SE526162C2 - Bleaching process, comprises oxygen delignification, chlorine dioxide bleaching, chelating agent treatment and peroxide bleaching steps - Google Patents

Abstract

The bleaching process is carried out at the same temperature and comprises an oxygen delignification step, a chlorine dioxide bleaching step, a chelating agent treatment step and a peroxide bleaching step. No washing of the pulp is carried out in between chlorine dioxide bleaching and treatment with the chelating agent. A process for treating bleached cellulose pulp comprises cooking the pulp and delignifying it with oxygen in at least one alkaline step, then washing it to obtain a pulp with a kappa value less than 15 and then bleaching in order to obtain pulp with the desired brightness. The entire bleaching sequence, including the oxygen delignification step, is carried out at a temperature greater than 90 deg.C. Final bleaching is carried out at a pulp consistency of 5-15 % in a sequence comprising the following steps : (i) transferring the washed pulp from the oxygen delignification step to a chlorine dioxide bleaching step, in which steam, chlorine dioxide and acid (for regulating pH) are added, the amount of chlorine dioxide corresponding to that needed to give a kappa value of 0.5-2.5, the amount of acid corresponding to that needed to give an initial pH of less than 5 and the amount of steam corresponding to that needed to give an initial temperature of 100 deg.C or more, the dwell time in this bleaching step being 60-200 (90-120) minutes ; (ii) treating the pulp directly, without any intermediate washing, with a chelating agent (preferably DTPA) and optionally an alkali to increase pH ; (iii) washing the pulp in a washing device (22) ; (iv) transferring the washed pulp to a pressurised peroxide bleaching step, in which alkali is added in an amount corresponding to that needed to give an initial pH of more than 9, peroxide is added in an amount of 5-25 kg/tonne pulp (kg/BDMT) and steam is added in an amount corresponding to that needed to give a temperature of at least 90 deg.C, the dwell time in this bleaching step being 30-200 (60-120) minutes ; and (v) washing the pulp in at least one washing device (32) to obtain a pulp with a brightness of more than 88 (90) % ISO.

Description

: # 40 20 30 526 162 Z In the production of fl uff pulp, the sequence DQ (PO) on softwood pulp is used in a mill in Sweden (Skutskär), which gives a brightness to the pulp produced at a level of 88% ISO. In this sequence, the usual technique for the chlorine dioxide step is applied, with temperatures around 60 ° C. This pulp, which is used for sanitary products such as diapers, napkins, etc., and since post-yellowing is not a significant problem in bleaching softwood pulp, the process is commercially viable. In another mill in Sweden (Skoghall), the sequence DQ (PO) was used on softwood pulp and the process was not driven as far as the pulp was used to produce kraft pulp with lower requirements for lightness in favor of strength (used for board production). In no use has the bleaching sequences DQ (PO) or (DQ) (PO) been used on oxygen-depleted leaf mass, as the leaf mass has a much greater tendency to post-yellowing.

Object and object of the invention The main object of the invention is to have a process for producing bleached cellulose pulp from a sulphate boiler, preferably with hard bleached leaf pulp, with as few process steps as possible, where the number of expensive washing machines is minimized, but where pulp with commercial brightness exceeds 90%. ISO can be obtained with good pulp quality, ie strength tensile index etc.

Another object is to obtain a bleaching process with a substantially constant temperature, where differences between the lowest and highest temparturi process are minimal, whereby the energy requirement can be significantly reduced relative to alternative processes where the temperature of the pulp after boiling is lowered more than 20-30 degrees, to then again raised 20-30 degrees.

Another purpose is to maintain a high efficiency in the included washing machines, which gives better process results or higher capacity in the process. Another object is that the brightness stability of the pulp, especially for hardwood pulp cooked in a sulphate boil, can be maintained high without having to implement special treatment steps to reduce post-yellowing.

List of drawings Figure 1, shows a complete treatment process according to the invention.

Detailed Description of Preferred Embodiments Figure 1 shows a complete set of devices for delignification and subsequent final bleaching.

The mass obtained after cooking and subsequent sieving is fed to a downcomer 1, where oxidized white liquor can be added (Ox. Wh. L).

PARTICIPATION (Oxygen I 02) The pulp is fed and pressurized to a first pressure level P1 with pump 2 to a first mixer 3 where oxygen is added. A first reaction zone with the residence time R1 is formed in a line loop 4, at a certain first pressure P1 and at a first temperature T1. These conditions, P1 / T1 / R1, establish optimal reaction conditions for the first delignition zone.

