SE515708C2 - Bleaching of bleached mechanical pulp with oxidizing bleach enhanced with borohydride - Google Patents

Abstract

The present invention relates to a method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said location, characterized in, that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.

Description

Among the bleaching agents that can be used for lignin-preserving bleaching, there are both reducing and oxidizing bleaching agents. Within the former group, the most commercially common is dithionite, usually sodium dithionite, which is also called hydrosulte. Within the latter group, the most commercially common is hydrogen peroxide.

The most common bleach in the described context is probably hydrogen peroxide. The great advantage of hydrogen peroxide is that it is a very effective, ie a highly brightening, bleaching agent. However, hydrogen peroxide bleaching usually requires the use of separate bleaching towers and also other bleaching equipment, which means that the fixed costs of hydrogen peroxide bleaching become and are high.

Ditionite, usually sodium dithionite, is not as effective from a bleaching point of view as hydrogen peroxide, but has the advantage that, in addition to using bleach, the bleach can also be added directly to the pulp suspension, for example in a storage plant or earlier of bleach and other bleaching equipment. The latter means a reduction in fixed costs. This by applying a reducing bleach, for example sodium dithionite, directly to the emerging pulp suspension just after the fr ber release is known from the Swedish patent application 99008 16-1. Transition metals, especially iron and manganese, are suitable for drying when bleaching mechanical pulp and CTMP with, for example, both hydrogen peroxide and dithionite. The presence of manganese ions in significant amounts is especially serious in bleaching such pulp with hydrogen peroxide. An increasing amount of manganese ions leads to an increasing decomposition of the added amount of hydrogen peroxide, which means that the part of the hydrogen peroxide which decomposes will not be used for eluting the mass. Furthermore, the decomposition products themselves are harmful, since they form chromophoric groups in the pulp, which counteract what is sought, namely an illumination of the pulp. When bleaching such a mass with dithionite, it is the iron ions that are especially dangerous. These transition metals are usually removed or neutralized from the pulp and pulp suspension by complexing the transition metals with complexing agents, for example in the form of ethylenediaminetetraacetic acid (EDTA) and / or diethylenetriaminepentaacetic acid (DTPA). It has also been proposed that, in addition to a complexing agent, a reducing chemical be added to the pulp suspension, such as, for example, sodium hydrogen salt or sodium salt. Success has also been achieved in treating the starting material, usually wood ice, with both complexing agents only and with both chemicals just mentioned.

The use of the reducing bleach boron hydride, usually sodium borohydride, for bleaching, for example, high yield pulp has also been suggested. U.S. Pat. No. 3,100,732 teaches the use of alkali metal borohydride and peroxide bleach in the same step. Two ways of performing such bleaching of the pulp are described.

According to one method, a common bleach solution is prepared which is fed to the pulp, i.e. sodium borohydride is dissolved in a peroxide solution, which may also contain water glass (sodium silicate), magnesium sulphate and sodium hydroxide. According to the second method, the mass is fed with a sodium borohydride solution at the end of the peroxide bleaching step in order to efficiently utilize the residual peroxide at the end of the bleaching step. In both methods, an increase in brightness was obtained compared to using only peroxide as a bleaching agent. A disadvantage of borohydride is its high price.

Description of the invention Technical problem In the manufacture of bleached mechanical pulp such as TMP, for example of fresh Scandinavian spruce wood, it is today normal to achieve a brightness of 80% ISO and possibly one or two percentage points above it. Achieving mass masses that are higher than those just stated, for example up to 85% ISO, is not possible with known technology.

The Solution The present invention provides a solution to this problem and relates to a process for bleaching mechanical and chemithermomechanical pulp comprising bleaching and exfoliating pulp suspension by bleaching reducing bleaching agent therein in a position just after the bleaching and the bleaching taking place at direct connection thereto conditions in the form of high temperature and minimized oxygen supply, characterized in that the bleached pulp as described is subjected to at least one further bleaching treatment including bleaching the pulp with an oxidizing bleaching agent, preferably peroxide bleaching agent, fortified with borohydride. 10 15 20 25 515 708 u. - ~ -. -. .

