EP0884415B1

EP0884415B1 - Method of washing pulp in connection with bleaching

Info

Publication number: EP0884415B1
Application number: EP98113684A
Authority: EP
Inventors: Kaj Henricson; Bertil Stromberg
Original assignee: Andritz Oy
Current assignee: Andritz Oy
Priority date: 1993-03-03
Filing date: 1993-05-25
Publication date: 2002-11-20
Anticipated expiration: 2013-05-25
Also published as: FI93232C; FI954093A; ES2186948T3; PT884415E; RU2126471C1; WO1994020673A1; CA2157109A1; NO953444D0; EP0687321A1; DE69332508T2; EP0687321B1; FI930954A; FI934036A; ES2130260T3; ATE228181T1; BR9307858A; EP0884415A1; FI93232B; AU5370594A; FI930954A0

Abstract

The present invention is directed to a method of washing pulp in connection with bleaching, in which method the pulp is bleached in a first bleaching arrangement, washed in a first washer, bleached in a second bleaching arrangement, and washed in a second washer in such a manner that the washing liquid is recirculated from said second washer to said first washer countercurrent to the pulp flow, characterized in introducing washing liquid to said second washer for washing the pulp coming from said second bleaching arrangement, displacing with said washing liquid at least two different filtrates, collecting said at least two different filtrates separately, and using said at least two filtrates separately upstream of said second washer as washing liquids.

Description

The present invention relates to a method of washing pulp in connection with bleaching. The invention is especially related to ozone bleaching of pulp without preceding removal of heavy metals.
Different regulations and marketing requirements have set higher and higher demands on producers of kraft pulp to decrease or completely eliminate organic chlorine compounds in pulp products and effluents from bleaching. In order to be able to fulfill these demands the use of chlorine gas or any other compound containing chlorine (e.g. chlorine dioxide) should be avoided. When avoiding the use of chlorine-based bleaching chemicals, it is extremely difficult to obtain the desired brightness especially if the manufactured pulp has been given acceptable strength requirements. Consequently, lignin must be removed, for example, with oxygen. By using multi-stage oxygen delignification advantages have been achieved in delignification and in selectivity, especially when chelating is added to limit the amount of harmful metallic ions, and especially when between stage washing is included in the process (see US-A-4,946,556). Practical hindrances, however, restrict both the delignification and the quality of the pulp produced merely by oxygen delignification, especially if followed by an ozone bleaching stage.
So far a common and typical purpose of chlorine-free bleaching methods has been to remove heavy metals from the pulp as completely as possible prior to ozone treatment, since heavy metals are known to destroy ozone as taught by EP-A-0 512 590. Typical bleaching sequences by which pulp has earlier been bleached, are, for example, OOAZEZPZ, OAZEZPZ, OOAZEZP and OAZEZP. These sequences thus include one or more oxygen bleaching stages (O), an A-stage (acid washing), an ozone stage (Z), an extraction stage (E), a second ozone stage (Z) and a peroxide stage (P), and possibly a third ozone stage (Z). In the acid stage (A) prior to the first ozone stage heavy metals are removed, which are flushed away, when a portion of the wash filtrate is removed. The extraction stage (E) may be an oxidizing peroxide extraction stage or a conventional oxidizing extraction stage. The ozone bleaching stages are preferably carried out with pulp having the consistency of about 5-18%.
