DE69839097T2 - BLEACHING PULP BY PERSONAL - Google Patents

Abstract

The invention relates to a process for the bleaching of chemical pulp and the use of a bleaching solution. According to the invention the pulp is treated in a plurality of different steps by using at least in one step a bleaching solution which contains a peracid, and the essential idea of the invention is that the peracid is used in a post-bleaching which is the last step of the bleaching process, the post-bleaching being carried out in the presence of one or more earth-alkali metal compounds. By using a peracid the pulp is brought to its final brightness, and the purpose of the earth-alkali metal added is to combat the lowering of the viscosity of the pulp. According to the invention the post-bleaching can be carried out after the bleach plant steps in a pulp flow pipe, a storage tower or in the paper machine at a paper mill.

Description

The The present invention relates to a process for bleaching chemical Pulp, the pulp in a variety of different steps is treated and containing a peracid solution in at least one of the steps is used. The invention additionally relates to Use of peracid-containing bleach solution in a method step according to the invention.

In one sooner Known Pulpedelignifizierverfahren is a raw pulp by pulping cellulose containing materials with suitable digestion materials a Treatment whereby the lignin is removed and a bleach, which involves the use of oxidative chemicals. The purpose of the conventional Bleaching of chemical pulp is the complete removal of lignin from a crude pulp obtained from a digestion process to induce. This bleaching can be through the use of chlorine or chlorine dioxide and subsequent alkaline extraction steps in which the lignin from the Pulp solved is carried out become. Currently The bleaching of a chemical pulp is increasingly common Bleaching without the use of elemental chlorine or chlorine compounds carried out. The first mentioned Bleaching will be ECF bleaching (ECF: elementarg chlorine free; elemental chlorine) and the latter TCF bleaching (TCF: totally chlorine free; completely chlorine-free). The chemical pulp is in bleached in successive steps. nowadays bleaching becomes common started with an oxygen delignification, after which the other Bleaching is carried out by various methods can. In TCF bleaching the delignification z. B. by the Use of ozone, peracetic acid or hydrogen peroxide under acid or alkaline conditions to be continued. In ECF bleaching, chlorine dioxide steps and then use alkaline extraction steps.

It gives an earlier Known method in which the last bleaching step for chemical Pulp is performed by the use of peracetic acid. peracetic acid The easiest way to do this is by mixing vinegar and hydrogen peroxide after which a so-called equilibrium solution is obtained which in addition to peracetic acid also contains unreacted starting substances. Also, by distillation a pure peracetic acid from this reaction solution getting produced. There are also other methods of manufacturing of peracetic acid known.

Peracetic acid (PAA) is a supreme selective bleaching chemical, by their use good strength properties the pulp can be maintained. Will be peracetic acid used is the optimal pH is about 4-7. At a lower pH, peracetic acid is a highly selective delignification chemical. At a higher pH, their bleaching effect is greater, their selectivity decreases but off.

The U.S. Patent 5,552,018 describes multi-step processes for equation and delignification of pulp. These include a sequence having a final peracetic acid step and a sequence having a peracetic acid step with the use of a magnesium salt and a phosphonic acid to stabilize the peracetic acid, followed by a final alkaline extraction step. Delignification was represented by reduced kappa numbers and increased brightness.

Also describes the U.S. Patent 4,222,819 a bleaching sequence, including a peracetic acid step with magnesium sulfate, followed by alkaline extraction of soluble lignin as the last step of the sequence.

By modern bleaching processes can with both ECT and TCF pulps slightly high degrees of brightness can be achieved. However, achieving can be of very high levels of brightness by conventional methods considerable Consume quantities of chemicals. Similarly, after a normal bleaching process the variation of the degree of brightness of the pulp and the pH of the pulp paper stock running on the paper machine runtime problems in the paper machine. The pulp brightness can also in storage towers be reduced, in which case the pulp significantly above the required degree must be bleached, hence the reduction Brightness for it is compensated.

A good way to solve these problems is to bleach. Suitable post-bleaching agents include peracetic acid and dithionite. The use of hydrogen peroxide is limited by the fact that it would require alkaline conditions to function properly. In terms of pH, dithionite and peracetic acid are extremely suitable for post-bleaching. Dithionite is commonly used in the bleaching of mechanical pulps but is not normally used in the bleaching of chemical pulps. The sulfur present therein can also cause problems. Nachblei Using peracetic acid is a very effective method of increasing brightness. The consumption of chemicals for bleached pulps is low and, moreover, the reaction is fast even at low consistency. In addition, the optimum pH range for bleaching is suitable for post-bleaching. However, the bleaching with peracetic acid of chemical pulps involves the problem that the selectivity is no longer the best within the optimal pH range for the bleaching result. In the case of a fully bleached pulp with a very low kappa number, the use of peracetic acid damages the fibers, causing the degradation of hydrocarbons and the decrease in pulp strength. This also causes the release of organic matter from the pulp, a factor that can interfere with the run of the paper machine.

