A PROCESS FOR TREATING A FIBER PULP WITH A CHEMICAL SOLUTION

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: A process for treating a fiber pulp with a chemical solution The present invention relates to a continuous-working treatment of a fiber pulp with a chemical solution by forming from a pulp suspension a pulp bed on a filter surface and by feeding the bed through one of several chemical treatment steps in which a chemical solution is displaced through the bed. When a process of this type is used for the bleaching of a cellulose material it is in general called displacement bleaching or dynamic bleaching. When it is desired to produce a cellulose fiber having a high degree of brightness, the fiber has to be bleached after the actual defibration process. For example, in order to obtain a sulfate pulp with a high degree of brightness, it normally has to be bleached in five steps by using chlorine and chlorine dioxide as the bleaching chemical and sodium hydroxide as the extraction chemical. So far the most common process in this connection has been first to mix the chemical solution with the pulp suspension, thereafter to direct the mix (pulp + chemicals) through the re¬ tention tank in order that the chemicals have time to diffuse sufficiently and to react with the fiber material. Thereafter the reaction products which have formed are washed off. In order to obtain sufficient diffusion, depending on the chem¬ ical, a relatively long retention time is used. For example, when chlorine dioxide is used, the normal retention time is 3 - 5 hours. For this reason the apparatus reguired is relatively large and expensive. It was observed long ago that by using the displacement method instead the mixing method it is possible substantially to shorten the retention time reguired. In the 1960s, industrial equipment suitable for this process began to be developed. Some of the expected advantages were indeed achieved, for example it was possible to decrease substantially the heat requirement and water consumption of the bleaching process as compared with the technology which had been used until then. On the other hand, the consumption of bleaching chemicals increased somewhat. However, the conventional technology was also developed at the same time, especially the mixers used in it. For this reason the system based on the displacement technology was no longer as competitive as had been expected. Today, displacement bleaching is used primarily when a fiber material easy to bleach is concerned. The bleaching process based on the displacement method, used so far, is briefly as follows: Through a pulp bed which has been formed there is displaced a chemical solution having a liquid volume approximately equal to the liquid content of the bed. Normally a quantity coefficient 1 - 1.1 (quantity coefficient = volume of chemical solution used, divided by the liquid content of the bed) . The concentra¬ tion of the active chemical in the displacing chemical solution is adjusted so that most of the active chemical is consumed in the treatment of the pulp bed. Before the removal of the above- mentioned chemical solution, and together with it the formed reaction products, from the bed by displacing them with the chemical solution used in the subsequent step, or in the case of the last bleaching step with washing water, the pulp bed is directed through a so-called stationary step, with a sufficient retention time, in order that the concentration profile pro¬ duced in the bed after the displacement should have time to level out under the effect of diffusion. It has been noted that, in order to achieve by this method a result as good as that achieved by the conventional method, the retention time of the above-mentioned stationary step should be in the same order as the retention times in the conventional method. The object of the present invention is to avoid the above- mentioned disadvantage by using a displacement process which requires no stationary step for achieving an even chemical treatment throughout the pulp bed. The bleaching process based on the displacement method is in many respects similar to the washing process based on the dis¬ placement method, but there are between them also great dif¬ ferences which it is important to take into account expediently in the further development of the bleaching process. In both processes there occurs a similar mixing of liquids due to dis¬ persion and to the fact that the liquid flow in the fiber bed is laminar. For this reason, when the liquid content of the pulp bed is displaced with another liquid, the displacement never occurs ideally as a plug, but there always occurs mixing between the liquids, which is disadvantageous in both the wash¬ ing process and the bleaching process. This disadvantageous mixing can be reduced in both processes by carrying out several successive displacements according to the countercurrent prin¬ ciple. However, the higher the number of displacement steps used, the higher the respective equipment costs. In the washing of pulp there occur no chemical reactions, and the principal purpose of the process is to separate from the pulp, as carefully as possible, the chemicals present in the solution in the pulp bed. In the washing of pulp, when the quantity coefficient of the displacement increases, the re¬ covery of chemicals improves, and for this reason a relatively high quantity coefficient is