After this first delignification zone, the mass is fed and pressurized to a certain second pressure P2 with pump 5 to a second mixer 6 where oxygen and steam are added. A second reaction zone with extended residence time R2 is formed in a reactor 7, at a certain second pressure P2 and at a second temperature T2. These conditions, P2fl'2 / R2, establish optimal reaction conditions for the second delignition zone.

The delignment preferably takes place in accordance with the DUALOXTM system marketed by Kvaerner Pulping AB, which system corresponds to patent application SE, A, 9902586-8 (= EP, A, 1067237), and which is characterized by the quantities P2fl'2 / R2 all exceeding the conditions in the first delignification zone P1 / T1 / R1, ie that P2> P1_ T2> Tisamt also that R2> R1. 02§ $ 0se.doc 20 25 30 526 162 lll DUALOXTM is a further development of Kvaerners Pulping AB's developed two-reactor oxygen delignment with a pioneer patent in SE, C, 467582, which two-reactor technology has received extensive market acceptance and acceptance of other suppliers' subsystems.

This process technology in DUALOXTM differs from the current technology where the pressurization is established in the first zone and where the pressure in subsequent reactors is lower. In this way, an optimal delignification process is established that is adapted to the initial and final delignification process.

After the delignification in reactor 7, the pulp is fed to a ventilating precipice 8, where the pulp is relieved from a certain overpressure.

Within the recovery tank, of course, other types of delignment steps can also be used, for example also a reactor's oxygen depletion system, but where a mass with kappa numbers below 15 is obtained.

WASHING AFTER DELIGINATION After delignification, wash the mass. For best effect, two washing apparatus series were used here, where the first washing apparatus is a so-called pressure diffuser 10. This apparatus feeds the pulp into the top where the pulp is led in an annular flow down towards the outlet. Wash water is fed to the formed downflowing pulp bed from the outside, after which the filtrate is displaced radially inwards into the site. The pressurized wash means that a high temperature wash can be obtained while maintaining a high temperature of the pulp from the previous oxygen degeneration into the washing apparatus.

After the pressure diffuser, the pulp is led to a washing press 11, preferably but not necessarily a washing press of the COMPACT PRESS type, corresponding to patent SE, C, 512753 (= US6306259). Here, a washing extension is carried out with washing liquid and final dewatering by the mass laid on two counter-rotating drums being led via a converging sweep towards a final press nip between the drums where final watering takes place.

The combination with these two different washing machines gives an optimal washing of the pulp from a washing point of view, which is very important in this early position of the treatment of the pulp. Even if each washing machine corresponds to an investment of SEK 30-50 million ( just over 3-5 million Euros), this investment cost is usually taken home in the form of lower operating costs in the form of lower chemical investments later in the bleaching process.

CHLORINE DIOXIDE STEPS After the oxygen wash 10,11, the pulp is led to a precipice 15 for further feeding and pressurization, by means of a pump 16, to subsequent chlorine dioxide steps. The pulp pressurized from the pump is led to a mixer 17 where chlorine dioxide (ClO 2) is charged to the pulp. After this chemical mixer, the pulp is led to a steam mixer 18, where steam is supplied to the pulp in order to heat it to the required temperature.

After this charge of chlorine dioxide and steam, the pulp is led to an upstream reactor 19, where the pulp is fed in from below and then fed out to the top.

METAL BINDING (Q-Binding) In the final phase of the chlorine dioxide step, which takes place under acidic conditions which are favorable for the release of metals from the pulp, chelating agents, here in the form of DTPA, are charged to the pulp trap receiving the pulp from the previous chlorine dioxide treatment in the reactor 19.

Since not all metals are harmful to subsequent peroxide steps, so-called useful metals such as calcium and magnesium (which reprecipitate on the pulp at pH above 5), an alkali additive (NaOH) can be added in connection with the addition of the chelating agent.

The appropriate pH which is thereby established to retain the useful metals must then be in the range of pH 5-7.

WASHING AFTER CHLORINE DIOXIDE (CHELATION / Q-treatment) The mass present in the precipitate 20, with its addition of chelating agent and alkali, is then fed with a pump 21 to a washing apparatus 22, which corresponds to the previously described washing apparatus 11.