It is characteristic of the above mentioned sequences that they include at least five washing stages, in other words bleaching stages alternate with washing stages, i.e. washers, by which the chemicals separated from the fibers as reaction products or otherwise in each bleaching stage are removed from the suspension. Since the washers form a considerable part of the investment costs in a bleaching plant, the number of the washers should, of course, be limited as much as possible, if it is only possible without risking the quality of the final product.
In the same connection chemicals necessary for some bleaching reactions, such as magnesium (Mg), can also be removed from the pulp, which requires addition of magnesium subsequent to the ozone treatment.
Other multi-stage bleaching sequences have been described for instance in EP patent applications EP-A-0 426 652, EP-A-0 512 978 and EP-A-0 402 335.
EP-A-0 426 652 discloses several bleaching sequences where ozone has been used in combination with peroxide and oxygen. For instance, sequences like EOP-Z-PE-Z-PE, EOP-Z-PE, O-Z-EO-Z-P, O-Z-EOP-Z-P and O-Z-EO-Z-D were discussed. However, the publication does not take into account the presence of heavy metals in the pulp in spite of the fact that at least one of the examples discusses bleaching of kraft pulp by means of a peroxide containing compound.
EP-A-0 512 978 discloses a multi-stage bleaching process using both ozone and peroxide for bleaching pulp. The document does not pay any attention to the removal of heavy metals from pulp before bleaching with peroxide.
EP-A-0 402 335 discloses a process for bleaching lignocellulose-containing pulps by means of peroxide containing substance. The main teaching of the document is to alter the trace metal profile of the pulp by means of a separate treatment with a complexing agent. In other words, the document teaches the importance of treating pulp prior to a peroxide stage in a separate metal removing stage. The document further teaches that the treatment comprises a washing stage between the treatment with complex formers and the peroxide stage.
US-A-4 310 384 describes ozone bleaching with stages before and after it. The pulp is washed after each stage with counterflow of the wash filtrate.
On the other hand, the previously used chlorine has also prevented a screening stage subsequent to bleaching from being combined with the bleaching plant, because in chlorine bleaching the screening and/or vortex cleaning treatment of pulp would lead to serious corrosion problems. Thus it has been necessary to carry out the vortex cleaning and/or screening as a separate stage. A conventional bleaching + screening plant has thus included four or five bleaching stages and one screening and/or vortex cleaning stage, which the washing stage separates from each other. The present invention compresses these five to six stages into three stages and thus almost halves the investment costs of a bleaching plant and a screening plant.
By utilizing the present invention as described in claim 1 it is possible to eliminate the above mentioned disadvantages occuring with the removal of heavy metals and the omission of screening in the prior art technique. At the same time a bleaching plant is provided, which includes only three washing stages.
The method in accordance with the present invention involves that pulp is bleached with a sequence beginning with ozone and without the removal of heavy metals preceding the ozone stage.
It is characteristic of another embodiment of the method in accordance with the present invention that pulp is bleached with a three-stage sequence (ZT) (EOP) (ZP) without an actual removal of heavy metals preceding the sequence, in which

