WO1998051855A1

WO1998051855A1 - Method of minimizing scaling problems in the manufacture of bleached cellulose pulp while using an essentially fully closed washing liquid flow pattern

Info

Publication number: WO1998051855A1
Application number: PCT/SE1998/000879
Authority: WO
Inventors: Jan Georg LIDÉN; Staffan Lars Sune Magnusson
Original assignee: Mo Och Domsjö Aktiebolag
Priority date: 1997-05-14
Filing date: 1998-05-12
Publication date: 1998-11-19
Also published as: SE9701778D0; SE9701778L; SE509444C2; AU7463998A

Abstract

Scaling problems in, for instance, washing apparatus occur when using fully closed liquid flow patterns in the manufacture of bleached cellulose pulp. The present invention provides a solution to this problem, and relates to a method for minimizing calcium caused scaling problems when washing and/or increasing the consistency of cellulose pulp suspensions and in other cellulose pulp suspension processes where the pulp suspension is treated in at least two stages, including at least one bleaching stage, alternately in a neutral/acidic and alkaline environment, wherein washing liquid (suspension liquid) is passed counter-currently such that the treatment process will be essentially fully closed with respect to the washing liquid (suspension liquid) flow pattern, characterized in that the major part of the calcium present in and on cellulose pulp fibres incoming to the treatment process is prevented from being released in the suspension liquid, by adding a water soluble, oxalate-containing chemical to the suspension liquid downstream of the neutral/acidic treatment stage and/or to the pulp suspension immediately prior to or in the neutral/acidic treatment stage.

Description

Method of minimizing scaling problems in the manufacture of bleached cellulose pulp while using an essentially fully closed washing liquid flow pattern

Technical field

The invention relates to a method of minimizing scaling problems in certain critical positions in the manufacture of bleached cellulose pulp where washing liquid (suspension liquid) is passed counter-currently, such that the manufacturing process will be essentially fully closed with respect to the flow pattern of washing liquid (suspension liquid). The motive lying behind the latter is to both reduce the consumption of fresh water and to reduce the amount of organic material released to the recipient when manufacturing bleached cellulose pulp.

By cellulose pulp is meant primarily chemical pulp, i.e. where lignocellulosic material of any kind whatsoever is chemically digested by means of an acid or an alkaline process. The sulphite method is an example of an acidic digestion process, while the sulphate method is an example of an alkaline digestion process.

Other known alkaline digestion processes are the polysulphide process and processes of the type soda ( sodium hydroxide) process in which catalysts are used, such as any quinone compound. Falling within the term sulphate method is, for instance, the use of high sulphidity, the use of counter- current cooking where white liquor is also delivered during an advanced stage of the cook, and use of a chemical treatment of the lignocellulosic material prior to the actual sulphate cook. Cellulose pulp is produced from a very large number of different lignocellulosic materials. A very common lignocellulosic material is wood, originating from both deciduous trees and coniferous trees, which is normally chopped to chip form prior to the pulp manufacturing, for instance the digestion (cooking) process.

The cellulose pulp is normally bleached in several stages, with the use of both oxygen-based bleaching agents and chlorine dioxide (D). Examples of oxygen-based bleaching agents are oxygen gas ( 0 ) , ozone ( Z ) and some per-compound (P), for instance hydrogen peroxide, sodium peroxide and different per-acids. The per-compound can be activated by polyoxoanions, for instance in the form of molybdate, in an acid environment. When using the above recited oxygen-based bleaching agents, and then particularly some form of per- compound, it is often necessary to subject the pulp to a sequestering or chelating agent treatment (Q) in the bleaching plant. Examples of typical chelating agents are ethylenediaminetetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA) and nitrilotriacetic acid (NTA).

Background art

One has, for environmental reasons, started to use the earlier mentioned oxygen-based bleaching agents more and more when bleaching, for instance, sulphate pulp. The use of these bleaching agents has enabled an increased closing of the liquid flow pattern. With closing is meant, that the

(washing) liquids in the bleaching plant are taken care of in an increased way. In the context described, it is also possible to utilize the excellent bleaching agent chlorine dioxide when used in a suitable, i.e. in not too large, additional amount. Described in the Swedish Patent Specification 9304173-9 (502 172) is a method for producing bleached cellulose pulp in which the digested lignocellulosic material is subjected serially to at least an oxygen gas deligni- fication process ( 0 ) , a chelating agent treatment ( Q ) and bleaching with a non chlorine-containing oxidative bleaching agent, for instance a per-compound (P), after first having washed the digested lignocellulosic material and optionally having screened the same. Subsequent to these various treatment stages, the pulp is liberated in a far going way from undesirable substances, by washing for example, and the washing liquid (the suspension liquid) is passed essentially strictly counter-current, such that the pulp manufacturing process will be essentially fully closed with respect to the flow pattern of the liquid. In order to succeed with this essentially fully closed liquid flow pattern, it is necessary to take care of the transition metals, primary manganese, that are present in the pulp and in the system in general. This is achieved by bringing practically all manganese to form a water soluble manganese complex with a chelating agent and by keeping the water soluble manganese complex intact in the suspension liquid counter-currently throughout the entire treatment chain, so as to end up in weak black liquor and, subsequent to evaporation, in a heavy black liquor which is burned in the recovery boiler and therewith destroyed.