PEROXIDE STEP 0230se.doc 0 0 I IOII C000 QIO 20 30 526 1s2 C; - After the chlorine dioxide wash 22, the mass is led to a precipice 25 where peroxide (H2O2) is charged. From the precipice 25, the pulp is fed and pressurized with a pump 26 to mixer 27. In this mixer, steam (MP-Steam) and oxygen (oxygen) are charged.

The pulp to which peroxide, steam and oxygen have been added is then passed to a subsequent peroxide stage in the form of an upstream reactor 29, where the pulp is fed in from below and then fed out to the top.

WASHING AFTER PEROXID After the peroxide step 29, the pulp is fed to a ventilating precipice 30 where overpressure on the pulp is vented. After the fall 30, the pulp is fed with a pump 31 to a final washing apparatus 32, which corresponds to the previously described washing apparatuses 11 and 22. The pulp out of the washing apparatus 32 gives the finished bleached pulp.

PROCESS LIQUID FLOWS Pure process water is led in a strict countercurrent, relatively mass flow, through the process.

The pure process water 40 (White Water) is taken into the process as washing liquid until the last wash 33. The filtrate 41 from the wash 33, which is used, is in turn led via the tank 60, partly to the precipice 30 as diluent and partly to the laundry 11 as washing liquid.

For the intermediate wash 22, hot water / condensate 42 is added to the process, which is normally available at the pulp mill. The filtrate 43 from the wash 22, which contains released metals / chelate, goes via tank 61 partly to the precipitate 20 as diluent and partly via heat exchanger 62 to the pulp before the precipitate 15 as diluent. In the washing position after the chlorine dioxide step where metals are washed out of the pulp in the washing apparatus 22, the process allows a limited bleeding of metal-rich fl itrate into drains (or special destruction systems).

The bleeding of metal-rich filtrate 44 from the filtrate 43 of the washing apparatus 22 can be limited to a volume corresponding to <5 ma / ton mass. 0230sc.doc 20 30 526 162 5 * The filtrate from the wash 11 is passed via tank 63 to the wash 10 as washing liquid, and the filtrate from the wash 10 is then passed in countercurrent in the process to the previous boiling (not shown) in a manner known per se.

PROCESS CONDITIONS The process conditions used in Figure 1 and which are necessary for the bleaching allow full brightness, so-called "90 pulp", to be obtained from a sulphate-cooked pulp which mainly consists of leaf pulp, ie. that more than 85% of the pulp consists of leaf pulp, which pulp has subsequently been oxygen delignified in at least one alkaline oxygen step, the pulp after the oxygen delignment being washed to obtain a pulp with a kappa number below 15.0 before the recoverable bleaching takes over to obtain pulp. desired brightness.

The hardwood pulp is more prone to yellowing than softwood pulp.

The process is adapted and assumes that the whole final bleaching takes place at a pulp consistency in the range 5-15% in a bleaching sequence called (DQ) * (PO), where the chelation Q is an option, and "*" denotes high temperature above 85 ° C or preferably above 90 ° C. The bleaching sequence can thus in its simplest form be called D * (PO). led the first bleaching step in the form of a chlorine dioxide step D * steam, chlorine dioxide and acid are added when pH correction is needed. The amount of chlorine dioxide invested must correspond to a kappa factor in the range 0.5-2.5. Acid is added in an amount that results in an initial pH below 4.0. Steam is added in an amount which results in a temperature of at least 90 ° C, preferably 100 ° C or more. The residence time in D * is in the range of 60-200 minutes, preferably 90-120 minutes. In this way a very hard driven chlorine dioxide step is obtained which at the same time gives a breakdown / release of the constituents in the pulp which otherwise cause post-yellowing, and where low levels of AOX in filtrate are obtained.

After D *, the mass is washed in a washing machine.

The washed pulp is then transferred to a pressurized peroxide step (PO), with at least 2 bar overpressure preferably 5-20 bar overpressure, in which (PO) step alkali is added in an amount resulting in an initial pH above 9.0, typically pH 10.5 -12.0. Peroxide is added to the (PO) step in an amount in the range of 5-25 kg / l 526 162 tons of pulp (kg / BDMT). Steam together with oxygen / OZ is added in an amount which results in a temperature of at least 90 ° C, and preferably above 100 ° C and at most up to 140 ° C. The oxygen is added in an appropriate amount in the range of 2-20 kg / BDMT.