in general used, although the re¬ covered liquid content therefore increases and respectively the evaporation requirement of the recovered solution increases. In displacement bleaching, on the other hand, in the manner it has been implemented so far, it is not advantageous to use a high quantity coefficient, since in that case the active chemi¬ cal losses in bleaching respectively increase, owing to the fact that the active chemicals displaced through the pulp bed are not recovered in this case. Both in washing and in bleaching, the displacement is based on the cross-flow method, in which the liquid flow through the bed during the displacement is perpendicular to the travel direc¬ tion of the bed. Since pulp fibers are compressible, the consistency of the pulp bed is not constant throughout the bed; it increases exponen¬ tially towards the filter surface. This profile is, of course, dependent on the type of the pulp concerned and on how large a pressure difference is used to produce the liquid flow. Normal¬ ly it rises from a consistency value of approx. 2 % to approx. 20 %, being on average approx. 10 %. From the viewpoint of the efficacy of the wash this phenomenon does not have a great significance, but in bleaching, in which it is important that all the fiber layers in the bed receive a sufficient quantity of active bleaching chemicals, it is of especially great significance, in particular since, in addi¬ tion, the concentration of the active chemicals in the solution decreases continuously as the solution flows toward the filter surface. This is due to the above-mentioned dispersion phenome¬ non and the fact that chemicals are consumed as they react with the pulp layers through which they flow. For this reason, in the displacement bleaching method so far used, the pulp layers which are at a greater distance from the filter surface are treated with a chemical solution which has a great excess of chemicals as compared with the requirement. Respectively, the layers close to the filter surface do not receive a sufficient amount of active chemicals, or receive no active chemicals, during the displacing, and so the bleaching of the pulp bed is quite uneven. It has been observed, however, that the use of excess active chemicals is not detrimental by causing undesirable secondary reactions, such as disintegration of cellulose and fibers, if the process conditions such as the temperature and the pH are maintained at the correct values. However, the excess chemicals must be removed from the pulp before the subsequent chemical treatment, during which the process conditions will again be different. When the displacement technique is used for bleaching pulp, the most important thing is that all the layers of the pulp bed are treated with a sufficient amount of chemicals and that, in or¬ der to ensure this, a solution is used in which the amount of active chemicals is substantially greater than what is required for complete treatment of the whole bed. The object of the present invention is to provide this kind of pulp bed treatment, but in such a manner that through losses of active chemicals there is not caused a higher consumption of chemicals than in a normal bleaching process. The main characteristics of the invention are given in the ac¬ companying claims. Briefly, the process according to the invention is as follows: A pulp bed is formed from a clean washed pulp to be bleached, and the washing liquid present in it is displaced by using as the displacing liquid the first bleaching solution (which con¬ tains as the active chemicals, for example, chlorine and/or chlorine dioxide), which contains active chemicals in excess, preferably approx. double the amount required in this step for the complete chemical treatment of the whole pulp bed. Im¬ mediately, without a delay after this displacement, displace¬ ment is continued with another solution or several solutions to the degree that the solution used in the first displacement and the active chemical quantity present in it has in its entirety or almost in its entirety been displaced from the bed. The filtrate displaced from the bed during these displacements is divided into two fractions so that the first fraction con¬ tains the above-mentioned washing liquid and the first dis¬ placed bleaching solution which has reacted with the bed and contains in the main only spent bleaching chemicals and bleach¬ ing reaction products. The second fraction of the filtrate con¬ tains most of the bleaching chemicals which have not been spent during the displacement. This solution fraction is recovered and reinforced with fresh bleaching chemicals in the same amount as is the consumption of the active chemical in the step, before it is reused in the same chemical treatment step. In the process according to the invention, one bleaching step contains not only one displacement but displacements with at least two solutions. Respectively, the filtrates are divided into at least two fractions. The bleaching steps described above may be included several in succession in the process. In the manner described above it is possible to select freely the quantity coefficient to be used in the displacement, and it may be substantially higher than one without increasing the losses of bleaching chemicals. On the contrary, as is the case in the washing process, there is obtained a bleaching process in which the efficacy and the use and