The predominant mass type of the previously listed masses is TMP. In the production of such pulp, the exposure usually takes place either in one refiner or in two successive refiners. Whether it is the rainer or after each rainer there is a steam separator, usually a cyclone of some kind, through which the resulting pulp suspension is passed. There, the pulp suspension is transported, for example by means of a screw conveyor, usually to a dissolver (which, however, is not absolutely necessary) and further to a storage vessel (latency cycles) and from there to a silage and from there to bleaching treatment. In some TMP factories there is a dissolver and the pulp suspension is usually transported by means of a pump, located just after the dissolver, through a pipe to the holding vessel.

Several alternative positions for adding the reducing bleaching agent, usually in the form of an aqueous solution, to the emerging pulp suspension.

For example, the bleach can be added to the pulp suspension in the previously mentioned screw conveyor. Another auxiliary position and a preferred one is the previously mentioned pump. By adding the bleach in the particular pump, you probably get an excellent distribution of the bleach in the pulp suspension. Diluent is usually fed to the dissolver and the bleach can be introduced into this diluent, which is later added to the pulp suspension. Furthermore, it is possible to charge the bleach directly into the dissolver. Of course, it is possible to divide the batch of bleach and add the pulp suspension bleach, for example, in two or two of the specified positions.

The above-mentioned provision "the emerging pulp suspension" included in the main claim is to be understood in a broad sense. What is meant is not only when the pulp suspension flows forward, for example in a pipe or pipe, but also when the pulp suspension is in vessels and containers, for example in the form of dissolvers and storage vessels, since the pulp suspension also then advances in the form of the pulp suspension being fed. in one place in the vessel and discharged in another place in the vessel.

Any known reducing bleach can be used. Examples of such bleaches are ditionite (which is sometimes referred to as hydrosul fi t and is preferred), borohydride, hydrazine and formamidine insulin acid. Ditionite is available on the market primarily as sodium dithionite, ie Na2S2O4. The bleach in question is preferably added to the pulp suspension in the form of an aqueous solution, the concentration of which is suitably in the range of 20-120 g / l. The rate of bleaching agent is determined, inter alia, by how difficult it is to bleach the pulp in question and how large an increase in brightness of the pulp is desired in the pre-bleaching in relation to the total increase in brightness of the pulp.

Parameters for the bleaching of the pulp in the form of temperature, time, pulp concentration, pH, etc. are mainly determined by the conditions which naturally prevail in the production of, as in the case described, thermomechanical pulp. At the above-mentioned additive positions for the bleach to the pulp suspension, that temperature is naturally very high, for example 80-95 ° C, and the pulp concentration is usually low, for example 2-4%. Among other things, as a result of the temperature being very high, the bleaching time becomes short, probably in the interval from a number of seconds up to a few minutes. The bleaching time is probably partly also dependent on the rate of fate of the pulp suspension when the bleach is added. As for the pH value, it is naturally in the range 4-7. In some cases it may be advisable to adjust the pH slightly in the form of adding either acid or alkali to the pulp suspension in the affected position. To achieve optimal bleaching results when using ditionite as a bleaching agent, the pH value should be from 4.5 and up.

In bleaching, a pH as high as 8.5 can be used, but such a high pH is less suitable for other reasons.

The amount of oxygen in the pulp suspension should be as low as possible with the bleach addition, preferably zero. From the ñber exposure position and in the future a bit in the pulp treatment chain, the pulp suspension is protected from lu fi contact and luñ mixture to a significant degree. At the same time, the temperature of the pulp suspension in that area is very high, which also contributes to an elective bleaching of the pulp.

The production of CTMP is very similar to the production of thermomechanical pulp, as briefly described above, and the additive positions of the bleach in bleaching this pulp are largely similar to those described above.

