FI85513B - FOERFARANDE FOER BEHANDLING AV FIBERMASSA MED KEMIKALIELOESNING. - Google Patents

Abstract

The process relates to continuous treatment of a fiber pulp with a chemical solution by forming on a filter surface a bed of a fiber material suspension and by feeding it through one or several chemical treatment steps in which the chemical solution is displaced through the bed. According to the method, there are used in each treatment step two different treatment liquids which are successively displaced through the bed. The first treatment solution is a chemical solution which contains a greater amount of active chemicals than is consumed in the reactions occurring in the pulp bed, preferably approx. twice the amount. The second treatment liquid is a solution which does not contain active chemicals in an amount significant in terms of the reaction. Its purpose is not only to displace that portion of the first treatment liquid from which the active chemicals have been spent but also that portion which still contains active chemicals upon leaving as a filtrate. Of the filtrate of the treatment step, that fraction is recovered which has the highest concentration of active chemicals and contains most of the active chemicals leaving the bed and is in its amount only so large that, when reinforced with a fresh chemical solution, it can be used in its entirety as the first treatment solution. The total liquid volume of the treatment liquids is substantially greater than the liquid volume present in the bed upon its leaving the treatment step - preferably 2- to 3-fold.

Description

1 85513

This invention relates to the continuous treatment of fibrous pulp with a chemical solution by forming a pulp bed through a filtration surface and feeding it with one or more chemical mixtures. When this type of process is used to bleach 11uIose material1, it is generally referred to as displacement bleaching or dynamic bleaching.

When it is desired to produce a pulp fiber having a high brightness, it has to be bleached after the actual defibering process. For example, in order to obtain a high-brightness pulp, it is normally required to be bleached in five steps: using chlorine and chlorine dioxide as the waxing chemical and sodium hydroxide as the extraction chemical.

• · · Until now, the most common method in this context has been to first mix the chemical into a pulp suspension, after which the mixture (pulp + chemicals) is passed through a residence time to allow the chemicals to diffuse sufficiently and react with the fibrous material. The reaction products formed are then washed off.

• · ·

To achieve sufficient diffusion, depending on the chemical, a relatively long residence time is used.

• · «·. ·» ·, Eg when using k 1 oor id ioks idia normal residence time ♦ · 'Γ. 25 is 3 to 5 hours. As a result, the equipment required • · • ’·· becomes relatively large and expensive.

• · ·

It has long been found that by using the displacement method1 instead of the mixing method, the required residence time can be substantially reduced by 2,85513. In the 1960s, the development of industrial equipment suitable for this process began. Indeed, the expected advantages were partially achieved, for example, the heat demand and water consumption of the bleaching process 5 could be substantially reduced compared to the technology used until then. Consumption of chemical chemicals, on the other hand, increased somewhat.

At the same time, however, the technology of the convention was further developed, in particular the mixers used therein. As a result, the solution based on displacement technology was no longer as competitive as expected. Today, displacement protection is mainly used in the case of a fibrous material which is easily bleached.

Briefly, the bleaching process based on the displacement method used so far is as follows:

The chemical is displaced through the formed pulp bed, the amount of liquid of which is approximately the same as that of the bed **: 20 liquid interior 1 work. Normally a volume factor of 1 to 1,1 is used (volume factor = the amount of Chemistry solution used divided by the amount of the liquid in the bed). The active chemical concentration of the displacing chemical solution is adjusted so that most of the active chemical is consumed in the treatment of the pulp bed. Before the above-mentioned chemical solution and the reaction products formed with it are removed from the bed by displacing it • «with the chemical used in the next lie to 1 1 a, or · in the case of the last silencing step, with washing water, 30 A pulp bed with a sufficiently delay is passed. through a static phase, in order to allow the concentration profile in the bed after displacement to level off due to the effect of diffusion e · ’.

3,85513

It has been found that in order for this method to achieve as good a result as the conventional method, the residence time of the above-mentioned stationary phase should be of the same order as the residence times in the conventional method.

