CA1044933A - Process for controlling the supply of liquid in continuously washing suspensions - Google Patents

Abstract

PROCESS FOR CONTROLLING THE SUPPLY OF LIQUID IN CONTINUOUSLY

WASHING SUSPENSIONS

ABSTRACT OF THE DISCLOSURE

A process is provided for controlling the supply of aqueous suspending liquid in continuously washing fibrous suspensions in aqueous suspending liquors containing dissolved impurities, to remove such impurities by ex-changing aqueous suspending liquors substantially free from such impurities for the aqueous suspending liquor, which comprises washing fibrous material of the suspension in aqueous suspending liquid substantially free from dissolved impurities; and forming a washed fibrous suspension in such liquid; withdraw-ing aqueous suspending liquor containing dissolved impurities; diluting the washed fibrous suspension by adding aqueous suspending liquid substantially free from dissolved impurities; measuring the amount of dissolved impurities remaining with the fibrous suspension after the washing has been completed by determining (1) the volumetric flow rate of the washed suspension; (2) the liquid content of the washed suspension; and (3) the content of dissolved impurities in the suspending liquid; and then controlling the volume amount of wash liquid added according to the washing losses to maintain washing losses within a predetermined limiting range.

Description

~Q~ 333 SPECIFICATION
Cellulose pulp is normally washed after separation of the pulping liquor at the conclusion o-E the digestion, before it is passed on to subsequent chemical treatment stages, such as bleaching. The pulping liquor contains substantial 5 quantities of dissolved impurities, which react with treating chemicals, and if these impurities are not removed, or the concentration thereof at least greatly reduced, subsequent chemical treatments applied to the pulp, par-ticularly bleaching, may be relatively inef~ective, because of the consumption of such chemicals ~y the impurities. The impurities therefore not only reduce 10 the bleaching e~ect, but may also require the addition of larger amounts of the treating agents, which are largel~ wasted. Dîssolved impurities present in the pulping liquor after digestion include the pulping chemi~als and the organic substances formed in the course of the pulping process which are water-soluble and become dissolved in the liquor.
The dissolved impurities accordingly accompany the cellulose pulp ,~
suspension, and are removed by the washing.
The impurities are valuable as a source of fuel, and therefore can be burned, utilizing the heat elsewhere in the pulp mill. Inorganic materials which are burned are recov~ered as smelts in the combustion residues, and 20 the smelt can be recycled as a source o pulping chemical values, particularly sulfur and all~ali. The dissolved water-soluble materials present in the pulping liquor and in the suspending liquor for the fibrous cellulose pulp suspension can be collectively referred to as the solids contentof the liquor, and the solids content is normally expressed as a percentage equal to the 25 total quantity of solids materials, i. e., organic and inorganic materials present, divided by the total quantity of pulping liquor.

~V(~4~3~3 Accordingly, the cellulose pulp washing system is designed to remove the dissolvecl impurities, and this is normally done by simply replacing the aqueous suspending liqu~r c~ntaining dissolved impurities with a fresh or relatively pure aqueous suspending liquid, substantially free from such 5 impurities, or at least having a lower colltent thereof than the aqueous suspension from the pulper or digester~
- Cellulose pulp-washing systems are highly specialized, and a special te~minology has been developed to refer to various aspects thereof. Several of the more important and more cornmonl~T encountered terms are defined 10 below:

~riginal black The pulping liquor which serves as a suspending medium liquor for the cellulose pulp in the digester, at the conclusion of the pulping process. This liquor contains dissolved pulping chemicals, and also inorganic and organic material produced as byproducts from the pulping reaction, includ-ing organic water-soluble material dissolved Erom the wood.

E~ecovered black The black liquor which is obtained subsequent to washing liquor or release liquor the pulp and conta~ning the dissolved solids present in the original black liquor. The recovered black liquor is passed to the evaporation stage, where the liquor is concentrated to a heavy blac~ liquor or thick black liquor.

~ashing losses The quantity of original black liquor dissolved solids which ren~ains with the washed cellulose pulp suspension, after the washing has been completed. In Kraft pulping, the washing losses are expressed as kilograms of sodium sulfate per ton of pulp. In sulfite pulping, the washing losses are expressed as kilograms of Na~O or MgO per ton of pulp, depending upon whelther sodium or ma~nesium base pulping liquor is used. In sulfite pulping, the washing losses can also be expressed as the total loss of solids~
including both inorganic and organic materials. The washing losses can also be e~pressed in terms of BOD7 or COD-loss. BOD7 ~measured in accordance with the standard analytical method SCAN-W 5:71) is an abbrevia-tion for biochemical o~ygen demand, ~. e., the consumption of biochemical ~xygen. The analytical procedure de-termines how much ~xygen as 2 the washing ~osses, i. e., the organic portion thereo~, consumes after discharge in the atmosphere after seven days at a temperature of 20C, measured biochemically. COD is an abbre~iation for "chemical oxygen demand'l, and refers to the amount of chemical o~ygen consumed. This determines how much o~ygen as 2 the organic portion and à portion of the inorganic materials consumes when discharged to the atmosphere, and measured chemically.

The washing losses vary according to the pulping process and the analytical technique used to determine it.
The washing loss determination, however made, is a direct measurement of the efficiency of the washing system.
Dilution factor The difference between recovered black liquor and original (DF) black liquor, i. e., the quantity of black liquor in excess of the quantity of original black liquor charged, to obtain the desired washing. Dilution i:actor is often expressed in - termS of t~n- or -~ubic meter of liquid per ton of pulp.
10 - - The term "dilution factor" can only be used in a closed washing system or sub system with four flows only as shown below:
Liquor out Liquo~ in .
<
Process > _~

. _ , Pulp suspension in Pulp suspension out e.g. a process with:
one pulp suspension stream in, one pulp suspension stream out, one iiquor stream in, one liquor stream out.
The definition of dilution factor (DF) applied to this scheme gives:
DF = Liquor out--Liquor in pulp suspension in per unit of pulp.
When making a l~quor balance over the system it can also be shown that:
DF = Liquor in--Liquor in pulp suspension out per unit of pulp~

The washing system described in Figure 1 corresponds to this scheme.
The corresponding numbers in Fi ure 1 are:
= pulp suspension in at 16a = pulp suspension out - 5 13 - liquor in 32 = liquor out.
Tt should be noted that the washing system ends at the doctor blade 16a and that no liquor fro~n the line 21 enters the washing system but is used only to mal~e it possible to determine the amount of liquor in the pulp that lea~es the washing system at 16a.