(ZT) refers to a bleaching stage with ozone, which also includes treatment of heavy metals and which stage is followed by washing and/or thickening,
(EOP) refers to a bleaching stage with peroxide or oxygen and peroxide in alkali conditions and the stage is followed by washing and/or thickening,
(ZP) refers to bleaching stage with ozone and peroxide without a between stage washing and which stage is followed by washing and/or thickening.

In each of the stages (ZT), (EOP), and (ZP) there is no interstage washing.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically illustrates a bleaching sequence in accordance with a first exemplary embodiment of the invention;
Fig. 2 schematically illustrates a bleaching sequence in accordance with a second embodiment of the invention;
Fig. 3 schematically illustrates the later part of a bleaching sequence in accordance with a third embodiment of the invention;
Fig. 4 schematically illustrates another bleaching sequence for comparative purposes, and especially portions thereof where acid and/or alkali may be added;
Fig. 5 schematically illustrates a recirculation method for washing filtrates in accordance with another embodiment of the invention;
Fig. 6 schematically illustrates a recirculation method for washing filtrates in accordance with the second embodiment of the invention illustrated in FIGURE 2; and
Fig. 7 schematically illustrates results of a research carried out with a DRUM DISPLACER™ washer.

DETAILED DESCRIPTION OF THE DRAWINGS

According to the embodiment of Fig. 1, a bleaching sequence is provided using a high consistency pulp tower 10, from which pulp is discharged and fed, for example, with an MC® (i.e. fluidizing) pump 12 into an ozone reactor 14, from which pulp is preferably discharged by means of a gas separator 16 into a first reaction tower 18. Pulp is preferably discharged from tower 18 into a washer 22 by means of an MC® pump 20. The washer may be a conventional DRUM DISPLACER™ washer or a conventional pressure diffuser. Pulp is preferably pumped from washer 22 by an MC® pump 24 into an oxygen reactor 26, and from reactor 26 into a second reaction tower 28. From tower 28 pulp is fed, preferably by an MC® pump 30, to a second washer 32 (preferably a DRUM DISPLACER™ washer), from which pulp is further pumped with an MC® pump 34 into a second ozone reactor 36 and further therefrom through a as separator 38 to a third reaction tower 40. From tower 40 pulp is pumped with an MC® pump 42 into a third washer 44 (preferably a DRUM DISPLACER™ washer).
As also seen in Fig. 1, ozone (in a carrier gas) is mixed with pulp by a mixer 60 prior to the first ozone reactor 14. Similarly, oxygen may be mixed prior to the oxygen reactor 26 with mixer 62 and the mixture of ozone and carrier gas prior to the second ozone reactor 36. The mixers 60, 62, 64 are preferably AHLMIXER™ type fluidizing mixers, which are able to mix very large amounts of gas into fiber suspensions, including medium consistency suspensions.
Furthermore Fig. 1 teaches how, in order to adjust the pH value of the pulp for the first ozone stage and the removal of heavy metals subsequent thereto, acid may be supplied into the pulp, for example, in the pump 12. Similarly, prior to the removal of heavy metals in the first reaction tower 18 complex formers, such as EDTA, and/or alkali may be added to the pulp. If too much magnesium is removed from the pulp by washer 22, it may be added, for example, with the alkali either in pump 24 and/or in the discharge from the oxygen reactor 26, or in any other appropriate way. Another possibility to adjust the pH of the pulp for the second ozone treatment is to feed acid subsequent to the second washer 32 in pump 34 or in some other suitable way. Also prior to feeding the pulp into the third reaction tower 38 alkali, peroxide and/or magnesium may be added into the pulp, as illustrated in Fig. 1.
All the reaction towers 18, 28 and 40 in Fig. 1 are shown as of the down flow type. Alternatively, they may be of the up flow type, as is shown in Fig. 2. The only significant difference between Figs. 1 and 2 is the flow direction of the reaction towers. In the Fig. 2 embodiment components functionally equivalent to or the same as the components in the Fig. 1 embodiment are illustrated by the same reference numeral, only preceded by a "1". Also pumps 20, 30 and 42 of Fig. 1 are replaced by pumps 120', 130' and 142' of Fig.2, because they have been relocated at the other side of the reaction tower, in other words instead of feeding washers 22, 32 and 44 as illustrated in Fig. 1 they feed pulp to the reaction towers 118, 128 and 140 in the embodiments of Fig. 2.
In the process in accordance with the invention pulp is cooked, for example with a continuous EMCC digester, sold by Kamyr Inc., of Glens Falls, New York, to a low kappa number, whereafter the kappa number is further decreased by oxygen bleaching to a value of about 14 or below. By utilizing efficient . cooking, hot alkali extraction and oxygen bleaching a kappa range of 14 - 5 is obtained both with soft wood and birch. Usually a kappa number of 14 is sufficient in order to carry out the final bleaching with chlorine-free bleaching chemicals and yet reach a full brightness defined by ISO 86 (preferably ISO 88). Thus also the method in accordance with the present invention may be utilized succesfully to finally bleach pulp prebleached to kappa number 14.
