chloride and sulfate 
salts in water, leaching and crystallization techniques can be employed. 
For example, the ash can be treated with water which dissolves the more 
soluble chlorides and potassium compounds forming a solution rich in these 
materials and a solid phase rich in sulfates. The aqueous and solid phases 
can be separated by conventional techniques such as filtration. 
The component rich in sulfates is conveyed via line 142 and added to the 
strong black liquor prior to combustion in recovery boiler 130. The 
component rich in chlorides can be discharged from separator 140 via line 
144 as for example to the sewer or can be treated to recover its chlorine 
value. For example, the component rich in chlorides can be conveyed via 
line 146 to purification unit 147 in which the chlorides are purified 
employing conventional techniques and is then conveyed to the chlorate 
generator conversion unit 148 via line 149 for conversion of chlorides to 
chlorates. The resulting chlorates can be conveyed via line 150 to 
chlorine dioxide generator 152 where chlorates are converted into chlorine 
dioxide which can be conveyed to chlorine dioxide bleaching stages 120 by 
some conveying means 154 for use in the acidic bleaching stages as will be 
described in more detail below, if chlorine dioxide is used as the active 
bleaching agent in the acidic bleaching stage. 
Alternatively, purified sodium chloride can be converted into an aqueous 
solution which can be used to regenerate the ion exchange resin which may 
be used in metals removal unit 162 as described in more detail below 
employing means not depicted in the figure. 
After the last post oxygen washing stage 108, the pulp preferably has a 
consistency of from about 2% to about 14%. The pulp can be stored in a 
storage tank (not depicted) until required for the first acidic bleaching 
stage 120 or conveyed directly to stage 120 via line 122 as depicted in 
the figure. In acidic bleaching stage 120, the pulp is bleached under 
acidic conditions with a bleaching agent such as elemental chlorine, 
ozone, chlorine dioxide, peracids and the like. In the preferred 
embodiments of the invention as depicted in the figure, the bleaching 
agent is ozone or chlorine dioxide comprising less than about 30%, 
preferably less than about 20%, more preferably less than about 10% and 
most preferably less than about 5% of the active bleaching agent is 
elemental chlorine. In the embodiments of the invention of choice, the 
active bleaching agent is ozone or chlorine dioxide which contains no or 
substantially no elemental chlorine (i.e. less than about 1% to about 5%). 
The application rates, pHs, times and temperatures used in the acidic 
bleaching stage may vary widely and any known to the art can be used. 
The bleached pulp is conveyed via line 156 to at least one post acidic 
bleaching stage washer or decker 158. All, or preferably a portion of the 
filtrate from washer 158 which contains non-process metals is conveyed via 
line 160 and 161 to metals removal unit 162 where all or a portion of the 
non-process metals are removed from the filtrate and discharged via line 
164. The untreated filtrate from washer 158 and the treated filtrate from 
metals removal unit 162 can be discharged from the process (not depicted) 
or can be recycled through the process. In the preferred embodiments of 
the invention, all or a portion of these filtrates are recycled. In the 
preferred embodiments, the use of such filtrate usually will be dictated 
by the requirements of the system, as for example the consistency of the 
pulp exiting washer 108 and whether such consistency is too high for 
optimum performance in the first acidic bleaching stage 120 because of 
process, apparatus limitations and other limitations. For example, the 
untreated filtrate can be recycled via line 160 to point 163 to dilute 
pulp from washer 108 when the consistency of such pulp is too high because 
of apparatus and process restrictions for use in acidic bleaching stage 
120. Similarly, if consistency need not be modified, untreated filtrate 
can be conveyed as wash water to one or more post-oxygen delignification 
washing stages, one or more brown stock washing stages or a combination 
thereof. Similarly, filtrate treated in unit 162 and containing a reduced 
amount of non-process metals can be used as wash water on washing stage 
158, on post oxygen delignification washing stage 106 and or 108 and or 
brown stock washing stage, discharged from the process or a combination 
thereof. As depicted in the figure, the filtrate containing reduced 
non-process metals is recycled to washer 158 as wash water via line 179, 
and the portion of filtrate from washer 158 not conveyed via line 160 to 
metals removal unit 162 is conveyed via line 160 to point 163 dilute pulp 
prior to conveyance to bleaching stage 120 because in the embodiment of 
the figure the consistency of the pulp from washer 108 is too high for use 
in acidic bleaching stage 120. Any suitable procedure for removing metals 
from an aqueous phase can be used in metals removal unit 162. In one 
preferred embodiment of the invention, the non-process metals are removed 
by precipitation caused by increasing the pH of the filtrate. For example 
as depicted in FIG. 6, filtrate from first acidic bleaching stage 120 and 
washer 158 having a pH of less than 7, usually from about 2 to about 4 is 
conveyed via line 161 to flash mixer 165 where a base such as sodium 
hydroxide and/or sodium bicarbonate is added via line 165(a) in sufficient 
amount to raise the pH of the filtrate to greater than 7, preferably from 
about 8 to about 11, which causes non-process metal salts such as 
hydroxides and/or carbonates which are insoluble under alkaline conditions 
to precipitate from the filtrate. The mixture of liquid phase and 
precipitates is conveyed to flocculator 167 via line 169 where the 
precipitate is allowed to flocculate, after which the mixture of aqueous 
phase and flocculated precipitate is conveyed via line 171 to 
sedimentation unit 173. In unit 173, the flocculated precipitate and the 
aqueous phase separate by sedimentation. The precipitate is removed from 
unit 173 via line 175 and discharged and the aqueous phase is conveyed via 
line 177 to washer 158 via line 179. 
