METHOD OF BLEACHING FIBRE PULP FOR THE PRODUCTION OF PAPER AND P APERBOARD

Abstract: At a method of bleaching pulp intended for paper or paperboard the hydrogen peroxide is added as mist in air or vapour to the suspension of the dry pulp, which at high speed flows out of the drier. Also the base used for adjusting the pH value can be added in this way. The disintegration of the peroxide which heretofore rendered dry bleaching difficult is avoided, and high degrees of brightness are achieved.

Description: Method of bleaching fibre pulp for the production of paper and paperboard This invention relates to a method of bleaching fibre pulps, which are obtained mechanically, chemi-mechanic¬ ally, che i-thermomechanically or semi-chemically from wood, especially hardwood, and are used for the making of paper or paperboard. These fibre pulps will be called in this specification shortly "pulps". At present, the aforementioned types of pulps are bleach¬ ed in most cases by using hydrogen peroxide as bleaching agent. The bleaching is carried out normally as follows: The pulp resulting from one of the said defibering or digesting methods preferably first is washed more or less completely, in order to remove as completely as possible soluble substances originating from the raw material or pulp or obtained at the defibration. The pulp then is mixed with bleaching agents, especially hydrogen peroxide, alkali and sodium silicate, whereby the bleaching is initiated, which normally is carried out at pH 8 to 9 and at appr. 6θ°C. The processed susp¬ ension of the pulp in water then has a concentration of about 10 to 30$ by weight. The bleaching requires a reaction time of some hours. It is known that at the bleaching the following steps are of advantage: 1) The pulp shall not contain large amounts of water- -soluble substances, which arose or were released at the pre-treatment. 2) The bleaching shall be carried out at a high concen¬ tration of the pulp in water. 3) The bleaching shall be carried out with a high per¬ oxide concentration. 4) The bleaching shall be carried out in two steps. First a normal bleaching operation is carried out where- after the residual chemicals and the soluble substances resulting at the bleaching are removed by a washing. In a second step bleaching is then carried out with new chemicals. It also has been tried to bleach the pulp after its dry¬ ing, in such a way, that bleaching chemicals were added to the dried papermaking pulp prior to its pressing to bales. Such a method is advantageous for several reas¬ ons, especially for the following ones: 1) The bleaching proceeds at a favourable high pulp concentration, because the dried pulps normally have a water content of about only 15% by weight. 2) The pulp normally is pressed very strongly prior to its drying, in order to obtain a dry matter content of about 50 by weight so that as much as possible energy will be saved at the subsequent drying. At the said pressing, the main part of the water-soluble byproducts is removed which arose or were released at the making of the pulp, and also the residual chemicals originat¬ ing from the digesting process are removed. 3) A "dry" bleaching would be suitable also as the second step in a two-step bleaching of pulp, which was bleached already previously in a conventional manner. The "dry" bleaching known per se. however, was neither economically nor technically successful, for the reasons as follows: 1) In order to avoid a repeat of the drying of the pulp, no substantial water amounts must be supplied. It is therefore necessary to operate with unusually high concentrations of bleaching agents. This implies, however, that alkali and peroxide meet at high concen¬ trations, and that peroxide, due to the high pH value produced, is decomposed relatively very rapidly, before it can exercise its bleaching effect on the pulp. When the amount of alkali is reduced, in order to reduce the decomposition of the hydrogen peroxide, the base would be neutralized rapidly by the pulp, and the bleachr ing would suffer a substantial loss of efficiency. 2) High concentrations of alkali bring about discoloration of the pulps, so that normally an increase of the concen¬ tration of bleaching agents is required, and a subsequent washing would be necessary for removing by-products and discolorations. 