The residence time in this peroxide step (PO) is in the range of 30-200 minutes, preferably 60-120 minutes.

After the peroxide step (PO), the pulp is washed with at least one washing apparatus to obtain a finished bleached pulp with the desired brightness. The washing apparatus can preferably be a washing press, or a diffuser wash where the diffuser wash is arranged at the top of a storage tower, where the storage tower can have a 20-180 minute residence time in atmospheric conditions (non-pressurized storage in addition to static pressure in the tower).

In a preferred embodiment of the process, the charge of chlorine dioxide in the D * step corresponds to a kappa factor in the range 1.0-2.0. The amount of chlorine dioxide charged, as well as the acid required for pH correction, corresponds to an amount of chlorine dioxide in the range of 10-20 kg per tonne of pulp (kg / BDMT), preferably about 1511.0 kg per tonne of pulp (kg / BDMT).

When bleaching pulps with a high metal content, or in processes where these metals have not been dissolved in any other way (eg by acid pre-bleaching steps, etc.), the pulp is treated after the chlorine dioxide treatment without intermediate washing with complexing agents, preferably DTPA, and if necessary alkali addition to raise pH , followed by washing of the mass. Alkalite is then added in an amount which results in a pH in the range 5-7.

In an advantageous implementation, the entire bleaching sequence takes place, including at least the last step in a previous two-reactor oxygen degeneration at a temperature above 90 ° C. Suitably the last (PO) step can take place at t00 ° C, the previous (DQ) * / D * at 95i5 ° C and the last step in the previous oxygen delignification at 90i5 ° C. The last wash liquid supplied to the last wash 32 is slightly colder, which gives a certain temperature drop on the obtained filtrate from the wash 32 which is then taken to the previous step (in countercurrent to the mass flow). However, the other hot water condensate which is added in earlier steps, for example as washing liquid in the wash 22, is high temperature around 80-95 ° C, and does not give a greater degree of cooling. Current washing steps can be run at a high temperature, which provides good washing of the mass.

The finished bleached pulp from the (DQ) * (PO) or D * (PO) sequence according to the invention can with these conditions obtain a brightness of at least 85% ISO, preferably above 88% ISO, and even more advantageously 90% ISO, starting from a leaf mass boiled in sulphate boiling with accompanying oxygen delignification down to kappa below 15.

COMPARATIVE EXAMPLES As a starting point for the comparison of the inventive (DQ) * (PO) sequence, a short variant of conventional DEDED bleaching sequence was used in the form of a DED bleaching sequence, which is driven so hard that only three steps are required with intermediate washing and where the extraction step is enhanced by oxygen, thus a D (EO) D.

The reference mass used throughout the comparison is a eucalyptus mass (leaf mass) which is first boiled in a sulphate boiling to a kappa number of 15.2 and viscosity of 1220 dma / kg and then delignified with oxygen down to a kappa number of 9.9 a viscosity of 941 dina / kg.

The process is applied to pulp which has first been boiled in a sulphate boil and then oxygen-delignified to kappa numbers below 10, from which pulp a final bleached "90 pulp" can be obtained.

For the D (EO) D sequence, the following conditions apply; .kg Investment H2SO4, kg / BDMT of active chlorine, UI 0230sc.doc 00 00 0 OOIIOOO 0 0 000 0000 0 0 0 0000 0000 000 526 162/0 Investment MgSO4, kg / BDMT Temp, “C min The total investment of active chlorine in the sequence D (EO) D as above becomes then; 2 3 CIÖzi 15.5 total For the (DQ) * (PO) sequence the following conditions apply; D 'ANDE Q 10 NaOH.

Activation active chlorine, Initiation H2SO4, kg / BDMT BV, Test 2 Test 3 1 Temp, “C 1 00. min, bar Investment MgSO4, kg / BDMT 0230sc.doc 526 162 / l To determine the brightness after forced yellowing, a treatment of sheets formed by the pulp was used, where the sheets are dried in an oven at 105 ° C for three hours and then allowed to cool in a conditioning room (92% humidity).

The total charge of active chlorine and peroxide in the sequence (DQ) * (PO) as above will then be; Conclusions of comparative tests With an inventive method of bleaching according to (DQ) * (PO), essentially the same brightness of the pulp can be obtained as a hard-driven DED bleaching sequence, which brightness is fully accepted for a commercially acceptable "90 pulp". "90 pulp" is a term for a conventional pulp grade that has a lower acceptance limit at 88 (sometimes 89)% ISO.