Degradation of the manganese complex in any position so as to cause manganese, for instance, to be reprecipitated on the pulp is prevented by bringing the pH of the suspension liquid subsequent to oxygen gas delignification and forward in the pulp treatment chain to the bleaching process with the non chlorine-containing oxidative bleaching agent, for instance hydrogen peroxide, to a value of at most 10, and by bringing the carbonate concentration of the suspension liquid to a value which is equal to or which exceeds a given lowest value, depending on the position in the cellulose pulp treatment chain. If the amount of manganese that accompanies the pulp into the aforesaid third treatment stage, i.e. the peroxide stage for instance, is not successfully reduced to a substantial degree, the pulp cannot be bleached in the aforedescribed way because the major part of the bleaching agent charged will immediately be destroyed by the manganese.

When the described method is applied in a sulphate pulp mill, scaling problem is arised in one, and sometimes in some, position(s). The most critical position is where the pulp suspension is washed after the Q-stage, preferably in two stages. The pH value of the pulp suspension is normally about 5 during the Q-stage and retains this pH value when the pulp suspension is delivered, for instance, to a subsequent first washing filter. As previously mentioned, the washing liquid is conveyed strictly counter-current, meaning that after the P-stage, the pulp is washed on, e.g., a washing filter either with fresh water or with suspension liquid emanating from a subsequent treatment stage in the described treatment sequence. Because the P-stage is carried out under relatively strong alkaline conditions, the washing liquid (suspension liquid) collected in a storage tank underneath the aforesaid washing filter will also be relatively strongly alkaline, with a pH value of at most 10. Part of the suspension liquid in the mentioned storage tank is conveyed counter-currently and used as washing liquid in a second wash filter downstream of the described Q-stage. The aforesaid washing liquid (suspension liquid) is collected in a storage tank underneath the washing filter concerned. Part of the suspension liquid in this storage tank is conveyed counter- currently and used as washing liquid in the first washing filter mentioned by way of introduction, downstream of the Q- stage. A storage tank is also provided underneath this washing filter, for collecting the washing liquid or the suspension liquid, part of this liquid being conveyed counter-currently, and so on. As before stated, the pulp suspension delivered to the first washing filter after the Q-stage has a pH value of about 5. The pulp suspension contacts in the washing filter a suspension (washing) liquid whose pH is below 10. The extent to which the pH value of this suspension liquid will be below 10 is dependent on a number of circumstances. A weakly acid pulp suspension and an alkaline suspension liquid meet at this washing filter. Scales are formed in and in the vicinity of the washing filter concerned, for reasons described in direct connection with what has just been stated. A certain degree of scales can also be formed in the subsequent second washing filter. A position that is particularly subjected to scaling is the washing stage immediately prior to the Q- stage, particularly if one in the Q-stage uses a pH signi- ficantly below 5. Scales can also be formed in the washing stage or stages immediately after the alkaline oxygen gas bleaching of the pulp. This is quite unusual, however.

The reason why scales are formed and why scaling problems arise is because the starting material used for the manufacture of the cellulose pulp, i.e. the lignocellulosic material in the form, e.g., of wood, contains calcium in various high amounts. Some of the original calcium can disappear from the pulp during its way to the bleaching plant, where the first bleaching stage in the described case is comprised of an alkaline oxygen gas bleaching stage. Both the pulp entering the oxygen gas bleaching stage and the pulp that leaves said stage contain significant quantities of calcium. When the pH of an alkaline pulp suspension, e.g. a suspension as well from a sulphate cook as from an oxygen gas bleaching stage, is lowered, some of the calcium present in and on the pulp fibres will be released and dissolved in the suspension liquid. The amount of calcium released is a function of the total calcium content of the pulp and of the pH, and the main rule is that the lower the pH, the more calcium released. If it in different positions will be so, that the suspension liquid besides a high content of calcium also contains high oxalate content and/or sulphate content and/or carbonate content, precipitation of calcium oxalate, calcium sulphate and calcium carbonate respectively will take place. Large quantities of oxalate are formed from the pulp in both acidic and alkaline, oxidizing bleaching stages. Sulphate most often originates from the sulphuric acid that is supplied to the pulp suspension or the suspension liquid as a matter of routine when desiring a low pH value in a pulp treatment stage. Examples of such stages are the Q-stage, D- stage and Z-stage. High concentrations of carbonate can also occur in certain positions.

The scales formed in washing apparatuses immediately after acidic treatment stages or bleaching stages consist of precipitates of primarily calcium oxalate, and possibly also of precipitates of calcium sulphate to some extent. In a strongly alkaline environment, for instance in washing apparatus immediately downstream of an oxygen gas bleaching stage, the scales are composed of precipitates of calcium carbonate.

When there is used a washing filter that includes a wire cloth, the precipitate will first fasten to the threads of the wire cloth and it is successively growing so as to gradually reduce the size of the open areas of the wire cloth. As the size of these open areas diminishes, it will be more and more difficult for the suspension liquid or washing liquid to pass through the wire cloth and it is ultimately necessary to switch-off the filter and clean the more or less clogged wire cloth with the aid of a chemical agent and/or a mechanical device. When washing presses or presses are used to liberate the pulp from undesirable substances, the large number of round holes included in the presses and their rollers and through which the washing liquid or suspension liquid shall pass gradually become blocked or clogged in an analogous manner. The scaling problems can become so serious as to cause even pipes or tubing of significant dimensions through which suspension liquid passes in the proximity of any washing apparatus to become completely blocked.