3) The unavoidable higher decomposition of the hydrogen peroxide is detrimental to the economy of the method. The present invention proposes a new bleaching method, which renders it possible to entirely eliminate the afore¬ said disadvantages. The method according to the invention is defined in claim 1, and the subclaims set forth preferred embodiments of the invention. According to the invention, a suspens¬ ion of hydrogen peroxide is produced in air or water vapour, i.e. a suspension of a liquid as disperse phase in a gas as continuous phase, which means in principle it is a mist. When for special reasons water vapour is not desirable, air is preferred as continuous phase, because it is thereby possible to keep the temperature of the suspension very low until immediately before its mixing with the pulp suspension. At the method according to the invention, hydrogen peroxide can be brought into direct contact with the pulp, without causing the per¬ oxide in the first place to meet too high a base concen¬ tration (too high a pH value) and to be decomposed there¬ by. A greater proportion of the hydrogen peroxide is then available for bleaching the pulp. At the method according to the invention, thus, the hydrogen peroxide can be combined in very high concentration with dry pulps and, consequently, this bleaching results in high, improved brightness degrees. At the hydrogen peroxide bleaching,in addition to this also a base is required, which has the object to deliver the necessary pH value, at which the hydrogen peroxide has optimum bleaching effect. According to the invention, different bases can be used. At a preferred embodiment, the base is added in the form of a suspension to the dried pulp. The suspension of the base preferably is produced separately, so that peroxide and base do not meet unnecessarily early. A pref¬ erable step further is, not to mix the suspensions of peroxide and bases with each other in air or water vapour, before they are added to the pulp suspension. When the peroxide suspension is added first to the pulp, and thereafter the suspension of the base is introduced into the pulp suspension, the bleaching can commence, and the peroxide enters into the pulp particles by diffusion as liquid, by diffusion of the vapour of the evaporated peroxide and by absorption. Thereby a substantially uniform distribution of the hydrogen peroxide in the pulp part-, icles is obtained before the particles get into contact with the base, which also gets into contact with pulp particles, which already contain the hydrogen peroxide. A premature decomposition of the hydrogen peroxide by contact with a high-concentration base, consequently, is prevented. An improved contact and a more uniform absorption of the hydrogen peroxide by the pulp can be obtained by electro¬ statically charging the hydrogen peroxide suspension prior to i.ts admixture to the pulp. The charge must be suitable with respect to attraction to the particles of the pulp. The same step can be taken for admixing the base suspension, the particles of which are to be charged so that they have a charge opposed to the pulp particles containing hydrogen peroxide. At the peroxide bleaching normally an alkali hydroxide is used as base, in order to obtain the pH most favourable for the bleaching. At the bleaching according to the invention, however, in this way a strong base at the high necessary concentrat¬ ions would cause a rapid premature decomposition of the hydrogen peroxide. This is prevented, according to the invention, in that a carbonate, or still better a bicarb¬ onate, of an alkali metal, especially sodium or ammonium, is used. This step can be improved still more by using a difficultly soluble or a slowly soluble substance. It was found that as difficultly soluble basic substances such are to be used which have a solubility lower than 0,015 g/100 g water at 20°C. Such difficultly soluble substances can be regarded to be especially magnesium hydroxide, magnesium oxide or basic magnesium carbonate. Magnesium carbonates and calcium-magnesium carbonates.