The bleaching process according to the invention, the ones mentioned above Solves problems, is characterized in that a peracid used in the post-bleaching which is the last step of the bleaching process, is on delignified pulp having a kappa number of 4 or less is carried out, wherein the post-bleaching in the presence of one or more alkaline earth compounds takes place and after the bleaching stages in a Pulpefließrohr, a Storage tower and / or a paper machine is running, where the peracid Chromophore groups in the pulp makes colorless. Such a bleaching can be advantageously carried out as a separate step on the paper mill, to which the pulp was transferred from a bleaching plant.

The Alkaline earth metals such as magnesium and calcium, which coexist with a peracid in the process according to the invention used, stabilize hydrocarbons, reducing the reduction the pulp strength is prevented. In other words, the strength the pulp stays fine and the release of organic material is insignificant. The brightness and the whiteness the pulp are also higher as without the use of alkaline earth metals. This lowers z. B. the need for optical brighteners in the paper machine. If this method is used, the pH of the paper stock, which is to be processed into paper, and the brightness and the whiteness the pulp more evenly, with in this case, the running characteristics of the paper machine considerably are improved.

The Addition of magnesium to the pulp, z. In steps on oxygen and peroxide, is state of the art. Equally is the use of magnesium in conjunction with peracetic acid delignification z. B. from the publication Liebergott, N., 81st meeting, technical section CPPA (1995), B 157-B 170, known. According to the publication protects Magnesium the viscosity the pulp. However, there is the question of delignification, which is carried out on a pulp with a high kappa number. This delignification step is followed by a separate bleaching during which the brightness on their final Degree is raised. While the delignification become transition metals as Fe and Mn are released from the pulp, and magnesium is prevented their harmful ones Effects. The addition of magnesium will improve the ratio from magnesium to transition metal used, in particular, the passage of the subsequent alkaline Peroxide step improved.

in the Contrary to the above, the invention relates to post-bleaching, that on an already delignified pulp with a low Kappa number of 4 or less and which is free of transition metals, is carried out where the purpose is the brightness and whiteness of the Pulp on the desired To raise degrees. In this case, the peracetic acid makes the chromophoric groups in the pulp colorless, mainly in Hydrocarbons.

One another difference between the peracetic acid delignification method the prior art and the present invention is according to a preferred embodiment in that in the invention the peracetic acid dose is considerable is smaller. In the method according to a preferred embodiment of Invention is surprising even a very small peracetic acid dose from 0.1-7 kg / tp, preferably 0.5-3 kg / tp is sufficient, with larger doses even harmful are.

In processes known from the literature which use peracetic acid, it has been observed that the optimum pH in terms of brightness is within a neutral or mildly alkaline range; the strength of the pulp suffers within a slightly acidic range. In the process of the present invention, it is possible to work within a pH range of 3-8, preferably 4-7, and at the same time, the lowering of the pulp strength is prevented. A preferred acidic pH range is essential to the runnability of the paper machine. It has been observed that by using an alkaline earth metal addition, it is possible to carry out post-bleaching within an acidic pH range without loss of pulp strength and without dissolving organic material from the pulp. In previously known methods, the best bleaching effect was achieved in mild alkaline conditions. As the stability of peracetic acid decreases, if the pH increases, using small amounts of peracetic acid, a wasteful consumption of the chemical lowers the brightness. Large doses of peracetic acid do not show the effect of wasteful consumption of the chemical. Consequently, it is possible by the process according to the invention to postpone pulp with considerably less chemical doses without loss of pulp strength than in previously known processes.

Another advantage obtained by using a peracid, e.g. Peracetic acid, is achieved within an acidic pH range, is that the acid liberated during the reaction serves as the acidifying agent chemical, after which the use of SO ₂ or H ₂ SO ₄ as the acidifying agent chemical is avoided. Consequently, the total amount of sulfur can be reduced and possible SO ₂ odor problems are avoided.

The used peracid in the process of the present invention is preferably peracetic acid. Other usable peracids shut down Performic acid, perpropionic or a slightly longer chain percarboxylic acid. The peracid can a so-called equilibrium solution or Equilibrium solution, z. B. a reaction mixture containing the acid, a peracid and Hydrogen peroxide, or a pure peracid solution may be used become. The method by which the peracid solution is prepared does not limit the use of the method; the peracid can z. B. by distillation an equilibrium solution or be prepared from an anhydride and hydrogen peroxide. The peracid can also be a monoper sulfuric acid (Caro's acidity) or a mixture of Caro's acid and a percarboxylic acid, z. B. peracetic acid be. Persulfates can also, either as such or as a mixture with any the aforementioned peracids be used.

in the According to the invention after-bleaching used alkaline earth metal may in particular calcium or magnesium be. these can the bleaching solution in the form of sulfate, acetate, carbonate, oxide or any Be added compound. It is possible in the procedure either Magnesium or calcium alone or both of them together in one any ratio to use. Calcium has the advantage over magnesium, that calcium does not precipitate extract-based and resin-based substances, which may be present at the wet end of the paper machine.