The object of the present invention is to avoid the above-mentioned drawback by using a displacement method in which no stationary step is required at all to achieve a uniform chemical treatment throughout the pulp bed.

10 The bleaching process based on the displacement process is in many respects similar to the bleaching process based on the displacement process, but there are also large differences between them, which are important to take into account appropriately when further developing the bleaching process.

15 In both cases, similar mixing of liquids occurs due to dispersion and the fact that the liquid flow ·. the fiber bed is laminar. For this reason, when displacing the liquid content of the pulp bed with another liquid, displacement el '· / · never occurs ideally as a stopper, but always occurs *: **; A mixture between 20 liquids, which is both washing and; * · *; unfavorable in the bleaching process. This disadvantageous mixing can be reduced in both processes by performing successive displacements on the countercurrent process11a. However, the more displacement steps used, the higher the hardware cost.

The washing of the pulp does not involve any chemical reactions and its main purpose is as accurate as possible. separates the chemicals contained in the pulp bed solution from the pulp.

_ * * 30 In pulp washing, as the displacement rate increases • "the recovery of chemicals is improved and therefore a relatively high volume factor is usually used, although ♦ 85513 the amount of liquid recovered increases and the need for evaporation of the recovered solution increases accordingly.

In displacement bleaching, on the other hand, as has hitherto been carried out, it is not advantageous to use a large quantity factor of 5, because then the active chemical losses of the bleaching increase correspondingly due to the fact that no active chemicals are taken or passed through the pulp bed.

In both washing and bleaching, displacement is based on a transverse flow method in which the flow of liquid through the bed during displacement is perpendicular to the direction of travel of the bed.

Because the pulp fibers are compressible, the consistency of the pulp bed is not constant throughout the bed, but rises exponentially as it travels toward the filtration surface. This profile, of course, depends on the type of mass ** involved and how large the pressure difference is used:. * To achieve fluid flow. Normally, it rises from a consistency value of about 2% to a consistency of about 20 K, being an average of about 10 X.

• · * · * *

From the point of view of washing efficiency, this phenomenon is not of great importance, but in bleaching, where it is important that all the fibrous layers of the bed receive sufficiently active bleaching chemicals, it is of particular importance, especially when the solution concentration is always active. decreases as it flows towards the filter surface. This is due to the above: * dispersion and the fact that chemicals are consumed as they react with the mass layers they flow through.

Therefore, in the displacement control method used heretofore, pulp layers 5 85513 farther from the filtration surface come to a treated chemical solution 1a with a large excess of chemicals compared to the need. Correspondingly, the layers close to the filtration surface do not receive enough 5 or no active chemicals during displacement, so that the bleaching of the pulp bed becomes quite uneven.

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However, it has been found that the use of additional active ingredients is not detrimental to cause undesired side reactions such as cellulose and fiber cleavage if process conditions such as temperature and pH are considered correct. However, the excess chemicals must be removed from the pulp before the next chemical treatment, in which case the process conditions are different.

15 When a displacement technique is used to bleach the pulp, it is most important that all layers of the pulp bed are treated with a sufficient amount of chemical and that a solution is used that substantially exceeds the amount required to completely treat the entire bed.

However, it is an object of the present invention to provide such a treatment of the pulp bed in such a way that no higher chemical consumption is generated through the losses of the active chemical than in the normal bleaching process.

The main features of the invention appear from the appended claims.

- * · The method according to the invention is briefly as follows; / · A pulp bed is formed from the cleaned »#" bleached pulp and the washing liquid contained therein is displaced • e · 30 using the first solution 6 85513 (containing, for example, chlorine and / or chlorine dioxide as active substance), which contains an excess of active chemicals, most preferably about twice as much as is required for a complete chemical bed of the entire pulp bed Immediately without delay after this displacement, displacement is continued with the second or more solutions until the total displacement used in the first displacement is almost 10% or less. out of bed.