recovery of chemicals are improved as a higher quantity coefficient is used. In this bleaching process, as was shown above, there may thus be one or several liquid zones in the same bed cross section, depending on how high the partial quantity coefficients are which have been used for the different liquids. The quantity coefficient of the recovered filtrate which contains the active chemical determines how effectively the unused active chemicals are recovered. The higher this quantity coefficient is, the better the recovery. Through this there is also determined the quantity coefficient for the chemical solution used for the displacement, since it has to be the same as the quantity coef¬ ficient of the recovered filtrate, added with the effect of the quantity of fresh chemicals. In terms of efficient exploitation of the bleaching chemicals, the recovery of the above-mentioned active chemicals effective¬ ly displaced through the bed is important in order that as little as possible of the solution of the step, containing ac¬ tive chemicals, should become mixed with the active chemical solution used in the subsequent treatment step. For this reason and owing to the fact that in a treatment step it is possible to use a high total quantity coefficient without thereby in¬ creasing the losses of active chemicals, the active chemical solution of the directly subsequent treatment step is not used for displacing the active chemical solution of the step con¬ cerned, but a solution as neutral as possible with respect to both treatment steps is used, for example preferably pure water at a suitable temperature. If, for example, a chlorine solution is displaced directly with a sodium hydroxide solution, owing to dispersion the solutions become mixed with each other in the boundary zone. Thus they partly cancel each other's positive effect in terms of bleaching and, in addition, disadvantageous secondary reactions are produced. Owing to the drastic pH change, for example, the chlorine solution may be converted to a hypochlorite solution. In some cases it may, however, be advantageous to use a fil¬ trate which contains only chemicals or reaction products which have become spent from the viewpoint of the bleaching process. The filtrate which is removed from the fiber bed through the filter surface during the chemical treatment by displacing it with the active chemical solution and the neutral solution used in the step changes with respect to its chemicals content as the pulp bed proceeds during the progress of the displacement. Its first fraction contains mainly chemicals used and spent in the previous step and the reaction products produced. The frac¬ tion then changes more slowly or faster, according to the strength of the dispersion caused by the bed, to a filtrate which mainly contains the active chemicals used in the treat¬ ment step in question, which chemicals have become non-active when reacting with the fiber material of the pulp bed, and reaction products which have been formed. This fraction then in a corresponding manner changes to a fraction which in the main contains active chemicals used in the treatment step which have not been spent through reaction and correspond to the excess of active chemical which is used in the treatment step. This frac¬ tion for its part changes, when the bed proceeds, in a cor¬ responding manner to a fraction which in the main contains chemicals which are derived from the neutral solution which is used as a displacement solution in this step immediately after the active chemical solution. If this solution, as is normally recommendable, is pure water, the above-mentioned fraction which contains active chemicals changes to nearly pure water before it again changes to a solution which contains spent chemicals used in the subsequent treatment step and the formed reaction products, unless the step in question is the last treatment step. The third filtrate fraction above, which contains active chemi¬ cals which have not been spent in the reactions in the ' treat¬ ment step, are recovered as carefully as possible and are re¬ used together with fresh added chemicals as a chemical solution in the same treatment step. The portion of this solution which can be recovered is, of course, the active chemical solution quantity of the step minus the quantity of fresh chemicals. Since the solution quantity to be recovered is limited in the above-mentioned manner and, in addition, owing to the disper¬ sion in the fiber bed, it is not possible to recover 100 per¬ cent the total active chemical quantity which is displaced through the bed; a small proportion of it ("tail") is always left in the filtrate fraction displaced before it from the bed, and respectively a second "tail" in the filtrate fraction dis¬ placed after it. The maximum recovery of active chemicals is obtained when it is possible to adjust the removal of the filtrate fractions so that the maximum concentration of active chemicals in both of the "tails" is the same, i.e. the concentration of active chem¬ icals in the filtrate is the same at both division points of the filtrate fractions. The filtrate could be divided into different fractions simply by dividing the filtrate chamber behind the screen surface by means of partitions sealing tightly to the screen. In this case it would, however, not be possible in all operating situations to achieve maximal recovery of chemicals, since