Described in Swedish Patent Specification 9401125-1 ( 502 706 ) is a method for producing bleached cellulose pulp that coincides to a great extent with the aforedescribed, patented method. The primary difference is in the actual treatment/bleaching sequence. In the case of this method, the cellulose pulp can serially in the bleaching plant be subjected to oxygen gas delignification (0), jointed chelating agent treatment and chlorine dioxide bleaching

(Q/D) and bleaching with a non chlorine-containing oxidative bleaching agent, for instance a per-compound (P). Instead of using a jointed Q/D-stage, the pulp may first be treated with a chelating agent (Q) and the pulp then be bleached with chlorine dioxide (D). Although not necessary, it is preferred to follow the Q-stage with a washing stage.

Scaling problems have also occurred in one and sometimes more position( s ) in a sulphate pulp mill in which this method is applied. When a Q/D-stage is used, scaling problems occur primarily in the wash immediately after this acidic treatment stage. When the Q-stage is followed by the D-stage, scaling problems occur primarily in the wash immediately after the Q- stage, which most often also is acidic. It has been shown to be a common feature, that when more than one acidic stage is included in a treatment/ bleaching sequence, it is in the washing stage after the first acidic stage that scaling problems primarily occur. This is, in itself, natural, since the suspension liquid in this position normally includes the maximum amount of released calcium. Scaling problems can also occur in washing apparatus located immediately upstream of an acidic treatment/bleaching stage. In this case, acidic/neutral and calcium-rich suspension liquid (washing liquid) is delivered to the normally alkaline pulp suspension in the washing apparatus, which results in the precipitation of calcium oxalate and/or calcium carbonate to some extent.

Several solutions to the problem of calcium- containing scales with increased closure of the liquid flow pattern in bleaching plants have been proposed in the literature.

One such proposal involves sampling the suspension liquid obtained when washing the cellulose pulp after an acidic treatment stage, and separately evaporating that liquid. The arosen concentrate can be dealt with in two ways. One way is, that the concentrate is mixed with the remaining quantitatively dominating spent liquor in a position during its evaporation from weak black liquor to heavy black liquor, which then is burned in the recovery boiler. The other way is that the concentrate is destroyed separately by burning in a furnace separated from the recovery boiler. In the case of separate evaporation where, in addition to calcium, organic material, manganese and magnesium are also removed from the pulp manufacturing process, there is also the risk of calcium precipitate formations, since the suspension liquid concerned will most often contain oxalate and/or sulphate. The solubility of calcium oxalate and/or calcium sulphate can be exceeded when the water content of the suspension liquid is lowered by evaporation. This results in scaling problems in the evaporation plant instead.

Another proposal involves precipitating calcium from acidic suspension liquid with the aid of an alkaline carbonate-containing liquid and to subsequently separate the formed precipitate. The purified suspension liquid is then returned to the liquid cycle.

A third proposal consists of separating divalent cations from acidic suspension liquid by means of ion exchange technique.

A fourth proposal consists of adding substances that prevent the re-precipitation of calcium released in the suspension liquid. Examples of such substances are different types of polymers, and simple inorganic substances in ionic form, such as phosphate.

In STFI Report BF, June 3, 1996, entitled "Precipitation in the Bleaching Plant: Part 1. The Solubility of Calcium Oxalate in Pure Solutions and in Bleaching Filtrates", the authors, Rune Radestrόm and Per Ulmgren, touch on the problems described above and also propose certain solutions. (STFI is an abbreviation for Svenska Traforskningsinstitutet, Swedish Pulp and Paper Research Institute) .

The following passage found on page 28 of the report is of interest:

"The content of carbonate ions is high in the E-filtrate and P-filtrate. At an enlargement of the contents around these stages, e.g. as a result of an increased closing of the system, primarily it will be calcium carbonate which precipitates, since pH > 8. A neutralization of these filtrates to apH < 8, e.g. by mixing with acidic neutral filtrate rich in calcium ions may cause calcium oxalate to precipitate. Calcium oxalate can also be expected to precipitate in the vicinity ofDI- and Z -stages, since these stages have high contents of both calcium and oxalate ions. The risk of precipitation in connection with the Q-stage also exists in a QP-sequence in strictly counter -current when the P-filtrate rich in oxalate ions is mixed with the Q-filtrate containing calcium ions.

There are, in principle, two ways of preventing the solubility of calcium oxalate being exceeded. The first alternative is to operate a completely open system as in the case of "yesterday's bleaching plant", which is no durable solution. The other way is to eliminate one of the dangerous components calcium ions or oxalate ions. It will probably be necessary to reduce the content of both calcium ions and oxalate ions. For instance, the calcium ions can be dissolved from the pulp in an acidic wash and the acidic filtrate rich in calcium ions and other metal ions is removed for purification ("kidney") (Caron, Williams, 1996) or recirculation to the smelt dissolver in the chemical recovery process (Ulmgren etal, 1994). Another way of preventing process problems is to control the precipitation to a specific site in the process where the precipitate can be readily dealt with, or by precipitating calcium oxalate in the pulp and, in this way, removing the precipitate from the process. In this latter case, however, problems can occur in the paper mill due to the carry over. "

Disclosure of the invention Technical problem

In conjunction with an increased closing of the liquid flow pattern in the manufacture of bleached cellulose pulp with the use of alternating neutral/acidic and alkaline treatment stages, scales tend to form e.g., in those washing apparatus that are in the immediate proximity of the stages, meaning that the washing apparatus must be cleaned frequently and therewith limit the production of bleached cellulose pulp.