During these displacements, the filtrate displaced out of the bed is divided into two parts, the first part containing the above-mentioned washing liquid and the first discarded bleach solution which has reacted with the bed 15, containing mainly spent bleaching chemicals and bleach reaction products. The second part of the filtrate contains most of the stabilizing chemicals that have not been worn during displacement. This portion of the solution is recovered and fortified fresh. reused in the same Kemi kaa 1 i kas i tte 1 y step.

• «• · *. *. In the method according to the invention, one stabilization step el a * contains only one displacement, but displacements 25 with at least two solutions. Correspondingly, the filtrates are divided into at least two parts ** * The steps described above may be · * · * several in succession in the process.

• · '**. As described above, it is possible to freely choose the amount factor to be used in the displacement and it can be substantially higher than one, without increasing the losses of the sink. On the contrary, as in the case of a washing process, a bleaching process is obtained, the efficiency of which and the utilization and recovery of chemicals are improved according to the higher the coefficient used.

5 In this bleaching process, as indicated above, the same bed cross-section may thus have one or more liquid zones, depending on how large sub-volume factors have been applied to different liquids. The amount factor of the recovered active chemical-containing filtrate 10 determines how efficiently the unused active chemical chemicals are recovered. The higher this coefficient, the better the recovery. This also determines the amount of the chemical solution used for displacement, since it must be the same as the amount of the recovered filtrate plus the amount of fresh chemicals.

For the efficient utilization of chemical chemicals, it is important for the recovery of the above-mentioned efficiently * *· - active chemicals to be mixed with as little of the active compound as possible in the following steps:: For this reason, and due to the fact that a large total amount factor can be used in the treatment step without increasing the losses of the active chemical, the active ingredient solution of the next treatment step is not used directly to displace the * 30 suitable pure water If, for example, the chlorine solution is directly displaced to sodium hydroxide 1, the solutions will mix in the middle due to the dispersion. In this way, they partially • · 4 m *** cancel out the positive effect of S 85513 on each other's bleaching and, in addition, unfavorable side reactions can occur. Due to the strong change in pH, for example, the chlorine solution may turn into a hypocrital solution.

In some cases, however, it may be advantageous to use a filtrate containing only the chemicals or reaction products consumed for the bleaching process.

The filtrate removed from the fiber bed through the filtrate surface during the core treatment step by displacing it with the active chemical solution and the neutral solution used in step 10 changes in its chemical content as the pulp bed progresses as the displacement progresses. Its first fraction contains mainly the chemicals used in the previous step and the reaction products formed. The fraction then changes more slowly or more rapidly, depending on the strength of the dispersion caused by the bed, to the filtrate, which mainly contains the active chemical used in the treatment step 1 e And which e \ t · have changed e i-Akti ivisi ks i in reaction with the pulp bed i: 20 with the fibrous material and the reaction products formed.

• e e: ··· This fraction is then similarly converted into a fraction, ”which mainly contains those used in the processing step. ·. Act i vikemi kaa e e and which are not consumed through the reaction and correspond to the excess of active chemical 25 used in the treatment step. This fraction, on the other hand, converts in a similar manner to a fraction which mainly contains chemicals derived from the neutral solution used as a displacement solution in this treatment step immediately after the active solution. If this solution is, “· Γ ··. as is normally recommended, pure water, the above fraction containing active chemicals becomes ** almost pure water before it is again converted into a solution containing 9 85513 chemicals consumed in the next treatment step and the reaction products formed if that step is not the last step. .

The third filtrate fraction above, which contains active chemicals not consumed in the reaction treatment step, is recovered as accurately as possible and reused as fresh solution in the same treatment step as the fresh addition chemicals.

The amount of this solution that can be recovered 10 is, of course, the amount of active chemical solution in the step minus the amount of fresh chemical.

Because the amount of solution to be taken is limited as mentioned above and, in addition, due to the dispersion of the fiber bed, the total amount of active core displaced through the bed 15 cannot be recovered 100X, but always a small part ("tail") remains above the bed. * · To the filtrate fraction and the corresponding "tail" thereafter to the displaced filtrate fraction.