Fibrous cellulose pulp suspensions are normally washed in one or more washing stages. Usually, three or four washing stages are used. When a multiplicity of washing stages are employed, the stages are arranged m counterflow, i. e., the fresh washing liquid is supplied to the last stage, and then progresses forwardly towards the first washing stage, in series along the line of washing stages. In this way, the washing liquor containing a pro-gressively greater proportion of dissolved impurities is utilized to wash the cellulose pulp fiber suspension containing a progressively lesser proportion of impurities, so that the washing liquor is re-used efficiently from stage to stage. In the final washing stage, the washing liquid, often pure water, can be expected to remove substantially all of the remaining dissolved impurities.
The spent washing liquor containing the impurities dissolved from the starting cellulose pulp suspension is then ccllected, and the solids content can be recovered as desired.
For optimum washing efficiency, it is obviously desirable to carry out the washing with the least possible amount of washing loss, and the least 4~33 ~ossible dilution of the recovered black liquor. The srnaller the washing loss, the cleaner the cellulose pulp, and the greater the proportion of pulping chemicals and organic substances recovered. The least possible dilution is desired because recovery of the dissolved chemicals then requires less energy in removal of the liquid.
Heretofore, there has been no practical method or continuously determining washing losses, so that washing losses can be regulated in a favorable manner. Normally, the washing losses are estimated, based on a sampling of the pulp suspension as it leaves the last washing stage, determiningthe solids content i. e. the content of dissolved organic and/or inorganic material, in the sample of suspending liquor of the washed suspension. Thus, if-a wash filter is used, a sample is taken of the suspension immediately after it lea~es the terminal wash filter in the series.
Normally, the washed pulp suspension has a solids content within the range from about 10 to about 15/c as it leaves the last wash filter stage, which means that the pulp suspension is in the form of a web from which pulp samples can readily be taken. Suspending liquid is squeezed from the sample, and the content of dissolved inorganic materi~1 is determined, in accordance with the standard procedure of SCAN C 30:74.
This test procedure determines analytically the amount of sodium in the sample, and is thus primarily usable in cellulose pulping processes in which a sodium compound is used as the basic pulping chemical. In pulping plants using some other metal compound as the base chemical, such as calcium and magnesium, the analytical method must be modi-Eied so that this metal is determined instead of sodium. In accordance with this method, the washing losses are then expressed as kilograms of sodium sulfate per ton of dry pulp.

From the value obtained, t is possible to obtain an indication of the total washing losses to be expected, i. e., the quantity of organic and inorganic material that is lost with the pulp. If the level of washing losses is found to be acceptable, no remedial measures need be taken. If however the level of 5 washing losses is exceedingly high, the amount of washing liquid charged to the last washing filtration stage is increased.
This method of control is very unreliable, however, since a con ~iderable amount of time elapses between the time when a sample is taken and the time when the amount of dissolved solids has been determined, and too much 10 or too little washing liquid~ may have been used, and the washing losses may have been unduly high, for some time. Moreover, processing of the pulp which passes through before the determination is completed may be inadequate, further down the line.
The sampling o the pulp web i~self leads to inaccuracies. Modern 15 washing machines produce very wide webs which can be nonuniform from one ~! part to another. Moreover, the pulp web is often rewetted just as it is removed from the drum, due to foaming of the liquid within the wash filter. When this happens, the sample has already been taken, and si~lce this liquid has a higher content of dissol~ed organic and inorganic material than the suspending liquid 20 present in the sample, the subsequent analysis will show a lower washing loss than the true value.
Variations in washing losses also can be caused by a number of different factors. For example, the amount of organic material charged together with the pulp to the washing stage may suddenly increase, due to the 25 fact that the quantity of organic material dissolved in the course of the pulping 5t3~
is higher than normal. Moreover, pulps from different pulping stages may be more difficult to wash than others, due to variations in the degree of delignifica-tion of the lignocellulosic material.
High washing losses are of course disad~antageous for many reasons.
5 The chemicals that are lost are economically important, and their loss increases the cost of operation. Hence, it is desirable that the washing losses be kept as low as possible, with only a reasonable dilution ~ctor. High washing losses also lead to problems in the subsequent treatment OI the pulp.
If an excessively large amount of dissolved impurities accompanies the pulp 10 ~- from the washing stage to the screening stage, and then on to the bleaching stage, there may be an unduly high consumption o~ bleaching chemicals and other treating chemicals.
There are two types o screening plants; open screening and relatively closed screening. When the pulp is screened through open screens, 15 large quantities of water are added to dilute the pulp to a concentration suitable for the screening system. When the pulp leaves the screening stage, the pulp is dewatered to a pulp concentration within the range from about 7 to about 15~/c, a major part of the white water obtained being discharged as e~f~uent. A large portion of the washing loss is dissolved in this white water, and therefore is 20 discharged as well, which can increase the pollution problem.
When a relatively closed screening apparatus is used, the washing losses, i. e., the dissolved impurities, to a large extent accompany the pulp to the bleaching stage. Here, more bleaching agent then is used, since the impurities also react with the bleaching agent. Unless the washing losses are 25 checked continuously, it is not possible to anticipate sudden increases in ; washillg losses, alld ~ccause of lhis cous~lml~tioll of l~leaching agent, I)leaclliTIg W~l be inade(3uate uniil addi~ ional l~leac}ling chelllical is charged to the sy~item.
If this SOl't Gf Situal;ion lS to l~e ea~pect~d, then it is necessary, for uniform blcachinD, to charge an excess o bleaching chemical to the bleaching 5 stage, so as to be sure that tlle desired deg~ee oI bleaclling of the pulp is obtainecl even if the washing losses are momentarily high. This naturally increases the consumption of bleaching chemical, and also the amount of bleaching chemical impurities that are discharged at the conclusion of the bleaching stage.
Since there is no method for continuously determinin, washing losses, 10 it has been very difficult for pulp manufacturing plants to maintain the amount of impuritles discharged from the plant below the predetermined minimum set by the en~rironmental control authorities. This naturally can seriously affect the opera-tion of the plant, and can lead to heavy legal fines in the event that the limits imposed are Yiolated, even though this be entirely accidental, and quite l~eyond the control of the mill operation. ~-`
In accordance with the present in~ention, it becomes possible to de~
termine continuously the content of dissolved impurities, i. e. ,the washing losfies P~
in the cellulose pulp suspension lea~ring the washing sy~tem, thereby making; it possible to control the washing losses that are obtained in the washing. The 20 process in accordance with the invention comprises controlling the supply of aqueous suspending liquid in continuously washing fihrous suspensions in aqueous suspending liquors containing dissolved impurities, to remoYe such impurities by exchanging aqueous suspendingr liquors substantially free from such impuri-ties for the aqueous suspending liquor, and comprises washing fibrous material 25 of the suspension in aqueous suspending liquid substantially iree from dissolved '! `~

'.