The solution

The present invention provides a solution to the aforementioned problem and relates to a method for minimizing calcium-caused scaling problem when washing and/or increasing the consistency of and further handling of the cellulose pulp suspension which in at least two stages, including at least one bleaching stage, is treated alternately in a neutral/ acidic and alkaline environment and wherein washing liquid (suspension liquid) is passed counter-currently so that the treatment process will be essentially completely closed with respect to the flow pattern of washing liquid ( suspension liquid), characterized in that the major part of the calcium present in and on cellulose pulp fibers entering the treatment process is prevented from dissolving in the suspension liquid by adding a water soluble, oxalate- containing chemical to the suspension liquid downstream of the neutral/acidic treatment stage and/or to the pulp suspension immediately prior to or in the neutral/acidic treatment stage. The invention is based on the idea concept of trying to retain the calcium in and on the pulp fibres to the greatest possible extent and to counteract the calcium leaving the pulp fibres and dissolving in the surrounding suspension liquid, even when there are used in the treatment process stages that range from a neutral to a strongly acidic environment. Given as a pH range, this means a pH from 7.5 down to 2. By alkaline environment is meant that the cellulose pulp is treated (bleached) at a pH value of 8 or higher. This is achieved by adding a substance, i.e. a water soluble, oxalate-containing chemical, that to a high extent blocks the release of the calcium from the pulp fibres.

Examples of suitable water soluble, oxalate- containing chemicals are oxalic acid and sodium oxalate. Of these two chemicals, oxalic acid is preferred because of its low price. Naturally, any suitable water soluble,oxalate- containing chemical can be used.

The water soluble,oxalate-containing chemical shall be added in an amount such that the mole ratio between oxalate and calcium in each position will be 0.8 or greater. As earlier pointed out, oxalate is formed from the pulp in both acidic and alkaline, ozidizing bleaching stages, and often in significant quantities. In other words, process associated oxalate is present in the suspension liquid. Thus, with respect to the aforesaid oxalate quantity in the form of a mole ratio, both the process associated oxalate and the added oxalate are included. Because the addition chemical concerned, preferably oxalic acid, commands a considerable kilo price, it is important to keep the amount added as small as possible. It can be mentioned in this context that had the process associated oxalate not been present, this invention would not have had any commercial interest, since the cost of the chemical addition would then have been unrealistically high. Although a very large addition of the water soluble, oxalate-containing chemical to the system is unharmful from the process aspect, with a view to process economy the addition of said chemical should be restricted so that the mole ratio between oxalate and calcium is less than 10. It is surprising that the mole ratio between oxalate and calcium in different positions need not always be the stoichiometric ratio or higher. It has also been observed that threshold values are found with respect to the calcium content of the suspension liquid which may not be exceeded when one has the intention to avoid scaling. By means of correct addition amount of e.g. oxalic acid one controls that the calcium content in the suspension liquid in the position concerned doesn't amount to the threshold value in question. The invention finds application in the treatment of unbleached cellulose pulp in accordance with a large number and partially different sequences.

The cellulose pulp suspension can be treated in series with oxygen gas ( 0 ) , chelating agent ( Q ) and peroxide ( P ) , or with some other non chlorine-containing oxidative bleaching agent. Impurities are removed from the pulp suspension after all given treatment stages, by washing and/or by increasing concistency in at least one stage. Any known apparatus can be used, and here exemplified by single- stage or two-stage diffusers, pressurized or not, belt (flat) washer, washing (drum) filter, washing press and other presses. As earlier mentioned, the washing liquid (the suspension liquid) is conveyed counter-currently, so that the treatment process will be essentially fully closed with respect to the liquid flow pattern. This means that substantially all suspension liquid, which can also be called bleaching spent liquor, is recovered and conveyed counter- currently and finally mixed with cooking spent liquor and the mixture is passed in the form of weak black liquor to the evaporation plant of the mill and is thereafter conveyed in the form of heavy black liquor to the recovery boiler of the mill for combustion and final destruction. Although not at all necessary, it is preferred to convey the washing liquid (the suspension liquid) strictly counter-currently.

In the aforesaid sequence, both the oxygen gas bleaching process and the peroxide bleaching process are effected under pronounced alkaline conditions in accordance with known technology, whereas the chelating agent treatment is effected in a neutral or acidic environment. The pH range is suitably 4 to 7 and the use of a pH of about 5 is probably the most common in practice. In the described case, the water soluble, oxalate- containing chemical shall be added preferably either to the suspension liquid at a position downstream of the Q-stage, or to the pulp suspension immediately prior to or in the Q- stage. The chemical may, of course, be added to the system in both of these positions. Since the suspension liquid is conveyed counter-currently in the system, addition of the chemical to the suspension liquid downstream of the Q-stage means, that also in that case the chemical is brought into contact with the pulp suspension before the pulp suspension is conveyed into the normally weak acid environment in the Q- stage.

Certain modifications of the aforedescribed treatment sequence are conceivable, and may even be preferred. For instance, two or more oxygen gas bleaching stages may be used initially, with or without intermediate washing of the pulp. Furthermore, the peroxide stage can be a pressurized stage. In this case, oxygen gas is normally delivered at an overpressure and such a stage normally being designated PO.

In another sequence, the cellulose pulp suspension is treated in series with oxygen gas (0), chelating agent plus chlorine dioxide (Q/D) and peroxide (P) or some other non chlorine-containing oxidative bleaching agent.

In this case, it is the Q/D-stage that is acidic. The water soluble, oxalate-containing chemical shall be delivered to the suspension liquid in the described sequence downstream of the Q/D-stage and/or be delivered to the pulp suspension immediately prior to or in the Q/D-stage. This additional procedure is analogous with the procedure described with reference to the first mentioned treatment sequence.

