CA2134730C - Chlorine-free bleaching with short sequence and filtrate recirculation - Google Patents

Abstract

A method of total chlorine-free bleaching to a brightness of ISO 82-90 uses an acid stage and a superatmospheric pressure peroxide stage. The acid stage (which may be the only acid stage) may comprise a Q stage or a Z
(ozone) stage, in which metals in the pulp are freed from pulp fibers and become dissolved in the surrounding liquid. The pulp is then subjected to fractional washing to produce at least two wash filtrates, the first filtrate having over 50% of the dissolved metals from the pulp. The first filtrate is ultimately returned (e.g. through brown stock washing) to the black liquor recovery system of the pulp mill so that the metals precipitate on organics in the black liquor recovery system due to the alkaline pH of the black liquor.
Other filtrates from the fractional washing are used to wash the pulp just before the acidic (e.g. ozone) stage. Acid and/or a chelating agent are added to the wash liquid during fractional washing so that the pH of the first filtrate is six or less, and magnesium may be added to the pulp while it has an acidic pH
just prior to the peroxide bleaching stage. From the fractional washing stage the filtrates may each have a flow volume of 0.5-7 cubic meters per ton of pulp. A further (ZP) stage may be added if it is desired to achieve a brightness of ISO 89 or above.

Description

..:..~, .. 21 343 Q

CHLORINE-FREIE BLEACHING WITH SHORT SEOUEhTC~
NOD FILTRATE RECIRCULATTON

BACKGI~ouNn arr. sumNraRy OF ~E ~~rrlO~r It is becoming increasingly more important to eliminate chlorine, typically used as a bleaching chemical, in the production of cellulose pulp, particularly in the production of chemical pulp such as kraft pulp: While it is desirable to eliminate the chlorine, it is also highly desirable to use as short a bleaching sequence as possible, with little or no waste liquid produced by the bleaching sequence.
According to the present invention, a method of total chlorine-free bleaching of cellulose puhp, to a brightness of 82-90 ISO, e.g. at least about ISO 84 (typically 84-86), and preferably to at least about ISO 88, is provided which is completely closed (that is there is a minimum or zero discharge of waste liquid from the bleaching sequence), and which has only one or two bleaching stages, so that the amount of equipment necessary is nunimized (while of course at the saJne time rrunimizing the number of streams and thereby the potential for liquid discharge).
According to the present invention, the bleaching chemicals utilized are either peroxide alone (the peroxide bleaching taping place at superatmospheric pressure), or first ozone and then followed by peroxide (at superatmospheric pressure) if an ISO above 86 is necessary. The invention utilizes fractional washing between an acid stage (which may be the ozone bleaching stage) and the peroxide stage, with the; first filtrate from the fractional washing containing over 50% (typically about '70-90%) of the metals dissolved in the liquid surrounding the pulp fibers during the acid treatment. The first filtrate is returned to the black liquor recovery system while at a pH of about 4-6 (if a chelating agent is used), or a pH of about 3 or less (if no chelating agent is used). The metals will ultimately precipitate on organics in the black liquor recovery system due to the alkaline pH thereof, and therefore the metals (such as transition metals) which consume peroxide bleaching chemical are removed 1 o and do not recirculate with the fibers to the bleach plant, but are taken care of in the chemical recovery system for the pulp mill.
Second, and if present third, filtrates from the fractional washing are led back to a washer immediately before the acid (e.g. ozone) stage. The flows are adjusted for optimum rmults, that is so that most of the metals are disposed of in the black liquor recovery system. Filtrates with some COD and a relatively low pH are provided just before the ozone stage to reduce the need for acid to reduce the pH u1 the ozone stage to the desired range (e.g. about 2.5-3.5) while providing organic compounds which improve performance in the ozone stage, and acid and/or chelating agent are added as necessary to the 2o fractional washing stage to maintain the desired pH at various parts of that stage.