3~
(, ' '~u~ities, and formi~ a wasl-ed îiblou~; suspellsion in such liquid; u/itllclrawillg aqueous suspending liquor containin~" dissolved impurities; diluting the washed fibrous suspensivn by addillg aqueous suspending liquid substantially free fI om dissolved impurities; measuring the amount of dissolved iml~urities rem~inillg 5 with the fibrous suspension ater the washinD has been completed by cletermining (1) the volumetric flow rate o~ the washed suspension; (2) the liquid content o the washed suspension; and,(3) the content of dissolved impurities in the suspend-ing liquid; and then controlling the volume amount of wash liquid added according - to the washing losses to maintain washing losses within a predetermine~ limiting 10 range.
- The process of the invention is applicable to any kind o~ fibrous cellulose pulp suspension, including chemical pulps, mechanical pulps, chemimechanical pulps, semichemical pUlpS7 and thermomechanical pulps, for example, sulfite pulps, sulfate pulps, and, pulps obta~ned from ~he oxygen alkali pulping of ligno-15 cellulosic material.
Figure 1 shows a washing system for a pulp mill c~pable of procluciogchemical pulp, utilizing wash filters in two stages.
The washing system of Figure 1 receives via line 1 the cellulose pulp directly from the digester, suspended in spent black liquor, containing dissolved 20 impurities, and it is collected in a storage or flow-equaliæing reservoir 2, pro-vided with a stirrer 3 to maintain the suspension uniform. A line 3a at the bottom o~ the reservoir is in flow connection with the filtrate tank 4 receiving washing liquor via line 4a from the interior of the filter drum 6 in the first washing stage W1. The filtrate liquor in the tank 4 contains an appreciable 25 proportion of the same types of dissolved solids present in the black liquor ,, ' ç~n~ering with the pulp via line 1. Filtlate liqlIor Irol~l tanlc 4 enterillg reservoir

2 via line 3a is used to dilute the pulp,aided by the stirrer 3~ and is purnped from the tank 4 by the pump P1 for the purpose. The diluted pulp suspension, thoroughly mi~ed by the stirrer 3, then is led by the lille 3b to the inlet bo~c 5.
The box 5 is in flow communication via line 5a with line 3a and the tar~; 4, andthe pulp suspension can therefore be further diluted with liquor from the tank 4~hile in the inlet box 5.
In normal operation~ the cellulose pulp suspension in black liquor entering the reservoir 2 has a pulp concentration of approximately 12~ fter dilution in t~o stages, first in the reservoir 2 and second in the inlet box 5~.the pulp concentration is reduced to approximately 1%.
The diluted pulp is led from the inlet box 5 by overflow into the trough 6a oE the first washing stage W1. A cylindrical drum 6 of wire mesh is rotated continuously clockwise while partially immersed in the pulp suspension in 7 trough 6a. As is conventional, suction is drawn on the interior of the cylinder 6 by means not shown, ~o that the suspending liquor is drawn through the wire mesh o~ cylinder 6, and the pulp iibers are drawn down onto the surface of the wire mesh, forming a pulp web 6b. The liquor (filtrate) is withdrawn from the interior of the cylinder by the line 4a, ancl passed to the tank 4.
The drum 6, rotating cloclcwise, carries the web 6b of pulp fibers up to and beneath the array of spray nozzles 7, where relatively fresh washing liquor from tank 8 is sprayed onto the pulp web 6b. The liquor is fed to these nozzles via line ~a ana pump P2 from the tank 8, and is the washing liquor from the second washing stage 12. This wash ing liqu~r has been utilized only once, in washing stage W2, and contains an appreciably lower content of dissolved .

4~33 solids than the liquor in tank 4. A part of this liquor also is drawn through the wire mesh of drum 6 by the suct~on, and passes by line 4a into the tank 4.
The washed pulp web is then scraped oEf the wire mesh by the doctor blade 9a, at the entry to the outlet box 9, and the separated pulp is collected 5 in the outlet box 9 in aggregates or clumps of fibers of varying sizes. The outlet box includes a screw conveyor 10, for mixing the particles with liquid from filtrate tank 8. The solids content of the pulp at this stage is from 12 to 18~c Outlet box 9 is in communication via line 9b and pump P3 with the 10 tar~ 8, and liquor from the tar~ 8 is used to dilute the pulp in the bo~ so that a pulp suspension is formed at a pulp concentration of approximately 1~c. This pulp suspension is then passed directly via line 8c to the inlet box 11 o the ~econd washing stage W27 where the pulp suspension is ~ed by overflow into trough 12a and is taken up on the cylindrical wire mesh drum 12 exactly as in 15 the first stage, by application of suction to the interior of the drum.
The liquor drawn through the mesh is brought to the ta~ 8 via the line 8b while a web 12b of pulp is formed on the surface of the drum. The pulp web is carried upwardly by the clockwise rotation of the drum to beneath the array of nozzles 14, which spray liquid thereon, conveyed thereto via 20 line 13. This liquid is normall~r pure water, or a steam condensate obtained at some other treatment stage in the pulp mill, for example condensation of steam from the evaporators in the black liquor recovery system. The wash water from the nozzles 14 is drawn through the pulp web 12b into the interior of the drum 12, and then carried by line 8b to the tar~ 8. The pulp web 12b 25 is brought against the doctor blade 16a at the inlet to the outlet box 16. The ~ licls contellt; of the pulp is from 10 to 1$~c at this sta~e, and the pulp ngain is stri~ped of the ~rum and collected in the outlet box in the form of aggrcgates or clumps of fihers of v~Lrying size. The outlet box 16 includes a conveyor screw 15 for mLxing of the particles with dilution liq-lid via line 22 and valve 5 23 fiom line 21.
If the pulp web is too difficult to dewater, it is not possible to add all the wanted wash water through the nozzles 14. If one tries to do this, the result will be that a larger amount of wash water is transported out o~ the washing system via the pulp web at 16a. In ~rder to improve this situation 10 a portion (less than 50~c) of the wash water added through 13 could be added directly in the inlet box 11 via line lla and valve llb, and serve as diluent.
E:ach of the filtrate tanks ~, 8 is provided with liquid le~el s~nsors 17, 18, the sensor 17 controlling val~re 19 in the washing liquor discharge line 32, via control line 17a, and the sensor 18 controlling val~e 2û in line ~b - 15 leading to the spray nozzles 7, via control line 18a. The recovered washing liquor in line 32, referred to as thin liquor, is passed to an evaporation stage via the line 32, for recovery of dissolved solids therein.
The waslling system employs the counterflow l~rinciple, in which the water from tlle last washing stage W2 is used in sequence up the series of 20 washing sta6es to the first washing stage W1, and then discharged. While only two washing stages are shown, it will be understood that one, two, three or more washing stages of like type can be in~erposed in series~ and in like 't interConnect ion between W1 and W2 .

., 13 ~, -'.