In a slightly modified sequence, the cellulose pulp suspension is treated in series with oxygen gas (0), chelating agent ( Q ) , chlorine dioxide ( D ) and peroxide ( P ) . This sequence includes two acidic stages, i.e. both the Q- stage and the D-stage. In this case, the water soluble, oxalate-containing chemical shall be added to the suspension liquid downstream of the Q-stage and/or the D-stage and/or shall be added to the pulp suspension immediately prior to or in the Q-stage and/or the D-stage. The precise manner in which the chemical is introduced into the system in this case is determined partially by the pH value that prevails in both the Q-stage and the D-stage. A very low pH value in the chlorine dioxide stage should be avoided (as is known, the addition of chlorine dioxide to the pulp suspension will cause the pH value to drop) and the pH value can be increased, for instance, to the level 3-4 by means of an alkali addition.

A fourth conceivable sequence is 0-Z-Q-P, where both the ozone (Z) stage and the chelating agent (Q) stage is acidic. Reference is made to the statement just above with regard to the positions at which the chemical is added. Too strongly acidic treatment stages shall be avoided and it is not completely clear what happens if the ozone bleaching is carried out at a pH of about 2. In such a case, it would seem appropriate to supply the pulp suspension with an alkaline solution after completion of the ozone bleaching process and prior to conveying the pulp suspension to the following washing stage. The pH value of the pulp suspension may be increased to 4, for instance. The aforedescribed treatment sequences produce bleached cellulose pulp having brightnesses that are sufficient for the intended final product in many cases, usually paper (or paperboard) of different kinds. The invention enables such bleached pulp to be produced with a fully closed liquid flow pattern. When desiring pulp of very high brightness, such as 90% ISO or thereabove, at least one further bleaching stage is necessary, for example two, possibly together with a further chelating agent stage. In those cases, a choice can be made between also allowing these treatment stages being included in a totally closed liquid flow system or of allowing said treatment stages to be included in an open system, i.e. to wash the pulp suspension with fresh water after respective treatment stage and allow the recovered, comparatively slightly contaminated, washing liquid to pass directly to the recipient or to be collected and subjected to an external purification process. There are a number of reasons why it may be beneficial to use an open liquid system for the terminating stage or stages in a very long treatment sequence.

The invention also finds application in treatment sequences that lack chelating agent (Q) stages and which thus include solely bleaching stages and possibly extraction stages. Examples of such sequences are 0-D-EO and D-EO, where EO designates an oxygen gas reinforced extraction (e.g. sodium hydroxide) stage. In these cases, the water soluble, oxalate- containing chemical shall be added to the suspension liquid downstream of the acidic chlorine dioxide stage (D) and/or be added to the pulp suspension immediately prior to or in the D-stage. In these sequences (similar in other sequences), excessively high chlorine dioxide charge shall not be used, since .a high chlorine dioxide charge can result in plugging problems in the recovery boiler due to the occurrence of high chloride contents in the chemical recovery system when the liquid system is substantially fully closed. One solution to this problem may be to separate sodium chloride from the electrostatic precipitator dust, although this means a costly investment.

As earlier mentioned, it is not necessary for the liquid flow pattern in the bleaching plant and backwards in the pulp manufacturing process to be strictly counter- current. The invention can also be applied when, for instance, choosing to utilize the suspension liquid (washing liquid) that is recovered in the washing stage after an acidic stage, for instance a Q-stage, and to treat that liquid flow separately in the aforedescribed manner, by evaporating the liquid and later burning the concentrate in a separate furnace distanced from the recovery boiler, or burning the concentrate in the recovery boiler mixed into the heavy black liquor. In this embodiment of the invention, the water soluble oxalate-containing chemical is preferably added to the pulp suspension immediately prior to or in the Q- stage. In this case, the chemical added will prevent scaling in the evaporating apparatus in addition to standard prevention of scaling in the washing stage immediately after the Q-stage. When choosing to use fresh water as the washing liquid in this position of the system, scaling is prevented only in the evaporation apparatus.

Advantages

The invention enables a substantially fully closed liquid flow pattern to be used in the manufacture of bleached cellulose pulp with a final brightness that is sufficient for the intended final product in many instances.

One of the advantages obtained with fully closed liquid flow pattern is that the amount of fresh water required is reduced to a minimum and that the emission of contaminated and oxygen consuming materials to the recipient disappears .

The unique advantage with the invention is that the scaling problems that are automatically dependent on a fully closed liquid circulation system are reduced to a minimum and possibly eradicated. This enables bleached cellulose pulp to be produced persistently in a highly environmentally friendly manner, i.e. day after day, week after week and possibly month after month without interruption.

Description of the drawing Figure 1 is a flowsheet illustrating part of the bleaching plant of a sulphate pulp mill. Best embodiment

Described below are two working examples, one of which is concerned with trials carried out on a full scale in a sulphate pulp mill. Process parameters in the various treatment stages are given in detail in conjunction with the account of these trials.

Example 1

Samples of coniferous sulphate pulp having a kappa number of 17 measured in accordance with the standard method SCAN-C1:77, and a viscosity of 950 cm³/gram measured according to the standard method SCAN-C15:62 were taken at a consistency of 15% from a washing filter in a sulphate pulp mill. Subsequent to digesting pine wood chips in a continuous digester and a subsequent digester wash, the pulp was screened and thereafter washed first on a washing filter and then in a washing press. The pulp was then oxygen gas bleached under alkaline conditions and washed on two washing filters in series and in a subsequent washing press. The pulp was then stored in a storage tower for a given period of time, and then washed on a washing filter where pulp samples were taken in accordance with what has been said in the introduction.