In the production of abrasive pulp, the pulp suspension is collected after the ñber exposure in a sanding trough, from where the pulp suspension is transported further. Suitable additive positions for the bleach are the abrasive tray and just after that. It is particularly suitable to apply the bleaching in question to a system pressurized with steam, i.e. to pressure grinding.

This bleaching of the pulp is described in detail in our Swedish patent application 9900816-1. This patent application also describes how the reject pulp stream is handled and preferably bleached before it is mixed into the main pulp stream. After sieving (usually) and possibly passing through storage vessels and / or storage towers, the pulp is subjected to at least one further bleaching treatment.

Either directly or in intermediate bleaching steps, the pulp is bleached with an oxidizing bleach, preferably peroxide bleach, fortified with borohydride.

The term fortified with borohydride includes both treating the pulp with borohydride in a pretreatment step, i.e. before the peroxide bleaching step itself, and adding boron hydride to the pulp at some point during the peroxide bleaching step itself including adding the borohydride to the pulp suspension together with the peroxide bleaching agent.

The peroxide bleaching step may itself be conventional (except when any addition of borohydride is added) and any known peroxide bleaching agent may be used. Examples of these are hydrogen peroxide, sodium peroxide, peracetic acid, permyric acid and peroxosulfuric acid (Caro's acid). The preferred peroxide bleach is hydrogen peroxide. The bleaching time is at least 30 minutes, preferably at least 60 minutes, and can for example amount to 2 hours, i.e. 120 minutes.

Parameters in general are; batch of peroxide, calculated as hydrogen peroxide, = 0.5 - 5%, calculated on absolute dry mass, temperature = 50-90 ° C, mass concentration = 5-3 8%, starting pH in the case of hydrogen peroxide = at least 10.

Of the above-described methods of reinforcing with borohydride, the use of a pretreatment of the pulp is the absolute preference.

Any known alkali metal borohydride can be added to the pulp. The most common is sodium borohydride. This can be added to the pulp as such. In practice, the commodity Borol, which is an aqueous solution containing 12% sodium borohydride and 40% sodium hydroxide, is the most widely used. It is advantageous to use one or more of the supplemental chemical (s) to the borohydride during the pretreatment step. In the event that the commodity Borol is used as a source of borohydride, it is not always necessary for an alkali, for example sodium hydroxide, to be added to the pulp, since the commodity contains a significant amount of sodium hydroxide. However, it is common for alkali in varying amounts to be added to the pulp, for example together with the commodity Borol. In the case that sodium borohydride as such is added to the pulp, it is necessary to also add alkali, for example sodium hydroxide. Further suitable additive chemicals are water glass (sodium silicate) and some magnesium compound, such as magnesium sulphate. The pretreatment of the pulp with at least borohydride takes place within a short period of time, for example at least 15 15 15 5 515 708 seconds and at most 500 seconds. Optimal results are obtained in the range of 60 to 180 seconds. The time period in question is counted from the time the borohydride is added to the pulp until the peroxide bleach is added to the pulp in subsequent peroxide bleaching steps. It is suitable that the borohydride, calculated as sodium borohydride, is added to the pulp in an amount of 0.05-0.3%, calculated on absolutely dry pulp. Suitable temperature is 20-95 ° C. The pulp concentration is not directly critical, but it is preferred from average concentration and upwards, i.e. from 6% to 38%. Since alkali is always present during the pretreatment of the pulp via the addition of the commodity Borol or via direct addition or via both methods, the pretreatment takes place under alkaline conditions, for example within the pH range 9-13.

In the case of the subsequent peroxide bleaching of the pulp, it is in some cases appropriate to add a supplementary chemical) and in some cases even necessary. If in the preferred case of hydrogen peroxide as bleach no extra alkali or too little alkali has been charged in the pretreatment step, it is necessary to add alkali, for example sodium hydroxide, in the peroxide bleaching step. It can also be good to add water glass and some magnesium compound, especially if these chemicals have not been added to the pulp at all or in too low an amount in the pretreatment step.