In another preferred embodiment of the invention, the non-process metals 
are removed by ion exchange techniques. For example, as depicted in FIG. 
7, filtrate from first acidic bleaching stage 120 and washer 158 is 
conveyed via line 161 to solid removal unit 185 where fibrous solids are 
removed. Any conventional process can be used to remove fibrous solids 
such as filtration, sedimentation, air flotation, centrifugation and the 
like. Separated fibrous solids are conveyed to the alkaline bleaching 
stage 166 via line 187 or disposed of in any suitable manner and the 
treated filtrate is conveyed to ion exchange feed tank 190 via line 189 
where the filtrate can be held until used. Alternatively, the filtrate can 
be conveyed directly to ion exchange unit 193 by means not depicted. When 
required for use, the filtrate is conveyed to unit 193 from tank 190 via 
line 191 under force of pump 192. In unit 189, all or a portion of the 
non-process metals are removed from the filtrate and the treated filtrate 
is discharged (not depicted), conveyed to washer 158 via line 179 or is 
conveyed to at least one post oxygen delignification washing stage (means 
not depicted) and used as wash water or is conveyed by means not depicted 
to a point in the process after the last post oxygen delignification 
washing stage 108 and prior to first acidic bleaching stage 120 (means not 
depicted), is used to dilute pulp prior to the acidic bleaching stage 120 
(means not depicted), conveyed to at least brown stock washing stage as 
wash water (means not depicted) or a combination thereof. 
As depicted in FIG. 7, a unit 193 has reached its capacity for removed 
metals, it can be treated with a regenerated stream introduced into unit 
193 via line 188 from storage tank 200 under force of pump 202. As the 
regenerate stream ions contained in the stream preferentially are 
exchanged with non-process metals retained in the cationic exchange resin 
forming a waste regenerant stream which comprises such metals which exits 
unit 193 via line 204. The waste regenerant stream may be discharged from 
the process via line 206 as for example to the sewer or may be conveyed 
via line 208 to regeneration unit 210 where all or a portion of the 
non-process metals are removed from the stream to form a stream containing 
reduced non-process metal content which is then conveyed to tank 200 via 
line 212 for further use in the process. Any process can be employed in 
unit 210 for removal of non-process metals. In the preferred embodiments 
of the invention as depicted in the figure, non process metals are removed 
by adjusting the pH to a value greater than 7 by adding base such as 
sodium hydroxide, sodium carbonate or a combination thereof which 
precipitates non-process metals, and separating the precipitated 
non-process metals by some suitable technique as for example 
sedimentation, filtration, centrifugation or the like. The aqueous stream 
having reduced metal content is then recycled as described above and the 
separated metal precipitate can be discharged via line 211 and disposed of 
in some suitable way. 
As depicted in FIG. 7, when additional regenerate solution is required, the 
solution having the required ions in the required amounts is formed in 
make down tank 214 from water and some regenerate source (as, for example, 
sodium salts such as sodium chloride as for example sodium chloride from 
chloride removal unit 140, sodium sesqui-sulfate from the chlorine dioxide 
generator (not depicted) or sodium sulfate) which is introduced into tank 
214 via line 216. The solution is then conveyed when needed to tank 200 
via line 218 under force of pump 220. 
After washing on washer 158, the pulp, preferably where the amount of 
non-process metals is such that the alkaline bleaching process adversely 
affect to an undue extent, is conveyed via line 168 to alkaline bleaching 
stage 166 where the pulp is bleached under alkaline conditions. While any 
suitable alkaline bleaching agents and conditions can be used, in the 
preferred embodiments of the invention, alkaline bleaching stage 166 is 
oxidative extraction with oxygen in the presence of base preferably 
carried out in the presence of peroxide, employing conventional oxidative 
extraction times, pressures and temperatures. 