The pulp was transferred to a laboratory, where it was centrifuged to a pulp consistency of about 33%. Eight beakers having a volumetric capacity of 3 litres were each placed in order.

100 grams of bone dry pulp, i.e. 300 grams of about 33% pulp, and 1700 g water containing dissolved cooking salt (NaCl) were added to each beaker, such as to obtain in each beaker a pulp suspension having a pulp consistency of 5% and a sodium chloride content of 50 mmole per litre.

Sodium oxalate was also added to four of these beakers so as to obtain a total content of 5 mmole oxalate per litre. One molar hydrochloric acid (HC1) solution was added in different quantities to these two series of pulp suspensions, each series including four beakers and its contents, so as to obtain a variation in pH of between 6 and 3.

The samples were allowed to stand for about 30 minutes at room temperature, whereafter some of the suspension liquid was removed and acidified with nitric acid (HN0₃) to a pH of about 0.5 without prior filtration. The calcium content was then determined by atom absorption spectrometry. This procedure therefore provided a measurement of both the calcium dissolved in the liquid phase and the presence of any calcium in the form of particulate calcium oxalate.

The measured results are set forth in Table 1 below.

Table 1

Sample No . PH Oxalate Addition Calcium content mmol/litre

1 6 . 1 No 1.39

2 5 . 0 1.76 3 3 4 4..00 " 2.18

4 3.0 2.42

5 6. 1 Yes 0.23

6 5 . 0 0.25

7 4. 0 0.42 8 8 3 3..00 " 0.33

In the series where no oxalate was added to the pulp suspension, the calcium content increased in the suspension liquid as expected, with a fall in pH. At pH 3, the calcium content of the liquid was 2.4 mmol per litre, which with a 5% pulp consistency corresponds to a release of slightly more than 1800 mg calcium per kg oven dried pulp.

When oxalate was present in the suspension liquid, the calcium content was only 0.4 mmole per litre, or lower, and was substantially independent of the pH level. This result indicates that the addition of an oxalate-containing chemical to the pulp suspension immediately prior to or in a neutral or acidic treatment stage is able to effectively prevent the release of calcium from the pulp.

Example 2

With the intention of testing whether or not the aforegiven result achieved in the laboratory could also be achieved under actual operating conditions, the following experiment was carried out on a full scale.

Birch wood chips were digested in batch digesters with sulphate cooking liquor, so as to obtain cellulose pulp. The pulp was initially screened and then subjected to the treatment stages apparent from the flowsheet in Figure 1. The screened pulp was conveyed to a belt washer 1, where the pulp was washed with suspension liquid passed counter-currently. The pulp suspension was then passed to a washing press 2, where the consistency of the pulp was raised to about 30%, simultaneously with liberating the pulp from impurities to a certain extent. During its way to the oxygen gas bleaching reactor 3, the pulp was supplied with alkali in the form of sodium hydroxide (NaOH) in an amount corresponding to 45 kg per tonne of pulp, and magnesium in the form of magnesium sulphate (MgS0₄) in an amount corresponding to 0.25 kg per tonne of pulp. By pulp is meant here and in the remainder of this example in connection with addition figures a pulp having a dryness of 90%. The pulp was diluted with suspension liquid immediately prior to entering the oxygen gas bleaching reactor 3, to obtain a pulp consistency of 10%. 14 kg of oxygen gas were charged to the pulp suspension in the reactor 3 for each tonne of pulp. The temperature was about 110°C, the time 90 minutes, and the pressure 0.3 Mpa at the top of the reactor. The pulp had a kappa number of 15 and a viscosity of 1100 cm /g prior to the oxygen gas stage. The kappa number fell to 9 and the viscosity to 780 cm /g after the oxygen gas delignification (bleaching). In this position, the pulp had a brightness of 60% ISO (measured in accordance with the standard method SCAN-C 11:75). The oxygen gas bleached pulp was passed to a washing press 4 and, after having been purified, was passed from there to a storage tower 5. Suspension liquid was supplied immediately upstream of the inlet to the storage tower 5, so as to obtain a pulp consistency of 10%. Subsequent to having passed the storage tower 5, the pulp was passed to a washing filter 6. After being washed in this position, the pulp was passed to a bleaching tower 7 in which the pulp was treated with both chlorine dioxide ( C10₂ ) and a chelating agent in the form of EDTA. Chlorine dioxide was added in an amount corresponding to 11 kg per tonne of pulp, calculated as active chlorine. EDTA was added in an amount corresponding to 2 kg per tonne of pulp. 1.1 kg of sulphuric acid (H₂S0₄) was also added for each tonne of pulp, so as to obtain a pH value of about 4.7 in the pulp suspension. The pulp consistency was 10% during this treatment stage, and the temperature of the incoming pulp was 70°C and the time 2 hours and 30 minutes. After this joint treatment stage, the pulp was passed to two serial washing filters 8 and 9. The pulp suspension was then passed at a pulp consistency of 10% to a terminating peroxide bleaching stage 10. 12 kg hydrogen peroxide (H₂0₂) per tonne of pulp and 7.5 kg sodium hydroxide (NaOH) per tonne of pulp were added to the pulp suspension prior to said suspension entering the bleaching tower 10. The temperature was 82°C and the time 3 hours. The pulp was passed from the bleaching tower 10 to a washing filter 11. In this position, i.e. after the washing, the pulp had a brightness of 85% ISO. The description hitherto has primarily been concentrated on the transport of the pulp suspension through the bleaching plant, in the flow sheet from left to right. The transport of the washing (suspension) liquid counter-currently through the bleaching plant, in the flow sheet from right to left will now be described.