- (dolomites) also can be used, but then the type to be used must be selected accurately, because the natural minerals in this combination often dissolve too slowly even in finest ground state. The natural calcium carbonates normally are too slowly soluble for being usable at this method. In general, however, some qualities of chalk can be used which have a relatively very rapid dissolving rate. Also other bases can be used which have a higher solub¬ ility, for example up to 0,1 g/100 g water at 20°C. These bases, however, should dissolve relatively slowly, as a matter of fact so slowly, that when 1 g of the subst¬ ance is intensely stirred in 100 g of water at 8θ°C during two minutes the pH value of the suspension does not exceed the value of 9,5, especially 9,0. As already mentioned, besides the carbonates and bicarb- onates of sodium and ammonium, magnesium hydroxide, basic magnesiu carbonates, magnesium oxide, magnesium carbonates or chalk with quick-soluble properties are suitable bases. In order to achieve a particularly suitable combination of solubility, dissolving rate and pH value, two or more of the aforementioned bases can be combined with each other. To such combinations applies as standard value and rule, that at the mixing of 0,03 mole of the basic substance or substance mixture with 100 g of prepared pH-neutral pulp and 2000 g of water in a laboratory apparatus according to the standard method SCAN-N2:64 during two minutes a pH value of between 7,5 and 9,0, preferably a pH value between 7,8 and 8,5, is achieved. The present invention is based on the supply ' of hydrogen peroxide as suspension in air or water vapour to the suspension of the pulp dried and suspended in air or water vapour. This does not imply that also the base must be added entirely or partially after the drying of the pulp. The base, on the contrary, can be added entire¬ ly or partially to the aqueous suspension of the pulp prior to its final dewatering and drying. This method has advantages as well as disadvantages. An advantage is that at the adding of the base to the aqueous suspension of the pulp the base is better dist¬ ributed. The reason is that the pulp suspension after drying does not only consist of single fibres, but also of fibre aggregates, which are pressed together more or less strongly. These aggregates often give rise to considerable difficulties at the papermaking with resp¬ ect to redefibration of the pulp stored. When the base is added as powder after the drying of the pulp, this powder cannot get into contact with the innermost parts of the said aggregates. It is, therefore, at times advantageous- to add at least a part of the necessary bases in the form of a difficultly soluble or slowly soluble powder to the pulp suspension prior to its drying. This addition should not be made too long a time before the drying, because the base commences to dissolve, is lost partially at the dewatering and in any case renders the pulp too strongly alkaline, which can cause discolouring at the subsequent drying. It was now found, that the aforementioned difficultly soluble or slowly soluble bases should be added at maximum 30 minutes prior to the drying of the aqueous pulp suspens¬ ion. The accurate time, of course, depends on the solubility and dissolving rate of the base. In order to prevent the base from dissolving too strongly, the addition should be made; hen the stock has a concentrat¬ ion of at least 5% by weight. It also was found that the base dissolves much more rapidly when it is intensely stirred with the stock. This intense admixture by forceful stirring should not last too long, especially not longer than three minutes. According to the invention, in order to facilitate the redefibration at the papermaking it is advantageous to add part of the base necessary for bleaching prior to the drying of the pulp. Thereby basic substance enters also into the inner zones of the subsequently forming fibre aggregates. There the pH is increased, the re- redefibration is facilitated, and the hydrogen peroxide, which subsequently by diffusion as liquid or gas enters into the interior of the aggregates, can even there have a bleaching effect due to the higher pH value. When base is added prior to the drying of the pulp, relatively inexpensive chemicals can be used.. Chalk is well suitable. The base added after the drying of the pulp as suspens¬ ion in air or water vapour rather should be a magnesium compound, because magnesium acts as a stabilizer for hydrogen peroxide.Magnesium also seems to have the special property to prevent the so-called hornification of the pulp. It further waa found, that preferably about half of the base required for the bleaching can be added prior to the drying of the pulp, mainly in order to improve the re- defibration of the bleached pulp obtained. It is, according to the invention, extremely important that the suspensions of the reagents, especially those of the hydrogen peroxide, after the drying of the pulp are added to its suspension in air or water vapour, whereby a rapid and uniform mixing with the pulp takes place. For this reason, it is very important that the pulp suspension in