* ··; The recovery of the maximum active chemical is obtained when it is possible to adjust the extraction of the filtrate fraction so that the maximum concentration of both "tails" of the active chemical 1 is the same, i.e. the active chemical concentration of the filtrate is the same at both distribution points of the filtrate.

* - * ‘25 The filtrate could be simply divided into different fractions by dividing the filtrate space behind the sieve surface by partitions condensing into the sieve. In this case, however, it would be possible to achieve * * »/ · maximum chemical recovery in all applications, because the position of the filtrate •“ 30 kernikaa1iprof 11I in the used device shifts, • · · • · * ... * when the difference between the displacement fluid flow rate and the filtration surface 10 8551 3 the relationship changes. In addition to the above, the location of the 4 chemical profiles between the spent chemicals in the reaction and the still active chemicals is affected by how much of the 5 active core materials used has been spent in the bed treatment.

This depends on e.g. the amount of residual lignin or similar chemical consuming portion of the pulp bed to be treated.

Utilizing the 10 device solutions disclosed in patent application 891661 to implement the method disclosed in this application, it is also possible to adjust the extraction of the filtrate fraction containing Akti 1 vernerns in the above-mentioned operating gear changes so that the recovery of active chemicals is always maximized.

15 The filter space is divided by partitions which leave a gap flat in relation to the filtrate surface, through which part of the filtrate fraction can pass to the previous or next filtrate fraction. When measuring the concentration of the chemical at the gaps of both partitions ... 20 separating the filtrate fraction which contains the active chemical from the other filtrate fractions and • · · 'adjust the ratio of these other filtrate fractions to such a' 1 the concentration platelets become the same, resulting in optimal chemical recovery, as shown above.

When using the device according to patent application Θ91661 to implement this solution, it is preferred that: *: the embodiment is that the main variables of the process are the same. order of magnitude than when applying the pulp washing device.

The thickness of the pulp bed is 20 to 100 mm, most preferably about 50 mm. The pressure difference to cause liquid displacement is 1 - ·. ’·· 4 meters from the water column. The speed of the sieve advances is 0.2 to 1 · ...: m / s, most preferably about 0.5 m / s. The total volume factor of the displacement of one treatment step “85513” is> 1.5, most preferably> 2. Of these figures, the volume factor of the active chemical solution is about half. Based on the values 5, the total delay of the fiber bed in one processing step will be 10 to 50 seconds, normally about 20 seconds.

Since the most preferred embodiment of this method is to use a high amount of coefficient in each displacement step, i.e. about twice as large as the displacement method used hitherto, and further divide each step into at least two sub-steps, the use of this bleaching method requires the method and an apparatus which allows several successive displacement steps to be carried out in the same device and that, when the steps are added, their marginal costs are relatively small and that the equipment costs per effective screen surface are also advantageous.

: 20 If not all of them can be performed in the same device, but the pulp track has to be transferred from one device to another, this mass transfer causes the pulp bed and its liquid to mix almost perfectly. In this case, in order to avoid mixing the different chemical solutions, the transfer should take place in such a way that the entire liquid content of the bed is essentially only one chemical solution, preferably water or a neutral solution.

This means that the volume factor of this chemical solution must be at least one, preferably higher.

»· *« · * '* In this method, the surface layer of the fiber bed is treated with approximately' ... 'twice the amount of Chemical 1 compared to the' 2 85513 base layer. However, this non-distribution does not affect the uniform chemical life of the bed, because If individual fiber layers of the bed are considered, active chemicals flow through each layer, the amount of which is infinitely proportional to the amount consumed by that bed to achieve a complete reaction.

Thus, a sufficiently high probability for the reaction between the curing chemical and the residual lignin in each fiber 10 is achieved, which is independent of the processing time, i.e. it is independent of the residence time of the bed in the curing step. It also follows that the pulp bed does not have to be thick, but the optimal thickness is determined by the fact that the dispersion of the liquid 15 caused by it becomes sufficiently small without the need for an unnecessarily large filter collar.