Fresh water was added to the washing filter 11 through the line 12. Contaminated liquid recovered from beneath the washing filter 11 was passed through the line 13 to the washing filter 9. The liquid recovered beneath the washing filter 9 was passed to the washing filter 8 through the line 14. The liquid recovered beneath the washing filter 8 was passed backwards and divided into two volumetrically equal flows. One flow was passed to effluent via the line 15 (broken line) and the other flow was passed to the washing filter 6 via the line 16. Fresh water was also added to this washing filter, through the line 17 (broken line) in an amount that corresponded to the amount of liquid passed to effluent through the line 15. The liquid recovered from beneath the washing filter 6 was passed to the washing press 4 through the line 18. Suspension (washing) liquid was delivered to the forwardly moving pulp suspension through the branch line 19 in accordance with what has earlier been described. The liquid recovered from beneath the washing press 4 was passed to the washing press 2 through the line 20. Suspension (washing) liquid was delivered to the forwardly moving pulp suspension through the branch line 21 in accordance with what has earlier been described. The liquid recovered from beneath the washing press 2 was passed to the belt washer 1, where the washing liquid meets the pulp suspension containing cooking spent liquor counter-currently. The recovered mixture of cooking spent liquor and bleaching spent liquor was passed in the form of weak liquor through the line 23 for evaporation, to obtain heavy black liquor that was later burned in the recovery boiler ( these latter stages are not shown in the Figure ) .

The aforedescribed method of manufacturing bleached cellulose pulp in accordance with the sequence O- Q/D-P including the liquid flow pattern, was applied in the sulphate pulp mill concerned at a time just prior to testing the application of the invention.

The test, or trial, was commenced by adding to the pulp suspension after it had left the washing filter 6, i.e. immediately upstream of the acid stage Q/D in position 7, an oxalic acid (H₂C₂0₄) solution in a quantity corresponding to 5 kg oxalate (C₂0₄ ^2~) per tonne of pulp, through the line 24. The addition of sulphuric acid to the pulp suspension in order to obtain the desired pH value in the Q/D-stage was stopped when the supply of oxalic acid to the pulp suspension was started.

Bleeding of suspension liquid to effluent via the line 15 was stopped three hours after starting the trial. The supply of fresh water to the washing filter 6 through the line 17 was stopped at the same time. As a result, the forwardly moving birch pulp suspension was treated in accordance with the sequence 0-Q/D-P with a fully closed liquid flow pattern and a suspension (washing) liquid flow pattern strictly counter-current, i.e. in accordance with a preferred embodiment of the invention.

A further change was made at the time described, this change comprising lowering the amount of oxalic acid added such as to correspond to 2.5 kg oxalate for each tonne of pulp. On the basis of measurements earlier made in this bleaching plant, it was possible to calculate that this amount of oxalate together with oxalate formed in the Q/D- stage and the P-stage would exceed the quantity of calcium in the system. The reason for this high initial oxalic acid addition at the beginning of the trial was because the return of calcium-rich suspension liquid from the washing filter 8 to the washing filter 6 via the line 16 caused enrichment of calcium in the system around the Q/D-stage 7. A high initial oxalic acid addition safely allows a stoichiometric excess of oxalate in relation to calcium to be obtained. This following lower oxalic acid addition then continued for 17 hours, whereas the fully closed suspension (washing) liquid flow pattern was maintained for a further 5 hours, i.e. after the supply of oxalic acid ceased.

During the trial period, i.e. immediately before adding oxalic acid and during the addition of oxalic acid and sometime after ceasing to add oxalic acid, pulp samples and suspension liquid samples were taken for determining calcium and oxalate contents.

The calcium content and the oxalate content of wet pulp from the washing filter 8 were determined by mixing a known quantity of pulp with water and hydrochloric acid ( HCl ) in a beaker so as to obtain a pH lower than 1. The beaker was heated to a temperature of 60°C for 60 minutes, whereafter the solution was filtered through a 0.45 μm membrane filter. Final determination of calcium and oxalate (C₂0₄ ^2~) was effected with atomic absorption spectrometry and ion chromatography respectively. Calcium and oxalate contents in suspension liquid taken from beneath the washing filter 8 with a temperature of 70°C and a pH of 5,5 were obtained by acidifying a known volume of suspension liquid with hydrochloric acid to a pH lower than 1, filtering the sample through a 0.45 μm membrane filter and, subsequent to appropriate dilution, making a final determination by means of atomic absorption spectrometry and ion chromatography respectively. The measurement results are set forth in Table 2 below.

Table 2

Time after Pulp Suspension liquid Concentration product trial start, Ca²⁺ A_. ^" Ca²⁺ >0₄ ^2" (mol² l²) in suspension hours mmole/kg mmole/kg mmole/litre mmole/litre liquid

[Ca²⁺] x [CaO₄ ^2~]

14 5.9 3.2 0.69 -5.66

-0.1 10

3.4 0.69 -5.63

1 10

3 0.8 1.4 10 -5.95

7 0.65 1.7 10 -5.96

11 42 39 0.98 1.4 10 -5.86

14 1.0 1.5 -5.8_ 10

17 41 40 0.85 1.1 10 -6.03 11 2.0 1.1 10 -5.66

24* 19

= four hours after terminating the addition of oxalic acid.