the place where other substances are added flows with high turbulence. This is achieved in that the pulp suspension is passed at high speed through a pipe. During the flow through this pipe, at least the peroxide suspension is admixed. The flow rate in the pipe should be at least 50 m/sec, preferably at least 150 m/sec. The suspension of the base can be introduced in the same pipe, or it can be added lateron to the pulp suspension, for example together with the cooling and transport air for the pulp suspension. The concentration of the pulp in suspension in the place where the peroxide suspension is added requires to be taken into regard. In a pulp suspension too concentrated, fibre flocculations or loose aggregates will form, which render a homogenous admixing of the peroxide difficult. According to the invention, it is advantageous when the pulp suspension contains at maximum 15 kg of dry pulp per cubic meter, preferably at maximum 8 kg/m and especially advantageously at maximum 5 kg/m . The invention can be applied with special advantage to pulps, which were obtained with a high yield of 80 to 95% from wood, calculated on dry matter. These pulp types normally often can be redefibred only with difficulty, after they have been dried, pressed and stored. The per¬ oxide bleaching after the drying, and especially also the possibility at this method of batching difficultly sol¬ uble bases more strongly,- without darkening the colour of the pulp, facilitate the redefibratioπ in the paper mills . These features are especially important in the case of pulps, which are obtained in high yields from hardwood, preferably aspen, p'oplar or birch-tree. The method according to the invention can also be the last step in a multi-step bleaching process. The steps preceding the bleaching according to the invention can be conventional bleaching processes in aqueous phase. By means of such bleaching methods comprising two or more steps a much stronger bleaching can be obtained, Bleaching Production of pulp from 30$ spruce and 70$ aspen. Chopped wood chips of the raw material were washed, so that possible adm± ' x urss of sand, stone etc. are removed. The wood chips was heated to 90°C, pressed and introd¬ uced into an alkaline sodium sulphite solution. The impregnation solution had a temperature of about 50°C. After completed impregnation the chips was cooked for about 30 minutes with steam at atmospheric pressure. Thereafter it was defibred in two steps. The resulting pulp was washed in order to remove solved constituents. The pulp was then conventionally bleached, with 2% hydrog¬ en peroxide, 0,1$ complexing agent (DDPA), 3% sodium silicate and about 1% sodium hydroxide. The pulp was then at low concentration (about 1% ) cleaned of splint¬ ers and dewatered to a concentration of 5% » After a partial washing the pulp was dewatered to about 9% con¬ centration. At this point the method according to the invention commences. The 9% pulp suspension was intens¬ ively mixed with 1% finely ground chalk, calculated on the dry weight of the pulp. The admixing of the chalk lasted about β seconds. The pulp was then continuously dewatered in a press, whereby a dry matter content of about 0 / 5 was achieved. The period between the admixing of chalk and the dewatering in the press lasted about two minutes. The solid pulp mat formed in the press was then again divided and fed into the driers. The drying time was about 20 seconds. A further break-up of the particles is effected by blowing the pulp at about 300 m/sec through a nozzle in a pipe, in which the pulp suspension flows at a rate of about 170 m/sec. Into this pipe a suspension of a hydrogen peroxide solution of about 50$ by weight in air was injected tangentially. The amount of hydrogen peroxide was about 1,8$, related to the weight of the dry pulp. The pulp suspension was thereafter transferred to cooling and drying air, the amount of which was about four times the weight of the pulp. In this cooling air about 0,8% by weight of magnesium oxide was suspended. This cooling and transporting air was then separated from the pulp in a cyclone. The pulp was pressed to bales and wrapped up. After storage of two days the pulp had a brightness of 84%, calculated after ISO. Compared therewith, a pulp produced in accurately the same way, but bleached only conventionally in aqueous phase, had abrightness of only 79% ISO. The pulp bleached according to the invention could after storage also be redefibrated more easily, compared with normally bleached pulp.