According to one aspect of the present invention a method of total chlorine-free bleaching to a brightness of 82-90 ISO, typically to at least about ISO 84 (e.g. 84-86) of chemical cellulose pulp produced in a pulp mill, including a black liquor re~wery system containing fluids at alkaline pH, the pulp having pulp fibers and surrounding liquid and having a kappa number of 14 or below, is provided. The method comprises the following steps: (a) Treating the 14 or below kappa number pulp in a metals removal stage (e.g. a first and only acid stage), so that metals in the pulp are freed from pulp fibers 3o and become dissolved in the surrounding liquid. (b) Subjecting the pulp from ~ 34,3 step (a) to fractional washing, to produce at lest first and second wash filtrates, the first filtrate having a substantial amount of dissolved metals therein. (c) Ultimately recir. culating the first filtrate to the black liquor recovery system so that the metals precipitate on organics in the black liquor recovery system due to the alkaline pH. And, (d) after step (b), peroxide bleaching the pulp at super~trnospheric pressure and an alkaline pT-~ to a brightness of at least ISO 82 (e.g. 82-86 ISO).
Step (a) may be pracrticed by ozone bleaching, in which case step (d) is practiced to a brightness of about ISO 88 (typically 84-89), or step (a) may 1 o alternatively be practiced in a Q stage. Step (b) is typically practiced to provide over 50% of the di:>solved metals in a first filtrate, e.g. about 70-90%.
There is typically the further step, immediately before step (a), of (e) washing the pulp, and step (b) is ty~~ically practiced to provide a third filtrate, the second and third filtrates used in step (e). All filtrate flow volumes may be in the range of 0.5-7 m3/tp; for one e~rnple, the first filtrate may have a flow volume of 0.5-4 cubic meters per ton of pulp, the second filtrate a flow volume of 3-6 cubic meters per ton of pulp, and the third filtrate a flow volume of about 7 cubic meters per tan of pulp. Step (b) may be further practiced by adding acid to the wash liquid used in. step (b) so that the first filtrate has a pH
of about 3 or less, or step (b) may be practiced by adding acid and chelating agent to the wash liquid so that the fn~st filtrate has a pH of about 4-6. If magne~siurn is removed from the pulp to such an extent that there is insufficient magnesium to act as a prot~~:tor in the peroxide stage, then magnesium is preferably added to the pulp (while it has an acidic pH) just before the practice 2s of step (d).
The peroxide stage is typically practiced at a pressure of about 4-10 bar, a temperature of about 80-1 i0°C, a pH of about 9-12, and a peroxide dosage of about 10-30 kg of peroxide per ton of pulp. The ozone stage is typically practiced ax a pH of about 2.5-3.5, a temperature of about 40-70°C, 3o and an ozone dosage of al~~ut 2-8 kglton of pulp.

i .,~ v 1~7~

Alternatively, instead of step (c) described above, the filtrate could have its pH adjusted to precipitate out the metals as described in U. S. patent number 5,401,362; issued on March 28, I9~5.
According to another aspect of the present invention a method of treating comminuted fibrous material is provided comprising the following steps: (a) digesting the material to produce pulp ~d black liquor; (b) oxygen delignifying the pulp; (c) steps (a) and (b) being practiced so as to produce pulp having kappa number 14 or less; (d) bleaching the pulp from step (c) to , ISO 84 or greater; and, wherein step (d) consists essentially of hydrogen peroxide bleaching at superatmospheric pressure.
According to yet another asp~t of the present invention a method of treating cellulosic commirmted fibrous material is provided comprising the following steps: (a) digesting the material to produce pulp and black liquor;
(b) oxygen delignifying the pulp; (c) steps (a) and (b) being practiced so as to produce pulp having kappa number 14 or less; (d) bleaching the pulp from step (c) to ISO 88 or greater; ~~nd wherein step (d) consists essentially of a single ozone stage and then a single peroxide stage at superatrnospheric pressure.
It is the primary object to provide for the effective total chlorine-free bleaching of paper pulp to an ISO 86 or greater with essentially complete recirculation of liquid in the bleach plant (i.e. little or no production of waste liquids), with a short bleach sequence. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.
BR~F DESCRB'T~ON OF 'III DRAWING
FIGURE 1 is a schematic view of exemplary stages and equipment for the practice of one exemplary embodiment of the method according to the present invention;