The post-bleaching invention can be preceded by any known bleach reaction known from bleach sequences. Post-bleaching is particularly advantageous for use after chlorine dioxide or peroxide bleaching. Also, acidification that may occur after bleaching does not limit the use of the process; it can be used on both non-acidified and acidified stock. The acidification may, for. B. with SO ₂ , sulfuric acid or a suitable for this purpose any acid can be performed. The operation of the invention will not be adversely affected by a possible washing of the pulp before or after the post-bleaching or by the absence of washing. The consistency of the pulp to be bleached may be 1-30%, and the temperature during the bleach may be between 30 and 100 ° C.

The bleached pulp can be brought directly to the paper machine or be dried to produce concentrated pulp.

The Procedure is for the use on sulphate and sulphite pulps, both softwood hardwood and various organosolv pulps suitable.

The Prior art processes in which a peracid and Magnesium may be used or in which a peracid in the Last steps of the bleaching sequence are characterized in that they are carried out in the normal way in the bleaching of the pulp mill. The aftergrowing according to the invention, in which the bleaching reaction even at a low consistency is very fast, is not tied to the bleach and requires consequently no investment costs for the bleaching. As already pointed out, the process according to the invention relates to After bleaching sequence of the bleaching performed after-bleaching outside of the actual Bleaching in a pulp flow tube while the transfer of the Pulp while Pulp storage in a storage tower or paper machine. Instead of the bleaching is the place where the Nachblimmen invention carried out is typically a bleached pulp storage tower or the Paper mill.

The alkaline earth metal compounds used may be added to the bleaching solution in any suitable process step. They can be added to the circulating water before the water arrives in the post-bleaching step, or they can e.g. B. in the dilution water from the paper machine kom where they can be added in any form. It is also possible to use chelating compounds simultaneously with alkaline earth metals.

In addition to The post-bleaching process described above comprises the invention the use of a solution, the one peracid and an alkaline earth metal for post-bleaching to a kappa number of 4 or less delignified chemical pulp, wherein the post-bleaching is carried out after the bleaching stages on a paper mill.

example 1

A birchwood sulfate pulp bleached using a sequence consisting of an oxygen step, chelation, oxygen + peroxide step, chlorine dioxide step, peroxide extraction, chlorine dioxide step (-OQ-Op-D-Ep-D) was separately post-bleached 50 ° C with a residence time of 30 minutes at a consistency of 5% with a peracetic acid dose of 3 kg / tp at two different pH levels (about 6.5 and about 4.5). The peracetic acid used was distilled. Calcium was added to the pulp in the form of acetate and magnesium in the form of sulfate. The figures in the table indicate the dose of the chemical concerned per one metric ton of pulp (kg of 100% chemical / one metric ton of pulp). Test number 0 stands for pulp that has not been trimmed. Vers initial pH final pH CaOAc kg / tp MgSO ₄ kg / tp Brightness% ISO Viscosity dm ³ / kg 0 - - - 90.3 930 1 6.5 5.4 - 91.0 766 2 4.5 4.3 - - 91.5 845 3 6.5 5.3 - 1 92.5 925 4 4.5 4.2 - 1 92.3 911 5 6.5 5.4 1 - 92.8 899 6 4.5 4.3 1 - 92.6 901 7 6.5 5.3 0.5 0.5 92.8 887 8th 4.5 4.4 0.5 0.5 92.5 896

As can be seen from the table, with the addition of alkaline earth the viscosity the pulp is much higher being held. The by the copper-ethylenediamine method viscosity is directly proportional to the strength of the pulp, in particular, if the same pulp sample is affected. It is reasonable to assume that the higher the viscosity the pulp is, the better its strength is.

A higher viscosity also means that the yield is better, and that less organic material released from the pulp has been. The ISO brightness has been increased by about 1% ISO through the alkaline earth addition improved.