~LO~LgL"333 The process in accordance with the invention to control washing losses is applied to this washing system following the last washing stage, W2.
In order to continuously determine washing losses, it is necessary to measure continuously the following three variables:

5 (a) The volumetric flow rate of the cellulose pulp suspension The flow of pulp can be measured directly, for example, by means of a flowmeter, for example, a magnetic flowmeter; other types of flowmeters can be used ~ `

T l d ntent of the- ulp suspension followin the last washin sta~e.
(b) he ~qul co p g g This parameter is dif~icult to measure. Any method of determining the liquid content o the pulp suspension which provides a reliable result can be used. It is however preferred to apply the method described below.

(c) The amount oE organic and inorganic water-soluble material dissolved in the suspending liquid.
.... .
The content of these materials can be determined analytically using a number of available procedures as described below.
The determination of the liquld content of the pulp suspension (b) can be carx ied out in either of two ways, depending upon whether it is known how much pulp enters the washing system at line 1.
Tn order to supply the correct quantity of washing liquid through the line 13 and thereby correctly control the dilution factor, the liquid content of the pulp, which leaves the washing systern when it is stripped from the filter 12 by the doctor blade 16a, must be determined .

Let il; ~)e assumed that this pulp has a solids content of 12~'~C. rhe p~llp in the outlet bo~ 1~ is then diluted with suspendill~ licluor through line 21 22, via ~alve 23. It has been found most suita~le to dilute the pulp here to a concentration within the range from about 1 to about 10~C, preferably fr~m ~, about 2 to about 5~c - Although th~ dilution can be carr ied out in one ste~, it is suitably carried out in t~o steps, once at the outlet box 8 via lines 21, 22 and again beyond the outlet bo2~ ~ia lines 21, 25 through valve 26. In this case, the first dilution in the outlet box 16 is a rough dilution, without applying precise measurement or control, and can be effected by the operator, using spot 10 judgment, and manual control of the valve 23. After this rough dilution, the diluted pulp is passed via line 2~, past the junction with line 25. Here, more suspending liq~lid is combined with the pulp, ~ut the amount of suspending ïiquid introduced tllrough tlle line 25 is controlled by the valve 26. This valve S
is in turn controlled by a pulp concentration measuring device 27, which auto-15 matically controls the amount of liquid added via line 25, to give the desiredpulp concentration. The pulp concentration in line Z4 normally is approxirnatel~

3~c-The quantity of diluting liquor required to obtain the desired pulpconcentration is measul ed continuously by the flowmeter 28, which is, for 20 example, of magnetic type, in the line 21. The total flow of pulp suspension departing from the system in line 24 is also measured continuously, by the flowmeter 29, which can be of the same type as the flowmeter 28. It should here be noted that nothing of the liquid entering through the line 21 and used for dilution of the washed pulp, is entering the actual washing system. It does not 25 affect the dilution factor of the washing system and is used only to dilute the already washed pulp. Information concerning the amount or volume of flow in lines 21 and 2~ can be collected by the signal converter 30, and th;s information ` ~4~33 together with the pulp concentration is usecl to continuously calcukate the liquid content of the washed pulp, e. g., when the pulp lea~es the last wash ilter 12.With the aid of the signal converter 30, the quantity of washing liquor supplied through the line 13 is then regulated via control valve 31, so as to 5 obtain constant dilution of the pulp.
- If the amount of cellulose pulp (calculated as absolutely dry pulp~
flowing through the washing system is known, for example, by measuring the amount of pulp entering via line 1 into the reservoir 2, there is no nleed to measure the pulp concentration, and the measuring device 27 can be omitted.
There is then a direct relationship between pulp concentration and the total flow o~ suspension in line 24. With a constant Elow of pulp, calculated as absolutelydry pulp, the amount of diluent liquid flowing through the line 21 can be con-trolled directly by the total flow of suspension in line 24, so as to maintain aconstant suspension flow. In th~s alternative approach, the flows in lines 21 and 24 are continuously measured b~ the flowmeters 28 and 29, as before.
The quantity of li~uid accompanying the washed pulp Erom the washing process can be calculated as follows:
Let:
Q = the total volume flow per unit of time.
Q24 = the total volume of suspension flow per unit of time in the line 24.
V = the totai liquid volume flow per unit of time.
V2l = the total liquid volume flow of diluent through line 21.
V2,l = the total liquid volume flow per unit of time through line 24.
y = the liquid content of the pulp when the pulp leaves the last pulp washing stage (at 16a~-m = the concentration of the pulp suspension in the line 24.

9L9~3 The pulp concentration measuring device 27 controls the flow of diluent V21, so that the conce~tration of the pulp suspension in line 24 has the specific value rn. The value of m is kr~wn from the ~pulp concentration meter 2~, and is normally 3~/c, but it can vary from 1 to 10'3~ as indicated previously.
5 By measuring the suspension flow Q24~ the flow of cellulose fibers (calculated as absolutely dry pulp) can be calculated as m x Q2'1' the liquid volume flow in line 24 V2, is then equal to:

~24 = Qa4 - m~ Q24 = ~l-m) Q2~
- When a liquid balance is established, the following reLationship is 10 Dbtained:
Vpulp~} Val - ~Ta~

pUlp ~Tæ~--V2l VpUlp ~ (1--m)Qæ~--V21 where m is known from the pulp-concentration meter 27 - - 15 Q2.~ iS measured in flowme~er 29 V'21 iS measured in flowmeter 28 Thus, Vpulp can be calculated and followed continuously.
As previously mentioned, the flow of cellulose fibers = m x Q2~1 In this way, information is obtained as to the quantity of liquid per quantit~ of cellulose fiber leaving the washing system with the washed pulp.
When the production of cellulose pulp is known, for example by means ~f measurement upstream of the washing system, it is not necessary to measure the concentration o~ the pulp, as previously mentioned, and consequently no flowmeter 27 need be provided.
The amount of li~uid accompanying the pulp from the washing filter 12 can be calculated in the following manner:

3~
-- - ~ V2l and Q~, ar e measur ed V2,~ 2~i - pul},~ productioll i The pulp protluction is expressed as unit volume/unit time as previo~sly shown;
VpUlp= ~T24-V21~ i-e- ~-~pulp = (Q2~--pulp p~oduction)--V2l p In order to measure the am~unt of inorganic and organic dissolved impurities tc) in the suspending liquid, these materials are reacted with an oxidizing chemical. Examples of oxidizing chemicals which can be used include hypochlorous acid, chlorine, for example, chlorine water, sodium or potassium hypochlorite, chlorine dioxide, hydrogen pero~ide, sodium and ~-potassium bichromate. The preferred oxidizing chemical is hypochlorous acid ~XOCI.
Tlle amount of lnorganic and organic substances present also c`an be ,.
measured directly without addition of an oxidizing chemical by using ion~
selective electrodes, photometry, flame photornetry, conductometry, or i~
density measuring techniques. In a preferred method there is added to the sample of liquid an excess of aqueous hypochlorous acid solution. A specified period of time is allowed to elapse, and then the excess remaining hypochlorous 20 acid determined analytically by means of iodotitration, polographic measure-ment, redox potential measurement, photometry, colorimetry, or similar processes.
-~ The most suitable analytical method is one in which the liquid sample is mixed with an excess of aqueous hypochlorous acid solution, and the amount of25 heat developed measured calorimetrically. Surprisingly, it has been found -~
',