The above means that the formation and maintenance of a homogeneous pulp bed is advantageous for this method. In addition, it is particularly important for this process: it is important to be able to distinguish between the two filtrates, the first of which contains mainly reaction products and the second of which is mixed before each other. In addition, it is required to be able to measure the chemical concentration immediately behind the filtrate surface and to determine and adjust the filtrate amounts in an optimal manner accordingly.

This method requires the fastest possible transfer of chemicals, based on a forced directed flow • not only around the fibers but also through them, Due to the fact that. 30 that the residence time is so short that no significant • · mass transfer can occur by diffusion. For this reason, '* efforts must also be made to keep the system biphasic, which requires that the system pressure does not I; 85513 may fall below the total equilibrium pressure of gases and vapors. In this case, as much of the gases and vapors as possible remain dissolved in the liquid phase, and no interfering gas bubbles or gas phase are formed in the bed during processing.

A method and an apparatus solution developed for this purpose, which well meet these requirements and conditions, are disclosed in patent application 891661. Using its method and equipment in this bleaching method 10, e.g. the following advantages over the conventional blending and displacement bleaching methods used so far:

From a process and technical point of view, the bleaching process becomes simple because it is possible to perform several bleaching steps in one device.

Energy consumption is low because the need to pump the liquid is small and there is no need for mixing.

'·' · - Consumption of solids becomes lower than in normal solids because it is possible to effectively keep the solution containing the active chemical from the bleaching process solutions containing reaction products that could otherwise advance the solids. if they were allowed to mix with them.

25 - Optimal process conditions can be maintained at different stages of bleaching and also poorly before bleaching: de 1 i gn i f i o iut fiber bundles are treated ____; amount of chemical 11a. These factors make it possible to achieve a high and uniform mass density and a low potency, and good strength properties are maintained.

Claims (5)

A process for sausage tuning treatment of fibrous pulp with a chemical solution by forming a bed on a filter surface of the fiber material suspension and feeding it through one or more chemical treatment steps, in which the chemical solution is forced through the bed, characterized in that each treatment step is used. various treatment liquids are gradually displaced through the bed, the first treatment liquid being a chemical solution containing more active chemicals, advantageously about two times more than that consumed by the reactions in the pulp bed and the second treatment liquid being a solution comprising the reaction does not contain significant amounts of active chemicals and its intention is, in addition to displacing the portion of the first treatment liquid whose active chemicals have been consumed, also the portion which still contains active chemicals upon its departure as filtrate, from which the fraction is recovered whose active chemical concentration is highest and which contains most of the active chemicals departing from the bed and which in its amount is only so large that it is enhanced with fresh chemical solution can be completely re-used as the first treatment liquid. . Process according to Claim 1, characterized in that - the total amount of liquid of two treatment liquids of each treatment step is substantially greater than the fluid amount of the fiber bed, since this departs from the treatment step, preferably 2-3 times. 30 3. A process according to claims 1-3, characterized in that the second treatment liquid used in the treatment step is water and or some other solution from the treatment stage, which is essentially compressed only containing reaction products. 4. A process according to claims 1-3, characterized in that the filtrate which separates from the bed in the chemical treatment stage of the bed is divided into three successive fractions, by: the middle fraction containing most of the active chemicals leaving the bed and used on the bed. new with fresh chemicals in the same treatment step, by measuring active chemical concentrations in the middle fraction's breakdown point to the first fraction and in its breakdown point to the third fraction and on the base thereof, controlling the amount ratio between the first and third removed fraction, in the bed the breakdown points are basically the same, so that a possible large part of the active chemical crowded from the bed is recovered for reuse. Process according to claims 1-4, characterized in that in the chemical treatment steps, the fibrous or finely divided material is moved horizontally and the liquid filtration and liquid exchange is carried out as a liquid flow through the bed in the horizontal direction, the filter is moved and the direction of movement is saturated. While the filter surface is constantly in the liquid and vertically horizontally with respect to the direction of movement by means of a hydrostatic pressure difference across the filter surface.