Claims: Claims 1. A method of bleaching fibre pulp for the production of paper or paperboard, comprising the steps of drying the pulp prior to bleaching to a water content of 3 to 30% by weight and using as bleaching agent an aqueous solution of hydrogen peroxide of at least 10% by weight, Q h a r a c t e r i z e d i n that the bleaching agent solution in the form of a suspension in air or water vapour is added to the pulp at that state of itβ manuf¬ acturing process when the pulp after its drying prevails as suspension in air or water vapour. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that a basic substance, which in addition to the hydrogen peroxide is required for the bleaching, is added to the pulp suspension in the form of a suspens¬ ion of the substance in air -or water vapour, which basic substance is a finely ground powder or an aqueous solution with a content of at least 25% by weight. 3. A method as defined in claim 1 or 2, c h a r a c t - e r i z e d i n that the bleaching agent suspension and the suspension of the basic substance are produced separately from each other and prior to their suspension in water vapour or air are not mixed with each other. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the said suspensions of the reagents are added separately from each other to the pulp in suspension, preferably in such a manner, that the predominant part of the basic suspens¬ ion is added to the pulp suspension first after the hydrogen peroxide suspension had been added to the pulp suspension. 5. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the hydrogen peroxide suspension prior to its adding to the pulp suspension is charged electrostatically with a charge, which effects electro¬ static attraction to the particles of the suspended pulp. 6. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the susp¬ ension of the base prior to its admixing to the pulp suspension is charged electrostatically with a charge, the sign of which is opposed to that of the charge of the hydrogen peroxide suspension. 7. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that as base a carbonate or bicarbonate of the sodium or ammonium is used. 8. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that a base is used, which in water of 20°C has a solubility of at maximum 0,015 g/100 g water. 9. Anet od as defined in one or more of the claims 1 " o β, c h a r a c t e r i z e d i n that a base is used, which in water of 20 C has a solubility of at max¬ imum 0,1 g/100 g water, and the dissolving rate of which in water due to the particle size or for other reasons is such, that at intensive mixing of 1 g of the base in 100 g of water of 80 C during 2 minutes the pH value of the resulting suspension does not exceed 9,5. 10. A method as defined in claim 9, c h a r a c t e r ¬ i z e d i n that as difficultly soluble bases finely ground powder of magnesium hydroxide, basic magnesium carbonates, magnesium oxide, magnesium carbonates, especially limestone, which by pre-treatment was made quick-soluble, or chalk with quick-solving properties are used. 11. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the base with respect to solubility, dissolving rate and/or grain size is chosen so, that at the mixing of 0,03 mole of the base with 100 g of mechanically processed pulp and 2000 g of water in a laboratory apparatus for redispers- ion according to Standard SCAN No 2:64 a pH value in the range of 7,5 to 9,0, preferably in the range of 7,8 to 8,5 is obtained. 12. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the base required for the bleaching is added entirely or partially to the pulp prior to its final dewatering and drying. 13. A method as defined in claim 12, c h a r a c t e r ¬ i z e d i n that the mixing of the base with the pulp is carried out at maximum 30 minutes prior to the drying of the pulp. 14. A method as defined in claims 11 and 12, c h a r ¬ a c t e r i z e d i n that the mixing of the base with the pulp by intensive stirring is carried out during at maximum 3 minutes, and the pulp during the admixing has a concentration in its suspension of at least 5% by weight. 15. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that a part, preferably about half of the necessary base, especially in the form of finely ground chalk, is added to the aqueous pulp suspension according to one or several of the claims 12,13 and 14, and that the remaining base preferably in the form of a basic magnesium compound is added as a powder suspension to the said suspension of the dried pulp in air or vapour. 16. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the dried pulp suspended in air or water vapour flows through a pipe at a rate of at least 50-m/s, especially at least 150 m/s, and that the bleaching agent suspension is fed into this pipe. 17. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that in the place where the bleaching agent suspension and/or the base suspension are added-to the pulp suspension, said latter has a concentration not exceeding 15 kg pulp per πr suspension medium, especially not exceeding 8 kg pulp/ m , preferably not exceeding 5 kg pulp/m . 18. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the pulp to be bleached is such, which was obtained with a yield of 80 to 95%, preferably 87 to 95%, related to the wood used as starting material and to substance free from water. 19. A method as defined in claim 18, c h a r a c t e r ¬ i z e d i n that the pulp entirely or at least to 40% by weight is produced_of hardwood, preferably birch- -tree, aspen or poplar. 20. A method as defined in at least one of the preceding claims, c h a r a c t e r i z e d i n that the pulp to be bleached has. been pre-bleached already in wet manner, at which pre-bleaching an aqueous suspension of the pulp is treated with bleaching agent.

Inventors: FLINCK KARL-EVERT, FLINCK, KARL-EVERT
Applicants: FLINCK KARL EVERT, FLINCK, KARL-EVERT
Application year: 1985
Main classification: D21C9
IPC codes: D21C9