4~33 ~hat when hypochlorous acid is used at a pH o 5. 5, the amount of precipitate and problems colmected therewith are much less7 thereby ~ro~iding more reliable results, than when using all~aline hypochlorite at a pH of 10.
Hypochlorous acid and the other oxidizing chemicals reEerred to above react with the organic substances present in the suspending liquid and with thatpart of the inorganic materials present in the form of sulfate and thiosul:fate.The remaining inorganic substances, probably the predominant portion, do not react with hypochlorous acid.
Accordingly, it is important to note that in the analytical techniques described above7- no in~ormation-is obtained regarding total washing losses, since this is not susceptible of determination analytically. Analysis according to the standardized procedure of SCAN C 30: 7~ will provide information solel relatin~r to those imp~ucities bound to sodium.
These analyses to some extent overlap one another, and therefore it is not possible to add the results of one analysis to the results of another andobtairl a total for the washing losses. On the other hand, it is possible for one to estimate approximately, with the aid of the analytical techniques, the total washing losses that can be expected from the values obtained according to the present invention, or according to the analytical procedure of SCAN C
30:73-Calorimetric techniques which can be used include those described inHultmanpatentNo, 3,888,726, patentedJunelO, 19~5, whichappliesthis technlque to the control of pulping chemicals added in the delignificatinn and/or bleaching of cellulose pulp.
The analytical methods described above also can be used to determine the solids content oi the thin liquor or filtrate liquor recovered subsequent to f ~ 3~ .
ast washi1lg si~ge, beEore i~ is S~llt on to the e~aporatic)n plant. Il1e sa1llemethod can also be uscd to analyze thick liquor subsequent to evaporation of the thill liquor and prior to c1larg;ng thick liquor to the co1nbustors. In thisway, it is possihle to estimate the fuel value of the thick liquor7 which may beof interest.
The manner and stagre at which the content of impurities of the suspend-ing liquid is determined is shown in ~. A filtered liquid sample of washed suspension lig.uid is taken continuously from line 24 via line 34 to a - continuously operating analyzer 35, for example, a calorimeter, where the con-tent of dissolved impurities is measured. Since the flowmeter 29 measures the tQtal flow o~ suspension in line 24, this is known, and is designated Q2L.
If the pulp production is known, then the amount of liquid in line 24, designated V2~" is as follows:
V2~, = Q~,--pulp.
When the pulp concentxation m is measured and re~llatec1 by means of the pulp concentration meter ~7, the amount of liquid in the line 24 is representedby the equation:
V2~ = Q2~ x (l--rn) The total outflow of dissolved impurities i.e., the washing losses, ao is then obtalned by multiplying the amo~mt of liquid by the content of dissolved impurities. This is carried out continuously in a computer 3q. If the diluent ~n line 21 and the washing liquid in line 13 is pure water, th~ washlng losses, i. e., the dissolved impurities remaining with the pulp due to incomplete wash-ing, will be equal to the quantity of dissolved impurities. In reality, however,~5 the washing liquid in line 2l is not water, but a liquid which is contaminated , .

' ~- ~ h ~oth orgal-ic an~l inor~anic material. Pure w~ter i~ normall~ used as tl1e ~vashi1lg liquid in line 13~ although it is also possible here as well to utili7.e a liquid which contains small amounts of organic and/or inorgal1ic material.
- Because o this, it is necessary to analyze the dissolved impurities cGntent of

5 the diluent and the washing liquid, when the washing liquid is not pure water. I
Figure 1 shows only the set-up for an analysis of the diluent. In carrying ou~ the analysis, a sample ~E liquid is continuously taken from the line 21 and passed through the line 33 to the continuously operating analyzer 35, -- - which is in the form of a calorin~eter, and in which the content of dissolYed 10 impurities is determined- As mentioned above, the flowmeter 28 measures the total liqnid flow in line 21, and this Elow is designated as V2l. The total amount of impurities introduced through line 21 is V2l multiplied by the sample content of dissolved impurities. The calculation is made continuously in the computer 36. When the washing liquid is pure water, the washing losses of 15 the pulp, designated Tm is equal to the quantity of impurities in the line 24, designated T24, minus the quantity of impurities in the line 21, designated T2l, divided by the pulp production P. Tn other words:
T = T2~p-T~

This calculation can be made continuously, using the computers 30, 36 20 and 37. A signal can then be sent to control the supply of washlng liquid to the washing stage W2, through the regulating valve 31.
When the washing liquid in line 13 contains dissolved impurities~
correspondin~ measurements are made; the flow in line 13 and the dissolved irnpurities content of the liquid are determined continuously. In this case, the 25 washing losses of the pulp are shown in the following equation:

`~
.' ' ' x~

() Tm_ 'r2.1--(--~Tl3) Thus, in accord~-nce ~Yith the process of the inYelltiOn, it is possible continuc~lsly to n~onitor tlle washing losses and to mal~e adj~lstments in the amount of washin~ uid added on the basis of these determinations. Thus, 5 fvr the first time it is possible to wash the pulp both economically and In ~
manner to minimize the discharge of undesirable impurities from the plant.
If, for example, the washing losses are undesirably high, the amount of fresh suspending liquid or washing liquid charged to the system via line 13 - is increased until the washing losses have been reduced to a desired level, i. e., 10 a level at which the washing losses can be tolerated, both from the standpoint of the recovery of chemicals and the discharge of waste ch~micals.
If the washing losses are unclesirably low, the supply OI fresh suspend-ing liquid to the system via line 13 is reduced until the washing losses have been increased to the c~esired value, taking into account the cost of evaporation of - , ~
15 liquids, due to excessive dilution, and the capacity of the washing system. In principle, it is desirable to maintain the washing losses as low as possible with respect to environmental problems and the purity of the manuEactured pulp, v while a~oiding excessively hi@h production costs, due to an excessively low washing loss.
The following Examples in the opinion OI the inventors represent preferred embodiments of the invention.
EXAMPLES 1 to 15 The process of the invention was applied in a washing system similar to that shown in _gure 1, but utilizing four wash filters of the type shown, in series. The system was then applied to the washing of birch Kraft pulp.