S
FIGURE 2 is an enlarged detailed view of the fractional washer between the ozone and peroxide bleaching stages of FIGURE 1;
FIGURE 3 is a schematic graphical representation of the filtrate concentration distribution in the fractional washer of FIGURE 2;
FIGURE 4 is a graphical representation comprising brightness to consumed hydrogen peroxide for pressurized and non pressurized treatments;
and FIGURE S is a detailed view of a second embodiment of the invention like that of FIGURE 1 only with an acid stage replacing the ozone stage.
DETAII~D DFSCRIPTION OF 1ITE DRAWING
An exemplary method/apparatus for the total chlorine-free bleaching of ceIIulosic pulp, to about an ISO 88 brightness, is illustrated in FIGURE I.
Wood chips used to produce the pulp ark typically slurried and then digested in a conventional cooking installation, shown generally by reference numeral IO
in FIGURE 1. The cooking installation 10 may be a batch digester, a series of batch digesters, a continuous digester, or the like. Since according to the invention it is necessary to deliver pulp to the bleaching sequence at a kappa of about 14 or less (e.g. 8 or less), the cooking apparatus 10 must be capable of producing pulp having a low enough kappa number so that after oxygen delignification the kappa number is 14 or less (e.g. 8 or less). One particular type of digester capable of this result is a digester sold by Andritz Inc. of Glens Falls, New York under the trademark "EMCC~", which can readily reduce the kappa number to 20 or below, and in fact under some circumstances to 14 or below by itself.

s After cooking at 10, the pulp is washed in a plurality of washing stages, such as the three washing stages 11 through 13 shown in FIGURE 1 (brown stock washing), illustrated schematically there as drum washers, however any type of efficient washing equipment is suitable, such as mufti or three stage washers, displacem~t washers, etc. The filtrate from the first washing stage, in line 14, is typically passed to be combined with the black liquor in a black liquor recovery system illustrated schematically at 15 in FIGURE 1, and the typical flow of filtrate in line 14 is about 12-14 cubic meters per ton of pulp.
'Ihe black liquor in system 15 is evaporated and then burned in a recovery boiler or furnace.
After washing in stages 11 through 13, the pulp is optionally, depending upon the ISO ultimate brightness required and the kappa number to which the pulp has been cooked by the cooking apparatus 10, oxygen delignified at stage 16 under conventional oxygen delignification conditions.
The pulp from the oxygen delignification stage 16, which then definitely has a kappa number below 14, typically about 8 or below, is then subjected to further washing, for example illustrated at stages 17, 18 in FIGURE 1, the stage 18 for example being a mufti-stage washer just prior to a first, and only, acid stage 19. In FIGURE 1 the acid stage is an ozone bleaching stage.
In the ozone stage 19 the bleaching takes place at an acid pH, and typically while the pulp is at medium consistency (e.g. about 6-15%).
Preferably the ozone stage 19, in which medium consistency pulp is treated, is as disclosed in EP 0397308 published November I4, 1990, or as shown in a paper entitled "Paperi ja Puu", April, 1992, "New Generation Kraft Pulping and Bleaching Technology", Henricson, and as is commercially available from Andritz Oy of Helsinki, Finland, and Andritz Inc. of Glens Falls, New York.
The reaction conditions during the ozone bleaching stage 19 are typically pH
about 2.5-3.5, a temperature of about 40-70°C, and an ozone dosage of about 2-8 kg/ton of pulp.