the location of the chemical profile of the filtrate in the equipment used shifts when the ratio between the displacement liquid flow rate and the filter surface travel speed changes. In addition to this, the location of the chemical profile between the chemi¬ cals spent in the reaction and the still active chemicals is affected by the proportion of the used active chemicals spent in the treatment of the bed. This proportion depends, among other things, on how much residual lignin or a similar chemicals-consuming constituent is present in the pulp bed which is being treated. By making use of the equipment system disclosed in our copending Finnish Pate Application891661 for the implementation of the process dis¬ closed in the present application it is possible also in the above-mentioned varied operating situations to adjust the with¬ drawing of the filtrate fraction which contains active chemi¬ cals in such a manner that the recovery of the active chemicals is always maximized. The filtrate chamber is divided by means of partitions which leave a uniformly wide slit in relation to the filter surface, through which slit part of the filtrate fraction can pass into the preceding or subsequent filtrate fraction. When the con¬ centrations of active chemicals are measured at the slits of both partitions separating the filtrate fraction which contains active chemical from the other filtrate fractions and the mu¬ tual withdrawing ratio of these other filtrate fractions is adjusted according to it so that both measured concentrations will be the same, an optimal recovery of chemicals is obtained, as shown above. When the equipment system according to our copending Finnish Patent implicati 891661 is used for implementing the present bleaching process, the most recommendable embodiment is that the principal vari¬ ables of the process are in the same order as when the equip¬ ment is used for washing pulp. The thickness of the pulp bed is 20 -100 mm, preferably approx. 50 mm. The pressure difference for accomplishing the liquid displacement is 1 - 4 meters of water head. The travel speed of the screen is 0,2 - 1 m/s, preferably approx. 0.5 m/s. The total quantity coefficient of the displacement of one treatment step is >1.5, preferably >2. The quantity coefficient of the active chemical solution is approx. one-half of these values. On the basis of the above values, the total retention time of the fiber bed in one treat¬ ment step will be 10 - 50 seconds, normally approx. 20 seconds. Since the most preferred embodiment of the present invention is to use in each displacement bleaching step a high quantity co¬ efficient, i.e. a coefficient approx. twice that in the dis¬ placement bleaching processes used so far, and in addition to divide each step into at least two partial steps, the use of this bleaching process requires that there be available a method and equipment required for implementing the method, en¬ abling several successive displacement steps to be implemented in one and the same apparatus and that, when steps are added their marginal costs are relatively low and also the equipment costs calculated per effective screen surface are economical. If it is not possible to carry out all bleaching steps in the same apparatus and the pulp web has to be transferred from one apparatus to another between the bleaching steps, this transfer of the pulp causes an almost complete mixing of the pulp bed and its liquid content. In order to be able in this case to avoid the mixing of the different chemical solutions with each other, the transfer must take place so that the liquid content of the entire bed is in the main of only one chemical solution, preferably water or a neutral solution. This means that the partial quantity coefficient of this chemical solution must be at minimum one, preferably higher. In the present process the surface layer of the fiber bed is treated with an approx. double amount of chemicals as compared with the bottom layer. This non-uniform distribution does not, however, affect the uniform chemical treatment of the bed, since, if we examine the individual fiber layers of the bed, through each layer there flow active chemicals the amount of which approaches infinite in ratio to the amount that the fiber layer concerned spends in order to achieve a complete reaction. Thus a sufficiently high probability is achieved in each fiber for a reaction between the bleaching chemical and the residual lignin, the probability being independent of the treatment period, i.e. it is independent of the retention time of the bed in the bleaching step. From this it also follows that the pulp bed need not be thick; its optimal thickness is determined on the basis of the fact that the liquid dispersion caused by the bed is sufficiently small without an unnecessarily large filter surface being required. The above means that the forming and maintaining of a homoge¬ nous pulp bed is advantageous for the present process. In addi¬ tion, it is especially important for this process that it is possible to separate from each other the two filtrate frac¬ tions, the first of which contains mainly reaction products and the second active chemicals, before they become mixed with each other. In addition it is required that it is possible to mea¬ sure the concentration of chemicals immediately behind the fil¬ ter surface and accordingly to determine and adjust the fil¬ trate rates