The treatment of the pulp described above, i.e. this special bleaching technique, is described in detail in a parallel application, with the same filing date as this patent application.

As previously stated, it is possible to mix the borohydride and the peroxide bleach and any supplemental chemicals in one and the same aqueous solution forming a bleaching liquid which is added to the pulp suspension leading to a bleaching treatment of the pulp suspension in accordance with the previously described peroxide bleaching step.

Furthermore, at the end of, for example, a conventional peroxide bleaching step, for example when the peroxide bleaching has taken place for 100-120 minutes, boron hydride and any supplementary chemical can be added to the pulp suspension.

The batch of borohydride, calculated as sodium borohydride, can also in these two cases be in the range 0.05 - 0.3%, calculated on absolutely dry mass.

As previously stated, just before bleaching with peroxide bleach fortified with borohydride, the pulp suspension can be bleached in another way in one or more steps. According to a preferred embodiment of the invention, the pulp bleached in the manner previously described is bleached with peroxide bleach at medium concentration, while the subsequent 515 70 515 708 .-. f ». ~ v. borohydride enhanced peroxide bleaching step is performed at high pulp concentration and furthermore the bleach liquor from the high concentration bleaching step, which contains residual peroxide such as residual hydrogen peroxide, is returned to the pulp processing process and mixed into the pulp just before or at the beginning of the average concentration bleaching step.

Advantages By bleaching mechanical pulp and CTMP in the manner described, i.e. in accordance with the invention, it is possible to achieve a brightness on the pulp of up to 85% ISO. Admittedly, the use of usually three different bleaches, compared to a conventional use of a bleach in usually one step, leads to an increase in the cost of the bleaching process as a whole, but it should be remembered that the market constantly demands increased brightness on lots of the types described and consequently also increased brightness on paper made from such masses. This demand naturally has the consequence that you are prepared to pay properly for what is demanded.

Furthermore, it is the case that even if three different bleaches are used in the process according to the invention leading to a significant bleach cost, the bleaching process is overall extremely cost-effective, partly due to the fact that the inventive steps are associated with low fixed costs.

Best Mode for Carrying Out Since the invention has already been described in detail and even in great detail with reference to two other patent documents, the restriction is made here that only one exemplary embodiment is presented.

Example 1 Thermomechanical pulp was prepared in a manner schematically represented in Figure 1 of Swedish patent application 9900816-1, to which reference is again made. The starting material for the pulp production was relatively fresh, barked spruce logs of Scandinavian origin. To the flowing pulp suspension in position 14 and more specifically in the pump (not shown in the figure) just after the dissolver 14 was added an aqueous solution containing sodium dithionite at a concentration of 60 g / l to such an extent that the batch of bleach was 6 kg per ton of dry pulp. . The temperature of the pulp suspension in that position was 88 ° C, its pH 515 708 .. »..- .... ;; - pH value 4.6 and its concentration 3%. The pulp thus bleached was filtered in the silage 19 so that an acceptance pulp stream and a reject pulp stream were obtained. The shrimp pulp stream was treated in accordance with the procedure of the said patent application and an aqueous solution containing sodium dithionite in a concentration of 60 g / l was fed to the shrimp pulp suspension stream in the pump (not shown in the figure) which is just the resolver 31 in such a fate. the batch of bleach was 6 kg per ton of dry pulp even in that position. The temperature of the pulp suspension in that position was 85 ° C, its pH 5.1 and its concentration 3%.

Mass was taken out on the plate 21 for further transport to the laboratory. In that position, the pulp, ie the bleached pulp, had a brightness of 71% ISO and a metal content, determined according to standard analytical methods, of 12.1 ppm (or mg per kg of absolutely dry pulp) manganese (Mn), 11.2 ppm iron (Fe) and 3.8 ppm copper (Cu). The pulp arriving at the laboratory was pressed to a pulp concentration of 35% and stored there in a cold room.

In the laboratory, four experiments were made with this mass, a reference experiment and three experiments in which the method according to the invention was simulated.