The ozone bleaching in stage 19 takes place almost instantaneously, and the bleached pulp from the stage 19 is then passed to the fiactional washer shown by refer~ce numeral 20 in FIGURES 1 and 2. The fractional washer 20 may be any conventional high washing efficiency mufti-stage washer, but preferably comprises an Andritz DD l~.~asher available from Andritz Inc. of Alpharetta, Georgia. The washer includes multiple inputs for washing liouid, as indicated ax 21 and 22 in FIGURES l and 2, an inlet 23 for pulp to be treated which then flows clockwise in the washer 20 as indicated by arrow 24 in FIGURE 2, a pulp outlet 25 into the line 26 leading to the peroxide stage 27, and a plurality of filtrate outlets. In the embodiment shown in FIGURES 1 and 2, first filtrate outlet is indicated by reference numeral 28, the second filtrate outlet by reference numeral 29, and the third filtrate outlet by reference numeral 30.
If the filtrate in line 28 is too acidic - that is if its pH is so low that it requires excessive caustic addition to the 02 stage 16, or causes lignin redeposition in the washer 13, the pH of the filtrate while in line 28 may be raised. This may be accomplished, for example, by adding any readily available alkali source, such as sodium hydroxide. It is nat desirable to raise the pH in line 28 so that it is no longer acidic, however.
FIGURE 3 shows the filtrate concentration distribution of an imaginary substance X in the filtrate of washer 20 as a function of distance from the feed end of the washer (pulp inlet 23). For example this indicates that if the concentration of the substance X in the filtrate in the beginning of the wash stage is 0.1 grams per liter, the concentration is 0.075 gel about half way through washing in the washer 20, and about 0.02 at the pulp discharge 25.
Thus the properties of the filtrates in the lines 28, 29, and 30 are substantially different, as are the temperatures. For example the first filtrate, in line 28, may have a temperature almost the same as that of the entering pulp, while the filtrate in the third filtrate line 30 is heated almost to the temperature of the wash liquid, while the filtrate in line 29 is somewhere therebetween. For s 21 373 ~
example the temperature of the filtrate in the line 28 may be about 52°C, while that in the line 30 about 66'°C (e.g. assuming that the pulp inlet temperature is about SO°C, the wash liquid temperature is about 80°C, and the pulp outlet temperature is about 76.5°C',, the consistency of the pulp about 10°/g and the DF equal to 2.5 m3/ADT (air dried ton of pulp)).
During treatment of the pulp in the ozone stage 19, where the pH is highly acidic, metals, particularly transition metals, in the pulp are dissolved, being freed from the pulp fibers and becoming dissolved in the surrounding liquid. 'Ihe vast majority of the metals are dissolved in the Z stage 19, and as long as the pH remains relatively low, they will basically stay in solution and not precipitate out. However if the pH is raised too high they will precipitate back onto the fibers, or other organics. Therefore in order to remove the metals -- which will disadvantageously consume hydrogen peroxide bleaching chemical in stage 27 - the first filtrate line 28 is led out of the bleaching sequence (defined by the stages 19, 2'~. However in order not to have too much waste Liquid, it is desirable that the filtrate in line 28 be recirculated, typiclly as indicated in FItJURE 1 the line 28 leading to wash liquid inlet 32 to the washer 13, thus flowing backwardly in the system to the filtrate Line 14, which leads to the black liquor recovery system 15. The black Liquor contains large amounts of organics, .and is highly alkaline, so that the metals will precipitate on organics in the black liquor, and be handled in the black liquor recovery system 15.
In order to ensure treat the metals in filtrate line 28 do not precipitate out during washing in fractional washer, preferably acid, in line 34, andlor chelating agent, in line 35, are added. For example, the chelating agent, such as EDTA, is added in order to maintain the dissolved transition metals in solution at a higher pH. Typically thc~ metals would start to come out of solution at a pH below 4, but by adding the chelating agent such as EDTA in line 35 the pH before the metals start to precipitate out can be raised to 4-6.
'Ihe acid in line 34 may be added to keep the pH low, e.g. below about 3.