The kraft pulp is bleached subsequent to the pulp washing and according to the invention without a separate removal of heavy metals, for example, with a sequence in accordance with Fig. 1, which is described more in detail below. Pulp may be treated, if so required, with enzymes prior to the sequence in accordance with the present invention. Pulp is brought from the high consistency pulp tower 10 to the first bleaching stage, which is a (ZT) stage. In the Z portion of the (ZT) stage pulp is bleached with ozone, the dosing being about 2-10 kg/adt, pH about 2- 7, and temperature about 40-70°C. The pH value of pulp is adjusted by adding acid to the bottom of the high consistency tower 10, pump 12 (or the discharge to pump 12 as seen in Fig. 1), or chemical mixer 60. The ozone having reacted, the residual gas is removed from the pulp preferably in a gas separator 16 and the treatment of heavy metals begins in the first reaction tower 18 in the T portion of the (ZT) stage.
The T portion of the (ZT) stage may be carried out, for example, in the following ways. The first alternative is to allow the pH value of the pulp to decrease to the range of 2 - 4, whereby the majority of the heavy metals are dissolved into the filtrate phase and may be washed off in the thickener or washer 22 following that stage. The disadvantage here is that the majority of the magnesium (Mg) is also discharged, so that it is possible that magnesium must be added to the pulp, mostly in the form of magnesium sulphate, for the oxygen and/or peroxide stages following later on in the sequence.
Another way to carry out the T portion of the (ZT) stage is to use complex formers, for example, EDTA. The T portion of the (ZT) stage is then carried out in the pH range of about 4-7 and it is advantageous also to have the pH of Z portion of the (ZT) stage preceding T portion above 4. In this way, manganese (which is harmful in the oxygen stages) may be washed off without the magnesium being discharged, so that less magnesium addition (or even no magnesium addition) is necessary in the oxygen and peroxide stages of the final bleaching.
As is described above, the actual metal treatment (T portion of the (ZT) stage) is not carried out prior to the Z portion. Previously this has been considered necessary. Tests with medium consistency ozone treatment have shown that the bleaching reactions with ozone are so rapid that heavy metals do not have time to destroy any significant amount of ozone.
If so required also enzymes may be added in the T portion of the (ZT) stage. Filtrate S₁ of the washer 22 subsequent to the (ZT) stage may be brought to pulp washing prior to the (ZT) stage, or passed to the sewer or to the recovery of cooking chemicals.
The (ZT) stage is followed in Fig. 1 by an (EOP) stage. In this stage the oxygen dose is about 2-6 kg/adt and the peroxide dose about 10-20 kg/adt. In some special cases it is possible to run the process completely without oxygen. Temperature in the (EOP) stage is about 60-95°C, pH about 9-12, and the duration is about 2-8 hours. If required, magnesium may be added as a protective chemical. The (EOP) stage is followed by washing, which gives filtrate S₂. The filtrate S₂ may be taken to pulp washing prior to or subsequent to the (ZT) stage, to sewer, or to recovery of chemicals.
The (EOP) stage is followed in Fig. 1 by a second ozone bleaching stage, i.e. an (ZP) stage. The ozone portion of the (ZP) stage is normally carried out in the processes in accordance with the prior art in cold, acid conditions in order to have the ozone react properly. Correspondingly, the P portion of the (ZP) stage is carried out according to the teachings of the prior art in hot, alkali conditions in order to have the peroxide react properly. Thus the combination thereof in an economically advantageous way according to the present concepts is conventionally considered impossible. However, it is possible to utilize the following conditions in the (ZP) stage:
In the Z portion of the (ZP) stage the ozone dose is small, below 3 kg/adt and the purpose of the ozone is only to activate. Although disadvantageous conditions are used and a part of the ozone reacts poorly, this is insignificant, because the dose is small. Ozone is thus mainly used for the activation of the bleaching stage. The temperature in the ozone stage may be 50-80°C, preferably, for example, 60-70°C. The pH is 4-10, preferably about 6- 10.
The dose in the P portion of the (ZP) stage is also small, usually less than 10 kg/adt. Normally about 3-7 kg/adt is sufficient. Thus the temperature in the peroxide stage may be dropped to the range of 60-80°C, preferably to 70-80°C. The pH is 9-11, preferably about 10. The duration is about 1-6 hours.
Thus the conditions of Z and P portions of the (ZP) stage are brought close to each other and washing and heating between the Z and P portions of the (ZP) stage are avoided. Moreover, small acid and alkali amounts are sufficient for the pH-control in the Z and P portions of the (ZP) stage. In some cases no between stage heating and/or acid is/are required.
After the (ZP) stage the pulp is washed and a filtrate S₃ is obtained. The filtrate S₃ may be used for the washing of pulp in connection with the earlier bleaching stages, discharged to the sewer, or led to the recovery of cooking chemicals.
According to yet another embodiment, shown in Fig. 3, the process in accordance with the present invention is significantly changed by repositioning of the equipment. For example, a vortex cleaner 66 and/or a screening plant may be added according to Fig. 3 to the last stage of the bleaching plant to precede the thickener/washer 68, which in this case does not have to be an MC® washer, as in the earlier embodiments. Pulp is diluted to the consistency range of about 0.5-1.5 % after the P-tower 140, when vortex cleaning or screening with a slotted screen is used. On the other hand, when screening with a perforated screen, a dilution to about 2-4% is usually sufficient. Subsequent to vortex cleaning or screening, the pulp is thickened and washed - usually with a suction filter 68. Previously pulp had to be diluted after washing to a screening consistency and thickened again after screening to a medium consistency.