During the test runs, pulp was extracted from the cold room and divided into batches of 20 grams of dry-thinking pulp for each run.

In the reference experiment, the pulp was subjected to a conventional hydrogen peroxide bleaching in the form of the twenty grams of pulp being distributed by hand so that the pulp was put into fluffed form. The mass was then stuffed into a plastic bag. To this was added an aqueous solution in a certain amount containing hydrogen peroxide in a batch of 4%, calculated on absolute dry mass, sodium hydroxide in a batch of 3.5%, calculated on absolute dry mass and water glass (sodium silicate) in a batch of 3.12%, calculated on absolutely dry mass. After adding said aqueous solution, the contents of the plastic bag, i.e. the pulp sample, were kneaded by hand so that the chemicals were evenly distributed in the specified amount of pulp. If the chemical mixture was mixed, the pulp concentration was 30%. The plastic bag with its contents was then placed in a water bath, at a temperature of 75 ° C, for a period of 90 minutes.

The bleaching was stopped by taking the plastic bag out of the water bath and transferring its contents to a glass container, in which the pulp sample was diluted with deionized water down to a pulp concentration of 0.6%. Furthermore, a sulfuric acid solution was added so that the pH of the pulp suspension dropped to 5. Before the sulfuric acid was added to the pulp suspension and during the dilution, samples were taken of the bleach liquor in which the final pH and residual hydrogen peroxide content was determined according to conventional analytical techniques. From the pulp sample, a sheet according to SCAN-C M 11:75 was made by hand on which the brightness was determined according to the analysis method SCAN-P 3:93. All brightness values mentioned in this embodiment are measured in the manner just described. The three bleaching experiments according to the invention were also performed on 20 grams of toned pulp.

In one of the experiments, called P 1, the pulp sample was first added to the plastic bag an aqueous solution containing sodium borohydride and sodium hydroxide in the form of the commodity Borol in an amount of sodium borohydride of 0.1%, calculated on absolute dry mass, plus extra sodium hydroxide in an amount of 1 l. 55%, calculated on absolutely dry mass and water glass in an amount of 3. 12%, calculated on absolutely dry mass. After extensive atomization of the chemicals by hand kneading, the closed plastic bag was placed in the aforementioned water bath, which maintained a temperature of 75 ° C. After 2 minutes, the plastic bag was taken out of the water bath and opened and an aqueous solution containing hydrogen peroxide in an amount of 4%, calculated on absolute dry mass, and sodium hydroxide in an amount of 1.55%, calculated on absolute dry mass, was added. After this addition of the aqueous solution, the pulp had a concentration of 30%. The plastic bag was sealed and put back into the said water bath and stored there for a period of 90 minutes. The handling of the sample was continued even in this experiment in the manner described above. The characteristic of this experiment was thus that the sodium borohydride was added to the pulp in a pretreatment step before the actual hydrogen peroxide bleaching of the pulp took place.

In a second experiment according to the invention, called Ps, the pulp sample was treated according to the reference experiment with the important difference that the pulp was added in addition to the chemicals specified in that experiment also the commodity Borol, so that the batch of sodium borohydride was 0.1%, based on absolutely dry pulp. Furthermore, the direct addition of sodium hydroxide was reduced to 3. 1%, calculated on absolutely dry mass. The characteristic of this experiment was thus that the sodium borohydride and the hydrogen peroxide were added to the pulp simultaneously.

In a third experiment according to the invention, called Pe, the pulp sample was treated in accordance with the reference experiment with the important difference that after 80 minutes of the hydrogen peroxide bleaching of the pulp, the plastic bag was taken out of the water bath and 515 708 - -. ~ u ~. ll was opened. The bag was fed to the commodity Borol so that sodium borohydride was fed to the pulp in an amount of 0.1%, calculated on absolutely dry pulp, and further to deionized water so that the pulp concentration dropped to 5%. In addition, sulfuric acid was added so that the pH of the pulp suspension dropped to the range 6.5-7.0. The plastic bag was sealed and the chemicals were leveled in the pulp sample by hand kneading. There, the plastic bag was again placed in the water bath for a period of 10 minutes. Also in this experiment, the direct addition of sodium hydroxide was reduced to 3.1%, calculated on absolutely dry mass. Thus, the hallmark of this experiment was that the sodium borohydride was added to the pulp only at the end of the hydrogen peroxide bleaching.