Bleach sequences according to DEDED are the dominant bleach sequence used.

This is obtained with a (DQ) * (PO) sequence with only two steps, unlike the D (EO) D sequence which requires three steps at an additional cost for investment of approximately SEK 40-50 million (approximately 5-6 million Euro) and associated increased operating costs for the laundry. Each additional wash requires the installation of a washing machine and associated equipment.

The pulp bleached by the (DQ) * (PO) sequence becomes at least as resistant to post-yellowing, with only about two% 100 units being lost in forced post-yellowing according to the standard procedure described above. 0230en.doc 526 162/1 The amount of chlorine dioxide used in the bleaching can be radically reduced, by about 30-50% for directly compatible pulps with corresponding brightness, which reduces any load with AOX in the filtrate. The invention can be used to advantage in a sequence where both the at least the last (or only) reactor in the oxygen delignification and the subsequent (DQ) * (PO) sequence take place at substantially the same temperature at or exceeding 90 ° C.

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Claims (1)

A process for the production of bleached cellulosic pulp comprising more than 85% of hardwood pulp, which assumes that the pulp is first boiled in a sulphate boil and oxygen-delignified in at least one alkaline oxygen step, the pulp after oxygenation being delignified. to obtain a pulp with a kappa number below 15.0 before bleaching takes over to obtain bleached pulp with desired brightness characterized in that the whole bleaching sequence including previous oxygen delignification takes place at a temperature above 90 ° C - that the whole final bleaching takes place at a pulp consistency in the range 5-15% in the following sequence; after the oxygen delignment, the washed mass is transferred to a chlorine dioxide step to which steam, chlorine dioxide and acid are added when pH correction is needed, where the amount of chlorine dioxide means an investment corresponding to a kappa factor in the range 0.5-2.5, acid in an amount resulting in a starting pH below 4.0, and steam in an amount which results in a temperature of 100 ° C or more, and where the residence time in this chlorine dioxide step is in the range of 60-200 minutes, preferably 90-120 minutes, -that the mass after the chlorine dioxide treatment without intermediate wash is treated with complexing agent, preferably DTPA, and if necessary alkali addition to raise the pH, -after which the pulp is washed in a washing apparatus, -after which the washed pulp is transferred to a pressurized peroxide step, in which alkali is added in an amount resulting in an initial pH over 9.0, peroxide is added in an amount in the range of 5-25 kg tons of pulp (kg / BDMT), and steam in an amount which results in a temperature of min 90 ° C, and where the residence time in this peroxide step is in the range 30-200 minutes, preferably 60-120 minutes, after which the pulp is washed with at least one washing apparatus to obtain a finished bleached pulp with a brightness above 88% ISO, and even more advantageously 90% ISO. 0230se4.doc 20 25 30 526 162 11 /. Process according to Claim 1, characterized in that the amount of chlorine dioxide in the chlorine dioxide treatment involves an investment corresponding to a kappa factor in the range 1.0-2.0. . Process according to claim 3, characterized in that in the treatment of complexing agents alkali is added in an amount which results in a pH in the range 5-7. . Process according to Claim 1, characterized in that the amount of chlorine dioxide charged, as well as the acid required for pH correction, corresponds to an amount of chlorine dioxide in the range of 10-20 kg per tonne of pulp (kg / BDMT), preferably about 15-_H, 0 kg per tonne of mass (kg / BDMT). . Process according to any one of the preceding claims, characterized in that the bleaching after the oxygen delignification to finished bleached pulp takes place in only two steps, with only two intermediate washing steps, where a washing step is arranged after the complexing treatment and a washing step arranged after the peroxide treatment. Process according to Claim 5, characterized in that the entire delignification process and the bleaching after cooking, ie oxygen delignification and final bleaching, only involve three washing machines. Process according to claim 5, characterized in that the entire delignification process and the bleaching after cooking, ie oxygen delignification and final bleaching, only comprises four washing machines, where two of these washing machines are arranged in series with or without intermediate storage towers, after the oxygen delignment. . Process according to one of Claims 5 to 7, characterized in that a maximum of 5 m3 of filtrate per tonne of pulp is taken from the bleaching to sewage, which is taken out of the filtrate after treatment with complex bidder. 0230se4.doc