k n., As an alternative to recirculating the filtrate in line 28 as illustrated in FIGURE l, it could be treated as shown in U.S. Patent 5,401,362 issued to the present applicant. That is, its pH could be adjusted (e.g. by adding lime, slaker grits, white liquor, etc.) so that it was greater than 9 (e.g. 10.5 or more), while providing a filtrate carbonate content of at least about 0.01 mole/liter (e.g.
0.5 mole/liter), to cause the dissolved transition metals to precipitate as solids. Then the solids could be filtered out (e.g. by drum filtering, precoat disc filtering, clarification followed by polishing filtering, etc.) to produce a transition metal content depleted filtrate, which filtrate then could be used elsewhere in the pulp mill, e.g. in liquor manufacture, or as a wash liquid in the sequence in where an alkaline wash liquid was desired.
Typically the equijpment of FIGURE 1 is utilized in the method according to the invention so that the majority of the dissolved metals exit the bleaching sequence in lint: 28. For example the particular area at which the line 28 is taken from the washer 20, as are the particular wash liquids (with a particular additive such a:> the acid from line 34 and chelating agent from line 35), so that the filtrate in line 28 has more than 50% of the dissolved metals from the pulp, e.g. about 70-90% of the dissolved metals. The volume of the flow in each of the lines 28-30 is preferably 0.5-7 m3/tp. As one example, the flow in line 28 is preferably in the range of 0.5-4 cubic meters per ton of pulp, the volume of flow in second filtrate line 29 is about 3-6 cubic meters per ton of pulp, and the flow in t~'hird filtrate line 30 is about 7 cubic meters per ton of pulp.
The filtrates in lines 29, 30 are used as the wash liquid in the washing stage 18 just before the ozone stage 19. The filtrate in line 29 has relatively low pH, but his a significant COD since it contains organic compounds, therefore it is used as the last washing liquid just prior to the Z stage 19.
The low pH plus the presence of organic compounds makes the liquid in line 29 a particularly desirable liquid to use as a wash liquid just before the stage 19 1~
since it lowers the pH (reducing the amount of acid necessary to add to stage 19 to obtain the desired pI-~, and since the organics improve the perforniance of the ozone stage. The filtrate in line 30, on the other hand, has a somewhat higher pH and more orgaruc compounds, therefore it is introduced upstream of 5 the filtrate in line 29. Al.>o, part of some filtrates may be taken from the bleaching sequence to use; in other parts of the mill, such as for cooking liquor preparation. The amount of filtrate used in other parts of the mill is typically relatively small, however, usually below 3 m3/tp.
While it is desirable to remove the transition metals that consume 10 peroxide bleaching chemical in peroxide stage 27, it is desirable to have a substantial amount of mal~esium in the pulp to act as a protector in the stage 27. If too much magnesi~.im is removed from the pulp in the acid stage 19, so that there is insufficient magnesium to act as a protector, then magnesium preferably in the form of magnesium sulfate (MgS04) is added while the pulp is still acidic, e.g. in line 35 with the chelating agent EDTA as seen in FIGURE 1. The magnesium will have higher solubility if added while the pulp is still acidic, before it is made alkaline for the proper reaction in the peroxide stage 27.