The pulp production was measured continuously in kilograms/minute upstr~ ~rn of tl~e washillg system, and there~ore no pul~) concentration m~ter 27 was used.
The concer~tratiol- oi the washed p~llp suspension leaving the last washillv stage in the series ~aried bet~veen 10 and 15/c during the test period.
This pulp suspension was diluted with white water from the screening system to a pulp concentration ranging from 3 to 4c/c during the test. The quantity of diluent in the line 21 was measured with the flowmeter 28, and information concerning the quantity of diluent V2l was registered continuously on a recorder.
In a similar manner, the flow of pulp suspensinn in line 24 was measured continuously by the flowmeter 29, so as to record Q2g. - Y^
In order to determine the quantity oE dissolved impurities in the pulp suspending liquid, a flow of suspending liquid was taken from line 24 and passed vla line 34 to the calorimeter 35. The flow of liquid was taken off through a filter placed in line 24, so that no cellulose fibers were present. Since the diluellt 5 comprised white water from the screening system, this system also contained small amounts of dissolved impurities. Because of this, a stream of liquid was t~en ., from the line 21 and passed through a line 33 to the calorilneter 35 as well.
.
The liquid samples were passed continuously througrh the calorimeter, which had two cells. In the calorimeter each liquid sample was ml~ecl with 0 an aqueous solution of hypochlorous acid HOCl having a concentration of 5 g/liter, calculated as active chlorine. Distilled water was used as a reference solution. The reaction loops in the respective cells were sufficiently long, that the samples had a residence time of l minute 20 seconds in the cell. The heat generated by the reaction of dissolved impurities in the liquid with the hypo- .
~,.
chlorous acid was con~rerted using a thermopile to an electric signal reg,istered .~
!:

,.

f"``
~- as mLllivolts continuous]y on a recorder.
Tcsts were also macle in which the liquid samplcs ~vere diluted with water tu,o or fi~e l~ es. Identical analytical results were obtained in all series.
Thus, two signals were ohtained from the calorimeter, one f-rom each liquid 5 sample. These signals in millivolts were found to be proportional to the amount of dissolved impurities present in the liquid, according to the following formula:

C- O.90g~ 0.10 where X equals the si~nal on the calorimeter in millivolts; and C equals the amount of dissolved impurities in the liquid, correspondin~
to the amount of oxygen consumed in grams/liter.
Wlth the aid of this formula, it is possible to calculate the ~luantity oE
dissolved impurit~es in the diluent, designated C2l, and in the pulp suspending 15 Iiquid, designated C2~. Since the flow of diluent V2l and the flow of pulp SUS-pension ~22~ were known, it was possible to calculate continuously the washing losses of the pulp, designated as Tm~ calculated in kilograms of oxygen con-sumed per ton of pulp, in accordance with the formula:
(Q2 ~--pulp production) x C~--V2 1 "
Tm ~pulp production ~0 Thus, ~n this way it is possible to calculate and record the washing losses of the pulp in a continuous manner.
During the test period, as a control, samples of the pulp suspension were also taken manually, just before the pulp web was removed from the filter drum 12. These pulp samples were analyzed for sodium in accordance 25 with SCAN C 30:73, and calculated as kilo~rams of sodium sulfate per ton of pulp.

~ .

4~
The samples were also analyzed for Ghemical oxygell demand COD
of the liquicl, according lo the methocl devis~d 'i)~7 It~dustrins Vaiten och LuEtv~rd Aktiebolag, based on ASI~ Test Designation D 1~62-60. In briefJ this l~leth~d requires reacting the dissolved impul~ities of the sample liquid with 0.250 N
5 potassium bichromate solution, K2Cr20q. This analysis gives information concerning the content of organic substances of the liquid sample, and also - the sulfides part of the inorganic substances. The content of solubilizec~
impurities is given as COD in grams of oxygen/liter, i. e., the amount of oxygen the substance will consume in order to be completely oxidi~ed.
10These two analyses were also applied to samples of the diluent.
The times at which the pulp samples were taken were noted, and the results obtained at the corresponding points of time in comparison with the results obtained using the method in accordance with the invention are ~ .
apparent from the following Table.

~ .
F~

, .

' ~

3)3 ~o ~
U~ ~ ~ r~ r~ r-~ ~ r~ J r~

_~ . . ~

d ~ c~o CD O ~ ~ CD O ~ O _ u~ O d~
r-l C';l r-l C~ ~ C`3 ~--1 C`J ~I C~ r-l r~ r~ cCi v Oi ci ci o c; o o o o ci ci c~ c ~ o 5~ . ' ~`
~ . ~ , , '~ ~ > ~ ~ OD
~ ~ _~ O r~ ~ ~1 ~ O r-~ O Lt~
i~:1 V oc3ooc;,oooo~ooooo ~ . . .... ..
~ ~3 ~ O 1~ O 0~ IO O O C~
'~t . " ~.
t'. ~

.~ ~ r~ t~ > C~ ' O a~ C`J C0 ~ a~ tN CC~ ~IC01~ r-i CD 00 U~ CD 0~
W 'U~ V O O Or;r; OC;r; O ~i 0 C; Ci O ~
~ t~ _ ', ,~ t ~3 ~ _ c--c~c~ O r~ O O d~ u~ C J u~ r-~
O '~ 1~ 1~15~ U~ ~1CO1~C~ ~D cr~ 1~ L~ 00 CD CO

C V Ci O O~i~--i . O C; r; tO _i C; O C; r~l , ~:1 ~r~
,a Q~ d . .
.. ~ c~ C~ O C~U~ ~ O U~ tP~
h ~3 . . ~ . . . . . . . .
.: ~ e ~q a~ O O ~ ~ ~ o~cn co co O O Q cr~
~3 o ~3 o o ~ o o o o o o c~ o o o o c~
~D tC~ O O O ' t~c~ CD t~ tf~
~ ~ C~ C~t C~ tP~ t r~

,a~

~i ,G ~ c~
r~ ~u~ t0~ r~

2~

33~
~- Cor~espondl~ results obtailled willl SC~N C 30: 70 manual analysis are given in Ta~)le 11:

Suspending Liquor from Pulp Sus~ension _ l~iluerlt __ Washin~ Losses Example COD Na~SO,~ COD N~ SO,~COD Na2SO~
No. g O2/l g71 g o~/l g,~l~g 2/ kg/ ton pulp __n pulp 1 . 0. 61 0. 65 0. 18 0. 19 13 13 2 0. 57 0. 60 0. 26 0. 28 13 13 3 0.57 0.63 0.02 0.02 19 20 1. 31 1. 50 0. 73 0. 8~ 18 19 1.17 1,31 0.4D~ 0.52 22 25