Example 2

A birchwood sulphate pulp bleached using the sequence -OQ-Op-D-Ep-D according to Example 1 was bleached at 50 ° C with a residence time of 30 min at a consistency of 5% with a peracetic acid dose of 3 kg / tp at two different pH levels. The magnesium sulphite addition was 1 kg / tp. The peracetic acid used was distilled. The brightness of the pulp before post-bleaching was 90.3% ISO and the viscosity 930 dm ³ / kg. No Mg addition _MgSO4 initial pH Viscosity dm ³ / kg Brightness% ISO Viscosity dm ³ / kg Brightness% ISO 3 890 91.4 913 92.0 3.5 868 91.4 912 92.1 4 850 91.5 907 92.2 4.5 839 91.5 911 92.3 5 799 91.6 912 92.4 5.5 766 91.7 915 92.5 6.5 750 91.8 925 92.5 8th 802 91.2 916 91.9

It it can be seen from the table that without a magnesium addition the viscosity the pulp considerably is lowered; At the lowest, the viscosity is within a pH range from about 5.5-6. Due to the addition of magnesium, the viscosity remains high regardless of the pH. The brightness is also much higher.

Example 3

A TCF-bleached softwood sulfate pulp bleached using oxygen, ozone and hydrogen peroxide was bleached at 70 ° C with a residence time of 240 minutes at a consistency of 10% using two different peracetic acid doses. The conditions and results of the post-bleaching are shown in Table 3. The brightness of the pulp before post-bleaching was 86.8% ISO and the viscosity was 642 dm ³ / kg and the kappa number was 1.7. Experiment No. 1 2 3 4 PAA, kg / tp 3 1.5 1.5 3 pH, initially 7.2 7.2 7.2 7.2 pH, final 6.3 6.6 6.8 5.6 MgSO _4, kg / tp 1 1 - - Residual PAA, kg / tp 0.7 0.5 0.1 0.1 Kappa 1.6 1.6 1.4 1.3 Viscosity, dm ³ / kg 633 625 572 564 Brightness,% ISO 89.1 88.9 88.4 88.6

As from the table, with the addition of magnesium, a higher Brightness was maintained and the viscosity remained significantly better. At the Post-bleaching with magnesium addition did not become an organic material solved, as can be seen from the kappa number and the viscosity of the pulp.

Example 4

A TCF bleached softwood sulfate pulp bleached using oxygen, ozone and hydrogen peroxide was bleached at 70 ° C with a hold time of 240 minutes at a viscosity of 10% using two different peracetic acid doses. The conditions and results of the post-bleaching are shown in Table 4. The brightness of the pulp before post-bleaching was 87.6% ISO and the viscosity was 623 dm ³ / kg and the kappa number was 1.8. Experiment No. 1 2 3 4 5 6 PAA, kg / tp 1.5 1.5 1.5 1.5 1.5 1.5 pH initially 7.2 5.5 5.5 6.1 6.5 7.2 pH final 6.9 4.9 4.9 5 5.5 6.9 MgSO _4, kg / tp 1 1 - - - - Residual PAA, kg / tp 0.4 0.4 0.4 0.4 0.4 0.2 Viscosity, dm ³ / kg 612 609 574 571 561 579 Brightness,% ISO 89.9 89.3 88.6 88.5 88.5 89.0

As The table shows, with the addition of magnesium a clear better final viscosity receive.

the It is clear to a person skilled in the art that various applications of the invention not limited to those are, which are shown above by way of example only; they can be inside the attached claims vary.

Claims (8)

Process for bleaching chemical pulp, the pulp being treated in a variety of different steps and wherein at least one step uses a bleaching solution which is a peracid contains where the peracid used in a post-bleaching, which is the last step bleaching operation on delignified pulp with a Kappa number of 4 or less is performed, wherein the bleaching takes place in the presence of one or more alkaline earth metal compounds and after the bleaching stages in a pulp flow tube, a Storage tower and / or a paper machine is running, where the peracid Chromophore groups in the pulp makes colorless. Method according to claim 1, characterized in that the whiteness content of the pulp is at least 85% ISO is before the after-bleaching, which is done with a peracid. Method according to one the aforementioned claims, characterized in that the amount of peracid used for post-bleaching becomes, 0,1-7 kg / tp, preferably 0.5-3 kg / tp, is. Method according to claim 3, characterized in that the peracid is peracetic acid. Method according to one the aforementioned claims, characterized in that the post-bleaching solution is a calcium compound contains such as Calcium acetate or calcium carbonate. Method according to one the aforementioned claims, characterized in that the post-bleaching solution is a magnesium compound contains such as For example, magnesium sulfate. Method according to one the aforementioned claims, characterized in that the pH of the post-bleaching solution within of the range 3-8, preferably 4-7, is. Using a solution containing a peracid and contains an alkaline earth metal, for the Post-bleaching of chemical pulp delignified to one Kappa number of 4 or less, with the after-bleaching being carried out after the bleaching stages in a paper mill and the peracid chromophore Makes groups in the pulp colorless.