in an optimal manner. The present process requires a maximally rapid transfer of chemicals, which is based on a forced, directed flow not only around the fibers but also through them, because the retention time is so short that no significant transfer of material can take place through diffusion. For this reason it is also neces¬ sary to aim at the system remaining a two-phase system, which requires that the pressure in the system will not be below the total equilibrium pressure of the gases and vapors. In this case a maximum proportion of the gases and vapors remains dis¬ solved in the liquid phase, and no disturbing gas bubbles or gas phase is formed in the bed during the treatment. The process and the apparatus developed for it, which well meet these prereσuisites and conditions, are disclosed in our copending Finnish Pa Application 891661 By using the method and apparatus of the said application for the present bleaching process, the follow¬ ing advantages, among others, are gained over the conventional processes which utilize mixing and over the displacement bleaching processes used thus far: In terms of process and equipment technology, the bleaching process becomes simple, since it is possible to carry out several bleaching steps in one apparatus. The consumption of energy is low, since the need for pumping liquid is small and there is no need for mixing. The consumption of bleaching chemicals is less than in nor¬ mal bleaching, since it is possible effectively to keep the solution which contains active chemicals separate from the solutions produced during the bleaching process which con¬ tain reaction products which could otherwise further consume bleaching chemicals if they were allowed to mix with them. It is possible to maintain optimal process conditions in the various bleaching steps and, also, fiber clusters which have been delignified poorly before the bleaching will be treated with a sufficient amount of chemicals. These factors provide the preconditions for accomplishing a high and even bright¬ ness of the pulp and a low dirt content, and good strength properties are maintained.

Claims: Claims 1. A process for 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 sev¬ eral chemical treatment steps in which a chemical solution is displaced through the bed, characterized in that in each treat¬ ment step there are used two different treatment liquids which are displaced successively through the bed, the first treatment liquid being 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, and the second treatment liquid being a solution which does not contain significant amounts of active chemicals in terms of the process and the purpose of which is not only to displace that portion of the first treatment liquid the active chemicals of which have been spent but also that portion which still con¬ tains active chemicals upon being removed as a filtrate, from which that fraction is recovered which has the highest con¬ centration of active chemicals and contains most of the active chemicals removed from the bed and is in quantity only so large that, when reinforced with a fresh chemical solution, it can in its entirety be reused as the first treatment liquid. 2. A process according to Claim 1, characterized in that the total liquid volume of the two treatment liquids of each treat¬ ment step is substantially greater than the liquid volume pres¬ ent in the fiber bed upon its leaving the treatment step, pref¬ erably 2- to 3-fold. 3. A process according to Claims 1 - 2, characterized in that the second treatment liquid used in a treatment step is water and/or displaced liquid from some other treatment step, which liquid in the main contains only reaction products. 4. A process according to Claims 1 - 3, characterized in that the filtrate which leaves the bed in the chemical treat¬ ment step of the bed is divided into three successive frac¬ tions, of which the middle fraction contains most of the active chemicals leaving the bed and is reused together with fresh chemicals in the same treatment step, by measuring the con¬ centration of active chemicals at the point of division between the middle fraction and the first fraction and at the division point between the middle fraction and the third fraction, and by adjusting on the basis of this measurement the volume ratio between the first and the third removed fractions in such a manner that the concentration of active chemicals at both divi¬ sion points is approximately the same, so that a maximal pro¬ portion of the active chemical amount displaced from the bed can be recovered for reuse. 5. A process according to Claims 1 - 4, characterized in that in the chemical treatment steps the fibrous or finely-di¬ vided material is transferred in the horizontal direction and the filtration of liquid and the displacement of liquid are carried out by means of a horizontal liquid flow through the bed, perpendicular to the direction of travel of the bed, in such a manner that the filtration and the displacement of li¬ quid are carried out while the filter surface is continuously in a liquid and in a vertical position in relation to the hori¬ zontal travel direction by means of a hydrostatic pressure dif¬ ference across the filter surface.

Inventors: OESTMAN HAAKAN, OESTMAN, HAAKAN
Applicants: HACKMAN PROSESSI OY AB, SALOMANS OY
Application year: 1990
Main classification: D21C9
IPC codes: D21C9, D21C11