After alkaline bleaching stage 166, the pulp is subjected to at least one 
post alkaline bleaching washing stage 166 preferably from 1 to about 6, 
more preferably from 2 to about 4 and most preferably 1 post alkaline 
bleach washing stages. As depicted in FIG. 5 there are 2 post alkaline 
bleaching stages. In this embodiment of the invention, the alkaline 
bleached pulp is conveyed from alkaline bleach stage 166 via line 170 to 
post alkaline bleach stage washer 118 and from washer 118 to optional 
washer 172 via line 174. The wash water from washer 118 is derived by 
countercurrently recycling the wash water from washer 172 to washer 118 
via line 176. If there are more than two post alkaline bleaching washing 
stages, for all or a portion of these stages other than the last post 
alkaline bleaching washing stage, wash water is preferably derived by 
countercurrently recycling wash water from a subsequent, preferably the 
next subsequent, post alkaline bleaching washing stage. In this manner, 
all or a portion of the filtrates from all or a portion (preferably all) 
post alkaline bleaching washing stage are combined in the filtrate from 
the first post alkaline bleaching washing stage which can then be directly 
or indirectly recycled countercurrently as wash water to a post oxygen 
delignification washing stage (preferably the last post oxygen 
delignification washing stage) as for example via line 116 to post oxygen 
delignification washer 108, and thereafter to brown stock washing and to 
the recovery system where chlorides can be removed from the process. Wash 
water for the last post alkaline bleaching washing stage 172 or for those 
preferred embodiments of the invention where there is only one post 
alkaline washing stage 118 can be obtained from any convenient source as 
for example fresh water or water recycled from the mill such as water 
recycled from a subsequent bleaching stage. For example, where the process 
includes no further bleaching stages the wash water can be non-bleach 
plant water obtained from other aqueous streams such as in the paper mill 
or from the fresh water system. If the process includes other bleaching 
stage, fresh non-bleach stage water can be used, or filtrates from one or 
more of these subsequent bleaching stages can be countercurrently recycled 
in the last post alkaline bleach stage washer as described below such that 
these filtrates are combined in the filtrate countercurrently recycled via 
line 116 to a post oxygen delignification stage washer 108 as wash water 
and ultimately to brown stock washing stages 100 and to the recovery 
system. A portion of the filtrate may also be countercurrently recycled 
via lines 116 and 228 to washer 158 as wash water. For example as depicted 
in FIG. 5, wash water from last post alkaline bleach stage washer 172 can 
be fresh water introduced via line 178 or filtrate conveyed from washing 
stage 180 of bleaching stage 182 via line 184. The filtrate from washing 
stage 180 can be discharged from the process or can be countercurrently 
recycled directly to washing stage 172 via line 220. Alternatively, if 
chlorine based bleaching agents are employed and the bleaching stage 
results in the production of chlorides can be treated to remove all or a 
portion of the chlorides by some suitable technique in unit 222, as for 
example an anionic ion exchange process, prior to use as wash water on 
washing stage 172, if deficiencies in the recovery system will not allow 
recovery of additional chlorides from a chlorine based bleaching stage 
182. 
The pulp can be processed from system and used for conventional purposes or 
the pulp can be subjected to one or more additional acidic and/or alkaline 
bleaching stages as for example further bleaching with one or more 
bleaching agents selected from the group consisting of peroxide, chlorine 
dioxide and ozone. Such additional bleaching stages may be without 
subsequent washing or may be followed by subsequent wash stage or stage(s) 
in which the wash water is either discharged from the process, is 
countercurrently recycled as wash water substantially as described above 
for washing stages 172 and 118 through at least one post oxygen 
delignification stage through brown stock washing and ultimately to the 
recovery system. For example, in the embodiment of FIG. 6, the washed 
alkaline bleached pulp is conveyed via line 186 to a second acidic 
bleaching stage 182 where the pulp is bleached substantially as described 
below for first acidic bleaching stage 120. After bleaching, the pulp can 
be subjected to further bleaching stage(s) without washing, or as depicted 
in FIG. 6 can be conveyed via line 226 to at least one post acidic 
bleaching washing stage 180 where the pulp is washed preferably with fresh 
water introduced via line 188. The fully processed pulp exits the 
bleaching sequence via line 190 for conventional use as for example in a 
paper making process and the washed water is countercurrently recycled via 
lines 184 and/or 220 as wash water to washing stage 172, discharged from 
the process via line 224 or a combination thereof. 
Many variations of the present invention will suggest themselves to those 
of ordinary skill in the art in light of the above-detailed description. 
All such obvious modifications are within the full intended scope of the 
appended claims.