In the peroxide stage 27, the, pulp is bleached under superatmospheric pressure, typically about 4-10 bar_ Peroxide bleaching under superatmospheric conditions (e.g. 75 psig) results in increased reaction rate and consumption of hydrogen peroxide, brightness, and kappa number reduction. This is, for example, graphically represented in FIGURE 4 where oxygen delignified Scandinavian soft wood having a kappa number of 7.8 and brightness of 41.8 percentage ISO was bleached with 5.0% hydrogen peroxide on BD pulp (as the only bleaching chemical). The graph line represented by reference numeral 38 illustrates peroxide bleaching at atmospheric pressure, 90°C, for three hours. CJraph line 39 illustrates peroxide bleaching at 5 bar pressure, 90°C, for one hour, and date point 40 illustrates c 11 ~ X73 treatment at 5 bar, 100°C, for one hour. This shows that just by supeWtnospheric pressure peroxide treatment of pulp with a kappa number below i4 a brightness of ISO 86 or greater can be obtained by a single bleaching stage. However, the starting kappa in this case was only 7.8 and the S peroxide dosage 50 kg. In a commercial mill starting kappa is often higher and the amount of peroxide may be only 10 to 30 kg, so that a more typical mill brightness level after the P-stage is about 82-85.
Typical conditions in the peroxide, superattnospheric, stage 27 are a pH
of about 9-12, a pressure of about 4-10 bar, a temperature of about 80-110°C, and a peroxide dosage of about 10-30 kg H202/ton of pulp. The consistency of the pulp during peroxide trf;atment is preferably about 10-18%. Oxygen may be added to the P-stage to improve its perfornlance or reduce consumption of peroxide.
As indicated in FIG'~URE 1, after the peroxide stage 27 the pulp passes in line 42 to another washer, such as a DD washer 43, where essentially fresh washing liquid is added in :(roes 44, 45, for example a volume of about b-7 cubic meters per ton of pulp in each of the-lines 44, 45. The filtrateJwashing liquor lines 21, 22 go from the post-peroxide bleaching washer 43 to flow in the lines 21, 22, the flow volume in lines 21, 22 also typically being about 6-m3/tp.
The sequence illustrated in FIGURE 1 is suitable for producing pulp having an iS0 88 or greater. However if pulp of about ISO 86 is all that is required, the ozone bleachvlg stage 19 may be replaced by another stage, such as the Q stage 119, illustrated in FIGURE 5. In FIGURE 5 the cooking and oxygen delignification stage are not illustrated, and the components in FIGURE 5 comparable to those in the FIGURE 1 embodiment are shown by the same reference numeral only preceded by a "1". All of the flows, pHs, etc.
are the same as in the FIG1:JRE 1 embodiment except that no ozone is added in the stage 119; rather there its merely a metals dissolving treatment at a pH
below 7 (e.g. pH about 3-6 if chelating agent is used, or about 2.5-3.5 if none is used). The filtrate in line 128 may pass to brown stock washing (as illustrated in FIGURE l) or directly to the black liquor stream in black liquor recovery system 115, ultimately to be evaporated and then burned in a recovery boiler, and treated to recover the chemicals therein.
The invention has been described as a QP sequence to reach a brightness of about iS0 84 and a (ZQ)P sequence to reach a brightness of about ISO 88. In the P-stage it is often advantageous to add oxygen. Other ways to improve the brightness, such as by adding enzymes, etc. may also be used. The brightness may ~be further improved by adding one or several additional bleaching stages. For example with the sequence (ZQKPOXZP) a brightness of over ISO 90 c:an be reached. An additional bleaching stage may also be required for pulps difficult to bleach. The filtrates from the final (ZP) stage may be used instead of pure water as wash liquid for the (PO) stage, so that the addition of a bleaclhing stage utilizing chlorine free bleaching chemicals does not adversely affect the ability to "close" a bleach plant.
It will thus be seen that according to the present invention an extremely simple totally chlorine-free bleaching sequence is proviued, with virtually no waste liquids from the bleach plant. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment it will be apparent to those of ordinary skill in the art that many modification:> may be made thereof within the scope of the invention which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and processes.