As will be evident, the pulp on the wash filter 8 (= 15% pulp consistency) was measured to contain 14 mmole calcium and 5.9 mmole oxalate per kg of pulp immediately before adding oxalic acid. Corresponding suspension liquid contained 3.2 mmole calcium and 0.69 mmole oxalate per litre. Three hours after commencing the addition of oxalic acid, each litre of suspension liquid had a calcium content of 0.8 mmole, i.e. a significant drop, and an oxalate content of 1.4 mmole, i.e. twice as much. The calcium content of the suspension liquid remained at 1 mmole per litre, or lower, during the whole of the oxalic acid addition period. The mole ratio between oxalate and calcium in the pulp was close to 1 during the addition period, which indicates that calcium was bound to oxalate in and on the pulp fibres to a great extent. Twentyfour hours after starting the trial and four hours after terminating the oxalic acid addition, it was observed how the content of calcium in the pulp had fallen and the content of calcium in the suspension liquid had risen. The analysis of oxalate in the same pulp samples showed that the mole ratio oxalate/calcium had fallen markedly below 1, which indicates that the substance quantity oxalate in the Q/D- stage had been too low (caused by the exclusion of an oxalic acid addition) to prevent calcium release. It is important to note that at the time before starting the trial, no scaling problems were observed in the washing filter 8 or in any other position. This in turn caused by persistently passing half of the suspension (washing) liquid recovered from beneath the washing filter 8 to effluent via the line 15, and suppplying a corresponding amount of fresh water to the washing filter 6. The calcium content of the suspension liquid immediately prior to beginning the trial was measured as 3.2 mmole per litre. This content corresponds to about 130 mg calcium per litre suspension liquid. It has been found in the earlier use of a fully closed liquid flow pattern that with a calcium content of the suspension liquid of 200 mg per litre, and sometimes perhaps slightly higher, preciptiation has started to appear resulting in the formation of scales in the washing apparatus. In the trial run in accordance with the invention, i.e. during the period in which oxalic acid was added to the pulp suspension and a fully closed liquid flow pattern was applied, the calcium content of the suspension liquid was 1 mmole per litre, or lower, which means 40 mg or less per litre of suspension liquid. This indicates that an at least five-fold safety margin was achieved when applying the invention during the trial period, despite the liquid flow pattern having been fully closed. This indicates that it should be possible to produce bleached cellulose pulp in accordance with the invention with a fully closed counter- current liquid flow pattern persistently, i.e. in the absence of scales in, e.g., washing apparatus.

Claims

1. A method of minimizing calcium caused scaling problems when washing and/or increasing the consistency of a cellulose pulp suspension and in other aspects of handling said cellulose pulp suspension, wherein said pulp suspension is treated in at least two stages, including at least one bleaching stage, alternately in a neutral/acidic environment and an alkaline environment, and wherein .washing liquid (suspension liquid) is passed counter-currently so that the treatment process will be essentially fully closed with respect to the washing liquid (suspension liquid) flow pattern, characterized in that the major part of the calcium present in and on cellulose pulp fibres entering the treatment process is prevented from dissolving in the suspension liquid, by adding a water soluble, oxalate- containing chemical to the suspension liquid downstream of the neutral/acidic treatment stage and/or to the pulp suspension immediately prior to or in the neutral/acidic treatment stage.

2. A method according to Claim 1, characterized by adding the water soluble, oxalate-containing chemical in an amount such that the mole ratio between oxalate and calcium in each position will be 0.8 or greater.

3. A method according to Claims 1-2, characterized in that the pH value in the neutral/acidic treatment stage lies in the range of 2-7.5; and in that the pH value in the alkaline treatment stage is at least 8.

4. A method according to Claims 1-3, characterized in that the cellulose pulp suspension is treated serially with oxygen gas ( 0 ) , chelating agent ( Q ) and peroxide ( P ) ; and in that a water soluble, oxalate-containing chemical is added to the suspension liquid downstream of the Q-stage and/or is added to the pulp suspension immediately upstream of or in the Q-stage.

5. A method according to Claims 1-3, characterized in that the cellulose pulp suspension is treated serially with oxygen gas (0), chelating agent plus chlorine dioxide (Q/D) and peroxide (P), and in that a water soluble, oxalate- containing chemical is added to the suspension liquid downstream of the Q/D-stage and/or is added to the pulp suspension immediately upstream of or in the Q/D-stage.

6. A method according to Claims 1-3, characterized in that the cellulose pulp suspension is treated serially with oxygen gas ( 0 ) , chelating agent ( Q ) , chlorine dioxide ( D ) and peroxide (P); and in that a water soluble, oxalate-containing chemical is added to the suspension liquid downstream of the Q-stage and/or the D-stage and/or is added to the pulp suspension immediately prior to or in the Q-stage and/or the D-stage.

7. A method according to Claims 4-6, characterized in that the oxygen gas treatment (0) of the cellulose pulp suspension is divided into two or more stages with or without intermediate washing of the cellulose pulp suspension.

8. A method according to Claims 4-7, characterized in that the cellulose pulp is treated with acid (A) at a pH of 2-4 after the oxygen gas treatment stage (0).

9. A method according to Claims 1-8, characterized in that the water soluble, oxalate-containing chemical is oxalic acid.