6 0. 90 0. 99 0. 27 0. 30 22 ~
q 0. 73 0. 80 0. 09 0. 10 18 20 8 1. 19 1. 31 0. 25 0. 26 25 28 9 O. 68 0~ 75 0. 08 0. 10 16 î~ ~
1. ~3 2. 10 0. 71 0. 82 31 35 ~3 11 0. 58 0. 63 0. 15 0. 17 14 15 ~ ' 12 0. 60 0. 66 0. 10 0. 11 ~6 lq 13 0. 89 0. 98 0. 25 0. 28 20 24 14 0. 72 0. q9 0. 12 0. 13 ~8 21 1. 28 1. ~2 0. 62 0. 68 25 29 - In Table 11 the pulp production and volurnetric rates of flo~v have not 25 been given, since these figures have already been given in Table 1. When the washing losses recorded in Table I are compared with corresponding values in Table n, it will be seen that there is an extremely good correlation between ' f~
~- ti~ ~alues shown in Table I and ol~t~ined in accor(lance ~vith the inYelltioll7 and the values shown in Tal~le II ~btained by analyses in accordance with SC~N C
30: 73.
As previously describecl, the preierred analytical reagent used to 5 determine the amount of solubilized substance ill the flows of sample liquid in the method according to the invention was hypochlorous acid, while the reagent used to det~rmine the amount of solubilized impurities (i. e. the organic substance and part of the inorganic substance) in the licluid ~amples taken manually was potassium bichromate. These two reagents have been ~und to 10 provide good results, with very good correlation. When making the analysis using calorimetric determinations, hypochlorous acid is to be preerred, however, for technical reasons .
It is possible to perform a regression analysis in which the calori-metric value in millivolts tmV) is plotted against the COD-value according to 15 tbe IVL-method in gram 2 per liter. With this analysis, the previously mentioned relationship:
- . ~rr c=0.90~ -~ 0.10 was obta~ined. Thus, in this way it has been possible to convert the indication in millivolts to an indication of the content of consuming chemical oxygen 20 substances in the suspending liquid. Tn addition, it is possible to compare the washing losses, i. e. the quantity of solubilized impurities, rneasured as COD
according to the IVL-method, with the washlntr losses measure~ as Na2SO4 according to the SCAN C 30:73 method. Such a comparison will show that the relationship between the q~lantity of Na given as Na2SO4 in grarns per liter or ~5 kilogram per ton of pulp, and COD given as grams oxygen per liter is approxi-mately 1:1. This relationship is empirical, and while it is applicable to 3~3 sulphate-pulp manufacturing plants"t may require some modification for special conditions in a gi~en plant.
The washing losses measured in accordance with the invention can be converted by means of the factor 1:1, however, and given in kilograms Na2SOg 5 per ton of pulp. When this is done, the follo~ing values are obt~ined:
TABLE lll Washing losses Example No. kg Na2SO ton_pulp .

7 20 3~

2rl As will be e~ident from the aforegoing, it is possible by using the in~rention to continuously determine the washing losses of a washed sulphate 25 cellulose pulp after washing of the pulp. The values obtained can be used to control the washing losses to the desired level.

~4~ 33 EXAMPLES 16 to 21 Tn a sulphite-pulp manufacturing plant, the cellulose pulp suspension in black liquor was washed in a number of vessels. Subsequent to washing the pulp suspension was passed to a retention vessel~ from which the pulp was 5 pumped continuously to a terminal washing filterO The measuring apparatus shown and described in Figure 1 were i~corporated, downstream o~ the terminal washing filter. In other respects, the same procedural steps as in E~ample l were carried out, with the one exception that the pulp being washed was sulphite pulp. The values obtained are given in Tables IV, V and Vl.

n5 ~ a3;~ , b~. . t~
O ~ CO O O
c~) ~ i.~ N N C~l C~ ~ o ~

e . V b~ ' , : . ' r.

!
.. . ~ ~ ~ , o o ~ o o ~ I br, . ~ ~

~ ~ ~ f- ~
~ , . .
o ," ,~, ::5 U ~D o o o o o o . ~ 5 ~ ~ O C~
O h ~ c~ c~ C~
~ 0~, O O O O O O
'Ei ~ ~.,, ,~ ~ ~ C~
s:4 E~ ~ c~ o o o ,`~
.', ' ~ f.~ 3 ~ 1 S.
. j,~

h ~3 O O O O O O
,~, br O O O O O O

, ' ~3 ~ O ~
&1~ '~ ~

: ~ . ; ' ' ~, The above ~alues ~vere oi:~tained w:len apl)lying t`he presellt invention.
T~le indi~ ual values were tal~en ~rom the recor :lers whicII contirluo-Isly followe(l the pulp-washillg procedure alld at those points of time at which the manually-tal~en samples were removed. The manually-taken and analyzed samples gave the following results:
T~B~ V

Suspending Liquor from Pulp ~uspension Diluent Washing Losses :E~xample COD N~æSC)~ COD Na SO,~COl:) Na2So4 o No. g 02/l g/l g 02/l gt~lkg O2/ k~/ ton pulp ton pulp 16 0. 74 ~. 23 0. 49 0. 1~ 38 12 lq 0. 68 0. 22 0. 43 0. 14 36 12 18 0. 62 0. ~0 0. ~7 0. 15 27 9 19 0. 80 0. 26 0. 72 0. 23 ~8 9 0. 36 0. 12 0. 18 0. 06 20 6 ~, . 21 0.36 0.12 0.12 0.04 19 6 .

There is extremely good correlation between the washing losses recorded in Table IV and the corresponding values in ~able V9 obtained according to SCAN C 30: 73 .
The values obtained when manually analyæing the COD value of the samples were placed against indications in rnillivolts obtained In calorimetric analyses in accordance with tlle invention, whereupon the following relationship - was obtained:
- k:
C a 2.18 X--0.05 in which X is the signal fron~ the calorimeter in millivolts, and c = the content of dissolved impurities of the liquor, corresponding to the amount of i , ( ,/ O:..`J 'il cons-lmed in g/l. l'he conlellts C24 an(l C2, ~iven in Table IV were calculate~l with the aid of this formula. When a comparison is nnade bet~veen this -ormula and the formula obtained when washing sulphate pulp according to Example 1, it will be ound that the formulae deviate greatly from one anotller.
5 This is explained l)y the fact that the dissolved impurit~es in the pulp-suspension liquid in sulphite plants gives a much lower heat when reacting witl~ hypochlorous acid than do the dis~olved impurities in sulphate pulp. It is desirable to deter-mine the aforementioned relationship between the millivolt indicatlon and the content of dissolved substance of the liquLd in grams per liter in each particular 10 case, i. e., in each sulphite-pulp manufacturing plant.
The relationship between washing losses expressed as COD and as sodium can l~e calculated ~rom Table V. The follow~ng relationship is obtallled:
Na~O = COD x 0. 32 Within the sulphite-pulp manufacturing industry, washing losses of ~1, 15 sodium are expressed as g/Na2O per liter or kg Na2O per ton of pulp. With the aid of this relationship, washing losses measured in accordance with the invention can be converted to kg Na20 per ton of pulp, ~s will be seen from the i -following Table:

TABLE ~1 Washing losses Example No. kg Na?O/ton of pulp 3~
( `i It will be apparent frorn the Ioregoing that when washing sulpllite p~
(as ~vhen waslling sulphate pulp~ using the present invention, it is possible to follow the wasllillg losses of the pulp continuously, and to express such losses in the usual manner. The obtained values can then be used to regulate tlle 5 washing losses to a desired level.
Although the description i~ primarily concerned with washinD chemical pulp, and to monitor continuously the results of the washing process, to measure the washing losses, the pxocess of the invention can also be applied to the washing of semichemical,- chemimechanical and mechanical pulp. In the 10 manufacture of mechanical pulp by defibration of wood no chemicals are used, hence the pulp is not normally washed subsequent to being manufacturecl. When mechanical p~llp is bleached, however, and it is deslrable to w~sh the pulp subsequent to bleaching the same, the present invention can then be applied to advantage-The in~ention is not restricted to the washing of cellulose pulp, but it can also be used for washing any form of fibrous suspension. Other regions in which the invention can be applied to advantage include the washing of sludge in purification plants, and in the washing of fibrous suspensions in sugar- f producing factories.
While the invention has been illustrated in the drawings by wash filters in two or more stages, the invention can be applied to the washing of fibrous suspensions in any kind of apparatus used for the continuous ~Yashin~, of fibrous suspensions, and especially cellulose pulp, for example, 1.
pressure washing and continuous digester washing processes, as descril~ed inRydholm, Pulping Processes, pages 722 to 733, inclusive, and in continuous diffuser washing, as in a Kamyr Continuous Diffuser. This type of continuous d ~tser llas a~ outer casil~o witllin ~vhich there are a number of concentric double-sided screen rings. Each screen ~ing is fastened to radial drainace arms with vertical liLting bars al; the ends, ~Yhich in turn are connectecl to hydrau.lic cylinders. The pulp enters in the bottom of the conical part of the 5 casing and moves upwards.
The automatically regulated hydraulic cylinders are lifting the screen unit with approximately the same speed as the pulp suspension is moving upwa~ds. At the end o~ the lift the extraction is momenta~ily shut off where-after the screen unit mal~es a rapid downward movement, clearing the screen 10 Surface.
Above the screen unit rotates a set of scraper-arms, on which the nozzles for distribution of wash liquor are fastened.
The wash liquid displaces the liquor in the pulp, which in turn is extracted through the conca~e and the convex sides of the screen rings.

The displacement liquor, thus collectedby the screens, is flowing down to the drainage arms and to a collectin,~ pipe or header outside the shell.
The washed pulp is discharged at 10~C with scraper plates erected l.
on the rota~ing arm to a common outlet in the same manner as a conventional upflow bleaching tower.

Alternatively the washed pulp is diluted to 5G/c consistency. Dilution liquid is added through nozzles, which are erected on the distribution arms.
Pulp and dilution liquid are mixed by the rotating arms . In~this case the pulp level is kept constant above the rotatingr arms and the pulp outlet in order to avoid air entrainment in the pulp suspension to be discharged.

t: `:

~.

Claims (10)

Having regard to the foregoing disclosure, the following is claimed as the inventive and patentable embodiments thereof:

1. A process for continuously washing fibrous suspensions in aqueous suspending liquors containing, dissolved impurities, to remove such impurities by exchanging aqueous suspending liquors substantially free from such im-purities for the aqueous suspending liquor, controlling the washing losses that are obtained in the continuous washing of the fibrous suspensions by determining continuously the content of dissolved impurities in the washed fibrous suspensions and controlling the supply of aqueous suspending liquid accordingly, which comprises washing fibrous material of the suspension in aqueous suspending liquid substantially free from dissolved impurities, and forming a washed fibrous suspension in such liquid; withdrawing aqueous suspending liquor containing dissolved impurities; diluting the washed fibrous suspension by adding aqueous suspending liquid substantially free from dissolved impurities;
measuring, the amount of dissolved impurities remaining with the fibrous suspension after the washing has been completed by determining (1) the volumetric flow rate of the washed suspension; (2) the liquid content of the washed suspension; and (3) the content of dissolved impurities in the suspending liquid; and then controlling the volume amount of wash liquid added according to the washing losses to maintain washing losses within a predetermined limiting range.

2. A process according to claim 1, in which the fibrous suspension is a cellulose pulp suspension selected from the group consisting of chemical pulps, mechanical pulps, chemimechanical pulps, semichemical pulps, and thermo-mechanical pulps.

3. A process according to claim 2, in which the pulp is a chemical pulp selected from the group consisting of sulfite pulps, sulfate pulps, and pulps obtained from the oxygen alkali pulping of lignocellulosic material.

4. A process according to claim 1, which comprises measuring the liquid content of the washed undiluted fibrous suspension by diluting the washed sus-pension with aqueous suspending liquid substantially free from dissolved impurities measuring the amount of such suspending liquid added thereto, and then determining the volumetric flow rate of the washed fibrous suspension.

5. A process according to claim 4, which comprises determining the fiber concentration of the washed diluted fibrous suspension.

6. A process according to claim 4, which comprises measuring the content of dissolved impurities of the suspending liquid substantially free from dissolved impurities; and subtracting this quantity from the quantity of dissolved impurities remaining in the suspending liquid after the washing.

7. A process according to claim 1, which comprises measuring the quantity of dissolved impurities in the aqueous suspending liquid substantially free from dissolved impurities that is used for washing, and subtracting this quantity of dissolved impurities from the quantity of dissolved impurities remaining in the suspending liquid after the washing.

8. A process according to claim 1, which comprises measuring the quantity of dissolved impurities in the suspending liquor by reacting said substance with an oxidant, and measuring the heat liberated calorimetrically.

9. A process according to claim 8, in which the oxidant is hypochlorous acid.

10. Apparatus for continuously washing fibrous suspensions in aqueous suspending liquors containing dissolved impurities, to remove such impurities by exchanging aqueous suspending liquors substantially free from such im-purities for the aqueous suspending liquor, controlling the washing losses that are obtained in the washing of the fibrous suspensions by determining con-tinuously the content of dissolved impurities in the washed fibrous suspensions, and controlling the supply of aqueous suspending liquid accordingly, which comprises means for washing fibrous material of the suspension in aqueous suspending liquid substantially free from dissolved impurities, and forming a washed fibrous suspension in such liquid; means for withdrawing aqueous suspending liquor containing dissolved impurities; means for diluting the washed fibrous suspension by adding aqueous suspending liquid substantially free from dissolved impurities; means for measuring the amount of dissolved impurities remaining with the fibrous suspension after the washing has been completed by determining (1) the volumetric flow rate of the washed suspension;
(2) the liquid content of the washed suspension; and (3) the content of dissolved impurities in the suspending liquid; and means for controlling the volume amount of fresh aqueous suspending liquor added according to the washing losses to maintain washing losses within a predetermined limiting range.