In the embodiment illustrated in Fig. 4 no EDTA is used, but the removal of metals is carried out with acid in a Z stage and by adding magnesium to the (EOP) and (ZP) stages. The addition may well be done in the (EOP) stage in an MC® pump 224, or in an oxygen mixer 262. This may also well be done in the (ZP) stage in an MC® pump 234, an ozone mixer 264, or in a peroxide mixer 70. The necessary total chemical amounts are given in Table 1. Thus the initial kappa number prior to bleaching is presumed to be 10.

Stage	Chemical consumption adt	Duration min.	Temperature °C
Z	O₃	0.5	2	40 - 50
E	O₂	0.4	60	80 - 90
P	H₂O₂	1.5	180	80 - 90
	MgSO₄	0.4
Z	O₃	0.1	2	70
P	H₂O₂	0.4	180	70
	MgSO₄	0.3

In the practice of the method set forth in Table I, about 20-30 kg of NaOH/adt, and 15-25 kg of H₂SO₄/adt is consumed, depending upon water usage.
In addition to the chemicals of Table 1 bleaching may be intensified by utilizing enzymes. Appropriate places for the enzyme treatment are:

HD-tower 210 prior to the (ZT) stage,
Drop leg 218 subsequent to the Z reactor,
Drop leg 72 between washer 222 and the (EOP) stage,
Drop leg 74 between washer 232 and the (ZP) stage.

The effluent flows from the bleaching plant may be decreased by recirculating the filtrates within the process according to Fig. 5. Figure 5 illustrates an oxygen delignification stage 80, which is followed by a two-stage washing 82. Pulp is transferred from the washing stage to the (ZT) stage 83, and from there via washing 84 to (EOP) stage 85, and from there via washing 86 to the (ZP) stage 87, which is followed by a washing stage 88. The amount of effluent, which is brought to the effluent clarification, discharge channel 90, is 0-5 m3/adt. Part of the effluent may alternatively be transferred to the manufacture of cooking chemicals, via discharge channel 92, to be used instead of fresh water. Thus the amount of effluent that must be treated is minimized.
The reutilization of filtrates illustrated in Fig. 5 may further be intensified by dividing the filtrates from the washer into two fractions with different pH according to Fig. 6. Fig. 6 utilizes the reference numbers of Fig. 5 with a preceding "1". The washers used in Fig. 6 are manufactured and marketed by A. Ahlstrom Corporation, and known as DRUM DISPLACER™ washers. For example, the pulp for the last washer 188 comes from the alkali (ZP) stage. Then the first outwashed filtrate 1881 is clearly alkaline and the filtrate 1882 coming out later is less alkaline or even neutral, because water 1880 flowing to the last washer 188 is generally neutral or slightly acid.
In this way two circulation waters 1881 and 1882 of different pH values are obtained, which may be used to adjust the pH of the pulp appropriate before the bleaching sequence, or a particular stage of the bleaching sequence. Filtrates 1881 and 1882 of the last washer 188 in Fig. 6 are brought to the preceding washer 186 in a way that drops the pH of the pulp prior to the (ZP) stage. Thus acid and alkali are saved in the bleaching plant. In the embodiment of Fig. 6 two alkali filtrates 1861 and 1862 are obtained from the washer 186 preceding the (ZP) stage, which are supplied to the washer 184 preceding the (EOP) stage. According to Fig. 6 preferably acid (H₂SO₄) 1840 is added to the filtrate 1862, whereby the first filtrate obtained from the washer, of which one portion 1841 is led to effluent clarification and the rest 1842 to the washer 182 preceding the (ZT) stage, is acid and the second filtrate 1843 alkaline.
The pH of the filtrates may also be adjusted by adding acid or alkali to them before they are used again. In some cases it may, for example, be necessary to add alkali to the filtrates prior to their being brought to the brown stock washing or it may be necessary to add acid in order to maintain the pH low during the removal of the metals in the (ZT) stage (point 1840). It is possible that heavy metals are thickened again and attach to the fibers, if the pH increases during washing. Thus heavy metals are entrained to the P portion of the (ZP) stage following the Z portion and disturb the peroxide stage. Preferably the pH value is maintained less than 4, or at least maintained at 4, during the removal of heavy metals.