Experimental parameters and results obtained are shown in Table 1 below.

Table 1 Parameters and results Experiment Reference P, P, Pc NaBH4,% 0 0.1 0.1 0.1 Na2SiO3, ° / o 3.12 3.12 3.12 3.12 NaOH,% 3.5 3.1 3.1 3.1 H2O2,% 4.0 4.0 4.0 4.0 Mass concentration,% 30 30 30 30; 5 Temperature, ° C 75 75 75 75 Time, minutes 90 90 90 80 + 1O Final pH 9.16 9.17 9.37 - Residual H2O2,% 0.56 1.00 0.60 1.12 Brightness,% ISO 81.6 83.6 82.4 82.2 As can be seen, all the pulp samples bleached in according to the invention gave luminosities which are higher than the luminosity of the reference mass. In the experiment where u. Nu v - n n. N m -. ~ | s - - i .u »i n. v» -. - »s -. «~ -. - " " .now. _, . -. now . .

A,. ,. .. i. i v 1 s n .- H. | '. «.- 12 a special pre-treatment step for the mass with borohydride has been used, the increase in brightness is particularly marked and more precisely 2% ISO.

Furthermore, it has been established in advance that at the time when the bleached pulp was taken out in the TMP factory and forwarded to the laboratory, the starting mass, ie just after the second cleaning step, brightness was unusually low, which leads to the final brightness of the pulp being lower than otherwise it should be the case.

Claims (11)

10 15 20 515 708 «. », .- 13 PATENT REQUIREMENTS 1. l. A method for bleaching mechanical and chemithermomechanical pulp, comprising bleaching and emerging pulp suspension after fiber release by reducing bleaching agent in a position just after fi berf exposure and that the bleaching takes place at conditions added to the addition position and in direct connection thereto. of high temperature and minimized oxygen supply, characterized in that the bleached pulp as described is subjected to at least one further bleaching treatment including bleaching the pulp with an oxidizing bleaching agent, preferably peroxide bleaching agent, fortified with borohydride. 2. A method according to claim 1, characterized in that borohydride is added to the pulp and allowed to react with it for a short period of time in the form of a pretreatment before the pulp is bleached with peroxide bleach. 3. A method according to claim 1, characterized in that borohydride is added to the pulp at the beginning of the peroxide bleaching treatment. 4. A method according to claim 1, characterized in that borohydride is added to the pulp at the end of the peroxide bleaching treatment. 5. A method according to claims 1-4, characterized in that the pre-bleached pulp is bleached with peroxide bleach and then with peroxide bleach fortified with borohydride. 6. A method according to claim 5, characterized in that the first peroxide bleaching step is performed at medium concentration and that the second peroxide bleaching step is performed at high pulp concentration and that bleaching liquor containing residual peroxide from the second bleaching step is recycled and mixed into the pulp before or in connection with it. understand the peroxide bleach step. 10 515 708. . 1 «.- 14 7. Process according to claims 1-6, characterized in that the pulp, in addition to borohydride, is fed to at least one of the chemicals alkali, water glass (sodium silicate) and magnesium compound. 8. A process according to claims 1-7, characterized in that the pulp, in addition to peroxide bleach, is fed to at least one of the chemicals alkali, water glass (sodium silicate) and magnesium turnings. 9. A process according to claim 1, characterized in that the reducing bleaching agent is sodium dithionite. 10. A process according to claims 1-9, characterized in that the borohydride consists of sodium borohydride, preferably in the form of the commodity Borol. 11. A process according to claims 1-10, characterized in that the peroxide bleach is hydrogen peroxide.