Claims (19)

1. A method of total chlorine-free bleaching to a brightness of 82-90 ISO chemical cellulose pulp produced in a pulp mill, including a black liquor recovery system containing fluids at alkaline pH, the pulp having pulp fibers and surrounding liquid and having a kappa number of 14 or below, comprising the steps of:
(a) treating the pulp in a metals-removal stage, at a pH below 7, so that metals in the pulp are freed from pulp fibers and become dissolved in the surrounding liquid;
(b) subjecting the pulp from step (a) to fractional washing, to produce at least first and second wash filtrates, the first filtrate having a substantial amount of dissolved metals therein;
(c) ultimately recirculating the first filtrate to the black liquor recovery system so that the metals precipitate on organics in the black liquor recovery system due to the alkaline pH; and (d) after step (b), peroxide bleaching the pulp at superatmospheric pressure and an alkaline pH, to a brightness of at least ISO 82.

2. A method as recited in claim 1 wherein step (a) is practiced by ozone bleaching the pulp, and wherein step (d) is practiced to a brightness of ISO 84-89, or wherein step (a) is practiced in a Q stage.

3. A method as recited in claim 2 wherein step (b) is practiced to provide over 50% of the dissolved metals in the first filtrate.

4. A method as recited in claim 2 wherein step (b) is practiced to provide about 70-90% of the dissolved metals in the first filtrate.

5. A method as recited in claim 3 comprising the further step, immediately before step (a), of (e) washing the pulp; wherein step (b) is practiced to provide a third filtrate, the second and third filtrates used in step (e).

6. A method as recited in claim 5 wherein step (b) is further practiced to provide as each of the first through third filtrates a flow volume of 0.5-7 cubic meters per ton of pulp.

7. A method as recited in claim 2 wherein step (b) is practiced by adding acid to wash liquid used in step (b) so that the first filtrate has a pH of about 3 or less.

8. A method as recited in claim 2 wherein step (b) is practiced by adding acid and chelating agent to the wash liquid used in step (b) so that the first filtrate has a pH of about 4-6.

9. A method as recited in claim 1 comprising the further step of adding magnesium to the pulp while it has an acidic pH just prior to the practice of step (d).

10. A method as recited in claim 1 wherein step (b) is practiced to provide about 70-90% of the dissolved metals in the first filtrate.

11. A method as recited in claim 10 comprising the further step, immediately before step (a), of (e) washing the pulp; wherein step (b) is practiced to provide a third filtrate, the second and third filtrates used in step (e) as washing liquid.

12. A method as recited in claim 11 wherein step (b) is further practiced to provide as each of the first through third filtrates a flow volume of 0.5-7 cubic meters per ton of pulp.

13. A method as recited in claim 2 wherein step (a) is an ozone bleaching step; and wherein said ozone and pressurized peroxide steps are the only bleaching steps.

14. A method as recited in claim 1 wherein said pressurized peroxide step is the only bleaching step.

15. A method as recited in claim 1 wherein step (a) is practiced by ozone bleaching the pulp and after step (d) there is at least one additional bleaching step using chlorine free bleaching chemicals to reach a final brightness of 89 ISO or above.

16. A method as recited in claim 1 wherein step (d) is practiced at a pressure of about 4-10 bar, a temperature of about 80-110 degrees C, a pH
of about 9-12, and a peroxide dosage of about 10-30 kg of peroxide per ton of pulp.

17. A method as recited in claim 1 wherein oxygen is added during the practice of step (d).

18. A method as recited in claim 1 wherein said step (a) comprises a first and only acid stage.

19. A method of total chlorine-free bleaching to a brightness of 82-90 ISO chemical cellulose pulp produced in a pulp mill, including a black liquor recovery system containing fluids at alkaline pH, the pulp having pulp fibers and surrounding liquid and having a kappa number of 14 or below, comprising the steps of:

(a) treating the pulp in a metals-removal stage, at a pH below 7, so that metals in the pulp are freed from pulp fibers and become dissolved in the surrounding liquid;
(b) subjecting the pulp from step (a) to fractional washing, to produce at least first and second wash filtrates, the first filtrate having a substantial amount of dissolved metals therein;
(c) adjusting the pH of the first filtrate so that it is greater than 9 so that the metals precipitate out, filtering the precipitated metals out of the pH-adjusted filtrate, and then using the thus greatly reduced metals content filtrate elsewhere in the pulp mill; and (d) after step (b), peroxide bleaching the pulp at superatmospheric pressure and an alkaline pH, to a brightness of at least ISO 82.