Example:

In an experiment a DRUM DISPLACER™ washer was used to wash the incoming 100°C pulp with 60°C water. The washed pulp was discharged from the washer at a temperature of 65°C. The temperatures of the exiting filtrates were 95°C and 75°C. Consequently, two significantly different filtrates were obtained from one washer. This may be utilized in a bleaching plant to obtain two filtrates having different pH values, because the pH value of the filtrate to some extent correlates to the temperature of the filtrate.
The experiment did not follow the pH value, only the temperature, but based on the temperature distribution the expected pH values are marked in Fig. 7 at respective places.

pH - incoming pulp 10

pH - exiting pulp 7.5

pH - filtrate I 9.5

pH - filtrate II 8.0
Such differences in the pH values between the filtrates are extremely significant when optimizing the chemical consumption of the bleaching plant. The pH values of the filtrates are close to the respective pH values of the entering and exiting pulp, preferably closer to these than to each other.
As may be seen from the above description, a new method has been developed for washing and bleaching pulp with chlorine-free chemicals in a short sequence without the removal of heavy metals preceding the bleaching sequence. The present invention also includes a new method of arranging the screening subsequent to the pulp bleaching in such a way that a separate washing between the last washing stage and screening/vortex cleaning is unnecessary, but only dilution to screening/cleaning consistency is needed.

Claims

A method of washing pulp in connection with bleaching, in which method the pulp is bleached with a sequence beginning with ozone and without the removal of heavy metals preceding the ozone stage (83) in a first bleaching arrangement comprising said ozone stage (83), optionally followed by a stage (85) with peroxide in alkaline conditions, or oxygen, washed in a first washer (84; 184), and optionally washed in a subsequent washer (86; 186) bleached in a second bleaching arrangement (87), and washed in a second washer (88; 188) in such a manner that the washing liquid is recirculated from said second washer (88; 188) to said first washer (84; 184) countercurrent to the pulp flow, introducing washing liquid (1880) to said second washer (88; 188) for washing the pulp coming from said second bleaching arrangement (87), characterized in displacing with said washing liquid at least two different filtrates (1881, 1882) having a different pH, collecting said at least two different filtrates (1881, 1882) separately, and using said at least two filtrates (1881, 1882) separately upstream of said second washer (88; 188) as washing liquids, and to adjust the pH of the pulp.
A method as recited in claim 1, characterized in collecting from said second washer a first filtrate and a second filtrate, and introducing said first and said second filtrates as first and second washing liquids to said first washer.
A method as recited in claim 2, characterized in that said second bleaching arrangement is a combined ZP-stage whereby said first filtrate from said second washer is alkaline and is introduced as a first washing liquid to said first washer.
A method as recited in claim 1, characterized in collecting from said second washer a first filtrate and a second filtrate having different pH, and introducing said first and said second filtrates as second and first washing liquids, i.e. in opposite order to said first washer.
A method as recited in claim 4, characterized in that said second bleaching arrangement is a peroxide bleaching stage whereby a first filtrate collected from said second washer is introduced into said first washer as the second/last washing liquid.
A method as recited in claim 4, characterized in that said second bleaching arrangement is an ozone bleaching stage whereby a first filtrate collected from said second washer is introduced into said first washer as the second/last washing liquid.
A method as recited in claim 1, characterized in having an additional bleaching arrangement (80) and an additional washer (82; 182) upstream of said first bleaching arrangement (83, 85).
A method as recited in claim 7, characterized in receiving at least two different filtrates (1842, 1843) separately from said first washer (184), and introducing said at least two different filtrates (1842, 1843) separately to said additional washer (182) for washing pulp coming from said additional bleaching arrangement (80).
A method as recited in claim 1, characterized in that said washers are single-stage washers.
A method as recited in claim 1, characterized in that said second bleaching arrangement includes a peroxide bleaching tower from where the pulp is introduced into said second washer.
A method as recited in claim 10, characterized in that said second bleaching arrangement includes an ozone bleaching stage connected upstream of said peroxide bleaching tower.
A method as recited in claim 1, characterized in that said first bleaching arrangement includes a peroxide bleaching tower
A method as recited in claim 7, characterized in that said additional bleaching arrangement (80) includes an oxygen bleaching stage.
A method as recited in claim 1, characterized in introducing fresh washing liquid to said second washer.
A method as recited in claim 1, characterized in introducing said at least two filtrates in the same order to said first washer as washing liquids.
A method as recited in claim 1, characterized in introducing said at least two filtrates in an opposite order to said first washer as washing liquids.
A method as recited in claim 1, characterized in adjusting the pH of at least one of said filtrates by means of adding acid or alkali prior to introducing said filtrate as washing liquid to said first washer.